Radiometric age determinations from Finnish Lapland and their bearing on the timing of Precambrian volcano-sedimentary sequences Edited by Matti Vaasjoki Geological Survey of Finland, Special Paper 33, 15– 43 , 2001.

HISTORY OF STRATIGRAPHICAL RESEARCH IN NORTHERN FINLAND

by Eero Hanski

Hanski, Eero 2001. History of stratigraphical research in northern Finland. Geological Survey of Finland, Special Paper 33,, 15–43.15–43. 66 figures.figures. AnAn overview overview on on the the results results of of stratigraphical stratigraphical research research carried carried out in northern Finland (Central Lapland, the Kuusamo and Peräpohja schist belts) since the early 20th century is presented. The traditional stratigraphical names, Lapponian and Kumpu and their derivatives, which have been widely used in northern Finland, were defined in the 1920s and 1930s when the stratigraphical interpretations were based on establishing cycles of sedimen- tation separated by diastrophisms and intrusions of granites with then unknown ages. Later results of isotopic and geological work have forced the investigators to revise their stratigraphical schemes and regional correlations, but the nomenclature rooted to the traditional names has continued their life until recently. Particularly, the chronostratigraphical position of the Lapponian and Kumpu rocks and their lithostratigraphical correlation with the traditional Karelian formations have been a controversial issue over the years. The review illuminates the reasons that led to this long-lasting lack of consensus and explains why the abandonment of the old names along with the adoption of the formal lithostratigraphical nomenclature was still neces- sary as late as in the 1990s.

Key words (GeoRef Thesaurus, AGI): stratigraphy, lithostratigraphy, chronostratigraphy, greenstone belts, schist belts, metamorphic rocks, Paleoproterozoic, Archean, research, history, Kuusamo, Peräpohja, central Lapland, northern Finland

Eero Hanski, Geological Survey of Finland,P.O. Box 77, FIN-96101 Rovaniemi, Finland. E-mail: [email protected]

INTRODUCTION We have all been hopelessly wrong at one time posed Archean and Paleoproterozoic terranes, field or another, and it would ill behove us to hold the investigations alone have not always produced unam- great pioneers of the Precambrian in any the less biguous results concerning the order of supracrustal esteem because we have the advantage of tech- sequences and their regional correlations. In the niques undreamt of by them. absence of fossil-bearing strata in the early Pre- cambrian, radiometric datings have become invalu- Arthur Holmes, 1963 able in providing indirect or direct age information for supracrustal rocks. It is obvious that the first U-Pb Field studies are the cornerstone of all stratigraphical zircon ages obtained in Finland quickly brought about interpretations and models of geological evolution. fundamental changes in the conceptual framework of However, in structurally complicated and poorly-ex- the Precambrian geology within the Fennoscandian

15 Geological Survey of Finland, Special Paper 33 Eero Hanski

Shield (Kouvo 1958, Wetherill et al. 1962). However, but this was not regarded as satisfactory by everyone despite the increasing number of radiometric data as reflected, for example, by the following statement produced since then, their major impact on solving by Marmo and Ojakangas (1984): “Division of the stratigraphical problems in northern Finland took place rock sequences on the Baltic Shield into groups relatively late. and formations apparently has not been done as The Precambrian rocks in Finland are divided into rigorously as the Code of Stratigraphic Nomen- three main units: 1) the Svecofennian terrane in the clature prescribes for North American rock se- southwest representing a juvenile Paleoproterozoic quences.” Nevertheless, this seemed to remain as a arc complex, 2) the Archean granite-gneisses and temporary practice before shifting to the more strict greenstone belts in the northeast, and 3) the lithostratigraphical system based on international re- Paleoproterozoic supracrustal sequences, assigned commendations in which the stratigraphical names commonly to the Karelian formations, which overlie are formed by combining local geographic names of and, at least for the most part, were originally depos- the type areas with the proper rank terms (Hedberg ited on the Archean sialic basement. The focus of this 1976, Salvador 1994). Applying this procedure, clas- paper is on the third unit, particularly the Karelian sification of the Karelian formations into formal metasedimentary and metavolcanic rocks occurring lithostratigraphical groups and formations gradually in Central Lapland and the Peräpohja and Kuusamo took over in the Kainuu and northern Karelia schist schist belts. belts in central and eastern Finland (Kontinen 1986, The Karelian supracrustal successions reach sev- Gehör & Havola 1988, Strand 1988, Laajoki 1991, eral kilometres in total thickness, are spread intermit- Pekkarinen & Lukkarinen 1991, Kohonen & Marmo tently over an area occupying the northeastern half of 1992). Lately, the same classification system has also the Fennoscandian Shield and record a geological been applied to rock successions in northern Finland, evolution of several hundreds of millions of years. Due both in the Peräpohja area and in Central Lapland to their wide development in space and time, the (Perttunen et al. 1995, Räsänen et al. 1995, Lehtonen stratigraphy of the Karelian formations and their et al. 1998). Most geologists now prefer to restrict the correlation between different areas have drawn much use of the traditional names to broad, informal discus- attention since the beginning of the 20th century. The sions only or, as done by Laajoki (1988, 1990), to the Karelian stratigraphy has often been described in concepts of tectofacies. terms of adjectival names such as Sariolian, Jatulian Besides the lithological correlation with traditional and Kalevian (e.g., Meriläinen 1980a,b). These poorly- Karelian stratigraphical units, the interpreted defined, informal names, derived from the Karelian geochronological position of the supracrustal rocks in folklore, are deeply ingrained in the Finnish geological Lapland has varied much, from Archean to Proterozoic, literature, but have been used in an inconsistent man- in the articles by different researchers. This has ner and have also turned out to be insufficient for evidently caused confusion among Finnish geologists accurate correlations (e.g., Laajoki 1986, 1988). The as well as foreign investigators trying to make a historical background of the definitions and subse- synthesis on the geological evolution of northern part quent development of these commonly used of the Fennoscandian Shield (e.g., Pharaoh & Brewer stratigraphical terms are not explained here, accounts 1990, Brewer & Pharaoh 1990, Goodwin 1991). One of that can be found in Simonen (1955, 1986) and of the reasons for differing views is the geological Laajoki (1986) and references in them. For the partly diversity of rocks in Lapland, making them partly different nomenclature used in the eastern part of the similar to other Karelian formations, but on the other Fennoscandian Shield, the reader is referred to Rus- hand, there exist some rock units, for instance thick sian papers such as Kratts et al. (1984), Zagorodny et piles of komatiites, which hardly have any counter- al. (1986) and Semikhatov et al. (1991). In northern parts in Karelian formations in eastern or central Finland, additional terms, the Lapponian and Kumpu Finland. Until recently, there has been a lack of formations and their derivatives, have been widely reliable geochronological data to constrain the ages of employed, and on their relation to other Karelian certain sedimentary and volcanic rock units, but the formations, several differing opinions have been pre- situation is gradually improving as shown, for example, sented in the geological literature over the years. by the accompanying articles in this volume. In the 1980s, there was a tendency in Finland to At this stage, when the new lithostratigraphical combine the traditional (mythological) names with names have just been adopted and consequently more lithostratigraphical rank terms (e.g., Marmo et al. perplexity may still arise, it is prudent to review the 1988, Luukkonen & Lukkarinen 1986, Silvennoinen development of stratigraphical concepts in northern 1985, Geological Map, Northern Fennoscandia 1987) Finland over the past century. It is hoped that this will

16 Geological Survey of Finland, Special Paper 33 History of stratigraphical research in northern Finland lead to a better understanding of the reasons for in the Kuusamo schist belt because these two entities difficulties which which were were encountered encountered by by the the previous previ- form geographically distinct areas and have usually ousinvestigators investigators and and hindered hindered the achievementthe achievement of theof been described separately using different stratigraphi- endurable and uniformly accepted stratigraphical cal terms. It is assumed that the reader has basic schemes in Lapland. In the following treatise, the knowledge of the geology of northern Finland, as it is Peräpohja schist belt is dealt with separately from the beyond the scope of this paper to present any detailed Central Lapland Greenstone Belt and its continuations geological review.

HISTORY OF STRATIGRAPHICAL STUDIES IN CENTRAL LAPLAND AND KUUSAMO

The Paleoproterozoic greenstone belt considered north across the Finnish-Norwegian border. This is here forms a volcano-sedimentary belt extending from the area where the terms ‘Lapponian’ and ‘Kumpu’ Kuusamo through Salla, Sodankylä, Kittilä and Kolari have generally been applied. to the western border of Finland and from Kittilä to the

The 1920s to 1940s

Attempts at rationalization of the stratigraphy of the existence of an unconformity between these Central Lapland were initiated by Hackman (1927) sedimentary formations and the “older schists”, in- in his pioneering study of the Kittilä-Sodankylä area. cluding the presence of pebbles and cobbles in the His stratigraphical concepts are shown schematically basal conglomerates and conglomeratic quartzites, in Figure 1. Hackman regarded the gneiss granites of the rock types of which were unmistakably the same Muonio, west of Kittilä, as the oldest rocks, on which as in the older underlying rocks of the area. These two sequences of supracrustal rocks had been depos- pebbles were partly composed of quartzites and ited. The older one of these sequences, which he phyllites similar to those occurring in the “older schists”, called the “older schists”, was divided into a lower and also metabasites (metavolcanics and metadiabases) and upper series with no apparent hiatus between and iron ores and jaspers, evidently from the Porkonen- them. The lower series comprised pelites, quartzites Pahtavaara area. Hackman also established that the and carbonate rocks and also the iron formations of Kumpu quartzites had different strikes and dips from the Porkonen-Pahtavaara area occurring in the midst the older rocks in places, indicating the presence of an of the Kittilä greenstones, while the upper series angular unconformity. consisted of quartzites, the “Sodankylä quartzites”, As mentioned above, Hackman (1927) regarded and the spilitic metabasites (now known as mafic metavolcanics) of the Kittilä area. In the southern part of the area, the metasediments of the upper series are intersected by granites which belonged to the large “post-Kalevian” Central Lapland granite area. Hackman (1927) considered it likely that the metavolcanics are also older than these granites. For the younger sedimentary sequence in the Kittilä- Sodankylä area Hackman (1927) coined the name “Kumpu quartzites”, after the type occurrence at Kumputunturi. These metasediments, which form laterally discontinuous bodies extending from the Kaarestunturi area in Sodankylä to Pyhätunturi east of Kittilä, comprised coarse-clastic quartzites and polymictic conglomerates with brownish purple col- our locally. One of the best representatives of the basal conglomerates was shown to be the Sirkka conglomerate on the northern flank of the fell Fig. 1. Stratigraphical sketch of Hackman (1927) for the Kittilä- Levitunturi. Hackman presented ample evidence for Sodankylä area in Central Lapland.

17 Geological Survey of Finland, Special Paper 33 Eero Hanski

the granites cutting the “older schists” as similar to the determinations were available, Precambrian sequences “post-Kalevian” granites that were known to have were classified on the basis of their relations to epochs penetrated Kalevian rocks in northern and eastern of diastrophism, as manifested by intrusions of gran- Finland. It was therefore logical to assign the “older ites, for example. Using this method, Sederholm (1932) schists” and the large volcanic region in Kittilä to the divided the Precambrian of Fennoscandia into four Kalevian. It is important to stress here that since the cycles of sedimentation (Fig. 2). The oldest, works of Frosterus (1902) and Ramsay (1902), the Katarchean rocks were represented by Svionian sedi- Kalevian schists were generally regarded as older mentary and effusive rocks in the Svecofennian belt of than the Jatulian rocks, and consequently Hackman’s southern Finland, which were cut by post-Svionian “older schists” were considered pre-Jatulian. granites. Younger than these granites were the Hackman (1927) could not draw any firm conclu- Younger Archean Bothnian sedimentary and volcanic sion on the relationship between the “post-Kalevian” rocks in southern and central Finland which were in granites and the Kumpu quartzites. He had observed turn intruded by post-Bothnian granites. The third neither granitic dikes cutting into the Kumpu quartzites cycle of sedimentation, the Karelidic cycle, comprised or their basal conglomerates nor any typical post- the Jatulian and Kalevian rocks, penetrated by post- Kalevian granites as boulders in the conglomerates. Kalevian granites. The conglomerates do have granitic fragments, but Sederholm (1932) assigned Hackman’s (1927) according to Hackman, they could not be traced back Kumpu quartzites to the Jatulian rocks. According to to their source. Yet he was convinced that there was him, the Kumpu formation was unconformably over- a great age difference between the Kumpu quartzites lying a sequence of greenstones and associated schists and the quartzites from the “older schists” and consid- which had a similar position to the Bothnian rocks ered it probable that the former were younger than the between the post-Svionian and post-Bothnian gran- post-Kalevian granites. In his stratigraphical scheme, ites. Hackman (1927) had correlated the same rocks Hackman correlated the Kumpu quartzites with the with the Kalevian schists, but this correlation was no Jatulian formations, although with reservations on longer tenable since, as the result of the work of account of some important observations related to Wegmann (1929), Väyrynen (1928) and Hausen lithological differences and the occurrence of mafic (1930), it had turned out that the Jatulian and Kalevian dikes within the respective formations. For example, rocks of eastern Finland belonged to one Karelidic he pointed out that Kumpu-type polymictic conglom- cycle of deposition and, furthermore, the Kalevian erates were absent from the Jatulian formations of the rocks were probably younger than the Jatulian ones. Rovaniemi-Tervola area in the Peräpohja schist belt, In contrast to the Karelidic cycle of deposition in where the rocks were very similar to the quartzites of eastern Finland, a great unconformity separated the his older sedimentary unit. Hackman also drew atten- different parts of the sedimentary rocks in Lapland, tion to the fact that intrusive metadiabases and which had been collectively referred to as “Karelian”. metagabbros were very common within Jatulian Sederholm thus considered that instead of one, at least metasediments in the Kuusamo and Rovaniemi-Tervola two cycles of deposition and diastrophism had oc- areas but were practically non-existent within the curred in Lapland. This was supported by the interpre- Kumpu quartzites. He knew only one example, from tation that in addition to the post-Jatulian (post-Kalevian) Kaarestunturi. Instead, he maintained that such rocks occurred widely as boulders in conglomerates of the Kumpu metasediments. It is worth noting here that Hackman (1918) had already earlier wondered about the absence of intersecting metadiabases in the Pyhätunturi conglomerates of the Pelkosenniemi area. The next contribution to the stratigraphy of Lapland was provided by Sederholm (1932) in his explana- tory notes to the Geological Map of Fennoscandia. As stated earlier, Sederholm (1932) was the first to introduce the later frequently used term ‘Lapponian’ for the widely distributed volcanic and sedimentary rocks of Central Lapland. To fully understand the terminological development, it is imperative to form an

idea of the methods of stratigraphical classification Fig. 2. Three oldest of the four cycles of sedimentation estab- employed at that time. As no radiometric age lished by by Sederholm’s Sederholm (1932). (1932).

18 Geological Survey of Finland, Special Paper 33 History of stratigraphical research in northern Finland granites, the lower supracrustal rocks were also cut by aluminous schists and black schists. Mikkola consid- an older plutonic suite, fragments from which are ered the eruption of the associated volcanic rocks to found in the basal conglomerates of the Kumpu be chronologically very close to the deposition of these metasediments. sediments. He was able to recognize two main types This reasoning of Sederholm thus negated the of volcanic rocks, one with ordinary basicity, the so- possibility of correlating the Central Lapland called Kittilä greenstones, and the other with a more greenstones and associated schists with either the basic chemical nature, the amphibole-chlorite rocks, Jatulian or Kalevian rocks. On the other hand, despite later to be known as komatiites (Mutanen 1976). the inferred similar position to the Bothnian schists, Although they were areally fairly well separated, Sederholm could not assign the Lappish sedimentary Mikkola thought that these two types of volcanic rocks and volcanic rocks to the Bothnian schists because of were genetically related. The jasper quartzites and differences in their primary characters. He concluded iron ores associated with the Kittilä greenstones in the that these two formations were of different ages but, Porkonen-Pahtavaara area were no longer regarded nevertheless, belonged to the same cycle of sedimen- as epiclastic sediments but as chemical precipitates tation. Therefore, Sederholm regarded it as necessary whose origin was closely linked with the volcanism. to adopt a special name, Lapponian, for these (pre- These metasediments were earlier documented as the Jatulian) supracrustal rocks of Lapland. oldest supracrustal rocks in Central Lapland (Hackman It is important to note here that Sederholm (1932) 1927, Sederholm 1932), but this view no longer fitted did not exclude the possibility that some quartzites of with Mikkola’s interpretations. northern Finland were even younger than the Jatulian Hackman (1927) had already demonstrated the proper. He was referring in this context to the Kumpu existence of an unconformity beneath the Kumpu formation, the correlation of which with the Jatulian formations, and Mikkola (1941) agreed with him, rocks of the Kemi and Kuusamo areas he regarded as regarding the evidence for the unconformity as con- not yet having been fully verified. vincing. In many areas, however, Mikkola had not The general geological mapping of Finland on a found clear field evidence for an unconformity be- scale of 1:400,000 by the Geological Commission tween Hackman’s older quartzites and the quartzites (since 1945 the Geological Survey of Finland) was of the Kumpu formations, and he therefore expanded initiated in the 1890s, and was later also extended to the younger group to include more quartzites, espe- Lapland. As the result of fieldwork extending from cially in the southern and eastern part of Hackman’s 1929 to 1935, Erkki Mikkola’s (1941) posthu- area, and even a considerable area of mica schists mously published explanation to three map sheets occurring around Oraniemi, south of Sodankylä. This (Tuntsajoki-Sodankylä-Muonio) covered the territory extension of the geographical distribution of the younger of Central Finnish Lapland in an E-W direction from group of sediments was accompanied by renaming the the Soviet border to the Swedish border. For decades unit as the Kumpu-Oraniemi Series. Mikkola ac- to come, this comprehensive treatise of Mikkola served knowledged a clear facies difference between the as a basic source of lithological and stratigraphical Kumpu quartzites and the other nearby quartzites, but information on Lapland. Many of the regional or local still regarded both as post-Lapponian. It was shown geological units recognized by him are still essentially later that the Oraniemi mica schists and many of the valid. quartzites included by Mikkola in the younger sediments The oldest geological unit distinguished by Mikkola are not part of the Kumpu sediments, and hence was the Tuntsa-Savukoski supracrustal series, occur- Hackman’s interpretation is closer to the scheme ring in eastern Lapland, which was composed mainly accepted by subsequent investigators (e.g., Rask of mica gneisses, quartzites and amphibolites. The 1978, Tyrväinen 1983). next chronological group of rocks was the gneissose As noted by Hackman (1927), the Sirkka conglom- granites and granite gneisses, which occupy large erates contain fragments of obviously intrusive gra- areas of eastern Lapland but are encountered in other nitic rocks, including gneissose granites and aplitic parts of the mapped territory as well. According to rocks of varying coarseness. Neither he nor Mikkola Mikkola, these gneisses definitely served as a base- (1941) could confidently trace the source of these ment for the supracrustal rocks, which he designated fragments, although Mikkola considered the aplites to as the Lapponian Series, applying the term of be possibly derived from the ‘syenite series’ (later to Sederholm. be called the Haparanda series by Ödman et al. 1949 Mikkola (1941) divided the Lapponian meta- and the Haaparanta Suite by Finnish geologists, e.g., sediments into quartzites and schists, with the latter Lehtonen et al. 1998) which forms a characteristic group being further divided into mica schists, highly igneous suite in western Lapland and the western part

19 Geological Survey of Finland, Special Paper 33 Eero Hanski

of the Peräpohja schist belt. place in Lapland as recorded in Jatulian successions The other issue concerning the granites is Mikkola’s elsewhere, the Jatulian-Lapponian correlation also observation that the westernmost Kumpu quartzites in seems to be excluded with regard to sedimentary the Muonio area are traversed by granitic dikes and rocks. A further indication of Mikkola’s unwillingness that these quartzites are associated with typical Sirkka to draw any firm conclusions on the regional correla- conglomerates that have been strongly migmatized. tions is revealed by the following passage: “But as the Two decades later, Eskola (1963) emphasized this author in any case considers the Lapponian for- observation, which, according to him, suggests that the mations, which have clearly a basement of gneissose Kumpu-Oraniemi Series belongs to the same orogenic granites etc. in E. Lapland, as joining more closely cycle as the Lapponian Series. the Karelides in respect to the age of deposition The correlation of the Lapponian Series with other and time of folding than other, more ancient stratigraphical units in Finland was regarded by Mikkola orogenic cycle of the Svecofennides which oc- as an important problem, but he conceded that differ- curred in Finland ...” Mikkola considered it possible ent conclusions may be arrived at depending on that the Kumpu-Oraniemi Series may correspond to whether volcanic or sedimentary rocks are concerned the Jatulian or Kalevian rocks farther south, based on in the first place. He tentatively correlated the the assessment that the underlying Lapponian rocks Lapponian metavolcanics with the volcanic rocks of are older than those stratigraphical units. However, eastern Finland. Wilkman (1921) had described abun- this correlation was not without obvious difficulties dant basic metavolcanics and more basic chlorite- due to the lithological deviations. Firstly, Mikkola amphibole rocks in the Nurmes map sheet area (D4), stressed the lack of carbonate rocks and black schists and according to Väyrynen (1933), these rocks were in the Kumpu-Oraniemi Series, which are typical of a similar to those occurring in the basement of the complete Jatulian succession. Secondly, the Kumpu- Jatulian system in the Pielisjärvi area (map sheet D3). Oraniemi Series hardly bears more resemblance to Other counterparts of the Lapponian metavolcanics characteristic flysch-type sediments of the Kalevian could be found in the Kuhmo-Suomussalmi greenstone sequences, such as occur, for example, in the classic belt, which in Mikkola’s opinion could also precede the area southwest of Lake Pielinen (Northern Karelia). Jatulian. Erkki Mikkola’s geological research in Lapland When considering the sedimentary rocks of the was interrupted by the outbreak of the war between Lapponian system, a different comparison had to be Finland and the Soviet Union in 1939. The commentar- performed by Mikkola, since the above-mentioned ies to the map sheets (Mikkola 1941), completed by his greenstone areas in the basement do not contain many colleagues Th. Sahama and K. Rankama, were pub- epiclastic sedimentary rocks. He admitted that the lished posthumously after Mikkola’s premature death lithological similarity of the quartzites in the Jatulian at the front in 1940 at the age of 35 years (see Eskola and Lapponian systems and their widespread regional 1941a,b). Several important chapters of the explana- development as basal units would strongly suggest a tion planned to be written by Mikkola, including that common period of deposition, but because no such dealing with tectonic development of southern Lapland, voluminous eruption of volcanic rocks as had taken were never realized (see Laitakari 1941).

The 1950s and 1960s

The next significant contribution to the discussion the detailed stratigraphical structure is a quite hope- on the Lappish stratigraphy was made by Väyrynen less task in most parts of the area. Nevertheless, one (1954) in his book ‘Suomen Kallioperä’ (The Bed- clearly discernible stratigraphical unit, the Kumpu- rock of Finland). This was largely based on the tunturi formation, can be shown to have been depos- accounts of Hackman (1927) and Mikkola (1941), but ited discordantly on the Kittilä greenstones. These he interpreted the available data in the light of the older greenstones and the associated metasediments wide knowledge of the Karelian formations which he did not show any signs of their mutual age relation- had achieved by studying these rocks in Kainuu and ships or stratigraphical positions. With regard to the Northern Karelia (Väyrynen 1928, 1933). sedimentological facies, however, the quartzites, Väyrynen (1954) pessimistically stated that igne- phyllites and dolomites represented mainly the same ous and sedimentogenic rocks lie in utter mutual formation types as occur in the Peräpohja schist belt, disorder in Central Lapland, and therefore unravelling for example, and they thus probably correspond to the

20 Geological Survey of Finland, Special Paper 33 History of stratigraphical research in northern Finland

Fig. 3. Correlation of the Lapponian and Kumpu formations in Central Lapland with the traditional Karelian formations as presented by various authors. (See also the column of Silvennoinen, 1992, in Fig. 4).

Jatulian rocks. Väyrynen pointed out that only the Kalevian rocks (Fig. 3). Referring to the Lapponian Kumputunturi-Sirkka metasediments had earlier been rocks, he stated: “The sedimentary formations in included in the Jatulian because the youngest sedi- Karelia, Kainuu and Lapland are everywhere mentary formations had usually been called Jatulian similar in character, and no geological evidence irrespective of their depositional type. of an age difference has been found.” Väyrynen (1954) hence regarded the older Differing from Väyrynen (1954), Simonen (1960a, supracrustal group in Central Lapland as correspond- p. 26) correlated the Lapponian succession in Lapland ing to the Jatulian (Fig.3), and consequently the with Kalevian micaceous schists (Fig. 3), which rep- Kumputunturi formation was bound to be post-Jatulian. resent a geosynclinal association deposited after the This correlation with the Jatulian formations explains Jatulian transgressive series. On the other hand, he why he did not need to use the term Lapponian at all. regarded the arkoses and conglomerates of the Kumpu- He placed the Kumputunturi formation in the Kaleva Oraniemi formation as molasse-type sediments de- in his stratigraphical scheme. Although the rock types posited at the end phases of the Karelidic orogenic of these two correlated units differ markedly in gen- cycle. With regard to the sedimentological facies, eral, Väyrynen offered the Kalevian Jaurakka con- Simonen’s opinion was compatible with that pre- glomerate in the Kainuu schist belt as an analogy for sented by Väyrynen (1954) a few years earlier, but he the Kumpu formation. Because of the relatively low assessed the Kumpu-Oraniemi Series as being post- metamorphic grade of the metasediments of the Kalevian rather than Kalevian, and hence maintained Kumputunturi formation, Väyrynen suggested that that it had no obvious counterparts in eastern Finland. the Central Lapland granites do not intersect the At this point, it is appropriate to review the revolu- Kumputunturi metasediments, and considered these tion in geological thinking which took place in Finland rocks to represent molasse-type sediments deposited in the late 1950s and early 1960s along with the during folding. T. Mikkola (1961) also emphasized appearance of the first radiometric dating results, the molasse-like nature of the Kumpu formation. In his especially by virtue of the pioneering work of O. comprehensive overview of the Precambrian of Fin- Kouvo. Until then, the main means for placing the land, Eskola (1963) shared Väyrynen’s view on the supracrustal belts in chronological order was the age correlation and held that it would seem natural to relations of the schists to granitic plutons of suppos- correlate the Lapponian supracrustals with the Jatulian edly different ages. As described earlier, this method rocks and the Kumpu-Oraniemi Series with the led Sederholm (1932) to regard the Svecofennian

21 Geological Survey of Finland, Special Paper 33 Eero Hanski

orogenic belt as considerably older than the Karelian situation is different, however, where the Lapponian orogenic belt, with both of them belonging to two Series is concerned. This series has usually been independent orogenic cycles. In addition, he consid- considered Proterozoic, but in the late 1970s and the ered the Lapponian to be part of the same cycle of 1980s several authors expressed opinions favouring a sedimentation as the Bothnian rocks of southern late Archean age. This issue will be addressed in some Finland. detail later on. When Kouvo (1958) and Wetherill et al. (1962) Returning to our consideration of the stratigraphy of published their U-Pb zircon ages for orogenic plutonic Central Lapland, geological research continued in the rocks from both of the above-mentioned orogenic sixties and seventies mainly in the form of thematic belts, it became immediately apparent that the prior studies carried out by university students and geolo- assumptions on the markedly different ages of the gists employed by prospecting companies. Among the post-Svionian, post-Bothnian and post-Kalevian gran- works touching upon wider stratigraphical aspects, ites were no longer valid, as the Svecofennian and the university thesis of Mäkelä (1968) concerning Karelian plutonic rocks proved to be contemporane- the Sirkka conglomerates and quartzites deserves ous, with ages of c. 1.8 Ga. Additional geochronological mentioning. He regarded these sedimentary rocks data provided by Kouvo and Tilton (1966) confirmed together with the metasediments of Kumputunturi the result further. The inescapable conclusion was proper as molasse-type accumulations. Apparently that the Svecofennian and Karelian formations be- relying more on contemporary theoretical models of longed to the same Svecofenno-Karelian orogenic the geological evolution than on field evidence, he also belt (Simonen 1960b). The other significant outlined a model for the whole stratigraphy of Central geochronological result was that the ancient pre- Lapland. While the age relationship between the Karelian gneissic basement and mantled gneiss domes lower quartzitic sedimentary rocks and the Kittilä in the Karelidic belt in eastern Finland yielded much metavolcanics had been more or less obscure up to older ages, in the range of 2.6-2.8 Ga (Wetherill et al. then, Mäkelä’s evolutionary model placed the volcanites 1962, Kouvo & Tilton 1966). above the majority of these sediments in the The Svecofennian and Lapponian supracrustal rocks stratigraphy. In addition, he distinguished certain had previously been assigned to the Archean (e.g., aluminous mica schists and conglomerates from the Simonen 1953, Mikkola 1941), and when classifying other sediments and classified them as flysch-type the Karelian supracrustal sequences, Sederholm (1932) accumulations deposited after the eruption of the had stated that these rocks are generally similar to Kittilä greenstones. These included the pelitic formations that have been regarded as Proterozoic in sediments of Oraniemi, which Mikkola (1941) had other regions, probably referring to the Huronian incorporated into his Kumpu-Oraniemi Series. Since formations in Canada. The advent of radiometric age then, the Oraniemi rocks, along with the geographical determinations gave new geochronological connota- name, have usually been excluded from the youngest tions to the words Archean and Proterozoic, although sedimentary group of Lapland. they continued to be used quite loosely until the Mäkelä (1968) surmised that the Central Lapland general international agreement in the late 1970s to granite area had been uplifted and eroded and had define the boundary at 2500 Ma (James 1978). The acted as a source for the flysch-type deposits. After early results indicated that the Karelian formations this stage, a long tranquil period had ensued before the were deposited between 2.6 and 1.8 Ga ago, and since deposition of the coarse-grained molasse-type Sirkka then they have been generally regarded as Proterozoic. and Kumpu sediments. One of the interesting obser- The Archean, pre-Karelian basement extends from vations reported by Mäkelä is the presence of southeastern Finland to eastern Lapland. Mikkola undeformed microcline granite pebbles in the Sirkka (1941) already regarded the Tuntsa-Savukoski Series conglomerates. On the other hand, he described in eastern Lapland as part of this basement complex, pegmatitic and aplitic dikes cutting quartzites on the and the Archean age of that series has never been hill of Aakenusvaara, for example. questioned since (see Juopperi & Veki 1988). The

The 1970s

Paakkola (1971) performed a detailed study of volcanic complex. He agreed with the explanation of manganiferous iron ores and their environment in the Mikkola (1941) concerning the origin of the ores as Porkonen-Pahtavaara area, located within the Kittilä related to submarine volcanic emanations. Paakkola’s

22 Geological Survey of Finland, Special Paper 33 History of stratigraphical research in northern Finland stratigraphical scheme for Central Lapland was es- Amphibole schist Fm. The uppermost Amphibole sentially the same as that proposed by Mäkelä (1968). schist Formation is composed of mafic tuffs interca- He fixed the Kittilä volcanic complex and its iron lated with dolomites and black schist and also mica formations in the pre-flysch stage of the Lapponian schists in the upper part of the formation. Silvennoinen geosyncline development and the volcanites as prod- could easily correlate this formation, coupled with the ucts of the initial magmatism of the Karelidic orogeny. underlying Dolomite Fm, with the marine Jatulian The systematic geological mapping of Lapland to a formation of Väyrynen (1933) in southeastern and scale of 1:400,000 was completed in Lapland in 1965, eastern Finland, implying that Kalevian rocks are but in the meantime the Geological Survey (GTK) had absent in the Kuusamo area. The lower parts of the initiated remapping of the bedrock of northern Finland succession were more difficult to correlate, especially to a scale of 1:100,000 in the early 1960s, with the Salla because of the presence of the lowermost volcanic greenstone area and the eastern part of the Kuusamo unit resting immediately on the basal conglomerate, an schist belt as the first target areas. As a result of this unknown situation at that time elsewhere in the Finnish mapping activity, Silvennoinen (1972) worked out a part of the Karelian belt. The correlation of the basal detailed stratigraphical subdivision for the Kuusamo conglomerate with the Sariolian formations could not schist belt. Earlier this region had been mapped by be confirmed because of the lack of a weathering Hackman and Wilkman (1929) who distinguished crust above it. older Kalevian mica schists, dolomites and metabasites A pronounced increase in interest in the Archean in the north and younger Jatulian quartzites, dolomites greenstone belts of the ancient Precambrian shields and metabasites in the south, with a discordance was manifested world-wide in the 1970s, and this between these two groups. The Jatulian rocks were trend also reached Finland during the second half of thought to be separated from the oldest rocks, the pre- the decade. The recognition of komatiites, one of the Kalevian gneisses and gneiss granites further south, most characteristic rock types of the Archean by a tectonic contact. greenstone belts, in Finnish Lapland (Mutanen 1976) The stratigraphical order of the main lithological was among the reasons that apparently led Gaál et al. units of the Kuusamo schist belt (older Kalevian, (1978) to suggest that the greenstone belts in Finnish younger Jatulian), as presented by Hackman and Lapland are Archean in age and thus comparable with Wilkman (1929), had already been rejected by Mikkola the Archean greenstone belts of eastern Finland (1941). Silvennoinen (1972) then demonstrated the (Kuhmo-Suomussalmi, Ilomantsi). Several separate existence of a conglomerate at the base of the Jatulian volcanite-dominated greenstone belts were deline- succession, against the old (Saamian) granite gneiss ated by these authors on the geological map of Lapland, complex, and he also divided the Karelian including those of Salla, Jauratsi and Kittilä. On the stratigraphical sequence into 11 formations, following other hand, the Kumpu formation and most of the the basal conglomerate, namely: the Greenstone Fm I, Lapponian epiclastic sedimentary rocks were re- Sericite quartzite Fm, Sericite schist Fm, Quartzite garded as younger than the greenstone belts and schist Fm, Greenstone Fm II, Siltstone Fm, Greenstone probably Proterozoic in age. Fm III, Rukatunturi quartzite Fm, Dolomite Fm and

The 1980s

A three-year research project was undertaken at ultramafic. The transition from sedimentary to vol- the University of Oulu in the late 1970s, one of the canic processes was a gradual one, as indicated by the principal tasks of which was to elucidate the structure presence of sedimentary interlayers within the and stratigraphy of the bedrock in the Kittilä-Sodankylä metavolcanics. Following earlier investigators, the area. After field studies conducted in several separate coarse-clastic Kumpu quartzites and conglomerates subareas, Kallio (1980) made a synthesis of the were distinguished as the youngest supracrustal rock mapping profiles and formulated a general strati- unit. graphical scheme for the supracrustal rocks. Accord- Kallio (1980) also published an age determination of ing to him, the evolution started with the deposition of c. 2213 Ma for an albitite. This sample had been taken an assembly of sedimentogenic rocks on the Archean from a contact with the adjacent quartzite and was basement, including orthoquartzites, greywackes and explained as having originated through a process of mica schists. These were followed by the eruption of adinolization from a greenstone with purported vol- volcanites, varying laterally from mafic (spilitic) to canic structures. According to Kallio, this age demon-

23 Geological Survey of Finland, Special Paper 33 Eero Hanski

strated that, contrary to the suggestions made not long several contributions from northern Finland in its before by Gaál et al. (1978), not all the volcanism in proceedings and stimulated discussion on the the Kittilä greenstone area could be of Archean age. stratigraphy of Karelian formations in general. However, due to the poor characterization of the Silvennoinen et al. (1980) published a generalized dated rock, the involvement of secondary processes in stratigraphical classification of northern Finland ex- its origin and the fact that similar ages have been pressed in lithostratigraphical terms. They distinguished obtained for mafic intrusive sills in the area (Tyrväinen Archean Lower and Upper Lapponian Groups, with 1983), it is difficult to assess how precisely the figure the boundary between the Archean and Proterozoic obtained is related to the actual age of the volcanism. aeons being set at 2600 Ma. These units were fol- A sufficient number of radiometric age determin- lowed by the Karelian Supergroup consisting of the ations, particularly on basic dike rocks, had accumu- Lower, Middle and Upper Jatulian Groups represent- lated in the 1970s for Simonen (1980) to be able to ing a peneplane stage of the Svecokarelian orogeny. conclude in his explanation to the Geological Map of The overlying Kalevian Groups of the last flysch and Finland (scale 1:1,000,000) that the Jatulian sedimen- molasse stages belonged to the Svecofennian tation took place approximately 2200-2000 Ma ago. Supergroup. The Lower and Upper Lapponian Groups The overlying Kalevian rocks were assessed as being were thought to have been deposited between 3100 younger than c. 2000 Ma but older than the synorogenic and 2600 Ma ago, while the time boundary between Svecokarelidic plutonic rocks of c.1900 Ma age. the Lower and Middle Jatulian Groups was consid- Simonen (1980) still regarded the Kumpu formation as ered to be the same as the age of the 2.45-Ga mafic a molasse-like deposit formed after the main layered intrusions (Fig. 4). The Lower and Upper Svecokarelian folding, but contrary to the situation 20 Lapponian Groups encompassed all the rocks in Cen- years earlier (Simonen 1960a), it sufficed for him to tral Lapland which had previously been called call the underlying volcanic and sedimentary rocks Lapponian, including the Kittilä and Salla greenstone Karelidic schists, without any attempts at a more complexes, and also the rocks of the Tuntsa-Savukoski detailed correlation or reference to the term ‘Lapponian’ area. On the other hand, the Jatulian rocks were (Fig. 3). represented in Central Lapland only by the Kumpu- A Finnish-Soviet symposium on Jatulian geology, tunturi quartzite conglomerate formation and its equiva- held in Finland in 1979 as a final meeting of a collabo- lents at Värttiövaara and Pyhätunturi. Applied to the rative endeavour that had started in 1973, yielded Kuusamo area, the scheme of Silvennoinen et al.

Fig. 4. Stratigraphical schemes presented for the Kuusamo schist belt since 1972 and correlation with rock formations on the Russian side of the border. Ages shown in million years.

24 Geological Survey of Finland, Special Paper 33 History of stratigraphical research in northern Finland

(1980) resulted in the following classification: the to the Kalevian Group, with a stratigraphical position basal conglomerate and the Greenstone Fm I be- above the Kumpu formation, while there still existed longed to the Lower Lapponian Group and the Sericite younger, molasse-stage conglomerates and siltstones quartzite Fm and the Sericite schist Fm to the Upper of Vesikkovaara, resting on the Latvajärvi meta- Lapponian Group, while the other seven formations volcanics in his scheme. In contrast, based on the higher up in the sequence were distributed among the drillings and field investigations of a project on the iron Lower, Middle and Upper Jatulian Groups (Fig. 4). ore mineralizations hosted by the Latvajärvi interme- The stratigraphical classification of Silvennoinen et diate metavolcanics, Puustinen et al. (1980) main- al. (1980) deviated in many respects from the views of tained that the Kumpu-type quartzites were deposited other contemporary researchers (e.g., Simonen 1980, after the eruption of the Latvajärvi volcanic rocks. see Fig. 3). Firstly, the term Lapponian was extended In a study aimed at explaining the genesis of the from Central Lapland to the Kuusamo schist belt, strata-bound skarn iron ores of Rautuvaara in western where the rocks had previously been described with Lapland, Hiltunen (1982) extended his investiga- the terms Jatulian and Kalevian (Hackman & Wilkman tions to the stratigraphical and structural aspects of 1929) or only Jatulian (Silvennoinen 1972). Secondly, the whole Rautuvaara (Kolari) area. He divided it into the Lapponian metasediments and metavolcanics were eastern and western areas and constructed a separate excluded from the Karelian rocks (Karelian lithostratigraphical column for each. The lowermost Supergroup), and thirdly, the Lapponian rocks were unit of the eastern section was the Niesakero- regarded as late Archean in age thus corresponding Kuertunturi quartzite complex. As the name implies, temporally to the Archean greenstone belts of eastern this is composed mainly of quartzites, but sillimanite- Finland, in analogy with the earlier suggestions made bearing and calc-silicate-bearing gneisses and by Mikkola (1941) and Gaál et al. (1978). The last amphibolites also occur. The Niesakero-Kuertunturi point gained support from geochronological data, as quartzite complex was overlain by the ore-bearing Silvennoinen et al. (1980) reported an age of 2790 Ma Rautuvaara Formation and higher up by the Kolari for an acid metavolcanic belonging to the Greenstone Greenstone Formation. The uppermost units were the Fm I in the western part of the Kuusamo schist belt. Luosujoki conglomerate and the Yllästunturi quartz- The stratigraphical column for the Kittilä area ite. presented by Rastas (1980) at the above-mentioned In the western area, the Niesakero-Kuertunturi symposium was in harmony with the general scheme complex was correlated with the Mustijärvi quartzite, of Silvennoinen et al. (1980). The Archean Lower while the Kolari Greenstone Formation corresponded Lapponian Group consisted of the amygdaloidal to the Siekkijoki Greenstone Formation. The Tapojärvi metavolcanics of Kaukonen, which were overlain by Quartzite Formation was in turn regarded as the sericite quartzites and mica schists of the Archean counterpart to the highest Yllästunturi quartzite in the Upper Lapponian Group. Higher up in the Upper east. Lapponian Group followed the metavolcanics and Hiltunen (1982) did not extend his classification to associated sedimentary rocks of the Kittilä greenstone the group or supergroup level but seemed to agree complex. The oldest unit of the greenstone complex, with Rastas’ (1980) comparison of the sillimanite- according to Rastas (1980), was an acid volcanite at bearing quartzites and mica schists of western Lapland Jeesiö for which a zircon age of c. 2700 Ma had been with the sedimentary rocks of the Upper Lapponian obtained. In the Kittilä area, the Karelian Supergroup Group in the Kittilä area. He also considered it possi- was represented by the lower Jatulian Luosujoki ble to correlate his volcanic formations with the Upper conglomerates and the middle Jatulian Kumpu forma- Lapponian greenstone complex of Kittilä. An Archean tion. The Jatulian age for the Kumpu formation was age had been put forward for these Lapponian rocks corroborated by an age of c. 2200 Ma measured for by Rastas (1980) and Silvennoinen et al. (1980) two an intersecting albite diabase at Värttiövaara years earlier. Hiltunen (1982) did not express any (Kaarestunturi). Rastas (1980) also reported a U-Pb decisive support for this view, but provided some zircon age of c. 1885 Ma for the Latvajärvi volcanic apparently supportive evidence in terms of zircon rocks, an areally limited occurrence of acid and dates. He had sampled two keratophyre layers in the intermediate volcanic rocks discovered west of the upper part of the Siekkijoki Greenstone Formation, village Kittilä at the beginning of the 1970s in connec- and although the analytical results were highly dis- tion with mapping by the GTK. The age of these rocks cordant, two zircon fractions suggested an Archean indicated that they were coeval with the nearby Kallo age for these rocks (the upper intercept of a two-point quartz monzonite massif belonging to the Haaparanta chord equivalent to 2738 Ma). (However, it is possible Suite. Rastas assigned the Latvajärvi volcanic rocks that the zircons are inherited in a similar manner as

25 Geological Survey of Finland, Special Paper 33 Eero Hanski

reported by Perttunen and Vaasjoki, 2001, this vol- It is necessary in this context to mention a Sm-Nd ume). isotope study performed by Krill et al. (1985) on The uppermost sedimentary rocks, the Luosujoki MgO-rich volcanic rocks from the Karasjok greenstone conglomerate, Yllästunturi quartzite and Tapojärvi belt in northern Norway. Although the belt lies beyond Quartzite Formation, were readily correlated by the geographical borders of the area under discussion Hiltunen with the Kumpu formation. If confirmed, this in this article, this isotopic study is of significance would be significant because, according to Hiltunen’s because the dated komatiites come from a belt re- map, a monzonite intrusion belonging to the 1.88 Ga garded as a direct northerly continuation of the Kittilä Haaparanta Suite cuts into the Tapojärvi Quartzite greenstone complex, and also correspond structurally Formation. This view is at variance with that of and geochemically to the komatiites occurring in Mikkola (1941), who had earlier regarded the Tapojärvi Finnish Central Lapland. Krill et al. obtained a Sm-Nd quartzite as older than the Kumpu quartzites. It should age of 2085±85 Ma for the Karasjok komatiites, which also be mentioned in this context that when comparing argued strongly for an early Proterozoic age for the the Rautuvaara rocks with those occurring on the Karasjok greenstone belt, and for the Kittilä greenstone Swedish side of the border, Hiltunen pointed out that belt as well (cf. Silvennoinen et al. 1980). even though sedimentary rocks younger than the In his stratigraphical classification of the Proterozoic Haaparanta Suite, belonging to Maattavaara Quarzite rocks of northern Finland, Silvennoinen (1985) used Group of Eriksson and Hallgren (1975), exist in the essentially the same nomenclature as in Silvennoinen Vittangi area, no such rocks have been identified in the et al. (1980) with some purely cosmetic changes Rautuvaara area. related to the spelling of the names; the Lapponian, The base of the Niesakero-Kuertunturi quartzite Jatulian and Kalevian Groups being replaced by the complex was not known in the Rautuvaara area, but Lapponia, Jatuli and Kaleva Groups and the Karelian it was inferred from the gneissose plutonic rock and Svecofennian Supergroups by the Karelia and fragments in the conglomerate of the lower part of the Svecofennia Supergroups, respectively (Fig. 4). More quartzite that the unit may have been deposited on a significant differences in the nomenclature included basement composed of granite gneisses. Such a base- the introduction of the term ‘Lapponia Supergroup’, ment was later recognized by Väänänen (1989, incorporating the Lower and Upper Lapponia Groups. 1992) in the south, not far from the border of the area Also, another previously unnamed supergroup was studied by Hiltunen (1982), and called the Venejärvi included in the stratigraphical column, the lowermost, Gneiss Complex. Hiltunen (1982) similarly found no Archean Tuntsa Supergroup. Silvennoinen (1985) still analogies to the lowermost Kaukonen amygdaloidal placed the Lower Lapponia Group entirely in the metavolcanics of the Kittilä area, assigned to the Archean (>2500 Ma) and the Upper Lapponia Group Lower Lapponian Group by Rastas (1980). The partly so, but commented that the chronostratigraphical Venejärvi Gneiss Complex was instead shown to be correlation was not yet fixed for these units. Other overlain by the amygdaloidal, komatiitic Teuravuoma changes included the lower time boundary of the volcanite unit, which was considered to correspond to Karelia Supergroup. Presumably influenced by the Lower Lapponian metavolcanics (Väänänen 1989, finding of a Jatulian basal conglomerate lying directly 1992). on c. 2.44 Ga mafic layered intrusions in the Peräpohja Let us turn our attention back to the east. Lauerma schist belt (Perttunen 1985), this boundary was re- (1982) was the first to criticize the inference of an garded as younger than the age of the intrusions Archean age for the lowermost supracrustal rocks of themselves, which were now taken as manifestations the Kuusamo schist belt, as proposed by Silvennoinen of Lapponian magmatic activity. Silvennoinen (1985) et al. (1980). He pointed out that ages between 2600 correlated the Lapponia Supergroup with the Sumi- and 2800 Ma had been obtained for granitoids and Sariolian formations occurring on the Soviet side of the pegmatites in the basement complex south of the border. It is also worth of mentioning that Silvennoinen Kuusamo schist belt and that no such granites had (1985, his Fig. 3) correlated the abundant mafic been seen to penetrate the schists. On the contrary, metavolcanics of the Salla area with the Greenstone the schists overlie the basement complex unconform- Formation I of the Lower Lapponia Group. We will ably, containing basal conglomerates with fragments come back to this important issue below when dis- from the basement in places. Lauerma (1982) also cussing studies carried out by Manninen (1991) in the emphasized that the corresponding rocks on the Soviet Salla area. side had been considered to be part of the Sumian- The stratigraphical scheme outlined by Silvennoinen Sariolian group (Kulikov et al. 1980), implying that (1985) was used as a base for classifying the rocks of they cannot be much older than 2450 Ma. northern Finland when the bedrock map of the

26 Geological Survey of Finland, Special Paper 33 History of stratigraphical research in northern Finland

Nordkalott map series was completed a couple of ate the prospects for finding iron ores in Finland years later as a joint effort between the Nordic yielded various maps, including a regional stratigraphi- geological surveys (Geological Map, Northern cal sketch map, and several articles dealing with the Fennoscandia 1987). Both the Lapponia and Karelia stratigraphy of Lapland and other areas (Ivanov et al. Supergroups were defined in this map as being of early 1986, Muradymov et al. 1986, 1988, Korsakova et al. Proterozoic age (Silvennoinen et al. 1986). 1988). Ivanov et al. (1986) incorporated the Lower Following the terminology of Silvennoinen (1985), Proterozoic formations in Finland into the Svecokarelian Kortelainen et al. (1986) considered the youngest supergroup, consisting of the Sariolian, Jatulian and quartzites and conglomerates in Lapland to belong to Kalevian-Svecofennian groups, and constructed the Karelia Supergroup. The inner stratigraphy of stratigraphical columns with inferred mutual correla- these well-exposed deposits, which have been inter- tions between several areas in the Karelian and preted as fluviatile, was described in the west, at Lapland blocks. The basal conglomerate and the Sirkka, by Kortelainen (1983) and in the east, at Greenstone Fm I of the Kuusamo schist belt were Pyhätunturi, by Räsänen and Mäkelä (1988). The included in the Lower and Upper Sariolian subgroups, latter distinguished four (Jatulian) formations in the respectively, while all the overlying formations were Karelia Group in their study area. Subsequent studies assigned to the Lower and Upper Jatulian subgroups. have revealed that these do not form a continuous, In contrast to the Kuusamo area, Ivanov et al. northerly facing succession and only two of them, the (1986) considered Central Lapland to be practically Isokuru and Pyhätunturi Formations, belong to the devoid of Sariolian rocks. Here the Lower Proterozoic sedimentary package regarded then as Jatulian formations begin with Lower Jatulian basal horizons, (Räsänen & Huhma 2001, this volume). This is represented mainly by polymictic conglomerates and worth mentioning here because the Pyhätunturi area arkoses together with greenish fuchsite-bearing quartz- has served as an important link when the rock se- ites. It is worth mentioning at this point that Silvennoinen quences of the Kuusamo area have been traced (1985) had emphasized the presence of green, chro- towards northwest to Central Lapland. mium-bearing sericite in Lower Lapponian meta- One of the keenest advocates of an Archean age sediments. Ivanov and his coworkers also included in for the Lapland greenstone belts was Saverikko the Lower Jatulian rocks the Kaarestunturi and (1987, 1988). While concurring with the earlier Pyhätunturi conglomerates and arkoses which previ- interpretation of Silvennoinen et al. (1980) that the ously had usually been linked with the Kumpu forma- greenstone belts running from Kolari via Kittilä and tion (e.g., Silvennoinen 1985). In their opinion, the Sodankylä to Salla were Archean in age, he went still upper, volcanogenic section of the Lower Jatulian further in proposing that the whole Kuusamo schist seemed to be missing in Central Lapland and the belt belonged to the Archean, too. He placed the Lower Jatulian metasediments were directly overlain Kumpu formation in the Proterozoic, with its basal by Upper Jatulian volcanic and sedimentary rocks. greywacke conglomerates correlated with the Sariolian They distinguished three Upper Jatulian type sections, and the overlying arkosic conglomerates and quartz- essentially dominated by sedimentary (Pelkosenniemi ites with the Jatulian, and subdivided the Lapponian structure), volcanogenic (Kittilä and Salla structures) rocks into three groups instead of the commonly used and volcanogenic-sedimentary rocks (Kolari struc- two-fold division (Fig. 3). The Lower Lapponian ture), respectively. The abundant volcanic rocks of group was represented by volcanic rocks, e.g. those the Kittilä area and the associated siliceous iron occurring in the Möykkelmä area of Central Lapland formations were thus regarded as Upper Jatulian (see and in the Salla greenstone complex, and by quartzites also Muradymov et al. 1986). As the uppermost and schists. The Middle Lapponian group comprised formations, Ivanov et al. distinguished molasse-like the Oraniemi association, an arkose-slate-quartzite Kumpu, Sirkka and Luosujoki conglomerates and sequence located between the Salla and Kittilä quartzites, the overlying Latvajärvi metavolcanics and greenstone complexes, the transition to the Upper metasediments and finally the molasse-like Lapponian group being marked by a graphitic slate unit Vesikkovaara conglomerates. They classified the overlain by pyroclastic komatiites in the Sattasvaara Latvajärvi rocks as middle Kalevian, separating the and Kummitsoiva areas. The Upper Lapponian group Lower and Upper Kalevian metasedimentary units. also included the large Kittilä greenstone complex. The Kalevian sequence was obviously adopted from A three-year joint Finnish-Soviet project (1980- Rastas (1980) but elevated to a higher position in the 1983) carried out by geologists from the North-West- stratigraphy. ern Regional Geological Department (SEVZAP- It is clear from the preceding discussion that Ivanov GEOLOGIA) and Rautaruukki Co. in order to evalu- et al. (1986) disagreed with those authors who had

27 Geological Survey of Finland, Special Paper 33 Eero Hanski

argued for an Archean age for the greenstone belts in (Lehtonen 1989)1989) with aiman aim of providing of providing information information on Kittilä and Salla (Gaál et al. 1978, Silvennoinen et al. theon generally the generally poorly poorly exposed exposed volcanic volcanic rocks of rocks north- of 1980), but the project members did not totally exclude ernnorthern Finland Finland by means by means of effective of effective sampling sampling (including (in- the presence of Archean supracrustal rocks in Central drillingcluding anddrilling trenching) and trenching) and geochemical and geochemical methods. meth- One Lapland. In fact, Muradymov et al. (1988) claimed ofods. its One goals of its was goals the was chemical the chemical characterization characteriza- of that Upper Archean formations are widespread in volcaniction of volcanic rocks at rocks different at different stratigraphical stratigraphical levels. lev-Be- northern Finland. As indicated by the Schematic foreels. Before the actual the actual commencement commencement of the of fieldworkthe fieldwork for Tectono-Geologic Map of the Baltic (Fennoscandian) thefor project,the project, a geological a geological map map of Central of Central Lapland Lapland was Shield, scale 1:2,500,000, accompanying Korsakova compiledwas compiled in collaboration in collaboration with Lapinwith Lapin Malmi Malmi Co. on Co. the et al. (1988), these Archean supracrustal rocks were basison the of basis all the of allgeological the geological data available data available at that at time that thought to occur partly in regions where komatiites are (Lehtonentime (Lehtonen et al. et al. 1984) 1984). A. A partly partly overlapping overlapping known to prevail. Muradymov et al. (1988) mentioned stratigraphical map map of ofwestern western Lapland Lapland was wasalso pub- also the Jauratsi and Sattasvaara structures, which they publishedlished the the following following year year (Lehtonen (Lehtonen et et al.al. 1985a) 1985a). assigned to the Upper Lopian, though without provid- These maps and the explanation to the first of them ing any obvious evidence for their age. (Lehtonen et et al. al.1985b) 1985b) used usedthe stratigraphical the stratigraphical nomen- Among the few age determinations that have so far nomenclatureclature of Silvennoinen of Silvennoinen (1985) (1985) but but without without any been achieved directly from volcanic rocks in northern geochronological connotation. connotation. Finland, one from the Peurasuvanto area north of the The work of the LVP in 1986 included a field Koitelainen layered intrusion deserves mentioning. investigation into a 2.2 Ga diabase at Värttiövaara, an Here Peltonen et al. (1988) described the oval-shaped body 40x100 m in diameter at the contact volcaniclastic Rookkiaapa Fm, comprising conglom- between the quartzites and conglomerates that had erates, tuffaceous arenites and interstratified traditionally been regarded as parts of the Kumpu pyroclastic layers which surround the Archean formation. This diabase may be the one mentioned by Tojottamaselkä granite gneiss dome (see also Pihlaja Hackman (1927) as the only one that he knew to occur & Manninen 1988). Zircons from these basement within the Kumpu quartzites. Later, the diabase has gneisses were dated by Kröner et al. (1981) to c. 3.1 been used as evidence of a Jatulian age for the Kumpu Ga, rendering these rocks some of the oldest in formation (Rastas 1980). The excavations carried out Finland. The overlying Rookkiaapa Formation con- by the LVP did not confirm an intersecting contact tains various clasts, such as TTG gneisses, quartzites, relation, as the contacts turned out to be strongly acid to intermediate metavolcanics, diabases, and sheared. The age of c. 2.2 Ga obtained for the diabase komatiites thought to have been derived from the has become an intriguing puzzle, as a younger age had basement gneisses or the Archean Tuntsa Supergroup, been recorded for detrital zircons from the surround- whereas the Lapponian aluminous schists and ing quartzite, and therefore a tectonic juxtaposition of ultramafic metavolcanics and the c. 2.44 Ga Koitelainen the diabase and quartzite was judged to be a viable intrusion are not represented among these clasts. alternative (Lehtonen 1987). According to Peltonen et al. (1988), an acid porphyry The small Möykkelmä granite gneiss dome within from an interbed of volcanic breccia yielded a U-Pb the Lapponian supracrustal rocks NNW of Sodankylä zircon age of 2526 Ma (published without errors), thus was selected as one of the first LVP target areas placing these rocks close to the Archean-Proterozoic because Tyrväinen (1983) had just reported the boundary. As this rock represents the lowermost level existence of volcanic rocks directly on the granite of the Lapponian succession, the other Lapponian gneiss basement in this area. Räsänen et al. (1989) rocks can be assumed to be younger. This result is documented a detailed study of the Möykkelmä clearly in conflict with the postulated widespread metavolcanics which occur between the basement occurrence of Archean greenstone belts in Central and the overlying Lapponian quartzites. A surface Lapland (e.g., Gaál et al. 1978), although given the profile and a drill core section revealed the presence heterogeneous nature of the host formation, there is a of two ultramafic and three mafic, subaerially erupted possibility that the sample may have contained a mixed volcanic units with a total thickness of c. 250 m. zircon population (see Manninen et al. 2001, this Geochemically, the ultramafic rocks turned out to be volume). This means that the date represents a komatiites and basaltic komatiites while the mafic maximum age for the lowermost Lapponian rocks, rocks ranged from basalts to andesites. Räsänen et al. which strengthens even further the conclusion of a (1989) showed that all these volcanic rocks display a Proterozoic age for the bulk of the Lapponian rocks. strong crustal signature in their chemical composition. The 5-year Lapland Volcanite Project (LVP) was In this respect, the Möykkelmä komatiites differed launched at the Rovaniemi Office of the GTK in 1984 significantly from the megascopically similar,

28 Geological Survey of Finland, Special Paper 33 History of stratigraphical research in northern Finland volcaniclastic komatiites occurring higher up in the in HFSE), located below the Lapponian sedimentary stratigraphy, e.g., in the Sattasvaara area. They also rocks but younger in age than the Archean granite verified the existence of volcanic rocks with chemical gneiss basement. characteristics of their own (enriched in LIL, depleted

The 1990s

As part of the Lapland Volcanite Project, cluded that this intrusion is roughly coeval with the Manninen (1991) studied the stratigraphy and Salla Formation. volcanite geochemistry of the Salla greenstone com- The middle Lapponi quartzite-dominated rocks oc- plex on the Finnish-Russian border. Various opinions cur west of the metavolcanics described above and had been offered earlier concerning the age and are separated from them by a tectonic contact. In the correlation of the Salla metavolcanics. The greenstones middle of these metasediments (Kelloselkä Fm, were candidates for counterparts of the Kittilä Matovaara Fm), there is a mafic volcanic formation greenstones in the comparison made by Mikkola (Tahkoselkä Fm) which can be traced farther in the (1941), mainly due to the large volume of volcanic south as merging with the Jatulian Greenstone Fm III rocks in both areas. The same correlation was also of Silvennoinen et al. (1980). The supracrustal rocks favoured by Ivanov et al. (1986). Meanwhile, Gaál et classified as Upper Lapponian are encountered in the al. (1978), Silvennoinen et al. (1980) and Saverikko northern part of the Salla greenstone complex and are (1987) had suggested an Archean age for the belt, composed of ultramafic and mafic pyroclastic rocks whereas Lauerma (1982) regarded it definitely as similar to those found in the Kummitsoiva and Proterozoic. As noted before, Silvennoinen (1985) Sattasvaara areas. TheyThey overlieoverly thethe middlemiddle Lapponian assigned it to his Lower Lapponia Group. In striking dolomites, graphite-bearing mica schists and jaspilites contrast, according to Kulikov et al. (1980), the corre- of the Aatsinginhauta Formation. sponding metavolcanics on the eastern side of the The area investigated by Manninen (1991) is a key border constitute the uppermost unit in the stratigraphy region because here the Central Lapland and Kuusamo of the Paanajärvi-Kuolajärvi belt. These authors placed schist belts join together. Several important aspects them in the Suisaarian group, which implies that they were reported by him. Firstly, the term Lapponi was would be post-Jatulian in age. not used in the sense defined e.g. by Silvennoinen Manninen (1991) divided the depositional sequence (1985), that is as being pre-Jatulian (beneath the in the Salla area into seven sedimentary and volcanic Karelian Supergroup), but instead the sequence of formations and organized them into three groups, the rocks forming the three Lapponi Groups was regarded Lower, Middle and Upper Lapponi Groups (Fig. 3). as correlative with other Karelian units ranging from The majority of the supracrustal rocks in the Salla the Sumian and Sariolian through Jatulian to even complex belong to the two lowermost volcanic Salla Ludikovian. Secondly, most of the volcanic rocks of and Mäntyvaara Formations, forming the Lower the Salla greenstone complex differed in their Lapponi Group. The metavolcanics of the former geochemical compositions drastically from the formation range geochemically from basaltic andesites metavolcanics from the Kittilä greenstone complex, to rhyolites and those of the latter from komatiitic so that the correlation between these two complexes basalts to high-Mg basalts. They have a significant could not be substantiated. Finally, as the bulk of the crustal signature in their composition, in the same Salla metavolcanics were correlated with the Sumian manner as recorded by Räsänen et al. (1989) for the and Sariolian metavolcanics, the Suisaarian connec- Möykkelmä metavolcanics. According to Manninen, tion as advocated by Kulikov et al. (1980) was ne- the volcanic structures and geochemical composition gated. Later, a dike cutting volcanic rocks in Salla has of the Salla and Mäntyvaara Formations find their been dated, demonstrating a pre-Suisaarian age for closest counterparts on the other side of the border in these rocks (Manninen & Huhma 2001, this volume). the Sumian and Sariolian metavolcanics, respectively. Silvennoinen (1972) had already recorded the pres- The Mäntyvaara Fm was also correlated with the ence of acid porphyry clasts in the basal conglomerate Greenstone Fm I in the Kuusamo area, while the Salla of the Kuusamo schist belt. In his explanation to the Fm was considered to be older. Not far from Salla, the Kuusamo and Rukatunturi map sheets, Silvennoinen 2.44 Ga Akanvaara layered intrusion is known to (1991) published U-Pb isotope results for zircons intersect felsic metavolcanics that are comparable to from three separate clasts, which taken together those of the Salla Formation. Manninen (1991) con- yielded an age of 2405±6 Ma. This can be regarded as

29 Geological Survey of Finland, Special Paper 33 Eero Hanski

a maximum age for the lowermost Lapponian as unwarranted because, as interpreted by Lehtonen supracrustal rocks in the Kuusamo area, thus finally et al. (1992), the Kumpu rocks are younger than the nullifying the proposed Archean age for the Lapponian Latvajärvi metavolcanics. Lehtonen et al. (1992) pro- rocks of the Kuusamo area (cf. Saverikko 1987). vided further evidence for post-Jatulian deposition by The next account of the stratigraphy of the Kuusamo reporting ages of 1913-2066 Ma for detrital zircons area was given by Silvennoinen (1992) in the from the Kumpu quartzites. Rastas (1980) had based proceedings of the Finnish-Soviet symposium on the his Jatulian connection partly on an age of 2044 Ma Paanajärvi-Kuusamo-Kuolajärvi area. An attentative obtained for a diabase which was assumed to have reader may note that in his description of the intersected the Kumpu quartzite at Sätkänävaara. supergroups, Silvennoinen (1992) had shifted the lower Excavations carried out by the LVP had nevertheless boundary of the Karelia Supergroup from its original revealed that cobbles from the diabase in question place at the base of the Quartzite schist Fm exist within the adjacent quartzite (Lehtonen et al. (Silvennoinen et al. 1980) to the base of the overlying 1992). Greenstone Fm II. In the correlation across the bor- In his investigation on the carbon isotope composi- der, presented in the legend to the appendix map tion of sedimentary carbonate rocks from the (Silvennoinen et al. 1992), the Lapponia Supergroup Fennoscandian Shield, Karhu (1993) recognized a was interpreted to be equivalent of the Sumi-Sariolian fairly systematic 13C/12C variation within individual Superhorizon, while the Karelia Supergroup corre- stratigraphical sequences of Karelian formations and sponded to the Jatuli and Ludikovia Superhorizons showed that this method can potentially be used as a (Fig. 4). new tool for stratigraphical correlation. When plotting Lehtonen et al. (1992) divided the Paleo- the carbon isotope composition against time for the proterozoic supracrustal rocks of Central Lapland into sedimentary rocks whose age could be constrained by five lithostratigraphical groups: the Lower, Middle and radiometric dating of associated intrusive rocks, a Upper Lapponi Groups, overlain by the Lainio and distinct positive isotope anomaly in the 13C range of Kumpu Groups. The type formations of the Lower +10±3 per mil could be observed in the time interval Lapponi Group are the Madetkoski and Möykkelmä 2.2 to 2.1 Ga, that is when Jatulian carbonate rocks Formations, comprising crustally contaminated were precipitated. From the standpoint of the present amygdaloidal and pyroclastic metavolcanics ranging review, it is interesting to note that Karhu also re- from komatiites to dacites. The mainly sedimentogenic ported generally anomalous carbon isotope composi- Middle Lapponi Group, with the Virttiövaara Fm as its tions for sedimentary carbonates within the Central type formation, includes traditional Lapponian quartz- Lapland quartzites that Lehtonen et al. (1992) had ites, arkosites, mica schists etc., while the Upper assigned to the Middle Lapponi Group. According to Lapponi Group is composed mostly of volcanogenic him, this result supports the correlation of the Middle rocks. The lowermost part contains pillowed and Lapponi Group with the Jatulian formations in Karelia pyroclastic komatiitic and picritic volcanic rocks, rep- (cf. Manninen 1991). On the other hand, carbonates resented by the Sattasvaara Formation, for example. associated with the voluminous volcanic rocks of the In the Kittilä-Sodankylä area, the Sattasvaara Forma- Kittilä greenstone complex (north of the ‘Sirkka tec- tion is overlain by a thick sequence of submarine basic tonic line’) displayed normal marine isotopic values, volcanic rocks subdivided into the lower Fe-tholeiitic suggesting that they lie at a higher or alternatively Kautoselkä Formation and the upper Mg-tholeiitic lower stratigraphical level compared with the Jatulian Vesmajärvi Formation, which are separated by the carbonate rocks. Karhu favoured the former option. BIF-bearing Porkonen Formation. Among other interesting outcomes of Karhu’s study The coarse-clastic sedimentary rocks deposited was the finding that the excursion of the carbon unconformably on the Lapponi Groups were split by isotope composition towards highly positive anomalies Lehtonen et al. (1992) into two separate units, the had already reached δ13C values of up to +8‰ in Lainio and Kumpu Groups, on the basis of differences carbonates from the Sericite Schist Formation in the in their deformation histories. The Lainio Group com- Kuusamo area, representing part of the Lapponia prises the sedimentogenic Ylläs and Vesikkovaara Supergroup of Silvennoinen (1985, 1992). Even higher Formations and the intervening metavolcanics of the ratios were recorded for carbonates from the overly- 1.9 Ga Latvajärvi Formation, while the overlying ing Karelia Supergroup with the youngest carbonates Kumpu Group is exemplified by the Levi Formation. belonging to the Upper Jatuli Group showing a ten- The rocks representing the Kumpu Group had previ- dency to decrease towards normal values. ously been included in the Middle Jatulian Group A recent and fundamentally different approach to (Rastas 1980), but this assignment was now regarded the stratigraphical classification of supracrustal rocks

30 Geological Survey of Finland, Special Paper 33 History of stratigraphical research in northern Finland in northern Finland was made in the final works Lainio Groups, represent the previous Kumpu forma- produced within the Lapland Volcanite Project, in- tions or the Karelia Supergroup, while the other five cluding a new stratigraphical map of Central Lapland groups are the heirs of the former Lapponian rocks or by Räsänen et al. (1995), scale 1:200,000, and a the Lapponi Supergroup. In addition to this correlation, report by Lehtonen et al. (1998) on the stratigraphy, Figure 5 shows the type formations for the groups and petrology and geochemistry of the Kittilä greenstone their approximate lithological compositions as taken area in Central Finnish Lapland. They renamed the from the legend to the Appendix map of Lehtonen et rock units at the group and formation levels using al. (1998). Field geological, lithological and geochemical geographical names together with lithostratigraphical characteristics of these units and their representative rank terms and hence totally refrained from using type formations were documented by Lehtonen et al. names like Lapponian or Jatulian that had been com- (1998), and the readers are referred to this work for mon earlier. Although the term Lapponian and its more detailed information. It only suffices here to derivatives have a long tradition behind them and are briefly summarize the most salient features of the five deeply ingrained in the Finnish geological literature, lowermost groups. The Salla and Onkamo Groups are this new approach was chosen because it conforms composed mostly of volcanogenic rocks showing better with the international recommendations for commonly structures indicative of subaerial eruptions. lithostratigraphical classification (Hedberg 1976, Sal- They vary in composition from intermediate to felsic vador 1994) and avoids the ambivalent meanings in the Salla Group and from ultramafic to mafic in the coined by various authors with time for many loosely Onkamo Group and can be correlated with the pre- defined traditional terms. Jatulian Sumi-Sariolian rocks in the eastern part of the For the Paleoproterozoic sedimentary and volcanic Fennoscandian Shield. These units are overlain by the rocks, Räsänen et al. (1995) and Lehtonen et al. Sodankylä Group comprising the wide-spread (1998) distinguished the following lithostratigraphical “Lapponian” quartzites and spatially associated minor groups, from oldest to youngest: the Salla, Onkamo, dolomites and mafic metavolcanics. These are the Sodankylä, Savukoski, Kittilä, Lainio and Kumpu best candidates for counterparts of the traditional Groups. The two last mentioned ones, the Kumpu and Jatulian quartzites in the Karelian schist belts. The

Fig. 5. Lithostratigraphical scheme of Lehtonen et al. (1998) for rocks in Central Lapland and its correlation with the Lapponian and Karelian Supergroups of Silvennoinen (1985, 1998).

31 Geological Survey of Finland, Special Paper 33 Eero Hanski

next stratigraphical unit, lying conformably on the compatible with the U-Pb zircon ages between 2012±5 Sodankylä Group, is the Savukoski Group, which and 2018±7 Ma obtained for felsic porphyries within contains pelitic metasediments and minor mafic tuffites the Kittilä Group. The field characteristics of these in its lower part and primitive volcanic rocks (komatiites porphyries provide evidence for their coeval emplace- and picrites) in its upper part. The volcanic rocks and ment with the surrounding mafic metavolcanics (Hanski associated chemical metasediments of the Kittilä et al. 1998, Rastas et al. 2001, this volume). Although greenstone area are assigned to the Kittilä Group. mafic volcanic rocks with an apparent pre-Jatulian Hanski (1997) presented evidence for the presence of age, such as the tholeiitic and komatiitic metavolcanics ophiolitic ultramafic rocks around Nuttio at the eastern of the Onkamo Group lying directly on the Archean margin of the Kittilä greenstone area. This, coupled basement at Möykkelmä (Räsänen et al. 1989), exist with structural, geochemical and isotopic data on in Central Lapland, the age determinations referred to mafic and felsic rocks of the Kittilä Group, suggests above indicate that the majority of the mafic volcanic that this rock complex forms an allochthonous unit rocks of the Kittilä greenstone area, belonging to the representing a block of ancient oceanic crust (Hanski Kittilä Group, are post-Jatulian in age, and are thus et al. 1998), and therefore its direct correlation with in striking contrast with the first definition and later the Karelian cratonic volcanogenic-sedimentary se- usage of the term Lapponian. quences is not warranted. The Jatulian-Kumpu correlation has become an Additional reasons, and perhaps the most compel- impossible alternative as a result of U-Pb dating of ling ones, for rejecting the traditional Lapponian- felsic porphyry and granite fragments and detrital Jatulian nomenclaturenomenclature in in Lapland Lapland were were provided provided by U-by zircons from the Lainio and Kumpu Group U-PbPb and and Sm-Nd Sm-Nd age age determinations. determinations. In In many metasediments. Hanski et al. (1997) obtained an age stratigraphical schemes ever since the first definition of 1927±6 Ma for a felsic porphyry cobble from the by Sederholm (1932) to the system by Silvennoinen Mantovaara conglomerate located 20 km NE of Sirkka, (1992, 1998), the Lapponian rocks have been re- demonstrating that these sediments were deposited garded as pre-Jatulian, which was also implicit in the later than this date. An even younger age of deposition correlation of the Lapponian with the Sumi-Sariolian is indicated by granitoid fragments from the Kellostapuli (Manninen 1991, Silvennoinen 1992). This implies that and Vesikkovaara conglomerates (Rastas et al. 2001, the radiometric age of the Lapponian rocks should be this volume) and detrital zircons from the greater than that of the Jatulian rocks. Deposition of Aakenustunturi and Värttiövaara quartzites (Hanski the Jatulian supracrustal rocks commenced well be- et al. 2000), as these contain rock fragments and fore 2.2 Ga, which is the age of the gabbro-wehrlite zircons most likely derived from the 1.88 Ga sills intruding into Jatulian quartzites (Hanski 1987). Haaparanta Suite plutonic rocks and Latvajärvi For- Against this background, it is significant that Hanski et mation metavolcanics. These results are in harmony al. (1997) reported a Sm-Nd isochron age of 1990±35 with the suggestion put forward already by Mikkola Ma for the mafic metavolcanics of the Vesmajärvi (1941) that there exist aplitic ‘syenite series’ clasts in Formation belonging to the Kittilä Group. This age is the Sirkka conglomerates.

HISTORY OF STRATIGRAPHICAL STUDIES IN THE PERÄPOHJA SCHIST BELT

Introduction

With the exception of interpretations based on the decades, with the rock associations being described as first investigations carried out at the beginning of this a typical Karelian succession composed of Jatulian century, the history of stratigraphical research in the and Kalevian rocks. The longest history of research, Peräpohja schist belt does not involve such contradic- stretching back for about a century, concerns the SW tory ideas on the general stratigraphical order of part of the schist belt, where the stratigraphical out- supracrustal rocks and their ages and regional corre- lines were also formulated. Later studies have shown lation as are revealed by the foregoing pages on that the geological structure of the belt becomes more research into the rocks of Central Lapland. Instead, a complicated towards the north and east, where there consensus on the character of the main lithological are areas with higher degrees of deformation and units and their mutual relationships has prevailed for metamorphism. These observations and the changes

32 Geological Survey of Finland, Special Paper 33 History of stratigraphical research in northern Finland in the general stratigraphical classification system in tions to the lithostratigraphical nomenclature (Perttunen northern Finland, have necessitated recent modifica- et al. 1995).

From the 1910s to the 1950s

The bedrock mapping to a scale of 1:400,000 started with quartzites. The other criteria included the rela- at the end of the 19th century formed the basis for the tionship to the granites as it was asserted that the first effort at evaluating the stratigraphy of the Jatulian rocks are nowhere observed to be cut by Peräpohja schist belt. In his explanation to the granites - which was obvious since this premise was Rovaniemi, Tornio and Ylitornio map sheets, Hackman included in the definition of Jatulian rocks. The tinge of (1914, 1918) identified the gneissose granites and circularity involved in this common definition was diorites in the southern part of the mapped area as the pointed out later by Sederholm (1932), for instance. oldest, pre-Kalevian rocks, subdividing the overlying The commonly held view of older Kalevian and supracrustal rocks into older Kalevian and younger younger Jatulian rocks was challenged for the first Jatulian rocks in accordance with the general opinion time in the Peräpohja area by Mäkinen (1916), but on the relation between the Kalevian and Jatulian his suggestions remained unacknowledged for many formations in Finland at that time. The Kalevian rocks years. He studied only the extreme SW part of the contained phyllites, mica schists and gneisses, quartz- Peräpohja schist belt, but was able to decipher a ites, dolomites, amphibole schists and metabasites, synclinal structure, revealing a consistent rock suc- while the Jatulian rocks were mainly composed of cession. The following quotation from Mäkinen (1916) quartzites, metabasites and conglomerates, including illuminates his observations: “Next to the basement the conglomerates of Pyhätunturi in the Pelkosenniemi there is a zone of quartzite, from 4 to 5 km broad, area. with a dip of 60o-70o inwards. The inner parts are The large ‘post-Kalevian’ granite massif occurring occupied by phyllites and mica schists with inter- on the northern side of the Peräpohja schist belt was calated layers of dolomite, all of which are con- described by Hackman (1914) as being fringed in formable to the quartzite in their strike and having many places by mica gneisses and veined gneisses fairly steep, almost vertical dips. The quartzite that have clear indications of being affected by the forms the basal layers of the sedimentary series granites. These gneisses were equated with the bet- and contains in its lowest parts detritus from the ter-preserved phyllites occurring within the schist subjacent basement”. Thus Mäkinen could not find belt. Also, some schistose and glassy quartzites were any support for either the previously presented order observed to be intersected by granites and were of deposition or an unconformity between the phyllitic therefore included in the Kalevian rocks. On the other Kalevian and the quartzitic Jatulian rocks. He was hand, the quartzites with a more clastic appearance inclined to regard the quartzites as basal layers of the were not observed to be intersected by granites and same, Kalevian sedimentary series. were therefore regarded as younger and included in Hausen (1936), in his attempt to clarify the the Jatulian. Further evidence for a difference in stratigraphical and tectonic evolution of the sedimen- metamorphic grade between the Kalevian and Jatulian tary series in the Peräpohja schist belt, established the rocks was provided by penetrating metabasites, those following sequence of deposition of supracrustal rocks. associated with Jatulian quartzites being argued to be A basal conglomerate (?) was deposited on the Archean less metamorphosed than those cutting only Kalevian basement of granites, granodiorites, and migmatites metasediments. Hackman (1914) also mentioned con- etc. and overlain by feldspar quartzites and sericite glomerates which could be regarded as Jatulian basal quartzites. These were followed by pure quartzites, conglomerates and contained fragments of a probable with probably contemporaneous amygdaloidal lava Kalevian age. These stratigraphical notions arrived at rocks. The next large unit comprised argillites with by Hackman (1914) regarding the Peräpohja schist intercalations containing a few quartzite conglomer- area were clearly influential in his subsequent treatise ates (Taivalkoski) and dolomites as well as “green on the Kittilä-Sodankylä area as discussed earlier in schists” of tuffaceous origin. The last unit to precipi- this paper (Hackman 1927). tate contained dolomites with interlayers of marls and The distinction between the Kalevian and Jatulian also psammites. It was possible for these dolomites to rocks was thus partly based on metamorphic grade be still overlain by some argillites higher up in the and, as admitted by Hackman (1914), was not always sequence. a straightforward matter, especially when dealing Much attention was paid later to the Taivalkoski

33 Geological Survey of Finland, Special Paper 33 Eero Hanski

conglomerate (Mäkinen 1949, Härme 1949, Väyrynen Central Lapland by E. Mikkola (1941) already pub- 1954), which was described for the first time by lished, A. Mikkola (1949) was able to make a good Hausen (1936). This occurs within a phyllitic environ- comparison between the supracrustal sequences of ment and contains mainly rounded pebbles of quartzite the Peräpohja and Central Lapland areas. According accompanied by lesser amounts of angular phyllite to him, the Lapponian Series contained the counter- fragments. Hausen (1936) classified the conglomer- parts of all the rocks to be found in the Peräpohja ate as intraformational and did not assign it any great schist belt. The greatest difference concerned the tectonic significance in terms of stratigraphical breaks. amount of carbonate rocks, which are not as abundant Besides working out the stratigraphical sequence, in Lapland as in Peräpohja. Correlation with the Hausen (1936) also provided it with a palaeo- Kumpu-Oraniemi Series was not so straightforward. sedimentological interpretation, based on beautifully Mikkola (1949) pointed out the absence throughout preserved primary sedimentary structures. Accord- the Peräpohja area of the polymictic conglomerates to ing to him, the whole series bears a transgressive be found in the basal formations of the Kumpu- character, the facies changing with stratigraphical Oraniemi Series. On the other hand, thick slate and height from epicontinental (deltaic-fluviatile) to littoral mica schist piles typical of the Peräpohja area were and shelf types. Hausen did not find any evidence for not known in the Kumpu-Oraniemi Series. a stratigraphical break within the sedimentary series Although the stratigraphical scheme of the Kuusamo- and therefore disagreed with the prior two-fold divi- Salla area proposed by Hackman and Wilkman (1929) sion into “Kalevian” and “Jatulian”, which was based had become obsolete, a correlation between the on the degree of metamorphism and the relation of the Peräpohja schist belt and that area seemed obvious to metasediments to younger granites. Instead, he con- Mikkola (1949), at least insofar as the occurrence of sidered the supracrustal rocks to be members of one similar rock types in both areas was concerned. A and the same series, the North-Bothnian, which could more direct parallel could be made with the Kainuu have been formed during the “Karelidic” cycle of schist belt, where counterparts for the Kivalo quartz- sedimentation of Sederholm (1932). ites were found among the Kainuan-type quartzites of The first results of post-war investigations in the Väyrynen (1933), though Mikkola erroneously men- Peräpohja schist belt appeared in two academic dis- tioned the absence of basal formations in the Peräpohja sertations based on fieldwork in partly overlapping area as one difference (cf. Perttunen 1980). Further- areas in the western and southwestern parts of the belt more, the transition from quartzites to varved slates (A. Mikkola 1949, Härme 1949). Mikkola (1949) and dolomites corresponded to the marine Jatulian in shared the opinion of Mäkinen (1916) that quartzites Väyrynen’s nomenclature. Finally, a correlation could form the lowermost known part of the sedimentary be drawn between the Kalevian schists and the fine- series, calling the well-preserved quartzites in the grained metasediments in the Peräpohja schist belt. A southern part of the belt the Kivalo quartzites and the complicating factor, however, concerned the quartz- more strongly metamorphosed, often fuchsite-bear- ites and conglomerates of the Jaurakka facies, mark- ing, anticlinal quartzites in the central part the north- ing an apparent unconformity at the base of the ern quartzites. He regarded these quartzites as be- Kalevian formations. This did not conform with longing to the same stratigraphical level, differing Mikkola’s perception of the absence of such from each other only in their degree of metamorphism. unconformities in the slates of the Kemi river basin. Higher up in the sequence, he distinguished a separate Härme (1949) and Mikkola (1949) agreed on black quartzite interstratified with slates and dolomites. many aspects of the geology of the Peräpohja schist In general, the slates were documented as lying belt, such as the general order of deposition of the directly on the quartzites without any unconformity. supracrustal rocks, the lack of major unconformities Mikkola (1949) paid special attention to the distin- within the Karelian sequence and the intrusive nature guishing of volcanic (tuffitic greenstone, agglomer- of the Haaparanta Suite. Härme (1949) described ates, amygdaloidal lavas, pillow lavas) and hypabyssal mafic igneous bodies located between the Archean rock types among those covered by the widely used basement and the schist belt which were later to be collective term ‘metabasite’. He also studied the called mafic layered intrusions, giving them the name contact relations of the Haaparanta Suite igneous ‘anorthosite-serpentine series’, but he could not as- rocks, varying from gabbros to granodiorites, and certain whether they were older or younger than the showed that these are definitely intrusive. Earlier they schist belt. had been regarded as being older than the schist belt The new discoveries made by Härme (1949) also (Hackman 1914, Mäkinen 1916). included two occurrences of polymictic conglomer- With the comprehensive description of the rocks of ates at the southern margin of the belt which turned out

34 Geological Survey of Finland, Special Paper 33 History of stratigraphical research in northern Finland to be rather enigmatic. These conglomerates were fragments in the Kivalo conglomerates (cf. Perttunen found within the space of a few tens of metres on the 1980) should be seen against the background of the Kivalo ridge, with rocks of the anorthite-serpentine general belief held at the time that the migmatite- series on one side and greenstones interpreted as forming microcline granites in the basement area hypabyssal diabases on the other. According to Härme belonged to late-kinematic granites of the Karelidic (1949), the fragments were mostly composed of orogenic cycle. migmatitic granite and ossipite (uralite gabbro) supple- Since the work of Hackman (1914), Väyrynen mented with greenstone, anorthosite, quartzite and (1954) was the first to propose a discordance be- slate (see also Enkovaara et al. 1953). These frag- tween the Kalevian phyllitic and Jatulian quartzitic ments were recorded as being of exactly the same rocks in the Peräpohja schist belt, but this time the type as found in outcrops around the conglomerates. order of these rocks was reversed. Väyrynen sug- These features coupled with the position of the con- gested that certain quartzites, like that occurring at glomerates between two mafic bodies of alleged Kallinkangas, have features resembling those of the intrusive origin and a low degree of deformation of the Jaurakka facies rocks in the Kainuu schist belt and conglomerates were taken by Härme as evidence of could be basal parts of the phyllite formation. He also the conglomerates being younger than the sedimen- referred to the Ukonköngäs conglomerate, described tary cycle of the schist belt. Furthermore, he corre- by Mikkola (1949), which is situated in the SE corner lated the migmatitic granites in the SE side of the schist of the Paakkola phyllite area and contains fragments belt with granite veins cutting through rocks of the of quartzite, dolomite and granite. The occurrence of anorthosite-serpentine and Haaparanta Suite. This led phyllites in axial depressions and their close relation to him to infer that the migmatitic granite fragments in tuffitic volcanic rocks which were thought to have the conglomerates were also much younger than the erupted during the folding of the Jatulian rocks, and Peräpohja sedimentary sequence. The conglomer- also the presence of the above conglomerates, led ates were thus assumed to have been deposited at a Väyrynen to imply some kind of discordance between very late stage, when these migmatitic granites were the two major units in the Peräpohja schist belt. subjected to erosion. The erroneous conclusion that However, later workers have never been able to Härme had reached about the nature of the granite provide any unequivocal support for this suggestion.

The 1980s to 1990s

The Geological Survey of FinlandFinland startedstarted mappingmapping Perttunen (1980) designated the basal conglomerate the Peräpohja schist belt to aa scalescale ofof 11:100,000 : 100 000 in the together with the overlying Runkaus Volcanic Forma- early 1960s under the leadership of V. Perttunen, and tion as forming the Lower Jatulian Group. Higher up the first summary of these investigations was pre- in the sequence lay the Kivalo Quartzite Formation sented by Perttunen (1980) in connection with the and the Jouttiaapa Volcanic Formation of the Middle Finnish-Soviet Jatulian symposium. He described a Jatulian Group. The Upper Jatulian Group was repre- basal conglomerate which grades downwards to a sented by the Kvartsimaa Quartzite, Tikanmaa Vol- disintegrated granite of the Archean basement com- canic and Rantamaa Dolomite Formations. The up- plex. Further field observations have verified that this permost formation of the Peräpohja schist belt was the is a true basal conglomerate and not younger than the Martimo Phyllite Formation assigned to the Kalevian schist belt as argued previously by Härme (1949). The Group. Following the general scheme of Silvennoinen granite boulders in the conglomerate were reported by et al. (1980), the Jatulian groups were united to the Perttunen (1980) to have an age of 2600 Ma, which Karelian Supergroup while the Kalevian Martimo was used as evidence for the Jatulian nature of the Phyllite Formation was taken being as part of the conglomerate as opposed to Lapponian (cf. Svecofennian Supergroup. Silvennoinen et al. 1980). Another discovery related to The detailed stratigraphy of the upper part of the the lower part of the sequence was the recognition of sequence had previously been somewhat obscure a mafic volcanic formation above the basal conglom- (Hausen 1936, Väyrynen 1954), but now Perttunen erate unit. This amygdaloidal metavolcanite, the (1980) had definitely classified the phyllitic rocks of Runkaus Volcanite Fm, is spatially closely associated the Martimo formation as the youngest supracrustal with a sill-like differentiated diabase and was not rocks. The tuffaceous greenschists that earlier had distinguished from the latter by earlier investigators. often been associated with the slates (phyllites) con- In his lithostratigraphical classification (see Fig. 6), stituted in Perttunen’s scheme an independent forma-

35 Geological Survey of Finland, Special Paper 33 Eero Hanski

Fig. 6. Stratigraphical schemes presented for the Peräpohja schist belt since 1980. Ages shown in million years.

tion separated from the phyllites of the Martimo ments from the latter were found within the conglom- Formation by the Rantamaa Dolomite Formation. erates of the mapped area. Later, he provided evi- According to Perttunen (1980), the quartzites of the dence for this erosional contact in terms of outcrop Middle Jatulian Group were penetrated by differenti- and drill core observations (Perttunen 1991), while ated albite diabase sills of an age c. 2200 Ma. This age Alapieti et al. (1989) described the finding of a together with that of 2600 Ma obtained for the granite PGM-bearing phenoclast within a conglomerate, ap- boulders within the basal conglomerate thus con- parently derived from an underlying layered intrusion. strained the time of deposition of the lower part of the These observations placed tighter time constraints on Jatulian succession. On the other hand, the minimum the beginning of Jatulian sedimentation in the area. age for the youngest supracrustal rocks of the schist Huhma etet al.al. (1990) (1990) reported anan Sm-Ndisochron isochron age of belt was provided by the age of c. 1900 Ma determined 2090±70age of 2090±70 Ma for the Ma metavolcanics for the metavolcanics of the Jouttiaapa of the for the intrusive rocks of the Haaparanta Suite. FormationJouttiaapa Formationand applied and the sameapplied method the same to the method Runkaus to The next account of Perttunen (1985) retained Formationthe Runkaus to Formation obtain a somewhat to obtain a imprecise somewhat age impre- of the same stratigraphical sequence but included super- 2330±80cise age of Ma. 2330±80 Nonetheless, Ma. Nonetheless, this was thisin good was in agree- good ficial modifications in the nomenclature at supergroup mentagreement with thewith dates the dates obtained obtained previously, previously, 2.44 2.44 Ga forGa to formation levels (see Fig. 6). The lowermost basal thefor underlyingthe underlying mafic mafic layered layered intrusions intrusions and and 2.2 2.2Ga forGa conglomerate and associated arkosic metasediments thefor differentiatedthe differentiated sills sills penetrating penetrating into into the thequartzites. quartz- were now termed the Sompujärvi Formation. In the Basedites. Based on field on geological field geological evidence, evidence,it was known it wasthat case of the other formation names, the attributes theknown age ofthat the the Runkaus age of theFormation Runkaus should Formation fall between should describing the rock types were left out, so that the thesefall between two figures. these two figures. former Runkaus Volcanic Formation was now called In a study related to the Lapland Volcanite Project, the Runkaus Formation, for example. One additional Perttunen (1989) characterized geochemically the formal difference was the labelling of the Archean basic volcanic and sill-like units in the Peräpohja schist basement complex as the Pudasjärvi Granite Gneiss belt using the stratigraphical scheme of Perttunen Complex. (1985). The Runkaus Formation displayed an obvious Perttunen (1985) reported an important observation influence of contamination with crustal material, according to which the conglomerates of the Sompujärvi whereas the Jouttiaapa and Tikanmaa Formations did Formation were deposited unconformably on the 2.44 not show any recognizable signs of sialic contamina- Ga mafic layered intrusions, even though no frag- tion. The Jouttiaapa Formation in particular has a very

36 Geological Survey of Finland, Special Paper 33 History of stratigraphical research in northern Finland depleted, MORB-like composition which renders it to decline back to more normal values. quite unique among volcanic rocks that have erupted The most recent report on advances in stratigraphical in an environment of epiclastic sedimentary rocks (see studies in the Peräpohja area was published in a also Huhma et al. 1990). In fact, it is difficult to find conference abstract by Perttunen et al. (1995). any geochemically analogous volcanic units in a simi- This work is related to the stratigraphical map of the lar setting elsewhere in the Fennoscandian Shield. The Peräpohja schist belt, scale 1:200,000. In their geochemical comparison further demonstrated that lithostratigraphical nomenclature, the authors aban- the 2.2 Ga differentiated sills did not have any geneti- doned the traditional Jatuli- and Kaleva-based names cally related counterparts among the overlying vol- at the group level and replaced them with geographical canic rocks, which was in contrast to some earlier names, in accordance with international stratigraphical suggestions (e.g., Härme 1949). guides (Hedberg 1976, Salvador 1994). This proce- On the basis of long-term, detailed field studies dure resulted in a two-fold subdivision of the carried out in the Peräpohja schist belt, as described supracrustal sequence into the lower Kivalo Group above, carbonate rocks were known to occur at and the upper Paakkola Group, corresponding to the various, well-defined stratigraphical levels, and there- former Jatulian and Kalevian rocks, respectively. fore this belt served as one of the key areas in Additional revisions were made at the formation level, Karhu’s (1993) carbon isotope study of sedimentary including the distinction for the first time of a volcanic carbonate rocks. The isotope analyses showed that formation composed of pillow lavas (the Väystäjä the lowermost carbonates from the Sompujärvi For- Formation) in the former Kalevian rocks (now the mation had already attained anomalous positive δ13C Paakkola Group). The stratigraphy and lithology of the values (+8.6‰) and this trend persisted higher up in type formations in the southern part of the Peräpohja the sequence until the uppermost Jatulian carbonate schist belt are shown in Figure 6. rocks of the Rantavaara Formation where they started

SUMMARY AND DISCUSSION

The stratigraphical research in Finland was based in been the general agreement since the 1920s that two the first half of the 20th century on the distinction of major supracrustal rock compartments, traditionally cycles of sedimentation separated by events of dias- called with the names Lapponian and Kumpu and trophism (Sederholm 1932). In the absence of radio- their derivatives, occur in Finnish Lapland, but the metric data, this was the only possible way to proceed, agreement generally has not extended beyond that. and many historical accounts have shown that this Especially, their relationship to the Karelian forma- method of deduction was not always devoid of circu- tions elsewhere in the shield has been under dispute lus in demonstrando. Later age determinations and (Fig. 3). The Karelian formations were originally geological research have brought about revolutions in proposed by Eskola (1921, 1925) as a collective term our understanding of the stratigraphical relations and for Jatulian, Kalevian and Ladogan deposits in east- the geological evolution of the Fennoscandian Shield. ern Finland and were later defined as comprising The rocks previously included in the Archean rocks Sariolian, Jatulian and Kalevian rocks in Finland have turned out to possess ages spanning a range of (Meriläinen 1980a,b, Simonen 1986). Since it had more than 1 billion of years and are now distributed appeared that the Kalevian are probably younger than between the Archean and Paleoproterozoic forma- the Jatulian rocks without any notable stratigraphic tions. It has been discovered that certain parts of the breaks between them, Sederholm (1932) decided to Finnish bedrock were generated in totally different unite the Jatulian and Kalevian rocks into the same geotectonic regimes (e.g., Karelian, Svecofennian), cycle of sedimentation, the Karelidic cycle. The and therefore direct correlations of their rock units arguments for Sederholm to exclude the Lapponian have become meaningless. Also in the course of rocks from the Karelidic cycle and give these rocks research, the age relations of certain stratigraphical a special name included 1) their inferred position units have been reversed (e.g., Kalevian and Jatulian). analogous to that of the rocks of the Bothnian cycle All these developments have rendered previously in southern Finland (though being lithologically differ- presented stratigraphical schemes obsolete, but many ent, both were interpreted as having been intruded by traditional stratigraphical names have remained in post-Bothnian granites) and 2) the presence of the common usage until the present day. great unconformity between the Lapponian forma- As the above historical overview reveals, there has tions and the overlying, Jatulian-correlated Kumpu

37 Geological Survey of Finland, Special Paper 33 Eero Hanski

quartzites. Along with these results a terminological crisis was A common feature of all the Karelian formations in unavoidable. This was conveniently circumvented by eastern Fennoscandia is that they were deposited on shifting to a totally new, formal lithostratigraphical the deeply eroded Archean cratonic basement (e.g., nomenclature (Räsänen et al. 1995, Lehtonen et al. Eskola 1963, Laajoki 1986). We now know that this is 1998). As was stated in the Introduction, a similar also true for many rock units assigned to the Lapponian stratigraphical reform had already been in process by Sederholm (1932). The correlation of the Lapponian elsewhere in Finland for some years. The previous rocks with Karelian formations seems to have been rocks of the Karelia Supergroup or Kumpu formations evident for many prominent geologists from the 1950s were assigned to the Lainio and Kumpu Groups, while to the 1970s in Finland (Fig. 3) and also in northern the Lapponian rocks or the rocks of the Lapponia Sweden (Geijer 1963). For example, Väyrynen (1954) Supergroup were divided between the Salla, Onkamo, did not mention the term Lapponian at all and there Sodankylä, Savukoski and Kittilä Groups (Lehtonen et were no difficulties for Eskola (1963) to include al. 1998). Abandonment of the traditional stratigraphical Mikkola’s Lapponian and Kumpu-Oraniemi series in names in formal classification of rock units has pro- the ‘karelides’. On the other hand, the molasse-like ceeded rather smoothly in northern and eastern Fin- nature of the Kumpu formations, deposited after the land, but not everywhere on the Fennoscandian Shield. main phase of Svecokarelidic folding, was advocated Particularly, there has been a major confrontation of especially by Simonen (1960a,b, 1971, 1980). The the old and new systems in the Kola Peninsula as Karelian-Lapponian and Kumpu-molasse connections evidenced by the vigorous public debate carried out were, however, broken at the turn to the 1980s (Fig. recently by Melezhik and Sturt (1998a,b), Smolkin 3) when the Lapponian rocks were regarded as (1998) and Sharkov and Smolkin (1998). Archean in age and correlated with Archean New research results from Finnish Lapland also greenstone belts, such as the Kuhmo-Suomussalmi include the proposal that the Kittilä Group represents belt in eastern Finland (Gaál et al. 1978, Silvennoinen an allochthonous piece of ancient oceanic crust (Hanski et al. 1980). Three Supergroups, Lapponia, Karelia 1997). This interpretation and the recognition of and Svecofennia, were established in Lapland, with Paleoproterozoic ophiolitic terranes elsewhere (e.g., only the Jatulian rocks (represented by the Kumpu Kontinen 1987, Scott et al. 1992) which may have formation in Lapland) included in the Karelia originated hundreds or thousands of kilometres from Supergroup (Silvennoinen 1985). Even though the their present sites, and whose geological history may post-Archean age for the Lapponian rocks has been have little or nothing in common with the continental widely accepted for some time (Silvennoinen 1986, mass to which they are now physically attached, has Gaál & Gorbatchev 1987), the division into the above- important implications for stratigraphical research. In mentioned supergroups has continued until very re- the least, they provide a serious warning against the cently (Silvennoinen 1998). practise of extending a ‘layer-cake’ stratigraphy over In principle, placing of the Lapponia Supergroup a whole craton without caution. The Kittilä Group and beneath the Jatuli Groups of the Karelia Supergroup is the overlying coarse-clastic, molasse-like meta- in agreement with the original definition of Sederholm sediments of the Lainio and Kumpu Groups possess (1932) according to which the Lapponian rocks are lithological characteristics and geological histories pre-Jatulian in age. However, isotopic and geological which hardly find parallels in other Karelian forma- studies carried out in the1980s and 1990s have pro- tions which partly explains why previous workers vided ample evidence that Sederholm’s “Jatulian” have had difficulties in squeezing them into the stand- Kumpu quartzites are much younger (< 1.88 Ga) than ard Karelian stratigraphy. the Jatulian supracrustal rocks elsewhere in the In retrospect, it is interesting to note that there were Fennoscandian Shield (Rastas et al. 2001, this vol- obvious reasons for the cautiousness that several ume, Hanski et al. 2000, 2001). On the other hand, the notable researchers had in taking the final stand in rocks previously called Lapponian seem to cover a regard to the correlation of the Lapponian and Kumpu very large range of ages. Most importantly, a large formations with other Karelian formations. Although part of the Lapponian volcanic rocks have turned out the Kumpu-Jatulian connection was regarded as a to be younger than Jatulian, approaching an age of c. viable alternative, many lithological observations were 2.0 Ga (Rastas et al. 2001, this volume, Hanski et al. at odds with this option. Hackman (1927) pointed out 1998). Furthermore, typical Jatulian quartzites and the absence of intrusive metadiabases and metagabbros hypabyssal layered sills within them as well as pre- in the Kumpu formations. Mikkola (1941) in turn Jatulian volcanic rocks correlated with Sumi-Sariolian emphasized the obvious lack of carboniferous and rocks are found among the “Lapponian” rocks. carbonate-rich interbeds in these rocks and on the

38 Geological Survey of Finland, Special Paper 33 History of stratigraphical research in northern Finland other hand, drew attention to the general lithological London: Institute of Mining and Metallurgy, 177-187. similarity between the Lapponian and Jatulian quartz- Brewer, T. S. & Pharaoh, T. C. 1990. Early Precambrian basic rocks of the Baltic Shield. In: Hall, R. P. & Hughes, D. ites. He also considered it possible that some granitoid J. (eds) Early Precambrian Basic Magmatism. Glasgow: fragments found in the Kumpu-Oraniemi series con- Blackie and Son, 273-293. glomerates were derived from the syenite series Enkovaara, A., Härme, M. & Väyrynen H. 1953. Oulu- (Haaparanta Suite) plutonic rocks. Even Sederholm Tornio. The General Geological Map of Finland, 1:400 000, Explanations to the Map of Rocks, Sheets C5-B5. Geologi- (1932) did not appear confident of the Kumpu-Jatulian cal Survey of Finland. 153 p. correlation although much of his stratigraphical inter- Eriksson, B. & Hallgren, U. 1975. Beskrivning till pretation hinged on this assumption. berggrundskartbladen Vittangi Nv, No, Sv, SO. Sveriges Not all the stratigraphical problems have yet been Geologiske Undersökning, Series Af, Nr 13-16. 203 p. (in Swedish with English summary). solved, but new tools, like NORDSIM in Stockholm, Eskola, P. 1921. Kidetieteen, mineralogian ja geologian alkeet. are now available for dating purposes. Although the Porvoo: Wenner Söderström Osakeyhtiö. 296 p. radiometric datings are of paramount importance in Eskola, P. 1925. On the petrology of eastern Fennoscandia. stratigraphical research, they always need critical I. The mineral development of basic rocks in the Karelian formations. Fennia 45 (19), 1-93. evaluation against the background of field evidence. Eskola, P. 1941a. Erkki Mikkola. Bulletin de la Commission The irony lies in the fact that some of the first zircon géologique de Finlande 126, xiv-xxiii. dates from northern Finland, namely the c. 2.2 Ga age Eskola, P. 1941b. Erkki Mikkola und der heutige Stand der präkambrischen Geologie in Finnland. Geologische for a diabase occurring among Kumpu-type quartzites Rundschau 32, 452-483. at Värttiövaara (see Rastas et al. 2001, this volume) Eskola, P. 1963. The Precambrian of Finland. In: Rankama, K. and the Archean age for a felsic rock from the (ed.) The Geologic Systems: The Precambrian, Vol. 1. western part of the Kuusamo schist belt (see Räsänen London: John Wiley & Sons, Ltd., 145-263. Frosterus, B. 1902. Bergbyggnaden i sydöstra Finland. Bul- & Huhma Vaasjoki 2001, 2001, this this volume volume) were) were among among the evi- the letin de la Commission géologique de Finlande 13. 13 p. evidencedence which which was wasutilized utilized to assign to assign the Kumpu the Kumpu forma- Gaál, G. & Gorbatchev, R. 1987. An outline of the Precambrian formationstions to the Middle to the Jatulian Middle Group Jatulian and Group all the andLapponian all the evolution of the Baltic Shield. Precambrian Research 35, 15- Lapponianrocks to the Archean rocks to (Rastas the Archean 1980, Silvennoinen (Rastas 1980, et al. 52. Gaál, G., Mikkola, A. & Söderholm, B. 1978. Evolution of Silvennoinen1980). Furthermore, et al. should 1980). the Furthermore, first age determinations should the the Archean crust in Finland. Precambrian Research 6, 199- firstfrom agethe Peräpohja determinations schist from belt have the Peräpohjacome from schist acid 215. beltdikes have containing come from inherited acid dikeszircons containing (see Perttunen inherited & Geological Map, Northern Fennoscandia 1:1 mill. 1987. zirconsVaasjoki (see 2001, Perttunen this volume, & Vaasjoki Hanski 2001, et al. this 2001 volume,, this Geological Surveys of Finland, Norway and Sweden, Hel- sinki. Hanskivolume ),et theal. 2001interpretations, this volume of), the the geological interpretations evolu- of Gehör, S. & Havola, M. 1988. The depositional environment thetion geologicalin that area would evolution have in been, that at arealeast temporarily, would have of the early Proterozoic Tuomivaara iron-formation and been,quite different. at least temporarily,Datings can be quite utilized different. to solve Datings prob- associated metasediments, eastern Finland. In: Laajoki, K. canlems, be but utilized they surelyto solve will problems, sometimes but createthey surely new ones will & Paakkola, J. (eds.) Sedimentology of the Precambrian Formations in Eastern and Northern Finland. Proceedings of sometimesand impose createnew challenges new ones for and further impose field geological new chal- the IGCP 160 Symposium at Oulu, January 21-22, 1986, lengesresearch, for asfurther exemplified field geological by the finding research, of unexpect- as exem- Geological Survey of Finland, Special Paper 5, 109-133. plifiededly young by the detritalfinding zirconsof unexpectedly in arkosites young from detrital the Geijer, P. 1963. The Precambrian of Sweden. In: Rankama, K. zirconsPeräpohja in schist arkosites belt from (Perttunen the Peräpohja & Vaasjoki schist 2001, belt (ed.) The Geologic Systems: The Precambrian, Vol. 1. London: John Wiley & Sons, Ltd., 81-143. (Perttunenthis volume &). VaasjokiThus the 2001,stratigraphical this volume schemes). Thus that the Goodwin, A.M. 1991. Precambrian Geology. London: Aca- stratigraphicalwe now espouse, schemes may againthat we be, now at least espouse, partly, may at demic Press Ltd. 666 p. againstake. be, at least partly, at stake. Hackman, V. 1914. Rovaniemi-Torneå-Overtorneå. The General Geological Map of Finland 1 : 400 000, Explanation Acknowledgements to the Map of Rocks, Sheets C6-B5-B6, Geological Survey of Finland. 86 p. (In Swedish) Hackman, V. 1918. Rovaniemi-Tornio-Ylitornio. The Gen- Drs. Ahti Silvennoinen and Matti Vaasjoki, both of eral Geological Map of Finland 1 : 400 000, Explanation to the Geological Survey of Finland, are thanked for their the Map of Rocks, Sheets C6-B5-B6, Geological Survey of constructive comments on the manuscript. Finland. 80 p. (In Finnish) Hackman, V. 1927. Studien über den Gesteins aufbau der Kittilä-Lappmark. Bulletin de la Commission géologique de Finlande 79. 105 p. REFERENCES Hackman, V. & Wilkman, W.W. 1929. 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