Stonemasonry in the Říčany Region – Žernovka Quarry, Part 2

The Říčany Pluton forms a body of a roughly oval section (13 x 9 km). As Katzer (1888) has already shown, it consists mainly of two concentrically positioned magmatic facies. The outer facie is rich in phenocrysts, the inner facie is not. Their contact has not been uncovered, but it is most likely gradual. The differences in modal composition and the content of the main elements are also minimal. The new U – Pb datings of zircon confirm that both facies intruded at the same time, about 337 million years (Trubač et al., in print). The central part of the body was broken through by a younger, now poorly exposed intrusion of a fine-grained two-mica leucogranite of the so-called Jevany type, and the southern edge of the pluton is lined by an approximately 1 km wide zone with numerous veins of pegmatite, microgranite and aplite rich in boron, tin, niobium, and tantalum.

The western contact of the Říčany Pluton with Neoproterozoic and Cambrian metasediements of the Teplá-Barrandian area is intrusive, with a distinct contact metamorphosis. The eastern edge is covered in permocarbon sediments whose basal conglomerates contain pebbles of the Říčany granite. As we will see in Žernovka Quarry, the central facie of the Říčany Pluton is characterised by the occurrence of large mafic microgranular enclaves, which are hybrid, potassium-rich and show other affinities to durbachitic magma. They are often associated with notable accumulation of K-feldspar phenocrysts. This association suggests complex interaction between alkaline and acidic magma, as well as near-solidified K-feldspar cumulates, presumably carried out in several stages.

Figure 1:
a – The position of the Variscan Central Bohemian Plutonic Complex (CBPC) in the Bohemian Massif.
b – An outline of CBPC and its division into magmatic suites.
c – A geological sketch of the Říčany Pluton with Žernovka Quarry (Janoušek et al., 2014).

For World of Stone provided by authors:
Barbora Dudíková Schulmannová, Vojtěch Janoušek, Tomáš Navrátil, Václav Rybařík and Pavel Vlašímský