The boundary between the Central Asian Orogenic belt and Tethyan tectonic domain deduced from Pb isotopic data

Abstract

A detailed comparison of the tectonic features of Central Asian Orogenic belt (CAOB) and Tethyan Tectonic domain (TTD) is of great significance to our understanding of the origin of global orogenic systems. Currently, there are many uncertainties in the general framework to fully define the tectonic properties of the CAOB and TTD. The Pb isotope data from Paleo-Asian Ocean (PAO) ophiolites in the CAOB and Tethyan ophiolites in the TTD allow us to conduct a detailed comparative study between these two global orogenic systems. Results of the study show the presence of an isotopic boundary between the different mantle domains and tectonic properties of the CAOB and TTD, with the Xinjiang region in the former representing the transition between the two systems. The distinctive 208Pb/204Pb isotope compositions of the PAO and Tethyan mantles suggest the existence of a long time-integrated lower Th/U reservoir beneath the CAOB compared to that beneath the TTD throughout the Paleozoic. Results thus suggest the distinct Pb isotope compositions of the PAO and Tethyan mantles are intimately related to the different magma–tectonic processes that formed the CAOB and TTD. Based on plate tectonic reconstruction, the Neoproterozoic to Paleozoic evolution of the accretionary margins of the CAOB mimics the modern circum-Pacific Ocean rim. In this scenario, the PAO had a low Th/U mantle isotopic signature and the subduction of PAO crust gave rise to the circum-Pacific type accretionary orogen. On the other hand, the Tethys oceans produced the high Th/U mantle isotopic signatures in an evolving collisional orogen. Significantly, the generally radiogenic and juvenile Hf isotopic signature of the CAOB is consistent with an accretionary orogenic setting for PAO whereas the relatively more unradiogenic Hf isotopic signature of TTD is consistent with a collisional orogenic setting for Tethys oceans. Thus, our study sheds some light on the PAO evolution as well as the plate tectonic reconstruction of Central Asian and Tethyan orogens.