Re-evaluation of polyphase kinematic and 40Ar/39Ar cooling history of Moldanubian hot nappe at the eastern margin of the Bohemian Massif

International Journal of Earth Sciences - A structural and geochronological 40Ar/39Ar study was performed in kilometre-scale middle and lower crustal lens-shaped domains dominated by a preserved...     

Superposition of Variscan ductile shear deformation on pre-Variscan mantled gneiss structure (Catherine dome,Erzgebirge, Bohemian massif)

Three tectonometamorphic events have been recognized in the crystalline rocks of the Catherine mantled gneiss Dome (CD) in the central Erzgebirge. The first tectonometamorphic event is characterized by the acquisition of a S1–S2 metamorphic foliation, development of F2 isoclinal partly syn-schistose folding and intrusion of early synkinematic granites. P-T conditions correspond to higher amphibolite facies metamorphism and culminate in anatexis. A diapiric intrusion of Cambro-Ordovician muscovite-biotite porphyritic granite has verticalized the S2 foliation and is responsible for the genesis of the classical mantled gneiss dome. Oblate strain and orthorhombic diffuse c-axis patterns are typical of the marginal parts of the dome.The whole rock sequence was subsequently affected by westward oriented heterogeneous Variscan ductile shearing under the conditions reaching amphibolite fades. A strongly non-coaxial deformation has partly overprinted the earlier fabrics in anatectic orthogneiss and produced augen-orthogneiss from porphyritic granite in the central part of the CD dome. Prolate to plane strain fabrics, oblique quartz c-axis patterns and a widespread occurrence of S-C fabrics are the most characteristic features of this phase.

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Zusammenfassung Im Kristallin des Katherinenberg mantled gneiss dome im mittleren Erzgebirge wurde eine dreiphasige tektono-metamorphe Entwicklungsgeschichte nachgewiesen.Das alte Stadium stellt bereits eine polyphase Entwicklung dar. Es wird durch die Anlage eines penetrativen metamorphen Lagenbaus (syn D₁ — syn D₂), durch F₂ früh Isoklinalfalten, sowie durch die Intrusion frühkinematischer z. T. anatektischer Granite charakterisiert. PT-Daten belegen amphibolitfazielle Bedingungen, die lokal zu Anatexis führen.Diapirartige Intrusionen kambro-ordovische, porphyrische Zweiglimmergranite bewirkten eine sehr unterschiedliche z. T. subvertikale Raumlage der regionalmetamorphen Einheiten. Diese Intrusionen des zweiten Stadiums sind verantwortlich für die Entstehung des eigentlichen mantled gneiss dome. Oblat Gefüge und diffuse Kreuzgürtel-Verteilungen der Quarz c-Achsen sind typisch für die Randbereiche dieses Gneisdomes.Während des jüngsten Stadiums prägen bzw. überprägen westvergente duktile Scherprozesse bei Bedingungen der niedriggradigen Amphibolitfazies die Gefüge. Schiefgürtel-Verteilungen der Quarz c-Achsen und S-C Gefüge charakterisieren diese jüngste, variszische Phase.

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Résumé Trois événements tectono-métamorphiques ont été reconnus dans les roches cristallines du dôme de gneiss coiffé de Catherine (Erzgebirge oriental). Le premier de ces événements est caractérisé par l'acquisition d'une foliation S_1–2, le développement de plis isoclinaux F₂ partiellement synschisteux et l'intrusion de granites syntectoniques précoces. Les conditions (P, T) correspondent au facies supérieur des amphibolites et atteignent l'anatexie. L'intrusion diapirique d'un granite à deux micas cambro-ordovicien a verticalisé la foliation S_1–2 et est responsable de la genèse du dôme de gneiss coiffé. Une déformation par aplatissement et des fabriques orthorhombiques diffuses des axes c du quartz caractérisent les zones marginales du dôme.L'ensemble des terrains a été ensuite affecté par un cisaillement ductile varisque hétérogène à vergence ouest, dans les conditions du facies inférieur des amphibolites. Une déformation fortement noncoaxiale a transposé partiellement les structures précoces des orthogneiss anatectiques et a transformé les granites porphyriques de la partie centrale du dôme de Catherine en orthogneiss oeillés. Cette phase est caractérisée par des fabriques obliques pour les axes c du quartz et par de fréquentes structures s-c.

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Evolution of mass-transfer during progressive oblique under-thrusting of the Variscan foreland: eastern Bohemian Massif

Abstract

Four ductile shear zones were sampled in the autochthonous Thaya basement and the Upper Bíte? nappe (Moravian unit) at the Eastern margin of the Bohemian massif. In both studied units, the tectono-metamorphic evolution and the chemical mass transfer are different. Two deformational events are recognised: the first deformation stage under amphibolite facies conditions is overprinted by a second event under greenschist facies conditions.

The first deformation affected the western margin of the Thaya basement and the whole Bíte? nappe: microstructures are characterised by dynamic recrystallisation of feldspars and quartz, and occurrence of myrmekites and grain-boundary migration of quartz. None or weak chemical mass transfer is related to this medium to high temperature deformation. This deformation corresponds to the thrusting of Moldanubdian units on the Brunovistulian units (Moravian nappes and autochthonous Thaya basement).

The second deformation generated shear zones in the until then preserved Thaya basement and reactivated both shear zones of the western margin of the Thaya basement and those of the Bíte? nappe. This deformation is retrograde and mainly associated with chemical mass transfer: a decrease of CaO, FeO, FeO/Fe₂O₃ and an increase of MgO, K₂O and H₂O. These chemical changes are related to greenschist metamorphic reactions leading to the destabilisation of feldspars and the crystallisation of white micas and Ca-silicates. The large chemical mass transfer is associated with the circulation of a large volume of fluids. A model of progressive fluid circulation correlated with Variscan prograde and retrograde metamorphism during the collision of Moldanubian and Brunovistulian units is proposed.     

Some remarks on fabric overprints and constrictional AMS fabrics in igneous rocks

During the ascent, emplacement and post-emplacement deformation of igneous rocks, two or more phases of deformation that overprint each other are often depicted. These overprints, when magnetic minerals are present, are recorded in magnetic fabric. In this contribution, overprints are studied by means of numerical modeling, following several basic scenarios common to igneous rocks. Biotite and amphibole that occur often together in igneous rocks are considered as carriers of the anisotropy of magnetic susceptibility. Modeling shows that (1) a constrictional fabric with a low degree of anisotropy as commonly recorded in magmatic rocks may result from a deformation overprint and not necessarily from an extensional/transtensional regime, and (2) that the constrictional AMS fabrics originates from orthogonal superimposition of a deformation event on an AMS fabric inherited from earlier magma emplacement history. Therefore, the interpretation of a constrictional fabric must be performed with caution. Numerical modeling may provide a suitable help in strengthening the interpretation of real magnetic fabric data.     

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

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

The significance of Late Devonian ophiolites in the Variscan orogen: a record from the Vosges Klippen Belt

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 (ε_Nd?=?+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.