ON THE AGES OF SOME GRANITOID AND SCHIST COMPLEXES IN NORTHERN FINLAND

RAIMO LAUERMA

LAUERMA, RAIMO 1982: On the ages of some granitoid and schist complexes in northern Finland. Bull. Geol. Soc. Finland 54, 1 -2, 85-100.

Contact observations and most radiometric datings indicate that the large complex of granitoid rocks and migmatites in Central Lapland is late Svecokarelian (Proterozoic) in age. The geological setting of the complex suggests, however, that it previously formed the basement for the Proterozoic sedimentation and a source of some of the sedimentary material. About 1800 Ma ago it apparently under- went partial anatexis, especially near the Proterozoic schist belts. The concept that some parts of the schist areas of Kuusamo, Salla and Central Lapland are of Archean age is disputable. The Salla schist area, which some authors maintain is an Archean greenstone belt, is obviously Proterozoic in age. U-Pb ages reported for zircons, titanites and monazites from granitoid rocks from six localities in Kuusamo, Salla, Kemijärvi and Rovaniemi areas range from 2738 Ma to 1770 Ma.

Raimo Lauerma, Geological Survey of Finland, SF-02150 Espoo 15, Finland.

Introduction rocks and migmatites were subjected to The present-day concept of the major reconnaissance only, whereas localities of features of the Precambrian bedrock in great geological importance, especially with Finland is based mainly on the general regard to the age relations of the rocks, were geological mapping at a scale of 1 : 400 000 investigated in detail and often mapped on a started in the 1890s and completed in 1980. large scale in the field. The reasons for a map of this type and the According to Sederholm's original plans underlying principles and goals are sum- (op.cit.), about 10 000 km2 should be mapped marized by Sederholm (1903). Investigations every year in southern Finland and at least were usually started in areas where the effects 5000 km2 in northern Finland. The areas of of the geological phenomena were more the map sheets B 6 Ylitornio, C 6 Rovaniemi readily explicable. The results from these (Hackman 1910 a, b) and D 6 Kuolajärvi investigations were then applied to more (Hackman & Wilkman 1925) totalling some complicated areas. Large, geologically mo- 40 000 km2, were investigated at a rate ap- notonous areas composed mainly of granitoid proaching that recommended by Sederholm. 86 Raimo Lauerma

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Owing to the limited man-power assigned to by U-Pb methods on zircons and titanites, this task, usually one or two geologists and a however, most of these granites are nowdays few field assistants, the inferiority of the field regarded as Archean (e.g. Simonen 1971, maps and the difficulty of working in large 1980 a, b). nearly unpopulated areas, these maps leave much to be desired. Hackman (1927, p. 97) proposed new investigations in some areas Age relations and mineral ages in the covered by the maps that he himself had Kuusamo area compiled and Mikkola (1941, p. 7) considered Neither granitic veins intruding the schists that all of the maps should be revised and nor basal conglomerates had been found on reprinted. the southern border of the Kuusamo schist The remapping of the Precambrian bed- area (Fig. 1) before the remapping of the rock in northern Finland was started in the area and, therefore, the contact between the early 1960s, this time at a scale of 1 : 100 000. underlying gneissose basement and the So far this mapping is limited to parts of the schists was supposed to be tectonic (Väyry- schist areas of Peräpohja, Kuusamo, Salla nen 1954, p. 191; Simonen 1964, p. 68). In the and Central Lapland (Fig. 1) and their im- late 1960s several occurrences of basal con- mediate environment. Consequently, knowl- glomerates were found along the contact. edge of the geology of these areas has been They have been described by Silvennoinen much enhanced in the last two decades, but it (1972). has remained nearly unchanged, part of it Samples for age determinations were taken since 1910, in the central parts of areas com- from a reddish, homogeneous granite and posed mainly of granitoid rocks and mig- two pegmatite dykes in the basement com- matites. plex near Soilu, about 10 km south of the Sederholm (1892) gave detailed instructions Kuusamo schist area (Fig. 1). Field observa- as to how the field observations for the tions indicate that these samples represent general geological mapping were to be the youngest granitic rocks in this area. carried out. Most rocks, especially plutonic The U-Pb age determinations reported in rocks and schists, were to be placed in one of the present paper were all made at the Geo- two categories, older or younger. In addition logical Survey of Finland. The ages were to contact relations the main criterion for calculated with the decay constants given by the age of rocks was the degree of metamor- Jaffey et al. (1971). Some of the age data phism and granitization. reported here have been published pre- The consequences of this mapping method viously in Annual Reports of the Geological in northern Finland are still visible in Sheet Survey of Finland (GSF, Ann. Rep.) using 2 of the Carte Géologique Internationale de older constants (until 1972) and/or without l'Europe (1966). On this map most of the calculated diffusion model ages. »young-looking», i.e., massive, mostly red- Analytical data on zircons from the Soilu dish non-cataclastic granitoid rocks in the area are given in Table 1; a concordia diagram Archean basement complexes south of the is given in Fig. 2. Their ages, 2738 Ma and schist areas of Peräpohja and Kuusamo and 2712 Ma, agree well with the ages obtained northeast of the schist area of Central Lap- for the granitoid rocks surrounding and land (Fig. 1) are designated as late Karelian intruding the greenstone belts of Suomus- (i.e. Proterozoic) granites. On the basis of salmi and Kuhmo (e.g. Kouvo & Tilton 1966, radiometric age determinations, mainly made Geological Survey of Finland, Petrological 88 Raimo Lauerma CM ^ ^ CO CO CO [> tn CO CO CO sr co +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 CM m oi io t IO O CM CO sr CO CD -H O) CO CO C- © sr Oi o r- m OO CMO ctno Osti IC O- CM D— LO CO CO Oi Oi CO Oi o Oi Oi O o Oi 00 r- o CS] CM C\> CM rt CM CM CM —' CM

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r-t C- CO Department. Reports on activities in years +1 +1 +1 a ^ o 1980 and 1981, Hyppönen 1982). © M © t- r- co Age relations of rocks in the Peräpohja area rt > .. looking» granitic rocks southeast of the co rocks (e.g. Härme 1949; Kouvo & Tilton 1966, CO pp. 438-439; Kujanpää 1971) may be attrib- 90 Raimo Lauerma

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Fig. 2. U-Pb concordia plot for zircons from the Archean basement complex near Soilu, Kuusamo. Map sheet 4522 04. Sample A415 is from a reddish, homogeneous granite by road No. 4641 (x = 7315.300, y = 436.500); sample A424 is from a red pegmatite dyke a few tens of centimetres wide penetrating the same granite, and sample A425 is from a light grey pegmatite dyke penetrating migmatitic rock about 100 m south of the Soilu canal (x = 7316.300, y = 438.400). Samples A415 and A424 fit the diffusion curve cor- responding to an age of 2738 Ma. Analogously, the isotope ratios of two zircon fractions from sample A425 were deciphered through a 2712 Ma diffusion model (Wasserburg 1963). The diffusion model ages are 2706 Ma for the fraction A425A and 2717 Ma for the fraction A425B.

uted to the effect of the heat of the mafic granites are younger than the supracrustal intrusions on their environment (Reino 1973). rocks in their vicinity. Numerous attempts have been made to Mineral ages from the Salla area date the rocks intruding the schists. Some of the rocks did not yield enough zircon, titanite A part of the Salla schist area and of the or monazite to be reliably dated with U-Pb granitoid and migmatite complex westward methods. Interpretation of a great deal of the of it w.as mapped in 1961-67 (Lauerma 1967 a, data obtained so far is ambiguous. Even so, b). The location of these map-sheet areas, the results do give some idea of the geological 4621 Salla and 3643 Kursu, is shown in Fig. 1. and geochronological problems encountered The main geological features are shown in in this area. The localities from which the Fig. 3. samples reported here were taken are shown No basal conglomerates have been found in Fig. 3. Analytical data are given in Table 1 in the area of these map sheets or elsewhere and some of the isotope ratios are plotted on in the proximity of the western border of the the concordia diagram in Fig. 4. Salla schist area. In many places granites Large outcrops of a rather homogeneous intrude the supracrustal rocks, and schist granite occur in Hatajavaara (Fig. 3). About inclusions have been found in the granitic 800 m north of it, in Paikanselkä, dykes of rocks. These observations suggest that the obviously the same granite intrude metadia- On the ages of some granitoid and schist complexes . 91

Fig. 3. Sources of the samples for age determinations from the Salla area. The base map is simplified from Lauerma (1967 a, b). 1. Mica schist and mica gneiss. 2. Metadiabase, amphibolite and hornblende gneiss. 3. Quartzite. 4. Dolomite and skarn. 5. Schist inclusions and schlieren. 6. Granite and granodiorite. 7. Dia- base dyke. 8. Sample locality.

base (Lauerma 1967a). The width of the not far from the chord for sample A641, and largest granite dyke found is difficult to the finest fraction corresponds to a diffusion estimate because the outcrop has been curve of 1872 Ma. This could be interpreted broken by frost action; it seems, however, by mixed population. Should the fractions to be about 20 m to 30 m. Some other dykes be oogenetic, extension of the trajectory gives are 1-2 m wide, and in a railway cut about an upper intercept of 2531 ± 14 Ma and a 1 km farther northeast the granite dykes are fairly high lower intercept of 1422 ± 5 Ma on only a few cm to some tens of cm wide. the concordia curve. Three granite samples, Nos. A126, A640 and Sample A640 is from a pink granite with A641, were taken from a quarry near the greyish schlieren, probably relics of mica northern margin of Hatajavaara. All of them gneiss inclusions. Three density fractions proved to be exceptionally poor in zircon. were analyzed. In all of them, the grain size Sample A126 is from the most homoge- is larger than 70 (im. Isotopic ratios are not neous granite, which contains some miarolitic far from the data points for sample A641. cavities about 5 cm to 10 cm in diameter If a three-point isochrone (actually two points filled mainly with potassium feldspar but only, because fractions A and B gave nearly which exhibit neither schlieren nor veins. identical isotope ratios) were calculated, the Three size fractions from the same density upper intersection would be 2148 Ma and the fraction (d > 3.8 g/cm3) were analyzed. The lower one 935 Ma. finer fractions trend to be more discordant Sample A641 is from a grey granite that and slightly richer in uranium. The three size contains some pegmatitic streaks and mica fractions are collinear. Fraction +70 um is gneiss inclusions with sharp boundaries. 92 Raimo Lauerma

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Fig. 4. Concordia diagram for zircons from the Hatajavaara granite (samples A126, A640 and A641) and from the Lätävaara migmatite (A125), Salla. The discordia array through data points B, C, G, D and E from the sample A641 (York 1966) gives an upper concordia intercept at 2216 ± 70 Ma (2 sigma error). When pooled, A640 and A641 yield an age of 2197 ± 64 Ma. For reference, the dashed lines corresponding to the diffusion model ages of 1872 Ma and 1808 Ma were calculated through data points A126C and A641F (see text). The three zircon fractions from the Lätävaara migmatite sample (A125) are broadly consistent with the 2216 Ma discordia age.

MOS2 was noted in the density fraction given was 2216 ± 70 Ma. Fraction A, which +4.6 g/cm3. The best fit chord based on four has a density of 4.2-4.6 g/cm3 and a grain size normal zircon fractions (B, C, G, D) and one of less than 70 gives a diffusion model zircon fraction preleached in hydrofluoric age of 1983 Ma. Fraction F consists of light acid (fraction E) gives 2216 ± 71 Ma and a coloured long crystals, hand picked from a lower intercept of 1030 ± 120 Ma. There is a population with a density of 4.0-4.2 g/cm3 general correlation between the degree of and a grain size of +70 |im. It is fairly rich in discordance and the uranium content (423- uranium and belongs to a diffusion chord 875 ppm U) from the fraction B to fraction of 1808 Ma. If all of the fractions from samples D. An attempt to remove the discordant com- A640 and A641, excluding fractions A641A ponent with HF treatment was only partly and A641F, are pooled, they yield an age of succesful. Much common lead was removed, 2197 ± 64 Ma. but the residue (fraction E) had an uranium Irrespective of the collinearity of most of content of only 325 ppm and was the most the zircon fractions from sample A641, the discordant of all fractions. coexistence of definitely younger zircons Fractions A and F of sample A641 were made it difficult to assess the apparent excluded when fitting the chord; the age isochron relationship. Even so, it is certain On the ages of some granitoid and schist complexes . 93 that the majority of the zircon population concordant than those from the Hatajavaara represents a minimum age of about 2200 Ma. granite. The isotopic ratios of fraction A fit on Sample A127 is from a small quarry near the chord of best fit for zircon fractions from road No. 950, about 1 km north of the lake sample A641 -Hatajavaara; those of fractions Kolmiloukkonen (Fig. 3), at a site where sev- B and C from Lätävaara are not far from the eral hornblende-bearing mica schist layers same chord. and granite sills intercalate. The mica schist Isotope geological studies on the granitic layers and the granite sills are both about rocks in Salla present two main problems. 5 m to 10 m thick. The granite also intrudes One is to date the intrusions of granites into the mica schist discordantly as veins a few cm the supracrustal rocks of the Salla schist belt; thick. The sample was taken from the middle the other is to establish the source or sources of a granite sill. of the zircon population in these granites and, The sample proved to be very poor in thus, the material from which the granites zircons. The only zircon fraction (d > 3.8 gl derived. cm3) analyzed so far is highly discordant and Most of the zircon data on the Hatajavaara shows a diffusion model age of 2137 Ma. The and Kolmiloukkonen granites indicate an

207Pb/206Pb age on m0nazjte from thg same anomalous age of about 2200 Ma. This age is sample is only 1803 Ma. The common lead by about 300 Ma older than the typical syn- content in this monazite is very low (206Pb/ orogenic Svecokarelian age of about 1880 Ma 204Pb = 23596). and by 400 Ma older than the lateorogenic The granitoid rocks near the western bor- events. Disturbances on the concordia array der of the Salla schist area are mostly homo- (Fig. 4) indicate, however, that this is not a geneous and contain few or no inclusions or simple oogenetic suite of magmatically crys- schlieren. About 10 km to 20 km farther tallized zircons. It is also possible, that due to westward in the western part of the map high discordance or mixed population, or sheet Kursu (Lauerma 1967 b) the rocks con- both, the fitted isochrone gives a minimum sist mainly of migmatites. There is also a age only. marked difference in the aeromagnetic pat- The Paikanselkä metadiabase north of tern of these two rocks. The migmatites give Hatajavaara (Fig. 3) was dated by U-Pb meth- rather strong aeromagnetic anomalies but the ods. The samples were taken from a rail- homogeneous granites only very weak or way cut about 4 km west of Salla railway none at all. station. When plotted on a concordia dia- The migmatitic rocks, especially those in gram, the isotope ratios of zircons and titanite the vicinity of Lätävaara (Fig. 3), resemble cluster in a rather small area and do not form those in many areas of the Archean basement a clear isochrone; even so, the preliminary of eastern Finland. Sample A125 was taken ages obtained remain within 2100 ± 50 Ma for age determination from the restite of such (M. Sakko, pers. comm.). migmatite near the southern end of Lätävaa- As described previously, dykes of the Hata- ra (map sheet 3643 02, x = 7415.100, y = javaara granite intrude the Paikanselkä 548.00). Three zircon fractions were analyzed. metadiabase. Therefore, the granite dykes The analytical data are given in Table 1, and cannot have intruded earlier than some 2100 the isotopic ratios are plotted in Fig. 4 on the Ma ago. The youngest zircons (1808 Ma) from same concordia diagram as those of the Hata- Hatajavaara and the monazite (1803 Ma) from javaara zircons. Despite their higher uranium Kolmiloukkonen indicate that it probably content, zircons from Lätävaara are more occurred about 1800 Ma ago. 94 Raimo Lauerma

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The source of the zircon population in the model age of 1819 Ma. Monazite from the granitic rocks in Salla is still unsolved. Other same sample gave an age of 1805 Ma. Analyti- isotopic techniques (e.g., Sm-Nd, Lu-Hf and cal data are given in Table 1. Rb-Sr) could further elucidate this problem North of the town of Rovaniemi granites and aid in establishing the material from and pegmatites intrude the quartzites of the which these granites derived. Peräpohja schist area. A granite sample, A368, was taken by J. Nuutilainen (Rauta- The age of the Central Lapland granitoid ruukki Co.) from the northern margin of the complex Rovaniemi airfield. Analytical data on two zircon fractions and one titanite are given in In addition to the rocks from Salla, two Table 1. Isotopic ratios, which are plotted on a other granites have been dated from the east- concordia diagram in Fig. 5, give an age of ern and southern part of the Central Lapland 1770 ± 8 Ma. granitoid area (Fig. 1). According to contact observations and Sample A642 was taken from a grey homo- most age determinations, the Central Lap- geneous granite near Pietariselkä, about 20 land granitoid complex is late Svecokarelian km east of the town of Kemijärvi (map sheet in age. Within a larger framework, however, 3641 10, x = 7401.050, y = 535.650, northern its geological setting suggests that it pre- side of a road cut on highway No. 5). The viously formed the basement for the Protero- surrounding rocks are mainly various migma- zoic sedimentation and was a source of some tites penetrated in places by pink granite. The of the sedimentary material. only zircon fraction (d = 4.0-4.2 g/cm3, grain The complex is surrounded in most direc- size > 160 um) analyzed gave a diffusion tions by Svecokarelian schist belts with On the ages of some granitoid and schist complexes . 95

grade into migmatites similar to the Archean basement gneisses in eastern Finland. In some places, e.g., in the vicinity of Unari (Fig. 1) the migmatites display conspicuously gently dipping or subhorizontal foliation and banding. According to Mikkola (1937, 1941) and Tyr- väinen (1979) there occur probably Archean basement rocks south of Sodankylä (Fig. 1), in the northern part of the Central Lapland granitoid area. The age problems of the Central Lapland granitoid and migmatite complex are partly the same as those of the mantled gneiss domes described by Eskola (1949). In these domes an old granitoid basement was covered by sediments and, together, they were up- heaved and granitized during the following orogeny. The age relations of the granitoid rocks and the overlying sedimentary cover can be solved in their contact zone, but in larger granitoid areas such as the Central Lapland granitoid complex farther away from the schist belts it is difficult to distinguish the possible Archean granitoid rocks from the 20° 25° Proterozoic ones by field investigations Fig. 6. Location of the granitoid areas of Central alone. Finland (1) and Central Lapland (2). The isotope geological difficulties of dating various events in the development of quartzites in their marginal parts. The schist mantled gneiss domes and in palingenic and belts of Peräpohja, Kuusamo and Salla each polymetamorphic rocks in general has been form a synclinorium, underlain on one side elucidated by many examples from the by an Archean basement and intruded on the Finnish Precambrian. Kouvo and Tilton other by granitoid rocks (the eastern part of (1966) have reported mineral ages from the the Salla schist area is located in the territory mantled gneiss domes in Kuopio, Central of the U.S.S.R.) The supracrustal rocks ap- Finland (Fig. 6). For the granitic cores of parently deposited on an Archean basement these domes the U-Pb data on zircons suggest even in those regions where they are now an age of 2800 Ma, whereas biotites in the penetrated by granitoid rocks. According to same rocks dated by Rb-Sr and K-Ar meth- Hausen (1936), the rocks near the northern ods give ages from 1715 Ma to 1800 Ma. Data margin of the Peräpohja schist area have on similar gneiss domes in the Kotalahti area, undergone regional anatexis. about 30 km south of Kuopio, published by Farther away from these schist areas, the Gaål (1980), indicate an age of about 2800 Ma penetrating, mostly massive granites often for the zircons, 1830 Ma to 1850 Ma for the 96 Raimo Lauerma titanites and an K-Ar age of about 1670 Ma to formed during the main folding stage of the 1730 Ma for the biotites. Svecokarelian orogeny. On their schematic New data on the Sotkuma gneissose grano- stratigraphic map (op.cit.), they have marked diorite dome, eastern Finland, give a zircon as Archean most of the areas designated by isochron showing the upper intersection of Hackman (1910 a, b) and Hackman & Wilk- concordia at 2682 Ma (O. Kouvo, pers. comm.) man (1925) as migmatitic mica gneisses. but monazite from the same sample gives an The greatest part of the Central Lapland almost concordant age of 1900 Ma. (GSF, granitoid area was mapped in a rather cursory Petro logical Dept. Ann. Rep. 1981). fashion about 45 to 85 years ago (Hackman Two zircon generations, long needles with 1910 a, b; Hackman and Wilkman 1925; Mik- an age of about 1900 Ma and darker, roundish kola 1937). Aeromagnetic maps, surveyed in crystals with an age of about 2600 Ma, have the 1960s, reveal new structural features in been found in the same granodiorite (sample many parts of the area. Geological remapping A32) from Kaavmkoski, eastern Finland of large areas and further isotopic studies are (Huhma 1976). In northern Finland two zircon needed before the possible Archean and generations with ages of 3110 ± 17 Ma and palingenic and anatectic parts of this grani- 3069 ±16 Ma in samples from the Tojotta- toid and migmatite complex can be delin- manselkä tonalitic gneiss have been reported eated. by Kröner et al. (1981). Titanite with an age of 2776 Ma and a monazite dated at 1902 Ma have been found within samples from the same Chronostratigraphic implications locality (GSF, Petrological Dept. Ann. Rep. On similar grounds as in the foregoing, viz., 1980; O. Kouvo, pers. comm.) a large complex of granitoid rocks and The examples given in the foregoing show migmatites surrounded by Proterozoic schist that concepts like »the age of a rock» and »the belts, areas of former Archean basement can age of a granitoid complex» are ambiguous, be proposed in other parts of Finland as well. especially when applied to palingenic and On the geological maps covering the whole of polymetamorphic rocks and based on one Finland (e.g. Simonen 1980 a) the most con- dated mineral only. spicuous of areas like this is the granitoid The first attempts to find isotopic evidence complex of Central Finland (Fig. 6). Its of Archean rocks or minerals in the Central boundaries are, however, vague and difficult Lapland granitoid area were made in 1960s on to delineate accurately on existing geological migmatites in Lätävaara, Salla. The interpre- maps and even more so in the field. tation of the age and source of the zircon Salop (1971) has proposed that this com- population in these rocks and in the granites plex is possibly composed of older, mainly of Hatajavaara and Kolmiloukkonen, de- Archean palingenic granites. The radiometric scribed in the foregoing, is, however, not age determinations made so far, however, in- unambiguous. cluding those on zircons, have all given Pro- The notion that some parts of the Central terozoic ages (GSF Ann. Rep. 1966-78). Lapland granitoid area are Archean in age has A recent trend in Finland has been to desig- gained currency in the last 10 to 15 years and nate many of the schist areas or parts of them is now accepted by many geologists. Accord- previously believed to be Proterozoic as ing to Silvennoinen et al. (1980, p. 158), this Archean. One of these is the Salla schist area, granite is composed mainly of the melting which Gaål et al. (1978) have designated as an products of the Archean basement complex Archean (> 2500 Ma) greenstone belt. Their On the ages of some granitoid and schist complexes . 97 opinion was partly based (op.cit., p. 206) on to Kulikov et al. (1980), to the Sumian - the U-Pb age of zircons from the Hatajavaara Sariolian group, which means that they granite, or, more properly, on the isotope cannot be much older than 2450 Ma (Meriläi- ratios of the zircons from sample A126, nen & Sokolov 1981, p. 13). reported in the GSF, Ann. Rep. 1973. As The crucial problem seems to be the posi- pointed out in the foregoing chapter, this tion and age of the acid metavolcanic rock in granite, like the granite of Kolmiloukkonen, the western part of the Kuusamo schist area, obviously intruded supracrustal rocks about or, more properly, the primary source or 1800 Ma ago; they do not, therefore, indicate sources of the zircon population in this rock. an Archean age for the Salla schist area. The same problem is also encountered else- On their stratigraphic map, Silvennoinen et where in northern Finland, where many al. (1980) have marked the quartzites in the supracrustal rocks have been proposed Salla schist area as Proterozoic and all the Archean (Gaål et al. 1978; Silvennoinen et al. other supracrustal rocks as Archean (> 2600 1980). The majority of the radiometric datings Ma). This view is apparently based on correla- have been made by U-Pb methods on zircons, tion with the chronostratigraphic model titanites and monazites separated from worked out by Silvennoinen (op.cit., Fig. 4) in various rocks. The ages obtained by these the Kuusamo schist area. In this model the methods for mafic rocks in the schist areas of lowest members of the supracrustal column Kuusamo, Salla and Central Lapland do not are of Archean age. Zircons separated from exceed 2450 Ma. The Archean ages reported an acid volcanic rock in the western part of are all from granitoid rocks, quartzites and the schist area have given an age of 2790 Ma acid metavolcanic rocks (GSF, Ann. Rep. (op.cit. p. 159) and this age has been used to 1969-77; GSF, Petrological Dept. Ann. Rep. date the extrusion of the lowermost, mainly 1980). Kallio (1980, pp. 57-60) has listed seven mafic volcanic rocks of the sequence. previously published ages and reported four new ones from the Kittilä greenstone area. This model is not consistent with some of They range from 1834 Ma to 2213 Ma. The the data available at present. The age deter- Sm-Nd determinations, recently initiated at minations of the granitoid rocks and pegma- the Geological Survey of Finland, will prob- tites in the basement complex south of the ably give more age data, especially on mafic Kuusamo schist area have given ages within rocks within and overlying the Archean base- 2600-2800 Ma (Kouvo & Tilton 1966; GSF ment complexes in northern Finland. Ann. Rep. 1971-77; GSF Petrological Dept. Ann. Rep. 1980,1981; Vidalet al. 1980; Hyppö- The existence of Archean greenstone belts nen 1982; Fig. 2 in this paper). The schists or other Archean mafic to ultramafic rocks in overlie unconformably the basement com- Finnish Lapland (north of the latitude 66° N) plex, and in places basal conglomerates con- is by no means excluded. It seems plausible taining pebbles from the basement rocks are that, as proposed by Gaål et al. (1978), some encountered. There are no reports of granitic zones of mafic to ultramafic rocks in the veins intruding the schists in the southern Tuntsa-Savukoski area (Fig. 1) belong to this border of the Kuusamo schist area. There- category. Their geological setting within an fore, also the lowermost volcanic rocks of the area composed of Archean granitoids and sequence are evidently younger than the their aeromagnetic patterns are very similar granitic rocks in the underlying basement. to those of the Suomussalmi and Kuhmo The same volcanic rocks in the Soviet part greenstone belts. Yet, the northernmost of the Kuusamo schist belt belong, according greenstone belts in Finland, delineated so far 98 Raimo Lauerma by reasonably detailed geological mapping nated it traditionally as Svecokarelian (i.e. and dated as Archean by radiometric meth- Proterozoic), Silvennoinen et al. (1980) as ods, are those of Suomussalmi and Kuhmo. partly Archean (> 2600 Ma), partly Pro- The occurrences of mafic rocks in Syöte, terozoic, and Gaål et al. (1978) as an Archean Porttivaara and Näränkävaara, south of the (> 2500 Ma) greenstone belt. As mentioned by Kuusamo schist area (Fig. 1), designated by Kulikov et al. (1980), different interpretations Gaål et al. (op.cit. Fig. 1) as metavolcanics of have also been presented by Soviet re- Archean greenstone belt association, are searchers. The greatest discrepancy at pre- mafic layered intrusions of the age group sent concerns the age of rocks on both sides 2430-2450 Ma (Simonen 1980 a, b). The data of the border between Finland and the reported by Alapieti (1982) have yielded an U.S.S.R., in the central part of the synclino- age of 2436 ± 5 Ma for these intrusions. rium. According to Silvennoinen et al. (op.cit.) The bedrock in the Finnish part of the Sal- the mafic metavolcanic rocks in the proximi- la schist area is poorly exposed and large ty of the border belong to the lowermost areas of it are totally covered by glacial drift Archean members of the combined Archean- and peat bogs. Owing to the subarctic cli- Proterozoic column and, according to mate, most of the outcrops are broken by Kulikov et al. (1980), to the uppermost frost action; this renders reliable tectonic ob- Suisarian group of the Proterozoic sequence. servations difficult or impossible in many In some parts of the Salla schist area the places. In this respect, the Salla region is only geological mapping carried out so far similar to the Central Lapland schist area. was at the beginning of this century. New Aeromagnetic maps were available when the investigations will undoubtedly alter the latest geological maps (Lauerma 1967 a, b) current concepts of the geology of this area. were compiled, but owing to the flight alti- From the data available at present, however, tude (150 m) and the flight line separation the Salla schist area seems to be Proterozoic (400 m), their resolution was not sufficient in in age. structurally complicated areas. The investiga- tions made to date (Hackman & Wilkman 1925; Lauerma 1967 a, b) give only a broad outline of the geology in the Finnish part Acknowledgements. All the age determinations of the Salla schist area. The Soviet part of the reported in this study were performed at the Geo- schist area has obviously been investigated in logical Survey of Finland by Dr. Olavi Kouvo and much greater detail (Kulikov et al. 1980). his co-workers. Dr. Kouvo also read the manuscript and gave valuable advice and criticism. Mrs. Elsa Analogies with other areas have been Järvimäki draw the figures and Mrs. Gillian Häkli, applied to the Salla schist area with conflict- B. A., corrected the English of the manuscript. I ing results. Simonen (1980 a, b) has desig- wish to express my sincere thanks to all of them.

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