Thesis (DSc)--Stellenbosch University, 2012 / ENGLISH ABSTRACT: This study is an investigation of the anatectic history of high-grade paragneisses from the Ancient
Gneiss Complex (AGC) in Swaziland. The work involved an integrated field, metamorphic,
geochemical, geochronological and structural study of metasedimentary granulites from three
separate, but spatially related areas of outcrop in south-central Swaziland, which were subjected to
multiple high-grade partial melting events throughout the Meso- to Neoarchaean. The project has
aimed to constrain the age(s) and conditions of metamorphism, so as to contribute to the
understanding of geodynamic processes in the Barberton and AGC granite-greenstone terranes, as
well as to investigate certain physical and chemical aspects of anatexis in the migmatites. The
metamorphic record retained in these rocks, constrained by phase equilibria modelling as well as
zircon and monazite SHRIMP and LA-ICP-MS geochronology, informs on the state of the mid- to
lower-crust of the southeastern Kaapvaal Craton during key events associated with early lithosphere
assembly and crustal differentiation. It also suggests that the region is comprised of more than one
high-grade terrane. Two of the areas investigated experienced high-temperature metamorphism at
ca. 3.23-3.21 Ga, in addition to a major 830-875º C, 6.5-7.6 kbar anatectic event at ca. 3.11-3.07
Ga. Intermediate and younger high-temperature events are recorded at ca. 3.18 Ga, ca. 3.16 Ga and
2.99 Ga. The timing of these metamorphic events coincided with the amalgamation of the eastern
domain of the proto-Craton via subduction and accretion of micro-continental fragments at ca. 3.23
Ga, including the Barberton Greenstone Belt (BGB) and AGC terranes, as well as discrete episodes
of crustal differentiation and potassic granitic magmatism between ca. 3.23 and 3.10 Ga. The third
area investigated holds no record of Mesoarchaean metamorphism, but instead experienced a 830-
855 ºC, 4.4-6.4 kbar partial melting episode at ca. 2.73 Ga. This broadly coincided with the
formation of a large continental flood basalt province, the ca. 2.71 Ga Ventersdorp LIP, and
widespread intracratonic granitic magmatism on the Craton towards the end of the Neoarchaean. An
explanation for the contrast in metamorphic record in the two terranes may be that the 2.71 Ga
granulites represent a much younger sedimentary succession, and that granulites from the older
terrane were left too restitic, after substantial partial melting during the Mesoarchaean, to record
subsequent high-grade events. Finally, this study documents the details of S-type granitic magma
production and extraction from a typical metapelitic source. Using the 2.73 Ga granulites from the
AGC as a natural field laboratory, a case is made for the selective entrainment of peritectic garnet to
the magma as a mechanism for generating relatively mafic, peraluminous S-type granite
compositions. The work demonstrates the evolution of entrained peritectic garnet in such magmas,
and is in strong support of a ‘peritectic phase entrainment’ process by which relatively mafic granite
magmas are produced from melts which, in theory, should be highly leucocratic. / AFRIKAANSE OPSOMMING: Hierdie studie ondersoek die anatektiese geskiedenis van hoëgraadse metasedimentêre gneise uit die
Ancient Gneiss Complex (AGC) in Swaziland. Die werk behels 'n geïntegreerde veld,
metamorfiese, geochemiese, geochronologiese en strukturele studie van metasedimentêre granuliete
van drie afsonderlike, maar ruimtelik verwante gebiede in suid-sentraal Swaziland, wat aan verskeie
hoëgraadse anatektiese gebeure onderworpe was gedurende die Meso-tot Neoargeïese tydsperiode.
Die studie is daarop gemik om die ouderdomme en die kondisies van metamorfose vas te stel, om
sodoende by te dra tot die begrip van die geodinamiese prosesse in die Barberton en AGC granietgroensteen
terrein, asook om sekere fisiese en chemiese aspekte van die anatektiese proses te
ondersoek. Die metamorfe rekord, bepaal deur mineraal ewewigsmodellering sowel as sirkoon en
monasiet SHRIMP en LA-ICP-MS geochronologie, belig die toestand van die middel-tot laer-kors
van die suidoostelike Kaapvaal Kraton tydens vroeë litosfeer samesmelting en differensiasie. Dit
stel ook voor dat die streek uit meer as een hoëgraadse terrein bestaan. Twee van die gebiede het
hoë-temperatuur metamorfose by 3.23-3.21 Ga ervaar, asook 'n hoof 830-875 ° C, 6.5-7.6 kbar
anatektiese gebeurtenis by 3.11-3.07 Ga. Intermediêre en jonger hoë-temperatuur gebeure was ook
by 3.18 Ga, 3.16 Ga en 2.99 Ga geregistreer. Die metamorfose van die gebied stem ooreen met die
samesmelting van die oos Kaapvaal Kraton domein deur subduksie en aanwas van mikro-kontinente
by 3.23 Ga, insluitend die Barberton en AGC terreine, asook diskrete episodes van kors
differensiasie en kalium-ryke graniet magmatisme tussen 3.23 en 3.10 Ga. Die derde gebied hou
geen rekord van Mesoargeïkum metamorfose nie. In plaas daarvan het dit 'n 830-855 ° C, 4.4-6.4
kbar anatektiese episode by 2.73 Ga ervaar, wat ooreenstem met die vorming van 'n groot
kontinentale vloedbasalt provinsie, die 2.71 Ga Ventersdorp Supergroep, en wydverspreide
intrakratoniese graniet magmatisme teen die einde van die Neoargeïkum. 'n Moontlike
verduideliking vir die kontras in metamorfe rekord in die twee terreine mag wees dat die 2.71 Ga
granuliete 'n jonger sedimentêre afsetting verteenwoordig, en dat granuliete van die ouer terrein te
restieties gelaat was na aansienlike anateksis in die Mesoargeïkum, om daaropvolgende hoëgraadse
gebeure te registreer. Ten slotte, hierdie studie dokumenteer die besonderhede van S-tipe graniet
magma produksie en ontginning van 'n tipiese metasedimentêre bron. Die 2.73 Ga granuliete word
gebruik as 'n natuurlike veld laboratorium om die selektiewe optel-en-meevoering van peritektiese
granaat tot die magma te ondersoek. Die werk toon die evolusie van peritektiese granate in sulke
magmas aan, en ondersteun lewering van relatiewe mafiese graniet magmas deur 'n ‘peritektiese
fase optel-en-meevoerings’ proses.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/20311 |
| Date | 03 1900 |
| Creators | Taylor, Jeanne |
| 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 | English |
| Type | Thesis |
| Format | 140 p. : ill., maps |
| Rights | Stellenbosch University |
Page generated in 0.0028 seconds