Thesis (MSc)--University of Stellenbosch, 2004. / ENGLISH ABSTRACT: The southern portions of the Early- to Mid-Archaean Barberton granitoid-greenstone
terrain of South Africa consists of a high-grade metamorphic granitoid-gneiss terrain that
is juxtaposed against the low-grade metamorphic supracrustal sequence of the Barberton
Greenstone Belt. The boundary of the two different crustal domains corresponds to the
Theespruit Formation, an amphibolite-facies, highly tectonized mélange of metabasites,
felsic volcanics and rare, aluminous clastic sediments that occurs along the granitoidgreenstone
margins. Amphibolite-facies lithologies in the Theespruit Formation are
characterized by strongly prolate mylonitic fabrics that formed in a constrictional tectonic
regime. Away from the granitoid-greenstone margin and towards the central parts of the
greenstone belt, these rocks grade to, and are overprinted by, greenschist-facies S-L
mylonites that formed during non-coaxial deformation. Both peak and retrograde
minerals define, and are aligned parallel to, the fabrics in these rocks, indicating that
shearing was initiated under peak metamorphic conditions and continued during
retrogression. S-C’ fabric relationships indicate that shearing occurred in an extensional
tectonic regime and that, during deformation, the gneiss terrain was uplifted relative to
the greenstone belt. Peak metamorphic assemblages of grt-st-bt-chl-pl-qtz and ky-st-btms-
pl-qtz in metasediments and grt-ep-hbl-pl-qtz in amphibolite constrain peak
metamorphic conditions of 7.4 ± 1.0 kbar and 560 ± 20 ºC that were attained during the
main accretionary episode in the Barberton terrain at 3229 ± 25 Ma. Peak assemblages in
all rocks are pre-tectonic and were deformed and re-equilibrated during retrogression,
resulting in these being minimum estimates of peak metamorphic conditions.
Petrographic evidence and retrograde pressure-temperature estimates indicate that
retrogression involved near-isothermal decompression of ca. 4 kbar prior to cooling into
the greenschist-facies. The style and timing of metamorphism in the Theespruit
Formation is similar to that of the granitoid-gneiss terrain, suggesting that the Theespruit
Formation shares a geological history with the gneiss terrain and that it is allochtonous to
the greenstone belt. The main deformational and fabric-forming event exhibited in the
Theespruit Formation occurred during the exhumation of the granitoid-gneiss terrain
subsequent to peak metamorphism. Consequently, the juxtaposition of this terrain againstthe greenstone belt was achieved by tectonic underplating and core complex formation at
ca. 3.23 Ga. The occurrence of high-grade constrictional mylonites that are overprinted
by low-grade non-coaxial mylonites as well as extension in an overall compressional
tectonic regime is consistent with exhumation by extensional orogenic collapse. Burial
of the high-grade terrain to depths of 25 – 30 km is only possible in a relatively cold and
rigid crustal environment, while the extremely low apparent geothermal gradients of ca.
20 ºC/km preserved in this terrain suggest that burial and exhumation occurred rapidly,
within a time-span of ca. 15 – 20 Ma. These parameters strongly suggest that
metamorphism occurred in response to a lateral plate tectonic process that was
operational in the Barberton terrain at 3230 Ma. / AFRIKAANSE OPSOMMING: Die suidelike dele van die Vroeg- tot Middel-Argaïese Barberton graniet-groensteen
terrein van Suid-Afrika bestaan uit ‘n hoë-graad metamorfe graniet-gneiss terrein wat die
lae-graad metamorfe groenstene van die Barberton Groensteen Gordel begrens. Die
grens tussen die twee verskillende kors-domeine hang saam met die Theespruit Formasie,
‘n amfiboliet-fasies, getektoniseerde melange van metabasiete, felsiese vulkaniese
gesteentes en skaars, alumineuse klastiese sedimente wat langs die graniet-groensteen
kontakte voorkom. Amfiboliet-fasies gesteentes in die Theespruit Formasie word
gekenmerk deur sterk prolaat milonitiese maaksels wat in ‘n vernouende tektoniese
omgewing gevorm het. Hierdie rotse word weg van die graniet-groensteen kontak en na
die sentrale dele van die groensteen gordel oordruk en vervang deur groenskis-fasies S-L
miloniete wat tydens nie-koaksiale vervorming gevorm het. Beide piek en retrograad
minerale definieer, en is georienteer parallel aan, die maaksel in die rotse, wat daarop dui
dat skuifskeur onder piek metamorfe toestande begin het en volgehou het tydens
retrogressie. S-C’ maaksels dui daarop dat skuifskeur in ‘n verlengende tektoniese
omgewing plaasgevind het en dat die gneiss terrein opgehef is relatief tot die
groensteengordel tydens vervorming. Piek metamorfe versamelings van grt-st-bt-chl-plqtz
en ky-st-bt-ms-pl-qtz in metasedimente en grt-ep-hbl-pl-qtz in amfiboliet bepaal piek
metamorfe toestande van 7.4 ± 1.0 kbar en 560 ± 20 ºC wat bereik is gedurende die
hooffase van akkresie in die Barberton terrein teen 3229 ± 25 Ma. Die piek metamorfe
versamelings in alle rotse is pre-tektonies en is vervorm en geherekwilibreer tydens
retrogressie, wat maak dat die beramings minimum skattings van piek metamorfe
toestande is. Petrografiese getuienis asook druk-temperatuur beramings dui daarop dat
retrogressie gepaard gegaan het met byna-isotermiese drukverligting van naastenby 4
kbar voor afkoeling tot in die groenskis-fasies. Die styl en tydsberekening van
metamorfose in die Theespruit Formasie is vergelykbaar met metamorfose in die granietgneiss
terrein, wat daarop dui dat die Theespruit Formasie ‘n geologiese geskiedenis met
die gneiss terrein deel en allochtoon is tot die groensteen gordel. Die hooffase van
vervorming en maakselvorming in die Theespruit Formasie het plaasgevind gedurende
die herontbloting van die graniet-gneiss terrein na piek metamorfose. Gevolglik is dieteenplasing van dié terrein teen die groensteen gordel vermag deur tektoniese
onderplasing en kernkompleksvorming teen ongeveer 3.23 Ga. Die verskynsel van hoëgraadse
vernoude miloniete wat oordruk word deur lae-graadse nie-koaksiale miloniete
asook verlenging in ‘n algeheel saamdrukkende tektoniese omgewing dui daarop dat
herontbloting plaasgevind het deur middel van verlengende orogenetiese ineenstorting.
Die begrawing van die hoë-graadse terrein tot dieptes van 25 – 30 km is net moontlik in
‘n relatief koel en star kors-omgewing, terwyl die uitermate lae geotermiese gradiente
van ongeveer 20 ºC/km wat in die terrein behoue gebly het daarop dui dat begrawing en
herontbloting vinnig geskeid het, binne ‘n tydsverloop van ongeveer 15 – 20 Ma. Hierdie
beperkings is ‘n sterk aanduiding dat metamorfose plaasgevind het as gevolg van ‘n
laterale plaattektoniese proses wat werksaam was in die Barberton terrein teen 3230 Ma.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/16309 |
| Date | 12 1900 |
| Creators | Diener, Johann F. A. |
| Contributors | Stevens, Gary, Kisters, Alex, University of Stellenbosch. Faculty of Science. Dept. of Earth Sciences. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | xvi, 210 leaves : ill. |
| Rights | University of Stellenbosch |
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