P-T conditions of garnet-staurolite-bearing schists from Variscan NE Sardinia (Italy)

In north-eastern Sardinia, in the southern side of the Posada-Valley which is a km-wide shear zone of regional relevance, we collected garnet-staurolite schists with plurimillimetric garnet and staurolite porphyroblasts. At the outcrop scale the schists appears as silver-coloured, strongly foliated rocks rich in mica characterized by the occurrence of reddish to brownish garnet porphyroblasts up to 0.8-1 cm in size. Garnet crystals are enveloped by the main S2 regional schistosity which is identified by the orientation of white mica and chlorite. The garnet porphyroblasts are characterized by an inclusion-rich nucleus (garnet core) surrounded by a garnet mantle almost free of inclusions. The inclusions within the garnet crystals, which mainly consist of quartz, rutile, ilmenite, chloritoid, mica and zircon, often follow an oriented spiral-shaped and/or sigmoidal arrangement recording an earlier S1 schistosity. Some smaller (less than 0.5 cm) and/or anhedral garnet grains lack the inclusion rich nucleus. The yellowish staurolite porphyroblasts (up to 0.5 cm in size) are often characterized by opaque minerals oriented inclusions. Chloritoid is found as oriented and isolated single crystals within the rock matrix. Paragonite locally grows on chloritoid, when this latter is included in the garnet. Accessory minerals in the schists are apatite and tourmaline. EMP X-ray mapping and rim-core-rim garnet compositional profiles reveal a strong compositional zoning of the garnet components and the occurrence of a very thin (only a few dozens of μm in size) outern garnet rim, not detectable with the polarizing microscope. The garnet core composition ranges between XCa (Ca/(Ca+Mg+Fe+Mn): 0.21-0.27; XMg (Mg/(Ca+Mg+Fe+Mn): 0.01-0.02; XFe (Fe/(Ca+Mg+Fe+Mn): 0.45-0.50; XMn (Mn/(Ca+Mg+Fe+Mn): 0.25-0.30. The garnet mantle is enriched in almandine (XFe 0.55-85), pyrope (XMg 0.018-0.075) and depleted in grossular (XCa 0.06-0.18) and spessartine (XMn 0.05-0.24) components as compared to the garnet core. The narrow outern rim shows an abrupt decrease in grossular (XCa 0.02-0.03) and increase in pyrope content (XMg 0.11-0.13). Plagioclase single crystals in the matrix are oligoclase/albite (Ab ca. 90 mol.%). Staurolite porphyroblasts show XMg ratio of 0.14-0.13 in the core and 0.11-0.10 in the rim. Ti and Mn contents in staurolite is 0.1 and 0.02-0.05 a.p.f.u, respectively. Chloritoid, independently from its microstructural position (i.e. inside the garnet or in the rock matrix) shows XMg ratio of 0.12-0.13. Biotite in the matrix has XMg ratio of 0.47. White mica shows different composition based on the microstructural position: white mica included in garnet has Si (a.p.f.u.) 6.38 and XMg 0.54, white mica single crystals in the matrix shows Si 6.11 a.p.f.u. and XMg 0.47, white mica in the micaceous layers has Si 6.04 a.p.f.u. and XMg 0.40. Chlorite shows XMg  0.4. Monazite in situ dating with EMP yielded ages comprised between 417 and 322 Ma (average 377 Ma) for monazite included in garnet and 402-330 Ma (average 357 Ma) for monazite in the rock matrix. Small rutile grains preserved in garnet contain up to 0.02 wt.% zirconium, whereas the rare rutile grains in the rock matrix have undetectable zirconium content. Application of the Zr-in-rutile thermometer, following the calibration after [1], to 35 rutile grains included in garnet gives an average temperature of 557°C. Preliminary results on thermodynamic modelling suggest that the schist underwent a clockwise P-T path with a prograde portion that reached ca. 1.0 GPa peak pressure and temperature up to 600°C. The rock underwent later low-grade re-equilibration and mylonitization. References: [1] Tomkins et al. (2007) J Metamorphic Geol 25:703–713