LAUSR

Peridotites and different types of eclogites occurring in the Monte Duria area (Adula–Cima Lunga unit, Central Alps, Italy) share a common eclogite-facies peak at P1⁄42 6–3 0 GPa and T1⁄4 710–750 C, constrained by conventional thermobarometry and thermodynamic modelling. High-pressure minerals are replaced both in peridotites and in eclogites by lower-P and high-T assemblages. In peridotites, the zirconium titanate srilankite occurs as micrometre-sized crystals in textural equilibrium with spinel, clinopyroxene and orthopyroxene in kelyphites developed between garnet and olivine. By using a new ZrO2–TiO2 solid-solution model, we provide evidence that srilankite is stable in peridotites relative to zirconþ rutile for T>810 C at an assumed P 0 9 GPa, consistent with estimates of T 850 C (at assumed P1⁄4 0 9 GPa) determined for symplectites made of sapphirineþspinelþAl-rich orthopyroxeneþ amphibole found in fractures within garnet. In eclogites, kyanite is replaced by symplectites made of anorthite-rich plagioclaseþ spinel6 sapphirine6 corundum, formed at T 850 C and P1⁄40 8–1 0 GPa, conditions that are coincident with the high-T overprint observed in the peridotites. Thermodynamic modelling coupled with a material-transfer study provides constraints for these sapphirine-bearing symplectites. In these micro-domains, the ‘inert’ components could not fully equilibrate with the surrounding rock, and the locally high Al content promoted the stability of the Al-rich phases (i.e. mosaic equilibrium). This is the first report from the Alps of eclogite-facies rocks of supposed Alpine age showing a granulite-facies metamorphic overprint, which is, in contrast, well documented in the Variscan belt. On these grounds, although the age of the high-pressure and hightemperature stages in the Monte Duria rocks is still not constrained, the possibility that they reached eclogitic and granulitic conditions in pre-Alpine times should be taken into account.