Analysis Of The Geochemical Characteristics And Structual Formation Environment Of The Volcanic Rocksin Mesozoic Era In Shumugou Area,Inner Mongolia

Affected by the Circum Pacific tectonic domain,from the Late Jurassic to Early Cretaceous the volcanic rocks developed well and experienced the Tamulangou period,the Manketouebo period,the Manitu period,the Baiyingao period,and the Longjiang period.In order to understand the age and genetic mechanism of Mesozoic large-scale magmatism in the Shumugou Area,and to get more comprehensive information for the study of the evolution history of regional tectonic setting,this paper has studied in detail the rock composition,age,and geochemistry of the Mesozoic volcanic rocks in the Shumugou area of Inner Mongolia,and discussed the origin of the volcanic rocks and the structural background.Through zircon U-Pb isotopic dating of 10 samples,the age of the Tamulangou Formation is 156.8±1.5Ma,the age of the Manketouebo Formation is 150.1±1.0Ma,and the age of the Manitu Formation is 142.9±1.6Ma.The age of the Baiyingao Formation is 139±1.7Ma,and the age of the Longjiang Formation is 120.2±2.2Ma.The main lithologic combination of the Tamulangou?the Manitu and the Longjiang Formation are neutral volcanic lava and clastic rocks,and the main lithologic combination of the Manketouebo and the Baiyingaolao Formation are acid volcanic lava and clastic rocks.There are mainly trachyteandesite and andesite,followed by dacite and rhyolite.Basaltic andesite is rare.The analysis of geochemical data of 46 samples in this area shows that the volcanic rocks of the Tamulangou Formation,the Manketouebo Formation,the Manitu Formation,and the Baiyin Gaolao Formation are calc alkalic series.The volcanic rocks of the Longjiang Formation is alkalic series.The total amounts of REE in the volcanic rocks of every formation are relatively high,and there are richment in LREE and loss of HREE.The Manketouebo and the Baiyingao Formation have obvious negative anomalies in Eu.The contents of Ni,Co,and Cr are extremely low.At the same time,they are rich in LILE,such as Rb,while they are short of HFSE,such as Nb?P?Ti.These characteristics indicate that the magma source of Mesozoic volcanic rocks in this area is not the primary magma of the mantle source,and is contaminated by the crustal material,and there is a complex crystallization differentiation during the evolution of the magma.Compared with volcanic rocks in other typical areas,the volcanic rocks in the this area are in good agreement with the main characteristics of Andean arcs.Through further geochemical analysis,it concludes that the formation environment of volcanic rocks is between the active continental margin and the intraplate transition zone,which reflects the change of the formation environment of volcanic rock is from extrusion to extension.Affected by the Pacific tectonics,during the late Jurassic,the subduction caused thickening of the crust and magmatic activity which produced a series of rocks that is similar to active continental marginal rocks,and formed the volcanic rocks of late Jurassic cycle;During the Early Cretaceous,regional tectonic settings transformed from compressive thickening to extentional thinning,and formed a series of volcanic rocks in the Early Cretaceous cycle.