Chronological And Geochemical Characteristics Of The Late Mesozoic Granitic Rocks In Aershan Region Of The Central Great Xing’an Range And Their Tectonic Significance

The Great Xing’an Range,located in the eastern part of the Central Asian orogenic belt,is the tectonic superimposed composite part of the three tectonic domains of the Paleo-Asian Ocean,the Circum-Pacific Ocean and the Mongolia-Okhotsk Ocean,with strong tectonic-magmatic activity,and as the best window for studying the Mesozoic tectonic-magmatic evolution in the Northeast.This paper discusses the diagenetic age,petrogenesis and tectonic setting of the Late Mesozoic granite in the Central Great Xing’an Range by using the methods of petrography,zircon U-Pb chronology and rock geochemistry,and mainly obtains the following understanding:The LA-ICP-MS zircon U-Pb geochronology results show that the petrogenic age of medium-fine grained granodiorite in the study area is 165±1Ma,the diagenetic ages belong to Middle Jurassic;the petrogenic ages of medium-fine grained syenogranite and medium-coarse grained monzogranite are 152±1Ma,147±1Ma and 148±1Ma,the diagenetic ages belong to Late Jurassic;the petrogenic age of fine-grained syenogranite is 138±1Ma,the petrogenic ages of quartz monzonite porphyry is 137±1Ma and 129±1Ma,the diagenetic age belong to Early Cretaceous.According to the research results of rock geochemistry in the study area,it is concluded that the Middle Jurassic and Late Jurassic granites samples in the study area are rich in silicon and alkali,poor in magnesium,iron and titanium,the low ratio of TFe O/Mg O and Zr/Hf,with medium to strong europium negative anomaly,They are enriched Rb,Th,U,La,Ce,Nd,Hf and depleted in Ba,Nb,Sr,P,Ti,the above reflects that it belongs to high differentiation I-type granites,and the magma originate from the partial melting source area of the lower crust;Early Cretaceous granite samples are rich in high silicon and potassium,relatively rich in aluminum,poor in titanium and magnesium,the low ratio of TFe O/Mg O and Zr/Hf,with weak to medium europium negative anomaly,they are relatively rich in Rb,Th,U,La,Ce,Hf and depleted in Ba,Nb,Sr,P,Ti,the above reflects that it belongs to medium to high differentiation I-type granite,and the magma originate from the partial melting source area of the lower crust.In this dissertation,the Late Mesozoic magmatism age of the Alshan in the Central Great Xing’an Range is measured to be from the Middle Jurassic to the Early Cretaceous.It is speculated that the Middle Jurassic granite was formed in the collision and compression tectonic setting,and the diagenetic tectonic setting was related to the southward subduction of the Mongolia-Okhotsk Ocean;the Late Jurassic to Early Cretaceous granites were formed in a postcollision to post-orogenic extension environment,and the diagenetic tectonic setting was related to the closure of the Mongolia-Okhotsk suture zone and its later evolution.There are 26 figures,7 tables and 87 references in this dissertation.