Colle del Baracun coesite occurrence, Bobbio Pellice, Metropolitan City of Turin, Piedmont, Italyi

Regional Level Types
Colle del Baracun coesite occurrence	Pass
Bobbio Pellice	Municipality
Metropolitan City of Turin	Metropolitan City
Piedmont	Region
Italy	- not defined -

Occurrence of coesite-bearing garnet micaschist belonging to the oceanic metasedimentary succession of the Lago Superiore Unit, Monviso meta-ophiolite complex.

The coesite-bearing garnet micaschist crops out within metre-thick discontinuous metapelitic levels showing different textural features, from fine-grained to conglomeratic terms. This rocks are included in a wedge-shaped stratigraphic unit, mainly composed of calc-schist. The calc-schist is mostly made of carbonates, quartz, and white mica, with subordinate textural relics of lawsonite, and it hosts lenticular intercalations, up to metres thick, of clast- to matrix-supported metabreccia with clasts of gabbroic material, which is laterally gradational into a metasandstone unit.

Intact coesite single crystals (10-60 μm) are hosted by garnets (Ghignone et al., 2023). According to the authors, the coesite-bearing garnet micaschist experienced metamorphic peak conditions at P=2.8-2.9 GPa and T=500-520°C, within the coesite stability field. Several mineral inclusions are present in the garnets. Besides non-re-equilibrated coesite and polycrystalline quartz after coesite, inclusions of amorphous carbon, rutile, zircon, and chloritoid have been observed. Garnets are zoned and their composition varies from Alm47-56 Sps32-43 Prp3-5 Grs3-6 in the core to Alm57-74 Sps7-32 Prp4-12 Grs2-6 in the mantle and Alm48-66 Sps16-23 Prp1-9 Grs2-3 s at the rim.

The lithotype is characterized by a pervasive foliation, mainly defined by white mica, wrapping garnet porphyroblasts. The eclogitic assemblage consists of coesite (<1%), garnet (~10%), phengite (~40%), chloritoid (~1%), and accessory rutile (~6%). Muscovite + paragonite ± biotite and albite + muscovite + paragonite lozenge-shaped pseudomorphs (<1%) after glaucophane and jadeite locally occur in fractured garnets. A retrograde mineral assemblage is represented by muscovite (~20%), albite (~10%), chlorite (~4%), biotite (<1%), and accessory titanite (<1%). In addition, minor amounts of amorphous carbon, quartz, apatite, zircon, allanite, and opaque iron oxides are observed. The larger garnet porphyroblasts (1-3 mm) hold a microfolded relict foliation defined by amorphous carbon, chloritoid and minor white mica. Potassium white mica is the most abundant mineral in the studied material, showing a strong zoning in phengite (Si = 3.40–3.65 apfu) and muscovite (Si = 3.17–3.33 apfu). Chloritoid occurs as 100–200 μm sized inclusions within garnet, and is absent in the matrix. Chlorite and biotite replace garnet along cracks and rims, but also locally occur in the matrix. Together with paragonite, they also occur within lozenge-shaped pseudomorphs after glaucophane (muscovite + paragonite ± biotite) and jadeite (paragonite + muscovite + albite). Albite is quite abundant in the rock matrix, replacing former paragonite-rich domains. Quartz is arely present in the rock matrix (~1%). Conversely, quartz and coesite are relatively abundant as inclusions in the garnet.

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References