Fluid and melt inclusions in the Mesozoic Fangcheng basalt from North China Craton: implications for magma evolution and fluid/melt-peridotite reaction

摘要

Melt inclusions and fluid inclusions in the Fangcheng basalt were investigated to understand the magma evolution and fluid/melt-peridotite interaction. Primary silicate melt inclusions were trapped in clinopy-roxene and orthopyroxene phenocrysts in the Fangcheng basalt. Three types of melt inclusions (silicate, carbonate, and sulfide) coexisting with fluid inclusions occur in clinopyroxene xenocrysts and clinopyroxene in clinopy-roxenite xenoliths. In situ laser-ablation ICP-MS analyses of major and trace element compositions on individual melt inclusions suggest that the silicate melt inclusions in clinopyroxene and orthopyroxene phenocrysts were trapped from the same basaltic magma. The decoupling of major and trace elements in the melt inclusions indicates that the magma evolution was controlled by melt crystallization and contamination from entrapped ultramafic xenoliths. Trace element patterns of melt inclusions are similar to those of the average crust of North China Craton and Yangtze Craton, suggesting a considerable crustal contribution to the magma source. Calculated parental melt of the Fangcheng basalt has features of low MgO (5.96 wt%), high A1_2O_3 (16.81 wt%), Sr (1,670 ppm), Y (>35 ppm), and high Sr/Y (>40), implying that subducted crustal material was involved in the genesis of the Fangcheng basalt. The coexisting fluid and melt inclusions in clinopyroxene xenocrysts and in clinopyroxene of xenoliths record a rare melt-peridotite reaction, that is olivine + carbonatitic melt] (rich in Ca) = clinopyroxene + melt_2 ± CO_2. The produced melt_2 is enriched in LREE and CO_2 and may fertilize the mantle significantly, which we consider to be the cause for the rapid replacement of litho-spheric mantle during the Mesozoic in the region.