Mafic rocks in a deep-crustal segment of the Variscides (the Góry Sowie, SW Poland): evidence for crustal contamination in an extensional setting

Coronitic metagabbronorites (so-called hyperites) and metabasites occur within gneisses, migmatites and minor granulites in the high-grade metamorphic Góry Sowie Block in the Sudetes (SW Poland). Incompatible trace-element and )Nd data, combined with field and petrographic evidence, suggest that three groups may be distinguished: (1) "enriched" amphibolites (Zr/Nb ca. 8, )Nd₅₀₀ +1 - +2.5), (2) "depleted" amphibolites (Zr/Nb >30, )Nd₅₀₀ +5 - +6), and (3) metagabbronorites (Zr/Nb 13-19, )Nd₅₀₀ scattered between +5.3 and -1). These characteristics favour an extensional within-plate setting, consistent with their field occurrence as small bodies scattered within metasedimentary and felsic metaigneous gneisses. In that setting, enriched mantle sources were partially melted to produce a range of basic magmas, showing affinities with continental tholeiites (within plate-type basalts). Presumably, in a more advanced stage of rifting, more depleted varieties, transitional to N-MORB, were emplaced. These mafic magmas were variably contaminated during their ascent in the crust. The metagabbronorites, with their well-preserved igneous textures, were previously interpreted as having been intruded after the main deformation and after the peak of amphibolite facies metamorphism (some authors suggesting a relation to the "circum Góry Sowie ophiolites"). However, the geochemical characteristics and P-T estimates suggest that the metagabbronorites are unrelated to the ophiolitic gabbros found in the neighbourhood of the Góry Sowie Block and, in contrast to them, have experienced a complex, polybaric P-T path.     

Late Glacial and Holocene depositional history in the eastern part of the Szczecin Lagoon (Great Lagoon) basin—NW Poland

Analyses on 27 sediment cores taken from the bottom of the Szczecin Lagoon allowed environmental reconstruction of the postglacial main stages of basin development, based on detailed sedimentological, geochemical, diatomological and malacological studies of selected key cores. Studies revealed that during the Late Glacial and Holocene this area developed in several stages. In the Late Glacial the whole study area constituted a low alluvial plain. At the turn from Younger Dryas to Holocene the alluvial plain was cut through by the Odra river to a level of 10–11 m below sea level (b.s.l.). Along with the first phases of the Holocene marine transgression at the southern Baltic Sea's coasts the accumulation of the limnic-swampy deposits began in this lower part of the Odra valley. At ca. 6–6.5 ka BP the transgression proceeded and Littorina Sea waters flooded the area. At that time the Szczecin Lagoon constituted a marine embayment in which series of sands, partly rich in malacofauna, was deposited. The development of the Swina barrier resulted in the isolation of the embayment from the direct inflow of Baltic Sea waters.     

Rill characteristics and sediment transport as a function of slope length during a storm event on loess soil

On the basis of detailed rill surveys carried out on bare plots of different lengths at slopes of 12 per cent, basic rill parameters were derived. Rill width and maximum depth increased with plot length, whereas rill amount and cross‐sectional area, expressed per unit length, remained similar. On smaller plots, all rills were connected in a continuous transport system reaching the plot outlet, whilst on larger plots (10 and 20 m long) part of the rills ended with a deposition areas inside the plots. Amounts of erosion, calculated from rill volume and soil bulk density, were compared with soil loss measured at the plot outlets. On plots 10 and 20 m long, erosion estimated from volume of all rills was larger than measured soil loss. The latter was larger than erosion estimated from volume of contributing rills. To identify contributing soil loss area on these plots, two methods were applied: (i) ratio of total soil loss to maximum soil loss per unit area, and (ii) partition of plot area according to the ratio of contributing to total rill volume. Both methods resulted in similar areas of 21·8–23·5 m² for the plot 10 m long and 31·2 m² for the plot 20 m long. Identification of contributing areas enabled rill (5·9 kg m^?2) and interrill (2·6 kg m^?2) erosion rate to be calculated, the latter being very close to the value predicted from the Universal Soil Loss Equation. Although rill and interrill rates seemed to be similar on all plots, their ratio increased slightly with plot length. Application of this ratio to compute slope length factor of the Revised Universal Soil Loss Equation resulted in similar values to those predicted with the model. The achieved balance of soil loss suggested that all the sediment measured at the plot outlet originated from contributing rills and associated contributing rill areas. The results confirmed the utility of different plot lengths as a research tool for analysing the dynamic response of soil to rainfall–runoff. Copyright © 2005 John Wiley & Sons, Ltd.     

Geochronology of a composite granitoid pluton: a high-precision ID-TIMS U–Pb zircon study of the Variscan Karkonosze Granite (SW Poland)

Resolving time differences between successive magmatic pulses in composite granitoid plutons is often a difficult task. High-precision CA-ID-TIMS zircon ages obtained from such a pluton, the Variscan Karkonosze Granite (NE part of the Bohemian Massif), provide evidence that the crystallization of the two main granite facies, porphyritic and equigranular, happened between 312.5 ± 0.3 and 312.2 ± 0.3 Ma, thus unresolvable at the 0.08–0.1 % precision level of a single ²⁰⁶Pb/²³⁸U age. This finding is at odds with most other previous dating attempts and asks for a re-evaluation of the previous scattered geochronological data. The main reasons for the scatter of the earlier dates obtained by various techniques can include analytical causes, the presence of older inheritance and disturbance of the U–Pb isotopic system, due to zircon metamictization (enhanced by high-U content in zircon) or late- and post-magmatic alteration.