Abstract The status of Pannotia as an Ediacaran supercontinent, or even its mere existence as a coherent large landmass, is controversial. The effect of its hypothesized amalgamation is generally ignored in mantle convection models claiming the transition from Rodinia to Pangaea represents a single supercontinent cycle. We apply three geodynamic scenarios to Pannotia amalgamation that are tested using regional geology. Scenarios involving quasi-stationary mantle convection patterns are not supported by the geological record. A scenario involving feedback between the supercontinent cycle and global mantle convection patterns predicts upwellings beneath the Gondwanan portion of Pannotia and the arrival of plumes along the entire Gondwanan (but not Laurentian) margin beginning at c. 0.6 Ga. Such a scenario is compatible with regional geology, but the candidates for plume magmatism we propose require testing by detailed geochemical and isotopic studies. If verified, this scenario could provide geodynamic explanations for the origins of the late Neoproterozoic and Early Paleozoic Iapetus and Rheic oceans and the terranes that were repeatedly detached from their margins.

Abstract A Cambro-Ordovician palaeogeographical restoration of the southwestern European margin of Gondwana is proposed based on the relative positions of Variscan tectonostratigraphic units. Four palaeogeographical proximal–distal transects are recognized and comprise: (i) the Cantabrian, West Asturian-Leonese, Central Iberian/Central Armorican and Ossa-Morena/North Armorican zones and domains of the Iberian and Armorican massifs, respectively; (ii) the South Armorican Domain and its lateral prolongation into the Thiviers-Payzac unit and the Occitan Domain, including the transect from the Axial, southern and northern Montagne Noire, and the Albigeois-southern Cévennes unit; (iii) the southern and northern sides of the Canigó Massif in the Eastern Pyrenees; and (iv) the External Zone and the External and Internal nappes of Sardinia. Two geodynamic scenarios are recognized controlled by the presence/absence of: (i) the Furongian–Early Ordovician (Toledanian or ‘lacaune normande’) break-up unconformity across the Ossa-Morena/North Armorican and Central Iberian/Central Armorican belts; (ii) the Early–Late Ordovician (Sardic) Phase across the Occitan and Pyrenean domains and SW Sardinia; and (iii) the migration of peaks in trilobite and cinctan (echinoderm) diversity. Other similar palaeogeographical shifts are recognized in zircon provenance patterns, the occurrence of climatically sensitive subtropical facies and mineral indicators across platform–basinal transects along the Gondwana margin. This multidisciplinary framework is proposed as a preliminary step in the quest to produce more tightly constrained Early Paleozoic reconstructions along southwestern Europe.

Abstract Protracted magmatism has long been recognized in the exotic South Portuguese Zone of southern Iberia where Early–Late Devonian volcano-sedimentary successions potentially record processes associated with ocean closure and continental collision associated with the formation of Pangaea. Collectively, these rocks represent a bimodal, predominantly submarine volcanic succession with massive sulfides spatially associated with basalts and rhyolites. Volcanic rocks are intruded by the voluminous Sierra Norte Batholith. Although the region hosts some of the world's largest ore deposits, the temporal and genetic relationships between the Iberian Pyrite Belt, the Sierra Norte Batholith and the evolution of the Variscan Orogen remain enigmatic. In an attempt to better understand these complexities, field investigations and targeted geochronology were completed throughout the volcanic and sedimentary rocks of the Iberian Pyrite Belt, and the plutonic rocks of the proximal Sierra Norte Batholith. Our results suggest that the emplacement of the pyrite belt was both pre- and syncollisional, and was initiated primarily by lithospheric delamination related to tectonic escape and crustal thinning of the lower plate. In this scenario, protracted magmatism occurred in both subaerial and subaqueous settings from c. 370 to 338 Ma.

Abstract It has long been recognized that the Late Palaeozoic evolution of SW Iberia preserves a record of terrane accretion, collision and suturing between Laurussia (South Portuguese Zone) and Gondwana (Ossa Morena Zone), which is one of the key events in the development of the Variscan orogen and the amalgamation of Pangea. The suture zone (Pulo do Lobo Zone) is classically considered to be an accretionary complex and is characterized by an assemblage of greenschist facies, polydeformed and imbricated meta-sedimentary rocks, mélanges and mafic complexes. However, recent work has shown some of the metasedimentary rocks and mélange were probaby derived from neither the upper nor the lower plates. Mafic complexes in the mélange have NMORB compositions, highly depleted Sm–Nd isotopic signatures and geochronological data imply that their protoliths probably formed prior to c. 354 Ma. Geochronological data also imply that components of the mafic mélange contain a volumetrically minor amount of ancient continental detritus. The Pulo do Lobo Zone together with the two bounding units (South Portuguese and Ossa Morena zones) were also intruded by c. 360–310 Ma composite plutons and related dykes ranging from gabbro to granite in composition. The oldest phases of these intrusions are syn- to late-tectonic with respect to the deformation. Taken together these recent observations suggest that much of the tectonic evolution of the Pulo do Lobo Zone post-dates the onset of collisional tectonics elsewhere in the Variscan orogen, suggesting that its evolution was dominated by subduction in relatively narrow tracts of oceanic lithosphere. This scenario may be broadly analogous to the complex Cenozoic tectonic evolution of the eastern Mediterranean oceanic tracts relative to the ongoing collision between the African, Eurasian and Arabian plates.

Abstract The Rheic Ocean formed during the Late Cambrian–Early Ordovician when peri-Gondwanan terranes (e.g. Avalonia) drifted from the northern margin of Gondwana, and was consumed during the collision between Laurussia and Gondwana and the amalgamation of Pangaea. Several mafic complexes, from the Acatlán Complex in Mexico to the Bohemian Massif in eastern Europe, have been interpreted to represent vestiges of the Rheic Ocean. Most of these complexes are either Late Cambrian–Early Ordovician or Late Palaeozoic in age. Late Cambrian–Early Ordovician complexes are predominantly rift-related continental tholeiites, derived from an enriched c. 1.0 Ga subcontinental lithospheric mantle, and are associated with crustally-derived felsic volcanic rocks. These complexes are widespread and virtually coeval along the length of the Gondwanan margin. They reflect magmatism that accompanied the early stages of rifting and the formation of the Rheic Ocean, and they remained along the Gondwanan margin to form part of a passive margin succession as Avalonia and other peri-Gondwanan terranes drifted northward. True ophiolitic complexes of this age are rare, a notable exception occurring in NW Iberia where they display ensimatic arc geochemical affinities. These complexes were thrust over, or extruded into, the Gondwanan margin during the Late Devonian–Carboniferous collision between Gondwana and Laurussia (Variscan orogeny). The Late Palaeozoic mafic complexes (Devonian and Carboniferous) preserve many of the lithotectonic and/or chemical characteristics of ophiolites. They are characterized by derivation from an anomalous mantle which displays time-integrated depletion in Nd relative to Sm. Devonian ophiolites pre-date closure of the Rheic Ocean. Although their tectonic setting is controversial, there is a consensus that most of them reflect narrow tracts of oceanic crust that originated along the Laurussian margin, but were thrust over Gondwana during Variscan orogenesis. The relationship of the Carboniferous ophiolites to the Rheic Ocean sensu stricto is unclear, but some of them apparently formed in a strike-slip regimes within a collisional setting directly related to the final stages of the closure of the Rheic Ocean.

Abstract Within the Appalachian–Variscan orogen of North America and southern Europe lie a collection of terranes that were distributed along the northern margin of West Gondwana in the late Neoproterozoic and early Palaeozoic. These peri-Gondwanan terranes are characterized by voluminous late Neoproterozoic ( c . 640–570 Ma) arc magmatism and cogenetic basins, and their tectonothermal histories provide fundamental constraints on the palaeogeography of this margin and on palaeocontinental reconstructions for this important period in Earth history. Field and geochemical studies indicate that arc magmatism generally terminated diachronously with the formation of a transform margin, leading by the Early–Middle Cambrian to the development of a shallow-marine platform–passive margin characterized by Gondwanan fauna. However, important differences exist between these terranes that constrain their relative palaeogeography in the late Neoproterozoic and permit changes in the geometry of the margin from the late Neoproterozoic to the Early Cambrian to be reconstructed. On the basis of basement isotopic composition, the terranes can be subdivided into: (1) Avalonian-type (e.g. West Avalonia, East Avalonia, Meguma, Carolinia, Moravia–Silesia), which developed on juvenile, c . 1.3–1.0 Ga crust originating within the Panthalassa-like Mirovoi Ocean surrounding Rodinia, and which were accreted to the northern Gondwanan margin by c . 650 Ma; (2) Cadomian-type (e.g. North Armorican Massif, Ossa–Morena, Saxo-Thuringia, Moldanubia), which formed along the West African margin by recycling ancient ( c . 2.0–2.2 Ga) West African crust; (3) Ganderian-type (e.g. Ganderia, Florida, the Maya terrane and possible the NW Iberian domain and South Armorican Massif), which formed along the Amazonian margin of Gondwana by recycling Avalonian and older Amazonian basement; and (4) cratonic terranes (e.g. Oaxaquia and the Chortis block), which represent displaced Amazonian portions of cratonic Gondwana. These contrasts imply the existence of fundamental sutures between these terranes prior to c . 650 Ma. Derivation of the Cadomian-type terranes from the West African craton is further supported by detrital zircon data from their Neoproterozoic–Ediacaran clastic rocks, which contrast with such data from the Avalonian- and Ganderian-type terranes that suggest derivation from the Amazonian craton. Differences in Neoproterozoic and Ediacaran palaeogeography are also matched in some terranes by contrasts in Cambrian faunal and sedimentary provenance data. Platformal assemblages in certain Avalonian-type terranes (e.g. West Avalonia and East Avalonia) have cool-water, high-latitude fauna and detrital zircon signatures consistent with proximity to the Amazonian craton. Conversely, platformal assemblages in certain Cadomian-type terranes (e.g. North Armorican Massif, Ossa–Morena) show a transition from tropical to temperate waters and detrital zircon signatures that suggest continuing proximity to the West African craton. Other terranes (e.g. NW Iberian domain, Meguma) show Avalonian-type basement and/or detrital zircon signatures in the Neoproterozoic, but develop Cadomian-type signatures in the Cambrian. This change suggests tectonic slivering and lateral transport of terranes along the northern margin of West Gondwana consistent with the transform termination of arc magmatism. In the early Palaeozoic, several peri-Gondwanan terranes (e.g. Avalonia, Carolinia, Ganderia, Meguma) separated from West Gondwana, either separately or together, and had accreted to Laurentia by the Silurian–Devonian. Others (e.g. Cadomian-type terranes, Florida, Maya terrane, Oaxaquia, Chortis block) remained attached to Gondwana and were transferred to Laurussia only with the closure of the Rheic Ocean in the late Palaeozoic.

Abstract Outcrops of pre-Mesozoic rocks in Spain form various massifs that relate to both Variscan (Late Palaeozoic) and pre-Variscan tectonic settings (Fig. 9.1 ). The largest one of these is the so-called Iberian Massif, an autochthonous massif across which an almost complete, undisturbed geotraverse of the European Variscan orogen has been preserved. Other massifs occur as variably reworked basement complexes in Alpine chains. These are: (i) the various pre-Mesozoic massifs of the Iberian and Catalonian Coastal ranges, that can basically be considered autochthonous with respect to the Iberian Massif; (ii) the basement massifs of the axial zone of the Pyrenees; and (iii) parts of the internal zones of the Betics. The latter two are essentially exotic with respect to the Iberian Massif. Several tectonic syntheses have been published so far on this orogen (e.g. Matte 1986 , 1991 ; Julivert & Martínez 1987; Dallmeyer & Martínez-García 1990; Martínez-Catalán 1990 a ; Ribeiro et al . 1990 c ; Quesada 1990 b , 1992; Quesada et al . 1991; Shelley & Bossière 2000 ). It is agreed that the European Variscan belt resulted from the oblique collision and interaction between Palaeozoic supercontinents (Gondwana, Laurentia and Baltica) and a number of continental microplates during Neoproterozoic through Palaeozoic times. These microcontinents included fragments of magmatic arcs formed previously during a process of continental convergence at the margins of the major Neoproterozoic continental masses. Such a process resulted in the so-called Cadomian, Avalonian or Pan-African orogeny, developed