Petrophysical investigations in the Southern Bohemian Massif (Austria): Data-acquisition, -organisation and-interpretation

Summary The petrophysical parameters Density (p), Susceptibility (SUS), as well as Natural Remanent Magnetisation (NRM) and Koenigsberger ratio (Q_n) were measured on approx. 2600 core samples (magmatic and a few metamorphic rocks) from the Southern Bohemian Massif. These and associated data have been organized into primary and subsidiary dBASE IV databases. The quantity and kind of information now available through the databases are described.40 sampled types of rocks have been statistically analysed and a detailed delimitation of different rocks as well as their varieties is attempted with the aid of various graphic software.The lowest susceptibility values (SUS < 0.1 × 10^–3 SI) were measured for the leucocratic Altenberg and Haibach granites, above it for the acid Eisgarn granite and for aplites. The only fine grained granite with a higher average (M = l.32 × 10^–3 SI) than the other granites is the Schlägl granite.Average values of NRM vary over a range of 10⁴ mA/m. The Altenberg and Haibach granites (fine-to medium-grained, two-mica leucogranites) are again (see SUS) in the group with the lowest values (< 1 mA/m). The average values of coarse-grained, older synorogenic granites (Finger andHöck, 1986), Weinsberg and Engerwitzdorf (medium-to coarse grained) granites, Schlieren granite and Rastenberg granodiorite are generally uniform (< 5 mA/m), with the exception of the stronger remanent magnetism of the Schlieren granite (25 mA/m).The Q_n, values of all investigated coarse grained granites are less than 0.25 (exception: Schlieren granite) whereas the fine-middle grained granites Peuerbach, Schaerding, Schrems and the fine grained granites in general all have Q_n > 1.The densities of all studied granite types vary only from 2600 kg/m³ to the upper limit of 2710 kg/m³ (average of rock types). Therefore consideration of only one petrophysical parameter does frequently not suffise for characterisation of a rock type. However, a combined study of NRM-SUS or p-SUS proved to be useful in many cases e.g. petrophysical distinction between Schrems granite and Mauthausen granite.

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Petrophysikalische Untersuchungen in der südlichen Böhmischen Masse (Österreich): Daten-Akquisition, -Organisation und -Interpretation

Zusammenfassung Die petrophysikalischen Parameter Dichte (p), Suszeptibilität (SUS), sowie Natürliche Remanente Magnetisierung (NRM) und Königsberger Faktor (Q_n) wurden an rund 2600 Bohrkernen (Magmatite und einige Metamorphite) aus der Böhmischen Masse ermittelt. Diese und damit im Zusammenhang stehende Daten wurden in einer dBASE IV Hauptdatenbank und gekoppelten Nebendatenbestanden organisiert. Es wird die Art von Information, die über die Datenbank nun zugänglich ist näher erläutert. 40 beprobte Gesteinstypen werden einer statistischen Analyse unterzogen und unter zu Hilfenahme diverser Graphiksoftware wird eine detailliertere Abgrenzung der einzelnen Gesteine und ihrer Varietaten versucht.Die geringsten Suszeptibilitätswerte (SUS < 0.1 × 10^–3 SI) wurden an Proben der leukokraten Altenberger und Haibacher Granite, darüber hinaus auch an Kernen des sauren Eisgarner Granits und der Aplite gemessen. Der einzige feinkörnige Granit mit einem überdurchschnittlichen Mittelwert (M = 1.32 × 10^–3 SI) im Vergleich zu anderen Graniten ist der Schlägl Granit.Die errechneten Mittelwerte der NRM streuen über einen Bereich von 10⁴ mA/m. Der Altenberger und der Haibacher Granit (fein- bis mittelkörnige Zweiglimmergranite) weisen auch hier wieder die geringsten Werte (< 1 mA/m) auf. Die Gruppe der grobkörnigen, älteren synorogenen Granite (Finger undHöck, 1986), nämlich Weinsberger und Engerwitzdorfer (mittel- bis grobkörnig), Schlierengranit und Rastenberger Granodiorit bleiben mit ihren NRM Werten unter 5 mA/m mit Ausnahme des offensichtlich stärker remanent magnetisierten Schlierengranits (25 mA/m).Alle untersuchten grobkörnigen Granite weisen Q_n Werte < 0.25 (Ausnahme: Schlierengranit) auf, während hingegen die fein- bis mittelkörnigen Peuerbacher, Schärdinger und Schremser Granit, sowie die Feinkorngranite im allgemeinen, alle Q_n > 1 erreichen.Die Dichten der verschiedenen Granite variieren nur von 2600 kg/m³ bis 2710 kg/m³ (Gesteinsmittelwerte). Dies zeigt, daß die Betrachtung nur eines einzigen petrophysikalischen Parameters in vielen Fällen nicht alleine ausreicht um ein Gestein petrophysikalisch eindeutig zu bestimmen. Vielmehr stellte sich für eine Charakterisierung der Gesteine eine kombinierte Untersuchung von NRM-SUS oder p-SUS oft als zielführend heraus, wie z.B. im Falle des Schremser Granits und des Mauthausener Granits.

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With 15 Figures     

A scheelite mineralization in calc-silicate rocks of the Moldanubicum (Bohemian Massif) in Austria

In the Moldanubicum (Bohemian Massif) of Austria a tungsten mineralization bound to calc-silicate rocks was discovered in the so-called Bunte Serie, a metasedimentary geologic unit now in amphibolite to higher amphibolite facies. The main constituents of the scheelite-bearing rocks are ferro-salite, meionite-rich scapolite, and quartz. The average tungsten content is estimated to be 1500 ppm; no further elements that would be characteristic for an exoskarn formation are enriched conspicuously. The whole series of calc-silicate rocks is intercalated in sillimanite gneisses. This series could be traced for about 5 km. The mineralization seems to be strata-bound — contacts with intrusive rocks were not observed. The field evidence is, therefore, easiest to reconcile with a synsedimentary origin, but the geology of the region is not known in sufficient detail to make this interpretation conclusive.

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Zusammenfassung Im österreichischen Anteil des Moldanubikums (Böhmische Masse) wurde eine Wolframvererzung entdeckt. Sie ist an Kalksilikatgesteine der Bunten Serie gebunden, einer metamorph-sedimentären Einheit, die in Amphibolit- bis höherer Amphibolitfacies vorliegt. Die Hauptkomponenten dieser Scheelit-führenden Gesteine sind Ferrosalit, Mejonit-reicher Skapolith und Quarz. Der durchschnittliche Wolframgehalt liegt bei 1500 ppm; weitere Elemente, die für einen Exoskarn charakteristisch sein würden, sind nicht angereichert. Die gesamte Kalksilikatgesteinsserie ist Sillimanitgneisen eingelagert. Sie ist auf etwa 5 km im Streichen verfolgbar. Die Mineralisation erscheint schichtgebunden — Kontakte zu Intrusivgesteinen wurden nicht beobachtet. Der Feld-befund ist daher am einfachsten mit einer synsedimentären Entstehung in Einklang zu bringen. Die Geologie dieses Gebietes ist jedoch im Detail zu wenig bekannt, um diese Interpretation als zwingend darzustellen.

     

Uranium,thorium and potassium in granulite facies rocks,Bohemian Massif,Lower Austria,Austria

Summary The granulite facies rocks of the Lower Austrian Moldanubian Zone of the Bohemian Massif were formed under conditions of the earth's lower crust. The U contents lie below 0.4 ppm. The rocks are also characterized by low Th contents (<3 ppm), although in some cases Th is higher in biotite-rich layers of banded granulites, which could be explained as a primary feature. Marginal parts of the granulite facies complexes occasionally exhibit retrograded U enrichments. The K contents of the granulites are generally lower than in the surrounding rock types of similar mineralogical composition (Gföhl gneisses).

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Uran, Thorium und Kalium in Gesteinen der Granulit-Fazies aus der Böhmischen Masses in Niederösterreich, Österreich

Zusammenfassung Die Granulitfaziesgesteine des niederösterreichischen Moldanubikums wurden unter Bedingungen der tiefen Kruste gebildet. Die U-Gehalte der Gesteine liegen zum größten Teil unterhalb 0,4 ppm. Sie weisen auch geringe Th-Gehalte (<3 ppm) auf. Th ist allerdings in gewissen biotitreichen Partien höher, was auch als primäre Anlage gedeutet werden kann. In randlichen Bereichen der Granulitfacieskomplexe ist gelegentlich U leicht angereichert. Die K-Gehalte der Granulite sind im allgemeinen geringer als in umgebenden Gesteinen mit ähnlicher mineralogischer Zusammensetzung (GfÖhler Gneis).

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With 4 Figures

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Dedicated to Prof. DDr.H. Wieseneder on the occasion of his 70th birthday.     

Cordierite from a high-temperature low-pressure shear zone of the south-western Bohemian Massif (Moldanubian terrain,Austria)

A medium-scale shear zone exposed in the gneiss rocks of the South-western Bohemian Massif (Moldanubian Zone) contains cordierite, whose Na p.f.u. is subject to a significant increase from the centre to the edge of the deformation area, whilst other elements only show negligible variations. Coexisting mineral phases of cordierite include garnet, biotite, and sillimanite. According to the results obtained from the garnet-cordierite Fe²⁺/Mg²⁺-exchange thermometer a decrease of peak temperature from 639 °C in the central mylonite to 593 °C in the marginal mylonite can be observed, which indicates significant shear heating. Lithological pressures were estimated by considering the position of cordierite-forming reactions in the P-T field and the stability of coexisting sillimanite. They are subject to a reduction from 0.35 GPa in the highest deformed mylonite to 0.31 GPa at the margin of the shear zone. According to the results of comprehensive petrographic and mineralogical studies the investigated shear zone underwent a Variscan HT-LP metamorphic event implying the formation of cordierite and an Alpine MT-LP event entailing the rotation and decomposition of the cordierite phase.     

Geodynamic and tectonic evolution of the southeastern Bohemian Massif: The Thaya section (Austria)

Summary The tectonostratigraphy within eastern sections of the Bohemian Massif includes two different terranes. A Proterozoic terrane is composed of the Moravo-Silesian parautochthon, the Moravian nappe complex and the Moldanubian Variegated and Monotonous complexes. A Paleozoic terrane includes the Gföhl Gneiss and the granulite klippen. Both terranes are separated by an oceanic suture zone which is represented by the Letovice ophiolite complex (Czech Republic) and the Raabs complex in Austria. The Raabs structural unit is interpreted to represent a tectonic melange of a dismembered ophiolite complex and metaandesites.The tectonic evolution of the southeastern Bohemian Massif includes: (1) Paleozoic extension predating late Variscan nappe stacking; (2) Variscan (c. 350-320 Ma) NE-directed nappe assembly by foreward propagation of thick-skinned nappes, whereas individual thrusts initiated within different crustal levels; (3) post-stacking Variscan W-E extension which was responsible for penetrative nappe internal deformations; and, (4) dispersion of units by a system of dextral strike-slip faults and genetically related thrust- and normal faults. The kinematic history during Variscan convergence is explained to have been related to oblique (dextral) transpression of Proterozoic against Paleozoic terranes.

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Geodynamische und tektonische Entwicklung der südöstlichen Böhmischen Masse: Das Thaya Profil (Österreich)

Zusammenfassung Eine Gliederung der südöstlichen Böhmischen Masse umfaßt zwei kontinentale Blöcke (Terranes). Das proterozoische Terrane besteht aus dem Moravo-Silesischen Parautochton, den Moravischen Decken und basalen Anteilen des Moldanubikums (Bunte Serie und Monotone Serie). Das paläozoische Terrane umfaßt den Moldanubischen Gföhler Gneis und die Granulitklippen. Beide Krustenblöcke werden durch eine ozeanische Sutur getrennt, die durch den Letovice-Ophiolith (Tschechien) und die Raabser Einheit (Österreich) repräsentiert ist. Die Raabser Einheit wird als eine tektonische Melange, bestehend aus einem Ophiolith und einer kalkalkalischen, andesitischen Suite gedeutet. Die tektonische Entwicklung läßt folgende Entwicklungsstufen erkennen: (1) Paläozoische Krustenextension vor der spätvariszischen Deckenstapelung; (2) Spätvariszische (ca. 350-320 Ma) nordostgerichtete Deckenstapelung, wobei jüngere Decken in Richtung des Vorlandes progradierten. Dabei wurden einzelne Deckenbahnen in unterschiedlichen Krustenniveaus betätigt; (3) Generelle West-Ost Extension und Entwicklung des penetrativen Gefüges nach der Deckenstapelung; (4) Verteilung der Einheiten durch gleichzeitige Aktivität von steilen nordost-streichenden Scherzonen und flachen Auf- und Abschiebungen. Die kinematische Entwicklung während der variszischen Gebirgsbildung ist auf schräge (dextrale) Plattenkonvergenz zwischen dem proterozoischen und dem paläozoischen Terrane zurückzuführen.

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With 10 Figures     

Formation of low-pressure reaction textures during near-isothermal exhumation of hot orogenic crust (Bohemian Massif,Austria)

Two types of aluminous paragneiss from the Loosdorf complex (Bohemian Massif, NE Austria) contain coarse-grained granulite assemblages and retrograde reaction textures that are investigated to constrain the post-peak history of the Gföhl unit in the southern Bohemian Massif. Both types have a peak assemblage garnet–biotite–sillimanite–plagioclase–K-feldspar–quartz–granitic melt ± kyanite ± ilmenite ± rutile, recording peak metamorphic conditions of $$0.9–1.1 GPa and $$780–820°C estimated by isochemical phase equilibrium modelling. The first sample type (Ysper paragneiss) developed (i) cordierite coronae around garnet and (ii) cordierite–spinel and cordierite–quartz reaction textures at former garnet–sillimanite interfaces. Calculated chemical potential relationships indicate that the textures formed in the course of a post-peak near-isothermal decompression path reaching $$0.4 GPa. Texture formation follows a two-step process. Initially, cordierite coronae grow between garnet and sillimanite. As these coronae thicken, they facilitate the development of local compositional domains, leading to the formation of cordierite–spinel and cordierite–quartz symplectites. The second sample type (Pielach paragneiss) exhibits only discontinuous cordierite coronae around garnet porphyroblasts but lacks symplectites. The formation of cordierite there also indicates near-isothermal decompression to 0.4–0.5 GPa and 750–800°C. This relatively hot decompression path is explained by the contemporaneous exhumation of a large HP–UHT granulite body now underlying the Loosdorf complex. The timing of regional metamorphism in the granulites and the southern Bohemian Massif in general is well constrained and has its peak at $$340 Ma. Monazite from Loosdorf paragneiss samples yield a slightly younger age of $$335 Ma. Although the ages overlap within error, they are interpreted to reflect near-isothermal decompression and exhumation resulting in the formation of the observed reaction textures.     

Morphology and growth trends of accessory zircons from various granitoids of the South-western Bohemian Massif (Moldanubicum,Austria)

As the main objective of the present study, the morphological development of accessory zircons from four granitoids (pearl gneiss, fine-grained granite, coarse-grained gneiss, and Weinsberg granite) of the South-western Bohemian Massif was described in detail. On the one hand, this was realized by the classical approach, including a statistical evaluation of external zircon morphologies with the typology scheme. On the other hand, direct insight into the growth of single crystals was established by the production of crystal sections parallel and perpendicular to the crystallographic c-axis and by their subsequent imaging with the electron microprobe. Regarding the second method, eventual morphological trends were represented as a function of the growth rates of single crystal forms. Except for the coarse-grained gneiss, zircon crystals of the investigated granitoids show similar morphological trends according to both methods, starting with a dominant {1 0 0} prism and equally sized pyramids. Final crystal habit, however, is marked by the predominance of {1 1 0} and {2 1 1}. Zircon crystals from the coarse-grained gneiss run through a completely different development with a change of the prism habit from {1 1 0} to {1 0 0} and a more or less static growth of {1 0 1} with only slight modifications in size. Comparison of the results with data from the literature underlines the role of magma chemistry, magma temperature, and cooling rate as the main factors responsible for growth trends of accessory zircon.     

Precambrian blacksands as precursors of magnetite and Ilmenite bearing chlorite-micaschists,Bohemian Massif,Austria

Summary A complex of Precambrian chlorite-micaschists in the Moravian zone of the Bohemian massif contains up to 20 vol.% of fine grained magnetite and ilmenite. The rocks are characterized by high Fe, Ti and V contents (averages: 13 wt %, 1.6 wt% and 200 ppm), showing a strong positive correlation. SiO₂ contents amount to 60 wt.%; together with relatively high K and low Ca and Mg values this is considered to be evidence for metapelites and -psammites. Relictic sedimentary layers of magnetite and chlorite, as well as relictic titano-magnetites were occasionally observed. Magnetite contains up to 0.5 wt.% V₂O₃ and Cr₂O₃ each, and up to 1.2 wt.% TiO₂. Ilmenites show up to 5 wt.% MnO.The unusual bulk chemistry of the metasediments, the relictic layered structures and the composition of the magnetites (in relation to magnetites of different genesis) suggest a clastic sedimentary origin of the ore minerals, eroded probably from some basic magmatic complex. Hence a Precambrian black-sand is proposed as a precursor rock and compared with different blacksands known from the literature.

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Präkambrische Blacksands als Precursors von Ilmenit führenden Chlorit-Glimmerschie fern der Böhmischen Masse, österreich

Zusammenfassung Eine Serie prdkambrischer Chlorit-Glimmerschiefer im Moravikum der Böhmischen Masse enthält bis zu 20 Vol.% feinkörnigen Magnetit und Ilmenit. Die Gesteine sind durch hohe Gehalte an Fe, Ti and V (durchschnittlich 13 Gew.%, 1.6 Gew.% und 200 ppm), die cine starke positive Korrelation untereinander aufweisen, charakterisiert. Die SiO₂-Gehalte betragen 60 Gew.%; dies wird, gemeinsam mit relativ hohen K- und niedrigen Ca- und Mg-Werten, als Hinweis auf Metapelite und -psammite angesehen. Gelegentlich wurden reliktische Lagen von Magnetit und Chlorit, wie auch reliktische Titano-Magnetite beobachtet. Der Magnetit enthält bis zu jeweils 0.5 Gew.% V₂O₃ und Cr₂O₃, und bis zu t.2 Gew.% TiO, Die Ilmenite zeigen bis zu 5 Gew.% MnO.Die ungewöhnliche Gesamtzusammensetzung der Metasedimente, die reliktisch gebänderten Strukturen und die Zusammensetzung der Magnetite (verglichen mit Magnetiten verschiedenster Genese) legen einen klastisch-sedimentären Ursprung der, wahrscheinlich von einem basischen Magmatit abgetragenen, Erzminerale nahe. Folglich wird der Einfluß eines präkambrischen Blacksands im Ausgangsgestein vorgeschlagen und mit unterschiedlichen Blacksands aus der Literatur verglichen.

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With 5 Figures     

Radiometric age of volcanics of the Cambrian “Krivoklat-Rokycany” complex (Bohemian Massif)

The volcanics from Krivoklat-Rokycany complex (Barrandian synclinal of Bohemia) are investigated by the Rb-Sr whole-rock isochron method (λ ⁸⁷Rb=1,47 10^?11y^?1). The age of 474±5 m. y. of these volcanics, stratigraphically overlain by the Tremadoc, implies that the Cambrian-Ordovician boundary, up to now estimated at 500 m. y. might be reset at about 470 m. y. The good quality of the isochron stresses the consanguinity of the different pétrographic terms of the Krivoklat-Rokycany volcanic series and the initial isotopic ratio⁸⁷Sr/⁸⁶Sr at 0,7041±0,0003 points out its infracrustal or mantle origin.     

Minor element- and carbonaceous material thermometry of high-grade metapelites from the Sauwald Zone, Southern Bohemian Massif (Upper Austria)

The Sauwald area is located at the southern rim of the Bohemian Massif and contains migmatites and high-grade metapelitic and granitic gneisses. These rocks were metamorphosed during the post-collisional high-T/low-P stage of the Variscan metamorphic event (~330 Ma). Metapelitic samples were taken from two localities near Kössldorf and Pyret in Upper Austria and the investigated samples contain the mineral assemblage garnet + cordierite + spinel + sillimanite + K-feldspar + quartz + biotite + muscovite + magnetite + graphite. An important aspect of this study is the evaluation of previously published P-T estimates for these high-grade metapelites (Knop et al. 1995; Tropper et al. 2006) involving Ti-in-biotite, Na-in-cordierite thermometry and the micro-Raman thermometer based upon the degree of crystallization of carbonaceous material. In the two samples studied three texturally and chemically different biotites are distinguished. Biotite inclusions in garnet have the highest Ti contents of 5–6 wt.% TiO₂. Matrix biotites contain 2–4 wt.% TiO₂ and biotites from late-stage muscovite-biotite symplectites contain <2 wt.% TiO₂. This corresponds to temperatures of 730–760°C (stage 1), 600–700°C (stage 2), and 550–610°C (stage 3). Since the Ti-in-biotite thermometer strongly depends on X _Mg of biotite, which is susceptible to changes during retrogression the calculated temperatures for stage 1 are interpreted as minimum temperatures of the peak metamorphic stage. The Na contents of the studied cordierites vary from 0.1 to 0.2 wt.% Na₂O. Application of the Na-in-cordierite thermometer yields temperatures in the range of 770–900°C; they are strongly dependent on the bulk Na₂O content of the samples. The micro-Raman geothermometer of graphite was applied to carbonaceous material, which occurs as inclusions in garnet and cordierite. It yielded a maximum temperature >650°C, i.e. above the calibration limit of this method. This study shows that the obtained temperature estimates agree well with the P-T estimates based on phase equilibrium thermobarometry (Knop et al. 1995; Tropper et al. 2006), thus illustrating the validity of these thermometers. Nevertheless in order to more precisely constrain the metamorphic evolution of these high-grade rocks, better constrained experimental calibrations of, for instance the Na-in-cordierite thermometer, are clearly needed.     

Constraints on the origin and the genesis of graphite-bearing rocks from the variegated sequence of the Bohemian Massif (Austria)

Summary Transitions between graphitic gneisses, graphite-bearing calcsilicate rocks and marbles of the Variegated Sequence (Bunte Serie) in the Moldanubian zone of the Bohemian massif were examined for their primary sedimentary signatures. The bulk chemistry of the graphitic gneisses is similar to those of common black shales. Chondrite-normalized rare earth element (REE) distribution patterns of graphite-bearing rocks resemble those of average pelitic sediments. Graphite ¹³C-values around –22%o (PDB) show primary organic isotopic characteristics in the silicate rocks and confirm their organogenic origin. Traces of metasomatic mobilizations were found for a number of elements such as Ba, Rb, Cs, the REEs and Mn.Carbon isotope thermometry using the calibration ofScheele andHoefs (1992) suggests a peak-temperature of 640–780°C for the graphite-bearing calcsilicates and marbles. Small scale isotopic studies at the interface between gneisses and marbles argue against a pervasive streaming CO₂-rich fluid derived from an external source and imply that the fluid was controlled more locally by lithology. Ore petrographic investigations showed iron-rich alabandite as an abundant phase within the calcsilicate rocks and marbles. The occurrence and metamorphic origin of alabandite within these rocks indicates high S- and low CO₂-, O₂-fugacities in local layers of the Variegated Sequence.

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Herkunft und Genese von graphitführenden Gesteinen der Bunten Serie aus der Böhmischen Masse (Österreich)

Zusammenfassung Übergänge zwischen Graphitgneisen, Kalksilikatgesteinen und Marmoren der Bunten Serie im Moldanubikum der Böhmischen Masse wurden auf primäre sedimentäre Merkmale hin untersucht. Die Gesamtzusammensetzung der Graphitgneise gleicht der von durchschnittlichen Schwarzschiefern. Chondrit-normierte Verteilungsmuster der Seltenen Erdelemente (SEE) der graphitführenden Gesteine spiegeln pelitische Sedimente wider. Kohlenstoffisotopenzusammensetzungen der Graphite mit ¹³C-Werten um –22 (PDB) aus den Graphitgneisen lassen eindeutig auf einen organogenen Ursprung der Graphite schließen. Hinweise auf metasomatische Mobilisierungen finden sich bei den Elementen Ba, Rb, Cs, den SEE und Mn.Graphit-Kalzit Isotopenthermometrie nachScheele undHoefs (1992) ergibt eine Peak-Temperatur zwischen 640–780°C für das prägende Metamorphoseereignis. Isotopengeochelnische Kleinbereichsuntersuchungen erlauben es, ein während der prägenden Metamorphose lithologische Grenzen pervasiv durchströmendes CO₂-reiches Fluid, das von einer externen Quelle abgeleitet wird, auszuschließen und weisen auf eine eher lokale, von der Lithologie abhängige, Prägung des Fluids hin. Im Zuge der erzpetrographischen Untersuchung wurde Alabandin in den Kalksilikatgesteinen und Marmoren gefunden. Die Gegenwart von Alabandin, der in diesen Gesteinen als metamorphe Bildung angesehen wird, kann als Anzeiger für bereichsweise höhere S- und niedere CO₂- bzw. O₂-Fugazitäten in lokalen Bereichen der Bunten Serie gewertet werden.

     

Petrography,geochemistry and geochronology of granite hosted rhyodacites associated with a disseminated pyrite mineralization (Arnolz,Southern Bohemian Massif,Austria)

Mineralogy and Petrology - The study focuses on a subvolcanic rhyodacite dyke intruding a fine grained biotite granite and paragneisses of the South Bohemian Massif, part of the Variscan Orogenic...     

Zircon U/Pb and Pb/Pb geochronology of the Rastenberg granodiorite,South Bohemian Massif,Austria

Summary A combined zircon typology, zircon Pb-Pb evaporation, and conventional U-Pb study of the late- to post-tectonic Rastenberg granodiorite yields the following results: Typological investigations show two distinguishable zircon populations. Type l: subtype S24 ofPupin, colourless to slightly pink, clear to turbid, often with cores, few to abundant inclusions, long prismatic; type 2: subtype S4 ofPupin, colourless to reddish or slightly pink, clear to slightly turbid, no visible cores, abundant inclusions, tabular habit, short prismatic.At least 4 different zircon-forming events can be distinguished: Inherited cores with ages around 623±22Ma and single ages > 1206Ma from type 1 zircons imply the reworking of rocks derived from Cadomian and Proterozoic to Archean crust. Ages around 353±9Ma from type 1 zircons are interpreted as timing a first magma formation or the onset of a long-lasting magma-generating event during the Variscan plutonism in the South Bohemian pluton. The actual intrusion of the granodioritic magma into the middle crust took place around 338±2Ma (type 2 and rims of type 1 zircons).Only type 1 zircons are found as inclusions in large K-feldspar phenocrysts providing evidence that these phenocrysts have grown before the 338Ma event and may be as old as 353 Ma.[]

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Zirkon U/Pb und Pb/Pb Geochronologie des Rastenberger Granodiorits, südliche Böhmische Masse, Österreich

Zusammenfassung Zirkon-typologische Untersuchungen, Einzelzirkon Pb-Pb-Evaporations- und konventionelle U-Pb-Altersbestimmungen an Gesteinen des spät- bis postkinematischen Rastenberger Granodiorits geben folgende Resultate: Typologisch lassen sich zwei Zirkonpopulationen unterscheiden: Typ 1: S24 Subtyp vonPupin, farblos bis leicht rosa, klar bis getrübt, häufig mit Kernen, wenige bis viele Einschlüsse, lang prismatisch; Typ 2: S4 Subtyp vonPupin, farblos bis rötlich oder leicht rosa, klar bis leicht getrübt, keine sichtbaren Kerne, viele Einschlüsse, flachtafeliger Habitus, kurzprismatisch.Mindestens 4 unterschiedliche Altersgruppen lassen sich unterscheiden: Ererbte Kerne von Typ 1 Zirkonen mit Altern um 623±22Ma und einzelnen Altern > 1206Ma sprechen für die Aufarbeitung von Gesteinen, die von cadomischen und proterozoischen bis archaischen Gesteinen im Krustenbereich des Südböhmischen Plutons abstammen könnten. Alter um 353±9Ma von Typ 1 Zirkonen werden einer ersten magmatischen Phase oder dem Beginn der Krustenaufschmelzung im Zuge der Bildung der variszischen Plutonite zugeordnet. Die eigentliche Intrusion des Granodiorits um 338±2Ma wird mit Typ 2 und mit Randpartien von Typ 1 Zirkonen erfaßt.In den großen K-Feldspat-Phänokristallen finden sich nur Zirkone vom Typ 1. Dies deutet darauf hin, daß die Phänokristalle vor dem 338Ma Ereignis gebildet wurden, eventuell also bis 353Ma alt sein können.

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With 6 Figures     

The Precambrian tectogenesis in the Bohemian Massif

The Cadomian and Variscan tectogeneses are two distinctive and easily distinguished cycles of a long-term geological process leading to the almost complete crustal consolidation of the Bohemian Massif. The internal parts of the massif are those of the main development of the Upper Proterozoic geosyncline and the intensive Cadomian metamorphism and plutonism.The Precambrian of the Bohemian Massif is divided into two principal regional chronostratigraphic units: the Moldanubian and the Brioverian; the age of the boundary between them is estimated at about 1000 Ma B. P. The Brioverian is subdivided into three units: the Lower, Middle and Upper. The Moldanubian is provisionally subdivided into two units: the Lower and the Upper.The maximum of the metamorphic activity of the Cadomian cycle falls approximately within the sedimentation interval of the Middle Brioverian, and that of the Variscan cycle in the Devonian. Compared with the Cadomian regional metamorphism which attained mostly amphibolite to granulite facies, the Variscan metamorphism generally did not exceed greenschist facies.The origin of the granitoid rocks of the massif is closely associated with the metamorphic processes of the two above-mentioned cycles. Cadomian granitoids are represented mostly by rocks of granodiorite-tonalite and durbachite types whereas the Variscan intrusives are dominated by rocks of granitic composition.

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Zusammenfassung Die assyntische und variszische Orogenese sind die wesentlichen strukturprägenden Ereignisse im Böhmischen Massiv. Die internen Teile des Massives sind aus der jung-proterozoischen Geosynklinale hervorgegangen und während der assyntischen Gebirgsbildung metamorphisiert und von Magmatiten durchdrungen worden. Das Präkambrium des Böhmischen Massives ist in zwei chronostratigraphische Einheiten zu gliedern: das Moldanubium und das Brioverian. Die Grenze liegt etwa bei 1000 Mill. Jahren. Das Brioverian läßt sich in drei Einheiten gliedern und das Moldanubium in zwei Einheiten.Das Maximum der metamorphen Überprägung während der assyntischen Orogenese fällt in die Sedimentationslücke im mittleren Brioverian; im variszischen Zyklus erfolgt die Metamorphose im Devon. Die assyntische Metamorphose erreicht Amphibolit- bis Granulitfazies, die variszische geht nicht über eine Grünschieferfazies hinaus.Die magmatischen Gesteine sind eng mit den beiden Orogenesen verbunden. Die assnytischen Magmatite sind vorwiegend Granodiorit-Tonalite und Durbachite, während die variszischen Magmatite aus Graniten bestehen.

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Résumé Les tectogenèses cadomienne et varisque sont deux cycles différents, bien discernables, d'un processus géologique de longue durée, qui a conduit à la cosolidation presque complète du Massif de Bohème. La partie interne du Massif est celle du développement principal du géosynclinal protérozoïque supérieur et du métamorphisme et du plutonisme cadomiens.Le Précambrien du Massif de Bohème se divise en deux unités chronostratigraphiques régionales: le Moldanubien et le Briovérien. On estime l'âge de la limite entre elles à 1000 millions d'années environ. Le Briovérien est subdivisé en trois unités: inférieure, moyenne et supérieure. Le Moldanubien est divisé provisoirement en deux unités: inférieure et supérieure.La culmination de l'activité métamorphique du cycle cadomien tombe à peu près dans l'intervalle de sédimentation du Briovérien moyen, celle du cycle varisque dans le Dévonien. Comparé avec le métamorphisme régional cadomien qui le plus souvent a atteint le faciès amphibolite, le métamorphisme varisque, en général, n'a pas dépassé le faciès schistes verts.L'origine des roches granitoïdes du Massif et associée avec les processus métamorphiques des deux sus-dits cycles. Les granitoïdes cadomiens sont représentés pour la plupart par des roches du type des granodiorites-tonalites et durbachites, tandis que parmi les roches intrusives varisques prédominent des roches de composition granitique.

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, , . - , , , . : Moldanubium Brioverian. 1 . Brioverian , a Moldanubium . Brioverian; . ; . . - , — qg.

     

Exhumation, strain localization, and emplacement of granitoids along the western part of the Central Bohemian shear zone (Bohemian Massif)

Emplacement of granitoid magmas and simultaneous exhumation of deeply buried rocks has been investigated along the western part of the Central Bohemian shear zone (CBSZ, Bohemian Massif). Combined structural, petrological and geochronological data of the steeply dipping shear zone suggest complex uplift and exhumation of deeply buried, high-temperature Moldanubian rocks, resulting in the juxtaposition against the supracrustal Teplá-Barrandian unit. Uplift of Moldanubian rocks from depths of probably more than 30 km was initiated after crustal stacking in Upper Devonian times. Syntectonic Lower Carboniferous emplacement of the Klatovy pluton into the pre-existing shear zone led to melt-controlled strain softening and localization. However, the major part of the total displacement of the CBSZ was accommodated within a late- to post-intrusive high-temperature shear zone in the uprising Moldanubian unit and a post-intrusive unexposed fault zone in the Klatovy pluton, respectively. During uplift of the Moldanubian rocks, strain was strongly partitioned into melt-bearing zones (Klatovy pluton, migmatites of the Moldanubian unit) resulting in a repeated shift of deformation in space and time.     

Fluid-rock interaction and thermal evolution during thrusting of an Alpine metamorphic complex (Tinos island, Greece)

This study examines the fluid-rock interaction and thermal evolution along a thrust that juxtaposes calcite-rich marbles of high P-T metamorphic unit of the Attic-Cycladic Massif (Greece) on top of a lower-grade dolomite marble unit. The Tertiary thrust represents a major phase of tectonic movement related to the decompression of the Alpine orogen in the Hellenides. The stable isotope signatures of the thrust plane and adjacent sections of the footwall and hanging wall rocks are characterized by significant carbon and oxygen isotope depletions. The depletion is most pronounced in calcite, but is almost entirely missing in coexisting dolomite. The isotopic patterns in the thrust zone can be explained by the infiltration of an externally derived water-rich H₂O-CO₂-CH₄ fluid [X _C (=X _{CO 2}+X _{CH 4})<0.05] at water-rock ratios on the order of 0.1 to 0.5 by weight. The fluid-induced calcite recrystallization is viewed as an important rheological control during thrusting. The temperature evolution of the footwall, hanging wall and mylonitic tectonic contact was determined by calcite-dolomite solvus thermometry. Histograms of calcite-dolomite temperatures are interpreted as indicating a heating of the footwall dolomite marble during the thrusting of the hotter upper plate. Conversely, the hanging wall marble unit was cooled during the thrusting. The calcite-dolomite thermometry of the thrust plane gives temperatures intermediate between the initial temperatures of the lower and upper marble units, and this leads to the conclusion that conductive heat transfer rather than fluid infiltration controlled the thermal evolution during thrusting. Received: 14 April 1998 / Accepted: 9 December 1998     

Experimental constraints on ultrapotassic magmatism from the Bohemian Massif (durbachite series, Czech Republic)

The equilibrium phase relations of a mafic durbachite (53 wt.% SiO₂) from the Třebíč pluton, representative of the Variscan ultrapotassic magmatism of the Bohemian Massif (338–335 Ma), have been determined as a function of temperature (900–1,100°C), pressure (100–200 MPa), and H₂O activity (1.1–6.1 wt.% H₂O in the melt). Two oxygen fugacity ranges were investigated: close to the Ni–NiO (NNO) buffer and 2.6 log unit above NNO buffer (∆NNO + 2.6). At 1,100°C, olivine is the liquidus phase and co-crystallized with phlogopite and augite at 1,000°C for the whole range of investigated pressure and water content in the melt. At 900°C, the mineral assemblage consists of augite and phlogopite, whereas olivine is not stable. The stability field of both alkali feldspar and plagioclase is restricted to low pressure (100 MPa) at nearly water-saturated conditions (<3–4 wt.% H₂O) and T < 900°C. A comparison between experimental products and natural minerals indicates that mafic durbachites have a near-liquidus assemblage of olivine, augite, Ti-rich phlogopite, apatite and zircon, followed by alkali feldspar and plagioclase, similar to the mineral assemblage of minette magma. Natural amphibole, diopside and orthopyroxene were not reproduced experimentally and probably result from sub-solidus reactions, whereas biotite re-equilibrated at low temperature. The crystallization sequence olivine followed by phlogopite and augite reproduces the sequence inferred in many mica-lamprophyre rocks. The similar fractionation trends observed for durbachites and minettes indicate that mafic durbachites are probably the plutonic equivalents of minettes and that K- and Mg-rich magmas in the Bohemian Massif may have been generated from partial melting of a phlogopite–clinopyroxene-bearing metasomatized peridotite. Experimental melt compositions also suggest that felsic durbachites can be generated by simple fractionation of a more mafic parent and mixing with mantle-derived components at mid crustal pressures.     

Metamorphic record of burial and exhumation of orogenic lower and middle crust: a new tectonothermal model for the Drosendorf window (Bohemian Massif, Austria)

Summary A continuous, but attenuated section through orogenic lower and middle crust overthrust by a second lower-crustal complex was distinguished at the eastern margin of the Bohemian Massif. This indicates the existence of two lower-crustal “autochthonous” extrusions into middle crust that is not compatible with the model of “allochthonous” lower crustal klippen remaining after flat thrusting of the Gf?hl nappe over large distances. The base of the lower crust is represented by a granulite body exhumed from c. 15 kbar and 800 °C. A hangingwall complex of layered amphibolites gradually passes into amphibolite bearing paragneisses (the Monotonous unit) and micaschists intercalated with marbles at the top (the Varied unit). The metamorphic grade and anatexis decreases upwards and the micaschists preserve a prograde path to c. 8 kbar and 700 °C. This sequence is overthrust by a second lower crustal strongly migmatitized complex, referred to as the Raabs complex, which is marked by an eclogite-bearing belt at the base. The garnet zoning of eclogite indicates burial from 10 kbar to min. 15 kbar. In all units, relics of a steep metamorphic fabric were identified, reworked by folding and a moderately west-dipping foliation. The conditions of 7–10 kbar and approximately 750 °C for the flat foliation were obtained in all units indicating that exhumation of the lower crust into a middle crustal level occurred earlier, probably during the development of steep fabrics. The intense flat fabric is interpreted as a result of thrusting of the whole assembly over the middle crustal Brunian indentor.