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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
ABSTRACT
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.
INTRODUCTION
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 www.ciw.edu/mantle/kaapvaal/.
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).
CRUSTAL GROWTH AND MODIFICATION
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.
TECTONIC SETTING OF CONTINENT FORMATION
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.
MANTLE ROOT
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.
BENEATH THE ROOT?
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.
SUMMARY
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.
ACKNOWLEDGMENTS
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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