Petrologic Characterization of Pelitic Schists 

in the Western Metamorphic Belt, 

Coast Plutonic-Metamorphic Complex, 

Near Juneau, Southeastern Alaska 
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Petrologic Characterization of Pelitic Schists 

in the Western Metamorphic Belt, 

Coast Plutonic-Metamorphic Complex, 

Near Juneau, Southeastern Alaska 

By Glen R. Himmelberg, David A. Brew, and Arthur B. Ford 

U.S. GEOLOGICAL SURVEY BULLETIN 2074 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1994 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Robert M. Hirsch, Acting Director 

For sale by 

USGS Map Distribution 

Box 25286 MS306 

Denver Federal Center 

Denver, CO 80225 

Any use of trade, product, or firm names in this publication is for descriptive purposes 

only and does not imply endorsement by the U.S. Government. 

Text and illustrations edited by James W. Hendley II 

Library of Congress Cataloging-in-Publication Data 

Himmelberg, Glen R. 

Petrologic characterization of pelitic schists in the western metamorphic belt, coast plutonic-metamorphic complex, near Juneau, southeastern Alaska I by Glen R. Himmelberg, David A. Brew, and Arthur B. 

Ford. 

p. em.- (U.S. Geological Survey bulletin; 2074) 

Includes bibliographical references. 

1. Schists-Alaska-Juneau Region. 2. Geology-Alaska-Juneau Region. I. Brew, David A. II. 

Ford, Arthur B. (Arthur Barnes), 1932- Ill. Title. IV. Series. 

QE75.B9 no. 2074 

[QE475.S3] 

557.3 s-dc20 

[552'.4] 

94-293 

CIP 
CONTENTS 

Abstract .................................................................................................................................... 1 

Introduction ..................................................................................................................... ·.···.... 1 

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

Western Metamorphic Belt..................................................................................................... 2 

Structural Relations................................................................................................................. 5 

Age Relations .............................................................................................................. · · · · · · · .. · · · 5 

Metamorphic Zones and Isograds . . ................ ..... .. . .......... ...... . ......... ....................... ............... 6 

Nature and Development of the Inverted Metamorphic Gradient .......................... 6 

Mineral Assemblages and Model Reactions at Isograds ............................. ............ 7 

Mineral Chemistry . . ............ ................. ......................... ........ .......... ........... .......... ............... .... 8 

Estimation of Fluid Composition ......................................................................................... 14 

Summary ................................................................................................................................ 15 

References Cited . . ............ ........ .......... ................ .... ... . ......... ..... . . . . ............ ...... ........ ...... . . . . ..... 16 

FIGURES 

1. Index map showing Coast plutonic-metamorphic complex ............. ....... ........ ........................... ....... ... . .. ..... .................. 2 

2. Map of Juneau area (parts of the Juneau A1, A2, B1, and B2 quadrangles) ............................................................... 4 

3. Schematic Al20 3-Fe0-Mg0 projections ....................................................................................................................... 12 

4. Petrogenetic grid for the Si02-Al20 3-FeO-MgO-K20-H20 system ........................................................................... 12 

TABLES 

1. Approximate time relations of plutonic, deformational, and metamorphic events...................................................... 3 

2. Observed mineral assemblages in the Juneau area ......................................................................................................... 8 

3. Mineral associations and localities of samples of pelitic and semi-pelitic rocks ........................................................ 9 

4. Representative analyses of chlorite from the Juneau area ........................................................................................... 13 

5. Representative analyses of plagioclase from the Juneau area ..................................................................................... 13 

6. Representative analyses of muscovite from the Juneau area ....................................................................................... 13 

7. Representative analyses of staurolite from the Juneau area ........................................................................................ 14 

8. Representative analyses of biotite from the Juneau area ............................................................................................. 15 

9. Representative analyses of garnet from the Juneau area ............................................................................................. 16 

III 

Petrologic Characterization of I, eli tic Schists in the 

Western Metamorphic Belt, Coast ]Plutonic-Metamorphic 

Complex, Near Juneau, Southeastern Alaska 

By Glen R. Himmel berg, David A. Brew and Arthur B. Ford 

ABSTRACT 

The metamorphic rocks exposed near Juneau, Alaska, 

are part of the western metamorphic belt of the Coast plutonic-metamorphic complex of western Canada and 

southeastern Alaska. This belt underwent a complex history of deformation, metamorphism, and plutonism that 

ranges in age from about 120 to 50 Ma as a result of tectonic overlap and (or) compressional thickening of crustal 

rocks during collision of the Alexander and Stikinia terranes. Most of the schists near Juneau were metamorphosed during the M5 metamorphic event about 70 m.y. 

ago and are part of a belt of schists of similar metamorphic age that extends for at least 400 km to the southeast. 

Distribution of pelitic mineral isograds, systematic 

changes in mineral assemblages, and silicate 

geothermometry of the Juneau schists indicate an inverted 

metamorphic gradient over a distance of about 5 km. 

Changes in mineral assemblages across isograds separating the metamorphic zones are in general agreement with 

discontinuous reactions in the ideal Si02-Al20rK20-

FeO-MgO-H20 system. Biotite and staurolite in low variance assemblages show changes in Mg/(Mg+Fe) ratios 

with metamorphic grade that are generally consistent with 

predicted changes in the ideal system, although exceptions exist. The observed metamorphic mineral assemblages and previously determined peak temperatures and 

pressures (530°C for the garnet zone to about 705°C for 

the upper kyanite-biotite zone, and 9 to 11 kbar) are consistent with recently published petrogenetic grids for 

pelitic rocks. Maximum mole fraction of H20 as constrained by distribution of species in the C-0-H system 

for graphitic schists ranges from about 0.93 for the garnet zone to about 0.90 for the upper kyanite-biotite zone. 

Metamorphic mineral growth was synchronous with the 

third of four recognized folding events. 

Manuscript approved for publication, October 14, 1993. 

INTRODUCTION 

The metamorphic rocks exposed near Juneau, Alaska 

are part of the western metamorphic belt of the informally 

named Coast plutonic-metamorphic complex (Brew and 

Ford, 1984) of western Canada and southeastern Alaska (fig. 

1). The western metamorphic belt and associated plutons developed as a result of tectonic overlap and (or) compressional thickening of crustal rocks during the collision of two 

large allochthonous terranes-the Alexander terrane on the 

west and tht~ Stikinia terrane to the east (Monger and others, 

1982; Brew and Ford, 1983). The western metamorphic belt 

is one of the major metamorphic features of southeastern 

Alaska (Brew and others, 1992). 

The western metamorphic belt ranges from a few kilometers to several tens of kilometers wide and is 

characterized in general by Barrovian metamorphism. 

The metamorphic grade increases to the northeast, with 

the highest grade adjacent to the plutonic part of the 

Coast plutonic-metamorphic complex. At the latitude of 

Juneau the western metamorphic belt is about 30 km 

wide. The mineral isograds, systematic changes in mineral 

assemblages, and structural relations indicate an inverted 

metamorphic gradient along the easternmost part of the 

belt, where the metamorphic grade goes abruptly from 

pumpellyite-actinolite facies to kyanite- and sillimanitebearing amphibolite facies in less than 5 km across strike. 

The development of the inverted metamorphic sequence 

and the geothermometry and geobarometry associated with 

its formation was recently discussed by Himmelberg and 

others (1991). This report extends that work to include more 

detailed data on mineral assemblages, metamorphic reactions, structural relations, age constraints, and mineral chemistry that were not included in the earlier paper. These data 

and discussions provide a more complete characterization 

of the western metamorphic belt in the Juneau area and, 

along with the earlier paper, will serve as a foundation to 

continually evaluate the evolution of western metamorphic 

belt as other data are obtained. 
2 PETROLOGIC CHARACTERIZATION OF PELITIC SCHISTS, NEAR JUNEAU, ALASKA 

ACKNOWLEDGMENTS 

We thank J.H. Dover and R.W. Tabor for helpful reviews of this manuscript and Dan Kremser for his help in 

using the electron microprobe at Washington University. 

WESTERN METAMORPHIC BELT 

The western metamorphic belt underwent a complex 

history of deformation, metamorphism, and plutonism that 

ranges in age from about 120 Ma to about 50 Ma (Crawford 

and others, 1987; Brew and others, 1989). The belt varies 

along and across strike in protolith and metamorphic grade. 

The belt also varies along strike in the nature of its contact 

with the dominant granitic part of the Coast plutonicmetamorphic complex. Although we cannot discuss the topic 

0 150 KILOMETERS 

fully here, our analysis of the regional variations indicates 

that the western metamorphic belt near Juneau contains all 

of the belt's subparts, whereas most other areas do not. The 

Juneau area is, therefore, considered to be representative of 

the whole regional belt. 

The protoliths for the western metamorphic belt were 

mainly the heterogeneous rocks of the Alexander terrane 

of Permian and Triassic age and the flysch and volcanic 

rocks of the Gravina overlap assemblage (Berg and others, 1972) of Late Jurassic through early Late Cretaceous 

age. In the Juneau area, intermixed pelitic and semipelitic 

metasedimentary rocks and mafic metavolcanic and 

intrusive rocks are dominant. Impure calcareous metasedimentary rocks, quartzite, and quartz diorite and granodioritic orthogneiss are also present. 

The regional history of the Coast plutonic-metamorphic complex was comprehensively summarized by Brew 

EXPLANATION 

Chugach terrane 

D Wrangellia 

cs.:J Alexander terrane 

~ Western metamorphic belt 

Great tonalite sill 

Coast plutonic-metamorphic 

~ complex (Yukon prong, Nisling, 

+ and Stikine terranes) 

..-......_ Coast Range Megalineament 

Figure 1. Index map showing Coast plutonic-metamorphic complex of Brew and Ford 

(1984), western metamorphic belt, and terrane boundaries, southeastern Alaska (modified 

from Himmelberg and others, 1991 ). 
WESTERN METAMORPHIC BELT 3 

Table 1. Approximate time relations of plutonic, 

deformational, and metamorphic events in the western 

metamorphic belt of the Coast plutonic-metamorphic complex 

(from Brew and others, 1989). 

Tune (Ma) 

Type 120 110 100 90 80 70 60 50 

Plutonic events Pl P2 P3 P4 P5 P6 

Deformational Dl 02 03 06? 

events. 04 04? 

05 05? 

Metamorphic Ml M2 M3 M4 M5 M5? M6 

events. M4' 

and others ( 1989) in terms of discrete deformational, 

metamorphic, and plutonic events (table 1 ). The earliest 

documented metamorphic event, M 1, occurred prior to 

110 Ma and was dynamothermal; the extent of the area 

affected and the metamorphic grade (probably up to 

greenschist facies) are not well known owing to later 

· metamorphic overprinting. In the Juneau area, metabasites along the westernmost part of the belt contain M 1 

mineral assemblages characteristic of the pumpellyiteactinolite facies (Himmelberg and others, in press). Metamorphic events M2 (110 Ma), M3 (1 00 Ma), and M6 (50 

Ma) were mostly thermal events that developed aureoles 

around discrete plutons emplaced during regionally extensive magmatic episodes. 

The M4 and M4' metamorphic events were in response to intrusion of widely separated distinctive granitic to dioritic plutons that range in age from 85 to 101 

Ma, and average 95 Ma (P3 plutonic event, Brew and others, 1989). Most of these plutons were statically emplaced 

and have narrow thermal aureoles (M4), but the larger 

bodies are syntectonic and are surrounded by closely 

spaced Barrovian isograds (M4 '). These plutons and associated metamorphic rocks occur along much of the 

length of the western metamorphic belt but are best developed south of the Juneau area. Near Juneau west of 

the Coast Range megalineament (Brew and Ford, 1978), 

the 95-Ma plutons and associated aureoles occur in the 

subgreenschist-facies metamorphic rocks. East of the 

megalineament, M4 mineral assemblages have been overprinted by the later M5 dynamothermal metamorphism. 

The Mount Juneau pluton (fig. 2) may belong to the P3 

event (Ford and Brew, 1977b); however, there are no vestiges of what would be M4 or M4 ' metamorphism associated with this pluton. 

The M5 metamorphic event was also dynamothermal 

and produced a belt of metamorphic rocks that is up to 

10 km or more wide and is well preserved for the northern 400 km of the 700-km-long belt. The M5 metamorphism was accompanied and followed by intrusion of 

tonalite to diorite plutons (P4 plutonic event, Brew and 

others, 1989) that make up the large composite sill, referred to most recently by Brew (1988) as the great 

tonalite sill, which extends along the eastern side of the 

western metamorphic belt along its entire length (fig. 1 ). 

The age of plutons in the great tonalite sill range from 

about 70 Ma to 55 Ma and average about 65 Ma (Brew 

and others., 1989). Final emplacement of the great tonalite 

sill plutons caused local re-equilibration and recrystallization of adj acent schists (M5' metamorphic event, 

Himmelberg and others, 1991). On the basis of isotopic 

ages of plutons and geologic relations, Brew and others 

( 1989) interpreted the age of the M5 and M5' metamorphism to be between about 70 Ma and 65 Ma (see discussion below). The M5 metamorphism, which is the focus 

of this paper, produced an inverted metamorphic sequence that dips to the northeast. 

Brew and Morrell (1983), Brew (1988), and Brew 

and others (1989) discussed younger plutonic and associated thermal metamorphic events to the east and west of 

the western metamorphic belt. 

Brew and others (1989) recognized six deformational 

events in the western metamorphic belt. D 1 and D3 were 

regional folding events that produced southwest vergent 

folds that plunge northwest and southeast. The D2 and 

D4 deformations were fold events localized about the 95-

Ma plutons and great tonalite sill, respectively. Subsequent deformation produced a persistent mylonite zone 

(D5 deformation event) that commonly forms the southwest contact of the great tonalite sill. The M5 metamorphism in the .Juneau area and elsewhere occurred during 

the D3 deformation event. 

Along its entire length, the western metamorphic belt 

is divided longitudinally by the prominent Coast Range 

megalineament (fig. 1) (Brew and Ford, 1978). The 

megalineament is a locally ductile fault zone (D6 deformation event, Brew and others, 1989) that typically is located near the western edge of the high-grade part of the 

metamorphic belt. At Juneau the megalineament is the 

Gastineau Channel fault (fig. 2) along which the M1 

metamorphic rocks are structurally juxtaposed against the 

M5 inverted metamorphic sequence (Himmelberg and 

others, in press). 

Throughout the length of the western metamorphic 

belt the metamorphic grade increases to the northeast. 

Disregarding M4 aureoles around 95-Ma plutons, the 

westernmost part of the belt at any latitude is generally 

of greenschist or subgreenschist facies; to the east, either 

at the Coast Range megalineament or several kilometers 

to the west of it, the metamorphic grade increases 

abruptly toward the great tonalite sill. To the east of the 

sill, the metamorphic grade remains high up to where the 

belt is cut off entirely by granitic rocks of the central granitic belt of the Coast plutonic-metamorphic complex 

(Brew and Ford, 1984 ). 

Few systematic variations in metamorphic grade have 

been identified along the strike of the western metamorphic belt in either the low-grade westernmost part or in 
4 PETROLOGIC CHARACTERIZATION OF PELITIC SCHISTS, NEAR JUNEAU, ALASKA 

the high-grade part east of the great tonalite sill. In the 

central part of the belt, however, where the grade increases abruptly toward the great tonalite sill, there are 

differences along strike in the highest grades preserved 

(Crawford and others, 1987; Stowell, 1989; Himmelberg 

and others, 1991; McClelland and others, 1991). Some 

differences may be due, in part, to variations in the original metamorphic conditions, but most are interpreted to 

be due to the effects of the MS" thermal metamorphism 

and to mylonitic deformation (DS) along the footwall of 

the great tonalite sill. DS deformation occurred during and 

after emplacement of the sill, and where it was greatest, 

Stephens Passage 

as at Berners Bay north of Juneau and along Work Channel near Prince Rupert, British Columbia, all of the record 

of MS metamorphism has been destroyed and only the 

record of the older M1, M4, and M4" metamorphism is 

preserved; however, vestiges of the MS" event are preserved adjacent to the mylonitic footwall of the sill. 

Where the mylonitic deformation has been the least, as 

near Wrangell, Alaska (Brew and others, 1984), and 

Juneau, all of the MS and MS" record is preserved. Between 

these two situations a regular progression in metamorphic 

grade is observed toward the sill, although isograds may be 

truncated by the sill (Ford and Brew, 1977b ). 

Turner Lake 

pluton 

Great tonalite 

sill plutons 

Mount Juneau 

pluton 

Greenstone, phyllite 

and schist 

134°00' 

EXPLANATION 

Contact, dotted 

where concealed 

64 

~ Strike and dip 

of foliation 

Metamorphic isograd, 

dotted where concealed 

Megalineament, dashed 

where concealed 

Figure 2. Map of Juneau area (parts of the Juneau A 1, A2, B 1, and B2 quadrangles) showing distribution of metamorphic facies 

and location ofisograds (modified from Ford and Brew, 1973, 1977a; Brew and Ford, 1977; Himmelberg and others, 1991). 
AGE RELATIONS 5 

STRUCTURAL RELATIONS 

Four phases of deformation have been recognized in 

the metamorphic rocks of the Juneau area1. The oldest 

phase produced the major schistosity (S 1), which is 

subparallel to compositional layering. The age of this earliest deformation is uncertain; however, on the basis of 

the timing of porphyroblast growth described below, it 

predates the M5 metamorphism, and we tentatively equate 

it to the D 1 deformation event of Brew and others (1989), 

although a D2 age cannot be ruled out. The second phase 

of deformation recognized in the Juneau area is D3 of 

Brew and others (1989). The D3 event folded the S1 

schistosity about gently plunging (l 0° to 20°) northwesttrending axes. F3 axial planes are generally steep (55° to 

75°) and development of an s3 axial plane foliation is 

variable. In the field the S3 foliation is generally defined 

by aligned biotite porphyroblasts and (or ) crenulation of 

S1. In thin section S3 shows all stages of progressive development (Bell and Rubenach, 1983) from the open 

crenulation of S1, through crenulation cleavage, to penetrative S3 schistosity. On the limbs of folds, S 1 and S3 

are subparallel and not readily distinguished. Except in fold 

hinges, the S1-S3 foliation and compositional layering strikes 

northwest and dips 25° to 75° to the northeast. Observed 

amplitudes of F3 folds range from a few centimeters to several tens of meters, and wavelengths range from a few centimeters to many tens of meters. No large-scale structures or 

minor folds ·associated with the D 1 deformation event have 

been clearly identified, although there is some suggestion 

that D1 and D3 may be nearly coaxial. 

M5 metamorphism occurred during the D3 deformation event. Inclusion trails in porphyroblasts, particularly 

biotite and garnet, record the stages of development of s3 

crenulation foliation, and where S 1 foliation in the rock 

matrix has been completely obliterated by S3, the 

porphyroblasts preserve the only evidence of the early sl 

foliation (Himmelberg and others, 1984a; Bauer and others, 1988). Similarly, where the late-stage shearing has 

obliterated evidence for S3 foliation in the matrix of some 

of the rocks, the porphyroblasts preserve the only evidence for both S1 and S3 foliations (Bauer and others, 

1988). The age of the D1 deformation event is unknown 

but pre-dates the M5 metamorphism. 

Evidence for the third phase of deformation in the 

Juneau area, equivalent to the D4 event of Brew and others (1989), is restricted to the footwall contact of the great 

1 Deformation and metamorphic events in the Juneau area 

are correlated where possible with the regional events recognized by Brew and others (1989) (table 1 ). To be consistent 

with their report, we use their notation and numbering system 

for the different events. 

tonalite sill where F3 folds are folded about steep axes, 

which are parallel to hornblende lineations in the sill. 

Axial planes of these folds parallel the dominant foliation in the metamorphic rocks and sill plutons. The fourth 

phase of deformation is a shearing event, which is evident from bilotite fish and shear bands in the metamorphic rocks (Bauer and others, 1988; Hooper and others, 

1990). Vve tentatively relate this shearing event to uplift 

associated with the final emplacement of the great tonalite 

sill and perhaps to early phases of movement on the Coast 

Range rnegalineament (D6). 

The plutons of the great tonalite sill contain a foliation that is parallel to the S3=S 1 foliation in the adjacent 

metamorphic rocks to the west. The dip of the foliation 

in the plutons ranges from 45° to 75° to the northeast; 

three-point d:ip determinations from individual plutons 

range from 26° to 60° and average 46°. 

AGE RELATIONS 

The timing of deformational, metamorphic, and plutonic events in the western metamorphic belt near Juneau 

is constrained by fossil data and by K-Ar and U-Pb isotopic-age data from plutons. These data provide limits 

within which geologic relations are used to interpret the 

sequence of events. 

The maximum age for all events near Juneau is provided by Cretaceous fossils from deformed and low-grade 

metamorphosed rocks. Three collections (D.L. Jones, 

written commun., 1969; J.A. Wolfe, oral commun., 1989), 

two from flyschoid clastic rocks west of the Coast Range 

megalineament and one from lithologically similar rocks 

east of it, indicate that rocks as young as Albian (late Early 

Cretaceous, 113 to 97.5 Ma, Geological Society of America, 

1984) and possibly Cenomanian (early Late Cretaceous, 97.5 

to 91 Ma, Geological Society of America, 1984) or younger 

are involved in the earliest deformation (D 1 ). 

Isotopic ages have been obtained from plutons representing the five plutonic events that occurred near Juneau. 

An unpublished U-Pb age determination on zircons from 

the Jualin pluton in the northern part of the Juneau area 

indicates that it is about 104 Ma (G.R. Tilton, written 

commun., 1985), which represents the P2 intrusive event 

of Brew and others (1989). The age of the P3 intrusive 

event of Brew and others ( 1989) in the Juneau area is 

constrained to about 94 Ma by unpublished U-Pb age determinations on zircons (G.R. Tilton, written commun., 

1985; Brew and others, 1992) from the Butler Peak stock 

on the west side of the Coast Range megalineament and 

by a confirming K-Ar age determination on hornblende 

from a compositionally similar pluton on northeastern 

Admiralty Island (N. Shew, written commun., 1987). J.L. 

Wooden (written commun., 1988) reported a U-Pb age of 
6 PETROLOGIC CHARACTERIZATION OF PELITIC SCHISTS, NEAR JUNEAU, ALASKA 

about 84 Ma and Gehrels and others (1991) reported aUPb age of about 72 Ma on zircons from the Mount Juneau 

pluton. The Mount Juneau pluton underwent deformation and metamorphism during the D3-M5 event (Ford 

and Brew, 1977b); the M5 isograd surfaces cut the pluton 

at a high angle, and there is mineralogic evidence of progressive metamorphic changes in the pluton from west to 

east. K-Ar determinations on coexisting biotite and hornblende from the Mount Juneau pluton yielded discordant 

ages of about 54 Ma and 66 Ma, respectively (J.G. Smith, 

written commun., 1974), which are interpreted to be the 

result of involvement in the later M5 metamorphism and 

proximity to the great tonalite sill. 

U-Pb age determinations on zircons from the Carlson 

Creek pluton, which is part of the great tonalite sill, 

indicate that its age is about 67 Ma (Gehrels and others, 

1984, 1991). U-Pb ages on zircons from the Mendenhall 

pluton, which also is part of the great tonalite sill, are 

about 61.5 Ma (Gehrels and others, 1991). Several K-Ar 

age determinations on coexisting biotite and hornblende 

from the Mendenhall pluton yield ages of about 55 Ma 

(F.J. Wilson, written commun., 1985; F.H. Wilson and N. 

Shew, written communs., 1986, 1987; N. Shew, written 

communs., 1987, 1988, 1989). Intrusion of the great 

tonalite sill plutons represent the P4 plutonic event of 

Brew and others ( 1989). 

The age of the M5 metamorphism in the Juneau area 

has not been determined directly by isotopic methods. 

K-Ar ages ranging from 50.4 to 56.1 Ma were obtained 

on hornblende and biotite in migmatitic gneisses on 

Blackerby Ridge and immediately south of Taku Inlet 

(Forbes and Engels, 1970; F.J. Wilson, written commun., 

1985). However the hornblende ages are older than those 

obtained on coexisting biotite; therefore, these ages probably reflect thermal resetting by the Eocene igneous intrusions (P6). 

The age of the M5 metamorphism can be indirectly 

constrained, however, by the isotopic ages of the great 

tonalite sill plutons and by geologic relations between 

the great tonalite sill and the metamorphic rocks. The 

most important of these relations are as follows: 

1. The M5 isograds are in general parallel to the great 

tonalite sill for long distances, which suggests that the 

metamorphism was imposed on the region by the regional 

emplacement of the great tonalite sill. However, the 

Mount Juneau pluton was affected by the D3-M5 event, 

which places an upper age limit on the metamorphism, 

and truncation of the M5 isograds by the great tonalite 

sill to the north indicates that sill intrusion continued after 

the isograds were established. 

2. The M5 metamorphism accompanied large-scale 

F3 folding and concomitant development of s3 foliation. 

On the limbs of folds S3=S1. Foliation within the great 

tonalite sill is approximately parallel to the S3=S1 foliation in the metamorphic rocks, which suggests that the 

great tonalite sill was emplaced, at least in part, during 

the F3 deformation event. 

3. Close to the western contact of the sill the F3 folds 

were locally refolded on steep axes (F 4) and hornblende 

lineations in the sill are parallel to these F 4 axes. The development of the F 4 structures postdates the development 

of M5 mineral assemblages, but the absence of new mineral assemblages superimposed on the M5 metamorphism 

indicates that the pressure-temperature (P-T) conditions at 

the time of emplacement of the great tonalite sill were not 

markedly different from those of the M5 metamorphism. 

These data suggest that the age of the M5 metamorphism is close to the age of emplacement of the 67±2-Ma 

and younger great tonalite sill rocks, therefore, Brew and 

others (1989) tentatively interpreted the M5 metamorphism age to be about 70 Ma. 

METAMORPHIC ZONES AND ISOGRADS 

NATURE AND DEVELOPMENT OF THE 

INVERTED METAMORPHIC GRADIENT 

Forbes ( 1959) conducted the first systematic study of 

the distribution of pelitic index minerals along a single 

transect across the western metamorphic belt near Juneau. 

In that transect he documented the first appearances of 

biotite, garnet, staurolite, kyanite, and sillimanite. More 

extensive mapping by Ford and Brew (1973, 1977a) and 

Brew and Ford ( 1977) confirmed and refined Forbes' 

original interpretation and extended the M5 isograds from 

Berners Bay on the north to the south side of Taku Inlet. 

On the basis of more detailed collections, we further revised the isograds in our study area (fig. 2). 

The metamorphic zones and isograds define an inverted metamorphic gradient. The isograd surfaces strike 

northwest and dip to the northeast (fig. 2) and are parallel 

or subparallel to the dominant metamorphic S3-S 1 foliation. Himmelberg and others (1991) reported that the 

present magnitude of the dip of the isograd surfaces 

ranges along strike from about 32° to 83° with an average dip of about 62°. However, their attitude has been 

modified by postmetamorphic Tertiary uplift of the Coast 

plutonic-metamorphic complex (Crawford and Hollister, 

1982; Donelick, 1986; Wood and others, 1987; Zen, 

1988; Brew and others, 1989), and a structural evaluation 

by Himmelberg and others (1991) suggested that the 

isograd surfaces were originally shallower and ranged in 

dip from about 42° to 47°. 

We also note, however, that the F1 and F3 folds are 

essentially coaxial, indicating a long history of southwestvergent folding. Continuation of general large-scale eastover-west movement after the peak of metamorphism 

could conceivably have rotated and overturned the 
METAMORPHIC ZONES AND ISOGRADS 7 

isograds from an originally steeper attitude. A continuation of east-over-west movement would also have affected the great tonalite sill plutons. The short time 

interval available for such a continuation of movement, 

namely from the 55-Ma emplacement age of the youngest tonalite sill plutons to the 50-Ma emplacement of the 

voluminous granodiorites of the Coast Mountains, argues 

against its having had a significant effect. 

Pressures and temperatures calculated by Himmelberg and others (1991) indicate that the inverted metamorphic gradient was developed under a pressure of 

about 9 to 11 kbar with a progressive increase in temperature from about 530°C for the garnet zone to about 

705°C for the upper kyanite-biotite zone. Himmelberg 

and others (1991) interpreted the inverted gradient to have 

formed during compression of a thickened wedge of relatively wet and cool rocks in response to heat flow associated with the upward movement of the regionally 

continuous tonalite sill along the east side of the metamorphic belt. Final emplacement of the tonalite sill, during uplift, under pressures of about 5 to 6 kbar caused 

widespread re-equilibration of garnet rim compositions 

and growth of chlorite. 

MINERAL ASSEMBLAGES AND MODEL 

REACTIONS AT ISOGRADS 

Mineral assemblages and phase relations in the pelitic 

rocks of the Juneau area can be analyzed on the Al20 3-

Fe0-Mg0 (AFM) projection of the "ideal" pelitic system 

Si02-Al20rK20-Fe0-MgO-H20 (Thompson, 1957). 

Listed in table 2 are mineral assemblages that are appropriate to the AFM projection. In addition to the minerals 

listed, all the assemblages contain quartz and muscovite 

and may contain plagioclase and the accessory minerals 

graphite, ilmenite, tourmaline, beryl, sphene, zircon, 

allanite, and a sulfide mineral. All assemblages also occur in muscovite-deficient quartz-biotite schists, and there 

are no assemblages in muscovite-deficient rocks that do 

not occur in muscovite-bearing rocks. Mineral assemblages of all samples studied are listed in table 3. Locations of samples are also given in table 3. 

The pelitic mineral assemblages allow us to recognize six distinct metamorphic zones (fig. 2; table 2). The 

beginning of each zone is marked by the first appearance 

of the particular index mineral for which it is named. The 

kyanite-biotite zone is subdivided into upper and lower at 

the staurolite-out isograd. The phase relations for the garnet through sillimanite zones are shown in figure 3. 

These phase relations differ slightly from those given by 

Himmel berg and others ( 1984b ), because a subsequent 

textural/structural study showed that much of the chlorite 

previously considered part of the peak metamorphic 

assemblage is instead of a later generation (Bauer and 

others, 1988). All five metamorphic zones are well exposed on Heintzleman and Blackerby Ridges near Juneau. 

To the northwest the higher grade zones are progressively 

truncated by the great tonalite sill, and to the southeast 

near Taku Inlet, only the biotite, garnet, and staurolitebiotite zones were recognized. However, H.H. Stowell 

(oral commun., 1992) recently reported finding kyanite 

on the northwest shore of Taku Inlet. 

The isograds separating the metamorphic zones are 

marked by the first appearance of the index minerals and 

can be modeled in the ideal pelitic system by the reactions discussed below (reaction coefficients are not given 

but can be obtained from Thompson, 1976). 

AFM mineral assemblages below and above the garnet isograd (table 2; fig. 3) suggest the model garnetforming reaction 

Chl+Ms+Qtz=Grt+Bt+H20 (1) 

This is a divariant continuous reaction; the first appearance of garnet is also dependent on bulk composition 

and thus may not represent a true isograd. 

Mineral assemblages above the garnet-chlorite join 

have not been observed in the Juneau area. The first appearance of staurolite is at the staurolite-biotite isograd 

which can be modeled by the discontinuous reaction 

Chl+Grt+Ms=St+Bt+Qtz+H20 (2) 

The complete reaction assemblage chl-grt-st-bt occurs near the isograd (table 2) in the lowest part of the 

staurolite-biotite zone. However, chlorite does not persist 

upward, and most of the staurolite-biotite zone is characterized by the assemblage staurolite-biotite-garnet, which 

maintains equilibrium by the continuous reaction 

St+Bt+Qtz=Grt+Ms+H20 (3) 

The stability curve of reaction 3 is nearly flat in P-T 

space; therefore, as pointed out by Holdaway and others 

(1982), increasing temperature causes none of the phases 

to react, grow, or change composition. 

In the Juneau area the chlorite-out isograd is nearly 

coincident with the staurolite-biotite isograd. The AFM 

topologies of Himmel berg and others (1984b) which 

showed chlorite persisting into the kyanite-biotite zone, and 

the chloritc:-out isograd as mapped by Ford and Brew (1973, 

1977a) and Brew and Ford (1977), as noted above, were 

based on what now is interpreted as late-generation chlorite. 

In other areas of progressive metamorphism 

(Carmichael, 1970; Guidotti, 1974; Novak and Holdaway, 

1981; Holdaway and others, 1982; Klaper and BucherNurminen., 1987) the formation of kyanite (or sillimanite) 

and the kyanite (or sillimanite) -biotite isograd results 

from the model discontinuous reaction 

Chl+St+Ms=Ky+Bt+Qtz+H20 (4) 
8 PETROLOGIC CHARACTERIZATION OF PELITIC SCHISTS, NEAR JUNEAU, ALASKA 

Table 2. Observed mineral assemblages in the Juneau area appropriate to Thompson's 

(1957) AI20 3-Fe0-Mg0 (AFM) projection. 

[All assemblages contain quartz and muscovite] 

Metamorphic zone Mineral assemblage 

Biotite-----------------Chlorite-biotite 

Gamet---------------.Chlorite-biotite 

Biotite-garnet 

Chlorite-biotite-garnet 

Staurolite-biotite------Biotite 

Chlorite-biotite 

Biotite-garnet 

Biotite-staurolite 

Chlorite-biotite-garnet 

Biotite-garnet -staurolite 

Chlorite-biotite-garnetstaurolite 

In the Juneau area, however, the absence of prograde 

chlorite in the staurolite-biotite zone, other than near the 

staurolite-biotite isograd, indicates that the first kyanite 

and the kyanite-biotite isograd did not form by reaction 

4, an indication which is consistent with the recent observations elsewhere of Holdaway and others (1988). In the 

Juneau area the first appearance of kyanite occurs in 

garnet-bearing rocks, and the kyanite-biotite isograd can 

be attributed to the model discontinuous reaction 

St+Ms+Qtz=Ky+Bt+Grt+H20 (5) 

which is divariant in nature because the "four phase" assemblage staurolite-kyanite-biotite-garnet persists over a 

significant field interval. Kyanite+biotite can also be 

formed by the continuous reaction 

St+Ms+Qtz=Ky+Bt+H20, (6) 

which yields rare biotite-staurolite-kyanite assemblages. 

Reaction 3 may also maintain equilibrium in the kyanitebiotite zone. The divariant reaction 

St+Qtz=Ky+Grt+H20 (7) 

is also possible, although the absence of kyanite-garnet 

assemblages without biotite suggests that bulk compositions were not appropriate for reaction 7. 

The staurolite-out isograd can be modeled by either 

reaction 5 or 6 going to completion. The limiting assemblage kyanite-biotite-garnet has not been observed in 

muscovite-bearing rocks in the Juneau area, but it is common in muscovite-absent rocks, some with a trace of 

staurolite, which might represent reaction 5 going to 

completion inK-deficient rocks. 

The absence of staurolite in sillimanite-bearing 

samples indicates that reactions 5 and 6 were completed 

before the reaction 

Ky=Sil, (8) 

which defines the sillimanite isograd. With further 

Metamorphic zone Mineral assemblage 

Lower kyanite-biotite------Biotite-garnet 

Biotite-staurolite 

Biotite-kyanite 

Biotite-garnet -staurolite 

Biotite-staurolite-kyanite 

Biotite-garnet -staurolitekyanite 

Upper kyanite-biotite ------Biotite-garnet 

Biotite-kyanite 

Sillimanite------------------Biotite-sillimanite 

Biotite-garnet -sillimanite 

Biotite-k anite-sillimanite 

increase in grade, garnet and muscovite react continuously to form sillimanite and biotite. 

Changes in the Mg/(Mg+Fe) ratios of biotite and 

staurolite with increasing metamorphic grade in threeand four-phase assemblages appropriate to the above reactions are generally consistent with the changes predicted by Thompson (1976), although exceptions do exist. 

Changes in garnet composition in the same assemblages 

are further complicated owing to complex zoning patterns 

and retrograde equilibration of some rim compositions. 

Himmel berg and others ( 1991) showed that the above 

sequence of reactions and the calculated pressure and 

temperature for the individual metamorphic zones are 

consistent with the petrogenetic grid of Spear and Cheney 

(1989) (fig. 4). 

MINERAL CHEMISTRY 

Mineral analyses were obtained for 30 samples of 

quartz-muscovite-bearing pelitic schists and for 10 

samples of muscovite-absent pelitic schists. Analyses of 

samples used for thermobarometry were previously reported by Himmel berg and others ( 1991 ). Additional 

analyses for low variance assemblages are given in tables 

4 through 9; however, the mineral chemistry summary 

given below is based on all analyses. Analyses of garnet 

zonation and plagioclase were obtained using the JEOL 

model 733 superprobe at Washington University, St. 

Louis, Mo.; all other analyses were obtained using the 

ARL EMX-SM microprobe at the University of MissouriColumbia. Matrix corrections were made by the method 

of Bence and Albee (1968) using'the correction factors of 

Albee and Ray ( 1970). Owing to the presence of graphite 

and ilmenite in most samples, iron was assumed to be 

Fe2+. Tabulated chlorite, muscovite, biotite, and staurolite analyses are averages of several grains per sample; 

plagioclase analyses are grain averages for unzoned 

samples and rim averages for zoned samples. Garnet core 

and rim or near-rim analyses were grouped and averaged 
MINERAL CHEMISTRY 9 

Table 3. Mineral associations and localities of samples of pelitic and semi pelitic rocks obtained near Juneau, Alaska. 

[In addition to the minerals listed, most assemblages contain graphite, tourmaline, and ilmenite. Beryl, allanite, zircon, sphene, and calcite 

may also be present. Chl, chlorite; Bt, biotite; Grt, garnet; St, staurolite; Ky, kyanite; Sil, sillimanite; Qtz, quartz; Ms, muscovite; Pl, 

plagioclase; X, mineral present; query (?), identification uncertain; space, mineral not detected. Where a mineral's presence is enclosed in 

brackets, it is interpreted to be a lower grade relict] 

Zone Chl Bt Grt St Ky Sil Qtz Ms PI Quadrangle Area Latitude Longitude Sample No. 

Bt ------- X X X Juneau B2 Heintzleman Ridge 58° 22' 55" 134° 30' 55" 79GH1A 

Bt ------- X X X X Juneau B2 Heintzleman Ridge 58° 22' 53" 134° 31' 10" 79GH2A 

Bt ------- X X Juneau B2 Heintzleman Ridge 58° 22' 50" 134° 31' 40" 79GH4A 

Bt ------- X ? X X X Juneau B2 Heintzleman Ridge 58° 22" 34" 134° 33'05" 79GH5A 

Bt ------- X X X X Juneau B2 Heintzleman Ridge 58° 23' 04" 134° 30'03" 79GH11B 

Bt ------- X X X Juneau B2 Blackerby Rridge 58° 20' 40" 134° 27' 37" 79GH63A 

Bt ------- X X X Juneau B2 Blackerby Ridge 58°21'03" 134° 27' 15" 81GH19A 

Bt ------- X X X X Juneau B2 Blackerby Ridge 58° 21' 02" 134° 27' 06" 81GH20A 

Bt ------- X X X Juneau B1 Sheep Mt. 58° 17' 14" 134° 19'22" 83DB93B 

Bt ------- X X X X Juneau B1 Sheep Mt. 58° 17' 20" 134° 18'50" 83GH8A 

Bt ------- X X X Juneau B1 Sheep Mt. 58° 17' 23" 134° 18'34" 83GH9A 

Bt ------- X X X X X Juneau B1 Sheep Mt. 58° 17' 23" 134° 18' 34" 83GH9B 

Bt ------- X X X Juneau B1 Powerline Ridge 58° 16' 46" 134° 16'39" 83GH5A 

Bt ------- X X X Juneau Bl Powerline Ridge 58° 16'41" 134° 16' 42" 83GH6A 

Bt ------- X X X Juneau Bl Powerline Ridge 58° 16' 38" 134° 16' 22" 83SK93A 

Bt ------- X X X Juneau B1 Powerline Ridge 58° 16' 45" 134° 16' 20" 83SK94A 

Bt ------- X X X Juneau B1 Hawthorne Peak 58° 16' 09" 134° 15'35" 83GH19A 

Bt ------- X X Juneau A1 Taku Inlet 58° 11' 24" 134° 05'05" 84SK262D 

Bt ------- X Juneau A1 Taku Inlet 58° 11' 20" 134° 05' 11" 84SK263A 

Bt ------- X X X X Juneau A1 Taku Inlet 58° 11' 10" 134° 05' 17'' 84SK265A 

Bt ------- X X X X X Juneau A1 Taku Inlet 58° 10' 47" 134° 05' 15" 84SK267A 

Bt ------- X X X X X Juneau A1 Taku Inlet 58° 10' 33" 134° 05'09" 84SK268A 

Grt ------ X X X X Juneau B2 Heintzleman Ridge 58° 23' 37" 134° 30' 00" 79GH15A 

Grt ------ X X X X Juneau B2 Heintzleman Ridge 58° 23' 52" 134° 29'57" 79GH17A 

Grt ------ X X X Juneau B2 Heintzleman Ridge 58° 23' 26" 134° 29' 14" 79GH19A 

Grt------ X X X X ? Juneau B2 Heintzleman Ridge 58° 23' 27" 134° 29'05" 79GH20A 

Grt ------ X X X X X Juneau B2 Heintzleman Ridge 58° 23' 27" 134° 29'05" 79GH20B 

Grt ------ X X X X X Juneau B2 Heintzleman Ridge 58° 23' 17" 134° 29' 40" 81DB15A 

Grt------ X X X X X Juneau B2 Heintzleman Ridge 58° 23' 19" 134° 29'48" 81DB16A 

Grt ------ X X X X X Juneau B2 Heintzleman Ridge 58° 23' 19" 134° 29'48" 81DB16B 

Grt------ X X X X X Juneau B2 Heintzleman Ridge 58° 23' 30" 134° 29'56" 81DB19A 

Grt ------ X X X Juneau B2 Heintzleman Ridge 58° 23' 30" 134° 29'56" 81DB19B 

Grt------ X X X X Juneau B2 Heintzleman Ridge 58° 23' 45" 134° 29' 59" 81DB20A 

Grt------ X X X X Juneau B2 Blackerby Ridge 58° 21' 05" 134° 26'59" 81GH18A 

Grt ------ X X X X Juneau B2 Blackerby Ridge 58° 21' 10" 134° 26' 28" 81GH21A 

Grt------ X X X X X Juneau B2 Blackerby Ridge 58° 21' 10" 134° 26' 28" 81GH21B 

Grt------ X X X X X Juneau B1 SheepMt. 58° 17' 34" 134° 17' 56" 83DB89B 

Grt------ X X X X Juneau B1 SheepMt. 58° 17' 25" 134° 18'20" 83GH10A 

Grt------ X X X X X Juneau B1 Sheep Mt. 58°17' 32" 134° 18' 10" 83GH11A 

Grt------ X X X X Juneau B1 Sheep Mt. 58° 17' 30" 134° 17' 17" 83GH12A 

Grt------ X X X X Juneau B1 Sheep Mt. 58°1 7' 27" 134° 17'05" 83GH13B 

Grt ------ X X X X Juneau B1 Powerline Ridge 58° 16' 55" 134° 16' 35" 83GH2A 

Grt------ X X X X Juneau B1 Powerline Ridge 58° 16' 56" 134° 16'40" 83GH3A 

Grt------ X X X X Juneau B1 Powerline Ridge 58° 16' 53" 134° 16'43" 83GH4A 

Grt------ X X X X Juneau B1 Powerline Ridge 58° 16' 49" 134° 16' 14" 83SK95A 

Grt------ X X X X Juneau B1 Hawthorne Peak 58° 16' 14" 134° 15'22" 83GH20A 

Grt------ X X X Juneau B1 Hawthorne Peak 58° 16' 23" 134° 14' 56" 83GH21A 

Grt------ X X X X X Juneau A1 Taku Inlet 58° 11' 39" 134° 04' 52" 84SK270A 

Grt------ X X X Juneau A1 Taku Inlet 58° 11' 39" 134° 04' 52" 84SK270C 

Grt ------ X X X X Juneau A1 Taku Inlet 58° 11' 55" 134° 04' 49" 84SK272A 

Grt ------ X X X X Juneau A1 Taku Inlet 58° 12' 38" 134° 07' 52" 85GH66A 

St-Bt ---- X X X X X Juneau B2 Heintzleman Ridge 58° 23' 29" 134° 29' 00" 79GH21A 

St-Bt ---- X X X X Juneau B2 Heintzleman Ridge 58° 23' 29" 134° 29' 00" 79GH21B 

St-Bt ---- X X X X X Juneau B2 Heintzleman Ridge 58° 23' 31" 134° 28' 57" 79GH22A 

St-Bt ---- X X X X X X X Juneau B2 Heintzleman Ridge 58° 23' 30" 134° 29' 00" 81SK11A 

St-Bt ---- X X X X Juneau B2 Heintzleman Ridge 58° 23' 30" 134° 29' 00" 81SK11B 

St-Bt ---- X X X X X Juneau B2 Heintzleman Ridge 58° 23' 40" 134° 29' 00" 81SK12A 

St-Bt ---- X X X X X Juneau B2 Heintzleman Ridge 58° 23' 35" 134° 28' 50" 81SK13A 

St-Bt ---- X X X X Juneau B2 Heintzleman Ridge 58° 23' 28" 134° 28' 50" 81GH5A 

St-Bt ---- X X X X X Juneau B2 Heintzleman Ridge 58° 23' 28" 134° 28' 50" 81GH5B 

St-Bt ---- X X X X X Juneau B2 Heintzleman Ridge 58° 23' 28" 134° 28' 50" 81GH5C 

St-Bt ---- X X X X X X Juneau B2 Blackerby Ridge 58° 21' 10" 134° 26'25" 79GH57A 

St-Bt ---- X X X X Juneau B2 Blackerby Ridge 58° 21' 09" 134° 26' 30" 79GH58A 

St-Bt ---- X X X X Juneau B2 Blackerby Ridge 58° 21'09" 134° 26' 30" 79GH58B 

St-Bt ---- X X X X X Juneau B2 Blackerby Ridge 58° 21' 17'' 134° 26' 04" 79GH84B 

St-Bt ---- X X X X Juneau B2 Blackerby Ridge 58° 21' 17'' 134° 26' 04" 79GH84C 

St-Bt ---- X X X X Juneau B2 Blackerby Ridge 58° 21' 20" 134° 25'55" 79GH85A 

St-Bt ---- X X X X X X X Juneau B2 Blackerby Ridge 58° 21' 30" 134° 24'49" 81DB21A 

St-Bt ---- X X X X Juneau B2 Blackerby Ridge 58° 21'21" 134° 25'45" 81GH23A 

St-Bt ---- X X X X X X Juneau B2 Blackerby Ridge 58° 21'22" 134° 25'38" 81GH24A 

St-Bt ---- X X X X ? Juneau B1 Powerline Ridge 58° 16'49" 134° 16' 14" 83GH7A 

St-Bt ---- X X X Juneau B1 Powerline Ridge 58° 17' 07" 134° 16' 35" 83GH14A 

St-Bt ---- X X X X Juneau B1 Powerline Ridge 58° 17' 08" 134° 15'53" 83GH15B 

St-Bt ---- X X X Juneau B1 Powerline Ridge 58° 17' 08" 134° 15' 53" 83GH15C 
10 PETROLOGIC CHARACTERIZATION OF PELITIC SCHISTS, NEAR JUNEAU, ALASKA 

Table 3. Mineral associations and localities of samples of pelitic and semipelitic rocks obtained near Juneau, Alaska-Continued. 

Zone Chi Bt Gtt St Ky Sil Qtz Ms PI Quadrangle Area Latitude Longitude Sample No. 

St-Bt ---- X X X X Juneau B1 Powerline Ridge 58° 17' 12" 134° 15'43" 83GH16A 

St-Bt ---- X X X X Juneau B1 Powerline Ridge 58° 17' 12" 134° 15'43" 83GH16B 

St-Bt ---- X X X X X Juneau B1 Powerline Ridge 58° 17' 39" 134° 15' 21" 84GH18A 

St-Bt ---- X X X X X Juneau B1 Powerline Ridge 58° 17' 39" 134° 15' 21" 83GH18B 

St-Bt ---- ? X X X X X Juneau B1 Powerline Ridge 58° 16'46" 134° 15' 40" 83GH22A 

St-Bt ---- X X X X X Juneau B1 Powerline Ridge 58° 17' 28" 134° 14' 55" 83GH28A 

St-Bt ---- X X X X Juneau B1 Hawthorne Peak 58° 16'22" 134° 14' 37" 83GH24B 

St-Bt ---- X X X Juneau B1 Hawthorne Peak 58° 16' 00" 134° 13' 00" 84SK254A 

St-Bt ---- X X X X X Juneau Bl Hawthorne Peak 58° 16' 00" 134° 12'58" 84SK254C 

St-Bt ---- X X X X X X Juneau Bl Hawthorne Peak 58° 15'55" 134° 12' 01" 84SK256A 

St-Bt ---- X X X X Juneau Bl Hawthorne Peak 58° 15'55" 134° 12' 01" 84SK256B 

St-Bt ---- X X X X Juneau Bl Hawthorne Peak 58° 15'55" 134° 12' 01" 84SK256C 

St-Bt ---- X X X X Juneau Bl Hawthorne Peak 58° 15'54" 134° 11' 42" 84SK257A 

St-Bt ---- X X X X Juneau Bl Hawthorne Peak 58° 15'54" 134° 11' 42" 84SK257B 

St-Bt ---- X X X X X Juneau Bl Hawthorne Peak 58° 15'58" 134° 11' 28" 84SK258A 

St-Bt ---- X X X X Juneau Bl Hawthorne Peak 58° 16'22" 134° 10' 16" 84SK261B 

St-Bt ---- X X X X X Juneau AI Taku Inlet 58° 14' 40" 134° 10' 07" 84GH25A 

St-Bt ---- ? X X X X X Juneau AI Taku Inlet 58° 14'40" 134° 10' 07" 84GH25B 

St-Bt ---- X X X X X Juneau AI Taku Inlet 58° 14'40" 134° 10'07" 84GH25C 

St-Bt ---- X X X X Juneau AI Taku Inlet 58° 14' 17'' 134° 10' 08" 84SK253A 

St-Bt ---- X X X X X X Juneau AI Taku Inlet 58°13' 20" 134 °08' 26" 85GH67A 

St-Bt ---- X X X X X X Juneau AI Taku Inlet 58° 13' 20" 134° 08'26" 85GH67B 

St-Bt ---- X X X X X Juneau AI Taku Inlet 58° 13' 20" 134° 08' 26" 85GH67C 

St-Bt ---- X X X X X Juneau AI Taku Inlet 58° 13' 35" 134° 08' 15" 85GH68A 

St-Bt ---- X X X X Juneau AI Taku Inlet 58° 13' 57" 134° 07' 15" 85GH69A 

St-Bt ---- X X X Juneau AI Taku Inlet 58° 13' 57" 134° 07' 15" 85GH69B 

Ky-Bt--- X X X X Juneau B2 Heintzleman Ridge 58° 23' 34" 134° 28' 50" 79GH23A 

Ky-Bt--- X X X X X X X Juneau B2 Heintzleman Ridge 58° 23' 34" 134° 28'50" 79GH23B 

Ky-Bt--- X X X X X Juneau B2 Heintzleman Ridge 58° 23' 39" 134° 28'43" 79GH24A 

Ky-Bt--- X X X X X Juneau B2 Heintzleman Ridge 58° 23' 42" 134° 28'41" 79GH25A 

Ky-Bt--- X X X X X Juneau B2 Heintzleman Ridge 58° 23' 42" 134° 28' 41" 79GH25B 

Ky-Bt--- X X X X X Juneau B2 Heintzleman Ridge 58° 23' 42" 134° 28'41" 79GH25C 

Ky-Bt--- X X X Juneau B2 Heintzleman Ridge 58° 23' 47" 134° 28' 34" 79GH27A 

Ky-Bt--- X X X X X Juneau B2 Heintzleman Ridge 58° 23' 48" 134° 28' 25" 79GH28A 

Ky-Bt--- X X X X Juneau B2 Heintzleman Ridge 58° 23' 55" 134° 28' 12" 79GH30A 

Ky-Bt--- X X X Juneau B2 Heintzleman Ridge 58° 23' 55" 134° 28' 12" 79GH30B 

Ky-Bt--- X X X X X Juneau B2 Heintzleman Ridge 58° 23' 54" 134° 28' 07" 79GH31A 

Ky-Bt--- X X X X X Juneau B2 Heintzleman Ridge 58° 23' 53" 134° 28'03" 79GH32A 

Ky-Bt--- X X X X X Juneau B2 Heintzleman Ridge 58° 23' 54" 134° 28' 00" 79GH33A 

Ky-Bt--- X X X X X Juneau B2 Heintzleman Ridge 58° 23' 56" 134° 27' 57" 79GH34A 

Ky-Bt--- X X X X X X X Juneau B2 Heintzleman Ridge 58° 23' 58" 134° 27' 44" 79GH35A 

Ky-Bt--- X X X X Juneau B2 Heintzleman Ridge 58° 24' 01" 134° 27' 35" 79GH36A 

Ky-Bt--- X X X X Juneau B2 Heintzleman Ridge 58° 24' 02" 134° 27' 23" 79GH38A 

Ky-Bt--- X X X X Juneau B2 Heintzleman Ridge 58° 24' 02" 134° 27' 23" 79GH38B 

Ky-Bt--- X X X X Juneau B2 Heintzleman Ridge 58° 24' 08" 134° 26'55" 79GH41A 

Ky-Bt--- X X X X Juneau B2 Heintzleman Ridge 58° 24' 09" 134° 26'42" 79GH42A 

Ky-Bt--- X X X X Juneau B2 Heintzleman Ridge 58° 24' 13" 134° 26' 29" 79GH43B 

Ky-Bt--- X X X X Juneau B2 Heintzleman Ridge 58° 24' 15" 134° 26'25" 79GH44A 

Ky-Bt--- X X X X X Juneau B2 Heintzleman Ridge 58° 24' 16" 134° 26' 20" 79GH45A 

Ky-Bt--- X X X X X Juneau B2 Heintzleman Ridge 58° 23'33" 134° 28'35" 81GH6A 

Ky-Bt--- X X X X X Juneau B2 Heintzleman Ridge 58° 23' 33" 134° 28' 35" 81GH6B 

Ky-Bt--- X X X X X Juneau B2 Heintzleman Ridge 58° 23' 33" 134° 28' 35" 81GH6C 

Ky-Bt--- X X X X X X Juneau B2 Heintzleman Ridge 58° 23' 43" 134° 28' 06" 81GH7A 

Ky-Bt--- X X X X X X Juneau B2 Heintzleman Ridge 58° 23' 43" 134° 28'06" 81GH7B 

Ky-Bt--- X X X X X Juneau B2 Heintzleman Ridge 58° 23'42" 134° 28' 22" 81GH8A 

Ky-Bt--- X X X X Juneau B2 Heintzleman Ridge 58° 23'51" 134° 28' 00" 81GH10A 

Ky-Bt--- X X X X X Juneau B2 Heintzleman Ridge 58° 23'55" 134° 27' 59" 81GH11A 

Ky-Bt--- X X X X X Juneau B2 Heintzleman Ridge 58° 24'00" 134° 27' 41" 81GH13A 

Ky-Bt--- X X X X X X Juneau B2 Heintzleman Ridge 58° 23'59" 134° 27' 39" 81GH14A 

Ky-Bt--- X X X X X X X Juneau B2 Heintzleman Ridge 58° 23' 59" 134° 27' 39" 81GH14B 

Ky-Bt--- X X X X X Juneau B2 Heintzleman Ridge 58° 24' 07" 134° 27' 07" 81GH15A 

Ky-Bt--- X X X X Juneau B2 Heintzleman Ridge 58° 24' 07" 134° 27' 07" 81GH15B 

Ky-Bt--- X X X X X Juneau B2 Heintzleman Ridge 58° 24' 08" 134° 27' 03" 81GH16A 

Ky-Bt--- X X X X X Juneau B2 Heintzleman Ridge 58° 24' 08" 134° 27' 03" 81GH16B 

Ky-Bt--- X X X Juneau B2 Heintzleman Ridge 58° 24' 10" 134° 26' 51" 81GH17A 

Ky-Bt--- X X X X Juneau B2 Heintzleman Ridge 58° 24' 10" 134° 26'51" 81GH17B 

Ky-Bt--- X X X X Juneau B2 Heintzleman Ridge 58° 23' 45" 134° 28' 45" 81SK14A 

Ky-Bt--- X X X Juneau B2 Heintzleman Ridge 58° 23'45" 134° 28' 45" 81SK14B 

Ky-Bt--- X X X ? X X Juneau B2 Heintzleman Ridge 58° 23'45" 134° 28' 45" 81SK14C 

Ky-Bt--- X X X X X Juneau B2 Heintzleman Ridge 58° 23'55" 134° 29' 50" 81SK15A 

Ky-Bt--- X X X X Juneau B2 Heintzleman Ridge 58° 24'00" 134° 29' 50" 81SK16A 

Ky-Bt--- X X X X Juneau B2 Heintzleman Ridge 58° 24'00" 134° 29' 50" 81SK16B 

Ky-Bt--- X X X X X X Juneau B2 Heintzleman Ridge 58° 24' 05" 134° 29' 40" 81SK17A 

Ky-Bt--- X X X X X X Juneau B2 Heintzleman Ridge 58° 24'05" 134° 29' 40" 81SK17B 
MINERAL CHEMISTRY 11 

Table 3. Mineral associations and localities of samples of pelitic and semi pelitic rocks obtained near Juneau, Alaska-Continued. 

Zone Chi Bt Grt St Ky Sil Qtz Ms PI Quadrangle Area Latitude Longitude Sample No. 

Ky-Bt--- X X X X Juneau B2 Heintzleman Ridge sse 24' 15" 134e 29' 20" 81SK19A 

Ky-Bt--- X X X X X Juneau B2 Heintzleman Ridge sse 24' ()()" 134e 29' 10" 81SK20A 

Ky-Bt--- X X X X X X Juneau B2 Heintzleman Ridge sse 23' 50" 134e 28'50" 81SK22A 

Ky-Bt--- X X X X Juneau B2 Blackerby Ridge sse 21'23" 134e 23' 18" 79GH78A 

Ky-Bt--- X X X Juneau B2 Blackerby Ridge sse 21' 23" 134e 23' 18" 79GH78B 

Ky-Bt--- X X X X Juneau B2 Blackerby Ridge sse 21' 18" 134e 23'55" 79GH80A 

Ky-Bt--- X X X X X Juneau B2 Blackerby Ridge sse 21' 16" 134e 24'03" 79GH81C 

Ky-Bt--- X X X X Juneau B2 Blackerby Ridge sse 21' 15" 134e 24' 25" 79GH82A 

Ky-Bt--- X X X X X Juneau B2 Blackerby Ridge sse 21' 17'' 134e 24'29" 79GH83A 

Ky-Bt--- X X X X Juneau B2 Blackerby Ridge sse 21' 23" 134e 25'35" 79GH88A 

Ky-Bt--- X X X X Juneau B2 Blackerby Ridge sse 21' 23" 134° 25'35" 79GH88B 

Ky-Bt--- X X X X X Juneau B2 Blackerby Ridge 58° 21'23" 134° 25'35" 79GH88C 

Ky-Bt--- X X X X X X X Juneau B2 Blackerby Ridge 58° 21' 23" 134° 25'35" 79GH88D 

Ky-Bt--- X X X X X X X Juneau B2 Blackerby Ridge 58° 21'24" 134° 25' 30" 79GH89A 

Ky-Bt--- X X X X X X X Juneau B2 Blackerby Ridge 58° 21' 24" 134° 25'30" 79GH89B 

Ky-Bt--- X X X X X Juneau B2 Blackerby Ridge 58° 21' 21" 134° 24' 58" 79GH91A 

Ky-Bt--- X X X X X X Juneau B2 Blackerby Ridge sse 21' 17'' 134e 24'30" 79GH92A 

Ky-Bt--- X X X X Juneau B2 Blackerby Ridge 58° 21' 18" 134° 24'36" 79GH93A 

Ky-Bt--- X X X X X X X Juneau B2 Blackerby Ridge 58° 21' 38" 134° 24' 45" 81DB22A 

Ky-Bt--- X X X X X X X Juneau B2 Blackerby Ridge 58° 21'38" 134° 24' 45" 81DB22C 

Ky-Bt--- X X X X X X Juneau B2 Blackerby Ridge 58° 21' 45" 134e 24' 41" 81DB23A 

Ky-Bt--- X X X X Juneau B2 Blackerby Ridge 58° 21' 47" 134e 24' 33" 81DB24A 

Ky-Bt--- X X X X Juneau B2 Blackerby Ridge 58° 21' 48" 134e 24' 26" 81DB25A 

Ky-Bt--- X X X X Juneau B2 Blackerby Ridge 58° 22'05" 134e 24' 07" 81DB27A 

Ky-Bt--- X X X X Juneau B2 Blackerby Ridge 58° 21' 46" 134e 23' 19" 81DB32A 

Ky-Bt--- X X X X Juneau B2 Blackerby Ridge 58° 21' 38" 134° 23' 04" 81DB33A 

Ky-Bt--- X X X X X X X Juneau B2 Blackerby Ridge sse 21'24" 134° 25' 30" 81GH25A 

Ky-Bt--- X X X X Juneau B2 Blackerby Ridge sse 21' 20" 134° 23' 20" 81SK29A 

Ky-Bt--- X X X X Juneau B2 Blackerby Ridge sse 21' 20" 134° 23' 20" 81SL29B 

Ky-Bt--- X X X X Juneau B2 Blackerby Ridge 58° 21' 20" 134° 23' 50" 81SK31A 

Ky-Bt--- X X X Juneau B2 Blackerby Ridge 58° 21' 15" 134° 24' 20" 81SK32A 

Ky-Bt--- X X X X X Juneau B2 Blackerby Ridge 58° 21' 17'' 134° 24'35" 81SK33A 

Sil------- X X [X] X X X Juneau B2 Heintzleman Ridge 58° 24' 18" 134° 26' 19" 79GH46A 

Sil------- X X [X] X X X Juneau B2 Heintzleman Ridge 58° 24' 18" 134° 26' 19" 79GH46B 

Sil------- X X [X] X X X Juneau B2 Heintzleman Ridge 58° 24' 20" 134° 26' 11" 79GH47A 

Sil------- X X X X X Juneau B2 Heintzleman Ridge 58° 24' 20" 134° 26' 11" 79GH47B 

Sil------- X X [X] X X X Juneau B2 Heintzleman Ridge 58° 24' 28" 134° 26'02" 79GH49B 

Sil------- X X X X Juneau B2 Heintzleman Ridge 58° 24' 33" 134° 25'58" 79GH50A 

Sil------- X X [X] X X X X Juneau B2 Heintzleman Ridge 58° 24' 40" 134° 25'57" 79GH51A 

Sil------- X X X X X X Juneau B2 Heintzleman Ridge 58° 24' 40" 134° 25'57" 79GH51B 

Sil------- X X X X X X Juneau B2 Heintzleman Ridge 58° 24'47" 134° 25'55" 79GH52A 

Sil------- X X X X X Juneau B2 Heintzleman Ridge 58° 24' 47" 134° 25'55" 79GH52B 

Sil------- X X Juneau B2 Heintzleman Ridge 58° 24' 57" 134° 25'27" 79GH94A 

Sil------- X X X X Juneau B2 Heintzleman Ridge 58° 24' 56" 134° 24' 31" 79GH99A 

Sil------- X X X X X Juneau B2 Heintzleman Ridge 58° 24' 57" 134 24 21 79GH100B 

. Sil------- X X X X Juneau B2 Blackerby Ridge 58° 21' 31" 134 22 00 79GH67A 

Sil------- X X X X Juneau B2 Blackerby Ridge 58° 21' 31" 134e 22' ()()" 79GH67B 

Sil------- X X X X Juneau B2 Blackerby Ridge 58° 21' 32" 134e 22'39" 79GH71B 

Sil------- X X X Juneau B2 Blackerby Ridge 58° 21' 28" 134° 22' 47" 79GH73A 

Sil------- X X X X Juneau B2 Blackerby Ridge sse 21'27" 134° 22' 50" 79GH74A 

Sil------- X X X X X Juneau B2 Blackerby Ridge 58° 21'26" 134° 22' 51" 79GH75A 

Sil------- X X X X X Juneau B2 Blackerby Ridge sse 21'25" 134° 23' ()()" 79GH76A 

Sil------- X X X X X Juneau B2 Blackerby Ridge 58° 21' 25" 134° 23' ()()" 79GH76B 

Sil------- X X X X Juneau B2 Blackerby Ridge sse 21' 25" 134° 23' ()()" 79GH76C 

Sil------- X X X X X Juneau B2 Blackerby Ridge 58° 21' 24" 134° 23' 04" 79GH77A 

Sil------- X X X X Juneau B2 Blackerby Ridge sse 21' 24" 134° 23' 04" 79GH77B 

Sil------- X X X X X Juneau B2 Blackerby Ridge 58° 21' 05" 134° 22' 30" 81SK23C 

Sil------- X X X Juneau B2 Blackerby Ridge 58° 21'55" 134° 23'05" 81SK24A 

Sil------- X X X X Juneau B2 Blackerby Ridge 58° 21' 25" 134° 22' 55" 81SK27A 

Sil------- X X [X] X X X X Juneau B2 Blackerby Ridge 58° 21' 25" 134° 23' 10" 81SK28A 

on the basis of chemical similarity; obvious rim retro- Plagioclase is present in most of the pelitic schists, 

grade analyses were not included. and plagioclase analyses are given in table 5. The comPrograde chlorite coexisting with quartz, white mica, position ranges from about An 12 to An40 and shows 

biotite, and plagioclase was analyzed in two samples from no relation to metamorphic grade. Compositionally 

the low-grade biotite zone (table 4). Mg/(Mg+Fe) ratios zoned plagioclase grains are not common; where zoning 

are 0.52 and 0.67, and (Fe, Mg)/ AI ratios are 1. 76 and is present the rims are enriched by 1 to 5 mole percent in 

1.67. There are no significant differences in composition the anorthite component. Orthoclase content does not exfrom grain to grain in the same sample. Late-generation ceed 2.5 mole percent and commonly is less than 1 mole 

chlorite in a sample from the staurolite-biotite zone has percent. 

an Mg/(Mg+Fe) ratio of 0.55 and an (Fe, Mg)/Al ratio of Muscovite in a given sample is chemically homoge1.57. neous, but the fact that its composition shows considerable 
12 PETROLOGIC CHARACTERIZATION OF PELITIC SCHISTS, NEAR JUNEAU, ALASKA 

variation between samples appears to reflect bulk rock 

composition rather than systematic changes with metamorphic grade (table 6). Phengite content, expressed by 

excess Si over the ideal trisilicic formula, ranges from 

Si=3.01 to Si=3.16 per 11 oxygens, and paragonite content (Na/(K+Na+Ba)) ranges from about 0.08 to 0.29. 

Many Juneau area muscovites have a high Ba content; 

Ba/(K+Na+Ba) ratio ranges from about 0.01 to about 

0.16, which is considerably higher than reported in other 

common pelitic schists (Guidotti, 1984). 

Staurolite was analyzed in nine samples, four of 

which are given in table 7. Problems of the staurolite 

formula were discussed by Zen (1981) and Holdaway and 

others (1988). We calculated cation proportions on the 

basis of 23 oxygens, assuming 1 mole H20 per formula. 

The addition of H20 would increase the analytical sums 

to close to 100 percent. The Mg/(Mg+Fe) ratio ranges 

only from 0.17 to 0.23 in eight of the analyzed samples; 

one sample has an Mg/(Mg+Fe) ratio of 0.28. ZnO content ranges from 0.17 to 3.85 weight percent. No optical 

zoning of staurolite was noted, and the microprobe analyses show no appreciable chemical variation within grains 

or between grains in the same sample. 

STAUROLITE-BIOTITE 

ZONE A 

(near isograd) 

STAUROLITE-BIOTITE 

ZONE A 

LOWER 

KY ANITE-BIOTITE 

ZONE 

UPPER 

KYANITE-BIOTITE 

ZONE 

SILLIMANITE 

ZONE 

Figure 3. Schematic Al20 3-Fe0-Mg0 (AFM) projections based 

on observed mineral assemblages in Juneau area. All assemblages 

include quartz and muscovite. A, Al20 3; F, FeO; M, MgO; Grt, 

garnet; Bt, biotite; Chl, chlorite; St, staurolite; Ky, kyanite; Sil, 

sillimanite (from Himmelberg and others, 1991). 

Biotite analyses are given in table 8. Structural formulas of biotite calculated on the basis of 11 oxygens 

consistently yield an octahedral site occupancy of less 

than 3 and an A-site occupancy of less than 1. Dymek 

(1983) demonstrated that octahedral occupancy in biotite 

is generally less than 3 cations per formula as a result of 

Ti and Al VI octahedral substitution not balanced by tetrahedral Al. Biotite ranges in Mg/(Mg+Fe) ratio from 0.31 

to 0.70. Many of the analyzed biotites occur in multivariant assemblages and the compositional variations 

clearly reflect bulk composition. In assemblages that are 

divariant relative to the ideal AFM projection, the biotite 

commonly changes in Mg/(Mg+Fe) ratio with metamorphic 

grade, consistent with the change predicted by Thompson 

(1976). In some instances, however, the variation between 

14 

12 

~ 10 c( 

ID 

0 

..J 

Si2 8 

~ 

u.i 

a: 

~ 6 1/) 

w 

a: 

c. 

OL---~---L---L--~----L---~--~--~--~ 

350 450 550 650 750 

TEMPERATURE, IN DEGREES CELSIUS 

Figure 4. Petrogenetic grid for the SiOTA120 3-FeO-MgO-K20-

H20 system from Spear and Cheney (1989) showing reaction 

curves appropriate for assemblages in Juneau pelitic schists. Shortdashed boundaries are for garnet+chlorite+biotite assemblages 

where Mni(Mn+Fe+Mg) is 0.1. Plotted points are preferred 

temperatures and pressures calculated from mineral equilibria for: 

lower kyanite-biotite zone (point 1), upper kyanite-biotite zone 

(point 2), and sillimanite zone (points 4, 5). Point 3 is inferred 

from pressure-temperature path calculation (Spear and Selverstone, 

1983) using zoned garnet in sillimanite zone. Point 6 is determined 

for a sample of staurolite-biotite zone and is interpreted to reflect 

reequilibration rather than peak metamorphism. Arrow is inferred 

prograde and retrograde metamorphic field gradient for Juneau 

pelitic schists (see Himmelberg and others, 1991, for discussion). 

Low-grade metamorphic field gradient {long dash) is extrapolated 

from data of Himmelberg and others (in press). Aim, almandine; 

And, andalusite; Ann, annite; AS, aluminosilicate; Bt, biotite; Chi, 

chlorite; Cld, chloritoid; Crd, cordierite; Fe, iron; Grt, garnet; Kfs, 

K-feldspar; Ky, kyanite; Ms, muscovite; Prl, pyrophyllite; Qtz, 

quartz; Sil, sillimanite; St, staurolite. 
MINERAL CHEMISTRY 13 

Table 4. Representative analyses of 

chlorite from the Juneau area. 

[Leaders(-), not detected] 

Biotite zone 

79GH5A 

Si02 -------------- 26.6 

Ti02 -------------- 0.04 

Al203 ------------ 20.8 

FeO 1-------------- 24.6 

MnO-------------- 0.40 

MgO -------------- 15.1 

CaO--------------- 0.02 

Na20 ------------- 0.08 

79GH11B 

27.2 

0.10 

22.2 

17.0 

0.10 

9.8 

0.03 

K20 --------------__ 0_.0_3 ____ _ 

Total---------- 87.67 86.43 

Formula normalized to 14 oxygens 

Si------------------ 2.788 2.767 

AliV --------------- 1.212 1.233 

AlVI --------------- 1.358 1.428 

Ti ----------------- 0.003 0.008 

Fe----------------- 2.156 1.446 

Mn ---------------- 0.036 0.009 

Mg ---------------- 2.359 3.002 

Ca----------------- 0.002 

Na ---------------- 0.016 0.006 

K------------------ 0.004 

Mg/(Mg+Fe) ----- 0.52 0.67 

1Total iron calculated as FeO. 

Table 5. Representative analyses of plagioclase from the 

Juneau area. 

[Ky, kyanite; Bt, biotite; An, anorthite; Ab, albite; Or, orthoclase] 

Lower Ky-Bt zone Sillimanite zone 

79GH28A 79GH33A 79GH46A 79GH52A 79GH100B 

Si02 ---- 63.2 62.9 62.2 62.7 59.0 

Al203 -- 22.8 23.0 23.4 23.1 25.5 

CaO----- 3.45 3.56 4.50 3.92 6.75 

Na20--- 9.62 9.57 9.03 9.34 7.71 

K20 ---- 0.12 0.10 0.16 0.09 0.30 

Total-- 99.19 99.13 99.29 99.15 99.22 

Formula normalized to 8 oxygens 

Si-------- 2.909 2.898 2.871 2.891 2.653 

AI------- 1.053 1.065 1.085 1.070 1.349 

Ca------- 0.170 0.176 0.223 0.194 0.325 

Na ------ 0.859 0.855 0.807 0.835 0.672 

K-------- 0.007 0.006 0.009 0.005 0.017 

An------ 16.4 16.9 21.4 18.7 32.1 

Ab ------ 82.9 82.5 77.7 80.8 66.3 

Or------- 0.7 0.6 0.9 0.5 1.7 

samples is greater than reasonable for the field interval 

involved and thus probably also reflects bulk rock compositional variation. The most Fe-rich biotite is greenish 

brown in color and occurs in a calcareous biotite-chloritemuscovite schist in the low-grade biotite zone. The most 

Mg-rich biotite coexists with the most Mg-rich staurolite 

analyzed. Biotite is chemically unzoned, and no compositional differences exist between biotite porphyroblasts 

and matrix biotite in the same sample nor between biotite 

in contact with garnet or staurolite relative to matrix 

biotite. Biotite enclosed in garnet in sample 79GH52A, 

Table 6. Representative analyses of muscovite from the Juneau 

area. 

[St, staurolite; Bt biotite; Ky, kyanite; Ms, muscovite; Pg, paragonite; Ba, 

barium mica] 

Sillimanite 

St-Bt zone Lower Ky-Bt zone zone --- 79GH22A 79GH25A 79GH25C 79GH28A 79GH33A 79GH52A 

Si02 ---- 46.9 46.2 47.3 46.4 46.4 46.8 

Ti02---- 0.43 0.54 0.51 0.57 0.76 1.27 

Al203 -- 36.1 35.3 36.0 34.2 35.0 34.6 

Fe01---- 0 .. 93 0.81 0.74 1.27 1.30 1.28 

MgO---- 0 .. 59 0.50 0.51 1.00 0.87 0.47 

BaO----- 2 .. 05 2.77 2.77 2.48 1.54 0.27 

Na20 --- 1.73 1.89 2.12 0.86 0.90 1.00 

K20 ---- 7 .. 80 7.38 7.19 9.15 9.43 10.00 

Total-- 96.53 95.39 97.14 95.93 96.20 95.69 

Formula normalized to 11 oxygens 

Si-------- 3.082 3.086 3.095 3.104 3.077 3.013 

AliV ----- 0.918 0.914 0.905 0.896 0.923 0.897 

AlVI _____ 1.879 1.865 1.871 1.805 1.820 1.806 

Ti ------- 0.021 0.027 0.025 0.029 0.038 0.064 

Fe------- 0.051 0.045 0.040 0.071 0.072 0.072 

Mg------ 0.058 0.050 0.050 0.100 0.086 0.046 

Ba------- 0.053 0.073 0.071 0.065 0.040 0.007 

Na ------ 0.220 0.245 0.269 0.112 0.116 0.129 

K-------- 0.653 0.629 0.600 0.781 0.799 0.847 

Ms ------ 0.705 0.665 0.638 0.816 0.837 0.861 

Pg ------- 0.238 0.259 0.286 0.117 0.121 0.132 

Ba------- 0.057 0.077 0.076 0.068 0.042 0.007 

1Total iron calculated as FeO. 

however, has a higher Mg/(Mg+Fe) ratio (0.50) than matrix biotite (0.45), even where the matrix biotite is in contact with garnet. The biotite enclosed in garnet also has a 

lower Ti02 content (1.63 percent) than the matrix biotite 

(2.57 percent). Ti02 content in all biotite analyzed ranges 

from about 1.12 to about 3.55 weight percent but not in 

relation to metamorphic grade as was reported by other 

workers (Guidotti, 1970; Fletcher and Greenwood, 1979; 

McLellan, 1985). Rare samples have a millimeter-scale 

layering with different color biotites in different layers; 

the biotite Mg/(Mg+Fe) ratios in adjacent layers differ by 

0.05 and Ti02 contents differ by more than 1 weight percent. Retention of the biotite compositional differences 

indicates that the scale of intergrain metamorphic diffusion was restricted to millimeters or less. 

Analyzed garnets are almandine rich (table 9). Using 

average rim and core compositions, the garnet components reported here and in Himmelberg and others (1991) 

range from about 46 to 75 percent almandine, 6 to 18 percent pyrope, 4 to 25 percent grossular, and 3 to 30 percent spessartine. The Mg/(Mg+Fe) ratio is generally 

between 0.1 and 0.2, although two samples have values 

of about 0.28. Most garnets are compositionally zoned, 

and compositions of individual points may fall outside the 

ranges given above. Maximum compositional zoning for 

a single grain is 15 mole percent almandine, 10 mole 

percent pyrope, 12 mole percent grossular, and 17 mole 

percent spessartine. Core to rim compositional zoning 

profiles for garnets in the garnet zone through the kyanite-
14 PETROLOGIC CHARACTERIZATION OF PELITIC SCHISTS, NEAR JUNEAU, ALASKA 

Table 7. Representative analyses of staurolite from the Juneau area. 

[St, staurolite; Bt, biotite; Ky, kyanite] 

St-Bt zone Lower Ky-Bt zone 

79GH2IA 79GH22A 79GH23B 79GH25A 79GH25C 79GH35A 

Si02 ---------- 28.2 27.7 28.3 27.5 27.8 27.8 

Ti02 ---------- 0.44 0.56 0.25 0.64 0.59 0.64 

Al203 -------- 53.3 53.5 52.5 

Fe01---------- 10.9 12.7 9.50 

53.6 52.5 52.9 

13.8 13.8 12.8 

MoO---------- 0.17 0.18 0.48 0.17 0.22 0.38 

MgO---------- 1.81 1.79 2.07 1.90 1.94 1.79 

ZoO----------- 2.79 1.46 3.85 0.41 0.61 1.12 

Total-------- 97.61 97.89 96.95 98.02 97.46 97.43 

Formula normalized to 23 oxygens 

Si-------------- 3.924 3.853 3.967 3.821 3.890 3.884 

Al------------- 8.740 8.771 8.674 8.777 8.658 8.712 

Ti ------------- 0.046 0.059 0.026 0.067 0.062 0.067 

Fe------------- 1.268 1.477 1.114 1.603 1.615 1.496 

Mn ------------ 0.020 0.021 0.057 0.020 0.026 0.045 

Mg ------------ 0.375 0.371 0.433 0.393 0.405 0.373 

Zo------------- 0.287 0.150 0.399 0.042 0.063 0.116 

Mg/(Mg+Fe)- 0.23 0.20 0.28 0.20 0.20 0.20 

1

Total iron calculated as FeO. 

biotite zone are typical growth-zoning patterns (Tracy, 

1982). The garnet profiles in the sillimanite zone are typical 

diffusion homogenized compositional patterns modified by 

post-growth retrograde reequilibration (Tracy, 1982). lllustrations of these patterns and a discussion of the specific 

details were given by Himmelberg and others (1991). 

ESTIMATION OF FLUID COMPOSITION 

The H20 content of the fluid phase present during 

the metamorphism of the Juneau pelitic schists can beestimated from the dehydration equilibria involved in the 

breakdown of muscovite and staurolite and from considering the distribution of species in the C-0-H system in 

the presence of graphite. French ( 1966) and Ohmoto and 

Kerrick ( 1977) have shown that, under typical metamorphic conditions with graphite present, H20, C02, and 

CH4 are the principal fluid species present and that because of the equilibrium 

(9) 

pure water is not stable. The mole fraction of H20 in 

fluids of graphite-bearing rocks is a slowly varying function ofT and Panda rapidly varying function of oxygen 

fugacity (f 02) (Ohmoto and Kerrick, 1977); if T, P, and 

f 02 are known then the mole fraction of H20 (X H2o) can 

be uniquely determined. An independent determination 

off 02 is not available for the Juneau pelitic schists; nevertheless, the maximum possible values of XH2

o for the 

various metamorphic zones can be determined by utilizing the data of Ohmoto and Kerrick (1977) for the temperatures and pressures estimated from garnet-biotite 

equilibrium. Maximum XH2

o decreases as temperature 

increases because reaction 9 is favored by increasing ternperature (Ohmoto and Kerrick, 1977). Using the average 

temperatures and pressures determined for each metamorphic zone, the maximum XH2

o ranges from about 0.93 

for the garnet zone to about 0.90 for the upper kyanitebiotite zone. If the Juneau pelitic rocks were metamorphosed as a closed system, then the values above are 

probably reasonable estimates because Ohmoto and 

Kerrick (1977) have shown that dehydration equilibria of 

graphite-bearing pelites in a closed system buffer XH2o 

to maximum possible values during metamorphism. 

The equilibrium 

NaA13Si30 10(0H)2+Si02=NaA1Si30 8+Al2Si05+H20 (1 0) 

paragonite quartz albite ky/sil 

was applied to the three samples in the lower kyanitebiotite zone. The expression for the end-member equilibrium was taken from Lang and Rice (1985), and activitycomposition relations were taken from Pigage and 

Greenwood (1982). Muscovite solid solution was modeled in two ways in order to determine the activity of 

paragonite. Model 1 assumes a simple binary solution between muscovite and paragonite; model 2 considers nonideal mixing with muscovite, paragonite, K celadonite, 

and N a celadonite as components (Pi gage and Greenwood, 

1982). Using the lower kyanite-biotite zone average temperature and pressure as determined from garnet-biotite and 

garnet-muscovite-plagioclase-biotite equilibria, respectively, values for X H2

o ranging from 0.89 to 0.64 were 

obtained for muscovite model 1, and values ranging from 

0.77 to 0.50 were obtained for muscovite model 2. The 

muscovite dehydration equilibrium was also applied to 

sillimanite-bearing samples using the end-member equilibrium expression calculated by McLellan (1985), but 

anomalously high values for XH2

o (greater than 1) were 

obtained. 

The dehydration equilibrium 

1/2Fe4Al18Si7.

50 44(0H)4+ 12.5/6Si02= 

staurolite quartz 

2/3Fe3Al2Si30 12+23/6Al2Si05+H20 (11) 

garnet kyanite 

is also applicable to assemblages in the lower kyanitebiotite-zone pelitic schists. The mole fraction of H20 

necessary to displace the equilibrium to the average temperature and pressure of the zone was calculated using 

the end-member equilibrium expression determined by 

Lang and Rice (1985), the activity-composition model for 

garnet proposed by Hodges and Spear (1982) and Hodges 

and Royden (1984), and activity-composition models for 

staurolite proposed by Pigage and Greenwood (1982) and 

Holdaway and others (1988). Calculated values for XH2

o 

range from 0.82 to 0.63 with no significant differences in 

values obtained using the two different activity-composition 

models for staurolite. 
SUMMARY 15 

Table 8. Representative analyses of biotite from the Juneau area. 

[Leaders(-), not detected. Gt, garnet; St, staurolite; Bt, biotite; Ky, kyanite] 

Gt zone St-Bt zone LowerKy-Bt zone Sillimanite zone 

79GH118 79GH22A 79GH23A 79GH238 79GH25A 79GH25C 79GH25C 79GH33A 79GH46A 79GH52A 79GHIOOB 

Si02 ------------ 38.6 38.2 38.8 38.8 37.8 37.0 36.6 36.6 36.5 35.9 37.3 

Ti02------------ 1.81 1.72 1.73 1.12 1.83 1.74 2.83 2.50 2.68 2.57 1.38 

Al203 ---------- 17.5 19.6 19.4 19.8 19.3 19.6 19.3 19.8 19.8 19.9 20.4 

Feo1 ----------- 14.2 17.2 11.7 11.6 16.7 17.0 16.0 17.3 18.4 19.5 14.3 

MnO------------ 0.06 0.17 0.15 0.07 0.06 0.35 0.24 0.10 0.13 0.10 

MgO------------ 13.8 11.4 15.0 15.2 11.4 11.2 11.2 10.6 9.35 8.90 13.4 

CaO ------------ 0.04 0.07 

N a20----------- 0.23 0.32 0.34 0.37 0.44 0.40 0.26 0.32 0.40 0.35 0.26 

K20 ------------ 8.42 8.63 8.63 8.29 8.27 8.56 8.91 9.03 8.82 9.30 9.34 

Total---------- 94.60 97.13 95.77 95.33 95.81 95.56 95.45 96.39 96.05 96.55 96.55 

Formula normalized to 11 oxygens 

Si --------------- 2.849 2.787 2.797 2.799 2.787 

AI IV------------- 1.151 1.213 1.203 1.201 1.213 

AI VI _____________ 0.374 0.471 0.444 0.487 0.468 

Ti --------------- 0.100 0.094 0.094 0.061 0.101 

Fe--------------- 0.878 1.047 0.705 0.701 1.029 

Mn-------------- 0.004 0.010 0.009 0.004 

Mg-------------- 1.520 1.236 1.611 1.632 1.248 

Ca -------------- 0.003 

N a-------------- 0.033 0.046 0.048 0.051 0.063 

K --------------- 0.793 0.802 0.793 0.763 0.779 

Mg/(Mg+Fe) --- 0.63 0.54 0.70 0.70 0.55 

1

Total iron calculated as FeO. 

The values obtained for mole fraction of H20 from 

the muscovite and staurolite equilibria are generally consistent but less than the values indicated by the C-0-H 

equilibria. Values of XH2o calculated from the muscovite 

and staurolite equilibria, however, are extremely sensitive 

to temperature, and considering the uncertainty in the 

temperature determinations, the mole fraction of H20 is 

probably better constrained by the distribution of species 

in the C-0-H system for graphitic schists. 

SUMMARY 

The western metamorphic belt underwent a complex 

history of deformation, metamorphism, and plutonism that 

ranges in age from about 120 Ma to about 50 Ma 

(Crawford and others, 1987; Brew and others, 1989). The 

protoliths for the western metamorphic belt were mainly 

the heterogeneous Alexander terrane rocks of Permian and 

Triassic age and the flysch and volcanic rocks of the 

Gravina overlap assemblage (Berg and others, 1972) of 

Late Jurassic through early Late Cretaceous age. In the 

Juneau area, the metamorphic rocks consist dominantly of 

intermixed pelitic and semipelitic metasedimentary rocks, 

and mafic metavolcanic and intrusive rocks. Impure calcareous metasedimentary rocks, quartzite, and quartz diorite and granodioritic orthogneiss are also present. 

Most of the schists and gneisses in the Juneau area 

are products of the M5 metamorphic event of Brew and 

others (1989), which they interpreted to have occurred between about 70 Ma and 65 Ma. The mineral isograds, systematic changes in mineral assemblages, and structural 

2.752 2.723 2.713 2.725 2.689 2.716 

1.248 1.277 1.287 1.275 1.311 1.284 

0.463 0.416 0.439 0.463 0.448 0.467 

0.097 0.158 0.139 0.150 0.145 0.076 

1.054 0.994 1.072 1.149 1.221 0.871 

0.004 0.022 O.oi5 0.006 0.008 0.006 

1.243 1.241 1.170 1.039 0.994 1.455 

0.005 

0.058 0.037 0.045 0.058 0.051 0.037 

0.811 0.845 0.853 0.839 0.889 0.868 

0.54 0.56 0.52 0.47 0.45 0.62 

relations indicate an inverted metamorphic gradient along 

the easternmost part of the belt, where the metamorphic 

grade goes abruptly from greenschist facies to kyaniteand sillimanite-bearing amphibolite facies in less than 5 

km across strike. The westernmost part of the belt consists of metabasites metamorphosed to pumpellyiteactinolite facies during the M 1 metamorphic event. The 

major schistosity, S 1, is subparallel to compositional layering and is interpreted to have formed during the regional D 1 deformational event. The D3 deformation 

event, which was synchronous with M5 metamorphism, 

folded the S1 schistosity about gently plunging northwest 

trending axes. F3 axial planes are generally steep and development of an S3 axial plane foliation is variable. On 

the limbs of folds sl and s3 foliation are subparallel and 

not readily distinguished. No large-scale structures or minor folds associated with the D 1 deformation event have 

been clearly identified, although there is some suggestion 

that the D1 and D3 deformation events may have been 

nearly coaxial. The isograd surfaces strike northwest and 

dip to the northeast, parallel or subparallel to the dominant metamorphic S 1

-S3 foliation. 

Pelitic mineral assemblages produced during the M5 

metamorphic event define six metamorphic zonesbiotite, garnet, staurolite-biotite, lower kyanite-biotite, 

upper kyanite-biotite, and sillimanite. Isograds separating 

the zones are in general agreement with discontinuous 

reactions in the ideal KMFASH system. Peak temperatures of metamorphism increase progressively from about 

530°C for the garnet zone to about 705°C for the upper 

kyanite-biotite zone. Silicate geobarometry suggests that 

the thermal peak metamorphism occurred under pressures 
16 PETROLOGIC CHARACTERIZATION OF PELITIC SCHISTS, NEAR JUNEAU, ALASKA 

I I I I I I I 

I I I I I I I 

I I I f I 

I I I I I 

1 : M : : : : '(i 

ss~o~C?oo o i:i)~::(~~~~E--

&l~~~~~~~ 0\0 0 OllillilliN 

~o~o.....:ooo 

~~~~~~&l:g 0\00\0Cf"lOOCf"lCf"l 

~0.....: o.....:ooo 

I I I I I I I I 

I I I I I I I I I I I I I I I I 

I I I I I I I I 

I I I I I I I I 

I I I I I I I I 

! ~· >' : : : ~: v;:.;::.;:~r.e~~~ 

r-- 0\0\ IIi 

~,....;cx).,o 

--N..qof 9 to 11 kbar. The sequence of reactions and the calculated pressure and temperature for the individual metamorphic zones are consistent with the petrogenetic grid 

of Spear and Cheney (1989). Changes in Mg/(Mg+Fe) 

ratio of biotite and staurolite with increasing metamorphic grade in three.:. and four-phase assemblages appropriate to reactions in the KMFASH system are generally 

consistent with the changes predicted by Thompson 

(1976), although exceptions do exist. Changes in garnet 

composition in the same assemblages are complicated, 

owing to complex zoning patterns and retrograde equilibration of some rim compositions. On the basis of the distribution of species in the C-0-H system for graphitic 

schists, the mole fraction of H20 during metamorphism 

probably ranged from about 0.93 for the garnet zone to 

about 0.90 for the kyanite-biotite zone. 

REFERENCES CITED 

Albee, A.L., and Ray, L., 1970, Correction factors for electron probe 

microanalysis of silicates, oxides, carbonates, phosphates, and sulfates: Analytical Chemistry, v. 42, p. 1408-1414. 

Bauer, R.L., Himmelberg, G.R., Brew, D.A., and Ford, A.B., 1988, 

Relative timing of porphyroblast growth, foliation development, 

and ductile shear in pelitic metamorphic rocks from the Juneau 

area, southeastern Alaska, in Galloway, J.P. and Hamilton, T.P., 

eds., Geological studies in Alaska by the U.S. Geological Survey 

during 1987: U.S. Geological Survey Circular 1016, p. 138-142. 

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

growth and crenulation cleavage development during progressive deformation: Tectonophysics, v. 92, p. 171-194. 

Bence, A.E., and Albee, A.L., 1968, Empirical correction factors for the electron microanalysis of silicates and oxides: 

Journal of Geology, v. 76, p. 382-403. 

Berg, H.C., Jones, D.L., and Richter, D.H., 1972, GravinaNutzotin belt-Tectonic significance of an upper Mesozoic 

sedimentary and volcanic sequence in southern and southeastern Alaska, in Geological Survey Research, 1972: U.S. 

Geological Survey Professional Paper 800 D, p. D1-D24. 

Brew, D.A., 1988, Late Mesozoic and Cenozoic igneous rocks 

of southeastern Alaska-A synopsis: U.S. Geological Survey Open-File Report 88-405, 29 p. 

Brew, D.A., and Ford, A.B., 1977, Preliminary geologic and 

metamorphic-isograd map of the Juneau B-1 quadrangle, 

Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-846, scale 1:31,680. 

---1978, Megalineament in southeastern Alaska marks 

southwest edge of Coast Range batholithic complex: Canadian Journal of Earth Sciences, v. 15, p. 1763-1762. 

---1983, Comment on "Tectonic accretion and the origin 

of the two major metamorphic and plutonic welts in the 

Canadian Cordillera": Geology, v. 11, p. 427-428. 

---1984, Tectonostratigraphic terranes in the Coast plutonicmetamorphic complex, southeastern Alaska, in BarschWinkler, S., and Reed ,K., eds., The United States Geological 

Survey in Alaska: Miscellaneous geologic research 1982: U.S. 

Geological Survey Circular 939, p. 90-93. 
REFERENCES CITED 17 

Brew, D.A., Ford, A.B., and Himmelberg, G.R., 1989, Evolution of the western part of the Coast plutonic-metamorphic 

complex, southeastern Alaska, U.S.A.: A summary, in 

Daly, S.R., Cliff, R.A., and Yardley, B.W., eds., Evolution 

of metamorphic belts: Geological Society Special Publication 43, p. 447-452. 

Brew, D.A., Himmelberg, G.R., Loney, R.A., and Ford, A.B., 

1992, Distribution and characteristics of metamorphic belts in 

the south-eastern Alaska part of the North America Cordillera: Journal of Metamorphic Geology, v. 10, p. 465-482. 

Brew, D.A., and Morrell, R.P., 1983, Intrusive rocks and plutonic belts of southeastern Alaska, U.S.A, in Roddick, J.A., 

ed., Circum-pacific plutonic terranes: Geological Society 

of America Memoir 159, p. 171-193. 

Brew, D.A., Ovenshine, A.T., Karl, S.M., and Hunt, S.J., 1984, 

Preliminary reconnaissance geologic map of the Petersburg 

and parts of the Port Alexander and Sumdum ! :250,000 

quadrangles, southeastern Alaska: U.S. Geological Survey 

Open-File Report 84-405, 2 sheets, 43 p. pamphlet. 

Carmichael, D.M., 1970, Intersecting isograds in the Whetstone 

Lake area, Ontario. Journal of Petrology, 11, 147-181. 

Crawford, M.L., Hollister, L.S., and Woodsworth, G.J., 1987, 

Crustal deformation and regional metamorphism across a 

terrane boundary, Coast Plutonic Complex, British Columbia: Tectonics, v. 6, p. 343-361. 

Donelick, R.A., 1986, Mesozoic-Cenozoic thermal evolution of 

the Atlin terrane, Whitehorse Trough, and Coast Plutonic 

Complex from Atlin, British Columbia to Haines, Alaska 

as revealed by fission track-geothermometry techniques: 

Troy, New York, Rensselaer Polytechnic Institute, M.S. 

thesis, 167 p. 

Dymek, R.F., 1983, Titanium, aluminum, and interlayer cation 

substitutions in biotite from high-grade gneisses, West 

Greenland: American Mineralogist, v. 68, p. 880-899. 

Fletcher, C.J.N., and Greenwood, H.J., 1979, Metamorphism and 

structure of the Penfold Creek area, near Quesnel Lake, British Columbia: Journal of Petrology, v. 20, p. 743-794. 

Forbes, R.B., 1959, The geology and petrology of the Juneau 

Ice Field area, southeastern Alaska: Seattle, University of 

Washington, Ph.D. dissertation, 261 p. 

Forbes, R.B., and Engels, J.C., 1970, K40/Ar40 age relations of 

the Coast Range batholith and related rocks of the Juneau 

ice field area, Alaska: Geological Society of America Bulletin, v. 81, p. 579-584. 

Ford, A.B., and Brew, D.A., 1973, Preliminary geologic and 

metamorphic-isograd map of the Juneau B-2 quadrangle, 

Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-527, scale 1:31,680. 

---1977a, Preliminary geologic and metamorphic-isograd 

map of the Juneau A-1 and A-2 quadrangles, Alaska: U.S. 

Geological Survey Miscellaneous Field Map MF-84'7., 

scale 1 :31,680. 

---1977b, Truncation of regional metamorphic zonation 

pattern of the Juneau , Alaska, area by the Coast Range 

batholith, in Blean, K.M., ed., The United States Geological Survey in Alaska: Accomplishments during 1976: U.S. 

Geological Circular 751-B, p. B85-B87. 

French, B.M., 1966, Some geological implications of equilibrium 

between graphite and a C-H-0 gas phase at high temperatures 

and pressures: Reviews of Geophysics, v. 4, p. 223-253. 

Gehrels, G.E., Brew, D.A., and Saleeby, J.B., 1984, Progress 

report on U/PB (zircon) geochronologic studies in the 

Coast plutonic-metamorphic complex east of Juneau, 

southeastern Alaska, in Reed, K.M. and Bartsch-Winkler, 

S., ed., The United States Geological Survey in Alaska: 

Accomplishments during 1982: U.S. Geological Survey 

Circular 939, p. 100-102. 

Gehrels, G.E., McClelland, W.C., Samson, S.D., Patchett, P.J., 

and Brew, D.A., 1991, U-Pb geochronology of Late Cretaceous and early Tertiary plutons in the northern Coast 

Mountains batholith: Canadian Journal Earth Sciences, v. 

28, p. 899-911. 

Geological Society of America, 1984, Decade of North American Geology Geologic Time Scale: Geological Society of 

America Map and Chart Series, MC-50. 

Guidotti, C.V., 1970, The mineralogy and petrology of the transition from the lower to upper sillimanite zone, in the Oquossoc 

area, Maine: Journal of Petrology, v. 11, p. 277-336. 

---1974, Transition from staurolite to sillimanite zone, 

Rangely quadrangle, Maine: Geological Society of 

America Bulletin, v. 85, p. 475-490. 

---1984, Micas in metamorphic rocks: Mineralogical Society of America Reviews in Mineralogy, v. 13, p. 357-467. 

Himmelberg, G.R., Brew, D.A., and Ford, A.B., 1991, Development of inverted metamorphic isograds in the western 

metamorphic belt, Juneau, Alaska: Journal of Metamorphic Geology, v. 9, p. 165-180. 

---in press, Low-grade metamorphism of the Douglas Island Volcanics: Earliest recognized metamorphic event in 

the western metamorphic belt near Juneau Alaska: Geological Society of America Special Paper. 

Himmelberg, G.R., Ford, A.B., and Brew, D.A., 1984a, Progressive metamorphism of pelitic rocks in the Juneau area, 

southeastern Alaska, in Coonrad, W.L., and Elliot, R.L., 

eds., The United States Geological Survey in AlaskaAccomplishments during 1981: U.S. Geological Survey 

Circular, 868, p. 131-134. 

---1984b, Reaction isograds in pelitic rocks of the Coast 

Plutonic-Metamorphic Complex near Juneau, in Reed, 

K.M., and Bartsch-Winkler, S., eds., The United States 

Geological Survey in Alaska-Accomplishments during 

1982: U.S. Geological Survey Circular 939, p. 105-108. 

Hodges, K.V., and Royden, L., 1984, Geologic thermobarometry of retrograded metamorphic rocks: an indication 

of the uplift trajectory of a portion of the northern Scandinavian Caledonides: Journal of Geophysical Research, v. 

89, p. 7077-7090. 

Hodges, K.V .. , and Spear, F.S., 1982, Geothermometry, geobarometry, 

and the Al2Si05 triple point at Mt. Moosilauke, New Hampshire: American Mineralogist, v. 67, p. 1118-1184. 

Holdaway, M.J., Dutrow, B.L., and Hinton, R.W., 1988, Devonian 

and Carboniferous metamorphism in west-central Maine: The 

muscovite-almandine geobarometer and the staurolite problem revisited: American Mineralogist, v. 73, p. 20-47. 

Holdaway, M.J., Guidotti, C.V., Novak, J.M., and Henry, W.E., 

1982, Polymetamorphism in medium- to high-grade pelitic 

metamorphic rocks, west-central Maine: Geological Society of America Bulletin, v. 93, p. 572-584. 

Hooper, R.J., Brew, D.A., Himmelberg, G.R., Stowell, H.H., 

Bauer, R.L., and Ford, A.B., 1990, The nature and signifi-
18 PETROLOGIC CHARACTERIZATION OF PELITIC SCHISTS, NEAR JUNEAU, ALASKA 

cance of post-thermal-peak shear zones west of the great 

tonalite sill near Juneau, Alaska, in Dover, J.H. and Galloway, J.P., eds., Geologic Studies in Alaska by the U.S. 

Geological Survey, 1989: U.S. Geological Survey Bulletin 1946, p. 88-94. 

Klaper, E.M., and Bucher-Nurminen, K., 1987, Alpine metamorphism of pelitic schists in the Nufenen Pass area, Lepontine 

Alps: Journal of Metamorphic Geology, v. 5, p. 175-194. 

Lang, H.M., and Rice, J.M., 1985, Geothermometry, geobarometry, and T-X(Fe-Mg) relations in metapelites, Snow 

Peak, northern Idaho: Journal of Petrology, v. 26, p. 889-924. 

McClelland, W.C., Anovitz, L.M., and Gehrels, G.E., 1991, 

Thermobarometric constraints on the structural evolution of 

the Coast Mountains batholith, central southeastern Alaska: 

Canadian Journal of Earth Sciences, v. 28, p. 912-928. 

McLellan, E., 1985, Metamorphic reactions in the kyanite and 

sillimanite zones of the Barrovian type area: Journal of Petrology, v. 26, p. 789-818. 

Monger, J.W.H., Price, R.A., and Tempelman-Kluit, D.J., 1982, 

Tectonic accretion and the origin of the two major metamorphic and plutonic welts in the Canadian Cordillera: Geology, v. 10, p. 70-75. 

Novak, J.M., and Holdaway, M.J., 1981, Metamorphic petrology, mineral equilibria, and polymetamorphism in the 

Augusta quadrangle, southcentral Maine: American Mineralogist, v. 66, p. 51-69. 

Ohmoto, H., and Kerrick, D.M., 1977, Devolatiliztion equilibria in graphitic systems: American Journal of Science: v. 

277,p. 1013-1044. 

Pigage, L.C., and Greenwood, H.J., 1982, Internally consistent 

estimates of pressure and temperature-The staurolite 

problem: American Journal of Science, v. 282, p. 943-969. 

Spear, F.S., and Cheney, J.T., 1989, A petrogenetic grid for 

pelitic schists in the system Si02-Al20 3-Fe0-Mg0-K20-

H20: Contributions to Mineralogy and Petrology, v. 101, 

p. 149-164. 

Spear, F.S., and Selverstone, J.E., 1983, Quantitative P-T paths 

from zoned minerals: Theory and tectonic applications: Contributions to Mineralogy and Petrology, v.83, p. 348-357. 

Stowell, H., 1989, Silicate and sulfide thermobarometry of lowto medium-grade metamorphic rocks from Holkham Bay, 

south-east Alaska: Journal of Metamorphic Geology, v. 7, 

p. 343-358. 

Thompson, A.B., 1976, Mineral reactions in pelitic rocks: I. 

Prediction of P-T-X (Fe-Mg) phase relations: American 

Journal of Science, v. 276, p. 401-425. 

Thompson, J.B., 1957, The graphical analysis of mineral assemblages in pelitic schists: American Mineralogist, v. 42, 

p. 842-858. 

Tracy, R.J., 1982, Compositional zoning and inclusions in 

metamorphic minerals, in Ferry, J.M., ed., Characterization 

of Metamorphism through Mineral Equilibria: Reviews in 

Mineralogy, v. 10, p. 355-393. 

Wood, D.J., Stowell, H.H., and Onstott, T.C., 1987, Uplift and 

cooling rates from thermochronology (

40 Ar/39 Ar) of the 

Coast plutonic complex sill, SE Alaska: Geological Society America Abstracts with Programs, v. 19, p. 896. 

Zen, E-An, 1981, Metamorphic mineral assemblages of slightly 

calcic pelitic rocks in and around the Taconic allochthon, 

southwestern Massachusetts and adjacent Connecticut and 

New York: U.S. Geological Survey, Professional Paper 

1113, 128 p. 

---1988, Tectonic significance of high-pressure plutonic 

rocks in the western Cordillera of North America, in Ernst, 

W.G., ed., Metamorphism and crustal evolution of the 

western United States (Rubey volume): Englewood Cliffs, 

N.J., Prentice-Hall, p. 41-67. 

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