Late Paleozoic-triassic Tectonic Evolution Of Eastern Segment Of The Southern Margin Of The Xing’an–mongolia Orogenic Belt: Evidence From Detrital Zircon U-Pb Geochronology And Igneous Rock Associations

This thesis studies the deposition ages and provenance of the Late Paleozoic sedimentary strata in central Jilin Province and the geochronology and geochemistry of the Middle Permian-Middle Triassic intrusive rocks in central-eastern Jilin Province by using LA-ICP-MS zircon U-Pb dating method. Besides, this thesis discusses the spatial-temporal variation of igneous rock associations, nature of magma sources, and their tectonic settings based on major and trace elements as well as zircon Hf isotope for these intrusive rocks. These chronological and geochemical data, together with regional geological data, provide constraints on the Late Paleozoic-Triassic tectonic evolution of eastern segment of the southern margin of the Xing’an–Mongolia Orogenic Belt(XMOB) and final closure time and style of the eastern segment of the Paleo-Asian Ocean. Main achievements are as follows: 1. Deposition ages of the Late Paleozoic strata in central Jilin ProvinceThe dating results of detrital zircons from quartz sandstones in the Middle Devonian Wangjiajie, the Middle-Late Devonian Xiaosuihe, the Early-Late Carboniferous Luquantun and the Middle-Late Permian Shoushangou, Fanjiatun and Yangjiagou formatio,together with paleontological evidence, the contact relationship of the overlying strata and their dating results, suggest that detrial zircons from the Wangjiajie Formation is deposited between 399~384 Ma; the Xiaosuihe Formation are deposited between 384~364 Ma, rather than the previously proposed Late Silurian-Early Devonian; the Luquantun Formation are deposited between 332~301 Ma, rather than the previously suggested Early Carboniferous; the Shoushangou, Fanjiatun and Yangjiagou formations are deposited between 265~256 Ma, rather than the previously proposed Early Permian for the Shoushangou and Fanjiatun formations. 2. Sedimentary formations and provenance of the Late Paleozoic strataThe Middle Devonian Wangjiajie Formation is composed mainly of carbonate and marine clastic formations, the dating results of detrital zircons from sandstone in this Formation indicate that the sediments predominantly came from the Phanerozoic geological terrane(440 and 399 Ma), with minor Neoarchean(~2500 Ma),Paleoproterozoic(2066~1648 Ma), and Neoproterozoic(~921 Ma) terrane. The majority of the Phanerozoic zircons within this unit has negative eHf(t) values(-21.4 to -6.1), implying that they could be sourced from igneous rocks in the northern margin of the North China Craton(NCC), whereas minor Phanerozoic zircons with positive eHf(t) values(-1.9 to +4.9) in this unit were most likely sourced from newly accreted crust within the XMOB. In addition, the presence of the Neoarchean and Paleoproterozoic detrital zircons, and their rounded to sub-rounded shape indicate that they underwent long distance transportation prior to deposition, and that they were sourced from the NCC. The majority of the Neoproterozoic(920~723 Ma) detrital zircons within our sample have eHf(t) values of -16.9 to -13.8, indicating that they could be sourced from the NCC.The Middle-Late Devonian Xiaosuihe and Early-Late Carboniferous Luquantun formations consist of neritic terrigenous clastic, carbonate, and spilite-keratophyre units. The dating results of detrital zircons from their sandstones show that the sediments from these strata were predominantly sourced from the Phanerozoic geological terrane(ages of detrial zircons from the Xiaosuihe Formation: ~433 and ~386 Ma; ages of detrial zircons from the Luquntun Formation: ~428, ~402, ~364 and ~332 Ma). Additionally, dominant detrital zircons have positive eHf(t) values(+5.5~+16.3), combined with their subhedral–euhedral shapes, suggesting that they did not experienced long distance transportation and were all sourced from the XMOB.The Middle-Late Permian Shoushangou and Fanjiatun formations are composed mainly of neritic terrigenous clastic, carbonate and volcanic clastic formations. However, the coeval Yangjiagou Formation consists of marine-terrigenous molasse and carbonate formations. The dating results of detrital zircons from their sandstones suggest that the sediments were predominantly sourced from a Phanerozoic and Neoproterozoic sources. Their subhedral–euhedral shapes indicate that they did not underwent long distance transportation prior to deposition, and their positive eHf(t) values(-5.5~+16.6) imply that they were most likely sourced from the XMOB. Additionally, minor Phanerozoic zircons have negative eHf(t) values(-15.9 to -7.2), together with the Neoproterozoic detrital zircons with eHf(t) values of -6.9 to -2.2, implying that they could be sourced from the NCC.3. Permian-Triassic magmatic events and rock associations in central-eastern Jilin ProvinceZircons from the Late Paleozoic-Triassic intrusive rocks in the central-eastern Jilin Province, NE China are euhedral-subhedral in shape and display fine-scale oscillatory growth zoning and striped absorption in CL images, together with their high Th/U ratios, implying their magmatic origin. LA-ICP-MS zircon U-Pb dating results indicate that the Permian-Triassic magmatic events can be subdivided into three stages, i.e., Middle Permian(263~260 Ma), Late Permian-Early Triassic(256~249 Ma) and Middle Triassic(245~244 Ma).The Middle Permian magmatism in the study area is dominated by biotite monzogranites. They are characterized by high Si O2, Al2O3 and Sr concentrations, low TFe2O3, Mg O, Y and Yb concentrations, enrichment in LREEs and LILEs, and depletion in HREEs and HFSEs, being an affinity to adakitic rocks, suggesting that their primary magmas could form by partial melting of a thickened mafic lower crust. Their εHf(t) values and Hf two-stage model ages range from +8.3~+9.3 and 609~692 Ma, respectively. Taken together, we conclude that the primary magma for these middle Permian monzogranites could be derived from partial melting of a thickened lower continental crust accreted in Neoproterozoic.The Late Permian-Early Triassic granitoids and gabbros in the central-eastern Jilin Province, NE China constitute a bimodal igneous rock association, suggesting that they could form under a post-collisional extensional environment related to collapse of thickened crust caused by middle Permian collision between the NCC and the Songnen-Zhangguangcai Range Massif.The Middle Triassic magmatism in the central Jilin Province, NE China is composed of biotite monzogranites. They are characterized by high Al2O3, Na2 O, and Sr concentrations, low Y and Yb concentrations, enrichment in LREEs and LILEs, and depletion in HREEs and HFSEs, being an affinity to adakitic rocks. Their εHf(t) values and Hf two-stage model ages range from +0.0~+5.5 and 790~1074 Ma, respectively, suggesting that they could be derived from partial melting of a thickened mafic lower crust. The Middle Triassic igneous rocks in the central Jilin Province consist chiefly of the garnet-bearing muscovite monzogranites. They have high Si O2, Al2O3 and alkali conctents, low TFe2O3 and Mg O contents, relatively flat rare earth element(REE) patterns and low total REE abundances(SREE), as well as negative Eu anomalies. Their εHf(t) values and Hf two-stage model ages range from +6.1~+8.0 and 716~824 Ma, respectively. Combined with the presence of the garnet and their peraluminous geochemical features, it is suggested that they are in chemistry the syn-collisional granites, implying that their primary magma could be derived from partial melting of a thickened mafic lower crust. 4. Late Paleozoic-Triassic tectonic evolution history and final closure of the Paleo-Asian Ocean in central-eastern Jilin ProvinceThe Late Paleozoic sedimentary formations and the provenance analysis indicate that the Middle-Late Devonian marine carbonate and clastic terrigenous formations reveal that a passive continental margin extensional setting occurred in the southern margin of the XMOB during Middle-Late Devonian; and that late Carboniferous neritic terrigenous clastic, carbonate, and spilite-keratophyre formations are an indicative of an extensional environment, consistent with the occurrence of late Carboniferous bimodal volcanic rocks and alkali granite in the central Jilin Province. The middle Permian adakitic rocks and Late Permian molasse formation in the central Jilin province indicate a happening of a lower crustal thickening related to the closure of the Paleo-Asian Ocean and a rapid uplift environment, respectively. The late Permian-Early Triassic magmatism in the central-eastern Jilin Province consist of granitoids and gabbros, a typical bimodal igneous rock association, representing a post-orogenic extensional environment; whereas Middle Triassic monzogranites with an affinity to adakites in the central-eastern Jilin Province reveal a crustal thickening and the final closure of the Paleo-Asian Ocean. The early-middle Permian in western-central Jilin, to middle Permian in central Jilin, and to Middle Triassic in eastern Jilin adakitic rocks gradually occur from the west to the east imply that the eastern segment of the Paleo-Asian Ocean closed in a scissors type and its final closure took place in the Middle Triassic.