AGE OF THE TOKKALEHTO METAGABBRO AND ITS ...volcanic cover of the late Archean basement of the...

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AGE OF THE TOKKALEHTO METAGABBRO AND ITS SIGNIFICANCE TOTHE LITHOSTRATIGRAPHY OF THE EARLY PROTEROZOIC KUUSAMO

SUPRACRUSTAL BELT, NORTHERN FINLAND.

PAUL M. EVINS and KAUKO LAAJOKI

EVINS, PAUL and LAAJOKI, KAUKO 2001. Age of the Tokkalehto metagab-bro and its significance to the lithostratigraphy of the early Proterozoic Kuusa-mo supracrustal belt, northern Finland. Bulletin of the Geological Society of Fin-land 73, Parts 1–2, 5–15.

The c. 30-km2 Tokkalehto metagabbro and associated sills and dykes intrudea large part of the western part of the early Proterozoic Kuusamo supracrustalbelt in northern Finland. These rocks have previously been mapped as a meta-volcanic unit, Greenstone Formation III, the type area of which occurs furthereast in the Kuusamo supracrustal belt. However, observations along the con-tacts of this unit (contact breccias, distinctive skarns and xenoliths) and withinthis unit (igneous layering and mafic enclaves) indicate that these rocks are in-trusive.

High resolution SIMS analysis of zircon from the metagabbro at Nurkamo-kumpu yields a crystallization age of 2216.2 ± 3.8 Ma. On the basis of this ageand the lithological and petrographical characteristics of the metagabbro, it be-longs to the c. 2.2 Ga karjalite or gabbro-wehrlite suite, which has a wide dis-tribution in eastern and northern Finland.

These results show that the strong aeromagnetic anomalies of the westernpart of the Kuusamo supracrustal belt do not represent Greenstone FormationIII metavolcanics. A subdivision of the Kuusamo supracrustal belt into easternand western halves is proposed along the Kitka Fault. Instead, these anomaliesrepresent c. 2.2 Ga gabbroic intrusions, which intruded metasediments of thewestern Kuusamo supracrustal belt.

Key words: metagabbro, intrusions, absolute age, U/Pb, magmatism, lithostratig-raphy, magnetic anomalies, Proterozoic, Tokkalehto, Kuusamo, Finland

Paul Evins and Kauko Laajoki: University of Oulu, Department of Geology,P.O. Box 3000, FIN-90401 Oulu, Finland.E-mails: [email protected], [email protected]

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6 Paul M. Evins and Kauko Laajoki

INTRODUCTION

The Kuusamo supracrustal belt is one of the keyareas where the early Proterozoic sedimentary-volcanic cover of the late Archean basement of theFennoscandian Shield can be studied. The Geo-logical Survey of Finland has mapped the belt atthe 1 : 100 000 scale (Silvennoinen 1973, 1982,1989, unpublished sheets 3544 and 4611). Silven-noinen (1972, 1991) has outlined the stratigraphyof the eastern part of the belt whereas that of thewestern part has been published only on small-scale maps (Silvennoinen et al. 1992, Korsman etal. 1997).

The type area of the Kuusamo supracrustal se-quence is the Rukatunturi area in the eastern part

of the belt. Here the sequence is over 2500 m thickand is subdivided by Silvennoinen (1972) into thir-teen formations including three metavolcanic unitsnamed Greenstone Formation I, II, and III. The c.200-m thick Greenstone Formation III is usuallystrongly magnetic and has been used as a key unitin mapping. This formation covers a large area ofthe western part of the Kuusamo supracrustal belton the small-scale maps of Silvennoinen et al.(1992) and Korsman et al. (1997). Our mapping,however, has not revealed any significant meta-volcanic unit corresponding to magnetic anoma-lies in this area. Instead, the strong positive mag-netic anomalies appear to represent a c. 30-km2

large metagabbro intrusion. This metagabbro isnamed after Tokkalehto where it is best exposed

Fig. 1. Map of the Kuusamo supracrustal belt (modified from Korsman et al. 1997) and distribution of the Tokka-lehto metagabbro (black). J = Jäkäläniemi. Area of detailed map in Fig. 3 is framed.

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7Age of the Tokkalehto metagabbro and its significance to the lithostratigraphy of...

and easily accessible by a forest road.The main purpose of this study is to describe

the intrusive characteristics of the Tokkalehtometagabbro and present its age as determined atthe NORDSIM laboratory in Stockholm. The sig-nificance of the Tokkalehto metagabbro to thelithostratigraphy of the Kuusamo supracrustal beltis then discussed.

GEOLOGICAL SETTING

The early Proterozoic Kuusamo supracrustal beltcovers a triangular region some 2500 km2 in areain northern Finland (Fig. 1). It is bordered by thelate Archean Kuhmo Basement Complex, the earlyProterozoic Salla Greenstone Belt, and the earlyProterozoic Central Lapland Granitoid Complexin the south, in the northeast, and in the northwest,respectively. The boundary with the Kuhmo com-plex is a nonconformity, the one with the Salla beltis tectonic (Manninen 1991, Ruotoistenmäki 1992,Airo 1999), and the boundary with the CentralLapland Granitoid Complex is tectonic (along thePosio Fault) and intrusive (Laajoki 1994, Kärki &Laajoki 1995, Evins et al. 1997). The Kuusamosupracrustal belt was subjected to several phasesof deformation and consequently its structure israther complex.

The stratigraphy of the Kuusamo supracrustalbelt is best known along its southeastern marginwhere the cover sequence starts with a basal brec-cia conglomerate (Fig. 2). The 2405 ± 6 Ma age(Silvennoinen 1991) of three quartz-porphyryclasts in the conglomerate gives the maximum ageof the beginning of sedimentation in Kuusamo.The overlying sequence is at least 2300 m thickand consists of three volcanic formations (Green-stone Formation I, II and III) with intervening sed-imentary units. Greenstone II and overlying silt-stone formations are cut by a 2209 ± 9 Ma (Sil-vennoinen 1991) metadiabase at Jäkäläniemi onthe Vasaraperä map sheet (Fig. 1). The 200-mthick Greenstone Formation III of massive, flood-basaltic metalavas is overlain by a 600- to 800-mthick sequence of clean quartzites with minor het-eroliths of the Rukatunturi Formation. The upper-most units, the 0- to 100-m thick Dolomite For-mation and the > 250-m thick Amphibole SchistFormation, are found only in restricted areas to thenorth.

In Posio to the west, the stratigraphy is not eas-ily resolved due to sparser outcrops, higher degreeof metamorphism, more complex folding, and sev-eral shear zones. Laajoki (2000) has establisheda tentative stratigraphy for the area that differs sig-nificantly from that of the eastern Kuusamo su-pracrustal belt (Fig. 2). The Ahola formation con-sists of sillimanite facies, turbiditic metasand-stones and metapelites deposited upon subaeriallavas of the Karkuvaara formation. The latter isusually correlated with Greenstone Formation I(Silvennoinen et al. 1992, Korsman et al. 1997).Quartzites of the Nilovaara formation overlay theAhola formation. The Nilovaara quartzites areoverlain by the distinctive Kirintökangas forma-tion, which consists of alternating members ofsandstone and heterolith of presumed tidal to shal-low-marine origin (Laajoki 1996). This formationeventually grades upwards into arkosic and mica-ceous quartzites and siltstones with sillimanite-grade mica schist interbeds in the westernmost partof the area. The Tokkalehto metagabbro intrudesthe upper part of this formation. The Himmerkin-lahti conglomerate member described by Laajoki(2000) rests unconformably atop quartzites of the

Fig. 2. Differences in lithostratigraphy between thewestern and eastern Kuusamo supracrustal belt.

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previous formation and grades into the siltstonesand quartzites of the “Uppermost quartzite”. Notethat the eastern and western parts of the Kuusa-mo belt cannot be directly correlated lithostrati-graphically with each other. They are consideredas separate tectonic units.

The Tokkalehto area lies between Posio and theeastern Kuusamo supracrustal belt (Fig. 1). Silven-noinen (1992) subdivides the supracrustal se-quence here into the Lapponia (lower) and Kare-lia supergroups. The latter includes in its lowerpart plateau basalts, which on the basis of the mapby Silvennoinen et al. (1992) are correlated withGreenstone Formation III. Korsman et al. (1997)subdivide the sequence into a lower quartzite-

arkosite-mica schist unit and upper quartzite unitseparated by a metavolcanic unit (units 60, 58, and57, respectively, on their map). The geology ofthis metavolcanic unit (here re-interpreted as ametagabbro) is described below.

GEOLOGY OF THE TOKKALEHTOMETAGABBRO

The Tokkalehto metagabbro intrudes arkose-,sericite-, fuchsite-, or ortho-quartzites and silli-manite- or garnet-muscovite-biotite schists of thewestern Kuusamo supracrustal belt. In the studyarea, the intrusive body trends northwestward,

Fig. 3. Detailed map of the main part of the Tokkalehto metagabbro (shown with its aeromagnetic signatures) inthe northwest with the location of sample # 888 and observation stations. T=Tokkalehto, N=Nurkamokumpu,L=Lauttalampi, H= Hanhilammi.

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generally parallel to layering (composite S0�/S1) inits host and to the structural grain of this part ofthe Kuusamo supracrustal belt (Fig. 3). It has beencomplexly folded by several phases of deforma-tion. Along with its host sediments, it forms alarge fold with a moderately southwest-dipping,northwest-trending axial plane. Numerous mylo-nite zones with strong southward plunging linea-tions are parallel to this axial plane and cutthrough the metagabbro. The hinge of the largefold is centered at Tokkalehto. Its northern limbfollows the road to Kuusamo and southern limbfollows the forest road to the southeast. In thenorthwestern corner of the area, near Lauttalam-pi, a series of northeast-trending shear zones jux-taposes high-grade garnetiferous schists and sil-limanite-bearing arkosites against the Tokkalehtometagabbro and metasediments of the Kuusamosupracrustal belt. These shear zones truncate themetagabbro and older, northwest-trending fabricsin the area. A northeast-trending S2 foliation is ax-ial planar to F2 folds, the largest of which is rep-resented by the deflection of the entire Tokkaleh-to metagabbro to a northeasterly direction southof the study area (Fig. 1). Near conformity be-tween the metagabbro and its host’s layering andinternal features described below imply that it wasoriginally intruded as a series of differentiatedsills.

Igneous features of the Tokkalehtometagabbro

The Tokkalehto metagabbro usually occurs as adark green to black, massive to foliated, medium-to coarse-grained amphibolite. Massive, coarse-grained varieties have a typical metagabbroic ap-pearance (containing amphibole, pyroxene andplagioclase in Fig. 4a) on both the northern (Fig.4b) and southern (Fig. 4c) limb of the intrusion.At several localities (Lauttalampi, Tokkalehto, andNurkamokumpu) the metagabbro grades into in-distinct, irregular, meter-scale regions of ultrama-fic rock. Some of these rocks may contain signif-icant amounts of sulfides as evidenced by theirrusty appearance. More felsic varieties are alsofound as c. 30-cm thick layers of diorite to

anorthosite dispersed throughout the gabbro atNurkamokumpu. These are marked by an increasein plagioclase in the rock and occasional cumu-late texture where layers can be distinguished (Fig.4d). However, in most cases distinct layers havenot formed (Fig. 4e). Instead, boundaries betweenfelsic and mafic material are irregular. These fea-tures may have formed due to magmatic differen-tiation or magma mingling. Mafic enclaves (up to5 cm) composed of polycrystalline amphibole inthe metagabbro at Tokkalehto and Nurkamokumpuprovide further evidence of magmatic differenti-ation (Fig. 4f). Fine-grained layers of amphiboliteand diopside-bearing granite and pegmatite veinsoccur within the metagabbro, usually near its con-tact with the host rock. Volcanic features, such asamygdules and lava pillows, have not been ob-served anywhere in the Tokkalehto metagabbro.All of the textures described above within theTokkalehto amphibolite demonstrate that the rockwas originally a mafic magma intruded at somedepth below the surface.

Contact relationships

Due to the sill-like geometry of the Tokkalehtometagabbro intrusion, cross cutting contacts areshallow and rarely seen in outcrop (Fig. 4c), how-ever, the intrusion isolates large blocks of the sur-rounding country rock (described below). Instead,contacts between the metagabbro and its metased-imentary host are marked by three types of contactzones. The most common type of contact zone oc-curs where metagabbro intrudes sericite quartzitecausing the growth of calc-silicate minerals, suchas diopside, predominantly in the quartzite. Thesecalc-silicate skarns are found throughout the areawithin 200 meters of the contact. When the meta-gabbro intrudes less silty metasediments such asmeta-arkose and orthoquartzite, albitization occursin the quartzite. At these contacts both the meta-gabbro and quartzite are strongly disrupted result-ing in a contact breccia. The third type of contactzone is almost completely exposed along a 200-meter transect across the northwestern boundary ofthe metagabbro along the Nurkamokumpu Road.Here the metagabbro intrudes a garnet-biotite me-

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Fig. 4. Igneous features of the Tokkalehto metagabbro. a) thin section of metagabbro sampled for U-Pb analy-sis; b) typical appearance on the northern limb, Station # 840; c) typical appearance on the southern limb wherethe metagabbro (top right) truncates layering in sericite quartzite (bottom left) at a shallow angle, Station # 784;d) cumulate layering at Nurkamokumpu, Station # 896; e) irregular boundary showing incomplete magmatic dif-ferentiation at Nurkamokumpu, Station # 899; f) mafic enclaves of polycrystalline amphibole in the metagabbroat Tokkalehto, Station # 844. Lens cap diameter is 6.5 cm.

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tapelite from a strongly deformed supracrustal pack-age that includes arkosite with pelite interbeds, sil-limanite-biotite metapelite with in situ granitic meltveins, calc-silicate gneisses, and medium-grainedmetavolcanic amphibolite with calc-silicate layers.Two hundred meters from the metagabbro, amphi-bole appears in the metapelite. Garnet clusters (ap-proximately 2 cm) begin to form in the metapeliteand grow larger as one approaches the metagabbro.The metapelite then grades into a c. 100-meter widezone of sulfide-bearing garnet-gedrite skarn withlarge (up to 20 cm) garnet clusters (Fig. 5a). Onceinside the metagabbro, several 2-meter wide me-tapelite xenoliths, composed mostly of amphibole,garnet and biotite, have been assimilated by the me-tagabbro. Further away from the contact, relictghosts of the metapelite xenoliths are representedby vague, irregular regions of cm-sized garnets andamphibole clusters in the metagabbro (Fig. 5b).These garnets and clusters eventually disappear intopure metagabbro at > 150 meters from the contact.

Several blocks of the metasedimentary wallrock are found within the Tokkalehto metagabbro.Of particular interest are the Hanhilammi, Tokka-lehto, and Lauttalampi inclusions (Fig. 3). The1000 x 300-meter, northwest-trending Hanhilam-mi inclusion consists of fuchsite quartzite, seric-ite quartzite, orthoquartzite, sillimanite rod arko-site, metapsammite, and biotite-muscovite me-tapelite. Garnet-gedrite skarn is found along partsof its contact with the metagabbro. The 100 x 50-

meter, northwest-trending Tokkalehto inclusion iscomposed of fuchsite quartzite that has beenstrongly albitized and brecciated almost beyondrecognition. The 700 x 300-meter, northeast-trend-ing Lauttalampi inclusion is cored by quartzitesurrounded by sillimanite-bearing metapelite. Thequartzite contains a 1-m wide sulfide vein. Dueto lithological similarities between the xenolithsand the nearby metasedimentary host rock to themetagabbro, we interpret these inclusions as nearlyin situ xenoliths.

U-PB GEOCHRONOLOGY

Sample

The dated sample is a lineated amphibolite froma recently blasted ditch along the NurkamokumpuRoad (X=7365.03, Y=3570.50 in the Finnish Uni-form Coordinate System) near the center of theintrusion. It is representative of the relatively ho-mogenous, massive, medium-grained amphibolitethat makes up the majority of the intrusion. It iscomposed of amphibole and often sericitized pla-gioclase with minor, relic pyroxene and quartz(Fig. 4a). Titanite is a common accessory miner-al in the rock. Amphibole and polycrystalline re-gions of plagioclase define a lineation in the sam-ple. The rock sample was large (~10 kg) and a fairamount of zircon was recovered.

Fig. 5. Characteristics of the contact between the metagabbro and surrounding metapelites. a) sulfide-bearinggarnet-gedrite skarn, Station # 752 (garnets appear light grey); b) metagabbro with garnets from a ghost of ametapelite xenolith, Station # 794. Lens cap diameter is 6.5 cm.

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Methodology

U-Pb isotopic measurements were conducted atthe NORDSIM Laboratory for Isotope Geology atthe Swedish Museum of Natural History in Stock-holm. The rock sample was crushed with a jaw-crusher and pulverized with a disk mill at the Uni-versity of Oulu. Zircon grains were separated fromthe resulting rock powder by heavy mineral en-richment on a Wilfley table and subsequent mag-netic separation on a Frantz isodynamic separa-tor and density separation using tetrabromethaneand di-iodinemethane at the Geological Museumin Oslo. The most transparent zircons with theleast fractures and inclusions under binocular mi-croscope were chosen for analyses. These werethen mounted in epoxy with the 91500 zirconstandard (Weidenbeck et al. 1995), and ground andpolished with progressively finer diamond paste.Imaging the zircon under backscatter electron andcathodoluminescence (CL) aided in identifyingpossible cores, overgrowths, and unwanted frac-tures. These images were used for the final selec-tion of spot analyses. Spots with a diameter of 30µm where analyzed with a Cameca IMS 1270 highmass-resolution, high-sensitivity ion-microprobeat the NORDSIM Laboratory following the ana-lytical procedure of Whitehouse et al. (1999). Datareduction was performed by Joakim Mansfeld atthe NORDSIM Laboratory using a modified Ex-cel spreadsheet (Whitehouse et al. 1997, 1999) andIsoplot/Ex by Ludwig (1998). Common Pb correc-tions were made assuming Stacey and Kramers’(1975) terrestrial Pb composition. Isotopic data arepresented in Table 1.

Results and interpretation

Nearly all zircons from the sample are large andprismatic (c. 200 µm long, 70 µm wide). Nonewere identified in thin section. All are remarka-bly transparent under plane light and display os-cillatory or sector zoning under CL; typical ofmagmatic growth (Pidgeon 1992) (Fig. 6). Nocores or overgrowths were visible. Analyses showa homogenous zircon population with one ex-tremely discordant outlier, together showing anupper intercept age of 2250 ± 54 Ma. Exclusionof the discordant point (analysis N514-145a in Ta-ble 1) yields a group of 9 concordant analyses. Theresulting zircon age, calculated according to Lud-wig (1998), is 2216.2 ± 3.8 Ma with a MSWD of1.6 (Fig. 7). A weighted average of the 9 concord-ant 207Pb/206Pb-ages yields an age of 2217.6 ± 7.4Ma with a MSWD of 1.8 at the 95% confidencelevel (Fig. 7). The two ages are the same withinerror. In view of the homogenous nature of theanalyzed zircon material (excluding the extreme-ly discordant analysis) the upper concordia age of

Fig. 6. Cathodoluminesence images of typical sector(left) and zoned (right) zircon from sample # 888. Num-bers refer to location of spot and number of ion-micro-probe analysis in Table 1. White bar is 0.1 mm long.

Table 1. Ion-microprobe (NORDSIM) zircon data for sample # 888. Errors shown for 207Pb/206Pb ratios are thegreater of counting vs. observed errors. Errors in Pb/U include a generally dominant factor propagated fromthe standard measurements.

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2216.2 ± 3.8 Ma is interpreted to represent thecrystallization age of the Tokkalehto metagabbro.

LITHOSTRATIGRAPHIC CONSEQUENCES

The contact relationships between the Tokkalehtometagabbro and its surroundings as well as mag-matic features within the intrusion described aboveindicate that the mafic rocks in the western Kuu-samo supracrustal belt around Tokkalehto are in-trusive. Consequently, these rocks cannot be cor-related with Greenstone Formation III in the Ru-katunturi type locality where Silvennoinen (1972,1991) describes it as clearly volcanic with abun-dant amygdules and other volcanic features. Theonly similarity between the Tokkalehto metagab-bro and Greenstone Formation III is their highaeromagnetic signatures.

The 2216 ± 3.8 Ma age of the Tokkalehto me-tagabbro is within error of the 2209 ± 9 Ma age(Silvennoinen 1991) reported for a metadiabasedirectly adjacent to the southern continuance of theTokkalehto metagabbro (previously shown asGreenstone Formation III) at Jäkäläniemi to thesouth of the Tokkalehto area (Fig. 1). Their equalage and spatial continuity imply that they are partof the same intrusive body. The Tokkalehto me-tagabbro may, however, represent a hypabyssalfacies of the Greenstone Formation III. Rocks thatare undoubtedly volcanic and part of GreenstoneFormation III seem to be restricted to the area eastof Kitka Lake (Fig. 1).

Because of the absence of Greenstone Forma-tion III and other substantial differences in stratig-raphy (Fig. 2), we propose that a major tectonicboundary, the Kitka Fault, lies along Kitka Lake(Fig. 1). West of the Kitka Fault, positive aero-magnetic anomalies do not represent GreenstoneIII and therefore cannot be used as stratigraphicmarkers. This implies that the metasedimentsfound in the center of the Tokkalehto metagabbromay not be part of the younger Rukatunturi For-mation as represented on maps by Silvennoinenet al. (1992) and Korsman et al. (1997), but morelikely represent older strata, possibly of Silven-noinen’s (1972) Sericite Quartzite Formation.

Observations of major lithological and sequentialdiscrepancies between the sediments in this areaand those of the Rukatunturi Formation supportthis conclusion and will be discussed in a laterpaper. The elimination of the Rukatunturi andGreenstone III Formations from the area west ofthe suggested Kitka Fault implies that the Kuusa-mo supracrustal belt has been split into an east-ern half with the complete stratigraphy describedby Silvennoinen (1972, 1981) and a western halfthat may be correlative either to Silvennoinen’s(1972) early Proterozoic Sericite Quartzite andGreenstone I Formations or to the Archean Cen-tral Puolanka Group (Kontinen et al. 1995) to thesouth. Parts of the Kuusamo supracrustal beltnorth of the Tokkalehto area have not beenmapped in detail, and it is possible that some ofthe metavolcanic units corresponding to positivemagnetic anomalies outlined on small-scale maps(Silvennoinen et al. 1992, Korsman et al. 1997)west and southwest of the Salla Belt represent the2.2 Ga metagabbro suite called karjalites (Vuollo& Piirainen 1992) or the gabbro-wehrlite associ-ation (Hanski 1986) that are distributed through-out eastern and northern Finland (Vuollo 1994 andreferences therein). The authors emphasize that

Fig. 7. U-Pb concordia and weighted mean diagramsof zircon data for sample # 888. Error ellipses repre-sent 1σ on the concordia age diagram. In the lowerright of this diagram is the weighted mean of 207Pb/206Pb-ages with 2σ error bars.

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these magnetic anomalies should not be used asstratigraphic markers.

CONCLUSIONS

Aeromagnetic anomalies in the western part of theKuusamo supracrustal belt previously represent-ed as metavolcanics of Greenstone Formation IIIare, instead, differentiated gabbroic intrusions, thelargest of which is the Tokkalehto metagabbro.These intrusions crystallized at 2216.2 ± 3.8 Ma.On the basis of its age and lithological character-istics, the Tokkalehto metagabbro belongs to thec. 2.2. Ga karjalite or gabbro-wehrlite mafic suitefound dispersed throughout eastern and northernFinland.

ACKNOWLEDGMENTS. Our deepest gratitudegoes out to Niina Ahtonen who carried out someof the analyses at NORDSIM. Joakim Mansfeldperformed the data reduction. Torbjörn Sundemounted the zircon and prepared the epoxy puckfor analysis. His help with all of the analyticaldevices at the NORDSIM laboratory was muchappreciated. Comments from Jouni Vuollo, Tor-björn Skiöld, Yrjö Kähkönen and Martin White-house were welcome to the manuscript. This studywas financially supported by the Academy of Fin-land and the Thule Institute, University of Oulu.NORDSIM contribution # 49.

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AGE OF THE TOKKALEHTO METAGABBRO AND ITS ...volcanic cover of the late Archean basement of the...

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