The pyroclastic breccia of the Cabezo Negro de Tallante (SE Spain): The first finding of carbonatite volcanism in the Internal Domain of the Betic Cordillera

The Cabezo Negro de Tallante volcano (SE Spain) is well known in literature for the presence of abundant mantle and crustal xenoliths in basaltic lithologies. Here we report field, petrographic, mineralogical and geochemical data of a poorly investigated pyroclastic deposit (breccia) and provide evidence for the presence of a primary magmatic carbonatite component. The matrix of the pyroclastic breccia contains carbonate and silicate portions. The carbonate portion is made up of calcitic ash with micro-lapilli of fine-grained carbonate, while globules and clasts of basaltic composition represent the silicate portion. A liquid immiscibility relationship between the carbonate and silicate portions is indicated by the presence of sharp, curved menisci between them, the presence of carbonate globules in silicate glass (and vice-versa) and the overall appearance as emulsion texture with bulging and pinch-off shapes. Carbonate globules in fresh silicate glass are also present as inclusions within xenolith crystals and bimodal carbonate mineralogy, consisting of either calcite (plus Mg-calcite) and dolomite, was observed in quenched liquids within mantle xenoliths. The whole-rock composition of the carbonate portion is within the compositional range of the carbonatitic rocks, but shows low concentrations of Sr, Ba, Nb, P and LREE if compared with typical carbonatite magmas. REE profiles of the carbonate portion are parallel to the patterns of associated alkaline basalts and fall in the range of both intraplate and convergent margin carbonatites. The carbonate portion shows high δ18O compared to primary igneous carbonates, a feature common to many extrusive carbonatites and plots slightly towards higher 87Sr/86Sr for a given 143Nd/144Nd in the Nd-Sr isotopic trend described by the associated rocks (alkaline basalts and mantle xenoliths) suggesting a contribution from crustal subducted material. These results support a magmatic origin for the carbonate portion and a contemporaneous eruption of carbonatic and silicatic magmas. The CO2 produced by the transformation of dolomitic melt into Mg-calcitic melt was the principal propellant in the high-energy explosive eruption at the Cabezo Negro de Tallante volcano.