Jiří Fiala

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.

Evidence from zircon dating for existence of approximately 2.1 Ga old crystalline basement in southern Bohemia, Czech Republic

Zircon ages are reported for three Moldanubian amphibolite grade orthogneisses from the southern Bohemian Massif obtained by conventional U/Pb analyses. For two of these orthogneisses, conventional U/Pb data are supported by ion microprobe single zircon ages or single grain evaporation data. The amphibolite grade orthogneisses, occurring in three small tectonic lenses within the Varied Group close to the South Bohemian Main Thrust, are of tonalitic, granodioritic or quartz dioritic composition.Conventional bulk size fraction and ion microprobe analyses of nearly euhedral zircons from a metatonalite, erroneously interpreted as a metagreywacke in a previous study, yielded an upper Concordia intercept age of 2048 ± 12 Ma. The well preserved euhedral grain shapes of the zircons suggest crystallization from a magmatic phase, and the upper Concordia intercept age is now interpreted as reflecting a magmatic event at that time. The age of this rock is compatible with the conventional zircon data and the (207Pb/206Pb)* single grain evaporation result from two further orthogneisses providing intrusion ages of 2 060 ± 12, 2 104 ± 1 and 2 061 ± 6 Ma, respectively. For one sample a concordant U/Pb age for sphene of 355 ± 2 Ma defines the age of amphibolite facies metamorphism. The upper Concordia intercept ages of three orthogneisses constitute the first direct evidence for the presence of early Proterozoic crust under the supracrustal cover in the southern part of the Bohemian Massif.

Zircon dating of North Bohemian granulites, Czech Republic: further evidence for the Lower Carboniferous high-pressure event in the Bohemian Massif

U-Pb zircon and rutile multigrain ages and 207Pb/206Pb zircon evaporation ages are reported from high-pressure felsic and metapelitic granulites from northern Bohemia, Czech Republic. The granulites, in contrast to those from other occurrences in the Bohemian Massif, do not show evidence of successive HT/MPLP overprints. Multigrain size fractions of nearly spherical, multifaceted, metamorphic zircons from three samples are slightly discordant and yield a U-Pb Concordia intercept age of 348 ± 10 Ma, whereas single zircon evaporation of two samples resulted in 207Pb/206Pb ages of 339 ± 1.5 and 339 ± 1.4 Ma, respectively. A rutile fraction from one sample has a U-Pb Concordia intercept age of 346 ± 14 Ma. All ages are identical, within error, and a mean age of 342 ± 5 Ma was adopted to reflect the peak of HP metamorphism. Because rutile has a lower closing temperature for the U-Pb isotopic system than zircon, the results and the P-T data imply rapid uplift and cooling after peak metamorphism. The above age is identical to ages for high-grade metamorphism reported from the southern Bohemian Massif and the Granulite Massif in Saxony. It can be speculated that all these granulites were part of the same lower crustal unit in early Carboniferous, being separated later due to crustal stacking and subsequent late Variscan orogenic collapse.

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.