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 GSA Today, v. 10, no. 2, February 2000

  February Table of Contents (including full issue in pdf format)


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Continental Growth, Preservation, and Modification in Southern Africa

R. W. Carlson, F. R. Boyd, S. B. Shirey, P. E. Janney, Carnegie Institution of Washington, 5241 Broad Branch Road, N.W., Washington, D.C. 20015, USA
T. L. Grove, S. A. Bowring, M. D. Schmitz, J. C. Dann, Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
D. R. Bell, J. J. Gurney, S. H. Richardson, M. Tredoux, A. H. Menzies, Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa
D. G. Pearson, Department of Geological Sciences, Durham University, South Road, Durham, DH1 3LE, UK
R. J. Hart, Schonland Research Center, University of Witwatersrand, P.O. Box 3, Wits 2050, South Africa
A. H. Wilson, Department of Geology, University of Natal, Durban, South Africa
D. Moser, Geology and Geophysics Department, University of Utah, Salt Lake City, UT 84112-0111, USA


To understand the origin, modification, and preservation of continents on Earth, a multidisciplinary study is examining the crust and upper mantle of southern Africa. Xenoliths of the mantle brought to the surface by kimberlites show that the mantle beneath the Archean Kaapvaal craton is mostly melt-depleted peridotite with melt extraction accompanying crust formation in the Archean. Eclogitic xenoliths from the craton record subduction of altered oceanic crust beneath the craton at ca. 3 Ga. Proterozoic age peridotite found beneath the surrounding Proterozoic accretionary belts provides evidence for crust-mantle coupling and long-term stability of the upper 150 km of the lithosphere. Petrologic examination of Archean ultramafic magmas (komatiites) from South Africa indicates that some komatiitic magmas contain substantial quantities of water (>4 wt%). This finding strengthens the possibility that the cratonic lithosphere formed initially in a subduction zone setting, the demise of which led to accretion of the arc crust and thickening of the lithospheric mantle to create a stable, thick, continental lithosphere. Geochronologic studies of lower crustal xenoliths from the craton show a prolonged thermal evolution of the lower crust extending to 1 Ga. This thermal evolution is also reflected in ca. 1 Ga ages of some eclogitic diamond inclusions from the lithospheric mantle.


To explore the causes of continent formation and preservation, a multidisciplinary study involving geology, geochemistry, geochronology, petrology, and seismology was initiated 3 years ago with support from the National Science Foundation Continental Dynamics Program. The study focuses on the current structure and geologic history of southern Africa. A general description of the project and list of participants can be found at The centerpiece of the study is an array of 55 portable broadband seismometers placed in 82 sites along a rectangular array from Cape Town to Zimbabwe (Fig. 1; Carlson et al., 1996; James et al., 1999). This report summarizes early results from the geochemical, geochronologic, and petrologic components of the Kaapvaal Craton Project.

Southern Africa provides an excellent field laboratory to study the history of ancient continents. The region (Fig. 1) comprises the Archean Kaapvaal and Zimbabwe cratons, separated by a late Archean metamorphic terrane, the Limpopo Belt (Tankard et al., 1982). Overall, the Kaapvaal craton is made up of a number of granite-greenstone terranes with distinctive igneous rocks, deformation histories, and tectonic styles that were welded together to form the core of the continent (de Wit et al., 1992). Surrounding the cratons are accretionary belts added in the middle to late Proterozoic. Of particular interest, the southern African lithosphere has been penetrated by hundreds of kimberlite diatremes (Fig. 1) that have brought xenoliths of lower crust and upper mantle to the surface (Gurney et al., 1991).


Whereas the geology of the shallow crust of the Kaapvaal craton is relatively well known, the deeper crust is not well exposed. One exception is the large circular Vredefort structure in the middle of the Kaapvaal craton (Fig. 2). It is widely regarded as a deeply eroded remnant of a 2.02 Ga impact crater (Kamo et al., 1996; Moser, 1997). Hart et al. (1990) suggested that the impact turned the crust on its side so that traversing from rim to center leads one from near-surface sediments through mid-crustal granite to granulite grade supracrustal rocks. At the center of the impact structure is a large positive gravity anomaly (Fig. 2) that has been drilled and found to consist predominantly of peridotite. Re-Os systematics of this peridotite are similar to those of the kimberlite-borne xenoliths from the Kaapvaal lithospheric mantle that have very low Re/Os and 187Os/188Os ratios, and Re-depletion model ages of 3.3 to 3.5 Ga (Tredoux et al., 1999). These data suggest that the deep crust-mantle transition is exposed in the Vredefort section.

Besides abundant mantle xenoliths, many southern African kimberlites contain excellent suites of lower crustal xenoliths. High-resolution U-Pb accessory-mineral geochronology of these lower crustal xenoliths furthers our understanding of the interrelationships between the surficial geologic record and nascent mantle geochronology. Sapphirine granulite xenoliths in the Lace, Voorspoed, and Star kimberlites, a southwest-trending alignment of kimberlites between the Vredefort structure and the city of Bloemfontein in the central craton, preserve evidence for a dramatic transient thermal pulse in the deep crust of the Kaapvaal craton. Thermobarometry of these garnet-quartz-sapphirine assemblages indicate extreme peak temperatures of <1100 °C at pressures from 1.0 to 1.5 GPa (Dawson et al., 1997). Zircon and monazite from these xenoliths give identical U-Pb dates of 2723 Ma, which are interpreted as dating early cooling and metamorphic zircon growth from the ultrahigh temperature metamorphism (Schmitz et al., 1998). This 2723 Ma episode of ultrahigh temperature metamorphism in the intracratonic lower crust appears to be synchronous with the initiation of Ventersdorp flood basalt volcanism (Armstrong et al., 1991).

In contrast to the Late Archean ages of central craton granulite xenoliths, the abundant garnet-bearing granulite and upper amphibolite facies xenoliths from the Markt kimberlite, at the southwestern edge of the craton, yield Mesoproterozoic metamorphic zircon U-Pb dates ranging from 1114 to 1092 Ma (Schmitz and Bowring, 1999). Similarly, metamorphic zircon and monazite in felsic to mafic granulite xenoliths from the northern Lesotho kimberlites, along the southern edge of the craton, have been dated at 1050-1000 Ma (Schmitz and Bowring, 2000). The new geochronology confirms that the lower parts of the thickened crust along the southern and eastern edge of the craton were modified in the Mesoproterozoic and indicates that the cratonic crust experienced a dynamic metamorphic history that significantly postdates the ostensible time of cratonization around 3.0 Ga.


Archean greenstone belts contain komatiite, an igneous rock that has an unusually high MgO content (22%–25%) compared to any volcanic rock observed today. Detailed mapping, geochemical, and petrographic study of komatiites in their type locality in the Komati formation in the Barberton Mountains, South Africa, provides several new clues to the origin and possible tectonic setting of formation of this magma type (Grove et al., 1996b). Some of the southern African komatiites retain part of their original igneous mineralogy (Fig. 3). The freshest Barberton komatiites have igneous olivine and/or pyroxene whose compositions are consistent with these komatiites preserving magmatic compositions (Parman et al., 1997). In addition, the mapping effort has led to the suggestion that some of the Komati units represent sills rather than flows (Grove et al., 1996b). In some of these sills, the composition of preserved igneous pyroxenes (Fig. 3) indicates that the magmas contained over 4 wt% water (Parman et al., 1997).

Wet primary komatiite magma is further supported by the appearance of spinifex crystallization textures as dissolved water in magma lowers nucleation rate and increases crystal growth rate, leading to the formation of the elongate, skeletal, olivine, and pyroxene crystals that typify the spinifex texture (Grove et al., 1996a). High water contents in primary komatiitic magmas could either imply substantially higher water content in the Archean mantle, or that the southern African komatiites formed in a convergent margin setting, the water being provided to the mantle source by dewatering of the subducted plate. In the latter case, the more Mg-rich nature of the komatiites compared to modern arc basalts could simply reflect hotter mantle temperatures, leading to higher degrees of melting in the Archean.

Supporting evidence for a convergent margin setting for komatiitic volcanism in the Kaapvaal craton comes from the Nondweni greenstone belt found ~200 km south of Barberton (Wilson and Versfeld, 1994a). The Nondweni sequence consists predominantly of mafic and ultramafic lavas with felsic volcanic rocks in a structurally intermediate unit (Wilson and Versfeld, 1994b). Compared to the Barberton komatiites, the Nondweni komatiites have relatively low MgO contents (<21 wt%) and higher silica contents (>50 wt%) and display pyroxene, rather than olivine, spinifex flows. Initial Nd isotopic compositions of the mafic and ultramafic lavas vary with lava composition in a manner that suggests progressive contamination of the differentiating lavas by felsic crust &126;3.5 b.y. old, like that now found just to the north (Wilson and Carlson, 1989). This result indicates that the Nondweni sequence formed in proximity to the preexisting Kaapvaal craton, not in an intra-oceanic setting.

Additional evidence for the importance of subduction in continent formation comes from eclogite xenoliths in on-craton kimberlites. Many eclogite xenoliths have oxygen isotopic compositions outside the normal range for mantle derived rocks (MacGregor and Manton, 1986; Shirey et al., 1999a), suggesting that some eclogite xenoliths represent the high-pressure equivalent of subducted ocean floor basalt. The correlation of Re abundance with oxygen isotopic composition in these xenoliths suggests that their Re-Os system was affected by hydrothermal alteration on the Archean seafloor (Shirey et al., 1999a). Curiously, all diamond-bearing eclogites from the Roberts Victor kimberlite analyzed so far have oxygen isotopic compositions overlapping mantle values. Also, diamond-bearing eclogites from the Roberts Victor and Newlands (Menzies et al., 1999) kimberlites show limited scatter about a 3 Ga Re-Os reference isochron, whereas diamond-free samples show considerable scatter on an isochron plot of Re-Os (Shirey et al., 1999a). These results may suggest that seafloor alteration oxidizes the oceanic crust sufficiently to retard diamond growth upon its subduction. Alternatively, the highly disturbed Re-Os systematics of diamond-free eclogites may indicate that diamond is lost from previously diamond-bearing eclogite by metasomatism and/or partial melting in the mantle (Shirey et al., 1999a).

The Archean age for diamond-bearing eclogites contrasts with Proterozoic ages for eclogitic diamond crystallization observed at several other kimberlite localities (Finsch, Orapa, Jwaneng, and Premier). These ages were based on the Sm-Nd isochron relationships between garnet and clinopyroxene inclusions (Richardson et al., 1999). Re-Os study of individual sulfide grains in diamonds from Orapa shows two age groups, one near the ca. 1 Ga age obtained for silicate inclusions and another giving a Re-Os isochron age near 3 Ga (Shirey et al., 1999b). These results clearly indicate more than one generation of eclogitic diamond growth in the Kaapvaal craton and suggest that subduction of oceanic crust, to depths within the diamond stability field, was occurring during formation of both the craton in the Archean and the surrounding accretionary belts in the Proterozoic.


As is typical of Archean cratons, preliminary seismic results from the Kaapvaal project (James et al., 1999) show that the Kaapvaal and Zimbabwe cratons are underlain by a thick, seismically fast "root" that extends to depths of at least 200–250 km. Samples of the upper 200 km of this root, brought to the surface by kimberlites, are predominantly peridotite that is highly depleted in those major elements (Ca, Al, Fe) that partition into melts (Boyd and Mertzman, 1987). Low Fe and low abundance of garnet resulting from less Al in the restitic peridotite (Boyd and McCallister, 1976) causes this residual peridotite to be less dense than fertile mantle at the same temperature. These characteristics gave rise to the idea of the "tectosphere" (Jordan, 1988) beneath cratons, consisting of a chemical boundary layer of melt-depleted peridotite that adds buoyancy and long-term stability to the overlying crust.

The antiquity of these mantle roots was first indicated by ancient (>2 Ga) ages for silicate and sulfide inclusions in diamonds (Kramers, 1979; Richardson et al., 1984). Re-Os dating of individual diamond sulfide inclusions confirms an Archean age for some diamonds (Pearson et al., 1998b; Shirey et al., 1999b). Other diamonds have sulfide inclusions with Re-Os ages ranging from mid-Proterozoic to Mesozoic (Pearson et al., 1998b; Shirey et al., 1999b). Thus, diamond growth in the lithospheric mantle was not restricted to the Archean, in accord with earlier results for silicate inclusions (Richardson et al., 1993), but appears to have occurred episodically, perhaps in association with subduction and/or magmatic underplating beneath the craton.

Walker et al. (1989) showed that Re-Os isotope systematics of whole rock peridotites track and potentially date the melt-depletion events important to lithosphere formation. In southern Africa, most peridotite xenoliths extracted from on-craton kimberlites give Archean Re-depletion model ages (Fig. 4), and show no clear trend in age versus depth of origin, at least to depths of 180–200 km (Walker et al., 1989; Pearson et al., 1995; Carlson et al., 1999). Thus, most of the upper 180–200 km of the Kaapvaal craton mantle root formed in the Archean and has been attached to the overlying crust since that time. This also is true of the mantle beneath the Limpopo belt, as indicated by Archean ages for xenoliths from the Venetia kimberlite, but not for the area beneath the 2.05 Ga Bushveld igneous complex (Eales and Cawthorn, 1996). Many of the mantle xenoliths from the Premier kimberlite, which penetrated the Bushveld complex, give ca. 2 Ga ages suggesting substantial modification of the mantle during intrusion of the Bushveld (Carlson et al., 1999).

The most obvious age differences in the mantle beneath southern Africa are seen in peridotite xenoliths from on- and off-craton (Fig. 4). Whereas the majority of xenoliths in on-craton kimberlites give Re-depletion model ages in excess of 2.5 Ga, all but one peridotite from off-craton kimberlites give model ages <2.4 Ga (Pearson et al., 1998a; Janney et al., 1999). These model ages overlap the oldest Nd and Pb model ages for the Proterozoic crust south and west of the craton. The rough correspondence between crustal and mantle lithosphere ages in the off-craton xenoliths show that thick lithospheric keels are not unique to Archean cratons but also can be formed, and remain attached, beneath Proterozoic continental crust.


The crystallization products of melts derived from deep in the lithosphere, or perhaps beneath the lithosphere, are widely believed to be represented by the Cr-poor megacryst suite commonly found in kimberlites (Gurney and Harte, 1980). The depth of megacryst crystallization varies across the craton into the surrounding mobile belts (MacGregor, 1975), and is reflected in the composition of megacrysts (Boyd and Nixon, 1980). Preliminary results from a new regional survey of megacryst compositions indicate a close correspondence to craton boundaries and significant variability within the craton. These variations correlate spatially with seismic velocity variations.

Megacrysts that precipitated before significant interaction with lithospheric mantle occurred can be used to fingerprint the compositional characteristics of the mantle at deep levels within, and perhaps below, the depleted root. Two isotopically and temporally distinct varieties of kimberlite, groups I and II of Smith (1983), contain megacryst suites with distinct major and trace element (Bell et al., 1995a, 1995b), radiogenic isotope (Smith et al., 1995), and delta18O (Schulze et al., 1998) compositions. Rare examples of isotopically intermediate kimberlites host megacrysts of correspondingly intermediate and mixed attributes (Bell, 1997; Bell and Mofokeng, 1998). The Sr, Nd, and Pb isotopic compositions of megacrysts from group I kimberlites indicate a source for these magmas in a widespread, compositionally uniform reservoir with low 87Sr/86Sr, high 143Nd/144Nd and 206Pb/204Pb similar to the isotopic component called HIMU that is found in ocean island basalts (Smith et al., 1995). Hf-Nd isotope systematics of these megacrysts indicate the influence of a unique component with a composition reflecting a long-term depletion in Lu/Hf relative to Sm/Nd (Nowell et al., 1999), possibly derived from the sublithospheric mantle.


Results from the Kaapvaal craton project highlight both stable and dynamic aspects of the history of continents on Earth. These findings clearly show that continents consist not only of their crustal provinces, but also include a thick section of underlying mantle that formed during a time interval similar to that of the overlying crust. Several aspects of our data could relate to a common petrogenetic process reflecting craton formation in a convergent margin setting. These include:

  • evidence that southern African komatiites derive from wet primary magmas;
  • lithospheric peridotites with compositions indicative of extremely high degrees of melt removal, possibly the residues of komatiite extraction;
  • Archean ages for melt depletion measured for the peridotites;
  • the presence of subducted Archean oceanic crust in the deep lithospheric mantle as sampled by eclogitic xenoliths.

In the southern African case, this process continued sporadically over ~500 m.y. and resulted in the creation of a lithospheric block that has survived at Earth's surface for over 3 b.y.

Once formed, the history of this continental block was not yet complete. Accretionary belts were welded to its margins in the Proterozoic, increasing the crustal thickness of the craton around its margins as shown by geochronological results from crustal xenoliths. As before, this episode of continent growth was not restricted to the crust. Both the relative youth of some Kaapvaal diamonds and the Proterozoic ages obtained for off-craton peridotite xenoliths show that continent formation and/or modification involved the underlying mantle to depths extending at least into the diamond stability field. The presence and characteristics of the kimberlite-borne megacrysts extend this interaction to the very base of the lithosphere. The results demonstrate the dynamic nature of the whole continent, from top to bottom, as it has interacted with the surrounding crust and mantle over Earth history.


This project would not be possible without the collaboration of academic and industrial colleagues and funding from National Science Foundation Earth Sciences Division-Continental Dynamics and South African industrial collaborators. We thank the many participants for their input and Roberta Rudnick and Ashish Basu for reviews.


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Manuscript received October 18, 1999; accepted December 9, 1999.

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