A 2058 Ma paleopole for the Kaapvaal Craton: Implications for late Rhyacian plate motion and duration of the Bushveld Complex

Abstract

The Rustenburg Layered Suite (RLS) of the Bushveld Complex, the largest ultramafic–mafic-layered intrusion on Earth, is associated with satellite intrusions and marginal sills with which it shares a common parental heritage (i.e. the B1, B2 and B3 parental magmas). The duration of the RLS emplacement is regarded as a focused event of less than 1 million years. Some of the more primitive satellite intrusions and marginal sills have been interpreted to predate the main Bushveld Complex. The precision and extent of existing age data, however, have not allowed confirmation of the relative timing of their emplacement. This study provides paleomagnetic support of a longer period of emplacement for the RLS and its marginal sills and satellite intrusions. Stepwise demagnetization of marginal sills with B1-geochemical affinity sampled regionally and associated with the exposed western and eastern limbs of the RLS reveals stable near antipodal components with improved grouping upon structural restoration of sills to their paleo-horizontal. 40Ar/39Ar dating of one of the studied sills indicates a crystallization age of 2058.5 ± 7.9 Ma. Although this age overlaps with the precise 2056–2055 Ma ages from the RLS, it is similar to seemly older ages reported from the lowermost part of the RLS and from other Bushveld-related units (e.g. pyroxenitic sills and Uitkomst Complex) with B1 geochemical affinity. A mean paleopole (λ = 26.63°N; ϕ = 036.02°E; A95 = 8.80; 20 sites) was calculated from tilt-corrected new and previous data from the 2058 Ma B1-marginal sills and Uitkomst Complex. This pole, which is assumed to be primary, is statistically distinguishable from the existing 2056–2055 Ma Bushveld Complex paleopole. This suggests a small, but appreciable amount of plate motion separating punctuated events of magma emplacement in the Kaapvaal Craton during the lateRhyacian Period. Our results support at least two episodes of magma emplacement in the formation of the RLS and its associated marginal intrusions. These are separated in time by a few million years (i.e. a B1-type magma pulse around 2058 Ma and the B2 and B3-type magma pulses following closely on each other around 2056–2055 Ma).