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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]


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).


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).


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


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


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-


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).


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.


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-


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.


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).


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!].


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).


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)


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,


(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


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


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.


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.


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