Provenance And Tectonic Significance Of The Permian Shangshihezi Formation At The Southwestern Margin Of The North China Craton

The North China Craton(NCC)is mainly dominated by the active continental margin in the Late Paleozoic.This craton is bounded by Ondor Sum subduction-accretion complex and Bainaimiao arc terrane of the Central Asian Orogenic Belt to the north,and the North Qinling Island Arc of the North Qinling Orogenic Belt to the south.Most areas of the NCC began to deposit after experiencing hiatus during the Middle Ordovician to Late Carboniferous.At the same time,the southwestern margin of the NCC did not accept deposits until the Middle–Late Permian,which made the Shangshihezi Formation unconformity overlying the Ordovician Majiagou Formation.Based on the previous study,the detritus source of the Shangshihezi Formation was discussed by petrography,geochemistry,detrital zircon U-Pb geochronology in the southwest margin of the NCC.Furthermore,the tectonic movement and constraints on plate interior movement were also analyzed in the orogenic belt around NCC.The petrographic and geochemical characteristics of sandstone indicate that the sediments of the Shangshihezi Formation in the study area are mainly litharenite and greywacke.The sediments are characterized by both short distance and long-distance transportation,and the sedimentary environment is mainly active continental margin.The major and trace element analysis show that the materials in the source area are mainly felsic rock with a strong tectonic environment,and the tectonic background is characterized by both active continental margin and continental island arc.The CL images and the trace element composition characteristics of detrital zircons indicate that the zircons in the Shangshihezi Formation are mainly magmatic.The zircon age consists of four age groups: 2.4?2.6 Ga,1.7?2.0 Ga,368?551 Ma and 254?352 Ma.The weighted average age of the six youngest detrital zircons was 256.8±3.6 Ma.Combined with biostratigraphic age,the maximum sedimentary age of the Shangshihezi Formation was the Middle–Late Permian.The sediments of the Shangshihezi Formationin in the study area came from multiple provenance areas.Comprehensive analysis suggests that the Neoarchean to Early Paleoproterozoic zircons were mainly derived from the basement rock of the NCC,with a minor contribution from North Qinling and North Qilian Orogenic Belt.The Middle–Late Paleoproterozoic zircons were derived from the basement rock of the NCC.The zircons with ages of 368?551 Ma were mainly sourced from island-arc volcanic rocks in the North Qinling and North Qilian Orogenic Belt,which junction part was the most likely source area.The Late Paleozoic volcanic rocks of the northern margin of the NCC provided the 254?352 Ma detrital zircons to study area.A comparison of the age spectrum of detrital zircons from the Shangshihezi Formation and other parts of the NCC shows that,except the Luoyang Basin in southeastern craton,the other regions all contain more zircon from the north margin of the NCC with the peak age of ?320 Ma,suggesting that the paleotopography of the NCC in this period is characterized by the southern tilting.This may be due to the impact of the closed and collision of the Paleo-Asian Ocean,the intense uplift and magmatism occurred at the northern margin of the NCC,which provided the clastics inside the NCC.Zircons with a peak age of ?450 Ma in the study area indicate that due to the further northward subduction of South Qinling Terrane,the North Qinling–North Qilian Orogen begins to uplift slightly during the Shangshihezi Formation period,and provides clastics for the study area.