Search

The mid-low-temperature thermal history of layered mafic intrusions is crucial to understanding the duration of hydrothermal systems and the formation of economically viable ore deposits within the large thermal anomaly of these magma bodies. Due to a lack of low-temperature chronometry, the mid-low-temperature thermal evolution of the world's largest layered intrusion, the Bushveld Complex, remains relatively unknown. We provide a comprehensive plagioclase 40Ar/39Ar thermochronologic study throughout the Rustenburg Layered Suite (RLS) of the Bushveld Complex in South Africa. 40Ar/39Ar step-heating analyses were applied to 20 plagioclase grains that encompass 12 stratigraphic levels and the entire ~ 7 km thickness of the Eastern Limb of the RLS. Individual plagioclase grains yield 40Ar/39Ar plateau ages ranging from 1997.9 ± 5.0 Ma to 2051.4 ± 5.3 Ma, and closure temperatures of 151 ± 15 °C to 310 ± 12 °C (assuming a 10 °C Ma−1 cooling rate). Plagioclase 40Ar/39Ar ages and closure temperatures, combined with 40Ar/39Ar biotite ages from the same samples, suggest that RLS cooling rates decrease abruptly (from ~ 1000 °C Ma−1 to ~ 10 °C Ma−1) near the 40Ar/39Ar closure temperature of biotite. The decrease in cooling rate is a product of the cessation of hydrothermal circulation (within < 1 Ma of zircon crystallization) associated with the emplacement of the RLS and is supported by numerical heat-loss simulations that produce the observed geochronology. The duration of hydrothermal circulation places age constraints on hydrothermal ore deposits within the host rocks (Transvaal and Witwatersrand Supergroups), as any ore deposits related to emplacement of the RLS must be coeval with 40Ar/39Ar biotite ages. [ABSTRACT FROM AUTHOR]

The Kuruman Kimberlite Province contains the oldest known kimberlites in the Kaapvaal Craton (∼1·6–1·8 Ga), but only limited geochronological and isotopic data exist for this province. To provide better constraints on the sources of the kimberlite magmas and the timing of magmatism, we have carried out in situ U–Pb dating and Sr–Nd isotopic analysis of groundmass perovskite from four of the Kuruman kimberlites (Bathlaros, Elston, Helpmekaar and Zero). We also have measured the Sr-isotope composition of groundmass apatite and carbonate in several of the kimberlites, for comparison with published whole-rock isotopic data. Attempts to date perovskite using in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were hindered by extensive Pb loss, even in apparently unaltered grains, yielding anomalously young emplacement ages. The assumption of a Mesoproterozoic common-Pb component allows the extraction of intercept ages from the least altered grains (Bathlaros, 1640 ± 13 Ma; Elston, 1663 ± 49 Ma; Helpmekaar, 1638 Ma; Zero, 1845 ± 45 Ma). The smaller sampling volume of secondary ion mass spectrometry and the ability to measure 204Pb, thus allowing more precise common-Pb corrections, produces concordant ages. These reveal that kimberlite intrusion occurred over a time span of ∼200 Myr (Bathlaros, 1647 ± 27 Ma; Zero, 1835 ± 25 Ma), with emplacement ages younging to the east. The results of this study suggest that alteration may disrupt the U–Pb system in perovskite. In situ Sr and Nd isotope analyses [LA-multi-collector (MC)-ICP-MS] of perovskite show a moderate range in values (87Sr/86Sr(i) = 0·70383–0·70507; &varepsilon;Nd(i) = –2·7 to 1·5), but similar mean values among the four kimberlites (87Sr/86Sr(i) = 0·70396–0·70425; &varepsilon;Nd(i) = –0·8 to 0·8). The Nd-isotope compositions of perovskite are similar to those of whole-rock samples of the host kimberlite (&varepsilon;Nd(i) = 1·0 to −3·0), whereas Sr in perovskite and apatite (87Sr/86Sr(i) = 0·70424–0·70427) is more radiogenic than the whole-rock values (87Sr/86Sr(i) = 0·70293–0·70382). However, groundmass carbonates generally yield lower values (87Sr/86Sr(i) = 0·70348–0·70405), closer to those of the kimberlite samples, indicating that the whole-rock Sr isotope compositions are largely controlled by carbonates, probably assimilated from the immediate country rocks. The isotopic data suggest that the Kuruman kimberlite magmas were initially derived from a deeper sub-lithospheric source region, but their distinctive compositional characteristics were imposed through assimilation, first of metasomatized sub-continental lithospheric mantle and later of crustal material, during magma ascent. [ABSTRACT FROM PUBLISHER]

Integrated electron nanobeam (EBSD, CL, EDS) and isotopic measurements (U-Pb, (U-Th)/He) of zircon from the collar and centre of the 80 km wide central uplift of the 2020 ±3 Ma Vredefort impact structure reveal new shock features in a microstructural progression related to impact basin formation and degree of U-Pb age resetting: (1) planar fractures in {1K0} and {1K2} orientation during initial shock wave compression; (2) curviplanar fractures in {1K1} orientation, now annealed, which host glassy inclusions of partial melt of the host rock; (3) microtwin lamellae in an orientation of 65° about [110], attributed to shock wave rarefaction; (4) nucleation of impact-age crystallites, possibly on microtwins, during post-shock heating by impact melt; and (5) crystal-plastic deformation linked to crater modification of the core of the central uplift. Planar fracturing and microtwinning ≥20 GPa in 'cold shock' zircon in granitoid at a radial distance of 25 km failed to reset zircon age. Single-grain ID-TIMS data extend between pre-impact age of 2077 ±11 Ma and a secondary Pb-loss event at ca. 1.0 Ga - the latter reflecting Kibaran igneous activity between 1.110 and 1.021 Ga. Age resetting by the impact event operated in an ~15 km wide 'hot shock' zone of impact-elevated temperatures ≥700 °C at the core of the central uplift. Mechanisms include internal recrystallization, defect-accelerated Pb diffusion via shock microstructures and melt films, and late crystal-plastic deformation. Igneous zircons from a 2019 ±2 Ma foliated norite impact melt yield a mean (U-Th)/He date of 923 ±61 Ma, indicating exposure of the present surface after this time. Des mesures intégrées de faisceaux électroniques à l'échelle nano (EBSD, CL et EDS) et des mesures (isotopes de U-Pb, (U-Th)/He), sur des zircons provenant du col et du centre du soulèvement central de 80 km de large de la structure d'impact de Vredefort 2020 ±3 Ma, révèlent de nouvelles caractéristiques de choc dans une progression microstructurale reliée à la formation du bassin sous l'impact ainsi qu'au niveau du réajustement de l'âge déterminé par U-Pb : (1) des fractures planaires dans l'orientation {1K0} et {1K2} durant la compression par l'onde de choc initial; (2) des fractures curviplanaires dans l'orientation {1K1}, maintenant recuites, et qui contiennent des inclusions vitreuses de fusion partielle de la roche encaissante; (3) des lamelles de micro-macles selon une orientation de 65 ° environ à [110] attribuées à la décompression de l'onde de choc; (4) la nucléation des cristallites datant de l'impact, possiblement sur des micro-macles, durant l'échauffement après le choc par fusion d'impact et (5) la déformation plastique des cristaux reliée à la modification du cratère du noyau du soulèvement central. La fracturation planaire et le micro-maclage ≥20 GPa dans un zircon de « choc thermique froid » dans un granitoïde à une distance radiale de 25 km n'a pas permis de réajuster l'âge des zircons. Des données ID-TIMS sur des grains uniques se répartissent entre un âge pré-impact de 2077 ±11 Ma et un événement secondaire de perte de Pb vers 1,0 Ga - ce dernier étant le reflet de l'activité ignée de Kibaran entre 1,110 et 1,021 Ga. Le réajustement de l'âge par l'événement d'impact s'est effectué dans une zone d'environ 15 km de large de « choc chaud » de températures élevées causées par l'impact, soit ≥ 700  °C, au cœur du soulèvement central. Les mécanismes comprennent une recristallisation interne, la diffusion, accélérée par des irrégularités, du Pb à travers les microstructures de choc et des films de fusion et la déformation plastique tardive des cristaux. Des zircons ignés provenant de la fusion par impact d'une norite feuilletée datant de 2019 ±2 Ma ont donné une date (U-Th)/He moyenne de 923 ±61 Ma, indiquant une exposition de la présente surface après ce temps. [ABSTRACT FROM AUTHOR]

Abstract: Sterkfontein Caves is the single richest early hominin site in the world with deposits yielding one or more species of Australopithecus and possible early Homo, as well as an extensive faunal collection. The inability to date the southern African cave sites accurately or precisely has hindered attempts to integrate the hominin fossil evidence into pan-African scenarios about human evolutionary history, and especially hominin biogeography. We have used U-Pb and U-Th techniques to date sheets of calcium carbonate flowstone inter-bedded between the fossiliferous sediments. For the first time, absolute age ranges can be assigned to the fossil-bearing deposits: Member 2 is between 2.8 ± 0.28 and 2.6 ± 0.30 Ma and Member 4 between 2.65 ± 0.30 and 2.01 ± 0.05 Ma. The age of 2.01 ± 0.05 Ma for the top of Member 4 constrains the last appearance of Australopithecus africanus to 2 Ma. In the Silberberg Grotto we have reproduced the U-Pb age of ∼2.2 Ma of for the flowstones associated with StW573. We believe that these deposits, including the fossil and the flowstones, accumulated rapidly around 2.2 Ma. The stratigraphy of the site is complex as sediments are exposed both in the underground chambers and at surface. We present a new interpretation of the stratigraphy based on surface mapping, boreholes logs and U-Pb ages. Every effort was made to retain the Member system, however, only Members 2 and 4 are recognized in the boreholes. We propose that the deposits formally known as Member 3 are in fact the distal equivalents of Member 4. The sediments of Members 2 and 4 consisted of cone-like deposits and probably never filled up the cave. The U-Th ages show that there are substantial deposits younger than 400 ka in the underground cave, underlying the older deposits, highlighting again that these cave fills are not simple layer-cakes. [Copyright &y& Elsevier]

We describe the geological, geochronological, geomorphological, and faunal context of the Malapa site and the fossils of Australopithecus sediba. The hominins occur with a macrofauna assemblage that existed in Africa between 2.36 and 1.50 million years ago (Ma). The fossils are encased in water-laid, clastic sediments that were deposited along the lower parts of what is now a deeply eroded cave system, immediately above a flowstone layer with a U-Pb date of 2.026 ± 0.021 Ma. The flowstone has a reversed paleomagnetic signature and the overlying hominin-bearing sediments are of normal polarity, indicating deposition during the 1.95- to 1.78-Ma Olduvai Subchron. The two hominin specimens were buried together in a single debris flow that lithified soon after deposition in a phreatic environment inaccessible to scavengers. [ABSTRACT FROM AUTHOR]

Abstract: Australopithecus robustus is one of the best represented hominin taxa in Africa, with hundreds of specimens recovered from six fossil localities in the Bloubank Valley area of Gauteng Province, South Africa. However, precise geochronological ages are presently lacking for these fossil cave infills. In this paper, we provide a detailed geological background to a series of hominin fossils retrieved from the newly investigated deposit of Cooper''s D (located partway between Sterkfontein and Kromdraai in the Bloubank Valley), including uranium-lead (U-Pb) ages for speleothem material associated with A. robustus. U-Pb dating of a basal speleothem underlying the entire deposit results in a maximum age of 1.526 (±0.088) Ma for Cooper''s D. A second U-Pb date of ca. 1.4Ma is produced from a flowstone layer above this basal speleothem; since this upper flowstone is not a capping flowstone, and fossiliferous sediments are preserved above this layer, some of the hominins might be slightly younger than the calculated age. As a result, we can broadly constrain the age of the hominins from Cooper''s D to between 1.5 and approximately 1.4Ma. Extinct fauna recorded in this comparatively young deposit raise the possibility that the Bloubank Valley region of South Africa represented a more stable environmental refugium for taxa relative to tectonically more active East Africa. The sediments of the deposit likely infilled rapidly during periods when arid conditions prevailed in the paleoenvironment, although it is unclear whether sediment deposition and bone deposition were necessarily contemporaneous occurrences. We reconstruct the paleoenvironment of Cooper''s D as predominantly grassland, with nearby woodlands and a permanent water source. The hominin teeth recovered from Cooper''s D are all from juveniles and can be confidently assigned to A. robustus. In addition, two juvenile mandibular fragments and an adult thoracic vertebra are tentatively attributed to A. robustus. [Copyright &y& Elsevier]

Investigates the uranium-lead zircon ages and geochemical evidence for the beginning of Neoproterozoic continental breakup in the Richtersveld Igneous Complex in South Africa. Geology and previous geochronology of the Richtersveld igneous complex; Determination of the crystallization age for the Klipbokkop rhyolite.

The Early Pleistocene site of Swartkrans in South Africa's Cradle of Humankind World Heritage Site has been significant for our understanding of the evolution of both early Homo and Paranthropus , as well as the earliest archaeology of southern Africa. Previous attempts to improve a faunal age estimate of the earliest deposit, Member 1, had produced results obtained with uranium-lead dating (U–Pb) on flowstones and cosmogenic burial dating of quartz, which placed the entire member in the range of >1.7/1.8 Ma and <2.3 Ma. In 2014, two simple burial dates for the Lower Bank, the earliest unit within Member 1, narrowed its age to between ca. 1.8 Ma and 2.2 Ma. A new dating program using the isochron method for burial dating has established an absolute age of 2.22 ± 0.09 Ma for a large portion of the Lower Bank, which can now be identified as containing the earliest Oldowan stone tools and fossils of Paranthropus robustus in South Africa. This date agrees within one sigma with the U–Pb age of 2.25 ± 0.08 Ma previously published for the flowstone underlying the Lower Bank and confirms a relatively rapid rate of accumulation for a large portion of the talus. [ABSTRACT FROM AUTHOR]

Newly exposed cave sediments at the Malapa site include a flowstone layer capping the sedimentary unit containing the Australopithecus sediba fossils. Uranium-lead dating of the flowstone, combined with paleomagnetic and stratigraphic analysis of the flowstone and underlying sediments, provides a tightly constrained date of 1.977 ± 0.002 million years ago (Ma) for these fossils. This refined dating suggests that Au. sediba from Malapa predates the earliest uncontested evidence for Homo in Africa. [ABSTRACT FROM AUTHOR]