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Kimberlites, megacrysts and related xenocrysts from southern and Central Africa: geochemistry and petrology
Get PDFKimberlites, megacrysts and related xenocrysts from twelve localities in central and southern Africa have been investigated with particular emphasis on megacryst petrogenesis, the evolution of the subcontinental lithospheric mantle (SCLM) beneath the northeastern Kasai Craton and the petrogenesis and sources of central African kimberlites. The geochemistry of southern and central African megacryst suites is generally consistent with their derivation from protokimberlitic magmas by a combined fractional crystallization-assimilation process. Through the comparison of the geochemistry of Group 1 and Group 2 kimberlite megacrysts, evidence is provided for a genetic link between megacrysts and their host kimberlites. Major element, trace element and Sr isotope similarities of Group 1 and Group 2 kimberlites and their respective megacrysts strongly suggest a close genetic relationship. The geochemistry and single mineral thermobarometry of peridotitic clinopyroxene xenocrysts suggests that the SCLM is cooler and more depleted beneath Tshibwe compared to that below Mbuji-Mayi, and this is consistent with the finding from this work that the age of Tshibwe is 84 Ma, approximately 14 Myr older than Mbuji-Mayi. A metasomatic event in this region between 84 and 70 Ma is the simplest explanation for the differences in geochemistry and geothermal gradient inferred for the SCLM sampled by the two kimberlites. The Tshibwe and Mbuji-Mayi kimberlites belong to two clusters that together constitute the Mbuji-Mayi kimberlite field in the northeastern Kasai craton, and this field, possibly combined with the kimberlites of the Kabinda field further to the east in the DRC was formed by a pulse of kimberlite magmatism, from roughly 85 to 70 Ma, that formed a lineament with a roughly east-west orientation. The apparently similar age and orientation of these kimberlites suggests that they were emplaced along an eastwest oriented zone of lithospheric weakness in the northeastern Kasai craton, tentatively termed the "Kasai corridor". This may connect with the "Lucapa corridor", a linear zone of concentrated kimberlite magmatism with a northeast-southwest orientation cutting across much of Angola. The whole-rock composition of central African kimberlites is fairly similar to southern African kimberlites except for having generally lower MREE/HREE ratios, lower overall concentrations of incompatible elements and isotopic compositions ranging from those typical of southern African Group 1 kimberlites to extreme DUPAL signatures with high 207Pb/204Pbi and 208Pb/204Pbi for moderate 206Pb/204Pbi values. Published data for Sr and Pb isotope compositions of southern and southwestern African peridotite xenoliths indicate that this DUPAL signature most likely does not originate from the lithospheric mantle, but rather is present in the underlying convecting mantle, as appears to be the case for the source for Brazilian kimberlites with similar extreme DUPAL isotope signatures. Data from this study, therefore, require a greater lateral distribution for the extreme DUPAL signatures than has been previously recognised, extending from southwestern Africa to the far southern Mid-Atlantic Ridge to Brazil. The prior, exclusive attribution of this signature, to the "African large low shear velocity province" may be premature
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
The geochemistry and petrogenesis of the Saltpeterkop carbonatite complex near Sutherland, Northern Cape, South Africa
Get PDFThe Saltpeterkop Carbonatite Complex is a Late Cretaceous (≈76 Ma) volcanic and shallow intrusive magmatic feature located approximately 20 km southeast of Sutherland in the Northern Cape. It is unusual among southern African carbonatite complexes in that it has not been deeply eroded, and retains a significant vestige of its original volcanic features. The main geologic expression of the Complex is a ≈1.5 km diameter tuff ring, located on top of prominently updomed and fractured Beaufort Group (Karoo) sediments, that appears to have formed as the result of a major diatreme-type eruption. The volcaniclastic breccias making up the tuff ring have been heavily altered and silicified by hydrothermal activity, and thick (mm to tens of cm) Fe oxide-rich crusts, which appear to represent the alteration products of Fe-rich carbonatites, are common in this area. Outside of the central ring structure are numerous shallow intrusions (dykes, sills and irregular shapes), satellite breccia pipes and pipe-shaped intrusions that host fresh to only moderately altered igneous rocks. The main igneous rock types include (in decreasing order of abundance): carbonatite, potassic trachyte, olivine melilitite and ultramafic lamprophyre. This thesis provides the first detailed petrographic and geochemical description of these rocks (e.g., major and trace elements) and attempts to explain several aspects of their petrogenesis. The olivine melilitites and ultramafic lamprophyres are the most primitive igneous rocks in the complex and have experienced only minor to moderate extents of differentiation, respectively. They apppear to have been derived by low-degrees of partial melting of a carbonated, likely phlogopite-bearing mantle source. The lamprophyres appear to have been derived by melting at shallower depths than the melilitites based on REE constraints. The carbonatites range from relatively primitive to highly differentiated and they form a nearly continuous compositional range with the ultramafic lamprophyres and melilitites. This seems to argue against a major role for liquid immiscibility in their origin. Their REE content (up to 2 wt.% total REE oxides) correlates with their extent of differentiation. The potassic trachytes are plausibly linked to melts of mafic lower continental crust that has been metasomatised by hydrous potassium-rich carbonatitic melts and which have experienced significant fractional crystallization and assimilation of upper crustal sedimentary rocks during emplacement
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Thermobarometry and geochemistry of peridotite xenoliths from the southwestern margin of the Kaapvaal craton, South Africa
Get PDFGlobally, there are significant contrasts, both thermally and chemically, between peridotite xenoliths exhumed from Archean, and post-Archean terranes. Studies of the thermal structure of the lithosphere, in combination with surface heat flow data, suggest that thermal gradients beneath cratonic regions (Archean blocks which stabilized > 2.5 Ga) are lower than those in off-craton regions, commonly attributed to thinner lithosphere in Proterozoic domains. Although this is true to an extent for southern Africa, the contrasts appear less distinct than Archean-Proterozoic lithosphere contrasts elsewhere, and the thermal structure reflects a regional, Mesozoic, disturbances which has been temporally linked to large-scale tectonic processes. Major element P-T results from peridotite xenoliths sampled during Group II kimberlite magmatism (~ 150 Ma), that erupted through the southwestern Proterozoic NamaquaNatal Province record geothermal gradients akin to those of cratonic terranes. In contrast, xenoliths sampled during the younger Group I kimberlite magmatism (~ 80 Ma) within the Namaqua-Natal Province record equilibration temperatures which are displaced to higher values throughout the pressure ranges. This study reports results from peridotite xenolith samples from seven kimberlites and related rocks which erupted within the Eastern Namaqualand and Namaqualand-Bushmanland-Warmbad area. Two of the localities studied here (Melton Wold and Markt) erupted during Group II kimberlite magmatism, and five localities (Hebron, Uintjiesberg, Gansfontein, Hoedkop and Schuitdrift) erupted during the younger Group I kimberlite magmatism. The results build on prior work, which focused on mineral and whole rock major element chemistry, platinum group elements and Re-Os isotope data, and provide an insight into lithosphere formation and modification in the Namaqua-Natal lithospheric mantle through mineral trace element analysis. These samples also provide an opportunity to further investigate the Mesozoic thermal evolution of the NamaquaNatal lithosphere through application of a REE-based thermobarometer. REE diffusion rates are typically 2 – 3 orders of magnitude lower than divalent major elements, making the REE-based thermobarometer a potentially useful tool for probing the contrasting thermal profiles exhibited in samples from the Group II and Group I kimberlites studied here. Major element-based thermobarometry results and the resulting FITPLOT paleogeotherms indicate that at the time of Grp II kimberlite magmatism the Namaqua-Natal lithosphere was ~ 200 km thick, with a 60 km “diamond window”, and had a geothermal gradient of 40 mW/m2 . In contrast, at the time of Grp I kimberlite magmatism ~ 60 km of lithospheric erosion may have occurred, accompanied by a shift in the thermal regime of the Namaqua-Natal lithosphere to a 45 mW/m2 geotherm. REE-based thermobarometry results produce a large amount of scatter in P-T space, even after a rigorous attempt at identifying well equilibrated clinopyroxene and garnet pairs. The presence of carbonatitic and silico-carbonate metasomatic signatures in these samples necessitates caution in the use of the REE-based thermobarometer when applied to xenoliths entrained by kimberlites. It is likely that the scatter observed in the results presented here is due to differing REE partitioning controls in systems containing carbonate in the melt to those of carbonate-free, silicate melts. HREE concentration in reconstructed whole-rocks and olivine Mg-numbers are consistent with 30% melt extraction in a shallow melting regime. Garnet and clinopyroxene trace element signatures indicate a shift in the style of metasomatism in the Namaqua-Natal lithosphere between Group II and Group I kimberlite magmatism. Zr/Hf versus Ti/Eu systematics reflect kimberlitic/silicate style metasomatism in the samples exhumed during Group II magmatism, whereas samples exhumed during Group I kimberlite magmatism reflect a carbonatitic style of metasomatism
Volcanology and geochemistry of selected kimberlites from the Lulo kimberlite field, Angola
No full textAbstract The area under study is the Lulo Kimberlite Field in the province of Lunda Norte, Angola, which is located 630 km east of the capital, Luanda. This 3000 km2 concession is one of the world's most abundant alluvial diamond fields, producing high value Type IIa diamonds, as well as hosting a significant number of kimberlite pipes. Within Angola, there are hundreds of known kimberlite bodies of which, it is estimated, far fewer than 50% have been studied and fewer than 5% have economic diamond grades (Pereira et al., 2003). With ongoing exploitation of predominantly high-grade alluvial deposits, depletion of the diamond reserves demands continual and improved exploration of primary sources. Alluvial diamond mining within the Lulo Field has yielded large, high value diamonds along the Cacuilo River. These diamonds are anhedral in morphology and show sharp edges with little abrasive signs of travel, indicating that their primary kimberlite source is likely nearby. With more than 560 geophysical anomalies having been identified in the Lulo Field, 164 have been drilled, with 141 shown to be kimberlitic (Lucapa Diamond Company, 2024). This study aims to conduct a reconnaissance survey of volcaniclastic kimberlites within these pipes to classify eruptive styles, providing insights into the internal geology of pipes with implications for diamond grade bulk sampling. A total of 52 kimberlite targets were sampled, yielding 83 thin sectionsfor petrographic analysis and eight representative hypabyssal kimberlites for bulk rock geochemistry, through collaboration with Lucapa diamonds. Each one of the 83 thin sections were described using a petrographic microscope and classified following the scheme of Scott Smith et al. (2018) and Webb & Hetman (2021), with 22 chosen as representative samples. Petrographic analyses of the thin sections allowed for the classification of the kimberlites, at the most primary form of subdivision, as either coherent or magmaclastic (Webb & Hetman, 2021). Within the magmaclastic subsection, pyroclastic kimberlites are classified as Fort à la Corne-type pyroclastic kimberlites (FPK) and abundant resedimented volcaniclastic kimberlites (RVK) are also observed interbedded with the FPKs. The classification of the Lulo kimberlites as FPKs is based on their clast supported texture, olivine-dominated magmaclasts, and ultra-fine serpentine and carbonate cement. The magmaclasts display varied morphologies and include cored and uncored varieties, with a wide range of olivine macrocrysts abundances and low proportion of crustal xenoliths further supporting the classification. Furthermore, the presence of RVKs is confirmed due to the predominant clastic texture, abundant quartz and commonly seen broken grains. The identification of kimberlites as predominantly FPK and RVK indicates an explosive eruptive style that led to pipe excavation and post-eruptive reworking. Comparisons with other kimberlite fields, such as the Voorspoed mine in South Africa and the Lac de Gras kimberlite field in Canada, reveals similarities in internal pipe geology to those in the Lulo field and thus are comparable to kimberlite pipes within Diavik and Ekati mines alike. Thus, the Lulo field kimberlites pipes are classified as Class 3 pipes following the scheme of Skinner and Marsh (2004) or Lac de Gras type following the scheme of Scott Smith (2008). Geochemical analysis shows the Lulo kimberlites share characteristics with South African Group I kimberlites, known for their diamondiferous nature. However, certain deviations, such as depleted MgO and enriched Al₂O₃ and Nb concentrations, suggest crustal contamination that is further supported by a contamination index > 1.0 for all bulk-rock samples. Exploration initiatives, including drilling and bulk sampling, are directed at determining diamond grade and identifying the primary source of the high-value, Type IIa alluvial deposits. Kimberlite pipes within the Lulo field, classified as Class 3 pipes, consist of multiple volcaniclastic units (FPK and RVK) within individual pipes, indicative of considerable variability in diamond content. This heterogeneity underscores the importance of detailed stratigraphic analysis to enhance the accuracy of grade estimation. Olivine macrocrysts, which serve as key indicators of diamond-bearing potential, range from 25 to 53 vol.% in FPKs, reflecting mantle-derived material with varying degrees of sorting (ranging from poorly to well sorted) across different pipes. As a comprehensive assessment across the full length of boreholes has not yet been conducted, olivine macrocryst abundance and sorting remains speculative. A thorough understanding of internal pipe geology and olivine distribution is essential for refining diamond exploration and sampling strategies. Moreover, for the RVKs, the proportion of externally derived material may dilute grades and should therefore be considered. Due to the favourable geological setting of the Lulo Mine and its geological and geochemical similarity to other diamond-rich kimberlite occurrences, it can be inferred that Lulo kimberlites hold significant potential for high-grade diamond deposits. However, current bulk sampling for diamond grades is primarily conducted at the surface for each pipe. Accurate diamond grade estimation requires understanding the internal geological variation within a kimberlite, as surface sampling alone may not represent the true diamond potential of deeper units. Therefore, understanding the internal geology of each pipe is essential, as it significantly influences diamond grades during bulk sampling campaigns aimed at discovering high-value stones. This study highlights the importance of integrating detailed petrographic, geochemical, and geological analyses to fully understand the factors controlling diamond grade, providing valuable insight for the future exploration of Angola's primary diamond sources
Megacryst suite from the Salpeterkop carbonatite complex, Sutherland, Northern Cape, South Africa: an in-depth geochemical study
Get PDFPresented here are major and trace element, stable (oxygen and hydrogen) and radiogenic (Sr-Nd-Pb) isotope analyses for a Cr-poor megacryst suite from the Salpeterkop complex, South Africa. The clinopyroxene, amphibole, phlogopite and ilmenite megacrysts all appear to be cogenetic, and based on known mineral relationships and intergrowths from xenoliths in the complex, the apparent order of mineral crystallisation is as follows: phlogopite → ilmenite → amphibole → clinopyroxene. Megacrysts of amphibole and phlogopite exhibit δD and δ18O values that are aligned with these grains having crystallised from melt originating from the upper mantle. Additionally, the amphibole and phlogopite megacrysts appear have experienced dehydration styled degassing, possibly related to their exhumation. Calculated P-T conditions have the megacrysts crystallising in the lower crust, under conditions ranging from 1 to 1.5 GPa (35 to 45 km depth) and 1000 to 1250 ℃. Calculated REE melts in equilibrium with the megacryst as well as radiogenic isotope results suggest that the Salpeterkop ultramafic lamprophyres are genetically related the the SPKC megacryst suite, however, the calculated parent melt to the megacryst appears to have mixed with a HIMU component. These findings primarily affect higher Mg-number megacrysts, suggesting that this assimilation or mixing occurred during initial stages of crystallisation. Lower Mg-number megacrysts lack the variations noted in their more primitive counterparts and present more tightly defined trends. A model of formation for the megacryst suite of the Salpeterkop complex sees grains having crystallised from an SPKC ultramafic lamprophyre-like melt originating from sublithospheric/asthenospheric conditions. During ascension the melt episodically assimilates material with a HIMU signature. The high Mg-number megacryst population crystallises from this melt at lower crustal depths. Soon after assimilation halts the megacryst parent melt homogenises (or re-homogenises), with grains to crystallise from this melt forming the low-Mg megacryst population
The petrogenesis and geochemistry of the Namaqualand olivine melilitite pipe cluster, western South Africa
Get PDFThe Namaqualand olivine melilitite pipe cluster is a 40 km by 10 km north/south oriented area with at least 10 closely spaced alkaline ultramafic subvolcanic pipes and diatremes containing olivine melilitite and nephelinite, spanning an age range of 35-56 Ma. A related carbonatite complex containing olivine melilitite is also found within this cluster, with an emplacement age of ≈ 55 Ma. The Namaqualand cluster represents the southernmost component of the Late Cretaceous to Paleogene-age Namaqualand-Bushmanland-Warmbad (NBW) lineament, representing a 400 km long NNE-SSW trending feature made of hundreds of diatremes containing ultramafic lamprophyre and kimberlite in addition to the rock types named above. The NBW lineament appears to be an ageprogressive feature, with ages increasing toward the northeast at a rate and direction roughly consistent with Late Cretaceous to Cenozoic African plate motion. This suggests that it could have formed from a single source, such as a mantle plume, with the earlier products being the Warmbad kimberlites, followed by the later diatremes of the Bushmanland cluster and the youngest being the Namaqualand olivine melilitites. This study focuses on the petrography and geochemistry of fresh igneous rocks sampled from 10 pipes in the Namaqualand cluster. The samples have unusual compositions for diatreme-hosted alkaline igneous rocks in that they are relatively differentiated, with whole rock Mg numbers of between 71 and 45. This suggests that many of these samples represent magmas that evolved though more than 50% fractional crystallization of mineral assemblages dominated by olivine but also containing significant melilite, nepheline, phlogopite, titanomagnetite and perovskite. However, least-squares fractionation modelling appears to only provide an approximate guide to the fractionating mineral assemblages. The concentrations of most incompatible trace elements in the Namaqualand melilitites are relatively uniform, suggesting a common source and petrogenesis. Low Pb concentrations in the Namaqualand melilitites, along with their SiO2- and Al2O3-poor major element compositions make it unlikely that they experienced significant (e.g., more than 5-10%) assimilation of local continental crust. The Namaqualand melilitites are characterized by extraordinarily high and variable Nb/Rb and U/Th ratios, and correlations with other elements indicate that these ratios have been affected by variable fractional crystallization of phlogopite and perovskite, respectively. However, this cannot explain the unusually high U concentrations and low Th/U ratios of the most primitive Namaqualand melilitites, which appear to be a source feature. Oxygen isotope ratios of olivine separates indicate that olivines from a large majority of the melilitite pipes have compositions indistinguishable from those from typical upper mantle peridotite (4.9-5.2‰). However, three melilitite pipes emplaced within the mid-Cretaceous Koegel Fontein igneous complex contain M. D. Kirchner University of Cape Town (2022) MSc Thesis olivines with exceptionally low δ18O values (i.e., down to +4.2 ‰). The crustal country rock surrounding the Koegel Fontein complex, as well as igneous rocks strongly contaminated by this crust, have exceptionally low δ18O values (down to -4 and -5.3‰, respectively). The unusually low -δ 18O values of these olivines could be explained by the assimilation of up to 10% of Koegel Fontein country rock crust having the lowest δ18O values measured. Larger amounts of crustal assimilation are not plausible as they would result in detectable increases in SiO2 and Pb/Ce ratios. It is unclear whether the crustal assimilation detected in the melilitites emplaced within the Koegelfontein complex is typical of most Namaqualand melilitites. The Namaqualand melilitites have radiogenic isotope compositions that overlap with those of melilitites and kimberlites from the Bushmanland and Warmbad clusters, as well as with southern African Group I kimberlites. However, the Namaqualand pipes are unique in that some samples have radiogenic isotope values approaching those of HIMU oceanic island basalts (e.g., from St. Helena, Mangaia, Tubuai), whereas the Bushmanland and Warmbad clusters display isotopic compositions that only extend to weaker HIMU signatures. The geochemical and age-distance patterns displayed by the NBW igneous rocks are most consistent with the action of a mantle plume passing beneath the western margin of southern Africa in the Late Cretaceous to Paleogene, resulting in the generation of the NBW lineament. This is consistent with the fact that the samples with the strongest HIMU signatures in the NBW lineament are those that have been emplaced on the thinnest lithosphere, nearest the continental margin. The HIMU signature dominant in the Namaqualand melilitites is presumably related to the plume source, which could contain ancient recycled oceanic crust. The formation of the African megalineament could theoretically be related to the same plume that caused the NBW lineament by triggering a zone of magmatism along a deep-seated zone of weakness in the lithospheric mantle between Southern Africa into the east African rift zone, however, more evidence would be needed to fully support this possibility
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