ALBERT

Description: Early Jurassic sheet-like intrusions (sills and dykes) are abundant in the Karoo Basin in South Africa, and were emplaced as a part of the Karoo Large Igneous Province. Here we discuss the evolutionary history of dolerite sills and dykes in different parts of the basin on the basis of new major and trace element analyses of dolerite samples collected from drill-cores (five sites spanning 1700 m of basin stratigraphy) and previously published data on sills and dykes in the Golden Valley Sill Complex (GVSC). In addition, we present Sr–Nd isotope data for selected samples. The dolerites are subalkaline tholeiitic basalts and basaltic andesites characterized by enriched trace element patterns, variable degrees of depletion in Nb–Ta relative to light rare earth elements, negative to positive Pb anomalies, and mild to moderate enrichment in initial Sr–Nd isotopic ratios. The aim of this study is to unravel the evolutionary history of the melts that gave rise to the dolerites. We propose that the primary melts were derived from sub-lithospheric mid-ocean ridge basalt (or ocean island basalt) source mantle and had acquired a weak subduction signature (relative depletion in Nb–Ta, mildly enriched Sr–Nd isotopic ratios) through interaction with metasomatized lithospheric mantle. In the deep crust the magmas underwent assimilation and fractional crystallization (AFC) processes involving up to 10% assimilation of granulites with strong arc-type geochemical signatures. The AFC processes may alternatively have taken place in the uppermost mantle. Distinct geochemical characteristics among the GVSC and drill-core units reflect different amounts of AFC. During and/or after intrusion into the sedimentary rocks in the Karoo Basin the magmas underwent a second stage of fractional crystallization (50–60%) and local contamination by their sedimentary wall-rocks. High U concentrations and U/Th ratios in some dolerites in the southwestern part of the Karoo Basin were probably caused by fluids released from shales rich in organic material (e.g. Ecca Group shales) during devolatilization and contact metamorphism. Contamination in a GVSC unit may reflect interaction with Ta–Th–U-rich minerals of the type found in stratiform uranium ore bodies in the Karoo Basin, or fluids that have interacted with such rocks. Considering that continental flood basalts are emplaced through continental crust and sedimentary basins, it is likely that other LIPs have similar evolutionary histories to that proposed for the Karoo Basin.