Научная деятельность

Geology

In the first part I wish to discuss the position of komatiites in the stratigraphic column of the most interesting greenstone structures in the Baltic Shield. Let us begin with Lopian greenstone belts that are common in the Baltic Shield over a time span of 3.1 - 2.7 Ga. Komatiitic rocks have no definite position in their stratigraphic columns, but they are typically confined to the lower and less commonly the middle parts of the columns. Cyclic units, composed of komatiites alternating with felsic volcanics and sediments are occasionally encountered.

Sheme showing the distribution of greenstone structures in Russian Karelia and Finland.

Based on a general gelogical map of the Baltic Shield (1985)

1 - Phanerozoic strata,
2 - Caledonides,
3 - Middle and Upper Proterozoic (Riphean, Jotnian and sub - Jotnian, 1600 - 700 Ma),
4 - 1750 - 1550Ma rapakivi granites,
5 - 1900 - 1750Ma Karelian (Svecokarelian) granites,
6 - 2000 - 1900Ma Svecofennian (Kalevian, Vepsian, Ludicovian) sedimentary and volcanogenic rocks and their analogues,
7 - 2500 - 2200Ma Middle and Lower Karelian (Ludicovian, Jatulian, Sariolian Sumian, Lapponian) rocks and their analogues,
8 - Archean granulite and gneiss complexes,
9 - 3100-2600Ma Upper Archean greenstone belts,
10 - Lower Archean (Saamian) and partly Upper Archean strata,
11 - block boundaries and faults,
12 - Upper Archean greenstone structures:
1 - Hautavaara, 2 - Manga, 3 - Koikary,
4 - Semch, 5 - Palaselga, 6 - Saezero,
7 - Bergaul, 8 - Sovdozero, 9 - Jalonvaara,
10 - Gimoly, 11 - Ilomantsi, 12 - Tipasjarvi,
13 - Kuhmo, 14 - Suomussalmi, 15 - Kostomuksha,
16 - Hedozero, 17 - Matkalahta, 18 - Toksha,
19 - Senegozero, 20 - Lake Kamennoye, 21 - South Vygozero,
22 - Korosozero, 23 - Parandovo, 24 - Pebozero,
25 - Keret, 26 - Hisovaara, 27 - Tikshozero,
28 - Kichany, 29 - Tersk - Allarechka, 30 - Kolmozero - Voronye.

Greenstone belts: Vedlozero - Segozero (1, 2, 3, 4, 5, 6, 7, 8), Jalonvaara - Ilomantsi (9, 11) , Kuhmo - Suomussalmi (12, 13 , 14), Gimoly - Kostomuksha (10, 15, 16) , Matkalahta (17), Sumozero - Kenozero (18 , 19 , 20 , 22) , South Vygozero (21) , Parandovo - Tikshozero (23, 24 , 25 , 26 , 27 , 28), Tersk - Allarechka (29) , Kolmozero - Voronya (30).


  • The above studies were basically done by the author in the Vedlozero-Segozero greenstone belt in Central Karelia ( N: 1, 2, 3, 4, 5, 6, 7, 8). It extends for 300 km from Lake Vedlozero to Lake Segozero and comprises the Hautavaara, Koikary, Palaselga and Sovdozero structures. Now I wish to briefly discuss their geological structure.
  • In the Hautavaara structure (N:1), the 2500-2700-m-thick basal unit is formed by lavas and tuffs of andesite composition as well as sediments. The unit is overlain by a 600- to 1500-m-thick komatiite-tholeiite assemblage with sedimentary interbeds. Resting on the assemblage are 500- to 900-m-thick terrigenous, chemically precipitated sediments and felsic volcanics as well as the second, 600-1000-m-thick level of tholeiitic basalts.
  • The base of the Koikary structure(N:3) is formed by a 800- to 2800-m-thick komatiite-tholeiite assemblage. It is overlain by a 940-m-thick pile of felsic volcanics and terrigenous rocks.
  • The entire 1500-1800-m-thick column of the Palaselga structure (Fig:5) is formed by the rocks of komatiite-tholeiite series.
  • In the Sovdozero structure(N:7), the base of the Lopian unit is formed by a 900- to 1000-m-thick komatiite-tholeiite series. Resting on it are 1000-m-thick volcano-sedimentary strata consisting of felsic tuffs, siltstones and iron formation.
  • Paleovolcanic edifices were reconstructed on the Koikary, Palaselga, Hautavaara and Kostomuksha structures.
    The age of Vedlozero-Segozero komatiites is estimated at 2935-3000 Ga, as shown by the isotope data obtained by the U-Pb zircon method for cross-cutting dykes.


    Type sections of Archean greenstone structures in the Baltic Shield


    Geochronological ages of rocks in the Vedlozero - Segozero greenstone belt.

    Rock and localityMethodAge (Ma)Reference
    Andesite, Palaselga stucturePb-Pb isochron whole-rock3000A.F.Loginov, 1982
    Dacite, subvolcano, Koikary stuctureU-Pb (zircon)2935+/-20E.V.Bibikova, 1983
    Andesidacite subvolcano, Ignoila, Hautavaara stuctureU-Pb (zircon)2995+/-20S.A.Sergeyev, 1989
    Rhyolite dike, Ignoila, Hautavaara stuctureU-Pb2854+/-14S.A.Sergeyev, 1989


    1. Introduction
    2. Geology
    3. Layered flows
    4. Structures
    5. Petrography
    6. Geochemistry
    7. Mantle sources
    8. References
    9. Researcher (Sergei Svetov)


    Архейские коматиты Карелии
    Последние изменения: 10 марта 2016