Nature Of The Mantle Plume Under The Emeishan Large Igneous Province

Many large igneous provinces(LIPs)are thought to be associated with mantle plumes and therefore are of great value to the study of Earth's mantle.The main lithology of LIPs are tholeiitic basalts and are considered significantly evolved before eruption.Therefore,LIPs are far less than OIBs used in the study of the nature of the mantle component.Picrites usually contain olivines with high Fo value(e.g.>90 in Emeishan picrites),melt inclusion hosted in the most magnesium olivines,which form at the early stage of crystallization,are generally accepted can represent nearly unevolved,primary melt compositions,and is considered can preserve more information about the mantle source.The magmatism of the Emeishan LIP occurred at ca.260 Ma.It is generally accepted to be related to mantle plume.In this study,we conduct major and trace element and Pb isotope analyses on olivine-hosted melt inclusions in the Lijiang picrites using EPMA,SIMS,LA–MC–ICP–MS,respectively.We also analysed the major and trace element and Sr–Nd–Pb–Hf isotope compositions of the Lijiang picrites.Using these data,we comprehensive discuss the nature of the mantle sources for the Lijiang high-Ti picrites(Ti/Y=645–654).The major and trace element and Pb isotope compositions of Lijiang picrite are homogeneous and depleted(⁸⁷Sr/⁸⁶Sr_i=0.70420–0.70429;?_Nd(t)=+3.27–+3.48;?_Hf(t)=+7.71–+8.04;²⁰⁸Pb/²⁰⁶Pb_i=2.077–2.088;²⁰⁷Pb/²⁰⁶Pb_i=0.833–0.843).However,through the melt inclusion study,it can be found that the major and trace elements,especially the Pb isotope compositions of melt inclusions are highly heterogeneous(²⁰⁸Pb/²⁰⁶Pb_i=2.015–2.201,²⁰⁷Pb/²⁰⁶Pb_i=0.812–0.882),which compositional ranges span almost half of the Pb isotope variability previously reported in OIBs worldwide.The observed compositional variations in the Lijiang melt inclusions are not a result of crustal contamination during the ascend of magma to the surface,but reflects the small-scale compositional heterogeneity of the mantle sources.The trace elements and Pb isotopes of melt inclusions have weak correlations,which indicating the mantle source of the Lijiang picrites was mainly created by the mixing of a FOZO-like depleted composition and an EM1-like enriched component.The enriched component is characterized by extremely high ²⁰⁸Pb/²⁰⁶Pb,low Nb/La and Nb/Zr ratios.In the primitive mantle normalized trace element patterns of the Lijiang melt inclusions,we can find an enriched composition and is considered can reflect the characteristics of the recycled component in the source of the Lijiang picrites.These melt inclusions are depleted in Ba,Nb,Ta,Zr,Hf,and have subdued negative Pb anomalies,and some of them even have slightly positive Pb anomalies.Compared to the pelagic sediments,an old recycled GLOSS II-like sediment was probably the main component that imparted the EM1 signature into the sources of the Lijiang picrites.The isotopic modeling result shows that mixture of different proportions of FOZO and an old(2.35 Ga)recycled component(subducted sediments+bulk oceanic crust)can produce a source appropriate for the components of the Lijiang picrites and melt inclusions,and can explain the heterogeneous Pb isotopic composition of the Lijiang melt inclusions.Although the recycling age cannot be defined precisely due to the dependence of the model results on the selected parameters,the models indicate an old recycling age is necessary.Moreover,given that the formation age of the Emeishan LIP of 260 Ma,this estimated recycling age is also consistent with the estimation in previous studies that a period of 1–2 Ga is needed for the recycled component in the mantle to evolve into the trace element and isotopic signature of oceanic island basalts.Furthermore,we compare the compositional characteristics of the Lijiang olivine-hosted melt inclusions with those of Dali,and present the results of Al-in-ol thermometer of olivines from Lijiang and Dali.The highest Al-in-olivine temperature yields 1380°C and 1433°C for Lijiang and Dali,respectively.We reverse calculated the mantle potential temperature beneath Lijiang and Dali using the Al-in-ol temperature.The estimated mantle potential temperature for Lijiang and Dali are1450°C and 1592°C,respectively.We infer that the Lijiang picrites might have originated from the mantle plume with lower thermal energy,corresponding to the waning stage of the near axis of the Emeishan mantle plume.Our study suggest that high-Ti magmas of the Emeishan LIP were derived from those parts or stages of the mantle plume with lower thermal energy.We argue that the spatial and temporal variation in the thermal energy of the mantle plume was a key control on the geochemical characteristics of mafic-ultramafic magmatisms in the Emeishan LIP.