High-grade metamorphism and partial melting in Archean composite grey gneiss complexes

Abstract

Much of the exposed Archean crust is composed of composite gneiss which includes a large proportion of intermediate to tonalitic material. These gneiss terranes were typically metamorphosed to amphibolite to granulite facies conditions, with evidence for substantial partial melting at higher grade. Recently published activity–composition (a−x) models for partial melting of metabasic to intermediate compositions allows calculation of the stable metamorphic minerals, melt production and melt composition in such rocks for the first time. Calculated P−T pseudosections are presented for six bulk rock compositions taken from the literature, comprising two metabasic compositions, two intermediate/d..