Permian-Early Jurassic Tectonic Evolution Of Kaiyuan-Yanji Area In The Eastern Segment Of The Northern Margin Of The North China Block

Kaiyuan-yanji area is located in the eastern segment of the northern margin of the North China Block?NM-NCB?.This area,which has experienced the evolution of the Paleo-Asian Ocean?PAO?tectonic domain and the superimposition and reconstruction of the Paleo-Pacific Ocean?PPO?tectonic domain since the Paleozoic,is a key region to study the closure of the PAO and the transition of the two tectonic domains.This thesis studies on the Permian-Early Jurassic igneous rocks in Kaiyuan-Yanji area and the Kaiyuan tectonic mélange.LA-ICP-MS and SHRIMP zircon U-Pb dating results and the published chronologic data indicate that the Permian-Early Jurassic magmatic activities in Kaiyuan-yanji area can be subdivided into four stages:Permian?285-252Ma?,Early Triassic-early Late Triassic?250-235 Ma?,middle Late Triassic?229-208Ma?and late Late Triassic-Early Jurassic?207-175Ma?.The rock assemblages,geochemical features,Lu-Hf and Sr-Nd isotopic data have been used to discuss their magma source and tectonic setting.Combing these new data with the spatial and temporal distribution characteristics of the igneous rocks and tectonic mélange in this region,the thesis has finally discussed the Permian-Early Jurassic tectonic evolution of the Kaiyuan-yanji area.The volcanic rocks from the Dashenhe Formation are composed mainly of andesite,dacite and rhyolite.These calc-alkaline volcanic rocks and the intrusive rock association composed of quartz diorites,granodiorites and monzogranites reflect that the eastern segment of the NM-NCB was in an active continental margin setting during the Early-Middle Permian.The Bajiazi quartz diorite pluton is representative as the Late Permian magmatic event.The quartz diorites belong to aluminous and mid-K to high-K calc-alkaline series,and display enrichment in LREEs and LILEs?Cs,Ba,K and Sr?and depletion in HFSEs?Nb,Ta and P?,showing the features of I-type granite.Their primary magma could be probably derived from partial melting of the basaltic lower crust.The quartz diorite together with the coeval diorites,granodiorites and monzogranites consist the rock association of the active continental margin setting.In addition,the Late Permian volcanic rocks associated with subduction were existed in the tectonic mélange of Seluohe and Qinglongcun.Combined with the presence of the calc-alkaline intrusive and volcanic rock association in the southern-western margin of the Khanka Massif,we propose that the PAO plate subducted beneath the North China Block and Khanka Massif during the Permian.The early Early Triassic magmatism in study area is represented by Fudong porphyritic granite,which belongs to aluminous-peraluminous and high-K calc-alkaline series.These porphyritic granites are characterized by high SiO₂,alkali and Sr concentrations,low Y and Yb concentrations,enrichment in LREEs,and depletion in HREEs and HfSEs?Nb,Ta,Ti and P?,being affinity of adakitic rocks.Their?_Hf?t?values and two-stage model ages range from-3.10⁺5.6 and 1473⁹15Ma,respectively,indicating that the primary magma could be derived from partial melting of the Meso-Neoproterozoic accreted thickened crust and were contaminated by ancient continental crust.Together with the coeval metamorphism of the Hulan Group and Wudaogou Group and the numerous syn-collision granites,suggesting the ocean basin of PAO closed and the continent-continent collision happened.Meanwhile,the presence of bimodal intrusive rocks,high-Mg andesite and basalt in the study area reflect that the delamination of subducted slab occurred in the early Early Triassic.The metamorphism of Zhaobeishan Formation and cessation of the marine deposition in the study area,we propose that residual sea disappeared in the Middle Triassic,which led to the final closure of the PAO.The Middle Triassic-early Late Triassic igneous rocks are composed mainly of the biotite monzogranites and granodiorites,which have low Sr and Yb concentrations and belong to mid-K to high-K calc-alkaline series.Their?_Nd?t?values and two-stage model ages range from-2.5⁺1.2and 1258⁹16Ma,respectively,implying that the primary magma could be generated by partial melting of the Meso-Neoproterozoic accreted thickened crust.The granodiorites are characterized by high Sr and low Yb concentrations,and negative?_Hf?t?values,being affinity of adakitic rocks,indicating that they formed by partial melting of a thickened ancient continental crust.Combined with the temporal and spatial distribution of the syn-collision granites in Kaiyuan-yanji area,the collisional orogenesis had been lasting until the early Late Triassic.The rhyolites exposed in Dalingou area are representative as the middle Late Triassic magamatic event in the study area.The rhyolites are rich in SiO₂ and alkali,poor in MgO,CaO and TFe₂O₃,with their?_Hf?t?values and two-stage model ages of+6.67⁺11.17 and 814⁵31Ma,respectively,suggesting that the primary magma derived from partial melting of the Neoproterozoic accreted crust.The geochemical features show that they belong to A-type rhyolite,implying that they formed in an extensional environment.Besides,there are a mass of the contemporary A-type granites,A-type rhyolites and basic-ultrabasic complex bodies distributed in the Lesser Xing'an-Zhangguangcai Ranges,western Liaoning province,Yanshan-Liaoning and study area,and the coeval deformation of Hulan group,revealing that the eastern segment of the NM-NCB was under a post-orogenic extensional setting during the middle Late Triassic.The late Late Triassic-Early Jurassic igneous rocks are composed of the monzogranites,granodiorites and rhyolitic ignimbrites in the study area.The monzogranites and granodiorites belong to high-K calc-alkaline series and I-type granite.They have high SiO₂ and Al₂O₃ contents,low MgO contents,and display an enrichment in LILEs?Cs,K and Th?and depletion in HFSEs?Nb,Ta,P and Ti?,showing the geochemical characteristics of active continental margin magmatic rocks.Their primary magma could be probably derived from partial melting of the thickened lower crust.The rhyolitic ignimbrites are characterized by high SiO₂ and alkali contents,low MgO,CaO and TFe₂O₃,enrichment in LILEs?Cs,K,U?,and depletion in HFSEs?Nb,Ta,P,Ti?,implying that they formed in an active continental margin environment.Their?_Hf?t?values and two-stage model ages range from+6.26⁺10.19 and 824⁵72Ma,respectively,suggesting that the primary magma derived from partial melting of the Neoproterozoic accreted crust.There are abundant coeval calc-alkaline intrusive and volcanic rock associations and adakitic rocks associated with subduction in the Kaiyuan-Yanji area,revealing that the PPO plate had subducted beneath the Eurasian continent during the late Late Triassic-Early Jurassic.The Kaiyuan tectonic mélange is composed of the Meso-Neoarchaean TTG gneisses block,Paleoproterozoic schists block,Permian intrusive and basic metamorphic rocks and marbles blocks,Early Triassic volcanics block and the matrix of low metamorphic clastic rocks.The protolith of the Wangxiaopu basic metamorphic rocks is basalt,which formed in a fore-arc extensional setting during Late Permian?258Ma?.The Early Triassic volcanic rocks?²50Ma?formed under the local extensional environment related to the delamination of subducted slab.The occurrence of Zhaobeishan Formation metamorphic event represents that the PAO finished the final closure in Kaiyuan area during the Middle Triassic?²45Ma?.In summary,the PAO had been going on bidirectional subduction during Permian,the eastern segment of the NM-NCB and the southern margin of the Khanka Massif were in an active continental margin setting.During the Early Triassic,the ocean basin of the PAO closed,the continent-continent collision happened,and the residual sea formed.In the Middle Triassic,the PAO finished the final closure along the Changchun-Wangqing-Hunchun belt.The collisional orogenesis had been lasting until early Late Triassic,and the study area was under the post-orogenic extensional setting during the middle Late Triassic.During the late Late Triassic-Early Jurassic,the Eurasian continent was affected by the subduction of the PPO.