Desxifrant l'orogènia varisca polifàsica en el NE d'Ibèria mitjançant la geocronologia de fragments de roques cristal·lines en dipòsits sinorogènics

The sediment archive of material eroded from an active tectonic region and stored in adjacent syn-orogenic basin can provide a valuable record of the tectonic evolution of foldbelts, especially whin polyphase deformation and metamorphism has obscured successive events in the orogenic crystalline massfis. In this regard, the syn-orogenic, Culm facies wild-flysch deposits of lower Carboniferous Visean age occur in several Variscan massifs in NE Iberia. Within the Visean Culm facies in several of these massifs, conglomerates occur which contain fragments of gneisses, variably-deformed granites, and schists. The matrix of thses conglomerates is itself affected by cleavage, and the Visean is locally crosscut by late Carboniferous granites. The presence of these pebbles implies a crystalline source area previous to the widespread mid-Carboniferous orogenic peak. Finding it has important consequences concernig the tectonic evolution of the massifs which contain the Visean conglomerates, and also concernig the evolution of the whole Variscan Chain in SW Europe. If the pebbles come from erosion of the underlying pre-Visean crystalline rocks this would imply that there is an unconformity between older rocks and the Visean Culms. This unconformity would imply an important pre-Carboniferous deformation and metamorphism, and then an intense period of erosion in a part of the chain, far from the internal terrains of the French Massif Central, where Devonian meamorphic ages have been found. An unconformity-based model would imply a new vision of the progression of deformation throughout the whole Variscan Chain. The exhumation of the crystalline rocks underlying the Visean in the massifs could be recorded in the ages of metamorphic minerals. Therefore, our on-going study of the metamorphic evolution of the Guilleries massif should include the U-Pb dating of monazite and zircon inclusions in zoned garnets, and in some of the earliest quartz lenses. (...)