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Svecofennian complexes
The Palaeoproterozoic greenstones are overlain by
volcanic and sedimentary rocks comprising several
different but stratigraphically related units. Regionally,
they exhibit considerable variation in lithological
composition due to partly rapid changes from
volcanic- to sedimentary-dominated facies. Stratigraphically
lowest in the Kiruna area are rocks of
the Porphyrite Group and the Kurravaara Conglomerate.
The former represents a volcanic-dominated
unit and the latter is a mainly epiclastic unit (Offerberg
1967) deposited as one or two fan deltas
(Kumpulainen 2000). The Sammakkovaara Group
in northeastern Norrbotten comprises a mixed volcanic-epiclastic
sequence that is interpreted to be stratigraphically
equivalent to the Porphyrite Group and
the Kurravaara Conglomerate, and the Pahakurkio
Group, south of Masugnsbyn. The Muorjevaara
Group in the Gällivare area is also considered to be
equivalent to the Sammakkovaara Group in the Pajala
area and is dominated by intermediate volcaniclastic
rocks and epiclastic sediments. In the Kiruna
area, these volcanic and sedimentary units are overlain
by the Kiirunavaara Group that is followed by
the Hauki and Maattavaara quartzites constituting
the uppermost Svecofennian units in the area.
In northern Finland, pelitic rocks in the Lapland
Granulite Belt were deposited after 1.94 Ga
(Tuisku & Huhma 2006). Svecofennian units are
mainly represented by the Kumpu Group in the
CLGB (Lehtonen et al. 1998) and by the Paakkola
Group in the Peräpohja area (Perttunen & Vaasjoki
2001). The molasse-like conglomerates and quartzites
comprising the Kumpu Group were deposited
in deltaic and fluvial fan environments after 1913
Ma and before c. 1800 Ma (Rastas et al. 2001).
The Kumpu rocks apparently are equivalent to the
Hauki and Maattavaara quartzites, and Porphyrite
Group rocks and the Kurravaara Conglomerate of
the Kiruna area.
With the present knowledge of ages and petrochemistry
of the Porphyrite and Kumpu Groups,
it is possible to attribute these rocks completely to
the same event of collisional tectonics and juvenile
convergent margin magmatism. This period of convergence
was manifested by the numerous intrusions
of Jörn- (south of the craton margin) and Haparanda-
(within the craton) type calc-alkaline intrusions, as
described by Mellqvist et al. (2003). Within a few
million years, this period of convergent margin magmatism
was followed by a rapid uplift recorded in
extensive conglomeratic units, more alkaline and
terrestrial volcanism (Vargfors-Arvidsjaur Groups
south of the craton margin and the Kiirunavaara
Group within the craton) and plutonism (Gallejaur-
Arvidsjaur type south of the craton margin, Perthite
Monzonite Suite within the craton). This took place
between 1.88 and 1.86 Ga and the main volcanic episode
probably lasted less than 10 million years.
The evolution after c. 1.86 is mainly recorded
by an extensive S-type magmatism (c. 1.85 Ga
Jyryjoki, and 1.81–1.78 Ga Lina-type and the Central
Lapland Granitoid Complex) derived from anatectic
melts in the middle crust. In the western part of the
shield, extensive I- to A-type magmatism (Revsund-
Sorsele type) formed roughly N-S trending batholiths
(the Transcandinavian Igneous Belt) coeval with the
S-type magmatism. Scattered intrusions of this type
and age also occur further east (e.g. Edefors in Sweden,
Nattanen in Finland). The period from c. 1.87
to 1.80 Ga possibly also involved a shift in orogenic
vergence from NE-SW to E-W in the northern part of
the Shield as suggested by Weihed et al. (2002).
Palaeoproterozoic magmatism
Early rifting and emplacement of
layered igneous complexes
The beginning of the rifting period between 2.51 and
2.43 Ga is indicated by intrusion of numerous layered
mafic igneous complexes (Alapieti et al. 1990,
Weihed et al. 2005). Most of the intrusions are located
along the margin of the Archaean granitoid
area, either at the boundary against the Proterozoic
supracrustal sequence, totally enclosed by Archaean
granitoid, or enclosed by a Proterozoic supracrustal
sequence. Most of the intrusions are found in Wtrending
Tornio-Näränkävaara belt of layered intrusions
(Iljina & Hanski 2005). Rest of the intrusions
are found in NW Russia, central Finnish Lapland and
NW Finland. Alapieti and Lahtinen (2002) divided
the intrusions into three types, (1) ultramafic–mafic,
(2) mafic and (3) intermediate megacyclic. They also
interpret the ultramafic–mafic and the lowermost part
of the megacyclic type to have crystallised from a
similar, quite primitive magma type, which is characterised
by slightly negative initial εNd values and
relatively high MgO and Cr, intermediate SiO 2, and
low TiO 2 concentrations, resembling the boninitic
magma type. The upper parts of megacyclic type intrusions
and most mafic intrusions crystallised from
an evolved Ti-poor, Al-rich basaltic magma.
Amelin et al. (1995) suggested two age
groups for the intrusions for Fennoscandian Shield,
the first with U–Pb ages at 2.505–2.501 Ga, and the
second at 2.449–2.430 Ga. All Finnish layered intrusions
belong to the younger age group. The intrusions
were later deformed and metamorphosed during the
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