The Origins Of High-Ti And Low-Ti Magmas In The Emeishan Large Igneous Provinces,Insights From Melt Inclusion Study And Trace Element Modeling

Large Igneous Province(LIP)is used to indicate an intraplate magma province with a distribution larger than 0.1 Mkm~2 that is mainly composed of mafic rocks.Most LIPs produce massive extrusive and intrusive rocks within short time(e.g.10Myrs),with the bulk of the magmatism occurring in less than 1 Myr(Ernst et al.,2005).High-Ti and low-Ti lava series occurred simultaneously in many large igneous provinces(LIP)but their origins remain controversial.A detailed geochemistry study was performed on melt inclusions and their host olivines in Dali picrites collected from the Emeishan LIP.With Second Ion Mass Spectrometry(SIMS),this study established analytcial method for trace elments in melt inclusions.The precisions for elements with concentrations higher than 10 ug/g are better than 10%(2SD),for elements with concentrations lower than 10 ug/g are better than 15%(for Er and Lu,they are about18%).The bias of all measured elemental concentration from their preferred values are within±20%.With laser ablation multi-collector inductively coupled plasma mass spectrometry(LA-MC-ICP-MS),analytical method for Sr isotopic ratio in melt inclusions were established.The analytical results of measured basaltic reference glasses are within 100 ppm with their preferred values.Although Dali picrate lavas have a limited Ti/Y variation(~356-404,Wu et al.,2018),olivine-hosted melt inclusions from the Dali picrites have Ti/Y ratios ranging from 213 to 741,covering the range from low-Ti to high-Ti lavas.Pb-Sr isotopes and trace element ratios(such as Nb/U and Nb/La)of the melt inclusions have limited variation and display little correlation with Ti/Y ratios,indicating that the sampled low-Ti and high-Ti lavas share similar sources.Trace element mass balance modeling indicates a mantle source comprising~84%peridotite from the lower mantle,~15%recycled MORB and~1%pelagic sediment for the Emeishan lavas.Modeling results for the Emeishan and the Karoo LIPs show that partial melts formed at a greater depth in the mantle have high Ti contents and Ti/Y,whereas partial melts formed at shallower depths have low Ti contents and Ti/Y.Mixing between melts formed at different depths with different degrees of partial melting can produce the continuous range of Ti/Y ratios observed.Our study illustrates the strong influence that melting pressure and degree play on Ti content and Ti/Y ratio of the derived melts.