Re-equilibration history and P–T path of eclogites from Variscan Sardinia, Italy: a case study from the medium-grade metamorphic complex

Retrogressed eclogites are hosted within the Variscan low- to medium-grade metamorphic complex near Giuncana, north-central Sardinia. These rocks are medium to fine grained with garnet and amphibole as the most abundant mineral phases along with clinopyroxene, plagioclase, quartz, biotite, chlorite, epidote, ilmenite, rutile and titanite. Four stages of mineralogical re-equilibration have been distinguished. The stage I is characterized by the occurrence of omphacite, epidote, quartz, amphibole, rutile and ilmenite in garnet poikiloblasts. The stage II is characterized by two types of symplectitic microstructures: (1) amphibole + quartz symplectite and (2) clinopyroxene + plagioclase +/- A amphibole symplectite. The first symplectite type replaces omphacite included in garnet, whereas the second one is widespread in the matrix. Biotite droplets and/or lamellae intimately growing with fine-grained plagioclase resemble biotite + plagioclase symplectite after phengite. The stage III is characterized by the widespread formation of amphibole: (1) as zoned porphyroblasts in the matrix, (2) as corona-type microstructure replacing garnet. Subordinate plagioclase (oligoclase) is also present in the amphibole corona. The stage IV is characterized by the local formation of biotite replacing garnet, actinolite, chlorite, albite and titanite. P-T pseudosections calculated with Perple_X give P-T conditions 580 < T < 660 A degrees C, 1.3 < P < 1.8 GPa for the stage I. After the stage I, pressure decrease and temperature increase led to the breakdown of omphacite with the formation of clinopyroxene + plagioclase +/- A amphibole symplectite at similar to 1.25-1.40 GPa and 650-710 A degrees C (stage II). P-T conditions of the amphibolite-facies stage III have been defined at 600-670 A degrees C, P = 0.65-0.95 GPa. P-T conditions of the latest stage IV are in the range of greenschist facies. The P-T path of the retrogressed eclogite hosted in the medium-grade micaschist and paragneiss of Giuncana recalls the P-T trajectory of retrogressed eclogite hosted in the migmatite complex of northern Sardinia. The eclogites from Giuncana do not preserve the prograde segment of the P-T path, but, similarly to the other Sardinian eclogites, they record a slight increase in temperature, though slighter, during exhumation. This suggests that the thermal flow responsible for the amphibolitic-granulitic event post-dating the eclogitic stage recorded in the Giuncana eclogite may be tentatively referred to the slab break-off also responsible for the production of the Mg-K igneous suite in Corsica.