Neoproterozoic-early Palaeozoic tectonostratigraphy and palaeogeography of the peri-Gondwanan terranes: Amazonian v. West African connections

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.

Does the Meguma Terrane Extend into SW England?

“…Pollock et al 2009;Barr et al 2012) that closely match those of Avalonian arc magmatism (e.g. Nance et al 2008). Only in the Silurian-Devonian Arisaig Group of Nova Scotia, which is thought to have been derived from Baltica with a progressively increasing input from Laurentia, do initial ε Nd values become strongly negative (-4.8 to -9.3) and T DM ages get older than 1.5…”

“…While differing in their tectonostratigraphic histories, each of these terranes is believed to have occupied a position off the northern (West African/Amazonian) margin of Gondwana in the Neoproterozoic/Cambrian (e.g. Nance et al 2008). However, the Meguma terrane is the only major terrane in the northern Appalachian orogen that has no obvious correlatives elsewhere in the Appalachians (Williams 1978(Williams , 1979 and no confirmed linkages to either the Caledonide or Variscan orogens of Western Europe (see Waldron et al 2009).…”

“…1) and, as with all peri-Gondwanan terranes, are believed to have been proximal to the northern (West African/Amazonian) margin of Gondwana in the late Neoproterozoic (e.g. Nance et al 2008;Bassett 2009). Despite their shared paleogeography, Avalonia crops out discontinuously from southeastern New England to eastern Newfoundland, whereas outcrop of the Meguma terrane is restricted to the southern mainland of Nova Scotia (e.g.…”

“…Keppie et al 1998;Thompson and Bowring 2000;Murphy et al 2004b, c;Barr et al 2012;Thompson et al 2012), which have been used to position the terrane during the Neoproterozoic/Cambrian in the proximity of Amazonia, the age provinces of which they broadly match (e.g. Nance et al 2008).…”

A new, high precision U–Pb date from the oldest known rocks in southern Britain

“…Together these make up part of the record of assembly of West Gondwana at around 765-660 Ma and 635-550 Ma (Nance, Murphy & Keppie, 2002;Nance et al 2008 and references therein). This included accretion of Avalonian-type and Ganderean-type terranes against a composite West AfricaAmazonia-Baltica margin of Gondwana (Murphy et al 2004;Nance et al 2008), recorded in southern Britain as the c. 667-650 Ma metamorphic event (Strachan et al 1996(Strachan et al , 2007. Available isotopic data suggest that the Avalonian terranes of southern Britain were assembled together by latest Precambrian to Early Cambrian times (Gibbons & Horák, 1996;Strachan et al 2007).…”