U-Pb dating of magmatic zircon and metamorphic baddeleyite in the Ligurian eclogites (Voltri Massif, Western Alps)

U-Pb geochronology with ion microprobe (SHRIMP) analysis has been carried out on eclogite-facies rocks of the Beigua Unit, an ophiolitic slice of the Voltri Massif, Western Alps. The investigated samples are eclogites and high-pressure metasomatic rocks (metarodingites and centimetre-sized Ti-clinohumite-bearing dykes). Zircon contained in an eclogitic metagabbro and a metarodingite preserves magmatic zoning patterns and trace element compositions. The zircon ages of 160±1 and 161±3 Ma are interpreted to date the crystallization of the gabbroic protoliths. Ti-clinohumite dykes in the same unit contain baddeleyite crystals in textural equilibrium with Ti-clinohumite, diopside, chlorite and magnetite, which form the eclogite-facies assemblage in these rocks. Baddeleyite also contains inclusions of such minerals, indicating its formation at high pressure. The baddeleyite has cathodoluminescence intensity and chaotic patterns similar to metamorphic zircon. It contains a significant amount of Hf (1.3–1.7 wt%), traces of Ti, Y, Nb, Ta, REE, U and Th. Its chondrite-normalised trace element pattern has strong enrichment in middle REE, positive Ce-anomaly and small negative Eu-anomaly. This represents the first report of baddeleyite formed during regional metamorphism, and suggests that this mineral could (re)crystallize easier than zircon under low-temperature, high-pressure conditions. The age of the baddeleyite is interpreted as likely dating the eclogite-facies metamorphism in the Beigua Unit at 33.6±1.0 Ma. This age is very close to the Early Oligocene age of the overlying Tertiary continental breccias and conglomerates, which contains clasts of high-pressure rocks. This sedimentary record, which is unique for Alpine high-pressure units, is direct evidence of fast exhumation of the Beigua eclogites. The young age for the HP metamorphism of the Beigua ophiolite makes a revision of either the palaeogeography prior to collision, or of the subduction setting in the entire region, necessary.