TIMING AND SOURCE OF MELTING AT THE EASTERN EDGE OF THE GURLA MANDHATA CORE COMPLEX, NW NEPAL HIMALAYA

Continent-continent collision leads to crustal thickening and enhanced heat production, which generate regional metamorphism, crustal melting, and localized mid-crustal weakening. This crustal melting can take the form of migmatites, felsic dyke networks, and large granitic plutons, which are all superbly exposed along the Chuwa Khola at the eastern edge of the Gurla Mandhata metamorphic core complex in the NW Nepal Himalaya.

The Chuwa Khola exposes stromatic metatexite, calc-silicate gneiss, augen orthogneiss, and schlieren-structured diatexite, all crosscut by a leucogranitic dyke and sill network that appear to culminate in the Chuwa granite at the structurally highest level. Distributed shear-sense indicators are consistent with top-to-the-southwest ductile extrusion of the metamorphic core observed elsewhere in the Himalaya. Geochemical analyses indicate the three melt types are peraluminous and granitic in composition. Rare earth element contents for all samples show a strong negative Eu anomaly, suggesting a crustal source of melting. In situ U-Th/Pb monazite petrochronology conducted on the Chuwa granite yields a mean crystallization age of ~18 Ma, with no ages younger than 16.5 ± 0.6 Ma. The dyke and sill network also yields a mean crystallization age of ~18 Ma, with a minimum age of 15.8 ± 0.6 Ma. While the southernmost migmatite samples yield similar migmatization age (~18 Ma), they also contain monazite ages as young as 12.8 ± 0.5 Ma. Although most analyzed monazite grains are not zoned, Y content of the monazite in the migmatite increases with decreasing age, suggesting garnet breakdown and retrograde metamorphism at ~15 Ma.

The dyke and sill network is interpreted as a feeder system to the overlying Chuwa granite, based on their similar ~18 Ma age and geochemical signature. The migmatization is interpreted to have occurred subsequently, as young as ~13 Ma, as a result of orogen-parallel extension at ~15-13 Ma. It is therefore suggested that the dyke and sill network, and the coeval Chuwa granite were emplaced during southward-directed crustal extrusion prior to orogen-parallel extension, while the Chuwa Khola migmatites record the transition from orogen perpendicular extrusion of the Himalayan metamorphic core to orogen parallel extensional exhumation at ~15-13 Ma.