The Las Chacras-Potrerillos batholith (Pampean Ranges,Argentina): structural evidences,emplacement and timing of the intrusion     

Siegesmund  S.  Steenken  A.  López de Luchi  M. G.  Wemmer  K.  Hoffmann  A.  Mosch  S. 《International Journal of Earth Sciences》2004,93(1):23-43

Within the southern part of the Sierra Pampeanas (the Sierra de San Luis, Argentina), a series of extensive intrusive bodies are regarded to post-date the Famatinian cycle but were emplaced during the Achalian, a period of heterogeneous deformation along crustal scale fault zones. The largest of those is the Las Chacras-Potrerillos batholith that is situated at the northern end of the transpressive, sinistral Guzmán shear zone. This composite pluton exhibits three sub-domains that comprise two granitoid sub-units each: The southern Potrerillos stock (muscovite-bearing red granite and biotite-bearing red granite) and the central (biotite porphyritic granite and giant porphyritic granite) and northern domain (equigranular granite and porphyritic granite) of the Las Chacras stock. The crystallisation ages of the biotite porphyritic granite is around 381 Ma (U/Pb on zircons and Pb/Pb on sphene), while the host rock was already cooled below 350 °C at 420 Ma. Thermal modelling approaches favour a pulsed intrusion with a duration of 1.5 Ma. The emplacement was followed by rapid cooling below the muscovite cooling temperature. Biotite cooling ages in different sub-units reflect either a long-lasting cooling history of approximately 30 Ma (which is supported by the modelling) or a reheating effect at around 350 Ma. Devonian-age determinations on the fault rocks and granitoids point to a syn-tectonic emplacement of the batholith. The pluton is interpreted to be positioned at the crossover of sinistral shear zones. The origin of this NNE directed extensional setting in a transpressive regime seems to be related to the transfer of displacement along a secondary set of NNW-trending sinistral faults. The final emplacement is due to a subsequent ballooning of the batholith following the direction of space creation. This model is based on the relative timing of the emplacement sequence and macroscopically visible planar fabrics in the field as well as magnetic fabric data. Our results indicate that the emplacement is syn-kinematic with respect to the Achalian deformation event.  相似文献   

The influence of pressure and composition on the viscosity of andesitic melts     

Christian Liebske  Harald Behrens  Rebecca A. Lange 《Geochimica et cosmochimica acta》2003,67(3):473-485

The effect of pressure and composition on the viscosity of both anhydrous and hydrous andesitic melts was studied in the viscosity range of 10⁸ to 10^11.5 Pa · s using parallel plate viscometry. The pressure dependence of the viscosity of three synthetic, iron-free liquids (andesite analogs) containing 0.0, 1.06, and 1.96 wt.% H₂O, respectively, was measured from 100 to 300 MPa using a high-P-T viscometer. These results, combined with those from Richet et al. (1996), indicate that viscosities of anhydrous andesitic melts are independent of pressure, whereas viscosities of hydrous melts slightly increase with increasing pressure. This trend is consistent with an increased degree of depolymerization in the hydrous melts. Compositional effects on the viscosity were studied by comparing iron-free and iron-bearing compositions with similar degrees of depolymerization. During experiments at atmospheric and at elevated pressures (100 to 300 MPa), the viscosity of iron-bearing anhydrous melts preequilibrated in air continuously increased, and the samples became paramagnetic. Analysis of these samples by transmission electron microscopy showed a homogeneous distribution of crystals (probably magnetite) with sizes in the range of 10 to 50 nm. No significant difference in the volume fractions of crystals was found in samples after annealing for 170 to 830 min at temperatures ranging from 970 to 1122 K. An iron-bearing andesite containing 1.88 wt.% H₂O, which was synthesized at intrinsic fO₂ conditions in an internally heated pressure vessel, showed a similar viscosity behavior as the anhydrous melts. The continuous increase in viscosity at a constant temperature is attributed to changes of the melt structure due to exsolution of iron-rich phases. By extrapolating the time evolution of viscosity down to the time at which the run temperature was reached, for both the anhydrous (at 1055 K) and the hydrous (at 860 K) iron-bearing andesite, the viscosity is 0.7 log units lower than predicted by the model of Richet et al. (1996). This may be explained by differences in structural properties of Fe²⁺ and Fe³⁺ and their substitutes Mg²⁺, Ca²⁺, and Al³⁺, which were used in the analogue composition.The effect of iron redox state on the viscosity of anhydrous, synthetic andesite melts was studied at ambient pressure using a dilatometer. Reduced iron-bearing samples were produced by annealing melts in graphite crucibles in an Ar/CO atmosphere for different run times. In contrast to the oxidized sample, no variation of viscosity with time and no exsolution of iron oxide phases was observed for the most reduced glasses. This indicates that trivalent iron promotes the exsolution of iron oxide in supercooled melts. With decreasing Fe³⁺/ΣFe ratio from 0.58 to 0.34, the viscosity decreases by ∼1.6 log units in the investigated temperature range between 964 and 1098 K. A more reduced glass with Fe³⁺/ΣFe = 0.21 showed no additional decrease in viscosity. Our conclusion from these results is that the viscosity of natural melts may be largely overestimated when using data obtained from samples synthesized in air.  相似文献   

Paleomagnetic study of Deception Island, South Shetland Islands, Antarctica     

Andrés Baraldo  Augusto E. Rapalini  Harald Böhnel  Mabel Mena 《Geophysical Journal International》2003,153(2):333-343

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Gyromagnetic remanence acquired by greigite (Fe₃S₄) during static three-axis alternating field demagnetization     

S. Hu  E. Appel  V. Hoffmann  W. W. Schmahl  & S. Wang 《Geophysical Journal International》1998,134(3):831-842

A magnetic study was carried out on lacustrine sediments from the Zoigê basin, Tibetan Plateau, in order to obtain a better understanding of palaeoclimatic changes there. Gyromagnetic remanence (GRM) acquisition is unexpectedly observed during static three-axis alternating field (AF) demagnetization in about 20 per cent of a large number of samples. X-ray diffraction (XRD) analysis on a magnetic extract clearly shows that greigite is the dominant magnetic mineral carrier. Scanning electron microscopy (SEM) reveals that the greigite particles are in the grain size range of 200–300  nm, possibly in the single-domain state. Greigite clumps of about 3  μm size are sealed by silicates. Fitting of XRD peaks yields a crystalline coherence length of about 15  nm, indicating that the particles seen in the SEM are polycrystalline.
  GRM intensities of most samples are of the same order as the NRM, while others show much stronger GRM although their magnetic properties are similar. Variation of the demagnetization sequence confirms that GRM is mainly produced perpendicular to the AF direction. The anisotropy direction can be derived from GRM, but more systematic studies are needed for detailed conclusions. An attempt to correct for GRM failed due to high GRM intensities and because smaller GRM acquisition was also found along the demagnetization axis. Behaviours of acquisition and AF demagnetization of GRM are comparable with those of NRM, ARM, IRM, indicating fine grain sizes of remanence carriers.  相似文献   

Tides and Earth Rotation     

Peter Brosche  Harald Schuh 《Surveys in Geophysics》1998,19(5):417-430

A short general explanation of tidal forces and tidal effects is given. The influences of Earth tides and ocean tides on the Earth's rotation vector are presented. Today, the theoretical models for periodic variations in the Earth's rotation and in polar motion can be compared with precise measurements done by modern space techniques. Secular changes of the Earth's rotation due to interactions within the Earth-Moon-system are also discussed.  相似文献   

Late and post-glacial history of meromictic Längsee (Austria), in respect to climate change and anthropogenic impact     

Roland Schmidt  Sybille Wunsam  Ursula Brosch  Jan Fott  Andrea Lami  Heinz Löffler  Aldo Marchetto  Harald W. Müller  Miroslava Pražáková  Bernd Schwaighofer 《Aquatic Sciences - Research Across Boundaries》1998,60(1):56-88

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Acceptance of emplacement hole positions — stage 1 thermomechanical study     

Harald H  kmark 《Engineering Geology》1998,49(3-4):215-222

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Emission of Biogenic Volatile Organic Compounds: An Overview of Field, Laboratory and Modelling Studies Performed during the `Tropospheric Research Program' (TFS) 1997–2000     

J.-P. Schnitzler  N. Bauknecht  N. Brüggemann  W. Einig  R. Forkel  R. Hampp  A. C. Heiden  U. Heizmann  T. Hoffmann  C. Holzke  L. Jaeger  M. Klauer  M. Komenda  R. Koppmann  J. Kreuzwieser  H. Mayer  H. Rennenberg  G. Smiatek  R. Steinbrecher  J. Wildt  W. Zimmer 《Journal of Atmospheric Chemistry》2002,42(1):159-177

The present paper summarises results on the emission of biogenicvolatile organic compounds (BVOC) achieved within the frame of thenational `German Tropospheric Research Programme' (TFS) between 1997 and2000. Field measurements were carried out at the meteorologicalmonitoring station `Hartheimer Wald' located in the vicinity of Freiburg(upper Rhine valley), Germany, within a pine plantation dominated byScots pine (Pinus sylvestris L.). The measured BVOC emissionrates were used to determine the daily and seasonal variation of BVOCemission and its dependence on important meteorological and plantphysiological parameters. In parallel, laboratory experiments usingyoung trees of pine (P. sylvestris), poplar (Populustremula ×P. alba) and pedunculate oak (Quercusrobur L.) were performed, and the influence of abiotic (e.g.,light, temperature, seasonality, flooding) factors on the biosynthesisand emission of BVOC was quantified. Based on these data, emissionalgorithms were evaluated and a process-oriented numerical model for thesimulation of the isoprene emission by plants was developed. Inaddition, newly calculated land use and tree species distributions wereused for the calculation of an actual BVOC emission inventory ofGermany.  相似文献   

Identification of greigite in lake sediments and its magnetic significance   总被引：5，自引：1，他引：5  

HU Shouyun  E. Appel  V.Hoffmann  W. Schmahl 《中国科学D辑(英文版)》2002,45(1):81-87

Detailed rock magnetic investigations and mineralogical analysis were conducted on lacustrine sediments recovered from Zoigê Basin, eastern Tibetan Plateau. Greigite, with fine and homogenous grain size, is found as a main magnetic carrier, which is suspected to be of biochemical origin. In contrast to some reports, greigite within the sediments is not oxidized even after exposure to air for a few years, which could be due to the fact that greigite is sealed by silicate.  相似文献   

Untersuchungen zur Adsorption von Spuren hydrolysierbarer Metallionen an Oberflächen     

H.-J. Hoffmann  J. Bosholm 《洁净——土壤、空气、水》1986,14(4):395-405

In static laboratory experiments there are determined quantities influencing the adsorption of selected metal ions on different surfaces (base metals, glass of instruments, quartz glass, plastic materials, precious metals). A great dependence on pH, ionic species, ionic concentration, material as well as temperature and presence of complexing agents is observed. Processes are proposed or confirmed by which the losses of material during the trace analysis are reduced.  相似文献