Origin And Deep Process Of Cenozoic Alkali-rich Porphyry In Taozi Xiang,Yanyuan,Sichuan

Copper resources is a scarce strategic resource,and the alkali-rich porphyry copper is one of the most important type of copper mines in China.The alkali-rich porphyry in Taozixiang,Yanyuan is located in the transitional zone between the western margin of the Yangtze Block and the Tibetan Plateau.Systematical petrological,petrogeochemical,and chronological studies on Taozixiang porphyry can help to understand the formation and evolution of Cenozoic mantle-derived alkaline magmatism in this area,as well as the origin of deep magma.These magmatic activities were caused by the collision of mantle material from the Tibetan Plateau with the eastward shifting of the Sanjiang area.The geological studies in this area reported in literature mainly focused on the metallogenic model and mineralization of porphyry copper deposits,with petrology research focusing on ore-bearing porphyry bodies.Very few studies were about the formation order of alkali-rich porphyry,the material sources,genesis and tectonic environment of magma.Some available age data are mutually exclusive,and the exact timing of the formation of alkali-rich porphyry in this area remain unclarified.The magmatic rocks exposed in Taozixiang consist of mostly quartz diorite,which can be divided into medium-fine-grained porphyry quartz diorite and quartz diorite based on their matrix size.The porphyry minerals consist mainly of feldspar,hornblende,and a small amount of biotite.These rocks have high silicon content and are rich in alkali.They can be divided into alkaline series and calcium alkaline series.They have high rare earth element concentration and are rich in light rare earths elements,and show modest europium anomalies.Regarding trace elements,they are rich in LILE(Pb,Ba,Sr)and depleted in HFSE(Nb,Ta,Ti).The zircon U-Pb dating results show age range of 29.89 Ma~32.44 Ma,with duration less than 4 Ma.The age of calcium-alkaline series rocks is slightly older than that of alkaline series rocks.The calculated TDM2 model age mostly fall in the range of 1030-1271 Ma.The zircon ?Hf(t)values show range of-3.08 to 0.92,with average of-1.06.The fraidronite vein intrusion in this area indicates that the alkali-rich porphyry in Taozixiang comes from the partial melting of the crust in the Jinning period,with the participation of mantle material.The rock chemical composition is similar to those from volcanic arcs,and it has the characteristics of adakite rocks.Zirconium saturation thermometer indicates 815°C as the average formation.The source rock contains garnet,and may contain a small amount of plagioclase and angular flash.These rocks should have been formed in the thickened mafic crust,caused mainly by partial melting,with a small amount of mantle material involved.It is suggested that within 65 Ma ago,the lithosphere became thickened after the continental collision,associated with,deep underneath,the collapse of the orogenic belt,the upwelling of the mantle material,and the partial melting of the thickened lower crust.With the large scale eastward sliding and the formation of pull-out basins in the Sanjiang area,a local depression was formed,making the way for the rapid intrusion of alkali porphyry and the formation of the porphyry copper deposit metallogenic system.