新疆北部后碰撞幔源岩浆活动与陆壳纵向生长

Postcollisional Mantle-Derived Magmatism and Vertical Growth of the Continental Crust in North Xinjiang

新疆北部后碰撞幔源岩浆活动强烈。Nd、Sr和Pb同位素资料表明,在330～250Ma的后碰撞期间,有大量的幔源花岗岩类和少量的镁铁—超镁铁杂岩在上地壳侵位。与加里东、海西和喜马拉雅等造山带起源于再循环陆壳的花岗岩类不同,新疆北部后碰撞岩浆岩一般表现出ε_(Nd)(t)值高、(^(87)Sr/^(86)Sr)值相对较低、Nd和Pb模式年龄年轻等特点。阿尔泰山和天山的一些后碰撞花岗岩类可能具有陆壳源区的特点或表现出地壳物质对幔源岩浆及其分异产物有不同程度的混染,东、西准噶尔花岗岩类很少甚至没有受到陆壳物质混染。新疆北部后碰撞花岗岩类和镁铁—超镁铁杂岩主要是幔源岩浆及其分异产物在上地壳侵位的结果。这些幔源花岗岩类代表了新生的初始地壳,其时代可代表地壳形成时代。在后碰撞阶段,新疆北部的陆壳以纵向生长为特征。

North Xinjiang is characterized by extensive development of postcollisional magmatism. Nd, Sr, and Pb isotopic data indicate that large amounts of mantle-derived granitoids and minor amounts of mafic-ultramafic complexes emplaced in the upper crust during late Carboniferous and Permian times (250 to 330 Ma). In contrast to the granitoids, with a recycled crustal origin, of the Caledonides, Hercynides, and Himalayas, the postcollisional magmatic rocks of North Xinjiang commonly show high εNd(t) values, relatively low (87Sr/86Sr); ratios, and young Nd and Pb model ages. Although the involvement of crustal materials in the mantle-derived magmas and their differentiates is obvious in some granitoids and mafic-ultramafic complexes to some extent and some granitoids from the Altai and Tianshan mountains have chiefly crustal sources, there are almost completely mantle-derived granitoids with no admixture of crustal materials in the East and West Junggar, which could be considered as representing the primitive original crust and whose ages could correspond to the crust-formation ages. Therefore, most granitoids from North Xinjiang are the representative of the Phanerozoic juvenile crust. Vertical growth of the continental crust in North Xinjiang could mainly occur in a postcollisional environment, resulting from the underplating of mantle-derived magmas at or near the base of the continental crust and from the emplacement of the differentiates of mantle-derived magmas in the upper crust.