The Geological Significance Of The Early Paleozoic Mélange In Chegendalai,inner Mongolia

Chegendalai mélange is located in the north domain of the boundary named Bayan Obo-Chifeng fault between the North China craton and the Xing'an-Mongolia orogenic belt,but its tectonic affinity is still unknown.The tectonic relationship between Chegendalai tectonic mélange and the Early Paleozoic arc magmatic rocks and the tectonic significance of Chegendalai tectonic mélange is still debated.Therefore,geochemistry and isotope geochronology data are used in this study to better understand the tectonic affinity Chegendalai mélange and the relationship with the arc magmatic rocks.Quartz diorite and its andesitic xenolith that occurring in the north domain of Bayan Obo area,which were developed in the Late Ordovician.Both of which show the characteristics of volcanic arc magmatic rocks that developed in the subduction zone,combined with its zircon U-Pb ages and geochemical data,their tectonic setting should be active continental margin.They are the products of the southward subduction of the Paleo-Asian oceanic crust.Chegendalai mélange consists of the fine-grained clastic sedimentary rocks as the matrix,metagabbros,diabases,and metaperidotites as the blocks.The diabase and some metagabbro blocks within the Chegendalai mélange are belonging to calc-alkaline basaltic rocks,and show the characteristics of continental island arc magmatic rocks that source from Bainaimiao island arc.Other metagabbro and metaperidotite blocks are components of the ophiolite that dismembered by late-stage tectonic movement.Through the geochemical analysis of Chegendalai ophiolite,SSZ type ophiolite is suggested.The crystallization age of our pyroxene peridotite sample is ca.440 Ma,and the zircon U-Pb age of our diabase porphyrite sample is 431±3 Ma,indicating that the Bainaimiao back-arc oceanic basin was developed during the Early Silurian.The emplacement age of Chegendalai ophiolite is the Middle Silurian,which reflects that the back-arc basin began to close at this time.The Ondor Sum subduction complex placed at the north to the study area,the Bainaimiao magmatic arc and back-arc basin constitute the Early Paleozoic trench-arc-basin system.Combining the Xibiehe Formation that deposited in the upper Silurian with the characteristics of molasses deposition that widely deposited in Bainaimiao arc,and the initial activity time of Xuniwusu thrusting that widely developed in Bainaimiao area,which suggests that the arc-continent collision occurred at the latest Silurian,and the evolution of the trench-arc-basin system were ended at this time.