Ashley Gumsley

Timing and tempo of the Great Oxidation Event

Significance We present U-Pb ages for the extensive Ongeluk large igneous province, a large-scale magmatic event that took place near the equator in the Paleoproterozoic Transvaal basin of southern Africa at ca. 2,426 Ma. This magmatism also dates the oldest Paleoproterozoic global glaciation and the onset of significant atmospheric oxygenation. This result forces a significant reinterpretation of the iconic Transvaal basin stratigraphy and implies that the oxygenation involved several oscillations in oxygen levels across 10−5 present atmospheric levels before the irreversible oxygenation of the atmosphere. Data also indicate that the Paleoproterozoic glaciations and oxygenation were ushered in by assembly of a large continental mass, extensive magmatism, and continental migration to near-equatorial latitudes, mirroring a similar chain of events in the Neoproterozoic. The first significant buildup in atmospheric oxygen, the Great Oxidation Event (GOE), began in the early Paleoproterozoic in association with global glaciations and continued until the end of the Lomagundi carbon isotope excursion ca. 2,060 Ma. The exact timing of and relationships among these events are debated because of poor age constraints and contradictory stratigraphic correlations. Here, we show that the first Paleoproterozoic global glaciation and the onset of the GOE occurred between ca. 2,460 and 2,426 Ma, ∼100 My earlier than previously estimated, based on an age of 2,426 ± 3 Ma for Ongeluk Formation magmatism from the Kaapvaal Craton of southern Africa. This age helps define a key paleomagnetic pole that positions the Kaapvaal Craton at equatorial latitudes of 11° ± 6° at this time. Furthermore, the rise of atmospheric oxygen was not monotonic, but was instead characterized by oscillations, which together with climatic instabilities may have continued over the next ∼200 My until ≤2,250–2,240 Ma. Ongeluk Formation volcanism at ca. 2,426 Ma was part of a large igneous province (LIP) and represents a waning stage in the emplacement of several temporally discrete LIPs across a large low-latitude continental landmass. These LIPs played critical, albeit complex, roles in the rise of oxygen and in both initiating and terminating global glaciations. This series of events invites comparison with the Neoproterozoic oxygen increase and Sturtian Snowball Earth glaciation, which accompanied emplacement of LIPs across supercontinent Rodinia, also positioned at low latitude.

New U–Pb geochronologic and palaeomagnetic constraints on the late Palaeoproterozoic Hartley magmatic event: evidence for a potential large igneous province in the Kaapvaal Craton during Kalahari assembly, South Africa

Abstract The volcanic Hartley Formation (part of the Olifantshoek Supergroup, which is dominated by red bed successions) in South Africa recorded depositional and tectonic conditions along the western Kaapvaal Craton during the late Palaeoproterozoic. It formed in association with red bed deposition elsewhere in the cratonic hinterland and along the craton’s northern margin. However, the exact correlation of the Olifantshoek Supergroup with these other red-bed successions is hindered by poor geochronological constraints. Herein, we refine the age and palaeopole of the Hartley Formation, and provide geochronological constraints for large-scale 1.93–1.91 Ga bimodal magmatism on the Kaapvaal Craton (herein named the Hartley large igneous province). We present new age constraints for the mafic and felsic phases of this event at 1923 ± 6 Ma and 1920 ± 4 Ma, respectively, which includes the first reported age dating of the Tsineng Dyke Swarm that has been linked to Hartley volcanism. A mean 1.93–1.91 Ga palaeomagnetic pole for the Hartley large igneous province at 22.7°N, 328.6°E with A95 = 11.7° represents a significant improvement on a previously published virtual geomagnetic pole. This improved pole is used to refine the late Palaeoproterozoic apparent polar wander path of the Kaapvaal Craton. This can assist in correlation of red-bed successions in southern Africa.

U–Pb baddeleyite geochronology and geochemistry of the White Mfolozi Dyke Swarm: unravelling the complexities of 2.70–2.66 Ga dyke swarms across the eastern Kaapvaal Craton, South Africa

Abstract On the south-easternmost Kaapvaal Craton, a NE-trending plagioclase-megacrystic dolerite dyke swarm, herein named the White Mfolozi Dyke Swarm (WMDS), has been identified. New U–Pb baddeleyite ages presented here indicate that the WMDS was emplaced within less than 10 million years, with our three most robust results yielding a weighted mean age of 2662 ± 2 Ma. The WMDS is coeval with the youngest dykes of a 2.70–2.66 Ga radiating dyke swarm already identified further north on the eastern side of the Kaapvaal Craton. This dyke swarm radiates out from the eastern lobe of the ca. 2.05 Ga Bushveld Complex. A clustering of ages from the WMDS and the 2.70–2.66 Ga radiating dyke swarm identify potential magmatic peaks at 2701–2692 Ma, 2686–2683 Ma and 2665–2659 Ma. Geochemical signatures of the dykes do not correlate with these age groups, but are rather unique to specific areas. The northern part of the eastern Kaapvaal Craton hosts relatively differentiated 2.70–2.66 Ga dolerite dykes that could have been derived from a moderately enriched mantle source, whereas the ca. 2.66 Ga WMDS from the southernmost area exhibit much more depleted signatures. In between these two margins, the central area hosts more andesitic 2.70–2.66 Ga dykes that may have assimilated substantial amounts of partly digested tonalite–trondhjemite–granodiorite crust from the basement. We investigate the evolution for the Kaapvaal Craton during a highly magmatic period that extends for over 60 million years from extensive Ventersdorp volcanism to the eruption of proto-basinal volcanic rocks at the base of the Transvaal Supergroup.

Paleomagnetism and U–Pb geochronology of the Black Range dykes, Pilbara Craton, Western Australia : a Neoarchean crossing of the polar circle

ABSTRACT We report a new paleomagnetic pole for the Black Range Dolerite Suite of dykes, Pilbara craton, Western Australia. We replicate previous paleomagnetic results from the Black Range Dyke itself, but find that its magnetic remanence direction lies at the margin of a distribution of nine dyke mean directions. We also report two new minimum ID-TIMS 207Pb/206Pb baddeleyite ages from the swarm, one from the Black Range Dyke itself (>2769 ± 1 Ma) and another from a parallel dyke whose remanence direction lies near the centre of the dataset (>2764 ± 3 Ma). Both ages are slightly younger than a previous combined SHRIMP 207Pb/206Pb baddeleyite weighted mean date from the same swarm, with slight discordance interpreted as being caused by thin metamorphic zircon overgrowths. The updated Black Range suite mean remanence direction (D = 031.5°, I = 78.7°, k = 40, α95 = 8.3°) corresponds to a paleomagnetic pole calculated from the mean of nine virtual geomagnetic poles at 03.8°S, 130.4°E, K = 13 and A95 = 15.0°. The poles reliability is bolstered by a positive inverse baked-contact test on a younger Round Hummock dyke, a tentatively positive phreatomagmatic conglomerate test, and dissimilarity to all younger paleomagnetic poles from the Pilbara region and contiguous portions of Australia. The Black Range pole is distinct from that of the Mt Roe Basalt (or so-called ‘Package 1’ of the Fortescue Group), which had previously been correlated with the Black Range dykes based on regional stratigraphy and imprecise SHRIMP U–Pb ages. We suggest that the Mt Roe Basalt is penecontemporaneous to the Black Range dykes, but with a slight age difference resolvable by paleomagnetic directions through a time of rapid drift of the Pilbara craton across the Neoarchean polar circle.

The Timing of the Palaeoproterozoic Great Oxidation Event using Dykes, Sills and Bolcanics of the Ongeluk Large Igneous Province, Kaapvaal Craton

The timing of the Palaeoproterozoic Great Oxidation Event using Dykes, Sills and Volcanics of the Ongeluk Large Igneous Province, Kaapvaal Craton

Constraining the Timing of the Molopo Farms Complex Emplacement and Provenance of Its Country Rock

layered, mafic-ultramafic intrusion straddling the southern border of Botswana with South Africa. It does not outcrop due to Cenozoic cover, but is believed to intrude the sedimentary succession of the Neoarchean to Paleoproterozoic Transvaal Supergroup. This is based on numerous intersections in exploration drillcore. The emplacement of the MFC is currently poorly constrained by an unpublished Rb-Sr date of 2044 ± 24 Ma. It is however, widely considered to be related to the Bushveld Large Igneous Province or LIP. Here the U-Pb zircon provenance of sedimentary country rock to the MFC as well as U-Pb baddeleyite geochronology of the maficultramafic rocks of the MFC are reported. Samples for this study originate from eight boreholes from both the southern and northern limbs of the MFC. The youngest concordant U-Pb ages of detrital zircon grain populations within 6 analysed samples are dominated by ages between 2018 ± 39 Ma and 2276 ± 19 Ma. These do not allow for conclusively distinguishing between characteristic provenance of either the Olifantshoek Group or the Transvaal Supergroup. U-Pb ID-TIMS dating of four fractions of baddeleyite yield a free regression upper intercept age of 2052±16 Ma, with a negative lower intercept and relatively high MSWD of 4. However, one of the analyses resulted in a younger Pb/Pb date compared to other fractions. A weighted mean of all the fractions is 2052±4 Ma, while the weighted mean of the three oldest fractions is 2055±3 Ma, which illustrates this problem. Rejecting the youngest analysis gives a free regression upper intercept date of 2060±6 Ma. The lower intercept points toward discordance associated with the Karoo LIP, and lowers the MSWD to 0.36. Upon forcing regression through 180 Ma, an upper intercept date of 2057±3 Ma, with a MSWD of 0.38, is achieved. This is interpreted as representing the emplacement age for the MFC, and is within error of the age of the Bushveld Complex (i.e., 2054.4 ± 1.3 Ma). Given the baddeleyite constraints it becomes clear that the sedimentary country rock of the MFC represents the Transvaal Supergroup. What remains unresolved is the possible presence of younger mafic intrusions (e.g., dolerite sills) in the area that are currently wrongfully identified as MFC – as suggested by preliminary geochemical data. Some of these intrusions may likely intrude into sedimentary successions younger than the Transvaal Supergroup, which would explain the presence of younger U-Pb detrital zircon age populations in some of our samples. Michiel O. de Kock, Livhuwani Ravhura, Clarisa Vorster, Nicolas J. Beukes, and Ashley P. Gumsley, 2016. Constraining the Timing of the Molopo Farms Complex Emplacement and Provenance of Its Country Rock. Acta Geologica Sinica (English Edition), 90 (supp.1): 78.