Petrogenesis of early paleozoic peralkaline rhyolites from the Macedo de Cavaleiros region (NE Portugal)     

M. Luisa Ribeiro 《International Journal of Earth Sciences》1987,76(1):147-168

The petrology and geochemistry of Variscan peralkaline gneisses from NE Portugal, discovered in 1962, is presented. Major and trace element data for the oversaturated sequence of the Macedo de Cavaleiros area depict some specific geochemical patterns: high Fe₂O₃ ^(t) (even higher than in the Ethiopian Main Rift), high REE concentrations and small Eu anomalies. Field and petrographical observations also suggest that liquid immiscibility has played an important petrogenetic role in the crystallization trends. The alternating pyroxenefeldsparhematite-rich bands observed in outcrop are interpreted as resulting from fluctuating fO₂ conditions. Petrological and geochemical data show that these pantelleritic and comenditic rocks are the end members of an alkaline-transitional basaltic sequence, by which they are overlain. An uppermost sequence of oceanic tholeiitic basalts completes the picture of an evolving rift environment, where mantle processes generate crustal thinning and eventually oceanic crust.

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Zusammenfassung Petrologie und Geochemie variszischer peralkaliner Gneisse aus Nordostportugal, 1962 entdeckt, werden dargestellt. Die Haupt- und Spurenelementanalysen dieser übersättigen Gesteine weisen Besonderheiten auf, wie z. B. die hohen Fe203-Gehalte, die selbst noch die des »Ethiopian Main Rift« übersteigen, die hohen Konzentrationen an Seltenen Erdelementen und die kleine negative Eu-Anomalie. Geländegeologie und Petrographie suggerieren, da\ die liquid-magmatische Entmischung eine wichtige petrogenetische Rolle im Kristallisationsablauf spielte. Im Aufschlu\ wird eine Wechsellagerung von Pyroxen-Feldspat- und Hämatit-reichen Schichten beobachtet, die hier als Folge von fO2-Schwankungen erklärt wird. Die petrologischen und geochemischen Daten zeigen, da\ die pantelleritischen und comenditischen Gesteine die letzten Glieder in einer Abfolge vom alkalinen übergangstyp darstellen... Die vulkanische Abfolge im Gebiet von Macedo de Cavaleiros endet mit tholeiitischen Basalten, die damit das Bild der magmatischen Entwicklung in einem Rift-Bereich, in dem Mantelprozesse eine Krustenverdünnung in einem Rift-Bereich, in dem Mantelprozesse eine Krustenverdünnung und die Bildung ozeanischer Kruste bewirkten, vervollständigen.

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Résumé L'auteur présente la pétrologie et la géochimie des gneiss varisques peralcalins du nord-est du Portugal, découverts en 1962. Les analyses des éléments majeurs et en traces dans ces roches sursaturées montrent quelques caractéristiques spéciales comme la haute teneur en Fe₂O₃ (t) (supérieure mÊme à celles de l'»Ethiopian Main Rift«), la haute concentration en terres rares et la faible anomalie négative de Eu. La géologie de terrain et les observations pétrographiques suggèrent aussi que l'imiscibilité entre liquides a joué un rÔle pétrogénétique important au cours de la cristallisation. Les alternances de bandes alternativement riches en pyroxène-feldspath et en hématite, observées sur les affleurements, sont interprétées comme le résultat de la fluctuation de fO₂. Les données pétrologiques et géochimiques montrent que les roches pantéléritiques et comenditiques représentent les derniers termes d'une séquence du type alcalintransitionel qui les surmonte. La séquence volcanique dans le secteur de Macedo de Calvaleiros se termine par des basaltes tholéiitiques qui complètent le cadre d'une environnement d'un zone de »rift« dans laquelle les processus mantéliques ont déterminé l'amincissement crustal et la formation d'une croûte océanique.

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Late Quaternary climatic history of the Chilean Channels based on fossil pollen and beetle analyses,with an analysis of the modern vegetation and pollen rain     

Allan C. Ashworth  Vera Markgraf  Carolina Villagran 《第四纪科学杂志》1991,6(4):279-291

Fossil beetles and pollen were examined from an intermorainal bog at Puerto Edén, Isla Wellington, Chile (latitude 49°08'S, longitude 74°25'W). Wood from near the base of the section has an age of 12 960 ± 150 yr BP. Occurrence of flightless beetle species in the basal peat sample is evidence that some members of the biota survived the last glacial maximum in refugia. The assumption that the Chilean Channels were entirely ice-covered is incorrect. Plants and insects that invaded the deglaciated terrain were those of an Empetrum heathland in which patches of Nothofagus forest were restricted to sheltered locations. The climate supporting the heathland is inferred to have been windier and probably drier than that of the present day. From 13 000 yr BP to 9500 yr BP Nothofagus forest expanded, possibly in response to less windiness and more available moisture. Neither the fossil beetle nor pollen data support a return to significantly colder conditions between 11 000 and 10 000 yr BP at the time of the Younger Dryas Stade. From 9500 to 5500 yr BP the climate was as wet as that of the present day, based on an increased representation of the pollen of moorland plants and of aquatic beetle species. From 5500 to 3000 yr BP the climate was drier, as indicated by the expansion of Empetrum heath and the reduction in mesic habitats. From 3000 yr BP to the present-day mesic habitats dominated as the climate returned to a wetter mode. The alternatively wetter and drier episodes are attributed to latitudinal shifts in the position of storm tracks in the belt of Southern Westerlies.  相似文献   

Permissible modes of non-thermal convection in planetary mantles     

David G. Ashworth 《Astrophysics and Space Science》1969,5(1):71-75

Boundary conditions are imposed upon the solutions of the conservation equations for non-thermal convective motion in a self-gravitating, homogeneous, non-rotating sphere of radiusR, consisting of a core extending to a fraction of the radius of the sphere and with a viscous mantle overlying the core. It is shown that convective modes are permissible in the mantle only for certain values ofn and .  相似文献   

A paleoenvironmental reconstruction of Laguna Babícora, Chihuahua, Mexico based on ostracode paleoecology and trace element shell chemistry     

Manuel R. Palacios-Fest  Ana Luisa Carreño  José R. Ortega-Ramírez  Guillermo Alvarado-Valdéz 《Journal of Paleolimnology》2002,27(2):185-206

Paleoecology of Laguna Babícora, Chihuahua, Mexico was reconstructed using ostracode faunal assemblages and shell chemistry. The paleolimnological record is used to show the magnitude of paleoclimatic changes in the area from 25,000 years to the present.Faunal assemblages consist of four species of the genus Limnocythere: L. sappaensis, L. ceriotuberosa, L. bradburyi and L. platyforma, all associated with Candona caudata, Candona patzcuaro and Cypridopsis vidua. A paleosalinity index developed from these assemblages indicates that the lake's salinity fluctuated frequently from oligo- to meso-haline conditions during the last 25,000 years. This pattern and low salinity range are in good agreement with modern TDS (here used as an indicator of salinity) values recorded from 26 wells and one spring from the area (258–975 mg l^–1). To estimate paleotemperature we examined the trace element content (Mg/Ca ratios) from individual valves of L. ceriotuberosa and L. platyforma, the two species most commonly recorded in Laguna Babícora.Shell Mg/Ca ratios of 204 specimens of these two species were used to estimate water temperature (Mg/Ca) by means of experimental standard coefficients. Our data show that paleowater temperature ranged from 5.6–21.3 °C (with 2 values ranging from 0.2–4.8 °C), which suggest a close correlation with atmospheric temperatures around the lake. These results are in good agreement with a modern mean winter temperature (3.5 °C) and mean summer temperature (20 °C) recorded in the area between 1970 and 1980.  相似文献   

Electron petrography of shock effects in a gas-rich enstatite-achondrite     

J. R. Ashworth  D. J. Barber 《Contributions to Mineralogy and Petrology》1975,49(2):149-162

The enstatite-achondrite Khor Temiki has been studied by high-voltage electron microscopy. The normal Khor Temiki lithology has a fine-grained matrix in which individual grains show the well-known effects of unshielded solar irradiation. Intensity of deformation varies greatly from grain to grain; this material has a varied history of impact deformation, and must have formed in an environment similar to that of the lunar regolith. The meteorite is traversed by veins of extra-dark material. This was produced in situ from the normal lithology by intense shock, sufficient to erase its irradiation record almost completely. Instead of the enstatite that dominates the bulk meteorite, optically twinned clinoenstatite is found to be the major mineral in such a shock-vein. It is highly defective, and its electron diffraction patterns contain diffuse orthopyroxene maxima. It is interpreted in terms of inversion of protoenstatite produced by the thermal effect of the shock. Recrystallization phenomena, and the occurrence of enstatite in close association with the clinoenstatite, are described. This occurrence of twinned clinoenstatite is contrasted with those due to quenching of primary protopyroxene in small bodies such as chondrules. The effects of shock have eliminated porosity from the vein material, and indurated it. Less severe such effects, outside the veins themselves, must have contributed to the lithification of the meteorite.  相似文献   

Staurolite at anomalously high grade     

J. R. Ashworth 《Contributions to Mineralogy and Petrology》1975,53(4):281-291

In the Huntly — Portsoy area, north-east Scotland, staurolite occurs in muscovite-free migmatized semipelites. This occurrence in the Sillimanite-potash feldspar Zone is petrographically distinct from lower-grade staurolite found nearby. Petrographic and chemical data indicate that the staurolite formed stably during the migmatization episode. It was stabilized by high zinc-content. Its formation was probably an early retrograde effect.  相似文献   

Electron petrography of shock-deformed olivine in stony meteorites     

J.R. Ashworth  D.J. Barber 《Earth and Planetary Science Letters》1975,27(1):43-50

We have studied olivine by high-voltage electron microscopy in meteorites showing shock effects, namely the ordinary chondrites Olivenza and Hedjaz, the ureilite Goalpara and the carbonaceous chondrite Allende. The observations are compared with published data on experimentally deformed and annealed olivine.Olivenza and Hedjaz have suffered low-temperature shock, which has produced high densities of [001] screw dislocations and cracks. Arrays of cracks are found to correspond to planar features which can be seen optically and to be responsible for mosaicism. Goalpara has suffered very heavy shock, of which the final result is extensive recrystallization of the olivine, and the new grains have distinctive low-density distributions of dislocations. Allende has not been shocked as a whole, but individual parts have deformational and heating histories dating from before the aggregation of the meteorite.  相似文献   

Lithification of gas-rich meteorites     

J.R. Ashworth  D.J. Barber 《Earth and Planetary Science Letters》1976,30(2):222-233

High-voltage electron microscopy has been used to study the fine structure of four gas-rich meteorites, with particular reference to the cementation and compaction processes that have affected the fine-grained matrix. The observed features are compared with similar effects in lunar breccias. Lithification is attributed to the passage of shock waves through porous aggregates, causing deformation whose intensity varied spatially on a small scale, the most intense deformation and heating effects being concentrated at the edges of large grains and in the matrix between them. It is inferred that relatively mild shocks have produced amorphous cement between matrix grains in the achondrite Khor Temiki and the chondrite Weston. Relatively intense shock is inferred for specimens of the chondrites St. Mesmin and Pantar. These have non-porous, completely crystalline matrices, and fine-grained black veins which fill cracks in relatively large deformed grains. Recrystallization of some deformed material is attributed to shock-heating, which was not sustained long enough to erase the irradiation record from all the relatively large grains. Matrix recrystallization without extensive melting constitutes a metamorphic event, and the observations indicate that shock was responsible for the metamorphism experienced by these chondrites at relatively late stages of the evolution of their parent bodies.  相似文献   

Shell‐shape and morphometric variability in Mytilus galloprovincialis from micro‐tidal environments: responses to different hydrodynamic drivers          下载免费PDF全文

Vanessa Moschino  Monica Bressan  Luigi Cavaleri  Luisa Da Ros 《Marine Ecology》2015,36(4):1440-1453

Tidal conditions differently influence inter‐tidal organisms in terms of general physiological and metabolic responses. In this study we investigated the morphological response in shells of Mytilus galloprovincialis native to different micro‐tidal coastal environments in the Northern Adriatic Sea. Our purpose was to highlight the ecophenotypic variability across tidal levels and to elucidate how tidal currents and waves produced by anthropogenic activities may play a part in modulating shell morphology. Three sampling sites were selected: an open‐sea area 15 km off‐shore and two sites within the lagoon of Venice, the first near one of its three inlets, and the other one in the proximity of Venice city centre. At each sampling site, organisms were seasonally collected at different depths within their vertical zonation, either in the inter‐tidal zone – i.e. at both the highest and lowest tide zonation limits, and subtidally. The mussel shells were analysed by investigation of their morphometric relationships (height/length and width/length ratios) and by elliptic Fourier analysis of the shell contours. Shell thickness and condition index were also evaluated for a better comprehension of energy allocation/partitioning. Estimates based on long‐term measurements, visual observation, wind statistics and wave growth laws allowed an evaluation of the forces acting on shells. At the open‐sea site, the observed phenotypic variability of both shell shape and thickness was clearly related to the tidal vertical zonation. At the two lagoon sites, the currents generated by tidal flow through the inlet and the waves caused by the frequent passage of boats influenced both shell shape and thickness. A trade‐off between protection and growth was apparent along the tide gradient, as emphasized by the differences in the partitioning and allocation of energy between shell and flesh production.  相似文献   

Sequence stratigraphic framework of a mixed turbidite-contourite depositional system along the NW slope of the South China Sea   总被引：1，自引：0，他引：1  

Luisa Palamenghi  Hanno Keil  Volkhard Spiess 《Geo-Marine Letters》2015,35(1):1-21

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