Enrichment Mechanism Of Sulfide In Cumulates In The Lower Crust Of Gangdese Belt,Tibet

Based on the comparative study of mineralogy,petrology,petrochemistry,isotope geochemistry,and sulfide droplet composition,the paper defines the petrogenesis and mineral cumulate process of the two types of cumulates The genesis of sulfide and the triggering mechanism of sulfide supersaturation.The pyroxene hornblende,hornblende,biotite hornblende of Late Cretaceous in Milin area and plagioclase hornblende of Early Jurassic in Dongga area all present cumulate structure,which indicates that they are all of magma cumulate origin.The Milin cumulate is rich in sulfide,which occurs in the form of sponge meteorite,disseminated and drop,and is dominated by pyrrhotite,pyrite and chalcopyrite,with a small amount of goethite,Galenite and pentlandite.The bead like sulfides are mainly wrapped by amphibole,and the combination is chalcopyrite+pyrrhotite+pentlandite,which shows that the sulfides are magmatic.The oxygen fugacity of sulfide-rich cumulates is calculated from the composition of amphibole and magnetite ilmenite mineral pairs(?NNO+0.1-?NNO+1.5)is lower than that of sulfide-poor cumulate(?NNO+2.0).The two sets of cumulates are characterized by low Si O₂,high Mg O and positive correlation between Si O₂and Mg O,indicating that they are the products of cumulate of mantle derived magma.The equilibrium melt compositions of two sets of cumulates retrieved from amphibole compositions show enrichment of LREE((La/Sm)_N=1.01-3.96),enrichment of large ion lithophile elements(Rb,Ba),and depletion of high field strength elements(Nb,Ta,Zr and Hf).Nb/Y=0.03-0.53 and Rb/Y=0.40-1.76 show that the rocks are derived from the wedge-shaped mantle metasomatized by fluids.The isotopic characteristics of sulfide-rich cumulates are(⁸⁷Sr/⁸⁶Sr)_i=0.7043-0.7044;?_Nd(t)=+3.67-+3.94 and(²⁰⁶Pb/²⁰⁴Pb)_i=18.464-19.177;(²⁰⁷Pb/²⁰⁴Pb)_i=15.618-15.681;(²⁰⁸Pb/²⁰⁴Pb)_i=38.590-39.351,and(⁸⁷Sr/⁸⁶Sr)_i=0.7036-0.7038;?_Nd(t)=+5.66-+5.82 and(²⁰⁶Pb/²⁰⁴Pb)_i=18.185-18.445,(²⁰⁷Pb/²⁰⁴Pb)_i=15.546-15.648,(²⁰⁸Pb/²⁰⁴Pb)_i=38.152-38.653,indicating that there are more ancient crustal materials mixed into the sulfide-rich cumulates than the sulfide-poor cumulates during mash.The decrease of oxygen fugacity caused by the injection of ancient crustal materials triggered the supersaturation of sulfides.The sulfide droplets encapsulated in amphibole can represent the composition of the initial sulfide melt.LA-ICP-MS test reveals that the initial sulfide melt is monosulfide solid solution(MSS)and sulfide liquid(L).With the decrease of temperature,the sulfide composition of present-day sulfide-rich cumulates is gradually formed.The rich pyrite and nickel bearing sulfide are significantly different from those of MORB and Copper-Nickel sulfide deposits in mid ocean ridge.The main reason is that the initial melt of cumulates has higher sulfur fugacity and oxygen fugacity.In conclusion,both Milin sulfide-rich cumulates and Dongga sulfide-poor cumulates originated from partial melting of mantle wedge metasomatized by plate subduction fluid.Due to the mixing of reductive ancient crust in mash process of island arc magma,oxygen fugacity decreased to varying degrees,resulting in different sulfide supersaturation.In the system of supersaturation of sulfides in the lower crust,the initial sulfide melt is dissolved in the form of monosulfide solid solution(MSS)and sulfide liquid(L).With the decrease of temperature,the sulfide assemblage in cumulate is formed gradually.