Crust-mantle interaction in the Bushveld Complex: Evidence from Sm-Nd isotopic data of the cumulate rocks

The Bushveld Complex is a Large Igneous Province of modest dimensions that probably formed through processes like those that created continental flood basalts. A large volume of mafic magma was emplaced in a short time and the parental magmas are probably related to the arrival of a Proterozoic mantle plume beneath the Kaapvaal lithosphere. However, the nature and source of the melting reservoirs remain unclear.

Here we present Nd and Sr isotopic compositions of 17 samples from a 4.7 km borehole section through the Lower, Critical and Main Zones of the Western limb of the Complex at Union Section. The Lower and Critical Zones have eNd between -5.3 to -6 and (87Sr/86Sr)i between 0.704 to 0.707, while the Main Zone has eNd between -6.4 and -7.9, and (87Sr/86Sr)i between 0.708 to 0.709. These data suggest the presence of an old enriched component throughout the Complex. Published Pd and Os isotopic data confirm the more pronounced signatures in the upper portions of the Complex.

Trace-element patterns of the cumulates and parental magmas closely resemble continental crust, but the lower portions of the Complex have higher contents of incompatible trace elements (LREE, Rb, Th), and more fractionated patterns, than the upper portions. This decoupling of isotopic and incompatible trace element patterns may be explained by melting of an ascending mantle plume and contamination of the magmas with variable amounts of old lower continental crust and younger upper crust. Melting of enriched sub-continental lithospheric mantle is considered less likely by us, because the Bushveld trace-element patterns are similar to patterns in both lower and upper crust, but very different from those in lithosphere-derived xenoliths. Another argument is the anhydrous nature of Bushveld cumulates which contrasts with the hydrous nature of melts produced by melting of metasomatised lithospheric mantle.