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БЪЛГАРСКО ГЕОЛОГИЧЕСКО ДРУЖЕСТВО, Национална конференция с международно участие „ГЕОНАУКИ 2010“BULGARIAN GEOLOGICAL SOCIETY, National Conference with international participation “GEOSCIENCES 2010”

The lava flow studied crops out in Eastern Rhodopes near to Studen Kladenets village and covers an area about 5 km2 wide with thickness up to 100 m. It is a part from the early volcanic activity of the Nanovitsa Volcanic Complex (Yordanov et al., 2008) and is situ-ated in the periphery of the Sveti Iliya paleovolcano. According to Ivanov (1960) it belongs to the so called 2nd Rupelian acid volcanic phase. The lava flow lies

The Studen Kladenets lava flow – petrology and structural featuresСтуденкладенецки лавов поток – петроложкa и структурнa характеристикaStoyan Georgiev1, Borislav Yordanov2, Yotzo Yanev1

Стоян Георгиев1, Борислав Йорданов2, Йоцо Янев1

1 Bulgarian Academy of Sciences, Geological Institute, Department of Geochemistry and Petrology, Acad. G. Bonchev Str., Bl. 24, 1113 Sofia, Bulgaria; E-mail: stoyang@geology.bas.bg; yotzo@geology.bas.bg2 Research Institute “Geology and Geophysics” AD, 23 Sitnyakovo Blvd., 1505 Sofia

Key words: physical volcanology, petrology, Eastern Rhodopes.

over acid pyroclastic rocks dated as Rupelian. It is in-tersected by acid domes and flows and is covered by acid pyroclastic rocks of the same age. The Arda River cuts in it a shallow (up to 10–15 m) but narrow (from 0.5 m) gorge called the Devil’s bridge.

The lava flow consists of dark coloured mas-sive latite with sparse phenocrysts and microphe-nocrysts of plagioclase (andesine–labradorite), augite

Fig. 1. Structural features of the Studen Kladenets lava flow: a, vent area of the lava flow (in the left corner – interpretation); b, feeder dyke (in the right corner – interpretation); c, inclined columnar jointing (entablature) due to the movement of the lava flow; d, relief geological map of the area (modified after Yordanov et al., 2008); e, thick upper colonnade of well developed fine prisms

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(Wo43.5En41.8Fs14.7), enstatite (Wo2.8En70.8Fs26.4) and bio-tite. The plagioclase and clinopyroxene often display glomeroporphyric texture. The plagioclase is zonal with inclusions of clinopyroxene. Some of the crystals have skeletal (or corroded) periphery with inclusions of vol-canic glass along the zones. The accessory minerals are Ti-magnetite, zircon and apatite. The groundmass has hyalopilitic texture and is built of irregularly distributed plagioclase and clinopyroxene microlites, ore minerals and devitrified volcanic glass. The rocks are altered by chlorite and smectite. Amygdales filled with quartz and chalcedony are often found. Various kinds of agates (mostly Uruguayan-type) are observed.

The analyses of the rocks often fall in the field of the trachydacite due to the presence of quartz and chalcedony amygdales and veins. The chondrite-nor-malized spidergram pattern is characteristic for the orogenic rocks of the destructive margins. Slightly pronounced Eu anomaly is observed.

The temperature and the pressure of crystallization of enstatite using the equilibrium with the whole rock (Putirka, 2008) are 1081 °C and 6.7 kbar and for the augite – 1088 °C and 3.8 kbar. This probably corre-sponds to early crystallization of the enstatite in the

magma chamber and later crystallization of the augite in the conduit in equal temperatures.

The lava flow structure could be determined using the columnar joints. It is well known that the colum-nar joints form during the cooling of the lava and in common are situated transversely to the cooling sur-faces. Thus the localized vent represents a semi-cir-cular feeder dyke (Fig. 1). The flow is extended in NE direction and the lobes are radially distributed to the feeder dyke. The columnar joints are sub horizontal in the feeder dyke, perpendicular to the vent walls and gradually become upright in the lava flow. Two zones of distinct columnar joint development are observed. The first one (lower colonnade) consists of coarse prisms (up to 1 m in diameter) due to the fast cool-ing. The second one (upper colonnade) is presented by fine, well-developed prisms (10–30 cm in diameter). The upper colonnade is considerably thicker (over 2/3 of the lava flow thickness) than the lower colonnade. Locally, a third zone of inclined prismatic jointing is developed between the two zones (due to the flow movement) – entablature which dips points the vent area. The inclined prisms gradually transit upright in the upper colonnade.

ReferencesIvanov, R. 1960. Magmatism of the Eastern Rhodope depres-. Magmatism of the Eastern Rhodope depres- Magmatism of the Eastern Rhodope depres-

sion. Part I–Geology. – Trud. Geol. Bulg., Ser. Geohim. i Polez. Izkop., 1, 311–387 (in Bulgarian with a German abstract).

Putirka, K. 2008. Thermometers and barometers for volcanic systems. – In: Putirka, K. D., F. Tepley (Eds.). Rev. in

Mineral. and Geochem., 69, 61–120.Yordanov, B., S. Sarov, S. Georgiev, S. Gerdzhikov, V. Valkov,

D. Kamburov, E. Raeva, V. Grozdev, E. Balkanska, L. Mos kovska, G. Dobrev, S. Sarov. 2008. Geological Map of Bulgaria in Scale 1:50 000, Map Sheet Studen Kladenets. Sofia.