华夏地块泛非期构造热事件研究

The reconstruction and evolution of supercontinents is one of the key issues in the study of Geology. The most important characteristic of each supercontinent reconstruction is the occurrence of global collisional orogeny during its assembly. The Pan-African tectonothermal events, which are the global collisional orogeny happened in the late Neoproterozoic-Cambrian period, corresponding to the assembly of Gondwanaland. The Pan-African tectonothermal events are recorded in most of the Gondwana continental blocks, whereas the direct evidences of these events, including Pan-African metamorphic and magmatic activities, are extremely lack in the South China Block (SCB), resulting in the understanding about the crustal evolution of SCB during Pan-African period and its position in the Gondwanaland are very controversial.. Recently, we first identified the Pan-African metamorphism and magmatism in the Zhenghe-Dapu Fault zone in the Cathaysia Block of SCB, which can provide important research objects to resolve above questions. Based on that, this project proposes to study those Pan-African metamorphic and magmatic rocks in the Cathaysia Block using an integrated multi-disciplinary approach, in order to reveal the nature of Pan-African tectonothermal event, and to understand the crust evolution of the Cathaysia Block during Pan-African period. Moreover, comparative studies between the Pan-African events in the Cathaysia Block and those in the Gondwana continental blocks will be conducted, in order to explore the position of the SCB in the Gondwanaland.

全球超大陆的重建和演化长期以来一直是地质学研究的热点。超大陆聚合过程中最显著的特征是形成全球性的碰撞造山事件。泛非期构造热事件，即晚新元古代-寒武纪期间发生的全球性造山事件，是冈瓦纳大陆聚合的重要标志。泛非事件在冈瓦纳大陆的众多陆块中都有记录，但该事件的直接证据(包括泛非期变质作用和岩浆活动)在华南板块却极度缺乏，导致华南泛非期的地壳演化以及华南在冈瓦纳大陆中的位置的认识存在很大争议。近年来，申请人在华夏地块政和-大埔构造带内的前寒武纪地层中厘定出泛非期变质岩和岩浆岩，这些泛非期地质体的厘定为解决上述问题提供了重要的研究对象。本项目拟在此基础上对泛非期地质体开展矿物学、岩石学、岩石地球化学、年代学和同位素地球化学等多学科综合研究，以揭示华夏地块泛非期构造热事件的性质，明确华夏地块在泛非期的地壳演化。并进一步开展该事件与冈瓦纳大陆泛非造山作用的对比研究，探讨华南板块在冈瓦纳大陆中的位置关系。

华夏地块政和-大埔断裂带内出露有基性-超基性岩，这些岩石经历了较强的后期改造作用，本人前一个基金项目的研究结果显示这些超基性岩部分为残余的岩石圈地幔，但是否属于蛇绿岩套的组成部分还需进一步研究。本次项目对与这些变质超基性岩共生的变质酸性岩开展了详细的全岩主微量元素、Sr-Nd同位素分析以及锆石年代学和原位Hf-O同位素分析工作。 研究发现其地球化学特征与典型大洋斜长花岗岩一致，由新生大洋壳重熔形成，类似于挪威西部的East-Karmoy型斜长花岗岩，表明斜长花岗岩源自大洋中脊附近活动剪切带中的角闪岩重熔，属于蛇绿岩套的组成部分。变质斜长花岗岩另外一个非常重要的特征：其岩浆锆石和变质锆石具有几乎相同的SIMS U-Pb年龄，分别为523-521 Ma和522-518 Ma，在误差范围内一致。近乎同时的岩浆作用和变质作用也倾向于斜长花岗岩形成于高温剪切带环境，并且斜长花岗岩的形成略晚于大洋地壳。ca. 520 Ma的年龄代表大洋地壳的形成年龄，而不是其侵位年龄。该斜长花岗岩的厘定进一步确定了政和-大埔断裂带内以基性-超基性岩为蛇绿岩的组成部分，并且属于MOR蛇绿岩而非SSZ蛇绿岩。该ca. 520 Ma MOR型蛇绿岩套的厘定，表明华夏地块可以以政和-大埔断裂带可以作为缝合带分为东、西华夏两个地块。该蛇绿岩的侵位显然发生在ca. 520 Ma之后，极有可能发生在早古生代晚期（加里东期），为华夏板块早古生代造山作用的性质提供了重要依据。