| This project is designed to understand and quantify the processes that modify sub-cratonic lithospheric mantle (CLM) and ultimately lead to disruption of cratonic crust. The Archaean formation age of sub-cratonic CLM demonstrates the potential long-term stability of the CLM. However, significant amounts of cratonic material has been destroyed or rifted apart. Our goal is to understand of how the composition, bulk density and rheology of the CLM is modified by tectono-magmatic events and then to model the dynamic behaviour of continents and hence their tendency to survive. CLM xenolith suites will be examined from two geodynamic settings that have been subjected to major tectono-magmatic events. The ultimate aim is to determine why a craton survives in one region (Bushveld Province) but is rifted in a second (Tanzania). The timing of this proposal is very opportune in that the Southern Africa seismic experiment has produced a detailed tomographic model of the Craton (James et al. 2001). This imaging clearly establishes that the CLM beneath the Bushveld region was extensively modified but has survived intact. The project will build on existing collaborative national and international multidisciplinary research of the Kaapvaal CLM (e.g. Simon et al. 2002) to establish how the CLM in "modified" regions (Tanzania, Bushveld) compares with "typical" sections of the Kaapvaal Craton. The goal will be achieved using suites of petrologically well characterised xenoliths. The timing of mineral growth in cratonic peridotites will be compared to the formation age of the lithospheric mantle. The innovative aspects of this proposal will be the combined detailed petrology, chemical and multi-isotope (Sr-Nd-Os-Hf) approach. The approach involves a combination of whole-rock microsampling and in situ analysis using state-of-the-art geochemical techniques (LA-MC-ICPMS, SIMS). The ultimate goal is to determine what proportion of the CLM retains primary characteristics and how much has been modified over time and how changes are distributed with depth. This information will allow the thermal and mechanical response of the CLM to be modelled and may help explain why cratons have responded in different ways over time. |
