Microgranitoid enclaves in granites—globules of hybrid magma quenched in a plutonic environment

Abstract

Microgranitoid enclaves (autoliths, cognate xenoliths, mafic inclusions) are common in high-level granitoid plutons. They have relatively fine-grained igneous (microgranitoid) microstructures1, and many show evidence of having flowed in a magmatic condition2. These features counter interpretations that the enclaves represent transformed solid fragments of diverse wall rocks3–5 or restite6–11. An alternative interpretation is that the enclaves represent globules of mafic magma that have mingled (‘commingled’) and quenched in the granitoid host magma12–16. A detailed review of the literature, occurrence, morphology and composition of micro-granitoid enclaves strongly supports this view, hut indicates that the enclave magmas range in composition from mafic to felsic, and that magma-mixing may be involved in their formation17. I summarize here the main features of microgranitoid enclaves and briefly outline the possible processes leading to their incorporation as magma globules in the host granitoid.