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Experimental evidence for diamond facies metamorphism in the Dora Maira massif

J. Hermann

The peak metamorphic conditions of subducted continental crust in the Dora Maira Massif (Western Alps) have been revised by combining experimental results in the KCMASH system with petrographic observations in whiteschists. Textural observations in whiteschists suggest that the peak metamorphic assemblage consisting of garnet + phengite + kyanite + coesite ± talc originates from the reaction kyanite + talc <-> garnet + coesite + liquid. In the experimentally determined petrogenetic grid, this reaction occurs above 45 kbar at ~730°C. At lower pressures talc reacts either to orthopyroxene and coesite or, together with phengite, to biotite, coesite and kyanite. The liberated liquid is probably supercritical in nature with equal amounts of H2O and dissolved granitic components. Experimentally determined Si-isopleths in the talc-phengite-kyanite-coesite field as well as in the garnet-phengite-kyanite-coesite filed further constrain peak pressures to ~42 kbar for the measured Si = 3.59 of phengite in the natural whiteschists. All these data provide evidence that the whiteschists reached diamond facies conditions.

The fluid absent equilibrium 4 kyanite + 3 celadonite = 4 coesite + 3 muscovite + pyrope has been empirically calibrated on the basis of garnet and phengite compositions in the experiments. Garnet-phengite thermometry in graphite bearing metapelites and kyanite-phengite eclogites, forming the country rocks of the whiteschists, yield temperatures of ~750°C. At this temperature, peak metamorphic pressures were calculated to 44 ± 3 kbar using the calibrated equilibrium. Therefore, the whole ultrahigh-pressure unit of the Dora Maira massif most likely experienced peak metamorphic conditions in the diamond stability field. The absence of metamorphic micro diamonds might be explained by kinetic problems of graphite to convert to diamond.