U-Pb-Hf isotopic data from detrital zircons in late Carboniferous and Mid-Late Triassic sandstones,and also Carboniferous granites from the Tauride and Anatolide continental units in S Turkey: implications for Tethyan palaeogeography

ABSTRACT

Zircons from Carboniferous sandstones (three samples) and Mid-Late Triassic sandstones (four samples) from the Tauride and Anatolide continental units were analysed for U-Pb-Hf isotopes. For comparison, zircons were also analysed from Carboniferous granites of the Afyon Zone, Anatolides (three samples). A NE African/Arabian source is inferred for both the Carboniferous sandstones of the Taurides (Alada?) and the Anatolides (Konya Complex). In contrast, the Carboniferous Karaburun Melange is characterised by a NW African provenance. A prominent Devonian population occurs in the Carboniferous Karaburun Melange, characterised by mainly positive ε_Hf(t) values that differ significantly from those of the Devonian granites of the Sakarya continental crustal unit (Pontides). Middle-Late Triassic Tauride sandstones include minor Palaeozoic and Early Mesozoic zircons. In contrast, Devonian and Carboniferous zircons are relatively abundant in Late Triassic sandstones of the Karaburun Peninsula. The Hf isotopic compositions of 25 Carboniferous-aged zircons from three samples of Mid-Late Triassic sandstone and one of Late Carboniferous age (one sample) overlap with the ε_Hf(t) values of Carboniferous arc-type granites in the Anatolides. Taking account of the available U-Pb and Lu-Hf isotopic data from comparative crustal units, the Devonian zircon populations from the melanges in the Karaburun Peninsula and the Konya Complex are inferred to have a westerly source (e.g. granitic rocks of Aegean region or central Europe). A tectonic model is proposed in which Palaeozoic Tethys sutured during the late Carboniferous in the west (Aegean region westwards), leaving an eastward-widening oceanic gulf in which sandstone turbidites accumulated, including Devonian zircons.     

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.     

In situ LA-ICPMS Isotopic and Geochronological Studies on Carbonatites and Phoscorites from the Guli Massif,Maymecha-Kotuy,Polar Siberia

In this study we present a fresh isotopic data, as well as U–Pb ages from different REE-minerals in carbonatites and phoscorites of Guli massif using in situ LA-ICPMS technique. The analyses were conducted on apatites and perovskites from calcio-carbonatite and phoscorite units, as well as on pyrochlores and baddeleyites from the carbonatites. The ⁸⁷Sr/⁸⁶Sr ratios obtained from apatites and perovskites from the phoscorites are 0.70308–0.70314 and 0.70306–0.70313, respectively; and 0.70310–0.70325 and 0.70314–0.70327, for the pyrochlores and apatites from the carbonatites, respectively.Furthermore, the in situ laser ablation analyses of apatites and perovskites from the phoscorite yield εNd from 3.6 (±1) to 5.1 (±0.5) and from 3.8 (±0.5) to 4.9 (±0.5), respectively; εNd of apatites, perovskites and pyrochlores from carbonatite ranges from 3.2 (±0.7) to 4.9 (±0.9), 3.9 (±0.6) to 4.5 (±0.8) and 3.2 (±0.4) to 4.4 (±0.8), respectively. Laser ablation analyses of baddeleyites yielded an eHf(t)d of +8.5 (± 0.18); prior to this study Hf isotopic characteristic of Guli massif was not known. Our new in situ εNd, ⁸⁷Sr/⁸⁶Sr and eHf data on minerals in the Guli carbonatites imply a depleted source with a long time integrated high Lu/Hf, Sm/Nd, Sr/Rb ratios.In situ U–Pb age determination was performed on perovskites from the carbonatites and phoscorites and also on pyrochlores and baddeleyites from carbonatites. The co-existing pyrochlores, perovskites and baddeleyites in carbonatites yielded ages of 252.3 ± 1.9, 252.5 ± 1.5 and 250.8 ± 1.4 Ma, respectively. The perovskites from the phoscorites yielded an age of 253.8 ± 1.9 Ma. The obtained age for Guli carbonatites and phoscorites lies within the range of ages previously reported for the Siberian Flood Basalts and suggest essentially synchronous emplacement with the Permian-Triassic boundary.     

Mineral dissolution kinetics as a function of distance from equilibrium – New experimental results

We revisit a fundamental question in mineral dissolution kinetics, namely: is the function of dissolution rate versus the distance from equilibrium continuous, or does the “switch” between two different reaction mechanisms cause a discontinuity, i.e., a kinetic bifurcation? Based on new insight from experimental results, including direct observations of retreating crystal surfaces with vertical scanning interferometry (VSI), we present evidence that a discontinuity does indeed exist. Through a carefully designed near-equilibrium albite dissolution experiment, we show how a non-steady-state dissolution rate observed on a crystal surface reflects reactivity inherited from earlier episodes of undersaturation. This outcome forces us to re-think the common practice of extrapolating overall dissolution rates measured far-from-equilibrium to near-equilibrium conditions.     

Different styles of metasomatic veining in ultramafic xenoliths from the TUBAF Seamount (Bismarck Microplate,Papua New Guinea)

Petrologic, geochemical and isotopic investigations on two ultramafic xenoliths with metasomatic veins from the TUBAF Seamount in the Bismarck Archipelago NE of Papua New Guinea reveal different styles of metasomatic overprinting. The first xenolith, a clinopyroxene–poor spinel lherzolite, was part of the depleted upper mantle. It contains an orthopyroxene-rich vein that formed by hydrous metasomatism at ~ 980 °C and ~ 1.5 GPa. The second xenolith is a clinopyroxene-dominated spinel olivine websterite that formed as a magmatic cumulate at the transition of the upper mantle to the oceanic crust. The websterite contains a vein with orthopyroxenes and clinopyroxenes, which give evidence for high-temperature crystallization at ~ 1300 °C and < 0.36 GPa. Both xenoliths were transported to the seafloor by a Quaternary trachybasalt in a fore-arc position. The vein minerals show a strong affinity to a supra-subduction zone or island arc setting. The REE pattern of the vein in the clinopyroxene–poor lherzolite strongly resembles the one from the host trachybasalt, with a high enrichment of the LREE and a strong to moderate enrichment of the MREE and HREE. Although broadly similar in shape, the REE pattern of the vein in the websterite shows a much weaker enrichment. The same applies to the trace-element patterns, although there are significant differences in the Eu, Zr, Hf and Nb concentrations. The isotope signatures of both veins suggest a derivation from a subducted slab that had been hydrothermally altered by seawater (high ⁸⁷Sr/⁸⁶Sr values).The contrasting crystallization temperatures of the vein minerals as well as their overall geochemical differences indicate that the metasomatic agents responsible for the vein in the websterite were mobilized from a previously depleted source at a much deeper mantle level than those forming the vein of the clinopyroxene–poor lherzolite. The metasomatic agents may also have been mobilized at different times and from different plates, i.e., the deeply subducted Solomon Sea Microplate (for the veins in the websterite) and the shallow dehydrating Pacific Plate (for the veins in the clinopyroxene–poor lherzolite).Metasomatic agents responsible for similar petrologic phenomena, i.e., modal or cryptic metasomatism, may have distinctly different origins and show contrasting histories. A strongly depleted lherzolite may totally lose its initial geochemical signature by the influence of an enriched metasomatic agent, whereas a primarily enriched ultramafic rock, e.g., a websterite, may strongly obscure the trace-element pattern of a less enriched metasomatic vein. Furthermore, the geochemistry of the ultramafic xenoliths may reflect polyphase cryptic and modal metasomatism related to veining and later transport by the hosting melt to the seafloor.     

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.