Thermobarometry, detailed petrography, electron microprobe analysis, and fluid inclusion microthermometry were used to quantify the history of a high grade Proterozoic terrane exposed within the northeastern Colorado Front Range. Pressure-temperature calculations identified two blocks from different crustal levels exposed adjacent to one another within the Poudre Canyon west of Fort Collins. They are the eastern, higher pressure, block (HPB; 734 degree C, 7.1 kbar) and the western, lower pressure, block (LPB; 655 degree C, 5.2 kbar). The blocks followed a clockwise uplift path and were juxtaposed by ductile shear near the end of the decompressive stage of uplift. Final assembly of the separate blocks to the same crustal level was constrained by mineralogic and textural considerations and by the isochores of identical low density carbon dioxide inclusions trapped by both blocks. Both blocks experienced peak metamorphism within the sillimanite-K-feldspar zone accompanied by partial melting, are characterized by stromatic migmatites, and show textural evidence for localized reactions related to decompression and cooling. Differences between the blocks include the type of melt-generating reactions and the pronounced late muscovite in the LPB. Gibbs' Method calculations indicate that local (outcrop scale) textural and mineralogical differences between HPB Mg-enriched pelites are caused by small differences in bulk composition and locally variable H2O content. The quartzo-feldspathic biotite gneisses in the LPB generated migmatites by wet melting, whereas the pelitic schists and gneisses in the HPB generated migmatites by dehydration melting of muscovite and biotite. Biotite dehydration melting enriched the leucosomes in HPB pelitic migmatites in K-feldspar and garnet. The minor presence of late muscovite in the HPB relative to the LPB was controlled by the different positions of their uplift paths relative to the muscovite breakdown reaction. / Ph. D.

Mineral textures, preserved in the metamorphosed sedimentary sequences that are exposed in orogenic hinterlands, are crucial to understanding the architecture and evolution of collisional mountain belts. In this thesis the textural record of anatexis and melt transfer in the Himalayan metamorphic core is decoded and the controls that these processes exert on the tectonic evolution of the Himalaya are explored. The problem is divided into two parts, corresponding to variations in protolith lithostratigraphy: melt source - the pelitic region where melt was first generated, and melt sink - the psammitic region where melt accumulated and crystallised. Dehydration melting of muscovite has long been recognized as a critical reaction for the generation of anatectic melt in the Himalaya, but a textural understanding of how this reaction progresses is limited by the inherent difficulties in identifying specific reaction products. Using samples collected from the Langtang area in central Nepal, a mechanistic model for muscovite dehydration melting was constructed, and a set of textural criteria were developed, which were used to distinguish peritectic K-feldspar from K-feldspar grains formed during melt crystallisation. Melt is transferred from the source to the sink in two stages: firstly along a pervasive network of mineral grain boundaries, and secondly via a channelised network of sills and dykes in the melt sink where it solidified as leucogranite. Variation in the primary mineral assemblage and appearance of leucogranite bodies reflect the degree of interaction that occurred between the melt and metasedimentary country rock, rather than a change in primary melt composition. The modal proportion of K-feldspar in the melt source requires vapour-absent conditions during muscovite dehydration melting and leucogranite formation, indicating that the generation of large volumes of granitic melts in orogenic belts is not necessarily contingent on an external source of fluids. The crystallisation of hydrous minerals in leucogranite consumes <15.5 % of water released by the breakdown of muscovite. These results indicate that anatexis efficiently dehydrates the middle crust and suggests that the continents have limited potential to store water over geological time.

Visando contribuir para o entendimento dos processos anatéticos em rochas metamórficas de composição semipelítica, este trabalho apresenta dados de campo e modelagem metamórfica, utilizados como ferramentas para a correlação entre biotita xisto e metatexito ricos em quartzo e plagioclásio, com granada, cianita, muscovita, rutilo e ilmenita, pertencentes à unidade Santo Antônio no Sistema de Nappes Andrelândia, sul de Minas Gerais. Com o levantamento de campo foi caracterizada a associação cianita + granada + biotita + plagioclásio + muscovita + rutilo + ilmenita + quartzo + leucossoma tonalítico em litotipos que variam de xisto homogêneo a metatexito protoestromático, com o estabelecimento do protobandamento favorecido pela geração e segregação limitada do leucossoma, que aumenta da base para o topo do pacote. A amostragem dos litotipos principais é feita a partir de três pedreiras, da base para o topo: IC01, em que predomina xisto com feições de fusão in situ subordinadas às porções granoblásticas e cristais idiomórficos de plagioclásio em veios de quartzo; IC03, dominada por xisto, na base, e metatexito no topo, com ampla distribuição de patches, bolsões e veios de leucossoma tonalítico in situ ou in source; e IC07, caracterizada pelo bandamento de porções de xisto e metatexito, em escalas de 1 cm a 1 m, com leucossoma in situ ou in source reforçando o bandamento composicional, ou também na forma de veios, patches e bolsões que podem truncar a foliação. Com a química de rocha total é evidente que não existem diferenças composicionais significativas entre xistos e metatexitos. A geração de pequenos volumes de leucossoma rico em Si e Na, compatível com as segregações tonalíticas observadas, e resíduo rico em Fe, Mg e K, associado às acumulações de biotita nas suas bordas, não mudou de forma significativa a composição das rochas. A similaridade entre os padrões de elementos terras-raras reconhecidos para os litotipos analisados indica que a anatexia não alcançou condições altas o suficiente para a dissolução e reprecipitação de fases ultraestáveis como monazita e zircão, hipótese sustentada também pela ausência de feldspato potássico no leucossoma. O cálculo de isopletas resulta em topologia semelhante para amostras de IC01 e IC03; o plagioclásio em amostras das três pedreiras apresenta intervalo composicional contínuo entre An15 e An48, com grãos progressivamente mais ricos em albita, partindo dos inclusos em granada, na matriz xistosa ou metatexítica, e no leucossoma. Esta progressão na quantidade de albita é também observada em cristais de IC01 para IC03 e IC07, respectivamente, sentido no qual, em contrapartida, diminuem as diferenças composicionais entre cristais de mesma amostra e diferentes sítios texturais. A variação composicional da muscovita mostra grupos com distintos teores de sódio, sugerindo trajetória metamórfica horária para o pacote metatexítico e a termobarometria otimizada aplicada a IC01 e IC03 resulta em temperaturas de 750 a 850 °C para o pico metamórfico da base para o topo da unidade, respectivamente, em condições de pressão elevada, maior que 8 kbar. Estas evidências caracterizam o metamorfismo invertido atuante na região e demonstram a importância o reconhecimento da anatexia para a interpretação petrogenética de rochas semipelíticas. / As a contribution to the understanding of partial melting processes in metamorphic rocks of semipelite composition, this work presents field data and metamorphic modeling. They were used as tools for the correlation between biotite schist and metatexites, which are rich in quartz and plagioclase, with garnet, kyanite, muscovite, rutile and ilmenite, both rocks are correlated to the St. Antônio unit, in the Andrelândia Nappe System, south of Minas Gerais, Brazil. From the rocks field descriptions, it was characterized as the main mineral assemblage kyanite + garnet + biotite + plagioclase + muscovite + rutile + ilmenite + quartz + leucosome with tonalite composition, in rock types ranging from the homogeneous schist to the proto-stromatic metatexite. The compositional banding was enhanced by the generation and limited segregation of leucosome, and its proportion is higher from structural bottom to top of unit. Samples of the main rock types were collected in three quarries, from bottom to top: IC01, schist with sparse in situ melting features that are limited to the granoblastic portions and idiomorphic plagioclase crystals in quartz veins; IC03, dominated by schist, at the base, and metatexite, on top, with wide distribution of patches, pockets and veins of leucosome with tonalite composition, in situ or in source; and IC07, characterized by the compositional banding, varying from schist to metatexite, in bands with thickness ranging from 1 cm up to 1 m, and either in situ or in source leucosome veins enhance the compositional banding, although some veins, patches and pockets truncate the main foliation. With the whole rock chemistry analyses it is evident there are no significant compositional differences between schist and metatexite. The generation of small volumes of silicate Na-rich liquid is compatible with tonalite leucosome, and residue rich in Fe, Mg and K, associated with accumulations of biotite at its edges, but has not changed significantly the bulk rock compositions. The similarity of the of rare earth elements patterns for both rock types indicates anatexis did not reach a high degree and dissolution and reprecipitation of ultra-stable phases, as monazite and zircon, did not happen, which is sustained by absence of potassium feldspar in leucosome. The pseudosection modeling and isopleths calculation result in similar topology for samples of IC01 and IC03. Plagioclase crystals of the three quarries present continuous compositional change, between An 15 and An 48, with progressively more albite-rich crystals starting from the ones that are enclosed in garnet, in schist, metatexite matrix, and leucosome. This increase in the amount of albite is also observed in crystals from IC01 to IC03 and IC07, respectively, direction which, in turn, decrease the compositional differences between crystals of the same sample and different textural sites. The compositional variation of muscovite allows separation in groups with different quantities of Na, suggesting a clockwork P-T path to the rock, and optimized thermobarometry applied to IC01 and IC03 results in temperatures of 750-850 ° C for the metamorphic peak from base to top of the unit, in high pressure conditions, higher than 8 kbar. This P-T variation, from bottom to top, characterizes an inverted metamorphism in the region and demonstrates the importance of the recognition of anatexis processes for petrogenetic interpretation of semipelite rocks.

Thesis advisor: Seth C. Kruckenberg / The Potters Pond migmatite domain (PPMD) is a heterogeneous zone of migmatites located ~10 km southwest of Cascade, Idaho within the western Idaho shear zone (WISZ). The PPMD is the only known exposure of migmatites within the WISZ over its ~300 km length, occurring where the shear zone orientation changes from 020° south to 000° north of the migmatite domain. Structural mapping within the PPMD has identified multiple generations of migmatite with varied structural fabrics. Leucosome layers were sampled from distinct migmatite localities and morphologies (e.g., metatexite, diatexite) to determine the timing and duration of partial melting in the PPMD. U-Pb age determinations of zircon by means of LA-ICP-MS document two periods of protracted migmatite crystallization during the Early and Late Cretaceous. Early Cretaceous (ca. 145 to 128 Ma) migmatite crystallization ages are coeval with the collision and suturing of oceanic terranes of the Blue Mountains province with North America, and the formation of the Salmon River suture zone (SRSZ). Migmatite crystallization ages from ca. 104 to 90 Ma are associated with Late Cretaceous dextral transpression in the WISZ. Field observations and geochronology of cross cutting leucosome relationships are interpreted to record deep crustal deformation and anatexis associated with formation of the SRSZ, subsequently overprinted by solid-state deformation and renewed anatexis during the evolution of the WISZ. These data are the first direct evidence of the synmetamorphic fabric related to the SRSZ east of the initial Sr 0.706 isopleth, and that the WISZ is a temporally distinct overprinting structure. / Thesis (MS) — Boston College, 2016. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

Visando contribuir para o entendimento dos processos anatéticos em rochas metamórficas de composição semipelítica, este trabalho apresenta dados de campo e modelagem metamórfica, utilizados como ferramentas para a correlação entre biotita xisto e metatexito ricos em quartzo e plagioclásio, com granada, cianita, muscovita, rutilo e ilmenita, pertencentes à unidade Santo Antônio no Sistema de Nappes Andrelândia, sul de Minas Gerais. Com o levantamento de campo foi caracterizada a associação cianita + granada + biotita + plagioclásio + muscovita + rutilo + ilmenita + quartzo + leucossoma tonalítico em litotipos que variam de xisto homogêneo a metatexito protoestromático, com o estabelecimento do protobandamento favorecido pela geração e segregação limitada do leucossoma, que aumenta da base para o topo do pacote. A amostragem dos litotipos principais é feita a partir de três pedreiras, da base para o topo: IC01, em que predomina xisto com feições de fusão in situ subordinadas às porções granoblásticas e cristais idiomórficos de plagioclásio em veios de quartzo; IC03, dominada por xisto, na base, e metatexito no topo, com ampla distribuição de patches, bolsões e veios de leucossoma tonalítico in situ ou in source; e IC07, caracterizada pelo bandamento de porções de xisto e metatexito, em escalas de 1 cm a 1 m, com leucossoma in situ ou in source reforçando o bandamento composicional, ou também na forma de veios, patches e bolsões que podem truncar a foliação. Com a química de rocha total é evidente que não existem diferenças composicionais significativas entre xistos e metatexitos. A geração de pequenos volumes de leucossoma rico em Si e Na, compatível com as segregações tonalíticas observadas, e resíduo rico em Fe, Mg e K, associado às acumulações de biotita nas suas bordas, não mudou de forma significativa a composição das rochas. A similaridade entre os padrões de elementos terras-raras reconhecidos para os litotipos analisados indica que a anatexia não alcançou condições altas o suficiente para a dissolução e reprecipitação de fases ultraestáveis como monazita e zircão, hipótese sustentada também pela ausência de feldspato potássico no leucossoma. O cálculo de isopletas resulta em topologia semelhante para amostras de IC01 e IC03; o plagioclásio em amostras das três pedreiras apresenta intervalo composicional contínuo entre An15 e An48, com grãos progressivamente mais ricos em albita, partindo dos inclusos em granada, na matriz xistosa ou metatexítica, e no leucossoma. Esta progressão na quantidade de albita é também observada em cristais de IC01 para IC03 e IC07, respectivamente, sentido no qual, em contrapartida, diminuem as diferenças composicionais entre cristais de mesma amostra e diferentes sítios texturais. A variação composicional da muscovita mostra grupos com distintos teores de sódio, sugerindo trajetória metamórfica horária para o pacote metatexítico e a termobarometria otimizada aplicada a IC01 e IC03 resulta em temperaturas de 750 a 850 °C para o pico metamórfico da base para o topo da unidade, respectivamente, em condições de pressão elevada, maior que 8 kbar. Estas evidências caracterizam o metamorfismo invertido atuante na região e demonstram a importância o reconhecimento da anatexia para a interpretação petrogenética de rochas semipelíticas. / As a contribution to the understanding of partial melting processes in metamorphic rocks of semipelite composition, this work presents field data and metamorphic modeling. They were used as tools for the correlation between biotite schist and metatexites, which are rich in quartz and plagioclase, with garnet, kyanite, muscovite, rutile and ilmenite, both rocks are correlated to the St. Antônio unit, in the Andrelândia Nappe System, south of Minas Gerais, Brazil. From the rocks field descriptions, it was characterized as the main mineral assemblage kyanite + garnet + biotite + plagioclase + muscovite + rutile + ilmenite + quartz + leucosome with tonalite composition, in rock types ranging from the homogeneous schist to the proto-stromatic metatexite. The compositional banding was enhanced by the generation and limited segregation of leucosome, and its proportion is higher from structural bottom to top of unit. Samples of the main rock types were collected in three quarries, from bottom to top: IC01, schist with sparse in situ melting features that are limited to the granoblastic portions and idiomorphic plagioclase crystals in quartz veins; IC03, dominated by schist, at the base, and metatexite, on top, with wide distribution of patches, pockets and veins of leucosome with tonalite composition, in situ or in source; and IC07, characterized by the compositional banding, varying from schist to metatexite, in bands with thickness ranging from 1 cm up to 1 m, and either in situ or in source leucosome veins enhance the compositional banding, although some veins, patches and pockets truncate the main foliation. With the whole rock chemistry analyses it is evident there are no significant compositional differences between schist and metatexite. The generation of small volumes of silicate Na-rich liquid is compatible with tonalite leucosome, and residue rich in Fe, Mg and K, associated with accumulations of biotite at its edges, but has not changed significantly the bulk rock compositions. The similarity of the of rare earth elements patterns for both rock types indicates anatexis did not reach a high degree and dissolution and reprecipitation of ultra-stable phases, as monazite and zircon, did not happen, which is sustained by absence of potassium feldspar in leucosome. The pseudosection modeling and isopleths calculation result in similar topology for samples of IC01 and IC03. Plagioclase crystals of the three quarries present continuous compositional change, between An 15 and An 48, with progressively more albite-rich crystals starting from the ones that are enclosed in garnet, in schist, metatexite matrix, and leucosome. This increase in the amount of albite is also observed in crystals from IC01 to IC03 and IC07, respectively, direction which, in turn, decrease the compositional differences between crystals of the same sample and different textural sites. The compositional variation of muscovite allows separation in groups with different quantities of Na, suggesting a clockwork P-T path to the rock, and optimized thermobarometry applied to IC01 and IC03 results in temperatures of 750-850 ° C for the metamorphic peak from base to top of the unit, in high pressure conditions, higher than 8 kbar. This P-T variation, from bottom to top, characterizes an inverted metamorphism in the region and demonstrates the importance of the recognition of anatexis processes for petrogenetic interpretation of semipelite rocks.

The heterogeneity of the continental crust has a first order control on the dynamics of plate tectonic processes and the compositions of the Earth in both time and space. Heterogeneity can be characterized at a variety of scales and in a multitude of tectonic environments, but it is the links between seemingly disparate tectonic settings and crustal levels that are critical in understanding construction of the continents. The focus of this dissertation work is to apply microtextural, microgeochemical, whole rock geochemical and traditional petrographic techniques to study features in both deep and shallow crustal igneous rocks. The goal of these efforts is to better understand the roles that magmatic processes, mafic-felsic magma interaction, and partial melting have on the evolution of continental crust. Two principal field areas were selected, the Gouldsboro pluton in coastal Maine and the Fehr granite in northern Saskatchewan, Canada, because they each represent end-members of the processes involved with the generation, modification, transport, and emplacement of magmas that build continental crust. Evidence for bimodal magmatism preserved in the Silurian age Gouldsboro pluton has led to a refined model for the construction of shallow crustal magma chambers. Research efforts focused on the Neoarchean Fehr granite and Paleoproterozoic Chipman dike swarm have contributed to the current understanding of the links between high temperature metamorphism (migmitization) and the production of new felsic magmas as well as the rheological and chemical influences of mafic-felsic magma interaction in the deep crust. The results of these combined field and laboratory efforts have demonstrated the important role of mafic-felsic magma interaction on the strength and composition of both deep and shallow continental crust and have contributed to the current understanding of the complex links between deep crustal heterogeneity and bimodal magmatism at shallow crustal levels.

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 (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.

The accretionary Mesoproterozoic Grenville Orogeny (ca. 1300 – 980 Ma) involving the Central Metasedimentary Belt is a key building block of the eastern Laurentian margin. A petrographic, mineralogical, geochemical and geochronological study of the migmatite complex in Otter Lake (QC) within the Marble Domain is used to resolve regional metamorphic and magmatic events primarily recorded in the leucosome accessory minerals (i.e. monazite). The relationship between the different stages of monazite and garnet growth and dissolution during the tectonic evolution of the orogenic history for the interpreted metasomatic (injected) and anatectic (in situ) monazite-bearing neosomes from this study supports published thermochronological work in the area and challenges the claim that the Central Metasedimentary Belt was not heated above 500 °C during the Ottawan phase. Instead, the region shows Grenville magmatic and anatectic events were overprinted by high-temperature, fluid-rich Ottawan-phase metamorphism recorded within both injected (monazite-bearing) and in situ (monazite- and garnet-bearing) neosomes.