Rb-Sr geochronology in favour of polymetamorphism in the Pan African Damara belt of Namibia (South West Africa)

New Rb-Sr whole rock age data are reported from two metasedimentary sequences of the Damara Supergroup in the central Pan African Damara belt of Namibia (South West Africa).Calc-granofels rocks of the Karibib Formation (Swakop Group) near Usakos are dated at 665±34 Ma (⁸⁷Rb=1.39×10^–11 a^–1) which is interpreted as reflecting a high-grade metamorphic event predating widespread granite intrusion.Pyroxene-bearing feldspathic gneisses of the Khan Formation (Nosib Group) from the Khan-Swakop River area east of Swakopmund show incomplete homogenization at 474±16 Ma. In view of similar ages obtained on the nearby Rössing alaskite granite and on biotites from a variety of rock assemblages this age is interpreted as reflecting a second Damaran metamorphic event rather than a specific stage in a long cooling history as previously thought.It is probable that the new ages characterize two distinct Pan-African tectono-metamorphic events previously named Katangan and Damaran episodes respectively (Clifford, 1967). The younger of these has affected large areas of south western Africa both within orogenic zones and on the neighbouring Kalahari Craton and may reflect crustal processes of sub-continental proportion during the closing stages of the Pan-African tectogenesis.

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Zusammenfassung Neue Rb-Sr-Gesamtgesteins-Isochronenalter von zwei sedimentären Abfolgen der spätpräkambrischen Damara-Supergruppe in der Zentralzone des Pan-Afrikanischen Damara-Orogens von Namibia (Südwest-Afrika) deuten auf eine mehrphasige metamorphe Geschichte hin.Kalk-Granofels-Gesteine der Karibib-Formation (Swakop-Gruppe) südöstlich von Usakos ergaben ein Alter von 665±34 M. J. (⁸⁷Rb=1,39×10^–11 a^–1) und wir interpretieren dieses Alter als das Resultat isotoper Homogenisierung während einer intensiven Regionalmetamorphose, die vor dem Eindringen weitverbreiteter Granite stattfand.Pyroxenführende und feldspathaltige Gneise der Khan-Formation (Nosib-Gruppe) aus dem Gebiet der Khan- und Swakop-Flüsse östlich von Swakopmund zeigen unvollständige Homogenisierung ihrer Sr-Isotope um 474±16 M. J. In Anbetracht ähnlicher Alter für den benachbarten Rössing-Alaskitgranit und für Biotite verschiedener Gesteinstypen aus dem gesamten Zentralbereich des Orogens interpretieren wir obiges Alter eher als den Höhepunkt einer zweiten Regionalmetamorphose als ein bestimmtes Stadium in der post-orogenen Abkühlungsgeschichte.Es ist wahrscheinlich, daß unsere Altersbestimmungen zwei deutlich voneinander getrennte pan-afrikanische tektono-metamorphe Phasen charakterisieren, vor allem wenn man sie in Zusammenhang mit publizierten Daten betrachtet, und wir nehmen an, daß diese den vonClifford (1967) postulierten Katanga- und Damara-Episoden entsprechen. Die jüngere dieser Episoden hat nicht nur die pan-afrikanischen orogenen Bereiche im südwestlichen Afrika, sondern auch weite Teile des Kalahari-Kratons erfaßt und mag auf tektogenetische Prozesse von nahezu kontinentalen Ausmaßen in und unter der Kruste im frühen Paläozoikum zurückzuführen sein.

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Résumé Deux séquences métasédimentaires du supergroupe du Damara ont été datées par la méthode Rb/Sr.Des gneiss calciques («granofels») de la formation de Karibib (groupe de Swakop), récoltés près d'Usakos ont été datés à 665±34 Ma (avec⁸⁷Rb=1.39× 10^–11 a^–1). Cet âge est interprété comme le résultat d'un événement métamorphique de degré é levé antérieur à la mise en place des grandes masses de granite.Des gneiss feldspathiques à pryoxène de la Formation de Khan (Groupe de Nosib), échantillonnés dans la région de la Khan River — Swakop à l'Est de Swakopmund, montrent une homogénéisation isotopique incomplète à 474±16 Ma. Des âges voisins ayant été obtenus pour l'Alaskite de Rössing qui affleure dans la même zone et pour les biotites de différentes roches, cet âge est interprété comme le résultat d'un second métamorphisme damarien plutôt que comme un stade de la longue histoire de refroidissement comme cela avait été imaginé auparavant.Ces nouveaux âges caractériseraient les deux épisodes tectonométamorphiques distincts de l'orogénie Pan-Africaine, qui avaient été appelés respectivement épisode katangien et épisode demarien parClifford (1967). Le plus récent de ces événements a affecté une grande partie de l'Afrique du S.W. aussi bien dans les zones orogéniques que dans le craton voisin du Kalahari; il peut refléter une évolution crustale d'échelle subcontinentale se produisant pendant les derniers stades du tectogène Pan-Africain.

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Rb/Sr - , - ; , .- , , 665±34 (⁸⁷Rb=1,39 10^–1a^–1); , . , , , , 474 ± 16 . - ; , , . , - - , , . , 1967 , . - - , ; , , .

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Contribution No. 27 of the South African Geodynamics Project, presented at the 9th Colloquium on African Geology, Göttingen, April 1977.     

A comparison of observed and thermodynamically predicted phase equilibria and mineral compositions in mafic granulites

Recently published activity–composition (a–x) relations for minerals in upper amphibolite‐ and granulite facies intermediate and basic rocks have expanded our ability to interpret the petrological evolution of these important components of the lower continental crust. If such petrological modelling is to be reliable, the abundances and compositions of phases calculated at the interpreted conditions of metamorphic equilibration should resemble those in the sample under study. Here, petrological modelling was applied to six granulite facies rocks that formed in different tectonic environments and reached different peak metamorphic pressure–temperature (P–T) conditions. While phase assemblages matching those observed in each sample can generally be calculated at P–T conditions that approximate those of peak metamorphism, a consistent discrepancy was found between the calculated and observed compositions of amphibole and clinopyroxene. In amphibole, Si, Ca and A‐site K are underestimated by the model, while Al and A‐site Na are overestimated; comparatively, in clinopyroxene, Mg and Si are generally underestimated, while Fe²⁺ and Al are typically overestimated, compared to observed values. One consequence is a reversal in the Fe–Mg distribution coefficient (K_D) between amphibole and clinopyroxene compared to observations. Some of these mismatches are attributed to the incorrect partitioning of elements between the predicted amphibole and clinopyroxene compositions; however, other discrepancies are the result of the incorrect prediction of major substitution vectors in amphibole and clinopyroxene. These compositional irregularities affect mineral modal abundance estimates and in turn the position and size (in P–T space) of mineral assemblage fields, the effect becoming progressively more marked as the modal abundance of hornblende increases; hence, this study carries implications for estimating P–T conditions of high‐temperature metabasites using these new a–x relations.     

The radius and other fundamental parameters of the F9 V star β Virginis

We have used the Sydney University Stellar Interferometer (SUSI) to measure the angular diameter of the F9 V star β Virginis (β Vir). After correcting for limb darkening and combining with the revised Hipparcos parallax, we derive a radius of  1.703 ± 0.022 R_⊙  (1.3 per cent). We have also calculated the bolometric flux from published measurements which, combined with the angular diameter, implies an effective temperature of  6059 ± 49 K  (0.8 per cent). We also derived the luminosity of β Vir to be   L = 3.51 ± 0.08 L_⊙  (2.1 per cent). Solar-like oscillations were measured in this star by Carrier et al. and using their value for the large frequency separation yields the mean stellar density with an uncertainty of about 2 per cent. Our constraints on the fundamental parameters of β Vir will be important to test the theoretical models of this star and its oscillations.     

Environmental isotope study on hydrodynamics of Lake Naini,Uttar Pradesh,India

Environmental isotopes (δ¹⁸O, δD and ³H) were used to understand the hydrodynamics of Lake Naini in the State of Uttar Pradesh, India. The data was correlated with the in situ physico‐chemical parameters, namely temperature, electrical conductivity and dissolved oxygen. The analysis of the data shows that Lake Naini is a warm monomictic lake [i.e. in a year, the lake is stratified during the summer months (March/April to October/November) and well mixed during the remaining months]. The presence of a centrally submerged ridge inhibits the mixing of deeper waters of the lake's two sub‐basins, and they exhibit differential behaviour. The rates of change of isotopic composition of hypolimnion and epilimnion waters of the lake indicate that the water retention time of the lake is very short, and the two have independent inflow components. A few groundwater inflow points to the lake are inferred along the existing fractures, fault planes and dykes. In addition to poor vertical mixing of the lake due to the temperature‐induced seasonal stratification, the lake also shows poor horizontal mixing at certain locations of the lake. The lake–groundwater system appears to be a flow‐through type. Also, a tritium and water‐balance model was developed to estimate the water retention time of well‐mixed and hydrologically steady state lakes. The model assumes a piston flow of groundwater contributing to the lake. The developed model was verified for (a) Finger Lakes, New York; (b) Lake Neusiedlersee, Austria; and (c) Blue Lake, Australia based on literature data. The predicted water retention times of the lakes were close to those reported or calculated from the hydrological parameters given in the references. On application of this model to Lake Naini, a water retention time of ～2 years and age of groundwater contributing to the lake ～14 years is obtained. Copyright © 2001 John Wiley & Sons, Ltd.     

The spatial organization of services in new land settlement: experiences and trends

Conclusion Despite its uniqueness, the vitality inherent in the rural structure of Israel is of particular interest to other developing regions with a predominant rural-agricultural population. Latin American countries, for example, are investing considerable resources in the development of new lands through construction of penetration roads and other infrastructure. Several countries have created regional development authorities; some have policies designed to attract private investments into virgin areas.¹³⁾ The general emphasis tends increasingly toward state-initiated and planned settlement, often in conjuction with agrarian reform programs — an approach dictated both by economic efficiency and welfare criteria.Past experience has shown that the rural population has to be organized in viable communities in order to become amenable to economic and cultural integration. Communities must likewise be spatially organized in an optimal way that will make it possible to provide them with amenities and so direct their production for the purpose of achieving economic status. At the same time, maximum flexibility must be preserved to fit varying developmental stages, since physical plans once carried into effect are extremely difficult to modify.In order to reduce the social and economic pull of existing urban centers it may be advisable to develop new settlement areas as self-sufficient enclaves, independent to some extent from the facilities existing in the region. The settlements would share the national infrastructure of communications and public services, but would gear their production to regional as well as extra-regional demand, bypassing the traditional local market place. Then, as the new communities consolidate as social and economic entities, the options for collaboration or competition with existing central places can be laid open on a more equitable basis.Admittedly such sheltered development may affect the role of the local intermediary and lessen the commercial activities of the urban sector, but it would also stimulate the development of an independent framework of handling, marketing, and an increasing degree of processing the settlements' produce. These complementary activities would help to retain part of the added value of the production and generate new sources of employment for successive village generations. The delay in the growth for the region's total output may well be worthwhile for achieving that social and economic transformation which in turn may lead to a more balance and sustained development of the entire region.¹⁴⁾     

Multisite campaign on the open cluster M67 – II. Evidence for solar-like oscillations in red giant stars

Measuring solar-like oscillations in an ensemble of stars in a cluster, holds promise for testing stellar structure and evolution more stringently than just fitting parameters to single field stars. The most-ambitious attempt to pursue these prospects was by Gilliland et al. who targeted 11 turn-off stars in the open cluster M67 (NGC 2682), but the oscillation amplitudes were too small (<20 μmag) to obtain unambiguous detections. Like Gilliland et al. we also aim at detecting solar-like oscillations in M67, but we target red giant stars with expected amplitudes in the range 50–  500 μmag  and periods of 1 to 8 h. We analyse our recently published photometry measurements, obtained during a six-week multisite campaign using nine telescopes around the world. The observations are compared with simulations and with estimated properties of the stellar oscillations. Noise levels in the Fourier spectra as low as  27 μmag  are obtained for single sites, while the combined data reach  19 μmag  , making this the best photometric time series of an ensemble of red giant stars. These data enable us to make the first test of the scaling relations (used to estimate frequency and amplitude) with an homogeneous ensemble of stars. The detected excess power is consistent with the expected signal from stellar oscillations, both in terms of its frequency range and amplitude. However, our results are limited by apparent high levels of non-white noise, which cannot be clearly separated from the stellar signal.