ABSTRACT

The Western Kunlun orogenic belt (WKOB) located south of the Tarim Basin and the north-western margin of the Tibetan Plateau, was previously considered a complex orogenic belt that closed along the Kudi-Qimanyute suture zone (KQSZ) and Mazar–Kangxiwar suture zone (MKSZ) from north to south during Proto- and Paleo-Tethys. The MKSZ between the South Kunlun and Tianshuihai terranes was interpreted as the southern boundary of the WKOB that formed during the subduction of the Paleo-Tethys oceanic crust. However, the evolution of the MKSZ in the Proto-Tethys Ocean remains controversial. We newly recognized seamount formation with pillow basalts and carbonate cap from Dongguashan group on Tianshuihai terrane. The pillow basalts had geochemical features of typical oceanic island basalts (OIBs). Zircon U–Pb dating revealed that this basalt had a crystallization age of 465 ± 6.6 Ma, with a gap of more than 10 Ma between the pillow basalts and fossils in the seamount. This implied that the basalt base reached the carbonate compensation depth. Accordingly, the seamount depositional age was restricted to the Late Ordovician. Detrital zircon showed that part of the clastic unit at the top of the Dongguashan group originated from the South Kunlun and Tianshuihai terranes, suggesting that the analysed sediments probably formed in the remnant of the Proto-Tethys Ocean and were deposited on the top of or accreted into the seamount during oceanic crust subduction. This discovery provides robust evidence of the MKSZ undergoing an evolution with Proto-Tethys. Moreover, our results supported the approach that an accretionary wedge, including the Late Ordovician seamount in the southern MKSZ, should be considered part the WKOB.

摘要

位于塔里木盆地以南、青藏高原西北缘的西昆仑造山带（WKOB）以前被认为是沿着库地-奇马约特缝合带（KQSZ）和马扎尔-康西瓦缝合带闭合的复杂造山带（MKSZ）在原始和古特提斯期间从北到南。南昆仑和天水海地体之间的MKSZ被解释为在古特提斯洋壳俯冲过程中形成的WKOB南边界。然而，原特提斯洋中 MKSZ 的演化仍存在争议。我们在天水海地体上新发现了东瓜山群的枕状玄武岩和碳酸盐盖层海山组。枕状玄武岩具有典型大洋岛玄武岩（OIBs）的地球化学特征。锆石 U-Pb 测年表明，该玄武岩结晶年龄为 465 ± 6.6 Ma，枕形玄武岩与海山化石之间的间隙超过 10 Ma。这表明玄武岩基底达到了碳酸盐补偿深度。相应地，海山沉积时代仅限于晚奥陶世。碎屑锆石显示，东瓜山组顶部的部分碎屑单元起源于南昆仑和天水海地体，表明分析的沉积物可能形成于原特提斯洋的残余物中，沉积在或增生的顶部。在洋壳俯冲过程中进入海山。这一发现为 MKSZ 与 Proto-Tethys 一起进化提供了强有力的证据。此外，我们的结果支持增生楔的方法，