Diachronous subduction of the Proto-Tethys Ocean along the northern margin of East Gondwana: Insights from SHRIMP and LA-ICP-MS zircon geochronology in the West Kunlun Orogenic Belt, Northwestern China

The geological evolution of the Proto-Tethys Ocean remains vague. The Tianshuihai Terrane (TSHT), a subterrane of the West Kunlun, distributed to the south of the Proto-Tethys Ocean during the Early Palaeozoic, records abundant information on the geological evolution of the Proto-Tethys Ocean. In this study, we reported SHRIMP and LA-ICP-MS zircon U–Pb ages, whole-rock major and trace element composition data, and Sr-Nd-Hf isotopes of a suite of monzogranites and the monzonitic-syenitic enclaves in Zankan and syenogranites in Laobing regions of the TSHT, West Kunlun Orogenic Belt. The syenogranites in the Laobing region yielded SHRIMP zircon U-Pb ages of 550.4 ± 6.4 Ma to 547.5 ± 5.3 Ma, which were the earliest age records of subduction-related magmatism in the TSHT during the Late Neoproterozoic-Early Palaeozoic. The host monzogranites in the Zankan area yielded SHRIMP and LA-ICP-MS zircon U-Pb ages of 542.6 ± 8.4 Ma to 540.5 ± 2.8 Ma, which is coeval with the monzonitic-syenitic enclaves ages of 533.8 ± 3.4 Ma to 534.7 ± 3.0 Ma. We speculated that an active margin developed along the TSHT during the Cambrian and the initial subduction of the Proto-Tethys oceanic slab must have occurred prior to the Early Cambrian (>550 Ma). The TSHT and the Southern Kunlun Terrane were distributed between the northern margin of the East Gondwana continent and the Tarim Block. Additionally, the coexistence of two branches of the Proto-Tethys Ocean represented by the Kangxiwa Fault and Kudi Ophiolite Belt during the Early Palaeozoic. Based on the chronological statistics of micro-continental blocks in the northern margin of the East Gondwana continent, subduction of the Proto-Tethys Ocean could be diachronous, initially originating in the northwestern part of the East Gondwana continent, and gradually propagating to the east of the East Gondwana continent.

原特提斯洋（Proto-Tethys Ocean）的地质演化历程至今仍不甚明晰。天水海地体（Tianshuihai Terrane, TSHT）是西昆仑造山带的次级地体，早古生代时期分布于原特提斯洋南侧，其保存有丰富的原特提斯洋地质演化相关信息。本研究针对西昆仑造山带天水海地体扎坎（Zankan）与老冰（Laobing）地区的岩石样品开展分析，获取了二长花岗岩及其所含二长-正长岩包体、老冰地区正长花岗岩的SHRIMP及激光剥蚀电感耦合等离子体质谱（LA-ICP-MS）锆石U-Pb定年数据、全岩主量与微量元素组成数据以及Sr-Nd-Hf同位素数据。老冰地区的正长花岗岩获得的SHRIMP锆石U-Pb年龄为550.4±6.4 Ma至547.5±5.3 Ma，这是新元古代晚期-早古生代时期天水海地体内与俯冲作用相关的岩浆作用的最早年龄记录。扎坎地区的寄主二长花岗岩获得的SHRIMP及LA-ICP-MS锆石U-Pb年龄为542.6±8.4 Ma至540.5±2.8 Ma，其形成时代与二长-正长岩包体的533.8±3.4 Ma至534.7±3.0 Ma年龄一致。研究认为，寒武纪时期天水海地体沿线发育活动大陆边缘，原特提洋洋壳板片的初始俯冲作用发生于早寒武世之前（>550 Ma）。天水海地体与南昆仑地体（Southern Kunlun Terrane）均位于东冈瓦纳大陆（East Gondwana continent）北缘与塔里木地块（Tarim Block）之间。此外，早古生代时期存在以康西瓦断裂（Kangxiwa Fault）与库地蛇绿岩带（Kudi Ophiolite Belt）为代表的原特提斯洋两支分支。基于东冈瓦纳大陆北缘微陆块的年代学统计结果，原特提斯洋的俯冲作用可能具有穿时性：初始俯冲起始于东冈瓦纳大陆西北部，随后逐渐向东冈瓦纳大陆东部扩展。