EARLY AND LATE CARBONIFEROUS MAGMATISM IN THE VARISCIDES OF THE CENTRAL PYRENEES: SOLUTION TO A TECTONIC CONTRADICTION AND A NEW MAGMATIC ENIGMA

Magmatic activity in the Variscan Pyrenees has previously been confined to the late Carboniferous (ca. 310-298 Ma). This poses tight temporal constrictions on the tectonometamorphic evolution of gneiss domes in the Pyrenees, confing the main Variscan deformation phase to the Westphalian-Stephanian. New geochronological studies of deep seated granite intrusions and migmatites within the core zone of gneiss and structural-metamorphic domes extend the early phase of deformation and high temperature metamorphism back to the Visean (ca. 340 Ma), the intrusive ages of the Bossòst and Soulcem granites of the Central Pyrenees. This discovery solves apparent contradictions of the earlier model, e.g. granite intrusions on one hand enhance the development of schistosity in orthogneisses, which, on the other hand, is also crosscut by granite plutons. The older intrusions can be distinguished from younger Bassiès and Mont Louis-Andorra plutons by high temperature contact metamorphism and migmatization. The new results show that HT metamorphism, the development of flat-lying schistosity, and the early phase of dome formation in the Central Pyrenees already occurred during the Visean. Though previously unknown in the Pyrenees, early Carboniferous magmatism (340-330 Ma) is reported from other European Variscan orogens, e.g. Montagne Noire, Massif Central. This raises the question of the origin of Visean magmatism. Recent plate tectonic reconstructions place the Variscan Pyrenees to the northern margin of the Palaeothetys or at an intracontinental subduction zone; both are in agreement with compressional tectonic setting associated with the Visean intrusions. Isotopical and geochemical studies are required to determine the nature of early Carboniferous magmas, and help solve a new and crucial puzzle in the evolution of the southern European Variscides.