The exhumation of high pressure ophiolites (Voltri Massif, Western Alps): Insights from structural and petrologic data on metagabbro bodies

Abstract

Serpentinites can enhance the exhumation of HP-rocks by lowering the bulk density of subducted material and by establishing a low-viscosity channel between the lower and upper plates; this mechanism has been proposed for a 100m-scale tectonic mélange outcropping inside the Voltri Massif (Ligurian–Piemontese Units, Western Alps, Italy). We want to test if the mechanism of the subduction channel is feasible at a larger scale, i.e. for the km-scale Voltri Massif, that consists of metaophiolites recording blueschist to eclogite facies peak conditions.We identified a study area in the eastern part of the massif made of highly sheared serpentinite which hosts lenses of metagabbros; a strong strain partitioning between host-rock and the lenses and between core and rims of the lenses occurs.We coupled P–T pseudosections, garnet–omphacite geothermometry and petrographic observations and we obtained clockwise P–T paths with almost isothermal decompression trajectory. The analyzed blueschist lens recrystallized at peak conditions of 10–15kbar and 450–500°C; the eclogitic ones reached peak metamorphic conditions ranging from about 21kbar and 450–490°C to 22–28kbar and 460–500°C; the studied bodies thus recorded different metamorphic peaks acquired at slightly different T conditions.Comparing our results with literature data, a large scatter in the metamorphic peak conditions is displayed, with peak temperatures up to 650°C and pressures ranging from 18 to 25kbar.(1) The presence of poorly deformed lenses (from m- to km-scale) of heterogeneous lithologies inside highly sheared serpentinite and metasediments country-rocks, (2) the strong strain partitioning and (3) the heterogeneous metamorphic peak conditions of the different lenses allow us to suggest that the Voltri Massif matches the requirements of a tectonic mélange; in particular the eastern sector may thus represent a “fossil” serpentinite-subduction channel, comparable to the small-scale one already described in the western sector of the Voltri Massif.