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Armorican Massif |
Department of Geology, Oxford Polytechnic, Oxford, OX3 OBP, UK
The North Armorican Massif (NAM) of NW France is the type locality of the late Precambrian Cadomian orogenic belt. The NAM largely escaped reworking during the Upper Palaeozoic Variscan cycle, with the result that earlier geological relationships are commonly preserved intact. A large part of the Cadomian belt of the NAM comprises variably deformed and metamorphosed supracrustal rocks of the Brioverian succession (Barrois 1895, 1896, 1908; Graindor 1957; Cogne 1962; Le Corre 1977; Rabu et al. 1982, 1983; Chantraine et al. 1982). The Cadomian orogeny, which takes its name from Cadomus (Caen) in Normandy, was defined by Bertrand (1921) as the late Precambrian orogeny which in the NAM resulted in the folding and uplift of the Brioverian succession, prior to the deposition of Lower Palaeozoic red-bed sequences. Events which have been ascribed to the Cadomian orogeny span the period c. 700–425 Ma, during which time the Brioverian was heterogeneously deformed and metamorphosed, and intruded by calc-alkaline igneous complexes. The evolution of the belt has been interpreted in terms of the gradual cratonization of calc-alkaline arc complexes and syn-orogenic sediments in an Andean-type continental margin (Brown et al. 1990; Dupret et al. 1990).
This contribution reviews briefly the geology of the Cadomian belt of the NAM (Fig. 1) and outlines some of the more controversial debates concerning the distribution and status of pre-Cadomian basement, the stratigraphy of the Brioverian succession, the nature of Cadomian magmatism, the kinematic significance of major structures and the geotectonic setting of the component parts of the
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