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The El Arenal metagabbros preserve coronitic shells of orthopyroxene ± Fe-oxide around olivine, as well as three different types of symplectite consisting of amphibole + spinel, clinopyroxene + spinel and, more rarely, orthopyroxene + spinel. The textural features of the metagabbros can be explained by the breakdown of the olivine + plagioclase pair, producing orthopyroxene coronas and clinopyroxene + spinel symplectites, followed by the formation of amphibole + spinel symplectites, reflecting a decrease in temperature and, possibly, an increase in water activity with respect to the previous stage. The metagabbros underwent a complex P–T history consisting of an igneous stage followed by cooling in granulite, amphibolite and greenschist facies conditions. Although the P–T conditions of emplacement of the igneous protolith are still doubtful, the magmatic assemblage suggests that igneous crystallization occurred at a pressure lower than 6 kbar and at 900–1100 °C. Granulitic P–T conditions have been estimated at about 900 °C and 7–8 kbar combining conventional thermobarometry and pseudosection analysis. Pseudosection calculation has also shown that the formation of the amphibole + spinel symplectite could have been favoured by an increase in water activity during the amphibolite stage, as the temperature of formation of this symplectite strongly depends on aH2O (

Two impure ultrahigh-pressure (UHP) marbles, a calcite marble with the peak assemblage Grt + Phe + Cpx + Rt + (Arg) and a dolomite marble with the peak assemblage Crn + Chl + Rt + Dol (±Arg), from the same lens from the polymetamorphic complex of the Brossasco-Isasca Unit (BIU) (southern Dora-Maira Massif) have been petrologically investigated and modelled by calculating P–T phase-diagram projections for H2O–CO2 mixed-volatile systems. Thermobarometric data obtained from the calcite marble suggest Alpine peak conditions in the diamond stability field (4.0 GPa at 730 °C), and allow reconstruction of the earlier portion of the Alpine retrograde P–T path, which is characterized by a significant decompression coupled with a moderate and continuous cooling to 650 °C at 2.50 GPa. The modelled fluid compositions at peak conditions point to 0.025 ≤ X(CO2) ≤ 0.10 and X(CO2) ≤ 0.0012 in the calcite marble and dolomite marble, respectively, suggesting fluid heterogeneity at the local scale and an internally buffered fluid evolution of the studied impure marbles. The lack of micro-diamond in the BIU marbles is explained by the very-low X(CO2) values, which favoured relatively high fO2-conditions, preventing the formation of diamond at the UHP peak metamorphic conditions.

A well-preserved phengite-amphibole eclogite (Br2F) from the UHP Brossasco-Isasca Unit (BIU) of the Dora-Maira Massif was studied in detail. The eclogite consists of the peak assemblage omphacite, garnet, phengite, rutile, and quartz. A porphyroblastic blue-green amphibole statically overgrows the eclogitic foliation defined by the preferred orientation of phengite flakes, and by the alignments of abundant accessory rutile grains. Both omphacite and phengite are partially replaced by fine-grained symplectites, consisting of clinopyroxene + albite and biotite + oligoclase, respectively. The metamorphic evolution of eclogite Br2F was reconstructed combining microstructural observations, conventional thermobarometry, and pseudosection analysis. A first pseudosection was calculated in the NKCFMASH system in the pressure range 5-45 kbar to model the peak and early retrogressive conditions, whereas a second pseudosection, calculated in the NCFMASH system, was used to model the albite + clinopyroxene symplectite...

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.

The Monviso Ophiolite Geopark (MOG) is one of the strategic geothematic areas chosen to represent the geodiversity of Piemonte within the research project “PROactive management of GEOlogical heritage in the PIEMONTE region”. The MOG is an extraordinary well preserved ophiolite body in the Italian Western Alps. It is one of the best known relics of oceanic crust in the Alps and formed during opening of the Mesozoic western Alpine Tethys and underwent high pressure metamorphism during Alpine subduction. The MOG encompasses the whole lithological spectrum of the Piemonte-Ligurian ophiolites, i.e. metamorphosed peridotite, gabbro, dolerite, basalt, and cover sediments. The MOG gives the almost unique chance for everybody to see and appreciate different portions of the ancient ocean along a mountain trail; from the Po river springs at Pian del Re, to Lago Fiorenza, Lago Lausetto and Lago Superiore, a path rises from 2,000 up to about 2,350 m a.s.l. and shows all different ophiolitic lithologies within few kilometers. The inventory of a number of different geosites at the MOG, whose conservation and development require different expertises, is considered as the first and essential starting point for the geoconservation of geological heritage and geosites.

The type and kinetics of metamorphic CO2-producing processes in metacarbonate rocks is of importance to understand the nature and magnitude of orogenic CO2 cycle. This paper focuses on CO2 production by garnet-forming reactions occurring in calc-silicate rocks. Phase equilibria in the CaO–FeO–Al2O3–SiO2–CO2–H2O (CFAS–CO2–H2O) system are investigated using P–T phase diagrams at fixed fluid composition, isobaric T–X(CO2) phase diagram sections and phase diagram projections in which fluid composition is unconstrained. The relevance of the CFAS–CO2–H2O garnet-bearing equilibria during metamorphic evolution of calc-silicate rocks is discussed in the light of the observed microstructures and measured mineral compositions in two representative samples of calc-silicate rocks from eastern Nepal Himalaya. The results of this study demonstrate that calc-silicate rocks may act as a significant CO2 source during prograde heating and/or early decompression. However, if the system remains closed, fluid–rock interactions may induce hydration of the calc-silicate assemblages and the in situ precipitation of graphite. The interplay between these two contrasting processes (production of CO2-rich fluids vs. carbon sequestration through graphite precipitation) must be considered when dealing with a global estimate of the role exerted by decarbonation processes on the orogenic CO2 cycle.

In the Kanchenjunga area (far eastern Nepal), as well as in the whole Himalayan chain, the juxtaposition of the high-grade mid-crustal Higher Himalayan Crystallines (HHC) onto the low grade Lesser Himalayan Sequence (LHS) is structurally marked by the Main Central Thrust Zone (MCTZ). On the base of original field mapping and mesoand micro-structural data, the MCTZ has been here identified as a crustal-scale ductile to ductile-brittle shear zone roughly centred on the Inverted Metamorphic Sequence (IMS), in which different rocks are more pervasively deformed and sheared with respect to adjacent rocks. The boundaries of the MCTZ are not represented by single thrusts. The lower boundary is marked by phyllonites and mylonitic schists located at the uppermost portions of the LHS at the contact with strongly mylonitic augen gneisses of the IMS. The upper boundary of the MCTZ is roughly located at the base of Grt-Kfs-Ky-Sil anatectic gneisses in the lower portion of the HHC, being itself characterized by pervasive ductile shearing. Structural field data combined with petrologic results clearly indicate that the MCTZ is internally imbricated, resulting in the juxtaposition of rock packages characterized by different P-T evolutions and T/depth gradients, separated by “metamorphic discontinuities” which do not always correspond to evident structural breaks. This is the case of the metamorphic discontinuity identified at the upper structural levels of the IMS and juxtaposing the upper IMS rocks, locally anatectic, characterized by higher T/depth gradients on the lower IMS rocks, characterized by lower T/depth gradients. A similar metamorphic discontinuity was previously reported westward (Milke Danda transect) thus suggesting that it could be of regional importance in the tectonometmaoprhic architecture of eastern Nepal Himalaya. Citation: 2012. Structural and metamorphic features of the Main Central Thrust Zone and its contiguous domains in the eastern Nepalese Himalaya . In: (Eds.) Michele Zucali, Maria Iole Spalla, and Guido Gosso, Journal of the Virtual Explorer, volume 41, paper 2, doi: 10.3809/jvirtex.2011.00294 Journal of the Virtual Explorer, 2012 Volume 41 Paper 2 http://virtualexplorer.com.au/

International audience; In the Schistes Lustres of Alpine Corsica (France) serpentinized mantle rocks are associated with continental basement and meta-volcanic/-seclimentary cover rocks. The relationships among these different lithologies are especially well exposed in the Monte San Petrone unit, where Alpine metamorphism reached lawsonite-eclogite conditions. The contact between serpentinites and slivers of continental basement, relatively flat-lying over several kilometers, is characterized by evidence of cataclastic deformation pre-dating Alpine High-Pressure ductile fabrics. The serpentinite/continental basement pair is stratigraphically overlain by metasediments with a typical Jurassic-Cretaceous supra-ophiolitic lithostratigraphy, with metaradiolarites passing upward to marbles and cal cschists. Noticeably, no evidence of cataclastic deformation is found in metasediments. These observations indicate that the lithostratigraphy of the Monte San Petrone unit was established during a pre-Alpine polyphase evolution, which culminated in extensive brittle deformation along a flat-lying detachment fault prior to the deposition of Jurassic sediments. We suggest that the inferred Mesozoic extensional tectonics were related to the opening of the Westem Tethys. The Mesozoic architecture of the Monte San Petrone area, which is typical of an Ocean-Continent Transition (OCT) zone, was preserved despite Alpine deformation and metamorphism, when the different lithologies (i.e. meta-ophiolites, continent-derived rocks and metasediments) underwent a common metamorphic evolution, culminating at T=490-550 degrees C and P= 2.2-2.6 GPa. Similar tectonostratigraphic associations are observed in other high-pressure terranes of Alpine Corsica, suggesting that inherited OCT-type domains may be common in Alpine-type orogens.

The Main Central Thrust Zone (MCTZ) is a key tectonic feature in the architecture of the Himalayan chain. In the Arun valley of the eastern Nepal Himalaya, the MCTZ is a strongly deformed package of amphibolite- to granulite-facies metapelitic schist and granitic orthogneiss. This package is tectonically interposed between the underlying, low-grade, Lesser Himalaya sequences and the overlying, high-grade and locally anatectic, Higher Himalayan Crystallines (HHC). The MCTZ is characterized by a well documented inverted metamorphism from the Grt–Bt zone, across the Ky-in, St-in and -out, Kfs-in, Ms-out and Sil-in isograds. Partial melting with local occurrence of migmatitic segregations has been rarely reported from the highest structural levels of the MCTZ. While it is widely accepted that thrusting along the MCT occurred during the Miocene, geochronological data constraining the timing of crustal anatexis in the upper portion of the MCTZ are still lacking. In order to understand the link between partial melting in the MCTZ and the Miocene activation of the MCT, we present the P – T –time evolution of a kyanite-bearing anatectic gneiss occurring at the highest structural levels of the MCTZ, along the Arun–Makalu transect (eastern Nepal). Microstructural observations combined with P – T pseudosection analysis show that dehydration partial melting occurred in the kyanite-field. After reaching peak conditions at about 820 °C, 13 kbar, the studied sample experienced decompression accompanied by cooling down to 805 °C, 10 kbar, which caused in situ melt crystallization. SHRIMP monazite and zircon geochronology provides evidence that the anatexis affecting the upper portion of the MCTZ occurred during Early Oligocene (∼ 31 Ma). These results demonstrate that in the upper MCTZ, at least in the eastern Himalaya, crustal anatexis was earlier than, and not a consequence of, decompression linked to exhumation along the MCT.

The island of Sifnos (Cyclades, Greece) is famous for its spectacular blueschist-facies rocks and eclogites. Their very well preserved high-pressure assemblages are characterized by the occurrence of Fe+ 3-rich minerals such as aegirine-rich pyroxene, riebeckite-rich amphibole, magnetite and deerite. Therefore, the common model system NCFMASH (Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O), in which all iron is assumed to be ferrous, is not sufficient to completely model these highly oxidized assemblages. We propose new constraints on the P–T evolution of the glaucophane schists and associated rocks from Vroulidia Bay (northern Sifnos) using P–T isochemical diagrams (pseudosections) modelled for the oxidized N(K)CFMASHO system [Na2O–(K2O)–CaO–FeO–Fe2O3–MgO–Al2O3–SiO2–H2O], and calculated using solid solution models involving Fe+ 3-end-members. The resulting P–T conditions could be fitted by a simple trajectory consisting of a smooth clockwise P–T loop, with two distinct high-pressure events at T = 450–500 °C, P > 2.0 GPa (assemblage A) and T = 525–565 °C, P > 2.1 GPa (assemblage B), respectively, followed by cooling and decompression. Geospeedometry based on 40Ar/39Ar data, however, allows constraint as to the duration of individual thermal excursions associated with these events. Should these limits be taken into account, the P–T envelope derived in this paper encompasses a more complex P–T history, with individual excursions that potentially involve relatively short periods of temperature increase ± pressure variation.

The metamorphic evolution of a granulitized eclogite from the Phung Chu Valley (Eastern Himalaya) was reconstructed combining microstructural observations, conventional thermobarometry and quantitative pseudosection analysis. The granulitized eclogite consists of clinopyroxene, plagioclase, garnet, brown amphibole, and minor orthopyroxene, biotite, ilmenite and quartz. On the basis of microstructural observations and mineral relationships, four metamorphic stages and related mineral assemblages have been recognized: (i) M1 eclogite-facies assemblage, consisting of garnet, omphacite (now replaced by a clinopyroxene + plagioclase symplectite) and phengite (replaced by biotite +plagioclase symplectite); (ii) M2 granulite-facies assemblage, represented by clinopyroxene, orthopyroxene, garnet, plagioclase and accessory ilmenite; (iii) M3 plagioclase + orthopyroxene corona developed around garnet, and (iv) M4 brown amphibole + plagioclase assemblage in the rock matrix. Because of the nearly complete lack of eclogitic mineral relics, M1 conditions can be only loosely constrained at > 1.5 GPa and > 580 degrees C. In contrast, assemblage M2 tightly constrains the peak granulitic stage at 0.8-1.0 GPa and > 750 degrees C. The second granulitic assemblage M3, represented by the plagioclase + orthopyroxene corona, formed at lower pressures (similar to 0.4 GPa and similar to 750 degrees C). During the subsequent exhumation, the granulitized eclogite experienced significant cooling to nearly 700 degrees C, marked by the appearance of brown amphibole and plagioclase (M4) in the rock matrix. U-Pb SHRIMP analyses on low-U rims of zircon from an eclogite of the same locality suggest an age of 13-14 Ma for the M3 stage. The resulting decompressional clockwise P-T path of the Ama Drime eclogite is characterized by nearly isothermal decompression from > 1.5 GPa to similar to 0.4 GPa, followed by nearly isobaric cooling from similar to 775 degrees C to similar to 710 degrees C. Modelling of phase equilibria by a calculated petrogenetic grid and conventional thermobarometry on a biotite-garnet-sillimanite metapelite hosted in the country rock granitic orthogneiss extends the inferred P-T trajectory down to similar to 630 degrees C and similar to 0.3 GPa.

Within a mica schist from the coesite-bearing Brossasco-Isasca Unit (Western Alps), microstructural analysis shows that Alpine garnet grains are aligned with the crenulated foliation. Garnet crystallographic orientation was analysed with electron backscatter diffraction (EBSD): the obtained crystallographic dispersion patterns and distribution patterns of misorientation axes suggest a strong parallelism of {110} garnet planes with a 56°W-dipping foliation. The data are interpreted as evidence for an epitaxial growth of garnet upon (001) biotite planes, sometime during and/or after dispersion of the biotite/garnet crystals from their initially foliation-parallel orientation by rotation about the Alpine crenulation axis. This interpretation is based on the comparison of the measured EBSD data with: (i) theoretical dispersion trajectories of garnet crystallographic data, (ii) numerically modelled pole figures, and (iii) numerically modelled misorientation axis distribution patterns. Our data suggest that epitaxial growth of garnet upon biotite is allowed by distortion of the pseudohexagonal basal oxygen ring structure on (001) biotite surfaces, and that distortion is driven by introduction of missing ions. Our data further suggest that the spatial distribution of precursor phases influences the distribution patterns of garnet within mica schists.

Identifying serpentine minerals in rocks is generally accomplished by means of Scanning Electron Microscopy - Energy Dispersion Spectrometry (SEM-EDS) and Transmission Electron Microscopy (TEM), both of which require complicated sample preparation. In this work, we evaluate the use of micro-Raman spectroscopy, which requires no sample preparation, in identifying the different serpentine minerals contained in thin sections of serpentinized peridotites, where the various phases occur in different microstructural positions. The micro-Raman spectra were obtained from samples previously characterized by optical microscopy, SEM-EDS and TEM. Micro-Raman spectroscopy proved to be a quick, easy and reliable method for the identification of serpentine minerals.

The variation of the Fe-redox state across a strongly zoned garnet crystal occurring in a FeTi-oxide metagabbro from a subduction setting has been investigated comparing micro-XANES and electron microprobe ("flank method") results. This sample represents an ideal monitor of the redox processes associated with devolatilization reactions occurring in the subducting slab and likely involved in the oxidation of the mantle wedge, as well as a challenge for the analytical methods. Since in the studied garnet the Fe 3+/ΣFe variations are small and occur on a micrometric scale, we exploited the X-ray microprobe available at the ESRF ID22 beamline to reach a spot size of 1.7 μm × 5.3 μm. Our study represents a first attempt to measure the Fe3+/ΣFe variations across a single crystal of garnet, ca. 400 μm in diameter, using the micro-XANES technique. The absolute Fe3+/ΣFe values obtained with micro-XANES and "flank method" are perfectly comparable, ranging from a maximum of about 0.1 in the garnet core to a minimum of 0.0 in the garnet rim. © 2012 The Royal Society of Chemistry.

DOI = 10.3126/hjs.v5i7.1241 Himalayan Journal of Sciences Vol.5(7) (Special Issue) 2008 p.34