Modification of cratonic lithosphere by major tectono-magmatic events: implications for the survival of Archaean cratons

Abstract

The majority of the Sub-Cratonic Lithospheric Mantle (SCLM) has been shown to be of Archaean age (Pearson, 1999). This implies that the SCLM plays a fundamental control in stabilizing and preserving overlying continental crust and isolating significant volumes of mantle from the convecting asthenosphere (Jordan, 1988). It is clear from the consideration of accretion models that, over time, Archaean crust has been destroyed or modified by erosion and/or major tectono-magmatic events. Determining the processes that have modified the CLM is therefore fundamental to our understanding of the evolution of continental crust and the mantle. The aim of this project is to understand the factors/ processes that have led to the local preservation/destruction of Archaean continents. For this reason CLM xenoliths suites will be examined from two geodynamic settings that have been subjected to major tectono-magmatic events. One craton survives (Bushveld Province) while the other becomes rifted (Tanzania). In Kaapvaal craton, recent geophysical and petrological studies show a large anomaly beneath Premier, a region affected by 2.05 Ga Bushveld magmatism. The 1.2 Ga Premier pipe provides a natural probe of the mantle in this area. Later, Northern Tanzania xenoliths will be studied as they are from a region where an Archaean craton (Chesley et al., 1999) is actively rifting. These xenoliths will provide information on how SCLM is modified by magmatic and metasomatic events before a craton no longer acts as a stable continental core. To assess if differences between these two tectonic settings are the result of SCLM modification, we need to understand what percentage of the SCLM has been modified by what processes and the changes with time. Our approach consists of petrology, major and trace element, and multi-isotope studies related to detailed spatial sampling of mineral domains. Whole rock and Re-Os analyses will provide strong mass balance constraints on compositions and ages of the CLM at different depths. Petrology studies will allow calculation of pressure corrected xenoliths densities. Because P estimates can be used to constrain the depth of origin of xenoliths, we can therefore assess the extent to which the CLM has been modified at different depths and then model the predicted thermal and mechanical response of the craton. Whole rock and mineral compositional data will be presented from Premier, allowing an estimate of composition with depth. Preliminary Re-Os, PGE and trace elements in minerals from the first part of the suite will be discussed. Isotopic analysis will be initiated later this year.