U-Pb and Rb-Sr dating of a polymetamorphic nappe terrain: The Caledonian Jotun nappe,southern Norway

The Caledonian Jotun nappe in the Tyin area of southern Norway has been investigated using U-Pb analysis of zircon and sphene and Rb-Sr measurements of minerals and whole rocks, with special reference to the reaction of the isotope systems to various kinds of metamorphic event. The nappe consists of Precambrian basement rocks and their cover, inversely thrust over the Baltic shield and its parautochthonous, presumably lower Paleozoic sediments during the Caledonian orogeny. While the nappe basement retained its pre-Caledonian structures, the nappe cover was penetratively deformed and metamorphosed to lower greenschist facies conditions.U-Pb analyses of zircon and sphene ofnappe basement rocks point to the crystallization of a syenitic to monzonitic magma at 1694± 20m.y., the intrusion of gabbros into the syenites and monzonites at 1252_?25⁺²⁸ m.y., and the metamorphism (upper greenschist-lower amphibolite facies) and deformation (gneissification and mylonitization) of the whole complex at 909± 16m.y. Although this latest event caused strong lead loss in all zircon populations, it had no influence on the Rb-Sr system on a whole rock scale. The Caledonian movements did not disturb the U-Pb system of zircon and sphene but strongly influenced the Rb-Sr system in certain minerals and zircon and/or its inclusions (K-feldspar and brown biotite partially, green biotite completely reset).In contrast to the nappe basement, zircons from thecover rocks show pronounced lead loss during the Caledonian metamorphism/deformation — U-Pb analysis give discordia lower intercept ages of 415± 21 m.y. and 395± 6m.y. Again, however, the Rb-Sr whole-rock system has not been reset in Caledonian time. Minerals from the same rocks which provided the zircons give Rb-Sr isochron ages of 390± 11m.y. and 386 ± 20 m.y.Comparison of U-Pb and Rb-Sr results from the nappe basement demonstrates that only the analysis of U-Pb systematics in zircon and especially sphene resolved the 900-m.y. event. Rb-Sr mineral data alone yield ambiguous results.The results from the nappe cover confirm that U-Pb analyses from penetratively deformed sediments are very useful in dating a metamorphic/tectonic event in lower greenschist facies conditions.From the measured cell dimensions of the zircon populations it is concluded that lead can be completely retained in zircon during recrystallization.     

Mean bulk densities of samples of dry atmospheric aerosol particles: A summary of measured data

Mean bulk densities of various samples of dry atmospheric aerosol particles sampled at different sites and during different seasons and weather situations range between 1.8 and more than 3 gm cm³.     

The Néel transition and magnetic properties of terrestrial,synthetic, and lunar ilmenites

The magnetic susceptibility of a terrestrial, synthetic and lunar ilmenite specimen has been measured from 4 to 300 K. All specimens had a single Néel temperature transition which ranged from 56 to 57.7 K. In one case the powdered specimen was partially aligned in the magnetic field prior to the susceptibility measurements and the Néel transition was observed to shift to 60 K indicating magnetic anisotropy. A study of several magnetic parameters calculated from the experimental data showed gross impurities in the terrestrial specimen, single-domain to multi-domain metallic iron in the synthetic specimen, and a small amount of superparamagnetic metallic iron in the lunar sample. No multidomain iron was observed in the lunar ilmenite. The results of electron spin resonance measurements were also in general agreement with these findings. Because of the absence of Fe³⁺ compared to most terrestrial samples it is suggested that the anisotropic magnetic parameters be determined on lunar ilmenite when a large enough single crystal becomes available.     

Natural shock behavior of almandite in metamorphic rocks from the ries crater,Germany

An extensive study of a big number of gneiss specimens with various shock features from the suevite allowed unravelling of the shock behavior of almandite garnets.Almandites in shocked metamorphic rocks show with increasing dynamic pressures strong irregular fracturing. differently oriented sets of planar fractures or elements, brown turbidity and nucleation of minute crystals of an unknown phase in solid garnets. At higher peak pressures garnet was found to break down to (1) orthopyroxene + spinel + glass, and to (2) spinel + glass due to fast shock-melting.Extensive quantitative electron microprobe studies of almandite garnets and their breakdown products were carried out. The breakdown products within the original grain boundaries of the garnets consist of an alumina-rich orthopyroxene (with up to 10 wt. % Al₂O₃), hercynite to pleonaste spinels and a silica and calcium-rich glass matrix. The chemical zonation of magnesium and manganese of the former garnets is inherited in the composition of the newly formed orthopyroxenes.Petrographic evidence and chemical composition suggest a fast breakdown of the almandite garnets after passing of shock waves at rapidly falling pressures and very high post-shock temperatures within the ejected gneissic rock material.     

The origin of the Nicaraguan depression

A conspicuous graben extends for 800 kilometers through El Salvador and western Nicaragua to the Caribbean Sea in northeastern Costa Rica. Like the smaller but structurally similar Semangko and Toba Depressions of northern Sumatra, the trough is clearly related to voluminous volcanic eruptions during Late Tertiary time. In the region around Lakes Managua and Nicaragua, where the depression is best defined and reaches its greatest dimension, a thick series of Tertiary sediments and volcanic rocks provides a means of interpreting the Cenozoic history of the region. Following a long period of intermittent volcanic activity and sedimentation, extensive sheets of andesitic and dacitic ignimbrites were erupted during Late Miocene time from fissure sources which appear to have been located near the now-subsided central portion of the graben. Near the coast, ignimbrites flowed across a flat lagunal shore overwhelming and burying the tropical vegetation and finally coming to rest in shallow water. Unusual textures and chaotic mixtures of pumice with sediments and silicified wood characterize the bases of many of the water-laid ignimbrites. Subsidence of the graben does not appear to have occurred concurrently with the ignimbrite eruptions but followed them closely near the end of the Miocene or the beginning of Pliocene time. Subsequent activity has been confined to relatively smaller eruptions from central vents near the boundary faults of the graben.     

Buchbesprechungen

Ohne Zusammenfassung     

Die lokale Beeinflussung eines stehenden Gewässers durch eine punktförmige Abwasserbelastung; ein direkter Nachweis der wachstumsbegrenzenden Wirkung des Phosphors im Vierwaldstättersee

Zusammenfassung Die Beeinflussung des Vierwaldst?ttersees (Horwer Bucht) durch den mit dem Abwasser von 10 000 Einwohnern belasteten Steinibach liess sich im Oberfl?chenwasser auf eine Distanz von mehr als 2 km verfolgen (Abb. 1a, 1b, 1c). Es zeigte sich, dass eine erh?hte Phosphorzufuhr eine verst?rkte biogene Nitratzehrung erm?glicht. Die Abh?ngigkeit der Pigmentkonzentration y von der Gesamtphosphorkonzentrationx liess sich durch die folgende Mitscherlich-Gleichung beschreiben: log(30-y)=log 30–0,012 (x−6) Damit wurde im Vierwaldst?ttersee erstmals unter v?llig natürlichen Bedingungen die Beeinflussung der Prim?rproduktion durch eine erh?hte Phosphorkonzentration nachgewiesen (Abb. 2). Die Erfolgsaussichten von Abwassersanierungsmassnahmen an Seen verschiedener Trophiegrade werden diskutiert.

---

Summary The influence of the ‘Steinibach” tributary, into which the wastewater of 10,000 inhabitants is discharged, on the Lake of Lucerne (Horwer Bucht) is detectable in the surface water over a distance of more than 2 km (Picture 1a, 1b, 1c). It was revealed that the increased phosphorus supply enables more intensive biological nitrate consumption. The dependence of the pigment concentration,y, on the total phosphorus concentration,x can be expressed by the following Mitscherlich equation: log (30−y)=log 30–0.012 (x−6). This was the first time that the influence of an increased phosphorus concentration on primary production was proved in the Lake of Lucerne under completely natural conditions (Picture 2). The possibilities of implementing successful water pollution control measures in lakes with various trophic levels are discussed.