196,002 research outputs found
Multilayer corona textures in the ultramafic amphibolite of Montiggiu Nieddu, NE Sardinia, Italy
No full textA lenticular-shaped, 40 m thick body of ultramafic amphibolite is hosted in a km-sized metabasite lens at Montiggiu Nieddu, 8 km NE of the town of Olbia, in the Variscan Migmatite Complex of NE Sardinia. This amphibolite outcrops as a dark-green to black mass and preserves relics of igneous minerals (mostly olivine and plagioclase). On the basis of modal contents and microstructures, three main compositional layers were distinguished (layers A, B, C; Scodina et al., 2018 and references therein). Layer B is characterized by the occurrence of impressive coronitic microtextures growing around the aforementioned igneous relics. Millimetre-sized olivine is surrounded by a thin layer of orthopyroxene followed by a discontinuous layer of clinopyroxene. Plagioclase is enveloped by a discontinuous thin layer of spinel and/or a symplectite of spinel + Al-rich clinopyroxene. This layer is surrounded by a thick garnet layer which is separated from the clinopyroxene + spinel symplectite by corundum lamellae. Coronitic garnet also contains spinel and/or corundum lamellae, as well as a spinel + Al-clinopyroxene symplectite. All these coronitic microstructures are overgrown by matrix amphibole and amphibole + spinel symplectite. The matrix amphibole is locally replaced by actinolite. The metamorphic evolution of the coronitic textures in layer B can be ascribed to three stages that followed an igneous stage represented by relics of olivine, plagioclase, clinopyroxene and orthopyroxene: Stage I is documented by the formation of coronitic orthopyroxene, clinopyroxene, spinel + Al-rich clinopyroxene symplectite, corundum and finally garnet from the igneous phases. The corona minerals are arranged according to low diffusion rates of Al and Ca, because minerals with high Al contents (spinel) formed close to plagioclase, whereas minerals with no, or little, Al and Ca contents (orthopyroxene) grew only in contact with olivine. Stage II is represented by the growth of brown and green amphibole, along or associated to symplectitic spinel, replacing the corona minerals. Stage III is documented by the local growth of late phases such as actinolite, chlorite and epidote. Applying conventional geothermobarometry and P-T pseudosection modelling, we constrained the P-T conditions of all stages. The ultramafic amphibolite experienced an anticlockwise P-T path, characterized by an igneous stage at 780 - 850 C and 0.2 – 0.6 GPa and subsequent cooling and increasing pressure to granulite facies conditions of 650 - 730 C and 1.0 - 1.4 GPa. Decompression towards amphibolite and greenschist facies conditions followed.
Scodina M., Cruciani G., Franceschelli M., Massonne H.-J. (2018). Anticlockwise P-T evolution of amphibolites from NE Sardinia, Italy: geodynamic implications for the tectonic evolution of the Variscan Corsica-Sardinia block. Lithos, 324-325, 763-775
Geology of the Montigiu Nieddu metamorphic basement, NE Sardinia (Italy)
No full textA geological map at 1:10000 scale of the metamorphic rock units cropping out in Montigiu Nieddu area (NE Sardinia) is presented. These metamorphic units belong to the High Grade Metamorphic Complex in the Inner Zone of the Variscan chain of Sardinia. The rocks include different types of migmatite, amphibolite, and retrogressed eclogites. The geological map and cross section show the complex tectonic and metamorphic setting of the area, whereas the metamorphic assemblages and deformation framework represent a valuable tool for the reconstruction of the P-T metamorphic evolution for the crustal sectors involved in the Variscan orogeny. The metamorphic rock units reported in the map derived from the lower and upper continental plates involved in the Variscan collision and were tectonically juxtaposed during the early Carboniferous exhumation stage
Anticlockwise P-T evolution of amphibolites from NE Sardinia, Italy: Geodynamic implications for the tectonic evolution of the Variscan Corsica-Sardinia block
No full textIn 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
Occurrence of foliated leucocratic bodies within Migmatite Complex of NE Sardinia: preliminary data
No full text
Geochemical constraints on petrogenesis of Ca-rich leucosomes from Porto Ottiolu migmatites (NE Sardinia)
No full textNature 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
No full textU–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
PETROGRAPHIC FEATURES OF FOLIATED LEUCOCRATIC BODY IN THE MIGMATITE COMPLEX FROM NORHEASTERN SARDINIA
No full textEarly stage P–T metamorphic evolution of retrogressed amphibolites from NE Sardinia, Italy
No full textThe Golfo Aranci area belongs to the Migmatite Complex of the Inner Zone of the Variscan Sardinian metamorphic basement. In this area, located a few kilometres north of the town of Olbia, a large lensoid amphibolite body, 2 km long in NE-SW direction and 100-150 m wide, crops out. Within this body two main lithologies can be distinguished: retrogressed amphibolites and ultramafic amphibolites. The retrogressed amphibolites are coarse-grained, dark-green rocks with a schistose to weakly massive aspect. Within these amphibolites centimetric-sized layers locally occur which are featured by millimetric porphyroblastic garnet. These layers, oriented parallel to the regional schistosity, consist of millimetric (up to 1 cm) euhedral and subhedral garnet porphyroblasts in a matrix of green amphibole, plagioclase, quartz and aggregates with a clinopyroxene + plagioclase fine-grained symplectite-type texture. Garnet porphyroblasts, which can reach 30% vol. in these layers, contain a large amount of inclusions of amphibole, plagioclase, quartz and rare clinopyroxene and show a thin coronitic rim made up of plagioclase and amphibole. Amphibole inside this rim and in the matrix can be zoned with a retrograde phase (actinolite) at its margin. Garnet porphyroblasts are almandine rich (56–59 mol%) and spessartine poor (1–7 mol%), with intermediate grossular (26–28 mol%) and pyrope (10–16 mol%) contents. From core to rim pyrope progressively increases from 10 to 16 mol%, spessartine contents decrease from 7 to 1 mol%. We calculated a P–T path applying pseudosection modelling in the NCKFMASHO+Ti+Mn system to garnet bearing layers in retrogressed amphibolites. Pseudosections related to the garnet rim were corrected for fractionation of elements in garnet. The garnet core grew at granulite facies conditions of T = 680–720°C and P = 0.7–1.0 GPa. These conditions fit the XNa ratio of 0.07 in clinopyroxene and the XCa ratio of 0.35 in plagioclase included in garnet well. The garnet rim grew at T = 680–700°C and P = 1.3–1.4 GPa. These conditions are compatible with the XCa ratio of 0.16 in plagioclase of the matrix, which likely represents the beginning of decompression after the end of garnet growth. The P–T pseudosection modelling for the retrogressed amphibolites demonstrates an anti-clockwise P–T path from the granulite- to the high-pressure granulite-facies with a pressure increase of 0.3–0.7 GPa and a slight decrease in temperature. It is likely that garnet porphyroblasts stopped their growth after reaching the
peak pressure and, thus, before the decompression characterized by the growth of amphibole. We can assume that after this decompression phase, the retrogressed amphibolites likely followed the same metamorphic retrograde path as the adjacent ultramafic amphibolites during further exumation
P–T conditions in mylonitic gneiss from Posada Shear Zone, NE Sardinia
No full textThe 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
- …
