Search

The coexistence of chloritoid and biotite in medium-pressure Barrovian terranes is quite uncommon, and the parameters controlling their equilibrium relations are still controversial. Various studies have already investigated the influence of pressure (P), temperature (T), bulk rock (X bulk) and fluid (X fluid) compositions on the stability of this assemblage. Here we apply forward thermodynamic modelling on amphibolite-facies metapelites from the upper portion of the Lesser Himalayan Sequence (eastern Nepal Himalaya) to test which parameters mostly influence the stability of the chloritoid + biotite assemblage. P–T isochemical phase diagrams calculated in the MnNKCFMASHTO system fail in reproducing the coexistence of chloritoid and biotite, predicting biotite appearance at higher temperatures than chloritoid breakdown. Neither the fluid composition (i.e. reduced H2O activity due to the presence of CO2) nor a more oxidated state of the system favours their coexistence, while slightly H2O-undersaturated conditions expand the biotite stability field toward lower temperatures, allowing the development of the chloritoid + biotite assemblage. Kinetic factors could have further contributed to the stability of this assemblage: thermal overstepping of the chloritoid-consuming and staurolite-producing reaction, induced by the difficulty in the staurolite nucleation and/or by the sluggishness of chloritoid dissolution, could have enhanced the metastable persistence of chloritoid at temperatures compatible with the presence of biotite. Being the kinetics efficiency intrinsically linked to the degree of fluid availability, the two factors (i.e. H2O-undersaturated conditions and kinetics of the chloritoid-consuming reaction) were likely complementary rather than mutually exclusive.

Post-collisional mountain-root collapse and subsequent massive partial melting occurred in the high-temperature (HT) ultrahigh-pressure (UHP) metamorphic terrane of the North Dabie complex zone (NDZ), central China. The NDZ was deeply subducted in the Triassic, producing widespread migmatites and various magmatic intrusions in the Cretaceous. Post-collisional metadiorites with distinctive large K-feldspar augen porphyroblasts, locally reported but rarely exposed in the NDZ, underwent a complex evolutional history. In this contribution, integrated studies including field investigation, petrographic observation and mineral analysis, zircon U-Pb geochronological and Hf isotopic analyses, and whole-rock elemental and Sr-Nd-Pb isotopic analyses of the metadiorites were carried out. Our results provide new constraints on the mountain-root collapse in the Dabie orogen. The metadiorites are enriched in large ion lithophile elements and light rare earth elements, whereas they are depleted in high field strength elements and heavy rare earth elements with significant Ba positive anomalies, a composition consistent with the lower continental crust. All the studied samples have moderately enriched initial 87Sr/86Sr ratios (0.707582–0.708099), low εNd(t) values (−15.3 to −20.4), and low initial Pb isotopic ratios (16.0978–16.8452, 15.3167–15.4544, and 37.1778–37.8397 for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb, respectively). However, they have highly negative εHf(t) values and Paleoproterozoic two-stage Hf model ages, which are only partially consistent with data from the associated UHP metamorphic rocks. Such features suggest the metadiorites resulted from a magma produced by mixing of Triassic UHP mafic lithologies and minor amounts of mantle-derived materials. Zircon morphological analysis and U-Pb sensitive high-resolution ion microprobe dating combined with conventional thermobarometry indicate that these upwelling melts crystallized at pressure-temperature (P-T) conditions of 5.4–5.7 kbar and 750–768 °C at ca. 130 Ma and subsequently suffered HT metamorphism at ca. 125 Ma. We conclude that the metadiorites’ precursors were derived from partial melting of the Triassic subducted Neoproterozoic mafic lower-crustal rocks, with addition of minor amounts of mantle-derived materials in the Early Cretaceous, in response to mountain-root collapse of the orogen. Based on petrographic textures and mineral compositions, it is moreover inferred that formation of the distinctive K-feldspar porphyroblasts is likely related to a two-stage process, i.e., crystallization derived from biotite breakdown after the formation of the metadiorite at T = 640–703 °C and P < 4.5 kbar and coarsening related to shear deformation.

Calcite-bearing sediments (calcareous pelites, marls, impure limestones) are among the most investigated sources of carbon in collisional settings (e.g. Groppo et al., 2017, 2021, 2022; Rapa et al., 2017). Dolomite- and magnesite-bearing sediments, however, can also be important constituents of evaporitic sequences deposited along passive margins and involved in collisional orogenic processes. So far, decarbonation reactions in dolomite- and magnesite-bearing rocks have been rarely investigated, and their contribution to the orogenic carbon cycle substantially neglected. As a contribution to the understanding of the influence of dolomite- and magnesite-bearing lithologies on the global Earth's carbon cycle, a petrologic study was focused on the Lesser Himalayan Sequence (LHS) in central Nepal. The LHS is a thick Proterozoic sedimentary sequence originally deposited on the northern margin of the Indian plate, metamorphosed during the Himalayan orogeny. Abundant dolomite- and magnesite–bearing lithologies occur in the Upper-LHS, whose protoliths can be grouped in: (1) a dolomitic series (dolostones, dolomitic marls, dolomitic pelites), and (2) a magnesitic series (sparry magnesite ores, magnesitic pelites). The magnesite deposits associated to dolomitic lithologies are interpreted as the evidence of evaporitic environments during the Proterozoic.The schists derived from dolomitic pelites show mineral assemblages similar to those of normal metapelites, but with significant amounts of Ca-rich minerals (e.g. plagioclase) and with biotite anomalously enriched in Mg. The schists derived from magnesitic pelites are, instead, characterized by uncommon assemblages such as orthoamphibole + kyanite + garnet + phlogopite. Thermodynamic forward modelling (P/T-X(CO2) pseudosections) applied to these schists allowed to: (1) understand the nature of the main decarbonation reactions; (2) constrain the P-T conditions at which these reactions occurred, and (3) estimate the amounts of dolomite/magnesite consumed during prograde metamorphism, and the correspondent amounts of released CO2. The main results are:the observed assemblages formed during a heating decompression stage, at P-T conditions of 620 ± 20°C, 8.5 ± 0.2 kbar, consistent with those registered by the associated metapelites; the observed peak assemblages are predicted to be stable in equilibrium with a CO2-bearing fluid, even in those samples where carbonates are no more preserved; the overall results point to an internally buffered P/T-X(CO2) evolution. The amount of carbonates consumed during prograde metamorphism varies in the range 7-20 vol%, corresponding to 3-10 wt% of CO2 These CO2 amounts are nearly double the CO2 released by calcareous pelites (Groppo et al., 2021). The main consequence of this study is that the CO2 productivity of dolomitic and magnesitic pelites is significant and that these lithologies could be relevant sources of CO2, possibly contributing to the diffuse Himalayan CO2 degassing (e.g. Girault et al., 2014, 2018). ReferencesGirault et al. (2014). Geoph. Res. Lett. 41, 6358–6366Girault et al. (2018). Nat. Comm. 9, 2956Groppo et al. (2017). J. Petrol. 58, 53-83.Groppo et al. (2021). J. metam. Geol. 39, 181-207.Groppo et al. (2022). Comm. Earth Environ, doi: 10.1038/s43247-022-00340-wRapa et al. (2017). Lithos, 292–293, 364–378.

The Tibetan Plateau, largely derived from the accretion of several Gondwana microplates to the southern margin of Asia since the late Palaeozoic, is the highest and largest topographic relief on Earth. Although the first order geodynamic processes responsible for its pre-Cenozoic evolution are quite well-known, many issues are still debated, among which is the timing of collision of each terrane with the southern margin of Asia. Even more uncertain is the pre-Palaeozoic history of these terranes, due to the lack of basement exposures. As a contribution to understanding the pre-Cenozoic evolution of the Tibetan Plateau, this paper focuses on the Aghil Range, a remote and poorly investigated area close to the Karakorum Fault between Kunlun and Karakorum (Xinjiang, China) in western Tibet. The tectono-metamorphic and magmatic evolution of the Aghil Range is investigated using a multidisciplinary approach that combines field mapping, petrology and geochronology (U Pb on titanite, zircon, monazite and xenotime using SHRIMP-RG). We demonstrate that the Aghil Range preserves a coherent slice of Neoproterozoic crystalline basement with a late Palaeozoic sedimentary cover deposited on a passive continental margin during the Gondwana break-up. This represents the westernmost exposure of Precambrian crystalline basement known so far in the Tibetan Plateau. Furthermore, petrological and geochronological results allow reconstructing the Mesozoic poly-metamorphic evolution of this sector of the Tibetan Plateau, which records the evidence of Middle Jurassic (ca. 170 Ma) and Late Cretaceous (66 Ma) collisional events, as well as of the Late Jurassic (ca. 150 Ma) early subduction of an accretionary complex developed on its southern margin. Evidence of Late Cretaceous subduction-related magmatism preceding the last collisional event is also recorded. These results allow tentative correlation of the different terranes of Central Tibet with those of the Pamir-Karakorum Range on both sides of the Karakorum fault.

Peculiar coesite + chloritoid + garnet ± glaucophane talcschists have been used to make at least four quern-stones, unearthed in the ruins of a villa rustica belonging to the Roman imperial period and located at Costigliole Saluzzo, Western Alps. The site of the villa rustica and the presence of coesite relics could suggest a possible provenance of these rocks from one of the already known ultra-high pressure ( UHP ) units of the Western Alps, i.e ., either the Brossasco-Isasca Unit (BIU) of the southern Dora-Maira Massif or the Lago di Cignana Unit (LCU) of the Piemonte Zermatt-Saas Zone. However, similar talcschists have never been reported from these units. Two samples of coesite-bearing, chloritoid + garnet ± glaucophane talcschist collected from two different specimens of quern-stones have been petrologically investigated with the aim of defining their peak pressure–temperature ( P-T ) conditions. The stability field of the coesite + garnet + talc + chloritoid + glaucophane assemblage has been constrained using isochemical phase diagrams modelled in the MnNCFMASHO system; prograde P-T conditions have been additionally constrained using the yttrium-in-garnet (YAG) geothermometer. Thermodynamic modelling tightly constrains peak P-T conditions at 480–510°C, 27–31 kbar. The unusual Mg-rich composition of the talcschists suggests that they originated by Mg-metasomatism of either a continental crustal protolith (granodiorite or Fe-rich metapelite) or an oceanic crustal protolith (altered oceanic crust). A mechanism similar to that proposed for the well-known pyrope-bearing whiteschists of the UHP BIU, i.e . influx of antigorite-derived fluids along shear zones during subduction, can be envisaged. Although the field occurrence of these coesite + chloritoid + garnet ± glaucophane talcschists is still unknown, the obtained results clearly show that these rocks cannot belong to the UHP BIU, whose peak P-T conditions are at significantly higher T and P (730°C, 40–43 kbar). Therefore, this finding opens the challenging hypothesis of the existence of a further, still unmapped, UHP Unit in the Western Alps that also experienced UHP metamorphism and fluid influx from underlying serpentinites during subduction. A detailed mapping and petrologic investigation of the tectono-metamorphic continental ( i.e ., southern Dora-Maira Massif) and oceanic ( i.e ., Monviso meta-ophiolite Complex) units in the proximities of Costigliole Saluzzo is required in order to further constrain the location and the dimension of this new UHP unit.

This contribution presents a geological mapping of the Quaternary succession of the Bule and low Alpetto tributary valleys (Po Valley, Piedmont), together with preliminary data on its petrography. The investigated area, covering an altitude range between 3015 and 1485 m a.s.l., is located in the Monviso Massif Ophiolite Complex, at the contact with marble and calcschist of the Dora-Maira Massif cover. The Quaternary sequence mainly consists of glacial, outwash, debris and landslide deposits, with local torrential/avalanche and colluvial bodies. The investigation on the petrographic composition of these sediments has allowed to recognize the occasional occurrence of jadeitite boulders, likely exploited during the Neolithic period for axe head production. Four possible jadeitite blocks have been found; one of them is described here for the first time, sizing up to 1 m3. In detail, one of the finds is a Neolithic site of jade polishing in the high Bule Valley that was referred to the middle of V millennium BC (Allisio, La Preistoria e l’alta Valle Po, Lar Editore, Perosa Argentina, 2012). The jadeitite blocks have been found in the debris (site A), landslide (site B), glacial (site C) and outwash (site d) sediments. The petrographic characterization of the new jadeitite samples might potentially shed light on the still controversial genesis of these peculiar rocks.

In this preliminary note, we describe our strategy for collecting, storing and representing new and old geological data acquired in several sectors of the central-eastern Himalaya where, since the 70's, researchers of the CNR- IGG Torino and the Department of Earth Sciences of the University of Torino addressed a significant part of their activity. All the geologic features (including rock samples, petrographic, petrologic and structural observations and measurements) are stored and harmonized in a suitable geodatabase using ArcGIS. Standard criteria are used to describe their typology and quality, facilitating future homogeneous data implementation and a correct data traceability. This geodatabase represents the fundamental starting point for the editing of a digital geological map of the central-eastern Nepal Himalaya.

Collection and sharing of geoheritage information are nowadays enhanced by using digital tools and IT (Information Technology) applications that allow homogeneous storing of data, building of sharable databases, and transfer of knowledge between experts and communities. In the frame of the PROGEO-Piemonte project (PROactive management of GEOlogical heritage in the PIEMONTE region), the geological and geomorphological trail in the Monviso Massif (Western Alps) is an example of how IT-applications may support management of geoheritage information. The trail particularly allows walking across the meta-ophiolite succession of an ancient ocean and on glacial landforms. Its realization was supported by different tools that allow (i) capturing of data via GPS handheld mobile device, (ii) managing of field data by means of GIS (Geographic Information Systems) applications, and (iii) virtual visualization of data by means of GIS-based 3D viewer. The different (virtual) views of the geology along the trail can be implemented by adding photos and bookmarks and easily shared through web mapping tools that ensure wide accessibility of geoheritage information.

International audience; The interplay between the processes controlling the mobility of H2O and C-bearing species during subduction zone metamorphism exerts a critical control on plate tectonics and global volatile recycling. Here we present the first study on fresh, carbonate-bearing, lawsonite eclogite-facies metabasalts from Alpine Corsica, France, which reached the critical depths at which important devolatilization reactions occur in subducting slabs. The studied samples indicate that the evolution of oceanic crustal sequences subducted under present-day thermal regimes is dominated by localized fluid-rock interactions that are strongly controlled by the nature and extent of inherited (sub)seafloor hydrothermal processes, and by the possibility of deep fluids to be channelized along inherited or newly-formed discontinuities. Fluid channelization along inherited discontinuities controlled local rehydration and dehydration/decarbonation reactions and the stability of carbonate and silicate minerals at the blueschist-eclogite transition. Fluid-mediated decarbonation was driven by upward, up-temperature fluid flow in the inverted geothermal gradient of a subducting oceanic slab, a process that has not been documented in natural samples to date. We estimate that the observed fluid-rock reactions released 20–60 kg CO2 per m3 of rock (i.e. ~0.7–2.1 wt% CO2), which is in line with the values predicted from decarbonation of metabasalts in open systems at these depths. Conversely, the estimated time-integrated fluid fluxes (20–50 t/m2) indicate that the amount of carbon transported by channelized fluid flow within the volcanic part of subducting oceanic plates is potentially much higher than previous numerical estimates, testifying to the percolation of C-bearing fluids resulting from devolatilization/dissolution processes operative in large reservoirs.

A detailed structural, lithological and petrological study of different transects in the Rasuwa district of central Nepal Himalaya allows the characterization of the tectonostratigraphic architecture of the area. It also facilitates constraining the P-T evolution of the different units within the Lesser (LHS) and Greater (GHS) Himalayan Sequences. Peak P–T conditions obtained for the studied metapelite samples using the pseudosection approach and the Average PT method highlight the existence of four different T/P ratio populations in different tectonometamorphic units: 80 ± 11 °C/kbar (LHS), 66 ± 7 °C/kbar (RTS), 73 ± 1 °C/kbar (Lower-GHS) and 101 ± 12 °C/kbar (Upper-GHS). Integration of structural and petrological data emphasizes the existence of three tectonometamorphic discontinuities bounding these units, characterized by top-to-the-south sense of shear: the Ramgarh Thrust, which separates the LHS (peak metamorphism at ∼600 °C, 7.5 kbar) from the overlying RTS (peak metamorphism at ∼635 °C, 10 kbar); the Main Central Thrust, which separates the RTS from the Lower-GHS (peak at 700–740 °C, 9.5–10.5 kbar with a prograde increase in both P and T in the kyanite stability field), and the Langtang Thrust, which juxtaposes the Upper-GHS (peak at 780–800 °C, 7.5–8.0 kbar with a nearly isobaric heating in the sillimanite stability field) onto the Lower-GHS. An increase in the intensity of deformation, with development of pervasive mylonitic fabrics and/or shear zones, is generally observed approaching the discontinuities from either side. Overall, data and results presented in this paper demonstrate that petrological and structural analysis combined together, are reliable methods adequate to identify tectonometamorphic discontinuities in both the LHS and GHS. Geochronological data from the literature allow the interpretation of these discontinuities as in-sequence thrusts.

Impure calcite marbles from the Precambrian metamorphic basement of the Wuhe Complex, southeastern margin of the North China Craton, provide an exceptional opportunity to understand the depositional processes during the Late Archean and the subsequent Palaeoproterozoic metamorphic evolution of one of the oldest cratons in the world. The studied marbles are characterized by the assemblage calcite + clinopyroxene + plagioclase + K-feldspar + quartz + rutile ± biotite ± white mica. Based on petrography and geochemistry, the marbles can be broadly divided into two main types. The first type (type 1) is rich in REE with a negative Eu anomaly, whereas the second type (type 2) is relatively poor in REE with a positive Eu anomaly. Notably, all marbles exhibit remarkably uniform REE patterns with moderate LREE/HREE fractionation, suggesting a close genetic relationship. Cathodoluminescence imaging, trace elements and mineral inclusions reveal that most zircons from two dated samples display distinct core-rim structures. Zircon cores show typical igneous features with oscillatory growth zoning and high Th/U ratios (mostly in the range 0.3-0.7) and give ages of 2.53 - 2.48 Ga, thus dating the maximum age of deposition of the protolith. Zircon rims overgrew during granulite-facies metamorphism, as evidenced by calcite + clinopyroxene + rutile + plagioclase + quartz inclusions, by Ti-in-zircon temperatures in the range 660-743 °C and by the low Th/U (mostly < 0.1) and Lu/Hf (< 0.001) ratios. Zircon rims from two dated samples yield ages of 1839 ± 7 Ma and 1848 ± 23 Ma, respectively, suggesting a Palaeoproterozoic age for the granulite-facies metamorphic event. These ages are consistent with those found in other Precambrian basement rocks and lower-crustal xenoliths in the region, and are critical for the understanding of the tectonic history of the Wuhe Complex. Positive Eu anomalies and high Sr and Ba contents in type 2 marbles are ascribed to syn-depositional felsic hydrothermal activity which occurred at 2.53 - 2.48 Ga. Our results, together with other published data and the inferred tectonic setting, suggest that the marbles' protolith is an impure limestone, rich in detrital silicates of igneous origin, deposited in a back-arc basin within an active continental margin during the late Archean and affected by synchronous high-T hydrothermalism at the southeastern margin of the North China Craton.

The Mortisa landslide is located in the urban area of Cortina d'Ampezzo, north-east Italy. The landslide unit consists of three mudslides; the affected area is 3500 m long, stretching from 1750 to 1300 m a.s.l. The landslide has been investigated since 1998 as its track crosses a national route and some buildings have been severely damaged by the movements.Through radiocarbon dating it was possible to reconstruct the dynamic of the events that led to the formation of the present-day Mortisa slope. The landslide body is composed by interdigitated layers of gravel in a silty clay matrix originated from subsequent earth and debris flows events since the Lateglacial.To assess the stability conditions of the Mortisa landslide via numerical modelling it is crucial to consider the mechanical influence of the gravel lenses in the dynamic of the whole landslide body. However, their position is almost unknown inside the clay matrix with just some certain positions, where stratigraphic data are available. Therefore, in order to calibrate the model on the monitoring data through back analysis, a new procedure has been implemented. A new code permits to generate stochastically several soil configurations which are automatically elaborated by the commercial software FLAC. The results are organized and filtered based on the monitoring data, hence it is possible to select the model that fits the data best. In this way it it possible to obtain a sound back-analysis model of the Mortisa landslide, to be used in order to design countermeasure works and mitigate the risk associated with the phenomenon.

Metamorphic degassing from active collisional orogens supplies a significant fraction of CO2 to the atmosphere, thus playing a fundamental role even in today’s Earth carbon cycle. Appealing clues for a contemporary metamorphic CO2 production in active orogens are represented by the widespread occurrence, along the whole Himalayan belt, of CO2 rich hot-springs mainly localized along major tectonic discontinuities. In contrast to these well-studied hot-springs, almost no chemical and isotopic data are actually available for cold-springs, especially for those located at high-altitude and in remote areas of the Himalayas. In the framework of the Ev-K2-CNR SHARE (Stations at High Altitude for Research on the Environment) Project, we have started a preliminary chemical and isotopic study on high-altitude cold-springs located at different structural levels in the eastern Nepal Himalayas. Chemical and isotopic data obtained from the high-altitude cold-springs are compared with those obtained by previous authors from hot-springs located along the MCT. The isotopic signature of stable isotopes of hydrogen and oxygen could help to identify the waters sources in the investigated Himalayan sectors, to individuate mixing phenomena between waters of different provenience and possible connection with different circulation nets. These first measurements on high-altitude springs from remote areas of eastern Nepal represent a first step towards a better definition of a reliable scenario of water resources availability and will contribute to the understanding of the water cycle in the studied area.

In those ultrahigh pressure (UHP) terranes that experienced protracted high/ultrahigh temperature (HT/UHT) exhumation histories, slow exhumation rates and a widespread anatexis, the UHP metamorphism is often elusive and difficult to be constrained. In the Dabie-Sulu orogenic belt of central-eastern China, which is the largest UHP terrane in the world, the migmatitic North Dabie complex Zone (NDZ) stands out for the widespread anatexis that widely overprinted the traces of eclogite-facies metamorphism, hampering a precise reconstruction of its P-T-(t) evolution. Different peak P-T conditions, varying between non-eclogitic to UHP conditions, have been proposed so far for various high-grade metamorphic rocks from the NDZ. Few attempts were also made to reconstruct its P-T evolution. Most of the proposed P-T paths, based on conventional thermobarometry, follow a clockwise trajectory at relatively HT (> 750 °C) and almost none of them infer the prograde portion. In this paper we present a detailed petrologic study on two eclogites from the Luotian dome of the NDZ: sample 11-7c2 shows a well preserved eclogitic assemblage (Grt + Cpx + Rt), whereas sample 11-9c1 is a Qtz-Ky-bearing eclogite pervasively retrogressed under granulite-facies conditions. The results of the thermodynamic forward modelling allowed to constrain, for the first time, the prograde portion of the NDZ P-T evolution; the decompression evolution at granulite-facies conditions has been also precisely constrained. However, due to the HT overprinting and to poorly reactive bulk compositions, this method alone is not sufficient to reconstruct the whole P-T trajectory of the NDZ: more specifically, unambiguous evidence of the attainment of UHP conditions have not been found. Different “unconventional” thermobarometric methods (such as those based on trace element and textural characterization of zircons) might be more suitable to decipher the HP/UHP history of this “really hot and slow” UHP terrane.

A number of studies suggest that mountain ranges have strong impact on the global carbon cycle; metamorphic degassing from active collisional orogens supplies a significant fraction of the global solid-Earth derived CO2 to the atmosphere, thus playing a fundamental role even in today’s Earth carbon cycle. The Himalayan belt, a major collisional orogen still active today, is a likely candidate for the production of a large amount of metamorphic CO2 that may have caused changes in long-term climate of the past, present and near future. Large metamorphic CO2 fluxes are facilitated by rapid prograde metamorphism of big volumes of impure carbonate rocks coupled with facile escape of CO2 to the Earth’s surface. So far, the incomplete knowledge of the nature, magnitude and distribution of the CO2-producing processes hampered a reliable quantitative modeling of metamorphic CO2 fluxes from the Himalayan belt. This study, integrated in the framework of the Ev-K2-CNR SHARE (Stations at High Altitude for Research on the Environment) Project, focuses on the metamorphic decarbonation processes occurring during the Himalayan collision. We hereby present preliminary results focusing on the distribution of different types of metacarbonate rocks in the Eastern Himalaya, their petrographic description and the first reported petrological data about the nature of the CO2-producing reactions in garnet-bearing calc-silicate rocks. These results represent a contribution toward a better understanding of the influence exerted by orogenic processes on climatic changes at global scale.

This paper presents a minero-petrographic study of the rocks employed in the building of stone bridges over the Po River in the city of Turin (Piemonte Region—NW of Italy). In all the bridges, the dimension stones are present in different quantities and elements, either representing the only building material or associated with several artificial materials. The total number of bridges on the Po River is six, three of which are made of natural stones (Princess Isabella Bridge, King Umberto I Bridge and King Vittorio Emanuele I Bridge) and three are built with artificial materials with minor dimension stones (Balbis Bridge, Queen Margherita Bridge and Sassi Bridge). The lithologies of the six bridges have been identified in order to find out the corresponding geological units and the original quarry site. The stones mainly consist of rocks from the western Alps, with a prevalence of metamorphic rocks such as gneisses, as well as intrusive igneous rocks. Sedimentary rocks are minor represented.

Retrogressed eclogites are hosted within the Variscan low- to medium-grade metamorphic complex near Giuncana, north-central Sardinia. These rocks are medium to fine grained with garnet and amphibole as the most abundant mineral phases along with clinopyroxene, plagioclase, quartz, biotite, chlorite, epidote, ilmenite, rutile and titanite. Four stages of mineralogical re-equilibration have been distinguished. The stage I is characterized by the occurrence of omphacite, epidote, quartz, amphibole, rutile and ilmenite in garnet poikiloblasts. The stage II is characterized by two types of symplectitic microstructures: (1) amphibole + quartz symplectite and (2) clinopyroxene + plagioclase ± amphibole symplectite. The first symplectite type replaces omphacite included in garnet, whereas the second one is widespread in the matrix. Biotite droplets and/or lamellae intimately growing with fine-grained plagioclase resemble biotite + plagioclase symplectite after phengite. The stage III is characterized by the widespread formation of amphibole: (1) as zoned porphyroblasts in the matrix, (2) as corona-type microstructure replacing garnet. Subordinate plagioclase (oligoclase) is also present in the amphibole corona. The stage IV is characterized by the local formation of biotite replacing garnet, actinolite, chlorite, albite and titanite. P–T pseudosections calculated with Perple_X give P–T conditions 580

One of the peculiarities of Turin (NW Italy) lies on the presence of monumental arcades which mainly consist of stone material. These arcades, characterized by more than 12 km of interconnected paths, represent one of the widest city promenades of Europe and are an architectural, aesthetic and socio-economic example unique in the world. This paper, analysing the urban axis of Via Roma (Rome Street), aims to study the material used in arcade construction. The main stones occurring in Via Roma have been identified and described from a petrographic and mineralogical point of view in order to find out the corresponding geological units and original quarry sites. The minero-petrographic study is accompanied by an architectural survey that was performed applying different methods, as well as the geometric mapping and the perspective rectification of span-types, of block terminations and of other architectonical elements, in plan and in elevation, of the arcades. This allows us to emphasize the merging of cultural and scientific interest for the stone materials used in the historical architecture of a town closely interconnected to the surrounding Western Alps orogenic chain.