Experimental investigation into the influence of stratification on the passive earth pressure

In this paper, the effect of inhomogeneous soil condition on the passive earth pressure is studied by using model tests. The model tests are carried out with two soil layers. The tests are compared with the corresponding results for homogenous soil. The shear band formation is identified with Particle Image Velocimetry measurements.     

Protoliths and phase petrology of whiteschists

Whiteschists appear in numerous high- and ultrahigh-pressure rock suites and are characterized by the mineral assemblage kyanite + talc (+-quartz or coesite). We demonstrate that whiteschist mineral assemblages are well stable up to pressures of more than 4 GPa but may already form at pressures of 0.5 GPa. The formation of whiteschists largely depends on the composition of the protolith, which requires elevated contents of Al and Mg as well as low Fe, Ca, and Na contents, as otherwise chloritoid, amphibole, feldspar, or omphacite are formed instead of kyanite or talc. Furthermore, the stability field of the whiteschist mineral assemblage strongly depends on XCO₂ and fO₂: already at low values of XCO₂, CO₂ binds Mg to carbonates strongly reducing the whiteschist stability field, whereas high fO₂ enlarges the stability field and stabilizes yoderite. Thus, the scarcity of whiteschist is not necessarily due to unusual P–T conditions, but to the restricted range of suitable protolith compositions and the spatial distribution of these protoliths: (1) continental sedimentary rocks and (2) hydrothermally and metasomatically altered felsic to mafic rocks. The continental sedimentary rocks that may produce whiteschist mineral assemblages typically have been deposited under arid climatic conditions in closed evaporitic basins and may be restricted to relatively low latitudes. These rocks often contain large amounts of the clay minerals palygorskite and sepiolite. Marine sediments generally do not yield whiteschist mineral assemblages as marine shales commonly have too high iron contents. Sabkha deposits may have too high CO₂ contents. Protoliths of appropriate geochemical composition occur in and on continental crust. Therefore, whiteschist assemblages typically are only found in settings of continental collision or where continental fragments were involved in subduction. Our calculations demonstrate that whiteschists can form by closed-system metamorphism, which implies that the chemical and isotopic composition of these rocks provide constraints on the development of the protoliths.     

Schuppung und Faltung im glazialtektonischen Experiment

Zusammenfassung Die wichtigsten glazialen Stauchungsvorgänge wurden experimentell nachgeahmt. Als Versuchsmaterial fanden erwärmte Sand-Paraffin-Gemenge mit eingelegten Schichten von Modellgips Verwendung. Bei Temperaturen über dem Schmelzpunkt des Paraffins besitzen sie ähnlich günstige Eigenschaften wie der meist bevorzugte Tonschlamm, bei Zimmertemperatur sind sie erhärtet und erlauben, im Gegensatz zum Tonschlamm, beliebige Profilschnitte und damit Einblicke in den inneren Bau.Die Experimente ergaben Zusammenhänge zwischen Veränderungen im Versuchsansatz und verschiedenen Störungstypen. Es entstanden:Schuppen bei Stauchung vor dem Modell einer Eiszunge,Falten bei Stauchung unter Belastung,Horizontal eingeschlichtete Streifen bei Bewegung des Modelles unter seiner Sohle.Die Beobachtungen stimmen weitgehend überein mit den Lagerungsstörungen in eisentstandenen Formen entsprechender Bildungsbedingungen.     

4He diffusion in specular hematite

In order to test the chronometer qualities of speculante for the (U + Th)/He dating method, ⁴He release experiments by stepwise heating of two specularites from the Rimbach mineralization locality in the southern Vosgues (France) have been carried out. The diffusion coefficients define linear Arrhenius plots within a temperature interval of 250 to 830 °C, which is suggestive of volume diffusion. Extrapolation of the diffusion behavior to 20° C yields diffusion coefficients (D₂₀ values) smaller than 10^?26 [cm² s^?1] for both hematites with activation energies at 116 [kJ/mole]. The results of our study suggest that specularite is a very helium retentive hematite variety which is capable of quantitatively retaining radiogenic helium over geologic periods of time.     

Uranium–lead ages of apatite from iron oxide ores of the Bafq District,East-Central Iran

Iron oxide–apatite (IOA) deposits, often referred to as Kiruna-type iron ore deposits, are known to have formed from the Proterozoic to the Tertiary. They are commonly associated with calc–alkaline volcanic rocks and regional- to deposit-scale metasomatic alteration. In the Bafq District in east Central Iran, economic iron oxide–apatite deposits occur within felsic volcanic tuffs and volcanosedimentary sequences of Early Cambrian age. In order to constrain the age of formation of these ores and their relationship with the Early Cambrian magmatic event, we have determined the U–Pb apatite age for five occurrences in the Bafq District. In a ²⁰⁶Pb/²³⁸U vs. ²⁰⁷Pb/²³⁵U diagram, apatite free of or poor in inclusions of other minerals plots along the Concordia between 539 and 527 Ma with four out of five samples from one deposit clustering at the upper end of this range. For this deposit, we interpret this cluster to represent the age of apatite formation, whereas the spread towards younger ages may reflect either minor Pb loss or several events of IOA formation. Apatite with inclusions of monazite (±xenotime) yields disturbed systems with inclusions having developed after formation of the iron ore–apatite deposits, possibly as late as 130–140 Ma ago. Obtained apatite ages confirms that (IOA) and the apatite-rich rocks (apatites) of the Bafq district formed coevally with the Early Cambrian magmatic (-metasomatic) events.     

Zircon fission‐track ages from Newfoundland—A proxy for high geothermal gradients and exhumation before opening of the Central Atlantic Ocean

Following Appalachian orogenesis, metamorphic rocks in central Newfoundland were exhumed and reburied under Tournaisian strata. New zircon fission‐track (ZFT) ages of metamorphic rocks below the Tournaisian unconformity yield post‐depositionally reset ages of 212–235 Ma indicating regional fluid‐absent reheating to at least ≥220°C. Post‐Tournaisian sedimentary thicknesses in surrounding basins show that burial alone cannot explain such temperatures, thus requiring that palaeo‐geothermal gradients increased to ≥30–40°C/km before final late Triassic accelerated cooling. We attribute these elevated palaeo‐geothermal gradients to localized thermal blanketing by insulating sediments overlying radiogenic high‐heat‐producing granitoids. Late Triassic rifting and magmatism before break up of Pangaea likely also contributed to elevated heat flow, as well as uplift, triggering late Triassic accelerated cooling and exhumation. Thermochronological ages of 240–200 Ma are seen throughout Atlantic Canada, and record rifting and basaltic magmatism on the conjugate margins of the Central Atlantic Ocean preceding the onset of oceanic spreading at ~190 Ma.     

Genetic relationship between silver–lead–zinc mineralization in the Wutong deposit,Guangxi Province and Mesozoic granitic magmatism in the Nanling belt,southeast China

More than 50 % of the world's total reserves of tungsten are in China and most tungsten deposits are located in the Nanling range in southeast China. This study explores the potential genetic relationship between tungsten–tin (W–Sn) mineralization and shallower Ag–Pb–Zn deposits in the Nanling range based on data from the Wutong deposit, Guangxi Province. The lead, oxygen, carbon, sulfur, and strontium isotopic compositions of minerals at Wutong indicate that a single crustal-derived fluid was responsible for mineralization. Wutong likely formed at relatively low temperatures (～200–300 °C) and low pressures, as indicated by the similarity between homogenization temperatures of fluid inclusions and those estimated from S isotopic compositions of minerals. The hübnerite age (92.3–104.4 Ma) indicates that the Wutong mineralization is likely related to nearby Late Yanshanian (Cretaceous) S-type granites derived from Proterozoic crust. This mineralization event coincides with the last W–Sn mineralization event and the Cretaceous peak of mineralization in the Nanling range.     

Redistribution of HFSE elements during rutile replacement by titanite

Titanite growth at the expense of rutile during retrograde hydration of eclogite into amphibolite is a common phenomenon. We investigated an amphibolite sample from the Tromsø eclogite facies terrain in Northern Norway to gain insight into the trace element distribution between rutile and titanite during incomplete resorption of the rutile by titanite. Patchy compositional zoning of Al, Ti, and F in titanite relates to the presence of a fluid with variable Ti/Al and/or F during its growth. Laser ablation ICP–MS and electron microprobe data for high field strength elements (HFSE: Nb, Zr, Ta, and Hf) of rutile resorbed by titanite indicate a pronounced enrichment of these elements in the rim of a large single rutile crystal (~8 mm) and a systematic decrease towards uniform HFSE contents in the large core. HFSE contents of smaller rutile grains (~0.5 mm) and rutile inclusions (<100 μm) in the titanite overgrowth are similar or higher than in the rims of large rutile crystals. Element profiles from the rim inward demonstrate that HFSE enrichment in rutile is controlled by diffusion. HFSE ratios in diffusion-altered rutile show systematic variations compared with the uniform core composition of the large rutile. Modelling of Zr and Nb diffusion in rutile indicates that diffusion coefficients in rutile in fluid-dominated natural systems must be considerably higher than those determined experimentally at 1 bar in dry systems. Variations of HFSE contents in the newly formed titanite show no systematic spatial distribution. HFSE ratios in titanite and the rims of rutile are different, indicating different solid/fluid distribution coefficients in these minerals. Element fractionation by diffusion into the relict rutile and during fluid-mediated growth of new titanite could substantially change the HFSE budget of these minerals and could affect their use for geochemical tracing and other applications, such as Zr-based geothermobarometry.     

Vesuvianite-new tool for the U-Pb dating of skarn ore deposits

Summary Vesuvianites from the Early Proterozoic Björntjärn tungsten skarn deposit in northern Sweden were dated with the U-Pb and Pb-Pb methods. Low²⁰⁶Pb/²⁰⁴Pb values make the U-Pb dating of vesuvianite rather sensitive to the common lead correction. However, the combination of Pb-Pb and U-Pb data on the same material permits the deduction of precise ages on Proterozoic vesuvianites. Vesuvianite can be used to date the formation of skarn mineralizations and possibly also the metamorphism and metasomatism of argillaceous limestones.

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Vesuvianit-ein neues Werkzeug zur U-Pb Datierung von Skarnerzlagerstätten

Zusammenfassung Vesuvianit von der Wolfram-Skarnerzlagerstätte Björntjärn in Nordschweden wurde mit der U-Pb and Pb-Pb Methode datiert. Das tiefe²⁰⁶Pb/²⁰⁴Pb Verhältnis des Vesuvianits macht die U-Pb Altersbestimmung kräftig von der Korrektur des gewöhnlichen Bleis abhängig. Die Kombination von Pb-Pb und U-Pb Altersbestimmung am gleichen Probenmaterial erlaubt jedoch die präzise Datierung proterozoischer Vesuvianite. Vesuvianit kann zur direkten Altersbesimmung von Skarnlagerstätten und möglicherweise der Metamorphose und Metasomatose von mergeligen Kalksteinen verwendet werden.

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With 4 Figures     

Stratigraphic evidence for an Early Aptian sea-level fluctuation: the Graua Limestone of south-eastern Ethiopia

The occurrence of a thin band of marine limestone (Graua Limestone) within a thick succession of fluviatile sandstones in south-eastern Ethiopia is direct evidence of flooding of part of the East African craton (Horn of Africa). According to the presence of abundant orbitolinid foraminifers (Palorbitolina lenticularis Blumenbach andPraeorbitolina cormyi Schroeder), the age of the Graua Limestone can be referred to the Early Aptian. Stratigraphy and palaeogeographic reconstructions for the Early Cretaceous in the surrounding regions (Kenya, Somalia, Yemen, Socotra, Oman, Syria, Lebanon, Israel, Egypt and Libya) show that the Early Aptian transgression was of regional extent. Our data seem to confirm that this transgression was of relatively short duration. This pulse cannot be related to tectono-eustatic mechanisms, which are too slow. A short-lived event should be invoked: either a regional tectonic pulse or the desiccation of the proto-South Atlantic.