THE ASSEMBLAGE DIASPORE PLUS QUARTZ IN METAMORPHIC ROCKS - A PETROLOGICAL, EXPERIMENTAL AND THERMODYNAMIC STUDY

The upper pressure limit of pyrophyllite is given by the equilibria (i ) pyrophyllite = diaspore + quartz and (ii) pyrophyllite = diaspore coesite. High-P experimental investigations carried out to locate equi librium (i) yield brackets between 497 degrees C/24.8 kbar and 535 deg rees C/25.1 kbar, and between 500 degrees C/23 kbar and 540 degrees C/ 23 kbar. Equilibrium (ii) was bracketed at 550 degrees C between 26.0 and 28.3 kbar. In the experimental P-T range, equilibria (i) and (ii) are metastable with respect to kyanite. A stable P-T grid is calculate d using thermodynamic data derived under consideration of the present experimental results. According to these data, the lower pressure limi t of the assemblage diaspore + quartz according to equilibrium (i) ran ge from about 12 kbar/300 degrees C to 20 kbar/430 degrees C (in the p resence of pure water). The upper stability of diaspore + quartz is li mited by the reaction diaspore + quartz = kyanite + H2O at about 450 d egrees C (nearly independent of pressure) and, to higher pressure, by the quartz = coesite transition. Equilibrium (ii) is metastable over t he whole P-T range. Natural occurrences of the diaspore-quartz assembl age in metamorphic rocks in Sulawesi, New Caledonia, Amorgos and the V anoise are characterized by minerals indicative of high-P such as ferr omagnesiocarpholite, glaucophane, sodic pyroxene and lawsonite. The me tamorphic P-T conditions of these rocks are estimated to be in the ran ge 300-400 degrees C, >8 kbar. These data are compatible with the deri ved P-T stability field of the diaspore + quartz assemblage. We conclu de that, in metamorphic rocks, diaspore + quartz is, as ferrocarpholit e, an indicator for unusual low-T/very high-P settings.