喜马拉雅造山带的高压超高压变质作用与印度-亚洲大陆碰撞

喜马拉雅造山带是印度与亚洲大陆碰撞作用的产物,正在进行造山作用,是研究板块构造的天然实验室.高压和超高压变质岩分布在喜马拉雅造山带的核部.这些变质岩具有不同的形成条件、形成时间和形成过程,为印度与亚洲碰撞带的几何学、运动学和动力学提供了重要的限定.含柯石英的超高压变质岩产出在喜马拉雅造山带的西段,它们形成在古新世与始新世之间(53～46Ma),为印度大陆西北边缘高角度超深俯冲作用的产物,并经历了快速俯冲与快速折返过程.在约5 Myr内,超高压变质岩从＞100km的地幔深度折返到了中地壳深度,且仅仅叠加角闪岩相退变质作用.高压榴辉岩产出在喜马拉雅造山带中段,形成时间约为45Ma,为印度大陆低角度深俯冲作用的产物,经历了至少20Myr的长期折返过程,叠加麻粒岩相退变质作用和部分熔融.高压麻粒岩产出在喜马拉雅造山带的东端,是印度大陆东北缘近平俯冲作用的产物,峰期变质作用时间约为35Ma,经历了约20Myr的长期折返过程,叠加了麻粒岩相和角闪岩相退变质作用,并伴随有多期部分熔融.因此,喜马拉雅造山带的变质作用具有明显的时间与空间变化,显示出大陆深俯冲与折返过程的差异性,以及大陆碰撞造山带形成机制的多样性.

Indian and Asian continental collision viewed from HP and UHP metamorphism of the Himalayan orogen

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.