P-T estimates of peak Bushveld metamorphism in the eastern Bushveld complex, Limpopo Province, South Africa : constraints from P-T pseudosections

Abstract

The Bushveld Igneous Complex (BIC) is the largest layered mafic intrusion in the world and contains the largest known deposits of vanadium, chromium and Platinum group elements on the planet, as well as large deposits of iron, nickel, copper, tin and fluorite. To aid and improve our understanding of the tectonics that prevailed during the emplacement of the Bushveld Complex relevant data can still be extracted from the metamorphic aureole of the Complex, not the least among which are accurate determinations of pressure conditions during peak metamorphism. A relatively large number of geothermobarometric investigations have been performed on the Bushveld Complex aureole. The summation of all the thermobarometric studies on the Bushveld Complex aureole produces a dataset with largely divergent pressure-estimates, ranging from 1.5 kbar to 5.5 kbar. This study’s main aim was to produce new thermobarometric data for the Eastern Bushveld Complex aureole. To this ends metapelites from the aureole were sampled between Lydenburg and somewhat northwest of Penge. Polished thin-sections were produced for a number of samples and studied under microscope. After XRF analyses were performed on a refined number of samples, pseudosections for these samples were produced using Perplex. Electron microprobe analyses were used to analyze mineral chemistries of five samples and the resultant data used to construct isopleths for these samples in Perplex. The isopleth data was then used to scrutinize and, where possible, refine PT-estimates. The principal results obtained from mineral equilibrium modeling were the pseudosections and isopleths of samples DY09-54 and DY09-56. These samples’ cumulate results suggest that the metapelites of their sampling locality, which lies roughly ~36 km northwest of Penge, reached 530-565 ºC and 2230-2960 bar during peak metamorphism. Modelled isopleths of MnO/(MnO+CaO+FeO+MgO) suggest that these estimates be refined to 550 ± 5 ºC and 2650 ± 20 bar. These pressure estimates agree well with the majority of barometric studies in the literature that post-date the nineteen-eighties. The pressure estimates of 2230-2960 bar suggest that DY09-54 and DY09-56 were at a crustal depth of 7.9-10.4 km during peak metamorphism, assuming that a roughly 1.5 km thick load of rock, mainly of the Rooiberg Group and/or the Lebowa Granite Suite, were situated above the Rustenburg Layered Suite and at the top of the pile that overlay the samples. In such a case the Rustenburg Layered Suite’s contribution to the load would have represented a 4.2-6.7 km thick pile of these mafic rocks and, assuming that the load of Pretoria Group strata in the floor to the Complex had a thickness of 2350 m, the base of the Rustenburg Layered Suite would have been at a crustal depth of 5.6-8.0 km during peak metamorphism and directly above samples DY09-54 and DY09-56. Modelled palaeogeotherms together with the peak-metamorphic crustal depths estimated for samples DY09-54 and DY09-56 suggest that at peak metamorphism the samples’ temperatures had been elevated by no less than 320-355 °C, assuming that no thermal metamorphic effect was active on the samples just prior to the intrusion of the Bushveld Complex. Copyright