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 characterise d by a intracontinental plate magmatic association of leucitites, meli litites, nephelinites and olivine basalts extruded during the late Mio cene to Quaternary. Most of the rocks represent relatively primitive m agmas and less than 45% have experienced small degrees (<25%) of cryst al 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 high ly incompatible elements and x3 for the moderately incompatible) mantl e source, composed of O1+Opx+Cpx+Gt+Phl. The leucitites appear to be d erived by low degrees of partial melting (similar to 4%) from a differ ent mantle source, characterized by higher Rb, Ba, K and Sr-87 enrichm ent and the presence of residual apatite. Phlogopite contributed to th e 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 characteristic s similar to the HIMU reservoir. These conclusions combined with geoph ysical observations indicate that the CVP magmatism could develop in t wo stages. In a first stage, a HIMU-like mantle diapir would trigger m agma generation in the overlying subcontinental lithosphere by melting of pervasive enriched streaks or veins with phlogopite+apatite, givin g 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 asthen ospheric diapir starts to melt giving rise to the basaltic liquids whi ch 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). Thi s volcanic region is linked with the rift system which developed from late Miocene to Quaternary times in western/central Europe and shows s trong geochemical similarities with this volcanism, in which both lith ospheric and asthenospheric components have been detected.