Carbonatites and associated nephelinites from São Vicente, Cape Verde Islands

The island of Sao Vicente has the most abundant carbonatite outcrops in the Cape Verde Islands. A\udfield survey of the main outcrops has shown that they consist of extrusive carbonatites, carbonatite\uddykes and small apophyses of intrusive carbonatite. These outcrops are spatially related to nephelinites.\udThe compositions of the extrusive carbonatites and dykes plot close to, and within, the\udmagnesiocarbonatite field. In contrast, the intrusive carbonatites are calciocarbonatites, with similar\udaverage strontium contents to those of extrusive carbonatites and dykes (around 4000 ppm), but\udremarkably low barium, niobium and total rare earth element concentrations. Whole-rock geochemistry\udindicates a strong affinity between the nephelinites and intrusive carbonatites, such that the latter could\udrepresent fractionation products of the same parental magma. This is in agreement with radiogenic\udisotope geochemistry, which shows a very restricted range of compositions in the Sr, Nd and Pb\udsystems. Fractionation from a common parental magma occurred in two main steps: high-temperature\udnephelinite crystallization and high-temperature carbonatite immiscibility. The carbonatitic melts\udcrystallized in two different environments, as follows: (1) a shallow intrusive environment, giving rise\udto the early calciocarbonatite cumulates; and (2) a vapour-dominated, extrusive environment,\udproducing the later magnesiocarbonatites