Two episodes of Late Paleozoic mafic magmatism in the western Tianshan Orogen: From Carboniferous subduction to Permian post-collisional extension

The evolution of the western Tianshan Orogen in the southern Central Asian Orogenic Belt involved prolonged accretion and collision processes in response to the closure of the Paleo-Asian Ocean during the Late Paleozoic. However, the timing for the tectonic transition from subduction to post-collision is still poorly constrained. Herein, we address this issue based on geochronological, geochemical and Sr-Nd-Hf isotopic investigations on representative mafic dikes from the Awulale Mountains in the heart of the western Tianshan. Zircon U-Pb dating suggest two episodes of mafic magmatism during the Late Carboniferous (ca. 324-310 Ma) and Early Permian (ca. 299-290 Ma). The Late Carboniferous gabbro dikes exhibit large variations in MgO (2.79-10.97 wt%) and Ni (7.58-244 ppm), and arc-like trace element patterns with high Th/Yb (0.55-2.26) ratios, relatively low epsilon Nd-(t) (+0.2 to +2.5) and variable epsilon Hf-(t) (+1.3 to +10.4) values, indicating that they were derived from fractional crystallization of a peridotite-bearing mantle source metasomatized by sediment-derived hydrous melts. In contrast, the Early Permian diabase dikes have slightly variable MgO (5.88-7.78 wt%) and Ni (59.90-104 ppm) with apparently low Th/Yb ratios (0.20-0.34, except for one sample: 0.86) and depleted epsilon Nd-(t) (+5.0 to +7.8) and epsilon Hf-(t) (+8.0 to +14.6) values, which indicates derivation from partial melting of a juvenile lithospheric mantle source with contributions from early subduction metasomatic components. In context with relevant published data, our study indicates that the Late Carboniferous magmatism of the Awulale Mountains was likely induced by breakoff of the North Tianshan oceanic slab during ongoing subduction; conversely, the Early Permian magmatism may have originated from delamination of the lower crust and lithospheric mantle in the subsequent post-collisional setting. Furthermore, the tectonic transition from Carboniferous subduction to Permian post-collisional extension of the western Tianshan suggests that terminal closure of the North Tianshan Ocean took place at Carboniferous-Permian boundary at similar to 300 Ma. (C) 2022 Published by Elsevier B.V. on behalf of International Association for Gondwana Research.

Automated summary

The evolution of the western Tianshan Orogen was characterized by prolonged accretion and collision processes. The Late Carboniferous magmatism in the Awulale Mountains was likely a result of ongoing subduction, while the Early Permian magmatism may have been caused by post-collisional delamination of the lower crust and lithospheric mantle. This study provides important insights into the geological evolution and tectonic transition processes of the western Tianshan.