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Vladimír Cajz, Vladislav Rapprich, Vojtěch Erban, Zoltan Pécskay and Miroslav Radoň

Late Miocene volcanic activity in the České středohoří Mountains (Ohře/Eger Graben, northern Bohemia)

First occurrences of superficial bodies of Late Miocene volcanic activity were found in the western part of the České středohoří Volcanic Complex (CSVC) and extended our knowledge of its volcanostratigraphy. Their K-Ar ages (9.59, 9.61 and 11.36 Ma) correspond to the age of alkaline basaltic rocks of the youngest known Intrusive Suite of this area. Unlike the previously known subvolcanic bodies of this system, the newly observed bodies are represented by superficial products: two scoria cones with remnants of lava flows and one exclusive lava flow produced from a lava cone. The magmas forming all three occurrences are basanitic. Their primitive chemical composition Sr (0.70347-0.70361) and Nd (0.51279-0.51284) isotope ratios are similar to the products of the first and third volcanic formation of the CSVC. The proved existence of superficial products of the youngest volcanic formation, together with clear superposition relations to sedimentary formations and the chemical character of the youngest magmas in the central part of the Ohře (Eger) Graben support the stratigraphic scheme of volcanic activity in the České středohoří Mts. The eruptive style of the youngest formation volcanoes was purely magmatic (Strombolian) with no phreatic influence.

Open access

Vladimír Cajz, Petr Schnabl, Zoltan Pécskay, Zuzana Skácelová, Daniela Venhodová, Stanislav Šlechta and Kristýna Čížková

Abstract

This paper presents the results of a paleomagnetic study carried out on Plio-Pleistocene Cenozoic basalts from the NE part of the Bohemian Massif. Paleomagnetic data were supplemented by 27 newly obtained K/Ar age determinations. Lavas and volcaniclastics from 6 volcanoes were sampled. The declination and inclination values of paleomagnetic vectors vary in the ranges of 130 to 174 and -85 to -68° for reversed polarity (Pleistocene); or 345 to 350° and around 62° for normal polarity (Pliocene). Volcanological evaluation and compilation of older geophysical data from field survey served as the basis for the interpretation of these results. The Pleistocene volcanic stage consists of two volcanic phases, fairly closely spaced in time. Four volcanoes constitute the Bruntál Volcanic Field; two others are located 20 km to the E and 65 km to the NW, respectively. The volcanoes are defined as monogenetic ones, producing scoria cones and lavas. Exceptionally, the largest volcano shows a possibility of remobilization during the youngest volcanic phase, suggested by paleomagnetic properties. The oldest one (4.3-3.3 Ma), Břidličná Volcano, was simultaneously active with the Lutynia Volcano (Poland) which produced the Zálesí lava relic (normal polarity). Three other volcanoes of the volcanic field are younger and reversely polarized. The Velký Roudný Volcano was active during the Gelasian (2.6-2.1 Ma) and possibly could have been reactivated during the youngest (Calabrian, 1.8-1.1 Ma) phase which gave birth to the Venušina sopka and Uhlířský vrch volcanoes. The reliability of all available K-Ar data was evaluated using a multidisciplinary approach.