Rapakivi granite problems: plagioclase mantles and ovoid megacrysts

R. H. Vernon*

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    14 Citations (Scopus)

    Abstract

    Mantling of alkali feldspar megacrysts by oligoclase (‘rapakivi texture’) generally can be interpreted as the result of magma mixing, although decompression is a viable interpretation, especially for high-level intrusions. Coexistence of mantled and unmantled crystals can be explained by transfer of mantled crystals (‘antecrysts’) from a mixed (hybrid) rock to a host granitoid devoid of mantled crystals, for example, by disintegration of microgranitoid enclaves. Processes capable of explaining multiple oligoclase shells include repeated increase and release of volatiles, and repeated replenishment by more mafic magma. The shells could be formed by transfer of megacrysts into and out of a magma-mixing zone during flow in dyke-like conduits or in the fronts of mafic flows moving across cumulate layers in plutons. Ovoid megacrysts, which occur especially in Proterozoic rapakivi granitoids, are difficult to interpret but are better explained by growth processes than by magmatic corrosion. The common presence of simple twinning, partial crystal faces, euhedral plagioclase inclusions and granophyre-like intergrowths with quartz favours normal magmatic growth. The common ovoid shapes with local facets could reflect incomplete development of crystal faces, owing to relatively rapid growth. Granophyre-like intergrowths in the ovoids, local granophyre occurring as megacryst rims and in the groundmass, and the common presence of miaroles suggest growth of the ovoids at relatively shallow depth, at conditions of delayed nucleation and consequent undercooling, resulting from accumulation and retention of fluid. Development of the ovoids is independent of plagioclase mantling.

    Original languageEnglish
    Pages (from-to)675-700
    Number of pages26
    JournalAustralian Journal of Earth Sciences
    Volume63
    Issue number6
    DOIs
    Publication statusPublished - 17 Aug 2016

    Keywords

    • decompression
    • granitoid
    • magma mixing
    • mantled xenocryst
    • microgranitoid enclave
    • ovoid
    • rapakivi

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