Evidence that some microgranitoid enclaves are felsic magma cumulates

R. H. Flood, S. E. Shaw

    Research output: Contribution to journalMeeting abstract

    Abstract

    Granite enclaves. Three main types of enclaves can be recognised
    in granitic rocks. Some, though not abundant, are metamorphic
    rocks (quartzites and gneisses). Some, that can be locally
    abundant, are extremely fine grained igneous rocks, have chilled
    margins, and are, with little doubt, mafic magma sheets or globules
    quenched in the felsic magma. The majority, are microgranitoid
    enclaves (MGE), that have medium grainsize igneous
    microstructures, have no chilled margins and virtually all are as
    mafic or more mafic than the host. While outcrops that record
    the sites of mafic magma injection and quenching have been
    well-documented, many enclaves of this type are commonly found
    in outcrop in association with the other two types as disordered
    accumulations. The site of enclave accumulation (that can include
    all three types) appears unrelated to the position in the magma
    chamber where they formed, and are therefore analogous to palaeontological
    ‘‘death assemblages.’’
    MGE as cumulates from felsic magmas. Observations relevant
    to the origin of MGE include: MGE in S-type granites are generally
    S-type; in general 87Sr/86Sr initial ratios are similar to the
    host; generally the MGE have the same minerals as the host but
    in different proportions; some have minerals that could not have
    crystallised from a magma of the bulk composition of the MGE;
    many MGE have epsilon 176Hf/177Hf values lower than the host
    [2]; some have abundant phenocrysts that are the same size and
    composition as those of the host or are minerals that might be
    expected to be near-liquidus phases in a magma of that composition.
    The recognition that many granitic plutons have significant
    isotopic variation, and that in at least one zoned pluton, it is the
    felsic core that is the most isotopically primitive leads to the suggestion
    that primitive isotopic ratios do not necessarily mean mafic
    compositions. It is argued that many MGE are not thermally
    quenched mafic magma globules but are cumulates formed near
    the roof of the magma chamber as rafts of crystals that heterogeneously
    nucleated on phenocrysts during pressure quench events.
    Original languageEnglish
    Pages (from-to)A178-A178
    Number of pages1
    JournalGeochimica et Cosmochimica Acta
    Volume70
    Issue number18 supplement
    DOIs
    Publication statusPublished - 2006
    EventGoldschmidt Conference (16th : 2006) - Melbourne, Australia
    Duration: 27 Aug 20061 Sep 2006

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