N. G. Vorob&#;eva  V. N. Sergeev  M. A. Semikhatov 《Doklady Earth Sciences》2006,410(7):1038-1043

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P–T evolution and reaction textures in retrogressed eclogites from Svetlik, the Moldanubian Zone (Czech Republic)     

S. W. Faryad  M. Perraki  S. Vrána 《Mineralogy and Petrology》2006,88(1-2):297-319

Summary Lenses and pods of mafic rocks from the Monotonous Unit near Svetlik are characterized by eclogite facies mineral assemblages; however some inclusion patterns (oriented quartz rods in clinopyroxene and cuboids of disordered graphite in garnet) that are usually known from ultra-high pressure rocks were also observed in one sample. Conventional thermobarometry yielded maximum P–T conditions of 2.0–2.5 GPa and 750 °C. Decompression and heating at amphibolite and granulite facies conditions resulted in the formation of at least five distinct types of symplectites. These include symplectitic intergrowth of ilmenite and clinopyroxene after titanite, described here for the first time from the Moldanubian Zone. In addition, symplectites of plagioclase and biotite with accessory amounts of spinel after tabular pseudomorphs (after phengite?) are also reported here. Mass balance relations indicate that symplectites of diopside + plagioclase after omphacite and plagioclase + spinel (sapphirine) after kyanite + garnet, formed by nearly isochemical reactions. All other symplectite-forming reactions were allochemical and were accelerated by the presence of fluid in the primary phases. Preserved zoning pattern in garnet with high compositional gradient in some samples suggests that the rocks were affected briefly by granulite facies overprint.  相似文献   

The lithosphere beneath the Sangilen Plateau, Siberia: evidence from peridotite, pyroxenite and gabbro xenoliths from alkaline basalts     

V. V. Egorova  N. I. Volkova  R. A. Shelepaev  A. E. Izokh 《Mineralogy and Petrology》2006,88(3-4):419-441

Summary Ultramafic and mafic xenoliths in Ordovician Agardag alkaline basalt dikes from the Sangilen Plateau, southeastern Siberia, provide samples from the upper mantle and crust beneath central Asia. Three major groups were distinguished among the xenoliths: Group I xenoliths are spinel lherzolites, Group II xenoliths are spinel-garnet clinopyroxenites, and Group III comprises gabbroic xenoliths with two subgroups: Group IIIa comprises garnet bearing gabbroids and Group IIIb is represented by garnet-free gabbroids. The spinel lherzolite xenoliths represent the uppermost lithospheric mantle beneath the Sangilen Plateau and have geochemical characteristics similar to those of primitive mantle. Spinel-garnet clinopyroxenite and gabbroic xenoliths are of igneous origin and represent fragments of intrusive bodies crystallized at depths close to the mantle-crust boundary, as well as in the lower and the upper crust. The gabbroic xenoliths are evidently the crystallization products of melts similar in major and trace element composition to parental magma of the Bashkymugur gabbronorite-monzodiorite intrusion. Gabbroic xenoliths from the Ordovician Agardag alkaline basalt dikes demonstrate the presence of intermediate magmatic chambers within the crust beneath the Sangilen Plateau during the Early Palaeozoic. The relatively high equilibration temperatures of the mantle and lower crust xenoliths in the Agardag alkaline basalt dikes are largely attributable to a plume occurring beneath the Sangilen Plateau during the Ordovician.  相似文献   

Peculiarities in the behavior of trace and rare earth elements during granitization and migmatization of the Elgakan complex (East Siberia, Nyukzha River)     

V. A. Glebovitsky  I. S. Sedova  L. M. Samorukova 《Doklady Earth Sciences》2006,411(8):1309-1312

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The role of nappe tectonics in the development of abnormally high formation pressure and economic metalliferous brines: A case study of the southern Siberian craton     

A. G. Vakhromeev  V. I. Sizykh 《Doklady Earth Sciences》2006,407(2):209-212

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Tectonic position of the Alshar Au-As-Sb-Tl deposit, Macedonia     

N. T. Kochneva  A. V. Volkov  T. Serafimovski  G. Tasev  I. N. Tomson 《Doklady Earth Sciences》2006,407(2):175-178

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Structural and nutritional heterogeneity of riparian vegetation in Patagonia (Argentina) in relation to seasonal grazing by sheep     

V.R. Utrilla  M.A. Brizuela  A.F. Cibils 《Journal of Arid Environments》2006,67(4):661-670

Riparian meadows of southern Patagonia are temporally and spatially heterogeneous habitats. They are thought to play a key role in regulating the dynamics of arid grazed steppes of the region. We conducted a 2-year study with sheep to determine the influence of two grazing conditions (deferment and spring grazing) on structural and nutritional parameters of three vegetation types apparently associated with a soil moisture gradient in a riparian meadow in southern Santa Cruz (Argentina). Spring deferment allowed forage accumulation in very moist and intermediate vegetation types by the beginning of the summer and had no detrimental impact on forage quality. Deferment had no effect on biomass accumulation of the drier vegetation type at our study site. Structural attributes of the vegetation appeared to affect spatial grazing patterns of sheep. Soil-related patchiness, rather than sheep grazing, appeared to control vegetation structure and nutritional value. The proportion of more mesic plant communities in riparian habitats of Patagonia could determine the outcome of plant–animal interactions in these systems. Practical recommendations of grazing capacity in paddocks, or more theoretical considerations of ecosystem dynamics of the Patagonian steppes need to explicitly consider the internal patchiness of riparian habitats.  相似文献   

Lichen-dominated soil crusts as arthropod habitat in warm deserts   总被引：2，自引：0，他引：2  

J.S. Lalley  H.A. Viles  J.R. Henschel  V. Lalley 《Journal of Arid Environments》2006,67(4):579-593

Soil crust lichens can be the dominant vegetation in arid lands, yet their importance as habitat to secondary producers is relatively unknown. This study examines the distribution of arthropod communities in the northern Namib Desert to evaluate whether a lichen-rich area is more or less productive than adjacent habitats in terms of the consumers each supports. Arthropods are diverse and highly endemic in the Namib Desert and lichens dominate this desert's extensive gravel plains. We sampled lichen-rich, dwarf shrub, and unvegetated sites and found distinct arthropod assemblages in the lichen-dominated sites, including species unique to lichen sites. Arthropod assemblages in two of the lichen sites were similar to those found in the dwarf shrub site. In a canonical correspondence analysis, crustose lichens and overall lichen cover were key in driving the variance in arthropod assemblages within the lichen sites. Furthermore, lichen morphotypes, overall lichen cover and species richness, were significantly correlated with the representation of arthropod subgroups and arthropod species richness. These findings provide evidence that lichen-dominated soil crusts in the Namib Desert are important supporters of secondary production, warranting more in-depth studies into the ecology and conservation of this lichen-rich habitat in warm deserts.  相似文献   

Reconstructing hydrological variability from testate amoebae analysis in Carpathian peatlands     

C. Schnitchen  D. J. Charman  E. Magyari  M. Braun  I. Grigorszky  B. Tóthmérész  M. Molnár  Zs. Szántó 《Journal of Paleolimnology》2006,36(1):1-17

Peatlands offer the potential for high resolution records of water balance over Holocene timescales, yet this potential is under-exploited in many areas of the world. Within Europe, peatlands are mostly confined to areas north of 55° N, but several areas of southern and eastern Europe contain small peatlands which may be suitable for palaeoclimatic reconstruction. In this paper we test the potential of peatlands in the Carpathian region for deriving quantified estimates of water table changes using testate amoebae analysis. A training set for palaeohydrological reconstruction from testate amoebae assemblages was obtained by collecting surface samples from 13 peatlands, including 9 from Hungary and 4 from Transylvania (Romania). Using a simple measure of mean annual water tables estimated from staining of PVC tape, we found that some peatlands were heavily influenced by runoff and groundwater, and were therefore not suitable as modern analogues of ombrotrophic climatically sensitive sites. The relationship between the testate amoebae assemblages in the modern samples and the environmental variables was explored using CCA. The CCA biplot showed that the most important variables are depth to water table and moisture content, confirming that hydrology is a key control on taxon distribution. pH was a secondary gradient. A transfer function for % moisture and depth to water table was established and applied to fossil assemblages from a sequence from Fenyves-tető, Transylvania, Romania. The reconstructed water table shows a number of variations which have parallels with other palaeoclimatic records from Europe and the North Atlantic prominent phases of higher water tables are associated with the periods 8000–8300 cal BP, 3000–2500 cal BP and after 600 cal BP. We suggest that these were periods of particular intensification of westerly airflow which affected eastern Europe as well as western and central Europe.  相似文献   

A Comparison of Sediment Varves (1950–2003 AD) in Two Adjacent Lakes in Northern Sweden     

V. Gälman  G. Petterson  I. Renberg 《Journal of Paleolimnology》2006,35(4):837-853

Koltjärnen and Nylandssjön are two closely situated lakes (<2 km apart) in northern Sweden. During the past century, distinct varved sediments have formed in these lakes. Nylandssjön has two varved, deep basins. Since lake and catchment characteristics superficially appear very similar for the two lakes and they are exposed to the same climate, one would expect the sediment varves to be similar. This investigation compares the varves in the two deep basins of Nylandssjön, and in the two lakes. The comparison of basins of Nylandssjön shows that varve thickness, water content and annual accumulation rates of organic matter and nitrogen are correlated for the period (1970–2003). The grey-scale curves are only clearly similar in about 50% of the varves. In the between-lake comparison varve thickness, water content and annual accumulation rates of organic matter and nitrogen are correlated for the period (1950–1996). However, the annual accumulation rates of dry mass, minerogenic matter and biogenic silica differ between the lakes, as well as within-varve successions in grey-scale. A general explanation to the differences is that the prerequisites for varve formation are not totally similar because of differences in catchment size, catchment- to-lake material fluxes, lake productivity and land-use influence. This study illustrates the complex relationships that exist between a lake, its catchment, in-lake productivity and formation of sediment varves. As a consequence, we must not apply a too simplistic view of the potential of varves as past climatic indicators, especially if the lakes are affected by land-use.  相似文献