Browsing by Author "Janney, Philip Edward"

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    A comparison of melt depletion, thermal structure and metasomatism of proterozoic mantle lithosphere in the Namaqua-Natal and Rehoboth Provinces of Southern Africa
    (2017) Shiimi, Ellwin Taleni; Janney, Philip Edward
    Major and trace element mineral data are presented for garnet and spinel bearing peridotite xenolith suites from four Late Cretaceous kimberlites to the west of the Kaapvaal craton in two Proterozoic terranes: Hoedkop and Pofadder in the north-western Namaqua-Natal Province and Rietfontein and Louwrensia (Gibeon) in the Rehoboth Province. These are used to constrain and compare the thermal structure, the extents of melt extraction and metasomatism of the two mantle terranes with each other and with the Archaean Kaapvaal craton. Xenoliths from the Rietfontein and Pofadder localities have never previously been studied in detail. The information from this study is crucial for understanding the thermal and chemical evolution of Southern Africa's off-craton lithospheric mantle and the various processes (e.g., kimberlite and other magmatism, continental breakup) that have affected it. Mineral thermobarometry from a variety of independent thermobarometers indicates that, at pressures less than about 4 GPa, xenoliths from the Rehoboth Province (Louwrensia and Rietfontein) lie on or very near the Kaapvaal geotherm and between the 40mWm⁻² and 45mWm⁻² conductive geotherms, demonstrating that the lithospheric mantle in the Rehoboth Province was thermally similar to that of the Kaapvaal craton during the Late Cretaceous. In contrast, peridotites from the Namaqua-Natal Province (Hoedkop and Pofadder) in this pressure range fall at temperatures approximately 100°C - 200°C warmer than the Kaapvaal geotherm at any given depth and lie between the 45mWm⁻² and 50 mWm⁻² conduction geotherms, suggesting a Phanerozoic thermal disturbance in these regions. At higher pressures, samples from both terranes (represented only by Louwrensia and Hoedkop) fall above the Kaapvaal geotherm. However, these deeper, higher temperature peridotites are mostly sheared and indicate localised melt-rock interaction, therefore are not likely representative of the general thermal state of the lithosphere. Peridotites from the four suites have similar mineral major element compositions and their compositional variations fall well within the range of other southern Africa off-craton xenolith localities. Most garnets have smooth REE patterns with LREE depletions and relatively enriched, flat MREE - HREE abundances and patterns, while most clinopyroxenes have negatively sloping REE patterns with maximum enrichments in Nd and Sm. The average extents of melting beneath both Proterozoic provinces are moderate, and the samples are more fertile, on average, than cratonic lithospheric mantle. Peridotites from Louwrensia (Gibeon) appear to have experienced the greatest extents of melt extraction, as garnets from this locality extend to the lowest Y contents and some display strongly sinusoidal REE patterns similar to cratonic subcalcic garnets, suggesting that all of the REE were initially strongly depleted prior to metasomatism. A notable difference between the two terranes is that peridotites from Gibeon and Rietfontein (Rehoboth Province) show well developed trace element equilibrium between clinopyroxene and garnet, whereas those from Hoedkop and Pofadder (NW Namaqua-Natal Province) display significant disequilibrium. Although all peridotites show evidence for metasomatic incompatible element enrichments, some peridotites from Hoedkop show strong disequilibrium in Rb, Ba, Th and Nb (with clinopyroxene being overly enriched in these elements) and all Pofadder peridotites show major disequilibrium, with clinopyroxene being overly enriched in Ba and Nb and overly depleted in the middle and heavy REE relative to garnet. Further, clinopyroxenes from Pofadder peridotites are unique in that they have linear REE patterns, with maximum enrichment in La, in contrast to clinopyroxenes from all other localities in this study, which show concave-down patterns in the light to middle REE. Differences in the likely nature of the metasomatising agents affecting the lithosphere of the two off-craton terranes are discussed, as well as implications for the region's geological evolution.
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    Geochemical and isotopic constraints on the source regions of phanerozoic carbonatites and associated alkaline rocks from the Zandkopsdrift complex of Namaqualand, South Africa, and the Marinkas Quellen, and Dicker Willem complexes in Namibia
    (2020) Ogungbuyi, Ibiyemi Prisca; Janney, Philip Edward; Harris, Chris
    This is a study of Phanerozoic carbonatites and related alkaline silicate rocks from Zandkopsdrift in Namaqualand, South Africa (55 Ma), and Marinkas Quellen (529 Ma), and Dicker Willem (49 Ma) in southern Namibia, all emplaced within the Proterozoic Namaqua Natal province. The aims of this thesis are to characterise their mantle source region, including the timing and nature of the metasomatism that affected their sources, and to constrain their petrogenesis, particularly the relationships between the carbonatites and associated silicate igneous rocks. These associated silicate igneous rocks include olivine melilitite, aillikite and alkaline lamrophyre at Zandkopsdrift, nepheline syenite and trachyte at Marinkas Quellen, and ijolite and trachyte at Dicker Willem. At both Marinkas Quellen and Dicker Willem, the trachytes appear to be derived primarily from fenitised country rock. The major and trace element characteristics, enrichment in LREE and other incompatible elements, large Zr-Hf-Ti depletions and high Zr/Hf ratios all appear to have been inherited from a metasomatised mantle source region rather than being the result of residual source mineralogy. The δ18O and δ13C values of carbonate in the study locations vary significantly. The δ13C values (-3.9 to -8.8 ‰) are within the range of mantlederived carbonatites, whereas the δ18O values are often significantly higher (+8.64 to +22.22 ‰, versus SMOW) for “mantle-derived” carbonatites. The higher δ18O values observed are most likely attributable to low-temperature, post-emplacement alteration by hydrous fluids. O-isotope thermometry of the silicate mineral pairs (clinopyroxene, amphibole, and biotite) gives close to magmatic temperatures (≈800 oC), whereas the equilibration temperature of calcite-silicate mineral pairs is lower. The carbonatite and associated igneous rock samples contain unradiogenic Sr and mildly radiogenic Nd isotope compositions below and above Bulk Earth/CHUR values respectively. This suggests that carbonatite magmas were generated from sources with long-lived Rb/Sr lower than, and Sm/Nd higher than, the primitive mantle. In ƐHf(t)-ƐNd(t) space, the carbonatites and associated silicate rocks plot as much as 8 ƐHf units below the terrestrial ƐNd-ƐHf array, indicating mixing of a source with moderate ƐNd and exceptionally unradiogenic Hf isotope compositions. The radiogenic Pb isotope composition of the carbonatites (206Pb/204Pbi ratios from 18.06 to 22.38), is consistent with a source having high U/Pb, akin to the HIMU mantle end member. The radiogenic isotopes of the carbonatites and the alkaline silicate rocks seem most consistent with a dominantly asthenospheric source, with minor contributions from lithospheric sources. There is little evidence supporting the derivation of carbonatites at the three complexes from parental hybrid carbonate-silicate magmas. Rather, the evidence seems most consistent with deriving the carbonatites directly from very low-degree mantle melts, which subsequently become variably differentiated, first by melt-rock interaction in the mantle and subsequently by fractional crystallization. The associated silica undersaturated silicate igneous rocks appear most likely to represent relatively primitive (melilitites) to differentiated (nepheline syenites) melts of metasomatic wehrlites that were formed by carbonatite-peridotite melt-rock interaction.
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    Megacryst suite from the Salpeterkop carbonatite complex, Sutherland, Northern Cape, South Africa: an in-depth geochemical study
    (2020) Peel, Chad; Janney, Philip Edward
    Presented 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.
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    The petrogenesis and geochemistry of the Namaqualand olivine melilitite pipe cluster, western South Africa
    (2023) Kirchner, Michael; Janney, Philip Edward
    The 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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    Thermobarometry and geochemistry of peridotite xenoliths from the southwestern margin of the Kaapvaal craton, South Africa
    (2022) Clark, Fiona; Janney, Philip Edward
    Globally, 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.