Problems of local granites

Mantling of alkali feldspar megacrysts through oligoclase (‘rapakivi texture’) usually may be interpreted as the result of magma blending, despite the fact that decompression is a possible interpretation, particularly for excessive-degree intrusions. Coexistence of mantled and unmantled crystals can be defined with the aid of transfer of mantled crystals (‘antecrysts’) from a mixed (hybrid) rock to a bunch granitoid devoid of mantled crystals, as an instance, through disintegration of microgranitoid enclaves. Processes able to explaining more than one oligoclase shells include repeated boom and release of volatiles, and repeated replenishment via more mafic magma. The shells will be shaped with the aid of transfer of megacrysts into and out of a magma-mixing region for the duration of glide in dyke-like conduits or within the fronts of mafic flows transferring across cumulate layers in plutons. Ovoid megacrysts, which arise particularly in Proterozoic rapakivi granitoids, are tough to interpret however are higher explained by means of boom procedures than via magmatic corrosion. The common presence of easy twinning, partial crystal faces, euhedral plagioclase inclusions and granophyre-like intergrowths with quartz favours regular magmatic increase. The common ovoid shapes with neighborhood aspects may want to replicate incomplete development of crystal faces, attributable to exceedingly rapid boom. Granophyre-like intergrowths in the ovoids, local granophyre going on as megacryst rims and inside the groundmass, and the common presence of miaroles advocate increase of the ovoids at notably shallow intensity, at conditions of not on time nucleation and consequent undercooling, due to accumulation and retention of fluid. Development of the ovoids is impartial of plagioclase mantling.

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I thank Roberto Dall’Agnol, Scott Paterson, Simon Turner and an nameless referee for helpful feedback, Lawford Anderson and Matthias Bräunlich for providing images of ovoid granitoids, and Simon Turner for imparting snap shots and skinny-sections of the Mannum Granite, as well as discussions inside the discipline. I additionally thank Dave Duncan for generous get entry to to his PhD thesis and skinny-sections, Ralph Bottrill (Mineral Resources Tasmania) and Izzy von Lichtan, (University of Tasmania) for help with samples and pix, and Jason Gittany of Gitani Stone, Seven Hills, New South Wales, for donating an first-rate polished slab of Finnish wiborgite. I thank Geoff Clarke, for get entry to to photographic gadget, and explicit my appreciation of Ilmari Haapala and colleagues, for showing me outcrops of the Finnish rapakivi granitoids.