Granulite facies overprint in garnet peridotites and kyanite eclogites of Monte Duria (Central Alps, Italy): clues from srilankite- and sapphirine-bearing symplectites

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 P = 2.6–3.0 GPa and T = 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 µm-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 zircon + rutile for T > 810 °C at an assumed P ≈ 0.9 GPa, consistent with estimates of T ≈ 850 °C (at assumed P = 0.9 GPa) determined for symplectites made of sapphirine + spinel + Al-rich orthopyroxene + amphibole found in fractures within garnet. In eclogites, kyanite is replaced by symplectites made of anorthite-rich plagioclase + spinel ± sapphirine ± corundum, formed at T ≈ 850 °C and P = 0.8–1.0 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 high-temperature 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.