The Palaeoproterozoic Komatiite-Picrite Association of Finnish Lapland

Abstract

The large range of chemical variation within intimately associated INTRODUCTION highly magnesian volcanic rocks in the Palaeoproterozoic Central Komatiitic lavas with high MgO contents are often Lapland Greenstone Belt prompted the construction of a new considered to represent high degrees of mantle melting classification scheme for MgO-rich volcanic rocks, based on an and consequently to approach their mantle sources in [Al 2 O 3 ] vs [TiO 2 ] diagram where the axes are the Al 2 O 3 and TiO 2 composition (e.g. Bickle et al., 1976). However, as pointed contents (in mole proportions) of the rocks projected from the olivine out by Arndt (1986), they can be a dirty window into the composition. This diagram places the Lapland rocks in the fields Archaean mantle because of some complicating factors. of Ti-enriched komatiites and picrites. Komatiitic rocks are depleted These include the high temperature of high-MgO liquids, in both light and heavy rare earth elements (LREE and HREE) which makes them susceptible to crustal contamination relative to middle REE (MREE) and possess relatively high TiO 2 during ascent or eruption even in the most LREE-depleted varieties, whereas picritic rocks Another feature of komatiites-variable chemistry in approach geochemically Hawaiian picrites. Seven clinopyroxene and terms of Al 2 O 3 /TiO 2 or heavy rare earth element (HREE) whole-rock pairs analysed for Sm-Nd isotopes yield an average age ratios-could have arisen from garnet fractionation, and of 2056 25 Ma for the komatiites. Uncontaminated komatiites if this fractionation took place during partial melting (e.g. and picrites have similar positive Nd values (+4) indicating Ohtani, 1990), then the komatiite composition may not generation from a mantle source with a long-term depletion in reflect that of the source mantle. Observations on ultra-LREE relative to MREE. Geochemical characteristics of the depleted and enriched melt inclusions in early crystallized komatiite-picrite association, including REE and Nb/Y-Zr/Y minerals from recent primitive oceanic basalts have systematics, indicate chemical heterogeneities in the source region, shown that melts produced by partial melting of upwelling which seem to have been created by complex depletion and enrichment mantle undergoing decompressional melting can be much processes shortly before or related to a dynamic melting process. The more varied in their trace element characteristics than high MgO contents of the rocks coupled with chemical similarity the erupted lavas themselves (Gurenko & Chaussidon, between the Lapland and Hawaiian picrites supports a mantle 1995). This feature has been explained by mixing of plume model for their genesis. Nevertheless, the geotectonic evolution liquids produced by a dynamic melting process from a appears to have proceeded without significant regional uplift shortly homogeneous mantle source (Gurenko & Chaussidon, before volcanism. 1995). Dynamic melting has also been suggested to account for the chemical variability of komatiites and picrites in Gorgona Island, Colombia (Arndt et al., 1997). Soon after the identification of the first komatiites in the late 1960s Al/Ti,