Pre-Variscan metagabbro from NW Sardinia, Italy: evidence of an enriched asthenospheric mantle source for continental alkali basalts

Small metagabbro bodies are enclosed in the metasedimentary sequence of NW Sardinia. The metagabbros represent the last magmatic episode before the continent-continent collision that built up the Variscan chain of north Sardinia. The metagabbros are composed of variable proportions of plagioclase and pyroxene igneous relies and metamorphic minerals. Major and trace element data, specifically high TiO2 and P2O5 and low K and Rb contents, as well as light rare-earth elements, Nb and Ta enrichment, suggest an alkaline affinity for the gabbro and emplacement in a within-plate tectonic setting. The gabbro was derived from an ocean island alkali basalt-like asthenospheric mantle source enriched with incompatible elements and uncontaminated by crustal or subducted materials. Non-modal modelling indicates a 5-7% partial melting of the asthenospheric mantle. Copyright (C) 2003 John Wiley Sons, Ltd.