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

The petrologic study of cordierite-bearing anatectic rocks is particularly relevant for the understanding of low-pressure anatectic processes in the continental crust. This paper focuses on the cordierite-bearing lithologies occurring at the higher structural levels of the Higher Himalayan Crystallines (eastern Nepal Himalaya). Three cordierite-bearing gneisses (samples 09-29, 07-26 and 05-17) from different geological transects in Eastern Nepal (from mount Everest to Kangchenjunga) are studied in detail. In all the studied samples cordierite is spectacularly well preserved. The three samples differ in terms of bulk composition, likely reflecting different sedimentary protoliths, though they all consist of quartz, K-feldspar, plagioclase, biotite, cordierite and sillimanite in different modal percentages. Garnet occurs in two samples, as a relict phase in very low modal amounts. A detailed petrographic study allows to recognize and interpret the relevant microstructures related to melt production and/or melt consumption and to distinguish between peritectic and cotectic cordierite. The P-T evolution of the studied samples is reconstructed combining microstructural observations, mineral chemical data and pseudosection modeling with the main aims of: (i) investigating the melt productivity, at low pressure conditions, of different prograde assemblages (from two-micas metapelite to biotite-metapelite), evaluating the effects of muscovite vs. biotite de-hydration melting on both mineral assemblages and microstructures, and (ii) understanding why the studied samples lack of retrograde re-hydration, showing spectacularly well preserved cordierite crystals. The results of the pseudosection modeling suggest that: (i) 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, being the two-mica metapelitic protoliths significantly more fertile at any given temperature than biotite gneisses over the same temperature interval; and (ii) 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 15-18 bar/°C.