Paleomagnetism and chronology of B-1 marginal sills of the Bushveld Complex from the eastern Kaapvaal Craton, South Africa

Abstract

The Rustenburg Layered Suite (RLS) of the Bushveld Complex in South Africa is the largest mafic–ultramafic-layered complex on Earth. The RLS is associated with marginal sills that penetrate into the ~2.2 billion-year-old sedimentary strata of the Pretoria Group. These sills are in contact and share some geochemical similarities with different zones of RLS and are classified in terms of chemical composition, which suggests their derivation from distinct parental magma compositions (so-called B-1, B-2 and B-3 parental magmas). Existing paleomagnetic constraints for the Bushveld Complex originate from the upper Critical to Upper zones of the RLS, which are associated with B-2 and B-3 marginal sills. Geochemically, verified B-1 marginal intrusions are here used as a proxy for constraining the paleomagnetism and chronology of the Lower and lower Critical zones of the RLS. We identified a dual-polarity magnetic component with a paleopole (Latitude = 13.1°N, Longitude = 44.0°E, A95 = 14.3, N = 7) that is very similar to the established Bushveld Complex poles. We further report 2058.4 ± 1.3 Ma and 2058.1 ± 6 Ma U–Pb baddeleyite ages from B-1 sills that record opposite magnetic polarities. The ca. 2058 Ma ages are older than the 2054.89 ± 0.37 Ma age recently reported from throughout the RLS, but near identical to a previously reported ages of the Marginal Zone and from the upper Critical Zone. The ages could be interpreted as distinct pulses of magma emplacement separated in time by up to 4 million years (i.e., B-1 type magma pulse around ca. 2058 Ma and the B-2 and B-3 types magma pulses following closely on each other around ca. 2054 Ma), but is unlikely when petrological models are considered.