ALKALI BASALTS AND LEUCITITES IN AN EXTENSIONAL INTRACONTINENTAL PLATE SETTING - THE LATE CENOZOIC CALATRAVA VOLCANIC PROVINCE (CENTRAL SPAIN)

The Calatrava Volcanic Province (CVP) of central Spain is characterised by a intracontinental plate magmatic association of leucitites, melilitites, nephelinites and olivine basalts extruded during the late Miocene to Quaternary. Most of the rocks represent relatively primitive magmas and less than 45% have experienced small degrees (<25%) of crystal fractionation. Melilitites, nephelinites and olivine basalts form a suite generated by variable degrees of partial melting (F=5-17%) of a nearly homogeneous enriched (up to x18 chondritic values for the highly incompatible elements and x3 for the moderately incompatible) mantle source, composed of O1+Opx+Cpx+Gt+Phl. The leucitites appear to be derived by low degrees of partial melting (similar to 4%) from a different mantle source, characterized by higher Rb, Ba, K and Sr-87 enrichment and the presence of residual apatite. Phlogopite contributed to the leucititic liquids in a higher proportion than in the basaltic suite whereas clinopyroxene participated in lower amounts. The geochemical characteristics of the primary liquids suggest a contribution from two source components: (1) a subcontinental lithospheric component which appears to be a strongly enriched garnet-lherzolite with phlogopite+/-apatite and (2) an asthenospheric diapir component with characteristics similar to the HIMU reservoir. These conclusions combined with geophysical observations indicate that the CVP magmatism could develop in two stages. In a first stage, a HIMU-like mantle diapir would trigger magma generation in the overlying subcontinental lithosphere by melting of pervasive enriched streaks or veins with phlogopite+apatite, giving rise to small amounts of leucititic liquids and to initial extension of the upper crust. In subsequent steps the extension developed, the lithosphere becomes stripped of its enriched components and the asthenospheric diapir starts to melt giving rise to the basaltic liquids which are virtually OIB-like magmas. This model is also supported by the available K-Ar radiometric ages as the leucitites were extruded first (similar to 7.6 Ma), followed by the basaltic suite (4.6-1.75 Ma). This volcanic region is linked with the rift system which developed from late Miocene to Quaternary times in western/central Europe and shows strong geochemical similarities with this volcanism, in which both lithospheric and asthenospheric components have been detected.