High viscosity and cooling rate in komatiites: Supervolcanic plume adjoining

mantle underplating at ~2.7 Ga in the Fennoscandian Shield? Matti Saverikko, Phil.Lic. Retired. Albergan esplanadi 4 a 10, FI-02600 ESPOO, Finland. Email: matti.saverikko@mbnet.fi

Saverikko, Matti 2017. High viscosity and cooling rate in komatiites: Supervolcanic plume adjoining mantle

underplating at ~2.7 Ga in the Fennoscandian Shield. 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://komati.mbnet.fi/Supervolcano.PDF.

Abstract Europe’s largest nickel deposit at Sakatti, in Finnish Lapland was surprise in the framework of Paleoproterozoic

plate-tectonic models prevailing in Lapland. Its location in a mantle-plume area within the richest gold province

in Europe justifies discussion about global mantle-plume epoch that coincided with the peak of gold production

~2,7 Ga. The mantle plume lies in the center of pyroclastic komatiite zone and erupted as supervolcano at the

border of mantle underplating that will explain high cooling rate in the komatiites, which rose through the thick

continental crust.

The Saamian craton was solid continent in the Mesoarchean but split into the Kola and Lapponia-Karelia

halves. The crustal spreading caused by mantle diapirism reached no more than an incipient stage in the rigid

plate in the Neoarchean. One of the main mantle pulses (2,72-2,66 Ga) initiated mantle underplating and rotated

Kola Peninsula in accord with granulitic overthrust.

The mantle underplating was also source for placer-gold accumalitions on the granulite belt in the drainage of

Rivers Ivalojoki and Lemmenjoki. The river canyons delineate tight fault-valley set in NE direction that in-

cludes numerous parallel quartz-gold veins and connects the Sakatti and Pechenga Ni-Cu ores in some kind of

crustal hinge-line zone.

Key words: Explosive komatiite volcanism. Supervolcano. Mantle underplating. Mantle plume. Lapland green-

stone belt. Fennoscandian/Baltic Shield.

High viscosity and cooling rate in komatiites: Supervolcanic plume adjoining mantle underplating at ~2.7 Ga in the Fennoscandian Shield?

Matti Saverikko, Phil.Lic. Retired. Albergan esplanadi 4 a 10, FI-02600 ESPOO, Finland. Email: matti.saverikko@mbnet.fi

Abstract Europe’s largest nickel deposit at Sakatti, in Finnish Lapland was surprise in the framework of Paleoproterozoic plate-

tectonic models prevailing in Lapland. Its location in a mantle-plume area within the richest gold province in Europe justi-

fies discussion about global mantle-plume epoch that coincided with the peak of gold production ~2,7 Ga. The mantle

plume lies in the center of pyroclastic komatiite zone and erupted as supervolcano at the border of mantle underplating

that will explain high cooling rate in the komatiites, which rose through the thick continental crust.

The Saamian craton was solid continent in the Mesoarchean but split into the Kola and Lapponia-Karelia halves.

The crustal spreading caused by mantle diapirism reached no more than an incipient stage in the rigid plate in the Neoar-

chean. One of the main mantle pulses (2,72-2,66 Ga) initiated mantle underplating and rotated Kola Peninsula in accord

with granulitic overthrust.

The mantle underplating was also source for placer-gold accumalitions on the granulite belt in the drainage of Riv-

ers Ivalojoki and Lemmenjoki. The river canyons delineate tight fault-valley set in NE direction that includes numerous

parallel quartz-gold veins and connects the Sakatti and Pechenga Ni-Cu ores in some kind of crustal hinge-line zone.

Key words: Explosive komatiite volcanism. Supervolcano. Mantle underplating. Mantle plume. Lapland greenstone belt.

Fennoscandian/Baltic Shield.

Highlights

• Komatiites as block-to-aa lavas and prevalent ejecta with glassy lapilli tuffs

• Supervolcanic plume at ~ 2,7 Ga instead of oceanic allochthon at 2,0-1,9 Ga

• Lapponian cratonic to mantle-activated rifting due to Archean mantle upwelling

• Radial breakups around center of domal uplift due to feeder of mantle underplating

• Hinge-line fault set across mantle underplating liberated Ni ores and Au in placer gold

1 Introduction The study is based on data-mining method about geotectonic–lithostratigraphic–paleogeographic cluster and

crowned by till geochemistry in residual regolith region [Geochemical Atlas of Northern Europe

(http://weppi.gtk.fi/publ/negatlas)] correlated with the deep-crustal seismic data on FIRE profiles. The valid data have

good correlation with the information in the user interfaces Mineral Deposits and Exploration

(http://gtkdata.gtk.fi/mdae/), Fennoscandian Mineral Deposits Application, Ore Deposits Database and Metal-

logenic Map (http://gtkdata.gtk.fi/fmd/) and Gateway to Finland’s Geological Information

(http://hakku.gtk.fi/en/locations/search/), produced or edited by Geological Survey of Finland. The user inter-

faces illustrate discrepancies from the dominant plate-tectonic ideas and other geologic considerations about the

Paleoproterozoic model of the Lapland greenstone belt. That’s why justification needs so many figures. Geo-

logic descriptions and arguments (Komatiitic Explosive Volcanism: http://koti.mbnet.fi/komati) according to

this antithesis are not disproved or referred to in the previous geologic opinions, which are mentioned later in

the text at appropriate context. English papers of Geological Survey are in web (http://hakku.gtk.fi/en/) and

those of Geological Society at http://www.geologinenseura.fi/english.html.

1.1 Archean – Karelian (2,5-2,0 Ga) mantle activities

Initial diapir magmatism took place 2,99-2,91 Ga at Solovetsky in the

White Sea region (Fig. 1) where the mantle diapir was active also 2,88-

2,80 Ga and 2,72-2,66 Ga (Arestova et al. 2003, 2012). The third (2,72-

2,66 Ga) diapir-magmatic stage reflected in the lower lithosphere (Lobach-

Zhuchenko and Levchenkov 1986) and is seen as tectonic peak 2,7-2,6 Ga

(Ez et al. 1984; Gorokhov 1984).

Fig. 1. Fennoscandia and the place-names mentioned in the text.

The mantle upwelling appeared in mantle-related endogenic processes 3,0-2,65 Ga that shifted to the west (Lo-

bach-Zhuchenko et al. 1986; Arestova et al. 2003) also in East Finland (Engel and Diez 1989), where the proc-

esses culminated as high tectono-metamorphic pulse at 2,7 Ga (Paavola 1986, 1988) along with large-scale iso-

topic homogenization at 2,75-2,66 Ga (Luukkonen and Lukkarinen 1986; Halliday et al. 1988). The strong

mantle convection rotated pronouncedly the Fennoscandian Shield 2,7-2,6 Ga (Mertanen et al. 1989) before the

crustal restabilization at 2,6 Ga (Silvennoinen 1985).

The numerous layered intrusions (2,50-2,41 Ga) had large thermo-tectonic impact on the craton as a whole be-

cause they formed shield-wide group indicating thermal spreading from the mantle diapir (Arestova et al.

2003). The mantle activities in Pechenga took place at the shallow level 2,33-2,20 Ga and deep level 2,05-1,92

Ga in super-plume processes (Bayanova and Skuf’in 2008) coeval with the high-grade metamorphism (2,15-1,9

Ga) in the granulite belt (Meriläinen 1976; Bernard-Griffiths et al. 1984; Kozlov et al. 1995). The periodical man-

tle activity is seen in the continent-wide clan of the mafic dike swarms emplaced rhythmically 2,45 Ga–2,32

Ga–2,2 Ga–2,1 Ga–1,98 Ga (Vuollo and Huhma 2005). The dike swarms 2,32-1,98 Ga mainly in the NW direc-

tion may be proof of uniform directions of the stress-strain forces in the crust during the extensive mantle

upwelling (Vuollo and Huhma 2005), which may have been liberated in the super-plume processes.

Finnish geology in Lapland does not recognizated the Solovetsky diapir itself as well as its thermal influence on

the Saamian basement giving ages of 2,83-2,68 Ga but enclosing a gneiss dome of 3,1 Ga age (Kröner et al.

1981). Also the thermal effect (2,50-2,41 Ga) associated with the layered intrusions, and the deep super-plume

activity (2,05-1,92 Ga) with the continent-wide swarm of mafic dikes get illusion about the deposition at 2,45-

1,92 Ga in the Lapland greenstone belt (Hanski et al. 2001; Hanski and Huhma 2005) – despite Archean ages of

the Lapponian supracrustal rocks (Papunen et al. 1977; Hiltunen 1982; Räsänen and Huhma 2001; Räsänen and

Vaasjoki 2001; Rastas et al. 2001). The Lapland greenstone belt and granulite belt originally have kept as Ar-

chean in age e.g. by Meriläinen (1976), Gaál et al. (1979), Silvennoinen et al. (1980) and Saverikko (1987).

Isotopic geology has to strive for understanding geological processes, not merely produce “ages” for rocks

(Olavi Kouvo, referred by Matti Vaasjoki 2001, Dedication. In: Geol. Surv. Finland, Spec. Paper 33).

2 High viscosity and cooling rate in komatiites

Pyroclasticity is quite exceptional quality in the komatiites but isolated volcanic centers of the pyroclastic ko-

matiites (Fig. 2) form arc-shaped zone in Lapland from Karasjok (Norway) to Kuolajärvi (Russia). Their high

viscosity and eruption mechanism are detectable by routine geologic methods in the Sattasvaara and Kummit-

soiva complexes (Fig. 3), the explanation of which are referred after Saverikko (1992a) and can be confirmed

on field-excursion sites (Saverikko 1992b).

Fig. 2. Pyroclastic ko-

matiite complexes form

arc-shaped zone parallel

to the granulite belt (Save-

rikko et al. 1985). The

“Sattasvaara-type” koma-

tiites appear at Kuolajärvi

in Russia, too (Kulikov et

al. 1980).

Fig. 3a. The main final extrusion phase in the Sattasvaara complex formed cinder cone within two or more explosion cra-

ters and beside flow piles of the block and aa lavas. Eruption structures at the SW slope are visible by zooming to the Place

name = Sattasvaara on Aerial image of the Excursion map (http://www.excursionmap.fi). The komatiite complex as a whole

is present in Figure 8. Shaded elevation model: http://hakku.gtk.fi/en/locations/search/.

Fig. 3b. The Kummitsoiva complex is flattened volcanic dome in shape like central-vent volcano (Saverikko 1983) but the

laser-scanning image points out that viscous lavas poured out through two vents. The vents are in line with the exposures

of serpentinite-peridotite lavas verifying fissure extrusions.

The main Lapponian ultramafic eruption phase occurred in the isolated volcanoes, around which the volcanic

complexes are mainly amphibole-chlorite rocks (MgO 30-18 wt.%). Large amounts of ejecta with crude or ab-

sent sorting imply violent volcanic explosions, which are verified principally magmatic in nature. The petro-

graphic evidence for a highly viscous magma is as follows:

1. Great explosiveness: the high viscosity retards the expansion of gas bubbles exsolving from the rising

magma and produces high internal pressures (McBirney 1973; Sparks 1978). Strombolian-type eruptions,

whose fingerprints are readily discerned in the amphibole-chlorite rocks (Saverikko 1983, 1985), are due to

disruption of pasty vesicular magma close to the surface (e.g. Williams and McBirney 1979).

2. Predominance of cinders (Fig. 4): they fall to the ground in an essentially solid state (Macdonald 1972) and

form relict cinder cone in the Sattasvaara hill.

3. Frequent glassy ejecta (Fig. 5): highly viscous magmas generally form entirely glassy pyroclasts (Fisher and

Schmincke 1984).

4. Dearth of spatter-like coarse ejecta among angular blocks.

5. Subordinate lavas are blocky flows (Fig. 6): the viscous lavas only split into blocks.

Their high cooling rate is established as follows:

1. Absence or rarity of spinifex textures: no a high enough cooling rate and degree of supercooling (Donaldson

1982).

2. The mainly unwelded ejecta and minor presence of chilled skin in the lava flows.

3. Glass inclusions (Ø 0,1-1 cm): increasingly slow cooling of the supercooled liquid (Carmichael 1979).

4. Rarity of the polyhedral jointing: thermal-related contraction too small (Williams and McBirney 1979).

5. The block lavas: those of basaltic flows already erupt at very low temperature (Williams and McBirney

1979).

6. Lack of visible thermal effect on the surroundings of terminal flows and plugs, i.e. the Nuttio ultramafics of

peridotitic komatiite (see Kontinen 1981).

Fig. 4. At left: An agglutinate belonging to the relict cinder cone at Sattasvaara (6. excursion site). At right: A cinderite

near the central-vent volcano at Kummitsoiva (31. excursion site): its scoriaceous texture is obvious in the upper part of

the photo.

Fig. 5. Vitric ash particles, clear in plane-polarized light, from the Sattasvaara (at left) and Kummitsoiva (at right) com-

plexes. At left: tiny broken bubbles in the glass shards (in red circles) exhibit bubble-wall shards formed by vesiculation

and bursting. At right: Vitric ash particles with smooth or rounded shapes, whose origin is difficult to prove (Hay et al.

1979), are typical of Strombolian ejecta (Williams and Mc Birney 1979). The vitric material is preserved in places as

unaltered glass (Martti Lehtinen, pers. comm. 2007). However, essentially unaltered glass is reported only in two other

places in the world (Arndt et al. 2008).

The volcanics are ultramafic komatiites (>18 wt.% MgO) and komatiitic basalts (18-9 wt.% MgO) i.e. amphi-

bole rocks. The former are differentiated also in lava flows into peridotite-serpentinite cumulates or peridotitic

komatiites, and aphanitic amphibole-chlorite rocks or pyroxene-peridotitic komatiites interconnected by the

MgO ~ 30 wt.%.

The peridotitic komatiites of the Moskuvaara initial phase were volcanic crystal mush displaying random spat-

ters and polyhedral jointing (13. excursion site) in large planar serpentinite-peridotite flow piles, which indicate

alternating cumulate sheets with different olivine contents. The effusive flood flows are penetrated by the am-

phibole rock in volcanic necks but also by a siliceous hotspring or fumarole (15. excursion site).

In the Sattasvaara complex proper, the magma poured out in Hawaiian to Strombolian-type eruptions along

with upwards increasing ultrabasicity related to the spreading of komatiitic volcanism (Fig. 6). Terminal vol-

canics were again crystal mush of serpentinite-peridotite in volcanic necks with or without autobrecciated crust

and associated flow of amphibole-chlorite rock (5. and 8. excursion sites). This Nuttio final phase was of dis-

tinctly high-magnesian composition and emplaced in long (75 km) podiforming belt (Peltonen 2005) to the Sot-

kaselkä komatiite complex.

Fig. 6. Stratigraphic column of the Sattasvaara complex on fault

block that subsided vertically several kilometers (Lehtonen et al.

1998). The second and third pillowed lava pulses of amphibole

rock disappear on the northern side of the fault scarp. Saverikko

(1985).

The low-pressure magma differentation that’s already seen

in outcrops (Fig. 7), and volatile enrichment in upper lev-

els of the magma body refers to zonal melt structures of a

static reservoir. Gravitational differentation was favorable

process also for the nickeliferous accumulations nearby

(Fig. 8).

Fig. 7. Komatiitic block lava (at left) is like a recent flow in the lower flow pile beside the Sattasvaara hill (7. excursion

site). The microscopic texture in plane-polarized light is jagged and spinose like megascopic structures of the aa-lavas (at

right) in the upper flow pile. The grey lithic clinkers include clear vitric droplets. Magmatic differentiation in shallow-

level magma chamber or lava pocket is visible in the peridotitic clinkers wrapped up and dropped in the amphibole-chlo-

rite rock. The block or aa lavas are impossible habitus to overheated komatiite magmas and quite exceptional as recent-

like flows of Archean origin.

Fig. 8.The Sattasvaara komatiite complex reaches the Koitelainen gabbro (Saverikko 1990, 2014) and extrusive ko-

matiites form xenoliths in the gabbro that had thermal effect in peridotitic komatiites (Mutanen 1997). The volcanic necks

in the W-E row detect the fault scarp, which is mentioned in the caption of Figure 6.

3 Archean domal uplift in solid continent

NW-trending rifts prevail in the Fennoscandian Shield constituting the main riftal system but radial swarm of

linear crustal openings forms another system (Fig. 9), which is not recognized in geotectonic evaluations of the

Shield. The radial framework of the greenstone belts is difficult to explain by crustal accretion model. Also,

there are not proved any direct evidence for crustal movements despite the anticlockwise rotation of Kola Pen-

insula; the Archean mantle diapirism at Solovetsky (Bylinski et al. 1977; Arestova et al. 2003) rotated the meg-

ablock since Late Archean (Bylinski et al. 1977) but perhaps prior to 2,56 Ga (Slabunov et al. 2006) or 2,5-2,4

Ga (Mertanen and Pesonen 2005). The continental spreading along Kantalahti Bay reached no more than an

embryonic stage of divergence but the anticlockwise rotation was sufficient to evolve quasi-collisional structure

as overthrust of the granulite belt (references in Saverikko 1990: 27-28).

The linear Lopian greenstone belts in Russia were intraplate tectonic basins (e.g. Musatov et al. 1984; Rybakov

1988; Kozhevnikov et al. 2006) and the Kuhmoan greenstone-belt chain in East-Finland is defined as intracon-

tinental rift (Papunen et al. 2009). The fan-shaped arrangement of the Archean greenstone belts is part of the

radial swarm indicating that the domal uplift was Archean phenomenon of the solid plate. In the Lapland granu-

lite belt the volcanism in the lower part took place at 2,8-2,7 Ga (Kozlov et al. 1995). Ductile structures along

the Archean greenstones between the main crustal blocks in Kola Peninsula, represent long-lived zones which

were able to Paleoproterozoic intracontinental rifting (Dobrzhinetskaya et al. 1995). The intracontinental rifting

at Imandra–Varzuga also started prior to 2,5 Ga along the longitudinal deep faults (Melezhik and Sturt 1994).

Fig. 9. The greenstone belts and other crustal breakups form radial swarm of domal uplift, which was in connection with

the shield-wide mantle diapir (Saverikko 1990). The Lapland greenstone belt hides the crustal fractures, which supple-

ment the fan-shaped arrangement of the greenstone belts. The greenstones in the Kola district are revised after Melezhik

and Sturt (1994), Dobrzhinetskaya et al. (1995) and Kozlov et al. (1995). The NNW-SSE rift graben at Pechenga is aula-

cogen in origin (Barnes et al. 2001). Compare to Geological Map of the Fennoscandian Shield 1:2.000.000.

In addition to the absence of real plate-tectonic motions between the crustal megablocks there are no deep-

crustal structures in Lapland such as descending plates or separate crustal thickening (von Knorring and Lund,

1989; Patison et al. 2006).

Thus, the domal uplift was Archean phenomenon of the solid plate that is emphasized by the Archean Kantalah-

ti rift (Belyaev et al. 1977) in the Belomoria/White Sea megablock inclined away from the uplift center (Save-

rikko 1987, 1988) because its southeastern part subsided already in the Archean times (Akudinov et al. 1972;

Bylinski et al. 1977). The updoming center appears inside the Tuntsa high-metamorphic terrain at the head of

Kantalahti Bay that is manifested also by till geochemistry (Chekushin et al. 2009) in ice-divide region of the

last glaciation (Fig. 10).

Fig. 10a. Co-Cr-Cu-Ni concen-

trations in fine-grain (< 2mm)

till estimate clearly the focus of

the domal uplift that is also pre-

sent as gravimetric minima

(Bouguer Anomaly Map of the

Fennoscandian Shield,

1:2.000.000 ) perhaps because

of the crashed crust above the

mantle plume. The pyroclastic

komatiite zone in Finnish-Nor-

wegian Lapland (Fig. 10b) is

visible, too. The concentration

means marked ultramafic or

mafic processes (Geochemical

Atlas of Northern Europe

Http://weppi.gtk.fi/publ/negatlas

).

Fig. 10b. The geochemistry of the soil C-horizon is strongly controlled by the bedrock such as the Fe-Ti-V-Co-Cr-Cu-Mn-

Ni-Sc concentration in Lapland, Finmark (N-Norway) and West Kola Peninsula, where the Co, Cr, Ni, Cu, V, Fe and Mn

also in stream sediments and fine-fraction of till mean marked mafic-ultramafic processes (Geochemical Atlas of

Northern Europe http://weppi.gtk.fi/publ/negatlas). The siderophile-calcophile element combination in larger circular

appearance is overlapped by the granulitic overthrust. The deep-reflection seismic sounding FIRE 4-4A transects the

area, the profile of which (Fig. 11) shows 10-15 km thick mantle underplating in the lowermost crust.

3.1 Mantle underplating and pluming

The Archean cratons were assembled above shallowly dipping detachments as is seen in the subhorizontal to

shallowly dipping seismic reflections in the crust and upper mantle which are interpreted as 2,8-2,6 Ga struc-

tural fabrics (van der Velden et al. 2006). Similar reflectivity characteristics, 2,85-2,70 Ga in age (Kontinen and

Paavola 2006), are described from the central and northern part of the Fennoscandian Shield (van der Velden et

al. 2006; Kukkonen et al. 2006). But the apparent difference is the high proportion of anomalously thick high-

velocity lower crust and deep Moho areas (Kukkonen et al. 2006), which have been suggested in general as

proof of underplated or tectonically stacked material (van der Velden et al. 2006).

In North Finland the bright subparallel reflectors of listric shape form 10-15 km thick and highly dense package

of younger fabrics and characterize the lowermost crust (Patison et al. 2006) displaying broken-up structures

(Elo 2006). Their distribution connected with concentration of mantle-related elements in regolith (Fig. 11)

postulates rather the mantle underplating than crustal stacking.

Mantle plumes squeezed high-density magma through crustal fissures up to volcanic eruptions. The feeding

fractures of the mantle plume (Fig. 12) delineate crustal megablocks, the array of which is evidence of rigidity

of the solid plate at that time. The Neoarchean cratons appear to be products of rigid plate behaviour (van der

Velden et al. 2006).

Fig. 11. The mantle-related fine material is dispersed in distinct circular area, which is untried constellation in Precam-

brian geology of Lapland. The deep-seismic sounding profile FIRE.4-4A shows the E4 wedge of distinct density and seis-

mic velocity between the previous crust and mantle (Patison et al. 2006). It forms the mantle underplating connected to

the focus of the domal uplift or the feeder associated with the Belomoria high-metamorphic belt to Solovetsky.

Fig. 12. High-density magma ridges fill

gaps between the megablocks (Saverikko

1988, 1990) within the mantle plume. The

course of the mantle diapir is seen in the

pyroclastic komatiite arc from Karasjok to

Kuolajärvi, which is crossed by two ridges.

The southeastern one is NW edge of the

Oraniemi aulacogen (1), which fed the Sat-

tasvaara mafic-ultramafic komatiites, ma-

fic-ultramafic cumulates with the Sakatti

Ni-Cu ore and the Koitelainen gabbro. The

komatiitic fissure extrusion and the fault

scarp appear in a row of the volcanic necks

and in pillowed lava field upon the vertical-

ly subsided fault block (Figs. 6 & 8) as is

confirmed by geophysical reconstructions

(Lehtonen et al. 1998). The rectangular

area is that of the general geological map

in Fig. 14. Bouguer Anomaly Map

(http://hakku.gtk.fi/en/locations/search/).

3.1.1 Mantle underplating and placer gold

Gold panning in Finnish Lapland is concentrated in the drainage of Rivers Ivalojoki and Lemmenjoki but also

in some other places (Stigzelius 1986) on the mantle-underplating domain. Gold accumulations in regolithic

material (Pulkkinen and Sarala 2009) are limited to that area, too.

Straight canyon of the meandering rivers Ivalojoki and Paatsjoki is track of the crustal fracture from the mantle

plume to the Pechenga nickel mine at Nikel in Russia (Fig. 13). The NE–SW-striking strikeslip shear zones

cross-cut the Kittilä terrane and continue as crustal-scale feature in Sweden (Patison et al. 2006; Niiranen et al.

2014). The Lemmenjoki river-valley belongs to steep fracture that is seen also in long line of eskers parallel to

the adjacent esker line and fault scarp of the straight NW shoreline of Lake Inarijärvi (Alalammi et al. 1986).

The Lemmenjoki crustal fracture is apparent also in gravimetric framework.

Fig. 13. The gold-rich mantle plume emplaced at the junction of two crustal fractures. The feeding fracture in NE direc-

tion continues superficially as the canyon of Rivers Ivalojoki and Paatsjoki in Russia. Another gold panning drainage is

the Lemmenjoki (Le) river-valley that is part of the crustal fracture apparent in the gravimetric framework. Aeroelectro-

magnetic Quadrature Component Map (http://hakku.gtk.fi/en/locations/search/), Bouguer Anomaly Map and Shaded

Elevation Map (http://gtkdata.gtk.fi/mdae/).

The rivers delineate tight fault-valley set in NE direction with or without small dislocations (Aerogeophysical

Low Altitude Map: http://gtkdata.gtk.fi/mdae/). The fault valleys include parallel swarm of quartz-gold veins

and continue in the Pechenga region (Karpuz et al. 1995). The fault set is reactivated and composed of syn-de-

positional faulting with co-magmatic dikes (2,31 Ga) in the central rift graben at Pechenga where the central

volcano was active 2,50-2,30 Ga, 2,30-2,20 Ga and 2,20-1,90 Ga ago the latest one feeding the Ni-Cu ore

(Smolkin et al. 1995). The Laanila dolerite dikes 1,04 Ga (Mertanen et al. 1996) across the granulite belt show

much later prolonged activity in the flexural fault set initiated at ~ 2,7 Ga the fissuring of which liberated gold

from the mantle underplating.

4 Explosive komatiite volcanism at ~2.7 Ga

Mantle upwelling associated with rifting commonly results in generation of significant volumes of mantle-de-

rived magmas, which are added to the crust (Olsen and Morgan 1995). The central and northern part of the

Fennoscandian Shield has anomalously thick lower-crust and deep Moho areas. The peak in production of con-

tinental crust at 2,7 Ga with crustal thickening was related to a catastrophic mantle overturn event, which gave

rise to a large number of mantle plumes at ~ 2,7 Ga (Condie and Benn 2006).

Europe’s richest gold province (Ojala 2007) in central Lapland is associated with the mantle plume (Saverikko

2014; Fig. 14) and postulates its connection with the global (2,70 ± 0,1 Ga) riftal greenstone-belt genesis with

high gold potential (Groves et al. 1987) the peak of gold production being at 2,70-2,65 Ga (Condie and Benn

2006). Also consistent with the worldwide mantle-plume event was a peak in deposition of BIF–black-shales

(Condie and Benn 2006) just like the widespread graphitic slate zone with BIFs at Jauratsi beside the Kummit-

soiva complex (Saverikko 1983) and carbonaceous greenschists with BIFs at Porkonen-Pahtavaara during the

pyroclastic komatiite volcanism (Saverikko 1985, 1990).

The mantle-activated rifting at 2,7-2,6 Ga was the most important Archean period of global magmatic activity

(Condie 1981) to which the explosive komatiite volcanism as part of the Lapponian mantle-activated rifting

(Saverikko1990) is linked together with the high-temperature plume magmatism at 2,72-2,66 Ga in the Solovet-

sky mantle diapir (Arestova et al. 2003).

Fig. 14. Gravimetric anomalies and gold deposits (http://gtkdata.gtk.fi/mdae/) in the Kittilä greenstone area.

Border of the high-density area is marked with yellow dashed line on the lithological map, which is simplified

from that of Lehtonen et al. (1998). The U-Pb age determinations may be from the same volcanic suite along

with the graphitic slates: the keratophyre sheet 2,74-2,40 Ga (Hiltunen 1982), albitized felsic volcanics 2,75-

2,72 Ga (Rastas et al. 2001) and felsic volcanic breccia 2,43 Ga (Manninen et al. 2001) close by the Koitelai-

nen gabbro 2,44 Ga (Mutanen and Huhma 2001) in its thermo-metamorphic zone; the 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).

Contaminated komatiitic-mafic volcanics upon the basement complex at Möykkelmä yielded an age of 2,66 Ga

(Räsänen et al. 1989).

The Sattasvaara complex and Koitelainen gabbro laccolith emplaced through the same crustal fracture (see Fig.

12) and the komatiites reach into the gabbro (see Fig. 8) which includes xenoliths of extrusive komatiites (Mu-

tanen 1997). The gabbro generated contact-metamorphic zone in the peridotitic komatiites (Mutanen 1997)

within the graphitic slates under the Sattasvaara complex proper (Saverikko 1985: 73). One of them is penetrat-

ed by fumarolic neck (Ø 2-5 m) of cherty albite rock (15. excursion site). The albite rocks within widespread

graphitic slate zone beneath the Kittilä greenstone complex are 2,75-2,72 Ga (Rastas et al. 2001) and an albite-

rock dike is 2,72 Ga in the pyroclastic komatiite zone at Karasjok in Norway (Meriläinen 1976).

5 Supervolcano

The mantle plume or its magma reservoir is apparent as sharp-featured gravimetric anomaly because of the

shallow-level position where the komatiitic magma reached high cooling rate. It lies below concentric As and

Sb concentrations in regolithic material, the round shapes of which are evidence of central volcano feeding gas-

es and emanations beside Sattasvaara. The supervolcano is obvious as As-Sb-Au-Co-Cu-Fe-S concentration

incorporated in sulphides (Koljonen 1992). The pyroclastic komatiite arc from Karasjok to Kuolajärvi is distin-

guishable 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 (Koljonen 1992) that postulates the connec-

tion between the supervolcano and pyroclastic komatiites. Figure 15.

The supervolcano formed Kittilä greenstone area present as volcanic basin that is metamorphosed only at

greenschist fasies (Hölttä et al. 2007) and surrounded by the Sattasvaara and Sotkaselkä volcanoes, Linkupalo

volcano-park (Lehtonen et al. 1998) and volcanic vent at Tepsa. Komatiitic rocks are exposed also inside the

volcanic plain as amphibole-epidote-chlorite rocks (Paakkola 1971), high-MgO or komatiitic basalts (Lehtonen

et al. 1998; Niiranen et al. 2014) and the Nuttio serpentinite plugs.

Fig. 15. Glasial erosion here in the regolithic area was very limited (< 20 m) and led to the mixing of saprolite

material into 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 supervolcano beside Sattasvaara. Geochemical maps: Sav-

erikko et al. (1983) and Koljonen (1992).

The Kittilä volcanic basin has terraced borders but rather flat bottom relief (Lanne 1979; Lehtonen et al. 1998;

Niiranen et al. 2014) excepting its lowermost 1,5 km part that covers only a small area at the depocenter in dis-

tinct depression (Niiranen et al. 2014) that looks like caldera. Calderas or cauldron subsidence may be reflected

by ghost-like circular structures in aeroelectromagnetic in-phase components. The komatiitic belt formed vol-

canic rim (Niiranen et al. 2014) on the heterogeneous graphitic slate zone which is present as sub-horizontal,

highly reflective unit below the Kittilä terrane, corresponding well to the base outlines (Niiranen et al. 2014).

The contacts between the electrical conductors and altered komatiitic-mafic greenstones were pathways for Au-

mineralising fluids (Airo 2007) associated with intense albitisation and carbonisation (Patison 2007). Tiny

amorphous carbon indicates extremely reduced conditions (Patison 2007). The widespread carbon-sulphide

concentrations and adinoles with fumaroles or hotsprings exhibit initial phase of volcanic gases and emanations

from the supervolcano. Figure 16.

Fig. 16. 3D-modelling (Niiranen et al. 2014) describes the Kittilä volcanic basin as terraced subsidence [A]

surrounded by komatiitic rim [B]. Flat surface of the volcanic plain is preserved in landforms and in magnetic

anomalies [D] but BIFs and carbonaceous greenschists are seen by electromagnetic quadrature components

[C] as exposed sub-horizontal layers in the volcanic complex lying on the graphitic slate zone. Electromagnetic

in-phase components [E] display cirquelike features like craters or calderas that appear to be moved to south-

east from the heart area [F]. Drawing B is compiled and modified after Niiranen et al. (2014). Aerogeophysi-

cal maps: http://hakku.gtk.fi/en/locations/search/ .

The extensive hydrothermal solutions required the shallow-level magma reservoir(s) at volcanic stage. The

Hawaiian-type effusions of komatiitic basalt preceded deposition of carbonaceous greenschists (Saverikko

1985) with the BIFs at Porkonen-Pahtavaara in rapidly subsiding volcanic basin (Paakkola and Gehör 1988).

That was followed by the Strombolian-type extrusions of ultramafic komatiites (Saverikko 1985, 1990) along

with the final spouts of volcanic gases and emanations present as the As-Sb concentrations in till. The final vol-

canic phase appears in the Vesmajärvi greenstones of spilitic suite with komatiites at the base concordantly on

the Porkonen-Pahtavaara BIFs (Paakkola 1971; Lehtonen et al. 1998). The Vesmajärvi greenstones stayed near

the volcanic focus surrounded by the Porkonen carbonaceous greenschists

(http://hakku.gtk.fi/en/locations/search/ “Bedrock of Finland 1:200.000”).

North Europe’s largest gold deposit, the Kittilä gold mine at Suurkuusikko lies close to the focus that is the cen-

tral vent. About 75 % of gold is in arsenopyrite and the rest is mainly in arsenian-pyrite, associated with the

intense carbonatisation and albitisation (Patison et al. 2007). The final volcanic spouts fed arsenian emanations

and Patison et al. (2007) say that the free carbon is sourced from carbon-rich sediments within argillaceous ho-

rizons intercalated with volcaniclastic material – called here the carbonaceous greenschists.

Hanski and Huhma (2005) keep the Kittilä greenstone basin as allochthonous crust that emplaced tectonically

onto the Archean basement because some volcanics show EMORB and OIB-like geochemical features and the

Nuttio serpentinite-belt as ophiolitic suite supports rest of the previous oceanic crust in the suture zone. The

komatiitic to picritic volcanism was not related to mantle-plume activity in consequence of the absence of pre-

ceding strong crustal uplift (Hanski and Huhma 2005).

In any way, the high-density magma reservoir from the mantle underplating emplaced in the junction of the two

crustal fractures and the Nuttio serpentinite plugs erupted as terminal phase at Sattasvaara through riftal fault

scarp (Fig. 17) that is steep and several kilometers up to 12 km deep (Lehtonen et al. 1998; Patison et al. 2006).

The Kittilä volcanic unit is ca. 9 km at the thickest and usually twice the thickness of typical oceanic crust but

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

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

reservoir remained in place but sliced only weakly in the N-NE direction (Fig.18). The tectonic tension de-

scribed to have directed, however to southeast discharged along the sub-horizontal layers of euxinic-exchalative

sediments making internal gliding folds (Saverikko 1992b: 1. excursion site) below and between the flat mafic-

ultramafic lava piles. The inferred circular structures and the Vesmajärvi greenstones didn’t move enough that

the dislocation could be called as allochthon in plate-tectonic meaning.

Fig. 17. The Nuttio serpentinites form line of plugs from the Sattasvaara to Sotkaselkä complex as terminal volcanic vents

at least at Sattasvaara. They erupted through riftal fault scarp (Edge.A and Edge.B) and aulacogenic fault scarp

(Edge.C) with 2-12 km subsidence. The rift-fault scarp (Lehtonen et al. 1998; Patison et al. 2006) directed the final vol-

canics i.e. Vesmajärvi greenstones to have dislocated 10-20 km to southeast.

Fig. 18. Mantle-magma reservoir remained in place and is only weakly sliced (Niiranen et al. 2014). The weak(?) thrusts

directed to north but the dislocations within the Kittilä complex directed to southeast.

6 Regional correlation

6.1 Lapponian deposition

Geochronologic opinion about Paleoproterozoic origin of the Lapland greenstone belt (Hanski et al. 2001; Han-

ski and Huhma 2005) evolved from minimum ages (>2,2-2,0 Ga) dated from intrusive rocks, and the 2,44 Ga

age of the Koitelainen gabbro at the bottom (Lehtonen et al. 1998; Rastas et al. 2001).

Firstly, the Koitelainen layered intrusion wedged into the pre-existent strata and includes Sattasvaara-type ko-

matiite as xenoliths, produced contact-metamorphic sings in the Moskuvaara komatiitic serpentinite-peridotites

and hosts chromium deposits liberated from melted high-aluminous schists (Mutanen 1997). The high-

aluminous metapelite of the Oraniemi suite (Tyrväinen 1983) in the 1-1,7 km thick deposit (Saverikko 1988)

reaches into the gabbro.

The Lapponian deposition started not until 2,45 Ga ago by the felsic volcanics with the obscure age determina-

tions of 2,43-2,41 Ga (Manninen et al. 2001; Räsänen and Huhma 2001) in the thermo-metamorphic contact

zone of the layered intrusions. Räsänen and Vaasjoki (2001) would update the GSF’s stratigraphic conception

so that Mesoarchean strata extend from Sodankylä or Oraniemi to southeast in consequence of the 2,80-2,77 Ga

datings from volcanics (Räsänen and Huhma 2001; Räsänen and Vaasjoki 2001) and the discordant mafic dike

at minimum age 2325 Ma in the Salla bimodal greenstones (Manninen and Huhma 2001), which belongs to the

~2,45 Ga dike swarm intimately associated with the layered intrusions (Vuollo and Huhma 2005). But there are

Mesoarchean age determinations from Lapponian volcanics in west from Sodankylä, too (Papunen et al. 1977;

Hiltunen 1982; Rastas et al. 2001; Räsänen et al. 1989; Lauri et al. 2016).

Secondly, the numerous mafic(-felsic) intrusions and dikes emplaced at 2,2-1,9 Ga widely in North Finland (Han-

ski et al. 2001). One of them is Haaskalehto-type diabase (2,2 Ga) that cuts also the post-Lapponian remnant at

Värttiövaara (Rastas 1980). Lehtonen et al. (1998) and Rastas et al. (2001) afterwards speculate tendentiously that

it’s merely enclosed by the post-Lapponian metasediments. Conglomeratic lenses contain paleoplacer gold disinte-

grated from the Kittilä greenstones and have no indication of epigenetic gold (Eilu et al. 2007).

In historical review the Lapponian greenstone-belt association is comparable to the Kuhmoan greenstone belt

but the Oraniemi succession belongs to the Kumpu(-Oraniemi) formation of post-Lapponian origin (Mikkola

1941). The Oraniemi rock suite however continues to west as Sodankylä quartzite between two volcanic se-

quences (Hackman 1927) and is Lapponian in origin, too. The discrepancies in lithostratigraphic interpretation

may be resolved by invoking Lower, Middle and Upper-Lapponian subdivision instead of the bipartite or five-

partite subdivision (Silvennoinen et al. 1980; Silvennoinen 1985; Lehtonen et al. 1985; 1998) because the Ora-

niemi suite of Middle-Lapponian separates the Salla and Kittilä greenstone associations (Saverikko 1987,

1988).

The sections in Fig. 19 differ from those of Lehtonen et al. (1985; 1998) and Niiranen et al. (2014) who don’t

separate the Lapponian quartzites by Mikkola (1941) i.e. Lower-Lapponian (ortho)quartzite–carbonate–schists

and the Oraniemi arkose–metapelite–quartzite suite. The bimodal-volcanic–arkose–slate–quartzite association

composed by the Salla greenstones and Oraniemi suite is valid as evolutionary key-stratotype. The bimodal-

volcanic–quartzite–arkose associations indicate cratonic rifting (Condie 1982) and the thick metapelite deposit

displays deeply weathered products from the Salla greenstones (Saverikko 1988) which may have fed also de-

trital zircon 2,80 Ga (Rastas et al. 2001) in the Sodankylä quartzite.

Fig. 19a. The stratigraphic records are compiled and discussed by Saverikko (1987) but those at Kolari are

reduced to displaying the sections in south and north (Väänänen 1998; 2002). The sections at Luosto (Haimi

1977) and Virttiövaara (Nikula 1988; Lehtonen et al. 1985) are added later. The bimodal-volcanic–arkose–

slate–quartzite association is colored as evolutionary key-stratotype. The Lower-Middle Lapponian cratonic

rifting is clearly traceable in general but especially by a flood-plain stage with tidal sedimentation at Oraniemi

and Kuusamo (Saverikko 1988), Luosto (Haimi 1977), and Virttiövaara (Nikula 1988). The peak in deposition

of BIF–black-shales that was associated with the mantle-plume event is present as the graphitic slate zone prior

to the Upper-Lapponian mantle-activated rifting in the whole area and also as the carbonaceous greenschists

in the Sattasvaara-Kittilä-Kolari area. The U-Pb age determinations: Hiltunen (1982), Rastas et al. (2001), Rä-

sänen and Huhma (2001), Mutanen and Huhma (2001), and Räsänen and Vaasjoki (2001). Obs! There are volcanics of 2,9-2,8 Ga age (Lauri et al. 2016) also in vicinity to Tärendö at the Norrbotten mega-

block (see Fig. 12). And a diabase (2,2 Ga) cuts also the Kumpu rock suite in Kittilä district at Värttiövaara (Rastas

1980) or is merely enclosed by the post-Lapponian metasediments after the later tendentious speculations of Lehtonen et

al. (1998) and Rastas et al. (2001)!

Fig. 19b. Lithostragraphic correlations are apparent within the greenstone-belt areas in Finnish-Norwegian-

Russian Lapland. The sequence in Sydvaranger–Pasvik belongs to the Pechenga greenstone belt. Compiled and

discussed by Saverikko (1990) but revised for part of the Pechenga(-Pasvik) greenstone belt. The Lower-Middle

Lapponian cratonic rifting is clearly traceable beneath the graphitic slate zone prior to the Upper-Lapponian

mantle-activated rifting at Kautokeino and Karasjok. The cratonic rifting appears at Karasjok in the

Oal’gejåkka sandstone of tidal sedimentation (Elvebakk et al. 1985) that is just like the Sodankylä (sericitic)

quartzite (Saverikko 1988; Nikula 1988). At Kautokeino the riftal stage accumulated the Masi quartzite with

fuchsite in economic abundances (Solli 1983) that resembles the (Sodankylä) quartzite with fuchsite quartzites

(Rastas 1980; Lehtonen et al. 1985; Nikula 1988). The Sattasvaara-type komatiites are deposited in the Bakkil-

varri formation at Karasjok, where Meriläinen (1976) described the albite-rock dike of 2,72 Ga age.

The petrographic similarities are stratigraphically important also in the post-Lapponian deposits which

are simplified here by calling them all in Finland as the Kumpu formation the characteristics of which are the

Kumpu quartzites of brownish to red color and the red jaspilites (Mikkola 1941). Similar metasediments are

typical of the Čaravarri metasandstone at Kautokeino (Siedlecka et al. 1985) and the Kuetsjärvi and Kolosjoki

formations at Pechenga (Smolkin et al. 1995). Observe the komatiitic rocks between the basement complex and

the quartzite–carbonate–schist suite as initial volcanic phase also in Norway! The age determinations at

Pechenga are after Melezhik and Sturt (1994) and Smolkin et al. (1995).

The Lapponian stratotype combines the strata of the Salla greenstone area, Oraniemi aulacogen and Kit-

tilä greenstone area. The Koitelainen gabbro lies at the edge of the aulacogen and the pre-existent rocks are

Sattasvaara-type komatiites on the platform and thick metapelite and subarkosic to micaceous quartzites i.e.

Sodankylä quartzites in the aulacogen.

6.2 Archean mantle activities and depositional evolution

Crustal fissuring liberated at 3.0-2,9 Ga komatiitic-mafic volcanism of the initial diapir magmatism 2,99-2,91

Ga in the White Sea region (Arestova et al. 2003, 2012). The pulse invaded at 3,01-2,95 Ga in Kuhmo (Papu-

nen et al. 2009) and it is seen in Lapland as slightly related mafic enclaves within arkose gneisses at Tankajoki

(Lehtonen et al. 1998), the 2,99 Ga komatiite–schist breccia (Papunen et al. 1977) and a contaminated member

(2,66 Ga) of the komatiitic-mafic volcanics (Räsänen et al. 1989) between the gneissose basement and the

Lower-Lapponian quartzites (Saverikko 1987). Archean komatiitic rocks appear also elsewhere between the

basement gneisses and the lowermost quartzitic quartz-feldspar-mica gneisses (Räsänen 1984; Räsänen and

Huhma 2001; Juopperi and Veki 1988; Väänänen 1998) – also in North Norway (Fig. 19b). Bottom-up astheno-

pheric mantle convections with plume ascent operated already much earlier in the Earth (Ernst 2009).

The Lower-Lapponian (ortho)quartzite–carbonate–schist blanket covers the volcanics and sialic weathering

crust just like quartz arenites in East Karelia (Thurston and Kozhevnikov 2000). The common association of the

quartz arenites and ultramafic rocks belongs to the global sedimentation period at 3,0-2,8 Ga (Donaldson and de

Kemp 1998).

The bimodal volcanic associations are cratonic extensional rock assemblaces (Eriksson et al. 1994). The second

2,88-2,80 Ga diapir-magmatic stage (Arestova et al. 2003) at the latest led to the radial crustal fracturing for the

bimodal volcanism after 2,82 Ga in East Karelia (Kozhevnikov et al. 2006), at 2,9-2,8 Ga in Tuntsa (Juopperi

and Vaasjoki 2001), 2,9-2,8 Ga in Norrbotten, N-Sweden (Lauri et al. 2016), 2,8-2,7 Ga under the granulites

(Kozlov et al. 1995) and 2,76 Ga at Olenegorsk (Slabunov et al. 2006) in Kola Peninsula, 2,81-2,79 Ga in Kuh-

mo speeded up with komatiitic volcanism 2,79-2,77 Ga ago (Papunen et al. 2009) and 2,80-2,77 Ga at Salla

(Fig. 19a) in Lapland with komatiitic members, too. Crustal subsidences for the Middle-Lapponian cratonic

rifting with the Oraniemi sedimentation took place at 2,77-2,75 Ga (Fig. 19a) and the riftal sedimentation at

Kuhmo dominated at 2,745 Ga (Papunen et al. 2009). The cratonic rifting occurred also at the same period in

the other shield areas (Blake and Groves 1987; Hartlaub et al. 2004) or started already at 3,0 Ga (Burke et al.

1985).

The third (2,72-2,66 Ga) diapir-magmatic stage in the Solovetsky mantle plume (Arestova et al. 2003) reflected

in the lower lithosphere (Lobach-Zhuchenko and Levchenkov 1986) resulting in the mantle underplating and

pluming in Lapland. It was connected with the strong mantle convections at 2,7-2,6 Ga (Mertanen et al. 1989).

The tectonic peak at 2,7-2,6 Ga (Ez et al. 1984; Gorokhov 1984) was tectono-thermal event (2,72-2,63 Ga) in

Kuhmo, which initiated the restabilization of the crust in East Finland (Käpyaho 2007). The Upper-Lapponian

mantle-activated rifting was part of the global similar rifting period at 2,7-2,6 Ga (Saverikko 1990) and world-

wide mantle-plume event at ~ 2,7 Ga in connection with the deposition of BIF–black-shales and the high gold

production.

6.3 Geotectonic evaluation

The Saamian craton was originally Mesoarchean solid continent but cratonized as rigid plate prior to the craton-

ic deposition and rifting at 2,9-2,8 Ga, like the cratons usually in the Neoarchean (van der Velden et al. 2006).

The continent split into Kola and Lapponia-Karelia halves by the same as the initial NW-trending breakup at

the Belomorian megablock uncovered pulsating mantle activities since 3,0 Ga in the Solovetsky large plume or

diapir (Arestova et al. 2003, 2012). The lithosphere overrode the mantle upwelling in the Lapponia domain

resulting in the mantle pluming and underplating. Worldwide lithospheric thickening at 2,8-2,6 Ga and culmi-

nated growth of the continental crusts were the cause of the catastrophic mantle overturn event with global

mantle-plume epoch ~2,7 Ga ago (Condie and Benn 2006).

Depositional basin of the Lapland greenstone belt was mainly 5-5½ km deep (Lanne 1979; Elo et al. 1989; Leh-

tonen et al. 1998) but ~ 9 km at the deepest (Niiranen et al. 2014) and the Oraniemi aulacogen was 3-3½ km

deep (Saverikko 1988). That indicates the magnitude of subsidence in Archean depositional basins, depending

on the lithospheric properties prevailing at that time (McKenzie et al. 1980). Up to that the crustal structure in

Lapland has reflective properties resembling to those of the Canadian and Australian Shields (Kukkonen et al.

2006). But the increasing amount of komatiites together with the decreasing one of terrigenous metasediments

upwards in the Lapland greenstone belt is reverse to the classic stratigraphic order in the Archean (Anhaeusser

1971). That was liable to controversial stratigraphic opinions about the Lapponian deposition but displays well

the evolving mantle upwelling through the Archean continental lithosphere.

The exogenic evolution proceeded from cratonic deposition through cratonic rifting to mantle-activated rifting

(Saverikko 1990). Also the granulite belt was continental rift (Barbey et al. 1984) upon the Archean continental

crust (Huhma 1986). Quartzofeldpar-gneiss–quartzite–mica-gneiss blanket at the base (Meriläinen 1976) im-

plies cratonic deposition and the overlying bimodal metavolcanics (Barbey et al. 1984) of 2,8-2,7 Ga age (Ko-

zlov et al. 1995) and the thick and cyclic pile of arkoses and pelites composing khondalites (Meriläinen 1976;

Barbey et al. 1980) imply Lapponian cratonic rifting.

The only crustal movement identified is the anticlockwise rotation of the Kola megablock prior to 2,56 Ga

(Slabunov et al. 2006) or 2,5-2,4 Ga (Mertanen and Pesonen 2005). The coeval overthrust of the granulite belt,

the disappearance of the allochthon proper and the rest of oceanic crust make it difficult to understand the idea

of oceanic convergence up to the closure at 1,91 Ga (Daly et al. 2001) and the subsequent granulitic overthrust

(Hanski and Huhma 2005) by the continental collision that is compiled by Lahtinen et al. (2005).

Instead of the oceanic closure the megablock rotation caused the crustal stacking in contrasting directions and

therefore crustal thickening (Fig.

20) that the obduction produced

astrobleme-like structures (Kor-

honen 1990).

Fig. 20. Block diagram (Patison et

al. 2006) based on the deep-crustal

seismic profile FIRE.4-4A displays

rift between the Belomoria and

Lapponia-Karelia crustal domains,

and uplifted tail of the Belomoria

megablock. In the crustal cross-

section attention is directed to the

riftal structures facing in con-

trasting directions to crustal thick-

ening. The tectonic thickening was

greatest in the surroundings of the

P6a block (see Fig. 11) disturbing

the block boundaries (Patison et al.

2006). The crustal inclination up-

wards directed the mantle under-

plating to spread to northwest.

The domal uplift in the Saamian

continent (Saverikko 1990) is not

recognized in the other studies.

The flexural fault set between

Rivers Lemmenjoki and Ivalo-

joki-Paatsjoki is bounded by the

Kittilä-Pechenga fracture on the opposite side to the uplifted domain. It may be prolonged hinge-line fault set

and much tighter than that of the other parallel river valleys in the tundra region associated with metallic miner-

al deposits (Fig. 21). The Kittilä-Pechenga fracture appears across the Kittilä volcanic plain in the SW-striking

strike-slip shear zones, which continue as crustal-scale feature in Sweden (Patison et al. 2006). Lahtinen et al.

(2005) propose to keep it collisional boundary between the Norrbotten craton (i.e. megablock) in the west and

the Karelian craton (i.e. domain) in the east without really concrete evidence.

Fig. 21. The mantle underplating reflects in metallogenic constellation that is concentrated in the tundra region along the

parallel river valleys i.e. fault valleys and esp. on the crustal fractures. The hinge-line fault set associated with the Kit-

tilä-Pechenga fracture was active repeatedly as is seen in the feeder fissures of ultramafic dikes (> 2,44 Ga), granite

domes (1,79-1,77 Ga) and the Laanila dikes (1,04 Ga), retouched on the aeromagnetic map. The Kemi-Murmansk frac-

ture is rift fault that bounds the Peräpohja schist belt on the subsided block (Patison et al. 2006) at the other side than the

domal uplift and fed layered intrusions (2,43 Ga) with Cr or Ni-Cu ± PGE. The Kummitsoiva (Ku) komatiite complex and

BIFs at Jauratsi (~ 2,7 Ga) delineate eruption fissure beside the Akanvaara layered intrusion with Cr-V. The Sokli car-

bonatite (365 Ma) and others may indicate late activities during Caledonian orogeny. Gold occurrences are included also

here. Bluish zone = Salla-Kittilä rift and other megablock boundaries in Figure 11, Yellowish zone = Gold-critical flex-

ure zone. Compiled after Metallic Mineral Deposit Map of the Fennoscandian Shield (1:2.000.000) and Geological Map

of the Fennoscandian Shield (1:2.000.000).

The supervolcano at Kittilä and central volcano at Pechenga (Smolkin et al. 1995; Bayanova and Skuf’in 2008)

erupted through the Kittilä-(Nikel-)Pechenga fracture where it crossed rifts like the western rift graben at

Pechenga. The Kummitsoiva komatiite complex formed at the junction of the Kemi-Murmansk fracture and the

Salla-Kittilä rift that is crossed also by the Oraniemi aulacogen (Saverikko 1988), where the Sattasvaara (Sa)

cinder cone piled up. The intersection of two crustal fractures formed conduits through which the magma

passed from the mantle underplating without uplifting effect. It may be more correct to speak about supervol-

canic plumes than mantle plumes proper that kind of plume was the feeder of the mantle underplating at the

center of domal uplift.

7 Summary

Explosive komatiite volcanism was globally exotic but common in North Finland, although the Fennoscandian

Shield underwent similar exogenic and endogenic processes to those in the other shields. Magmatic explosions

concentrated on a long arc-shaped zone during the global mantle pluming event at ~ 2,7 Ga. The pyroclastic

komatiites originated from magma reservoirs at the border of mantle underplating, the stay in which may have

caused the ultramafic magma to get cool enough for viscous komatiite lavas as they rose to shallow level in the

thick continental crust.

The mantle underplating was related to the shield-wide linear mantle diapir. Its feeder was in good correlation

with domal uplift and was the mantle plume proper. The intersection of two crustal fractures formed conduits

through which the separated ultramafic magmas passed from the mantle underplating without uplifting effect.

The main plume at Kittilä erupted at ~ 2,7 Ga as supervolcano through the fracture that fed later the long-lived

(2,3-1,9 Ga) central volcano at Pechenga. The fracture may have been the hinge on which flexural fault set de-

veloped feeding the Sakatti Ni-Cu ore, too and quartz-gold veins as the source for placer gold.

The thermal effect of the diapir got geochronologic fingerprints at 2,50-2,41 Ga in the Archean rocks, which

together with the tectono-metamorphic events (2,2-1,9 Ga) get illusion about the Karelian (2,5-2,0 Ga) origin of

the Lapponian deposition. Because of the mantle diapir and mantle underplating the Lapponian cratonic to man-

tle-activated rifting was continental rifting of mantle-active initiation by Bott (1995), and the rifting processes

in the domal uplift were quite unlike those of repeated subsidence of marginal basins (Ojakangas et al. 2001) at

the Karelian continent.

Anticlockwise rotation of the Kola megablock was sluggish plate-tectonic motion within the Saamian craton.

Asthenospheric convections spread and rotated the megablock since the Late Archean. Repeated bottom-up

mantle convections produced frontal spreading of the thermal pulses shifting widely in the lithosphere, which

speeds up speculation about compensating top-down mantle convections. Internal heating in the lithospheric

plate played an important role in controlling plate dynamics in the Archean (Korenaga 2006). In sluggish plate

tectonics top-down convections after 2,7 Ga started asthenopheric convection cell increasing in lateral dimen-

sions along with enlargement of the capping lithosphere plates (Ernst 2009). That may have been the situation

at 2,8-2,3 Ga produced by the asthenopheric convection cell in action up to 1,9 Ga ago under the Saamian–

Karelian continent as explanation for the repeated subsidence in the marginal basins of the evolving continent

plate (Saverikko 1992c).

The mantle underplating, supervolcanic plume and domal uplift in the Saamian craton are previously unknown

or unaccepted phenomena, which are worth of further investigations. In addition, the Archean stratigraphy and

geotectonics in the Fennoscandian Shield appear to need more interdisciplinary examination and appreciation

between the geosciences and real acceptation of different interpretations in the future. Otherwise, the data-

mining method is impossible to improve geological knowledge behind the enthusiasm for pure stratigraphy and

isotopic geology to produce merely “ages” for rocks.

Acknowledgements. Prof. Nils Edelman (deceased), in Åbo Akademi asked 1983-84 my opinion about the link

between the granulitic and komatiitic arcs. The impressive question was left without answer because of the au-

thor’s ignored Synopsis(!) for academic dissertation 1991-92 (http://koti.mbnet.fi/komati). K.H. Renlunds

Stiftelse gave me a grant (2.500 €) to publish now the answer on open-access principle after the previously un-

accepted manuscripts; Mantle underplating at ~2,7 Ga in Lapland and satellite pluming with Ni-ore deposits at

Sakatti and Pechenga [Techtonophysics 2016] - Hinge-line fault set across mantle underplating – source for

the placer gold in Lapland? [Geologi/Geol.Soc.Finland 2015] - High viscosity and cooling rate in komatiites:

Extrusions from mantle underplating at ~2,7 Ga in North Finland? [Precambrian Research 2015] - Highly

Viscous Komatiite Lavas Due to Their High Cooling Rate as Exotic Evidence of (Archaean) Plate Tectonics in

the Lapland Greenstone Belt, Finland. [Precambrian Research 2012].

Yellowstone Park inspired my wife’s friend Ms. Becky Bingham in Sandy, Utah so much that it gave me the

idea of Kittilä’s “Yellowstone” against the Dogma about allochthonous oceanic crust and suture zone.

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