Geologic Features And Provenance Analysis Of Late Paleozoic-Early Mesozoic Sediment Strata At Qinghai Daowei Area In The Northern Margin Of The West Qinling

Permian Ganjia Formation and Early Triassic Longwuhe Formation widespread in northern margin of West Qinling,yet for which there were few researches on stratigraphic division,sedimentary environment,sedimentary source and basin prototype.Taking Permian Ganjia formation and Early Triassic Longwuhe formation located at Qinghai Daowei area in the northern margin of the West Qinling as the research object,devide lithostratigraphic unit base on field observation,surveying geological section and geologic mapping.This article discussed the depositional era,sedimentary characteristics,sedimentary facies and environments,basin fill sequence,provenance,basin tectonic attribute and basin prototype base on integrated information about petrology,sedimentology,detrital composition analysis,petrogeochemistry and detrital zircon isotope chronology.And late Paleozoic-early Mesozoic tectonic evolution of the northern margin of West Qinling has been established.The main results achieved in this work are listed as follows:1.The late Paleozoic-early Mesozoic sedimentary strata in Xunhua Daowei area are Permian Ganjia formation and early Triassic Longwuhe formation.Ganjia formation can be divided into 4 segments: First segment(Pg~1)is coarse clastic rocks with slate rock segment,transition from fan-deltaic plain deposits up to intertidal carbonate sediments.Second segment(Pg~2)is clastic rocks with sandstone segment,neritic shelf marginal basin sediment.Third segment(Pg~3)is bioherm limestone segment,intertidal zone of neritic shelf carbonate platform sediments.Fourth segment(Pg~4)is thin clastic rocks with limestone segment,bathyal-neritic shelf marginal basin sediments.Longwuhe formation can be divided into 5 segments: First segment(T1l~1)is coarse clastic rocks with slate rock segment,belong to inner turbidite fan sedment.Second segment(T1l~2)is thin clastic rocks with thin-bedded sandstone and limestone segment,belong to central turbidite fan sedment(no watercourse parts).Third segment(T1l~3)is thin layer slate,minddle-minddle thin layer sandstone segment,belong to central turbidite fan sedment(with watercourse parts).Fourth segment(T1l~4)is slate with minddle-minddle thick sandstone segment(interbed),belong to central turbidite fan sedment,may be on the lower continental slopes.Fifth segment(T1l~5)is silty sand(muddy)slate segment,belong to external turbidite fan sedment,distal turbidite and deep-water basin transition zone.2.Obtained the youngest detrital zircon U-Pb ages in sandstone sample from Ganjia formation first segment is 299 Ma,representative deposit limit of Ganjia formation.combine the fossils information,determined the Ganjia formation sedimentary age to Permian.The youngest detrital zircon U-Pb age in Longwuhe formation is 258 Ma,representative deposit limit of Longwuhe formation,Guyi diorite emplaced in Longwuhe formation,crystallization age is 246.7±2.1Ma,representative deposit ceiling age,so dedimentary age of the Longwuhe formation can be restricted to the 258~247Ma.Combine previous fossil information,Longwuhe formation depositional age can be determined to early Triassic.3.Source rock of Ganjia formation has low weathering degrees,belong to first cycle sedmentation,sediment sources mainly derived from Early Palaeozoic dissected arc in Qilian orogenic belt,dominated by acidic rocks,and a small amount from the material recorded related to Middle Proterozoic geothermal events and the Neoproterozoic convergence and cracking of Rodinia supercontinent.Sedimentary basins prototype is neritic shelf in the passive continental.4.Longwuhe formation is the first cycle sedments,with weake chemical weathering in source area,and the paleocurrent deriction is NWW-SEE,sedments source include Early Palaeozoic dissected arc in Qilian orogenic belt,series material record related to the extention and subduction of Paleo-Tethys Ocean,and the metamorphic basement of Qilian orogenic belt,especially the Hualong group in the study area could be a direct sources.Sources area was dominated by acidic rocks,and a few basite,a area of mixture distribution maight affected by basement uplift.sedimentary basin prototype should be back-arc basin.5.Combined this work,the regional geological data and previous research,the tectonic evolution of northern margin of West Qinling and adjacent area on late Paleozoic-early Mesozoic can be divided into three stages: 1)Devonian-Permian paleo-Tethys Ocean splitting-tension phase;2)Late Middle Permian-Early Triassic northward subduction,back-arc rift basin expansion phase;3)Middle-Late Triassic,back-arc basin close,collision and post-collision phase.