Petrology and Geochemistry of Metabasalts from the 1.95 Ga Jormua Ophiolite, Northeastern Finland

The Jormua Ophiolite exposes a unique fragment of Red Seatypeoceanic crust formed in a setting related to continental break-up1950 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 E_Nd(1.95Ga)–0.6, indicative of their derivation from an enrichedsource. The remaining dykes and all lavas belong to the second,E-MORB-like ‘main basalt suite’, which is characterizedby high mg-number and Cr contents, flat REE patterns, Zr/Nb= 6–17, chondritic Th/Ta and only moderately depletedisotopic signatures E_{ND (1.95 Ga)}+1.9]. Most ‘main suite’samples cannot be related solely by fractional crystallizationto a common parental magma. Rather, they represent distinctmelt fractions that underwent variable amounts of chromite +olivine plagioclase fractionation during ascent. A significantpart of the compositional diversity of the ‘main basaltsuite’ can be explained by mixing a depleted source witha relatively uniform proportion of an enriched component similarto that represented by the OIB-like ‘early dykes’.It is probable that during the latest stages of continentalrifting the OIB-type melts metasomatized the upper part of thedepleted asthenospheric mantle, which became the source of the‘main basalt suite’ soon after the old continentallithosphere was ruptured. The complete absence of any evidencefor a subduction-related component in the basalts implies thatJormua is not a back-are ophiolite. ^*Fax: +358 0 462 205. e-mail: petri.peltonen{at}gsf.fi