Martin B. Klausen

U–Pb zircon (SHRIMP) ages for the Lebombo rhyolites, South Africa: refining the duration of Karoo volcanism

U–Pb SHRIMP ages are reported for three rhyolite flows from the Lebombo rift region of the Karoo volcanic province. Two flows are interbedded with the Sabie River Basalt Formation and a third sample is from the overlying rhyolitic Jozini Formation. The interbedded rhyolites yield ages of 182.0 ± 2.1 and 179.9 ± 1.8 Ma, whilst the overlying Jozini Formation rhyolite yields an age of 182.1 ± 2.9 Ma. Combined with existing 40Ar/39Ar geochronology, the new SHRIMP data fine-tunes the chronology of the Karoo volcanic province and indicates the 12 km succession of volcanic rocks in the Lebombo rift were erupted in 1–2 million years and lends considerable support to the links between the Pleinsbachian–Toarcian extinction event and the global environmental impact of Karoo volcanism.

Baddeleyite U–Pb ages and gechemistry of the 1875–1835 Ma Black Hills Dyke Swarm across north-eastern South Africa: part of a trans-Kalahari Craton back-arc setting?

Abstract Eleven new baddeleyite U–Pb crystallisation ages and associated whole-rock geochemistry on NE–NNE-trending tholeiitic dykes cutting across the north-eastern corner of the Archaean Kaapvaal Craton, the overlying Transvaal basin and the Bushveld and Phalaborwa igneous complexes collectively define a 1875–1835 Ma Black Hills Dyke Swarm (BHDS). Dyke ages do not discriminate between dyke trends or geographic location, but subdivide the BHDS into an older set of four more primitive dykes (MgO = 9.4–6.8 wt.%) and a younger set of seven dykes with more differentiated compositions (MgO = 5.6–4.2 wt.%). Despite being emplaced over a c. 40 Myr period, major element compositions are remarkably consistent with a single inversely modelled bulk fractionating assemblage of 57.5% plagioclase, 29.5% augite and 13.0% olivine. This fractionating assemblage requires an additional assimilation of bulk continental crust (at a low r-value of 0.2) for reversed modelling of parental rare earth elements. Even though this crustal assimilation indicates that primary magmas could potentially have been derived from a spinel-bearing ambient primordial and asthenospheric mantle source, anomalously low Nb and high Pb values for the more primitive older dykes may also have been inherited from a sub-continental lithospheric mantle source. The ages for the BHDS bridge a gap between c. 1889 and 1867 Ma mafic sills and c. 1830 Ma rhyodacitic pyroclasts, interbedded in the top of a ~3 km-thick Sibasa basalt sequence, which combine into a continuous c. 1.89–1.83 Ga igneous province. Similar geochemical signatures are consistent with all sills, volcanic rocks and BHDS feeders collectively belonging to a very voluminous and coherent igneous province, which arguably formed behind active Magondi and Okwa-Kheis arcs, along the western margin of the proto-Kalahari Craton.

New U–Pb age and paleomagnetic constraints from the Uitkomst Complex, South Africa: clues to the timing of intrusion

Abstract The Uitkomst Complex is considered to be coeval with, and genetically linked to the Rustenburg Layered Suite (RLS) of the Bushveld Large Igneous Province. This study reports new paleomagnetic, geochemical and geochronological results from the Uitkomst Complex and a crosscutting dyke at the Nkomati Mine. Primary magnetisations for the complex and the dyke are statistically indistinguishable. This, together with the geochemical signature of the dyke, suggests a late-magmatic link with the Uitkomst Complex. A Virtual Geomagnetic Pole calculated for the complex at Longitude 28.7°N and Latitude 58.5°E (dp = 6.2; dm = 9.4; N = 3) differs from the 1900 Ma and younger-aged poles for the Kaapvaal Craton. It, however, shares similarities with the poles from the 2058 to 2054 Ma RLS and other ~2.0 Ga paleopoles for the Kaapvaal Craton. Moreover, a new U–Pb baddeleyite age of 2054 ± 7 Ma given by a coarse-grained gabbroic sample from Nkomati’s underground mine provides a minimum age constraint on the crystallisation of the Uitkomst Complex. This date is nominally older than the 207Pb/206Pb zircon age of 2044 ± 8 Ma previously reported for this complex, and also near identical to the new age of 2054.89 ± 0.37 Ma from the Critical Zone of the RLS. Here, it is suggested that the Uitkomst Complex was emplaced at the same time as, if not before the Critical Zone. Data presented here have bearings on the location of the Uitkomst Complex in the time frame model recently proposed for the Bushveld magmatism.

Paleomagnetism and chronology of B-1 marginal sills of the Bushveld Complex from the eastern Kaapvaal Craton, South Africa

Abstract The Rustenburg Layered Suite (RLS) of the Bushveld Complex in South Africa is the largest mafic–ultramafic-layered complex on Earth. The RLS is associated with marginal sills that penetrate into the ~2.2 billion-year-old sedimentary strata of the Pretoria Group. These sills are in contact and share some geochemical similarities with different zones of RLS and are classified in terms of chemical composition, which suggests their derivation from distinct parental magma compositions (so-called B-1, B-2 and B-3 parental magmas). Existing paleomagnetic constraints for the Bushveld Complex originate from the upper Critical to Upper zones of the RLS, which are associated with B-2 and B-3 marginal sills. Geochemically, verified B-1 marginal intrusions are here used as a proxy for constraining the paleomagnetism and chronology of the Lower and lower Critical zones of the RLS. We identified a dual-polarity magnetic component with a paleopole (Latitude = 13.1°N, Longitude = 44.0°E, A95 = 14.3, N = 7) that is very similar to the established Bushveld Complex poles. We further report 2058.4 ± 1.3 Ma and 2058.1 ± 6 Ma U–Pb baddeleyite ages from B-1 sills that record opposite magnetic polarities. The ca. 2058 Ma ages are older than the 2054.89 ± 0.37 Ma age recently reported from throughout the RLS, but near identical to a previously reported ages of the Marginal Zone and from the upper Critical Zone. The ages could be interpreted as distinct pulses of magma emplacement separated in time by up to 4 million years (i.e., B-1 type magma pulse around ca. 2058 Ma and the B-2 and B-3 types magma pulses following closely on each other around ca. 2054 Ma), but is unlikely when petrological models are considered.

Mesoproterozoic dykes in the Timmiarmiit area, Southeast Greenland: evidence for a continuous Gardar dyke swarm across Greenland’s North Atlantic Craton

Abstract During the Proterozoic, several mafic dykes with variable trends and mineralogies intruded the Archaean basement of Southeast Greenland. Some of the younger ENE-trending dykes are interpreted to represent a prolongation of the Mesoproterozoic Gardar Province, and have been termed Timmiarmiit dykes. Extrapolations of their trends across the inland ice sheet coincide with the northernmost so-called brown dykes (BD’s) which are part of the Gardar Province. Baddeleyite U–Pb ID-TIMS analyses for three ENE-trending Timmiarmiit dykes give ages of 1277 ± 4, 1275 ± 3 and 1268 ± 4 Ma, which are slightly younger than the oldest (BD0 = 1284–1279 ± 3 Ma, Upton 2013) and only dated generation of dykes in the Gardar Province, and thereby indirectly provide a possible age for the two younger dyke generations (BD1 and BD2). The Timmiarmiit–Gardar correlation is strengthened by a rigorous multivariate statistical analysis, on the basis of all major and trace elements. Thus, a coherent ENE-trending trans-Greenlandic dyke swarm is constituted. The major and trace element data of the Timmiarmiit dykes show that they crystallised from comparably evolved mantle-derived magma with minor crustal contamination and indicate a strong contribution of a metasomatised subcontinental lithospheric mantle component in the evolution of melts. This component was probably influenced by supra-subduction zone metasomatism during the Palaeoproterozoic Ketilidian orogeny. The data presented here, in addition to recent plate reconstruction models, give new evidence for a petrogenetic link between rift-related Mesoproterozoic magmatism in North America, South Greenland and Central Scandinavia which possibly formed in response to back-arc basin formation.

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.

New advances in using large igneous provinces (LIPs) to reconstruct ancient supercontinents

1 Department of Geology, Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden; Ulf.Soderlund@geol.lu.se 2 Department of Geosciences, Swedish Museum of Natural History, Box 50 007, SE-104 05 Stockholm, Sweden 3 Department of Earth Sciences, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa; klausen@sun.ac.za 4 Department of Earth Sciences, Carleton University, Ottawa, Ontario, K1S 5B6 Canada 5 Faculty of Geology and Geography, Tomsk State University, 36 Lenin Ave, Tomsk, 634050, Russia; Richard.Ernst@ ErnstGeosciences.com 6 Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, K1A 0E8, Canada; wouter.bleeker@canada.ca Article

A centennial reappraisal of the Vredefort pseudotachylytes: shaken, not stirred by meteorite impact

Major pseudotachylyte zones constitute a spectacular component of the renowned c. 2.023 Ga Vredefort impact structure, South Africa, but it has always been difficult to explain how they were formed. In his original account, in 1916, Shand interpreted the pseudotachylyte as due to cataclasis and frictional heating but pointed out two enigmas that have remained since: there were no associated major faults, and the pseudotachylyte volumes he observed were far greater than in similar rocks located within faults elsewhere on Earth. New observations show that the Vredefort pseudotachylyte zones were indeed formed by cataclasis and frictional heating, not by faulting but owing to impact-induced seismic shaking initiated around temporarily loosened blocks in dendritic fracture systems. Progressive cataclasis of such loose blocks by intense, high-frequency oscillations of the country rock at the beginning of the cratering process led to size reduction, rounding and comminution, and frictional melting of feldspars and biotite in the comminuted parts. Most pseudotachylyte was thus not injected from anywhere but produced in situ. The process of seismic shaking is well known from impacts on the Moon and asteroids, terrestrial earthquakes and nuclear tests but has largely been overlooked in terrestrial cratering, except in the theoretical concept of acoustic fluidization.