Geochemical Constraints on Leucogranite Magmatism in the Langtang Valley, Nepal Himalaya

Publisher Website
Google Scholar
Abstract
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–aH2O. Tourmaline leucogranites with high Rb/Sr ratios represent low-fraction-melts (F˜ 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 aH2O 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 uppercrustal stress regime during late–orogenic extensional collapse of the thickened crust.

This publication has 0 references indexed in Scilit: