Middle-late Permian high-K adakitic granitoids in the NE Alxa block, northern China: Orogenic record following the closure of a Paleo-Asian oceanic branch?

How and when the Paleo-Asian Ocean was finally closed still is a hot topic open to debate. Here we presented geochmnological (laser ablation inductively coupled plasma mass spectrometry, LA-ICP-MS, and 40Ar/39Ar), geochemical, and Sr-Nd-Hf isotopic data for the granitoids from the NE Alxa block in the southern margin of the Central Asian Orogen Belt, northern China, in an attempt to constrain the closure of the Paleo-Asian oceanic branch in this region. These granitoids were dated at 268-255 Ma. They show distinct adakitic signatures of high Sr/Y (19-100) and (La/Yb)(N) (13-50) ratios, and low Y (3.98-16.50 ppm) and Yb (0.24-1.46 ppm) concentrations. They are also characterized by relatively high K2O/Na2O ratios (0.7-1.37), and low MgO (0.22-0.75 wt%), Cr (1.41-9.96 ppm), and Ni (0.36-6.31 ppm) contents and variable Mg+ values (26-50). In addition, they display negative epsilon(Hf)(t) (-16.46 to -7.48) and epsilon(Nd)(t) (-14.2 to -9.83) and high (Sr-87/Sr-86); (0.7072-0.7196) and Th (7.33-14.10 ppm), indicating that they were most likely generated by partial melting of thickened continental lower crust up to similar to 59 km thick. This thickened crust was built on the early Permian transitional Alxa crust (similar to 32 km thick), likely caused by the rifting related to the upwelling of mantle plume. Consequently, the NE Alxa block could have witnessed a tectonic transition during the mid-Permian from rifting to arc-continent collision and resultant crustal thickening following the closure of the Paleo-Asian oceanic branch. The subsequent upwelling of asthenosphere in response to the delamination of the orogen during the middle-late Permian provided thermal source for the adakitic magmatism.