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SPATIAL-TEMPORAL ZONATION OF GRANITOID COMPLEX PETROPHYSICAL CLASS

DISTRIBUTION IN EAST PRE-AZOV (THE UKRAINIAN SHIELD) AS REFLECTION OF CHANGE IN GEODYNAMIC SETTINGS OF THEIR FORMATION


Shabatura A.V.

Kiev State University named after Taras Shevchenko, Kiev, Ukraine, sand@univ.kiev.ua


The East Pre-Azov is one of the most considerable and interesting areas of the Ukrainian Shield. On comparatively small territory there are polytype and hetero-aged magmatic, metamorphic and metasomatic formations.The long history of this region development have caused a variety of compositions, structures, physical properties of rocks, etc.

The so called lateral zonation of the Early Proterozoic magmatism, which can be the proof of the Andian type active continental margin existence was established by numerous researchers (Kalyaev, 1995; Glevassky, Kalyaev, 2000). The successive latitudinal change in matter composition, geochemical and metallogenic peculiarities (Shcherbak et al., 2003; Shcherbak, Ponomarenko, 2000; Shcherbakov, 2000), including the petrophysical classes of rocks, etc. has been noted. For Pre-Azov region, the azimuthal extension of overthrust coincides with direction of updating the isotopic data (from north-west to south-east) (Shcherbak, Ponomarenko, 2000) (Fig. 1.).

Fig.1. The zonation of granitoid complex petrophysical class distribution in East Pre-Azov: unconsolidated rocks are in white colour, – compressed rocks - grey, metamorphic rocks- yellow, basic and ultrabasic rocks – in black one; areas of high oxygen fugacity (fO2) -with the green shading, the boundaries of conditional rocks’ depth (b – abyss, c –middle, m - small, om – very small - hypabyssal) - red line with intersection and codes, petrophysical class zones of development (main representatives: I – elanchiksky and kalmiusky granosyenites, II – dmitrovsky granite; III – kalmiusky quartz-syenite, tokmaksky granodiorite; IV – anadolsky granite and maksimovsky leucogranite; V – kremnevsky syenite; VI – obitochnensky tonalite, VII – kremenevsky quartz-syenite) - Roman numerals.


The determination of petrophysical classes as indicators of change in geodynamic (only collisional and postcollisional regimes are more or less obviously presented) is based on studying the conformable changes in a number of petrophysical parameters: bulk density, calculated from mineral composition, and crystal-chemical density, elastic properties and their correlations, acoustic hardness of rocks, bulk and effective porosities and also some petrochemical indexes (Tolstoi et al., 2003).

The first petrophysical class is presented by the high-porous, low-density, by high fO2, hypabyssal formation with high fO2. The second one is represented by the low-density, very high-porous, radioactive and greatly friable, plastically deformed formations at volumous extension, with low fO2. The third class is similar to the first one, except the higher crystal-chemical density and very low index of oxygen chemical activity. The fourth class is the most heterogeneous one that is differed by considerable variance of practically all petrophysical parameters, as well mesoabyssal depths. The fifth class is characterized by the promoted crystal-chemical density, lower fO2 and middle porosity. The sixth class is high-density and high elastic, hypabyssal rocks with low fO2. The seventh class is high-density, hypabyssal, low-oxidized rocks with considerable anisotropies of elastic properties.

The east part of the Pre-Azov megablock was welded to the existing Paleo-Archean continent in the subsequent stages of formation, and the synkinematic diorites (obitochnensky complex), the anatectic granitoids and migmatites (anadolsky complex), which made up the basic canvas of region were already formed in the Paleo-Proterozoic (Kalyaev, 1995; Glevassky, Kalyaev, 2000). Paleo-Proterozoic intrusive formations tectonically associated with deep faults were intruded in 2 episodes of post-orogenic tectono-magmatic activation: 1) Chernigovsky complex of carbonatites, Khlebodarovsky complex of intrusive charnockites, Saltychansky complex of intrusive granites; 2) South Kalchitsky complex of gabbro-syenites, Kamennomogilsky complex of subalkaline granites, Octyabrsky alkaline complex (Shcherbak et al., 2003; Shcherbak, Ponomarenko, 2000; Shcherbakov, 2000). The geodynamic regime was caused by paleotectonics of the region, and its specific features of geodynamics are the following: a) there are uplifted south-western and northern-east blocks, with the relatively subsided ones among them; b) cross-like diagonal area of consolidated rocks in the matrix of the unconsolidated formations; c) presence of meridionally oriented belts of oxygen high chemical activity and latitudinal change in values of density and other petrophysical properties, etc.

The above features allow to identify paleogeodynamical settings (active continental margin and subsequent collisional area), which are the reflections of the ocean plate overthrust differentiated by rates and direction under the Pre-Azov megablock. Also, more careful petrophysical analysis will allow to reconstruct their paleotectonical elements: plutonic belts, rear areas of extension that have different physical and matter characteristics.

References

Glevassky E.B., Kalyaev G.I. (2000) Tectonics of Pre-Cambrian UShCh // Mineralogical journal. 22. N2/3.

Kalyaev G.I. (1995) Paleotectonic reconstructions // Mineralogical journal. 17. N6.

Shcherbak N.P., Ponomarenko A.N. (2000) Age succession of volcanism and granitoid magmatism processes in the Ukranian schield // Mineralogical journal. V.22. N2/3.

Shcherbak N.P., Bibikov E.V., Skobelev V.M., Shcherbak D.N. (2003) Evolution in time and metallogenic specialization of the Ukranian schield Early Pre-Cambrian crust (3,7 – 1,7 Ga) // Mineralogical journal. V.25. N4.

Shcherbakov I.B. (2000) Evolution of the Ukranian schield magmatism // Mineralogical journal. V.22. N2/3.

Tolstoi M.L., Gasanov Yu.L., Kostenko N.V. (2003) та ін. Петрогеохімія і петрофізика гранітоїдів Українського щита та деякі аспекті їх практичного використання: Довідник-навчальний посібник. К.: ВПЦ «Київський університет», 2003. 329с.