The Tectonic Environment Characteristics Of The Great Xing'an Mountains South Area In The Late Mesozoic Era-The Contrast Between Mid-ridge And East Area

The Malegenba and Chaoyanggou granites locat in the main ridge of Da Hinggan Mountains,whereas Budunhua and Yezhugou granites locat in the eastern,all of them intruded between Xilamulun river fault and Hegenshan fault.It is critical to study their characteristics and the differences of these four granites from late Jurassic to early Cretaceous.In this contribution,we present petrology,major and trace elements geochemistry,and zircon U-Pb ages research of these four different granites,in order to constraintthe magmatic activity background of late Jurassic to early Cretaceous of Da Hinggan Mountains..According to the diagrams of the A/CNK-A/NK,AR-SiO₂,SiO₂-K₂O and thin section identification,the rocks of main ridge of Da Hinggan Mountains belong to high potassium calc-alkaline-quasi aluminum granite,whereas rocks of the eastern slope show the characteristics of calcium alkaline to high potassium calc alkali-quasi aluminum to peraluminous TTG granites.In situ LA-ICP-MS zircon U-Pb dating of Chaoyanggou and Budunhua granites from main ridge yield an age of154±1 Ma and 154.1±1.6 Ma,respectively,both of which belong to the late Jurassic,whereas Malegenba and Yezhugou granites form eastern slope yield an age of144.62±0.74Ma and 140.2±2.7Ma respectively,both of which belong to the early Cretaceous.Granites of main ridge were more depleted in Ba,Nb,Sr,P,Ti,Eu elements than that of the eastern slope.?Zr+Nb+Ce+Y?and 10⁴Ga/Al discriminant diagrams,along with the A/CNK,Na₂O and P₂O₅ content,show that the granites of main ridge and eastern slope belong to I-type granite,and we come to the concludes when combined with zircon saturation temperatures,thatgranites of main ridge are highly fractionated I-type granite,while that of eastern slope are I-type granites form thickened crustal.Discriminant diagrams also show that granites of main ridge origin from a syn-collision environment,whereas that of eastern slope show an arc volcano environment.During late Jurassic to early Cretaceous,the East Asian continent experienced a high-speed oblique subduction striking NNW from the Izanagi plate,which produced a compresso-shear environment along the margin of the East Asian continent.Collisional orogeny process of the Mongolia-Okhotsk orogenic belts ended in late Jurassic,crustal extension of Da Hinggan Mountains probably formed within the following extensional regime in post-collision setting,and the Pacific plate probably had a relatively less effect on the south of Da Hinggan Mountains,however.Therefore,the south of Da Hinggan Mountains region probably under the combination effect of high-speed oblique subduction striking NNW from the Izanagi plate and the closure of the Mongolia-Okhotsk ocean during the late Jurassic to Early Cretaceous.The granites of main ridge were probably formed by the mixture of mantle-derived magma and the partial melts of the lower crust underplating by which,and located along the fault zone of main ridge of the Da Hinggan Mountains,whereas the granites of the eastern slope formed under the process of collision thickened at that time.