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Strelecska hura

Basaltic magma of Střelečská hůra raised along fault of WSW-ENE direction. Basaltic dykes extend from Střelečská hůra along this fault as far as Mladějov, where another small volcano has been formed – “Na hrádku”. The superficial part of the Střelečská hůra volcano has been eroded similarly like surrounding Cretaceous sediments. The abandoned quarry, currently used as firing range, exposes subsurface facies of a monogenetic volcano. Conduit of the volcano is represented by vent breccia penetrated by branched feeder dyke. The dyke is 10 m thick in its widest part. The breccia consists of angular clasts of basaltic rock and sediments of Cretaceous and Permian age. Some of the basaltic fragments are slightly vesicular. The basanite of the dyke has irregular columnar to boulder disintegration. The phenocrysts of olivine and clinopyroxene are enclosed in fine-grained groundmass. The olivine crystals use to be significantly altered. The groundmass consists of clinopyroxene, olivine, nepheline, analcite and plagioclase. The groundmass is locally also altered. Xenocrysts of quartz are relatively abundant and partly replaced by mixture of clinopyroxene and glass. Velká hůra is also important mineralogical site. Druses of carbonates and zeolites occur locally in the breccia. Natrolite is frequent forming veins up to 4 mm wide. Natrolite used to be accompanied by mesolite and thomsonite. Thomsonite forms also monomineral druses or veins. Phillipsite can be also found. Calcite is very frequent mineral in the breccia.

GPS position

N 50° 28.934', E 15° 16.268'



Geopark UNESCO Český ráj
Jiří Vlasák