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The Cadomian Orogeny comprises a series of complex sedimentary, magmatic and tectonometamorphic events that spanned the period from the mid-Neoproterozoic (c. 750 Ma) to the earliest Cambrian (c. 540-530 Ma) along the periphery of the super-continent Gondwana (peri-Gondwana, Fig. 3.1). Modern data demonstrate broad continuity between Cadomian events and the later opening of the Rheic Ocean during Cambrian-Ordovician times (Linnemann et al. 2007). Due to very similar contemporaneous orogenic processes in the Avalonian microcontinent, the collective terms ‘Avalonian-Cadomian’ Orogeny and ‘Avalonian-Cadomian’ Active Margin have often been used in the modern literature (e.g. Nance & Murphy 1994; Fig. 3.1). Rock units formed during the Cadomian Orogeny are commonly referred to collectively as ‘Cadomian Basement’. Peri-Gondwanan terranes, microcontinents and crustal units in Central, Western, Southern and Eastern Europe, in the Appalachians (eastern USA and Atlantic Canada), and in North Africa were affected by the Cadomian Orogeny. This orogenic event is also apparently present in Baltica because of the 'Cadomian affinity' of late Precambrian orogenic events in the Urals and in the Timanides on the margin of Baltica (Roberts & Siedlecka 2002).

The Cadomian Orogeny sensu stricto was first defined in the North Armorican Massif in France on the basis of the unconformity that separates deformed Precambrian rock units from their Early Palaeozoic (Cambro-Ordovician) overstep sequence (see below). This unconformity is commonly referred to as the ‘Cadomian unconformity’ (Fig. 3.2). However, it cannot be precluded that the youngest metasedimentary rocks affected by

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