Partial Melting of Aluminous Metagreywackes in the Northern Sierra de Comechingones, Central Argentina

Abstract

We describe a suite of metamorphic and migmatitic rocks from the Sierra de Comechingones (Sierras Pampeanas of Central Argentina) that include unmelted gneisses, migmatites and refractory granulites. The gneisses are aluminous greywackes metamorphosed in the amphibolite grade and are likely to have been the protoliths for the higher-grade migmatites and granulites. Mineralogical characteristics and major and trace element compositions show that metatexite migmatites, diatexite migmatites and granulites are all melt-depleted rocks. The migmatites (both metatexites and diatexites) have undergone H2O-fluxed melting and lost ∼20% melt, whereas the granulites have lost as much as 60% melt, formed chiefly by dehydration-melting of biotite. The granulites are rocks from which essentially all the granitic components have been extracted, whereas the migmatites cooled and solidified while still retaining a granitic fraction. The only evidence for the presence of melt in the study area, apart from migmatitic leucosomes, are dikes and sills of leucogranite rich in cumulate K-feldspar. These leucogranite bodies may represent the pathways along which melt was drained from the exposed rocks. Our study shows that in the Sierra de Comechingones both migmatites and granulites represent source regions of anatectic magmas, which were ‘frozen in’ at different stages of development.