ABSTRACT Early Ordovician to late Permian orogenies at different plate-boundary zones of western Pangea affected continental crust derived from the plates of North America (Laurentia), Europe (East European Craton including Baltica plus Arctida), and Gondwana. The diachronic orogenic processes comprised stages of intraoceanic subduction, formation and accretion of island arcs, and collision of several continents. Using established plate-tectonic models proposed for different regions and time spans, we provide for the first time a generic model that explains the tectonics of the entire Gondwana-Laurussia plate-boundary zone in a consistent way. We combined the plate kinematic model of the Pannotia-Pangea supercontinent cycle with geologic constraints from the different Paleozoic orogens. In terms of oceanic lithosphere, the Iapetus Ocean is subdivided into an older segment (I) and a younger (II) segment. Early Cambrian subduction of the Iapetus I and the Tornquist oceans at active plate boundaries of the East European Craton triggered the breakup of Pannotia, formation of Iapetus II, and the separation of Gondwana from Laurentia. Prolonged subduction of Iapetus I (ca. 530 –430 Ma) culminated in the Scandian collision of the Greenland-Scandinavian Caledonides of Laurussia. Due to plate-tectonic reorganization at ca. 500 Ma, seafloor spreading of Iapetus II ceased, and the Rheic Ocean opened. This complex opening scenario included the transformation of passive continental margins into active ones and culminated in the Ordovician Taconic and Famatinian accretionary orogenies at the peri-Laurentian margin and at the South American edge of Gondwana, respectively. Rifting along the Avalonian-Cadomian belt of peri-Gondwana resulted in the separation of West Avalonian arc terranes and the East Avalonian continent. The vast African/Arabian shelf was affected by intracontinental extension and remained on the passive peri-Gondwana margin of the Rheic Ocean. The final assembly of western Pangea was characterized by the prolonged and diachronous closure of the Rheic Ocean (ca. 400–270 Ma). Continental collision started within the Variscan-Acadian segment of the Gondwana-Laurussia plate-boundary zone. Subsequent zipper-style suturing affected the Gondwanan Mauritanides and the conjugate Laurentian margin from north to south. In the Appalachians, previously accreted island-arc terranes were affected by Alleghanian thrusting. The fold-and-thrust belts of southern Laurentia, i.e., the Ouachita-Marathon-Sonora orogenic system, evolved from the transformation of a vast continental shelf area into a collision zone. From a geodynamic point of view, an intrinsic feature of the model is that initial breakup of Pannotia, as well as the assembly of western Pangea, was facilitated by subduction and seafloor spreading at the leading and the trailing edges of the North American plate and Gondwana, respectively. Slab pull as the plate-driving force is sufficient to explain the entire Pannotia–western Pangea supercontinent cycle for the proposed scenario.

The Lausitz Group in the easternmost part of Saxo-Thuringia, Germany, forms the largest exposure of partly anchimetamorphic Cadomian basement in Germany. In common with adjoining units to the west, it was deposited in a convergent-margin basin of northern peri-Gondwana. Sedimentary features within the turbiditic graywacke successions suggest continuous accumulation during a deepening stage that followed basaltic-andesitic arc-volcanic activity. The turbidites are irregularly intercalated with tuffaceous graywacke, which was derived from reworked, but only slightly older, basic volcanic material. Petrological and geochemical data reveal a broadly uniform source area for the graywackes with a dissected magmatic arc signature containing exhumed pre-Cadomian basement. Graywackes of the Lausitz Group together with earlier Cadomian basin successions and parts of the pre-Cadomian basement became mobilized during Cadomian subduction-related anatexis. During basin closure, the graywackes were weakly folded with a northerly vergence. Subsequently, they were contact-metamorphic overprinted by granodioritic intrusions, which mark the end of the Cadomian orogeny and show both Cadomian and pre-Cadomian crustal signatures. Superimposed very low-grade S-C textures in gray-wackes of the eastern Lausitz area are due to Variscan processes.

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.

Ca. 500 Ma orthogneisses and bimodal suites are widespread along the northern part of the Bohemian Massif (central European Variscides) and are interpreted to document intense magmatism during a continental break-up episode along the northern periphery of Gondwana. Based on geological setting, and geochemical and isotopic evidence, these felsic igneous rocks record the generation of: (1) magmas of pure or predominantly crustal derivation, represented by minor extrusives and much more voluminous orthogneisses similar to S-type granitoids; (2) subordinate magmas of exclusively mantle origin (ranging from within-plate alkali trachytes to oceanic plagiogranites) corresponding to felsic derivatives of associated basalts; and (3) magmas of hybrid origin, produced either as a result of large degrees of contamination of mantle-derived magmas ascending through the crust, or alternatively, generated by partial melting of mixed sources, such as interlayered sediments and mafic rocks or graywackes containing a juvenile component. The high-temperature dehydration melting process responsible for the generation of the most abundant rock-types necessitated the advection of mantle heat, in a context of continental lithosphere extension, as documented by broadly coeval basaltic magmatism at the scale of the igneous province. The large volumes of felsic magmas generated during the 500-Ma anorogenic event are interpreted to result from the combination of a hot extensional tectonic regime with the widespread availability in the lower crust of fertile lithologies, such as metagraywackes. This in turn reflects the largely undifferentiated nature of the crustal segment accreted some 50–100 m.y. earlier during the Cadomian orogeny.

The Léon domain adjacent to the Cadomian realm in the North Armorican domain appears to be a displaced crustal block, as its metamorphism and rock types bear a resemblance to the South Armorican domain of the internal Variscan belt. The amphibolite-facies Conquet-Penze Micaschist unit overlies the high-grade Lesneven Gneiss unit in the central part of the Léon. Timing and conditions of the metamorphic evolution have been evaluated. At the base of the Lesneven Gneiss unit, a high-pressure eclogite-facies stage (700 °C at >13 kbar) was followed by a high-temperature event (800 °C at 8 kbar), which is characterized by the crystallization of garnet-cor-dierite assemblages in aluminous paragneisses. Maximal temperatures in the upper parts of the Lesneven Gneiss unit were 630 °C at 6 kbar. Zoned garnet in assemblages with staurolite recorded prograde P-T paths from 490–610 °C at 5–8 kbar in the upper and at 6–9 kbar in the lower parts of the Conquet-Penze Micaschist unit. Garnet Y, heavy rare earth elements, and Li are low in high-grade gneisses and display strong zonations in the micaschists. A younger population of monazite with a broad range of Y contents displays Th-U-Pb ages between 340 and 300 Ma. It crystallized subsequent to formation of foliations S 1 -S 2 and Variscan peak metamorphic assemblages. In contrast, an older population of Cadomian monazite at 552–517 Ma is uniformly rich in Y, suggesting an earlier crystallization than garnet, however, at elevated temperatures. The findings do not support a South Armorican provenance of the Léon domain. The Léon units appear as part of a Cadomian crust at the northern margin of the former Armorican microplate. During a Variscan collision, this crust was strongly overprinted by underthrusting toward the southeast or east beneath the Central Armorican domain and by later uplift accompanied by Late Carboniferous dextral shear tectonics. The features are typical of the Variscan Saxo-Thuringian zone, which faced the Rheic Ocean to the north.

The aim of this article is to present a compilation of available information on the Évora Massif based on structural mapping, whole-rock geochemistry, recognition of metamorphic mineral assemblages, and geothermobarometry. In our view, trans-current movements responsible for strong orogen-parallel stretching were dominant and had a major role in the geodynamic evolution of this part of Ossa-Morena zone (southwest Iberian Massif). Cadomian and Variscan orogenic events separated by a period of intense rifting were the cause for the composite distribution of zones with contrasting metamorphic paths, the structural complexity, the variety of lithological associations, and the sequence of deformation events and magmatism. The proposed geodynamic reconstruction for this segment of the northern Gondwana continental margin includes three main stages in chronological order: (1) Neoproterozoic accretion and continental magmatic arc developing, dismantling, and reworking, followed by late-“orogenic” magmatism; (2) Lower Paleozoic crustal thinning, block tilting, and mantle upwelling, induced by generalized rifting, leading to the formation of marine basins with carbonate platform sediments and thick accumulations of volcaniclastic and terrigenous sediments, contemporaneous with normal and enriched mid-oceanic ridge basalt–type magmatism; and (3) Upper Paleozoic transpressional orogenesis resulting from obliquity of convergence and the geometry of the involved blocks. The third stage includes the tectonic inversion of Lower Paleozoic basins, crustal thickening, the exhumation of high- to medium-pressure rocks and partial exhumation of high-grade metamorphic lithologies (controlled by local transtension and major detachments), the formation of synorogenic basins filled with volcanic-sedimentary sequences, and finally, the emplacement of late Variscan granodiorites and granites.