摘要
The Himalayan orogen formed in response to ongoing continental convergence between the India and Asia that began as ca. 55Ma,and is the largest active orogen in existence and provides a nature laboratory for studying plate tectonics. High-pressure ( HP) and ultrahigh-pressure ( UHP) metamorphic rocks occur in the core segment of Himalayan orogen. The formation conditions, times and processes of these HP and UHP rocks provide important constraints on the geometry,kinematic and dynamic evolution of the India-Asia collisional zone. In the western Himalayan orogen,coesite-bearing UHP rocks formed during the early stage of steep subduction of the Indian northern margin at the time of the Paleocene to Eocene boundary ( 53 ～ 46Ma) . These UHP rocks experienced the rapid exhumation with a duration time of ca. 5Myr from the mantle depth of > 100km to the middle crustal level,resulted in the amphibolite-facies retrograde metamorphism. In contrast,in the central Himalayan orogen,HP eclogites formed by the shallower subduction of the Indian continent at ca. 45Ma. These HP rocks experienced a slow exhumation with a duration time of up to 20Myr from the lower crust to the middle-upper crust level,resulted in the overprinting of granulite-facies metamorphism and partial melting. Similarly,in the eastern Himalayan orogen,HP granulites formed by the shallower subduction of the Indian continent at ca. 35Ma. These HP rocks also experienced a slow exhumation with a duration time of up to 20Myr,resulted in the overprinting of granulite-facies retrograde metamorphism and multistage of partial melting. Therefore,the metamorphism of whole Himalayan orogen shows distinct temporal and spatial change,indicating difference of the continental subduction and exhumation processes,and diverse of the mechanism of continent-continent collisional orogeny.