On the basis of immobile trace elements and Nd isotope signatures, the Barrandian meta-basalts may be ascribed to two major groups, extracted from contrasting mantle sources: A depleted group, with strong light rare earth element depletion, elevated Zr/Nb ratios (>30), and highly radiogenic Nd isotopes (ϵNd 600 from +7.8 to + 9.3). Multi-element patterns normalized to normal mid-ocean ridge basalt all show negative anomalies of Nb, and to a lesser degree, Zr and Ti. Eight samples may define a 605 ± 39-Ma whole-rock isochron with ϵNd i of +8.8 ± 0.2. An enriched group, comprising both mildly enriched (Zr/Nb 12–18) and strongly enriched (Zr/Nb 4–7) samples, with ϵNd 600 ranging from +8.2 to +3.8. The depleted group is interpreted to reflect generation from depleted mantle sources fluxed by subduction-related components, probably in an intraoceanic back-arc basin. In contrast, the younger enriched group is typical of the within-plate style of mantle enrichment and documents the extinction of the subduction-related component. The switch from suprasubduction zone to within-plate magmatism suggests that new mantle material flowed into the former arc and back-arc system sources. This flow might have occurred simply as a result of ocean-ward migration of the subduction zone. Alternatively, the subduction fluxing might have stopped as a result of impingement of a spreading ridge with the intraoceanic trench, leading to mutual annihilation, a switch to a transform plate boundary, and opening of a slab window that allowed the inflow of new mantle and the generation of late-stage, within-plate enriched basalts. In terms of modern analogues, the Neoproterozoic of the Barrandian and other Cadomian regions of western Europe resemble arc and back-arc systems from the western Pacific region, where large intraoceanic subduction systems fringe major continental masses with a complex mosaic of microplates and magmatic arcs, including intervening basins floored either by oceanic crust or attenuated continental crust.

Nd-Sr-Pb isotope data are used to characterize the sources of Late Neoproterozoic and Early Paleozoic siliciclastic rocks of the Teplá-Barrandian unit of the Bohemian Massif. Geochemical and isotopic signatures of samples from different stratigraphic levels reflect changing sources and weathering conditions through time and allow a correlation with shifting geotectonic regimes. Late Neoproterozoic rocks were deposited in a magmatic arc–related setting within the Avalonian-Cadomian belt at the periphery of West Gondwana. Fine-grained graywackes yield crustal residence ages (T DM ) of 2.17–1.49 Ga, documenting contributions of old crust. Their ϵNd 570 values, as well as Pb and Sr isotopic compositions, reflect mixing of detritus derived from old crust with a Neoproterozoic magmatic arc component. The change in the geo-tectonic regime to transtension/rifting occurred during the terminal Neoproterozoic and is documented by more radiogenic ϵNd T values (−6.0 to +1.0) and younger T DM (1.65–1.12 Ga) of the Cambrian sediments. Besides the involvement of a post-Neoproterozoic juvenile source, the Lower Cambrian basin was also fed from an old upper crustal domain, as indicated by their high 207 Pb/ 206 Pb values. In contrast, Middle Cambrian siliciclastic rocks are mainly derived from the Cadomian basement. In the Ordovician pelites, ϵNd T values of −9.6 to −8.3 and radiogenic Sr and Pb isotopic compositions reflect an increasing input of material derived from the cratonic hinterland. Their T DM values range from 2.02 to 1.88 Ga. The uniform geochemical and isotopic compositions of the Ordovician samples indicate efficient mixing of the detritus prior to deposition in a mature rift or shelf environment at the Gondwanan margin.

Abstract The lower Paleozoic succession of central Europe exposed in the Bohemian Massif is a classic area of geology with a long-standing tradition of research dating back to the eighteenth century. The Ordovician rocks form parts of sections in several units that sit on the Cadomian basement. These sedimentary and volcano-sedimentary fills of partial depressions in the basement are relics of the system of rift basins in the Gondwanan margin reflecting the rifting of the Rheic Ocean. The Ordovician sections are related to the subsidence period during the extensional regime accompanied by volcanism. They are underlain by Neoproterozoic or Cambrian rocks and continue up usually without breaks. After closure of the Rheic Ocean owing to the Gondwana–Laurussia collision, the Ordovician successions were incorporated into the Variscan Orogen belt and preserved in denudation relics such as the Bohemian Massif and its units. Ordovician strata with Gondwanan shelf affinities can be traced along the Variscans from Spain to central Europe, and are reflected in the regional stratigraphic scale based mainly on the succession in the Prague Basin. The Ordovician fill of this accumulation centre, together with relics of another preserved in the Schwarzburg Anticline, represents the main exposures in the Bohemian Massif. The individual features of the Ordovician successions, such as facies developments, fossil associations and volcanism, make them model areas both for understanding the palaeogeographic and geotectonic evolution of the peri-Gondwanan margin and a stratigraphic standard for a cool-water regime.

Abstract The Cadomian Orogeny comprises a series of complex sedimentary, magmatic and tectonometamorphic events that spanned the period from the mid-Neoproterozoic ( c . 750 Ma) to the earliest Cambrian ( c . 540-530 Ma) along the periphery of the super-continent Gondwana (peri-Gondwana, Fig. 3.1 ). Modern data demonstrate broad continuity between Cadomian events and the later opening of the Rheic Ocean during Cambrian-Ordovician times ( Linnemann et al. 2007 ). Due to very similar contemporaneous orogenic processes in the Avalonian microcontinent, the collective terms ‘Avalonian-Cadomian’ Orogeny and ‘Avalonian-Cadomian’ Active Margin have often been used in the modern literature (e.g. Nance & Murphy 1994 ; Fig. 3.1 ). Rock units formed during the Cadomian Orogeny are commonly referred to collectively as ‘Cadomian Basement’. Peri-Gondwanan terranes, microcontinents and crustal units in Central, Western, Southern and Eastern Europe, in the Appalachians (eastern USA and Atlantic Canada), and in North Africa were affected by the Cadomian Orogeny. This orogenic event is also apparently present in Baltica because of the 'Cadomian affinity' of late Precambrian orogenic events in the Urals and in the Timanides on the margin of Baltica ( Roberts & Siedlecka 2002 ). The Cadomian Orogeny sensu stricto was first defined in the North Armorican Massif in France on the basis of the unconformity that separates deformed Precambrian rock units from their Early Palaeozoic (Cambro-Ordovician) overstep sequence (see below). This unconformity is commonly referred to as the ‘Cadomian unconformity’ (Fig. 3.2 ). However, it cannot be precluded that the youngest metasedimentary rocks affected by

Abstract Around 88% of the history of the Earth occurred during the Precambrian period, which can be subdivided into the Archaean and the Proterozoic eons (Figs. 2.1 & 2.2 ). The Archaean eon (Greek archaia — ancient ones; 4.56-2.5 Ga) comprises the Eo-Palaeo-, Meso-and Neoarchaean eras. For the early Archaean the term Hadean is also used (Greek hades — unseen or hell; 4.56-3.8 Ga) (Fig. 2.1). The Proterozoic eon (Greek proteros — first, zoon — creature; 2.5-0.542 Ga) is composed of the Palaeo-, Meso-and Neoproterozoic eras (Fig. 2.2). The latter eras can be subdivided into different periods defined by the International Commission on Stratigraphy on the basis of geochronological data and characteristic features such as particular geotectonic settings and events ( Gradstein et al. 2004 ). Palaeoproterozoic periods include the Siderian (Greek sideros — iron; 2.5-2.3 Ga), the Rhyacian (Greek rhyax — steam of lava; 2.3-2.05 Ga), the Orosirian (Greek orosira — mountain range; 2.05-1.8 Ga) and the Statherian (Greek statheros — stable; 1.8-1.6 Ga). The Calymmian (Greek calymma — cover; 1.6-1.4 Ga), Ectasian (Greek ectasis — extension; 1.4-1.2 Ga), and Stenian (Greek stenos — narrow; 1.2-1.0 Ga) are the Mesoproterozoic periods, while the Neoproterozoic is subdivided into the Tonian (Greek tonas — stretch; 1.0-0.85 Ga), Cryogenian (Greek cryos — ice, genesis — birth; 0.85-0.635 Ga), and finally Ediacaran (0.635-0.542 Ma). This latter is named after the Ediacara Hills (Flinders Ranges, Australia) and characteristically contains the Ediacara biota which represents the dawn of evolved life-forms. The Ediacaran period