扬子北缘早新元古代镁铁质岩浆岩成因及其对罗迪尼亚超大陆聚合的限制

Rodinia, the supercontinent assembled in late Mesoproterozoic to Neoproterozoic, includes nearly all the known continents. The place of South China in Rodinia, i.e., located in the interior or at the margin of supercontinent, lies to the Southeast of Australia and west of Laurentia or to the North of west Australia and north India, is hotly debated. The magmatic records from the Jiangnan Orogen and the Cathaysia Block implies that the Cathaysia and Yangtze Blocks amalgamated in succession to the supercontinent after the major body of the supercontinent had formed. In this situation, the Yangtze Block becomes the margin of the supercontinent with its north margin facing out. On the other hand, the ~750 Ma rift magmatism suggests that the north margin of the Yangze Block is where it ruptured from the supercontinent. This paradox urges a comprehensive understanding of tectonic setting in the north margin of the Yangtze Block during the assembly of Rodinia. The recently reported mafic rocks with ages of 0.9-1.0 Ga in the Mian-Lue Zone and the South Qinling terrane provides an opportunity to study this question. In this proposal, the early Neoproterozoic rocks in three different places, together with associated felsic rocks, will be studied for their geochronology and geochemistry. The data will be used to discuss their petrogenesis, the nature of their mantle source and the geodynamic background. The detailed subduction and accretion history of the northern Yangtze Block in the early Neoproterozoic will be constrained, shedding new light on the assembly of Rodinia and the place of South China in the supercontinent.

形成于中元古代晚期到新元古代的罗迪尼亚超大陆包含了几乎所有的大陆。但是，华南在超大陆中的位置，是位于超大陆的中央还是边缘，是位于澳大利亚东南和劳伦大陆的西部还是位于西澳大利亚和印度的北部，一直存在争议。来自江南造山带和华夏的岩浆岩记录，要求华夏和扬子于超大陆主体形成之后，先后拼合到超大陆之上，扬子北缘成为超大陆的最边缘，而扬子北缘~750Ma的大陆裂谷岩浆活动要求扬子北缘是扬子从超大陆裂解的地方。如何理解这一矛盾，需要我们充分认识扬子北缘在超大陆聚合过程中的岩浆活动性质和构造体制。在扬子北缘勉略带和南秦岭地区发现的0.9-1.0 Ga的基性岩，为研究这一问题提供了可能。本项目将以三个不同地方的早新元古代基性岩及其相关的长英质岩石为研究对象，精确测定它们的岩浆活动时间，限定其成因、地幔源区性质和构造背景，建立扬子北缘在新元古代早期的俯冲和增生历史。

本项目对扬子北缘勉略带和南秦岭地区的新元古代早期和中期的不同基性岩进行了岩石学和地球化学研究，厘定了这些镁铁质岩浆的岩石成因、源区性质和动力学背景，建立扬子北缘新元古代时期从早期的俯冲增生经过弧陆碰撞演化到大陆裂谷的构造演化历史，限定了华南在罗迪尼亚超大陆中的位置。查明了勉略带中三岔子地区的基性火山岩是一套早新元古代的蛇绿岩，形成于洋内俯冲活动，大洋弧岩浆的活动时间为950-930 Ma，900 Ma左右的斜长花岗岩的形成标志着蛇绿岩的仰冲就位。对勉略带中长坝地区的一个复杂露头的研究发现，这个露头中既发育了985-950 Ma的大洋弧的中下地壳，又发育了830 Ma的长英质岩浆，代表了大陆弧或者弧陆碰撞的产物。我们在扬子西北缘还识别出一个新的低δ18O岩体，780-750 Ma的铁瓦殿岩体，该岩体与扬子北缘其他低δ18O岩浆一起构成了一条横贯整个华南北缘的新元古代低δ18O岩浆带，说明华南从Rodinia超大陆裂解是沿着其北缘进行的，并且可以与其他陆块同时期的低δ18O岩浆进行对比，可以制约华南在罗迪尼亚超大陆中的位置。