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

A well-preserved set of mid-Miocene scoria-and tuff-cones and their feeders crops out in the Jicin 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.