Metamorphic Minerals

Texture and Microstructure

Metamorphic Rocks

Retrocessed Eclogite (Porto Ottiolu)

Eclogite: A high P/T metamorphic rock composed of more than 75% vol. garnet (often pyrope or Mg-rich almandine) and omphacite (Na-Ca-Al-Mg clinopyroxene). Both must be present and neither component more than 75% vol. Eclogite never contains plagioclase. Rutile, kyanite, and quartz are typically present.

Porto Ottiolu (Punta de li Tulchi) eclogites

The Sardinian metamorphic chain (Fig.1) belongs to the southern part of the European Variscides. From south to north the following tectono-metamorphic zones have been distinguished within the Hercynian segment of Sardinia: the External Zone (SW Sardinia), the Nappe Zone, subdivided in External (south and central Sardinia) and Internal Nappe Zone (central-northern Sardinia), and the High Grade Metamorphic Complex (HGMC) (northern Sardinia), which is the Axial Zone of the Hercynian chain. The Internal Nappe Zone is separated from the HGMC by the Posada-Asinara shear zone which has been interpreted as a paleosuture between the Gondwana and Armorica plates. The basement of NE Sardinia consists of a suite of multideformed gneisses, various types of migmatites, orthogneisses, metabasites that preserve relics of granulite and eclogite facies assemblages, and widespread intercalations of calc-silicate rocks and rare marbles. The age of the metasedimentary sequence is unknown; an Ordovician emplacement age has been suggested for the orthogneisses and for the metabasites with eclogite facies relics.

Fig.1: Tectonic sketch map of the Variscan Belt in Sardinia. Modified from Carmignani et al., 2001.

The metabasites with eclogite relics generally occur as elongated boudins parallel to E–W regional schistosity. Except for some hectometric bodies outcropping at Golfo Aranci, Punta Tittinosu and Punta de li Tulchi, eclogite-bearing rocks have decimetric-to-metric dimensions. Among all eclogite outcrops, only that of Punta de li Tulchi still preserves its original polyphased metamorphic features. At Punta de li Tulchi (Fig.2), eclogites form a 100 m long, 20-30 m thick lens, enclosed within nebulitic migmatites. Near the main body, decimeter-sized eclogite nodules are embedded within the migmatites. Two main lithotypes have been distinguished (Fig.3): garnet-pyroxene-rich layers and amphibole-plagioclase-rich layers.

Garnet-pyroxene-rich layers show garnet porphyroblasts up to 5 mm in size hosting relict omphacitic pyroxene, amphibole, ilmenite, rutile and quartz inclusions. The matrix consists of plagioclase and clinopyroxene crystals forming a symplectitic structure. Garnet borders in contact with symplectites are surrounded by radial kelyphites.
Amphibole-plagioclase-rich layers consist of plagioclase, pale-green and brown amphibole, garnet, quartz, ilmenite, sphene, rare biotite and chlorite. The rock is made up of aggregates of plagioclase and amphibole forming "white pods". Relics of garnet within the white pods suggest that these pods could represent evolution of garnet-pyroxene-rich layers, in which kelyphytic coronas completely replace the garnet.

Both garnet-pyroxene and plagioclase-amphibole layers exhibit a slightly convex light REE pattern, a small negative Eu anomaly, and a flat heavy REE pattern. Textural data and chemical features suggest that the protolith was probably an Fe-gabbro. U-Pb dating yields an age of 410 Ma, which have been interpreted to reflect a protolith age. The following sequence of metamorphic re-equilibration has been recognised in the eclogitic rocks of Punta de Li Tulchi:

Pre-Eclogite stage (?): The association of euhedral tschermakite and zoisite included in the core of garnet is difficult to interpret: it might be the evidence of a pre-eclogitic epidote-amphibolite stage.

Eclogite stage: The eclogite stage is documented by the occurrence of inclusions of omphacitic pyroxene in garnet porphyroblasts. A probable mineral assemblage at the eclogite stage comprises garnet, omphacite, rutile, zoisite and quartz.

Granulite stage: The destabilisation of eclogite facies minerals is documented by the widespread occurrence of oligoclase-andesine plagioclase + calcic clinopyroxene symplectite, and by the formation of orthopyroxene and calcic plagioclase. The probable mineral paragenesis at the granulite stage includes garnet, calcic clinopyroxene, orthopyroxene, Ca-poor and Ca-rich plagioclase, rutile, ilmenite, zoisite and quartz.

Amphibolite stage: This stage is characterised by the widespread formation of the amphibole and plagioclase keliphytes between garnet and clinopyroxene-plagioclase symplectite. The mineral paragenesis at the amphibolite stage was hornblende, plagioclase, cummingtonite, ilmenite, titanite and biotite.

Greenschist stage: Minerals which formed at this stage are actinolite, chlorite, epidote s.s., REE-epidote and titanite. They replaced garnet, clinopyroxene, hornblende and biotite.

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Fig.2: Geological sketch map between Porto Ottiolu and Punta de li Tulchi. and A-B cross-section.

Fig.3: Punta de li Tulchi eclogites. AP = Amphibole-plagioclase-rich layers; GP = Garnet-pyroxene-rich layers.

Bibliography

• Carosi, R., Frassi, C., & Montomoli, C. The Variscan basement on Northern Sardinia (Italy): Field guide to the excursion in the Baronie region.
• Cruciani, G., Franceschelli, M., Loi, M., & Elter, F. (2001): Migmatites, eclogitic, and granulitic rocks in NE Sardinia. Geology and mineral deposits of Northern Sardinia, WRI-10 Pre-Session Field Trip Guide, 12.

Garnet crystals with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 1x (Field of view = 9mm)	Garnet crystals with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 1x (Field of view = 9mm)	Garnet crystals with plagioclase + spinel + amphibole corona, amphibole and omphacite (symplectitic mixture of diopside and albite). XPL image, 1x (Field of view = 9mm)
Garnet crystals with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 1x (Field of view = 9mm)	Garnet crystals with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 1x (Field of view = 9mm)	Garnet crystals with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 1x (Field of view = 9mm)
Garnet crystals with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 1x (Field of view = 9mm)	Garnet crystals with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 1x (Field of view = 9mm)	Garnet crystals with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 1x (Field of view = 9mm)
Garnet crystals with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 1x (Field of view = 9mm)	Garnet crystals with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 1x (Field of view = 9mm)	Garnet crystals with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 1x (Field of view = 9mm)
Garnet crystals with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 1x (Field of view = 9mm)	Garnet crystals with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 1x (Field of view = 9mm)	Garnet crystals with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 1x (Field of view = 9mm)
Garnet crystals with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 1x (Field of view = 9mm)	Garnet crystals with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 1x (Field of view = 9mm)	Garnet crystals with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 1x (Field of view = 9mm)
Garnet crystals with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 2x (Field of view = 7mm)	Garnet crystals with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 2x (Field of view = 7mm)	Garnet crystals with plagioclase + spinel + amphibole corona, amphibole and omphacite (symplectitic mixture of diopside and albite). XPL image, 2x (Field of view = 7mm)
Garnet crystals with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 2x (Field of view = 7mm)	Garnet crystals with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 2x (Field of view = 7mm)	Garnet crystals with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 2x (Field of view = 7mm)
Garnet crystals with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 2x (Field of view = 7mm)	Garnet crystal with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 10x (Field of view = 2mm)	Garnet crystal with plagioclase + spinel + amphibole corona, amphibole and omphacite (symplectitic mixture of diopside and albite). XPL image, 10x (Field of view = 2mm)
Garnet crystal with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 10x (Field of view = 2mm)	Garnet crystal with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 10x (Field of view = 2mm)	Garnet crystal with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 10x (Field of view = 2mm)
Garnet (pink), amphibole (brown) and omphacite (kelyphitic mixture of diposide and albite). PPL image, 2x (Field of view = 7mm)	Garnet (isotropic), and omphacite (kelyphitic mixture of diposide and albite). XPL image, 2x (Field of view = 7mm)	Garnet (pink), amphibole (brown) and omphacite (kelyphitic mixture of diposide and albite). PPL image, 2x (Field of view = 7mm)
Garnet (isotropic), and omphacite (kelyphitic mixture of diposide and albite). XPL image, 2x (Field of view = 7mm)	Garnet (pink), amphibole (brown) and omphacite (kelyphitic mixture of diposide and albite). PPL image, 2x (Field of view = 7mm)	Garnet (isotropic), and omphacite (kelyphitic mixture of diposide and albite). XPL image, 2x (Field of view = 7mm)
Garnet crystal with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 10x (Field of view = 2mm)	Garnet crystal with plagioclase + spinel + amphibole corona, amphibole (orange) and omphacite (symplectitic mixture of diopside and albite). XPL image, 10x (Field of view = 2mm)	Garnet crystal with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). PPL image, 10x (Field of view = 2mm)
Omphacite → kelyphitic mixture of diposide and albite. XPL image, 10x (Field of view = 2mm)	Omphacite → kelyphitic mixture of diposide and albite. PPL image, 10x (Field of view = 2mm)	Omphacite → kelyphitic mixture of diposide and albite. XPL image, 10x (Field of view = 2mm)
Omphacite → kelyphitic mixture of diposide and albite. PPL image, 10x (Field of view = 2mm)	Omphacite → kelyphitic mixture of diposide and albite. XPL image, 10x (Field of view = 2mm)	Omphacite → kelyphitic mixture of diposide and albite. XPL image, 10x (Field of view = 2mm)
Omphacite → kelyphitic mixture of diposide and albite. XPL image, 10x (Field of view = 2mm)	Omphacite → kelyphitic mixture of diposide and albite. XPL image, 10x (Field of view = 2mm)	Omphacite → kelyphitic mixture of diposide and albite. PPL image, 10x (Field of view = 2mm)
Omphacite → kelyphitic mixture of diposide and albite. XPL image, 10x (Field of view = 2mm)	Omphacite → kelyphitic mixture of diposide and albite. PPL image, 10x (Field of view = 2mm)	Omphacite → kelyphitic mixture of diposide and albite. XPL image, 10x (Field of view = 2mm)
Rutile crystal with titanite corona (enclosed in a garnet). PPL image, 10x (Field of view = 2mm)	Rutile crystal with titanite corona (enclosed in a garnet). PPL image, 10x (Field of view = 2mm)	Rutile crystal with titanite corona (enclosed in a garnet). XPL image, 10x (Field of view = 2mm)
Rutile crystal with Titanite corona (enclosed in a garnet). PPL image, 10x (Field of view = 2mm)	Rutile crystal with titanite corona (enclosed in a garnet). PPL image, 10x (Field of view = 2mm)	Rutile crystal with titanite corona (enclosed in a garnet). XPL image, 10x (Field of view = 2mm)