The Chronology And Litho-geochemistry Characteristics Of The Volcanic Rocks From The Upper Volcanic Rock Series In Xinchang Area Of Zhejiang Province

Cenozoic volcanic rocks in southeastern coast has been one of hotspots in research study at home and abroad,this paper selected the study area which is located in Xinchang,Zhejiang province,the geotectonic belongs to late Mesozoic volcanic rock with the most obviously symbol in Fujian and Zhejiang region of southeastern China.It was widely distributed in the area with multiphasic volcanic activity product of volcanic sedimentary rock series.According to the widelyspread distribution of volcanic rocks in the geological characteristics and the nature of the regional unconformity between volcanic formation of China's southeast coastal areas,the former divided late Mesozoic volcanic rock strata are into the upper and lower volcanic series,this paper choose "the upper volcanic series" as the target of system investigation in geochemistry and chronology research,and what obtained as the following:Xinchang Volcanic structural depression belongs to ? volcanic tectonic,which is composed of Cangyan volcanic eruption center,Bamao resurrection volcano and Dongming resurrection breaking volcanic three ? of volcanic structure.The upper volcanic series consists of three layer: The Guantou layer,the Chaochuan layer and the Fangyan layer.The Guantou layer is a set of fluvial vitreous tuff with eruptive facies in lacustrine glutens-tuff sandstone-shale.Chaochuan layer is a set of fluvial and lacustrine red fine detrital sedimentary rocks containing brecciated vitreous tuff of eruptive facies.Fangyan layer of the quartic formation is a thick layer of massive conglomerate with 1-2 lamellar fluidic breccies-bearing crystal tuff and rhyolite;the lower part of the second section is a rhyolite bearing breccies-breccies-bearing tuff;the upper part is a crater lacustrine tuff-sandstone-mudstone;in the center of the crater can be seen bulbous rhyolite and lump-gathering breccia.The U-Pb dating method of zircon LA-ICP-MS indicates that the zircons measured are of magmatic origin,and the ages of the two Guantou layer measured are 105.7±1.9Ma and 110.5±2.2Ma,corresponding to the late Early Cretaceous.The geochemical characteristics shows that the rocks are high in K and aluminum,relatively alkali rich,low in Mg,Ti and Na,and belong to the series of high potassium,calcium and alkaline.In terms of trace elements,the large ion lithophilic elements(LILE),Rb,K,Nd and Pb are enriched,and the depleted high-field strong elements Nb,Ta and Ti,which is inclined to the right,showing that the petro-geochemical characteristics of continental arc volcanic rocks.The REE shows obvious negative europium anomaly,LREE is enriched,HREE is relatively flat,and the differentiation degree of LREE/HREE and LREE decreases,suggesting that the differentiation degree of magma decreases gradually with the formation changes.Combined with the chronology and geochemistry characteristics,the tectonic discrimination diagram shows that the volcanic rocks of "the upper volcanic series" were formed in the continental arc tectonic environment and subjected to the stretching stage of Eurasian continent under the subduction of Pacific plate.It was formed in the stage of relatively strong arc magmatism and the magmatism has undergone intense differentiation evolution.