Origin And Tectonic Significance Of Hongliuhe Ophiolite Belt In Beishan,Southern Margin Of Central Asian Orogenic Belt

Beishan orogenic belt,located in the southern margin of the Central Asian Orogenic Belt,is the key area to study the tectonic evolution of the Paleo-Asian Ocean splitting-expansion-subduction-closure and the Central Asian Orogenic Belt.There are many ophiolite belts in the area,of which the Hongliuhe-Yushishan-Yueyashan ophiolite belt is the early Paleozoic ophiolite formed in Beishan area,and it is also the most complete and largest ophiolite belt in Beishan area.In this paper,we take the Hongliuhe-Yushishan-Yueyashan ophiolite belt as the research object and carry out a detailed study of the time-space distribution,rock association,petro-geochemistry,zircon U-Pb chronology and Sr-Nd isotope of ophiolite belt based on detailed field geological survey and laboratory research.The result revealed the genesis,age and tectonic background of Hongliuhe-Yushishan-Yueyashan ophiolite belt,and determined the initial subduction time of the southern part of the Paleo-Asian Ocean for the first time.Meanwhile,the tectonic evolution model of the southern margin of the Paleo-Asian Ocean in the Early Paleozoic was preliminarily constructed.The main achievements and understandings are as follows:1.The Hongliuhe-Yushishan-Yueyashan ophiolite is mainly composed of serpentinized peridotite?pyroxenite?,gabbro,basalt and siliceous rocks.It is a relatively complete set of ophiolite suites,exposed in front of Huaniushan-Shuangyingshan continental margin arc,and tectonically intruded on the Precambrian crystalline basement,which is in obvious fault contact with the strata.Zircon U-Pb ages of gabbro in Hongliuhe,Yushishan and Yueyashan ophiolites are525.2±4.6Ma,535.3±3.6Ma and 538.9±3.1Ma,respectively,representing the age of ophiolite formation.2.The SiO₂ content of ultramafic rocks in Hongliuhe-Yushishan-Yueyashan ophiolite belt is37.26%⁴8%,which is characterized by low Al₂O₃?2.81%⁵.82%?,low TiO₂?0.02%⁰.28%?,low K₂O?0.01%⁰.04%?,low Na₂O?0.13%⁰.6%?,rich MgO?24.4%³3.1%?and Fe₂O₃?7.25%¹2.3%?.The values of Mg^#?68.55⁷4.91?consistent with the original mantle?68⁷2?and all belong to ultramafic cumulation?UC?;gabbro and basalt have SiO₂ content ranging from 45.23%to 53.6%,with low MgO?4.23%¹5.9%?,low K₂O?0.03%¹.9%?,low Na₂O?0.45%⁴.15%?,low TiO₂?0.06%¹.46%?,high Al₂O₃?10.24%²1%?and high CaO?6.02%¹6.7%?and belong to the transition region of calc-alkaline and tholeiitic series.Low MgO and Mg^#indicate that they have experienced a certain degree of fractional crystallization.The chondrite-normalized REE partition curve is similar to that of N-MORB,and in the primitive mantle-normalized trace element spider diagrams,it shows the characteristics of enrichment of large ion lithophile elements?LILE,such as Rb,Ba,K,Sr?,loss of high field strength elements?HFSE,such as Nb,Ta,Ti?.The Nb/U ratio?7-13?is close to the IAB range,indicating that the ophiolite has both N-MORB and IAB geochemical characteristics.The ⁸⁷Sr/⁸⁶Sr_?t?values of gabbro and basalt are between 0.703264and 0.705432,while the value of?Nd?t?is between+2.91 and+12.25.MgO and Co,Ni of gabbro and basalt are weakly positively correlated,while SiO₂,Al₂O₃,CaO and V are weakly negatively correlated,indicating the fractional crystallization of olivine.By comparing the composition of gabbro-basalt and the pre-arc,island-arc and back-arc volcanic rocks of the Izu-Bonin-Mariana subduction system in the discrimination diagram of Zr-Zr/Y,Zr-Ti,Mg-^TFeO-Al₂O₃,Hf-Th-Nb,Ta/Yb-Th/Yb,Th/Nb-Ce/Nb,it is found that the characteristics of gabbro and basalt in the Hongliuhe-Yushishan-Yueyashan ophiolite are consistent with those of the pre-arc basalt,indicating that they are SSZ-type ophiolite formed in the pre-arc environment.3.Based on the chronology and genesis of the Hongliuhe-Yushishan-Yueyashan ophiolite and the previous research results,this paper considers that the Paleo-Asian Ocean crust subducted southward initially in the early Cambrian?525Ma⁵38Ma?due to plate subsidence,forming magma with both N-MORB and IAB geochemical characteristics in front of the arc.The magma formed a new forearc oceanic crust through fractional crystallization and ascending emplacement,which closed in the Late Silurian and eventually accreted in the forearc environment of Huaniushan-Shuangyingshan continental margin arc,forming the Hongliuhe-Yushishan-Yueyashan ophiolite.