Zila alterovaneho bazaltu („diabasu“)
na severnim uboci Divcich hradu v Praze-Radlicich

A new rock outcrop was uncovered in a temporary open cut for the house construction on the northern slope of the Dívčí hrady Hill in Praha-Radlice (GPS 50° 3’ 19.080” N; 014° 23’46.200” E). The outcrop was completely hidden in the weathered Ordovician sediments (Figs 1 and 2). A closer research of the locality revealed, that the rock outcrop is formed by a sill of the grey-blue altered basalt (locally known as the “diabase”), which intruded into clayey shales of the Bohdalec Formation (Ordovician, Prague Basin). This intrusion is not marked on existing geological maps (Fig. 3) as it does not manifest itself morphologically.

The sill of altered basalt has two tectonically separated parts. The eastern, smaller part of the intrusion (4 × 2.5 m in size, Fig. 4) is tectonically separated from the western part, which is larger (10 × 4.5 m in size) and fenced. The western part is bounded on both sides by noticeable faults with steep and polished surfaces striking NNE-SSW (288–291°/87°). As obvious in both parts of the outcrop, the basalt form a concordant sill (Fig. 5), which intruded along the primary layering of the shales of the Bohdalec Formation striking ENE-WSW (173°/54°). The true thickness of the sill fluctuates, also due to the tectonic segmentation, from about 1.5 m to 4.5 m (Fig. 6).

From the petrographic point of view, the altered basalt is fine to medium-grained, finely darkly dotted with chloritized clusters up to 3 mm in size (decomposed mafic minerals, possibly vesicles). Mica is not visible. Samples are often intersected by white calcite veins. During the weathering processes, the color of the rock changed from grey-blue to grey-green, and brownish weathering rind also formed closer to the surface of fragments (Fig. 7). Vesicular structure is locally developed in both parts of the outcrop (Fig. 8). In the thin section, strongly carbonatized plagioclase and chloritized pseudomorphs after mafic minerals (pyroxene, olivine?) predominate (Figs 9 and 10). Accessory minerals are represented by titanite, pyrite and chalcedony.

Intrusive rocks (dykes, sills; mainly of basalts) penetrating Paleozoic sediments in the area of Dívčí hrady were focus of a number of studies, especially in the early 1970s. At that time, a biotite minette dyke cutting through the Lower Devonian Dvorce-Prokop Limestones was found in the NE part of the hill (Fiala 1971; Fiala – Chlupáč 1973). This minette dyke is located about 800 m SE from the newly described outcrop of the altered basalt (see the lower right corner of Fig. 3). The minette is petrographically different, its color is light and creamy, and it contains abundant biotite flakes (Fig. 11).

In the 1980s, the geological map of the northern part of Prague (Praha-sever) at a scale of 1 : 25 000 was published (Králík et al. 1983). Several dykes and sills of altered basalt that intruded the Ordovician and Silurian Formations were drawn on this map also in the Dívčí hrady area (Fig. 3). However, petrography of these rocks was not described in detail in the explanatory notes. Dyke of biotite minette and intrusions of Silurian altered basalt have been recently found also in the exploratory gallery for the Radlice tunnel (Chmelař et al. 2016).

Although the age of the altered basalt from the newly uncovered rock outcrop is not yet constrained by geochronological dating, its petrographic character corresponds to the Silurian basalts (i.e. altered intrusive basalts or “diabases”), which have been described in the SW part of Prague mainly from the lower part of the Silurian strata (especially from the Liteň and Kopanina Formation – see Fiala 1970; Kříž in Králík et al. 1984; Štorch in Chlupáč et al. 1998).

References

Fiala, F. (1970): Silurské a devonské diabasy Barrandienu. – Sbor. geol. Věd, Geol. 17, 7–89.

Fiala, F. (1971): Ordovický diabasový vulkanismus a biotitické lamprofyry v Barrandienu. – Sbor. geol. Věd, Geol. 19, 7–97.

Fiala, F. – Chlupáč, I. (1973): Minetová žíla v barrandienském devonu a její význam. – Čas. Mineral. Geol. 18, 1, 47–55.

Chlupáč, I. – Havlíček, V. – Kříž, J. – Kukal, Z. – Štorch, P. (1998): Palaeozoic of the Barrandian (Cambrian to Devonian). – Czech Geol. Surv. Prague.

Chmelař, R. – Síla, L. – Tůma, P. – Březi na, B. (2016): Poznatky z inženýrskogeologického průzkumu pro tunel Radlice (Knowledge from Engineering Geological Survey for Radlice Tunnel). – Tunel 25, 1, 40–50.

Králík, F. – Brunnerová, Z. – Čuta, J. – Havlíček, V. – Chlupáč, I. – Klein, V. – Kříž, J. – Odehnal, L. – Šefrna, L. – Šimek, R. – Tomášek, M. – Zoubek, J. (1984): Vysvětlivky k základní geologické mapě ČSSR 1 : 25 000, list 12-243 Praha-sever. – 144 str. Ústř. úst. geol. Praha.

Králík, F. – Havlíček, V. – Chlupáč, I. – Kříž, J. – Valečka, J. – Zelenka, P. – Zoubek, J. (1983): Základní geologická mapa ČSSR 1 : 25 000, list 12-243 Praha-sever. – Ústř. úst. geol. Praha.