Insight into early craton nucleation and stabilization – a case study from crustal and mantle xenoliths from the Star kimberlite mine, East Kaapvaal craton
Final Report Abstract
Cratons are the oldest sections of our continents that have survived over several billions of years without modification by subsequent geological processes. Samples from deep inside these cratons can only be recovered from sparse localities where young volcanic pipes have pierced through the craton and delivered rock fragments (xenoliths) and individual crystals (xenocysts, including diamonds) to the surface. In southern Africa they are typically found in volcanic rocks called kimberlite, which are relatively young compared to the age of the craton. The xenoliths may be derived from the continental crust of the craton or from its underlying lithospheric mantle section. Previous studies have investigated either crustal or mantle samples. Here, we studied a vertical profile from the deep lower crust to the base of the lithospheric mantle from a single diamond mine (Star mine on the East Block of the Kaapvaal Craton). The lower crust is represented by a type of metamorphic rock that experienced the most extreme temperatures observed in any crustal rock, so-called ultra-high temperature (UHT) granulites. The UHT granulites from Star Mine in this study showed a very similar composition and history to the samples from the Lace diamond mine 200 km to the north for which conditions of >1050 °C at the base of the crust had been deduced, followed by subsequent cooling at depth. This must have been the metamorphic history of the Star granulites as well judging from the strong similarities of the two sample sets. We also deduced subsequent cooling of the lower crust down to 500 °C. A new method of radiometric dating was recently established in our lab, the U-Pb method on metamorphic garnet for which the material is probed with a finely-focused laser beam. This method was employed here and a range of ages was recovered, falling into two groups: an older group between 3.1 and 2.9 Ga (7 samples), and one younger sample of 2.7 Ga. These groups coincide with major tectonometamorphic events in the Kaapvaal craton: The oldest ages coincide with the time of the assembly of the East Block at 3.1 Ga. This major event was followed by several structural-magmatic events between 2.99-2.88 Ga, and finally a wide-spread volcanic event, the Ventersdorp large igneous province, at 2.73 Ga that covered a large part of the Kaapvaal Craton. The latter was also dated by Schmitz and Bowring (2003) who reported a narrow age range between 2.73–2.70 Ga based on U-Pb ages of U-rich minerals in crustal samples. The age range recorded in garnet (and other minerals) requires clarification by further work, including a) the determination of garnet U-Pb ages from further samples that are available to us; b) the analysis of zircon that enables a simultaneous determination of formation ages and temperatures. A further unknown is the protolith of the crustal samples, which could be sedimentary or magmatic. Tight constraints may be obtained from the carbon isotope composition of graphite and from the sulfur isotope composition and the PGE patterns of the ubiquitous sulfides. The mantle-derived samples of the Star Mine show that the mantle was partially melted to a high degree and subsequently infiltrated by fluids or melts rich in particular trace elements. The conditions of last residence of the rock samples in the mantle were between 700 and 800 °C along a conductive geothermal gradient at depths between 75 and 90 km, whereas the xenocrysts stem from depths between 120 and 150 km. The ages of all but one mantle sample are dominantly between 2.9 and 3.0 Ga, thus overlapping with the ages determined for the crustal samples. We, therefore, propose that melt extraction occurred in subcratonic lithospheric mantle beneath the eastern Kaapvaal craton at the same time as melting and UHT metamorphism occurred in the crust. The combined processes between 3.1 to 2.7 Ga in both mantle and curst eventually led to the final tectonic stability of the eastern Kaapvaal Craton.
