Petrology and geochemistry of metabasalts from the 1·95 Ga Jormua Ophiolite, Northeastern Finland

Abstract

The Jormua Ophidite exposes a unique fragment of Red Sea-type oceanic crust formed in a setting related to continental break-up 1950 Ma ago. Two distinct types of basalt are represented: the 'early dykes' and the 'main basalt suite'. 'Early dykes' have fractionated (H) REE patterns, OIB-like trace element patterns, low Zr/Nb (≈6) and εNd(1·95 Ga) ≈ -0·6, indicative of their derivation from an enriched source. The remaining dykes and all lavas belong to the second, E-MORB-like 'main basalt suite', which is characterized by high mg-number and Cr contents, flat REE patterns, Zr/Nb = 6-17, chondritic Th/Ta and only moderately depleted isotopic signatures [εNd(1·95 Ga ≈ + 1·9]. Most 'main suite' samples cannot be related solely by fractional crystallization to a common parental magma. Rather, they represent distinct melt fractions that underwent variable amounts of chromite + olivine ± plagioclase fractionation during ascent. A significant part of the compositional diversity of the 'main basalt suite' can be explained by mixing a depleted source with a relatively uniform proportion of an enriched component similar to that represented by the OIB-like 'early dykes'. It is probable that during the latest stages of continental rifting the OIB-type melts metasomatized the upper part of the depleted asthenospheric mantle, which became the source of the 'main basalt suite' soon after the old continental lithosphere was ruptured. The complete absence of any evidence for a subduction-related component in the basalts implies that Jormua is not a back-arc ophiolite.