Multi-stage metamorphic re-equilibration in eclogitic rocks from the Hercynian basement of NE Sardinia, Italy

Eclogitic rocks are hosted within gneisses and migmatites of the Hercynian basement of NE Sardinia. They are characterized by two compositional layers: garnet-pyroxene rich-layers and amphibole-plagioclase layers. The former contain structural, mineralogical and compositional relics of eclogite facies re-equilibration. Four stages of evolution have been identified: an eclogite stage, a granulite stage and a retrograde amphibolite to greenschist stage. A possible pre(?)-eclogite stage is documented by inclusions of euhedral tschermakitic amphibole + zoisite within the core of garnet. This early stage was followed by an increase in pressure under which the eclogite climax developed (T up to 700 °C, P = 13-15 kbar), as documented by omphacite inclusions towards the rim of garnet. Characteristic mineral reactions after the eclogitic stage are: omphacite → diopside + plagioclase (symplectite) and garnet → orthopyroxene + plagioclase. These reactions testify the presence of a granulite stage during which the peak of metamorphism was reached (T up to 870 °C, P ≈10 kbar). The orthopyroxene → cummingtonite + quartz and garnet + diopside → hornblende + plagioclase (kelyphite) transformations indicate extensive amphibolite retrogression (T = 550-650 °C, P = 3-7 kbar). Finally, actinolite and chlorite developed (greenschist stage) at falling temperature and pressure (T = 300-400 °C, P < 2-3 kbar).