Doctoral Degrees (Earth Sciences)

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Browsing Doctoral Degrees (Earth Sciences) by browse.metadata.advisor "Buick, I. S."

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    Petrogenesis of S-type granites : the example of the Cape Granite Suite
    (Stellenbosch : University of Stellenbosch, 2010-03) Villaros, Arnaud; Stevens, Gary; Buick, I. S.; University of Stellenbosch. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH SUMMARY: S-type granite intrusions are extremely common in the continental crust and form from the partial melting of metasediments. Compositions of S-type granite range from leucogranite to granodiorite and have trace element contents that globally increase with increasing maficity (Fe + Mg). Models proposed for the formation of S-type granite do not answer satisfactorily all petrological and compositional requirements. In this study, S-type granite of the Cape Granite Suite (CGS), South Africa is used to discriminate between potential sources of compositional variation. Experimental studies show that melt produced from the partial melting of sediment is exclusively leucocratic. On this basis, the entrainment of up to 20 wt.% of peritectic garnet within S-type melt can be established to produce the observed major element variations. S-type CGS locally contains garnet. This garnet is in equilibrium with granite composition at P-T conditions (5kb and 750 C for the core of the garnet and 3kb and 720 C for the rim) well below conditions recorded by xenoliths from the same granite (10 kb and 850 C from a metabasite). From this result it seems that the originally entrained garnet no longer exists in the Stype CGS and it have been replaced by newly formed minerals (garnet, cordierite and biotite). Considering the short time necessary to emplace granites (about 100 000 years), it appears that garnet has been compositionnally re-equilibrated through a dissolution-precipitation process. The study of trace element variations in S-type CGS shows that most leucocratic compositions are undersaturated in Zr and Ce compared to predictions from experimental models for dissolution of accessory zircon and monazite in their source regions. Thus, S-type melts are likely to be formed in disequilibrium with respect to accessory phase stability. As a result the observed increase in trace element content with increasing maficity indicates that accessory minerals such as zircon and monazite are co-entrained with peritectic garnet in melt to produce the observed trace element variation in S-type granite. Trace element disequilibrium in the CGS S-type granitoids requires particularly short times of residence of melt within the source region. Together, these results provide for the first time, a fully comprehensive model for major and trace elements variations. Compositional variation in CGS S-type granite results from source processes by a selective entrainment of peritectic and accessory minerals. After entrainment, these minerals are likely to be re-equilibrated within the magma, through a dissolution-reprecipitation process. In addition, it appears that the construction of large S-type granitic bodies occurs through successive addition of magma batches of different composition that originates directly from the source region.