The Study Of Chronology And Geochemistry Of Gabbros In East Of Shangdan Structural Belt

As a boundary between the North China plate and the Yangtze plate,the Shangdan structural belt is product of the subduction of the Shangdan Ocean and reduced to the process of closure.The main Shangdan structural belt is a set of greenschist-lower amphibolite facies volcanic-sedimentary rocks,such as ophiolitic melange,metasedimentary,meta-volcanic rocks,marble and all that.There are also a series of acidic to ultrabasic intrusive rocks,including gabbros,diorites and granites.Many of them had lenticular linear distribution of basic-ultrabasic intrusive bodies.Most of these intrusive rocks are preserved with magmatic structures and formed in the early Paleozoic,The existing researches have different understanding of the forming reason and evolution of these gabbros,and lack corresponding research on the basic-ultrabasic intrusive rocks in Danfeng area.The formation ages,structural setting and forming reasons of these intrusive rocks have important geological significance to explore the tectonic evolution of the Early Paleozoic in the North Qinling orogenic belt.In this paper,based on the previous systematic studies,the Gu Chenggou gabbro body in the Danfeng area of the East Qinling is selected as the research object,and the detailed field geology,petrology,geochemistry and chronology researches are carried out.Get the following views:(1)The Gu Chenggou gabbro body in the Danfeng area is small and in the form of lenticular parallel to the Shangdan fault zone and intrudes into the volcanic rocks of the Danfeng Group.The pluton is mainly composed of gabbros,and a small amount of gabbro-diorite and diorite.From the edge of the rock to the center,the higher the basic degree of the rock and more coarse of mineral grain of the rock.The centerof the Gu Chenggou gabbro body has low degree of metamorphism.The edge of the gabbro body has strong metamorphism and develop penetrative structural foliation of gabbro mylonite.(2)The dating of LA-ICP-MS zircon U-Pb shows that the weighted average age of the Gu Chenggou gabbro body is 425.1±1.7Ma(MSWD=0.10)and 424.3±1.3Ma(MSWD=0.16).The formation age is Silurian Wenlockian.(3)The Gu Chenggou gabbro body have the calc-alkaline to sub-alkaline of Low potassium and rich sodium property,and the rare earth elements(REE)are the right-graded fractionation mode,LREE/HREE = 8.03-11.95,LaN/YbN= 9.82-17.64,8Eu 0.93-1.31,Eu anomaly is not obvious or weak positive anomaly.Richment of the large ion lithophile elements(LILE)Cs,K,Sr,Ba and depletion of the high field-strength dement(HFSE)U,Th,Nb,Ta,Hf,Zr.and the Th/Ta value is 6.00-13.67.The geochemical characteristics of the Gu Chenggou gabbro body is not belong to"ophiolite genesis" of Danfeng Group.The geochemical characteristics show that the Gu Chenggou gabbro body is formed in the active continental margin environment.The magma of gabbro derived from the plate subduction caused by the mantle wedge part of the melting.In addition to addition of components from subduction,the magma in the process of rising by the contamination of crust.(4)The evolution of the Shangdan structural belt from the late Jinning period to the Caledonian period is established by combining the achievement of Gu Chenggou gabbro body and other basic-ultrabasic intrusive rocks of the North Qinling.At 600 Ma,the Shagndan ocean was in the quiet period.At about 515 Ma,the Shagndan oceanic crust began to subduction northward.About 440Ma,the Shagndan oceanic crust entered the deep subduction stage,part of the initial plate collision occurred.About 422Ma,Shangdanocean closed,into the land-land collision stage.