B1 basaltic lavas in Vestfold–Jeløya area, central Oslo rift: derivation from initial melts formed by progressive partial melting of an enriched mantle source

The earliest (B1) lavas in Vestfold and Jeløya, in the central part of the Permo-Carboniferous Oslo rift have been divided into four main groups: high Ti (HT) basalts (TiO2>4.2 wt.%), low Ti (LT) basaltic rocks (TiO2<3.7 wt.%), phonotephrites and ignimbrites. They are all highly enriched in strongly incompatible elements and have high La/Yb ratios. Initial εNd and εSr ratios of whole-rock samples and clinopyroxene separates (assumed age=295 Ma) are +6.0 to −1.6 and −17 to +57, respectively. Their trace element and isotopic compositions imply that, during ascent, the magmas were subjected to varying degrees of fractional crystallization, mixing between evolved and mafic magmas, and crustal contamination in the upper crust. Crustal contamination was most likely caused by rocks similar to the metasediments now exposed in the Bamble sector, south of the Oslo rift. The preserved lava sequences represent interfingering of lavas from at least three eruption centers. After extrusion, the lavas suffered post-magmatic alteration that led to mobilization of Rb, U and K, and increased 87Sr/86Sr ratios. Interpretation of magmatic signatures, inferred to have been preserved through contamination and alteration, allow us to make the following conclusions concerning the mantle sources of the Vestfold and Jeløya B1 basalts. The formation of the HT-basalts involves highly Ti–P-rich, garnet-bearing mantle source rocks, e.g. clinopyroxenites, wehrlites, websterites, and amphibolites. The HT and LT basalts may have originated by progressive partial melting of a veined, enriched, garnet–phlogopite-bearing peridotite at the base of the lithosphere. Progressive melting of this complex source initially produced magnesian lavas with high REE and HFSE contents, followed by less enriched, more voluminous lavas. The early basalts were derived from an isotopically depleted source similar to the PREMA source. Contribution of older lithospheric material to the later basalts produced more enriched Sr–Nd isotopic signatures. As some of the most primitive basalts found within the European Permo-Carboniferous rifting event, these lavas are key to understanding the nature of the sublithospheric mantle beneath Europe at this time.