Carbonatite genesis: A geochemical link between carbonatite and silicate magmas from Brava, Cape Verde Islands.

摘要

The pyroclastic sequence on Brava, Cape Verde Islands, includes the only reported occurrence of morphologically youthful carbonatite eruptives from an oceanic island. This study is based on analyses of samples collected during a 1985 field investigation. Whole-rock major element geochemistry and petrology has allowed identification of 5 rock types among the lavas and pyroclastics from Brava. These are olivine nephelinite, nephelinite, phonolite, peralkaline phonolite and carbonatite. Major element modelling based on bulk-rock geochemistry and microprobe analyses of phenocryst phases shows a coherent fractionation trend from the most primitive to the most evolved silicate rocks. This trend is only viable if carbonatite liquid is included as a fractionating phase. The most notable feature of the trace element geochemistry is overall depletion of the rare earth elements (REE) and more intense light rare earth element (LREE) depletion in the phonolites. Trace element covariations suggest that removal of apatite from the more primitive magmas, followed by the immiscible separation of carbonatite liquid from the more evolved magmas, can produce this effect. Quantitative trace element modelling corroborates this interpretation.