2090-2070 Ma A-type granitoids in Zanhuang Complex: further evidence on a Paleoproterozoic rift-related tectonic regime in the Trans-North China Orogen
The Xuting pluton, located at the north section of the Zanhuang Complex, contains potassic and sodic granite in both of which shallow level emplacement is shown by graphic texture feldspar and quartz intergrowth. Two potassic granite and one sodic granite samples yielded zircon 207Pb/206Pb ages of 2066 ± 17 Ma, 2092 ± 14 Ma, and 2071 ± 14 Ma, respectively, suggesting that the Xuting pluton emplaced between 2070 Ma and 2090 Ma, which is coeval with the Paleoproterozoic Gantaohe Group dominated by mafic volcanic and sedimentary rocks. The potassic granite is enriched in SiO2, total FeO, REEs (except Eu), Zr, Nb, Ga and Y, and depleted in MgO, CaO, Al2O3, Sr, V, Cr and Ni, with high TFeO/MgO ratios, consistent with the features of A-type granite. Except for the high MgO and low K2O contents, the sodic granite also presents most of the features of A-type granites. The zircon saturation temperatures from the potassic and sodic granites range from 855 to 998 °C and 853 to 933 °C respectively, indicating that they are from the high temperature magma. The potassic granites have the εHf(t) ranging from -5.9 to +1.33 and Hf model ages of 2.5-2.8 Ga, which are identical with the ages of Neoarchean TTG gneisses in the Zanhuang Complex. We propose that the potassic granite in the Xuting pluton derived from the partial melting of Neoarchean TTG gneisses. The sodic granites also present negative εHf(t) values from -4.81 to -0.76 and Hf model ages of 2.5-2.8 Ga, suggesting that they come from the partial melting of the Neoarchean crust. However, with relative enrichment in MgO and total FeO, and lower K2O and Rb, the sodic granite was derived from a mafic source in the lower crust. The likely heat source was high heatflow associated with mantle upwelling. The A-type Xuting pluton and mafic volcanic rocks in the Gantaohe Group constitute a bimodal lithologic association from a Paleoproterozoic continental rift environment. 2.2-2.0 Ga magmas are widely distributed along the Trans-North China Orogen (TNCO), and also present a bimodal character. We propose that 2.2-2.0 Ga bimodal magmas throughout the TNCO were likely formed in a Paleoproterozoic rifting setting. Considering the widespread 2.2-2.0 Ga magmatic rocks in the North China Craton (NCC), we infer that the magmatism likely occurs in tensional environment after the initial amalgamation of the NCC in the late Archean.