Thesis (PhD (Earth Sciences))--University of Stellenbosch, 2010. / 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CGS and it have been replaced by newly formed minerals (garnet, cordierite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. / AFRIKAANSE OPSOMMING: S-tipe granietinstrusies is baie algemeen in die kontinentale kors en vorm deur
die gedeeltelike smelting van metasedimente. Samestellings van S-tipe graniete
strek vanaf leukograniet tot granodioriet en het spoorelementsamestellings wat
global toeneem met ’n toenemende mafiese component (Fe + Mg). Modelle wat
voorgestel is vir die formasie van S-tipe graniete beantwoord nie bevredigend al
die petrologiese en komposisionele benodigdhede nie. In hierdie studie word S-tipe
graniete van die Kaapse Graniet Suite (CGS), Suid Afrika, gebruik om te diskrimineer
tussen potensiele bronne van komposisionele variasie. Eksperimentele studies
wys dat smelt, geproduseer van die gedeeltelike smelting van sedimente, uitsluitlik
leukokraties is. Op hierdie basis kan bewys word, dat die optel-en-meevoering van
tot 20 wt% van peritektiese granaat in S-tipe smelt, die waargeneemde hoofelement
variasies kan produseer. S-tipe CGS bevat lokale granaat. Hierdie granaat is in
ekwilibrium met die graniet samestelling by P-T kondisies (5kb en 750circC vir die
kern van die granaat en 3kb en 720circC vir die rand) ver onder kondisies waargeneem
by xenoliete van dieselfde granite (10kb en 850circC van ’n metabasiet). Van
hierdie resultaat kan afgelei word dat die oorspronklike opgetel-en-meegevoerde
graniet bestaan nie meer in die S-tipe CGS en dat dit vervang is deur nuutgevormde
minerale (granaat, kordieriet en biotiet). As in ag geneem word die kort tyd wat nodig is om graniete in te plaas (omtrent 100 000 jaar), wil dit voorkom dat
granaat se samestelling geherekwilibreer word deur ’n oplossings-presipitasie proses.
Die studie van spoorelement variasies in S-tipe CGS wys dat meeste leukokratiese
samestellings is onderversadig in Zr en Ce in vergelyking met voorspellings deur
eksperimentele modelle vir die oplossing van bykomstige zircon en monasiet in hulle
brongebiede. Dus is S-tipe smelte meer geneig om gevorm te word in disekwilibrium
in verhouding tot bykomstige mineraalstabilileit. Met die gevolg is dat die
waargenome toename in spoorelementinhoud met toename in mafiese component
wys dat bykomstige minerale, soos zirkoon en monasiet, word saam opgetel-enmeegevoer
met peritektiese granaat in smelt om die waargenome spoorelement
variasie in S-tipe graniete te verklaar. Spoorelement disekwilibrium in die CGS
S-tipe granitoide benodig veral kort tye van residensie van die smelt binne die
brongebied. Saam gee hierdie resultate vir die eerste keer ’n algehele antwoord vir
hoof- en spoorelement variasies. Variasie in samestelling in CGS S-tipe graniete is
die resultaat van bronprosesse deur ’n selektiewe optel-en-meevoer van peritektiese
en bykomstige minerale. Na die optel-en-meevoer van hierdie minerale word hulle
geherekwilibreer binne die magma deur ’n oplossings-presipitasie proses. Addisioneel
wil dit voorkom of die konstruksie van groot S-tipe granietliggame plaasvind
deur opeenvolgende toevoegings van magma lotte van verskillende samestellings wat
direk uit die brongebied kom.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/4015 |
| Date | 03 1900 |
| Creators | Villaros, Arnaud |
| Contributors | Stevens, G., Buick, I. S., University of Stellenbosch. Faculty of Science. Dept. of Earth Sciences. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 208 p. : ill., maps |
| Rights | University of Stellenbosch |
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