Petrogeochemical Characteristics Of Laiyang Group's Sandstones In The Zhucheng Basin And The Tectonic Setting Analysis Of Provenance

Zhucheng Basin is a extensional basin formed in Early Cretaceous period.And there are few studies on the relationship between Zhucheng Basin and Jiaonan Orogen.prototypes of Zhucheng Basin and tectonic setting remain controversial..This paper is based on field study,sandstone detrital composition statistics,petrogeo-chemical analysis and comparison,zircon U-Pb geochronology,discusses the debris source direction of the Laiyang Group's sandstones in the Zhucheng Basin,weather-ing characteristics of the provenance and tectonic setting.The conclusions are as follow:(1)The major element data of the LWZ Fm and QGZ Fm in Zhucheng Basin revealed that the source undergo full weathering during the QGZ formation.Comparing the chemical composition of Laiyang Group's sandstones with the average chemical composition of the continental crust,most oxide mass percent is lower than the average chemical composition of the continental crust or is closely to the latter.Only the Na2O content is higher than the latter.(2)The zircon age of the granitic gneiss material in the conglomerates at the base of the Laiyang Group is 790±8.4 Ma,close to the ages of the Neoproterozoic granitic gneisses in the Jiaonan Orogen(739?819 Ma).Clearly,these ages differ significantly from those of the Lower Cretaceous intrusions,revealing that the Neoproterozoic granitic gneisses in the Jiaonan Orogen may be the main provenance of the Laiyang Group in the Zhucheng Basin.(3)In the primitive mantle-normalized spider diagram of the trace element,Laiyang Group is relatively enriched in the large ion lithophile elements of Rb,Ba,K and rare earth elements La,Ce and is depleted in the inactive elements P,Ti,Y,Yb and Lu,with intermediate amounts of Zr and Hf and various Sr contents.In the chondrite-normalized diagram of the Zhucheng Basin sediments,the Laiyang Group is relatively enriched in the light REEs,with negative Eu anomalies.In the North American Shale Composite normalization of the Zhucheng Basin sediments,light REEs content is basically the same as the North American shale,there was a slight loss of HREEs content,the content of Eu is slightly higher,which indicates that the sandstone of LWZ Fm and QGZ Fm of Laiyang group has experienced a relatively sufficient sedimentary cycle.(4)The LWZ Fm 's sandstones and the QGZ Fm s sandstones undergo weakly weathering.The CIW value of the sandstones in LWZ Fm is 47,the CIW value of the sandstones in QGZ Fm is 50.And it is similar to the average chemical weathering index of the upper crust,suggesting the source of the sandstones has the characteristics of rapid uplift.The chemical alteration and paleo-weathering indices(CIA?CIW?PIA and ICV)suggest that the source rocks generally did not experienced weathering.Jiaonan Orogen undergo the rapid uplift in Laiyang Group's period,accompanied by the extensive intrusion activity of the early cretaceous rock mass.(5)The source of sandstones has the characteristics of active continental margins and continental island arcs.From the perspective of sedimentary sequence,the source transformed from active continental marginal tectonic environment to continental island arcs tectonic environment.The Jiaonan Orogen maybe the main source of Laiyang Group's sandstones in Zhucheng Basin.(6)The rapid subsidence of the Zhucheng Basin during the formation of the Laiyang Group and the rapid uplift of the Jiaonan Orogen both resulted from a single regional extensional event.This is a performance of the thinning of the North China craton lithosphere in this region and a result of the regional extension events.