SHRIMP Dating Of The Key Horizons Of The Precambrian And The Neoproterozoic Tectono-palaeogeography Of Southwest Sichuan,China

Southwest Sichuan is located in the southwestern margin of the Yangtze Paleocontinent.It is characterized by a large number of Mesoproterozoic and Neoproterozoic shallow metamorphic rocks.It has the basic characteristics of sedimentary strata,magmatic activity and deformation metamorphism in the orogenic belt.Based on the theories and methods of Stratigraphy,Isotope chronology,Geochemistry and Rock tectonics,the correlation framework of the main stratigraphic units in the study area is optimized by zircon U-Pb dating of key horizons.On this basis,the Neoproterozoic plate tectonic framework was reconstructed and the relationship between the Tianbaoshan Formation and the Jiangnan Orogenic Belt was discussed by deeply analyzing the formation age,magmatic activity characteristics and prototype basin attributes of the Tianbaoshan Formation.Major new developments have been made as follows:1.The SHRIMP zircon U-Pb age of the rhyolitic melted tuff in the upper and middle part of the Tianbaoshan Formation in northern Huili is 1007.3±8.1 Ma,which confirms that the age of the Tianbaoshan Formation can be extended to the early Neoproterozoic.2.In the northern part of Yanbian County,the original correlation of the outcrop along the Yalong River becomes a set of acid volcanic rocks with tuffaceous mudstone series of the Lieguliu Formation.The SHRIMP zircon U-Pb age of volcanic rocks is 703.0±4.1 Ma,combining with regional stratigraphic correlation,the volcanic rock series belongs to the bottom of the Guanyinya Formation.Its age is 703.0±4.1Ma,which is the bottom age of Guanyinya Formation.3.Based on the geochemical characteristics,sedimentary assemblages and lateral variations of the Tianbaoshan Formation,it is determined that the Tianbaoshan Formation was formed in the magmatic arc and its back-arc basin.At that time,Southwest Sichuan should be located in the southern margin of the uplift plate,which belongs to the subduction orogenic belt.4.Zircon Lu-Hf isotope studies show that the magma source area of intermediate-acid volcanic rocks in the Tianbaoshan Formation is dominated by ancient crust remelting and neogenic crust.5.Based on the new achievements of the paper,combined with the previousresearch results and regional geological survey data,the Tectonic-Palaeogeographic pattern of the Kangdian paleo-continent area in the late pre-Nahua Period was reconstructed.The tectonic evolution in this area can be divided into 1730-1650 Ma continental rift stage,1650-1126 Ma passive continental margin development stage and 1126-820 Ma convergent orogenic stage.Furthermore,the West extension direction of the Jiangnan orogenic belt is discussed.