Metamorphic Facies Map of Southeastern Alaska- Distribution, Facies, and Ages of Regionally Metamorphosed Rocks

By CYNTHIA DUSEL-BACON, DAVID A. BREW, and SUSAN L. DOUGLASS

REGIONALLY METAMORPHOSED ROCKS OF ALASKA

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 149'7-D

Prepared in cooperation with the Alaska Department of Natural Resources, Division of Geological and Geophysical survey^

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1996 U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Gordon P. Eaton, Director

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 edited by Jan Detterman Illustrations edited by Jan Detterman and Dale Russell, prepared by Mike Newman and Glen Schumacher

Library of Congress Catalog in Publication Data

Dusel-Bacon, Cynthia. Metamorphic facies map of southeastern Alaska: Distribution, facies, and ages of regionally metamorphosed rocks 1 by Cynthia Dusel-Bacon, David A. Brew, and Susan L. Douglass. p. cm. -- (Regionally metamorphosed rocks of Alaska) (U.S. Geological Survey Professional Paper ; 1497-D) "Prepared in cooperation with the Alaska Department of Natural Resources, Division of Geological and Geophysical Surveys." Includes bibliographical references (p. - 1. Supt. of Docs. no. : 1 19. 16: 1497-D 1. Rocks, Metamorphic--Alaska. 2. Metamorphism (geology)--Alaska. I. Brew, David A. 11. Douglass, Susan L. 111. Alaska. Division of Geological and Geophysical Surveys. IV. Title. V. Series. VI. Series: U.S. Geological Survey professional paper ; 1497-D. QE475.A2D884 1995 552'.4'09798--dc20 95-34356 CIP

For sale by U.S. Geological Survey, Information Services, Box 25286, MS 306, Federal Center, Denver, CO 80225 CONTENTS

Page Abstract ------.----Dl Detailed description of metamorphic map units-Continued Introduction ...... 1 Western metamorphic belt ...... ------Acknowledgments ----.------4 Admiralty Island and adjacent mainland area ------Summary of the major metamorphic episodes that Lpp (eK) ------.------affected southeastern Alaska ...... 7 GNS (eK) ...... Detailed description of metamorphic map units ...... 9 GNS AMP (eK) ...... Southern Prince of Wales Island and adjacent islands ---- 9 Kupreanof, Etolin, and Revillagigedo Islands and GNS (Oc) + LPP (DS) ...... 9 Cleveland Peninsula area ...... AMP (Oc) + LPP (DS) ...... 10 Lpp/GNS (mK) ...... Lpp (DS) ...... 10 LPP/GNS (mK) + GNL+I (IK) ...... GNS (DS) ...... 10 GNS (K) ...... GNS (DS) + GNS (K) ...... 11 AM1 (IK) ...... Glacier Bay and Chichagof and Baranof Islands area ----- 11 Mainland belt ...... AMP (eKR) ------..11 GNS (eTIJ) -.------GNS (eK17;) ------.12 GNL+I (IK) + GNI (eTIK) ...... AMP (eKlg) ...... 12 LPP (eTIK) ...... 1 - -.- GNS (eKIT) ...... 13 GNI (eTIK) ...... Lpp (eKeJ) -.------13 GNI AM1 (eTIK) ...... AMP (eK) ...... 14 AM1 (eTIK) ...... LPP/GNS (eTI'E;) ------.------14 AM1 L (eTIK) ...... LPp GNS (eTIJ) ------..------15 Tectonic interpretation of metamorphism in the LPP GNS (eTIJ) + GNL (eT) ...... 16 western metamorphic belt during Late LPP GNS (eTIJ) + AML (eT) ...... 17 Cretaceous and early Tertiary time ------GNS/AMp (eTK) ...... 18 References cited ...... AMP (eTK) ...... 19 LPP (eTIK) ------.19

ILLUSTRATIONS

[Plates are in pocket]

PLATE1. Metamorphic facies map of southeastern Alaska 2. Metamorphic-mineral locality map of southeastern Alaska Pdge FIGURE1. Map showing area of this report and other reports in the series of metamorphic studies of Alaska------D2 2. Map showing regional geographic areas in southeastern Alaska that are discussed in text ...... 3 3. Diagram showing schematic representation of metamorphic-facies groups and series in pressure-temperature space and their letter symbols ...... 4 4. Map showing general sources of metamorphic data for the metamorphic facies map of southeastern Alaska ------6

TABLES

REGIONALLY METAMORPHOSED ROCKS OF ALASKA

-- METAMORPHIC FACIES MAP OF SOUTHEASTERN ALASKA - DISTRIBUTION, FACIES, AND AGES OF REGIONALLY METAMORPHOSED ROCKS

By CYNTHIADUSEL-BACON, DAVID A. BREW,and SUSANL. DOUGLASS

ABSTRACT sively developed aureoles around some of the plutons within amphibolite-facies rocks in the central and southern part of the Nearly all of the bedrock of southeastern Alaska has been 90-Ma metamorphic belt. Other 90-Ma plutons that intrude low- metamorphosed to some degree. Much of it has been metamor- grade rocks have narrow low-pressure aureoles. Relict andalu- phosed under medium- to high-grade conditions during episodes site that has been replaced by kyanite occurs in aureoles within that were associated with widespread plutonism. amphibolite-facies rocks near the northern end of the 90-Ma The two oldest known metamorphic episodes in southeastern belt, near northernmost Wrangell Island. The early formation of Alaska occurred during an early Paleozoic and a middle Paleo- andalusite, indicative of low-pressure conditions, is hard to rec- zoic orogeny and affected probable arc-type volcanic, sedimenta- oncile with the high to intermediate pressures inferred for the ry, and plutonic rocks in the area of southern Prince of Wales associated garnet- and epidote-bearing plutons. Island. During Late Cambrian to Early Ordovician time, rocks The final metamorphic episode or phase was mostly synkin- were penetratively deformed, flattened, and recrystallized under ematic with, but may have slightly preceded, the lastest Creta- greenschist- to amphibolite-facies conditions. The subsequent ceous and early Tertiary mesozonal intrusion of a 600-km long, metamorphic episode, which was Silurian to earliest Devonian northwestward-trending composite body herein referred to as in age, occurred under prehnite-pumpellyite-facies conditions the "great tonalite sill." The northern two-thirds of the meta- and was not accompanied by penetrative deformation. morphic belt produced in Alaska during this episode is com- The predominant period of metamorphism and plutonism oc- posed of an intermediate-pressure (Barrovian) sequence in curred during the interval of Early Cretaceous to early Tertiary which metamorphic grade increases northeastward. Isograds time. The oldest documented metamorphic episode during this marking the first appearance of biotite, garnet, staurolite, kya- interval took place in northern southeastern Alaska and appar- nite, and sillimanite are generally parallel to the sill and in- ently was associated with the intrusion of elongate bodies of verted. Kyanite has not been reported in the southern part of highly foliated, 120- to 110-Ma tonalite and diorite. Low-grade the belt, and metamorphic pressures may not have been as high metamorphism of mid-Cretaceous age produced a weakly to as in the north. Intermediate and felsic epizonal plutons in- moderately developed metamorphic fabric in rocks extending truded the eastern part of the belt in Eocene time during the from Kupreanof Island in the north to the peninsula north and final phase of this metamorphic episode and produced low-pres- west of Revillagigedo Island (Cleveland Peninsula) and perhaps sure aureoles and zones of migmatite. to Revillagigedo Island in the south. This episiode predated the intrusion of mafic-ultramafic bodies that have yielded K-Ar ages of 110-100 Ma. Low-grade metamorphism of melange and flysch INTRODUCTION north of Cross Sound and on Chichagof and Baranof Islands, southwest of the Peril Strait fault, also took place sometime during the Early Cretaceous to early Tertiary interval. Meta- This report identifies and describes the major, morphism of the melange occurred in a subduction environment regionally developed metamorphic episodes that af- and may have begun as early as latest Jurassic time. fected southeastern Alaska throughout its evolu- Metamorphism during the next episode or phase was associ- tion. It is one of a series of four reports that pre- ated with the intrusion of garnet- and epidote-bearing plutons of early Late Cretaceous age (approximately 90 Ma). Experi- sents the metamorphic history of Alaska (fig. 1). mental data on the composition of magmatic garnet and the Metamorphic rocks are assigned to metamorphic- pressure required to crystallize magmatic epidote have been facies units, which are shown on a 1:1,000,000- used to infer a minimum 13-15 kb initial depth and a 6-10 kb scale colored map (pl. I), on the basis of the occur- final depth for crystallization of the magma body. Kyanite, in- rence of pressure- and temperature-sensitive min- dicative of intermediate-pressure conditions, occurs in exten- erals and the age of metamorphism. Plutonic rocks are also categorized on the basis of their age and the relation of their intrusion to metamorphism. Manuscript approved for publication March 26, 1987. By means of detailed unit descriptions, this report D2 REGIONALLY METAMORPHOSED ROCKS OF ALASKA summarizes the present (about mid-1987) state of are divided into three facies groups based on in- knowledge of the metamorphic grade, pressure con- creasing temperature: (1)laumontite and prehnite- ditions, age of protoliths and metamorphism, and, pumpellyite facies (LPP), shown in shades of gray in some cases, the speculated or known tectonic and tan; (2) greenschist facies (GNS), shown in origin of regional metamorphism. Metamorphic shades of green; and (3) epidote-amphibolite and units are discussed in the same order as that used amphibolite facies (AMP), shown in shades of red for the map explanation. Within each geographic and orange. Where possible, the greenschist-facies area (fig. 2), units are discussed in chronologic or- and the epidote-amphibolite- and amphibolite- der, from oldest to youngest; units of the same facies groups are divided into three facies series metamorphic age or age range are discussed in or- based on pressure. A high-, intermediate-, or low- der of increasing metamorphic grade. pressure-facies series is indicated by an H, I, or L The metamorphic-facies determination scheme in place of the final letter in the symbol used for (fig. 3, table 1) on which the map (pl. 1) is based the previously mentioned facies groups. was developed by the Working Group for the Car- In this compilation, the scheme of Zwart and tography of the Metamorphic Belts of the World others. (1967) is expanded. Specifically, combina- (Zwart and others, 1967). This scheme is based on tions of letters and symbols are used to indicate pressure- and temperature-sensitive metamorphic metamorphic conditions transitional between dif- minerals that are petrographically identifiable by ferent facies groups and series. Where two facies most geologists. Regionally metamorphosed rocks groups or facies series occur together but have not

FIGURE1.-Map showing area of this report (shaded) and other reports in the series of metamorphic studies of Alaska. A, Dusel-Bacon and others (1989); B, Dusel-Bacon and others (1996); C, Dusel-Bacon and others (1993). SOUTHEASTERN ALASKA

50 25 0 50 KILOMETERS I I I I

FIGURE2.-Regional geographic areas in southeastern Alaska that are discussed in text. D4 REGIONALLY METAMORPHOSED ROCKS OF ALASKA been differentiated, the designation of the more herein have been calculated or recalculated using abundant facies is given first, and the two designa- the decay constants of Steiger and Jager (1977). tions are separated by a comma. Where the meta- Metamorphic mineral assemblages for most morphic grade of a unit was transitional between metamorphic-facies units (table 2) follow the de- two facies groups, the lower grade designation is tailed descriptions of the metamorphic units and given first, and the two designations are separated are keyed to the metamorphic-mineral locality map by a slash. Where the pressure changes within a (pl. 2). facies series, an arrow is used to show the direc- General sources of metamorphic data used to com- tion of change. As a further expansion, a symbol pile the metamorphic facies map (pl. 1) are shown on for either the metamorphic age or minimum and figure 4. Complete citations for published sources are maximum limits of the metamorphic age is given given in the references. Additional sources are re- in parentheses following the facies symbol. In two ferred to in the detailed unit descriptions. instances, the numerical subscript "1" is used to differentiate between map units that have the same metamorphic grade and age but that have ACKNOWLEDGMENTS different protoliths and are believed to have differ- ent metamorphic histories. Where two metamor- We wish to thank the numerous geologists from phic episodes have affected the rocks, the symbol the U.S. Geological Survey, the State of Alaska De- gives the facies and age of each metamorphic epi- partment of Natural Resources, Division of Geo- sode, beginning with the oldest episode. Protolith logical Surveys, and several universities who freely and metamorphic age designations are based on communicated their thoughts and unpublished the Decade of North American Geology Geologic data to this report. R.A. Loney, M.L. Crawford, Time Scale (Palmer, 1983). Radiometric ages cited G.E. Gehrels, R.D. Koch, and R.L. Elliott were par-

1 FAClES GROUPS I FACES SERIES I TEMPERATURE - TEMPERATURE I

High-pressure-facies- series

FIGURE3.-Schematic representation of metamorphic-facies groups and series in pressure-temperature space and their letter symbols used in this report (modified from Zwart and others, 1967). Stability fields of AI,SiO, polymorphs andalusite (anda.), kyanite (ky.), and sillimanite (sill.) shown by dashed lines. SOUTHEASTERNALASKA

Table 1.-Scheme for determining metamorphic facies

[Modified from Zwart and others, 19671

Facies Diagnostic minerals Forbidden minerals Common Remarks symbol and assemblages and assemblages minerals and assemblages

- LAUMONTITE AND PREHNITE-PUMPELLYITE FACIES

LPP Laumontite +quartz, Pyrophyllite, analclme + "Chlorite", saponite, Epidote, actinolite, and prehnite + purnpellyite. quartz, heulandlte dolomite + quartz. "sphene" possible in ankerite + quartz, prehnite-pumpellyite kaolinite, montmoril- facies. lonite, albite, K-feldspar. "white mica"

GREENSCHIST FACIES

GNS Staurolite, andalusite. Epidote, chlorite. cordierite, piagioclase chloritoid, albite. (An>lO), laumontite + muscovite, calcite. quartz, prehnite + dolomite, actinolite pumpellyite talc

Low- and intermediate-pressure greenschist facies GNL and GNI Hornblende. glaucophane. Blotite and manganiferous crossite. lawsonite. garnet possible; stilpno- jadeite + quartz. melane mainly restricted aragonite to intermediate-pressure greenschist facies.

High-pressure greenschist (blueschist) facies GNH Glaucophane, crossite Almandine. paragonite, Subcalcic hornblende aragonite. jadeite + stilpnomelane (barroisite) may occur in quartz h~ghesttemperature part of this facies.

Low-temperature subfacies of high-pressure greenschist facies GNH Above minerals plus (w~th purnpellyite and (or1 stipple) lawsonite

EPIDOTE-AMPHIBOLITE AND AMPHlBOLlTE FACIES

AMP Staurolite Orthopyroxene + Hornblende. plag~o- clinopyroxene. clase. garnet, biotite. actlnolite + calcic muscovite. diopside. playioclase +quartz. K-feldspar. rutile, cal- glaucophane. cite, dolomite, scapolite

Low-pressure amphibolite facies AML Andalusite + staurolite. Kyanite Cordierite. sillimanite. Pyralspite garnet rare In cordierite + orthoamphl- cummingtonlte. lowest possible pressure bole. part of this facies.

Intermediate- and high-pressure amphibolite facies AM1 and AMH Kyan~te+ staurollte Andalusite Sillimanite mainly re- stricted to intermediate- pressure amphibolite facles.

TWO-PYROXENE FACIES

2PX Orthopyroxene + Staurollte, orthoamphi- Hypersthene, clinopyrox- Hornblende possible clinopyroxene bole. muscov~te.epidote. ene, garnet, cordler~te. Kyanite may occur in high- zois~te anorthite, K-feldspar. er pressure part of this sillimanite, biotite, facies and periclase and scapolite, calcite. dolo- wollastonite in low- mite. rut~le. pressure part. REGIONALLY METAMORPHOSED ROCKS OF ALASKA

FIGURE4.-General sources of metamorphic data for the metamorphic facies map of southeastern Alaska (pl. 1).Numbers refer to sources of data listed in explanation. Ring patterns around numbers correspond to boundary pattern used to delineate that area. SOUTHEASTERN ALASKA D7 ticularly helpful in this regard. Drafting and tech- Island (Klakas orogeny of Gehrels and others nical assistance were provided by M.A. Klute, E.O. (1983) and Gehrels and Saleeby (198713)). Rocks Doyle, and K.E. Reading. R.A. Loney and M.L. metamorphosed for the first time during this epi- Crawford made valuable suggestions that helped sode include basaltic to rhyolitic metavolcanic improve the original version of this manuscript. rocks, metasedimentary rocks, metachert, and met- The expert and patient map and text editing by alimestone of late Early Ordovician to Early Siluri- J.S. Detterman is especially appreciated. an protolith age and quartz dioritic metaplutonic rocks of Middle Ordovician to Early Silurian age (Eberlein and others, 1983; Gehrels and Berg, SUMMARY OF THE MAJOR METAMORPHIC 1984). Metamorphic grade is lowest (prehnite-pum- EPISODES THAT AFFECTED SOUTHEASTERN pellyite facies) on southern Prince of Wales Island ALASKA where the rocks are not penetratively deformed, and relict sedimentary and volcanic textures are The oldest metamorphic episode in southeastern widespread. Metamorphic grade increases south- Alaska occurred during Late Cambrian and Early westward to lower greenschist facies and eastward Ordovician time under greenschist- and amphibolite- to greenschist facies and, locally, epidote- facies conditions and produced greenstone, mafic amphibolite facies. The Wales Group is presumed schist, pelitic schist, phyllite, marble, and intermedi- to have undergone weak retrograde metamorphism ate metaplutonic rocks, all of which are assigned to during this episode. the Wales Group. These rocks crop out on southern The dominant period of orogenic activity in Prince of Wales Island and adjacent islands and southeastern Alaska took place from Early Creta- have been described most recently by Gehrels and ceous to early Tertiary time and consisted of mul- Saleeby (1987b). In most areas, protolith features tiple episodes of plutonism and dynamothermal are obscured by penetrative metamorphic recrystalli- metamorphism. In the northern half of southeast- zation and a high degree of flattening. ern Alaska, metamorphism of units that have A weakly to moderately developed Silurian to metamorphic-age designation "eK" apparently was earliest Devonian metamorphic episode was part of associated with the intrusion of elongate bodies of the subsequent orogenic event that affected the highly foliated tonalite and diorite of Early Creta- rocks in the vicinity of southern Prince of Wales ceous age (120-110 Ma; Loney and others, 1967; Decker ind Plafker, 1982). ear Glacier Bay and on Chichagof Island, the Early Cretaceous meta- EXPLANATION morphic sequence consists of metasedimentary and 1. George Plafker and Travis Hudson (unpublished mapping, metavolcanic rocks of Silurian to Devonian proto- 1978) lith age that were metamorphosed under amphibo- 2. Brew (1978) lite- or hornblende-hornfels-facies conditions. In 3. Plafker and Hudson (1980) these areas, structural trends in metamorphic 4. MacKevett and others (1974) 5. Redman and others (1984) rocks parallel those of the Early Cretaceous plu- 6. Barker and others (1986) tons. On Admiralty Island and the adjacent main- 7. Brew and Ford (1985) land, the Early Cretaceous metamorphic sequence 8. Ford and Brew (1973) comprises metasedimentary, metavolcanic, and 9. Brew and Ford (1977) metaplutonic rocks of Ordovician to Early Creta- 10. Ford and Brew (1977a) 11. Brew and Grybeck (1984) ceous protolith age. Metamorphic grade in the area 12. Lathram and others (1965) of Admiralty Island increases progressively toward 13. Johnson and Karl (1985) the Early Cretaceous plutons; in the highest grade 14. Loney and others (1975) part of the sequence, large areas of dynamother- 15. D.A. Brew and D.J. Gr~beck(unpublished mapping, 1969) mally metamorphosed phyllite, schist, and gneiss 16. Brew and others (1984); S.L. Douglass (unpublished meta- morphic facies map, 1985) merge with contact aureoles of the plutons (Loney 17. R.L. Elliott and R.D. Koch (unpublished mapping, 1979) and others, 1967). 18. Eberlein and others (1983) Low-grade metamorphism of mklange and flysch 19. Gehrels and Berg (1984); G.E. Gehrels (written commun., north of Cross Sound and on Chichagof and 1985) Baranof Islands southwest of the Peril Strait fault 20. Berg and others (1978; 1988) 21. M.L. Crawford (written commun.. 1984) took place prior to the intrusion of Eocene plutons and associated regional thermal metamorphism. Deposition of the mklange is interpreted to have D8 REGIONALLY METAMORPHOSED ROCKS OF ALASKA occurred, in part, during Late Jurassic to Early indeed problematic. However, within lower grade Cretaceous time (Brew and others, 1988), and rocks to the north and west of the amphibolite- deposition of the flysch is inferred to have occurred facies rocks on Wrangell Island and to the south sometime during the Cretaceous. Metamorphism of and west of the amphibolite-facies rocks on the melange occurred in a subduction environment Revillagigedo Island, 90-Ma plutons that have nar- and may have begun as early as latest Jurassic row, low-pressure aureoles also occur (not shown or time. East of the Chatham Strait fault, low-grade described in this report). metamorphism of presumed mid-Cretaceous age The final metamorphic episode or phase of a pro- produced a weakly to moderately developed meta- longed metamorphic cycle, may have slightly pre- morphic fabric. This episode predated the intrusion ceded but was mostly synkinematic with the of mafic-ultramafic bodies that have yielded K-Ar lastest Cretaceous and early Tertiary mesozonal ages of 110-100 Ma (Lanphere and Eberlein, 1966; intrusion of a 600-km-long composite body here re- Brew and others, 1984). ferred to as the "great tonalite sill." Intrusion of In many areas, the low-grade fabric developed the sill has been dated by U-Pb zircon methods at during mid-Cretaceous time was subsequently about 69 Ma near Juneau in the north, at about 62 overprinted by metamorphism associated with the Ma near Petersburg in the central part of the sill's early Late Cretaceous (about 90-Ma) intrusion of extent (Gehrels and others, 1984), and at about intermediate plutons that contain primary garnet 58-55 Ma east of Revillagigedo Island in the south and epidote. Zen and Hammarstrom (1984b), using (Berg and others, 1988). The northern two-thirds of experimental data on the composition of magmatic the metamorphic belt shown in Alaska is composed garnet and on the pressure required to crystallize of an intermediate-pressure (Barrovian) sequence magmatic epidote, propose that the magma for whose metamorphic grade increases northeast- these plutons began to crystallize at a minimum ward. This sequence comprises metasedimentary pressure of 13-15 kb (about 40-50 km in depth) and metavolcanic rocks, metalimestone, metachert, and finally crystallized at about 6-10 kb (about schist, amphibolite, gneiss, and migrnatite that crop 20-30 km in depth). out along the mainland of southeastern Alaska from The degree and regional extent of recrystalliza- Skagway to the area east of Wrangell Island. Min- tion associated with this plutonism decreases to eral isograds marking the first appearance of biotite, the north and west as does the apparent pressure garnet, staurolite, kyanite, and sillimanite trend during metamorphism. In the central and southern north-northwest, generally parallel with the elongate part of the belt (on Wrangell Island, Cleveland tonalite sill. Isogradic surfaces dip moderately to Peninsula, and northern Revillagigedo Island), steeply northeast (Ford and Brew, 1973; 1977a; kyanite, which is indicative of intermediate- Brew and Ford, 1977) and, hence, are inverted. The pressure metamorphic conditions, is common in southern one-third of the belt in Alaska is made up staurolite+garnet+sillimanite-bearing pelitic schist of high-grade migmatite, massive to foliated or adjacent to the larger 90-Ma plutons. In that area, gneissic batholiths, and smaller plutons that enclose metamorphic grade increases toward the plutons metamorphic screens and roof pendants of and sillimanite and kyanite isograds are concentric paragneiss. Kyanite has not been reported in the to large plutons. Geobarometric data from two southern part of the belt, and metamorphic pres- samples of garnet-kyanite schist from northern Re- sures may not have been as high as in the north. villagigedo Island indicate a final equilibration Evidence for the association of metamorphism and pressure for mineral rims of about 7 to 9 kb, which plutonism during this episode consists of the in- falls in the range of pressures proposed for the crease in metamorphic grade toward the sill, the garnet-epidote-bearing plutons. general parallelism between the sill and isograds Farther north near northernmost Wrangell Is- that define the Barrovian sequence, and the parallel- land, pelitic schist from the aureoles of small 90- ism of foliation, contacts, and locally developed linea- Ma plutons contains relict andalusite that has tion of the sill with structural elements in the adja- been replaced by static (radial) kyanite or aggre- cent metamorphic rocks. During the final phase of gates of intergrown kyanite and staurolite. The this episode, intermediate and felsic epizonal plu- combination of the high- to intermediate-pressure tons intruded the eastern part of the belt during magmatic and crystallization history inferred for Eocene time and produced low-pressure aureoles the plutons and the occurrence of relict andalusite, and zones of migmatite. which is indicative of low-pressure conditions (less The tectonic environment of the widespread than 3.8 kb; Holdaway, 1971), in their aureoles is plutonometamorphic episode(s) that occurred along SOUTHEASTERN ALASKA D9 the western edge of the Coast Mountains in early (Buddington and Chapin, 1929; Herreid and others, Late Cretaceous and early Tertiary time was domi- 1978; Eberlein and others, 1983), and most recent- nated by crustal thickening caused by the accretion ly as the informally designated "Wales metamor- of an outboard terrane(s) to the west. Petrologic phic suite" (Gehrels and Saleeby, 1987b). Prelimi- and isotopic data from metamorphic and plutonic nary U-Pb data on zircon indicate Middle and (or) rocks near Prince Rupert, British Columbia, which Late Cambrian protolith ages for interlayered are -considered to be correlative with Alaskan rocks metaplutonic bodies and, therefore, Late Protero- in the area of Revillagigedo Island and the main- zoic and (or) Cambrian protolith ages for the meta- land to the east, have been interpreted to indicate sedimentary and metavolcanic rocks (Gehrels and that crustal thickening resulted from west-directed Saleeby, 1987b). tectonic stacking of crustal slabs along east-dipping The most abundant and widely distributed rock thrusts, in places possibly lubricated by the intru- type is greenschist (chlorite+albite+epidote sion of the epidote-bearing plutons discussed above fquartz* actinolite); more siliceous and argillaceous (Crawford and others, 1987). Rapid (1 mmlyr) variants contain sericite rather than chlorite. vertical uplift in the eastern half of the plutono- Metamorphosed silicic volcanic rocks (quartz ke- metamorphic belt is proposed to have followed be- ratophyre) are commonly associated with the tween about 60 and 48 Ma, when the tonalite sill greenschist and have a distinctive blastoporphy- was intruded at deep levels during the early stages ritic texture defined by quartz eyes and pheno- of uplift, and the felsic and intermediate plutons crysts or glomeroporphyritic clots of albite in a were intruded at high levels during the late stages fine-grained matrix of quartz and albite. On the of uplift (Hollister, 1982; revised in Crawford and basis of petrographic and chemical evidence, others, 1987). According to Crawford and others Herreid and others (1978) proposed that although (1987), the intrusion of the tonalite sill facilitated much of the albite in the metamorphosed kerato- uplift by weakening the crust and serving as a phyre and spilite is of igneous origin, some of the melt-lubricated shear zone. albite present in those rocks, like that present in metasedimentary phyllites and greenschists, is of metamorphic origin. DETAILED DESCRIPTION OF METAMORPHIC In most areas protolith features are obscured by MAP UNITS penetrative metamorphic recrystallization and a high degree of flattening. Locally preserved proto- SOUTHERN PRINCE OF WALES ISLAND AND ADJACENT lith features include relict pillows and pyroclastic ISLANDS fragments in basaltic and andesitic metavolcanic rocks and rhythmic and graded bedding in meta- sedimentary rocks (Gehrels and Saleeby, 1987b). Dynamothermally metamorphosed greenschist- Schistosity in these and the other rocks of this unit facies greenschist, greenstone, pelitic schist and generally is parallel to compositional layering phyllite, quartz-sericite schist, and marble crop out (Herreid and others, 1978; Eberlein and others, on southern Prince of Wales Island (Gehrels and 1983). Shallow-plunging upright folds, which have Saleeby, 198713) and in an area (too small to show kilometer-scale wavelengths, and asymmetric out- on pl. 1) on the southernmost tip of Gravina Island crop-scale folds, which probably formed as parasitic and the small islands adjacent to it (Gehrels and structures on limbs of the regional folds, do not others, 1987). On Prince of Wales Island, this unit have an axial planar foliation and are interpreted also includes small areas of amphibolite-facies to have formed during the waning stages or after schist and metaplutonic rocks in the northeastern the main phase of deformation and metamorphism (Herreid and others, 1978) and southwestern (Gehrels and Saleeby, 1987b). (Gehrels and Saleeby, 1987b) parts of the unit. Metamorphism of the Wales Group took place Protoliths are mafic to intermediate volcanic rocks during the Late Cambrian and Early Ordovician and volcaniclastic strata, locally interlayered lime- Wales orogeny of Gehrels and Saleeby (1987a, b) as stone, and minor dioritic bodies, all of which are indicated by the following data: (1) metaplutonic interpreted to have formed in a volcanic arc rocks of Middle and (or) Late Cambrian age are environment (Gehrels and Saleeby, 1987b, and ref- metamorphosed and deformed (Gehrels and erences cited therein). The metavolcanic and meta- Saleeby, 1987b); (2) uppermost Lower and Middle sedimentary rocks have been described as the Ordovician marine clastic strata of the Descon For- Wales Series (Brooks, 1902), the Wales Group mation that occur near and probably overlie the Dl0 REGIONALLY METAMORPHOSED ROCKS OF ALASKA

Wales Group are only weakly metamorphosed and Middle Ordovician to Early Silurian plutonic rocks lack the penetrative metamorphic fabric character- (shown as a weakly metamorphosed pluton, pl. I), istic of the Wales Group (Eberlein and others, which include diorite, quartz diorite, granodiorite, 1983; Gehrels and Saleeby, 1987b); and (3) K-Ar quartz monzonite, granite, and subordinate gabbro, data indicate that the Wales Group was involved in pyroxenite, trondhjemite, quartz syenite, and mafic a regional thermal event that cooled to argon- dikes (Eberlein and others, 1983; Gehrels and blocking temperatures about 483 Ma (Turner and Saleeby, 1987b). The plutonic rocks show no visible others, 1977). This unit does not appear to have effects of the low-grade metamorphic episode, but been retrograded by subsequent low-grade meta- they are presumed to have been metamorphosed morphism, but geologic relations indicate that it because the proposed age of low-grade metamor- was probably affected by the same low-grade meta- phism postdates the protolith ages of these rocks. morphic episode during Silurian and earliest Devo- Low-grade metamorphism of metasedimentary and nian time that is recorded in the adjacent Ordovi- metavolcanic rocks is indicated in the southern cian and Silurian rocks of unit LPP (DS). area by fine-grained, felty aggregates of chlorite, albite, and epidote; this metamorphism decreases northward (G.E. Gehrels, oral commun., 1985). AMP (OC)+ LPP (DS) This unit is not penetratively deformed, and relict sedimentary and volcanic textures are widespread Amphibolite-facies schist and gneiss that crop (Eberlein and others, 1983). out on southern Dall Island are interpreted by G.E. A Silurian and earliest Devonian metamorphic Gehrels (oral commun., 1987) to compose higher age is proposed on the basis of (1) strata of Early grade equivalents of the greenschist-facies rocks of Silurian age are metamorphosed but overlying the Wales Group to the east (unit GNS (OC)+ LPP strata of middle Early Devonian age and younger (DS)) and to have been metamorphosed initially are unmetamorphosed (Gehrels and others, 1983) during the Late Cambrian and Early Ordovician and (2) geologic relations in greenschist-facies Wales orogeny described above. Protoliths are ma- units GNS (DS) and GNS (DS) + GNS (K) (dis- rine clastic strata, basaltic and felsic volcanic cussed below) that are presumed to have been rocks, and subordinate limestone (Gehrels and metamorphosed during the same episode that af- Berg, 1984) of probable Late Proterozoic and (or) fected unit LPP (DS). This metamorphic episode is Cambrian age (G.E. Gehrels, oral commun., 1987). considered to have been part of a metamorphic, Characteristic metamorphic mineral assemblages deformational, and mountain-building event re- include sillimanite, garnet, muscovite, biotite, and ferred to as the Klakas orogeny by Gehrels and hornblende (Eberlein and others, 1983; G.E. others (1983) and Gehrels and Saleeby (198713). Gehrels, oral commun., 1985). This unit does not appear to have been retrograded by subsequent low-grade metamorphism, but geologic relations in- GNS (DS) dicate that it was probably affected by the same low-grade metamorphic episode during Silurian This unit comprises lower greenschist-facies and earliest Devonian time that is recorded in the greenschist, semischist, phyllite, and slate, as well adjacent Ordovician and Silurian rocks of unit LPP as metalimestone on Long and Dall Islands (DS). (Eberlein and others, 1983; Gehrels and Berg, 1984). Protoliths for these rocks include Silurian limestone, Lower Ordovician through Lower Siluri- LPP (DS) an sedimentary and volcanic rocks of the Descon Formation, and Middle Ordovician to Silurian in- This unit comprises (1) uppermost Lower Ordo- trusive rocks (Eberlein and others, 1983; Gehrels vician to Lower Silurian weakly metamorphosed and Berg, 1984). A Silurian and earliest Devonian subgreenschist- (probably prehnite-pumpellyite-) metamorphic age is proposed for this unit because facies metagraywacke, metamudstone, argillite, rocks of Silurian age are involved in the metamor- metavolcanic rocks of basaltic, andesitic, and felsic phism and are cut by an undeformed latest Siluri- composition, and minor metachert of the Descon an to earliest Devonian (408k10-Ma) pyroxenite in Formation and age-equivalent rocks; (2) Silurian the Dixon Entrance quadrangle (Eberlein and 0th- metalimestone on northern Dall Island (Eberlein , ers, 1983; G.E. Gehrels, unpub. mapping, 1984). and others, 1983; Gehrels and Berg, 1984); and (3) I Metamorphism apparently decreases in grade to SOUTHEASTERN ALASKA Dl1 the north into unit LPP (DS) (Eberlein and others, of the episode (Gehrels and others, 1983, 1987). 1983). The early episode of greenschist-facies and, locally, epidote-amphibolite-facies metamorphism of the lower Paleozoic rocks of this unit is therefore con- GNS (DS) + GNS (K) sidered to have occurred during the Silurian and earliest Devonian metamorphic, deformational, and Polymetamorphosed greenschist-facies and, lo- mountain-building episode referred to as the cally, epidote-amphibolite-facies metavolcanic and Klakas orogeny by Gehrels and others (1983, metasedimentary rocks, metadiorite (shown as 1987). strongly metamorphosed plutons, pl. 1) and meta- Near Cape Fox, the metamorphosed Late Siluri- trondhjemite (shown as weakly metamorphosed an trondhjemite pluton has been more completely plutons, pl. 1) (Berg, 1972, 1973; Berg and others, recrystallized than have been the other plutons of 1978; Gehrels and others, 1983, 1987) crop out on that age and composition within this unit: almost Gravina, Annette, and Duke Islands and near Cape all primary biotite has been replaced by chlorite, Fox to the southeast. Volcanic and sedimentary metamorphic. epidote is abundant, and parts of the protoliths are Ordovician to Early Silurian in age; pluton, particularly the northern part, have been crosscutting intrusive rocks yield Ordovician and reduced to quartzofeldspathic semischist and schist Early Silurian (diorite) and Late Silurian (trond- that are difficult to distinguish from the enclosing hjemite) U-Pb igneous crystallization ages on zir- schistose country rocks (Berg and others, 1978, con (Gehrels and others, 1987). Greenschist-facies 1988). Much of the metamorphic recrystallization rocks include greenstone, fine-grained greenschists of this pluton may have occurred during the second and quartz-rich schists (perhaps originally inter- metamorphic episode. bedded quartz keratophyres and spilites), impure A Cretaceous age for the second metamorphic quartzites, phyllite, marble, massive hornblende- episode is indicated by the presence of lower green- and quartz-metadiorite, metadioritic migmatite schist-facies metamorphic mineral assemblages and agmatite, and metabasites that probably rep- and fabrics in overlying Devonian, Triassic, Juras- resent dikes and sills associated with the dioritic sic, and Cretaceous rocks (adjacent unit GNS (K)). intrusive rocks (Berg, 1972, 1973; Berg and others, This lower greenschist-facies metamorphism pro- 1978; Gehrels and others, 1987). duced retrograde metamorphic effects in the higher Cataclastic textures are common throughout this grade parts of this unit (Berg, 1972, 1973). A K-Ar unit, and, although the rocks are locally schistose, age determination of about 77 Ma on biotite from a they generally are only weakly foliated (Berg, metamorphosed diorite on the Metlakatla Penin- 1972, 1973). Metadiorite and metavolcanic and sula of Annette Island (Berg, 1973) gives a local metasedimentary rocks generally have a north- minimum metamorphic age for this episode. northwest-striking foliation (Gehrels and others, 1983). Metamorphic mineral assemblages contain, depending on their original rock types, combina- GLACIER BAY AND CHICHAGOF AND BARANOF ISLANDS tions of chlorite, epidote-clinozoisite, albite, color- AREA less to moderately pleochroic green amphibole, sericite, quartz, calcite, dolomite, and traces of AMP (eKR) brown biotite (Berg, 1972, 1973). Minor retrogres- sive metamorphism is apparent in the higher grade Amphibolite- or hornblende-hornfels-facies am- rocks of this unit (Berg, 1972, 1973). phibolite, gneiss, hornblende schist, and biotite The first and most intense metamorphism of schist, locally intercalated with thin layers of mar- these rocks predated the deposition of the overly- ble and calc-silicate granofels (Loney and others, ing marine strata of middle Early Devonian age 1975; Johnson and Karl, 1985), crop out in the (Gehrels and others, 1983). On Annette Island, west-central part of the Sitka quadrangle. Proto- Late Silurian trondhjemite dikes crosscut the folia- liths are probably mafic volcanic rocks and marine tion of metadioritic rocks that were metamor- sedimentary rocks (Johnson and Karl, 1985). These phosed during the first episode, but the dikes have protoliths predate the Middle and Late Jurassic the same late-metamorphic deformational features (168- to 155-Ma) K-Ar ages on biotite and horn- as their wallrocks, indicating that the trondhjemite blende from crosscutting diorite intrusions (Loney dikes and plutons were intruded before the final and others, 1967) and are thought to be Paleozoic deformation that occurred during the latter stages or Mesozoic in age. The most abundant rock type is Dl2 REGIONALLY METAMORPHOSED ROCKS OF ALASKA a quartz-andesine-biotite-hornblende schist that cite, and sodic plagioclase. Some chloritic schist and commonly contains almandine garnet; variations in phyllite also contain biotite and epidote and trace the amount of quartz and feldspar versus mafic amounts of sphene or garnet, particularly near plu- minerals are common and give the rocks a striking tons. Higher grade hornblende- or staurolite-bearing banded appearance. Calc-silicate granofels contains assemblages also occur near intrusive bodies the assemblages bytownite-diopside-clinozoisite- (MacKevett and others, 1974). Large areas of ther- calcite-pyrite-sphene and quartz-calcite-diopside- mally metamorphosed rocks are shown on the meta- grossularite. morphic facies map (contact aureole symbol, pl. l). The age and origin of this metamorphic episode Temperature and pressure(?) conditions of regional are unknown. Loney considers that the metamor- dynamothermal metamorphism probably increased phism of this unit is unrelated to the intrusion of to the northeast because along the northeast margin the Jurassic plutons because the direction of an of this unit schist and phyllite is in gradational con- overall increase in metamorphic grade (generally tact with gneiss and amphibolite of the amphibolite- northwest to southeast in the vicinity of Salisbury facies unit described immediately below. Sound) bears no relation to the distance from the The age of the metamorphism is not well known. plutons (R.A. Loney, oral commun., 1985). How- It is bracketed between the inferred Permian age of ever, Johnson and Karl (1985) report that this unit the youngest protolith proposed by MacKevett and grades into the dioritic rocks of Jurassic and Ju- others (1974) to be present in the Skagway area and rassic or Cretaceous age and that the metamorphic the Early Cretaceous (about 120- to 110-Ma) age of rocks become more migmatitic close to the plutons. crosscutting intermediate plutons that postdate dy- The relations described by Johnson and Karl imply namothermal metamorphism (MacKevett and others, a genetic connection between plutonism and meta- 1974). Geologic evidence from the apparent continua- morphism. Following the interpretation of Johnson tion of the metamorphic sequence about 105 km to and Karl, these metamorphic rocks may be part of the northwest in Canada suggests that the regional the basement of the Jurassic and Early Cretaceous metamorphism and deformation occurred during lat- arc that is now represented by the more volumi- est Triassic to Early Cretaceous time and may have nous late Early Cretaceous granitic rocks of been associated with latest Jurassic to earliest Cre- Chichagof Island, as suggested by D.A. Brew and taceous (150- to 130-Ma) plutonism. According to S.M. Karl (oral communs., 1985). Because of the R.B. Campbell and C.J. Dodds (written communs., uncertainty in the metamorphic history of this 1986), unit, we have shown the age of metamorphism to be sometime during Paleozoic to Early Cretaceous The younger of the 130- to 150-Ma plutons seem clearly to post- time to allow for the possibilities that metamor- date the metamorphism and deformation in the northeast, where they produce distinct contact metamorphic aureoles. The phism occurred long before plutonism (assumming older plutons of this group to the southwest may have been in- the oldest possible protolith age for the rocks) or truded during the metamorphism and deformation; they are that metamorphism accompanied plutonism. commonly elongate parallel with the regional structural grain, but they clearly have local crosscutting contacts and probably in part postdate those events. Upper Triassic strata probably rest unconformably on Paleozoic rocks but, nevertheless, appear to GNS (eKIX) be equally deformed and metamorphosed; thus, the widespread deformation and metamorphism seem to be post-Late Triassic Greenschist-facies chlorite schist, mica schist, and pre-earliest Cretaceous. phyllite, slate, metalimestone, and subordinate im- pure quartzite crop out in the central part of the Assuming continuation of the metamorphic unit and Skagway quadrangle. Protoliths include mafic vol- the continuity of the general geologic relations, we canic rocks, tuff, and quartzofeldspathic, pelitic, have assigned Late Triassic and Early Cretaceous calcareous, and carbonaceous sedimentary rocks metamorphic-age brackets to this unit. that have been correlated with rocks of Silurian to Permian age (MacKevett and others, 1974; Redman and others, 1985; Gilbert and others, 1987). Rocks AMP (eKIX) of this unit are structurally complex; they have been multiply deformed and locally are complexly Quartz-biotite gneiss, amphibolite, hornblende folded (MacKevett and others, 1974). schist, and minor phyllite and marble (MacKevett Schist and phyllite are fine grained and are com- and others, 1974; Redman and others, 1985; Gil- posed primarily of chlorite, quartz, muscovite, cal- bert and others, 1987) crop out in the central part SOUTHEASTERN ALASKA Dl3 of the Skagway quadrangle. Protoliths are pre- that these rocks were originally part of, or were co- sumed to range in age from Silurian to Permian extensive with, the Wrangellia terrane of Jones and include granitic rocks, from which the gneiss and others (1977) (Plafker and Hudson, 1980; probably was derived, and mafic volcanic rocks and Davis and Plafker, 1985). dikes, sedimentary rocks, and limestone, from Metamorphism is known to have occurred some- which the other rocks were derived (MacKevett time between the Late Triassic (Carnian?) age of and others, 1974). Rocks of this unit are strongly the protoliths and the late Early Cretaceous age of foliated; gneissic rocks are locally folded or crosscutting plutons (MacKevett and others, 1974). cataclastically deformed (MacKevett and others, Low-grade metamorphism may have been caused 1974). by heating as a result of the intrusion of Creta- Probable orthogneiss contains abundant quartz ceous dioritic or granodioritic rocks that crop out +oligoclase+biotite+muscovitefpotassium-feldspar. adjacent to the metabasalt along its east margin. Less abundant minerals include epidote, calcite, The origin of the near-vertical foliation in the and chlorite (largely as alteration products), northern part of this unit is unknown. opaque minerals, and rare garnet and staurolite (MacKevett and others, 1974). Arnphibolite is com- posed primarily of hornblende, andesine, minor to LPP (eKeJ) abundant chlorite and biotite, and minor quartz, opaque minerals, calcite, epidote, and sphene. This unit comprises weakly metamorphosed Si- Hornblende-biotite-quartz schist locally contains lurian and (or) Devonian metagraywacke, argillite, small amounts of chlorite and garnet (MacKevett phyllite, slate, semischist, metalimestone, meta- and others, 1974). Mineral assemblages indicate conglomerate, and mafic to intermediate metavol- conditions of the lower amphibolite facies. canic rocks; Mississippian metalimestone and The age and origin of metamorphism are shale; Permian phyllite, slate, semischist, meta- the same as those for unit GNS (eKI3) discussed limestone, metavolcanic rocks, and metachert; and above. Lower Jurassic(?) metachert and argillite (Lathram and others, 1959; Loney and others, 1975; Brew, 1978; C.D. Blome, written commun., 1987). The GNS (eKIli), rocks appear to be very weakly recrystallized but are lacking minerals that are diagnostic of a par- This unit comprises weakly metamorphosed ticular low-grade metamorphic facies. Upper Devo- metabasalt and, on the west side of the Chilkat nian andesitic and basaltic rocks on northern Peninsula directly south of Haines, associated car- Chichagof Island have experienced widespread bonaceous argillite, metasiltstone, volcaniclastic chloritization and epidotization and locallized metasandstone, and metalimestone (MacKevett albitization (Lathram and others, 1959). Metamor- and others, 1974; Plafker and Hudson, 1980; phic minerals developed in the matrix of gray- Redman and others, 1984; Davis and Plafker, wackes on Chichagof Island include chlorite, epi- 1985). dote, white mica, albite, and quartz (Devonian Metabasalt between Klukwan and Haines is Cedar Cove Formation) and chlorite and calcite (Si- characterized by near-vertical foliation that strikes lurian Point Augusta Formation) (Loney and oth- northwest, approximately parallel to the fault that ers, 1975). Permian rocks that crop out northeast occurs along the west margin of the unit of Muir Inlet in Glacier Bay and Lower Jurassic(?) (MacKevett and others, 1974; Redman and others, metachert and argillite in the north-central part of 1984). Metabasalt of the Chilkat Peninsula south the Chilkat Range appear to have experienced the of Haines is not penetratively deformed, and pri- same general degree of recrystallization as the Si- mary textures and structures are typically well lurian and Devonian rocks (D.A. Brew, unpub. preserved (Plafker and Hudson, 1980; Davis and data, 1985, 1986). Plafker, 1985). Metabasalt is recrystallized to The age and origin of low-grade metamorphism lower greenschist-facies assemblages that include are unknown. Metamorphism is known to be brack- primarily chlorite, actinolite, epidote, clinozoisite, eted between the Early Jurassic(?) protolith age of albite, white mica, and sphene. Geochemical data the youngest rocks that are clearly metamorphosed from metabasalt on the Chilkat Peninsula and the and the late Early Cretaceous age of crosscutting Carnian(?) age of associated weakly metamor- plutons. A gradual increase in metamorphic grade phosed limestone have been interpreted to indicate between the low-grade rocks of this unit and the Dl4 REGIONALLY METAMORPHOSED ROCKS OF ALASKA amphibolite-facies rocks (AMP (eK)) does not ap- clase or andesine, biotite, and minor muscovite. pear to be present on Chichagof Island; this sug- These rocks also contain minor amounts of alman- gests that, unlike the higher grade rocks whose dine garnet, but whether it is a metamorphic or rel- metamorphism is considered to have been associ- ict igneous mineral is not known (Loney and others, ated with late Early Cretaceous plutonism, low- 1975). Mineral assemblages found in calcareous grade metamorphism in that area may have oc- granofels and marble are calcite+diopsidef curred prior to plutonism. In the Glacier Bay area, grossularite~wollastonite, calcite+serpentine+brucite evidence of overprinting by the locally adjacent +spinel+scapolite, and calcite+cummingtonite. AMP (eK) metamorphism is not present, however, Metamorphism of this unit is considered to have and the low-grade metamorphism may be a more taken place during intrusion of the late Early Cre- distant expression of the same event. taceous plutons. The general parallelism between the foliate fabric of the plutons, pluton-wallrock contacts, and structures in the wallrocks suggests AMP (eK) that plutonism, folding, and thermal and dynamo- thermal metamorphism all took place as part of a This unit constitutes a diverse assemblage of continuum that occurred under roughly the same amphibolite-facies and hornblende-hornfels-facies stress conditions. K-Ar ages of about 110-120 Ma pelitic and semipelitic schist and gneiss, marble, for the plutonic rocks (Loney and others, 1967; and amphibolite and minor amounts of lower grade Decker and Plafker, 1982) provide a minimum age greenstone and greenschist. Protoliths are known for the late Early Cretaceous metamorphic episode. to be sedimentary and volcanic rocks of Silurian and Devonian age (Loney and others, 1975; Brew, 1978). These rocks are considered to be part of the Alexander terrane (Brew, 1978; Decker and Plafker, 1982). The unit is extensively intruded by Transitional prehnite-pumpellyite- to green- elongate bodies of highly foliated tonalite and dio- schist-facies greenstone, greenschist, and meta- rite of late Early Cretaceous age (M.A. Lanphere, limestone of presumed Late Triassic age (Goon Dip unpub. data, 1980; Decker and Plafker, 1982) that Greenstone and Whitestripe Marble) and uncon- generally trend north-northwest; in the northern formably(?) underlying metasedimentary and meta- part of the unit, west of Muir Inlet, these elongate volcanic rocks of Paleozoic and (or) Mesozoic age bodies trend west-northwest and east-west (Brew, (Loney and others, 1975; Johnson and Karl, 1985) 1978). Structural trends in metamorphic rocks par- compose this low-grade unit. The presumed Late allel those of the Cretaceous plutons. The unit is Triassic age of the greenstone, greenschist, and also intruded by a much lesser volume of un- metalimestone is based on their correlation with foliated Tertiary granitoids. lithologically similar rocks of that age in the Wran- North of Cross Sound, characteristic metamor- gellia terrane (Jones and others, 1977). The rocks phic mineral assemblages in metasedimentary crop out adjacent to and east of the Border Ranges rocks are calcic plagioclase+hornblendefquartz fault (Plafker and others, 1976; Decker and fbiotitef chlorite and calcic plagioclase+quartz Johnson, 1981). +potassium-feldspar+hornblende+biotite+diopside Mafic protoliths of the presumed Late Triassic (Seitz, 1959). Mafic rocks generally have been rocks consist of commonly amygdaloidal basaltic recrystallized into well-foliated and locally banded flows, sills, and flow breccias. Most massive green- gneiss composed of hornblende+quartz+calcic stone has been recrystallized to epidote, actinolite, plagioclasefgarnet and poorly foliated to unfoliated chlorite, albite, prehnite, calcite, pyrite, and hornblende-plagioclase rock of Rossman (1963) and sphene; relict amygdules commonly are filled with (Brew, 1978). quartz and epidote, accompanied by chlorite or On Chichagof Island, rocks are intensely folded, prehnite. Greenschist is composed of albite, chlo- and a complete gradation in metamorphic textures rite, and epidote. Pumpellyite has not been re- between granofels or hornfels and foliated rocks is ported in any mafic assemblage, but this may present (Loney and others, 1975). A characteristic reflect the lack of familiarity with this mineral by metamorphic mineral assemblage in mafic rocks is those doing the early, detailed petrography of these andesine+hornblende+quartzfbiotitefdiopside. rocks (Rossman, 1959; Loney and others, 1963) Quartzofeldspathic granofels and schist, derived rather than inappropriate chemical or physical con- from igneous rocks, contain quartz, microcline, oligo- ditions for the formation of this mineral (R.A. SOUTHEASTERN ALASKA Dl5

Loney, oral comun., 1985). Marble is locally stylo- the Chugach terrane of Plafker and others (1977). litic and is composed of nearly pure calcite and, lo- Blocks within the melange are Triassic or Jurassic, cally, accessory chlorite, sericite, graphite, quartz, Late Jurassic (Tithonian), and Early Cretaceous albite, and pyrite (Johnson and Karl, 1985). (Valanginian) in age (Loney and others, 1975; The low-grade rocks of Paleozoic or Mesozoic age Plafker and others, 1976; Decker and others, 1980; that crop out discontinuously along the east edge Johnson and Karl, 1985; Brew and others, 1988). of this unit (unit MzRs of Loney and others (1975) Deposition of the melange matrix is interpreted by and Johnson and Karl (1985)) include metachert, Brew and others (1988) to have taken place, in metasandstone, metatuff, metalimestone, and slaty part, during the Late Jurassic (Tithonian) and pre- argillite, all of which are intercalated with foliated sumably to have continued during at least the greenstone and greenschist. In most places, the Early Cretaceous (age of the youngest blocks) eastern part of this assemblage is extensively in- (Decker, 1980, Johnson and Karl, 1985; Brew and truded by foliated granitic rocks of Jurassic and others, 1988). The depositional age of the bedded (or) Cretaceous age and by diabase and gabbro rocks is unknown but is considered to be Creta- sills. Metamorphic mineral-assemblage data are ceous on the -basis of correlation with lithologically not available for these rocks, but general lithologic similar rocks in the Valdez Group and Yakutat descriptions of them suggest that they are either of Group to the northwest (Plafker and others, 1977; prehnite-pumpellyite-facies or lower greenschist- Brew and Morrell, 1979a). Eocene plutons intrude facies grade. These rocks are described as being both the melange and the bedded rocks and estab- more deformed and as having a slightly higher lish a minimum age for their protoliths. In the metamorphic grade than the overlying Goon Dip Fairweather Range north of Cross Sound, the unit Greenstone. Although available stratigraphic evi- includes graywacke, phyllite, and minor slate, dence suggests that these underlying rocks uncon- phyllite, argillite, graywacke semischist, and meta- formably underlie the Goon Dip Greenstone conglomerate (Brew, 1978; Brew and Morrell, (Rossman, 1959; Loney and others, 1975), they are 1979a). On Yakobi, western Chichagof, and lithologically distinct from the Permian basement Baranof Islands, the unit includes these same lith- rocks in stratigraphically comparable areas else- ologies as well as greenstone and greenschist and where within the Wrangellia terrane (Johnson and large areas of melange. The melange is composed Karl, 1985). of kilometer-scale blocks of greenstone, greenschist, Little is known about the age and tectonic origin metatuff, metagraywacke, meta-argillite, meta- of the low-grade metamorphism of this unit. Maxi- chert, metalimestone, phyllite, quartzite, and mum and minimum metamorphic age constraints graywacke semischist that are separated from one are indicated by the Triassic protolith age of the another by shear zones and faults; smaller blocks youngest rocks and the early Tertiary(?) age of these same lithologies are set in a matrix of (Johnson and Karl, 1985) of the postmetamorphic moderately metamorphosed and penetratively granitoids that intrude them. Most of the Jurassic sheared graywacke, argillite, and metatuff (undi- plutons (B.R. Johnson, oral commun., 1985) are fo- vided Kelp Bay Group, Khaz Formation, Freeburn liated and locally highly altered and probably pre- assemblage, Waterfall Greenstone, Pinnacle Peak date metamorphism. It is not known with certainty Phyllite, and related rocks) (Loney and others, that metamorphism of the pre-Late Triassic rocks 1975; Plafker and others, 1976; Johnson and Karl, of this unit took place during the same episode 1985; Decker, 1980). that affected the overlying Late Triassic rocks, but The part of this unit that is located between for the time being this seems to be the most rea- faults that occur in the area of Tarr Inlet and sonable assumption. Brady Glacier (Tarr Inlet suture zone of Brew and Morrell (1979b)) comprises a similar sequence of structurally complicated, diverse, low-grade phyl- lite, slate, metaconglomerate, metachert, green- stone, greenschist, and marble that have been This unit consists of undifferentiated prehnite- proposed to correlate with (1) the melange unit de- pumpellyite- and lower greenschist-facies bedded scribed above (Kelp Bay Group and related rocks of turbiditic metasedimentary rocks and metavolcanic the Chugach terrane) (Decker and Plafker, 1982), rocks (Sitka Graywacke) and a more inboard (2) Permian and (or) Triassic rocks of the Wrangel- melange unit (Kelp Bay Group). They are included lia terrane (Brew and Morrell, 1979b), or (3) the in the flysch and melange facies, respectively, of melange unit in the southern part of the suture Dl6 REGIONALLY METAMORPHOSED ROCKS OF ALASKA zone and the Wrangellia rocks in the northern part metamorphic minerals are identifiable in hand (D.A. Brew, unpub. data, 1985). Foliated, sheared, specimen for the first time. Glaucophane and and highly altered diorite of late Early Cretaceous phlogopite occur in textural grades 3, 4, and 5; bar- age (shown as a weakly metamorphosed pluton, roisite occurs only in textural grades 4 and 5; and pl. 1) invades the Tarr Inlet zone (Brew, 1978; fuchsite and talc occur only in textural grade 5. Decker and Plafker, 1982), and the fabric and al- These textural grades were used by Decker, to- teration in these rocks is probably related to the gether with vo1canic:sedimentary rock ratios, to low-grade metamorphism that affected the rest of subdivide the low-grade rocks into subduction this unit. units. Each unit is described as a piece of oceanic Characteristic metamorphic mineral assembla- crust that records its own subduction history which ges are those of the prehnite-pumpellyite facies differs from that of neighboring units. Metamor- and, locally, those of the lower greenschist facies. phic textures and mineral assemblages reflect fac- Metasedimentary rocks in the Fairweather Range tors such as subduction rate, depth of burial, uplift contain chlorite, white mica, and quartz, minerals rates, and thermal history (Decker, 1980, p. 105). that are only diagnostic of low-grade metamor- The age of low-grade metamorphism is probably phism. Metamorphosed turbidites on Yakobi, Early Cretaceous to early Tertiary, but a Late Ju- Chichagof, and Baranof Islands commonly contain rassic age is possible. The maximum metamorphic metamorphic chlorite and microcrystalline epidote age is unknown, but the oldest dated part of the aggregates; lesser amounts of quartz, white mica, melange matrix (on northern Baranof Island) con- albite, calcite, and prehnite (which typically occurs tains fossils of Tithonian age (Brew and others, in veinlets or clots) (Loney and others, 1975; John- 1988), and metamorphism may have closely fol- son and Karl, 1985); and rare pumpellyite (Decker, lowed deposition of that part of the melange. An 1980). Metamorphic assemblages and textures in early Tertiary minimum metamorphic age is indi- the rocks of the more inboard melange facies are cated by the overprinting of low-grade assemblages more variable. Mineral assemblages within indi- in the melange and the bedded rocks by thermal vidual blocks are those of the prehnite-pumpellyite, metamorphism associated with Eocene plutons lower greenschist, and blueschist(?) facies and indi- (Loney and others, 1975; Loney and Brew, 1987). A cate low-temperature and low- to intermediate- or mid-Cretaceous metamorphic age for melange on perhaps high-pressure conditions (Decker, 1980). Chichagof Island is suggested by 106- to 91-Ma Decker (1980) analyzed part of the melange unit K-Ar ages on actinolite and sericite from interlay- on western Chichagof Island and divided it into the ered metavolcanic and metasedimentary rocks following five-part textural-grade scheme that is (Decker and others, 1980). Metamorphism of this based on both structural features and increasing unit most likely occurred during subduction of the degree of recrystallization in argillite and thinly Chugach terrane beneath the adjacent continental layered volcanic rocks: (1) incipiently recrystal- margin that, in southeastern Alaska, was made up lized, generally massive rocks that have slaty of the adjacent (composite) Wrangellia-Alexander cleavage and metavolcanic rocks that contain chlo- terrane. rite, sericite, epidote minerals, actinolite needles, and rare prehnite and pumpellyite; (2) layered rocks that are weakly foliated, originally massive LPP,GNS (eTIJ) + GNL (eT) rocks that are still massive, and metavolcanic rocks that are characterized by finely disseminated This unit consists of polymetamorphosed meta- chlorite, actinolite, and epidote minerals; (3) graywacke, slate, amphibolite, greenschist, and subphyllitic rocks that are distinctly foliated; meta- minor phyllite and schist whose second meta- sedimentary rocks that contain detrital grains that morphism was low- to medium-temperature, low- have been moderately recrystallized and have new pressure thermal metamorphism that was associated quartz, sericite, graphite, and carbonate; and meta- with Eocene plutonism. Albite-epidote hornfels as- volcanic rocks that contain chlorite, epidote-group semblages are superimposed over the prehnite- minerals, actinolite, and, locally, blue amphibole pumpellyite- to lower greenschist-facies assemblages and phlogopite; (4) phyllitic rocks that are com- previously developed on a regional extent sometime pletely recrystallized, metasedimentary rocks that during latest Jurassic to early Tertiary time contain new micaceous minerals, and metavolcanic (described above under unit LPP,GNS (eTIJ)). These rocks that contain actinolite and micaceous miner- rocks are described in detail by Loney and others als; and (5) fine-grained schistose rocks in which (1975). Protoliths are sedimentary and volcanic rocks SOUTHEASTERN ALASKA Dl7 of the Sitka Graywacke west of the northwest-south- Rocks east of the northwest-southeast-trending east-trending fault on Baranof Island and melange of fault on Baranof Island consist of amphibolite and the Kelp Bay Group that contains blocks of Triassic greenschist that are interlayered with subordinate or Jurassic, Late Jurassic, and Early Cretaceous age amounts of phyllite and biotite schist. Metamor- east of the fault, as described above for unit phism within this part of the unit increases both to LPP,GNS (eTIJ). The first appearance of biotite, the north and to the south as a result of proximity to which was determined mainly in hand specimen, has two different plutons in those directions. Typical been used to define the contact between the regional- metamorphic assemblages are quartz-albite-epidote- ly metamorphosed prehnite-pumpellyite- to lower actinolite (amphibolite) and quartz-albite-epidote- greenschist-facies rocks and the polymetamorphosed chlorite (greenschist). The rocks clearly show the rocks of this unit. remnants of an earlier cataclastic foliation. On Kruzof Island and on southern Baranof Is- Evidence that the second metamorphic episode is land, low-grade regional assemblages containing related to Eocene plutonism consists of the relation calcite, sphene, epidote, chlorite, albite, and musco- between metamorphic zonation and proximity to vite have been replaced progressively toward the known or inferred plutons and the fact that two of pluton by the low-pressure assemblage epidote- three K-Ar ages from greenschist-facies rocks of chlorite-albite-muscovite-biotite-plagioclase on into this unit fall in the same age range (43 to 48 Ma) the plagioclase-quartz-biotite-andalusite and as was determined for the plutons (Loney and quartz-plagioclase-biotite-cordierite-tourmaline- others, 1975). garnet assemblages of the adjacent unit LPP,GNS (eTIJ) + AML (eT). Metamorphism during Eocene time is considered LPP,GNS (eTIJ) + AML (eT) to have taken place under low-pressure conditions that may have been transitional into medium- This unit comprises polymetamorphosed meta- pressure conditions. On southern Baranof Island, graywacke and slate hornfels, biotite schist and metamorphism was accompanied by kinematic ef- gneiss, and greenschist whose second metamorphism fects that intensified the preexisting foliation, pro- was medium- to high-temperature, low-pressure duced new foliation, and developed new folds and thermal metamorphism associated with Eocene plu- lineations about axes subparallel to those of the ear- tonism. Hornblende-hornfels-facies assemblages are lier folds. Loney and others (1975) and Loney and superimposed over the prehnite-pumpellyite- to Brew (1987) suggest that this area, referred to by lower greenschist-facies assemblages previously de- them as "lineated and schistose Sitka Graywacke," veloped on a regional extent sometime during latest may have been produced during emplacement of an Jurassic to early Tertiary time (described under unit inferred large igneous body that may be present at LPP, GNS (eTIJ)). These rocks are described in detail depth between the large pluton on Baranof Island by Loney and others (1975) and Loney and Brew and the smaller, elongate pluton near the southern (1987). Protoliths are sedimentary and volcanic rocks tip of the island. This unit along the northern mar- of the Sitka Graywacke (west of the northwest- gin of the large pluton on Baranof Island contains southeast-trending fault on Baranof Island) and Kelp the following metamorphic mineral assemblages that Bay Group (east of the fault), as described for unit appear to indicate metamorphism was transitional LPP, GNS (eTIJ). between the albite-epidote-hornfels (or greenschist) On Kruzof Island, the contact between this unit facies and the hornblende hornfels facies: plagio- and the adjacent lower grade equivalents of these clase+quartz+biotite+epidotefchloritef sphenekcalcite; rocks is defined by the first appearance in the field plagioclase+actinolite+calcite+epidote; biotite+quartz of readily visible garnet or cordierite in biotite- +plagioclase+almandine+albite(?)+sphene+epidote; orthoclase-plagioclase hornfels (Loney and others, and plagioclase+quartz+biotite+cordierite(?)+mus- 1975). covite. Andalusite, indicative of low-pressure condi- On Baranof Island, this unit south of the largest tions, along with cordierite and garnet, are present pluton is lineated and schistose, having experi- in the aureole adjacent to the pluton (area too small enced the kinematic effects presumed to have been to show as a separate higher grade unit, pl. 1). Sedi- produced during emplacement of an inferred large mentary textures in these rocks are well preserved subsurface pluton connecting the largest pluton and relatively unchanged to within about a hundred shown with the elongate one near the southern tip meters of the contact with the pluton (Loney and of the island (Loney and others, 1975; Loney and others, 1975). Brew, 1987). Diagnostic metamorphic minerals Dl8 REGIONALLY METAMORPHOSED ROCKS OF ALASKA that mark the lower grade boundaries of this unit quartz+biotitef garnetfplagioclase+muscovite on southern Baranof Island are plagioclase, alman- (Hudson and others, 1977; Brew, 1978). Mafic dine garnet, and cordierite; andalusite occurs in a metavolcanic rocks in the Skagway quadrangle are higher grade zone within this unit. Staurolite coex- fine to medium grained, variably segregated, and ists with andalusite and garnet in small areas in contain the following assemblages: chlorite+white the higher grade parts of this unit, and sillimanite mica+albite+epidote+quartz+sphene; plagioclase occurs both east of the elongate pluton near the tip +biotite+chlorite+quartz+actinolite+calcite; and of the island and on the south side of the largest hornblende+epidote+plagioclase+sphene (George pluton (Loney and others, 1975). Narrow zones of Plafker, written commun., 1984). pyroxene hornfels-facies rocks (too small to show Exact timing of metamorphism is unknown. A on pl. 1) occur within this unit adjacent to the Eo- Cretaceous maximum age of metamorphism is pro- cene plutons. Metamorphic mineral assemblages in posed on the basis of the age of the protoliths. Meta- these rocks contain quartz+plagioclase+orthoclase+ morphism is known to predate the intrusion of cross- sillimanitefbiotitefenstatite (Loney and others, cutting Oligocene (28f8 Ma) gabbroic plutons north- 1975; Loney and Brew, 1987). west of Cross Sound (Loney and Himmelberg, 1983), The area of this unit east of the northwest- and probably also predates the intrusion of Eocene southeast-trending fault consists of polymetamor- plutons of intermediate composition in the south- phosed biotite schist and gneiss, minor amphibolite, western Skagway quadrangle (K-Ar age on horn- and quartzite. The broadness of the aureole south of blende from one pluton is about 51 Ma and that the large pluton suggests that the pluton underlies from another is about 45 Ma; G. Plafker, oral much of the southern part of this unit. Rocks are commun., 1989). A 67-Ma age on hornblende from characterized by calcic plagioclase and almandine amphibolite in the Nunatak Fiord area, 55 km garnet, followed by fibrolitic sillimanite and stauro- northeast of Yakutat, may indicate a latest Creta- lite in progressively higher metamorphic zones. With ceous metamorphic age (Barker and others, 1985). progressive metamorphism, grain size increases and The origin of the metamorphic episode that a relict cataclastic foliation becomes indistinct and is resulted in the greenschist- to amphibolite-facies replaced by a single schistosity (Loney and others, metamorphism of this unit and the adjacent 1975). The presence of staurolite, sillimanite, and al- amphibolite-facies unit described below is unknown. mandine garnet in these rocks and the absence of It is doubtful that metamorphism was associated cordierite and andalusite suggest that pressures with Tertiary plutonism because no clear cut spatial within this large aureole may have been intermedi- relation between higher grade metamorphic condi- ate between those of the hornblende-hornfels facies tions and proximity to Tertiary plutons seems to be and the amphibolite facies. present. D.A. Brew infers that this medium-grade Evidence for relating the second metamorphic metamorphic episode was the higher temperature episode to Eocene plutonism is discussed above. equivalent of the prehnite-pumpellyite- and lower greenschist-facies metamorphic episode (unit LPP,GNS (eTIJ)) that affected the rocks north of Cross Sound. However, a relatively high thermal gradient is required to explain the abrupt transition Transitional greenschist- to amphibolite-facies from prehnite-pumpellyite- and lower greenschist- biotite schist and semischist derived from gray- facies rocks to amphibolite-facies rocks observed in wacke and argillite crop out in the Fairweather the western part of the Fairweather Range. This Range (Brew, 1978), and amphibolite and quartzo- high thermal gradient is incompatible with the low feldspathic biotite schist crop out farther to the thermal gradient implied by the subduction-related northwest (George Plafker, unpub. mapping, 1980). metamorphic history proposed by Decker (1980) for Protoliths are interpreted to be sandstone and the melange part of unit LPP,GNS (eTIJ) south of mudstone turbidites and volcanic rocks of Creta- Cross Sound on western Chichagof Island (discussed ceous age on the basis of lithologic similarity to the above). The two areas also are not analogous in that Valdez Group, Sitka Graywacke, and part of the Decker's study concerned a simple, single gradient Yakutat Group (Plafker and others, 1977; Brew situation inboard of the flysch unit, whereas the and Morrell, 1979a; D.A. Brew and George Plafker, Fairweather Range contains a complex situation in oral communs., 1985). which the metamorphic grade increases abruptly Biotite schist is fined grained, variably banded, within the flysch unit from both the east and west granoblastic to strongly schistose, and composed of towards a maximum near the crest of the range. SOUTHEASTERN ALASKA Dl9

AMP (eTK) ing unmetamorphosed Oligocene volcanic rocks (Brew, 1978). A latest Cretaceous and (or) early Ter- Amphibolite-facies biotite gneiss and hornblende tiary metamorphic age is provisionally assigned to schist and gneiss (amphibolite) (Brew, 1978; George these rocks on the basis of the youngest probable Plafker, unpub. mapping, 1980) crop out in the protolith age (Maastrichtian) and the fact that cor- Fairweather Range. Protoliths are interpreted to be relative low-grade rocks of the Yakutat Group that sandstone and mudstone turbidites and volcanic occur farther to the north in the Yakutat-St. Elias rocks of Cretaceous age on the basis of lithologic Mountains area appear to increase in grade into similarity to the Valdez Group, Sitka Graywacke, amphibolite-facies rocks (Dusel-Bacon and others, and part of the Yakutat Group (Plafker and others, 1993) that yield a K-Ar age on hornblende of 65 Ma 1977; Brew and Morrell, 1979a; D.A. Brew and (Hudson and others, 1977). George Plafker, oral communs., 1985). The rocks are well foliated and amphibolite is also locally well line- ated. Biotite gneiss is composed primarily of biotite, WESTERN METAMORPHIC BELT quartz, plagioclase, and locally garnet. Amphibolites hornblende+plagioclasefgarnetfbiotite contain ADMIRALTY ISLAND AND ADJACENT MAINLAND AREA fchlorite (Brew, 1978; George Plafker, written com- mun., 1984). LPP (eK) Constraints on the age of metamorphism and questions about its origin are the same as those Weakly metamorphosed argillite, slate, meta- described for the unit immediately above. graywacke, metaconglomerate, intermediate to mafic metavolcanic rocks, metachert, and minor metalimestone and phyllite compose this low-grade LPP (eTIK) unit (Buddington and Chapin, 1929; Loney, 1964; Lathram and others, 1965; Brew and Grybeck, This unit comprises laumontite+quartz- and (or) 1984). Protoliths are sedimentary and volcanic prehnite-pumpellyite-facies late Mesozoic flysch rocks. On Admiralty Island, protoliths include and melange of the Yakutat block (Plafker, 1983), rocks of Ordovician age (Carter, 1977) and Permian which consist of large blocks of greenstone, phyl- through Early Cretaceous age (Lathram and oth- lite, metagraywacke, argillite, and metachert and ers, 1965). On the adjacent mainland to the east, thick sequences of slate, silty metalimestone, limy protoliths are correlated with rocks that range in metasiltstone, and minor greenstone (Brew, 1978). age from Permian through Cretaceous (Buddington Also included is a sheared and chloritized elongate and Chapin, 1929; Brew and Grybeck, 1984). Meta- diorite body of probable Cretaceous age (Brew, morphic minerals developed in this unit suggest 1978; shown as a weakly metamorphosed pluton, conditions of the prehnite-pumpellyite facies and pl. 1). The unit crops out in the vicinity of Cape include chlorite, sericite, and calcite in Permian Fairweather. Northwest of the map area, near metasedimentary rocks; quartz, albite, calcite, chlo- Yakutat, correlative bedded sequences are Late rite, sericite, and pyrite in Triassic argillite; and Cretaceous in age, and disrupted blocks are Late chlorite, calcite, epidote, prehnite, and white mica Jurassic and (or) Early Cretaceous in age (George in Jurassic and Lower Cretaceous metagraywacke Plafker, written commun., 1985; Dusel-Bacon and (Loney, 1964). others, 1993). Metamorphism is known to postdate the Early Petrographic study of correlative sandstones to Cretaceous depositional age of the youngest low- the north in the Yakutat-St. Elias Mountains area grade rocks of this unit and to predate the early has indicated that metamorphic minerals devel- Tertiary (Paleocene to Miocene) age of overlying oped in these rocks include laumontite, prehnite, unmetamorphosed sedimentary rocks. A late Early quartz, chlorite, sphene, pumpellyite, and white Cretaceous age of metamorphism is proposed be- mica (Hudson and others, 1977). Similar assembla- cause the low-grade rocks of this unit grade into ges are inferred to exist in this map area. greenschist- and amphibolite-facies rocks (units Metamorphism is known to postdate the Late Cre- GNS (eK) and GNS,AMP (eK)) whose major meta- taceous protolith age of the bedded sequences in the morphism was associated with the intrusion of in- Yakutat area (George Plafker, oral commun., 1984) termediate plutons that yield K-Ar mineral ages of and the probable Cretaceous age of diorite near about 120 and 110 Ma (Loney and others, 1967). Lituya Bay and to predate the deposition of overly- Whether the area of this unit on the mainland east D20 REGIONALLY METAMORPHOSED ROCKS OF ALASKA of Admiralty Island was affected by this episode or (Lathram and others, 1965). However, because meta- whether some, if not all, of the low-grade metamor- morphic grade apparently increases toward this pluton phism there was part of the eastward-increasing and because the contact aureoles merge with large areas metamorphic episode that occurred during latest of dynamothermally metamorphosed phyllite, schist, Cretaceous or early Tertiary time is not clear. and gneiss (Loney and others, 1967), we consider that the intrusion of this pluton was associated with regional metamorphism on Admiralty Island. A late Early Creta- GNS (eK) ceous age for this episode is indicated by K-A. ages of about 120 and 110 Ma on minerals from the batholith This unit comprises phyllite, greenschist, green- (Loney and others, 1967). Absolute age constraints on stone, slate, marble, and metaquartz diorite; it also in- metamorphism are provided by the Early Cretaceous cludes poorly exposed migmatitic rocks and feldspathic protolith age of the youngest affected rocks and the schist south of the largest pluton on the island. The ma- early Tertiary (Paleocene) age of overlying, unmetamor- jority of sedimentary and volcanic protoliths ranges in phosed sedimentary rocks. age f?om Ordovician to Triassic (Loney, 1964; Lathram and others, 1965; Carter, 1977); Upper Jurassic and Lower Cretaceous sedimentary protoliths are present along the east side of the northern peninsula of the is- land (Lathram and others, 1965). This unit is generally This unit comprises undifferentiated greenschist-fa- schistose or phyllitic. Common metamorphic minerals cies (or albite-epidote hornfels-facies) and amphibolite- are characteristic of greenschist-facies or perhaps albite- facies (or hornblende hornfels-facies) rocks, including epidote horrdels-facies conditions and include chlorite, quartz-albite-chlorite epidoWbiotite schist, hornblende- albite, white mica, and epidote. In the southeastern part albite-epidote hornfels, micaceous schist, metachert, of this unit, phyllite, greenschist, and calcareous rocks of marble, slate, phyllite, quartz-andesine-hornblende-bi- the Garnbier Bay Formation are massive appearing and otite amphibolite, quartz-andesine-diopside-microcline are phyllonitic; these rocks contain the assemblages gneiss, and andesine-garnet-pyroxene gneiss; it aIso in- quartz-muscovite-chlorite-albite, quartz-talc-calcite, and cludes migmatitic rocks north of the largest pluton on the chlorite-epidote-calcite. Augite porphyroclasts in island (Lathram and others, 1965). Protoliths are the metavolcanic rocks in that area have overgrowths of am- same as those described for the unit above. The northern- phibole that consist of colorless tremolite except for the most area of this unit is poorly exposed, but a small part in contact with the augite, which consist of a bluish granite body, whose boundary with the surrounding (sodic) amphibole (Loney, 1964). gneiss is gradational across several tens of meters, crops On the northern peninsula of the island, metaquartz out near the center of the area (Lathram and others, diorite is heated, well foliated, and sheared. The trend 1965). The spatial association between this unit and late of the foliation is parallel to that of the country rock Early Cretaceous plu-tons and an apparent decrease in (Barker, 1957). Barker proposed that the pluton was in- metamorphic grade away from the plutons suggest that truded &r the formation of the schistosity in the coun- meta-morphism was associated with plutonism, try rock and that aRer solidification the intrusive rock as is discussed above for lower grade was deformed and metamorphosed in a later stage of equivalents of this late Early Cretaceous metamorphic the orogeny. However, neither the age of the igneous episode. protolith nor of the country rocks has been determined, and possibly either (1) the pluton intruded unmetamorphosed country rocks and was subsequently KUPREANOF, ETOLIN, AND REVILLAGIGEDO ISLANDS AND metamorphosed along with the country rocks or (2) it CLEVELAND PENINSULA AREA was intruded synkmematically with regional metamor- phism. Recent structural studies support the first inter- pretation and suggest that the pluton was intruded as a sill-like body, was subsequently deformed and metamor- Prehnite-pumpellyite- to lower greenschist-facies phosed with the country rocks, and was later sheared by greenschist, greenstone, phyllite, slate, meta- movements on the Chatham Strait fault to the west agglomerate, and minor argillite, semischist, (D.A. Brew, unpub. data, 1985). Intermediate intrusive metaconglomerate, metalimestone, and m.etachert rocks of the largest batholith on the island are poorly to (Eberlein and others, 1983; Brew and others, 1984) well foliated, but the trend of the foliation relative to crop out from Kupreanof Island in the north to the that of the country rocks has not been studied in detail peninsula north and west of Revillagigedo Island SOUTHEASTERN ALASKA D2 1

(Cleveland Peninsula) in the south. Protoliths in- sillimanite are developed occur adjacent to 90-Ma clude clastic sedimentary rocks, intermediate to intermediate plutons (IKg) and 20-Ma intermediate mafic volcanic rocks, limestone, and chert. Proto- plutons (ITg). The adjacent unit LPPIGNS (mK) + liths for much of this unit are considered to be GNL+I (IK) differs from this unit in that the 90-Ma temporally correlative with the unmetamorphosed aureoles are regional in character and also contain strata of the Seymour Canal Formation (Brew and porphyroblasts of either unreacted andalusite or others, 1984) that has yielded Late Jurassic to andalusite that has been replaced by kyanite and Early Cretaceous (Berriasian) fossils on Admiralty staurolite. The geographic limits of the area af- Island (Loney, 1964) and similarly aged fossils on fected by this episode are not known with cer- southwestern Etolin Island (Berg and others, tainty, and they may have extended toward the 1972), including an Albian or Cenomanian ammo- east into western Revillagigedo Island (unit GNS nite (Brew and others, 1984). Conodonts from lime- (K)). stone layers in west-central Kupreanof Island indi- cate a Late Triassic protolith age for at least part of this unit (Brew and others, 1984). Protolith ages LPP/GNS (mK) + GNL+I (IK) of this unit in the northeastern part of the Craig quadrangle were previously considered to be late Rocks included in this unit are polymetamor- Paleozoic or Mesozoic (Eberlein and others, 1983) phosed greenschist, phyllite, semischist, greenstone, and probably mainly Jurassic and Cretaceous metavolcanic rocks, and minor metagraywacke and (Berg and others, 1976; Eberlein and others, 1983). slate (Brew and others, 1984). Protoliths generally Recent studies (C.M. Rubin, oral commun., 1987) are similar to those described for the unit to the indicate that both rocks of both early Paleozoic and west (LPPIGNS (mK)); they are thought to be Late Jurassic and (or) Cretaceous protolith age are Jurassic to mid-Cretaceous in age and to be equiva- present. lent to the Seymour Canal Formation (Brew and Rocks have been dynamothermally metamor- others, 1984, p. 23). The rocks were regionally meta- phosed and locally are very folded and faulted. Meta- morphosed under prehnite-pumpellyite- to lower sedimentary rocks are generally poorly foliated, but greenschist-facies conditions during mid-Cretaceous fine-grained variants have good cleavage; greenschist time (see description for unit LPPIGNS (mK) for and greenstone appear less deformed and metamor- discussion of metamorphic age constraints). Subse- phosed and locally contain abundant relict pyroxene quently, these rocks were recrystallized under phenocrysts (Brew and others, 1984). On low-pressure evolving to intermediate-pressure northwestern Kupreanof Island, rocks have been greenschist-facies conditions during the metamorphic metamorphosed to albite-muscovite-chlorite subfacies episode associated with the early Late Cretaceous of the greenschist facies (Muffler, 1967); elsewhere (90 Ma) intrusion of the locally garnet- and (or) typical metamorphic mineral assemblages include epidote-bearing intermediate plutons of the epidote-albite-actinolite-quartz-white mica-chlorite, Admiralty-Revillagigedo belt of Brew and Morrell as well as biotite in the northeastern part of the (1983) (Brew and others, 1984; Douglass and Brew, Craig quadrangle (Eberlein and others, 1983). The 1985). Original sedimentary textures and structures prehnite-pumpellyite-facies rocks are not yet well are generally obscured, but some relict igneous py- documented. roxene is preserved in some greenstones (Brew and Regional low-grade metamorphism is known to others, 1984). predate the intrusion of Alaskan-type mafic- The first metamorphic episode produced a weak ultramafic bodies that have yielded K-Ar ages of to well-defined planar fabric, and the second epi- 100-110 Ma (Lanphere and Eberlein, 1966; Clark sode produced porphyroblastic, decussate, and and Greenwood, 1972; Brew and others, 1984; granoblastic textures (Brew and others, 1984). Douglass and Brew, 1985). The late Early Creta- Typical mineral assemblages developed during the ceous minimum metamorphic age indicated by this first episode include chlorite, quartz, white mica, dating is close to the Albian or Cenomanian proto- plagioclase, clinozoisite, epidote, and less com- lith age of the youngest rocks included in this unit monly biotite. on Etolin Island. Allowing for minor age uncertain- The second metamorphic episode occurred ties in both the isotopic- and fossil-age determina- primarily under thermal conditions, but, locally, it tions, a mid-Cretaceous metamorphic age is indi- occurred under dynamothermal conditions. It pro- cated for this unit. Hornblende-hornfels-facies ther- duced porphyroblasts of andalusite that in most mal aureoles in which garnet and rarely areas are partly to totally replaced by static (radi- D22 REGIONALLY METAMORPHOSED ROCKS OF ALASKA

al) kyanite (locs. 27 and 28, Petersburg quad- 1 and Jurassic and Cretaceous metagraywacke, rangle, table 2, pl. 2), which indicates an increase metalimestone, argillite, metaconglomerate, and in pressure during crystallization. At other locali- metatuff. Weakly metamorphosed diorite and ties, andalusite is not replaced by kyanite but, in- quartz diorite of presumed Jurassic or Cretaceous stead, is either partly replaced by staurolite (loc. age also occur within this unit on Annette Island 32, Petersburg quadrangle, table 2, pl. 2) or almost (Berg and others, 1978, 1988). On Duke Island, the totally replaced by white mica (loc. 40, Petersburg unit consists of weakly metamorphosed pyroxene quadrangle, table 2, pl. 2). Evidence of relict anda- gabbro that has yielded an apparent Late Triassic lusite (pleochroic pink andalusite in cores of the re- (226f3-Ma) U-Pb age on zircon (Gehrels and oth- placed porphyroblast and chiastolite crosses within ers, 1987). To the east on Cape Fox, the unit con- kyanite) is the same as that described for the aure- sists of metasedimentary and metaigneous rocks of oles near northern Wrangell Island in the adjacent, unknown protolith age (Berg and others, 1978, higher grade equivalent unit AM1 (IK). Other miner- 1988). al assemblages that formed in aureoles developed The intensity of regional metamorphism in- during the second episode include garnet, biotite, creases gradually northward and eastward from in- plagioclase, quartz, staurolite, and fibrolite cipiently slaty rocks on western Gravina Island to (Douglass and Brew, 1985). The degree of meta- upper greenschist-facies rocks near their apparent morphism generally increases toward the plutons. gradational contact with the amphibolite-facies Away from the aureoles, typical mineral assembla- rocks (unit AM1 (IK) described below) according to ges developed during the second metamorphic Berg and others (1978, 1988) and R.L. Elliott and episode include biotite, clinozoisite, quartz, plagio- J.G. Smith (oral communs., 1985). On Annette Is- clase, actinolite, and white mica. land, a typical metamorphic mineral assemblage in The regional extent of the two metamorphic epi- metasedimentary and mafic metavolcanic rocks sodes described for this unit is imprecisely known, consists of sericite, quartz, chlorite, calcite, and owing to overprinting of these metamorphisms by epidote and (or) clinozoisite; on Gravina Island, re- subsequent metamorphic episodes that affected ad- gional deformation and metamorphism generally jacent areas; for this reason, the boundaries of this are less intense (Berg, 1972, 1973; Berg and oth- unit are only approximate. ers, 1988). On Revillagigedo Island and the Cleve- land Peninsula to the west, metamorphic foliation dips to the northeast, and the metamorphic se- GNS (K) quence is cut by southwest-vergent thrust faults (Berg and others, 1988; Rubin and Saleeby, 1987). In general, this unit consists of greenschist- Metamorphic mineral assemblages on Revil- facies flyschoid metasedimentary rocks, intermedi- lagigedo Island contain quartz, feldspar, biotite, ate or mafic metavolcanic and metavolcaniclastic muscovite, pyrite, actinolite, and chlorite in meta- rocks, and relatively sparse intermediate or mafic sedimentary rocks and actinolite, plagioclase, metaplutonic rocks. On Revillagigedo Island, on quartz, biotite, pyrite, and epidote and (or) clino- Cleveland Peninsula, and on the mainland near zoisite in metavolcanic rocks; garnet occurs in both the mouth of Boca de Quadra, common rock types metasedimentary and metavolcanic rocks in the are phyllite, semischist, pelitic schist, greenschist, northern part of the island. Porphyroblastic miner- minor marble, quartz-muscovite schist, slate, als that replace older, fabric-forming metamorphic phyllitic grit or metaconglomerate, metadiorite, minerals or that randomly overprint the earlier metagabbro, and greenstone (Berg and others, fabric occur in narrow (less than 1-km-wide) con- 1978, 1988). Few fossils have been found within tact aureoles around many of the Cretaceous plu- this area, but most protoliths are considered to be tons and in a wider aureole around the Tertiary Paleozoic or Mesozoic in age. Fossils of latest pluton on Revillagigedo Island (J.G. Smith, oral Middle Triassic and late Early Permian age have commun., 1985; Berg and others, 1988). These nar- been found within this unit on southwestern row aureoles are similar to those described above Revillagigedo Island (Silberling and ethers, 1982). within unit LPPIGNS (mK). On Gravina and Annette Islands, the unit consists Widespread development of feather schist occurs of Lower or Middle Devonian marble, phyllite, and locally on southern and southeastern Revillagigedo metagraywacke; Upper Triassic metalimestone, Island and across east Behm Canal on the main- metaconglomerate, semischist, meta-agglomerate, land (Berg and others, 1978, 1988). This distinctive metatuff, metavolcanic breccia, and greenstone; rock type contains bowtie-like porphyroblastic SOUTHEASTERN ALASKA D23 sheafs of actinolite as much as 2 m in length, al- According to the second hypothesis, the regional mandine garnet idioblasts as much as 2 cm in di- greenschist-facies metamorphism and fabric is a ameter, and coarsely crystalline muscovite. The higher grade equivalent of the low-grade metamor- metamorphic origin of this distinctive texture is phism and fabric (diagnostic of unit LPP/GNS (mK)) not known. that occurs to the northwest. There, it is crosscut A Cretaceous age is indicated for the regional by both 100- to 110-Ma mafic to ultramafic plutons greenschist-facies metamorphic episode because and by 90-Ma primary epidote-bearing intermedi- metamorphism is known to postdate the Early Cre- ate intrusions. This hypothesis questions the taceous age of the jroungest protoliths on Gravina gradual northeastward increase in metamorphic and Annette Islands and to predate the approxi- grade reported by Berg and others (1978, 1988) mate 90-Ma age of crosscutting, unmetamorphosed and infers that the metamorphism on southwestern plutons that intrude this metamorphic unit (Smith Revillagigedo Island is part of the episode that oc- and Diggles, 1981). The 90-Ma plutons, which were curred before 100-110 Ma rather than the episode mentioned above, are a critical element in under- that slightly preceded or was synchronous with standing the history of metamorphism in this area; 90-Ma plutonism. According to this interpretation, they define the Admiralty-Revillagigedo belt of the effects of the approximately 90-Ma metamor- Brew and Morrell (1983). Many of the plutons in phic episode were (1) the development of narrow this belt contain primary (magmatic) epidote and aureoles around some of the 90-Ma plutons that in- garnet (Zen and Hammarstrom, 1984a). The cross- trude this unit and (2) an extensive prograde aure- cutting age relations and the development of nar- ole around the large pluton within unit AM1 (IK) on row contact aureoles around the postmetamorphic northern Revillagigedo Island. This interpretation 90-Ma plutons are most clearly seen on the south- is based on: (1) extending the interpretation made western part of Revillagigedo Island (J.G. Smith, from observations in the Petersburg quadrangle to oral commun., 1986). Isotopic and metamorphic the northwest (Brew and others, 1984; Douglass data are insufficient to determine the exact timing and Brew, 1985), (2) a reconnaissance of north- of regional metamorphism, and two different hy- western Revillagigedo Island by D.A. Brew (unpub. potheses about the metamorphic history of this data, 1983), and (3) recent detailed mapping by unit have been inferred from the available data. M.L. Crawford (oral commun., 1988) that indicates According to the first hypothesis, metamorphism an abrupt rather than a gradational increase in preceded but was part of the same thermal episode metamorphic grade at the boundary between units that culminated in the intrusion of early Late Creta- GNS (K) and AM1 (IK). This second hypothesis ceous (90-Ma) primary garnet- and epidote-bearing makes this unit essentially the same as unit plutons of intermediate composition. The primary LPPJGNS (mK) + GNL+I (IK) to the northwest. evidence for this origin is the apparent gradual northeastward increase in metamorphic grade (Berg and others, 1978, 1988; R.L. Elliott and J.G. Smith, AM1 (IK) oral communs., 1985) within this greenschist-facies unit. This increase is inferred to continue into the This unit comprises intermediate-pressure and, in related kyanite-bearing amphibolite-facies rocks (AM1 a small area in the northern part, low-pressure OK)) that are spatially related to a large 90-Ma plu- evolving to intermediate-pressure amphibolite-facies, ton on northern Revillagigedo Island that is inter- kyanite-staurolitef sillimanite-bearing pelitic schist, preted to have been intruded synmetamorphically quartzofeldspathic schist and gneiss, amphibole (Berg and others, 1988). According to this first hy- schist and gneiss, and minor marble and calc-schist pothesis, the narrow contact aureoles around some of (Berg and others, 1978, 1988; Brew and others, the 90-Ma plutons formed postkinematically and 1984). The unit also includes migmatitic rocks on overprint the regional foliation developed earlier southwestern Wrangell and eastern Etolin Islands during this same early Late Cretaceous metamorphic that consist primarily of agmatite and irregularly episode. The location of the eastern and southern banded gneiss; metamorphic melasomes in these limits of the earlier (before 100- to 110-Ma) low- rocks are fine- to medium-grained biotitekgarnet grade metamorphic episode (LPP/GNS (mK)) that oc- fsillimanite-bearing hornfels, schist, and semischist curs to the west and northwest is unknown. It pre- (Brew and others, 1984). On Revillagigedo Island, sumably occurs somewhere within this unit and is the unit also includes a small body of metadiorite overlapped by the effects of greenschist-facies meta- and a larger body of intermediate-composition meta- morphism associated with the 90-Ma plutonism. plutonic rocks. East of Revillagigedo Channel, the D24 REGIONALLY METAMORPHOSED ROCKS OF ALASKA

unit consists of a heterogeneous assemblage of pelitic tallization of kyanite postdated that of andalusite schist, hornblende schist, and minor semischist, have been described for the Chiwaukum Schist of phyllite, and marble (Berg and others, 1978, 1988). the North Cascades, Washington (Evans and Berti, This unit is suffiently recrystallized so that neither 1986, and references cited therein) and the Settler the original textures nor structures remain. No fos- Schist of the Coast Ranges, southern British Co- sils have been found in these rocks. Protoliths in- lumbia (Hollister, 1969; Pigage, 1976, and other clude clastic sedimentary rocks, intermediate to references given in Evans and Berti, 1986). In mafic volcanic and volcaniclastic rocks, and lime- these areas in Washington and British Columbia, stone. On the basis of similarities with fossiliferous andalusite and kyanite formed during a single strata from nearby areas, protoliths of this unit are metamorphic episode that accompanied intrusion of considered to include Jurassic and (or) Cretaceous Late Cretaceous plutons. flysch, Permian and Triassic limestone, and intru- The boundaries between this metamorphic unit sive rocks of presumed Jurassic or Cretaceous age and adjacent units in the vicinity of Wrangell Is- (Berg and others, 1978, 1988; Brew and others, land are approximate, and some parts of this unit 1984). may have been affected by younger metamorphic On and near Wrangell Island, intermediate- episodes, as well as by the earlier (mid-Cretaceous) pressure metamorphic conditions are indicated for low-grade metamorphism that affected rocks to the the latest and dominant phase of the metamorphic north and to the west. Along the southwesternmost episode by the presence of kyanite and sillimanite margin of the large pluton on central Wrangell Is- in aureoles around 90-Ma garnet- and epidote- land, parts of this unit preserve a weak to well- bearing plutons and in the adjacent more regional- defined planar fabric that was produced during the ly extensive metamorphic rocks. The location of the earlier low-grade metamorphism (described under sillimanite and kyanite isograds around the large unit LPPIGNS (mK)). 90-Ma plutons on central Wrangell Island and the On Revillagigedo Island, metamorphic mineral as- increase in metamorphic grade toward the plutons semblages show an increase in grade from the is evidence that this metamorphism was associated southwest to the northeast, beginning in greenschist- with that particular plutonic episode. Typical min- facies rocks (GNS (K)) and continuing into this eral assemblages in pelitic aureole schists (shown amphibolite-facies unit. Common mineral assembla- by the contact aureole symbol, pl. 1) and in equiva- ges that show increasing grade in mafic rocks are lent rocks farther from the large pluton (unit IKg hornblende+quartz+clinozoisite/zoisite+plagio- with triangle pattern, pl. 1) are staurolite+ clasefbiotitekcalcite, followed by garnet+biotite+ garnet+biotite+quartz+plagioclase+muscovitef hornblende+quartz+plagioclase and quartz+biotite+ kyanitefsillimanite (Douglass and Brew, 1985); hornblende+garnet+plagioclase+diopside. Calcareous calcareous rocks contain hornblende+calcic plagio- rocks contain, in increasing metamorphic grade, clase+quartz+garnet+diopside. quartz+plagioclase+sphene+zoisite+ In addition to the widespread occurrence of kya- tremolite+biotite+muscovite followed by nite in the area of Wrangell Island, relict andalu- hornblende+garnet+sphene+calcic site has been observed in pelitic schist from plagioclase+quartz+diopside. No relict andalusite has aureoles of 90-Ma plutons that intrude this unit been reported from this part of the unit. The most (loc. 35, Petersburg quadrangle, table 2, pl. 2) and common assemblage in pelitic rocks is bio- from aureoles within unit LPPJGNS (mK) + GNL+I tite+garnet+quartz+plagioclasekkyanitef staurolite (IK) to the north. Samples from these aureoles con- (M.L. Crawford, written commun., 1984). Sillimanite tain relict porphyroblasts of andalusite that have occurs in pelitic rocks in a 1-km-wide zone along the been replaced by static (radial) kyanite or, in some southern margin of the batholith that crops out near locations, mineral aggregates of intergrown kyanite the north end of Behm Canal, and staurolite and and staurolite. Although largely replaced, the relict kyanite occur in pelitic rocks in a lower grade zone andalusite is easily identified by the presence of that is several kilometers wide and is located beyond relict chiastolite cross-shaped inclusions or by the sillimanite zone (Berg and others, 1978, 1988). areas of pink andalusite in their cores. This Some of these kyanite-bearing rocks contain late crystallization sequence of the Al,SiO, polymorphs fibrolitic sillimanite and muscovite that may have appears to indicate an increase in pressure from formed during a late-kinematic phase of the dynamo- low- to intermediate-pressure conditions in the thermal metamorphic and plutonic episode and (or) northern part of this unit during intrusion and during subsequent decompression of the region (M.L. metamorphism. Similar textures in which the crys- Crawford, oral commun., 1985). Limited petrographic SOUTHEASTERN ALASKA D25 data from pelitic rocks along the northeastern mar- phic unit, including the one on Wrangell Island gin of the pluton do not indicate the existence of around which the metamorphic isograds mark an similar sillimanite and kyanite zones; in that area, increase in metamorphic grade toward the pluton, rocks in which kyanite and sillimanite coexist and in are probably the main cause of the metamorphism which either kyanite or sillimanite is the sole and deformation in the adjacent rocks and were Al,SiO, polymorph are present at approximately emplaced during the waning stages of the meta- equal distances from the pluton (R.L. Elliott, unpub. morphism and deformation (Brew and others, 1984; data, 1979). Douglass and Brew, 1985; Berg and others, 1988). Geothermometric and geobarometric data from The synkinematic batholith near the northern end two samples of garnet-kyanite schist in the Ketchi- of Behm Canal and the smaller pluton that in- kan quadrangle (locs. 2 and 6, pl. 2) indicate trudes the greenschist-facies rocks to the south (to- equilibration temperatures and pressures of 600°C, nalite of Bushy Point) yield U-Pb ages on zircon, 7.5-8.5 kb and 575-600°C, 8.5-9.2 kb, respectively interpreted as their emplacement ages, of approxi- (M.L. Crawford, written commun., 1983). These mately 90 and 94 Ma, respectively (C.M. Rubin, data are based on garnet-biotite equilibration tem- oral commun., 1987; Rubin and Saleeby, 1987). The peratures (calibration of Ferry and Spear, 1978) Moth Bay pluton, which intrudes the greenschist- and plagioclase-garnet-A12Si05-quartzequilibration facies rocks on the southern tip of Revillagigedo Is- pressures. The temperatures and pressures are de- land, also may be synkinematic (J.G. Smith, oral rived from analysis of the outer parts of the grains commun., 1985), as suggested by the facts that the and, hence, represent final crystallization equili- igneous minerals of the pluton are commonly bration for mineral rims rather than peak meta- slightly altered and bent or fractured and that, al- morphic conditions (M.L. Crawford, written com- though parts of the pluton are relatively massive, mun., 1983). Similar moderately high final crystal- it grades into fine-grained blastomylonite and my- lization pressures have been proposed for slightly lonite schist along some margins (J.G. Smith, oral postmetamorphic or late synmetamorphic primary commun., 1985; Berg and others, 1988). The Moth garnet- and epidote-bearing 90-Ma plutons that in- Bay pluton has yielded a 40Ar/39Arplateau age on trude this unit and the associated greenschist-fa- hornblende of 96-97 Ma (Sutter and Crawford, cies rocks. In their study of the pluton at Bushy 1985) and a U-Pb age on zircon of 101 Ma Point on northwestern Revillagigedo Island, Zen (Saleeby, 1987). Intrusion of the southernmost 90- and Hammarstrom (1984b), applying experimental Ma primary garnet- and epidote-bearing plutons data on the composition of magmatic garnet and on within this unit, as well as most of those in the pressures of crystallization of magmatic epidote, pro- adjacent greenschist-facies unit on Revillagigedo posed that the magma began to crystallize at a mini- Island, postdates the regional metamorphism and mum pressure of 13-15 kb (depth of about 40-50 deformation of the country rocks. km) and finally crystallized at about 6-10 kb (depth K-Ar age determinations on amphibolite- and as- of about 20-30 km). Similar final crystallization sociated greenschist-facies rocks show a decrease pressures are proposed for the other approximately in maximum apparent ages northward and east- 90-Ma primary epidote-bearing plutons that form the ward on Revillagigedo Island (Smith and Diggles, Admiralty-Revillagigedo belt of Brew and Morrell 1981; Berg and others, 1988). 40Ar/39Arplateau (1983) (Zen and Hammarstrom, 1984a). The combi- ages on hornblendes from plutonic and metamor- nation of the high- to intermediate-pressure mag- phic rocks on Revillagigedo Island, as well as from matic and crystallization history inferred for the plu- metamorphically similar rocks to the south in the tons and the occurrence of relict andalusite, indica- area of Prince Rupert, British Columbia, show an tive of low-pressure conditions (less than 3.8 kb; age pattern similar to that shown by conventional Holdaway, 1971), in their aureoles in the Wrangell K-Ar ages; they range from more than 90 Ma in Island area is indeed problematic. Also significant to the western part of the island to about 56-57 Ma this problem is the fact that narrow, low-pressure near the eastern boundary of this metamorphic aureoles are present around some of the 90-Ma epi- unit (Sutter and Crawford, 1985). 40Ar/39Arage dote-bearing plutons that intrude lower grade rocks spectra suggest that the observed age pattern was to the north and west of the amphibolite-facies rocks caused by cooling associated with differential uplift on Wrangell Island and to the south and west of the and not by partial loss of 40Ar during the later amphibolite-facies rocks on Revillagigedo Island. heating event that was associated with rocks of the The 90-Ma batholiths (unit IKg with triangle Coast Mountains to the east (Sutter and Crawford, pattern, pl. 1) at the northern end of this metamor- 1985). Uplift of the block consisting of these D26 REGIONALLY METAMORPHOSED ROCKS OF ALASKA amphibolite-facies rocks and the associated I strata that are believed to range from early Middle greenschist-facies rocks was greatest (exposing Jurassic to middle Late Jurassic in age (Alldrick, gneissic parts of the batholith along northern 1985). Metamorphism is known to predate the em- Behm Canal) and occurred latest in the eastern placement of Eocene plutons (eTg) that produced part of the block. The eastern boundary of this thermal aureoles as much as 2 km wide along their block is spatially related to the Coast Range contacts with the metagranodiorite (Berg and oth- megalineament (Brew and Ford, 1978), a topo- ers, 1988). graphic, structural, and geophysical feature that appears to have been near the western limit of large-scale regional uplift of southeastern Alaska GNLtl (IK) + GNI (eTIK) and adjacent parts of British Columbia beginning in early Tertiary time (Hutchison, 1970, 1982; Greenschist-facies garnet-chlorite-biotite schist Crawford and Hollister, 1982, 1983). and semischist and minor phyllite, slate, marble, and calcareous tremolite-chlorite-bearing granofels (Brew and others, 1984) crop out in the eastern Pe- MAINLAND BELT tersburg quadrangle. The protoliths and latest Cre- taceous and (or) early Tertiary metamorphic his- GNS (eTIJ) tory of this unit are similar to that described below for the rocks to the northwest (GNI (eTIK)), This unit consists of greenschist-facies metatuff but, in addition, this unit was deformed (Hunt and breccia, semischist, phyllite, marble, and crosscut- Brew, 1986) and probably was metamorphosed sig- ting metagranodiorite and minor metaquartz dio- nificantly during the earlier metamorphism that rite (Smith, 1977; Berg and others, 1977, 1978, was associated with the intrusion of the 90-Ma plu- 1988). The metaplutonic rocks are the Texas Creek tonic belt. Low-pressure evolving to intermediate- Granodiorite and yield Late Triassic and Early Ju- pressure metamorphic conditions are indicated for rassic K-Ar ages (approximately 210 to 202 Ma) on the first episode because kyanite occurs &ong with hornblende (Smith and Diggles, 1981; Berg and and replaces andalusite in the aureoles of the others, 1988) and Early Jurassic concordant U-Pb nearby 90-Ma plutons. It is generally difficult to ages (approximately 195 to 193 Ma) on zircon associate a given assemblage with one or the other (Alldrick and others, 1986). Other protoliths in- episode because metamorphic minerals produced clude andesitic volcanic and volcaniclastic rocks, during both greenschist-facies episodes are the flysch, and minor limestone (Berg and others, same. Well-foliated rocks developed in the first epi- 1988). Late Paleozoic and early Mesozoic protolith sode were recrystallized during the later Barrovian ages are suggested by the occurrence of a Permian metamorphism as were parts of some of the 90-Ma fossil in limestone to the northwest that is prob- plutons (Brew and others, 1984). Part of the west- ably correlative with marble of this unit (R.L. ern part of this unit also may have been metamor- Elliot, oral commun., 1983) and by a probable ge- phosed during the mid-Cretaceous low-grade re- netic relation between the andesitic rocks of this gional metamorphism recorded in the rocks of unit unit and the Late Triassic and Early Jurassic LPPIGNS (mK) to the west. Some evidence (Brew Texas Creek Granodiorite (Berg and others, 1988; and others, 1984) indicates that the eastern limit Alldrick, 1985). of that metamorphism was close to this area in Rocks are characterized by lower greenschist- which the 90-Ma metamorphism is overprinted by facies assemblages that contain actinolite, chlorite, the latest Cretaceous and (or) early Tertiary Barro- albite, quartz, clinozoisite-epidote, calcite, and vian metamorphism. opaque minerals. Relict volcanic and plutonic textures are common, and rocks generally are not schistose or otherwise penetratively deformed LPP (eTIK), (Smith, 1977; Berg and others, 1988). The Texas Creek Granodiorite is locally characterized by cata- Prehnite-pumpellyite-facies basaltic metatuff clastic or mylonitic textures. Regional metamor- breccia, in places interlayered with metagraywacke phism and cataclasis postdate the emplacement of and metapelite, crops out in the Juneau quad- the granodiorite plutons. Field relations in the ad- rangle and forms the lowest grade part of an east- joining area in British Columbia, Canada, suggest ward-increasing Barrovian metamorphic sequence that the metamorphism postdates sedimentary that developed during latest Cretaceous and (or) SOUTHEASTERN ALASKA D27 early Tertiary time (Ford and Brew, 1973, 1977a, ermost greenschist facies (or perhaps prehnite- b; Brew and Ford, 1985). Fossils indicate that the pumpellyite facies) to upper greenschist facies. age of the protoliths are as young as Early(?) Cre- Clastic and other relict textures are present only in taceous (Ford and Brew, 1973). A weakly metamor- the lowest grade part of this unit, where an early phosed mid-Cretaceous pluton of intermediate foliation, presumably formed during the low-grade composition is also included in this unit. Charac- late Early Cretaceous regional episode, is locally teristic metamorphic minerals include albite, white detectable (Brew and others, 1984). As metamor- mica, chlorite, pumpellyite, epidote, actinolite, and, phic grade increases, these rocks exhibit well- locally, prehnite and stilpnomelane (Ford and defined crenulation cleavage and transposition Brew, 1977b). layering; most rocks included in this unit are well The eastern boundary of this unit is defined by foliated and lineated. the first appearance of green biotite that occurs The lower greenschist-facies rocks are character- only slightly upgrade from the last appearance of ized by the assemblage albite+white mica+chlo- pumpellyite and prehnite. The western and south- rite+quartz+epidote, and the upper greenschist- ern boundaries of this unit with previously meta- facies rocks .are characterized by the assemblages morphosed prehnite-pumpellyite-facies rocks (units muscovite+chlorite+biotite+albite+quartzf garnet LPP (eKeJ) and LPP (eK), respectively) are approxi- and clinozoisite+albite+quartz+chlorite+calcite mate. The effects of low-grade metamorphism dur- +muscovite+biotite+actinolitefsphene (Brew and ing the previous episode(s) may have extended into others, 1984). The eastern (high-grade) boundary of the area of low-grade Barrovian metamorphism, this unit generally separates rocks in which the and the metamorphic effects of the later episode chlorite-garnet tieline persists from rocks to the may have continued farther to the south into the northeast in which this tieline has been broken, area shown as unit LPP (eK). and the tieline between biotite and staurolite or The possible tectonic origins that have been pro- aluminum silicate has been established. This posed to explain the development of this Barrovian boundary is an isograd and is northeast dipping metamorphic sequence are discussed in the final and, hence, is inverted. The isograds in this unit section of this report. and the related unit AM1 (eTIK) (Ford and Brew, 1973; 1977a; Brew and Ford, 1977) are the only in- verted isograds that have been mapped to date in GNI (eTIK) the Coast plutonic-metamorphic complex of Brew and Ford (1984). Farther south within the Cana- This elongate unit forms the intermediate-grade dian part of the complex, however, an older in- part of the northeastward-increasing Barrovian verted metamorphic sequence is interpreted to metamorphic sequence and comprises dynamother- have formed during metamorphism related to the mally metamorphosed, intermediate pressure, intrusion and west-directed thrusting of a 98-Ma lower to upper greenschist-facies phyllite, slate, epidote-bearing pluton (Crawford and others, semischist, greenschist, chlorite-biotite schist, and 1986). That pluton is part of the plutonic belt dis- greenstone. Studies of these rocks and their meta- cussed earlier for units GNS (K) and AM1 (IK). morphic history have been made in the Juneau Metamorphism is considered to be slightly quadrangle by Forbes (1959), Ford and Brew (1973, prekinematic and primarily synkinematic with the 1977a), Brew and Ford (1977), and Himmelberg latest Cretaceous and (or) early Tertiary mesozonal and others (1984a, b, 1985, 1986); in the Sumdum intrusion of quartz diorite, tonalite, and minor and Taku River quadrangles by Brew and Grybeck granodiorite of the great tonalite sill (Ford and (1984); and in the Petersburg quadrangle by Brew Brew, 1977b; Brew and Ford, 1981; Brew and and others (1984) and Douglass and Brew (1985). Morrell, 1983), shown as the 600-km-long synkin- Protoliths are considered to have been primarily ematic intrusive body just east of the Coast Range clastic sedimentary rocks; mafic to intermediate megalineament (Brew and Ford, 1978). Intrusion of volcanic, intrusive, and volcanogenic sedimentary the sill has been dated by U-Pb zircon methods at rocks; and minor limestone and chert. Fossils about 69 and 62 Ma in the Juneau and Sumdum found in these rocks near Juneau indicate Permi- quadrangles, respectively, (Gehrels and others, an(?) and Late Triassic protolith ages; Jurassic and 1984) and at about 55-58 Ma in the Ketchikan Cretaceous protoliths also may have been involved. i quadrangle to the south (Berg and others, 1988). The metamorphic grade of this unit increases Evidence for this genetic relation is the increase in from southwest to northeast and ranges from low- metamorphic grade toward the sill, general paral- D28 REGIONALLY METAMORPHOSED ROCKS OF ALASKA lelism between the sill and isograds that define the morphic isograds by the sill and parallelism between Barrovian metamorphic sequence, and parallelism foliation in the sill with that in the metamorphosed of foliation, contacts, and locally developed linea- wallrocks suggest that the intrusion of the.sill tion of the sill with structural elements in the ad- accompanied a late stage of the regional metamor- jacent metamorphic rocks. phism, namely after the thermal maximum but be- The inverted isograds are interpreted to have fore the end of penetrative deformation (Ford and formed during northeast-southwest compression Brew, 1977b; Brew, 1983; Brew and others, 1984). that both folded and intensified earlier folds in the The latest Cretaceous and early Tertiary age of the mixed pelitic, volcanic, and carbonate protoliths. sill establishes a similar age range for Barrovian The isograds are not folded, which indicates that metamorphism, as discussed for the adjacent units. the metamorphism they record postdates the peak folding deformation. Isograds in equivalent amphibolite-facies rocks of this sequence (unit AM1 (eTIK) in the Juneau quadrangle are truncated by the sill (shown by the truncation of the kyanite iso- This unit forms the highest grade part of the grad near Juneau and about 20 km to the north, Barrovian metamorphic sequence and comprises pl. I), whose foliation, however, parallels that in amphibolite-facies schist, paragneiss, orthogneiss, the metamorphosed wallrocks. This suggests that and migmatite. Common rock types include pelitic, the intrusion of the sill complex accompanied a semipelitic, and quartzofeldspathic schist and late stage of the regional metamorphism, namely gneiss; hornblende schist; and subordinate am- after the thermal maximum but before the end of phibolite, quartzite, marble, and calc-silicate. penetrative deformation (Ford and Brew, 1977b; These rocks have been described by Redman and Brew, 1983; Brew and others, 1984). The inverted others (1984) and Barker and others (1986) in the isograds are thought to have formed during the Skagway quadrangle; by Forbes (1959), Ford and late stages of folding and compression as a thermal Brew (1973, 1977a), Brew and Ford (1977), and envelope advanced ahead of the great tonalite sill Himmelberg and others (1984a, b, 1985, 1986) in during its upward and southwestward movement the Juneau and Taku River quadrangles; by Brew in the crust; the relatively cool and wet rocks in and Grybeck (1984) in the Sumdum and Taku the footwall of the advancing thermal envelope River quadrangles; and by Brew and others (1984) were progressively metamorphosed (Himmelberg in the Petersburg quadrangle. Detailed mapping of and others, 1991). migmatite at localities in the Petersburg, Taku River, and Juneau quadrangles has indicated that elongate zones of intensely deformed migmatite in those areas are composed of various combinations of hornblende-biotite quartzofeldspathic schist and This areally restricted, elongate unit is composed gneiss, calc-silicate schist and gneiss, marble, and of the undifferentiated, intermediate-pressure assorted plutonic and metaplutonic rocks of the greenschist- and amphibolite-facies rocks of the great tonalite sill and younger intrusive units Barrovian metamorphic sequence whose metamor- (Karl and Brew, 1984; Brew and others, 1984). phic grade increases northeastward. The rock types Metamorphic protoliths are considered to have and metamorphic history are the same as that de- been primarily clastic sedimentary rocks, including scribed for the two nearby metamorphic units GNI a minor amount of limestone, and mafic to inter- (eTIK) and AM1 (eTIK), but the map scale (pl. 1) is too mediate volcanic, intrusive, and volcanogenic sedi- small to allow differentiation of these two units. Iso- mentary rocks. No fossils have been found in these grads have not been mapped within this undifferen- rocks, but protolith ages are considered to be, in tiated unit, which occurs north of Juneau, but a part, Permian, Triassic, Jurassic, and Cretaceous rapid eastward increase in metamorphic grade is in- on the basis of regional correlations (Brew and dicated by the mineral assemblages observed (D.A. Ford, 1984a, b, 1985). Brew, unpub. data, 1978). Immediately southeast of Metamorphic grade within this unit and within this undifferentiated unit spacing of metamorphic the entire sequence increases to the northeast. The isograds increases rapidly. The isograds in that area southwesternmost boundary of this unit is defined are truncated by the contact with the great tonalite by the first appearance of staurolite. Mineral iso- sill (shown by the truncation of the kyanite isograd, grads marking the first appearance of staurolite, pl. 1) (Ford and Brew, 1977b). Truncation of meta- kyanite, and sillimanite (as well as biotite and gar- SOUTHEASTERN ALASKA D29 net in the adjacent lower grade rocks of this facies and Endicott Arms indicates general pressures that series shown as unit GNI (eTIK)) trend north- are consistent with values of 3.8-4.5 kb at 575°C cal- northwest, generally parallel to the tonalite sill. As culated from silicate mineral equilibria of staurolite- noted in the discussion of unit GNI (eTIK), isogradic zone rocks in the same general area (Stowell, 1985). surfaces dip moderately to steeply northeast (Ford Farther north in the Juneau quadrangle, and Brew, 1973, 1977a; Brew and Ford, 1977) and geobarometry by several methods indicates pressures are .either vertical or inverted. Field, petrographic, of about 9 kb for the same rocks, but the data ex- and mineral chemistry studies of the progressive hibit large scatter (G.R. Himmelberg, oral commun., metamorphism near Juneau indicate that 1987; Brew and others, 1987). metamorphic index-mineral isograds mark the oc- This unit is generally well foliated and lineated; currence of particular reactions and, thus, are true foliation in gneissic rocks is locally anastamosing or reaction isograds (Himmelberg and others, 1984b). lenticular. Within the Coast Range megalineament In the Petersburg quadrangle, characteristic zone, rocks are locally cataclastic or mylonitic, and metamorphic mineral assemblages in pelitic rocks many contain late-kinematic, retrograde greenschist- include quartz+muscovite+plagioclase+biotite+ facies minerals, primarily chlorite, white mica, and garnet+staurolite_+kyaniteand, as metamorphic calcite, whose development apparently accompanied grade increases, sillimanite+biotite+quartz+plagio- shearing (Brew and others, 1984; D.A. Brew, unpub. clase+garnet+ilmenite and sillimanite+potassium data). feldspar+muscovite+biotite+garnet+qu~oclase Kyanite- and sillimanite-bearing metamorphic in rocks that crop out east of the Coast Range mineral assemblages similar to those in the Juneau megalineament (Brew and others, 1984; Douglass and Petersburg areas have recently been reported and Brew, 1985). Mafic rocks contain hornblende+ adjacent to the southern extension of the great tonal- biotite+quartz+garnet+plagioclasef clinopyroxenef ite sill north of Prince Rupert, B.C. (Gareau, 1988; potassium feldspar, and calcareous rocks contain S.A. Gareau, oral commun., 1988); it is possible that calcite+wollastonite+quartz~diopside~scapolite.In this unit is present south of unit AMI,L (eTIK) that the Petersburg quadrangle, fibrolitic sillimanite crops out in the Bradfield Canal, Ketchikan, and only rarely coexists with kyanite-bearing gneiss Prince Rupert quadrangles and that is on strike with west of the megalineament, and prismatic silliman- unit AM1 (eTIK). ite only occurs east of it (Brew and others, 1984; Metamorphism is considered to be slightly pre- Douglass and Brew, 1985). Isogradic surfaces in kinematic and synkinematic with the lastest Creta- this area appear to steepen northeastward toward ceous and (or) early Tertiary mesozonal intrusion of the Coast Range megalineament, which locally quartz diorite, tonalite, and minor granodiorite of marks the sillimanite isograd (Brew and others, the tonalite sill, as discussed previously for unit GNI 1984, p. 33). (eTIK). The deeper levels now exposed in the western Farther to the north in the Juneau quadrangle, part of the Coast plutonic-metamorphic complex of characteristic metamorphic mineral assemblages, in Brew and Ford (1984a) clearly have been uplifted increasing metamorphic grade, are quartz+musco- relative to those in the eastern part. Brew and Ford vite+biotite+garnet+staurolite_+plagioclase_+chlorite, (1978) considered the deeper crustal levels now ex- quartz+muscovite+biotite+garnet+ posed to be due to differential uplift focused near the staurolite+kyanite+plagioclase, Coast Range megalineament. The area west of the and quartz+muscovite+biotite+garnet+sillimanite megalineament was uplifted less than the area im- +plagioclase (Himmelberg and others, 1984a). mediately to the east, and the easternmost side of Garnet-biotite geothermometry for sillimanite-zone the complex was uplifted less than the areas adja- rocks indicates equilibration temperatures of about cent to the megalineament. The net result was a tilt- 690°C (calibration of Thompson (1976)) or 750°C ing of the complex during early Tertiary time. (calibration of Ferry and Spear (1978)) (Himmelberg Crawford and others (1987) speculated that uplift of and others, 1984a). the complex in British Columbia was a result of the Minimum equilibration pressures of about 3.8 kb weakening of the crust by anatexis and the develop- are indicated for the entire Barrovian sequence on ment of melt-lubricated shear zones, particularly the the basis of the Al,SiO, triple point of Holdaway zone represented by the great tonalite sill, along (1971) above which andalusite is unstable. Prelimi- which rapid vertical movement (tectonic surge) took nary sphalerite geobarometry of three massive place. sulphide deposits within the megalineament zone in The eastern part of this metamorphic unit was in- the Sumdum quadrangle near the mouth of Tracy truded during Eocene time by epizonal plutons of in- D30 REGIONALLY METAMORPHOSED ROCKS OF ALASKA termediate composition (unit eTg) (Brew and others, 1988) and by a nearly concordant U-Pb age of Morrell, 1983). These Eocene plutons are surrounded 139 Ma on zircon from non-migmatitic leucogneiss by mostly low-pressure, high-temperature metamor- near Redcap Mountain, British Columbia, some 50 phic rocks and by migmatites. The original eastern km southeast of the International Boundary (Hill, limit of the Barrovian metamorphism is obscurred by 1984). On the basis of chemical data from ortho- the Eocene intrusions, but limited evidence indicates gneiss in the area of Boca de Quadra, Barker and that the low-pressure metamorphism around the Eo- Arth (1984) suggested that in that area this unit cene plutons was superimposed over the previous represents the root of an andesitic arc complex. intermediate-pressure metamorphism. Metamorphic mineral assemblages in paragneiss The tectonic setting of this Barrovian metamor- generally consist of quartz+muscovite+biotite+ phic episode is discussed in the final section of this plagioclase+garnet+sillimanite (Berg and others, report. 1978, 1988; M.L. Crawford, written commun., 1984). Cordierite occurs locally in pelitic rocks in the Ketchikan quadrangle (Berg and others, 1988) and in the Bradfield Canal quadrangle (R.L. Elliott, unpub. data, 1979). Cordierite-bearing assemblages This unit comprises a heterogeneous complex of (c, pls. 1,2) in the Bradfield Canal quadrangle com- migmatite, massive to foliated or gneissic batholiths, monly contain the assemblage cordierite+siUimanite+ and smaller plutons that enclose metamorphic hercynite+garnet+plagidase+bioti~rthd~u~~~vite screens and roof pendants of paragneiss (Berg and (R.L. Elliott, unpub. data, 1979). Paragneiss and as- others, 1988). Dominant rock types consist of pelitic sociated rocks also have been reported to contain gneiss and schist and subordinate quartzfeldspathic minor amounts of the metamorphic minerals gneiss, migmatite, and orthogneiss derived from potassium-feldspar, clinopyroxene, and hornblende granodiorite, quartz monzonite, and quartz diorite. (Berg and others, 1988). Cummingtonite occurs in Minor rock types consist of marble and calc-silicate mafic schist in the Bradfield Canal quadrangle (table gneiss, pegmatite, quartzite, amphibolite, and aplite 2 and pl. 2; R.L. Elliott, written commun., 1985). (Berg and others, 1978, 1988). These rocks compose Metamorphic mineral assemblages indicate part of what has been termed the Coast Plutonic amphibolite-facies metamorphism. Small amounts of Complex by Roddick and Hutchison (1974), the retrograde minerals, actinolite, albite(?), chlorite, Central Gneiss Complex by Hutchison (1970, 1982), epidote, and sericite attest to widespread incipient and the Coast plutonic-metamorphic complex by retrograde metamorphism (Berg and others, 1988). Brew and Ford (1984a). Intermediate- to low-pressure amphibolite-facies The paragneiss and associated rocks have been conditions are indicated by mineral-assemblage data complexly folded and may have been recrystallized and geobarometric and geothermometric calculations more than once (Berg and others, 1978, 1988). In determined for one sample in the northeastern many areas, paragneiss grades downward and later- Ketchikan quadrangle. Kyanite has not been ob- ally into gneissic granodiorite; elsewhere, it is in served within this unit in Alaska. Andalusite (to- sharp contact with plutonic rocks or passes gradu- gether with sillimanite and garnet) occurs at one ally into them through a zone of migmatite (Berg locality within this unit adjacent to a 50-Ma pluton and others, 1978, 1988). Protoliths of the paragneiss (loc. 3, Bradfield Canal quadrangle, table 2, pl. 2) sequence consist of argillaceous clastic marine sedi- (R.L. Elliott, written commun., 1985) and indicates ments and minor amounts of limestone, chert(?), and that low-pressure metamorphic conditions, at least felsic to intermediate or mafic volcanic or volcani- locally, accompanied Eocene plutonism. Pressure and clastic rocks. A Paleozoic or Mesozoic protolith age temperature calculations on a sample of sillimanite- for the paragneiss sequence has been proposed on garnet-biotite-quartz-plagioclase schist (loc. 6, the assumption that some of the pelitic paragneiss Ketchikan quadrangle, table 2, pl. 2) indicate meta- may correlate with the amphibolite-facies, regionally morphic equilibration conditions of 3.5-4.5 kb and metamorphosed upper Paleozoic or Mesozoic meta- 650 "C (calibration of Ferry and Spear (1978)) (M.L. sedimentary rocks on eastern Revillagigedo Island Crawford, unpub. data, 1983). (Brew and Ford, 1984b; Berg and others, 1988). An Intrusion of the foliated quartz diorite (part of the Early Cretaceous protolith age for at least some of great tonalite sill of Brew and Ford (1981); discussed the orthogneiss has been indicated by a U-Pb age of in the description of unit GNI (eTIK)), which forms 127 Ma on zircon from migmatitic gneiss along Boca the western margin of this metamorphic unit, oc- de Quadra in the Ketchikan quadrangle (Arth and curred sometime during amphibolite-facies metamor- SOUTHEASTERN ALASKA D3 1

phism. The quartz diorite commonly is at least (Crawford and Hollister, 1982, 1983; Crawford and weakly foliated, and in the extreme case it is gneiss- others, 1987). As discussed above for unit AM1 (eTIK), ic. This foliation generally strikes north or northwest intrusion of the synmetamorphic elongate sills along and is parallel to the outcrop trend and to the inter- the west margin of the Central Gneiss Complex is nal structure of the adjoining metamorphic rocks. interpreted to have had a causal relation to the pro- Much of the quartz diorite also has cataclastic tex- posed drastic uplift by greatly increasing crustal tures, such as undulose quartz and granulated grain thickness in the region (Hollister, 1982), weakening boundaries (Berg and others, 1988). A Late Creta- the crust, and serving as a lubricant in the major ceous and (or) early Tertiary emplacement age for I shear zone along which the large and rapid vertical the quartz diorite and the rest of the elongate belt of uplift took place (Hollister and Crawford, 1986; similar plutonic rocks is indicated by U-Pb ages on Crawford and others, 1987). zircon of about 58-55 Ma in the Ketchikan quad- The boundary between this poorly studied meta- rangle (Berg and others, 1988) and of about 62 and morphic unit (AMI,L (eTIK)) in Alaska and the litho- 69 Ma in the Juneau and Sumdum quadrangles, re- logically similar unit farther northwest (AM1 (eTIK)) spectively (Gehrels and others, 1984). 1 has been drawn to separate the northern rocks in Few detailed mapping and petrologic studies have which kyanite occurs (and, hence, intermediate- been conducted within this metamorphic unit, but it pressure conditions are indicated) from rocks of this appears to be similar to rocks across the Interna- unit in which kyanite has not been identified and for tional Boundary to the south in British Columbia which intermediate- to low-pressure conditions are (M.L. Crawford, oral commun., 1985). In that area, indicated by thermobarometric data on one sample metamorphic reactions, thermobarometric data, and and the limited occurrence of cordierite. Future iden- isotopic data indicate that the Central Gneiss Com- tification of kyanite within unit AMI,L (eTIK) would plex in the Prince Rupert area (between about lat not be unexpected, however, because relict kyanite 54" and 55") was uplifted and eroded at a rate of (Crawford and Hollister, 1982, 1983) and unreacted about 1 mmlyr between about 60 and 48 Ma, begin- kyanite (S.A. Gareau, oral commun., 1988) have ning at a depth of about 20 km and terminating at been identified in rocks equivalent to this unit in about 5 km (Hollister, 1982; revised in Crawford and British Columbia. Although the proposed metamor- others, 1987). The emplacement of the elongate 60- phic histories of both of these units involve the asso- Ma Quotoon pluton, which is the continuation of the ciation of metamorphism with the Late Cretaceous (Alaskan) great tonalite sill, apparently occurred at and (or) early Tertiary intrusion of the tonalite sill, deep levels during the early stages of uplift, and em- differences in interpretation, such as the duration placement of intermediate and felsic plutons along and number of separate metamorphic episodes, be- the east margin of the metamorphic unit occurred at tween the Alaskan work to the north (unit AM1 high levels during the end stages of upliR. According (eTIK)) (Brew, 1983; Douglass and Brew, 1985) and to the Canadian work, metamorphism continued Alaskan work in this area (unit AMI,L (eTIK)) (R.L. throughout the period of uplift under evolving pres- Elliot, unpub. data, 1980), as well as Canadian work sure and temperature conditions (Hollister, 1982; to the south (for example, Hollister, 1982; Crawford Crawford and HoIIister, 1982, 1983). On the basis of and Hollister, 1982, 1983; Crawford and others, similarities of style and conditions of metamorphism 19871, also warrant at least a temporary boundary exhibited in rocks exposed west of the mega- between these two similar units. lineament in British Columbia to those suggested by early metamorphic relicts found in rocks exposed east of the megalineament, Crawford and Hollister TECTONIC INTERPRETATION OF METAMORPHISM IN THE (1982, 1983) suggested that early metamorphism of WESTERN METAMORPHIC BELT DURING LATE CRETACEOUS the Central Gneiss Complex (which is correlative AND EARLY TERTIARY TIME with this metamorphic unit) may have been synchro- nous with the approximately 90-Ma metamorphism The tectonic environment of the widespread of the western terrane (represented in the Ketchikan plutonometamorphic episode that occurred along the area by units GNS (K) and AM1 (IK)). After much of western edge of the Coast Mountains in early Late the hinged uplift of the western terrane took place, Cretaceous and early Tertiary time (affecting units which began about 90 Ma and continued to about 56 of the western metamorphic belt with age designa- Ma (Sutter and Crawford, 1985), a structural break tions of (IK) and (eTIK)) was dominated by crustal (the megalineament) between the western terrane thickening due to the accretion of an outboard and the Central Gneiss Complex was established terrane(s) to the west. Exactly what was accreted is D32 REGIONALLY METAMORPHOSED ROCKS OF ALASKA

not agreed upon, however. Monger and others (1982, Alaska: the roots of a high-K, calc-alkaline arc?: Physics of 1983) proposed that the plutonometamorphic belt de- the Earth and Planetary Interiors, v. 35, p. 191-198. veloped as a welt resulting from the accretion of the Barker, Fred, Arth, J.G., and Stern, T.W.,, 1986, Evolution of the Coast batholith along the Skagway traverse, Alaska' and amalgamated Wrangellia and Alexander terranes British Columbia: American Mineralogist, v. 71, p. 632-643. against the previously accreted Stikinia and other Barker, Fred, McLellan, E.L., and Plafker, George, 1985, Partial terranes in Cretaceous and Tertiary time. Brew and melting of amphibolite to trondhjemite at Nunatak Fiord, Ford (1983) interpreted stratigraphic and paleomag- St. Elias Mountains, Alaska cabs.]: Geological Society of netic evidence to suggest that the Alexander and America Abstracts with Programs, v. 17, no. 7, p. 518-519. Berg, H.C., 1972, Geologic map of Annette Island, Alaska: U.S. Stikinia terranes were one and the same and that a Geological Survey Miscellaneous Geologic Investigations rift developed in the megaterrane as it migrated Map 1-684, 8 p., scale 1:63,360. northward. According to their model, that rift was 1973, Geology of Gravina Island, Alaska: U.S. Geological filled with Gravina belt (Berg and others, 1972) Survey Bulletin 1373, 41 p., scale 1:63,360. flysch and volcanic rocks during Late Jurassic and Berg, H.C., Elliott, R.L., and Koch, R.D., 1988, Geologic map of the Ketchikan and Prince Rupert quadrangles, southeastern Early Cretaceous time. Brew and Ford (1983) pro- Alaska: U.S. Geological Survey Miscellaneous Investigations posed that the plutonometamorphic belt formed as a Map 1-1807, scale 1:250,000. result of the closure of the rift and the resultant Berg, H.C., Elliott, R.L., Koch, R.D., Carden, R.B., and Wahl, crustal thickening during accretion of the Chugach F.A., 1976, Preliminary geologic map of the Craig D-1 and terrane that lies to the west and southwest of the parts of the Craig C-1 and D-2 quadrangles, Alaska: U.S. Geological Survey Open-File Report 76-430, scale 1:63,360. Alexander and Wrangellia terranes. Berg, H.C., Elliott, R.L., Smith, J.G., and Koch, R.D., 1978, Geo- Workers in the southern extension of this meta- logic map of the Ketchikan and Prince Rupert quadrangles, morphic belt near Prince Rupert, British Columbia, Alaska: U.S. Geological Survey Open-File Report 78-73A, concur with the terrane accretion model of Monger scale 1:250,000. and others (1982). They have proposed that the Berg, H.C., Elliott, R.L., Smith, J.G., Pittman, T.L., and Kimball, A.L., 1977, Mineral resources of the Granite crustal thickening resulted from west-directed tec- Fiords Wilderness Study Area, Alaska: U.S. Geological Sur- tonic stacking of crustal slabs along east-dipping vey Bulletin 1403, 151 p. thrusts, in places possibly lubricated by intrusion of Berg, H.C., Jones, D.L., and Richter, D.H., 1972, Gravina- melt (parent magma of intermediate, epidote-bearing Nutzotin belt-tectonic significance of an upper Mesozoic plutons and sills) generated at the base of the crust sedimentary and volcanic sequence in southern and south- eastern Alaska, in Geological Survey research 1972: U.S. (Hollister and Crawford, 1986; Crawford and others, Geological Survey Professional Paper 800-D, p. Dl-D24. 1987). Thrusts possibly correlative with those that Brew, D.A., 1978, Geology, in Chap. B, Geology, geophysics, and were synchronous with 100- to 90-Ma plutonism geochemistry, of Brew, D.A., Johnson, B.R., Grybeck, near Prince Rupert have been identified on Revil- Donald, Griscom, Andrew, Barnes, D.F., Kimball, A.L., Still, lagigedo Island in Alaska (M.L. Crawford, oral com- J.C., and Rataj, J.L., Mineral resources of the Glacier Bay National Monument Wilderness Study Area, Alaska: U.S. mun., 1987). Geological Survey Open-File Report 78-494, p. B1-B21, pl. lA, scale 1:125,000. 1983, Metamorphism and deformation associated with in- trusive suites, Coast plutonic-metamorphic complex, south- REFERENCES CITED eastern Alaska Labs.]: Geological Association of Canada, Program with Abstracts, v. 8, p. A8. 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Table 2.-Metamorphic mineral-assemblage data

Metamorphic Quadrangle Locality Rock Assemblage Metamorphic facies Metamorphic-facies Sample name and No. Contributors typeZ (AS) or mineral indicated by unit in which No., if reference No. (plate 2) ' occurrence assemblage3 given assemblage occur^^,^ available (plates 1.2) (oc) assemblage4

Skagway 109 E.M. MacKevett GN S GNS (eKI X) ....do AMP AMP (eKI 1) ..-.do ;.-.. AMP ------do,------..-do .--.-- GN S GNS (eKI X)

Mt. Fairweather 116 D.A. Brew AMP AMP (eTK) 116 .-.-do AMP GNSIAMP (eTK) 66ABd709 116 ..--do AMP AMP (eK) 116 ....do GN S LPP (eKeJ) 116 .---do AM L GNSIAMP (eTK) 116 ----do AMP AMP (eTK) 116 ..-.do AMP GNSIAMP (eTK) 116 ----do AMP AMP (eK) 116 ----do AMP ------do------116 ..--do ..---- AMP 116 ----do AMP 116 ----do,----- AMP 116 ----do ,----- AMP

Juneau 117 A.B. Ford CP +HO+BIfQEEP+_CAfGA+C~DI+_SH AM I AM1 (eTIK) 117 ----do QZ+BIfWMf PLf CBf GAfSTfKYfiI AM1 ------do 117 G.R. Hirnmelberg QZ+MU+BI+GA+SI+PL AMP ------do.------117 ----do MU+BI+SI+CN+PL AMP ------do ------117 ----do QZ+MU+BI+GA+ST+KY+PL AM I ------do.------117 ----do QZ+MU+CL+BI+GA+ST AMP ------do 117 -.--do QZ+MU+BI+GA+ST+PL AMP ------do 117 ----do QZ+MU+CL+BI GN S GNI (eTIK) 117 ----do QZ+MU+BI+GA+PL GN S ------do,------117 ----do ,----- QZ+WM+CL+PL+CD GN S ------do 68ABd145A 117 ----do ,----- WM+CL+PL+CD+BI+AMf QBCA GN S ------do 68ABd159A,B 117 ----do QZ+MU+CL+BI GN S ------do 117 -...do QZ+MU+BI+EP+CA+DO GN S ------do 117 A.B. Ford AB+CL+ACfCZfCAfEPfBlf WMf ZOf QZ GN S ------do 117 ----do QZ+CB+AB+CL+WM GN S ------do,------117 ----do HO+PLfBIf QEGAfSHfCA AM I AM1 (eTIK) 117 ----do QZ+BI+PLfGAfSTfKYfSIfWMfCB AM1 ------do 117 ----do CA+_DO+_QZ+PLfDIfSCfHOfSH+_TR AM1 AM1 (eTIK) 117 ----do AC+PL+CL+_EPfCZ+CAfQZfSHfBI GN S GNI (eTIK) 117 .....do QZ+BI+WM+CB GN S do ,------117 ....do .----- AB+CL+EPfACf QBPUfSHf CA LPP? LPP (eTIK), 117 R.A. Loney CL+EP+AB+QZ GN S GNS (eK) 117 ....do .----- WM+CL+ABfQZ GN S ------do,------117 ...-do AB+QZ+BI+CLfEP+GA+HO GN S ------do------117 D.A. Brew AB+CA+CL+EP LPP? LPP (eKeJ) 56ALy26 117 ....do .-.--- CL+EP+MU+AB+QZ LP P ? ------do,------117 ....do CA+DI+GA+WO AMP AMP (eK) 117 R.A. Loney CP+HO+QEBI AMP GNS,AMP (eK)

Sitka 124 D.A. Brew EP+QZ+AB+ZO+SH+AC+HO GN S LPP,GNS (eTeK) 124 ----do QZ+CP+KF+BI+MU+GA AMP AMP (eK) 124 ----do EP+CL+CA+MU+AC LPP? LPP (eKeJ) 124 ....do CA+Dlf GAf EPf WOfTR AMP AMP (eK) 124 ----do CA+CL+QZ+CM LPP? LPP (eKeJ) 124 ----do,..... MU+AB+EP+CL+CA+AC LP P ? ------do,------124 ----do CL+EP+AB+QZ LPP? ------do,------124 ----do CP+HO+BI+QZ AMP AMP (eK) 124 ---.do ,.-..- QZ+EP+ZO+CLfPH+CA GN S LPPJGNS (eTl X) 124 CL+ZO+EP+PH+LU+AB+ACfTR LP P LPPIGNS (eTeK) 124 CL+ZO+EP+QZ+FS+PH GN S ------do 124 QZ+EP+ZO+CL+BI GN S ------do------REGIONALLY METAMORPHOSED ROCKS OF ALASKA

Table 2.-Metamorphic mineral-assemblage data-Continued

Assemblage Metamorphic Quadrangle Locality Rock (AS) or Metamorphic facies Metamorphic-facies Sample name and No. Contributors type2 occurrence mineral indicated by unit in which No., if reference No. (plate 2) ' (oc) assemblage3 given assemblage occur^^,^ available (plates 1,2) assemblage4

D.A. Brew GN H LPPIGNS (eTeK) ----do GN S ------do .----do, ----- GN S LPPIGNS (eTI R) ....-do LPP? LPP,GNS (eTeK) ....do GN S LPPIGNS (eTI R) .---do GN S ------do ..--do ,----- GN S ------do ------do GN S LPP,GNS (eTeK) ----do GN S ------do,------do GN S ------do ----do GN H ------do ----do LPP? ------do ---.do GN S LPPIGNS (eTI R) ----do LPP? LPP,GNS (eTeK) .---do LP P ? ------do ----do GN S LPPIGNS (eTI R) .---do GN S LPP,GNS (eTeK) ---.do AMP AMP (eKI 1) ----do AMP ------do,------...do AMP ------do ..--do .---.. LP P LPP,GNS (eTeK) R.A. Loney GN S GNS (eK) .--do, ----- GN S ------do D.A. Brew LP P LPP,GNS (eTeK) ----do LP P ------do.------..--do GN S LPP,GNS (eTeK) + GNL (eT) AMP LPP,GNS (eTeK) + AML (eT) LP P LPP,GNS (eTeK) LP P ------do LPP? LPP,GNS (eTeK) AMP LPP,GNS (eTeK) + AML (eT) AMP LPP,GNS (eTeK) + AML (eT) AMP ------do.------AMP ------do AMP ------do AMP ------do

Sumdum 125 1 R.A. Loney GN S LPP (eK) 125 2 ----do LPP? ------do 125 3 S.L. Douglas~ GN S GNI (eTIK) 125 4 ----do GN S ------do - 125 5 ----do AMP AM1 (eTIK) 125 6 ----do AMP ------do 125 7 ----do AMP ------do 125 8 ---.do AMP ------do 125 9 ..--do AMP ------do,------125 10 ..-.do AMP GNI (eTIK) 125 11 .---do ....-- GN S ------do 125 12 ----do AMP GNI (eTIK) 125 13 ..--do GN S ------do 125 14 -.-.do .----. AMP AM1 (eTIK) 125 15 L.J.P. Muffler GN S LPPIGNS (mK) 125 16 ----do GN S ------do.------125 17 S.L. Douglas~ GN S GNI (eTIK) 125 18 ----do AM I AM1 (eTIK) 125 19 ----do AMP ------do 125 20 ----do LP P LPPIGNS (mK) 125 2 1 ----.do AMP AM1 (eTIK) 125 22 ----do AMP ------do

Port Alexander 130 1 D.A. Brew AS QZ+AB+CL+MU+EPk PHf SH LPP,GNS (eTeK) 130 2 ---.do AS (2) CP+QZ+CL+BI+MU+EP?SHf CAfTO ------do - - SOUTHEASTERN ALASKA D39

Table 2.-Metamorphic mineral-assemblage data-Continued

Assemblage Metamorphic Quadrangle Locality Rock (AS) or Metamorphic facies Metamorphic-facies Sample name and No. Contributors type2 occurrence mineral indicated by unit in which No., if reference No. (plate 2) (0'3 assemblage' given assemblage occurs4 available (plates 1,2) assemblageJ

--

130 3 D.A. Brew LPP,GNS (eTeK) + GNL (eT) 130 4 ----do ------do

130 5 ----do LPP,GNS (eTeK) 130 6 -..-do ,---.. LPP,GNS (eTeK) + GNL (eT) 130 6 .---do GN S ------do,------

130 7 ..--do AMP LPP,GNS (eTeK) + AML (eT) 130 8 ----do AMP ------do,------130 9 ----do AMP ------do 130 10 ....do .---.. AMP LPP,GNS (eTeK) + GNL (eT) 130 I 1 ----do QZ+CP+HO+BIf GMTO AMP LPP,GNS (eTeK) + AML (eT) 130 12 ----do AML LPP,GNS (eTeK) + GNL (eT) 130 13 --..do ------do ------130 14 ----do,----- LPP,GNS (eTeK) + AML (eT) 130 15 ..--do ------do------130 16 --.-do LPP,GNS (eTeK) + GNL (eT) 130 17 ----do .-.--- GN S ------do 130 18 ----do.-...- AML ------do 130 19 ....do LP P LPP,GNS (eTeK) 130 20 ----do GN S LPP,GNS (eTeK) + GNL (eT) 130 2 1 ....do ------do 130 22 ----do ------do 130 23 --..do LPP,GNS (eTeK) + AML (eT)

------do,---em-- 130 • 24 ----do AMP 130 25 ...-do GN S LPP,GNS (eTeK) + GNL (eT) 130 26 ----do GN S ------do 130 27 ----do AMP LPP,GNS (eTeK) + AML (eT)

Petersburg 131 1 S.L. Douglass GN S 131 2 ....do GN S GN S 131 LPPIG NS 131 GA+EP+BI+QZ+PL+HO+CL GN S GNI (eTIK) 131 GA+EP+QZ+CA+PL+CL GN S ------do 131 MU+PL+CL+BI+QZ+GA+EP GN S ------do 131 AC+SH+CL+EP+PL+MU+CA+QZ GN S ------do.------131 (I) KY+ST+GA+PL+QZ+BI+MU AM I AM1 (eTIK) (2) BI+EP+CL+QZ GN S 131 (I ) KY+GA+PL+HO+QZ+BI+MU AM I (2) CL+EP+BI+SH GN S 131 QZ+MU+KY+ST+PL AM I ------do 131 CL+WM GN S GNI (eTIK) 131 SI+BI+QZ+CP+GA+CO(?) AMP AM1 (eTlK) 131 ST+KY+GA+CP+QZ+BI AM I ------do 131 ( 1) KY+MU+BI+ST+QZ+CP+GA AM1 AMI (eTlK) (2) BI+PL+QZ+CL+MU+SH GN S 131 CZ+MU+GA+CL+BI+PL+QZ GN S GNI (eTIK) 131 QZ+WM+EP+CL GN S ------do,------131 CL+CA+SH+WM+CB+EP+QZ+PL+BI GN S ------do,------131 KY+GA+BI+CP+QZ AM I AM1 (eTIK) 131 CP+QZ+SI+MU+BI AMP ------do 131 GA+CP+QZ+BI+MU+KY AM I ------do 131 (1) CP+KF+BI+GA+SI AMP ------do (2) CL+WM+BI GN S D40 REGIONALLY METAMORPHOSED ROCKS OF ALASKA

Table 2.-Metamorphic mineral-assemblage data-Continued

Assemblage Metamorphic Quadrangle Locality Rock (AS) or Metamorphic facies Metamorphic-facies Sample name and No. Contributors type2 occurrence mineral indicated by unit in which No., if reference No. (plate 2) ' (oc) assemblage' given assemblage occur^^,^ available (plates 1,2) assemblage4

131 S.L. Douglass AMP GN S 131 ..--do GN S GN S 131 ----do GN S GNL+I (IK) + GNI GN S (eTIK) 131 ----do GN S ------do GN S 131 ----.do AMP AMP 131 ....do AMP ------do 131 --...do GN S ------do,------GN S 131 -----do,..--- AM I LPPIGNS (mK) + GNL-11 (IK) 131 -----do,..--- AM I ------do.------131 --..do LP P lG ------do.------NS GN S 131 ----do GN S GN S 131 ....do LPPIG NS GN S 131 .---do AMP ------do 131 ----do AML AM1 (IK) 131 ---.do AM1 ------do------131 ----do AM I ------do - -- - 131 ----do AM I ------do,------131 ----do AMP ------do,------131 ....do LPPIG LPPIGNS (mK) + NS GNL4 (IK) AMP 131 ----do LPPIG NS GN S 131 .---do AMP ------do 131 ----do AMP AM1 (IK) LPPlG NS 131 -...do AM I LPPlG NS 131 -...do AMP 131 .---do AM I

131 ----do ,....- AM I LP P lG NS 131 ---do AM I LPPlG NS 131 .-.-do AM I LPPIG NS 131 ....do ,---.- (I) QZ+HO+GA+DI AM I (2) CA+QZ+AM+GA+ZO+SH GN S 131 .---do (1) QZ+BI+CP+ST AMP (2) CL+MU LP P lG NS 131 -----do AM I 131 .---do AMP

Bradfield Canal 132 R.L. Elliott AML AM1 (eTIK) 132 ----do AMP AMI,L (eTIK) 132 ---.do AML ------do 132 .---do AM I AM1 (IK) 132 -----do AMP ------do------SOUTHEASTERN ALASKA D41

Table 2.-Metamorphic mineral-assemblage data-Continued

Assemblage Metamorphic Quadrangle Locality Rock (AS) or Metamorphic facies Metamorphic-facies Sample name and No. Contributors type2 occurrence mineral indicated by unit in which No., if reference No. (plate 2) (oc) assemblage3 given assemblage occur^^,^ available (plates 1,2) assemblage4

132 R.L. Elliott AMP AM1 (IK)

132 ----do AMP ------do.------132 ..--do ,--.-- AMP AMI,L (eTIK) 132 -----do AMP ------do,------132 -----do AMP ------do.------132 .....do AMP AM1 (IK) 132 --.--do ,----- AMP AMI,L (eTIK) 132 ----do AMP ------do I32 ----do ,----- AMP ------do

132 ----do AM I AM1 (IK) 132 ----do AM I ------do,------

132 -...do AM I ------do ------132 ..--do ,-..-. AM I ------do

132 ----do AMP AMI,L (eTIK) 132 ----do AMP ------do,------132 ....do ,----- AMP ------do 132 ----do AMP AM1 (IK) 132 ---.do AMP AMI,L (eTIK) 132 ----do AMP ------do,------132 ----do AMP ------do 132 ..--do .-.--. AMP ------do 132 ....do AMP ------do 132 ----do ,...-- AMP ------do 132 --..do AMP ------do 132 H.C. Berg GN S GNS (eTIJ) 132 R.L. Elliott AMP AMI,L (eTIK)

Craig 136 G.D. Eberlein AB+CL+EP+CA+ QZ GNS? LPP (DS) 136 ---- 40 AB+CL+EPfCA_+AC GN S ------do,------136 ..--do PL+CL+EP+WM+BI+CA GN S GNS (O€) + LPP (DS) 136 ...-do AC+CL+AB+EP+CA+QZ GN S ------do,------136 T.K. Bundtzen (1) HO+CP+GA+QZ AMP ------do 136 G.D. Eberlein CA+AM+BI GN S ------do 136 ..--do AB+EP+CLfMU+CA GN S ------do 136 ..--do QZ+WM+CBfCA+AM+ CL GN S ------do,------136 ....do .----. ZO+WM+AM+EP+CLfQZ+FS GN S ------do.------136 ..--do .---.. QZ+AB+CA+CL+WM GN S ------do,------136 ...-do .---.. QZ+AB+CL+CA+EP GN S ------do 136 T.K. Bundtzen AC+AB+BI+CL+QZ GN S LPP (DS) 136 ..---do TR+AB+BI+WM+CL GN S ------do 136 G.D. Eberlein QZ+ABfWM GN S ------do,------136 ----do QZ+AN+WM+CL+SH GN S ------do,------136 -.-.do ,----- QZ+AB+CLf WMfEPf SH GN S ------do.------136 .....do CL+AB+EPf WMf QZfSH GN S ------do 136 --.-.do. ----- AB+CZ+CL+WM+QZ+EP GN S LPP (DS) 136 ----do QZ+ABf SH+BI+CAfAM GN S GNS (0€ + LPP (DS) 136 ---.do ,----- CLLQZ GN S ------do 136 ----do QZ+AB+CL+EP+HOH?A GN S 136 ----do CL+CA GN S 136 ----do AB+EP+CL+QZ+AC GN S 136 -...do QZ+WM+CL+EP GN S 136 ----do QZ+AB+EP+CL+SH GN S 136 .-.-do .----. AB+CL+EP+PL+SHkQZ GN S

Ketchikan 137 M.L. Crawford AM I AM1 (IK) 137 ----do AM1 ------do.------137 -...do AM I ------do 137 ....-do AM I ------do,------137 J.G. Smith AM I ------do,------137 ..--do AM I ------do 137 M.L. Crawford AM I ------do 137 ----do AM I ------do - --- - 137 H.C. Berg AMP AMI,L (eTIK) 137 M.L. Crawford AMP ------do D42 REGIONALLY METAMORPHOSED ROCKS OF ALASKA

Table 2.-Metamorphic mineral-assemblage data-Continued

Assemblage Metamorphic Quadrangle Locality Rock (AS) or Metamorphic facies Metamorphic-facies Sample name and No. Contributors type2 occurrence mineral indicated by unit in which No., if reference No. (plate 2) ' (oc) assemblage3 given assemblage occur^^,^ available (plates 1,2) assemblage4

M.L. Crawford AMP AMI,L (eTIK) J.G. Smith GN S GNS (K) M.L. Crawford AMP AMI,L (eTIK) J.G. Smith AMP ------do M.L. Craw ford AMP ------do.------J.G. Smith AMP AM1 (IK) ----do AMP ------do ,------M.L. Crawford GNSIA ------do MP M.L. Crawford QZ+GA+EP+BI+CL+MU GN S GNS (K) -----do MU+SH+CA+DO+QZ+ZO+CL GN S ------do .---do .----- HO+QZ+PL+ZO AMP AM1 (IK) J.G. Smith QZ+BI+MU+GA AMP ------do------....do PL+HO+EP+BI+GMPY AMP ------do H.C. Berg AB+AC+CL+EP+CZ+QZ+MU+PH GN S GNS (K) ----do (1) AC+CL+EP+AB+MU+QBCAfHE GN S GNS (DS) + GNS (K) (2) GN S AB+MU+EP+CZ+CA+QZ+HE+KF+CM+PH ( 1) AB+CL+EP+CZ+AC+QBMU+CABI GN S (1) CL+MU+AB+QZ+BI+CA*AC+KF GN S (2) AB+QZ+EP+CBCUMUkPH+CA+AC GN S J.G. Smith QZ+MU+BI+GA GN S GNS (K) ----do MU+BI+PL+QZ+GA AMP ------do ....do HO+PL+EP+SH+BI AMP ------do M.L. Crawford BI+GA+MU+KY+ST+QZ+PL AM1 AM1 (IK) ....do GA+BI+HO+QZ+PL AMP ------do ,------J.G. Smith CP+QZ+BI+GA+KY AM I ------do ....do CP+BI+HOf QWA AMP ------do -...do CP+QZ+BI+GA+SI AMP AMI,L (eTIK) M.L. Crawford SI+GA+QZ+BI AMP AM1 (IK) -----do,----- (I) QZ+PLffiA+KY +MU+SI AM1 ------do ---.-do KY+ST+GA+PL+BI+MU+QZ AM I ------do

Prince Rupert 142 1 H.C. Berg GN S GNS (K) 142 2 J.G. Smith GN S GNS (DS) + GNS (K) 142 3 ...-do AMP AM1 (IK) 142 3 -----do AMP ------do 142 4 M.L. Crawford AMP AMI,L (eTIK) 142 5 J.G. Smith AMP ------do 142 6 M.L. Crawford AM I AM1 ((IK) 142 7 ----do AM1 ------do

'Localities numbered consecutively within each 1:250,000-scale quadrangle 'Rock types: BA, basic; CA, calcic; OT, other; PE, pelitic. 3Metamorphic minerals, arranged in order of decreasing abundance:

AB , albite (An 0-10) calcic plagioclase HO, hornblende PY, pyrophyllite AC, actinolite (An 11-100) HR, hercynite QZ quartz AM, amphibole crossite KF, potash feldspar SC, scapolite AN, andalusite cummingtonite KY, kyanite SH, sphene BI, biotite clinopyroxene LU, laumontite SI, sillmanite CA, carbonate clinozoisite MI, mica ST, staurolite CB. carbonaceous and ( diopside MU, muscovite TA, talc graphitic material dolomite OP, orthopyroxene TO, tourmaline chloritoid epidote PH, prehnite TR, tremolite chlorite feldspar PL, plagioclase WM, white mica clay minerals garnet (An 0-100) WO, wollastonite corundum glaucophane PU, pumpellyite ZO, zoisite cordierite ematite PX, pyroxene

(1) First phase of a polymetamorphic episode or early phase of a single evolving metamorphic episode (shown by arrow). (2) Second phase of a polymetamorphic episode, late phase of an evolving metamorphic episode (shown by arrow), or subsequent recrystallization due to an event not shown on map.

4Refer to text for explanation of symbols. "n a few cases, the area of the metamorphic-facies unit in which assemblage occurs is too small to show on map.

i?U.S. GOVERNMEMT PRINTING OFFICE: 1996 - 774-045 / 20029 REGION NO. 8