(Clarke Memorial Lecture, 1983) Restite, xenoliths and microgranitoid enclaves in granites.

Abstract

Microstructural evidence of restite is meagre or absent in high-level metaluminous (I-type) granitoid plutons, and is limited in high-level peraluminous (S-type) granitoids.The microgranitoid enclaves (autoliths, ''cognate xenoliths") that occur abundantly in both metaluminous and peraluminous granitoids are probably formed by quenching of magma in the plutonic environment.The enclaves are best explained by mingling and quenching of globules of more mafic magma in the host granitoid magma, although some may be solid fragments of quenched magma.Several quenching processes are feasible, provided incorporation of partly liquid material is postulated.However, mineralogical and microstructural evidence of hybridism suggests that many microgranitoid enclaves may be the result of magma mixing, either in the same reservoir as the host granitoid or elsewhere.Mingling of magma blobs with the granitoid magma is consistent with all the characteristic features of microgranitoid enclaves.