Geochronology,Geochemistry And Geological Significance Of The Solonker Mélange Ophiolite,Inner Mongolia

The Solonker ophiolitic mélange is located in the west end of Solonker suture in the Xingmeng orogenic belt.In order to deepen our understanding of the late Paleozoic tectonic setting of the Xingmeng orogenic belt,a synthesis of the petrology,geochronology and geochemistry of the ophiolite component and the clastic sedimentary component in solonker ophiolitic mélange belt are reported on the basis of field geological investigation.The formation age,genetic type and tectonic background of ophiolite are clarified,the material source and tectonic environment of clastic sedimentary rocks are analyzed,the tectonic evolution of Xingmeng orogenic belt in Late Paleozoic and the indication significance of ophiolite mélange to the metallogenic potential of study area are discussed.Solonker ophiolite is distributed in nearly east-west direction,with complete rock types,mainly including serpentinized harzburgite,dunite,basalt,gabbro,diabase and radiolarian silicalite.Ophiolite is characterized by large and small blocks mixed in matrix.The basalts and gabbros in Solonker ophiolite are dominated by separate crystallization during their formation and are rarely contaminated by the crust.Basalts are originated from 5% ? 10% partial melting of the spinel lherzolite depleted mantle.gabbros are the source of garnet bearing spinel lherzolite depleted mantle,with greater melting degree than basalts.Meanwhile,both of them were metasomatized by subduction fluid during their formation,showing dual geochemical characteristics of N-MORB and island arc magmatic rocks,indicate that the Solonker ophiolite belongs to the supra-subduction zone(SSZ)type and may occur in expanding environment of the intra-oceanic back-arc basin.Solonker SSZ-type ophiolite represents the product of intra-oceanic subduction in the early Permian,it shows that the closing time of the Paleo-Asian Ocean(PAO)should be after the early Permian.From Benbatu formation of Upper Carboniferous to Zhesi formation of Middle Permian,the content of quartz in detrital framework components of sandstone is relatively reduced,the composition of lithic fragments and feldspar is relatively increased,and the sorting and rounding degree of grain is gradually reduced.Therefore,it is speculated that the sedimentary source area of Middle Permian is relatively close,or the water body is gradually shallow,and the sediment has not been transported for a long distance.According to the age distribution of detrital zircons,the Upper Carboniferous Benbatu Formation sandstone contains a small amount of northern accretionary orogenic belt sources,and contains no or few sources information of the North China Craton basement,while the Middle Permian Zhesi formation sandstone contains a large amount of North China Craton and the North accretionary orogenic belt provenance supply,reflecting the evolution of the ocean basin is close to the end stage during Middle Permian.The geochemical characteristics of each group of sandstones show that they were deposited on the active continental margin,indicating that the area was in a subduction related environment during the Late Carboniferous to the Middle Permian,and the closure of the Paleo-Asian Ocean(PAO)was after the Middle Permian.Based on the comprehensive analysis of regional strata,magmatic rocks,syn-collisional and post-collisional granites,biogeography,paleomagnetism and other data,it is concluded that the Paleo-Asian Ocean(PAO)finally closed in the Late Permian to the early Triassic along Solonker-Xar Moron.In addition,combined with the mineral data of the study area,it is considered that the metallogenic potential of ophiolitic mélange in this area is great.