Two metamorphic cycles recorded by monazite in eclogite-facies gneisses (Southern Armorican Massif, France): A Cambro-Ordovician continental crust involved in eo-Variscan subduction.

Abstract

We present new U-Th/Pb ages obtained for xenotime and monazite from the polycyclic eclogite-facies para- and ortho-gneisses of the Les Essarts Unit (Vendée, southern Armorican Massif, France), which have recorded an HT-LP cycle ending with a first retrogression and a subsequent HP eclogite-facies metamorphism similar to that of the neighbouring eclogites. Some paragneisses and orthogneisses are only slightly deformed and retrogressed, showing the structure of nebulitic migmatite or metagranite, respectively; both show well-preserved complex coronitic and pseudomorphic microstructures, due to the eclogite-facies metamorphism. Monazite I and xenotime crystallised during the HT stage providing an opportunity to date the early HT metamorphic event in the paragneiss and the emplacement of granite in the orthogneiss. U/Pb ages obtained from monazite I and xenotime of the cordierite-bearing migmatitic paragneisses range between 510 and 480 Ma (Late Cambrian-Early Ordovician). These ages may correspond to the crystallisation and/or re-equilibration of monazite and xenotime during the prograde stage of the HT cycle, close to the T peak. Consistent monazite and xenotime U/Pb ages around 496 Ma in the orthogneiss represent the age of the granite protolith. During subsequent HP overprint in the gneisses, numerous coronas developed at the expense of the early HT parageneses, in particular plagioclase. In both paragneiss and orthogneiss, monazite I in contact with HT plagioclase reacted to form apatite + zoisite + monazite II coronas. The tiny monazite II crystals could be dated in a paragneiss sample and gave a concordant Th-U/Pb age of 395 ± 6 Ma, interpreted as the age of eclogite-facies HP metamorphism. This age is in agreement with those obtained in HP metamorphic rocks of the Upper Allochthon Unit of the Iberian-Armorican Arc (Cabo-Ortegal, Bragança, Audierne) representing the first evidence of convergence in the Variscan cycle.