P-T EVOLUTION OF THE YAMBE PERIDOTITE, NORTHERN NEW CALEDONIA: HIGH-P ULTRAMYLONITE DEVELOPMENT DURING THE EMPLACEMENT OF 'PROGRADE SUBDUCTION ZONE' PERIDOTITE

The Pouébo terrane of NE New Caledonia mostly contains metabasic eclogite and glaucophanite that experienced c.40Ma metamorphism. Eocene high-P metamorphism was contemporaneous with the SW directed obduction of the New Caledonian Ophiolite; an extensive ultramafic nappe that dominates outcrop in the south of the Island. In the North of the island ultramafic lithologies are found only as discrete lensiod masses tectonically interleaved with the high-P rocks. The largest of these ultramafic bodies is enveloped by intensely foliated metabasites of the Pouébo terrane. The base of the peridotite pod is marked by a distinctive (1m thick) band of high-P mylonite to ultramylonite. Textural evidence within mylonitic lithologies suggests the ultramafic bodies were tectonically emplaced during the prograde burial of the Pouébo terrane under conditions of T<510oC and P<16kbar. Peridotite bodies were sourced from the overriding New Caledonian Ophiolite and emplaced within the Pouébo terrane resulting in the development of S1 blueschist facies ultramylonites. Once emplaced the ultramafic fragments were transported to depths of ?60km, where coarse-grained garnet-omphacite-rich assemblages developed under static conditions within the enclosing mylonites at conditions of T=600oC and P?19kbar. Near isothermal decompression and partial hydration resulted in the development of hornblende-albite-omphacite-garnet bearing assemblages within selected layers of the mylonite sequence at P?14kbar, followed by subsequent cooling and partial hydration under greenschist facies conditions.