We present improved age constraints for the world-class Zinkgruvan Zn-Pb-Ag and Cu deposit: one of the largest Zn deposits of the Fennoscandian shield, and one of the earliest large, basin-hosted Zn deposits that formed from oxidized saline brines. Secondary Ion Mass Spectrometry (SIMS) U-Pb dating on zircon is used to constrain at least two phases of c. 1.9 Ga volcanism in the Zinkgruvan area, separated by a period of fluvial sedimentation, all of which predated formation of the stratiform Zn-Pb-Ag mineralization. A 1908 & PLUSMN; 4 Ma age for a rhyolitic rock of the first volcanic phase is the oldest recorded U-Pb zircon age of a volcanic rock in the Bergslagen lithotectonic unit (BLU) where Zinkgruvan is located. Similarly, two identical ages of 1898 & PLUSMN; 5 Ma for volcanic rocks belonging to the second volcanic phase indicate that the local volcanic activity, which formed the stratigraphic footwall, ended earlier in the Zinkgruvan area than in other parts of the BLU, where intense explosive felsic volcanic and intrusive activity until c. 1891 Ma has been demonstrated. This, along with a hybrid siliciclasticvolcaniclastic (tuffitic) character of the Zinkgruvan ore host, confirms earlier interpretations that the Zinkgruvan deposit formed in an actively subsiding basin, distal to active volcanic centers in the BLU in the time range 1.90-1.89 Ga. Our results support models suggesting that basinal brine-driven hydrothermal systems in sedimentary basins distal to volcanic centers could form world-class Zn deposits as early as c. 1.90 Ga.

Malingen is the 0.7km wide minor crater associated to the 10 times larger Lockne crater in the unique Lockne-Malingen doublet. The craters formed at 458Ma by the impact of a binary asteroid related to the well-known 470Ma Main Belt breakup event responsible for a large number of Ordovician craters and fossil meteorites. The binary asteroid struck a target sequence including similar to 500m of sea water, similar to 80m of limestone, similar to 30m of dark mud, and a peneplainized Precambrian crystalline basement. Although the Lockne crater has been extensively studied by core drillings and geophysics, little is known about the subsurface morphology of Malingen. We performed magnetic susceptibility and remanence, as well as density, measurements combined with gravity, and magnetic field surveys over the crater and its close vicinity as a base for forward magnetic and gravity modeling. The interior of the crater shows a general magnetic low of 90-100nT broken by a clustered set of high-amplitude, short wavelength anomalies caused by bodies of mafic rock in the target below the crater and as allogenic blocks in the crater infill. The gravity shows a general -1.4mgal anomaly over the crater caused by low-density breccia infill and fractured crystalline rocks below the crater floor. The modeling also revealed a slightly asymmetrical shape of the crater that together with the irregular ejecta distribution supports an oblique impact from the east, which is consistent with the direction of impact suggested for the Lockne crater.

Here, we present a compilation of 44 metamorphic pressure-temperature (P-T) estimates from 31 localities in the Svecofennian province of eastern and central Sweden. Based on these P-T estimates, which were obtained using the average P-T method of the computer programme THERMOCALC, we calculated an apparent metamorphic field gradient of 54 +/- 4 degrees C/km for the Svecofennian province. This is typical for low-medium P/T (Buchan) metamorphism and supports tectonic models that imply Svecofennian crustal growth by accretion of volcanic arc systems. In general, estimated P and T conditions range from 0.2 to 0.6 GPa and from 400 to 800 degrees C, respectively; i.e., from greenschist to granulite facies conditions. Metamorphic grade is generally higher, reaching upper amphibolite or granulite facies in northern and southwestern parts of the Svecofennian province, whereas metamorphism in Bergslagen was at greenschist to lower amphibolite facies conditions. The higher metamorphic temperatures recorded by rocks in the southwestern part of the province might relate to magmatic activity associated with the Transscandinavian Igneous Belt (TIB). Higher pressure, epidote amphibolite facies metamorphic conditions in the western part of the province probably reflect Sveconorwegian overprinting. Finally, local upper amphibolite and granulite facies conditions probably reflect contact metamorphism.

Ulfanderssonite-(Ce) is a new mineral (IMA 2016-107) from the long-abandoned Malmkarra iron mine, one of the Bastnas-type Fe-rare earth element ( REE ) deposits in the Bergslagen ore region, central Sweden. It is named for Ulf B. Andersson, a Swedish geologist and petrologist. In the type specimen, the mineral occurs with vastmanlandite-(Ce), bastnasite-(Ce), phlogopite, talc, magnetite, pyrite, fluorbritholite-(Ce) and scheelite. Ulfanderssonite-(Ce) forms pinkish, translucent subhedral grains, 100–300 μm, in aggregates up to 2 mm. Fracture is uneven, and there is an indistinct cleavage parallel (001). Mohs’ hardness is 5–6, D calc = 4.97 g cm −3 . Optically, ulfanderssonite-(Ce) is nonpleochroic, biaxial negative, with 2 V meas = 55° and n calc = 1.82. The ideal composition is Ce 15 CaMg 2 (SiO 4 ) 10 (SiO 3 OH)(OH,F) 5 Cl 3 . Electron microprobe and LA-ICP-MS chemical analyses yielded (in wt%) La 2 O 3 11.87, Ce 2 O 3 30.98, Pr 2 O 3 3.99, Nd 2 O 3 17.14, Sm 2 O 3 2.81, Eu 2 O 3 0.18, Gd 2 O 3 1.15, Dy 2 O 3 0.30, Tb 2 O 3 0.10, Y 2 O 3 1.11, CaO 2.26, FeO 0.02, MgO 1.97, P 2 O 5 0.08, SiO 2 19.13, H 2 O calc 1.07, F 1.09, Cl 2.89, O=(F, Cl) −1.10, sum 97.04. The five strongest powder X-ray diffraction lines are [ I (%) d obs (A) ( hkl )]: 100 2.948 (−421), 47 2.923 (204), 32 2.660(−225), 26 3.524 (40-1), 25 1.7601 (6-23). Ulfanderssonite-(Ce) is monoclinic, Cm , with a = 14.1403(8), b = 10.7430(7), c = 15.498(1) A, β = 106.615(6)° and V = 2256.0(2) A 3 for Z = 2. The crystal structure has been solved by direct methods and refined to R 1 = 2.97% for 5280 observed reflections. It consists of a regular alternation of two layers, designated A and B , along the c -axis: A ( ca . 9 A thickness), with composition [(Ce 8 Ca), MgSi 7 O 22 (OH,F) 4 ] 8+ , and B ( ca . 6.5 A with composition [Ce 7 MgSi 4 O 21 (OH,F) 2 Cl 3 ] 8− ; the A layer is topologically and chemically closely related to cerite-(Ce). A FTIR spectrum shows strong absorption in the region 2850–3650 cm −1 , related to the presence of O–H stretching bands. Ulfanderssonite-(Ce) is interpreted as a primary mineral at the deposit, along with the more common fluorbritholite-(Ce), formed by a magmatic-hydrothermal fluid with REE, Si, F and Cl ion complexes reacting with dolomite marble. The presence of ulfanderssonite-(Ce) is direct evidence of a Cl-rich mineral-forming aqueous solution, normally not reflected in the composition of skarn minerals in Bastnas-type deposits.

The Bastnäs-type deposits, with mineral assemblages of Fe oxides, Ca-Mg silicates, rare earth element (REE) silicates, REE fluorocarbonates, and Cu-Fe-Mo-Bi sulfides, are associated with marble horizons in a strongly Na, K, and/or Mg altered, metavolcanic succession, over a distance of at least 80 km in a SW-NE trending zone in western Bergslagen. Two subtypes occur: (1) enriched (relative to the other type) in light REE (LREE) and Fe, exemplified by the Bastnäs and Rödbergsgruvan deposits, and (2) enriched in heavy REE (HREE), Y, Mg, Ca, and F, represented by deposits in the Norberg district. Bastnäsite hosts primary fluid H2O-CO2 inclusions with salinities of 6–29 eq. wt% CaCl2 and with total homogenization temperatures (Th tot) of ca. 300–400 °C. Subtype 2 has late-stage fluorite with fluid inclusions that show 1–16 eq. wt% NaCl and Th tot of ca. 90–150 °C. Molybdenite Re-Os ages obtained from three deposits are 1,904 ± 6, 1,863 ± 4, and 1,842 ± 4 Ma. Nd isotopic data from five different REE minerals yielded no defined isochron, but a range in εNd (1.88 Ga) of +0.2 to +1.6. The oxygen isotope values (δ18OSMOW) of dolomite and calcite from the associated REE-mineralized skarn and recrystallized carbonate assemblages lie in the range 6.1–8.6 ‰, overlapping with those of the host marbles. Carbon isotope values (δ13CPDB) show typical magmatic signatures of −6.7 to −4.4 ‰, while the host marbles group around ca. −2.4 ‰. The sulfur isotope (δ34SCDT) values of associated sulfides range between −10.8 and +0.2 ‰. The combined evidence suggests REE mineralization, beginning at 1.9 Ga, from mainly Svecofennian, juvenile magmatic (>400 °C) fluids carrying Si, F, Cl, S, CO2, and the REE in addition to other metals; mineralization occurred through reactions with dolomitic layers in the supracrustal units coevally with regional metasomatic alteration associated with fluid circulation through an extensive active volcano–plutonic complex.

Geochemistry and Sr-Nd isotope geochemistry of ca. 1.8 Ga, mafie intrusions of the Trans-scandina vian lgneous Belt (TIB-1) in the Fennoscandian shield were studied in southeastern Sweden. These rocks show LILE-LREE--enriched, HFSE-depleled, calc-alkaline, continental are signatures in the north, grading to slightly less enriched, oceanic affinities southward.epsilon(Nd) (1.80 )values range from +2.0 to +0.7 and Sr-87/Sr-86(Sr(1.80) from 0.7022 to 0.7029 (with one outlier a. 0 and 0.7033). without correlation to fractionation (e.g., Mg#) of crustal contamination, indicating sources that me mildly depleted. The most depleted ratios occur ill the south, trending with the geochemistry toward more enriched compositions northward. The sources represent depleted mantle wedge material that Was sub jected to enrichment not long before (T-DM ca. 2.0 Ga). i.e.. during the preceding are subduction (2. 1-1.82 Ga). and/or (luring the TIB-I magmatism itself, 1) ' y 11 V11yd fluids With I sediment and/or melt input increasing northward. The TIB-1 magmatism occurred above a south(west) ll'I'etlretreating subduction zone along the continental margin of the juvenile Svecofennian continent at 1.81-1.76 Ga.

Re–Os dating of molybdenite from small deposits is used to define crustal domains exhibiting ductile versus brittle behaviour during gravitational collapse of the Sveconorwegian orogen in SW Scandinavia. A 1019±3 Ma planar quartz vein defines a minimum age for brittle behaviour in central Telemark. In Rogaland–Vest Agder, molybdenite associated with deformed quartz and pegmatite veins formed between 982±3 and 947±3 Ma in the amphibolite-facies domain (three deposits) and between 953±3 and 931±3 Ma west of the clinopyroxene-in isograd (two deposits) in the vicinity of the 0.93–0.92 Ga Rogaland anorthosite complex. The data constrain the last increment of ductile deformation to be younger than 0.95 and 0.93 Ga in these two metamorphic zones, respectively. Molybdenite is the product of an equilibrium between biotite, oxide and sulfide minerals and a fluid or hydrated melt phase, after the peak of 1.03–0.97 Ga regional metamorphism. Molybdenite precipitation is locally episodic. A model for gravitational collapse of the Sveconorwegian orogen controlled by lithospheric extension after 0.97 Ga is proposed. In the west of the orogen, the Rogaland–Vest Agder sector is interpreted as a large shallow gneiss dome, formed slowly in two stages in a warm and structurally weak crust. The first stage at 0.96–0.93 Ga was associated with intrusion of the post-collisional hornblende–biotite granite suite. The second stage at 0.93–0.92 Ga, restricted to the southwesternmost area, was associated with intrusion of the anorthosite–mangerite–charnockite suite. Most of the central part of the orogen was already situated in the brittle upper crust well before 0.97 Ga, and did not undergo significant exhumation during collapse. In the east of the orogen, situated against the colder cratonic foreland, exhumation of high-grade rocks of the Eastern Segment occurred between 0.97 and 0.95 Ga, and included preservation of high-pressure rocks but no plutonism.

Abstract The Oskarshamn-Jönköping Belt in southeastern Sweden is a geographically well-defined area comprising calc-alkaline intrusions and volcanic rocks together with units of coarse-grained clastic metasedimentary rocks. Deviating from the general composition is the Fröderyd Group with basalts of MORB character. The belt is surrounded by the 1.81-1.77 Ga Transscandinavian Igneous Belt. A conglomerate clast from the central part of the Oskarshamn-Jönköping Belt yielded a U—Pb zircon age of 1829±8. This age confirms the c. 1.83-1.82 Ga formation age of the belt. Sm—Nd whole rock analyses of various rock types throughout the Oskarshamn-Jönköping Belt show that no substantially older (>100 m.y.) continental material contributed to these rocks. Furthermore, the high positive εNd values of many of the analysed rocks points to the depleted mantle as the main component of their source. The Sm—Nd analyses also suggest that while some of the felsic units were formed by a high degree of magmatic fractionation from a mantle derived melt others were formed as the result of remobilisation of older Svecofennian crust. The new U—Pb data together with previously published ages imply that the formation of the Oskarshamn-Jönköping Belt was a rather quick process, possibly completed in c. 10 m.y. The most likely model of genesis for the area, based on the new U—Pb and Sm—Nd data, is formation at a continental margin subduction zone to the present south-west of a slightly older Svecofennian continent. In such a model the Fröderyd Group represents either a fore-arc setting or a back-arc rift.

The timing of continental building in the Sveconorwegian orogen of SW Scandinavia is evaluated with zircon U-Pb geochronclogy. ID-TIMS, LA-ICPMS and SIMS data are reported for 21 samples of orthogneiss, metarhyolite and metasandstone in S Norway, with emphasis on the Suldal area. The Sveconorwegian orogen is divided into a reworked Fennoscandian 1.80-1.64 Ga parautochthonous segment, the Eastern Segment, and two allochthonous terranes. The Idefjorden terrane is interpreted as a composite 1.66-1.52 Ga arc formed at the margin or near the margin of Fermoscandia. The western terrane, including the Telemark, Hardangervidda, Suldal and Rogaland-Vest Agder sectors, is named Telemarkia. U-Pb zircon data indicate that Telemarkia was built during a short magmatic event between 1.52 and 1.48 Ga, and was located at the margin of a Palaeoproterozoic craton, possibly Fermoscandia. No basement older than 1.5 Ga can be positively identified. In the early stage of the Sveconorwegian orogeny, Telemarkia collided with the Idefiorden terrane. The Bamble-Kongsberg sector, characterized by a mixed lithology and 1.13-1.10 Ga early-Sveconorwegian high-grade metamorphism, is interpreted as the original collision zone between these terranes.

Clinopyroxenes from pyroxenite, ijolite and nepheline syenite from the main intrusion of the Alnö complex define two sub-parallel compositional trends with respect to Na, Ca and FeTOT plotted against alkali-pyroxene fractionation index (Na–Mg). Both trends define a smooth fractionation of increasing Na and FeTOT and decreasing Ca with increasing Na–Mg, but one set of samples contain clinopyroxenes that constantly plot at higher Na and lower FeTOT and Ca (at similar Na–Mg) than the rest of the samples. Clinopyroxenes with higher Ca and FeTOT and lower Na (trend 1) co-exist with substantial amounts of Ti-andradite (up to 70 vol.%), while the sample set defining the more Na-rich trend (trend 2) lack co-existing Ti-andradite. Clinopyroxenes from both trends show fractionated REE patterns with a distinct difference in HREE content, reflecting the content of co-existing Ti-andradite. The rocks of the first Ti-andradite-bearing trend crystallized slightly prior to the rocks of the second trend, probably from a primitive, Ca- and Ti-rich nephelinitic magma. Crystallisation of pyroxenite and melteigite occurred under low aSiO2 and high aCaO and aTiO2 as evidenced by the presence of perovskite and sometimes substantial amounts of magnetite. Subsequent increase in aSiO2 is evidenced in the overgrowth of perovskite by titanite, which in turn is overgrown by Ti-andradite. Nepheline syenitic residuals crystallized under higher aSiO2 and aNa2O and lower aCaO and aTiO2, which reduced Ti-andradite into an accessory phase and produced more Si- and Na-rich clinopyroxenes. Some of these residuals probably also mixed with new primitive magma producing a hybrid magma that crystallised the more Na-rich and Ca- and FeTOT-poor clinopyroxenes of trend 2. The complete lack of Ti-andradite in these rocks indicates different crystallisation conditions and also a different magma composition.

The Sveconorwegian and Grenville orogenic belts display widespread 1.19–1.13Ga Early Grenvillian continental magmatism including A-type granitoids. In the Sveconorwegian province, S Norway, bimodal 1.17–1.14Ga metavolcanic rocks of the Telemark sector are part of this magmatism. Volcanic rocks in low- to medium-metamorphic grade are interlayered with immature and locally conglomeratic clastic metasediments and covered by a thick metasedimentary sequence. Minor unconformities are reported. New zircon U–Pb data are presented and integrated in a revised stratigraphy of the Telemark supracrustal rocks. A metarhyodacite at the base of the Nore group yields a crystallisation age of 1169±9Ma and displays 1.7–1.5Ga inherited zircon grains (SIMS data). A metarhyolite situated below sandstone of the Heddal group yields a crystallisation age of 1159±8Ma. In the cover sequence, a metasandstone of the Heddal group has detrital zircon grains in the intervals 2.86–2.41 and 1.94–1.11Ga (34 analysed grains) and a metasandstone of the Kalhovd formation in the intervals 2.85–2.74 and 2.00–1.05Ga (41 analysed grains). These metasediments were deposited after 1121±15Ma and 1065±11Ma, respectively and were transformed by 1.01Ga Late Sveconorwegian deformation and metamorphism. The metasedimentary rocks contain a significant amount of regionally derived clasts. Two deformed A-type granite metaplutons yield zircon U–Pb intrusion ages of 1146±5Ma (Eiddal) and 1153±2Ma (Haglebu, ID–TIMS data). The 1.19–1.13Ga magmatism is distributed in the western part of the Sveconorwegian province, in the Telemark, Bamble and Rogaland–Vest Agder sectors, indicating that these sectors were part of a single plate at that time, which is characterised by a thin lithosphere today. The A-type geochemical signature of the felsic magmatism and the continental lithosphere signature of the associated mafic volcanism point to a continental non-compressional tectonic regime. The overlap in time between widespread 1.19–1.13Ga continental magmatism, intermontane basin formation and Early Sveconorwegian 1.15–1.12Ga granulite-facies metamorphism recorded in the Bamble sector suggest a thermal pulse linked to upflow of asthenospheric mantle. Deposition of the cover of clastic sediments between 1.12 and 1.01Ga possibly reflects thermal subsidence after the 1.19–1.13Ga event and before the Late Sveconorwegian (1.03–0.95Ga) orogenic phases. An analogy between the 1.19–1.13Ga evolution of the Sveconorwegian province and the Cenozoic formation of the Basin and Range province in USA is discussed.

Ferriallanite-(Ce), previously reported as "cerine" or iron-rich "allanite", is the most common lanthanide mineral next to cerite-(Ce) at the Bastnas Fe-Cu-REE deposit, Skinnskatteberg, Vastmanland, Sweden. It is closely associated with cerite-(Ce), bastnasite-(Ce), bastnasite-(La), tomebohmite-(Ce), quartz, fluocerite-(Ce) and sulfide minerals. The Fe2O3 and FeO contents of ferriallanite-(Ce) are in the ranges 12.4-15.4 and 7.5-11.8 wt.%, respectively, and the dominant chemical variations are related to Fe2+ <----> Mg and Cc <----> La substitutions. The concentrations of Th and U are extremely low, <3 ppm. A complete, averaged structural chemical formula for one sample, with unit-cell parameters a 8.941(4), b 5.799(2), c 10.151(6) Angstrom, beta 114.7(6)degrees, V 478.2(5) Angstrom(3), is (A)[(Ce0.51La0.41,Nd0.10Pr0.03Y0.01SM0.01)(Sigma1.07)Ca-0.95] (M3)[Fe0.792+Mg0.21] (M2)[Al0.13Fe0.173+] (M1)[Fe0.923+Mg0.08] (T)[Si2.96Al0.04]O-12(OH0.90F0.10), based on electron-microprobe data and Mossbauer spectroscopy. The Bastnas samples are distinct from the type ferriallanite-(Ce) from Ulyn Khuren, Mongolia, mainly in their higher REE and Mg contents.

Recent models suggest that Laurentia and Baltica were contiguous during the Mesoproterozoic and shared a long-lived active continental margin, subsequently reworked during the Grenvillian orogeny. Around 1.25 Ga, the geological record is dominated by dyke-swarm intrusion, continental rift basin formation, A-type felsic magmatism, and arc – back-arc basin development. It points to a dominantly extensional tectonic regime over most of the craton and the Grenvillian margin, suggesting a retreating subduction boundary at that time. In the westernmost allochthonous domain of the Sveconorwegian Orogen, southern Norway, the Sæsvatn–Valldal supracrustal sequences are interpreted as rift or pull-apart basins. They formed at and after 1.27 Ga, in a continental setting, at the margin of Baltica. This interpretation is based on geological, geochemical, and new secondary ion mass spectrometry (SIMS) zircon U–Pb data. A subvolcanic quartz porphyry at the base of the Sæsvatn sequence yields a 1275 ± 8 Ma intrusion age. Metarhyolite samples in the lower part of the sequences yield equivalent extrusion ages of 1264 ± 4 Ma (Sæsvatn sequence) and 1260 ± 8 Ma (Valldal sequence). The metarhyolite units are overlain by sequences of metabasalt and metasandstone. An angular unconformity between the metarhyolites and overlying rocks is locally observed and possibly reflects rift tectonics during formation of the basin. A sample of arkosic metasandstone at the top of the exposed Sæsvatn sequence yields a few Archaean detrital zircon grains and a large spectrum of 2.2–1.2 Ga Proterozoic grains. These data point to a varied continental provenance and constrain sedimentation to later than 1211 ± 18 Ma.

There are several benefits which would result from the development of an in situ analytical technique for ultra trace elemental analysis of quartz, including rapid screening of possible high-purity quartz resources, by eliminating the need to remove solid and liquid inclusions by expensive dressing techniques prior to chemical analysis of structural impurities. Information on the petrogenetic history of the quartz can also be obtained from the distribution of trace elements. The main purpose of this paper is to describe an analytical method for estimating the concentrations of structural bounded trace elements in quartz.A double focusing sector field inductively coupled plasma mass spectrometry (ICP-MS, Finnigan MAT model ELEMENT), with the CD-1 Guard Electrode and a 266-nm UV laser ablation system was used in the development of the method for in situ analysis of quartz.The following elements are included in the analytical protocol: Al, Ba, Be, Cr, Fe, Ge, K, Li, Mg, Mn, Pb, Rb, Sr, Th, Ti, U. Analyses were carried out in low mass resolution (m/Deltam = 300), except for Mg, Ti, Cr, Fe (medium mass resolution, MR, m/Deltam approximate to 3500) and K (high mass resolution, HR, m/Deltam > 8000). The isotope Si-29 was used as an internal standard at low resolution, and Si-30 at medium and high resolution. External calibration was done by using the international reference materials: NIST 612, NIST 614, NIST 616, NIST 1830 from the National Institute of Standards and Technology (NIST), BCS 313/1 from the Bureau of Analysed Samples (BAS), the rhyolite RGM-1 reference sample from the United States Geological Survey (USGS), Reston and the certified reference material "pure Substance No. P silicon dioxide SiO2 from the Federal Institute for Material Research and Testing, Berlin, Germany (BAM). Because of the absence of an SiO2 blank, the BAM no. I SiO2, was used for the estimation of detection limits. Detection limits for most of the elements are between 0.2 and 0.01 mug g(-1.) Analysis time and laser spot size were adjusted so that the raster did not exceed 300 X 300 mum on a 200-mum-thick section. New data for the international reference materials BCS 313/1 (BAS) and NIST 1830 and the standards BR-KI and BR-FR2 are reported. To improve the lower limit of quantification and analytical uncertainty at low concentrations, it is important to have calibration curves with well defined intercepts. This can be achieved by the use of certified standards, with trace element concentrations lower than the BAM no.1 SiO2 or a sample blank. (C) 2002 Elsevier Science B.V. All rights reserved.

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-km2 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 (1σ) 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 (1σ) 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.