Wolfgang Dörr

Neoproterozoic to Early Cambrian history of an active plate margin in the Teplá–Barrandian unit—a correlation of U–Pb isotopic-dilution-TIMS ages (Bohemia, Czech Republic)

Abstract The Tepla–Barrandian unit (TBU) of the Bohemian Massif shared a common geological history throughout the Neoproterozoic and Cambrian with the Avalonian–Cadomian terranes. The Neoproterozoic evolution of an active plate margin in the Tepla–Barrandian is similar to Avalonian rocks in Newfoundland, whereas the Cambrian transtension and related calc-alkaline plutons are reminiscent of the Cadomian Ossa–Morena Zone and the Armorican Massif in western Europe. The Neoproterozoic evolution of the Tepla–Barrandian unit fits well with that of the Lausitz area (Saxothuringian unit), but is significantly distinct from the history of the Moravo–Silesian unit. The oldest volcanic activity in the Bohemian Massif is dated at 609+17/−19 Ma (U–Pb upper intercept). Subduction-related volcanic rocks have been dated from 585±7 to 568±3 Ma (lower intercept, rhyolite boulders), which pre-dates the age of sedimentation of the Cadomian flysch (Stěchovice Group). Accretion, uplift and erosion of the volcanic arc is documented by the Neoproterozoic Dobřis conglomerate of the upper part of the flysch. The intrusion age of 541+7/−8 Ma from the Zgorzelec granodiorite is interpreted as a minimum age of the Neoproterozoic sequence. The Neoproterozoic crust was tilted and subsequently early Cambrian intrusions dated at 522±2 Ma (Těsovice granite), 524±3 Ma (Vsepadly granodiorite), 523±3 Ma (Smržovice tonalite), 523±1 Ma (Smržovice gabbro) and 524±0.8 Ma (Orlovice gabbro) were emplaced into transtensive shear zones.

Accretion of first Gondwana-derived terranes at the margin of Baltica

Abstract In central Europe, three crustal units, i.e. the Małopolska, the Łysogóry and the Bruno-Silesia, can be recognized by basement data, faunas and provenance of clastic material in the Cambrian clastic rocks. They are now situated within the Trans-European Suture Zone, a tectonic collage of continental terranes bordering the Tornquist margin of the palaeocontinent of Baltica, but during the Cambrian their position in relation to each other and to Baltica was different from today. These units are exotic terranes in respect to Baltica and are interpreted as having been derived from the Cadomian margin of Gondwana. Their detachment is probably related to the final break-up of the supercontinent Rodinia at c. 550–590 Ma. New detrital zircon and muscovite age data provide evidence that Małopolska was derived from the segment of the Cadomian orogen that bordered the Amazonian Craton. It must have already separated from Gondwana in Early Cambrian time (some 40–50 Ma before Avalonia became detached and began its rapid drift). The accretion of Małopolska to Baltica occurred between late mid-Cambrian and Tremadocian times. Both palaeontological and provenance evidence demonstrate that Małopolska and not Avalonia was the first terrane to join the Baltica palaeocontinent. This event initiated the progressive crustal growth of the European lithosphere, which continued during Phanerozoic times and led to the formation of modern Europe.

Emplacement depths and radiometric ages of Paleozoic plutons of the Neukirchen–Kdyně massif: differential uplift and exhumation of Cadomian basement due to Carboniferous orogenic collapse (Bohemian Massif)

Abstract The igneous complex of Neukirchen–Kdyně is located in the southwestern part of the Tepla–Barrandian unit (TBU) in the Bohemian Massif. The TBU forms the most extensive surface exposure of Cadomian basement in central Europe. Cambrian plutons show significant changes in composition, emplacement depth, isotopic cooling ages, and tectonometamorphic overprint from NE to SW. In the NE, the Vsepadly granodiorite and the Smržovice diorite intruded at shallow crustal levels ( 20 km). The Teufelsberg (Certův kamen) diorite, on the other hand, forms an unusual intrusion dated at 359±2 Ma (concordant U–Pb zircon age). K–Ar dating of biotite of the Teufelsberg diorite yields 342±4 Ma. These ages, together with published cooling ages of hornblende and mica in adjacent plutons, are compatible with widespread medium to high-grade metamorphism and strong deformation fabrics, suggesting a strong Variscan impact under elevated temperatures at deeper structural levels. The plutons of the Neukirchen area are cut by the steeply NE dipping Hoher–Bogen shear zone (HBSZ), which forms the boundary with the adjacent Moldanubian unit. The HBSZ is characterized by top-to-the-NE normal movements, which were particularly active during the Lower Carboniferous. A geodynamic model is presented that explains the lateral gradients in Cambrian pluton composition and emplacement depth by differential uplift and exhumation, the latter being probably related to long-lasting movements along the HBSZ as a consequence of Lower Carboniferous orogenic collapse.

The Carboniferous to Jurassic evolution of the pre-Alpine basement of Crete: constraints from U-Pb and U-(Th)-Pb dating of orthogneiss, fission-track dating of zircon, structural and petrological data

Abstract The pre-Alpine evolution of the external Hellenides is poorly constrained because of the Alpine impact which largely erased the older orogenic imprints. Only a few outcrops with pre-Alpine basement exist, one of which is located in eastern Crete. The pre-Alpine basement, part of the Phyllite-Quartzite Unit, is composed of four sub-complexes, which are different in protolith age, type and age of metamorphism, and postmetamorphic cooling history. The lowermost, Kalavros crystalline complex (KCC) underwent Permian amphibolite-facies metamorphism related to top-to-the-NE shearing. The KCC exhibits a four-stage garnet zonation and a late, high-temperature event associated with the growth of K-feldspar. The KCC is overlain by the Myrsini crystalline complex (MCC), which underwent Carboniferous amphibolite facies metamorphism associated with top-to-the-north shearing. Late cooling of the MCC is documented by Jurassic fission track ages of zircon. The Chamezí crystalline complex underwent upper greenschist-facies metamorphism related to top-to-the-north shearing. In addition, the Vaí crystalline complex, in an uncertain structural position, is characterized by Triassic emplacement of granite, followed by amphibolite-facies top-to-the-NW shearing and cooling, as is indicated by Jurassic fission-track ages of zircon. A preliminary tectonic model is presented, which invokes south-directed subduction, collision and accretion of the crystalline complexes to the northern margin of Gondwana.

The Saxothuringian-Rhenohercynian boundary underneath the Vogelsberg volcanic field: evidence from basement xenoliths and U-Pb zircon data of trachyte

The present study is focusing on Variscan basement xenoliths, which are present inside pyroclastic rocks of the Miocene Vogelsberg volcanic field (VVF). The investigated samples have been collected from quarries situated at the SW margin of the Vogelsberg (Nickel quarry, Nidda Ober-Widdersheim) and at the NE margin of the Vogelsberg (Rauher Berg quarry, Alsfeld-Brauerschwend). Moreover, we investigated different types of trachyte, which are present (1) as xenoliths inside an alkali basaltic tuff breccia of the Nickel quarry, (2) as trachytic lava from an outcrop at the Hauserhof (Nidda OberWiddersheim), and (3) as part of a trachytic/phonolitic lava dome drilled in the central VVF near Schotten-Sichenhausen. Apart from one garnet-bearing sample, the investigated basement xenoliths consist of phyllite and basic to felsic metavolcanics. Both the metamorphic index minerals and the quartz deformation microfabrics suggest that these rocks underwent greenschist facies metamorphism, and thus can be attributed to the Northern Phyllite Zone of the Variscan chain. These findings allow tracing the Northern Phyllite Zone from the Rhenish Massif via the VVF to the Harz Mountains. Variscan basement underneath the VVF is also documented by inherited U-Pb zircon ages obtained from the Hauserhof trachyte (366 ± 8.5 Ma and 294 ± 3 Ma) and from the trachyte of the Sichenhausen-Eschwald drilling (323 ± 12 Ma to 344 ± 6 Ma and ages ranging from the Middle Proterozoic to Cambrian/Ordovician boundary). The Permian and Carboniferous ages correlate with widespread late-Variscan volcanic activity in Central Europe at ~290 Ma, and with granitoid intrusions along the Rheic suture during the main phase of the collision of the Rhenohercynian and Saxothuringian domains at ~340 Ma, respectively. For the Latest Devonian ages (366 ± 8.5 Ma) several scenarios are possible. Middle Proterozoic to Cambrian/Ordovician ages are regarded as reworking of pre-Variscan sedimentary and magmatic material. The new ages and the garnetbearing xenolith, the metamorphic grade of which does not fit with the greenschist facies rocks of the Northern Phyllite Zone, suggest that rocks of both the Northern Phyllite Zone and the Mid-German Crystalline Zone are present underneath the volcanic rocks of the VVF. The U-Pb zircon ages of trachytic xenoliths of the Nickel quarry suggest the emplacement of a trachytic lava dome at 66.7 ± 0.4 Ma. Similar ages have been obtained from camptonitic dykes and trachyte intrusions within Permian and Triassic sediments exposed at the southwestern margin of the VVF. The large amount of trachyte together with a significant positive magnetic anomaly in the southwestern VVF might indicate a magma chamber that developed at shallow crustal levels below the southwestern part of the present Vogelsberg during the Upper Cretaceous. The new results have been used to develop a model for the early evolution of the southwestern Vogelsberg. Kurzfassung: Die vorliegende Arbeit befasst sich mit variszischen Grundgebirgsxenolithen, die sich in vulkanischen Gesteinen des tertiaren Vogelsberg-Vulkangebiets (VVG) befinden. Die untersuchten Proben stammen aus Steinbruchen am SW-Rand des Vogelsberges (Steinbruch Nickel, Nidda Ober-Widdersheim) sowie am NE-Rand des Vogelsberges (Steinbruch Rauher Berg, Alsfeld-Brauerschwend). Auserdem untersuchten wir verschiedene Arten von Trachyt, der (1) als xenolithische Einschlusse in einer alkalibasaltischen Tuffbrekzie des Steinbruchs Nickel, (2) als trachytische Lava an einem Aufschluss nahe des Hauserhofs (Nidda Ober-Widdersheim) und (3) als Teil eines trachytisch-phonolitischen Lavadoms in einer Bohrung nahe Schotten-Sichenhausen vorkommt.