Chert In The Ophiolite Mélange Within The Middle Part Of The Xingmeng Orogenic Belt And Its Tectonic Significance

The Xingmeng Orogenic Belt(XMOB)is located in the southeast of the Central Asian Orogenic Belt(CAOB).Since the Early Paleozoic,the Paleo-Asian Ocean(PAO)has experienced a series of island-arcs,fore-arc or back-arc basins,micro-continents,and remnants of oceanic crust.There are still debates on major scientific issues such as the closure of the PAO and the tectonic evolution process of the XMOB.Some scholars believe that the XMOB was formed by the closure of an ocean and orogenic,Others believe that the XMOB was formed during the Early Paleozoic and the tectonic evolution of the Late Paleozoic belongs to two different tectonic cycles.Because the chert blocks in the ophiolitic mélange belt carry abundant information about the paleo-ocean environment during their formation,a comparative study of the tectonic setting of the formation of the chert blocks developed in different ophiolitic mélange belts in the middle section of the XMOB is carried out,can provide strong evidence for the structural evolution of the XMOB.By summarizing the results of previous studies and combining the isotope age results obtained in this paper,it is believed that the formation age of chert in the Solonker-Mandula ophiolite mélange zone is Early-Middle Permian.The formation age of the chert in the Ondor Sum ophiolitic mélange should also be the Early-Middle Permian.In this paper,the zircon U-Pb of the gabbro rock block obtained in the Xiaoweitang ophiolite mélange.The age is 281±2.6 Ma.The age of the youngest Harmonic zircon in the siliceous mudstone is 239±1 Ma.Combined with the Middle Permian radiolarian fossils in the Xingshuwa ophiolitic mélange,it is considered that the chert in the Xiaoweitang ophiolite mélange zone were formed in the Middle-Late Permian.The chert in the Sonid Zuoqi-Diyanmiao ophiolite mélange zone were formed in the Late Carboniferous-Early Permian.The zircon U-Pb ages of the intrusive quartz diorite obtained in the Meilaotewula ophiolite mélange zone are 287.0±2.4 Ma and 293.8±1.8 Ma.According to the field contact relationship and the Middle-Late Devonian radiolarian found in the chert block in the Hegenshan ophiolitic mélange zone,it is believed that the formation age of the chert in the Erenhot-Hegenshan ophiolitic mélange zone is earlier than 293.8 Ma,and should be the Middle-Late Devonian.The geochemical characteristics of the chert in the ophiolite mélange zone in the middle part of the XMOB show that there are three areas: Solonker-Mandula-Weijing,Ondor Sum-Xiaoweitang,and Erenhot-Hegenshan-Meilaotewula,the chert in the above three ophiolite mélange zones have the characteristics of high content of Al and Ti elements,low content of Fe and Mn elements,and a flat distribution curve of rare earth elements.It is affected by biological processes and terrigenous materials in the continental margin environment.The chert in the Sonid Zuoqi-Diyanmiao ophiolite mélange zones has higher Fe and Mn content,lower Al and Ti content,and the distribution map of rare earth elements shows a left-leaning pattern with relatively depleted light rare earths.It is formed under the influence of magma hydrothermal action in an environment close to the mid-ocean ridge.Based on the analysis of the formation age,genesis and tectonic environment of the chert in the ophiolite mélange zone in the middle section of the XMOB,combined with previous research results,it is believed that the Solonker-Mandula ophiolite mélange zone travels eastward through the Ondor Sum,and it is connected to the Xiaoweitang ophiolite mélange zone,representing a Late Paleozoic ocean basin,and the ophiolite mélange zone has the characteristics of gradually getting younger from west to east,reflecting that the PAO may have experienced a scissors-like closure from west to east.The Erenhot-Hegenshan ophiolitic mélange zone was formed in the Late Devonian,and the XMOB is the product of two tectonic cycles.