Characteristics,Age And Tectonic Environment Of Ophiolite Tectonic Melange Belt On The Area Yueyashan,inner Mongolia

The tectonic background of Beishan area is very complicated as it was located in the combination of the North China plate,kazakhstan plate and Tarim plate.There are four ophiolite belts from the north to the south of this region,which are named Hongshishan-Baiheshan-Pengboshan ophiolite belt,Jijitaizi-Xiaohuangshan ophiolitebelt,Hongliuhe-Xichangjingophiolitebeltand Huitongshan-Zhangfangshan ophiolite belt.The Yueyashan area,which is the research area of this thesis was located in the south of the Inner Mongolia Ejinaqi Alashan about 130km.Yueyashan ophiolite tectonic melange belt was was in the eastern part of Hongliuhe–Xichangjing ophiolite belt and nearly located in Yueyashan-Xichangjing area.We surely found out that Yueyashan ophiolite suite was exposed completely,and characterized by ultrabasic complex,gabbro,basalt and radiolarian siliceous rocks from the bottom to the top.The north part of ophiolite suite formed an ophiolite tectonic melange belt as an result of intense tectonic mélange.The ophiolite tectonic melange consisted of strong mylonitization and strong serpentinization pyroxene peridotite,basalt matrix and plagioclase granite,cumulate gabbro,pyroxenite,peridotite,hornblendite,Olivine gabbro,radiolarian siliceous rocks and so on.Combined with its petrochemistry,geochemistry,rare earth elements and trace elements characteristics,we come to the conclusion that plagioclase granite in our research area might belong to the part of the Yueyashan ophiolite suite.The low contents of TiO₂,Fe₂O₃+FeO and Na₂O,high contents of MgO and CaO of the studied gabbros from Yueyashan ophiolite suite may point out low degree of magmatic differentiation evolution of their source.According to the content of TiO₂,KO₂ and P₂O₅,the characteristics of basalt in Yueyashan ophiolite suite was similar to the mid-ocean ridge basalt?MORB?.All studied peridotites and pyroxenites were characterized by depletions in light rare earth elements?REE?,low degree fractionation of light and heavy rare earth elements,significantly positive Eu anomalies,left-leaning distribution patterns of REE,which may imply that their were the products of cognate magma evolution sequence.In spite of the complicated characteristics of the gabbros and basalts,they both reach an agreement with the features of normal mid-ocean ridge basalt?N-MORB?.The experimental results of trace element also prove out that the ophiolite type of this area belongs to the mid-ocean ridge basalt?MORB?type,which were produced in the plate eruption of a deep-sea environment.In addition,its characteristics and N-MORB type ophiolite were showing a high degree of consistency.We carryied out the research of zircon U-Pb isotopic dating of ophiolite suite rock samples.At the same time,we actualized U–Pb isotope experiments of rock samples deriving from theVolcanic arc of Xieshan--Dongqiyishan on the north side of ophiolite suite.The isotope dating confirmed that the formation age of Yueyashan ophiolite suite was about 530 Ma,the equivalent of early Cambrian.We could also come to the conclusion that ocean basin subduction of the era of mass from the south to the north was 421.0±15Ma-442.4±1.5Ma,the equivalent of Silurian.Based on the study of stratigraphic formation environment on both sides of the ophiolite belt and its significance to the limit of stratigraphic unit,we concluded that Hongliuhe--Xichangjing ophiolite belt,which separated the Kazakhstan plate to the north and Tarim plate to the south represented the ancient ocean closed plate suture zone.