The Research Of Early-middle Mesozoic Igneous Rocks In Eastern Jilin And Heilongjiang Provinces And Its Tectonic Implications

Eastern Jilin and Heilongjiang provinces is located in the superposition of the Paleo-Asian Ocean tectonic domain and the Paleo-Pacific Ocean tectonic domain,and exprienced a complicated tectonic evolution.Predecessors have studied the genesis of multiple intrusive rock masses and volcanic rocks in this region.And explored the regional tectonic evolution.However,when the impact of the Paleo-Asian Ocean tectonic domain stopped,and when the Paleo-Pacific Plate began to subduct under the Eurasian Plate is still controversial.The reason of may be related to the lack of reseraching of the temporal and spatial distribution characteristics of the igneous rocks in the early and middle Mesozoic in the study area.This article takes the igneous rocks of the Early-Middle Mesozoic in Yanbian area and a NNW ductile shear zone in the north of Helong City as the research subject.We use the LA-ICP-MS zircon U-Pb dating method to measure the intrusion time of igneous rocks in Yanbian area,and sort out the spatial and temporal distribution characteristics of intrusive rocks of Early-Middle Mesozoic in research area.The main elements and trace elements of these rock masses were analyzed to discuss the origin of the magma and the geotectonic environment when it formed.We analyzed the deformation characteristics and measured the age of the deformation of the ductile shear zone,and analyzed the possible causes of the deformation.According to the LA-ICP-MS results and the published data in the study area,the Middle Permian-Early Jurassic intrusive rocks in the Yanbian area can be divided into four stages:the Middle Permian-early Middle Triassic?263²46 Ma?;early Middle-early Late Triassic?245²30 Ma?;middle Late Triassic?229²15 Ma?and middle Late Triassic-late Early Jurassic?214¹75 Ma?.Intrusive rocks of middle Late Triassic-late Early Jurassic?214¹75 Ma?was characterized by the domination of calc-alkali rocks.The geochemical characteristics of Laolongkou gabbro?213 Ma?and Maying gabbro?203 Ma?were enriched in large ion lithophile elements?LILEs?and depleted in high field strength elements?HFSEs?.The petrogenesis of this two gabbros was related to the partial melting of the mantle wedge caused by the subduction and dehydration of oceanic crust.And there is hardly any contamination of crustal material during magmatic emplacement.The intermediate and acidic rocks such as Kaishantun granodiorite-quartz diorite?198 Ma?,Hualongdao?187 Ma?and Daduchuan quartz diorite?197 Ma?are frequently characterized by containing diorite enclaves,geochemical characteristics also show that it has been affected by mantle material.The andesitic tuff lava?198 Ma?of the Tuopangou formation located on the North-West of Laolongkou reservoir also shows subduction-related geochemical characteristics.Many cotemporary volcanic rocks in the study area are calcium alkaline,indicating the volcanic arc tectonic environment.The igneous rock assemblage of this period is gabbro-diorite-quartz diorite–granodiorite-granite,which belongs to the active continental margin igneous rock assemblage.The Late Triassic-Early Jurassic?214-175 Ma?igneous rocks in Yanbian area was classified and mapped with a time span of 10 Ma.It was found that as the rocks westward distribution the intrusive time gets newer,suggesting that the scope of the plate influence expands westward along with the subduction.It also shows that the subduction of Paleo-Pacific plate to Eurasian continental crust at the late Late Triassic.The impact of Paleo-Pacific subduction on Yanbian area is not only reflected in the type and distribution of intrusive rocks,but also the deformation of the rocks.In this paper,a NNW strike ductile shear zone in the Helong area was studied.The newborn amphibole was selected to be the object for ⁴⁰Ar-³⁹Ar isotope dating,and the results is 184Ma.Combined with the SW dip direction of the gneissosity,the S dip direction fold hinge and the positive slippage supported by feldspar porphyroclasts and asymmetrical folds composed by felsic strip,suggested this shear zone was mainly caused by NE-SW extension,Based on the above evidence,this paper argues the formation of this ductile shear zone should be related to the SW subduction of the Paleo-Pacific plate.In summary,during Middle Permian-early Middle Triassic,Yanbian area was controlled by the bi-directional subduction of the Paleo-Asian Ocean plate.The continental collision environment was started at early Middle Triassic?²45 Ma?.At Late Triassic?²29Ma?Yanbian area entered the post-collision stage.In the late Middle Triassic?²14Ma?Paleo-Pacific Ocean tectonic domain kicked off,the Paleo-Pacific plate began to subduct below the Eurasian plate.