Geochemical Characteristics And Tectonic Significance Of The Jijitaizi Ophiolite In Beishan

Restoring the Paleozoic oceanic-continental transformation process in Beishan is one of the key issues in the study of the Central Asian orogenic belt,and the corresponding ophiolite belt is an important part of it.Based on the field geological survey and previous research results,this paper studies the LA-ICP-MS zircon U-Pb geochronology,geochemistry and Sm-Nd isotope system of the important components of the Qintaizi ophiolite with high accuracy,reveals the type,nature,formation age,rock association,genesis and tectonic background of the ophiolite.It provides important petrological and chronological basis for inversion of Paleozoic tectonic evolution history in Beishan area and even in Central Asia orogenic belt.The main achievements and understandings are as follows:1.The ²⁰⁶Pb/²³⁸U age of gabbro in the Jijitaizi ophiolite ranges from 479 Ma to482 Ma,with a weighted average of 479.5+3.9 Ma?MSWD=0.0091?.It represents the formation age.2.Pyroxene The Jijitaizi ophiolite is mainly composed of pyroxene peridotite,plagioclase peridotite,peridotite,gabbro and basalt.The basalt has relatively stable SiO₂?47.71wt.%⁵4.96wt.%??Al₂O₃?13.38wt.%¹5.38wt.%?,and TiO₂?0.93wt.%¹.25wt.%?content,low MgO content?6.21wt.%⁹.66wt.%?,Mg^#?28⁴6?,a wide range of K₂O?0.19wt.%¹.32wt.%?and CaO?4.82wt.%⁸.51wt.%?,indicating that basalt magma has undergone a certain degree of separation and crystallization.The basalt is slightly enriched in LREE,?La/Yb?_N=1.53³.62,which is close to Ping.The HREE distribution of flat type is characterized by slight,negative Eu anomaly to no anomaly,with?Eu=0.88¹.06.Standardized cobweb maps of basalt primitive mantle show that the rocks are enriched with Rb,Th,U,Ba and depleted with Nb,Ta,Zr,Hf and Ti in varying degrees.The ratio of basalt(¹⁴³Nd/¹⁴⁴Nd)_t ranges from 0.5190 to 0.2628,and?Nb_?t?=1.9².6=1.9-2.6.The basalts in Zi ophiolite have both N-MORB and IAB geochemical characteristics,and are SSZ-type ophiolites.The gabbro in Qinqitaizi ophiolite has a small range of SiO₂?50.39wt.%⁵1.66wt.%?,TiO₂?0.77wt.%⁰.95wt.%?and MgO?6.04wt.%⁶.73wt.%?contents,with high Al₂O₃?15.82wt.%¹6.79wt.%?,CaO?8.93wt.%¹0.4wt.%?,rich in sodium and poor in potassium,Na₂O?4.18wt.%⁴.63wt.%?and K₂O?0.27wt.%⁰.44wt.%?.Gabbro samples are flat in REE chondrite standardized partition map,?La/Yb?_N=0.93¹.10,without Eu anomalies and?Eu=0.98¹.09.In the original mantle standardized diagram of trace elements,samples Rb,Th,U,K show relative enrichment,Nb-Ta,Zr-Hf depletion,Sr shows positive anomalies may be related to plagioclase cumulation,and the overall characteristics are SSZ-type ophiolite.It indicates that gabbro and peridotite in the Jijitaizi ophiolite should be the product of cumulation.3.Based on previous research results and regional geological background,it is considered that during the Ordovician-Late Silurian period,the Paleo-Asian Ocean represented by Hongliuhe-Niujuanzi ophiolite subducted northward to form the Gongpoquan continental margin arc and triggered back-arc secondary expansion.The oceanic crust of the back-arc basin was formed by partial melting of the mantle material under the influence of subduction fluids.Under the action of subduction and migration in Devonian,the Jijitaizi-Xiaohuangshan Ocean began to subduct southward and closed in Early Carboniferous,finally forming the Jijitaizi-Xiaohuangshan ophiolite belt.