GEOCHEMICAL CONSTRAINTS ON THE BIMODAL ORIGIN OF HIGH HIMALAYAN LEUKOGRANITES

Major and trace element and Rb-Sr isotope systematics of the Manaslu leucogranite, Central Nepal, have been examined to constrain the role of mineral fractionation and fluids in peraluminous granite petrogenesis. Biotite and tourmaline are, for the most part, mutually exclusive, with a predominance of two-mica leucogranites over tourmaline leucogranites. The Sr-87/Sr-86 initial isotopic ratios (Sr-i) indicate that leucogranitic melts were derived from two different sources, the two-mica leucogranites having a metagreywacke origin (with Sr-i<0.752 and epsilon Nd < - 15) and the tourmaline leucogranites a metapelitic one (Sr-i>0.752; epsilon Nd> - 15). Such a bimodal nature of the source zone does not directly influence the magmatic evolution, except that probably the higher initial boron content in the metapelitic rocks may increase the Na2O/K2O ratio. In contrast, the amount of water present during melting principally controls in part anatectic processes and element behaviour. Water-saturated conditions probably occured during melting of metagreywackeous rocks and favoured crystallization of two-mica leucogranites whereas water-absent conditions prevailed during melting of metapelitic layers and favoured biotite, plagioclase and monazite fractionation in the source zone and tourmaline crystallization in the leucogranite.