GEOCHEMICAL CONSTRAINTS ON LEUCOGRANITE MAGMATISM IN THE LANGTANG VALLEY, NEPAL HIMALAYA

A complex of crustally derived leucogranitic sills emplaced into sillimanite-grade psammites in the upper Langtang Valley of northern Nepal forms part of the Miocene High Himalayan granite association. A series of post-tectonic, subvertical leucogranitic dykes intrude the underlying migmatites, providing possible feeders to the main granite sills. The leucogranite is peraluminous and alkali-rich, and can be subdivided into a muscovite-biotite and a tourmaline-muscovite facies. Phase relations suggest that the tourmaline leucogranites crystallized from a water-undersaturated magma of minimum-melt composition at pressures around 3-4 kbar. Potential metasedimentary protoliths include a substantial anatectic migmatite complex and a lower-grade mica schist sequence. Isotopic constraints preclude the migmatites as a source of the granitic melts, whereas trace-element modelling of LILEs (Rb, Sr, and Ba), together with the Nd and Sr isotopic signatures of potential protoliths, strongly suggest that the tourmaline-bearing leucogranites have been generated by fluid-absent partial melting of the muscovite-rich schists. However, REE and HFSE distributions cannot be reconciled with equilibrium melting from such a source. Systematic covariations between Rb, Sr, and Ba can be explained by variations in protolith mineralogy and P-T-a(H2O). Tourmaline leucogranites with high Rb/Sr ratios represent low-fraction melts (F approximately 12%) efficiently extracted from their protoliths under conditions of low water activity, whereas the heterogeneous two-mica granites may result from melting under somewhat higher a(H2O) conditions. The segregation of low-degree melts from source was probably by deformation-enhanced intergranular flow and magma fracturing, with the mechanisms of migration and emplacement controlled by variations in the upper-crustal stress regime during late-orogenic extensional collapse of the thickened crust.