Termination of the Paleo-Asian Ocean in the Beishan orogen, NW China: Constraints from detrital zircon U-Pb age and Hf isotope analysis of turbidites

Abstract

The termination of the Paleo-Asian Ocean, including duration and architecture, is controversial and has led to different reconstructions of the Central Asian orogenic belt. Deep-marine turbidites are of great significance in constraining the evolution of the oceanic basin. The youngest marine turbidites in the Beishan orogen in the central part of the southern Central Asian orogenic belt are separated by the Liuyuan mélange into two sections, the South turbidites and North turbidites. Detrital zircon grains from South turbidites yielded a maximum depositional age of 255 Ma. Those from North turbidites yielded a maximum depositional age of 254 Ma. Most of the Precambrian zircons of South turbidites have concentrated εHf(t) values (−6 to −2.6), whereas those of North turbidites range from −12.6 to +10.4, indicating different provenances. The North turbidites also have more positive whole-rock εNd(t) values (−0.5 to +1.1) than the South turbidites (−4.2 to −2.4), suggesting more juvenile or less evolved crustal components in the source. Detailed comparison of the detrital zircon ages and Hf-Nd isotopic characteristics with igneous and metamorphic rocks in the south Beishan area distinguishes two sources. These findings constrain the Liuyuan suture as the latest suture in the Beishan region, occurring no earlier than 254 Ma. The youngest peak age of detrital zircons of the youngest marine sedimentary rocks in the southern Central Asian orogenic belt becomes younger eastward from 288 Ma to 247 Ma, characterizing the closure duration of the Paleo-Asian Ocean from west to east in ∼40 m.y. This study finds that the youngest turbidites in the Beishan orogen not only record sedimentary processes in response to the final episode of orogenic evolution in the southern Central Asian orogenic belt, but they also constrain how and when an archipelago-type accretionary orogen terminated.