Petrogenesis and geodynamic implications of two episodes of Permian and Triassic high-silica granitoids in the Chinese Altai, Central Asian Orogenic Belt

Abstract

Tectonic transition from post-collision to intraplate settings is a crucial process in orogenic belts. To characterize such a transition in the Chinese Altai, we conduct an integrated study on Permian to Triassic granitoids. Zircon U–Pb dating reveals that the granitoids can be subdivided into two stages: early Permian (291–286 Ma) and late Triassic (216–209 Ma). Granitoids of both two stages exhibit high SiO2, low Mg#, low Cr and Ni contents, and variable Sr–Nd–Hf isotopes, all of which indicate a multiple crustal origin. Generally, the Triassic granitoids have similar εNd(t) but relatively higher εHf(t) values than those of the Permian granitoids. Moreover, the Triassic granitoids possess relatively higher ASI and more variable Al2O3/TiO2, CaO/Na2O, Rb/Sr and Rb/Ba values than those of the Permian granitoids, suggesting that the Triassic magmatic sources were much variable and complicated with higher proportions of sediments. By integrating regional data, two magmatic peaks at Permian and Triassic can also be recognized. From early Permian to Triassic, the SiO2, Na2O + K2O and Th contents, ASI and 87Sr/86Sr, Al2O3/TiO2, Rb/Sr and Rb/Ba values for the felsic rocks gradually increase, implying more and more strongly reworking of continental crust. Also, the Rb, Y + Nb and Yb + Ta concentrations for felsic rocks progressively increase from early Permian to Triassic, demonstrating a geodynamic evolution from syn/post-collision to intraplate settings. Furthermore, we propose that the post-collision and intraplate tectonism, especially the latter, could facilitate the generation of high-silica granites, which drive maturation of continental crust in accretionary orogenic belts.