Petrogenesis of Permian Emeishan Basalts in Jiulong,Shangri-la

This paper studied petrological and geochemical characteristics of Emeishan basalts from Jiulong of Shangri-la and discussed the petrogenesis of these rocks.These rocks belong to alkaline basalt series,SiO2,MgO and Na2O+K2O contents vary from 41.84 to 50.14%,2.59%~16.13%,and 0.76%~9.91%,respectively,where Na2O contents are higher than K2O contents.The basalts exhibit flat chondrite-normalized patterns with moderate HREE depletion(LREE/HREE=4.71-9.58,(La/Yb)N=5.47~13.66),no Eu anomalies(δEu=0.97~1.12).The primitive mantle-normalized spidergrams show that the basalts enrich,relatively,HSFEs(Ta,Nb,Zr,Hf and Ti),LREEs,P,Th and U,deplete K and Y,and vary in the contents of Ba and Sr.The ratio of Ti/Y(530~726) indicates that the Emeishan basalts are high-Ti basalts.Based on above geochemical features,we propose that these basalts were derived from partial melting of the lower mantle of the garnet mantle peridotites in the continental intra-plate extensional tectonic setting.Meanwhile,the parental magma underwent intense mineral fractional crystallization,including olivine,clinopyroxene,apatite,magnetite and rutile,whereas there had little crustal contamination during magma ascent.It was considered that the Emeishan basalts represent a major magmatic event triggered by the mantle plume,that is,decompressional melting of the upwelling mantle plume produced basaltic melts.