Study On The Formation Mechanism Of The Panzhihua Basic-ultrabasic Dyke In The Emeishan Large Igneous Province,SW China

The Emeishan large igneous province was formed by the mantle plume magmatism in the late Permian.There are several world-classes Fe-Ti-V oxide deposites such as Panzhihua Hongge Baima and Taihe in the Emeishan large igneous procince.At present,the genesis of these magmatic deposits is still controversial.Basic-untrabasic dyke is the early product of Emeishan magamatism produced by mantle plume,therefore,the study of basic-untrabasic dyke plays an important role in understanding the magma origin and evolution of the Emeishan large igneous province.Based on the analysis and study of petrography,major oxides,trace elements and Sr-Nd istopic compositons of the Panzhihua basic-ultrabasic dyke and marbles at Zhujiabaobao,the genesis of the Panzhihua dykes are discussed,which is helpful to understand the magma origin and evolution of Emeishan large igneous province.Several important conclusions can be made from this study.They are summarized below:1.Primitive mantle-normalized trace elements and Chondrite-normalized rare earth elements patterns of the Panzhihua basic-ultrabasic dyke are similar to the coeval picrites and basalts in the nearby Ertan area,indicating that Panzhihua dyke has the same magma source as Emeishan high-Ti basalt.Besides Sr,the trace elements in the samples of surrounding rock brucite marble are all 2-3 orders of magnitude lower than the basic-unltrabasic dyke samples containing olivine.On the Chondrite-normalized rare earth elements patterns diagram,basic-unltrabasic dyke and marble samples show LREE enrichment and HREE is depletion.Nb and Ta positive and negative anomalies are shown in different samples.2.In the Nb/Yb versus Th/Yb diagram,most of the Panzhihua dyke samples fall into the range of MORB-OIB series and Emeishan picrate,and some of them fall between the mantle and crustal end-menber,indicating that the magma of the basic-ultrabasic dyke in Panzhihua area underwent low degree of contamination by crustal materials.3.The ratios of(⁸⁷Sr/⁸⁶Sr)_i of zhujiabaobao Panzhihua dyke samples have a wide range,varing from 0.704076 to 0.707852,with an average value of 0.7050947,which are higher than that of oceanic island basalts(OIB)and mid-oceanic ridge basalts(MORB),However,the?_Nd(t)values have a narrow range,(-1.9 to 1.69).Country rocks marble have relatively high(⁸⁷Sr/⁸⁶Sr)_i ratios(0.705947 to 0.708350)and low?_Nd(t)of-0.77.Sr-Nd isotopes indicate that the Panzhihua dykes have similar magma source to the coeval high-Ti basalt Panzhihua and Dali areas.The mixing calculations show that the assimilation of brucite marble by basaltic magmas similar to the average compositions of the coeval high-Ti basalts was up to 30 wt%.4.The shows that the basic-ultrabasic dyke is the result of the local assimilation of brucite marble by the basalt magma.The assimilation of carbonate rocks by basaltic magma may be a key factor in the formation of Panzhihua V-Ti-Fe oxide ore deposit.