The metamorphic and anatectic history of Archaean metapelitic granulites from the South Marginal Zone, Limpopo Belt, South Africa.

Nicoli, Gautier (2015-04)

Thesis (DSc)--Stellenbosch University, 2015.

Thesis

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.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/97041
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