Subglacial tunnel valleys in the Alpine foreland: an example from Bern, Switzerland

The morphology of the Alpine and adjacent landscapes is directly related to glacial erosion and associated sediment transport. Here we report the effects of glacio-hydrologic erosion on bedrock topography in the Swiss Plateau. Specifically, we identify the presence of subsurface valleys beneath the city of Bern and discuss their genesis. Stratigraphic investigations of more than 4,000 borehole data within a 430 km²-large area reveal the presence of a network of >200 m-deep and 1,000 m-wide valleys. They are flat floored with steep sided walls and are filled by Quaternary glacial deposits. The central valley beneath Bern is straight and oriented towards the NNW, with valley flanks more than 20° steep. The valley bottom has an irregular undulating profile along the thalweg, with differences between sills and hollows higher than 50–100 m over a reach of 4 km length. Approximately 500 m high bedrock highlands flank the valley network. The highlands are dissected by up to 80 m-deep and 500 m-broad hanging valleys that currently drain away from the axis of the main valley. We interpret the valleys beneath the city of Bern to be a tunnel valley network which originated from subglacial erosion by melt water. The highland valleys served as proglacial meltwater paths and are hanging with respect to the trunk system, indicating that these incipient highland systems as well as the main gorge beneath Bern formed by glacial melt water under pressure.     

Syn-orogenic extension in the Peloritani Alpine Thrust Belt (NE Sicily,Italy): Evidence from the Alì Unit

Structural and petrological analyses on the Alì Unit, in the Peloritani Thrust Belt, document the first evidence for Alpine exhumation associated with syn-orogenic extension in this part of the Calabria-Peloritani Arc. The Alì Unit displays ductile structures occurred during three Alpine deformation phases (D_a1, D_a2, D_a3). D_a1 and D_a3 developed in a contractional context, whereas D_a2 was generated in an extensional regime. The present-day tectonic contact between the Alì Unit and the overlying Mandanici Unit is interpreted as a low-angle extensional detachment responsible for the metamorphic break between the two units. Structural overprinting relationships indicate that the development of D_a2 structures and related tectonic exhumation occurred during syn-convergence extension, and were followed by further nappe stacking in the Peloritani Belt. To cite this article: R. Somma et al., C. R. Geoscience 337 (2005).     

Geochemistry of metabasites from NE Sardinia, Italy: nature of the protoliths, magmatic trend, and geotectonic setting

Summary The metabasites of Montiggiu Nieddu occur as a lenticular body in the Hercynian migmatite complex of NE Sardinia and consist of two major lithological associations: ultramafic amphibolites and plagioclase-banded amphibolites, which are genetically related by processes of cumulate differentiation of an original tholeiitic magma. The ultramafic amphibolites consist of relics of igneous phases (anorthite, olivine, orthopyroxene, and clinopyroxene) and metamorphic minerals (orthopyroxene, clinopyroxene, plagioclase, garnet, chlorite, and amphibole) in variable proportions. Based on the distribution of the relics of igneous minerals three main compositional layers (A, B, C) have been identified in the ultramafic amphibolites. The plagioclase-banded amphibolites consist of dark-green and white bands made up of plagioclase, amphibole, garnet, and rare pyroxene in variable proportions. No relics of magmatic minerals were observed. The trace element patterns of the Montiggiu Nieddu metabasites are characterised by selective enrichment of incompatible elements of low ionic potential, such as Sr, Rb, Ba, and Th, and low abundance of K, Cr, Ni, and elements of high ionic potential (from Ta to Yb), which define a relatively flat trend similar to MORB composition. Ti, Y, and heavy rare earth elements are all positively correlated with Zr. This correlation suggests that no significant transport of these elements took place during metamorphism. The metabasites of Montiggiu Nieddu are characterised by low Ti, Ti/V, and Ti/Zr content, low rare earth and high-field strength element concentrations, and moderate selective enrichment in large-ion lithophile elements. They also show geochemical features similar to those of supra subduction zone tholeiites. The CaO-MgO diagram displays a segmented trend, which provides powerful evidence in favour of crystal-liquid separation during magmatic evolution. Received December 4, 2000; revised version accepted January 29, 2001     

Phase petrology and P-T conditions of mafic blueschists from the Meliata unit, West Carpathians, Slovakia

Abstract Blueschists occurring as layers in calcite marbles of the Meliata unit occur along the so-called Roznava tectonic line situated in the southern part of the Gemericum, Slovakia. Mineral assemblages and compositions from seven blueschists localities and one occurrence of amphibolite facies rocks overprinted by blueschist metamorphism were investigated. The most common minerals in the blueschists are blue amphibole, epidote and albite. Some Fe²⁺- and Al-rich rocks also contain garnet and chloritoid, respectively. Na-pyroxene with a maximum 50% jadeite component was also found. The blue amphiboles correspond mostly to crossite and also to glaucophane and ferroglaucophane in some samples. Almandine- and spessartine-rich garnet has very low MgO content (<3 wt%). The Si content in phengite ranges between 3.3 and 3.5 pfu calculated on the basis of 11 oxygens. The zoning patterns of blue amphibole, garnet and chloritoid suggest their formation during a prograde stage of metamorphism. The P-T conditions of metamorphism are estimated to be about 380–460° C and 10–13 kbar. Pressures of 7.5–8.5 kbar and temperatures of 350–370° C were obtained for some actinolite- and aegirine-rich rocks. Apart from chlorite, other mafic minerals formed during retrograde metamorphism are biotite and occasionally also actinolite.     

Amphibole zonation in metabasites as a guide to the evolution of metamorphic conditions

In low grade metabasites the amphibole components tremolite, glaucophane, edenite and tschermakite have their activities controlled by interactions with the excess components albite, clinozoisite, chlorite, quartz and H₂O vapor. Three types of reaction are involved, (i) Those in which only components of condensed phases take part: isopleths of equilibrium constant are straight lines in the P-T plane. (ii) Dehydration reactions in which entropy change due to change in Al coordination is of the same sign as that due to dehydration: isopleths of constant K are positive at low pressure and negative at high pressure. (iii) Dehydration reactions in which entropy change due to Al coordination change is opposite in sign to that of dehydration: isopleths of constant K loop in the P-T plane with positive slopes at low and at high pressure. Zonation in naturally occurring amphiboles records the evolution of metamorphic conditions in particular rocks. In an example from the eastern Alps (Austria) early conditions calculated as 15 kb, 200 ° C evolve upgrade to 6 kb, 525 ° C implying concurrent heating and erosion. The record of evolving conditions may span some 30 Ma of geological history.     

Gold-telluride-sulfide association in the Sandaowanzi epithermal Au-Ag-Te deposit,NE China: implications for phase equilibrium and physicochemical conditions

The Sandaowanzi gold-telluride deposit, with a total reserve of ?≥?25 t of Au and an average grade of 15 g/t, is located in the Great Hinggan Range Metallogenic Belt in NE China. This deposit is the first reported case of a dominantly Au (±Ag)-telluride deposit in this area and it reveals highly economic bonanza Au- and Ag-telluride ores. Ore bodies principally occur in quartz veins and stockworks and minor in disseminations hosted by trachyandesites and andesitic breccias. Four paragenetic stages of mineralization are identified, demonstrating an early deposition of sulfides and subsequent precipitation of tellurides, which are mainly composed by petzite, sylvanite and to a lesser extent, hessite, calaverite, altaite, unnamed telluride (Au_1.8Ag_0.2Te), krennerite, empressite, stützite and coloradoite. Abundant telluride assemblages identified from Sandaowanzi ores are mostly attributed to breakdown of early tellurium-bearing phases (i.e., γ- and χ-phases) during cooling. The deposition of substantial Au-Ag-Te minerals are constructed under physicochemical conditions of T?=?240 to 280 °C, pH?=?4.39 to 5.64, logfO₂?=–44.8 to –41.8, logfTe₂?=–9.75 to –9.43, logαAu⁺ _(aq)/αAg⁺ _(aq)?=??6.87 to –6.56, and gold is mostly scavenged from a HTe^?-dominant ore-forming fluid. The unusually high Te concentrations in the Sandaowanzi epithermal system are likely attributed to alkaline to calc-alkaline magmatic degassing.     

The gabbro–eclogite transformation: an oxygen isotope and petrographic study of west Alpine ophiolites

Both magmatic and eclogitic parageneses are preserved in the gabbros of western Alpine ophiolites. Samples with relic magmatic mineralogies display partial transformation to eclogitic assemblages along cracks and grain boundaries. Gabbros with eclogitic mineralogies contain zoned pseudomorphs after olivine, comprising talc-rich cores with kyanite, Mg-chloritoid and omphacite in outer cores and garnet rims. The compositional zonation of these olivine pseudomorphs closely parallels that shown by olivines in hydrothermally altered ocean-floor gabbros. The eclogitic gabbros are hydrous, containing paragonite, zoisite and other water-bearing minerals, and it has been suggested that water was introduced during high-pressure metamorphism. However, the similarity of olivine alteration patterns to those of ocean-floor gabbros suggests that hydration and local metasomatism leading to the stability of aluminous minerals in olivine sites occurred during hydrothermal alteration prior to subduction. Oxygen-isotope systematics are consistent with this proposal: Alpine gabbros with magmatic relics have a mean δ¹⁸O value of 5.7±0.7, similar to that of unaltered oceanic crust, whereas eclogitic gabbros have a mean δ¹⁸O value of 4.8±0.9.This statistically significant difference is consistent with the eclogitic samples having undergone high-temperature ocean-floor alteration. The preservation of magmatic and hydrothermal δ¹⁸O values in ocean-floor gabbros that have been metamorphosed at 2–2.5 GPa (60–75 km) implies that the deeper levels of ocean crust have not experienced pervasive fluid flow during subduction or subsequent exhumation. Magmatic assemblages were preserved despite an overstep of eclogitization reactions by at least 0.6–1.1 GPa implying that equilibrium was not attained in undeformed parts of the system because of slow diffusion in water-deficient rock volumes.     

Identifying trends in groundwater quality using residence time indicators: an example from the Permian aquifer of Dumfries, Scotland

The Permian sandstone and breccia aquifer of Dumfries has an important role in supplying water to the principal town in southwest Scotland. The area comprises mainly pastoral farmland with some industry and fish farming. Ongoing development of the aquifer has revealed the existence of complex groundwater flow through fractures and increasing nitrate concentrations. To further investigate these issues, the age and quality of groundwater throughout the aquifer has now been assessed using standard hydrogeochemical techniques together with CFCs and SF₆ as residence time indicators. The aquifer consists of sandstone- and breccia-dominated units: the Locharbriggs Sandstone in the east and the Doweel Breccia in the west. Groundwater throughout the aquifer is of Ca–Mg–HCO₃ type and moderately mineralised; pH is near neutral. The observed groundwater chemistry is the product of maritime rainfall modified by the dissolution of carbonate material in the breccia, sandstone and surficial deposits. CFC and SF₆ concentrations are interpreted on the basis of mixing between older (>50 years) and recent (1990s) components. Although there is generally a higher proportion of older water within the Locharbriggs Sandstone compared to the Doweel Breccia, stable isotope evidence suggests that the older water component in the interbedded sandstones of the breccia is of much greater antiquity, possibly containing an element of palaeowater. Concentrations of nitrate across the aquifer can be directly related to the amount of recent recharge. Modern groundwater contains approximately 9 mg l^–1 NO₃-N and pre-1950s groundwater has approximately 2 mg l^–1 NO₃-N. Nitrate concentrations measured at individual boreholes are explained by the relative proportions of modern and pre-1950s groundwater. If current practices continue, the concentrations of nitrate measured across the Dumfries Basin will rise as the proportion of pre-1950s groundwater diminishes.

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Resumen El acuífero de areniscas y brechas Pérmicas de Dumfries desempeña un papel importante en el abastecimiento a la ciudad principal del Sudoeste de Escocia. El área comprende sobretodo tierras de pastoreo y granjas, junto con algunas industrias y piscifactorías. El desarrollo iniciado en el acuífero ha revelado la existencia de un complicado flujo subterráneo a través de las fracturas y de un aumento progresivo en la concentración de nitrato. Con el fin de profundizar en el conocimiento de estos temas, se ha determinado la edad y calidad de las aguas subterráneas en todo el acuífero por medio de técnicas hidrogeoquímicas estándar y de indicadores del tiempo de residencia como los clorofluorcarbonados (CFCs) y el fluoruro de azufre (SF₆). El acuífero está formado por las areniscas de Locharbriggs, al Este, y las brechas de Doweel, hacia el Oeste. Las aguas subterráneas son de tipo bicarbonatadas cálcico-magnésicas y presentan una mineralización moderada, mientras que el pH es prácticamente neutro. La química de las aguas subterráneas es el resultado del aerosol marino modificado por la disolución de materiales carbonatados en las brechas, areniscas y depósitos superficiales. Las concentraciones de CFCs y SF₆ se interpretan a partir de una mezcla de aguas entre una componente antigua (más de 50 años) y una reciente (de la década de 1990). Aunque en general se da una mayor proporción de aguas antiguas en las areniscas de Locharbriggs que en las Brechas de Doweel, los isótopos estables sugieren que la componente de aguas antiguas en las intercalaciones de areniscas que aparecen en la formación de brechas es mucho más antigua, y que, posiblemente, se trate de paleoaguas. Las concentraciones de nitrato en el acuífero pueden ser relacionadas directamente con la aportación de recarga reciente. Las aguas subterráneas modernas contienen aproximadamente 9 mg l^–1 de nitrato, mientras que las aguas subterráneas anteriores a 1950 tienen unos 2 mg l^–1. Se puede explicar las concentraciones de nitrato en pozos individuales por las proporciones relativas de aguas subterráneas modernas y anteriores a 1950. Si las prácticas actuales continúan, se producirá un aumento de las concentraciones de nitrato en la cuenca de Dumfries debido a la disminución de la proporción de aguas antiguas.

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Résumé L'aquifère des grès et brèches du Permien de Dumfries joue un rôle important dans l'alimentation en eau de la principale ville du sud-ouest de l'Écosse. La région est principalement soumise à de l'élevage pastoral avec quelques industries et des piscicultures. La progression de l'exploitation de cet aquifère a révélé l'existence d'un écoulement souterrain complexe en fractures et des concentrations en nitrate croissantes. Dans le but d'étudier ces deux points plus en détail, l'âge et la qualité de l'eau souterraine dans tout l'aquifère ont été évalués en utilisant des techniques hydrogéochimiques conventionnelles avec les CFC et SF₆ comme marqueurs du temps de séjour. L'aquifère est constitué par des unités dominées par des brèches et des grès: les grès de Locharbriggs à l'est et les brèches de Doweel à l'ouest. L'eau souterraine dans tout l'aquifère a un faciès bicarbonaté calcique et magnésien et est moyennement minéralisée; le pH est proche de la neutralité. Le chimisme observé de l'eau souterraine résulte des apports marins par la pluie modifiés par la dissolution du matériau carbonaté des brèches, des grès et des formations superficielles. Les concentrations en CFC et SF₆ sont interprétées sur la base d'un mélange entre des composantes ancienne (>50 ans) et récente (années 1990). Bien qu'il y ait en général une plus forte proportion d'eau ancienne dans les grès de Locharbriggs que dans la brèche de Doweel, les isotopes stables suggèrent que la composante d'eau ancienne dans les grès interstratifiés des brèches est beaucoup plus ancienne, contenant probablement un élément d'eau fossile. Les concentrations en nitrate dans l'aquifère peuvent être directement reliées au taux de recharge récente. L'eau souterraine moderne contient approximativement 9 mg l^–1 en NO₃-N et l'eau souterraine d'avant 1950 environ 2 mg l^–1 en NO₃-N. Les concentrations en nitrate mesurées dans des forages individuels sont expliquées par des proportions relatives d'eaux souterraines moderne et antérieure à 1950. Si les pratiques actuelles continuent, les concentrations en nitrate mesurées dans le bassin de Dumfries augmenteront alors que la part de l'eau souterraine d'avant 1950 diminuera.

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Paragonite–hornblende assemblages and their petrological significance: an example from the Austroalpine Schneeberg Complex, Southern Tyrol, Italy

ABSTRACT Paragonite-bearing amphibolites occur interbedded with a garbenschist-micaschist sequence in the Austroalpine Schneeberg Complex, southern Tyrol. The mineral assemblage mainly comprises paragonite + Mg-hornblende/tschermakite + quartz + plagioclase + biotite + ankerite + Ti-phase + garnet ± muscovite. Equilibrium P–T conditions for this assemblage are 550–600°C and 8–10 kbar estimated from garnet–amphibole–plagioclase–ilmenite–rutile and Si contents of phengitic muscovites. In the vicinity of amphibole, paragonite is replaced by symplectitic chlorite + plagioclase + margarite +± biotite assemblages. Muscovite in the vicinity of amphibole reacts to form plagioclase + biotite + margarite symplectites. The reaction of white mica + hornblende is the result of decompression during uplift of the Schneeberg Complex. The breakdown of paragonite + hornblende is a water-consuming reaction and therefore it is controlled by the availability of fluid on the retrogressive P–T path. Paragonite + hornblende is a high-temperature equivalent of the common blueschist-assemblage paragonite + glaucophane in Ca-bearing systems and represents restricted P–T conditions just below omphacite stability in a mafic bulk system. While paragonite + glaucophane breakdown to chlorite + albite marks the blueschist/greenschist transition, the paragonite + hornblende breakdown observed in Schneeberg Complex rocks is indicative of a transition from epidote-amphibolite facies to greenschist facies conditions at a flatter P–T gradient of the metamorphic path compared to subduction-zone environments. Ar/Ar dating of paragonite yields an age of 84.5 ± 1 Ma, corroborating an Eoalpine high-pressure metamorphic event within the Austroalpine unit west of the Tauern Window. Eclogites that occur in the Ötztal Crystalline Basement south of the Schneeberg Complex are thought to be associated with this Eoalpine metamorphic event.     

Metamorphic history and geodynamic significance of high-grade metabasites from the ophiolitic mélange beneath the Yarlung Zangbo ophiolites, Xigaze area, Tibet

Blocks of highly foliated amphibolite are locally embedded within a serpentinite mélange underlying the Yarlung Zangbo ophiolites in the Xigaze area of southern Tibet. The ophiolites are remnants of an Early Cretaceous back-arc basin within the Permo-Cretaceous Tethys Ocean, which are exposed along in the Yarlung Zangbo Suture Zone (YZSZ). These amphibolites are interpreted as fragments of a dismembered dynamothermal sole. Three types of amphibolite are present: (1) common amphibolite with assemblages of Hbl + Pl ± Ep ± Ap ± Ttn, (2) clinopyroxene-bearing amphibolite with Hbl ± Pl ± Cpx ± Ep ± Ttn ± Qtz ± Ap and (3) garnet–clinopyroxene-bearing amphibolite characterized by the assemblages Hbl + Cpx + Grt + Pl ± Rt and Grt + Hbl + Pl (corona assemblage). In all three types, plagioclase is pseudomorphed by late albite–prehnite. Retrograde cataclastic veins containing assemblages of Prh + Ab + Ep ± Chl are also present. P–T estimates indicate that the amphibolites reached peak metamorphic conditions of 13–15 kbar and 750–875 °C. Partial replacement of pyrope-rich (up to 35 mole%) garnet by Al-tschermakite (Al₂O₃ up to 21 wt%) reflects a high pressure (≈18 kbar, 600 °C) metamorphic event followed by rapid exhumation. Soon after exhumation, the amphibolites were intruded by very fine-grained diabase dykes that were then hydrothermally altered. The field relationships and metamorphic history of the amphibolites indicate formation during inception of subduction within a back-arc basin prior to obduction of the ophiolites onto the Indian passive margin.     

斑岩型Au矿床的包裹体标志:以黑龙江金厂金矿矿床为例

通过对黑龙江金厂金矿床和河北蔡家营铅．锌．银矿床的对比研究,发现斑岩中包裹体的演化和成分特征反映了成矿流体的演化、成矿能力和矿床成因,对这类矿床的找矿评价也具有一定的参考价值。金厂金矿床中代表岩浆演化各个阶段的包裹体类型完全,且包裹体富含盐类、成矿物质和挥发组分,表明该区岩浆结晶分异作用完善、具有较强的成矿能力,能形成斑岩型矿床。而河北蔡家营铅-锌-银矿床,斑岩中只有硅酸盐玻璃包裹体和后期盐水溶液包裹体,反映该岩浆分异演化程度不高、分异作用极不完善。含盐度较高而且大量挥发组分的存在可能是后期热液活化萃取围岩成矿物质的重要因素。     

Interplay between equilibrium and kinetics in prograde metamorphism of pelites: an example from the Nelson aureole, British Columbia

The distribution of metapelitic mineral assemblages in the Nelson aureole, British Columbia, generally conforms to what is predicted from phase equilibria. However, in detail, the sequence and spacing of isograds, mineral textures and mineral compositions and mineral chemical zoning do not. Two of the main disequilibrium features in the aureole are: (i) delay in the onset and progress of several reactions, i.e. overstepping in temperature; and (ii) unreactivity of staurolite and especially garnet porphyroblasts when they are reactants in prograde reactions. The thermal overstepping is ascribed to difficulty of nucleation of the product porphyroblasts and sluggishness of dissolution of porphyroblasts when they are reactants. The extent to which these kinetic barriers delay the onset of reaction is related to the reaction affinity of each reaction, defined herein as the Gibbs free‐energy difference between the thermodynamically stable, but not‐yet‐crystallized, products and the metastable reactants. For oversteps in temperature (ΔT), reaction affinity is, in turn, related to the difference in entropy (ΔS) between these two states through the relation A = ΔT * ΔS. Mineral reactions which release large quantities of H₂O, such as chlorite‐consuming reactions, have a higher entropy change per unit of temperature overstep, and therefore a higher reaction affinity, than those which release little or no H₂O, such as the chlorite‐free staurolite‐consuming reaction. Thermal overstepping is consequently expected to be less for the former than for the latter, as was estimated in the aureole where 0 to 30 °C overstepping was required for garnet, staurolite and andalusite growth from a muscovite + chlorite‐bearing precursor rock and ～70 °C overstepping was required for the growth of Al₂SiO₅ from a staurolite‐bearing, chlorite‐free precursor. In all cases, reaction progress was strongly influenced by the presence or absence of fluid, with presence of fluid lowering kinetic barriers to nucleation and growth and therefore the degree of thermal overstepping. Textural features of rocks from the nearly coincident garnet, staurolite and andalusite isograds are suggestive of a fluid‐catalysed ‘cascade effect’ in which reaction took place in a narrow temperature interval; several competing muscovite + chlorite‐consuming reactions, some metastable, appear to have occurred in parallel. Metamorphic reaction, fluid release and possibly fluid presence in general in the aureole were episodic rather than continuous, and in several cases well removed from equilibrium conditions. The extent to which these findings apply to regional metamorphism depends on several factors, a major one being enhanced deformation, which is expected to lower kinetic barriers to nucleation and growth.     

Determination of the thicknesses of oil films in micro- and nano-pores in tight oil sands: an example from the southern Songliao Basin,NE China

An experimental method of field-emission environmental scanning electron microscope (ESEM) coupled with the energy dispersive spectrum (EDS) to determine the thickness of oil films in the micro- and nano-pores of tight oil sands was proposed to overcome the limitations of current methods. First, we got the data of the detection range volumes, elemental mass percentages, and elemental atomic percentages using the EDS, and then calculated the volumes of oil films in the detection range according to the elemental mass percentages and the elemental atomic percentages of carbon. After that, we calculated the surface areas of oil films using the pore size data (lengths, widths, and depths) detected by ESEM. The thicknesses of oil films in the pores equaled the volumes divided by the surface areas. An analysis of tight oil samples from the Quan 4 Segment of in the southern Songliao Basin, NE China confirmed that the oil in the micro- and nano-pores had two main forms: oil films and oil droplets. The thicknesses of the oil films ranged from about 400 nm to 2 μm, while the thicknesses of the oil droplets ranged from about 200 to 700 nm.     

Gold scavenged by bismuth melts: An example from Alpine shear-remobilizates in the Highiş Massif, Romania

Summary Gold mineralization occurs in the Şoimuş Ilii vein, the main Cu prospect in the Highiş Massif, Western Apuseni Mts., Romania. The Highiş Massif is part of the Highiş Biharia Shear Zone, a 320–300 Ma Variscan greenschist belt, with a 114–100 Ma Alpine overprint. In Highiş, phyllonites enclose an igneous core consisting of an Early Permian basic complex intruded by Middle Permian granitoids. The vein is hosted within basalt hornfels at its contact with the 264 Ma Jernova granite. Gold is not only present as native gold, but also as jonassonite (ideally AuBi₅S₄). The latter occurs as inclusions 1–30 μm in size in chalcopyrite; microanalysis gives the empirical formulae Au_1.02(Pb_0.47Bi_4.51)_4.98S_4. The two Au minerals are spatially associated with Bi–(Pb) sulfosalts (oversubstituted bismuthinite, cosalite) and sulfotellurides/selenides (ingodite, ikunolite and laitakarite) in blebs/patches, mainly hosted in chalcopyrite. This Au–Bi–Te association overprints an earlier, chalcopyrite-quartz assemblage, occurring as trails along discrete zones of brecciation that crosscut former mineral boundaries. Curvilinear and cuspate boundary textures within the blebs/patches suggest deposition in a molten form. Mineral associations in combination with phase relations indicate that the Au–Bi–Te association formed as a result of melting of pre-existing native Bi (and possibly sulfosalts) at 400 °C under sulfidation conditions. These melts incorporated Au, Pb, Te and S as they moved in the vein during shearing and were locked within dilational sites. Native Bi occurs as coarse aggregates along vein margins, but in the Au–Bi–Te association, it is present only as small droplets in shear gashes, never together with other Bi- and Au-minerals. The Bi-derived melts are part of an internal remobilizate which also includes chlorite and adularia. Minerals in the system Au–Bi–Te were deposited from a neutral low reducing fluid during Alpine shearing in the Early Cretaceous. The fluid also assisted solid-state mobilisation of chalcopyrite and cobaltite. This study illustrates the significant potential of Bi, a low melting-point chalcophile element (LMCE), to act as Au scavenger at temperatures as low as 400 °C.     

Petrogenesis and tectonic significance of Jurassic IAT magma types in the Hellenide ophiolites as deduced from the Rhodiani ophiolites (Pelagonian zone, Greece)

The Rhodiani ophiolites are represented by two tectonically superimposed ophiolitic units: the “lower” Ultramafic unit and the “upper” Volcanic unit, both bearing calcareous sedimentary covers. The Ultramafic unit consists of mantle harzburgites with dunite pods and chromitite ores, and represents the typical mantle section of supra-subduction zone (SSZ) settings. The Volcanic unit is represented by a sheeted dyke complex overlain by a pillow and massive lava sequence, both including basalts, basaltic andesites, andesites, and dacites. Chemically, the Volcanic unit displays low-Ti affinity typical of island arc tholeiite (IAT) ophiolitic series from SSZ settings, having, as most distinctive chemical features, low Ti/V ratios (< 20) and depletion in high field strength elements and light rare earth elements.The rare earth element and incompatible element composition of the more primitive basaltic andesites from the Rhodiani ophiolites can be successfully reproduced with about 15% non-modal fractional melting of depleted lherzolites, which are very common in the Hellenide ophiolites. The calculated residua correspond to the depleted harzburgites found in the Rhodiani and Othrys ophiolites. Both field and chemical evidence suggest that the whole sequence of the Rhodiani Volcanic unit (from basalt to dacite) originated by low-pressure fractional crystallization under partially open-system conditions. The modelling of mantle source, melt generation, and mantle residua carried out in this paper provides new constraints for the tectono-magmatic evolution of the Mirdita–Pindos oceanic basin.     

High-pressure metamorphism in metabasites from the Betic Cordilleras (S.E. Spain) and its evolution during the Alpine orogeny

The Nevado-Filábride complex is the lowest tectonic unit of the Betic Zone sensu stricto (ss) of the Betic Cordilleras (S.E. Spain). The upper series of this complex consists of a metamorphosed sequence intruded by basic and ultrabasic igneous rocks. High-pressure metamorphism in the eclogite and blueschist facies is recorded in the metabasites, but this was partially obliterated by further successive metamorphic stages in the almandine-amphibolite and greenschist facies.Coronitic and granoblastic eclogites appear side by side in the large stocks of basic rocks. The coronitic eclogites originate from coarse-to medium-grained olivine gabbros, and the granoblastic eclogites from fine-grained basic rocks (dolerites and porphyritic basaltic rocks). Higher chemical mobility and rate of diffusion, as well as the availability of fluids during the eclogite facies metamorphism, are responsible for the greater degree of recrystallization found in the granoblastic eclogites. The availability of fluids during this metamorphic stage was controlled by the difference in the hydration of the protolith and by variable proximity to surrounding water-rich metasediments.The minerals in the eclogites are chemically homogeneous, suggesting that they are almost completely equilibrated, even in the coronitic eclogites. The estimated equilibrium P-T conditions were found to be the same (approximately 550° C at 12 kbar pressure) in both coronitic and granoblastic eclogites, and it has, therefore, been deduced that the coronitic eclogites do not represent the first and lower-grade step of a prograde metamorphism in which the granoblastic eclogites are the higher-grade step.No relationship was found between shearing and eclogite crystallization. Nevertheless, a first fabric/foliation developed in the later blueschist facies stage, and syntectonic growth of the minerals was detected in glaucophane-bearing rocks.The further metamorphic evolution of the metabasites from high-to intermediate-pressure conditions is documented by the formation of minerals belonging to albiteepidote and almandine-amphibolite facies assemblages. The application of the amphibole zonation model, in order to deduce the P-T path, does not give realistic values.High-pressure metamorphism is related to an early subduction event in the Betic Cordilleras, with a later more-or-less isothermal uplift to shallower levels.     

Phase equilibrium, geothermobarometric and xenotime age dating constraints on the Alpine metamorphism recorded in chloritoid schists from the southern part of the Tisia Mega-Unit (Slavonian Mts., NE Croatia)

The chloritoid schists from the Slavonian Mts., which are attributed to the basal part of Devonian to Permian “Hercynian Semimetamorphic Complex,” represent a very rare lithology, not only in the Tisia Mega-Unit outcrops in Croatia, but also in the wider area. The investigated outcrop in the Kutjeva?ka Rijeka transect (Mt. Papuk) encompasses chloritoid-bearing metapelitic and metapsammitic lithologies. Both contain K-white mica, chlorite, chloritoid (10–15 vol.%), quartz and minor K-feldspar, plagioclase (albite), opaque minerals and pyrophyllite, together with accessory zircon, rutile, xenotime. The Th–U–Pb age dating on xenotime grains within the K-white mica + chlorite + quartz matrix and on inclusions found inside the chloritoids gave an average age 120 ± 36 Ma. Peak metamorphic conditions during the Alpine chloritoid-forming event reached 3.5–4 kbar and 340–380 °C, based on phengite barometry, chlorite–chloritoid thermometry and intersection of chlorite and chloritoid isopleths in the KFMASH quantitative phase diagram. The post-tectonic character of lath- and rosette-shaped chloritoids with respect to two foliations in the rock, together with the older age of 219 ± 81 Ma obtained on Yb-rich xenotime core domain(s), implies a possible existence of older low-grade metamorphic phase(s). The chemistry of the chloritoid schists bears the signature of upper continental crustal felsic rocks as potential protoliths, probably the felsic rocks of the nearby Papuk Complex of Slavonian Mts. The evidence presented here for the chloritoid-bearing low-grade metamorphic rocks from the Slavonian Mountains clearly show that the prograde Alpine metamorphic event had a more significant influence on the evolution of the southern part of Tisia Mega-Unit than previously considered.     

A thermodynamic theory of the Grüneisen ratio at extreme conditions: MgO as an example

The Grüneisen ratio, γ, is defined as γy=αK _TV/C_v. The volume dependence of γ(V) is solved for a wide range in temperature. The volume dependence of αK _T is solved from the identity (? ln(αK _T)/? ln V)T ≡ δ _T-K′. α is the thermal expansivity; K _T is the bulk modulus; C _V is specific heat; and δ _Tand K′ are dimensionless thermoelastic constants. The approach is to find values of δ _T and K′, each as functions of T and V. We also solve for q=(? ln γ/? ln V) where q=δ _T -K′+ 1-(? ln C _V/? ln V)T. Calculations are taken down to a compression of 0.6, thus covering all possible values pertaining to the earth's mantle, q=? ln γ/? ln V; δ _T=? ln α/? ln V; and K′= (?K _T/?P)T. New experimental information related to the volume dependence of δ _T, q, K′ and C _V was used. For MgO, as the compression, η=V/V ₀, drops from 1.0 to 0.7 at 2000 K, the results show that q drops from 1.2 to about 0.8; δ _T drops from 5.0 to 3.2; δ _T becomes slightly less than K′; ? ln C _V/? In V→0; and γ drops from 1.5 to about 1. These observations are all in accord with recent laboratory data, seismic observations, and theoretical results.