Cadomian orogeny

The Cadomian Orogeny was a tectonic event or series of events in the late Neoproterozoic, about 650–550 Ma, which probably included the formation of mountains. This occurred on the margin of the Gondwana continent, involving one or more collisions of island arcs and accretion of other material at a subduction zone. The precise events, and geographical position, are uncertain, but are thought to involve the terranes of Avalonia, Armorica and Iberia. Rocks deformed in the orogeny are found in several areas of Europe, including northern France, the English Midlands, southern Germany, Bohemia, southern Poland and the southwest Iberian Peninsula. The name comes from Cadomus, the Latin name for Caen, northern France. L Bertrand gave the orogeny its name in 1921, naming it after Cadomus the Gaulish name for Caen in Normandy. He defined the end as being marked by Lower Palaeozoic red beds.

The interpretation is that the belt was formed as oceanic crust subducted below the Armorica land mass in a similar way to the Andes. Sediments deposited on the continental margin were pushed up onto the continent, at the same time as intrusions of calc-alkaline magmas occurred.

The pre Cadomian basement rocks consist of Orosirian or Paleoproterozoic Icart gneiss dated at close to 2,018 million years ago. These are exposed on Guernsey and at Cape La Hague in Brittany. This is termed the Icartian succession. Near Trégor the gneiss in the Cadomian Perros-Guirec complex has been U-Pb dated to 1,790 million years ago. Sark also has a gneiss basement. Vidal et al. studied Strontium isotope ratios and concluded that the basement is not widely present below the Cadomian orogeny, and Rabu et al. argued that the existing surface gneiss are fragments detached from the African craton. However it is possible that a basement of gneiss exists at depth below Normandy and Brittany. This gneiss was formerly known as Pentevrian. However it is now known that the Pentevrian type rocks do not predate the Cadomian orogeny, and this name is no longer appropriate for the Icart Gneiss.

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