Mesoscopic structures resulting from crystal accumulation and melt movement in granites

Abstract

Several mesosocopic structures are consistent with mechanical accumulation of crystals and movement of melt in granite magmas, as well as compaction and shear of crystal-melt aggregates, concentrations of microgranitoid enclaves indented by megacrysts, and concentrations of crystals of the same mineral with different crystallisation histories. Evidence for crystal and enclave accumulation is shown clearly in mafic and silicic layered intrusions (MASLI-type granite plutons), for example, the Kameruka Granodiorite, Bega Batholith, south-eastern Australia.

Crystal accumulations with interstitial liquid may become mobile in a magma chamber, owing to instabilities in the host magma caused by seismic and replenishment events or thermal and buoyancy variations. This remobilised material may intrude other parts of the chamber, as well as earlier-formed cumulates and even wall-rocks, as dykes, tubes, troughs and pipes. Marked concentrations of accessory and mafic minerals may also develop in these structures. Interstitial melt may also be extracted from accumulated aggregates, intruding and disrupting the aggregates. Spectacular examples of these various structures are preserved in the Tuolumne Batholith, California. Detailed mechanisms for the formation of many of the structures are not well understood, though the formation of cumulates in vertical layers suggests that sorting and filter pressing during flow and resulting strain of crystal mushes may play important roles.