Metal Fertilization(Cu,Au) Of Juvenile Lower-crust And Petrogenesis Of Adakite-like Rock In Beiya-Liuhearea

Beiya-Liuhe area locate in western edge of Yangtze Craton,and had undergone multiple episodesof magma-tectonic activities since Proterozoic.A number of porphyry deposits,related to Cenozoic potassic intrusions,were generated along Jinshajiang-Ailaoshan mineralization belt.The porphyry deposits,formed in margin of Yangtze Craton,show broad similarities to arc and collision analogues in many respects,such as in the metallic associations,mineralization styles,alteration zones.The ore-forming magmas usually have geochemical affinities with adakite,but the petrogenesis are different from arc and collision-related porphyry deposits.Previous study mainly focuses on the metallogenetic modeland source of thedeposits but were short of the research for mechanism of metal fertilization and difference between fertile and barren intrusions.Concentrating on these deficiency of previous study,this work selected deep-crustal xenoliths and adakite-like rock to reveal the evolution of lower-crust,mechanism of metal fertilization and difference between fertile and barren adakite-like rocks in study area.The lower-crust of Beiya-Liuhe area is mafic juvenile lower-crust,including garnet pyroxenite,garnet amphibole-pyroxenite and garnet amphibolite.In Neoproterozoic?809-773Ma?,the lower crust underwent magmatic underplating and metamorphism at same time during slab subduction.In late Permian?259-251Ma?,the lower crust also underwent magmatic underplating and metamorphism at same time under mantle plume-lithosphere mantle interaction.S-poor,Au-enriched juvenile lower-crust was formed by arc magmas,which was derived from a mantle wedge metasomatized by fluids expelled from the subducting slab in Neoproterozoic.Amphibolite,occurredin overlying crust,was metasomatizedby the fluids which were released from the Neoproterozoic lower crustal cumulates andtransported metal?Cu,Au?during later prograde metamorphism.Compare with Beiya giant porphyry-skarn Au-Cu deposits and the Cenozoic giant porphyry Au deposits in the eastern Rocky Mountains,western USA.Reactivated paleo-magmatic arcs at the edge of cratons wasreplenished by metal components and have the potential to form porphyry Au deposits.Liuhe orthophyre,entrained a lot of deep crustal xenoliths,was derived from partial melting of ancient metasomatic SCLM,and mingled with juvenile lower-crustal material.The orthophyre have high water contents and low oxygen fugacity,induced by subduction of relatively reduced Paleo-Tethyan oceanic lithosphere adjacent to western edge of Yangtze Craton.Due to the oxygen fugacityof Liuhe melt is lower than critical value of fertile magma,the orthophyre have no potential to form porphyry deposits.All Cenozoic adakite-like were derived from juvenile lower-crust.Beiya and Machangqing high fO₂,hydrous,fertile magmas were generated by intense water-fluxed melting of Neoproterozoic garnetamphibole-pyroxenite,which was triggered coeval H₂O-eniched lamprophyre.Meanwhile,anhydrous,low fO₂ barren magmas were generated bymoderatewater-fluxed melting of juvenile lower crust duringcoeval hot mafic magmasunderplating.