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Originally unpublished appendix to my unaccepted academic dissertation at 1991. 1 (26)
A supplementary review to Saverikko, Matti, 1992: Komatiitic Explosive Volcanism, Volcanoes, and Its Tectonic
Significance in Northern Finland, the Fennoscandian (Baltic) Shield.
(Early) Precambrian Convection Cell In The Fennoscandian Shield?
by Matti Saverikko
SAVERIKKO, MATTI, 1992: (Early) Precambrian convection cell in the Fennoscandian Shield?
A supplementary review to Saverikko, Matti, 1992: Komatiitic Explosive Volcanism, Volcanoes, and
Its Tectonic Significance in Northern Finland, the Fennoscandian (Baltic) Shield.
Http://koti.mbnet.fi/komati/convectioncell.pdf.
The Fennoscandian (Baltic) Shield comprises an Archean granite–greenstone terrain and an
early Proterozoic crustal segment of discrepant geosynclinal nature and obscure early history.
The Saamian sialic crust, which cratonized at 3.1-3.0 Ga and split into a mosaic of
megablocks, was involved as a coherent plate in domal uplift and broke up along a mantle
diapir. Mantle-activated rifting at 2.7-2.6 Ga was accelerated with widespread explosive
volcanism of komatiites. The divergence remained at the incipient stage and the cratonic area
restabilized at 2.6 Ga.
The early Proterozoic segment, which differentiated from mantle to crust at 1.9-1.8 Ga, now
exists mainly as a continental granitoid province containing about 10% completely associated
Archean crustal material. A long intraplate basin with volcanic borders of rocks of mixed and
contaminated composition is here called the Birkala mobile belt. This belt was very active
1.90-1.89 Ga ago but shows signs of prolonged sagduction by older ( 2.75 Ga) crustal
material.
The late Archean linear mantle diapir and the Birkala mobile belt form a pair of tectonic
belts of shield dimensions, implying a mantle-convection cell (< 1000 km on the ground
level). The hypothesis requires that convection-cell mechanism was feasible only under
broken continental nuclei when the free high-thermal energy escaped from surrounding
"oceanic" provinces. The mantle-convection cell was exceedingly long in duration (2.8-2.3
Ga) and resulted by mantle currents which were active at 3.0-1.9 Ga in periodic systems
linked to global-scale mantle activities.
Key words. Mantle convection. Endogenic processes. Exogenic processes. Plate tectonics. Archean.
Proterozoic. Finland. Fennoscandia. Baltic Shield.
Originally unpublished appendix to my unaccepted academic dissertation at 1991. 2 (26)
A supplementary review to Saverikko, Matti, 1992: Komatiitic Explosive Volcanism, Volcanoes, and Its Tectonic
Significance in Northern Finland, the Fennoscandian (Baltic) Shield.
1 Introduction
The Precambrian bedrock in Fennoscandia is
composed of an Archean basement complex in
the northeastern part of the Shield and an early
Proterozoic crust in the southwestern part.
Contradictory interpretations of the strati-
graphy, particularly of the greenstone belts in
the Archean domain, have been presented;
although the Finnish bedrock (Fig. 1) provides
the most complete geologic profile across the
Fennoscandian (Baltic) Shield, yet Finnish
geologists are not unanimous about its
chronostratigraphy. This essay examines the
geochronological events, in particular those
tentatively attributed to an isotopic resetting
associated with previously overlooked mantle
activities, and proposes an unused plate-
tectonic pattern to the Fennoscandian Shield: I
need to explain the role of a linear mantle rise
(Saverikko 1990) in the Precambrian evolution
of the Shield!
Fig. 1. Lithostratigraphic features of the
Finnish bedrock, compiled after Simonen
(1980), Gaál (1986), Luukkonen and
Lukkarinen (1986), and Saverikko (1987).
Greenstone belts: 1. Lapland, 2. Kuhmo –
Suomussalmi, 3. Ilomantsi. The southwestern
border of the Raahe(R)–Ladoga(L) tectonic
belt is marked with a dashed line.
Originally unpublished appendix to my unaccepted academic dissertation at 1991. 3 (26)
A supplementary review to Saverikko, Matti, 1992: Komatiitic Explosive Volcanism, Volcanoes, and Its Tectonic
Significance in Northern Finland, the Fennoscandian (Baltic) Shield.
2 Finnish Precambrian in brief
2.1 Archean domain
The Archean granite-gneiss complex
includes unknown quantities of the Saamian
granitoids dated at 3.1-2.8 Ga (Simonen 1980,
Kröner et al. 1981, Luukkonen and Lukkarinen
1986, Paavola 1986, Huhma 1987) and,
indirectly, at 3.2 Ga from a detritus of Archean
metasediments (Huhma 1987). Nd-isotopic
data on the rare protolith indicate derivation
from the mantle at least 3.5 Ga ago (Jahn et al.
1984). The Saamian sialic crust, in Russia if
not elsewhere, was cratonized at 3.1-3.0 Ga
(Musatov et al. 1984), as suggested also by
cratonic sedimentation after 3.0 Ga in northern
Finland (Saverikko 1987). The high scatter in
2.8-2.6 Ga dates (Simonen 1980) results from
geochronological overprints in the above-
mentioned rocks (e.g. Kröner et al. 1981,
Paavola 1986). Granitoids generated at that
time occur in and around the Archean
greenstone belts (Gaál 1986, Luukkonen and
Lukkarinen 1986), some of them being gene-
tically linked to the adjacent volcanics (Martin
1987). Hence, the Archean granitoid complex
contains the Saamian body and a younger
granitic contribution.
The Cwenan greenstone-belt genesis, 3.0-
2.5 Ga in maximum age span (Saverikko
1987), has been dated at 2.9-2.5 Ga in the
Kuhmo–Suo-mussalmi area (Luukkonen and
Lukkarinen 1986, Vaasjoki 1988). The
greenstone-belt associations in Lapland and
Kuhmo–Suomussalmi (Ilomantsi included) are,
broadly speaking, similar (Saverikko 1990): the
tripartite supracrustal sequences are composed
of bimodal volcanics in the lower part and of
multimodal volcanics in the upper part, with
shallow-water clastic sediments, Fe-rich
tholeiitic basalts, and graphitic debris in
between (e.g. Taipale et al. 1983, Barbey and
Martin 1987, Saverikko 1987, Tuukki et al.
1987). The paleoresidue of the Saamian sialic
crust is preserved only in Lapland, where it is
found together with basal arkoses and the cra-
tonic quartzite–carbonate–schist suite; also
unique to Lapland are the pyroclastic koma-
tiites prevailing in the upper volcanic complex
(Saverikko 1987).
The Lapland greenstone belt extends into
northern Sweden and Norway (e.g. Saverikko
1987, 1990), where it is considered early Prote-
rozoic in age (e.g. Witschard 1984, Krill et al.
1985, Skiöld 1987).
High-grade metamorphic rocks in Lapland
form an arc-shaped granulite belt of the lower-
middle Lapponian (see Saverikko 1987). The
bimodal-volcanic–arkose–slate association
(Barbey et al. 1984) is 2.8-2.5 Ga old and may
have been uplifted at 2.5 Ga (Meriläinen 1976).
The identification by Bernard-Griffiths et al.
(1984) of a 1.9 Ga old volcanic rock at the
major thrust plane of the arc (Barbey et al.
1984) has encouraged Barbey et al. (1984) and
Barbey and Martin (1987) to argue that the
granulites are of early Proterozoic age.
Depositional evolution in Lapland advanced
from cratonic sedimentation through cratonic
rifting to mantle-activated rifting mainly in ter-
restrial settings (Saverikko 1987). The strata of
the granulite belt were the result of turbidity-
current deposition in an intraplate trench after
the cratonic rifting (Barbey et al. 1980, 1984).
The Kuhmo–Suomussalmi belt evolved in con-
tinental to oceanic trench environments of
intraplate rifting (Martin et al. 1984). The
intraplate trenches are also found in Russia
(Musatov et al. 1984, Rybakov 1988), where
Salop (1983, ps. 99, 136) proposed a
correlation between the Lopian and Sumian
sequences and the Kuhmoan and Lapponian
sequences, although not as suggested by
Originally unpublished appendix to my unaccepted academic dissertation at 1991. 4 (26)
A supplementary review to Saverikko, Matti, 1992: Komatiitic Explosive Volcanism, Volcanoes, and Its Tectonic
Significance in Northern Finland, the Fennoscandian (Baltic) Shield.
Saverikko (1990). The Lopian and the
Kuhmoan/Lapponian sequences are considered
by Muradymov et al. (1988) to be directly com-
parable in stratigraphy.
Paleomagnetic data imply that the
continental areas have formed a coherent plate
since at least 2.7 Ga (Pesonen and Neuvonen
1981). Nonetheless, the data are insufficient for
the drawing of separate paths for crustal blocks
(Pesonen et al. 1989), even though Mertanen et
al. (1989) suggest that no large-scale
movements have taken place between the
basement segments of Lapland and southern
Karelia. Preferably, the radial swarm of
greenstone-belt trenches (Fig. 2) should be
interpreted as an aulacogen net, a viewpoint
that is consistent with domal uplift of the
compact plate at the start of the Cwenan
diastrophism, i.e. at 3.0 Ga! Continental rifting
at 3.0 Ga has been recognized elsewhere, too
(Burke et al. 1985), and the Archean conti-
nental crust in general has been interpreted as a
mosaic of sialic megablocks (Kröner 1981).
.
Fig. 2. Continental breakup and mantle diapir in association with domal uplift in the Archean conti-
nent (Saverikko 1990). The Skellefte(–Raahe–Ladoga) tectonic belt is regarded as the marginal rift,
Originally unpublished appendix to my unaccepted academic dissertation at 1991. 5 (26)
A supplementary review to Saverikko, Matti, 1992: Komatiitic Explosive Volcanism, Volcanoes, and Its Tectonic
Significance in Northern Finland, the Fennoscandian (Baltic) Shield.
although the Kiruna(–Jällivaara) fault system (Witschard 1984) is also in agreement with Öhlander et
al. (1987).
The basic factor in the Cwenan tectonic re-
gime in Lapland was linear mantle diapirism of
shield dimensions (Fig. 2), apparent as the
large Solovetski mantle plume (Bylinski et al.
1977), a swarm of small ultramafic bodies
(Papunen and Idman 1982), the upper
Lapponian pyroclastic komatiite zone (Fig. 3)
and an auriferous province (Saverikko 1990).
The center of domal uplift correlates well with
the linear mantle diapir.
Fig. 3. The chain of komatiitic volcanoes ex-
posed in a zone of pyroclastic complexes
(Saverikko et al. 1985).
Bylinski et al. (1977) and Efimov et al.
(1977) described counterclockwise rotation of
the Kola megablock in association with the
Solovetski mantle plume after the Late
Archean; the divergence, which is revealed in
the Kantalahti rift, led to southwesterly overth-
rusting of the granulite belt in the Early
Proterozoic. The overthrust has also been
regarded as a continent-collisional structure at
2.0-1.9 Ga (e.g. Barbey et al. 1984, Marker
1985, Krill 1985), despite the lack of any deep-
crustal proof of a descending plate or tectonic
crustal thickening (von Knorring and Lund,
1989). Further, the inferred suture (Barbey et
al. 1984) includes a wedging-subsidence fault
system of the Kantalahti rift which ruptures
circular megastructures in the granitoid
basement (Bylinski et al. 1977), demonstrating
preferably the crustal split in this region.
Tectonic restabilization 2.6 Ga ago (Silven-
noinen 1985) appears to have continued until
early Karelian times, 2.5-2.3 Ga, when a
Sariolan conglomerate–arkosite–argillite–
greenstone association was laid down onto the
weathered granite–greenstone terrain as local
sediments of half-graben to platformal type
deposits (Pekkarinen 1979, Meriläinen 1980,
Marmo et al. 1988); the sandy-argillic suite
was partly glaciogenic (Marmo and Ojakangas
1984, Marmo et al. 1988). The widespread but
rare, thin exposures (see: Luukkonen and
Lukkarinen 1986, Saverikko 1987) may
indicate extensive crustal fissuring with minor
fault-block subsidences and subordinate
volcanism. The Sariolan lithogenesis was
similar to the lithosphere-activated rifting of
Condie (1982, pp.175-177).
A late Karelian (2.3-2.0 Ga) transgressive
sedimentation followed the erosional period,
and Jatulian quartzites and a Marine-Jatulian
slate–dolomite–black-slate association
deposited as an extensive sheet with distinct
depositional subprovinces (e.g. Ojakangas
1965, Pekkarinen 1979, Meriläinen 1980,
Perttunen 1985, Kontinen 1986, Luukkonen
and Lukkarinen 1986). This anorogenic period
was accompanied by mafic volcanism (Meriläi-
nen 1980, Simonen 1980, Aro and Laitakari
Originally unpublished appendix to my unaccepted academic dissertation at 1991. 6 (26)
A supplementary review to Saverikko, Matti, 1992: Komatiitic Explosive Volcanism, Volcanoes, and Its Tectonic
Significance in Northern Finland, the Fennoscandian (Baltic) Shield.
1987).
2.2 Raahe–Ladoga tectonic belt
The Raahe–Ladoga tectonic belt (Fig. 4) is
composed of major NW-trending dextral
wrench faults, the Main Sulfide Ore Belt, a
negative gravity anomaly, and a deep-seated
fracture system; its Svecokarelian (2.5-1.75
Ga) origin is suggested by the Svecofennian
(2.0-1.75 Ga) intrusions, volcanics, and fault-
block movements, and by the Jatulian (2.3-2.0
Ga) eruption fissures, and Sariolan (2.5-2.3 Ga)
graben faults (see references in: Gaál 1986,
Huhma 1986). It is clear that, as a tectonically
active continental margin (Gaál 1982), the belt
separated the Archean and early Proterozoic
crustal segments (Huhma 1986). However, the
proof of Archean tectono-magmatic activities
has been disregarded: the Archean greenstones
of subalkaline to calc-alkaline nature
(Kähkönen et al. 1986, Luukkonen and
Lukkarinen 1986, Tuukki et al. 1987) and the
block-faulting processes linked to an Archean
carbonatite (Talvitie 1971, Puustinen and
Kauppinen 1989) and to vertical movements
(Paavola 1984, 1986) suggest tectonic activity
at the margin as early as during the Archean.
The marginal rift was part of the Archean rift
swarm with NW trend (Saverikko 1990).
Fig. 4. The Raahe–Ladoga tectonic belt bound-
ing the Svecofennian crustal segment with vol-
canic arcs, in the northeast.
The Karelian (2.5-2.0 Ga) tectonic evolution
of the belt, too, has remained obscure largely
on account of the Kalevan graywacke–slate
cover deposited at 2.0-1.9 Ga. The Kalevan
metasediments contain materials derived from
both Karelian epicontinental supracrustals and
the Archean basement (Huhma 1987); arkosic
conglomerates and other phenoclastics at the
base indicate marginal rifting and base-of-slope
environment (Honkamo 1985, Ward 1987,
references in Luukkonen and Lukkarinen
1986). The sedimentation, which was partly
controlled by the N-trending fractures of the
Archean basement (Bowes et al. 1984), was
characterized by high relief and rapid erosion
(Huhma 1987). The Kalevan province with
distinct depositional basins (Ward 1988) and
local Jatulian-Kalevan unconformities
(Simonen 1980) may indicate continental shelf
environments of basin-and-range nature
(Saverikko 1990).
Originally unpublished appendix to my unaccepted academic dissertation at 1991. 7 (26)
A supplementary review to Saverikko, Matti, 1992: Komatiitic Explosive Volcanism, Volcanoes, and Its Tectonic
Significance in Northern Finland, the Fennoscandian (Baltic) Shield.
The Raahe–Ladoga marginal rift extends to
Sweden in the form of the Skellefte fault
system and metallogenic province (Adamek
and Wilson 1979, Lund 1987). The
contribution of a Svecofennian suture to its
development has been advocated by some
authors, especially by Gaál (1986) and by Gaál
and Gorbatschev (1987), largely in view of the
limited subhorizontal thrusting of supracrustal
cover, the tectonic crustal thickening, and the
volcanism. Park (1985) suggested an accretion
model similar to that of the Phanerozoic.
According to Vaasjoki and Sakko (1988), the
Raahe–Ladoga belt underwent different plate-
tectonic processes, when a Proterozoic oceanic
plate collided with the Archean continent
between 1.93 and 1.85 Ga ago. However,
Welin (1987) has abandoned the plate-tectonic
paradigm, because no seismic evidence for a
subducted plate has been found. [Obs! A
descendent infracrustal fault surface is found
by sounding south of Skellefte (P. Heikkinen,
oral commun., 18.4.1991)]. Instead he
proposes an intracontinental rift development.
As is stated later in this paper, the Sveco-
fennian sialic segment evolved mainly at 1.90-
1.87 Ga. Mafic-ultramafic plutons of that age
in the Raahe–Ladoga belt display chemo-
petrologic affinity of continental intrusions
emplaced at moderate pressure (Mäkinen
1987). After this crustal event, probably during
the uplift of some fault blocks (Korsman et al.
1988) 1.88-1.84 Ga ago (Neuvonen et al.
1981), the tectonic belt was active between the
sialic/continental segments. The collision
proper with the Archean segment may have
been a later event, coinciding with reactivation
of NW-trending strike-slip faults that
controlled the emplacement of some granitoids
1.84-1.83 Ga ago (Nironen 1989), and with the
tectonic crustal thickening at 1.82-1.80 Ga
(Hölttä and Korsman 1986, Vaasjoki and
Sakko 1988); the metamorphic effect at 1.82-
1.80 Ga became weaker into the Archean
segment (Haudenschild 1990). The
subhorizontal thrust, which is evident in
distinct allochthons (Koistinen 1981), appears
to have been related to the fault-block
movements, because only the block adjacent to
the allochthons is rotated relative to its
surroundings in the Raahe–Ladoga belt
(Neuvonen et al. 1981).
2.3 Svecofennian (2.0-1.75 Ga) domain
According to Welin (1987), this crustal seg-
ment formed a bowl-shaped basin for the
Svecofennian sea (Fig. 5). Its Archean-Karelian
(> 2.0 Ga) geologic history is unknown.
Thickening of the strata (Lundqvist 1987,
Lundström 1987) and paleocurrent patterns
(Ojakangas 1986) are directed on a depocenter
which may have been located north of the
Åland Islands. The geosynclinal sequence has a
maximum thickness of 8-10 km within the vol-
canic belt in southern Finland and on the west
coast of the Gulf of Bothnia (Simonen 1980,
Lundqvist 1987).
In general agreement with Simonen (1953),
Hietanen (1975) stated that the belt was a vol-
canic island arc, that continued over to
Sweden. However, the volcanic province in
central-southern Sweden is discontinuous
eastwards, being in some way related to a
continental crust (Lundström 1987). Oen
(1987) has described a graben-basinal
development, with the spreading-subsiding
trough directed to the inferred depocenter. The
mainly felsic volcanism at 1.90-1.86 Ga (Oen
1987, Welin 1987) was associated with this
intraplate destruction zone with no evidence of
an oceanic crust or its subduction (Welin
1987). Hence, the volcanic arc is limited to
southern Finland, though its strike may turn to
the north along a nickeliferous magmatic belt
(Kahma 1973) that continues in the Ni-Cu
province south to Skellefte in Sweden (Nilsson
1985) (see Fig. 6).
Originally unpublished appendix to my unaccepted academic dissertation at 1991. 8 (26)
A supplementary review to Saverikko, Matti, 1992: Komatiitic Explosive Volcanism, Volcanoes, and Its Tectonic
Significance in Northern Finland, the Fennoscandian (Baltic) Shield.
The arc framework is indirectly confirmed
by a combined zone of younger rapakivi
granites and associated mafic rocks (see Figs. 4
and 6), as these rock assemblages generally
constituted elongated arrangements in post-
orogenic rifting propagated by linear mantle
processes (Emslie 1978). The mafic rocks
appear as dikes and small intrusions within and
beside the rapakivi granites (Aro and Laitakari
1987) and in a 4 km thick sheet beneath the
largest massif (Tuomi 1988). The basic
magmatism was tholeiitic: the dikes are
chemically similar to continental flood basalts
and the gabbroid–anorthosite suite is typical of
the Proterozoic massive anorthosites (Rämö
1991). The rapakivi granites, which derived
from reworked 1.9 Ga Svecofennian crust
(Huhma 1986, Rämö 1991), indicate mantle–
crust interaction along the line of crustal
sinking. Previous, but weaker, action of the
mantle is evidenced by (hot)spot-like centers
of low-pressure granulitization (see Westra
1988) and by small komatiite exposures in
southernmost Finland (Ehlers et al. 1986,
Schreurs et al. 1986, Laine 1988). Komatiites
are found also south of Skellefte, in Sweden
(Nilsson 1985).
Fig. 5. Svecofennian supracrustal provinces in
Finland and Sweden, completed after Welin
(1987).
The supracrustal sequence is thickest (Simo-
nen 1980, Lundqvist 1987) within the volcanic
belt, which appears as synclines with
subvertical axial planes and gently plunging
fold axes (see references in Kähkönen 1989). A
marine trench is suggested by a mainly
subaqueous paleoenvironment (Kähkönen
1989), basaltic volcanics of oceanic affinity
(see below), lithologic provinces with different
Originally unpublished appendix to my unaccepted academic dissertation at 1991. 9 (26)
A supplementary review to Saverikko, Matti, 1992: Komatiitic Explosive Volcanism, Volcanoes, and Its Tectonic
Significance in Northern Finland, the Fennoscandian (Baltic) Shield.
metallogenic patterns (Kinnunen and Saltikoff
1989), and mafic-ultramafic rocks with chemo-
petrologic signs of low-pressure marine
intrusions (Mäkinen 1987).
The volcanic belt is characterized by calc-
alkaline acidic to intermediate volcanics with
predominant pyroclastics; tholeiitic rocks with
pillow structures are subordinate (Gaál 1986).
The rocks in chemo-stratigraphic units show a
wide dispersion in chemical composition and
vary from basalts to rhyolites, from subalkaline
types to alkaline types, and from low-K rocks
to very high-K rocks (Kähkönen 1989). The
volcanic-sedimentary evolution occurred at
1.90-1.89 Ga (Patchett and Kouvo 1986,
Kähkönen et al. 1989). The major phases of
deformation, metamorphism, and migmatiza-
tion also occurred at those times (Hopgood et
al. 1983).
At first sight, the island-arc hypothesis
seems to be confirmed by the presence of
andesitic volcanics with the mixed and conta-
minated composition typical of island-arc
affinity (Patchett and Kouvo 1986). These vol-
canics were coupled with a thick volcaniclastic
apron (Laitala 1973) similar to arc-basinal
sediments (cf. Okada 1980). The volcanics in
the belt host massive sulfide ores, just like
those of island arcs (Latvalahti 1979, K.
Mäkelä 1980, Hangala 1987) and related to
early rifting episodes (U. Mäkelä 1989).
Additionally, a melange was developed at the
southern margin of the volcanic belt (Edelman
and Jaanus-Järkkälä 1983), and the synkine-
matic rocks of the granitoid complex of central
Finland (Nurmi and Haapala 1986), on the
opposite side of the belt, display many of the
features of subduction-related material (Huhma
1986, Front and Nurmi 1987); they also imply
diapiric emplacement closely related to major
regional compression (Nironen 1985).
Hietanen (1975) maintained that the oceanic
crust descended to the northeast – an idea
shared by many others as well.
The evidence against the island-arc
hypothesis is stronger, however. The volcanic
belt exhibits zonality in that the volcanogenic
margins are separated by the Tampere schist
area. The "inner-arc" abounds with calc-alka-
line intermediate volcanics that resemble
present-day mature arcs or arcs near or at active
continental margins (Kähkönen 1987). The
lowermost pillowed lavas of mantle origin
(Vaasjoki and Huhma 1987) are mid-ocean
ridge basalts (K. Mäkelä 1980) or marginal-
basin basalts (Kähkönen 1989). The basaltic
rocks do not usually represent primary mantle
melts but have undergone fractionation of
olivine ± Cr-spinel ± pyroxene (Kähkönen
1989). The low ISr values of the metavolcanics
suggest that their protoliths separated from
upper mantle sources within a short time
interval (Kähkönen et al. 1989).
The "outer-arc" contains tholeiitic amphibo-
lites with close geochemical similarity to the
within-plate basalts and calc-alkaline basalts at
continental margins (Ehlers et al. 1986, Ehlers
and Lindroos 1986). The mafic-ultramafic vol-
canic association also displays MORB or OIB
affinity (Ehlers et al. 1986, Schreurs et al.
1986).
The LREE-depleted asthenospheric mantle
reservoir beneath the present shield area was
similar to the source of MORB today (Huhma
1986). Volcanics with geochemical characteris-
tics of mid-ocean ridge and/or within-plate ba-
salts occur also elsewhere, outside the volcanic
belt (e.g. Vaarma 1990).
The Tampere schist area, which is here re-
garded as the basinal fill of the central trough,
is a Bothnian deep-sea province (Kinnunen and
Saltikoff 1989) and consists of intervolcanic
turbidites of submarine fans with westerly pa-
leocurrents (Ojakangas 1986). Rifting events
(Ehlers et al. 1986, Colley and Westra 1987,
Kähkönen 1987), which may have occurred in
local or temporal extensional tectonic regimes,
were not restricted to a single episode in the
Originally unpublished appendix to my unaccepted academic dissertation at 1991. 10 (26)
A supplementary review to Saverikko, Matti, 1992: Komatiitic Explosive Volcanism, Volcanoes, and Its Tectonic
Significance in Northern Finland, the Fennoscandian (Baltic) Shield.
evolution of these convergent-like plate
margins (Kähkönen 1987). The extensional
basins exhibit rifting-spreading events not
necessarily related to subduction (Kähkönen
1989). The lithologic zones with characteristic
metallogenic patterns display paleotectonic
provinces which do not correspond to present-
day plate-tectonic belts (Kinnunen and
Saltikoff 1989).
A further argument against the island-arc
hypothesis is that the Svecofennian crustal seg-
ments on either(!) side of the volcanic belt have
nNd from -1 to +3, suggesting an Archean crus-
tal contribution to the new mantle-derived
material, and implying substantial mantle-to-
crust differentiation at 1.9-1.8 Ga (Huhma
1986, Patchett and Kouvo 1986, Patchett et al.
1987). Moreover, despite the low-pressure
granulitic areas (Hölttä 1986, Schreurs and
Westra 1986), the paired high-P/high-T
metamorphic pattern (and high-pressure
granulites in general) is lacking in the volcanic
belt (Gorbatschev and Gaál 1987). The mafic-
ultramafic intrusions were also emplaced under
low-pressure conditions (Mäkinen 1987), and
at least some granitoids were first emplaced
passively through faults and fractures (Nironen
1989). The melange at the southern margin
may be an intrusive breccia (Edelman et al.
1986).
Differing from present-day convergent plate
margins in the rapidity of crustal growth
(Kähkönen 1987), the Svecofennian granitic
crust developed between 1.90 and 1.86 Ga
(Front and Nurmi 1987, Patchett et al. 1987),
but mostly from 1.89-1.87 Ga (Patchett and
Kouvo 1986). During the major volcanism, the
new continental crust beside the volcanic arc
was more than 20 km thick (Kähkönen 1987).
Thus, the Bothnian volcanic belt present in
the form of shallow-to-deep sea provinces
(Kinnunen and Saltikoff 1989) evolved within
one or between two or more sialic/continental
segments of newly formed crust and underwent
"oceanic" crustal development before the
segments rewelded. A high-thermal gradient
over large areas implies a relatively thin
differentiated acidic-to-intermediate crust with
a substantial heat source over large areas
(Westra 1988). As was noted by Branigan
(1987), the belt displayed signs of an ensialic
mobile belt during the subsequent long period
of mantle–crust interaction and crustal
adjustment by shearing (Hubbart and Branigan
1987). These post-orogenic processes after
1.83 Ga led to granitic magmatism with
forceful injection and refilling of collapsed
magma chambers (Ehlers and Bergman 1984)
and, at 1.70-1.54 Ga, to polyphase intrusion of
rapakivi granites
(Vaasjoki 1977). The rapakivi granites and re-
lated mafic rocks derived from anorogenic
magmas of partial melting in the upper-mantle
and Svecofennian continental lower-crust
(Rämö 1991, Branigan 1989).
3 Convection cell
The apparent conformity in strike between
the late Archean linear mantle diapir and the
early Proterozoic mobile belt (Fig. 6) suggests
genetic linkage, despite the ostensible
difference in their temporal and spatial
appearance (Fig. 7). If a mantle upwelling of
shield dimensions occurred, the rising mantle
currents must have been compensated by
descending currents of regional
extent responding to the crustal downwarp. The
inferred sagduction zone is here called the
Birkala mobile belt, without any chronostrati-
graphic connection. (Birkala was a region near
to Tampere from which the Swedish-Fin-
Originally unpublished appendix to my unaccepted academic dissertation at 1991. 2 (26)
A supplementary review to Saverikko, Matti, 1992: Komatiitic Explosive Volcanism, Volcanoes, and Its Tectonic
Significance in Northern Finland, the Fennoscandian (Baltic) Shield.
nish/"Svecofennian" people of the Viking Age
taxed and robbed Lappish/"Lapponian" or
"Saamian" people to the north).
Fig. 6. The Birkala mobile belt, which manifests itself as geologic features 1.90-1.89 Ga in age and as
younger rapakivi granites, may also show older crustal sagduction to compensate for Archean mantle
upwelling to the north. Layered intrusions (see Papunen et al. 1985) indicate frontal spreading of the
thermal zones (dotted areas) associated with the rising mantle currents.
Originally unpublished appendix to my unaccepted academic dissertation at 1991. 12 (26)
A supplementary review to Saverikko, Matti, 1992: Komatiitic Explosive Volcanism, Volcanoes, and Its Tectonic
Significance in Northern Finland, the Fennoscandian (Baltic) Shield.
Fig. 7. Crustal evolutionary periods in Fenno-
scandia and their connection with mantle
activities attributed to a convection cell of one
or more cycles.
3.1 Rising mantle currents
The linear mantle diapir, which manifests it-
self as the Archean mid-continental chain of
komatiitic volcanoes and the Solovetski mantle
plume, must have extended its thermo-tectonic
influence over the surroundings. The
continental province in Russian Karelia
possesses attributes of the mantle-related
endogenic processes that shifted from east to
west at 3.0-2.65 Ga (Lobach-Zhuchenko et al.
1986) when the greenstone-belt genesis in the
Kuhmo–Suomussalmi area in Finland started
with small-scale mantle convection and
continued with a rise of the mantle–crust
boundary (Engel and Diez, 1989): the
volcanism repeatedly discharged felsic to
ultramafic material with increasing MgO con-
tents of komatiites owing to advancing crust
and mantle melting (Taipale 1988). At the
western continental margin the tectono-
metamorphic period started at 3.0 Ga and
culminated at 2.7 Ga (Paavola 1986, 1988).
The widespread dates of 2.8-2.6 Ga determined
as overprints in the Saamian granitic crust
define also strong heat-flow episodes.
The mantle activity was apparent in one of
the most pronounced continental rotation at
2.7-2.6 Ga (Mertanen et al. 1989). The crustal
restabilization at 2.6 Ga (Silvennoinen 1985)
marks the end of the mantle-activated rifting
and of the associated strong heating that
probably brought about the limited granitic
diapirism accompanied by large-scale isotopic
homogenization (Halliday et al. 1988) at 2.75-
2.66 Ga (Luukkonen and Lukkarinen 1986),
and the low-pressure granulite metamorphism
at 2.65 Ga (Lobach-Zhuchenko et al. 1986).
The continued mantle activity is visible in
the layered intrusions, 2.45-2.44 Ga old, which
contain chromite, (vanadium)magnetite, and
PGE-bearing Cu-Ni ores (Piirainen et al. 1974,
Alapieti 1982, Söderholm and Inkinen 1982,
Lahtinen 1985, see Öhlander et al. 1987) and
Originally unpublished appendix to my unaccepted academic dissertation at 1991. 13 (26)
A supplementary review to Saverikko, Matti, 1992: Komatiitic Explosive Volcanism, Volcanoes, and Its Tectonic
Significance in Northern Finland, the Fennoscandian (Baltic) Shield.
which may derive from the upper mantle (Gaál
1986); the mantle activity is supported by
initial 87
Sr/86
Sr features of granites in the
surroundings (Halliday et al. 1988). The linear
distribution of the intrusions in the Archean
craton (Fig. 6) may indicate frontal shifting of
the rising mantle currents to the southwest. The
layered intrusions are also numerous in the
northeast, where they form an elongated
province with a WNW trend (Papunen et al.
1985). The mantle activity appears to have
slackened at the same time beneath the
paleotectonic mantle-diapir area in Lapland,
where only one layered gabbro complex
(Mutanen 1981) was emplaced. Diminishing in
strength, the mantle upwelling continued in
extended areas until 2.45-2.44 Ga. The
convective drift was nevertheless strong
enough to rotate the Kola megablock slowly
counterclockwise, causing the granulite belt to
be overthrust to the southwest (Bylinski et al.
1977) and uplifted at 2.5 Ga (Meriläinen 1976).
The thermal shifting advanced, as demon-
strated by isotopic homogenization (Halliday et
al. 1988) and K-Ar biotite blocking ages of 2.4-
2.2 Ga (Kallio et al. 1986) in the southwestern
periphery of the continental area in Finland.
This heat conduction culminated in the
remobilization of mantled gneiss domes in the
Raahe–Ladoga marginal rift at 2.5-2.3 Ga
(Brun 1980, Martynova 1980). The Birkala
mobile belt, in turn, showed sings of
sagduction in the form of the disappearance of
crust with an age of at least 2.3 Ga (see the
next section). This mantle-convection episode
was responsible for a clockwise rotation of the
shield at 2.4-2.2 Ga (see Mertanen et al. 1989).
3.2 Descending mantle currents
The unknown early history of the Sveco-
fennian (2.0-1.75 Ga) domain calls for specula-
tive introduction to the crustal processes before
1.90 Ga.
Although no Archean-Karelian (> 2.0 Ga)
crustal units have been found in the
Svecofennian domain, the newly formed crust
has a ubiquitous Archean component, which
accounts for an estimated 10% of the total
material (Patchett and Kouvo 1986, Patchett et
al. 1987). Owing to its uniform appearance, it
has been inferred that the main input of the
recycled Archean crustal material was in the
form of sediments (Patchett and Kouvo 1986,
Patchett et al. 1987). For this to be true, the
detritus would have to have been transported as
much as 500 km from the actual exposed
Archean crust. The distance is realistic for the
transportation of marine sediments (e.g.
Reineck and Singh 1980, p.474). However,
only a few synkinematic granitoids have
Al/(Ca/2+Na+K) ratios high enough to accord
with a pelitic component in their source (Front
and Nurmi 1987). The granitoids with the
highest content of Archean components, i.e.
about 20% (Patchett et al. 1987), are spatially
connected with the deposition areas of the
Bothnian (1.9-1.75 Ga) graywacke–slates,
which typically contain 25-50% Archean
material (Huhma 1985, Claesson 1987,
Patchett et al. 1987).
A few remains of micro organisms, mainly
stromatolites, in the early Proterozoic sea(s)
since the Marine-Jatulian stage (Tynni 1971,
Matisto 1974, Perttunen 1985, Tynni and Sara-
pää 1987) suggest that amounts of pelagic de-
bris, too, were inappreciable relative to the
other detritus.
A more likely agent for transporting the in-
ferred sediment cover of constant volume may
have been the Sariolan (2.5-2.3 Ga) continent-
wide glaciation proposed by Ojakangas (1985).
This requires the existence of a pre-Sariolan,
i.e. Archean, crust. The glaciomarine nearshore
deposits were composed of a sandy-argillic
association with a diamictite member (Marmo
and Ojakangas 1984, Marmo et al. 1988).
The inferred Archean crust and the absence
Originally unpublished appendix to my unaccepted academic dissertation at 1991. 14 (26)
A supplementary review to Saverikko, Matti, 1992: Komatiitic Explosive Volcanism, Volcanoes, and Its Tectonic
Significance in Northern Finland, the Fennoscandian (Baltic) Shield.
of clayey and pelagic detritus are features that
permit me to argue that the Archean
component, either partially or predominantly,
originated from a thin, solid crust that was
assimilated with the evolving early Proterozoic
crust; the Archean component was thoroughly
mixed, and the fair homogenization outside the
Bothnian sedimentary areas may have been due
to active, near-surface tectonic processes
(Patchett et al. 1987). Owing to the mixing, it
would be futile to look for a depleted mantle of
the conventional type, with åNd from +4 to +5,
beneath the Svecofennian domain (Patchett and
Kouvo 1986, Patchett et al. 1987).
In the Birkala mobile belt, Patchett and
Kouvo (1986) discerned features of mixing and
contamination with Archean material in several
volcanics 1.89 Ga old. Vaasjoki and Huhma
(1987) reported a common lead age of 1.99 Ga
for a volcanic suite of mantle-derived and
contaminated material that now lies at the
lowermost known supracrustal level
(Kähkönen 1989). Kähkönen (1986) described
a conglomerate with quartzitic and felsitic
pebbles near this level, and inferred the
existence of rocks older than 1.90-1.89 Ga.
Similarly, Huhma (1987) reported a mean
crustal residence age of 2.22 Ga for the
Tampere metasediments, which, according to
Kouvo and Tilton (1966), contain 2.30-Ga-old
detrital zircons sufficiently coarse grained to be
evidence of a local crust at least this old. The
incorporation and digestion of slightly younger
material is corroborated by small inherited
zircon components 1.96 Ga and 1.91 Ga in age
within the 1.90 Ga old trondhjemites (Patchett
and Kouvo 1986). The only direct evidence for
the hypothetical mixing of Archean material is
the 2.75 Ga old relict zircon cores found within
a rapakivi granite 1.59 Ga old (Vaasjoki 1977).
The Birkala mobile belt was thus active be-
fore the Bothnian (1.9-1.75 Ga) volcanism, at
least since the Karelian times, when the 2.3 Ga
old crust disappeared. (If this crust disappeared
from sight, why could not the Archean crust
have done so, too, or dragged into the Birkala
mobile belt during sagduction). The surviving
evidence is poor as is that for the Archean
material at least 2.75 Ga old. It is worth noting
that the main stage of mantle diapirism in
Lapland, which is manifest in the mantle-
activated rifting, followed the cratonic rifting at
2.79-2.70 Ga (Saverikko 1987, 1988). The
mantle upwelling, beginning as early as 3.0 Ga
ago, appears to have reached such a high-
energy level after 2.8 Ga that the descending
mantle currents were probably in operation.
3.3 Duration
Providing the above concept is valid, the
Precambrian mantle-convection was a slow
process of very long duration, starting to evolve
3.0 Ga ago, accelerating at 2.8-2.6 Ga, and
continuing until at least the beginning of late
Karelian times (2.3 Ga) or of Svecofennian
times (1.96-1.90 Ga). But the slow rate and
protracted duration of Archean(-Proterozoic)
tectonic events are demonstrated by the mantle-
related tectono-metamorphic megapulse at 3.0-
2.7/2.65 Ga in the continental area (Lobach-
Zhuchenko et al. 1986, Paavola 1986, 1988)
and by the rotation of the Kola megablock after
the Late Archean and during Proterozoic times
(Bylinski et al. 1977, Efimov et al. 1977) when
the crustal opening stayed at an embryonic
stage of the Wilson cycle (see Saverikko 1990).
3.4 Mantle reactivation
The diminution in mantle activity permitted
the early Karelian (2.5-2.3 Ga) lithosphere-acti-
vated rifting, but the reactivity characterized
the late Karelian (2.3-2.0 Ga) period and
affected the entire Archean continent. The
process was thermally mild (Mertanen et al.
1989) despite its demonstration by the
Originally unpublished appendix to my unaccepted academic dissertation at 1991. 15 (26)
A supplementary review to Saverikko, Matti, 1992: Komatiitic Explosive Volcanism, Volcanoes, and Its Tectonic
Significance in Northern Finland, the Fennoscandian (Baltic) Shield.
numerous small mantle-derived intrusions of
2.2-2.1 Ga old gabbro–wehrlite association
(Hanski 1986) and the 2.0-1.9 Ga old mafic
granulites of upper-mantle origin (Bernard-
Griffiths et al. 1984) emplaced in the Archean
paleotectonic belts, and by the 2.2-1.95 Ga old
diabases originating from the mantle (Huhma
1986) and occurring in recently opened joints
throughout the continental area forming
parallel dense swarms mostly in a NW
direction (Aro and Laitakari 1987).
The Jatulian (2.3-2.0 Ga) sedimentation,
first alluvial and then marine, marked a
transgression of regional extent and the end of
mantle uplifting; the significant presence of
coarse-clastic arkosic detritus implies sharp
uplifts linked to rapid crustal collapse
(Pettijohn 1975, p.167). It was associated with
a high-kinematic activity in the continental
drift (Mertanen et al. 1989) and the crustal
extension was propably caused by lateral
stress-strain forces.
The continental margin underwent (half-
)riftal processes after 2.1 Ga (Kontinen 1986,
Koistinen 1986, Ward 1987), accompanied by
emplacement of the Jormua ophiolite at 1.96
Ga (Kontinen 1987) and of the Outokumpu op-
hiolitic rock suite with strata-bound Cu-Co-Zn
ores at 1.97 Ga (Koistinen 1981). The area as a
whole displays evolutionary records in agree-
ment with the non-plate-tectonic hotspot model
of Lambert (1981), as the medium-size mantle
plumes evolved during the main stage of devel-
opment in the continental setting (see
Saverikko 1990). Epicontinental deposition of
the Kalevan thick turbidity graywacke–slate
successions, 2.0-1.9 Ga ago, required high
differences in the vertical movements of crustal
blocks, which may have responded to limited
mantle upwelling.
Another hotspot-like product, in my
opinion, is the host rock of the Pechenga Ni-Cu
ores in Russia, i.e. the serpentinite–pyroxenite–
gabbro complex of mantle parentage
(Gorbunov et al. 1985, Hanski 1986) that lies
in a triple-junction setting (see Gorbunov et al.
1985, Saverikko 1990). Its comagmatic
relatives are 1.99(-1.97?) Ga old ultramafic
volcanics (Hanski et al. 1991) which display
chemical characters of within-plate basalts
(Hanski and Smolkin 1989). The hotspot
activity appeared thus only at the continental
periphery, owing either to its low intensity or to
the great thickness of the proximal crust.
The Svecofennian orogeny (2.0-1.75 Ga)
had a penetrative effect far into the Archean
terrane (Mertanen et al. 1989) and invoked
recrystallization and isotopic resetting of the
Saamian granitoid basement and the greensto-
ne-belt associations (see Simonen 1980,
Kröner et al. 1981, Siedlecka et al. 1985,
Paavola 1986, Mertanen et al. 1989), finely
demonstrated by convulsive granulite
metamorphism (Bernard-Griffiths et al. 1984)
and by basement reactivation (Witschard
1984).
The basement reactivation in a continental
continuation of the Birkala mobile belt in
northern Sweden is worth noting (see Fig. 6).
The distinct Archean granitoid areas preserved
there provide signs of geologic events at 2.83
Ga and 2.67 Ga; the contribution of Archean
material to the granitic remobilization at 1.89-
1.84 Ga decreases toward the Archean
continental margin (Öhlander et al. 1987), in
front of which only a minimal Archean
component occurs (Skiöld and Öhlander 1989)
and the mantle efflux now occurs as komatiites
of the 1.88 Ga old Ni-Cu ore province south of
Skellefte (Nilsson 1985, see Lundqvist 1987).
Negligible amounts of komatiitic magma also
discharged in the Finnish side of the Bothnian
volcanic belt (Ehlers et al. 1986, Schreurs et al.
1986, Laine 1988). The mantle-crustal
reactivation of the sialic basement took place in
association with the Bothnian "oceanic" crustal
opening of the Birkala mobile belt.
The Svecofennian crustal segment, which
Originally unpublished appendix to my unaccepted academic dissertation at 1991. 16 (26)
A supplementary review to Saverikko, Matti, 1992: Komatiitic Explosive Volcanism, Volcanoes, and Its Tectonic
Significance in Northern Finland, the Fennoscandian (Baltic) Shield.
developed mainly 1.90-1.87 Ga ago, was
uplifted coevally as shown by regression of the
Svecofennian sea at 1.90-1.85 Ga (Lundqvist
1987) and by a widespread randomly oriented
joint group of mafic dikes 1.90-1.80 Ga old
(Aro and Laitakari 1987). This crustal uplift is
consistent with
the Bothnian regional-scale magmatic
underplating caused by imbrication tectonics
and/or low-angle subduction as postulated by
Westra (1988) to explain the thermal history
and crustal thickness of that time in the Birkala
mobile belt: the crust thickened from 5-10 km
to 60-75 km during the Early Proterozoic.
4 Discussion
This essay attributes controversial age deter-
minations within the Archean domain to
thermal overprinting. The geotectonic
constraint is based on the findings of recent
investigations, which thus supersede most of
the previous plate-tectonic interpretations of
the Fennoscandian Shield. Although the studies
partly deal with the geochemical affinities of
Precambrian igneous rocks, it is beyond the
scope of this essay to deliberate on the
relevance of geochemical comparison between
the (early) Precambrian and Phanerozoic rocks.
Instead, the Archean linear mantle diapir, in
good agreement with domal uplift, and the co-
herency of the Archean continental plate
should be taken into account in future plate-
tectonic evaluations of the Fennoscandian
Shield. Furthermore, in view of the lack of
solid evidence for (early) Precambrian
subducted plates and other implications of
buoyancy-subduction dualism, the subduction
hypotheses may be in need of amendment. The
island-arc systems are not readily applicable to
the (Archean) craton-scale framework, or, more
precisely, to the aulacogenic nature of the
greenstone-belt trenches (cf. Windley 1984, ps.
87, 355).
On a global scale, the inferred (early)
Precambrian convection cell under the
Fennoscandian Shield is consistent with
Archean plate tectonics, along with Archean
sea-floor spreading (e.g. Helmstaed et al. 1986)
and the theoretical availability of the Archean
ultramafic oceanic crust for subduction (Arndt
1983). Admittedly, it is difficult to believe
without detailed specific studies that the mantle
convection was in continuous operation during
2.8-2.3 Ga (or even from 3.0 Ga to 1.9 Ga) and
that the continental plate stayed in position
with respect to the convection cell despite
strong mantle currents and the Precambrian
drift of the Fennoscandian Shield.
The Fennoscandian Shield underwent three
major driftal periods: a counterclockwise rota-
tion along with only moderate latitudinal shifts
at 2.7-2.6 Ga, a clockwise rotation at 2.4-2.2
Ga, and a counterclockwise rotation
accompanied by a considerable latitudinal shift
at 2.2-1.9 Ga, which all reflect extensional
tectonic regimes and may implicate shifting of
the Shield as an independent plate before
getting together with other shields (Mertanen et
al. 1989).
Because of the extremely thin or (partly) ab-
sent crust, might it not be possible that, in Ar-
chean "oceanic" provinces, high-thermal proc-
esses accompanied by the movements of
mantle-derived material resembled bubbling
porridge? And were the conditions for
descending mantle currents to form part of the
convection cell only suitable under continental
nuclei? The main continental ruptures in the
crustal screen should have liberated
asthenospheric thermal pressure (Fig. 8), with
the result that the mantle upwelling would have
remained stationary in relation to the continent
despite free migration of the continental
nucleus. Also, the lateral extent of a convection
cell, which in the present instance is inferred to
Originally unpublished appendix to my unaccepted academic dissertation at 1991. 17 (26)
A supplementary review to Saverikko, Matti, 1992: Komatiitic Explosive Volcanism, Volcanoes, and Its Tectonic
Significance in Northern Finland, the Fennoscandian (Baltic) Shield.
be much smaller than those of today, depended
on the size of the overlying continent.
Fig. 8. The exogenic features of the Fen-
noscandian Shield discussed in the text may be
attributed to the above mantle-convection
development of the (early) Precambrian: a
continental nucleus creates a thermal-pressure
shadow on the underside, making convection-
cell mechanism feasible as the crustal screen
breaks up and brings the asthenosphere into
disequilibrium. Descending mantle currents
lower the high geothermal gradient at the
continental edges and lead to crustal over-
growth. At the same time the convection cell is
enlarged along with expansion of the crustal
screen. The lithospheric periphery of crustal
weakness is liable to sagduction due to period-
ic acceleration of mantle convection by in-
creasing heat-producing processes.
Mantle convection operated periodically in
connection with global mantle activity. The ac-
celeration, which was due to continental rifting
(2.8-2.7 Ga) and mantle-activated rifting (2.7-
2.6 Ga), is consistent with a late thermal event
at 2.9-2.7 Ga in the North Atlantic Archean
craton (see Windley 1984, pp.21-22) and with
the major world-wide greenstone-belt
development at 2.7-2.6 Ga (see Condie 1981,
p.43). The later events – the emplacement of
numerous layered intrusions (2.45-2.44 Ga),
the termination(?) of convection cell activity
(2.3-2.2 Ga), and the Bothnian "oceanic"
crustal opening along with mantle-to-crust
differentiation at 1.90-1.87 Ga – are also
reflected in a rhythmic discharge of mafic dike
swarms and plateau basalts in the Canadian
Shield 2.5-1.9 Ga ago (see Baragar 1977).
Acknowledgement. I wish to thank Prof. J.
Kalliokoski of Michigan Technological
University for encouraging me to write this
paper. Special thanks are due to Drs. O.
Kouvo, M. Vaasjoki, and H. Huhma, of the
Geological Survey of Finland, for critically
reading the manuscript and adding spice to my
work by presenting contrary opinions.
Originally unpublished appendix to my unaccepted academic dissertation at 1991. 18 (26)
A supplementary review to Saverikko, Matti, 1992: Komatiitic Explosive Volcanism, Volcanoes, and Its Tectonic
Significance in Northern Finland, the Fennoscandian (Baltic) Shield.
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