New Mexico Geological Society Annual Spring Meeting — Abstracts

The Jicin Volcanic Field, Czech Republic preserves a well-preserved set of Middle Miocene scoria- and tuff-cones and their feeders. Zebin Hill is a tuff cone that has been quarried to reveal the volcanoes feeder system. This edifice offers the opportunity to understand how magma is transported through a monogenetic pyroclastic cone. Rock types include a coarse-grained basal phreatomagmatic layer and a stratified upper wall facies both of which are penetrated by feeder dikes. The character of magmatic fragments suggests quench fragmentation during a phreatomagmatic eruption. Anisotropy of magnetic susceptibility (AMS) data were collected at twenty-one sites from feeder dikes and the main conduit of the volcano. Magnetic susceptibility intensity ranges from 100E-3 to 300E-3 SI indicating that the dominant magnetic mineral is a ferromagnetic phase with little contribution from paramagnetic minerals. AMS ellipsoids shapes are both oblate and prolate with the shape varying across the intrusions. Curie point estimates yield a spectrum of results indicating a mixture of magnetic minerals from high-Ti titanomagnetite, iron sulfide, and low-Ti titanomagnetite. The AMS inferred magma flow directions yield flow away from the central vent area and subhorizontal flow towards and away from the axial conduit; both upward and downward magma flow is evident at some sites. Paleomagnetic data reveal a high coercivity normal polarity magnetization that varies spatially between the sampled sites. We interpret these data to indicate that subvolcanic deformation occurred during the emplacement of the dikes; although we can not rule out the possibility that the dispersion between sites represents secular variation. We find this unlikely given the short lived emplacement time scale (< 10E1 yrs) for the growth on monogenetic volcanoes. The simple external structure of cinder cones hides a rather complex magmatic plumbing system that continues to evolve during the life of the volcano. The growth of many volcanoes occurs not due to a simple central axis feeder system but rather through interplay of local structures, magmatic effects, and constructs evolution throughout the life time of the volcano. The comparison between structural observations, paleomagnetic, and AMS show that these methods provide valuable complementary data on dike propagation and subsequent magma flow.