| Low grade metamorphic rocks of the Neoproterozoic Banxi Group disconformatively overlain by the Sinian and Paleozoic are extensively exposed in the Jiangnan-Xuefeng structural belt.At present it is generally considered that the Xuefeng structural belt is a Caledonian uplift at southeast margin of the Yangtze continent.In this paper,we mainly studied geological structure of the Kaili-Congjiang-Rongshui-Rongan-Dushan belt at the southwest margin of the Jiangnan-Xuefeng structural belt,and discussed the tectonics according to spacial changes of sedimentary facies of different ages and evolution of sedimentary basins.In addition, geochemical test of magmatic rocks and isotope geochronological analysis were also conducted.On the basis of these studies,some primary conclusions were summarized as following:1.Outcrops of ages ranging from the Neoproterozoic Banxi Group to the Middle Triassic are fully exposed at the southeastern Upper Yangtze continent and southwest margin of the Jiangnan-Xuefeng structural belt,wile the Upper Triassic,Lower-Middle Jurassic,and Upper Cretaceous are partially exposed.However,the Paleogene and Neogene are invisible in the study area.The study area can be sub-divided into four structural layers,in chronological order,the pre-Sinian,Sinian and Lower Paleozoic,Devonian-Triassic,and post-Triassic structural layers,which reflect four main geologically developmental stages in the study area, respectively.Stratigraphically,the depocenter migrates from southeast to northwest.2.It is notable that the study area is divisible by the Kaili-Dushan structural belt. Eastern and western wings of this belt show distinctive character of deformation.Vertical uplift occurs in western Kaili-Dushan structural belt,causing deletion of the Upper Silurian and parallel unconformity between the Devonian and Lower Paleozoic.In the west,Yanshan movement exhibits strong folding,causing extensive deletion of the Jurassic and Cretaceous as well as angular unconformity between clastic rocks of the Upper Cretaceous Maotai Group and the underlying strata.In the east,Yanshan movement exhibits strong extension,causing the occurrence of extensive granites in South China.The Paleogene and Neogene is absent from the study area.According to the development of Quaternary fluvial terrace and dissolved cavern,Himalayan movement mainly exhibits vertical movement in the study area.3.Deformations of a variety of age developed in the study area are,in chronological order,inherited from the younger stages of deformation.Multiple,long-term activated large faults and revived faults have together destroyed structure of the basement and controlled deposition of the capping bed. 4.A U-Pb age of 836±5.1 Ma is obtained using La-ICP-MS dating method from zircon. The zircon-host granitic rock is sampled from Hekou of Xiutang,eastern Guizhou.In southeastern Guizhou-northern Guangxi area,early-middle stage activities of Neoproterozoic acidic magmatism occurred after collision of the Jiangnan orogenic belt,which may have occurred during tension stage of the magmatism.This magmatism is part of the large-scaled, southeastern Guizhou-northern Guangxi acidic magmatism.5.Four stages of structural evolution occurred in the Jiangnan-Xuefeng structural belt:(1)Formation Rodinia ancient land stage(Early Qingbaikou):the Yangtze and Cathaysia continents merged together;Jiangnan orogenic belt formed.(2) Detachment and re-assemblage of the Rodinia ancient land stage(Early Qingbaikou-Silurian):depression(Qingbaikou-Middle Ordovician) and uplift and erosion (Late Ordovician-Silurian) of the Jiangnan-Xuefeng structural belt.Four sub-stages can be identified:late Qingbaikou-Sinian(820-680Ma) Jiangnan-Xuefeng depression basin, Sinian-Middle Ordovician Jiangnan-Xuefeng depression basin,Late Ordovician-Silurian folding and deformation of the eastern study area,and uplift and erosion of the western study area.(3) Devonian-Triassic:Inner-Yangtze Craton depression and superimposed evolution of the NW-trending Ziyun-Luodian-Nandan depressed trough.Four sub-stages can be identified: Tension of the Ziyun-Luodian-Nandan depressed trough(Devonian) and depression of other area,formation of inner-craton carbonate platform(Carboniferous-Permian),shallow water basin under regression(Early-Middle Triassic),and transition of oceanic to terrestrial environments(Late Triassic).(4) Basin and orogenic belt evolution stage(Yanshan-Himalayan):Three sub-stages can be identified:Jiangnan-Xuefeng inner-continent deformation during Late Jurassic-Early Cretaceous(early Yanshan),tension and thinning of lithosphere and A-type magmatism during Late Cretaceou-Paleogene(late Yanshan to early Himalayan),and inner-continent deformation during Neogene afterward(late Himalayan).6.On the basis of filed observation,detailed investigations of strata and previous publications,we suggest,as proposed by previous workers,that the Xuefeng structural belt is the result from a Caledonian uplift of the southeastern margin of the Yangtze Continent. |
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