新疆准噶尔地区花岗岩中微粒闪长质包体特征及后碰撞花岗质岩浆起源和演化

Microgranular dioritic enclaves in Junggar granites and their implications for the origin and evolution of post-collisional granitic magmatism in North Xinjiang

新疆准噶尔地区也布山、庙儿沟两个晚古生代后碰撞准铝过铝质花岗岩体中 ,广泛发育大量的暗色微粒闪长质包体。岩石学、矿物学、主量元素和微量元素地球化学研究表明 ,包体与其寄主岩存在明显的亲缘关系。东准噶尔也布山黑云母花岗岩体中的暗色微粒包体与寄主岩有相似的地球化学成分 ,表明它是与寄主花岗岩相同成因的同源包体 ,是来自上地幔的基性岩浆经过高度演化、结晶分异的产物 ;西准噶尔庙儿沟二长花岗岩体中含钾长石斑晶的微粒包体则主要是由幔源的下地壳基性岩部分熔融形成的残余体 ,被酸性岩浆携带并发生成分上的同化和混染 ,最后在上地壳侵位的产物。同准噶尔碱性花岗岩一样 ,载荷包体的准铝过铝质花岗岩是晚古生代后碰撞阶段构造岩浆活动的岩石类型之一 ,其形成和演化标志了准噶尔地区后碰撞幔源岩浆底侵作用导致大陆地壳垂向生长的过程。

A large number of microgranular dioritic enclaves occur in some post-collisional metaluminous-peraluminous granites, which include Yebushan and Miao'ergou occurring respectively in eastern and western Junggar of North Xinjiang. Petrological and geochemical researches reveal the cogenetic relationship between the enclaves and the host granites. The microgranular enclaves having similar geochemical composition to host biotite granite in Yebushan were generated by the fractional crystallization of mantle-derived basic magma. By contrast, the enclaves hosted in Miao'ergou monzogranite seem to be restites resulting from partial melting of highly evolved basic rocks underplated in the lower crust during earlier mantle-derived magmatism. The enclaves carried by ascending acidic magma were assimilated and finally emplaced in the upper crust. Consequently, these enclaves in common with Junggar peralkaline granite are products of post-collisional magmatic activity in late-Paleozoic. The formation and evolution of both enclaves and host granites in Junggar region prove that the process of vertical growth of continental crust resulted from the underplating of post-collisional mantle-derived magmas in late-Pale-ozoic.