Abstract
Equilibration temperatures on eclogites and eclogitic micaschists from the Sesia-Lanzo zone are determined by using two techniques : Fe-Mg partition data in coexisting phases (clinopyroxene and garnet ; phengite and garnet) and oxygen stable isotope measurements on quartz, clinopyroxene, white mica and rutile. Two groups of temperatures have been observed : around 520 °C for the eclogites and around 800 °C for the micaschists. Several hypotheses have been considered to interpret this result : — the role of the bulk chemistry of the rock : the phengite, massively present in the micaschists, fixes the magnesium and consequently the KD Fe-Mgcpx-gt decreases, which leads to an overestimation of the temperature estimation ; — an evolution of the rocks through several stages : a high temperature one, followed by a low temperature (down to 350 °C) one, suggested by the isotopic fractionation measures ; — an isotopic general re-equilibration of assemblages around 500 °C. The formation pressure of those rocks determined, from the jadeite content in clinopyroxene, is around 13 ± 1 kbar.
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