WEAKENING OF KAAPVAAL SCLM BY PLUME RELATED METASOMATISM AT ~2.06 GA: HF ISOTOPE EVIDENCE FROM THE BUSHVELD LARGE IGNEOUS PROVINCE

Plume-related weakening by thermomechanical erosion and metasomatic refertilization of the subcontinental lithospheric mantle (SCLM) is thought to render cratons more susceptible to destruction during the course of normal plate-mantle interaction. Here, zircon Hf isotope data are used to understand the nature of plume-lithosphere interaction during emplacement of the Paleoproterozoic aged Bushveld Large Igneous Province (B-LIP) into Archean continental crust. Most of the intrusions in the B-LIP, which comprises a diverse array of >30 magma bodies that intruded the Kaapvaal Craton (KC) at ~2.06 Ga, have relatively unradiogenic and internally homogeneous εHf _{(2.06 Ga)} values. The intrusion-specific average εHf _{(2.06 Ga)} values range from -21.2 ± 5.2 to -2.7 ± 2.8, consistent with published values for the Bushveld and Phalaborwa complexes (two prominent intrusions in the B-LIP with εHf _{(2.06 Ga)} = -8.6 ± 2.6 and -7.5 ± 2.4, respectively). Because the most radiogenic Hf isotope compositions in the B-LIP are within error of εHf _{(2.06 Ga)} = 0, it is likely that the heat source was a plume(s) from the deep mantle. Many of the more unradiogenic values in the B-LIP can be reconciled with melt generation in subduction modified and metasomatically refertilized subcontinental lithospheric mantle (SCLM). Evidence for these processes include the occurrence of highly unradiogenic (εHf _{(2.06 Ga)} < -40) subdomains in B-LIP aged zircons from a metasomatically altered xenolithic fragment of the SCLM associated with a basement inlier near the geographic center of the B-LIP. These highly unradiogenic values suggest that ancient (>3.8 Ga) crustal material residing in the KC-SCLM became mobilized during arrival of the plume at ~2.06 Ga. Pervasive metasomatic alteration leading to weakening of the SCLM beneath parts of the KC is known to have occurred in the Mesozoic, but these results suggest that the KC has also withstood pervasive metasomatism at ~2.06 Ga. With regional evidence for earlier plume-related magmatic activity at ~2.7 Ga in the same location as the B-LIP and the fact that both plume arrivals occurred in close proximity to a tectonic suture (the Limpopo Belt), there may be some geodynamic significance to the fact that this part of the KC has remained intact.