Timing, petrogenesis, and setting of granites from the southern Beishan late Palaeozoic granitic belt, Northwest China and implications for their tectonic evolution

The mid-Permian Jianquanzi and Jiuquandihongshan monzogranites have highly fractionated potassium-rich calc-alkaline characteristics with negative εNd( t ) (−4.3) and very high initial 87 Sr/ 86 Sr (0.71949), reflecting a continental crustal component. The compositionally diverse Jiujing pluton and the granodiorite and high-Sr monzogranite phases display adakite-like compositions with relatively low εNd( t ) (−0.1 and −2.2) and high initial 87 Sr/ 86 Sr (0.70822 and 0.70913).

The Jiujing low-Sr monzogranite has higher initial 87 Sr/ 86 Sr (0.73464) and lower εNd( t ) (−2.8), indicating a significant continental crustal component in its genesis. This work, combined with the regional geology and previous studies, suggest that the early to middle Permian southern Beishan plutons formed in a post-collisional environment, probably an intracontinental rift environment linked to asthenospheric upwelling in response to the break-off of a subducted slab.

In the late Permian, the southern Beishan area was in a compressive tectonic regime and thickening of the continental crust resulted in the formation of the Jiujing adakite-like granite.