The Tectono-Magmatic Evolution And Lithosphere Dynamics Of Ri-Qing-Wei Basin

Eastern Shandong Offshore Ri-Qing-Wei Basin is situated in Sulu Orogenic Belt according to geotectonic division,which is a NE-SW trending rift basin.Ri-Qing-Wei Basin is bounded by Wulian – Qingdao – Rongcheng Faults at NW and by Qianliyan Fault at SE.Ri-Qing-Wei Basin and Jiaolai Basin is separated by Jiaonan Uplift at NW and separated by Qianliyan Uplift at NE.Sedimentary petrology,structural geology,geochemistry and zircon geochronology are comprehensively applied to the outcrops in the offshore areas of Ri-Qing-Wei Basin in order to investigate the Lower Cretaceous compositions,geotectonic setting,formation age,structural characteristics and tectonic evolution,moreover,combined with the evolution of Cretaceous prototype basins,the cooling history of metamorphic core complex,the time of magmatism in Dabie-Sulu Orogenic Belt and its vicinity to discuss the lithosphere dynamics of the studied area.The acquired conclusions are as follows:The Laiyang Group of Ri-Qing-Wei Basin was characterized by gravit flow deposits and siliceous clastic rocks.According to the stratigraphic contact relationship,the Laiyang Group can be further divided into Upper Laiyang subgroup,Middle Laiyang subgroup and Lower Laiyang subgroup.The strata in Lingshan Island was constituted by Upper Laiyang subgroup and Qingshan Group,the strata in Taolin Area was comprised of Upper,Middle and Lower Laiyang subgroup and Qingshan Group,the strata,exoposed in Qingshan bay,Yakou and Baxiandun,was constituted by Middle and Lower Laiyang subgroup.The sedimentary geochemistry analysis which was carried out on turbidite in Lingshan Island revealed that the turbidite was composed of first cycle sediments,its provenance consisted of felsic material and had a low degree chemical weathering.The geotectonic setting of turbidite in Lingshan Island was probably continental island arc and active continental margin.The geochemical characteristic value of turbidite indicate that the depositional environment was marine reductive environment.The REEs of turbidite are classifiedly and statiscally analysed by the method of statistics,whose results reveal that the most possible provenance of Lingshan Island turbidite are from Jiaonan Group and Penglai Group.The geochemistral analysis carried out on siliceous clasitc rocks illustrate that the formation of this type of rock was linked with the hydrothermal activity.According to the occurrence and lithologhic characteristics,the genetic model may be related to the hydrothermal origin and terrigenous clasts.The evolution of Cretaceous stress field was systematically analysed by the structures such as folds,slickensides,joints and dykes in the outcrops of Cretaceous strata.In the early stage of Early Cretaceous,the basin was dominanted by NW-SE trending extensional stress field;in the middle stage of Early Cretaceous,the basin was dominated by NWW-SEE trending compressional stress field;by the time of late stage of Early Cretaceous,the stress field became NWW-SEE trending extensional,in the late stage of Early Cretaceous to the early stage of Late Cretaceous,the basin was dominated by NW-SE trending compressional stress field,by the time of the ending of Late Cretaceous,the stress field became NE-SW trending compressional.Zricon LA-ICP-MS dating was carried out on the typical igneous rocks in Ri-Qing-Wei Basin,the results of the dating reveal that these magmatic rocks were formed at the time of Early Cretaceous(125~122Ma).This paper also investigates the petrogenesis of typical igneous rocks,felsic alkaline rocks were generated from alkaline magma which were formed in mid-crust and underwent fractional crystallization;basic rocks were generated from the baisc magma,which were form by the process of partial melting and underwent fractional crystallization;there was genetic relationship between the basic rocks and diorite,it is speculatedthat the diorite was formed by the baisc magma which underwent more fractional crystallization;the granitoid was formed by the partial melting of the lower crust and frational crystallizaiton.The deep mechanism of the burst of the magmatic activities in the short time interval was most likely that the lithosphere mantle and partial lower crust were delaminated into the asthenosphere.On the basis of the development of the sedimentary strata,paleostress field evolution and magma activities,the tectonic evolution Ri-Qing-Wei Basin was divided into 3 extesion periods and 3 times periods.In the time of early stage of Early Cretaceous(135~125Ma),the basin was dominated by the NW-SE trending extention and the marine rift basin was fromed.In the time of middle stage of Early Cretaceous(125~122Ma),the basin was dominated by NWW-SEE trending extrusion and changed from a marine basin to a terrestrial basin.In the time of late stage of Early Cretaceous,the basin was dominated by NWW-SEE trending extension and a large volume of volcanic rocks were formed during this period.In the time of the ending of Early Cretaceous to the beginning of Late Cretaceous(95~85Ma),the basin was dominated by NW-SE trending extrusion,severely uplifted and denudation.By the time of Late Cretaceous,there may be some Wangshi Group deposition at the middle structural belt of Ri-Qing-Wei Basin.In the time of Paleocene,the basin was underwent NE-SW trending extrusion andfurther shrank.A synthetic analysis,including sedimentary sequence,the cooling history of MCC and the time of magmatic activities in Dabie-Sulu Orogenic Belt and its vicinity,indicates that the de-rooting process was the possilbe deep mechanism which generated the Cretaceous basins and the subsequent each paleostress field was related to the subduction of Izanagi plate and Pacific plate.