Thesis (DSc)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: Anatexis is the first step in granite genesis. Partial melting in the lower crust may produce leucoratic features of unusual chemical compositions, very different from the final products
of crustal differentiation. Therefore, the links that exists between some migmatites
and crustal-derived granites can be ambiguous.
This study is an investigation of the anatectic history of a high-grade terrain: the Southern
Marginal Zone of the Limpopo Belt (SMZ), north to the Kaapvaal Craton in South
Africa. The work involved an integrated field, metamorphic, geochemical and geochronogical
study of the metasedimentary granulites from two separate quarries in the northern
zone of the Southern Marginal Zone, the Bandelierkop quarry and the Brakspruit quarry,
where Neoarchean high-grade partial melting features can be observed.
The project has aimed to address two main issues: (1) to accurately constrain the pressuretemperature
conditions and the age of the metamorphic episode in the SMZ, with implication
for the geodynamic processes near the end of the Archean, (2) to investigate
the fluid-absent partial melting reactions that control formation of K2O-poor leucosomes
and to understand the chemical relationships in the system source-leucosome-melt–S-type
granite.
The P-T-t record retained in the Bandelierkop Formation metapelites, constrained by
phase equilibria modelling as well as zircon LA-ICP-MS geochronology, gives an insight
into crustal differentiation processes in the lower crust. Rocks in both quarries indicate
high-temperature metamorphism episodes with peak conditions of 840-860 oC and 9-11
kbar at c. 2.71 Ga with formation of leucosomes (L1) during the prograde path. Minor
leucocratic features (L2) were produced during decompression to 6-7 kbar. The end of the
metamorphic event is marked by the granulites/amphibolites facies transition (< 640 oC)
at c. 2.68 Ga. The maximum deposit age for the detrital zircons in the metapelites (c.
2.73 Ga) indicates a rapid burial process ( 0.17 cm.y1). Those evidences strongly support
that the Southern Marginal Zone contains sediments deposited in an active margin
during convergence, and that the metapelites were metamorphosed and partially melted
as a consequence of continental collision along the northern margin of the Kaapvaal Craton
at c. 2.7 Ga.
The leucocratic features generated along this P-T-t path display an unusual chemistry
with low K2O and FeO+MgO content and high CaO content. The combination of field
observations, chemical mapping and geochemical analyses leads to the conclusion the major
part of the leucosomes (L1) crystallized prior to syn-peak of metamorphism concurrent
with melt extraction from the source.
This study documents the details of leucosomes formation using field observations in
the Southern Marginal Zone and numerical modelling. This work demonstrates that the
formation of K2O-poor leucosome in the metasedimentary lower crust is controlled by
the difference in volume of equilibration and heterogeneities within the migmatites. The
partial melting of the source coupled with melt loss and water diffusivity within the melt
transfer site is a potential mechanism to explain the chemical link in the sytem residuum–
melt–S-type granite. / AFRIKAANSE OPSOMMING: Anateksis is die eerste stap in granietgenese. Meganismes wat in die onderste kors aan
die werk is, is verantwoordelik vir korsdifferensiasie en bepaal die chemiese samestelling
van die graniet.
Hierdie studie het’n ondersoek behels van die anatektiese geskiedenis van ’n ho egraadse
terrein: die suidelike randstreek van die Limpopo-gordel, noord van die Kaapvaal-kraton
in Suid-Afrika. Die werk het ’n ge integreerde veld- , metamorfiese, geochemiese en
geochronologiese studie van die metasedimentêre granuliete van twee afsonderlike groewe
in die noordelike sone van die suidelike randstreek (SRS), die Bandelierkop-groef en die
Brakspruit-groef, waar Neoarge iese ho egraadse gedeeltelike smeltkenmerke waargeneem
kan word, ingesluit.
Die projek was gerig op die ondersoek van twee belangrike kwessies: (1) om die drukâtemperatuurtoestande
en die ouderdom van die metamorfiese episode in die SRS akkuraat te
beheer, met implikasie vir die geodinamiese prosesse naby die einde van die Arge ikum, en
(2) om die reaksies onder gedeeltelik gesmelte toestande wat die vorming van migmatiete
beheer, te ondersoek en die chemiese verwantskappe in die stelsel bron - leukosoom - smelt
- S-tipe graniet te begryp.
Die P-T-t-rekord wat in die Bandelierkop-formasie metapeliete behoue is, ingeperk deur
modellering van fase-ekwilibria asook sirkoon LA-ICP-MS-geochronologie, gee insig in korsdifferensiasieprosesse
in die onderste kors. Rotse in albei groewe dui op metamorfismeepisodes
teen hoë temperature met piektoestande van 840â860 oC en 9â11 kbar teen
ongeveer 2.71 Ga met vorming van leukosome (L1) gedurende die progradeerpad. Geringe
leukokratiese eienskappe (L2) het tydens dekompressie tot 6â7 kbar ontstaan. Die einde
van die metamorfiese voorval word gekenmerk deur die fasiesoorgang van granuliete /
amfiboliete (<640 oC) by ongeveer 2.68 Ga. Die maksimum afsettingsouderdom vir die
detitrale sirkone in die metapeliete (ongeveer 2.73 Ga) dui op Å snelle begrawingsproses (
0.17 cm.y1). Daardie bewyse bied sterk ondersteuning daarvoor dat die SRS sedimente
bevat wat gedurende konvergensie in Å aktiewe rand afgeset is, en dat die metapeliete
gemetamorfoseer en gedeeltelik gesmelt het as gevolg van kontinentbotsing langs die noordelike
rand van die Kaapvaal-kraton teen ongeveer 2.7 Ga.
Die leukokratiese eienskappe wat langs hierdie P-T-t-pad opgewek word, toon Å ongewone
chemiese samestelling met lae K2O en FeO+MgO-inhoud en ho e CaO-inhoud. Die
kombinasie van veldwaarnemings, chemiese kartering en geochemiese ontledings lei tot die
gevolgtrekking dat die grootste deel van die leukosome (L1) gekristalliseer het voor die
syn-piek van metamorfisme tesame met smeltekstraksie van die bron. Hierdie studie het
die besonderhede van leukosoomformasie met behulp van veldwaarnemings in die SRS
en numeriese modellering opgeteken. Hierdie werk toon aan dat korsdifferensiasie in die
metasedimentêre onderste kors deur Å ander volume van ekwilibrasie en heterogeniteite in
die migmatiete beheer word. Die gedeeltelike smelting van die bron gepaard met smeltverlies
en waterdiffusiwiteit tot in die smeltoordragterrein is ’n potensiele meganisme om die
chemiese skakel in die stelsel residuum-smelt-S-tipe graniet te verklaar.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/97041 |
| Date | 04 1900 |
| Creators | Nicoli, Gautier |
| Contributors | Stevens, Gary, Moyen, Jean-Francois, Stellenbosch University. Faculty of Science. Dept. of Earth Sciences. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 117 pages : illustrations |
| Rights | Stellenbosch University |
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