Petrology and associated woody vegetation of the Koedoesfontein Complex in the Vredefort dome

Abstract

The Vredefort dome impact crater hosts several Neoarchaean (2800 Ma) to Mesoproterozoic (1000 Ma) peralkaline to ultramafic intrusive bodies that are exposed amongst its core and collar. The Koedoesfontein Complex (KC), in particular, forms part of an alkaline-dioritic suite of post-Transvaal, syn-Bushveld (~2050 to 2060 Ma) intrusions including the Schurwedraai-, Lindequesdrift-, Roodekraal- and Rietfontein- Complexes, as well as the Winddam wehrlite. Due to the current level of erosion, the geology of the KC is relatively less well understood and has been historically understudied, probably because of its relatively small size, poor outcrop visibility and dense vegetation cover. These characteristics make the Complex difficult to interpret, however, since several plant species are well established amongst the diverse geology, it posed a unique opportunity for a multidisciplinary approach to re-evaluate the petrology of the underlying intrusions and to simultaneously study their effects on the hosted woody vegetation. With regards to the geology, a number of small dyke structured outcrops of the KC were identified and described during previous studies. These consist of ultramafic wehrlite and dioritic lamprophyre (termed spessartite), as well as a few dispersed outcrops of sodic (alkali) granite. More recently, investigation of the KC revealed a broader textural and mineralogical spectrum within the spessartite body, as well as the presence of small unmapped sills of mafic gabbro, hornblendite, olivine ± clinopyroxenite and quartz alkali syenite. These intrusive rock types are hypothesised to be related to the known syn-Bushveld intrusions of the KC, and to collectively form a layered series which is derived from magmatic differentiation. The KC was mapped, and a complete set of rock samples were taken across the entire intrusive sequence. The rock samples were prepared for the production of thin sections, as well as X-ray diffraction and X-ray fluorescence analysis to determine the mineral composition and geochemistry. Structural evidence, petrographical observations and geochemical trends of the known and unmapped outcrops, in their entirety, indicate that the intrusions collectively form a coherent stratigraphical, mineralogical and geochemical sequence that supports the notion for differentiation. It is therefore concluded that the KC represents a layered differentiated intrusion quite similar and probably closely related to the nearby Rietfontein Complex. With regards to the botany, the broad difference in mineral composition of the intrusions, as well as the abundant quartzite and shale country rocks, are believed to cause abrupt transitions in soil chemistry that ultimately affect the floristic, physiognomic and biochemical characteristics of the hosted woody vegetation. Ultramafic intrusions, in particular, are known to produce soils that are rich in magnesium, chrome, cobalt and nickel, and may cause adverse affects on species that are not adapted to these environments. In spite of various efforts to study the ultramafic-adapted vegetation (also known as “serpentine flora”) of southern Africa, little is known about the effect these particular intrusions have on the soil and vegetation components of the Vredefort dome. The KC was established to collectively form a mineralogical gradient ranging from purely mafic- to felsic-dominant silicates. Three primary components (geology, soil and woody vegetation) were sampled along this gradient and analysed to determine the effect of these intrusions on the residual soil chemistry, and the floristics, physiognomy and biochemistry of the supported woody vegetation. Weathering of the ultramafic to felsic alkaline rock types greatly affected the soil’s chemical properties. Major chemical variation in the soil, including calcium and sodium availability and the total chromium and nickel content, is correlated with the difference in abundance of calcium-, sodium-, iron- and magnesium-rich silicate minerals. Significant variations in woody species composition (particularly across members of Senegalia and Vachellia) have been established between soils originating from ultramafic, mafic, dioritic and felsic rock types, and can be utilised as indicators for different geological substrates. The ability of V. karroo to outcompete S. caffra and V. robusta on ultramafic intrusions was confirmed by higher species performance index values for this substrate. It is concluded, therefore, that V. karroo is an indicator for ultramafic rocks types at the KC. The dominance of V. karroo on ultramafic rock types was hypothesised to be due to a higher tolerance of magnesium, chrome, cobalt and nickel by means of element exclusion. Results are in support of this notion, and it is concluded that V. karroo is able to more effectively suppress its magnesium, chrome, cobalt and nickel content compared to other species such as Grewia. flava, S. caffra and V. robusta