.ANORTHOSITIC SILLS OF THE SOUTHERN ADIRONDACK

MOUNTAINS OF NEW YORK STATE

by

Theresa Anne,Beddoe

Thesis submitted to the Graduate Faculty of the

Virginia Polytechnic Institute and State University

in partial fulfillment of the requirements for the degree of

MASTER OF SCIENCE

APPROVED:

D.R. Wones

M.C. Gilbert

in

Geological Sciences

b.X. Hewitt, ChairmaJl

Y.W. Isachsen

June, 1981

Blacksburg, Virginia

J .A. Spee~

J.M. McLelland

ACKNOWLEDGEMENTS

I would like to thank

and for editing the manuscript,

State Geological Survey and

for directing the study

of the New York

of Colgate University

for time spent in the field and for reviewing the manuscript, and .

of the New York State Geological Survey for the use of

sodium cobaltinitrite and other helpful contributions including

reviewing the manuscript.

I would like to thank and for the

use of their analytical facilities and for his

expertise during analysis for the completion of some of the analyses and

for reviewing the manuscript. I would also like to thank

and for

their helpful contributions. To go special thanks for

taking a deep interest in the project and for providing invaluable

insights. I would like to thank

and and

for serving on my thesis committee.

My deepest thanks go to for being my constant field

assistant; to , who, at 70 years of age, was

the best field guide one could ask for; to for her

help in the field and for zealously typing the manuscript.

Finally, my special thanks go to my family for their constant

support and understanding, and to for her continuous

encouragement and friendship, and for reviewing the manuscript.

ii

This research was supported by graduate teaching and research

assistantships from Virginia Polytechnic Institute and State University

and from NSF grant EAR77-23225 AOl to , by a grant to

of the New York State Geological Survey, the New

York State Conservation Department, and a Grant-in-Aid of Research from

Sigma Xi, The Scientific Research Society.

iii

TABLE OF CONTENTS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements • List of Figures. ...................................................

ii

v

List of Tables ..................................................... vi

List of Plates ••••••••••••••• .vii

Introduction....................................................... 1

Analytical Procedures •• . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . General Geo 1 ogy ••• ................................................. Petrography ••••••••••

Mineral Chemistry •••.•••••

Whole Rock Chemistry •• . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metamorphic Petrology •.•

Intensive Parameters ••••••••

Metamorphic Assemblages.

Igneous Petrology.

Parent Magma •••••.•...•••• . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Crystallization Sequence.

Intrusive History • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bibliography .•.•••• . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8

10

15

27

45

54

54

57

65

65

66

68

76

Appendix 1: Modes. . . . • . . . . . . . . . . • • . • . • . . . . . . . • . • . . . • • • . . . . . . • . . • . . 83

Appendix 2:

Appendix 3:

Vi ta •..•••

Abstract

Chemical Analyses .•

Mineral Analyses •.••••••••.••.•••...••••••••••••••.

94

97

. . . . . . . . . . . . . . . . . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 50

iv

LIST OF FIGURES

FIGURE PAGE

1. Southern Adirondacks, New York State--Location Map •..••.•.••••• 3

2. Lake Pleasant Quadrangle, Northern Half--Geological Map •••••.•• 5

3. Modal Plagioclase Plot for the Speculator Sheet ..•.••••...••••• 17

4. Plagioclase Compositions ........................•.............. 29

5. Pyroxene Compositions •••••••••••.••••••••••••.•••••••.•••••.••• 31

6. Arnphibole Compositions ......................................... 33

7. Speculator Sheet Amphibole Compositional Trends ••.•.••••••••••. 37

8. Biotite Compositions •••••••••.•.•••••••••.•••..•••.•••.•••••••. 39

9. Speculator Sheet Biotite Compositional Trends •••.....•..••••••. 41

10. Garnet Compositions ••••.•••••••••••••••••.••••.••••••••..•••••. 44

11. Whole Rock Chemistry vs. Distance from the Base of the Sill •••• 48

12. Harker Variation Diagrams •..••••••••••.•••••...•.••••••••.••.•• 50

13. AFM Variation Diagram for the Sills and the Oregon Dome ••.••••• 53

14. ACFM Diagram illustrating the garnet corona-producing reaction. 60

15. AFM Variation Diagram for the Snowy Mountain and

Thirteenth Lake Dome • •••••••••••••••••••••••••••••••••••.•••••• 73

Al. Sample Map for the lS Traverse ................................. 87

A2. Sample Map for the 2S Traverse . ................................ 89

A3. Sample Map for the 3S and 4S Traverses •••••••.•••••••.•••.••.•• 91

A4. Sample Map for the Wells Traverses . ............................ 93

v

LIST OF TABLES

TABLE PAGE

1. Average Chemical Compositions ••••.••..••..•••.•••••••••.••..••• 46

2. Metamorphic Geothermometry ••..•••••.••.•.••..•••.•.••••...••... 55

vi

LIST OF PLATES

PLATE PAGE

1. Speculator Sheet Garnet with Coronas and

Spongy Tenantville Garnet •.•..••••.•...•••••.••.•••••.•••.•..•• 21

vii

INTRODUCTION

A large body of knowledge exists on the major anorthositic

intrusives of the Adirondack Grenville Province (i.e. Miller, 1918;

Balk, 1930; Buddington, 1936 and 1939; Isachsen, 1969), yet little is

known of the petrology and geochemistry of the small leucocratic

intrusives of the anorthositic series (Buddington, 1969). The petrology

of two of these small bodies, the Speculator Sheet and the Wells Sill,

is the subject of this investigation. Both of these bodies crop out in

the Lake Pleasant 15' Quadrangle of the southern Adirondacks (Figure 1)

and have intruded into the Rooster Hill/Little Moose Mountain Formations

(Figure 2). The contact relationships between the Speculator and Wells

bodies and the metasedimentary country rocks are important in light of

the current controversy concerning the relationship of the major

anorthositic massifs with the overlying metasediments.

Walton and de Waard (1963) have proposed two major orogenic cycles

for the evolution of the Adirondacks: intrusion of anorthosite,

beveling by erosion, deposition of sediments on an anorthositic

basement, then final metamorphism by the Grenville Orogeny. Their

careful mapping of the eastern and south-central Adirondacks revealed a

persistent unit of marble calc-silicate rock forming the first unit of

the metasediments in contact with the massif. Based on these data, they

suggested that the massif was a pre-Grenville basement for the

deposition of the overlying metasediments and that the subsequent

metamorphism and deformation during the Grenville orogeny was solely

responsible for the present structural relationships between the two

1

2

FIGURE 1

Southern Adirondacks, New York State

showing the three anorthositic massifs (the Oregon Dome,

the Thirteenth Lake Dome and the Snowy Mountain Dome)

and two sills (the Speculator Sheet and the Wells Sill)

studied. (Isachsen and Fisher, 1970)

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Lake Pleasant Quadrangle, Northern Half

(Mclelland, in preparation)

shows the relationship of the Speculator and

Wells Sills to the Glens Falls Syncline

and gives a key to sample maps

in Appendix 1.

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6

units. Mapping of the anorthositic Snowy Mountain Dome of the south

central Adirondacks (de Waard and Romey, 1963 and 1969) substantiated

their hypothesis.

In contrast to these conclusions, subsequent developments indicate

that the contact between the metaigneous and metasedimentary units is a

primary intrusive contact (Buddington, 1939). Husch et al. {1975) and

Isachsen et al. (1975) cite the intrusive relationships of the Wells and

Speculator anorthosite masses, which lie stratigraphically above the the

base of Walton and de Waard's (1963) supracrustal sequence, as evidence

against an unconformity. Hills and Isachsen (1975) obtained a whole-

rock Rb-Sr isochron date of 1199 +/- 14 my with an initial Sr ratio of

0.70452 for the monzonite (mangerite) and metanorite of the Snowy

Mountain Dome, which agrees well with other dates from Adirondack

metaigneous rocks (i.e. 1200 my for the Marcy massif using a Nd-Sm whole

rock-plagioclase isochron; Ashwal et al., 1981). Based on the close

correspondence of these dates, they conclude that the Snowy Mountain

Dome is intrusive into the country rock and does not unconformably

underlie them.

It is therefore important to establish the geochemical and

petrologic fingerprints of these sills, to determine if their history is

linked to that of the major anorthositic massifs, which have been

interpreted as a basement complex by Walton and de Waard (1963). By

determining their relationship, a positive contribution may be made to

this controversy.

In addition to the intrusive history of the sills, their subsequent

7

metamorphism must be investigated to understand the changing

assemblages throughout the sills and the occurrance of fine reaction

rims of orthopyroxene and plagioclase around some of the garnets.

Such reaction rims have been discussed in the literature for almost

a century (i.e. Holland, 1896; Fermor, 1912; Eskola, 1920; Shand, 1945;

Subramaniam, 1956). Holland (1896, p. 29) first proposed that the

reaction rim represented a stage in the formation of garnet from the

primary mafic minerals, and Fermor (1912) believed that the rims formed

by the destruction of garnet because of a rapid reduction of temperature

accompanied by a gradual lowering of pressure. These points of view

have been the two most often advocated.

Some recent workers in other areas (i.e. Kornprobst, 1977; Horrocks,

1980) have interpreted these coronas to be the result of lower pressures

and/or higher temperatures. In the Adirondacks, de Waard (1965, 1967)

used them to distinguish the high pressure field from the low pressure

field of the granulite facies. De Waard (1965) states that the rims

most probably form from the reaction hbd + gar + qtz = opx + plag + H2o and thus that garnet is a disappearing phase in granulite facies

terranes in rocks that also contain hornblende and quartz. The

selective occurrences of garnet rims in the rocks of the Wells and

Speculator bodies conflict with this interpretation.

ANALYTICAL PROCEDURES

McLelland's map (in preparation, Figure 2) of the Lake Pleasant 15 1

quadrangle was used as a base map for extensive sampling of the two

sills. 81 samples were collected from the Speculator Sheet and 39 thin

sections were described. 51 samples were collected from the Wells Sill

and 33 thin sections were described. The sample locations are shown in

Appendix 1. Twelve samples were collected from the Oregon Dome from the

Siamese Pond trail in the Thirteenth Lake 15 1 quadrangle. Eight thin

sections were described.

Modes, also presented in Appendix l, were determined optically from

point counts of each thin section. Grain size ranged from less than 0.5

mm to 6 mm and the number of points counted were sufficient for the

reported values to have a statistical accuracy of better than :2% (Van

Der Plas and Tobi, 1965).

Mineral analyses were performed with an ARL-SEMQ microprobe with an

operating voltage of 15kv; mineral standards for the six and nine

element analyses were Goschener titanite (Ti), Rockport fayalite (Fe),

Marjalahti olivine (Mg), synthetic tephroite (Mn), Tiburon albite (Na,

Al, Si), Ingamells' orthoclase (K), anorthite glass (Ca), synthetic

barium apatite (Ba, Cl), Tiebaghi chromite (Cr), Sillbole humite (F),

pure anhydrite (S), Durango apatite (P), and spec pure strontianite

(Sr}. Matrix correction factors used were those of Bence and Albee

(1968}.

Rock analyses were performed using a Phillips automated wavelength-

dispersive, X-ray fluorescence spectrometer. Five samples from the 2S

8

9

traverse of the Speculator Sheet and seven samples from the lW traverse

of the Wells Sill were collected for analysis. Their locations are

shown in Appendix 1. Five samples from the Oregon Dome were collected

along the Siamese Pond Trail. Major element abundances were analyzed on

fused glass disks using the technique described by Norrish and Hutton

(1969) and later modified by Harvey et al. (1973). Trace element

abundances were determined on pressed powder pellets using the matrix

correction methods of Reynolds (1963). Rock standards for the major

elements were USGS standards PCC-1, BCR-1, GSP-1, AVG-1, and G-2.

Precisions are better than 1% of the amount present for Si02, Al 2o3, + Ti02, Fe2o3, Cao, and K2o; about 3% for MgO and Na2o; and about - 0.01%

absolute for MnO and P2o5• GSP-1 was used as the standard for the trace element analyses (Rb and Sr). Precisions are approximately 1% for Rb

and Sr.

GENERAL GEOLOGY

The Adirondack Grenville Province of northern New York State

consists of intensely deformed and faulted granulite-facies metamorphic

rocks, which are cored by a 3100 km2 body of anorthosite (Marcy massif).

The study area in the southern Adirondacks is dominated by metasediments

and a number of smaller anorthositic bodies (Figure 1).

The second largest (greater than 300 km2 - Isachsen and Fisher,

1970) anorthositic massif of the province, the Oregon Dome, covers the

southwest corner of the Thirteenth Lake 15 1 Quadrangle and the northern

tips of the Lake Pleasant and Harrisburg 15 1 Quadrangles. The dome

consists of a large outer rim of anorthositic gabbro with an inner core

of anorthosite and intrudes biotite-hornblende granitic gneiss and

leucogranitic gneiss (Krieger, 1937).

To the north, the Thirteenth Lake Dome is cored by anorthosite-

norite and surrounded by a comagmatic charnockite (Letteney, 1969). To

the northwest of the Oregon Dome, in the north-central Indian Lake 15 1

Quadrangle, the Snowy Mountain Dome also has a core of anorthosite that

grades outward to charnockite (de Waard, 1969).

The two anorthositic sills of the study intrude the syenite gneiss

of the Rooster Hill/Little Moose Mountain Formations in the northern

half of the Lake Pleasant 15 1 Quadrangle (Figure 2; sample locations are

in Appendix 1). The Oregon Dome also crops out in the extreme

northeastern corner of this quadrangle. Mclelland (in preparation;

Figure 2) has shown that the dome intrudes the Lake Durant Formation

within the quadrangle. As this formation stratigraphically underlies

10

11

the Rooster Hill/Little Moose Mountain Formations, the Oregon Dome also

stratigraphically underlies the two sills.

The Speculator Sheet crops out five kilometers south of Speculator

in the center of the Glens Falls syncline, which plunges to the east-

southeast (Mclelland and Isachsen, 1980). The sheet intruded the

country rock before the formation of the syncline; therefore, the strike

of the sheet parallels the trace of the fold and its dip is 10-15°

toward the axis of the fold. The structural relationships concerning

the southern limb of the fold are not completely clear. The whole of

the Speculator Sheet dips to the south; it may be that the rest of the

sheet is faulted away (Mclelland, personal colTU'llunication) but further

work will be needed to substantiate this. The exposed thickness of the

sheet is approximately 500 feet.

Miller (1916) produced a detailed report on the geology of the Lake

Pleasant Quadrangle. He described the Speculator Sheet as "a basic

phase of syenite, usually non-quartzose and of dioritic or gabbroic

composition." He did state that "the rock looks much like certain

garnet-bearing phases of anorthosite described by both Cushing and

Kemp." Bartholom~ (1956) likewise mapped the quadrangle as part of his

study of Hamilton County, New York. He mapped the Speculator Sheet as

gabbroic anorthosite and anorthosite intrusive into syenite gneiss.

The sheet consists of gabbroic anorthosite and anorthosite that

appear grey to white in outcrop and weather buff to white. The mafic

minerals define a slight to strong lineation. The rock becomes more

mafic at the stratigraphic base; where highly garnetiferous, the outcrop

12

is colored red. In the extreme eastern part, the country rock is cut by

several thin mafic dikes. Metasedimentary inclusions of quartzite and

fine-grained well-foliated gneiss have been observed by others

(Bartholome, 1956); intrusions of gabbroic anorthosite occur in the

underlying syenite gneiss.

The Wells Sill intrudes along the southern limb of the Glens Falls

syncline, close to the contact of the Rooster Hill/little Moose Mountain

Formations with the subjacent Tomany Mountain/Blue Mountain Lake

Formations. It extends east-southeastward to Tenantville, a total

distance of about 22 kilometers (Goldberg, 1975; and Mclelland, personal

conmunication), striking N59W and dipping to the northeast at 27-34°.

The section of the sill studied in detail is fault-bounded on the west

and east and crops out on and in the vicinity of the Silver Bells ski

hill to the southeast of the town of Wells. Bartholome (1956) mapped

the sill as gabbroic anorthosite intruding syenite gneiss. It intrudes

the Rooster Hill/Little Moose Mountain Formations, which on the north

side of the sill is a locally-charnockitic syenite gneiss, and on the

south side is charnockite (Mclelland, personal communication).

The sill is composed of three different facies: a gabbro, an

anorthositic gabbro, and a fine-grained gabbroic anorthosite named the

Tenantville facies by Mclelland (personal communication) for it crops

out extensively in the Tenantville area (Harrisburg 15' Quadrangle).

The two. leucocratic facies of the sill have a stratigraphic thickness of

about 300 feet; including the gabbroic facies, the sill has a total

thickness of about 500 feet.

13

The gabbro is the most extensive lateral unit and is medium-grained

and equigranular with diffuse modal banding in outcrop due to higher

concentrations of plagioclase. It is dark to black when fresh and

generally weathers a deep rust or black. Its abrupt contact with the

intrusive Tenantville facies is well exposed at the top of the ski hill

and also at the northwest base of the hill, where the contact has been

emphasized by weathering.

Anorthositic gabbro covers an area of about one square kilometer on

the ski hill and forms the core of the sill. It appears grey to white

in outcrop and weathers buff to white. The mafic minerals define a

distinct lineation. Pyroxene rimmed with hornblende and the occasional

rimmed garnet are most striking.

The Tenantville facies surrounds and is gradational into the

anorthositic gabbro. It is intrusive into the gabbro facies and

contains many inclusions of the gabbro. The contacts between the gabbro

and the Tenantville facies are quite sharp and well-defined; the gabbro

inclusions show no signs of assimilation by the anorthositic gabbro. At

the top of the ski hill, the Tenantville facies has a number of

metasedimentary inclusions of two different types: well-assimilated

amphibolite and a few carbonate rocks. The Wells sill is cut by a few

pegmatitic quartzofeldspathic veins, one of which is best exposed in a

stream bed on the north side of the hill. In outcrop, the Tenantville

facies is white and fine-grained. It weathers white and has no

foliation. The large spongy garnets are distinctive in outcrop.

The syenite gneiss of the Rooster Hill/Little Moose Mountain

14

Formations is white with rust-colored staining in outcrop. It is

medium-grained, with large microperthitic augen imparting a

conglomerate-like texture to the rock. The mafic minerals define a

strong lineation. (See Mclelland and Isachsen, 1980.)

PETROGRAPHY

The Speculator Sheet

The well-lineated gabbroic anorthosite of the sheet contains 50 to

99% plagioclase, a to 23% hornblende, a to 24% orthopyroxene, 0 to 24% clinopyroxene, a to 16% garnet, 0 to 9% opaque minerals, a to 3% biotite, and traces of apatite, zircon, and hematite. The modal

plagioclase content of the samples collected along the 2S, 3S, and 4S

traverses (see Appendix 1 for locations) are plotted against vertical

distance from the base of the sill in Figure 3. This section shows the

typical scatter observed in all the traverses but illustrates the

increasing leucocratic nature towards the stratigraphic top of the sill

that is typical of the body.

The equant plagioclase crystals of the gabbroic anorthosite are

generally 2 mm long, and display albite and pericline twinning, as do

all the plagioclase crystals from the anorthositic bodies studied. Some

of these crystals form mosaic aggregates that are pseudomorphs after

originally euhedral plagioclase 10 - 15 mm long and 2 - 5 mm thick. The

centers of these aggregates rarely contain a relict core crystal of

plagioclase (3-4 mm). The largest of these relicts exhibit Spry•s

(1969) mortar texture with fine (less than 0.5 mm) recrystallization

around the edges. Antiperthitic blebs are rare and minute (less than 1

micron); randomly oriented opaque needles are common. Patchy alteration

to sericite or muscovite is common.

The major mafic mineral is pyroxene. Clinopyroxene is usually

dominant over orthopyroxene though abundances of the two vary

15

16

FIGURE 3

Modal Plagioclase for Speculator Sheet Traverses

2S, 3S, and 4S presented as a function of

stratigraphic feet above the lower contact.

500 +-(.) c +-c: 400 0 (.)

'-CD 300 ~ 0

~200 0 ..a c +- 100 CD CD

'+-

17

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1· •

35• \ "· .----------- ./· ·:--.. ,/ -;::•2S \ Vs " o.____.~~_._~'~_._~_.__..__..~

20 40 60 80 100 modal 0/o plagioclase

18

substantially. Pyroxene occurs in subequant to elongate crystals, 0.5 -

2 rrm in size with a few relict orthopyroxene phenocrysts reaching up to

8 rrm in length. Clinopyroxene exhibits occasional orthopyroxene

exsolution along the (100) plane, especially in the larger grains

(greater than 1 mm). Rare exsolution of opaque minerals occurs along a

plane which makes a small angle to (100). Orthopyroxene shows a fine

twinning which has, in some cases, initiated alteration to amphibole

along the twin plane. The pyroxene crystals are sometimes altered to

hornblende along crystal boundaries or in patches in the interior of the

crystal. Some have reaction rims of hornblende or are altered to

chlorite •

. The hornblende occurs in elongate crystals generally less than 0.5

mm in length; a very few larger crystals extend up to 2 rrm.

Biotite occurs in both small (less than 0.5 rrm) crystals and larger

elongate crystals up to 2 rrm by 1 mm in size. Biotite and hornblende

define the foliation. It alters to a chlorite or to an unidentified

fibrous material. Locally, small (less than 0.5 mm), lath-shaped

titanite (retrograde?) grows along its cleavages.

Zircon and apatite are ubiquitous accessories. Apatite is found in

subhedral crystals, sometimes growing to large sizes (6 mm by 1.5 mm).

Hematite occurs as staining along crystal boundaries and fractures, and

in aggregates of opaque minerals. One large (greater than 1 rrm)

euhedral crystal of titanite was observed in the anorthosite of the

Speculator Sheet. The opaque phases are ilmenite with very rare

magnetite and pyrite. They usually occur in small crystals (less than

19

0.5 nm) or occasionally form irregular aggregates less than 1.5 mm in

size. These opaque phases are commonly rimmed by metamorphic titanite

and only rarely by garnet. They are often associated closely with other

mafic phases, at times surrounding and interpenetrating them.

Deep pink garnet occurs in porphyroblasts of varying shapes up to 12

mm in diameter, generally free from inclusions but cut by parallel

fractures. The distribution of the garnet varies widely; there are

substantial fluctuations in garnet content with the more mafic rocks

being those that are more garnetiferous. As compared to those in the

gabbroic veins, the garnets in the anorthositic gabbro facies display a

striking dactylic intergrowth that surrounds the porphyroblasts in a

regular and undeformed reaction rim that grows larger as the central

garnets decrease in size. In some rocks, the central garnet has

completely disappeared. The rim consists of thin (20 micron) stringers

of orthopyroxene and rarely hornblende (Plate 1, a and b) which radiate

outward approximately normal to the face of the garnet, penetrating

surrounding plagioclase or separated by equally thin stringers of calcic

plagioclase (An70_90). Where large plagioclase grains occur adjacent to

the garnet they display a sharply defined calcic rim 20-30 microns

thick. Opaque oxides often occur around and between the orthopyroxene

stringers (Plate 1, c).

The mafic dikes that cut the eastern country rock contain 28 to 70%

hornblende, 26 to 42% plagioclase, 1 to 17% orthopyroxene, 0 to 8%

garnet, 0 to 4% clinopyroxene, and traces of biotite, opaque minerals,

apatite and hematite (see Appendix 1; 4S2, 4S3, 4S5). The gabbros of

20

PLATE 1

a) Speculator Sheet garnet with an

orthopyroxene - plagioclase corona

b) Speculator Sheet garnet corona with hornblende

c) Speculator Sheet garnet corona with opaques

d) Tenantville spongy garnet

22

the basal portion of the sheet differ from these mafic dikes because

they contain pyroxene as the dominant mafic phase and they are more

leucocratic and coarser grained (i.e. Appendix 1: 4S8, 4S9, 4Sl0).

The Wells Si 11

a) Gabbro

The medium-grained, gneissic gabbro contains 39 to 49% plagioclase,

9 to 38% hornblende, 2 to 19% orthopyroxene, 1 to 15% clinopyroxene, 0

to 11% biotite (with 1 to 3% conman), 0 to 15% quartz, 0 to 9% opaque

minerals, 0 to 3% garnet, and traces of apatite, hematite and zircon.

The quartz usually occurs in veins; hematite staining is often present

along grain boundaries and fractures.

Plagioclase occurs in small (less than 0.5 nm) granulated crystals

and exhibits pericline and albite twinning. Antiperthitic blebs are

rare. The grains are equant with little indication of preferred

orientation.

The main mafic mineral is usually hornblende which occurs in

elongate crystals up to 2 mm long. These segregate into gneissic bands

of subparallel crystals with large interstitial opaques.

Orthopyroxene and clinopyroxene occur in 0.5 to 1.0 nm subequant to

elongate crystals with a few larger relict phenocrysts (about 2 mm).

The large crystals of both pyroxenes exhibit exsolution similar to the

pyroxenes of the Speculator Sheet. The pyroxenes conmonly show

alteration to amphibole along grain boundaries and in patches within the

pyroxene.

The biotite crystals are rarely longer than 1 nm and are sometimes

23

bent. A few are broken. The opaque minerals are primarily ilmenite

with little magnetite and rare pyrite. These may form irregular

aggregates up to 2 mm long. Hematite is often present in these

aggregates. Garnet, when it does occur, is very pale pink to colorless,

and either forms small euhedral crystals 0.5 mm in size, or it forms in

contact with the opaque aggregates. It has few inclusions, which are

most often apatite or quartz.

b) Anorthositic Gabbro

The anorthositic gabbro contains 50 to 81% plagioclase, 3 to 39%

hornblende, 1 to 16% clinopyroxene, 0 to 10% orthopyroxene, 0 to 8%

biotite, 0 to 24% quartz, 0.2 to 6% opaque minerals, 0 to 2% apatite and

traces of zircon and hematite. Quartz occurs in veins. The

anorthositic gabbro is compositionally and texturally similar to the

anorthositic gabbro of the Speculator Sheet, except that: (1) Garnet is

much less common. (2) Hornblende is sometimes the dominant mafic phase.

(3) The plagioclase content of the rock does not change in any

systematic manner; nor is the base of the unit more gabbroic. (4) The

pyroxenes tend to form clots which are surrounded by hornblende.

c) The Tenantville Facies

The fine-grained Tenantville facies contains 72 to 85% plagioclase,

0 to 18% hornblende, 0 to 6% clinopyroxene, 0 to 6% orthopyroxene, 0 to

3% biotite, 0 to 3% garnet, 0 to 18% quartz, 0 to 3% opaque minerals,

and traces of apatite, zircon and hematite. The quartz occurs mainly in

veins.

The Tenantville facies of the Wells Sill is generally more

24

leucocratic and finer grained than the anorthositic gabbro, with

individual crystals rarely exceeding 0.5 ITITI in size and always less than

2 rrm.

Plagioclase occurs as small (less than 0.5 ITITI) rounded grains; some

aggregates form pseudomorphs of original plagioclase phenocrysts 10-15

mm in size. Rarely, a large (5-6 ITITI) relict plagioclase will remain

intact in the center of the aggregates. The plagioclase exhibits both

albite and pericline twinning, has rare antiperthitic blebs and is more

sericitized in this facies than in any other.

Hornblende is the most common mafic mineral, and occurs in small

elongate crystals or blocky crystals 1.5 mm in size.

Pyroxene is a minor phase and may be altogether absent (Appendix 1;

5W7). Where it does occur, it is most often clinopyroxene with minor

orthopyroxene or has been completely altered to chlorite. It forms

small (less than 0.5 111Tl) rounded to irregular crystals. It exhibits no

exsolution; alteration to hornblende is rare.

Biotite occurs in small (less than 1.5 ITITI) elongate crystals. The

growth of titanite along its cleavages is characteristic. Some biotite

crystals (less than 1.0 nm in size) are riddled with opaque inclusions;

these are most probably a secondary alteration of the opaque phase.

Light pink garnets form in small (less than 0.5 mm) euhedral

crystals which have a tendency to form aggregates of what can be termed

a 11 spongy 11 garnet up to 4 l11TI in size. These spongy garnets contain

inclusions of plagioclase, pyroxene, hornblende, biotite, and opaque

minerals and have no reaction rims (Plate l, d).

25

Apatite and zircon are corrmon accessories; hematite is rare. Opaque

minerals are ilmenite with rare magnetite and pyrite, and form

aggregates generally less than 2 nm long which contain frequent

inclusions of titanite.

The Oregon Dome

The strongly lineated anorthositic gabbro contains 60 to 83%

plagioclase, 3 to 18% clinopyroxene, 0.2 to 7% hornblende, 1 to 7%

orthopyroxene, 0.2 to 7% garnet, 0.2 to 6% opaque minerals, 0 to 2%

biotite, and traces of apatite, zircon and hematite.

This facies of the dome is similar to the anorthositic gabbro and

gabbroic anorthosite of the sills except for grain size, the nature of

the garnets and the opaques. In the Oregon Dome, grain size is larger

overall, with single crystals of minerals commonly achieving lengths of

2-4 nm and larger. The garnets are pink and have no coronas of

orthopyroxene and plagioclase; they are anhedral with many inclusions

and are sometimes closely associated with mafic phases (opaques and

hornblende). There is a consistent thin rim of plagioclase between the

mafic phase and the garnet. Magnetite is also more conman in the dome

than in the sills.

The Country Rock

The finely lineated syenite gneiss of the Rooster Hill/Little Moose

Mountain Formations in contact with the Speculator Sheet contains 60 to

70% patch perthite, 10 to 15% plagioclase, 10 to 15% hornblende, 5 to

10% quartz, 1 to 3% opaque minerals, 1 to 3% clinopyroxene, 1 to 2%

biotite, and traces of orthopyroxene, zircon and apatite.

26

The coarsely-exsolved patch perthite occurs in small to medium sized

(less than 2 mm), equant to irregular crystals. Hematite staining is

heavy along crystal and exsolution boundaries. Elongated and flattened

quartz crystals occur as large 4.0 rrm laths. Most hornblende occurs in

small (0.5 mm) laths though there are a few 2 rrm irregular fractured

crystals. The plagioclase occurs as small (less than 0.5 rrm) rounded

crystals. Biotite occurs as small (less than 0.5 rrm) laths that are

altered to a red, amorphous mineral. Opaque minerals are small equant

crystals forming irregular aggregates up to 1.5 rrm long in very mafic

areas. Equant to rectangular clinopyroxene crystals are usually less

than 0.5 rrm and always less than 2 rrm in size. Patchy alteration of the

pyroxene to hornblende is corrmon. Orthopyroxene is extremely rare,

occurring as small, isolated, equant to rectangular crystals. Euhedral

zircons and euhedral apatite laths are small (less than 0.5 rrm).

Hematite staining is pervasive, especially on perthite crystals and

biotites and other mafic phases.

MINERAL CHEMISTRY

Mineral analyses were obtained from 17 thick sections of the

Speculator Sheet, two thick sections of the mafic veins that cut the

eastern end of the Sheet (4S2, 4S5), four sections each of the three

facies of the Wells Sill, and six sections of the anorthositic gabbro

facies of the Oregon Dome. The number of analyses of each mineral

species in each slide ranged from one to ten with an average of five,

depending on the abundance of the mineral. A second set of amphibole

and biotite analyses was taken to determine fluorine and chlorine

contents. Analyses are presented in Appendix 3.

Plagioclase

Plagioclase compositions for the two sills and the Oregon Dome are

presented in Figure 4. Within the Wells Sill, the plagioclase becomes

more calcic from the gabbro (An42 _47 ) to the anorthositic gabbro

(An 50_54 ) to the Tenantville facies (An52_56 ). Plagioclase composition

for the Speculator Sheet ranges from An44 to An87 • The three samples

with high anorthite content (An70_87 ) are from rocks which have a large

volume of rimmed garnets. The composition of plagioclase in garnet

coronas ranges from An74 to An87 , except for one sodic analysis

resulting from zoning in large plagioclases adjacent to garnet with no

or very minute rims. Data on the Oregon Oome 1 s anorthositic gabbro

indicates a composition ranging from An47 to An53 • Orthoclase content

in all the plagioclases is less than 2%.

Pyroxene

Pyroxene compositions are presented in Figure 5. The tie lines

27

28

FIGURE 4

Plagioclase Compositions for the Oregon Dome,

Wells Sill, and Speculator Sheet.

Oregon Dome: small circles - anorthositic gabbro

Wells Sill: squares - gabbro facies

triangles - anorthositic gabbro

large circles - Tenantville facies

Speculator Sheet: filled triangles - gabbroic anorthosite

x - from garnet coronas

squares - gabbroic veins

U> c: 0 ·-+-U> 0 a. G> E E 0 0 (.) c

c Cl) 0 U> O> 0 G> .... 0 0 0 C'I 0 -a..

... 0

... 0

29

c

30

FIGURE 5

Pyroxene Compositions for the Oregon Dome,

Wells Sill and Speculator Sheet.

Symbols are as in Figure 4.

en c 0 ..... en 0 a. E 0 (.)

Cl> c Cl> )( 0 ... ~

Cl..

Cl> E 0 0 c: 0 C'I Q) ... 0

"'O :c

en Cl)

Q)

3:

0 v

31

-Q) Cl> .c en ... 0 -0 :J (.) Cl> 0.. en

0 v

32

between coexisting clinopyroxene and orthopyroxene are generally

subparallel, both within and between the three bodies. The

clinopyroxenes in the Speculator Sheet and in the Tenantville and

anorthositic gabbro facies of the Wells Sill range in composition from

Wo46Fs12En42 to Wo48Fs19En33 ; however, those of the Oregon Dome and the

gabbro facies of the Wells Sill tend to be more iron-rich, ·with

compositions in the Oregon Dome ranging from Wo44Fs19En 37 to

Wo46Fs22 En32 and in the gabbro facies from Wo46 Fs17En37 to Wo46Fs21 En33 •

The same relationship is seen in the orthopyroxenes, with the Speculator

Sheet and the Tenantville and anorthositic gabbro facies of the Wells

sill ranging from wo1Fs34En65 to wo1Fs47En52 , whereas the Wells gabbro

and Oregon Dome range from wo1Fs46En53 to Wo1Fs51 En48 • Na, Ti, and Mn

are present only in trace amounts (less than 0.05 moles of oxide) in

both pyroxenes. The clinopyroxenes of the Speculator Sheet contain

approximately 11% of calcium Tschermak 1 s molecule while those of the

Wells Sill contain only 8%. The clinopyroxenes of the Oregon Dome

contain 12% of the molecule. The orthopyroxenes of the Speculator Sheet

contain 4% of magnesium Tschermak 1 s molecule as do those of the Wells

Sill, whereas the Oregon Dome contains slightly over 5%.

Amphibole and Biotite

Compositional data for the amphiboles in the three bodies are given

in Figure 6 according to Leake 1 s (1978) classification for calcic

amphiboles. Amphiboles for all three bodies are ferroan pargasitic

hornblende. A few amphiboles, usually from rocks affected by garnet

corona - producing reactions, are ferroan pargasite. There is a strong

33

FIGURE 6

Amphibole Compositions for the Oregon Dome,

Wells Sill and Speculator Sheet

according to Leake (1978).

Symbols are as in Figure 4.

Cal

cic

Am

phib

ole

Com

posi

tions

Ca+

Na

> 1

.34

; N

a<

0.6

7;

Na+

K >

0.5

0;

Ti<

0.5

0

Si

7.5

7.0

6.5

6.0

- Q) LL + 1.0

~ 0

.5

- ....... Ct ~

0

i >--- - I

I I

Ede

nitic

E

deni

te

Hor

nble

nde

Ferr

o-Fe

rro

-E

deni

te

Ede

nitic

H

ornb

lend

e

I

I

Par

gasi

tic

Par

gasi

te

Hor

nble

nde •••

~~i ..

.... Fe

rroan

~· ~

Parg

a site

6

.oce

. Fe

rroa

n 1

Par

gasi

tic

Hor

nble

nde

Ferr

o-Fe

rro

-P

arga

site

P

arga

sitic

H

ornb

lend

e

I I

I

I

w

-+::>

35

negative correlation between the amounts of iron and octahedral aluminum

as there is a positive correlation between titanium and tetrahedral

aluminum, as is illustrated in Figure 7. This is a reflection of

titanium and tschermakitic substitutions: 0.48 Al atoms/formula unit

were substituted in the Y site by tschermakitic substitution, and 0.11

and 0.17 Ti atoms/formula unit were substituted by Ti+4 + 2AlIV = 2Si + Mg 2+ and Ti+4 + 2Na = Mg + 2Ca, respectively. Analyses for fluorine (see Appendix 3) indicate as much as a 7.5% (= F/(F +OH+ Cl))

substitution of fluorine for (OH) in the Speculator Sheet while the

greatest substitution in Wells is 4.5%.

Biotite compositional data for the two sills and the Oregon Dome are

illustrated in Figure 8. The biotites in all three bodies show only a

small range in composition. Titanium substitutions are reflected by a

correlation between the amounts of Ti and octahedral vacancies as in the

upper plot of Figure 9. In the lower plot of Figure 9, the x-axis

corrects for the Al tschermakitic substitution and the Y axis corrects

for the Ti vacancy substitution. The data should fall along the solid

line which indicates Ti tschermakitic substitution; however, they fall

along a parallel line because 0.09 atoms/formula unit of Ti were

substituted by a third substitution. These substitutions are: 0.12 Al

atoms/formula unit were substituted in the Y site by tschermakitic

substitution, and 0.175, 0.34 and 0.095 Ti atoms/formula unit were

substituted by Ti+4 + 2AlIV = 2Si + Mg+2, Ti+4 +a vacancy = 2Mg+2 and Ti02 = Mg(OH) 2, respectively. Analyses for fluorine indicate as much as 8.25% (= F/(F +OH+ Cl)) of fluorine has been substituted for (OH) in

36

FIGURE 7

Amphibole compositional trends for the

Speculator Sheet:

tetrahedral aluminum vs. titanium and

Fe/(Fe+Mg) vs. octahedral aluminum

(atoms/formula unit).

37

·\ • c:::t" c:::t" r lQ • • • •

• o_ ' 0 -r ::::> IQ ::::> • '+-• • '+- ........... ........... \ 0 0 -• (!) - {. (!) ~ • \ ~ ·- ~ •• r- -•

38

FIGURE 8

Biotite Compositions for the Oregon Dome,

Wells Sill and Speculator Sheet

projected on the biotite quadrangle.

Symbols as in Figure 4.

39

eastonite siderophyl lite 3.0 ------.-,---~,~-----.-, ---......

"' -c -:l 0 :l E ~

0 .... ........ 2.5 -"' E 0 -0

>

40

FIGURE 9

Biotite compositional trends for the Speculator Shee

titanium vs. octahedral vacancies, and

titanium minus octahedral vacancies vs.

tetrahedral minus octahedral aluminum

(atoms/formula unit). Solid line indicates

trend of Ti tschermakitic substitution.

-::J '+--............

0.7

.So.5

I-

41

0.3 ________ ......_ __ ,_____ 0.2 0.3 0.4 0.5

:J0.4 '+--............

0 -r;i 0.2 D I

o!lI (a/fu)

I- 0.0 ____ ......._ __ ....__ __ _..___ 2.0 2.2 2.4 2.6

Alnz::.-Alm (a/fu)

42

the Speculator Sheet biotites, whereas the maximum substitution in the

Wells Sill is 4.0%.

Garnet

The garnet compositions are summarized in Figure 10. The Mn and Ca

molecular contents for the garnets are uniform at about Sp2_3 and

Gr17_21 respectively; however, the amounts of Fe and Mg vary

considerably. Garnets which do not have coronas have Py13Alm65 to

Py19Alm60 (i.e. the Wells Sill and Oregon Dome plots, figure 10).

Those with reaction rims range from Py19Alm61 to Py33Alm45 (see

Speculator Sheet plots, Figure 10).

43

FIGURE 10

Garnet Compositions for the Oregon Dome,

Wells Sill and Speculator Sheet.

Symbols as in Figure 4.

~--

........ -

.--v....-~~~-~-'"~·,--

~ A

lm

+ Sp

10

Gar

net

Com

posi

tions

Py

Ore

gon

Dom

e 4o)

0

21 f

20

30 A

lm

10

20

+ Sp

Py

Py

Wel

ls S

ill

o} S

pecu

lato

r Sh

eet

.;::. .;::.

4

.A

AA

.a

.. 21

t ..

a§>

a

30

Alm

10

20

30

40

50

Gr

+ Sp

WHOLE ROCK CHEMISTRY

Chemical analyses and norms are presented in Appendix 2; average

compositions are presented in Table 1 for five analyses of the

Speculator Sheet gabbroic anorthosite, three analyses of the Wells

gabbro, one analysis of the Wells Tenantville facies, three analyses of

the Wells anorthositic gabbro and five analyses of the Oregon Dome

anorthositic gabbro.

The data from the Speculator Sheet and Wells Sill are presented in

Figure 11 as a function of stratigraphic distance above the lower

contact with the metasediments. Variation of major oxide data in the

Speculator Sheet is minor, and is a function of the mafic content of the

rock (see Figure 3); as the rock becomes more leucocratic towards the

top of the exposure, Si02, Al 2o3, Cao and Na2o increase while Fe203,

MnO, and MgO decrease.

Variation within the Wells Sill is slightly more complex. The mafic

zone on the upper and lower sides of the sill is defined by high MgO,

Fe203, MnO, Ti02, and P2o5 values and low Si02, Al 2o3,. Na2o, and K2o values; however, the point for the basal gabbro facies (100 feet, Figure

11) is high in silica because of a fine quartz veining in the basal

section of the sill. The three data points from the anorthositic gabbro

are rich in Si02, A1 2o3, Cao, Na2o, and K2o and poor in Fe2o3, MnO, MgO,

Ti02, and P2o5• They approximate the values for the Speculator Sheet

rocks. The one value for the Tenantville facies lies at a greater Si02 value and lower Fe2o3 and MgO values than the anorthositic gabbro, but the rest of the data are comparable to the anorthositic gabbro.

45

TABL

E l

AVER

AGE

CHEM

ICAL

COM

POSIT

IONS

SPEC

ULAT

OR

WELL

S SI

LL

0.0.

Pa

rent

Ma

gma

fade

s Ga

bAn

Gab

Tena

nt

AnGa

b An

Gab

* W

eight

%

Si02

52

.54

49,7

8 57

.43

53.4

9 49

.49

54.0

5 Ti

02

0.40

2,

03

0.61

0.

61

3.02

0.

53

Al20

3 24

.91

15 .0

1 22

,06

23.8

8 19

.34

25.4

4 Fe

203

4. 12

14

.24

3,89

4.

41

10.5

7 2.

35

MnO

0.06

0.

20

0,03

0.

03

0. 11

Mg

O 2 .

15

5.33

1.

38

l,74

2.73

1.

30

+::-

cao

10,8

5 9,

07

8.32

9,

61

9.71

10

.20

O'\

Na20

3.

72

2. 14

3.

06

4.20

2.

84

4.54

K2

0 0,

54

0.85

1.

36

1.27

0.

72

0.90

P2

05

0.08

0.

24

0.20

0.

14

0.39

0.

09

H20

0.53

0,

87

0,59

0.

81

0.89

-

i

Tota

l 99

.90

99.7

6 98

.93

l 00.

19

99.8

1 99

.58

ppm

Rb

3.6

12.2

18

. l

22.2

0.

7 Sr

52

0.5

258.

5 32

9.7

413.

8 64

3.2

* Bu

dd1 n

gton

197

2

-~-._,,-.,.,.._~ -

-~,,,.-,-

_,.,,._~-

'~<"--"--'"""'~:_-

____

",."-,_--,=-.,,_~_ •. ~

~-~---

-~----~

----=~-~

-,---,~~-~

47

FIGURE 11

Oxide weight% variation of the sills as a

function of stratigraphic feet above the lower

contact of the sill with the metasediments.

Symbols are as in Figure 4.

IC> N 0 0 "'o a. .

0

N 0 0 c ~ 0

0

N

0 N-~

0

C\I N 0 I- 0

0 ...

"' 0 z N

= 0 0 °' -(.) CD CD ,._

.s::; CJ)

0

"' 0 :l Cll

N 0 0 "'o a. ci

I _o_ ·~,

49

FIGURE 12

Harker Variation Diagrams for the major

elements (weight%) and Rb and Sr (ppm).

Symbols are as in Figure 4.

50

Harker Variation Diagrams

30 16 0 i. Al 20 3 Fe203 ~ 0 26 12

MtA 0 0 i. 0

22 0 0 8 0 i. & A i.

18 9 4 ~a 0

0 i. 14 0 0 0

i. coo 12 a r:j ~ ~{;. 0 MgO 10 A

0 a ~ 4 ·6) 0 0 6 ~ MAA 0

6 Na20 i. 0 4

CJ>(}J ~ 4 Q) Ti02 0 0 '8~ 2 0 2 0

K20* 2 0 0

ell~ 0 0 60

0 Rb 0 o0 i. Sr 600 40

0 0 i. 0 0 ~

400 6 AA ~A 20 A 0 0 o• 0 0

200 0 0 0 %i-M. 0 48 52 56 60 48 52 56 60

Si02 weight 0/o Si 02 weight 0/o

51

Figure 12 shows the same data plotted on Harker variation diagrams.

Although the scatter is large, several trends are evident. With

increasing silica, which is the result of an increase in plagioclase

content, Al 2o3, Cao, Na2o, and K2o increase, and Fe2o3, MgO, and Ti02 decrease. Once again, the values for the Tenantville facies and the

quartz-veined gabbro plot on the high silica side of the diagram, but

their oxide components are similar to the anorthositic gabbro and Wells

gabbro, respectively. The Wells gabbro has a trend that shows a much

smaller change of oxide content with Si02•

The Rb and Sr data plotted in Figure 12 show a tendency to increase

and decrease respectively with increasing Si02• Rb content, as well as

K2o content, is lower in the Oregon Dome and Speculator Sheet as

compared to the Wells Sill.

In most cases, the rocks from the Wells Sill span all the values

from the other two bodies with the gabbro samples more mafic than the

Oregon Dome compositions and the anorthositic gabbro samples being very

similar to rocks from the Speculator Sheet. Though all the rocks

studied are calcic (Peacock, 1931), the AFM ternary variation diagram

(Figure 13) indicates that the Speculator Sheet and the Wells

anorthositic gabbro and Tenantville facies have a differentiation trend

showing only moderate iron enrichment, whereas the Oregon Dome and the

Wells gabbro have a differentiation trend of more extreme iron

enrichment. Nockold's and Allen's calc-alkalic (1953) and alkalic

(1954) differentiation trends are provided for reference.

52

FIGURE 13

AFM Ternary Variation Diagram for the

Speculator Sheet, the Wells Sill, and

the Oregon Dome. A =Na2o + K20; F = Total Fe + MnO M = MgO (weight%). Symbols as in Figure 4;

filled small circle is the composition of the

hypothetical parent magma for the Adirondack

anorthosite (Buddington, 1974); solid line is

the average calc-alkalic differentiation trend

(Nockolds and Allen, 1953); dashed line is the

average alkalic differentiation trend

(Nockolds and Allen, 1954).

53

lJ....

METAMORPHIC PETROLOGY

Intensive Parameters

The intensive parameters for the metamorphism of the two sills and

the Oregon Dome are similar; significant variations were not found

between the three bodies.

Estimates of geothermometry in Table 2 indicate a maximum

metamorphic temperature of 650 - 700° C, which agrees well with the

feldspar and oxide geothermometry presented by Bohlen and Essene (1977),

whose four samples from this area yield temperatures of 690 - 700° C.

The results of the clinopyroxene - orthopyroxene geothermometry are high

because of one of two reasons. First, they could be a reflection of

igneous temperatures, with reequilibration between the two minerals upon

cooling ceasing effectively at temperatures below 850° C. Second, _they

could be a reflection of the fact that both orthopyroxene -

clinopyroxene geothermometers are designed for less Fe-rich pyroxenes at

much higher temperatures than the pyroxenes in these rocks. Bishop's

(1979) ilmenite - clinopyroxene geothermometer indicates a maximum

temperature of 600 - 700° C with a late reequilibration at 425 - 525° C,

which could have caused the late formation of titanite in and around the

ilmenite. This is consistent with the conclusions of Jaffe, Jaffe and

Ashwal (1977) who determined a period of late reequilibration at 500 -

700° C in the Marcy Quadrangle, based on low pigeonite exsolution in

host augite throughout the quadrangle.

The stability of plagioclase (Ghent, 1977) places a limiting maximum

metamorphic pressure on the sills of 13 kb. Thompson (1976b) indicates

54

55

TABLE 2

GEOTHERMOMETRY

Estimate Mineral Reference oc Pair 850t25 opx .. cpx Wood and Banno (1974) 855±25 opx .. cpx We 11 s ( 1977)

625±25 cpx-hbd Kretz and Jen (1978) 655±100 gar-bio Thompson (1976b} 693±100 gar-bio Goldman and Albee (1977} 696±100 gar-bio Ferry and Spear (1978) 683±50 cpx-gar Raheirn and Green (1974) 654±100 cpx-gar Dahl (1980) 650±50 cpx .. ilm Bishop (1979)

475±50 cpx-ilm Bishop (1979)

56

that the country rock assemblage Mg-rich biotite and sillimanite is

stable at minimum pressures of 5.8 to 6.4 kb at the temperatures of

metamorphism of the area. The presence of sillimanite in metasediments

throughout the Adirondacks in itself gives an upper limiting pressure of

6, 8, and 10 kb at temperatures of 600, 700, and 800° C respectively

(Holdaway, 1971 and Richardson et al., 1968). Determinations of

pressure in other areas of the Adirondacks have yielded pressure

estimates from 7 to 10 kb (Jaffe, Jaffe and Ashwal, 1977; De Witt and

Essene, 1975). By linear extrapolation of experimental data on garnet

assemblages to crustal conditions (Kushiro and Yoder, 1966; Green and

Ringwood, 1967 and 1972; Raheim and Green, 1974), Whitney (1978)

determined that in silica-saturated rocks in the Adirondacks, garnet of

compositions reported for the Adirondack Highlands (Mclelland and

Whitney 1977; (FeO/MgO)gar/(FeO/MgO)cpx = 6.7 - 8.2) forms at temperatures of 715° ! 25° C and pressures of 7.1!1.4 kb. Martignole's and Schrijver's (1971) analysis of garnet-quartz

symp1ectites in rocks around the Adirondack and Morin anorthosite masses

suggested a load pressure of the order of 8 - 10 kb.

It is concluded that the sills were metamorphosed at a pressure of

about 8 kb, which corresponds to a depth of roughly 25 km.

Metamorphic oxygen fugacity may be estimated in two ways. The

analysis of three magnetite-ilmenite pairs yields an estimate of log f 0 2

= -14 to -17.5 for a temperature of 650 to 700° C from Buddington and Lindsley (1964), though there may be a problem because of the

(retrograde?) formation of titanite, as was noted previously. Wanes

57

(1981) has reassessed data for the assemblage titanite - magnetite -

quartz, all three of which are minor constituents of the sills and the

Oregon Dome. His data yield an estimate of a minimum log f 0 of -16.8 2

to -18.5 for a temperature of 650 - 700° C. A log f 0 greater than the 2

Wanes (1981) estimate and less than the magnetite-ilmenite estimate is

indicated for the metamorphism of these rocks.

The biotite fluorine data was used to estimate metamorphic fluid

composition, using the experimental data of Munoz and Ludington (1974,

Figure 5). A log fH 0/fHF = 3.5 - 5 is indicated for 650 - 700° C and a 2 F/(F +OH+ Cl) = 5 - 10%~ Troll and Gilbert (1972) state that

phlogopite extracts more fluorine from a vapor under comparable

conditions and at a faster rate than does tremolite. The hornblende

data generally support these conclusions, with maximum substitution

approximately 1% less than that in biotite.

Metamorphic Assemblages

The stability of garnet within the Adirondack granulite facies has

been a matter of debate for some time (Buddington 1963 and 1966; de

Waard, 1965 and 1967b; Whitney, 1978; Whitney and Mclelland, 1973;

Mclelland and Whitney, 1977). Changes in rock composition, temperature,

and load and water pressure have been cited to explain the occurrence

and non-occurrence of garnet. A study of the garnet petrology in the

Wells and Speculator Sheets provides some valuable data for resolving

the controversy because of the lack of a systematic temperature or

pressure gradient over the area, and because both unrimmed and rimmed

garnets occur in the same rock type.

58

The five mineral assemblages found in the Wells and Speculator Sills

(in addition to quartz, plagioclase, ilmenite and magnetite) are:

(1) Hornblende-Garnet-(Biotite) H-G-(B)

(2) Orthopyroxene-Clinopyroxene-(Garnet) 0-C-(G)

(3) Hornblende-Clinopyroxene-Orthopyroxene-Garnet-(Biotite) H-C-0-

G-(B)

(4) Hornblende-Orthopyroxene-Clinopyroxene-(Garnet) H-0-C-(G)

(5) Hornblende-Biotite-Orthopyroxene-Clinopyroxene-(Garnet) H-B-0-

C-(G)

Assemblage (3) H-C-0-G-(B) occurs in both the gabbro and Tenantville

facies of the Wells Sill (2Wl, 2W6, 5W4). Microprobe data indicate that

the ratio Fe/(Fe+Mg) = XFe decreases in the order Gar-Hbd-Opx-Cpx for

assemblages (1), (2), and (3). The mineral chemistry, textural evidence

of the rimming of pyroxene by hornblende, patchy interior alteration of

pyroxene to hornblende, and the garnet coronas suggest the reaction

hbd + qtz = opx + cpx + gar + an + ab + H2o can be used to describe the stability of hornblende. Assemblage (1) H-

G-(B) is found only in the Tenantville facies of the Wells Sill (1W15,

5W7). The assemblage (2) 0-C-(G) occurs only in pockets of anorthosite

in the Speculator Sheet (2S20) where the garnet is isolated from the

system by a corona.

Assemblage (5) H-B-0-C-(G) is found in the anorthositic gabbro of

both sills and also in the Wells Sill gabbro (i.e. 1S30, 2Sl7, 1W2,

1W9). It is the most common assemblage in the sills. Microprobe data

indicate that XFe decreases in the order Hbd-Opx-Bio-Cpx. The mineral

59

FIGURE 14

ACFM Diagram with mineral compositions projected

from plagioclase, quartz and water onto the

Cao - FeO - MgO plane. The three phase

triangle sweeps to the left with

metamorphism, forming the garnet coronas.

0 c (.)

60

61

chemistry and textural evidence of the close association of biotite with

other mafic phases suggests the following reaction controls the

coexistence of hornblende and biotite,

hbd + bio + qtz = an + ab + opx + cpx + or + H2o where anorthite, albite and orthoclase form a plagioclase solid

solution. Assemblage (4) H-0-C occurs in the anorthositic gabbro of

both sills (3S9, 4S8, 1Wl2).

Geothermometry indicates no regular change of temperature for the

different assemblages; there is a non-systematic variation of 65° that

is within a reasonable uncertainty for the geothermometer and is

therefore not considered to be significant. As there is no evidence to

substantiate a temperature, pressure or f 0 gradient (though changes 2

that are within the errors of the estimations used here may occur) the

factor controlling the assemblage is believed to be the fugacity of

water. There is a significant variation from assemblages with two

hydrous mineral phases to totally anhydrous assemblages. This is

consistent with Buddington (1963) who determined that the variety of

mineral assemblages in the orthogneisses between any two isograds is

best interpreted in terms of development under a wide range of PH 0• 2 The metamorphism of these rocks, either on cooling or during the

Grenville Orogeny, not only involved recrystallization of mineral

phenocrysts such as plagioclase and pyroxene and the development of a

foliation but also involved an addition of mobile components. Late

stage magmatic water or fluid from the surrounding country rock affected

the original igneous mineral assemblages. Hornblende was produced by

62

hydration of original pyroxene by reactions similar to opx + cpx + an +

ab + H2o = hbd + qtz. Biotite resulted from a mobilization of potassium from the potassic content of the original plagioclase and possibly from

the surrounding quartz syenite. The biotite present in these rocks

probably accounts for the low potassic content of this plagioclase

(Orl.O-l.S) as compared to Ashwal~s (1978) primary plagioclase

composition of Or6, as well as accounting for the relative rarity of

antiperthite. Both biotite and hornblende have been stabilized at

granulite conditions by the substitution of fluorine (Holloway, 1977).

The calcium content of the plagioclase is affected by the formation

of garnet (becomes more albitic), or subsequent garnet coronas (becomes

less albitic). With increasing grade of metamorphism, hydrous phases

react to form garnet and pyroxene and to release water. At maximum

metamorphic conditions garnet and clinopyroxene react to form the garnet

coronas, forming the anhydrous mineral assemblage 0-C-(G).

Ashwal et al. (1981) suggest that the cooling of the Adirondack

Highlands continued 200-300 my after the magmatic event, and may have

been associated with relatively rapid uplift after isobaric cooling from

magmatic temperatures to ambient lower crustal temperatures (about 800°

C), producing the metamorphic mineral assemblages. The present data are

not inconsistent with such a history. The assemblages could be the

result of two different thermal histories: (1) Cooling from magmatic

temperatures produces hydrous mineral assemblages. A subsequent rise in

temperature accompanied by decreasing f H 0 produces garnet and 2 eventually garnet coronas. (2) Cooling from magmatic temperatures to

63

Grenville temperatures produces hydrous mineral assemblages. Uplift

then causes dehydration, producing garnet and finally the garnet

coronas.

Bartholome (1956) and also Buddington (1966), with particular

reference to the Speculator Sheet, believe that the rims formed

contemporaneously with the garnet, drawing an analogy between the

orthopyroxene - plagioclase rims in this gabbroic anorthosite with the

hornblende rims of garnets in the amphibolite of the Gore Mountain

Barton garnet mine. The observations of this study are inconsistent

with this hypothesis. Garnets are found in every stage from totally

unrimmed to spherical knots of myrmekitic orthopyroxene and plagioclase

grains suggesting complete replacement of the garnet. In contrast to

the rimmed garnets of the anorthositic gabbro, those of a derivative of

the anorthositic gabbro, the Tenantville facies, have no rims. The

composition of the garnet is the critical factor determining if it

reacts: only those garnets that have a pyrope content greater than 18

or 19% show development of coronas; calcium content is not critical.

This is illustrated by the Na-absent ACFM diagram of Figure 14, where

the mineral compositions are projected from plagioclase onto the Cao -

FeO - MgO plane of the tetrahedron. All those garnets with an

Fe/(Fe+Mg) ratio less than 0.77 will be involved in the reaction gar +

cpx + qtz = opx + an, as the three phase triangle sweeps to the left with metamorphism. The effect of ilmenite would be to decrease the

amount of ferromagnesian silicate phases and to increase the amount of

quartz needed for reaction (Mclelland and Whitney, 1977). Those garnets

64

with Fe/(Fe+Mg) greater than 0.77 will not participate in the reaction

at these metamorphic conditions. The hornblende that occurs in some

rims is a retrograde product of the original orthopyroxene, formed by

increasing f H 0 with decreasing metamorphic grade. Opaque oxides which 2 occur in the rim are an indication that ilmenite or hornblende played a

role in the formation of that garnet. The zoning of plagioclase

adjacent to the rims is a result of diffusion of calcium away from the

garnet during formation of the rim.

Therefore, in these rocks, Fe-rich garnet is not a disappearing

phase, but is part of the characteristic assemblage of the granulite

facies, whereas Mg-rich garnet is a disappearing phase. The garnet's

reaction to orthopyroxene and plagioclase is a function of the

composition of the garnet and of f H 0, and not of pressure, temperature 2 or rock composition. Those garnets without coronas simply contain

enough iron to be stable at conditions at which coronas were formed

around the more magnesian garnets.

IGNEOUS PETROLOGY

Parent Magma

There have been innumerable proposals hypothesizing a parent magma

composition for anorthosite, as outlined in Table 1 of de Waard (1969).

It was Buddington (1936) who first proposed a magma of a gabbroic

anorthosite composition, based on the concept that the anorthosite

series and the spatially associated acid rocks - syenite, monzonite,

quartz monzonite or granite (syenite, mangerite, adamellite and

charnockite; see Streckeisen, 1973) - are separate intrusions

(Buddington, 1969; Anderson, 1969; Davis, 1969 and 1971; Ashwal and

Seifert, 1980). The generation of such a magma has been discussed

extensively in the literature (Emslie, 1980; Isachsen, 1969) and

experimental data have demonstrated mechanisms of generation under both

hydrous and anhydrous conditions, though a magma formed under hydrous

conditions would immediately become undersaturated with continued

temperature rise so that no gas phase would be present after the

earliest stages of generation (Yoder, 1954 and 1965; Yoder, 1969).

Anhydrous experiments in the systems anorthite - diopside - silica

(Clark et al., 1962), plagioclase (Ab40An60) - diopside - enstatite

(Emslie, 1970) and anorthite - diopside - forsterite (Presnall et al.,

1978) all indicate a dramatic shift of the piercing point toward

anorthite or plagioclase with increasing pressure up to 20 kb. This

suggests that partial melting at high pressure of plagioclase bearing

mafic and ultramafic source materials can produce aluminous melts which

would crystallize large amounts of plagioclase upon intrusion to

65

66

shallower depths (Ashwal, 1978). The high temperatures required for

generation of aluminous melts under anhydrous conditions (approximately

1400° Cat 15 kb - Emslie, 1970 and Presnall et al., 1978} are

consistent with crystallization temperatures obtained by Ashwal (1978 -

1200-1300° C at 15-20 kb}. However, further experimental work is

necessary to completely substantiate the anhydrous mechanisms of

generation of a gabbroic anorthosite magma. Evidence for a gabbroic

anorthosite parent magma ranges from the occurrence of satellitic sheets

of gabbroic anorthosite that intrude the country rocks bordering the

main Adirondack anorthosite massif (Buddington, 1972) to trace element

data (Ashwal and Seifert, 1980}. The existence of such a magma for the

Nain complex has been demonstrated by Wiebe (1979).

Buddington (1972) postulated a chemical composition of that

hypothetical magma composed of 60% liquid of gabbroic anorthosite

composition and 40% andesine crystals (see Table 1). This composition

is very similar to that of the anorthositic gabbro facies of the

Speculator and Wells Sills (Figure 13). These facies therefore may

represent a cumulate from a magma that may have also given rise to some

of the larger anorthositic bodies.

Crystallization Sequence

Feldspar accumulation from an essentially dry gabbroic anorthosite

magma has also been demonstrated experimentally (Yoder, 1969). Ashwal

(1978) has proposed a crystallization sequence for a magma of gabbroic

anorthosite composition. Plagioclase of composition An 50Ab44or6 (based

on the spectrographic analyses of Isachsen and Moxham (1969) and the

67

reintegrative microprobe analyses of Kay (1977)) crystallized first, and

when the magma was about 65% crystallized, plagioclase was joined by

augite and pigeonite (Wo36En41Fs23 and wo11En 57Fs32 respectively,

determined by microprobe reintegration of coarsely exsolved pyroxenes).

After the magma was about 80% crystalline, hemo-ilmenite and magnetite

formed, and at 92%, apatite crystallized. Such a crystallization

sequence would drive Buddington•s (1972) parent magma composition from

lower left to upper right on the AFM diagram of Figure 13 to yield the

other rocks in the sills except for the Wells gabbro.

Though the application of any crystallization sequence to the sills

is hypothetical at best because of severe complications due to

subsequent metamorphism, the textural evidence observed for the Wells

and Speculator Sills supports Ashwal's hypothesis. The Tenantville

facies and the anorthositic gabbro of the Wells Sill and the Speculator

Sheet contain large pseudomorphs of primary plagioclase phenocrysts.

There are large crystals of both orthopyroxene and clinopyroxene in the

Speculator Sheet and the Wells anorthositic gabbro, the latter of which

exhibits exsolution of orthopyroxene, though there is no evidence of

primary pigeonite. It is most probable that the magma had been emplaced

before crystallization of pyroxene because of the lack of pyroxene

megacrysts in the Tenantville facies. Opaque minerals form irregular

interstitial aggregates; apatite forms both small and very large

euhedral crystals.

Ashwal (1978) estimated conditions of initial crystallization of 15

to 20 kbar and 1200 - 1300° C from the sodic composition of his primary

68

plagioclase and his primary pyroxene compositions. At the level of

final emplacement (about 25 - 30 km), metamorphic reconstitution of the

primary mineral assemblages produced abundant garnet, hornblende and

minor biotite.

IntY'Usive History

Comparison of the mineral chemistry of the two sills and the Oregon

Dome reveals relationships between the various intrusive facies that are

most clearly illustrated by the whole rack data as presented in the

Harker diagrams of Figure 12. Though metamorphism has affected the

mineral chemistry, comparisons may still be made. Plagioclase

compositions (Figure 4) for the Wells anorthositic gabbro and

Tenantville facies and the Speculator sheet are calcic (An50_70) except

for those from garnet coronas (more calcic) and those from a few very

mafic rocks (less calcic). Wells gabbro and Oregon Dome compositions

are less calcic than these (An43_50). The ferromagnesian minerals of

the Wells gabbro and Oregon Dome are more iron-rich than those of the

anorthositic gabbro facies of the two sills. Clinopyroxene and

orthopyroxene (Figure 5) average compositions are Wo47Fs15En38 and

Wo1Fs40En 59 for the gabbroic anorthosites and wo45Fs21En34 and

wo1Fs49En 50 for the Wells gabbro and the Oregon Dome. The metamorphic

mafic minerals also show the same relationships: hornblende (Figur~ 6),

biotite (Figure 8), and, most clearly, garnet (Figure 10) of the Wells

gabbro and the Oregon Dome are more iron-rich than the metamorphic

minerals in the Speculator Sheet and Wells anorthositic gabbro.

However, in the Wells Tenantville facies, the mafic metamorphic minerals

69

tend to be slightly more iron enriched than those of the Wells

anorthositic gabbro. These data indicate that the minerals of the Wells

gabbro and the Oregon Dome coexisted with magmas that were very similar

in composition. Likewise, the minerals of the Speculator Sheet and

Wells anorthositic gabbro seem to have coexisted with magmas of similar

composition, but which were markedly different from those of the Wells

gabbro and Oregon Dome.

The mineral chemistry dictates the major element chemistry, and

similar relationships may be ascertained from Figure 12. The

anorthositic gabbro of the Wells Sill and the gabbroic anorthosite of

the Speculator Sheet have very similar analyses and trends for most of

the major elements. One analysis of anorthosite from the Speculator

Sheet shows either higher or lower values for the oxides than the

remaining four, which is expected because of the lack of mafic minerals

and the preponderance of plagioclase. The one Tenantville analysis

occurs at high Si02 because of the significant amounts of vein quartz

present. The other oxides occur in similar abundances as those for the

anorthositic gabbro and gabbroic anorthosite of the two sills (Figure

13), but the point does not lie on the trend of the Wells anorthositic

gabbro. Such a correlation is also evident in Figure 11, where the data

are plotted against stratigraphic distance from the base of the sill.

The gabbro from the Wells Sill and the Oregon Dome may also be

compositionally compared. They have similar Si02 content, except for

the one sample of the Wells gabbro which is finely veined with quartz.

The Wells gabbro is more mafic than any other rock in this study. It

70

has more MgO and Fe2o3 and less Al 2o3, Cao, and Na2o than any other facies. This mafic enrichment of the gabbro is what gives the arcuate

shape to the Wells Sill profiles in Figure 11. The Oregon Dome shows a

more mafic composition than the anorthositic facies of the two sills:

it is enriched in iron and titanium and depleted in Al 2o3 and Na2o.

However, the Wells gabbro has a different trend than the Oregon Dome on

the Harker variation diagram, and its genesis cannot be directly linked

to that of the Oregon Dome. The Oregon Dome has a trend which parallels

that of the Speculator and Wells anorthositic gabbro which is dictated

by the amount of cumulous plagioclase in the rock. The Wells gabbro has

a shallower trend, but a trend based on two points (the high silica

point should be disregarded) is hypothetical at best.

The similarities in mineral chemistry between the Speculator Sheet

and the anorthositic gabbro and Tenantville facies of the Wells Sill are

emphasized in the AFM diagram of Figure 13 that shows that they have a

differentiation trend showing only moderate iron enrichment. The only

points that deviate from this trend are the three analyses for the Wells

gabbro facies, which, along with the Oregon Dome, have a trend of more

extreme iron enrichment, contrary to the rest of the sills. The

composition of the Wells gabbro on the AFM diagram more closely

approximates that of the Oregon Dome than the compositions of the rest

of the Wells Sill and the Speculator Sheet. While these differences are

emphasized on the AFM diagram, it must be remembered that both the Wells

and Speculator bodies are calcic (Peacock, 1931).

Goldberg (1975, 1977) analyzed the rare-earth element abundances of

71

five small anorthositic bodies, including the Wells-Tenantville Sill,

and discovered large variations in rare-earth element content and in the

extent of the europium anomaly. Based on these data he could draw no

conclusions concerning the pre-Grenville history of the rocks of the

anorthositic series.

However, the geochemical evidence from this study suggests two

compositional episodes of anorthositic intrusive activity in the

southern Adirondacks, one slightly earlier than the other characterized

by iron enrichment on an AFM diagram, and the other showing more

moderate iron enrichment. Data from the Snowy Mountain Dome (de Waard

and Romey, 1969b), presented in Figure 15, indicate that this body has

an iron enrichment trend; whereas the Thirteenth Lake Series (Letteney,

1969) has a trend similar to that of the Speculator Sheet and Wells

anorthositic gabbro.

Ashwal et al. (1981) produced an age determination of about 1200 my

with an initial Nd ratio of 0.51053 for the Marcy massif using

neodymium-samarium isotopes. This agrees well with the date of 1199 +

14 my for the mangerite rocks of the Snowy Mountain Dome obtained by

Hills and Isachsen (1975). This indicates that the intrusive activity

characterized by iron enrichment trends in the southern Adirondacks

occurred concurrently with the activity in the Highlands, but does not

discount the possibility that the intrusive activity characterized by

more moderate iron enrichment is a slightly younger phenomenon. This is

indicated by the crosscutting of the gabbro facies of the Wells Sill

(which shows indication of iron enrichment in the mineral chemistry) by

72

FIGURE 15

AFM Ternary Variation Diagram for

the Snowy Mountain Dome - open circles

and the Thirteenth Lake Dome - closed circles

(de Waard and Romey, 1969b; Letteney, 1969).

A= Na2o + K20; F =Total Fe+ MnO; M = MgO (weight%). Lines are as in Figure 13.

73

• • • • •

•

•

74

the Tenantville facies.

In the southern Adirondacks, the data indicate that the first

intrusive episode led to the development of two different masses, the

Oregon Dome and the Snowy Mountain Dome. As this magma solidified and

cooled, it developed a mafic residual liquid that was expelled into the

country rock along a plane of weakness, perhaps as a result of stresses

caused by the start of the second period of intrusion. This liquid

solidified quickly, leaving the fine-grained slightly gneissic gabbro

exposed in the Wells-Tenantville Sill.

The second period of magmatic activity formed the Thirteenth Lake

Dome and at the same time sent two intrusions of gabbroic anorthosite

into the superjacent country rock. The first of the satellitic

intrusions formed the Speculator Sheet. As this thick sill of

stationary magma cooled slowly against a country rock already warmed by

its proximity to the still cooling Oregon Dome, some settling of mafic

phases caused a mo~e gabbroic rock at the base of the unit. Poorly

cemented aggregates of large plagioclase crystals caused the formation ' of more leucocratic pockets; some of these aggregates cause anomalous

occurrences of more leucocratic material at the base of the sill.

The second of the satellitic intrusions reached the zone of weakness

initially penetrated by the mafic differentiate of the Oregon Dome/Snowy

Mountain Dome magma. At the exposure in Wells, the anorthositic gabbro

magma penetrated the gabbro and formed a core. The Tenantville facies

formed as a chill margin against the gabbro (which is more than twice

the distance from the Oregon Dome as is the Speculator Sheet), as

75

suggested by the number of suspended plagioclase crystals and the

absence of other large crystals such as orthopyroxene. In Tenantville,

the mass of magma that penetrated the gabbro was too small to form a

slowly-cooled core of anorthositic gabbro; the chilled facies is the

only leucocratic facies present. The small positive europium anomaly

for the Tenantville facies found by Goldberg (1975, 1977) would be

expected from a liquid with suspended plagioclase phenocrysts.

These intrusive relationships do not support the theory of de Waard

and Romey (1963 and 1969a) that the Snowy Mountain Dome, and therefore

also the Oregon Dome, formed a basement upon which the overlying

sediments were deposited, for the geochemical history of the sills is

intimately linked with that o~ the three massifs in the southern

Adirondacks. It is therefore concluded that, at least in this area of

the Adirondacks, the contacts between metaigneous and metasedimentary

bodies are primary intrusive contacts, and are not the product of

sedimentation on an anorthosite basement.

76

BIBLIOGRAPHY

Anderson, A.T., Jr. (1969} Massif-type anorthosite: a widespread Precambrian igneous rock IN Isachsen, Y.W., ed. (1969) Origin of Anorthosite and Related Rocks, New York State Museum and Science Service Memoir 18: 47-56.

Ashwal, L.D. (1978) Petrogenesis of massif-type anorthosites; crystallization history and liquid line of descent of the Adirondack and Morin complexes: unpublished Ph.D. thesis, Princeton University: 136 p.

Ashwal, L.D., Seifert, K.E. (1980) Rare-earth-element geochemistry of anorthosite and related rocks from the Adirondacks,