|
Konstantin Esipchuk, Institute Geochem, Mineral. and Ore Formation N P. Semenenko, Ukr. Ac. Sc. (Ukraine)
Olexander Ponomarenko, Institute Geochem, Mineral. and Ore Formation N P. Semenenko, Ukr. Ac. Sc. (Ukraine)
Vitaly Kalinin, Institute Geochem, Mineral. and Ore Formation N P. Semenenko, Ukr. Ac. Sc. (Ukraine)
|
|
There are known rather large (up to 100 km 2 ) Octiabrsky massif and more than 10 manifestations of alkaline rocks in the Ukrainian Shield (Krivdyk, Tkachuk, 1990). Almost all of them are spatially associated with the occurences of large gabbro-anorthosite-rapakivi-granite and gabbro-sienyte plutons or confined to the sutural zones that seperate megablocks of the Ukrainian Shield. Long time ago has been postulated the idea about the certain relation of Octiabrsky massif (East-Nearazovian part of the shield) plate tectonics and, in particular, with the subduction of Middle Dnieper plate under the Near-Azovian one (Glevassky, 1989).
The processes of recycling of large volumes of oxidized continental crust into the mantle results in non-homogeneous mantle composition as to oxidation-reduction mode. The new zones with the enrichement in water, oxygen and carbonic acid are formed in mantle. It is these zones within which melts of low degrees of fusion, favorable for transportation of plenties of alkalis and not coherent elements are formed (Wenland, Harrison, 1979; Kogarko, 2006).
The Earlyproterozoic age of the alkaline rocks of the Ukrainian Shield, gabbro-anorthosite-rapakivi-granite and the gabbro-sienyte plutons (1820-1740 Ma) is well correlated with intensive functioning of plate tectonic mechanisms, since Archean-Proterozoic boundary (2700-2500 Ma).
As have been already mentioned, formation of alkaline and subalkaline rocks of Near-Azovian region (Octiabrsky and Yuzhno -Kalchinsky massifs) as well as Malotersiansky massif of nephelinic and alkaline syenites and Starobogdanovsk manifestation of syenites should be connected with the processes of Neoarchean-Paleoproterozoic subduction of Middle-Dnieper plate into the mantle, local enrichment of the mantle by volatile and not coherent elements and the subsequent occurrence here of alkaline magma enriched with rare and rare-earth elements.
The formation of Korsun'-Novomyrgorodsk and Korosten gabbro-anorthosite-rapakivi-granite plutons and associated occurences of syenites, fenites and melteigites as well as intraplate Proskurovsk manifestations of iolites, malignites, nephelinic and without-nephelinic syenites are likely to be also connected with the subduction of Dniester-Bugsk plate under Ingulsky (Kirovograd) and Northwest (Volynsk) ones.
|
