METABASITES WITH ECLOGITE FACIES RELICS FROM VARISCIDES IN SARDINIA, ITALY: A REVIEW

Abstract

Metabasites with eclogite facies relics occur in northern Sardinia as massive to strongly foliated lenses or boudins embedded within low- to medium-grade rocks (Anglona) and migmatites ( NE Sardinia). U-Pb zircon dating yielded 453 +/- 14, 457 +/- 2 and 460 +/- 5 Ma as the protolith ages; 400 +/- 10 and 403 +/- 4 Ma have been interpreted as the ages of the HP event and 352 +/- 3 and 327 +/- 7 Ma as the ages of the main Variscan retrograde events. A pre-eclogite stage is documented by the occurrence of tschermakite, zoisite relics within garnet porphyroblasts (Punta de li Tulchi) and an edenite-andesine inclusion within a relict kyanite porphyroblast (Golfo Aranci). Four main metamorphic stages have been distinguished in the eclogite evolution: ( 1) eclogite stage, revealed by the occurrence of armoured omphacite relics within garnet porphyroblasts. The Golfo Aranci eclogites also include kyanite, Mg-rich garnet and pargasite; ( 2) granulite stage, producing orthopyroxene and clinopyroxene plagioclase symplectites replacing omphacite. At Golfo Aranci, the symplectitic rims around relict kyanite consist of sapphirine, anorthite, corundum and spinel; ( 3) amphibolite stage, leading to the formation of amphibole-plagioclase kelyphites between garnet porphyroblasts and pyroxene - plagioclase symplectites and to the growth of cummingtonite on orthopyroxene.