Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)

Phonolite and trachyte dykes on Huseň Hill in the Doupovské hory Mts.

 

Vladislav Rapprich, Zoltán Pécskay, Jan Matějů

Geoscience Research Reports 52, 2019, pages 11–16
Map sheets: Kadaň (11-22)

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Published online: 18 February 2019

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Abstract

A network of trachyte and phonolite dykes penetrating phlogopite-phyric ankaramite was exposed in an excavation of a water-lock on Huseň Hill in the Doupovské hory Volcanic Complex. Trachytes and phonolites are rather scarcely exposed in this Volcanic Complex, compared to other volcanic areas of the Bohemian Massif. The occurrence of highly alkaline sodalite-phyric phonolite on the Huseň Hill has already been known from a small disintegrated outcrop, but its relationship to surrounding rocks remained unclear. The excavation (8 × 12 m) dug out in 2016 exposed a system of trachyte to phonolite dykes penetrating in several directions (NNW-SSE, E-W, ENE-WSW) phlogopite-phyric ankaramite lava of the Doupovské hory Volcanic Complex edifice of the Lower Oligocene age (31.7 Ma). The observed small dyke-swarm consists of one dyke composed of vesicular trachyte and trending E-W to WNW-ESE, two dykes of sodalite-phyric phonolite running NNW-SSE and ENE -WSW, associated with a larger body (lava or sill) of the same composition, and one strongly altered dyke of E-W strike. The thickness of individual dykes varies between 30 cm and 1 m. The phonolite dykes contain abundant phenocrysts of sodalite (up to 1 mm in size), while phenocrysts of biotite and amphibole are less common. The latter one being of rounded shape indicating magmatic corrosion. Larger crystals of sodalite have bluish core. The emplacement age of the freshest dyke D was determined using the K-Ar (bulk-rock) method at ca 37 Ma. However, the established age does not seem to be compatible with the local geological structure, most likely due to frequent sodalite, possibly containing excess argon. The age of the Doupov Intrusive Complex (ca 29 Ma) was also assumed for dykes observed on the Huseň Hill, due to their genetic relationships. The thickness of these dykes is by a few orders smaller than the common size of trachyte and phonolite bodies in the Eger Rift, suggesting these differentiated rocks to have been emplaced at high temperatures reducing the effective viscosity. Orientation of the observed dykes coincides with regional faults (the NW-SE to NNW-SSE trending Liboc Fault and the NE-SW to ENE-WSW oriented Střezov Fault) constraining the Permo-Carboniferous Žihle Basin in the basement, and the Obrovice Fault crosscutting the volcanic complex along the E-W to WNW-ESE direction.
 

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