Le pointement de péridotite à grenat-spinelle de La Croix-Valmer (Maures centrales): un cumulat d'affinité océanique impliqué dans la subduction éohercynienne ?

Abstract

A garnet-spinel metaperidotite body has been found in the leptyno-amphibolitic complex of the central pan of the Maures Massif (Hercynian Belt, south eastern France). Based on bulk rock and mineral chemistry, a mantle tectonite origin can be ruled out for this rock. Instead, it appears to be an igneous cumulate that crystallized at low P, in the olivine-plagioclase stability field, and was later brought down to mantle depths (P = 16–18 kbar, T = 850–860 °C). Although Sm-Nd isotopes preclude a strictly comagmatic origin, the clear oceanic affinity of neighbouring metagabbros (ε Nd 500 + 7.7 and + 8.7) suggests a very evolved extensional tectonic setting, having led to the break-up of the continental lithosphere and the formation of a rifted margin. The high pressure metamorphic overprint recorded by the peridotite is interpreted as reflecting the subduction of a passive margin, at the final stage of consumption of an oceanic domain. This event probably occurred in Early Hercynian times, as documented elsewhere in the west european Hercynides.