Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)
Petrographic investigation of Tertiary volcanics on map sheet 02-224 Varnsdorf and 02-242 Dolní Podluží, northern Bohemia
Abstract
Although the studied area is situated on the periphery of the North Bohemian alkaline sub-province, the amount of volcanic bodies is very large here. The rock set is markedly bimodal (Fig. 1) with phonolitic + trachytic and basaltoid sub-sets. According to chemical classification (Fig. 2), phonolites are much more abundant than trachytes. The scatter of chemistry of the basaltoid sub-set is larger in comparison with the former one. It consists mainly of tephrites and basanites, less of foidites, alkali basalts, trachybasalts, trachyandesites and polzenites. Not very frequent volcanoclastics are confined to basaltoids and they split into tuffs + tuffites and pipe breccias. Subvolcanic rocks of plutonitic appearance, known from the central part of the sub-province, as well as alkaline lamprophyres and trachyandesites are missing. Ultramafic nodules occur scarcerly. Nepheline and/or analcime are the most common foids, minerals of the sodalite group are present in subordinate amount and leucite is absent, even when K2O abundances are comparatively high (Fig. 3).
