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Geochronological constraints on the emplacement and deposition ages of pre-Variscan rocks. For orthogneisses, the emplacement age corresponds to the Rb-Sr whole rock isochron age (circles) or the the youngest U-Pb, U-Th-Pb age obtained on magmatic zircon or monazite. For paragneisses, a maximum deposition age is provided by the youngest inherited age. Cambrian and Ordovician orthogneisses do not appear associated with a given nappe but are found throughout the French Massif Central.
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