Petrogenesis and tectonic implications of the early Mesozoic granitoids in the northern Alxa region, Central Asian Orogenic Belt

The northern Alxa region is located in the central segment of the southern Central Asian Orogenic Belt. Many controversies and deficiencies still exist regarding the magma source characteristics, petrogenesis and tectonic regimes during the late Palaeozoic - early Mesozoic period within this region. This study presents whole-rock compositions and zircon U-Pb and Lu-Hf isotopic data for three early Mesozoic I- and A-type granitic plutons occurring in the northern Alxa region. The Haerchaoenji and Chahanhada I-type granitoids yielded zircon Pb-206-U-238 ages of 245 +/- 5 Ma and 245 +/- 2 Ma, respectively. The variable positive zircon epsilon(Hf)(t) values between +1.8 and +11.8, with young T-DM ages of 425-837 Ma, indicate that these I-type granitoids were mainly derived from juvenile crustal materials. The Wulantaolegai pluton has a zircon Pb-206-U-238 age of 237 +/- 2 Ma and is classified as having high-K calc-alkaline A-type affinity. Furthermore, the positive zircon epsilon(Hf)(t) values of the Wulantaolegai granite range from +3.3 to +8.7 with young T-DM ages of 545-778 Ma, suggesting the involvement of a juvenile crustal source as well. Furthermore, the major-element compositions of the Chahanhada and Wulantaolegai granites suggest the input of metasedimentary components. Geochemically, the Haerchaoenji and Chahanhada I-type granitoids show an arc affinity, while the Wulantaolegai granite exhibits a post-collisional affinity. However, with regional data, we suggest that the Haerchaoenji and Chahanhada I-type granitoids were also emplaced in a post-collisional setting, and the arc affinity was probably inherited from recycled subduction-related materials. These lines of evidence obtained in this study enable us to argue that the Palaeo-Asian Ocean in the central segment of the Central Asian Orogenic Belt closed before Middle Triassic time.