Thesis (PhD)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: The sodic and leucocratic Tonalite, Trondhjemite and Granodiorite (TTG) granitoid series of
rocks characterise Paleo- to Meso- Archaean felsic continental crust, yet are uncommon in the
post-Archaean rock record. Consequently, petrogenetic studies on these rocks provide
valuable insight into the creation and evolution of Earth’s early continental crust. The highpressure
(HP)-type of Archaean TTG magmas are particularly important in this regard as their
geochemistry requires that they are formed by high-pressure melting of a garnet-rich eclogitic
source. This has been interpreted as evidence for the formation of these magmas by anatexis
of the upper portions of slabs within Archaean subduction zones. In general, TTG magmas
have been assumed to arise through fluid-absent partial melting of metamafic source rocks.
Therefore, very little experimental data on fluid-present eclogite melting to produce Archaean
TTG exist, despite the fact that water drives magmatism in modern arcs. Consequently, this
study experimentally investigates the role of fluid-present partial melting of eclogite-facies
metabasaltic rock in the production of Paleo- to Meso-Archaean HP-type TTG melts.
Experiments are conducted between 1.6 GPa and 3.0 GPa and 700 ºC and 900 ºC using
natural and synthetic eclogite, and gel starting materials of low-K2O basaltic composition.
Partial melting of the natural and synthetic eclogite occurred between 850 ºC and 870 ºC at
pressures above 1.8 GPa, and the melting reaction is characterised by the breakdown of sodic
clinopyroxene, quartz and water: Qtz + Cpx1 + H2O ± Grt1 = Melt + Cpx2 ± Grt2. The
experimental melts have the compositions of sodic peraluminous trondhjemites and have
compositions that are similar to the major, trace and rare earth element composition of HPtype
Archaean TTG. This study suggests that fluid-present eclogite melting is a viable petrogenetic model for this component of Paleo- to Meso-Archaean TTG crust. The nature of
the wet low-K2O eclogite-facies metamafic rock solidus has been experimentally defined and
inflects towards higher temperatures at the position of the plagioclase-out reaction. Therefore,
the results indicate that a crystalline starting material is necessary to define this solidus to
avoid metastable melting beyond temperatures of the Pl + H2O + Qtz solidus at pressures
above plagioclase stability. Furthermore, this study uses numerical and metamorphic models
to demonstrate that for reasonable Archaean mantle wedge temperatures within a potential
Archaean subduction zone, the bulk of the water produced by metamorphic reactions within
the slabs is captured by an anatectic zone near the slab surface. Therefore, this geodynamic
model may account for HP-type Archaean TTG production and additionally provides
constraints for likely Archaean subduction. The shape of the relevant fluid-present solidus is
similar to the shape of the pressure-temperature paths followed by upper levels of the
proposed Archaean subducting slab, which makes water-fluxed slab anatexis is very
dependant on the temperature in the mantle wedge. I propose that cooling of the upper mantle
by only a small amount during the late Archaean ended fluid-present melting of the slab. This
allowed slab water to migrate into the wedge and produce intermediate composition
magmatism which has since been associated with subduction zones. / AFRIKAANSE OPSOMMING: Die reeks natruimhoudende en leukokraties Tonaliet, Trondhjemiet en Granodioriet (TTG)
felsiese stollingsgesteentes is kenmerkend in die Paleo- tot Meso-Argeïkum felsiese
kontinentale kors, maar is ongewoon in die post-Argeïese rots rekord. Gevolglik,
petrogenetiese studies op hierdie rotse verskaf waardevolle insig in die skepping en evolusie
van die aarde se vroeë kontinentale kors. Die hoë-druk (HD)-tipe van die Argeïkum TTG
magmas is veral belangrik in hierdie verband as hulle geochemie vereis dat hulle gevorm
word deur hoë druk smelting van 'n granaat-ryk eklogitiese bron. Dit word interpreteer as
bewys vir die vorming van hierdie magmas deur smelting van die boonste gedeeltes van die
blaaie in Argeïese subduksie sones. TTG magmas in die algemeen, is veronderstel om op te
staan deur middel van water-afwesig gedeeltelike smelting van metamafiese bron rotse.
Daarom bestaan baie min eksperimentele data op water-teenwoordig eklogiet smelting om
Argeïkum TTG te produseer, ten spyte van die feit dat water magmatisme dryf in moderne
boë. Gevolglik is hierdie studie ‘n eksperimentele ondersoek in die rol van water-teenwoordig
gedeeltelike smelting van eklogiet-fasies metamafiese rots in die produksie van Paleo- tot
Meso-Argeïkum HD-tipe TTG smelte. Eksperimente word uitgevoer tussen 1.6 GPa en 3.0
GPa en 700 ºC en 900 ºC met behulp van natuurlike en sintetiese eklogiet, en gel begin
materiaal van lae-K2O basaltiese samestelling. Gedeeltelike smelting van die natuurlike en
sintetiese eklogiet het plaasgevind tussen 850 ºC en 870 ºC te druk bo 1.8 GPa, en die
smeltings reaksie is gekenmerk deur die afbreek van natruimhoudende klinopirokseen, kwarts
en water: Qtz + Cpx1 + H2O ± Grt1 = Smelt + Cpx2 ± Grt2. Die eksperimentele smelte het die
komposisies van natruimhoudende trondhjemites en is soortgelyk aan die hoof-, spoor- en seldsame aard element samestelling van HD-tipe Argeïkum TTG. Hierdie studie dui daarop
dat water-teenwoordig eklogiet smelting 'n lewensvatbare petrogenetiese model is vir hierdie
komponent van Paleo- tot Meso-Argeïkum TTG kors. Die aard van die nat lae-K2O eklogietfasies
metamafiese rock solidus is eksperimenteel gedefinieër en beweeg na hoër temperature
by die posisie van die plagioklaas-out reaksie. Daarom dui die resultate daarop dat 'n
kristallyne materiaal nodig is om hierdie solidus te definieër en metastabiele smelting buite
temperature van die Pl + H2O + Qtz solidus druk bo plagioklaas stabiliteit te vermy. Verder
maak hierdie studie gebruik van numeriese en metamorfiese modelle om aan te dui dat die
grootste deel van die water geproduseer deur metamorfiese reaksies binne die blaaie bestaan
vir redelike Argeïkum mantel wig temperature binne 'n potensiële Argeïkum subduksie sone,
en word opgevang deur 'n smelting sone naby die blad oppervlak. Daarom kan hierdie
geodinamies model rekenskap gee vir HD-tipe Argeïkum TTG produksie en dit bied ook die
beperkinge vir waarskynlik Argeïese subduksie. Die vorm van die betrokke waterteenwoordig
solidus is soortgelyk aan die vorm van die druk-temperatuur paaie gevolg deur
die boonste vlakke van die voorgestelde Argeïkum subderende blad, wat water-vloeiing blad
smeltingbaie afhanklik maak van die temperatuur in die mantel wig. Ons stel voor dat
afkoeling van die boonste mantel met slegs 'n klein hoeveelheid gedurende die laat Argeïese,
die water-vloeiing smelting van die blad beëindig. Dit het toegelaat dat die blad water in die
wig migreer en intermediêre samestelling magmatisme produseer wat sedert geassosieer
word met subduksie sones.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/80214 |
| Date | 03 1900 |
| Creators | Laurie, Angelique |
| Contributors | Stevens, Gary, 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 | 66 p. : ill., maps |
| Rights | Stellenbosch University |
Page generated in 0.0023 seconds