Post-orogenic shoshonitic magmas of the Yzerfontein pluton,South Africa: the ‘smoking gun’ of mantle melting and crustal growth during Cape granite genesis?

The post-orogenic Yzerfontein pluton, in the Saldania Belt of South Africa was constructed through numerous injections of shoshonitic magmas. Most magma compositions are adequately modelled as products of fractionation, but the monzogranites and syenogranites may have a separate origin. A separate high-Mg mafic series has a less radiogenic mantle source. Fine-grained magmatic enclaves in the intermediate shoshonitic rocks are autoliths. The pluton was emplaced between 533 ± 3 and 537 ± 3 Ma (LA-SF-ICP-MS U–Pb zircon), essentially synchronously with many granitic magmas of the Cape Granite Suite (CGS). Yzerfontein may represent a high-level expression of the mantle heat source that initiated partial melting of the local crust and produced the CGS granitic magmas, late in the Saldanian Orogeny. However, magma mixing is not evident at emplacement level and there are no magmatic kinships with the I-type granitic rocks of the CGS. The mantle wedge is inferred to have been enriched during subduction along the active continental margin. In the late- to post-orogenic phase, the enriched mantle partially melted to produce heterogeneous magma batches, exemplified by those that formed the Yzerfontein pluton, which was further hybridised through minor assimilation of crustal materials. Like Yzerfontein, the small volumes of mafic rocks associated with many batholiths, worldwide, are probably also low-volume, high-level expressions of crustal growth through the emplacement of major amounts of mafic magma into the deep crust.     

A Methodology for the Definition of Geotechnical Mine Sectors Based on Multivariate Cluster Analysis

This paper offers a new method for the definition of geotechnical sectors in open pit mines based on multivariate cluster analysis. A geological-geotechnical data set of a manganese open pit mine was used to demonstrate the methodology. The data set consists of a survey of geological and geotechnical parameters of the rock mass, measured directly in several points of the mine, structured initially in twenty-eight variables. After the preprocessing of the data set, the clustering technique was applied using the k-Prototype algorithm. The squared Euclidean distance was used to quantify the proximity between numerical variables, and the Jaccard's coefficient of similarity was used to quantify the proximity between the nominal variables. The different cluster results obtained were validated by the multivariate analysis of variance. The identification of cluster structures was achieved by plotting them on the mine map for spatial visualization and definition of geotechnical sectors. These sectors are spatially contiguous and relatively homogeneous regarding their geological–geotechnical properties, indicated by a high density of points of the same group. It was possible to observe a great adherence of the proposed sectors to the mine geology, demonstrating the practical representativeness of the clustering results and the proposed sectors.

     

Winter and autumn daily precipitation patterns in Catalonia,Spain

Summary A principal component analysis (PCA), based on a network consisting of 60 pluviometric gauges and their daily precipitation data, is attempted in order to describe the main winter and autumn patterns governing precipitation in Catalonia (NE Spain). This PCA procedure is applied to the interstation correlation matrix and rotated component loadings are then deduced and extensively interpreted. The PCA results are then used in a clustering process (Average Linkage), leading to two rainfall divisions, one for each season, which are then compared.With 5 Figures     

Tourmaline composition and boron isotopes record lateritic weathering during the Great Oxidation Event

Identifying evidence of oxidative weathering in the geological record is essential to trace the evolution of Earth's atmosphere oxygenation. Metamorphosed residues of lateritic weathering have been identified as two rock types in the 2.1‐Ga‐old Cercadinho Formation, Piracicaba Group, Quadrilátero Ferrífero of Minas Gerais. One is tourmaline–hematite–sillimanite–kyanite quartzite; the other is rutile–tourmaline–hematite–muscovite phyllite. Both rocks have abundant tourmaline with δ¹¹B values between about ?17‰ and ?13‰. The Cercadinho tourmaline is roughly parallel to the povondraite–“oxy‐dravite” join of meta‐evaporitic tourmaline, in its more aluminous segment, offset to higher contents of iron. These compositional and isotopic characteristics of the Cercadinho tourmaline indicate that continental evaporitic brines interacted with aluminium‐ and iron‐rich residues of lateritic weathering. The abundance of disseminated tourmaline, a mineral poorly reported from palaeosols worldwide, implies a boron‐rich brine overprint on the lateritic profile before the onset of metamorphism, reflecting a climatic change from humid to arid conditions in a continental setting. The recognition of lateritic weathering in the Cercadinho Formation contributes to the amount of evidence for increased levels of atmospheric oxygen between 2.22 and 2.06 Ga ago.     

Are changes in the structure of nematode assemblages reliable indicators of moderate petroleum contamination?

This study assesses through a multiple before-after-control-impact (MBACI) design the effects of diesel oil on the structure of nematode assemblages in unvegetated tidal flats of a subtropical estuary. Oil-exposed treatments were contrasted with controls for a duration of four successive days before and after an experimental spill in three distinct areas of the Paranaguá Estuarine Complex (Southern Brazil). No significant differences were observed in nematode total density, number of taxa and the overall assemblage structure between the control and impact treatments from before to after the experimental spill. This reinforces the idea that, despite being good indicators of environmental stress, free-living marine nematodes are able to tolerate low concentrations of hydrocarbons and to survive in moderately contaminated areas. We also show that robust experimental designs are useful to avoid confounding expected natural variability with the effects of a mild impact.     

Implications of principal stress axes and eigenvalue ratios on critical orientation of fractures. Application to two tectonic regions in Alaska

The principal stress axes and eigenvalue ratios of the stress tensors from two active seismic regions in Alaska (eastern Aleutian Arc and eastern Gulf of Alaska) are computed. These results are obtained using focal mechanisms of earthquakes with magnitudes ranging from 5.5 to 8.2 and show for both regions slightly different compressional states with an orientation of the pressure axis of N26°W for the eastern Gulf of Alaska and N27°W for the east Aleutian Arc. The eigenvalue ratios for the eastern Gulf of Alaska and east Aleutian Arc are 0.84 and 0.99, respectively. The algorithm used in computations allows us to detect some focal mechanisms which are incoherent with respect to the obtained stress tensors. These incoherences are explained in terms of some tectonic features of the region. The orientations of possible pre-existing fractures which would need an unrealistically high maximum shear stress to start a slip on their fault planes are also investigated. These critical orientations depend on a constitutive frictional law as well as on eigenvalue ratios and eigenvectors of the stress tensors. The orientation, which is given in our case by the angle between the normal vector to a fracture and the regional tensional axis, is approximately 59° for the eastern Aleutian Arc. This angle ranges from 49° to 59° for the eastern Gulf of Alaska. It must be pointed out that fractures which need unrealistically high shear stress to start slip on their fault planes are defined by a very narrow band of possible angles between tensional direction and the normal vector to fault plane.     

Elastic-anelastic Regional Structures for the Iberian Peninsula Obtained from a Rayleigh Wave Tomography and a Causal Uncoupled Inversion

The elastic and anelastic structure of the lithosphere and asthenosphere of the Iberian Peninsula is derived by means of tomographic techniques applied to local phase and group velocities and local attenuation coefficients of Rayleigh wave fundamental mode. The database consists of surface wavetrains recorded at the broadband stations located in the Iberian Peninsula on the occasion of the ILIHA project. Path-averaged phase and group velocities and attenuation coefficients were previously obtained by standard filtering techniques of surface wavetrains and, subsequently, local dispersion curves were computed according to the Yanovskaya-Ditmar formulation. First, a principal component analysis (PCA) and the average linkage (AL) clustering algorithm are applied to these local values in order to classify the Iberian Peninsula in several rather homogeneous domains from the viewpoint of the similarity of the corresponding local dispersion curves, without previous seismotectonic constraints. Second, averaged phase and group velocities and attenuation coefficients representing each homogeneous region are used to derive the respective elastic and anelastic models of the lithosphere and asthenosphere. This purpose is achieved by using the uncoupled causal inversion of phase and group velocities and attenuation coefficients. The main features of the homogeneous regions are discussed by taking as reference the Hercynic, Alpine and Neogene domains of the Iberian Peninsula, and two questions affecting the reliability of the elastic-anelastic models are revised. First, the coherence of the shear-velocity and Q_β⁻¹ models obtained by causal uncoupled inversion for each region is analysed. Second, the influence of the causal phase and group velocities on the shear-velocity models is evaluated by comparing elastic and anelastic models derived from causal uncoupled inversion with those deduced from non-causal inversion.     

Some Statistical Characteristics of Monthly and Annual Pluviometric Irregularity for the Spanish Mediterranean Coast

Summary  Seven series of monthly pluviometric amounts, sometimes exceeding recording periods of 100 years and compiled by the Instituto Nacional de Meteorología (Spain), are used to study the irregularity of the pluviometric regime along the Spanish Mediterranean coast and nearby Atlantic coast. First of all, three statistical functions (gamma, log-normal and a combination of Poisson and gamma distributions) and moment-ratio diagrams are used to model the monthly and annual empirical distributions of precipitation amounts, each distribution being tested by means of the Kolmogorov-Smirnov test. It is noteworthy that, whereas most of the monthly cases require the gamma distribution, the pluviometric behaviour of the summer months is well described by the Poisson-gamma distribution. Moreover, both the log-normal and the gamma distributions satisfactorily model empirical annual amounts. Consequently, rainfall amounts are not identically distributed along a year for each gauge tested. Second, temporal trends deduced for annual and seasonal amounts are computed and their statistical significance evaluated. The most notable fact is that, although some linear trends are close to 1 mm/year, their significance levels exceed the assumed threshold value and, excepting the winter season for Barcelona, they are considered non-significant from a statistical point of view. Finally, by again using monthly and annual amounts, three temporal irregularity indexes are computed for each pluviometric series, the temporal disparity of the rainfall patterns of the Mediterranean region being enhanced as a result. It addition to the temporal irregularity, a change with latitude is observed both in the parameters of the statistical distributions and the temporal irregularity indexes for the rain gauges analysed. The two most southerly rain gauges constitute a special case in comparison with the remaining stations, because they also receive the Atlantic influences due to their proximity to this ocean. Received February 25, 1999/Revised August 2, 1999     

Proto-Adamastor ocean crust (920 Ma) described in Brasiliano Orogen from coetaneous zircon and tourmaline

Proto-Adamastor ocean bathed Rodinia and successor continental fragments from 1.0–0.9 Ga up to 0.75 Ga, and evolved into world Adamastor Ocean at 0.75–0.60 Ga. Mesoproterozoic oceanic crust is poorly preserved on continents, only indirect evidence registered in Brasiliano Orogen. We report first evidence of ophiolite originated in proto-Adamastor. We use multi-technique U-Pb-Hf zircon and δ¹¹B tourmaline isotopic and elemental compositions. The host tourmalinite is enclosed in metaserpentinite, both belonging to the Bossoroca ophiolite. Zircon is 920 Ma-old, ?_{Hf(920 Ma)} = +12, Hf_TDM = 1.0 Ga and has ‘oceanic’ composition (e.g., U/Yb < 0.1). Tourmaline is dravite with δ¹¹B = +1.8‰ (Tur 1), 0‰ (Tur 2), ?8.5‰ (Tur 3). These characteristics are a novel contribution to Rodinia and associated world ocean, because a fragment of proto-Adamastor oceanic crust and mantle evolved at the beginning of the Brasiliano Orogen.