2024 April LIP of the Month

Neoarchean lavas of the Ventersdorp Large Igneous Province, South Africa: Sr-Nd-Hf isotopic and trace element evidence for a long-lived plume beneath a stationary African continent

Khulekani B. Khumalo ^a, Lewis D. Ashwal ^a, Ben Hayes ^a, Linda M. Iaccheri ^a, P. Gerhard Meintjes ^b, Susan J. Webb ^a , khumainbk@gmail.com

^a School of Geosciences, University of the Witwatersrand, Private Bag 3, WITS 2050, Johannesburg, South Africa

^b Department of Geology, University of the Free State, Bloemfontein 9300, South Africa

Extracted and modified from: Khumalo, K.B., Ashwal, L.D., Hayes, B., Iaccheri, L.M., Meintjes, P.G. and Webb, S.J., 2024. Neoarchean lavas of the Ventersdorp large Igneous Province, South Africa: Sr-Nd-Hf isotopic and trace element evidence for a long-lived plume beneath a stationary African continent. Earth-Science Reviews, 104752, https://doi.org/10.1016/j.earscirev.2024.104752

Introduction

Large Igneous Provinces (LIPs) such as the Ventersdorp Supergroup represent significant episodes of magmatism, covering vast areas with volumes exceeding 0.1 million cubic kilometres and occurring predominantly within a short geological timeframe - mostly less than 10 million years (Ernst, 2014; Ernst and Youbi, 2017). LIP magmatism has been attributed to mantle plumes with the melting of either the asthenospheric mantle (Crow and Condie, 1988; Hatton, 1995; White, 1997; Eriksson et al., 2002; Campbell, 2005, 2007; Ashwal, 2021; Hayes et al., 2024) or the subcontinental lithospheric mantle (SCLM) (Hawkesworth et al., 1984; Marsh et al., 1992; Jourdan et al., 2009; Humbert et al., 2019), or both, with or without crustal contamination. The Ventersdorp Supergroup, situated on South Africa’s Kaapvaal Craton, is one of the world’s oldest extensive basalt provinces, offering a unique window into Archean continental flood basalts (Figure 1). Despite the abundance of over 600 deep boreholes from gold exploration and mining operations, scientific studies on these resources remain disproportionately sparse. Our understanding of the mantle sources, spatial distribution, and temporal extent of the Ventersdorp Supergroup remains inadequate, highlighting the critical need for further detailed isotopic analysis to enhance the existing data. Prior to this research, isotopic analysis for the Ventersdorp Supergroup was notably deficient. By enriching the current dataset with comprehensive isotopic data, we can facilitate a more rigorous evaluation of the existing models that propose the origins of Ventersdorp magmatism. In this study, we use the new data to constrain the magma sources, petrogenesis and plume longevity, and we discuss the possible effects of crustal contamination during their transport through the crust.

Properties of the Ventersdorp LIP

The Ventersdorp Supergroup is a Neoarchean volcano-sedimentary sequence with an average thickness of nearly 3 km, spreading across about 225,000 km² on the Kaapvaal Craton (Crow and Condie, 1988; Van der Westhuizen et al., 2006; Meintjes and van der Westhuizen, 2018a, 2018b). Its stratigraphy is complex, with the sequence being divided into three groups: the basal Klipriviersberg Group composed of basalts and basal komatiitic lavas; the medial Platberg Group with its diverse volcanic and sedimentary formations; and the upper Pniel Sequence that includes basaltic and andesitic formations (Figure 2). This stratigraphic complexity is mirrored by significant variations in thickness across different formations, influenced by erosion and tectonic activity, making precise estimates of thickness and volume challenging. Nevertheless, the comprehensive drilling data available provide robust minimum estimates for these parameters, highlighting the substantial magmatic output of the Ventersdorp Supergroup.

Secondary Processes

The lavas of the ~2.7 Ga Ventersdorp Supergroup show signs of alteration, possibly resulting from events like the 2.6–2.7 Ga Limpopo orogeny and the 2.0 Ga Kaapvaal Craton-wide metamorphic event (Figure 3; Duane et al., 2004; Van Reenen et al., 2019). These alterations have impacted the isotopic compositions of the lavas, especially the Rb–Sr isotopic system. Hydrothermal fluids, potentially derived from the overlying Transvaal Supergroup, have interacted with the lavas, affecting elements like Rb, Ba, and K, and mobilizing rare earth elements. Despite these secondary processes, the Sm–Nd and Lu–Hf isotopic systems remain robust, providing reliable insights into the mantle sources and magmatic petrogenesis of the Ventersdorp Supergroup.

How Many LIPs?

Recent geochronological studies suggest that the Ventersdorp Supergroup comprises multiple magmatic provinces, rather than a single LIP, due to the significant time spans between formations (Cornell et al., 2017; Gumsley et al, 2020). The new dates reveal that as much as 100 million years may have elapsed between the bottom and top of the Ventersdorp stratigraphic package, which is far too long for a single LIP. The Klipriviersberg Group (2.78 Ga, 0.142 х 10⁶ km³), Platberg Group (2.72 Ga, 0.022 x 10⁶ km³), and Allanridge Formations (2.68 Ga, 0.042 x 10⁶ km³), with varying ages and magmatic volumes, may represent three separate LIPs, consistent with the wide range of ages and the presence of inter-formation unconformities. However, only the Klipriviersberg Group meets both the areal and volumetric criteria to be classified as a LIP, challenging traditional definitions based on short eruption intervals. Alternatively, the three components of the Ventersdorp Supergroup are magmatic products originating from a single long-lived plume. The spatial overlap of these components may have resulted from the African continent having remained stationary over a period of approximately 100 million years, from 2.78 to 2.68 billion years ago. This scenario suggests that the initial, large-volume magmatic outputs of the Klipriviersberg Group were sourced from the hot, central sections of the plume head. The later, lower-volume magmatic products from the Platberg and Allanridge lavas likely originated from the narrower plume tail.

Magmatic Processes

The geochemical signatures of the Ventersdorp lavas suggest that several types of magmatic processes were minor to absent (see Earth Science Reviews 252 (2024) 104752 – Khumalo et al). There is no evidence of fractional crystallization, in as much as expected correlations in Mg# and incompatible elements are absent. Similarly, the data do not show signs of assimilation of continental crust, or combined assimilation-fractional crystallization (AFC) processes having affected the lavas. Additionally, variations in degrees of partial melting of mantle sources do not align with the isotopic data (Figure 4). Instead, the evidence suggests that the Ventersdorp magmatic rocks originate from multiple mantle sources. The variations suggest complex interactions rather than single-process magmatism, pointing towards a scenario varying degrees of source mixing.

Mantle Sources

The geochemical data from Ventersdorp lavas indicate mixing between a depleted mantle source, evidenced by high ε_Nd (+2.12) and high ε_Hf (+3.88) values of Westonaria komatiites and picrites, and potentially a lithospheric source. The isotopic and elemental patterns suggest a dynamic mixing between an asthenospheric or plume component and possibly lithospheric components, challenging the notion of simple mantle source contributions and highlighting complex source heterogeneity. Another alternative is a heterogeneous plume with compositionally distinct materials that occur as domains. This is illustrated in Figure 5, which shows all Ventersdorp units, scaled in terms of volume, and colour-coded as per their mantle sources, either from the central hotter parts of a deep mantle plume, or from subduction-modified mantle in plume margins or tails, or both.

Implications for the Vaalbara Supercraton

Correlations between the Ventersdorp Supergroup and similar geological formations of the Fortescue Group in Australia support the hypothesis of a Neoarchean supercraton, Vaalbara, which suggests a once adjoined landmass comprising the Kaapvaal and Pilbara Cratons (Cheney, 1996; de Kock et al., 2012). However, new Nd isotopic data introduce complexities in these correlations, indicating the need for further research to reveal the precise relationships and histories of these ancient cratonic blocks. The volcanics of the Mt. Roe, Hardley, Bamboo Creek, Kylena and Maddina Formations of the Fortescue Group (Figure 6) have similar isotopic values throughout the whole stratigraphy (Nelson et al., 1992; Mole et al., 2018) and are similar to those of the Platberg and Allanridge units but are significantly lower than those of the Klipriviersberg Group. Komatiitic rocks of the Pyradie Formation (Mole et al., 2018) do resemble Westonaria picrites and komatiites but occur much higher in the Fortescue stratigraphy rather than occupying a basal level, as in the Ventersdorp stratigraphy. These similarities and differences in lithostratigraphy and isotopic compositions may reflect differences in basin architecture and structural evolution.