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套长英质岩浆岩组成，即Erinpua花岗岩和Malani火成岩套。这两次事件发生的时间正值世界范围的裂谷时期，与罗迪尼亚的解体有关，当时印度位于超级大陆的边缘。为了解释北西印度地体的新元古代演化及其与中国南部(扬子地块和卡塔西亚地块)邻接地体的关系，人们提出了不同的构造-岩浆模式(弧、弧后背景、羽流活动)。然而，这些模型与我们先前项目的结果并不一致。结合文献资料，我们的研究结果显示了西北印度带具有正负端值的区域的空间差异，并表明推断的线型将两个不同的岩石圈块体分开。有迹象表明，这一岩石圈边界已经形成于早期岩浆事件(Erinpua花岗岩，约890至820 Ma)，我们怀疑前马拉尼地壳伸展和沿大陆边缘裂谷是更合适的情景。我们预计，马拉尼型岩石的同位素比值只是模拟了先前存在的地壳结构，这将对解释这一较年轻(770至750 Ma)岩浆活动的构造环境具有重要意义。为了验证这一假设，我们想要分析从克拉通到裂谷边缘的横断面样品的锶和钕同位素，以便更好地约束从负值到正值的过渡。此外，我们想要测量同一地区出现的马拉尼镁铁质岩墙的锶、钕和铅同位素，这可以提供对地幔下部的洞察。

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.