ECLOGITIC METAMORPHISM IN CARBONATE ROCKS - THE EXAMPLE OF IMPURE MARBLES FROM THE SESIA-LANZO ZONE, ITALIAN WESTERN ALPS

The impure marbles of the internal Sesia-Lanzo Zone underwent a multi-stage metamorphic evolution of Alpine age and retain early-Alpine eclogitic assemblages, partially recrystallized under blueschist to greenschist facies conditions. These high-P assemblages consist of carbonates, phengite, quartz, omphacite, grossular-rich (locally spessartinic) garnet, zoistite and Al-rich titanite. Retrogressive stages are characterized by the growth of glaucophane, paragonite, phlogopite, tremolite and albite. Halogen-rich biotite and amphibole are also present. P-T estimates of the early-Alpine metamophism have been calculated from these unique high-P assemblages, in order to test the applicability of some calibrations to impure carbonate systems. In particular, some Gt-Cpx calibrations and the phengite geobarometer give results (T = 575 +/- 45-degrees-C at 15 kbar for the eclogitic climax and T less-than-or-equal-to 500-degrees-C at PH2O less-than-or-equal-to 9 kbar for early-Alpine retrogressive stages) which are within the range obtained from the surrounding lithologies. Phase relationships in P-T-XCO2 space indicate that mineral assemblages in the impure marbles coexisted with H2O-rich fluids (XCO2 < 0.03) during their entire Alpine evolution.