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

Abstract

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 40 Ar/ 39 Ar 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.

Base of the Toarcian Stage of the Lower Jurassic defined by the Global Boundary Stratotype Section and Point (GSSP) at the Peniche section (Portugal)

“…The magnetostratigraphy in the Karoo volcanic rocks sampled along the Lebombo volcanic rifted margin (Riley et al, 2004, and references therein), dated to the PLB/TOA, shows a reversed/normal polarity succession characterized by three normal magnetozones. The intermediate magnetozone corresponds to a very thick interval (~4 km) within the Sabie River Basalt Formation.…”

Neo-Tethys geodynamics and mantle convection: from extension to compression in Africa and a conceptual model for obduction

“…3), from the Central Atlantic Magmatic Province (CAMP) at 200 Ma (Marzoli et al 1999), the Karoo event some 183-182 Ma ago (Riley et al 2004;Svensen et al 2012), the Etendeka LIPS between 135 and 130 Ma (Turner et al 1994;Dodd et al 2015), the Madagascar-Agulhas LIPS around 100 Ma within a greater southeast African LIP (Gohl et al 2011), and finally the Afar volcanism ϳ45 to 30 Ma ago (Hofmann et al 1997;Ershov and Nikishin 2004) further north and the subsequent northward migration of intraplate volcanism across the western Arabian plate and eastern Anatolia (Courtillot et al 1999;Faccenna et al 2013b;Gaina et al 2013). The Madagascar-Agulhas LIP, in particular, has been present offshore South Africa from ϳ140 to 95 Ma (Gohl et al 2011).…”

Episodicity of Mesozoic terrane accretion along the Pacific margin of Gondwana: implications for superplume-plate interactions

“…The province erupted largely from ~205 Ma to 199 Ma (Marzoli et al 1999;Nomade et al 2002) with early olivine dolerites at ~209 Ma in the Nova Scotia region South America and the Antarctic Peninsula, which constitutes one of the largest silicic provinces on Earth (Pankhurst et al 2000). Basaltic eruption appears to have been concentrated in a brief period at around 183-182 Ma (Riley & Knight 2001;Riley et al 2004), with rhyolitic magmatism in three pulses over a more extended period between 188 Ma and 153 Ma (Pankhurst et al 2000). Extensive bimodal basaltrhyolite magmatism in the period 186-170 Ma, hosting economically important precious and base-metal mineral deposits, is also recorded from British Columbia (Evenchick & McNicoll 2002;MacDonald et al 1996).…”

“…We propose that these were superplume-driven and we present a unifying mechanism and conceptual model for the interaction between large plates and superplume arrival from depth, focussing on deformation on the Gondwana/Pangaea margin during the Late Triassic-Early Jurassic and deformation on the margin of the palaeo-Pacific basin during the mid-Cretaceous. Both midCretaceous and Late Triassic-Early Jurassic deformation events were temporally associated with a short-term global increase in the rate of ophiolite obduction (Vaughan & Scarrow 2003), and were associated with major, concentrated magmatic episodes (e.g., Larson 1997;Riley et al 2004), changes in geomagnetic field behaviour (Biggin & Thomas 2003), and large-scale environmental perturbation (Hesselbo et al 2002;Larson & Erba 1999). Time and timing is obviously very important, particularly when relating event chronologies determined from the stratigraphic record with those determined radiometrically.…”