大别超高压变质岩折返过程中的部分熔融作用

PARTIAL MELTING IN EXHUMATION OF ULTRAHIGH PRESSURE METAMORPHIC ROCKS, DABIESHAN MOUNTAINS, CHINA

研究表明，在大别超高压变质岩的折返过程中，超高压变质岩一方面要经受中下地壳所特有的角闪岩相退变质作用，另一方面要在合适环境下发生部分熔融作用．超高压变质岩在中下地壳的减压退变和部分熔融作用，是一个重要的物理化学过程，它可以降低岩石的强度，提高岩石的可变性，促进挤压（ 碰撞） 体制向伸展体制的转变．超高压变质岩折返过程减压退变和部分熔融作用的确定，进一步支持了“原地”观点．它不仅说明了超高压榴辉岩和围岩片麻岩（ 即所谓的超高压片麻岩） 及面理化含榴花岗岩（ 非高压围岩） 之间的演化关系，而且可为正确地建立大别超高压变质岩折返的动力学模型提供科学的依据．一般来说，供给地壳岩石重熔作用的热量不能仅依赖地壳本身，还必须有其他支撑，我们推断由陆陆碰撞所造成的岩石圈增厚及岩石圈的拆沉作用和岩浆底侵作用。

The research shows that the ultrahigh pressure metamorphic rocks in Dabieshan mountains experienced both the retrograde metamorphism of the amphibolite facies characteristics of the middle_lower crust, and the partial melting in the appropriate decompression environment when they were exhumed from the mantle depth to the middle_lower crust. The retrometamorphism and partial melting of the ultrahigh pressure rocks associated with the changes in thermal state in the middle_lower crust, is an important physical and chemical process. This process may result in a large decrease in the integrated yield strength of the rocks, in the enhancement of the deformability of the rocks, and in the promotion of the transition from the squeezing (collision) to extensional regimes. The determination of the retrometamorphism and partial melting of the ultrahigh pressure rocks when the amphibolite facies were exhumed from the mantle depth to the middle_lower crust has further supported the “in_site” model for the exhumation of the ultrahigh pressure rocks in Dabieshan. This determination not only accounts for the evolutionary relationship between the ultrahigh pressure eclogite and wall_rock gneiss (i.e. the so called ultrahigh pressure gneiss) and plane_schistose granite with eclogite (non_high_pressure wall rocks), but also provides us with a scientific basis for the correct establishment of the dynamic model for the exhumation of the ultrahigh pressure metamorphic rocks in the Dabieshan mountains. Generally speaking, purely conductive heat transfer from the crust itself was probably insufficient to provide the temperature conditions required for partial melting, so that additional heat sources must have been added. Therefore, it can be inferred that the partial melting and extensional flow were driven by delamination and magmatic underplating of thickened lithospheric mantle following the continental oblique collision.