Z. Vejnar’s research while affiliated with The Czech Academy of Sciences and other places

Clastic metasedimentary rocks from the Tepla Crystalline Complex (western Bohemian Massif) were analysed for major and trace elements, Sr and Nd isotopes. The metamorphic grade of these rocks of presumed Neoproterozoic protolith age increases from SE to NW from very low-grade to amphibolite-facies conditions. Geochemistry indicates that the sedimentary protoliths for the whole sequence consisted of immature (pelitic) greywackes chiefly derived from an ensialic island arc. No significant changes in composition from the lowest to the highest grade or across the strike of isograds were observed. Chemical variations between original slates and greywackes within a single locality often considerably exceed the variation among samples of different metamorphic grades or of different geographic positions. The prevailing REE spectra with distinct negative Eu anomalies show a close similarity with those of modern turbidites from ensialic island arcs. Several samples without any Eu anomaly resemble the REE patterns of less differentiated island arc andesites. LREE leaching under oxidizing conditions is suggested by several REE patterns with positive Ce anomalies. The Sm-Nd model ages T-DM of samples with Ce positive anomalies are higher (T-DM 1.8-2.0 Ga) than those of all other samples (T-DM = 1.1-1.5 Ga). Initial Sr isotopic ratios for all samples are fairly constant and compatible with an assumed dominance of isotopically less evolved detrital material. Geochemical characteristics of the clastic metasediments of the Tepla Crystalline Complex are thus consistent with a model of incorporation and preservation of arc-derived sediments in a Cadomian accretionary wedge.

The crystalline basement of the Cheb Tertiary Basin is comprised of muscovite granite of the Smrčiny Pluton and crystalline schists of the Saxothuringian Unit. With increasing depth (as seen from the 1190 m deep drill hole HV-18) this crystalline schist exhibits rapid metamorphic gradation, with the characteristic development of garnet, staurolite, and andalusite zones of subhorizontal arrangement. The dynamic MP-MT and static LP-MT crystallization phases were followed by local retrograde metamorphism. The moderately dipping to subhorizontal S 2 foliation, which predominates in the homogeneous segments, is followed by subvertical S 3 cleavage. The vertical succession of psammo-pelitic, carbonitic, and volcanogenic rock sequences, together with geochemical data from the metabasites, indi-cates a rock complex representing an extensional, passive continental margin setting, which probably originated in the Late Cambrian to Early Ordovi-cian. On the contrary, the geochemistry of the silicic igneous rocks and of the limestone non-carbonate components point to the compressional setting of a continental island arc. This disparity can be partly explained by the inheritance of geochemical characteristics from Late Proterozoic rocks in the source region.

The West Bohemian shear zone (WBSZ) forms a steep collapse structure along which east-side-down normal movements led to the juxtaposition of the relatively cold Cadomian basement of the Tepla-Barrandian unit against high grade Moldanubian rocks. Synkinematic plutons straddle the WBSZ. The Mut&#51nin pluton intruded into Moldanubian crust at a depth of 23dž km as derived by using Al-in-hornblende barometry. The Tepla-Barrandian Babylon pluton intruded at <12 km depth as indicated by phengite barometry and petrogenetic considerations. Both emplacement depths, together with mineral cooling ages, result in a minimum vertical displacement of 10 km between 340 and 320 Ma. This large throw could be explained by over-thickened crust that was weakened from below. The alkaline signature of the Mut&#51nin diorite indicates that mantle melting was important to thermally weaken the crust at 340 Ma. The cold Tepla-Barrandian upper crust sank into its weak, partly molten Moldanubian substratum, resulting in elevator-style movements, not only along the WBSZ, but also along the Hoher Bogen and Central Bohemian shear zone. All these ductile normal shear zones were active simultaneously during the Lower Carboniferous and dip steeply towards the Tepla-Barrandian unit that probably formed a highly elevated plateau at this time.

The Teplá – 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 Teplá – 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 Teplá – 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/À19 Ma (U – Pb upper intercept). Subduction-related volcanic rocks have been dated from 585F7 to 568F3 Ma (lower intercept, rhyolite boulders), which pre-dates the age of sedimentation of the Cadomian flysch (Š těchovice Group). Accretion, uplift and erosion of the volcanic arc is documented by the Neoproterozoic Dobříš conglomerate of the upper part of the flysch. The intrusion age of 541+7/À8 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 522F2 Ma (Těšovice granite), 524F3 Ma (Všepadly granodiorite), 523F3 Ma (Smržovice tonalite), 523F1 Ma (Smržovice gabbro) and 524F0.8 Ma (Orlovice gabbro) were emplaced into transtensive shear zones. D 2002 Elsevier Science B.V. All rights reserved.

The igneous complex of Neukirchen–Kdyně is located in the southwestern part of the Teplá–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 Všepadly granodiorite and the Smržovice diorite intruded at shallow crustal levels (<ca. 7 km depth) as was indicated by geobarometric data. K–Ar age data yield 547±7 and 549±7 for hornblende and 495±6 Ma for biotite of the Smržovice diorite, suggesting that this pluton has remained at shallow crustal levels (T<ca. 350 °C) since its Cambrian emplacement. A similar history is indicated for the Všepadly granodiorite and the Stod granite. In the SW, intermediate to mafic plutons of the Neukirchen–Kdyně massif (Všeruby and Neukirchen gabbro, Hoher–Bogen metagabbro), which yield Cambrian ages, either intruded or were metamorphosed at considerably deeper structural levels (>20 km). The Teufelsberg (Čertův kámen) 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.

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.

U–Pb zircon dating of three metagranitoids, situated within a tilted crustal section at the northwestern border of the Tepl Barrandian unit (Tepl crystalline complex, TCC), yields similar Cambrian ages. The U–Pb data of zircons of the Tepl orthogneiss define an upper intercept age of 513 +7/–6&#117Ma. The 207Pb/206Pb ages of 516ᆞ and 511&#4510&#117Ma of nearly concordant zircons of the Hanov orthogneiss and the Lestkov granite are interpreted to be close to the formation age of the granitoid protolith. Similar to the Cambrian granitoids of the southwestern part of the Tepl Barrandian unit (Doma&#151lice crystalline complex, DCC) the Middle Cambrian emplacement of the TCC granitoids postdates Cadomian deformation and metamorphism of the Upper Proterozoic country rocks, but predates Variscan tectonometamorphic imprints. Structural data as well as sedimentological criteria suggest a dextral transtensional setting during the Cambrian plutonism, related to the Early Paleozoic break-up of northern Gondwana. Due to strong Variscan crustal tilting, the degree of Variscan tectonometamorphic overprint is strikingly different in the dated granitoids. It is lowest in the weakly or undeformed Lestkov granite, located in the greenschist-facies domain. The Tepl orthogneiss in the north underwent pervasive top-to-NW mylonitic shearing under amphibolite-facies conditions. There is no indication for a resetting of the U–Pb isotopic system of the Tepl orthogneiss zircons that could be attributed to this imprint. Radiation damages accumulated until recent have probably caused lead loss.