Geochemical temporal evolution of Brava Island magmatism: Constraints on the variability of Cape Verde mantle sources and on carbonatite-silicate magma link

摘要

Elemental and isotopic (Sr, Nd and Pb) analyses of silicate and carbonatite rocks from Brava, the southernmost island of the Cape Verde archipelago, are reported here to assess the nature, origin and location of the main contributors to magma composition and to evaluate the chemical evolution of magma through time. In contrast to other islands of the archipelago where a continuous range is usually observed, Brava is clearly characterized by two groups of distinct isotopic compositions. Rocks from the Upper Unit (<. 0.5. Ma) are less Nd- and Pb-radiogenic and more Sr-radiogenic than rocks belonging to the Basal Complex (~. 3 to 1.4. Ma), which are also characterized by more unradiogenic He signatures. Here, the chemistry of the Basal Complex is mainly explained by the mixing of a high-μ (HIMU)-type local end-member, which represents an ancient (~. 1.3. Ga) recycled oceanic crust, with the lower mantle. For the Upper Unit, elemental and isotopic signatures suggest the involvement of an additional local end-member akin to the EM1-type. Such differences in the contributing end-members to the Upper Unit and Basal Complex sources mirror those usually assigned to the southern and northern Cape Verde islands, respectively. This temporal evolution is discussed in light of plume interaction with a shallow EM1-type domain, which is considered to represent the subcontinental lithosphere floating in the asthenosphere. Brava carbonatites define two distinct groups with similar isotopic ranges as those observed for the coeval alkaline silicate rocks. This observation suggests that, in each of the volcano-stratigraphic units, carbonatite and silicate magmas are ultimately derived from the same sources. We propose that calciocarbonatites from either the Basal Complex or the Upper Unit resulted from nephelinite-carbonatite liquid immiscibility, while the subordinate Basal Complex magnesiocarbonatites represent residual liquids after calcite fractionation from carbonatite magma.