Zur chemischen Variabilität regionalmetamorph gebildeter Plagioklase,Epidote und Granate

Plagioclases, Fe-rich epidotes and garnets coexisting in metamorphic rocks of basaltoid and granitoid substrata have been analyzed by standard microprobe techniques. The results presented are based on 34000 data on plagioclases, nearly 24400 on coexisting epidotes as well as of 5000 garnet data. The chemical variations within the singular plagioclase grain and in the range of a thin section are very comprehensive. Adjoining microfields (10–20 μm distance) exhibit differences in the An-contents between 30 and 80% rel. These observations are valid for nearly 78% of the examined plagioclases. Only in the higher amphibolite facies the An-variations are reduced to an average value of 32% rel. Merely the high temperature plagioclases with a compositional range of An 68-An 79 coexisting with clinopyroxene are largely homogeneous. Their variation is amounting to 6,5% An rel. The epidote chemisms reveal a dependance on both of the different substrata and their oxidation status. The maximum value of the Fe (III) substitution on the M₁ position in the epidote structure is reaching 97 molar per cent at the higher oxidized meta-granitoids and merely 87% at the metabasaltoids. The variations of the Fe/Al-proportions in the microfields are amounting to 5–20% as to the granitoid substrata and to 5–47% rel. as to the basaltoid substrata. In both of the substrata the garnets are very rich of almandine (40–70%). Besides variable amounts of grossularite and pyrope components they also contain 10–20% (basaltoids) and 1–20% (granitoids) spessartine. The most significant element variations are observed for the element Mn in garnets of granitoid origin. These variations attain 46% within the garnet grains. Only for the garnets of the basic substrata from the amphibolite facies one can state a homogenization. The reaction inertia in achieving metamorphic equilibrium is demonstrated by a selected metabasite sample of the amphibolite facies. It seems that only at temperature >600°C a homogenization of the plagioclases and garnets is initiated so that at low temperature a thermometry by means of An-contents in the plagioclases or Mn/Fe-relations in the garnets is not possible.     

Effects of different types of mechanical disturbances on a charophyte dominated macrophyte community

Charophytes are unique and endangered macroalgae. Mechanical stress is known to damage charophyte communities, however, virtually nothing is known how such disturbance affect seasonal succession and recovery of charophyte communities. We examined the response of a shallow water charophyte community to different types and timing of disturbances by conducting seasonal in situ field experiment in a brackish water ecosystem. The disturbances associated with mechanical removal of vegetation or sediment had the largest impact on the charophyte community. Disturbances that only partly damaged the plants had minor effects and communities were characterized by fast recovery. The timing of disturbance had significant effects on the charophyte community with stronger effects when disturbed at the early stage compared to mid-stage of seasonal succession.     

Site occupation and electric field gradient in acentric neptunite-measurements and evaluations concerning powder- and single crystal – Mössbauer spectroscopy and x-ray diffractometry

Mössbauer measurements on neptunite (KNa₂Li(Fe,Mn,Mg)₂Ti₂Si₈O₂₄) at 400?K reveal the distribution of Fe-ions on the crystallographic sites in agreement with neutron diffraction results published elsewhere. Even the previously postulated small amount of Fe-ions on the Ti(2) site has been detected, combined with a charge transfer which is in agreement with optical absorption investigations by other authors. A qualitative site occupation model is able to explain the different features of our observations. Single crystal Mössbauer measurements with the k-vector of incident γ-rays parallel to the crystallographic b-axis (space group Cc) of neptunite at different temperatures yield the angle β between the main component of the electric field gradient (EFG), V _zz and b. This angle is in close accordance with a calculated value of β for the Fe(1) position from electron density maps. The latter also reveal an absolute value of V _zz which is in satisfactory agreement with V _zz derived from Mössbauer spectroscopy.     

Die Bedeutung des Rückstandes von Druck-Lösungsvorgängen für stratigraphische Abfolgen,Wechsellagerung und Lagerstättenbildung

Zusammenfassung Druck-Lösungserscheinungen sind nicht auf stylolithische Kontakte beschränkt. Die Mehrzahl der Druck-Lösungsflächen ist glatt und nicht als solche erkennbar. Bei Abwesenheit von Stylolithisierung wird Druck-Lösungsrückstand meistens fälschlich als primäres Sediment angesprochen.Druckgelöstes Material wandert von Becken aus. Auf Schwellen führt es zu erhöhtem Stoffangebot für chemische und organische Ablagerung. Druckunlösliches Material konzentriert sich passiv in den Becken.Die Unterschiede zwischen benachbarten lithologischen Horizonten werden in stratigraphischen Abfolgen während der Diagenese über Druck-Lösungstätigkeit verstärkt. Wechsellagerungen können entstehen, indem relativ reine, fast monomineralische Horizonte (Kalkspat, Dolomit, Quarz) von Druck-Lösung kaum betroffen werden, während etwas mehr verunreinigte Schichten, besonders bei Gegenwart von Tonmineralien, bis auf ihren Lösungsrückstand zusammenschrumpfen können.Die während Faltung und Schieferung fortdauernde Druck-Lösungstätigkeit bewirkt weiteren Gesteinsschwund.Hauptsächlich synsedimentäre Lagerstätten können über Druck-Lösungsvorgänge während Diagenese und Orogenese eine weitere Anreicherung erfahren, indem Wertmineralien im Lösungsrückstand zurückbleiben. Verstärkte Anreicherungen sind in Richtung der Becken und Faltenschenkel zu erwarten.

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Pressure solution phenomena are not restricted to stylolitic contacts. The majority of pressure solution planes is smooth and not recognizable as such. Pressure solution residue in the absence of stylolitization is often misinterpreted as a primary sediment.Material dissolved under nonuniform pressure migrates from basins to ridges, where it leads to increased precipitation and organic deposition. The material that is insoluble under unilateral pressure thus becomes more concentrated in the basins.The differences between neighbouring lithological units in stratigraphic series are increased during diagenesis by pressure solution activity. Intercalations may result from relatively pure, almost monomineralic layers (e. g. calcite, dolomite, quartz) remaining relatively unaffected by pressure solution processes, while more impure layers, especially those containing clay minerals, may shrink to its solution residue.During folding and cleavage, continuing pressure solution activity results in further loss of rock matter.Mainly synsedimentary ore deposits may be further concentrated by pressure solution processes, both during diagenesis and orogenesis, leaving valuable minerals as part of the solution residue. Elevated concentrations are to be expected towards the basins and limbs of folds.

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Résumé Les phénomènes de dissolution par pression inégale ne sont pas limités aux contacts stylolithiques. La plupart des surfaces de dissolution par pression linéaire est lisse et sans caractères particuliers. En l'absence de stylolithisation, les résidus sont considérés souvent à tort comme des sédiments primaires.Le matériel dissous par pression linéaire migre du centre des bassins vers ses bords; cela conduit à un accroissement des dépÔts chimiques et organiques sur les seuils. Le matériel insoluble se concentre passivement au centre des bassins.Les différences entre horizons lithologiques voisins dans les séries stratigraphiques sont accentuées pendant la diagénèse par l'activité de dissolution par pression inégale. Il peut se former des alternances ou des horizons relativement »purs«, presque monominéraux (calcite, dolomite, quartz), qui ne sont pas touchés par la dissolution par pression inégale, tandis que des couches moins »pures« en raison de la présence en particulier de minéraux argileux, peuvent Être réduites à des résidus de dissolution.L'activité persistante de la dissolution par pression linéaire au cours du plissement et de la schistosité entraÎne une nouvelle diminution de matière dans la roche.Ce sont surtout les gisements synsédimentaires qui peuvent subir un enrichissement supplémentaire pendant la diagénèse et l'orogénèse, puisque ce sont les minéraux de valeur qui s'accumulent dans les résidus de dissolution. Ces enrichissements peuvent Être particulièrement renforcés en direction du centre des bassins et des flancs des plis.

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Multistage Variscan magmatism in the central Tauern Window (Austria) unveiled by U/Pb SHRIMP zircon data

U/Pb SHRIMP ages of nine Variscan leucocratic orthogneisses from the central Tauern Window (Austria) reveal three distinct pulses of magmatism in Early Carboniferous (Visean), Late Carboniferous (Stephanian) and Early Permian, each involving granitoid intrusions and a contemporaneous opening of volcano-sedimentary basins. A similar relationship has been reported for the Carboniferous parts of the basement of the Alps further to the west, e.g. the “External massifs” in Switzerland. After the intrusion of subduction-related, volcanic-arc granitoids (374?±?10?Ma; Zwölferkogel gneiss), collisional intrusive-granitic, anatectic and extrusive-rhyolitic/dacitic rocks were produced over a short interval at ca. 340?Ma (Augengneiss of Felbertauern: 340?±?4?Ma, Hochweißenfeld gneiss: 342?± 5?Ma, Falkenbachlappen gneiss: 343?±?6?Ma). This Early Carboniferous magmatism, which produced relatively small volumes of melt, can be attributed to the amalgamation of the Gondwana-derived “Tauern Window” terrane with Laurussia–Avalonia. Probably due to the oblique nature of the collision, transtensional phenomena (i.e. volcano-sedimentary troughs and high-level intrusives) and transpressional regimes (i.e. regional metamorphism and stacked nappes with anatexis next to thrust planes) evolved contemporaneously. The magmas are mainly of the high-K I-type and may have been generated during a short phase of decompressional melting of lithospheric mantle and lower crustal sources. In the Late Carboniferous, a second pulse of magmatism occurred, producing batholiths of calc-alkaline I-type granitoids (e.g. Venediger tonalite: 296?±?4?Ma) and minor coeval bodies of felsic and intermediate volcanics (Heuschartenkopf gneiss: 299?±?4?Ma, Peitingalm gneiss: 300?±?5?Ma). Prior to this magmatism, several kilometres of upper crust must have been eroded, because volcano-sedimentary sequences hosting the Heu- schartenkopf and Peitingalm gneisses rest unconformably on 340-Ma-old granitoids. The youngest (Permian) period of magma generation contains the intrusion of the S-type Granatspitz Central Gneiss at 271?±?4?Ma and the extrusion of the rhyolitic Schönbachwald gneiss protolith at 279?±?9?Ma. These magmatic rocks may have been associated with local extension along continental wrench zones through the Variscan orogenic crust or with a Permian rifting event. The Permian and the above-mentioned Late Carboniferous volcano-sedimentary sequences were probably deposited in intra-continental graben structures, which survived post-Variscan uplift and Alpine compressional tectonics.     

Bau und Entwicklung des Harznordrandes bei Bad Harzburg

Zusammenfassung Konglomeratische Oberkreidesandsteine im Raum Bad Harzburg wurden auf ihren Geröll- und Leitmineralgehalt hin untersucht. Aus der Leitmineralverteilung ergibt sich die stratigraphische Stellung der Emscher- und Senonvorkommen zueinander; aus der Geröllführung geht hervor, daß der Brockengranit bereits im Oberemscher im Erosionsniveau lag und die Aufrichtungszone mesozoischer Gesteine am Harznordrand auch im Schimmerwaldgebiet bis zum Quadratensenon übertage vorhanden war, obwohl sie heute hier nicht mehr zu finden ist. Ursache dieser Erscheinung ist ein Schollenabbruch aus dem Dach des Ilsenburggranits, der an der Wende Granulaten-Quadratensenon erfolgte und zu einer lokalen Überfahrung der Aufrichtungszone geführt hat. Die geologische Neuaufnahme des Paläozoikums nördlich des Ilsenburggranites stellt die heutigen Lagerungsverhältnisse dieses Gebietes klar; sie werden bei der Rekonstruktion der ursprünglichen Lagerung und des Bewegungsvorganges zugrunde gelegt, dessen Ergebnis die teilweise Überdeckung des Vorlandes ist. Ein ähnlicher Vorgang liegt auch am benachbarten Okervorsprung der Harznordrandlinie vor.Unter Berücksichtigung der Ergebnisse neuer Tiefbohrungen im Subherzyn wird eine Auffassung von Ablauf und Ursachen tektonischer Vorgänge in diesem Raum entwickelt, die der herrschenden Meinung in zahlreichen Punkten widerspricht:Eine echte Faltung des Subherzyns im Mesozoikum, die bisher vermutete starke Nordbewegung der Harzscholle und eine ausschließliche Bindung tektonischer Vorgänge in diesem Gebiet an orogene Phasen wird abgelehnt.Auslösender Vorgang aller tektonischen Erscheinungen im Subherzyn ist eine vertikale Differentialbewegung der Harz- und Vorlandscholle, die sich seit Ausgang des Paläozoikums gleichsinnig, aber mit örtlich und zeitlich wechselnder Intensität, abspielt.     

Sr-gorceixite,a weathering product in rich iron ores from the Córrego do Feijão Mine,Minas Gerais,Brazil

A sulfate-bearing Sr-Ba phosphate (Sr-gorceixite) formed by supergene alteration in rich iron ores from Minas Gerais, Brazil, has been studied by means of microprobe techniques. Analytical data and the structural formula are presented.     

Submarine hydrothermal activity and gold-rich mineralization at Brothers Volcano, Kermadec Arc, New Zealand

Brothers volcano, of the Kermadec intraoceanic arc, is host to a hydrothermal system unique among seafloor hydrothermal systems known anywhere in the world. It has two distinct vent fields, known as the NW Caldera and Cone sites, whose geology, permeability, vent fluid compositions, mineralogy, and ore-forming conditions are in stark contrast to each other. The NW Caldera site strikes for ??600?m in a SW?CNE direction with chimneys occurring over a ??145-m depth interval, between ??1,690 and 1,545?m. At least 100 dead and active sulfide chimney spires occur in this field and are typically 2?C3?m in height, with some reaching 6?C7?m. Their ages (at time of sampling) fall broadly into three groups: <4, 23, and 35?years old. The chimneys typically occur near the base of individual fault-controlled benches on the caldera wall, striking in lines orthogonal to the slopes. Rarer are massive sulfide crusts 2?C3?m thick. Two main types of chimney predominate: Cu-rich (up to 28.5?wt.% Cu) and, more commonly, Zn-rich (up to 43.8?wt.% Zn). Geochemical results show that Mo, Bi, Co, Se, Sn, and Au (up to 91?ppm) are correlated with the Cu mineralization, whereas Cd, Hg, Sb, Ag, and As are associated with the dominant Zn-rich mineralization. The Cone site comprises the Upper Cone site atop the summit of the recent (main) dacite cone and the Lower Cone site that straddles the summit of an older, smaller, more degraded dacite cone on the NE flank of the main cone. Huge volumes of diffuse venting are seen at the Lower Cone site, in contrast to venting at both the Upper Cone and NW Caldera sites. Individual vents are marked by low-relief (??0.5?m) mounds comprising predominately native sulfur with bacterial mats. Vent fluids of the NW Caldera field are focused, hot (??300°C), acidic (pH????2.8), metal-rich, and gas-poor. Calculated end-member fluids from NW Caldera vents indicate that phase separation has occurred, with Cl values ranging from 93% to 137% of seawater values. By contrast, vent fluids at the Cone site are diffuse, noticeably cooler (??122°C), more acidic (pH?1.9), metal-poor, and gas-rich. Higher-than-seawater values of SO₄ and Mg in the Cone vent fluids show that these ions are being added to the hydrothermal fluid and are not being depleted via normal water/rock interactions. Iron oxide crusts 3?years in age cover the main cone summit and appear to have formed from Fe-rich brines. Evidence for magmatic contributions to the hydrothermal system at Brothers includes: high concentrations of dissolved CO₂ (e.g., 206?mM/kg at the Cone site); high CO₂/³He; negative ??D and ??¹⁸O_H2O for vent fluids; negative ??³⁴S for sulfides (to ?4.6??), sulfur (to ?10.2??), and ??¹⁵N₂ (to ?3.5??); vent fluid pH values to 1.9; and mineral assemblages common to high-sulfidation systems. Changing physicochemical conditions at the Brothers hydrothermal system, and especially the Cone site, occur over periods of months to hundreds of years, as shown by interlayered Cu?+?Au- and Zn-rich zones in chimneys, variable fluid and isotopic compositions, similar shifts in ³He/⁴He values for both Cone and NW Caldera sites, and overprinting of ??magmatic?? mineral assemblages by water/rock-dominated assemblages. Metals, especially Cu and possibly Au, may be entering the hydrothermal system via the dissolution of metal-rich glasses. They are then transported rapidly up into the system via magmatic volatiles utilizing vertical (??2.5?km long), narrow (??300-m diameter) ??pipes,?? consistent with evidence of vent fluids forming at relatively shallow depths. The NW Caldera and Cone sites are considered to represent stages along a continuum between water/rock- and magmatic/hydrothermal-dominated end-members.     

Nuclear and incommensurate magnetic structure of NaFeGe2O6 between 5?K and 298?K and new data on multiferroic NaFeSi2O6

The compound NaFeGe₂O₆ was grown synthetically as polycrystalline powder and as large single crystals suitable for X-ray and neutron-diffraction experiments to clarify the low temperature evolution of secondary structural parameters and to determine the low temperature magnetic spins structure. NaFeGe₂O₆ is isotypic to the clinopyroxene-type compound aegirine and adopts the typical HT-C2/c clinopyroxene structure down to 2.5?K. The Na-bearing M2 polyhedra were identified to show the largest volume expansion between 2.5?K and room temperature, while the GeO₄ tetrahedra behave as stiff units. Magnetic susceptibility measurements show a broad maximum around 33?K, which marks the onset of low-dimensional magnetic ordering. Below 12?K NaFeGe₂O₆ transforms to an incommensurately modulated magnetic spin state, with k?=?[0.323, 1.0, 0.080] and a helical order of spins within the M1-chains of FeO₆ octahedra. This is determined by neutron-diffraction experiments on a single crystal. Comparison of NaFeGe₂O₆ with NaFeSi₂O₆ is given and it is shown that the magnetic ordering in the latter compound, aegirine, also is complex and is best described by two different spin states, a commensurate one with C2??/c?? symmetry and an incommensurate one, best being described by a spin density wave, oriented within the (1 0 1) plane.