Northwest Namibia Research Articles

Barrovian and Buchan type metamorphism in the Pan-African Kaoko belt, Namibia: implications for its geotectonic position within the framework of Western Gondwana

The polyphase deformed Pan-African Kaoko belt, Namibia, which is part of the late Neoproterozoic mobile belt system of western Gondwana, consists of pre-Pan-African basement gneisses, Pan-African volcano-sedimentary cover sequences and large granitoid intrusions. The eastern part of the Kaoko belt comprises a nearly complete Barrovian sequence of medium to high temperature/medium pressure metapelitic rocks, which range in grade from greenschist- to upper amphibolite-facies conditions. From east to west, north-northwest to northwest trending subparallel garnet-, staurolite-, kyanite-, and kyanite-sillimanite-muscovite metamorphic zones can be distinguished, whereas the western Kaoko belt is dominated by a high temperature/low pressure Buchan-type metamorphism with sillimanite-K-feldspar- and garnet-cordierite-sillimanite-K-feldspar metamorphic zones. The two metamorphic terranes are tectonically separated by the Puros shear zone. The pressure-temperature path for each metamorphic zone was inferred by using observed mineral assemblages and reaction textures, a combination of various geothermobarometers and, in particular, pseudosections. Peak metamorphic conditions of the Barrovian and Buchan zones indicate peak pressure-temperature ratios of ~20°C/km and 50°C/km, respectively. The Barrovian-type terrane is probably linked to a fold-and-thrust belt, the Buchan-type terrane may be linked to a magmatic arc setting. Similarities between the Kaoko belt in northwest Namibia and the central Ribeira belt in South America might point to a common geological history.

Subject Disobedience: The Colonial Narrative and Native Counterworks in Northwestern Namibia, c.1920–1975

This article concentrates on the ambiguities and contradictions in the colonial archive on North West Namibia (a region also known as Kaokoland), and on the way these were exploited by the “stubborn traditionalists” inhabiting it. Its aim is to place the emergence of postcolonial identities and subjectivities in the region in an historical perspective. To do so, it takes a look at the depoliticized discourse on livestock and development in which South West African rule framed its politics of identity. This case study investigates how precisely this discourse fuelled political resistance in the region. However, the efforts by elders and so‐called commoners to counter indirect rule and apartheid hardly ever took the form of overt rebellion or explicit political protest. It concerned instead a more subtle and defiant form of counterworks, a very specific local modernity rooted in local subjectivity and experience that happened to be quite efficient.

North-west Namibian desert-dwelling elephant project

"The elephant involved in the Kunene region study are for the most part resident outside protected areas and within communal lands. Since both the elephant and human populations are increasing there is a growing problem of land use conflict. Although this desert population has been photographed and discussed many times little scientific research has been carried out. This current project, which in September 2002 radio-collared 8 animals, as part of the programme to identify and monitor the elephants in the Kunene region of north-west Namibia should rectify this situation."

How special are Etendeka mesas? Flora and elevation gradients in an arid landscape in north-west Namibia

Seven mesas (i.e. flat-topped, isolated mountains) and the Grootberg mountain in arid north-west Namibia were investigated with regard to floristic relationships in relation to environmental gradients. Relationships between mesas and their surrounding, species richness and number of mesa specialists were used as measures to explore the influence of elevation on these floristic parameters. Also soil properties were investigated along these elevation gradients. The overall questions were designed to investigate the importance, i.e. ‘special status’, of these mesas in terms of contributing to species diversity and to elucidating ecological processes in this arid landscape. With regard to special floristic status, the tops of mesas supported different plant communities than their surrounding plains, while slopes indicated various levels of intermingling with plain vegetation. Differences between mesas and plains became more pronounced with increasing elevation. There were, however, no clear vegetation belts related to elevation. Plant species richness and number of mesa specialists showed a trend of increasing numbers with elevation, but these trends were statistically poorly supported, largely due to high variability between samples. Do soil gradients and elevation, as two important environmental variables, affect this ‘special’ status? Pronounced soil gradients were evident for many soil properties. These in combination with altitudinal effects of increased moisture and lower temperatures were believed to directly influence plant species composition and richness. The soil gradients may also indicate nutrient flow from the mesas to the surrounding lowlands, a process of ecological importance, particularly should degradation due to over grazing become a problem in the lowland areas.

Integrating Conservation and Development: A Namibian Case Study

Resume Integrer conservation et developpement: etude de cas en Namibie Cette recherche examine les initiatives de gestion des ressources naturelles locales (GRNL)--en anglais community-based natural resource management (CBNRM)--prises par les gardiens de troupeaux semi-nomades Himba et Herero, situes dans la partie isolee de la Namibie du nord-ouest, et par les chasseurs-amasseurs au nord-est. Cette recherche montre que la conservation comme les personnes peuvent beneficier de la GRNL et sontient que les nomades de Namibie ont ete avantages par cette approche de conservation dans le contexte d'un Etat africain moderne en voie de developpement. Resumen Integrar la preservacion y el desarrollo: un estudio de csos de la Namibia Este estudio de casos examina las iniciativas de gestion de los recursos naturales locales (GRNL)--en ingles community-based natural resource management (CBNRM)--tomadas por los pastores semi-nomadas en el noroeste remoto de Namibia y por los cazadores-recolectores en el noreste. Este estudio de casos muestra que tanto la conservacion como las personas pueden beneficiar de la GRNL y afirma que los nomadas de Namibia son favorecidos por este metodo de conservacion en el contexto de un estado moderno africano en vias de desarrollo. Introduction This case study presents a practical, field-based, implementation perspective--that of a Namibian Non-Government Organisation (NGO) providing technical support to more than 30 rural communities to take advantage of new, enabling, community-based natural resource management (CBNRM) legislation in Namibia. More than half of these communities fall within the definition of mobile peoples, as pastoralists or hunter-gatherers. The paper briefly outlines the development of what is today a national CBNRM programme and examines an approach to conservation and development that has evolved over 20 years. This approach was able to turn around massive commercial and subsistence poaching, including desert-adapted elephant and black rhino in northwest Namibia in the 1980s. Game increases have continued over the past decade-and-a-half and today wildlife and tourism are thriving in this region and there is widespread grassroots support for CBNRM. The national programme is expanding beyond wildlife to include fish, forest and other plant resources, and now involves communities in nine of Namibia's 13 regions, 12 NGOs and government. As importantly, CBNRM has enabled some of Namibia's most marginalised people--including and perhaps especially, the nomads--to develop and start implementing a vision for their future, as elucidated below. It is a vision that links wildlife conservation and development so that both people and their natural resources are beneficiaries. The wildlife figures (Figure 1) and expanded area under conservation status in Namibia (Figure 2) speak for themselves. [FIGURES 1-2 OMITTED] At the same time rural people are benefiting from: * More rights over their resources and the tools to exercise those rights * Improved natural resource management * Social empowerment via increased capacity, skills and knowledge * The growth of grassroots democracy as CBNRM promotes opportunities for the voices of remote rural people to be heard at national and regional forums * Diversification of rural economies The tangible successes of this programme, as highlighted by the game increases and start-up income generation (Figure 3), should not obscure the complexities and challenges of making CBNRM work on the ground. CBNRM is the long, slow and difficult route. It can and often does challenge or change previously existing power structures--traditional leaderships, government bureaucrats, politicians and other elite groupings--by extending rights over valuable resources to ordinary people. Thus community and other conflicts are to be expected and, we believe, are a necessary step to be negotiated. …

Population genetic data for eight STR loci in the south of Africa

Population data for short tandem repeat (STR) loci, today’s method of choice for human identification and paternity testing in forensic genetics, are available for numerous populations from the major ethnic groups. Nevertheless, the allele frequency data for Sub-Saharan Africans in particular are extremely generalized and little information is available for particular populations. In this study, we present the allele frequency and the forensic efficiency values for eight STR polymorphisms (i.e. TH01, vWA, ACTBP2, FGA, D21S11, D3S1358, D8S1179, D18S51 in addition to the Amelogenin locus for gender identification) in two small population samples from Southern Africa (42 unrelated Himbas from Northwest Namibia, and 72 unrelated black South Africans from the Cape Town area). The purpose of this work was to establish a database for forensic purposes including paternity testing and to increase our knowledge of STRs in African populations.

Description of a new cryptic species of African sand frog, Tomopterna damarensis (Anura: Ranidae), from Namibia

Sand frogs of the African genus Tomopterna Duméril and Bibron, 1841 comprise seven described cryptic species. In this paper we describe a new species from Khorixas, north west Namibia. It is considered a distinct form based on the advertisement call, morphology and mitochondrial DNA sequences.

The sulfur isotopic composition of Neoproterozoic seawater sulfate: implications for a snowball Earth?

The present study employs a method for analysis of the sulfur isotopic composition of trace sulfate extracted from carbonates collected in Namibia in order to document secular variations in the sulfur isotopic composition of Neoproterozoic oceanic sulfate and to assess variations in the sulfur cycle that may have accompanied profound climatic events that have been described as the snowball Earth hypothesis. The carbonates in the Otavi Group of Northwest Namibia contain 3–295 ppm sulfate. Positive excursions, to a high of 40‰ (CDT), occur above the lower (Chuos Formation) and upper (Ghaub Formation) glacial intervals in the Rasthof and Maieberg cap carbonates, respectively. Positive excursions at the top of the Rasthof Formation (reaching 51‰) and within the overlying Gruis Formation (34‰) do not appear to correspond to glaciation. The δ34Ssulfate values within the Ombaatjie Formation exhibit shifts over relatively short stratigraphic intervals (tens of meters), varying between ∼15 and 25‰. Cap carbonates from Australia exhibit positive δ34Spyrite trends with amplitudes similar to those of Namibian δ34Ssulfate, although, more data are necessary to firmly establish these δ34S trends as global in nature. δ34Ssulfate excursions found in Namibian cap carbonates are consistent with the snowball Earth hypothesis in that they appear to reflect nearly complete reduction of sulfate in an isolated, anoxic global ocean, although, there are other mechanisms that may have facilitated these large shifts in δ34Ssulfate. Regardless, the low sulfate concentrations in Otavi carbonates, the high amplitude variability of the δ34Ssulfate curve, and the apparently full reduction of sulfate (as implied from δ34Spyrite data), even in strata low in Corg, suggest that Neoproterozoic oceanic sulfate concentrations were much lower than modern values. Additionally, the buildup of ferrous iron and banded-iron formations during the Sturtian glacial event would indicate that Fe supply exceeded sulfide availability during the glacials and/or that all sulfide was fixed and buried. This could be construed as further evidence in support of low oceanic sulfate (and sulfide) at this time.

Isotope constraints on the origin of Pan-African granitoid rocks in the Kaoko belt, NW Namibia

The geodynamic evolution of the Kaoko belt, northwest Namibia, which is part of the late Neoproterozoic mobile belt system of western Gondwana, is considered to be the result of collision between the Congo (Africa) and Rio de la Plata (South America) cratons. In the coastal area of this belt, Pan-African dioritic to granitic melts intruded at ~650 Ma and between ~580 and ~550 Ma into sedimentary sequences of the Kuiseb Formation. Field observations suggest that the later intrusive episode coincided with the peak of high-grade metamorphism in this region, which is characterised by granulite facies conditions and anatexis. Franz et al. (1999) state that both intrusive episodes can be connected with high-grade metamorphic events. The δ18O values of the Pan-African granitoids range from +8.5 to +10.3‰ and tend to be slightly higher than those of the Palaeoproterozoic and late Archaean basement gneisses (+7.6 to +9.9‰ and +7.4 to +9.0‰, respectively). The Pan-African granitoids have low to moderate initial 87Sr/86Sr ratios between ~0.7036 and ~0.7121. Sm-Nd isotopic data of the Pan-African granitoids have moderate initial ϵNd values ranging from −1.2 to −6.8. They are distinct from Palaeoproterozoic as well as late Archaean orthogneisses from the adjacent Congo Craton, which reveal much lower ϵNdvalues between −18 and −28 during the time of emplacement of the Pan-African granitoids. Thus, the derivation of these granitoid melts exclusively from much older crust can be precluded. Although the Pan-African granitoids and the Pan-African high-grade metasedimentary rocks have similar initial ϵNd values of −1.2 to −6.8 and also similar mean crustal residence ages between 1.2 and 1.6 Ga, the derivation of the granitoids from these metasedimentary rocks alone seems also improbable since the granitoids reveal significantly lower initial 87Sr/86Sr ratios than the metasediments. It is here suggested that the Nd, Sr and O isotopic features of the Pan-African granitoids reflect mixing between upper crustal material (metasediments) and remelted basement gneisses from lower crustal levels.

A major perturbation of the carbon cycle before the Ghaub glaciation (Neoproterozoic) in Namibia: Prelude to snowball Earth?

A large (11–15‰) negative shift in δ13C is observed in shallow water carbonates directly beneath Neoproterozoic glacial deposits (or correlative disconformity) in northwest Namibia ascribed to a snowball Earth. Reproducibility and stratigraphic concordance of this anomaly in 16 sections across the ancient continental shelf support a primary origin, and field relations show it predates the fall in sea level associated with the Ghaub glaciation. We crudely estimate the duration of the isotopic shift as ∼0.6 × 106 years from a simple thermal subsidence model. Similar or larger δ13C anomalies are found directly beneath Neoproterozoic glacial units in Australia, Canada, China, Scotland, and Svalbard. After considering conventional interpretations for negative δ13C anomalies, we conclude that a prolonged methane release to the atmosphere is most consistent with the timescale, magnitude, and geological context of the anomaly in Namibia. Counterintuitively, an anomalous methane flux that is sustained for 100s kyr may be consistent with a snowball glaciation.

The fairy circles of Kaokoland (Northwest Namibia) – is the harvester termite Hodotermes mossambicus the prime causal factor in circle formation?

Summary In a research article, published in “Basic and Applied Ecology” in 2000, the authors T. Becker and S. Getzin described dynamics and characteristics of the so-called fairy circles in Kaokoland (North-West Namibia). They stated that the foraging activity of the harvester termite Hodotermes mossambicus Hagen, which is widely distributed in Southern Africa, results in conspicuous, circular, vegetationless patches (fairy circles), surrounded by densely packed grass tussocks. The authors concluded that a) foraging and nesting characteristics of H. mossambicus determine the occurrence of the fairy circles b) grass species richness and assemblage influences the harvesting pattern of H. mossambicus c) differences in the intensity of foraging of H. mossambicus may cause different diameters of the circular forms d) the diameter of the circles is limited by the temperature sensitivity of H. mossambicus Becker and Getzin's statements regarding basic termite biology are based on literature data published in the late 1950's and the author's fieldwork was restricted to few weeks in March/April 2000. The objective of the present opinion paper was to write a comment upon the findings of Becker and Getzin and to encourage future research, which will result in a sound theory about the origin of the fairy circles in Namibia. In einem wissenschaftlichen Beitrag im Journal “Basic and Applied Ecology” (2000) beschrieben T. Becker und S. Getzin Dynamik und Eigenschaften der sogenannten Feenkreise in Kaokoland (Nord-West Namibia). Sie berichteten, dass die Ernteaktivitat der im sudlichen Afrika weit verbreiteten Termite Hodotermes mossambicus kreisrunde, vegetationslose, von dichten Grasbuscheln umgebene Flecken (Feenkreise) verursacht. Die Autoren kamen zu dem Schluss, dass a) Ernteaktivitat und Nesteigenschaften von H. mossambicus das Auftreten der Feenkreise bestimmen b) Artenreichtum der Graser sowie deren Ansammlung das Erntemuster von H. mossambicus beeinflussen c) Intensitatsunterschiede in der Ernteaktivitat von H. mossambicus zu unterschiedlich grosen Feenkreisen fuhren kann d) der Durchmesser der Feenkreise duch die Temperaturempfindlichkeit von H. mossambicus beeinflusst wird Becker und Getzin's Befunde zur Termitenbiologie stutzen sich auf Literaturdaten aus den 1950'er Jahren und die Feldarbeit der Autoren war auf wenige Wochen im Marz/April 2000 beschrankt. Das vorliegende “Opinion Paper” kommentiert die Befunde von Becker und Getzin und will zu weiterer Forschung ermutigen, die zu einer wohlbegrundeten Theorie zur Entstehung der Feenkreise fuhren kann.

Reply to Discussion: Vreeland Diamictites - Neoproterozoic glaciogenic slope deposits, Rocky Mountains, northeast British Columbia

Hoffman et al. (1998b) developed a modified ‘snowball’ Earth hypothesis to explain the association of Neoproterozoic low-latitude glaciation with the deposition of “cap carbonate” rocks bearing highly depleted carbon isotopic values. According to this hypothesis, a runaway albedo feedback caused the world’s oceans to freeze for millions of years. During this time interval, the continental ice cover was thought to have become thin and patchy because of the virtual elimination of the hydrological cycle (Kirschvink, 1992; Hoffman et al., 1998b). The Vreeland diamictites record vigorous erosion by an extensive ice sheet at or near the pressure melting point (McMechan, 2000). Their deposition and that of several other thick (>800 m) Neoproterozoic glaciogenic deposits, including the Mineral Fork diamictites in Utah (Christie-Blick, 1983), Rapitan diamictites in northwestern Canada (Yeo, 1981), the Sturtian diamictites in South Australia (Young and Gostin, 1989), the Hangeerqiaoke diamictites in northwest China (Wang et al., 1981), the Port Askaig diamictites in southwest Scotland (Spencer, 1971), and the Chuos diamictites in northwest Namibia (Hoffman et al., 1998a), show that substantial areas of vigorous contential glaciation occurred in the Neoproterozic. The existence of these deposits is incompatible with the hypothesized thin and patchy continental ice cover of the ‘snowball’ Earth model (Christie-Blick et al., 1999; McMechan, 2000). Rather than taking the sedimentary record as evidence of warmer climates and partially open oceans, Hoffman (in Hoffman and Schrag, …

Vegetation along climate gradients in Kaokoland, North-West Namibia

Vegetation along climate gradients in Kaokoland, North-West Namibia

Transient high temperatures in mantle plume heads inferred from magnesian olivines in Phanerozoic picrites.

Both scaled laboratory experiments and numerical models of terrestrial mantle plumes produce 'balloon-on-a-string' structures, with a bulbous head followed by a stem-like tail. Discussions have focused on whether their initial upwelling heads are hotter than the tails or cooler, as a result of entrainment of ambient mantle during ascent, and also on whether initial plume upwelling is a newtonian or non-newtonian process. The temperature of the mantle delivered to the base of the lithosphere is a critical parameter in such debates. Dry continental magmas can normally contribute little to this topic because their hottest (ultramafic) examples can be expected to be trapped, owing to their density, beneath the Moho. Here we report a rare case in which olivine (with 93.3% forsterite; Mg2SiO4) phenocrysts, precipitated from an unerupted komatiitic melt (approximately 24% MgO) of the Tristan mantle plume head 132 Myr ago, were carried to upper-crust levels in northwest Namibia by less Mg-rich (9.6-18.5% MgO) magmas. We infer that the hidden melt, generated when the plume impinged on the base of the lithosphere, originated in the mantle with a potential temperature of approximately 1,700 degrees C. This is approximately 400 degrees C above ambient and much hotter than the temperatures previously calculated for steady-state Phanerozoic mantle plumes. Published data show that the same conclusion can be reached for the initial Iceland and Galapagos plumes.

Oxide-sulphide relationships in sodalite-bearing metasomatites of the Epembe-Swartbooisdrif Alkaline Province, north-west Namibia

The south-eastern part of Kunene Intrusive Complex (KIC), Namibia/Angola, is host to volumetrically significant, and economically important, concentrations of sodalite in the area around Swartbooisdrif, north-west Namibia. The mineralisation was formed by metasomatic exchange with carbonatites of the Epembe–Swartbooisdrif Alkaline Province. This process led to the breakdown of ore minerals initially present in various rock types of the KIC and caused the formation of new opaque phases in the sodalite-bearing metasomatites. A detailed investigation of textures and chemical compositions of the Fe–Ti oxides and sulphides has allowed evaluation of the complex ore-forming processes related to the polyphase magmatic and metasomatic history of the sodalite deposit. The predominant opaque phases in the various rock types of the KIC are ilmenite and (titano)magnetite, which are highly concentrated in the so-called magnetite plugs. It is clear from the textural evidence that most of the ilmenite and (titano)magnetite, although of orthomagmatic origin, recrystallised under subsolidus conditions. Conformably, their respective chemical compositions and phase relations represented in the system FeO–1/2Fe2O3–TiO2 point to re-equilibration at temperatures below 600 °C. Ilmenite and (titano)magnetite were affected by later deformation and decomposed by various reactions, related to, or outlasting, the metasomatic process. Oxidation of ilmenite led to the formation of symplectitic aggregates of rutile and secondary magnetite. Carbonatisation of the Fe–Ti oxides produced rutile and the siderite and rhodochrosite components in ankerite. Pyrite, in part together with rutile and secondary magnetite, was formed by sulphidation of the Fe–Ti oxides. Conspicuous aggregates of granular or lamellar intergrowths of pyrite with hematite and/or magnetite are interpreted as products of contemporaneous sulphidation and oxidation of former igneous pyrrhotite. Rarely observed pyrrhotite with pentlandite lamellae is probably not an igneous relic, but was formed during the metasomatic event. Smaller amounts of chalcopyrite, bornite, digenite–chalcocite, galena, ferroan siegenite, millerite and polydymite testify to different cooling stages during or after metasomatism. Applying the phase relations in the simplified system Fe–Co–Ni–S–O, we were able to reconstruct a semi-quantitative T–f(S2)–f(O2) path for the ore-forming processes.

A new record ofCletocamptus confluens(Schmeil 1894) (Copepoda Harpacticoida) from a small pond in north-west Namibia

Specimens of Cletocamptus Schmankewitsch 1875, collected in a small pond in north-west Namibia, were identified and redescribed as C. confluens (Schmeil 1894). The morphology of both sexes is illustrated in detail and compared to the existing descriptions. The new record expands considerably the known distribution of C. confluens. A map showing the locations of this widespread species is presented.

The fairy circles of Kaokoland (North-West Namibia) origin, distribution, and characteristics

Embedded in species-poor grasslands, fairy circles are circular or sub-circular patches devoid of any vegetation. Characteristically, the circumference of each circle shows a band of more densely packed taller tussocks within a shorter, more sparse grassland matrix. The average diameter of the circles is between 5‐8 m. Restricted to sites showing deep sandy deposits, in Namibia, fairy circles occur in a broken belt in the pro-Namib from southern Angola to the Orange River (Republic of South Africa). The distribution of fairy circles is significantly limited to areas with an average of 50‐100 mm of annual precipitation. Based on remote sensing methods and expeditions, a map of the distribution area of fairy circles in Kaokoland (NW Namibia) was drawn up. The map indicates that fairy circles are more widely spread over the western parts of Kaokoland than previously reported. The two different active biological hypotheses of the fairy circles’ origin comprise 1. the allelopathic interaction between Euphorbia damarana and the contemporary herbaceous vegetation (Theron 1979) and 2. the involvement of harvester termites (Moll 1994). Both hypotheses are discussed and a model about the origin and dynamic of fairy circles is presented. This model includes the foraging behaviour of the harvester termite Hodotermes mossambicus which is the prime causal factor in forming the fairy circles of Kaokoland. Eingebettet in artenarme Graslander, bilden Feenkreise runde bzw. kreisformige vegetationslose Flachen. Als typisches Merkmal besitzen diese Flachen einen Rand, der aus dicht stehenden grosen Grashorsten gebildet wird und sich deutlich von der sparlichen Grasland-Matrix abhebt. Der Durchmesser der Feenkreise liegt haufig zwischen 5 und 8 m. Beschrankt auf Gebiete mit tiefgrundigen, sandigen Substraten, treten Feenkreise in einem unterbrochenen Streifen in der Vornamib vom sudlichen Angola bis zum Orange (Republik Sudafrika) auf. Das Areal der Feenkreise ist hier auf Gebiete mit einer durchschnittlichen Jahresniederschlagsmenge von 50‐100 mm beschrankt. Auf der Grundlage fernerkundlicher Verfahren und Gelandeuntersuchungen wurde eine Karte des Verbreitungsgebietes von Feenkreisen im Kaokoland (NW Namibia) erstellt. Die Karte zeigt, dass Feenkreise im Kaokoland eine grosere Verbreitung haben, als es zuvor bekannt war. Die beiden aktuellen Hypothesen zur Entstehung der Feenkreise umfassen 1. die allelopathische Interaktion zwischen Euphorbia damarana und der rezenten krautigen Vegetation (Theron 1979) und 2. die Beteiligung einer auf Gras als Nahrung spezialisierten Termitenart (Moll 1994). Beide Hypothesen werden diskutiert und ein Modell der Entstehung und Dynamik von Feenkreisen

Geochemical characteristics of Cretaceous carbonatites from Angola

The Early Cretaceous (138–130 Ma) carbonatites and associated alkaline rocks of Angola belong to the Paraná-Angola-Etendeka Province and occur as ring complexes and other central-type intrusions along northeast trending tectonic lineaments, parallel to the trend of coeval Namibian alkaline complexes. Most of the Angolan carbonatite-alkaline bodies are located along the apical part of the Moçamedes Arch, a structure representing the African counterpart of the Ponta Grossa Arch in southern Brazil, where several alkaline-carbonatite complexes were also emplaced in the Early Cretaceous. Geochemical and isotopic (C, 0, Sr and Nd) characteristics determined for five carbonatitic occurrences indicate that: (1) the overall geochemical composition, including the OC isotopes, is within the range of the Early and Late Cretaceous Brazilian occurrences from the Paraná Basin; (2) the La versus La Yb relationships are consistent with the exsolution of CO i2-rich melts from trachyphonolitic magmas; and (3) the 143 Nd 144 Nd and 87 Sr 86 Sr initial ratios are similar to the initial isotopic ratios (129 Ma) of alkaline complexes in northwest Namibia. In contrast, the Lupongola carbonatites have a distinctly different 143 Nd 144 Nd initial ratio, suggesting a different source. The Angolan carbonatites have SrNd isotopic compositions ranging from bulk earth to time-integrated depleted sources. Since those from eastern Paraguay (at the western fringe of the Paraná-Angola-Etendeka Province) and Brazil appear to be related to mantle-derived melts with time-integrated enriched or B.E. isotopic characteristics, it is concluded that the carbonatites of the Paraná-Angola-Etendeka Province have compositionally distinct mantle sources. Such mantle heterogeneity is attributed to ‘metasomatic processes’, which would have occurred at ca 0.6–0.7 Ga (Angola, northwest Namibia and Brazil) and ca 1.8 Ga (eastern Paraguay), as suggested by Nd-model ages.

Introduction

Tropical open woodlands account for about 40% of the total area of open and closed woodland and forest in the tropics but have so far not received the attention that they deserve in the monitoring and modelling of global environmental change. This Special Issue explores some of the challenges that need to be faced if this neglect is to be overcome. With the exception of Furley's paper on the cerrados of Brazil, the main geographical focus is Sub-Saharan Africa ( Fig. 1). The papers in this issue derive from a workshop held at the University of Leeds on 10 December 1997 with funding from the European Union's Framework 4 Co-operation With Developing Countries (INCO-DC) Programme. The editors would like to express their thanks to those involved in reviewing the papers featured in this issue. Open woodlands neare Linguère in northern Senegal (Photo: D. P. Bradley) Modelling changes in land cover in open woodlands and relating the changes to their underlying human causes is, as Grainger shows, more difficult than modelling deforestation in tropical closed forests, since only a proportion of the total change involves outright clearance (deforestation) and the same area of woodland can be exploited for multiple uses by multiple users. This includes the harvesting of non-timber forest products for subsistence uses, as described by Schreckenberg. Modelling is also limited by poor data availability on woodland area and rates of change, partly resulting from constraints on the use of remote sensing techniques to monitor change. Lambin examines these constraints and suggests how to overcome them. Several papers provide empirical insights into the processes of woodland degradation—the term now commonly employed to describe lesser changes in woody cover that do not involve complete clearance. Dougill and Trodd show how grazing and the use of fire may change the balance between different vegetation layers. Sullivan finds little evidence to support the notion that woodland degradation declines with distance from settlements and indeed questions the scientific basis of continuing fears of degradation driven by over-exploitation of resources by local herders in her study area (north-west Namibia). Although above-ground biomass and biodiversity may well decline when open woodland is converted to cultivated land, Mortimore, Harris and Turner report that total land productivity does not necessarily decrease. A number of papers draw attention to the importance of spatio-temporal variability. Fuller reports considerable intra-annual variation in tree canopy cover in open woodlands. Homewood and Brockington argue that, unlike deforestation, it would be wrong to expect degradation in open woodlands to maintain a consistent trajectory, owing to an inherent variability to which humans may make only a minor contribution. Uncertainty engendered by the absence of reliable data can easily lead to commonly held perceptions having little basis in reality. Ribot warns against perceptions of degradation biased by localised phenomena or colonial prejudice (a theme also evident in Sullivan's paper). Homewood and Brockington argue that spatial bias in biodiversity exploration can lead to problems in conservation assessment and planning. Furley reports that, according to recent research, the biodiversity of tropical open woodlands is not quite as inferior to that of tropical closed forests as previously thought. Our hope is that the papers in this special issue may be of use to those engaged in empirical research in the field by highlighting particular data needs on which advances in understanding may be based. Just as importantly, we hope that they succeed in highlighting some of the important unresolved theoretical debates with respect to these ecosystems. In particular, these seem to focus on the tension between: (i) received views of degradation and the ecological thinking on which these views are based, and; (ii) the ideas of continuing flux, and non-equilibrium, in the light of which the significance of human impacts is often argued to be of lesser importance.

Volume Texture of a Deformed Quartzite Observed With U-Stage Microscopy and Neutron Diffractometry

The volume texture of a naturally deformed quartzite from the Kaoko belt, North-West Namibia, has been analysed by both universal stage microscopy and neutron diffraction. Universal stage microscopy is restricted to the determination of the base pinacoid preferred orientation in quartzite. For a more complete description of the texture, the orientations of additional crystal planes, such as first and second order prisms as well as positive and negative rhombs, must be known. Neutron methods allow the evaluation of pole figures of all Bragg reflecting planes, of which those of the first order prisms being considered to be the most active slip planes, are of particular interest. Drawbacks of neutron diffraction, i.e. the faking of an eventually absent inversion centre and lack of resolution, can be overcome by pole figure inversion and subsequent calculation of desired pole figures. Both, universal stage microscopy and neutron diffraction yield well comparable results, of course only with respect to the pole figure of the c-axis.