U-Pb Zircon Calendar for Namaquan (Grenville) Crustal Events in the Granulite-facies Terrane of the O'okiep Copper District of South Africa

The O'okiep Copper District is underlain by voluminous 1035–1210Ma granite gneiss and granite with remnants of metamorphosedsupracrustal rocks. This assemblage was intruded by the 1030Ma copper-bearing Koperberg Suite that includes jotunite, anorthosite,biotite diorite and hypersthene-bearing rocks ranging from leuconoriteto hypersthenite. New sensitive high-resolution ion microprobeage data demonstrate the presence of 1700–2000 Ma zirconas xenocrysts in all of the intrusive rocks, and as detritalzircon in the metasediments of the Khurisberg Subgroup. Thesedata are consistent with published Sm–Nd model ages ofc. 1700 Ma (T_CHUR) and c. 2000 Ma (T_DM) of many of the intrusivesthat support a major crust-forming event in Eburnian (Hudsonian)times. In addition, U–Th–Pb analyses of zirconsfrom all major rock units define two tectono-magmatic episodesof the Namaquan Orogeny: (1) the O'okiepian Episode (1180–1210Ma), represented by regional granite plutonism, notably theNababeep and Modderfontein Granite Gneisses and the Concordiaand Kweekfontein Granites that accompanied and outlasted (e.g.Kweekfontein Granite) regional tectonism [F₂(D₂)] and granulite-faciesmetamorphism (M₂); (2) the Klondikean Episode (1020–1040Ma), which includes the intrusion of the porphyritic RietbergGranite and of the Koperberg Suite that are devoid of regionalplanar or linear fabrics. Klondikean tectonism (D₃) is reflectedby major east–west-trending open folds [F₃(D_3a)], andby localized east–west-trending near-vertical ductilefolds [‘steep structures’; F₄(D_3b)] whose formationwas broadly coeval with the intrusion of the Koperberg Suite.A regional, largely thermal, amphibolite- to granulite-faciesmetamorphism (M₃) accompanied D₃. This study demonstrates, interalia, that the complete spectrum of rock-types of the KoperbergSuite, together with the Rietberg Granite, was intruded in ashort time-interval (<10 Myr) at c. 1030 Ma, and that therewere lengthy periods of about 150 Myr of tectonic quiescencewithin the Namaquan Orogeny: (1) between the O'okiepian andKlondikean Episodes; (2) from the end of the latter to the formalend of Namaquan Orogenesis 800–850 Ma ago. KEY WORDS: U–Pb, zircon; O'okiep, Namaqualand; granite plutonism; granulite facies; Koperberg Suite; Namaquan (Grenville) Orogeny     

The household economy of pastoralists and wage-labourers in the Richtersveld, South Africa

The Richtersveld is situated in an arid environment and its inhabitants are confronted with various hazards. Based on data collected during 22 months of ethnographic fieldwork, the livelihood strategies of Richtersvelders are explored. Besides a thorough examination of the diversified economy, the internal institutional arrangements for pooling and redistributing resources within households are analysed in this paper. Encouraging household members to engage in different activities and to share their income—as well as skills to manage the household budget properly—are integral to diversification and reducing the vulnerability of households. This study has shown that even in a community marked by considerable dependence on wage labour, almost three-quarters of the households are directly involved in stock farming. Although pastoralism still plays an important role for households, it has shifted from being the core economic activity to being an insurance against unemployment and contributing to subsistence. Stock farming serves to build up resilience through the diversification of economic activities inside the household. In the South African context, the relevance of activities dependent on land has implications not only for rural development, but also for post-apartheid land and agrarian reform.     

Diversity to decline-livelihood adaptations of the Namaqua Khoikhoi (1800–1900)

An environmental history of the Leliefontein community of Namaqualand, Northern Cape provides a detailed case of the nexus between social and ecological stresses shaping livelihood change. By combining an historical proxy precipitation data set with a livelihood change study the value of historical research in integrated studies of past human-environment systems is illustrated. The identification of effective livelihood adaptation to extreme climatic conditions is examined, illustrating the tradeoffs made between adaptation and ‘coping’ strategies which were unsuccessful over the long term. During the course of the 19th century the Namaqua Khoikhoi population changed from a sustainable nomadic pastoral community to a poverty stricken rural community with a diversity of livelihood strategies. For the Namaqua increased livelihood diversity – usually an effective adaptation in times of stress – instead of promoting resilience, contributed to their material decline. Widespread transhumance between different climatic regions is shown to have been a successful adaptation to climatic extremes, but external economic exposure and restricted access to land become drivers of decline. The ‘double exposure’ framework used in contemporary studies, proved useful in accounting for this decline as it can accommodate both environmental and economic stressors.     

Low-P / high-T metamorphism in the Okiep Copper District, western Namaqualand, South Africa

The Okiep Copper District, part of the 1.2–1.0 Ga high-grade terrane in western Namaqualand, is composed of a mid-Proterozoic supracrustal sequence and several pre- to post-orogenic intrusive suites affected by two high-grade events (M2a/M2b, M3) of Kibaran and one low-grade event (M4) of Pan-African age. Peak assemblages in quartz-bearing pelites are characterized either by garnet+cordierite coexisting with sillimanite/biotite, or by biotite+sillimanite±garnet; a difference controlled by bulk composition and variation in water activities (0.1–0.7) during dehydration melting. Maximum P–T conditions were reached during M2a coevally with the major deformational event (D2a) and are estimated at 750–820  °C and 5–6  kbar. A counterclockwise P–T  path is indicated by regionally occurring pseudomorphs of sillimanite after andalusite and by prograde reaction textures preserved as relics in M2a porphyroblasts. Two stages of retrograde metamorphism are distinguished: M2a garnet+cordierite-bearing assemblages were retrogressed to biotite+sillimanite+quartz (M2b) along discontinuous foliation planes and shear zones (D2b). Retrograde M3 corona assemblages formed at similar P–T  conditions (580–660  °C and 5.8±0.5  kbar) to the M2b assemblages but M3 crystallization postdates penetrative D2 deformation, intrusion of 1.06 Ga granitoids and formation of associated W–Mo deposits. It is concluded that: (a) Kibaran high-grade metamorphism in the Okiep Copper District is thermally punctuated and (b) reaction textures documenting apparent isobaric cooling of this low- P high- T  terrane must be interpreted with caution.     

Management of the old field vegetation in the Namaqua National Park, South Africa: conflicting demands of conservation and tourism

Springtime mass displays of wild flowers are especially prominent on old fields and other disturbed sites in Namaqualand, an arid region in the north–western part of South Africa. This springtime floral spectacle draws thousands of tourists annually. However, there is a potential conflict between biodiversity conservation and the mass flowering that is a valuable source of income to the region. To develop a management plan for the old field vegetation in the Namaqua National Park, the questions of whether to disturb or not, how often to disturb and what type of disturbance should be applied are of vital importance. The aim of this paper is to report on experimental work to determine the impact of disturbance on mass flowering displays and biodiversity. Results show that although species diversity increased with time since last cultivation, the mass effect of the flowering display diminished. The loss of the mass effect appears to be primarily caused by the increased abundance of a short–lived perennial species ( Leysera gnaphalodes ) that suppressed the flowering display of the prominent annual species. To maintain mass displays as a tourist attraction, regular disturbance by tilling therefore seems essential. Initially, a four–year rotational tilling programme is proposed to produce a mosaic of fields of different ages to ensure that there will always be some old fields producing a spectacular and uncluttered display. Frequency of disturbance should thereafter be determined by an adaptive management policy. If the management of the old field vegetation is seen in the context of conserving total landscape ecodiversity, this includes conserving cultural bio–landscapes and their intrinsic values. Viewed in this context, human–induced disturbance in the small area set aside for mass displays in the Namaqua National Park (less than 0.05% of the park) becomes justified.     

Partial melting and the formation of granulite facies assemblages in Namaqualand, South Africa

Abstract Dehydration-melting reactions, in which water from a hydrous phase enters the melt, leaving an anhydrous solid assemblage, are the dominant mechanism of partial melting of high-grade rocks in the absence of externally derived vapour. Equilibria involving melt and solid phases are effective buffers of aH₂,o. The element-partitioning observed in natural rocks suggests that dehydration melting occurs over a temperature interval during which, for most cases, aH₂o is driven to lower values. The mass balance of dehydration melting in typical biotite gneiss and metapelite shows that the proportion of melt in the product assemblage at T± 850°C is relatively small (10–20%), and probably insufficient to mobilize a partially melted rock body. Granulite facies metapelite, biotite gneiss and metabasic gneiss in Namaqualand contain coarse-grained, discordant, unfoliated, anhydrous segregations, surrounded by a finer grained, foliated matrix that commonly includes hydrous minerals. The segregations have modes consistent with the hypothesis that they are the solid and liquid products of the dehydration-melting reactions: Bt + Sil + Qtz + PI = Grt ° Crd + Kfs + L (metapelite), Bt + Qtz + Pl = Opx + Kfs + L (biotite gneiss), and Hbl + Qtz = Opx + Cpx + Pl + L (metabasic gneiss). The size, shape, distribution and modes of segregations suggest only limited migration and extraction of melt. Growth of anhydrous poikiloblasts in matrix regions, development of anhydrous haloes around segregations and formation of dehydrated margins on metabasic layers enclosed in migmatitic metapelites all imply local gradients in water activity. Also, they suggest that individual segregations and bodies of partially melted rock acted as sinks for soluble volatiles. The preservation of anhydrous assemblages and the restricted distribution of late hydrous minerals suggest that retrograde reaction between hydrous melt and solids did not occur and that H₂O in the melt was released as vapour on crystallization. This model, combined with the natural observations, suggests that it is possible to form granulite facies assemblages without participation of external fluid and without major extraction of silicate melt.     

Rehabilitation of Rangelands in Paulshoek, Namaqualand: Understanding vegetation change using biophysical manipulations

In Namaqualand, heavy grazing of low-lying areas has resulted in the reduction of palatable perennial shrubs and the dominance of the unpalatable shrub, Galenia africana. This study investigates the process of rehabilitation and how it can be used to gain an understanding of the ecological factors that may be sustaining altered vegetation states. We intervened to change the biophysical environment through the construction of microcatchments and brushpacks to act as traps for water, seed and organic material combined with the introduction of seeds and seedlings were implemented. Two years after implementation brushpacked microhabitats improved natural recruitment of herbaceous species compared to bare areas at two out of three sites. Microhabitats did not influence natural recruitment of perennial seedlings. Emergence of palatable perennial seed was not influenced by microhabitat manipulation. Seedlings transplanted under adult G. africana shrubs showed higher survival rates at two out of three sites compared to seedlings in unmodified areas.     

Namaqualand's climate: Recent historical changes and future scenarios

A brief outline of some issues concerning global climate change research is presented before discussing local-scale changes in Namaqaland's rainfall. Using a gridded data set derived through interpolation of station records, trends in observed rainfall for the period 1950–1999 are discussed. To assess what changes may occur during the 21st century, the downscaled results of six different General Circulation Model projections are presented. The historical trends show some clear spatial patterns, which depict regions of wetting in the central coastal belt and the north-eastern part of the domain, and extensive drying along the escarpment. Reasonably good agreement is shown by the different downscaled projections. These suggest increased late summer convective precipitation in the north-east, but extensive drying along the coast in early and mid winter consistent with the poleward retreat of rain-bearing mid-latitude cyclones.     

Rb-Sr and Sm-Nd Mineral Isochron Ages of the Metamorphic Rocks in the Namaqualand Metamorphic Complex, South Africa

Rb-Sr and Sm-Nd isotopic studies were carried out for metamorphic rocks in the Namaqualand Metamorphic Complex, South Africa. The metamorphic rocks give the Rb-Sr mineral isochron ages (whole-rock - biotite - felsic fractions) of 844±85 Ma and 811.6±6.6 Ma for the lower granulite zone and of 776.5±5.4 Ma for the upper granulite zone. The rocks yield the Sm-Nd mineral isochron ages of 1071±18 Ma (whole-rock - garnet - felsic fractions) and 1067±158 Ma (whole-rock - hornblende - biotite rich fraction - felsic fractions) for the lower granulite zone and of 1052.0±3.6 Ma and 1002.5±1.4 Ma (whole-rock - garnet - felsic fractions) for the upper granulite zone. These age data suggest that the granulite facies metamorphism took place at 1060-1000 Ma, and that the rocks cooled down at 850-780 Ma. The Sr and Nd isotopic compositions of metamorphic rocks are different between the lower and upper granulite zones.     

The role of facilitation in seedling recruitment and survival patterns, in the Strandveld Succulent Karoo, South Africa

Several studies in arid ecosystems have demonstrated that seedling establishment and survival were greater underneath the canopies of shrubs than in the open spaces between shrubs. These results led to the formulation of the hypothesis that seedling recruitment and survival were dependent on the shelter provided by shrub species (nurse-plant hypothesis). Seedling emergence and survival beneath and between the canopies of selected perennial shrub species were investigated at five localities in the Strandveld Succulent Karoo, each dominated by a different shrub species. Seedling emergence and survival were also examined at three localities dominated by annual species. In general, species richness and seedling densities were significantly higher in open areas than underneath shrubs, while seedling survival percentages did not differ significantly between microsites. Therefore, no evidence could be found to support the hypothesis that seedling recruitment and survival were facilitated by the presence of shrub species. Although most species will be able to establish in the absence of shrubs, the presence of woody species may have other advantages when restoring vegetation in mined areas.