Neoproterozoic geodynamics in NW India – evidence from Erinpura granites in the South Delhi Fold Belt

Erinpura Granites form the basement for the Neoproterozoic Malani Igneous Suite in NW India. Based on this study, the Erinpura granites can be divided into Erinpura-East (gneissic fabric), a belt parallel to the southern sector of the South Delhi Fold Belt, and Erinpura-West (magmatic fabrics). EPMA dating on monazites gives a time frame of 890 to 860 Ma for crystallization of both types. The geochemically homogeneous peraluminous S-type granites with ε_Nd values of −2.1 to −10.8 are interpreted as melting products of Archaean crust. REE pattern follows the pattern of the average continental crust, but Erinpura-East samples are more fractionated with steeper HREE depletion indicating melting in a deeper crustal level. A thermal pulse between 835 and 820 Ma constrains the timing of deformation in granite-gneisses during uplift along thrust planes, coeval with shear-bound exhumation of high-grade metamorphic rocks and initiated by delamination of the lower crust in this southern sector of the South Delhi Fold Belt. This is in contrast to the northern sector of the SDFB with arrested orogenic development and without considerable delamination or erosion of the lower crust. Latest movement related to the 200 Ma tectono-magmatic history overlaps with initiation of rifting during the Malani igneous event. A change from S-type to A-type granites and shift of isotopic signatures to ε_Nd values of −2.8 to −1.7 indicate substantial contribution of asthenospheric material in the Malani melting process.

埃尔因普拉花岗岩（Erinpura Granites）构成了印度西北部新元古代马拉尼火成岩套（Malani Igneous Suite）的基底。基于本研究成果，埃尔因普拉花岗岩可划分为两类：其一为埃尔因普拉东带，发育片麻状构造，其展布与南德里褶皱带（South Delhi Fold Belt, SDFB）南部区段平行；其二为埃尔因普拉西带，具有岩浆构造。对独居石开展的电子探针显微分析（EPMA）定年结果显示，两类花岗岩的结晶时代均为890 Ma至860 Ma。地球化学性质均一的过铝质S型花岗岩的ε_Nd值介于-2.1至-10.8之间，被认为是太古代地壳熔融的产物。其稀土元素（REE）配分模式与大陆地壳平均值相符，但埃尔因普拉东带的样品分异程度更高，重稀土元素（HREE）亏损更为显著，指示其形成于更深的地壳熔融环境。835 Ma至820 Ma期间的热脉冲限定了花岗岩-片麻岩沿逆冲面抬升过程中的变形时限，该时限与高级变质岩受剪切约束的抬升剥露时代一致，其触发机制为南德里褶皱带南部区段的下地壳拆沉作用。这与南德里褶皱带北部区段的造山演化停滞、未发生显著下地壳拆沉或剥蚀的情形形成鲜明反差。与200 Ma构造-岩浆活动历史相关的最新构造运动，与马拉尼火成事件期间的裂谷作用起始时间相重合。从S型花岗岩向A型花岗岩的转变，以及同位素特征向ε_Nd值-2.8至-1.7的偏移，表明软流圈物质在马拉尼熔融过程中发挥了实质性的贡献作用。