Tectonometamorphic evolution of the internal variscan belt. Examples from eastern/western Alps, Bohemian massif, Massif central and southern Brittany

Abstract : Pressure-temperature-time-deformation (P-T-t-d) paths from metamorphic rocks provide consid erable insight into the tectonothermal history of single crystalline segments in an orogenic belt. Most precise information about the P-T-t-d- space evolution of a terrain can be obtained from single metapelite and metabasite samples by relating mineral zonations and successive mineral equilibria to linear and planar (L-S) structures of progressive deformation. When continuous reactions are con­ sidered in such rocks, each step of garnet or amphibole growth zonations represents a finite temporal and spatial domain of equilibration within the assemblages and allows to evaluate P and T or P-T changes for each deformational step by geothermobarometry based on cation exchange and amphibole end member equilibria (Triboulet & Audren 1985; Triboulet 1992; Schulz 1993). This microstructural-petrological approach has been applied to several segments of the internal Variscan belt. The charac­ teristic shapes of the P-T-t-d paths from each segment allowed a thermotectonic interpretation by comparison with numerically modelled P-T paths.