Temperature and age constraints on the metamorphism of the Tethyan Himalaya in Central Nepal: A multidisciplinary approach

Metasediments of Devonian to Triassic age of the Tethyan Himalaya (TH) from several areas in central and western Nepal, between western Dolpo and Marsyandi Valley, were sampled for thermo-metamorphic studies (illite and chlorite “crystallinity”, vitrinite reflectance, calcite–dolomite and chlorite-chloritoid geothermometers, K/Ar dating on illite-rich fractions and zircon fission track thermochronology). This paper reports, for the first time, the occurrence of chloritoid in the TH out of the range of granite contact aureoles. It also presents the first zircon fission track dating performed on TH metasediments. The peak temperatures of metamorphism have been around 250–300 °C, 320–350 °C, 330–370 °C and 400–450 °C in the western Dolpo, Hidden Valley, Manang and Marpha areas, respectively. In the Manang and Hidden Valley areas, illite K/Ar data are interpreted as ages of recrystallized K-white micas newly formed during metamorphism at around 25–30 Ma. In the Marpha area, illite K/Ar and zircon fission-track ages (12–15 Ma) are consistent representing cooling ages after metamorphic overprinting of higher grade than in the other areas. The joint investigation of the organic maturation and zircon FT chronology yields insight on the thermal calibration of zircon reset. The Rmax of 5.7% to ∼8.0% indicates a temperature range of ca. 315–325 °C which is in the partial annealing zone of the zircon FT thermochronometer. Our results might be explained by the presence of a thick thrust sheet once existing above the study area.