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

Abstract

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=2·6-3·0GPa 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 asmicrometre-sized crystals in textural equilibriumwith 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=0·9GPa, consistent with estimates of T≈850°C (at assumed P=0·9GPa) 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+spinel6sapphirine6corundum, formed at T≈850°C and P=0·8-1·0GPa, 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 thesemicro-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.