Cr-rich magnesiochloritoid eclogites from the Monviso ophiolites (Western Alps, Italy)

Cr-rich magnesiochloritoid in the eclogitized ophiolites of the Monviso mas sif occurs in the least differentiated rocks of the gabbroic sequence (troc tolites to melatroctolites). Chloritoid (X-Mg=0.63-0.85; Cr less than or eq ual to 0.55, atoms) co-exists with omphacite, talc and garnet. Minor, syn-e clogitic minerals are chromite, rutile and sometimes magnesite and Cr-Ti ox ides. Coronitic textures, indicative of a static recrystallization, characterize the analysed samples. Layers of variable mineral composition develop among igneous plagioclase, olivine, clinopyroxene and spinel. The minerals in the coronitic layers display sharp compositional zonings. The igneous minerals are commonly not preserved; their presence in the original assemblage is i nferred from the mineralogical composition of the pseudomorphs. Syn-eclogitic volatile components are indicated by the development of OH-be aring minerals (e.g. chloritoid & talc) and carbonates (e.g. magnesite), an d supported by the presence of coarse-grained and fibrous mineral growths. The complex pseudomorphic replacements of igneous minerals suggest that the se rocks changed their mineralogical composition prior to the eclogite faci es recrystallization, most likely during ocean-floor metamorphism. It is su ggested that syn-eclogitic fluids formed by breakdown reactions of pre-eclo gitic volatile-bearing minerals. Geothermobarometry indicates that the investigated rocks recrystallized at a depth corresponding to 2.4 GPa and temperatures of 620 +/- 50 degrees C. The attainment of high-pressure conditions is supported by the presence of magnesiochloritoid, magnesite and garnet with high pyrope content (up to 58 mol%). P-T estimates point to a very low thermal gradient (about 9 degrees C km(-1)), comparable to that deduced in the adjacent Dora-Maira ultra-hig h pressure unit.