Geochemical characteristics, zircon U-Pb age of SSZ ophiolite in the Baiheshan area of the Beishan orogenic belt, Inner Mongolia, and its indication for the evolution of the Paleo-Asian Ocean

The Hongshishan-Baiheshan ophiolite mélanges belt is the northernmost ophiolite mélanges belt in the Beishan area.However, most of the researchers have been focused on the Hongshishan ophiolite mélanges but paid very insufficient attention to the Baiheshan ophiolite.The authors analyzed geochemistry and zircon U-Pb chronology of gabbros in order to understand the ocean basin tectonic environment and formation time.The results show that gabbro has high content of Al₂O₃, MgO and LILE but poor content of P₂O₅ and HFSE.Meanwhile, the gabbro have bulk Sr-Nd isotope composition of (⁸⁷Sr/⁸⁶Sr)_i=0.70418~0.70711, (¹⁴³Nd/¹⁴⁴Nd)_i=0.512392~0.512568, and ε_Nd(t)=+ 4.61~+ 7.28.These data show that the magma of gabbro might have come from depleted mantle source area and the formation process of gabbro was affected by the subduction fluid.The LA-MC-ICP-MS zircon U-Pb dating yielded a ²⁰⁶Pb/²³⁸U age of 344.6±1.8 Ma for the Baiheshan ophiolite.The petrogeochemical analysis shows that gabbro formed SSZ type ophiolites in the subducting plate above the immature back-arc basin.The result suggests that Baiheshan ophiolite mélanges belt might have been a Carboniferous limited ocean formed in the Queershan-Yuanbaoshan arc and may have been the branch ocean basin of the Paleo-Asian Ocean in the Beishan area.According to the studies of the Late Carboniferous-Early Permian Baishan magmatic arc located in the southern part of the Hongshishan-Baiheshan ophiolite mélanges belt, the authors hold that the evolution of the Paleo-Asian Ocean lasted at least to the end of the Late Paleozoic.