Xenoliths, Kaapvaal Craton

Bedini R. M., Blichert-Toft J., Boyet M., and Albarede F. (2002) Lu-Hf isotope geochemistry of garnet-peridotite xenoliths from the Kaapvaal craton and the thermal regime of the lithosphere. Geochim. Cosmochim. Acta (Spec. Suppl.) 66S1, A61. 

Irvine G. J., Pearson D. G., and Carlson R. W. (2001) Lithospheric mantle evolution in the Kaapvaal craton A Re-Os isotope study of peridotite xenoliths from Lesotho kimberlites. Geophys. Res. Lett. 28, 2505—2508. 

Pearson, D. G., Carlson, R. W., Shirey, S. B., Boyd, F. R. Nixon, P. H. 1995. Stabilisation of Archaean lithospheric mantle a Re-Os isotope study of peridotite xenoliths from the Kaapvaal Craton. Earth and Planetary Science Letters, 134(3-4), 341-357. 

Schmitz, M. D. Bowring, S. A. 2000. The significance of U-Pb zircon ages from lower crustal xenoliths of the southwestern margin, Kaapvaal Craton, southern Africa. Chemical Geology, 172, 59-76. 

Schmitz, M. D., Bowring, S. A. Robey, J. v. A. 1998. Constraining the thermal history of an Archean craton U-Pb thermochronology of lower crustal xenoliths from the Kaapvaal craton, southhem Africa. In 7th International Kimberlite Conference, Extended Abstracts. University of Cape Town, Cape Town, 766-768. 

Table 2. Summary of Re Os model age information for peridotite xenolith suites on and around the Kaapvaal Craton...

Fig. 11. Location map of kimberlites on and around the Kaapvaal Craton from which peridotite xenolith suites have been dated by Re-Os isotopes (after Carlson et al. 2000 Irvine et al. 2001). Data sources Pearson et al. (1994, 1995a, 1998) Pearson (1999) Carlson et al. (1999) Janney et al. (1999) Irvine et al. (2001). Kaapvaal Craton is outlined in dark grey with the Archean Limpopo Metamorphic Belt shaded in light grey.

To date, Re-Os isotope studies of peridotite xenolith suites, taken to be representative of the lithospheric mantle beneath continents, reveal that although some lithospheric mantle began to form beneath continents in Palaeo- to Mesoarch-aean time, major crustal differentiation events continued until Neoarchaean time when, at least in southern Africa, significant volumes of lithospheric mantle formed and stabilized the newly accreted continent. On the Kaapvaal Craton, major magmatic events such as that which formed the Bushveld intrusion, substantially modified or introduced new material into the lithospheric mantle (Carlson et al. 1999) and the lower crust (Schmitz Bowring 2000), but the craton retained its integrity for another 2 Ga. 

Richardson and colleagues working at MIT in the 1980s showed that diamond inclusions in mantle xenoliths from beneath the >3.0 Ga Kaapvaal Craton in South Africa were extremely ancient. This discovery demonstrated for the first time a link between the age of the continental crust and its underlying lithospheric mantle (Richardson et al., 1984, 1985). 

One particular class of Archaean mantle xenoliths has received special attention. These are eclogites recovered from the subcontinental lithosphere. Richardson et al. (2001) showed that some Archaean eclogite xenoliths have the trace element and Os-isotopic characteristics of a basaltic protolith and an isotopic history which shows a significant time gap between basalt generation and eclogite crystallization. These properties are typical of subducted ocean floor and indicate that the subcontinental lithosphere beneath the Kaapvaal Craton contains fragments of 2.9 Ga subducted ocean floor. 

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