Geochemistry and geochronology of early Palaeozoic seamount in Western Kunlun orogenic belt and the tectonic implications

Abstract

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.