Regional Thermal Metamorphism and 

Deformation of the Sitka Graywacke, 

Southern Baranof Island, 

Southeastern Alaska 

U.S. GEOLOGICAL SURVEY BULLETIN 1779 

Regional Thermal Metamorphism and 

Deformation of the Sitka Graywacke, 

Southern Baranof Island, 

Southeastern Alaska 

By ROBERT A. LONEY and DAVID A. BREW 

U.S. GEOLOGICAL SURVEY BULLETIN 1779 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1987 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center, Box 25425 

Denver, CO 80225 

Library of Congress Cataloging-in-Publication Data 

Loney, Robert Ahlberg, 1922-

Regional thermal metamorphism and deformation of the 

Sitka Graywacke, southern Baranof Island, southeastern 

Alaska. 

U.S. Geological Survey Bulletin 1779 

Bibliography 

Supt. of Docs. No.: 119.3: 1779 

1. Metamorphism (Geology)-Aiaska-Baranof Island. 2. 

Rock deformation-Alaska Baranof Island. 3. Sitka 

Graywacke (Alaska). I. Brew, David A. II. Title. Ill. Series. 

QE75.B9 No. 1779 557.3 s 87-600001 

[QE475.A2] [552' .4'097982] 
CONTENTS 

Abstract 1 

Introduction 1 

Acknowledgments 2 

Geologic setting 2 

Structure 3 

Metamorphism 3 

General description 3 

Whole-rock composition 5 

Garnet composition 5 

Metamorphic isograds and zones 5 

Discussion 12 

Summary 16 

References cited 16 

FIGURES 

1-3. Maps showing: 

1. Location of the southern Baranof Island study area 2 

2. Location of structural domains and geometry of bedding, foliation, fold 

axes, and lineations 4 

3. Major geologic units, metamorphic isograds and zones related to the 

Crawfish Inlet and Redfish Bay plutons, and location of samples 

analyzed in this study 6 

4. Schematic AFM projection showing the composition of analyzed garnet, 

the adjusted bulk compositions of pelitic rocks, and estimated compositions 

of other important mineral phases 12 

5. Schematic AFM diagrams based on mineral assemblages in pelitic 

metamorphic rocks listed in table 6 13 

6. Petrogenetic grid for muscovite-quartz- and potassium feldsparquartz-bearing systems 14 

TABLES 

1. Metamorphic mineral zones in the aureoles south of Crawfish Inlet as defined 

by Loney and others (1975, p. 79-81) and their correlation with isograds and 

zones in this paper 7 

2. Major oxide and semiquantitative spectrographic analyses of metamorphosed 

coarse-grained rocks of the Sitka Graywacke, southern Baranof Island, 

Alaska 8 

3. Major oxide and semiquantitative spectrographic analyses of metamorphosed 

fine-grained rocks of the Sitka Graywacke, southern Baranof Island, 

Alaska 9 

4. Statistical comparison of metamorphosed coarse- and fine-grained rocks of 

the Sitka Graywacke, southern Baranof Island, Alaska 10 

5. Garnet and whole-rock analyses of schists from the metamorphosed Sitka 

Graywacke, southern Baranof Island, Alaska 11 

6. Observed mineral assemblages in the pelitic metamorphic rocks of southern 

Baranof Island appropriate to the AFM projection (fig. 5) 12 

Ill 

Regional Thermal Metamorphism and Deformation of the 

Sitka Graywacke, Southern Baranof Island, 

Southeastern Alaska 

By Robert A. Loney and David A. Brew 

Abstract 

Souihern Baranof Island is underlain by at least 

1,200 km of regional thermally metamorphosed rocks, 

making it one of the largest regions of such 

metamorphism reported to date. Turbidites of the 

Sitka Graywacke of Late Jurassic and Early 

Cretaceous age belonging to the flysch facies of the 

Chugach terrane were metamorphosed to albiteepidote hornfels, hornblende hornfels, amphibolite, and 

perhaps pyroxene hornfels facies by Tertiary (about 42 

to 47 Ma) granitic plutons. The protolith was 

compositionally limited, and a full sequence of 

metamorphic minerals documenting the temperature 

increase is present; further studies could establish the 

detailed pressure-temperature-composition history of 

the region. 

Prior to the regional thermal metamorphism, the 

Sitka Graywacke was dynamothermally metamorphosed to prehnite-pumpellyite-metagraywacke or 

lower greenschist facies. This event was synchronous 

with regional folding with northwest-striking 

subvertical axial planes and axial-plane foliation and 

axial orientations ranging from a moderate southeast 

plunge through horizontal to a moderate northwest 

plunge. Two generations of folds and foliations of 

about the same style and axial orientation are actually 

present, but because of their similarity they cannot be 

separated consistently and are therefore considered 

two phases of a single deformation. Subsequent 

emplacement of the plutons had little effect on the 

structural geometry. 

Based on major-oxide and trace-element analyses 

of 20 specimens from throughout the region, there 

appear to be no systematic changes in composition 

from lower to higher grades. The analyses show that 

the coarse-grained metagraywackes are semipelites, 

and that the fine-grained metamudstones are pelites 

that are compositionally within the system Si02-

Al2o3-Mg0-FeO-K2o-H2o. 

Individual aureoles are zoned, in general, inward 

from low to high temperature and perhaps from low to 

higher pressure. Local synkinematic metamorphism 

produced intensely foliated and lineated schists 

subparallel to the preexisting general structural 

orientation, and these schists are interpreted to be 

related to an early phase of the intrusive event. Seven 

previously mapped metamorphic mineral zones were 

evaluated relative to reaction isograds and bulk 

compositional control based on mineral assemblages in 

pelitic schists. Zones L1 to L3 are the biotite zone, 

the L3-L4 boundary is the garnet isograd, L4 is the 

garnet zone, the L4-H1 boundary is the andalusitecordierite-biotite isograd, H1 and HZ are the 

andalusite-cordierite-biotite zone, the H2-H3 boundary 

is the sillimanite isograd, and H3 is the sillimanite 

zone; in addition, a higher garnet-cordierite zone was 

newly recognized. The restricted occurrence of 

staurolite is probably due to the generally 

inappropriate bulk composition. 

The sequence of mineral assemblages in the 

aureoles was used to define a pressure-temperature 

gradient that ranges from about 3. 9 kbars at 550 °C to 

about 4.4 kbars at 790 °C. These pressures are 

equivalent to a depth of metamorphism of from 13.6 to 

15.4 km. 

INTRODUCTION 

Previous geologic studies on Ba.ranof Island 

(Loney and others, 1975, p. 62j5, 73-85; fig.1) 

indicated that at least 1,200 km of pelitic and 

semipelitic rocks had been subjected to clearly zoned 

regional thermal metamorphism associated with 

granitic plutons of Tertiary age (42-47 Ma, Loney and 

others, 196 7). Inasmuch as this is one of the world's 

largest known areas of regional metamorphism 

(Miyashiro, 1973; Turner, 1981), we have reexamined 

the original interpretations, which were based on about 

120 thin sections and reconnaissance mapping (Loney 

and others, 1975) in light of 20 additional specimens 

and chemical analyses; our goals in this report are to 

summarize the present understanding of this unique 

region and to encourage more detailed studies. Our 

study is limited both by the reconnaissance nature of 

the fieldwork and by our lack of detailed compositional 

data on the minerals in the different assemblages; we 

have nevertheless pressed the interpretations as far as 

possible lacking such information. 

1 
Figure 1. Index map of southeastern Alaska, showing 

the southern Baranof Island study area. 

The protolith of the metamorphic rocks is the 

Sitka Graywacke of Late Jurassic and Early 

Cretaceous age (Loney and others, 1975). It consists 

dominantly of turbidites, probably all derived from the 

same provenance terrane, that have consistent 

textures, mineralogy, and (as is shown here) a 

restricted range of chemical composition. Prior to the 

intrusion of the granitic bodies, the rocks were folded 

and underwent low-grade regional metamorphism, 

resulting in a homogeneous structural and lithologic 

environment. The granitic bodies were generally 

passively intruded both along and transverse to the 

dominant north-northwest-trending older structures, 

which makes it possible to study metamorphic changes 

along and across the general strike of the 

compositional layers. 

The field studies suggested that the turbidites 

had simple and consistent chemical compositions, and 

this in turn suggested that it would be possible to test 

if the metamorphism were isochemical. To do this (1) 

all but one of the seven different zones defined in our 

previous investigations were revisited and sampled to 

obtain both the coarsest grained rock type (generally 

sandstone protolith) and the finest grained type 

present (generally mudstone protolith); (2) the samples 

were analyzed for 10 major elements by X-ray 

fluorescence techniques, for FeO by chemical 

techniques, and for 45 trace elements by emission2 

spectrographic techniques; and (3) the results of the 

chemical analyses were analyzed statistically for 

central tendency and dispersion, with the sandstones 

and mudstones treated separately. In general, the 

analyses show the sandstones (metagraywackes) to be 

uniformly semipelitic and the mudstones to be 

uniformly pelitic. 

The mineral assemblages resulting from the 

temperature and pressure changes associated with the 

Tertiary intrusions indicate that albite-epidote 

hornfels, hornblende hornfels, amphibolite, and perhaps 

transitional pyroxene hornfels facies (Turner, 1981) are 

present. The wide distribution of pelitic rocks of 

uniform composition resulted in the wide development 

of mineral assemblages in the Si0...2-AI2o3-Mg0-Fe0-

K20-H 0 system (Thompson, 195rl). This uniformity 

allowe~ construction of coherent mineral-assemblage 

zones, which were evaluated relative to isograds and 

to model metamorphic reactions. These zones were 

correlated with the previously mapped metamorphic 

mineral zones (see fig. 5 and tables 1 and 6; Loney and 

others, 1975). In a discussion section, the pressuretemperature (P-T) gradient of the Baranof 

metamorphic facies series is interpreted with regard 

to the sequence of mineral assemblages in this 

region. At the present time, both our lack of detailed 

mineral composition data and the unsolved 

experimental problems prevent certain estimates of PT conditions. Nevertheless, we have used the available 

information to infer ranges for those conditions. 

Acknowledgments 

We wish to thank G.R. Himmelberg for his 

review of the revised manuscript and for helpful 

discussions on metamorphic petrology; A.B. Till, B.R. 

Johnson, and two anonymous reviewers for their 

perceptive reviews of the original manuscript; and two 

anonymous reviewers of the revised manuscript for 

some helpful comments. We also acknowledge the 

helpful advice of L.S. Hollister (an associate editor for 

the Bulletin of the Geological Society of America), 

who enlightened us on the proper vehicle for the 

publication of reconnaissance studies of metamorphic 

terranes. 

GEOLOGIC SE'ITING 

The protolith for the metamorphic rocks 

consisted of semipelitic and pelitic sediments: very 

thick bedded lithic and arkosic wacke, interbedded 

thin- to medium-bedded wacke and shale, and 

rhythmically thin-bedded wacke and shale turbidite. 

Metavolcanic rocks and metacherts occur sparsely in 

the southeastern part of Baranof Island. All these 

rocks are part of the Sitka Graywacke (Loney and 

others, 1975). Based on the study by Zuffa and others 

(1980) of the rock-fragment component in selected 

graywacke samples from southern Baranof Island, the 

clasts were probably derived from a terrane composed 

dominantly of volcanic rocks but also containing a 

significant amount of sedimentary rocks and lesser 
amounts of plutonic and low-grade metamorphic 

rocks. The earlier study (Loney and others, 1975) 

concluded that, compositionally, the graywackes are 

(in order of decreasing abundance) feldspathic, lithic, 

and arkosic (Gilbert, in Williams and others, 1954). 

They typically contain 10-30 percent potassium 

feldspar (orthoclase and minor microcline), 5-30 

percent plagioclase (oligoclase to andesine), 30-6 0 

percent lithic fragments, and minor amounts of 

myrmekite, biotite, muscovite, apatite, augite, 

hypersthene, zircon, garnet, and hornblende. The shale 

or argillite beds interbedded with the graywacke are 

compositionally similar to the graywacke matrix 

material. 

Sometime prior to the contact metamorphism 

and probably accompanying the structural events 

described below, the Sitka Graywacke underwent lowgrade regional dynamothermal metamorphism. In 

general, the mineral assemblages related to this 

"background" metamorphism are indicative of the 

prehnite-pumpellyite-metagraywacke facies (Turner, 

1981), but in many places they are indistinguishable 

from those of the lower greenschist facies. These 

lower grade rocks are characterized by widespread 

chlorite. 

STRUCTURE 

The rocks of southern Baranof Island have been 

folded twice (F 1 and F 2); evidence for these episodes 

is found only in the vicinity of the Tertiary plutons. 

Folds (B1) in bedding (S) have northwest-striking 

subvertical axial planes and axial-plane foliation (S1) 

and axial orientations ranging from a moderate 

southeast plunge through horizontal to a moderate 

northwest plunge. The metamorphic terrane is divided 

into two structural domains (fig. 2): domain I, in which 

B1 folds plunge mainly to the northwest, and domain II, 

in which they plunge mainly to the southeast. The 

southeast-plunging domain contains a number of 

northwest-plunging fold axes that are concentrated 

largely in four narrow eastward-trending subdomains 

too small to show with separate plots but that are 

outlined within the domain (dashed regions in fig. 2). 

The second folding episode (F 2) also affected 

these rocks, and the strong preferred orientation of 

planar and linear elements in these two domains is in 

part due to a pronounced s2 foliation that maintains a 

remarkably uniform orientation throughout the 

metamorphic terrane. The s2 foliation cuts the B1 folds and s1 foliation and appears to be subparallel to 

axial planes of 82 folds that deform bedding and s1 

foliation. In contrast to the commonly cataclastic 

nature of the s1 foliation, the s2 foliation is defined 

mainly by the planar preferred orientation of 

micaceous minerals and by strain-slip cleavage. 

Although there are two generations of folds and 

foliation in the metamorphic terrane, the geometry of 

the planar and linear structures of both generations is 

about the same, and they share almost the same axis 

of folding. The two generations indicate either two 

distinct phases of the same episode of compression or 

two distinct episodes of compression with similar 

orientations. Because of their close similarity of 

orientation, the two generations were not consistently 

separated in the field. They are here considered two 

phases of a single deformation that occurred after the 

deposition of the Sitka Graywacke in Late Jurassic and 

Early Cretaceous time and either with or before the 

Tertiary pluton ism. 

The areas in which these deformational effects 

are most apparent have been mapped as "lineated, 

schistose, and semischistose graywacke and slate" on 

figure 3. The lineations in these areas consist of 

mineral alignments markingS intersections, very small 

scale folds or crenulations, and local rodding. The 

schistosity varies from a close-spaced cleavage to a 

well-developed platy metamorphic mineral 

schistosity. The contacts of this lineated terrane with 

the adjacent nonlineated hornfels are gradational over 

a few hundred meters, except in the area near Whale 

Bay, where sporadic lineations occur over a broad 

area. 

Local recrystallization of the s1 cataclastic 

foliation and synchronous growth of micaceous 

minerals along the s2 foliation show that the 

metamorphic conditions related to the Tertiary 

plutonism in southern and eastern Baranof Island 

started sometime during the F 2 folding (Loney and 

others, 1975, p. 63, 64). The metamorphism appears to 

have continued after the F 2 folding stopped, as 

indicated in the inner zones of some contact aureoles, 

where metamorphic crystallization has tended to 

obscure the F 2 structures. The Tertiary plutons 

appear to be postkinematic because they are distinctly 

crosscutting and not highly foliated. It is possible, 

therefore, that the metamorphism that occurred 

during the F 2 folding preceded the actual intrusion of 

the plutons to their present level. 

METAMORPHISM 

General Description 

The aureoles developed in the Sitka Graywacke 

by the Tertiary granitic rocks are zoned, in general, 

inward from low to high temperature. The pressure of 

metamorphism of the low-temperature, outer part 

(albite-epidote hornfels and (or) greenschist facies) 

was low, whereas the pressure of the hightemperature, inner zones may have been higher (that 

is, transitional between hornblende hornfels facies and 

amphibolite facies; Turner, 1981). The distribution of 

the mineral assemblages indicative of this zonation is 

complicated, varying from pluton to pluton and, around 

the Crawfish Inlet, varying from the north side to the 

south. The area of concern in this study is on the 

south side of the pluton at Crawfish Inlet (table 1; fig. 

3). The aureole here is broad and contains the best 

developed zoning, which is interpreted to have been 

produced by a single episode of thermal 

metamorphism. The apparent higher pressures 

suggested by the mineral assemblages closer to the 

plutons may have been induced by local effects of the 

diapirlike intrusions. This progressive low- to hightemperature, low- to medium-pressure metamorphism 

was accompanied and closely preceded by the 

deformation (F 2> described earlier. 

The areal distribution of the lineated and 

schistose rocks is related to the exposed plutons and 

3 
their contact metamorphic aureoles in that similar 

rocks do not occur elsewhere within the Sitka 

Graywacke, even where isolated plutons are present. 

The lineated rocks do not occur immediately adjacent 

to igneous masses except locally near the pluton at 

Redfish Bay and at Whale Bay, but they do, in general, 

form a band connecting the plutons at Crawfish Inlet 

and at Redfish Bay. The configuration of the 

metamorphic zones shows a close relationship to the 

I 

I 

DATA FOR DOMAINS 

46 poles; 39 axes; contours 20-15-

1 0-5-( 1) percent of 1 percent area 

56° 

45' 

pluton at Redfish Bay and a loose relationship to the 

pluton at Crawfish Inlet (fig. 3), and most of the 

lineated terrane is directly along and to the southeast 

of the shortest distance connection between the two 

plutons. All of the mapped igneous-metamorphic rock 

contacts on the south and southwest sides of the pluton 

at Crawfish Inlet are steep, but several small cupolas 

indicate the probable presence of a larger granitic 

body at depth. 

II 147 poles; 1 05 axes; contours 25-20-

15-( 1) percent of 1 percent area EXPLANATION 

II 

DIAGRAMS 

(Equal area, lower hemisphere projections) 

North 

I-Domain 

MAP 

LI; 

~~-~ / 

~-- Domain boundary or limit 

of study and number 

Dotted where covered by water 

limit of pole distribution ( 1) 

0 5 10 MILES 

0 5 10 KILOMETERS 

Figure 2. Southern Baranof Island, showing structural domains and geometry of bedding, foliation, fold axes, and 

lineations (from Loney and others, 1975, pl. 3). Dashed boundaries indicate regions containing northwest-plunging 

fold axes within the southeast-plunging domain. 

4 
Although not conclusive, these relationships 

suggest that the two igneous masses are connected at 

depth and that the lineated terrane was produced 

during the emplacement of this inferred large igneous 

body. These suggestions are supported by the 

abruptness of the gradational transition between 

lineated and nonlineated terranes, and by the greater 

distance between the metamorphic isograds south of 

the pluton at Crawfish Inlet than north. As 

speculation, it is possible that this inferred concealed 

body is a pluton intruded during F2 folding that is 

nowhere exposed. Later, after the folding had ceased, 

the presently exposed plutons intruded through it to 

their present level. Alternatively, the inferred body 

may represent an early phase of a series of continuous 

intrusions that culminated in the exposed plutons, or it 

may represent a magma chamber from which the later 

magma moved upward. In any case, the lineated and 

schistose unit is interpreted to be a part of the contact 

aureole of an unexposed, slightly(?) earlier, 

synkinematic pluton. If this pluton is the initial phase 

of the Tertiary plutonism, as is suggested above, then 

the F 2 folding died out during the Tertiary plutonism. 

If this interpretation is correct, the lineated terrane 

lies in the southeastward-dipping roof of a large 

concealed pluton and is analogous to the southwarddipping, deeply eroded roof of the pluton elsewhere on 

Baranof Island at Kasnyku Lake (Loney and others, 

1975, p. 85-87). 

Whole-Rock Composition 

Before attempting to apply Thompson's (1957) 

analysis of mineral assemblages in pelitic schists, we 

first examine the bulk compositions of rocks of the 

aureole and compare them with those of typical pelitic 

schist. The 20 samples collected for chemical analysis 

were studied petrographically, but not all the mineral 

assemblages described by Loney and others (1975) were 

found in samples collected for this chemical study. 

Brief descriptions of the samples are given in tables 2 

and 3. The mineral assemblages in all the samples are 

compatible with those in the biotite to sillimanite 

zones of the present work (zones Ll to H3 as defined 

by the previous work), even though diagnostic minerals 

were lacking in some specimens. 

With the one exception described below and with 

due consideration of the limited number of samples 

available, the major oxides of the coarse- and finegrained samples fall respectively into two distinct 

groups. Inspection of the data (tables 2 and 3) and 

plots (not included here) for the oxides compared with 

the metamorphic zones shows (1) the general 

uniformity of oxide content within each group, and (2) 

the absence of systematic changes between the higher 

and lower metamorphic zones. 

The uniformity within each group is confirmed by 

the statistical analysis (table 4). The fine-grained 

specimens are significantly lower in Si02 and 

significantly higher in AI2o3, MgO, FeO, and Ti02 

than are the coarse-grained samples. The 

interpretation of the apparent absence of systematic 

changes between the zones is severely limited by the 

low (1 or 2) number of samples in each zone. 

The exception referred to above concerns 

specimen 79DB178B (table 3); it was collected as a 

fine-grained rock interlayered with a coarser grained 

rock and was thought to represent a mudstone 

originally interlayered with graywacke. The majoroxide composition of 79DB178B is clearly closer to 

that of the coarse-grained rocks, and petrographic 

study suggests that the sample was originally a finegrained sandstone that has been "milled" to finer grain 

size, rather than being an original mudstone. For 

these reasons, the sample was not included in the 

statistical analysis. 

The trace-element compositions of the coarseand fine-grained rocks (tables 2 and 3) are not 

significantly different, and inspection of the data 

shows no systematic change from the lower 

metamorphic zones to the higher. 

The foregoing discussion indicates that the 

interbedded metagraywacke and metamudstone units 

are remarkably uniform in composition throughout the 

aureoles. The bulk composition of fine-grained rocks 

compares favorably with the mean values for the 

compositions of metamorphic rocks of pelitic origin 

given by Mehnert (1969). For example, his mean 

values for Si02 range from 59.64 to 61.18 percent (avg 

60.43 percent); for AI2o3 from 16.4 percent to 17.41 

percent (avg 16.90 percent); and for K2o from 3.25 

percent to 4.14 percent (avg 3.62 percent). Finally, 

the thinly bedded nature of these rocks ensures that 

both pelitic and semipelitic rocks are distributed more 

or less uniformly throughout the aureole. 

Garnet Composition 

Although the garnets are known to be zoned (on 

the basis of primitive microprobe studies made in the 

1960's), we have no reliable compositional data on the 

zoning. The available bulk composition information 

listed in table 5 is intended only to show that the 

garnets from pelitic schists (62ABdl49 and 62ABdl52, 

containing 91.6 and 86.5 percent almandine + pyrope, 

respectively) have a composition much more 

appropriate for the AFM projections (Thompson, 1957) 

than do those from semipelitic rocks (62ABd147, 66.4 

percent almandine+ pyrope). 

Metamorphic Isograds and Zones 

Loney and others (1975, p. 80-81) mapped 

metamorphic zones (Ll to H3) on the south side of the 

pluton at Crawfish Inlet and around the Redfish Bay 

pluton (fig. 3). The mineral assemblages upon which 

these zones are based and the inferred facies were 

discussed in detail by these authors and are 

summarized, together with the zones as defined in this 

paper, in table 1. The present work evaluates the 

earlier defined zones relative to reaction isograds or 

bulk-composition controls, and the map (fig. 3) shows 

only those zone boundaries that are true isograds in 

the sense that they represent changes of mineral 

facies diagrams. 

To minimize the effects of bulk composition, 

Thompson (1957) proposed that isograds be defined on 

the basis of discontinuous reactions, and as an aid to 

5 
6 

·· ... . ........ ~~ BIORKA ·· ... :-....... . 

ISLAN~D. · .. :., · .. :>~:· } · ... 

I .·· 

KJ,hi vx : Te M.· 

+ 7J .. KJsh .· 

EXPLANATION 

Intrusive granitoid rocks (Miocene and 

Oligocene) at Gut Bay 

Intrusive granitoid rocks (Eocene) 

El Ultramafic rocks 

KJsg 

Sitka Graywacke 

KJs, graywacke and argillite, minor conglomerate 

KJsh,hornfels in contact aureoles 

KJss, linea ted, schistose, and semischistose 

graywacke and slate 

KJsg ,greenstone 

B Khaz Formation 

Phyllite, quartzite, greenschist, and graywacke 

semischist 

T. p,phyllite. some quartzite, 

greenschist, graywacke 

semischist, and metachert 

1> g.greenschist and greenstone; some phyllite, 

graywacke semischist, 

56° 15' and metachert 

0. 

::I 

0 

0 

,.., 

~ "' 

0. 

~ Schist, gneiss, amphibolite, and greenschist 

JT> a, amphibolite and 

greenschist 

J'fl s, biotite schist and 

gneiss; some amphibolite, quartzite, phyllite, and hornfels 

Fault 

u 

6 

N 

Sl 

~ 

OV> 

z=> 

<~:o 

ULU 

~~ <{LU 

::;u """" 

Dashed where approximately located; dotted where concealed by water 

Formational contact 

Dotted where concealed by water 

Metamorphic isograd 

0 79DB175 

Sample locality discussed in text 

0 

0 

5 10 MILES 

5 10 KILOMETERS 
Table 1. Metamorphic mineral zones in the aureoles south of Crawfish Inlet 

as defined by Loney and others (1975, p. 79-81) and their correlation with 

isograds and zones in this paper 

[FA, first appearance; A, absence. Identification: H, hand lens; P, 

petrographic methods; An 10+ or An 10_, plagioclase composition. Rocks also 

contain quartz, muscovite, biotite, and plagioclase] 

This paper Loney and others (1975) 

Biotite zone (Bz) 11 FA biotite H; An10_ 

12 FA epidote and tourmaline P; An 10_ 

13 A chlorite and calcite P; An10_ 

Garnet isograd --------------------

Garnet zone (Gz) 

Andalusite-cordierite-biotite 

isograd--------------------------

Andalusite-cordierite-biotite 

zone (ACz) 

Sillimanite isograd----------------

Sillimanite zone ( Sz) 

Garnet-cordierite isograd----------

Garnet-cordierite zone (GCz) 

14 

H1 

H2 

H3 

studying these reaction isograds (Winkler, 1979, p. 66) 

he introduced the AFM projection of the "ideal" pelitic 

system Si02-Al20 3-MgO-FeO-K20-H20. On the AFM 

projection, discontinuous reactions may be indicated 

by distinct changes in the tieline configuration without 

necessarily implying that the reaction actually 

occurred in the rocks concerned. 

The mineral assemblages from the rocks of the 

aureole appropriate to the AFM projection are listed in 

table 6. In addition to the minerals listed, all 

assemblages contain quartz and muscovite, and they 

may also contain plagioclase, orthoclase, and 

accessory ilmenite, magnetite, graphite, zircon, 

apatite, tourmaline, epidote, and sphene. All phases in 

an assemblage are inferred to be in equilibrium on the 

basis of textural criteria. The order in which minerals 

are listed has no significance. The compositions of the 

two analyzed garnets from pelitic schists (table 5) are 

plotted on a schematic AFM projection (fig. 4). Also 

plotted on this projection is the field of compositions 

for the pelitic schists analyzed (table 3), which was 

used to control the mineral assemblage construction. 

According to Thompson (1957), rock compositions may 

be plotted on the AFM projection provided that the 

content of the mineral phases not appearing in the 

Figure 3. Southern Baranof Island, showing major 

geologic units, metamorphic isograds and zones related 

to the Crawfish Inlet and Redfish Bay plutons, and 

locations of samples analyzed. Bz, biotite zone; G, 

garnet isograd; Gz, garnet zone; AC, andalusitecordierite-biotite isograd; ACz, andalusite-cordieritebiotite zone; S, sillimanite isograd; Sz, sillimanite 

zone; GCz, garnet-cordierite zone. (Modified from 

Loney and others, 1975, fig. 25.) 

FA garnet + cordierite P; An10+ 

FA andalusite H and garnet H; An 10+ 

FA staurolite H; An 10+ 

FA sillimanite + orthoclase P; An 10+ 

Not recognized 

projection are first subtracted from the bulk 

composition. Plagioclase is the only such mineral of 

significance in these rocks; therefore, an amount of 

Al2o3 equivalent to the amount of CaO and Na2o was 

subtracted from each rock analysis plotted on the AFM 

projection (fig. 4). 

AFM diagrams constructed from observed and 

compatible mineral assemblages (table 6) are shown in 

figure 5. The first appearance of garnet marks the 

garnet isograd and the boundary between the biotite 

and the garnet zones (tables 1 and 6; fig. 5A). The 

garnet isograd cannot be modeled by a discontinuous 

reaction, and we lack the critical textural evidence to 

work out the details of the reaction. However, it is 

likely the formation of garnet was related to 

continuous reactions such as those given by 

Chakraborty and Sen (196 7), 

Chl + Bt + Qtz = Gt + Bt + W, 

and by Thompson (1976), 

Chl + Mu + Qtz = Gt + Bt + W. 

(1) 

(2) 

The first occurrence of garnet, then, is probably bulk 

compositionally dependent, and the relatively 

widespread occurrence of the garnet-biotite-chlorite 

assemblage reflects the relatively uniform bulk 

composition of the pelitic rocks of the garnet zone. 

Although generally a minor constituent of the 

garnet zone, rocks containing the assemblage chloritebiotite-cordierite form a relatively wide (4 km 

maximum), vaguely defined zone east of Redfish Bay 

in which garnet-bearing rocks are sparse. A 

reevaluation of the evidence in this area resulted in a 

major widening of the original garnet zone at the 

expense of the andalusite-cordierite-biotite zone to 

the west (fig. 3). 

7 
8 

Table 2. Major oxide and semiquantitative spectrographic analyses of metamorphosed 

coarse-grained rocks of the Sitka Graywacke, southern Baranof Island, Alaska 

[X-ray spectroscopic analyses by L. Estos and S. Ramage. FeO and powder density determinations by P. 

Klock. Emission spectrographic analyses by C. Heropoulos; data reported by procedure described in 

Meyers and others (1961). See table 1 for zone abbreviations. Concentrations below procedural 

detection limits are indicated by the less than (<) values. Trace elements looked for but not 

detected (minimum detection limit, in parts per million, in parentheses): Ag (0.7), As (100.0), Au 

(7.0), Bi (7.0), Cd (7.0), Ce (50.0), Eu (1.0), Ge (7.0), Hf (50.0), In (1.50), Li (100.0), Mo (2.0), 

Nd (20.0), Pd (1.0), Pr (20.0), Pt (5.0), Re (7.0), Sb (15.0), Sm (50.0), Ta (50.0), Te (300.0), Th 

(150.0), Tl (3.0), U (150.0), W (10.0)] 

Zone 

Bz(Ll) Bz(L2) Bz(L3) Gz(L4) ACz(H2) Sz(H3) 

Specimen 

79DB182A 79DB173A 79DB181A 79DB178A 79DB179A 79DB177A 79DB180A 79DB175A 79DB176A 79DB183A 

Lat (56° N.) 

35' 22" 

Long (134 ° W. ) 

01, 52" 

Si02 •• 

Al2o3 • 

Fe2o3 • 

FeO ••• 

MgO ••• 

CaO ••• 

Na2o •• 

K20 ••• 

Ti02 •• 

P20s·· 

MnO ••• 

70.37 

14.86 

.74 

2.97 

1.65 

2.00 

3.97 

2.09 

.52 

.10 

.06 

Total 99.33 

Powder 

density 2.72 

23' 50" 

36' 11" 

70.63 

13.38 

.71 

3.61 

1.90 

2.54 

3.42 

1.85 

.58 

.13 

.07 

98.82 

2.72 

34 '24" 28'07" 32'40" 25'13" 33'40" 

56'36" 48, 56" 46' 54" 50' 50" 50' 37" 

Major oxide analyses (weight percent) 

69.34 

14.62 

.so 

3.02 

1.59 

2.19 

3.69 

2.39 

.so 

.10 

.05 

97.99 

2.80 

66.42 

15.50 

.86 

4.34 

2.42 

2.21 

3.97 

1.89 

.7S 

.1S 

.08 

98.S9 

2.76 

6S.97 

15.76 

.87 

3.80 

2.44 

2.36 

3.47 

2.62 

.69 

.13 

.06 

98.17 

2.76 

71.69 

14.27 

.58 

2.81 

1.54 

2.16 

3.64 

1.83 

.so 

.11 

.06 

99.19 

2.72 

68.39 

1S.78 

.62 

3.64 

2.15 

1.89 

3.07 

2.42 

.58 

.10 

.07 

98.71 

2.72 

20' 47" 

40' 17" 

67.17 

14.44 

.77 

4.07 

2.39 

2.91 

3.52 

1.82 

.68 

.13 

.08 

97.98 

2.72 

Semiquantitative spectrographic analyses (parts per million) 

B ••••• 10 

Ba •••• 700 

Be •••• 1.S 

Co.... 10 

Cr.... 30 

Cu.... 30 

Ga.... 20 

La.... 70 

Mn •••• 500 

Nb.... 10 

Ni.... 1S 

Pb. ••• 30 

Sc.... 15 

Sn.... 3 

Sr •••• 300 

v ••••• 150 Y..... 1S 

Yb.... 2 

Zn.. •• 70 

Zr.... 100 

s 

700 

1 

20 

30 

10 

20 

30 

700 

10 

20 

20 

20 

< 2 

soo 

150 

20 

3 

100 

100 

Specimen descriptions: 

10 

700 

1 

10 

30 

20 

20 

30 

300 

15 

1S 

30 

10 

s 

300 

100 

15 

2 

70 

70 

2 

700 

1 

10 

30 

20 

20 

so 

soo 

7 

10 

20 

15 

3 

300 

150 

1S 

2 

70 

100 

5 

1SOO 

1.5 

7 

30 

20 

30 

50 

500 

1S 

7 

20 

15 

3 

700 

1SO 

15 

2 

100 

70 

10 

soo 

1 

7 

30 

20 

20 

50 

500 

20 

15 

30 

15 

3 

300 

100 

15 

2 

70 

200 

1S 

700 

1 

10 

20 

30 

20 

50 

500 

15 

10 

30 

1S 

3 

300 

100 

15 

2 

100 

70 

1S 

soo 

1 

20 

so 

30 

30 

so 

1000 

10 

30 

so 

20 

< 2 

300 

150 

20 

3 

150 

150 

14 '04" 

47, 43" 

68.78 

1S .18 

.53 

3.73 

2.11 

1.69 

1.88 

3.81 

.so 

.11 

.07 

98.39 

2.84 

7 

1500 

2 

10 

20 

20 

30 

70 

500 

20 

20 

30 

15 

5 

300 

100 

20 

3 

100 

100 

40'30" 

08'25" 

69.97 

14.61 

.47 

2.96 

1.68 

1.98 

4.09 

2.44 

.49 

.11 

.06 

98.86 

2.72 

20 

1000 

1 

1S 

so 

so 

30 

30 

500 

15 

20 

30 

20 

5 

300 

200 

15 

2 

100 

100 

79DB182A.Fine-grained epidote-opaque-muscovite-biotite-plaglioclase-quartz metagraywacke. 

79DB173A.Fine- to very fine grained calcite-chlorite-epidote-biotite-plagioclase-quartz semischist. 

79DB181A.Fine-grained calcite-opaque-muscovite-biotite-plagioclase-quartz metagraywacke. 

79DB178A.Fine- to very fine grained epidote-opaque-muscovite-biotite-plagioclase-quartz semischist. 

79DB179A.Fine- to very fine grained ilmenite-muscovite-biotite-plagioclase-quartz semischist. 

79DB177A.Fine-grained tourmaline-epidote-opaque-muscovite-biotite-plagioclase-quartz semischist. 

79DB180A.Fine-grained tourmaline-epidote-opaque-muscovite-biotite-plagioclase-quartz semischist. 

79DB175A.Fine- to very fine grained andalusite(?)-muscovite-ilmenite-tourmaline-epidote-garnetbiotite-plagioclase-quartz semischist. 

79DB176A.Fine- to very fine grained opaque-muscovite-biotite-plagioclase-quartz schist. 

79DB183A.Fine-grained K-feldspar-opaque-muscovite-biotite-plagioclase-quartz hornfels/metagraywacke. 
Table 3. Major oxide and semiquantitative spectrographic analyses of metamorphosed finegrained rocks of the Sitka Graywacke, southern Baranof Island, Alaska 

[X-ray spectroscopic analyses by L. Espos and S. Ramage. FeO and powder density determinations by P. 

Klock. Emission spectrographic analyses by C. Heropoulos; data reported by procedure described in 

Meyers and others (1961). See table 1 for zone abbreviations. Concentrations below procedural 

detection limits are indicated by the less than (<) values. Trace elements looked for but not 

detected (minimum detection limit, in parts per million, in parentheses): Ag (0.70), As (100.0), Au 

(7.0), Bi (7.0), Cd (7.0), Ce (50.0), Eu (1.0), Ge (7.0), Hf (50.0), In (1.50), Li (100.0), Mo (2.0), 

Nd (20.0), Pd (1.0), Pr (20.0), Pt (5.0), Re (7.0), Sb (15.0), Sm (50.0), Ta (50.0), Te (300.0), Th 

(150.0), Tl (3.0), U (150.0), W (10.0)] 

Zone 

Bz(Ll) Bz(L2) Bz(L3) Gz(L4) ACz(H2) Sz(H3) 

Specimen 

79DB182B 79DB173B 79DB181B 79DB178B 79DB179B 79DB177B 79DB180B 79DB175B 79DB176B 79DB183B 

Lat (56° N .) 

35'22" 23'50" 34'24" 28'07" 32'40" 25' 13" 33'40" 20' 47" 14'04" 40'30" 

Long (134° W.) 

01'52" 38' 11 II 56' 36" 48' 56" 46' 54" 50'50" 50'37" 40' 17" 47'43" 08' 25" 

Major oxide analyses (weight percent) 

Si02 •• 63.36 61.03 60.39 70.59 60.04 59.36 60.42 62.63 63.61 59.93 

Al2o3 • 17.09 16.88 18.32 13.93 18.97 17.80 18.66 17.87 17.80 18.64 

Fe2o3 • .70 1.13 .80 .70 .82 .97 1.12 .61 .89 .85 

FeO ••• 4.75 5.64 5.51 3.74 4.75 5.35 4.18 5.34 5.22 4.87 

MgO ••• 2.78 2.97 3.03 2.09 3.03 3.49 2.75 2.92 3.01 2.68 

CaO ••• 2.58 2. 77 2.44 1.75 2.62 3.00 2.17 2.46 2.00 2.05 

Na2o •• 2.83 2.38 2.50 2.02 3.60 2.70 2.29 2.63 2.64 2.78 

K20 ••• 3.41 2.95 3.95 2.60 2.97 2.38 3.62 3.48 3.26 4.39 

Ti02 •• .75 .91 .78 .68 .84 .81 .89 .77 .80 .76 

P20s •• .19 .21 .17 .13 .13 .21 .17 .18 .21 .16 

MnO ••• .08 .10 .07 .06 .08 .11 .08 .12 .09 .09 

Total. 98.52 96.97 97.96 98.29 97.85 96.18 96.35 99.01 99.53 97.20 

Powder 

density 2.76 2.76 2.84 2.}6 2.76 2.88 2.76 2.76 2.84 2.76 

Semiquantitative spectrographic analyses (parts per million) 

B. • • • 20 15 30 15 30 so 30 70 20 

Ba ••• 1000 1000 1500 1500 1000 1000 1000 700 1000 1000 

Be ••• 1 1.5 1 1.5 1 1 < 0.7 1.5 1.5 1 

Co ••• 10 30 15 20 10 15 7 30 15 15 

Cr ••• 30 70 so so so 30 so so so so 

Cu ••• 30 70 70 100 so 70 30 70 70 so 

Ga ••• 20 30 30 20 30 30 30 30 30 30 

La ••• so 70 30 < 7 20 so so 70 so 30 

Mn ••• 700 1000 500 500 500 700 500 1500 700 500 

Nb ••• 15 10 10 10 10 10 15 10 15 15 

Ni ••• 20 so 30 30 10 20 10 so 30 20 

Pb ••• 30 30 30 20 30 30 20 so 20 30 

Sc ••• 20 30 20 15 15 15 20 20 15 20 

Sn ••• 3 < 2 7 3 3 5 3 < 2 5 5 

Sr ••• 200 300 300 200 300 300 300 300 300 300 

v •••• 200 200 200 150 200 200 150 200 200 200 

Y •••• 15 30 20 15 15 15 15 30 15 15 

Yb ••• 2 5 2 2 2 3 2 3 3 2 

Zn ••• 100 150 100 100 100 150 70 200 100 100 

Zr ••• 70 200 70 70 100 70 70 150 70 100 

Specimen descriptions: 

79DB182B. Very fine grained opaque-muscovite-epidote-plagioclase-quartz-biotite metamudstone. 

79DB173B. Very fine grained epidote-ilmenite-biotite-chlorite(?)-plagioclase-quartz phyllite. 

79DB181B. Very fine grained tourmaline-ilmenite-plagioclase-quartz-biotite-muscovite metamudstone. 

79DB178B. Fine- to very fine grained opaque-muscovite-biotite-plagioclase-quartz semischist. 

79DB179B. Fine- to very fine grained tourmaline-epidote-ilmenite-muscovite-biotite-plagioclase-quartz 

79DB177B. 

79DB180B. 

79DB175B. 

79DB176B. 

79DB183B. 

schist. 

Very fine grained ilmenite-muscovite-biotite-plagioclase-quartz schist. 

Very fine grained tourmaline-muscovite-ilmenite-biotite-quartz schist. 

Fine- to very fine grained epidote-garnet-opaque-chlorite-muscovite-biotite-plagioclasequartz semischist. 

Very fine grained staurolite-garnet-muscovite-biotite-plagioclase-quartz schist. 

Very fine grained opaque-cordierite(?)-plagioclase-quartz-muscovite-biotite schist. 

9 
The assemblage chlorite-biotite-cordierite 

requires a magnesian bulk composition (Burnell and 

Rutherford, 1984). Hess' (1969) petrogenetic grid 

shows that the assemblage chlorite-biotite-cordierite 

can be associated in the same P-T fields as the garnetzone assemblage chlorite-biotite-garnet, and therefore 

it may be considered compatible with the garnet zone 

in the absence of andalusite and staurolite. 

The andalusite-cordierite-biotite isograd is 

marked by the first appearance of andalusite and by 

the disappearance of chlorite (figs. 5A and 5B; table 

6). The breaking of the chlorite-garnet tieline 

commonly involves the appearance of staurolite, 

marking the staurolite isograd and zone (Hess, 1969; 

Carmichael, 1970; Novak and Holdaway, 1981). 

However, here staurolite does not appear until higher, 

within the andalusite-cordierite-biotite zone, and is 

bulk compositionally controlled, reflecting, where 

present, a more iron- and aluminum-rich composition. 

According to Hess' (1969; fig. 6) petrogenetic 

grid, the transition from the garnet zone to the 

andalusite-cordierite-biotite zone (figs. 5A and 58) 

should involve the following sequence of discontinuous 

reactions (see also Thompson, 1976): 

Gt + Chl + Mu = St + Bt + Qtz + W, (3) 

St + Chl + Mu = Bt + And + Qtz + W, (4) 

Chl + Mu + Qtz = Bt + Crd + And + W. (5) 

The assemblage chlorite-biotite-cordierite is listed by 

Hess (1969) as an assemblage that may be compatible 

with the products of reaction (4), and thus it could be 

compatible with andalusite-biotite assemblages. 

However, here andalusite occurs first with cordieritebiotite (reaction 5), and the assemblage chloritebiotite-cordierite would be eliminated by this 

reaction. The assemblage biotite-cordierite-andalusite 

(table 6) is given by Hess (fig. 6) as a possible product 

Table 4. Statistical comparison of metamorphosed coarse- and fine-grained rocks of the 

Sitka Graywacke, southern Baranof Island, Alaska 

[Specimen 79DB178B (table 3) was not included in the statistical calculations because it was 

determined petrographically to have been a sandstone originally, not a mudstone like the other 

fine-grained specimens. Number of coarse-grained samples is 10, and of fine-grained samples 

is 9. Geometric mean for semiquantitative spectrographic results rounded to appropriate 

units; major elements in weight percent; minor elements in parts per million] 

Standard 

Range He dian He an deviation 

Coarse Fine Coarse Fine Coarse Fine Coarse Fine 

Si02 •••• 65.97-71.69 59.36-63.61 69.06 60.04 68.87 61.20 1.80 1.50 

Al2o3 ••• 13 .38-15.78 16.88-18.97 14.74 17.87 14.84 18.00 .71 .67 

Fe2o3 ••• .47- .87 .61- 1.13 .67 .85 .67 .88 .14 .16 

FeO ••••• 2.81- 4.34 4.18- 5.64 3.63 5.22 3.50 5.06 .so .44 

MgO ••••• 1.54- 2.44 2.68- 3.49 2.01 2.97 1.99 2.96 .34 .22 

cao ••••• 1.69- 2.91 2.00- 3.00 2.18 2.46 2.19 2.45 .33 .31 

Na2o •••• 1.88- 4.09 2.29- 3.60 3.58 2.64 3.47 2.71 .60 .35 

K20 ••••• 1.82- 3.81 2.38- 4.39 2.24 3.41 2.32 3.38 .57 .56 

Ti02 •••• .49- .75 .75- .91 .55 .80 .58 .81 .09 .05 

P2o5 •••• .10- .15 .13- .21 .11 .18 .12 .18 .02 .03 

MnO ••••• .OS- .08 .07- .12 .07 .09 .06 .09 .02 .02 

B. • • • • • • 2-20 7-70 10 30 7 30 

Ba •••••• 500-1500 700-1500 700 1000 700 1000 

Be •••••• 1-2 .7-1.5 1 1 1 1 

Co •••••• 7-20 7-30 10 15 10 15 

Cr •••••• 20-50 30-70 30 so 30 so 

Cu •••••• 10-50 30-70 20 70 20 so 

Ga •••••• 20-30 20-30 20 30 20 30 

La •••••• 30-70 20-70 so 50 50 50 

Mn •••••• 300-1000 500-1500 500 700 500 700 

Nb •••••• 7-20 10-15 15 10 15 10 

Ni •••••• 7-30 10-50 15 20 15 20 

Ph •••••• 20-50 20-50 30 30 30 30 

Sc •••••• 10-20 15-30 15 20 15 20 

Sn •••••• 2-5 2-7 3 3 3 3 

Sr •••••• 300-700 200-300 300 300 300 300 

v ••••••• 100-200 150-200 150 200 150 200 

y ••••••• 15-20 15-30 15 15 15 20 

Yb •••••• 2-3 2-5 2 2 2 3 

Zn •••••• 70-150 70-200 100 100 100 100 

Zr •••••• 70-200 70-200 100 100 100 100 

10 
of reaction (5), which models the disappearance of 

chlorite (Burnell and Rutherford, 1984). Reactions (4) 

and (5) may have occurred simultaneously on southern 

Baranof Island (see the section "Discussion"). 

The sillimanite isograd is marked by the first 

appearance of sillimanite (fibrolite) and the 

disappearance of staurolite (figs. 5B and 5C); the 

isograd is based on the reaction 

And =Sill. (6) 

The disappearance of staurolite can be modeled by the 

following discontinuous reaction from Thompson (1976; 

see also, Hess, 1969): 

St + Mu + Qtz = Gt + Sill + Bt + W. (7) 

This reaction may have proceeded simultaneously with 

reaction (6). For the details of the transition from the 

staurolite zone to the sillimanite zone in similar (but 

not identical) rocks and conditions, see Guidotti 

(1974). In the sillimanite zone, sillimanite occurs 

mostly as fibrolite needles on biotite, and it is the only 

AI2Si05 mineral present in most specimens. Only a 

few specimens contain both andalusite and sillimanite, 

and in these andalusi te occurs as separate 

porphyroblasts not in contact with the fibrolite, which 

tends to be associated with biotite. 

These andalusite and sillimanite occurrences may 

be interpreted in two ways: (1) the rocks attained the 

pressure-temperature (P-T) conditions of the 

sillimanite stability field, and andalusite is an unstable 

mineral, or (2) fibrolite crystallized metastably at a 

lower temperature in the andalusite field (fig. 6). 

The first interpretation is supported by 

Holdaway's (1971) experimental evidence that 

fibrolite, which he considers to be metastable, raises 

the andalusite-sillimanite boundary about 100 °C 

higher than that determined by Weill (1966). He states 

that in order to form fibrolite from andalusite, 200 °C 

or more of overstepping is required, whereas to form 

fibrolite from muscovite dehydration would require no 

Table 5. Garnet and whole-rock analyses of schists from the metamorphosed Sitka 

Graywacke, southern Baranof Island, Alaska 

[Garnet analyses (in weight percent) by R.N. Elsheimer, C.O. Ingamells, M. Cremer 

(H2o), S. Neil (FeO), and R.E. Mays (Ti02); rapid rock analyses by P. Elmore, S. 

Botts, L. Artis, and H. Smith; n, index of refraction; D, density (in grams/cubic 

centimeter); a, unit cell (in a~gstroms); garnet end-me~ber molecules (in percent) 

after Rickwoodl (1968) 

Garnets Whole rock 

Sample No ••• 62ABd147 62ABd149 62ABd152 62ABd147 62ABd149 62ABd152 

Si02 •••••••••• 

A1 2o3 ••••••••• 

Fe2o3 ••••••••• 

FeO ••••••••••• 

MgO ••••••••••• 

CaO ••••••••••• 

Na2o •••••••••• 

K2o ••••••••••• 

Total H2o ••••• 

Ti02 •••••••••• 

P2os •••••••••• 

MnO ••••••••••• 

co2 ••••••••••• 

Total 

36.9 

20.6 

1.96 

24.58 

2.0 

5.6 

.06 

.OS 

.01 

.15 

.01 

7.6 

99.52 

1.805 

4.10 

11.5922 

End-member molecules 

Andradite 

Pyrope 

Spessartine 

Grossular 

Almandine 

Schor lomite 

1.4 

8.1 

17.4 

14.4 

58.3 

.4 

Sample descriptions: 

36.9 

20.5 

6.69 

27.63 

3.8 

1.2 

.03 

.04 

.10 

.12 

.03 

2.1 

99.14 

1.804 

4.19 

11.5326 

1.3 

15.5 

4.9 

2.0 

76.1 

.2 

37.1 

20.6 

4.36 

27.64 

3.9 

2.2 

.02 

.01 

.oo 

.15 

.01 

3.1 

99.09 

1.805 

4.15 

11.5439 

.9 

15.8 

7.1 

5.1 

70.7 

.4 

66.2 

14.3 

1.3 

5.4 

2.1 

2.8 

2.4 

2.5 

1.39 

.77 

.30 

.53 

.08 

100.07 

55.5 

20.0 

1.0 

7.3 

3.8 

2.6 

3.9 

2.5 

2.44 

1.1 

.22 

.16 

.06 

100.58 

62ABd147 Garnet-biotite semischist Gz(L4) zone, Larch Bay, Baranof Island. 

62ABd149 Garnet-biotite schist ACz(H1) zone, Larch Bay, Baranof Island. 

62ABd152 Garnet-biotite schist ACz(H1) zone, Larch Bay, Baranof Island. 

62.7 

16.7 

.83 

4.7 

3.2 

2.5 

3.3 

2.8 

1.82 

.80 

.28 

.11 

.10 

99.84 

11 
more than 9 °C of overstepping. Thus, according to 

Holdaway (1971) it is more common to grow fibrolite 

on micas upon heating while andalusite in the same 

rock breaks down. According to these data, the 

presence of fibrolite in the aureoles should indicate 

that the rocks have attained at least the temperature 

range of the sillimanite field. 

The second interpretation is based mainly on 

Hollister's (1969) work on the field relations in the 

contact metamorphic terrane of the Kwoiek area of 

British Columbia. In this terrane, both fibrolite and 

coarse-grained sillimanite occur. The fibrolite 

developed on biotite and andalusite, whereas 

sillimanite developed from kyanite. Hollister 

concludes that fibrolite formed at a lower temperature 

than the coarse-grained sillimanite, probably 

metastably in the andalusite stability field. This 

interpretation is counter to Holdaway's (1971) evidence 

that an overstepping of temperature is necessary for 

the andalusite-fibrolite transition. 

In summary, we favor interpretation (1) because 

Holdaway's (1971) mineral assemblages and textural 

relations are closer to those at southern Baranof Island 

than are those of Hollister (1969) at Kwoiek. The 

latter, especially the presence of kyanite, seem more 

complex and seem to represent higher pressure 

metamorphism. Finally, we use "sillimanite" for 

fibrolite in comparing mineral assemblages of southern 

Baranof Island with those of other metamorphic facies 

series, no ambiguities have been noted, and our 

mineral assemblages are those to be expected in 

sillimanite-bearing rocks. 

A mineralogic characteristic of the sillimanite 

zone is the appearance of orthoclase accompanied by a 

decrease in muscovite (table 6). (This isograd is not 

shown on figure 3.) This finding suggests that at least 

part of the sillimanite zone exceeded the temperature 

for the initiation of the reaction (Storre, 1972) 

Mu + Qtz = Kspar + Sill + W. (8) 

Table 6. Observed mineral assemblages in the pelitic 

metamorphic rocks of southern Baranof Island 

appropriate to the AFM projection (fig. 5) 

[All assemblages contain quartz and muscovite, and a few may also 

contain orthoclase, indicated (Or). Chl, chlorite; Bt, biotite; 

Gt, almandine-rich garnet; St, staurolite, Crd, cordierite; And, 

andalusite; Sill, sillimanite] 

Garnet zone 

Chl-Bt 

Bt-Gt 

Chl-Bt-Gt 

Chl-Bt-Crd 

Andalusite-cordierite-biotite zone 

Bt-And 

Bt-Crd-And 

Bt-Gt 

Bt 

Crd 

Bt-Gt-St 

Bt-St-And 

12 

Sillimanite zone 

Bt-Sill (Or) 

Bt-Crd (Or) 

Bt-Gt-Sill (Or) 

Bt-Gt-And-Sill 

Garnet-cordierite zone 

Bt-Gt-Crd-Sill (Or) 

A mineral assemblage indicative of a zone higher 

than the sillimanite zone, the garnet-cordierite zone, 

occurs in migmatitic gneiss in a small roof pendant 

near the northeast contact of the Crawfish Inlet 

pluton, about 5 km south of the head of Gut Bay (GCz, 

fig. 3). The gneiss contains, in addition to muscovite, 

orthoclase, and quartz, the assemblage biotite-garnetcordierite-sillimanite. When plotted on the AFM 

diagram (projected from potassium feldspar), the 

garnet-cordierite tieline breaks the sillimanite-biotite 

tieline and defines the garnet-cordierite isograd 

(dashed line, fig. 5D). According to Thompson (1976, 

fig. 3b) garnet-cordierite-potassium feldspar is the 

high-temperature product of the discontinuous 

reaction 

Bt +Sill= Gt + Crd + Kspar. (9) 

The presence of the four-phase assemblage (table 6) 

suggests an incomplete reaction, but the assemblage 

might also be due to substitution by other components 

(particularly in the garnet) or by the XH 0 being 

buffered by the reaction. 2 

DISCUSSION 

The sequence of mineral assemblages in the 

southern Baranof Island thermal aureoles can be 

plotted with good agreement on Hess' (1969) 

petrogenetic grid, which was constructed for similar 

pelitic rocks (fig. 6). A P-T gradient was established 

for the Baranof facies series by finding the appropriate 

mineral assemblages on his grid. Hess, in plotting the 

...... ~ 

I I 

I I 

\ I ,_ 

Bt 

M 

Figure 4. Schematic AFM projection (projected from 

muscovite) showing composition of analyzed garnet 

(black dot includes specimens 62ABd149 and 

62ABd152, table 5), adjusted bulk compositions (see 

text) of pelitic rocks (dashed line encloses field of 13 

analyses, tables 3 and 5), and estimated compositions 

of other important mineral phases. See table 6 for 

abbreviations. 
P-T gradient for the Pyrenean facies series, states 

that the absence of the chlorite-biotite-andalusite 

assemblage requires that the gradient passes on the 

low-pressure side of the invariant point (G,H). The 

Baranof rocks also lack the chlorite-biotite-andalusite 

assemblage, but the disappeHrance of chlorite is 

accompanied by the appearance of the assemblage 

andalusi te-cordieri te-bioti te instead of staurolitebiotite-cordierite and suggests that the P-T gradient 

passes on the high-pressure side of (G,H). How far 

A 

A. Garnet zone (L4) 

A 

And 

B. Andalusite-cordierite-biotite zone 

(H 1 and H2) 

Figure 5. Schematic AFM diagrams based on mineral 

assemblages in pelitic metamorphic rocks listed in 

table 6. Dashed lines indicate existence of four-phase 

assemblages; X marks observed assemblages. All 

above (G,H) is uncertain; it depends on whether the 

reactants of reaction (4) are absent because the 

gradient lies close to (G,H), or because of 

inappropriate bulk composition. The Baranof gradient 

shown is based on the first assumption; if the second 

obtained, the curve would be higher and the slope more 

like that of the Pyrenean facies series. 

The low end of the Baranof gradient lies in the 

P-T field in which the mineral assemblage found in the 

garnet zone, garnet+chlorite, is stable (fig. 6). 

A 

Silt 

C. Sillimanite zone (H3) 

D. Garnet-cordierite zone 

assemblages include quartz and muscovite. Figure 5D 

projected from orthoclase; all others from muscovite. 

See table 6 for abbreviations. Zone designations L4, 

Hl, H2, and H3 refer to table 1. 

13 
According to Hess, the associated assemblage chloritebiotite-cordierite is stable in this field. 

Proceeding upgrade on the grid, the gradient cuts 

the boundary represented by reaction (3) in which the 

garnet-biotite tieline is broken and the staurolitechlorite tieline is formed (staurolite isograd). 

However, on Baranof Island staurolite appears to be 

absent at this point, and instead andalusite appears, 

accompanied by cordierite-garnet, the products of 

reaction (5) (andalusite-cordierite-biotite isograd). As 

noted previously, reaction (3} probably did not occur 

because of inappropriate bulk composition. This may 

also be the reason why the products of reaction (4) 

seem to be absent, but if the P-T gradient lies close to 

(G,H) (fig. 6), the P-T field of this reaction would be 

very small. Since reaction (5) is a terminal reaction 

for chlorite, the nearly simultaneous appearance of 

5 ~~ .lJ: H• 

6 

CJ) 

Ill 

a: 

0 

<:{ ~ ..J 5 3 ~ 700 750 

~ 

w' 

a: 

::::> 

CJ) 4 CJ) 

w 

:~ a: 

CL 

3 

2 

500 

andalusite and disappearance of chlorite is further 

evidence that reaction (5) is responsible for the 

appearance of andalusi te. 

The first appearance of staurolite in the upper 

part of the andalusite-cordierite-biotite zone does not 

correspond to a specific reaction in that region of 

Hess' grid; instead it lies in the large P-T field in 

which the assemblages biotite-staurolite-andalusite 

and biotite-garnet-staurolite are stable. It is likely 

that reaction (3) did not occur, and hence the 

staurolite isograd is not seen because of inappropriate 

bulk composition. 

Sillimanite appears and staurolite disappears 

nearly simultaneously, and thus the P-T gradient must 

pass near the intersection of the boundary between the 

andalusite and sillimanite fields and the staurolite-out 

reaction curve (reaction (7)). This control point 

---

'i> ~ ~If 

~· 

~ 

H ~ 

600 700 

TEMPERATURE, IN DEGREES CELSIUS 

Figure 6. Petrogenetic grid for muscovite-quartz- and 

potassium feldspar-quartz-bearing systems (modified 

from Hess, 1969, fig. 3). Ky, Si, and A indicate 

position of alternative Al2Si05 triple point from 

Holdaway (1971); dash-dot arrow, pressuretemperature gradient of Baranof facies series; circled 

numbers refer to reactions in text; dashed arrow, 

pressure-temperature gradient of Pyrenean facies 

series; double lines, AI2Si05 equilibria; heavy lines, 

equilibria in Fe-free system, reaction (1), and Fe14 

bearing system, reaction (2); light lines, calculated 

equilibria; insert shows invariant point (Ch,S) on 

univariant line marked by equilibrium A + B = Cd + G. 

A, andalusite; B, biotite; Ch, chlorite; Ch', Mgchlorite; Cd, cordierite; G, garnet; H, hypersthene; Ky, 

kyanite; Mu, muscovite; ph, phlogopite; Q, quartz; Sa, 

sanadine; Si, sillimanite; S, staurolite. Small unlabeled 

diagrams indicate AFM fields and tielines present in 

different parts of the grid. 
requires a steeper P-T gradient than that of the 

Pyrenean facies series. If the disappearance of 

staurolite is bulk compositionally controlled and occurs 

before the staurolite-out reaction curve, then the 

gradient would have to be even steeper in order to 

reach the sillimanite field. 

The next reaction from the grid that is 

recognized is the muscovite-out reaction (8). This is 

manifest by a decline in muscovite and an increase in 

orthoclase in the sillimanite zone and higher (table 

6). The reaction places only a broad control on the 

position of the P-T gradient in the grid (fig. 6). 

The highest grade assemblage, biotite-garnetcordierite-sillimanite, corresponds to Hess' hightemperature reaction, producing cordierite-garnet 

(reaction (9)) on the low-pressure side of the invariant 

point (Ch,S). This assemblage controls the hightemperature end of the Baranof P-T gradient, but a 

negative slope is needed in order to reach this field. 

Whether or not this reflects a real decrease in pressure 

is unknown, but Gil lbarguchi and Martinez (1982) 

report a decrease in both pressure and temperature in 

the highest zone, possibly due to late deformation. 

Overall, the Baranof P-T gradient ranges in 

temperature from 550 to 790 °C and in pressure from 

3.9 to 4. 75 kbars (fig. 6). The pressure decreases 

above 700 °C, attaining a value of about 4.4 kbars in 

the cordierite-garnet field at 790 °C. 

As indicated by the foregoing discussion, the 

petrogenetic grid of Hess (1969) seems internally 

consistent with regard to the sequence of metamorphic 

mineral assemblages from southern Baranof Island. 

However, the position of the grid in P-T space is 

another matter. This is discussed here because it 

indicates the uncertainty that attends the use of 

petrogenetic grids, regardless of whether generalized 

or specific mineral composition data are used. One of 

the principal P-T controls of the grid is the system 

kyanite-andalusite-sillimanite of Richardson and 

others (1969), the triple point of which is at 5.5 kbars 

and 6 22 °C. It is well known, however, that values for 

this triple point obtained by various workers have 

ranged widely. For example, the triple point of 

Holdaway (1971), which has been favored in more 

recent papers (Holdaway, 1978; Anderson and others, 

1977; Day and Kumin, 1980; Grambling, 1981; Hod~es 

and Spear, 1982), is located at 3. 75 kbars and 501 C 

(fig. 6). The use of this triple point to control the grid 

would considerably lower both the temperature and the 

pressure values in the grid. 

Another P-T control on the grid concerns the 

formation of cordierite. As mentioned in the 

descriptions of the mineral assemblages, recent 

workers have studied the reactions involving cordierite 

with variable results. Burnell and Rutherford (1984) 

experimentally produced the terminal reaction for 

chlorite (reaction (5)) and obtained data that 

constrained the univariant assemblage chloritecordierite-biotite AI2Si05 as lying between points 

640 °C at 4 kbars and 612 °C at 2 kbars. Although 

these pressures are comparable to those on the grid, 

the temperatures are much higher than those for the 

chlorite-bearing assemblages (fig. 6). 

The widespread occurrence of the assemblage 

cordierite-almandine rich garnet-sillimanite is possible 

only at high temperatures and at a restricted range of 

intermediate pressures (Wynne-Edwards and Hay, 1963; 

Winkler, 1979, p. 230). The reaction through which 

cordierite is converted to garnet, sillimanite, and 

quartz is pressure sensitive, and the Mg content of 

cordierite should be a good geobarometer (Martignole 

and Nantel, 1982, p. 313). However, cordierite has two 

properties for which the effects on the reaction have 

not been properly evaluated. These are (1) the effects 

of water activity related to the variable hydration of 

cordierite, and (2) the effects of cordierite 

order/disorder (Essene, 1982, p. 180-181). The 

experimental and theoretical models proposed by 

various workers do not agree by wide margins. 

However, the models based on the experiments of 

Holdaway and Lee (1977) and those based on the 

calculations by Newton and Wood (1979) and 

Martignole and Sisi (1981) do agree approximately on 

pressures for intermediate values of water pressure; 

the pressures for intermediate Fe-Mg cordierite range 

from 3.25 to 3. 7 kbars for 650 °C. These pressures and 

temperatures are lower than those of the grid. 

In contrast, the pressure calculations of Gil 

lbarguchi and Martinez (1982), based on compositions 

of garnet, biotite, and cordierite from the garnetcordierite-sillimanite gneisses in the Iberian Massif, 

Spain, are generally higher, ranging from 4 to 7 

kbars. These variations resulted from using different 

geobarometers and different assumptions. Using the 

Newton and Wood (1979) garnet-cordierite 

geobarometer, Gil lbarguchi and Martinez found 

pressures ranging from 4 kbars (PH 0 = 0) to 5 kbars 

(PH 0 = Pt). They state that their ~ost reliable result ca~ from calculating the P-T conditions, using the 

experimental data of Holdaway and Lee (1977), for the 

coexistence of the seven-phase assemblage in the 

following reaction: 

Bt +Sill+ Gt + Qtz = Crd + Kspar + W, (10) 

which yielded 4.3 kbars, 695 °C for XH 0 = 0.5. This 

pressure is close to the value 4.4 kbars 2 (at 790 °C) 

for the garnet-cordierite field at the upper end of the 

Baranof P-T gradient on Hess' grid (fig. 6). 

It should be obvious from the above discussion 

that no general solution to the P-T petrogenetic grid 

problem is at hand. The pressures obtained by the 

various workers for the garnet-cordierite-sillimanite 

assemblage range from 3.5 to 7.0 kbars, about 

equivalent to 11 to 25 km depth. Most of the 

calculated pressures are more in harmony with the 

higher pressures of Hess' (1969) grid than the lower 

ones that would be produced if his grid were readjusted 

by using the lower triple point of Holdaway (1971). 

The best conclusion is that of Hess (1969), that the 

value of the petrogenetic grid is not found in the 

pressure-temperature estimates of the various facies 

but in the systematic analysis of various facies series, 

based on certain critical mineral assemblages. 

Finally, in Loney and others (1975, p. 90) it was 

stated that the experimental evidence, largely from 

Al2Si05 triple points, indicates a deeper minimum 

depth of metamorphism (11 km) of the type on Baranof 

Island than does field evidence for the depth of the 

overburden (3 km). Although the present analysis is 

based on a different approach, this statement still 

holds. 

15 
SUMMARY 

1. The Sitka Graywacke in which the contact 

aureoles were developed consists dominantly of 

interbedded semipelitic metagraywacke and pelitic 

metamudstone with a consistent composition 

throughout the aureoles. 

2. The intrusion of the Tertiary plutons that 

produced the contact aureoles in southern Baranof 

Island was postkinematic and passive. The 

me~amorphic rocks underlie an area of at least 1,200 

km and are the result of a single episode of thermal 

metamorphism. An initial and unexposed stage of the 

intrusive episode may have occurred during the last 

phase of intense folding (F 2) that affected Baranof 

Island in general. 

3. The previously mapped metamorphic zones and 

their boundaries (Loney and others, 1975; table 1) have 

been evaluated relative to model reactions using AFM 

projections and found to correspond to now-accepted 

reaction isograds. The boundary of the staurolite zone 

was determined to be bulk compositionally controlled 

and is not a reaction isograd. 

4. The sequence of mineral assemblages in the 

aureoles shows good agreement with Hess' (1969) 

petrogenetic grid, on which we define a P-T gradient 

that ranges from about 3.9 kbars at 550 °C in the 

garnet zone, through a maximum pressure point of 

about 4.9 kbars at 690 °C in the sillimanite zone near 

the muscovite-out reaction curve, and ending at about 

4.4 kbars at 790 °C in the garnet-cordierite zone (fig. 

6). The Baranof P-T gradient as defined by Hess' 

(1969) grid represents depths of metamorphism ranging 

from 13.6 to 15.4 km. 

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16 

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17 


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