The 2 Ga peraluminous magmatism of the Jacobina — Contendas Mirante Belt (Bahia) Brazil): Major and trace-element geochemistry and metallogenic potential

The Jacobina — Contendas Mirante belt represents a Transamazonian (2 Ga), N-S, 500-km long, elongated orogenic domain in the central part of the São Francisco craton, Bahia state. Numerous syntectonic to post-tectonic peraluminous leucogranites were emplaced along the major structures of the belt. Their mineralogical and geochemical and some of their metallogenetic characteristics are very similar to their Hercynian and Himalayan equivalents. However, their average peraluminous index varies from one granitic pluton to another and biotite is, on average, slightly more magnesian in the Transamazonian leucogranites. Higher oxygen fugacity is indicated by the general occurrence of magnetite, the stability of allanite and sometimes epidote in most of the plutons and by biotite chemistry. The peraluminous magmatism of the Jacobina-Contendas Mirante belt results from crustal partial melting during a continental collision event at 2 Ga.Trace-element geochemistry implies variable source composition and/or melting conditions for the different granitic plutons and some different facies within the same plutonic unit. The scarcity of ilmenite, the general occurrence of magnetite, and the relatively low peraluminous index of some of these granites suggest that graphite-beating sediments are not a significantly source material. From their mineralogical and geochemical characteristics, acid meta-igneous rocks such as the Sete Voltas TTG suite of presumed Archaean age, seem to represent a suitable source for these granites.Sn, W, Li, F and Be enrichment of most Transamazonian leucogranites is much weaker than in the mineralized Variscan equivalents. The Caetano-Aliança and Riacho das Pedras granites represent the most specialized granitic bodies. Beryl (emerald), molybdenite and scheelite mineralizations are related to some of these granites which intrude ultrabasic formations: the Campo Formoso and the Carnaiba granites. In many of these granites, uranium content is comparable to values measured in mineralized Hercynian leucogranites. The occurrence of hexavalent uranium minerals, mineralization and episyenitic alteration are favourable criteria for finding Variscan-type uranium ore deposits.     

Meta-igneous origin of Hercynian peraluminous granites in N.W. French Massif Central: implications for crustal history reconstructions

Seventy samples of Hercynian peraluminous granites (Guéret, Millevaches and Saint Sylvestre massifs) and metamorphic units of the Limousin area were analysed for Rb–Sr and Sm–Nd. The source rocks of the peraluminous granites can be found in the metamorphic rocks of Limousin, among them meta-igneous rocks were largely predominent over meta-sedimentary rocks in the source of the three granites. Millevaches and Guéret granites were generated by the partial melting of rocks comprising meta-volcanics and meta-sediments, whereas the Saint Sylvestre granite was produced exclusively by the melting of late Precambrian granites. This leads to confusing T _DM ^Nd values, the confusion being amplified by the segregation of monazite during the petrogenetic evolution of the peraluminous granites, which leads to dramatic fractionation in Sm/Nd ratios. The data of the present study tend to demonstrate that peraluminous granites do not give a good representation of isotopic mean crustal estimates. Late Precambrian time seems, however, to have been a period of extensive crustal generation in Western Europe.     

Love-Rayleigh wave incompatibility and possible deep upper mantle anisotropy in the Iberian peninsula

Love and Rayleigh wave phase velocities are analyzed with the goal of retrieving information about the anisotropic structure of the Iberian lithosphere. The cross-correlation method is used to measure the interstation phase velocities between diverse stations of the ILIHA network at periods between 20 and 120 s. Despite the 2-D structure of the network, the Love wave data are too few to enable an analysis of phase velocity azimuthal variations. Azimuthal averages of Love and Rayleigh wave phase velocities are calculated and inverted both in terms of isotropic and anisotropic structures. Realistic isotropic models explain the Rayleigh wave and short-period Love wave phase velocities. Therefore no significant anisotropy needs to be introduced in the crust and down to 100 km depth in the upper mantle to explain our data. A discrepancy is observed only at long periods, where the data are less reliable. Love wave data at periods between 80 and 120 s remain 0.15 km/s faster than predicted by isotropic models explaining the long-period Rayleigh wave data. Possibilities of biases in the measurements due to interferences with higher modes are examined but seem unlikely. A transversely isotropic model with 8% of S-wave velocity anisotropy in the upper mantle at depths larger than 100 km can explain the whole set of data. In terms of a classical model of mantle anisotropy, this corresponds to 100% of the crystals perfectly oriented in the horizontal plane in a pyrolitic mantle. This is a rather extreme model, which predicts at time delay between 0 and 2 seconds for split SKS.     

A New Method for Collecting Core Samples Without a Drilling Rig

A new piston sampler allows the collection of high-quality core samples from sand, silt or clay, up to depths of 18 meters. The sampler is operated by a one- or two-person crew without a drilling rig. The sampler and ancillary equipment fit easily into a half-ton truck, making this a highly portable sampling system. Other advantages include minimal mechanical disturbance and precisely known sample depth. Casing is not required to maintain an open corehole below the water table and drilling fluid is not used in the corehole, so the solids and pore water of the sample should not be contaminated by foreign fluids. High-quality samples for physical, geochemical, and microbiological characterization of the subsurface are easily obtained with this new device.     

Ignimbrites of basaltic andesite and andesite compositions from Tanna,New Hebrides Arc

Tanna island is part of a large volcanic complex mainly subsided below sea-level. On-land, two series of hydroclastic deposits and ignimbrites overlie the subaerial remains of a basal, mainly effusive volcano. The ‘Older’ Tanna Ignimbrite series (OTI), Late Pliocene or Pleistocene in age, consists of ash flows and ash- and scoria-flow deposits associated with fallout tephra layers, overlain by indurated pumice-flow deposits. Phreatomagmatic features are a constant characteristic of these tuffs. The ‘younger’ Late Pleistocene pyroclastics, the Siwi sequence, show basal phreatomagmatic deposits overlain by two successive flow units, each comprising a densely welded layer and a nonwelded ash-flow deposit. Whole-rock analyses of 17 juvenile clasts from the two sequences (vitric blocks from the phreatomagmatic deposits, welded blocks, scoriaceous bombs and pumices from the ignimbrites) show basaltic andesite and andesite compositions (SiO₂=53–60%). In addition, 296 microprobe analyses of glasses in these clasts show a wide compositional range from 51 to 69% SiO₂. Dominant compositions at ∼54, 56, 58.5 and 61–62% SiO₂ characterize the glass from the OTI. Glass compositions in the lower – phreatomagmatic – deposits from the Siwi sequence also show multimodal distribution, with peaks at SiO₂=55, 57.5, 61–62 and 64% whereas the upper ignimbrite has a predominant composition at 61–62% SiO₂. In both cases, mineralogical data and crystal fractionation models suggest that these compositions represent the magmatic signature of a voluminous layered chamber, the compositional gradient of which is the result of fractional crystallization. During two major eruptive stages, probably related to two caldera collapses, the OTI and Siwi ignimbrites represent large outpourings from these magmatic reservoirs. The successive eruptive dynamics, from phreatomagmatic to Plinian, emphasize the role of water in initiating the eruptions, without which the mafic and intermediate magmas probably would not have erupted. Received: February 19, 1993/Accepted October 10, 1993     

Current status,distribution, life cycle and ecology ofRhithrogena germanica Eaton, 1885 in Switzerland: Preliminary results (Ephemeroptera,Heptageniidae)

Rh. germanica is a European species that, due to human impacts, has progressively disappeared from its original territories and is now considered rare. In SwitzerlandRh. germanica has disappeared from the Aare, Broye and Kleine Emme rivers and is actually found in a limited reach of the River Rhine and some of its tributaries (Limmat, Sihl, Thur and Töss rivers).Rh. germanica is a univoltine species, hatching probably in summer. The emergence occurs from February to the beginning of April. It takes place on the water surface. Laboratory observations have shown that the subimaginal stage lasts four days.Rh. germanica occurs on different substrate types, ranging from stones and cobbles to pebbles and gravel. The larvae are more abundant in zones of erosion with coarse and porous sediments than in depositional zones with fine sediments and silts. They are most abundant in riffles with water velocities from 20 to 150 cm/sec. The species tolerates moderately organic-polluted waters.The particular strategy concerning growth and emergence ofRh. germanica is also discussed.     

Trends of thought concerning meteorological research

Summary Certain problems of synoptic meteorology relating to the nature of the large quasi-stationary cyclonic and anticyclonic centers aloft are examined from the standpoint of an hypothesis which seeks to connect their maintenance with that of the smaller transient disturbances. The suggested link between these two scales of eddies is in many ways similar to that operating in the general circulaton in order to account for the sustained existence of the circumpolar mean zonal motions, according to the more modern concepts. Newer views concerning the dynamcis of large convective processes, and the essential nonlinearity of the laws governing motions in the atmosphere, figure importantly in the arguments advanced.

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Zusammenfassung Gewisse Probleme der synotischen Meteorologie im Zusammenhang mit der Natur von grossen halb-stationären zyklonischen und antizyklonischen Zentren in der höheren Atmosphäre werden von einem hypothetischen Standpunkt aus untersucht; und es wird versucht, deren Bestehen in Zusammenhang zu bringen mit dem Vorhandensein von kleineren, vorübergehenden Störungen. Der vorgeschlagene Zusammenhang zwischen diesen beiden Arten von Wirbel-Systemen ähnelt in vielen Beziehungen der allgemeinen Zirkulation, die verantwortlich ist für das fortwährende Bestehen der zirkumpolaren durchschnittlichen zonalen Strömungen, wie sie zufolge modernerer Auffassungen erklärt werden. Neuere Standpunkte, die die Dynamik von grossen konvektiven Vorgängen in der Atmosphäre und die nicht-linearen Gesetze im Hinblick auf die atmosphärischen Strömungs-Gesetze betreffen, werden hauptsächlich in der folgenden Abhandlung untersucht.