Neoproterozoic Magmatic Evolution And Its Constraints On Mineralization In The Eshan Granite Batholith,Yunnan

The Eshan Granite is located in the Eshan-Shiping district of Yuxi City,Yunnan.It is one of the largest exposed granitoid bodies?approximately 200km²?in the southwestern margin of the Yangtze Craton,South China.The pluton was emplaced in a regime dominated by extension following the subduction of Cathaysia block beneath Yangtze block,a collisional amalgamation that occurred during the Si Bao Orogeny.This paper describes geology,whole-rock geochemistry,Rb-Sr,Sm-Nd,Lu-Hf,O isotope compositions,geochronology,fluid inclusions of intrusions in the Eshan Granite area to reveal the magmatic evolution,and metallogenic relationship.?1?The extensive Eshan Granites of Yunnan are made up of three intrusive units distinguished by their field contact relations;in descending order of age they are the Pojiao Unit,the Lüzicun Unit and the Mokela Unit.The major rock type in the Pojiao Unit is porphyritic biotite monzogranite;in the Lüzicun Unit,porphyritic biotite?two-mica??tourmaline?granite;and in the Mokela Unit,biotite alkali-feldspargranite.The Pojiao Unit and Lüzicun Unit contain petrographically and geochemically similar rocks but contact relationships show that the latter is younger.The Mokela Unit mainly consists of dykes intruding the other two and has petrographic and geochemical differences.The geochemistry results show the compositional range of SiO₂ of 60.2wt.%–78.41 wt.%,with corresponding K₂O contents 3.37wt.%–5.27 wt.%.All the samples are peraluminous with alumina saturation index?A/CNK?ratios of 0.9 to1.7.FeO^t contents 0.18wt.%–5.26 wt.%,magnesium index?Mg^#?ratios of 43.0 to5.3,amd the differentiation index?DI?ratios of 71.12 to96.92.Primitive mantle normalized trace element diagram shows characteristic obvious negative anomalies in Nb,Ta,Ti,P,and positive anomalies in Rb,Th,U,K and Zr?Hf,REE which are consistent with the characteristics of the continental crust.Chondrite-normalized REE patterns are characterized by high strong LREE enrichment and HREE depletion.in descending order of age,with a gradually strong negative Eu anomaly.The three Units granite samples have similar REE and trace element compositions suggesting a possible link between them.But some different elemental ratios indicate that they are not simple products of crystallization differentiation of a homologous magma.Study of contact relationships between different rock types,hand specimens and petrography combined with unit and superunit mapping permits.Seven representative samples from the three units were chosen to be LA-ICP-MSU-Pbtested,including porphyritic biotite monzogranite of the Pojiao Unit,porphyritic biotite?tourmaline?granite of the Lüzicun Unitand the biotite alkali-feldspar granite of the Mokela Unit.The zircon LA-ICP-MSU-Pb analysis results shows the ages of Eshan Granite in different sequences are determined to be 852±9Ma⁸54±11Ma,842±8Ma and823±8Ma,indicates clearly that the Pojiao Unit has an early Neoproterozoic crystallization age,the Lüzicun Unit has a middle Neoproterozoic crystallization age,the Mokela Unit has late Neoproterozoic crystallization ages.?3?Comparison of orogenic cycle magmatism in the view of continental collision,subduction,exhumation and mountain collapse in collisional orogenic belts,and Geochemical and rare earth element differentiation characteristics.Post-collisional magmatism in Eshan occurred-30Ma later than continental collision at ca.880Ma,when the Yangtze craton was suturing to Cathaysia?Ling et al.,2003?,and shows characteristics of multi-stage episodic magmatism:Magmatism began with large-scale melting of a greywacke and amphibolite source derived from Mesoproterozoic epic metamorphic volcano-sedimentary rocks at ca.854-852Ma.Followed by stretching of plate recession,A later batch of melts of residual mafic substancefromPojiao UnitandMesoproterozoicepicmetamorphic volcano-sedimentary rocks with crustal contamination and assimilation–fractional crystallization was intruded at ca.842Ma,and lasted for over 20Ma until theliquid immiscibility of silicate.The Mokela-Unitgranite crystallized from residual melts during the cooling emplaced stage,experienced silicate liquid immiscibility and the separation of conjugate silicate melts,i.e.ailica-rich vs.iron-rich at ca.823Ma.It's marked the final stage of exhumation.?4?Study of constraints of tectonic and magma evolution on metallogenic elements source and formation of ore-forming fluid.Thirteen mineralization points in the study area have been explore.Results shows during the cooling emplaced stage,that rich metallogenic elements have been enriched in the residual melts,experienced silicate liquid immiscibility and the separation of conjugate silicate melts,i.e.ailica-rich vs.iron-rich.These metal-rich melts crystallize locally or transfer to the fracture which formed by tension of recession plate.According to the magmatic tectonic evolution model,metallogenic model is established from deep to shallow of Eshan granite.