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.