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Characteristics and Petrogenesis of Alaskan-Type Ultramafic-Mafic Intrusions, Southeastern Alaska

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Characteristics and Petrogenesis of Alaskan-Type Ultramafic-Mafic Intrusions, Southeastern Alaska Characteristics and Petrogenesis of Alaskan-Type Ultramafic-Mafic Intrusions, Southeastern Alaska By Glen R. Himmelberg and Robert A. Loney U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1564 Field relations, petrography, rock chemistry, mineral chemistry, and interpretation of origin and emplacement of Alaskan-type intrusions UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1995 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY Gordon P. Eaton, Director For sale by U.S. Geological Survey, Information Services Box 25286, Federal Center Denver, CO 80225 Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government. Text and illustrations edited by Mary Lou Callas Gra@cs by authors Layout by Mary Lou Callas Library of Congress Cataloging-in-Publication Data Himmelberg, Glen R. Characteristics and petrogenesis of Alaskan-type ultrarnafic-mafic intrusions, southeastern Alaska I by Glen R. Himmelberg and Robert A. Loney. p. cm. - (U.S. Geological Survey professional paper : 1564) "Field relations, petrography, rock chemistry, mineral chemistry, and interpretation of origin and emplacement of Alaskan-type intrusions." Includes bibliographical references (p. - ). Supt. of Docs. no.: 119.16:1564 1. Intrusions (Geology)-Alaska. 2. Rocks, Ultrabasic-Alaska. 3. Geochemistry-Alaska. I. Loney, Robert Ahlberg, 1922- . 11. Title. 111. Series. QE611.5.U6H56 1995 55 1.8'8'097982-4~20 95-2236 CIP CONTENTS Abstract ................................................................................................ Introduction ..................................................................................... ------ Regional setting and age ............................................................................. Field relations and petrography ............................................................... ------ Relation of Alaskan-type ultramafic intrusions to regional structure and metamorphism--- General statement ............................................................................... Union Bay complex ............................................................................. Kane Peak complex ............................................................................. Blashke Islands complex ................................................................. ------ Rock &emistry ................................................................................. ------ Major elements .................................................................................. Trace elements----------------------------------------------------------------------------------- Mineral chemistry .................................................................................... Discussion ............................................................................................. Crystallization conditions of ultramafic-mafic rocks ....................................... Nature of the parent magma ................................................................... Intrusive mechanism and zonal structure .......................................................... Conclusions ........................................................................................... References cited ...................................................................................... FIGURES Map of southeastern Alaska showing locations of Alaskan-type ultramafic-mafic intrusions and major tectonostratigraphic units ........................................................................................... Geologic map of ultramafic complex at Red Bluff Bay ........................................................ Geologic map of ultramafic complex at Union Bay ............................................................ Hypothetical cross section through ultramafic complex at Union Bay ....................................... Equal-area lower hemisphere plots of structural data in Union Bay area .................................... Equal-area lower hemisphere plots of olivine X, Y, and Z axes in ultramafic complex at Union Bay -- Geologic map of ultramafic complex at Kane Peak ............................................................ Equal-area lower hemisphere plots of structural data in metaturbidites in Kane Peak area --------------- Equal-area lower hemisphere plots of olivine X, Y, and Z axes in ultramafic complex in Kane Peak area .................................................................................................................... Geologic map of ultramafic complex at Blashke Islands ....................................................... Equal-area lower hemisphere plots of olivine X, Y, and Z axes in ultramafic complex at Blashke Islands ................................................................................................................ Rare-earth element patterns of ultramafic and gabbroic rocks of Alaskan-type intrusions at Blashke Islands, Kane Peak, Union Bay, Salt Chuck, and Port Snettisham ........................................... Rare-earth element patterns of ultramafic xenoliths, plutonic gabbros, and a tholeiite flow associated with Aleutian island-arc volcanism ............................................................................... CONTENTS 14-18. Plots of chemical parameters of Alaskan-type intrusions: 14. Compositions of clinopyroxene and orthopyroxene in ultramafic rocks and gabbro .................... 28 15. Weight percent A1203against Mg/(Mg+Fe2+)of clinopyroxene in ultramafic rocks and gabbro ------ 24 16. Percent AllVagainst weight percent Ti02 for clinopyroxene in clinopyroxenite and gabbro ----------- 29 17. AllVagainst cations in A-site for hornblende in ultramafic rocks and gabbro ............................ 33 18. Cr/(Cr+A1+Fe3+) against Fe2+/(~g+Fe2+)for chromian spinel in ultramafic rocks ...................... 33 TABLES 1. Major rock types and characteristics of Alaskan-type ultramafic-mafic bodies ............................................ 2. Chemical compositions of Alaskan-type ultramafic and mafic rocks ........................................................... 3. Trace-element contents of Alaskan-type ultramafic and mafic rocks ............................................................ 4-9. Analyses of minerals in Alaskan-type ultramafic and mafic rocks: 4. Olivine ................................................................................................................................................... 5. Orthopyroxene ................................................................................................... ................................... 6. Clinopyroxene ............................................................................................................................ .......... 7. Hornblende ............................................................................................................................................ 8. Biotite .................................................................................................................................................... 9. Chromian spinel .................................................................................................................................... 10. Analyses of plagioclase in Blashke Islands Alaskan-type gabbro ................................................................. Characteristics and Petrogenesis of Alaskan-Type Ultramafic-Mafic Intrusions, Southeastern Alaska By Glen R. Himmelbergl and Robert A. Loney2 ABSTRACT Clinopyroxene in the later differentiates has a substantial esseneite component that is a result of the hydrous, oxidiz- ing nature of the magma. Rare-earth-element (REE) patterns Alaskan-type ultramafic-mafic intrusions occur along of the ultramafic rock samples markedly show signatures of a belt that extends from Duke Island to Klukwan in south- cumulus origin involving accumulation of dominantly oli- eastern Alaska and fall into two age groups400 to 440 Ma; vine, clinopyroxene, and hornblende. The absence of a posi- and 100 to 118 Ma. Most of the intrusions occur in the tive Eu anomaly and the relatively flat REE pattern of most Alexander terrane or in the Gravina overlap assemblage, but gabbro samples suggest that the gabbros are not cumulates they are not restricted to these terranes. The Alaskan-type but probably represent static crystallization of a differenti- ultramafic bodies range in size from sills only a few meters ated liquid that had undergone substantial removal of oli- thick to intrusions about 10 km in maximum exposed di- vine, clinopyroxene, and some hornblende. The markedly mension. Most of the bodies consist of magnetite-bearing similar REE abundance levels and patterns for the same rock hornblende clinopyroxenite or hornblendite, however many types in all the bodies studied suggest that all the bodies were of the larger ones also include dunite, wehrlite, olivine derived by differentiation of closely similar parent magmas clinopyroxenite, and, in some cases, gabbro. The Blashke under near-identical conditions. Islands and Union Bay bodies are markedly concentrically The magnesium-rich olivine in Alaskan-type dunite and zoned; dunite in the core is surrounded progressively out- wehrlite is consistent with crystallization from an ward by wehrlite, olivine clinopyroxenite, clinopyroxenite, unfractionated mantle-derived primary melt. The exact com- hornblende clinopyroxenite, and gabbro. In the other
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