P–T conditions in mylonitic gneiss from Posada Shear Zone, NE Sardinia

The Posada Valley shear zone is part of a regional-scale mylonitic belt that runs from Posada (NE Sardinia) to Asinara Island (NW Sardinia) separating the Variscan Medium Grade Metamorphic Complex to the south from the Migmatite Complex to the north. In the Posada Valley, a dextral top–to–the SE shear belt developing ductile and brittle–ductile D2 mylonites has been recognized. In southern Gallura the dextral shear movement follows a sinistral top–to–the NW shear belt, coeval to the initial D2 post–collisional phase. At Punta Orvili, a few kilometers north from Posada village, a sequence of mylonitic gneiss with subordinate calc–silicate nodules and metabasite lenses crop out. The main foliation at the mesoscale is the S2 schistosity striking N 60–80°and dipping 30°–50° SE. Locally, the S2 schistosity is affected by centimetre size, sinistral strike–slip greenschist shear zones, striking N 30° and dipping 60° SE. Three mineral lineations have been recognized on the S2 schistosity: a feldspar+quartz lineation trending N30°–50° and plunging 20°–30° SW; a quartz lineation trending N 40°, plunging 30° SW; a biotite±chlorite lineation trending N 20° and plunging 15°–30° SW. The mylonitic gneiss consists of quartz, plagioclase, K-feldspar, biotite, white mica, fibrolite, ± garnet. Centimetric garnet porphyroblasts, locally observed, are Alm58–68, Pyr8–12, Grs2–3, Sps16–31. The mylonitic gneiss is characterized by the widespread occurrence of millimetre– to centimetre–sized nodules enveloped by the S2 foliation. The nodules occur in four textural types: (i) quartz–rich, (ii) K-feldspar–rich, (iii) plagioclase–rich, and (iv) sillimanite–rich nodules. The K-feldspar–rich nodules are made up of polygonal aggregates of submillimetric K-feldspar crystals or by elongated, anhedral K-feldspars up to 5mm in length associated with smaller quartz–feldspathic grains. Perthite exsolutions and very thin albite rims are often observed in K-feldspar. The plagioclase–rich nodules consist of plagioclase (An20) aggregates with subordinate quartz and K-feldspar. The feldspar rich–nodules probably document an incipient melting affecting the rock prior of the mylonitic deformation. Application of the garnet–biotite geothermometer and GASP geobarometer to some selected mylonitic gneiss yielded P–T conditions of P ~ 6 kbar, T = 540–620°C. These P–T conditions most likely refers to the development of the D2 deformation, i.e. to the development of the sinistral top–to–the NW shear deformation. In conclusion, the Posada shear zone is first characterized by a medium– to high–grade sinistral shear movement followed by dextral shear movement as shown by Carosi et al. (2012) in southern Gallura.