Late Neopaleozoic Tectonic Evolution History Of The Hegenshan Oceanic Plate,NE China:Evidence From Isotopic Chronology And Geochemistry

The Inner Mongolia-Daxinganling Orogenic Belt(MDOB)is the eastern Chinese extension of the Central Asian Orogenic Belt.Oceanic crust subduction and microcontinental block collision had occurred in the Paleozoic,and forming extensive tectonic-magmatic activity and multiple suture zones.The Hegenshan ophiolitic complex is one of the most important suture zone in the MDOB.The ophiolitic suite developed along the suture zone,and in addition,Paleozoic magmatic rocks are also widely distributed on both sides of the suture zone.The study of the tectonicmagmatism in the Hegenshan suture zone is very important for understanding the formation and evolution history of the Hegenshan Ocean.This paper has undertaken U-Pb dating,major element,trace element and Hf isotope analyses of magmatic rocks located in the Hegenshan ophiolitic complex belt and Huliyasuta continental margin in the central Inner Mongolia,in order to determine the activity stages of the magmatic activity,petrogenesis and tectonic evolution history of Hegenshan Ocean.1.The Paleozoic magmatic activities consist of 4 stages in the Wuliyasutai continental margin northern the Hegenshan suture zone: the 315 Ma granite,the 301~303 Ma dolomite and trachyte,the 293 Ma granite,244 Ma trachyandesite.These magmatic rocks were formed on the active continental margin and originated from the new lower crust,which is the product of subduction of young oceanic crust.2.The Paleozoic magmatic activities consist of 5 stages in the W the Hegenshan suture zone: the 307 Ma granite,the 300~303 Ma quartz diorite,the 277~280 Ma granite and quartz diorite,244 Ma granite.Exception of 244 Ma granodiorite,the other magmatic rocks are of the origin of the new lower crust and are subduction products of the oceanic crust.The 244 Ma granodiorite is the product of the same collision period.3.The northward subduction of the Paleo-Asian Ocean may be occurred in late Proterozoic.The Hegenshan Ocean may have opened in the middle Ordovician to the early Silurian.The subduction of the oceanic crust of the Hegenshan ocean to the north should begin around 315 Ma.The most active period of subduction is about 300 Ma and continued to 260 Ma.The closing time should be between the Late Permian-early Triassic.