Results in 2006
IAA 300130612– Combined magnetostratigraphic studies of Cenozoic volcanics, Bohemian Massif
V. Cajz, P. Pruner, M. Chadima, P. Schnabl, J. Ulrych, M. Konzalová,
F. Hrouda, , F. Holub, V. Rapprich (PřFUK)
Sampling for magnetic propertieswas performed from 33 individual sites from the České středohoří Mts. Petrographic characteristics of the studied rock types were described in thin sections. They included olivine basalt, basanite, bostonite, camptonite, monzodiorite, phonolite, tephrite, trachybasalt, and trachyte.
The nature of the magnetic carriers was investigated using different rock magnetic techniques. Magnetic susceptibility (k), low-field variation of magnetic susceptibility (kHD), NRM, SIRM were measured for each specimen. Later, several specimens representing each sampling site and/or particular rock type were analyzed using temperature variations of magnetic susceptibility, IRM acquisition and back-field demagnetization, thermal demagnetization of the tree-component IRM, and hysteresis parameters. In most of the specimens the low coercivity ferrimagnetic mineral corresponding to titinomagnetite with variable Ti-content was identified. In some cases minor amounts of hematite and/or pyrrhotite may be present. The increasing amount of substituted Ti in titanomagnetite, as revealed by low-field variation of magnetic susceptibility, decreases the Curie temperature of the studied rock and may influence some hysteresis parameters. Consequently, the unblocking temperature (when TRM was acquired) varies significantly according to the rock type studied.
Stable characteristic remanent magnetization (ChRM) directions were obtained by stepwise alternating field (AF) and thermal (Th) demagnetization methods and subsequent principal component analysis. In most cases both methods yielded comparable results. The mean direction was calculated by combining the means from 24 sites (715 specimens). The site selection was based on the following criteria: number of specimens with stable ChRM greater than 10, precision parameter K >10.0, and α95 < 10.0º. Both normal- and reverse-polarity sites were present, the angle between respective means is 170.34º. After transposition of reverse-polarity sites, the common mean direction is: D = 3.9º, I = 64.6º, k = 41.32, A95 = 4.7º. This gives a virtual geomagnetic pole (VGP) of 85.09ºN, 160.88ºE (dp = 6.06º, dm = 7.55º) with a corresponding paleolatitude of 46.48º, which agrees well with the Late Tertiary apparent polar wander path (APWP) for Europe (see Figure). For the purpose of the anisotropy of magnetic susceptibility we studied 23 individual sites yielding more than 500 oriented specimens. The studied rock types included olivine basalt, bostonite, camptonite, phonolite, tephrite, trachybasalt. Quantitative parameters of magnetic anisotropy (kmean, P, T) were evaluated with regard to individual sites and different rock types. Magnetic susceptibility values, kmean, vary according to rock type from about 3000 × 10–6 (phonolites) to more than 120000 × 10–6 SI (camptonite). The degree of anisotropy, P, is generally relatively low, P < 1.10, except for some phonolite specimens where P reaches the maximum values of 1.25. This fact reflects the differences in viscosities of the respective. Consequently, different mechanisms orienting magnetic minerals should be expected in various rock types studied. The shape of anisotropy ellipsoid ranges from slightly prolate to neutral and oblate.
Several different orientations of the principal AMS directions in relation to dike or sill orientation can be found. An inverse fabric with magnetic lineations and magnetic foliations perpendicular to the dike margins was found in camptonite dike. Normal magnetic fabric with magnetic foliations and magnetic lineations subparallel to the dike margins was found in trachybasalt and bostonite dikes. In the former the magnetic lineations are subvertical whereas in the later subhorizontal magnetic lineations can be observed. Comparing all studied sites it seems that the type of magnetic fabric is lithology-dependent (e.g. bostonite possessing normal fabric and camptonite possessing inverse fabric). Whether predominant occurrence of normal/inverse fabric reflects the presence of MD/SD grains or results from different orientation mechanism of magnetic minerals remains unclear. This question must be answered before any successful geological interpretation of magnetic fabric can be made.
Published results:
Cajz, V., Mrlina, J., Adamovič, J., Mach, K., Chadima, M., 2006: Structural setting of the České středohoří Mts. volcanic centre, Bohemian Massif, central Europe. – LASI II Physical geology of subvolcanic systems: Laccoliths, sills and dykes. 1.- 3.4. 2006, Portree, Isle of Skye, p. 73-76.
Chadima, M., Cajz, V., 2006: Magnetic fabric of dikes and sills of the České Středohoří Mts. – preliminary results. – 10th „Castle Meeting“ New Trends in Geomagnetism, Paleo, Rock and Environmental Magnetism, 3.- 8.9.2006 2006, Castle of Valtice. Travaux Géophysiques, p. 19-20.
Chadima, M., Schnabl, P., Cajz, V., Marske, J., Herrero-Bervera, E., 2006: A new paleomagnetic pole for Central Europe as derived from selected Tertiary volcanic rocks of the Bohemian Massif. – 10th „Castle Meeting“ New Trends in Geomagnetism, Paleo, Rock and Environmental Magnetism, 3.- 8.9. 2006, Castle of Valtice. Travaux Géophysiques, p. 22-23.