Metamorphism

Abstract

Structural, petrological, and geophysical data collected during the last decade have shown that the Variscan nappes of the French Massif Central are the result of subduction and collision tectonics which evolved during ca. 150 Ma (Cogné 1976; Burg and Matte 1978; Autran and Cogné 1980; Bard et al. 1980; Kornprobst et al. 1980; Matte 1983, 1986; Pin and Peucat 1986; Robinson et al. 1988; Ledru et al. 1989). Thus, the tectono-metamorphic history of the French Paleozoic orogen is polyphase (Santallier 1983 b). In such a context, single estimates of the pressure and temperature (P-T) conditions that prevailed during regional metamorphism provide little information on the orogenic history of a nappe belt (England and Richardson 1977; Thompson and England 1984; Davy and Gillet 1987). A dynamic approach of metamorphism is required to define the pressure-temperature-time (P-T-t) paths of each tectono-metamorphic unit stacked in the orogenic zone (see, for example, the high-pressure and low-temperature metamorphism in the western Alps, Caby et al. 1978; Lardeaux et al. 1982; Rubie 1984; Gillet et al. 1986). The purpose of this chapter is to discuss the P-T-t evolution of the main nappes of the French Massif Central, in order to delineate the major factors that control the development of regional metamorphism in the internal part of the Variscan chain.