Neocrystallization or cooling? 40Ar/39Ar ages of white micas from low-grade mylonites

E L S E V I E R Chemical Geology 143 (1997) 181-203

CHEMICAL GEOLOGY

INCLUDING

ISOTOPE GEOSCIENCE

Neocrystallization or cooling? 4°mr/39mr ages of white micas from low-grade mylonites

W.J. Dunlap *

University of California, Los Angeles, Earth and Space Sciences, Los Angeles, CA 90024, USA

Received 7 October 1996; revised 21 July 1997

Abstract

4°AT/39AF data for white micas from quartzite mylonites from a thrust system are evaluated with respect to whether the minerals record cooling ages or, alternatively, ages of deformation-induced neocrystallization. To address these interpretations three types of intormation are needed: (1) the effective diffusion length scale controlling volume diffusion of argon in white micas; (2) an estimate of the diffusion parameters for volume diffusion of argon; and (3) the thermal history. Completely recrystalli2"ed quartzites exhibit smooth grain size distributions for white micas, with good exponential or power law fits, suggesting that the grains grew under similar conditions. The microstructures indicate that the grains grew over a narrow range in time near the end of the deformation history. Measured grain size distributions provide a maximum limit for the diffusion length scale. Thermal histories are estimated by 4°Ar/39At diffusion experiments on K-feldspars using the multi-diffusion-domain method of Lovera [Lovera, O.M., Richter, F.M., Harrison, T.M., 1989. The 4°Ar/39Ar thermochronometry for slowly cooled samples having a distribution of diffusion domain sizes. J. Geophys. Res. 94, 17917-17935.]. These results are consistent with the expected temperature-time path for the thrusting environment, and predict maximum temperatures of 250-350°C during the time span recorded by the white micas. Experimentally and empirically calibrated diffusion parameters for white micas are applied to a cylindrical diffusion geometry to explore the question of cooling ages versus neocrystaUization ages. Forward modeling of the passage of the white micas through the closure window for argon diffusion, using the thermal histories derived from the K-feldspars, indicates that the white micas record neocrystallization rather than cooling, and supports the idea that the white micas record the timing of when ductile deformation ceased. ~, 1997 Elsevier Science B.V.

Keywords: Ar/Ar; muscovite; thermal diffusivity; crystallization; deformation; recrystallization

1. Introduction

Although the potential of the 4°At/39AE method for tectonic studies is great, the tectonic implications

* Present address: The Australian National University, Research School of Earth Sciences, Canberra, ACT 0200, Australia. Tel.: +61 (6) 249-3247; Fax: +61 (6) 249-3683; e-mail: Jim.Dunlap @ anu.edu.au

of white mica data from low-grade (i.e., greenschist facies) deformed rocks is not always critically evaluated. The vast majority of interpretations of such data is in terms of cooling through a closure temperature (Dodson, 1973) for argon diffusion ( ~ 350°C on average), with the inherent assumption that all grains in a dated aggregate have closed to diffusion. An alternative is that some or all grains in a dated aggregate have crystallized below their closure tern-

0009-2541/97/$17.00 ~.) 1997 Elsevier Science B.V. All rights reserved. Pll S0009-2541(97)00113-7

182 W.J. Dunlap/Chemical Geology 143 (1997) 181-203

perature, and thus do not record closure to diffusion at all. The timing of mineral crystallization in rela- tion to the timing of closure to argon diffusion is pivotal to the interpretation of the results (cf. Chopin and Maluski, 1980; Desmons et al., 1982; Hunziker, 1986). In the last 10 years the dating of white mica crystallization (rather than cooling) by the 4°mr//39Ar method has found great interest (e.g., Kligfield et al., 1986; Hunziker et al., 1986; Zingg and Hunziker, 1990; Dunlap et al., 1991, West and Lux, 1993). Clearly, tectonic models are critically dependent on whether mineral crystallization or mineral closure has been dated (i.e., Cliff, 1985; Hunziker, 1986). Quantitative analysis presented here indicates that the interpretation of 4°mr//39Ar data from white micas in low-grade deformed rocks requires knowledge of the thermal history and the kinetics of argon diffusion.

White micas that grow during low-grade deformation and metamorphism in many cases record closure to volume diffusive loss of argon. If a homogeneous population of white micas grow above their closure temperature for argon diffusion, and the effective diffusion length scale and diffusivity of argon are similar for all the grains, then in theory, upon cooling to below the closure temperature, all the grains will have similar argon concentrations and concentration gradients. However, white micas in low-grade deformed rocks represent a heterogeneous system, in general. Individual grains in a mica population grow over a range in time and temperature, and the closure temperatures and effective diffusion length scales for argon diffusion must certainly vary from grain to grain. The nature of the recrystallization process essentially guarantees that such heterogeneity will exist. For example, if deformation occurs after a grain grows it may experience changes in, among other things, mineral chemistry, density of defects and dislocations, and grain size (due to cleaving or segmentation). All of these changes will affect the minerals' ability to retain argon (cf. Goodwin and Renne, 1991; Lee, 1995; Dahl, 1996). Furthermore, grains or parts of grains can dissolve into ambient fluids, thus altering the age and grain size character- istics of the mica population. It is not surprising then that age spectra for white micas from low-grade deformed rocks are not as flat or concordant as one might hope they would be. In fact single grain

studies of white micas might be the best approach, except that the small grain volumes ( < ~ 50 ~m 3) that characterize low-grade deformed rocks make such studies difficult.

In this paper a 4°Ar/39mr data set for white micas from greenschist facies mylonites is critically as- sessed in terms of whether the results represent cooling ages or neocrystallization ages. The white micas were sampled in a restricted area within a thrust system in central Australia, where it is clear that all the rocks were deformed at a similar structural level in the crust (Dunlap and Teyssier, 1995; Dunlap et al., 1995). Since the temperature-time (T-t) history is so critical to this analysis, a series of argon diffusion experiments and multi-domain models for K-feldspars have been performed to constrain the T-t history of the sampling area.

2. Mica growth, argon loss, and the closure window

For white micas that crystallize during deformation the 4°At/39Ar age retained is most strongly dependent on, the timing of mineral growth, the T-t history during passage through the 'closure window' for argon diffusion (see below), the effective diffusion length scale and geometry, and the kinetics of volume (lattice) diffusion of argon. In general, however, many of these parameters are difficult to estimate. The relative timing of mineral growth can in some cases be estimated from textural arguments. T-t histories can be constrained by a variety of independent means, one of which is utilized in this study. The appropriate diffusion geometry, diffusion length scales and kinetic parameters governing argon diffusion in whites micas are currently under debate.

The bulk diffusivity of argon within the mica lattice may be addressed in terms of ionic porosity (Dahl, 1996), a concept which incorporates not only the major element chemistry but also such important features as defects, vacancies and minor element concentrations (e.g., Na in white micas, F in triocta- hedral micas). Each of these affects the interlayer bond length, which appears to be the most important factor affecting argon diffusivity. Yet, unless large variations in crystal chemistry are present, variations in ionic porosity will be perhaps an order of magni-

w.J. Dunlap~Chemical Geology 143 (1997) 181-203 183

tude less important than, for instance, the microstructural features which limit lattice diffusion length scale; features that act as higher diffusivity path- ways, ones too large or too irregular to be defined in terms of a lattice (di,,;location networks, kink boundaries, subgrains, etc.).

Robbins (1972) induced argon loss from white micas in a hydrothermal apparatus and concluded that loss was via diffusion mainly along the c-axis. The results indicate that an activation energy of 40 kcal /mol and a frequency factor of 6.03 × 10 -7 cm2/s (or 1.38× 10 -4 cm2//s for a cylindrical model) might be appropriate for muscovite. How- ever, the results are biased by the experimental techniques. By using a blender to reduce the size of a single crystal, the muscovite must have been exten- sively damaged by intragranular delamination of the cleavage and tearing at fragment margins (my own sizing experiments confirm this). In addition, grain growth and dissolution occurred during the hydrothermal treatmentS. The damage, dissolution and growth processes maximize argon loss, artificially increasing the diffusivity. Thus, even though the data can be fit by a slab raodel, there is significant scatter in the results and it is difficult to draw conclusions. Moreover, white micas are more retentive than indicated by the results (i.e., closure temperatures are too low; Hames and Bowring, 1994).

The recent experiments by Hames and Bowring (1994) give a clear indication that the cylindrical model of diffusion is the most appropriate for white micas. Their laser spot fusion data for several cleaved layers of a single crystal of muscovite show that 4°Ar concentration decreases outward from grain centers to grain margins in all cases and that no evidence exists for systematic age variation along the c-axis. They conclude that the physical grain size of the muscovite is the effective length scale for diffusion, and they prefer a lower diffusivity for white mica (they reassess Robbins data), which is consistent with empirical obseJwations (E a = 52 kcal /mol and D O = 0.04 cm2/s). ]in the end it must be concluded that the cylindrical model is the most appropriate for white micas. However, the appropriate kinetic parameters are not well constrained by the data of Robbins (1972).

There is substantial evidence to indicate that the effective diffusion dimension in micas is smaller

than the physical grain size, although admittedly it is likely to be a large fraction of the physical grain size. One of the first indications of this came from the work of Harrison et al. (1985), who suggested an effective diffusion dimension of 150 ixm for biotite. Further confirmation came from the work of Wright et al. (1991), which showed that for biotite grains as large as 600 lxm in radius the effective diffusion radius was no larger than about 225 txm. Although Hames and Hodges (1993), Hodges et al. (1994), and Hames and Bowring (1994) have argued for an effective diffusion dimension for argon in white micas at the physical grain size, it is hard to recon- cile this possibility with the mounting evidence from their own and other laser spot fusion studies. Irregu- lar patterns of spot fusion ages, mapped on cleavage planes of single grains of white mica, lend strong support to the notion that argon diffusion in micas is controlled by a domainal structure (fig. 5 of Scaillet et al., 1990; fig. 1 of Hames and Hodges, 1993; fig. 3 of Hodges et al., 1994). Higher diffusivity path- ways, which are probably common in micas, appear to truncate the largest domains in the interiors of grains. The interpretation that is preferred here, one which is consistent with all of the available data, is that the dominant diffusion length scale in micas is a large fraction of the physical grain size, but with smaller length scales also controlling diffusion. It seems obvious that only perfect crystals have an effective diffusion dimension equal to the physical grain size.

The concept of a closure temperature (Dodson, 1973) for micas is a useful one. If closure temperatures were known then well constrained cooling curves could be constructed. However, closure temperatures for micas are not known to within ~ + 50°C, due to a lack of knowledge of governing diffusion parameters, effective diffusion length scales and cooling rates. When grain aggregates are dated the heterogeneity of the population would dictate that no single closure temperature is appropriate for the whole. In fact, for a given cooling rate aggregates of like grains probably exhibit a normal distribution of closure temperatures. In a cooling system at any given temperature within this distribution a proportion of the grains will be effectively closed while the rest remain open. If such a population of grains is held within this temperature range for a


geologically significant period of time and then cooled to below the lowest closure temperature, a range of argon concentration gradients are preserved. In such cases, although it is convenient, it is not appropriate to interpret the data in terms of cooling through a single closure temperature.

From a tectonic perspective, one of the limitations of the closure temperature concept is that minerals continue to lose significant argon by diffusion even at temperatures of a few tens of degrees below their closure temperature. A more appropriate concept is one of a 'closure window' (Fig. 1A). The closure window is constructed by calculating the temperature above which no significant argon will accumulate, and the temperature below which no significant argon will be lost, even if the mineral is maintained at that temperature for the time scale of interest. When the appropriate time scale is selected the closure window is then uniquely defined in temperature-time space. The same concept can be extended to include grain aggregates. The usefulness of the closure window concept is that white micas which evolve in the middle and upper crust may spend large amounts of time in the closure window (but below their closure temperature), or they may enter and leave the closure window several times during their evolution.

It is clear that minerals which spend extended periods in the closure window lose a substantial proportion of their radiogenic argon, even well below the closure temperature. If grains crystallize within the closure window, and only undergo monotonic cooling subsequent to their formation, it is not immediately obvious whether they will record cooling ages or crystallization ages (Fig. 1B). Clearly, grains that crystallize at temperatures above that of the closure window (within time interval t s) will record cooling ages, whereas those that crystallize at temperatures below that of the closure window (interval t c) will record crystallization ages. It can be shown that grains that crystallize within the closure window (interval t b) and cool monotonically will for all intensive purposes always record crystallization ages. On the other hand, grains that pass through their closure temperature and thereafter remain isothermal within the closure window for a significant period of time are not likely to retain a cooling age or a crystallization age. As shown in Fig. 1 C, for monotonic cooling (linear) over a wide range of

400

350

300

E 250 I-

200

150

(A)

" • J "Closure Temperature"

"Closure

I i

ta t b t c Time Interval

Growth in t a

Growth in t b

Growth in t c

(B) t a t b t c Time Interval

1.00 i J ~ i t I I I i I

2 0.96 /

' / ~ 0.94

:~ 0,92 ~ "Closure Temperature"

0.9o ~ -~o -~o -~o -8o

(C) Temperature (°C) at Growth (relative to Tc)

Fig. 1. Illustration of the closure window concept. (A) closure window for continuous cooling determined by closure temperature, the cooling rate and the temperature below which diffusive loss of argon will not affect 4°Ar/39At ages on the geological time scale of interest. In the case of reheating, the cooling rate does not help to determine the closure window. (B) Schematic illustration of argon accumulation in white micas where grains that crystallize in interval (a) do not accumulate argon until interval (b), grains that crystallize in interval (b) retain some argon, and grains that crystallize in interval (c) never lose significant argon. (C) Shows the percentage of original crystallization age retained by white micas that grow in a cooling system and near their closure temperature (kinetics from Robbins, 1972).

W.J. Dunlap/Chemical Geology 143 (1997) 181-203 185

cooling rates the ratio of measured age to growth age will virtually always be greater than about 0.96, and in most cases it will be greater than 0.99. These curves vary only slightly with changes in diffusion parameters (Robbins, 1972, used here), diffusion length scale and geometry, and time scale of interest, yet the curves shown here can effectively be considered the worst case (i.e., lowest ratios). These simple calculations show that a crystallization age will be retained if the mineral grows below its closure temperature and subsequently cools monotonically.

3. Regional setting

The study area is the Ruby Gap duplex of central Australia, which is composed of at least five thrust sheets, each containing a distinctive quartzite marker

horizon (Fig. 2), the Heavitree Quartzite. The quartzite was undeformed ( ~ 0.5 km thick) prior to thrusting and resting unconformably on the basement, but it was intensely mylonitized during the Paleozoic Alice Springs Orogeny. The Ruby Gap duplex is bounded above by a megathrust zone which has carded greenschist to amphibolite facies rocks over the duplex, inducing up to greenschist facies deformation and metamorphism within the duplex itself (Dunlap, 1992). In a regional synthesis of previous isotopic studies in the region Dunlap and Teyssier (1995) concluded that the rocks immediately above the megathrust had cooled from maximum temperatures of about ~ 350-450°C in the Paleozoic. Reconstructions of duplex evolution indicate that the thrust system formed by a forward sequence of thrust fault propagation (Dunlap et al., 1997).

The T- t path that the rocks within the duplex experienced during Paleozoic overthrusting is likely

|

Plateau-like age, ~ I'~'~1-'~-1 I-~ neocrystallized white mica ~ E ~ 7 ~ 20~ ~

Maximum Temperature ~ of Kspar Model ~ 1~3Cl I-~ from 335-310 Ma

Approximate Age of White Mica Porphyroclasts

Fig. 2, Cross-section of the Ruby Gap duplex, a thrust system in central Australia. The Ruby megathrust is the uppermost thrust surface. Each thrust sheet contains the Heavitree Quartzite marker horizon, which outlines five laterally continuous thrust sheets (Sheets 1-5 in circles). Plateau-like ages of white micas are in rectangles; K-feldspars temperatures in ovals; ages of white mica porphyroclasts in sigmoid symbol.


to have been relatively simple, given the straightfor- ward geometry of the system. Dunlap et al. (1995) have documented the thermotectonic history of the region around the duplex through a combined structural and isotopic dating study. Prior to deformation the quartzite horizon was subhorizontal, lying at ~ 6 km depth on undisturbed basement, and buried be- neath a NeoProterozoic to Paleozoic sedimentary sequence. By about 350 Ma the Ruby megathrust (Fig. 2) had formed and displacement on it drove the lower plate rocks of the Ruby Gap duplex to deeper levels. As this process continued, the rocks of the upper plate were exhumed and largely eroded. The lower plate experienced increasing temperatures and pressures. Strain was accommodated largely by dislocation creep processes in quartz, which resulted in nearly complete recrystallization of the quartzite and generation of a pervasive micaceous cleavage. Be- tween 340 and 310 Ma each of the thrust sheets containing the quartzite were partitioned from the downgoing lower plate of the megathrust into the uplifting and cooling upper plate. During this cycle, each thrust sheet passed through its temperature maximum (which was different for each sheet) and then probably cooled monotonically thereafter.

4. Microstructure and petrogenesis of analyzed samples

White mica bearing quartzite mylonites and K- feldspar bearing basement rocks were collected from across the thrust system to determine whether the age gradients reported by Dunlap et al. (1991) reflect progressive closure to argon diffusion or, alternatively, trends in deformation ages (neocrystallization ages). In this section the microstructure of the K- feldspars is discussed briefly, while that of the white micas is documented in some detail.

K-feldspars in this study are from granitic, quart- zofeldspathic or pelitic basement gneisses. Three samples are from the lower plate rocks of the Ruby Gap duplex and four are from the gneisses of the upper plate (Fig. 2). The microstructure of the K- feldspars suggests that no significant modification of the grains has occurred since the Paleozoic. In the lower plate, deformation features range from breccia- tion to minor recrystallization around the margins of

porphyroclasts, possibly due to grain growth from extremely fine-grained (< 5 Ixm) cataclasite. Only the cores of porphyroclasts were used from these samples. In upper plate rocks, the K-feldspars have undergone internal deformation ranging from warp- ing of the lattice (undulatory extinction) to the development of deformation bands, subgrains and completely recrystallized margins. However, the K- feldspars selected from the upper plate were taken from pods of gneiss that had experienced only minimal Paleozoic strains; all of the deformation occurred at high (> 350°C) temperature. Thus, all of the K-feldspars dated here were essentially unaffected by deformation processes during argon accumulation below about 350°C.

The white micas dated in this study are from mylonitized Heavitree Quartzite (boxes in Fig. 2), many of which are completely recrystallized. One additional white mica is from pelitic gneiss in the megathrust zone immediately overlying the duplex. There is up to ~ 10% Proterozoic detrital white mica in the mode of the undeformed quartzite. In the quartzites that are not entirely recrystallized some detrital white mica has survived the deformation, and this contaminant is apparent in thin-section and isotopic analyses (see below). Cleavage-forming white micas present in the completely recrystallized (> ~ 98%) quartzite mylonites have grown during the deformation, possibly from a fluid phase. These inferences are confirmed by this study and by pre- liminary microstructural, chemical and isotopic studies on both populations of white micas (Dunlap et al., 1991, 1995; Dunlap, 1992).

Fig. 3 is a series of photomicrographs that illus- trate the deformation and disaggregation of the Pro- terozoic detrital white micas and the growth and evolution of the Paleozoic cleavage-forming white micas. There is an overall increase in the grain size of the recrystallized quartz and cleavage-forming white micas from a few tens of micrometers in the south to as much as a couple of hundred micrometers in the north (Table 1). This transition has been interpreted to reflect an increase in temperature (or strain rate) from south to north, and to reflect a temperature gradient during deformation (Dunlap et al., 1997).

The nearly undeformed quartzite from the lower- most thrust sheet, Sheet 1 of the duplex (see Fig. 2


Fig. 3. Photomicrographs of micaceous Heavitree Quartzite from study area; long axes are 1200 I~m in A-E and 630 Ixm in F. (A) Nearly undeformed quartzite (sample 731) contains quartz grains and Proterozoic detrital white micas. Sericite films at quartz grain margins are Paleozoic in age. (B) Mica-rich bedding-parallel shear zone in Quartzite of Sheet 1 (sample 731). Detrital white mica is disaggregated and neocrystallized white micas have grown between broken quartz. (C) Quartzite deformed by subgrain rotation recrystallization of quartz (sample 100B). White mica population contains both deformed detrital grains (arrows) and neocrystailized grains. (D) Quartzite deformed by grain boundary migration recrystallization (sample 760A). White micas form a homogeneous population of isolated grains defining cleavage. (E) Quartzite fi'om northern part of duplex (sample 249A); here quartz and white micas are coarsest-grained. Migrating grain boundaries have trapped white micas inside recrystallized quartz grains. (F) Quartzite from northern part of duplex (sample 533). Small white micas are found in the interior of recrystallized quartz grains; evidence is exhibited of grain boundary pinning by white micas (arrows).

Tab

le 1

Mic

rost

ruct

ural

an

aly

sis

and

grai

n si

ze d

istr

ibut

ions

for

qua

rtzi

te

Sam

ple

Nr.

Q

uart

z G

S

% u

x %

ux

Mic

a G

S

1 tr

of

mic

a G

S

Mic

a m

in/m

ax

S

iev

e L

M

1 tr

of

LM

N

E

xp

P

wr

Ag

e (~

m)

WR

m

ica

(l~

m)

(~m

) (I

Lm

) (t

xm)

(~m

)

943A

25

.2

15

2 14

.2

6.0

10

-45

1

0-5

3

19

6 31

5 0.

99

0.97

>

500

91

9C

68.2

6

10

81.2

67

.1

10

-38

0

75

-12

5

161

17

115

na

na

> 3

50

921

85.0

3

0 48

.2

39.4

1

0-2

80

7

5-1

25

14

3 23

45

7 0.

96

0.66

33

6

917A

58

.8

2 0

22.2

19

.0

5-1

30

5

3-7

5

87

14

421

0.98

0.

74

335

924

99.7

0

0 36

.7

34.2

1

0-2

80

5

3-7

5

97

17

342

0.95

0.

90

334

943C

60

.9

8 2

19.4

11

.6

10

-65

7

5-1

25

*

na

186

0.98

0.

97

329

520

99.0

5

5 25

.3

19.2

1

0-1

10

3

8-7

5

73

14

414

0.99

0.

94

325

952

168

0 0

26.2

28

.6

5-1

95

1

0-7

5

47

26

344

0.88

0.

95

323

733

50.0

0

0 13

.2

10.5

5

-75

1

0-4

0

23

10

381

0.99

0.

86

322

953

94.3

0

0 27

.0

23.3

5

-13

5

53

-75

10

1 20

32

4 0.

98

0.78

32

1 76

4 10

4 0

0 32

.0

40.7

5

-27

5

38

-78

77

17

35

9 0.

74

0.98

32

0

797

160

0 0

55.2

65

.0

10

-50

0

75

-18

0

168

37

457

0.88

0.

84

318

926

167

0 0

42.4

44

.7

10

-32

0

75

-12

5

145

19

481

0.89

0.

97

314

533

317

0 0

39.8

36

.4

10

-22

0

75

-15

0

143

24

491

0.97

0.

92

311

927

* *

250

est

0 0

na

na

na

na

160

est

na

na

na

na

309

g~

L,o

GS

= m

easu

red

grai

n si

ze (

diam

eter

);

%

ux

WR

= e

stim

ated

%

o

f w

ho

le r

ock

tha

t re

mai

ned

un

recr

yst

alli

zed

; %

ux

m

ica

= e

stim

ated

%

of

detr

ital

wh

ite

mic

a (i

.e.,

un

recr

ysta

lliz

ed);

Mic

a m

in/m

ax

= r

ange

of

op

tica

lly

mea

sure

d g

rain

len

gth

s (I

xm);

Sie

ve =

sie

ve

size

of

pure

(>

98

%)

wh

ite

mic

a; L

M =

mea

n

gra

in s

ize

for

part

of

popu

lati

on g

reat

er t

han

1.5

tim

es m

inim

um

and

max

imu

m s

iev

e o

pen

ing

s; N

= n

umbe

r o

f gr

ains

mea

sure

d; E

xp

= c

orr

elat

ion

co

effi

cien

t o

f b

est

neg

ativ

e ex

po

nen

tial

fit

; P

wr

= c

orre

lati

on c

oeff

icie

nt o

f b

est

po

wer

law

fit

; A

ge

= p

late

au-l

ike

seg

men

t o

f ag

e sp

ectr

um;

* =

all

gra

ins

mea

sure

d a

re s

mal

ler

than

sie

ve

size

; *

* =

sam

ple

fro

m g

neis

s;

est

= e

stim

ated

; na

= n

ot a

vail

able

.

i ixd

W.J. Dunlap / Chemical Geology 143 (1997) 181-203 189

for thrust sheet location), is characterized by slightly strained quartz grains and variably altered grains of detrital white mica up to a few millimeters across (Fig. 3A). Sheet 1 also contains some mica-rich bedding-parallel shear zones in the quartzite, such as that shown in Fig. 3B. In these zones, the detrital white micas have undergone dissolution and disaggregation, whereas the cleavage-forming white micas are new grains (i.e, neocrystallized; see between broken quartz grain).

The quartzite of Sheet 2 is characterized by partial recrystallization, higher strains and a better developed cleavage of neocrystallized white mica. Fig. 3C shows the characteristic microstructure of mica-rich Sheet 2 quartzite, where quartz grains are flattened and elongated parallel to a foliation defined by neocrystallized whites micas and deformed and ro- tated remnants of detrital white micas (see arrows). The white micas are either isolated between fine grains of recrystallized quartz (neocrystallized white micas) or they form mica-rich seams which are composed of both neocrystallized white micas and remnant detrital grains. The quartz has deformed primarily by intracrcstalline subgraln rotation, lead- ing to homogeneous flattening of grains (regime 2 of Hirth and Tullis, 19'92). It is clear from thin-section analysis that the white micas in the quartzite of Sheet 2 are an intimately mixed and heterogeneous population of both neocryslzllized grains and remnant detrital grains.

The quartzites in the uppermost thrust sheets, Sheets 3-5, are completely recrystallized to a homogeneous matrix of polygonal quartz grains and disseminated cleavage-forming white micas (Fig. 3D). Recrystallization of quartz has likely proceeded by subgrain rotation recrystallization and relatively rapid grain boundary migration (regime 3 of Hirth and Tullis, 1992). There is no evidence that detrital white mica has survived this intense recrystallization process (except sample 919C, see below). The cleavage-forming white micas are homogeneous in composition (Dunlap et al., 1995) and grain size distribution (see below). The micas appear to pin quartz grain boun&a-ies and do not generally pene- trate the quartz grains. Along the northern margin of the duplex in Sheet 5, however, the recrystallized quartz grains are much larger and some of the smaller cleavage-forming micas can be found in the interiors

of quartz grains (Fig. 3E). Quartz-quartz grain boundaries appear to have migrated past grains of white mica and entrapped them. Yet, even in these rocks there is evidence for pinning of the migrating grain boundaries, such as shown in Fig. 3F (arrows). Note that one grain boundary appears to wrap around a mica grain. These microstructures formed at higher temperatures than those shown in Fig. 3D.

The textures shown in Fig. 3 are characteristic of dynamic deformation (rather than static annealing) and indicate that the latest increments of a potentially long history of deformation are recorded by the microstructures. The deformation history of these rocks spanned perhaps tens of millions of years (i.e., Shaw et al., 1992; Dunlap et al., 1995). The fact that dynamic foliations are preserved, particularly in the quartz, indicates that the microstructures were frozen when deformation ceased. Furthermore, the delicate textures of the thin (a few ixm along c-axis) cleavage-forming white micas (Fig. 3D) were not signifi- cantly affected by deformation after the grains grew. There is limited evidence for kinking, folding or boudinage of the white micas. Thus, these micas grew late in the deformation history.

Grain size distributions (GSD) for the cleavage- forming white micas have been determined from lineation-parallel and foliation-perpendicular thin- sections (Table 1). For each sample 300 to 500 grain lengths were measured parallel to cleavage, rounding to the nearest 5-10 ~m depending on the grain size and the resolution of the microscope. It is difficult to measure the smallest grains by this method, thus there is potential for under-representation in the GSD. In a couple of cases the extremely fine grain size precluded measurement of 300 grains within a reasonable time. The grain size of the recrystallized quartz was determined by the method of linear intercept analysis (Dehoff and Rheins, 1968), where grain boundaries are counted along thin-section traverses and the results converted to mean grain sizes. Gal- wey and Jones (1963) concluded that the relative frequency of crystal sizes (of garnets) determined from a two-dimensional section differs only slightly from the true distribution in three dimensions, with grain lengths slightly underestimated in comparison with the true distribution. It seems likely that the GSDs measured here are also good estimates of the true distributions.


A measure of the mean grain sizes of the white mica populations is needed for modeling of the argon data. During crushing (disk mill), breakage of grains perpendicular to cleavage was minimal, but splitting along cleavage was prevalent. Thus, true cleavage-parallel grain dimensions are largely preserved in the separates. As sieves were employed, the grain size distribution of a given separate is not the true distribution. The aspect ratio of the white micas measured in the cleavage is about 2:1 on average, but may range from 1:1 to 4 :1 or more. Sieving tends to size populations according to the smallest dimension; high aspect ratio grains still pass the sieve even if they are longer than the opening is wide. For modeling purposes, a limited mean grain size (LM) was calculated as coarser than 1.5 times the minimum sieve opening and smaller than 1.5 times the maximum sieve opening. Breakage of grains perpendicular to cleavage, although not prevalent, will reduce LM. LM is considered a good approximation of the characteristic dimension of the white mica separates. It is clear from Table 1 that sieving has eliminated the largest part of the white mica populations, preferentially retaining only the largest grains. LMs range from 19 to 168 txm, increasing from south to north and also upward through the duplex (Table 1 and Fig. 2).

Aside from one outlier, all of the GSDs of the white micas approximate negative exponential or power law distributions (Table 1), and the fits are equally good in some cases. Exponential best fits of grain length histograms are negative in semi-log space, exhibiting a linear correlation of the form N = N0exp( - bL), where N is the number of crystals of a given size range, L is the length of crystals, b is the slope and N O is the intercept at L = 0. Power law best fits of grain length histograms are negative in semi-log space, exhibiting a correlation of the form N = A(L) -d, where A is a constant of propor- tionality and d is the slope (i.e., a line in log-log space). Fig. 4 shows the three different types of GSDs found for the white micas. Quartzites that are not completely recrystallized may or may not show a smooth GSD. Sample 919C contains the highest estimated percentage of detrital white mica ( ~ 10% of total white mica). A histogram of grain lengths suggests that the population is multi-modal (Fig. 4a), and correlation of the data is poor. In contrast, all

other analyzed samples exhibit GSDs that are highly correlated in either semi-log space (R = 0.74-0.99) or in log-log space (R = 0.66-0.98). Fig. 4b and Fig. 4c show a histogram of grain lengths and a semi-log plot, respectively, for sample 917A, both of which are characteristic of the negative exponential GSDs. Fig. 4d and Fig. 4e show a histogram of grain lengths and a semi-log plot, respectively, for sample 926, both of which are characteristic of samples that exhibit the best power law fits. Two trends are apparent from this analysis. Samples that are finer- grained and not completely recrystallized tend to yield better exponential fits, whereas those that are coarser-grained tend to have equally good or better power law fits. In addition, the slopes of the best exponential fits becomes less negative from the finer- to the coarser-grained rocks.

Do the GSDs tell us anything about the evolution of the white micas? The microstructures may represent 'steady state' conditions, but this is difficult to assess. For purposes of this paper, consider a simple model of mica evolution where grains undergo birth (grains form from a stable nucleus), subsequently grow, and may also die (leave the population). Under greenschist facies conditions, grains likely grow from a fluid phase situated primarily along grain boundaries. Grains may become unstable and dissolve back into solution, such that a cyclic process of mica precipitation/dissolution exists during deformation.

Marsh (1988) has shown that GSDs in rocks are characteristically negative exponential and that a bal- ance between grain birth, growth and death predicts a log-linear distribution. When GSDs are in steady state where rates of birth, growth and death are constant, the resulting GSD will be negative exponential. GSDs are curved when one of these rates is not constant in time, or when rates change and the population has not reached a new equilibrium state. For instance, for constant birth and death rates, but with growth rate proportional to grain size the resulting steady state GSD will obey a power law.

Cushman and Ferry (1988) found that GSDs from contact metamorphosed rocks are characteristically negative exponential. In contrast, slowly cooled high-grade metamorphic rocks yielded smoothly curved GSDs with peak frequencies in the middle of the distribution. They suggested that the deficit of the smallest grains was due to annealing (Ostwald

W.J. Dunlap~Chemical Geology 143 (1997) 181-203 191

18

16

14

12

~ 10 n er 8

6

4

2

0 10

(a)

80

70

60

50 | 40 g30

20

10

0 5

(b)

40 70 100 130 160 190 220 250 280

Length (microns)

100

~10

1 20 35 50 65 80 95 110 125 140 0

Length (microns) (c)

20 40 60 80 100 120 140

Length (microns)

160,

140,

120

100

g ~ 60

40

20

0

(d)

Sample 926 100

~.io r~

1

10 40 70 100 130 160 190 220 250 280 Length (microns) (e)

50 100 150 200 250 300 350 Length (microns)

Fig. 4. White mica GSDs from mylonifized Heavitree Quartzites. Three types of GSDs are found: (a) multi-modal (sample 919C); (b, c) negative exponential (sample 917A); and (d, e) power law distributions (sample 926).

ripening) and modification of an originally linear GSD. None of the GSDs for white micas in this study show significant curvature for the finest grain sizes in semi-log space. Thus, it does not seem likely that significant annealing has occurred after primary crystallization of the white micas.

From the finer- to the coarser-grained rocks, the decrease in the slope of the regressions and the apparent increase in the number of the largest grains relative to the rest of the population suggests that

growth of larger grains was favored. In comparing these naturally deformed quartzites with experimentally deformed Heavitree Quartzite, Dunlap et al. (1997) concluded that grain boundary migration in quartz was relatively rapid in the coarser-grained rocks. If the operation of relatively rapid grain boundary migration in quartz is responsible for the observed increase in grain size of quartz, and if the components needed for concomitant grain growth of white micas were transported primarily along grain

192 w.J. Dunlap/Chemical Geology 143 (1997) 181-203

boundaries, then it might be expected that size-dependent growth (i.e., power law) of white micas would be enhanced when grain boundary migration in quartz was most rapid.

Clearly, the development of the microstructures is a complex process where many different factors influence white mica growth. Several important pieces of information arise from the above analysis. The analyzed samples are characterized by dynamic textures where the white micas have grown during the deformation and remained unaffected by post-de- formational grain growth or loss. The operation of relatively rapid grain boundary migration broadens the GSDs and promotes growth of the largest white mica grains.

5. 4°Ar/39Ar results for cleavage-forming white micas

Nineteen white micas from quartzite mylonites from the Ruby Gap duplex have been dated by the 4°Ar//39At method. One additional white mica from a retrogressed gneiss was also dated (sample 927). Fifteen of the mineral separates have been dated by the 4°At/39Ar step-heating method. For one of these samples, whole-rock and a quartz-rich separate were

also step-heated. Four of the white micas from the quartzites had previously been dated by Dunlap et al. (1991); these data are described as briefly as possible.

Four of the step-heated white micas are from partially recrystallized quartzites (stretched detrital quartz clasts > 3% in mode), which exhibit the type of microstructure shown in Fig. 3C (cf. Hirth and Tullis, 1992). The remaining twelve samples come from quartzites that exhibit completely recrystallized microstructures typified by Fig. 3D.

The four samples of partially recrystallized quartzite, samples 520, 943A, 943C and 919C, come from the most southerly sampled localities in Sheets 1-4, respectively (Fig. 2). Further south the quartzite is progressively less deformed and the cleavage more poorly developed. The age spectra for samples 943A and 919C, shown in Fig. 5, yield the oldest age steps and are the most strongly discordant of all the micas. The discordance appears to be due to contamination by Proterozoic detrital white mica that survived deformation. Sample 943A contains two generations of white mica, a cleavage-forming white mica (< ~ 30 txm) distributed mainly among the recrystallized quartz, and much larger grains ( ~ 50-130 p,m) that are abraded, deformed and internally segmented. The larger grains ( ~ 2% by volume of the white micas)

u & 450-

J 400- a .

300-

250 0

943A Muse ~ I ~

I

T

I

919C Musc /

919C WR - - 919C Qtz

L

20 40 60 Cumulative %~Ar released

80 100

Fig. 5. Age spectra for white mica samples 943A and 919C. Results for whole-rock and 'quartz' for 919C are also shown.


are almost certainly remnant detrital grains that have survived the deformation process. Interestingly, these grains are not apparent in the GSD, but they were concentrated in the 10-53 p.m sieve fraction, as indicated by the age spectrum.

A similar case can be made for sample 919C, which has a multi-modal GSD (Fig. 4). The coarsest white micas are concentrated in the 75-125 ~m size fraction, and both remnant detrital grains and neocrystallized grains are present. The rising age spectrum for 919C reflects this mixed population in both the range of apparent ages (298-455 Ma) and the elevated apparent age of the plateau-like portion ( ~ 365 Ma). A step-heat of a small ( ~ 50 mg) whole-rock chip of sample 919C mylonite yields a result only roughly ,dmilar to that of the white mica separate. The total gas ages of the two step-heats are dissimilar (375 Ma far white mica, 350 Ma for chip), and the plateau-like segment for the chip is younger ( ~ 340-350 Ma). A 'quartz' separate was prepared from sample 919C to date the other major compo- nent of the rock, but the separation was unsuccessful in that two populations of white micas were observed within the quartz fragments. The white micas rather than the quartz would be expected to contain the vast majority of the potassium and argon. A step-heat of the 'quartz' separate gave a discordant age spectrum with a total gas age of 349 Ma (Fig. 5) and a plateau-like segment similar to the result for the chip.

The results for partially recrystallized samples 943C and 520 are not as easy to interpret as those for samples 943A and 919C, mainly because the results are in general agreement with the remainder of the dated white micas. The plateau-like segments of the bulk of the dated white micas are shown in Fig. 6. White mica from sample 943C gives an age spectrum with an initial step at 276 + 1 Ma, followed by a steadily rising plateau-like segment averaging 329 _ 1 Ma over about '75% of gas release, and finishing with steeply rising apparent ages (all plateau-like ages are step-size weighted means with error limits of 1 s.d. on the weighted means rather than on the analytical precisions; plateau-like segments may range several Ma). The GSD has a mean grain size of 19.4 ~m, but the mineral separate interval is 75-125 ~m. Thus, if any detrital white mica survived the recrystallization process then it is likely to

be concentrated in the separate. It is possible, however, that the level of contamination from detrital white mica is trivial, and that the plateau-like segment reflects a range of ages of neocrystallized grains.

Sample 520 yields an age spectrum that is markedly saddle-shaped in comparison with the other white micas (Fig. 6). The first step containing 5% of gas release gives an apparent age of 273 + 1 Ma. From the second step at 340 Ma successive steps decrease to a minimum apparent age of 325 Ma and subsequently rise to a maximum apparent age of 345 Ma (Dunlap et al., 1991). The GSD for sample 520 has a mean grain size of 25.3 Izm, while the separate interval is 38-75 Izm, indicating that the largest grains have been concentrated. The discordance in the age spectrum may be the result of a mixed population of detrital and neocrystallized white micas. Alternatively, the plateau-like portion in the middle of gas release may contain geologically meaningful information.

Eleven white micas from completely recrystallized quartzites yield age spectra with plateau-like segments from 311 to 336 Ma (Figs. 2 and 6). A comparison of the GSDs and mineral separate inter- vals indicates that the coarser neocrystallized grains have been concentrated in the separates (Table 1). Samples 953, 764, and 733 are from Sheet 3. The age spectrum for sample 733 reaches a minimum age of 3 0 2 + 4 Ma in the second step, develops a plateau-like segment of 322 + 2 Ma over 67% of gas released, and rises monotonically thereafter. Sample 764 exhibits an age spectrum that is flat for 89% of gas released and defines an age of 3 2 0 _ 1 Ma for the plateau-like segment (Dunlap et al., 1991). The age spectrum for sample 953 has a minimum apparent age of 306 _ 1 Ma in the first step, and defines a plateau-like segment in the saddle at 320 d- 1 Ma. The plateau-like ages for Sheet 3 samples are identical. Samples 917A and 921 are from Sheet 4. Sam- ple 917A yields an age spectrum that is discordant, with a weighted mean age of 335 ___ 3 Ma. Sample 921 gives an age spectrum that is saddle-shaped, excluding the first two steps, where steps 5 -9 in the saddle combine for a plateau-like age of 336 _ 1 Ma. The plateau-like segments for Sheet 4 samples are identical. Six white micas have been dated from Sheet 5, samples 924, 797, 952, 533 and 926 from


(a) 342 -

338 -

334 -

330 -

326 -

322 -

318

=

3 1 4

310 927

306 0

334] ~ 330-

g 326- ¢ 1

t =

~ 322- Q . e~

< 318 •

I I i i

20 40 60 80 100

Cumulat ive %agAr released

917A

314 •

310.

306- 0 20 40 60 80 100

Cumulative %agAr released

Fig. 6. Age spectra for white micas from the study area, plotted on two diagrams for clarity: (a) samples 921,924, 520, 952, 764, 926, and 927; and (b) samples 917A, 733, 943C, 953, 797, and 533, from oldest to youngest, respectively.

quartzite mylonites, and sample 927 from basement gneiss. The age spectrum for sample 924 exhibits a plateau-like portion at 333 + 1 Ma. Sample 797 gives an age spectrum that rises more or less continuously from the second step to define a plateau-like portion at 3 1 8 _ 4 Ma (Dunlap et al., 1991). Sample 952 yields an age spectrum with a fiat plateau-like seg-

ment at 323 _ 1 Ma. The age spectrum for sample 533 defines a plateau at 311 + 1 Ma (Dunlap et al., 1991). Sample 926 yields a plateau-like segment at 314 + 1 Ma, and sample 927 gives a plateau-like segment at 309 _+ 1 Ma. Over the length of Sheet 5 the plateau-like ages show decreasing age toward the north.


6. 4°Ar/39Ar difftLsion experiments and thermal modeling of K-feldspars

The multi-domain method of thermal modeling (Lovera et al., 19891) has been applied to 4°Ar/39Ar data collected during argon diffusion experiments on K-feldspars. The thermal histories (T- t ) obtained through this method are used to constrain the closure window for diffusive loss of argon from the cleavage-forming white nucas.

Because K-feldspars remain stable when heated under vacuum (Fitz Gerald and Harrison, 1993), it is possible to obtain Arrhenius parameters over the course of step-heating (Lovera et al., 1991). By mid-1994, optimal experimental procedures were es- tablished and rigorously adhered to during the experiments (Lovera et al , 1993). When the data are fitted via the multi-domain method an estimate of the T-t history that the rock has experienced (over ~ 150- 350°C) can be obtained. The method assumes a discreet distribution of non-interacting diffusion domains of, in this case, slab geometry that best de- scribe the stepwise release of 39Ar. In order to find the best-fit domain distribution for the Arrhenius parameters the method usually requires at least four domains. However, the resultant thermal histories are insensitive to the number of domains, as long as the appropriate minimum number is used (e.g., Lovera et al., 1993). Once the best-fit domain distribution is obtained, trial thernlal histories are chosen and forward modeling of the data yields a model age spectrum. Fine-tuning of the model age spectrum is done by adjusting the T-;.~ .history until a good fit with the laboratory-derived age spectrum is obtained. The method has in nume, rous cases produced geologically reasonable T-t histories (i.e., Copeland et al., 1995; Dunlap et al., 199.'i). The accuracy of the method with respect to temperature depends largely on the ability to estimate the activation energy. In this study, diffusion of argon is assumed to be governed by a single activation energy, which is obtained by regressing a line through the lowest temperature portion of the Arrhenius array. It should be noted that the multi-domain method predicts that the slope of the linear array defined by the gas released over the first few steps is proportional to the activation energy, but this is only true if all of the domains degas in fixed proportions.

Seven T- t models have been produced from 4°AE//39Ar step-heating data on K-feldspars, an example of which is shown in Fig. 7 for sample 795 K-feldspar (see also data repository). The preferred T- t paths are summarized in Fig. 8. Two of the models (samples 627 and 639) were previously reported by Dunlap et al. (1995). As shown in Fig. 2, three samples are from the lower plate of the megathrust (samples 627, 940 and 795), and four samples are from the upper plate of the megathrust (samples 639, 798, 927 and CAR).

At the beginning of the age spectrum for sample 795 K-feldspar ( ~ 0-6% of 39At release), the apparent age pattern is characterized by spikes and troughs. This pattern is the result of isothermal duplicate treatment, where excess argon likely to be sited in fluid inclusions is preferentially lost during the first of each isothermal pair of steps. This treatment was imposed on the sample in order to resolve the age of the lowest temperature domains in the troughs. The goal of the treatment was to use chlorine-correlated corrections for excess argon to further resolve the age of the lowest temperature domains (Harrison et al., 1994), but the resultant correction was in this case only minor and not considered critical for the present study (as is the case for the remainder of the analyzed samples).

Samples 627, 940 and 795, from within the Ruby Gap duplex, have experienced maximum temperatures of about 250-300°C during white mica crystallization at 311-336 Ma (Fig. 8). Somewhat higher temperatures ( ~ 350°C max.) are recorded prior to this time (sample 795). All of the samples from the lower plate were within ~ 6 + 2 km of the surface prior to overthrusting, thus the T- t models are required to start between 200 ° and 250°C prior to ~ 420 Ma. Sample 940 K-feldspar yields an age spectrum with apparent ages ranging from about 240 to 385 Ma, The preferred model contains seven domains with an activation energy of 44.8 kcal/mol, although a reasonable fit would be obtained with only five domains. The resultant T-t path indicates an initial period of cooling from ~ 350 to 250°C which started at about 380 Ma, followed by near-isothermal conditions during the time span recorded by the micas (311-336 Ma). By about 285 Ma cooling ensued again to below ~ 160°C. The 627 K-feldspar of Dunlap et al. (1995), from the same structural


340

320

300

t~ 280

260 ,<

240

220

7 9 5 K - f e l d s p a r

ca ,- , . : -

. . . . .

..,- ................. ~ii~ "

. . . . . . . Modeled +/- 20

I ~ 4 I I ~ I 20 0 6 0 80

Cumulative % 39Ar released 100

-4

-5

-6 v

-7 o

-8

-9 6

o 6 ' ' ' b o 0

- - Modeled

I i I 8 10 ll2

1 0 0 0 0 / ! " (°K) 14

2 . 0

1 .5

1.0

0 0 . 5

0 . 0

-0 .5

e 4 0 0

~300

, i i i ~ i

d

J -

240 260 280 300 Age (Ma)

1 I ~ I I 100 ~ I '* I 0 40 60 80 1 O0 220 320 340 360

C u m u l a t i v e % 39Ar r e l e a s e d

Fig. 7. Results of thermal modeling of K-feldspar 795 from lower plate of the Ruby megathrust: (a) measured laboratory degassing and best-fit model age spectrum (sensitivity is indicated by + 20°C models); (b) Arrhenius plot for measured and modeled results; (c) log r / r o plot of both measured and modeled results; see Lovera et al. (1989) for details; (d) model thermal history, dashed where inferred. See text for further details.

level as sample 940, yields similar results (Fig. 8). Initial cooling in this model starts at about 420 Ma followed by near-isothermal conditions ( ~ 250°C) until about 300 Ma when the final phase of cooling begins. The third K-feldspar from within the duplex, sample 795, is from the most northerly part of the duplex and structurally lies just one-hundred meters below the Ruby megathrust. The preferred model consists of eight domains with activation energies of 48.0 kca l /mol (Table 2). The T - t model is valid starting at about 330 Ma and 280°C, but immediately prior to that time the model is required to be hotter. Moreover, a peak in temperature is expected for thrust sheets that are first overridden and then exhumed toward the surface (e.g,, Davy and Gillet, 1986). If the temperature maximum lasted a few

million years then the peak would be about 350°C (a narrower peak requires higher temperatures). The flat top of the temperature peak must be no older than about 350 Ma, otherwise model age steps become too old at 80-90% gas release. Post-330 Ma the model must stay essentially isothermal ( ~ 285°C), but by about 290 Ma relatively rapid cooling pro- ceeds to below about 210°C. As an illustration of the sensitivity of this type of modeling, slightly hotter and colder models are shown in Fig. 7a,d (modeled + 20°C). The three K-feldspars analyzed from the lower plate yield similar timing for final cooling to temperatures below that where argon diffusion is significant, and the results indicate a northward-increasing temperature gradient during duplex formation.


400

350

300

- |

~! 250 ( 1 ) ¢ ' L

E I--

200

Thermal Histories

150

. I , I

I

l

I I

I

i i & - t '

639 . - " ] ~ Car

927 - ; ~ / /

1 ~ 79'8B t

795

940

I I

1%00 250 300 350 400 450 Age (Ma)

Fig. 8. Composite plot of thermal histories for rocks from the upper and lower plate of the Ruby megathrust. Dashed where inferred.

K-feldspar samples from above the Ruby megathrust experienced higher temperatures during thrusting than those from the lower plate, due to exhuma- tion from the midcrust. Few ages older than about 320 Ma are recorded in these K-feldspars (Fig. 8), indicating that they remained hotter than ~ 300- 350°C until about 320 Ma. Sample 798B, an excep- tion to the above generalization, exhibits excess argon contamination and is difficult to model. How- ever, the model exhibits rapid cooling at 290 Ma, which is consistent with all of the other K-feldspar models, and is the simplest interpretation. K-feldspar

Table 2 Domain size distribution

Domain Log D O c m 2 / s Volume fraction Domain size (relative)

795 K-feldspar, E a = 48.0 kca l /mol 1 9.197 0.01313 0.00003 2 7.522 0.06556 0.00021 3 6.643 0.17816 0.00058 4 5.757 0.09336 0.00161 5 4.320 0.08046 0.00841 6 3.525 0.22021 0.02101 7 2.711 0.30190 0.05363 8 0.170 0.04722 1.00000

639 of Dunlap et al. (1995) provides a model of relatively rapid cooling starting at about 320 Ma over a temperature range of ~ 330-230°C. This result indicates that, for monotonic cooling, uplift from the middle crust started at temperatures above that of argon retention. K-feldspar 927 yields an age spectrum with maximum apparent ages of about 280 Ma. The preferred model contains ten domains with activation energies of 36.6 kcal/mol, and predicts rapid cooling starting at about 280 Ma. The activation energy and frequency factor are particularly unretentive for K-feldspar. Nevertheless, the results indicate that the upper plate remained hotter than ~ 300°C until about 280 Ma, whereupon rapid cooling started. K-feldspar sample CAR was collected some 30 km north of the Ruby megathrust (Fig. 2). The preferred model contains eight domains with activation energies of 45.3 kcal/mol. The resultant T-t model is consistent with prolonged slow cooling between about 400 and 300 Ma restricted to a temperature range of about 260 + 20°C, with relatively rapid cooling commencing at about 295 Ma (Fig. 8). The form of the T- t curve may be correct, but the temperatures may be too low or the apparent ages too high, or both, considering the regional thermal

198 w.J. Dunlap/Chemical Geology 143 (1997) 181-203

history (cf. Dunlap and Teyssier, 1995). Results for samples 927 and CAR are not used to model argon loss from the micas. In summary, the T - t histories derived from upper plate K-feldspars suggest that rocks above the Ruby megathrust have remained below the partial retention window for argon retention in micas since about 280-290 Ma. Prior to this time the rocks of the upper plate were almost certainly much hotter ( > ~ 330°C).

7. Discussion

The 4°AF/39A£ results for white micas and K- feldspars offer considerable potential for advancing our understanding of whether micas from low-grade mylonites are likely to record cooling ages or neocrystallization ages. Several problems stand in the way of this goal, but they may be surmountable. Contamination of white mica populations by relict grains is perhaps the most difficult problem to over- come, but in many cases it is detectable. A quantitative measure of the thermal history that samples have experienced is necessary if modeling of argon diffusion in white micas is to be carried out. In this respect, the reliability of the K-feldspar thermal models is critical. Another important factor in this type of analysis is knowledge of the diffusion kinetics of argon in white micas, which is perhaps the greatest unknown. An attempt to discern cooling ages from neocrystallization ages is made in the section following the discussion.

'Contamination' of white mica populations must be avoided if the apparent ages of white micas are to be interpreted in terms of tectonic processes. How- ever, low-grade deformed rocks commonly contain several generations of white micas that are impossi- ble to separate by sizing, except in rare cases (e.g., Huon et al., 1994). In the present study, the contaminant is any detrital white mica that has survived the deformation process. In the cases of samples 919C and 943A the contaminant is obvious in the anomalously old age spectra. Although the GSD for sample 919C reflects the inhomogeneity of the white mica population (multi-modal GSD), that of sample 943A does not (negative exponential distribution; R = 0.99). In the case of sample 943A it is possible that deformation has reduced the grain size of the detrital

grains and blended them with the neocrystallized white micas. For samples 943C and 520 it is possible that the level of contaminant is minimal; however, the discordant nature of the age spectra suggests otherwise. The partially recrystallized microstructure of these rocks suggests that some detrital grains may have been shielded from the deformation process. Contamination of 1% by volume is difficult to detect petrographically, but the elevated ages affect the tectonic interpretation.

Grain boundary migration in quartz is an important factor in the destruction of the detrital contaminant during deformation. The completely recrystallized rocks have undergone significant grain boundary migration such that any given point within the rock could have seen tens or even hundreds of grain boundaries sweep by (Karato et al., 1980; Hirth and Tullis, 1992). This process is efficient at destroying detrital grains while promoting growth of new white micas to form the cleavage. Destruction of detrital grains is most efficient when quartz forms a high percentage of the mode (e.g., > 95%). Micaceous seams (many grains thick) shield some micas from the deformation process by localizing the deformation. Clearly, the best samples to date are those that are characterized by disseminated individual white micas in a matrix of recrystallized quartz (as in Fig. 3D). In such samples contamination is unlikely.

The reliability of the T - t histories derived from the K-feldspars needs to be addressed; however, it is beyond the scope of this paper to cover all the experimental and theoretical subtleties. The precision of the method in analytical terms is about _ 15°C. The accuracy of the method in terms of temperature depends largely on the ability to measure the activation energy, which is generally less than +2 kcal/mol for regression of the low-temperature Ar- rhenius array. Extrapolation of these activation energies to the temperatures of argon accumulation in nature presents further uncertainties. An underesti- mate of the activation energy will lower the temperatures in the model T - t histories by about 10°C for every 1 kcal/mol. The activation energy determined for K-feldspar 927 is 36.6 kcal/mol, which is quite low relative to K-feldspars in this and similar studies (e.g., Lovera et al., 1993), and therefore not useful for analysis of the micas. However, the ages evolved are the youngest measured 4°Ar/39AF ages in this


region, which is consistent with the low overall retentivity of the sample.

Excess argon evolved in the earliest gas release from K-feldspars c~aa be corrected for in certain cases (Harrison et al., 1994). Excess argon evolved in the middle and later portions of gas release is often manifested as a hump in the age spectra of K-feldspars (cf. Foster et al., 1990). Without such humps, and without other independent evidence for anomalously old ages, it is difficult to constrain the level of contamination. When apparent age steps are elevated by excess argon the models must be propor- tionally cooler in order to match the age spectra. The effect will be to shift the T- t history down and to the right on Fig. 8. In the K-feldspars analyzed here, samples 798B and CAR are clear candidates for excess argon contamination (they have hump shapes in their age spectra). In these samples, the timing of the initial phase of rapid cooling is the oldest of the group, and the temperatures derived from the modeling are possibly too low for exhumed midcrustal rocks. By comparison, K-feldspars 927 and 639 from similar structural levels attest to temperatures higher than about 360°C prior to about 300 Ma.

8. White micas: closure ages or neocrystallization ages?

Using both the most reliable T- t histories for the K-feldspars and the estimated grain sizes for the white micas it is possible to calculate whether the white micas record cooling ages or neocrystallization ages. In this analysis published activation energies and frequency factors for argon diffusion in white micas are applied to a cylindrical diffusion geometry. It is shown below that a choice of one set of kinetic parameters over the., other completely changes the interpretation of wtfite mica data from low-grade mylonites.

For this analysis the T - t histories of K-feldspars 940 and 795 are considered to be both reliable and representative of the thermal histories seen by the white micas in the Ruby Gap duplex. During deformation, a northward-increasing geothermal gradient would be expected, and this is reflected in these two thermal histories. The most northerly white mica samples may have experienced slightly higher tem-

peratures than those in the model for K-feldspar 795. Note that the plateau-like segments of the age spectra for the white micas (Fig. 2) all fall after the thermal peak in the T- t models. This is not surprising given the likelihood that deformation continued after the thrust sheets were partitioned into the ac- tively exhuming and cooling upper plate. Moreover, the ages of the white micas do not record peak metamorphic conditions.

For the kinetics of argon diffusion, two end-mem- ber activation energies and associated frequency factors are chosen. The 'unretentive' model is taken as that derived by Robbins (1972), with an activation energy of 40 kcal/mol and a frequency factor of 1.38 x 10 -4 S -1 . The 'retentive' model is that of Hames and Bowring (1994), where the activation energy is 52 kcal /mol and frequency factor is 0.04 cm2/s. These kinetic parameters may in fact bracket the range of diffusivities likely to govern volume diffusion of argon in white micas.

Fig. 9 shows the results of thermal modeling of the white micas, using the two K-feldspar-derived T- t histories. Apparent age is plotted against diffusion length scale (grain radius) for both the unreten- rive (A, B) and the retentive (C, D) diffusion parameters. The effective diffusion length scale for micas is close to the physical grain size, and the LM grain sizes of the white micas are considered appropriate (Table 1). Contours of white mica crystallization age show the apparent age of grains of various size that all crystallize at the same time (400, 360, 330, 310 Ma), but may undergo subsequent argon loss depending on their diffusion length scale. Any grains that retain their crystallization age are effectively closed to argon loss via volume diffusion at the time they crystallize (neocrystallized: to the right of the dashed lines in C and D). Note that the data show no correlation between grain size and apparent age, which is consistent with neocrystallization ages rather than cooling ages; the finest-grained populations are in fact older than the coarser ones. This becomes doubly evident when it is realized that the cooling rates are extremely slow (<< 10°C/Ma), which should accentuate differences in closure age due to variations in diffusion length scale if volume diffusion is limiting the argon concentration. In Fig. 9A the unretentive diffusion parameters are combined with the hotter thermal history of K-feldspar 795.


According t o this model, all of the measured white micas should be younger than about 300 Ma and should yield similar ages, regardless of when they crystallized. The lack of fit suggests that the unretentive model of Robbins (1972) is inappropriate. When the cooler T- t history of K-feldspar 940 is applied (Fig. 9B) the model is more consistent with the results, but only some of the data for the white micas fall in the field of cooling ages (between the upper and lower curves). The unretentive model of Robbins (1972) is only consistent with cooling ages. When the retentive model is combined with the hot thermal history of K-feldspar 795 (Fig. 9C) virtually all of the data fall between the 400 Ma and 310 Ma crystallization age contours. Note that for the white micas from the vicinity of K-feldspar 795, those most relevant to this case (filled squares), most fall in or near the field of neocrystallization. When the cooler thermal history is combined with the retentive model all of the data fall in the field of neocrystal-

lization (Fig. 9D). The retentive model yields exactly the opposite results to the unretentive model, with most of the white mica ages representing neocrystallization ages.

The analysis presented here potentially relies on the idea that the white micas crystallized during the latest phase of the deformation. If the white micas crystallized over an extended period, say 310-380 Ma, then the potential exists for a mixed population of grains; some grains would record closure to volume diffusion of argon while others would record neocrystallization. Given that most of the dated white micas come from mylonites with textures like Fig. 3D, however, it is likely that they crystallized over a narrow range in time (a few Ma).

The insensitivity of the data to diffusion length scale, in such a slowly cooling system, indicates that the white micas are recording neocrystallization. For instance, white micas 924, 921 and 917A yield plateau-like ages that are essentially indistinguish-

~J

"5

<C

Unretentive + "hot" Kspar 795

2 0 4 0 6 0 8 0 1 0 0

Retentive + "hot" Kspar 795 3 6 0 - "

C 400 Ma i

3 5 0 . . . . . . . . . . . . . . . . . -- / ~ - - . . . . . . . . . .

3 4 0 - 0 M a X

33o- "7"-~ -'~'~

3 2 0 - "

3 1 0 -

, - ~ NeccP/stalltzation Ages 300 : : ' : ; : : : ; : : : ; : : : =,

0 2 0 4 0 6 0 8 0 1 0 0

Diffusion L e n g t h Sca le (p,m)

3 5 0 -

3 4 0 -

3 3 0 -

3 2 0 -

3 1 0 -

3 0 0 -

2 9 0 -

2 8 0 = 1 • • m • , • i , • •

Unretentive + =cold" Kspar 940

, • , i , • ' i

0 2 0 4 0 6 0 8 0 1 0 0

Retentive + "cold" Kspar 940

; i ...... ~ ........... ~" ........... ; ........... .............. : 3 5 0 - Neoc•stallization Ages 3 4 0 . . . . . . . . . . . - ~ ' ; . . . . . . . i ................ . . . . . . . . . . . . . . . . . .

, *

3 3 0 . . . . . . . . . ~ . . . . . . . . . . . . ~ " ~ ~ ~ 3 3 0 M a ~

3 2 0 - ~" .... p ¢

3 1 o - 31OM~ : ~ " " = ~

3 0 0 : : ' i ' , : : l l l l l i 0 2 0 4 0 6 0 8 0 1 0 0

Diffusion Leng th Sca le ( l a n )

Fig. 9. Grain size versus age plots showing the plateau-like ages and measured grain sizes of the white micas. Squares are for each measured white mica; ellipses represent the range of age in the plateau-like segment of the age spectrum and one standard deviation in limited mean grain size. Solid curves in each plot are generated using the diffusion parameters of either Robbins (1972) or Hames and Bowring (1994) and a thermal history derived from either K-feldspar 940 or 795. Each solid curve is a contour of the age of crystallization for white mica (400, 360, 330, and 310 Ma); grains only see the portion of the thermal history subsequent to crystallization. Neocrystallization ages plot right of the dashed lines in (C) and (D).

W.J. Dunlap~Chemical Geology 143 (1997) 181-203 201

able (Fig. 2). Their proximity in the field indicates that they experienced nearly identical thermal histo- ties, yet the results show no dependency on grain size. The same is true for samples 733, 764 and 953 from Sheet 3 where a change in grain radius by almost a factor of 2 does not result in any change in plateau-like age. Convincing evidence also comes from samples 952 a:ad 797B, which experienced the same thermal history (sampled from one outcrop). Although the 952 white mica is finer-grained by a factor of 3 it yields a plateau-like age that is clearly older than that for 797B white mica. The overall data set is best interpreted as neocrystallization ages, recording the latest increments of the deformation history.

9. Conclusions

problem is tractable. The method used here to test the interpretation of white mica 4°Ar/39Ar data can in principle be used to determine, through a series of such studies, what is the acceptable range in diffusion parameters for white micas. The approach requires both thermal modeling to obtain an appropriate T - t history and measurement of several grain size fractions of white micas over restricted sampling areas.

The interpretation of neocrystallization ages is the simplest and is probably correct, given the insensitivity of the data to grain size and the fact that cooling rates were quite slow (<< 10°C/Ma). Because dynamic textures have been dated, where the white micas grew late in the deformation history, the apparent ages approximate the timing of when ductile deformation ceased (e.g., Dunlap et al., 1991).

Given that white micas are, in general, less retentive than amphiboles and more retentive than K- feldspars, the characteristic diffusivity of argon in white micas should be lower than that proposed by Robbins (1972), but perhaps similar to or higher than that preferred by Hames and Bowring (1994). If this is in fact the case, then the geochronological commu- nity is confronted with the problem that 4°Ar/39Ar data sets for white micas from low-grade mylonites, most of which are interpreted in terms of cooling, may in fact contain information relating to neocrystallization.

The diffusion parameters that best characterize argon diffusivity in white micas are difficult to ascer- tain, but the results presented here give a partial solution to the problem. It is clear from the analysis that a choice of one set of diffusion parameters over the other completely changes the interpretation of the results. Knowledge of the dominant diffusion length scale, and the frequency factor and activation energy for volume diffusion of argon are the most important factors affecting the interpretation of 4°Ar/39Ar data for white micas from low-grade mylonites. Subtle chemical variations would seem to be much less important in controlling argon diffusion, since micas in thoroughly recrystaUized rocks (of one bulk composition) tend to exhibit limited chemical variation. If the dominant diffusion length scale is in fact close to the physical grain size of the micas, then the

Acknowledgements

This work has benefited greatly from interaction with Mark Harrison, Oscar Lovera and Marty Grove. Dunlap is grateful for a National Science Foundation Postdoctoral Fellowship which facilated this study.

Appendix A. Analytical procedures

Analytical procedure for 4°Ar/39Ar analysis at The Australian National University is covered by Dunlap et al. (1995). The remaining samples were analyzed at the University of California, Los Ange- les. For these samples, the following analytical procedures were used. The samples were irradiated for 80 h (without cadmium shielding) in the Ford Reac- tor, University of Michigan, in order to attain a neutron fluence similar to previously run samples. Aside from using a different time-temperature schedule for step-beating, treatment of the data was essentially identical to that done at The Australian National University. Temperature was progressively increased over the course of step-beating experiments and was monitored by a thermocouple at the base of a tantalum crucible within a double vacuum resistance furnace. Experiments were started at or above 400°C and concluded at 1500°C with the


temperature held at each temperature step for a variable amount of time depending on the desired step size. The detailed step-heating schedules are listed in the supplementary data tables. At each extraction temperature, the gas released was exposed to Zr-A1 getter pumps to remove active gases, and the argon was subsequently isotopically analyzed using gas source mass spectrometers operated in the static mode (VG1200 and VG3600). Measurements performed with the VG3600 utilized both a Daly photomultiplier (mass discrimination ~ 0.993) and a Faraday cup (Ar sensitivity of 2 × 10 -15 mol /mV; mass discrimination ~ 0.999). The typical gain of the photomultiplier was about 110; this was monitored during each analysis by measuring mass 39 on both detectors. Measurements performed on the VG1200 made use of an electron multiplier (Ar sensitivity of about 5 × 10-~7 mol /mV). Getter section blanks were normally below 5 × 10 -16 mol on mass 40 over l0 min. Blanks were essentially atmo- spheric, as well as could be determined, and the mass 39 blank (10 rain) was normally below 2 × 10 -t8 mol. Furnace blanks below 1000°C were generally well below 5 × 10 -15 mol of 4°Ar. Correction factors were determined from KESO 4 and CaF 2 in- cluded with the samples during irradiation, and the J factors were determined from Fish Canyon Sanidine (27.8 Ma; Cebula et al., 1985) by interpolative curve fitting. The 4°K abundance and decay constants rec- ommended by the IUGS Subcommission on Geochronology were used (Steiger and Jaeger, 1977).

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Neocrystallization or cooling? 40Ar/39Ar ages of white micas from low-grade mylonites