Polymodal occurrence of early mafic differentiate associated with mid-proterozoic calc-alkaline plutons of meghalaya

摘要

Early mafic differentiate of quartz diorite to quartz monzodiorite composition related to Mid-Proterozoic, late- to post-tectonic calc-alkaline (diorite-granodiorite) plutons in Meghalaya occur in three different modes, viz. (1) dykes in country rock gneisses along tectonic locales, (2) endogenous inclusions in the diorite-granodiorite intrusive plutons, as early differentiated product through fractional crystallization or liquid immiscibility, and (3) xenoliths in Upper Proterozoic-Early Palaeozoic granite plutons (Nonpoh granite). These appear to be basic precursors of the granitoids. The mafic inclusions were subjected to metasomatism by granitoids over an extended period starting with the formation of the granitic differentiates of calc-alkaline intrusions and ending with the placement of the later granitic plutons. Petrology and chemistry of the matic rocks suggest a possible basic parentage.