Thermal evolution of the Greater Himalaya, Garhwal, India

The hanging wall of the Main Central Thrust (MCT) in Garhwal, India (roughly 79°N–80°E; 30°N–31°N), exhibits an inverted metamorphic gradient: sillimanite ± potassium feldspar assemblages near the top of the hanging wall, or Greater Himalayan sequence, are underlain by kyanite grade rocks near the fault. Textural relationships in pelitic samples from the Alaknanda and Dhauli river valleys indicate that the “inversion” is the product of two distinct metamorphic events: an early Harrovian event (M1), which affected the entire Greater, Himalayan sequence and a later Buchan event (M2), the effects of which are most obvious in the upper part of the sequence. Rim thermobarometry, garnet inclusion thermobarometry, and thermodynamic modeling of garnet zoning reveal that the basal portions of the metamorphic sequence experienced peak M1 conditions of >900 K and >960 MPa (roughly 36 km depth) before following an “erosion controlled” uplift path (e.g., England and Richardson, 1977). M2 metamorphic temperatures in the upper part of the sequence also exceeded 900 K, but maximum pressures (317–523 MPa) indicate paleodepths of only 12–19 km. Calculated pressure-temperature paths indicate that M2 was characterized by temperature increases of >80 K and roughly 5 km of tectonic burial We attribute M1 to tectonic burial of the Greater Himalayan sequence during the early stages of India-Eurasia collision. We believe that the uplift and cooling path of the sequence was interrupted in late Oligocene(?) - Miocene time by a second burial and heating event (M2) related to thrust imbrications in southern Tibet. This burial was coincident with the generation of leucogranites, which are abundant near the top of the Greater Himalayan sequence but are virtually absent near the MCT. Field relations do not constrain whether the leucogranites were derived from some presently unexposed portion of the Greater Himalayan sequence and were injected at their present structural level, or were melted in situ. If the granites were injected, then they may have provided some of the heat necessary for M2 metamorphism. Although our data suggest no direct relationship between the Main Central Thrust (as mapped in Garhwal) and metamorphism in the Greater Himalaya, anatectic melting of an unexposed portion of the Greater Himalayan sequence could have been associated with movement along a blind thrust with characteristics similar to the mapped MCT in central Nepal (cf. Le Fort, 1981). If the granites were produced by in situ M2 melting, then we must appeal to a heat source within the upper part of the Greater Himalayan sequence such as locally high concentrations of heat-producing elements (cf. Pinet and Jaupart, 1987).