Chemical, petrological and mass balance constraints on the textural evolution of pelitic enclaves

Abstract

Reaction histories in xenoliths are commonly based on textural analysis alone and are often highly disputed. In this paper we propose a more rigorous approach that combines textural analysis, reaction balance calculations and quantitative P– T pseudosections using model reactions. Examples highlighting the power of this approach are: disequilibrium behaviour of plagioclase in a partial melting reaction; and the relation between biotite–plagioclase intergrowths replacing garnet and biotite patches overgrowing ilmenite. An important message is that textural analysis in xenoliths, and probably also in other rock types, must be complemented by chemical (compositions and reaction balance) and petrological (coherent P– T projections or pseudosections) constraints to distinguish between alternative generic possibilities. This method sheds new light on a discussion on the melt volume produced during biotite dehydration melting in migmatites.