Les âges U/Pb de zircons et le polycyclisme des gneiss de Brest et des formations encaissantes (Bretagne)
Abstract
Response to mesozonal metamorphism of the U/Pb System of zircons from detritic origin has been investigated in rocks of the Pays de Léon where geochronological and geological studies have been carried out recently (Chauris and Michot, 1965; Chauris, 1967; Bishop et al., 1968).
The rocks selected are the « Quartzophyllades de l’Elorn », a non metamorphic Brioverian sediment, the « Micaschistes du Conquet » and the « Gneiss de Brest » which are considered (partially for the last rock) as resulting from the action of Cadomian and Variscan metamorphisms on similar Brioverian sediments.
The zircons of the quartzophyllades appear to form a detritic population of discordant U/Pb ages, part of this population being Pentevrian or older (> 1500 M.Y.).
The zircons of the metamorphic rocks have been open U/Pb Systems, undergoing lead loss and partial recrystallisation at the approximate time of 690 M.Y. (Rb/Sr whole rock age of the Gneiss de Brest; Bishop et al., 1968) and 300 M.Y. (Rb/Sr whole rock age and U/Pb zircon age of the post-Variscan magmatic intrusions of St Renan-Kersaint, Tregana, Aber-Ildut; same authors and Deutsch and Chauris, 1965). However, the action of the mesozonal metamorphism on these rocks did not obliterate completely the ante-Pentevrian origin of part of their zircons, origin similar to that of the zircons of the quartzophyllades.
The tentative conclusions exposed in this paper agree with the interpretation by Grauert and Arnold (1968) of U/Pb ages of zircons in rocks of the Swiss Alps, from a polycyclic region of mesozonal facies.