Shah, Afroz (2010) Tectono-metamorphic evolution of Big … · 2011. 2. 11. · Afroz Ahmad Shah July 2010 iii . PhD Thesis Shah A.A. ELECTRONIC COpy DECLARATION 1, the undersigned,

This file is part of the following reference:

Shah, Afroz (2010) Tectono-metamorphic evolution of Big Thompson Canyon region, Colorado Rocky Mountains, USA.

PhD thesis, James Cook University.

Access to this file is available from:

http://eprints.jcu.edu.au/11911

http://eprints.jcu.edu.au/11911

Tectono-metamorphic evolution of Big Thompson Canyon region,

Colorado Rocky Mountains, USA

Volume I

Thesis submitted by

Afi'oz Ahmad SHAH MTech. (Civil Engg.) IIT Kanpur India

In July 2010

for the degree of Doctor of Philosophy in the School of Earth Sciences James Cook University

PhD Thesis Shah A.A.

CONTENT OF VOLUME 1

STATEMENT OF ACCESS ................... " ............................................................................ ii

STATEMENT OF SOURCES ............................................................................................ iii

ELECTRONIC COPY ......................................................................................................... iv

ACKNOWLEDGMENTS .............................. , ...................................................................... v

INTRODUCTION AND THESIS OUTLINE ................................................................... vii

- SECTION A -................................................................................................................ 1-32

Isogmd migmtion with time during orogetlesis.

-SECTION B -................................................................................................................. 1-32

Pressure-Tempemtllre-time-DefoI'nUltion (PTtD) paths derived from FIAs, P-T

pselldosectiolls allli ZOlled gal'llets

-SECTIONC- .............................................................................................................. 1-33

90 million years of orogenesis, 250 millioll yem's of quiescence and thell further orogenesis with no c//(/1/ge ill PT: significance for the role of deformation ill pOlphyroblast growth

- SECTION D - .............................................................................................................. 1-20

The problem, sigllificallce, allli metamorphic implications of 90 millioll years of

polyphase staurolite growth

CONCLUSIONS ............................................................................................................... 1-7


STATEMENT OF SOURCES

DECLARATION

I declare that this thesis is my own work and has not been submitted in any form for

another degree or diploma at any university or other institution of tertiGlY education.

Information derived ji-om the published and unpublished work of others has been

acknowledged in the text and a list of references is given

Afroz Ahmad Shah July 2010

iii


ELECTRONIC COpy

DECLARATION

1, the undersigned, the author of this thesis, declare that the electronic copy of this thesis

provided to the James Cook University Libra/Y is an accurate copy of the print thesis

submitted, within the limits of the technology available.

Afroz Ahmad Shah July 2010

iv


ACKNOWLEDGMENTS

I must stmi by thanking Professor Tim Bell for his brilliant supervision skills

peppered with the effervescent humour, which keeps the research at SAMRI going. Tim

has been instrumental in changing my ways of understanding structural and metamorphic

geology. The research conditions within the SAMRI group at James Cook University are

par-excellence. This international group has more than just research to offer! Social,

intellectual and group culture is what most of the other research institutes around the world

lack and I was lucky enough to get all of it here. This project was funded by International

Earth Science Scholarship (IESS) from the School of Emih and Environmental Sciences at

James Cook University.

The staff at the Advanced Analytical Centre at James Cook University provided

superb technical assistance in data collection. In particular, I would like to thank Dr Kevin

Blake for his support on the electron microprobe. and Darren Richardson for his help

during thin section preparation.

Special thanks Dr. Mike Rubenach for help with monazite dating and for endless

discussions about metamorphic petrology and many other aspects of geology. I sincerely

thank all my colleagues in SAMRI for providing an intellectual, social and friendly

atmosphere, enriched with stimulating ideas on just about anything under the sun! I will

surely miss them all, thanks tons! I wish to express my gratitude to the following, who in

their various ways provided warmth, assistance, inspiration, andlor ideas: Ahmed Abu

Sharib, Asghar Ali, Clement Fay, Chris Fletcher, Hui Cao, loan, Jyotindra Sapkota, Matt

Bruce, Mark Rieuwers, Mark Munrooh, Raphael Quentin de Gromard and Shyam Ghimire.

v


My sincere thanks to Ashraf Bhat, Firdoos, Jaiby, Jainky, Julia Miller, Lubna, Oana,

Quidsiya, Rafia, Raj, Shrema and Tajamul Bhat for their encouragement throughout this

research.

Special thanks to Elizaveta Moiseeva for her help during my presentation which

greatly improved a pre-completion talk at JCU. Her help at Beijing (China) for my

presentation is greatly acknowledged.

I am much thankful to Prof. IN. Malik (lIT Kanpur, India), for consistent

encouragements throughout this research.

I wish to acknowledge with love and thanks the support and patience of my parents,

Hafiza Akhtar and Mohammad Naseem, my brothers Gowher Naseem Shah and Bilal

Ahmad Shah and my adorable sister Sabiya Naseem for all their encouragement

throughout my stay in Australia. I must thank all my cousins, especially Aasim, Mohsina,

Mugeesa Qudsia and Sahreena and everybody directly or indirectly related to this research.

In the end, I must thank Sheeba, for her encouragement and discussions that have greatly

improved my ways of expression. Thanks for being such a great motivation during the

crucial stage of this work.

vi


INTRODUCTION AND THESIS OUTLINE

The geologically continuous nature of relative plate motion, which drives

orogenesis, versus the limited number of deformations preserved in the matrix of rocks,

has always been an anomaly that requires explanation. Porphyroblast studies suggest that a

large amount of orogenic history has been destroyed in the matrix and may reveal that

deformation and metamorphism is continuous, although partitioned both spatially and

temporally, for periods of a few 100 million years (e.g., Bell et aI., 2004; Bell & Newman,

2006). Reactivation (shearing) of the bedding or compositional layering in successive

deformations destroys previously developed foliations in the matrix. However,

porphyroblast growth prior to reactivation generally preserves these foliations, plus the

metamotphic histoty, from destruction. If the P-T and bulk composition conditions are

suitable for porphyroblast growth, this history is generally trapped within porphyroblasts,

which appear to grow during regional metamorphism only where the deformation occurs

(e.g. Bell and Rubenach, 1983; Bell and Johnson, 1989; Bell and Hayward, 1991, Cihan

2004; Cihan and Parsons; 2005).

Foliation intersection axes preserved within porphyroblasts (FlAs) enable linkage

of the structural and metamorphic histories from sample to regional scales through

consistent changes in their trend from the core to median to rim (e.g., Bell et aI., 1998,

1981). Significantly, regionally consistent successions ofFIA trends reflect changes in the

direction of bulk shotiening, which must in tum reflect changes in the direction of relative

vii


plate motion. The period of time over which each FIA trend developed has been dated by

electron microprobe and ion microprobe techniques using U-Th-Pb in monazite inclusions,

correlated directly with shear sense from inclusion trail asymmetry, as well as with phases

of porphyroblast growth.

Combining this data with the construction of P-T pseudosections (phase diagrams

specific to a palticular sample's bulk composition that provide a map of stable mineral

assemblages in P-T space) and thermobarometric calculations using THERMO CALC

allows P-T-t-d paths to be determined and correlated from sample to sample and region to

region. Thus quantitative integration of metamorphism, structure and tectonics within an

orogenic belt has become an exciting possibility. This contribution provides an example of

such an integrated approach using microstructural, F1As, thermodynamic modelling

(MnNCKFMASH) and chemical relationships to investigate in detail the metamorphic and

deformation processes and the progression in the distribution of various index mineral

phases (garnet, staurolite, andalusite and cordierite). The Big Thompson Canyon region,

Colorado Rocky Mountains, USA forms patt of a Proterozoic orogenic belt in the

southwestern United States and provides a classic example ofa large high-T-low-P terrane

and the problems associated with the tectonic interpretation of such a regime (e.g.,

Williams and Karlstrom, 1996). These rocks have been affected by strongly partitioned

deformation and provide an ideal place for developing new concepts on the role of

deformation pattitioning in deformation, metamorphism and tectonism.

This thesis consists of four sections (A-D), with each written in the matmer of

papers for submission to journals, although it is not intended that section B be submitted in

its present form. A palt of section A has been published in the Acta Geologica Sinica and

viii


Section C has been submitted to the Precambrian research. Section D has been submitted

to the Journal of Geological Society of India and was co-authored by Dr. loan Sanislav and

myself, and we both agree that conceptually, literally, and graphically we each contributed

50% to that paper. Volume I contains the paper text and Volume II contains the

accompanying tables, figures, and appendices.

SECTION A

This section explains the FIA (foliation intersection/inflection axes preserved

within porphyroblasts) technique and its application to all samples are used in this research.

FIAs were measured in garnet, staurolite, andalusite and cordierite porphyroblasts

preserving older foliations as inclusions trail microstructures. Their relative timing was

established based on microstructural and textural relationships. A progression of FIAs

trending successively NE-SW, E-W, SE-NW and NNE-SSW reveals four periods of

staurolite and garnet growth and two growth phases each of cordierite and andalusite.

Isograds associated with each of these FIAs shift progressively across the orogen. This

migration took place relative to a heat source to the WNW. Early formed staurolite was

altered to cordierite and andalusite during development of the last 2 of these FIA sets

(trending SE-NW and NNE-SSW). The lack of relationship of these isograds to pluton

boundaries to the north and south suggests that these igneous rocks were not the primary

heat source for metamorphism. The youngest of these isograds lie sub-parallel to those

mapped by previous workers

SECTIONB

ix


This patt of the thesis involves the use of microstructural and FlA evidence to track

down the overall prograde mineral succession in these metasediments. It suggests that

garnet, staurolite, andalusite and cordierite porphyroblasts grew in an overall prograde path,

where the growth of garnet was always followed by the formation of staurolite for each

FlA. For the last 2 periods of FlA development the growth of staurolite was also followed

by the development of andalusite and cordierite. Inclusions of earlier minerals within the

younger phases SUppOlt the porphyroblastic mineral sequence obtained through FIAs.

Thermodynamic modeling in the MnNCKFMASH system reveals that the episodic

growth of these phases occurred over a similar bulk compositional range and PT path for

each FIA in the succession. The intersection of Ca, Mn, and Fe isopleths in garnet cores for

3 samples, containing FIA set 1, set 2 and set 3, trending NE-SW, E-W and SE-NW

respectively, indicate that these rocks never got above 4kbars throughout the Colorado

Orogeny. They remained around the same depth until the onset of younger orogeny at

~ 1400 Ma, when the pressure decreased slightly as porphyroblasts formed with inclusion

trails preserving FIA set 4 and trending NNE-SSW. A slightly clockwise P-T path

occurred for both orogenies.

SECTIONC

In this section in-situ EPMA (electron probe microanalysis) dating of monazite

grains preserved as inclusions within foliations defining FlAs contained within garnet,

staurolite, andalusite and cordierite porphyroblasts is described, interpreted and explained.

Monazite grains contained within matrix foliations were also analysed and compared with

the dates obtained from porphyroblasts where the FIAs were used to determine the relative

x


age succession. This data have provided a chronology of ages for extended periods of

deformation and metamorphism in the region. Garnet and staurolite formed episodically

for a period of -1 00 Ma during the Colorado orogeny, ceased and then recommenced -300

million years later during the Belthoud Orogeny

SECTIOND

This part of thesis compares two spatially and temporally separated terranes with

similar histories of porphyroblast development and associated granite emplacement at

similar levels in the crust. These areas are a Paleozoic portion of West Central Maine and

the Proterozoic Colorado Front Range, where multiple periods of staurolite growth

occurred over a lengthy period of time. The metastable preservation of early staurolite

porphyroblasts through successive deformation and metamorphic events highlights the

impOltance of deformation on porphyroblast nucleation and growth. These two areas show

that the reactions involved in the formation of metamorphic minerals are episodic during

prograde metamorphism, starting or stopping, as a function of deformation pattitioning and

strain localization. Accessory minerals such as monazite can be dated and their ages can be

better integrated in the deformation and metamorphic history of a region when they are

well constrained by microstructural measurements. The similarities between the two

regions indicate that such processes may be common for most of the orogenic belts.

Xl


References

Bell, T. H., and Hayward, N., 1991. Episodic metamorphic reactions during orogenesis:

the control of deformation partitioning on reaction sites and reaction duration.

Journal of Metamorphic Geology, 9, 619-640.

Bell, T. H., and Johnson, S. E., 1989. Porphyroblast inclusion trails: the key to orogenesis.

Journal of Metamorphic Geology, 7, 279-310.

Bell, T. H., and Newman, R., 2006. Appalachian orogenesis: the role of repeated

gravitational collapse. In: Butler, R., Mazzoli, S. (Eds.), Styles of Continental

Compression. Special Papers of the Geological Society of America, 414, 95-118.

Bell, T. H., and Rubenach, M. J., 1983. Sequential porphyroblast growth and crenulation

cleavege developement during progressive deformation. Tectonophysics, 92, 171-

194.

Bell, T. H., Ham, A. P., and Kim, H. S., 2004. Partitioning of deformation along an orogen

and its effects on porphyroblast growth during orogenesis. Journal of Structural

Geology, 26, 825-845.

Bell, T.H., 1981. Foliation development: the contribution, geometry and significance of

progressive bulk inhomogeneous shortening. Tectonophysics, 75, 273-296.

Bell T.H., Hickey, K.A, Upton, 0.1.0., 1998. Distinguishing and correlating multiple

phases of metamorphism across a multiply deformed region using the axes of spiral,

staircase and sigmoidally curved inclusion trails in garnet. Journal of Metamorphic

Oeology 16, 767-794.

xii


Cihan, M., 2004. The drawbacks of sectioning rocks relative to fabricorientations in the

matrix: a case study from the Robertson River Metamorphics (North Queensland,

Australia). Journal o/Structural Geology, 26, 2157-2174.

Cihan, M., and Parsons, A. 2005, The use of porphyroblasts to resolve the history of

macro-scale structures: An example from the Robertson River Metamorphics, north

eastern Australia: Journal o/Structural Geology, 27 p. 1027- 1045

Williams, M.L., Karlstrom, K.E., 1996. Looping P-T paths, high-T, low-P middle crustal

metamorphism: Proterozoic evolution of the southwestern United States. Geology 24,

1119-1122.

xiii

Section A

ISOGRAD MIGRATION WITH TIME DURING OROGENESIS

Isogl'ad migration with time during orogenesis ..... SHAHA.A

Isograd migration with time during orogenesis

1.0. ABSTRACT ............................................................................................................................. 1

2.0. INTRODUCTION .................................................................................................................. 2

3.0. GENERAL GEOLOGY AND TECTONICS OF THE REGION .............................. 4

4.0. FOLIATIONS ......................................................................................................................... 6

5.0. METHODS .............................................................................................................................. 6 5.1. FIA measurements ..................................................................................... , ................ , ................... 6

6.0. RESULTS .................................................................................................................................. 7 6.1. Determination of multiple FlAs for relative timtng .............................................................. 8

7.0. INTERPRETATION OF THE FIA SUCCESSION ............................................................... 9

8.0. EVIDENCE OF EARLIER MINERAL INCLUSIONS WITHIN YOUNGER PHASES .10 8.1. Inclusions of garnet within staurolite, andalusite alUl cordierite .......................................... 10 8.2. Replacement of staurolite by andalusite ................................................................ , ........ , ........... 11 8,3, Replacement of staurolite by cordierite .. , .............. , ...................................................... , .. , ...... , .. ,,11 8.4, Replacement of andalusite by cordierite ' .................. , ................................................................ , 11

9.0. DATING OF FIAS AND THE SUCCESSION CONFIRMATION ......................... 12

10.0. FIAS DERIVED ISOGRADS ............. , ........................... " .. , .... , ......... , .......... , .............. , .. 12 10'], Staurolite isogr(f(ls for F1As 1, 2, 3 and 4 ................................................................................. 12 10.2, AlUlalusite alU/ cordierite isogmds ...................................................... , .... , ................................. 12

11.0. LATE ANDALUSITE AND CORDIERITE GROWTH ......................................... 13

12.0. DISTRIBUTION OF SILLIMANITE ...... " ....................... " ....... , ................ , ....... " ....... 13

13.0. AXIAL PLANE TRENDS OF REGIONAL FOLDS ................................................ 14

14.0. INTERPRETATION ........ , ................... , ........... ,,, ... , .. , ............. , ............................. " .... , ..... 14 14.1. Porphyroblast rotation or otherwise, ...... " ................ " .......... , .... ,,, ............ , ............................ " .. 14 14,2, Staurolite Isograds ",,"""""""'''''''''' '" """",,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, "" .. """""",,,,,,,,,,,,,,, """"" "" 15 14,3. Alii/all/site isogradfor FIAs 3 alll/4 """,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, .. ,,,,,,,,,,,,,,,,,,,,,,,,,,,,, .. ,,,,,,,,,, 15 14,4. Cordierite isogradsfor FIAs 3 and 4."""""""""""""""""""""""""""""""""""""""""""" 15 14,5. Rare matrix overprinting cordierite alii/ (lJulalusite """""""".""""""""""""""""""""""" 16 14,6, Sillimaflite"" """""'''''''''''''' """ """"", """'"'''''''''''''''''' """"".,,"" ", """""""''',,'''''',, .. ,,'' ",,,.,," 16 14,7. Relationship between FIA trends alii/ axial traces of folds """""""""""""""""" ... """""" 16 14,8, Foliatiofl trend alii/ Heat Flux """"""""",,,,,,,,,,,,,,,,,,,,,,,,.,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, 17

15.0. Discussion .......................................................................................................................... 18 15,1. Multiple phases of staurolite growth """"""""""""'" """""""""""",,",,""""",,"", ""'" 18 15,2. Multiple phases of andalusite and cordierite growth""""""""""""""""""."""""", 19

i

Isow'ad mif!ration with time during orogenesis ..... SHAHA.A

15.3. Comparison with earlier work ............................................................................................. 20 15.4. Deformation Partitioning ...................................................................................................... 20 15.5. Relevance to granite emplacement .................................................................................... 21 15.6. Significance ofthe shift in the isograd fl'Om FIA to FIA ............................................... 22 Acknowledgements ................................................................................................................................ 23

16.0 REFERENCES .................................................................................................................... 24

ii

Isograd migratioll with time during orogenesis ..... SHAHA.A

1.0. ABSTRACT

A progression of FIAs (foliation intersection/inflection axes preserved within

porphyroblasts) in the foothills of the Colorado Rocky Mountains reveals four periods of

staurolite growth and two growth phases each of cordierite and andalusite; these FIA based

isograds migrated ~2.5 kms across the orogen. This progression of mineral development

occurred about FIAs trending successively NE-SW, E-W, SE-NW and NNE-SSW.

Granitoids were emplaced during both periods of orogenesis in the surrounding region but

have no direct relationship to the isograds. Isograd migration took place away from a heat

source to the WNW and combined with the lack of relationship to pluton boundaries to the

north and south suggests that the latter rocks were not the heat source for metamorphism

but rather product of it. A final period of andalusite, cordierite and fibrolitic sillimanite

grew over the matrix foliation and consequently, no FIA was determined for it; the isograds

for this last period of mineral growth lie sub-parallel to those mapped by previous workers.

A strong correlation between the distribution of FIA trends and the axial trace of all

folds present in the area suggests that pockets of low strain occur due to deformation

partitioning, in spite of, or perhaps because of, the 4 successive changes in bulk shortening

direction. These pockets preserve folds in the orientation in which they formed from

subsequent rotation or destruction. They provide remarkable confirmation of the veracity of

the FIA data and indeed reveal that such data can be used to determine successions of fold

development from regional maps at which scale many overprinting criteria cannot be

applied.

Al

Isograd migration with time during orogenesis ..... SHAHA.A

2.0. INTRODUCTION

The stlUctural aud metamorphic evolution of low-pressure, high-temperature

Proterozoic rocks in the Colorado frontal ranges, which contain an abundance of granites, is

similar in character to other orogenic belts around the world. Such belts include the eastern

Lachlan Fold Belt (Collins and Vernon, 1992), the Pyrenees (Mezger and Passchier, 2003)

and a granulite belt in the Ordovician of central Australia (Hand et aI., 1999). A number of

mechanisms have been proposed for the origin of such settings including compression of

previously extended continental crust (Etheridge et aI., 1987; Oliver et aI., 1991), mantle

delamination (Loosveld and Etheridge, 1990), heat advection via intrusions due to high

mantle heat flow (Vernon et aI., 1990; Rubenach, 1992; Sandiford et aI., 1995; Rubenach

and Bakel', 1998), radiogenic heating due to emichment of heat-forming elements in the

upper crust (Mildren and Sandiford, 1995; Sandiford et aI., 1998; McLaren et aI., 1999);

and the burial of heat-producing stratigraphic sequences (Hand and Rubatto, 2002).

The relationship between metamorphism and granite emplacement has always been

somewhat of an enigma (e.g. Scaillet 1995; Brown and Dallmeyer, 1996). The question of

which comes first has always been a point of debate and this, to some extent still remains

umesolved, although many would favour that the heat generated by orogenesis eventually

promotes melting and granite emplacement (Brown 1994a, 2001; Solar et aI., 1998; Hutton

1997; Brown and Solar, 1998). The relationship between isograds and igneous bodies is

one approach that has been used to try and determine their inter-relationship. Previously,

single isograds have been determined for a particular mineral species because quantitative

distinctions in the timing of mineral growth from sample to sample within a particular

A2

Isograd mif!ratioll with lillie during orogenesis ..... SHAHA.A

mineral phase have not been possible. With the advent of techniques for the quantitative

measurement of inclusion trails within porphyroblasts, different periods of growth of a

porphyroblastic phase can be identified and confidently correlated across a region (Bell et

a!., 1998; Bell and Mares, 1999). Consequently, one can potentially distinguish isograds for

different periods of growth of a single mineral phase.

The area described herein forms part of a Proterozoic orogenic belt in the

southwestern United States and provides a classic example of a large high-T-low-P terrane

and the problems associated with the tectonic interpretation of such regimes (Williams and

Karlstrom, 1996). The only indication of deformation events that occur before the peak of

metamorphism, and how they affected the thermal structure of an orogenic belt, comes

from combined quantitative micro and macro structural studies. Microstructural studies are

also important in distinguishing between events at the peak of metamorphism and those

significantly post-dating it (Thompson and Ridley, 1987). This research has used the

combined approach of integrating geochronological, metamorphic and structural studies

(using FlAs).

The technique used requires the measurement of the orientation of foliation

inflection or intersection axes preserved by inclusion trails within porphyroblasts, which

are called FIAs. The measurement of these structures within garnet, staurolite andalusite

and cordierite porphyroblasts from the Colorado foothills of the Rocky Mountains has

revealed a succession of 4 FIAs within these rocks. This has enabled examination of

whether or not the distribution of staurolite, cordierite and andalusite isograds changed with

time. That is, whether they shifted with time as each FIA set developed. This paper reveals

the role of granitic emplacement versus isograd migration with time, and their significance

to the overall interpretation of PTtd paths.

A3

Isograd mirrration with time during orogenesis ..... SHAHA.A

3.0. GENERAL GEOLOGY AND TECTONICS OF THE REGION

Figure I shows the topography of this region and its location on the eastern edge of

the Rocky Mountains in Colorado; a regional geological map in shown in Fig. 2. Figure 3

shows a detailed geological map with sample location. The geological history of Colorado

has been well studied (e.g. Finch, 1967; Peter Bird, 1988; KarIstrom et aI., 1997) because

of its spectacular mountains and mineral wealth (Finch, 1967). The basement is composed

mainly of PaleoProterozoic rocks (2.5-1.6Ga, Sim et ai, 2001) with some rocks having a

MesoProterozoic affinity (1.6-0.9 Ga, Sim et aI., 2001, Reed et ai, 1987). The Central

Plains Orogen forms the eastern limit of the Colorado province (Sims and Peterman, 1986).

To the south it is thought to extend as far as northern New Mexico and probably beyond

(Robettson et aI., 1993; KarIstrom et aI., 1997).

The rocks mainly consist of qUattz-feldspar gneiss, biotite gneiss, amphibolite and

migmatite (Reed et aI., 1987). They were generally deformed under amphibolite facies

conditions (Braddock et aI., 1970). Igneous intrusions at ~1.7, ~1.4 and 1.1 Ga reshaped the

PaleoProterozoic rock distribution (Tweto, 1987). The composition of the oldest 01.7 Ga)

and most dominant of these intrusions is intermediate with a calc-alkaline affinity. It is

generally believed that these igneous bodies were emplaced synchronous with regional

deformation during the Colorado orogeny, but a few formed post-tectonically (Reed et aI.,

1987,1993) at ~1.4 Ga. These intrusives are undeformed except locally on their margins,

and were referred as A-type or "anorogenic" plutons, mainly because of their composition

(Anderson and Cullers, 1999). Recently some workers have questioned their anorogenic

A4

IsogJ'ad mif!ration with time during orogenesis ..... SHAHA.A

classification and have argued that they were formed during deformation associated with

the Berthoud orogeny (Nyman et aI., 1994).

The two major deformation/metamorphic events previously recognized reached

temperatures consistent with sillimanite-grade metamorphism (Sims and Stein, 2003)

although metamorphic conditions were very heterogeneous during these episodes. During

the Colorado orogeny, several areas recorded a transition in metamorphic grade from the

chlorite zone to the onset of migmatization (Braddock and Cole, 1979; Selverstone et aI.,

1997). Thermobarometric analyses by Selverstone et al. (1997) and Shaw et al. (1999)

suggested that there were two intervals of heating during the Proterozoic Colorado and

Belihoud orogenies. These were separated by an interval of cooling and decompression

(Sims and Stein, 2003). In the Big Thomson Canyon area (Fig. 3), thermobarometric

analysis of metapelites by Selverstone et al. (1997) indicated that "early metamorphism at

pressures of 7-10kbar (~24-35 km depth). was followed by continuing porphyroblast

growth (garnet and staurolite) down to pressures of 4-6 kbar 0- 14-21 km)". According to

Selverstone et al. (1997), the whole region near the Big Thomson Canyon was reheated

around 1.4-Ga. Geobarometric calculations on late garnet have shown that these rocks lay

at ~ 10km depth when this reheating occurred, similar to that at the end of the Colorado

orogeny (Selverstone et aI., 1997). The portion of the geological map by Selverstone et al.

(1997) reproduced in Fig. 4 shows a succession of isograds from staurolite through

andalusite, sillimanite (and K-feldspar further west from this area) from SE to NW across

the area.

A5

Isograd mif!ration with time during orogenesis ..... SHAHA.A

4.0. FOLIATIONS

The rocks have been multiply deformed. Within the matrix 3 foliations could be

recognized. The most common is the main matrix schistosity S I that lies parallel to

compositional layering So (Fig. SA). Two younger matrix foliations oblique to bedding (S2

and S3) were also observed (Fig. SB). However, S3 was only recognized locally (Fig. SA).

Foliations preserved as inclusion trails in porphyroblasts are commonly truncated by SI

parallel to So and in such cases generally appear to predate this foliation (see below). Figure

SB (a), shows the first matrix schistosity (SI) parallel to bedding (So). In Fig. SB (b), D2 is

crenulating So/lSI.

5.0. METHODS

5.1. FIA measurements

Hayward (1990) and Bell et al. (199S, 1998) described a technique for analysing the

geometries of inclusion trails within porphyroblasts. It involves the measurement of the

FIA, which is achieved by cutting a minimum of eight vertical thin sections around the

compass. These are required to measure a FIA trend within a 10° range, with one cut every

30° around the compass and two cut 10° apatt between the sections where the asymmetry

of the inclusion trails flip (Fig. 6a and b). Because the blocks for thin sectioning are cut

from a horizontal slab 2.8 cm thick, the thin sections are always vertical with the strike

parallel to the long edge and the way up marked with a single barb, as shown in Fig. 6.

FIA measurements are made relative to both geographic coordinates and a line

perpendicular to the earth's surface. The measurement is thus independent of assumptions,

inferences, or interpretations about the timing of the inclusion trails relative to any other

A6


structures present in the rock and whether or not the porphyroblast has rotated (Hayward,

1990; Bell et ai., 1995, 1998; Bell and Welch, 2002). Where the FIA trends vary from the

core to the rim of the porphyroblasts, a relative timing and thus a FIA succession can be

established (e.g. Fig. 5a,b; Bell et ai., 1998). The accumulated error associated with

determining the trend of the FIA in each rock is random, and is estimated to be +/ - 8° in

both situations when one uses a COCLAR compass (see Bell et ai., 1998). A very

significant microstructural advantage results from cutting sections this way because the

inclusion trails contained in many porphyroblasts can be observed from a large range of

orientations, providing a much better record of the complete inclusion trail geometry than

available from cutting just one or two thin sections.

5.2. Porphyroblast inclusion trail microstructures

Gamet, staurolite, andalusite and cordierite porphyroblasts preserve earlier

foliations as inclusion trails (e.g. Figs. 7 and 8). Gamet and staurolite porphyroblasts are

most common and generally contain excellent inclusion trails (e.g. Fig. 7). Many samples

preserve a very well developed schistosity (e.g. Fig. 7) or a differentiated crenulation

cleavage (e.g. Figs. 9 and 10). These foliations are most commonly straight with curvature

at their extremities (e.g. Figs. 8 and 9). Porphyroblast inclusion trails are commonly

truncated (e.g. Fig. 7c and d) by the matrix foliations but some are not (e.g. Fig. 8).

6.0. RESULTS

A total of 67 oriented samples were collected from the field and were reorientated

in the laboratory. These rocks contain porphyroblasts of gamet, staurolite, cordierite and

andalusite, which contained inclusion trails well enough developed for FIA measurement.

A7


800 oriented thin sections were prepared from these samples. A total of 13 8 FIA and

pseudo-FIA trends were determined (Table 1) and are shown as trends on a map for each

sample location in Fig. 11 and as a rose diagram in Fig. 12a. A total of 64 and 53 FIAs

were measured in garnet and staurolite porphyroblasts respectively (Fig. 12b and c). The

combined FIA trend data for garnet and staurolite is shown in Fig. 12d. The other

porphyroblastic phases in which FlAs were measured were andalusite and cordierite, with

fourteen measurements in the former and seven in the latter. Their trends are given in Table

1, and shown in map view in Fig. 11, and on a rose diagram in Fig. 12e and f.

6.1. Determillatioll o/mllltiple FIAs/or relative timillg

Samples C52, C67, C69, C76 and C96B preserve a different FIA in the core versus the rim

in garnet (Table 1). These multi-FIA samples enable the relative timing of successive FIAs

to be determined (e.g., Bell et aI., 1998; Sayab, 2005). For example, a FIA trend determined

from the core of the porphyroblast must be older than a FIA in the rim of the same sample.

Recent work by Bell et al. (1998), Ham (1999), Cihan (2000), Newman (2001) and Sayab

(2005) has shown that in some samples, porphyroblasts inclusion trails (Si) are truncated by

the matrix foliation whereas in others they are continuous with the matrix. This further

strengthens the relative timing criterion established by using core/rim FIAs. Generally,

porphyroblasts containing truncated trails are older compared to those continuous with

matrix foliation (e.g. Johnson & Vernon, 1995). Most of the samples which preserve

inclusion trails within porphyroblast were truncated by matrix foliations (e.g. Figs. 7 and

14), but some do not (e.g. Fig. 8). Many samples preserve differentiated crenulation

cleavages that have been overgrown by the porphyroblasts where the asymmetry of the

crenulated cleavage can be determined (e.g. Figs. 9, 10, 13 and 14). The crenulated

A8


cleavages consist of quartz and ilmenite grains, while the differentiated crenulation

cleavages predominantly contain ilmenite grains. The intersection between the crenulated

and crenulation cleavages can be determined, when viewed in three dimensions, and is

called a pseudo-FIA (P-FIA). The actual FIA is formed during porphyroblast growth and in

these samples is defined by the curvature of the differentiated crenulation cleavage (Table1

and Figs. 14 and 15).

7.0. INTERPRETATION OF THE FIA SUCCESSION

Figure 2 shows that porphyroblastic samples from which FlAs could be measured

were present relatively unifotmly across the eastern half of the region only becoming

patchily distributed to the west and this is significant for the construction of isograds (see

below). Garnet is present in the easternmost samples but staurolite is not. Furthermore,

garnet porphyroblasts are commonly overgrown by staurolite. Both features suggest that in

this region garnet grew before staurolite and this characteristic (see Table 2) provides one

of foul' criteria that were used to detelmine and test the relative timing of successive FIAs.

The few samples containing changes in FIA trend from the cores to the rims of garnet

porphyroblasts provide a second criterion, a third was provided by samples containing

pseudo (P-FIAs) versus actual FlAs (e.g. Figs. 12 and 13) in garnet and staurolite

porphyroblasts (Table 3) and a fourth was provided by those porphyroblasts with trails

truncated by the matrix foliation versus those whose trails were continuous with it. The FIA

succession indicated by arranging changes in FIA trend from garnet to staurolite in the

same sample in Table 2 so that they do not conflict is supported by that in Table 3 and by

those whose trails are truncated versus continuous with the matrix foliation. This

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succession is from FIA 1 (trending NE-SW), to FIA 2 (trending E-W), to FIA 3 (trending

SE-NW), and FIA 4 (trending NNE-SSW). Table 4 shows all the samples for which FIAs

have been determined and classified into FIA sets. Samples containing only one FIA trend

have been placed on this table based on that trend and secondly, whether the inclusion trails

are truncated (FIA 1) or continuous (FIA 4) with the matrix foliation. FIA 1 and 2 is

preserved only in garnet and staurolite porphyroblasts. FIA set 3 is mainly present in garnet

and staurolite porphyroblasts. However, a few samples contain this FIA set in andalusite

and cordierite porphyroblasts (Table 1). FIA set 4 can be found in each of the

porphyroblastic phases. This indicates a progression in the timing of growth as orogenesis

proceeded of garnet, staurolite, andalusite and cordierite.

8.0. EVIDENCE OF EARLIER MINERAL INCLUSIONS WITHIN YOUNGER

PHASES

8.1. Illelusiolls of gamet with ill staurolite, alU/a/usite (II/{/ cordierite

Several samples preserve remnants of older minerals included within younger phases,

during an overall prograde metamorphic succession. Figure l6a, shows an example of a

euhedral garnet porphyroblast included within a big poikiloblastic staurolite. A few

examples of garnet inclusions preserved within andalusite and cordierite are also observed

(e.g. Fig. 16b and c). These mineral relationships have suggested an earlier appearance of

garnet, and before the formation of staurolite, andalusite and cordierite.

AJO

Isograd mif!ralion with time during orogenesis ..... SHAHA.A

8.2. Rep/(lCemellt of st(llll'olite by (lnd(l/l/site

Staurolite porphyroblast in a couple of examples were found to be partially andlor

completely replaced by either andalusite or cordierite (e.g. Fig. 17). Traces of staurolite are

included in some big poikiloblastic andalusite (e.g. Fig. 17a and b). Inclusions preserved

with in the staurolite are texturally and geometrically distinct from those within andalusite

pOlphyroblasts. FIA measurements were impossible for andalusite in this sample, as very

few slides contain this mineral. However, staurolite preserves FIA set 4 inclusion trails.

8.3. Rep/(lcement of st(llll'olite by cOl'diel'ite

Some samples (e.g. C52 ) preserve evidence of direct replacement of staurolite by

cordierite. Traces of staurolite are included in some big cordierite poikiloblasts. Inclusions

trails preserved with in the staurolite are texturally and geometrically distinct from those

within cordierite (e.g. Fig. 17c and d). Cordierite preserves FIA set 4 and its inclusion trails

completely truncate those of staurolite (e.g. Fig. 17d). FIA measurements were impossible

for staurolite in this sample, as very few slides contain this mineral.

8.4. Rep/(lcemellt of (tlU/(I/lisite by conliel'ite

Inclusions of andalusite were found within the porphyroblasts of cordierite in

sample C77 (Fig 18a and b). Cordierite preserves a crenulated (FIA set 3) and a crenulation

cleavage. Andalusite only preserves one foliation, which is roughly consistent with the

orientation of the crenulated cleavage within the cordierite. This suggests that the

andalusite formed before or contemporaneous with the formation ofFIA set 3. Remnants of

inclusions within andalusite, which are mainly composed of staurolite, quartz, muscovite,

minor opaques and ilmenite, are geometrically and texturally distinct from those found

All

Iso,?rad migration with time during orogenesis ..... SHAHA.A

within the matrix and the cordierite porphyroblast. No staurolite inclusions were observed

within the cordierite.

9.0. DATING OF FIAS AND THE SUCCESSION CONFIRMATION

Monazite grains were dated within the foliations defining each FIA set (Thesis

Section C) with FIAs set 1,2 and 3 forming from around 1760 to 1670 Ma and FlA set 4

around 1420 Ma.

10.0. FIAS DERIVED ISOGRADS

10.1. Staurolite isograds for FIAs 1, 2, 3 and 4

The samples which contain FIA 1,2, 3 and 4 preserved in staurolite porphyroblasts

are shown in Fig. 1. The total distribution of FIAs measured from this mineral (Fig. 12c)

resulted from four periods of staurolite growth that show a consistent succession where

relative timing criteria are available (Tables 2 and 3). Staurolite isograds can be defined for

these FlAs as shown in Fig. 19. A slightly clockwise and eastward shift in the staurolite

isograds from FlA 1 to FIA 4 is apparent.

10.2. Afl(/a/usite and cOl'dierite isograds

Andalusite and cordierite were the last minerals to grow in these rocks. Two FIA

sets (3 and 4) were observed in these mineral phases. Cordierite porphyroblasts

predominantly preserve FIA set 4. Foliations defining the latest FIA sets preserved in both

of these mineral phases were always continuous with those in the matrix and deflections of

these trails from porphyroblast to matrix are very slight (e.g. Fig. 8). The andalusite and

Al2


cordierite isograds for FIAs 3 and 4 are shown in Fig. 20 (a, b). A slightly clockwise shift

with time in both of their isograds, similar to that for the staurolite isograds, is apparent.

11.0. LATE ANDALUSITE AND CORDIERITE GROWTH

A few samples contain rare porphYl'oblasts of andalusite (C92A, C55B, CIOl) or

cordierite (C66, C68B, C96B, C70). Cordierite or andalusite was only present in one thin

section of 8 to a maximum of 18 that were cut from each of these samples and so a FIA

could not be measured for them. In sample C92Arare andalusite was present in 3 out of 8

slides. In all of these samples the porphyroblasts overgrew the matrix with no deflection of

the foliation from inside to outside of the porphyroblast suggesting that they formed at the

very end of deformation (e.g. Figs. 17 and 22). The location of these samples and the two

isograds that they suggest is shown in Fig. 20 (c, d).

12.0. DISTRIBUTION OF SILLIMANITE

Sample C77 contains fibrolitic sillimanite within andalusite and cordierite that

contain FIA set 4. Fibrolitic sillimanite also occurs in some of the samples (e.g. Fig. 21a

and b) mentioned above that contain rare andalusite and cordierite. The fibrolitic sillimanite

in all of these samples appears to postdate FIA 4 and overprints the matrix foliation (e.g.

Fig. 22a and b). The sillimanite indicated by this distribution is shown in Fig. 20d.

Al3


13.0. AXIAL PLANE TRENDS OF REGIONAL FOLDS

The axial traces of all folds present in the area were measured from regional

geological maps (Fig. 23). They were then plotted on a rose diagram as shown in Fig. 23.

Note the similarity between this figure and that containing the FIA trends (Fig. 24).

14.0. INTERPRETATION

14.1. Porphyroblast rotation or otherwise

A consistent succession of 4 FIA sets as shown in Fig. 12 would be impossible if

the porphyroblasts had rotated. This is shown in Fig. l3. Set 1 FIA forms almost 45° from

FIA set 2. If FIA set 2 formed by rotation of the porphyroblasts around an axis with this

trend, FIA set 1 would have been rotated on a small circle by up to 90° (the maximum

curvature of inclusion trails defining FIA set 2) as shown in Fig. 25a. FIA set 3 lies 80°

from FIA set 1. An enormous spread of FIA set 1 data results when the spread of FIA 1 due

to rotation about FIA 2 is in tum rotated about FIA 3 (Fig. 25b) by up to 135° (the

maximum inclusion trail curvature about FIA 3). If these axes were further rotated about

FIA set 4, the spread of FIA trends would span the whole stereonet (Fig. 25c). The

succession of FIA trends show in Fig. 12 indicates that this is not the case. This does not

eliminate rotation as a mechanism for the development of FIA set 1 but it makes it highly

unlikely. Furthermore, with the recent computer-modeling discovery of the phenomenon of

gyro stasis, a mechanical explanation for the non rotation of porphyroblasts is now available

(Fay et aI., 2008). They name this for the process driving the phenomenon of lack of

rotation of rigid objects

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14.2. Staurolite Isograds

The migration of the staurolite isograd eastwards from FIA 1 to FIA 4 (Figs. 20a, b,

c and d) indicates that the source of heat for the metamorphism that generated this

succession of events lay to the west. Each of the four periods of staurolite growth involved

more than one phase of growth. Most porphyroblasts contain a single FIA but some

preserve an earlier formed differentiated crenulation cleavage from which a pseudo-FIA

(that predates porphyroblast growth) could be measured (Fig. 14). Consequently, these

rocks record evidence of more than 4 phases of staurolite growth that could not have been

distinguished without the FIA data preserved by their inclusion trails.

14.3. Am/a/I/site isograd for FIAs 3 and 4

The andalusite isograds for FIAs 3 and 4 shown in Fig. 20a, rotate clockwise as they

migrate eastwards. This is similar to the staurolite isograds, suggesting that they were a

product of migration ofthe heat source ii-om WNW to ENE with time.

14.4. Cordierite isogr(l(/s for FIAs 3 ami 4

The cordierite isograds for FIAs 3 and 4 shown in Fig. 20b, rotate clockwise as they

migrate eastwards. The migration of the cordierite isograds from FIA to FIA mimics that of

the staurolite and andalusite isograds, suggesting that they were a product of migration of

the heat source from WNW to ENE with time.

A15


14.5. Rare matrix overprilltillg cordierite amI (IIulalusite

Some samples contain cordierite or andalusite in only one thin section out of the 8

to 18 cut per sample (Sample C92A contains andalusite in 3 out of 8 thin sections). These

rare porphyroblasts overgrew the matrix foliation without any deflection of inclusion trails

associated with deformation during their growth. The distribution ofthese porphyroblasts is

significantly different to those for which FIAs could be measured and spreads eastwards

across the area as shown in Fig. 20c. The isograds that they indicate run at a high angle to

those isograds indicated by the FIA succession. They bear a close relationship to the

sillimanite isograd mentioned and so their significance is interpreted in that section.

14.6. Sillimallite

The sillimanite isograd shown in Fig. 20d runs almost olthogonal to the FIA 4

isograds determined for cordierite and andalusite. However, it nllls sub parallel to the

cordierite and andalusite isograds indicated for the youngest period of growth of these 2

mineral phases (Fig. 20c). This suggests that the growth of sillimanite and this youngest

phase of cordierite and andalusite were associated with same heat source. This phase of

metamorphism not only overprinted FIA sets 1 through 4, but the heat source causing it had

migrated significantly relative to that which accompanied development of the earlier

formed porphyroblasts.

14.7. Relatiollship betweell FlA trellds alld axial traces offolds

Comparison of Fig. 24a and b shows a strong correlation between the distribution of

FIA trends and the axial trace of all folds present in the area. The four different peaks in the

trends of axial traces exactly match the four FIA trends. Bell et al. (2004) and Bell and

A16

Isograd miwation with time during orogenesis ..... SHAHA.A

Newman (2006) argue that FIAs forming perpendicular to bulk shortening directions and

successions of FIAs record changes in this direction that occurred during orogenesis. Folds

are a product of the same process but are expected to be rotated with time by the

overprinting effects of successive deformation in contrast with the FIAs, which are

unaffected by subsequent ductile deformation (e.g. Bell et aI., 1995; Fay et aI., 2008). The

correlation of fold axial traces with the FIA trends recorded here is, therefore, quite

remarkable. It suggests that pockets of low strain occur due to deformation partitioning, in

spite of, or perhaps because of, the 4 successive changes in bulk shortening direction. These

pockets preserve folds in the orientation in which they formed from subsequent rotation or

destruction. They provide remarkable confirmation of the veracity of the FIA data and

indeed reveal that such data can be used to determine successions of fold development from

regional maps at which scale many overprinting criteria cannot be applied.

14.8. Foliation trend and Heat Flux

Figures 2 and 3 show that no granite is present on the WNW side of the isograds.

Figures 19 and 20a, b reveal that the staurolite, cordierite and andalusite isograds migrated

eastwards at a high angle to SO.1 with clockwise rotation of the FIA trend 1 to 2 to 3 to 4.

This suggests that heat flux through rock was controlled by the orientation of SO,I rather

than the FIA trend which is controlled by the concurrently developing sub-vertical

foliation. That is, reactivation of the heterogeneity provide by SO,1 provided a faster

pathway for heat than a newly developing cleavage. This also accords with ENE migration

of a heat source that lay to the WSW during orogenesis. The most recently formed

andalusite, cordierite and sillimanite isograds (Fig. 20c and d) lie roughly parallel to each

Al7

Isorp'ad mirp'ation with time during orogenesis ..... SHAHA.A

other and the distribution of the -1400 Ma granitoid plutons to the SW but orthogonal to

isograds defined by FlAs.

15.0. Discussion

Prior to the development of FIA studies, there was no way to distinguish different

periods of growth of the same porphyroblastic phase unless changes in inclusion density,

composition or orientation were very obvious. With the advent of a quantitative approach

using FlAs, many different periods of growth of the same porphyroblastic phase can now

be distinguished (Bell 1998). In the example documented herein, four different periods of

growth of garnet and staurolite have been distinguished and the isograds for four of the

periods of staurolite and two each of cordierite and andalusite growth have been mapped.

15.1. Multiple phases of staurolite growth

Isograds can potentially be obliterated by further heating (e.g. Braddock and Cole

1979). Shaw (1999) and Selverstone (1997; Fig. 4) claimed that the staurolite isograd

resulted from two generations of growth. They had isotopic age evidence for 2 periods of

tectonism separated by - 300 million years but did not attribute these two periods of growth

to periods of tectonism that far apati. Rather, they suggested that andalusite growth took

place at the end of the phase of tectonism that produced the staurolite (-1.7Ga) and then

reoccurred some 300 million years after staurolite growth (-1.4Ga). The ability to

quantitatively measure FIAs and distinguish many periods of growth of the same

porphyroblastic phase has changed this. The recognition of a succession of 4 FlAs

preserved within the staurolite porphyroblasts in this region plus at least 2 phases of

staurolite growth within each FIA set, is significantly different to most current concepts

about how reactions take place during deformation and metamorphism. Quantitative

A18

[sogmd migration with lime during orogenesis ..... SHAHA.A

measurement of inclusion trails within porphyroblasts reveals that the reactions from which

porphyroblasts such as staurolite grow do not simply start and go to completion. Rather

they take place over and over again, once the PT and bulk composition are suitable and are

a function of some other factor that previously has not been considered as significant. Bell

and Hayward (1991), Spiess and Bell (1996) and Bell & Bruce (2007) have shown that the

initiation of crenulation hinges controls where porphyroblast nucleation and growth takes

place during regional metamorphism and that growth ceases as soon as a differentiated

crenulation cleavage begins to develop. They argue that this results from strain softening

and the cessation of micro fracture and thus access of the components need for the reaction

to take place to the porphyroblast crystal faces. Consequently, when another deformation

initiates such that crenulations can begin to form again (which requires shortening near

orthogonal to the previous event) the same porphyroblastic phase can regrow because the

reaction was forced previously to cease. Such regrowth of staurolite took place in this

region during 4 different events, the first and last of which were spaced 250 million years

apart!

15.2. Multiple phases of andalusite and cordierite growth

A transition from andalusite to cordierite during FIAs 3 and 4 is suggested by a

concomitant increase in the latter phase and accords with the overall migration of staurolite,

andalusite and cordierite during the development of FlAs 1 tlu'ough 4 from W to E across.

This succession suggests growth of cordierite after andalusite during metamorphism

accompanied by deformation, which matches with the PT pseudosection.

The younger cordierite, andalusite and sillimanite isograds that are nearly

orthogonal to those defined by the FIA succession (compare Fig.2Ia-b and 2Ic-d). This

A19

Isograd mif!ration with lillie during orogenesis ..... SHAHA.A

suggests growth of andalusite was followed by cordierite, confirmed by the PT

pseudosections. They clearly resulted from a heat source to the SW instead of the WNW.

15.3. Comparison with earlier work

The isograds mapped by Selverstone (1997) shown in Fig. 4 resemble the youngest

isograds determined through this research. This is to be expected because they were unable

to distinguish the different phases of growth of staurolite, andalusite and cordierite that the

FIA approach allowed. Multiple isograds of garnet, staurolite, andalusite and cordierite that

migrated with time, suggest a much longer history of deformation and a prolonged

metamorphism than previously reported (e.g. Cavosie and Selverstone, 2003; Selverstone,

1997). The complete absence of kyanite and presence of andalusite and sillimanite isograds

suggests a low pressure metamorphic sequence and illustrates that high pressures were

never attained by these metasediments. One reason for their migration could be the

progressive upwards migration of melt.

15.4. Deformation Partitioning

Rocks are subjected to heterogeneities at all scales (e.g. Bell 1981; Williams 1994;

Bell 2004). In a given outcrop some aspects of the deformation history can have affected

some pOliions while others will be affected by other different events. Microstructural (Bell

and Hayward 1991; Spiess and Bell 1996; Bell and Bruce 2007) and FIA data (Bell 1998,

2008) strongly suggest that porphYl'Oblast nucleation and growth is a function of the

partitioning of deformation at the scale of a porphyroblast. This provides a simple

explanation and allows one to develop an intimate understanding of the role of deformation

in porphyroblast nucleation and growth and an understanding of why reactions should take

A20


place at different times from sample to sample in the same outcrops. Thus assumptions that

mineral growth is merely a function of pressure, temperature and bulk composition (e.g.

Thompson 1957) need to be revised as deformation partitioning has a strong role to play in

the growth of porphyroblastic mineral phases.

Different distributions of FlAs on a histogram from FIA to FIA for garnet,

staurolite, andalusite and cordierite (Fig. 26) reveal changes in the partitioning of

deformation within a region. Garnet dominates in FIA set 1 and staurolite in FIA set 4 (Fig.

26a). The total distribution in each FIA set reveals that there are more samples which

preserve FIA set 3 trails as compared to those which contain FIA set 1,2 or 4 (Fig. 26b).

Garnet contains the maximum FIAs followed by staurolite, cordierite and andalusite (Fig.

26c). Differences from region to region reveal changes in the role of deformation

partitioning at a large scale (e.g. Bell 2004; Bell and Newman 2006; Sayab 2007). The

disbursement of FIAs 1, 2, 3 and 4 in garnet and staurolite porphyroblasts across the area

strongly suggests that the P, T and bulk composition were suitable for growth of these

mineral phases from FIA 1 onwards. Yet staurolite porphyroblasts containing FIA 1 have

only been found in seven samples (Fig. 11). This is more likely to be the result of a change

in the pattern of deformation partitioning during the development of this FIA set rather than

a change in the P and T conditions (e.g. Be112004).

15.5. Relevance to granite emplacement

No broad first-order spatial relationship exists between granites and medium grade

rocks although granites exposed elsewhere in the region are enriched in radiogenic heat

producing elements. The only plutonic rocks known to have intruded during the peak of

metamorphism in the Colorado orogeny are pegmatite swarms close to the Big Thompson

A2I

Isof!l'ad migration with time during orogenesis ..... SHAHA.A

River (Fig. 3). However, the isograd patterns show no relationship to pegmatite contacts.

The emplacement of pegmatites may have contributed to the thermal budget but were not

the primary cause of the low-pressure metamorphism during the Colorado and Berthoud

orogenies as they lie too far away (e.g. Foster and Rubenach, 2006).

The eastward migration of the staurolite isograds with time revealed in Fig. 20 and

their near orthogonal orientation to SO,l suggests that the flux of heat was controlled by the

orientation of foliation parallel to the compositional layering. Intense shear along

compositional layering, referred to as reactivation, is thought to dominate over the

development of new oblique foliations (Ham & Bell, 2004). Heat transference may occur

by advection due to fluid flow along the layering during this process. However, heat

transference by conduction could be enhanced by diffusion associated with the dissolution

and diffusion of elements that accompanies cleavage seam development and reactivation

(Bell and Cuff 1989).

15.6. Significance of the shift in the isograd fi'om FIA to FIA

Heat flux, like deformation, will be heterogeneous and partitioned through the crust

if foliation development is important to diffusion is evident from the distribution of

staurolite isograds. The eastward shift with time signifies that heat progressively increased

although the overall migration is minimal. The succession of three FIAs has developed over

period of ~ 1 00 Ma for the Colorado orogeny. Other five fold FIA successions that have

been dated lasted 100 (Bell and WeIch 2002) to 200 (Cihan 2006) million years. If such

time lengths are involved, why was the eastward migration of index minerals so limited.

One possibility is that the isograds have steep dips rather than gentle ones. The garnet

isograd in the Homestake mine is sub-vertical through the mine centre and formed that way

A22


(Bell and Bell unpublished data). Steep dips of the isograds in this region would readily

explain the minimal migration over a potentially a long time period.

Acknowledgements

I would like to thank Prof. Tim Bell for his critical comments. Special thanks to Dr. Mike

Rubenach for help and critical discussion about isograds. I would also like to thank Prof.

Jane Selverstone for sending me the manuscripts of the Colorado region.

A23


16.0 REFERENCES

Anderson 1. L., Cullers R. L., 1999. Paleo- and Mesoproterozoic granite plutonism of

Colorado and Wyoming, in Frost, C.D., ed. Rocky Mountain Geology, 34, 149c 164.

Bell, T. H., 1981. Foliation development: The contribution, geometry and significance of

progressive bulk inhomogeneous shortening. Tectonophysics, 75, 273-296

Bell, T. H., Bruce, M.D., 2007. Progressive deformation partitioning and deformation

histOlY: Evidence from millipede structures Journal of Structural Geology, 29,18-35.

Bell, T. H., Cuff, C., 1989. Dissolution, solution transfer, diffusion versus fluid flow and

volume loss during deformation/metamorphism. Journal of Metamorphic Geology, 7,

425-448.

Bell, T. H., Hayward, N., 1991. Episodic metamorphic reactions during orogenesis: the

control of deformation partitioning on reaction sites and duration. Journal of

Metamorphic Geology, 9, 619-640.

Bell, T. H., Forde, A., Wang, J., 1995. A new indicator of movement direction during

orogenesis: Measurement technique and application to the Alps. Terra Nova, 7, 500-

508.

Bell T.H., Hickey, K.A, Upton, O.J.O., 1998. Distinguishing and correlating multiple


staircase and sigmoidally curved inclusion trails in garnet. Journal of Metamorphic

Geology, 16,767-794.

Bell, T. H., Mares, V. M., 1999. Correlating deformation and metamorphism around arcs in

orogens. American Mineralogist, 84, 1727-1740.

A24


Bell, T. H., Welch, P. W., 2002. Prolonged Acadian Orogenesis: Revelations from FIA

Controlled Monazite Dating of Foliations in Porphyroblasts and Matrix. American

Journal o/Science, 302, 549-581.

Bell, T. H., Ham, A P., Hickey, K. A, 2003. Early formed regional antiforms and

synforms that fold younger matrix schistosities: their effect on sites of mineral

growth. Tectonophysics, 367,253-278.

Bell, T. H., Ham, A. P., Kim H. S., 2004. Pa11itioning of deformation along an orogen and

its effects on porphyroblast growth during orogenesis. Journal 0/ Structural Geology,

26,825-845.

Bell, T. H., Newman, R. L., 2006. Appalachian Orogenesis: the role of repeated

gravitational collapse. In: Styles of Continental Contraction, eds S. Mazzoli and

R.W.H. Butler. Geological Society 0/ America Special Paper, 414, 95-118.

Bell, T. H., Bruce, M. D., 2007. Progressive deformation partitioning and deformation

history: Evidence from millipede structures. Journal o/Structural Geology, 29, 18-35.

Bird, P., 1988. Formation of the Rocky Mountains, western United States. Science, 22, 670-

671.

Braddock, W. A, Nutalaya, P., Gawarecki, S., and Coffin, G., 1970. Geologic map of the

Drake quadrangle, Larimer County, Colorado. United States Geological Survey

Geologic quadrangle map, GQ 829, scale 1 :24,000.

Braddock, W. A, Cole, J. C., 1979. Precambrian structural relations metamorphic grade

and intrusive rocks along the northeast flank of the Front Range in the Thompson

Canyon, Poudre Canyon, and Virginia Dale areas. Field Guide, Geological Society 0/

America Rocky Mountain section, 106-120.

A25


Brown, M., 1994a. The generation, segregation, ascent and emplacement of granite magma:

The migmatite-to-crustally-derived granite connection in thickened orogens. Earth

Science Reviews, 36, 83± 13

Brown, M., Dallmeyer, R. D., 1996. Rapid Variscan exhumation and role of magma in core

complex formation: Southern Brittany metamorphic belt France. Journal of


Brown, M., Solar, G. S., 1999. The mechanism of ascent and emplacement of granite

magma during transpression: a syntectonic granite paradigm. Tectonophysics, 312, 1-

33.

Cavosie, A.J., Selverstone, J., 2003. Early Proterozoic oceanic crust in the northern

Colorado Front Range: Implications for crustal growth and initiation of basement

faults. Tectonics, 22,1015, doi: 10.l029/200lTC001325

Chamberlain, K. R., 1998. Medicine Bow orogeny: Timing of deformation and model of

crustal structure produced during continent-arc collision ~ 1.78 Ga, southeastern

Wyoming. Rocky Mountain. Geology, 33, 259-277

Cihan, M., Evins, P., Lisowiec, N., Blake, K., 2006. Time constraints on deformation and

metamorphism from EPMA dating of monazite in the Proterozoic Robertson River

Metamorphics, NE Australia. Precambrian Research, 145, 1-2.

Collins, W. J., Vernon, R. H., 1992. Palaeozoic arc growth,deformation & migration across

the Lachlan Fold Belt, southeastern Australia. Tectonophysics, 214,381 - 400.

Condie, K. C., Martel, C., 1983. Early Proterozoic metasediments from north-central

Colorado: Metamorphism, provenance, and tectonic setting. Geological Society of

American Bulletin, 94, 1215-1224.

A26


Duguet, Manuel., Le Breton, Nicole., Faure, Michel., 2007. P T paths reconstruction of a

collisional event: The example of the Thiviers-Payzac Unit in the Variscan French

Massif Central. Lithos, 98, 1-4, 210-232.

Etheridge, M. A., Rutland, R W. R, Wyborn, 1. A., 1987. Orogenesis and tectonic process

in the Early to Middle Proterozoic of northern Australia. American Geophysical

Union Geodynamic Series, 17,131-147.

Fay, C., Bell, T. H. and Hobbs, B. E., 2008. Porphyroblast rotation versus non-rotation:

conflict resolution! Geology, 30, No 4.

Finch, Warren., 1967. Geology of epigenetic uranium deposits in sandstone in the United

States: U.S. Geological Survey Professional Paper, 538, 121.

Foster, D. R W., Rubenach, M. J., 2006. Isograd pattern and regional low-pressure, high

temperature metamorphism of pelitic, mafic and calc-silicate rocks along an east-west

section through the Mt Isa Inlier. Australian Journal of Earth Sciences, 53, 167-186.

Ham, A. P., Bell, T. H., 2004. Recycling of foliations during folding. Journal of Structural

Geology, 26, 1898-2009.

Hand, M., Fanning, M., Sandiford, M., 1995. Low-P high-T metamorphism & the role of

high-heat producing granites in the northern Amnta Inlier. Geological Society of

Australia Abstracts, 40, 60 - 61.

Hand, M., Rubatto, D., 2002. The scale of the thermal problem in the Mount Isa Inlier.

Geological Society of Australia Abstracts, 67, 173.

Hutton, D. H. W., 1988. Granite emplacement mechanisms and tectonic controls:

Inferences from deformation studies: Royal Society of Edinburgh Transactions. Earth

Sciences, 79, 245-255.

A27


Karlstrom, K. E., Dallmeyer, R. D., Grambling, J. A., 1997. 40Ar/39Ar evidence for 1.4 Ga

regional metamorphism in New Mexico: implications for thermal evolution of

lithosphere in the southwestern U.S. Journal of Geology, 105,205-223.

Loosveld, R. J. H., Etheridge, M. A., 1990. A model for low pressure facies metamorphism

during crustal thickening. Journal of Metamorphic Geology, 8, 257 - 267

Ludwig, K. R., 1998. IsoplotlEx: a geochronological toolkit for Microsoft Excel. Berkeley

Geochronological Center, Special Publication, No.1, 43.

McLarens, S., Sandiford, M., Hand, M., 1999. High radiogenic heat-producing granites and

metamorphism: an example from the western Mount Isa Inlier, Australia. Geology,

27,679 - 682.

Mezger, J. E., Passchier, C. W., 2003. Polymetamorphism & ductile deformation of

staurolite-cordierite schist of the Bossost dome: indication for Variscan extension in

the Axial Zone of the central Pyrenees. Geological Magazine, 140, 595 - 612.

Mildren, S. D., Sandiford, M., 1995. Heat refraction and low-pressure metamorphism in the

northern Flinders Ranges, South Australia. Australian Journal of Earth Sciences, 42,

241-247

Nyman, M., Karlstrom, K. E., Graubard, C., Kirby, E., 1994. Mesozoic contractional

orogeny in western North America: Evidence from ca. 1.4 Ga plutons. Geology, 22,

901-904.

Oliver, N. H. S., Holcombe, R. J., Hill, E. J., Pearson, PJ., 1991. Tectono-metamorphic

evolution of the Mary Kathleen Fold Belt, northwest Queensland: a reflection of

mantle plume processes? Australian Journal of Earth Sciences, 38, 425 - 455.

A28


Paul, K. Sims., Holly, J. Stein., 2003. Tectonic evolution of the Proterozoic Colorado

province, Southern Rocky Mountains: A summary & appraisal. Rocky Mountain

Geology, 38, 183-204.

Reed, J. C. Jr., Bickford, M. E., Premo, W. R, Aleinikoff, J. N., Pallister, J. S., 1987.

Evolution of the Early Proterozoic Colorado Province: Constraints from U-Pb

geochronology. Geology, 15, 861-865.

Reed, J. C. Jr., Bickford, M. E., Tweto, 0., 1993. Proterozoic accretionary terranes of

Colorado and southern Wyoming. In Reed, J. C., Jr.; Bickford, M. E.; Houston, R S.;

Link, P. K.; Rankin, D. W.; Sims, P. K.; & Van Schmus, W. R, eds. Precambrian:

conterminous U.S. Boulder, Colo. Geological Society of America, 211-228.

Robelison, J. M., 1993. Precambrian geology of New Mexico, in Reed, J.C. & others, eds.,

Precambrian--Conterminous U.S. Boulder, Colorado. Geological Society of America,

The Geology of North America, C-2,228.

Rubenach, M. J., 1992. Proterozoic low-pressure/high temperature metamorphism and an

anticlockwise P-T -t path for the Hazeldene area, Mount Isa Inlier. Journal of

MetamOlphic Geology, 10,333-346.

Rubenach, M. J., Barker, A. J., 1998. Metamorphic and metasomatic evolution of the Snake

Creek Anticline, Eastern Succession, Mt Isa Inlier. Australian Journal of Earth

Sciences, 45, 363-372.

Sandiford, M., Fraser, G., Arnold, J., Foden, J., Farrow, P., 1995. Some causes and

consequences of high-T, low-P metamorphism in the eastern Mt Lofty Ranges, South

Australia. Australian Journal of Earth Sciences, 42, 233-240.

Sandiford, M., Hand, M., Mclarens, S., 1998. High geothermal gradient metamorphism

during thermal subsidence. Earth and Planetary Science Letters, 163, 149-165.

A29

Isograd migration with time dUl'ing ol'ogenesis ..... SHAHA.A

Sayab, M., 2005. Microstructural evidence for N-S shortening in the Mount Isa Inlier (NW

Queensland, Australia): the preservation of early W-E-trending foliations in

porphyroblasts revealed by independent 3D measurement techniques. Journal of

Structural Geology, 27, 1445-1468.

Scaillet, B., Pichavant, M., and Roux, J., 1995, Experimental crystallisation of leucogranite

magmas. Journal of Petrology, 36, 663-705.

Selverstone, J., Hodgins, M., Shaw, C., A1einikoff, J. N., Fanning, C. M., 1997. Proterozoic

tectonics of the nOlihern Colorado Front Range. In Bolyard, D.W., & Sonnenberg, S.

A., eds. Geologic history of the Colorado Front Range. Denver. Rocky Mountain

Association of Geology, 9-18.

Selverstone, J., Aleinikoff, J., Hodgins, M., Fanning, C.M., 2000. Mesoproterozoic

reactivation of a Paleoproterozoic transcurrent boundary in the Colorado Front Range:

Implications for ca. 1.7 and 1.4 Ga tectonism. Rocky Mountain Geology, 35,136-162.

Shaw, C. A., Snee, L.W., Selverstone, J., Reed, J.C., 1999. 40Ar/39Ar thermochronology

of Mesoproterozoic metamorphism in the Colorado Front Range. Journal of Geology,

107,49-67.

Spiess, R, Bell, T. H., 1996. Microstructural controls on sites of metamorphic reaction: a

case study of the inter-relationship between deformation and metamorphism.

European Journal of Mineralogy, 8, 165-186.

Sims, P. K., Peterman, Z. E., 1986. Early Proterozoic Central Plains Orogen-a major buried

structure in the nOlih-central United States. Geology, 14,488-491.

Solar, G. S., Pressley, RA., Brown, M., Tucker, RD., 1998. Granite ascent in convergent

orogenic belts. Testing a model. Geology, 26, 711-714.

A30

Isograd mifTration with time during orogenesis ..... SHAHA.A

Spear, F. S., 1995. Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths.

Mineralogical Society of America Monograph, 2nd, 799.

Stowell, H. H., Taylor, D. L., Tinkham, D.L., Goldberg, S.A., Ouderkirk, K.A., 200l.

Contact metamorphic P-T - t paths from Sm-Nd garnet ages, phase equilibria

modelling and thermobarometry:garnet ledge, southeastern Alaska, USA. Journal of

Metamorphic Geology, 19, 645- 660.

Thompson, J. B. Jr., 1957. The graphical analysis of mineral assemblages in pelitic schists.

American Mineralogist, 42, 842-858.

Thompson, A. B., Ridley, J. R., 1987. Pressure-temperature-time (P-T-t) histories of

orogenic belts. Philos. Transactions in Royal Society of London, A321, 27--45.

Tilley, C. E., 1924. The facies classification of rocks. Geological Magizine, 61, l67-17l.

Tweto, O. L., 1987. Rock units of the Precambrian basement in Colorado. U.S. Geological

Survey Professional. Papers, 1321-A, 54.

Vernon, R. H., Clarke, G. L., Collins, W. J., 1990. Local, mid-crustal granulite facies

metamorphism and melting: an example from the Mount Stafford area, central

Australia. In: Ashworth J. R. & Brown M. eds. High-Temperature Metamorphism and

Crustal Anatexis, Unwin Hyman, London, 272 - 319.

Williams, M. L., 1994. Sigmoidal inclusion trails, punctuated fabric development, and

interactions between metamorphism and deformation. Journal of Metamorphic

Geology, 12, 1-21

Williams, M.L., Karlstrom, K.E., 1996. Looping PoT paths, high-T, low-P middle crustal

metamorphism: Proterozoic evolution of the southwestern United States. Geology, 24,

1119-1122.

A31


Williams, M.L., Jercinovic, M.J., Terry, M.P., 1999. Age mapping and dating of monazite

on the electron microprobe: Deconvoluting multistage tectonic histories. Geology, 27,

1023-1026

A32

Section B

PRESSURE-TEMPERATURE-time-DEFORMATION (PTtD) PATHS DERIVED FROM FIAS, P-T PSEUDO SECTIONS AND ZONED GARNETS

PRESSURE-TEMPERA TURE-lime-DEFORMATION. ............................................. SHAH A.A

PRESSURE-TEMPERATURE-time-DEFORMATION (PTtD) PATHS DERIVED

FROM FIAS, P-T PSEUDOSECTIONS AND ZONED GARNETS

PRESSURE-TEMPERATURE-time-DEFORMATION (PTtD) PATHS DERIVED FROM FlAS, poT PSEUDOSECTIONS AND ZONED GARNETS 1

1.0. ABSTRACT

2.0. INTRODUCTION

3.0. REGIONAL GEOLOGY AND TECTONICS

4.0. QUANTITATIVE MEASUREMENTS OF DEFORMATION USING FIAS

5.0. THE FlA SUCCESSION

6.0. PORPHYROBLAST TIMING 6.1. Via FIAs 6.2. Replacenient oJ staurolite by cordierite and andalusite 6.3. Replacement oJ andalusite by cordierite

7.0. TIMING OF METAMORPHIC INDEX MINERALS NOT PRESERVING FlAS 7.1 Garnet inclusions within staurolite, andalusite and cordie rite 7.2. Staurolite inclusions within andalusite and cordierite 7.3. Garne~ andalusite and staurolite inclusions within cordierite 7.4. Sillimanite timing

8.0. DATA FOR THERMO BAROMETRY 8.1. P-T pseudosections 8.2. Garnet Core Isopleths 8.3. Major elements

9.0. INTERPRETATION 9.1. The role oJ deJormation partitioning in porphyroblastgrowth 9.2. Multiple phases oJindex minerals 9.3. Mineral succession and migrating prograde metamorphic path 9.4. PoT pseudosections and garnet nucleation 9.5. Multi-equilibrium thermobarometry and Average PT 9.6. Pressure-Temperature-time-DeJormation (PTtD) path

10.0. DISCUSSION 10.1. Pseudosection and Textural agreement 10.2. Garnet temperature overstepping 10.3. Metamorphic and tectonic relationships 10.4. FIA data and the AUSWUS model

ACKNOWLEDGEMENTS

REFERENCES

B

2

2

4

5

6

8 8 8 9

9 9

10 10 11

11 11 12 13

16 16 17 18 19 21 22

23 23 24 24 26

26

28

1

PRESSURE-TEMPERATURE-time-DEFORMATION. ............................................. SHAH A.A

1.0. ABSTRACT

FlAs (foliation intersection/inflection axes preserved within porphyroblasts) in gamet,

staurolite, andalusite and cordierite from PreCambrian rocks in the nOlthern Colorado Rocky

Mountains reveal 4 periods of growth about axes trending NE-SW, E-W, SE-NW and NNE

SSW during an overall prograde path. The growth of garnet was always followed by

staurolite for each of the 4 FIA sets with andalusite and cordierite following staurolite during

development of the last 2 sets. Thermodynamic modelling in the MnNCKFMASH system

reveals that well documented episodic growth occurred over a similar bulk compositional

range and PT path for each FIA in the succession. The intersection of Ca, Mn, and Fe

isopleths in gamet cores containing FIA set 1 indicate that growth of this mineral phase

began at 540-550°C and 3.8-4.0 kbars. Similarly, the intersection of Ca, Mn, and Fe isopleths

in garnet cores containing FIA sets 2 and set 3 indicate that these rocks never got above

4kbars throughout the Colorado Orogeny. Indeed, they remained at approximately the same

depth with the onset of the much younger Berthoud orogeny, when the pressure decreased

slightly as porphyroblasts formed with inclusion trails preserving FIA set 4. A slightly

clockwise P-T path occurred for both orogenies.

2.0. INTRODUCTION

The calculation of Pressure-Temperature-time and Deformation (P-T-t-D) information for

different events preserved by key index minerals is an important step towards understanding

the defOl'mation and metamorphic history of orogenic belts. Matrix foliations are routinely

reactivated or re-used during younger deformations (e.g. Bell, 1998; Davis, 1993, 1995; Davis

and Forde, 1994; Ham & Bell, 2004) causing them to re-equilibrate to subsequently developed

B 2

PRESSURE-TEMPERA TURE-time-DEFORMATION .............................................. SHAH A.A

P-T conditions (Hickey and Bell, 1996). Deciphering plausible P-T-t-D paths is commonly

difficult and relative to the history of burial, deformation and uplift that the rocks have

experienced, past microstructural approaches provide little information on the topology of a P

T-t-D path associated with porphyroblast growth. Porphyroblasts commonly preserve structural

and metamorphic history that predates what remains in the matrix and provide a means of

accessing the P-T-t-D path for a region (e.g. Spear, 1995). The construction of a quantitative

path is now possible using successions of foliation intersection axes preserved within

porphyroblasts (FIAs) in combination with thermodynamic modeling. Information on the P-T

conditions for the nucleation of garnet for a particular F1A forming event can be determined. If

garnet has grown during the development of a succession of FIA sets, multiple periods of

growth ofthis phase can be distinguished that could not previously be identified.

Thus a FIA approach enables one to track the distribution of minerals along an overall

prograde path at a level of detail that was not previously possible. It should provide insights

into interactions between structural and metamorphic processes that have not previously been

within reach. For example, where one cannot distinguish multiple phases of growth of a single

mineral phase, isograds marking the first appearance of that phase may include several periods

of growth that occurred at very different times. This would disguise any shift in the isograd

with time. Indeed, this has been shown to happen within the area described herein. This

contribution integrates F1As and thermodynamic modelling (MnNCKFMASH) to investigate in

detail the overprinting effects of such a progression on the distribution of garnet, staurolite,

andalusite and cordierite. The area described herein forms part of a large high-T -Iow-P terrane

within the Proterozoic orogenic belt in the southwestern United States (Williams and

Karistrom, 1996).

B 3

PRESSURE-TEMPERATURE-time-DEFORMATJON. ............................................. SHAH A.A


The Colorado province is believed to have formed from addition of juvenile rocks to the

southern margin of the Archean Wyoming craton (Selverstone et ai., 1997; Sims and Stein,

2003). The rocks exposed in the Big Thomson Canyon region, Colorado are mainly

metasediments and granitoids (Figs. 1 and 2). The mature character of the metasediments

suggests deposition in a forearc setting (Condie and Mallell, 1983) although Reed et ai.

(1987) argued for a back-arc setting at around 1.8-1.7 Ma. Recent detrital zircon ages suggest

a maximum deposition age of 1758+26 Ma for the Big Thompson sequence (Selverstone et

ai., 2000). Multiple igneous intrusions at -1.7, -1.4 and 1.1 Ga (Tweto, 1987) are of

intermediate compositions with a calc-alkaline affinity were emplaced synchronous with

regional deformation during the Colorado orogeny, but a few formed post-tectonically (Reed

et ai., 1987).

The metasediments appear to have been repeatedly deformed, metamorphosed and

intruded by plutons in two major orogenic episodes at -1750-1700 Ma and -1400 Ma (e.g.

Karlstrom, et aI., 1997). The rocks show an increase in metamorphic grade towards the west

and north and three stages of folding and cleavage development. The first

deformation/metamorphism is regarded as having occurred at -1750 Ma and resulted in

large-scale isoclinal folds (F I) and a regional axial cleavage S I. The second and third stage of

folding (F2 and F3) occurred around 1726- Ma ago, when these rocks were intruded by the

Boulder Creek granodiorite and related rocks. One period of metamorphism (Ml) has been

associated with these events during which garnet and staurolite grew. A second metamorphic

event (M2), which was stronger than fil'St, resulted in the formation of up to sillimanite grade

mineral assemblages, although metamorphic conditions were very heterogeneous throughout

these episodes. A number of areas record a transition in metamorphic grade from the chlorite

zone to the onset of migmatization during the Colorado orogeny (Selverstone et ai., 1997).

B 4

PRESSURE-TEMPERATURE-time-DEFORMATION .............................................. SHAH A.A

4.0. QUANTITATIVE MEASUREMENTS OF DEFORMATION USING FIAS

In the past geologists have used one, sometimes two and rarely three thin sections from a

sample to study processes of deformation and metamorphism. The measurement of one FIA

requires 8 spatially oriented thin sections to be cut from a single sample. This approach

reveals much more information than just one or two thin sections per sample (e.g., Sayab,

2005) and if this had been done with these rocks, most of the deformation and metamorphic

history would have been never recognized. Samples in close proximity to each other can

preserve a different portion of the history because deformation partitions so heterogeneously

through an outcrop (e.g. Bell et aI., 1998). In many samples, porphyroblasts are absent in

some slides and if these were the only ones cut then the history they contain would have been

lost. The ease with which 3-D information can be obtained on foliation orientations in

porphyroblasts and the matrix and the access via a microprobe and whole rock XRF to PT

and age information is extraordinary once 8 thin sections are available per sample.

A total of 77 oriented samples were examined, 800 oriented thin sections prepared

and 145 FIA and pseudo-FIA trends determined (Table I, Fig. 2). A minimum of eight

vertically oriented thin sections around the compass from each rock sample allows the

measurement of one FIA trend through locating the switch in inclusion trail asymmetry

(clockwise or anticlockwise) within the porphyroblasts (e.g. Bell et aI., 1998). The relative

timing of a consistent FIA succession for a region can be established from samples

preserving FIAs that vary in trend from the porphyroblast core to rim (e.g. Bell et aI., 1998).

Figure 3a shows a rose diagram of all FIA trends measured within four porphyroblastic

phases. Sixty-four FIAs are preserved in garnet (Fig. 3b) and sixty-one in staurolite

porphyroblasts (Fig. 3c). FIA data from both phases are combined in Fig. 4d. FIAs were also

measured in samples containing cordierite and andalusite porphyroblasts; fOUlieen from the

former and six from the latter (Figs. 3e and 4t). Five samples preserve a different FIA in the

B 5

PRESSURE-TEMPERATURE-lime-DEFORMATION. ............................................. SHAH A.A

core versus the rim in garnet porphyroblasts (Table 1 and Fig. 4a, b, c). A few samples

contain porphyroblasts that overgrew a differentiated crenulation cleavage where the

asymmetry of the crenulated cleavage can be determined (Table 1 and Figs. 4d and e). The

crenulated cleavages consist of qUaJ1z and ilmenite grains, while the differentiated

crenulation cleavages predominantly contain ilmenite grains. The orientation of the axis of

the crenulated cleavage can be determined using the asymmetry of its curvature into the

differentiated crenulation cleavage in the same manner in which a FIA is measured and is

called a pseudo-FIA (P-FIA) because it did not form during porphyroblast growth.

5.0. THE FIA SUCCESSION

Figure 5 shows the location and trend of all the FlAs that have been measured. Garnet is

present in the easternmost samples but staurolite is not. Furthermore, garnet porphyroblasts

are commonly overgrown by staurolite. These features suggest that garnet grew before

staurolite and this characteristic provides one of four criteria that were used to determine the

relative timing of the FIA sllccession. The few samples containing changes in FIA trend from

the core to the rim of garnet porphyroblasts provide a second criterion. A third was provided

by samples containing pseudo (p-FIAs) verslls actual FlAs (e.g. Fig. 4) in garnet and

staurolite porphyroblasts (Table 1). A fourth was provided by those porphyroblasts with trails

truncated by the matrix foliation versus those whose trails were continuous with it (e.g. Figs.

6 and 7). The FIA trends for garnet (Fig. 4b), staurolite (Fig. 4c), and both phases combined

(Fig. 4d) are shown as rose diagrams. Four peaks are apparent. The FIA succession consistent

with all the data ( Shah, Section A) from first formed onwards is FIA 1 (trending NE-SW),

FIA 2 (trending E-W), FIA 3 (trending SE-NW) and FIA 4 (trending NNE-SSW). Figure 5

shows the trends of all F1As and pFlAs across the region grouped according to FIA set and

type and marked according to porphyroblastic phase. FIA sets 1 to 4 are present both in both

garnet and staurolite porphyroblasts. FIA sets 3 and 4 are also present in cordierite and

B 6

PRESSURE-TEMPERA TURE-time-DEFORMATION ............................................. SHAH A.A

andalusite (Fig. 4e and t). This progression accords with the normal prograde

garnet7staurolite7andalusite7cordierite succession of porphyroblastic phases in rocks of

this bulk composition (see below).

5.1. Garnet: Generally, garnet porphyroblasts are euhedral and contain excellent inclusion

trails (Fig. 6a and b). Internal foliations (Si) in these porphyroblasts in most samples are

truncated by the external foliations (Sc) but in some samples they are continuous with the

matrix. FIA sets 1 through 3 in these porphyroblast were always truncated by the matrix

foliations while those preserving set 4 are continuous.

5.2. Staurolite: Staurolite porphyroblasts range in shape from poikilitic euhedral, anhedral to

subhedral and contain excellent inclusion trails that are mainly sigmoidal (Figs. 6c,d and 7).

The relationships between internal (Si) and external foliations (Sc) vary. Most samples

preserving FIAs 1 through 3 contain porphyroblasts where the internal foliations are

truncated by the matrix foliation (Fig. 6c). Some samples contain inclusion trails that are

continuous with the foliations preserved in the matrix with most of these belonging to FIA set

4 (Fig. 7). Some samples preserve evidence of replacement of staurolite by andalusite or and

cordierite (e.g. Fig. 8).

5.3. Andalusite: Andalusite porphyroblast are generally poikiloblastic and extremely large in

some samples (> 2cm to 4 mm). Inclusion trails vary from poorly preserved to excellent (Fig.

6e,t). Shapes of the grains are usually anhedral to subhedral and rarely euhedral. In few

samples andalusite has replaced the micaceous part of the matrix completely and has retained

quartz, opaques, biotite and some graphitic inclusions. Two generations of andalusite growth

are preserved via FIAs with most containing F1A 3. Porphyroblasts containing FIA set 4 have

inclusion trails that look very similar to and are continuous with foliations in the matrix (Fig.

6e). Andalusite porphyroblasts contain sillimanite in a few samples and partially replace

B 7

PRESSURE-TEMPERA TURE-time-DEFORMATION. ............................................. SHAH A.A

staurolite in some others (e.g. Fig. 8a). In some samples andalusite pseudomorphs are well

preserved.

5.4. Cordierite: Cordierite porphyroblasts ranging from euhedral, anhedral to subhedral are

mostly poikiloblastic and contain well-preserved inclusion trails (Fig. 6g,h) that are usually

truncated by the matrix foliation in samples preserving FIA set 3 but continuous in those

preserving FIA set 4 (e.g. Fig. 6g,h). Some cordierite porphyroblasts are corroded and

changed mainly to coarse-grained muscovite. Examples of staurolite being replaced by

cordierite and coarse-grained muscovite are common (e.g. Fig. 9).

6.0. PORPHYROBLAST TIMING

6.1. Via FIAs

Four phases of garnet and staurolite growth can be distinguished using the FIA succession (

Shah, Section A). Staurolite always grows after garnet (Fig. 4). This is shown for samples

(C64 and C78B; Table I), which contain a pFIA (parallel to FIA I) plus FIA 2 within garnet

porphyroblasts and staurolite containing FlA 3. Sample (C65A), contains FIA I in garnet and

FlAs 2 and 3 within staurolite (Table. I). In one sample (C44), FIA 2 is preserved within

garnet and FlAs 3 and 4 within staurolite. One sample (C76) contains FlAs 2 and 3 within

garnet, FlA 3 in andalusite and FIA set 4 in cordierite porphyroblasts. This succession

suggests garnet was followed by staurolite, andalusite and finally cordierite in these rocks.

6.2. Replacement of staurolite by cordierite and andalusite

Both cordierite and andalusite replace staurolite as shown in Figs. 8 and 9. This took place

during FlAs 3 and 4 for andalusite and cordierite. Several samples preserve evidence of

staurolite being replaced by cordierite (Figs. 8c, d and 9 (sample C52)) and coarse-grained

muscovite commonly develops during this process. Staurolite in sample C52, which is locally

partially replaced by cordierite preserves FIA set 2; cordierite that does not replace staurolite

contains FIA set 4. Figure 10 (Sample C98B) shows garnet paltially included within

B 8


staurolite. The inclusions in garnet are geometrically and texturally distinct from those

preserved within staurolite or cordierite porphyroblasts. FIA measurements were impossible

for garnet in this sample, as very few slides contain this mineral but it is partially overgrown

by staurolite and cordierite porphyroblasts and predates them (e.g. Fig. 10).

6.3. Replacement of andalusite by cordierite

Inclusions of andalusite that extinguish simultaneously under cross polars occur within

cordierite in sample e77. A crenulation cleavage in the cordierite preserves a pFIA parallel to

FIA set 3. The PIA was difficult to determine in the cordierite because of the size and

poikiloblastic nature of the latter phase but the crenulation cleavage was continuous with the

foliation in the matrix. The inclusion trails in the andalusite appear weakly crenulated

suggesting that it formed during FIA set 3; they are composed of staurolite, quartz,

muscovite, minor opaques and ilmenite and are geometrically and texturally distinct from

those found within the matrix and the cordierite porphyroblast.

7.0. TIMING OF METAMORPHIC INDEX MINERALS NOT PRESERVING FIAS

7.1 Garnet inclusions within staurolite, andalusite and cordierite

For a few samples FIAs could not be measured in celtain mineral phases and their relative

timing was achieved microstructurally. Figure 11 a, shows an example of a euhedral garnet

porphyroblast included within a poikiloblastic staurolite. The staurolite grains are optically

continuous and contain FIA set 3 (Table 1). A FIA could not be measured for garnet because

it was only present in a few slides but it is locally overgrown by staurolite. Examples of

garnet porphyroblasts included within andalusite and cordierite were also observed (e.g. Fig.

11 b, c). These relationships indicate the growth of garnet prior to staurolite, andalusite and

cordierite.

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7.2. Staurolite inclusions within anda/usite and cordierite

Some staurolite porphyroblasts were found to be partially and/or completely replaced by

andalusite or cordierite (e.g. Figs. 12 and 13). Traces of staurolite are included in some

poikiloblastic andalusite. The scattered grains of andalusite are optically continuous (e.g. Fig.

12). In sample C68A, I thin section out of 16 contains cordierite that has partially replaced

staurolite; the cordierite grew on the edge of the staurolite porphyroblast where it was

wrapped by a differentiated cleavage. No changes in foliation orientation occur from inside to

outside cordierite. In sample C68B, cordierite occurs in I thin section out of 8 where it has

replaced staurolite. Cordierite overgrows the matrix foliation with no deflection of this

foliation on its boundary. In sample C66, lout of 18 thin sections contains cordierite that

overgrew the matrix foliation with no deflection of this structure at its boundary. In sample

C96B, cordierite has replaced staurolite. FIA measurements were impossible for cordierite in

this sample (e.g. Fig. 13). Sample C92 contains andalusite porphyroblasts in 3 out of 7 thin

sections. In most they overgrew fold hinges in the matrix without any deflection of the

foliation from inside to outside of the porphyroblasts. Sample C55B contains porphyroblasts

of andalusite in 2 out of 18 thin sections. Sericitized masses occur in 5 of these slides that

could have been andalusite or staurolite. These microstructures suggest the above-described

transformation of staurolite into andalusite and cordierite took place very late during the

orogenic history.

7.3. Garnet, anda/usite and staurolite inclusions within cordierite

Inclusions of garnet and andalusite are found within porphyroblasts of cordierite in a few

samples (e.g. C93A Fig. l4a). In C76 (Fig. l4b), staurolite and andalusite inclusions are

preserved within cordierite porphyroblasts. The timing of staurolite versus andalusite could

not be determined. Remnants of inclusions within the andalusite, which are mainly composed

of quartz, muscovite, biotite, minor opaques and some ilmenite grains, are geometrically and

B 10


texturally distinct from those found within both cordierite and the matrix. Staurolite growth

may have been followed by the formation of andalusite and cordierite (see below).

7.4. Sillimanite timing

Sillimanite is always younger than any porphyroblastic phase preserved in these rocks (e.g.

Fig. 15). In sample C77 it grew within andalusite that preserves FIA set 4. The other samples

where sillimanite has been found are those containing rare examples of cordierite (C68A,

C68B, C96B, CliO) or andalusite (CSSB, C67, C66, CIOI, C92; Fig. 2) porphyroblasts.

8.0. DATA FOR THERMOBAROMETRY

8.1. P-T pseudosections

Major element bulk rock analyses for seven samples selected to cover the isograd transition

were measured using the Siemens SRS3000 XRF at the Advanced Analytical Centre (ACC),

of James Cook University. A representative portion of each of these samples was crushed to

obtain SO-100mg of powder for XRF analysis. Table 2 shows the elemental data of these

samples. Pseudosections were then calculated in the MnO-Na20-Cao-K20-FeO-MgO-Ah03-

Si02-H20 (MnNCKFMASH) system using THERMOCALC 3.23 (Holland and Powell,

1985; 1990; 1998; Powell et aI., 1998), the dataset of Holland and Powell (1990) and the

mixing models of Tinkham et al. (2001). MnO was included in the model, since it has long

been recognised as expanding the stability fields for garnet (e.g. Tinkham et aI., 2001; Kim

and Bell, 200S). In all rocks, plagioclase was observed to be present in matrix phases and as

inclusions within other minerals. On the basis of compositional and mineralogical criterion,

MnNCKFMASH system modelling was considered suitable for these rocks (e.g. Fig. 16). In

all calculations, plagioclase and quartz were assumed to be in excess and kyanite was

omitted, as no sample enclosed this mineral. The water activity was assumed to be 1. The

limitations of these procedures have been published in several aIlicles (e.g., Vance and

Mahar, 1998; Tinkham et aI., 2001).

B II


8.2. Garnet Core Isopleths

Compositional zoning patterns in garnet porphyroblasts have been used for decades to

understand and interpret the metamorphic conditions that lead to their growth zoning patterns

(e.g. Tracy, 1982; Spear et aI., 1990; Carlson, 1999). The P-T conditions of garnet core

nucleation can be estimated using a tight intersection in PT space of its XMn (spessaltine), Xc.

(grossular), and XFe (almandine) isopleths (e.g., Vance and Mahar, 1998; Evans, 2004;

Sayab, 2005; Cihan et aI., 2006). X-ray maps for 4 of the above 7 samples (e.g. Figs. 17 and

18), one from each of the 4 FIA sets (Table 3), preserve normal growth zoning (e.g. Hirsch et

aI., 2003). One sample (C83) preserves slight resorption at the outer edge of the garnet rim

(Fig. 17b), shown by its pyrope and spessartine content.

Sample CIl7E (FIA set 1) contains garnet, staurolite, biotite, muscovite, plagioclase and

quartz with minor zircon, chlorite, apatite or monazite. Compositional isopleths for XMn

(0.1260), XFe (0.7555) and Xc. (0.0361) in the garnet core intersect tightly in the chl-bi-g-mu

stability field close to the st-in line (Fig. 19a; the small triangle formed by their intersection

lies well within the area of overlap of the 1 (J errors). It suggests garnet nucleation at 540-

550° C and 3.8-4.0 kbar (Fig. 20).

Sample C84 (FIA set 2) contains garnet, staurolite, biotite, muscovite, plagioclase and quartz

with minor chlorite, zircon, xenotine or monazite. In some thin sections, fine-grained

muscovite pseudomorphs after garnet and staurolite are common. Compositional isopleths for

XMn (0.1150), XFe (0.7458) and Xc. (0.0509) in garnet cores intersect tightly in the chl-bi-g

mu stability field (Fig. 19b; the small triangle formed by their intersection lies well within the

area of overlap of the 1 (J errors). It suggests garnet nucleation at 525-537°C and 3.40-3.65

kbar (Fig. 20).

Sample C82 (FIA set 3) contains staurolite, garnet, biotite, quartz and muscovite and minor

chlorite, ilmenite, zircon, apatite or monazite. Compositional isopleths for XMn (0.1418), XFe

B 12


(0.7246) and Xc. (0.0458) in the core of a garnet porphyroblast tightly intersect in chl-bi-g

mu stability field (Fig. 19c; the small triangle formed by their intersection lies well within the

area of overlap of the I a errors). It suggests garnet nucleation at 525-535°C and 3.3-3.6kbar

(Fig. 20).

Sample C54C (FIA set 4) contains garnet, cordierite, plagioclase, biotite, quartz, and

muscovite and minor chlorite, ilmenite, zircon, apatite or monazite. Fine-grained muscovite

pseudomorphs after staurolite are common and the latter phase is locally replaced by

cordierite. Compositional isopleths for XMn (0.0905), XFe (0.7918) and Xc. (0.0422) in garnet

cores tightly intersect in the chl-st-bi-g-mu stability field (Fig. 19d; the small triangle formed

by their intersection lies well within the area of overlap of the I a errors). It suggests garnet

nucleation at 537-550° C and 3.3-3.71 kbar (Fig. 20).

8.3. Major elements

For each of the above 4 samples major element compositions were determined using an

average of nine spots with a beam diameter of 31lm at 15kY and 20nA (for garnet rims a

beam diameter of I Ilm was used).

Garnet: The composition and stoichiometry of garnet cores and rims are shown in Tables 3

and 4. These samples were taken from sillimanite free zones. The calculated end-members

and structural formulae of their analyses are also shown. These calculations are based on

cations per 12 oxygen. The oxide totals are close to 100 weight percent with one analysis

value greater than 101 weight percent. The cation sums generally approach the ideal value of

8. The almandine content in their cores is less than in rims. The grossular, pyrope and

spessartine contents are greater in the cores. The overall chemistry of the analysed garnets

suggests a typical prograde zoning (e.g. Hirsch et aI., 2003). All samples preserve prograde

growth zoning signatures. The pyrope and spessartine content at the outer edge of the garnet

rim of sample C83 (Fig. 17b) suggests slight resorption.

B 13


Plagioclase: Table 5 shows the electron-microprobe derived chemical compositions of

plagioclase with the calculated atomic proportion and end-members. Structural formulae

were calculated on the basis of 8 oxygen atoms. The oxide totals generally are around 100 to

98 weight per cent and the cation sums are always close to the ideal value of 5. Small

deviations from ideal NaSi=CaAI substitution shown by some analyses may depend on

inaccuracy of the analyses and/or reflect the effect of alteration. Plagioclase can be

considered a solid solution of albite and anorthite. The K content is generally very low (0.001

to 0.003 atoms p.f.u.). All samples, have plagioclase as an oligoclase with An content fi'om

An 25 to An 29 (Table 5). Only one sample has plagioclase as andesine (in matrix) with An

content about An 31.220 (Table 5).

Staurolite: Table 6 shows the electron-microprobe derived chemical compositions of

staurolite with the calculated atomic proportions. Structural formulae were calculated on the

basis of 46 oxygen atoms. For the Si-O tetrahedra AIlv was assigned by calculating the

difference between 8 and Si. Si ranges from 7.144 to 7.717 atom p.f.u and the corresponding

increase in AIlv ranges from 0.283 to 0.856 atom p.f.u. The octahedral sites are filled with

AlvI (calculated as the difference between the Aholal and AIlv), Ti and Mg. The octahedral Al

ranges from (17.493 to 18.344 atom p.f.u). The majority of the Fe-O tetrahedra are occupied

by Fe (in average 3.46 atoms p.f.u) followed by Mg. The Mn content usually is low, with the

highest value of 0.09 atoms p.f.u. in one sample. Ca is very low, ranges from 0.001-0.004

atoms p.f.u.

Mica: Tables 7 and 8 show the electron-microprobe derived chemical compositions of

muscovite and biotite from with the calculated atomic proportions. Their structural formula

was calculated on the basis of 22 oxygen atoms. There is no significant variation in Si and

corresponding AJ'v content, calculated as the difference between 8.0 and the number of Si

atoms. All micas are saturated with Ti, because of their coexistence with ilmenite and

B 14

PRESSURE-TEMPERATURE-time-DEFORMATION. .............................................. SHAH A.A

sagenitic rutile. The presence of aluminous phases (garnet, staurolite, andalusite and

sillimanite), has caused Al saturation in these micas. Fe3+ was not calculated although the

presence of ilmenite and graphite would restrict its palticipation to a minimum level (Guidotti

and Sassi, 1998a, 1998b). The end member calculation was performed as described in

Holdaway et al. (1988).

The Alvl content in muscovite grains varies from 3.954 to 4.085 atoms per formula

unit (p.f.u). The octahedral sites are predominantly occupied by Al ions (-95%). The

remainder contain Fe and roughly equal proportion of Ti and Mg components. The Mn

content of all muscovite grains is very low « 0.01 cations p.f.u). Octahedral site occupancy

(ideally 4.00 atoms p.f.u.) varies from 4.259 to 4.328 atoms p.f.u. suggesting minor

divergence from dioctahedral micas in all analyses. The twelve-fold interlayer site is largely

filled by K (varies from 0.873-1.129 atoms p.f.u ) and minor amounts of Na (0.13-0.234

atoms p.f.u). The amount of Ca is negligible in all samples, except sample C75, which

contain 0.002 atoms p.f.u. The total occupancy of the interlayer site ranges from 1.006 to

1.363 atoms p.f.u. The H30Kl and NH4K.l substitutions and vacancies in twelve-fold sites

due to charge balance constraints caused by other substitutions in tetrahedral, octahedral and

twelve-fold sites could explain the deficiencies in twelve-fold sites in muscovite (e.g.

(Guidotti and Sassi, 1998a; Guidotti and Sassi, 1998b).

The octahedral sites in biotite are predominantly filled by Fetot,l (2.94-3.518 atoms

p.f.u.) and Mg (1.879-1.336 atoms p.f.u.). Octahedral Al in biotite (which is defined as the

difference between Altot,l and Allv) is roughly constant with an average of 0.97±0.02 atoms

p.f.u., which suggests operation of AI-Tschermaks substitution (Fe, Mg)SiAIIV.lAllv.l. Biotite

in all samples is saturated in Altotal due to coexistence with aluminous phases (garnet,

staurolite, andalusite, sillimanite). Ti contents in biotite range from 0.071 to 0.097 atoms

p.f.u. with no clear correlation with the contents of Fe and Mg. However, they roughly show

B 15

PRESSURE-TEMPERATURE-!ime-DEFORMATION. ............................................. SHAH A.A

an increasing trend with metamorphic grade. Concentrations of Mn are typically low,

between 0.004 and 0.020 atoms p.f.u. The sum of the octahedral cations analysed is below

the theoretical value of six atoms p.f.u. It is always above 5.70, implying no significant

amount of Fe3+ in the biotites. Supposedly, two atoms p.f.u. are allocated to the twelve-fold

site in biotite. The interlayer site in the analysed biotite is dominated by K (1.677-1.877

atoms p.f.u.) with a smaller amount of Na (0.02-0.086 atoms p.f.u.) and Ca (> 0.05 atoms

p.f.u.).

The impOltant compositional trends in muscovite and biotite can be summarised as follows:

I. Si4+ decreases (hence, AllY increases) systematically with increase in grade.

2. Alv1 increases fairly systematically, except for sample C75, where it decreases.

3. On the average, Ti and :EVl increases with increase in grade.

9.0. INTERPRETATION

The consistent FJA succession ( Shah, Section A) and matching progression in ages ( Shah,

Section C) reveal that none of the porphyroblasts rotated during any of the deformations that

postdated their growth

9.1. The role of deformation partitioning in porphyroblast growth

Multiple phases of growth of these porphyroblasts occurred during each of the different bulk

shortening directions defined by the F1As because:

I. None of the samples preserve all the 4 sets of FlAs. Most of them contain 1 to 2 sets of

F1As and rarely 3 sets within a particular mineral phase. Therefore the growth of these

phases was controlled by the partitioning of deformation across the outcrop and where it

did not partition at a suitably fine scale, no porphyroblast growth occurred.

2. F1As are defined by the foliations that resulted from periods of bulk shortening; if one or

more of the set of 4 F1As measured is missing from a sample then it is most likely that

B 16

PRESSURE-TEMPERA TURE-time-DEFORMATION ............................................. SHAH A.A

foliations did not form for that direction of bulk shortening in that vicinity once the PT

conditions for porphyroblast growth had been achieved

3. Samples preserving inclusion trails of FIA set 1 through 3 are always truncated by matrix

foliations while those preserving FIA set 4 are mostly continuous with the matrix

foliation.

4. C68A and C68B are adjacent samples (Fig. 2) preserving different aspects of the

deformation and metamorphic history. C68B contains FIA sets 1 and 2 in garnet

truncated by the matrix foliation. C68A contains FIA set 4 in staurolite continuous with

the matrix foliation. The early foliation history preserved in C68B was destroyed in the

matrix before the growth of staurolite in C68A.

5. FlAs, which are defined by porphyroblast inclusion trails that are truncated by matrix

foliations in all the following samples except C44, reveal that garnet and staurolite grew

episodically because:-

a. C44 preserves FlA set 1 in the garnet and FIA sets 3 and 4 in staurolite

b. C78B preserves FIA sets 1 and 2 in garnet and FIA set 3 in staurolite

c. C65A preserves FIA set 1 in garnet and FlA sets 2 and 3 in staurolite

d. C64 preserves FIA sets 1 and 2 in garnet and FIA set 3 in staurolite

9.2. Multiple phases of index minerals

The differentiation of mUltiple generations of index mineral phases in these rocks resulting

from the measurement ofFlAs enables the following interpretations:-

Garnet: Multiple phases of garnet growth have long been recognized, as this mineral can

grow due to a variety of reactions as a rock passes through PT space (e.g., Spear, 1995).

These rocks have undergone a minimum offour periods of garnet growth revealed by the FIA

succession and some of these involved at least 2 phases (Shah, Section A). These mUltiple

phases of growth are not due to a reaction pathway involving many different reactions.

B 17

PRESSURE-TEMPERATURE-time-DEFORMATION ............................................. SHAH A.A

Rather, they appeal' to be due to the role of crenulation initiation versus crenulation cleavage

development on porphyroblast growth versus cessation reactions (e.g. Spiess & Bell, 1996;

Bell et aI., 1998, 2003, 2004; Bell & Bruce, 2006, 2007). Figure 3b suggests that the

formation of garnet decreased with time based on the number of samples containing FlAs 1

through 4 across the area possibly due to a reduction in the chemical elements needed for

garnet producing reactions with time (e.g. Fig. 19).

Staurolite: Prior to the quantitative measurement of FlAs, multiple phases of growth of

staurolite could only be distinguished where rare truncational core rim microstructures were

found. A minimum of 4 periods of growth of this mineral occurred as the succession of FlAs

developed. This cannot be explained by a reaction pathway through several different

staurolite producing reactions. Rathel', the cessation and regrowth of this porphyroblastic

phase resulted from the role of crenulation initiation versus crenulation cleavage development

in statting and stopping the reaction (Bell & Bruce, 2006, 2007). Multiple phases of garnet

growth occurred in some samples prior to staurolite growth (Table I). No examples of

staurolite growth followed by garnet growth have been found.

Timing of andalusite growth relative to cordierite: PT pseudosections generally indicate that

andalusite should grow before cordierite. The FIA analysis suggests that this was the case in

these rocks as FIA 3 is more common than FIA 4 in andalusite and FIA 4 is more common in

cordierite than andalusite. Microstructural relationships, such as andalusite preserved within

and replaced by cordierite (e.g. Fig. 14), strongly support this interpretation.

9.3. Mineral sllccession and migrating prograde metamorphic path

The succession of FIA sets preserved within garnet, staurolite, andalusite and cordierite

porphyroblasts from this area, combined with isograd migration (Shah, Section A) from FIA

to FIA indicate a migrating prograde metamorphic path. The metamorphic grade increases

towards the west from the garnet zone defined by the staurolite, andalusite, cordierite and

B 18


sillimanite isograds (Shah, Section A). The FIA succession indicates that biotite, gamet,

staurolite, andalusite, cordierite, and sillimanite grew sequentially similar to other Low-

Pressure High-Temperature (LPHT) regions. Supporting this, traces of gamet have been

found in staurolite but not the reverse (e.g. Fig. 14), suggesting that growth of garnet always

predated that of staurolite in these pelites (Spear, 1995). Inclusions of garnet have also been

found within andalusite and cordierite phases. Similarly, staurolite can be seen as inclusions

within cordierite or andalusite porphyroblasts (e.g. Fig. 8). In some samples, inclusions of

andalusite are preserved within the porphyroblasts of cordierite (Shah, Section A), which

suggests that andalusite must have grown earlier than cordierite.

9.4. P-T pseudosections and garnet nucleation

P-T Pseudoseclions THERMOCALC software is an extensively used rock thermodynamic modelling tool

designed to tackle mineral equilibria problems by using mathematical calculations involving

solid solutions and by solving simultaneous non-linear equations which eventually allows the

construction of phase diagrams (e.g. Powell et aI., 1998). This tool allows one to build a

pseudosection using the bulk composition and mineralogy of a rock sample. In such a phase

diagram system, the P-T proj ection shows all the stable invariant points and univariant lines

for a given chemical system (e.g. MnNCKFMASH). The boundaries between different

mineral assemblage fields in such a P-T diagram represent the location where the mode of a

single phase goes to zero. However, this rule is violated when univariant reaction lines are

stable, along which the mode of two phases concurrently goes to zero (e.g. Tinkham et aI.,

2001).

Assumptions 1. It is generally assumed that all mineral assemblages considered for construction of a P-T

pseudosection are in equilibrium. This may not be true, as disequilibrium mineral

crystallization can take place.

B 19


2. The bulk composition of a rock remains unchanged during metamorphism. This is

generally accepted as a fact during the bulk of the modelling processes although it actually is

not true. Several studies have reported a significant loss of materials (mostly Si02) during

metamorphic processes of pelitic rocks (e.g. Ague, 1991; Williams et aI., 2001). Such an

error can partly be rectified by assuming quartz is in excess during THERMOCALC

modelling (e.g. Vance and Mahar, 1998).

A more concerning issue is that the bulk composition of a rock essentially gets modified

when porphyroblasts such as garnet starts to grow. Such minerals take material into their

strllcturallattice, which becomes isolated from the rock reaction system (Vance and Holland,

1993; Marmo et aI., 2002). Therefore, the effective bulk composition of such a reacting

system may not be suitably modelled by using XRF data (the effective bulk composition is

the composition of the volume of rock in which the equilibration of minerals is achieved and

maintained at a given stage in the metamorphic evolution, e.g. Stiiwe, 1997). The outer rim of

a garnet pOl'phyroblast is considered to be in equilibrium with the matrix phases and therefore

the contribution of its core is essentially removed from the whole rock bulk composition (e.g.

Marmo et aI., 2002). However, most of the garnets studied herein were flooded with

inclusions. Therefore, the material isolated within garnet may not be that different from the

bulk composition. This suggests that the effective bulk composition is not significantly

changed by garnet growth (e.g. Vance and Mahar, 1998).

Since it is difficult to accurately estimate the different effective bulk compositions

experienced by a sample, using the whole rock XRF analyses as a first approximation when

building pseudosections is a suitable approach (Stliwe and Powell, 1995).

3. It is assumed that fluid (H20) is present in excess. It is true during prograde

metamorphism, where the bulk of it is produced by various reactions. However, during

B 20

PRESSURE-TEMPERATURE-lime-DEFORMATION. ............................................. SHAH A.A

retrograde metamorphism, care must be taken while constructing and interpreting a P-T

pseudosection (e.g. Guiraud et aI., 2001).

The P-T conditions of nucleation of garnet cores can be estimated using a tight

intersection on these pseudo sections between XMn (spessartine), Xc. (grossular), and XFe

(almandine) isopleths. Garnet nucleated in samples C117B, C83 and C82, which preserve

FIA sets 1, 2 and 3 at 540-550° C and 3.8-4.0 kbar, 525-537°C and 3.40-3.65 kbar and 525-

535°C and 3.3-3.6kbar respectively. It always nucleated within the chl-bt-mu-g stability field

(e.g. Figs. 20) and allowing for error appears to have followed a slightly clockwise path (Fig.

20), which would explain the growth of andalusite before cordierite. Only during FIA set 4,

which formed ~250 million years later, did garnet nucleate within the chl-bi-mu-g-st stability

field (Fig. 19d; 537-550° C and 3.3-3.71 kbar). Garnet nucleation is possibly overstepped in

temperature because the garnet core isopleths for Ca, Fe and Mn always intersect ~20°C

above the garnet-in line obtained from the sample pseudosection.

9.5. Multi-equilibrium thermo barometry and Average PT

Garnet that grew dlll'ing FIAS I, 2 and 3 always contains inclusion trails that are truncated by

the matrix foliation. Only garnet that grew during the development ofFIA 4 contains inclusion

trails that are continuous with the matrix foliation. Garnet is a very refractory mineral and

diffusion of Ca, Fe and Mg within its rim only takes place at temperatures around 620°C

(Tracy, 1982). Re-equilibration with the matrix of garnet that grew during the development of

FIAs I, 2 and 3 is likely to be difficult at temperatures lower than 620°C. Prior to being able to

distinguish periods of garnet growth using FIAs, garnet rim and matrix mineral compositions

(e.g., staurolite, plagioclase, biotite and muscovite) would have been measured in order to

calculate the P-T conditions of the matrix using the average P-T mode in THERMOCALC

(Powell et al. 1998). So this was attempted to see what an approach without a FIA control

would suggest for PT conditions (Tables 4-6). The activities of these phases were calculated

B 21


with AX-software using simple activity-composition models for natural rock-forming minerals

(Holland and Powell, 1990). For each sample, average P-T conditions with their uncertainty

(+1- sigma) values are derived from an independent set of equilibria. The correction coefficients

"cor" values between the pressure and temperature are positive and higher than 0.7, which

suggests that all uncertainty ellipses are moderately flattened and their main axis has a positive

slope (powell and Holland, 1985; 1988). For each sample, the diagnostic parameter called

"sigfit" is lower than the cutoff value. Therefore it can be suggested that a solution for P-T has

been found, which is consistent with the input data within their unceltainties.

All four samples yield matrix P-T conditions in the range of 560-620 and 3-4kbars

which puts the final mineral assemblages in the stability field of sillimanite (Fig. 21). Yet very

few samples preserve late stage sillimanite growth. Where this did occur it postdated the

development of FIA 4 suggesting a late heating event. The average T of the matrix calculated

by this approach was 40 to 60°C hotter than the core for each FIA and essentially did not

overlap (allowing for error) for FIA 3 versus FIAs 1,2 and 4. The pressure dropped for FIA 1,

rose slightly for FIAs 2 and 4 and significantly for FIA 3. This suggests the refi'actory nature

of garnet causes significant problems with the average PT approach and conditions derived

from them should be used very cautiously.

9.6. Presslire-Temperatlire-time-Deformation (PTtD) path

The P and T changed little (a weak ACW path) through the 100 million year

development of FIAs 1 through 3 and slightly through the ~250 million gap before FIA 4

developed (Fig. 22). The lack of mineral growth, monazite nucleation and FIA formation in

between the two cycles suggests that the unit was exhumed to shallower crustal levels, where

none of this could happen 01' that no deformation partitioned through this region during the

intervening period. In the former case these rocks would have been buried again to the

similar crustal conditions to those recorded at the end of the ~ 1700 Ma orogeny. This is

B 22


suggested by the presence of FlA set 4 inclusion trails, PT conditions of garnet cores

preserving this FIA set and is also confirmed by the monazite growth during that period. The

post FIA controlled late growth of andalusite, cordierite and sillimanite suggests an increase

in temperature and drop in pressure as ductile deformation ceased, probably due to uplift and

granite emplacement. This was followed by the exhumation of these metasediments to the

present PT conditions.

10.0. DISCUSSION

General significance ofFIA measurement approach for metamorphism

Without measurement of the FlAs and recognition of the succession of periods of

minel'al growth that they disclose, the mUltiple periods of growth of garnet, staurolite,

andalusite and cordierite that occurred in this region would not have been distinguished.

Previous mapping shows the effects of mapping isograds without such controls; the isograds

simply equate with the youngest period of metamorphism which happened to be the only

time that sillimanite formed and occurred some 350 million years after tectonism began.

Being able to distinguish isograds within successive periods of growth of the same mineral

phase has allowed the migration of isograds across the region with time to be defined and its

tectonic implications assessed (e.g. Shah, Section A).

10.1. Pseudosection and Textural agreement

Figure 16 shows MnNCKFMASH P-T pseudosections for stable mineral assemblages

for 7 samples. Darker shades are used for higher variances. In general, garnet should initially

be produced in these rocks either by a reaction between chl-mu or chl-mu-bt. Staurolite was

initially produced by the consumption of chlorite. It has a smaller stability field than that for

garnet in all samples (e.g. Fig. 16). The appearance and disappearance of the mineral

assemblages shown in these pseudosections agrees with their observed geometrical and

B 23

PRESSURE-TEMP ERATURE-time-DEFORMATION. ............................................. SHAH A.A

textural relationships suggesting a prograde progression from chlorite-biotite-garnet

staurolite andalusite to cordierite.

10.2. Garnet temperature overstepping

The P-T conditions indicated by the intersection of compositional isopleths for the

samples analysed containing FIA sets 1 through 4 never lie on the garnet in-line. Their

intersection lies within the garnet stability field but at a higher temperature, even taking into

account the unceltainty value of the garnet isopleths given by THERMOCALC.

Possible explanations for this could be:-

1. The highest Mn content found within garnet porphyroblasts may not be the exact core in 3-D

(its core; e.g. Fig. 17).

2. The presence of ilmenite, garnet resorption or post growth diffusion influenced the Mn

content ofthe core (Vance and Mahar, 1998).

However, the fact that garnet nucleation was overstepped in every case suggests that growth of

this phase was not initiated until well above the temperature conditions of equilibrium (e.g.

Deguet et aI., 2007, Stowell et aI., 2001). Nucleation of any mineral phase requires that P-T and

bulk composition should be appropriate for that phase to grow. Howevel; deformation is also

known to play a vital role in formation of different minerals, particularly porphyroblastic

phases (Bell et aI., 1998; Williams, 1994, Cihan et aI., 2006) through its control on sites for the

access of nutrients needed for nucleation and growth (Spiess & Bell, 1996). Therefore, another

explanation for this phenomenon is that garnet growth did not occur until deformation

partitioned through the rocks and, because they had reached a higher temperature before this

occurred, overstepping occurred.

10.3. Metamorphic and tectonic relationships

The date obtained for a palticular FIA set averages ages for all samples containing

that FIA trend. This is a powerful approach as it allow one to group ages to tightly constrain a

B 24

PRESSURE-TEMP ERATURE-time-DEFORMATION ............................................. SHAH A.A

period of deformation and metamorphism. However, multiple foliations would have

developing over most of all of the extended period of time during that a particular direction

of bulk shOitening (relative plate motion - e.g., Bell & Newman. 2006) occurred. This must

be kept in mind when considering the relationship between the observations made herein and

the tectonism that was taking place. Magmatic events occurred in this region from 1750 to

1700 and at 1400 Ma (Tweto, 1987). Yet FIAs 1,2 and 3 developed over a period of time

that extended from at least 1760 Ma to 1674 Ma and so the emplacement of granite occurred

after porphyroblast growth began at this crustal level and continued afterwards. Previous

assessments of the age of Colorado Orogeny in this region range from 1780 to 1700 Ma (Sim

et aI., 2003).

The NW-SE directed shOitening required to produce the SW-NE trend of FIA set 1

accords well with the present orientation of the Cheyenne Belt which is regarded as a suture

zone. The presence of shear zones with the same trend as of FIA set 1 suggests these might

have been formed during the initial accretion process. The N-S directed shortening indicated

by the W-E trend of FIA set 2 suggests that the W-E trending axial traces of the regional

folds (e.g. Fig. 2) formed at this time or were rotated from SW-NE trends into this

orientation. Growth of garnet and staurolite during this FIA set, with a monazite inclusion

age around 1726 Ma, overlaps the intrusion of 1726± 15 Ma trondhjemites in the area

(Selverstone et aI., 1997) that lie sub-parallel to bedding. They could have been emplaced

during compressional or gravitational collapse phases during FIA set 2 as subsequent

deformation would have rotated them into parallelism with the compositional layering. The

SW-NE directed shOitening required to produce the NW-SE trend of FIA set 3 probably

formed folds with NW-SE trending axial traces of the folds in the region (Fig. 2).

B 25


10.4. FIA data and the AUSWUS model

Orogenesis ceased for some 250 million years and then recommenced around 1415±16Ma

when FIA set 4 formed (details of monazite ages in sections C). Cordierite and andalusite

porphyroblasts formed before and after this 250 million year period at similar PT conditions

(Figs. 22 and 23). Indeed, Hui (2009) working in the Arkansa River regin of south central

Colorado has dated a succession of FIAs from 1506 to 1366 Ma that includes the FIA 4

described herein. The lack of a significant change in PT conditions suggests that the

deformation effects of compressive relative plate motion were partitioned elsewhere for 250

million years but without a significant change in the far field deviatoric stress that operated.

Such a scenario could possibly have maintained this region at the same depth. This region

was chosen to test the AUSWUS model for the correlation of PreCambrian in the US and

Australia (Karl strom et aI., 1999). This model was previously proposed my M. Brookfield in

1993 on the basis of matching major lineaments. AUSWUS brings together the remarkably

similar geology of Mojavia, California-Nevada; and the Broken Hill block, Australia (Burrett

and BetTy, 2000)

Our data suggest that the deformations are so different in age to those in NE Australia

that the AUSWUS model appeared unlikely. However, deformation in NE Australia occUl"red

throughout this 250 million year gap from 1670 to 1500 Ma with none occurring for some

100 million years before of afterwards (Rubenach et aI., 2008; T. H. Bell pers. comm., 2010).

Therefore, the lack of change in PT conditions in the Colorado front range may actually

provide support for the AUSWUS model than negating it.

ACKNOWLEDGEMENTS

I would like to thank Professor Tim Bell for his critical comments. Special thanks to Dr Mike

for his help with monazite dating and critical discussions. I would also like to thank Professor

B 26


Jane Selverstone (University of New Mexico, New Mexico) for sending me the manuscripts

of Colorado region.

B 27


REFERENCES

Bell, T. H., Hickey, K. A, Upton, G. J. G., 1998. Distinguishing and correlating multiple


staircase and sigmoidally curved inclusion trails in garnet. Journal 0/ Metamorphic

Geology, 16, 767-794.

Bell, T. H., Ham, A. P., Hickey, K. A., 2003. Early formed regional antiforms and synforms

that fold younger matrix schistosities: their effect on sites of mineral growth.

Tectonophysics, 367, 253-278.

Bell, T. H., Ham, A. P., Kim H. S., 2004. Partitioning of deformation along an orogen and its

effects on porphyroblast growth during orogenesis. Journal 0/ Stn/ctural Geology, 26,

825-845.

Bell, T. H., and M. D. Bruce., 2006. The internal inclusion trails geometries preserved within

a first phase ofporphyroblast growth. Journal o/Structural Geology, 28,236-252.

Bell, T. H., Newman, R. L., 2006. Appalachian Orogenesis: the role of repeated gravitational

collapse. In: Styles of Continental Contraction, eds S. Mazzoli and R.W.H. Butler.

Geological Society 0/ America Special Paper, 414, 95-118.

Bell, T. H., Bruce, M. D., 2007. Progressive deformation partitioning and deformation

history: Evidence from millipede structures. Journal o/Structural Geology, 29, 18-35.

Burrett, C., Berry, B., 2000. Proterozoic Australia-Western United States (AUSWUS) fit

between Laurentia and Australia. Geology, 28, 103-106.

Cao H (2009) Chemical U-Th-Pb Monazite dating of deformations versus pluton

emplacement and the Proterozoic history of the Arkansas River region, Colorado, USA.

Acta Geol Sin-EngI83:917-926

B 28


Condie, K. c., Martel, C., 1983. Early Proterozoic metasediments from north-central

Colorado: Metamorphism, provenance, and tectonic setting. Geological Society of

American Bulletin, 94, 1215-1224.




Davis, B. K., 1993. Mechanism of emplacement of Cannibal Creek Granite with special

reference to timing and deformation history of the Aureole. Tectonophysics, 224, 337-362.

Davis, B. K., 1995. Regional-scale foliation reactivation and re-use during formation of a

macroscopic fold in the Robertson River metamorphics, north Queensland, Australia.

Tectonophysics, 242, 293-311.

Davis, B. K., Forde, A., 1994. Regional slaty cleavage formation and fold axis rotation by re

use and reactivation of foliations: Fiery Creek Slate Belt, North Queensland.

Tectonophysics, 230,161-179.


collisional event: The example of the Thiviers-Payzac Unit in the Variscan French Massif

Central. Lithos, 98, 1-4,210-232.

Guidotti, C. V., and Sassi, F. P.,1998a. Miscellaneous isomorphous substitutions in Na-K

white micas: a review with special emphasis to metamorphic micas. Rendiconti di Scienze

Fisiche, Matematiche e Naturali, Accademia Nazionale dei Lincei, 9, 57-78.

Guidotti, C. V., and Sassi, F P., 1998b. Petrogenetic significance of Na-K white mica

mineralogy: recent advances for metamorphic rocks. European Journal of Mineralogy, 10,

815-854.


Geology, 26, 1898-2009.

B 29


Hirsch, D. M., Prior, D. J., Carlson, W. D., 2003. An overgrowth model to explain multiple,

dispersed high-Mn regions in the cores of garnet porphyroblasts. American

Mineralogist, 88, 131-14l.

Holdaway, M. J., Dutrow, B. L. and Hinton, R. W. (1988). Devonian and Carboniferous

metamorphism in west-central Maine: the muscovite almandine geobarometer; the

staurolite problem revisited. American Mineralogist,73, 20-47.

Holland T .J. B., Powell, R., 1985. An internally consistent thermogynamic dataset with

uncet1atinites and correlations: 2. Data and results. Journal of Metamorphic Geology, 3,

343-370.

Holland, T. J. B., Powell, R., 1990. An enlarged and updated internally consistent

thermogynamic dataset with uncertatinites and correlations: the system K20-Na20-CaO

MgO-FeO-Fe203-Ab03-TiOrSi02-C-H2-02. Journal of Metamorphic Geology, 8, 89-

124.

Holland, T. J. B., Powell, R., 1998. An internally consistent data set for phases of

petrological interest. Journal of Metamorphic Geology, 16, 309-343.

Powell, R., Holland, T., Worley, B., 1998. Calculating phase diagrams involving solid

solutions via non-linear equations, with examples using THERMOCALC. Journal of


Karlstrom, K. E., Dallmeyer, R. D., Grambling, J. A., 1997. 40Ar/39Ar evidence for 1.4 Ga


lithosphere in the southwestern U.S. Journal of Geology, 105, 205-223.

Karlstrom, K. E., Williams, M. L., MCLelland, J., Geissman, J. W., and Ahiill, K.-I., 1999,

Refining Rodinia: Geologic evidence for the Australia-Western U.S. connection in the

Proterozoic: GSA Today, 9, 1-7

B 30


Kim, H., and Bell, T. H., 2005. Combining compositional zoning and foliation intersection

axes (FlAs) in garnet to quantitatively determine early P-T-t paths in multiply deformed

and metamorphosed schists: north central Massachusetts, USA. Contributions to

Mineralogy and Petrology, 149, 141-163.

Reed, l C., Jr., Bickford, M. E., Premo, W. R., Aleinikoff, IN., and Pallister, J., 1987.


geochronology. Geology, 15, 861-865.

Rubenach, M. R., Foster, D. R. W., and Evins. P., 2008. Tectonothermal evolution of the

Eastern Fold Belt, Mt Isa Inlier. Precambrian Research, 163, 81-107.



porphyroblasts revealed by independent 3D measurement techniques. Journal 0/

Structural Geology, 27,1445-1468.

Selverstone, l, Aleinikoff, J., Hodgins, M., Fanning, C.M., 2000. Mesoproterozoic

reactivation of a Paleoproterozoic transcurrent boundary in the Colorado Front Range:

Implications for ca. 1.7 and 1.4 Ga tectonism. Rocky Mountain Geology, 35, 136-162.

Selverstone, l, Hodgins, M., Shaw, C., Aleinikoff, J. N., & Fanning, C. M., 1997.

Proterozoic tectonics of the northern Colorado Front Range. In Bolyard, D.W., and

Sonnenberg, S. A., eds. Geologic history of the Colorado Front Range. Denver, Rocky

Mountain Association o/Geology, 9-18.

Sims, Paul K., Stein, Holly J., 2003. Tectonic evolution of the Proterozoic Colorado

province, Southern Rocky Mountains: A summary and appraisal. Rocky Mountain

Geology, 38, 2 183-204

Spear, F. S., 1995. Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths.

Mineralogical Society of America Monograph 2nd, 799.

B 31

PRESSURE-TEMP ERATURE-lime-DEFORMATION. ............................................. SHAH A.A

Spiess, R., Bell, T.H., 1996. Microstructural controls on sites of metamorphic reaction: a case

study of the inter-relationship between deformation and metamorphism. European

Journal of Mineralogy, 8, 165-186.

Stowell, H.H., Taylor, D.L., Tinkham, D.L., Goldberg, S.A., Ouderkirk, K.A., 2001. Contact

metamorphic P-T - t paths from Sm-Nd garnet ages, phase equilibria modelling and

thermobarometry:garnet ledge, southeastern Alaska, USA. Journal of Metamorphic

Geology, 19, 645- 660.

Tinkham, D.K., Zuluaga, C.A., Stowell, H.H., 2001. Metapelite phase equilibria modelling in

MnNCKFMASH: The effect of variable AI203 and MgO/(MgO + FeO) on mineral

stability. Geological Material Research, 3, 1--42.

Tracy, R.J, 1982. Compositional zoning and inclusions in metamorphic minerals. Reviews in

Mineralogy, 10, 355-397.

Tweto, o. L., 1987. Rock units of the Precambrian basement in Colorado. U.S. Geological.

Survey Professional Paper, 1321-A, 54.

Vance and Mahar, E., 1998. Pressure-temperature paths from P-T pseudosections and zoned

garnets: potential, limitations and examples from the Zanskar Himalaya, NW India,

Contributions to Mineralogy and Petrology, 114, 101-118.

Williams, M.L., 1994. Sigmoidal inclusion trails, punctuated fabric development, and

interactions between metamorphism and deformation, Journal of Metamorphic

Geology, 121-21

Williams, M.L., Karlstrom, K.E., 1996. Looping P-T paths, high-T, low-P middle crustal

metamorphism: Proterozoic evolution of the southwestern United States. Geology, 24,

1119-1122.

B 32

Section C

90 MILLION YEARS OF OROGENESIS, 250 MILLION YEARS OF QUIESCENCE AND THEN FURTHER OROGENESIS WITH NO CHANGE IN

PT: SIGNIFICANCE FOR THE ROLE OF DEFORMATION IN PORPHYROBLAST GROWTH

90 million years of orogenesis. 250 million years ". ". "". SHAHA.A

90 million years of orogenesis, 250 million years of quiescence and then further orogenesis with no change in PT: significance for the role of deformation in porphyroblast growth 1

1.0. Abstract 1

2.0. Introduction 2

3.0. Regional geology and tectonics 3

4.0. Methods 4 4.1. FIA data 4

5.0. Dating of FIA sets 6

6.0. Sample description, results and age interpretation 7 6.1. Sample Cl17 7 6.2. Sample C43 7 6.3. Sample C84 8 6.4. Sample Cl08 8 6.5. Sample C83 9 6.6. Sample C75 9 6.7. Sample C65A 9 6.8. Sample CllO 10 6.9. Sample C77 10 6.10. Sample C55A 11 6.11. Sample C51B 11

7.0. Dating of matrix foliations 12 7.1. Sample C75 (FIA 1 and 2 in staurolite) 12 7.2. Sample C84 (FIA 2 in garnet and 3 in staurolite) 12 7.3. Sample C65 (FIA 2 in garnet and 3 in staurolite) 13 7.4. Sample C43 (FIA I in garnet and 2 in staurolite) 13 7.5. Sample C83 (FIA 2 in garnet and 3 in staurolite) 13 7.6. Sample C51B (FIA 3 in cordierite) 14 7.7. Sample C77 (FIA 3 in cordierite) 14 7.8. Sample CJ08 (FIA 1 in garnet and 2 in staurolite) 14 7.9. Sample ClIO (FIA 4 in andalusite) 14

8.0. Compositional mapping of monazite grains 15

9.0. Interpretation and Discussion 15 9.1. Combining the age data within FIA sets 15 9.2. The significance of FIAs for determining monazite ages 16 9.3. Assessing the spread of the age data from matrix relative to thatfor the F1A sllccession 17 9.4. The porphyroblast ages versus matrix ages 17 9.5. Limitation of FIAs approach for monazite dating 18 9.6. Role of deformation and its significance for porphyroblast growth 18 9.7. Deformation and metamorphism 19 9.8. Regional tectonic implications of Colorado and Berthoud Orogenies 22 9.9. Episodic but continuous porphyroblastic growth during -1700 Ma orogeny 23 Acknowledgements 24

10. References 25

i

90 million veal's of orogenesis. 250 million years ...... .... . SHAHA.A

90 MILLION YEARS OF OROGENESIS, 250 MILLION YEARS OF

QUIESCENCE AND THEN FURTHER OROGENESIS WITH NO CHANGE IN PT:

SIGNIFICANCE FOR THE ROLE OF DEFORMATION IN PORPHYROBLAST

GROWTH

1.0. ABSTRACT

In-situ dating of monazite grains preserved as inclusions within foliations defining FlAs

(foliation inflection/intersection axes preserved within pOlphyroblasts) contained within

garnet, staurolite, andalusite and cordierite porphyroblasts provides a chronology of ages

that matches the FIA succession for the Big Thompson region of the northern Colorado

Rocky Mountains. FIA sets I, 2 and 3 trending NE-SW, E-W and SE-NW formed at

1760.5±9.7, 1719.7±6.4 and 1674±1l Ma respectively. For 3 samples where garnet first

grew during just one of each of these FlAs, the intersection of Ca, Mg, and Fe isopleths in

their cores indicate that these rocks never got above 4kbars throughout the Colorado

Orogeny. FUlthermore, they remained around approximately the same depth for ~250

million years to the onset of the younger Belthoud Orogeny at 1415±16 Ma when the

pressure decreased slightly as porphyroblasts formed with inclusion trails preserving FlA

set 4 trending NNE-SSW. No porphyroblast growth OCCUlTed during the intervening ~250

million years of quiescence, even though the PT did not change over this period. This

confirms microstructural evidence gathered over the past 25 years that crenulation

deformation at the scale of a porphyroblast is required for reactions to re-initiate and enable

further growth.

c 1

90 million years of orogenesis. 250 million years ...... ... .. SHAHA.A

2.0. INTRODUCTION

Multiply deformed and metamorphosed rocks commonly have ambiguous

information housed within their matrix, since each new deformation usually erases the

earlier history preserved (e.g. Ham and Bell, 2004). This primarily occurs because the

matrix generally decrenulates and rotates the early-formed foliations in parallelism with the

compositional layering (e.g. Bell, 1986; Davis, 1995; Bell et a!., 2004; Cihan, 2004).

Deformation partitioning strongly affects such kinds of processes from regional to

porphyroblastic scales and makes it difficult to correlate such processes from one region to

another (Bell et a!., 2004; Cihan and Parsons, 2005). The preservation of multiple early

matrix events as inclusion trails within porphyroblasts has greatly increased our

understanding of complex inclusion trail relationships, which otherwise could not be

interpreted 01' were misleading (Ham and Bell, 2004).

Accurate measurement of the foliation inflection/intersection axes preserved within

different porphyroblastic phases (FIAs) have made it possible to decode lengthy and

complex histories of deformation and metamorphism in orogens around the world (e.g. Bell

and Bell 1999; Bell and Bruce, 2007 ). More than ten years of research and data have

already been published using this technique from tectonically complex regions around the

world (e.g. Bell et a!., 1998; Bell & Chen, 2002; Bell et a!., 2003; Cihan, 2004; Bell et a!.,

2005; Kim and Bell, 2005; Sayab, 2005; Bell & Bruce, 2007).

The integration of detailed microstructural studies and FIA data with garnet isopleth

thermobarometry/MnNCKFMASH pseudosection constlUction can provide complete

pressure-temperature-time deformational trajectories of an area (e.g., Vance and Mahar,

1998; Kim and Bell, 2005; Cihan et a!., 2006).

c 2

90 million years oforogenesis. 250 million years ...... .... . SHAHA.A

Detailed P-T-D paths significantly improve our understanding of large-scale orogenic

processes but the absolute timing of these events remains a vital tool for unravelling the

tectonic evolution of the region. Geometrically and texturally controlled dating methods are

critical for constraining the ages of deformed and metamorphosed sediments and their

textures and foliations (e.g., Rasmussen et aI., 2001; Foster et aI., 2000; 2002; Williams and

Jercinovic, 2002; Cihan et aI., 2006). In pelites and psammites monazite is omnipresent at

amphibolite facies and above and is the typical mineral of choice for in situ geochronology

in such rocks (Dahl et aI., 2005; Williams et aI., 2007).

Dating of monazite using high precision electron microprobe U-Th-Pb techniques

(EPMA) was chosen to link different metamorphic and deformational events (e.g., Montel

et aI., 1996; Shaw et aI., 2001; Dahl et aI., 2005; Goncalves, 2005; Cihan et aI., 2006)

because the bulk of the monazite grains analysed were 25 ~m or smaller in size. Dating of

monazite inclusions within different FIA sets (Bell and Welch, 2002) can potentially

provide a robust tool for understanding and unravelling lengthy and complex orogenic

histories (Cihan et aI., 2006; Sayab, 2005). Integration of FIAs with this approach could

provide a strong basis for studying the complex pressure-temperature-time-deformation

(PT -t-D) paths that rocks appear to have followed. This paper reports the results obtained

from adapting this approach to the rocks collected in and around the Big Thompson region

of Colorado (Fig. 1).


The rocks exposed in the Big Thomson Canyon region, Colorado, USA, are mainly

metasediments and granitoids (Fig. 2). Condie and Martell (1983) suggested the

metasediments represent mature sediments deposited in a forearc setting. Reed et al. (1987)

c 3


argued that they were possibly deposited in a back-arc setting between two ~ 1.8-1.7 Ma

magmatic arc systems. Recent detrital zircon ages suggest a maximum age of 1758+26 Ma

for deposition of the Big Thompson sequence (Selverstone et ai., 2000). These sediments

were repeatedly deformed, metamorphosed and intruded by various plutons (e.g., Braddock

and Cole, 1979; Selverstone et ai., 1997; Sims and Stein 2003) during the Colorado (~1700

Ma) and Belthoud (~1400 Ma) Orogenies (Tweto, 1987; Nyman et ai., 1994; Karistrom, et

ai., 1997). The rocks show an increase in metamorphic grade towards the west and north

and three stages offolding and cleavage development (Cavosie and Selverstone, 2003).

The first deformation/metamorphism occurred before 1750 Ma and resulted in

large-scale isoclinal folds (F 1) and a regional axial cleavage S 1. The second and third stage

of folding (F2 and F3) occurred around 1750 Ma ago, when these rocks were intruded by

the Boulder Creek granodiorite and related rocks. Only 1 period of metamol]Jhism has been

associated with these events (Ml) during which garnet and staurolite grew. The second

metamorphic event (M2), which was stronger than first, resulted in the formation of up to

sillimanite grade mineral assemblages (Sims and Stein, 2003), though metamorphic

conditions were very heterogeneous throughout these episodes. A number of areas recorded

an entire transition in metamorphic grade from the chlorite zone to the onset of

migmatization during the Colorado orogeny (Braddock and Cole, 1979; Selverstone et ai.,

1997).

4.0. METHODS

4.1. FIA data

A total of 67 oriented samples were examined for the present research. 800 oriented thin

sections were prepared and a total of 138 FIA and pseudo-FIA trends were determined

c 4


(Table 1, Fig 2). These measurements were achieved by cutting a minimum of eight

vertically oriented thin sections around the compass from each rock sample (Fig. 3) and

then locating the switch in inclusion trail asymmetry (clockwise or anticlockwise) within

the porphyroblasts (e.g. Bell et aI., 1998). Garnet, staurolite, andalusite and cordierite

porphyroblasts preserve earlier foliations as inclusion trails. These foliations are most

commonly straight with curvature at their extremities (e.g. Fig. 4a). Porphyroblast inclusion

trails are commonly truncated (e.g. Fig. 4a) by the matrix foliations but some are not (e.g.

Fig.4b).

A relative timing and thus a FIA succession can be established from samples

preserving a FlA trend that varies from core to the rim of the porphyroblast (e.g. Bell et aI.,

1998). All FIA measurements are plotted on rose diagram and are shown in Fig. Sa. A total

of 64 and S3 FlAs were measured in garnet and staurolite porphyroblasts respectively (Fig.

Sb and c). The combined FIA trend data for garnet and staurolite is shown in Fig. Sd .. The

other porphyroblastic phases in which FIAs were measured were andalusite and cordierite,

with seven measurements in the former and fourteen in the latter. Their trends are given in

Table 1 and are shown on a rose diagram in Fig. Se and f. A few samples maintain

differentiated crenulation cleavages that have been overgrown by the porphyroblasts where

the asymmetry of the crenulated cleavage can be determined. The crenulated cleavages

consist of qUattz and ilmenite grains, while the differentiated crenulation cleavages

predominantly contain ilmenite grains. The intersection between the crenulated and

crenulation cleavages can be determined, when viewed in three dimensions, and is called a

pseudo-FIA (pseudo-FlA). The actual FIA is formed during porphyroblast growth and in

these samples is defined by the curvature of the differentiated crenulation cleavage. All

measured trends were plotted on a rose diagram as shown in Fig. S.

c 5


5.0. DATING OF FIA SETS

To determine the age of the four FIA sets measured in the area (Thesis Section A), 30

samples were selected for monazite dating. Polished thin sections were made and carbon

coated for use in the JEOL JXA-8300 Superprobe. Only 11 samples out of the 30 selected

contained monazite grains large enough for precise age calculations. Detailed pre-dating

maps were produced from each polished thin section to accurately locate monazite grain

and their textural setting. The analytical procedure is outlined in Table 2. The samples were

analysed with a 1-2 micron meter diameter beam at 15kv and 200nA. The collimators were

opened to a maximum (3mm) and the PHA settings were optimized as well. In all these

measurements, 1trz matrix corrections were performed using standard Pb, U, Th, and Y

concentrations in combination with the preset values for other elements (P 33.3, La 14.5,

Ce 26, Pr 2.6, Nd 10.3, Sm 1.5, Gd 1.48, Dy 0.82, Si 0.25, Ca 0.55 wt. % oxides).

Interference cOlTections of Th and Y on Pb Ma and Th on U MP were executed as in Pyle

et al. (2002). An internal standard monazite from Manangotry in Madagascar of 545+/-2Ma

(Paquette et aI., 1984) was analysed three times before and after each analytical session.

Chemical ages were calculated as described in Montel et al. (1996). Geologically

significant age information can be derived by assuming low amounts of common Pb (e.g.,

Pan'ish, 1990; Gaidies et aI., 2008) and slow diffusion rates for Th, U and Pb in monazite

(Cherniak et aI., 2004). The samples were chosen based on FIA set and the grains were

isolated and clustered according to their age, textural setting and whether any chemical

zonation was present (Cihan, et aI., 2006). This would potentially reduce any error and

make the age information reliable (Montel et aI., 1996; Pyle et aI., 2005; Gaidies et aI.,

2008). Dates and elTors were determined by mean age with standard errors at the 95%

c 6

90 million years of orogenesis. 250 million years ...... .... . SHAHA.A

confidence level for a cluster of spots analysis within a single age domain or grain. Ages

were then calculated for all the grain populations analyzed and plotted using software

Isoplot (Ludwig, 1998).

Three samples contained monazites grams big enough to extract valuable age

information in garnet porphyroblasts. Six contained suitable monazite grains in staurolite

porphyroblasts. Two contained suitable monazite grains in andalusite plus cordierite.

6.0. SAMPLE DESCRIPTION, RESULTS AND AGE INTERPRETATION

Unless otherwise stated monazite inclusions lie with the foliation defining the FIA set for

that mineral phase. All rocks contain biotite, muscovite, plagioclase and quartz with

accessory phases ilmenite and apatite. Quartz and apatite and rarely muscovite, biotite,

chlorite inclusions are always present within both garnet and staurolite porphyroblasts.

Monazite is always present within staurolite but not necessarily in garnet phases ..

6.1. Sample C117

Garnet (FIA set 1) and staurolite (FIA set 2) inclusion trails are always truncated by the

matrix foliation. Extra minor phases include zircon and xenotine. Two monazite inclusions

within garnet have given a mean age spread of 17S6±22 Ma (see Appendix C; Fig. 6).

Age interpretation: Garnet in this sample preserves the oldest deformation event recorded

in the area. The date obtained ii-om this porphyroblast agrees with the other samples

containing the same FIA sets.

6.2. Sample C43

Garnet and staurolite pOlphyroblasts contain FIA sets 1 and 2, respectively. Foliations in

the matrix always truncate those in the porphyroblasts. Staurolite in some thin sections is

c 7


replaced by andalusite and fine-grained muscovite. One monazite grain in staurolite was

dated at 1724±19 Ma (see Appendix C). No monazite grains were found in garnet.

Age interpretation: The age obtained in staurolite accords with the dates obtained for other

samples containing the same FIA set.

6.3. Sample C84

Garnet and staurolite inclusion trails are always truncated by the matrix foliation. Extra

accessory phases include magnetite, zircon, xenotine and monazite. A total of 2 monazite

grains were dated from this sample. One within garnet with an age spread of 1762±21 Ma

(FIA set 1) and other grain within staurolite (FIA set 3) with an age spread of 1683±36 Ma

(see Appendix C).

Age interpretation: The 1762±21 Ma monazite age in garnet accords with the dates

obtained from other samples and fits well with its FIA (see below). The younger grain

stored within staurolite is ellipsoidal and aligned parallel to the foliation defined by the

inclusion trails and should give a representative age for FIA 3

6.4. Sample CI08

Garnet and staurolite porphyroblasts contain FIA sets 1 and 2 respectively. The foliations

within both phases are tluncated by those in the matrix. Extra accessory phases include

apatite and zircon. Two monazite grains within staurolite are dated at an average age of

1721±14 Ma (see Appendix C). No monazite grains were found in garnet.

Age interpretation: The age obtained in staurolite accords with the dates obtained for other

samples containing the same FIA set.

c 8

90 million years o(orogenesis. 250 million years ...... .... . SHAHA.A

6.5. Sample C83

Garnet and staurolite contain FIA sets 2 and 3 respectively. Their inclusion trails are

truncated by the matrix foliations. Length fast chlorite with light green to colourless

pleochroism and grey to greyish green birefringence, suggesting that it is rich in Mg,

appears to be prograde (e.g., Albee, 1962). One monazite grain within staurolite has an age

of 1681±27 Ma (see Appendix C; Fig. 7). No monazite was detected within garnet.

Age interpretation: The age obtained within staurolite foliation accords with the dates

acquired ii-om other samples for FIA set 3.

6.6. Sample C75

This contains cordierite as an additional matrix mineral phase and zircon as an extra

accessory phase. Garnet contains FIA set 1 and staurolite contains a pseudo-FIA belonging

to set 1 FIA plus FIA set 2. The foliations within staurolite and garnet are truncated by

those in the matrix. Two monazite grains were dated in staurolite with an age spread of

1765±23 (see Appendix C; Figs. 8 and 9) and 1712±25 Ma (see Appendix C; Fig. 10). No

monazite grains were found in garnet.

Age interpretation: The 1712±25 Ma age obtained from the staurolite monazite gram

accord with the ages procured from other samples for this FIA set. The older monazite

grain (1765±23 Ma) lay Olthogonal to the foliation preserved as inclusion trails and is

consistent with the ages for FIA set 1 acquired from other samples (see Appendix C). It is

interpreted to represent relics of an earlier formed foliation.

6.7. Sample C65A

This contains additional matrix (andalusite) and minor phases (xenotine). Garnet preserves

FIA set 1 and staurolite contains a pseudo-FIA belonging to set 2 plus FIA set 3. The

c 9

90 million veal's of orogenesis. 250 million years ...... .... . SHAHA.A

foliations within these porphyroblasts are truncated by those in the matrix. Andalusite has

locally replaced staurolite. Fine-grained muscovite-rich pseudomorphs after staurolite are

common. Four monazite grains analysed from this sample lie in two different foliations

within staurolite (e.g. Fig. 11). Three monazite grains within the crenulated cleavage

foliation defined by inclusion trails have an average age of 1717.6±9.5Ma. The fourth

monazite grain, aged 1665±24Ma, was detected within the main foliation of staurolite.

Age interpretation: The dates obtained agree with the ages obtained from other samples for

FIA sets 2 and 3 (see Appendix C).

6.8. Sample CHO

Also contains andalusite and cordierite porphyroblasts. Extra accessory phases are

dominated by magnetite, xenotine, zircon and baddeleyite. Andalusite preserves FIA set 4

and its inclusions trails are continuous with the matrix foliation. Inclusions within

andalusite include staurolite and cordierite. A single monazite grain found in andalusite

gave an age of 1432+39 Ma (see Appendix C) for the foliation preserved as inclusion trails.

Age interpretation: The age recorded accords with the dates acquired from other samples

with this FIA trend. This is the youngest FIA set observed.

6.9. Sample C77

This sample contains andalusite and cordierite porphyroblasts, but no garnet and staurolite

porphyroblasts, and the extra accessory phases of magnetite and xenotine. Inclusion trails

in cordierite are continuous with the matrix foliation and preserve FIA set 4. Cordierite

contains a pseudo-FIA belonging to set 3 and FIA set 4. Andalusite contains inclusion trails

defining FIA set 3 that are truncated by foliations within both the matrix and the youngest

foliation in cordierite. Inclusions in both porphyroblastic phases include staurolite and

c 10

90 million years of orogenesis. 250 million years ...... "' .. SHAHA.A

garnet although the latter is rare. Three monazite grains enclosed within cordierite (2) and

andalusite (I) porphyroblasts were dated. One monazite grain within a crenulated cleavage

seam gives a pseudo-FIA set 3 age of 1678±17 Ma within cordierite. The andalusite

porphyroblast preserves the same foliation as FIA 3. The 1760±18 and 1726±18 Ma ages

were derived from their monazites (see below).

Age interpretation: The two dates obtained from monazite grains preserved within pseudo

FIA (set 3) and andalusite FIA (set 3) accord with the dates obtained from other samples

for this event. The two earlier ages acquired from monazite grains within the main foliation

of cordierite are relics of an older foliation that lies oblique to the main foliation defined by

the inclusion trails. Their dates are compatible with the ages obtained from FIA sets 1 and

2, preserved within other samples.

6.10. Sample CSSA

This sample also contains cordierite plus minor xenotine. Garnet preserves inclusion trails

defining FIA set 3 that are truncated by the foliation in cordierite and the matrix. Cordierite

contains FIA set 4 trails that are continuous with those present within the matrix. Staurolite

is also included in cordierite. A single monazite dated at 1666±26 Ma from this sample is

located within garnet. No monazite grains were found in cordierite.

Age interpretation: The 1666±26 Ma age in garnet accords with the dates obtained from

other samples bearing this FIA set.

6.11. Sample CS1B

This sample also contains cordierite porphyroblasts with inclusion trails defining FIA set 4

that are continuous with foliations preserved within the matrix. Staurolite and andalusite are

also present as inclusions. Two grains of monazite dated at an average age of 1412± 17 Ma

c 11


lie within the foliation preserved within the cordierite. Another monazite was dated at

1762±32 Ma, within the same foliation.

Age interpretation: The date acquired from two monazites in cordierite accords with the

dates obtained from other samples for FIA set 4 (see Appendix C). An older age from one

monazite is consistent with an earlier foliation aligned to FIA set 1 and represents its relics.

7.0. DATING OF MATRIX FOLIATIONS

The foliations within porphyroblasts are completely truncated by those within the matrix

phases in all samples except C110. Consequently, monazite ages in the matrix cannot be

used to date FIAs. They were dated to see what relics of the deformation history

determined from the FIA succession were preserved in the matrix and whether there was

any evidence for deformation occurring between the Colorado and Belihoud orogenies.

7.1. Sample C75 (FIA 1 and 2 in staurolite)

Three foliations in the matrix (Sea-Sec) are shown in Fig. 8d. A 1664±38 Ma age, was

derived from a monazite grain lying sub-parallel to Se2. Another monazite grain that lay

orthogonal to this foliation gave an age of 1762±35 Ma (Fig. 12). This sample preserves

FIA sets 1 and 2 within staurolite porphyroblasts.

Interpretation: The older monazite grain (1762±35 Ma) accords with the dates acquired for

FIA set 1 (see Appendix C).). The younger matrix age is coeval with dates acquired for FIA

set 3 suggesting that the matrix was reused or reactivated during the development of this

FIA set.

7.2. Sample C84 (FIA 2 in garnet and 3 in staurolite)

The dominant foliation in the matrix (Sel) contains a single monazite grain that lies sub

parallel to it that has an age of 1729±23 Ma.

c 12

90 million years of orogenesis. 250 million years", ... ..... SHAHA.A

Interpretation: The single matrix age accords with the date for FIA set 2 (see Appendix C).

and is interpreted to represent a relic of an earlier-formed foliation.


Two monazite grains in the main matrix foliation (Se.) have ages 1 742±29 Ma and 166S±23

Ma (e.g. Figs. 13 and 14). Both lie sub-parallel to Se.'

Interpretation: The matrix ages accord with dates acquired for FIA sets 1 and 3 respectively

(see Appendix C) and are interpreted to represent relics of earlier-formed foliations

preserved within the strain shadows of porphyroblast.


A single monazite grain parallel to the main matrix foliation (Sci) has an age of 1724±37

Ma. This sample preserves FIA sets I and 2 within garnet and staurolite porphyroblasts.

Interpretation: The matrix age obtained (l724±37 Ma) accords with the dates acquired for

FIA set 2 (see Appendix C), which suggests reactivation of the matrix during these events.


A single monazite grain parallel to the main matrix foliation (Sci) of this sample has an age

of 1723±34 Ma (see Appendix C). This sample preserves PIA set 2 within garnet and set 3

in staurolite porphyroblasts.

Interpretation: The matrix age accords with the dates acquired for FIA set 2 rather than

those for FIA set 3. This suggests that this grain was not deformed and recrystallized during

the development of PIA 3 prior to staurolite growth. Some crystallographic orientations of

grains relative to a developing strain field can make a patiicular mineral phase very

competent and hard to deform (e.g. Mancktelow, 1981). This grain could reflect such a

phenomenon.

c 13


7.6. Sample C51B (FIA 3 in cordierite)

A single monazite grain lying Olthogonal to the main matrix foliation (Sci) in this sample

has an age of \685±29 Ma (see Appendix C). This sample preserves FIA set 3 within

cordierite porphyroblasts.

Interpretation: This age accords with the date acquired for FIA set 3 suggesting the

monazite grew at this time and was preserved through modification of the matrix by a

subsequent deformation event.

7.7. Sample C77 (FIA 3 in cordierite)

A single monazite grain lying in the youngest matrix foliation (Sc3) in this sample has an

age of \677±\9 Ma (see Appendix C). This sample preserves FIA set 3 within cordierite

porphyroblasts.

Interpretation: The matrix age accords with the date for FIA set 3 suggesting it formed

during the same overall period of bulk shortening.

7.8. Sample CI08 (FIA 1 in garnet and 2 in staurolite)

A single monazite grain sub-parallel to the main matrix foliation (Sci) has an age of

\675±24 Ma (see Appendix C). This sample preserves FIA sets 1 and 2 within garnet and

staurolite porphyroblasts.

Interpretation: The matrix age accords with the dates acquired for FIA set 3, which suggests

subsequent reactivation of the matrix foliation at this time.

7.9. Sample CllO (FIA 4 in andalusite)

A single monazite grain sub-parallel to the main matrix foliation (Sel) has an age of

1438±30 Ma (see Appendix C). This sample preserves FIA set 4 within andalusite

porphyroblasts.

c 14


Interpretation: The matrix age accords with the dates acquired for FIA set 4. The foliations

preserved within the andalusite porphyroblast are very similar to and continuous with the

matrix.

8.0. COMPOSITIONAL MAPPING OF MONAZITE GRAINS

Samples containing monazite were compositionally mapped for Th, Y, U, Pb and Ce using

the JEOL JXA-8300 Superprobe. Most were devoid of any apparent chemical zoning (e.g.

Figs. 9 and 10). One monazite in the matrix of sample C65 showed chemical zoning in both

Th and Y (Fig. 14). Dating of 1742±29 and 1668±48 Ma suggests this was a product of

FIAs 1 and 3 (see below). Sample C75 showed a single example of a monazite with slight

zoning in Thpreserved within a staurolite porphyroblast (Fig. 10). A mean age of 1712±25

Ma was analyzed from this grain.

9.0. INTERPRETATION AND DISCUSSION

9.1. Combining the age data within FIA sets

Monazite grains are common within staurolite, cordierite and andalusite porphyroblasts but

rare in garnet. Of the eleven samples investigated, five contained inclusion trails defining

FIA set 1; six monazite grains were identified and 93 analyses completed defining an age of

1760.5±9.7 Ma (Table 3; Fig. 15). Five samples contained inclusion trails defining FIA set

2; eight monazite grains were identified and 136 analyses completed defining an age of

1719.7±6.4 Ma (Table 3; Fig. 16). Five samples contained inclusion trails defining FIA set

3; five monazite grains were identified and 56 analyses completed defining an age of

I 674± 11 Ma (Table 3; Fig. 17). Three samples contained inclusion trails defining FIA set

4; three monazite grains were identified and 28 analyses completed defining an age of

c 15


1415±16 Ma (Table 3; Fig. 18). These ages, (1760.5±9.7, 1719.7±6.4, 1674±11 and

1415±16 Ma), respectively, confirm the FIA 1,2,3,4 succession established using core/rim

criteria plus the previously recognized (Tweto, 1987; Nyman et aI., 1994; Karlstrom, et aI.,

1997) separation of orogenesis into 2 distinct periods 250 million years apart (Shah, Thesis

Section B).

9.2. The significance ofFIAs for determining monazite ages

Texttu'ally controlled dating of monazite inclusions has recently been used by many

petrologists around the world to date foliation ages (Williams et aI., 1999, Shaw et aI.,

2001, Dahl et aI., 2005). A range of ages will always be present in the matrix due to the

potential for the preservation of monazite grains within the strain shadows of successively

grown porphyroblasts and this is exemplified by Table 3. Depending on the timing of

porphyroblast growth a similar range can be preserved ii-om the influence of younger

events. The most critical phase in using an absolute microstructtu'al dating method is to

acctu'ately identify monazite grains within a particular textural and structtu'al setting. FIA

provide such a setting and offer a robust opportunity to extract in situ information from

individual monazite grains preserved within an independently determined relative

timeframe.

The accord between FIA set and age recorded herein is remarkable (Table 3). Only

one sample (e77) contains "anomalous" older ages which can be attributed to the earlier

events as just mentioned. The recognition of pseudo-FlAs and FIAs provided tight control

over what FIA sets were preserved in each sample. Without this level of control on the

distribution of FlAs, the - 100 million year range in ages obtained (not including the far

younger -1400 Ma ages) would have been attributed to noise. Instead it accords perfectly

with the independently obtained succession ofFIA sets!

c 16

90 million years o(orogenesis. 250 million years." ". "". SHAHA.A

9.3. Assessing the spread of the age data from matrix relative to that for the FIA

succession

Monazite grains are common within the matrix and randomly distributed. They are

generally preserved within muscovite and biotite grains (e.g. Figs 12 and 13). A total of 9

samples out of 30 investigated contained monazite crystals in the matrix. Two samples each

contain a monazite grain, from which a total of 14 analyses were completed defining an age

of 1749±23 Ma (Table 3; Fig. 19). Four samples contained four monazite grains from

which 28 analyses were obtained defining an age of 1726±17 Ma (Table 3; Fig. 20). Six

samples contained six monazite grains from which 59 analyses were completed defining an

age of 1674±11 Ma (Table 3; Fig. 21). One sample contained a single monazite grain from

which 7 analyses were completed defining an age of 1438±29 Ma (Table 3; Fig. 22). These

ages accord to some degree with dates obtained from the porphyroblasts preserving FlA

sets 1, 2, 3 and 4 and clearly reflect those events in spite of the fact that there is no real

control on the significance.

9.4. The porphyroblast ages versus matrix ages

In all the samples that were dated foliations defined by inclusion trails in porphyroblasts are

tlUncated by matrix foliations except in sample ClIO. Therefore, monazite ages in the

matrix have no relevance to the dating of FlAs. In most samples monazite grains in the

matrix foliation gave the same or younger ages than those within the porphyroblasts. Ages

range from 1749±23 to 1674±11 Ma (Table 3) with one sample preserving a monazite with

an approximate FIA 1 age of 1762 Ma and another containing a relic from a foliation

within the matrix that predated porphyroblast growth. The younger ages were always a

product of reuse or reactivation of old foliations (e.g. Bell et aI., 2003) or the development

of new ones. Consequently, only the ages obtained from monazite grains preserved within

c 17


porphyroblasts where a FIA control on the significance of that age were used to time

deformation and metamorphism. However, as mentioned above, it is apparent that most if

not all of the ages associated with the succession of FIA development are preserved within

the matrix. Yet an approach that involves dating monazite grains within the matrix can only

ever provide an average age that does not distinguish when deformation commenced or

when porphyroblast growth ceased.

9.5. Limitation of FIAs approach for monazite dating

The monazite grains within a foliation overgrown by a porphyroblast can have

formed when that schistosity formed or be relics from earlier formed deformation events

(e.g. Sample C5IB). However, if they are small « 20 micron long), ellipsoidal and aligned

parallel to the foliation preserved within the porphyroblasts that defines the FIA, then they

should give the age of that foliation. Since each collapse phase produces a gently dipping

foliation (Bell & Newman, 2006), FlAs are actually controlled by the steeply dipping

foliation forming events. Consequently, the possibility must be kept in mind that if the

foliation containing monazite is gently dipping, the age obtained could be older than the

when the FIA measured from that sample formed.

9.6. Role of deformation and its significance for porphyroblast growth

Nucleation of any mineral phase requires that P-T and bulk composition should be

appropriate for that phase to grow. However, deformation is also known to playa vital role

in formation of different minerals, patiicularly porphyroblastic phases (e.g. Bell et a!.,

1986; Williams, 1994, Cihan et a!., 2006) through its control on sites for the access of

nutrients needed for nucleation and growth (Spiess and Bell, 1996). The FIA controlled

monazite dating described herein reveals -90 million years of continuous

deformation/metamorphism followed by -250 million years of quiescence before

c 18

90 million years of orogenesis. 250 million years ...... .. ". SHAHA.A

orogenesis recommenced for ~ 20 million years with little or no change in PT conditions

(Shah, Thesis Section B). What kept this region at similar crustal levels during the 250

million years of quiescence?

The PT conditions and the bulk composition were clearly suitable for the growth of

porphyroblasts during the ~250 Ma between the development of FlAs 3 and 4. Yet no

porphyroblastic phases grew during this time and there is no microstructural evidence for

any foliations developing. The latter fact is confirmed by dating of monazite grains within

the matrix. They reflect the FIA succession and provide no evidence for any deformation

between at least 1665 and 1438 Ma. It is now well established that deformation and

concurrent metamorphism form cleavage seams by dissolution as well as provide a large

range of components essential for the nucleation and growth of porphyroblasts (Bell &

Cuff, 1989; Spiess & Bell, 1996). Deformation provides sites for nucleation and growth,

and a means of overcoming the energy barrier for nucleation, in the form of the energy

removal from, for example, crenulation hinges (Bell et aI., 1986). The lack ofporphyroblast

growth for this extended ~250 million year period can be attributed to the lack of

crenulation development (Bell et aI., 2003). When deformation recommenced around ~

1415Ma, porphYl'oblasts also began to grow again forming FIA set 4 inclusion trails within

garnet, staurolite, andalusite and cordierite porphyroblasts strongly supporting the role of

crenulation deformation in porphyroblast growth (e.g. Bell et aI., 1986; Cihan et aI., 2006).

9.7. Deformation and metamorphism

The three stages of folding and two stages of metamorphism reported previously from this

region were determined from matrix foliation relationships. A much longer history of

deformation and metamorphism is preserved by the porphyroblasts. The preservation of

four FIA trends with changing directions of shortening from (NE-SW to E-W to SE-NW to

c 19


NNE-SSW), which range in age from 1760.5±9.7 to 1415±16 Ma has considerable

implications for tectonics of this region. The first regional folding episode initiated during

FIA set 1 at 1760.5±9.7 Ma and trended NE-SW and approximately coincides with the

regional trend of the Cheyenne belt. This is regarded as the suture zone along which the

rocks of Colorado and Wyoming province accreted about 1790-1650 Ma ago (Sims and

Stein, 2003). Pressure and temperature at this time was about 540-550° C and 3.8-4.0 kbars,

as proposed by the garnet isopleth geothermobarometry. Intersections of Ca, Mg, and Fe

isopleths in garnet core, which preserved FIA set 1, indicate that these rocks never got

above 4kbars during the Colorado Orogeny (Shah, Thesis Section B).

During the formation ofFIA set 2 (centred around 1719.7±6.4 Ma), the pressure and

temperature changed slightly to 3.40-3.65kbar and 525-537°C. F2 regional folds were

formed during this stage. Foliation evidence from this period or orogeny is rarely preserved

as crenulations in the matrix, due to the effects of rotation and reactivation in the many

subsequent deformations. Trondhjemite dikes, which now have a sill like character, were

emplaced at ~ 1726±15 Ma (Selverstone et ai., 1997). Intrusion froming W-E trending

dikes could occur along the W-E trending vertical foliation that generated this FIA set

during gravitational collapse stages of orogenesis when this vertical foliation would have

created a plane of weakness that failed (e.g., Bell & Newman, 2006). Subsequent

reactivation of the compositional layering would have progressively rotated most of them

into sub-parallelism with the bedding and disguised any crosscutting relics of the dikes

along which they intruded.

Development of the FIA set 3 also during the Colorado Orogeny. The SE-NW trend

of this FIA set was created by NE-SW shortening. Previously formed folds were refolded

and a new F3, generation were created (Fig. 3). The pressure-temperature conditions

c 20


indicated by garnet isopleths conditions for this period were 3.3-3.6kbar and 525-535°C.

More staurolite porphyroblasts also grew at this time. In particular, it marks the first

appearance of andalusite and cordierite porphyroblasts. Some staurolite porphyroblasts

containing FIA set 3 have been partially replaced by andalusite or cordierite suggesting a

decrease in pressure accompanied their development. Absolute dating of monazite grains

enclosed within the inclusions of these four porphyroblastic phases provide an age of

1674±1l Ma for this period of FIA development which appears to end the Colorado

Orogeny. These rocks remained undisturbed for about 250 Ma.

Monazite grains enclosed within the foliations of andalusite and cordierite

porphyroblasts containing FIA set 4 give an age of 1420±14 Ma for this period of

orogenesis. The tight intersection of Ca, Mn and Fe isopleths in garnet cores indicate that

the pressure conditions during this period of orogenesis were similar to those observed at

the end of the Colorado orogeny. Large-scale heating event associated with granite

emplacement and some deformation at this time is regionally known as the Berthoud

Orogeny (Sims and Stein, 2003). FIA set 4 trends NNE-SSW and resulted from NNW -SSE

directed shortening. Garnet, staurolite, cordierite and andalusite also grew during this

period of orogenesis. Slightly higher temperatures were recorded during this period of

orogenesis than previously. Some andalusite and cordierite formed by replacing staurolite

porphyroblasts but most grew as completely new grains. After this period of orogenesis

these rocks were retrogressed, presumably during exhumation. This is revealed by

pseudomorphs after staurolite, garnet, cordierite and andalusite (Shah, Thesis Section B).

c 21


9.8. Regional tectonic implications of Colorado and Berthoud Orogenies

The metasediments exposed in and around the Big Thompson region of Colorado represent

mature sediments deposited in a fore-arc (Condie and Mattell, 1983) or back-arc setting

(Reed et ai., 1987). Detrital zircon ages suggest a maximum age of 1758±26 Ma for

deposition of the Big Thompson sequence (Selverstone et aI., 2000). Previous workers

(e.g., Selverstone et ai., 1997; Chamberlain, 1998; Shaw et ai., 1998; 2001; Williams et ai.,

1999) suggested protracted metamorphism and deformation associated with the ~ 1700 Ma

orogeny and local effects due to the ~ 1400 Ma orogeny.

The results obtained by electron microprobe monazite dating (Shaw et ai. 2001) in the

Homestake shear zone (Fig. 1), revealed that it preserves a pre-1400 Ma deformation

history. They explained that a northeast-striking strongly foliated region was transformed

into a mylonite zone around 1680-1630 Ma. Deformation and concentrated shearing

occurred under high temperature conditions. This orogenic phase was later overprinted by

high temperature conditions associated withthe 1400 Ma episode. A low pressure-high

temperature metamorphic sequence dominated by late staurolite, andalusite, cordierite and

garnet porphyroblastic phases was identified by Shaw et ai. (1999). They argued that these

were formed during the cooling stage (using 40Ar/39Ar geochronology of muscovite and

biotite) of the ~ 1400 Ma orogenic history and thus overprint earlier events allied to

Colorado orogenesis. Stein and Sims (2001) dated PreCambrian-hosted metamorphic

molybdenites from the Colorado mineral belt in the central Front Range. They have

interpreted Re-Os isochron age results of 1430-23 Ma as the time of the peak thermal

episode in the area.

c 22


The evidence presented here for deformation and metamorphism around 1758.8±9Ma

suggests orogenesis commenced around the time of sedimentation. This correlates with the

beginning of contractional deformation along the Cheyenne belt, during which time the

Colorado province was accreted onto the Archean Wyoming province (Chamberlain, 1998).

Orogenesis was essentially continuous for about -100 Ma and then ceased. FIA data

reveals that deformation during 1420±14 Ma Betthoud Orogeny was pervasive, with well

preserved foliations that are continuous with the matrix foliation. However, not a single

monazite grain of Betihoud age was found within garnet or staurolite porphyroblasts that

could be associated with this event. They were only found in andalusite and cordierite

suggesting a slight change in T, P conditions from those forming staurolite was required for

further growth of this phase.

9.9. Episodic but continuous porphyroblastic growth during -1700 Ma Orogeny

The histOlY of relative plate motion in the Alps shows that it changes on average

every 15 million years (e.g., Platt et aI., 1989). Since FIAs directly reflect the direction of

relative plate motion (Bell et aI., 1995; Bell & Newman, 2006), each one will develop over

a period averaging 15 million years but ranging in length from 5 to 30 million years.

Throughout these periods of time, foliations develop alternately in sub-vertical and sub

horizontal orientations. A FIA can be defined by a succession of at least 7 foliations or as

few as just 2 (Bell & Newman, 2007) as the measurement techniqueis independent of the

number of foliations that define it. Consequently, FIAs are correlated from sample to

sample and outcrop to outcrop rather than foliations eliminating the natural heterogeneity in

foliation development resulting from the pattitioning of deformation. Monazite dating

averages the age of some of the foliations from the few samples that are dated that define a

FlA. Clearly, these can have formed over a period of time ranging anywhere from 5 to 30

c 23


million years and all samples containing a FIA would have to be dated to fully define the

range of foliation ages present, which is not a practical approach. Consequently, the age

determined for a FIA will lie anywhere within the range of time that it developed over,

rather than within the middle of that period. This averaging of the ages makes the

deformation involved look more spaced or episodic than what it actually was. Although

plate motion driving orogeny is continuous, its impact on the rocks affected is episodic in

the upper crust because deformation periodically switches from foliations developing sub

vertically to sub-horizontally. This episodic character is increased when changes in

direction of plate motion take place because that upsets for a time the balance between

crustal thickening and collapse. Combined with the spread of possible foliations that define

a FIA, the episodicity appears greater when the ages of successive FlAs are determined

than what it actually is.

Acknowledgements

I would like to thank Prof. Tim Bell for his critical comments. Special thanks to Dr. Mike

Rubenach for help with monazite dating and critical discussions. I would also like to thank

Prof. Jane Selverstone for sending me the manuscripts ofthe Colorado region.

c 24


10. REFERENCES

Adshead-Bell, N. S., Bell, T. H., 1999. The progressive development of a macroscopic

upright fold pair dlU'ing five near-Olthogonal foliation-producing events: complex

microstructures versus a simple macrostructure. Tectonophysics, 306, 121-147.

Bell, T. H., 1986. Foliation development and refraction in metamorphic rocks: reaction of

earlier foliations and decrenulation due to shifting patterns of deformation

partitioning. Journal of Metamorphic Geology, 4, 421-444.

Bell, T. H., Hickey, K. A., Upton, G. J. G., 1998. Distinguishing and correlating multiple


staircase and sigmoidally clU'ved inclusion trails in garnet. Journal of Metamorphic

Geology. 16, 767 - 794.

Bell, T. H., Chen, A., 2002. The development of spiral-shaped inclusion trails dlU'ing

multiple metamorphism and folding. Journal of Metamorphic Geology. 20, 397-

412.

Bell, T. H., Welch, P. W., 2002. Prolonged Acadian Orogenesis: Revelations from FIA

Controlled Monazite Dating of Foliations in Porphyroblasts and Matrix. American

Journal of Science, 302, 549-581.

Bell, T. H., Ham, A. P., Hickey, K. A., 2003. Early formed regional antifOl'ms and


growth. Tectonophysics, 367, 253-278.

Bell, T. H., Ham, A. P., Kim H. S., 2004. Partitioning of deformation along an orogen and

its effects on porphyroblast growth during orogenesis. Journal of Structural

Geology, 26, 825-845.

C 25


Bell, T. H., Ham, A. P., Hayward, N., Hickey, K.A., 2005. On the development of gneiss

domes. Australian Journal of Earth Science, 52, 183-204.

Bell, T. H., Newman, R. 1., 2006. Appalachian Orogenesis: the role of repeated

gravitational collapse. In: Styles of Continental Contraction, eds S. Mazzoli and

R.W.H. Butler. Geological Society of America Special Paper, 414, 95-118.

Bell, T.H., Bruce, M.D., 2007. Progressive deformation partitioning and deformation

history: Evidence from millipede structures. Journal of Structural Geology, 29, 18-

35.

Braddock, W.A., Cole, J.C., 1979. Precambrian structural relations metamorphic grade, and

intrusive rocks along the northeast flank of the Front Range in the Thompson

Canyon, Poudre Canyon, and Virginia Dale areas: Field Guide. Geological Society

of America, 106-120.

Cavosie, A., Selverstone, J., 2003. Early Proterozoic oceanic crust in the nOithern Colorado

front Range: Implication for crustal growth and initiation of basement faults.

Tectonics, 22, 10-23.

Chamberlain, K.R., 1998. Medicine Bow orogeny: Timing of deformation and model of

crustal structure produced during continent-arc collision ~1.78 Ga, southeastern

Wyoming. Rocky Mountain Geology, 33, 259-277.

Cherniak, D. J., Watson, E. B., Grove, M., Harrison, T. M., 2004. Pb diffusion in monazite:

a combined RBS/SIMS study. Geochimica et Cosmochimica Acta, 68, 829-840.

Cihan, M., 2004. The drawbacks of sectioning rocks relative to fabric orientations in the

matrix: a case study from the Robertson River Metamorphics (Northern

Queensland, Australia). Journal of Structural Geology, 26, 2157-2174.

C 26


Cihan, M., Parsons, A., 2005. The use of porphyroblasts to resolve the history of macro

scale structures: an example ii'om the Robertson River Metamorphics, North

Eastern Australia. Journal o/Structural Geology, 27,1027-1045.




Condie, K. C., C. Martel., 1983. Early Proterozoic metasediments from north-central

Colorado: Metamorphism, provenance, and tectonic setting. Geological Society

America Bulletin, 94, 1215 - 1224.

Dahl, P. S., TelTy, M.P., Jercinovic, MJ., Williams, M.L., Hamilton, M.A., Foland, K.A.,

Clement, S.M., Friberg, L.M., 2005. Electron probe (Ultrachron) microchronometry

of metamorphic monazite: Umaveling the timing of polyphase thermotectonism in

the easternmost Wyoming Craton (Black Hills, South Dakota). American

Mineralogist, 90, 1712-1728.

Davis B. K., 1995. Regional-scale foliation reactivation and re-use during formation of a

macroscopic fold in the Robertson River metamorphics, north Queensland,

Australia. Tectonophysics 242, 293-311


collisional event: The example of the Thiviers-Payzac Unit in the Variscan French

Massif Central. Lithos, 98,1-4,210-232.

Evans, T. P., 2004. A method for calculating effective bulk composition modification due

to crystal fractionation in garnet-bearing schist: implications for isopleth

thermometry, Journal 0/ Metamorphic Geology, 22 547-557.

Foster Gavin., Kinny, P., Prince, C., Vance, D., N. HalTis., 2000. The significance of

c 27


monazite U-Th-Pb age data in metamorphic assemblages; a combined study of

monazite and garnet chronometry. Earth and Planet. Science Letters, 181, 327-340.

Foster Gavin., H. D. Gibsonc., Randy, Parrisha., Matthew, Horstwood., James, Fraserd.,

Andy, Tindlee., 2002. Textural, chemical and isotopic insights into the nature and

behaviour of metamorphic monazite. Chemical Geology, 191, 1-3,183-207.

Gaidies, F., Krenn, E., de Capitani, C., Abalt, R., 2008. Coupling forward modelling of

garnet growth with monazite geochronology: an application to the Rappold

Complex (Austroalpine crystalline basement). Journal of Metamorphic Geology,

26,775-793.

Goncalves, P., Williams, M. L., Jercinovic, M. J., 2005. Electron microprobe age mapping.

American Mineralogist, 90, 578-585.


Geology, 26,1898-2009.

Holland T. J. B., Powell, R., 1985. An internally consistent thermogynamic dataset with

uncettatinites and correlations: 2. Data and results. Journal of Metamorphic

Geology, 3, 343-370.

Holland, T. J. B., Powell, R., 1990. An enlarged and updated internally consistent

thermogynamic dataset with uncertatinites and correlations: the system KzO-NazO

CaO-MgO-FeO-Fez03-Ah03-Ti02-SiOz-C-H2-02. Journal of Metamorphic

Geology, 8, 89-124.

Holland, T. J. B., Powell, R., 1998. An internally consistent data set for phases of

petrological interest. Journal of Metamorphic Geology, 16, 309-343

C 28

90 million years oforogenesis. 250 million veal'S". ". "". SHAHA.A

Karlstl'Om, K. E., Dallmeyer, R. D., Grambling, J. A, 1997. 40Ar/39Ar evidence for 1.4 Ga


lithosphere in the southwestern U.S. Journal o/Geology, 105, 205-223.

Kim, H., and Bell, T. H., 2005. Combining compositional zoning and foliation intersection

axes (FlAs) in garnet to quantitatively determine early P-T-t paths in multiply

deformed and metamorphosed schists: north central Massachusetts, USA

Contributions to Mineralogy and Petrology, 149, 141- 163.

Ludwig, K. R., 1998. IsoplotlEx: a geochronological toolkit for Microsoft Excel. Berkeley

Geochronological Center, Special Publication, 1, 43.

Mancktelow, N.S, 1981. Strain variation between qumtz grams of different

crystallographic orientation in a naturally deformed metasiltstone. Tectonophysics,

78,73-84.

Montel, J.M., Foret, S., Veschambre, M., Nicolette, C., Provost, A, 1996. Electron

microprobe dating of monazite. Chemical Geology, 131, 37-53.

Nyman, M., Kar/strom, K. E., Graubard, C., Kirby, E., 1994. Mesozoic contractional

orogeny in western Notth America: Evidence from ca. 1.4 Ga plutons. Geology, 22,

901-904.

Paquette, J. 1., Nedelec, A., Moine, B., Rakotondrazafy, M., 1984. U-Pb, single zircon Pb

evaporation, and Sm-Nd isotopic study of a granulite domain in SE Madagascar,

Journal of Geology, 102, 523-538.

Parrish, R. R., 1990. U-Pb dating of monazite and its application to geological problems.

Canadian Journal of Earth Sciences, 27,1431-1450.

Platt, J. P., Behrmann, J. H., Cunningham, P. C., Dewey, J. F., Helman, M., Parish, M.,

C 29

90 million years o(orogenesis. 250 million years ... "'''''' SHAHA.A

Shepley., M. G., Wallis, S., Weston, P. 1. 1989. Kinematics of the Alpine arc and

the motion of Adria, Nature, 337, 158-61.

Powell, R., 1985. Geothermometry and geobarometry: A discussion. Journal of the

Geological Society of London, 142,29-38.

Powell, R, Holland, T. 1. B., 1985. An internally consistent thermodynamic dataset with

unceliainties and correlations: 1. Methods and a worked example. Journal of


Powell, R, Holland, T. J. B., 1988. An internally consistent thermodynamic dataset with

unceltainties and correlations: 3. Applications to geobarometry, worked examples

and a computer program. Journal of Metamorphic Geology, 6, 173-204.

Powell, R, Holland, T. J. B., 1994. Optimal geothermometry and geobarometry. American


Powell, R., Holland, T., 1994. Optimal geothermometry and geobarometry. American


Powell, R., Holland, T., Worley, B., 1998. Calculating phase diagrams involving solid

solutions via non-linear equations, with examples using THERMOCALC. Journal

of Metamorphic Geology, 16, 577-:588.

Pyle, 1.M., Spear, P.S., Rudnick, RI., McDonough, W.P., 2002. Monazite-xenotime-garnet

equilibrium in metapelites and a new monazite garnet thermometer. Journal of

Petrology, 42, 2083-2107.

Pyle, 1.M., Spear, P.S., Wark, D.A., Daniel, C.G., Storm, L.C., 2005. Contributions to

precision and accuracy of chemical ages of monazite. American Mineralogist, 90,

547-577.

Rasmussen, B., Pletcher, I.R, McNaughton, N.J., 2001. Dating low-grade metamorphic

c 30


events by SHRIMP U-Pb analysis of monazite in shales. Geology, 29,963-966.

Reed, J. C., Jr., Bickford, M. E., Premo, W. R., Aleinikoff, J. N., and Pallister, J., 1987.


geochronology. Geology, 15,861-865.



porphyroblasts revealed by independent 3D measurement techniques. Journal 0/


Selverstone, J., Aleinikoff, J., Hodgins, M., Fanning, C.M., 2000. Mesoproterozoic

reactivation of a Paleo proterozoic transcurrent boundary in the Colorado Front

Range: Implications for ca. 1.7 and 1.4 Ga tectonism. Rocky Mountain Geology, 35,

136-162.

Selverstone, J., Hodgins, M., Shaw, c., Aleinikoff, J. N., Fanning, C. M., 1997. Proterozoic

tectonics of the nOlthern Colorado Front Range. In Bolyard, D.W., and Sonnenberg,

S. A., eds. Geologic history of the Colorado Front Range. Denver, Rocky Mountain

Association Geology, 9-18.

Shaw, C. A., Snee, L. W., Selverstone, J., and Reed, J. C. 1999. 40Ar/39Ar

thermo chronology of Mesoproterozoic metamorphism in the Colorado Front Range.

Journal o/Geology, 107, 49-67.

Shaw, CA., Karistrom, K.E., Williams, M.L., Jercinovic, M.J., McCoy, A.M., 2001.

Electron microprobe monazite dating of ca. 1.71 - 1.63 Ga and ca. 1.45-1.38

deformation in the Homestake shem' zone, Colorado: Origin and early evolution of a

persistent intracontinental tectonic zone. Geology, 29 , 739-742

Sims, Paul K., Stein, Holly J., 2003. Tectonic evolution of the Proterozoic Colorado

c 31

90 million veal's o(orogenesis. 250 million years ". ". "". SHAHA.A

province, Southern Rocky Mountains: A summary and appraisal. Rocky Mountain

Geology, 38, 2 183-204

Spiess, R., Bell, TH., 1996. Microstructural controls on sites of metamorphic reaction: a

case study of the inter-relationship between deformation and metamorphism.


Stowell, H.H., Taylor, D.L., Tinkham, D.L., Goldberg, SA, Ouderkirk, KA, 2001.

Contact metamorphic P-T - t paths ii-om Sm-Nd garnet ages, phase equilibria

modelling and thermobarometry:garnet ledge, southeastern Alaska, USA. Journal

of MetamOlphic Geology, 19, 645- 660.

Tinkham, D.K., Zuluaga, C.A., Stowell, H.H., 2001. Metapelite phase equilibria modelling

in MnNCKFMASH: The effect of variable Al203 and MgO/(MgO + FeO) on

mineral stability. Geological Material Research, 3, 1--42.

Tweto, O. L., 1987. Rock units of the Precambrian basement in Colorado. U.S. Geological

Survey Professional. Papers, 1321-A, 54.

Vance and Mahar, E., 1998. Pressure-temperature paths from P-T pseudosections and

zoned garnets: potential, limitations and examples from the Zanskar Himalaya, NW

India. Contributions to Mineralogy and Petrology, 114,101-118.

Williams, M.L., 1994. Sigmoidal inclusion trails, punctuated fabric development, and

interactions between metamorphism and deformation. Journal of Metamorphic

Geology, 12, 1-21

Williams, M.L., Jercinovic, M.J., Terry, M.P., 1999. Age mapping and dating of monazite

on the electron microprobe: Deconvoluting multistage tectonic histories. Geology,

27,1023-1026.

Williams, M.L., Jercinovic, M.J., 2002. Microprobe monazite geochronology: Putting

C 32

90 million years o(orogenesis. 250 million years ...... .... . SHAHA.A

absolute time into microstructural analysis. Journal of Structural Geology., 24 ,

1013-1028.

Williams, M.L., Jercinovic, M.J., Callum J., Hethell'ington., 2007. Microprobe monazite

geochronology: understanding geologic processes by integrating composition and

chronology. Annual Review Of Earth And Planetmy Sciences, 35, 137-75

c 33

Section D

THE PROBLEM, SIGNIFICANCE, AND METAMORPHIC IMPLICATIONS OF 90 MILLION YEARS OF POLYPHASE STAUROLITE GROWTH

PhD Thesis SHAHA.A.

- SECTIONC-

THE PROBLEM, SIGNIFICANCE, AND METAMORPHIC

IMPLICATIONS OF 60 MILLION YEARS OF POLYPHASE

STAUROLITE GROWTH

1. ABSTRACT ................................................................................................................................................. 4

2. INTRODUCTION ....................................................................................................................................... 4

3. LOCATION AND SETTING ..................................................................................................................... 6

3.1. NORTHERN ApPALACHIANS .................................................................................................................... 6

3.2. COLORADO FRONTAL RANGE ................................................................................................................. 6

4. DATA ............................................................................................................................................................ 8

4.1. FlA ......................................................................................................................................................... 8

4.2. MONAZITE DATA ..................................................................................................................................... 8

4.3. THERMOCALC ......................................................................................................................................... 9

5. INTERPRETATION ................................................................................................................................... 9

6. DISCUSSION AND SIGNIFICANCE ..................................................................................................... 11

ACKNOWLEDGEMENTS ................................................................................................................................ 14

7. REFERENCES .......................................................................................................................................... 15

D2

PhD Thesis SHAHA.A.

1. ABSTRACT

Microstructural measurements of FIAs (foliation intersection/inflection axes preserved in

porphyroblasts) in staurolite reveal at least 3 periods of growth in the Proterozoic Colorado

frontal range and 5 in Palaeozoic Western Maine. Monazite inclusions in staurolite have an

absolute age of 1 765±23 Ma (FIA I), 1718.8±6.9 Ma (FIA 2), and 1674±16 Ma (FIA 3) in

Colorado, and 408±10 Ma (FIA 2), 388±8.8 Ma (FIA 3), 372.1±5.5 Ma (FIA 4), and

352.7±4.2 Ma (FIA 5) in Maine, confirming the multiple periods of deformation and

metamorphism indicated by the FIA succession in each region. Thermodynamic modeling

in the NCMnKFMASH system reveals that this episodic growth of staurolite occurred over

a similar bulk compositional range and PT path. Multiple phases of growth by one reaction

in the same and adjacent rocks in both regions strongly suggest that PT and X are not the

only factors controlling the commencement and cessation of metamorphic reactions. The

FIAs preserved by the staurolite porphyroblasts indicate that each stage of growth in both

areas occurred during deformation and indicate that the local partitioning of deformation at

the scale of a porphyroblast was the controlling factor on whether or not the reaction took

place.

2. INTRODUCTION

The basis of quantitative metamorphic petrology is the inference that the prograde

mineral assemblages within a rock are in chemical equilibrium (Bickle and Baker, 1990).

D4

PhD Thesis SHAHA.A.

This conclusion, though difficult to prove absolutely, seems a logical consequence of the

enormous amounts of geological time available for individual deformation/metamorphism

events. The extraordinary lengths of time available for reactions means that for a cel1ain

bulk composition and PT environment, once local equilibrium is achieved, it ought to go to

completion. Thus bulk composition, PT, fluids and time are generally assumed to be the

only controls on whether or not a reaction will take place. Workers such as Carlson et ai.

(1995), Baxter and DePaolo (2000, 2002 and 2004) and Baxter (2003) have shown that

several problems exist with this approach. However, newly developed approaches to

studying the metamorphic and structural interrelationships have revealed another

significant problem.

It is known that deformation affects fluid and heat flow (Lasaga et ai., 2000) and

can control porphyroblast nucleation. It does this by reducing the nucleation energy barrier

to a reaction (Carlson et ai., 1995; Hirsch, 2007) and providing rapid access and removal of

materials to a reaction site (Bell at ai., 1986). However, existing models for most orogenic

terranes commonly lack detailed time constraints on direct links between deformation and

metamorphism. The measurement of foliation intersection/inflection axes in

porphyroblasts (FlAs) allows detailed access to the timing of growth of the same as well as

different mineral phases. Multiple generations of garnet growth over lengthy time periods

have been recognized and dated via monazite grains preserved within the inclusion trails

that have been used to define a succession ofFIAs (Bell and Welch, 2002).

Garnet is a mineral that can grow from a variety of reactions over a large range of

TP conditions and so the recognition of multiple phases of growth of this phase did not

cause any conceptual problems. However, staurolite is not, since staurolite forming

D5

PhD Thesis SHAHA.A.

reactions typically have a lower vanance than those producing garnet. This study

demonstrates that the same behaviour that occurs for garnet can occur for staurolite in

rocks where there is little or no change in bulk composition. This has considerable

implications for the current assumptions used by most metamorphic petrologists. To

demonstrate the widespread nature of this problem, two differently aged and located fold

belts in the PreCambrian rocks in the Rocky Mountain Foothills in Colorado and the

Palaeozoic rocks of the Appalachians in western Maine are used

3. LOCATION AND SETTING

3.1. Northern Appalachians

The western Central Maine Belt (Fig. 1) was deformed and metamorphosed at

lower amphibolitic facies during the Acadian orogeny. At least three different

metamorphic events affected the region with peak metamorphic conditions occurring

around 400 Ma (Guidotti and Johnson, 2002). The first event produced chlorite, the second

And+St+BtGli, and the third St+Grt+Sill plus pro and retrograde pseudomorphs of earlier

formed porphyroblasts (Guidotti and Johnson, 2002). The Siluro-Devonian metasediments

were intruded syn-tectonically by the 389 to 370 Ma Mooselookmeguntik pluton. A strong

pervasive matrix foliation striking NE is crosscut by an axial plane crenulation cleavage.

3.2. Colorado Frontal Range

The Colorado front (Fig. 2) contains Early Proterozoic schists and gneisses that

record multiple episodes of deformation and metamorphism that varied on a regional scale

D6

PhD Thesis SHAHA.A.

during two main periods of orogeny around 1700 and 1400 Ma with associated igneous

intrusions (Selverstone et aI., 1997). In the northern Front Range most of the rocks reached

the amphibolite facies and extensive migmatization occurred at higher grades. Peak

metamorphic conditions produced Glt+St+And±Sill and have been broadly constrained

between 1.75-1.70 Ga (Nesse, 1984). A second metamorphic event was associated with

regional heating throughout the Front Range and locally produced new growth of garnet,

andalusite, and staurolite (Shaw et aI., 1999). The metasedimentary sequence was affected

by two main plutonic events, one around 1.7 Ga and the second one around 1.4 Ga

(Cavosie and Selverstone, 2003). The rocks show an increase in metamorphic grade

towards the west and nOlth and three stages of folding and cleavage development. The first

deformation/metamorphism is regarded as having occurred before 1750 Ma and resulted in

large-scale isoclinal folds (Fl) and a regional axial cleavage S 1. The second and third stage

of folding (F2 and F3) occurred around 1750 Ma ago, when these rocks were intruded by

the Boulder Creek granodiorite and related rocks. Only 1 period of metamorphism has

been associated with these events (Ml) during which garnet and staurolite grew. The

second metamorphic event (M2), which was stronger than first, resulted in the formation of

up to sillimanite grade mineral assemblages, though metamorphic conditions were very

heterogeneous throughout these episodes. A number of areas recorded an entire transition

in metamorphic grade from the chlorite zone to the onset of migmatization during the

Colorado orogeny (Selverstone et aI., 1997).

D7

PhD Thesis SHAHA.A.

4. DATA

4.1. FIA

Garnet and staurolite FIAs were measured by the asymmetry method described in

Hayward (1991) and Bell et al. (1995, 1998). A relative succession through time was

obtained using both garnet and staurolite core to rim changes in FIA trend that were

consistent for each area. The FIAs and monazite ages in staurolite porphyroblasts from the

Western Maine (Table I) were measured by Sanislav (2009) as part of his PhD thesis at

James Cook University. Table 2 shows all the samples in which FIAs have been measured

from the Colorado Front Range. A detailed description is provided in Section A and B of

this thesis (e.g. Fig. 3). The trends for each staurolite FIA in the succession are shown on

the rose diagram in Fig. 4.

4.2. Monazite data

A minimum of two samples containing one of each of the FIAs in staurolite from

the succession was used to measure the U, Th, Pb and Y contents of monazite grains. The

ages were determined using a JEOL JXA8200 microprobe housed at the Advanced

Analytical Centre, James Cook University. Table 3 show the trace element analyses of the

monazite grains included in staurolite porphyroblasts from Central Maine used to calculate

the ages. Section C of the thesis provides a detailed description of the monazite dating of

the staurolite porphyroblasts from the Colorado Front Range (Appendix-D). Figure 5a-f

and Fig. 6a-f show the probability curve and calculated age, for West Central Maine and

Colorado Front Range respectively, as weighted averages for each FIA set using ISOPLOT

D8

PhD Thesis SHAHA.A.

3 (Ludwic, 2003).

4.3. Thermocalc

A pseudosection was modeled in the NCMnKFMASH (Figs. 7 and 8) system for

each area by using the computer software Thermocalc (Holland and Powel, 1998) and the

average composition of the pelites from both regions determined from XRF analyses. The

bulk chemishy was determined by XRF at the Advance Analytical Centre, James Cook

University for samples from West Central Maine (Table 4) and Colorado Front Range

(Table 5).

S. INTERPRETATION

The rocks from each area have gone through a similar reaction history, as shown by

the pseudo sections, in reaching the regional PT conditions. In rocks from the Colorado

Front Range staurolite stability extends to lower pressures, about 0.3 kbars less than in

Western Maine, while the St+And stability field is slightly larger. Garnet and staurolite in

both areas appear to be in textural and chemical equilibrium. They have sharp intimate

contacts with one another and when garnet is included within staurolite it has euhedral

boundaries with no signs of retrogression prior to being included. In some samples,

cordierite and andalusite have locally replaced and/or partially pseudomorphosed staurolite.

In those few samples where sillimanite is present, staurolite porphyroblasts are replaced by

coarse muscovite and biotite. This suggests that staurolite mainly grew at the expense of

chlorite and that little biotite was consumed during the reaction. Thermodynamic modeling

D9

PhD Thesis SHAHA.A.

in the NCMnKFMASH system confirms this, with the reactions being:

Ms+Chl+Qtz = St+Bt+H20 (1),

if Grt was not consumed during St growth, or:

Ms+Chl+Grt+Qtz = St+Bt+H20 (2)

if Grt was pattially consumed during St growth. Synchronous growth of Grt and St can be

explained by the following reaction:

Chl+Ms = Mg-St+Mg-richer Bt+Mg-richer Chl+Grt+Na-richer Ms+H20 (3),

proposed by Guidotti (1974) which describes the petrological observations.

At 3-4kb and -520°C these reactions imply the coexistence of the metastable

mineral assemblage Grt+St+Bt+Chl (Figs. 7 and 8). In many satnples staurolite

porphyroblasts preserve chlorite as inclusions suggesting that this reaction did not go

entirely to completion.

Monazite dating shows that the succession of FIA sets accords with a progression

in ages from 1765 to 1674 Ma in the Rockies and 410 to 350 Ma in the Appalachians. The

consistency of this progression with the microstructurally determined FIA succession

confirms the value of FlA successions for accessing the combined structural and

metamorphic history of an orogen (e.g., Aerden, 1994; Bell et ai., 1998; Sayab, 2005; Yeh,

2007). Thus, 5 periods of staurolite growth occurred in the Palaeozoic rocks of Western

Maine and 3 in the Archaean rocks of Colorado.

It appears that many rocks in both regions did not grow staurolite during the first or

even second period of growth of this phase. Furthermore, some grew staurolite at one time

and then grew it again up to 60 million years later. This does not accord with current

concepts of reaction progress whereby once the P-T conditions for mineral growth occur

DlO

PhD Thesis SHAHA.A.

for a particular bulk composition, the reaction goes to completion. The only variables

outside of P-T -x that can explain this are the timing of growth relative to deformation and

the role of deformation in nucleation and growth itself. Bell et al. (2003) showed that

porphyroblasts commonly nucleate and grow into sites of progressive coaxial shortening

surrounded by anastomosing zones of predominantly non-coaxial shortening. When the

growing porphyro blasts reach the surrounding shear zones their growth ceases. It does not

recommence until a new deformation occurs at high angle to the earlier one and

repartitions around the porphyroblast margins, allowing new growth to occur in the strain

shadow (e.g., Spiess and Bell, 1996). This suggests that staurolite growth did not occur

unless the succession of deformation events that formed a specific FIA had taken place.

That is, partitioning of deformation through an outcrop at the scale of a porphyroblast is a

controlling factor on determining sites of metamorphic reaction and whether it occurs (Bell

et aI., 2004).

6. DISCUSSION AND SIGNIFICANCE

This is the first quantitative demonstration that staurolite can grow episodically by

the one reaction in rocks of similar bulk composition. Interpretations of metamorphic

reactions are commonly based on laboratory experiments, which suggest that reaction rates

at high temperature are very fast (Walther and Wood, 1984) and that local equilibrium is

achieved (Bickle et aI., 1997). However, field measurements ofreaction rates indicate that

the speeds of reactions in metamorphic rocks are orders of magnitude lower (Baxter and

DePaolo, 2000; 2002). They also suggest that disequilibrium is a common phenomenon

during mineral growth even for reactions that went to completion (Carlson, 2002). These

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PhD Thesis SHAHA.A.

results require the existence of a factor that controls porphyroblast nucleation and growth

that is distinct from bulk chemistry, PT, fluids and time. A number of studies have pointed

to the role of deformation in determining sites of metamorphic reaction and whether or not

a patticular reaction will take place (e.g., Spiess & Bell, 1996). Bell (1981) showed that

deformation is partitioned into zones of lower strain, dominated by coaxial deformation,

surrounded by anastomosing, higher strain zones, where the non-coaxial component is

dominant. Porphyroblasts preferentially nucleate in zones of lower strain (e.g., crenulation

hinges) prior to crenulation cleavage development (Bell et aI., 2003). A similar conclusion

was reached by Baxter and DePaolo (2004), who pointed out that certain reactions may

occur within discreet portions of the rock, while others react little or not at all. These

studies reveal that the reactions involved in the formation of metamorphic minerals are

episodic during prograde metamorphism, starting or stopping, as a function of deformation

pattitioning and strain localization (e.g., Etheridge and Hobbs, 1974; Wintsch, 1985; Bell

and Hayward, 1991; Dempster and Tanner, 1997;Whitmeyer and Wintsch, 2005). It has

been proposed that chemical and textural equilibrium during metamorphism can occur

locally (e.g., Bickle and Baker, 1990; Cartwright and Valley, 1991), while the overall rock

matrix was not in equilibrium (Kretz, 1973). This would allow reactions to go to

completion locally, but overall, the potential for new reactions to take place could be

preserved until the deformation repartitioned and new reactive sites were created.

Until recently porphyroblast matrix relationships were mainly used to define the

timing of porphyroblast growth relative to the matrix fabric (e.g., Zwart, 1962; Passchier

and Coelho, 2005). Such an approach is simplistic and ignores the fact that the matrix can

be re-used and reactivated many times during orogenesis (Bell et aI., 2003; Worley and

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PhD Thesis SHAHA.A.

Wilson, 1996). It also only allows one or two generations of growth of the same

porphyroblastic phase to be recognized. The FIA method provides a quantitative tool to

distinguish and con-elate multiple generations of porphyroblasts over large distances (Bell

and Mares, 1999). Combined with in situ dating of the inclusion trails that define a FIA set,

this provides detailed insights into tectono-metamorphic histories that were previously

inaccessible. Similar monazite ages to those recorded in Maine have been determined by

Pyle et aI., (2005) in the south western New Hampshire region of the NOlthern

Appalachians. Their ages (400±10; 381±8; 372±6; 352±14), combined with our data from

the Western Maine, confirm the -60 Ma of continuous tectono-metamorphism for this

portion of the Appalachians. Peak metamorphic conditions have commonly been regarded

as being contemporaneous along an orogen and late polymetamorphism was interpreted to

result from local pluton driven thermal activity (e.g., De Yoreo et aI., 1989). Orogeny is

not the result of a single event that culminates with peak metamorphic conditions, but

rather, a series of continuous but episodic events driven by tectonic plate motion (e.g., Bell

and Newman, 2006).

Available age data from metamorphic terranes is commonly derived from accessory

minerals that are present in the matrix. Such ages provide limited information and are

difficult to precisely integrate with the tectono-metamorphic history because of

reactivation of compositional layering plus local protection of grains in the strain shadows

of large competent minerals. More recent studies using other approaches (Bell and Welch,

2002; Pyle et aI., 2005) have revealed that accessory minerals such as monazite can grow

episodically during metamorphism. They showed that distinct populations yielding

significantly different ages can be separated, and this work confirms their approach. The

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PhD Thesis SHAHA.A.

episodic growth of staurolite porphyroblasts revealed by this study is significant for

metamorphism and reveals a significant role for deformation that needs to be taken into

account.

Acknowledgements

1 would like to thank Prof. Tim Bell for his critical comments. Special thanks to Dr. Mike

Rubenach and loan Sanislav for their help with monazite dating and critical discussions. 1

would also like to thank Prof. Jane Selverstone for sending me the manuscripts of the

Colorado region.

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PhD Thesis SHAHA.A.

7. REFERENCES

Aerden, D. O. A. M., 1994. Kinematics of orogenic collapse in the Variscan Pyrenees

deduced fi'om microstructures in porphyroblastic rocks from the Lys·Caillaouas

Massif. Tectonophysics, 236, 139·160.

Baxter, E. F., 2003. Natural Constraints on Metamorphic Reaction Rates. In Vance D,

Muller W, Villa I (eds) Geochronology: Linking the Isotopic Record with

Petrology and Textures. Geological Society, London, Special Publication, 220,

183·220.

Baxter, E. F., and DePaolo, D. J., 2000. Field measurement of slow metamorphic reaction

rates at temperatures of 500·600°C. Science, 288, 1411· 1414.

Baxter, E. F., and DePaolo, D. J., 2002. Field measurement of high temperature bulk

reaction rates II: Interpretation of results from a field site near Simplon Pass,

Switzerland. American Journal of Science, 302, 465·516.

Baxter, E. F., and DePaolo, D. J., 2004. Can metamorphic reactions proceed faster than

bulk strain? Contributions to Mineralogy and Petrology, 146, 657·670.

Bell, T. H., 1981. Foliation development: the contribution, geometry and significance of

progressive bulk inhomogeneous shortening. Tectonophysics, 75, 273·296.

Bell, T. H., and Hayward, N., 1991. Episodic metamorphic reactions during orogenesis: the

control of deformation pattitioning on reaction sites and duration. Journal of

Metamorphic Geology, 9, 619·640.

Bell, T. H., and Mares, V. M., 1999. Correlating deformation and metamorphism around

DIS

PhD Thesis SHAHA.A.

orogenic arcs. American Mineralogist, 84,1727-1740.

Bell, T. H., and Newman, R., 2006. Appalachian orogenesis: the role of repeated

gravitational collapse. In: Butler, R., Mazzoli, S. (Eds.), Styles of Continental

Compression. Special Papers of the Geological Society of America, 414, 95-118.

Bell, T. H., and Welch, P. W., 2002. Prolonged Acadian Orogenesis: Revelations from FIA

controlled monazite dating of foliations in porphyroblasts and matrix. American

Journal of Science, 302,549-581.

Bell, T. H., Fleming, P. D., and Rubenach, M. J., 1986. Porphyroblast nucleation, growth

and dissolution in regional metamorphic rocks as a function of deformation

patiitioning during foliation development. Journal of Metamorphic Geology, 4, 37-

67.

Bell, T. H., Forde, A. and Wang, J., 1995. A new indicator of movement direction during

orogenesis: measurement technique and application to the Alps. Terra Nova, 7,

500-508.

Bell, T. H., Ham, A. P., and Hickey, K. A., 2003. Early formed regional anti forms and


growth. Tectonophysics, 367, 253-278.

Bell, T. H., Ham, A., and Kim, H. S., 2004, Partitioning of deformation along an orogen

and its effects on porphyroblast growth during orogenesis. Journal of Structural

Geology, v. 26, p. 825-845.

Bell, T. H., Hickey, K. A., and Upton, G. J. G., 1998. Distinguishing and correlating

multiple phases of metamorphism across a multiply deformed region using the axes

of spiral, staircase and sigmoidally curved inclusion trails in garnet. Journal of

D16

PhD Thesis SHAHA.A.


Bickle, M. J., and Baker, J., 1990. Advective-diffusive transpOlt of isotopic fronts: An

example from Naxos, Greece. Earth and Planetary Science Letters, 97, 78-93.

Bickle, M. J., Chapman, H. J., Feny, J. M., Rumble, D., and Fallick, A. E., 1997. Fluid

flow and diffusion in the Waterville Limestone, south-central Maine: Constraints

from strontium, oxygen and carbon isotopic profiles. Journal of Petrology, 38,

1489-1512.

Carlson, W. D., 2002. Scales of disequilibrium and rates of equilibration during

metamorphism. American Mineralogist, 87,185-204.

Carlson, W. D., Denison, S., and Ketcham, R. A., 1995. Controls on the nucleation and

growth of porphyroblasts: Kinetics from natural textures and numerical models.

Geological Journal, 30, 207-225.

Cartwright, I., and Valley, J. W., 1991. Steep oxygen-isotope gradients at marble

metagranite contacts in the northwest Adirondack Mountains, New York, USA:

products of fluid-hosted diffusion. Earth and Planetary Science Letters, 107, 148-

163.

Cavosie, A., and Selverstone., J., 2003. Early Proterozoic oceanic crust in the northern

Colorado Front Range: Implications for crustal growth and initiation of basement

faults. Tectonics, 22, 10-1-10-23.

De Yoreo, J. J., Lux, D. R., Guidotti, C. V., Decker, E. R., and Osberg, P. H., 1989. The

Acadian thermal history of western Maine. Journal of Metamorphic Geology, 7,

169-190.

Dempster, T. J., and Tanner, P. W. G., 1997. The biotite isograd, Central Pyrenees: a

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PhD Thesis SHAHA.A.

deformation-controlled reaction. Journal of Metamorphic Geology, 15, 531-548.

Etheridge, M. A., and Hobbs, B. E., 1974. Chemical and deformational controls on

recrystallization of mica. Contributions to Mineralogy and Petrology, 43,111-124.

Guidotti, C. V., and Johnson, S. E., 2002. Pseudomorphs and associated microstructures of

western Maine, USA. Journal of Structural Geology, 24, 1l39-1156.

Hayward, N., 1991. Orogenic processes, deformation and mineralization history of

portions of the Appalachian orogen, USA, based on microstructural analysis.

Unpublished PhD thesis, James Cook University.

Hirsch, D. M., 2007. Controls on porphyroblast size along a regional metamorphic field

gradient. Contributions to Mineralogy and Petrology, 155,401-415.

Holland, T. J. B., and Powell, R, 1998. An internally consistent thermodynamic data set

for phases of petrological interest. Journal of Metamorphic Geology, 16, 309-343.

Kretz, R, 1973. Kinetics of the crystallisation of garnet at two localities near Yellowknife.

The Canadian Mineralogist, 12, 1-20.

Lasaga, A. C., Luttge, A., Rye, D. M., and Bolton, E. W., 2000. Dynamic treatment of

invariant and univariant reactions in metamorphic systems. American Journal of

Science, 300, 173-221.

Ludwic, K. R, 2003. Isoplot 3.0 - A geochronological toolkit for Microsoft Excel.

Berkeley Geochronology Center Special Publication No.4

Nesse, W. D., 1984. Metamorphic petrology of the nOliheast Front Range, Colorado: The

Pingree area. Geological Society of America Bulletin, 95,1158-1167.

Passchier, C.W., Coelho, S., 2006. An outline of shear sense analysis in high-grade rocks.

Gondwana Research, 10, 66-76.

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PhD Thesis SHAHA.A.

Pyle, J. M., Spear, F. S., Cheney, J. T., and Layne, G., 2005. Monazite ages in the

Chesham Pond Nappe, SW New Hampshire, U.S.A: implications for assembly of

central New England thrust sheets. American Mineralogist, 90, 592-606.



porphyroblasts revealed by independent 3D measurement techniques. Journal of


Selverstone, J., Hodgins, M., Shaw, C., Aleinikoff, and J. N., Fanning, C. M., 1997.

Proterozoic tectonics of the northern Colorado Front Range. In Geologic history of

the Colorado Front Range (Bolyard, D.W., and Sonnenberg, S. A, eds.). Denver,

Rocky Mountain Association of Geologists, 9-18.

Shaw, C. A, Snee, L. W., Selverstone, J., and Reed, J. C., Jr., 1999. 40Arr Ill'

thermo chronology of Mesoproterozoic metamorphism in the Colorado Front Range.

Journal of Geology, 107, 49-67.

Spiess, R., and Bell, T. H., 1996. Microstructural controls on sites of metamorphic reaction:

a case study of the inter-relationship between deformation and metamorphism.


Walther, J. V., and Wood, B. J., 1984. Rate and mechanism in prograde metamorphism.

Contributions to Mineralogy and Petrology, 88, 246-259.

Whitmeyer, S. J., and Wintsch, R. P., 2005. Reaction localization and softening of

texturally hardened mylonites in a reactivated fault zone, central Argentina. Journal

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PhD Thesis SHAHA.A.

0/ Metamorphic Geology, 23, 411-424.

Wintsch, R. P., 1985. The possible effects of deformation on chemical processes in

metamorphic fault zones. Advances in Physical GeochemistlY, 4, 251-268.

Worley, B. A., and Wilson, C. J. L., 1996. Deformation partitioning and foliation

reactivation during transpressional orogenesis, an example from the Central

Longmen Shan, China. Journalo/Structural Geology, 18, 395-411.

Yeh, M. W., 2007. Deformation sequence of Baltimore gneiss domes, USA, assessed from

porphyroblast Foliation Intersection Axes. Journal o/Structural Geology, 29,881-

897.

Zwart, H. J., 1962. On the determination of polymetamorphic mineral associations, and its

application to the Bosost area (Central Pyrenees). Geologische Rundschau, 52, 38-

65.

020

CONCLUSIONS Shah A.A.

CONCLUSIONS

E 1


CONCLUSIONS

This contribution has demonstrated that by adapting a mUltidisciplinary

approach to deformation and metamorphism via combining microstructural

measurements, geochronology, thermo barometry and phase diagram modeling,

complex tectono-metamorphic histories can be precisely revealed and interpreted. It

provides an example of such an integrated approach to investigate in detail the

metamorphic and deformation processes and the progression in the distribution of

various index mineral phases (garnet, staurolite, andalusite and cordierite) within the

Big Thompson Canyon region, Colorado Rocky Mountains, USA, which forms part

of a Proterozoic orogenic belt in the southwestern United States and provides a

classic example of a large high-T-low-P terrane and the problems associated with the

tectonic interpretation of such a regime (e.g., Williams and Karlstrom, 1996). These

rocks have been affected by strongly partitioned deformation and provide an ideal

place for developing new concepts on the role of deformation partitioning in

deformation, metamorphism and tectonism. The principal conclusions of each section

are summarized below:

SECTION A

A progression of FlAs (foliation intersection/inflection axes preserved within

porphyroblasts) in the foothills of the Colorado Rocky Mountains reveals four periods

of staurolite growth and two growth phases each of cordierite and andalusite; these

E 2


FIA based isograds migrated -2.5 kms across the orogen. This progression of mineral

development occurred about FlAs trending successively NE-SW, E-W, SE-NWand

NNE-SSW. Granitoids were emplaced during both periods of orogenesis in the

surrounding region but have no direct relationship to the isograds. Isograd migration

took place away from a heat source to the WNW and combined with the lack of

relationship to pluton boundaries to the nOlth and south suggests that the latter rocks

were not the heat source for metamorphism but rather product of it. A final period of

andalusite, cordierite and fibrolitic sillimanite grew over the matrix foliation and

consequently, no FIA was determined for it; the isograds for this last period of

mineral growth lie sub-parallel to those mapped by previous workers.

A strong correlation between the distribution of FIA trends and the axial trace

of all folds present in the area suggests that pockets of low strain occur due to

deformation partitioning, in spite of, or perhaps because of, the 4 successive changes

in bulk shOltening direction. These pockets preserve folds in the orientation in which

they formed from subsequent rotation or destruction. They provide remarkable

confirmation of the veracity of the FIA data and indeed reveal that such data can be

used to determine successions of fold development from regional maps at which scale

many overprinting criteria camtot be applied.

SECTIONB

FlAs (foliation intersection/inflection axes preserved within porphyroblasts)

in garnet, staurolite, andalusite and cordierite from Precambrian rocks in the northern

Colorado Rocky Mountains reveal 4 periods of growth about axes trending NE-SW,

E 3


E-W, SE-NW and NNE-SSW during an overall prograde path. The growth of garnet

was always followed by staurolite for each of the 4 FIA sets with andalusite and

cordierite following staurolite during development of the last 2 sets. Thermodynamic

modelling in the MnNCKFMASH system reveals that well documented episodic

growth occurred over a similar bulk compositional range and PT path for each FIA in

the succession. The intersection of Ca, Mn, and Fe isopleths in garnet cores

containing FIA set 1 indicate that growth of this mineral phase began at 540-550°C

and 3.8-4.0 kbars. Similarly, the intersection of Ca, Mn, and Fe isopleths in garnet

cores containing FIA sets 2 and set 3 indicate that these rocks never got above 4kbars

throughout the Colorado Orogeny. Indeed, they remained at approximately the same

depth with the onset of the much younger Berthoud orogeny, when the pressure

decreased slightly as porphyroblasts formed with inclusion trails preserving FlA set 4.

A slightly clockwise P-T path occurred for both orogenies.

SECTIONC

In-situ dating of monazite grains preserved as inclusions within foliations

defining FlAs contained within garnet, staurolite, andalusite and cordierite

porphyroblasts (foliation inflection/intersection axes preserved within porphyroblasts)

has provided a chronology of ages for extended periods of deformation and

metamorphism in the Big Thompson region of the Colorado Rockies in the USA. The

first period of tectonism, revealed by FIA set 1, trends NE-SE, and occurred around

1758.8±9 Ma. Garnet nucleated at 540-550°C and 3.8-4.0 kbars. The intersection of

Ca, Mg, and Fe isopleths in garnet cores for 3 samples, containing FIA set 1, set 2

E 4


(l758.8±9, 1720.5±6.1 Ma) and set 3 (l674±7.6 Ma), trending NE-SW, E-W and SE

NW respectively, indicate that these rocks never got above 4kbars throughout the

Colorado Orogeny. They remained around the same depth until the onset of younger

orogeny at 1420±14 Ma when the pressure decreased slightly as porphyroblasts

formed with inclusion trails preserving FIA set 4 and trending NNE-SSW.

SECTIOND

The lengthy time of tectono-metamorphism experienced by Paleozoic Western

Maine and by the Proterozoic Colorado Front Range resulted in multiple periods of

staurolite growth during subsequent deformation events. Five FIA sets were measured

from staurolite porphyroblasts in Western Maine and three from the Colorado Front

Range. Monazite inclusions in staurolite have an absolute age of l760±12 Ma (FIA 1),

1720.4±6.8 Ma (FIA 2), 1682±18 Ma (FIA 3) in Colorado and 408±10 Ma (FIA 2),

388±8.8 Ma (FIA 3), 372.l±5.5 Ma (FIA 4), 352.7±4.2 Ma (FIA 5) in Maine

confirming the multiple periods of deformation and metamorphism indicated by the

FIA succession in each region.

The metastable preservation of early staurolite porphyroblasts through

successive deformation and metamorphic events highlights the imp011ance of

deformation on porphyroblast nucleation and growth. These two areas show that the

reactions involved in the formation of metamorphic minerals are episodic during

prograde metamorphism, starting or stopping, as a function of deformation

partitioning and strain localization. Accessory minerals such as monazite can be dated

E 5


and their ages can be better integrated in the deformation and metamorphic history of

a region when they are well constrained by microstructural measurements.

Recommendations for further investigations

Further work will involve linking metamorphic and deformation processes with the

various pluton emplacement events. There are currently no detailed geochronological

data to confirm that metamorphic episodes were progressive over a span of -1 00 Ma

during the -1700 Ma Colorado orogeny, except those mentioned above (Shah, 2009,

2010). This research reveals that this orogeny took place over SO million years and

resulted from 3 distinct changes in the direction of horizontal bulk shortening over

that time. Orogenesis ceased and recommenced 260 million years later. These 4

periods of tectonism and metamorphism involved more than eight individual

deformation events that were partitioned by affected the breadth of the are studied.

Further work will be undertaken using a laser ablation-inductively coupled plasma

mass spectrometry (LA-ICP-MS) U-Pb zircon/monazite geochronological approach

on both metasediments and plutonic rocks to link deformation, metamorphism and

pluton emplacement mechanisms (e.g. Gabudianu et al.,2009; Cesare et aI., 2002;

Lombardo et aI., 2002; Rubatto et aI., 2001). The results so far suggest that

deformation and metamorphism began well before the plutons arrived at

17S9.7±9.SMa during the Colorado orogeny. However, just as deformation and

metamorphism pmtition, we now know that melting partitions (Rubatto et aI., 2001,

2001a. 2001b; Bauer et aI., 2007). Dating granite along strike from zones where one

FIA set dominates may reveal similar variations in age to those discovered by

Rubatto et al (2009; 2007) in partial melts horizons at depth in the Alps. Such

E 6


discoveries would significantly improve interpretation of causal and feedback

relationships between deformation, metamorphism events and pluton emplacement.

Furthermore, the overprinting foliation asymmetries preserved by the porphyroblasts

will provide previously unused quantitative orientation and shear sense data for

determining the mechanism of pluton emplacement.

References

Bauer C, Rubatto D, Krenn K, Proyer A, Hoinkes G (2007) A zircon study fi·om the Rhodope

Metamorphic Complex, N-Greece: Time record of a multistage evolution. Lithos

99:207-22.

Cesare B, Rubatto D, Hermann J, Barzi L (2002) Evidence for Late Carboniferous

subduction-type magmatism in mafic-ultramafic cumulates of the SW Tauern

window (Eastern Alps). Contrib. Mineral. Petrol. 142:449-464

Gabudianu Radulescu I, Rubatto D, Gregory C, Compagnoni R (2009) The age of HP

metamorphism in the Gran Paradiso Massif, Western Alps: a petrological and

geochronological study of "silvery micaschists". Lithos 110:95-108.

Shah A. A, 2009a. FIAs (Foliation Intersection/inflection Axes) preserved in porphyroblasts,

the DNA of deformation: A solution to the puzzle of deformation and metamorphism

in the Colorado, Rocky Mountains USA. Acta Geologica Sinica 83, 801-840.

Shah Afroz A, 2009b. Decoding the regional -1700 Ma deformational history from the rocks

of Big Thompson Canyon region Colorado using monazite dating of foliations

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preserved within garnet and staurolite porphyroblastic phases, Acta Geologica Sinica

(in press).

Lombardo B, Rubatto D, Castelli D (2002) Ion microprobe U-Pb dating of zircon from a

Monviso metaplagiogranite: implications for the evolution of the Piedmont-Ligurian

Tethys in the Western Alps. Ofioliti 27:109-118.

Rubatto, D., Williams, LS., Buick, LS., 2001. Zircon and monazite response to prograde

metamorphism in the Reynolds Range, central Australia. Contrib. Mineral. Petrol.

140,458-468.

Rubatto D, Schaltegger U, Lombardo B, Compagnoni R (2001a) Complex Paleozoic

magmatic and metamorphic evolution in the Argentera Massif (Western Alps)

resolved with U-Pb dating. Schweiz. Mineral. Petrogr. Mitt. 81:213-228.

Rubatto D, Williams IS, Buick IS (2001b) Zircon and monazite response to prograde

metamorphism in the Reynolds Range, central Australia. Contrib. Mineral. Petrol.

140:458-468

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Documents

Shah, Afroz (2010) Tectono-metamorphic evolution of Big … · 2011. 2. 11. · Afroz Ahmad Shah July 2010 iii . PhD Thesis Shah A.A. ELECTRONIC COpy DECLARATION 1, the undersigned,