EARLY TERTIARY ECLOGITE-FACIES METAMORPHISM IN THE MONVISO OPHIOLITE

Although crucial for the construction of tectonic models of the Alps t he timing of high pressure metamorphism is still poorly determined and controversial. It is likely to vary from one tectonic unit to another depending on when each became involved in subduction. This in turn re lates to palaeogeographic position with respect to the active ocean ba sin. Well defined, reliable geochronological data are too few to test this hypothesis. This paper extends the database by determining Sm-Nd mineral isochrons on two samples from the Monviso Ophiolite in the Pie monte Zone, carefully selected to minimize the problems of Sm-Nd datin g of eclogites encountered elsewhere in the Alps. The dated samples ha ve eclogite facies mineral assemblages typical of the Lago Superiore u nit of the ophiolite; mineral compositions are similar to previously r eported samples and indicate pressures of around 2 GPa and temperature s of 400-500 degrees C. Sm-Nd isochron ages of 60 +/- 12 and 62 +/- 9 Ma are defined by garnet and clinopyroxene, while the Rb-Sr age on phe ngite which is part of the high-P assemblage is 40 +/- 1 Ma. The new d ata fit an emerging pattern of ages in which high-P metamorphism in th e oceanic realm is Early Tertiary, with slightly older ages in the ove rlying Sesia Zone and younger, Oligocene ages in the underlying intern al basement massifs which only became involved in subduction when clos ure of the Piemont ocean was complete.