The epoch of layered intrusions 2.45-2.43 Ga at Mesoarchean aulacogens – gap between greenstone belts in Pechenga and Lapland, Fennoscandia.

Matti Saverikko, Senior citizen, Albergan esplanadi 4 A 10, FI-02600 ESPOO, Finland. Email: matti.saverikko@mbnet.fi

Saverikko, Matti 2017. The epoch of layered intrusions 2.45-2.43 Ga at Mesoarchean aulacogens – gap

between greenstone belts in Pechenga and Lapland, Fennoscandia. Explanation to the poster, Pulkkinen,

E. & Saverikko, M.: Kittilä supervolcano – a gold source in Archean underplating. Trade Show in 11th

Fennoscandian Exploration and Mining 31 Oct. – 2 Nov. 2017, Levi, Finland. The file

http://koti.mbnet.fi/komati/ArcheanAulacogens.PDF .

Abstract

Quite a number of the layered intrusions 2,45-2,43 Ga emplaced on edges of the Mesoarchean aulacogens

in the Saamian craton, Fennoscandian Shield. That is geochronologic reference level between the green-

stone areas in Finnish-Russian Lapland.

The Mt. General’skaya intrusion 2,45 Ga in Pechenga and that of Koitelainen 2,44 Ga in central Lap-

land constitute chronostratigraphic backbone between the Pechenga and central-Lapland greenstone areas.

The both intruded into reactivated crustal fractures of the Saamian gneissose basement but the Koitelai-

nen gabbro wedged also into the Lapponian succession whereas the Pechenga greenstone suite as a whole

covered the Mt. General’skaya gabbro.

The Koitelainen gabbro contains Lapponian xenoliths and upper chromitite layer in the roof zone con-

taminated from Middle-Lapponian Al-rich metapelite and got thermal effects to Upper-Lapponian ko-

matiite-greenstones associated with the felsic volcanics 2,75-2,72 Ga. Only post-Lapponian metasedi-

ments are interbedded within the Pechenga greenstones where the third 2,20-1,90 Ga volcanic stage was

coeval with the numerous diabases spread everywhere in North Finland.

The Koitelainen layered intrusion impresses that the central-Lapland greenstone area is not Paleo-

proterozoic basin but Mesoarchean depocenter of the Lapland greenstone belt branching into Swedish,

Norwegian and Russian Lapland. It continues to southwest in the Peräpohja schist belt bordering to au-

lacogenic fault scarp with long chain of the 2,43 Ga layered gabbro sheets. One of them at Tornio is

wedged into the strata and the Akanvaara gabbro intruded in the Lapponian succession. The epiclastics

don’t include practically any fragments from the considerable gabbro sheets. The Mesoarchean green-

stone-belt association in the aulacogens is highly gold-productive unlike the Karelian (2,5-2,0 Ga) depos-

its on the continental margin.

Key words. Mafic layered intrusion. Mt. Generalskaya. Koitelainen. Archean aulacogen. Pechenga

greenstone belt. Lapland greenstone belt. Fennoscandian Shield.

Provocative Replies

• Koitelainen gabbro overlaps Lapponian greenstone-belt suite of Mesoarchean

• Radial aulacogens of domal uplift in solid Archean continent

• Oraniemi, Kuusamo and Peräpohja aulacogens filled before Karelian 2,5-2,0 Ga

• Pechenga greenstones 2,5-1,9 Ga covered Mt. General’skaya gabbro

1 Introduction

The epoch of mafic, layered intrusions 2,45-2,43 Ga is chronostratigraphic level correlating the green-

stone belts in Fennoscandian Shield. The Pechenga greenstone area is much smaller than that of central

Lapland and clearly defined because of its known ore potential. It seems to have formed an idea of Paleo-

proterozoic evolution model also to the Lapponian greenstones (Hanski et al. 2001; Hanski and Huhma

2005) which were originally kept as Mesoarchean (e.g. Gaál et al. 1978; Silvennoinen et al. 1980). That

is sealed by the layered intrusion at Mt. General’skaya (2,45 Ga), which correlates to that at Koitelainen

(2,44 Ga) in the same kind of paleotectonic environment (Fig. 1). Up to that a new “Pechenga nickel ore”

is exposed at Sakatti near the Koitelainen intrusion in the central-Lapland greenstone area.

All the same, the intrusions exhibit reasons for reinterpretation of that tie between the exogenic evolution

in Lapland and at Pechenga in Kola Peninsula.

The Oraniemi aulacogen is the first Archean one to be observed in what is now the northern hemisphere

(Windley 1995: 341) but the aulacogens in radial swarm of domal uplift is not explained in the plate-tec-

tonic accretion models in Fennoscandian Shield. Author’s home page has an attempt

(Http://koti.mbnet.fi/komati/ConvectionCell.pdf) to understand the domal uplift as result of mantle diapir

which spread two frontal elongated belts of the layered intrusions (Papunen et al. 1985) indicating ther-

mal spreading associated with the asthenospheric convections.

Fig. 1. The greenstone belts and other crustal breakups form radial swarm of domal uplift (Saverikko

1990). The greenstone belts in Kola Peninsula are compiled after Melezhik and Sturt (1994), Dobrzhinet-

skaya et al. (1995), and Kozlov et al. (1995). The Lapland greenstone belt hides the crustal fractures, two

of which delineate the [Or] Oraniemi aulacogen (Saverikko 1988) with the Koitelainen intrusion [K] at

the NW border. Ku = Kuusamo aulacogen. The NNW-SSE rift graben at Pechenga also is aulacogen in

origin (Barnes et al. 2001) including the Mt. General’skaya [G] intrusion. The radial appearance of the

greenstone belts is evidence of Archean solid continent and difficult to explain by the plate-tectonic ac-

cretion model. Compare to Geological Map of the Fennoscandian Shield 1:2.000.000.

(http://www.gigapan.com/gigapans/171438)

1.1 Surroundings of the Mt. General’skaya intrusion

The Pechenga greenstone area forms subhorizontal body, which is gently declined but upthrust in NE di-

rection (Buyanov et al. 1995). The greenstones accumulated by central volcano (Bayanova and Skuf’in

2008) in a unique riftal framework during the three volcanic cycles: 2,50-2,30 Ga, 2,30-2,20 Ga and 2,20-

1,90 Ga ago prior to the orogenic stage at 1,90-1,85 Ga (Smolkin et al. 1995). The greenstones are situ-

ated at intersection of the western and central rift grabens (Fig. 2).

Fig. 2a. In Kola Peninsula, the Archean bimodal greenstones at Bjørnevatn and Olenegorsk 2,76 Ga are

in line with the reactivated faults. The bimodal greenstones (2,8-2,7 Ga) beneath the granulite belt (Ko-

zlov et al. 1995) form large complex also in Finnish territory (since Meriläinen 1976; Barbey et al.

1984). The Polmak–Pasvik–Pechenga and Imandra–Varzuga greenstone belts 2,5-1,8 Ga (Melezhik and

Sturt 1994) belong to the radial swarm of the crustal openings. The map is compiled and simplified after

Melezhik and Sturt (1994), Karpuz et al. (1995) and Kozlov et al. (1995).

Fig. 2b. Stratigraphic and lithologic features of the Pechenga greenstone area after Melezhik and Sturt

(1994).

The western rift graben is aulacogen in origin (Barnes et al. 2001) and part of the triple junction bounded

by syndepositional faulting (Melezhik and Sturt, 1994). The faults appear to have been active since Late

Archean times in line with the Mesoarchean bimodal greenstones at Bjørnevatn (Karpuz et al. 1995) and

Olenegorsk 2,76 Ga (Slabunov et al. 2006).

The central rift graben belongs to the deep-seated fault set in SW direction some of which transect the

NW-trending faults and are partly Paleoproterozoic in origin as reactivated basement fabrics (Karpuz et

al. 1995) and comagmatic dikes 2,31 Ga (Smolkin et al. 1995). The tail of the Mt. General’skaya intru-

sion 2,45 Ga (Balashov et al. 1993) appears to be fracture fill in the same basement fabrics.

1.2 Surroundings of the Koitelainen intrusion

The Koitelainen intrusion (Fig. 3) is laccolith that slopes gently eastwards on the Lapponian succession

and Saamian basement dome (Puustinen 1977). The subhorizontal strata form thin flat-lying cover on the

Pomokaira platform (Lehtonen et al. 1998; Patison et al. 2006). By geophysical reconstructions the plat-

form in west is subsided as terraced basin which is filled by the large volcanic plain of the Kittilä green-

stones (Lanne 1979; Lehtonen et al. 1998) so that the Sattasvaara komatiitic belt underneath rests as rim

around the Kittilä complex (Niiranen et al. 2014).

Fig. 3. General geological map of the Pomokaira platform. It is covered by Lapponian succession in thin

flat-lying layer, which surrounds the gabbro intrusion. The gently sloping intrusion and the subhorizontal

strata uncover dome-like areas of the Saamian gneiss complex. The high-aluminous metapelite as a strat-

igraphic key horizon is emphasized among the Lapponian metasediments, which contain also quartzites

and schists. Post-Lapponian remnants are conglomerate-sandstone-slate association.

NW-trending rift (Patison et al. 2006) conjoins with the slope of the basement (Niiranen et al. 2014). The

platform is also crossed by the Oraniemi aulacogen in northeast direction (Saverikko 1988, 1990) the

stepwise trough of which contains the gabbro intrusion at the edge (Fig. 4a). Lehtonen et al. (1998) don’t

explain the subvertical faults with 2-3 km dip slip, which are proof of the syn-depositional rifting.

The thin-layered strata with subhorizontal bedding make it difficult to understand the stratigraphy on the

platform and to connect the thin cover to the 3-3,5 km thick aulacogen fill (Saverikko 1988) and 5-5,5 km

thick Kittilä greenstone complex (Lanne 1979; Lehtonen et al.1998) in the 9,5 km deep basin (Niiranen et

al. 2014). In that case the Koitelainen intrusion is helpful with the numerous drill holes and by containing

xenoliths and displaying contact-metamorphic influence in the footwall and hanging-wall rocks.

Fig. 4a. The tectonic settings of the Sattasvaara komatiite complex that is located at the junction of the

NW-trending rift and the Oraniemi aulacogen. The geophysical sounding profiles bear evidence shown in

the block diagram. The cinder cone at Sattasvaara piled up at the junction and was the final volcano in

the complex, which however seems to be compiled by fissure eruptions. The Koitelainen intrusion lies on

the crustal fracture, which appears to have been also its feeder (Fig. 4b). The gabbro is fractured and

faulted on that place and a komatiitic feeder dike beside is evidence of the basement fissuring in that di-

rection.

Fig. 4b. Location of the Koitelainen intrusion [K] on the Bouguer anomaly map (Mineral Deposits and

Exploration: http://gtkdata.gtk.fi/mdae/). The curved crustal fracture begins from Sattasvaara [S] at the

junction of the NW-trending rift and the Oraniemi aulacogen but underlaps the granulite belt

The basement dome is surrounded by metamorphosed subarkoses, micagneisses, and felsic and mafic

volcanics, which all are lying beneath the Koitelainen layered intrusion. The pre-existent sequence con-

tinues on the laccolith as thick pile of the high-aluminous metapelite and komatiites with minor mafic

volcanics and subarkosic to micaceous quartzites. Mafic volcanics and extrusive komatiites occur as xen-

oliths. Up to that, extensive komatiitic peridotite-serpentinite layers contain spots of augite as thermal

effects of the intrusion. Mutanen (1997).

The high-aluminous metapelite at Oraniemi (Tyrväinen 1983) in 1-1,7 km thick deposit (Saverikko 1988)

reaches the gabbro (Fig, 5) and is overlain by the subarkosic to micaceous quartzites like the Sodankylä

arkosic to sericitic quartzite (Saverikko 1988). The laccolith wedged under the metapelite from which the

upper chromitite layer is contaminated in the roof zone (Mutanen 1997). The extrusive mafic-ultramafic

komatiites of the Sattasvaara complex also reach there and dropped as xenoliths into the gabbro whereas

the gabbro had thermal effect in the komatiitic peridotite-serpentinite as initial lava flows of the Sat-

tasvaara phase in the graphitic slate zone (Saverikko 1985).

The Oraniemi aulacogen fill and the Sattasvaara komatiitic belt (Niiranen et al. 2014) belong to the pre-

existent strata into which the Koitelainen intrusion emplaced spreading under the high-aluminous metape-

lite. The complicated flat-lying and thin sedimentary cover between the platform and gabbro laccolith

may be mainly Lapponian quartzites by Mikkola (1941) that means cratonic (ortho)quartzite-carbonate-

schist association (Saverikko 1987).

2 Lithostratigraphic comparison

The Pechenga greenstone area evolved on the fractured and reactivated crust of the Saamian basement

where the Mt. General’skaya gabbro already filled a crustal fracture. The volcano-sedimentary sequence

as a whole is younger than the gabbro intrusion 2,45 Ga and overlies discordantly the Bjørnevatn bimodal

volcanics of Mesoarchean origin, too.

The Koitelainen laccolith also intruded into crustal fracture but spread into the Lapponian sequence at

such a level that the Sattasvaara komatiitic belt, the Oraniemi arkose-pelite-quartzite suite and the under-

lying strata are older than the layered intrusion 2,44 Ga. But could the Kittilä greenstones form pre-exis-

tent rock unit on the Sattasvaara belt, too? The lithostratigraphic answer to that is only partly explained by

the post-Lapponian remnants on the greenstone complex, the specific characteristics of which are the

Kumpu quartzite as brownish to red (arkosic) quartzites and the red jaspers (Mikkola 1941). Similar rocks

are typical of the Kuetsjärvi and Kolosjoki formations in Pechenga (Smolkin et al. 1995). Stratigraphic

correlation of the Kittilä greenstones with the Ahmalahti greenstones (2,33 Ga) under the Kuetsjärvi for-

mation needs chronostratigraphic evaluation indeed, because the Kittilä complex shows signs of tectonic

gliding along the graphitic interlayers that gives impression of Paleoproterozoic allochthon (Hanski and

Huhma 2005; Lahtinen et al. 2005).

Fig. 5. The Koitelainen intrusion (Mutanen and Huhma 2001) and its situation in the Oraniemi–Sattas-

vaara area (Saverikko 1985, 1988). Cross-section is revised after Mutanen (1997).

3 Chronostratigraphic evaluation

Peltonen et al. (1988) describe a felsic volcanic breccia too near the west contact of the Koitelainen gab-

bro. Its U-Pb age of 2,43 Ga is from the heterogeneous data which provide Pb-Pb ages of 2,53-2,51 Ga

and >2,7 Ga (Manninen et al. 2001). The layered intrusions had a large thermo-tectonic impact in sur-

roundings during the long-term (2,50-2,41 Ga) heating period (Arestova et al. 2003) also in Finland (ref-

erences in Mertanen and Pesonen 2005).

The komatiitic peridotite-serpentinite layer contains pyroxene idioblasts as contact-metamorphic relicts in

the south contact zone of the Koitelainen intrusion (Saverikko 1985) and is penetrated by komatiitic-

basalt neck and a fumarole or hotspring (Fig. 6) with cherty beds changing into felsic ignimbrite and al-

bite-chlorite rock (Saverikko 1992: 15. excursion site). The rocky area is a few kilometers to east from the

specific outcrops of adinolized rocks by Mikkola (1941) associated with albite rocks within the graphitic

slates at Matarakoski on the other bank of River Kitinen. The albite rocks in the adinolization zone (Fig.

7) give U-Pb ages of 2,75-2,72 Ga (Rastas et al. 2001) in Kittilä district.

The Lapponian succession shows Mesoarchean ages of 2,80 Ga (Räsänen and Vaasjoki 2001) and 2,78-

2,77 Ga (Räsänen and Huhma 2001) in the Salla bimodal greenstone complex in eastern Lapland. And on

the west side, greenstones of 2,9-2,8 Ga age appear in Norrbotten, Swedish Lapland (Lauri et al. 2016).

The Lapponian greenstones are cut by the mafic dikes and intrusions of 2,2-1,9 Ga (Hanski et al. 2001)

but the Haaskalehto diabase 2,2 Ga penetrated also a post-Lapponian remnant at Värttiövaara (Rastas

1980). Afterwards, Lehtonen et al. (1998) and Rastas et al. (2001) speculated tendentiously that it is merely

enclosed by the post-Lapponian metasediments! Those belong to the continent-wide clan (Vuollo and Huh-

ma 2005) and are in attractive temporal connection with the third (2,20-1,90 Ga) volcanic stage in

Pechenga (Smolkin et al. 1995).

Fig. 6. Close-up from the fumarolic vent (Ø 3-5 m) that

penetrates serpentinite-peridotite sheet south of the Koite-

lainen gabbro. Chert layers pinch and swell in an irregular

manner stratified via interstitial feeders (white arrow) at

the distal position (B) but are apparently even-bedded and

laterally uniform at the proximal position (C), where the

rock is associated with ignimbritic quartz-albite rock.

Fig. 7. Gravimetric anomalies and gold deposits

in the Kittilä greenstone area (Mineral Deposits

and Exploration: http://gtkdata.gtk.fi/mdae/).

Border of the high-density area is marked by

yellow dashed line on the lithologic map, which

is simplified from that of Lehtonen et al. (1998).

The dated keratophyre 2,74-2,40 Ga (Hiltunen

1982), albitized felsic volcanics 2,75-2,72 Ga

(Rastas et al. 2001) and felsic volcanic breccia

of the obscure U-Pb age 2,43 Ga with Pb-Pb

age >2,7 Ga (Manninen et al. 2001) may be the

same volcanic suite along with the graphitic

slates beneath the Sattasvaara komatiitic belt

and Kittilä greenstones. Ni = Sakatti nickel-ore

deposit.

The richest gold province in Europe is in the Kittilä greenstone complex (Ojala 2007) on restricted grav-

imetric anomaly (Fig. 7) present like mantle plume (Saverikko 2014). The prevalent graphitic metasedi-

ments and numerous BIFs (i.e. Porkonen-Pahtavaara) adjust to the global mantle-plume event at ~ 2,7 Ga

with a peak in the BIF-black_shale deposition and with high gold potential the production of which

speeded up at 2,70-2,65 Ga, described by Condie and Benn (2006).

Fig. 8. The regolith here is local (< 20 m) and saprolite material mixed into the till (Hall et al. 2015). As-

Sb concentration along with Au, Co, Cu, Fe and S are incorporated in sulphides (Koljonen 1992) at the

center of the gravimetric anomaly (Mineral Deposits and Exploration: http://gtkdata.gtk.fi/mdae/) that

may be evidence of central volcano. The pyroclastic komatiite arc from Karasjok to Kuolajärvi (Saverik-

ko et al. 1985) is distinguishable in the local till also from the greenstones because of clearly elevated

concentration of Mg, Co, Cr, Fe, Ni, Sc, Ti and V but shows up distinctly in the scarcity of barium (Koljo-

nen 1992). That postulates the connection between the supervolcano and pyroclastic komatiites. Geo-

chemical maps by Saverikko et al. (1983) and Koljonen (1992).

The sharp features of the gravimetric anomaly mean shallow-level magma chamber that made possible

the hydrothermal solutions for the extensive carbon-sulfide accumulations and adinolisation due to

hotsprings prior to the main volcanic stage. Arsenic and antimonic concentration in regolith (Fig. 8) also

is track of the hydrothermal solutions and volcanic emanations but in cloudy form most probably due to

the final eruptions. The final phase appears in the Vesmajärvi spilitic greenstones with komatiites at the

base concordantly on the Porkonen-Pahtavaara BIFs (Paakkola 1971; Lehtonen et al. 1998) in rapidly

subsiding volcanic basin (Paakkola and Gehör 1988). The known tectonic gliding (Fig. 9) through the

euxinic-exhalative interlayers don’t show allochthonous dislocations because the Vesmajärvi greenstones

and the cloudy As-Sb concentration stayed near the epicenter. The Kittilä greenstones form parautochthon

and preceded the Koitelainen intrusion (2,44 Ga), and also the Ahmalahti greenstones (2,33 Ga) in

Pechenga.

Fig. 9. The Kittilä volcanic basin has terraced borders but rather flat bottom relief (Lanne 1979; Lehto-

nen et al. 1998; Niiranen et al. 2014) around the deepest 1,5 km part in distinct small depression at the

eruptive center (Niiranen et al. 2014). The volcanic unit is usually twice the thickness of typical oceanic

crust and thins out esp. towards the W and SE margins so that the keel-shaped unit (Niiranen et al. 2014)

conjoins with the riftal fault scarp and pyroclastic komatiite arc the heart of which is the mantle plume.

The final volcanic phase appears in the Vesmajärvi greenstones the core of which stayed near the volcan-

ic focus (http://hakku.gtk.fi/en/locations/search/ “Bedrock of Finland 1:200.000”). The tectonic gliding

directed along the riftal fault scarp (Edge.B, Edge.A) that is steep and 2-12 km deep (Lehtonen et al.

1998; Patison et al. 2006) feeding the Nuttio serpentinite plugs which erupted as terminal phase at Sat-

tasvaara (Saverikko 1985). Peltonen (2005) kept the plugs as ophiolitic suite and Hanski and Huhma

(2005) further supposed them to be rest of oceanic crust at the suture zone.

4 Kuusamo and Peräpohja aulacogens

The layered intrusions of the same epoch emplaced in South Lapland where the steep gabbro sheets are in

long line at the edge of the Saamian basement (Fig. 10) beside the Peräpohja schist belt that resembles

stratigraphically the central-Lapland greenstone area (Hanski et al. 2001; Patison et al. 2006). The strata

are kept as Paleoproterozoic because the numerous diabases 2,2-2,1 Ga (Perttunen and Vaasjoki 2001) are

part of the same continent-wide clan (Vuollo and Huhma 2005) among the schists. Also a few volcanics

yield Sm-Nd isochron ages of 2,09 (Huhma et al. 1990) resembling that of 2,08 Ga from the pyroclastic

komatiites at Karasjok, Norway (Siedlecka et al. 1985).

The Peräpohja and Kuusamo schist belts are interconnected according to the lithologic similarities with

stromatolites as specific characteristics (Härme and Perttunen 1964; Pekkala 1985). Gaál (1986) attempt-

ed to see the schist belts as the halves of the disjointed Koillismaa aulacogen named after the Koillismaa

layered intrusions 2,44 Ga (Alapieti 1982) at the border of the Kuusamo schist belt besides the Oulanka

intrusions at 2,44-2,42 Ga (Glebovitsky et al. 2001) in Russia. The stromatolites may be discrepancy

elsewhere in the Lapponian succession but the Oraniemi and Kuusamo aulacogen-fills are stratigraphy-

cally compatible and appear to have deposited discontinuously in the course of analogical tectono-

sedimentary evolution (see Saverikko 1988) on the Salla bimodal greenstones with felsic member of 2,80

Ga age at Kuusamo (Räsänen and Vaasjoki 2001). Kulikov et al. (1980) describe Sattasvaara-type ko-

matiites lying stratigraphically above the aulacogen fill also at Kuolajärvi–Paanajärvi district, in Russia.

Fig. 10. The layered intrusions in South Lapland. The Akanvaara gabbro is steeply SE-dipping intrusion

in the Lapponian succession beside the Kummitsoiva komatiite complex (Saverikko 1983). The stars show

the places of the U-Pb age determinations from the felsic volcanic (Räsänen and Huhma 2001).

The Lapponian succession spread out largely in the Central-Lapland granite province of poorly known

constellation (Fig. 11). The Siekkijoki migmatites of arkose-pelite suite at least resolve into the domal-

shaped province, which is directed by the increasing metamorphic gradients from greenschist to amphibo-

lite facies (see Perttunen and Vaasjoki 2001; Hölttä et al. 2007). That is more widely roofed by the supra-

crustals (e.g. Evins et al. 2002; Patison et al. 2006) and exposed partly as gneissose granite basement (e.g.

Tyrväinen 1983; Corfu and Evins 2002). Patison et al. (2006) describe in underground distinct Archean

crustal blocks and the other granitoids are mainly reworked products of the granite-gneiss basement in

intracratonic tectonic settings (Nironen 2005; Ahtonen et al. 2007).

Fig. 11a. The Central-Lapland granite province is surrounded by similar greenstone-belt associations

(Yleistetty kallioperä 1:1.000.000. http://gtkdata.gtk.fi/Maankamara/index.html: Layers/Karttatasot);

greenish to grey volcanics, yellow sandy and bluish pelitic sediments. The distinct basins are aulacogens

at Oraniemi and Kuusamo (Saverikko 1988). The Peräpohja schist belt and Lapland greenstone belt, the

both in the riftal troughs have obscure mutual boundary over the granite province (Patison et al. 2006)

which includes also supracrustal remnants (Fig. 11b) as is seen on Aerogeophysical Low Altitude Map

(http://gtkdata.gtk.fi/mdae/) converted by gradual red tones.

Fig. 11b. The Fe-Ti-V-Co-Cr-Cu-Mn-Ni-Sc concentration in distinct circular area is dispersed in the soil

C-horizon, stream sediments and fine till that means marked mafic-ultramafic processes in the local bed-

rock (Geochemical Atlas of Northern Europe http://weppi.gtk.fi/publ/negatlas). The deep-seismic sound-

ing profile FIRE.4-4A shows the E4 wedge of distinct density and seismic velocity between the previous

crust and mantle (Patison et al. 2006). The 10-15 km thick wedge displays broken-up structures (Elo

2006) just like mantle underplating. The central-Lapland granite province reflects Archean crustal thick-

ening, which was greatest in the surroundings of the P6a block disturbing block boundaries (Patison et

al. 2006). The half-riftal blocks E1a and E1b under the Peräpohja schists are facing in contrasting direc-

tion to the crustal stacking than the (half-)rifts of Lapland greenstone belt and granulite belt. The crustal

stacking is logical by the rotation of the Kola megablock (references in Saverikko 1988, 1990).

The Tornio intrusion is wedged into the pre-existent strata (Söderholm and Inkinen 1982) and the exten-

sive suite of the other layered gabbros hasn’t actually supported any loose material into the adjacent gra-

nitic sandy gravel (Perttunen 1985; Perttunen and Vaasjoki 2001). Alapieti et al. (1989) found only one

PGM-bearing phenoclast in the conglomerate on the Suhanko layered intrusion, which lies subhorizontal-

ly on the platform (Iljina and Salmirinne 2011) including remnants of Archean greenstone-belt suite. That

may be explained by rifting processes (Alapieti et al. 1990) in the radial crustal fracture present as steep

fault plane of the declined crustal wedge E1a+E1b (Patison et al. 2006). The fracture fed the Kemi and

Penikat layered intrusions 2,43 Ga (Perttunen and Vaasjoki 2001) and that at Suhanko but intruded also

the Akanvaara gabbro 2,43 Ga (Mutanen and Huhma 2001) into the Lapponian succession (Fig. 10)

where the stepped basin in SW direction (Lanne 1979) includes Sattasvaara-type komatiites on the gra-

phitic slates with the BIFs at Jauratsi around the central vent of the Kummitsoiva komatiites (Saverikko

1983). The 2,41 Ga age determinations from felsic volcanics too near the gabbro remained obscure be-

cause the zircon with high U content provides discordant analyses (Räsänen and Huhma 2001).

Räsänen and Vaasjoki (2001) reinterpret GSF’s stratigraphic conception about the Lapponian deposition

of Paleoproterozoic origin so that Mesoarchean strata extend from Sodankylä area (incl. Oraniemi) to

Kuusamo. The layered intrusions at Koitelainen and Akanvaara intruded in the pre-existent Mesoarchean

strata that is observed also by the Tornio intrusion and deduced by the Kemi-Penikat intrusions.

5 Discussion

The aulacogens seem to have been specialty for the emplacement of the layered intrusions 2,45-2,43 Ga

at its best where the stepwise edge of the Saamian basement is crossed by reactivated crustal fractures.

The focus is concentrated to the layered gabbros at Mt. General’skaya in Pechenga and at Koitelainen in

central Lapland from the first half (2,45-2,44 Ga) of the epoch. The second half at 2,44-2,43/2,42 Ga oc-

curred in the Kuusamo and Peräpohja aulacogens.

The Lapponian succession is thought to be temporally compatible to that in Pechenga (2,50-1,90 Ga) be-

cause of the obscure datings of 2,43-2,41 Ga from the felsic volcanics near the layered intrusions and be-

cause of the diabases 2,20-1,90 Ga emplaced into the Lapponian succession. The disputable points are

1. the contact-metamorphic influence of the Koitelainen and Akanvaara gabbros in the thermal period

(2,50-2,41 Ga) defined by Arestova et al. (2003) as continent-wide effect, and

2. the numerous mafic intrusions and dikes 2,2-1,9 Ga overall in North Finland (Hanski et al. 2001) which

were coeval with the third volcanic phase 2,20-1,90 Ga in Pechenga (Smolking et al. 1995) speeded up

by the deep super-plume activity 2,05-1,92 Ga (Bayanova and Skuf’in 2008). However, the Haaskalehto

diabase 2,2 Ga cuts the post-Lapponian remnant at Värttiövaara, too (Rastas 1980).

The Mt. General’skaya gabbro 2,45 Ga intruded into the Saamian gneissose basement and is overlain by

the Pechenga greenstone belt. The Koitelainen gabbro 2,44 Ga overlies the gneissose basement but over-

laps also Lapponian succession that the footwall and hanging-wall rocks show contact-metamorphic

signs. The thermal effects are detected in the Upper-Lapponian komatiite-greenstone association and the

contamination results from the Middle-Lapponian Al-rich metapelite of the Oraniemi aulacogen-fill. That

matches with the albite rocks (2,75-2,72 Ga) of adinolization zone on the Oraniemi rock suite (Fig. 12).

Fig. 12. The Kittilä greenstones and Sattasvaara ko-

matiitic belt lie on the graphitic slate zone but have tuf-

faceous graphitic-slate assemblage also as interlayer.

The retouched stratigraphic sections are compiled after

Saverikko (1987) at Sattasvaara–Oraniemi and Tärendö,

and after Niiranen et al. (2014) at Kittilä–Kolari. The

Oraniemi arkose-pelite-quartzite suite is not distin-

guished from the Lower-Lapponian cratonic quartzite-

carbonate-schist suite in Kittilä. The albite rock 2,75-

2,72 Ga (Rastas et al. 2001) was associated with adi-

noles (Mikkola 1941) and graphitic slates. The Paha-

kurkio rock suite may also be Mesoarchean because

Lapponian greenstones nearby in Swedish Lapland are

dated at 2,9-2,8 Ga (Lauri et al. 2016).

The only lithostratigraphic connection between the Pechenga and central-Lapland greenstone areas is

concealed in the post-Lapponian remnants the special characteristics of which are the Kumpu quartzites

of brownish to red color and the red jaspers. Identical rocks are also typical of the Kuetsjärvi and Kolos-

joki formations in Pechenga.

The post-Lapponian remnants rest on the Kittilä greenstones glided tectonically to southeast through the

graphitic slate interlayers. The dislocation may not be allochthonous in plate-tectonic meaning because

the final volcanics stayed on the high-density mantle-magma body near the central volcano from which

the volcanic gases and solutions appear as cloudy As-Sb concentration in the regolithic material.

The high-grade gold province and prevalent BIF-black_slate association are characteristics of the global

mantle-plume event at ~ 2,7 Ga as Mesoarchean crown of the Lapponian deposition in the Central-

Lapland or simply – Lapland greenstone belt in its whole extent. The Mesoarchean intracratonic strata

cover the Kuusamo and Peräpohja aulacogens where also the greenstone-belt associations control high

gold-productivity (Eilu et al. 2007) unlike the Karelian (2,5-2,0 Ga) deposits on the continental margin

(Patison et al. 2008).

Acknowledgements. This essay does honor to my deceased supervisors Kalervo Rankama, Raimo Lau-

erma, Juhani Nuutilainen, Tapio Koljonen and to my instigated advisers Reijo Salminen, Kari Airas, Ai-

mo Hiltunen and Rauno Hugg. The top-level public support by GSF with aerogeophysical and till-

geochemical facilities is precious to independent geological investigations – and economic prospecting.

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