An investigation of archean and paleoproterozoic metamorphic processes in the Limpopo belt via detailed analysis of reaction textures in metapelites from the Bandelierkop Formation, Southern Marginal Zone of the Limpopo belt
Date
2021-12
Authors
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Journal ISSN
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Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: The well exposed granulite facies rocks in the Southern Marginal Zone (SMZ) of the Limpopo Belt
(LB), South Africa, represent an exceptional window into the lower crust where partial melting
processes can be directly observed and investigated. Previous studies have demonstrated that these
rocks experienced a single granulite facies metamorphic event at ca. 2.71 Ga under conditions of
~850 ᵒC and 11 kbar along with partial melting via a sequence of biotite incongruent melting
reactions, which produced melt together with peritectic garnet, orthopyroxene, cordierite,
sillimanite and plagioclase. In rare cases, peritectic garnets host an abundance of
euhedral plagioclase inclusions with variable compositions. Despite their potential that
they present in understanding details of the melting processes they have not been a focus of a
previous study. This thesis focuses on these inclusions in order to understand the
anatectic processes responsible for the generation of the metasedimentary migmatites in the
SMZ, and importantly also characterises their retrograde metamorphic history. Specifically, this
work integrates geochemical, geochronological and petrological investigations of metapelitic
granulites from two localities in the Bandelierkop Formation (the Bandelierkop quarry and the
Brakspruit quarry), where Neoarchean high-grade metamorphic textures have been preserved.
Electron beam microanalysis show that some of the euhedral plagioclase inclusions have high
anorthite content (up to An83) compared to the majority of plagioclase inclusions, as well as the
matrix plagioclase, in these rocks. This is interpreted to reflect disequilibrium melting of
plagioclase in the source due to slow diffusion in plagioclase. The variable composition
of these euhedral plagioclase crystals suggests their entrainment by the melt from different
localised domains and the entrapment at the sites where peritectic garnet was growing from the
melt. The preservation of these garnets must be due to rapid and efficient melt loss from the
residual source, possibly aided by short residency of these granulites at high temperature due to
fast exhumation.
The peak metamorphic assemblage is partially retrogressed under amphibolite facies conditions. The
detailed petrological investigations of the retrogressed metapelitic granulites, combined with
LA-ICP-MS dating on rutile suggests that the SMZ of the LB has experienced a previously
unrecognised Paleoproterozoic regional amphibolite facies event. This Paleoproterozoic event is 600
Myrs younger than the age of peak metamorphism in the SMZ. Within the Kaapvaal Craton, it relates
to the ca. 2.15 Ga U-Pb age from metamorphic monazite and titanite in the Ghaap group, Chuniespoort
Group and the Pretoria Group of the Transvaal Supergroup and the ca. 2.1 monazite age from the
Central Rand Group of the Witwatersrand, all of which have been interpreted to indicate
craton-scale
fluid flow associated with thrusting and folding in the Kaapvaal Craton.
Globally, the ca. 2.15 Ga event correlates to the ca. 2.22–2.15 Ga tectonic driven fluid flow and
the Ophthalmain orogeny in the Pilbara Craton in Western Australia. It can be concluded
that the SMZ of the LB, is a polymetamorphic terrain, with an older granulite facies
anatectic metamorphic event at ca. 2.71 Ga and a younger
amphibolite facies Paleoproterozoic overprint at ca. 2.1 Ga.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
Description
Thesis (PhD)--Stellenbosch University, 2021.
Keywords
Limpopo Belt (South Africa and Zimbabwe), Facies (Geology), Granulite facies rocks -- South Africa -- Limpopo Belt, Metapelites, UCTD