Controls of post-Gondwana alkaline volcanism in southern Africa

Andrew Moore^1,2, Thomas Blenkinsop³ & F. Cotterill⁴

¹Department of Geology, Rhodes University, Grahamstown 6140, South Africa

²African Queen Mining Ltd., Box 66, Maun Botswana, andy.moore@info.bw

³School of Earth and Environmental Sciences, James Cook University, Townsville, QLD4811, Australia, Thomas.Blenkinsop@jcu.edu.au (corresponding author)

⁴AEON - African Earth Observatory Network, and Department of Geological Sciences,
and Department of Molecular and Cell Biology, University of Cape Town, Rondebosch 7701, South Africa, fenton.cotterill@uct.ac.za

The simplicity and elegance of the original “plume” concept to account for intra-plate volcanism (Wilson, 1963; Morgan, 1972) undoubtedly underpinned an early widespread acceptance of the model. A consequence was that alternatives, such as the Membrane Tectonics concept of Turcotte & Oxburgh (1973), failed to attract significant support, and plumes continue to be invoked as a trigger for volcanic activity, despite increasing criticism of the model. Here we summarize aspects of the timing and distribution of post-Gondwana alkaline volcanism in southern Africa which are not readily reconciled with a mantle plume trigger.

Figure 1 shows emplacement ages of post-Gondwana alkaline volcanic pipes and sub-volcanic pipe clusters in southern Africa. The dataset used excludes radiometric dates for single zircons, as these reflect igneous events in the mantle which may predate the emplacement age by 10 Ma or more, and thus only place an upper limit on the actual liming of volcanic activity (Moore et al., 2008). Collectively, the emplacement ages summarized in Figure 1 show that southern Africa experienced sporadic alkaline volcanism during the Triassic and Jurassic. The beginning of the Cretaceous (145.5 Ma) marked the start of an episode of major kimberlite volcanic activity on the sub-continent that continued into the mid-Cretaceous (~95 Ma), with a pronounced peak at 117 Ma. Most of the kimberlites emplaced during this period belong to the Group II population recognized by Smith (1983). Following a short (~5 Ma) period of quiescence, there was a renewed episode of volcanism in the Upper Cretaceous (~90-70 Ma), dominated by the emplacement of the Group I kimberlite suite of Smith (1983). This Upper Cretaceous volcanic episode was followed by a short (~5-10 Ma) period of relative quiescence (~70 to 60 Ma) and thereafter a minor though distinct early Tertiary period of activity (~60-55 Ma). The latter includes the Bushmanland olivine melilitites and para-kimberlites in the west of South Africa (Figure 2).

The Eocene was characterized by volcanic quiescence on the sub-continent, and was followed by a Late Eocene/Oligocene volcanic event, represented by a small number of pipes from a number of centres in the west of the country, possibly including the Chameis Bay pipes. These episodes of alkaline volcanism in southern Africa can be closely correlated with continent wide volcanic activity in the early Cretaceous (135-110 Ma) Late Cretaceous (90-70 Ma), and Late Palaeogene (~40 Ma; Bailey, 1993).

Figure 1: A summary of volcanic ages, offshore unconformities (data from McMillan, 2003) and Atlantic spreading histories (Nürenberg & Muller, 1991). CNPE = Cretaceous Normal Polarity Episode. Indian Spreading History (from McMillan, 2003 and Reeves & de Wit, 2000): 1 – Initial rifting between Africa and Antarctica; 2 – Commencement of spreading; 3 & 4: Changes in Indian spreading regime recognized by Reeves & de Wit (2000). Atlantic spreading history (from Nürnberg & Muller, 1991; Dingle & Scrutton, 1974): 1- Rifting extends into southern Atlantic ocean; 2 – Commencement of opening of Atlantic (drift sequence) (M4); 3: Estimated time of separation of Falkland plateau and Agulhas bank, based on assumed spreading rates; 4 – Major shift in pole of rotation of African/South American plates; 5 – Beginning of progressive shift in pole of rotation of African/South American plates. Note that Reeves & de Wit (2000) suggested that Atlantic spreading commenced earlier (~136 Ma) than the timing inferred by Nürnberg & Muller (1991) and McMillan (2003) (~127 Ma). From Moore et al. (2008). Click here or on Figure for enlargement.

Figure 2: Distribution, and representative ages of alkaline volcanic pipes in southern Africa. While zircon ages have been included where other data are unavailable, these only provide an upper limit on the timing of emplacement, as discussed in the text. From Moore et al. (2008). Click here or on Figure for enlargement.

The regional distribution of post-Gondwana alkaline pipes across southern Africa is illustrated in Figure 2, together with representative ages. Group I and Group II kimberlites were erupted in the same general areas in the interior of the country, but at different times. Subsequent volcanic events define an overall trend of younging from the interior towards the continental margins of southern Africa. Superimposed on these patterns is a major igneous lineament extending from NE Zambia to the west coast of South Africa, defined by alkaline pipe clusters that become younger towards the southwest (Figure 3). There is also a very systematic trend of decreasing average bulk rock MgO contents and maximum Forsterite content of olivine phenocrysts in volcanic pipe clusters from the interior to the margins of the sub-continent (Moore et al., 2008).

Figure 3: The alkaline volcanic pipe lineament that extends from the west coast of South Africa into the Zambezi and Luangwa rifts of Zimbabwe and Zambia. The dates available for volcanic rocks on the lineament indicate a systematic increase in age to the NE. Note that the zircon ages only provide an upper limit on the emplacement age, as discussed in the text. The inset shows earthquake epicentres from Reeves (1972) and a microearthquake study by Scholz et al. (1976). The Kalahari seismicity axis, approximately coincident with the lineament, was identified by Reeves (1972). Epicentres to the north west of the lineament in Botswana lie in the Okavango Rift Zone (Kinabo et al., 2007). Faults are marked in northern Botswana and the Mid Zambezi valley of Zimbabwe and Zambia from Scholz et al. (1976), Bailleuil (1979), and Kinabo et al. (2007). Ok = Okavango Delta; Mk = Makgadigadi Pans.

Key to pipe clusters: 1. Namaqualand olivine melilitites; 2. Bushmanland olivine melilitites; 3. post-Karoo diatremes; 4. Pofadder kimberlites; 5. Ariemsvlei kimberlites; 6. Noeniputs kimberlite; 7. Rietfontein kimberlite; 8. Southern Botswana kimberlites; 9. Kolongkweneng kimberlites; 10. Tsabong-Molopo kimberlites; 11. Khekhe fissure; 12. Mabuasehube kimberlites; 13. Kokong kimberlites 14. Kikao kimberlites; 15. Khutse kimberlites; 16. Gope kimberlites; 17. Orapa 19: Binga kimberlites and Katete carbonatite; 20. Sengwa kimberlites; 21. lower Luangwa (Kaluwe) carbonatites; 22. Kapamba lamproites; 23. Mushinje kimberlites; 24. Isoka kimberlites. From Moore et al. (2008). Click here or on Figure for enlargement.

Basins on the margins of southern Africa are characterized by episodic sedimentation patterns, with series of major unconformities, reflecting episodes of continental tectonic instability, separated by periods of uninterrupted sedimentation (McMillan, 2003; Figure 1). The initiation of each major set of unconformities can be correlated with plate reorganizations, and shows a systematic relationship to episodes of volcanic activity. Thus, disruption of Gondwana commenced with rifting in the east of Southern Africa at about 156-150 Ma, and deposition of a thick offshore rift sequence (McMillan, 2003). This preceded an Early Cretaceous volcanic episode by some 10 Ma (Figure 1). Ocean opening in the Atlantic and Indian commenced at ~126 Ma, roughly 10 Ma prior to the peak in alkaline activity at 117 Ma. A series of major offshore unconformities mark the early “drift” sequence in the sedimentary sequences in the basins on the continental shelf (Figure 1). An abrupt decline in alkaline volcanism on the sub-continent at ~115 Ma heralded a period or relatively uninterrupted sedimentation in the offshore basins (Figure 1).

Final separation of the Falkland plateau from the Agulhas bank at ~103 Ma initiated a renewed series of unconformities during the period ~100-63 Ma, separated by a short period of continuous sedimentation at ~85-78 Ma. The latter commenced at about the time of a major shift in the pole or rotation of the Atlantic ocean at Chron 34 (~84 Ma). A Mid/Upper Cretaceous to early Tertiary volcanic episode commenced some 10 Ma following the renewed series of depositional breaks in the offshore sedimentary record at ~100 Ma. This period of volcanism is interrupted by a short (5-10 Ma) but distinct period of quiescence which is closely comparable in time to the short episode of uninterrupted sedimentation in the offshore sequences that followed Chron 34. The minor Eocene/Oligocene volcanic episode followed some 5-10 Ma after a change in spreading regimes in the Indian Ocean (Reeves & de Wit, 2000) and an increase in mid-ocean ridge spreading rates in the Atlantic ocean (Nürenberg & Müller, 1991; Figure 1).

Implications for plume triggers

Bailey (1993) pointed out that in order to account for episodic volcanic events “A melt production step seems necessary, but this step must be small, rapidly surmountable, and easily reversible. These requirements rule out melting by extraneous thermal input, or as a consequence of large-scale lithosphere independent mass flows” (i.e. plumes). Hypothetical mantle plume triggers therefore do not readily explain why Group I and Group II kimberlites should have been emplaced in the same general areas at discrete time intervals. The trend of younger volcanic events towards the margins of southern Africa could possibly be ascribed to a channelling of a plume head away from a rising heat source beneath the centre of the continent. However, it would be necessary to explain why magmatism stopped above the primary heat source where temperatures would be expected to be greatest. Furthermore, the plume hypothesis is challenged to explain synchronous alkaline magmatic activity across wide areas of Africa (Bailey, 1992)

The major lineament of alkaline pipes illustrated in Figure 3 is superficially comparable to lines of volcanoes, such as the Hawaiian chain, which have been ascribed to plate motion over a stationary mantle plume. However, while there is a generalized increase in the age of volcanism along the lineament, it is not systematic. There is overlap in the ages of volcanism at different localities (e.g., the closely comparable ages of the Orapa kimberlites in Botswana and Katete carbonatite in Zimbabwe (Figure 3), which are separated by some 400 km). Furthermore, it would be anticipated that the major change in the pole of rotation of the African and South American plates in the upper Cretaceous (Nürnberg & Muller, 1991) would be marked by a clear inflection in the volcanic lineament, which is not observed.

The data summarized in Figure 1 demonstrate that episodes characterized by major breaks in the sedimentary sequences in the basins surrounding southern Africa, reflecting periods of tectonic instability, can be closely linked to changes of spreading regimes at the Atlantic and Indian ocean ridges. Episodes of volcanic activity follow these events by short time intervals (~10 Ma), suggesting initiation by tectonic triggers. Initiation of alkaline volcanism is readily explained in terms of the elegant model proposed by Bailey (1980). He suggests that the decrease in confining pressure related to a tectonic trigger would initiate upward rise of volatile components. This would lower the peridotite solidus, triggering melting, as illustrated in Figure 4. An extension of this model (Figure 4) provides a framework to account for the systematic chemical trends defined by alkaline pipe clusters from the interior to the margins of southern Africa.

Figure 4 Mantle solidus and melting model with shield and off-craton geotherms. Melting is triggered when the mantle solidus is lowered by rising volatiles. The depth of melting in each case is speculative, and dependent on the depth of origin of the volatiles. In shield areas, melting could occur in the depth interval A, if volatiles originate from depths below this depth, and follow the path A-A’. Adiabatic rise of such melts at temperatures above the solidus would not involve crystallization until they intersect the solidus inflection. Once magmas rise above this inflection, further crystallization would be inhibited, and early forming olivine phenocrysts would be partially resorped. At shallow depths (A’), rapid loss of volatiles would initiate a fluidized system, leading to rapid emplacement (of kimberlites) accompanied by mixing of liquid and early crystals (A’-A”). Average bulk rock compositions of such mixtures would approach primitive compositions.

It is assumed that sources of volatiles are shallower in areas of steeper geotherms, with melting taking place at shallower depths (B). Such melts would rise adiabatically, commence crystallization once they intersect the solidus, and thereafter follow a P-T path below solidus temperatures, causing crystallization and magma evolution. The result would be CO₂-saturation of the magma, and separation of an immiscible carbonate liquid (B’), thus preventing fluidization. The Sandkopsdrift carbonatite, which is associated with the Namaqualand olivine melilitites could be explained by this process (Moore & Verwoerd, 1985). Loss of a carbonate, coupled with decreasing pressure (B’-B’’) would cause rapid olivine crystallization (Moore & Erlank, 1979) resulting in the eruption an evolved final magma composition. From Moore et al. (2008).

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