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The contact zone between two major allochthonous lithotectonic units in the French Massif Central (FMC) is characterized by the presence of corundum-bearing amphibolites associated with serpentinites, flaser-gabbros, eclogites and granulites. These unusual amphibolites are best preserved in the Western FMC, where they are found within the lower oceanic crust of the Limousin ophiolite. Mineralogical observations and thermodynamic modelling of the spinel-corundum-sapphirine-kyanite amphibolites in the CMASH system show that they were formed at peak P-T conditions around 800°C ⁄ 10 kbar in response to near isothermal burial followed by a retrogressive anticlockwise path. Metamorphic reactions are controlled both by modification of P-T conditions and by local chemical changes linked to fluid infiltration. Pargasite growth has been enhanced by infiltration of Ca-and Al-rich fluids whereas kyanite-and sapphirine-forming reactions are partly controlled by local inputs of MgO-SiO 2 components, most probably during infiltration metasomatism. By analogy with worldwide ophiolites (Oman, Tethyan, Appalachian) and published numerical models, subduction of a still-hot oceanic ridge is proposed to form these Al-rich amphibolites from plagioclase-rich troctolites. The trace-element composition of high-Ti, fine-grained amphibolites (former fine-grained Fe-Ti gabbros) adjacent to the corundum-bearing ones, further indicates that the oceanic crust was initially created at a mid-ocean ridge (rather than within a back-arc basin), followed by the emplacement of supra-subduction zone-type magmas, probably due to intraoceanic subduction close to the ridge.

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Humite minerals, including Ti-rich, hydroxyl-dominant chondrodite and clinohumite, occur in Paleozoic antigorite serpentinite in the La Cabana area, in the Chilean Coastal Cordillera (similar to 38 degrees 30'S-73 degrees 150'W). This may be the first report from South America. Humite minerals are intergrown with Mn-rich olivine hosting antigorite blades in textural equilibrium, indicating a metamorphic origin. A comparison with previous results from piston-cylinder experiments and petrological studies of other high-P serpentinites constrains the formation conditions of the humite + olivine + antigorite assemblage to ca. 2.0-2.5 GPa and < 600 degrees C. Thus, the assemblage is interpreted as having formed during cold subduction of a segment of oceanic lithosphere to a depth >60 km, suggesting that the Paleozoic serpentinites were entrained into the mantle at higher P-T conditions than those experienced by the spatially associated olivine-lizardite metadunites and enclosing metasedimentary rocks (subducted to < 30 km). During exhumation along the subduction channel, high-P serpentinites together with metadunites underwent tectonic mingling with metasediments of the accretionary prism, preserving their signature of distinct metamorphic trajectories. This could be similar to the tectonic evolution of blueschists and high-P amphibolites found as isolated blocks in the metasediments of the Chilean Coastal Cordillera.