Il giacimento piombo-zincifero di Raibl in Friuli (Italia)

Sans résumé

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Il giacimento piombo-zincifero di Raibl in Friuli (Italia)

     

Hydrologic and geochemical survey of the lake “Specchio di Venere” (Pantelleria island,Southern Italy)

Hydrological and geochemical studies performed on Lake Specchio di Venere on Pantelleria island (Italy) indicate that this endorheic basin has been formed through upwelling of the water table, and that it is continuously fed by the thermal springs situated on its shores. The lake is periodically stratified both thermally and in salinity, albeit this stratification is rather unstable over time, since meteorological events such as strong rain or wind can determine the mixing of its waters. Periodical analyses of the lake water chemistry show large variations of the salt content due to the yearly evaporation-rain dilution cycle. These processes are also responsible for the saline stratification during steady meteorological conditions. The mineralogical characterisation of the bottom sediments shows the almost exclusive presence of neoformation minerals, mainly carbonates, formed in response to the pH gradient between spring- (pH ≈ 6) and lake-waters (pH ≈ 9). Finally, the CO₂ partial pressures in the lake water slightly exceeding the atmospheric one, are due to the large amounts of CO₂ brought to the lake through the bubbling free gas phase of the thermal springs. Nevertheless the high pH value of the lake water, its small volume and its periodical mixing prevent dangerous built up of this gas.     

Extreme-Mg olivines from venancite lavas of Pian di Celle volcano (Italy)

Olivines with a forsterite minal of up to 99.5 mol % were discovered in ultra-potassium lavas of Pian di Celle volcano (Italy). Based on high-precision analyses of these olivines and the available literature data, a new type of extreme-Mg olivines with a forsterite content of >96 mol % was distinguished. These olivines could have formed as a result of oxidation of lower Mg olivines, crystallization from the carbonatite melt, or they may represent earlier crystallized phenocrysts in skarn. Oxidized olivines are easily diagnosed on the content of typomorphic admixtures, in particular, high MgO and NiO contents. It is difficult to recognize olivines from skarns and carbonatites due to the lack of high-precision analytical data. Extreme-Mg olivines from Pian di Celle (Italy) are close to the carbonatite subtype of olivines.     

On the age of the Vedrette di Ries (Rieserferner) massif and its geodynamic significance

The Rb/Sr whole rock method was used to carry out radiometric analyses on about forty samples from the Vedrette di Ries massif. The samples were arranged in five sample groups. The age of this massif was found to be Middle Oligocene, and the best-defined isochron (30±3 m. y.) was obtained with a group of nine samples from the Upper Anterselva Valley. The values of the initial Sr isotopic ratio range between 0.709 and 0.727. This fact suggests that these melts were the result of the anatexis of different crustal materials, with some contributions from components with low (⁸⁷Sr/⁸⁶Sr)i values. A genetic link is proposed between these Oligocene melts and the Tauern metamorphism.

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Zusammenfassung Für eine radiometrische Gesamtgesteinsanalyse von etwa vierzig Proben des Rieserferner-Massivs, hat man sich der Rb/Sr Methode bedient.Die Proben wurden in fünf Gruppen zusammengestellt.Daraus ergibt sich, daß das Alter dieses Massivs Mittel-oligozän ist.Die beste Isochrone (30±3 M. Jahre) wurde mit einer Gruppe von neun Proben, die aus dem oberen Antholzertal ergaben, erreicht.Die Werte der initialen Sr⁸⁷/Sr⁸⁶-Isotopenverhältnisse sind zwischen 0,709 und 0,727.Das regt zum Denken an, daß diese Schmelze das Ergebnis von Anatexis von verschiedenen Krustengesteinen mit der Hilfe von Komponenten mit niedrigen Werten (⁸⁷Sr/⁸⁶Sr); waren.Zwischen diesen Oligozänschmelzen und der Tauernmetamorphose wird eine genetische Bindung vorgeschlagen.

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Résumé Pour analyser radiométriquement environ quarante échantillons du massif de Riesenferner (Vedrette di Ries) on a employé la méthode Rb/Sr sur roche totale. Les échantillons ont été réunis en cinq groupes. Il en résulte que l'âge de ce massif remonte à l'Oligocène moyen. L'isochrone la meilleure (30±3 m.y.) a été obtenue avec un groupe de neuf échantillons provenant de la haute vallée d'Antholz (Anterselva). Les valeurs des rapports initiaux (⁸⁷Sr/⁸⁶Sr) se placent entre 0,709 et 0,727: Cela suggère que ces liquides ont été le résultat d'une anatexie portant sur différents matériaux de la croûte, avec contribution de composants à basses valeurs (⁸⁷Sr/⁸⁶Sr). On propose un lien génétique entre ces magmas de l'Oligocène et le métamorphisme des Tauern.

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40- -. . , — . (30±3 ) 9- , . Sr⁸⁷/Sr^8b 0,709 0,727. , . .

     

Carbonate precipitation in the alkaline lake Specchio di Venere (Pantelleria Island,Italy) and the possible role of microbial mats

Alkaline lakes like the hydrothermally affected lake Specchio di Venere (Pantelleria Island, Central Mediterranean) are typical geological settings harbouring calcified microbial mats. The present work is focused on the discrimination between biotic and abiotic processes driving carbonate precipitation in this lake, using hydrochemical, mineralogical and isotopic data. Hydrochemical analyses demonstrate that the lake is nearly 10−fold supersaturated with regard to aragonite and seasonally reaches hydromagnesite supersaturation. Microscopic observations depict organosedimentary laminated structures consisting of microbial communities and aragonitic precipitates, which are rather disseminated in pores than directly linked to microorganisms. Oxygen isotopic data indicate that authigenic carbonate crystallisation from evaporating water is the dominant precipitation process, further suggested by the absence of textural evidence of diagenetic processes. Conversely, the observed δ¹³C values reflect an influence of extracellular polymeric substances (EPS) on carbon fractionation during the precipitation process, due to the selective sequestration of ¹²C in the biomass. The above considerations suggest that at lake Specchio di Venere the carbonate precipitation is mainly of inorganic nature, but a minor role played by biologically influenced processes in microbial mats is not excluded.     

The Cima di Vila (Zinsnock) intrusion,eastern alps: Evidence for crustal melting,acid-mafic magma mingling and wall-rock fluid effects

Summary The Alpine (29 Ma) stock of Cima di Vila (CdV, Zinsnock), South Tyrol/Alto Adige, Eastern Alps consists of dominant granodiorites with a few tonalites. Late granitic and aplitic dykes cut the pluton, which also contains several microgranular mafic enclaves. Major, trace element and Sr isotopic data reported for samples of various lithologies indicate that mafic, intermediate and acidic magmas in the CdV intrusion are not all strictly comagmatic. Crustal melting of lower continental crust appears to explain better the mineralogical and geochemical characteristics of granodiorites. Mingling between salic and mantle-derived mafic magmas is responsible for the genesis of tonalites, whereas granites and aplites are the products of separation of residual liquid fromin situ crystallized granodioritic magma. Studies of mafic enclaves and adjoining host rocks indicate that mafic and acidic magmas interacted extensively during the emplacement and crystallization of the granodioritic mass. This interaction produced selective enrichments of some elements (Rb, K, Ba) in the mafic magmas at the expense of the nearby acid magmas. Partial crystallization of the mafic magma and equilibration between the residual liquid and the host granodioritic magma produced a depletion of LREE contents in the enclaves. Sr isotopic data indicate that equilibrium between acidic magma and enclaves was not reached, in spite of the large chemical exchanges between mafic blobs and host granodiorites. Late stage fluids from the wall-rocks had an important role in determining Sr isotopic composition of the latest crystallized granitic and aplitic melts.

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Die Cima di Vila (Zinsnock) Intrusion, Ostalpen: Krustenaufschmelzung, sauermafische Magmenmischung und Nebengesteins-Fluideffekte

Zusammenfassung Der appidische (29 Ma) Cima di Vila (Zinsnock) Pluton (CdV) in südtirol besteht hauptsächlich aus Granodioriten and wenigen Tonaliten. Späte granitische und aplitische Gänge durchsetzen den Pluton, der auch einige mikrogranulare malische Enklaven enthält. Haupt-, Spuren- and Sr-Isotopenanalytik belegen, daß die mafischen, intermediären and sauren Magmen in der CdV Intrusion nicht alle streng comagmatisch sind. Aufschmelzung der unteren kontinentalen Kruste erklärt die mineralogischen und geochemischen Charakteristika weitaus besser. Die Tonalite werden als Mischung saurer und mafischer (Mantel-) Magmen erklärt, während die Granite und Aplite Produkte einer Separation von Restschmelzen von einemin situ kristallisierten granodioritischen Magma repräsentieren.Eine Studie der mafischen Enklaven und der Nebengesteine zeigt außerdem eine intensive Interaktion der mafischen und der sauren Magmen während der Platznahme und Kristallisation des Granodioritkörpers. Diese Interaktion hatte eine selektive Anreicherung einiger Elemente (Rb, K, Ba) in den mafischen Magmen zur Folge.Teilkristallisation des mafischen Magmas und die Interaktion der Restschmelzen mit dem granodioritischen Wirtsmagma resultierten in einer LREE-Abreicherung in den Enklaven. Sr-Isotope zeigen, daß, trotz der weitldufigen Austauschprozesse zwischen dem sauren Magma and den Enklaven, kein Gleichgewicht errreicht wurde. Späte Fluide aus den Nebengesteinen spielten eine wichtige Rolle für die Sr- Isotopenzusammensetzungen in den spät kristallisierten granitischen und aplitischen Schmelzen.

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With 7 Figures     

Electromagnetic methods to characterize the Savoia di Lucania waste dump (Southern Italy)

The aim of this work is the joint application and integration of non-invasive geoelectrical methods for studying the landfill of Savoia di Lucania (Southern Italy). This landfill for its engineering features and small dimensions (70 m × 30 m × 6 m) represents an optimal test site to assess a geophysical survey protocol for municipal solid waste landfills investigation and monitoring. The landfill of Savoia di Lucania has been built with a reinforced concrete material and coated with a high-density polyethylene (HDPE) liner. Three electrical resistivity tomographies (ERT), two self-potential (SP) map surveys and one induced polarization (IP) section have been performed, both in the surrounding area and inside the waste landfill. The geophysical investigations have well defined some buried boundaries of the landfill basin and localized the leachate accumulation zones inside the dumpsite. Comparison of our results with other engineering and geological investigations could be the key for evaluating the integrity of the HDPE liner. Finally, the joint use of the ERT, IP and SP methods seems to be a promising tool for studying and designing new monitoring systems able to perform a time-lapse analysis of waste landfill geometry and integrity.     

Andalusite-bearing veins at Vedrette di Ries (eastern Alps, Italy): fluid phase composition based on fluid inclusions

Abstract Andalusite-bearing veins formed during contact metamorphism in the aureole of the Vedrette di Ries tonalite. In the veins, quartz crystals that are completely armoured by andalusite or that occur in strain shadow areas contain three generations of fluid inclusions: low-salinity H₂O-CO₂-CH₄ mixtures with CH₄/(CO₂+ CH₄) ± 0.35 (type A); low-salinity aqueous fluids (type B); H₂O-free, CO₂-CH₄ fluids with the same carbonic speciation as A (type C). Carbonic types A and C typically have a dark appearance, which is attributed to graphite coatings on inclusion walls. Microstructural analysis of the host quartz and calculated densities indicate that type A inclusions were likely trapped during vein formation. These inclusions underwent strain-assisted re-equilibration during cooling that resulted in density increases without change of composition. After the rocks had cooled below about 350 ° C, type C inclusions appear to have formed from one of the immiscible fractions after unmixing of the H₂O-CO₂-CH₄ fluid mixtures. Aqueous type B inclusions, apparently trapped between 225 and 350 ° C, could represent an independent fluid, or could be the H₂O-rich fraction of unmixed type A fluids. Taking account of the uncertainties, the composition and density of the complex type A inclusion fluids are in good agreement with the properties of primary fluids calculated from the petrological data. The fluid inclusion data support the model of vein formation by hydrofracturing as a result of dehydration of graphitic metapelites. These new results also demonstrate the importance of considering strain in the interpretation of metamorphic fluid inclusions.     

Structure of a normal seismogenic fault zone in carbonates: The Vado di Corno Fault,Campo Imperatore,Central Apennines (Italy)

The Vado di Corno Fault Zone (VCFZ) is an active extensional fault cutting through carbonates in the Italian Central Apennines. The fault zone was exhumed from ∼2 km depth and accommodated a normal throw of ∼2 km since Early-Pleistocene. In the studied area, the master fault of the VCFZ dips N210/54° and juxtaposes Quaternary colluvial deposits in the hangingwall with cataclastic dolostones in the footwall. Detailed mapping of the fault zone rocks within the ∼300 m thick footwall-block evidenced the presence of five main structural units (Low Strain Damage Zone, High Strain Damage Zone, Breccia Unit, Cataclastic Unit 1 and Cataclastic Unit 2). The Breccia Unit results from the Pleistocene extensional reactivation of a pre-existing Pliocene thrust. The Cataclastic Unit 1 forms a ∼40 m thick band lining the master fault and recording in-situ shattering due to the propagation of multiple seismic ruptures. Seismic faulting is suggested also by the occurrence of mirror-like slip surfaces, highly localized sheared calcite-bearing veins and fluidized cataclasites. The VCFZ architecture compares well with seismological studies of the L'Aquila 2009 seismic sequence (mainshock M_W 6.1), which imaged the reactivation of shallow-seated low-angle normal faults (Breccia Unit) cut by major high-angle normal faults (Cataclastic Units).     

The Vedrette di Ries (rieserferner) plutonic complex: Petrological and geochemical data bearing on its genesis

Data on Co, Sc, Rb, Sr, Th, Zr, Ta, Hf and REE contents are reported for tonalites, granodiorites, granites and one diorite from the Alpine plutonic complex of Vedrette di Ries in the Eastern Alps. Co and Sc range respectively between 19.6-1.2 ppm and 32-3.3 ppm showing a good negative correlation with the Differentiation Index (D.I.). Rb values range between 61–197 ppm displaying a positive correlation with D.I. All the samples have variably fractionated light and heavy REE with La_N/Sm_N=1.7–5.55 and Tb_N/Yb_N=0.58–1.65 and no important europium anomalies. On the basis of the HREE fractionation two groups of rocks can be distinguished: one showing higher Tb_N/Yb_N values which range between 1.27–1.65 and are positively correlated with the D.I. and a second group with Tb_N/Yb_N values less than unity which decrease with the differentiation. The rocks with high Tb_N/Yb_N ratio also display a positive Sr versus D.I. variation whereas the samples with flat or upward concave HREE patterns (i.e. with Tb_N/Yb_N<1) define negative Sr versus D.I. trends. Th, Ta, Hf, Zr and LREE show an overall tendecy to increase and then to decrease with the differentiation, with a large scattering of values.The data obtained fit the hypothesis that the entire rock series under study is the product of a two-stage crystal/liquid fractionation process starting from one parent magma of tonalitic or dioritic composition. During the first stage, which occurred at high pressure, the separation of hornblende+garnet produced the liquids displaying the positve Tb_N/Yb_N and Sr versus D.I. correlation. These liquids during their rise through the crust would have undergone a second stage fractionation with separation of hornblende and plagioclase evolving toward low-Tb_N/Yb_N and Sr-poor composition.     

Petrogenetic relationships between peralkaline rhyolite dykes and mafic rocks in the post-Variscan gabbroic complex from Bocca di Tenda (northern Corsica, France)

The Lower Permian complex from Bocca di Tenda (Corsica island, France) consists of a gabbroic sequence crosscut by chilled dykes ranging in composition from basalt to trachyandesite and peralkaline rhyolite. The gabbroic sequence is mostly composed of olivine gabbronorites, quartz gabbronorites/diorites locally displaying high ilmenite amounts, and hornblende-rich tonalites. The quartz gabbronorites/diorites and the hornblende-rich tonalites have similar initial ε_Nd values (+0.9 to ?1.1) and record a fractional crystallization process driven by separation of plagioclase, pyroxene, and ilmenite. The olivine gabbronorites have slightly higher initial ε_Nd than the quartz gabbronorites/diorites and the hornblende-rich tonalites, thereby documenting that the early evolution of the melts that gave rise to the gabbroic sequence was controlled by concomitant fractional crystallization and crustal assimilation. The trachyandesite dykes are rare and rich in dark mica. The selected trachyandesite has initial ε_Nd of +0.4, which is slightly lower than the ε_Nd of the basalt dykes. The basalt and the trachyandesite dykes are most likely genetically related through a process of fractional crystallization controlled by segregation of plagioclase, clinopyroxene and minor ilmenite, and assimilation of crustal material. The peralkaline rhyolites have initial ε_Nd values ranging from +0.3 to ?0.3. Whole-rock chemical variations and trace element compositions of Na-amphibole (arfvedsonite) indicate that the peralkaline rhyolite dykes record a process of fractional crystallization mainly controlled by separation of alkali feldspar and minor ilmenite and arfvedsonite. A plausible petrogenetic hypothesis for the genesis of the peralkaline rhyolite melts implies a protracted process of fractional crystallization from the trachyandesitic melts. This fractionation process would be initially ruled by separation of plagioclase, dark mica, and minor ilmenite. An alternative hypothesis for the origin of the peralkaline rhyolite melts implies partial melting of nearly coeval amphibole-rich mafic intrusives, which formed by crustally contaminated mantle-derived melts. The genesis of the peralkaline rhyolites is in any case correlated with mantle-derived melts that experienced extensive crustal contamination.     

Sodium-rich dachiardite from Alpe di Siusi,Italy

Dachiardite (a zeolite) occurs as aggregates of thin, parallel fibers in the basalt of Alpe di Siusi, Bolzano, Italy. It is strongly sodic while dachiardite from Elba is mainly calcic. Indexed X-ray diffraction data, unit cell dimensions, chemical formula, density and optical properties are reported.     

Extensional deformation structures within a convergent orogen: The Val di Lima low-angle normal fault system (Northern Apennines,Italy)

A low-angle extensional fault system affecting the non metamorphic rocks of the carbonate dominated Tuscan succession is exposed in the Lima valley (Northern Apennines, Italy). This fault system affects the right-side-up limb of a kilometric-scale recumbent isoclinal anticline and is, in turn, affected by superimposed folding and late-tectonic high-angle extensional faulting.The architecture of the low-angle fault system has been investigated through detailed structural mapping and damage zone characterization. Pressure-depth conditions and paleofluid evolution of the fault system have been studied through microstructural, mineralogical, petrographic, fluid inclusion and stable isotope analyses. Our results show that the low-angle fault system was active during exhumation of the Tuscan succession at about 180°C and 5 km depth, with the involvement of low-salinity fluids. Within this temperature - depth framework, the fault zone architecture shows important differences related to the different lithologies involved in the fault system and to the role played by the fluids during deformation. In places, footwall overpressuring influenced active deformation mechanisms and favored shear strain localization.Our observations indicate that extensional structures affected the central sector of the Northern Apennines thrust wedge during the orogenic contractional history, modifying the fluid circulation through the upper crust and influencing its mechanical behavior.     

Joint analysis of SAR interferometry and electrical resistivity tomography surveys for investigating ground deformation: the case-study of Satriano di Lucania (Potenza, Italy)

We have jointly applied microwave remote sensing imaging and ground-based geophysical methodologies for investigating ground deformation. In particular, the Differential Interferometric Synthetic Aperture Radar (DInSAR) technique referred to as Small BAseline Subset (SBAS) algorithm and the Electrical Resistivity Tomography (ERT) method have been applied. The former is a tool for investigating large areas, allowing us to detect deformation phenomena and to analyse their spatial patterns and temporal evolution; the latter permits us to reconstruct, with high spatial resolution, the geometry of near-surface geological structures and to estimate the electrical resistivity of outcropping terrains. The starting point of our analysis has been a DInSAR survey carried out on an area of about 400 km² centred around the town of Potenza (Basilicata region, Italy), for which SAR data acquired by the ERS-1/2 sensors in the time interval 1992–2000 were available. The DInSAR analysis allowed us to detect an unknown ground deformation effect which involves the urban area of Satriano di Lucania, located close to Potenza. An on-site ERT survey was then carried out in order to characterize in detail the physical properties of the terrains involved in the main deformation pattern detected via the DInSAR processing. The integration of the DInSAR and ERT measurements permitted us to investigate areas where the deformation have space-variant characteristics and allowed us to formulate a hypothesis on the origin of the detected displacement processes that, although not fully conclusive, is consistent with the overall DInSAR and ERT analysis.     

P–T conditions for the eclogitic re-equilibration of the metaophiolites from Val d'Ala di Lanzo (internal Piemontese zone, Western Alps)

Abstract A study has been made of the high-pressure early-Alpine re-equilibration in the eclogites and metasedimentary cover of the Val d'Ala di Lanzo ophiolite. All of the main high-pressure minerals have been analysed and their compositions used to determine re-equilibration temperatures. The minimum conditions proposed ( P = 1.3 GPa, T = 450–460°C) are also indicated by the presence of a jadeite+quartz-bearing metagranite.
The temperatures are compared with those reported for similar eclogites from the Voltri Group, the Aosta Valley and the Valais. Comparison of recalculated temperatures shows that the temperature (and probably the pressure) of the eclogitic re-equilibration increased in the Aosta Valley and the Valais, in keeping with what has been observed in the internal Penninic basement of the Gran Paradiso and Monte Rosa crystalline massifs.     

Chemical zoning and crystallization mechanisms in the magma chamber of the Pomici di Base plinian eruption of Somma-Vesuvius (Italy)

Products of the Pomici di Base plinian eruption of Somma-Vesuvius consist of pumice and scoria fall deposits overlain by lithic-rich phreatomagmatic deposits. The plinian fall, which represents most of the magma volume involved in the eruption, ranges in composition from trachyte (SiO₂ = 62.5 wt%) to latite (SiO₂ ≈ 58 wt%) in the lower one-third of the deposit, whereas the upper two-thirds of the total thickness consists of latitic scoriae with fairly uniform composition (SiO₂ ≈ 55–56 wt%). All the products have very low content of phenocrysts (from 4 wt% in trachyte pumice to 1 wt% in the latite scoriae), most of which are not in equilibrium with the host rock. Minerals not in equilibrium, both in trachytic and latitic rocks, consist of discrete crystals of sanidine and plagioclase wetted by trachytic glass and felsic aggregates with interstital trachytic glass. Trends of major and trace elements are consistent with crystal-liquid fractionation processes and rule out syn-eruptive mixing processes between latitic and trachytic magmas. We suggest that discrete crystals and crystal aggregates not in equilibrium with the host rock represent fragments of the crystallising boundary layer at the upper walls of the magma chamber, which were wrenched and admixed into the magma during the ascent. This process diversifies the mineral assemblage and increases the crystal content of the rocks. We propose that diffusive crystallization processes operating at the wall of the chamber allowed the formation of a two-fold layered reservoir with a more mafic, homogeneous lower body and a more evolved, compositionally graded upper body. Around one-quarter of crystals adhering to the upper part of the magma chamber were admixed into the magma during the eruption. The absence of significant syn-eruptive mixing processes and the major role played by diffusive crystallization are consistent with a low aspect ratio magma chamber (width/height <1). Received: 23 March 1998 / Accepted: 11 December 1998