Evidence of two metamorphic cycles preserved in garnet from felsic granulite in the southern Variscan belt of Corsica, France

Abstract

The Solenzara-Fautea unit is composed of Variscan migmatites and garnet-kyanite granulitic gneisses with pyroxenite boudins outcropping between the villages of Fautea and Solenzara in southeast Corsica. We investigated felsic granulite samples exposed at the seashore south of the village of Tarcu to improve existing pressure-temperature (P-T) estimates of the metamorphic evolution. Relictic garnet (Grt1) is overgrown by a second generation of garnet (Grt2) that also appears as small, euhedral crystals in the matrix. The P-T history of the gneiss was reconstructed on the basis of garnet composition and isochemical phase diagrams. Grt1 formed at 1.3–1.75 GPa, 650–700 °C, whereas Grt2 reflects higher temperature conditions between 750 and 800 °C at 0.9 GPa. The P-T conditions obtained for Grt2 formation are supported by the application of Zr-in-rutile thermometry yielding average temperatures of 708–719 °C at 0.9 GPa. Temperatures calculated with ternary feldspar thermometry on reintegrated feldspar compositions are in the range 745–812 °C. Although a precise P-T path could not be obtained, the microstructural evidences and new P-T estimates support two cycles on which each garnet type was formed by prograde metamorphism. We hypothesize that the felsic granulites were part of a series of metamorphic slices of the Variscan Corsica-Sardinia block which were involved into a sequence of deep crustal movements. According to existing geochronological data, the two metamorphic cycles are Variscan and occurred within the time frame 365–335 Ma.