The Chamrousse ophiolite (Western Alps): a window of an oceanic stage at the onset of the Variscan orogeny

The Chamrousse ophiolite in the External Crystalline Massifs (Western Alps) has long been considered one of the oldest and best-preserved Variscan ophiolites (496±6 Ma; Menot et al., 1988). However, new in situ U–Pb zircon geochronology challenges the existence of a Cambro-Ordovician Ocean at Chamrousse.

Zircon from metabasite and metatrondjhemite unit, previously interpreted as the ophiolite volcanic layer, yields Cambro-Ordovician ages (460–520 Ma). The occurrence of few Proterozoic inherited grains and the trace element composition of zircon suggest a continental setting. In contrast, zircon from ultramafic, gabbro, and basaltic dike samples indicates a Devono-Carboniferous magmatic pulse (350–360 Ma). Their oceanic chemical signature suggests this age is that of ophiolite.

The Cambro-Ordovician base of the Chamrousse complex formed in a continental rifting environment within the northern Gondwana margin. The ophiolite itself likely represents a marginal basin developed in a back-arc setting, contemporaneously with other Devono-Carboniferous ophiolites of the Variscan belt.