Caledonian Episodes In Youjiang Basin, Southeastern Margin Of The Yangtze Plate

The Caledonian movement,which occurred in the late Early Paleozoic,had a profound impact in the South China Plate.In order to understand the nature of the Caledonian episodes that occurred in the southeast margin of the Yangtze Plate,several representative unconformities between the Lower Devonian and Cambrian,including regions of Longlin and west Damingshan in Guangxi province and Guangnan in Yunnan province,are selected to study the intensive Caledonian orogeny.Based on systematic geological investigations,the Caledonian episodes were rebuilt by analysis of orthographic projection,stratigraphic distribution and structural style.The rebuilding results reveal the Caledonian E-W-striking fold in west Damingshan region,NE-SW-striking fold in Guangnan region,and NE-SW-striking anticline in Longlin region.A gradually decreases deformational strength from south to north was revealed by fold styles.The E-W-striking fold belts involved the Cambrian strata in west Damingshan region are formed by top-to-the north thrusting of the Yunkai Block during the Late Cambrian-Early Ordovician.The NE-striking folds involved the Cambrian in Longlin and Guangnan regions might be produced by SE-NW principal pressure stress field during the Late Ordovician extrusion of the Cathaysia plate into the Yangtze plate.The composition of the Cambrian and Devonian in the Longlin area is similar.The standardized distribution curves of rare earth element chondrites are all right-inclined.High CIA value indicates high provenance chemical weathering degree.ICV value is low and the maturity of the components is high.The tectonic setting of the Cambrian and Devonian sedimentary basins is passive continental margin,and their provenance is quartzitic sedimentary source area.the Cambrian sedimentary rock in Damingshan area has a similar provenance,charactered by felsic and quartziferous rock.The Devonian detrital zircon age spectrum sampled from Longlin region reveal Devonian terrigenous sedimentary environment of the Yangtze Craton continental margin.Terrigenous debris are mainly came from the Neoproterozoic basement of the Yangtze plate.The regional unconformity is constrained at 394-474 Ma.The peak at 980 Ma of the Cambrian detrital zircon age spectrum in the western Daming region implies a possible Grenville orogenic belt,and thus the Cambrian detrital zircon source in the Daming region is mainly from the Cathaysia plate.Since Caledonian magmatic activities have not been clearly observed in Devonian sedimentary rocks in the Longlin region and the Damingshan-Dayaoshan regions,indicating that the Caledonian tectonic magmatic events in the southeast margin of the Yangtze plate are not strong.Therefore,we suggest that the Caledonian movement in the southeastern margin of the Yangtze plate shows character of soft orogenesis.