The cordierite-bearing anatectic rocks of the higher Himalayan crystallines (eastern Nepal): low-pressure anatexis, melt productivity, melt loss and the preservation of cordierite

Cordierite-bearing anatectic rocks inform our understanding of low-pressure anatectic processes in the continental crust. This article focuses on cordierite-bearing lithologies occurring at the upper structural levels of the Higher Himalayan Crystallines (eastern Nepal Himalaya). Three cordierite-bearing gneisses from different geological transects (from Mt Everest to Kangchenjunga) have been studied, in which cordierite is spectacularly well preserved. The three samples differ in terms of bulk composition likely reflecting different sedimentary protoliths, although they all consist of quartz, alkali feldspar, plagioclase, biotite, cordierite and sillimanite in different modal percentages. Analysis of the microstructures related to melt production and/or melt consumption allows the distinction to be made between peritectic and cotectic cordierite. The melt productivity of different prograde assemblages (from two-mica metapelite/metagreywacke to biotite-metapelite) has been investigated at low-pressure conditions, evaluating the effects of muscovite v. biotite dehydration melting on both mineral assemblages and microstructures. The results of the thermodynamic modelling suggest that the mode and type of the micaceous minerals in the prograde assemblage is a very important parameter controlling the melt productivity at low-pressure conditions, the two-mica protoliths being significantly more fertile at any given temperature than biotite gneisses over the same temperature interval. Furthermore, the cordierite preservation is promoted by melt crystallization at a dry solidus and by exhumation along P-T paths with a peculiar dP/dT slope of about 1518bar degrees C1. Overall, our results provide a key for the interpretation of cordierite petrogenesis in migmatites from any low-P regional anatectic terrane. The cordierite-bearing migmatites may well represent the source rocks for the Miocene andalusite-bearing leucogranites occurring at the upper structural levels of the Himalayan belt, and low-P isobaric heating rather than decompression melting may be the triggering process of this peculiar peraluminous magmatism.