The Variscan collage and orogeny (480–290 Ma) and the tectonic definition of the Armorica microplate: a review

Abstract

The Variscan belt of western Europe is part of a large Palaeozoic mountain system, 1000 km broad and 8000 km long, which extended from the Caucasus to the Appalachian and Ouachita mountains of northern America at the end of the Carboniferous. This system, built between 480 and 250 Ma, resulted from the diachronic collision of two continents: Laurentia–Baltica to the NW and Gondwana to the SE. Between these two continents, small, intermediate continental plates separated by oceanic sutures mainly have been defined (based on palaeomagnetism) as Avalonia and Armorica. They are generally assumed to have been detached from Gondwana during the early Ordovician and docked to Laurentia and Baltica before the Carboniferous collision between Gondwana and Laurentia–Baltica. Palaeomagnetic and palaeobiostratigraphic methods allow two main oceanic basins to be distinguished: the Iapetus ocean between Avalonia and Laurentia and between Laurentia and Baltica, with a lateral branch (Tornquist ocean) between Avalonia and Baltica, and the Rheic ocean between Avalonia and the so‐called Armorica microplate. Closure of the Iapetus ocean led to the Caledonian orogeny: a belt resulting from collision between Laurentia and Baltica, and from softer collisions between Avalonia and Laurentia and between Avalonia and Baltica. Closure of the Rheic ocean led to the Variscan orogeny by collision of Avalonia plus Armorica with Gondwana. A tectonic approach allows this scenario to be further refined. Another important oceanic suture is defined: the Galicia–Southern Brittany suture, running through France and Iberia and separating the Armorica microplate into North Armorica and South Armorica. Its closure by northward (or/and westward?) oceanic and then continental subduction led to early Variscan (430–370 Ma) tectonism and metamorphism in the internal parts of the Variscan belt. As no Palaeozoic suture can be detected south of South Armorica, this latter microplate should be considered as part of Gondwana since early Palaeozoic times and during its Palaeozoic north‐westward drift. Thus, the name Armorica should be restricted to the microplate included between the Rheic and the Galicia–Southern Brittany sutures.

SHRIMP dating of zircons from high-grade metasediments of the Schwarzwald/SW-Germany and implications for the evolution of the Moldanubian basement

“…This fits well to the concept of a Neoproterozoic active margin setting along the entire length of the future Palaeozoic microcontinents at the northern margin of Gondwana (e.g. Tait et al 1997Tait et al , 2000So¨llner et al 1997;Matte 2001;von Raumer et al 2002;Stampfli et al 2002 and references therein). In Fig.…”

“…The subsequent motion of the microplates and opening or closure of ocean basins has been modelled by alternative concepts (e.g. Scotese and McKerrow 1990;Tait et al 2000;Matte 2001;Cocks and Torsvik 2002;. For a critical discussion e.g.…”

Does the Meguma Terrane Extend into SW England?

“…2), which lies outboard of the Rheic suture and is widely assumed to be underlain by Avalonian basement (e.g. Franke 2000;Matte 2001). However, autochthonous pre-Devonian basement is not exposed and Shail and Leveridge (2009) noted that the basement in SW England might have originated as a separate (non-Avalonian) Gondwana-derived terrane.…”

“…We then use this approach to explore (i) whether the proposed continuation of the Meguma terrane into southern Iberia can be extended to that part of southern Britain (the Rhenohercynian Zone) with which the South Portuguese Zone is traditionally correlated (e.g. Frank 1989;Frank et al 1995;Matte 2001;von Raumer et al 2003), and (ii) what the implications of such a linkage might be to the potential presence of the Meguma terrane in the Harlech Dome region of North Wales and the Brabant Massif of the European Low Countries.…”

Potassic late orogenic Stephanian volcanism in the Southwest French Massif central (Decazeville, Figeac, Lacapelle-Marival basins): an example for mantle metasomatism along strike-slip faults?

“…The southern part of the Variscan orogeny results from continental collision following a northward subduction, whose remnants are found in the French Massif central [Matte, 1986[Matte, , 2001]. However, the southernmost part of the French Massif central as well as the Pyrénées belong to the subducting plate, and its underlying mantle was not in a suprasubduction position, and could not be metasomatised by a Variscan subduction.…”