Early Tertiary eclogite facies metamorphism in the Monviso Ophiolite |
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| Authors: | R A CLIFF A C BARNICOAT & S INGER |
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| Institution: | Department of Earth Sciences, University of Leeds, Leeds LS2 9JT , UK |
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| Abstract: | Although crucial for the construction of tectonic models of the Alps the 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 relates to palaeogeographic position with respect to the active ocean basin. 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 Piemonte Zone, carefully selected to minimize the problems of Sm–Nd dating of eclogites encountered elsewhere in the Alps. The dated samples have eclogite facies mineral assemblages typical of the Lago Superiore unit of the ophiolite; mineral compositions are similar to previously reported samples and indicate pressures of around 2 GPa and temperatures of 400–500 °C. Sm–Nd isochron ages of 60±12 and 62±9 Ma are defined by garnet and clinopyroxene, while the Rb–Sr age on phengite which is part of the high- P assemblage is 40±1 Ma. The new data fit an emerging pattern of ages in which high- P metamorphism in the oceanic realm is Early Tertiary, with slightly older ages in the overlying Sesia Zone and younger, Oligocene ages in the underlying internal basement massifs which only became involved in subduction when closure of the Piemont ocean was complete. |
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| Keywords: | eclogite facies geochronology Western Alps |
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