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Abstract The compositional variation of phengite and chlorite pairs in rocks sampled across the metamorphic zonation of three different blueschist belts (Cycladic, Sambagawa and Schistes Lustrés) is characterized from the regional to the thin‐section scale. The different sample suites show different compositional trends, but similar trends are observed at the regional and thin‐section scale in the same unit. At the thin‐section scale, several local chlorite–phengite equilibria involving minerals of different compositions can be identified. These observations suggest that at temperature T < ∼550°C, equilibration of chlorite and phengite compositions with varying pressure and temperature is controlled by crystallization/recrystallization processes rather than by changing the composition of older grains by lattice diffusion. In some instances, the relative time of growth of the different phyllosilicate generations can be determined using microstructural criteria. The observed compositional variations are interpreted and quantified in terms of pressure (P) and temperature (T) variations using new thermodynamic solution models accounting for the Tschermak, di/trioctahedral, and pyrophyllitic substitutions. Chlorite–phengite local equilibria constrain the shape of the exhumation P–T paths of the rocks under consideration. The assemblage chlorite–phengite–paragonite–albite–quartz–H 2 O can be used to constrain different P–T paths for Tinos and Sambagawa blueschists. Copyright © 2000 John Wiley & Sons, Ltd.
Vidal et al. (Sat,) studied this question.