Early Paleozoic tectonic evolution of the northern West Junggar (NW China): Constraints from Early Cambrian–Middle Silurian felsic plutons of the Chagantaolegai ophiolitic mélange

The early Paleozoic tectonic evolution of the Junggar Ocean (a major branch of the southern Paleo-Asian Ocean (PAO)) remains a topic of debate. This study mapped the Chagantaolegai ophiolitic mélange (COM) in the northern West Junggar (NWJ) on a large scale and identified several tectonically juxtaposed lithotectonic units, including the ophiolite suite and felsic plutons. The ophiolite suite mainly comprises serpentinized ultramafic rock, pyroxenolite, gabbro, dolerite, plagiogranite, basalt, volcanic rock, and radiolarian chert. Zircon U–Pb ages for two plagiogranite samples yielded ages of 515 ± 4 Ma and 513 ± 6 Ma, constraining the existence of the Junggar Ocean to the Early Cambrian at least. The felsic plutons can be subdivided into two groups. Group I (503–481 Ma), which intruded into the COM, shows low-K, calc-alkaline features with remarkable depletion of Nb, Ta, and Ti, resembling rocks formed in an intra-oceanic arc. Group II (435–428 Ma) was found on both sides of the COM and displays high-K, calc-alkaline series characteristics, similar to the Silurian to Early Devonian A-type granites in the NWJ, implying a post-collisional environment. Based on a combination of existing observations and our new observations and data with regional geological evidence, our new data indicate that the NWJ underwent a transition from the tectonic processes of seafloor spreading and subduction to arc–arc collision and post-collisional extension during the Early Cambrian–Early Devonian.