Field relationships and petro-chemical investigation of mafic sills and dykes in the vicinity of the Uitkoms complex, Mpumalanga South Africa
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Date
2011-05
Authors
Pecher, Marie-Luise
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
Numerous mafic sills and dykes intruded into the Lower Transvaal sediments and the Archaean
Basement in the vicinity of the Uitkomst Complex, which is assumed to be a satellite intrusion of the
Bushveld Complex. Investigations on mafic sill intrusions near the Eastern Bushveld Complex described
sills of pre-, syn- and post-Bushveld age and assigned the syn-Bushveld sills to the corresponding
marginal rocks of the Bushveld Complex. Purpose of the presented combined diploma thesis was to map the mafic sills and dykes in the vicinity of
the Uitkomst Complex as basis for a petrographical and geochemical characterization. The geological
mapping as well as the petrographic description distinguishes three groups of mafic intrusive rocks:
microgabbro sills, gabbronorite sills and gabbronoritic basement dykes. The group of elongated
microgabbro sills shows a widespread spatial as well as stratigraphic distribution within the study area,
whereas the gabbronorite sills form huge sill bodies within a main stratigraphic position between the
Upper Timeball Hill Shale and the Klapperkop Quartzite. The gabbronoritic basement dykes intruded
into the Archaean granite gneiss and represent the oldest of the investigated mafic rocks. Evaluation of the obtained geochemical data from about 160 samples verifies the classification into the
three main groups. Based on incompatible element contents and element ratios different magma
derivations are interpreted for the gabbronoritic sills, basement dykes and microgabbro sills. The
basement dykes and gabbronorites sills are derived from a primitive partial melt, whereas the
microgabbro sills show an evolved magma composition. Additional contamination with crustal material
also changed the magma composition of each group as reflected by changing Ti/Zr ratios within the
groups. The gabbronorite sills and basement dykes are probably contaminated with SiO2-rich material
from quartzites and granite gneiss (highest SiO2 and Zr values), whereas the microgabbro sills show
possible contaminations with dolomitic material (highest variation in CaO contents).The comparison to Bushveld related marginal rocks shows a similar composition of the gabbronorite sills
and basement dykes to the B1 quenched textured micropyroxenites, whereas the microgabbro sills
indicate little correlation with the composition of one of the Bushveld marginal rocks (B1 to B3).
Furthermore the microgabbro sills are nearly conformable in composition of selected trace elements
with the Basal Gabbro chilled margins at Uitkomst and Slaaihoek, which is supported by a possible
palaeomagnetic age relationship in some extent. The geochemical fingerprints of the investigated sills
and dykes compared to the Uitkomst Complex have identified no direct correlation between them and
show only low contents of exploration relevant elements for microgabbro sills, gabbronorite sills as well
as basement dykes.
Description
Keywords
Dikes (Geology)--South Africa--Mpumalanga, Sills (Geology)--South Africa--Mpumalanga, Sills (Geology)--South Africa--Mpumalanga, Geochemistry--South Africa--Mpumalanga, Gabbro--South Africa--Mpumalanga, Dissertation (M.Sc. (Geology))--University of the Free State, 2011