Sr and O isotope constraints on source and crustal contamination in the high-K calc-alkaline and shoshonitic neogene volcanic rocks of SE Spain

摘要

The Neogene volcanic province of SE Spain (NVPS) is characterized by calc-alkaline (CA), high-K calc-alkaline (KCA), shoshonitic (SH), ultrapotassic (UP), and alkaline basaltic (AB) volcanic series. All these series, except the AB, have high LILE/LREE, LILE/HFSE and B/Be ratios and high but variable Sr, Pb and O isotope compositions. The KCA and SH lavas contain metapelitic xenoliths whose mineralogical and chemical composition are typical of anatectic restites. The geochemical characteristics of CA, KCA, SH and UP series suggest that they originated from the lithospheric mantle, previously contaminated by fluids derived from pelagic sediments. Additionally, the presence of restite xenoliths in the KCA and SH lavas indicates some sort of interaction between the mantle-derived magmas and the continental crust. Trace element and isotope modeling for the KCA and SH lavas and the restites, point towards the existence of two mixing stages. During the first stage, the lithospheric mantle was contaminated by 1-5% of fluids derived from pelagic sediments, which produced a fertile source heterogeneously enriched in incompatible elements (particularly LILE and LREE), as well as in 87Sr/86Sr, without significant modifications of the δ18O values. In the second stage, the primary melts derived from this metasomatized mantle, which inherited the enrichment in LILE, LREE and 87Sr/86Sr, interacted with crustal liquids from the Betic Paleozoic basement during their ascent towards the surface. This mixing process caused an increase in δ18O values and, to a lesser extent, in 87Sr/86Sr ratios. However, the incompatible trace elements abundances only change slightly, even for high mixing rates, due to their similar concentrations in both components. We suggest the following geodynamic scenario to account for the global evolution of this area: (1) a Late Cretaceous to Oligocene subduction scheme during which mantle metasomatism took place, shortly followed by Upper Oligocene to Lower Miocene continental collision, and (2) a Middle to Upper Miocene extensional event triggering partial melting of the previously metasomatized mantle and the extrusion of the CA and associated magmas.