Statistische Untersuchungen an Zirkon und Apatit in Anatexiten des südlichen Bayerischen Waldes

Zusammenfassung In die im südlichen Bayerischen Wald weitverbreitete Serie der Perlgneise ist zwischen Deggendorf und Regen ein granodioritischer Gesteinskörper eingelagert. Feldgeologische und mikroskopische Untersuchungen machen eine metamorphe Entstehung dieses Paragranodiorits wahrscheinlich. Zur Gewinnung weiterer gesteinsgenetischer Kriterien werden die morphologische Ausbildung, das Längen-Breiten-Verhältnis, das Korn-in-Kom-Gefüge und die statistische Verteilung der Korngrößen von Zirkon und Apatit in beiden Gesteinen herangezogen. Hieraus ergeben sich zusätzliche Argumente für eine nichtmagmatische Entstehung des Paragranodiorits. Ein Teil dieser Untersuchungen kann nur am Dünnschliff durchgeführt werden.

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A granodioritic rock and its relationship to the surrounding biotite-gneiss in the metamorphic basement complex of northeastern Bavaria (Moldanubikum) was studied. Field work and microscopic investigations show strong evidence for metasomatic origin of the para-granodiorite. A detailed study of zircon and apatite both in the para-granodiorite and the gneiss is made in order to obtain further criteria. Morphology, elongation ratios, and the grain-in-grain fabric of zircon point to non-magmatic origin of both rocks. Size frequency distribution of zircon in gneiss is lognormal and probably due to sedimentary sorting. The skewed distribution curve of zircon in para-granodiorite is explained by metamorphic growth of sedimentary zircons which may have been lognormally distributed. Apatite in para-granodiorite on the other hand shows lognormal distribution of grain size. As crystallization of apatite is posttectonic, it seems that random growth conditions in the metamorphic stage may also produce lognormal size frequency distributions. Thin section study is emphasized as some data cannot be obtained from mounted zircon concentrates.

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Résumé Dans la série du gneiss en forme de perles, fréquent dans la Bayerischer Wald du Sud (Bavière), on peut trouver entre Deggendorf et Regen un interlit de roche granodioritique. Les recherches géologiques sur le terrain et au microscope rendent vraisemblable l'origine métamorphique de cette Paragranodiorite. Pour obtenir d'autres caractéristiques lithogénétiques on a fait appel à la texture morphologique, aux relations de longueur à largeur, à la texture grain-en-grain et à la granulométrie statistique du zircon et de l'apatite. Il se dégage ainsi d'autres arguments pour une origine non-magmatique de la Paragranodiorite.

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Eine analytische Lösung der Vorticitygleichung unter Einschluß orographischer Effekte

Zusammenfassung Ausgehend von der sogenannten kinematischen Randbedingung und mit Hilfe bestimmter Approximationen im Windfeld wird eine, dem VerfahrenNeamtans folgende, analytische partikuläre Lösung der Vorticitygleichung unter Einschluß orographischer Störeffekte eines west-östlich streichenden, idealisierten Gebirgszuges abgeleitet. Dem Modellcharakter der Untersuchung entsprechend, wird die Topographie dieses Gebirgszuges mit Hilfe einer Sinusfunktion dargestellt. Die Ergebnisse der Rechnung zeigen, zumindest innerhalb gewisser Grenzen, große Ähnlichkeit mit den in der Natur beobachteten Effekten.

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Summary Using the kinematic boundary condition and some approximations regarding the wind field an analytical particular solution of the vorticity equation is derived which is analogous to theNeamtan solutio but includes the disturbing effects of orography resulting from an idealized chain of mountains extending in west-easterly direction. Corresponding to the model character of the investigation the topography of this chain of mountains is represented as a sine function. The results of the calculations show great similarity to the effects observed in nature, at least whithin certain limits.

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Résumé On établit ici une solution analytique de l'équation tourbillonnaire en partant de conditions de bords cinématiques et à l'aide de certaines approximations dans le champ du vent. Cette analyse suit le procédé deNeamtan. Elle tient en particulier compte des perturbations dues à l'orographie d'une chaîne de montagne idéale orientée d'ouest en est. Conformément au caractère utilisé, la topographie de cette chaîne de montagne est représentée à l'aide d'une fonction sinusoïdale. les résultats des calculs montrent, au mois à l'intérieur de certaines limites, une grande similitude avec les effects observés dans la nature.

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Photometric observations of visual double stars by the area scanning technique

UBV magnitude differences in visual binaries are presented and their accuracy is discussed. Typical observed image profiles of double stars, including sample profiles of Antares, are shown. This contribution will appear jointly with the earlier one (p. 20) inLowell Obs. Bull., No. 154. The work has been supported by NSF Grant GP-6983.     

基于透射电镜的香肠构造流变计研究

采用透射电镜(TEM)对采自奥地利东阿尔卑斯和北京西山的香肠构造中的香肠体和相应基质的样品进行了超微构造分析。结果表明细粒基质中的位错密度较粗颗粒香肠体中的位错密度明显低很多。故认为香肠体的变形机制以位错蠕变为主,而基质的变形机制则以扩散蠕变为主。这从一个侧面为香肠构造流变计的这一假设提供了超微观研究的依据。     

Raexcess/Ba growth rates and U-Th-Ra-Ba systematic of Baltic Mn/Fe crusts

We analyzed ²³⁸U, ²³⁴U, ²³²Th, ²³⁰Th, and ²²⁶Ra by thermal ionization mass spectrometry (TIMS) and Ba by inductively coupled plasma optical emission spectrometry (ICP-OES) on eight Mn/Fe crusts from the Mecklenburg Bay (SW Baltic) and on one from the Bothnian Bay (N Baltic) to test the ²²⁶Ra_ex/Ba ratio as potential geochronometer. ²²⁶Ra_ex/Ba ratios decrease as a function of depth within the concretions in all analyzed profiles. Calculated diffusion coefficients are relatively low (∼9 · 10⁻⁷ cm²/yr for Ra and 5 · 10⁻⁷ cm²/yr for Ba) and suggest that diffusion is negligible for the Ra and Ba record. In addition, ²²⁶Ra_ex/Ba ages are consistent and independent from the growth rate and growth direction within a crust. Thus, the decline in ²²⁶Ra_ex/Ba ratio is most likely due to radioactive decay of ²²⁶Ra_ex, although the influence of varying oxic conditions has still to be evaluated. ²²⁶Ra_ex/Ba growth rates range from 0.021 to 0.0017 mm/yr and tend to be lower than those calculated and based on stratigraphic methods (1 to 0.013 mm/yr). ²²⁶Ra_ex/Ba ages of concretions from shallow water environment (20 m depth, Mecklenburg Bay/SW Baltic) cover a time interval from 990 ± 140 yr to 4310 ± 310 yr BP corresponding to the stabilization of the sea level close to the present position about 5500 to 4500 yr ago. One sample from greater depth (70 m, Bothnian Bay-/N Baltic) showed a higher ²²⁶Ra_ex/Ba age of 6460 ± 520 yr BP.     

Boron isotope composition of geothermal fluids and borate minerals from salar deposits (central Andes/NW Argentina)

We have measured the boron concentration and isotope composition of regionally expansive borate deposits and geothermal fluids from the Cenozoic geothermal system of the Argentine Puna Plateau in the central Andes. The borate minerals borax, colemanite, hydroboracite, inderite, inyoite, kernite, teruggite, tincalconite, and ulexite span a wide range of δ¹¹B values from −29.5 to −0.3‰, whereas fluids cover a range from −18.3 to 0.7‰. The data from recent coexisting borate minerals and fluids allow for the calculation of the isotope composition of the ancient mineralizing fluids and thus for the constraint of the isotope composition of the source rocks sampled by the fluids. The boron isotope composition of ancient mineralizing fluids appears uniform throughout the section of precipitates at a given locality and similar to values obtained from recent thermal fluids. These findings support models that suggest uniform and stable climatic, magmatic, and tectonic conditions during the past 8 million years in this part of the central Andes. Boron in fluids is derived from different sources, depending on the drainage system and local country rocks. One significant boron source is the Paleozoic basement, which has a whole-rock isotopic composition of δ¹¹B=−8.9±2.2‰ (1 SD); another important boron contribution comes from Neogene-Pleistocene ignimbrites (δ¹¹B=−3.8±2.8‰, 1 SD). Cenozoic andesites and Mesozoic limestones (δ¹¹B≤+8‰) provide a potential third boron source.     

Tertiary to recent oblique convergence and wrenching of the Central Dinarides: Constraints from a palaeostress study

The late Eocene to Neogene tectonic evolution of the Dinarides is characterised by shortening and orogen-parallel wrenching superposed on the late Cretaceous and Eocene double-vergent orogenic system. The Central Dinarides exposes NW-trending tectonic units, which were transported towards the Adria/Apulian microcontinent during late Cretaceous–Palaeogene times. These units were also affected by subsequent processes of late Palaeogene to Neogene shortening, Neogene extension and subsidence of intramontane sedimentary basins and Pliocene–Quaternary surface uplift and denudation. The intramontane basins likely relate to formation of the Pannonian basin. Major dextral SE-trending strike-slip faults are mostly parallel to boundaries of major tectonic units and suggest dextral orogen-parallel wrenching of the whole Central Dinarides during the Neogene indentation of the Apulian microplate into the Alps and back-arc type extension in the Pannonian basin. These fault systems have been evaluated with the standard palaeostress techniques. We report four palaeostress tensor groups, which are tentatively ordered in a succession from oldest to youngest: (1) Palaeostress tensor group 1 (D₁) of likely late Eocene age indicates E–W shortening accommodated by reverse and strike-slip faults. (2) Palaeostress tensor group 2 (D₂) comprises N/NW-trending dextral and W/WSW-trending sinistral strike-slip faults, as well as WNW-striking reverse faults. These indicate NE–SW contraction and subordinate NW–SE extension related to Oligocene to early Miocene shortening of the Dinaric orogenic wedge. (3) Palaeostress tensor group 3a (D_3a) comprises mainly NW-trending normal faults, which indicate early/middle Miocene NE–SW extension related to syn-rift extension in the Pannonian basin. The subsequent palaeostress tensor group 3b (D_3b) includes NE-trending, SE-dipping normal faults indicating NW–SE extension, which is likely related to further extension in the Pannonian basin. (4) Palaeostress tensor group 4 (D₄) is characterised by mainly NW-trending dextral and NE-trending sinistral strike-slip faults. Together, with some E-trending reverse faults, they indicate roughly N–S shortening and dextral wrenching during late Miocene to Quaternary. This is partly consistent with the present-day kinematics, with motion of the Adriatic microplate constrained by GPS data and earthquake focal mechanisms. The north–north-westward motion and counterclockwise rotation of the Adriatic microplate significantly contribute the shortening and present-day wrenching in the Central Dinarides.     

Ar/Ar mineral ages from basement rocks in the Eastern Kunlun Mountains, NW China, and their tectonic implications

⁴⁰Ar/³⁹Ar dating and estimates of regional metamorphic P–T conditions were carried out on the basement rocks of the Eastern Kunlun Mountains, Western China. Samples from the Jinshuikou, Xiaomiao, Kuhai, Wanbaogou, and Nachitai groups revealed distinct metamorphic events and four age groups. The age group in the range from 363 to 439 Ma is interpreted to represent cooling after Middle Silurian–Late Devonian granulite(?) and amphibolite facies metamorphism, which is dominated by low–middle pressure/high temperature conditions. This tectono-thermal event is related to the closure of an oceanic basin or marginal sea. An age group of 212–242 Ma represents cooling after Triassic metamorphic overprint, which is probably associated with magmatic intrusions. This thermal event, together with the Permo-Triassic ophiolite zone along the South Kunlun Fault, relates to the closure of a major ocean (between India and Eurasia) and the eventual N-ward accretion of the Qiangtang block in Permo-Triassic times. The significance of the age group of 104–172 Ma may be related to the ductile deformation along the Xidatan fault due to the northward-directed accretion of the Lhasa block. Biotites from Nachitai record a partial isotopic resetting at ca. 32 Ma that is interpreted to represent a late-stage exhumation caused by further crustal shortening.     

Spatial distribution of surface conditions and runoff generation in small arid watersheds, Zin Valley Badlands, Israel

High rill density may be regarded as indicative of frequent and integrated runoff along a drainage network. However, field observations of soil development and rill geometry in small, first-order catchments (0.1 to 1 ha) of the Zin Valley Badlands, northern Negev, Israel, suggest a pattern of partial area contribution and frequent flow discontinuities along hillslopes and channels. Changing soil properties, associated with an increase of slope angle and slope length, appear to be responsible for high infiltration on the slopes, leading to nonuniform runoff generation within small drainage basins. Runoff observation, sprinkling tests, and soil analysis along ridges and sideslopes were carried out to test this hypothesis. The results confirmed that infiltration capacity on the sideslopes is significantly higher than on the ridges. Under current rainfall conditions, only extreme rainfall intensities are sufficient to generate runoff along entire slopes. The discontinuous nature of most runoff events causes erosion on ridges and deposition on slopes, which enhances soil development on the valley sideslopes, creating a positive feedback on infiltration rate and depth. This demonstrates that the links between within-storm rainfall conditions and spatial distribution of soil characteristics have to be incorporated into our understanding of landscape development in badlands.