Pressure-temperature-time-deformation paths in metamorphic rocks and tectonic processes, as exemplified by the Variscan orogeny in South Brittany, France.
Résumé
To relate metamorphic conditions to tectonic processes, three main complementary steps may be followed. First, extract information from mineral assemblages which occur in metamor phic rocks in order to construct quantitative P—T—t paths. Second, correlate successive metamorphic fabrics with progressive evolution of P and T through time, and construct P—T— t—deformation paths in order to understand thermal/deformation histories. Third, model the thermal evolution of orogenic belts, par ticularly the case of continental collisions. Fol lowing these steps, and in the light of recent thermal models, this paper discusses large-scale structural/microstructural (Audren 1987) and geothermobarometric data (Triboulet & Audren 1985a, 1988) obtained on highgrade metamor phic rocks from the River Vilaine area in South Brittany. Geological setting and tectonic/metamorphic evolution The River Vilaine area in South Brittany (France) is part of the western axial zone of the European Hercynian belt (Fig. la). The meta morphic rocks consist of a volcano-detrital se quence of interbedded mica-schists (abundant) and amphibolites (scarce) and are interpreted as having formed on an active continental mar gin (oceanic/continental sedimentary associ ation) (Triboulet & Audren 1985b). During Palaeozoic times these rocks underwent a com plex tectonic and metamorphic two-stage evol ution, corresponding to successive Devonian and Carboniferous orogenic events that have been clearly separated through time and space (Audren 1987).