政和-大埔断裂带基性-超基性岩记录的华夏板块大地构造演化信息研究

There have long been debated about the petrogenesis of the mafic-ultramafic rocks which generally occurred in large-scale fault zones. These rocks can represent the relics of an ophiolitic suite, or may be within plate igneous rocks which form in an intracontinental rift setting. On the other hand, mafic-ultramafic rocks can be affected by late tectonothermal events, thus they can record significant information on tectonic evolution. There are large amount of mafic-ultramafic rocks occurred in the Zhenghe-Dapu fault zone, Cathaysia Block. Studies on their petrogenesis, tectonic setting as well as late tectonothermal events these rocks experienced can offer better understanding of the tectonic evolution of Cathaysia Block. In this project, we propose to carry out a detailed geological and geochemical study of the mafic-ultramafic rocks in Zhenghe-Dapu fault zone. Through mineralogy, major and trace element studies as well as Nd-Os isotope analyses of the mafic-ultramafic rocks, we will elucidate whether they are the relics of an ophiolitic suite, which will help to understand their petrogenesis and tectonic affinity. Through zircon and monazite U-Pb dating and zircon Hf-O isotopes and trace element analyses, we will constrain the nature of the tectonothermal events which these rocks experienced. When considered in the context of Pan-African Orogeny of Gondwanaland, the nature of the tectonothermal events will led to significant implications for the relationship between South China Block and Gondwana supercontinent.

大型断裂带内发育的基性-超基性岩的岩石成因及侵位方式一直是地学研究的热点，它们可能是构造蛇绿混杂岩的组成部分，或是裂谷背景下幔源岩浆的产物。另外，基性-超基性岩易受到后期构造热事件的影响，能记录构造热事件发生的时间及温压条件，从而为探讨构造热事件的性质提供制约。政和-大埔断裂带内出露有大量基性-超基性岩，对其岩石成因、构造属性及后期经历的构造热事件的研究能更好地了解华夏板块大地构造演化。本申请项目拟对该断裂带内的基性-超基性岩进行详细的野外调查，通过岩相矿物学和Nd-Os同位素研究，明确其是否含有蛇绿岩套的组成部分，并进一步探讨其岩石成因及构造属性; 通过对基性-超基性岩中变质锆石和独居石U-Pb定年，结合锆石Hf-O同位素、微量元素研究，明确基性-超基性岩所记录的构造热事件的性质, 在此基础上开展华夏板块与冈瓦那陆块经历的泛非事件的对比研究，为华南板块与冈瓦那大陆的关系提供依据。

华夏地块政和-大埔断裂带内的基性-超基性岩的构造属性一直存在很大争议。本项目对政和-大埔断裂带两侧的基性-超基性岩开展了锆石年代学和原位Hf同位素分析以及全岩主微量元素、Sr-Nd同位素和Re-Os同位素分析测试工作。分析结果显示基性岩具有不同的地球化学特征，第一类基性岩富集 LILE 和 LREE，略亏损 HFSE，具有略低 87Sr/86Sr和略负εNd（t）值的特征，其岩浆来源于富集型地幔，形成于板内构造环境。另一类基性岩稀土元素分布比较平滑，微量元素分配图具有N-MORB 特征，具有低 87Sr/86Sr和高εNd（t）值的特征，其岩浆来源于亏损型地幔，形成于新元古代。而超基性岩包含有不同的岩性，同样显示出不同的地球化学特征，其中蛇纹石化的橄榄岩具有球粒陨石的Os同位素组成，其187OS/188OS比值为0.11621-0.12008，非常低的187Re/188Os比值为0.031-0.129。其地球化学和Re-Os同位素特征，表明其为残余地幔橄榄岩，属构造成因而非岩浆成因。超基性岩角闪岩也分两类，第一类角闪岩显示分异的LREE稀土配分模式，富集HFSE，Sr-Nd和Re-Os同位素特征显示其原岩具有OIB的特征，来源于富集型地幔。第二类角闪岩糜棱岩化比较强烈，总体上具有低 87Sr/86Sr和高143Nd/144Nd值，显示岩浆来源于亏损的地幔源区，另外具有较低的Nb/Yb、Th/Yb和Ti/V的值，显示大洋岛弧拉斑玄武岩的特征。上述未经历糜棱岩化的角闪岩和经历糜棱岩化的角闪岩都纪录了泛非期的变质事件，其中前者的LA-ICPMS锆石U-Pb年龄为533±7Ma，而后者的SIMS锆石U-Pb年龄为522±3Ma。.整体上，我们认为该断裂带内的基性-超基性岩具有不同的来源，有的为陆内软流圈地幔上涌的产物，有的来源于残余的岩石圈地幔，有的为大洋岛弧的物质成分，在空间关系上显示出混杂岩的特征。结合厘定出的泛非期变质事件，我们认为政和-大埔断裂带可能为板块拼贴的缝合带，而非前人认为的裂谷带。这些基性-超基性岩石在泛非期发生了变质作用，说明华夏板块参与了冈瓦那大陆聚合过程。...