Characteristics Of Olivine Phenocrysts Of The Picritics In The Emeishan Large Igneous Province And Its Implications For Mantle Source

Large Igneous provinces(LIPs)are characterized by voluminous volcanic sequences erupting over a short period of time(1-5 Ma),and their formation is generally related to mantle plumes.The Emeishan Large Igneous province(ELIP)in the western part of the Yangtze craton,is the first internationally admitted large igneous province in China,and it is considered to be a typical mantle plume-derived LIP.The picrites formed at relatively high temperatures in the ELIP,providing one of the important evidence supporting for the role of mantle plume.However,some recent studies suggested that the source of the ELIP picrites involved variable proportions of pyroxenite or eclogites.Especially the nickel contents and Fe/Mn in olivine phenocrysts have been suggested to reflect the pyroxenite omposition of the mantle source of the host magma.The presence of eclogites or pyroxenite in the mantle source region can lead to overestimates of temperature as the thermometers assume a peridotite-dominated source.One of the important challenges is therefore to clarify whether the mantle source of ELIP picrite is dominated by peridotite or pyroxenite.In this thesis,we report trace element data on olivine phenocrysts in the Dali picrites and Lijiang picrites from the ELIP(e.g.,Ti,Cr,Co,Zn,Cu and Li).The Dali olivines are Ni-rich,and characterized by high Mn O values which is higher than those picritic lava from Hawaii.The Ni content in Lijiang olivine is lower than that of Dali picrite,and the variation range of Mn O content is consistent with that of Dali picrite.Both Dali and Lijiang olivines are characterized by high(>1.4)100×Mn/Fe value.For other mantle source indicators,both Dali and Lijiang picrites are characterized by olivine phenocrysts with low(<13)10000×Zn/Fe value,high(>15)Mn/Zn ratios and relative low Li concentration,indicating a peridotite-dominated source.Since the olivine–melt Ni partition coefficient(K_DNiol/melt)will decrease at high temperatures and pressures,the picrites derived from peridotite melting at high pressure,and that crystallized olivines at lower pressure,can generate high concentrations of Ni in olivine phenocrysts,excluding the necessity of a metasomatic pyroxenite contribution.Here we calculate primary magma composition using PRIMELT3 software for those Emeishan picrites as they represent definite liquid compositions.The results show that those picrites have similar primary magma characteristics,and the different Ti composition of may be due to the different melting degree.Additionally,based on the Al-in-olivine thermometer,the largest olivine crystallization temperature for Dali and Lijiang picrites were calculated at 1491±65°C and 1447±63°C,respectively.And the mantle potential temperature(T_P)were calculated at 1559°C and 1497°C respectively.Our results are 150°C higher than that of the MORB using the same method,and are consistent with the role of a mantle thermal plume for ELIP.