K-feldspar megacrysts in granites - Phenocrysts, not porphyroblasts

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    Abstract

    K-feldspar megacrysts in granitoid plutons have been interpreted as either phenocrysts or porphyroblasts. Most of the microstructural, mineralogical and chemical evidence (e.g., shape, alignment, concentration, Ba content, zoning, inclusions, and twinning) favours a phenocryst origin. The main features that have been used to support a porphyroblast origin are occurrence of megacrysts: (1) across aplite vein boundaries, (2) in country rocks, and (3) in or across boundaries of microgranitoid enclaves (mafic inclusions). However, these features can be explained by the phenocryst hypothesis. In particular, megacrysts in microgranitoid enclaves can be explained by growth or mixing in magma before a globule of that magma or a fragment of the resulting igneous rock was incorporated as an enclave. All available evidence favours or is consistent with a phenocryst origin for K-feldspar megacrysts in granitoid rocks and their enclaves. The large size of the megacrysts is evidently due to nucleation difficulties for K-feldspar in granitic melts. Though K-feldspar is commonly the last mineral to begin crystallizing in granitic magmas, abundant melt is still present at that stage, allowing sufficient space for the megacrysts to grow. The reason for the common lack of megacrysts in volcanic rocks may be that the phenocrysts do not grow large enough to be called "megacrysts" until the magma contains such a high proportion of crystals that it cannot erupt.

    Original languageEnglish
    Pages (from-to)1-63
    Number of pages63
    JournalEarth Science Reviews
    Volume23
    Issue number1
    DOIs
    Publication statusPublished - 1986

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