Geochemistry and geochronology of 1.7-1.8 Ga peraluminous A-type granites and granite-gneiss from the Mahakoshal Basin, Central Indian Tectonic Zone (CITZ): implications for an accretionary orogen for the evolution of CITZ

Here, we report petrography, whole-rock geochemistry, and zircon U-Pb geochronology to understand the origin, source, geodynamic setting, and evolution of A-type granite and granite gneiss as well as establish their emplacement age from Mahakoshal Basin. Mineralogically, Sidhi granite gneiss and Madanmahal granite of the Mahakoshal Basin dominantly consist of quartz, K-feldspar, and plagioclase. Geochemically, Sidhi granite gneiss and Madanmahal granite are ferroan in nature. The studied rocks are akin to peraluminous A-type granites and have average zircon saturation temperatures of 955°C and 977°C, respectively. The studied rock types are derived from partial melting of pre-existing crust indicated by (Y/Nb)N >0.18, (Th/Nb)N >2, (Ce/Pb)N <1, and (La/Nb)N >2 accompanied by the Eu, Ba, Sr, Ti, and Nb negative anomalies. The zircon U-Pb geochronology yielded emplacement ages of 1723 ± 16 Ma for the Sidhi granite gneiss and 1801 ± 64 Ma for the Madanmahal granite. Based on similar geochemical characteristics such as rare earth element patterns, Eu anomalies, and Ce/Pb, La/Nb, and Th/Nb ratios, we propose that the Sidhi granite gneiss and Madanmahal granite are derived from the same source i.e. Bundelkhand gneisses and granites, and formed in a post-orogenic rift environment of the accretionary orogen setting.

本研究报道了采自马霍萨尔盆地（Mahakoshal Basin）的A型花岗岩（A-type granite）及花岗片麻岩（granite gneiss）的岩石学（petrography）、全岩地球化学（whole-rock geochemistry）与锆石U-Pb年代学（zircon U-Pb geochronology）数据，旨在阐明其成因、物源、地球动力学背景与演化过程，并限定其侵位年龄。矿物学上，马霍萨尔盆地的西德花岗片麻岩（Sidhi granite gneiss）与马达恩马哈尔花岗岩（Madanmahal granite）主要由石英、钾长石（K-feldspar）及斜长石（plagioclase）组成。地球化学特征显示，两类岩石均属富铁质（ferroan）岩石，类似于过铝质（peraluminous）A型花岗岩，其锆石饱和平均温度（zircon saturation temperature）分别为955℃与977℃。研究区岩石均起源于先存地壳的部分熔融（partial melting），该结论得到以下地球化学指标的佐证：(Y/Nb)N>0.18、(Th/Nb)N>2、(Ce/Pb)N<1、(La/Nb)N>2，且伴随铕（Eu）、钡（Ba）、锶（Sr）、钛（Ti）及铌（Nb）的负异常。锆石U-Pb年代学结果显示，西德花岗片麻岩的侵位年龄为1723±16 Ma，马达恩马哈尔花岗岩的侵位年龄为1801±64 Ma。基于相似的地球化学特征（如稀土元素（rare earth element）配分模式、铕异常以及Ce/Pb、La/Nb、Th/Nb比值），本研究认为西德花岗片麻岩与马达恩马哈尔花岗岩具有相同的物源——本德尔坎德片麻岩与花岗岩（Bundelkhand gneisses and granites），且形成于增生造山带（accretionary orogen）背景下的造山后裂谷环境（post-orogenic rift environment）。