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

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 H 2 o and X H 2 o variations have been calculated from coexisting biotite‐garnet pairs, and from the equilibria: paragonite (in white mica) + quartz ± albite (in plagioclase) + Al silicate + H 2 O; 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 S 1 ‐S 2 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.