Kinematics of crustal stacking and dispersion in the south-eastern Bohemian Massif

The south-eastern Bohemian Massif consolidated during the Late Variscan orogeny by the oblique collision of two continental crustal blocks after closure of an oceanic realm. One microcontinent comprises portions which are now distributed among Moravian and Moldanubian units and which are characterized by Late Proterozoic tectonothermal events, especially by granitoid intrusions. The other microcontinent includes the Gföhl gneiss and granulites (Gföhl nappe) of probable Early Palaeozoic protolith ages. Both continental blocks are separated by an ophiolite-like assemblage, which is preserved in portions of the Raabs unit.Oblique crustal stacking is accompanied by north-eastward propagation of nappes in a dextral transpressive regime. Exhumation of previously thickened crust is achieved by equally oriented bulk extension but partitioned in distinct displacement paths. Coeval stacking and extension at different crustal levels is suggested.Correspondence to: H. Fritz     

Late Paleozoic volcanic rocks of the Intra-Sudetic Basin, Bohemian Massif: petrological and geochemical characteristics

Late Paleozoic volcanic rocks in the Intra-Sudetic Basin of the Bohemian Massif in the Czech Republic can be subdivided into two series: (I) a minor bimodal trachyandesite-rhyolite series of Upper Carboniferous age with initial ⁸⁷Sr/⁸⁶Sr of ca. 0.710 and ε_Nd values of −6.1 also characteristic of volcanics of the near Krkonoše Piedmont Basin (0.707 and −6.0, Ulrych et al., 2003) and (II) a major differentiated basaltic trachyandesite-trachyandesite-trachyte-rhyolite series of Lower Permian age with lower initial ⁸⁷Sr/⁸⁶Sr of ca. 0.705-0.708 and ε_Nd values ranging from −2.7 to −3.4/−4.1/. The newly recognized volcanic rocks of trachytic composition indicate that the rocks were formed by magmatic differentiation of similar parental melts rather than constituting a bimodal mafic-felsic sequence from different sources. Both series are generally of subalkaline affinity and calc-alkaline character with some tholeiitic tint (FeO/MgO vs. SiO₂, presence of orthopyroxene). The magmatic activity occurred in cycles in a layered chamber, each starting primarily with felsic volcanics and ending with mafic ones. The mafic rocks represent mantle-melt(s) overprinted by crust during assimilation-fractional crystallization. The Sr-Nd isotopic data confirm a significant crustal component in the volcanic rocks that may have been inherited from the upper mantle source and/or from assimilation of older crust during magmatic underplating and shallow-level melt fractionation.     

Late orogenic extension in the Bohemian Massif: petrostructural evidence in the Hlinsko region

Abstract

The tectonic contact between low-grade metase-dimentary series and high-grade rocks in the Hlinsko region (Bohemian Massif) is commonly interpreted as a thrust of the Barrandian sediments over the upper Moldanubian nappe.

The sediments occur in an E-facing synform that contains a tonalitic laccolith on its eastern boundary with the Moldanubian, and is truncated by a granodiorite pluton to the west. The synform represents a late deformational folding event related to the granodiorite intrusion. NW-oriented normal shear in the tonalite is indicated by S-C microstructures. Kinematic criteria associated with the major foliation and lineation development in the metasediments also indicate a north-westward, normal shear. In addition, Moldanubian gneiss display late shear bands due to north-westward, normal shear. Consequently, the presumed thrust is a low-angle, normal shear zone.

Low-pressure type metamorphism (3 < P < 4 x 10² MPa) coeval with the major deformational phase in pelites of the Hlinsko synform is attributed to both the tonalite aureole and the extensive HT metamorphism (under P > 6 x 10² MPa) that has affected the underlying Moldanubian.

The possibly polyphase normal fault is consistent with the meta-morphic pressure jump between the metasediments and the Moldanubian.

We suggest that the tonalite intruded syntectonically within the normal ductile shear zone active during waning stages of the Variscan orogeny.     

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.     

Tectonometamorphic evolution of the Bohemian Massif: evidence from high pressure metamorphic rocks

The Variscan orogenic belt, of which the Bohemian Massif is a part, is typically recognized for its characteristic low pressure, high temperature metamorphism and a large volume of granites. However, there are also bodies of high pressure rocks (eclogites, garnet peridotites and high pressure granulites) which are small in size but widely distributed throughtout the Massif. Initially the high pressure rocks were considered to be relicts of a much older orogenic event, but the increasing data derived from isotopic and geochronological investigations show that many of these rocks have Palaeozoic protoliths. Metamorphic ages from the high pressure rocks define no single event. Instead, a number of discrete clusters of ages are found between about 430 Ma and the time of the dominant low pressure event at around 320–330 Ma.Most of the eclogite and granulite facies rocks are assigned to allochthonous nappes that arrived close to the end of the low pressure event, but before final granite intrusion. The nappes contain a mixture of different units and the relationship between rocks with high pressure relicts and host gneisses with no apparent signs of deep burial is still problematic. Some of the high pressure rocks retain evidence of multiple stages of partial re-equilibration during uplift. Moreover, it can be shown in certain instances that host gneisses also endured a multistage metamorphic development but with a peak event convergent with one of the breakdown stages in the enclosed rocks with high pressure relicts. It thus appears that the nappe units are composite bodies probably formed during episodic intracrustal thrusting. Fluids derived from prograde dehydration reactions in the newly under thrusting slab are taken to be the catalysts that drove the partial re-equilibrations.On the scale of the whole Massif it can be seen within the units with high pressure relicts that the temperature at the peak recorded pressure and that during the breakdown are variable in different locations. It is interpreted that regional metamorphic gradients are preserved for given stages in the history and thus the present day dismembered nappe relicts are not too far removed from their original spatial distribution in an original coherent unit. From the temperature information alone it is highly probable that the refrigerating underthrusting slab was situated in the north-west. However, this north-west to south-east underthrusting probably represents the major 380–370 Ma event and is no guide to the final thrusting that emplaced the much thinned nappe pile with high pressure relicts.Granite genesis is attributed to the late stage stacking, during the final Himalayan-type collision stage, of thinned crust covered by young, water-rich, sediments — erosion products of the earlier orogenic stages. Regional metamorphism at shallow depths above the voluminous granites was followed by final nappe emplacement which rejuvenated the granite ascent in places. Correspondence to: P. J. O'Brien     

Probable anticlockwise P,–T evolution in extending crust: Hlinsko region, Bohemian Massif

ABSTRACT In the Hlinsko region (Variscan Bohemian Massif, Czech Republic) a major extensional shear zone separates low-grade metasedimentary series (Hlinsko schists) and high-grade rocks of the Moldanubian terrane (Svratka Crystalline Unit). During late-Variscan extension, a tonalite intruded syntectonically into the normal ductile shear zone, and caused contact metamorphism of the overlying schists. Concurrent syntectonic sedimentation of a flysch series took place at the top of the hangingwall schists. In order to decipher the detailed petrological evolution of the Hlinsko unit situated in the hangingwall of this tectonic contact, a phase diagram approach and petrogenetic grids, calculated with the thermocalc computer program, were used. The crystallization/deformation relationships and the paragenetic analysis of the Hlinsko schists define a P–T path with an initial minor increase in pressure followed by cooling. Calculated pseudosections constrain this anticlockwise P-T evolution to the upper part of the andalusite field between 0.36 and 0.40 GPa for temperatures ranging from 570 to 530°C. A low a_H2O is required to explain the presence of andalusite-biotite-bearing assemblages, and could be related to the presence of abundant graphite. In contrast, the footwall rocks of the Svratka Crystalline Unit record decompression from around 0.8 GPa at a relatively constant temperature, followed by cooling. Thus, the footwall and the hangingwall units display opposite, but convergent P–T histories. Decompression in the footwall rocks is related to a rapid exhumation. We propose that the inverse, anticlockwise P–T path recorded in the hangingwall pelites is related to the rapid, extension-controlled sedimentation of the overlying flysch series.     

Brittle deformation events at the western border of the Bohemian Massif (Germany)

Investigations of brittle deformation structures, present within the crystalline rocks of the Bavarian Oberpfalz, reveal a complex late to post-Variscan crustal evolution. Upper Carboniferous (mainly Westphalian) granites were emplaced into semibrittle to brittle rocks of the ZEV (zone of Erbendorf-Vohenstrauß) and the EGZ (Erbendorf greenschist unit), respectively. From both the alignment of the granites and the direction of granite-related tension gashes a north-east-south-west extension must be assumed for the period of magmatic activities. Apart from the granite intrusions, rapid crustal uplift (about 1.5 km/my) led to an increase in the geothermal gradient from < 30 °C/km (late Variscan pre-granitic) to > 40 °C/km (late Variscan post-granitic). The increased geothermal gradient persisted during the succeeding reverse faulting which results from late Carboniferous (probably Stephanian) east-west and northeast-south-west compression. Although not evidenced directly in the area considered, strike-slip faults seem to have played an important part during the late Variscan crustal evolution, particularly in the Early Permian. The strike-slip events indicate further crustal shortening and indentation under north-south compression.A similar indentation was present in Cretaceous time. After a weak phase of Early Cretaceous reverse faulting, which results from north-south compression, strike-slip faults formed under north-west-south-east and north-south compression. All these faults, in particular the strike-slip faults, seem to be related to the Cretaceous and lowermost Tertiary convergence of the Alpine/Carpathian orogeny.A late stage of crustal extension, characterized by a radial stress tensor (₂ = ₃), is indicated through high angle normal faults which probably formed during the subsidence of the adjacent Neogene Eger Graben.     

Variscan Sm-Nd and Ar-Ar ages of eclogite facies rocks from the Erzgebirge, Bohemian Massif

Abstract The Erzgebirge Crystalline Complex (ECC) is a rare example where both‘crustal’eclogites and mantle-derived garnet-bearing ultramafic rocks (GBUs) occur in the same tectonic unit. Thus, the ECC represents a key complex for studying tectonic processes such as crustal thickening or incorporation of mantle-derived material into the continental crust. This study provides the first evidence that high-pressure metamorphism in the ECC is of Variscan age. Sm-Nd isochrons define ages of 333 ± 6 (Grt-WR), 337± 5 (Grt-WR), 360± 7 (Grt-Cpx-WR) (eclogites) and 353 ± 7 Ma (Grt-WR) (garnet-pyroxenite). ⁴⁰Ar/³⁹Ar spectra of phengite from two eclogite samples give plateau ages of 348 ± 2 and 355 ± 2 Ma. The overlap of ages from isotopic systems with blocking temperatures that differ by about 300 ° C indicates extremely fast tectonic uplift rates. Minimum cooling rates were about 50° C Myr-¹. As a consequence, the closure temperature of the specific isotopic system is of minor importance, and the ages correspond to the time of high-pressure metamorphism. Despite textural equilibrium and metamorphic temperatures in excess of 800° C, clinopyroxene, garnet and whole rock do not define a three-point isochron in three of four samples. The metamorphic clinopyroxenes seem to have inherited their isotopic signature from magmatic precursors. Rapid tectonic burial and uplift within only a few million years might be the reason for the observed Sm-Nd disequilibrium. The ε_Nd values of the eclogites (+4.4 to +6.9) suggest the protoliths were derived from a long-term depleted mantle, probably a MORB source, whereas the isotopically enriched garnet-pyroxenite (ε_Nd–2.9) might represent subcontinental mantle material, emplaced into the crust prior to or during collision. The similarity of ages of the two different rock types suggests a shared metamorphic history.     

Variscan stress regime rotation: Insights from the analysis of kink folds in the northern margin of the Bohemian Massif,South Poland

We aimed to determine variations in stress regimes during the youngest Variscan deformations in the northern part of the Bohemian Massif. For this purpose, we calculated the orientation of the principal stress and strain axes for kink folds observed in the metamorphic envelope of the Karkonosze Granite, using two methods: 1) the traditional method, incorporating structural diagrams (for conjugate kink folds only), and 2) butterfly diagram analysis. The use of both methods enabled us to determine the stress regime, based not only on conjugate but also on monoclinal kink bands. The obtained results prove that butterfly diagram analysis, when applied to monoclinal kink folds, yields reliable results, especially when calibrated using the internal friction angle (Ф) calculated for the conjugate structures.We identified two generations of kink folds: 1) an older one, developed under sublatitudinal shortening and most probably related to the Early Carboniferous terminal stages of the northwest-directed thrusting of the metamorphic units, and 2) a younger one; produced by north-south Variscan Carboniferous compression, and the emplacement and subsequent doming of the Karkonosze Granite. This is the first study on brittle-ductile structures observed commonly in the metamorphic units of the Bohemian Massif, showing their relation to the granitoid intrusion and complementing the tectonic models that usually omit kink folds.     

Permo-Carboniferous volcanism in late Variscan continental basins of the Bohemian Massif (Czech Republic): geochemical characteristic

Extensive Permo-Carboniferous volcanism has been documented from the Bohemian Massif. The late Carboniferous volcanic episode started at the Duckmantian–Bolsovian boundary and continued intermittently until Westphalian D to Stephanian B producing mainly felsic and more rarely mafic volcanics in the Central Bohemian and the Sudetic basins. During the early Permian volcanic episode, after the intra-Stephanian hiatus, additional large volumes of felsic and mafic volcanics were extruded in the Sudetic basins. The volcanics of both episodes range from entirely subalkaline (calc-alkaline to tholeiitic) of convergent plate margin-like type to transitional and alkaline of within-plate character. A possible common magma could not be identified among the Carboniferous and Permian primitive magmas, but a common geochemical signature (enrichment in Th, U, REE and depletion in Nb, Sr, P, Ti) in the volcanic series of both episodes was recognized. On the other hand, volcanics of both episodes differ in intensities of Nb, Sr and P depletion and also, in part, in their isotope signatures. High ⁸⁷Sr/⁸⁶Sr (0.707–0.710) and low ε_Nd (−6.0 to −6.1) are characteristic of the Carboniferous mafic volcanics, whereas low ⁸⁷Sr/⁸⁶Sr (0.705–0.708) and higher ε_Nd ranging from −2.7 to −3.4 are typical of the Permian volcanics. Felsic volcanics of both episodes vary substantially in ⁸⁷Sr/⁸⁶Sr (0.705–0.762) and ε_Nd (−0.9 to −5.1). Different depths of magma source or heterogeneity of the Carboniferous and Permian mantle can be inferred from variation in some characteristic elements of the geochemical signature for volcanics in some basins. The Sr–Nd isotopic data with negative ε_Nd values confirm a significant crustal component in the volcanic rocks that may have been inherited from the upper mantle source and/or from assimilation of older crust during magmatic underplating and ascending of primary basic magma. Two different types of primary magma development and formation of a bimodal volcanic series have been recognized: (i) creation of a unique magma by assimilation fractional crystallization processes within shallow-level reservoirs (type Intra-Sudetic Basin) and (ii) generation and mixing of independent mafic and felsic magmas, the latter by partial melting of upper crustal material in a high-level chamber (type Krkonoše Piedmont Basin). A similar origin for the Permo-Carboniferous volcanics of the Bohemian Massif is obvious, however, their geochemical peculiarities in individual basins indicate evolution in separate crustal magma chambers.     

Brunovistulian terrane (Bohemian Massif, Central Europe) from late Proterozoic to late Paleozoic: a review

The Brunovistulian terrane represents a microcontinent of enigmatic Proterozoic provenance that was located at the southern margin of Baltica in the early Paleozoic. During the Variscan orogeny, it represented the lower plate at the southern margin of Laurussia, involved in the collision with the Armorican terrane assemblage. In this respect, it resembles the Avalonian terrane in the west and the Istanbul Zone in the east. There is a growing evidence about the presence of a Devonian back-arc at the margin of the Brunovistulian terrane. The early Variscan phase was characterized by the formation of Devonian extensional basins with the within-plate volcanic activity and formation of narrow segments of oceanic crust. The oldest Viséan flysch of the Rheic/Rhenohercynian remnant basin (Protivanov, Andelska Hora and Horní Benesov formations) forms the highest allochthonous units and contains, together with slices of Silurian Bohemian facies, clastic micas from early Paleozoic crystalline rocks that are presumably derived from terranes of Armorican affinity although provenance from an active Brunovistulian margin cannot be fully excluded either. The development of the Moravo–Silesian late Paleozoic basin was terminated by coal-bearing paralic and limnic sediments. The progressive Carboniferous stacking of nappes and their impingement on the Laurussian foreland led to crustal thickening and shortening and a number of distinct deformational and folding events. The postorogenic extension led to the formation of the terminal Carboniferous-early Permian Boskovice Graben located in the eastern part of the Brunovistulian terrane, in front of the crystalline nappes. The highest, allochthonous westernmost flysch units, locally with the basal slices of the Devonian and Silurian rocks thrusted over the Silesicum in the NW part of the Brunovistulian terrane, may share a similar tectonic position with the Giessen–Harz nappes. The Silesicum represents the outermost margin of the Brunovistulian terrane with many features in common with the Northern Phyllite Zone at the Avalonia–Armorica interface in Germany.     

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.     

Partially retrograded eclogites of the Münchberg Massif, Germany: records of a multi-stage Variscan uplift history in the Bohemian Massif

Abstract Eclogites with a wide range in bulk composition are present in the Münchberg Massif, part of the Variscan basement of the Bohemian Massif in north-east Bavaria. New analyses of the primary phases garnet, omphacite, phengite and amphibole, as well as the secondary phases clinopyroxene II, various amphiboles, biotite/phlogopite, plagioclase, margarite, paragonite, prehnite and pumpellyite, reveal a complex uplift history. New discoveries were made of samples with very jadeite-rich primary omphacite as well as a secondary omphacite in a symplectite with albite. Various geothermobarometric techniques, together with thermodynamic databases (incorporating separately determined activity–composition values) and experimental data have clustered the minimum conditions for the primary assemblages to the P–T range 650 ± 60° C, 14.3 ± 1 kbar. However, jadeite (in omphacite)–kyanite–paragonite (in phengite) and zoisite–grossular (in garnet)–kyanite–quartz relationships suggests pressures of 25–28 kbar at the same temperatures. The fact that the secondary omphacite–plagioclase assemblage yields pressures within a few hundred bars of the minimum pressures for the plagioclase-free assemblages strongly suggests that the minimum values are serious underestimates.
Zoning, inclusion suites and breakdown reactions of primary phases, in addition to new minerals formed during uplift, define a polyphase metamorphic evolution which, from geochronological evidence, occurred solely within the Variscan cycle. The complex breakdown in other Bohemian Massif eclogites and the distinct variation in their temperatures during uplift suggest a multi-stage thrusting model for the regional evolution of the eclogites. Such an evolution has significance with respect to incorporation of mantle slices into crustal sequences and fluid derivation from successively subducted units, possibly driving the breakdown reactions.     

Garnet pyroxenite and eclogite in the Bohemian Massif: geochemical evidence for Variscan recycling of subducted lithosphere

High-temperature, high-pressure eclogite and garnet pyroxenite occur as lenses in garnet peridotite bodies of the Gföhl nappe in the Bohemian Massif. The high-pressure assemblages formed in the mantle and are important for allowing investigations of mantle compositions and processes. Eclogite is distinguished from garnet pyroxenite on the basis of elemental composition, with mg number <80, Na₂O > 0.75 wt.%, Cr₂O₃ < 0.15 wt.% and Ni < 400 ppm. Considerable scatter in two-element variation diagrams and the common modal layering of some eclogite bodies indicate the importance of crystal accumulation in eclogite and garnet pyroxenite petrogenesis. A wide range in isotopic composition of clinopyroxene separates [_Nd, +5.4 to –6.0; (⁸⁷Sr/⁸⁶Sr)_i, 0.70314–0.71445; ¹⁸O_SMOW, 3.8–5.8%o] requires that subducted oceanic crust is a component in some melts from which eclogite and garnet pyroxenite crystallized. Variscan Sm-Nd ages were obtained for garnet-clinopyroxene pairs from Dobeovice eclogite (338 Ma), Úhrov eclogite (344 Ma) and Nové Dvory garnet pyroxenite (343 Ma). Gföhl eclogite and garnet pyroxenite formed by high-pressure crystal accumulation (±trapped melt) from transient melts in the lithosphere, and the source of such melts was subducted, hydrothermally altered oceanic crust, including subducted sediments. Much of the chemical variation in the eclogites can be explained by simple fractional crystallization, whereas variation in the pyroxenites indicates fractional crystallization accompanied by some assimilation of the peridotite host.     

Magmatic stoping as an important emplacement mechanism of Variscan plutons: evidence from roof pendants in the Central Bohemian Plutonic Complex (Bohemian Massif)

The presence of numerous roof pendants, stoped blocks and discordant intrusive contacts suggests that magmatic stoping was a widespread, large-scale process during the final construction of the Central Bohemian Plutonic Complex, Bohemian Massif. The measured total length of the discordant contacts that cut off the regional cleavage and were presumably formed by stoping corresponds to about half of all contacts with the upper-crustal host rocks. In addition, at least some of the straight, cleavage-parallel intrusive contacts may also have recorded complex intrusive histories ending with piecemeal stoping of thin cleavage-bounded host rock blocks into the magma chamber. Based on the above, we argue that the fast strain rates required for emplacement of large plutons of the Central Bohemian Plutonic Complex into brittle upper crustal host rocks over relatively short-time span could not have been accommodated entirely by slow ductile flow or slip along faults. Instead, the emplacement was largely accommodated by much faster thermal cracking and extensive stoping independent of regional tectonic deformation. Finally, we emphasize that magmatic stoping may significantly modify the preserved structural patterns around plutons, may operate as an important mechanism of final construction of upper-crustal plutons and thus may contribute to vertical recycling and downward transport of crustal material within the magma plumbing systems in the crust.     

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.     

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.     

Long-term landscape evolution of the northeastern margin of the Bohemian Massif: apatite fission-track data from the Erzgebirge (Germany)

To study the relative and absolute timing of post-Variscan cooling and denudation processes in the Erzgebirge of the Mid-European Variscides, eight samples for apatite fission-track (AFT) analysis were collected from a ~1,300 m drill-core. The fission-track data reveal two stages of accelerated cooling through the apatite partial annealing zone (APAZ; i.e., 110±10–60 °C) in the Late Jurassic-Late Cretaceous and in the late Cenozoic, respectively. Late Jurassic-Late Cretaceous cooling corresponding to denudation of 1.5–5.9 km has been related to wrench tectonics along the Elbe Zone during Triassic-Jurassic Pangea breakup. Late Cenozoic exhumation of 2.1–5.6 km, and the increase of the geothermal gradient from 17±5 °C km^–1 (Oligocene/Miocene) to 25–27 °C km^–1 (recent) is likely connected to the formation of the Eger Graben starting from the Oligocene, as a result of the late Alpine orogenic phases.     

Diffusion-controlled development of silica-undersaturated domains in felsic granulites of the Bohemian Massif (Variscan belt of Central Europe)

Plagioclase rims around metastable kyanite crystals appear during decompression of high-pressure felsic granulites from the high-grade internal zone of the Bohemian Massif (Variscan belt of Central Europe). The development of the plagioclase corona is a manifestation of diffusion-driven transfer of CaO and Na₂O from the surrounding matrix and results in isolation of kyanite grains from the quartz- and K-feldspar-bearing matrix. This process establishes Si-undersaturated conditions along the plagioclase–kyanite interface, which allow crystallization of spinel during low-pressure metamorphism. The process of the plagioclase rim development is modeled thermodynamically assuming local equilibrium. The results combined with textural observations enable estimation of equilibration volume and diffusion length for Na and Ca that extends ∼400–450 and ∼450–550 μm, respectively, around each kyanite crystal. Low estimated bulk diffusion coefficients suggest that the diffusion rate of Ca and Na is controlled by low diffusivity of Al across the plagioclase rim.     

2-D seismic tomographic and ray tracing modelling of the crustal structure across the Sudetes Mountains basing on SUDETES 2003 experiment data

The seismic data obtained during SUDETES 2003 experiment are analysed, and detailed crustal structure for profiles S02, S03 and S06 is presented using three different 2-D techniques: (1) “smooth” tomography of refracted waves travel times, (2) ray tracing of reflected and refracted waves, and (3) joint velocity and depth of reflector tomographic inversion. In spite of different interpretation techniques used, the models of the crustal structure show common characteristic features. The low velocity (V_p < 4 km/s) sedimentary layer was documented in the northeastern part of the study area. The topmost basement has in general a velocity of 5.8–6.0 km/s, and velocities at ca. 20 km depth are 6.15–6.25 km/s. The strong reflecting boundaries were found at 20–23 and 25–28 km depth with a velocity contrast about 0.4 km/s, and the highest velocities in the lowermost crust are 6.8–7.2 km/s. In general, the crust of the Bohemian Massif is slightly thicker (33–35 km) than in the northern part of the area. Velocities beneath Moho are relatively low, of 7.95 km/s. On the basis of well recorded reflected waves, mantle reflectors were discovered in the depth interval ca. 40–70 km. Apart of new results for the geology and tectonics of the area, some conclusion could be made about different techniques used. In the 2-D case the “clasical” ray tracing method with using all correlated phases gives the most adequate model of the structure, because of full, manual control of the model creation. The “smooth” first arrival travel times tomography, although very fast, is not satisfactory enough to describe the complex structure. So, the best candidate in 3-D case seems to be travel time tomography for both refracted and reflected waves in multi-layers models.