J.R.: Blueschist and eclogite transformation in Fe-Ti gabbros: a case from the western Alps ophiolites

The study of the alpine metamorphism of three suites of Fe-Ti metagabbros occurring in the western Alps ophiolites has shown a set of reactions governed by T, P, XH2O, and diffusion. T-P estimates point to 35O±5O°C at a minimum of 9-10 kb for the Queyras blueschist rocks and to 450-500 °C at a minimum of 12-13 kb for the Lanzo and Rocciavre eclogitized rocks. These variations are the result of different r-P-time trajectories during subduction/obduction events of alpine age. In the Fe-Ti metagabbros, little-deformed volumes showing a crystallization history controlled by local equilibrium are bounded by mylonites. Water-poor and water-rich volumes alternate during eclogitization of the Rocciavre suite. The persistence in the little-deformed rocks of prealpine metastable relics, of corona structures and of chemical gradients, demonstrates that a complete high pressure equilibration is inhibited by slow reaction kinetics and slow diffusion. Only in the mylonites has the catalytic effect of deformation favoured an approach to bulk-rock metamorphic equilibration. In the eclogitized coronitic rocks the apparent O2 release from the alteration of the magmatic opaques plays an important role in reaction rates; increasing extent of eclogitization may be enhanced either by the release of free O2 from the rocks, or by a process in which new H2O formed by the combination of oxygen with hydrogen introduced into the system