Sedimentological And Tectonic Characteristics Of The Ordovician In The Southern Part Of Central Hunan,China

The central and south Hunan is in the middle section of the Caledonian orogenic belt in southern China where ordovician sedimentary evolution is relatively complex.The study of transformation of Ordovician and prototype basin in this area is significant to understand the Caledonian orogenic belt.This paper adopted the petrology,the sedimentary stratigraphy,rock geochemistry and U-Pb zircon test methods based on geological field work and predecessors' research results.Some comprehensive and systematic understanding of the ordovician sedimentary environment,provenance direction and structural evolution process has been achieved as follows:1.The sedimentary facies in the study area are as follows:Baishuixi and Qiaotingzi formation's are shelf sea.Yanxi formation's is stranded basin deposit.Tianmashan formation's is turbidity current phase.2.The geochemical data of Baishuixi formation and Qiaotingzi formationwere first achieved which indicated that the sediments sourcecame from passive continental margin.Combining with paleocurrent direction analysis,it predicted that the sediments source in this period mainly came from Yangtzeblock in north-westdirection,through short distance transportation,and rapid accumulation.The prototype basin is passive continental margin in the same time.3.Contrastiveanalysis of detrital zirocon age result of Tianmashan formation and the old strata of Yangtzeand Cathaysian blocks,the upper ordovician of the study area had higher affinity with Cathaysian block.Combining with paleocurrent direction analysis,it predicted that the sediments source in upper ordovicianmainly came from Cathaysian block in south-eastdirection,through short distance transportation,and rapid accumulation.The proto-basin is passive continental margin in the same time.4.The clastic rock in southern margin of Yangtze blocks may be the component of Gondwanaland,and the detrital zircon age spectrum shows a higher affinity with the Himalaya block.