Emplacement history of the Trosky basanitic volcano (Czech Republic): paleomagnetic, rock magnetic, petrologic, and anisotropy of magnetic susceptibility evidence for lingering growth of a monogenetic volcano

Abstract

A well-preserved set of mid-Miocene scoria-and tuff-cones and their feeders crops out in the Jičín Volcanic Field, Czech Republic. The Trosky volcano is a scoria cone that has been eroded to reveal the volcano's feeder system. This edifice offers the opportunity to improve the understanding how magma is transported through a monogenetic pyroclastic cone. Physical volcanology, petrology, anisotropy of magnetic susceptibility (AMS) and paleomagnetic data were combined to study of the erosional remnant of the Trosky volcano. Selective erosion has exposed spectacular remnants of a twin scoria cone intruded by late volcanic spines. These spines host a medieval castle that is a landmark of the Bohemian Paradise area in northeast Czech Republic. Paleomagnetic and AMS samples were collected from 29 sites, including the conduits, lava flows, and dikes intruding the conduits. The AMS data reveal magmatic flow directions that were variable, vertical (upward and downward) as well as subhorizontal (into and away from the volcano). Paleomagnetic data from the conduits and lava flows yield reverse polarity directions that are statistically indistinguishable from the expected mid-Miocene reverse polarity field direction. The dikes, however, show both normal and reverse polarity magnetizations that are statistically distinct from the expected field direction. We documented significant compositional variability of lavas erupted from the Trosky volcano, in contrast to the uniform composition of later plugs and dikes. The variability of lavas (olivine-rich, olivinepoor, clinopyroxene-rich and olivine-clinopyroxene equal types) suggests magma storage in a zoned shallow magma chamber (containing olivine-or clinopyroxene-depleted/enriched zones). The combined results and the presence of both normal and reverse polarity magnetizations from the dikes crosscutting the volcano indicate that this monogenic system was long-lived. Taken together, the simple external structure of monogenetic volcanoes often hides a rather complex magmatic plumbing system that dynamically evolves during their lifespan.

Fingerprints of magma mingling processes within the Miocene Zebín tuff cone feeding system (Jičín Volcanic Field, Czech Republic)

“…The distribution of the Cenozoic volcanoes follows the above-mentioned dominant fault systems as they controlled the magma ascent (Vaněčková et al 1993). The E-W trending tectonic grain, however, was the leading factor that influenced the geometry of the dikes and magma feeder conduits at shallow crustal depths (Rapprich et al 2007;Petronis et al 2015). Volcanic activity in the Jičín Volcanic Field occurred during two separate episodes: (1) Early Miocene (19-16 Ma) and (2) Early Pliocene (5-4 Ma).…”

“…2 View of Zebín Hill from southeast. Nearby Trosky Volcano (Petronis et al 2015) is also indicated. Photo by A. Tauchman.…”

“…Apart from the main volcanic complexes (see e.g., Ulrych et al 2002;Rapprich and Holub 2008;Cajz et al 2009;Holub et al 2010;Skála et al 2014;Ackerman et al 2015), several volcanic fields were formed also on the shoulders of the rift (Awdankiewicz 2005;Rapprich et al 2007;Büchner and Tietz 2012;Valenta et al 2014;Büchner et al 2015;Petronis et al 2015;Tietz and Büchner 2015;Awdankiewicz et al 2016;Ulrych et al 2016;Haase et al 2017). Our study focused on the Zebín Volcano, located in the Jičín Volcanic Field (Rapprich et al, 2007;Petronis et al 2015; Fig. 1) on the southeastern shoulder of the Eger Rift, where Miocene volcanic rocks were emplaced into, or erupted onto, Upper Cretaceous marine sediments of the Bohemian Cretaceous Basin and continental Permo-Carboniferous strata of the Krkonoše Piedmont Basin.…”

“…This assumption is also supported by the scarcity of lithics. A different eruptive scenario is interpreted at Trosky Volcano (Petronis et al 2015; Figs 2 and 4) in a contrasting paleo-environmental setting. While Trosky Volcano erupted upon well-drained and dry sandstones, the eruptions of Zebín took place in wetlands on impermeable marlstones and claystones (Fig.…”

“…Recent studies show, however, that even small basaltic volcanoes may display a complex sub-volcanic structure (e.g., Petronis et al 2013Petronis et al , 2015Blaikie et al 2014;Shields et al 2016), long-lived magmatic emplacement, and unpredictable eruption scenarios including the production of mafic pyroclastic flows (Rosi et al 2006;Di Roberto et al 2014). From petrological point of view, individual monogenetic volcanoes are usually characterized by simple composition, although compositionally-simple monogenetic volcanoes may be clustered in volcanic 2.…”