Provenance And Its Geological Significance Of Triassic Clastic Rocks In Central Jilin Province Of Northeast China

This thesis mainly discussed the deposition ages of the Triassic clastic rocks in central Jilin Province by using LA-ICPMS U-Pb dating technique.Through the study of the geochemistry characteristics of the major and trace elements of the clastic rocks,combined with a variety of geochemical diagrams and detrital zircon chronology provenance analysis,discussed its provenance characteristics and tectonic setting.These chronological and geochemical analysis,combined with regional geological data and previous study,provide constraints on the Permian-Triassic tectonic evolution of eastern segment of the southern margin of the Xing'an-Mongolia Orogenic Belt(XMOB)and final closure time of the eastern segment of the Paleo-Asian Ocean in central Jilin Province.CL images of zircons show that the zircons studied in this thesis are mostly subhedral granular,and some show subrounded granular and prismatic.Most zircons have clear internal structure,typical developed magma concussion growth zoning or with a streak-like absorption features,combined with its high Th/U ratios,suggesting they are magmatic zircons.Combined with zircon LA-ICPMS U-Pb dating results,detrial zircon age composition of the overlying strata and the contact relationship of the intruded rocks and their dating results,indicated that the deposition age of Shoushangou formation is early-middle Triassic,not early Permian and late Permian from previous researches.The youngest 206Pb/238U weighted average age of detrital zircons of 4 samples from the Fanjiatun formation is 249Ma,249Ma,247Ma and 244Ma(22 in total,accounting for about 7%),combined the existence of quartz and feldspar fragments,Fanjiatun formation deposited in middle Triassic,not early Permian and late Permian from previous researches.The youngest 206Pb/238U weighted average age of detrital zircons of 6 samples from the Piaohechuan formation is 249Ma,243Ma,246Ma,247Ma,247Ma and 236Ma(49 in total,accounting for about 14%),combied with the age(222Ma)of intruded mafic and ultramafic gabbro,the Piaohechuan formation deposited between middle Triassic and late Triassic,not early Paleozoic,early Permian and late Permian-early Triassic from previous researches.The youngest 206Pb/238U weighted average age of detrital zircons of 4 samples from the Dongnancha formation is 250Ma,246Ma,242Ma and 242Ma(13 in total,accounting for about 4%),combined with the age(222Ma)of intruded mafic and ultramafic gabbro,the Dongnancha formation deposited between middle Triassic and late Triassic,not early Paleozoic from previous researches.Through geochemical and provenance analysis of the Shoushangou,Fanjiatun,Piaohechuan and Dongnancha formations,classification of geochemistry diagrams show that most samples of the 4 formations plot in the greywacke field,and some samples plot in arkose field.Primitive mantle(PM)-normalized trace element spider diagram and Chondrite-normalized REE patterns show that the Nb,Ta,Sr,P and Ti contents of all samples of the 4 formations are depleted,and all samples yield sub-parallel,enrich light rare earth element and relatively flat heavy rare earth element,combined negative Eu anomalies,the geochemical characteristics of the 4 formations are similar to those of the island arc-active continental margin.Combined CIA-ICV diagram,A-CN-K diagram,source area attribute discrimination diagrams and tectonic discrimination diagrams show that the provenance sediments of Shoushangou formation experienced a low degree of weathering,and the provenance consists of both relatively mature rocks and relatively immature rocks.The provenance of Shoushangou formation is predominately derived from intermediate to felsic igneous sources,and shows the addition of ancient sedimentary rocks.The provenance sediments of Fanjiatun formation experienced a low degree of weathering,and the provenance consists relatively immature rocks.The provenance of Fanjiatun formation is predominately derived from intermediate to felsic igneous sources.The Fanjiatun formation formed in the middle stage of collision orogeny.The provenance sediments of Piaohechuan formation experienced a low degree of weathering,and the provenance consists of both relatively mature rocks and relatively immature rocks.The provenance of Piaohechuan formation is predominately derived from intermediate to felsic igneous sources,and shows the addition of ancient sedimentary rocks.The provenance of the Dongnancha formation undergoes a relatively low degree of weathering,and the maturity is relatively high.The provenance of Dongnancha formation is predominately derived from intermediate to felsic igneous sources,and shows the addition of ancient sedimentary rocks.The Triassic clastic rocks could form in transformation stage between the collision orogeny to extensional environment.Based on the U-Pb geochronology and Hf isotopic composition of detrital zircons,the provenance analysis of the Triassic clastic rocks in the central Jilin Province,combined with provenance properties analysis of the Shoushangou,Fanjiatun,Piaohechuan and Dongnancha formations show that Neoproterozoic to Neoproterozoic detrital zircons were sourced from the ancient crystalline basement of North China Craton(NCC),Neoproterozoic to Neoproterozoic detrital zircons were sourced from Northeast block,Cambrian to Carboniferous detrital zircons were sourced from the magmatic rocks within the accretional belt of eastern of the northern margin of NCC and the magmatic rocks and Pan African metamorphic basement of Northeast block,and Permian to middle Triassic detrital zircons were sourced from the metamorphic volcanic rocks and granites within the accretional belt of eastern of the northern margin of NCC.The main sediment of the Triassic clastic rocks in central Jilin Province is derived from a bidirectional provenance,mainly sourced from intermediate-felsic igneous rocks within the accretional belt of eastern of the northern margin of NCC,and a small amount sourced from the Northeast block and NCC.Based on the analysis of the Permian-Triassic tectonic evolution history of the eastern segment of the southern margin of the XMOB show that the Paleo-Asian Ocean plate subducted to the NCC during early Permian-late Permian.The Paleo-Asian Ocean finally closed during early Triassic-middle Triassic,and occuring collisional orogeny,and the study area is under the extensional environment during late Triassic.The eastern segment of the Paleo-Asian Ocean finally closed during early Triassic-middle Triassic in central Jilin Province.