Early‐Middle Triassic high Sr/Y granites in the northern margin of the North China Craton: Petrogenesis and tectonic implications

Abstract

Radiometric dating results and geochemical and isotopic data obtained in this study for two granitic plutons in the northern margin of the North China Craton (NNCC) provide important information on the tectonic environment of the North China Craton (NCC) in the Early to Middle Triassic. The two plutons, Wanlongchang and Baiyanaobao, were dated being formed at 247 Ma (our previous study) and 237 Ma (this study), respectively. These two granites are characterized by low initial 87Sr/86Sr ratios (0.7055–0.7073), εNd(t) values (−9.9 to −15.0), and zircon εHf(t) values (−8.7 to −27.0), suggesting their magmas originated from ancient lower crust. Both granites display high Sr/Y ratios (most >30) and (La/Yb)N (most >30), but low Cr (8.31–40.9 ppm), Ni (3.59–13.8 ppm), MgO (0.12–1.17%) contents, and low Mg# values (26–39), which suggests that the melts that formed the two granites were derived from thickened lower crust. When elemental concentration data and compiled Sr/Y ratios in the previous works are evaluated, it is revealed that many Early‐Middle Triassic felsic rocks in this region have high Sr/Y ratios characters. Combining tectono‐magmatic investigations, we further proposed that the Early‐Middle Triassic crustal thickening may have resulted from the collision between the NCC and the South Mongolia Terranes after the final closure of the Palaeo‐Asian Ocean.