Geochronology and Geochemistry of the Early Permian A‐type Granite in the Hongol Area, Central Inner Mongolia: Petrogenesis and Tectonic Implications

Abstract

We performed geochronological and geochemical analyses of the A‐type granite in the Hongol area, central Inner Mongolia, to determine its age, petrogenesis and tectonic setting, which are significant for clarifying the Late Paleozoic tectonic evolution of the Xing'an Mongolian Orogenic Belt (XMOB). The rock type of the A‐type granite in the Hongol area is alkali‐feldspar granite, and it constitutes a western part of the Baiyinwula–Dongujimqin A‐type granite belt. Zircon U–Pb geochronology yields 206Pb/238U ages ranging from 293 to 286 Ma for the alkali‐feldspar granite, indicating this granitic pluton formed in the Early Permian. The alkali‐feldspar granite is high in silica (SiO2=75.13wt%–80.17wt%), aluminum (Al2O3=10.59wt%–13.17wt%) and alkali (Na2O+K2O=7.33wt%–9.11wt%), and low in MgO (0.08wt%–0.39wt%) and CaO (0.19wt%–0.70wt%). It is obviously enriched in LILEs such as Rb, Th and K, depleted in HFSEs such as Nb, Ti, La and Ce, with pronounced negative anomalies of Nb, Ti, P, Eu, Sr and Ba. Its Sr–Nd–Pb isotopic compositions show positive εNd(t) (+0.72–+3.08), low TDM2 (805–997 Ma), and high radioactive Pb with (206Pb/204Pb)i of 18.710–19.304, (207Pb/204Pb)i of 15.557–15.604 and (208Pb/204Pb)i of 37.887–38.330. Petrological characteristics and geochemical data suggest that the alkali‐feldspar granite in the Hongol area belongs to aluminous A‐type granite. This A‐type granite formed in a post‐collisional extensional setting and was generated by the partial melting of felsic rocks in the middle‐lower crust resulting from post‐collisional slab breakoff. It is suggested that the Paleo‐Asian Ocean was closed before the Permian in central Inner Mongolia.