The Palaeoproterozoic komatiite-picrite association of Finnish Lapland

Abstract

The large range of chemical variation within intimately associated highly magnesian volcanic rocks in the Palaeoproterozoic Central Lapland Greenstone Belt prompted the construction of a new classification scheme for MgO-rich volcanic rocks, based on an [Al2O3] vs [TiO2] diagram where the axes are the Al2O3 and TiO2 contents (in mole proportions) of the rocks projected from the olivine composition. This diagram places the Lapland rocks in the fields of Ti-enriched komatiites and picrites. Komatiitic rocks are depleted in both light and heavy rare earth elements (LREE and HREE) relative to middle REE (MREE) and possess relatively high TiO2 even in the most LREE-depleted varieties, whereas picritic rocks approach geochemically Hawaiian picrites. Seven clinopyroxene and whole-rock pairs analysed for Sm Nd isotopes yield an average age of 2056 +/- 25 Ma for the komatiites. Uncontaminated komatiites and picrites have similar positive epsilon (Nd) values (+4) indicating generation from a mantle source with a long-term depletion in LREE and relative to MREE. Geochemical characteristics of the komatiite-picrite association, including REE and Nb/Y Zr/Y systematics, indicate chemical heterogeneities in the source region, which seem to have been created by complex depletion and enrichment processes shortly before or elated to a dynamic melting process. The high MgO contents of the rocks coupled with chemical similarity between the Lapland and Hawaiian picrites supports a mantle plume model for their genesis. Nevertheless, the geotectonic evolution appears to have proceeded without significant regional uplift shortly before volcanism.