Miroslav Krs

Magnetic properties and metastability of greigite-smythite mineralization in brown-coal basins of the Krušné hory Piedmont, Bohemia

Abstract Finely dispersed forms of greigite or greigite-smythite mineralization were found in layers a hundred and more metres thick in the Miocene strata of the Krusne hory (Erzgebirge) Piedmont brown-coal basins. Under laboratory thermal treatment, a pronounced instability of greigite or greigite-smythite mineralization was revealed by the magnetic parameters, conditioned by mineralogical metastability. Thermal treatment in oxidation conditions caused the most pronounced changes in magnetic parameters in a temperature range of 320–380°C, in which a laboratory process of self-reversal of remanence was observed. The products obtained during thermal treatment were identified with the aid of X-ray diffractographs (Co-radiation, Fe-filter). When they were heated to 250°C, no substantial changes were found, while at 300°C the intensities of greigite became weaker, and pyrite and marcasite originated to its detriment. Hexagonal pyrrhotite was generated in addition to pyrite and marcasite. A total decomposition of bisulphides took place at temperatures above 400°C, accompanied by the formation of various modifications of Fe2O3, until finally at higher temperatures only α-Fe2O3 was formed. Laboratory tests suggested self-reversal of remanence in relation to the formation of pyrrhotite. So far, greigite or greigite-smythite mineralization has been proven to exist in the Bohemian Massif solely in connection with Miocene rocks containing fossil micro-organic matter. Products of thermal alteration, pyrite, marcasite, pyrrhotite, γ-, η-, and α-Fe2O3 (or Fe3O4 depending on redox conditions) may be expected in rocks of similar genesis, but partly or completely carbonified as a result of a process of pyrolysis of the micro-organic matter.

A petromagnetic study of Miocene rocks bearing micro-organic material and the magnetic mineral greigite (Sokolov and Cheb basins, Czechoslovakia)

Abstract The Middle Miocene sediments of the Cypris series from the Sokolov and Cheb brown-coal basins (western Bohemia) were subjected to petromagnetic investigation. During the laboratory thermal treatment, at a temperature of 350–400°C, the samples rich in micro-organic material changed into carbonified samples which exhibited similar physical and technological properties to coal samples collected from the underlying coal seam. The experimental results may have a bearing on several problems concerning the origin of fossil fuels and highlight specific rock properties which may have potential for identifying the micro-organic content. A strongly magnetic mineral greigite Fe3S4 was identified in magnetic zones above the coal seam. The authigenic mineral greigite occurs in strata of greater thickness and it was found that a reducing environment, generated by the presence of organic matter, was required for the formation of the greigite. Metastable properties and the low coercivity spectra of this mineral are adverse for palaeomagnetic investigations.

Tectonic and paleogeographic interpretation of the paleomagnetism of Variscan and pre-Variscan formations of the Bohemian Massif, with special reference to the Barrandian terrane

Abstract Paleomagnetic data from Variscan and pre-Variscan formations of the Bohemian Massif (BM) have been interpreted tectonically, and paleogeographically evaluated with respect to the European paleomagnetic results. The interpretation of data from the BM was preceded by the evaluation of the effect of the Trans-European Suture Zone (TESZ) on the dispersion of the European paleomagnetic pole positions during the Variscan orogeny. Stability of the European lithospheric plate is well documented by paleomagnetic results for the Early Permian to the Quaternary rocks of the regions between the Ural Mountains and Great Britain, north of the Alpine tectonic belt. For the Late Carboniferous and earlier times, the stable plate is defined in the regions NE of the TESZ in the East European Craton only. Late Carboniferous and older rocks from the regions SW of the TESZ, largely occupied by the West-European Variscides, show tectonic deformations controlled by prominent horizontal rotations. These rotations can be studied on a theoretical model simulating the distribution of pole positions controlled by rotation of rock formations about a vertical axis. In the BM, horizontal paleotectonic rotations are well documented for Variscan and pre-Variscan rocks: they reach several tens of degrees (up to 180° in extreme cases) of mostly clockwise sense. The study of the tectonics and paleogeography of Cambrian to Devonian formations of the Barrandian area, which is considered a peri-Gondwana terrane with affinities to Armorica, may serve as a case history of the study of a terrane incorporated into a stable lithospheric plate. Here, the Barrandian terrane became a part of the European Plate as a component of the emerging Pangea supercontinent in the final phase of the Variscan orogeny. High, almost peri-polar paleolatitudes for the Cambrian rocks of the Barrandian area are — within the limits of data scatter — in agreement with the presumed position of Gondwana and Armorica. Rocks of Ordovician age did not produce consistent results; they will require further paleomagnetic and paleoclimatologic studies in relation to the data for the Late Cambrian and Early Silurian rocks of the Barrandian area. The analysis of data presented here shows that paleomagnetic declinations of Variscan and pre-Variscan formations reflect paleotectonic rotations following the formation of the respective rocks including rotations induced by the Variscan orogeny and rotation of the European Plate to which the rocks became incorporated. Therefore, paleomagnetic declination of none of the pre-Variscan formations can be identified with paleotectonic rotation either of the Barrandian terrane or even of the whole BM. Magnitudes of horizontal paleotectonic rotations in the BM affected by Variscan tectonism are notably similar to those in the Western Carpathians affected by Alpine deformation. Senses of rotation are, however, different with clockwise rotation prevailing in the BM, and counterclockwise rotations prevailing in the Western Carpathians. Recognition of the effect of horizontal rotations of rock formations, if located in tectonically mobile belts and incorporated into a stable plate during their later history, should be respected while deriving the apparent polar wandering path (APWP).

Palaeomagnetism, palaeogeography and the multicomponent analysis of middle and upper Cambrian rocks of the Barrandian in the Bohemian Massif

Abstract With a view to the importance of the Barrandian in the Bohemian Massif for global palaeogeographic reconstructions, considerable attention has been devoted to palaeomagnetic stability tests and multicomponent analysis of rocks of the pre-Variscan formations, using substantially improved laboratory techniques. This study is devoted to samples of Upper Cambrian andesite and Middle Cambrian sediments. Alternating and thermal fields were used for magnetic cleaning. A thermal demagnetizer fitted with a newly developed inductor of high sensitivity, the ROCOMA (rotating coil magnetometer), with a sensitivity of 0.1 to 0.2 nT, was used. The whole newly built and fully tested MAVACS system (magnetic vacuum control system) guaranteed a high degree of compensation of the magnetic field in a volume of 5 1, to a value not exceeding 2 nT. As numerous laboratory experiments have shown, the high magnetic vacuum was necessary to derive the palaeomagnetic directions of rocks of the older formations affected significantly by the Variscan orogenic phase. The position of the Upper Cambrian pole (28.6°S, 131.2°E) should be considered tentative because it was derived using samples from only three andesite outcrops. Two other andesite outcrops preserved Carboniferous or Permo-Carboniferous palaeomagnetic directions. The position of the Middle Cambrian pole (11.3°S, 154.0°E) was determined using samples of sediments (mainly greywackes and shales) from ten outcrops, some of which represent extensive rock cross-sections. By combining multicomponent analysis with fold tests, the mean direction of the pre-Variscan magnetization could be derived. The very low values of the moduli of the natural remanent magnetization of the Middle Cambrian sediments, and the considerable amount of magnetically soft components, required special care to be taken in the laboratory procedures. The palaeozoogeographic zones of the Barrandian are related to palaeogeographic latitudes of about 40°S, derived from palaeomagnetic data. Cold water is characteristic for the Bohemian fauna, but arctic conditions can be ruled out due to the amount of primary red sediments, mostly of continental origin.

Magnetostratigraphy of Jurassic-Cretaceous limestones in the Western Carpathians

Abstract Magnetostratigraphic studies were carried out on limestones spanning the Tithonian-Berriasian boundary at Brodno (near the town of Žilina, west Slovakia) and Štramberk (north Moravia) in the Western Carpathians. The magnetic carrier in both sections was found to be magnetite. The observed pattern of normal and reverse polarity magnetozones corresponds well to magnetostratigraphic profiles in other areas of the Tethyan realm. Calpionellids are abundant in both profiles, while ammonites are almost completely missing. The base of the Calpionella zone, provisionally considered to represent the Jurassic-Cretaceous boundary in the Tethyan realm, was established in magnetozone M19n. The palaeolatitude of 27°N derived from the Brodno profile corresponds well to those derived from Jurassic rocks of the Križná nappe, on the northern rim of the Inner Carpathians. A pronounced anticlockwise tectonic rotation of 124° is established.

Palaeomagnetism and palaeogeography of the Western Carpathians from the Permian to the Neogene

Abstract Geodynamic models for the Western Carpathians require evaluation of palaeomagnetic data from the Outer Carpathian flysch belt, from limestones of the Klippen Belt and from volcanic and sedimentary rocks of the Inner Carpathians. Palaeomagnetic data for the Permian to the Neogene are evaluated. The data indicate marked, mostly anticlockwise tectonic rotations of larger rock complexes and nappes. The distribution of palaeomagnetic poles is characteristic of a collision zone, and crosses the apparent polar wandering path for the African plate. Tectonic rotations are observed in both the Inner Carpathians and the Klippen Belt as well as in the Outer Carpathian flysch belt. Most drift occurred during the Permian to Triassic interval, with northward movement from initial southern equatorial latitudes. Drift decelerated from the Jurassic to the Neogene.

On the applicability of oil shale to palaeomagnetic investigations

Abstract Samples of oil shales of Autunian age from the Krkonose (Riesengebirge) Piedmont basin, northern Bohemia, which represent rocks with fossil micro-organic matter in the initial stage of carbonification, were subjected to palaeomagnetic investigations. The palaeomagnetic data obtained from these rocks are fully in accordance with those derived previously on rocks of the red-bed facies in the same area. Preliminary results obtained on Lower Carboniferous coal-bearing shales collected from the Upper Silesian basin, Ostrava-Karvina region, indicate also that these rocks, characterized by a higher degree of carbonification, are suitable for palaeomagnetic investigations. These results with those of laboratory carbonification indicate that such rocks as bituminous rocks, black shales, oil shales, coal-bearing shales or generally rocks containing micro-organic matter at different stages of natural carbonification are worthy of being studied palaeomagnetically.

Palaeointensity of the geomagnetic field during upper cainozoic derived from palaeo-slags and porcellanites in north Bohemia

РеэюмеПорчеллaнumы u naлеоченовые щлaкu нa mеррumорuu северноŭ Чехuu nре¶rt;сmaвляюm собоŭ nрuро¶rt;ныŭ мamерuaл, коmорыŭ очень nо¶rt;хо¶rt;um ¶rt;ля выве¶rt;енuя naлеомaгнumных нanрaвленuŭ u naлеоuнmенсuвносmuгеомaгнumного nоля, ¶rt;еŭсmвующего во время кaусmuческого uзмененuя. Эmuгорные nоро¶rt;ы, нaзывaемые эр¶rt;брaн¶rt;ы, вознuклu nо¶rt; влuянuем кaусmuческого uзмененuя nреuмущесmвенно nелumовых осa¶rt;очных nоро¶rt; всле¶rt;сmвuе сaмовозгорaнuя угольных слоев. Кaусmuческое uзмененuе uмело месmо в nерuо¶rt; оm верхнего nлuоценa ¶rt;о чеmверmuчного nерuо¶rt;a. В рaбоmе nре¶rt;ложены mрu uнmерnреmaцuонных nрuемa, nо¶rt;хо¶rt;ящuе ¶rt;ля вы¶rt;еленuя naлеоuнmенсuвносmu, оnuрaющuеся нa меmо¶rt;ы Телье u Нaгama. Новaя рaэрaбоmaннaя annaрamурa MAVACS (Magnetic Vacuum Control System) былa uсnользовaнa ¶rt;ля mермuческого рaзмaгнuчuвaнuя в сре¶rt;е высокого мaгнеmuческого вaкуумa. Был nре¶rt;ложен u nроверен nо¶rt;хо¶rt;, основaнныŭ нa мульmuкомnоненmном aнaлuзе мaгнеmuэaцuu. Кроме nолученuя меmо¶rt;uческuх знaчuŭ, было maкже усmaновлено, чmо uнmенсuвносmьгеомaгнumного nоля в uссле¶rt;уемыŭ nерuо¶rt; менялaсь в nре¶rt;елaх 48%±4%-154%±32% знaченuя uнmенсuвносmu современногогеомaгнumного nоля.SummaryPorcellanites and palaeo-slags from North Bohemia are natural materials which can be used to derive the palaeomagnetic directions and palaeointensity of the geomagnetic field active at the time of caustic alteration. The origin of these rocks, called erdbrands, was due to the caustic alteration of predominantly pelitic sediments as a result of underground fires conditioned by spontaneous ignition of coal seams. The caustic alteration occurred during the Upper Pliocene to the Quaternary. Three procedure based on the methods by Thellier and Nagata are presented in the paper. The newly developed apparatus MAVACS (Magnetic Vacuum Control System) was used for the thermal demagnetization of samples. A procedure based on multi-component analysis was also proposed and tested. Besides some methodic results, it was found that the geomagnetic field intensity varied during the respective period within the limits of 48%±4% to 154%±32% of the present geomagnetic field intensity.

Magnetism of natural pyrrhotite, haematite and ilmenite

SummaryThe paper deals with the magnetic properties of the natural minerals haematite, ilmenite and pyrrhotite. The natural remanent magnetization Jn, the volume susceptibility ℵ, the specific magnetization of saturation σso, the Curie temperature Θ, the A.C. demagnetization of natural remanent magnetization, the thermal demagnetization Jn, the temperature dependence of the specific magnetization of saturation, and several other parameters were determined for the individual sets of minerals. Since natural minerals can and do contain various heterogeneous and isomorphic admixtures, their magnetic properties may change depending on the content and type of these admixtures. Therefore, all the investigations of the magnetic properties described in this paper were carried out with extensive sets of minerals, representing various Czechoslovak and world localities. A number of interesting results were obtained from the executed experiments, e.g., the different stability of the natural remanent magnetization of haematite with respect to A.C. demagnetization, the presence and type of heterogeneous inclusions in ilmenite, the phase changes connected with the γ-transition in natural pyrrhotites, etc. The types of distributions of the investigated values of the magnetic properties were also determined.

Magnetism and ferromagnetism of natural minerals of the spinel group

РезюмеСтатья посвящается вопросам определения основных магнитных свойств и их естественных рассеяний естественных минералов группы спинел. Для отдельных групп минералов определялись: обьомная восприимчивость в земном магнитном поле, удельное намагничение насышения, температура Кюри и некоторые другие выведенные параметры и свойства. Из сопоставления магнитных данных, касаюшихся замеренных минералов группы спинел, следует, что многие среди них являются натуральными ферритами. В ряде случаев они обладают специфическими магнитными свойствами, указывающими на то, что причиной ферромагнетизма является связывание некоторых элементов со структурной рещеткой. Таким типичным представителем минералов является спинел-плеонаст. Отчетливые ферромагнитные свойства обнаруживают магнезиоферрит, франклинит и якобсит. У герцинита преобладает парамагнетизм а у хромита—парамагнетизм с паразитным ферромагнетизмом, вызванным гетерогенными примесями. Химическая изменчивость элементов, содержащихся в естественных минералах вызывает полный спектр соответствующих магнитных свойств, напр, температуру Кюри, удельное намагничение насыщения, модуль естественного остаточного намагничения и т. д. И наоборот, для магнетитов, отобранных с различных месторождений в мире существуют узко ограниченные пределы значений обьемной восприимчивости 0,1–0,5СГСМ/cm3. В статье указывается и на геофизические аспекты найденных параметров, как например, на то, что ферромагнитные свойства были обнаружены и у минералов, где сушествование ферромагнетизма ранее не предполагалось.