Geochronology And Geochemistry Of The Late Paleozoic-Early Mesozoic Igneous Rocks In The Eastern Segment Of The Northern Margin Of The North China Block

This thesis studies the geochronology and geochemistry of the Late Paleozoic-EarlyMesozoic igneous rocks in the central Jilin Province, NE China, with the aim of providingconstraints on the Late Paleozoic-Early Mesozoic tectonic evolution in the eastern segment ofthe northern margin of the North China Block (NCB). Main achievements are as follows:1. Late Paleozoic–Early Mesozoic magmatic events in the eastern segment of thenorthern margin of the NCBZircons from the Late Paleozoic-Early Mesozoic igneous rocks in the central JilinProvince, NE China are euhedral-subhedral in shape and display fine-scale oscillatory growthzoning and striped absorption as well as their high Th/U ratios, implying their magmatic origin.LA-ICP-MS and SIMS zircon U-Pb dating results indicate that the Late Paleozoic-EarlyMesozoic magmatic events can be subdivided into five stages, i.e., Early Permian (293–279Ma), Middle Permian (~270Ma), Late Permian (262–255Ma), Early Triassic (249–247Ma),and Late Triassic (~222Ma).2. Early Permian–an active continental margin settingThe volcanic rocks from the Daheshen Formation are representative as the Early Permianmagmatic event in the central Jilin Province, and are composed mainly of the trachytic dacite,dacite, and rhyolite. They are rich in SiO2and alkali contents, poor in MgO, with their εHf(t)values and Hf two-stage model ages of+0.23~+10.37and1240~785Ma, respectively, suggesting that their primary magmas could be mainly derived from partial melting of theMeso-Neoproterozoic accretted lower crust. The volcanic rocks from the Daheshen Formationchemically belong to mid-K and high-K calc-alkaline series, and display an enrichment inLREEs and LILEs and depletion in HFSEs, implying that they could have formed under anactive continental margin setting, i.e., the subduction of the Paleo-Asiatic oceanic platebeneath the northern margin of the NCB happened in Early Permian.3. Middle Permian–continent-continent collisional settingThe Middle Permian igneous rocks consist chiefly of the FNG garnet-bearingmonzogranites. They have high SiO2, Al2O3and alkali conctents, low TFe2O3and MgOcontents, relatively flat rare earth element (REE) patterns and low total REE abundances(REE), as well as negative Eu anomalies. Their εHf(t) values and Hf two-stage model agesrange from–17.1~–14.1and2.37~2.18Ga, respectively, indicating that their primary magmacould be probably derived from partial melting of the ancient lower continental crust of theNCB. The presence of the garnet and their peraluminous geochemical features suggest thatthey are the syn-collision granites. Combined with the presence of the late Middle Permianmolasse in central Jilin Province, we propose that the garnet-bearing monzogranites couldform under an initial collision setting between the northern margin of the NCB and theSongnen-Zhangguangcai Range Massif.4. Late Permian–post-collisional extension settingThe Late Permian igneous rocks are composed mainly of the gabbros and the granitoids.The gabbros are characterized by low SiO2contents, high Mg#values, positive εHf(t) values,enrichment in LILEs, and depletion in HFSEs, implying that their primary magma could bederived from partial melting of a depleted lithospheric mantle modified by subducted slab-derived fluids. The granitoids have high SiO2, low MgO, Cr, and Ni contents, and positiveεHf(t) values, suggesting that their primary magma could be derived from partial melting of ajuvenile lower crust. The Late Permian gabbros and granitoids constitute a bimodal igneousrock association, suggesting that they could formed under a post-collisional extensionalenvironment related to collapse of thickened crust caused by Middle Permian collision between the northern margin of the NCB and the Songnen-Zhangguangcai Range Massif.5. Early Triassic–final closure of the Paleo-Asian OceanThe Early Triassic magmatism in the study area is dominated by biotite monzogranites.They are characterized by high Al2O3, Na2O, and Sr concentrations, low Y and Ybconcentrations, enrichment in LREEs and LILEs, and depletion in HREEs and HFSEs, beingaffinity of adakitic rocks, suggesting that they formed by partial melting of a thickened maficlower crust. This thickening process of the mafic lower crust implies that a Late Triassiccompressional tectonic setting occurred in the central Jilin Province. Together with theoccurrence of the coeval metamorphism of the Hulan Group in the eastern part of the studyarea and cessation of the marine deposition in the area, we propose that the final amalgamationof the northern margin of the NCB and the Songnen-Zhangguangcai Range Massif took placein the Early Triassic, which led to the final closure of the Paleo-Asian Ocean.6. Late Triassic–post-orogenic extensional settingLate Triassic intrusive rocks in the study area are mainly composed of harzburgites. Theirlow SiO2, high MgO, Cr, and Ni concentrations, and positive εHf(t) values, suggest that theirprimary basaltic magma could be derived from partial melting of a depleted lithospheric mantle.Late Triassic mafic-ultramafic intrusive rocks and coeval granitoids constitute a typicalbimodal igneous rock association in both the study area and adjacent regions, implying thatthey formed under a post-orogenic extensional environment related to the final amalgamationof the NCB and the XMOB.