A Sm-Nd isotopic study of 500 Ma old oceanic crust in the Variscan belt of Western Europe: the Chamrousse ophiolite complex,Western Alps (France)

We report the results of a Sm-Nd isotopic study of whole-rock samples from the Chamrousse ophiolite complex, in the Variscan basement of the Western Alps, France. Our data point to a 497±24 (2) Ma age, in excellent agreement with U-Pb results (Ménot et al. 1984) on subordinate plagiogranites. Strong isotopic heterogeneity and time evolution of magmas from relatively less depleted ( Nd= +5) to very depleted ( Nd= +9) sources are also implied. Available data favour a mixing model between N-MORB and E-MORB type end-members, but a third component with distinct supra-subduction zone affinity is also required. These results preclude any tectonic setting such as intracontinental rift, ensialic marginal basin, or major ocean ridge, but substantiate an oceanic back-arc environment. They also demonstrate that the well-documented Upper Cambrian-Lower Ordovician extensional phase here reached a true oceanic stage.