A new lithostratigraphic framework and unified nomenclature for the Nsuze Group of the Nkandla sub-basin, southern Kaapvaal Craton, South Africa

During the early 20th century the term Insuzi Series, later reclassified as the Nsuze Group of the Pongola Supergroup, was proposed for a volcano-sedimentary succession exposed in the upper Nsuze River valley in central KwaZulu-Natal, South Africa. Subsequently, however, there has been little consensus on lithostratigraphic frameworks within the type area, and limited correlation with the exceptionally well-defined stratigraphy within the main Pongola basin. Recent mapping, combined with newly acquired high-resolution aeromagnetic data, satellite imagery, and available published geochronological data suggest that previously published schemes within the Nkandla sub-basin require revision. Utilising important regional marker units, as well as the stratigraphic positions of distinct sedimentary facies within the otherwise volcanic Nsuze Group, a working model is proposed. Lithostratigraphic units are well represented in the Mhlatuze and Nkandla inliers with examples from these areas given prominence. Where exposed, potential correlates within the Nsuze nappe complex are discussed. Within the proposed scheme the siliciclastic Mantonga Formation forms the base of the Nsuze Group, nonconformably overlying basement granitoids of the Kaapvaal Craton within the Mhaltuze Inlier. Mafic volcanics of the Nhlebela Formation overlie the Mantonga Formation in the inlier. These two lower units are, however, not exposed elsewhere in the sub-basin. The sedimentary White Mfolozi Formation forms the base of the succession in the Nkandla Inlier. Diamictites and stromatolite-bearing carbonate lithologies unique to this unit are utilised for regional third-order correlations with the type-area in the White Mfolozi Inlier. Mafic volcanics of the Agatha Formation overlie the White Mfolozi Formation in all exposures, but are most extensively developed within the Mdlelanga syncline of the Nkandla Inlier. Sedimentary and volcaniclastic lithologies of the Mkuzane Formation cap the Nsuze Group in the Mhlatuze and Nkandla inliers. Thickness of this formation is, however, highly variable having been subjected to pre-Vutshini Formation erosion. Through detailed reinterpretation of the stratigraphy of the Nkandla sub-basin we present a third order, (formation) scale, lithostratigraphic scheme encompassing all the formational units of the currently accepted stratigraphy within the main Pongola basin. This working model has the potential for lower-ranking units to be identified and be placed at their appropriate stratigraphic levels in future.

A reinterpretation of the Archaean stratigraphy south of Nkandla, southern Kaapvaal Craton, South Africa: Geophysical and stratigraphic constraints on a sheared granitoid-greenstone remnant

A new lithostratigraphic framework based upon a review of historic data, field mapping and remote sensing, including aerial photography, high-resolution airborne aeromagnetic and radiometric data, is proposed for the Archaean geology along the southeastern margin of the Kaapvaal Craton, South Africa. A synthesis of new and existing data reveals that previously accepted lithostratigraphic schemes require complete revision, with reinterpretations identifying multiple major shear zones and previously unidentified granitoid successions along the margin of the craton. In this new lithostratigraphic framework, lithologies of the Southern Syncline previously correlated with the Nsuze Group of the Pongola Supergroup, are redefined as greenstone lithologies associated with the Ilangwe Greenstone Belt. The geology of the Nkandla region can be subdivided into five distinct geophysical domains including: (i) an extension of the Ilangwe Greenstone Belt, (Domain 1) which is subdivided into; a lower volcanic succession, the Thathe Formation, comprising pillow and amygdaloidal volcanics; the adjoining Sabiza Formation, comprising pillow volcanics exposed in the southeast of the study area; the volcano-sedimentary Mtshwili Formation, which overlies the Thathe and Sabiza formations, consisting of quartz (sericite) schist, phyllite, metavolcanics and iron formation; the Nomangci Formation, which occurs as a region of highly deformed quartz-kyanite-sericite schists, and the Simbagwezi Formation, which comprises maroon to green phyllites and schists in the north of the study area. (ii) granitoids of the Impisi Granitoid Suite (Domain 2) which border the greenstone succession to the north, intruding the Nomangci and Simbagwezi formations. (iii) a southern complex of sheared granitoids termed the Umgabhi Granitoid Suite (Domain 3), which intrudes the Thathe, Sabiza and Mtshwili formations. (iv) The two remaining domains, comprise the Mesoproterozoic Mfongosi and Ntingwe Groups (Domain 4) and Mesoarchaean volcano-sedimentary sequences of the Pongola Supergroup (Domain 5).

A LA-ICPMS zircon record of magmatic crystallization and compositional alteration in meta-igneous rocks of the eastern Kaapvaal Craton

Archaean zircons from the Kaapvaal Craton, South Africa, were analyzed by Laser Ablation (LA)-ICP-MS to obtain a coupled record of U-Th-Pb isotope ratios and selected trace elements with the aim to develop insights into physico-chemical conditions during igneous zircon crystallization and subsequent compositional alteration. Four rock samples previously dated by SIMS U-Pb using zircon were selected: 3.56 Ga Ngwane Gneiss, 3.55 Ga Theespruit felsic metavolcanic, 3.50 Ga Steynsdorp Gneiss and 2.98 Ga Nhlangano Gneiss. LA-ICP-MS U-Pb zircon ages agree with published SIMS U-Pb ages within analytical uncertainty. Assessment of the magmatic crystallization histories was based on near-concordant grains, and discordant grains were used to examine post-igneous element mobilization and alteration. Time-resolved laser drilling experiments allowed distinction of concordant and discordant zircon domains, but also revealed systematic changes in REE + Ti geochemistry, U + Th content, discordance and metamictization. Th/U and Zr/Hf, coupled with REE patterns, effectively distinguish compositional zircon types that reflect variable degrees of igneous differentiation and melt compositions. Eu/Eu* values indicate significant feldspar fractionation in some magmas. Averaged crystallization temperatures of magmatic zircons, as derived from the Ti-in-zircon thermometer, define a narrow range of 650 to 750°C for (near-)concordant grains, consistent with general constraints on temperatures at zircon saturation for felsic magmas, and testifying to a closed-system behavior of Ti (and other trace elements). Systematic deviations from primary igneous trace element signatures are strongly correlated with radiation damage. Specifically, Th/U and, to some extent, Zr/Hf decrease, and Ti increases with increasing U (+Th) content and isotopic disturbance (discordance).

A new lithostratigraphic framework for portions of the Pongola Supergroup within the Nkandla sub-basin, southern Kaapvaal Craton, South Africa; insights into Mozaan Group stratigraphy

A revised lithostratigraphic framework for Mozaan Group-equivalent strata within the Nkandla sub-basin is presented based on new field data, remote sensing and genetic sequence stratigraphic interpretations. Although previous literature has suggested that no Mozaan Group lithologies were deposited within the sub-basin, reinterpretations presented here indicate that 90% of the lithostratigraphy developed within the main basin occurs within the Nkandla and Mhlatuze inliers. Mozaan Group units previously defined as the Vutshini and Ekombe formations are correlated with stratigraphy from the lowermost Sinqeni Formation to the Gabela Formation. Although thinner than units within the type area in the main basin, thicknesses of the Sinqeni Formation are comparable to those observed within the White Mfolozi Inlier. A ~1 000 m composite reference profile is measured within the Mdlelanga Syncline of the Nkandla Inlier. Further profiles were measured for sequences in the Gem-Vuleka Syncline of the Nkandla Inlier, as well as within the Mhlatuze Inlier. These latter profiles, however, host only lower Mozaan Group strata. In all sections the basal portion of the sequence comprises two quartz arenite units, separated by a ferruginous shale, which hosts minor iron formation interbeds. This predominantly coarse-grained lower sequence is overlain by a shale-dominated succession with multiple sandstone interbeds. A prominent coarse-grained quartz arenite unit forms a distinct marker in the middle portion of the sequence. This is overlain by a sequence of shales and sandstones with two prominent igneous units present. Genetic sequence stratigraphic interpretations indicate cyclical deposition of dominantly shallow marine sediments with condensed sections, marked by iron formations or ferruginous shales, denoting periods of marine highstand along the southeastern margin of the Kaapvaal Craton. The evidence of Mozaan Group stratigraphy within the Nkandla sub-basin supports a passive margin tectonic model whereby deposition occurred in an arcuate shallow continental margin which opened to the southeast. The extension of Mozaan Group strata into the Nkandla sub-basin suggests that the Mozaan Basin likely formed a single depository rather than separate sub-basins as previously proposed.

Evolution of ultrapotassic volcanism on the Kaapvaal craton: deepening the orangeite versus lamproite debate

Orangeites are a significant source of diamonds, yet ambiguity surrounds their status among groups of mantle-derived potassic rocks. This study reports mineralogical and geochemical data for a ca. 140 Ma orangeite dyke swarm that intersects the Bushveld Complex on the Kaapvaal craton in South Africa. The dykes comprise distinctive petrographic varieties that are linked principally by olivine fractionation, with the most evolved members containing minor amounts of primary carbonate, sanidine and andradite garnet in the groundmass. Although abundant groundmass phlogopite and clinopyroxene have compositions that are similar to those of cratonic lamproites, these phases show notable Ti-depletion, which we consider a hallmark feature of type orangeites from the Kaapvaal craton. Ti-depletion is also characteristic for the bulk rock compositions and is associated with strongly depleted Th-U-Nb-Ta contents at high Cs-Rb-Ba-K concentrations. The resultant high LILE/HFSE ratios of orangeites suggest that mantle source enrichment occurred by metasomatic processes in the proximity of ancient subduction zones.The Bushveld-intersecting orangeite dykes have strongly enriched Sr-Nd-Hf isotopic compositions (initial 87Sr/86Sr = 0.70701-0.70741; εNd = −10.6 to −5.8; εHf = −14.4 to −2.5), similar to those of other orangeites from across South Africa. Combined with the strong Ti-Nb-Ta depletion, this ubiquitous isotopic feature points to the involvement of ancient metasomatized mantle lithosphere in the origin of Kaapvaal craton orangeites, where K-rich metasomes imparted a ‘fossil’ subduction geochemical signature. Previous geochronology studies identified ancient K-enrichment events within the Kaapvaal cratonic mantle lithosphere, possibly associated with collisional tectonics during the 1.2-1.1 Ga Namaqua-Natal orogeny of the Rodinia supercontinent cycle. It therefore seems permissible that the cratonic mantle root was preconditioned for ultrapotassic magma production by tectonomagmatic events that occurred along convergent plate margins during the Proterozoic. However, reactivation of the K-rich metasomes had to await establishment of an extensional tectonic regime, such as that during the Mesozoic breakup of Gondwana, which was accompanied by widespread (1000 × 750 km) small-volume orangeite volcanism between 200 and 110 Ma.Although similarities exist between orangeites and lamproites, these and other potassic rocks are sufficiently distinct in their compositions such that different magma formation processes must be considered. In addition to new investigations of the geodynamic triggers of K-rich ultramafic magmatism, future research should more stringently evaluate the relative roles of redox effects and volatile components such as H2O-CO2-F in the petrogeneses of these potentially diamondiferous alkaline rocks.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5440652