华北东部岩石圈减薄中的下地壳过程:岩浆底侵、置换与拆沉作用

Lower crust processes during the lithosphere thinning in eastern China:magma underplating,replacement and delamination.Acta Petrologica Sinica,

最近研究表明,华北中生代岩石圈减薄不仅是岩石圈地慢减薄,而且下地壳也发生了一定程度的减薄和置换。本文强调下地壳过程,如岩浆底侵、置换和拆沉作用是理解岩石圈减薄机制的关键因素之一。火山岩及其捕虏体的信息似乎支持在华北东部南、北缘存在局部造山带型的下地壳与岩石圈的拆沉作用。但是华北东部整体上的减薄机制难以用造山带的拆沉模式来解释。华北东部克拉通内部的火山岩中的下地壳捕虏体有两类,一类是经历了麻粒岩相变质的底侵辉长岩和辉石岩以及榴辉岩相变质的石榴辉石岩,形成时代约在140～120Ma;另一类是经过了中生代变质叠加的前寒武纪麻粒岩。中生代华北东部曾发生过大规模的岩浆底侵作用,现今的下地壳很可能已大部分不是前寒武纪的下地壳,它们由中生代变质的辉长岩、镁铁质岩石以及经历了很强烈改造的前寒武纪下地壳麻粒岩组成。此外,在华北东部普遍存在着化学成分类似于“埃达克岩”的中生代高锶花岗岩类岩石,它们的形成与岩浆底侵作用和镁铁质下地壳的部分熔融有关,其熔融残留应是榴辉岩或石榴角闪岩。尽管如此,要通过下地壳部分熔融形成一个厚的密度很大的榴辉岩层,由它的重力不均衡来带动80～120公里厚的岩石圈地幔一起拆沉进入软流圈的机制很难发生在华北克拉通内部。岩浆底侵和置换作用是下地壳过程非常重要的形式,与岩石囤的减薄具有密切联系,其具体机制尚不完善。

Our recent study shows that the lower crust was thinned together with lithosphere mantle in eastern North China Block （NCB） at Mesozoic. This paper reveals that lower crust process is a key point to understand tectonic mechanism of lithosphereic thinning. The geochemical data from volcanic rocks and their xenoliths possibly support that a delamination of lower crust and lithosphere mantle occurred in northern and/or southern margins of the NCB only, linking to a post-collisional orogenic process. However, the delamination difficultly happens in the whole of eastern NCB and result in a loss of 80 - 120 km thickness of lithosphere. The lower crust xenoliths in Mesozoic and Cenozoic basahs within the NCB have two kinds. One is granulite-facies metamorphosed gabbros/two-pyroxinenites and eclogite-facies metamorphosed garnet pyroxenites, which formed at 140 - 120Ma. Another kind is Precambrian granulites that were strongly overprinted by Mesozoic metamorphism. A large-scale lower crust replacement took place at Mesozoic beneath the NCB. In other words, the recent lower crust of the NCB is not Precambrian one. Most of the present lower crust is composed of Mesozoic meta-gabbros and pyroxinenites. Otherwise, a lot of late Jurrasic granites in the NCB are similar to adakite in geochemistry, which possibly derived from partial melting of lowermost crust and survived a remnant of eclogite or garnet-amphibolite. A hot upwelling mantle is necessary for replacement of lower crust and partial melting of lowermost crust. Although a certain surviving eclogites could sink into mantle, it is unimaginable there was a thick enough eclogite layer that can drag 80 - 120km thick lithosphere together to founder into asthenosphere beneath the NCB. On the contrary, magma underplating perhaps is one of choosers. Forming of the present lower crust through underplating and its replacement to the old one are closely related to lithosphere mantle thinning. And the both of lower crust thinning/replacement and lithosphere mantle thinning are controlled by a same dynamic mechanism, and are cause and effect by each other.