HIGH-MAGNESIAN VOLCANIC ROCKS OF THE PRECAMBRIAN IN RUSSIAN FENNOSCANDIA

Kulikov V.S., V.V. Kulikova

Karelian Research Centre of RAS, Petrozavodsk, Russia, kulikov@post.krc.karelia.ru

 

1. A broad range of high-magnesian volcanic rocks of different age (Palaeoarchaean, Mesoarchaean, Palaeoproterozoic, Mesoproterozoic) and belonging to different series (boninite, komatiite, tholeiite-picrite, meimechite, kamafugite) was recorded in the eastern part of the Fennoscandian shield (Russian Fennoscandia). These rocks and their intrusive comagmatic rocks are linked to deposits and ore occurrences of copper, nickel, chromites, EPG, diamonds and other minerals. Recent data on the geochemistry and geochronology of high-magnesian magmatic rocks (Puchtel et al., 1997, 1998; Smolkin, 1994) enable their correlation and identification of their connection to certain geological events of the regional and global scale.

2. The shortcomings of modern classifications of high-magnesian volcanites (A classification of igneous rocks.., 1989; Petrographic code, 1992; Jensen, 1976) do not allow adequate division of the examined formations in the Northwest Russia. The authors suggest new binary classification diagrams: lg (Al2O3:TiO2)- lg MgO (Kulikova, 1996) and Al2O3/TiO2 – MgO. The latter one is suitable for the rocks with the MgO content >9%. Seriality is determined by the Al2O3/TiO2 ratio: for the boninite series – over 30; komatiite - 30-10; tholeiite-picrite – 10-3.5; meimechite (kimberlite) and kamafugite (lamproite) series – less than 3.5. The boundaries of the rock kinds within the series in the basic-ultrabasic composition area with respect to MgO content (recalculated for anhydrous residue) obtained by statistical treatment of the representative body of the data (several thousand tests) are 9, 14 and 24% (Kulikov, 1991).

3. The formation of large amounts of magmatic rocks of the komatiite and tholeiite-picrite series in the Precambrian in Fennoscandia is due to the cyclic manifestation of the mantle plumes (or their separate jets) which rise from the lower mantle-core (?) boundary. The most powerful, multi-stage and lengthy plumes have a period of about 215 Ma, which is equal to the duration of the standard galactic year (Kulikova, Kulikov, 1997). No less than 4 such superplumes (Sumian, Jatulian, Ludicovian, Vepsian) noted for specific features of manifestation are distinquished in the Palaeoproterozoic. The separation of the plumes seems to be related to the effect of external cosmic factors, when the planet receives extra energy as the Solar system passes certain sections of the galactic orbit through jets of cosmic matter and energy (Barenbaum, 1997). Changes in the nature of the planetary mantle convection and the type of convective cells are also due to the effect of galactic factors.