Geochronology and Geochemistry of the Permian Granitoids from the Alxa Area, Inner Mongolia, China: Constrains on the Permian Evolution of the Central Asian Orogenic Belt

Abstract

We report new SHRIMP zircon U-Pb ages, zircon Lu-Hf isotopic and whole rock geochemical data from Permian granitoids located in the Alxa area of Inner Mongolia, China. In combination with published geochronological and geochemical data, the granitoids in the region can be divided into two age groups: ca. 285 Ma and ca. 269 Ma. The granitoids of the first group are mainly composed of calc-alkaline to high-K calc-alkaline, weakly peraluminous I-type granodiorites with ɛ_Hf(t) values of -19.6 to -4.3, which demonstrates evidence of crustal reworking; the granitoids of the second group, however, mainly consist of A-type granites that are high-K calc-alkaline to shoshonite, metaluminous to weakly peraluminous, and have high 10,000 × Ga/Al ratios (2.59–3.12) and ɛ_Hf(t) values ranging from –11.3 to –2.7, all of which demonstrates a mixed crust-mantle source. We interpret the granitoids of the first group to have formed during the subduction of Central Asian oceanic crust and the second group to have formed by the asthenospheric upwelling caused by the formation of slab windows during late ocean ridge subduction.