Article

Rb–Sr and K–Ar mineral data of the KTB and the surrounding area and their bearing on the tectonothermal evolution of the metamorphic basement rocks

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Abstract

&#112K–Ar and Rb–Sr age determinations on muscovites and biotites and K–Ar age determinations on amphiboles are presented for leucocratic gneisses, amphibolites and (meta-)igneous rocks from the Zone of Erbendorf Vohenstrauss (ZEV) and the KTB boreholes located in the northeastern ZEV and leucocratic gneisses from the Erbendorf Greenschist zone (EGZ). The investigations were carried out to contribute to the knowledge of the tectonothermal evolution of the area and to the response of the isotope systems to retrograde processes including recent elevated in- situ temperatures in the boreholes. A memory of an early Ordovician igneous event is given by Rb–Sr ages of 480&#455–475LJ Ma obtained on very coarse-grained relic igneous muscovites of metapegmatites from the ZEV. This memory is missing in micas from peg matoids of the drill site. Coarse-grained muscovites of gneisses from ZEV and EGZ surface outcrops and of gneisses and pegmatoids from the boreholes yielded maximum K–Ar ages of 377&#453–371Dž Ma related to the end of the Devonian regional metamorphism in both units. Consistent K–Ar and Rb–Sr apparent ages of some muscovites from surface outcrops point to a fast cooling of the metamorphic rocks due to rapid tectonic uplift. The lack of a depth-related gradient in model ages of coarse-grained KTB muscovites is explained by post-Variscan stacking processes. For minerals from intermediate to greater depths of the KTB, a strong age dependence on grain size and disturbances of the isotope systems due to various late- to post-Variscan retrograde processes can be demonstrated. A strong decrease in K–Ar model ages with increasing depth as obtained for biotites from the deepest section of the HB borehole probably indicates time-integrated response of the isotope system to elevated in situ temperatures.

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... This metamorphic stage of the end of deformation (371–376 Ma) is closely bracketed by other geochronological data, at least in the ZEV: U–Pb monazite ages around 380 Ma (Teufel 1988 ) are likely to represent peak-metamorphic conditions. Rb/Sr and K/Ar amphibole and mica ages of 380-371 Ma are interpreted as cooling ages (Ahrendt et al. 1997). The Rb–Sr data sets of some metapegmatites allow calculation of reliable initial Sr isotopic ratios. ...
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Granitic metapegmatites of two crystalline units of the western Bohemian massif, the Zone of Erbendorf-Vohenstrauss in Germany (ZEV), and the Zone of Teplá-Domazlice in the Czech Republic (ZTD) have been dated by means of U-Pb and Rb-Sr methods. Ages interpreted to reflect emplacement and crystallisation of the pegmatites were found consistently to be approximately 480Ma, as constrained by U-Pb analyses of primary magmatic zircon, monazite, garnet and columbite, and by Rb-Sr analyses of large pegmatitic muscovites. Later Devonian amphibolite-facies metamorphism caused ductile shearing of the pegmatites, leading to partial recrystallisation of pegmatitic material. A metamorphic, fine-grained generation of muscovite yielded consistent Rb-Sr ages of 371-376Ma for both the ZEV and the ZTD, interpreted as dating the end of deformational activity. The Rb-Sr system of the large pegmatitic muscovites turned out to remain closed up to metamorphic temperatures of >600 °C. Deformation at elevated temperatures is identified as the dominant mechanism for opening of the Rb-Sr system of primary muscovites: apparent ages grade towards Devonian ages as the muscovites become more deformed and fragmented. The data derived from the metapegmatites point to a similar or common tectonometamorphic evolution for the ZEV and the ZTD since the Ordovician. Furthermore, the magmatic formation of granitic pegmatites implies an upper crustal position of the intruded rocks in Lower Ordovician times, clearly documenting the existence of two distinct metamorphic cycles. In contrast to this, in the neighbouring part of the Moldanubian, only unmetamorphosed granitic pegmatites with intrusion ages around 320Ma are observed.
... The ZEV is comprised of various metasediments and metabasites, some of which contain relicts of a HP, eclogite-facies, metamorphism (O'Brien and Carswell, 1993). Previous geochronology, using the U-Pb zircon and Sm-Nd Grt-WR methods suggest the eclogite-facies event occurred at around 398 Ma (Von Quadt, 1990) and subsequent amphibolite-facies metamorphism has been constrained to 400-370 Ma based on K-Ar hornblende, biotite, and muscovite methods (Ahrendt et al., 1997;Kreuzer et al., 1989;Teufel, 1988). However, the limited exposures and poor preservation of eclogite-facies assemblages have precluded more detailed study of the ZEV HP rocks. ...
Article
Lu-Hf and Sm-Nd garnet-whole rock geochronology combined with petrographic observations, minero-chemical variations, thermodynamic modelling and structural data was used to constrain the P–T–t–d evolution of eclogites from the Mariánské Lázně Complex (Bohemian Massif). Boudins of mostly isotropic eclogite with relict steep eclogite-facies fabric are affected by steep migmatitic foliation, which is followed on a regional scale by the development of almost pervasive, predominantly SE-dipping, extensional foliation. The structural succession shows continuous transition from eclogite to garnetiferous migmatitic amphibolite and to amphibolite migmatite. A least retrogressed sample of eclogite shows clusters of fine-grained inclusion-poor garnet, omphacite relicts surrounded by a fine-grained clinopyroxene-plagioclase symplectite with minor amphibole, biotite-plagioclase intergrowths after white mica, kyanite with plagioclase-spinel coronas and accessory rutile. Rare potassic white mica occurs as inclusions in omphacite. A more retrogressed eclogite, with no omphacite or kyanite relicts, contains inclusion-poor garnet surrounded by amphibole-plagioclase corona in a matrix dominated by plagioclase-amphibole symplectite with minor clinopyroxene. In places, the symplectite is overgrown by coarse-grained amphibole. Peak P–T conditions, inferred from combined conventional thermobarometry and phase-equilibria modelling and based on inclusions of white mica (up to 3.33 Si p.f.u.), matrix omphacite (Jd33–36) and garnet core (Alm33–38Prp38–42Grs22–25Sps1) compositions are ~25 kbar at 650–750 °C. A HT overprint occurred at ~14–18 kbar and >800 °C based on coexisting clinopyroxene (Jd18–24), plagioclase (An18–35), and amphibole (Na(B) <0.20; Al(C) = 0.60–1.15) in symplectite after original omphacite and phase-equilibria modelling of garnet mantle compositions. Lu-Hf and Sm-Nd garnet geochronology has been applied to both samples, an older age (c. 390 Ma) obtained by the Lu-Hf method is interpreted as the timing of HP metamorphism, while c. 15 Ma younger ages were obtained by the Sm-Nd method. As temperatures for the HT overprint exceed the empirically and experimentally determined closure temperature of the Sm-Nd system the Sm-Nd ages are interpreted to date cooling following the HT overprint. Combined together, contrasting eclogite and amphibolite-facies migmatite fabrics, the mineral textures, calculated P–T conditions, and distinct Lu-Hf and Sm-Nd ages, provide a complete P–T–t–D path characterised by rapid (~15 Ma) transition from HP subduction, crustal thickening to extensional HT shearing. This unconventional exhumation path does not fit to models of monocyclic exhumation in a subduction channel proposed for the Münchberg and ZEV Devonian HP units to the west. In contrast, the post-peak extensional low angle shearing shortly after subduction and collision resembles more the geodynamic model typical for the Iberian subduction system both in time scales and sequence of tectonic events.
... 300 °C. Comparable K–Ar-and Ar–Ar ages of white mica have been reported from the zone of Erbendorf-Vohenstrauss (ZEV; Kreuzer et al. 1989; Wemmer and Ahrendt 1994; Henjes-Kunst et al. 1994; Ahrendt et al. 1997) where the time of the metamorphic overprint is dated at 398 Ma (Sm–Nd whole rock/garnet; Quadt 1997) and 380 Ma (U–Pb monazite; Teufel 1988; Grauert et al. 1994). Ordovician ages have recently been derived by Rb–Sr dating of white mica of the TCC. ...
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Chapter
EinleitungArbeiten zur methodischen Erweiterung Erfassung, Datierung und Interpretation von Ungleichgewichten der Isotopenverteilung von Strontium und NeodymUntersuchungen zur Frage der Anwendbarkeit geochronologischer Methoden Einfluss von Gesteinsdeformation, Rekristallisation und Temperung auf IsotopenverteilungenEinfluss wasserreicher fluider Phasen auf die Isotopenverteilung in GesteinenInterpretation von U-Pb-Altern akzessorischer MonaziteHomogenitätsbereiche und „Trennwände“ für den IsotopenaustauschUntersuchungen zu geochronologischen Fragestellungen einiger ausgewählter Gebiete Charnockite IndiensGebänderte Metasedimente NordwestargentiniensGneise und Relikte einer HP-HT-Metamorphose im zentralen SchwarzwaldAlter detritischer Zirkone im nordwest-mitteleuropäischen Paläozoikum (Rheinisches Schiefergebirge, Ardennen, Brabanter Massiv)Variszische Plutonite des HarzesAlkaligesteine des Urals, RusslandMagmen des KaiserstuhlsMetamorpher Kernkomplex der Insel Thasos, Nord-GriechenlandIsotopengeochemie des Stoffaustauschs in der Fenitaureole des Iivaara-Alkaligesteinskomplexes, FinnlandBeiträge zum Kontinentalen Tiefbohrprogramm der Bundesrepublik (KTB) Die Tiefbohrung und ihr Umfeld„Widersprüche“ in der geochronologischen Information aus der Tiefbohrung und dem UmfeldArbeiten zur Lagerstättengenese Blei-Isotopie von Galeniten aus dem Bergbaugebiet der Anden ZentralperusStrontium-Isotopie hydrothermaler Gangminerale in Lagerstätten WestdeutschlandsBeiträge zur Archäometallurgie Blei-Isotopenuntersuchungen an bronzezeitlichen VerhüttungsproduktenBlei-Isotopie mittelalterlicher GläserLiteraturProjekte des ZLG seit 1976Verzeichnis der Veröffentlichungen mit Ergebnissen des ZLG (ohne Kurzfassungen, Dissertationen und Habilitationsschriften)Verzeichnis der Dissertationen und Habilitationsschriften mit Ergebnissen des ZLGMitglieder der Senatskommission für Geowissenschaftliche Gemeinschaftsforschung Erfassung, Datierung und Interpretation von Ungleichgewichten der Isotopenverteilung von Strontium und Neodym Einfluss von Gesteinsdeformation, Rekristallisation und Temperung auf IsotopenverteilungenEinfluss wasserreicher fluider Phasen auf die Isotopenverteilung in GesteinenInterpretation von U-Pb-Altern akzessorischer MonaziteHomogenitätsbereiche und „Trennwände“ für den Isotopenaustausch Charnockite IndiensGebänderte Metasedimente NordwestargentiniensGneise und Relikte einer HP-HT-Metamorphose im zentralen SchwarzwaldAlter detritischer Zirkone im nordwest-mitteleuropäischen Paläozoikum (Rheinisches Schiefergebirge, Ardennen, Brabanter Massiv)Variszische Plutonite des HarzesAlkaligesteine des Urals, RusslandMagmen des KaiserstuhlsMetamorpher Kernkomplex der Insel Thasos, Nord-GriechenlandIsotopengeochemie des Stoffaustauschs in der Fenitaureole des Iivaara-Alkaligesteinskomplexes, Finnland Die Tiefbohrung und ihr Umfeld„Widersprüche“ in der geochronologischen Information aus der Tiefbohrung und dem Umfeld Blei-Isotopie von Galeniten aus dem Bergbaugebiet der Anden ZentralperusStrontium-Isotopie hydrothermaler Gangminerale in Lagerstätten Westdeutschlands Blei-Isotopenuntersuchungen an bronzezeitlichen VerhüttungsproduktenBlei-Isotopie mittelalterlicher Gläser
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The Moldanubian-Saxothuringian boundary∗ marks a prominent subduction zone in the internal parts of the European Variscides. In the Oberpfalz area deep reflection seismic profiles show a high density of various reflectors mainly in the upper crust indicating a complex polyphase deformation during the collision process.Seismic structures and related data from surface geology suggest that the Moldanubian-Saxothuringian collision consisted of two main phases. The first phase is marked by extensive NW-directed overthrusting of Moldanubian onto Saxothuringian crust along SE-dipping master décollements, some of which can be traced down to the Moho. During an advanced stage progressive overthrusting led to the development of wedge structures associated with SE-directed backthrusting and backfolding in the upper structural levels. Subsequently some of these wedges and their related décollements were folded to form the main NE-striking anticlines. The first phase of crustal imbrication correlates with a regional low-pressure-high-temperature metamorphism (at about 320 Ma) decreasing in grade northwestward. The second phase is characterized by a reorientation of the main direction of crustal shortening from NW-SE to SW-NE which is interpreted as the result of the N-directed indentation of the Vindelician terrane into the Moldanubian. This event is recorded by conjugate systems of overthrusting and subsequent strike-slip faulting in the Oberpfalz and the Bavarian Forest and the Black Forest-Vosges area. In the area of the southern German block between these basement outcrops the indentation front is marked by the arc-shaped axes of gravimetric and magnetic anomalies and Permo-Carboniferous troughs.
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Brittle faults and veins are very widespread in the crystalline rocks of the northern Oberpfalz, particularly in the 4000 m deep Kontinentale Tiefbohrung (KTB) pilot well. In the late Variscan (Late Carboniferous), subvertical tension gashes were formed under NE-SW extension. A genetic relationship between the vein development and late-phase magmatic activity of the late Variscan granites is obvious. Subsequently, but still during the Late Carboniferous, graphite-enriched reverse faults developed under E-W and NE-SW compression. In the deeper part of the KTB pilot hole these faults were formed within the brittle-ductile transition regime of the paragneisses. Since, during the late Variscan, the geothermal gradient was remarkably elevated, the brittle-ductile regime was situated at a relatively high crustal level where cataclasis, crystal plasticity and diffusion-controlled deformation mechanisms were active simultaneously.
Article
The Leuchtenberg granite (Oberpfalz, NE Bavaria) displays a continuous differentiation trend ranging from mildy peraluminous, coarse-grained, porphyritic biotite granites (BG) to strongly peraluminous, medium- to fine-grained, garnet-bearing muscovite granites (GMG). The Rb–Sr and K–Ar age determinations of whole-rock and mineral samples from the granite and associated intermediate rocks (redwitzites) have revealed two divergent age gradients: Rb–Sr wholerock dates decrease and initial 87Sr/86Sr ratios increase for successively more evolved subsets of the granite. All BG samples (87Rb/86Sr=2–16) yield a date of 3262 Ma with a low initial 87Sr/86Sr ratio of 0.707780.00013 (1), while all GMG samples (87Rb/86Sr=70 to 1000) yield a younger date of 3172 Ma with an enhanced initial 87Sr/86Sr ratio of 0.71460.0039. The K–Ar measurements on biotites and muscovites give closely concordant dates for the GMG (326–323 Ma) and the southern lobe of the BG (324–320 Ma). The northern lobe of the BG, including the redwitzites, shows a well-defined trend of decreasing K–Ar dates from 320 Ma to 300 Ma towards the northwest. Critical consideration of both isotope systems leads to the conclusion that the Rb–Sr system of the GMG was disturbed by a later hydrothermal event. The ca. 326 Ma whole-rock Rb–Sr date for the BG is not in conflict with any of the K–Ar mineral dates and is taken as approaching the crystallization age of the Leuchtenberg granite. The K–Ar age progression within the northern lobe of the BG indicates that this part either cooled down over a protracted period of some 20 Ma or experienced reheating at ca. 300 Ma. The study highlights the potential of combined Rb–Sr and K–Ar dating in deciphering detailed chronology on the scale of a single igneous intrusion.
Article
The polymetamorphic Moldanubian (MO) of the northeasthern margin of the Bohemian Massif has been thrust to the north onto the mainly Paleozoic sedimentary Saxothuringian of the Fichtelgebirge (FG). These two units have undergone polyphase deformation and the last regional event to affect both units was a low-pressure metamorphism in which temperatures decreased towards the north.In contrast, the nappe units of the Erbendorf-Vohenstrauss Zone (ZEV) and the Erbendorf Greenschist Zone (EGZ), which partly cover the border of the Moldanubian and the Saxothuringian, and the Münchberg nappe pile (MM), which lies on the Saxothuringian, were in parts subjected to a late medium-pressure metamorphic event.The ZEV, the EGZ, the MO and the FG are intruded by Late Carboniferous granites.Conventional K-Ar analyses, mainly of hornblendes and muscovites from the autochthonous FG and MO, the units beneath the nappes, have yielded exclusively Carboniferous dates. The oldest dates point to a regional cooling of the rocks which outcrop at the present-day surface at about 330-320 Ma, i.e., at the Early-Late Carboniferous boundary. The Late Carboniferous cooling history was largely governed by the thermal influence of the post-kinematic granites (320-295 Ma), especially in the FG and the northern MO.The high-grade metamorphic rocks in the western part of the ZEV and in the upper three nappes of the MM mostly yield dates around 380 Ma i.e., Early Devonian. The results show a relatively wide scatter. Moreover, biotites frequently appear to be older than the coexisting muscovites. Both observations indicate that the rocks underwent a later thermal influence. Whether some groups of older dates (e.g., 400 Ma) are due to excess argon or to inherited argon is still open to discussion.Slightly scattered muscovite dates around 366 Ma were obtained for the prasinite-phyllite series, one of the lower nappes of the MM. A single hornblende from the EGZ gave the same age. These two nappes have, therefore, probably been affected by a Late Devonian thermal and/or tectonic event.The muscovite dates obtained from the Paleozoic Bavarian lithofacies, the lowermost nappe of the MM∗, and the hornblende dates from the eastern part of the ZEV are indistinguishable from those of the autochthonous units FG and MO.