Sedimentary Environment And Provenance Of Shitoukoumen Formation,Changchun:Evidence From Siliceous Rocks And Detrital Zircons

The Shitoukoumen Formation in central Jilin Province is composed of Early Carboniferous,Late Carboniferous and Early Permian units,which are in fault contact with each other,showing the tectonic attribute of accretion wedge on the continental margin.This paper presents U–Pb ages and Hf isotope data for detrital zircons and whole-rock geochemical data for siliceous rocks in the Shitoukoumen Formation,central Jilin Province,northeast China,with the aim to determine the origin and sedimentary environment of siliceous rocks,as well as the depositional ages and provenance of Shitoukoumen Formation.The above data,combined with regional research data,have used detrital zircon trace elements to reconstruct the crustal thickness curve of the northern margin of the North China Craton（NCC）,further revealing the Paleozoic to early Mesozoic evolution history of ocean and land in the northern margin of the NCC.The siliceous rocks appear in the early Permian units in layers,which are divided into red and green,and both contain radiolarian fossils.The red siliceous rocks are in fault contact with the early Carboniferous basalts.The geochemical characteristics indicate that they are of hydrothermal origin,affected by basic volcanic hydrothermal fluid,and the sedimentary environment is close to the mid-ocean ridge.The green siliceous rocks occur between tuff and siltstone in layered form.Geochemical characteristics indicate that they are of non-hydrothermal origin and their sedimentary environment is close to the continental margin.The zircon U-Pb geochronological analysis of sandstone,tuff and siliceous rocks in the early Permian units shows that the youngest U-Pb of detrital zircons is～270 Ma,which is the lower limit of the units.In addition,there are several main zircon populations:late Paleozoic（298 Ma）,early Paleozoic（400 Ma,428 Ma,442 Ma,468 Ma and 520 Ma）as well as a small quantity of zircons with Middle Proterozoic-Paleoproterozoic（1029～2692 Ma）age.The majority of early Permian detrital zircons yield relatively highε_Hf（t）values（+8.7～+13.2）,implying that they were derived from the Xing’an–Mongolia Orogenic Belt（XMOB）,the residual zircons have relatively lowε_Hf（t）values（-5.9～-13.0）,indicating that they were sourced from the northern margin of the NCC.Theε_Hf（t）values of early Paleozoic detrital zircons are between-3.11 and+10.13,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.The detrital zircons in the early Permian sedimentary rocks are generally characterized by light rare earth depletion,the heavy rare earth elements are enriched and obvious positive Ce anomaly and negative Eu anomaly.The trace element composition indicates that most zircons are derived from continental crust igneous rocks in the arc-related/orogenic tectonic settings,and only two Cambrian zircons are derived from oceanic crust igneous rocks in the arc-related/orogenic tectonic settings.The age distribution of detrital zircon indicated that the Shitoukoumen Formation was in a convergent sedimentary environment.There is a reverse grain sequence sedimentary sequence in the field of sedimentary rocks of Shitoukoumen Formation,which reflects the sedimentary characteristics of the near continental margin turbidite,indicating that its sedimentary environment may be the deep-sea sedimentary fan formed by gravity flow near the sea.Collect regional geological data and combined with this paper,calculate the crustal thickness of the study area using the Eu/Eu^*value of detrital zircon and the whole rock（La/Yb）_N value of igneous rocks.The fitting results are shown in ca.500～450 Ma,the crust experienced a significant thickening process（30～50 km）,which was related to the arc-continent collision of the island arc belt and the northern margin of the North China Plate in the Early Paleozoic.At ca.330～280 Ma,the crust first experienced a significant thinning process.The crust thickness decreased from 56 km to 40 km,and then gradually thickened until ca.245 Ma reached the peak value（58km）.The change of crustal thickness during this period may be the crustal thinning caused by the high angle subduction of the Paleo-Asian Ocean in the Early Carboniferous,and the continuous subduction of the oceanic crust made the crust continuously thickened,and finally the continent-continent collision orogenic occurred in the Middle Triassic.