December 2012 LIP of the Month

The ~2.44 Ga LIP in the Fennoscandian shield

Laura S. Lauri1, Perttu Mikkola2 and Tuomo Karinen3

1Geological Survey of Finland, P.O. Box 77, FI-96101Rovaniemi, Finland,

2Geological Survey of Finland, P.O.Box 1237, FI-70211 Kuopio, Finland

3Mustavaaran Kaivos Oy, Kaarnatie 36, FI-90530 Oulu, Finland


The nucleus of the Fennoscandian shield formed during the Archean Eon between 3.6 Ga and 2.7 Ga (Hölttä et al., 2008 and references therein). By the end of the Archean the continental crustal fragment comprising the present Karelian and Belomorian domains and most probably also the Kola domain had formed a stable craton that may have been joined to the Archean cratonic domains of the present Canadian shield.

At the Archean–Proterozoic transition the Fennoscandian craton experienced two distinct periods of extension during which voluminous amounts of mafic magma intruded the crust, forming mafic layered intrusions (Fig. 1). The first Paleoproterozoic magmatic event in the Fennoscandian shield is seen only in the Kola Domain, where ca. 2.5 Ga mafic layered intrusions are found in the central and northern part of the Kola Peninsula (Amelin et al., 1995). If the age for the first event is correct it is coeval with the Mistassini dykes of the Superior craton and dykes of the North Atlantic craton (e.g., Ernst and Bleeker, 2010; Nilsson et al. 2010, and references therein). The younger, shield-wide magmatic event took place at ca. 2.44 Ga, forming a large igneous province with both mafic and silicic intrusions, diabase dykes and coeval volcanic rocks (Lauri et al., 2012a and references therein). The younger magmatic phase in Fennoscandia is coeval with other large igneous provinces in other cratons, such as the ~2.48-2.45 Ga Hearst-Matachewan dykes and the East Bull Lake intrusion of the southern Superior craton (Heaman, 1997; Corfu and Easton, 2000; Buchan and Ernst, 2004; Easton et al., 2010; Ernst and Bleeker, 2010), and the 2.45 Ga Weeli Wolli- Woongarra event of the Pilbara craton (Barley et al. 1997); and is slightly older than the 2.42 Ga Scourie dykes of Scotland (Heaman and Tarney, 1989).

Figure 1 Simplified geologic map of the northern part of the Fennoscandian shield (modified from Lauri et al., 2012a). 2.45-2.39 Ga A-type intrusions: 1 - Kynsijärvi, 2 - Nuorunen, 3 - Pussisvaara, 4 - Rasinkylä, 5 - Ryysyranta, 6 - Tuliniemet, 7 - Rasimäki.

The ~2.44 Ga LIP

The global magmatic event at ~2.44 Ga is perhaps best documented in the Fennoscandian shield, which hosts numerous A-type granite plutons coeval with the more voluminous mafic layered intrusions, diabase dykes and associated volcanic rocks with early Paleoproterozoic ages (2.50 – 2.39 Ga). The following descriptions are largely based on the review of Lauri et al. (2012a) and references therein.

Silicic intrusions

At least seven granitic and syenitic intrusions and numerous silicic dykes with ages between 2.45–2.39 Ga (Luukkonen, 1988; Buiko et al., 1995; Vaasjoki et al., 1999; Lauri and Mänttäri, 2002; Mikkola et al., 2010) are known in the Karelian province of the Fennoscandian shield. Only two of the intrusions, the syenitic Kynsijärvi in Finland and the granitic Nuorunen in Russia, are situated in the immediate vicinity of mafic layered complexes (Fig. 1). Near the other granitic plutons further south the ~2.44 Ga mafic magmatism is present only as diabase dykes (Vuollo and Huhma, 2005 and references therein).

The ~2.44 Ga silicic intrusions are small plutons that sharply cross-cut their Archean country rocks (Fig. 2). The largest known intrusion of this type in Finland is the Tuliniemet pluton in Kuhmo that is almost 10 km in diameter (Luukkonen, 1988), all other intrusions are smaller. According to Luukkonen (1988), the largest Tuliniemet intrusion shows rapakivi texture in the central parts. The smaller intrusions are commonly porphyritic in texture. Most of the ~2.44 Ga intrusions are undeformed. Only the westernmost intrusions (Rasinkylä, Pussisvaara and Rasimäki) have been somewhat deformed during the Svecofennian orogeny at 1.89-1.87 Ga (Mikkola et al., 2010).

Figure 2 Lithologic maps of some of the 2.44 Ga felsic plutons in the Fennoscandian shield. (a) lithologic map and (b) aeromagnetic grayscale image of the Kynsijärvi syenite, (c) Pussisvaara, Rasinkylä and Ryysyranta plutons (Mikkola et al., 2010), (d) Tuliniemet intrusions (Luukkonen, 1988). Coordinates in (a), (b) and (d) in Finnish KKJ.

The ~2.44 Ga granitic intrusions are nearly devoid of xenoliths and inherited zircons. The main minerals are porphyritic K-feldspar, quartz, plagioclase, and biotite. Typical accessory minerals include zircon, apatite, allanite, and fluorite. The intrusions may be divided into magnetic (Nuorunen, Pussisvaara, Ryysyranta, Rasinmäki and the Kynsijärvi syenite) and non-magnetic (Tuliniemet, Rasinkylä) groups; the magnetic intrusions contain accessory magnetite (Mikkola et al., 2010). The Kynsijärvi syenite differs from other intrusions in containing mesoperthitic alkali feldspar, quartz, and ferro-edenitic amphibole as the major minerals (Lauri and Mänttäri, 2002). Secondary stilpnomelane is also present in Kynsijärvi, whereas the most common secondary minerals in the granitic intrusions are muscovite and chlorite. The granite porphyry dykes have euhedral phenocrysts of K-feldspar (microcline), plagioclase (oligoclase), and blue quartz in a fine-grained groundmass (Luukkonen, 1988).

Geochemically the ~2.44 Ga silicic intrusions are silica-rich (> 70 wt.%), high-Fe# (0.76–0.96) rocks that are weakly peraluminous to metaluminous. They have high contents of Zr, Ga, Nb, and REE except Eu (Fig. 3). The intrusions are classified as within plate granites in the diagram of Pearce et al. (1984) and A-type in the diagrams of Eby (1992) and Dall’Agnol and Oliveira (2007). All ~2.44 Ga silicic intrusions have initial Nd isotope compositions that suggest their being derived from local Archean crust (Fig. 4).

Figure 3 The ~2.44 Ga felsic intrusions classified in the diagrams of (A) Pearce et al. (1984), ORG - ocean ridge granites, VAG - volcanic arc granites, syn-COLG - syn-collisional granites, WPG - within plate granites, (B) Eby (1992), and (C) Dall'Agnol and Oliveira (2007). (D) K/Na ratio, (E) Chondrite-normalized REE diagram, and (F) Primitive mantle normalized spider diagram of the ~2.44 Ga felsic intrusions. Normalizing values in (E) from Taylor and McLennan (1985) and (F) from Sun and McDonough (1989). Data on Bushveld granites from Kleemann and Twist (1989) in A-D and Hill et al. (1996) in E-F. Modified from Lauri et al. (2012a).

Figure 4 Initial epsilon-Nd vs. age diagram of the ~2.44 Ga felsic and mafic intrusions. Modified from Lauri et al. (2012a).

Mafic intrusions

Mafic layered intrusions of ~2.44 Ga age are found in northern Finland, Russian Karelia and Kola Peninsula (Fig. 1). Most of the intrusions were emplaced along the contact between the Archean basement and the overlying supracrustal rocks that seem to be of nearly similar age as the mafic layered complexes (see Lauri et al., 2012a and references therein). The roof contacts of some intrusions have been eroded and many of the intrusions have been fragmented into separate blocks by tectonic movements.

The mafic layered intrusions of ~2.44 Ga age may be structurally divided into the marginal series and the layered series, which may show a discordant contact with each other. Some of the intrusions seem to have crystallized from a single batch of magma whereas others show multiple injections of primitive magma, which Alapieti and Lahtinen (2002) called “megacyclic units”. The megacyclic units are seen as the reappearance of ultramafic cumulates within the more evolved rock types in the crystallization sequence.

The parental magma compositions of the mafic layered intrusions may be approximated from the chilled margins (Table 1) or associated diabase dykes of similar age (Karinen, 2010). For the ~2.44 Ga intrusions of the Fennoscandian shield, two compositionally different parental magma types have been proposed (Lahtinen et al., 1989; Alapieti et al., 1990; Saini-Eidukat et al., 1997; Vogel et al., 1998); the first type is characterized by relatively high MgO and Cr content, intermediate SiO2 and low TiO2 and referred to as boninitic. The second, tholeiitic type, has low MgO and Cr and intermediate SiO2 contents. Nd isotope composition of the mafic layered intrusions has been constrained to slightly below chondritic values at ~2.44 Ga (Fig. 4; Lauri et al., 2012a and references therein).

Table 1. Parental magma compositions of the ~2.44 Ga mafic layered intrusions in Finland.













































































































1 – average Penikat MCU1 (Penikat intrusion; Alapieti et al., 1990), 2 – average Narkaus MCU1 (Portimo complex; Alapieti et al., 1990), 3 – lower chilled margin, sample 259-TTK-00 (Koillismaa intrusion, Porttivaara block; Karinen, 2010), 4 – lower chilled margin, sample U0144l-72 (Koillismaa intrusion, Kuusijärvi block; Juopperi, 1976), 5 – average Penikat MCU4 (Penikat intrusion; Alapieti et al., 1990).

The ~2.44 Ga diabase dykes have been divided into five subgroups with based on their composition and the trends of the dykes (Vuollo and Huhma, 2005 and references therein). Diabase dykes of ~2.44 Ga age are widespread all over the Archean Fennoscandia. They have been studied especially for the paleomagnetic reconstructions of the Fennoscandian shield (Mertanen et al., 1999, 2006).

Volcanic rocks

Volcanic rocks with ~2.44 Ga ages are widespread all over the northern part of the Fennoscandian shield, following the areal distribution of the mafic layered intrusions. In Finland the earliest Paleoproterozoic volcanic rocks are grouped into the Salla group (Lehtonen et al., 1998), which consists of intermediate and silicic volcanic rocks that have been extruded on top of the Archean gneissic basement. The type locality for the Salla group is in the Salla area in eastern Lapland, where they form a sequence of well-preserved metavolcanic rocks (Manninen, 1991). The hanging-wall volcanic rocks of the Koillismaa, Akanvaara and Koitelainen mafic layered intrusions have been correlated with the Salla group and the age of the volcanic phase has been constrained to 2.49-2.44 Ga based on several age determinations (Manninen et al., 2001; Räsänen and Huhma, 2001; Lauri et al., 2003, 2012b). Some ~2.44 Ga volcanic rocks are also found in Russia associated with the Oulanka and Imandra layered intrusions (Amelin et al., 1995).

In the Koillismaa area the volcanic rocks of the Salla group form the roof of the mafic layered intrusion (Lauri et al., 2003). The stratigraphic sequence in the Koillismaa area starts with a thick, possibly pyroclastic rhyodacite unit overlain by less voluminous rhyolitic and andesitic lavas. Contrary to the coeval mafic and silicic intrusions the ~2.44 Ga volcanic rocks are not strictly bimodal in composition, instead they form a series of intermediate and silicic rocks ranging from basaltic andesite to rhyolite (Fig. 5). The rhyodacite in Koillismaa (Sirniövaara fm in Fig. 5) shows some A-type geochemical features such as high total alkali content, Fe/Mg, Ga/Al, Zr, REE, and a negative Eu anomaly (Lauri et al., 2003). Initial Nd isotope composition of the volcanic rocks in Koillismaa is close to the values measured from the layered intrusions, with initial εNd values at 2440 Ma ranging between -1.1 and -2.5 (Lauri et al., 2006).

Figure 5 Volcanic rocks of the Salla group in Koillismaa classified in the diagram of Winchester and Floyd (1977). Light blue field: Salla group rocks in the Salla greenstone belt (Manninen, 1991) Modified from Lauri et al. (2003).


The current interpretation of the tectonic evolution of the Fennoscandian shield during the Archean-Proterozoic transition is that the earliest Paleoproterozoic extensional event was first seen as volcanic eruptions that were followed by the emplacement of the mafic layered complexes between the Archean gneisses and the overlying volcanic rocks. The rifting does not seem to have proceeded to actual seafloor spreading stage at this time.


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