The solubility of water in NaAlSi3O8 melts: a re-examination of Ab−H2O phase relationships and critical behaviour at high pressures

The solubility of water in melts in the NaAlSi₃O₈–H₂O system at high P and T was deduced from the appearance of quenched products and from water concentrations in the quenched glasses measured by ion probe, calibrated by hydrogen manometry. Starting materials were gels with sufficient water added to ensure saturation of the melts under the run conditions. Experiments were carried out for 10–30 h in an internally heated argon pressure vessel (eight at 1400° C and 0.2–0.73 GPa and three at 0.5 GPa and 900–1200° C) and for 1 h in a piston-cylinder apparatus (three at 1200° C, 1–1.3 GPa). No bubbles were observed in the glasses quenched at P<0.5 GPa or from T<1300° C at 0.5 GPa. Bubble concentration in glasses quenched from 1400° C was low at 0.5, moderate at 0.55 GPa and very high at 0.73 GPa and still higher in glasses quenched in the piston cylinder. Water concentration was measured in all glasses, except for the one at 0.55 GPa, for which it was only estimated, and for those at 0.73 GPa because bubble concentration was too high. Inferred water solubilities in the melt increase strongly with increasing P at 1400° C (from 6.0 wt% at 0.2 GPa to 15 at 0.55 GPa) and also with increasing T at 0.5 GPa (from 9.0 wt% at 900° C to 12.9 at 1400° C). The T variation of water solubility is fundamental for understanding the behaviour of melts on quenching. If the solubility decreases with T at constant P (retrograde solubility), bubbles cannot form by exsolution on isobaric quenching, whereas if the solubility is prograde they may do so if the cooling rate is not too fast. It is inferred from observed bubble concentrations and from our and previous solubility data that water solubility is retrograde at low P and prograde at and above 0.45 GPa; it probably changes with T from retrograde below to prograde above 900° C at 0.5 GPa. Moreover, the solubility is very large at higher pressures (possibly>30 wt% at 1.3 GPa and 1200° C) and critical behaviour is approached at 1.3 GPa and 1200° C. The critical curve rises to slightly higher P at lower T and intersects the three-phase or melting curve at a critical end point near 670° C and 1.5 GPa, above which albite coexists only with a supercritical fluid.     

Quantitative P-T paths from zoned minerals: Theory and tectonic applications

An analytical approach to the analysis of zoning profiles in minerals is presented that simultaneously accounts for all of the possible continuous reactions that may be operative in a given assemblage. The method involves deriving a system of simultaneous linear differential equations consisting of a Gibbs-Duhem equation for each phase, a set of linearly independent stoichiometric relations among the chemical potentials of phase components in the assemblage, and a set of equations describing the total differential of the slope of the tangent plane to the Gibbs free energy surface of solid solution phases. The variables are the differentials of T, P, chemical potentials of all phase components, and independent compositional terms of solid solution phases. The required input data are entropies, volumes, the compositions of coexisting phases at a reference P and T, and an expression for the curvature of the Gibbs functions for solid solution phases. Results derived are slopes of isopleths (dP/dT, dX/dT or dX/dP) which can be used to contour P-T diagrams with mineral composition.To interpret mineral zoning, T and P can be expressed as functions of n independent composition parameters, where n is the variance of the mineral assemblage. The total differentials of P and T are differential equations that can be solved by finite difference techniques using the derivatives obtained from the analytical formulation of phase equilibria.Results calculated from Zone I and Zone IV garnets of Tracy et al. (1976) indicate that Zone I garnets grew while T increased (T+72° C) and P decreased sharply (P–3 kb). Zone IV garnets zoned in response to decreasing T (T–17° C) and P (P–1 kb). A P-T path calculated for a zoned garnet from the Greinerschiefer series, western Tauern Window, Austria, also indicates growth during decompression (–3kb) and heating (T+15° C). A P-T path calculated for the Wissahickon schist (Crawford and Mark 1982) indicates growth during cooling and compression (T–25 C, P+2.2 kb). The calculated P-T paths differ according to structural environment and can be used to relate mineral growth to tectonic processes.     

Polymorphism in sapphirine

Sapphirine-1Tc, a polymorph of sapphirine, was found in granulites near Wilson Lake, Labrador. It is triclinic with unit cell data: a=10.04, b=10.38, c=8.65 Å, =107°33, =95°07, =123°55. The X-ray diffraction patterns of several crystals of sapphirine from Wilson Lake were investigated and evidence was found of solid state reactions that lead from the ordered polymorph-1Tc to a disordered arrangement of domains with the structure of sapphirine-2M, that is normal sapphirine.     

Late Archean metamorphism in the Buksefjorden region,Southwest Greenland

Standard state thermodynamic data extracted from experimental studies and applied to mineral assemblages in orthogneisses, metasedimentary gneisses and metabasites show that conditions of late Archean (2,850 m.y.) upper amphibolite facies were P _solid7.0 kb, T630° C, and rose to P _solid10.5 kb, T810° C in adjacent granulite facies. The estimates of solid pressure for the granulite facies suggest a late Archean crustal thickness of ca. 35 km, comparable to present day continental crust. Upper amphibolite facies assemblages were in equilibrium with about one half P _solid, while granulite assemblages equilibrated at much lower , varying from about one tenth P _solid in quartzofeldspathic gneisses to one third P _solid in more basic layers.     

Sulfur isotope ratios of Icelandic rocks

Sulfur isotope ratios have been determined in 27 selected volcanic rocks from Iceland together with their whole rock chemistry. The ³⁴S of analyzed basalts ranges from –2.0 to +0.4 with an average value of –0.8 Tholeiitic and alkaline rocks exhibit little difference in ³⁴S values but the intermediate and acid rocks analyzed have higher ³⁴S values up to +4.2 It is suggested that the overall variation in sulfur isotope composition of the basalts is caused by degassing. The small range of the ³⁴S values and its similarity to other oceanic and continental basalts, suggest that the depleted mantle is homogeneous in its sulfur isotope composition. The ³⁴S of the depleted mantle is estimated to be within the range for undegassed oceanic basalts, –0.5 to +1.0     

Calibration of Al/Si order variations in anorthite

New single crystal diffraction data for natural and heat-treated anorthite crystals (Angel et al. 1990) allow the determination of their states of Al/Si order in terms of a macroscopic order parameter,Q _OD , for the transition. Numerical values ofQ _OD obtained from estimates of site occupancies are shown to vary with the scalar spontaneous strain, _s , as _s Q _OD ² , and with the ratio of the sums of typeb (superlattice) reflections and typea (sublattice) reflections asI _b/I _a Q _OD ² . An empirical calibration for pure anorthite is obtained giving varies between 0.92 and 0.87 in samples equilibrated at T1300° C, but then falls off relatively rapidly with increasing temperature, reaching 0.7 near the melting point ( 1557° C). The observed temperature dependence does not conform to the predictions of the simplest single order parameter models; coupling ofQ _OD withQ of the transition is suspeeted.     

Formation of Mud-Volcanic Fluids in Taman (Russia) and Kakhetia (Georgia): Evidence from Boron Isotopes

Temperatures of the formation of mud-volcanic waters are determined based on concentrations of some temperature-dependent components (Na–Li, Mg–Li). Estimates obtained for the Taman and Kakhetia regions are similar and range from 45 to 170°, which correspond to depths of 1–4.5 km. The calculated temperatures correlate with the chemical (Li, Rb, Cs, Sr, Ba, B, I, and HCO₃) composition of water and ¹³ (₂) and ¹³ (CH₄) values in spontaneous gases. The isotope values indicate that mechanisms of the formation of ¹³-rich gases, i.e., gases with high ¹³ values (up to +16.0 in ₂ and –23.4 in CH₄) in mud-volcanic systems of Taman and Kakhetia are governed by fluid-generation temperatures rather than the supply of abyssal gases. The ¹¹ value was determined for the first time in mud-volcanic products of the Caucasus region. This value ranges from +22.5 to +39.4 in the volcanic water of Georgia, from –1.2 to +7.4 in the clayey pulp of Georgia, and from –7.6 to +13.2 in the clayey pulp of Taman. It is shown that the ¹¹ value in clay correlates with the fluid-generation temperature and ¹¹ correlates with ¹³ in carbon-bearing gases. These correlations probably testify to the formation of different phases of mud-volcanic emanations in a single geochemical system and suggest the crucial role of temperature in the development of isotope-geochemical features.     

A late Precambrian (∼ 710 Ma) high volcanicity rift in the southern Eastern Desert of Egypt

The late Precambrian Shadli Metavolcanics of SE Egypt constitute a slightly metamorphosed bimodal sequence that has been previously interpreted as manifesting volcanic activity at an island arc. We report the first Rb-Sr geochronologic, trace element (including REE), and Nd isotopic data for these rocks. Two types of basalt are recognized, the stratigraphically lower suite having compositions like N-MORB ferrobasalt while the overlying basalt is similar to slightly fractionated E-MORB. The two basalt types were derived from melting of a strongly depleted source, most likely within the upper 60–75 km of the upper mantle. The origin of the felsic melts is problematic, and these could either have fractionated from a mafic melt or resulted from melting of juvenile crust. The mafic and felsic lavas yield a Rb-Sr isochron age of 712±24 Ma that probably represents the time of volcanic eruption. The trace element characteristics of both mafic and felsic members of the Shadli Metavolcanics show few of the hallmarks of subduction-related melts, and we reject the hypothesis that these formed at an island arc. Instead, the field and geochemical data are most consistent with the hypothesis that these rocks originated in a magmatic rift, where the eruption of large volumes of lava accompanied large-scale lithospheric extension. This inference suggests that the tectonic setting of the important 700–715 Ma crust-forming event in NE Africa and Arabia needs to be critically reexamined.

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Zusammenfassung Die spätpräkambrischen Shadli-Metavulkanite in der südöstlichen Eastern Desert von Ägypten sind eine schwach metamorphe bimodale Basalt-Rhyodazit-Abfolge, die bisher als Ausdruck eines Inselbogen-Vulkanismus gedeutet wurde. Zwei Basalt-Typen können aufgrund ihrer unterschiedlichen chemischen Zusammensetzung unterschieden werden: der stratigraphisch untere Typ ist ein N-MORB ähnlicher Ferrobasalt, während der überlagernde Typ Charakteristika eines leicht fraktionierten E-MORB aufweist. Die beiden Basaltvarietäten werden von stark an inkompatiblen Elementen verarmten Schmelzen aus den oberen 60–75 km des Mantels abgeleitet. Die Entstehung der sauren Metavulkanite ist nicht eindeutig geklärt; sowohl Fraktionierung aus einer mafischen Schmelze als auch Aufschmelzung juveniler Kruste vom Inselbogen-Typ sind denkbar. Die mafischen und felsischen Gesteine definieren zusammen ein Rb-Sr-Isochronenalter von 712 ± 24 Ma, das wir als den Zeitraum der Eruption deuten. Die Spurenelement-Verteilung der Shadli-Metavulkanite weist keine der charakteristischen Merkmale von Subduktionsmagmatismus auf, und wir sehen daher keinen direkten Zusammenhang mit einer Inselbogen-Entwicklung. Wir interpretieren den Shadli-Vulkanismus als Resultat eines Riftprozesses in junger kontinentaler Kruste, ähnlich dem Rio Grande-Rift oder dem Afar-Dreieck, wo starke Lithosphärendehnung die Förderung großer Lavamengen ermöglichte. Diese Interpretation stellt das einfache Schema einer panafrikanischen Krustenbildung durch Inselbogen-Addition im arabisch-nubischen Schild in Frage und erfordert eine Neubewertung bisheriger Modellvorstellungen.

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Résumé Les roches volcaniques faiblement métamorphisées du Précambrien tardif de Shadli dans le Sud-Est de l'Egypte se caractérisent par une séquence bimodale qui était jusqu'ici interprétée comme résultant d'un volcanisme d'arc insulaire. Nous présentons ici les premières données isotopiques (Rb-Sr, Nd) obtenues pour ces roches ainsi que des données d'éléments en trace (y compris les Terres Rares). Deux types principaux de basaltes peuvent être distingués: les basaltes qui se trouvent dans la partie inférieure de la colonne stratigraphique, ont une composition comparable aux ferrobasaltes de type N-MORB, tandis que les roches se trouvant dans la partie supérieure de la séquence s'apparentent plutôt aux E-MORB légèrement fractionnés. Les deux types de basalte proviennent de la fusion partielle d'un manteau fortement appauvri, et ce à une profondeur probable de 60 à 75 km dans le manteau supérieur. L'origine des laves acides pose, quant à elle, quelques problèmes: elles peuvent s'être formées par cristallisation fractionnée à partir d'un liquide basique, ou alors elles sont le produit de la fusion partielle d'une croûte juvénile. La combinaison des données Rb-Sr obtenues pour les roches acides et basiques permet l'obtention d'une isochrone définissant un âge de 712 ± 24 Ma. Cet âge est interprété comme datant les éruptions volcaniques. Les données d'éléments en traces obtenues pour les laves basiques et acides ne présentent aucune des caractéristiques associées au volcanisme d'arc insulaire. Par conséquent, nous rejetons l'hypothèse selon laquelle les roches volcaniques de Shadli se seraient formées dans un contexte d'arc insulaire. Les données géochimiques et de terrain seraient plutôt en accord avec une hypothèse selon laquelle les roches volcaniques métamorphiques de Shadli se seraient formées dans une zone de rift où de très volumineux épanchements de laves auraient succédé à une extension à grande échelle de la lithosphère. Cette interprétation des données nous conduit à suggérer un réexamen critique du contexte tectonique entourant la période de formation crustale se situant entre 700 a 715 Ma, dans le nord-est de l'Afrique ainsi qu'en Arabie.

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Shadli, - , — , . : , N — MORB MORB, . , , 60–75 . ; , . , Rb/Sr, 712 ± 24 . , . Shadli ; , Shadli, , , , . - ; .

     

Über Phasen- und Materiezustände in den tieferen und tiefsten Erdzonen (Ergebnisse der modernen Hochdruck-Hochtemperatur-Forschung zum geochemischen Erdbild)

Zusammenfassung Die modernen Hochdruck-Hochtemperatur-Verfahren ermöglichen z. Z. bereits die Simulierung aller Druck-Temperatur-Bedingungen aller Tiefenzonen der Erde. Mit ihnen sind in knappen 5–10 Jahren Daten gewonnen worden, die erstmals begründete Auskunft geben über die möglichen Zustände und Eigenschaften der gesamten Erdmaterie. Hierbei ergaben sich folgende allgemeine kristallchemische Gesetzlichkeiten: Steigender Druck bewirkt sukzessive Packungsverdichtung mit steigenden K-Zahlen, dgl. sukzessive Valenzänderungen von ionogen über unpolar nach metallisch. Bei genügender Druckhöhe ist jede Stoffart in den metallischen Zustand überführbar. Die hierfür benötigten Drücke sind von Stoff zu Stoff sehr verschieden. So sind die oxidischen Verbindungen von Mg, Si, Al und Ca bis 1 Mbar noch weitgehend semipolar bis halbmetallisch, die Chalkogenide bereits vollmetallisch.Für Petrologie und Geochemie ergibt sich damit die Aufgabe, die möglichen Phasen- und Materiezustände zunächst der häufigeren Elemente der Erde, zu denen hier bewußt auch C, N, P und H gezählt werden, für die Haupttiefenzonen der Erde experimentell festzulegen. Das so gewonnene Erdbild ähnelt dem vonGoldschmidt, doch ergeben sich für Oxid- und Chalkogenidzone stark geänderte Tiefenlagen und Valenzzustände und für den Erdkern stark geänderter Chemismus und Phasenbestand. So besteht der äußere Erdkern hiernach wesentlich aus druckmetallisch gewordenen Chalkogeniden der innere aus Schwermetall-Karbiden,-Nitriden, -Phosphiden und vermutlich reichlich -Hydriden.

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Modern high pressure — high temperature-methods now allow the simulation of any pressure-temperature-condition of the earth. In the last 5–10 years data have been gained by these methods giving well-found information on possible states and properties of matter. From this two general rules concerning solid matter as a function of pressure have been stated: — Rising pressure leads to structures with rising packing and coordination-numbers and to successive change of the state of valence from ionic to homopolar and to metallic. Thus any matter may be transferred to the metallic state by application of sufficient pressure. Yet the amount of pressure needed for this transformation differs widely with the different types of matter. So the oxides of Mg, Si, Al and Ca are still mostly semipolar at 1 Mbar whereas the chalcogenides are already metallic. Petrologists and geochemists should now try to determine by experiments the possible states of phases and properties of matter of the major elements of the earth (including C, N, P and H) for the different zones of depth. This leads to a conception of the earth similar to the picture of Goldschmidt, yet to strong differences concerning depth-range, phases and states of valence of the oxide- and chalcogenide-zone and concerning chemical composition and phases of the core. Thus the outer core consists largely of chalcogenides which have become metallic by pressure, and the inner core chiefly consists of heavymetal carbides, nitrides, phosphides and, so may be guessed, large quantities of hydrides.

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Résumé Les procédés modernes de haute température — haute pression permettent d'imiter les conditions de pression et de températures des zones profondes de toute la terre. Depuis 5 à 10 ans on a gagné à l'aide de ces méthodes des renseignements fondés au sujet des états et des propriétés possibles de toute la matière terrestre.Pour la matière solide on trouve deux règles générales: 1. pression croissante entraîne des structures avec des nombres croissants de coordination et 2. avec un changement successif dans l'état de valence en direction ionique homéopolaire métallique. À l'aide d'une pression suffisante on peut donc transformer chaque substance à l'état métallique. Mais les pressions nécessaires diffèrent beaucoup pour les diverses substances. Ainsi les oxides de Mg, Si, Al et Ca sont encore sémipolaires jusqu'à 1 Mbar, tandis que les chalcogénides sont déjà métalliques.Du point de vue de la pétrologie et de la géochemie il en résulte la tâche d'établir à l'aide d'expériences les états de phases et les propriétés de la matière dans les diverses zones profondes de la terre, d'abord pour les éléments dominants, y compris C, N, P et H.Le modèle ainsi obtenu ressemble à celui deGoldschmidt. Cependant on trouve des différences considérables par rapport à l'emplacement et à l'état de valence pour la zone des oxides et des chalcogénides et aussi par rapport à la composition chimique et aux phases du noyau. Ainsi le noyau extérieur de la terre consiste en chalcogénides transformés à l'état métallique par la pression et le noyau intérieur en carbures, nitrures, phospures et - vraisemblablement - en hydrures.

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, . . , : . , ; — , , .

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Nach Vorträgen, gehalten auf der Jahrestagung der Geolog. Vereinigung in Mainz, März 1967 und auf der Geowissenschaftl. Tagung in Berlin, Oktober 1967.     

Ammoniten-Ökologie am Beispiel des Oberjura

Zusammenfassung In vielen Fällen beeinflussen äußere Faktoren das Vorkommen der Ammoniten. Im Oberjura spielt die Wassertiefe eine große Rolle, daneben tritt die Wassertemperatur. Voraussetzungen der ökologischen Analyse sind bodenbezogenes Leben der Tiere und postmortale Autochthonie der Schalen. Methoden und Fehlerquellen werden besprochen. Aus der ökologischen Bindung vieler Ammoniten ergeben sich Folgen, vor allem für Stratigraphie, Phylogenie und Ontogenie.

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In many cases external factors determine the occurence of the ammonites. In the Upper Jurassic the depth of the sea is of great importance, as well as the temperature of the water. Ecological analysis requires life of the animals close to the sea floor, and that the shells are autochthonous.Methods and sources of errors are discussed. The dependence on environment of many ammonites has consequences, especially for stratigraphy, phylogeny and ontogeny.

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Résumé Souvent des facteurs extérieurs déterminent la présence des ammonites. Dans le Jurassique Supérieur la profondeur de la mer joue un grand rôle, aussi bien que la température de l'eau. L'analyse écologique doit supposer que les animaux vivent près du fond de la mer et que les coquilles sont autochtones. Les méthodes et les sources d'erreurs sont discutées. La dépendance des ammonites de leur milieu a des conséquences avant tout pour la stratigraphie, la phylogénie et l'ontogénie.

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Transformation kinetics of polycrystalline aragonite to calcite: new experimental data,modelling, and implications

Experimental data are used to model the transformation rate of polycrystalline aragonite (grain diameter 80 m) to calcite. Optimized values for nucleation and growth rates were obtained by numerically fitting the overall transformation rates from 280° to 380°C and 0.10 MPa to an expression for a grain-boundary-nucleated and interface-controlled transformation. The nucleation rate is 4–5 orders of magnitude faster than for calcite nucleated within aragonite grains, and the growing in rate is slower below 300°C than for calcite growing in aragonite single crystals. The activation enthalpy for growth in polycrystalline aggregate is 247kJ/mol compared to 163 kJ/mol for growth in single crystals. Permanent deformation of the phases limits the elastic strain energy due to the 7% volume change and reduces the coherency of the calcite/aragonite interace. Theoretical expressions are used to extrapolate the data for nucleation and growth to other temperatures, and data from 0.10 to 400 MPa are used to evaluate the effect of pressure on the grain-boundary nucleation rate. Because of permanent deformation of the phases, the effective strain energy for nucleation is 0.55 kJ/mol, which is less than a quarter of the value for purely elastic deformation. These data are used to predict the percent transformation for various P-T-t paths; without heating during uplift partial preservation of aragonite in dry blueschist facies rocks can occur if the calcite stability field is entered at 235° C, and the kinetic data are also consistent with published P-T-t paths which include heating during uplift. The predicted percent transformation is relatively insensitive to variations in the initial grain size of the aragonite, but strongly dependent on the effective strain energy.     

A K-Ar and 40Ar/39Ar study on white micas from the Brixen Quartzphyllite,Southern Alps

The Brixen Quartzphyllite, basement of the Southern Alps (Italy), consists of metasediments which had suffered progressive deformation and low grade metamorphism (p _max4 kbar, T _max375±25° C) during the Palaeozoic. It has been excavated by pre-Permian erosion, buried again beneath a pile of Permo-mesozoic to Cainozoic sediments (estimated T _max150° C), and is now exposed anew due to late Alpine uplift and erosion. The behavior of the K-Ar system of white micas is investigated, taking advantage of the narrow constraints on their thermal history imposed by the geological/stratigraphic reference systems.The six structurally and petrographically differing samples come from a single outcrop, whose position is roughly two kilometers beneath the Permian land-surface. White mica concentrates from five grain size fractions (<2 , 2–6 , 6–20 , 20–60 , 60–75 ) of each sample have been analyzed by the conventional K-Ar method, four selected concentrates additionally by the ⁴⁰Ar/³⁹Ar stepwise heating technique; furthermore, Ar content and isotopic composition of vein quartz were determined.The conventional ages of the natural grain size fractions (20–60 , 60–75) are in the range 316±8 Ma, which corresponds to the ⁴⁰Ar/³⁹Ar plateau age of 319.0±5.5 Ma within the error limits. The finer grain size fractions yield significantly lower ages, down to 233 Ma for fractions <2 . Likewise low apparent ages (down to 83 Ma) are obtained for the low temperature ⁴⁰Ar/³⁹Ar degassing steps.There is no correlation between microstructural generation of white mica prevailing in the sample and apparent age. This favours an interpretation of the 316±8 Ma values as cooling age; progressive deformation and metamorphism must be respectively older and their timing cannot be resolved by these methods. The data preclude any significant influence of a detrital mica component as well as of excess argon.The lower ages found for the fine grain-size fractions (respectively the low-T degassing steps) correspond to a near-surface period (p-T-minimum); the values are geologically meaningless. The effect is interpreted to result from partial Ar loss due to reheating during Mesozoic-Cainozoic reburial. A model based on diffusion parameters derived from the outgassing experiments and Dodson's (1979) equation yields a closure temperature of 284±40 °C for a cooling rate of 18° C/Ma. Furthermore, this model suggests that the observed argon loss of up to 5% may in fact have been induced by reheating to 150 °C for 50 Ma.     

Marine hdrothermal alteration at a Kuroko ore deposit,Kosaka, Japan

Isotopic compositions were determined for quartz, sericite and bulk rock samples surrounding the Uwamuki no. 4 Kuroko ore body, Kosaka, Japan. ¹⁸O values of quartz from Siliceous Ore (S.O.), main body of Black Ore B.O.) and the upper layer of B.O. are fairly uniform, +8.7 to +10.5. Formation temperatures calculated from fractionation of ¹⁸O between sericite and quartz from B.O. and upper S.O. are 250° to 300° C. The ore-forming fluids had ¹⁸O values of +1 and D values of –10, from isotope compositions of quartz and sericite.Tertiary volcanic rocks surrounding the ore deposits at Kosaka have uniform ¹⁸O values, +8.1±1.0 (n=50), although their bulk chemical compositions are widely varied because of different degrees of alteration. White Rhyolite, which is an intensely altered rhyolite occurring in close association with the Kuroko ore bodies, has also uniform ¹⁸O values, +7.9±0.9 (n=19). Temperatures of alteration are estimated to be around 300° C from the oxygen isotope fractionation between quartz and sericite. Paleozoic basement rocks phyllite and chert, have high ¹⁸O values, +18 and +19. The Sasahata formation of unknown age, which lies between Tertiary and Paleozoic formations, has highly variable ¹⁸O, +8 to +16 (n=4). High ¹⁸O values of the basement rocks and the sharp difference in ¹⁸O at their boundary suggest that the hydrothermal system causing Kuroko mineralization was mainly confined within permeable Tertiary rocks. D values of altered Tertiary volcanic rocks are highly variable ranging from –34 to –64% (n=12). The variation of D does not correlate with change of chemical composition, ¹⁸O values, nor distance from the ore deposits. The relatively high D values of the altered rocks indicate that the major constituent of the hydrothermal fluid was sea water. However, another fluid having lower D must have also participated. The fluid could be evolved sea water modified by interaction with rocks and the admixture of magmatic fluid. The variation in D may suggest that sea water mixed dispersively with the fluid.     

Satellitenbildauswertung zur Feststellung von Trinkwasserreserven im Rheinischen Schiefergebirge

Zusammenfassung Im östlichen Rheinischen Schiefergebirge konnten innerhalb der im Allgemeinen gering permeablen paläozoischen Festgesteinskomplexe Bereiche guter Wasserwegsamkeit festgestellt werden. Sie sind vornehmlich an Nord-Süd oder Ost-West gerichtete Zonen junger (tertiärer) Tektonik gebunden und vorwiegend feldgeologisch nicht kartierbar. Mit Hilfe der Satellitenbildauswertung lassen sich diese Bereiche gut erfassen, da sie als Lineare abgebildet werden.Die Erfassung und Abgrenzung der Zonen junger Tektonik stellt eine wichtige Grundlage zur Sicherung der Trinkwasserversorgung für Gemeinden im Rheinischen Schiefergebirge dar. Sie ist außerdem ein Hilfsmittel zur Beurteilung von Fragen des Grundwasserschutzes.An Hand eines LANDSAT-1 Bildes wurde eine Linearkarte angefertigt, die mit einem umfangreichen Abflußmeßprogramm verglichen wurde. Dabei ergab sich, daß die von Linearen durchzogenen Teilgebiete normalerweise überdurchschnittliche Abflußspenden haben. Auch die in diesen Bereichen gelegenen Tiefbrunnen weisen überdurchschnittliche Förderraten auf.Über die jungen tektonischen Schwächezonen besteht eine gute Verbindung zwischen Oberflächen- und Grundwasser. Daraus resultiert die Gefahr einer möglichen Grundwasserkontamination durch verunreinigtes Oberflächenwasser. Dies bezieht sich besonders auf in Linearbereichen liegende Mülldeponien. So konnte zum Beispiel eine Verunreinigung von oberflächennahem Grundwasser durch die Schlammdeponie Hirzenhain nachgewiesen werden.Die Linearkarte stellt eine Basis für vorbeugende Maßnahmen des Trinkwasserschutzes dar, da mit ihrer Hilfe eine Ausweisung bzw. Neuabgrenzung von Trinkwasserschutzgebieten möglich ist, wie am Beispiel der Tiefbrunnen von Biedenkopf ausgeführt wird.

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Linear zones of good permeability were recognized within the paleozoic strata of the Rheinisches Schiefergebirge, where the permeability of these intensively folded strata normally is low. These zones are related to young tectonic events of Tertiary. They follow North-Southern or East-Western directions and they can generally not be detected by methods of field-geology. The zones were mapped by interpretation of satellite-images where they show up as linear structural elements.The method can be used to back groundwater research and to prevent groundwater contamination. A LANDSAT-1 image was used for the structural interpretation. The resulting map was compared to a program of run-off measurements. Run-off rates above the average were proved at areas effected by linear elements. Also the production-rates of waterwells in these areas was proved to be above the average.Good communication between surface-water and ground-water has been recognized. This includes the possibility of groundwater pollution for example of refuse-deposits. Contamination of this kind has been found at several localities.

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Résumé Dans le Rheinische Schiefergebirge oriental, on a pu déterminer au sein du complexe paléozoïque, en général peu perméable, des domaines à bonne perméabilité due à l'existence de joints et de fissures.Ces domaines sont avant tout liés à des zones à tectonique récente (tertiaire), de direction Nord-Sud ou Est-Ouest, pratiquement impossibles à cartographier pour la plupart d'entre elles. L'interprétation des photos prises par satellite permet de les bien définir car ils apparaissent sous forme de lignes. La considération et la délimitation des zones à tectonique récente constitue une base importante pour assurer l'alimentation en eau potable des communes dans le Massif schisteux rhénan, et, en outre, un moyen de porter un jugement sur les questions concernant la protection des nappes souterraines. Une carte de ces lignes a été établie à partir d'un photo de LANDSAT-1; elle fut ensuite comparée à un vaste programme de mesures de l'écoulement. Cette étude montra que les régions traversées par de telles lignes ont normalement des débits au-dessus de la moyenne. Il en est de même en ce qui concerne les puits creusés dans ces domaines.Dans les zones à tectonique récente faible, il existe une bonne liaison entre les eaux de surface et les nappes souterraines, d'où le danger de contamination possible de ces mêmes nappes par les eaux de surface polluées. Cela concerne particuli-rement les dépôts d'immondices; a titre d'exemple, il a été possible de prouver la pollution des nappes souterraines à partir de la surface à la suite du dépôt de boues à Hirzenhain.La carte des lignes représente une bonne base pour décider des mesures de protection des eaux potables, puisqu'il est possible, grâce à elle, de faire une nouvelle délimitation des domaines de protection des eaux potables, comme par exemple, au puits de Biedenkopf.

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, , , . , N—S, E—W. , .. . . . LANDSAT-1 , , . , . . .

     

Scapolite-bearing and related calc-silicate layers from the alpujarride series. (Betic cordilleras of Southern Spain). A discussion on their origin and some comments

Generalized development of calc-silicate bands in several alpujarride units of the central and western segments of the Betic Zone, is interpreted as the result of metamorphic differentiation from original calcium-rich sedimentary beds in the sequence. Presence of Scapolite suggests that this phenomenon may have been made possible by the catalizing action of high anion concentrations in the pore fluid medium during metamorphism, probably caused by the existence of evaporic material in the original sediments. This hypothesis, which is further supported by anomalous high Ca, Na and K contents of the enclosing metapelites, leads to the correlation of the present calc-silicate bearing metamorphic formations with well-known permo-werfenian evaporitic series in other units of the Betic Zone. The use of this new criterion of stratigraphic correlation would indicate that highly metamorphosed permo-triassic sequences have often been misidentified as more ancient formations, thus causing an underestimation of the importance of alpine HT-LP metamorphism in the alpujarride tectonic domain.

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Zusammenfassung Das häufige Auftreten von Kalksilikatbändern (manchmal mit Skapolith) in einigen alpujarriden Einheiten, besonders im zentralen und westlichen Abschnitt der betischen Kordillere, wird erklärt als Ergebnis einer metamorphen Differentiation von ursprünglich Ca-reichen Schichten. Die Anwesenheit von Skapolith legt eine hohe Anionenkonzentration der intergranularen fluiden Phasen während der Metamorphose nahe, bedingt durch evaporitisches Material der ursprünglichen Sedimente. Diese Hypothese, zusätzlich gestützt durch den extrem hohen Gehalt der umgebenden Pelite an löslichen Kathionen (Ca, Na, K), erlaubt die Korrelation der Kalksilikatbänder-haltigen Formationen mit aus anderen betischen Einheiten bekannten evaporitischen Serien des Permowerfenïan. Auf die stratigraphische Korrelation angewendet, legt dies nahe, daß öfters hochmetamorphe, permotriadische Serien fälschlicherweise als älter betrachtet wurden, was zu einer Unterschätzung der alpinen Metamorphose (Niederdruck-Hochtemperatur) im alpujarridem Bereich führte.

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Resumen El desarrollo bastante generalizado de bandas de silicatos calcicos, entre los cuales se encuentra a veces una escapolita, en varias unidades alpujárrides de los segmentos central y occidental de la Zona Bética, es interpretado como el resultado de un proceso de diferenciación metamórfica a partir de lechos sedimentarios originalmente ricos en calcio. La presencia de escapolita sugiere que este hecho ha sido probablemente posible gracias a la existencia de una alta concentración de aniones en el fluido metamórfico intergranular, condicionada por la existencia de material evaporitico en los sedimentos originales. Esta última hipótesis que es ulteriormente apoyada por un contenido anómalamente alto de cationes solubles (Ca, Na, K) en las metapelitas encajantes, permite correlacionar las formaciones que contienen los lechos de silicatos calcicos con conocidas series evaporiticas de edad permowerfenense en otras unidades de la Zona Bética. La aplicación de este nuevo criterio de correlación estratigráfica sugiere que series permotriásicas altamente metamorfizadas han sido a menudo erróneamente interpretadas como más antiguas, hecho que ha conllevado a una substimación de la importancia del metamorfismo (BP-AT) alpino en el dominio tectónico alpujárride.

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Résumé Le developpement très fréquent de roches litées à silicates calciques dans quelques unités alpujarrides des regions centrale et occidentale de la Zone Bétique, est interpreté comme la conséquence d'un processus de différenciation métamorphique à partir de lits riches en calcium dans les sequences sédimentaires originelles. La présence de scapolite suggére que ce phénomène a été rendu possible par l'intervention d'un cataliseur naturel, tel que des hautes concentrations en anions dans le milieu fluide intergranulaire pendant le métamorphisme, occasionées probablement par l'existence de materiel évaporitique dans les sédiments originels.Cette hypothese, qui se trouve additionellement supportée par des teneurs élevées en cations solubles des métapelites encaissantes, permet d'assimiler les formations qui contiennent les lits à silicates calciques avec des séries évaporitiques permowerféniennes, bien connues dans d'autres unités des zones internes des Cordilléres Bétiques. L'application de ce nouveau critère de corrélation stratigraphique indiquerait que des séquences permotriasiques fortement affectées par le métamorphisme ont été souvent à tort interprétées comme plus anciennes, amenant ainsi a une sous-estimation de l'importance du métamorphisme alpin (BP-HT) dans le domaine tectonique alpujarride.

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- — — , , , , . , . , : , Na, , , - Permowerfenian, . , - , — — .

     

Stable isotope geochemistry and phase equilibria of coesite-bearing whiteschists,Dora Maira Massif,western Alps

Peak metamorphic temperatures for the coesite-pyrope-bearing whiteschists from the Dora Maira Massif, western Alps were determined with oxygen isotope thermometry. The ¹⁸O(smow) values of the quartz (after coesite) (¹⁸O=8.1 to 8.6, n=6), phengite (6.2 to 6.4, n=3), kyanite (6.1, n=2), garnet (5.5 to 5.8, n=9), ellenbergerite (6.3, n=1) and rutile (3.3 to 3.6, n=3) reflect isotopic equilibrium. Temperature estimates based on quartz-garnet-rutile fractionation are 700–750 °C. Minimum pressures are 31–32 kb based on the pressure-sensitive reaction pyrope + coesite = kyanite + enstatite. In order to stabilize pyrope and coesite by the temperature-sensitive dehydration reaction talc+kyanite=pyrope+coesite+H₂O, the a(H₂O) must be reduced to 0.4–0.75 at 700–750 °C. The reduced a(H₂O) cannot be due to dilution by CO₂, as pyrope is not stable at X(CO₂)>0.02 (T=750 °C; P=30 kb). In the absence of a more exotic fluid diluent (e.g. CH₄ or N₂), a melt phase is required. Granite solidus temperatures are 680 °C/30 kb at a(H₂O)=1.0 and are calculated to be 70°C higher at a(H₂O)=0.7, consistent with this hypothesis. Kyanite-jadeite-quartz bands may represent a relict melt phase. Peak P-T-f(H₂O) estimates for the whiteschist are 34±2 kb, 700–750 °C and 0.4–0.75. The oxygen isotope fractionation between quartz (¹⁸O=11.6) and garnet (¹⁸O=8.7) in the surrounding orthognesiss is identical to that in the coesitebearing unit, suggesting that the two units shared a common, final metamorphic history. Hydrogen isotope measurements were made on primary talc and phengite (D(_SMOW)=-27 to-32), on secondary talc and chlorite rite after pyrope (D=-39 to -44) and on the surrounding biotite (D=-64) and phengite (D=-44) gneiss. All phases appear to be in nearequilibrium. The very high D values for the primary hydrous phases is consistent with an initial oceanicderived/connate fluid source. The fluid source for the retrograde talc+chlorite after pyrope may be fluids evolved locally during retrograde melt crystallization. The similar D, but dissimilar ¹⁸O values of the coesite bearing whiteschists and hosting orthogneiss suggest that the two were in hydrogen isotope equilibrium, but not oxygen isotope equilibrium. The unusual hydrogen and oxygen isotope compositions of the coesite-bearing unit can be explained as the result of metasomatism from slab-derived fluids at depth.     

Changes in stable isotope ratios of metapelites and marbles during regional metamorphism,Mount Lofty Ranges,South Australia: implications for crustal scale fluid flow

The Mount Lofty Ranges comprises interlayered marbles, metapsammites, and metapelites that underwent regional metamorphism during the Delamarian Orogeny at 470–515 Ma. Peak metamorphic conditions increased from lowermost biotite grade (350–400°C) to migmatite grade (700°C) over 50–55 km parallel to the lithological strike of the rocks. With increasing metamorphic grade, ¹⁸O values of normal metapelites decrease from 14–16 to as low as 9.0, while ¹⁸O values of calcite in normal marbles decrease from 22–24 to as low as 13.2 These isotopic changes are far greater than can be accounted for by devolatilisation, implying widespread fluid-rock interaction. Contact metamorphism appears not to have affected the terrain, suggesting that fluid flow occurred during regional metamorphism. Down-temperature fluid flow from synmetamorphic granite plutons (¹⁸O=8.4–8.6) that occur at the highest metamorphic grades is unlikely to explain the resetting of oxygen isotopes because: (a) there is a paucity of skarns at granite-metasediment contacts; (b) the marbles generally do not contain low-XCO₂ mineral assemblages; (c) there is insufficient granite to provide the required volumes of water; (d) the marbles and metapelites retain a several permil difference in ¹⁸O values, even at high metamorphic grades. The oxygen isotope resetting may be accounted for by along-strike up-temperature fluid flow during regional metamorphism with time-integrated fluid fluxes of up to 5x10⁹ moles/m² (10⁵ m³/m²). If fluid flow occurred over 10⁵–10⁶ years, estimated intrinsic permeabilities are 10^-20 to 10^-16m². Variations in ¹⁸O at individual outcrops suggest that time-integrated fluid fluxes and intrinsic permeabilities may locally have varied by at least an order of magnitude. A general increase in XCO₂ values of marble assemblages with metamorphic grade is also consistent with the up-temperature fluid-flow model. Fluids in the metapelites may have been derived from these rocks by devolatilisation at low metamorphic grades; however, fluids in the marbles were probably derived in part from the surrounding siliceous rocks. The marble-metapelite boundaries preserve steep gradients in both ¹⁸O and XCO₂ values, suggesting that across-strike fluid fluxes were much lower than those parallel to strike. Up-temperature fluid flow may also have formed orthoamphibole rocks and caused melting of the metapelites at high grades.This paper is a contribution to IGCP Project 304 Lower Crustal Processes     

Die quartäre ‚Andesit“-Formation in der Hochkordillere Nord-Chiles

Zusammenfassung Im Anschluß an die Darstellung der känozoischen Rhyolith-Formation im Mittelabschnitt der Anden (Zeil &Pichler 1967) geben wir in dieser Arbeit einen Überblick über die quartäre Andesit-Formation in der Hochkordillere Nord-Chiles.Die Kulmination der Anden wird von Stratovulkanen und Lavaströmen geprägt, die im Pleistozän und Holozän entstanden sind. Die Vulkane erreichen Höhen von mehr als 6700 m und sitzen auf einem im Mittel 3900 bis 4000 m hohen Altiplano. Diese Andesit-Formation ist im Großen in die ältere Struktur des Anden-Bogens eingepaßt, folgt aber im Detail jüngeren Frakturen, die vermutlich als Folge des Aufstiegs der Schmelzen entstanden. Die rezente Tätigkeit ist gering.Geländearbeiten, 24 neue chemische Analysen, Bestimmungen der Spurenelemente und petrologische Untersuchungen zeigen, daß während der quartären vulkanischen Tätigkeit keine Andesite und Basalte, sondern Latitandesite bis Rhyodacite, vorwiegend aber quarzführende Latitandesite gefördert wurden.Die Entstehung der intermediären Schmelzen in der Hochkordillere kann nach unserer Auffassung am besten durch partielle oder vollständige Aufschmelzung von Teilen der unteren Sial-Kruste erklärt werden. Für die Bildung der Andesite der Anden durch Kristallisations-Differentiation von simatischem basaltischem Magma fanden wir keine Hinweise. Der Vergleich mit den vulkanischen Förderprodukten der im östlichen Pazifik gelegenen Juan Fernández-Inseln zeigt die deutlichen Unterschiede zwischen ozeanischem und kontinentalem orogenem Vulkanismus.Mit den bis jetzt bekannten geophysikalischen Daten im Mittelabschnitt der Anden stehen unsere Ergebnisse im Einklang.

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Following the demonstration of the Cenozoic Rhyolite Formation in the central section of the Andes (Zeil &Pichler 1967), this paper deals with the Quaternary Andesite Formation in the High Cordillera of Northern Chile.The culmination of the Andes is marked by stratovolcanoes and lava flows dating from Pleistocene and Holocene time. The volcanoes reach up to more than 6700 metres in altitude and are perched upon an altiplano of an average altitude of 3900 to 4000 metres. On the whole, this Andesite Formation fits into the older structure of the bending of the Andes; in detail, however, it is following younger fractures which were presumably formed by the rising of the melts. Recent activity is scarce.Field work, 24 new chemical analyses, trace element determinations and petrological investigations have shown that during Quaternary volcanic activity no andesites and basalts, but latite-andesites to rhyodacites, and predominantly quartz-bearing latite-andesites were erupted.The origination of intermediate melts in the High Cordillera may, in our opinion, be explained by partial or complete melting of parts of the lower sialic crust. As to the creation of the andesites of the Andes by crystallization differentiation of simatic basaltic magma, we did not find any indications. The comparison with the volcanic rocks of the Juan Fernández Islands situated in the eastern Pacific, shows the distinct differences between oceanic and continental orogenous volcanism.Our results are in agreement with the geophysical data so far known on the central section of the Andes.

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Résumé Faisant suite à la démonstration de la formation Rhyolithe cénozoique dans la partie centrale des Andes (Zeil &Pichler 1967), les auteurs présentent un aperçu sur la formation «Andésite» quaternaire dans la Haute Cordillère du nord de Chili.La culmination des Andes est marquée par des strato-volcans et des coulées de lave originant du Pleistocène et de l'Holocène. Les volcans atteignent des altitudes de plus de 6700 mètres et ils sont perchés sur un altiplano d'une altitude d'environ 3900 à 4000 mètres. En gros, cette formation «Andésite» est arrangée le long de la structure plus ancienne de la courbure des Andes. cependant en détail elle suit des fractures plus récentes développées présumablement par suite de l'ascension des bains de fusion. L'activité récente est minime.Des travaux de champs, 24 analyses chimiques nouvelles, des déterminations de traceurs et des recherches pétrologiques ont démontré que pendant l'activité volcanique quaternaire il n'y avait pas d'éruption d'andésites ni de basaltes, mais c'étaient des latite-andésites à rhyodacites, et surtout des latite-andésites quartzifères qui ont été projetées.L'origine des bains de fusion intermédiaires dans la Haute Cordillère peut être expliquée, à notre opinion, par une fusion partielle ou complète de parties de la croûte sialique inférieure. Quant à la formation des «andésites» des Andes par une différentiation de cristallisation d'un magma basaltique, nous n'en avons pas trouvé d'indications. La comparaison avec les produits d'émission volcanique des îles de Juan Fernandez situées au Pacifique oriental, montre les différences distinctes entre le volcanisme orogène océanique et continental.Nos résultats s'accordent bien avec les dates geophysicales connues à présent sur la partie centrale des Andes.

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. . 3900–4000 , 6700 . , , , , , . . , - , , , , . .

     

Synthesis and some properties of iron- and vanadium-bearing kyanites, (Al,Fe3+)2SiO5and (Al,V3+)2SiO5

Iron- and vanadium-bearing kyanites have been synthesized at 900 and 1100° C/20 kb in a piston-cylinder apparatus using Mn₂O₃/Mn₃O₄- and MnO/Mn-mixtures, respectively, as oxygen buffers. Solid solubility on the pseudobinary section Al₂SiO₅-Fe₂SiO₅(-V₂SiO₅) of the system Al₂O₃-Fe₂O₃(V₂O₃)-SiO₂ extends up to 6.5 mole% (14mole %) of the theoretical end member FeSiO₅(V₂SiO₅) at 900°C/20 kb. For bulk compositions with higher Fe₂SiO₅ (V₂SiO₅) contents the corundum type phases M₂O₃(M = Fe³⁺, V³⁺) are found to coexist with the Fe³⁺(V³⁺)-saturated kyanite solid solution plus quartz. The extent of solid solubility on the join Al₂SiO₅-Fe₂SiO₅ at 1 100°C was not found to be significantly higher than at 900° C. Microprobe analyses of iron bearing kyanites gave no significant indication of ternary solid solubility in these mixed crystals. Lattice constants a ₀, b ₀, c ₀, and V₀ of the kyanite solid solutions increase with increasing Fe₂SiO₅- and V₂SiO₅-contents proportionally to the ionic radii of Fe³⁺ and V³⁺, respectively, the triclinic angles ,, remain constant. Iron kyanites are light yellowish-green, vanadium kyanites are light green. Iron kyanites, (Al_1.87 Fe _0.13 ³⁺ )SiO₅, were obtained as crystals up to 700 m in length.     

The structure of the eastern border of the Andes in north-western Argentina

In the eastern border area of the Andes of north-western Argentina two structural units are represented: the Cordillera Oriental and the Sierras Subandinas.The Cordillera Oriental represents a structural unit of faulted blocks strongly uplifted in relation to the Sierras Subandinas. In the Cordillera Oriental, the metamorphic basement has been partially folded together with the sedimentary cover. In some cases the cover has been detached away from its substratum and folded independently of it.It was generally accepted that the Sierras Subandinas coincide with big asymmetrical anticlines produced in the cover rocks by tilting of rigid basement blocks. The detailed geological mapping of the region situated between the parallels 24° 45 and 26° latitude south and the meridians 64° 30 and 66° west has provided data that permit to modify considerably this scheme.The existence of important strike-slip faults running highly oblique to the main regional structural pattern and controlling the shape of the Sierras Subandinas folds has been verified. These highly oblique faults with strike-slip movements, that affect the whole region, are probably ancient lineaments reactivated during Andean diastrophism in Upper Pliocene to Early Pleistocene time.

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Zusammenfassung Im östlichen Grenzgebiet der Anden im nordwestlichen Argentinien liegen zwei strukturell unterschiedliche Einheiten vor, die Cordillera Oriental und die subandinen Sierren (Sierras Subandinas).Die Cordillera Oriental besteht aus Leistenschollen eines leicht metamorphisierten Sockels. Sie ist als Ganzes im Verhältnis zu den subandinen Sierren stark herausgehoben worden. In der Cordillera Oriental wurde dieser Sockel zum Teil zusammen mit seinen sedimentären Deckschichten verfaltet, zum Teil wurden sie aber auch von ihrem Unterlager abgeschert und unabhängig von ihm gefaltet.Bisher wurde allgemein angenommen, daß die subandinen Sierren aus großen assymmetrischen Falten aufgebaut werden, die in den Deckschichten durch Kippung starrer Blöcke im tieferen Untergrund hervorgerufen wurden. Die genaue geokartographische Aufnahme eines Gebietes zwischen den Breitenkreisen 24° 45 und 26° südl. Breite und den Längenkreisen 64° 30 und 66° westl. Länge erbrachte Daten, die es erlauben, diese Vorstellung erheblich zu modifizieren.Weit durchhaltende Seitenverschiebungen, die die übergeordneten Strukturen des gesamten Gebietes schräg durchziehen und die Gestalt der Falten in den Sierren erheblich beeinflussen, wurden nachgewiesen. Sie stellen wahrscheinlich alte, präandine Lineamente dar, die während der Anden-Orogenese im Jungtertiär bis Altpleistozän reaktiviert Wurden.

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Resumen En el borde oriental de los Andes del Noroeste Argentino están representadas dos unidades estructurales: la Cordillera Oriental y las Sierras Subandinas.La Cordillera Oriental es una unidad constituida por bloques fallados y, en conjunto, elevada con respecto a las Sierras Subandinas. En la Cordillera Oriental el basamento metamórfico ha sido parcialmente plegado junto con la cobertura sedimentaria. En algunos casos lo cobertura ha sido despegada de su substrato y plegada independientemente.En general se aceptaba que las Sierras Subandinas coincidian con grandes anticlinales asimétricos producidos en la cobertura sedimentaria por el basculamiento de bloques rígidos de basamento situados en el subsuelo. La cartografía geológica-estructural detallada de la región comprendida por los paralelos 24° 45 y 26° S y los meridianos 64° 30 y 66° W ha proporcionado datos que permiten modificar considerablemente este esquema.Se ha comprobado la existencia de importantes fallas transcurrentes, fuertemente oblicuas con respecto a la estructura regional, que han tenido profunda influencia sobre la forma de los pliegues subandinos. Estas fallas oblicuas son probablemente lineamientos antiguos en los que el diastrofismo Andico (Plioceno-Pleistoceno) ha producido considerables desplazamientos paralelos al rumbo de los mismos.

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Résumé Dans la bordure orientale des Andes du Nord-ouest de l'Argentine, deux unités structurales différentes sont présentes: les Sierras subandines et la Cordillère orientale.La Cordillère orientale est composée d'un socle faiblement métamorphique faillé en blocs fortement surélevés par rapport aux Sierras subandines. Dans la Cordillère orientale, ce socle métamorphique a été en partie plissé avec sa couverture sédimentaire; mais parfois celle-ci a été décolée de son substratum et plissé indépendentement.Jusqu'à présent, on admettait que les Sierras subandines sont constituées de grands anticlinaux assymétriques produits dans la couverture par l'ascension de blocs faillés du socle. La cartographie géologico-structurale detaillée de la région comprise entre les parallèles 24° 45 et 26° de latitude sud et les méridiens 64° 30 et 60° ouest conduit à modifier considérablement ce schéma.D'importants décrochements très obliques par rapport à la direction de la structure régionale et qui ont fortement influencé la forme des plis subandins sont probablement des accidents anciens rajeunis pendant le diastrophisme andin au cours du Pliocène supérieur et du Pléistocène inférieur.

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- : Cordillera Oriental Sierras Subandinas. . . ; . , , . 24°45 26° 64°30 66° . , , . , , , , .