Chemical compositions of phenocryst-hosted melt inclusions from the Sihetun basalt:Implications for the magma evolution

Melt inclusions trapped by phenocrysts in basalt can be used to infer the chemical characteristic of basalt source and the process of magma formation and evolution.Single inclusions hosted in olivine and clinopyroxene phenocrysts in the Mesozoic Sihetun basalts were analyzed by LA-ICPMS.The results show that,although inclusions show trace element patterns similar to the bulk rocks,melt inclusions have much larger compositional variations than bulk lava in terms of both major and trace elements.CaO and Cr2O3 contents and CaO/Al2O3 ratio of inclusion in Cpx correlate positively with Mg# of Cpx,reflecting the effect of Cpx crystallization on the magmatic evolution.The positive correlation(R = 0.94) of Sr-Al2O3 for inclusions indicate plagioclase crystallization during the magmatic evolution.The weak correlations of Ni-MgO and CaO/Al2O3-MgO in inclusions suggest that olivine crystallization was probably small.The compositional variations of inclusions can be interpreted by Ol,Cpx and Pl crystallization,suggesting that the crustal components as revealed by whole rock elemental and isotopic compositions could have inherited from the source rock instead of magmatic mixing.Combining with the previous work,we suggest that these basalts could have been derived from partial melting of(olivine) pyroxenites formed by peridotite + eclogite-derived reaction.On the other hand,some high Mg#-and high SiO2-melt inclusions in high-Mg# Cpx suggest assimilation by high Si rocks during the basalt eruption.Therefore,melt inclusions in high-Fo olivine phenocrysts could better reflect the characteristics of primitive than primary melt calculated by adding olivine to the bulk lava.