Jialin Wang

Geochronology and geochemistry of volcanic rocks in the Arbasay Formation, Xinjiang Province (Northwest China): implications for the tectonic evolution of the North Tianshan

ABSTRACT The extensively exposed late Carboniferous volcanic and volcaniclastic successions along the northern margin of the North Tianshan are called the Arbasay Formation. We present field-based mapping, petrography, zircon cathodoluminescence (CL) images, and U–Pb dates, as well as whole-rock geochemical data for these rocks, in order to constrain their formation age and petrogenesis, and understand the geodynamic setting. Conspicuously, the Arbasay Formation shows typical basalt–andesite–dacite–rhyolite volcanic series, and is dominated by andesites with a small amount of basalts and rhyolites based on the geological profile. U–Pb isotopic dating using the LA-ICP-MS method on zircons reveals that the volcanic rocks in the Arbasay Formation formed at 308–305 Ma, that is, late Carboniferous, rather than early Permian as previously proposed. Geochemically, the volcanic rocks mainly belong to the calc-alkaline series and have arc-like geochemical compositions. They are enriched in LREEs ((La/Yb)N = 2.9–7.5) and LILEs (K, Rb, Ba) and depleted in HFSEs (Nb, Ta, Ti). In the tectonic discrimination diagrams, the basalts mainly fall into the area of continental arc. Given our U–Pb dating results, geochemical characteristics, and the regional geological framework, we propose that the late Carboniferous volcanic rocks originated from the arc-related setting, not the intracontinental rift-related setting. They are possibly the major constituents of a continental arc that is formed with the southward subduction of the North Tianshan Oceanic lithosphere.

Two-stage evolution of the Cenozoic Kunbei fault system and its control of deposition in the SW Qaidam Basin, China

The structural relationship between the Qaidam Basin and Qimen Tagh-Eastern Kunlun Range holds important implications for evaluating the formation mechanism of the Tibetan Plateau. Various models have been proposed to reveal the structural relationship, although controversies remain. To address these issues, we analysed the seismic and lithologic data of the Kunbei fault system (i.e. the Kunbei, Arlar and Hongliuquan faults), which lies to the north of the Qimen Tagh-Eastern Kunlun Range within the SW Qaidam Basin. Based on the regional geological framework and our kinematic analyses, we propose that the Cenozoic tectonic evolution of the Kunbei fault system can be divided into two stages. From the Early Eocene to the Middle Miocene, the system was characterized by left-lateral strike-slip faults and weak south-dipping thrust faults based on the flower structure in the seismic section, which is an apparent strike-slip deformation that was identified in the −1510-ms time slice and the root-mean-square amplitude attribute slice. This strike-slip motion was generated by the uplift of the Tibetan Plateau caused by the onset of the Indian-Eurasian collision. Since the Middle Miocene, the Kunbei fault system has undergone intense south-dipping thrusting, and a nearly 2.2-km uplift has been observed in the hanging wall in the Arlar fault. The south-dipping thrusting is the far-field effect of the full collision that occurred between the Indian-Eurasian plates. The lake area in the SW Qaidam Basin has been shrinking since the Middle Miocene and presents widespread delta and fluvial deposits, which are consistent with the proposed tectonic evolution.