New Early Devonian paleomagnetic data from NW France: Paleogeography and implications for the Armorican microplate hypothesis

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Abstract
To constrain the Early Devonian paleogeography of the Armorican Massif, and determine the tectonic relationships between the Armorican and Bohemian Massifs (the Armorican microplate hypothesis), a paleomagnetic study of Lochkovian/Pragian sediments from the Pointe de l'Armorique, Brittany, NW France has been carried out. After removal of a present‐day overprint, a higher unblocking temperature component of magnetization is identified (the B direction) which yields an overall mean direction of 222°/+35°, k = 121.6, α95=4.2° after bedding correction, 11 sites. No fold test is possible, but the presence of a thin sill allowed sampling for a contact test, the results of which are positive. Thus the B direction of magnetization is considered to be primary in origin and closely bracketed with the rock age. The resulting paleopole (315°E, 13°S) corresponds to a paleolatitude of 19°S for the Armorican Massif, placing it adjacent to the southern margin of Baltica and Avalonia. This clearly demonstrates for the first time that this sector of the Rheic Ocean had closed by Early Devonian times, and that the Armorican Massif was in its present‐day orientation with respect to Baltica. These results are significantly different to existing paleomagnetic data from Bohemia which indicate large scale (˜140°) rotation between the earliest and latest Devonian, and thus provide important information for the pre‐ and syncollisional tectonic relationships of these Massifs. As it is unlikely that such large scale differential crustal rotations could have occurred in an intraplate setting, it is proposed that the Armorican and Bohemian Massifs did not form a single coherent microplate, but belonged to a fairly loose mosaic of terranes prior to collision with Avalonia. Thus, the concept of an Armorican microplate is no longer tenable, and the term Armorican terrane assemblage (ATA) is proposed in reference to the Gondwana derived Paleozoic terranes now situated within the European Variscan fold belt south of the Rhenohercynian Zone (Avalonia).