The calc-alkaline Miřetín Pluton (newly dated at 346 Ma ± 5 Ma; an U-Pb age obtained by laser-abl... more The calc-alkaline Miřetín Pluton (newly dated at 346 Ma ± 5 Ma; an U-Pb age obtained by laser-ablation ICP MS method on zircons) is a NNE-SSW elongated intrusive body emplaced into the upper-to mid-crustal rocks of the Polička Unit (eastern Teplá-Barrandian Zone; Bohemian Massif). Its composition reveals similarities to other calc-alkaline granitoids, which are mostly interpreted as products of magma mixing between the basic magmas derived from mantle wedge above a subduction zone with crustally-derived acid melts. The conditions of magma crystallization estimated at 653-681 °C and 0.29-0.43 GPa roughly correspond to peak metamorphic evolution of the host volcano-sedimentary rocks of the northwestern part of the Polička and Hlinsko units. The Miřetín Pluton was emplaced into a NNE-SSW oriented transpressional domain, well recognized on a regional scale along the eastern margin of the Teplá-Barrandian Zone. During, or shortly after the magma emplacement, the Miřetín Pluton was affected by pervasive submagmatic to high-T solid-state deformation, reflecting an additional strain increment of regional transpression in a narrow zone of thermal softening. Sharply superimposed low-T solid-state fabric preserved along the western part of the Pluton was connected with normal shearing between the Polička Unit at the bottom and the overlying Hlinsko Unit after 335 Ma.
In the Jizera granite of the Krkonoše–Jizera Plutonic Complex, northern Bohemian Massif, contrast... more In the Jizera granite of the Krkonoše–Jizera Plutonic Complex, northern Bohemian Massif, contrasting patterns of magmatic K-feldspar fabrics and brittle fractures characterize different structural levels of the pluton. The uppermost exposed level at ∼800–1,100 m above sea level is dominated by flat foliation that overprints two steep foliations. In contrast, K-feldspar shape-fabric in an underground tunnel (∼660 m above sea level) shows complex variations in orientation and intensity. Magnetic fabric carried by coaxial contributions of biotite, magnetite, and maghemite is homogeneous along the examined section of the tunnel, and is decoupled from the K-feldspar fabric. The Jizera granite is crosscut by two regional sets of subvertical fractures (∼NE–SW and ∼NW–SE) and by near-surface exfoliation joints. The multiple fabrics are inferred to reflect a complex magmatic strain history at different structural levels of the pluton, bearing little or no relationship to the fracture network. In contrast to the original concept of Hans Cloos (“granite tectonics”), we conclude that no simple genetic relationship exists between fabrics and fractures in plutons. An alternative classification of fractures in plutons thus should avoid relationships to magmatic fabrics and should instead consist of cooling, syntectonic, uplift, and post-uplift fractures.
The ∼340 Ma Knížecí Stolec durbachitic pluton was emplaced as a deep-seated cone-sheet-bearing ri... more The ∼340 Ma Knížecí Stolec durbachitic pluton was emplaced as a deep-seated cone-sheet-bearing ring complex into the Křišt’anov granulite body (Moldanubian Unit, Bohemian Massif). Prior to the emplacement of the durbachitic magma, the steep sub-concentric metamorphic foliation in the granulite formed due to intense ductile folding during high-grade retrograde metamorphism. Subsequently, the durbachitic pluton intruded discordantly into the granulite at around
Page 1. ORIGINAL PAPER Magmatic history and geophysical signature of a post-collisional intrusive... more Page 1. ORIGINAL PAPER Magmatic history and geophysical signature of a post-collisional intrusive center emplaced near a crustal-scale shear zone: the Plechy´ granite pluton (Moldanubian batholith, Bohemian Massif) Krytof ...
Skarns in the Svratka Unit, in the neighbouring part of the Moldanubian Zone and in the Kutná Hor... more Skarns in the Svratka Unit, in the neighbouring part of the Moldanubian Zone and in the Kutná Hora Complex were studied with respect to their metamorphic evolution, major-and trace-element geochemistry, oxygen isotopic composition and zircon ages. Skarns form competent lenses and layers in metamorphosed siliciclastic rocks and preserve some early deformation structures and several equilibrium assemblages representing the products of successive metamorphic reactions. The main rock-forming minerals, garnet and clinopyroxene, are accompanied by less abundant magnetite, amphibole, plagioclase, epidote ± quartz. In the Svratka Unit the early prograde M 1 , prograde/peak M 2 , and retrograde M 3 metamorphic stages have been distinguished. Metamorphic conditions in skarns of the Moldanubian Zone are limited to a relatively narrow interval of amphibolite facies. The prograde and retrograde events in the Kutná Hora Complex skarns probably took place under amphibolite-facies conditions. The presence of magnetite and the increasing proportion of the andradite component in the garnet indicate locally increased oxygen fugacity. Skarn geochemistry does not show systematic differences in the skarn composition among the three units. The regional variations are exceeded by differences among samples from individual localities. The Al 2 O 3 /TiO 2 , Al 2 O 3 /Zr, TiO 2 /Nb ratios point to the variable proportion of the detrital material, combined in skarn protoliths with CaO and FeO, the major non-detrital components. The skarns exhibit elevated abundances of Cu, Zn, Sn and As. The Eu/Eu* ratio varies in the range of 0.5-8.6, the total REE contents vary from 8 to 345 ppm. The lowest ΣREE values (< 100 ppm) occur in skarns with magnetite mineralization. The wide intervals of ΣREE and Eu/Eu* values are interpreted to indicate variations in the temperature and redox conditions among layers of the same locality and at various localities. The oxygen isotope compositions of garnets, pyroxenes and amphiboles from skarns of the Svratka Unit exhibit a range of δ 18 O = 0.1 to 4.1 ‰. In situ (laser-ablation ICP-MS) U-Pb dating of zircon from one of the Svratka Unit skarn bodies yielded a wide range of ages (0.5-2.6 Ga), supporting the detrital origin of this zircon population. The skarn protoliths were probably rocks of mixed detrital-exhalative origin deposited on the sea floor. The geological position of skarns, with their structural and metamorphic record, probably reflect tectono-metamorphic evolution shared with that of their host rocks. The geochemical characteristics, including oxygen isotopic compositions and the presence of detrital zircons with a wide range of ages exclude metasomatic, and point to a sedimentary-exhalative mode of origin for the studied skarns.
In this paper are reviewed structural, petrological and geochronological data from the main units... more In this paper are reviewed structural, petrological and geochronological data from the main units at the NE periphery of the Moldanubian Zone, i.e. Kutná Hora Complex, Svratka Unit, Polička and Zábřeh units, as well as the Strážek Unit of the Moldanubian Zone. In this domain of the Bohemian Massif, the lower-and upper-crustal units are dominated by metamorphic fabrics produced during the Variscan orogeny. The mid-to upper-crustal Svratka, Polička and Zábřeh units are affected by ~MP/MT "long-lived" (~350-339 Ma) tectonometamorphic event reflecting ~WNW-ESE right-lateral strike-slip shearing (transpressional to transtensional tectonics). These regional fabrics are in the Polička and Zábřeh units related with syn-tectonic emplacement and crystallization of calc-alkaline intrusions (Zábřeh Intrusive Complex, Miřetín nad Budislav plutons). In the three structurally high units in the Kutná Hora Complex, Orlice-Sněžník and the Strážek units the strike-slip, "long-lived" tectonics is rather localized; the high-pressure, high-temperature events followed by heterogeneous and polyphase exhumation of deep-seated rocks to mid-crustal levels are preserved. Ultrapotassic rocks (durbachites) of the Strážek Unit, dated at ~339 Ma, intruded posttectonicaly.
ABSTRACT Metagranites and migmatites of the Svratka and the Orlice-Sněžník units, northeastern Bo... more ABSTRACT Metagranites and migmatites of the Svratka and the Orlice-Sněžník units, northeastern Bohemian Massif, exhibit a number of similarities in their lithological, mineralogical and geochemical features. Both of these units were affected by intense migmatitization accompanied by intrusion of peraluminous granites during the Cambrian to Ordovician. The chemical composition and mineralogy of all the studied rocks correspond to crustal melts. From the geochemical point of view, both rock groups (migmatites and metagranites) exhibit progressive geochemical fractionation (increase in the SiO2, Rb, XFeO, W and Sn contents, accompanied by a decrease in the concentrations of Ba, Sr, Mg and Ca) as a result of varying degrees of partial melting and fractional crystallization. The main rock-forming minerals are influenced by metamorphic recrystallization under amphibolite-facies conditions during the Variscan orogeny.
In the porphyritic Jizera granite, Bohemian Massif, three distinct types of lattice-preferred ori... more In the porphyritic Jizera granite, Bohemian Massif, three distinct types of lattice-preferred orientations of biotite grains were revealed in schlieren-delineated magmatic structures using the electron backscatter diffraction (EBSD) method. (1) Biotite basal planes (001) reorient from schlieren-subparallel near the schlieren base to schlieren-perpendicular in the upper part of the schlieren. Both orientations share subhorizontal ∼N–S to ∼NNE–SSW-trending a axes. (2)
ABSTRACT [1] Field relationships combined with new U-Pb zircon geochronology suggest that the sha... more ABSTRACT [1] Field relationships combined with new U-Pb zircon geochronology suggest that the shallow-level Krkonoše-Jizera plutonic complex, northern Bohemian Massif, was assembled successively from bottom to top, starting with emplacement of the separately evolved S-type Tanvald granite (317.3 ± 2.1 Ma), followed by at least two voluminous batches of the I-type porphyritic Liberec (319.5 ± 2.3 Ma) and Jizera (320.1 ± 3.0 Ma and 319.3 ± 3.7 Ma) granites. The intrusive sequence was completed by uppermost, minor intrusions of the equigranular Harrachov (315.0 ± 2.7 Ma) and Krkonoše granites. The I-type granites exhibit an unusually complex pattern of superposed feldspar phenocryst and magnetic fabrics as revealed from the anisotropy of magnetic susceptibility (AMS). The outer Liberec granite preserves margin-parallel foliations and lineations, interpreted to record emplacement-related strain captured by cooling from the pluton floor and walls. In contrast, the inner Jizera, Harrachov, and Krkonoše granites were overprinted by synmagmatic strain resulting from dextral movements along regional strike-slip faults cutting the opposite ends of the plutonic complex. Late-stage felsic dikes in the Liberec and Jizera granites reorient from horizontal to vertical (lineation-perpendicular) attitude in response to changing the least principal stress direction, whereas mafic schlieren do not do so, representing only randomly oriented small-scale thermal-mechanical instabilities in the phenocryst framework. In general, this case example challenges the common approach of inferring pluton-wide magma flow from interpolated foliation, lineation, and schlieren patterns. More likely, magmatic fabrics in large plutons record complex temporal succession of superposed strains resulting from diverse processes at multiple scales.
This paper presents new structural, anisotropy of magnetic susceptibility (AMS), petrological, an... more This paper presents new structural, anisotropy of magnetic susceptibility (AMS), petrological, and geochronological data to examine the link between LP-HT metamorphism and S-type granite formation in the Moldanubian unit, Bohemian Massif. We first describe the intrusive relationships of an S-type granite to its host cordierite-bearing migmatites, superbly exposed in the Rácov locality, northeastern Moldanubian batholith. The knife-sharp contacts and rectangular stoped blocks establish that the migmatites cooled and were exhumed above the brittle-ductile transition prior to the granite emplacement. The U-Pb monazite geochronology combined with P-T estimations constrain the age and depth of migmatization at~329 Ma and~21 km (T ≈ 730°C). The migmatite complex was then exhumed at a rate of 6-7 mm y −1 to a depth of b 9 km where it was intruded by the granite at~327 Ma. These data indicate that the hot fertile metapelitic middle crust in this part of the Moldanubian unit, newly defined as the Pelhřimov complex, underwent rapid isothermal decompression at~329-327 Ma, giving rise to biotite melting and generation of large volumes of S-type granite magma. We propose that the rapid~329-327 Ma exhumation of the Pelhřimov complex may have been partly assisted by the crustal-scale Přibyslav mylonite zone, which delineates the underlying western edge of the Brunia microplate underthrust beneath the eastern half of the Moldanubian unit during the early Carboniferous. The front edge of Brunia thus acted as a rigid backstop at depth, localizing the exhumation of the Pelhřimov complex and separating the hot fertile middle crust to the west from the already cooled overthrust complexes to the east. The magnetic fabric of the granite around the migmatite blocks further reveals that the Pelhřimov complex was shortened vertically and extended in the~WNW-ESE direction during and after its exhumation, implying that the SSE-directed underthrusting of Brunia along the eastern margin of the Bohemian Massif was replaced by vertical thinning and~WNW-ESE stretching of the Moldanubian crust. As a general conclusion, we suggest that even for extremely rapid crustal exhumation, S-type granite magmas can be formed at greater depths by isothermal decompression of the metapelitic host, and then ascend almost instantaneously to the already exhumed (and cooled) shallow parts of the same metamorphic core complex. This model may explain the short time spans for the extensive migmatization and associated S-type granite formation, crustal exhumation, and granite emplacement, as well as the presence of "cold", discordant granite-migmatite contacts in once "hot" migmatite terrains.
In this study, we present structural and AMS data from the w335 Ma ultrapotassic Jihlava syenitoi... more In this study, we present structural and AMS data from the w335 Ma ultrapotassic Jihlava syenitoid pluton, which intruded the lower-to midcrustal orogenic root (Moldanubian Unit) in the Bohemian Massif, Central European Variscides. The emplacement of the pluton was accommodated by multiple processes, such as ductile host-rock shortening, formation of sheeted zones by magma wedging, magmatic stoping, and possibly host-rock displacement within a wide transtensional zone. Magmatic fabrics preserved in the pluton reflect both intrusive processes and regional strain. Margin-parallel and wENEeWSW foliations, which probably formed by strain during emplacement of inner magma pulses, were overprinted by tectonic strain within a zone of distributed wrench-dominated dextral transtension. This zone probably accommodated exhumation of different segments in the eastern part of the Moldanubian Unit during pluton emplacement. In contrast to existing models, we emphasize that the Jihlava pluton, as well as other ultrapotassic plutons widespread in the Moldanubian Unit, are structurally highly variable bodies emplaced by multiple intrusive processes. Our case study illustrates how careful documentation of structural relations around these ultrapotassic plutons may constrain the kinematic framework and local exhumation histories in different segments of the orogenic root during and shortly after the w340 Ma mechanical event in the Central European Variscides.
Geological Society, London, Special Publications, 2014
ABSTRACT This paper summarizes the current knowledge on the nature, kinematics and timing of move... more ABSTRACT This paper summarizes the current knowledge on the nature, kinematics and timing of movement along major tectonic boundaries in the Bohemian Massif and demonstrates how the Variscan plutonism and deformation evolved in space and time. Four main episodes are recognized: (1) Late Devonian–early Carboniferous subduction and continental underthrusting of the Saxothuringian Unit beneath the Teplá–Barrandian Unit resulted in the orogen-perpendicular shortening and growth of an inboard magmatic arc during c. 354–346 Ma; (2) the subduction-driven shortening was replaced by collapse of the Teplá–Barrandian upper crust, exhumation of the high-grade (Moldanubian) core of the orogen at c. 346–337 Ma and by dextral strike-slip along orogen-perpendicular NW–SE shear zones; (3) following closure of a Rhenohercynian Ocean basin, the Brunia microplate was underthrust beneath the eastern flank of the Saxothuringian/Teplá–Barrandian/Moldanubian ‘assemblage’; this process commenced at c. 346 Ma in the NE and ceased at c. 335 Ma in the SW; and (4) late readjustments within the amalgamated Bohemian Massif included crustal exhumation and mainly S-type granite plutonism along the edge of the Brunia indentor at c. 330–327 Ma, and peripheral tectonothermal activity driven by strike-slip faulting and possibly mantle delamination around the consolidated Bohemian Massif&#39;s interior until late Carboniferous–earliest Permian times.
All GSW Journals, GSW + GeoRef. ... that 90 % of crystal boundaries are lubricated by melt at eve... more All GSW Journals, GSW + GeoRef. ... that 90 % of crystal boundaries are lubricated by melt at even less than 10 % melt volume in the system, we envision ... If this is true, multiple magmatic fabrics may serve as a sensitive indicator of evolving mechanical coupling between pluton and ...
The calc-alkaline Miřetín Pluton (newly dated at 346 Ma ± 5 Ma; an U-Pb age obtained by laser-abl... more The calc-alkaline Miřetín Pluton (newly dated at 346 Ma ± 5 Ma; an U-Pb age obtained by laser-ablation ICP MS method on zircons) is a NNE-SSW elongated intrusive body emplaced into the upper-to mid-crustal rocks of the Polička Unit (eastern Teplá-Barrandian Zone; Bohemian Massif). Its composition reveals similarities to other calc-alkaline granitoids, which are mostly interpreted as products of magma mixing between the basic magmas derived from mantle wedge above a subduction zone with crustally-derived acid melts. The conditions of magma crystallization estimated at 653-681 °C and 0.29-0.43 GPa roughly correspond to peak metamorphic evolution of the host volcano-sedimentary rocks of the northwestern part of the Polička and Hlinsko units. The Miřetín Pluton was emplaced into a NNE-SSW oriented transpressional domain, well recognized on a regional scale along the eastern margin of the Teplá-Barrandian Zone. During, or shortly after the magma emplacement, the Miřetín Pluton was affected by pervasive submagmatic to high-T solid-state deformation, reflecting an additional strain increment of regional transpression in a narrow zone of thermal softening. Sharply superimposed low-T solid-state fabric preserved along the western part of the Pluton was connected with normal shearing between the Polička Unit at the bottom and the overlying Hlinsko Unit after 335 Ma.
In the Jizera granite of the Krkonoše–Jizera Plutonic Complex, northern Bohemian Massif, contrast... more In the Jizera granite of the Krkonoše–Jizera Plutonic Complex, northern Bohemian Massif, contrasting patterns of magmatic K-feldspar fabrics and brittle fractures characterize different structural levels of the pluton. The uppermost exposed level at ∼800–1,100 m above sea level is dominated by flat foliation that overprints two steep foliations. In contrast, K-feldspar shape-fabric in an underground tunnel (∼660 m above sea level) shows complex variations in orientation and intensity. Magnetic fabric carried by coaxial contributions of biotite, magnetite, and maghemite is homogeneous along the examined section of the tunnel, and is decoupled from the K-feldspar fabric. The Jizera granite is crosscut by two regional sets of subvertical fractures (∼NE–SW and ∼NW–SE) and by near-surface exfoliation joints. The multiple fabrics are inferred to reflect a complex magmatic strain history at different structural levels of the pluton, bearing little or no relationship to the fracture network. In contrast to the original concept of Hans Cloos (“granite tectonics”), we conclude that no simple genetic relationship exists between fabrics and fractures in plutons. An alternative classification of fractures in plutons thus should avoid relationships to magmatic fabrics and should instead consist of cooling, syntectonic, uplift, and post-uplift fractures.
The ∼340 Ma Knížecí Stolec durbachitic pluton was emplaced as a deep-seated cone-sheet-bearing ri... more The ∼340 Ma Knížecí Stolec durbachitic pluton was emplaced as a deep-seated cone-sheet-bearing ring complex into the Křišt’anov granulite body (Moldanubian Unit, Bohemian Massif). Prior to the emplacement of the durbachitic magma, the steep sub-concentric metamorphic foliation in the granulite formed due to intense ductile folding during high-grade retrograde metamorphism. Subsequently, the durbachitic pluton intruded discordantly into the granulite at around
Page 1. ORIGINAL PAPER Magmatic history and geophysical signature of a post-collisional intrusive... more Page 1. ORIGINAL PAPER Magmatic history and geophysical signature of a post-collisional intrusive center emplaced near a crustal-scale shear zone: the Plechy´ granite pluton (Moldanubian batholith, Bohemian Massif) Krytof ...
Skarns in the Svratka Unit, in the neighbouring part of the Moldanubian Zone and in the Kutná Hor... more Skarns in the Svratka Unit, in the neighbouring part of the Moldanubian Zone and in the Kutná Hora Complex were studied with respect to their metamorphic evolution, major-and trace-element geochemistry, oxygen isotopic composition and zircon ages. Skarns form competent lenses and layers in metamorphosed siliciclastic rocks and preserve some early deformation structures and several equilibrium assemblages representing the products of successive metamorphic reactions. The main rock-forming minerals, garnet and clinopyroxene, are accompanied by less abundant magnetite, amphibole, plagioclase, epidote ± quartz. In the Svratka Unit the early prograde M 1 , prograde/peak M 2 , and retrograde M 3 metamorphic stages have been distinguished. Metamorphic conditions in skarns of the Moldanubian Zone are limited to a relatively narrow interval of amphibolite facies. The prograde and retrograde events in the Kutná Hora Complex skarns probably took place under amphibolite-facies conditions. The presence of magnetite and the increasing proportion of the andradite component in the garnet indicate locally increased oxygen fugacity. Skarn geochemistry does not show systematic differences in the skarn composition among the three units. The regional variations are exceeded by differences among samples from individual localities. The Al 2 O 3 /TiO 2 , Al 2 O 3 /Zr, TiO 2 /Nb ratios point to the variable proportion of the detrital material, combined in skarn protoliths with CaO and FeO, the major non-detrital components. The skarns exhibit elevated abundances of Cu, Zn, Sn and As. The Eu/Eu* ratio varies in the range of 0.5-8.6, the total REE contents vary from 8 to 345 ppm. The lowest ΣREE values (< 100 ppm) occur in skarns with magnetite mineralization. The wide intervals of ΣREE and Eu/Eu* values are interpreted to indicate variations in the temperature and redox conditions among layers of the same locality and at various localities. The oxygen isotope compositions of garnets, pyroxenes and amphiboles from skarns of the Svratka Unit exhibit a range of δ 18 O = 0.1 to 4.1 ‰. In situ (laser-ablation ICP-MS) U-Pb dating of zircon from one of the Svratka Unit skarn bodies yielded a wide range of ages (0.5-2.6 Ga), supporting the detrital origin of this zircon population. The skarn protoliths were probably rocks of mixed detrital-exhalative origin deposited on the sea floor. The geological position of skarns, with their structural and metamorphic record, probably reflect tectono-metamorphic evolution shared with that of their host rocks. The geochemical characteristics, including oxygen isotopic compositions and the presence of detrital zircons with a wide range of ages exclude metasomatic, and point to a sedimentary-exhalative mode of origin for the studied skarns.
In this paper are reviewed structural, petrological and geochronological data from the main units... more In this paper are reviewed structural, petrological and geochronological data from the main units at the NE periphery of the Moldanubian Zone, i.e. Kutná Hora Complex, Svratka Unit, Polička and Zábřeh units, as well as the Strážek Unit of the Moldanubian Zone. In this domain of the Bohemian Massif, the lower-and upper-crustal units are dominated by metamorphic fabrics produced during the Variscan orogeny. The mid-to upper-crustal Svratka, Polička and Zábřeh units are affected by ~MP/MT "long-lived" (~350-339 Ma) tectonometamorphic event reflecting ~WNW-ESE right-lateral strike-slip shearing (transpressional to transtensional tectonics). These regional fabrics are in the Polička and Zábřeh units related with syn-tectonic emplacement and crystallization of calc-alkaline intrusions (Zábřeh Intrusive Complex, Miřetín nad Budislav plutons). In the three structurally high units in the Kutná Hora Complex, Orlice-Sněžník and the Strážek units the strike-slip, "long-lived" tectonics is rather localized; the high-pressure, high-temperature events followed by heterogeneous and polyphase exhumation of deep-seated rocks to mid-crustal levels are preserved. Ultrapotassic rocks (durbachites) of the Strážek Unit, dated at ~339 Ma, intruded posttectonicaly.
ABSTRACT Metagranites and migmatites of the Svratka and the Orlice-Sněžník units, northeastern Bo... more ABSTRACT Metagranites and migmatites of the Svratka and the Orlice-Sněžník units, northeastern Bohemian Massif, exhibit a number of similarities in their lithological, mineralogical and geochemical features. Both of these units were affected by intense migmatitization accompanied by intrusion of peraluminous granites during the Cambrian to Ordovician. The chemical composition and mineralogy of all the studied rocks correspond to crustal melts. From the geochemical point of view, both rock groups (migmatites and metagranites) exhibit progressive geochemical fractionation (increase in the SiO2, Rb, XFeO, W and Sn contents, accompanied by a decrease in the concentrations of Ba, Sr, Mg and Ca) as a result of varying degrees of partial melting and fractional crystallization. The main rock-forming minerals are influenced by metamorphic recrystallization under amphibolite-facies conditions during the Variscan orogeny.
In the porphyritic Jizera granite, Bohemian Massif, three distinct types of lattice-preferred ori... more In the porphyritic Jizera granite, Bohemian Massif, three distinct types of lattice-preferred orientations of biotite grains were revealed in schlieren-delineated magmatic structures using the electron backscatter diffraction (EBSD) method. (1) Biotite basal planes (001) reorient from schlieren-subparallel near the schlieren base to schlieren-perpendicular in the upper part of the schlieren. Both orientations share subhorizontal ∼N–S to ∼NNE–SSW-trending a axes. (2)
ABSTRACT [1] Field relationships combined with new U-Pb zircon geochronology suggest that the sha... more ABSTRACT [1] Field relationships combined with new U-Pb zircon geochronology suggest that the shallow-level Krkonoše-Jizera plutonic complex, northern Bohemian Massif, was assembled successively from bottom to top, starting with emplacement of the separately evolved S-type Tanvald granite (317.3 ± 2.1 Ma), followed by at least two voluminous batches of the I-type porphyritic Liberec (319.5 ± 2.3 Ma) and Jizera (320.1 ± 3.0 Ma and 319.3 ± 3.7 Ma) granites. The intrusive sequence was completed by uppermost, minor intrusions of the equigranular Harrachov (315.0 ± 2.7 Ma) and Krkonoše granites. The I-type granites exhibit an unusually complex pattern of superposed feldspar phenocryst and magnetic fabrics as revealed from the anisotropy of magnetic susceptibility (AMS). The outer Liberec granite preserves margin-parallel foliations and lineations, interpreted to record emplacement-related strain captured by cooling from the pluton floor and walls. In contrast, the inner Jizera, Harrachov, and Krkonoše granites were overprinted by synmagmatic strain resulting from dextral movements along regional strike-slip faults cutting the opposite ends of the plutonic complex. Late-stage felsic dikes in the Liberec and Jizera granites reorient from horizontal to vertical (lineation-perpendicular) attitude in response to changing the least principal stress direction, whereas mafic schlieren do not do so, representing only randomly oriented small-scale thermal-mechanical instabilities in the phenocryst framework. In general, this case example challenges the common approach of inferring pluton-wide magma flow from interpolated foliation, lineation, and schlieren patterns. More likely, magmatic fabrics in large plutons record complex temporal succession of superposed strains resulting from diverse processes at multiple scales.
This paper presents new structural, anisotropy of magnetic susceptibility (AMS), petrological, an... more This paper presents new structural, anisotropy of magnetic susceptibility (AMS), petrological, and geochronological data to examine the link between LP-HT metamorphism and S-type granite formation in the Moldanubian unit, Bohemian Massif. We first describe the intrusive relationships of an S-type granite to its host cordierite-bearing migmatites, superbly exposed in the Rácov locality, northeastern Moldanubian batholith. The knife-sharp contacts and rectangular stoped blocks establish that the migmatites cooled and were exhumed above the brittle-ductile transition prior to the granite emplacement. The U-Pb monazite geochronology combined with P-T estimations constrain the age and depth of migmatization at~329 Ma and~21 km (T ≈ 730°C). The migmatite complex was then exhumed at a rate of 6-7 mm y −1 to a depth of b 9 km where it was intruded by the granite at~327 Ma. These data indicate that the hot fertile metapelitic middle crust in this part of the Moldanubian unit, newly defined as the Pelhřimov complex, underwent rapid isothermal decompression at~329-327 Ma, giving rise to biotite melting and generation of large volumes of S-type granite magma. We propose that the rapid~329-327 Ma exhumation of the Pelhřimov complex may have been partly assisted by the crustal-scale Přibyslav mylonite zone, which delineates the underlying western edge of the Brunia microplate underthrust beneath the eastern half of the Moldanubian unit during the early Carboniferous. The front edge of Brunia thus acted as a rigid backstop at depth, localizing the exhumation of the Pelhřimov complex and separating the hot fertile middle crust to the west from the already cooled overthrust complexes to the east. The magnetic fabric of the granite around the migmatite blocks further reveals that the Pelhřimov complex was shortened vertically and extended in the~WNW-ESE direction during and after its exhumation, implying that the SSE-directed underthrusting of Brunia along the eastern margin of the Bohemian Massif was replaced by vertical thinning and~WNW-ESE stretching of the Moldanubian crust. As a general conclusion, we suggest that even for extremely rapid crustal exhumation, S-type granite magmas can be formed at greater depths by isothermal decompression of the metapelitic host, and then ascend almost instantaneously to the already exhumed (and cooled) shallow parts of the same metamorphic core complex. This model may explain the short time spans for the extensive migmatization and associated S-type granite formation, crustal exhumation, and granite emplacement, as well as the presence of "cold", discordant granite-migmatite contacts in once "hot" migmatite terrains.
In this study, we present structural and AMS data from the w335 Ma ultrapotassic Jihlava syenitoi... more In this study, we present structural and AMS data from the w335 Ma ultrapotassic Jihlava syenitoid pluton, which intruded the lower-to midcrustal orogenic root (Moldanubian Unit) in the Bohemian Massif, Central European Variscides. The emplacement of the pluton was accommodated by multiple processes, such as ductile host-rock shortening, formation of sheeted zones by magma wedging, magmatic stoping, and possibly host-rock displacement within a wide transtensional zone. Magmatic fabrics preserved in the pluton reflect both intrusive processes and regional strain. Margin-parallel and wENEeWSW foliations, which probably formed by strain during emplacement of inner magma pulses, were overprinted by tectonic strain within a zone of distributed wrench-dominated dextral transtension. This zone probably accommodated exhumation of different segments in the eastern part of the Moldanubian Unit during pluton emplacement. In contrast to existing models, we emphasize that the Jihlava pluton, as well as other ultrapotassic plutons widespread in the Moldanubian Unit, are structurally highly variable bodies emplaced by multiple intrusive processes. Our case study illustrates how careful documentation of structural relations around these ultrapotassic plutons may constrain the kinematic framework and local exhumation histories in different segments of the orogenic root during and shortly after the w340 Ma mechanical event in the Central European Variscides.
Geological Society, London, Special Publications, 2014
ABSTRACT This paper summarizes the current knowledge on the nature, kinematics and timing of move... more ABSTRACT This paper summarizes the current knowledge on the nature, kinematics and timing of movement along major tectonic boundaries in the Bohemian Massif and demonstrates how the Variscan plutonism and deformation evolved in space and time. Four main episodes are recognized: (1) Late Devonian–early Carboniferous subduction and continental underthrusting of the Saxothuringian Unit beneath the Teplá–Barrandian Unit resulted in the orogen-perpendicular shortening and growth of an inboard magmatic arc during c. 354–346 Ma; (2) the subduction-driven shortening was replaced by collapse of the Teplá–Barrandian upper crust, exhumation of the high-grade (Moldanubian) core of the orogen at c. 346–337 Ma and by dextral strike-slip along orogen-perpendicular NW–SE shear zones; (3) following closure of a Rhenohercynian Ocean basin, the Brunia microplate was underthrust beneath the eastern flank of the Saxothuringian/Teplá–Barrandian/Moldanubian ‘assemblage’; this process commenced at c. 346 Ma in the NE and ceased at c. 335 Ma in the SW; and (4) late readjustments within the amalgamated Bohemian Massif included crustal exhumation and mainly S-type granite plutonism along the edge of the Brunia indentor at c. 330–327 Ma, and peripheral tectonothermal activity driven by strike-slip faulting and possibly mantle delamination around the consolidated Bohemian Massif&#39;s interior until late Carboniferous–earliest Permian times.
All GSW Journals, GSW + GeoRef. ... that 90 % of crystal boundaries are lubricated by melt at eve... more All GSW Journals, GSW + GeoRef. ... that 90 % of crystal boundaries are lubricated by melt at even less than 10 % melt volume in the system, we envision ... If this is true, multiple magmatic fabrics may serve as a sensitive indicator of evolving mechanical coupling between pluton and ...
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Papers by Kryštof Verner