Geology Of The Diyanmiao Ophiolite In Xi U Jimqin Banner,Inner Mongolia And Its Tectonic Significance

The Inner Mongolia-Daxinganling Orogenic Belt?IMDOB?is situated at the southeastern margin of the Central Asian Orogenic Belt?CAOB?and is a key area for understanding the Paleozoic tectonic evolution of the CAOB.The IMDOB is a giant accretionary orogen between the Siberian craton and the North China craton?NCC?and is characterized by a series of ophiolitic belts.The study of these ophiolitic belts is so limited that its attribute and evolutional time are still controversial,which has constituted an obstacle for understanding the evolution of IMDOB.Newly discovered Diyanmiao ophiolite is situated at the eastern part of The Erenhot-Hegenshan ophiolite belt?EHOB?.According to field geological investigation and the results of petrology,geochemistry and geochronology,we found Diyanmiao ophiolite outcrops completely,including harzburgite,beded–massive gabbro,anorthosite,spilite,pillow basalt,and chert from bottom to top,locally with plagiogranite vein invading in pillow basalt.According to geochemical characteristics,the lavas of Diyanmiao ophiolite are divided into three groups,the first group belong to boninite series,which is characteristic of enrichment in Si?SiO₂=52.71%,61.22%?,Mg?MgO=6.81%,10.88%?and depletion of Ti?TiO₂=0.48%,0.51%?,HREE,HFSE;the second are similar with island arc tholeiite?IAT?,with low Ti?TiO₂=0.55%⁰.78%?,high Mg?MgO=5.20%¹2.41%?,and the Chondrite-normalized REE patterns of the second group are of characteristics of LREE depletion,similar to N-MORB,while the spider diagram shows the enrichment of the Large Iron Lithosphile Elements and depletion of High Field Strength Elements-the unique feature of the arc volcanic rocks;the third are similar with the ocean island basalt?OIB?,characteristics by high Ti?TiO₂=1.86%,1.91%?,Mg?MgO=5.25%,5.46%?and P?P₂O₅=0.23%,0.27%?,and high Th/Ta radios?2.32,2.53?.According to the existence of boninite and IAT,we suggest that the Diyanmiao ophiolite was likely formed in the arc and fore-arc setting.The plagiogranites are characterized by high SiO₂,low A1₂O ₃,with intensively rich in Na and poor in K.Furthermore,they also display low?REE concentration and little depleted-flat LREE pattern,indicating they should belong to the typical ocean plagiogranite formed by basaltic mama's crystallization from depleted mantle source and is a part of the ophiolite.LA-ICP-MS U-Pb dates on Zircon of massive gabbro,beded gabbro and plagiogranite yield ages of 359.1±6.4Ma,330 Ma and 328.2±1.5 Ma,respectively,indicating a late-carboniferous age of Diyanmiao ophiolite.All the research results above provide important evidence in exploring the tectonic evolution of Inner Mongolia-Daxinganling Orogenic Belt?IMDOB?.