Abstract U–Pb ages of detrital ( n = 2391) and magmatic ( n = 170) zircon grains from the Harz Mountains were obtained by LA-ICP-MS for provenance studies and absolute age dating. Results point to a complete closure of the Rheic Ocean at c. 419 Ma. A narrow Rhenish Seaway then re-opened in Emsian to mid-Devonian time ( c. 390–400 Ma). Devonian sedimentary rocks of the Harz Mountains were deposited on the northwestern (Rheno-Hercynian) and on the southeastern (Saxo-Thuringian) margins of the Rhenish Seaway. A new U–Pb zircon age from a plagiogranite (329 ± 2 Ma) within a harzburgite makes the existence of oceanic lithosphere in the Rhenish Seaway probable. The Rhenish Seaway was completely closed by Serpukhovian time ( c. 328 Ma). Existence of a terrane in the seaway is not supported by the new data. Provenance studies and spatial arrangement allow reconstruction of the thin- to thick-skinned obduction style of the Harz Mountains onto the southeastern margin of East Avalonia (Rheno-Hercynian Zone) during the Variscan orogeny. Detrital zircon populations define Rheno-Hercynian and Saxo-Thuringian nappes. Intrusion of the granitoid plutons of the Harz Mountains occurred in a time window of c. 300 to 295 Myr and constrained the termination of Variscan deformation.

Abstract The recent discovery of Ediacaran ophiolites in the SW Iberian Massif has made it possible to pinpoint the evolution of the Cadomian basement of Europe. The Calzadilla and Mérida ophiolites (gabbroic protoliths dated at c. 600 and 594 Ma, respectively) have geochemical characteristics typical of supra-subduction zone ophiolites. They are interpreted as originating during the initial opening of a forearc basin with boninitic magmatism (Calzadilla), followed by the formation of a back-arc basin with arc-tholeiites (Mérida). Widening of the back-arc led to the rifting and drifting of a section of the active continental margin (Cadomia). Closure of these oceanic domains initiated rapid contraction, culminating in the collision of Cadomia with Gondwana ( c. 590–540 Ma). The application of a PANALESIS model to this palaeogeographic setting confirms the plausibility of Cadomian rifting and the likely opening of broad oceanic domains. It also confirms the final collision of Cadomia with Gondwana, although the synthetic and regional data disagree in the precise chronology of the convergence and collision of Cadomia with the West Africa Craton. This work shows that the evolution of the Cadomian basement is much more complex than traditionally considered.

Abstract The Early Cambrian palaeogeographical enigma arises when tectonic reconstructions are made using palaeoclimatic v. palaeomagnetic data that result in possibly contradictory tropical, mid-latitude, and south polar locations for major continents. For example, NW Africa and Cadomia may have lain in a tropical zone (0° to ±30° latitude) based on the presence of archaeocyath reefs, minor evaporites, and carbonate platforms at c. 520 Ma ± 5 Ma or, alternatively, NW Africa and Cadomia may have lain in a south polar zone (90° to 60° south latitude) based on palaeomagnetic constraints. Greater Avalonia may have evolved independently from NW Africa if a dropstone constraint implying polar latitudes at c. 530 Ma and a palaeomagnetic constraint implying c. 50° latitude at c. 505 Ma are accommodated. We show here how counterclockwise rotation of Gondwana during the Cambrian about an interior axis may solve the enigma. Gondwanan apparent polar wander becomes consistent with tropical conditions inferred for NW Africa when adjusted to accommodate constraints placing the south pole near Peru for c. 540–520 Ma. Concurrent counterclockwise rotation of Baltica and Gondwana during the Middle Cambrian may have facilitated separation of Greater Avalonia from Baltica across dextral shear zones.

Abstract The late Neoproterozoic–Cambrian interval is characterized by global-scale orogenesis, rapid continental growth and profound changes in Earth systems. Orogenic activity involved continental collisions spanning more than 100 myr, culminating in Gondwana amalgamation. Avalonia is an example of arc magmatism and accretionary tectonics as subduction zones re-located to Gondwana's periphery in the aftermath of those collisions, and its evolution provides significant constraints for global reconstructions. Comprising late Neoproterozoic ( c. 650–570 Ma) arc-related magmatic and metasedimentary rocks, Avalonia is defined as a composite terrane by its latest Ediacaran–Ordovician overstep sequence: a distinctive, siliciclastic-dominated cover bearing ‘Acado-Baltic’ fauna. This definition implies that Neoproterozoic Avalonia may consist of several terranes, and so precise palaeomagnetic or provenance determination in one locality need not apply to all. On the basis of detrital zircon and Nd isotopic data, Avalonia and other lithotectonically related terranes, such as Cadomia, have long been thought to have resided along the Amazonian–West African margin of Gondwana between c. 650 and 500 Ma – Avalonia connected to Amazonia, and Cadomia to West Africa. These interpretations have constrained Paleozoic reconstructions, many of which imply that the departure of several peri-Gondwanan terranes led to the Early Paleozoic development of the Rheic Ocean whose subsequent demise in the Late Paleozoic led to Pangaea's amalgamation. Since these ideas were proposed, several new lines of evidence have challenged the Amazonian affinity of Avalonia. First, there is evidence that some Avalonian terranes may have been ‘peri-Baltican’ during the Neoproterozoic. Baltica was originally excluded as a potential source for Avalonia because, unlike Amazonia, arc-related Neoproterozoic rocks were not documented. However, subsequent recognition of Ediacaran arc-related sequences in the Timanides of northeastern Baltica invalidates this assumption. Second, detailed palaeontological and lithostratigraphic studies have been interpreted to reflect an insular Avalonia, well removed from either Gondwana or Baltica during the Ediacaran and early Cambrian. Third, recent palaeomagnetic data have raised the possibility of an ocean (Clymene Ocean) between Amazonia and West Africa in the late Neoproterozoic, thereby challenging conventional reconstructions that show the ‘peri-Gondwanan’ terranes as a contiguous belt straddling the suture zone between these cratons. In this contribution, we critically re-evaluate the provenance of the so-called ‘peri-Gondwanan’ terranes, the contiguity of the so-called ‘Avalonian–Cadomian’ belt and the validity of the various plate tectonic models based on the traditional interpretation of these terranes. In addition, we draw attention to critical uncertainties and the challenges that lie ahead.

In the British Isles, 11 major Caledonian (pre-Variscan) terranes can be identified. The northern (Laurentian) miogeocline is composed of two terranes that were probably originally basement to the Cambro-Ordovician passive margin. These were deformed, uplifted, and amalgamated by across-strike shortening in the Grampian Orogeny (ca. 500 Ma) and later between 460 and 410 Ma. The southern miogeocline (Midland platform) is composed of Gondwanaland continental crust and shelf sediments, and appears to contain at least two terranes that were amalgamated prior to the Early Cambrian. Between these two miogeoclines lie a number of terranes that were finally docked in the site of the closed Iapetus by middle Devonian time. Docking deformation was apparently by sinistral transpressional shear, and it occurred at about 400 Ma. This may be equated with the Acadian orogeny of North America. Analysis of these terranes indicates that they are fragments of: (a) the northern miogeocline; (b) the Laurentian continental foreland; (c) arcs, back arcs, and fore arcs of Lower and Middle Ordovician age; and (d) a Silurian successor basin. They appear to indicate that initial closure and cessation of oceanic crustal subduction occurred in Iapetus by Late Ordovician time. The end-Silurian to pre–Middle Devonian sinistral strike-slip may be interpreted as either the oblique convergence of Gondwanaland/Cadomia into the angle formed by the British and mid-European Caledonide trends or as the clockwise rotation and convergence of Gondwanaland/Cadomia against this Laurasian landmass. The latter model may mean that a clockwise-rotating Gondwanaland drove the Caledonian orogeny in the three arms of Iapetus from at least Middle Ordovician times onward. The subsequent counterclockwise rotation of Gondwanaland in the Variscan implies that the Caledonian and Variscan/Alleghanian orogenies are the results of fundamentally distinct plate-tectonic events.

Abstract The Paleozoic Variscan orogen in Europe has a markedly circuitous trace for which several different origins have been postulated, including deformation around promontories on the colliding continental margins, extrusion within the collision zone, folding of a ribbon continent and collision with a substantial dextral component. Adopting the latter assumption, unfolding of the large, steeply plunging folds (‘oroclines’) of Iberia and the Moroccan Meseta requires more than 4000 km of dextral lateral translation of Laurussia with respect to Gondwana. Constraints on the age of the folding require that this lateral translation occurred in mid-late Carboniferous time. Significant dextral translation of Laurussia with respect to Gondwana late in the Variscan collision is supported by palaeomagnetic data for the two supercontinents, although the exact timing of this relative motion is not well constrained due to large uncertainties in the palaeomagnetic pole positions. Our palinspastic reconstruction of the major Variscan folds of Iberia places the rocks of the South Portuguese Zone of western Iberia adjacent to the Rhenish Massif; for instance, as part of Avalonia. By the same token, the Sehoul Block in the Moroccan Meseta probably originated at the western end of Cadomia, although nothing in our analysis precludes it also being derived from Avalonia.

Abstract Saxo-Thuringia is classified as a tectonostratigraphic terrane belonging to the Armorican Terrane Collage (Cadomia). As a former part of the Avalonian–Cadomian Orogenic Belt, it became (after Cadomian orogenic events, rift-related Cambro-Ordovician geodynamic processes and a northward drift within Late Ordovician to Early Silurian times), during Late Devonian to Early Carboniferous continent–continent collision, a part of the Central European Variscides. By making use of single zircon geochronology, geochemistry and basin analysis, geological processes were reconstructed from latest Neoproterozoic to Ordovician time: (1) 660–540 Ma: subduction, back-arc sedimentation and tectonomagmatic activity in a Cadomian continental island-arc setting marginal to Gondwana; (2) 540 Ma: obduction and deformation of the island arc and marginal basins; (3) 540–530 Ma: widespread plutonism related to the obduction-related Cadomian heating event and crustal extension; (4) 530–500 Ma: transform margin regime connected with strike-slip generated formation of Early to Mid-Cambrian pull-apart basins; (5) 500–490 Ma: Late Cambrian uplift and formation of a chemical weathering crust; (6) 490–470 Ma: Ordovician rift setting with related sedimentation regime and intense igneous activity; (7) 440–435 Ma: division from Gondwana and start of northward drift. The West African and the Amazonian Cratons of Gondwana, as well as parts of Brittany, were singled out by a study of inherited and detrital zircons as potential source areas in the hinterland of Saxo-Thuringia.