Crystallization and hybridism in microgranitoid enclave magmas: Microstructural evidence

Abstract

Microstructural evidence indicates that many microgranitoid enclaves (“mafic inclusions”) in granitoids crystallize from globules of magma that have undergone previous magma mixing (hybridism). The evidence includes (1) rounded xenocrysts (“ocelli”) of quartz rimmed with fine‐grained aggregates of early formed minerals, especially mafic minerals; (2) K‐feldspar megacrysts, commonly rimmed with plagioclase (rapakivi structure); and (3) corroded cores, dendritic overgrowths, and calcic “spikes” in plagioclase. Initial quenching of a magma globule in felsic magma involves rapid temperature equilibration, which causes a high rate of nucleation of minerals precipitating from the enclave magma at the time, followed by a period of slow cooling, during which the nucleation and growth rates of later‐crystallizing minerals are similar to the much slower rates prevailing in the granitic melt. This may result in markedly poikilitic microstructures, involving inclusion of the earlier‐formed, finer‐grained minerals in quartz and/or K‐feldspar, which are common in microgranitoid enclaves.