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  • 1
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Zircon and monazite from granulite- to amphibolite-facies rocks of the Vosges mountains (central Variscan Belt, eastern France) were dated by ion-microprobe and conventional U-Pb techniques. Different granulites of igneous (so-called leptynites) and sedimentary origin (kinzigites) and their leucosomes were dated at 334.9 ± 3.6, 335.4 ± 3.6 and 336.7 ± 3.5 Ma (conventional age 335.4 ± 0.6 Ma). Subsequent growth stages of zircon were distinguished by secondary electron (SEM) and cathodoluminescence (CL) imaging: (1) subsolidus growth producing round anhedral morphologies and sector zoning; (2) appearance of an intergranular fluid or melt phase at incipient dehydration melting that first resulted in resorption of pre-existing zircons, followed by growth of acicular zircons or overgrowths on round zircons consisting of planar growth zoning; (3) advanced melting producing euhedral prismatic zircons with oscillatory zoning overgrowing the sector zones. Two further lithologies, the Kaysersberg granite and the Trois-Epis units, were both formerly considered as migmatites. The intrusion of the Kaysersberg granite was dated at 325.8 ± 4.8 Ma. The Trois-Epis unit was found to be the product of volume recrystallization of a former granulite, which occurred under amphibolite-facies conditions 327.9 ± 4.4 Ma ago. The amphibolite-facies overprint of the Trois-Epis zircons led to the complete rejuvenation of most of the zircon domains by annealing and replacement/recrystallization processes. Annealing is assumed to occur in strained lattice domains, which are possibly disturbed by high trace element contents and/or large differences in decay damage between adjacent growth zones. Investigation of cathodoluminescence structures reveals that the replacement occurs along curved chemical reaction fronts that proceed from the surface towards the interior of the zircon. The monazite U-Pb system still records the age of high-grade metamorphism at around 335 Ma. The chemical reagent responsible for the rejuvenation of zircon obviously left the monazite unaffected.
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  • 2
    ISSN: 1525-1314
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: 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.
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  • 3
    ISSN: 1525-1314
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: A structural, metamorphic and geochronological study of the Staré Město belt implies the existence of two distinct metamorphic events of similar peak P–T conditions (700–800 °C, 8–10 kbar) during the Cambro-Ordovician and the Carboniferous tectonometamorphic events. The hypothesis of two distinct periods of metamorphism was suggested on the basis of structural discordance between an undoubtedly Carboniferous granodiorite sill intrusion and earlier Cambro-Ordovician fabrics of a banded amphibolite complex. The analysis of crystal size distribution (CSD) shows high nucleation density (N0) and low average growth rate (Gt) for Carboniferous mylonitic metagabbros and mylonitic granodiorites. The parameter N0 decreases whereas the quantity Gt increases towards higher temperatures progressively approaching the values obtained from the Cambro-Ordovician banded amphibolite complex. The spatial distribution of amphibole and plagioclase shows intense mechanical mixing for lower-temperature mylonitic metagabbros. In high-temperature mylonites a strong aggregate distribution is developed. Cambro-Ordovician amphibolites unaffected by Carboniferous deformation show a regular to anticlustered spatial distribution resulting from heterogeneous nucleation of individual phases. This pattern, together with CSD, was subsequently modified by the grain growth and textural equilibration controlled by diffusive mass transfer during Carboniferous metamorphism. The differences between the observed textures of the amphibolites are interpreted to be a consequence of the different durations of the Carboniferous and Cambro-Ordovician thermal events.
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  • 4
    Publication Date: 2007-10-08
    Description: Deformation mechanisms of amphibole and plagioclase were investigated in two metagabbroic sheets (the eastern and western metagabbros from the Stare M[e]sto belt, eastern Bohemian Massif), using petrology, quantitative microstructural and electron back-scattered diffraction methods. After the gabbroic pyroxene was replaced by amphibole, both gabbroic bodies became progressively deformed. The eastern metagabbros were deformed under temperature of c. 650 {degrees}C and the western metagabbros under c. 750 {degrees}C. Subgrain rotation and dislocation creep, characterized by strong crystallographic and shape preferred orientations, operated in plagioclase of the eastern belt during the early stages of deformation. Subsequent randomizing of plagioclase crystallographic preferred orientation is interpreted to be due to grain boundary sliding in the mylonitic stage. Large (50-150 {micro}m) grain sizes during the mylonitic stages are interpreted to be due to low strain rates. Amphibole is stronger and deforms cataclastically, leading to important grain size reduction when the bulk rock strength drops substantially. In the western belt, plagioclase deformed by dislocation creep accompanied by grain boundary migration (possibly chemically induced) while heterogeneous nucleation and syndeformational grain growth in conjunction with dislocation creep were typical for amphiboles.
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  • 5
    ISSN: 1437-3262
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Description / Table of Contents: Abstract The entire pile of nappes in the eastern margin of the Bohemian massif is characterized by two stages of Variscan nappe emplacement each exhibiting a different kinematic and metamorphic evolution. The older emplacement (D1) probably occurred around 350-340 Ma ago and was synmetamorphic. The nappes show a typical systematic superposition of higher grade metamorphic units over lower grade ones. Thus, the crystalline complexes showing a HT-MP Barrovian imprint (Svratka allochthonous unit and Moldanubicum) were thrust over an intermediate unit affected by MTMP recrystallization (Bíteš orthogneiss and its country rock), and at the base of the D1 nappe pile the Inner Phyllite Nappe (Biý Potok Unit) is characterized by LT/LP metamorphism. The second stage of tectonic evolution (D2) is characterized by a thin-skinned northward-oriented nappe emplacement that occurred under LT-LP conditions dated at 320-310 Ma. The whole nappe sequence formed during the first tectonometamorphic period (D1) was transported northward over the autochthonous »Deblín polymetamorphic and granitic complex« of Upper Proterozoic age and its Devonian sedimentary cover with very low metamorphism. During this second tectonic event the Brno granite massif (580 Ma) was only marginally incorporated in the Variscan nappe tectonics which resulted in kilometer-scale cover and basement duplexes. The tectonic evolution of the nappe pile ended with stage D3, represented by large- to medium-scale east-vergent folds with limited displacement.
    Abstract: Résumé L'empilement des nappes a la bordure orientale du Massif de Bohème est caractérisé par deux stades de mise en place présentant différentes évolutions cinématiques et métamorphiques. La tectonique majeure de mise en place des nappes crustales intervient lors d'un métamorphisme de type barrowien, calé autour de 350-340 Ma. L'empilement qui en résulte montre une superposition systématique d'unités à fort degré de métamorphisme sur des unités moins métamorphiques. Ainsi les complexes cristallins, montrant des reliques de métamorphisme de haute à moyenne pression-haute température (unités cristallines de Svratka et du Moldanubien), chevauchent une unité intermédiaire affectée par un métamorphisme de moyenne à basse pression-moyenne température (l'orthogneiss de Bíteš et son encaissant). A la base de cette pile édifiée durant la tectonique D1, l'unité des phyllites internes (unité de Bílý potok) est caractérisée par un métamorphisme de basse témperature-basse pression. Le second stade D2 de l'évolution tectonique est caractérisé par une tectonique pelliculaire à vergence nord datée à 320-310 Ma. L'empilement résultant de D1 est ainsi transporté vers le nord, au dessus du complexe autochtone d'âge protérozoïque supérieur (groupe de Deblín) et sa couverture sédimentaire dévonienne très faiblement métamorphisée. Le massif granitique de Brno (580 Ma) n'est que marginalement incorporé à cette tectonique de nappe varisque. Ceci se traduit par des duplex socle-couverture d'échelle plurikilométrique. L'évolution tectonique s'achève lors d'une troisième phase, marquée par de grands plis à vergence est. Le déplacement associé est alors d'amplitude limitée.
    Notes: Zusammenfassung Der Deckenbau am Ostrand der Böhmischen Masse erfolgte in zwei aufeinanderfolgenden Stadien, die sich sowohl in ihrer Kinematik als auch in ihrer Metamorphoseentwicklung deutlich voneinander unterschieden. Die ältere Phase (D1 ca. 350-340 Ma) ist durch synmetamorphe Überschiebungen charakterisiert. Sie führt zu einer metamorphen Inversion der überschobenen Deckeneinheiten, so daß generell hohe metamorphe Einheiten schwach metamorphe tektonisch überlagern. Der Svratka Komplex und das Moldanubikum als hangendste Decken sind durch MP/HT Paragenesen vom Barrow-Typ gekennzeichnet. Beide Einheiten sind auf den MP/MT-metamorphen Biteš-Gneis und seine Rahmengesteine überschoben. Die Bílý potok Einheit als liegende Decke zeigt nur noch eine LP/ LT Regionalmetamorphose. Das jüngere Stadium (D2 ca. 320-310 Ma) ist durch eine Thin-skinned Tektonik mit nordvergentem Deckentransport unter LP/LT Bedingungen charakterisiert. Der gesamte, invers metamorphe D1-Deckenstapel wird dabei nach N über den autochtonen Deblín Komplex bzw. seine devonische Sedimenthülle überschoben. Das Brno Granit Massiv (580 Ma) wird nur randlich in diesen variszischen Deckenbau einbezogen. Die tektonische Entwicklung endet mit einem mittel bis großräumigen E-vergenten Faltenbau (D3 phase).
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  • 6
    ISSN: 0031-9201
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Geosciences , Physics
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  • 7
    Publication Date: 2013-08-30
    Description: Geochronology, structural and anisotropy of magnetic susceptibility data from the northern Vosges batholith, which belongs to the Northern Vosges–Mid-German Crystalline Rise arc, show contrasting emplacement modes of southern granodiorites and northern granites. The ENE–WSW-trending fabrics of granodiorites (346–334 Ma) are parallel to the metamorphic cleavage affecting the host rocks developed during regional compression. The NNW–SSE-trending fabrics of younger granitoids ( c . 330 Ma) reveal an extensional emplacement mode, associated with a normal shear zone separating the two magmatic suites. Palaeomagnetism shows that the switch from a compressive to an extensional regime coincides with a regional counterclockwise rotation. The 330–325 Ma extension is further supported by palaeomagnetic and seismic data indicating southeastward tilt of the whole batholith. Finally the system is rotated clockwise without any structural overprint. Data from the Early Carboniferous northern Vosges magmatic arc and remote Bohemian Massif suggest that this evolution is valid for the whole eastern branch of the European Variscan belt, for which the following model is proposed: (1) Late Devonian–Early Carboniferous east–west shortening of the Variscides above the Rhenohercynian subduction zone; (2) axial NNW–SSE shortening of the assembled Variscan orogen associated with (3) ‘internal’ sinistral rotation of inherited Rhenohercynian transform faults and shortening of intervening blocks; (4) east–west extension and dextral ‘external’ rotation of blocks between dextrally reactivated transforms. Supplementary materials: Analytical procedures for geochronology, magnetism, anisotropy of magnetic susceptibility and palaeomagnetism are available at www.geolsoc.org.uk/SUP18635 .
    Print ISSN: 0016-7649
    Topics: Geosciences
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  • 8
    Publication Date: 2012-06-01
    Description: The present work examines the lithological, structural, geochemical and geochronological records from the Klippen Belt located in the southern Vosges Mountains (NE France). The Klippen Belt is represented by discontinuous exposures of serpentinized harzburgite, ophicalcite, gabbro, gneiss and polymictic conglomerate overlain by deep marine pelitic sediments. Structural data and Bouguer anomalies reveal that the Klippen Belt coincides with a significant discontinuity now occupied by a granitic ridge. Gabbro geochemistry indicates a MOR-type affinity similar to recent slow-spreading ridges, but positive Ba, Sr, Th or U anomalies do not exclude the influence of fluids expelled from a subduction zone. A Sm–Nd isochron age of 372 ± 18 Ma is thought to reflect gabbro emplacement from a highly depleted mantle source (εNd = +11.3), and U–Pb zircon ages from a gneiss sample indicate that the basement found in the Klippen has a Neoproterozoic origin. Combined data indicate the formation of a deep basin during Late Devonian rifting. The Klippen lithologies could testify for the presence of an ocean–continent transition environment subsequently inverted during the Early Carboniferous. Basin inversion during the Middle Visean was probably controlled by rift-related structures, and resulted in folding of the sedimentary successions as well as exhumation along thrust zones of deep parts of the basin represented by the Klippen Belt. Based on correlations with the neighbouring Variscan massifs, it is proposed that the southern Vosges sequences represent a back-arc basin related to the North-directed subduction of the southern Palaeotethys Ocean. This geodynamic reconstruction is tentatively correlated with similar ophiolitic remnants in the northern part of the French Massif Central (Brévenne) and with the evolution of the southern Black Forest. The Late Devonian ophiolites are interpreted as relicts of small back-arc marginal basins developed during general closure of the Palaeozoic subduction systems. ©2011 Springer-Verlag
    Print ISSN: 1437-3254
    Electronic ISSN: 1437-3262
    Topics: Geosciences
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  • 9
    Publication Date: 2006-06-02
    Description: Processing of gravity and magnetic maps shows that the basement of the Upper Rhine Graben area is characterized by a series of NE–SW trending discontinuities and elongated structures, identified in outcrops in the Vosges, Black Forest, and the Odenwald Mountains. They form a 40 km wide, N30–40° striking, sinistral wrench-zone that, in the Visean, shifted the Variscan and pre-Variscan structures by at least 43 km to the NE. Wrenching was associated with emplacement of several generations of plutonic bodies emplaced in the time range 340–325 Ma. The sub-vertical, NE–SW trending discontinuities in the basement acted as zones of weakness, susceptible to reactivation by subsequent tectonism. The first reactivation, marked by mineralizations and palaeomagnetic overprinting along NE–SW faults of the Vosges Mountains, results from the Liassic NW–SE extension contemporaneous with the break-up of Pangea. The major reactivation occurred during the Late Eocene N–S compression and the Early-Middle Oligocene E–W extension. The NE–SW striking basement discontinuities were successively reactivated as sinistral strike-slip faults, and as oblique normal faults. Elongated depocenters appear to form in association with reactivated Variscan wrench faults. Some of the recent earthquakes are located on NE–SW striking Variscan fault zones, and show sinistral strike-slip focal mechanisms with the same direction, suggesting also present reactivation. ©2006 Springer-Verlag
    Print ISSN: 1437-3254
    Electronic ISSN: 1437-3262
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  • 10
    Publication Date: 1999-02-18
    Description: Zircon and monazite from granulite- to amphibolite-facies rocks of the Vosges mountains (central Variscan Belt, eastern France) were dated by ion-microprobe and conventional U-Pb techniques. Different granulites of igneous (so-called leptynites) and sedimentary origin (kinzigites) and their leucosomes were dated at 334.9 ± 3.6, 335.4 ± 3.6 and 336.7 ± 3.5 Ma (conventional age 335.4 ± 0.6 Ma). Subsequent growth stages of zircon were distinguished by secondary electron (SEM) and cathodoluminescence (CL) imaging: (1) subsolidus growth producing round anhedral morphologies and sector zoning; (2) appearance of an intergranular fluid or melt phase at incipient dehydration melting that first resulted in resorption of pre-existing zircons, followed by growth of acicular zircons or overgrowths on round zircons consisting of planar growth zoning; (3) advanced melting producing euhedral prismatic zircons with oscillatory zoning overgrowing the sector zones. Two further lithologies, the Kaysersberg granite and the Trois-Epis units, were both formerly considered as migmatites. The intrusion of the Kaysersberg granite was dated at 325.8 ± 4.8 Ma. The Trois-Epis unit was found to be the product of volume recrystallization of a former granulite, which occurred under amphibolite-facies conditions 327.9 ± 4.4 Ma ago. The amphibolite-facies overprint of the Trois-Epis zircons led to the complete rejuvenation of most of the zircon domains by annealing and replacement/recrystallization processes. Annealing is assumed to occur in strained lattice domains, which are possibly disturbed by high trace element contents and/or large differences in decay damage between adjacent growth zones. Investigation of cathodoluminescence structures reveals that the replacement occurs along curved chemical reaction fronts that proceed from the surface towards the interior of the zircon. The monazite U-Pb system still records the age of high-grade metamorphism at around 335 Ma. The chemical reagent responsible for the rejuvenation of zircon obviously left the monazite unaffected. ©1999 Springer-Verlag Berlin Heidelberg
    Print ISSN: 0010-7999
    Electronic ISSN: 1432-0967
    Topics: Geosciences
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