Field work 2015, the Baltic Shield

Fig. 1. A fragment of canonical map of the USSR; The Precambrian is shown in bright-red colors

For the last couple years I have been studying the Precambrian exposed within the East European craton (platform), so-called Baltic Shield (or sometimes called Fennoscandian) (Fig. 1). My current research project is related to the early Paleoproterozoic igneous activity and associated hydrothermal processes on the Baltic Shield. This summer I needed to visit several localities in the region (Fig. 2):

• The Kiy Island in the White Sea, located near the city of Onega with ~2.45 billion-year-old (hereinafter, Ga) gabbro-norite

• The 2.45 Ga Vetreny Belt rift on its southern edge, where its overlain by Phanerozoic sediments

• The Vetreny Belt at its northern inclination

• The Shueretskoe occurrence of garnet- and gedrite-bearing rocks

To visit all these localities we flew from Moscow to Arkhangelsk, drove to Onega, from Onega we took a ferry to Kiy Island and spend there a couple days. After Kiy Island, we visited Severoonezhsk and then Mt. Golec (Sumskyi Posad, Karelia) to study the Vetreny Belt rift. After we finished working on the Vetreny Belt rift, I moved north, to the Shueretskoe (station Shueretskaya) occurrence of garnet. 

Fig. 2. Generalized route taken during the field work

The Kiy Island, the White Sea

Fig. 3. The Kiysky monastery, build in 17th century

The Kiy Island is known for the orthodox cathedral that was built by patriarch Nikon, a reformer of the Russian Orthodox Church in the 17th century (Fig. 3). The island itself is composed of 2.45 Ga metamorphosed ultramafic-mafic intrusion (Fig. 4). The metamorphism occurred much later, at 1.8 Ga.

Fig. 4. Intensely metamorphosed part of the gabbro-norite complex

Fig. 5. Bright-green zoisite in metagabbro-norite 

Metamorphosed gabbro-norite is the most common variety of rock exposed here. Some parts of the complex are less metamorphosed and contain original igneous textures and minerals. Other parts were severely metamorphosed and deformed (Fig. 4). There are also zones of metasomatic rich in zoisite, garnet, kyanite and pink corundum (Fig. 5). These rocks are interesting for us because the might have formed in subglacial environments (Bindeman et al., 2014).  After two days spent on the Kiy Island, we drove a car to Severoonezhsk, a city of 5000 people, located in ~200km from Arkhanglesk. 

The Vetreny Belt rift, Akhanglesk Region

Fig. 6 Adapted from Melezhik and Hanski (2004); Igneous activity on the Baltic Shield in the early Proterozoic 

Also located on the Baltic Shield (Fig. 6), the Vetreny Belt is a notorious geological object. It is more than 250 km long and is composed of “remarkably fresh” komatiitic basalts that are ~2.45 billion years old (Puchtel et al, 1996). The early Paleoproterozoic basalts can be found untouched by metamorphic processes here, whereas the majority of the Baltic Shield was recrystallized during the Svecofennian orogeny 1.8 billion years ago. So, komatiitic basalts from the Vetreny Belt rift are unique rocks that still contain volcanic, perhaps, the oldest non-recrystallized volcanic glass in the world (Sharkov et al, 2004). It was a great opportunity to study the Vetreny Belt rift here because of several open-pit quarries located around the city of Severoonezhsk (Fig. 7,8).

Fig. 7. Unbelievably fresh-looking 2.45-billion-year-old pillow structures in a fabulous saw-cut outcrop

Fig. 8. Meanwhile, massively-structured basalts are taken here for gravel industry

After a few days spent in Severoonezhsk, we took a taxi to Plesetsk, from where we rode the train that took us to Sumskiy Posad in 10 hours. From Sumskiy Posad we had a car waiting for us, that drove us for about 10 hours to get to the next locality (Fig. 9).

The Vetreny Belt rift, Karelia

Fig. 9. UAZ-452 (also called bread loaf) is traditionally used by geologists in Russia car

The northern edge of the rift is exposed at Mt. Golets, a mountain located 250 km away from the where we were previously. The literature describes several basaltic flows here. What interesting is that Kulikov et al. (2008) describes Mt. Golets (гора Голец) and located nearby Ruiga intrusion (интрузив Руйга) to be related as a volcanic-plutonic system. Another interesting feature of local rocks is spinifex textures in komatiitic basalts. The texture is usually formed by elongated skeletal crystals of olivine. It is believed that this type of structure is formed due rapid quenching of extremely hot high-Mg basalt. Here, at the Mt. Golec, basalts spinifex textures are formed by skeletal crystals of olivine as well as clinopyroxene (Fig. 10).

Fig. 10. Spinifex texture in komatiitic basalt, the Vetreny Belt rift

After another 5 days of working here we were delivered to Belomorsk, were I took a train north, to Shueretskoe village. Unfortunately, we were misinformed and got off at the next station, Kem’ (Кемь). We decided to take a boat from Kem’ that took us right to the deposit of garnet. Karelian region is far north enough that it never gets dark in July (Fig. 11).

Fig. 11. 3AM, we unsuccessfully trying to reach the shore; The White Sea

Shueretskoe deposit of garnet, Karelia

Fig. 12. Gigantic crystals of garnet in biotite-gedrite matrix

As we moved north, we entered the Belomorian metamorphic belt. This area of the Baltic Shield was heavily metamorphosed ~1.8 Ga. The Shueretskoe deposit of garnet is notorious for having large amounts of gigantic crystals of garnet in gedrite matrix (gedrite is also not the most common mineral; Fig. 12). As described in the soviet literature (Glebovitsky and Bushmin, 1983), the Shueretskoe deposit was formed in a metasomatic way, when silicification (“окварцевание”, leaching out everything possible and replacing it with quartz; not petrification-related) of the host rocks (amphibolites) accompanied with formation of rocks rich in metals such as Fe, Mg and alkali such as K. Silification led to formation of “quartzites” (in metasomatic sense) with kyanite and garnet. Residual fluids (?) rich in Fe and Mg were redistributed and deposited in form of almandine-biotite schists and gedrite-almandine rocks with gigantic crystals of almandine (Fig. 13). The amount and sizes of garnet here are fascinating. Crystals of garnet larger than 20 cm in diameter can be found here (Fig. 14). The deposit is also interesting because it is the southernmost point of the low-δ18O zone described in Bindeman et al (2014). We stayed in this bay of the White Sea for a couple days to study the garnet-rich rocks in a quarry and visited a small mount called Terbestrov that contains another occurrence of gigantic garnets. After spending 4 days there, hiking around and taking the boat, we left the region on the train that was headed to Moscow.

Fig. 13. Outcrop that was inspiring for geologist attempting to explain the origin of the rocks by silisification and redistribution of Fe and Mg; Pink material is all garnet

 Fig. 14. A crystal of garnet found in tails
 

Fig. 15. One of the most extensive marshes I have ever stepped on. The way to get to huge garnets!

REFERENCES

1. Bindeman, I.N., Serebryakov, N.S., Schmitt, A.K., Vazquez, J.A., Guan, Y., Azimov, P. Ya., Astafiev, B. Yu., Palandri, J. & Dobrzhinetskaya, L. (2014). Field and microanalytical isotopic investigation of ultradepleted in 18O Paleoproterozoic “Slushball Earth” rocks from Karelia, Russia. Geosphere 10, 308-339. 

2. Glebovitsky, V.A. and Bushmin, S.V. Post-migmatitic metasomatism (Russian lang.). Leningrad, Nauka, 1983. 

3. Kulikov, V.S., Bychkova, Ya. V., Kulikova, V.V., Kostitsyn, Yu. A., Pokrovsky, O.S. and Vasiliev M.V. (2008) The Ruiga intrusion: a typical example of a shallow-facies Paleoproterozoic peridotite-gabbro-komatiite-basaltic association of the Vetreny Belt, SE Fennoscandia. Petrology 16, 531-551. 

4. Melezhik, V.A., Prave, A.R., Hanski, E.J., Fallick, A.E., Lepland, A., Kump, L.R., Strauss, H. (2013). Reading the archive of Earth oxygenation. Vol. 1, Paleoproterozoic of Fennoscandia as context for Fennoscandian Arctic Russia-Drilling Early Earth Project. Springer, 490p.

4. Puchtel, I.G., Hofmann, A.W., Mezger, K., Shchipansky, A.A. and Kulikov, V.S. (1996). Petrology of a 2.41 Ga remarkably fresh komatiitic basalt lava lake in Lion Hills, central Vetreny Belt, Baltic Shield. Contrib. Miner. Petrol. 124, 273-290. 

5. Sharkov, E.V., Trubkin, N.V., Krassivskaya, I.S., Bogatikov, O.A., Mokhov, A.V., Chistyakov, A.V. & Evseeva, K.A. (2004). Structural and compositional characteristics of the oldest volcanic glass in the Early Proterozoic boninite-like lavas of southern Karelia. Petrology 12, 227-243.