喜马拉雅造山带两种不同类型榴辉岩与印度大陆差异性俯冲

Two Contrasting Eclogite Types in the Himalayan Orogen and Differential Subduction of Indian Continent

印度与亚洲大陆新生代碰撞-俯冲形成的喜马拉雅造山带核部由高压和超高压变质岩组成.超高压榴辉岩分布在喜马拉雅造山带西段,由石榴石、绿辉石、柯石英、多硅白云母、帘石、蓝晶石和金红石组成.超高压榴辉岩的峰期变质条件为2.6～2.8GPa和600～620℃,其经历了角闪岩相退变质作用和低程度熔融.超高压榴辉岩的进变质、峰期和退变质年龄分别为～50Ma、45～47Ma和35～40Ma,指示一个快速俯冲与快速折返过程.高压榴辉岩产出在喜马拉雅造山带中-东段,由石榴石、绿辉石、多硅白云母、石英和金红石组成.高压榴辉岩的峰期变质条件为>2.1GPa和>750℃,叠加了高温麻粒岩相退变质作用与强烈部分熔融.高压榴辉岩的峰期和退变质年龄可能分别是～38 Ma和14～17 Ma,很可能经历了一个缓慢俯冲与缓慢折返过程.喜马拉雅造山带两种不同类型榴辉岩的存在表明,印度与亚洲大陆约在51～53Ma碰撞后,印度大陆地壳的西北缘陡俯冲到了地幔深度,导致表壳岩石经历了超高压变质作用,而印度大陆地壳的东北缘平缓俯冲到亚洲大陆之下,导致表壳岩石经历了高压变质作用.

The core of the Himalayan orogen, resulting from the Cenozoic collision between the Indian and Asian continents, consists of high-pressure (HP) and ultrahigh-pressure metamorphic rocks. The ultrahigh-pressure (UHP) eclogites occur in the western segment of the Himalayan orogen, and contain garnet, omphacite, coesite, phengite, zoisite/epidote, kyanite and rutile. The UHP eclogites record a peak metamorphic condition of 2.6-2.8 GPa and 600-620℃, and a late stage of amphibolite-facies retrogression and slight partial melting. The prograde, peak and retrograde metamorphic times of the UHP eclogites are^50 Ma,~45-47 Ma and^35-40 Ma, respectively, indicating that the UHP eclogites underwent a rapid subduction and rapid exhumation. The HP eclogites occur in the east-central segment of the Himalayan orogen, and contain garnet, omphacite, phengite, quartz and rutile. The HP eclogites have a peak metamorphic condition of > 2.1 GPa and > 750℃, and experienced a late stage of granulite-facies retrogression and extensive anataxis. The peak and retrograde metamorphic times of the HP eclotites are^38 Ma and^14-17 Ma, respectively, indicating that the HP eclogites underwent a slow subduction and slow exhumation. The presence of two contrasting eclogite types in the Himalayan orogen shows that, after the India collided with Asia at around 51-53 Ma, the north-western margin of Indian continental crust deeply subducted into the mantle, and underwent UHP metamorphism, and while the north-eastern margin of Indian continental crust shallowly subducted beneath the Asian continent, and experienced HP metamorphism.