The stratigraphy and geochemistry of the Rietgat formation between Allanridge and Bothaville, Free State Province

[摘要] An extensive lithological and geochemical appraisal of the Rietgat Formation wasundertaken. This involved the collection and analysis of 389 borehole core samples inthe area between Allanridge and Bothaville, as well as 23 outcrop samples from theT'Kuip hills. Given the relative visual and petrographic homogeneity of the lavas of theRietgat Formation, the purpose of the exercise was to establish whether these rocksmay be sub-divided on the basis of their geochemistry. Five distinct lava units wereidentified 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 (theUpper Sedimentary Horizon and Lower sedimentary Horizon) were also recognised.Using a selection of incompatible elements, (P20S, Ti02, Zr, Nb, V and Y) in additionto limited REE data, it was proven .with reasonable confidence (in the absence ofisotopic data) that the lavas of the Rietgat Formation represent the partial melting of an .enriched mantle. source, followed by subsequent fractionation of this material. TheREE 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 5distinct lava units were in fact consanguinous and were the product of an RTF-typemagma chamber in the lower crust.Petrographic and XRD data indicate lower greenschist facies metamorphism of thelavas. Fluid inclusion studies determined that the quartz veining in the lavas is of lowertemperature and lower pressure than that characterised by the metamorphic facies. Theveining is therefore interpreted as being of later origin.Interpreted sedimentary processes facilitate the identification of graben structures dueto the presence of clastic-wedge type sedimentation in the vicinity of fault scarps. Fieldand geochemical evidence in combination are consistent with rifling and associatedmagmatism of the Rietgat Formation.

[发布机构] University of the Free State