Abstract:

According to the sedimentation,magmatism,tectonics and isotopic data available from pre-Sinian basement rocks exposed along the margin of the upper Yangzi platform,it is suggested in view of stratigraphieal division that.1) the pre-Sibao (1700—1900 m.y.) magnisium-and iron-rich sediments,represented by Kongling group,are distributed along Sichuan-Jianghan region and controlled by the island chain composed of basic and ultrabasic rocks,they were subjected to regional metamorphism,migmatization and granization and finally turned to continental crust through consolidation;2) during Sibao period,the Sichaun-Jianghan continental crust largely emerged from water,with a miogeosyncline and a eugeosyncline lying parallel to north side which are similar to Atlantic type margin,while island-are type geosynclne,similar to Pacific type,along its south side.The Sibao movement (1400 m.y.) strongly influenced the south part and turned it into consolidated basement of continental crust;3) during Jinning period,the Jiangnan islandare continued its activities.At the same time,the back-are region,including north Guangxi and west Hunan,as well as Jianghan and east Sichuan,was evoluted into a miogeosyncline developed on a sialic crust.Carbonates and clastics,intercalated with intermediate-acidic volcanic rocks,formed along Kangdian Axis region with intermediate-acidic magnatic complexes placed in Kangdian Axis and Mr.Longmen area,implying an intensely active continental margin.The sedimentation in the ancient Qinlin Sea was continued from the preceeding eugeosyncline,but due to the compressive forces of Jinling movement (800 m.y.),exerted toward both the north-east and north-west,the entire basement of the upper Yangzi platform drifted northward,it was then finally closed and welded into Huabei platform.Thus the upper Yangzi platform completely cratonized and converted into continental crust;4) during Chengjiang period,a large part of the upper Yangzi platform recieved sedimentation,while the Kangdian Axis and Mr.Longmen mountain-building arc still continued its action,leading to further basement consolidation of the present area.Based on the evolution characteristics,the degree and style of consolidation,the texture of the basement,the platform discussed can be divided into.a) Sichuan-Jianghan region which accordingly subdivided into strongly consolidated Sichuan basin(At),with single tectostratigraphic unit,and eastern Sichuan and Jianghan region (A₂),with binary tectostratigraphic units,the upper one being slightly cratonized and the lower one being strongly cratonized;b) north Guangxi,west Hunan and east Yunnan region with also binary tectostratigraphic units,the upper one being slightly cratonized and the lower one moderately cratonized: c) the Kangdian Axis region with triple tectostratigraphic units with distinct lateral heterogeneity;and d) WudangBikou region with single tectostratigraphic unit.