Isotopic and trace element constraints on the genesis of the Faeroe lava pile

Abstract

Basaltic lavas from the Faeroe Islands form three stratigraphic series which define two geochemical groups. Both the lower and middle series are LREE enriched ((La/Yb) e.f.: 2–3) and are characterized by convex LREE profiles; in contrast, the upper series comprises both depleted ((La/Yb) e.f.: 0.45–0.6) and enriched lavas. This twofold geochemical division is also evident from the incompatible trace elements such as Zr, Nb, Hf and Ta and the compatible trace elements Cr, Ni, Sr and Y. Nd-Sr-Pb isotopic measurements show that the basalts are contaminated by crustal materials, implying the presence of Precambrian sialic basement underneath the Faeroes block, a conclusion supported by geophysical data [35,36]. The uncontaminated end-members, for the LREE-depleted basalts ( 87Sr/ 86Sr) 0 ∼ 0.7026 and εNd 0 + 10 and for the LREE-enriched basalts ( 87Sr/ 86Sr) 0 ∼ 0.7034 and εNd 0 + 9, require two different mantle source regions thus posing serious problems for petrogenetic models such as dynamic partial melting which have been proposed for the Faeroes. We interpret the LREE-depleted basalts as partial melts of the oceanic asthenosphere whilst the LREE-enriched basalts may result either from the partial melting of deep mantle blobs or of the subcontinental lithosphere during upwelling of the asthenosphere.