Geological Characteristics And U-Pb Dating Of Yuhuashan Super Unit In Yuhuashan Basin Jiangxi Province

During Mesozoic era, a large number of volcanic rocks formed in southeast China,which have a significant impact on Mesozoic mineralization, combined with the specialrock types, It has been studied by many scholars. JiangXi province Yuhuashan volcanicbasin is located on the north of western Fujian-eastern Jiangxi magma activity area. Only10kilometers away from Xiangshan volcanic basin, with the same tectonic structures, andboth in the area of Hangzhou-Jiangxi uranium mineralization. Many scholars have hadthorough studies on Xiangshan basin, but much less then for Yuhuashan. So, I chooseYuhuashan super unit and Taoxi unit as the objects of my study, which are important partsof Yuhuashan volcanic basin. Through the completely study on geological features androck ages, combined with geochemical characteristics with each unit, try to investigate therock formations and tectonic environments.Yuhuashan super unit consists of three parts: Lingli uni（tK1L）、Jincan uni（tK1J）andTaxi unit（K1D）. This located in the north of Yuhuashan basin. Occupies1/3of the wholeexposed volcanic area. The main lithologies contain extrusive–invasion rock facies:porphyroclastic lava and haplophyre. Besides, discovery of ignimbrite firstly proved thatYuhuashan volcanic basin has experienced the complete magma process:eruption-extrusive-invasion activity. Different from the old thought of “graniteporphyry”.With the stroma structure, we can divide porphyroclastic lava into three different rockfacies: border facies (K1L)、transition phase(K1J)and center phase(K1D). The same mineralcomponent, The same constitution and the same sources. Taoxi unit main lithologies isgraniteporphyry with large phenocrystal, reflect the different formation from Yuhuashansuper unit. Later age but the same source.Use the high-precision LA-ICP-MS U-Pb dating system on the four units rocks. Theresults shows: Yuhuashan super unit formation age is about137Ma, and Taoxi unit is 132.72±0.84Ma. Significantly later than former. Yuhuashan basin, the main volcanicformation age are Mesozoic, the Early Cretaceous. Consistent with age in the three unitswithin the error description,Yuhuashan large-scale volcanic intrusive activity may be aconcentrated, short-term. Yuhuashan Basin, large-scale volcanic activity stopped at about137Ma, and then became into the invasion-intrusive magmatism stage.Yuhuashan super unit has the similar geochemical characteristics with Taoxi unit.High silicon、potassium-rich、weakly peraluminous、calc-alkaline series、right-wing REE、HREE fractionation is significantly and negative δEu obviously, combined with traceelement characteristics and O isotopic evidences indicates that Yuhuashan and Xiangshanvolcanic complexs belong to the S-type granitic volcanic-intrusive rocks in the genesis.Diagenetic material is mainly derived from partial melting of the crust, Pb isotopiccompositions indicate the source of mantle-derived lead by adding obvious, tectonicenvironment with the characteristics of the orogenic belts.Combined with previous studies, in the Mesozoic era, Yuhuashan basin tectonicenvironment is about the edge of mainland activities, which is caused by the Pacific platebeneath to the Eurasian plate subduction, abatement of the lithosphere and the intrusion ofbasaltic magma, resulting in partial melting of overlying crustal material, provided theoriginal magma for Yuhuashan volcanic complex, mantle activity only provide heat, thereis no significant component to join.