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 εNd 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 εNd values of −2.8 to −1.7 indicate substantial contribution of asthenospheric material in the Malani melting process.艾琳普拉花岗岩（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。基于本项研究，艾琳普拉花岗岩可划分为两类：其一为艾琳普拉东带，发育片麻状构造，展布方向与南德里褶皱带（South Delhi Fold Belt, SDFB）南部区段平行；其二为艾琳普拉西带，发育岩浆构造。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为890至860 Ma。地球化学特征均一的过铝质S型花岗岩（S-type granite），其εNd值介于-2.1至-10.8之间，被认为是太古宙地壳熔融的产物。其稀土元素（Rare Earth Elements, REE）配分模式与大陆地壳平均值一致，但艾琳普拉东带样品的分异程度更高，重稀土元素（Heavy Rare Earth Elements, HREE）亏损更为显著，指示其形成于更深的地壳熔融环境。835至820 Ma期间的热事件限定了沿冲断面抬升过程中花岗片麻岩的变形时代，该事件与高级变质岩受剪切约束的折返作用同期，且由南德里褶皱带南部区段的下地壳拆沉作用触发。这与南德里褶皱带北部区段的地质特征截然不同：该区域造山作用发展停滞，未发生显著的下地壳拆沉或侵蚀作用。与200 Ma构造-岩浆演化历史相关的最新构造活动，与马拉尼火成事件期间的裂谷作用启动时间相重合。从S型花岗岩到A型花岗岩（A-type granite）的转变，以及同位素特征向εNd值-2.8至-1.7的偏移，指示软流圈物质在马拉尼熔融过程中起到了实质性贡献。