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Eclogitic metamorphism in carbonate rocks: the example of impure marbles from the Sesia‐Lanzo Zone, Italian Western Alps 1
Author(s) -
CASTELLI D.
Publication year - 1991
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.1991.tb00504.x
Subject(s) - phengite , geology , geochemistry , grossular , titanite , metamorphism , blueschist , albite , metamorphic rock , omphacite , pseudomorph , isochron , pyroxene , eclogite , quartz , paleontology , olivine , tectonics , subduction
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, 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‐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° C at 15 kbar for the eclogitic climax and T ≤ 500° C at P H 2 O ≤ 9 kbar for early‐Alpine retrogressive stages) which are within the range obtained from the surrounding lithologies. Phase relationships in P‐T‐X CO 2 space indicate that mineral assemblages in the impure marbles coexisted with H 2 O‐rich fluids ( X CO 2 <0.03) during their entire Alpine evolution.