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.     

Contrasting tectono-metamorphic evolution of orogenic lower crust in the Bohemian Massif: A numerical model

The Bohemian Massif, located at the eastern margin of the European Variscan belt, is characterised by an exceptional accumulation of felsic high-pressure granulites. The petrological, structural and geochronological studies of this region revealed systematic differences between the tectonometamorphic evolution of the southern (Moldanubian) and northern (West Sudetes) parts of the orogen. Two contrasting tectonic scenarios have been proposed: gravity-driven vertical mass exchanges followed by continental indentation in the Moldanubian domain, and crustal-scale folding leading to gneiss dome formation in the West Sudetes. We present a numerical model in order to correlate the apparent differences between these two regions with the variations in the dynamics of the modelled system. We model two colliding blocks: an orogenic root, where a felsic lower crust is overlain by a mafic layer and a middle crust, and a continental indentor. We examine the role of the rate of convergence of the two blocks, radiogenic heat production within the felsic lower crust and efficiency of erosion. The prograde part of the metamorphic evolution is controlled by the rate of convergence and the peak temperature depends on the heat production. The retrograde evolution is controlled mostly by erosional processes. In the models, where the material is weakened due to the heating in the felsic lower crust, the gravitational instability of the mafic and felsic layers causes their complete vertical exchange followed by a flow above the indentor. In colder and/or faster models, the thickening is dominated by the buckling of the mafic layer. These two styles of deformation, i.e. gravity-dominated and fold-dominated models, correspond to the structures observed in the Moldanubian and the West Sudetes. Moreover, the calculated pressure–temperature paths of the felsic lower crust are in agreement with available data.     

Dating granulite-facies structures and the exhumation of lower crust in the Moldanubian Zone of the Bohemian Massif

Deformation of granulite-facies rocks in the Moldanubian Zone of the southern Bohemian Massif is expressed in two intersecting planar fabrics - steeply disposed (S₁) and flat-laying (S₂) - which correspond to two deformation stages (D₁) and (D₂). The existing Sm-Nd garnet ages from banded granulite gneisses, new U-Pb zircon data from deformed granite intrusions within the granulite gneisses, and the P-T and field structural relations constrain the ages and P-T conditions of the two deformation phases. The early deformation (D₁) was associated with a HP-HT metamorphic stage with a minimum age of ca. 354 Ma which was followed by a near-isothermal decompression. A concordant U-Pb zircon age of 318ǃ Ma dates the emplacement of intrusions of deformed granite into the granulite gneisses and constrains deformation phase (D₂). This phase was associated with an LP-HT metamorphism dated in the region at ca. 340-330 Ma. The available structural and isotopic data indicate that granulites in the southern Bohemian Massif were exhumed from lower to middle crust during compression. The structural relations and P-T-t data for the studied granulites are consistent with their exhumation by near-vertical extrusion of the softened orogenic root.     

Vertical extrusion and middle crustal spreading of omphacite granulite: a model of syn-convergent exhumation (Bohemian Massif, Czech Republic)

The exhumation of eclogite facies granulites (Omp–Plg–Grt–Qtz–Rt) in the Rychleby Mts, eastern Czech Republic, was a localised process initiated by buckling of crustal layers in a thickened orogenic root. Folding and post‐buckle flattening was followed by the main stage of exhumation that is characterized by vertical ductile extrusion. This process is documented by structural data, and the vertical ascent of rocks from a depth of c. 70 to c. 35 km is documented by metamorphic petrology. SHRIMP ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²⁰⁶Pb evaporation zircon ages of 342 ± 5 and 341.4 ± 0.7 Ma date peak metamorphic conditions. The next stage of exhumation was associated with sideways flat thrusting associated with lateral viscous spreading of granulites and surrounding rocks over indenting adjacent continental crust at a depth of c. 35–30 km. This stage was associated with syntectonic intrusion of a granodiorite sill at 345–339 Ma, emplaced at a crustal depth of c. 25 km. The time required for cooling of the sill as well as for heating of the country rocks brackets this event to a maximum of 250 000 years. Therefore, similar ages of crystallization for the granodiorite magma and the peak of eclogite facies metamorphism of the granulite suggest a very short period of exhumation, limited by the analytical errors of the dating methods. Our calculations suggest that the initial exhumation rate during vertical extrusion was 3–15 mm yr^?1, followed by an exhumation rate of 24–40 mm yr^?1 during further uplift along a magma‐lubricated shear zone. The extrusion stage of exhumation was associated with a high cooling rate, which decreased during the stage of lateral spreading.     

Prograde and retrograde metamorphic fabrics – a key for understanding burial and exhumation in orogens (Bohemian Massif)

In the Orlica–?nie?nik Dome (NE Bohemian massif), alternating belts of orthogneiss with high‐pressure rocks and belts of mid‐crustal metasedimentary–metavolcanic rocks commonly display a dominant subvertical fabric deformed into a subhorizontal foliation. The first macroscopic foliation is subvertical, strikes NE–SW and is heterogeneously folded by open to isoclinal folds with subhorizontal axial planes parallel to the heterogeneously developed flat‐lying foliation. The metamorphic evolution of the mid‐crustal metasedimentary rocks involved successive crystallization of chlorite–muscovite–ilmenite–plagioclase–garnet, followed by staurolite‐bearing and then kyanite‐bearing assemblages in the subvertical fabric. This was followed by garnet retrogression, with syntectonic crystallization of sillimanite and andalusite parallel to the shallow‐dipping foliation. Elsewhere, andalusite and cordierite statically overgrew the flat‐lying fabric. With reference to a P–T pseudosection for a representative sample, the prograde succession of mineral assemblages and the garnet zoning pattern with decreasing grossular, spessartine and X_Fe are compatible with a P–T path from 3.5–5 kbar/490–520 °C to peak conditions of 6–7 kbar/～630 °C suggesting burial from 12 to 25 km with increasing temperature. Using the same pseudosection, the retrograde succession of minerals shows decompression to sillimanite stability at ～4 kbar/～630 °C and to andalusite–cordierite stability at 2–3 kbar indicating exhumation from 25 km to around 9–12 km. Subsequent exhumation to ～6 km occurred without apparent formation of a deformation fabric. The structure and petrology together with the spatial distribution of the metasedimentary–metavolcanic rocks, and gneissic and high‐pressure belts are compatible with a model of burial of limited parts of the upper and middle crust in narrow cusp‐like synclines, synchronous with the exhumation of orogenic lower crust represented by the gneissic and high‐pressure rocks in lobe‐shaped and volumetrically more important anticlines. Converging P–T–D paths for the metasedimentary rocks and the adjacent high‐pressure rocks are due to vertical exchanges between cold and hot vertically moving masses. Finally, the retrograde shallow‐dipping fabric affects both the metasedimentary–metavolcanic rocks and the gneissic and high‐pressure rocks, and indicates that the ～15‐km exhumation was mostly accommodated by heterogeneous ductile thinning associated with unroofing of a buoyant crustal root.     

Tectonic evolution of a continental magmatic arc from transpression in the upper crust to exhumation of mid-crustal orogenic root recorded by episodically emplaced plutons: the Central Bohemian Plutonic Complex (Bohemian Massif)

The Central Bohemian Plutonic Complex (CBPC) consists of episodically emplaced plutons, the internal fabrics of which recorded tectonic evolution of a continental magmatic arc. The ~354–350 Ma calc-alkaline plutons were emplaced by multiple processes into the upper-crustal Teplá-Barrandian Unit, and their magmatic fabrics recorded increments of regional transpression. Multiple fabrics of the younger, ~346 Ma Blatná pluton recorded both regional transpression and the onset of exhumation of mid-crustal orogenic root (Moldanubian Unit). Continuous exhumation-related deformation during pluton cooling resulted in the development of a wide zone of sub-solidus deformation along the SE margin of the CBPC. Finally, syn-exhumation tabular durbachitic pluton of ultrapotassic composition was emplaced atop the intrusive sequence at ~343–340 Ma, and the ultrapotassic Tábor pluton intruded after exhumation of the orogenic root (~337 Ma). We suggest that the emplacement of plutons during regional transpression in the upper crust produced thermally softened domain which then accommodated the exhumation of the mid-crustal orogenic root, and that the complex nature of the Teplá-Barrandian/Moldanubian boundary is a result of regional transpression in the upper crust, the enhancement of regional deformation in overlapping structural aureoles, the subsequent exhumation of the orogenic root domain, and post-emplacement brittle faulting.     

Tracing exhumation record in high-pressure micaschists: A new tectonometamorphic model of the evolution of the eastern part of the Fore Sudetic Block,Kamieniec Metamorphic Belt,NE Bohemian Massif,SW Poland

The Kamieniec Metamorphic Belt comprises a volcano-sedimentary succession exposed within a collision zone between the Saxothuringian and Brunovistulian crustal domains of the European Variscides. The studied rocks recorded two metamorphic episodes. The first episode, M₁, occurred at conditions of c. 485 ± 25 °C and 18 ± 1.8 kbar related to burial within a subduction zone. The subsequent episode, M₂, was linked to the final phases of exhumation to mid-crustal level, associated with pressure and temperature (P–T) conditions ranging from c. 520 ± 26 °C and 6 ± 0.6 kbar through 555 ± 28 °C and 7 kbar ± 0.7 to ~590 ± 30 °C and 3–4 ± 0.4 kbar. The documented deformation record is ascribed to three events, D₁ to D₃, interpreted as related to the burial and subsequent exhumation of the Kamieniec Metamorphic Belt. The D₁ event must have witnessed the subduction of the Kamieniec Metamorphic Belt rock succession whereas the D₂ event was associated with the exhumation and folding of the Kamieniec Metamorphic Belt in an E-W-directed shortening regime. A subsequent folding related to the D₂ event was initiated at HP conditions, however, the planar fabric produced during a late stage of the D₂ event, defined by a low-pressure mineral assemblage M₂, indicates that the D₂ final stage was synchronous with the onset of the M₂ episode. Consequently, the entire D₂ event seems to have been associated with the exhumation of the Kamieniec Metamorphic Belt to mid crustal level. The third deformation event D₃, synchronous with the M₂ episode, marked the last stage of the exhumation, and was linked to emplacement of granitoid veins and lenses. The latter resulted in heating and rheological weakening of the entire rock succession and in the formation of non-coaxial shear zones.     

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     

Thermal and mechanical behaviour of the orogenic middle crust during the syn‐ to late‐orogenic evolution of the Variscan root zone,Bohemian Massif

We combine structural observations, petrological data and ⁴⁰Ar–³⁹Ar ages for a stack of amphibolite facies metasedimentary units that rims high‐P (HP) granulite facies felsic bodies exposed in the southern Bohemian Massif. The partly migmatitic Varied and Monotonous units, and the underlying Kaplice unit, show a continuity of structures that are also observed in the adjacent Blanský les HP granulite body. They all exhibit an earlier NE?SW striking and steeply NW‐dipping foliation (S3), which is transposed into a moderately NW‐dipping foliation (S4). In both the Varied and Monotonous units, the S3 and S4 foliations are characterized by a Sil–Bt–Pl–Kfs–Qtz–Ilm±Grt assemblage, with occurrences of post‐D4 andalusite, cordierite and muscovite. In the Monotonous unit, minute inclusions of garnet, kyanite, sillimanite and biotite are additionally found in plagioclase from a probable leucosome parallel to S3. The Kaplice unit shows rare staurolite and kyanite relicts, a Sil–Ms–Bt–Pl–Qtz±Grt assemblage associated with S3, retrogressed garnet?staurolite aggregates during the development of S4, and post‐D4 andalusite, cordierite and secondary muscovite. Mineral equilibria modelling for representative samples indicates that the Varied unit records conditions higher than ~7 kbar at 725 °C during the transition from S3 to S4, followed by a P?T decrease from ~5.5 kbar/750 °C to ~4.5 kbar/700 °C. The Monotonous unit shows evidence of partial melting in the S3 fabric at P?T above ~8 kbar at 740–830 °C and a subsequent P?T decrease to 4.5–5 kbar/700 °C. The Kaplice unit preserves an initial medium‐P prograde path associated with the development of S3 reaching peak P?T of ~6.5 kbar/640 °C. The subsequent retrograde path records 4.5 kbar/660 °C during the development of S4. ⁴⁰Ar–³⁹Ar geochronology shows that amphibole and biotite ages cluster at c. 340 Ma close to the HP granulite, whereas adjacent metasedimentary rocks preserve c. 340 Ma amphibole ages, but biotite and muscovite ages range between c. 318 and c. 300 Ma. The P?T conditions associated with S3 imply an overturned section of the orogenic middle crust. The shared structural evolution indicates that all mid‐crustal units are involved in the large‐scale folding cored by HP granulites. The retrograde P–T paths associated with S4 are interpreted as a result of a ductile thinning of the orogenic crust at a mid‐crustal level. The ⁴⁰Ar–³⁹Ar ages overlap with U–Pb zircon ages in and around the HP granulite bodies, suggesting a short duration for the ductile thinning event. The post‐ductile thinning late‐orogenic emplacement of the South Bohemian plutonic complex is responsible for a re‐heating of the stacked units, reopening of argon system in mica and a tilting of the S4 foliation to its present‐day orientation.     

Fluid-rock interaction in the Mo-bearing Nebelstein greisen complex,Bohemian Massif (Austria)

Summary In the Nebelstein area, molybdenite-bearing greisens occur together with peraluminous leucogranites. In the compositional change of the granites to the greisens, there is an almost complete loss of Na, combined with a decrease in Ca, Mg, Sr, and Ti concentrations. The progressive alteration is reflected by lower homogenization temperatures and increasing salinity in aqueous fluid inclusions. The fluid regime prior to greisenization was water-dominated with low salt contents, while the early stage of the greisen development was characterized by a mixed fluid containing carbon dioxide and water. This was succeeded by a moderate saline aqueous fluid which caused the mineralization by exchange of metal ions for Na⁺(Ca²⁺, K⁺). A negative correlation between salt content in fluid inclusions and Na₂O concentrations in the bulk rocks supports this model. Mass balance calculations for this interaction yield a minimum fluid-rock ratio of approximately 2 : I. Greisenization took place at a minimum pressure of 180 MPa (1.8 kb) and in a temperature range between 200 and < 400 °C.

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Fluid-Gestein-Wechselwirkung in dem Molybdänit führenden Greisenkomplex Nebelstein, Böhmische Masse (Österreich)

Zusammenfassung Die Molybdänglanz führenden Greisengesteine des Nebelsteins sind an peraluminöse Granite gebunden. Bei der Alteration der Granite ist für den Übergang Biotitgranit zu Greisen eine weitestgehende Verarmung an Na zu beobachten, gleichzeitig nehmen auch die Gehalte an Ca, Mg, Sr und Ti ab. Die fortschreitende Greisenbildung dokumentiert sich in den wäßrigen Flüssigkeitseinschlüssen durch steigende Salinität bei sinkenden Homogenisierungstemperaturen. Die fluide Phase war vor der Greisenbildung H₂O dominiert und niedrig salinar. Der Beginn der Alterationsprozesse ist durch CO₂ und H₂O hältige Fluide gekennzeichnet. Danach folgt ein Anstieg der Salinität, der auf den Austausch von Metallchloridlösungen gegen Na⁺, K^- und Ca^2- zurückgeführt wird. Dies läßt sich durch eine negative Korrelation der Salinität in den Flüssigkeitseinschlüssen mit dem Na-Gehalt der Gesteine belegen. Daraus wurde die Volumsbeziehung der den Granit durchströmenden fluiden Phase relativ zum Gestein mit mindestens 2 : 1 abgeleitet. Die Mineralisation fand bei einem Minimaldruck von 1,8 kb in einem Temperaturbereich von 200 - < 400 °C statt.

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This paper was presented at the IGCP 291 Project Symposium Metamorphic Fluids and Mineral Deposits, ETH Zürich, March21–23,1991.     

Kinematic and rheological model of exhumation of high pressure granulites in the Variscan orogenic root: example of the Blanský les granulite, Bohemian Massif, Czech Republic

Summary A large-scale relict domain of granulite facies deformation fabrics has been identified within the Blansky les granulite body. The granulite facies mylonitic fabric is discordant to the dominant amphibolite facies structures of the surrounding retrograde granulite. The complex geometry of retrograde amphibolite facies fabric indicates a large-scale fold-like structure, which is interpreted to be a result of either crustal-scale buckling of an already exhumed granulite sheet or active rotation of a rigid granulite facies ellipsoidal domain in kinematic continuity with the regional amphibolite facies deformation. We argue that both concepts allow similar restoration of the original granulite facies fabrics prior to the amphibolite facies deformation and “folding”. The geometry of the granulite facies foliations coincides with the earliest fabrics in the nearby mid-crustal units suggesting complete mechanical coupling between the deep lower crust and the mid-crustal levels during the vertical movements of crustal materials. Microstructures indicate grain-size sensitive flow enhanced by the presence of silicate melts at deep crustal levels and a beginning of an exhumation process of low viscosity granulites through a vertical channel. The amphibolite facies fabrics developed at middle crustal levels and their microstructures indicate significant hardening of feldspar-made rigid skeleton of the retrograde granulite. Increase in the strength of the granulite allowed an active buckling or a rigid body rotation of the granulite sheet, which acted as a strong layer inside the weaker metasediments.     

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     

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.     

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.

     

Two generations of Variscan garnet: Implications from a petrochronological study of a high-grade Avalonia-derived paragneiss from the Drosendorf unit,Bohemian Massif

Garnets in metapelitic paragneisses from the southern Drosendorf unit in the Austrian part of the Bohemian Massif exhibit two episodes of growth during the Variscan orogeny, which can be distinguished on textural and chemical grounds. The first garnet (grt1) records evidence of high-grade metamorphism in the Late Devonian (Frasnian–Famennian), while the second garnet (grt2) formed by a second high-grade event in the Early Carboniferous (Visean). Both garnet generations contain abundant inclusions, of which monazite, rutile and crystallised melt droplets are particularly useful for reconstructing P–T–t conditions. The Late Devonian age (373 ± 9 Ma) for the first episode of garnet growth was obtained from chemical dating of monazite inclusions in grt1. Metamorphic conditions during the first episode of garnet growth are estimated to have been between 0.7 and 0.8 GPa at 680–700 °C and 0.95–1.10 GPa at 745–785 °C. There followed a phase of cooling and exhumation, after which the second garnet (grt2) were formed beginning under amphibolite facies conditions and continuing prograde to peak conditions of 0.95–1.10 GPa and 745–785 °C, which are similar to those of the first garnet forming event. Subsequently, the rocks experienced near isothermal decompression to 0.5–0.8 GPa. Chemical dating of both monazite inclusions in grt2 and the matrix provide a Visean age (343 ± 3 Ma).A study of detrital zircons in these paragneisses revealed zircon forming events at around 1.2, 1.5 and 1.8 Ga, suggesting an Avalonian provenance. The lack of zircons younger than 1 Ga and the presence of Cadomian metamorphic monazite relics (652 ± 15 Ma) indicates an Early Neoproterozoic deposition age for the sedimentary protolith likely. Our documentation of a Late Devonian high-grade metamorphic event in rocks derived from Avalonian corroborates tectonic models which assume that frontal parts of the Armorican terrane had already docked with Avalonia by this time.     

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.     

Multiple zircon growth during fast exhumation of diamondiferous, deeply subducted continental crust (Kokchetav Massif, Kazakhstan)

Diamondiferous rocks from the Kokchetav Massif, Kazakhstan, represent deeply subducted continental crust. In order to constrain the age of ultra high pressure (UHP) metamorphism and subsequent retrogression during exhumation, zircons from diamondiferous gneisses and metacarbonates have been investigated by a combined petrological and isotopic study. Four different zircon domains were distinguished on the basis of transmitted light microscopy, cathodoluminescence, trace element contents and mineral inclusions. Mineral inclusions and trace element characteristics of the zircon domains permit us to relate zircon growth to metamorphic conditions. Domain 1 consists of rounded cores and lacks evidence of UHP metamorphism. Domain 2 contains diamond, coesite, omphacite and titanian phengite inclusions providing evidence that it formed at UHP metamorphic conditions (P>43 kbar; T~950 °C). Domain 3 is characterised by low-pressure mineral inclusions such as garnet, biotite and plagioclase, which are common minerals in the granulite-facies overprint of the gneisses (P~10 kbar; T~800 °C). This multi-stage zircon growth during cooling and exhumation of the diamondiferous rocks can be best explained by zircon growth from Zr-saturated partial melts present in the gneisses. Domain 4 forms idiomorphic overgrowths and the rare earth element pattern indicates that it formed without coexisting garnet, most probably at amphibolite-facies conditions (P~5 kbar; T~600 °C). The metamorphic zircon domains were dated by SHRIMP ion microprobe and yielded ages of 527LJ, 528NJ and 526LJ Ma for domains 2, 3 and 4 respectively. These indistinguishable ages provide evidence for a fast exhumation beyond the resolution of SHRIMP dating. The mean age of all zircons formed between UHP metamorphic conditions and granulite-facies metamorphism is 528Dž Ma, indicating that decompression took place in less than 6 Ma. Hence, the deeply subducted continental crust was exhumed from mantle depth to the base of the crust at rates higher than 1.8 cm/year. We propose a two-stage exhumation model to explain the obtained P-T-t path. Fast exhumation on top of the subducted slab from depth >140 to ~35 km was driven by buoyancy and facilitated by the presence of partial melts. A period of near isobaric cooling was followed by a second decompression event probably related to extension in a late stage of continental collision.     

Strain coupling between upper mantle and lower crust: natural example from the Běstvina granulite body, Bohemian Massif

Strain patterns within mantle rocks and surrounding coarse‐grained felsic granulites from the Kutná Hora Crystalline Complex in the Variscan Bohemian Massif have been studied in order to assess their strain coupling. The studied rock association occurs within low‐strain domains surrounded by fine‐grained granulite and migmatite. The Doubrava peridotite contains closely spaced and steeply dipping layers of garnet clinopyroxenite, which are parallel to the NE–SW‐striking, high‐temperature foliation in nearby granulites, while the Úhrov peridotite lacks such layering. The Spa?ice eclogite is not associated with peridotite and shows upright folds of alternating coarse‐ and fine‐grained varieties bearing NE–SW‐striking axial planes. Electron back‐scattered diffraction measurements revealed full strain coupling between clinopyroxenites and coarse‐grained granulites in the S₁ fabric that is superposed on the S₀ fabric preserved in peridotites. The B‐type olivine lattice preferred orientation (LPO) characterizes the S₀ fabric in peridotites and its reworking is strongly controlled by the presence of macroscopic clinopyroxenite layering. The S₁ in clinopyroxenites and coarse‐grained granulites is associated with the LS‐type clinopyroxene LPO and prism <c> slip in quartz respectively. While the S₁ fabric in these rock types is accompanied invariably by a sub‐vertical stretching lineation, the S₁ fabric developed in reworked Úhrov peridotite is associated with strongly planar axial (010) type of olivine LPO. The peridotites with the S₀ fabric are interpreted to be relicts of a fore‐arc mantle wedge hydrated to a various extent above the Saxothuringian subduction zone. The prograde metamorphism recorded in peridotites and eclogites occurred presumably during mantle wedge flow and was reaching UHP conditions. Strain coupling in the S₁ fabric between clinopyroxenites and granulites at Doubrava and upright folding of eclogites at Spa?ice document a link between tectonic and magmatic processes during orogenic thickening, coeval with intrusions of the arc‐related calcalkaline suites of the Central Bohemian Plutonic Complex (c. 360–345 Ma). Juxtaposition of peridotites and granulites could be explained by a rheological heterogeneity connected to the development of clinopyroxenite layering in the upper mantle and a previously published model of a lithospheric‐scale transpressional arc system. It invokes vertical shearing along NE–SW trending, sub‐vertical foliations in the upper mantle that could have led to an emplacement of mantle bodies into the granulitized, orogenic root in the sub‐arc region. Clearly, such a transpressional arc system could represent an important pathway for an emplacement of deep‐seated rocks in the orogenic lower crust.     

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...     

Emplacement depths and radiometric ages of Paleozoic plutons of the Neukirchen–Kdyn massif: differential uplift and exhumation of Cadomian basement due to Carboniferous orogenic collapse (Bohemian Massif)

The igneous complex of Neukirchen–Kdyn is located in the southwestern part of the Teplá–Barrandian unit (TBU) in the Bohemian Massif. The TBU forms the most extensive surface exposure of Cadomian basement in central Europe. Cambrian plutons show significant changes in composition, emplacement depth, isotopic cooling ages, and tectonometamorphic overprint from NE to SW. In the NE, the V epadly granodiorite and the Smr ovice diorite intruded at shallow crustal levels (<ca. 7 km depth) as was indicated by geobarometric data. K–Ar age data yield 547±7 and 549±7 for hornblende and 495±6 Ma for biotite of the Smr ovice diorite, suggesting that this pluton has remained at shallow crustal levels (T<ca. 350 °C) since its Cambrian emplacement. A similar history is indicated for the V epadly granodiorite and the Stod granite. In the SW, intermediate to mafic plutons of the Neukirchen–Kdyn massif (V eruby and Neukirchen gabbro, Hoher–Bogen metagabbro), which yield Cambrian ages, either intruded or were metamorphosed at considerably deeper structural levels (>20 km). The Teufelsberg ( ert v kámen) diorite, on the other hand, forms an unusual intrusion dated at 359±2 Ma (concordant U–Pb zircon age). K–Ar dating of biotite of the Teufelsberg diorite yields 342±4 Ma. These ages, together with published cooling ages of hornblende and mica in adjacent plutons, are compatible with widespread medium to high-grade metamorphism and strong deformation fabrics, suggesting a strong Variscan impact under elevated temperatures at deeper structural levels. The plutons of the Neukirchen area are cut by the steeply NE dipping Hoher–Bogen shear zone (HBSZ), which forms the boundary with the adjacent Moldanubian unit. The HBSZ is characterized by top-to-the-NE normal movements, which were particularly active during the Lower Carboniferous. A geodynamic model is presented that explains the lateral gradients in Cambrian pluton composition and emplacement depth by differential uplift and exhumation, the latter being probably related to long-lasting movements along the HBSZ as a consequence of Lower Carboniferous orogenic collapse.