Possible geodynamic histories for the formation of Pre-Cambrian sub-alkaline granitoids of the Eastern Azov Sea Region of the Ukrainian shield
Sheremet E. M., Setaja L. D.
Ukrainian State Research and Design Institute of Mining Geology, Rock Mechanics and Mine Surveying (UkrNIMI) of the National Academy of Sciences of Ukraine (NANU), Donetsk, Ukraine EvgSheremet@yandex.ru
Today there are several hypotheses relating to the origin of Proterozoic magmatism of the Ukrainian shield and its accompanying fault-block tectonics, which try to link together the majority of the known facts. These views are given below.
To explain genesis of granitoids of Proterozoic magmatism of the Ukrainian shield a more applicable is C. Wilson [Wilson, 1965) and W. Morgan (Morgan, 1972) hypothesis of mantle plumes. According to this hypothesis, intra-plate magmatism is due to spots of heated athenosphere motionless relative to lithosphere and connected with mantle plumes that rise as a result of convection from the mantle boundary and Earth core. At the turn of 1.9-1.7 milliard years ago a stage of the formation of Proterozoic continental crust started owing to the occurrence of deep mantle flows rising as a result of convection from the mantle boundary and core. Mantle fluids brought heat and migratory components; formed astenoliths, and those, in their turn, were initiators of basalt melts embedded into subsurface of the earth’s crust and somehow influenced the re-melting of enclosing substratum.
Formation of sub-alkaline granite-granosyenites in the Azov Sea Region had to occur in conditions close to the conditions of formation of charnockites, that is, these are high-temperature rocks of hypersolvus-type that have been forming at large depth in dry environment. The chemism of rocks under consideration has much in common with chemism of rapakivi: high Fe content in rocks and dark-coloured minerals, low content of calcium oxide and others. Probably, the differences between them are due to different content of water and fluid phase in initial melts. We can assume that mother rocks were formations metamorphically altered in granulite phase of metamorphism.
There is a hypothesis of the occurrence of the Azov Sea Region fault-block tectonics and magmatism which is based on plate tectonics concept (Azarov N.Ya, 2005).
At the heart of the accepted here geodynamic model is the plate tectonics concept of the geologic evolution of the Priazovie megablock as the Proterozoic active continental remote area generated at the epi-Archaean crust. The Orekhovo-Pavlogradskaya suture zone (OPSZ) is considered as collision juncture of the united (after the intake of the oceanic lithosphere has come to an end and the basin that divided micro-continents has closed down) at the end of the paleo-Proterozoic Middle Pridneprovsky (the Trans-Dnieper Region) and Priazovie megablocks – fragments of the respective micro-continents.
The intake of the oceanic lithosphere under the Priazovie megablock was accompanied by transformation of: crust substance by fluids and magmatism of the oceanic lithosphere being metamorphized in the zone of subduction, epi-Archaean continental crust and also crust and mantle of the hanging plate itself. These points are proved by contrast of geological evolution of the neighbouring geoblocks in Proterozoic, by the structure and material composition of OPSZ and, in particular, by lateral magmatic zoning of Proterozoic magmatism which in the Priazovie megablock is similar to the characteristic one – from calc-alkali magmatites (the West Priazovie – obitochnensky, saltychansky and other complexes) to sub-alkaline (the East Priazovie – anadolsky, khlebodarovsky, yuzhnokalchiksky, oktyabrsky, kamennomogilsky complexes) – with rejuvenation from frontal to rear parts of the zoning in the interval of
2.4 to 2.1 – 2.0 milliard years ago.
It seems that as far as the Zavaritsky-Benioff zone (OPSZ) plunged into the mantle depths its activation took place which led to the rise of geoisotherms and deep fluids to the upper levels of the earth’s crust and, as a consequence, to melting of the substratum. Its deepest plunging has resulted in activation of non-depleted mantle, increased inflow of alkaline elements and formation of sub-alkaline complexes of the East Priazovie.
In conformity with nuclear model of lithosphere evolution (Glukhovsky, 1994), its vertical and horizontal growth was time-asynchronous (Pavlovsky, 1975) and space-non-uniform. Most of the continental crust has been formed in Archean (more than 2.7 milliard years ago). Its formation almost completed (up to 97 %) to the mark of 1.6 milliard years ago (Lowe, 1992). Pre-Cambrian processes have manifested themselves intensely within sialic nuclear cores with thick continental crust (down to 50 km) and less intensely – within inter-nuclear regions with the formation of less thick enderbite-basite and basite crust (down to 30 km). Crust formation in Pre-Cambrian has completed as long as 2.0-1.0 milliard years ago. Global processes of thermal tectonogenesis have led to the conversion of previously formed crust and to the formation of deep thrust faults along which anorthosites and their associated high-baric rock complexes in the form of granulite belts embedded into nuclear outer parts (Glukhovsky, 1992). In Ye. Pavlovsky’s opinion (Glukhovsky, 1994), heterogeneous sialic cores are traces of the Earth’ plumes that existed at the earlier stages of its evolution. The raise of the mantle diaper is accompanied by increase in thermal flux intensity.
At the territory of the Ukrainian shield, we can consider as riftogeneous structures, most likely of Archaean occurrence, suture zones that delimit megablocks of the Ukrainian shield. They have activated in Low-Proterozoic. Suture zones of the Ukrainian shield ‑ Orekhovo-Pavlogradskaya, Krivorozhsko-Kremenchugskaya and Golovanevskaya – are located along the edge of paleonuclears. It is possible that suture zones are related to polychrome riftogeneous structures and/or proto-plate flexures along which, due to tectogenesis global processes, either gabbro-anortosites, rappakivi granites in the North-West and Central (Kirovogradsky) megablocks of the Ukrainian shield or gabbro and syenite-granosyenite sub-alkaline rocks in the Priazovie megablock had been embedding into outer parts of the nuclears (in our case these are megablocks of the Ukrainian shield.
Azarov N.Ya., Antsiferov A.V., Glevassky Ye.B., Sheremet Ye.M. et al. Geologic-geoelectrical model of the Orekhovo-Pavlogradskaya suture zone of the Ukrainian shield. Kiev: Naukova dumka, 2005. 190 p. (in Russian).
Glukhovsky M.Z., Moralev V.M., Petrova Z.I. Geochemical features and tectonic nature of Archaean metabasites of the Sunnaginsky dome of the Aldan shield // Dokl. AN. 1992. Vol. 324. No. 3. P. 626-631. (in Russian).
Glukhovsky M.Z., Moralev V.M., Kuzmin M.I. Hot belt of Earth and its evolution // Geotectonics. 1994. No. 5. P. 3-15.
Lowe D. Major Events in the Geological Development of the Precambrian Earth / The Proterozoic Biosphere. A. Multidisciplinary Study. Cambridge, Univer. Press, 1992. P. 67-75.
Morgan W. Deep mantle convection plume and plate motion // Bull. Amer. Assoc. Petrol. Geol. 1972. Vol. 56. No. 2. P. 203-212.
Pavlovsky Ye.V. The origin and evolution of the earth's crust of the continents // Geotectonics. 1975. No. 6. P. 3-14. (in Russian).
Wilson C. A new class of faults and their bearing of continental drift // Nature. 1965. No. 4995. P. 343-347.