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Evolution of Proterozoic mantle lithosphere: petrological and
geophysical evidence from the Kaapvaal – Namaqua boundary
D. Bell 1,3 , U. Weckmann 2 ,P. Janney 1 , M. de Wit 3
1. SESE, Arizona State University, Tempe, AZ, USA, David.R.Bell@asu.edu
2. GeoForschungsZentrum Potsdam, Germany, uweck@gfz-potsdam.de
3. AEON, University of Cape Town, South Africa, Maarten.deWit@uct.ac.za
ABSTRACT
The subcontinental lithospheric mantle (SCLM) beneath Proterozoic and Archean crustal provinces differs in
composition, temperature, and thickness. To shed light on the origins of these differences we combined
petrological data from xenocrysts, megacrysts, and xenoliths in kimberlites with a lithospheric resistivity model
derived from a new high-resolution magnetotelluric (MT) traverse, to examine the transition between Archean and
Proterozoic lithosphere at the SW margin of the Kaapvaal Craton. Highly resistive SCLM persists up to 100 km
west of the Brakbos fault zone, confirming inferences from garnet xenocrysts of craton-like mantle beneath
Proterozoic crust (Kobussen et al. 2008). Xenolith thermobarometry also suggests craton-like mantle temperatures
in this region prior to Mesozoic disturbance (Bell et al. 2003; Janney et al. 2010). Surface and body-wave seismic
tomography studies reveal craton-like high-velocity domains within the SCLM beneath the Namaqua Belt.
Average whole-rock Mg#, Al2O3 contents, and Os isotope model ages of Kaapvaal and Namaqua mantle xenoliths
differ, but many individual off-craton xenoliths overlap the cratonic population and suggest complex multistage
history. Clinopyroxene megacrysts from the Pofadder kimberlites suggest interaction of deep melts with mantle
lithosphere similar to that beneath the Archean craton at Kimberley. The MT study reveals prominent vertical
zones of low resistivity within the high-resistivity SCLM in the boundary region. These may be metasomatized
zones that formed during the focused ascent of melts generated beneath the SCLM and have been inferred from
xenocryst compositions (Begg et al. 2009). Some kimberlite eruptions occur earlier in the boundary region than
would be predicted from their broader regional eruptive patterns, trace elements suggest some have mixed source
regions, and evolved megacrysts indicate local magma infiltration to shallow levels. These results support models
of (1) melt-refertilized Archean SCLM beneath Proterozoic crust, and (2) the focusing of mantle–derived magmas
into ancient shear zones that serve as repeating loci of tectonic reactivation. This model allows for tectonic
obduction of relatively young Namaqua arc crust across subducting Archean SCLM during arc-continent collision,
as is presently observed in the Banda Arc (Fichtner et al., 2010) [335 words]
KEYWORDS: mantle, lithosphere, kimberlite, conductivity, megacryst, xenolith, subduction-accretion.
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