Thesis (MSc)--Stellenbosch University, 2014 / ENGLISH ABSTRACT: The compositions of tonalite-trondhjemite-granodiorite (TTG)-series rocks from the
Barberton Granite-Greenstone Terrain (BGGT) of Southern Africa vary in terms of major
element concentrations (leuco-trondhjemites to tonalites), trace elements (e.g. Sr, Rb, REE)
and degree of HREE depletion [(La/Yb)N ~ 0.50 - 121]. These geochemical parameters have
been interpreted to have significance for the geodynamic processes that produced TTG
magmas, particularly the depth of the source. In the Nelshoogte pluton, trondhjemite
emplacement occurred prior to and during (D2)-tectonism, and hornblende-bearing
tonalites/granodiorites occur as intrusive plugs within the trondhjemite. The geochemical
range portrayed by the ca. 3230 Ma Nelshoogte pluton covers almost the whole range
portrayed by all the TTG rocks of the BGGT, formed in the time interval 3550 to 3230 Ma.
U-Pb zircon dating reveals no significant differences in apparent ages between the
trondhjemites and tonalites of the Nelshoogte pluton; all lie between ca. 3240 and 3220 Ma.
A/CNK- and Ti-maficity correlations within the Nelshoogte pluton show behaviour typical of
I-type granitic rocks, confirming the metamafic character of the source and the involvement
of a peritectic garnet and clinopyroxene component in the magmas. The REE and traceelement
patterns resemble those of both ca. 3450 Ma TTG plutons and other ~ 3230 Ma TTG
plutons in the BGGT. This suggests that these different generations of TTG magmas were
generated from similar sources and by similar processes. The geochemical details are
consistent with a metamafic source which underwent melting at high pressure. This TTG
source was fundamentally different from the more potassic source/s that produced the
younger 3100 Ma granites. Additionally, high Zr/Sm and low Nb/Ta ratios in the Barberton
TTG rocks suggest that the Nelshoogte pluton originated as a group of chemically distinct
magmas, in a similar way to other ~ 3200 Ma TTGs. The protoliths of ~ 3450 Ma plutons and
~ 3200 Ma plutons might typically be rutile-bearing eclogite and/or 10 to 30 % garnet
bearing amphibolite.
The geochemical features of the Barberton TTGs suggest similar source compositions, but
different pressures of partial melting. For the ca. 3230 Ma plutons, the rocks of the Badplaas
pluton and the Nelshoogte trondhjemites/tonalites correspond to the high-pressure magmas,
as reflected by high Sr content, as well as high Sr/Y and (La/Yb)N ratios. In contrast, the
Kaap Valley TTGs and Nelshoogte granodiorites reflect the melting of a similar source at
moderate pressure. Furthermore, the Hf isotope data; Hft (+ 0.1 to + 1.9) and TDM model ages (3330 - 3230 Ma), suggest that the source rocks are ~ 20 to 100 Myr older than the
pluton. / AFRIKAANSE OPSOMMING: Die samestellings van tonaliet-trondhjemiet-granodioriet (TTG)-reeks gesteentes vanaf die
Baberton Graniet-Groensteen terrien (BGGT) van Suiderlike Afrika verskil in terme van
hoofelement konsentrasies (leuko-trondhjemiete tot tonaliete), spoorelemente (bv. Sr, Rb en
seldsameaarde-elemente(SAE)) en ook van swaar seldsameaarde-element (SSAE) uitputting
[(La/Yb)N ~ 0.50 - 121]. Deur interpritasie is daar vasgestel dat hierdie geochemiese
beperkings betekenis dra vir die geodinamiese prosesse wat TTG magmas produseer het, en
veral vir die diepte van die bron. In die Nelshoogte pluton het trondhjemiet inplasing voor en
gedurende (D2)-tektonisme voorgekom en horingblende-draende tonaliet/granodioriet kom
voor as intrusiewe proppe binne-in die trondhjemiet. Die geochemiese reeks wat uitgebeeld is
deur die ~ 3230 Ma Nelshoogte pluton ondersteun byna die hele reeks wat uitgebeeld is deur
al die TTG gesteentes van die BBGT wat gevorm het gedurende die tydsinterval 3550 tot
3230 Ma. U-Pb sirkoon datering openbaar geen merkwaardige verskille in ouderdomme
tussen die trondhjemiete en tonaliete van die Nelshoogte pluton nie. Al die ouderdomme lê
tussen 3240 en 3220 Ma.
A/CNK- en Ti-mafisiteit korrelasies binne die Nelshoogte pluton toon die tipiese gedrag van
I-tipe granitiese gesteentes, en bevestig sodoende die metamafiese karakter van die bron,
asook die betrokkenheid van ‘n peritektiese granaat en klinopirokseen component in die
magmas. Die seldsameaarde-element en spoorelement patroone lyk soos dié van albei ~ 3450
Ma plutons en ander ~ 3230 Ma TTG plutons in die BGGT. Dit stel voor dat hierdie
verskillende generasies van TTG magmas genereer was vanaf soortgelyke bronne en ook
deur soortgelyke prosesse. Die geochemiese besonderhede stem ooreen met ‘n metamafiese
bron wat smelting teen hoë druk ondergaan het. Hierdie TTG bron het fondamenteel verskil
van die meer kalium-ryke bron/ne wat die jonger 3100 Ma graniete prodiseer het. In ‘n
eenderse manier as ander ~ 3200 Ma TTG gesteentes, stel hoë Zr/Sm en lae Nb/Ta
verhoudings in die Baberton TTG gesteentes verder voor dat die Nelshoogte pluton ontstaan
het as ‘n groep chemies verskillende magmas. Die protoliete van ~ 3450 Ma plutons en ~
3200 Ma plutons mag dalk tipies rutiel-draende eklogiet en/of 10 tot 30% granaat draende
amfiboliet wees.
Die geochemiese aspekte van die Baberton TTGs stel eenderse bron komposisies, maar
verskillende drukke van gedeeltelike smelting voor. Vir die ~ 3230 Ma plutons stem die
gesteentes van die Badplaas pluton en die Nelshoogte trondhjemiete/tonaliete ooreen met die hoë druk magmas, soos weerspieël deur hoe Sr inhoud, sowel as hoe Sr/Y en (La/Yb)N
verhoudinge. In kontras weerspiel die Kaap Valley TTGs en Nelshoogte granodioriet die
smelting van ‘n eenderse bron teen matige druk. Verder stel die Hf isotoop data; Hft (+ 0.1
to + 1.9) en TDM model ouderdomme (3330 - 3230 Ma) voor dat die bron gesteentes ~ 20 to
100 Mjr ouer is as die pluton.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/86589 |
| Date | 04 1900 |
| Creators | Matsumura, Risa |
| Contributors | Stevens, Gary, Clemens, J. D., Stellenbosch University. Faculty of Science. Department of Earth Sciences. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | xi, 108 p. : ill. |
| Rights | Stellenbosch University |
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