Insights into the metasomatic history of Kaapvaal SCLM from a Hf isotope study of the ~2.06 Ga Bushveld Large Igneous Province

Abstract:

The Bushveld Large Igneous Province (B-LIP) comprises a diverse array of >30 magma bodies that intruded the Kaapvaal Craton (KC) at ~2.06 Ga. To determine whether the B-LIP formed in response to the arrival of a plume(s) from the deep mantle or from melting of the depleted upper mantle during foundering of an eclogitized residue at the base of the lithosphere, we have measured zircon Hf isotope compositions for many of the bodies in the B-LIP. Most of the intrusions have relatively unradiogenic and internally homogeneous εHf _{(2.06 Ga)} values (intrusion-specific average εHf _{(2.06 Ga)} 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, as opposed to delamination-driven decompression melting of the depleted upper 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). Support for this model comes from B-LIP aged zircons in a metasomatically altered xenolithic fragment of the SCLM associated with a basement inlier near the geographic center of the B-LIP. Some domains in these zircons grew in the presence of a medium with a highly unradiogenic Hf isotope signature. This signature suggests that ancient (>3.8 Ga) crustal material residing in the KC-SCLM at the time of B-LIP magmatism 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 plume related metasomatic weakening during the Paleoproterozoic.