A variety of xenoliths from the lower crust to mantle transition occur in Quaternary mafic intraplate lavas of the Bayuda volcanic field of northern Sudan. The lower-crust xenoliths include plagioclase- and garnet-bearing mafic granulite. Ultra-mafic garnet-bearing pyroxenite, websterite, hornblendite, and distinct peridotite xenoliths are from the upper lithospheric mantle. Sr, Nd, and Pb isotope signatures distinguish between ultramafic and granulite xenoliths. The latter show a strong compositional affinity to juvenile Neoproterozoic crust. The Pb isotope composition of the ultramafic xenoliths resembles the distinct high-μ signature ( 206 Pb/ 204 Pb >19.5) of their host basanite. These xenoliths may represent cumulates of late Mesozoic to Quaternary mafic intraplate magmatism. The felsic upper crust in a schematic lithospheric profile of the Bayuda area includes predominantly granitoids, migmatites, and metasedimentary rocks that represent reworked old cratonic or juvenile Neoproterozoic rocks. The deep lower crust is represented by mafic granulite, likely cumulate rocks from Neoproterozoic juvenile magmatism. The crust-mantle transition is characterized by ultramafic cumulate rocks possibly from the late Mesozoic to Quaternary magmatism. The peridotites of the same xenolith suites represent typical lithospheric mantle with variable degrees of depletion by melt extraction.

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

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

The Saxo-Thuringian zone of the European Variscides contains the record of the Cadomian and Variscan orogenies and a Paleozoic marine transition stage. The classical view of a relatively simple, double-vergent folded sedimentary basin at the end of the Early Carboniferous is challenged by the widespread occurrence of Late Devonian to Early Carboniferous high-pressure metamorphic units tectonically juxtaposed with low-grade Paleozoic successions. Here we demonstrate that the subdivision of the Saxo-Thuringian zone in three principal units (autochthonous domain, wrench and thrust zone, and allochthonous domain) and their heterogeneous overprint by two regional deformation events during the Variscan orogeny explain the entire geological record. Late Devonian to Early Carboniferous subduction of continental crust inside the allochthonous domain affected a Cadomian basement and sediments deposited on the same continental shelf as the one preserved in the autochthonous domain. Strain partitioning during this regional D1 process led to the formation and evolution of a wrench and thrust zone surrounding the autochthonous domain. The latter was only affected by regional D2 deformation, which was related to regional dextral transpression, rapid exhumation of the subducted rocks of the allochthonous domain, and final filling and subsequent folding of the Saxo-Thuringian flysch basin that covers the autochthonous domain and the wrench and thrust zone. The Saxo-Thuringian zone is interpreted as a fragment of Peri-Gondwana that never separated from Gondwana to move as an independent terrane and that borders to the Old Red continent, represented by the Rheno-Hercynian zone, along a strike-slip dominated segment of the Rheic suture. The juxtaposition of the Saxo-Thuringian zone with the adjacent areas is discussed as a continuous subduction and/or accretion process representative for the entire Variscan orogen.

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 Drill core samples of garnet-clinopyroxene granulite at Tirschheim and a reference sample at Waldheim (Saxon Granulite Massif, Germany) endured the same P-T conditions, but developed variable mineral assemblages due to differences in bulk chemistry, reaction progress, deformation and retrogression. Titanite formed during peak-metamorphic conditions of 22–24 kbar and 1020–1050 °C. Dating titanite from the various samples should yield the same age for all. The observed age variation, which exceeds the duration of the entire metamorphic cycle, originates from the contrasting preservation of isotopic inheritance during peak metamorphism and from post-peak re-equilibration. (1) Pb inheritance observed in some peak-metamorphic titanite demonstrates that geochronologically relevant elements are redistributed among remaining reactants and reaction products during prograde metamorphism and that the sequence of metamorphic reactions does not result in isotopic homogenization. Instead, metamorphic minerals inherit the radiogenic signatures of the precursor minerals and may in extreme cases approach the age of the precursor mineral. (2) Titanite that formed at peak-metamorphic conditions is characterized by high A1 contents and X F ≈ 0.8−1. Texturally comparable titanite that re-equilibrated during cooling (reduced Al contents and X F ) yields too young U-Pb ages. The age of such re-equilibrated titanite does not correspond to the age of the event indicated by the texture.

Abstract A doubly vergent orogenic wedge system within the Central European Variscides developed during Carboniferous collision of two continental fragments, the northwestern edge of the Saxo-Thuringian upper plate and the Rheno-Hercynian passive margin in the lower plate. The resulting thrust system in the upper plate above the SE-dipping subduction zone retains the memory of the mode of deformation partitioning and material flow pattern in its internal architecture, its kinematic, metamorphic and geochronological record, and its reflection seismic image. New data indicate a stepwise SE-ward progradation of the NW Saxo-Thuringian fold belt with two stages of shortening between about 340 and 335 and between 320 and 310 Ma above a NW-dipping basal detachment. The NW Saxo-Thuringian fold belt is reinterpreted as a retro-wedge that was kinematically coupled to the Rheno-Hercynian pro-wedge and subduction system. The two steps in retro-wedge growth are linked to (a) the onset of collision with the Rheno-Hercynian margin causing upper-plate uplift and (b) a widespread late-orogenic stage of wedge thickening. The retro-wedge accumulated mostly diffuse shortening of > 100 km versus the shortening by imbrication of 180–200 km in the Rheno-Hercynian lower plate. Material advection and orogenic architecture were strongly affected by asymmetric erosional removal towards the lower-plate foreland and by transient mechanical properties of the wedge system.