Geochemistry,Geochronology And Implications Of The Sandstones In Upper Ordovician In Central Hunan

This paper studies upper Ordovician sandstones in Central Hunan and systematically analysises the geochemitry features,detrital zircon U-Pb chronology and its trace elements. Through the analysis of the main and rare elements,got samples CIA values are between 74.1 ~77.5,mean value is 75.CIW values are between 85.03~98.73,the average value is about 90.78.ICV values are between 0.53 ~ 1,the mean value is around 0.82.Log(Si O2/Al2O3)—Log(Fe2O3/K2O) 、Al2O3-Fe2O3 and Al2O3-SiO2 etc diagrams. All these indicate that sample sandstones in survy region have low compositional maturity and the source rocks suffered strong chemical weathering.w(Sr)/w(Ba)、w(V)/w(Ni)、w(V)/w(V+Ni) and U、Mo content show that upper Ordovician sandstones in Central Hunan sedimented in hypoxic offshore environment.The sandstones main elements features indicate that the tectonic setting of Central Hunan in late Ordovician is mainly PCM and ACM.The source region tectonic environment discrimination diagrams show ACM.F1-F2 and trace REE diagrams indicate that the source rocks are mainly felsic acidic magmatic rocks.Their chemical composition is close to the average upper crust.The source rocks formed from ancient sedimentary rocks.Detrital zircon U- Pb dating got 65 valid ages,including three of the zircon ages are 195±3Ma,236±3Ma and 274±11Ma which are hydrothermal reformation zircons.This phenomenon indicats that there was a massive hydrothermal convection wave in late Ordovician in Central Hunan.Five groups of harmonic age respectively is 1090±31Ma(n=8),1345±55Ma(n=7),1690±45Ma(n=4),2415±48Ma(n=6) and 2577±24Ma( n=5),which recorded much information about Proterozoic rocks basement.And 10 zircon ages show Archean basement,their ages are between 2600Ma~3200Ma.Zircon age spectrum diagram shows three age peaks ~ 1.2 Ga, ~ 1.9 Ga,~ 2.5 Ga, and a lot of detrital zircons with 500 ~ 850 Ma.Zircon rare earth elements distribution patterns are similar to shell source zircons.Trace element diagrams show that the vast majority of detrital zircons originated from the continental island arc environment.Through comprehensive analysis this paper holds a view that Central Hunan has higher sources affinity with Huaxia plot rather than Yangtze, and the sedimentary materias of late Ordovician strata mainly come from Huaxia plot. Early Paleozoic Caledonian movement caused Xuefeng mountains rapidly uplifted, and suffered strongly weathering.The Central Hunan area was an arc backshore shallow basin at that time and accepted a lot of terrigenous detrital sediments.