BLUESCHIST, ECLOGITE AND DECOMPRESSION ASSEMBLAGES IN THE ZERMATT-SAAS OPHIOLITE, NEWS FROM SUBDUCTED TETHYS OF THE CENTRAL ALPS

The Zermatt-Saas ophiolite complex of the Central Alps represents an almost complete fragment of Mesozoic Tethys oceanic lithosphere. The ophiolite was subducted during early phases of the Alpine orogeny and the mafic rocks were transformed to eclogite and blueschist associations. Pillow basalts exhibit locally preserved volcanic structures. Blueschist assemblages rim eclogite pillow cores. Deformed metabasalts show an intimate association of eclogite and blueschist. Fragments of the ophiolite returned to shallow levels were overprinted by regional low-grade greenschist facies metamorphism and multi-phase deformation. Omphacite-garnet-glaucophane-clinozoisite-dolomite-Mg-chloritoid-talc-paragonite-phengite-chlorite form characteristic coeval blueschist-eclogite assemblages. Garnet contains inclusions of Cld, Gl and Clz. Large omphacite crystals are overgrown by a distinct rim of Jd-rich omphacite. Uplift of the high-P ophiolite rocks was accompanied by development of abundant symplectites and other fine-grained reaction products formed by decompression reactions in a fluid-deficient regime. Particularly notable is the formation of margarite, paragonite, barroisite and preiswerkite. The later mineral, a very rare Na-biotite, formed as a result of the decomposition of chloritoid + paragonite. Two spinel phases (magnetite + MFA-spinel) are additional reaction products. Omphacite breakdown produced Cpx-albite-barroisite symplectites. The presence of preiswerkite is, to our knowledge, the first reported occurrences from Cld-Pg meta-basalts. Calculated equilibrium assemblage phase diagrams for meta-basalt compositions indicates PT-conditions of ~20-25 kbar and ~600-650°C for the subduction-related metamorphism. These conditions coincide with the upper PT limit of antigorite. Antigorite-serpentinites constitute the largest volume of rocks within the ophiolite. We suggest that the PT conditions recorded by the exhumed mafic rocks are causatively related to the conditions of antigorite breakdown in the serpentinite. The release of large amounts of dehydration water in the subducted serpentinite slab facilitated the exhumation of the high-P mafic rock fragments of the Zermatt-Sass ophiolite.