Geochemistry of the Quaternary alkali basalts of Garrotxa (NE Volcanic Province, Spain):: a case of double enrichment of the mantle lithosphere

The area of Garrotxa (also known as the Olot area) represents the most recent (700,000-11,500 y) and better preserved area of magmatic activity in the NE Volcanic Province of Spain (NEVP). This region comprises a suite of intracontinental leucite basanites, nepheline basanites and alkali olivine basalts, which in most cases represent primary or nearly primary liquids. The geochemical characteristics of these lavas are very similar to the analogous petrologic types of other Cenozoic volcanics of Europe, which are intermediate between HIMU, DM and EM1. Quantitative trace element modeling, suggests derivation from an enriched mantle source by degrees of melting that progressively increased from the leucite basanites (similar to4%) to the olivine basalts (similar to 16%). However, the relatively more variable Sr-Nd-Pb isotope signature of the magmas suggests the participation of at least two distinct components in the mantle source: (1) a sublithospheric one with a geochemical signature similar to the magmas of Calatrava (Central Spain) and other basalts of Europe; and (2) an enriched lithospheric component with a K-bearing phase present. The geochemical model proposed here involves the generation of a hybrid mantle lithosphere source produced by the infiltration of the sublithospheric liquids into enriched domains of the mantle lithosphere, shortly before the melting event that generated the Garrotxa lavas. The available geological data suggest that the first enrichment event of the mantle lithosphere under the NEVP could be the result of Late Variscan mantle upwelling triggered by the extensional collapse of the Variscan orogen during the Permo-Carboniferous. By Jurassic/Cretaceous time, large-scale NNE-directed sublithospheric mantle channeling of thermally and chemically anomalous plume material was placed under the Iberian Peninsula and Central Europe. However, the geodynamic conditions in the NEVP did not favor magmatism, which could not take place until the Cenozoic after extension started. This favored the second enrichment event of the mantle lithosphere by entrainment and storage of liquids generated in the sublithospheric plume material. After a relatively short period of time, as extension progressed, it triggered melting in the enriched portions of the mantle lithosphere during the Quaternary, generating the Garrotxa volcanism. (C) 2000 Elsevier Science B.V. All rights reserved.