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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 εNd values (+4) indicating generation from a mantle source with a long-term depletion in LREE 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 related 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. [ABSTRACT FROM PUBLISHER]

The Palaeoproterozoic Ni–Cu sulphide deposits of the Pechenga Complex, Kola Peninsula, occur in the lower parts of ferropicritic intrusions emplaced into the phyllitic and tuffaceous sedimentary unit of the Pilgujärvi Zone. The intrusive rocks are comagmatic with extrusive ferropicrites of the overlying volcanic formation. Massive lavas and chilled margins from layered flows and intrusions contain <3–7 ng/g Pd and Pt and <0·02–2·0 ng/g Ir, Os and Ru with low Pd/Ir ratios of 5–11. The abundances of platinum group elements (PGE) correlate with each other and with chalcophile elements such as Cu and Ni, and indicate a compatible behaviour during crystallization of the parental magma. Compared with the PGE-depleted central zones of differentiated flows (spinifex and clinopyroxene cumulate zones) the olivine cumulate zones at the base contain elevated PGE abundances up to 10 ng/g Pd and Pt. A similar pattern is displayed in intrusive bodies, such as the Kammikivi sill and the Pilgujärvi intrusion. The olivine cumulates at the base of these bodies contain massive and disseminated Ni–Cu-sulphides with up to 2 μg/g Pd and Pt, but the PGE concentrations in the overlying clinopyroxenites and gabbroic rocks are in many cases below the detection limits. The metal distribution observed in samples closely representing liquid compositions suggests that the parental magma became sulphide saturated during the emplacement and depleted in chalcophile and siderophile metals as a result of fractional segregation of sulphide liquids. Relative sulphide liquid–silicate melt partition coefficients decrease in the order of Ir > Rh > Os > Ru > Pt = Pd > Cu. R-factors (silicate-sulphide mass ratio) are high and of the order of 104–105, and they indicate the segregation of only small amounts of sulphide liquid in the parental ferropicritic magma. In differentiated flows and intrusions the sulphide liquids segregated and accumulated at the base of these bodies, but because of a low silicate–sulphide mass ratio the sulphide liquids had a low PGE tenor and Pt/Ir and Cu/Ir ratios similar to the parental silicate melts. During cooling the sulphide liquid crystallized 40–50% of monosulphide solid solution (mss) and the residual sulphide liquid became enriched in Cu, Pt and Pd and depleted in Ir, Os and Ru. The Cu-rich sulphide liquid locally assimilated components of the surrounding S-rich sediments as suggested by the radiogenic Os isotopic composition of some sulphide ores (γOs > 100). Most of the massive and breccia ores represent mixtures of mss and residual Cu-rich sulphide liquid whereas chalcopyrite-rich veins formed when the Cu-rich sulphide liquids were squeezed out into the surrounding sediments. [ABSTRACT FROM PUBLISHER]