Carbonatites and associated nephelinites from Sao Vicente, Cape Verde Islands

Abstract

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