Central Lapland Greenstone Belt Research Articles

Origin of ultra-nickeliferous olivine in the Kevitsa Ni–Cu–PGE-mineralized intrusion, northern Finland

The 2,058 ± 4 Ma mafic-ultramafic Kevitsa intrusion is located in the Central Lapland greenstone belt, northern Finland. It is hosted by a Paleoproterozoic vol- cano-sedimentary sequence that contains komatiitic vol- canic rocks and sulfide- and graphite-rich black schists. Economic Ni-Cu-(PGE) sulfide mineralization occurs in the middle part of the ultramafic lower unit of the intrusion. Two main types of ore are distinguished, ''normal'' and ''Ni-PGE'' ores. The normal ore is characterized by * 2t o 6 vol% disseminated sulfides and average Ni and Cu grades of 0.3 and 0.42 wt %, respectively (Ni/Cu \ 1). The Ni- PGE ore has broadly similar sulfide contents, but a higher Ni grade and lower Cu grade. As a result, the Ni/Cu ratio reaches 15, much higher than in the normal ore. The Ni- PGE ores occur as irregular, discontinuous, lense-like bodies in the ultramafic rocks. Notably, the olivines in the Ni-PGE ore contain extremely high Ni contents of up to 14,000 ppm, which is significantly higher than the Ni content of olivine in other mafic-ultramafic igneous rocks globally (up to *5,000 ppm) and in harmony with the associated Ni-rich sulfide assemblage containing pent- landite, millerite and pyrite. Microprobe mapping of olivine from the Ni-PGE ore suggests relatively low and homogeneous S contents and homogeneous distribution of Ni, Mg, Fe, which is inconsistent with the presence of sulfide inclusions in the olivine grains, or diffusion of Ni from interstitial sulfides into the olivine grains. We there- fore conclude that Ni substitutes for Mg in the olivine lattice. The clinopyroxenes from the Ni-PGE ore also have unusually high Ni concentrations reaching 1,500 ppm and show a positive correlation with the nickel content of the associated olivine. The Nicpx/Niolivine is *0.1 to 0.2 cor- responding to high T partitioning of Ni between clinopy- roxene and olivine. KD of 20 can account for the partitioning of nickel between olivine and the sulfide phase, consistent with magmatic equilibration. These data suggest that the olivine, clinopyroxene, and sulfides all crystallized from a basaltic magma with an unexceptionally high Ni content ranging from 300 to 1,100 ppm. The Ni-PGE ores are spatially associated with ultramafic xenoliths. Olivine in these ultramafic xenoliths have relatively high Fo con- tents (up to 90 mol %) and high Ni contents (up to 5,200 ppm) suggesting that the xenoliths formed from a komatiitic parental magma. It is proposed that assimilation by the Kevitsa magma of massive or semi-massive sulfides associated with komatiitic rocks elevated the Ni content of the magma and resulted in the formation of Ni-PGE ores and related extremely Ni-rich olivines.

Chrome spinel-hosted melt inclusions in Paleoproterozoic primitive volcanic rocks, northern Finland: Evidence for coexistence and mixing of komatiitic and picritic magmas

The Paleoproterozoic ca. 2.05Ga ultramafic volcanic rocks occurring in the Central Lapland Greenstone Belt are classified as Ti-enriched komatiites and picrites. Some of the rocks are high in Cr (ca. 4000ppm) and contain fresh chrome spinel grains hosting melt inclusions up to 50μm in size. These inclusions enabled us to study the compositional features of the magmas at the time of melt entrapment. Chrome spinels are divided into the low-Ti and high-Ti types reflecting crystallization from komatiitic and picritic parental magmas, respectively. There is a good correlation between the major element compositions of the melt inclusions and their host spinels, with picritic melts being found in high-Ti spinels and komatiitic melts in low-Ti spinels. MgO contents measured in the inclusions range between 10 and 22wt.%. Trace element analyses performed by LA-ICP-MS and SIMS revealed two contrasting magma compositions, which are either depleted or highly enriched in incompatible elements and produce convex-upwards or LREE-enriched chondrite-normalized REE patterns, respectively. Due to the shielding effect of the host spinel, melt inclusions provide information on the original mobile trace element contents (Li, K, Ba, Sr, Be) and LILE/HFSE ratios of the magmas.Interestingly, spinel grains carrying both depleted and enriched melt inclusions were discovered in a single komatiitic sample, demonstrating the coexistence and mixing of two different primitive magma types. The whole-rock and melt-inclusion geochemical data together with previous Nd and Os isotopic results indicate an asthenospheric origin of the magmas with insignificant crustal contamination or interaction with the subcontinental lithospheric mantle. We suggest that adiabatic upwelling of a Paleoproterozoic mantle plume was able to simultaneously produce komatiitic and picritic magmas from an originally depleted source via dynamic melting involving re-fertilization and re-melting processes and re-melting processes, though the details of these processes are difficult to constrain in a quantitative manner. These processes should be taken into account when using the geochemistry of Precambrian primitive magmas for assessing the temporal evolution of mantle plume reservoirs.

Radial Basis Functional Link Nets Used as a Prospectivity Mapping Tool for Orogenic Gold Deposits Within the Central Lapland Greenstone Belt, Northern Fennoscandian Shield

Among the more popular spatial modeling techniques, artificial neural networks (ANN) are tools that can deal with non-linear relationships, can classify unknown data into categories by using known examples for training, and can deal with uncertainty; characteristics that provide new possibilities for data exploration. Radial basis functional link nets (RBFLN), a form of ANN, are applied to generate a series of prospectivity maps for orogenic gold deposits within the Paleoproterozoic Central Lapland Greenstone Belt, Northern Fennoscandian Shield, Finland, which is considered highly prospective yet clearly under explored. The supervised RBFLN performs better than previously applied statistical weights-of-evidence or conceptual fuzzy logic methods, and equal to logistic regression method, when applied to the same geophysical and geochemical data layers that are proxies for conceptual geological controls. By weighting the training feature vectors in terms of the size of the gold deposits, the classification of the neural network results provides an improved prediction of the distribution of the more important deposits/occurrences. Thus, ANN, more specifically RBFLN, potentially provide a better tool to other methodologies in the development of prospectivity maps for mineral deposits, hence aiding conceptual exploration.

Combined conceptual/empirical prospectivity mapping for orogenic gold in the northern Fennoscandian Shield, Finland

Despite its exploration immaturity, the Central Lapland Greenstone Belt has geodynamic, geological and gold-endowment characteristics to indicate that it is potentially the premier orogenic-gold province in Finland. Previous GIS-based prospectivity mapping of the Central Lapland Greenstone Belt used only empirical geophysical and geochemical datasets based on the weights-of-evidence integration method. In this study, such empirical data are combined with conceptual geological parameters derived from a 1:200 000 scale bedrock map of northern Finland and use the weights-of-evidence, logistic regression and fuzzy-logic methods for integration. A targeting model is developed based both on the empirical data and conceptual parameters defined from understanding of the processes essential to produce large deposits in an orogenic-gold mineral system. Key spatially referenced layers are derived from the empirical data and from critical proxies for essential processes in the targeting model, and the spatial association between these key layers and a training set of known gold deposits and prospects is quantified. Key parameters are then reclassified into binary layers according to maximum spatial association with the training set, and these layers are then integrated into prospectivity maps using both the weights-of-evidence and logistic regression methods. The highest probability class in all models defines <1% of the Central Lapland Greenstone Belt, a remarkable reduction in highest-priority target zones. The resulting maps are evaluated using a set of gold prospects excluded from the original training set as validation sites. The resultant prospectivity maps provide subtle, but significant, improvement on the maps derived solely from empirical data, especially where logistic regression is applied. Proxies for strain gradients appear particularly important. The analysis demonstrates that homogeneous, high-quality, small-scale geological maps, integrated using a robust targeting model, can enhance the predictive capacity of prospectivity maps derived from GIS-based modelling.

Precise and accurate 186Os/ 188Os and 187Os/ 188Os measurements by Multi-collector Plasma Ionisation Mass Spectrometry, part II: Laser ablation and its application to single-grain Pt–Os and Re–Os geochronology

The abundance of platinum group alloy (PGA) grains available from some alluvial deposits combined with their great chemical resistance to conventional acid attack generates the need for an alternative method of obtaining routine, rapid yet precise Os isotopic data. Laser ablation multi-collector ICP-MS (LA-MC-ICP-MS) is ideally suited for this purpose and has been applied, in a relatively limited extent, to Os-rich samples by previous workers (e.g., Hirata, T., Michinari Hattori, M., and Tsuyoshi Tanaka, T., 1998. In-situ osmium isotope ratio analyses of iridosmines by laser ablation-multiple collector-inductively coupled plasma mass spectrometry. Chem Geol., 144, 269–280.; Malitch, K.N., 2004. Osmium isotope constraints on contrasting sources and prolonged melting in the Proterozoic upper mantle: evidence from ophiolitic Ru–Os sulfides and Ru–Os–Ir alloys. Chem Geol. 208. 157–173.; Walker, R.J., Brandon, A.D., Bird, J.M., Piccolia, P.M., McDonougha, W.F., R.D., Asha, R.D., 2005. 187Os– 186Os systematics of Os–Ir–Ru alloy grains from southwestern Oregon. Earth Planet. Sci. Lett. 230, 211–226.). The wide variety of PGA minerals yield a broad spectrum of Pt, Re and Os contents which translate into a wide spread in parent–daughter ratios that is an attractive proposition for geochronology using the Re–Os and Pt–Os decay schemes. Here we show that single PGA grains, can contain sufficient internal Re, Pt and Os elemental variability and coupled variations in parent/daughter and daughter isotope compositions that they present an opportunity to obtain ‘isochron’ ages from two different isotope decay systems. We present a rapid (40 s acquisition time), precise and accurate LA-MC-ICP-MS methodology suitable for applying Pt–Os and Re–Os geochronology approaches to single PGA grains, for use in dating chromitite deposits and identifying and dating multiple sources in alluvial PGA deposits. Because of isobaric interferences from Pt, PGA analyses by LA-MC-ICP-MS are best corrected for instrumental mass fractionation using the 189Os/ 188Os ratio. We demonstrate that, within our analytical uncertainties, mass bias effects for Os isotope ratios during LA-MC-ICP-MS are similar in nature and magnitude to mass bias variations observed during normal solution-mode analysis. While robust elemental interference corrections can be applied for Re and W on the Os mass spectrum during solution analyses (Nowell, G.M., Luguet, A., Pearson, D.A., Horstwood, M.S.A., 2008-this volume. Precise and accurate 186Os/ 188Os and 187Os/ 188Os measurements by Multi-Collector Plasma Ionisation Mass Spectrometry (MC-ICP-MS) part I: solution analyses. Chem Geol.) we show that the accurate correction for the Re interference on Os during laser ablation is problematic at 187Re/ 188Os ratios > ∼ 0.5 which means that Re-corrected 187Os/ 188Os ratios for some PGAs and high Re/Os sulfides must be regarded as problematic. Our interfering element correction approach can also be used for the correction for Os on 190Pt for obtaining 190Pt/ 188Os ratios, removing the need for an independent Pt monitor isotope. Accurate and precise geochronology by LA-MC-ICP-MS can be further limited by poorly quantified fractionation of the parent/daughter elemental ratio during ablation. Although additional work is required to understand the nature and exact extent of inter-element fractionation of the Pt/Os and Re/Os ratios during laser ablation analysis of Pt- and Re-rich PGA grains, our best estimate is that fractionation of the Pt/Os during laser ablation is around 5% or less. We present multi-grain isochrons and single-grain internal ‘isochrons’ for the Re–Os and Pt–Os isotope systems in PGA grains from Lapland. In all cases, the Pt–Os isotope system gives significantly more precise ‘isochrons’ than the Re–Os system. We interpret this discordance between the two systems as being most likely due to problems inherent with Re interference corrections at elevated Re/Os ratios. We show that a suite of Pt-rich PGA grains from the Central Lapland greenstone belt yields a spectrum of internal Pt–Os isochron ages that coincides well with pulses of formation of chromitite-bearing ophiolitic material within this magmatic terrane and we take this to indicate the record of chromitite formation in different magmatic units. The single-grain Pt–Os and Re–Os geochronology technique shows great promise in the dating of chromitite deposits in ophiolites and in the discrimination of multiple source regions within alluvial PGA deposits.

Magnetic signatures related to orogenic gold mineralization, Central Lapland Greenstone Belt, Finland

A number of lode–gold occurrences are hosted by hydrothermally altered greenstones along the southern boundary of the Palaeoproterozoic Central Lapland Greenstone Belt. The hydrothermally altered and mineralised zones are related to a major thrust and shear zone system that extends much across northern Finland. Spatial correlation between mineralized zones, brittle structural features and chemical alteration was explored and identified from high-resolution aeromagnetic data, in combination with airborne electromagnetic and gamma-ray spectrometric data and coupled with petrophysical and palaeomagnetic studies. The most prominent magnetic, ductile signatures formed during the Svecofennian Orogeny (1900–1800 Ma), resulting in elastic, curved, continuous magnetic patterns. These elastic anomaly patterns were disturbed by tectonic stress from S–SW, resulting in parallel, regularly oriented fracture families and thrust faults normal to the main stress direction. According to aeromagnetic, palaeomagnetic and structural evidence, the thrust zone was active during the latest stage of the orogenic event, but was also reactivated at a later date. Airborne gamma-ray data reveals zones of potassic alteration in the ultramafic rock units in the vicinity of cross-sections of these two fault sets. Chemical and mineralogical changes during alteration and metamorphism strongly affected the mafic and ultramafic host rocks throughout the deformation zone. The strong potassium enrichment and coinciding destruction of magnetic minerals resulted in enhanced potassium concentration and reduction of magnetic anomaly amplitudes. Palaeomagnetic results indicate that the remanent magnetization for the altered ultramafic rocks along the thrust zone is of chemical origin (CRM) and was acquired at 1880–1840 Ma, which is presumed also to be the age of the chemical alteration related to gold mineralization.

Quantitative Analysis of Scale of Aeromagnetic Data Raises Questions About Geologic-Map Scale

A recently published study has shown that small-scale geologic map data can reproduce mineral assessments made with considerably larger scale data. This result contradicts conventional wisdom about the importance of scale in mineral exploration, at least for regional studies. In order to formally investigate aspects of scale, a weights-of-evidence analysis using known gold occurrences and deposits in the Central Lapland Greenstone Belt of Finland as training sites provided a test of the predictive power of the aeromagnetic data. These orogenic-mesothermal-type gold occurrences and deposits have strong lithologic and structural controls associated with long (up to several kilometers), narrow (up to hundreds of meters) hydrothermal alteration zones with associated magnetic lows. The aeromagnetic data were processed using conventional geophysical methods of successive upward continuation simulating terrane clearance or ‘flight height’ from the original 30 m to an artificial 2000 m. The analyses show, as expected, that the predictive power of aeromagnetic data, as measured by the weights-of-evidence contrast, decreases with increasing flight height. Interestingly, the Moran autocorrelation of aeromagnetic data representing differing flight height, that is spatial scales, decreases with decreasing resolution of source data. The Moran autocorrelation coefficient scems to be another measure of the quality of the aeromagnetic data for predicting exploration targets.

Geology, geochemistry, fluid inclusion characteristics, and U–Pb age studies on iron oxide–Cu–Au deposits in the Kolari region, northern Finland

Several iron oxide–copper–gold deposits are known in the Kolari region, in the western part of the Central Lapland greenstone belt, northern Finland. They are hosted by clinopyroxene-dominated skarns that were formed near to contact zone between ca. 1860 Ma Haparanda Suite intrusions and > 2050 Ma Savukoski Group supracrustal rocks. All deposits are located within or next to shear and fault zones forming parts of the major, NNE-trending, Kolari shear zone. Three of the Fe–Cu–Au deposits, Kuervitikko, Cu-Rautuvaara, and Laurinoja were studied; all contain significant amounts of Cu (0.1% to 4.5%) and Au (0.1 to 6.6 ppm). At Laurinoja and Kuervitikko, Cu and Au are hosted by ironstone and skarn. At Cu-Rautuvaara, the host rock is a magnetite-disseminated albitite. The deposits have a distinct metal association of Fe–Cu–Au ± Ag, Bi, Ba, Co, Mo, Sb, Se, Te, Th, U, LREE. The wall and host rocks are intensely altered and display a deposit-scale zonal pattern. The distal alteration zone is characterised by albite ± biotite, K-feldspar, and scapolite and the proximal zone by clinopyroxene–magnetite ± amphibole, scapolite, calcite, and sulphides. Mass balance calculations indicate that Al 2O 3, TiO 2, and Zr were immobile during alteration. The calculations also indicate that significant quantities of Fe 2O 3, CaO, CO 2, S, Cu, Au, Bi, and Te were added to the proximal altered rocks. The main gains in the distal altered rocks are in Na 2O, K 2O, and Ba. Fluid inclusion data suggest that the fluids which circulated in the rocks during the main mineralisation event and subsequent brittle fracturing were highly saline (≤ 56 wt.% NaCl) H 2O ± CO 2 fluids. The temperature during the main mineralisation event was between 450 and 550 °C and the pressure was 1.5 to 3.5 kbar. Based on U–Pb age data of magmatic zircon from altered hanging wall diorite and granite that brecciates the ore, the age of the deposits is between 1864 ± 5 and 1766 ± 5 Ma. The 1797 ± 5 Ma age of zircon in skarn, combined with the 1810 to 1780 Ma ages of the metamorphic titanite in altered wall rocks and skarn, suggest that the deposits were most likely formed ca. 1800 Ma. This age has been interpreted to be broadly contemporaneous with the D 3 thrusting event in the Kolari region during which the Kolari shear zone was activated. The data presented in this work are inconsistent with previously proposed models for the Kolari deposits that they are metamorphosed syngenetic iron formations, or skarns related to ca. 1860 Ma monzonite intrusions. Instead, we propose that the Kolari Fe–Cu–Au deposits are of metasomatic replacement-type, and are controlled by the Kolari shear zone structures related to post-peak metamorphic D 3 thrusting event in northern Finland. Our data suggest that the Kolari deposits best fit into the category of epigenetic iron oxide–copper–gold mineralisation.

A Review of Gold Mineralization Styles in Finland

A wide range of styles of gold mineralization has been recognized in Finland, including orogenic, metamorphosed epithermal, skarn- or epigenetic ironstone-hosted, intrusion-related (nonskarn), massive sulfide-hosted, paleoplacer, and placer deposits. Gold production in the past was principally as a byproduct from the mining of Paleoproterozoic base-metal massive sulfide deposits. This situation has changed during the last decade with the closure of some base metal mines and the commencement of production from recently discovered, gold-only deposits. Gold mineralization occurred during distinct episodes of crustal evolution in Finland. Several late-Archean greenstone belts in the Karelian craton in eastern and northern Finland contain orogenic gold deposits dated at about 2.75 to 2.70 Ga, suggesting a coherent and pervasive tectonic and metallogenic event. In the Paleoproterozoic, low-grade ( The main accretionary stage of the Svecofennian orogeny in Finland culminated with metamorphism and deformation between 1.89 and 1.86 Ga. During that time, intrusion-related Au-Cu mineralization took place in both syntectonic porphyry stocks and their wall rocks in and close to the Archean-Proterozoic margin. Orogenic gold mineralization took place at or slightly after the metamorphic peak throughout the Svecofennian domain, possibly also overprinting massive-sulfide and intrusion-related occurrences. It is notable that the largest orogenic deposits are located within discrete plutons, rather than hosted by supracrustal sequences. This may be a consequence of the rheological contrasts between plutons and the supracrustal lithologies. The Paleoproterozoic Lapland domain in the northern part of the country records a protracted and complicated evolution involving multiple episodes of rifting, magmatism, sedimentation, and alteration, culminating at around 1.9 Ga with the collision of the Karelian craton, the Svecofennian fold belt to the south, and the Kola craton to the north. These deformational events led to orogenic gold mineralization throughout the Proterozoic greenstone belts of northern Finland at about 1.89 to 1.86 Ga, during and soon after the peak of metamorphism and deformation. Skarnlike or epigenetic ironstone-hosted deposits were also formed during the orogeny, in the western and southwestern parts of the Lapland domain, probably somewhat later, at around 1.86 Ga. The most favorable sites for mineralization are regionally extensive zones of early albitization that predate gold mineralization. The deposition of molasselike sediments during the terminal stages of the Svecofennian orogeny and weathering of both orogenic and syngenetic sulfide mineralization, also prior to 1.8 Ga, produced placer accumulations (now paleoplacers) in the conglomerates of the uppermost formation of the Central Lapland greenstone belt. Placer deposits of controversial origin and provenance have also been worked for many years in the glaciofluvial sediments of northernmost Finland. The areas with greatest potential for the discovery of new orogenic gold deposits in Finland are the late Archean and Paleoproterozoic greenstone belts in eastern Finland and Lapland, respectively. Some Fe-Cu-Au skarns or epigenetic ironstone-hosted deposits have been mined in the past, and there may be potential for the discovery of larger iron oxide-copper-gold systems in the western Central Lapland greenstone belt. For intrusion-related (nonskarn) mineralization, the most promising areas are close to the Raahe-Ladoga Suture, and the potential for additional metamorphosed and deformed epithermal mineralization exists in the Tampere schist belt. The significant number of gold occurrences already known in these areas highlights the potential for new discoveries.

Geology and Geochronology of the Suomujärvi Complex: a new Archaean gneiss region in the NE Baltic Shield, Finland

Recent mapping in the eastern Central Lapland Granitoid Complex and dating of zircons at the NORDSIM facility was performed in order to highlight the existence of an 892-km 2 area of Archaean gneisses not previously identified in southeast Lapland, Finland. This Archaean enclave is composed of northeast-trending, km-scale bands of orthogneisses with varying mafic content and is named the Suomujärvi Complex. Along its southeastern boundary lies the Aholanvaara volcano-sedimentary package. The evolution of the Suomujärvi Complex began at ca. 2810 Ma with intrusion of the homogeneous Jumisko biotite tonalite–granodiorite in the west. This lithology becomes increasingly heterogeneous to the east where muscovite and an inherited ca. 2870 Ma old rock component represented by zircon cores are present. These orthogneisses were later intruded by the 2755±18 Ma (1 σ) protolith of the Miehinkävaara amphibole and biotite bearing diorite–tonalite gneisses producing diffuse contacts characterized by gradual increases in mafic minerals. All of the orthogneisses were then migmatized/metamorphosed. After ca. 2730 Ma the Aholanvaara volcano-sedimentary package was deposited. Its main quartzite unit was dominantly derived from an unknown 2731±17 Ma (1 σ) source but it also contains a ca. 3.4 Ga detrital component. The absence of 2800–2820 Ma detrital material in these sediments indicates they were not derived locally from the Suomujärvi Complex. Instead, they share affinities with supracrustal packages further south in Posio, east in the Belomorian Province and north in the Central Lapland Greenstone belt. Widespread, voluminous intrusion of granites occurred in the area surrounding the Suomujärvi gneisses and left a marked metamorphic overprint on the gneisses during the ca. 1.8 extensional collapse of the Svecofennian and Lapland-Kola orogens. Multi-scale folding produced subhorizontal, southwest-plunging, tight to isoclinal, upright folds and the dominant axial planer, northeast-trending fabric in the area. This fabric is cut by a northwest-trending fabric in the southwestern part of the area. Geological characteristics of the Suomujärvi Complex and ages of the orthogneisses within it indicate that it most likely represents the southwestern margin of the Archaean Belomorian block.

Early Proterozoic nappe formation: an example from Sodankylä, Finland, Northern Baltic Shield

The Central Lapland Greenstone belt comprises rift-related metavolcanic and metasedimentary rocks representing one of the largest supracrustal belts in the Baltic Shield. The Sodankylä area in the central part of the belt represents a complex thrust duplex within a nappe overlying Belomorian Archaean basement and autocthonous Luirojoki calc-silicate rocks. Here, an early D1 schistosity is axial planar to at least three coaxial generations of southward-verging, subhorizontal, E–W-plunging D1 folds associated with major southwards thrusting. D2 is represented by broad, map-scale, upright, NE-trending folds in the south and crenulations in the north. Staurolite-grade metamorphism represented by post-tectonic andalusite + staurolite + kyanite assemblages occurred after D2 folding. Later D3 deformation was limited to local NW-trending folds and sinistral faults. The internal nappe-like structure of the Central Lapland Greenstone belt suggests that it represents the foreland of a large collisional complex cored by the Lapland Granulite belt.

The Palaeoproterozoic Komatiite-Picrite Association of Finnish Lapland

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 [Al 2 O 3 ] vs [TiO 2 ] diagram where the axes are the Al 2 O 3 and TiO 2 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 TiO 2 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 e Nd 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.