Eclogitic metamorphism in carbonate rocks: the example of impure marbles from the Sesia‐Lanzo Zone, Italian Western Alps1

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Abstract
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‐Passemblages consist of carbonates, phengite, quartz, omphacite, grossular‐rich (locally spessartinic) garnet, zoisite 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‐Testimates of the early‐Alpine metamophism have been calculated from these unique high‐Passemblages, 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° C at 15 kbar for the eclogitic climax andT≤ 500° C atPH2O ≤ 9 kbar for early‐Alpine retrogressive stages) which are within the range obtained from the surrounding lithologies.Phase relationships inP‐T‐XCO2space indicate that mineral assemblages in the impure marbles coexisted with H2O‐rich fluids (XCO2<0.03) during their entire Alpine evolution.