Variscan metamorphism and K-Ar dates in the Variscan fold belt of S Brittany and NW Spain

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Abstract
The metamorphism in Lower Palaeozoic rocks in the Variscan fold belt of S Brittany and NW Spain is not evenly distributed but forms HT, LP thermal domes with migmatites and granites, separated by rocks with lower grades of metamorphism. 41 mica dates, ranging from 332 to 262 Ma, obtained from granites, decrease in age westwards along the fold belt through S Brittany into NW Spain. These dates, interpreted as reflecting diachronous cooling of the Variscan fold belt, are correlated with age and intensity of the Variscan metamorphism: the oldest dates correspond to possibly earlier and lower grades of metamorphism in S Brittany while the youngest dates coincide with a later thermal dome in SW Spain. This age pattern supports a simple anticlockwise rotation of the Iberian Peninsula relative to France to form the Bay of Biscay.