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991.
Dieter F. Mertz Paul R. Renne Michael Wuttke Clemens Mödden 《International Journal of Earth Sciences》2007,96(2):353-361
The fauna of the Enspel (Westerwald) and the neighbouring Kärlich (Neuwied basin) fossil deposits correspond to the Upper Oligocene Mammal Paleogene (MP) reference level 28 and 28–30, respectively. Basaltic flows and a trachyte tuff terminating and predating the fossil deposit sedimentation allow to numerically calibrate the MP reference levels by radioisotope dating. Laser fusion 40Ar/39Ar step heating on volcanic feldspars yield a time interval of 24.9–24.5 Ma for reference level MP28 at Enspel and a maximum age of 25.5 Ma for the time interval MP28–MP30 at Kärlich. Interpolation between the time intervals determined for the Enspel reference level MP28 and the age of the global Oligocene/Miocene boundary of 24.0 ± 0.1 Ma taken from literature results in time intervals of 24.5–24.2 Ma and 24.2–23.9 Ma for the younger reference levels MP29 and MP30, respectively. These intervals of ≤ 0.4 m.y. for MP reference levels of the latest Oligocene are short relative to older Oligocene MP reference levels 21–27 between 34 and 25 Ma. Since subdivision into MP reference levels essentially is based on assemblages of mammal taxa and on evolutionary changes in tooth morphology of mammals short MP time intervals during the latest Oligocene indicate a rapid evolutionary change relative to the early Oligocene. 相似文献
992.
Hugues Raimbourg Bruno Goffé Laurent Jolivet 《Contributions to Mineralogy and Petrology》2007,153(1):1-28
Caledonian eclogite-facies metamorphism partially reworking Grenvillian granulite-facies anorthosite allows us to study the processes of garnet reequilibration at high pressure and to reconstruct the evolution of the unit near metamorphic peak conditions. Our results indicate that eclogite-facies metamorphism happened in two successive phases: first, inherited granulitic garnet was fractured and reequilibrated from their boundaries (crystal or fracture rims); then eclogite-facies minerals were crystallised in the fractures as overgrowths on inherited garnets. The reequilibration of inherited garnets is achieved through Fe2+Mg−1 exchange, whereas eclogite-facies garnets crystallised during the subsequent phase are notably richer in Ca than un- and re-equilibrated granulitic garnet. Pseudosection construction shows that this lack in Ca reequilibration cannot be related to variations in thermodynamic conditions (a
H2O, reacting system composition) between the two phases. From the compilation of the available data, the reequilibration of granulitic garnet seems to be controlled by the inefficient intra- and inter-granular transport properties of Ca compared to Fe2+ and Mg. While these kinetic factors confine garnet reequilibration to Fe2+Mg−1 exchange, the extent of reequilibration along this exchange vector is controlled by partitioning with adjacent omphacite. On the contrary to the diffusional reequilibration of granulitic garnet that lasted for several My according to our modelling of the diffusional relaxation, the strong compositional gradients between eclogite-facies and reequilibrated garnets, which are almost unaffected by diffusional reequilibration, provide evidence that rapid exhumation followed the crystallisation of eclogite-facies minerals. We propose that the movement reversal itself, from burial to exhumation, and associated deformation and fluid flow, triggered this crystallisation event. The resulting evolution near metamorphic peak conditions is therefore strongly asymmetrical: on the one hand, the prograde diffusional relaxation profiles indicate slow movement during the last stages of burial, whereas the unaffected retrograde overgrowth indicates fast exhumation rates. 相似文献
993.
Fukun Chen Xiang-Hui Li Xiu-Li Wang Qiu-Li Li Wolfgang Siebel 《International Journal of Earth Sciences》2007,96(6):1179-1194
The Baoshan block of the Tethyan Yunnan, southwestern China, is considered as northern part of the Sibumasu microcontinent.
Basement of this block that comprises presumably greenschist-facies Neoproterozoic metamorphic rocks is covered by Paleozoic
to Mesozoic low-grade metamorphic sedimentary rocks. This study presents zircon ages and Nd–Hf isotopic composition of granites
generated from crustal reworking to reveal geochemical feature of the underlying basement. Dating results obtained using the
single zircon U–Pb isotopic dilution method show that granites exposed in the study area formed in early Paleozoic (about
470 Ma; Pingdajie granite) and in late Yanshanian (about 78–61 Ma, Late Cretaceous to Early Tertiary; Huataolin granite).
The early Paleozoic granite contains Archean to Mesoproterozoic inherited zircons and the late Yanshanian granite contains
late Proterozoic to early Paleozoic zircon cores. Both granites have similar geochemical and Nd–Hf isotopic charateristics,
indicating similar magma sources. They have whole-rock T
DM(Nd) values of around 2,000 Ma and zircon T
DM(Hf) values clustering around 1,900–1,800 and 1,600–1,400 Ma. The Nd–Hf isotopic data imply Paleoproterozoic to Mesoproterozoic
crustal material as the major components of the underlying basement, being consistent with a derivation from Archean and Paleoproterozoic
terrains of India or NW Australia. Both granites formed in two different tectonic events similarly originated from intra-crustal
reworking. Temporally, the late Yanshanian magmatism is probably related to the closure of the Neotethys ocean. The early
Paleozoic magmatism traced in the Baoshan block indicates a comparable history of the basements during early Paleozoic between
the SE Asia and the western Tethyan belt, such as the basement outcrops in the Alpine belt and probably in the European Variscides
that are considered as continental blocks drifting from Gondwana prior to or simultaneously with those of the SE Asia. 相似文献
994.
995.
Coupling of oceanic and continental crust during Eocene eclogite-facies metamorphism: evidence from the Monte Rosa nappe,western Alps 总被引:1,自引:0,他引:1
Thomas J. Lapen Clark M. Johnson Lukas P. Baumgartner Giorgio V. Dal Piaz Susanne Skora Brian L. Beard 《Contributions to Mineralogy and Petrology》2007,153(2):139-157
High precision U–Pb geochronology of rutile from quartz–carbonate–white mica–rutile veins that are hosted within eclogite
and schist of the Monte Rosa nappe, western Alps, Italy, indicate that the Monte Rosa nappe was at eclogite-facies metamorphic
conditions at 42.6 ± 0.6 Ma. The sample area [Indren glacier, Furgg zone; Dal Piaz (2001) Geology of the Monte Rosa massif: historical review and personal comments. SMPM] consists of eclogite boudins that are exposed
inside a south-plunging overturned synform within micaceous schist. Associated with the eclogite and schist are quartz–carbonate–white
mica–rutile veins that formed in tension cracks in the eclogite and along the contact between eclogite and surrounding schist.
Intrusion of the veins at about 42.6 Ma occurred at eclogite-facies metamorphic conditions (480–570°C, >1.3–1.4 GPa) based
on textural relations, oxygen isotope thermometry, and geothermobarometry. The timing of eclogite-facies metamorphism in the
Monte Rosa nappe determined in this study is identical to that of the Gran Paradiso nappe [Meffan-Main et al. (2004) J Metamorphic Geol 22:261–281], confirming that these two units have shared the same Alpine metamorphic history. Furthermore,
the Gran Paradiso and Monte Rosa nappes underwent eclogite-facies metamorphism within the same time interval as the structurally
overlying Zermatt-Saas ophiolite [∼50–40 Ma; e.g., Amato et al. (1999) Earth Planet Sci Lett 171:425–438; Mayer et al. (1999) Eur Union Geosci 10:809 (abstract); Lapen et al. (2003) Earth Planet Sci Lett 215:57–72]. The nearly identical P–T–t histories of the Gran Paradiso, Monte Rosa, and Zermatt-Saas units suggest that these units shared a common Alpine tectonic
and metamorphic history. The close spatial and temporal associations between high pressure (HP) ophiolite and continental
crust during Alpine orogeny indicates that the HP internal basement nappes in the western Alps may have played a key role
in exhumation and preservation of the ophiolitic rocks through buoyancy-driven uplift. Coupling of oceanic and continental
crust may therefore be critical in preventing permanent loss of oceanic crust to the mantle. 相似文献
996.
This paper presents results of meticulous laboratory testing and numerical simulations on the effect of reinforcement on the
low-strain stiffness and bearing capacity of shallow foundations on dry sand. The effect of the location and the number of
reinforcement layers is studied in the laboratory, whereas numerical simulations are used to study the reinforcement-foundation
interaction. Laboratory tests show an increase of 100, 200, and 275% not only in bearing capacity but also in low-strain stiffness
(linear load–displacement behaviour) of a square foundation when one, two, and three layers of reinforcement are used, respectively.
The specimen preparation technique is found to be crucial for the repeatability and reliability of the laboratory results
(less than 5% variability). Numerical simulations demonstrate that if reinforcements are placed up to a depth of one footing
width (B) below the foundation, better re-distribution of the load to deeper layers is achieved, thus reducing the stresses and strains
underneath the foundation. Numerical simulations and experimental results clearly identify a critical zone between 0.3 and
0.5B, where maximum benefits not only on the bearing capacity but also on the low-strain stiffness of the foundation are obtained.
Therefore, soil reinforcement can also be used to reduce low-strain vibrations of foundations. 相似文献
997.
We report on the use of X-ray diffractometry under controlled conditions of temperature and relative humidity (RH-XRD) for
the investigation of NaCl deliquescence in the pore space of glass filter frits, which were used as model substrates. The
study confirms that RH-XRD is an appropriate experimental technique for the in situ observation of phase transformation in
porous materials. It is used for an investigation of both the deliquescence kinetics and the deliquescence humidity within
pores of different median pore diameter. Several major influences affecting deliquescence rates in the pore space close to
the surface of a porous material are discussed. It appears that quite short-term variation of ambient relative humidity, e.g.,
typical daily fluctuations, might induce damaging deliquescence–crystallization cycles within the pore space of building materials.
In agreement with theoretical considerations it was found that confinement of NaCl crystals in pores with median diameters
in the range 1.4–70 μm does not affect the deliquescence humidity of the salt. 相似文献
998.
Julien Berger Olivier Féménias Nicolas Coussaert Jean-Claude C. Mercier Daniel Demaiffe 《Contributions to Mineralogy and Petrology》2007,153(5):557-575
Ultramafic and mafic xenoliths of magmatic origin, sampled in the Beaunit vent (northern French Massif Central), derive from
the Permian (257 Ma) Beaunit layered complex (BLC) that was emplaced at the crust-mantle transition zone (∼1 GPa). These plutonic
xenoliths are linked to a single fractional crystallisation process in four steps: peridotitic cumulates; websteritic cumulates;
Al-rich mafic cumulates (plagioclase, pyroxenes, garnet, amphibole and spinel) and finally low-Al mafic cumulates. This sequence
of cumulates can be related to the compositional evolution of hydrous Mg basaltic magma that evolved to high-Al basalt and
finally to andesitic basalt. Sr and Nd isotopic compositions confirm the co-genetic character of the various magmatic xenoliths
and argue for an enriched upper mantle source comparable to present mantle wedges above subduction zones. LILE, LREE and Pb
enrichment are a common feature of all xenoliths and argue for an enriched sub-alkaline transitional parental magma. The existence
of a Permian magma chamber at 30 km depth suggests that the low-velocity zone observed locally beneath the Moho probably does
not represent an anomalous mantle but rather a sequence of mafic/ultramafic cumulates with densities close to those of mantle
rocks. 相似文献
999.
W. N. Mussel E. Murad J. D. Fabris W. S. Moreira J. B. S. Barbosa C. C. Murta W. P. Abrahão J. W. V. De Mello V. K. Garg 《Physics and Chemistry of Minerals》2007,34(6):383-387
We studied a chalcopyrite from a Cu ore deposit in Rio Grande do Sul, Brazil, by Mössbauer spectroscopy at room temperature and 110 K. Supporting methods to check for sample purity and to characterize further sample properties were slow-scanning X-ray powder diffraction and optical and microprobe analyses of polished sections of selected grains. Chemical analyses obtained using a scanning electron microscope equipped with an energy-dispersive X-ray spectrometer showed the sample to consist of homogeneous and essentially stoichiometric chalcopyrite (CuFeS2). Mössbauer spectra taken at both the above temperatures consist of asymmetric magnetically ordered patterns with unequal intensities of the line pairs 1–6 and 2–5, pointing to the existence of non-equivalent or multiple Fe sites. Least-squares fittings evidenced that the resonance intensity ratio of subspectrum with lower quadrupole shift (indicative of a more symmetric environment) to that with higher shift is 69:31, at room temperature, and 68:32, at 110 K. Rietveld refinement of the XRD data indicates the existence of tetragonal [a = 0.52855(1) and c = 1.0412(1) nm] and cubic [a 0 = 0.5273(2) nm] modifications in a proportion of 74:26, in good agreement with the Mössbauer data. The saturation magnetization of the sample was 32.7 J/(T kg), confirming the oxidation state of Fe as trivalent and pointing to little to no spin canting. 相似文献
1000.
Jan Pleuger Sybille Roller Jens M. Walter Ekkehard Jansen Nikolaus Froitzheim 《International Journal of Earth Sciences》2007,96(2):229-252
The boundary zone between two Penninic nappes, the eclogite-facies to ultrahigh-pressure Zermatt-Saas zone in the footwall
and the blueschist-facies Combin zone in the hanging wall, has been interpreted previously as a major normal fault reflecting
synorogenic crustal extension. Quartz textures of mylonites from this fault were measured using neutron diffraction. Together
with structural field observations, the data allow a refined reconstruction of the kinematic evolution of the Pennine nappes.
The main results are: (1) the contact is not a normal fault but a major thrust towards northwest which was only later overprinted
by southeast-directed normal faulting; (2) exhumation of the footwall rocks did not occur during crustal extension but during
crustal shortening; (3) the Sesia-Dent Blanche nappe system originated from a continental fragment (Cervinia) in the Alpine
Tethys ocean, and the Combin zone ophiolites from the ocean basin southeast of Cervinia; (4) out-of-sequence thrusting played
a major role in the tectonic evolution of the Penninic nappes.
An erratum to this article can be found at 相似文献