HT, pre-Alpine relics in a spinel-bearing dolomite marble from the UHP Brossasco-Isasca Unit (Dora-Maira Massif, western Alps)

A dolomite marble from the polymetamorphic complex of the UHP eclogite-facies Brossasco-Isasca Unit (southern Dora-Maira Massif), which preserves relics of a HT pre-Alpine, Spl-Dol-Ilm-bearing assemblage partially converted into a Crn + Chl + (Arg) + Rt UHP assemblage during the Alpine evolution, has been petrologically investigated. Its pre-Alpine P-T-X(CO2) evolution is discussed using P-T phase diagram projections for H2O-CO2 mixed volatile systems. The P-T-X(CO2) modelling suggests that the Dol + Crn = Cal + Spl + F decarbonation reaction is the most likely spinel-producing reaction along the pre-Alpine prograde path. In the mixed-volatile model system, a X(CO2) > 0.60 has been inferred as the equilibrium fluid composition of this univariant assemblage. Comparison of the P-T-X(CO2) pre-Alpine and Alpine evolutions shows that CO2-rich fluids involved in the pre-Alpine reactions are substantially different from the acqueous fluids produced during the UHP Alpine stage.