Subalkaline formations of the Pervomaysk explosion pipe (Northern Kyvy Rig area).

    Velikanov Y.F., Velikanova O.Y.

    M.P. Semenenko Institute of geochemistry, mineralogy and ore formation of NASU, Kiev, Ukraine

Subalkaline effusive rocks were established in the Pervomaysk explosion pipe situated in the Northern part of the Kryvbas, in the junction of sublatitudinal Devladovo regional fault zone and submeridionally striking Kryvy Rig – Kremenchug regional fault zone, i.e. in the tectonically weak position due to numerous differently oriented tectonic faults resulting in the block-mosaic structure which stimulated injection of intrusive-effusive substrates during tectonic-magmatic activization interrupting craton phase developing of the region.

The structure origin is interpreted ambiquously. V.A. Tihonov with co-workers [9] distinguished breccia pipe composed by tectonic and eruption breccias, but the last is cemented by greatly altered glass in which pyroxenes, olivine and magnetite have been determined. A.N. Nikolsky [6], V.L. Masaytis with co-workers [5], G.K. Eremenko [3], A.A. Walter with co-workers [1] and Y.P. Gurov [2] are followers of an astroblem genesis of the structure. N.P. Semenenko with co-workers [7, 8] consider this structure as an explosion tube resulted from tectonic-magmatic activization which interrupted platform phase of the region development.

Samples of the investigations were selected during underground mines documentation in the Pervomaysk mine in which fresh volcanogenic rocks (explosive breccias, lava-breccias and lavas) forming explosion tube, some hundreds m in diameter, among greatly metamorphised Precambrian rocks were found and investigated. In the underground mines these formations were found down the depth 920 m. In relation to the host rocks the volcanogenic formations are cutting bodies deepening near vertically.

Explosive breccias in contrast to tectonic breccias are composed by pieces of granitoids of basement and metamorphic rocks of different suites of the Kryvy Rig series moved up from the lower horizons of the section and intruded in the microschists and microgneisses of the uppermost Gleyevatsk suite as a result of explosion.

Lavas are green-gray dense or blebby rocks. Cavities in the blebby lava varieties represent 20-40 % of the rock values, in part often are often filled with quartz-carbonate substrate. Shapes of the amygdales and cavities are oval, roundish, awkward but size from 0.5-1.0 to 5-7 mm. Blebby textures were resulted from magma degassing during cooling at subsurface conditions. Lava structure is porphyric or aphyric. The phenocrysts are usually represented by intensively altered plagioclase, hornfelse and biotite. The whole mass structure is interstitial, hyalopelitic, cryptocrystalline, felsitic. At high magnificence observations the whole mass is composed by irregularly oriented thin elongated blades of plagioclase, thin allocations of K-feldspar, chlorite needles, fine glass and undetermined at highest magnifications finest dust-like masses.

In contrast to lavas lavabreccias contain pieces of the host rocks, difficultly determined light-gray fine-grained rocks apparently representing earlier solidified lavas, pieces of differently devitrified volcanic glasses and different minerals at 10-30 % of the total rock volume. The size of the pieces are from some mm to 2-3 cm. The rock pieces were fritted at different intensity. The edges of pieces and cementing lava are unclear. Dependently on the fritting intensity their contours became difficultly distinguishable due to merging with the host mass.

The major petrogenic components suggesting about rock character are their siliceous, aluminous, ferriferous and alkaline parameters. From chemical analyses silica content in in the range 54,09..68,80 %, alumina content – 10,0..16,28 %, total iron oxides content – 2.72..7.27 %; K₂O content – 3.96-9.50 %, Na₂O 0.22-4.25 %; but the amount of K₂O always exceeds Na₂O content.

The diagram of volcanogenic rocks of T.I. Frolova and M.A. Petrova [4] the investigated rocks are situated in the fields of trachibasalts, trachiadesites and trachites.

Subalkaline rocks of the Northern Kryvy Rig area are similar with andesite-trachiandesite complex of the Donbas.

Variations of chemical composition of the rocks are probably resulted from multiple pulsing volcanic ejections.

Isotopic investigations of the metamorphically unaltered volcanogenic rocks have shown that their formation occurred at the border of Paleozoic and Mesozoic. K-Ar isotopic determinations given age 250±10 mln.y. Similar age 280±10 mln.y. was determined by A.A. Valter at al. [1] but at suggestion that they are impactites.

From semiquantitative spectral analyses the lavas and lava-breccias contain Pb, V and Sr at about 0,03 %, Cu – up to 0,02 %, La, Zn, Ni and Zr – up to 0,01 %.

 

References:

1. Valter A.A., Rabenko V.A., Kotlovskaya F.I. Ternv astroblem – new the deepest eroded crater of the Ukrainian Shield. // Reports of AS of Ukrainian SSR. – Ser. Б. – 1981. – № 2. – P. 3-7.

2. Gurov Y.P. Stoshovite of the Ternv astroblem // Mineral. Journal, 1982, 4. – № 2. – P. 75-76.

3. Yeremenko G.K. Terny astroblem in the Northern Kryvy Rig area // Reports of the AS of USSR. – 1980. –   253. – № 2. – P. 449-451.

4. Yefremova S.V., Stafeev K.G. Petrochemical methods of the rock investigations. – M.: Nedra, 1985. – 511 p.

5. Masaytis V.L., Mashak M.S., Sokolova. Hyperbaric phases of silica in the Terny astroblem // Reports of AS of USSR. – 1980. –   255. – № 3. – P. 709-713.

6. Nikolsky A.N. On genesis of tridymite-hisingerite rocks of the Pervomaysk deposit in Kryvy Rig district.// Reports of AS of USSR. – 1979. –   249. – № 2. – P. 436-439.

7. Semenenko N.P., Yaroshuk E.A., Velikanov Y.F. et al. Connection of Mesozoic platform volcanism with zones of activation in Precambrian of the Northern Kryvy Rig area // Geochemistry and Ore Formation. – 1985. – 13. – P. 3-13.

8. Semenenko N.P., Velikanov Y.F., Gladkyh V.I. et al. Late Palaeozoic volcanism in Mid Fore-Dnieperian // Geol. Journal. – 1995. – № 2. – P. 9-13.

9. Tihonov V.A., Karpenko V.S., Kudlayev A.R. et al. Breccia pipe in the Northern Kryvy Rig area. – Geology of the ore deposits. – 1968. – № 3, 10. – P. 17-28.