DYNAMIC MAGNETIC FABRICS ASSOCIATED WITH CHANGES IN THE MAGMA COOLING DIRECTIONS: A CASE STUDY OF THE TLUSTEC VOLCANO, CZECH REPUBLIC

The Tlustec Kopec, or Hill, is a 1 km² volcano rising 300 m above the Bohemian Cretaceous basin. Tlustec volcano is located in the southern portion of the larger Lusatian Volcanic Field (LVF) at the NE termination of the Eger Rift. In this tri-border region of Poland, Czech Republic, and Germany, the LVF contains volcanoes that erupted within 3 main structural zones: 1) the Elbe Zone, 2) the Lausitz Anticline Zone, and 3) the Görlitz Syncline Zone. Quarrying for basalt dimension stone along 4 benches exposed nearly 100 m of vertical section through the volcano structure. The construct is remarkably uniform in rock type (olivine basalt) but displays a conspicuously disordered set of columns that fan, curve, and terminate against each other and display variably propagating cooling directions. We conducted geochemical and geophysical analyses to assess the relative role of chemistry and geologic setting on column formation. Five bulk geochemical analyses suggest a homogeneous composition of the body, classified as transitional between basanite and trachybasalt (SiO₂ 43.1-44.8 wt. %). The volcano shows a slightly evolved character with moderate and narrow ranges of MgO (6.3-7.3 wt. %), CaO (9.8-12 wt. %), and compatible elements (Cr 140-185 ppm, Ni 54-80 ppm). Forty-two sampling sites were collected across the construct for comprehensive rock magnetic, anisotropy of magnetic susceptibility (AMS), and paleomagnetic data to compare magnetic fabrics to cooling directions. Preliminary AMS data reveal variable fabric orientations that, at times, are consistent with macroscopic cooling directions. Preliminary paleomagnetic data show spatial variations in remanence orientations that suggest subvolcanic deformation associated with volcano growth. One electrical resistivity profile was conducted over the summit of the volcano to image its internal structure. The data were measured using the ARES resistivity system (GF Instruments, Ltd.) with a 5 m electrode spacing along the profile. The data suggest the contact of the basaltic rock with country-rock sandstones is steeply plunging underneath the basalt. This geometry can be interpreted as deeper parts of a lava-lake filling a phreatomagmatic crater. Initial conclusions support emplacement geometry as the controlling factor in column formation at Tlustec Kopec.