Quantitative petrogenetic constraints on the Pliocene alkali basaltic volcanism of the SE Spain Volcanic Province

Abstract

Alkali basalts of Pliocene age are the last episode of volcanism in the SE Spain Volcanic Province, postdating a complex series of Miocene calc-alkaline to ultrapotassic rocks. This volcanism is represented by small outcrops and vents NW of Cartagena that has been interpreted as a volcanic episode similar to the contemporaneous monogenetic alkaline basaltic volcanism of the Iberian Peninsula and Western/Central Europe. However, their geochemical signature is characterised by relatively higher 87Sr/ 86Sr ratios as well as distinct trace element anomalies which, at different scale, are only found in the spatially related calc-alkaline to ultrapotassic volcanism. Quantitative modelling of these data demonstrate that the geochemical signature of the Pliocene alkali basalts of Cartagena can be explained by the interaction between primitive melts generated from a sublithospheric mantle source similar to that identified for other volcanic regions of Spain, and liquids derived from the overlying lithospheric mantle. This interaction implies that the alkali basalts show some geochemical features only observed in mantle lithosphere-derived melts (e.g. Sr isotope enrichment and Th–U–Pb positive anomalies), while retaining an overall geochemical signature similar to other Iberian basalts (e.g. Rb–K negative anomalies). This model also implies that beneath the SEVP, enriched (metasomatized) portions were still present within the lithospheric mantle after the Miocene magmatic episodes. Comparison of this model with those already developed for other alkaline basaltic volcanic regions of western/central Europe supports the idea that the interaction of primitive magmas derived from a common sublithospheric mantle source with liquids derived from the overlying regionally heterogeneous lithospheric mantle is a relatively frequent scenario in the European realm.