Differences in isotopic signature of Neoproterozoic crust of NW India - evidence for a lithospheric boundary?

印度西北部新元古代地壳主要由两个长英质岩浆套组成，即Erinpura花岗岩和Malani火成岩套。这两个事件发生的时间都在世界范围内的裂谷时期，与罗迪尼亚分裂有关，印度位于超大陆的边缘。为了解释NW印度地体的新元古代演化及其与华南邻近地体（扬子地块和华夏地块）的关系，提出了不同的构造-岩浆模式（弧、弧后背景、地幔柱活动）。然而，这些模型与我们之前项目的结果并不一致。我们的研究结果与文献数据相结合，显示了NW印度的空间区分为正和负eNd值区域，并表明推断的界线分隔了两个不同的岩石圈块体。有迹象表明，这个岩石圈边界在早期岩浆期(Erinpura花岗岩，我们认为，马拉尼型岩石的同位素比率只是模拟了一种预先存在的地壳结构，这将对解释这一较年轻（770至750 Ma）岩浆活动的构造背景具有重要意义。为了验证这一假设，我们想分析从克拉通到裂谷边缘的样带样品的Sr和Nd同位素，以获得从负向正eNd值转变的更好约束。此外，我们希望测量出现在同一地区的马拉尼基性岩脉的Sr， Nd和Pb同位素，这可以深入了解下地幔。

The Neoproterozoic crust of NW India is mostly composed of two felsic magmatic suites, the Erinpura granites and the Malani Igneous Suite (MIS). Timing of the two events fall into the time period of worldwide rifting, related to Rodinia break-up, whereby India was situated at the periphery of the supercontinent. Different tectono-magmatic models have been proposed (arc, back-arc setting, plume activity) in order to explain the Neoproterozoic evolution of the NW Indian terrane and its relationship to neighboring terranes in South China (Yangtze and Cataysia blocks). However, these models are not consistent with the results from our previous project. Our results in combination with literature data show a spatial discrimination of the NW Indian into regions with positive and negative eNd values and suggest that the inferred lineament separate two different lithospheric blocks. There is indication that this lithospheric boundary formed already during the earlier magmatic episode (Erinpura granites, ca. 890 to 820 Ma) and we suspect pre-Malani crustal extension and rifting along a continental margin as a more appropriate scenario We anticipate that the isotopic ratios of the Malani type rocks just mimic a preexisting crustal configuration which would have important implications for interpretation of the tectonic setting for this younger (770 to 750 Ma) magmatic activity. In order to verify this hypothesis we would like to analyse Sr and Nd isotopes of samples from transects from the craton to the rifted margin, to get better constrains on the transition from negative to positive eNd values. Moreover, we would like to measure Sr, Nd and Pb isotopes from Malani mafic dykes that appear in the same region and which can give insights into the underlying mantle.