Abstract

Five zircons and one apatite from plutonic rocks of the ophiolitic sequence from the Piemontese zone (western Alps, southeast France) have been dated by the fission-track method. The ages obtained indicate that the minimum age of the Alpine oceanic crust in this zone is between 192 ± 6 and 212 ± 8 m.y. (Late Triassic–Liassic). Consideration of the available data on fission-track retention conditions in zircons and pressure-temperature conditions allowed by the high-pressure mineralogic assemblages of the rocks leads to the conclusion that in the external part of the Piemontese zone, the temperature-duration conditions of the high-pressure Alpine metamorphism were inadequate to significantly reset the zircons' fission-track clock, suggesting a short-lived metamorphic event.

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