印度西北部Marwar超群沉积构造属性及其对Rodinia超大陆中印度-扬子陆块关系的制约

The assembly and dispersal of supercontinent has been a hot topic in the past three decades. One of the most controversial issues is the possible linkage between Indian and the Yangtze Block in the configuration of Rodinia. Similar magmatic-sedimentary-tectonic systems are expected to occur in both continents if the proposed linkage is correct. Considering the formation and evolution of sedimentary basins are closely linked with several scientific issues, such as the nature of sedimentary sources and tectonic processes, comparison on coeval sedimentary basin from India and Yangtze is effective way to test the linkage. In recent years, Yangtze Block has been intensively studied, whereas the construction of the India in Rodinia supercontinent is poorly constrained. Therefore, well preserved sedimentary succession known as the Marwar Supergroup in the NW India provide excellent opportunity to study the crustal and tectonic evolution of the NW India and thus the comparison between India and Yangtze in term of sedimentary basin. In this project, we will carry out detailed studies on sedimentology, geochronology and geochemistry of accessory minerals, such as zircon, from sedimentary rocks, together with traditionally methods, like bull rock major and trace elements and modal point counting of clastics, in an order to understand the depositional time, nature of source rocks and tectonic setting of the Marwar Supergroup. These new results, in combination with the comparison among synchronous sedimentary strata in the Yangtze Block and other continents, will provide new insights into the crustal and tectonic evolution of the NW India and the role of India and Yangtze in the reconstruction of the Rodinia supercontinent.

印度和扬子陆块在Rodinia超大陆中的位置及相互关系是近年来的研究热点，但争议不断。对比在Rodinia聚合裂解过程中二者的岩浆-沉积-构造体系是否一致，是解决争论的关键因素。沉积盆地很好地保存了与源区有关的岩浆构造事件和造山运动等地质过程，从而为判断超大陆中陆块相邻与否提供重要的研究对象。近年来有关扬子陆块的研究深入丰富，而有关印度的研究则相对缺乏。印度西北部广泛出露的Marwar超群为研究该区沉积构造演化，以及进行印度-扬子对比提供了重要的研究素材。本项目拟以Marwar超群为研究对象，运用沉积学，锆石年代学和单矿物原位微量元素分析，结合全岩主微量元素、Nd同位素和沉积岩碎屑统计等方法，深入研究Marwar超群的沉积时限，源区特征和构造属性，探讨印度西北部新元古-早古生代的构造属性和演化。通过区域和全球同期沉积地层对比，为印度和扬子陆块在Rodinia超大陆中的相互关系提供重要约束。

新元古代是Rodinia-Gondwana超大陆转换的重要时期。分布于印度西北部的Marwar超群及其下覆Malani火成岩序列是该时期地质演化的重要见证者，是研究印度-扬子陆块在该时期可能存在的亲缘性以及超大陆重建的重要和理想研究对象。本项目对印度西北部Marwar超群，Malani火成岩及其伴生的碎屑岩进行重点研究。在详细的野外观测基础上，采集代表性岩石样品，运用系统的分析测试手段，从而约束Marwar超群的沉积物源特征和沉积环境，及其与下覆Malani大火成岩省序列的形成时代，以及该火山-沉积序列形成的地球动力学背景。野外观测表明Marwar超群以未变形变质的长石石英砂岩和灰岩为主要组分。碎屑岩中发育的板状交错层理和波痕指示古水流方向为北东东-南东东方向，而碎屑成分由单颗粒石英(83-89%)和长石(9-17%)组成。结合碎屑锆石2.6-2.4 Ga, 1.9-1.7 Ga和0.9-0.8 Ga的年龄峰值，Marwar超群的沉积物源主要来自印度克拉通内部的结晶基底岩石，并沉积于印度大陆西北(现代方位)的被动大陆边缘构造环境下。由此我们证实印度西北部Marwar超群并没有接受东非-南极洲造山带提供的沉积物，从而揭示了印度西北部（即东冈瓦纳西缘）与西冈瓦纳陆块之间一直存在未闭合洋盆的地质演化过程。SIMS锆石U-Pb定年结果指示下覆于Marwar超群的Malani火成岩主要形成于776.8-758.5 Ma。同时，本项目首次在Malani火成岩省中报道了低O同位素(δ18O锆石 = 4.12 至 -1.11‰)岩浆事件，提出在马达加斯加、塞舌尔、印度西北部和扬子陆块西、北缘存在一个新元古代低氧岩浆带的认识。再结合Malani火成岩省中OIB型和MORB型玄武质岩共生的特点，提出新元古代马达加斯加、塞舌尔、印度西北部和扬子陆块西、北缘低氧酸性岩及共生的玄武质岩石形成于弧后拉张的环境，受控于Rodinia超大陆的裂解的认识，证实这些陆块在Rodinia超大陆重建中的亲缘性。最后，通过对比华南扬子和华夏陆块同时代的火山-沉积建造，提出印度北部-华夏陆块至少在Ediacaran时期已经相邻的新观点。