Ordovician amphibolite-facies metamorphism in Hainan Island: A record of early Paleozoic accretionary orogenesis along the northern margin of East Gondwana?

Abstract

• Polyphase metamorphism is observed in the Baoban Complex, Hainan Island. • SIMS zircon U-Pb geochronological dating yielded a metamorphic age of 462 Ma. • The peak metamorphic conditions are 715–650 °C/0.91–0.86 GPa. • The Ordovician metamorphism is associated with accretionary event along the northern Gondwana margin. Understanding the metamorphic evolution of Hainan Island is crucial to constraining its geodynamic history. Here, we report evidence for a hitherto unknown early Paleozoic amphibolite-facies event that affected the Baoban Complex in southwestern Hainan Island and establish its role in the tectonic history of the northeast margin of Gondwana. SIMS and LA-ICP-MS zircon U-Pb dating reveals a polyphase metamorphic history, consisting of three age groups. Group 1 performed on the outermost zircon rim with the lowest concentration of HREE and U, gives a weighted average age of 462 ± 7 Ma that corresponds to the metamorphic stage of the crystallization of garnet. Group 2 displays moderate HREE and low Th/U ratios (less than 0.079). Concordant U-Pb ages range from 1371 to 1017 Ma which record metamorphism in the Mesoproterozoic. Group 3 consists of typical igneous-derived detrital zircons with ages ranging from 3095 to 1380 Ma. The age spectrum is consistent with the siliciclastic protolith lithologies of the Baoban Complex. P-T conditions for the garnet-forming stage (Di + Grt + An + Hb + Q) were estimated by geothermobarometry at 715–650 °C and 0.91–0.86 GPa, which is consistent with 708 ± 44 °C conducted on zircons of Group 1 through Ti-in-zircon thermometry. P-T estimates and zircon U-Pb ages reveal that the amphibolite-facies metamorphism occurred at 462 Ma. The end-Mesoproterozoic metamorphism overlapped in age with the assembly of Rodinia. Combing with regional tectonothermal history, we infer that the Ordovician amphibolite-facies metamorphism formed during accretionary orogenesis along the northern margin of East Gondwana.