Petrogenesis Of Lamprophyres In Sichuan And Yunnan:Mg Isotopic Evience

Lamprophyre is a kind of mantle-derived alkaline magmatic rock with porphyritic structure and rich in volatile and incompatibility elements.It is widely distributed in Sichuan-Yunnan region of the western margin of the Yangtze Craton and closely related to the distribution of polymetallic deposits in the region.Lamprophyre is an ideal tool to study the characteristics of mantle sources in Sichuan-Yunnan region and an important means to explore the metallogenic mechanism of polymetallic deposits.Previous studies on the classification and formation age of lamprophyre in Sichuan-Yunnan area have made some achievements,but the study on the characteristics of mantle source,rock origin and its correlation with polymetallic deposits needs to be supplemented.In this paper,the characteristics of mantle source,petrogenesis and its relationship with polymetallic deposit of lamprophyre in Sichuan-Yunnan area were analyzed and discussed by using Mg isotope as the main research method and combining with the petrography,major and trace elements and Sr-Nd-Pb isotope data of samples.The lamprophyre samples in the Sichuan and Yunnan areas are enriched in LILE（K,Rb,Ba）and LREE（La,Ce,Nd）,and depleted in HFSE（Nb,Ta,Ti）and HREE（Ho,Tm,Yb）,which is similar to the characteristics of arc magmatic rocks and formed in subduction environment.According to the Sr-Nd-Pb isotopic data of the samples,it is found that they have high Sr and low Nd isotopic characteristics,which fall in the range of EMI and EMII,indicating that the lamprophyre in Sichuan-Yunnan area originates from the enriched mantle.A large number of studies have shown that Mg isotopes can be used to study the characteristics of magmatic mantle sources,especially to effectively identify subduction materials in mantle sources.Theδ²⁶Mg values of lamprophyre samples in Sichuan-Yunnan area range from-0.44±0.07 to-0.07±0.05‰,andδ²⁶Mg values in Beiya area range from-0.35±0.03‰,which are lower than mantle value(δ²⁶Mg=-0.25±0.07‰).Theδ²⁶Mg values in Jianchuan,Yao’an and Yanyuan areas are-0.22±0.03‰,-0.15±0.03‰and-0.20±0.03‰,respectively,which are similar to or higher than the mantle value.In the Mg-Sr isotopic end-member mixing simulation,The samples from Jianchuan,Yao’an and Yanyuan areas are close to the trend line of clastic sediments,which indicates that they are influenced by clastic sediments.The lamprophyre samples from Beiya area fall near the simulated line of carbonate sediments（calcite）.However,the Ca O content of the samples from Beiya area is significantly lower than that of the other three areas,which rules out the metasomatism of subduction sedimentary carbonates.Meanwhile,the high Ti/Ti*and low Ca O/Al₂O₃characteristics of the samples from Beiya area indicate that they were metasomatized by carbonated eclogite.In terms of time,the lamprophyre diagenetic age（31～36Ma）in Sichuan-Yunnan area is consistent with the polymetallic metallogenic age（30～40Ma）in the western margin of Yangtze Craton.Spatially,the lamprophyre and metallogenic parent magma have the same source area.In addition,subduction fluids and sediments metasomatized the mantle source area,which has the potential to form and enrich volatile materials（H₂O,metals,etc.）,and may provide ore-forming materials for polymetallic mineralization.