| LA-ICP-MS zircon U-Pb dating method and in-situ zircon Hf isotopic analysis has been applied to late Permian to Triassic stratum from central-eastern Jilin Province,northeast China in this thesis,with the aim to constrain the depositional ages and provenance of these sedimentary units.Besides,this thesis discusses the timing of termination of the collisional orogency related to the closure of the ancient ocean in this area based on genesis analyses of metamorphic overgrowth zircons within these late Permian to Triassic sedimentary units.Geochronological and Hf isotopic data in this paper,together with previous regional geological data,sheds light on the timing of final closure of the Paleo-Asian Ocean within central-eastern Jilin Province,and the late Permian-Triassic regional tectonic evolution of the eastern segment of the northern margin of the NCC?North China Craton?.1.Deposional ages of the late Permian to Triassic sedimentary unitsU-Pb dating results of detrital zircons from sedimentary rocks in the late Permian to Triassic Dajianggang,Huangyingtun,Xiaohekou,Kedao,Dasuangou and Miaoling formations in central-eastern Jilin Provinces,together with paleontological fossil testimonies,and the latest U-Pb dating results of the overlying strata as well as the late-satge intrusions,demonstrate that the Dajianggang Formation was deposited during the middle Late Triassic;the Huangyingtun Formation was deposited during the late Permian-Early Triassic,rather than the previously proposed early Paleozoic or early Permian;the Xiaohekou Formation was deposited during the Late Triassic;the Kedao Formation was deposited during the Middle-Late Triassic;the Dasuangou and Miaoling formations were deposited during the late Permian.2.Provenance of the late Permian to Triassic sedimentary unitsU-Pb dating results of detrital zircons from clastic rocks within the late Permian Dasuangou and Miaoling formations reveal several main zircon populations:late Paleozoic?280 and 259 Ma?,early Paleozoic?473,443 and 419 Ma?,as well as a small quantity of zircons with Paleoproterozoic-Mesoproterozoic?18551439 Ma?ages.The majority of late Paleozoic detrital zircons yield relatively high?Hf?t?values?+5.5+15.8?,implying that they were derived from the Khanka Massif,the residual zircons have relatively low?Hf?t?values?-3.0-6.3?,indicating that they were sourced from the northern margin of the NCC.?Hf?t?values of the early Paleozoic detrital zircons are between-2.7and+10.4,consistent with Hf isotopic compositions of zircons from igneous rocks within early Paleozoic continental margin accretionary belt of the northern margin of the NCC,suggesting that they were derived from the early Paleozoic island arc igneous rocks in this area;Paleoproterozoic-Mesoproterozoic detrital zircons are rounded to sub-rounded in shape,implying long distance transportation,suggesting that they were derived from the Precambrian basement of the NCC.U-Pb dating results of detrital zircons from late Permian-Early Triassic Huangyingtun Formation display that this formation contains Phanerozoic?251,282,309,366,390,466and 503 Ma?and few Neoarchean-Paleoproterozoic?2.51.8 Ga?ages,as well as metamorphic zircon overgrowth age of 233 Ma.250 Ma magmatic events are widely distributed within both the northern margin of the NCC and the eastern Central Asian Orogenic Belt?CAOB?;early Permian detrital zircons yield relatively high?Hf?t?values?+6.7+12.2?,similar to Hf compositions of zircons from early Permian volcanic rocks within the northern margin of the NCC,additionally,they also contain zircons with negative?Hf?t?values?-4.7?,implying that they were derived from the northern margin of the NCC;Late Devonian-Carboniferous detrital zircons are characterized by extremely low negative?Hf?t?values?-22.2-13.0?,indicating a source from the NCC;Middle Devonian detrital zircons are only found within sample adjacent to the suture line,with U-Pb ages similar to ages of inherited zircons from Late Devonian rhyolites in the north of the suture line,implying that they were derived from the eastern CAOB;Cambrian-Ordovician detrital zircons yield U-Pb ages consistent with early Paleozoic magmatic events within the northern margin of the NCC;Neoarchean-Paleoproterozoic magmatism was mainly distributed in the NCC,therefore those cotemporary ancient zircons winthin this formation were derived from the NCC.Detrital zircon dating results of the Middle-Late Triassic Kedao Formation from eastern Jilin Province reveal three magmatic age populations?241,251 and 262 Ma?,and three magmatic age clusters?210,223 and 233 Ma?.Magmatic zircons within this formation yield?Hf?t?values of-6.0+7.7,similar to zircons from coeval igneous rocks in the northern margin of the NCC,indicating a dominant material source for this formation.Late Triassic Dajianggang Formation contains detrital zircons with age peaks at 217,248,265,910 Ma and 1.82.5 Ga.Late Triassic felsic-intermediate igneous rocks are widely distributed within the Zhangguangcai Range to the north,but rarely outcropped in the northern margin of the NCC,suggesting that the Late Triassic detrital zircons were derived from the eastern CAOB;265248 Ma magmatic events are widely distributed in both the northern margin of the NCC and the Songnen-Zhangguangcai Range Massif;910 Ma detrital zircons were probably derived from the Jiamusi Massif and Neoarchean-Paleoproterozoic detrital zircons were likely sourced from the NCC.Late Triassic Xiaohekou Formation contains detrital zircons with Phanerozoic?245,266,283,315 and 406 Ma?ages,as well as large amounts of ancient Neoarchean-Paleoproterozoic zircons.Late Paleozoic-Early Mesozoic detrital zircons within this formation yield distinct negative?Hf?t?values as well as Neoarchean-Paleoproterozoic TDM2 ages,suggesting that Xiaohekou Formation is dominated by material derived from the North China Craton.3.Timing of final closure of the Paleo-Asian Ocean in central-eastern Jilin Province and the late Permian-Triassic tectonic evolution of the eastern segment of the northern margin of the NCCAnalysis results of detrital zircon provenance show that late Permian-Early Triassic sedimentary units in the study area have bidirectional sources,together with previous studies,we propose that final closure of the Paleo-Asian Ocean basin in central-eastern Jilin Province took place during the late Permian at latest.During late Permian-Early Triassic,a shrinking ocean basin enabled material from both sides of the basin to mix and be deposited as sedimentary rocks,generating spatial and temporal changes in provenance within the late Permian to Early Triassic sedimentary rocks in the study area.Subsequent collisional orogenesis caused a transition from late Permian paralic to Early Triassic terrestrial sedimentary rocks in sedimentation,crustal thickening and the generation of molasse deposits in this area.The collisional orogenesis in central-eastern Jilin Province terminated before 233 Ma,after that,regional magmatism was characterized by the outcrop of alkaline granitoids,A-type granites and rhyolites,and mafic–ultramafic intrusions,indicating that this region and adjacent areas were dominated by an extensional tectonic setting after the final closure of the ocean basin. |
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