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Several mafic-ultramafic layered intrusions were emplaced in the NE Fennoscandian Shield during a magmatic episode at 2.44 Ga. The Paleoproterozoic Näränkävaara layered intrusion, northern Finland, is one of the largest ultramafic bodies in the Fennoscandian Shield, with a surface area of 25 km x 5 km and a magmatic stratigraphic thickness of ~3 km. The intrusion comprises a 1.3 km-thick peridotitic-dioritic layered series (2436 ± 5 Ma) with two peridotitic reversals, and a 1.5-2 km thick basal dunite series mainly composed of olivine adcumulates (dated here). The intrusion has been studied since the 1960's, but several questions regarding its structure and petrogenesis remain. The basal dunite shows several lithological features typical of komatiitic rather than intrusive olivine cumulates; namely, >1 km-thick ôextremeö olivine adcumulates, some showing textures with bimodal grain sizes, oscillating variations in Mg# with stratigraphic height, and poikilitic chromite. With Archean greenstone belts nearby, it was previously hypothesized that the basal dunite series could represent an Archean komatiitic wall rock to the Paleoproterozoic layered series. However, our new U-Pb ID-TIMS baddeleyite age of 2441.7 ± 0.9 Ma for the basal dunite series shows that the basal dunite and layered series of the Näränkävaara intrusion are co-genetic. New whole-rock Sm-Nd isotope data from key stratigraphic units (initial εNd at 2440 Ma of -3.5 to -1.7) indicate that the intrusion was constructed from repeated emplacement of LREE-enriched high-MgO basaltic magmas that were mantle-derived and contaminated by crust, similarly to other Fennoscandian 2.44 Ga intrusions. The parental magmas show similar compositions regardless of stratigraphic position, suggesting that most wall rock contamination and homogenization had occurred before emplacement, with in situ contamination being a relatively minor process. The open-system features of the basal dunite suggest that it may have formed (at least partly) as a feeder channel cumulate, possibly related to the ~100 km long Koillismaa-Näränkävaara Layered Igneous Complex. The Näränkävaara parental magmas show variably depleted metal ratios and could have potential for orthomagmatic mineral deposits, given the availability of S-rich wall rocks. [ABSTRACT FROM AUTHOR]

The Paleoproterozoic Näränkävaara layered intrusion, northern Finland, has a surface area of 25 km x 5 km and a stratigraphic thickness of ~3 km. The main body of the intrusion includes a 1.5-2 km thick basal dunite series and a 1.3 km thick peridotitic-dioritic layered series, the latter with two peridotitic reversals related to magma recharge. In addition, a series of poorly known elongate poikilitic harzburgitic intrusions (the northern peridotites) are found along the NE contact between the intrusion and the granite-gneiss basement complex. We investigate new mineral and whole-rock geochemical data from the northern peridotites, with the aim of clarifying their petrogenetic relationship to the main layered body of the intrusion. The northern peridotites form a 200-400 m thick cumulate series grading from olivine orthocumulates (OC) at the northern basement complex contact to olivine-orthopyroxene heteradcumulates (HAC) towards the main intrusion body in the south. The OC show whole-rock and mineral chemical trends consistent with origin as rapidly cooled olivine-melt mixtures. The HAC have crystallized in situ from a relatively Cr- and SiO2-rich magma. Based on lithological and stratigraphical correlations, the northern peridotites are linked to the emplacement of the magma that caused the first reversal in the layered series: marginal orthocumulates were formed at the initial emplacement of a new pulse of LREE-enriched siliceous high-MgO basaltic (SHMB) magma into the Näränkävaara chamber, followed by heteradcumulate formation from a fractionating magma with added external SiO2 and fluid. Ubiquitous granite-gneiss xenoliths and felsic veins in drill core suggest assimilation may have been a local process. The northern peridotite parental magma shows undepleted metal ratios suggesting no sulfide saturation occurred prior to emplacement. [ABSTRACT FROM AUTHOR]

Dehydration melting of metasupracrustal rocks at mid- to deep-crustal levels can generate water undersaturated granitic melt. In this study, we evaluate the potential of ∼1.89–1.88 Ga metasupracrustal rocks of the Precambrian of southern Finland as source rocks for the 1.86–1.79 Ga late-orogenic leucogranites in the region, using the Rhyolite-MELTS approach. Melt close in composition to leucogranite is produced over a range of realistic pressures (5 to 8 kbar) and temperatures (800 to 850 °C), at 20%–30% of partial melting, allowing separation of melt from unmelted residue. The solid residue is a dry, enderbitic to charnoenderbitic ganulite depleted in incompatible components, and will only yield further melt above 1 000–1 050 °C, when rapidly increasing fractions of increasingly calcic (granodioritic to tonalitic) melts are formed. The solid residue after melt extraction is incapable of producing syenogranitic magmas similar to the Mid-Proterozoic, A-type rapakivi granites on further heating. The granitic fraction of the syenogranitic rapakivi complexes must thus have been formed by a different chain of processes, involving mantle-derived mafic melts and melts from crustal rock types not conditioned by the preceding late-orogenic Svecofennian anatexis. [ABSTRACT FROM AUTHOR]

Several mafic-ultramafic layered intrusions were emplaced in the Fennoscandian Shield during wide-spread mantle-sourced magmatism at 2.5–2.4 Ga. The Näränkävaara intrusion (surface area 5 × 30 km2), northeastern Finland, contains a 1.5–2 km thick basal dunite (not dated), and a 1.5 km thick layered series (2436 ± 5 Ma). A newly discovered marginal series between the basal dunite and the layered series indicates that the basal dunite is older, and highlights the need for further study on their relationship. Along its southern basement contact, the basal dunite contains a 200–300 m thick zone of olivine ortho- and mesocumulates, but the bulk of it is composed of olivine adcumulates and lesser olivine-orthopyroxene heteradcumulates. Based on whole-rock geochemistry, the basal dunite is divided into a low-Fe zone (average FeOt 10.2 wt% and Ni 2250 ppm) and a high-Fe zone (average FeOt 12.5 wt% and Ni 1700 ppm). Both zones have high MgO (32–47 wt%) and varying Cr (830–5160 ppm) and Al2O3/TiO2 (16–26). Textural and geochemical layering is similar along the 30 km strike of the basal dunite. A LREE-enriched high-MgO basaltic parental magma composition (13–18 wt% MgO) is inferred for the basal dunite from olivine–melt mixing trends in orthocumulates. The dunite exhibits at least two geochemical reversals as well as abundant low-porosity adcumulates, poikilitic chromite, and bimodal olivine, suggesting formation in a high-volume open magmatic system. Significant similarity in major and trace element compositions with the Näränkävaara layered series and the Burakovsky intrusion and Vetreny belt extrusives in Russian Karelia suggests that the basal dunite belongs to the Fennoscandian 2.5–2.4 Ga mafic layered intrusions. An Archean komatiitic origin for the dunite body cannot be completely ruled out, however. Distinct Ni-depletion in olivine is found in the basal dunite from the low-Fe zone to the high-Fe zone (3200 versus 2200 ppm). This depletion does not correlate with Fo contents, which suggests that it is not related to olivine fractionation. The basal dunite may thus have potential for Ni-(Cu-Co-PGE) sulfide mineralization. [ABSTRACT FROM AUTHOR]

About 20 mafic-ultramafic layered intrusions in the northern Fennoscandian shield were emplaced during a widespread magmatic event at 2.5–2.4 Ga. The intrusions host orthomagmatic Ni-Cu-PGE and Cr-V-Ti-Fe deposits. We update the magmatic stratigraphy of the 2.44-Ga Näränkävaara mafic-ultramafic body, northeastern Finland, on the basis of new drill core and outcrop observations. The Näränkävaara body consists of an extensive basal dunite (1700 m thick), and a layered series comprising a peridotitic–pyroxenitic ultramafic zone (600 m thick) and a gabbronoritic–dioritic mafic zone (700 m thick). Two reversals are found in the layered series. The composition of the layered series parental magma was approximated using a previously unidentified marginal series gabbronorite. The parental magma was siliceous high-Mg basalt with high MgO, Ni, and Cr, but also high SiO2 and Zr, which suggests primary magma contamination by felsic crust. Cu/Pd ratio below that of primitive mantle implies PGE-fertility. The structural position of the marginal series indicates that the thick basal dunite represents an older wallrock for the layered intrusion. A subeconomic reef-type PGE-enriched zone is found in the border zone between the ultramafic and mafic zones and has an average thickness of 25 m with 150–250 ppb of Pt + Pd + Au. Offset-type metal distribution and high sulfide tenor (50–300 ppm Pd) and R-factor (105) suggest reef formation by sulfide saturation induced by fractional crystallization. The reef-forming process was probably interrupted by influx of magma related to the first reversal. Metal ratios suggest that this replenishing magma was PGE-depleted before emplacement. [ABSTRACT FROM AUTHOR]

The Kiviniemi mafic intrusion, near the eastern margin of the Paleoproterozoic Central Finland Granitoid Complex, is both spatially and temporally associated with post-kinematic Fe-Ti-P-enriched Svecofennian orogenic mafic magmatism. The main rock types in this small (~ 15 ha) intrusion are garnet-bearing fayalite ferrodiorite, leucoferrodiorite, ferromonzodiorite, and pyroxene diorite. The garnet-bearing fayalite ferrodiorite and leucoferrodiorite contain 50–281 ppm Sc, 275–5600 ppm Zr, and 58–189 ppm Y (n = 42), delineating a mineralized deposit some 2.5 ha in extent. Overall, these rocks show an evolved (iron-enriched) tholeiitic character; low values of Ni (< 20–40 ppm), Cr (< 20 ppm), and Cu (< 20–80 ppm); and high contents of Zn (213–700 ppm). The rock-forming minerals in the ferrodioritic rocks are (ferro)hedenbergite, plagioclase (~ An40), ferropargasite and ferroedenite, almandine garnet, and fayalite (Fo1–4). Accessory minerals include zircon, ilmenite, fluorapatite, biotite, pyrite, pyrrhotite, potassium feldspar, grunerite, and clinoferrosilite. Some relict cumulate textures have been preserved, but primary magmatic features have largely been overprinted by strong recrystallization and corona formation. The main carriers of Sc are amphibole, clinopyroxene, and apatite. The remarkably strong enrichment of Sc in ferromagnesian silicates and apatite, rather than in specific Sc-minerals, implies magmatic enrichment. Post-kinematic mafic intrusions in central Finland constitute a bimodal association with co-existing granitoid counterparts. The Kiviniemi mafic intrusion is associated with a coarse megacrystic granite and the two rock type display mingled contacts, indicative of contemporaneity of the two magmas. This conclusion is in accord with the coincident U-Pb zircon ages for the ferrodiorite, at 1857 ± 2 Ma (multigrain ID-TIMS) and the megacrystic granite, at 1860 ± 7 Ma (single-crystal LA-MC-ICP-MS). The initial εNd value of the ferrodiorite and the granite are + 0.1 and − 2.5, respectively. These Nd isotope compositions probably reflect a chondritic mantle source for the ferrodiorite and suggest incorporation of some Archaean crustal material into the granite in the course of magmatic evolution. The resource estimation calculated for Kiviniemi intrusion by using 40 g/t Sc cut off value is 13.4 Mt of rock with an average grade of 162.7 g/t scandium, 1726 g/t zirconium, and 81 g/t yttrium. [ABSTRACT FROM AUTHOR]

Na-metasomatic augite and aegirine-augite episyenites are hosted by subalkaline amphibole granites in the 1.644 Ga Suomenniemi rapakivi granite complex, southeastern Finland. In an examined drill core, episyenites are bordered by up to 50-cm-wide zones of Na-enriched augite-bearing granite in which alkali feldspar forms rims on plagioclase, and aggregates of augite and magnetite have formed by fluid-induced dehydration of hastingsite and biotite. In the episyenites, quartz has been partially removed, alkali feldspar (An<1Ab50Or50) and plagioclase have been partially recrystallized to granoblastic polygonal aggregates, and clinopyroxene (augite or aegirine-augite) has coarsened and been enriched in the aegirine component. Mass balance modeling implies addition of mainly Na, depletion of Si, F, Rb, Ba, Sr and 10–20% volume loss (by quartz dissolution) in the episyenitized zones in the drill core. Quartz-depletion and recrystallization may have been caused by several superimposed stages of alteration, probably related to voluminous dike- and A-type granite magmatism in the Suomenniemi area 10–15 m.y. after the emplacement of the Suomenniemi complex. Because of the coarse recrystallization textures and near-solidus recrystallization temperatures, distinguishing these fenite-like metasomatic rocks from igneous syenites is not trivial. In epizonal A-type granite complexes, high-temperature episyenites may be more common than currently thought. [ABSTRACT FROM AUTHOR]

U–Pb zircon single grain dating using the LA-MC-ICP-MS technique was utilized to determine the age of the host rocks of the Juomasuo Au–Co–Cu deposit located in the late Archean Kuusamo supracrustal belt. Even though the dated samples have diverse geochemical signatures that imply felsic volcanic and sedimentary precursors, the U–Pb data revealed heterogeneous detrital zircon populations for all samples. The host rocks are thus considered to belong to reworked sedimentary/volcano-sedimentary sequences. The maximun depositional ages of the samples fall in the 2.75–2.65 Ga age window, and no Paleoproterozoic ages were recovered in the examined zircon grains (151 analytical spots in total). In addition, a younger population (2.65–2.60 Ga) of internally featureless, BSE-pale/CL-dark zircon and zircon domains, was found in the mineralized sequences. These homogenized zircon grains resemble zircon formed in postmagmatic solid–state processes, in which zircon is recrystallized in metamorphic–hydrothermal conditions. This metamorphic-hydrothermal event most probably occurred in the original provenance area of the metasedimentary rocks. [ABSTRACT FROM AUTHOR]

The Vaasa batholith, western Finland, is a large, peraluminous granitoid pluton that crystallized at 1.88-1.87 Ga during the culmination of the Svecofennian orogeny. The batholith has gradual contacts, through metatexites and diatexites, with the enveloping metasedimentary rocks of the Bothnian Belt. We present ID-TIMS U-Pb age data on monazite from granitoids and xenoliths of the Vaasa batholith and combine these with published U-Pb zircon ages in order to shed further light on the evolution of the Vaasa batholith. The apparent monazite ages for seven of the examined samples are 1870-1863 Ma, and 1855±3 Ma for one further sample from the southern part of the batholith. Combined with pre-existing data, the monazite ages of the granitoids are 9 to 18 Ma (face values) or 3 to 9 Ma (external errors considered) younger than the U-Pb zircon crystallization ages from respective samples. Our new data suggest slow cooling for the Vaasa batholith - the closure/saturation temperature of the monazite U-Pb system was probably reached in ~10 m.y. after the crystallization of magmatic zircon in the examined rocks. [ABSTRACT FROM AUTHOR]

Abstract: In this paper we present new LA-MC–ICPMS U–Pb and Lu–Hf isotope data on zircon from three Svecofennian leucogranites of alleged S-type affinity from southern Finland: the Jaani, Perniö and Puruvesi granites. The results show that these granites were emplaced at about 1.86Ga, 1.84Ga and 1.79Ga, respectively. The Jaani and Puruvesi plutons also contain inherited zircon with Paleoproterozoic and Archean ages, whereas zircon from the Perniö pluton has a much more homogeneous (Paleoproterozoic) age distribution. The plutons show significant variation in initial Hf isotope composition: The 1.84Ga Perniö pluton has a relatively narrow range of initial ε Hf values between −2 and +4, the 1.86Ga Jaani pluton has a much wider range from −8 to +8. Igneous zircon of the 1.79Ga Puruvesi pluton is on average considerably less radiogenic, with all initial ε Hf values below chondritic (−18 to −1). The differences in the Hf isotope composition suggest that significantly different (metaigneous to metasedimentary) source rocks contributed to the melts that produced these granites, and thus they should not be considered exclusively S-type granites. [Copyright &y& Elsevier]

The Svecofennian domain comprises a c. 800 km by 800 km area in Finland and Sweden, mainly composed of Palaeoproterozoic granitoids, schists and gneisses. In the central part of the domain in western Finland, the Vaasa complex hosts a sequence of high-grade metamorphic rocks and granites formed by crustal anatexis during the culmination of the ∼1.9 Ga Svecofennian orogeny. The Vaasa complex consists of a c. 8000 km 2 granitoid batholith surrounded by diatexitic and metatexitic migmatites as well as metasedimentary rocks. In order to enhance the perception of the crystallisation history, provenance, and maximum sedimentation age of the protoliths involved, nine granitoid-xenolith pairs and a palaeosome-neosome pair were collected from the main body of the Vaasa batholith and surrounding rocks, and zircon from them was U–Pb dated using the LA-MC-ICPMS method. The U–Pb zircon crystallisation ages of the granitoids are 1.88–1.87 Ga, which is in accordance with the few previous results from the Vaasa region, and confirms a relatively swift crystallisation of the granite core throughout the Finnish part of the complex. The samples from the central part of the batholith have younger zircon ages than those from batholith margin. The inherited zircon age distribution pattern of the granitoids resembles that of the metasedimentary xenoliths. Our new comprehensive data, combined with whole-rock geochemistry from previous studies, render an in situ origin of the Vaasa granite probable. The maximum sedimentation age of the supracrustal precursors of the Vaasa complex is 1.92 Ga. Some of the metasedimentary xenoliths from the Vaasa complex host zircon that is coeval with their host granitoids and register incipient melting. Based on these ∼1.88–1.86 Ga zircon grains, the maximum sedimentation age would be deemed too young. Therefore, our data calls for caution when determining maximum sedimentation ages of high-grade metasedimentary rocks. [ABSTRACT FROM AUTHOR]