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UNACCEPTED MANUSCRIPT FOR MY DOCTOR THESIS 1 (22)
Komatiitic Explosive Volcanism, Volcanoes, and Its Tectonic Significance in Northern Finland, the Fennoscandian (Baltic) Shield.
by Matti Saverikko
Saverikko, Matti, 1992. Komatiitic explosive volcanism, volcanoes, and its tectonic significance in
northern Finland, the Fennoscandian (Baltic) Shield. Http://koti.mbnet.fi/komati/Synopsis.pdf.
The present study describes numerous well-developed volcanic structures, some of which, ei-
ther alone or together with others, have not previously been described from komatiites (e.g. ejec-
ta and their prevalence, block lavas, vesicles), and which are attributed to processes of komatiit-
ic explosive volcanism as specific evidence for an Archaean craton.
As the komatiites investigated are closely related to each other in space and time, they are
thought to be derived from the same magma system. Trace-element or REE data are not availa-
ble but a low-pressure magma differentiation is recognized in petrography: the volcanics are ul-
tramafic komatiites (>18 wt% MgO, anhydrous basis) and komatiitic basalts (18-9 wt% MgO,
anhydrous basis), the former being cumulates and (porphyro-) aphanitic lavas interconnected by
the MgO content of ca 30 wt% (anhydrous basis).
In the least-altered variants, the cumulates are dunitic to peridotitic rocks, the (porphyro-)
aphanitics are amphibole-chlorite rocks, and the komatiitic basalts are amphibole(-plagioclase)
rocks. The compositional transition from ultramafic to mafic manifests itself petrographically in
the conversion of colourless amphibole (tremolite) to pleochroic amphibole (actinolite), reflect-
ing clearly in lithologic characteristics.
Pyroclastics are frequent in the ultramafic komatiites of (porphyro-)aphanitic texture, which,
in this context, are called pyroxene peridotitic komatiites1 (18-30 wt% MgO, anhydrous basis).
The pyroclasts developed in minor amounts in the komatiitic basalts, too, whereas they are al-
most nonexistent in the ultramafic komatiites of cumulate texture called peridotitic komatiites2
(>30 wt% MgO, anhydrous basis). The (porphyro-) aphanitic komatiites of high viscosity under-
went magmatic shattering due to the high cooling rate; the absence of spinifex textures is one
piece of evidence of the unique "low" eruption temperature.
Isolated volcanoes of komatiitic explosive volcanism developed in terrestrial environments at
intersections of the Kemin-Lappi rift and aulacogens. They constituted a linear mid-continental
chain with signs of a shield-wide mantle diapir. Ascent of the ultramafic magma ridge with high
density contrast to granitic country rocks indicates vigorous mantle upwelling through continen-
tal crust during extensional tectonic regime. A domal uplift concentrated on the komatiite centre
giving the appearance of a mantle plume.
There is no doubt that these komatiites interest in gold prospecting. Although the auriferous
province in Lapland is poorly known, the pyroclastic komatiites host gold showings, which, how-
ever, are known in larger numbers on the surroundings. Their near-surface magma chamber may
have been a heat source for mobilization of the gold in or into earlier Lapponian rocks.
Key words: Komatiite. Pyroclastic. Viscosity. Cooling rate. Eruption mechanism. Magmatic differentiation.
Tectonics. Lapponian. Archaean. Finland. Fennoscandia. Baltic Shield.
Matti Saverikko, Hakamäki 4 G 97, SF-02120 Espoo, Finland.
1 They were known by the name of basaltic komatiite (Saverikko 1983) or (basaltic)komatiite (Saverikko 1985).
2 They go also under the name of komatiite proper (Saverikko 1983) or komatiite (Saverikko 1985).
Albergan esplanadi 4 A 10, FI-02600 Espoo, Finland.
Email: [email protected]
UNACCEPTED MANUSCRIPT FOR MY DOCTOR THESIS 2 (22)
Komatiitic Explosive Volcanism, Volcanoes, and Its Tectonic Signif-icance in Northern Finland, the Fennoscandian (Baltic) Shield
by Matti Saverikko
1 SYNOPSIS
1.1 Introduction
My Doctor's Thesis "Komatiitic Explosive Vol-
canism, Volcanoes, and Its Tectonic Significance in
Northern Finland, the Fennoscandian (Baltic) Shield"
is composed of the following papers:
1. Saverikko, M., 1990. Komatiitic explosive
volcanism and its tectonic setting in Finland, the
Fennoscandian (Baltic) Shield. Bull. Geol. Soc.
Finland 62, 1, 3-38.
2. Saverikko, M., 1987. The Lapland greenstone
belt: Stratigraphic and depositional features in
northern Finland. Bull. Geol. Soc. Finland 59, 2,
129-154.
3. Saverikko, M., 1985. The pyroclastic komatiite
complex at Sattasvaara in northern Finland. Bull.
Geol. Soc. Finland 57, 1-2, 55-87.
4. Saverikko, M., Koljonen, T. & Hoffrén, V., 1985.
Palaeogeography and palaeovolcanism of the
Kummitsoiva komatiite complex in northern Fin-
land. Geol. Surv. Finland Bull. 331, 143-158.
5. Saverikko, M., 1983. The Kummitsoiva komatiite
complex and its satellites in northern Finland.
Bull. Geol. Soc. Finland 55, 2, 111-139.
6. Appendix: Guide Map for Field Excursion in
Central Lapland: Special Reference to Pyroclastic
Komatiites.
The pyroclasticity of the komatiites in Finland, as
a unique phenomenon, was shown with the aid of
numerous volcanic structures in an excellent state of
preservation. These rocks offer unequalled geochem-
ical targets for the research into the komatiites of
well-preserved greenstone belts disintegrated from
and deposited on a sialic crust. First of all in this
context, their comagmatic origin, or magmatic
differentiation at a high crustal level calls for de-
tailed geochemical investigation (nota bene; Henrik-
sen 1983), whereas the high viscosity of the ko-
matiites and their eruption mechanism appear to be
detectable by routine geological methods.
A tectonic constraint of Shield dimensions is de-
scribed, and an occurrence of Archaean aulacogens,
their radial swarming partly in the form of Precam-
brian greenstone belts, the domal uplift of a coherent
continental plate, and its good correlation with a
mantle diapir are discussed. [Additional information
about them are giving in (1) Saverikko, M., 1988:
The Oraniemi arkose-slate-quartzite association: an
Archaean aulacogen fill in northern Finland. Geol.
Surv. Finland Spec. Paper 5, 189-212, and (2) Sav-
erikko, M., in review: (Early) Precambrian convec-
tion cell in the Fennoscandian Shield?. Bull. Geol.
Soc. Finland.] These features have gone virtually
unrecognized in previous tectonic studies of the
Fennoscandian Shield. Attention should be paid to
the radial arrangement of the greenstone-belt trench-
es and crustal fractures, when applying the plate-
tectonic paradigm to the Precambrian evolution of
the Fennoscandian Shield.
Palaeogeographic reconstructions to date have
lacked regional stratigraphic correlations. The litho-
stratigraphy is now clarified taking the middle
Lapponian rock suite as the key horizon. It was
previously overlooked and its stratigraphic position
has been disputed3; the chronostratigraphy is based
on a lithostratigraphic reinterpretation and geological
knowledge, not on geological prejudice.
In Archaean crustal evolution, deposition of the
Finnish greenstone belts is regarded as 3.0-2.5 Ga in
maximum time span and is included in a tectonic
period which is proposed to be called Cwenan
diastrophism in the Fennoscandian Shield.
3 These principles of my Thesis may justify me to pass over also
the tectono-stratigraphic hypotheses sufficiently documented in
Finnish geology, which did not take them into account (cf.
Saverikko, in review).
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1.2 Geological outline
The Precambrian bedrock in Fennoscandia (Fig.
1) is composed of an Archaean domain in the north-
eastern part of the Shield and an early Proterozoic
domain in the southwestern part. The Finnish bed-
rock (Fig. 2) provides a geological profile across the
Fennoscandian (Baltic) Shield, yet Finnish geologists
are far from unanimous about its chronostratigraphy.
Pyroclastic komatiites are found in most Precam-
brian shields but they constitute only a minor feature
in ultramafic volcanic rock piles. The exception is a
komatiite zone of distinct explosive origin in the
Archaean granite–greenstone terrain of the Fen-
noscandian Shield (Fig. 3). This study deals with
structures, stratigraphy, and origin of the explosive
volcanism in that zone.
1.2.1 Stratigraphy and deposition
The Lapponian komatiitic eruption phases were
included in the Cwenan diastrophism (3.0-2.5 Ga;
see Fig. 2) proposed by Saverikko (1987; 1990). The
explosive eruptions appear to have taken place in
three volcanic cycles (Fig. 4), attaining regional di-
mensions as a magmatic phenomenon during the last
cycle along with accelerated mantle-activated rifting
within the cratonic environment in northern Finland,
Norway and Russia (Saverikko 1987; 1990). Note
Fig.1. The Finnish bedrock provides the most
complete geological profile across the Fen-
noscandian Shield.
Fig. 2. The Raahe–Ladoga tectonic belt separates
the Archaean granite–greenstone terrain from the
Proterozoic crustal segment. The Archaean do-
main is overlapped by Karelian-Kalevan terri-
genous metasediments. Compiled by Saverikko
(1990, in review). The Lapponian greenstone-belt
association and the granulite belt, in the north,
are considered Karelian(-Kalevan) in origin, too
(e.g. Barbey et al. 1984, Silvennoinen 1985).
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Fig. 3. In the north, the Lapland greenstone belt includes komatiites in an arc-shaped zone (see arrows)
(Saverikko et al. 1985, Saverikko 1990), those of the upper Lapponian being more frequently in the south-
western part. The prevalence of lower-middle Lapponian metasediments of quartzite–slate(–carbonate)
association is indicative of a continental palaeoenvironment (Saverikko 1987, 1988) as is further confirmed
by the adjacent belt of granulites, mainly of arkose–slate parentage (Barbey et al. 1984).
that if the Lapponian supracrustal sequence was re-
garded as Proterozoic in age, the principal chrono-
stratigraphic features (Fig. 5) discussed by Saverikko
(1987; 1990) should be in want of amendment.
The first Lapponian komatiitic phase, with few
exposures, refers to the initial volcanism, which
resulted in an eruptive outburst when the komatiitic
greenstones discharged onto the Saamian (3.5-3.0
Ga) sialic crust (Saverikko 1987; 1990). Subsequent
cratonic sedimentation of quartzite–carbonate–schist
association preceded a bimodal volcanism which
included minor komatiitic eruptions, too: the rare
volcaniclasticity of the komatiites is attributed to
water-induced explosions, mainly in a terrestrial
environment (Saverikko 1990). As a result of craton-
ic rifting, sequences of arkose–slate–quartzite asso-
ciation (Saverikko 1988) were superimposed on
these volcanic complexes of the stable platform
(Saverikko 1987), forming the bimodal-volcanic–
arkose–quartzite
UNACCEPTED MANUSCRIPT FOR MY DOCTOR THESIS 5 (22)
Fig. 4. The stratigraphic order of the Lapland greenstone belt is the opposite of that of the classic Archae-
an greenstone–belt sequence (Anhaeusser 1971), as terrigenous metasediments are in excess in the lower
and middle Lapponian and komatiites do not prevail until in the upper Lapponian; the supracrustal se-
quence denotes the acceleration of mantle-activated rifting in the craton (Saverikko 1987). The composite
stratotype differs from those previously proposed in that the Lapponian sequence is subdivided into three
successions, the middle Lapponian consisting virtually of Oraniemi arkose–
slate–quartzite suite; there was just the question of stratigraphic position of
the Oraniemi suite (see Saverikko 1986).
Fig. 5. The chronostratigraphy of the Lapland greenstone belt has been
interpreted in disputable orders. But the lithostratigraphic records in Lapland
are well intercorrelated if the middle Lapponian metasediments (previously
overlooked) are used as a stratigraphic key horizon (Saverikko 1987, 1990).
The upper Lapponian komatiites are 2.7-2.44 Ga in maximum age span (e.g.
Kröner et al. 1981), albeit giving a Sm-Nd age of 2.08 Ga at Karasjok, Norway
(Krill et al. 1985). Assimilation of a continental crust may have modified the
isotopic pattern of the high-temperature lava suites, thus rendering the whole-
rock Sm-Nd dating of Archaean lavas highly suspect (Huppert andSparks
1985, Cattell 1987).
association of Condie (1982). Tectonic quiescency,
demonstrated by thin but extensive euxinic-
exhalative strata, separated the following mantle-
activated riftal period, which was characterized by
the main explosive komatiite volcanoes (Saverikko
1987; 1990).
The komatiite volcanoes lie in an arc-shaped zone
(Saverikko et al. 1985). A komatiite complex at
Kummitsoiva was a central-vent (Ø > 0.5 km) volca-
no, whose large cone is eroded partly into separate
exposures around lateral vents (Saverikko 1983;
1990); large volcanic cones with associated block
lavas are highly indicative of a continental environ-
ment (Macdonald 1972, p.93). Another complex at
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Sattasvaara includes a relict cinder cone of fair
reconstruction (see Fig. 14) in addition to a number
of small volcanic necks (Saverikko 1985).
The upper Lapponian komatiitic volcanism was
controlled by block faulting, and depositional condi-
tions varied from subaerial to subaqueous in a coastal
environment; eruption fissures are visible in lava
dykes and chains of terminal vents (see Saverikko
1990). The komatiite complexes possess attributes of
violent eruptions and earthquakes due to the block
faulting (Saverikko 1983; 1985).
1.2.2 Tectonic setting
In accordance with the Archaean global tectonics
by Kröner (1981) and Katz (1985), the continental
crust in the Fennoscandian Shield consisted of
separate megablocks which moved relative to one
another forming riftal tracks suitable for the devel-
opment of greenstone belts; vertical, rotational and
inclinational movements are established.
The alignment of the pyroclastic komatiite com-
plexes is suggested by Saverikko (1987; 1990) to indi
cate a mantle diapir, the strike of which is also de-
lineated by a gradational metamorphic temperature
maximum (see Hörmann et al. 1980) and by other
metamorphic gradients apart from or adjacent to the
granulite province (see Mattila 1974, Isomaa 1978,
Rask 1978, Krill 1985); the metamorphic zonation in
the northernmost part, in Norway, is inverted owing
to the overthrust of the granulites (Krill 1985).
According to Bylinski et al. (1977), the mantle
upwelling in Russia is exposed as the Solovetski
mantle plume and has been associated with crustal
splitting since the Late Archaean (Fig. 6). The
divergence
Fig. 6. Mantle upwelling was connected with an extensional tectonic regime caused by counterclockwise
rotation of the Kola megablock since the late Archaean: the Kantalahti Archaean rift consists of the central
deep-fault zone intersecting circular megastructures of the Saamian granitoid basement, but the original
fault system is much wider in the form of wedging subsidence within the Belomoria (White Sea) megablock
(Akudinov et al. 1972, Bylinski et al. 1977). The advancing rotation caused overthrusting of the Anar
megablock (see Saverikko 1990) when the granulite belt and an adjacent bedrock in the west formed imbri-
cation structures (Bylinski et al. 1977) similar to those of continent–continent collision (Barbey et al. 1984,
Marker 1985).
reached no more than an embryonic stage of the Wil-
son cycle if the plate-tectonic paradigm is applied
(Saverikko 1990). In association with this divergence
the Fennoscandian Shield as a whole got into pro-
UNACCEPTED MANUSCRIPT FOR MY DOCTOR THESIS 7 (22)
nounced counterclockwise rotation at 2.7-2.6 Ga
(Mertanen et al. 1989). In general, the extensional
tectonic regime was responsible for komatiitic
volcanism at a time when some forces were pulling
the continental nuclei apart (Nisbet 1982).
Continental rifting is associated with hot-spots,
which are one of the main causes of crustal doming
(Bott 1981). But an updoming alone cannot always
explain the formation of rifts (Fuchs et al. 1981), and
the major rift systems have not generally formed
under one and the same tectonic regime (Illies 1981).
In the Fennoscandian Shield, NW-trending rifts
constitute one system and a radial swarm of linear
crustal openings another system (Saverikko 1990).
The Solovetski mantle plume is attributed to a
large plume during the main to final stage of the hot-
spot–continent interaction described by Lambert
(1981); the komatiite centre (Figs. 7-8) may display
another mantle plume of smaller size (Saverikko
1990). A radial swarm of aulacogens and crustal
fractures (Fig. 9), which reveals that the linear
mantle upwelling with shield dimensions involved
domal uplift (Saverikko 1990), was unrecognized in
previous tectonic evaluations of the Fennoscandian
Shield; most of them referred genesis of these green-
stone-belts/trenches to island-arc systems (sic!).
Development of the Archaean aulacogens contrasts
with the opinion held by Windley (1984, ps.87, 355)
and Condie (1982, p.248) that lithospheric properties
prevailed during the Archaean.
Fig. 7. The lithology of the bedrock in an area where komatiites prevail can be correlated with regional till
geochemistry (Pulkkinen 1983, Saverikko et al. 1983) classified by statistical methods (Ahlsved et al.
1983), because an ice divide of the last glaciation lies in the middle of Finnish Lapland (Salonen 1986). The
komatiite zone is apparent in a total-dissolution geochemical Mg-Cr-Ni province characterized by the scarci-
ty of barium; arsenic and antimony anomalies discriminated by neutron-activation analyses (NAA) may
indicate a volcanic center (see: Geochemical Atlas of Finland (in press), Geological Survey of Fin-
land/Department of Geochemistry). This refers to komatiitic volcanism in consequence of associated par-
tial-leaching geochemical Fe-Mn-V-Ti patterns (See: GSF/DG, opt. Cit.) probably caused by the vanadium-
bearing titanium-iron deposits (see Silvennoinen 1984) and manganiferous banded iron formations (Paak-
kola 1971) associated with the upper Lapponian komatiites (Saverikko 1985). In this way, the mantle
diapirism was revealed in late Lapponian times along the southwestern margin of the Belomoria megablock
(see Fig. 6).
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Fig. 8. The komatiite centre delineated after Figure 7 is also revealed through gravimetric sounding:
Bouguer anomalies are at their highest where the upper Lapponian komatiites are covered with spilitic
greenstones. The anomaly pattern conforms to local crustal structures outsketched by Saverikko (1990).
The gravimetric patterns are from the Gravity Anomaly Map, Northern Fennoscandia, 1:1 mill. Geodetic
Institutes and Geological Surveys of Finland, Norway and Sweden, 1986. ISBN-91-7158-374-2.
UNACCEPTED MANUSCRIPT FOR MY DOCTOR THESIS 9 (22)
Fig. 9. Linear mobile belts of the Lopian greenstone belts in Russia were intraplate tectonic basins (Musatov
et al. 1984). They form a radial swarm of Archaean aulacogens together with the crustal fractures and
intracontinental trenchesof the Finnish greenstone belts. Thus, domal uplift in obvious correspondence with
the mantle diapir is manifested; furthermore the Belomoria megablock (see Fig. 6) is inclined (Saverikko
1987, 1988) as a consequence of preferential subsidence in the southeastern part (Akudinov et al. 1972,
Bylinski et al. 1977). After Saverikko (1990).
1.2.3 General geochemistry
The main-element geochemistry was applied to
regard simple geochemical differences between the
komatiitic members (also between the lavas, pyro-
clastics and epiclastics) which are petrographically
distinguished from one another.
Vulnerability to explosive eruptions during the
late Lapponian times was promoted by the preva-
lence of pyroxene peridotitic komatiite with high
viscosity (Saverikko 1983; 1985). The komatiitic
rock species (Fig. 10, Tables 1-7) are here defined
slightly differently from the conventional classifica-
tion by Arndt and Nisbet (1982); the nomenclature is
adopted from Pihlaja and Manninen (1988). The
cosmetic difference is due to the separation of the
ultramafic komatiites into distinct cumulate flows
and (porphyro-)aphanitic flows.
Peridotitic komatiites (>30 wt% MgO, anhydrous
basis) are coarse-grained olivine-pyroxene cumulates
with random volcanic features or with almost non-
existent pyroclasts. Pyroxene peridotitic komatiites
(30-18 wt% MgO, anhydrous basis) are present as
fine-grained, tremolite-chlorite rock often displaying
structures of ejecta and fragmentary lava flows with
or without basal cumulates. Komatiitic basalts (18-9
wt% MgO, anhydrous basis) are now actinolite-
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Fig. 10. Frequency distribution of MgO in the upper Lapponian komatiites (see Tables 1-7). Primarily the
porphyritic lavas consisted of high-magnesian cumulates and looser crystal mush, separated from each
other by the MgO content of 30 wt% (anhydrous basis)..
Fig. 11. Compositional fields of the upper Lapponian komatiitic lavas presented in Figures 12-13.
hornblende(-plagioclase) rocks of compact lavas with
infrequent ejecta. See Saverikko (1990).
All these komatiitic members display pyroclastic
structures and can be considered as distinct volcanic
rocks (Fig. 11); they also produced volcaniclastic
detritus (Saverikko 1990). The MgO content of 30
wt% (anhydrous basis) marks the boundary between
the pyroxene peridotitic komatiites and dense cumu-
lates of peridotitic komatiite composition which
discharged as a crystal mush in accordance with their
(almost?) completely accumulative texture also in
lava driplets (see Saverikko 1985; 13. excursion site
in Appendix). The pyroxene peridotitic komatiites in
the form of porphyro-aphanitic rocks with 31-34
wt% MgO may be basal cumulates, whereas two of
the three peridotitic komatiites with 26-29 wt% MgO
are pyroclastics of crystal-lithic-vitric tuff. In addi-
tion, the peridotitic komatiites with distinct MgO
contents of 46-48 wt% – the Nuttio ultramafics of
volcanic origin inferred by Papunen et al. (1977) –
may be appropriate to terminal flows of a residual
crystal mush. A cumulate (24 wt% MgO) differs
clearly from the other komatiitic basalts.
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The glass in komatiitic rocks may be enriched in
aluminum (Nisbet et al. 1977). As aphanitic lavas
and pyroclastics contain very variable quantities of
glassy material, they can not be readily distinguished
from each other or from the epiclastics with CMA or
Jensen Cation Plot diagrams (Fig. 12-13). The degree
of contamination caused by the sialic crust is un-
known as there are no trace element or REE data.
Fig. 12. Compositional features of the upper
Lapponian komatiites in MgO-CaO-Al2O3 dia-
grams. Data compiled from Sarapää (1980),
Henriksen (1983), and Saverikko (1983, 1985).
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Fig. 13. Jensen Cation Plot diagrams of the upper Lapponian komatiites. Data compiled from Sarapää
(1980), Henriksen (1983), and Saverikko (1983, 1985). [Nota bene! Compositional estimations on the dia-
grams presented from the Kummitsa and Sattasvaara complexes (Saverikko opt. cit.) were incorrectly
plotted by using oxides instead of cations!].
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1.3 High viscosity and cooling rate
The main, or third, Lapponian komatiitic eruption
phase occurred in isolated volcanoes, around which
the volcanic complexes are mainly pyroxene peri-
dotitic komatiites. Their large amounts of ejecta with
crude or absent sorting imply violent volcanic explo-
sions which are verified principally magmatic in na-
ture (Saverikko 1983; 1985). The petrographic evi-
dence for a highly viscous magma is as follows:
1. High abundance of ejecta. Great explosivity
is usually ascribed to the high viscosity that
retards the expansion of gas bubbles exsolv-
ing from the rising magma and produces high
internal pressures (McBirney 1973, Sparks
1978). The magma fragmented deep in the
ground may also bring about enormous ex-
plosions (Sparks 1978), but Strombolian-
type eruptions, whose fingerprints are readily
discerned in the pyroxene peridotitic ko-
matiite (Saverikko 1985; 1983), are due to
disruption of pasty vesicular magma close to
the surface (e.g. Williams and McBirney
1979, p.235).
2. Predominance of cinders (31. excursion site
in Appendix) which, according to Macdon-
ald (1972, p.128), fall to the ground in an es-
sentially solid state. Cinderites are not un-
common, and they form a relict cinder cone
in the Sattasvaara hill (Fig. 14; 6. excursion
site in Appendix).
3. Frequent glassy ejecta. The formation of vol-
canic glass depends on the cooling rate and
viscosity of a stable silicate liquid; thus high-
ly viscous magmas generally form entirely
glassy pyroclasts (Fisher and Schmincke
1984, p.76). See Figure 15.
4. Dearth of spatter-like coarse ejecta.
5. Subordinate lavas are blocky flows (Fig. 16;
7., 28. and 32. excursion sites in Appendix)
rather than pahoehoe flows (see Macdonald
1972, p.96).
The viscosity of lava is usually determined by its
chemical composition, pre-existing solid fragments,
the amount and condition of gas, and the eruption
temperature. In komatiite magma the low silica
content and weak Si-O polymerization increase
fluidity as does also the gas dissolved in magma up
to a certain limit, even when present as bubbles.
Owing to the proof(?) of low volatile content and
small number of pre-existing solid fragments, i.e. cu-
mulus minerals (except in the basal cumulates) and
accidental or accessory clasts, the viscosity of the
pyroxene peridotitic komatiite is suggested by Save-
rikko (1983; 1985) due to the "low" eruption temper-
ature. This could explain the absence of spinifex tex-
tures, which occur in the komatiites of the earlier
Lapponian volcanic cycles (see Saverikko 1990).
Low-temperature lavas may not have a high enough
cooling rate and degree of supercooling, both of
which are necessary for the development of spinifex
texture (Donaldson 1982).
Increasingly slow cooling of the supercooled liq-
uid results in the equilibrium transformation of the
liquid to glass (Carmichael 1979). Glass inclusions
(Ø 0.1-1 cm) frequently trapped in the komatiite
magma, even in a basal cumulate (Saverikko 1985),
and the absence or minor presence of glassy skin in
the lava flows (9. excursion site in Appendix) may
indicate that the critical cooling took place in the
Earth's crust before discharge.
The lavas of lowered temperature appear to have
been cooled by heat conduction rather than turbulent
convection; this is manifested with the abundance of
glass, whose general absence in the ultramafic
magma is due to a kinetic phenomenon (Carmichael
1979) and with that the highly viscous magma
impedes all convection (see e.g. Best 1982, p.251).
According to Huppert et al. (1984), the turbulent
convection should also have influenced on the
formation of spinifex textures. The high cooling rate
is established by
(1) the absence of spinifex textures,
(2) the mainly unwelded ejecta of the liquid of
high optimal temperature [T 1 360 oC at 1
atm; Nisbet (1982)],
(3) the block lavas which, after Williams and
McBirney (1979, p.112), are usually
intermediate to silicic flows but may also be
basaltic flows erupted at very low
temperature,
(4) rarity of the polyhedral jointing characteristic
of the komatiites (see Arndt et al. 1979), be-
cause thermal-related contraction was
smaller than the tensile strength (see Wil-
liams and McBirney 1979, p.115), and
(5) no visible thermal effect on the surroundings
of terminal flows and plugs, i.e. the Nuttio
ultramafics of peridotitic komatiite, (see
Kontinen 1981).
UNACCEPTED MANUSCRIPT FOR MY DOCTOR THESIS 14 (22)
Fig. 14. Relict cinder cone and cinderites at Sattasvaara. The remnant hill may have remained standing
because of the presence of welded pyroclastics: unwelded ejecta were eroded in the surroundings. The
flow direction of two lavas (see arrows) radiates from the hill. Completed after Saverikko (1985).
Fig. 15. Vitric ash particles, clear in plane-polarized light, from the Sattasvaara (at left) and Kummitsoiva
(at right) complexes (Saverikko 1985, 1983). At left: tiny broken bubbles in the glass shards exhibit bub-
blewall 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, p.234)
UNACCEPTED MANUSCRIPT FOR MY DOCTOR THESIS 15 (22)
Fig. 16. A block-lava flow resembling recent ones
near Sattasvaara has retained its original posi-
tion (Saverikko 1985). The flow morphology is
exposed and so the flow appears to have been
directed from the vent at Sattasvaara (see the
right-hand arrow in Fig. 14). The block lavas have
been quite exceptional among the komatiite flows
or the Archaean lava piles in general.
1.4 Magmatic differentiation
The principal complex (at Sattasvaara) in the ko-
matiite centre forms a gradual series including all
three komatiitic rock species. Although trace-element
or REE data were not available (in northern Finland),
main-element geochemistry and petrographic charac-
teristics (Saverikko 1983; 1985; 1987) suggest that
the komatiitic members constitute a comagmatic
suite as the ultramafic komatiites do at Karasjok in
Norway (Henriksen 1983). Their volcanic layering
(Fig. 17) may be relative to that of some completely
pyroclastic deposits showing systematic chemo-
petrological changes due to compositional zoning of
magma chambers; the felsic ash-flow sheets of
caldera-forming eruptions from the same magmatic
system commonly become more mafic with time,
because near-surface magma chambers tend to be
compositionally zoned and become more mafic with
depth (Smith 1979).
Compositional and thermal gradients existed in
the silicic liquid before phenocryst segregation and
developed largely independently of crystal–liquid
equilibria: the main part of the zonal melt structure is
due to thermo-gravitational convection–diffusion
processes without the marked contribution of crystal
fractionation or contamination (Hildreth 1979). On
the contrary, Henriksen (1983) proved (at Karasjok
in Norway) that fractionation of olivine and chromite
or Cr-spinel was the main factor in the magmatic
differentiation of the ultramafic komatiites. Also,
komatiitic basalts may have originated from the
komatiite parent magma by crystal fractionation and
crustal assimilation (Cattell 1987): the roughly 12%
contamination required may have been obtainable
during ascent of the magma body through a thick
continental crust (Cattell 1987), because even ko-
matiite lavas are prone to significant (up to 10%)
contamination during flow, irrespective of the flow
rate (Huppert et al. 1984). Even the lower Lapponian
komatiites furnish evidence for strong crustal con-
tamination (Räsänen et al. 1989).
Fig. 17. Volcanic successions of the upper Lap-
ponian komatiites intercorrelated with the aid of
their stratigraphic position over the graphitic slate
zone (Saverikko 1990, 1987). The Sotkaselkä and
Sattasvaara komatiite complexes are included in
the komatiite center.
Owing to the high position of the magma body in
the komatiite centre, the confining pressure was low
enough for gravitational differentiation and concen-
tration of volatiles at the top of the chamber (Fig.
18). The internal layering of the magma body
evolved before, and during, komatiitic eruptions as is
further evidenced by
(1) the initial (peridotitic komatiite) flows, i.e.
Moskuvaara ultramafics which constituted
the crystal mush,
(2) their alternating cumulate flows with clearly
different olivine contents in the form of ser-
pentinitic-peridotitic flow piles (13. excur-
sion site in Appendix),
(3) an interlayer of pyroxene peridotitic ko-
matiite within the komatiitic basalt succes-
sion,
(4) crystal ash of pyroxene peridotitic komatiite,
UNACCEPTED MANUSCRIPT FOR MY DOCTOR THESIS 16 (22)
(5) coarse-grained lithosomes in the lavas of py roxene peridotitic komatiite (Kallio et al.
1980), and
(6) distinctly high-magnesian composition of the
terminal flows and plugs of peridotitic ko-
matiite cumulates known as the Nuttio ul-
tramafics. The initial cumulates of peridotitic
komatiite (Moskuvaara ultramafics) derive
from a wehrlite–peridotite magma layer de-
pleted in sulphides whereas the Nuttio ul-
tramafics indicate a dunite magma layer with
UNACCEPTED MANUSCRIPT FOR MY DOCTOR THESIS 17 (22)
or without sulphide droplets (see Papunen et
al. 1977), from which the only(?)
known chro-
mite(-magnetite) showings (Paakkola and
Lanne 1979) may originate. Several lapilli of
(secondary?) magnetite or haematite-magnet-
ite are generated in preceding final eruptions
of the pyroxene peridotitic komatiites (Save-
rikko 1985).
Fig. 18. Petrographic characteristics of the upper Lapponian komatiites and their relation to eruption types,
physical properties of the magma, and magmatic layering in the magma chamber. The komatiite complexes
(see Fig. 17) are thought to be generated from the same magma system. Compiled after Saverikko (1983,
1985).
1.5 Eruption mechanism
An ultramafic magma is very dense, 2.7-2.9
g/cm3 (Nisbet 1982), and its density contrast to
granitic rocks hampers its ascent through a thick
sialic crust. Thermo-dynamic lifting capacity may
have lost its importance when the thermal instability
was reduced by the heating effect of mantle plumes
within the thick crust and by the cooling of the
ultramafic magma. Intrusion and extrusion of the
komatiitic magma in the continent may have been
promoted mainly by an extensive mantle upwelling
and an extensional tectonic regime, both of which are
stated above.
In the Fennoscandian Shield, the Saamian craton
underwent a high-metamorphic period due to mantle
activities at 3.0-2.65 Ga (Lobach-Zhuchenko et al.
1986, Paavola 1986; 1988) and the coeval crustal
UNACCEPTED MANUSCRIPT FOR MY DOCTOR THESIS 18 (22)
opening remained at the embryonic stage of the
Wilson cycle (Saverikko 1990), which were associ-
ated at 2.7-2.6 Ga with a pronounced rotation of the
Shield (Mertanen et al. 1989).
Emplacement close to the Earth's surface is sug-
gested by magmatic differentiation accompanied by
the concentration of volatiles in the products of
earlier eruptions (komatiitic basalts), that is, in the
upper part of the magma chamber, and by the contin-
ued cooling, which increased the viscosity of the
residual magma (ultramafic komatiites) extruded
later. This was reflected in eruption mechanisms.
With the exception of the initial flood eruptions of
peridotitic-komatiite crystal mush, the magma poured
out in Hawaiian to Strombolian-type eruptions along
with increasing ultrabasicity related to the spreading
of komatiitic volcanism; the terminal flows were
again crystal mush of peridotitic komatiite (Saverik-
ko 1985) but with a distinctly high-magnesian com-
position.
Amygdules and scorias (cinders) in the komatiitic
basalt and pyroxene peridotitic komatiite provide
evidence for a wet ultramafic magma. In general,
vesicles were very rare in the komatiitic lavas (see
Arndt et al. 1979) – the water is only slightly re-
leased in the vesicles in the high-temperature lavas
(Williams and Mc Birney 1979, p.125). However,
eruptions after prolonged quiescence usually produce
gas-rich magma in their opening stage and thereafter
the volatile content declines and viscosity increases
as deeper parts of the magma chamber are tapped; it
is uncertain whether the volatiles concentrated in the
upper part of the reservoir are from deeper horizons
or from adjacent sources of meteoric water, but the
degree of enrichment is a function of time (Williams
and McBirney 1979, p.75).
Water solubility diminishes with increasing ba-
sicity of a magma but the silica content is less impor-
tant than pressure and temperature (see Williams and
McBirney 1979, pp.31-32). The most general mecha-
nism of gas concentration may be decompression
during nearly isothermal rise of the magma (Fisher
and Schmincke 1984, p.50). Also, the crystallization
of anhydrous minerals enriches volatiles to concen-
trate in the melt (Burnham 1979). These both may
have been potential agents in the present instance.
The Hawaiian-type eruption is characterized by
discharge of fluid (basaltic) lava. The pahoehoe
flows of komatiitic basalt contain amygdules (Ø 2-
0.1 cm) of the static-vesicles and flow-vesicles of
Whitford-Stark (1973) as further proof of low viscos-
ity; some lava flows also have a frothy surface
(Saverikko 1985). The Strombolian-type eruption
usually involves semifluid magma. The fragmentary
flows of pyroxene peridotitic komatiite are weakly
vesiculated with a few amygdules (Ø 0.1-2 cm)
(Saverikko 1983; 1985; 5. excursion site in Appen-
dix), which may be attributed to the initially low
content of volatiles in these lavas, since the discharge
was usually followed by enormous explosions that
did not liberate gases until later (Saverikko 1985).
Explosions of Strombolian and Hawaiian-type
eruptions are caused by rapidly expanding gases
(Fisher and Schmincke 1984, p.97). The inapprecia-
ble volcaniclastic breccias of komatiitic basalt appear
to have been generated by phreato-magmatic shatter-
ing, whereas the prevalent pyroxene peridotitic
komatiite ejecta of block (bomb) to ash-size range
were products of magmatic explosions (Saverikko
1985, Saverikko et al. 1985).
The growth of gas bubbles is mainly controlled by
diffusion in slowly rising magmas and by decompres-
sion in rapidly rising ones; decompressional growth
becomes increasingly important at shallow depths
(Sparks 1978). The more rapid the rise the smaller
the final bubble dimensions; these are not affected by
the depth of starting vesiculation and evolve mostly
well before ejection: diameters of 5-0.1 cm are
typical of basaltic explosive eruptions (Sparks 1978).
The appearance of amygdules (Ø 2-0.1 cm) in the ko-
matiites, which usually were vesicle-free (see Arndt
et al. 1979), may indicate a slow rise of the ultra-
mafic magma.
The residual pressures in the gas bubbles, which
overcome those in the liquid, lead to a volcanic
explosion when the bubbles burst (Fisher and
Schmincke 1984, pp.56-57). Because only a fraction
of the water content is released in the vesicles in
basic lavas at high temperature (Williams and
McBirney 1979, p.125), the gas pressure may not
have been critical in the komatiitic basalt; Nisbet
(1982) estimates their temperature to have been up to
1 360 oC at 1 atm. Nevertheless the residual pres-
sures are very high in the highly viscous magmas (as
in the pyroxene peridotitic komatiite), in which the
viscous pressure is the major factor resisting expan-
sion of the bubbles when the gas pressure builds up
rapidly in a magmatic froth (Sparks 1978). The
bursting of the bubbles due to their internal residual
pressure disrupts the magma froth and proceeds to
ejection (Sparks 1978).
The main factors regulating the komatiitic erup-
tion mechanism may have been increased viscosity
related to advanced cooling and decompression in the
reservoir as part of the differentiated magma dis-
charged. Opening of feeders down to the different
magma layers (Fig. 19) may have resulted in the
distinct successions of the komatiite complexes.
UNACCEPTED MANUSCRIPT FOR MY DOCTOR THESIS 19 (22)
Summary
The reverse komatiitic layering of the upper Lap-
ponian, with upwards increasing ultrabasicity, corre-
lates well with the Lapponian supracrustal sequence,
whose stratigraphic order is the opposite of that of
the classic Archaean greenstone-belt sequence. The
lithology and tectonic framework of the Lapland
greenstone belt imply advanced mantle upwelling in
the Saamian continent, but the upper Lapponian
komatiite succession ascended from a magma reser-
voir which was separated from a mantle diapir and
was emplaced at a shallow level of the sialic crust.
The mantle diapir in the Fennoscandian Shield is
exposed in the large Solovetski mantle plume (in
Fig. 19. Idealized volcanic feeder pattern and its
inferred relation to the differentiated magma res-
ervoir to explain different volcanic successions of
the komatiite complexes.
Russia) and a shield-wide(?)
linear zone of the pyro-
clastic komatiites. The pyroclastics are peculiar to
the differentiate of pyroxene peridotitic komatiite,
that is, the non-cumulate ultramafic komatiite. The
required viscosity appears to have been caused
primarily by the "low" eruption temperature, which
may have resulted from loss of heat in the mantle-
derived magma body during its increasingly slow
ascent through the thick continental crust. This
produced glass in significant quantities controlled by
the cooling rate of supercooled liquid incapable of
developing spinifex textures.
The halt of the magma body at a shallow level in
the crust permitted zonal melt fractionation, gravita-
tional differentiation, infracrustal quenching to form
glass, and concentration of volatiles at the top of the
chamber. The magmatic differentiation (associated
with assimilation?) may have reached a climax
before the discharge and thus the ultramafic parent
magma produced distinct cumulate (>30 wt% MgO,
anhydrous basis) and non-cumulate (30-18 wt%
MgO, anhydrous basis) ultramafic komatiites, and
komatiitic basalts (18-9 wt% MgO, anhydrous basis).
The compositional zonation in the magma body
displays zonal melt structures like those of a static
reservoir. Their development refers to stable-conti-
nental conditions prevailing in that time; crustal con-
vulsions such as a continent–continent collision
would have destroyed the magmatic layering. Ascent
of the very dense ultramafic magma may have called
for an upthrust from the mantle itself during an
extensional tectonic regime. A mantle diapir as
extensive as in this instance may be attributed to
(early) Precambrian mantle convection (Saverikko, in
review).
Consequently, as the high-magnesian magmas,
whose fluidity requires extra-high temperature,
appear to have cooled down and developed a unique
viscosity during their rise through a (thick) continen-
tal crust,
their explosive eruptions of magmatic origin were
special features of the cratonic palaeoenvironment in
the (Early) Precambrian.
ACKNOWLEDGEMENTS. - The finishing work was
aided financially by a grant from the Outokumpu
Oy Foundation. Mrs. Gillian Häkli and Mrs.
Kathleen Ahonen helped me to correct the Eng-
lish of the manuscripts during 1983-1991. I
express my sincere gratitude to them. My very
special thanks go to my mother for her support
during these long and uneasy years.
UNACCEPTED MANUSCRIPT FOR MY DOCTOR THESIS 20 (22)
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