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161.
Age constraints on the evolution of the Austroalpine basement to the south of the Tauern Window 总被引:1,自引:0,他引:1
Siegfried Siegesmund Till Heinrichs Rolf L. Romer Daniel Doman 《International Journal of Earth Sciences》2007,96(3):415-432
The Austroalpine basement to the south of the Tauern Window once was part of the northern margin of Gondwana. It includes
the “Altkristallin” and the phyllitic Thurntaler Complex. In the Altkristallin (AMU, MPU), suites of arc-related metamafic
sequences occur together with calc-alkaline metagranite. SHRIMP U–Pb dating of zircon from calc-alkaline metagranite associated
with an eclogitic amphibolite give an age of 470 ± 3 Ma interpreted as the age of protolith emplacement. In the Thurntaler
Complex, metaporphyroids occur together with tholeiitic as well as alkaline within-plate basalt-type metabasite. The metaryholites
of this association give a crystallization SHRIMP age of 477 ± 4 Ma, which suggests contemporaneity of arc-related and extensional
settings in the Austroalpine basement units. The age data demonstrate widespread magmatic activity associated with the Early-Ordovician
amalgamation at the end of the 550–470 Ma subduction–accretion–collision cycle. The Pb–Pb and U–Pb systematics of step-wise
leached staurolite and kyanite from the peak-metamorphic assemblage of the Altkristallin indicate that (1) step-wise leaching
of staurolite and kyanite yields the age of inclusions rather than the host; (2) zircon inclusions in staurolite suggest an
Ordovician or older age for the precursor of the staurolite-schists; (3) the weighted average of the 206Pb/238U data of the various leaching steps yields a Variscan age for the inclusions (ilmenite, biotite, and andesine). Since these
inclusions are part of the metamorphic mineral assemblage, this age provides a minimum estimate for staurolite growth, i.e.,
metamorphism. Thus, the Pb–Pb and U–Pb systematics of staurolite provide evidence for a Variscan metamorphism of the Austroalpine
basement, e.g., MPU, AMU and Thurntaler Complex, to the south of the Tauern Window. 相似文献
162.
1 Introduction
Recent improvements in the precision of Li and B isotope measurements have demonstrated the potential of these elements in tracing a wide range of geological processes. The Li and B isotope systematics of ultrahigh-pressure (UHP) metamorphic rocks provides a unique opportunity to investigate the behaviour of Li and B during fluid-rock interaction at high temperatures and very high pressures and to constrain the fluid budget and the recycling of subducted crustal materials into the mantle during UHP metamorphism. 相似文献
163.
164.
We have investigated the potential of hübnerite for U-Pb dating. Hübnerite forms typically at medium to low-temperatures in a wide range of pneumatolytic-hydrothermal mineral deposits, particularly porphyry molybdenum and Sn-specialized granites. Hübnerite from the Sweet Home Mine (Alma, Colorado) formed in a Pb-rich, U-poor environment, but still developed relatively radiogenic Pb isotopic compositions. The low Pbcommon contents in hübnerite (0.075 to 0.155 ppm) demonstrate that Pb is efficiently excluded from the crystal lattice. In contrast, U may substitute for Mn. The U-Pb data of hübnerite scatter. Most of the scatter originates from samples with 206Pb/204Pb values below 50, where Pbblank contributes up to 30% to Pbtotal. Using the least radiogenic galena Pb, samples with 206Pb/204Pb values above 70 have overlapping 206Pb∗/238U and 207Pb∗/235U values and yield a 206Pb/238U age of 25.7 ± 0.3 Ma (2σ). Late stage apatite from the Sweet Home Mine yields a 206Pb/204Pb-238U/204Pb isochron corresponding to an age of 24.8 ± 0.5 Ma (2σ). A comparison of the U-Pb hübnerite ages with literature 40Ar/39Ar ages on earlier sericite and the U-Pb age on later apatite suggests that (i) hübnerite yields accurate U-Pb ages and (ii) the evolution of the Sweet Home mineralization from greisen-type mineralization to medium-temperature hydrothermal vein mineralization took place in a few hundred thousand years at most. Aqueous low-N2-bearing and aqueous inclusions in the dated hübnerite have homogenization temperatures between 325 and 356 °C and moderate salinity (up to 6.7 wt% NaCl equiv.). Thus, hübnerite represents one of the rare examples of a mineral that can be dated accurately and carries petrological information. 相似文献
165.
166.
Rolf Brahde 《Solar physics》1972,26(2):318-334
A numerical method for correction of stray light in solar observations has been developed. In particular a regular sunspot, where the circular contours of penumbra and umbra are projected as ellipses, has been studied. When a specified set of values for the stray light parameters is given, and also tentative values for the relative intensities of penumbra and umbra, the integration of stray light can be performed in any point. The result will be the observable intensity if the conditions were as given by these initial values.By means of limb observations the stray light parameters may be improved, and finally a variation of the penumbra- and umbra intensities in the computation, enables a determination of these quantities by comparison with observations.The method is tested on observations of the transit of Mercury, May 9, 1970. Calculation of isophotes with Mercury close to the limb shows the black drop phenomenon; which thus may be explained as an effect of stray light only.It is also shown that the Wilson effect on a sunspot cannot be produced by stray light alone. 相似文献
167.
Rolf Sternberg 《GeoJournal》2000,50(1):25-36
This article presents an evaluation of the current status of economic geography in general and German economic geography in particular with respect to research topics. A comparison with Anglo-American economic geography, more specifically, its actual research fields are also undertaken on the basis of a bibliometrical analysis of German and international journals dedicated to economic geography. On the basis of this analysis suggested actions are formulated, which could contribute to improved performance of research in German economic geography. 相似文献
168.
Giovanni Leonelli Manuela Pelfini Giovanna Battipaglia Matthias Saurer Rolf T. W. Siegwolf Paolo Cherubini 《Boreas: An International Journal of Quaternary Research》2014,43(3):600-607
With the aim of evaluating the influence of glacial meltwater signature on tree‐ring stable isotopes, we analysed δ18O and δ13C in the tree rings of Larix decidua Mill. specimens growing in the area of an ice‐contact lake (Lago Verde, at Miage Glacier, European Alps). Additionally, we analysed δ18O in the glacial meltwater of the lake and of the glacier stream and compared it with the δ18O of precipitation predicted by a spatial model. We found that tree‐ring cellulose of trees fed by glacial meltwaters (LVW site) is significantly more depleted in δ18O than at a control site LVM (?0.91‰) fed only by precipitation, thus reflecting the measured higher depletion of glacial meltwaters with respect to local precipitation. δ13C values did not show significant differences in mean values between the two sites but an anomalous correlation with summer temperature was found at the LVW site, probably due to the different responses of trees stomatal conductance. Over the 30‐year period of analysis, four years at LVW (1992, 1995, 2003 and 2009) were markedly depleted in δ18O. These years are those when the highest summer temperatures were recorded in the area (the ones during which glacier ablation usually increases and more depleted meltwaters fill the lake), with the exception of 1995 during which high water levels occurred following the year with the second highest summer temperature (1994). Overall, our analysis demonstrates that tree‐ring δ18O, driven by the glacial meltwater signature in the lake, can be used for detecting past major glacier runoff events. The proposed approach could also be used for quantifying past glacier runoff and for defining past distribution areas of glacial meltwaters in glacier forefields, thus contributing to past environmental reconstructions and to hazard assessment. 相似文献
169.
Sanjay K. Srivastava Sameer Saran Rolf A. de By V. K. Dadhwal 《International journal of geographical information science》2014,28(3):427-454
Innumerable forest fire spread models exist for taking a decision, but far less focus is on the real causative factors which initiate/ignite fire in an area. It has been observed that the majority of the forest fires in India are initiated due to anthropogenic factors. In this study, we develop a geo-information system approach for management of forest fire in Mudumalai Wildlife Sanctuary, Tamil Nadu, India, with the objective to develop a forest fire likelihood model, integrating GIS and knowledge-based approach for predicting fire-sensitive initiation areas considering major causative and anti-causative factors. Amongst the various causative factors investigated, it was found that wildlife-dependent factor (antler collection and poaching) contributed significantly to fire occurrence followed by management-dependent factors (uncontrolled tourism and grazing), with much less influence of demographic factors. Similarly, anti-causative factor (stationing of anti-poaching/ fire camps) was considered as quite significant.
The likelihood model so developed, envisaging various factors and flammability, accounted for different scenarios as a result of pair-wise comparison on an ordinal scale in a knowledge matrix. The inferential statistics computed indicated the robustness of the model and its insensitivity to moderate changes. It makes it possible for this forest fire likelihood model to predict and prevent a forest fire in an effective and scientific manner because it can assume forest fire likelihood in real time and present in proper time. 相似文献
170.
Rolf?DachEmail author Johannes?B?hm Simon?Lutz Peter?Steigenberger Gerhard?Beutler 《Journal of Geodesy》2011,85(2):75-91
In recent years, several studies have demonstrated the sensitivity of Global Navigation Satellite System (GNSS) station time
series to displacements caused by atmospheric pressure loading (APL). Different methods to take the APL effect into account
are used in these studies: applying the corrections from a geophysical model on weekly mean estimates of station coordinates,
using observation-level corrections during data analysis, or solving for regression factors between the station displacement
and the local pressure. The Center for Orbit Determination in Europe (CODE) is one of the global analysis centers of the International
GNSS Service (IGS). The current quality of the IGS products urgently asks to consider this effect in the regular processing
scheme. However, the resulting requirements for an APL model are demanding with respect to quality, latency, and—regarding
the reprocessing activities—availability over a long time interval (at least from 1994 onward). The APL model of Petrov and
Boy (J Geophys Res 109:B03405, 2004) is widely used within the VLBI community and is evaluated in this study with respect to these criteria. The reprocessing
effort of CODE provides the basis for validating the APL model. The data set is used to solve for scaling factors for each
station to evaluate the geophysical atmospheric non-tidal loading model. A consistent long-term validation of the model over
15 years, from 1994 to 2008, is thus possible. The time series of 15 years allows to study seasonal variations of the scaling
factors using the dense GNSS tracking network of the IGS. By interpreting the scaling factors for the stations of the IGS
network, the model by (2004) is shown to meet the expectations concerning the order of magnitude of the effect at individual stations within the uncertainty
given by the GNSS data processing and within the limitations due to the model itself. The repeatability of station coordinates
improves by 20% when applying the effect directly on the data analysis and by 10% when applying a post-processing correction
to the resulting weekly coordinates compared with a solution without taking APL into account. 相似文献