The stratigraphy and geochemistry of the Rietgat formation between Allanridge and Bothaville, Free State Province
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Date
2001-02
Authors
Crozier, Jeremy
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
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.
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Keywords
Dissertation (M.Sc. (Geology))--University of the Free State, 2001, Paleontology, Stratigraphic, Paleontology -- South Africa -- Free State