Causes and consequences of protracted melting of the mid-crust exposed in the North Himalayan antiform

Abstract

Gneiss domes exposed by the North Himalayan antiform in southern Tibet provide evidence for protracted melting of the mid-crust during convergent tectonics. Anatexis was initiated during underthrusting and recurred during the southwards extrusion of a low-viscosity crustal sheet. Sr–Nd isotope systematics establish that the cores of the domes, dominated by Cambrian gneiss, form a window of the High Himalayan Crystalline Series (HHCS; Indian Plate). Combined trace-element and isotopic analyses from 4 two-mica granites that intrude both core and the metasedimentary mantle of the domes indicate an origin from melting of the High Himalayan Crystalline Series by the incongruent breakdown of muscovite and, to a lesser extent, biotite, at temperatures from 680 to 760 °C. U–Pb dating of accessory phases extends the known period of granite magmatism from the Late Oligocene to Mid-Miocene. A Ca-rich, garnet-bearing leucogranite of Oligocene age (27.5 Ma) provides a rare insight into the pre-Miocene history of the underthrust Indian plate, reflecting prograde melting at greater depths during a period of crustal thickening. The origin of the Miocene granites (15–10 Ma) can be modeled from the exhumation of an extruding channel at a rate of ca. 1 mm/year, constrained by Ar-isotope data. Melting over a protracted time scale along the leading edge of the Indian Plate, less than 50 km from the suture, accelerated horizontal shortening within the collision zone and provided a positive-feedback mechanism between magmagenesis and deformation.