Anticlockwise P-T evolution of amphibolites from NE Sardinia, Italy: Geodynamic implications for the tectonic evolution of the Variscan Corsica-Sardinia block

In the Migmatite Complex from NE Sardinia, a large lensoid body of coarse-grained, dark-green amphibolite with a schistose to weakly massive aspect crops out. Within this amphibolite centimetre-sized layers locally occur which contain millimetric porphyroblastic garnet. We investigated the amphibolite and the layers applying microstructural analyses and thermodynamic modelling in the NCKFMASH+Ti+Mn system in order to reconstruct the pressure-temperature (P-T) metamorphic evolution. The amphibolite underwent a burial path, recorded by the compositional zoning of garnet, that started at pressures of 0.8 GPa and showed only a slight increase in temperature leading to peak P-T conditions. The garnet rim records peak P-T conditions of 1.3-1.4 GPa at 690-740 °C. As the early exhumation of the amphibolites occurred already at lower temperatures than the burial, an anticlockwise P-T path results which is in contrast to the typical clockwise P-T paths reported for several high-pressure metamorphic rocks from NE Sardinia. We interpret the anti-clockwise path by the location of the studied rocks in the lowermost part of the upper plate and their burial to depths of around 45 km during the Variscan continental collision between Laurussia and Gondwana. This process could have affected some rock slices of the upper plate only owing to tectonic erosion by the downgoing plate. The subsequent uplift occurred in an exhumation channel where these slices were continuously cooled by the upper portion of the lower continental plate

Nature and age of pre-Variscan eclogite protoliths from the Low- to Medium-Grade Metamorphic Complex of north–central Sardinia (Italy) and comparisons with coeval Sardinian eclogites in the northern Gondwana context

U–Pb zircon data on retrogressed eclogites sampled in the Giuncana locality from the Sardinian Medium-Grade Metamorphic Complex yielded a weighted age of 454 ± 6 Ma. This is in agreement with U–Pb zircon ages of 453–460 Ma obtained from eclogites from the High-Grade Metamorphic Complex. The Giuncana eclogites are very similar to the other well-known Sardinian eclogites. All of the Sardinian eclogites show positive K, Rb, Ba, U and Pb anomalies and negative Nb, La, Ce and Sr anomalies. Th is depleted in the Giuncana eclogites and enriched in those from Punta de Li Tulchi and Punta Tittinosu. All these data reveal clear crustal contamination of the Sardinian Ordovician mantle. REE patterns typical of normal mid-ocean ridge basalt (N-MORB) characterize all of the Sardinian eclogites. The supply of crustal and calc-alkaline materials to the Sardinian mantle during the Ordovician is further confirmed by the fact that most Sardinian eclogites plot on the left side and well above the mantle array in a Th/Yb v. Nb/Yb diagram. In the general Variscan framework of northern Gondwana, the Sardinian eclogites are witness to the most recent back-arc basins generated by the northward opening of the Rheic Ocean

Geothermobarometry of Al-silicate-bearing migmatites from the Variscan chain of NE Sardinia, Italy: a P-T pseudosection approach

This paper investigates Al-silicate-bearing migmatite from NE Sardinia by using the P-T pseudosection approach with the aim to determine the P-T conditions of partial melting and those of melt crystallization. P-T pseudosections were calculated in the NCKFMASH system within the P-T range 500-800°C, 0.1-1.5 GPa by using the average compositions of metapelitic greywacke, average mesosome and average trondhjemitic leucosome, respectively. The P-T pseudosections calculated for the average metapelitic greywacke and for the average mesosome, contoured for melt volume %, Si/Al and Na/K molar ratios in melt point to P–T conditions 700-740°C, 1.1-1.3 GPa which are indicative of partial melting. The P-T pseudosection calculated for the average composition of trondhjemitic leucosomes, contoured for kyanite and biotite modal content and for XMg ratio in biotite indicates P-T conditions of 660-730°C, 0.75-0.90 GPa for the crystallization of the melt. The comparison between the Na/K and Si/Al ratios in leucosomes, and the same ratios modeled for the anatectic melt by an haplogranitic melt model is thus a powerful tool for the reconstruction of P-T conditions of partial melting also in pelitic rocks, provided that leucosomes represent pure melts and are not contaminated by restitic phases or feldspar cumulates

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

Petrographic features of foliated leucocratic body in the Migmatite Complex from northeastern Sardinia

At Punta Bados, a few kilometers north of Olbia city, an elongated foliated leucocratic body striking N160°-170°, characterized by coarse-grained garnets up to 4-5 cm in diameter is hosted in a sequence of igneous and sedimentary derived migmatite and fibrolite-rich metapelites with minor metabasite and metaultramafics (Cruciani et al., 2002, 2008). The leucocratic body shows an alternation between poorly and moderately foliated domains, as well as a great variability in garnet size and abundance. Rock grain size is strongly variable reaching up, locally, a pegmatitic texture. The contact between the leucocratic body and the metapelites is marked by a 30 cm-thick cataclastic band with S-C structures and kinematic indicators suggesting a dextral shear component. It mainly consists of quartz, plagioclase, microcline, muscovite, biotite and garnet, with zircon, apatite and tourmaline as accessory minerals. Plagioclase is mainly unzoned albite (An~5). Garnet is unzoned to poorly zoned with composition Alm75, Pyr10, Sps15, Grs<1mol.%. It is free from inclusions and/or from other microstructural relics. Muscovite occurs as centimetre-sized crystals with Si content near to 6.15 a.p.f.u. and XNa = 0.10. The whole-rock composition of a few representative samples shows following major element content ranges: SiO2: 72.8-74.7; Al2O3: 15.0- 15.4; Fe2O3tot: 0.5-0.7; MnO: 0.05-0.08; MgO: 0.1-0.7; CaO: 0.4-2.0; Na2O: 4-5; K2O: 3.2-4.8 wt.%. The rocks are peraluminous with normative composition matching those of granite/trondhjemite. Preliminary data suggest that barium ranges from 71 to 634 ppm, whereas Rb and Sr vary between 36-252 and 54-327 ppm, respectively. Rare earth element (REE) content of the leucocratic body is ~50 ppm with moderately fractionated pattern for light and medium REE, and flat for heavy REE (CeN/YbN ~1.5, LaN/SmN ~1.5, GdN/YbN ~1.0) with no Eu anomaly. Field geological data and petrographic features indicate that the leucocratic felsic body of Punta Bados resembles the foliated leucogranites described in Golfo Aranci and Arzachena areas (northern Sardinia) interpreted as anatectic bodies emplaced during the late deformative event related to the exhumation of the metamorphic basament