A Chill Sequence to the Bushveld Complex: Insight into the First Stage of Emplacement and Implications for the Parental Magmas

A chilled margin of komatiite and Mg-rich basaltic andesite in the western Bushveld Complex, South Africa

“…Because the borehole was stopped after intersecting a mere 30 cm of quartzite, it cannot be ruled out that the latter represents a xenolith. However, as the thickness of the basal compositional reversal (~300 m, Teigler 1990a) resembles that in other layered intrusions and because the intersected chill/quench sequence has a similar width (90 cm) and broadly similar stratigraphy (a basal aphanitic chill, overlain by spinifex-textured and medium-grained rocks) as that described by Wilson (2012) from the eastern Bushveld Complex, we argue that the quartzite represents the floor of the intrusion. Alternatively, it could represent a very large xenolith, but this would not alter our interpretations.…”

“…It has a broadly similar composition as the komatiite, but slightly higher Cr (1844 ppm) and Ni (584 ppm) contents, and mostly slightly Rudnick and Fountain (1995), except for Nb (Barth et al 2000). B1 samples are from Barnes et al (2010), Clapham samples are from Wilson (2012), komatiite represents median of Al-undepleted komatiites with <0.3 % S from GEOROC. Tielines indicate AFC of upper crust by komatiite, with tick marks denoting proportion of crystallization higher incompatible trace element contents (27 ppm Zr, 0.98 ppm Nb, 0.75 ppm Th), with the exception of the LREE.…”

“…No conclusive geochemical traits have yet been identified that rule out one of these two models. The most recent contribution on the topic has been by Wilson (2012) who identified an 8 m sequence of high-Fo (up to 91.2 %) olivine overlying a magnesian andesitic chilled margin at Clapham in the eastern Bushveld Complex. The author went on to propose that these olivines crystallized from a komatiitic magma with approximately 18-19 wt% MgO.…”

“…Fortunately, the Bushveld Complex is one of few mafic-ultramafic intrusions where the parent magma composition could be studied in considerable detail, based on a suite of fine-grained, relatively unaltered sills exposed in the floor of the Complex (Davies et al 1980;Cawthorn et al 1981;Sharpe 1981;Barnes et al 2010). Chilled margins also occur, albeit relatively rarely, and their study has only begun relatively recently (Wilson 2012). Here, we present compositional data from a relatively unevolved chill sequence which overlies quartzite at Union Section in the western Bushveld Complex (Fig.…”

Microtextural characterisation of the Lower Zone in the western limb of the Bushveld Complex, South Africa: evidence for extensive melt migration within a sill complex

“…More recent studies by Wilson (2012) and Maier et al (2016a) concluded that the most likely parental magma to the LZ would have been a komatiitic one with 18-19 wt% MgO derived from the asthenosphere, which assimilated some continental crust.…”