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