Late Cretaceous Magmatism In Sinongduo Area,Tibet:Implications For Neo-tethys Subduction

There are several linearly distributed and formed in different periods volcanic-magmatic rock belts in the Gangdese Belt,southern,Tibet.Up to now,a variety of different types of deposits have been discovered in this area.Most of the intrusions which emplaced in Miocene and associated with the porphyry copper deposits show adakitic affinities.The Late Cretaceous adakite are widespread along the Gangdese Belt.However,the genesis of these rocks has been debated for decades,and researches mainly focus on Petrology,Geochemistry and Geodynamic processes.And,study on the productivity of these intrusions is limited.Only Kelu and Sangbujiala Cu–Au skarn deposits has been reported in recent years(ca.90Ma).To provide a further understanding of these issues,here,we present new zircon U-Pb geochronology and zircon trace element data,whole-rock geochemical and Sr–Nd–Pb isotopic data for the biotite monzogranite from Sinongduo-Jiaduobu area,Xietongmen County,Tibet.New age data indicates that the adakite emplaced in the Late Cretaceous(83.1±1.6Ma).These rocks are characterized by relatively high contents of Si O₂(60.21%?62.54%),Mg O(2.19%?3.02%),Mg^#(41.25?50.73),low content of Y(15.9?17.8ppm),Yb(1.46?1.73ppm)and high Sr/Y(35.0?47.6),La/Yb(29.6?38.6)ratio,showing High-Si O₂adakitic rock affinity.These rocks belong to high-K calc-alkaline and metaluminous series(A/CNK=0.84?0.92),and rich in LREE,LILE(large ion lithophile element)and depleted in HREE,HFSE(high field strength element),such as Nb and Ta.Five samples from the Sinongduo adakite have whole rock(⁸⁷Sr/⁸⁶Sr)_i of 0.707914?0.708193,?_Nd(t)values of-4.5?-3.8,(²⁰⁶Pb/²⁰⁴Pb)_i ratios of18.4781?18.6029,(²⁰⁸Pb/²⁰⁴Pb)_iratios of 38.7664?39.0015?(²⁰⁷Pb/²⁰⁴Pb)_i ratios of15.6673?15.6866.Different from the typical adakite dervied from the melting of slab,it shows the characteristics of strong crustal materials addition.The zircon trace elements data shows high,Ce/Ce*,10000*(Eu/Eu*),moderate Eu/Eu*ratios shows that magma has a higher oxygen fugacity,which is different from the poor-ore adakite in the Gangdese belt.Our research shows that the rock formation is due to the subduction of the Neo-Tethys mid-ocean ridge.During the subduction,a slab window is formed.The hot mantle material flows up through the slab window and contacts the slab to form thermal circulation.The slab near the mid-ocean ridge which partly melts to form an adakitic melt.During the ascending process,the melt passes through the overlying mantle wedge and undergoes metasomatic reaction with it,increasing the content of Mg O,Mg^#,Cr,and Ni in the melt.And ascending emplacement formed the Late Cretaceous adakitic intrusions in the region.