felsic volcanics in the course of ore formation. In the Kuhmo belt Fig. 1, quartz arenites with
2.8 – 3.0 Ga detrital zircons Hyppo¨nen, 1983 were described from the Hietapera-Kivivaara area
Piirainnen, 1988, where they form part of a felsic volcanic-sedimentary unit in the Juurikkaniemi
Formation
which consists
of metarhyolites,
metadacites, volcanic breccia, lapilli tuffs, tuffites and tuffaceous turbidites. The age of this unit is
estimated at 2798 9 15 Ma Hyppo¨nen, 1983. Northwards, in the Moisiovaara area Fig. 1,
immature sericitic quartz arenites associated with polymictic conglomerates lie between tholeiites
and komatiites. An old tonalite – trondhjemite – granodiorite complex and felsic rocks from the
Kuhmo belt are regarded as sources of 2996 9 60 and 2803 9 238 Ma detrital zircons Hyppo¨nen,
1983 as well as clasts in congomerates Lu- ukkonen, 1988. In the Hisovaara greenstone belt
Fig. 2, quartz arenites are described in associa- tion with underlying andesites as well as overlying
felsic
volcanics and
sedimentary rocks
Kozhevnikov, 1992; Kozhevnikov et al., 1992. According to Thurston 1990, so far no andesites
have been found in platform assemblages. When distinguishing the types of assemblages most re-
cently proposed for Archean greenstone belts, the andesite-quartz arenite association revealed in the
Hisovaara greenstone belt is considered a fairly rare type of assemblage with a ‘continental’ style
of volcanism whose depositional environment is ‘open to speculation’ Thurston, 1994. However,
knowledge of such an assemblage type may be useful in discussing models for the tectonic evolu-
tion of greenstone belts and in comparative analy- sis and correlation of Archean cratons.
2. Geological setting
2
.
1
. Regional setting The Hisovaara greenstone structure is a frag-
ment of
the Archean
Parandovo-Tikshozero greenstone belt which extends for 300 km along
the Belomoride-Karelide boundary, i.e. along the
Fig. 1. Map showing Archean greenstone belts in the Fenno-Karelian craton after Rybakov and Kulikov, 1985. Revised after: Lobach-Zhuchenko, 1988; Kozhevnikov, 1992; Glebovitsky, 1993. Numbered localities represent occurrences of Archean quartz-
rich metasediments.
Fig. 2. Geological map of the Archean Hisovaara greenstone belt.
boundary between the Karelian granite-green- stone province and the Belomorian collision zone
Glebovitsky, 1973; Volodichev, 1990; Glebovit- sky, 1993; Slabunov, 1993; Lobach-Zhuchenko et
al., 1995; Glebovitsky et al., 1996 Fig. 1.
2
.
2
. Structural geology and metamorphism The Hisovaara greenstone structure is a syn-
form thrown into composite folds, composed largely of supracrustal rocks and surrounded by
crosscutting granitoids Fig. 2. It displays evi- dence for multiple folding events Systra and Sko-
rnyakova,
1986; Shchiptsov
et al.,
1988 subsequently generalized into three deformation
stages Kozhevnikov, 1992. The rocks have suf- fered polymetamorphism of Archean and Sve-
cofennian 1.8 – 2.0 Ga ages, the latter taking place in a high pressure regime with the following
parameters: T = 580 – 640°C, P = 6.5 – 7.5 kbar Glebovitsky and Bushmin, 1983. Archean hy-
drothermal processes associated with felsic mag- matism Kozhevnikov, 1992, 1995 are apparent
together with Svecofennian metasomatic rocks represented by retrograde metamorphism T =
300 – 350°C Bushmin, 1978; Glebovitsky and Bushmin, 1983. The complicated tectonic and
metamorphic history of the Hisovaara greenstone belt is largely due to its proximity to the 2.70 –
2.68 Ga Belomorian collision zone Lobach- Zhuchenko et al., 1995 and later Proterozoic
events 1.95 – 1.75 Ga Bibikova, 1995.
2
.
3
. Lithologic assemblages Several major assemblages of supracrustal
rocks are distinguished in the Hisovaara green- stone belt Kozhevnikov, 1992. The northern and
southern flanks of the synform differ substantially in character, largely as a function of lateral facies
variation, and the types of crosscutting intrusive rocks etc. Table 1. Because the andesite-quartz
arenite association is revealed only on the north- ern flank of the structure, only units on the north
flank are described below in detail. The unit ter- minology of Kozhevnikov 1992 is retained in
subsequent sections of this paper for ease of ac- cess to the Russian literature.
2
.
3
.
1
. Lower mafic assemblage The lower essentially volcanic assemblage is
comprised from the base upwards of cumulate peridotitic komatiites, tholeiitic massive and rarer
pillow basalts, basaltic to pyroxenitic komatiites and ferrobasalts. This sequence is characterized
by thick up to 10 m massive and fairly uniform flows with scarce thin-bedded tuff horizons, the
absence of interflow sediments, and amygdaloidal textures all indicative of a mafic plateau type of
volcanic setting Thurston, 1994. The U-Pb zir- con age of felsic dykes cutting this assemblage was
estimated by O.A. Levchenkov to be 2803 9 35 Ma Kozhevnikov, 1992.
2
.
3
.
2
. Second 6olcanic-sedimentary assemblage The second assemblage consists of volcanics,
volcano-sedimentary, and chemical sedimentary rocks of intermediate to felsic composition. Its
lowermost unit is comprised of calcalkaline andes- ites. They are overlain by quartz arenites, and at
point B Fig. 2 there is an alternation of andesite and quartz arenite. Resting on the quartz arenites
is a thick sequence of felsic rocks including lavas, ash-flows, tuffaceous turbidites and chemical sedi-
ments with a clastic component that show compli- cated lateral relationships. Intense metasomatic
and deformational processes strongly distort and sometimes obliterate the primary textures and
compositions of these strata making interpreta- tion difficult. Where these processes are least in-
tense, there are some indications of graded ash flows and pyroclastic breccias as well as flows
with massive and flow top breccia textures. The volcano-sedimentary rocks have some features in-
dicative of graded rhyolitic turbidites with alu- mina-enriched
upper parts.
Transitional clastic-chemical sedimentary rocks represented by
carbonaceous schists sulfidic argillites, alumino- silicates and cherty rocks occur as thin horizons
and lenses
among felsic
volcanosedimentary rocks. The uppermost 100 m of this sequence
consists of thin, graded carbonaceous and carbon- ate-bearing silty sandstones.
2
.
3
.
3
. Third rudaceous assemblage Coarse clastic rocks dominate a third assem-
blage forming two wide zones in the centre of the
P .C
. Thurston
, V
.N .
Kozhe 6
niko 6
Precambrian
Research
101 2000
313 –
340
Table 1 Some characteristics of associations at the northern and southern flanks of the Hisovaara greenstone belt that illustrate its asymmetric geological structure
Assemblages Southern flank
Northern flank Intensely foliated tholeiitic basalts and minor tuffs. Thickness50.15 km.
Komatiitic, tholeiitic and ferrotholeiitic lavas and scarce Lower Mafic assemblage
tuffs. Thickness 0.5–1.7 km. Andesitic lavas and pyroclastic flows with indications of
Second Predominantly bedded andesite tuffs. Thickness 0–300 m. Quartz-rich
volcanic–sedimentary sediments not found. Felsic volcanics not characteristic. Coarse-bedded
subaerial–subaqueous volcanism. Thickness 100–700 m. assemblage
tuffs with oxide- and silicate-facies BIF horizons. Carbonaceous schists Quartz-rich arenite horizons with hummocky and trough
cross-bedding. Thickness+n–40 m. Rhyolitic lava and not found. Individual beds and units laterally persistent in thickness,
graded bedded ashflows, graded rhyolitic turbidites. bedding parallel in plan view, no indications of cross-bedding and the
Carbon-bearing sulfidic argillites, conglomerates, absence of sharp lateral transitions are characteristic.
alumino-silicate and siliceous rocks closely associated with felsic volcanics. Thickness+0–100 m. Graded
thin-laminated carbon- and carbonate-bearing siltstones. Variable thickness of individual beds and units as well
as complex lateral transitions are characteristic of the association. Indications of cross-bedding are common in
the quartz arenites.
Third Rudaceous Polymictic conglomerates with dominantly felsic volcanic pebbles of the
Thick oligomictic conglomerate ? or volcaniclastic rock belt.
assemblage units with tuffmatrix ? in the east closely associated
with felsic lava breccia. Massive and pillowed tholeiitic basalts with thin
Upper Mafic assemblage Massive and pillowed tholeiitic basalts.
komatiite horizons ? at the base. Locally microclinized tonalites rimming the structure in
Intrusive rocks Plagiomicrocline and garnet-muscovitic microcline granites rimming the
structure in the south. Scarce rhyolite dykes. Andesite-basalt–dacite sills the north. Rhyodacite-rhyolite dykes and stocks.
not found. Gabbro, gabbro-pyroxenite, granodiorite and komatiite sills Andesite-basalt–andesite-dacite sills. Gabbro,
and dykes. gabbro-diorite and komatiite sills and dykes.
Fig. 3. Fragmental andesite with 15 – 30 cm fragments of massive andesite with fragments of uniform mineralogic com-
position but varying in colour from grey to green. North shore of Lake Verkhnee; hammer 35 cm long.
tion pattern with the sills and dikes concentrated on the northeast side of the belt Fig. 2, Table 1.
As the goal of the present paper is to character- ize and interpret a quartz arenite – andesite associ-
ation, uncommon for Archean greenstone belts, these units are described in more detail below.
3. Quartz arenite-bearing assemblage
