The stratigraphy and geochemistry of the Rietgat formation between Allanridge and Bothaville, Free State Province
An extensive lithological and geochemical appraisal of the Rietgat Formation was undertaken. This involved the collection and analysis of 389 borehole core samples in the area between Allanridge and Bothaville, as well as 23 outcrop samples from the T'Kuip hills. Given the relative visual and petrographic homogeneity of the lavas of the Rietgat Formation, the purpose of the exercise was to establish whether these rocks may be sub-divided on the basis of their geochemistry. Five distinct lava units were identified according to their P20S/Zr characteristics and accordingly named Units 1-5. Conventional discrimination diagrams were applied to the data and elucidated a .rhyodacitic-andesitic-basaltic affinity for the lava series. Two sedimentary units (the Upper Sedimentary Horizon and Lower sedimentary Horizon) were also recognised. Using a selection of incompatible elements, (P20S, Ti02, Zr, Nb, V and Y) in addition to limited REE data, it was proven .with reasonable confidence (in the absence of isotopic data) that the lavas of the Rietgat Formation represent the partial melting of an . enriched mantle. source, followed by subsequent fractionation of this material. The REE characteristics of the lavas are consistent with contamination and fractionation, which have been explained here ill tern is uf all APC process. It is proposed that the 5 distinct lava units were in fact consanguinous and were the product of an RTF-type magma chamber in the lower crust. Petrographic and XRD data indicate lower greenschist facies metamorphism of the lavas. Fluid inclusion studies determined that the quartz veining in the lavas is of lower temperature and lower pressure than that characterised by the metamorphic facies. The veining is therefore interpreted as being of later origin. Interpreted sedimentary processes facilitate the identification of graben structures due to the presence of clastic-wedge type sedimentation in the vicinity of fault scarps. Field and geochemical evidence in combination are consistent with rifling and associated magmatism of the Rietgat Formation.