Avance del proyecto según la GCS

Mafic dyke swarms preserved on the Karelia craton are poorly defined. The age spectrum of 2.4-2.5 Ga events recorded on the craton is in the process of being evaluated (Kulikov et al. 2010). Preliminary analysis of the ages of Palaeoproterozoic mafic dykes preserved on the Karelia craton indicates that the craton experienced at least 3-4 mafic igneous events between 2.4-2.5 Ga which broadly overlap the ages of mafic dykes preserved on the other cratons studied in this work. Unlike the events on other cratons, very few attempts have been made to delineate the dated Karelia dykes into separate swarms which share a consistent trend, appearance, mineralogy and chemistry.

One of the few Palaeoproterozoic swarms to be identified is the Viianki dyke swarm (Fig. 2.26), which has been identified as the likely feeder to the ~2.44 Ga layered mafic intrusions on the craton (Vogel et al. 1998b). Vogel et al. (1998b) characterised the Viianki swarm as a northwest trending swarm of tholeiitic basalts and andesites. The Viianki dykes are likely to be equivalent to the Karelia dykes described by Mertanen et al. (1999) based on the similar ages, trends and geographic

48 locations of dykes reported by the two sets of workers. Mertanen et al. (1999) describes the Karelia dykes as northwest trending and subvertical, ranging in thickness from 6 cm - 200 m with compositions which range from Fe-tholeiitic and tholeiitic to calc-alkaline. Neither of the two studies describe the mineralogy of the dykes in detail, but Mertanen et al. (1999) notes that the dykes are generally unaltered and fresh.

While the work of Vogel et al. (1998b) and Mertanen et al. (1999) represent two of the very few attempts at identifying unique swarms on the Karelia craton, the existence of other Palaeoproterozoic dyke swarms on the craton is extremely likely.

U-Pb and other dating methods have shown that mafic dykes were intruded into the Karelia craton throughout much of the Proterozoic and doubtless, following more work on both dated and currently undated dykes, more mafic dyke swarms will be identified. As such dating work is beyond the scope of the current study, the following discussion will primarily focus on the Karelia/Viianki dyke swarm as defined by Mertanen et al. (1999) and Vogel et al. (1998b) This swarm will henceforth be referred to as the Viianki dyke swarm, so as to avoid any confusion with other dyke swarms preserved on the Karelia craton.

2.2.7.2. Age

Vogel et al. (1998b) estimate the age of the Viianki dykes as ca. 2440 Ma. Vogel et al. (1998b) does not report a direct date for the Viianki dykes, but rather suggests that the Viianki dykes provide the most likely parental magmas for some of the Palaeoproterozoic layered intrusions on the Karelia craton including Kemi, Koitelainen, Akanvaara, Burakovsky and Tornia, which range in age from 2433±8 Ma – 2449±2 Ma (see summary in Vogel et al. (1998b) and Lauri et al. (2012)).

When describing the Karelia dykes (thought to be equivalent to the Viianki dykes), Mertanen et al. (1999) reports a U-Pb baddeleyite age of 2446±5 Ma and several Sm-Nd ages with much larger errors of between 2422±35 Ma – 2476±30 Ma.

2.2.7.3. Tectonic Setting

Vogel et al. (1998b) studied the geochemistry of the Viianki dykes and their proposed analogues on the Superior Craton and found that the two sets of magmas could be petrogenetically related to each other through different degrees of crustal

49 contamination of a common magma, or, through differing degree of partial melting of a common mantle source. Vogel et al. (1998b) also notes that the chemical signature recorded by the Viianki dykes is consistent with the magma derivation from an upper mantle source, which had experienced removal of MORB during the Archaean and subsequent enrichment by subduction fluids; and does not indicate any input of material from a mantle plume.

Fig. 2.26. Geological map showing the extent of the Viianki dyke swarm. Dyke traces are schematic after Vogel et al. (1998b) and Kulikov et al. (2010).

50 Mertanen et al. (1999) dispute earlier continental reconstructions (Heaman 1997) which place Karelia to the south of the Superior craton at ~2.45 Ga and which interpret the coeval magmatism on the two cratons as the product of a mantle plume.

Mertanen et al. (1999) instead argue that the palaeomagnetism of the Viianki dykes does not allow for the dykes to be the southward continuation of the Matachewan swarm as suggested by others (Buchan and Ernst 2004; Ernst and Buchan 2004;

Söderlund et al. 2010) as available palaeomagnetic data for the Karelia craton places the craton to the north and east of the Superior Craton at ~2.45 Ga. Mertanen et al.

(1999) argue that such a reconstruction [as permitted by palaeomagnetic data (Bates and Halls 1990)] produces a Matachewan-Viianki dyke architecture which is incompatible with the two swarms having originated from the same point-source.

Bleeker (2003) suggests that the palaeomagnetic data used by Mertanen et al. (1999) is complex and largely uncertain and that allowing the palaeomagnetic data to dictate the ~2.45 Ga Superior-Karelia reconstruction dismisses far more robust geological data. Instead, the reconstruction provided by Bleeker et al. (2008) places the Karelia craton to the south of the Superior craton and interprets the Viianki dyke swarm to be the southward extension of the Matachewan dyke swarm and the whole Viianki-Matachewan composite swarm to have been intruded during mantle-plume induced continental rifting.

2.2.7.4. Ni-Cu-PGE mineralisation

The Viianki dykes are not known to host any Ni-Cu-PGE mineralisation. However, Vogel et al. (1998b) use geochemistry to show that the Viianki dykes are the most likely candidates to have fed some of the ~2.45 Ga mafic layered intrusions on the Karelia craton which themselves are known to host significant quantities of chromite and PGE-enriched base metal sulphides (Iljina and Hanski 2005).

2.2.8. Seidorechka Formation