An extension of the Rooiberg Group and Upper Zone of the Bushveld Complex in the northernmost Waterberg segment

Abstract

The Waterberg segment of the northern Bushveld Complex within the Southern Marginal Zone of the Limpopo Belt hosts an ∼ 850 m thick erosional remnant of a previously unrecognized Paleoproterozoic sequence of contact-metamorphosed basalt to basaltic andesite unconformably overlying Upper Zone ferrodiorite of the Rustenburg Layered Suite. The volcanic rocks are overlain by ∼ 1.5 km thick package of 1.95–2.05 Ga Waterberg Group sandstones. We report on data obtained from the NP1 borehole that intersects this sequence in the Waterberg segment north of the Hout River shear zone.The metabasalts are correlated with Dullstroom basalts and basaltic andesites of the Rooiberg Group based on their similar mineralogy, petrography, major, trace element compositions and supported by homogeneous and relatively non-radiogenic Sr-in-plagioclase (0.7042–0.7066, mean = 0.7052 at 2.056 Ga) with respect to more radiogenic Sr-in-plagioclase of 0.7064–0.7073 in underlying ferrodiorite, consistent with the known Upper Zone values. Both metabasalts and ferrodiorite show similar εNdt from − 5.4 to –5, typical of the Bushveld lineage, whereas anomalously low εHft from −13 to −18 could be a regional feature of the Waterberg segment. Contact-metamorphosed basalts enclose layers and xenoliths of metasedimentary hornfelses at the base. No intervening granite occurred between the Upper Zone and overlying volcanic rocks suggesting that numerous coarse-grained to granophyric fayalite-magnetite bearing felsic veins in the metavolcanics were derived from Upper Zone residual melts of monzonitic to granitic compositions. These felsic veins are rich in Zr-Hf and depleted in REE relative to cognate Upper Zone faylite-magnetite-pyroxene-plagioclase cumulates, while zircon from one of the topmost veins yielded an age of 2054.4 ± 3.7 Ma concordant with other ages for the Bushveld Complex within analytical uncertainty. Our study suggests a larger extension of Rooiberg basaltic volcanism than previously thought, supporting the comparable volumes of magmas for the volcanic and plutonic counterparts of the Bushveld Complex.