A petrographic and geochemical study of mafic sills within the Transvaal Supergroup, east of the Bushveld Igneous Complex
Date
2017-12
Authors
Journal Title
Journal ISSN
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Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: The marginal sills that intruded into the Transvaal Supergroup have long been used
to infer the parental magmas of the Rustenburg layered suite (RLS) due to their
geochemical similarities which they share with the different zones of the RLS. New
geochemical and petrographical data on mafic-ultramafic sills hosted within the
sedimentary rocks across most of the Transvaal Supergroup, as well as below it (i.e.,
into the Archean basement) reveal a large group of ‘boninitic norites’ (BN) with primary
mineral assemblage of euhedral orthopyroxene, subhedral plagioclase and interstitial
plagioclase as well as tholeiitic dolerites (TDs) that have a primary mineral
assemblage of euhedral plagioclase, clinopyroxene, and accessory Fe–Ti oxides.
There are also some ultramafic sills that clearly formed through the accumulation of
mainly olivine crystals, suggesting in situ fractional crystallisation within the sills. The
slightly ultramafic and orthopyroxene-rich BN-sills are not believed to be cumulates,
however, as testified by ‘spinifex-textured’ examples of similar compositions that may
be regarded as quenched magmas.
In many cases both BN and TD type sill samples are severely altered or even
metamorphosed into mainly amphibole-rich mineral assemblages (as well as
serpentinized harzburgites), where normative calculations become the best way to
interpret protolith modal compositions. It is not obvious to classify more
altered/metamorphosed sills as being pre-Bushveld and more pristine samples as synto
post-Bushveld, because local alteration, as opposed to regional contact
metamorphism, may also have played a role.
Truly boninitic sills have relatively high MgO, Si, LREE and lower Fe-Ti and were
emplaced throughout the sedimentary sequence, together with graphic quartzfeldspar
dominated norites, which through their geochemistry are believed to have
been more differentiated magmas within the same BN-suite. A comparison of
geochemical signatures confirms a link between the BN rocks and the early B1
magmas of the Bushveld Complex. However, the wider stratigraphical distribution of
BN sills argue against these clearly Bushveld related magmas to only have been
injected from the large magma chamber that formed the RLS, but rather that many BN
sills also were injected prior to the magma chamber’s establishment as well as
replenished it during the deposition of the RLS.
The geochemical characteristics of the tholeiitic dolerites (high Ti, low Si, Mg and LILE)
do not match other proposed Bushveld-related B2 and B3 parental magmas, These
TD sills also seem to cluster within the Silverton Formation, the Archean basement,
as well as the Malmani Group (mainly cutting the Uitkomst Complex), and could
therefore have formed during an entirely different magmatic event. A geochemical
match suggests that most basement sills were fed by an intersecting Rykoppies dyke
swarm, during the early deposition of the Wolkberg Group. A similar geochemical
comparison cannot be made with overlying Transvaal Supergroup hosted lavas, where
it seems plausible that many of the Silverton-hosted TD sills may have formed during
the deposition of the Machadodorp Formation. However, geochemical similarities with
the ~1.1 Ga Diabete dyke swarm also opens up for many of the TD suite being post-
Bushveld (especially those that cut throughout the Uitkomst Complex).
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar
Description
Thesis (MSc)--Stellenbosch University, 2017.
Keywords
Sedimentary rocks, Petrographical data, Geochemical data, Sills (Geology), UCTD