Provenance And Its Geological Implication For The Upper Triassic Xujiahe Formation In Western Sichuan Basin

Sichuan Basin is a polycle sediementary basin in the Yangtze block.In this study,sandstone petrology,paleocurrent and detrital zircon U–Pb ages are combined to discuss the provenance of Upper Triassic strata Xujiahe Formation of the Sichuan Basin.Based on this,integrated the data of sedimentological,the geological survey,mainly achieved understanding as follows:1.On the basis of the analysis in the regional geological structure and lithostratigraphic units,the U-Pb age spectrum and the spectrum of Hf isoplote are established.(1)Early to late upper Triassic samples from northern Sichuan Basin in Guangyuan show four populations: the Hercy-indosinian(243~298 Ma),the Caledonian(437~494 Ma),the late of Jinning period(785~865 Ma)and the Lvliangian(1817~1935 Ma).Corresponding with ?Hf(t)values are-25.9~-0.8,-9.7~+10.6,-9.5~+11.8,-14.8~-0.8,respectively.(2)The ones come from the middle part of western Sichuan Basin in Doujiangyan exhibit four similar consistent component populations: the Hercy-indosinian(243~244 Ma),the Caledonian(435~478 Ma),the late of Jinningian(772~865 Ma)and the Lvliangian(1805~1976 Ma),Corresponding with ?Hf(t)values are-7.9,-6.6~-7.6,-27.3~+9.2,-15.0~-1.6,respect ively.2.Upper Triassic Xujiahe Formation in Guangyuan and Doujiangyan sandstone samples of early to late age groups zircon trace element analysis,showed that the basement rock types of Lvliangian period are synite,dolerite and granite,the late of Jinning period mainly is synite and basalt,the basement rocks of Caledonian and the Hercy-indosinian are acidis rocks that granite and synite.3.The upper Triassic strata were continental including coal clastic sedimentary,characteristics of sandstones from the north part show low maturity,but relatively high in the middle part,and from the north to the middle the particle size became smaller.Combining with the palaeocurrent direction,the direction mainly came from north to south,north-south west to the east.Therefore,the source of western Sichuan Basin is mainly composed of the north orogenic belt.4.Bonded the reported data of sedimentological,the petrology of sandstone,the ages of detrital zircon U–Pb and the Hf isotopic,we propose that detrital zircons in the Xujiahe Formation yield consistent characteristics and the sediments in western Sichuan Basin were derived from the same source areas.By comparative study of the zircon U-Pb ages and Hf isotopic composition with the adjacent orogenic belt,a large population of detrital zircons show the characteristic of Qinling Orogenic whose U-Pb ages are 230~300 Ma,420~490 Ma and 1800~1900 Ma.The group of 770~890 Ma correspond to the data in Yangtze block,which indicate that the detrital zircons derive from Yangtze block.5.The sediments of the Upper Triassic in Songpan-Ganzi have the common characteristics of U-Pb ages and Hf values between the western Sichuan Basin and the provenance of the sandstones are consistent which indicate that the sediments of the Upper Triassic in Songpan-Ganzi and the western Sichuan Basin have the same provenance which mainly came from the north margin that Yangtze block and Qinling Orogenic Belt.The analyses show that Longmen Shan belt,which located between Sichuan and Songpan-Garze Basin,had not been uplifted during Late Triassic.