Petrology and Geochemistry of Eclogite Xenoliths from the Colorado Plateau: Implications for the Evolution of Subducted Oceanic Crust |
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Authors: | USUI, TOMOHIRO NAKAMURA, EIZO HELMSTAEDT, HERWART |
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Affiliation: | 1 PHEASANT MEMORIAL LABORATORY FOR GEOCHEMISTRY AND COSMOCHEMISTRY, INSTITUTE FOR STUDY OF THE EARTH'S INTERIOR, OKAYAMA UNIVERSITY AT MISASA, TOTTORI, 682-0193, JAPAN 2 DEPARTMENT OF GEOLOGICAL SCIENCES AND GEOLOGICAL ENGINEERING, QUEEN'S UNIVERSITY, KINGSTON, ONT., K7L 3N6, CANADA |
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Abstract: | Eclogite xenoliths from the Colorado Plateau, interpreted asfragments of the subducted Farallon plate, are used to constrainthe trace element and SrNdPb isotopic compositionsof oceanic crust subducted into the upper mantle. The xenolithsconsist of almandine-rich garnet, Na-clinopyroxene, lawsoniteand zoisite with minor amounts of phengite, rutile, pyrite andzircon. They have essentially basaltic bulk-rock major elementcompositions; their Na2O contents are significantly elevated,but K2O contents are similar to those of unaltered mid-oceanridge basalt (MORB). These alkali element characteristics areexplained by spilitization or albitization processes on thesea floor and during subduction-zone metasomatism in the fore-arcregion. The whole-rock trace element abundances of the xenolithsare variable relative to sea-floor-altered MORB, except forthe restricted Zr/Hf ratios (36·937·6).Whole-rock mass balances for two Colorado Plateau eclogite xenolithsare examined for 22 trace elements, Rb, Cs, Sr, Ba, Y, rareearth elements, Pb, Th and U. Mass balance considerations andmineralogical observations indicate that the whole-rock chemistriesof the xenoliths were modified by near-surface processes afteremplacement and limited interaction with their host rock, aserpentinized ultramafic microbreccia. To avoid these secondaryeffects, the Sr, Nd and Pb isotopic compositions of mineralsseparated from the xenoliths were measured, yielding 0·704530·70590for 87Sr/86Sr, 3·1 to 0·5 for Nd and 18·92819·063for 206Pb/204Pb. These isotopic compositions are distinctlymore radiogenic for Sr and Pb and less radiogenic for Nd thanthose of altered MORB. Our results suggest that the MORB-likeprotolith of the xenoliths was metasomatized by a fluid equilibratedwith sediment in the fore-arc region of a subduction zone andthat this metasomatic fluid produced continental crust-likeisotopic compositions of the xenoliths. KEY WORDS: Colorado Plateau; eclogite xenolith; geochemistry; subducted oceanic crust |
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Keywords: | : Colorado Plateau eclogite xenolith geochemistry subducted oceanic crust |
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