Geochronological enigma of the HP–UHP rocks in the Himalayan orogen

Abstract

Abstract The subduction of Indian plate lithosphere during its collision with Asian plate in the Eocene resulted in a regional metamorphic belt along the strike of the Himalayan orogen. High-/ultrahigh-pressure (HP/UHP) metamorphic rocks (eclogites and host gneisses) confirm the metamorphic event in western Himalaya (Kaghan c. 46 Ma and Tso Morari at c. 47 Ma) at mantle depths (>90 km: coesite-stable). In contrast, HP/UHP rocks have not been reported from central and eastern Himalaya and only highly retrogressed eclogites and granulites ( c. 25 to 13 Ma) occur. The presence of UHP rocks in western Himalaya and highly retrogressed eclogites and granulites in central and eastern Himalaya was regarded as evidence for a diachronous India–Asia collision. Despite the along-strike regional homogeneity in major lithotectonic units of the Himalayan orogen, metamorphic diachroneity is enigmatic. It is unlikely to have a subduction-related prolonged progressive metamorphic event. In contrast, the age difference and preservation of UHP phases in the west and their transformation into granulites in central and eastern Himalaya could be associated with their prolonged residence times at crustal levels in the central and eastern Himalaya whereas the rocks exhumed rapidly in the west. The higher thermal events relating to melting of the subducting Indian lithosphere in central and eastern Himalaya evidenced from ultra-potasic volcanics in southern Tibet probably decompressed the early metabasites into granulitized eclogites, even resetting their geological clock, which is why eclogites and granulites in the east show younger ages compared with their UHP counterparts in the west.