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 Ma, 457 2 Ma 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 Ma 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. Tschermakite to Mg-hornblende, plagioclase, cummingtonite, ilmenite, titanite and biotite are coexisting phases; 4) Greenschist to sub-Greenschist stage, defined by the appearance of actinolite, chlorite, epidote ss, titanite, sericite and prehnite. The following P-T ranges have been estimated for the different stages. Eclogite stage: 550°-700°C; 1.3-1.7 GPa; Granulite stage: 650°-900°C; 0.8-1.2 GPa, clustering in the range 1.0-1.2 GPa; Amphibolite stage: 550°-740°C; 0.3-0.7 GPa; Greenschist stage: 300°-400°C; 0.2-0.3 GPa. Comparable ranges characterise the other Variscan massifs in Europe: Eclogite stage: T=530°-800°C; P from 0.7-1.1 to 1.7 ± 0.3 GPa; Granulite stage : T=760°-870°C and P from 7.2-9.9 to 1.1-1.4 GPa, clustering around 1.0-1.2 GPa. Whole-rock chemistry: Sardinian eclogites are N- to T-MORB; European ones N- to E-MORB or calc-alkaline.
Metabasites with eclogite facies relics from Variscides in Sardinia, Italy: a review
FRANCESCHELLI, MARCELLO;CRUCIANI, GABRIELE;
2007-01-01
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 Ma, 457 2 Ma 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 Ma 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. Tschermakite to Mg-hornblende, plagioclase, cummingtonite, ilmenite, titanite and biotite are coexisting phases; 4) Greenschist to sub-Greenschist stage, defined by the appearance of actinolite, chlorite, epidote ss, titanite, sericite and prehnite. The following P-T ranges have been estimated for the different stages. Eclogite stage: 550°-700°C; 1.3-1.7 GPa; Granulite stage: 650°-900°C; 0.8-1.2 GPa, clustering in the range 1.0-1.2 GPa; Amphibolite stage: 550°-740°C; 0.3-0.7 GPa; Greenschist stage: 300°-400°C; 0.2-0.3 GPa. Comparable ranges characterise the other Variscan massifs in Europe: Eclogite stage: T=530°-800°C; P from 0.7-1.1 to 1.7 ± 0.3 GPa; Granulite stage : T=760°-870°C and P from 7.2-9.9 to 1.1-1.4 GPa, clustering around 1.0-1.2 GPa. Whole-rock chemistry: Sardinian eclogites are N- to T-MORB; European ones N- to E-MORB or calc-alkaline.
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