No description available.

No description available.

The Archean (~3.0 Ga) Buhwa Greenstone Belt, and surrounding granite-gneiss terrane, is the least understood major greenstone belt in the Archean Zimbabwe Craton, despite occupying a critical position between an early Archean continental nucleus and the Limpopo Belt. The cover succession in the Buhwa Greenstone Belt, which was probably deposited on the margin of this nucleus, is divisible into shelfal and basinal facies associations separated by a transitional facies association. The shelfal association consists mostly of quartzarenite and shale, but also contains a thick succession of iron-formation. Geochemical characteristics of the shales indicate that the source terrane consisted of several lithologies including tonalite, mafic-ultramafic volcanic rocks, and granite that underwent intense chemical weathering. Basinal deposits consist dominantly of greenstones, with less abundant chert and ironformation. The cover succession, which was deposited on a stable shelf transitional to deep water, has no stratigraphic equivalents elsewhere on the Archean Zimbabwe Craton. However, time and lithologic correlatives in the central zone of the Limpopo The Archean (-3.0 Ga) Buhwa Greenstone Belt, and surrounding granite-gneiss terrane, is the least understood major greenstone belt in the Archean Zimbabwe Craton, despite occupying a critical position between an early Archean continental nucleus and the Limpopo Belt. The cover succession in the Buhwa Greenstone Belt, which was probably deposited on the margin of this nucleus, is divisible into shelfal and basinal facies associations separated by a transitional facies association. The shelfal association consists mostly of quartzarenite and shale, but also contains a thick succession of iron-formation. Geochemical characteristics of the shales indicate that the source terrane consisted of several lithologies including tonalite, mafic-ultramafic volcanic rocks, and granite that underwent intense chemical weathering. Basinal deposits consist dominantly of greenstones, with less abundant chert and ironformation. The cover succession, which was deposited on a stable shelf transitional to deep water, has no stratigraphic equivalents elsewhere on the Archean Zimbabwe Craton. However, time and lithologic correlatives in the central zone of the Limpopo ~2.9 Ga in southern Africa. At ~2.9 Ga, the northern margin of the greenstone belt experienced kilometerscale, oblique-slip dextral shearing. This shear zone and the surrounding margins of the greenstone belt were later intruded by the ~2.9 Ga Chipinda batholith, which ranges from granitic to tonalitic in composition. A number of events occurred during the time period spanning 2.9-2.5 Ga and current geochronology cannot separate their order; some are known to be coeval. Crustal shortening to the northwest, which resulted in map-scale folding of the cover succession (and surrounding batholith) and greenschist-facies metamorphism, occurred along a set of discrete high-angle reverse-sense shear zones in response to uplift the Northern Marginal Zone of the Limpopo Belt over the Zimbabwe Craton. Two suites of potassic granites were intruded into the area near the end of reverse shearing. Analysis of a conjugate fault pair that is developed within one of the potassic granite suites, yields a principal compressive stress consistent with continued northwest-directed crustal shortening. The region was stabilized by ~2.5 Ga, with intrusion of the Great Dyke of Zimbabwe. It is possible that the last events to affect the area, which include sinistral shearing, transecting cleavage development, and northwest-striking open folding, took place during the 2.9-2.5 Ga time intervaL These structures post-date regional folding and metamorphism, but because of limited magnitude and extent, do not show obvious cross-cutting relationships with other rocks or structures. A tenable alternative is that these late structures formed at ~2.0 Ga. an age that is proving to be of great significance in the evolution of the Limpopo Belt and along parts of the southern margin of the Zimbabwe Craton. / Ph. D.

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.

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.

Combined Lu-Hf, Sm-Nd and Rb-Sr isotopic studies of continental crustal rocks were undertaken to assess the relative effects of secondary crustal processes on isotopic systematics of whole-rock systems. The processes studied include ductile deformation, and three cases of hydrothermal alteration, involving fluids of varying composition. The Rb-Sr system proved to be easily disturbed during all secondary processes, while Sm-Nd and Lu-Hf systems were, for the most part, resilient. These results show that Nd or Hf isotopic information obtained from old rocks that have undergone typical crustal deformational and alteration events can be counted on to be equally reliable. Nd and Hf isotopic analyses were performed on four suites of Early Archean felsic gneiss complexes from Greenland, Labrador, Swaziland, and Michigan to explore questions associated with Early Archean crustal growth. The Sm-Nd isotopic data yield initial ∊(Nd) values that are mostly consistent with published age data for the suites. Calculations show limited scatter may be attributed to subtle changes in the Sm/Nd ratio or Nd isotopic composition. The Hf isotopic results are more variable and complex than the Nd results. The relevance of the studies on isotopic mobility in the first part of this work is that they have demonstrated that Nd and Hf isotopes are equally resilient during a range of secondary crustal processes. Given the robustness of the Nd isotopic data from the Archean samples, however, it seems unreasonable to attribute the much wider variation in Hf isotopic data to post-Archean isotopic disturbances. Differences in initial Hf isotopic ratios from differing magma sources seem called for. Nd and Hf whole-rock analyses of a Late Archean pristine garnet-bearing granitoid complex from northern Canada point out the importance of garnet in fractionating Lu/Hf ratios, and in developing anomalous ∊(Hf) signatures in potential source regions. Calculations show that even short-lived upper mantle/lower crustal heterogeneities, products of previous partial melting events involving garnet fractionation, can develop the range of positive and negative ∊(Hf) values seen in the Early Archean samples.

The mapped area, measuring about 400m2, is situated along the southern margin of the Archaean Kaapvaal Craton south of Melmoth in KwaZulu-Natal and comprises greenstones and metasediments forming a narrow, linear E-W trending and dominantly northerly inclined belt flanked to the north and south by various granitoids and granitoid gneisses which have been differentiated for the first time in this study. This belt is here referred to as the ILANGWE GREENSTONE BELT. The lIangwe Belt rocks are grouped into the Umhlathuze Subgroup (a lower metavolcanic suite) and the Nkandla Subgroup (an upper metasedimentary suite). The former consists of: (a) the Sabiza Formation: a lower amphibolite association occurring along the southern margin of the greenstone belt; (b) the Matshansundu Formation: an eastern amphibolite-BIF association; (c) the Olwenjini Formation: an upper or northern amphibolite-banded chert-BIF association. whereas the latter is sub-divided into: (a) the Entembeni Formation: a distinctive phyllite-banded chert-BIF association occurring in the central and the eastern parts of the belt; (b) the Simbagwezi Formation: a phyllite-banded chert-amphibolite association occurring in the western part of the belt, south-east of Nkandla; (c) the Nomangci Formation: a dominantly quartzite and quartz schist formation occurring in the western part of the belt, south-east of Nkandla. The contacts between the six major tectonostratigraphic formations are tectonic. In the eastern sector of the lIangwe Belt, the lowermost metasedimentary formation, the Entembeni Formation, cuts across both the Sabiza and Matshansundu Formations (the lower formations of the Umhlathuze Subgroup) in a major deformed angular unconformity referred to as the Ndloziyana angular unconformity. In the central parts of the belt, the Entembeni Formation structurally overlies the Olwenjini Formation in what seems to be a major local unconformity (disconformity). In the western sector of the belt, the Simbagwezi Formation occurs as a structural wedge between the lower and upper formations of the Umhlathuze Subgroup. That is, it structurally overlies the Sabiza Formation and structurally underlies the Olwenjini Formation. The uppermost metasedimentary unit, the Nomangci Formation occurs as a complex series of finger-like wedges cutting and extending into the Simbagwezi Formation and in each case showing that the Nomangci Formation structurally underlies the Simbagwezi Formation. This structural repetition of lithological units is suggestive of normal dip-slip duplex structures. Palimpsest volcanic features, such as pillow structures and minor ocelli, indicate that many of the amphibolitic rocks represent metavolcanics, possibly transformed oceanic crust. This is also supported by limited major element geochemistry which suggests that the original rocks were ocean tholeiites. Evidence suggests that the talc-tremolite schists and the serpentinitic talc schists represent altered komatiites. The nature of the metasediments (represented by banded metacherts, quartzites and banded iron formations) and their similarity to those of the Barberton, Pietersburg and Nondweni greenstone complexes suggests that they were formed in relatively shallow water environments. The lIangwe magmatism is represented by different types of granitoids and granitoid gneisses and basic-ultrabasic intrusive bodies. Based on similar geochemical and mineralogical characteristics and on regional distribution, mutual associations and contact relationships, these granitoids and granitoid gneisses can be divided into three broad associations, viz: (a) The Amazula Gneiss - Nkwa/ini Mylonitic Gneiss - Nkwalini Quartzofeldspathic Flaser Gneiss Association: a migmatitic paragneiss and mylonitic to flaser gneiss association of older gneisses of Nondweni age occurring in several widely separated areas and intruded by younger granitoids. (b) The early post-Nondweni Granitoids comprising the Nkwalinye Tonalitic Gneiss (a distinctive grey gneiss intrusive into the greenstones and older gneisses) and the Nsengeni Granitoid Suite (an association of three granitoid units of batholithic proportions flanking the greenstone belt and intrusive into the greenstones, older gneisses and Nkwalinye Tonalitic Gneiss). (c) The late post-Nondweni Granitoids comprising the Impisi-Umgabhi Granitoid Suite, a batholithic microcrystic to megacrystic association of five granitoid phases/units occurring to the north and south of the greenstone belt and intrusive into the greenstones, older gneisses and early post-Nondweni granitoids. Limited major element geochemistry suggests that the granitoids and granitoid gneisses are of calc-alkaline origin and are of tonalitic, granodioritic, adamellitic and granitic composition. An igneous derivation from material located possibly at the lower crust or upper mantle is suggested. At least three major episodes of deformation (01, O2 and 03) have been recognized in the greenstones. During 01, a strong penetrative S1 tectonic foliation developed parallel to the So primary layering and bedding. This period was characterized by intense transpositional layering, recumbent and isoclinal intrafolial folding with associated shearing,thrusting and structural repetition of greenstone lithologies. These processes took place in an essentially horizontal, high strain tectonic regime. The first phase of deformation (OG1) in the migmatitic and mylonitic gneisses was also characterized by recumbent and isoclinal intrafolial folding and is remarkably similar to the 01deformational phase in the lIangwe greenstones. Structural features of the first phase of deformation suggest that it was dominated by formation of fold nappes and thrusts and was accompanied by prograde M1 medium-grade middle to upper amphibolite facies metamorphism. During D2 deformation, the subhorizontal D1 structures were refolded by new structures with steeply inclined axial planes. This resulted in the formation of superimposed Type 3 interference folding in the amphibolitic rocks and large-scale, E-W trending, doublyplunging periclinal folds in the metasediments. These periclinal folds have steeply inclined and overturned limbs and are characterized by narrow, closed elliptical outcrop patterns well-defined by extensive banded ironstones and metacherts. The second phase of deformation in the granitoids (DG 2) was characterized by steeply plunging and steeply inclined small-scale tight to isoclinal similar folds. Large-scale folds are not present in the granitoids. Evidence suggests that the second phase of deformation was a major compressional event which resulted in the large-scale upright, flattened flexural folds. It was accompanied by widespread regional greenschist metamorphism and the intrusion of the early postNondweni granitoids. The third phase of deformation produced steeply plunging small-scale folds on the limbs and axial planes of the pre-existing large-scale F2 folds and upright open folds in the granitoid terrain. This episode was characterized by the emplacement of the late postNondweni granitoids (along the D2 greenstone boundary faults) and is associated with two significant events of prograde M3 upper greenschist facies metamorphism and retrograde M3 lower greenschist facies metamorphism. Post-D3 deformation is characterized by late cross-cutting faults and the emplacement of younger basic - ultrabasic bodies. / Thesis (Ph.D.)-University of Natal, 1997.

Thesis (DSc)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: Anatexis is the first step in granite genesis. Partial melting in the lower crust may produce leucoratic features of unusual chemical compositions, very different from the final products of crustal differentiation. Therefore, the links that exists between some migmatites and crustal-derived granites can be ambiguous. This study is an investigation of the anatectic history of a high-grade terrain: the Southern Marginal Zone of the Limpopo Belt (SMZ), north to the Kaapvaal Craton in South Africa. The work involved an integrated field, metamorphic, geochemical and geochronogical study of the metasedimentary granulites from two separate quarries in the northern zone of the Southern Marginal Zone, the Bandelierkop quarry and the Brakspruit quarry, where Neoarchean high-grade partial melting features can be observed. The project has aimed to address two main issues: (1) to accurately constrain the pressuretemperature conditions and the age of the metamorphic episode in the SMZ, with implication for the geodynamic processes near the end of the Archean, (2) to investigate the fluid-absent partial melting reactions that control formation of K2O-poor leucosomes and to understand the chemical relationships in the system source-leucosome-melt–S-type granite. The P-T-t record retained in the Bandelierkop Formation metapelites, constrained by phase equilibria modelling as well as zircon LA-ICP-MS geochronology, gives an insight into crustal differentiation processes in the lower crust. Rocks in both quarries indicate high-temperature metamorphism episodes with peak conditions of 840-860 oC and 9-11 kbar at c. 2.71 Ga with formation of leucosomes (L1) during the prograde path. Minor leucocratic features (L2) were produced during decompression to 6-7 kbar. The end of the metamorphic event is marked by the granulites/amphibolites facies transition (< 640 oC) at c. 2.68 Ga. The maximum deposit age for the detrital zircons in the metapelites (c. 2.73 Ga) indicates a rapid burial process ( 0.17 cm.y1). Those evidences strongly support that the Southern Marginal Zone contains sediments deposited in an active margin during convergence, and that the metapelites were metamorphosed and partially melted as a consequence of continental collision along the northern margin of the Kaapvaal Craton at c. 2.7 Ga. The leucocratic features generated along this P-T-t path display an unusual chemistry with low K2O and FeO+MgO content and high CaO content. The combination of field observations, chemical mapping and geochemical analyses leads to the conclusion the major part of the leucosomes (L1) crystallized prior to syn-peak of metamorphism concurrent with melt extraction from the source. This study documents the details of leucosomes formation using field observations in the Southern Marginal Zone and numerical modelling. This work demonstrates that the formation of K2O-poor leucosome in the metasedimentary lower crust is controlled by the difference in volume of equilibration and heterogeneities within the migmatites. The partial melting of the source coupled with melt loss and water diffusivity within the melt transfer site is a potential mechanism to explain the chemical link in the sytem residuum– melt–S-type granite. / AFRIKAANSE OPSOMMING: Anateksis is die eerste stap in granietgenese. Meganismes wat in die onderste kors aan die werk is, is verantwoordelik vir korsdifferensiasie en bepaal die chemiese samestelling van die graniet. Hierdie studie het’n ondersoek behels van die anatektiese geskiedenis van ’n ho egraadse terrein: die suidelike randstreek van die Limpopo-gordel, noord van die Kaapvaal-kraton in Suid-Afrika. Die werk het ’n ge integreerde veld- , metamorfiese, geochemiese en geochronologiese studie van die metasedimentêre granuliete van twee afsonderlike groewe in die noordelike sone van die suidelike randstreek (SRS), die Bandelierkop-groef en die Brakspruit-groef, waar Neoarge iese ho egraadse gedeeltelike smeltkenmerke waargeneem kan word, ingesluit. Die projek was gerig op die ondersoek van twee belangrike kwessies: (1) om die drukâtemperatuurtoestande en die ouderdom van die metamorfiese episode in die SRS akkuraat te beheer, met implikasie vir die geodinamiese prosesse naby die einde van die Arge ikum, en (2) om die reaksies onder gedeeltelik gesmelte toestande wat die vorming van migmatiete beheer, te ondersoek en die chemiese verwantskappe in die stelsel bron - leukosoom - smelt - S-tipe graniet te begryp. Die P-T-t-rekord wat in die Bandelierkop-formasie metapeliete behoue is, ingeperk deur modellering van fase-ekwilibria asook sirkoon LA-ICP-MS-geochronologie, gee insig in korsdifferensiasieprosesse in die onderste kors. Rotse in albei groewe dui op metamorfismeepisodes teen hoë temperature met piektoestande van 840â860 oC en 9â11 kbar teen ongeveer 2.71 Ga met vorming van leukosome (L1) gedurende die progradeerpad. Geringe leukokratiese eienskappe (L2) het tydens dekompressie tot 6â7 kbar ontstaan. Die einde van die metamorfiese voorval word gekenmerk deur die fasiesoorgang van granuliete / amfiboliete (<640 oC) by ongeveer 2.68 Ga. Die maksimum afsettingsouderdom vir die detitrale sirkone in die metapeliete (ongeveer 2.73 Ga) dui op Å snelle begrawingsproses ( 0.17 cm.y1). Daardie bewyse bied sterk ondersteuning daarvoor dat die SRS sedimente bevat wat gedurende konvergensie in Å aktiewe rand afgeset is, en dat die metapeliete gemetamorfoseer en gedeeltelik gesmelt het as gevolg van kontinentbotsing langs die noordelike rand van die Kaapvaal-kraton teen ongeveer 2.7 Ga. Die leukokratiese eienskappe wat langs hierdie P-T-t-pad opgewek word, toon Å ongewone chemiese samestelling met lae K2O en FeO+MgO-inhoud en ho e CaO-inhoud. Die kombinasie van veldwaarnemings, chemiese kartering en geochemiese ontledings lei tot die gevolgtrekking dat die grootste deel van die leukosome (L1) gekristalliseer het voor die syn-piek van metamorfisme tesame met smeltekstraksie van die bron. Hierdie studie het die besonderhede van leukosoomformasie met behulp van veldwaarnemings in die SRS en numeriese modellering opgeteken. Hierdie werk toon aan dat korsdifferensiasie in die metasedimentêre onderste kors deur Å ander volume van ekwilibrasie en heterogeniteite in die migmatiete beheer word. Die gedeeltelike smelting van die bron gepaard met smeltverlies en waterdiffusiwiteit tot in die smeltoordragterrein is ’n potensiele meganisme om die chemiese skakel in die stelsel residuum-smelt-S-tipe graniet te verklaar.

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