Timing of HP metamorphism in the Schistes Lustrés of Alpine Corsica: New Lu–Hf garnet and lawsonite ages

Abstract

Alpine Corsica is crucial in understanding Western Mediterranean geodynamics. Available paleotectonic models attribute the high-pressure terrains of Corsica to the Western Alps (east dipping subduction), or to the Apennines (west-dipping subduction) or to a flip from one to the other subduction zone. Constraints on the timing of high-pressure metamorphism in Alpine Corsica are sparse and ages range from Late Cretaceous to Late Eocene. This represents a major point of debate. Here we present high-precision garnet Lu–Hf ages for three samples from the highest grade, Lawsonite-eclogite unit yielding ages of 34.23±1.6Ma; 34.11±0.89Ma, 34.65±0.55Ma. In addition to the well-established garnet dating, we employed lawsonite Lu–Hf analyses on a lawsonite-bearing (garnet-free) metagabbro from the neighboring lawsonite-blueschist unit that yielded an age of 37.5±1.3Ma. Because metamorphic temperatures of the eclogite-facies samples (ca. 500–550°C) are below the Lu–Hf closure temperature in garnet and since the eclogitic garnet displays a core-to-rim increase in Mn concentrations typical of prograde zoning, we attribute the above Lu–Hf ages to prograde subduction metamorphism. Similarly, the age of lawsonite in the blueschist-facies unit can be safely referred to prograde-peak conditions based on the available clockwise metamorphic path. The slightly older age of the lawsonite-blueschist sample suggests a diachronous timing in the achievement of high-pressure conditions in the eclogite and in the blueschist facies units. Since both units are thought to derive from a former ocean–continent transition zone, our geochronological data provide precise time constraints to the subduction history of Alpine Corsica in the frame of the Western Mediterranean geodynamics. Our data provide Late Eocene ages for the subduction event in Corsica, which are significantly younger than previously published Cretaceous ages. This calls for partial revision of the current evolutionary models on the Corsican segment of the Alpine belt.