PALEOMAGNETISM AND AGE RELATIONS OF THE ROCKS IN THE MAIN SULPHIDE ORE BELT IN CENTRAL FINLAND
K.J. NEUVONEN, KALEVI KORSMAN, OLAVI KOUVO and JORMA PAAVOLA
NEUVONEN, K. J., KORSMAN, K., KOUVO, O. and PAAVOLA, J. 1981: Paleomagnetism and age relations of the rocks in the Main Sulphide Ore Belt in central Finland. Bull. Geol. Soc. Finland 53—2, 109—133. Paleomagnetic data on the Main Sulphide Ore Belt in Finland are coherent but differ clearly from those obtained on the Archean basement area immediately north of it. The coherent magnetization in ithe Belt could have been caused by (i) axial uplift, (ii) a plate suture along the Belt or (iii) a short-time igneous pulse. The last interpretation is accepted since it agrees with the radiometric age data. Thus the paleomagnetic measurements in the area date the same thermal events as do the radiometric U-Pb age determinations, and the shape of the APW curve is thereby verified. A paleomagnetic pole determined for dykes intersecting the Archean basement block lies off the accepted APW curve which possibly need to be corrected. The magmatic pulse between 1880— 1840 Ma may have been associated with a plate collision or with a deep fluid convection producing the sulphide ore precipitation within the Belt. K. J. Neuvonen, Dept. of Geology and Mineralogy, University of Turku, SF-20500 Turku 50, Finland Kalevi Korsman and Olavi Kouvo, Geological Survey of Finland, SF-02150 Espoo 15, Finland. Jorma Paavola, Geological Survey of Finland, Box 237, SF-70101 Kuopio 10, Finland.
Introduction Very little, however, is known, about the geological background of this belt. It is part Paarma and Marmo (1961) were the first of the Svecokarelian tectonic province but to point out that a large number, if not most, lies on the boundary with the Archean base- of the ore deposits known in Finland are ment gneiss, which runs roughly subparallel located in belt about 100 km wide running in to the northeastern edge of the Belt but is in a NW-SE direction from Lake Ladoga to the places more than 100 km northeast of it. town of Raahe (Fig. 1). This belt lies parallel According to Penttilä (1963), the area shows to common fault lines in the same area seismic activity. Talvitie (1971) made a de- (Härme 1961) and to a deep negative gravi- tailed study of the seismotectonics of the metric anomaly (Honkasalo 1962) bordering Kuopio region (Fig. 1), which is cut by frac- the belt on its SW side. Mikkola and Niini tures and fault lines belonging to the sulphide (1966, 1968) divided the zone into three metal- ore zone. He observed the most pronounced logenic provinces and noticed an abundance lineaments in directions 0°—5°, 270°—275°, of basic (subsilisic) intrusions within the belt associated with the ore bodies. In his dis- 305° and 325°. The latter two are roughly, cussion on the metallogenic features of Fin- parallel to the Main Sulphide Ore Belt. From land. Kahma (1973) refers to this, belt as the seismotectonic interpretation and precise lev- »Main Sulphide Ore Belt». elling he concluded (op. cit, p. 37) that the 110 K. J. Neuvonen, Kalevi Korsman, Olavi Kouvo and Jorma Paavola
REMANENT MAGNETIZATION BETWEEN TOWN RAAHE AND LAKE LADOGA
RAAHE
Basic intrusions Iisalmi Diabase type 1 Table 9 ' Varpaisjärvi vPoles 9 and / Table 6 Pole 7 ,
Basement complex PYHÄSALMI Varpaisjärvi K,uf \\ \ V , Table 5 / , Pole 11 /
VARPAISJÄRVI PIHTIPUDAS Tonalite dikes Ylivieska and other NILSIÄ Nilsiä Pohjanmaa gabbros/ , Table 8 Poles 5 and 6 / \Pole 8
OUTOKUMPU
Basic intrusions Kiuru vesi-Pielavesi Table 4 / \ Poles 3 and 4 , % ItMUKltESI
Lamprophyre dikes Haukivesi Table 3 J \ Poles 1 and 2 / Lake Ladoga
HELSINK
150 km
Fig. 1. Remanent magnetization between the town of Raahe and Lake Ladoga (the Main Sulphide Ore Belt of Kahma 1973). 1. Basement complex within the Main Sulphide Ore Belt. 2. Approxi- mate area of the Main Sulphide Belt. 3. Subsilicic intrusions within the Belt. 4. Major fracture. 5. Area of the detailed maps in Fig. 2. and Fig. 3. Remanent magnetization shown on the Lambert equal area projection nets. The average direction shown as a tirangle. Table numbers refer to Tables in the text and pole numbers to the poles in Fig. 8. Paleomagnetism and age relations of the rocks in . . . 111 gravity trough in the 325° direction is pro- The lithology of the area bably »the manifestation of a larger zone of Lithologically, the Belt is not well defined crustal weakness». According to him, the and its borders cannot be drawn exactly. It crustal block east of the town of Kuopio (see consists of series of metasediments and meta- Fig. 1) is subsiding and causing the earth- volcanics deformed, recrystallized and folded quakes recently registered in the area. In the during the Svecokarelian (1800—1900 Ma) Iisalmi area (Fig. 1) Kauppinen (1973) has orogeny. These schists and gneisses are pene- mapped several blocks which he assumed to trated by various of intrusive rocks mainly have moved vertically in respect to each of synkinematic character. An area typical other. Parkkinen (1975) made a deformation of the Belt is met with in the southeastern analysis of a mafic intrusion in the south- part of the zone in the Haukivesi area (Figs. eastern part of the Belt. In his opinion, the 1 and 2). The geology of this area will be wrench zones parallel to the Belt are caused described below as representative of the by the oldest regional stress in a NE-SW whole Belt. direction. Marttila (1976) adopted the concep- For comparison, paleomagnetic samples tion of geosynclinal sedimentation of the were studied from the Nilsiä—Varpaisjärvi supracrustal series in the Kiuruvesi area area (Fig. 3) northeast of the Belt. This area northwest of Kuopio. He interpreted the NW- is part of the Presvecokarelian basement striking Belt as a subsiding rift valley system complex, Archean in age. The lithology of connected with subparallel faults, shear this area will also be described below. zones and extrusive and intrusive magma- tism. The Haukivesi area
The purpose of the present paper is to The Haukivesi area is located in the south- report on the paleomagnetic data obtained eastern part of the Raahe—Ladoga zone (Fig. for the Belt area and to compare them with 1). The geology of the area has been described paleomagnetic data on the Archean basement in detail by Hackman (1933), Gaål and Rauha- region just north of it. The aim is to throw mäki (1971) and Korsman and Pääjärvi (1980). more light on the obscure question of the The area is characterized by highly meta- origin of the Belt. The paleomagnetic mea- morphic supracrustal rocks and by hyper- surements are concentrated on two areas: sthene-bearing plutonic rocks with sporadic One is in the central part of the Main Sul- basic (subsilicic) dykes (Fig. 2). phide Ore Belt in the Haukivesi area, the The subracrustal complex consists of other is just north of the Belt in the Archean garnet-cordierite-sillimanite gneisses, hyper- basement complex in the Nilsiä—Varpais- sthene gneisses and hypersthene amphib- järvi area (Fig. 1). The lithology and paleo- olites. The garnet-cordierite-sillimanite magnetic data obtained will be described and gneisses are migmatized by trondhjemites. discussed below. Additional paleomagnetic The metamorphism of these rocks took place results already available from the north- under conditions of granulite facies but, western end of the Belt and from the Iisalmi according to Paavola (1976), the pressure was area (see Fig. 1) will be included in the Dis- not very high during the recrystallization. cussion. Radiometric age data and lead iso- Agglomeratic and pillow lava structures in tope values will also be considered in drawing some of the amphibolites and gneisses in the conclusions about the origin and evolution of area suggest a volcanic origin for these rocks. the Belt. The remanent magnetization found in these 112 K. J. Neuvonen, Kalevi Korsman, Olavi Kouvo and Jorma Paavola
E32
mg 3
S 4
S5
E3 6
L^ 7
d^ 8
28°10' 5 Km 28° 30* Fig. 2. Simplified geological map of the lake Haukivesi area. 1. gabbro, 2. hypersthene quartz diorite and diorite, 3. granodiorite, 4. high-grade gneiss and amphibolite, 5. biotite gneiss, 6. diopside amphibolite, 7. lamprophyre dyke, 8. metadiabase dyke. After Korsmän and Pääjärvi (1980). rocks is weak and unstable. Consequently, There are two types of subsilicic dyke rocks paleomagnetic investigation was not per- of different ages in the Haukivesi area. The formed on the rocks of the supracrustal older ones are metadiabase dykes 1 to 2 m complex. wide broken by later tectonic faults. These The supracrustal rocks are penetrated by dykes penetrate supracrustal rocks and dio- hypersthene-bearing plutonic rocks. They ritic and more basic plutonites but they have consist of granodiorites, quartz diorites, dio- not been observed to cut the granodiorite. rites and minor occurrences of gabbros and Their magnetite content is low and only one uitramafic rocks. The quartz diorites and diabase sample was found to carry stable diorites are foliated and have gradual con- magnetization. tacts towards the hypersthene gneisses and amphibolites. The magnetite content being The N-S-striking lamprophyre dykes de- low, only a few samples of these rocks were scribed by Hackman (1933) represent the suitable for paleomagnetic studies although youngest rock in the Haukivesi area. The a great number of samples were collected. dykes vary in thickness from a few centi- The hypersthene-bearing granodiorite in metres to some metres. They are fine-grained the area is porphyritic. It has intrusive con- porphyritic rocks with plagioclase, biotite tacts and brecciates quartz diorites and dio- and clinopyroxene as phenocrysts. Amygdules rites. Hence, it must be somewhat younger filled with carbonate and biotite indicate than those typically synorogenic rocks. No crystallization at a relatively low crustal samples carrying hard magnetization were depth. The magnetite content of these dykes found in this rock. is usually quite high and the rock is there- Paleomagnetism and age relations of the rocks in . . . 113 fore suitable for paleomagnetic measure- the rocks in this area belong to the Archean ments. basement complex. It is divided into two Granulite facies metamorphism in the Hau- parts by the N-S-trending Proterozoic (Sveco- kivesi area is closely connected with the karelian) Tahkomäki quartzite ridge (Fig. 3). hypersthene-bearing intrusions. The age dif- On the eastern side of the quartzite ridge, ference indicated by the contact relations in the Lake Syväri area, the basement com- mentioned above between the diorites and prises mainly of banded and migmatitic granodiorites cannot be very great. A radio- paragneisses of diverse compositions. These metric Pb-U age of about 1880 Ma for both gneisses are quartz dioritic or dioritic in com- was reported by Gaål and Rauhamäki (1971). position with mica- and hornblende gneiss The isotopic age of the lamprophyres (1837 and amphibolite horizons. Narrow cordierite- Ma, see Fig. 5) is only slightly less than that gneiss bands also occur. This paragneiss com- of the quartz diorite and granodiorite and plex is weakly magnetized and was not granodiorite. It seems likely that the geolog- studied paleomagnetically. ical evolution of the Haukivesi took On the northwestern side of the Tahkomäki place relatively fast between 1880 and 1840 quartzite ridge, in the Jonsa area, northeast Ma. The area seems to have cratonized soon of Varpaisjärvi (Fig. 3), the basement differs after the emplacement of the lamprophyre from other Archean granite gneiss types of dikes about 1840 Ma ago. Finland particularly in its orthopyroxene In the Pielavesi area, northwest of Hauki- content. The rock here is highly metamor- vesi, metamorphism and plutonic intrusion phosed and forms a clearly limited, separate evidently took place under conditions similar block of basement. It has a high magnetite to those in the Haukivesi area. Granulite content and is seen as a strong magnetic facies and hypersthene-bearing rocks are anomaly on the aeromagnetic map. The common. No lamprophyre dykes have been remanent magnetization of the rocks of the reported but diabase dykes closely associated complex being very high, it is suitable for with gabbro-diorite rocks are numerous. paleomagnetic investigations. Some of these rocks contain remanent magne- These basement rocks in the Jonsa area are tization strong and hard enough for paleo- cut by diabase dykes (Fig. 3) running approxi- magnetic measurements. mately in an E-W or SE-NW direction. The Closer to the Bothnian Sea, at the north- width of the dykes varies from a few centi- western end of the Belt metamorphism has metres to about 150 metres. The contacts be- taken place mainly under conditions of am- tween the diabase and the wall rock are phibolite facies. Plutonic gabbros and diori- always very sharp, and chilled margins are tes from this area contain large amounts of usual. The diabase dyke rock is very homo- magnetite, thus making it possible to conduct geneous, massive and fine- or medium-grain- paleomagnetic investigations. ed. Ophitic texture is common. The best pre- served dykes are composed of partly zoned plagioclase (An 40-60) and augite with some The Nilsiä—Varpaisjärvi area orthopyroxene. In some dykes brownish horn- North of Kuopio, the boundary between blende is the dominant dark mineral. the Svecokarelian rocks and the Archean Wilkman (1924 and 1938) divided the dia- basement runs subparallel to the Belt (Fig. base dykes in the area into enstatite-augite 1). The Nilsiä—Varpaisjärvi area lies about and hornblende dykes. Although they are 50 km north of Kuopio and the majority of somewhat different in their primary com- 114 K. J. Neuvonen, Kalevi Korsman, Olavi Kouvo and Jorma Paavola
Fig. 3. Simplified geological map of the Nilsiä—Varpaisjärvi area. Archean rocks: 1. granite gneiss, 2. dominantly paragneiss, 3. pyroxene amphibolite (gabbroidic), 4. orthopyroxene. Proterozoic rocks: 5. quartzite, 6. conglomerate, 7. mica schist, 8. metadiabase, 9. diabase dyke, 10. tonalite dyke, 11. fault and fracture. position, he considered the latter to be altera- do not, however, correspond to the two petro- tion products of the former. graphically different types suggested by Fresh and unaltered diabase dykes carry a Wilkman (op. cit.). strong and hard remanent magnetization No radiometric dating is available for these suitable for paleomagnetic studies. According dykes. In their chemistry and general east— to the directions of remanent magnetization west trend the dykes are similar to the Jatu- found in these dykes, they can be divided lian (2150 Ma) metadiabase dykes frequently into two different types (see p. 126), which seen to cut the Presvecokarelian basement Paleomagnetism and age relations of the rocks in . . . 115 in eastern Finland. Consequently, these dia- tion of sulphide lead is surprisingly homo- base dykes in the Varpaisjärvi area are geneous and includes polymetallic sulphide thought to belong to the Jatulian age group. deposits such as Vihanti (Rouhunkoski, 1968), A second set of intersecting dykes occurs in Pyhäsalmi (Stacey et al. 1977; Helovuori, the Nilsiä area (Fig. 3). These dykes are fine- 1979) and others (Vaasjoki 1981). This isotopic grained, massive and homogeneous. The con- composition of sulphide lead has been found tacts with the wall rock are sharp and in the immediate vicinity of the border of the apophyses are common. The dykes are from Archean craton, which consists of the evolu- a few centimetres to 4—5 metres wide. Their tionary groups known as Jatulian. It thus direction varies between N15°W and N85°W incorporates the Main Sulphide Ore Belt in a (345°—275°). The dip is subvertical. lithostratigraphic horizon. For the present, The mineral composition of these dykes this unique isotopic composition of sulphide includes plagioclase, quartz, biotite and horn- lead has not been found at the southern end blende with accessory epidote, sphene, apatite of the Belt. Chances of finding it, however and magnetite. Plagioclase occurs locally as do exist. intensely zoned (An 35.25) laths, 1.5—2 mm Aho (1979) and Helovuori (1979) have shown in size. According to the classification of that the isotopic composition of the total lead Streckeisen (1976), these dykes have a tonali- of volcanic rocks from the Pihtipudas area tic or quartz dioritic composition. They can just southwest of the Belt and from the Pyhä- be correlated with the tonalitic dykes de- salmi ore field within the Belt fit separate scribed by Huhma (1975 and 1981) from North isochrons showing the same age within the Karelia, with an age between 1860 and 1830 limits of experimental uncertainty. Of an Ma. These dykes have a weak but rather hard essential orogenetic significance is the fact remanent magnetization. that the point derived from the isotopic com- position of sulphide lead in the Pyhäsalmi ore deposit lies on the isochron defined by Radiometric datings the lead isotopic composition of Pyhäsalmi Kouvo and Kulp (1961) showed that there volcanites, whereas the Pihtipudas sulphide is a distinct difference between the isotopic lead lies on the isochron of Pihtipudas vol- composition of sulphide lead in the narrow canites. belt west of the Archean craton and that The plumbotectonic model published by found in the craton and in the large Sveco- Doe and Zartman (1979) indicates a mantle fennian area west of the Belt. They inferred origin and continental environments for the that either this may suggest an earlier history sulphide deposit in Pyhäsalmi, in the central for the Karelian rocks or it may represent part of the belt, whereas the lead from Pihti- anomalous lead with an unusual crustal envi- pudas just SW of the belt (Aho 1979), lies ronment. This lead, predating the time of the near to the orogenic curve (Stacey et al. 1977). Svecokarelian synorogenic stage, was charac- The amount of radiogenic lead isotopes terized by a low /(-value. Interlaboratory generally increases towards west from the comparisons were later performed between boundary of the Archean craton, and the the U.S. Geological Survey laboratories in common lead data show distinct boundaries Denver and the Geological Survey of Finland; along both sides of the belt. The isotopic these showed that the low /.«-value is definite- data on sulphide leads indicate an age of 2100 ly real and not due to incorrect measurements Ma for the Outokumpu ore on the north- of ->06Pb/204Pb ratios. This isotopic composi- eastern boundary of the belt, 1970 Ma for the 5 116 K. J. Neuvonen, Kalevi Korsman, Olavi Kouvo and Jorma Paavola
i-H t- CJ3 Pyhäsalmi ore in the central part of the belt + 1+1+1 +1+1 +1+1 and 1800 Ma for the Pihtipudas occurrence CO (N CM M I> CO I> r- t- lO CO 05 OJ O CD CD CO CO CO OS just southwest outside the belt proper (Stacey CM M M et al. 1977). A comprehensive study of this subject has been undertaken by Vaasjoki +1 +1 +1 +1 +1 +1 +1 (1981). CD (M 00 CO ^ ^ Oi CO LO CD CO CO C- D- CD CD lO CD CO CO 00 The age relations of the belt region com- (M (M (M pared with those outside the belt have been
rf CO CO Tf broadly defined in a number of U-Pb investi- +1+1+1 +1+1+1+1 gations. Except for the Archean craton there Oi C- (M HCOM^ MNin Oi E— lO LO is no reliable age difference between the LO CD RF CD CO CO CO N N N intrusions in side and outside the belt. In this work the U-Pb isotopic relationships O>J of zircons from the belt intrusions and tr- M H 05 (O OS CD CO CO N Ifi N ^ CD O ^ CO CO M Archean intrusions in the Varpaisjärvi area were studied in order to determine more precisely the age relations of the different W CO I> m H m co (M CD CO co co o io paleomagnetic pole positions obtained. fi-O CO CO CM ^ OI OI CO The isotope ratios of the most concordant CO oi zircon fractions from eleven intrusions define Oi C— 00 O CO RH TF1 QO. Ci o m M oi c- t> a summary chord with an upper concordia o m CD W M CD CO o o H O M H o o o o o o o o intercept at 1892 ± 4 Ma (calculated accord- ö ö ö ö ö ö ö ind to York 1966; 2). The results are all from the belt intrusions extending from the Parik- •o kala gabbro at the southeastern end of the cy •O £ H OJ 00 io t> co o belt to the Käpylä microcline granite at Vi- CM CO M ^ IFI ^ S Oi O rf m o co o O^ C/CTS5 CO CM lO O N CO ^ hanti, in the northwest. (M QJ The U-Pb data (Table 1, Fig. 4) show ex- •3 £ cellent linearity for the three density frac- cp a W> tions of zircons from a guartz dioritic pegma- ,2ä" XJ P-i toid (A843) and for a single, almost concord- co S K» ant, zircon fraction heavier than 4.6 g. cm"3 from a hypersthene-bearing quartz dioritic S a co co Oi c- a rock (A844) in the Varpaisjärvi area. The mCO CcOo CcOo CM O fitted diascordia line has an upper intersec- tion with concordia at 2680 ± 3 Ma (York CU 1966: 2 sigma). c N o '53 It has been found, especially in Finnish AA «i » 03 Lapland, that lead-lead total rock systems S s V, V h are disturbed and thus record a time of ß 5*3 CO M -ö O CD co-c O 8 S metamorphism, despite the quite good co- Va N AA o aaAA„i H> "O T3 ^ £ "Ö "O CO. linear arrays formed. However, as stated earlier, the lamprophyre dykes represent the o a C Fig. 4. Concordia dia- 0,6 gram and U-Pb rations for zircon samples from the Varpaisjärvi quartz dioritic pegmatoid (A843) and hypersthene-bearing quartz dioritic rock (A844); Iso-Uski lampro- phyre (A987); Parikkala gabbro (A884; Nykänen, unpublished); Ylivieska gabbro (A380; Pesonen and Stigzelius, 1972); Koivu järvi gabbro-diori- te (A311; Helovuori, 1979); Tuli-Toiviainen mafic pegmatoid (A678; Marttila 1976); Kotalahti mafic pegmatoid (A701; Gaål 1980); Voinsalmi hypersthene granite (A382; Gaål et al., 19); Jusko qtz. diorite (A834; Helovuori, 1979); Voin- salmi hypersthene qtz. 17,0 diorite (A383; Gaål et al., 207Pb / 235U 19); Käpylä granite (A899; unpublished); Vaarasi a hti hypersthene granite (A049; Wetherill et al., 1962); Lammasaho granodiorite (A600; Marttila, 1976). Each zircon fraction has a letter corresponding to the fractions listed in Table 1. from which zircons were separated yielded 1966 (2o). The age yielded by lead-lead data three apatite fractions: dark brownish, yellow are consistent with numerous U-Pb and Rb- and transparent. Isotopic analyses of two Sr ages measured on late- and post-orogenic apatites and four whole rock types (Table 2) granitoids in Finland (e.g. Neuvonen 1970; of lamprophyre dyke in Iso-Uski are plotted Korsman and Lehijärvi 1973; Vaasjoki 1977; on a lead-lead isochron diagram (Fig. 5). A Meriläinen 1976). In this connection mention petrographic description of these varieties of should be made of the observation by Aho Iso-Uski lamprophyre has been given by (1979) on the discordant zircon-titanite pair Laukkanen (in prep.), who has also examined in Pihtipudas granitoids where titanites and the different apatites used for isotopic work. monazite have as low ages as 1800 Ma. The data on different rock types and apatites Dykes like lamprophyres are found to bring define a well fitted isochron based mainly on up inherited material originating from wall total rock groups and two points of apatites rocks or representing early crystallization. with linear regression as follows: r = 0.99983, Zircons separated from a block weighing m = 0.112027 ± 1.3058E-03. The data given about 40 kg of the Iso-Uski lamprophyre dyke in Fig. 5 were calculated according to York yielded 5 mg of large zircon crystals (fraction Table 2. Analytical data on whole rock samples and two apatites from the Iso-Uski lamprophyre dyke. Sample No. •206pb/204pb 207pb/204pb 208pb/204pb JML6 whole rock 17.770 ± 0.034 15.504 ± 0.044 36.210 ± 0.109 JML7 whole rock 18.018 ± 0.048 15.550 ± 0.052 36.351 ± 0.117 JML8 whole rock 18.190 ± 0.017 15.559 ± 0.023 36.411 ± 0.060 JML5 whole rock 18.229 ± 0.005 15.562 ± 0.009 36.455 ± 0.035 A987 dark apatite 20.564 ± 0.017 15.812 ± 0.017 36.671 ± 0.037 A987 light coloured apatite 26.444 ± 0.010 16.484 ± 0.012 39.019 ± 0.031 118 K. J. Neuvonen, Kalevi Korsman, Olavi Kouvo and Jorma Paavola Fig. 5. -'06pb/ao4pb ver- sus 207pb/204pb in the Iso-Uski whole rock samples and two apa- tites. Each sample or fraction has a number or letter corresponding to analytical data listed in 206Pb / 204Pb Table 2. A987C), 3.5 mg of heavy zircon (density the following view of the Proterozoic history + 4.6 g.cm"3) and about 3 mg of zircon 4.2— of the 1900 Ma group: 4.6 g.cm"3. These zircons have been described by Laukkanen (in prep.). U-Pb data on the Age (Ma) zircon fractions are given in Table 1, and Rock body Locality ± 2 error Reference (concordia isotope ratios are plotted on the insert graph intercept) in Fig. 5. A tentative interpretation of these fairly concordant results in that they fit dif- Parikkala Kesus- Nykänen, 1886 ± 1 ferent diffusion curves (Wassenburg 1963) as gabbro maa in prep. Saari follows: fraction A 1905 Ma (K = 2.0000E- Voinsalmi Ranta- Gaål et ai., 1884 ± 13 11/a); fraction B 1900 Ma (K = 3.20834E- intrusions salmi 1971 and un- published 11/a) fraction C 1919 Ma (K = 4.47080E- Kotalahti Leppä- Gaål 1980 1882 ± 13 11/a). Owing to the small sample size the diorite and virta mafic pegm- limits of errors for single ages are quite large atoid (e.g. 207Pb/206Pb ages (Ma): A, 1905 ± 22; Koivujärvi Piela- Helovuori 1892 ± 14 gb. diorite vesi 1979 and un- B, 1898 ± 12; 1915 ± 13). Despite the limits published of the errors it is clear that a pooled zircon Vaaraslahti Piela- unpublished 1884 ± 9 hy. granite vesi chord for the Iso-Uski lamprophyre is con- Jusko qtz. Pyhä- Helovuori 1893 ± 3 sistently offset by 10—20 Ma with respect to diorite salmi 1979 Kettuperä Pyhä- Helovuori 1932 ± 3 the chord for the zircons from the belt intru- gneiss salmi 1979 sions and by 70 Ma with respect to the lead- Käpylä Vihanti unpublished 1886 ± 5 granite lead isochron of whole rocks and apatites. Tuli-Toiviai- Kiuru- Marttila 1981 1886 ± 5 From field evidence it is clear that the zircons nen mafic vesi pegmatoid are not coeval with the crystallization of the Ylivieska Yli- Pesonen and 1884 Iso-Uski whole rock. This effort to establish gabbro vieska Stigzelius 1972 (nearly the qualified tectonic framework provides concor- dant) Paleomagnetism and age relations of the rocks in . . . 119 The data presented provide an internally Three rock types yielding reliable paleo- consistent framework, which includes the magnetic results were collected for the base- Pihtipudas intrusions outside the belt. An ment region in the Varpaisjärvi—Nilsiä area: older event is represented by the Kettuperä 1) hypersthene-bearing dioritic basement gneiss described by Helovuori (1979). gneiss, 2) diabase dykes cutting the basement, The ages were all calculated according to and 3) tonalite dykes representing the 9 _1 238 the following decay constants (10" a ): U: youngest rock in the area. For comparison, 235 0.155125; U: 0.984850. additional samples were drilled from plutonic The K-Ar and Rb-Sr ages for biotite from subsilicic intrusive rocks and diabase dykes the Vaaraslahti pyroxene granite show fair in the Iisalmi region northwest of Varpais- agreement at 1760 Ma and 1730 Ma (Wetherill järvi (Fig. 1). et al. 1962). Similar K-Ar mica ages have The samples were drilled and oriented in been found for other granitoids and schists the field with a sun compass on glacially in S. Finland, irrespective of their strati- polished, fresh surfaces. The stability of the graphic position, and thus provide a minimum age for the last regional cooling event. remanence was tested by progressive demag- netization up to 90 or 100 mT and checked by heating. As a rule no secondary component was detected besides the VRM: Usually, only Paleomagnetic measurements those samples were accepted which yielded a circle of confidence less than 20° and a low Rocks suitable for paleomagnetic measure- 0 -value (Pesonen 1973) as positive tests for ments were found both inside and outside ß3 stability. the Belt area. Within the Belt, in the Hau- kivesi area, three different rock types with hard enough remanence were drilled: 1) hypersthene-bearing quartz diorites, 2) dia- Remanent magnetization of the intrusive base dykes, and 3) lamprophyre dykes. Con- rocks in the Haukivesi area tact relations indicate that the lamprophyre The only rock type which gave really reli- dykes are the youngest in the area. They are also the most suitable for paleomagnetic able results in the form of stable remanent work because of their hard and stable magne- magnetization in the Haukivesi region were tization. The quartz diorites and diabase the lamprophyre dykes encountered on small dykes are weakly magnetized and thus only islands in the lake Haukivesi (Fig 2). The in three samples was remanent magnetiza- 26 samples drilled at 12 sites all yielded a tion detected with the instrument available stable direction of remanent magnetization (Forster-type fluxgate spinner, Forster 1966) (Table 3 and Fig. 1). The samples were despite the large collection of oriented sam- cleaned with af- and heat-treatments; both ples drilled. Fortunately, the same rock types methods gave a similar end point (Fig. 6). In with more stable magnetization are found in contrast, only two samples from a large col- the Pielavesi—Kiuruvesi area. Additional lection of gabbro, diorite and quartz diorite paleomagnetic data on the Belt area are avail- samples had hard magnetization. Their mag- able thanks to the work of Pesonen and Stig- netic directions join with the direction mea- zelius (1972) on Ylivieska and other areas at sured on a narrow diabase dyke from the the northwestern end of the Sulphide Ore same area and are shown in Table 3. Belt. Although, as suggested by the cutting con- Table 3. Remanent magnetization of basic intrusions in the Haukivesi area. K> Remanent magnetization after cleaning o Sample Rock type Locality N n No. Deel, Intensity Incl. Ct95 k R Demagn. Remarks (deg.) (deg.) (A/m) (deg.) 7333 Lamprophyre dyke Pikku Pöljä, 1 12 347 32 .873 4 112.7 11.902 40 mT Af E Rantasalmi 7705 Iso-Uski, 1 10 357 37 .212 1 1973 9.995 80 mT Af Savonlinna 7707 Selkä-Anttonen, 2 10 + 10 352 33 .118 (2 + 8) 340 1.997 80 mT Af Rantasalmi oB 7801 Nuottaluoto, 2 10 + 9 347 29 .146 (2 + 2) 811 1.998 45 mT Af Rantasalmi 7802 Tiheinen, 2 9 + 9 346 41 .078 (8 + 2) 55.3 1.981 45 mT Af E Rantasalmi 7803 Kirvesluoto, 2 10 + 6 349 38 .035 (5 + 5) 1518 1.999 45 mT Af E Rantasalmi 7804 Tiheinen, 3 8 + 10 + 10 343 40 .062 7 296.3 2.993 50 mT Af N Rantasalmi 7805 Iso-Uski, 2 8 + 10 346 41 .242 (1 + 1) 1254 1.999 500°C Heated o Savonlinna s. 7806 2 9 + 10 341 45 mT Af Iso-Uski, 43 .312 (2 + 2) 13120 1.999 a Savonlinna o 5 s 7807 Selkä-Anttonen, 10 + 5 354 45 .285 8 91.8 4.956 450°C Heated s Rantasalmi o 1 o 7808 Maa-Anttonen, 10 346 37 .121 9 31.2 9.971 45 mT Af s S Rantasalmi a 7809 Maa-Anttonen, 2 10 + 10 347 39 .382 (5 + 3) 300.2 1.996 450°C Heated W Rantasalmi Pole 225° E, 48° N C •e Average lamprophyres 28.43° E 62.13° N 12 — 348 38 — 3.1 192.4 11.942 dm = 3.7 e s dp = 2.2° e o 7329 Hy-qu-diorite Torasalo, 1 11 346 53 .180 7 49.1 10.797 40 mT Af Rantasalmi 7330 Porosalmi, 1 10 350 30 .124 23 5.3 8.309 40 mT Af Rantasalmi 7703 Diabase dyke Näätäsaari, 1 10 349 49 0.013 7 45.9 9.804 50 mT Af Rantasalmi Pole 225° E, 53° N Average diorites and diabase 28.28° E 62.15° N 3 — 349 44 — 19 44.6 2.955 dm = 24° dp = 15° Total average Haukivesi area 28.40° E 62.13° N 15 — 348 39 — 3.4 130.8 14.893 Mean paleomagnetic pole position 225.4° east, 49.0° north, <5m = 4.1°, ép = 2.4° N = number of cores, n = number of specimens, a = circle of confidence at 95 °/o probability, k = estimate of the precision parameter, R = resultant of the unit vectors Paleomagnetism and age relations of the rocks in . . . 121 Fig. 6. Demagnetization of the lamprophyre dyke rock. Specimen 7804.15 demagnetized with alter- nating magnetic field (af) treatment. Specimen 7809.21 demagnetized by heating. tact relations, the dyke rocks must be some- Remanent magnetization of the subsilicic what younger than the plutonic rocks, the intrusions in Kiuruvesi—Pielavesi area directions of the remanent magnetization do not differ markedly. The paleomagnetic pole The Kiuruvesi—Pielavesi area, which lies positions calculated on the basis of these di- about 150 km northwest of the Haukivesi rections are plotted on the APW path given area is characterized by the same type of for the Baltic Shield by Pesonen and Neuvo- hypersthene-bearing plutonic rocks and simi- nen (1981) in Fig. 8 as pole numbers 1 and 2 lar diabase dykes as those encountered in the (subsilicic intrusions and lamprophyre dykes, Haukivesi area. These rocks are, however, respectively). more strongly magnetized than those in the 122 K. J. Neuvonen, Kalevi Korsman, Olavi Kouvo and Jorma Paavola Haukivesi area; hence, the paleomagnetic C o o results are more reliable. The paleomagnetic O) a> C/3 pole position measured on gabbro from this •M •M >> >> area has been reported by Pesonen and Neu- Q Q vonen (1981) to be 238.2° E and 35.9° N; it is plotted here in Fig. 8 as pole number 3. A G co H very similar pole was calculated from the S s a —• values measured on two gabbro samples and Q four diabase sites sampled in the Kiuruvesi— Pielavesi area by the present authors (Table M a 4). The paleomagnetic pole position given by these samples is 235.2° E and 43.1° N (pole number 4 in Fig. 8). The hypersthene gabbros in this area are assumed to be synorogenic and somewhat less than 1900 Ma in age (Mart- CSC tila 1976), see Table 1. The diabase dykes E are evidently very similar in age, although Paleomagnetism of the subsilic intrusions a 'o at the northwestern end of the sulphide OoJ tuO j) PS ore belt Pesonen and Stigzelius (1972) carried out a paleomagnetic investigation of the gabbros and related intrusive rocks at the north- western end of the Belt (Pohjanmaa region). They studied the remanent magnetization of a large noritic gabbro in Ylivieska and of :c0 .„- eight other gabbro-diorite bodies in the Poh- c/3 >> c/3 >> c/3 janmaa region which, according to Salli <1>> c/3 X> tow ^> to* 2 g 2 g S CO 3t n> >M > CO > »X 3 ti (1965), form a comagmatic synorogenic series. o 3 ai 3 CD 3 : •J Hard and stable magnetization was found in o> Ylivieska and in five other intrusions in the O) a M h co area (Fig. 1). The pole site given by the Yli- e ä^ co co O vieska gabbro is 242.2° E and 43.3° N and is XJ J3 .a c,rH CO .OCO J2 CJ o CO cfl O) CO OcCoD S c n other Pohjanmaa gabbros give a paleomag- S-2 netic pole at 239.1° E, 37.9° N; it is shown as S-£ u 83 cd ** rt o number 6 in Fig. 8. Cfi Cu 2 Paleomagnetism and age relations of the rocks in . . . 59 According to Pesonen and Stigzelius (1972), C the deviation observed in the magnetic direc- tions in the area can be assumed to be caused by paleosecular variation in the ancient geo- magnetic field. (M^MtDOM^O) rt COCOlOD-COC005£> 03050303 03 05 0303 Remanent magnetization in the olojoioiiflioidio Varpaisjärvi—Nilsiä area COt-COCD^OC''^ Paleomagnetic measurements were carried HIOHMNCOCON out on the hypersthene-bearing basement complex, diabase dykes and tonalite dykes in ro ttO woinincoojMM the Varpaisjärvi—Nilsiä area. Drilled at a number of sites NE of Varpaisjärvi (Fig. 3), CO the basement rock yielded unaltered, well a CO > ODt-NHD-iflhO preserved rock types with stable and hard c-5 cöinm-H^iococo rocks (313.0° E and 63.9° N) differs greatly OOOOCDCDtOtO from all other paleopoles measured for the Baltic Shield. This pole was used by Pesonen and Neuvonen (1981) as a 2680 Ma old key M M M M 0) a pole in their construction of the APW path >i for the Baltic Shield. This pole is shown as ttO netized although some unaltered samples CO contain hard and stable magnetization (Fig. $ w 7). The af-cleaned directions measured on M e C . in ö e § •CO CO HS o3 Ö.3 CO these diabase dikes are listed in Table 6. The CO C Ol ICO .rt w 0) :c0 B •9 'S § 11 Jj s i:ng5 paleomagnetic pole 187.8° E and 47.1° N, is rt Fig. 7. Demagnetization of samples from Varpais- järvi area. Specimen 7818.15 hypersthene dio- rite demagnetized with af-treatment. Specimen 7826.26 hypersthene dio- rite about 7 cm from the diabase contact (at-treait- ment). Specimen 7718.15 cutting diabase demagne- tized with af-treatment. Clegg 1962 and Pesonen 1978) was carried that the magnetizations measured on both the out. As seen in Table 7, the declination and diabase and the unaffected basement rock inclination values are unaffected about 100 m are of primary TRM. from the contact of a diabase dyke over 25 m Stable magnetization was also measured wide. When approaching the contact the on some samples from the tonalite dykes in direction of remanent magnetization gradu- Nilsiä, in the southern part of the Lake Syvä- ally moves towards the direction of the dyke ri area (see Fig. 3 and p. 115). The direction rock. Consequently, it can be assumed that of remanent magnetization of these dykes is the heat transferred from the dyke has re- given in Table 8. The remanent magnetiza- magnetized the wall rock. The gradual tion is weak but hard, and the average direc- change in wall rock magnetization confirms tion for the five dykes differs but little from Table 6. Diabase dykes of the Varpaisjärvi area, type 1. Remanent magnetization after AC-cIeaning bampie Locality N n Deel, Incl. Intensity Demagn. No. 0195 k R Remarks (deg.) (deg.) (A/m) (deg.) (mT) 7718.1 Hanhimäki 1 10 12 25 0.047 2 503.6 9.982 40 Af 7720.1 Saarnakallio 1 10 13 44 0.028 9 32.7 9.724 50 » 7723.1 Härkäharju 1 10 15 35 0.242 1 1784 9.995 50 » 7816.3 Nieminen 1 8 15 54 0.031 10 30.1 7.770 50 Baked contact, Af 7817.1—2 Nieminen 2 10 + 10 27 45 0.067 (9) 1068 1.991 40 Af 7821.1—2 Hanhimäki 2 9 + 10 8 32 0.028 (5) 198.3 1.995 50 » 7824.1—2 Joutsenus 2 10 + 10 10 34 0.145 (2) 729.9 1.999 50 » 7826.2 Jonsanharju 1 3 20 32 0.081 5.2 555.7 2.996 50 » 7827.1—2 Saarnakallio 2 10 + 10 9 44 0.027 (5) 276.4 1.996 40 » 7829.1—2 Suomäki 2 10 + 10 9 37 0.212 (4) 781.6 1.998 30 » 7831.1—2 Palomäki 2 10 + 10 24 41 0.020 (12) 35.3 1.971 40 » Average lat. 27.91° E 11 15 38 — 5.4 73.35 10.863 long. 63.38° N Paleomagnetic pole position 187.8° E, 47.1° N, (5m = 6.4°, <5p = 3.8° (Pole number 7 in Fig. 8) Notations as in Table 3 Table 7. Remanent magnetization of the hypersthene-bearing basement rock in contact with the diabase dyke. Direction of the magnetization after AC-cleaning Sample ... Distance from ' rLocality .. „„ „ WMN Decl IncL No Locality the contntaactt - Intensity a95 . R Demagn. (deg.) (deg.) (A/m) (deg.) K a (mT) 7818.1 Nieminen, > 100 m 6 291 65 .977 4.3 247.7 5.980 80 Varpaisjärvi 7826.3 Jonsanharju, 1.2 m 6 354 34 .152 2.8 561.7 5.9911 50—80 Varpaisjärvi 7816.1 Nieminen, c. 1 m 10 359 42 9.7 25.8 9.652 50 Varpaisjärvi 7816.2 c. 1 m 10 5 37 4.0 143.7 9.937 50 7816.3 » 0.2 m 8 15 54 .031 10.3 10.2 7.768 50 7826.2 Jonsanharju, 7 cm 3 20 32 .081 5.2 555.7 2.9964 50 Varpaisjärvi Notations as in Table 3 126 K. J. Neuvonen, Kalevi Korsman, Olavi Kouvo and Jorma Paavola that measured for the subsilicic intrusions e •. o oo O) LO to H o in the Haukivesi area (Table 3). As mentioned G S &H earlier on p. 114, the magnetization of two of 0) — CO l> I> rH rt o o 223.6° E and 47.3° N is given in Fig. 8 as LO CD CO CM number 8. a oi LO ^ + + Remanent magnetization of subsilicic CO o rocks in the Iisalmi region (M Tf CM Oi C- Hypersthene-bearing subsilicic rocks of an w P N^rtNO O C-5 O O O O O S < S5 intrusive character occur in the region north- c w o east of the town of Iisalmi near the north- o. eastern boundary of the sulphide ore belt (Fig. 1). Samples for comparison were drilled 011 £ for paleomagnetic measurements from gab- bros, diorites and diabase dykes. The mag- 0 00 l> H CD M > CO CO 01 ai LO LO LO T^ netic directions observed in these rocks are QS CO CO CO CO CO CO CO given in Table 8 and Fig. 1. There is a rather large scatter in the directions of the remanent magnetization measured. The magnetic ori- entation in the cutting diabase dikes is slight- + + o o ly different from that in the plutonic gabbros and diorites. Although not verified by radio- metric datings, this might be caused by a real difference in the time of emplacement. Paleo- magnetic poles 9 and 10 calculated on the basis of the Iisalmi intrusions lay close to the assumed APW path in Fig. 8 but the age dif- ference between the gabbros and diabase dykes has not been asserted. :cd :CÖ „sa 'c/3 'c/3 a.S2 - c ISS ^i? &ä «a •*-o> u s S J2 M Ui 00 'cfl co CU „ .. s g 4) —• 4) P* > Incl. Intensity Demag. Sample N Deel, «95 k R No. Rock type Locality (deg.) (deg.) (A/m) (deg.) (mT) 7339 Pyroxene Holopanlampi, 10 306 50 2.554 2.4 401.0 9.978 30 diorite Iisalmi 7340 — » — Tervamäki, 11 305 22 1.570 5.6 67.4 10.852 30 Iisalmi 7343 — » — Valkiamäki, 12 326 46 0.535 2.5 305.8 11.964 30 Iisalmi 7344 Diorite Saarela, 9 338 39 0.142 6.3 67.4 8.881 30 Iisalmi 7345 Pyroxene Syrjäpuro 12 338 29 1.390 2.5 295.9 11.963 30 TJ diorite Iisalmi rt 7767 Gabbro Central prison, 10 352 26 2.779 4.6 113.1 9.920 50 Sukeva 1 Ell HBQ IB Average gabbro-diorites 6 328 36 — 16.1 18.16 5.724 Pole site Pole 9 in 27.31° E 63.62°N 249° E 42° N Fig. 8 7772 Diabase Moilanen, 3 344 66 .175 2.9 1786 2.998 40 Sonkajärvi OJ 7773 Diabase Pienimäki, 3 345 44 .326 3.4 1284 2.998 40 D(CO Sonkajärvi 7774 Diabase Saarimäki, 3 334 64 .222 8.7 201.0 2.990 40 Sonkajärvi 7775 Diabase Halmemäki, 3 334 46 .010 9.3 176.7 2.989 100 Sonkajärvi Average diabase dykes 4 339 55 — 14.0 43.77 3.931 Pole site Pole 10 in 27.50° E 63.73° N 241°E 59°N Fig. 8 sr 3 On Notations as in Fig. 3 sr to -a 128 K. J. Neuvonen, Kalevi Korsman, Olavi Kouvo and Jorma Paavola the paleomagnetic data between the belt area either side of the fault line. Such a differ- and the region north of it. ence has not been observed in the paleomag- The directions of remanent magnetization netic data collected from and north of the measured inside the belt vary very little (Fig. Belt area. Furthermore, the type of differ- 1). This coherency is well seen in the cluster ence in paleomagnetism between these areas formed by the paleomagnetic poles of the cannot be explained by simple lateral dis- rocks in the belt area (Fig. 8). In contrast to placement. On the contrary, there must be this grouping, the paleomagnetic poles cal- a fundamental dissimilarity in the geological culated from the data collected in the Var- and thermal history of these two adjuncted paisjärvi—Nilsiä and Iisalmi areas are dis- areas to be able to produce such a big dif- tributed over a wide area (Stars in Fig. 8). ference in the nature of the paleomagnetic Any explanation given for the origin and behaviour as described above and illustrated evolution of the belt has to take this paleo- in Fig. 8. magnetically fundamental difference into If the Belt was formed as a result of the consideration. convergence of two crustal plates and if the Three tectonic models that differ in prin- magnetization of these plates took place be- ciple have been proposed to explain the ori- fore the collision, they should show some- gin of the Main Sulphide Ore Belt. As pointed what different orientations in remanence. out by several authors (e.g. Gaål and Rauha- Hence, they should also show different paleo- mäki 1971 and Parkkinen 1975) the large magnetic pole positions. As discussed by number of transcurrent faults and shear McElhinny and McWilliams (1977), the dif- zones testifies to a large lateral displacement ference observed is not, however, simply re- along the Belt. The Belt might well have lated to the amount of relative motion but developed as a result of a sequence of lateral also to the geometry of the movement in- displacements and shears. On the other hand, volved. On the other hand, the displacement Kahma (1973) and later Bowes (1980) have must be very large to be detected paleomag- proposed a tectonic model involving the con- netically. vergence of two crustal plates with the Belt To see convergent plate motion in the representing the suture zone of this collision. paleomagnetic results, we should have paleo- According to this model, the sulphide-bearing magnetic pole data available on both the volcanic- sedimentary sequence in the Belt is Archean plate northeast of the Belt and the a remnant of a volcanic island arc system. Presvecokarelian block southwest of it. This In the third model, proposed by Marttila is not the case since no such old remnant is (1976) it is suggested that a subsiding rift known on the southwestern side of the Belt. valley system with a folded and highly The divergence in magnetization between the metamorphosed volcanogenic-sedimentary Belt and the basement block in the Varpais- pile developed along the axial zone. järvi area north of it cannot be caused by the The paleomagnetic data available should, motion of two approaching plates. Further- in principle, be useful for finding out which more, the tonalite dykes cutting through the of the models proposed are acceptable. If Archean basement in Nilsiä have the same for instance, the Lake-Ladoga — Raahe magnetic direction and the same paleomag- structure were due mainly to large lateral netic pole position as do the intrusive rocks transcurrent displacements, it would be seen of the same age (about 1850 Ma) within the as a difference in the declination and/or in- Belt (Fig. 8). This indicates that no very clination of the remanent magnetization on large lateral displacement has taken place Paleomagnetism and age relations of the rocks in . . . 129 Fig. 8. Paleomagnetic pole positions determined from the Main Sulphide Ore Belt (diamonds) and basement areas north of it in the Varpaisjärvi—Nilsiä and Iisalmi regions (stars) plotted on the apparent paleomagnetic polar wandering path of Pesonen and Neuvonen (1981). Pole No 1. Subsilicic intrusions in the Haukivesi area, Table 3. No 2. Lamprophyre dykes in the Haukivesi area, Table 3. No 3. Subsilicic intrusions in the Pielavesi area, Pesonen and Neuvonen (1981). No 4. Subsilicic intrusions in the Pielavesi—Kiuruvesi area, Table 4. No 5. Ylivieska gabbros (Pesonen and Stigzelius 1972). No 6. Pohjanmaa gabbros (Pesonen and Stigzelius 1972). No 7. Type 1 diabase dykes in Varpaisjärvi, Table 6. No 8. Type 2 tonalite dykes and diabase dykes, in the Nilsiä—Varpaisjärvi area, Table 8. No 9. Gabbros in the Iisalmi area, Table 9. No 10. Diabase dykes from Iisalmi area. Table 9. No 11. Hypersthene- bearing isolated basement complex in the Varpaisjärvi area, Table 5. between the Belt and the Archean craton coherent magnetization would have taken area since Svecokarelian time. place when the rock units passed the blocking The simplest tectonic model to explain the or Curie-temperature in the crust (zero-line observed paleomagnetic character of the of Neuvonen 1961). areas now studied is axial uplift or subsiding If however, this type of uplift magnetiza- of the Belt in relation to the basement com- tion takes place in rock units that are of simi- plex in the northeast. The homogeneous and lar age but belonging to two adjacent blocks coherent remanence measured on the erosion with a different rate of uplift, the rocks surface in the Belt area could have been should show different magnetic directions on created in the rocks of the rising Belt in a the erosion surface. The tonalite dykes in manner similar to that described by Beckman Nilsiä (Table 8) and possibly also the subsi- et al. (1977) and Morgan (1976) from Green- licic intrusions in Iisalmi (Table 9) have the land (see also Ueno and Irving 1976). The same age as the intrusive rocks in the Belt 130 K. J. Neuvonen, Kalevi Korsman, Olavi Kouvo and Jorma Paavola area. The paleomagnetic directions are also The radiometric age data derived from the similar, and the paleomagnetic pole positions Belt area indicate a short period of igneous for all these rocks (poles 8, 9 and 10 and all activity about 1880—1840 Ma ago. the diamond-poles) plot in the same area in This period is also recorded in the mag- Fig. 8. This has important consequences for netization of the rocks of the Belt and is the interpretation of the paleomagnetic re- illustrated as a clustering of the paleomag- sults obtained. netic poles in Fig. 8. Although the conver- gence of two crustal plates cannot explain As the first conclusion we can postulate the magnetization observed in the area that the paleomagnetic measurements in the studied, high-temperature metamorphism and area date the very same events as do the the intensive igneous activity associated with radiometric U-Pb age determinations. This is it might well be caused by plate suturing. a very important fact since many Sveco- On the other hand, the high heat flow and karelian intrusive in this same area (about high temperature metamorphism connected 220°—250° E and 30°—60° N) rocks in other with magmatic activity along the Belt can localities of the Fennoscandian Shield (Peso- well explain both the paleomagnetic and the nen and Neuvonen 1981), plot for example radiometric data without any suturing. As intrusive rocks in northern Sweden (Cornwell proposed by Bowes (1980) plate collision and 1968), subsilicic plutonites in SW Finland the associated subduction associated, would, (Neuvonen 1974) and effusive and dyke rocks however, nicely complete the whole evolution in Soviet Karelia (McElhinny and Cowley picture of the Main Sulphide Ore Belt. This 1977). All these rocks are about or slightly model would explain the volcanogenic-sedi- younger than 1900 Ma. Therefore, there is mentary pile and the associated sulphide ores every reason to consider this spot as one of as the remnant of a volcanic island arc sys- the Key Points of the APW curve of the tem. The same explanation was earlier adop- Fennoscandian Shield. ted by Kahma (1973). An even more precise explanation for the ore deposits of the Belt Because the pole positions of the tonalite would, however, be that of deep fluid con- dykes and the Iisalmi intrusion are similar to vection as described e.g. Hutchinson et al. those of the Belt rock, the homogeneity ob- (1980). served in the magnetization within the Belt The paleomagnetic pole position calculated is not likely to have been caused by any plate for type 1 diabase dykes in Varpaisjärvi (pole motion, whether vertical or horizontal. Any number 7 in Fig. 8) lies far off the adopted such movement would have been recorded on APW curve. The Archean basement block these rocks and would have resulted in a intersected by these dykes was evidently in difference in the magnetic directions. Fur- an isolated position during the Proterozoic thermore, there has evidently not been any time, since the high grade Archean mineral marked crustal uplift associated with the belt parageneses are preserved and the paleo- since Svecokarelian time, since such a magnetic directions are still measurable. The phenomenon would certainly have been rec- positive baked contact test (p. 124) indicates orded in the magnetization of the Belt rocks. that the magnetic directions measured for It is also evident that the age of magnetiza- both the basement and the cutting dykes are tion of the Belt does not accord with the fis- primary. As proposed by Pesonen and Neu- sion track ages measured by Lehtovaara vonen (1981) a more complex polar wandering (1976). curve with a large post-orogenic loop could Paleomagnetism and age relations of the rocks in . . . 131 be drawn for the age interval 1850—1750 Ma Acknowledgements — The authors are indebted to fit this diabase pole as well. On the other to many persons who helped them during their work in the field and in the laboratory. Special hand, as mentioned earlier (p. 115), field evi- thanks are due to Mrs. Ritva Ääri for the dence suggests that these diabase dikes are magnetic measurements, to Mrs. Hanna Koskinen more readily correlated with the Jatulian for most of the drawing, to Mrs. Tuula Wan for dykes about 2150 Ma old. Therefore, discus- typing the manuscript and to Dr. Lauri Pesonen for critically reading and correcting it. sion on the shape of the APW curve must be postponed until more paleomagnetic work The assistance given by the National Research Council for Science towards collecting the has been done on new, radiometrically dated samples studied paleomagnetically is gratefully rocks aged between 1900 and 2500 Ma. acknowledged. References Aho, L. 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