Abstracts of International conference Geochemistry of magmatic rocks-2009 . School «Geochemistry of Alkaline rocks»
Features of Proterozoic alkaline and subalkaline magmatism in Ukrainian shield: dependence on geodynamic position
Sheremet E. M.
The Late Proterozoic subalkaline and alkaline magmatism of Ukrainian shield has manifested itself approximately at the same time (1.9 – 1.7 milliard years ago) in different megablocks united with each other by suture zones. The latter are considered from the position of plate tectonics as sutures of closed in the past intermicrocontinental basins. Fragments of the Early Pre-Cambrian microcontinents among which those basins were located are the present megablocks of Ukrainian shield. From east to west one can distinguish: the Orekhovo-Pavlograd suture zone located between the megablocks of the Azov Sea Region and the Middle-Dnieper Region; the West Ingulets-Krivoi Rog suture zone located between the megablocks of the Middle-Dnieper Region and the Ingul Region; the Golovanevsko-Yadlovo-Traktemirov suture zone located between the megablocks of the Ingul Region and the Belotserkovian-Bug Region; the Nemirov-Kocherovsk suture zone located between the megablocks of the Belotserkovian-Bug Region and the Volhynia-Podolsk Region.
Subalkaline and alkaline magmatism of the megablock of the Azov Sea Region (the Khlebodar granite–granosyenite complex, the South Kalchik gabbro-syenite complex, the Oktyabr nepheline-granosyenite complex, the Kamennye Mogily complex of subalkaline less-common metal granites) was formed at the final stage of sinking of the paleosubduction zone (the Orekhovo-Pavlograd suture zone). The absorption of the oceanic lithosphere under the megablock of the Azov Sea Region was accompanied by transformation of its epi-Archean continental crust as it penetrated deep into the mantle. Heat and fluids that generated melts escaped from the excited mantle. This is confirmed by lateral magmatic zoning of the Proterozoic magmatism from calcareous-alkaline magmatites to subalkaline-alkaline ones, their ages being rejuvenated from front to rear parts of the zoning.
Subalkaline Upper Proterozoic (1.75 milliard years) granitoids in the megablock of the Ingul Region are represented by rapakivi-granite Korsun-Novy Mirgotod pluton. We have a good reason to believe that its generation was connected with the final stage of formation of the Proterozoic granite-gneiss megablock of the Ingul Region which was arisen in place of the oceanic basin that separated microcontinents – megablocks of the Belotserkovian-Bug Region the Middle-Dnieper Region. West Ingulets-Krivoi Rog and the Golovanevsko-Yadlovo-Traktemirov suture zones are sutures of that oceanic basin. Collision histories accompanied by the junction of the above megablocks began formation in the Proterozoic of structural-material complexes of the Ingul Region megablock directly after closure of the basin. While oceanic sediments sunk into the mantle, more deep-laid parts of it had been generating. The deepest fluids have led to formation of the basic-based melts (anorthosites, norites, monzonitoids) and subalkaline acid rapakivi-granites (the Korosten pluton).
Subalkaline Upper Proterozoic (1.76 – 1.73 milliard years) magmatites in the megablock of the Volhynia-Podolsk Region are represented by the Korosten and the Perzhanian complexes. The Korosten pluton is composed of association consisting of rapakivi granites, rapakivi-like granites, granite-porphyrys, anorthosites, gabbronorites, syenites, alkaline syenites and others. The Perzhanian complex is represented by subalkaline granites. The Korosten pluton is a kind of a cap over the north part of the Nemirov-Kocherovsk suture zone which is a boundary structure between the Volhynia-Podolsk Region megablock and the Belotserkovian-Bug Region megablock. The latter does not manifest subalkaline magmatism as younger as milliard years. It can be expected that in place of the Nemirov-Kocherovsk suture zone there was an oceanic basin indirect confirmation of the existence of which are carbonaceous rocks in its north part. Closure of that basin was accompanied by plunging of oceanic sediments towards the Volhynia-Podolsk Region megablock and forming, as a consequence, of palingenic melts due to activation of the mantle. At the final stage of closure of the oceanic basin deep parts of the activated mantle have led to generation of subalkaline melts responsible for formation of the Korosten and the Perzhanian complexes.
Thus, manifestations of the Upper Proterozoic subalkaline magmatism in different megablocks of Ukrainian shield depend on the same reasons, namely, on the excitation of relatively deep parts of the mantle due to sinking of oceanic sediments deeply down as a result of collision of geotectonic histories. It appears that this is one of possible version for explanation of identity of petrochemical, geochemical, accessory-mineralogical compositions of the Late Proterozoic magmatites in separate megablocks of Ukrainian shield.