Issue 3: 1996

Abstract

The geochemical evolution of anorthosite residual magmas in the Laramie anorthosite complex, Wyoming

  • The geochemical evolution of anorthosite residual magmas in the Laramie anorthosite complex, Wyoming

    • JN. Mitchell, JS. Scoates2, CD. Frost1 and A. Kolker3 1Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071, USA, 2Departement des Sciences de la Terre et de L'Environnement, Universite Libre de Bruxelles Cp160/02, Avenue F.D. Roosevelt 50, B-1050 Brussels, Belgium, 3Department of Geology, University of Nebraska, Lincoln, NE 68588, USA and To whom correspondence should be addressed at: Ceramic Science and Technology, Mail Stop K762, Los Alamos National Laboratory, Los Alamos, NM 87545, USA

    ABSTRACT

    Olivine- and pyroxene-bearing Fe-enriched dioritic rocks in the 1434 Ma Laramie anorthosite complex are interpreted to represent variably fractionated and contaminated magmas residual after the crystallization of anorthosite. Geochemical characteristics of this suite include the following: high contents of TiO2, Fe2OT3, and P2O5; high incompatible trace element contents; rare earth element patterns with a large range of Eu anomalies; and isotopic compositions that reflect the geographic location of individual samples, with ISr increasing and [epsilon]Nd decreasing from south to north. After extraction from anorthosite, fractionation of ferrodioritic residual magmas resulted in secondary residual monzodioritic melts and complementary oxide-rich ferrodiorite cumulates. Geographic trends in isotopic composition reflect an increasing Archean crustal component from south to north. Dioritic dikes and cumulates with isotopic compositions similar to associated anorthosites were derived locally. Large isotopic discrepancies between some diorites and their hosting anorthosites reflect preferential contamination of residual magma during ascent and emplacement of mantle-derived plagioclase-rich diapirs, followed by subsequent extraction and isolation of Fe-enriched interstitial melt. Strong isotopic contrasts between anorthosite and associated Fe-enriched rocks in anorthosite complexes do not preclude a direct relationship between them and reflect the diversity and complexity of processes during their petrogenesis.

    Keywords: anorthosite; ferrodiorite; geochemistry; Laramie anorthosite complex; residual magma

    Pages: 637 - 660

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