Early-Paleozoic geodynamics of the Altai orogen: Constraints from geochemical and zircon U-Pb-Hf isotopic study of paragneissic rocks from the southern Chinese Altai

The high-grade metamorphic complexes in the Chinese Altai were previously regarded as the Precambrian basement and thus important for unravelling tectonic evolution of the Altai orogen. This study reports detailed filed investigation, zircon U-Pb-Hf isotopic and whole-rock geochemical data for the paragneissic rocks from Northern Fuyun Complex (NFC), southern Chinese Altai. Detrital zircons from the paragneisses have a predominant early Paleozoic age population (ca. 535-435 Ma), with minor Neoproterozoic and sparse Mesoproterozoic to Archean ages. The geochemical analyses together with the euhedral shape of the detrital zircons suggest that their sedimentary protoliths mainly came from felsic-intermediate igneous rocks with low maturity. In combination with the cumulative distribution curves of zircon age spectra, the variable zircon εHf(t) values (-25 to +13), as well as the immature geochemical compositions, we infer that the protoliths were most likely deposited on an active continental margin in the early Paleozoic and sourced mainly from proximal igneous rocks, which are comparable to the Habahe Group. Similar detrital zircon age spectra of early Paleozoic sequences from the Chinese Altai, Mongolia Altai and Khovd Zone support the existence of a giant accretionary wedge developed along the western margin of the Ikh-Mongol Arc system, resulting from continuous northeast-dipping oceanic subduction. This research was financially supported by the National Key R&D Program of China (2017YFC0601205), Hong Kong RGC GRF (17302317 and 17303415) and NSFC Projects (41730213 and 41190075).