The Lapland Greenstone Belt: Stratigraphic and Depositional Features in Northern Finland Matti Saverikko

Home , Argillite, Arkose, Cap of the North, Gneiss, Lapland (Finland)

THE LAPLAND GREENSTONE BELT: STRATIGRAPHIC AND DEPOSITIONAL FEATURES IN NORTHERN FINLAND MATTI SAVERIKKO

SAVERIKKO, MATTI, 1987: The Lapland greenstone belt: Stratigraphic and depositional features in northern Finland. Bull. Geol. Soc. Finland 59, Part 2, 129—154. The Lapland greenstone belt formed in a continental environment, as a result of rifting of the 3.1—3.0 Ga Saamian craton. The basin subsided to a depth of five kilometres between 3.0—2.5 Ga and contains a sequence which is subdivided into Lower, Middle and Upper Lapponian groups, with the Middle Lapponian metasediments separating the Lower Lapponian Salla—Jauratsi and Upper Lap- ponian Kittilä greenstone successions. After a folding episode the Lapponian supergroup was covered with the Kumpu conglomerate—metasandstone—slate suite, occurring as Karelian, i.e. Sariolan-Jatulian, deposits. A few erosional remnants are capped with Svecofennian rocks 1.9 Ga old. The Lower Lapponian consists of basal arkose, the Möykkelmä greenstones, a cratonic quartzite—carbonate—schist suite and the Salla greenstone complex(es) preserving evidence of a lava plateau. The Salla bimodal metavolcanic rocks and the Middle Lapponian metasediments, which, at their thickest, constitute the Ora- niemi arkose—slate—quartzite association (incl. redbeds), evidently indicate cratonic rifting that terminated with the growth of isolated lava fields of Fe-rich tholeiitic basalt. In the Upper Lapponian, euxinic-exhalative metasediments underlie the Sat- tasvaara, Sotkaselkä and Kummitsoiva komatiite complexes and the Kittilä greenstones, which are present as complicated but distinct eruption centres and contain a varying amount of ejecta and volcaniclastic greywacke—slate intercalations. The komatiite-dominant succession evidently represent mantle-activated rifting and the linear mantle upwelling. The Lapland greenstone belt and adjacent granulitic belt appear to constitute a paired metamorphic terrain, where the overthrusting of the granulites is explain- ed by mutual motions of the distinct cratonic megablocks. Key words: greenstone belts, metamorphic rocks, Hthostratigraphy, chronostratig- raphy, evolution, Archean, Proterozoic, Lapland, Finland. Matti Saverikko: Department of Geology, University of Helsinki, P.O. Box 115, SF-00I71 Helsinki, Finland.

Introduction of quartz-dioritic, granodioritic and granitic composition (Simonen 1980) but also contains The Finnish Precambrian can be broadly sub- supracrustal greenstone belts (Gaål et al. 1978); divided into the Archaean basement, Svecokare- the granulitic arc in the north, regarded by Me- lian fold belt, rapakivi granites and Jotnian sed- riläinen (1976) as Archaean, is considered by Bar- imentary rocks (Fig. 1). The Archaean basement bey et al. (1984) a Karelian high-grade meta- is dominated by cataclastic gneissose granitoids morphic terrain. 130 Maiti Saverikko The gneissose granitoids commonly give ages of 2.8—2.6 Ga, even though many rocks are 3.2—2.8 Ga old (Simonen 1980; Kröner et al. 1981; Luukkonen and Lukkarinen 1986; Paavo- la 1986; Huhma 1987) pointing to a Saamian origin (Silvennoinen 1985). On the other hand, granitoids of 2.8—2.4 Ga age also occur (Mar- tin 1987) and the basement has been subjected to polyphase deformation and metamorphism (e.g. Kröner et al. 1981; Luukkonen 1985; Paa- vola 1986) resulting in a very complicated structural history. It is referred to generally as the Pre- Svecokarelian granite-gneiss complex, with an older Saamian body of obscure extent. The Kuhmo—Suomussalmi greenstone belt in the east forms distinct synforms between granitoid diapirs (Gaål et al. 1978) occupied by the Kuhmoan supergroup (Simonen 1971; Luukko- nen and Lukkarinen 1986). Deposition occurred at 3.0—2.5 Ga (Piirainen 1985) within continental to oceanic rift environments (Martin et al. 1984). In the north, metavolcanic rocks accumulated in the Kittilä and Salla—Jauratsi greenstone areas (Fig. 2), constituting part of the Lapponian supergroup (Sederholm 1932; Silvennoinen et al. 1980) deposited on the disintegrated granite- gneiss complex (Mikkola 1941) and underwent high-grade metamorphism in the Tuntsa—Sa- vukoski ultrametamorphic terrain (Gaål et al. 1978). This Lapland greenstone belt is disrupted by granites whose geochemistry indicates sub- stantial recycling of Archaean crustal material (Lauerma 1982; Huhma 1986). The Lapponian deposition at 3.1—2.6 Ga (Silvennoinen et al. 1980), and perhaps continuing into the Early Proterozoic (Silvennoinen 1985), took place in a cratonic rift (Saverikko 1985; Saverikko et al. 1985; Silvennoinen 1985; Manninen and Pihlaja 1988) or geosynclinal basin environment (Mäke- lä 1968; Paakkola 1971; Piirainen 1975).

Fig. 1. Stratigraphic outline and Precambrian subprovinces I Orogenic plutonic rocks in Finland, compiled after Meriläinen (1980), Silvennoinen et al. (1980), Simonen (1980) and Gaål and Gorbatschev + -f I Rapakivi granites (1987). The map area in Figure 2 is marked with a dotted line. The Lapland greenstone belt: stratigraphic and depositional features in northern Finland 131

LAPLAND GREENSTONE BELT

"fezsi -r / -69" Jauratsi

SaUa

tm!

LITHOLOGIC MAP of

North Finland

100 Km

[ :!;;y; ^ ; I METAV0LCAN1CS (greenstones, komatiitic rocks etc.)

QUARTZITIC ROCK, | | MICA SCHIST

^ j MIGMATITES (mica gneiss, arkose gneiss, amphibollte)

I'/','?! GRANITIC GNEISSES , GRANULITES

+ I GRANITE , GABBROID

GRANODIORITE, QUARTZ DIORITE and MONZONITE

Fig. 2. Geological sketch map of the Lapland greenstone belt (incl. the Tuntsa—Savukoski ultrametamorphic terrain) and the granulite complex. Minor continuations of the Petsamo/Pechenga greenstone belt (U.S.S.R.) extend into northernmost Finland. The map is modified after Meriläinen (1976), Silvennoinen et al. (1980) and Simonen (1980).

The Karelian supergroup is composed of the whereas the Jatulian quartzites were deposited in Sariolan and Jatulian deposits (Gaål and Gor- a shallow epicontinental sea (Ojakangas 1965) batschev 1987). The Sariolan coarse-clastic rocks and are in turn overlain by the Marine-Jatulian are of limited occurrence and record stable plat- dolomite—black-slate association; the Karelian formal, riftal or glacial palaeoenvironments sequence includes also mafic metavolcanic rocks, (Pekkarinen 1979; Gaål 1980; Marmo and Oja- (Meriläinen 1980; Simonen 1980). kangas 1984; Laajoki 1986a; Marmo et al. 1986), The platformal strata are overlapped by the 132 Maiti Saverikko Svecofennian geosynclinal sequence (Simonen 0 m Scale 1980) including the Kalevan slate—greywacke association (Gaål and Gorbatschev 1987). These 1 km- latter formed in a continental shelf setting so that the terrigenous sediments derived from the Sariolan-Jatulian deposits and the Archaean basement (Huhma 1987); arkosic conglomerates Arkosic and other phenoclastics at the base indicate mar- conglomerate ginal rifting and base-of-slope environment (Honkamo 1985; Perttunen 1985; Ward 1987; Greywacky references in Luukkonen and Lukkarinen 1986). conglomerate

The Svecofennian domain proper consists of Schist newly mantle-derived material with only a minor »tssii i component from older continental crust (Huh- Quartzitic ma 1986). The crustal segment displays direct rock evidence of volcanic arcs, subduction and colli- Carbonaceous sion (references in Gaål 1986; Huhma 1986) and greenschists the bulk of the epiclastics in the thick volcanic- Graphitic sedimentary sequence was eroded from the slate zone recently formed crust (Huhma 1986, 1987). The Svecokarelian fold belt was intruded by m Amphibolite anorogenic rapakivi granites recording the onset Komatiitic of continental rifting (Nurmi and Haapala 1986), xsSm&n rock produced some fault-bounded troughs (Kahma iimi 1978) in which the Jotnian arkose—shale redbed Greenstone association was deposited (Simonen and Kouvo SAAM IAN Felsic 1955). metavolcanic

Fig. 3. Composite stratotype from the key area is based on the stratigraphic columns presented in Figure 5 (see p. 141). Lithologic characteristics The Salla greenstone complex is doubtless much thicker than in measured cross-sections because of its enormous extent. The Lapponian sequence is considerably thinner, The Lapponian supergroup underlies erosional and lacking in some, particularly Middle Lapponian units, remnants of Karelian deposits and consists at the northeast margin of the greenstone belt (see Mattila greenstones and two metasedimentary assem- 1974; Isomaa 1978; Saverikko 1978, 1983). blages (Hackman 1927), of which, according to Mikkola (1941), the younger Oraniemi arkose— schist—quartzite suite should be linked rather with the Kumpu(tunturi) conglomerate—meta- 1927; Mikkola 1941; Silvennoinen et al. 1980) sandstone—slate suite of Karelian origin. The re- may be resolved by invoking, instead of a bipar- mainder of the Lapponian is lithostratigraphical- tite subdivision, Lower, Middle and Upper Lap- ly comparable to the Kuhmoan supergroup (Mik- ponian groups (Fig. 3). kola 1941). However, the Oraniemi rock suite has The key area (Fig. 4) includes the northern part also been regarded as part of the Lapponian of the Salla—Jauratsi greenstone area and the sequence (Silvennoinen 1985). These discrepan- southeastern corner of the Kittilä greenstone cies in stratigraphic interpretation (cf. Hackman area. The following lithologic and petrographic 25 30' 28;30"

2830' Fig. 4. Lithostratigraphic map of central and eastern Finnish Lapland. The key area is marked with a dashed line; geology has been revised after Haimi (1977), Kallio et al. (1980), Saverikko (1978, 1985, 1988a), Saverikko and Manninen (1981) and Manninen and Pihlaja (1988). 134 Maiti Saverikko descriptions are compiled from various publica- at Kivivuotsonselkä (Räsänen 1983, 1984) may tions, theses and research reports. also be included in this assemblage. Basal arkose, up to 20 m thick, appears to cover the basement except in the east, where an Saamian gneiss complex arkose quartzite and quartz-pebbly conglomerate The Saamian gneiss complex is composed are separated by a distinct contact from the base- of gneissose granitoids that form a weakly de- ment. The basal arkose passes into Lower Lap- formed to veined and intensely folded rock ponian quartzite. assemblage, including granoblastic or cataclastic The Lower Lapponian quartzite—schist sheet rocks of granitic, granodioritic, trondhjemitic, begins with a quartzite blanket, 0.1—0.3 km tonalitic and quartz-dioritic composition. The thick at the margin overlain by and interdigi- granitoids also contain concordant enclaves and tating with a somewhat thicker schist unit at the palaeosomes of amphibolite, mica gneiss, horn- depocentre. The metasediments include also in- blende gneiss or conglomeratic arkose gneiss tercalate with uralitized mafic lithologies in the particularly prominent to the east of the vil- east and southeast. lage of Savukoski. Granitic streaks and feldspar The feldspathic quartzite is thinly stratified and metacrysts show incipient migmatization in both contains quartz-arenitic deposits and interlami- the orthogneisses and paragneisses. The gneiss nae, and sporadic quartz-pebbly accumulations. complex was weathered prior to Lapponian depo- The fine-grained quartzite is usually granoblas- sition and palaeoresidue is widespread. tic but the matrix in blastoclastic rocks is feldspathic with minor carbonate and mica. The rock grades into quartz-feldspathic schist or sericitic Lower Lapponian group schist that is fuchsitic in places. Peculiar to the quartzite are quartzose laminae, and dissemina- Basal arkose or quartzite usually lie directly on tions and bands of calc-silicate minerals. The weathered basement, since the lowermost, green- calc-silicate rock forms several thin and extensive stone unit beneath them is rarely exposed. The interlayers and dolomite is present in scattered upper part of the group consists of the extensive lenses or ribbons. Small-scale cross-beds, slump Salla greenstones. folds, ripple marks and mud cracks are found in The Möykkelmä greenstones are poorly the least foliated rocks. known, but in the north there are two exposures The schist consists mainly of biotite and mus- that are largely tuffaceous amphibole-mica rock covite, and bedding is seen as thin quartz- with gneiss fragments towards the base. When feldspathic or calc-silicatic bands. Pelitic (A1203 appreciable amounts of bombs and lapilli are < 36.6 wt °7o, Fe20, tot < 21.3 wt °7o) interbeds present, the lapilli tuff passes to amphibole rock occur within the semipelitic schists as bands with agglomeratic or blocky tuff texture. Py- alternately rich or poor in aluminium-silicate por- roclastics and infrequent lavas of komatiitic com- phyroblasts (Niemelä 1976). Garnet and calcium- position (MgO 13—21 wt <7o) envelop and under- silicate minerals together with dominating bio- lie tholeiitic greenstones of andesitic to basaltic tite are the most typical minerals, although cor- composition (Räsänen et al. 1986), now present dierite is a distinctive minor constituent. as coarsely vesicular amphibole-plagioclase The Salla greenstone complex, 1 —1.5 km in rocks. The volcanic pile is 250 m thick at the type measured thicknesses, consists virtually entirely locality at Möykkelmä, from which similar rocks of mafic greenstones: phaneritic rocks are in ex- can be traced northwards (Manninen and Pihla- cess of evidently effusive greenstones including ja 1988), while a spinifex-textured komatiitic rock aphanitic lavas and minor pyroclastics. Felsic The Lapland greenstone belt: stratigraphic and depositional features in northern Finland 135 rocks make up an extensive interlayer, up to 1 are quite the exception. The major minerals in km thick, and a few graphitic or dolomitic beds both rocks are quartz, albite-oligoclase, sericite (h. 1 — 10 m) and some small dolomite inclusions and biotite. The tuffs contain plagioclase also as are contained in both felsic and mafic rocks. coarse-grained crystal ejecta and show chemical Several gabbros, hornblendites and serpentinites evidence of possible terrigenous material (Man- are also present in the complex. ninen 1981). Magnetite that is partly martiniti- The mafic greenstones of aphanitic texture are zed has been enriched into several deposits in the lavas rich in amygdules concentrated into pecu- lavas, including ferruginous chert. Minor sulph- liar flow structures, and pyroclastics in which ides occur in rare fragmentary lavas. tuffs grade into slate and rare quartzitic or dolomitic beds. The phaneritic rocks are massive to Middle Lapponian group slightly oriented both in the differentiated lava flows and in sills or dykes intruding the other The Middle Lapponian group is composed of greenstones. The main minerals in the lavas and the Oraniemi arkose—slate—quartzite associa- pyroclastics are hornblende-actinolite, albite- tion (Saverikko 1988a) with amphibolites at the andesine and epidote. Biotite can also be a ma- top. The Lower-Middle Lapponian marker is a jor mineral, and sphene is a minor component. volcaniclastic greywacke—slate blanket (50 m Where the texture is not granoblastic, the phan- thick), varying from phenoclast-supported con- eritic rock is blastophitic, the aphanitic rock is glomerate to pebbly rock (Haimi 1977; Kröner microporphyritic, and the tuff is blastoclastic to et al. 1981; Saverikko 1988a). Comparable rocks nematoblastic. While some lava flows have trem- (Silvennoinen 1972; Rastas 1980; Veki 1985; Vir- olitic or talcose basal cumulates, not all the ul- ransalo 1985) are encountered at an analogous tramafics are differentiates, since an amphibole- stratigraphic level outside the key area. chlorite rock of komatiitic composition, for in- The marker is characterized by the amphibol- stance, is associated with tuffs (Manninen 1981), ic matrix in which some hornblende-actinolite and a lithic-vitric coarse tuff is mineralogically grains of good roundness exhibit authigenic of komatiitic affinity; these rocks underlie the overgrowth; epidote and quartz occur as well. felsic rocks. The volcaniclastic debris changes to clayey, and The felsic rocks are lavas of intermediate to the rock devoid of the phenoclasts is usually acidic composition but also acidic tuffaceous thinly stratified. The highly rounded pebbles rocks: calc-alkaline dacites and rhyolites and and cobbles are granitoid, cordierite-staurolite tholeiitic andesites are recognized from scant geo- schist, hornblende gneiss, vein quartz, quartzite, chemical data (Manninen 1981). carbonate rock, felsic rock, and greenstone The intermediate lavas are phaneritic and mas- mineralogically similar to the matrix. The matrix sive or aphanitic and amygdaloidal in texture. is tholeiitic basalt in chemical composition and is The cumulophyric or porphyritic lavas are some- therefore considered tuffaceous in origin (Kröner times weakly propylitized but the mineral as- et al. 1981). semblage is mainly albite-oligoclase, quartz, The Orakoski arkose is stratified in a homo- hornblende and biotite. Magnetite is a common geneous deposit (h. 0.9—1.2 km) of arkose minor constituent and sulphides occur in bands quartzite except for redbeds at the base and a few or disseminations. arkosic conglomerate lenses, known as the Huh- The acidic rocks are largely tuffs, present as takumpu conglomerate, in the lower part of the laminated or banded rocks, and minor lavas that deposit. The arkose quartzite is clearly clastic and are massive rocks with or without small amyg- the arenaceous detritus derives from cataclastic dules and plagioclase phenocrysts; pillow lavas granitoids: the clasts are polygonal quartz, 136 Maiti Saverikko mortar-structured K-feldspar and plagioclase (An both are variously convoluted. Highly rounded 10—35), and minor rock particles. The mainly isolated quartz pebbles occur sporadically. arenitic mixture of medium, coarse and very The sericite quartzite consists of laminae alter- coarse sands together with sericitic matrix form nately rich in quartz or sericite and of sericitic beds, 5—25 cm thick, which are distinguished by flasers associated with quartzose lenticular beds. greenish micaceous or grey metalliferous in- The matrix is fuchsitic in places and although terfaces. Current bedding is ubiquitous in the monomictic detritus dominates, feldspar clasts form of channel-fill cross-beds with channel- occur near the arkose quartzite: the well round- lag gravel or festoon cross-beds of large scale. ed sand grains floating in the sericitic matrix are The Huhtakumpu conglomerate contains highly distinctive to the Sodankylä quartzite (Hackman rounded pebbles and cobbles of vein quartz, 1927), and in the fines, plagioclase (An 10—15) granitoids and rarely slate in a rhythmically up- is in excess of K-feldspar, although the contents ward-coarsening pile interbedded by the arkose of both are small. Mud cracks with or without quartzite. haematite accretions occur, too. The metapelite sheet (1 —1.7 km thick) com- The Oraniemi assemblage has erosional rem- prises slate with andalusite, staurolite and kya- nants at Luosto, Jauratsi and Puu-Matovaara. nite, grading through schist into sillimanite The exposure at Luosto includes the marker, the gneiss. The rock is enriched in aluminium terminus of the Orakoski arkose and haematite- (A1203 16.9—30.1 wt <%) and iron (Fe203"" bearing metasediments relative to the Sodanky- 6.1 — 12.2 wt %) (Rask 1978, Leppänen 1985), lä quartzite (see Haimi 1977). At Jauratsi the accounting for the characteristic abundance of arkosic conglomerate contains pebbles and cob- aluminium-silicate porphyroblasts and magne- bles of granitoid, felsic rock, schist, greenstone, tite grains throughout the rock. Chloritoid and calc-silicate rock and fuchsitic quartzite within plagioclase porphyroblasts are also typical but the arkosic to greywacky matrix that is faintly they are not invariably present. The negligible pigmented with haematite. The deposit at Puu- quantities of calcium and manganese are insuf- Matovaara is sericite quartzite rich in carbonate ficient for the formation of garnet (Rask 1978). but it includes minor arkosic or quartzose inter- Uniform stratification occurs as laminae and thin beds. The quartzite is striated with calc-silicate planar beds usually exhibiting graded bedding. minerals and contains an intraformational con- The Sodankylä quartzite formation is sericite glomerate towards the base. quartzite about 1 km thick but the marginal zones Amphibolite from isolated lava fields is most- are more arkosic, overlying a thin calc-silicate ly structureless, albeit flow structures and flow- rock and a greenstone unit of limited extent. top breccias are present, and lava pillows and in- The arkose quartzite is coarse to fine-grained frequent amygdules are found, too. The grano- arenite or wacke of arkosic composition, and the nematoblastic rock consists of hornblende and detritus with a higher degree of roundness is oligoclase-andesine with epidote and biotite. mineralogically similar to that in the Orakos- Quartz and sphene can also be major constituents ki arkose, but numerous argillic clasts have and uralitized phenocrysts imply cumulative squashed into a quartz-feldspar-sericite pseudo- pyroxene. The top of the flow pile is generally matrix. Heavy minerals are tourmaline, apatite, banded, containing several dark horizons with zircon and epidote, exhibiting secondary over- disseminated graphite and interlayers of graphitic growth or high roundness. Composite bedsets slate and jasper. Garnet is a typical mineral of succeed in rhythmic pattern and are comprised the banded amphibolites. The massive rocks are of laminated to ripple-bedded fine sand or silt Fe-rich tholeiitic basalts in composition (Sara- and parallel-bedded to cross-bedded sand, which pää 1980; Manninen 1981) but the banded rocks The Lapland greenstone belt: stratigraphic and depositional features in northern Finland 137 are enriched in aluminium and depleted in mag- titic komatiite (MgO 18—30 wt %) and green- nesium (Sarapää 1980): the banded rocks can be stone in the upper part, also including a few flows considered to derive from detritus rather than and necks of peridotitic komatiite (MgO > 30 from primary ejecta, that should explain the wt %); the greenstones prevail in the west (Paak- enrichment in quartz and feldspar. kola 1971). The upper part is separated by carbonaceous greenschists with volcaniclastic conglomerates at the top, and it is comparable with Upper Lapponian group the isolated komatiite complexes at Kummitsoi- Graphitic and jaspery interbeds within the va and Sotkaselkä (Kallio et al. 1980; Saverikko banded amphibolites (Kallio et al. 1980; Saverik- 1983). The komatiite complexes are about 2 km ko and Manninen 1981; Kerkkonen 1982) are ac- thick. cepted here as marking the Middle-Upper Lap- The komatiitic basalt is amphibole rock usually ponian transition. Similar banded amphibolites with plagioclase as disseminations or laminae. (Latvalahti 1973; Meriläinen 1976; Hiltunen The nematoblastic to microporphyritic rock con- 1982) in comparable lithologic surroundings also sists of actinolite, saussuritized plagioclase and occur outside the key area. Because the amphib- minor epidote or zoisite together with chlorite; olites are absent over large areas, the boundary the microlithic groundmass surrounds uralitic or stratotype is in practice the lower contact of the chloritic pseudomorphs and ocelli or varioles. graphitic slate zone. The phaneritic to aphanitic rocks are pillowed to The graphitic slate zone consists typically of massive lavas usually with amygdaloidal or slates with variable graphite content. The graph- weakly autobrecciated flow margins. The lavas ite-poor slates sometimes contain andalusite enclose limited lithic pyroclastics and a blanket and staurolite, but sulphide disseminations and of monomictic volcanic conglomerate. laminae, garnet and amphibole needles in or out- The carbonaceous greenschists make up heter- side radial aggregates are most typical to the slate ogeneous strata, the main part of which is chlo- zone. The interbedded members are greywackes, ritic slates lacking evidence for primary ejecta calc-silicate rocks, dolomites and mafic vol- or intensely decomposed detritus, except for caniclastics along with scattered greenstones. some greywackes and mafic lapilli tuffs. Po- Serpentinitic or peridotitic rocks of komatiitic rous carbonate-chlorite-talc rock representing origin (Saverikko 1985) and strongly albitized or palaeoweathering lies at the base, and the slates carbonatized felsic rocks that were originally are rich in sericite in the lower part, where a few autobrecciated or massive lavas and pyroclastics, sericite-quartzite or coarse-grained quartzite in- in association with adinoles also occur (Pulkki- terbeds are present. The amount of graphite in- nen et al. 1983). A few polymictic conglomerates creases upwards and varies in beds so that black are found in greywacky or haematitic slates but slate is present as interbeds in the upper part. The individual pebbles are scattered throughout many dark dust can also be sideritic in places. The metasediments. The banded iron formations with jaspery and sulphidic laminae, dolomite and sulphide slates at Jauratsi (Rieck et al. 1967) are calcific limestone lenses, and the banded iron also included in the slate zone, which commonly formations with sulphide slates at Porkonen— contains chert and jasper in streaks and nodules. Pahtavaara (Paakkola 1971) are also enveloped Kittilä greenstone supercomplex is composed by the carbonaceous chlorite slates. largely of the Sattasvaara komatiite complex (Sa- The volcaniclastic conglomerate has accumu- verikko 1985, Manninen and Pihlaja 1988), lated in an irregular blanket (h. 50 m) on the which is komatiitic basalt (MgO 9—18 wt %) in komatiitic basalt and the carbonaceous green- the lower and middle parts but pyroxene perido- schists, whereas it occurs at Kummitsoiva in a 138 Maiti Saverikko prismatic? body (thickness < 0.3 km) overlying rocks. The spilitic greenstones are tholeiitic ba- the graphitic slates The phenoclasts (0 < 60 cm) salt and alkali-basalt in chemical composition derive from amphibole rock, greenstone or am- (Sarapää 1980). phibolite, jaspilite, chert, dolomite, calc-silicate rock and greywacke. The coarse ejecta are in the form of cored lapilli of amphibole-chlorite rock Kumpu conglomerate—metasandstone—slate or spilitic greenstone. The green greywacky suite matrix is rich in hornblende or contains mainly The Kumpu association is preserved in ero- chlorite, albite and quartz but passes into a sional remnants of depressiona! or synclinal de- chlorite-talc-amphibole assemblage together with posits (thickness 0.8—2.7 km) in which the basal increasing tuffaceous material. greywacky conglomerates form wedge-shaped The pyroxene peridotitic komatiite, now re- sheets (thickness < 0.3 km) (Haimi 1977; Räsä- presented by tremolile-chlorite rock, is pyroclas- nen 1977; Manninen 1981). The rocks apparent- tic and consists of lapillistone to tuff, with py- ly clastic are traced in following stratigraphic roclastic breccias and agglomerates being insig- order. nificant; reworked fabrics are not uncommon. The greywacky conglomerate is stratified in Autobrecciated flows are block lavas or contain thick and thin phenoclast-supported beds with flow-top breccia, but the more massive lavas may upward-fining interbeds of pebbly greywacke. be weakly vesicular and form a few lava pillows. The highly rounded clasts consist of greenstone, The spinifex texture disappears, but relict phe- graphitic metasediments, amphibolite, carbonate nocrysts of pyroxene and olivine are present in rock, jaspilite, chert, albite diabase, felsic rock, the basal cumulates and in dispersed crystal sericite quartzite, arkose quartzite, calc-silicate ejecta. Chlorite is a product after devitrificaton rock, greywacke, gabbro, granitoid, granite and of glassy droplets and vitric ejecta. conglomerate. Sorting is poor or absent except The spilitic greenstone constitutes lava flows in one marginal deposit containing greenstone displaying flow and pillow structures, fragmen- fragments within a green matrix rich in chlorite tary features, cooling cracks and coarsely vesic- and amphibole, and conglomerate bed having ular margins. The lavas are principally amphi- quartz pebbles and fuchsitic matrix. Bedding is bole-albite rocks but sometimes epidote and obscure except within the distinct greywacke in- chlorite are important minerals. The mineral terbeds which display large to small-scale cross- assemblage in the most intensely spilitized rocks beds, graded bedding, scour-and-fill structures is albite-chlorite-epidote, albite-biotite or albite- and depositiona! disconformities. The sandy det- paragonite. The rocks may also be carbonatized ritus is volcaniclastic lithic wacke, passing into to various degrees. The amphibole is hornblende calclithic as the amounts of carbonate and seri- or actinolite, and the albite occurs as microlites, cite increase. The colour is greenish to brownish phenocrysts and varioles or ocelli. Several pha- due to the prevailing hornblende, chlorite or bi- neritic greenstones of blastophitic texture and otite. Typical heavy minerals are magnetite, hae- poor in plagioclase may be differentiates in the matite and tourmaline. Sericitic or dolomitic in- lava flows, but sills are also present as in the terbeds are rare. graphitic slate zone beneath the volcanic super- The arkosic conglomerate (thickness < 0.2 km) complex. The lavas include tuffaceous intercala- is pile of thick gravel beds and interbedded with tions in which the beds of jasper, jaspilite, and passing upwards into arkose quartzite. The graphite or dolomite make it difficult to distin- phenoclasts (0 < 40 cm) are derived from guish the mafic tuffs from the carbonaceous quartzites (sericitic, arkosic, jaspery, quartz- greenschists, if indeed they really are distinct arenitic and fuchsitic), jaspilite, spilitic green- The Lapland greenstone belt: stratigraphic and depositional features in northern Finland 139 stone, slate, greywacke, graphitic metasediments, which the Lapponian supracrustal rocks are granitoid, gabbro and diabase. The sandy detri- stratified. tus is arkosic and the grains may be covered with The Lower Lapponian quartzite—schist sheet haematite dust so that some of the lowermost in- includes calc-silicate rock and dolomite, con- terbeds of arkosic quartzite are brownish to red, stituting a quartzite—carbonate—schist associa- as is characteristic of the Kumputunturi quartz- tion that is typical of cratonic sedimentation (Pet- ite (Hackman 1927). A few cobbles of the Kum- tijohn 1975, p. 573). The mud cracks and ripple putunturi quartzite are found in the upper part marks imply a littoral environment, further in- of the deposit. Scour-and-fill structures and dicated by the carbonatic sediments (see Veizer large-scale cross-beds are frequent. 1973) and the quartz-arenitic interlaminae, which The arkose quartzite is a coarse to fine-grained may have accumulated under tidal conditions rock with sporadic pebbles of quartzites, quartz, (Klein 1971, 1972) even though the more solid slate and quartz-feldspathic rock. Alignments quartzose bodies argue rather for intense weath- of pebbles indicate bedding in the otherwise mas- ering combined with desert aeolian or surf action sive rock. The clasts are quartz, K-feldspar and (Pettijohn et al. 1973, p. 227). However, the oligoclase-albite and the sericite-quartz matrix schist member shows typical structures of turbid- includes biotite in places. ity currents (Räsänen 1986), and highly decom- The sericite quartzite is of quartz wacke or posed debris has been transported as clay into more rarely arkosic wacke, with the matrix being semipelitic material in the depocentre. sericite, chlorite, quartz and carbonate. As mat- The cratonic sedimentation was preceded by rix content increases, the rock passes into slate, deep-seated fracturing in the sialic crust, accom- and conversely, with decreasing matrix, becomes panied by volcanism of the Möykkelmä koma- a quartz arenite. Quartzose laminae may accen- tiitic greenstones. The brittle crustal deformation tuate 0.1—50 cm thick planar beds. Ripple may have speeded up at the end of the sedimenta- marks, sometimes linguoid in shape, large-scale tion stage, because numbers of block faults cross-beds mainly in planar sets, and mud cracks developed: the laterally extensive Salla greenstone are frequent, whereas load cast and slump struc- complex is intruded by greenstone dykes, and the tures are less common. The highly to poorly major felsic interlayer calls for swarms of erup- rounded grains with or without secondary over- tion fissures to feed viscous magma into the lar- growth are quartz (also chert), albite, K-feldspar ge area. Smooth and coarsely vesicular flow and oligoclase. The most common heavy min- structures, demonstrating subaerial eruptions erals are tourmaline, zircon and magnetite but (Macdonald 1972, ps. 68, 76), distinguish the there are occasionally enough haematite to tinge greenstone complex although sporadic carbona- a brownish or red colour in particular as a dusty ceous or carbonatic intercalations, rare lava pil- cover around the clasts. The sericite quartzite in- lows and sparse terrigenous accumulations are cludes one or two thin greywacke interlayers with attributes of limited subaqueous conditions. In individual pebbles of quartz, quartzite and slate. extent, framework and petrographic features the complex bear a strong resemblance to the plateau basalts described by Baragar (1977) and Williams and McBirney (1979, pp. 266—272), albeit lying Depositional evolution greatly in a graben (Saverikko 1988a). The numerous exposures of palaeoresidue This volcanism was followed by a prominent developed in the Saamian quartz-feldspathic period of erosion, that is evident in the Lower- gneisses, along with basal arkoses, are pertinent Middle Lapponian marker bed; the widespread indicators of a sialic continental environment in conglomeratic greywacke—slate blanket exhibits 140 Maiti Saverikko fabrics pertinent to debris flows grading into tur- (Saverikko 1983; 1985; Manninen and Pihlaja bidity currents (Saverikko 1988a). Such sediment 1988), the Kittilä greenstone supercomplex in- transport may have been connected with sharp cluding the Sattasvaara komatiitic rocks, and the uplift of the granitic basement blocks, to which Sotkaselkä and Kummitsoiva komatiite com- conspicuous arkose accumulations such as the plexes developed in Hawaiian to Strombolian Orakoski arkose are usually related (Pettijohn et eruptions with or without volcanic mud flows and al. 1973, p. 184). Thin redbeds at the base repre- weakly phreatic or phreatomagmatic features sent a granite wash from remainder hills of the (Saverikko 1983, 1985). The subsidence was Saamian gneisses. The coarse-clastic deposit of strong in the west (Paakkola 1971; Kallio 1980; the Orakoski arkose is characterized by channel- Saverikko 1985), and at Kittilä, outside the key fill cross-beds with channel-lag gravel (Saverik- area, the jaspers surrounded by pillow lavas show ko 1988a), peculiar to alluvial fans of braided primary features similar to those of submarine rivers (Reineck and Signh 1980, p. 302). precipitates in geosynclinal basins (Kinnunen The thick uniform sheet of pelite with planar 1982). The vertical block movements caused con- graded-bedded strata (Rask 1978; Saverikko vulsionary deposition (Mäkelä 1968) that is evi- 1988a) is strong evidence of turbidity currents in denced by sporadic extrabasinal quartzites within deeper water and of an unstable tectonic envi- the carbonaceous greenschists, and volcaniclas- ronment. Its compositional maturity, or alumi- tic conglomerates. nous clayey parentage (see Saverikko 1988a), is Prior to deposition of the Kumpu conglom- much like that of the palaeosoils upon mafic erate—metasandstone—slate suite, the Lappo- metavolcanic rocks (Reimer 1985). In this man- nian sequence was folded (Lehtonen et al. 1985a) ner, the available amount of clayey debris may and faulted, because the basal wedge-shaped have been derived from the Salla greenstone com- greywacky conglomerate deposits appear to have plex of high relief. levelled off depressional irregularities. The The Sodankylä quartzite—slate association coarser clasts are locally well rounded, but the was deposited as arkosic sand into the compos- strata in general correspond in structure to talus ite bedsets like those found on flood plains (Mc- fans (see Reineck and Signh 1980, pp. 302—303). Kee et al. 1967), beach shelfes (Reineck and Signh The sericite-rich top of one deposit, which shows 1980, pp. 382—386) or in braided channel bars a zonal arrangement of chlorite and carbonate (Coleman 1969), and as silty to muddy debris in (Räsänen 1977), may signify palaeosoil develop- a tidal and/or storm-dominant environment (Ni- ment prior to deposition of the overlying gravel- kula 1985; Saverikko 1988a). Hence, emergence ly arkosic sands. before the stratification might be apparent. The bulk of the Kumpu sediments are arkosic Distinct eruptions of Fe-rich tholeiitic basalts to quartzose sands with numerous polymictic preceded a depositional change to euxinic- gravel beds in the lower part. The mineralogical exhalative conditions which is accepted as the and textural maturity increases upwards suggest- Middle-Upper Lapponian boundary. The gra- ing advanced transgression together with en- phitic slate zone forms a heterogeneous sequence largement of the source area over the margins of like those in semi-obstructed depositional basins the Lapponian province, where quartzitic rocks (Krumbein and Sloss 1963, p. 506). The effusive prevail. The first sandy detritus intermingled with to explosive eruptions occurred in the basin local gravel is represented by Kumputunturi bounded probably by fault scarps and volcanic quartzite, and some degree of lateritization in the barriers, discharging subordinate amounts of provenance area is indicated (Mikkola 1941). The felsic to ultramafic material. succeeding massive beds of arkose quartzite may At the rifting margin of the continental basin imply a submarine fan (see Walker 1967, 1984) The Lapland greenstone belt: stratigraphic and depositional features in northern Finland 141 but the washout structures, ripple marks, mud (Fig. 5). The Middle Lapponian rocks form a cracks, large-scale cross-beds and quartzose in- widespread horizon beneath which the metavol- terlaminae have developed in a littoral or coas- canic rocks corresponding to the Salla greenstone tal plain environment. Opposing palaeocurrent- complex are known variously as the ?Upper directions (Räsänen 1977; Lehtonen et al. 1985a) Veikkavaara, the Siekkijoki, the Kaukonen may demonstrate local rifting at least at Kaares- greenstones and Greenstone I. The volcanic tunturi, northwest of Sodankylä. complexes are not always bimodal in composition, and mafic or felsic metavolcanic rocks prevail in places; graphitic or carbonate interlayers Regional geological setting are present outside the key area as well. Beneath the greenstones the Lower Lapponian quartzite— The Lapponian sequence strikes across north- carbonate—schist suite is known in Sweden as ern Finland from Sweden to the Soviet Union the Tjärrö quartzite (Witschard 1984), which

FINLAND

SWEDEN U.S.S.R.

1 km- I "7" I Saamian gneiss complex, f I Quartzite-carbonate-schist , t ' " I Quartzite, mainly, |QQQ| Conglomerate, I I Volcaniclastic or phyllitic rock, usu. conglomeratic, | | Metapelite , BjMB; Metasilt- stone, ! I Greenstone, Felsic metavolcanic, J^l® Komatiitic rock, PS Amphibolite, £== Graphitic slate zone, ÉSP1 Carbona- ceous greenschists, h—-H Graphitic interlayers, E-:::*:*:*l Psammitic inter-

layers, Ka»., i Conglomerate-metasandstone-slate , |G«ST. | = Greenstone . Fig. 5. The Lapponian supergroup is subdivided into Lower, Middle and Upper Lapponian groups from northwest to southeast. The stratigraphic records are drawn as follows: Tärendö (Padget 1970; Niiniskorpi 1986), Kolari (Hiltunen 1982), Muonio (Lehtonen 1984), Kittilä—Kaukonen (Rastas 1980, 1984), Kaarestunturi (Saverikko 1985), Oraniemi (Saverikko 1988a), Kummitsoiva (Saverikko 1980; Manninen 1981), Kuusamo (Silvennoinen 1972) and Kuolajärvi—Paanajärvi (Hackman and Wilkman 1929; Kulikov et al. 1980). 142 Maiti Saverikko

SWEDEN is interstratified within the Veikkavaara green- TARENOO stone complex (Padget 1970), indicating that the ?Lower Veikkavaara greenstones correlate with the Möykkelmä greenstones. The Middle Lapponian group forms disjointed sedimentary deposits. The Oraniemi arkose— slate—quartzite association overlies the subsided Lappmark megablock (Saverikko 1988a) but it appears commonly as the Sodankylä quartzite in the west (Hackman 1927) until, in Sweden, it is found again as a distinct association in the Pa- hakurkio rock suite (Fig. 6). The Pahakurkio succession is considered post-Lapponian (e.g. Witsc- Legend hard 1984), but the previous lithostratigraphic I -Y V I SAAMIAN GNEISS COMPLEX * • • ^ BASAL ARKOSE (al AND CONGLOM- criteria may call for revision (Niiniskorpi 1986). ERATE (bl, I GREENSTONE, QUARTZITE, ^ j MICACEOUS (USU A quartzite—greenstone—metasiltstone suite at SERICITIC) QUARTZITE . SERICITE SLATE OR BOTH. METASILTSTONE , Kuusamo in eastern Finland was laid down in the METAPELITE , |oo o q1 CONGLOMERATE , DOLOMITE OR CALC- SILICATE ROCK --- MARKER BED I GRAPHITIC SLATE ZONE AND

Kitka—Oulanka synclinal basin (Hackman and AMPHIBOLITE Wilkman 1929), where the sedimentary history Fig. 6. Middle Lapponian type sections at Tärendö (Pad- (Silvennoinen 1972; Pekkala 1985) was compa- get 1970; Niiniskorpi 1986), Oraniemi (Saverikko 1988a) rable to that described earlier (Saverikko 1988a); and Kuusamo (Silvennoinen 1972). however, Silvennoinen (1985) regards this suite as Jatulian. Isolated amphibolitic lava fields are also present elsewhere as in the key area (Latva- paragneisses are here considered to be incorpo- lahti 1973; Hiltunen 1982). rated in the Tuntsan gneiss suite, in accordance In the Upper Lapponian the graphitic slate zo- with the current usage. ne is irregularly covered by metavolcanic rocks The granulite complex and the Lapland green- and the Kittilä greenstone supercomplex known stone belt pass into each other despite the over- locally as the Kolari or Vuontisjärvi greenstones. thrust of the granulites upon the greenstones (Me- The Tuntsa—Savukoski high-grade metamor- riläinen 1976). The granulites have a bulk sedi- phic terrain is problematic in its stratigraphic mentary parentage (Barbey et al. 1980, 1982, relationship to both the Saamian and Lapponian 1984; Hörmann et al. 1980) although metavol- provinces. Lithostratigraphic features suggest canic rocks prevail in Soviet territory (Suslova that the paragneiss suite, excluding the Tuntsa 1976). The primary strata were a thick cyclic pi- migmatite unit (Mikkola 1941; Joupperi 1983), le of arkosic and pelitic sediments with subordi- forms part of the Lapponian supergroup (Fig. 7), nate volcaniclastic greywackes, felsic volcanics, and the fundamental unconformity dividing these quartzites, graphitic and calc-silicatic sediments paragneisses into two parts (Piirainen 1985) coin- and tholeiitic basalts of massive or banded tex- cides well with the Lower-Middle Lapponian hi- ture (Barbey et al. 1980, 1984); Suslova (1976) atus. Piirainen (1985) also considers the migma- also reports pyroxene peridotitic komatiites in the tite unit to be Lapponian, perhaps, because the Soviet Union. Orthogneisses or gneisses of un- Lapponian felsic metavolcanic rocks have under- clear nature may derive from calc-alkaline basalts gone limited migmatization (Virransalo 1985), to rhyolites with abundant andesites (Barbey et which is seen in an early stage at Jauratsi as well al. 1980; Hörmann et al. 1980). The igneous (Manninen 1981). At any rate, all the Saamian rocks are chemically similar to those found in the The Lapland greenstone belt: stratigraphic and depositional features in northern Finland 143

LEGEND

METASANDSTONE

CARBONACEOUS GREENSCHISTS -"XH

KOMATttTlC ROCK

GRAPHITIC SLATE ZONE

AMPH1BOLITE

ME TAPELITE

QUARTZITE

CONGLOMERATE OOOO

FELSIC MIGMATITE T'

- METAVOLCANIC . L \

GREENSTONE

QUARTZITE- CARBONATE- SC HI S T

PARAGNEISSES

RTHOGNEISSES. USU GRANITIC [ l ie. 7. Lapponian rocks in the Tuntsa—Savukoski ultrametamorphic terrain. The stratigraphic columns at Kummitsoiva (Saverikko 1980, Man- ninen 1981), NE-Savukoski (Virransa- lo 1985) and Nuolusvaara (Veki 1985) are not excatly known to scale. Juop- peri and Veki (1986) regard all the supracrustal rocks beneath the Nuolus- vaara conglomerate as pre-Lapponian/ Tuntsan. SAAMIA N

Archaean greenstone belts (Suslova 1976; Hör- lish a Svecofennian age from a metavolcanic in mann et al. 1980), and the sequence is in the pa- the mafic charnockite suite. rentage like the Lapponian. For example, the The Karelian rocks, i.e. the Kumpu rock as- quartzofeldspar-gneiss—quartzite—mica-gneiss sociation, are at their most abundant in the west suite at the base (Meriläinen 1976) resembles the (Fig. 8). A few remnants are capped with the Sve- Lower Lapponian metasediments and, in the cofennian Latvajärvi felsic metavolcanics and southwestern margin, it is accompanied by a bi- Vesikkovaara conglomerate—metasandstone— modal metavolcanic complex showing signs of metasiltstone suite (Rastas 1980). In practice the crustal contamination (Barbey et al. 1984). This Kumpu rocks have been included in the Jatuli- volcanic-sedimentary gneiss assemblage underlies an, but the basal greywacky conglomerates (Mä- the khondalitic suite of arkosic and pelitic meta- kelä 1968; Haimi 1977; Räsänen 1977; Rastas sediments (Meriläinen 1976; Barbey et al. 1980) 1980; Kesola 1981; Manninen 1981) are petro- laid down in sediment gravity flow-dominated graphically quite unlike the overlying succession conditions (Barbey et al. 1984). Thick and cyclic of polymictic conglomerate with arkosic ground- sequence of khondalites may correspond to the mass and arkosic to quartzose metasandstones. Middle Lapponian metasediments, whereas the In addition, they appear to be riftal in nature and graphitic and calc-silicatic metasediments, felsic to have undergone chemical weathering at the metavolcanic rocks and pyroxene peridotitic ko- top, all signs pointing preferably to Sariolan matiites suggest an Upper Lapponian affinity. characteristics (Meriläinen and Sokolov 1981; Further, Bernhard-Griffiths et al. (1984) estab- Marmo et al. 1986); neither are there any clear 5 144 Maiti Saverikko

LITHOSTRATIGRAPHIC MAP

I::::| KUMPU AND VAKKO (IN SWEDEN) ROCK SUITES 2.5 Gg Upper I —1 KITTILÄ GREENSTONE COMPLEX. GRAPHITIC SLATE ZONE ANO L—J I AMPHIBOLITE Fig. 8. Geological sketch map of west- 1 1 ORANIEMI AND PAHAKURKIO (IN SWEDEN) ROCK SUITES Middle ern Lapland, modified after Padget MARKER BED (1970, 1977), Silvennoinen et al. KAUKONEN GREENSTONE COMPLEX, VEIKKAVAARA Lower i 1 ~ „ —1 GREENSTONE COMPLEX (1980), Hiltunen (1982) and Lehtonen Lapponia n 1 1 OUART2ITE - CARBONATE • SCHIST | (|fJ gWEDEN) et al. (1985a, b). The Sirkka conglom- 1 /Q) SAAMIAN GNEISS COMPLEX erate is tentatively separated from the Kumpu rock suite (see Figure 10). GABBRO. I +11+1 GRANITES. |P • | MONZONITE , INFERRED FAULT. discrepancies between their ages (Fig. 9) and the conglomerate. The latter has also been linked Sariolan stage at 2.5—2.3 Ga. with the Kumpu rocks because of its position on The greywacky conglomerate has been arbi- a thin blanket of sericite quartzite, although anal- trarily connected with the Sirkka conglomerate, ogous quartzitic portions are contained in the which is characterized by detritus derived from carbonaceous greenschists (Paakkola 1971; Lat- jaspilitic iron deposits and other Lapponian rocks valahti 1973; Räsänen 1977; Kesola 1981; Kor- (Hackman 1927; Mikkola 1941). But the volca- telainen 1983). Up to that, since the contact niclastic conglomerates (Saverikko 1980, 1983, between the Sirkka conglomerate and the Kum- 1985) or greywacky conglomerates of volcani- pu rock suite proper is tectonic in nature (Mä- clastic origin (Mäkelä 1968; Kesola 1981; Korte- kelä 1968; Ohlson 1969; Kortelainen 1983), the lainen 1983), which are associated with the Up- marginal conglomerates of the Kumpu deposits per Lapponian carbonaceous greenschists (Fig. should not be correlated with the Sirkka con- 10), are petrographically similar to the Sirkka glomerate without more careful investigation! The Lapland greenstone belt: stratigraphic and depositional features in northern Finland 145 Chronostratigraphic evaluation KAARESTUNTURI The granite-gneiss basement complex shows commonly datings of 2.8—2.6 Ga (Simonen VARTTIÖVAARA 1980), that is suggested by Silvennoinen (1985) as the age of cratonization. In the key area, however, the gneiss complex has been intruded by a tonalitic dome 3.1 Ga old (Kröner et al. o 1981) which belongs to the lithosphere developed during the Saamian diastrophism (Salop 1983, pp. 22—72) and cratonized at 3.1—3.0 Ga (Mu- Lappon i an satov et al. 1984). The lower time limit of the Lapponian is not precisely known but the mentioned age of 3.1 Ga Ir '] Conglomerate grey wacky (a). arkosiclb), p . ' j Arkose quart* and that of 2.99 Ga (Fig. 11) determined from ite. [ I Sericite quartzite a brecciated schist associated with ultramafic Fig. 9. Stratigraphic record of the Kumpu association at rocks (Papunen et al. 1977) may bracket the time Kaarestunturi, northwest of Sodankylä (Räsänen 1977); the interval within which the Lapponian started. The discordant diabases (Rastas 1980) are correlated after Niku- latter age is dated outside the key area and is la (1985). considered less significant in this context because of the absence of stratigraphic information about the dated rock. Thus, the onset of the Lapponi- an appears to be related to the cratonization at or without connection with the dyke, two or more 3.1—3.0 Ga, that is generally evidenced by the felsic metavolcanic rocks of 2.70 Ga age (Rastas deposition of quartz arenites (Eriksson and Do- 1980; O.Kouvo, pers. commun. 1986) are in- naldson 1986) such as the Lower Lapponian cluded in the Upper Lapponian graphitic slate quartzite. The oldest age, 2.79 Ga, with known zone. Hence, a felsic rock of 2.53 Ga age (Pelto- stratigraphic position is from a felsic rock (Sil- nen et al. 1986) may penetrate rather than be in- vennoinen et al. 1980) of the Salla greenstone terstratified within the Lower-Middle Lapponian complex which also gives a disputable age of 2.4 marker bed poorly unearthed. Ga (M. Vaasjoki, pers. commun. 1986) for lo- The upper time limit of the Lapponian is also cal basal conglomerate (Silvennoinen 1972) lying obscure, but the 2.44 Ga Koitelainen gabbro lac- at a higher stratigraphic level than the felsic rock. colith (Puustinen 1977) is younger than the Up- The 2.4 Ga age may indicate isotopic reset ap- per Lapponian komatiites at Sattasvaara (see Sa- propriate to emplacement of the nearby gabbro verikko 1985). The Kittilä greenstone supercom- intrusions and thereby to the regional plutonic plex or its spilitic rocks have been considered ear- epoch at 2.45—2.43 Ga in northern Finland (Si- ly Proterozoic (e.g. Gaål 1986), mainly on the ba- monen 1980). sis of diabases 2.20—1.95 Ga old, but these dia- The Möykkelmä greenstones at the base in- bases crosscut the complex and some of them include komatiitic rocks, one of which is dated trude the Kumpu rock suite, too (Rastas 1980). at 2.66 Ga. Räsänen (1984) regards this as a Layered gabbro sills of similar age (Silvennoinen minimum age, a view that appears to be verified et al. 1980; Tyrväinen 1983) are sandwiched al- by an albite-rock dyke 2.72 Ga old, that in- so within the Middle Lapponian strata, which trudes at least the Lower Lapponian quartzite have been intruded by a gabbro of 2.32 Ga age at Karasjok, in Norway (Meriläinen 1976). With (Manninen 1981). 146 Maiti Saverikko

KUMMITSOIVA

LEVI- TUNTURI (TS

, KOLARI SIRKKA Luosujoki

- -'J

Scale n

1 km

Legend rfa^lrW"« Volcaniclastic conglomerate; monomict ic (a), polymictic (b), Graywacky conglomerate, [•lvi Sirkka conglomerate, \ Graphitic slate zone, Carbonaceous greenschists, -^Band-

ed iron formation, Greenstone, Komatiite, |[o Komatiitic basalt, Amphibolite,

I.' . '. Quartzite , Sericite quartzite, Metasands tone-slate. Fig. 10. Problematic stratigraphic position of the Sirkka conglomerate shown with respect to the Lapponian supergroup and Kumpu rock suite. The stratigraphic columns and principal references are: Kolari (Hiltunen 1982), Sirkka—Levitunturi (Kortelainen 1983), Kaarestunturi (Räsänen 1977), Sattasvaara (Saverikko 1985) and Kummitsoiva (Manninen 1981; Save- rikko 1983).

In conclusion, the span of 3.0—2.5 Ga is as- As a chronostratigraphic enigma, the granu- sumed for the Lapponian; the ages of Lower, lite complex is regarded as Archaean (2.8—2.5 Middle and Upper Lapponian cannot be resolved Ga) by Meriläinen (1976) or as Karelian (2.4—2.0 with the geochronological data available. The Ga) by Barbey et al. (1984). Bernhard-Griffiths bulk of the Kumpu rock suite is older than 2.2 et al. (1984) have not recorded crustal events Ga and its minimum age may be 2.0 Ga, point- earlier than 2.0—1.9 Ga ago but do not deny the ing to the Sariolan-Jatulian age. At Kittilä, it is possible existence of older events in the granu- capped with the Latvajärvi felsic metavolcanic lite complex. Ages of 2.0—1.9 Ga are found in rock, 1.89 Ga old, and the Vesikkovaara con- the mafic charnockites of plutonic origin (Meri- glomerate with granitic phenoclasts of 2.06— läinen 1976, Bernhard-Griffiths et al. 1984) which 1.92 Ga age (Rastas 1980). are emplaced between fault blocks (Meriläinen The Lapland greenstone belt: stratigraphic and depositional features in northern Finland 147 Datings (in Ga) in Lapland 1976) and hence show the minimum age of the sequence. The associated tectono-metamorphic convulsion is geochronologically verifiable in the khondalitic metasediments (Bernhard-Griffiths et al. 1984). The thick khondalite suite overlies the cratonic volcanic-sedimentary gneiss assemblage of ?Lower Lapponian origin that is older than a granite body of 2.36 Ga age (Meriläinen 1976). These Tana River gneisses contain anorthosite bodies 1.9 Ga old and a coeval metavolcanic rock (Bernhard-Griffiths et al. 1984) along the major thrust plane of the granulite arc (Barbey et al. 1984).Their tectonic situation appears to indicate concentration of magmatic activity in the reac- tivated marginal fault set of the depositional palaeobasion: as such, the geochronological knowledge available cannot be used to substan- tiate the original age of the granulite arc proto- liths.

Discussion The Saamian sialic crust including the Tunt- san terrigenous paragneisses developed before the Lapponian volcanic-sedimentary evolution 3.0— 2.5 Ga ago. Cratonization at 3.1—3.0 Ga was apt for the deposition of the Lower Lapponian cratonic quartzite—carbonate—schist suite and the Salla greenstone complex of stable platformal volcanism. The Salla bimodal metavolcanic rocks and separated bodies of the Oraniemi .' * J METASANDSTONE arkose—slate—quartzite suite of Middle Lappo- |ÖOO| CONGLOMERATE nian constitute a bimodal-volcanic—quartzite— TKGSJJ CARBONACEOUS GREENSCHISTS arkose association which is indicative of crato- KOMATIITIC ROCK nic rifting (Condie 1982, p. 232); the develop- GRAPHITIC SLATE ZONE ment of thin redbeds was feasible because of a AMPHJBOLITE sufficiently oxygenic atmosphere already in the M E TA PE LITE

QUARTZITE Archaean (Windley and Simpson 1984). Tectonic

F, '.,V FELSIC METAVOLCANIC stability implied by the euxinic-exhalative strata distinguishes cratonic rifting from plainly mantle- ||F:II:R:II;;;]] GREENSTONE activated riftal processes (see Condie 1982, pp. QUARTZ1TE-CAR BO NATE-SCHIST 175—177) in which the Upper Lapponian K:^T] SAAMIAN GNEISS COMPLEX komatiite-dominant volcanic piles discharged at Fig. 11. Age determinations mentioned in the text and chronostratigraphic diagram distinct eruption centres in subaqueous-subaerial of the Lapland greenstone belt. conditions. 148 Maiti Saverikko The nearly 5 km thick Lapponian sequence (see saic of separate megablocks which moved rela- also Lanne 1979) indicates the magnitude of sub- tive to one another forming riftal tracts suitable sidence in Archaean depositional basins, depend- for the development of greenstone belts (Kröner ing on the lithospheric properties prevailing at 1981; Katz 1985). Bylinski et al. (1977) explain that time (McKenzie et al. 1980). In contrast, the the overthrust in question by anticlockwise ro- sequence, in which the komatiites are most dom- tation of the Kola megablock (Fig. 13) since Late inant in the upper part, and the metasediments Archaean. The granulitic metasediments may be are more abundant in the lower and middle parts, the fill of an intracontinental trench as was shows the reverse stratigraphic order to that in thought already by Barbey et al. (1980). Other the classic Archaean greenstone belt (Anhaeus- tectonic movements are as follows: the Lappmark ser 1971). At any rate, the Upper Lapponian megablock is subsided (Saverikko 1988a), and metasediments form a volcaniclastic greywacke— the White Sea megablock may be inclined in con- argillite association which was typical of the sequence of preferential subsidence in the south- Archaean, when coarse-clastic sediments in turn eastern part (Akudinov et al. 1972, Bylinski were rare (Lowe 1982), although cratonic riftal et al. 1977) while the northwestern part rose metasediments 3.0—2.5 Ga old appear also in requiring that the aulacogens (Saverikko 1988b) South Africa (Burke et al. 1985; Eriksson and are opened away from the centre of uplift. Donaldson 1986). It is noteworthy that Bylinski et al. (1977) de- Further information from the Salla bimodal scribe a mantle plume or diapir (Fig. 13) in associ- volcanic complex and the Middle Lapponian sed- ation with the Belomorian/White Sea high-grade iments may manifest the genetic linkage between metamorphic terrain which is linked to that at the Lapland greenstone belt and the granulite arc Tuntsa—Savukoski. The mantle upwelling may which is part of the bimodal-volcanic—arkose— be traced in Finland by an ultramafic intrusive pelite association (Barbey et al. 1984). They pass zone (Papunen and Idman 1982) and a conform- transitionally into each other (Meriläinen 1976), able line of the Upper Lapponian pyroclastic while the granulitized supracrustal sequence in komatiite centres (Saverikko et al. 1985); a gra- Soviet territory is also 3 to 5 km thick (Suslova dational metamorphic temperature maximum 1976). The geochronological data available is (Hörmann et al. 1980) also confirms the inferred not incompatible with an Archaean age for the mantle hot-line. In this respect, the granulite arc granulites. appears to indicate a concordant palaeotectonic With respect to the plate tectonic paradigm, basin adjacent to the mantle upwelling. the southwesterly overthrust of the granulite arc The Archaean origin of the Lapponian is fi- has been regarded as an attractive candidate for nally manifested by stratigraphic similarities to continental collision suture (Barbey et al. 1984, the Kuhmoan supergroup (Saverikko 1988b), in Marker 1985). In fact, the only direct evidence which the shallow-water metasediments separate is that for the earlier continental rifting (Barbey the two volcanic supercomplexes (Piirainen 1985; et al. 1984) and the Archaean continental crust Barbey and Martin 1987) formed in the con- (Huhma 1986) beneath the granulitized epicontinental to oceanic riftal troughs (Martin et al. tinental sediments (Barbey and Martin 1987); 1984). Laajoki (1986b) suggests a possible Sa- also, the proof of subducted oceanic crust is riolan age for the Lapponian supergroup, but the lacking. Palaeomagnetic data does not support basal greywacky conglomerates of the Kumpu significant microplate motions (Pesonen and association show Sariolan characteristics unless Neuvonen 1981) but the sialic crust may have that of glacial drift (Mikkola 1941) despite the been fragmented into megablocks (Fig. 12); the glaciation of continental dimension (Ojakangas Archaean continental crust in general was a mo- 1985). The Lapland greenstone belt: stratigraphic and depositional features in northern Finland 149

fP.chVngi

SVECOFENNIA ' /V 100 km MQj°r fQUlt i- EMU KARELIAN ROCKS, GRANULITES.

I LAPPONIAN ROCKS. •Qfl MIDDLE LAPPONIAN METASEDIMENTS. Rift, half-rift or inferred megablock boundar I I SAAMIAN BASEMENT. \- ', GRANODIORITE, QUARTZ DIORITE and M

GRANITE MEGABLOCK BOUNDARY. Fig. 12. Megablocks in northern Finland (see Saverikko 1988a). The distribution of the granulites relates more to the Middle Lapponian than to the Karelian metasediments.

The Lapland greenstone belt together with the granulitic belt resembles the Lopian greenstone belts, in adjacent U.S.S.R., which are similarly characterized by bimodal metavolcanic rocks (Musatov et al. 1984) and the flysch-like terrigenous metasediments at the middle level (Soko- lov and Heiskanen 1985); the Lopian orogenic megacycle occurred between the Saamian and the Karelian, too (Kratz et al. 1984). Because the Lapponian and Kuhmoan provinces are part of the old-Scandinavian Cwenaland, it is here proposed that this greenstone-belt genesis should

100km Fig. 13. Proposed mantle diapir in the northern part of the I I M A T I IT IC CENTRE. ULTRAMAFIC INTRUSIVE Baltic/Fennoscandian Shield. Megablock boundaries are 0 drawn after Bogdanov et al. (1973), Zhuravlev et al. (1980), ZONE. MANTLE PLUME. C ORE OF MANTLE UPWEILING, Witschard (1984) and Saverikko (1988a). ^^ MEGABLOCK BOUNDAR PALEOZOIC COVER. 150 Maiti Saverikko be called the Cwenan diastrophism or orogenic Kuhmoan with the Saamian and hence under- megacycle; the Lopian and Lapponian as words stand the Saamian system contrary to general are ethnologic synonyms. Luukkonen and Luk- opinion (Van Eysinga 1975; Salop 1983, p. 14). karinen (1986), on the other hand, associate the

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