1,738 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
Metamorphic evolution and P–T path of the Posada Valley amphibolites: new insights on the Variscan high pressure metamorphism in NE Sardinia, Italy
No full textThe Posada Valley amphibolites in the Inner Zone of Variscan Sardinia are characterized by garnet porphyroblasts containing epidote, titanite, K–feldspar, quartz, clinopyroxene, amphibole and plagioclase concentrated in the garnet core. The matrix is made up of amphibole and plagioclase and fine–grained clinopyroxene and plagioclase. The amphibolites recorded an HP stage, a metamorphic re–equilibration, and a third stage under greenschist facies. P–T conditions of stage I (530–650°C/0.9–1.3GPa for garnet core and 570–690°C/1.0–1.4GPa for garnet rim) were obtained by P–T pseudosection modelling. P–T conditions for stage 2 (T=600–700 °C/P=0.5–0.8 GPa) were obtained applying the same approach to the most retrogressed samples. The Posada Valley amphibolites underwent a cooler evolution, with a thermal peak lower than that recorded by the northernwards eclogites from the High–Grade Metamorphic Complex of Sardinia.
The proposed geodynamic scenario starts with subduction of oceanic crust under the peri–Gondwanan terrane in Upper Devonian. The Posada Valley amphibolites were subducted up to maximum depths of 50 km. Subsequently they reached the subduction channel or were detached from the slab before the eclogite–facies conditions were reached
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
Garnet zoning in kyanite-bearing eclogite from Golfo Aranci: new data on the early prograde P-T evolution in NE Sardinia, Italy
No full textKyanite-bearing eclogites from the Migmatite Complex of Golfo Aranci area, NE Sardinia, have been investigated using microstructural analyses and thermodynamic modelling in order to define their early metamorphic evolution and P-T path. The eclogites underwent a clockwise P-T path recorded by the different composition of garnet core, mantle and rim. P-T pseudosection modelling allowed to estimate P-T conditions in the range T = 580-630 °C and P = 1.5-2.00 GPa for the formation of garnet core, between T = 620-690 °C and P = 2.0-2.3 GPa for garnet mantle and within T = 650-700 °C and P = 1.4-2.1 GPa for garnet rim. The first part of the P-T path is characterised by a prograde increase in P and T in the eclogite facies that occurred between the growth of the garnet core and mantle. After the peak pressure, recorded by the garnet mantle composition, the rocks underwent moderate temperature increase and significant pressure decrease until they reach the peak temperature near to the eclogite/upper granulite facies transition, recorded by the garnet rim composition. The P-T trajectory proposed for the early stage of the metamorphic evolution of the kyanite-bearing eclogites is similar to already existing observations from the Sardinian Low Medium Grade Metamorphic Complex, and introduce new constraints on their prograde eclogite facies evolution. The role of the new prograde path is then discussed in the light of the Variscan orogen
Multilayer corona textures in the high-pressure ultrabasic amphibolite of Mt. Nieddu, NE Sardinia (Italy): equilibrium versus disequilibrium
No full textRocks with coronitic textures around igneous relics of olivine and plagioclase were sampled from the ultrabasic amphibolite of Mt. Nieddu being part of the so-called Migmatite Complex of northeast Sardinia. These textures are characterized by layers of orthopyroxene and clinopyroxene around olivine, and a symplectite of clinopyroxene+spinel and garnet around plagioclase; all these minerals were overgrown by amphibole. We applied conventional geothermobarometry and pressure-temperature (P-T)
pseudosection modelling of microdomains in order to constrain the main steps of the evolution of the sampled rocks. The igneous crystallization occurred at 0.2-0.5 GPa and minimum T of 780-850 °C. Garnet is the last coronitic phase formed at estimated
pressures of 1.3-1.7 GPa at 680-730 °C. As similar conditions were previously determined for the pressure peak on an adjacent rock, we conclude that during corona formation chemical equilibrium was reached probably over a much wider range than of mm-sized microdomains. Slow reaction kinetics is responsible for the partial preservation of the igneous phases
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
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
Geochemistry of Giuncana eclogite, North Sardinia (Italy) and comparison with coeval Sardinian eclogites
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
Garnet-rich veins in ultramafic amphibolites from NE Sardinia, Italy
No full textIn the Golfo Aranci area, located a few kilometres north of the town of Olbia, a large amphibolite lens crops out in which an ultramafic body approximately 100 m long and 50 m thick occurs. These ultramafic amphibolites are characterized by a massive to weakly-schistose structure and a medium grain size. Within this ultramafic body three main compositional layers can be distinguished. The uppermost layer (about 25 m thick) shows a dark green to black colour and is featured by a high amount of millimetric to centimetric garnet
crystals, which can be cumulated in ellipsoidal nodules (up to 15 cm in diameter) or centimetric thick veins. These veins are characterized by a well-defined structure from the margin to the centre: the boundary between the vein and the hosting amphibolites is defined by an irregular symplectitic microstructure of spinel
and amphibole, up to 2 mm thick. The inner margin domain of the vein consists of unzoned garnet (up to 3
mm in diameter) which contains inclusions of amphibole, spinel, chlorite and corundum. Garnet-rich veins locally show a central white area in which zoned garnet occurs in a matrix. This Matrix consists of two epidote species: one is represented by casually oriented elongated euhedral crystals (7.0–7.6 wt% Fe2O3) and the other by anhedral crystals with lower contents of iron (0.3–0.9 wt% Fe2O3). Within the matrix patches with sub-rounded regular forms can be also found. These patches, that likely represent a preexisting mineral according to their shape, are made of epidote, spinel, corundum and margarite. Garnet porphyroblasts in the centre of the vein are euhedral to subhedral and show a noticeable compositional zoning and contain epidote inclusions and small chlorite veins. From core to rim four zoning stages can be defined: core, mantle, inner rim and outer rim. The garnet core is almandine rich (49 mol%), with intermediate grossular (22 mol%) and pyrope (27 mol%) contents. Towards the mantle the grossular content (52 mol%) increases and the pyrope (6 mol%) and almandine (41 mol%) contents decrease. The garnet inner rim is grossular (42 mol%) and almandine (36 mol%) rich. The pyrope content in the entire rim is 22 mol%. Almandine and grossular contents in the garnet outer rim are 40 and 37 mol%, respectively). Spessartine contents remain constantly low at 1-2 mol% throughout the whole garnet. Preliminary thermodynamic modelling with pseudosections allowed us to reconstruct the P–T path segment recorded by garnet growth. P–T pseudosections were calculated in the NCKFMASHO+Ti+Mn system using the vein core as bulk-rock composition. The P–T conditions (around T = 600°C and P = 1.5 GPa) for the garnet core were obtained by using the host rock as bulk rock composition, due to the fact that mol% values for Ca, Fe and Mg in the garnet are over or under the corresponding range obtained by calculations with the vein core as bulk composition. This could mean that the garnet core grew before the vein formation. Therefore core-to-mantle P–T conditions deserve further insights and should be considered with caution. The P–T trajectory based on the compositional change of garnet from the mantle to the rim is similar to other metabasic rocks from north Sardinia: after the peak pressure the rock experienced a strong pressure decrease and a moderate temperature increase to granulite-facies conditions The rim conditions (T = 640–680°C and P = 0.8–0.9 GPa) point to a subsequent slight P–T decrease towards the amphibolite facies, in which the exhumation of the rock continued during the Variscan orogeny
Geochemical and geochronological dataset of rutile from a Variscan metabasite in Sardinia, Italy
No full textA c. 500 m wide and 1.5 km long body consisting of basic to ultrabasic rocks, metamorphosed up to granulite-facies and retrogressed to amphibolite-facies conditions during the Variscan orogeny, crops out near Olbia (NE Sardinia, Italy). Among abundant samples, one, collected from a garnet-rich centimetric layer, was chosen for a detailed analysis of rutile; chemical analyses of rutile were performed with the electron microprobe on petrographic thin sections, whereas U/Pb ages were determined by LA-ICP-MS on rutile mounted in epoxy resin. Chemical analyses show that rutile included in other minerals (Rt inc) commonly show higher SiO2 and FeO contents and lower Nb2O3 and ZrO2 contents if compared with rutile in the matrix of the garnet-rich layer (Rt mat). Cr2O3 concentrations are quite similar in both types of ru- tile. Rt mat commonly shows a greater variability in minor elements, especially Nb2O3 (0.049–0.284 wt.%) and SiO2 (0.019 - 0.193 wt.%) whereas Rt inc compositions are more homogeneous except for FeO (0.251–0.562 wt.%). The U-Pb isotopic data provided discordant ages and defined a lower intercept in the Tera-Wasserburg diagram of 273 ±13 Ma. Few compilations of geochemical and geochronological data on rutile in Variscan metabasites can be found in literature, thus these data represent a new insight on a mineral phase the significance and scientific interest of which are rising in the last years. Future studies on the origin and ages of emplacement and metamorphism (either prograde or retrograde) of this kind of rock, widespread in the Variscan chain, will benefit from these data as a term of comparison
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