Metabasites with eclogite facies relics have been described at Punta de li Tulchi, Costa Smeralda, Anglona and Golfo Aranci (Figure 3). The origin and age of these rocks are still controversial and highly debated in literature. In particular, except for Cappelli et al. (1992), who suggest an age of 957 ± 93 Ma, U-Pb zircon data (Table 1) point to protolith ages of about 460 Ma (Cortesogno et al., 2004; Palmeri et al., 2004; Giacomini et al., 2005a).
The metabasites with eclogite relics at Punta de li Tulchi consist of brownish to greenish layers, from a few cm up to 50 cm thick. Two main lithotypes have been distinguished by Franceschelli et al. (1998): garnet-pyroxene-rich and amphibole-plagioclase-rich layers (Figure 10a). The complex evolution of the eclogite-bearing rocks may be summarised as follows:
Figure 10. Field photography
A: Alternation of garnet-pyroxene (GP) layers and amphibole-plagioclase (AP) layers in the eclogite outcrop at Punta de li Tulchi (NE Sardinia).
B: Diopsidic clinopyroxene and Na-plagioclase symplectite resulting from destabilisation of omphacites; in the upper and lower left corners, note the kelyphytic structure around garnet. One polar.
Pre-eclogite stage: The occurrence of euhedral tschermakite and zoisite in the garnet core might be interpreted as a pre-eclogite amphibolite-epidote stage, as suggested by Chang Whan Oh and Liou (1990). Amphibole composition plotting in the field of medium pressure amphiboles supports this hypothesis.
Eclogite stage: Maximum jadeite content in omphacitic pyroxene is about 30%; reasonable assumptions led Franceschelli et al. (1998) to hypothesize an original content of 50% jadeite. Syn-eclogite garnet was probably almandine-rich (55-60%), with 15-20% pyrope and 20-25% grossularite content. The mineral assemblage of the eclogite stage was probably garnet, omphacite, rutile, zoisite and quartz.
Granulite stage: The destabilisation of eclogite stage minerals is documented by the formation of sodic plagioclase-clinopyroxene symplectites (Figure 10b), and the crystallisation of orthopyroxene, calcic plagioclase and new garnet. Probable mineral paragenesis at the granulite stage includes garnet, Na- and Ca-rich plagioclases, clinopyroxene and orthopyroxene as major minerals.
Amphibolite stage: The amphibolitisation of the granulite assemblage during exhumation of the chain led to the replacement of orthopyroxene by cummingtonite and the formation of coronitic rims of amphibole and plagioclase around garnet, plagioclase and clinopyroxene-plagioclase symplectites.
Greenschist stage: The mineral assemblage is: actinolite, chlorite, epidote ss., REE epidote and titanite, replacing garnet, clinopyroxene, hornblende and biotite.
In a recent paper, Cortesogno et al. (2004) have distinguished the eclogites enclosed in the Migmatite Complex, eclogites A, from those embedded in a medium-grade, mostly metapelitic Gneiss Complex, eclogites B. Metabasites with relics of eclogites A appear in Costa Smeralda (NE Sardinia) as rotated boudins within andalusite-cordierite-bearing metatexites or within diatexites derived from ortho- and para-gneisses showing a sillimanite +K-feldspar-bearing assemblage. Hectometric metabasite slices with eclogite B relics occur within staurolite + garnet + kyanite-bearing micaschists and paragneisses in the Anglona region, within the Posada-Asinara shear zone. Omphacite is preserved only as armoured relics in garnet; its former presence is suggested by symplectite pseudomorphs that have completely replaced the originally subhedral isoriented omphacite crystals. The decompression event caused the breakdown of omphacite into symplectite aggregates, the growth of kelyphytic rims and finally the attainment of retrograde amphibolite facies P-T conditions, with patchy growth of fine-grained to acicular amphibole. A slightly different framework is proposed by Giacomini et al. (2005a) for the banded amphibolites and amphibolitised eclogites embedded within the Golfo Aranci metamorphic basement (NE Sardinia, north of Olbia). The banded amphibolites are foliated and layered rocks with variable proportions of amphibole and plagioclase. The amphibolitised eclogites consist of clinopyroxene-garnet-amphibole rocks with compositional layering defined by alternating garnet-rich and garnet-poor bands. Eclogite relics are more frequent in garnet-rich layers and mainly represented by omphacite relics surrounded by clinopyroxene-plagioclase symplectite and, towards the garnet grains, by a rim of bluish-green amphibole. The most important novelty with respect to Punta de li Tulchi eclogites is the appearance of an inclusion-free kyanite displaying composite coronitic texture. The latter texture consists of outer coarse-grained plagioclase + clinopyroxene + orthopyroxene symplectite, an intermediate polycrystalline plagioclase domain and an inner symplectitic rim made up of sapphirine + anorthite ± corundum ± spinel passing to spinel + anorthite ± corundum in the innermost part, in contact with relict kyanite. Garnet-poor layers rarely show eclogitic relics and consist mainly of clinopyroxene-plagioclase symplectites, quartz and scattered garnet porphyroblasts. In extreme cases, the amphibolitisation process completely transformed the original rock into an aggregate of amphibole, plagioclase and quartz ± ilmenite, with few surviving relics of clinopyroxene-plagioclase symplectites. Sapphirine is a highly-peraluminous variety similar to that found by Godard and Mabit (1998) in the kyanite-bearing eclogites at Pays de Léon, France. For the final amphibolitisation stage, Giacomini et al. (2005a) hypothesize that zoisite breakdown provided a limited amount of water, but that major amounts of fluids were provided by metapelite dehydration processes.