PALEOMAGNETIC, GROUND MAGNETOMETRY, AND ANISOTROPY OF MAGNETIC SUSCEPTIBILITY STUDIES BEARING ON EMPLACEMENT OF ZEBíN HILL, JIčíN VOLCANIC FIELD, BOHEMIAN PARADISE, CZECH REPUBLIC

The Jičín Volcanic Field, Czech Republic proffers a relatively well-preserved set of Middle Miocene scoria- and tuff-cones and their feeders. Zebín 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. In addition, a reconnaissance high-resolution ground magnetometry survey was also conducted. 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. Preliminary ground magnetometry data indicate that both normal and reverse polarity rocks are present at Zebín Hill. Detailed paleomagnetic analysis is underway and will discern the polarity of the various dikes and aid with recognizing sub-volcanic deformation. Combining detailed paleomagnetic, ground magnetometry, AMS, and structural studies provide the needed data to constrain the evolution of volcanic systems. The simple external structure of monogenetic volcanoes hides a rather complex magmatic plumbing system that dynamically evolves during the life of the volcano. As we show, the well-exposed roots of Zebín Hill reveals that the growth of a volcano occurs not due to simple central axis feeder systems but rather through interplay of local structures, magmatic effects, and construct evolution throughout the lifetime of the volcano.