Quaternary extrusive calciocarbonatite volcanism on Brava Island (Cape Verde): A nephelinite-carbonatite immiscibility product

International audienceThe Cape Verde volcanic archipelago, located in the oceanic portion of the African plate some 500 km west of the Senegal coast, is renowned for the occurrence of carbonatites on at least 5 of its 10 islands. In this study we report the occurrence of about twenty new small outcrops of extrusive carbonatites on Brava Island (64 km2), the south-westernmost island of the archipelago. These new occurrences are studied from geological, petrographic, mineral chemistry and whole rock (elemental and isotopic) geochemical points of view, allowing for a discussion of their petrogenesis and emphasising their geological and geochemical peculiarities in the context of the Cape Verde carbonatites. Most of the extrusive carbonatitic formations correspond to pyroclastic rocks, comprising magmatic and/or phreatomagmatic ash and lapilli fall deposits and one probable pyroclastic flow. Lava flows occur at one locality. The predominance of pyroclastic facies demonstrates the significant explosivity of these magmas characterised by very low viscosity. Independent of the modes of emplacement, all samples are calciocarbonatites and exhibit a remarkable compositional uniformity, considering that they represent several different eruptions and present a wide geographical dispersion. Brava extrusive carbonatites belong to the younger (probable Holocene – Pleistocene) volcanic sequence of the island. This feature is unique in the context of Cape Verde geology, because in the other islands (including Brava) of the archipelago carbonatites are commonly assigned to the basal complexes, having formed during a fairly early stage of the emerged evolution of volcanic construction. Compared with the older intrusive sövitic rocks occurring at Middle Unit of Brava Island, extrusive facies are more iron and manganese rich and yield higher contents of trace elements like Ba, Th, U, Nb, Pb and REE, but somewhat lower Sr abundances. New initial Sr and Nd isotope data (0.703557–0.703595 and 0.512792–0.512816, respectively) determined in extrusive calciocarbonatitic rocks are clearly different from those obtained in intrusive rocks (0.703340–0.703356 and 0.512910–0.512912, respectively), which demonstrates that the studied rocks were ultimately the product of a source distinct from those that produced the older intrusive carbonatites. Brava extrusive carbonatites yield isotope signatures that are clearly distinct from all other Cape Verde carbonatites, but akin to the Southern Cape Verde silicate rocks. We propose that the extrusive carbonatites resulted from an immiscibility process that also produced conjugate melts of nephelinitic composition. 2009 Elsevier Ltd. All rights reserved