Polymetamorphic evolution of a micaschist from the ultrahigh-pressure terrane of the southern Dora Maira Massif, Western Alps

For a better process understanding of the subduction of the Ligurian Ocean and the adherent European plate under microcontinent Adria including the exhumation of deeply subducted rocks, we have investigated a micaschist from the ultrahigh-pressure (UHP) terrane of the southern Dora Maira Massif. This micaschist crops out about 1 km north of the hamlet of Masueria and contains quartz (40-45 vol%), phengite (almost 30 vol%), garnet (17 vol%), which is strongly variable in size (diameter of 300 µm to almost 1 cm), kyanite (7 vol%), pseudomorphs after jadeite (4 vol%) and different accessory minerals. The compositions and textural relations of the minerals were carefully studied with an electron microprobe. After determination of the bulk-rock composition of the micaschist, which points to a pelitic protolith, thermodynamic modelling with PERPLE_X was undertaken to reconstruct the metamorphic evolution of this rock.

The early mineral assemblage found as inclusions in extended cores of large garnet grains being chemically fairly homogeneous consists of quartz, chloritoid, staurolite, paragonite and kyanite. This assemblage formed at pressure-temperature (P-T) around 12.5 kbar and 600 °C, before relatively large volumes of garnet, after significant overstepping of its P-T limits, overgrew these minerals under release of considerable amounts of water. A relatively narrow rim developed around the inclusion-rich garnet core as the result of early subduction of the rock to depths corresponding to pressures of 20 kbar accompanied by slight heating. Only paragonite reacted to jadeite + kyanite at pressures of 26 kbar before UHP conditions were reached. This reaction resulted in another pulse of water released. Nevertheless, the subsequent burial at UHP to P-T conditions of 34±2 kbar and 715±35 °C, at which phengite with Si contents of 3.47 per formula unit (pfu) equilibrated, occurred under water-absent conditions so that possibly no coesite formed from quartz as the result of overstepping the coesite-quartz transition. The retrograde path is only characterized by the formation of phengite with Si contents lower than 3.4 pfu around UHP phengite and the replacement of jadeite mainly by albite rods. The latter reaction occurred at pressures below 18 kbar, a retrograde path provided that is characterized by slight cooling down to pressures of about 15 kbar as suggested by previous researchers of the Dora Maira UHP terrane. The described retrogression occurred in absence of free H₂O, but defornation caused the partial recrystallization of UHP phengite by phengite with lower Si contents.

The studied polymetamorphic micaschist does not indicate a polycyclic metamorphism. A flat subduction to 45 km (~12.5 kbar) was followed by steep subduction to 110 km. During subduction, pulses of hydrous fluid changed the rock during prograde metamorphism in the pressure range of about 11 to 26 kbar clearly. At UHP and during early retrogression (down to ~15 kbar), changes took place only by deformation as virtually no hydrous fluids were released in the rock or infiltrated it.