Zircon U-Pb geochronology, geochemistry and petrogenesis of the Permian A-type granite in the Honggeer area, central Inner Mongolia
ZHANG Xiang-xin;WEI Rui-hua;GAO Yong-feng;XU Sheng-chuan;LEI Shi-he;College of Resource Science, Hebei GEO University;
The alkali-feldspar granite in the Honggeer area, central Inner Mongolia, constitutes a key part of the Baiyinwula-Dongujimqin A-type granite belt. The results of LA-ICP-MS zircon U-Pb dating indicate the alkali-feldspar granite in the Honggeer area formed in the Permian and has an age of 286~294 Ma. The alkali-feldspar granite is high in silica(SiO_2 = 75.13%~80.17%), alkali(Na_2O + K_2O = 7.33%~9.11%), and iron(total Fe_2O_3 = 1.01%~2.31%) and low in MgO(0.09%~0.27%) and CaO(0.12%~0.52%). It is obviously enriched in LILEs such as Rb, Th, K, and Pb and depleted in HFSEs such as Nb, Ti, La, and Ce with relative enrichment in Zr and Hf. It has low ∑REE contents(33~165 μg/g) and shows weak REE fractionation with weak LREE enrichment and obvious negative Eu anomalies(δEu = 0.06~0.13), resulting in nearly flat REE patterns of slightly right-dipping seagull-type in the chondrite-normalized REE diagram. Its Sr-Nd-Pb isotopic compositions show positive εNd(t)(+1.08 ~ +3.18), low tDM2(805~965 Ma), and highly radioactive Pb with(206 Pb/204 Pb)i of 18.710~19.280,(207 Pb/204 Pb)i of 15.557~15.597, and(208 Pb/204 Pb)i of 37.887~38.330. The petrological and geochemical characteristics suggest that the alkali-feldspar granite in the Honggeer area belongs to aluminous A-type granite and was produced by the partial melting of mafic rocks in the middle-lower crust resulting from the subducted slab breakoff in a post-collisional extensional setting. The A-type granite in the Honggeer area probably indicates that the Paleo-AsianOcean was closed at the Permian in central Inner Mongolia.