A Dissertation Submitted tc the Faculty of Science,
University of the Witwatersrand, Johannesburg,
for the degree of Doctor of Philosophy.
Johannesburg 1981 / An area of about 200 'sq km has been mapped at a scale of
1 : 25 000 between Messina and Tshipise. Subsequent laboratory
work has included petrographic, whole-rock and mineral analysis
in order to describe the Precambrian rock-types and lithologies,
their structure, and their metomorphic history.
The Precambrian lithologies underlying the study area
consist of grey banded basement gneisses of granodioritic
composition, together with a large variety of supracrustal rocktypes.
These include quartzo-feldspathic gneiss, Singeleletype
granitoid gneiss, garnet-cordierite-sillimunite gneiss,
sapphirine-bearing rock, garnet-orthopyroxene-plagioclase
symplectite, pyroxenitic amphibolite, quartzite, banded magnetite
quartzite, amphibolite, calc-silicate gneiss and marble. These
supracrustal rocks may belong to a geosynclinal-type series of
depv .» Led or extruded lithologies. Intrusive rocks of the
Messina Layered Intrusion consist of gabbroic and anorthositic
gneiss. Metapyroxenites and serpentinites also occur. Both
ancient deformed and you ger fabric-free mafic dykes transect
this stratigraphy.
Polyphase deformation has produced complex and intense
folding of the area. Early isoclinal and ductile folds, now
manifest as tight intrafolial folds, have been refolded around
later structures. Most fold hinges plunge moderately to the
south-west. Considerable flattening, attenuation and alongstrike
boudinaging of the units occurs in the region, probably
as a result of regional simple shear The assymetry of the
folds in the region suggest that this simple shear was left
lateral.
Peorce-type variation diagrams for data from the Messina
Layered Intrusion show plagioclase fractionation trends, and
support the argument that these rocks are of plutonic igneous
origin. The anorthcsites were the earliest cumulates, with
the gabbros forming by subsequent fractionation. Rayleigh's
law indicates that about 70 per cent fractionation has occurred
in these rocks. The parental liquid appears to have been
anomalously enriched in rubidium.
The supracrustal units have experienced a high-grade
metamorphism between about 3 100 m.y. ago and 2 400 m.y. ago.
The P-T conditions for this metamorphism range from about 9 kbar
and 900 C at the 'peak' of the metamorphism, to about 4 kbar
and 650°C, and thus represents a retrogression within the field
of medium pressure granulites. Earlier high-pressure granulite
metamorphism is indicated by assemblages reported from other
regions in t.ie Central Zone of the Limpopo Mobile Belt. These
data suggest that the supracrustal rocks were subjected to burial
into regions of the lower crust up to 40 km depth, and geothermal
gradients between 15°C/km and 35°C/km were experienced. Water
activities were low durin.j this high-grade metoeorphism, with
water making up not more than 10 per cent of the fluid
present during this event. The onset of relative tectonic
stability and the end of high-grade metamorphism was achieved
by about 2 200 m.y. 0go.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/18194 |
| Date | 06 August 2015 |
| Creators | Horrocks, Peter Charles Brammer |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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