MIGMATITES AND LEUKOGRANITES OF TERTIARY AGE FROM THE HIGH HIMALAYAN CRYSTALLINS OF ZANSKAR (NW INDIA) - A CASE-HISTORY OF ANATEXIS OF PALEOZOIC ORTHOGNEISSES

In the High Himalayan Crystallines of Zanskar (NW India), migmatites and peraluminous leucogranitic melts were produced by partial melting not only of paragneisses but also of Palaeozoic orthogneisses. Anatexis occurred at T = 650-720-degrees-C and P = 4-7 kbar and is related to a decompression path at increasing T of Oligocene-Miocene age. Under vapour-absent conditions anatexis of orthogneisses occurred in response to dehydration melting involving muscovite and produced very low amounts (almost-equal-to 5%) of leucogranitic melt. This melt segregated "in situ" as homogeneous leucoanatexites. In heterogeneous diatexites, the leucosomes formed by disequilibrium melting probably at T in excess of the H2O-saturated solidus. Extensive anatexis and melt segregation into dykes or bodies require infiltration of an aqueous fluid. A model is proposed in which large rock-volumes showing low-degrees of vapour-absent melting are associated with ductile shear zones infiltrated by H2O and showing high-degrees of vapour present melting. With respect to the eastern Himalayas, the relative scarcity of leucogranites in Zanskar depends on: 1- the lack of a high-T, low- P stage; 2- the abundance of dry igneous (i.e. orthogneiss) protoliths relative to more fertile metasedimentary magma sources.