Petrogenesis of Devonian and Permian Pegmatites in the Chinese Altay: Insights into the Closure of the Irtysh–Zaisan Ocean

Abstract

Owing to tectonic, magmatic, and metamorphic controls, pegmatites associated with different spatiotemporal distributions exhibit varying mineralisation characteristics. The petrogenesis of pegmatites containing rare metals can improve the understanding of geodynamic processes in the deep subsurface. In order to understand the difference of petrogenesis between Devonian and Permian pegmatites, zircon U-Pb geochronological and Hf-O isotope analyses were performed on samples of the Jiamanhaba, Amulagong, and Tiemulete pegmatites from the Chinese Altay. According to the results obtained, the Amulagong and Tiemulete pegmatites were formed during the Devonian, and samples that were analysed yielded zircon U-Pb ages of 373.0 ± 7.8 and 360 ± 5.2 Ma, respectively. Samples from these pegmatites produced εHf(t) values of 2.87–7.39, two-stage model ages of 900–1171 Ma and δ18O values of 9.55‰–15.86‰. These results suggest that the pegmatites were formed via an anatexis of mature sedimentary rocks deep in the crust. In contrast, the Jiamanhaba pegmatite was formed during the Permian, and its samples produced εHf(t) and δ18O values of 2.87–4.94 and 6.05‰–7.32‰, respectively, which indicate that the associated magma contained minor amounts of mantle/juvenile materials. The petrogenesis of pegmatites containing rare metals can reveal tectonic settings of their formation. A combination of data that were generated in the present study and existing geochronological and Hf-O isotope data for felsic igneous and sedimentary rocks in the Chinese Altay shows that the εHf(t) sharply increased while the δ18O suddenly decreased between Late Carboniferous and Early Permian. These changes highlight a tectonic transformation event during this critical period. This tectonic event promoted mantle–crustal interactions, and thus, it was probably linked to assemblages of the Altay orogen and the Junggar Block. The present study provides evidence that the Irtysh–Zaisan Ocean probably closed during the Late Carboniferous (~300 Ma).