Continuous reactions between biotite, garnet, staurolite, kyanite-sillimanite-andalusite and P-T-time-deformation path in micaschists from the estuary of the river Vilaine, South Brittany, France

Abstract Microprobe analysis of the continuous chemical evolution of coexisting biotite-garnet and biotite-garnet-staurolite has been undertaken from interbedded micaschists of the volcanodetrital group of the Vilaine. A thermobarometric study using pertinent mineralogical equilibria reveals a complex P-T evolution, continuous throughout time, from high pressure, medium temperature (kyanite zone) to medium pressure, high temperature (sillimanite zone), then low pressure, medium temperature (andalusite zone). The T, P, f_H2o and X_H2o variations have been calculated from coexisting biotite-garnet pairs, and from the equilibria: paragonite (in white mica) + quartz ± albite (in plagioclase) + Al silicate + H₂O; and, 3 anorthite ± grossular + 2 Alsilicate + quartz. The P-T evolution is correlated with the continuous change in composition of minerals (using P–X_Mg and T–X_Mg diagrams) and with the evolution of assemblages. This continuous P-T-time evolution, correlated with the successive formation of S₁-S₂ foliations, allows us to propose a P-T-time-deformation path for the micaschists and to relate the growth of its mineral components to tectonic processes.