Late Paleozoic Tectonic Evolution Of The Orogenic Belt In Southern Of Zhalantun Area,Inner Mongolia

This thesis studies the deposition ages and provenance of the Late Paleozoic sedimentary strata and the geochronology and geochemistry of the Late DevonianMiddle Permian igneous rocks in Southern of Zhalantun area with the method of LA-ICP-MS zircon U-Pb dating.Furthermor,this thesis discusses the spatial-temporal variation of igneous rocks associations,nature of magma sources and their tectonic settings based on major and trace elements for these igneous rocks.These chronological and geochemical data,together with regional geological data,provide constraints on the Late Paleozoic tectonic evolution of Southern of Zhalantun area.The dating results of detrital zircons from quartz sandstones in Daminshan,Hongshuiquan,Baoligaomiao,Gegen`aobao,Dashizhai and Zhesi,together with paleontological evidences,the contact relationship of the overlying strata and their dating results,indicate that the sedimentary ages of the Formations mentioned above are respectively the Late Devonian(Daminshan),the Early Carboniferou(Hongshuiquan)the Late Carboniferous(Baoligaomiao and Gegen`aobao),the Early Permian(Dashizhai)and the middle Permian(Zhesi).The Late Devonian Daminshan Formation is composed mainly of marine clastic formations,the dating results of detrital zircons from sandstone indicate the sediments predominantly came from the Paleozoic igneous arc and minor older basement of Songnen massif.Associated with the features of fossils and geochemistry of siliceous rocks in Daminshan Formation,the Daminshan Formation may represented forearc sediments.The early Carboniferous Hongshuiquan Formation is composed mainly of flysch formation,the dating results of detrital zircons from sandstone indicate the sediments predominantly came from the Paleozoic igneous arc and a few older basement of Songnen massif.Associated with the features of fossils in Hongshuiquan Formation,the Formation may represented backarc-interarc basin sediments.The Late Carboniferous Baoligaomiao Formation is composed mainly of terrigenous deposit formation,the dating results of detrital zircons from sandstone indicate the sedimental source is single and predominantly came from the Late Paleozoic igneous arc and a few older basement of Songnen massif.Associated with the features of fossils in Baoligaomiao Formation,the Baoligaomiao Formation may represented backarc basin sediments.The Late Carboniferous Gegen`aobao Formation is composed mainly of marine terrigenous deposit formation,the dating results of detrital zircons from sandstone indicate the sediments predominantly came from the Paleozoic igneous arc and a few igneous arc of Early Paleozoic and older basement of Songnen massif.Associated with the features of fossils and rock association in Gegen`aobao Formation,the Formation may represented backarc basin sediments.The early Permian Dashizhai and Shoushangou Formation and middle Permian Zhesi Formation are composed mainly of neritic marine deposit formation,the dating results of detrital zircons from sandstone indicate the sediments predominantly came from the Late Paleozoic igneous arc,the Formation may represented rifted basin sediments.Zircons from the igneous rocks in the southern of zhalantun area display fine-scale oscillatory growth zoning and are euhedral-subhedral in shape 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 magmatic events can be subdivided into four stages: The Late Devonian,early Carboniferous,Late Carboniferou,early-middle Permian.The Late Devonian magmatism in the study area is dominated by volcanic rock in Daminshan Formation.They are characterized by high Ti O2,Fe O,Ti and Zr concentrations,low Y and Sr concentrations,and have similar values of Ti/Yb,Zr/Yb and La/Nb,suggesting that they formed in an ocean island environment within the oceanic plate.The early Carboniferous granitoids are characterized by high Si O2 and Al2O3 concentrations and low Mg#,Ni and Co concentrations,enrichments in LREES and LILES,and depletion in HREEs and HFSEs,suggesting that their primary magmas could form by partial melting of lower crust.Taken together,we conclude that they formed in syn-collision to late orogenic setting.The Late Carboniferous basic rocks are characterized by low Si O2,Sc,Co and Ni concentrations and High Mg O,Fe O,Fe2O3 concentrations,,suggesting that their primary magmas could form by partial melting of mantle.The values of Nb/U and Ta/U indicate the primary magmas have been significantly contaminated by crustal components.The volcanic rocks in Gegen`aobao Formation are characterized by low Y and Yb concentrations and high Sr,enrichment in LREEs and depletion in HREEs and weakly negative Eu anomaly.The volcanic rocks are being an affinity to adakitic rocks.Taken together,we conclude that The Late Carboniferous igneous rocks formed in a thickened lower continental crust accreted in Neoproterozoic.The early Permian granitoids and middle-basic intrusive rocks 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 collision between the Songnen massif and Xing`an massif.The Late Paleozoic sedimentary formations and the provenance analysis indicate that the Late Devonian marine sedimentary formations and alkali basalt reveal that an active continental margin forearc basin setting,and that early Carboniferous marine sedimentary formations indicate a subduction-collison setting,consistent with the occurrence of calc-alkaline to high-k calc-alkaline arc magma suites and A-style rhyolite in backarc basin,the Late Carboniferous neritic terrigenous clastic and adakitic volcanic rocks suggest that they formed in an collision and rapidly uplift environment.The early Permian A-style and similar bimodal igneous rocks represent a post-orogenic extensional environment and form a marine sedimentary.