拉萨地块南缘侏罗纪岩浆作用东西段差异及其对雅江特提斯洋演化的制约

Variable rock assemblage and forms of arc magmatism is controlled by the subduction-angle of oceanic slab in the convergent plate boundary. The widespread igneous rocks distributed in southern Lhasa terrane are response to the slab-subduction of Yarlung Tethys Ocean. However, there is obviously different rock assemblage along the W-E trend, such as basaltic- intermediate volcanic rocks of Sangri gourp and Yeba formation mainly distributed in the East instead of intermediate-acid intrusive rock in the West. On the other hand, our new researches indicated that the geochemical and isotopic compositions of intermediate-acid intrusive rocks, which are around the Xiongcun giant porphyry Cu-Au deposit, are affinity to those of Aleutian island arc. Above features of Jurassic igneous occurred in southern Lhasa terrane, combining with the obviously different source composition and distribution of volcanic rocks of Yeba formation and Sangri group, indicate whether there is variable subduction-angle of Yarlung Tethys oceanic slab along the W-E trending? And whether Xiongcun porphyry Cu-Au deposit was developed in an island-arc? These topics, however, have not been well constrained. Based on the previous researches, this project is planning to conduct systematical geochronological, petrologcial, elemental, and Sr-Nd-Hf-B isotopic geochemical studies on the Jurassic W-E and N-S trending igneous rocks occurred within southern Lhasa terrane. These new data, combined with the previous results, when comparing with the magmatism-mineralization of the typical continental marginal arc (Chilean type)/ intra-oceanic arc (Mariana type), will help us to explore the tectonic settings of above variations of rock assemblage and mineralization along the W-E trending in southern Lhasa terrane. This project is expected to provide constraints on the evolutionary history of not only Yarlung Tethys Ocean, but also the accretion processes of southern Lhasa terran in Jurassic.

汇聚板块边缘大洋板片俯冲角度的差异会导致与之响应的弧岩浆作用有不同的岩石组合和表现形式。拉萨地块南缘广泛发育有与雅江特提斯洋北向俯冲相关的并在东西向上具有明显差异的侏罗纪岩浆作用，如其东部以桑日群和叶巴组中-基性火山岩为主，而在其西部则以中酸性侵入岩为主；另外，我们的初步研究显示在雄村大型斑岩Cu-Au矿床周围发育有一系列与阿留申洋内弧相似的中酸性侵入岩。上述拉萨地块南缘侏罗纪岩浆岩分布特征，结合桑日群和叶巴组火山岩在分布位置和源区性质的不同，是否指示雅江特提斯洋北向俯冲过程中在东西向上有不同俯冲角度并在其内部存在洋内弧？目前尚不清楚。本研究拟通过对拉萨地块南缘东西向分布的侏罗纪岩浆岩详细的年代学、地球化学和Sr-Nd-Hf-B同位素的综合分析，结合前人研究成果，探讨研究区控制岩浆作用时空分布差异性的动力学机制，据此对侏罗纪时期雅江特提斯洋北向俯冲形式以及拉萨地块南缘增生构造演化加以约束。

活动大陆弧岩浆作用组成的变化是俯冲板片与上覆岩石圈地幔相互作用的产物，本项目通过青藏高原冈底斯弧南缘东西向分布的侏罗纪岩浆岩形成时代、地球化学组成及其分布特征综合研究后认为，他们主要形成于早-中侏罗纪（192-168 Ma）时期并分布在雅江缝合带以北和北纬29.48度以南较窄的范围之内。虽然他们在东西向上表现形式不一致（如东部主要为中-基火山岩，西部主要为中酸性侵入岩），但其地球化学组成特征指示冈底斯弧在早侏罗世时期已经发展成为一个具有正常的地壳厚度陆缘弧，并随着时间的演化而逐渐增生。同时随着新特提斯洋东西向上差异性俯冲导致青藏高原南部形成于中生代晚期的源于岩石圈地幔的镁铁质岩在东西向上具有明显不同的地球化学组成，指示因新特提斯洋东西向的差异性俯冲导致其交代的青藏高原南部岩石圈地幔在东西向具有分段性特征。