ALBERT

Description: 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.

Description: 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 Acatlan 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.