Evidence for pre-orogenic,Early Devonian rifting in the Variscan belt: stratigraphy and structure of the Palaeozoic cover of the Mauges Unit (Upper Allochthon,Armorican massif,France)

The Palaeozoic sediments from the Mauges Unit (Armorican Massif, France) are the best-preserved pre-orogenic sequences belonging to the Upper Allochthon. Two coherent sequences are identified. The southern unit (Chateaupanne Unit) represents the cover of the Proterozoic basement and consists of Ordovician sediments unconformably overlain by Emsian carbonates followed by Emsian to earliest Eifelian immature sandstones. The northern unit (Tombeau Leclerc Unit) consists of an Hirnantian to Emsian condensed sequence, in reverse position, that has been thrust over the southern unit. The Devonian unconformity is interpreted as evidence for an Early Devonian extension, recorded by normal faults affecting both the Early Devonian limestones and the underlying Ordovician series. This crustal extension, recorded here for the first time, is possibly related to the opening of a back-arc basin (Saint-Georges-sur-Loire) associated with the subduction of an ocean located further south (Galicia-Brittany-Massif Central Ocean).     

The Saint-Georges-sur-Loire olistostrome,a key zone to understand the Gondwana-Armorica boundary in the Variscan belt (Southern Brittany,France)

In the southern part of the French Armorican massif, the Ligerian domain is located along the boundary between Gondwana and Armorica. Lithological, geochemical and structural data on the Saint-Georges-sur-Loire Unit, which is the northern part of the Ligerian domain, allow us to distinguish two sub-units. A southern sub-unit, formed by various blocks (chert, limestone, sandstone, rhyolite, mafic rocks) of Silurian to Middle Devonian age included as olistoliths in a Middle-Late Devonian terrigeneous matrix, overthrusts a sandstone-pelite northern sub-unit. Both units experienced two deformation events. The first one is a top-to-the-NW thrusting and the second one is a left-lateral wrenching. The Saint-Georges-sur-Loire Unit is an accretionary prism formed during the Late Devonian closure of the Layon rift, coeval with the main phase of the Variscan orogeny. The Layon rift, which according to the mafic olistoliths was partly floored by oceanic crust, appears as a buffer structural zone that accounts for the lack in Central Brittany of any tectonic or sedimentary echo of the closure of the Medio-European Ocean. The tectonic evolution of the Saint-Georges-sur-Loire Unit supports a polyorogenic model for this part of the Variscan Belt.     

Late Devonian-Early Carboniferous peak sulphide mineralization in the Western Hercynides

The Late Devonian-Early Carboniferous (Dinantian) within the Western Hercynides is marked by the formation of volcanic-hosted massive sulphide deposits: Chessy and Chizeuil in the Brévenne and Somme successions of the French Massif Central; Bodennec and La Porte-aux-Moines in the Châteaulin Basin of the French Armorican Massif; Rio Tinto, Neves-Corvo, Tharsis, etc., in the Volcano-Sedimentary formation of the Iberian Pyrite Belt; and Ketara, Draa Sfar and Hajar in the Jebilet-Guemassa district of the Moroccan Southern Meseta. Although these deposits show a slightly diachronous emplacement in response to a progressive migration of the metalliferous event from Late Devonian in France to Dinantian in Morocco, it is nevertheless possible to define an overall metalliferous ‘‘peak” around 350 Ma. The mineralization of the Armorican, Iberian and Moroccan sectors took place in epicontinental domains of the outer zone of the Hercynian belt, whereas that of the northeastern Massif Central occurred within the inner zone of the belt. This difference is registered by variations both in the geochemical characteristics of the ores (Sn in the outer zone and Mo-Ni in the inner zone) and in their lead isotopic signatures (clear mantle participation exclusively in the inner zone). In many cases the ores appear to be closely related to the felsic member of a bimodal magmatic association, although the massive sulphide deposits in the outer zone are more commonly associated with sedimentary rocks whereas those in the inner zone are hosted by felsic volcanic rocks. Another feature that should be noted is that the host sequences of the massive sulphide deposits commonly seem to be underlain by chaotic formations (notably with olistoliths) reflecting the beginning of Hercynian orogenic activity in the outer zone. It can be concluded that the peak mineralization took place within tensional domains developed during a period of plate convergence, and that it occurred around 350 Ma after a major period of Devonian compression but before the Carboniferous continental closure.     

Comment on D?rr and Zulauf: Elevator tectonics and orogenic collapse of a Tibetan-style plateau in the European Variscides: the role of the Bohemian shear zone. Int J Earth Sci (Geol Rundsch) (2010) 99: 299–325

The present comment disproves the tectonic model of a late Devonian/early Carboniferous Tibetan-style collisional plateau in the Teplá-Barrandean (TB) part of the Bohemian Massif, which later collapsed by thermal weakening of the underlying crust. Contrary to this model, the TB neither reveals major crustal thickening nor uplift and erosion, and eastern continuations of the TB were, during the relevant time-span, areas of open marine sedimentation. Late Devonian/early Carboniferous marine sediments widespread also in the Armorican and Central Massifs of France testify to low topography in central parts of the Variscan orogen. Notional traces of a Permo-Carboniferous ice cap on the French Massif Central do not support the plateau model, because they are questionable and much younger than the inferred plateau stage of the TB. The relative uplift of high-grade metamorphic rocks to the NW and the SE of the TB is not due to sinking of an elevated TB, but, instead, to the hydraulic and buoyant expulsion of HP material from the Saxo-Thuringian and Moldanubian subduction channels. The rise of lower-grade HT rocks along the southwestern margin of the Bohemian Massif was effected by late Carboniferous transpression. The high temperature and the resulting low viscosity of the rising materials were probably not caused by Variscan mantle delamination, but relate to lithospheric thinning and heating at the tip of the westward propagating Tethys Rift.     

The northwest-directed “Bretonian phase” in the French Variscan Belt (Massif Central and Massif Armoricain): A consequence of the Early Carboniferous Gondwana–Laurussia collision

In the Variscan French Massif Central and Armorican Massif, the tectonic significance of a widespread NW–SE-trending stretching lineation, coeval with medium pressure–medium temperature metamorphism, is an open question. Based on a structural analysis in the southern part of the Massif Central, we show that this top-to-the-NW shearing is a deformation event, referred to as D2, which followed a D1 top-to-the-south shearing Devonian phase, and was itself re-deformed by a Late D3 Visean–Serpukhovian southward-thrusting event. We date the D2 phase at 360 Ma (Famennian–Tournaisian boundary). In the Armorican Massif, D2 is the “Bretonian phase” recorded in the metamorphic series and sedimentary basins. Geodynamically, D2 is related to a general northwestward shearing during the Laurussia–Gondwana collision, which occurred after the closure of the Rheic Ocean, as indicated by the emplacement of the Lizard ophiolitic nappe in Britain. The left-lateral Nort-sur-Erdre fault accommodated the absence of ductile shearing in Central Armorica.     

Paleozoic evolution of the External Crystalline Massifs of the Western Alps

The External Crystalline Massifs (ECMs) of the Alps record, during the Paleozoic, the progressive closure of oceanic domains between Gondwana, Armorica and Avalonia in three contrasting tectonic domains. The eastern one shows the Early Devonian closure of the Central-European Ocean between Armorica and Gondwana along a northwest dipping subduction zone. The western domain is marked by Lower Ordovician rifting followed by Mid-Devonian obduction of the back-arc Chamrousse ophiolite. The central domain underwent Late Devonian to Dinantian extension in a back arc setting associated with southeast dipping subduction of the Saxo-Thuringian Ocean. Based on tectonostratigraphic correlations, we propose that the western domain shows an affinity to the Barrandian domain while the eastern and central domains correspond to the north-eastward extension of the Moldanubian zone, to the south of the present-day Bohemian Massif. From Mid-Carboniferous to Permian, the eastern and central domains of the ECMs, including the internal parts of the Maures Massif, Sardinia and Corsica were stretched towards the south-west along the ca. 1500 km long dextral ECMs shear zone preceding the opening of the Palaeo-Tethys ocean.     

Fault rocks as indicators of progressive shear deformation in the Guingamp region, Brittany

The North Armorican Shear Zone is a major structural feature running from the island of Moiene in the west to Moncontour in the east of the Armorican Massif. In the region of Guingamp it cuts through a Precambrian migmatite complex and granitoid rocks of both Precambrian and Hercynian age. A variety of fault rocks are present in this part of the shear zone, and are thought to represent a time sequence in which deep level, ductile deformation gave way to higher level brittle displacements. Mylonite and cataclasite series rocks, and pseudotachylites are described and their conditions of formation considered. The Hercynian Quintin granite post-dates the main movement of the shear zone but is itself dextrally displaced during the late stages of shear movement.     

Imagerie sismique de la zone de collision hercynienne dans le Sud-Est du Massif armoricain (projet Armor 2/programme Géofrance 3D)

The structure of the Hercynian collision zone in the southeast of the Armorican Massif is illustrated by a 70-km long deep seismic profile acquired in September 2000. The profile images a previously unknown south-dipping thrust that brought the Champtoceaux Domain on top of the Central Armorican Domain during Carboniferous times. Dextral strike-slip motions along the South Armorican Shear Zone, which is downward cut by the thrust zone, are partly coeval with northward thrusting. A major discontinuity, hidden by the thrust front, is also imaged in the lower crust between the Champtoceaux area and the Central Armorican Domain. These new data lead to a structural and kinematic re-interpretation of this part of the Hercynian collision zone. To cite this article: A. Bitri et al., C. R. Geoscience 335 (2003).     

Geology and geodynamic evolution of the high-P/low-T Maksyutov Complex, southern Urals, Russia

The high-pressure/low-temperature Maksyutov Complex is situated in the southern Urals between the Silurian/Devonian Magnitogorsk island arc and the East European Platform. The elongated N-S-trending complex is made up of two contrasting tectono-metamorphic units. Unit 1 consists of a thick pile of Proterozoic clastic sediments suggested to represent the passive margin of the East European Platform. The overlying unit 2, composed of Paleozoic sediments, volcanic rocks, and a serpentinite mélange with rodingites, is interpreted as a remnant of the Uralian Paleo-ocean. Devonian eastward subduction of oceanic crust beneath the Magnitogorsk island arc resulted in an incipient blueschist-facies metamorphism of unit 2 indicated by lawsonite pseudomorphs in the rodingites. While unit 2 was accreted to the upper plate, subduction of the continental passive margin caused the high-pressure metamorphism of unit 1. Buoyancy-driven exhumation of unit 1 into the forearc region led to its juxtaposition with unit 2 along a retrograde top-to-the-ENE shear zone. Further exhumation of the Maksyutov Complex into its present tectonic position was accomplished by later shear zones that were active as normal faults and are exposed along the margins of the complex. At the western margin a top-to-the-west shear zone juxtaposed a low-grade remnant of a Paleozoic accretionary prism (Suvanyak Complex) above the Maksyutov Complex. Along the eastern margin a top-to-the-east shear zone and the brittle Main Uralian Normal Fault emplaced the Maksyutov Complex against the Magnitogorsk island arc in the hanging wall.     

Ductile Thrusting Recorded by the Garnet Isograd from Blueschist-Facies Metapelites of the Ile de Groix, Armorican Massif, France

Mineral assemblages in the blueschist-facies metapelites fromthe Ile de Groix (Armorican Massif, France) permit the distinctionof two main units. The Upper Unit is characterized by: (1) highmodal proportions of garnet; (2) larger grain size; (3) therarity of graphite-bearing layers; (4) a single, although composite,foliation S₁. A Lower Unit is defined by: (1) low modal proportionsof garnet; (2) smaller grain size; (3) an abundance of graphite-bearinglayers; (4) a pervasive crenulation cleavage S₂. In the UpperUnit, coexisting garnet and chloritoid are more magnesian andless manganiferous than in the Lower Unit. The differences inmodal proportions and chemistry of coexisting minerals reflectdifferent P–T conditions. The P–T history of theblueschist-facies metapelites is estimated using a simplifiedpetrogenetic grid in the NFMASH system and thermodynamic calculations,which suggest peak P–T conditions at about P = 16–18kbar, T = 450–500°C and P = 14–16 kbar, T =400–450°C in the Upper and Lower Units, respectively.Peak P–T conditions were followed by a nearly isothermaldecompression for both units at slightly different temperatures(of the order of 50°C). The contact between the two units,i.e. the garnet isograd, is interpreted as a greenschist-faciesductile thrust. Thrusting of the higher-grade unit, i.e. theUpper Unit, over the Lower Unit occurred after the high-pressureevent, i.e. during the exhumation of both units. The observedsuperposition of higher-grade rocks over lower-grade rocks arguesagainst models where the exhumation history is entirely controlledby crustal-scale vertical shortening (i.e. extension). KEY WORDS: Armorican Massif; blueschist facies; Ile de Groix; metapelites; P–T path; garnet isograd     

The Southern part of the Armorican orogeny: A result of crustal shortening related to reactivation of a pre-Hercynian mafic belt during Carboniferous time

This synthesis of detailed geological and geophysical information relates deep crustal structure to deformation and the emplacement of granites in the South Armorican Massif and the adjacent continental shelf. Near the continental margin a zone of geophysical anomalies is regarded as a nearly complete line of suture between a southern continent and America. A major mafic body in the trace of the supposed suture in interpreted as a relic of probable oceanic crust which has apparently acted as a deflector of regional strain.It is proposed that late in the Hercynian collision history, during late Carboniferous time, compressional strain acted across the mafic body and can be directly related to generation of younger regional structures in the metasediments and to localization of granite emplacement.     

Lamproites as indicators of accretion and/or shallow subduction in the assembly of south‐western Anatolia,Turkey

The geochemistry and mineralogy of lamproites from south‐western Anatolia can be used as a snapshot of the lithospheric composition beneath the Menderes Massif. High and near‐constant K₂O contents, the presence of mantle xenocrystic phlogopite and olivine, highly magnesian olivine phenocrysts and Cr‐rich spinel inclusions all indicate that the lithospheric mantle was phlogopite‐bearing ultradepleted harzburgite at the time of lamproite eruption (20–4 Ma). This mantle assemblage most probably originated in a complex multistage process, including (intra‐oceanic) supra‐subduction zone depletion during the final stages of southern Neotethyan ocean closure, and accretion of the forearc oceanic lithosphere as shallowly subducted material to the already assembled Anatolia. The data presented here support shallow subduction of the oceanic lithosphere as a cause of the uplift of the Menderes Massif, in contrast to the traditional core‐complex model. Terra Nova, 00, 000–000, 2010     

Single subduction zone for the generation of Devonian ophiolites and high‐P metamorphic belts of the Variscan Orogen (NW Iberia)

Within the Variscan Orogen, Early Devonian and Late Devonian high‐P belts separated by mid‐Devonian ophiolites can be interpreted as having formed in a single subduction zone. Early Devonian convergence nucleated a Laurussia‐dipping subduction zone from an inherited lithospheric neck (peri‐Gondwanan Cambrian back‐arc). Slab‐retreat induced upper plate extension, mantle incursion and lower plate thermal softening, favouring slab‐detachment within the lower plate and diapiric exhumation of deep‐seated rocks through the overlying mantle up to relaminate the upper plate. Upper plate extension produced mid‐Devonian suprasubduction ocean floor spreading (Devonian ophiolites), while further convergence resulted in plate coupling and intraoceanic ophiolite imbrication. Accretion of the remaining Cambrian ocean heralded Late Devonian subduction of inner sections of Gondwana across the same subduction zone and the underthrusting of mainland Gondwana (culmination of NW Iberian allochthonous pile). Oblique convergence favoured lateral plate sliding, and explained the different lateral positions along Gondwana of terranes separated by Palaeozoic ophiolites.     

Indosinian tectonics in Vietnam

In Vietnam, the Triassic Indosinian collision affected coevally the Truong Son belt and the Kontum Massif,which were not independent tectonic units, but parts of the same Gondwana-derived Indochina continental block. This thermotectonic event took place synchronously throughout Vietnam, during the Lower Triassic 250–240-Ma time interval, as demonstrated by numerous geochronological data, combining Ar–Ar and U–Pb dating methods. Structural and kinematic investigations, in the Indosinian metamorphic rocks, reveal that the collisional process resulted from a consistent northwest-striking convergence of Indochina with respect to the adjacent blocks. It is suggested that this motion was taken up by a pair of opposite subduction zones: to the north, beneath South China, and to the west, beneath western Indochina, along the Song Ma and Po Ko sutures, respectively. Tectonic markers, calc-alkaline subduction-related volcanism and granitic intrusions and the generation of high-pressure rocks that have been recently discovered support this geodynamic setting, at least along Po Ko. Along the northwest-trending Song Ma zone, the obliquity of the convergence during subduction and subsequent collision resulted in the development, within the Truong Son Belt, of a set of subparallel dextral mylonitic shear zones, under amphibolite-facies metamorphism. The intermediate segments remained weakly metamorphic or even almost devoid of metamorphism. Along Po Ko, the convergence was near-orthogonal, with a left-lateral strike-slip component; the ongoing continental subduction resulted in the reworking of the Kontum granulitic basement and the development of Indosinian HP granulitic conditions; the subsequent extension-related exhumation operated approximately in the same northwestwards direction. This Indosinian evolution, applied on a continental crust that had been probably affected, as in South China, by a Caledonian-related event, as judged by the general unconformity of the Lower Devonian sediments, the widespread occurrence of magmatic crystallisation ages of ca 450 Ma (Ordovician-Silurian), and by the approximately similar age of the primary granulitic episode in the Kontum Massif. The similarities of the Devonian facies over central, northern Vietnam and South China imply a land connection, possibly as a consequence of a Caledonian collision along Song Ma, but this does not preclude a further oceanic opening and a closure during the Indosinian.     

The western end of the Eoalpine High‐Pressure Belt (Texel unit,South Tyrol / Italy)

Eclogites in the Texel Unit (Eastern Alps; South Tyrol, Italy) represent the westernmost outcrops of the E–W striking Eoalpine High‐Pressure Belt (EHB). East of the Tauern Window, the EHB forms part of a Cretaceous intracontinental south‐dipping subduction/collision zone; however, the same nappe stack displays a northwest dip at its western end. This prominent change in dip direction gave rise to discussions on the general setting of the Eoalpine collision. Based on our own observations and literature data, we present a new tectonic model for the western end of the EHB. Due to the special situation of this area at the tip of the Southalpine indenter, originally south(east) dipping structures became overturned, and former thrusts appear as normal faults (e.g. Schneeberg fault zone) while former normal faults presently display thrust geometries (e.g. Jaufen fault). Thus, we explain the current configuration with a coherent Eoalpine subduction direction.     

Fammenian–Tournaisian dextral ductile shear in the French Variscan belt

The South Armorican Shear Zone consists of a set of faults that runs across the southern Armorican Massif and extends eastwards to the Massif Central. One of its branches, the Cholet Shear Zone of South Brittany, can be correlated with the North-Millevaches–La Courtine Shear Zone in the Massif Central. It was active immediately after the regional Frasnian anatexis (372–368 Ma) as a right-lateral strike-slip fault. The horizontal offset, which can be estimated between 110 and 170 km, was achieved before the emplacement of non-deformed Late Tournaisian calc-alkaline and peraluminous granites (355–350 Ma). This newly established age of activity (Fammenian–Tournaisian) of the Cholet–La Courtine Shear Zone (CCSZ) has to be taken into account in geodynamical reconstructions of the Variscan belt of western Europe. To cite this article: C. Cartannaz et al., C. R. Geoscience 338 (2006).     

Petrotectonic Evolution of the Maksyutov Complex,Southern Urals,Russia: Implications for Ultrahigh-Pressure Metamorphism

The Maksyutov Complex consists of two juxtaposed lithotectonic units—Unit #1 of probable Late Proterozoic formation age, and Unit #2, apparently generated in Cambro-Ordovician time. The eclogite-facies metamorphism of Unit #1 occurred prior to 370-380 Ma, when this unit was subjected to blueschist-facies overprinting. Unit #2 displays the effects of a somewhat similar blueschist- or high-pressure greenschist-facies recrystallization, indicating that it may have been metamorphosed contemporaneously with Unit #1. Our field work and geochemical studies have focused on the Sakmara River area. Preliminary conclusions are as follows: (1) Unit #1 was subjected to metamorphic temperatures of 620 ± 70° C and minimum pressures of 1.5 GPa, or 2.7 GPa if the previously reported interpretation of coesite pseudomorphs from similar rocks exposed near the village of Shubino, 75 km to the south (Chesnokov and Popov, 1965), is correct. Peak metamorphic pressures would have reached at least 3.2 GPa if blocky graphite described in this report from a Sakmara River eclogitic mica schist has replaced neoblastic diamond; (2) Unit #2 experienced much lower maximum metamorphic pressures, on the order of 0.5 to 0.6 GPa; (3) Unit #2 was variably but intensely metasomatized, indicating the presence of an aqueous fluid during the Early Devonian blueschist/greenschist-facies metamorphism; (4) tectonic parallelism of the lithostratigraphic units and their bounding sutures, combined with P-T conditions of recrystallization, suggest assembly of the Maksyutov Complex in an intra-oceanic subduction zone. This process was followed by exhumation and suturing against the more easterly Middle Paleozoic unmetamorphosed ophiolitic (oceanic) basement and superjacent calc-alkaline Magnitogorsk island arc. The Late Proterozoic-Ordovician Mugodzhar and Ilmen microcontinents subsequently were thrust beneath the eastern edge of the Devonian Magnitogorsk Arc. Collision of the entire complex with the Ordovician-Lower Carboniferous continentalmargin Suvanjak-Sakmara accretionary complex, lying to the west on the Russian Platform, also occurred during Middle Paleozoic time. Finally, (5), the tectonic imbrication of the several units within and adjoining the Maksyutov Complex was itself truncated and deformed into N-S parallelism by postulated Late Paleozoic postcollisional strike-slip movement (Dobretsov et al., in review).     

Palaeozoic history of the Armorican Massif: Models for the tectonic evolution of the suture zones

The Armorican Massif (western France) provides an excellent record of the Palaeozoic history of the Variscan belt. Following the Late Neoproterozoic Cadomian orogeny, the Cambro-Ordovician rifting was associated with oceanic spreading. The Central- and North-Amorican domains (which together constitute the core of the Armorica microplate) are bounded by two composite suture zones. To the north, the Léon domain (correlated with the “Normannian High” and the “Mid-German Crystalline Rise” in the Saxo-Thuringian Zone) records the development of a nappe stack along the northern suture zone, and was backthrusted over the central-Armorican domain during the Carboniferous. To the south, an intermediate block (“Upper Allochthon”) records a complex, polyorogenic history, with an early high-temperature event followed by the first generation of eclogites (Essarts). This intermediate block overthrusts to the north the Armorica microplate (Saint-Georges-sur-Loire), to the south: (i) relics of an oceanic domain; and (ii) the Gondwana palaeomargin. The collision occurred during a Late Devonian event, associated with a second generation of eclogites (Cellier).     

Palaeozoic amalgamation of Central Europe: new results from recent geological and geophysical investigations

Multidisciplinary studies of geotransects across the North European Plain and Southern North Sea, and geological reexamination of the Variscides of the North Bohemian Massif, permit a new 3-D reassessment of the relationships between the principal crustal blocks abutting Baltica along the Trans-European Suture Zone (TESZ). Accretion was in three stages: Cambrian accretion of the Bruno–Silesian, Lysogory and Malopolska terranes; end-Ordovician/early Silurian accretion of Avalonia; and early Carboniferous accretion of the Armorican Terrane Assemblage (ATA). Palaeozoic plume-influenced metabasite geochemistry in the Bohemian Massif explains the progressive rifting away of peri-Gondwanan crustal blocks before their accretion to Baltica. Geophysical data, faunal and provenance information from boreholes, and dated small inliers and cores confirm that Avalonian crust extends beyond the Anglo-Brabant Deformation Belt eastwards to northwest Poland. The location and dip of reflectors along the TESZ and beneath the North European Plain suggest that Avalonian crust overrode the Baltica passive margin, marked by a high-velocity lower crustal layer, on shallowly southwest-dipping thrust planes forming the Heligoland–Pomerania Deformation Belt. The “Variscan orocline” of southwest Poland masks two junctions between the Armorican Terrane Assemblage (ATA) and previously accreted crustal blocks. To the east is a dextrally transpressive contact with the Bruno–Silesian and Malopolska blocks, accreted in the Cambrian, while to the north is a thrust contact with easternmost Avalonia, deeply buried beneath younger sedimentary cover. In the northeast Bohemian and Rhenohercynian Massifs Devonian “early Variscide” deformation dominated by WNW and NW-directed thrusting, records closure of Ordovician–Devonian seaways between detached “islands” of the ATA and Avalonia.     

Geodynamic evolution of the European Variscan fold belt: palaeomagnetic and geological constraints

The Variscan fold belt of Europe resulted from the collision of Africa, Baltica, Laurentia and the intervening microplates in early Paleozoic times. Over the past few years, many geological, palaeobiogeographic and palaeomagnetic studies have led to significant improvements in our understanding of this orogenic belt. Whereas it is now fairly well established that Avalonia drifted from the northern margin of Gondwana in Early Ordovician times and collided with Baltica in the late Ordovician/early Silurian, the nature of the Gondwana derived Armorican microplate is more enigmatic. Geological and new palaeomagnetic data suggest Armorica comprises an assemblage of terranes or microblocks. Palaeobiogeographic data indicate that these terranes had similar drift histories, and the Rheic Ocean separating Avalonia from the Armorican Terrane Assemblage closed in late Silurian/early Devonian times. An early to mid Devonian phase of extensional tectonics along this suture zone resulted in formation of the relatively narrow Rhenohercynian basin which closed progressively between the late Devonian and early Carboniferous. In this contribution, we review the constraints provided by palaeomagnetic data, compare these with geological and palaeobiogeographic evidence, and present a sequence of palaeogeographic reconstructions for these circum-Atlantic plates and microplates from Ordovician through to Devonian times.