Late Eocene lawsonite-eclogite facies metasomatism of a granulite sliver associated to ophiolites in Alpine Corsica

Abstract

The metamorphic history of two samples of continental slivers from Alpine Corsica is reconstructed on the basis of detailed petrology, trace and major element geochemistry of the bulk rock and of metamorphic minerals, and U-Pb dating of zircon.A lawsonite fels with a Ca-rich and Si, Na, K-poor bulk composition, and a garnet-bearing schist were investigated. They both exhibit strongly zoned garnet crystals characterised by cores with high Mg# (0.25-0.33), low grossular component (0.03-0.09) and REE patterns with a negative Eu anomaly. U-Pb dating of zircons from the lawsonite fels yielded scattered inherited core ages from 912 to 575. Ma and a first metamorphic rim that is dated at 292.5 ± 3.3. Ma. Ti-in zircon thermometry, the trace element pattern of the zircon and the composition of the garnet cores indicate that both samples experienced Permian upper amphibolite- to granulite-facies metamorphism. These high-grade metamorphic rocks are associated to serpentinites, metabasalts and metasediments that are typical of the Alpine Tethys, indicating that the continental slivers were incorporated into an ocean-continent transition during rifting.Alpine eclogite-facies metamorphism is characterised by low temperature conditions as demonstrated by lawsonite crystallisation in both samples. The lawsonite fels preserved the most complex history with two stages of garnet growth corresponding to a first prograde path, possibly in the lawsonite-blueschist facies, and a second stage at lawsonite-eclogite facies conditions. This second stage was marked by strong Ca-metasomatism that is responsible for the peculiar composition of the sample and the massive crystallisation of lawsonite. The strong metasomatism is also responsible for the partial recrystallisation of zircon and the formation of Alpine rims. The age of these zircon rims (34.4 ± 0.8. Ma) represents the first U-Pb age for Alpine high-pressure metamorphism in Corsica.