A Sillimanite Gneiss Dome in 

the Yukon Crystalline Terrane, 

East-Central Alaska: 

Petrography and GarnetBiotite Geothermometry

By CYNTHIA DUSEL-BACON and HELEN L. FOSTER

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1170-E

Petro'graphic, geothermometric, and structural data are used 

to support the hypothesis that a 600-km2 area of 

pelitic metamorphic rocks is a gneiss dome

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1983
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary

GEOLOGICAL SURVEY

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data

Ousel-Bacon, Cynthia.

A sillimanite gneiss dome in the Yukon crystalline terrane, east-central Alaska.

(Shorter contributions to general geology) (Geological Survey Professional Paper 1170-E)

Bibliography

Supt. of Docs, no.: I 19.4/2:1170-E

1. Gneiss Alaska. 2. Intrusions (Geology) Alaska. I. Foster, Helen Laura, 1919- . II. Title. III. Series. IV. Series:

United States: Geological Survey. Professional Paper 1170-E. 

QE475.G55D87 1983 552'.4 83-600031

For sale by the Distribution Branch, U.S. Geological Survey, 

604 South Pickett Street, Alexandria, VA 22304
CONTENTS

Page

Abstract-...--.-----..........-...-......-..---..-.-...................-..-.....-...---^

Introduction .-.. ... ...... ... .. .... .... ...... ........... ..... ....... ... ... .-. .. ... ... ....... 1

Acknowledgments .................................................................................................................................. 2

Geologic setting................................................................................................................................................ 2

Field relations and macroscopic characteristics of the sillimanite gneiss and associated pelitic 

schist........................................................................................................................................................ 3

Petrographic description of the aluminum-silicate-bearing rocks of the Salcha River area .... 6

Sillimanite gneiss.................................................................................................................................... 6

Equigranular feldspathic rocks .......................................................................................................... 10

Pelitic schist.............................................................................................................................................. 10

Geothermometry based on coexisting garnet and biotite in the pelitic schist and sillimanite

gneiss ...................................................................................................................................................... 14

Discussion .......................................................................................................................................................... 17

Metamorphic reactions and isograds in pelitic schist and sillimanite gneiss ........................ 17

Staurolite+biotite isograd.............................................................................................................. 17

Staurolite-out isograd .................................................................................................................... 20

Sillimanite-andalusite isograd .................................................................................................... 21

Proposed origin of equigranular aluminum-silicate-bearing rocks......................................... 21

Physical conditions of metamorphism .............................................................................................. 21

Conclusions........................................................................................................................................................ 23

References cited .............................................................................................................................................. 23

ILLUSTRATIONS

FIGURE 1.Index map showing location of study area in east-central Alaska................................................................................................ ........... E2

2. Geologic map of the sillimanite gneiss and vicinity showing distribution of key minerals and metamorphic isograds .......... 4

3. Photograph showing marble nodule in sillimanite gneiss............................................ ............................................................................. 5

4. Photograph showing contact between granitic rock and sillimanite-biotite gneiss.............................................................................. 6

5. Photomicrographs showing textural relations in sillimanite gneiss, equigranular feldspathic rock, and pelitic schist .......... 11

6. Simplified geologic map of pelitic schist and sillimanite gneiss showing location of garnet-biotite samples .............................. 19

7. Diagram showing phase equilibria pertaining to study area.......................................-.---....-...--.--.-.-.-..-.-.-.-.-.-.------- 22

TABLES

Page

TABLE 1. Major-element data and CIPW norms for samples of sillimanite gneiss ................................................................................................ E6

2. Representative mineral assemblages of aluminum-silicate-bearing rocks ............................................................. ............................ 8

3. Representative chemical analyses of biotite ............................................................................................. ........... . .. . . ... . .. 15

4. Representative chemical analyses of garnet.......................................-...................................-.....-..--...-- -.---..--- . -.... 16

5. Distribution coefficients for the Fe/Mg ratio in coexisting garnet and biotite and equilibration temperatures (in degrees

Celsius) calculated at 0.4-GPa total pressure ......................................................................................... ........-....... ....... ..... ....... 18

ill

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

A SILLIMANITE GNEISS DOME IN THE

YUKON CRYSTALLINE TERRANE, EAST-CENTRAL ALASKA: 

PETROGRAPHY AND GARNET-BIOTITE GEOTHERMOMETRY

By CYNTHIA DUSEL-BACON and HELEN L. FOSTER

ABSTRACT

A topographically high, nearly circular area of sillimanite 

gneiss crops out over approximately 600 km2 in the Big Delta 

quadrangle, Yukon crystalline terrane, east-central Alaska. Foliation is generally horizontal or subhorizontal in the center of the 

gneiss body and dips outward in all directions. Felsic dikes, some of 

which are pegmatitic, occur in the central area of gneiss. Quartzite 

and marble are locally infolded in the gneiss, particularly near its 

margins. The gneiss is partly bordered on the north and east by 

pelitic schist interlayered with lesser amounts of quartzite, marble, 

and amphibole schist. The ages of the protoliths of these rocks are 

believed to be Paleozoic or possibly Precambrian; major metamorphism probably took place between Mississippian and Middle Triassic time. Two isograds within the pelitic schist are concentric to 

the gneiss body and indicate an increase in metamorphic grade 

toward the gneiss. The outermost isograd is defined by the appearance of staurolite with biotite, at the expense of chlorite and muscovite. Upgrade of this isograd, several combinations of the aluminum silicate polymorphs (andalusite, kyanite, and sillimanite) 

+garnet+staurolite are present in quartz-biotite-muscovite schist. 

Textural relations suggest that these three polymorphs all formed 

during the same prograde metamorphic event in which pressuretemperature conditions were near the triple point. The innermost 

isograd, defined by the disappearance of staurolite, closely coincides with the schist-gneiss contact. Increase in metamorphic grade 

in the gneiss (quartz+biotite+sillimanite+orthoclase+plagioclase± 

muscovite± garnet and locally quartz+biotite+sillimanite+cordierite+orthoclase +plagioclase +muscovite) is indicated by an increase 

in modal orthoclase and decrease in muscovite toward the center of 

the body, consistent with the breakdown of muscovite+quartz. 

Migmatitic textures, although rare, may be evidence of local partial melting of the gneiss. The relative distribution of Fe and Mg 

between garnet and biotite indicates equilibration at about 535° to 

600°±30°C for pelitic schist north of the gneiss body and 655° to 

705°±30°C for sillimanite gneiss. Petrographic data from poorly 

exposed rocks south and west of the gneiss also suggest an increase 

in pressure and (or) temperature conditions toward the gneiss body; 

such an increase is indicated by an isograd which separates 

andalusite-sillimanite-bearing schist and gneiss to the southwest 

from sillimanite gneiss to the northeast. Evidence that the sillimanite gneiss is a dome consists of: (1) the topographic expression and 

radial drainage pattern, (2) the outward dip of foliation, (3) an 

increase in metamorphic grade toward the center of the body, and 

(4) an increase in garnet-biotite temperatures inward. Minor intrusion of synmetamorphic dikes or small granitic bodies is suggested 

by the presence of equigranular, aluminum-silicate-bearing rocks 

at several localities in the proposed dome which contain textural

features indicative of both igneous and metamorphic processes. The 

staurolite-out and andalusite-sillimanite isograds define the northern and eastern and the southwestern margins, respectively, of the 

proposed gneiss dome.

INTRODUCTION

Sillimanite gneiss crops out in a 600 km2 area in 

the central part of the Big Delta quadrangle, eastcentral Alaska, near the northwestern margin of the 

Yukon crystalline terrane (fig. 1). On an early geologic map of the Yukon-Iknana region (Mertie, 1937), 

the gneiss and most of the surrounding schists were 

included in a single geologic unit, the Birch Creek 

Schist (of former usage). The general geology of the 

area is shown at a scale of 1:63,360 on a reconnaissance geologic map by Foster and others (1977a) and 

at a scale of 1:250,000 on a reconnaissance geologic 

map (Weber and others, 1978). The possibility that the 

area of sillimanite gneiss might be a gneiss dome was 

reported initially by Foster and others (1977b).

In this paper we present new petrographic and 

petrologic data on the sillimanite gneiss and surrounding pelitic schist, and we interpret these data in 

terms of the metamorphic history of these rocks. In 

particular, we address the hypothesis that the large, 

topographically high area of sillimanite gneiss is a 

gneiss dome. This is the first gneiss dome in the 

Yukon crystalline terrane to be reported in the literature, but reconnaissance mapping in the Alaskan part 

of the crystalline terrane (Weber and others, 1978) has 

identified other areas of gneiss which appear roughly 

circular in plan view and which also may prove to be 

gneiss domes.

This particular body of sillimanite gneiss, flanked 

by pelitic schist, was chosen for study because of the 

abundance of mineral assemblages containing the 

pressure-temperature-sensitive aluminum silicates 

and the relatively good exposure of the gneiss. The 

distribution of aluminum silicates in thin sections of
E2 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

pelitic schist and sillimanite gneiss enables the estimation of metamorphic pressure and temperature 

conditions, and garnet-biotite pairs in these same 

rocks provide an additional means of determining 

metamorphic temperatures. Because kyanite, andalusite, and sillimanite occur together in single thin 

sections of pelitic schist from four different localities, 

we also have attempted to determine whether or not 

these Al2SiOs polymorphs crystallized during a single 

metamorphic episode and thus whether their occurrence indicates triple-point conditions in the schist. 

Study of the sillimanite gneiss dome can provide a 

basis for subsequent comparisons with other such 

structures in Alaska and the Canadian Cordillera.

This paper is an outgrowth of geologic mapping 

of the Big Delta quadrangle for the U.S. Geological 

Survey's Alaska Mineral Resource Assessment Program (Foster and others, 1979). Most of the f ieldwork, 

all of which was of a reconnaissance nature, was done 

in 1976 and 1977; the laboratory work was accomplished mainly in 1979 and 1980.

ACKNOWLEDGMENTS

We wish especially to recognize the many contributions of Florence R. Weber to the mapping of the 

Big Delta quadrangle and her initial recognition of 

sillimanite gneiss in the area. We are also indebted to 

Charles R. Bacon for microprobe analyses, aid in 

interpretation of the data, and support throughout the 

study. Discussions of metamorphic petrology and 

petrography with Jo Laird and Anna Hietanen have 

been particularly valuable.

GEOLOGIC SETTING

The sillimanite gneiss occurs in the Yukon crystalline terrane (Tempelman-Kluit, 1976), a geologically complex metamorphic and igneous terrane that 

is bounded by two major right-lateral strike-slip fault 

systems, the Tintina to the north and the Denali to the 

south (fig. 1). Tempelman-Kluit (1976) has proposed 

that much of this terrane is allochthonous, and

130° 120°

Yukon crystalline -•

terrane 

Big Delta quadrangle

FIGURE 1. Location of sillimanite gneiss dome, Yukon crystalline terrane, and Big Delta quadrangle in east-central Alaska.
SILLIMANITE GNEISS DOME, YUKON CRYSTALLINE TERRANE, ALASKA E3

according to Foster and Keith (1974), it is possibly a 

continental fragment that moved in over oceanic 

crust along an ancient suture bordering the North 

American plate. Churkin and others (1982) suggest 

that the terrane may be composed of at least four 

subterranes.

The metamorphic rocks of the Yukon crystalline 

terrane range in metamorphic grade from low greenschist to amphibolite facies. They have been intruded 

by Mesozoic and Tertiary granitic plutons, and small 

areas are overlain by Mesozoic and Tertiary volcanic 

and sedimentary rocks. Several masses of ultramafic 

rocks, including alpine-type serpentinized peridotite 

and dismembered ophiolite assemblages, have been 

tectonically emplaced.

Most of the metamorphic rocks are derived from 

protoliths with continental affinities, although some 

greenstones and other volcanic and ultramafic rocks 

occur which have oceanic, mantle, or island-arc affinities. Quartzitic rocks are most abundant, but there 

are also interlayered marble, pelite, calc-silicate 

rocks, and mafic schist. Although pelitic gneiss and 

schist are not generally abundant in much of the 

Yukon crystalline terrane, in the central part of the 

Big Delta quadrangle such rocks occupy a significant 

area, including the region discussed in this paper. 

Lesser amounts of pelitic rocks occur in other parts of 

this quadrangle and in the Circle quadrangle to the 

north. Metaigneous rocks, particularly ortho-augen 

gneiss (Aleinikoff and others, 1981), crop out, but in 

most areas are subordinate to metasedimentary 

rocks. Areas of augen gneiss (Weber and others, 1978; 

Foster, 1970, 1976; Tempelman-Kluit and Wanless, 

1980; and J. K. Mortensen, written commun., 1982) 

that probably represent deformed batholiths define a 

crude northwesterly-trending belt across the Yukon 

crystalline terrane.

The ages of the protoliths are poorly known 

because of the lack of fossils, the scarcity of radiometric age data, and the difficulty of interpreting those 

data. Much of the metamorphic terrane is believed to 

have Paleozoic protoliths but Precambrian protoliths 

may also be present (Foster, 1976). Although the presence of Precambrian protoliths is uncertain, some 

rocks contain detrital material of Proterozoic age as 

indicated by U-Pb data for zircon separates from 

quartzites (Aleinikoff and others, 1983b).

The timing and number of metamorphic events 

that have affected various parts of the terrane are 

uncertain. In Alaska, or at least that part of it in 

which the sillimanite gneiss body occurs, the presence 

of an unmetamorphosed granitic pluton of Triassic 

age, about 210 m.y. old (Aleinikoff and others, 1981a), 

that intrudes the terrane and contains xenoliths of

metamorphic rocks suggests that major metamorphism of the terrane may have occurred before Middle Triassic time. Radiometric dating of a deformed 

batholith of augen gneiss (Aleinikoff and others, 

1981b) that crops out about 50 km southeast of the 

sillimanite gneiss indicates that major metamorphism occurred either during or after the intrusion of 

the augen gneiss protolith in Early Mississippian 

time. Recent U-Pb dating of zircons from the sillimanite gneiss described in this paper (Aleinikoff and 

others, 1983a) has yielded results that could be interpreted to indicate that metamorphism of the sillimanite gneiss was roughly synchronous with Mississippian intrusion of the augen gneiss. The U-Pb zircon 

ages also indicate an early Proterozoic provenance 

age (2.0 to 2.3 b.y.) for the sedimentary protolith of the 

sillimanite gneiss.

FIELD RELATIONS AND MACROSCOPIC 

CHARACTERISTICS OF THE SILLIMANITE 

GNEISS AND ASSOCIATED PELITIC SCHIST

The sillimanite gneiss occurs in a topographically 

high, nearly circular area south of the Salcha River 

(fig. 2). Maximum relief in this area is about 1,200 m, 

and drainage is outward in all directions from the 

center of the gneiss body. Exposure of the gneiss is 

fairly good above the tree and brush line, which is 

about 1,000 m in elevation. Altiplanation terraces and 

patterned ground are well developed on high ridges, 

and coarse frost-riven rock rubble generally covers 

the surface; outcrops are scattered widely. Foliation, 

defined by the alinement of biotite flakes and elongate 

quartz grains, is moderately to poorly developed and 

is characteristically nearly horizontal in the central 

part of the area of gneiss. The nearly horizontal attitudes result from recumbent isoclinal folding on both 

a mesoscopic and macroscopic scale. Dips of foliation 

are roughly outward in all directions from the center 

of the gneissic area and are rarely more than 30°. The 

few attainable measurements of fold axes indicate 

that they have a general westerly strike with no consistent direction of plunge. Diopside-bearing marble 

and quartzite, once interlayered with the pelitic 

rocks, now occur only locally as discontinuous layers 

and blocks in the sillimanite gneiss (fig. 3). Brittle, 

cataclastic deformation of the gneiss has also 

occurred, but its effects are not so readily visible in 

the field as are those of ductile deformation. The 

gneiss is faulted in places (fig. 2), but faults are not 

mapped in detail.

The rocks throughout the sillimanite gneiss body 

are much alike in mineral composition and texture. 

Major-element analyses and norms for two samples of
SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 145°30'

64°45' —

10 KILOMETERS

FIGURE 2.-Geologic map of the sillimanite gneiss body and vicinity (after Weber and others, 1978) showing distribution of key minerals

and metamorphic isograds.
SILLIMANITE GNEISS DOME, YUKON CRYSTALLINE TERRANE, ALASKA E5

sillimanite gneiss that are representative of the gneiss 

body are shown in table 1. Chemical compositions of 

these rocks are compatible with the sedimentary proEXPLANATION

20V

O

D 

A

QUATERNARY

TERTIARY AND 

CRETACEOUS

PERMIAN

PALEOZOICf?)

PALEOZOIC

PALEOZOIC AND 

(OR) PRECAMBRIAN

SURFICIAL DEPOSITS Alluvium, 

colluvium, loess, and sand

GRANITIC ROCKS Granodiorite 

to quartz monzonite

GREENSCHIST-FACIES ROCKS-As 

mapped, divided into:

Peridotite, greenstone

Cataclastic quartzofeldspathic rocks

Semischist, greenschist, quartzite, 

and marble

Carbonaceous quartzite, argillite, 

marble and calc phyllite

EPIDOTE-.AMPHIBOLITE AND 

AMPHIBOLITE-FACIES ROCKSAs mapped, divided into:

Metamorphosed diorite

Pelitic schist, quartzite, amphibole schist, 

and marble

Sillimanite gneiss

Orthogneiss and paragneiss (includes 

biotite gneiss, amphibole gneiss, 

augen gneiss, and quartzite)

Metamorphosed peridotite and diorite

*Note: Age relations between metamorphic units are unknown. Ages given 

here for metamorphic rocks are protolithic ages. Metamorphic 

ages for these rocks are uncertain but major metamorphism is 

probably post-earliest Mississippian to pre-Middle Triassic.

_ Contact

. Fault Dotted where concealed 

_ Probable thrust fault Sawteeth on upper plate 

. Lineament observed on aerial photographs Probable 

fault Dotted where concealed

Strike and dip of foliation

Inclined

Horizontal 

Bearing and plunge of fold axis or mineral lineation

Inclined

Horizontal 

Key minerals Symbols may be combined; numbered

locations refer to table 2 and figure 5A -L

Sillimanite

Kyanite

Andalusite

Staurolite

Garnet, shown only where it occurs with one of the 

minerals above

Cordierite

Sillimanite-bearing equigranular feldspathic rock 

(may also contain other key minerals)

tolith proposed for the gneiss: that is, K is greater 

than Na and Mg greater than Ca; also, normative 

quartz is close to or greater than the 50-percent value 

considered by Mason (1966, p. 249) to be indicative of 

a sedimentary protolith. The gneiss is medium dark 

brown in color and fine to medium grained. Biotite, 

quartz, orthoclase, and plagioclase generally occur as 

scattered subequant grains that average a millimeter 

in size and give the rock a characteristic "salt and 

pepper" appearance. The sillimanite is rarely coarse 

enough to be seen with a hand lens, but where visible 

it appears as fine, milky-white threads. Because of the 

subequant shape of biotite flakes and a lack of pronounced mineral segregation, gneissic banding is 

commonly poorly defined. Biotite-rich layers and 

clots of biotite occur locally, and swirly banding is 

particularly evident in the biotite-rich layers. Migmatitic streaking, suggested by textural and compositional differences on the order of a few centimeters, 

was observed at a few areas well within the gneiss 

body.

Unfoliated granitic-appearing rock, generally 

found only as rubble, is present in several areas in the 

central part of the gneiss body. The equigranular texture of the gneiss in these places makes it difficult to 

distinguish igneous from metamorphic textures. At a 

few localities, granitic rock and sillimanite-biotite

FIGURE 3. Marble nodule (above hammer) in sillimanite gneiss 

from north-central part of gneiss body. Subhorizontal and horizontal foliation and recumbent folds are indicated by fracture 

pattern and subtle banding on weathered, lichen-covered 

outcrop.
E6 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

TABLE 1. Major-element data and CIPW norms for samples of sillimanite gneiss

-^Major elements (weight percent)

A1 2°3 —————— ' 

bFe2 03 —————— •

P 2 05— — — ...

CLOI ————— •

Q— -— — — —

Ab ———————— •

Ap— — — — —

81ADB35A

____ 6 21

9 lp

. ————————————————— .41

. ——————————————— .76

I

rp

-HNorms (weight percent)

. ——————————————— 5.71

. ——————————————— 1.75

. ——————————————— g.58

. ——————————————— 5.55

. ——————————————— 4.08

. ——————————— - ——— i.4i

81ADB36A

9.65

1.20

2 pQ

.52

.07

.04

pi

99.47

59.48

3.54

3.03

.17

1.00

1.97

.96

-^Determined by quantitative X-ray spectroscopy. 

ilotal iron as FejjC^-

-£|_oss on ignition.

-Calculated on the basis of Fe3+=2 Fez+ .

gneiss have a diffuse, irregular contact in outcrop; the 

granite may represent leucosomes in migmatitic 

gneiss (fig. 4) from synmetamorphic injection of the 

igneous material (to be discussed below). Felsic dikes 

and quartzofeldspathic, tourmaline-bearing pegmatite dikes cut the gneiss locally, particularly in the central area of gneiss. Potassium feldspar content in the 

gneiss is high in areas where these felsic dikes or granitic rocks occur. Xenoliths of biotite gneiss, marble, 

and quartzite (several centimeters to a meter in size) 

are partially assimilated in felsic and granitic dikes 

in the east-central part of the sillimanite gneiss body. 

Toward the northern margin of the gneiss, a 

gradual increase in muscovite content is accompanied 

by a decrease in feldspar. Micaceous schist and finegrained gneiss are interlayered just south of the Salcha River. North of the Salcha River the pelitic rocks 

are nearly all schistose and are interlayered with 

quartzitic schist, quartzite, marble, amphibolebearing schist, and quartzofeldspathic schist.

Foliation in the schist generally dips an average 

of about 20° in an easterly direction. Lineation is 

formed by crenulation of the foliation, commonly seen 

in micaceous layers, and mineral alinement is somewhat inconsistent in trend and plunge but most commonly is' oriented within 20° of north and plunges less 

than 5° to the north or south. Schists are only well 

exposed along crests of ridges and in scattered outcrops along the Salcha River and its tributaries.

Quartz-biotite-muscovite schist, commonly containing pink garnets up to a centimeter in diameter, is 

the most common rock type north of the Salcha River. 

Also present in many layers in the schist are darkblue-gray andalusite porphyroblasts 1 or 2 cm long 

and small porphyroblasts, generally less than half a 

centimeter long, of dark-brown staurolite, black 

tourmaline, and dark-silvery-blue kyanite. Sillimanite, present in trace amounts in many schist samples, 

is too small to be detected in hand specimen.

The eastern margin of the sillimanite gneiss is 

the most complex, and it is bordered by a feldspathic 

gneiss interlayered with marble and amphibolebearing biotite gneiss. These bordering rocks are 

mapped as a separate geologic unit, although they 

may be a part of the gneiss body. A small, foliated, 

Paleozoic diorite plug occurs west of the headwaters 

of the South Fork of the Salcha River (fig. 2). Several 

kilometers east of the plug, a small serpentinite mass 

apparently is infolded in feldspathic gneiss. East of 

the South Fork of the Salcha River, the sillimanite 

gneiss or other gneissic rocks grade into pelitic schist 

whose mineralogy and structures are similar to those 

on the north side of the sillimanite gneiss body. The

FIGURE 4. Irregular, interfingering contact between granitic rock 

(right) and sillimanite-biotite gneiss from south-central part of 

gneiss body.
SILLIMANITE GNEISS DOME, YUKON CRYSTALLINE TERRANE, ALASKA E7

extent of the gneiss on the west and south is poorly 

known because the area is largely covered by loess, 

alluvium, sand, brush, and trees, but sillimanitebearing rocks are known to occur as much as 40 km 

southwest of Gilles Creek (fig. 2).

North and east of the pelitic schist unit are greenschist facies rocks (Weber and others, 1978) that consist predominantly of quartzofeldspathic cataclastic 

rock, semischist, greenschist, quartzite, and marble 

(fig. 2). These greenschist facies rocks are probably in 

thrust contact with higher grade rocks which flank 

the sillimanite gneiss. A thrust is suggested by the 

zone of intensely cataclasized quartzofeldspathic rock 

north of the schist (fig. 2) and mylonitic textures in 

greenschist facies rocks east of the pelitic schist unit, 

apparent termination of isograds, and abrupt changes 

in lithology.

A few Cretaceous or Tertiary, unfoliated felsic 

plutons crop out in the area surrounding the gneiss 

(fig. 2). No contact metamorphic aureoles have been 

observed around these plutons. Incipient granoblastic 

texture and sericitization of white mica, noted in several thin sections from rocks near the large pluton 

west of the North Fork of the Salcha River, suggest 

that only minor thermal metamorphism has resulted 

locally.

PETROGRAPHIC DESCRIPTION OF THE

ALUMINUM-SILICATE-BEARING ROCKS OF THE

SALCHA RIVER AREA

Petrographic data for the aluminum-silicatebearing lithologies sillimanite gneiss, equigranular 

feldspathic rock, and pelitic schist which occur in 

the Salcha River area are given in table 2, and sample 

locations are shown in figure 2. Only petrographic 

data for selected samples of sillimanite gneiss and 

pelitic schist that are considered representative of the 

aluminum-silicate-bearing rocks of the area are presented. However, petrographic data on all available 

samples (7) of the equigranular rocks are given 

because of significant differences among the samples.

SILLIMANITE GNEISS

Quartz, orthoclase, and plagioclase (oligoclase to 

andesine) are the most abundant felsic constituents of 

the gneiss, and together generally compose onequarter to one-half of the rock (table 2). Quartz most 

commonly occurs as small, flattened lenticles 1 or 2 

mm in length, alined along the foliation. Grains characteristically have undulose extinction and sutured 

margins, indicating that there was little recrystallization after deformation. Orthoclase (identified by Xray diffraction) is present as small (1 to 2 mm in 

diameter) untwinned xenoblastic grains. Perthitic 

intergrowths are very rare. Plagioclase is similar in 

size to quartz and orthoclase, but it is generally subidioblastic. In some samples of gneiss, plagioclase is 

elongate and xenoblastic, showing iron staining along 

abundant fractures. Polysynthetic twins are common 

and are offset by the fractures in some thin sections. 

Concentric zoning is indicated in a few grains by differential extinction. Sericitization of feldspar grains 

generally is minor.

Biotite is the dominant mica, making up half or 

more of the rock and giving the overall dark aspect to 

the gneiss. In thin section, the grains show red-brown 

to tan pleochroism. They are lens-shaped or subidioblastic or idioblastic, depending on the relative 

amounts of recrystallization following cataclasis. 

Subequant biotite grains with ragged margins are 

common. Biotite is slightly altered locally to a mixture of chlorite and rutile or, rarely, sericite. More 

extensive alteration has been observed in only a few 

thin sections, where "islands" of biotite occur in a 

"sea" of sericite.

In some of the more schistose samples, particularly those near the northern gneiss-schist contact 

(fig. 2), muscovite is interleaved with biotite. Where 

,muscovite is present, it generally makes up only a few 

percent of the rock (table 1) and consists of small, 

ragged grains.

Sillimanite occurs as fibrolite mats, as small 

colorless needles, and as well-formed prisms (fig. 5A). 

All three forms are commonly observed in the same 

thin section. Some bundles of fibrous sillimanite 

reach up to % cm in length and develop cross fractures 

similar to the transverse partings of a single prism 

(fig. 5B). The average length of the sillimanite fibers 

or grains is 1 to 2 mm, but grains 5 mm long are 

common in the center of the gneiss body. Thin, wavy 

streams of fibrolitic sillimanite commonly define the 

foliation. In many thin sections, sillimanite is intergrown with biotite, giving the impression that it is 

replacing biotite. All ranges of the degree of intergrowth of the two minerals, from minor amounts of 

sillimanite needles at the edges of biotite grains to 

abundant needles and mats of sillimanite within biotite, can be observed in thin section (fig. 5C). In the 

most advanced stages of what appears to be replacement by sillimanite, biotite remains as indistinct 

remnants, commonly with pale, "bleached," tan 

pleochroism. Although sillimanite is associated most 

commonly with biotite, in some samples it is concentrated between quartz or feldspar grain boundaries. 

Rarely, it is found as inclusions within quartz, orthoclase, and cordierite.
TABLE 2.—Representative mineral assemblages of aluminum-silicate-bearing rocks (mineralogy of equigranular feldspathic sample

text also included)

[Mineral identification made using standard petrographic techniques; abundances estimated from amount present in thin section. M, maj

mineral present in amounts greater than 30 volume percent; m, minor mineral present in amounts approximately 5 to 30 volume percen

tr, trace mineral present in amounts less than 5 volume percent; x, present as accessory mineral; -, not found]

Mineral constituents

Sample

locality

(fig. 2)

1

1

2

3

4

5

6

7

8

10

12

13

14

15

16

17

18

19

20

21

23

24

Field number

77AFr43A

77AFr43F

77AFr46E

77AFr4019B

76AFr272A

77AFr4179

77AFr2044A

75AWr710

77AWr363

72AFrl48

77AFr3009

77AFr274

77AFr206B

77AFr269C

76AFr216

76AFr4112

76AFr206

76AFr264

76AFr257

76AFr4118

77AFr2008B

76AFr4114

Rock name

Pelitic schist

—— do. ———

—— do. ——

—— do. ———

—— do. ——

—— do. — —

...--do. —————

—— do. ———

—— do. ——

—— do. ———

Gneiss

Pelitic schist

Graniticappearing rock

—— do. — —

Gneiss

Graniticappearing rock

Pelitic schist

—— do. — —

Gneiss

—— do. ——

—— do. ———

—— do. — —

Texture

Lepidoblastic

—— -do.—— -

Lepidoblastic,

porphyroblastic

Lepidoblastic

Lepidoblastic,

porphyroblastic

Lepidoblastic

Lepidoblastic,

porphyroblastic

Lepidoblastic

—— do. ——

—— do. — —

Lepidoblastic,

cataclastic

Lepidoblastic

Equigranular

—— do. — ~

Lepidoblastic,

cataclastic

Equigranular

Lepidoblastic

Lepidoblastic,

cataclastic

—— do. ———

—— do. — —

Lepidoblastic,

homeoblastic

Lepidoblastic,

cataclastic

4_>

ire

rs

m

M

M

M

m

m

m

M

m

M

M

M

M

m

M

M

M

M

M

m

m

M

QJ

.,—

O

CO

m

M

m

m

m

m

m

M

M

M

M

M

tr

tr

M

tr

M

M

M

M

M

M

QJ

0

U

*

M

M

M

M

M

m

M

M

m

tr

tr

M

tr

tr

-

tr

m

m

tr

-

tr

_

QJ

re 

o

en 

re

D_

m

m

tr

m

m

m

m

m

tr

tr

m

m

M

M

tr

m

m

m

m

m

m

m

OJ 

S~ +-> QJ

a -E z V) <T3 (/) OJ 

-O E 3 -M

OJ i — fO C 

*i- i— -0 ro

£ £ 5 S

tr tr M tr

. . . .

- - tr tr

. . . .

- tr m tr

tr

- tr -

- tr -

- tr m tr

- tr m

tr m

- tr -

M tr tr -

M tr tr -

m tr

M tr tr -

tr m

- m - -

m tr

m M - -

M m - -

M tr - -

QJ QJ

•i- QJ -ri— +J SO 4-> -I- QJ

1_ QJ i- -i-

=3 C 0 T3

re i- <— t4-> re -c o

</l O O O

tr - - -

- m - -

tr m tr

tr tr tr -

tr tr

tr tr -

tr m

- tr -

tr tr -

tr - tr -

_

- m - -

- tr -

.

- tr -

- tr -

. . - -

- tr -

- tr -

- - - -

- tr -

- tr -

Accessory minerals

QJ

O O

i- O

•i- E

X

X

-

X

X

X

-

-

X

X

X

X

_

-

X

X

X

X

X

-

X

X

QJ 

l/> i^ O

QJ re QJ 4-

re 3 •*-> r

c- o 3 a

0 1— OL <

X - - 

X - - 

X X - 

XXXX - - 

X - - 

X X - 

X - - 

X - - 

X - - 

X - X 

X - - 

X - - 

X - - 

X - - 

_

X X - 

X - - 

X - - 

X X - 

X - 
25

26

28

28

29

30

31

32

33

34

35

36

37

38

39

40

41

76AFr236

77AFr4090

77AFr4102B

77AFr4103

72AWr49B

77AWrl50D

72AWr46

77AWrl63

64APw3

77AFr4118

77AFr4112

77AFr4111

77AWrl85A

77AWrl94B

77AFr4123B

77AFr4140

77AFr3020

——— do. ———

——— do. ———

——— do. ———

——— do. ———

——— do. ———

——— do. — —

Graniticappearing rock

Gneiss

Graniticappearing rock

Gneiss

Graniticappearing rock

Gneiss

—— do. ———

——— do. — —

Graniticappearing rock

Gneiss

Gneiss

Lepidoblastic

Homeoblastic,

crudely

lepidoblastic

Lepidoblastic

Lepidoblastic,

intensely

deformed

Lepidoblastic

Lepidoblastic,

homeoblastic

Equigranular with

slight alinement

of micas

Cataclastic,

lepidoblastic

Equigranular

Lepidoblastic

Equigranular with

slight preferred

orientation

Lepidoblastic

—— do. ——

—— do. — —

Equigranular

Lepidoblastic,

cataclastic

——— do. — —

M

m

tr

tr

m

m

M

M

m

m

m

m

M

m

m

M

m

M

m

M

M

m

M

tr

M

tr

M

tr

M

M

M

tr

m

M

tr

tr

tr

.

tr

tr

tr

m

tr

-

tr

-

tr

-

tr

tr

-

m

M

tr

tr

tr

m

m

tr

tr

-

M

tr

tr

m

m

M

tr

m

M

M

m

M

M

M

m

M

M

M

M

tr

M

M

tr

M

tr - - - tr - tr

tr ----- -

m ------

M ----- M

m ----- M

tr - - - tr - -

tr ----- -

tr ----- -

tr ----- -

m ----- -

tr tr - - - - -

m ------

m ----- tr

tr ----- tr

tr ----- -

tr ----- -

m ------

X

X

X

X

X

X

X

X

X

X

.

X

X

X

_

X

X

X - - 

X - 

X - - 

X 

X - - 

X - - 

X X - 

X - - 

X - - 

X - - 

X - - 

- - 

X - - 

X - - 

X - X 

X - - 

X - - 
E10 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Andalusite, rather than sillimanite, is the only 

Al2Si05 polymorph in one sample of biotite gneiss 

from the southwestern part of the gneiss body (fig. 2). 

It occurs in a thin micaceous layer several millimeters wide as elongate xenoblasts 1 to 3 mm long, and it 

shows pink pleochroism in places. Nearby samples of 

gneiss contain sillimanite and no andalusite. Andalusite, together with sillimanite, is present in several 

rocks which have equigranular textures; those rocks 

are discussed in the next section.

Cordierite was identified optically in a total of 

eleven samples of sillimanite-biotite-orthoclase gneiss, 

most of which came from the central part of the 

gneiss body (fig. 2). Petrographic identification was 

confirmed by the electron microprobe. Typical 

cordierite-bearing assemblages are given in table 1. 

Cordierite occurs most commonly as elongate xenoblastic grains several millimeters in length which 

include trains of minute sillimanite needles and small 

biotite flakes (fig. 5Z)), but more equant, smaller 

grains of Cordierite are not unusual. Incipient alteration of Cordierite to pinite is present to varying 

degrees in some thin sections, but overall, the cordierite is relatively fresh. Local alteration of cordierite to 

a serpentine-group mineral was observed in several 

grains in two thin sections.

Garnet, zircon, tourmaline, apatite, and rutile, in 

decreasing order of abundance, occur as accessory 

minerals (table 2). Garnets are generally small idioblasts or granules less than 0.5 mm in diameter 

around which there is little deflection of biotite folia. 

Zircons are smaller than the other accessory minerals 

and are most common as small inclusions with 

pleochroic halos in biotite. Tourmaline is present only 

locally as small (1 mm long) subidioblasts with bluishgreen to yellow pleochroism, which only rarely are 

zoned. Minute, nearly opaque blebs and needles or, 

rarely, yellowish to reddish-brown granules of rutile 

occur in chlorite which has replaced biotite. The only 

opaque mineral positively identified by means of the 

microprobe is ilmenite.

EQUIGRANULAR FELDSPATHIC ROCKS

In seven different localities within the gneiss 

body or, in the case of one sample, in schist just north 

of the body, rocks that have equigranular, igneousappearing textures are found to contain sillimanite, 

sillimanite+andalusite, or sillimanite+andalusite+ 

garnet in thin section. Textural features indicative of 

both igneous and metamorphic processes are present 

in these rocks. The locations of these samples are 

shown on figure 2 (localities 14, 15, 17, 31, 33, 35, and 

39) and their complete mineral assemblages in table

2. All of the samples contain more than 30 percent 

modal orthoclase, which is not an unusual amount for 

the areas in which these rocks are found; nearby sillimanite gneiss is also high in orthoclase (see table 2 

and fig. 2). In most of the aluminum-silicate-bearing 

equigranular rocks, the development of sillimanite is 

similar to that in the foliated rocks, in which fibrolite 

is intergrown with biotite (localities 17, 31, 33, and 

35), or, as in a few schist samples, has grown as a 

fringe around andalusite (fig. 5E) (localities 17, and 

35) or as needles within it (locality 15) indicating that 

the rocks have been metamorphosed. A very subtle 

alinement of biotite and muscovite grains was 

observed in two samples (localities 14 and 15), and 

andalusite prisms as well are crudely alined in the 

sample from locality 35. Incipient cataclastic foliation, suggested by a planar fabric of sutured contacts 

and granulation between grains, also occurs in the 

equigranular rocks from localities 14 and 15.

However, there are important differences between 

the equigranular aluminum-silicate-bearing rocks 

and surrounding sillimanite gneiss. Unlike the sillimanite gneiss, many of the more equigranular rocks 

are characterized by: (1) a total or nearly total lack of 

preferred mineral orientation (localities 17, 31, 33, 39, 

and 14, 15, 35, respectively), (2) grains of andalusite, 

as well as sillimanite (localities 14, 15, 17, and 35), (3) 

halos of muscovite which rim several grains of andalusite (localities 14 and 15) and garnet (locality 14), (4) 

oscillatory zoning in plagioclase laths (fig. 5F) (localities 17, 31, and 33), (5) highly perthitic potassium 

feldspar (localities 14,15,17, 31, and 33), or (6) development of myrmekite along the margins of some plagioclase grains (localities 14, 15, 17, and 33).

At three localities, field relations suggest an intrusive relationship: samples from localities 14 and 15 

were collected from rocks which cut the surrounding 

sillimanite gneiss, and the sample from locality 31 

was taken from a knoll of granitic-appearing rock 

which crops out in the center of the gneiss body. Three 

other samples (localities 17, 35, and 39) were collected 

from areas of granitic-appearing rubble in which 

more gneissic rubble is also present. Information 

about the geologic setting of the sample from locality 

33 is not available. The origin of these aluminumsilicate-bearing, equigranular rocks is discussed later 

in this paper.

PELITIC SCHIST

Biotite and muscovite in well-formed books are 

the major constituents of the schistose rocks. Sheaves 

of both micas, 1 to 3 mm long, typically are interleaved and form continuous folia several millimeters
SILLIMANITE GNEISS DOME, YUKON CRYSTALLINE TERRANE, ALASKA Ell

wide. In a small number of thin sections, a minor 

amount of biotite or muscovite lies at an angle to the 

foliation. Chloritization of biotite or sericitization of 

muscovite is minor. Chlorite occurs as discrete crystals in a small number of samples; it is more common 

in the northern part of the area mapped as schist (see 

table 2 and fig. 2).

Alternating with the mica folia are bands composed of quartz, plagioclase (oligoclase or andesine), 

and potassium feldspar. These minerals are more 

coarsely grained in the schist than in the gneiss, with 

crystals commonly up to 5 mm in length. Quartz is 

strained and shows sutured grain boundaries between 

polycrystalline segments of elongate bands and 

lenses. Subhedral plagioclase laths display wellformed polysynthetic twins, some of which have 

deformed lamellae. Potassium feldspar is rare in the 

schists and where present consists of untwinned small 

xenoblasts. Feldspar is generally fresh and has only 

minor sericitization or kaolinitization.

Subidioblastic or idioblastic garnet porphyroblasts averaging 1 or 2 mm but reaching as much as 1 

cm in diameter are common in the pelitic schist. A 

few of the large garnets have rotational and snowball 

structures, evidenced by a spiral pattern of ilmenite 

and (or) quartz inclusions, which indicate syntectonic 

crystallization (Spry, 1969, p. 253). Most, but not all, 

of the garnet porphyroblasts deflect mica folia. A

small percentage of garnets have a weblike appearance caused by the large number of quartz inclusions 

which have grown to cut off segments of the crystal. 

Alteration of garnet to chlorite is minor. In a few thin 

sections, small lens-shaped knots composed of biotite, 

muscovite, quartz, and fibrolite replace garnet (fig. 

5G). Small granules of relict garnet are present in 

some of these knots. Fresh euhedral garnets, partially 

replaced garnets, and wholly replaced garnets may 

occur together in a single thin section.

Strongly colored yellow pleochroic porphyroblasts of staurolite are seen in most thin sections of 

pelitic schist. The crystals are typically stubby subidioblastic prisms or small granules, 1 to 2 mm in 

length, some of which poikilitically enclose minute 

quartz grains. Staurolite is commonly concentrated 

in layers, associated with and next to garnet or kyanite porphyroblasts, with which it is in apparent textural equilibrium (fig. 5H). Staurolite does not generally deflect the foliation. In one sample, a band of 

staurolite crystals is bowed around a large garnet 

porphyroblast. These textural relations may indicate 

either that the crystallization of garnet predates that 

of staurolite or that garnet formed later than staurolite and pushed out the preexisting foliation that contained the staurolite porphyroblasts (see Misch, 1971, 

for a discussion of the "crystallization force" of 

porphyroblasts).

FIGURE 5 ON PAGES 12-13

FIGURE 5. Photomicrographs showing textural relations in sillimanite gneiss, equigranular feldspathic rock, and pelitic schist. Andalusite (A); biotite (bi); cordierite (co); fibrolite (F); garnet (G); kyanite (ky); muscovite (mu); orthoclase (or); plagioclase (pi); quartz (qu); 

sillimanite (sil); staurolite (st). A, Prismatic sillimanite in garnetiferous sillimanite gneiss from north-central part of gneiss body 

(locality 22, fig. 2). Sillimanite occurs adjacent to garnet, in quartz folia, and within and around biotite. Note transverse partings in 

largest sillimanite prisms. Plane-polarized light. B, Sheaflike mats of fibrolite with transverse partings (locality 27, fig. 2) in 

sillimanite gneiss. In this sample (from central area of gneiss body), fibrolitic sillimanite occurs adjacent to quartz grains rather than 

biotite, as more commonly is the case. Plane-polarized light. C, Finely acicular sillimanite that appears to be replacing biotite in 

cordierite-sillimanite gneiss from central part of gneiss body (locality 28, fig. 2). Plane-polarized light. D, Xenoblastic cordierite from 

central part of gneiss body (locality 29, fig. 2). Note dark alteration (chlorite?) along cracks and preferential concentration of minute 

grains of sillimanite and biotite within cordierite. Crossed polarizers. E, Zoned andalusite grain rimmed by fringe of fibrolitic 

sillimanite in equigranular feldspathic rock (locality 17, fig. 2). Plane-polarized light. F, Oscillatory zoning in plagioclase grain in 

same rock as E. Crossed polarizers. G, Nearly total replacement of garnet porphyroblast in staurolite-3-aluminum silicate-quartzmica schist north of the Salcha River (locality 4, fig. 2). Deflection of foliation around knot of remnant garnet granules, biotite, quartz, 

minor muscovite, and fibrolite outlines area of former porphyroblast. Crossed polarizers. H, Association of staurolite, kyanite, and 

garnet in 3-aluminum-silicate-bearing schist east of the gneiss body (locality 9, fig. 2). Staurolite shares common grain boundary with 

kyanite, evidence of their equilibrium relationship. Plane-polarized light. I. Acicular fringe of sillimanite that has grown on margin 

of small andalusite xenoblast in staurolite-bearing quartz-mica schist east of South Fork of the Salcha River (locality 10, fig. 2). 

Andalusite and sillimanite present in trace amounts only. Opaque mineral is probably ilmenite. Plane-polarized light. J, Kyanite 

included in and growing on margin of andalusite porphyroblasts in garnetiferous staurolite-kyanite-andalusite-biotite schist east of 

South Fork of the Salcha River (locality 11, fig. 2). Textural relations imply that kyanite crystallized before andalusite. Planepolarized light. K, Irregularly shaped patch of staurolite adjacent to fibrolitic sillimanite in sample of garnet-staurolite-3-aluminum 

silicate-mica schist, north of the Salcha River near its North Fork (locality 8, fig. 2). Appearance of staurolite in photo and its spongy 

appearance due to numerous quartz inclusions elsewhere in thin section suggest staurolite may be unstable. Fibrolitic sillimanite is 

associated with biotite, from which at least part of it may have formed. The proximity of unstable staurolite and apparently newly 

crystallized sillimanite also allow the possibility that staurolite may have been consumed in a sillimanite-producing reaction. Planepolarized light. L, Fibrolitic sillimanite and andalusite growing on opposite ends of the same biotite crystal in sample of figure 5G. 

Note continuation of biotite in matrix into andalusite xenoblast. Fibrolitic sillimanite is present in trace amounts and appears to 

replace biotite. Plane-polarized light.
E12 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

0.5 mm

B

0.5mm 

_J

D

1.0mm 

I

0.5mm 

I

>**<«•»

FIGURE 5.—Continued.
SILLIMANITE GNEISS DOME, YUKON CRYSTALLINE TERRANE, ALASKA E13

G

1.0mm 

I

H

0.5mm

0.1 mm

0.5 mm 

I

1.0mm

FIGURE 5.—Continued.
E14 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Andalusite is generally the largest and most 

abundant aluminum silicate polymorph in the quartzmica schists; it occurs as colorless or faintly pink 

elongate xenoblasts, 1 or 2 cm long, enclosing abundant needlelike biotite flakes that are parallel to foliation. Inclusions of staurolite are present in a few 

andalusite grains. In several thin sections, andalusite 

forms a small mass in the middle of muscovite folia, 

with an irregular contact between the two minerals. 

Andalusite is rarely in direct contact with any of its 

polymorphs, but in the three staurolite-sillimaniteandalusite-bearing schist samples which are found 

within l/2 km of one another just east of the South Fork 

of the Salcha River (fig. 2), a narrow fringe of acicular sillimanite has grown around part of an andalusite porphyroblast (fig. 57), and in another locality 

just a few kilometers away, kyanite occurs within and 

adjacent to an andalusite porphyroblast (fig. 5J).

Kyanite is the next most abundant aluminum silicate; it occurs as small, stubby idioblasts or subidioblasts, less than 2 mm long, which are generally surrounded by or adjacent to biotite or muscovite. In one 

thin section, kyanite has grown in the middle of a 

large crystal of muscovite. Kyanite, which is typically 

elongate in a plane parallel or nearly parallel to the 

foliation, does not deflect micaceous foliation. Only in 

one thin section do kyanite prisms lie at an angle to 

the foliation, and even in this thin section, most of the 

prisms do not.

Fibrolitic sillimanite is present in small or trace 

amounts in most thin sections of pelitic schist. There 

is a general increase in the amount of sillimanite in 

the schist toward the schist-gneiss contact. As in the 

samples from the gneiss body, fibrolitic or finely acicular sillimanite has nucleated adjacent to and within 

biotite grains, generally parallel to foliation. In a few 

samples, minor amounts of sillimanite occur within 

some grains of quartz or feldspar. In one thin section 

(locality 8, fig. 2), needles of sillimanite occur next to a 

small, irregularly shaped bleb of staurolite (fig. 5K). 

These textural relations may indicate staurolite was 

consumed partially in a sillimanite-producing 

reaction.

Several different combinations of the Al2Si05 

polymorphs are commonly only millimeters apart in 

the samples. Andalusite, kyanite, and sillimanite 

occur together in single thin sections from four separate localities (see fig. 2), 6 to 15 km apart in a linear 

zone. The modes of occurrence of the aluminum silicates and the textures and mineralogy in these samples are identical to those described above. Although 

no aluminum silicate polymorphs in any of the four 

thin sections directly touch one another, in one thin 

section (fig. 5L) both sillimanite and andalusite are

present on opposite sides of a biotite grain, and in 

another sample sillimanite and kyanite are also seen 

on opposite sides of a biotite grain. These associations 

suggest that these pairs of aluminum silicates are 

both stable with biotite and probably are also stable 

with one another.

Tourmaline prisms averaging 2 mm in length, 

commonly with zonal structure, are present in many 

pelitic schist samples. Pleochroic colors of gray, 

bluish gray, and olive suggest that the mineral is the 

iron-rich tourmaline, schorlite. Other accessory minerals, in order of decreasing abundance, are zircon, 

apatite, rutile, ilmenite, hematite, and sphene. Small 

amounts of magnetite and graphite may also be present, but all the opaques identified thus far by microprobe have been ilmenite.

GEOTHERMOMETRY BASED ON COEXISTING

GARNET AND BIOTITE IN PELITIC SCHIST AND

SILLIMANITE GNEISS

The distribution of Fe and Mg between garnet 

and biotite provides a way to determine, independent 

of the implications of the distribution of aluminum 

silicate minerals, the temperatures at which the minerals equilibrated in the pelitic schist and sillimanite 

gneiss. Similar geothermometry has been applied 

successfully to many other pelitic rocks (e.g., Ghent 

and others, 1979; Baltatzis, 1979; and Labotka, 1980, 

to list a few of the recent papers). We viewed the geothermometric results in terms of three problems: (1) 

Is there a systematic increase in temperature toward 

the center of the gneiss body which would support the 

hypothesis that it is a gneiss dome? (2) Was the 

temperature in the sillimanite gneiss body high 

enough to produce in situ formation of granitic melt? 

(3) Are equilibrium temperatures indicated for the 

schist samples that contain all three aluminum 

silicates?

Garnet-biotite pairs were analyzed by electron 

microprobe in 11 samples of aluminum-silicatebearing rocks from 9 different localities. Complete 

analyses were made of 29 garnet-biotite pairs; only 

two of the four samples containing all Al2Si05 polymorphs had garnet-biotite pairs with apparent equilibrium textures. Each value represents an average of 

at least six closely spaced points. In all cases but one, 

biotite and garnet were in physical contact with one 

another and were analyzed very near their mutual 

contact.

Representative microprobe analyses of biotite are 

presented in table 3 and of garnet in table 4. Temperatures based on the calibrations of Goldman and Albee 

(1977), Thompson (1976), and Ferry and Spear (1978)
SILLIMANITE GNEISS DOME, YUKON CRYSTALLINE TERRANE, ALASKA 

TABLE 3.—Representative chemical analyses ofbiotite

[Analyses by electron microprobe, using natural and synthetic mineral standards and the Z.A.F. matrix correction 

program FRAME (Yakowitz and others, 1973). C. R. Bacon, analyst]

E15

Sample- number 

(see fig. 16 

for location)

Crystal rim

S *n

A1 2 03 ————— -

M«n

CaO ——————— -

Na20 —————— - 

Kn

MnO ——————— -

r i ____ __ _.

H

nC

Less 0 ————— -

Total—­

Si —— - —— — -

Al _________ .

Fe ——————— -

MJK.

Ca ————— ——

Ma ——————— -

Ti ——————— .

Mn ——————— -

n

F — - ———

Temperature °&

75AWr 

784A

1A

. __ 35 7

- —— 20.4

O1 O.

8 on

- —— .04

• —— .16

9 1 Q

- —— .13

• — - .01

- —— .27

3 QC

1 9

• —— 100.94

- —— 3.603

• —— 2.670

1

QCO

• —— .006

- —— .046

- —— 1.755

- — .017

- —— .003

1 9Q

t —— 549

77AFr 

46 E

1

35 4

20.3

20.0

8.29

.07

.17

8 KQ

1.17

.04

.02

3?

.14

98.07

Mineral

3.660

1 QQ1

.011

.050

1.696

.135

nnc

.005

.155

541

76AFr 

197B

2

36.7

19.8

20.4

8.94

.02

.21

8 79

1.31

.15

.02

oc

3

QC

.11

i nn 90

formulae on

2 CC9

1.994

.003

.061

1.664

.147

.019

.005

540

76AFr 

272C

j*2

34 9

21 3

91 Q

7.72

04

.16

8 73

1.40

.14

.02

.22

.10

100.17

the basis

3.796

2

7C7

1.740

.006

.047

1.684

m Q

.005

.105

601

75AFr 

981

1

?1 3

20.0

9.05

.02

.31

8 QQ

.90

• \J\J

3 Q9

99.85

of 24 an ions

3.777

2.030

.003

.090

1.724

.102

.010

.000

592

76AFr 

241

1

35 3

20 5

7.42

.03

.19

2 QQ

.01

.17

.07

1 nn QQ

3.677

2

C7O

1.659

.005

.055

1 ono

325

.003

noi

655

77AWr 

150D

2

34 9

9n Q

19.2

7.99

.03

.17

9.16

2.41

9Q

.00

25

3 Q9

.11

QQ on

5.302

2.440

1.810

.005

.050

1.775

275

.036

.000

.120

678

76AFr 

4115A

1

35 0

20.1

21.1

7.76

.01

9.74

2.97

.27

.00

.22

3 Q7

.09

101.10

5.269

3

CCC

2

CC7

1.742

.002

.044

1 Q71

.336

.034

.000

.105

721

-^Biotite inclusion in garnet 2.

-b-Total Fe as FeO.

-^O calculated assuming full hydroxyl site occupancy and no ferric iron, with OH equal to 2-F-C1.

-^Equilibration temperature for coexisting biotite and garnet calculated from Ferry and Spear (1978).
E16 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

are presented in table 5. The highest temperature 

garnets tend to have relatively low Ca and high Mn 

values, and the Ti in biotite seems to increase with 

temperature. All of the samples fall within or close to 

the compositional limits of the calibration of the geothermometer given by Ferry and Spear (1978).

Cores and rims of three garnet porphyroblasts 

were analyzed. In the two garnets for which the highest temperatures were calculated (77AWrl50D and 

76AFr4115A, table 4), there was a decrease between 

cores and rims in Fe, Mg, and Ca and an increase in

Mn, whereas for the lower temperature sample 

(77AFr46E), all the trends were reversed except for 

that of Ca. Insufficient data preclude generalization 

about the zoning trends which may be present in the 

garnets. Inclusions in garnet, identified by microprobe, are (in decreasing order of abundance): quartz, 

ilmenite, rutile, and rare pyrite.

Estimated temperatures (table 5) for garnetbiotite pairs (in direct contact and in apparent textural equilibrium) within a single one-inch-diameter polished thin section agreed to better than 20°C in almost

TABLE 4.—Representative chemical analyses of garnet

[Analyses by electron microprobe, using natural and synthetic mineral standards and the Z.A.F. matrix correction program FRAME 

(Yakowitz and others, 1973). C. R. Bacon, analyst]

Sample number 

(see fig. 16 

for location)

Crystal

Si02 ————————

A1 2°3 ———————— 

FeQb— — — — —

MgQ —————————————

CaO —————————

Na2 0 —————————

TiQ2 ————————— 

MnO ——————————

Total ————

Si ——— .... ... —

AT __________ _____ __

Fe

Mg ——————————————

Ca— — -— — —

Na ——————————

Ti ——————————

Mn ——————————

Temperature °CC—

75AWr 

784A

1 rim

T7 Q

- 21 3

- 2.22

- 4.62

.02

no

CO

-101.37

3 OQ/1

- 4.606

525

"JQC

.006

- .011

nQT

549

77AFr 

46 E

1 core

37.3

21.2

1.20

6

"7O

.02

.09

.75

101.78

5.940

OQC

1.147

.006

.011

.101

77AFr 

46 E

1 rim

37.6

21.4

3.99

.02

.10

.33

101.19

Mineral

5.982

4.012

4.710

.557

con

.006

.012

.044

541

76AFr 

197B

2 rim

37.3

21.1

31.7

2.22

.01

.02

5.69

100.77

f ormul ae

5.988

3.992

4.256

.531

.470

. uuo 

.002

.774

540

76AFr 75AFr 

272C 981

12 1 rim

37.5 37.6

21 3 21 2

33.6 34.9

2.28 2.95

4.17 3.39

.02 .01

.06 .05

2 CQ on

101.51 100.40

on the basis of 24

5.964 6.007

3.992 3.991

4.469 4.663

.711 .580

.006 .003

.007 .006

348 041

601 592

76AFr 

241

1 rim

37.1

On Q

31.4

2.51

1.27

.01

.05

7.69

100.83

oxygens

5.979

3.950

4 232

.603

.219

.003

nnC

1.050

655

77AWr 

150D

2 core

•57 p

91 Q

32.0

4.04

1 OC

.01

.05

100.79

5.979

4.064

4.233

953

.315

.003

.006

433

77AWr 

150D

2 rim

37.3

21.4

Tfi Q

2.99

1.31

.01

.03

6.42

100.26

5.986

4 134

.715

.225

.003

.004

Q7T

678

76AFr 

4115A

1 core

37.5

21.4

31.5

3.10

1.42

.01

.02

5

"7O

100.73

5.990

4.029

4.208

7TQ

".243

.003

.002

7Q9

76AFr 

4115A

1 rim

37.6

21.2

31.1

2.94

1.54

.02

.04

6.66

101.10

5.998

3.986

4.149

.699

oco

.006

.005

.900

721

-Garnet analyzed adjacent to biotite inclusion.

-k-Total Fe as FeO.

-Equilibration temperature for coexisting biotite and garnet calculated from Ferry and Spear (1978).
SILLIMANITE GNEISS DOME, YUKON CRYSTALLINE TERRANE, ALASKA E17

all determinations and to within 10°C in over half of 

the determinations. No reliable temperature estimates were obtained for the two samples (76AFr272A 

and 77AWr387C) that contain all three aluminum silicate polymorphs, because the garnet-biotite pairs 

analyzed were not totally fresh and appeared not to be 

in equilibrium with each other and the surrounding 

phases. The textural and mineralogical similarity 

between these two andalusite-kyanite-sillimanitebearing samples and the five other samples of pelitic 

schist (three of which contain more than one Al2Si05 

polymorph) for which temperature data were consistent (table 5) suggests that equilibrium conditions in 

which andalusite, kyanite, and sillimanite are stable 

together may indeed have existed locally.

Agreement between garnet-biotite temperatures 

calculated from Thompson's and Ferry and Spear's 

calibration is generally good, agreeing to within 15°C 

for over half the determinations and to within 30°C 

for all but a few determinations. Agreement is not as 

good, however, between the former two calibrations 

and that of Goldman and Albee; most temperatures 

calculated to the latter's calibration are lower by 

about 30°C to 80°C. In general, there is better agreement between Goldman and Albee's calibration and 

the other two calibrations (table 5) in pelitic schist 

than there is in the higher temperature sillimanite 

gneiss. Ghent and others (1979) reported similar good 

agreement between temperatures calculated from the 

calibrations of Thompson (1976) and Ferry and Spear

(1978) and poor agreement between those two calibrations and that of Goldman and Albee (1977). Goldman 

and Albee (1977) suggest that the source of a possible 

problem with their calibration might be inaccuracies 

in the experimental calibration of oxygen-isotope 

fractionation (with which their calibration is correlated) at higher temperatures. Ghent and others

(1979) argue that the poor agreement could also be 

due to cessation of 18 0/160 exchange at lower temperatures than those permitting Fe/Mg exchange.

The temperatures reported here for garnetbiotite equilibration are based on an assumed pressure of 0.4 GPa, but because exchange equilibria are 

virtually independent of variation in pressure, the 

temperatures change by only about 5° per GPa for 

these samples. Estimates of the analytical precision of 

our data, based entirely on microprobe analytical 

error, suggest that an uncertainty of ±30° is very conservative for relative temperatures in this study. 

Absolute temperatures, however, may be subject to 

greater uncertainty. The rounded averages of 

temperature data for garnet-biotite pairs considered 

to be in equilibrium are given for the three calibrations in separate columns in table 5 headed "Averaged

G-A, T, and F-S." In order to simplify discussion, and 

because the averaged temperature data for the Ferry 

and Spear and the Thompson calibrations differ very 

little in their absolute values, the averaged temperatures calculated from Ferry and Spear's calibration 

are referred to in the text and their areal distribution 

is shown in figure 6.

The highest temperature (705°±30°C) was determined for a sample of typical, fine-grained, "salt 

and pepper" sillimanite gneiss from the center of the 

body (76AFr4115A, fig. 6, tables 2-4). Samples of similar gneiss to the west and south yielded slightly lower 

temperatures of 655° and 670°±30°C. Equilibration 

temperatures reported for samples of garnet-staurolite-aluminum-silicate-bearing schist (535° to 600°± 

30°C) are appreciably lower than the values determined for samples of sillimanite gneiss.

DISCUSSION

METAMORPHIC REACTIONS AND ISOGRADS IN PELITIC 

SCHIST AND SILLIMANITE GNEISS

STAUROLITE+BIOTITE ISOGRAD

The northernmost isograd shown in figure 2 

marks the first appearance of staurolite with biotite. 

North of the staurolite+biotite isograd, rocks are 

lower in grade (epidote-amphibolite facies, Thompson 

and Norton, 1968), consisting of muscovite+chlorite+ 

quartz±biotite ±plagioclase±garnet assemblages. 

Because chlorite is common north (downgrade) of this 

isograd and is rare south of it (see table 1), the 

staurolite-producing reaction which defines this isograd is believed to be one in which staurolite forms at 

the expense of chlorite, most likely by the reactions: 

chlorite+muscovite —• staurolite+

biotite+quartz+tbO (1) 

(modified from equation 26, Thompson and Norton, 

1968), or

chlorite+muscovite+almandine — staurolite+

biotite+quartz+H20 (2) 

(equation 22, Thompson and Norton, 1968).

The first appearance of staurolite with biotite 

coincides with the first appearances of kyanite, andalusite, and sillimanite. Pelitic schist that occurs upgrade of this isograd will be referred to as being in 

the staurolite-aluminum silicate zone. Within this 

zone, andalusite appears to have developed at the 

expense of muscovite according to the reaction: 

chlorite+muscovite+quartz — andalusite

(or kyanite)+biotite+H20 (3) 

(modified from equation 16, Thompson and Norton, 

1968). The presence of needlelike grains of biotite
TABLE 5.—Distribution coefficients for the Fe/Mg ratio in coexisting garnet and biotite and equilibration temperatures (in deg

calculated at 0.4-GPa total pressure 

[KD calculated from microprobe analyses of garnet and biotite adjacent to mutual contacts. Temperatures calculated from Goldman a

Thompson (1976) (T), and Ferry and Spear (1978) (F-S). Averaged (Avg) temperatures are rounded averages of temperature data f

considered to be in equilibrium. Uncertainty in temperatures due to analytical error estimated to be +30°C. Key minerals, lis

of abundance: mu, muscovite; st, staurolite; a, andalusite; ky, kyanite; sil, sillimanite; trace amounts indicated by parenth

Sample number 

(see fig. 16 

for location)

77AWr387C

75AWr784A

77AFr46A

77AFr46E

76AFrl97B

76AFr272A

76AFr272C

Crystal number

Key minerals Rock type Garnet

mu, st, ky, Schist 1

a, sil 3

mu, st -do.- 1

1

2

3

mu, a, st -do.- 1

2

2

mu, a ky -do.- 1

st 2

mu, st (sil) -do.- 1

1A

2

mu, a, st, -do.- 1

sil, ky 2

m, a, sil -do.- 1

2

2

Biotite

1

3

-b-l

1A

2

3

1

2

2A

1

2

1

1A

2

1

2

1

12

2A

XMn

0.006

.013

.014

.014

.023

.105

.013

.083

.038

.007

.035

.125

.121

.128

.033

.074

.064

.057

.029

XCa

0.157

.208

.131

.131

.141

.121

.062

.082

.056

.114

110

.078

.072

.078

.076

.075

.092

.117

.127

Garnet

XFe

0.530

.522

.587

.590

.571

.575

.597

.583

.545

.575

.557

.559

.560

.561

.645

.637

.603

.613

.618

Temperat

XTi

0.023

.020

.029

.033

.036

.035

.030

.025

.013

.023

.020

.017

.041

.025

.031

.040

.029

.027

.025

XAI VI

0.203

.212

.194

.193

.191

.195

.228

.212

.207

.216

.191

.218

.190

.197

.185

.193

.206

.207

.193

KD

0.273

.102

.240

.164

.177

.151

.244

.193

.184

.160

.155

.158

.169

.160

.326

.203

.304

.192

.249

G-A

1771

1479

1642

520

1554

527

1607

1572

1543

519

514

541

-^536

534

1689

1545

1736

568

1644

Avg G-A T

1718

1464

1675

525 568

1586

548

1681

1611

1598

520 562

554

540 559

-^583

562

1784

1625

1757

570 609

1687
SILLIMANITE GNEISS DOME, YUKON CRYSTALLINE TERRANE, ALASKA E19

s

«of .aj1 of

within andalusite xenoblasts (fig. 5L) may be due to 

the fact that both these phases are products of reaction 3. Alternatively, these biotite grains, which are 

parallel to and continuous with adjacent folia of biotite and muscovite, may be unreacted remnants of 

biotite that was formerly interleaved with the muscovite that reacted to form andalusite.

Textural evidence for the paragenesis of kyanite 

in the staurolite-aluminum silicate zone consists only 

of the one thin section in which kyanite is surrounded 

by muscovite and which suggests that the kyanite 

may have formed from muscovite, possibly also by 

reaction 3. Staurolite appears to be stable with kyanite and no textural evidence of its involvement in a 

kyanite-producing reaction was observed.

Rare textural evidence for the development of 

fibrous sillimanite in pelitic schist (fig. 2) within this 

same zone suggests two possible sillimanite-producing 

reactions that may have occurred locally. The tiny 

needles of sillimanite that form a fringe around small 

andalusite porphyroblasts, mentioned earlier and 

shown in figure 57, may indicate that some sillimanite 

formed by a polymorphic transition from andalusite. 

Textural evidence for the development of sillimanite 

at the expense of staurolite (see fig. 5K) in a garnetstaurolite-andalusite-kyanite-sillimanite-bearing

Schist i

(Paleozoic and (or) Free

Sillimanite gneiss

(Paleozoic and (or) Precambrian)

FIGURE 6.—Simplified geologic map of pelitic schist and sillimanite gneiss showing location of samples for which garnetbiotite pairs were analyzed and rounded averages of equilibration temperatures calculated from Ferry and Spear 

(1978) (table 5).
E20 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

schist (locality 8, fig. 2) suggests the reaction

staurolite+muscovite+quartz — sillimanite+

almandine+ biotite+H20 (4) 

(equation 29, Thompson and Norton, 1968) may have 

occurred locally in the staurolite-aluminum silicate 

zone. However, because garnet+biotite+staurolite, 

garnet+biotite+staurolite+sillimanite, and staurolite+ 

biotite+sillimanite all coexist with quartz and muscovite (and, in some samples, either andalusite or kyanite or both, shown in fig. 2), it is unlikely that this 

reaction can be used to explain all the occurrences of 

sillimanite in the pelitic schist. The development of 

sillimanite within the staurolite-aluminum silicate 

zone is not clearly the result of any one reaction.

With the exception of the one area east of the 

South Fork of the Salcha River where a fringe of sillimanite on andalusite may indicate a polymorphic 

transition, all three Al2Si05 modifications appear to 

be stable in the pelitic schist within the staurolitealuminum silicate zone. Evidence for metamorphic 

conditions being at or near the triple point in this zone 

consists of: (1) the close spatial association of andalusite, kyanite, and sillimanite in the 4 samples in which 

they occur together; (2) the occurrence of sillimanite 

and andalusite and sillimanite and kyanite in contact 

with the same small intervening biotite crystals; (3) 

the numerous rocks in which combinations of aluminum silicates occur together (of the 10 samples that 

contain kyanite, all but 2 also have andalusite, and 4 

of the 10 have sillimanite); and (4) the absence of any 

obvious disequilibrium textural relations between 

polymorphs (with the one exception noted above). The 

close association between kyanite and andalusite, in 

which kyanite occurs within and adjacent to a large 

andalusite grain (fig. 5J), suggests that these two 

minerals formed during the same metamorphic 

event. This relation, as well as the presence of a fringe 

of sillimanite on a few andalusite crystals (see fig. 5J), 

also suggests that the order of crystallization of aluminum silicates was "normal" with rising temperature 

at pressures at or slightly below the triple point; kyanite formed first, followed by andalusite and finally 

by sillimanite. Textural evidence does not appear to 

indicate a second, lower pressure andalusite-producing event separate from a higher pressure or lower 

temperature kyanite-producing metamorphic event. 

Although the discrepancy in geothermometric data 

from two samples containing all three Al2Si05 polymorphs might suggest possible complexities in thermal history, the good agreement between garnetbiotite equilibration temperatures in the other schist 

samples in this zone (see table 4 and fig. 6), together 

with the textural data cited above, argues for equilibrium conditions.

STAUROLITE-OUT ISOGRAD

Whereas the development of sillimanite at the 

expense of staurolite is not considered to have been 

significant in pelitic schist within the staurolitealuminum silicate zone, this reaction was probably 

very important in the sillimanite gneiss and pelitic 

schist south of this zone. Staurolite is absent from all 

samples of sillimanite gneiss and pelitic schist just 

north of the gneiss, and sillimanite+biotite+garnet is a 

common assemblage. These mineralogic data have 

been used to map an isograd, shown on figure 2, on the 

basis of the disappearance of the staurolite as it is 

consumed in the sillimanite-producing reaction 4. 

This isograd defines the north boundary of a zone in 

which sillimanite and garnet are the key minerals, 

and for that reason the rocks upgrade of this isograd 

are here referred to as the sillimanite-garnet zone. 

The staurolite-out isograd outlines the north and east 

margin of the proposed gneiss dome.

The common association of sillimanite with biotite in gneiss (fig. 5C) and schist in the sillimanitegarnet zone suggests that some sillimanite formed at 

the expense of biotite, as has been proposed by several 

authors in their studies of other areas (Hietanen, 

1956, p. 23; Tozer, 1955, p. 313; Woodland, 1963, p. 

359, 367; Pitcher and Read, 1963, p. 267, 278). This 

textural relation could also be explained by a reaction 

of which biotite is a product, along with sillimanite, 

rather than a reactant. Thompson and Norton (1968, 

p. 323) cite reaction 4, discussed above, as one of the 

reactions which very probably "are responsible for 

much of the common association of fibrolitic sillimanite with muscovite and biotite."

The absence of staurolite from all samples of 

pelitic schist and gneiss in the sillimanite-garnet zone 

is good evidence that reaction 4 accounts for much of 

the sillimanite present in these rocks. There is no textural evidence in the gneiss of this zone of sillimanite 

having formed from andalusite, but the occurrence of 

fringes of sillimanite around grains of andalusite in 

two equigranular, granitic-appearing rocks may 

imply that this reaction also took place in the gneiss.

The consistent decrease in muscovite, accompanied by an increase in potassium feldspar in the 

sillimanite-garnet zone toward the center of the sillimanite gneiss body (fig. 2; table 2), together with the 

high equilibration temperatures determined for 

samples of sillimanite gneiss (fig. 6; table 5), indicates 

that some sillimanite must have formed by the breakdown of muscovite in which:

muscovite+quartz — K-feldspar+

sillimanite+H20 (5) 

(reaction 3, Evans, 1965). Because trace amounts of
SILLIMANITE GNEISS DOME, YUKON CRYSTALLINE TERRANE, ALASKA E21

muscovite are present in most samples of sillimanite 

gneiss, the breakdown of muscovite appears to be 

incomplete in most of the rocks, and it has not been 

possible to map an isograd defined by this reaction.

SILLIMANITE-ANDALUSITE ISOGRAD

The southernmost isograd (fig. 2), which outlines 

the southwest margin of the proposed dome, has been 

drawn to pass through two localities in which sillimanite and andalusite coexist in pelitic schist south of 

the dome, and between two localities, in one of which 

sillimanite occurs in biotite gneiss north of the isograd and in the other of which andalusite occurs in 

biotite gneiss south of it. If sillimanite and andalusite 

are in equilibrium in the rocks in which they both 

occur, then this isograd may define the area in which 

the physical conditions of metamorphism were such 

that the univariant polymorphic reaction andalusite-* 

sillimanite may have occurred.

PROPOSED ORIGIN OF EQUIGRANULAR ALUMINUM 

SILICATE-BEARING ROCKS

The best explanation of the textural and mineralogical features of the equigranular rocks previously 

described is that they resulted from synmetamorphic 

intrusion of small granitic bodies or dikes. Local plutonism may have occurred as the paragneiss was 

being metamorphosed under the temperature and 

pressure conditions of the andalusite field. The 

andalusite and possibly also the garnet grains in these 

equigranular rocks may represent xenocrysts that 

were incorporated in a granitic melt that crystallized 

before the polymorphic transition from andalusite to 

sillimanite took place in the surrounding paragneiss.

Evidence for a plutonic origin of at least some of 

the equigranular rocks consists of the oscillatory zoning in plagioclase (fig. 5F) observed in several samples, the fact that equigranular rocks crosscut sillimanite gneiss at two localities, and, perhaps, the more 

perthitic nature of the potassium feldspar relative to 

sillimanite gneiss in most samples and the presence of 

myrmekite in several samples.

Evidence that deformation and prograde metamorphism continued subsequent to intrusion of the 

equigranular rocks consists of the development of 

crude mineral alinement, incipient cataclastic foliation, and development of some of the sillimanite needles from biotite or andalusite or both (fig. 5E). As 

andalusite was not found in any samples of sillimanite 

gneiss in the sillimanite-garnet zone in which the 

equigranular rocks occur, it seems reasonable to conclude that the andalusite grains, presumed to be

xenocrysts, were metastable under sillimanite-grade 

conditions. The halos of muscovite that rim several 

grains of andalusite and garnet in two samples could 

be interpreted as evidence of the instability of andalusite and garnet in the granitic melt. It is not clear 

whether the granitic melt in any of the areas where 

equigranular rocks were observed was derived in situ 

from partial melting of the gneiss or whether the 

magma was derived at greater depth and injected 

synmetamorphically. Garnet-biotite equilibration 

temperatures for the gneiss are high enough to be 

compatible with partial melting, but the inclusion of 

andalusite xenocrysts in some of the samples suggests 

that the granitic rocks were intruded before the peak 

of metamorphism.

PHYSICAL CONDITIONS OF METAMORPHISM

The mineral assemblages in the pelitic schist and 

gneiss described in this study are considered to have 

been produced by a single metamorphic episode, 

although prior low-grade metamorphic events are not 

precluded. The only suggestion of prior metamorphic 

events in the thin sections studied is the partial 

replacement of garnet by biotite, quartz, muscovite, 

and feldspar seen in a few samples. This texture 

might be considered an indication of two prior metamorphic events: one in which garnet was formed, and 

a subsequent retrograde event in which garnet was 

replaced. However, because there appears to be no 

other textural evidence for such previous events and 

because unreacted and partially replaced garnets 

occur along with the relict garnets in the same thin 

section, it seems more likely that this texture resulted 

from local late-stage retrograde metamorphism. The 

rare examples of coarse mica flakes lying at an angle 

to foliation may have grown during the last stages of 

metamorphism, after major deformation of the rocks.

Experimental phase equilibria pertinent to this 

study are shown in figure 7. These equilibria, 

together with the temperatures calculated from the 

relative distribution of Fe and Mg between coexisting 

garnet and biotite (table 5), have been used to estimate the range of temperature and pressure conditions that accompanied metamorphism of the pelitic 

schist and sillimanite gneiss (shown as a stippled 

pattern in figure 7).

Temperatures of 535° to 600°±30°C obtained 

from coexisting garnet and biotite in pelitic schist in 

the staurolite-aluminum silicate zone are consistent 

with the fact that muscovite and quartz coexist in 

these samples, whereas K-feldspar is rare (see table 

2), and there is no evidence of partial melting of the 

schist. Staurolite is also present in many of these
E22 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

samples (table 5). Petrologic data from natural assemblages (Hietanen, 1973; Carmichael, 1978, among 

others) indicate that the triple point is within the stability field of staurolite in pelitic schist; that conclusion is consistent with our interpretation that andalusite, kyanite, and sillimanite are in equilibrium 

within the staurolite-aluminum silicate zone. Experimental determination of the upper stability limit of 

staurolite+quartz (e.g., Richardson, 1968) does not 

agree with many of the petrologic data reported in the 

literature and that experimentally determined limit 

has been shown to be located at implausibly high 

temperatures (see Thompson, 1976; Carmichael, 1978; 

Ghent and others, 1979; Hutcheon, 1979; and Ferry, 

1980 for discussion of the problems of staurolite equilibrium). For this reason, the experimentally determined staurolite stability field is not shown in figure 

7. The temperature range of 535° to 600°±30°C is 

based on the rounded averages of temperatures calculated according to Ferry and Spear's calibration 

(1978). However, use of the equilibration temperatures for pelitic schist calculated from Goldman and 

Albee's calibration (1977) of 520° to 570°+30°C (table 

4) results in better agreement between the estimated 

metamorphic temperatures in the schist and the 

triple-point temperature of about 500°C (Holdaway, 

1971).

0.9 ,-

± 0.5 -

0.1

300 700 800

TEMPERATURE, IN DEGREES CELSIUS

FIGURE 7.—Phase equilibria pertaining to study area. A^SiOs triple point (Holdaway, 1971); breakdown of muscovite (mu)+ 

quartz (qz) (Chatterjee and Johannes, 1974); melting interval 

of muscovite granite with excess H20 (Wyllie, 1977). Best estimate of final pressure and temperature of mineral equilibration in pelitic schist and sillimanite (sil) gneiss shown by 

stippled pattern. Andalusite (a); k-feldspar (kf).

Pressure during metamorphism of the schist in 

the staurolite-aluminum silicate zone would be around 

0.36 GPa, on the basis of Holdaway's triple point 

(1971). The absence of kyanite from pelitic schist in 

the andalusite-sillimanite zone southwest of the proposed dome suggests that pressure conditions may 

have been slightly less or temperatures slightly 

greater in this area, relative to conditions in the 

staurolite-aluminum silicate zone north and east of 

the gneiss body. The disappearance of andalusite 

northeast of the andalusite-sillimanite isograd appears to indicate an increase in pressure and (or) 

temperature conditions northeastward toward the 

gneiss body, similar to an increase in metamorphic 

grade toward the gneiss that is documented in pelitic 

schist north and east of the sillimanite gneiss.

During metamorphism of the sillimanite gneiss, 

temperature conditions increased toward the center 

of the body from 655°±30°C to 705°±30°C. The scarcity of muscovite near the center of the gneiss dome 

and the ragged appearance of the muscovite, suggestive of instability in rocks in which potassium feldspar is also present, agree with an estimated maximum temperature limit near and probably above that 

at which the breakdown of muscovite+quartz occurs 

(Chatterjee and Johannes, 1974). The temperature 

and mineral-assemblage data, combined with experimental data (fig. 7) and the migmatitic appearance 

of the gneiss at several localities, suggest that some in 

situ melting of the gneiss may have occurred. Pegmatite and felsic dikes, which are common locally in 

the gneiss body, are also possible evidence of partial 

melting.

The pressure during metamorphism of the gneiss 

body is more difficult to estimate. An upper pressure 

limit of about 0.8 GPa is required because of the 

absence of kyanite in all the assemblages. A more 

reasonable pressure estimate is provided by the combination of the assemblages cordierite+biotite+ 

Al2Si05 and garnet+biotite+Al2Si05, which, according to Hess (1969), suggests a range of 0.3 GPa (at 

590°C) to 0.6 GPa (at 710°C). Stability relations of 

Fe-Mg cordierite with potassium feldspar determined for conditions of muscovite-quartz instability 

in high-grade pelitic rocks (Holdaway and Lee, 1977) 

indicate that a pressure range of about 0.2 to 0.5 GPa 

(depending on the distribution of Fe and Mg between 

cordierite and biotite and the estimated PH?O) cor" 

responds with the temperature range determined for 

the sillimanite gneiss.

The mineral assemblages that consist of several 

combinations of the aluminum silicates, staurolite 

and garnet in the quartz-mica schist are characteristic of the Idahoan-type metamorphic facies series de-
SILLIMANITE GNEISS DOME, YUKON CRYSTALLINE TERRANE, ALASKA E23

scribed by Hietanen (1967) and the low-pressure 

intermediate group facies series of Miyashiro (1961). 

The assemblages sillimanite+orthoclase±muscovite± 

garnet and sillimanite+cordierite+orthoclase+muscovite in the gneiss are also compatible with those facies 

series.

CONCLUSIONS

The possibility that the sillimanite gneiss body is 

a gneiss dome was previously postulated (Foster and 

others, 1977b) on the basis of mapped relations, such 

as the distribution of high-grade metamorphic rocks. 

The petrography and geothermometry presented in 

this paper support the gneiss dome hypothesis. Evidence that the large body of sillimanite gneiss and 

schist (between the staurolite-out and andalusitesillimanite isograds) is a gneiss dome consists of the 

following: (1) the topographic expression of a circular 

area with a radial drainage pattern, (2) the general 

outward dip of foliation (see fig. 2), (3) the increase in 

metamorphic grade from a staurolite-aluminum silicate zone on the north and east and possibly an andalusite sillimanite zone on the southwest to a sillimanite-garnet zone (fig. 2) and the breakdown of 

muscovite+quartz in the central part of the body, and 

(4) the increase in equilibration temperatures from 

those for pelitic schist flanking the proposed dome 

(535° to 600°±30°C) to those for sillimanite gneiss in 

its core (705°±30°C).

The distribution of key minerals and of equilibration temperatures, calculated on the basis of the relative distribution of Fe and Mg between biotite and 

garnet in pelitic schist and sillimanite gneiss, indicates an increase in metamorphic temperature from 

the flanks of the proposed gneiss dome to its core. 

Pelitic schist that flanks the gneiss on the north and 

east equilibrated at temperatures in the range of 

about 500° to 630°C. Locally, metamorphic conditions 

were probably close to those under which all three 

aluminum silicates are stable. Rocks composing the 

gneiss dome equilibrated at temperatures in the 

range of about 650° to possibly as much as 730°C. 

Partial melting of the pelitic rocks in the center of the 

dome may have occurred locally. Synmetamorphic 

intrusion of equigranular aluminum-silicate-bearing 

rocks probably took place while metamorphic conditions were in the temperature-pressure field for the 

formation of andalusite as the stable aluminum silicate polymorph.

Throughout much of the gneiss dome, the sillimanite gneiss is not truly migmatitic; this, on the 

basis of comparison with some gneiss domes in the 

Shuswap Metamorphic Complex of British Columbia

(Reesor, 1970), could imply that deep structural levels 

of the gneiss dome core are not exposed. The apparent 

gradual transition from sillimanite gneiss to pelitic 

schist on the north and east also suggests that the 

sillimanite gneiss represents a fairly high structural 

level of the dome and may be composed wholly or in 

part of mantling rocks.

The relationship of the greenschist facies rocks to 

the pelitic schist that flanks the sillimanite gneiss on 

the north and east is probably that of a thrust fault. 

North of the dome, pelitic schist is separated from 

greenschist facies rocks by a zone of cataclastic rocks, 

and regional relations suggest that the thrusting is 

probably not directly related to the rise of the gneiss 

dome.

Other gneiss domes may be present in the Yukon 

crystalline terrane. For instance, an unstudied area 

about 60 km northeast of the sillimanite gneiss dome, 

of topographically high amphibolite facies gneiss and 

schist, may have a domal structure. At present, the 

sillimanite gneiss dome cannot be related definitely to 

structural trends or thermal patterns in the Yukon 

crystalline terrane. However, the fact that the gneiss 

dome occurs on line with the general trend of augen 

gneiss batholiths in east-central Alaska and Yukon 

Territory suggests the possibility that metamorphism 

of the gneiss dome and intrusion of augen gneiss 

occurred in response to the same set of thermal and 

tectonic conditions. This genetic relationship is also 

suggested by U-Pb isotopic studies that indicate a 

Mississippian intrusive age for the augen gneiss 

batholith southeast of the gneiss dome (Aleinikoff and 

others, 1981a) and a possible, albeit poorly constrained, Mississippian metamorphic age for the 

gneiss dome (Aleinikoff and others, 1983a). Further 

study of the para-and ortho-gneisses in the Yukon 

crystalline terrane is necessary in order to evaluate 

the significance of the general trend defined by the 

augen gneiss batholiths and to better understand the 

structural and thermal history of this terrane.

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