青藏高原新生代地壳变形对同碰撞岩浆侵位的制约

Cenozoic deformation of the Tibetan Plateau:Constraints on emplacement of syn-collisional magma.

同碰撞岩浆作用是青藏高原新生代构造-岩浆活动的一种重要形式,各种类型岩浆岩广泛分布于高原内不同的构造单元中,其中有的来自地幔,不过规模都不大,它们是如何穿过异常厚的地壳侵位到近地表或喷出地表,这还是个未解之谜。根据构造分析,这些岩浆岩均侵位于新生代向斜构造中,例如侵入于高原东南边缘的楚雄复向斜、兰坪-思茅复向斜和老君山向斜的碱性岩,侵入于高原南缘的北喜马拉雅拉轨岗日向斜的片麻状花岗岩以及喷出于高原北部巴颜喀拉和雁石坪复向斜的安山-玄武岩。文中通过一个力学模式,说明这些岩浆岩的侵位受控于地壳应力场特征,即:向斜构造的下部承受的是张应力,地壳发生减薄和张裂,下地壳或上地幔熔融物质以此为通道发生向上的侵位。不过,因向斜顶部地壳承受的是挤压应力,地壳发生挤压缩短,所以只有少量岩浆能侵位到近地表并发生变形。与此相反,背斜构造的下部地壳产生的是挤压应力,阻止了下伏地壳内岩浆的侵入。因此,岩浆的侵位一般不会沿背斜发生。这一力学机制解释了为什么青藏高原规模很小的同碰撞岩浆能穿过异常厚的地壳沿一系列向斜构造侵位到近地表或喷出地表的原因。

Syn-collisional magmatism is an important form of Cenozoic tectonic activity of the Tibetan Plateau, resulted in a variety of magmatic rocks, some of which were originated from the upper mantle. They occurred within the different tectonic elements of the plateau, most of which are very small in size. As how they emplaced into the surface or sub-surface through such thick crust remains as a mystery. Based on deformation analysis of the crust of the plateau, most of Cenozoic intrusive and extrusive rocks are found occurred along the syncline that formed coevally. Such as those occurred along the Chuxiong basin, Lanping-Simao basin and Lao Junshan syncline within the southeastern margin of the plateau, and those occur along the Laguigangri syncline within the southern margin of the plateau, as well as those occur along the Xoh Hil and Qiantang blocks within the northern plateau. A mechanic model is adapted to illustrate the process of emplacement of the magma under control by stress field status of the crust. As it＇s shown, the lower part of the syncline undergoes tension, by which the magma intrude into the upper crust along extensional fractures, whereas the lower part of the anticline undergoes contraction, which proved the fact that the magma emplace upward into the upper crust. Therefore, the magma is not likely to be found along the anticline. This mechanic model gives a reasonable explanation for the occurrence of emplacement of some small-sized syn-collisional magma through thick crust into the surface or sub-surface.