Early Mesozoic Tectonomagmatism And Geodynamic Setting In Southern Jilin Province

Since Mesozoic,the North China Craton has been affected by a series of tectonic events,including the closure of the Paleo-Asian Ocean,the subduction and collision between the Yangtze Craton(YC)and the North China Craton(NCC),and the subduction of the Paleo-Pacific Plate.Southern Jilin Province is located in the northeast of the NCC,and the intense Early Mesozoic magmatism and crustal tectonic deformation provide strong evidence for the study of the transition of the Early Mesozoic tectonic regime.In this paper,the Early Mesozoic magmatism and the Hunjiang composite syncline and thrust nappe structures in southern Jilin Province are taken as the research objects.By means of LA-ICP-MS zircon U-Pb geochronology,geochemistry and zircon Hf isotope,and through the comprehensive analysis of sedimentary strata,magmatic activities and tectonic deformation,the deformation stage,evolution process and geodynamic mechanism of the Early Mesozoic crustal structure are discussed.The coupling relationship between the Early Mesozoic magmatism,sedimentation and tectonic deformation in the study area limits the onset of the destruction of NCC,and solves the key scientific theoretical problem of the transformation mechanism and process of the Early Mesozoic tectonic regime in southern Jilin Province.The main observations and conclusions are as follows:1.According to the LA-ICP-MS zircon U-Pb dating results in this paper and the previous chronological research results,the Early Mesozoic magmatism in southern Jilin Province can be subdivided into two stages:Late Triassic(227?217 Ma)and EarlyMiddle Jurassic(181?163 Ma).2.The petrogenesis and tectonic setting of the Late Triassic magmatism in southern Jilin Province were determined.The Late Triassic magmatic activities are represented by volcanic rocks of the Changbai Formation,alkali feldspar granite,porphyritic quartz monzoite,biotite monzoite,monzogranite and monzodiorite,which are mainly characterized by high-K calc-alkaline,metaluminous-weakly peraluminous I-type granite.Among them,rhyolitic cyrstal tuff of the Changbai Formation and alkali feldspar granite have characteristics of high differentiation.The analysis results of major elements,trace elements and zircon Hf isotopes show that the rhyolitic crystal tuff of the Changbai Formation was derived from the juvenile lower crust.The alkali feldspar granite was derived from the ancient lower crust.The monzonitic granite was derived from the juvenile crust with contribution from ancient crustal materials.The monzodiorite was derived from the enriched lithospheric mantle caused by subduction.The porphyritic quartz monzonite and biotite monzonite belong to adakitic rocks.The porphyritic quartz monzonite was derived from partial melting of thickened ancient lower crust,and the biotite monzonite was derived from partial melting of delaminated lower crust.During the Late Triassic,the southern Jilin Province experienced a strong crust-mantle interaction,which was related to the delamination of the subducted slab and lithospheric delamination after the subduction and collision between YC and NCC.Combined with the A-type granite,alkaline rock and mafic rock developed in Liaodong Peninsula,it shows that the southern Jilin Province(late Indosinian)is in a strong extensional tectonic setting during the Late Triassic.3.The petrogenesis and tectonic setting of Early-Middle Jurassic magmatism in southern Jilin Province were determined.Early-Middle Jurassic magmatism is represented by biotite monzogranite and garnet muscovite granite.Both belong to high-K calc-alkaline series and I-type granite.The biotite monzogranite belongs to adakitic rock,and the analysis results of major elements,trace elements and zircon Hf isotopes show that the biotite monzogranite was derived from the partial melting of thickened ancient lower crust.The samples of biotite monzogranite are enriched in LILEs and LREEs,and depleted in HREEs and HFSEs,exhibiting arc-like geochemical characteristics,suggesting that the biotite monzogranite formed in an active continental margin setting.The garnet muscovite granite shows a spectacular tetrad effect in REE distribution pattern.The analysis results of major elements,trace elements and zircon Hf isotopes show that the garnet muscovite granite was derived from ancient crustal materials and was strongly affected by hydrothermal fluids in the later stage.Combined with the Early-Middle Jurassic Andean type active continental margin rock assemblages distributed in the NE direction in the eastern part of Jilin and Heilongjiang,the northeast of North China Craton and Korean Peninsula,it shows that the southern Jilin Province was under the subduction of the Paleo-Pacific Plate during the Early-Middle Jurassic.4.The structural deformation characteristics,tectonic deformation stages,evolution process and geodynamic mechanism of Hunjiang composite syncline and thrust nappe structures are identified.The Hunjiang composite syncline is generally characterized by a northeast-trending inverted folds.According to the age of the youngest strata involved in the deformation of Hunjiang composite syncline and the structural transformation relationship by the Longtou pluton of Late Triassic,it is judged that its formation age is Early-Middle Triassic(early Indosinian).The thrust nappe structure is characterized by a imbricate thrust fault,thrust from southeast to northwest,with the shortest nappe distance of about 8 km.Based on the structural contact relationship between molasse formation of Xiaohekou Formation of Upper Triassic and thrust nappe,and the angle unconformity of Cretaceous strata on thrust nappe structure,its formation age is divided into two stages.The first stage is the same period with Hunjiang composite syncline,which is EarlyMiddle Triassic(early Indosinian),and the second stage is Jurassic,which is likely to be formed in the Middle Jurassic.The two Early Mesozoic crustal tectonic deformations in southern Jilin Province are the response of the subduction and collision between the YC and the NCC,and the subduction of the Paleo-Pacific Plate.5.The initial time of the decrationization of the NCC and the Early Mesozoic tectonic evolution model of the southern Jilin Province.The eastern and northeastern parts of the NCC were in a strong post-collisional extensional setting during the Late Triassic,and the decrationization(lithospheric thinning)of the NCC began in the Late Triassic.During the Early-Middle Triassic(early Indosinian),the southern Jilin Province was in a nearly north-south compression environment caused by the subduction and collision between the YC and the NCC,and it was in a strong extensional tectonic environment caused by the delamination of the subducted slab and lithospheric delamination after the subduction and collision between YC and NCC during the Late Triassic(late Indosinian).The transition from the subduction and collision between the YC and the NCC to the Paleo-Pacific tectonic domain was completed during the Early-Middle Jurassic,and the southern Jilin Province was in a compressive environment under the subduction of the Paleo-Pacific Plate during the whole Jurassic.