华北克拉通破坏过程中下地壳的改造--捕虏体证据

Phanerozoic modification of the lower crust beneath the North China Craton: constraints from granulite xenoliths

华北克拉通出露的前寒武纪麻粒岩地体和显生宙不同时期火山岩中的麻粒岩捕虏体是地质历史上不同时期下地壳的代表性样品,对它们的对比研究为华北克拉通下地壳的演化提供了重要制约。本文在前人工作基础上,主要从地体麻粒岩和不同时代火山岩中捕虏体麻粒岩的锆石年代学和Hf同位素组成特征角度,指出在华北克拉通岩石圈破坏过程中,不仅岩石圈地幔发生了减薄和改造,岩石圈地幔之上的下地壳也发生了不同程度的减薄和改造,这种改造存在区域上的差异,包括改造的时间、改造的程度等。下地壳的改造方式主要以幔源岩浆的底侵为主,古老地壳物质的重熔也是可能的一种方式。

Granulite terrains exposed on the North China craton are the exhumed Precambrian lower crust, whereas the granulite-facies xenoliths entrained in volcanic rocks are representatives of the Phanerozoic lower crust. The comparative studies of granulite terrains and granulite xenoliths provide important constraints on the evolution of the lower crust beneath the North China Craton. Based on data available on the granulite terrains and granulite xenoliths, this paper discusses the modification of the lower crust beneath the North China Craton during the Phanerozoic mainly from the perspectives of zircon U-Pb ages and their Hf isotopic compositions. The lower crust was also significantly modified in company with the destruction of the mantle lithosphere since the Mesozoic, with the time and degree of modification varying from region to region. Magmatic underplating played the most important role in modifying the pre-existent ancient crust, while the remelting of the ancient crust was probably another way to modify the lower crust.