Academic literature on the topic 'Greenstone'

Tonalite-trondhjemite–granodiorite (TTG) gneisses and mela nocratic to ultramafic greenstones dominate the Arc haean basement of southern West Greenland. The greenstones are likely to represent different original environments, which is important as the mineral deposits they may host depend on this. For example, massive sulphide deposits associated with gold and base metals are commonly volcan og enic, while chrome, nickel and platinum group elements are more commonly associated with layered intrusions (Robb 2005). Cur rent investigations by the Geological Survey of Denmark and Greenland (GEUS) in southern West Greenl and are therefore focused on the origin of greenstones and their relationship to associated TTG gneisses. Here, we report on work in progress on greenstones within the Tasiusarsuaq terrane (Fig. 1; Friend et al. 1996). They differ from many other greenstone belts in southern West Green land in their spatial association with the TTG gneisses. Unlike the Isua, Ivisârtoq and Storø greenstone belts in the central and northern Nuuk region, the Tasiusarsuaq greenstones are not proximal to terrane boundaries but form dismembered blocks and slivers within the terrane (Fig. 1). Contact relationships to the gneisses are almost exclusively tectonic, and primary textures are, with rare exceptions, ob literated by amphibolite to granulite facies metamorphism.

The Golden Mile at Kalgoorlie represents a giant Archean hydrothermal gold system localized by ductile shear zones and hosted mainly by a differentiated tholeiitic sill. Chlorite, carbonate, and pyrite alteration zones cover the whole mineralized area (1 km × 3 km), and calculations suggest that for the Golden Mile (production around 1200 t Au), the amounts of components added to these alteration zones are 340 Mt CO2, 20 Mt K, and 5 Mt S. If one adopts a metamorphic-replacement model for gold mineralization in which all ore components derive from devolatilization of greenstones at amphibolite facies or above, these data suggest that a source area involving a 5 km thick greenstone slab of area 8 km × 8 km could produce the necessary CO2, K, S, H2O, and Au. This is considered a reasonable volume of greenstone belt, and under such a model the minimum spacing of large gold deposits would be approximately 20 km along strike.Neither special Au-enriched source rocks nor unreasonably large volumes of greenstone belt are required to produce a giant gold deposit. Instead, the most critical parameters are suitable structural environments providing focussing of fluids and multiple channelways through specific Fe-rich, low-tensile-strength host rocks to ensure efficient depositional mechanisms.

Abstract A Palaeoarchaean greenstone fragment and associated granitoid gneisses from an area south of Ulundi in KwaZulu-Natal is described. The fragment consists of an association of garnetiferous amphibolite and calc-silicate that was intruded at 3388 ± 4 Ma by tonalite and at 3275 ± 4 Ma by trondhjemite. Strong ductile deformation of the greenstones and granitoids under amphibolite facies conditions (7 kbar and 600 to 650°C) took place prior to uplift and emplacement of a granite batholith at ~3.25 Ga ago in which the granitoid gneiss-greenstone domain is now found. Magmatism 3.27 to 3.25 Ga ago was a direct response to regional metamorphism and anataxis, and gave rise to stabilization of the southeastern Kaapvaal Craton at that time, earlier than other parts of the craton. Deposition of quartz-arenites on stable granitic basement took place <3.1 Ga ago. Contrasting ages in magmatic pulses and regional metamorphism reflect a different crustal growth history of the eastern and southeastern part of the Kaapvaal Craton.

Greenstone belts are long, curvilinear accumulations of mainly volcanic rocks within Archean granite−greenstone terranes, and are subdivided into two geochemical types: komatiite−tholeiite sequences and bimodal sequences. In rare instances where basement is preserved, the basement is unconformably overlain by platform to rift sequences consisting of quartzite, carbonate, komatiite and/or tholeiite. The komatiite−tholeiite sequences consist of km-scale thicknesses of tholeiites, minor intercalated komatiites, and smaller volumes of felsic volcanic rocks. The bimodal sequences consist of basal tholeiitic flows succeeded upward by lesser volumes of felsic volcanic rocks. The two geochemical types are unconformably overlain by successor basin sequences containing alluvial–fluvial clastic metasedimentary rocks and associated calc-alkaline to alkaline volcanic rocks. Stratigraphically controlled geochemical sampling in the bimodal sequences has shown the presence of Fe-enrichment cycles in the tholeiites, as well as monotonous thicknesses of tholeiitic flows having nearly constant MgO, which is explained by fractionation and replenishment of the magma chamber with fresh mantle-derived material. Geochemical studies reveal the presence of boninites associated with the komatiites, in part a result of alteration or contamination of the komatiites. Within the bimodal sequences there are rare occurrences of adakites, Nb-enriched basalts and magnesian andesites. The greenstone belts are engulfed by granitoid batholiths ranging from soda-rich tonalite−trondhjemite−granodiorite to later, more potassic granitoid rocks. Archean greenstone belts exhibit a unique structural style not found in younger orogens, consisting of alternating granitoid-cored domes and volcanic-dominated keels. The synclinal keels are cut by major transcurrent shear zones. Metamorphic patterns indicate that low pressure metamorphism of the greenstones is centred on the granitoid batholiths, suggesting a central role for the granitoid rocks in metamorphosing the greenstones. Metamorphic patterns also show that the proportion of greenstones in granite−greenstone terranes diminishes with deeper levels of exposure. Evidence is presented on both sides of the intense controversy as to whether greenstone belts are the product of modern plate tectonic processes complete with subduction, or else the product of other, lateral tectonic processes driven by the ‘mantle wind.’ Given that numerous indicators of plate tectonic processes – structural style, rock types, and geochemical features − are unique to the Archean, it is concluded that the evidence is marginally in favour of non-actualistic tectonic processes in Archean granite−greenstone terranes.RÉSUMÉLes ceintures de roches vertes sont des accumulations longiformes et curvilinéaires, principalement composées de roches volcaniques au sein de terranes granitique archéennes, et étant subdivisées en deux types géochimiques: des séquences à komatiite–tholéite et des séquences bimodales. En de rares occasions, lorsque le socle est préservé, ce dernier est recouvert en discordance par des séquences de plateforme ou de rift, constituées de quartzite, carbonate, komatiite et/ou de tholéiite. Les séquences de komatiite-tholéiite forment des épaisseurs kilométriques de tholéiite, des horizons mineurs de komatiites, et des volumes de moindre importance de roches volcaniques felsiques. Les séquences bimodales sont constituées à la base, de coulées tholéiitiques surmontées par des volumes mineurs de roches volcaniques felsiques. Ces deux types géochimiques sont recouverts en discordance par des séquences de bassins en succession contenant des roches métasédimentaires clastiques fluvio-alluvionnaires associées à des roches volcaniques calco-alcalines à alcalines. Un échantillonnage à contrôle stratigraphique des séquences bimodales a révélé la présence de cycles d’enrichissement en Fe dans les tholéiites, ainsi que des épaisseurs continues d’épanchements tholéiitiques ayant des valeurs presque constante en MgO, qui s’explique par la cristallisation fractionnée et le réapprovisionnement de la chambre magmatique par du matériel mantélique. Les études géochimiques montrent la présence de boninites associées aux komatiites, résultant en partie de l’altération ou de la contamination des komatiites. Au sein des séquences bimodales, on retrouve en de rares occasions des adakites, des basaltes enrichis en Nb et des andésites magnésiennes. Les ceintures de roches vertes sont englouties dans des batholites granitoïdes de composition passant des tonalites−trondhjémites−granodiorites enrichies en sodium, à des roches granitoïdes tardives plus potassiques. Les ceintures de roches vertes archéennes montrent un style structural unique que l’on ne retrouve pas dans des orogènes plus jeunes, et qui est constitué d’alternances de dômes à cœur granitoïdes et d`affaissements principalement composés de roches volcaniques. Les synclinaux formant les affaissements sont recoupés par de grandes zones de cisaillement. Les profils métamorphiques indiquent que le métamorphisme de basse pression des roches vertes est centré sur les batholites, indiquant un rôle central des roches granitoïdes durant le métamorphisme des roches vertes. Les profils métamorphiques montrent également que la proportion de roches vertes dans les terranes granitiques diminue avec l’exposition des niveaux plus profonds. On présente les arguments des deux côtés de l’intense controverse voulant que les ceintures de roches vertes soient le produit de processus moderne de la tectonique des plaques incluant la subduction, ou alors le produit d’autres processus tectoniques découlant du « flux mantélique ». Étant donné la présence des indicateurs des processus de tectonique des plaques – style structural, les types de roches, et les caractéristiques géochimiques – ne se retrouvent qu’à l’Archéen, nous concluons que les indices favorisent légèrement l’option de processus tectoniques non-actuels dans les terranes granitiques de roches vertes à l’Archéen.

AbstractFormations on the northwestern flank of the Barberton Greenstone Belt have hosted over 85% of all the gold recovered from the ca. 3550 to 3000 Ma Barberton Supergroup since early discoveries in 1872. This sector of the greenstone belt also happens to coincide with a complex tectonic architecture resulting from successive stages of folding and faulting superimposed onto a complex lithostratigraphy. Of particular importance has been the influence of two diapiric granitoid intrusions that caused added structural complexity following their emplacement ca. 3227 to 3250 Ma. Of these the larger Kaap Valley Pluton invaded the area north of present day Barberton town causing the separation of the greenstones into a northern arm (Jamestown Schist Belt) and a southern sector which remained attached to the main greenstone belt (Moodies Hills). The ballooning pluton produced vertical as well as horizontal flattening stresses, the latter reactivating earlier high-angle faults and resulting in subhorizontal strike-slip movements, particularly along the Barbrook Fault Zone, which acted as a right-lateral strike-slip fault. Formations north of this fault were buckled, following progressive deformation in the region known as the Sheba Hills, into major synclinal folds (Eureka and Ulundi Synclines) with folded axial planes that dip steeply to the south, southeast or east. The second granitoid intrusion (Stentor Pluton), which has been extensively modified by subsequent magmatic events, caused significant flattening of greenstone belt rocks in the northeastern part of the Barberton Greenstone Belt (Three Sisters region) as well as in other areas rimming the granitic body. Combined, the two plutons produced a wide range of interference and reactivated structures particularly affecting a triangular region extending from the Jamestown Schist Belt into the area occupied by the New Consort Gold Mine and areas to the east. This paper attempts to outline, in the simplest manner, the geological and structural evolution of the main gold-producing region of the Barberton Goldfield. The principal aim is therefore to highlight the structural influence of the diapiric plutonism and the manner in which the plutons contributed significantly to the horizontal reactivation of pre-existing regional faults, which in turn, resulted in the progressive deformation of a heterogeneous lithological terrane.

Zircon ages from three granitoid plutons are the first to be reported from the La Grande greenstone belt. Two of the dated samples are from highly tectonized, early tectonic plutons that at the present level of erosion are just outside the greenstone belt proper. Their zircon ages of ca. 2740 Ma are emplacement ages or alternatively represent the age of maximum deformation of the greenstone belt. The third sample is from a mildly deformed late tectonic pluton within the greenstone belt. Its zircon age of ca. 2670 Ma probably represents the emplacement age. The above dates and the relationships of the dated plutons to the greenstone belt as a whole suggest that the bulk of the volcanism in the La Grande belt is older than 2.7 Ga. This limiting age indicates that the age of the La Grande "supracrustals" is similar to those of the other greenstone belts in the Superior Province.

The golden generation have a wealth of experience and knowledge from throughout their lifetimes that younger generations wish to retain. In our technology filled world an obvious means of collecting this information is electronically. Digital library col- lections are increasingly used by libraries and large institutions to record their large amounts of information but they can also be used for personal collections. Seniors are often willing and keen to impart their years of experience upon people of the younger generation but time is not always on their side as they grow older. Throughout a lifetime a person could collect large amounts of papers, diaries, pho- tos and media but the time it takes to organise these documents can be long and exhausting and the person's health is not always at its best in old age. Greenstone is a suite of software for creating digital libraries, which are organised collections of documents. Greenstone has the ability to distribute collections either using a server or CD-ROM, and provides advanced searching and organization tools. While Greenstone is a versatile and useful tool in creating digital collections, its in- terface is not designed for senior users. Seniors are commonly perceived to have more physical and mental disadvantages as they get older. These disadvantages can dramatically affect how usable seniors find a piece of software. The aim of this thesis is to investigate how usable the current Greenstone interface is for use by seniors and to re-design the interface so that Greenstone may be more easily used by senior users. This thesis focuses upon what types of documents and descriptive data seniors would like to include in a collection about their life. This is to ascertain exactly what parts of the interface must be improved when it comes to metadata and classifiers. The results of this investigation also helped in the creation of a customised metadata set for senior users use.

This thesis examines the origin and relative timing of carbonate alteration zones in the poly-deformed Doolena Gap Greenstone Belt in the East Pilbara Terrane, the type locality of Archaean dome-and-keel-terranes. The key findings are: [1] shear-assisted carbonate alteration occurred throughout the entire tectonic history of the greenstone belt; and [2] weak pre- and syntectonic carbonate minerals make up 40 to 60% of the examined greenstone rocks. These outcomes imply that carbonate minerals likely controlled the strength of Archaean lithosphere.

The Belingwe greenstone belt in southern Zimbabwe contains one of the most well preserved Archean volcanic successions in the world. The komatiites in this succession have been studied in great detail, but the associated basalts have received much less attention. A detailed study of these basalts in the Zeederbergs Formation has revealed the existence of a previously unrecognised lava type which has important implications for the petrogenesis of the suite. The Zeederbergs Formation and the underlying Reliance Formation form the 2.7 Ga Ngezi Group volcanics, which are underlain by thin, generally shallow water sediments of the Manjeri Formation. These in turn rest unconformably on 3.6 Ga and 2.9 Ga granitoid gneisses in the east and on older (2.9 Ga) greenstones in the west. The nature of the basal contact of the Ngezi Group volcanics on the sediments of the Manjeri Formation has been the subject of recent controversy, and is interpreted here as comformable. The type section of the Zeederbergs Formation in the Ngezi River is logged and described in detail for the first time. Combined with correlation of geochemical marker horizons in other sections this has led to a re-evaluation of the vertical thicknesses of the Zeederbergs Formation - estimated here to be approximately 3km. Study of the geochemical stratigraphy has revealed a horizon of basalts with low Zr/Nb and high CaO/Al₂O₃ compared to the rest of the formation. The basalts in this horizon are called Type II (as opposed to the Type I basalts which make up the majority of the formation). Examination of the petrography of the Zeederbergs Formation basalts has revealed that no subdivision into different rock types on petrographic grounds is practical. The lavas are generally fine grained, sparsely phyric and altered to hydrated low greenschist assemblages. The 'spheroids' in the lavas are considered in some detail, and are thought to represent products of spherulitic devitrification.

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Este trabalho trata da geologia e petrogênese do "greenstone belt" Identidade, situado entre as cidades de Xinguara e Rio Maria, SE do Estado do Pará. Os dados obtidos permitiram discutir a evolução geodinâmica do terreno granito - "greenstone" da região de Rio Maria, inserindo-a no contexto da Província Mineral de Carajás (PMC), SE do cráton Amazônico. O "greenstone" em lide compõe um cinturão "sinformal" direcionado WNW-ESE, correspondendo a um pacote metavulcãnico, com xistos ultramáficos (UM), basaltos (BAS) e gabros (GB) na base, e, no topo, rochas hipabissais dacíticas (DAC - ca. 2,94 Ga, Pb/Pb). O conjunto foi intrudido por metaplutônicas Mesoarqueanas, os tipos mais precoces sendo quartzo dioríticos, seguidos sucessivamente por granodioritos (com enclaves máficos), trondhjemitos / tonalitos e leucogranitos. O embasamento gnáissico (GN - aflorante a norte e reconhecido por conter uma fábrica mais antiga Sn-1/D1), o "greenstone" e os metagranitóides foram intrudidos no final do Paleoproterozôico por enxames de diques riolíticos (ca. 1,60 Ga, Rb/Sr) e diabásicos. O "greenstone" apresenta estruturas e texturas ígneas reconhecíveis, porém obliteradas em regiões de contato com metagranitóides e em zonas de cisalhamento. As ultramáficas ocorrem como tremolititos, tremolita - talco xistos e talco xistos; o anfibólio é bastante alongado e fino, comumente em arranjos paralelos, interpretados como fantasmas de texturas "spinifex". Os basaltos são maciços ou almofadados, freqüentemente variolíticos. Mostram diferentes graus de recristalização, sendo identificados restos de texturas hialofiticas, pilotaxíticas e traquitóides. Clinoanfibólio (hornblenda actinolítica), epídotos e plagioclásio (albita - andesina) são os minerais mais abundantes. Os gabros são maciços a porfiriticos, distinguindo-se relíquias de texturas subofiticas e granofiricas. Os dacitos são porfiríticos, com fenocristais de quartzo e plagioclásio (oligoclásio), além de hornblenda e nódulos máficos (biotita, clorita, opacos, epidotos, titanita, apatita) nas variedades menos evoluídas. Dentre os metagranitóides, os leucogranitos e trondhjemitos contêm biotita cloritizada, enquanto granodioritos e parte dos tonalitos portam biotita ou biotita + hornblenda (também em quartzo dioritos). O "greenstone" e os metagranitóides foram afetados por uma deformação dúctil, heterogênea, que evoluiu para zonas miloníticas. A estruturação da área é marcada por uma fábrica planar (Sn//Sm/D2) direcionada WNW-ESE a E-W, de mergulhos divergentes. Lineações de estiramento E-W, WNW-ESE ou NW-SE, meso e microestruturas assimétricas S-C, peixes de micas e de clinoanfibólios, e rotações de porfiroclastos a e 15 indicaram uma megaestrutura resultante de um binário com encurtamento NW-SE. A geometria atual do "greenstone" seria derivada de transpressão dextrógira, com o "greenstone" compondo uma estrutura em flor positiva. O regime transpressivo favoreceu a criação de regiões transtrativas, onde se alojaram plútons graníticos no NW, além de clivagens de crenulação extensional (Sn+i/D2) no SW. A quantificação da deformação revelou encurtamento da ordem de 60%, extensão subhorizontal, paralela ao "trend" do "greenstone", de 68 a 500%, e extensão vertical de 101 a 280%. O elipsóide de deformação variou de oblato a prolato, com mudanças de densidade e rotação do eixo de estiramento máximo (X) nas zonas miloníticas. A inversão da deformação permitiu reconstruir a forma original do "greenstone", que seria também alongada WNW-ESE, embora de excentricidade menor que a atual. Estes dados, juntamente com a petrofábrica do eixo c do quartzo, sugeriram que a deformação progressiva envolveu mecanismos de cisalhamento puro e simples, sendo o arcabouço final resultante deste último. Falhas e fraturas rúpteis diversas, afetando também diques riolíticos e diabásicos, marcaram o último evento (D3). As paragêneses minerais do metamorfismo principal (Mn/M2) originaram-se de recristalização estática, pré-tectônica, que modificou parte das texturas e quase totalmente a mineralogia das rochas do "greenstone". Formaram-se anfibólio verde azulado (hornblenda actinolítica), epídotos (pistacita predominante), titanita e quartzo em BAS e GB; tremolita, talco e clorita em UM. Saussuritização e sericitização de plagioclásio, biotitização de anfibólio, cloritização de biotita e transformação de hornblenda em titanita verificaram-se nos metagranitóides. A coexistência de hornblenda + plagioclásio (An> 17) e/ou hornblenda actinolítica + epidotos + clorita em rochas metabásicas mostrou que o evento supra foi de pressão baixa e temperaturas transicionais entre as fácies xisto verde e anfibolito. Este episódio essencialmente térmico refletiu o aquecimento crustal produzido pelo plutonismo do final do Mesoarqueano, tendo obliterado as associações prévias do metamorfismo de fundo oceânico. Ligeiramente concomitante a francamente subseqüente, houve um evento de recristalização dinâmica extensiva (Mm/M2) na fácies xisto verde, particularmente em zonas de cisalhamento e contatos litológicos. Em tais locais, existem evidências de aporte de fluidos (blastomilonitos xistosos e abundantes veios de quartzo) e remobilização da maioria dos elementos químicos (Al, Fe, Ca, K, Na, Rb, Sr, Zr). Em condições PT ainda menores, deu-se finalmente a ação de um evento discreto, relacionado com crenulações e formando clorita, epídotos e quartzo (Mn+1/M2). O evento M2, bem como aquele detectado somente em GN (M1 em fácies anfibolito), foram de natureza dúctil, o que os distinguiu nitidamente do último episódio (D3/M3). Este foi posicionado no final do Paleoproterozóico, tendo caráter hidrotermal e associado á feições rúpteis de alto nível crustal. A evolução progressiva do metamorfismo M2, com pico térmico precoce ao pico da deformação, sugeriu uma trajetória P-T-t anti-horária, correspondente á evolução metamórfica de bacias marginais fanerozóicas. Algumas análises químicas de rochas metavulcânicas permitiram a definição de séries magmáticas e discussão de modelos petrogenéticos. Reconheceram-se três séries geoquímicas, a saber, da mais antiga para a mais nova, komatiítica (UM), toleitica (BAS e GB) e cálcio-alcalina (DAC). A primeira corresponde a komatiitos peridotíticos, com MgO>18% em peso (base anidra), com um "trend" de enriquecimento em Al, tal como em Geluk e Munro, e menos cálcico do que Barberton. Os padrões de terras raras leves são irregulares, com razões (La/Sm)N entre 0,42 e 4,2 e anomalias negativas de Eu. Os terras raras pesadas pareceram menos afetados por processos pós-eruptivos, sendo planos ou ligeiramente fracionados (1,0<(Gd1Yb)N<2,3). Modelos quantitativos foram de dificil execução em virtude da remobilização de vários elementos, porém, em termos qualitativos, foi possível estimar cumulados ricos em olivina e ortopiroxênio. Dentre os toleítos, BAS e GB apresentaram padrões geoquímicos muito similares entre si. Ambos são toleítos de baixo potássio, comparáveis a toleítos arqueanos empobrecidos. Os elementos terras raras são quase planos, com valores 10X o condrito, e anomalias fracas ou inexistentes de Eu. Modelos preliminares sugeriram cumulados semelhantes para BAS e GB, compostos essencialmente de clinopiroxênio e plagioclásio. De acordo com alguns cálculos geoquímicos, a fonte dos magmas que originaram os komatiitos e toleítos seria o lherzolito a granada. Os DAC apresentaram características geoquímicas afins à metavulcânicas e metaplutônicas cálcio-alcalinas tanto modernas quanto arqueanas, seguindo o "trend" trondhjemítico. A diferenciação magmática teria decorrido por fracionamento de plagioclásio>quartzo>hornblenda>K-feldspato, com quantidades accessórias de biotita, magnetita, titanita, alanita e zircão. A fonte do magma dacítico seria crustal do tipo toleíto metamorfisado em fácies granada anfibolito e ligeiramente enriquecido em terras raras leves. No modelo geodinâmico proposto, já existia um embasamento gnáissico antes de 2,96 Ga. Entre 2,96 e 2,90 Ga, a conjugação de alto gradiente geotérmico com extensão litosférica provocou o rifteamento continental, formando bacias marginais, onde se daria a extrusão de komatiitos e toleítos. Em torno de 2,94(?)-2,90 Ga, geraram-se os DAC através de fusão de crosta oceânica em zonas de subducção, evoluindo por fracionamento a baixas pressões. Os mesmos mecanismos geradores dos DAC também seriam responsáveis pelo plutonismo cálcio-alcalino, culminando com a inversão estrutural do "greenstone", espessamento crustal e forma final do terreno granito - "greenstone" (transpressão dextrógira ca. 2,88-2,86 Ga). A região sofreu ainda um episódio de (rea)quecimento, detectado a nível de minerais, sem deformação e metamorfismo correlatos, ao final do Eoarqueano (2,69-2,50 Ga), e intrusão de enxames de diques riolíticos (1,60 Ga, Rb/Sr) e diabásicos ao final do Paleoproterozóico. A correlação com o conhecimento atual da PMC permitiu admitir que o terreno granito - "greenstone" de Rio Maria já estava configurado quando da implantação do Supergrupo Itacaiúnas (ca. 2,76 Ga) e da granitogênse alcalina na Serra dos Carajás. Assim, a transpressão sinistrógira que inverteu aquele supergrupo corresponderia a um evento posterior e bem distinto da transpressão dextrógira da região de Rio Maria.
This thesis deals to the geology and petrogenesis of the Identidade greenstone belt, located between Xinguara and Rio Maria towns, SE of Pará state. The data of this area permitted the discussion of the tectonic evolution of the gravite greenstone terrain of the Rio Maria region in the context of the Província Mineral de Carajás, SE of the Amazonian craton. The greenstone studied compose a synformal belt in the WNW-ESE direction, corresponding to one metavolcanic pile, formed predominantly by ultramafic schists (UM), basalts (BAS) and gabbros (GB) at the base, and hypabyssal dacitic rocks (DAC - ca. 2.94 Ga, Pb/Pb) at the top. The whole was intruded by metaplutonic rocks of Mesoarchean ages, the older one being quartz diorites, followed successively by granodiorites, trondhjemites / tonalites and leucogranites. The gneissic basement (GN - outcroping toward north and recognized for having an older fabric Sn-1/D1), the greenstone and the metagranitoids were intruded by hypabyssal rhyolitic (ca. 1.60 Ga, Rb/Sr) and basic dykes at the end of the Paleoproterozoic. The greenstone presents igneous structures and textures still recognized, although obliterated near the contacts with the metagranitoids and shear zones. The ultramafics occur as tremolitites, tremolite - talc schists and talc schists; the amphibole is very elongated and thin, commonly in parallel arrays, interpreted as ghosts of spinifex textures. The basalts are massive or pillowed and frequently variolitic. They show different degrees of recrystallization, with some relicts of hyalophitic, pilotaxitic and traquitoid textures. Clinoamphibole (actinolitic hornblende), epidotes and plagioclase (albite - andesine) are the most abundant minerais. The gabbros may be massives to porphyritics (plagioclase phenocrysts), still with some relicts of subophitic and granophyric textures. The dacites are porphyritic, with phenocrysts of quartz and plagioclase (oligoclase), besides hornblende and mafic clots (biotite, chlorite, opaque minerais, epidotes, sphene, apatite) in the less evolved samples. Concerning the metagranitoids, the leucogranites and trondhjemites have chloritized biotite, whereas the granodiorites and some tonalites comprise biotite or biotite + hornblende (also in quartz diorites). The greenstone and the metagranitoids were affected by one event of heterogeneous, ductile deformation, that evolved to mylonitic zones. The structural framework of the area is marked by a planar fabric (Sn//Sm/D2) in the WNW-ESE to E-W direction, with moderate to strong dips in a divergent fan. E-W, WNW-ESE or NW-SE stretching lineations, meso and asymmetric S-C microstructures, mica and clinoamphibole fishes, and rotation of o and i porphyroclasts indicated one megastructure resulting from a binary system with NW-SE shortening direction. The actual geometry of the greenstone would be derived from a dextral transpression, with the greenstone forming a positive flower structure. The transpressional regime favored the grow of transtensional cites and subsequent emplacement of granitic plutons on the NW contact, and extensional crenulation cleavage (Sn+1/D2) on the SW of the greenstone. Strain measurements displayed a ca. 60% shortening, subhorizontal extension of ca. 60 to 500% parallel to the greenstone trend, and vertical extension of ca. 101 to 280%. The strain ellipsoid may be oblate to prolate, with changes in density and rotation of the axis of maximum stretching (X) toward the mylonitic zones. The inversion of the deformation permitted the reconstruction of the original shape of the greenstone, that would be also elongated WNW-ESE, but with lesser eccentricity than today. These data, together with the quartz petrofabric, suggested that the deformation has been accommodated by pure and simple shear mechanisms, the final framework resulting essentially from the later. The last event (D3) are represented by faults and fractures which also affected the felsic and basic dykes. The paragenesis of the main metamorphic event (Mn/M2) is represented by static recrystallization, which modified some textures and almost ali minerais within the greenstone. The minerais formed phases were bluish green amphibole (actinolitic hornblende), epidotes, sphene and quartz in BAS and GB; tremolite, talc and chlorite in UM. The metagranitoids show transformations of plagioclase (saussurite, fine white mica), amphibole (to biotite and/or sphene) and biotite (to chlorite). The coexistence of hornblende + plagioclase (An>17) and/or actinolitic hornblende + chlorite in metabasic rocks shows that this event was of low pressures and temperatures in the transitional field of the greenschist and amphibolite facies. This episode should reflect a regional crustal heating produced by the plutonism at the end of the Mesoarchean, that obliterated the previous associations of ocean floor metamorphism. Slightly coeval to subsequently, it occurred one event of extensive dynamic recrystallization (Mm/M2) in the greenschist facies, specially within shear zones and lithological contacts. In these places, there are evidences of fluid incoming (schistose blastomylonites and abundant quartz veins) and remobilization of chemical elements (Al, Fe, Ca, K, Na, Rb, Sr, Zr). Finally, under lower PT conditions, it occurred a less expressive event related to crenulation cleavages and forming chlorite, epidotes and quartz (Mn+1/M2). The M2 event, as well as the one detected only in GN (M1 under amphibolite facies), was of ductile nature and cleary distinguished from the last one (D3/M3). The later was placed at the end of the Paleoproterozoic, being of hydrothermal character and associated to high crustal structures. The progressive evolution of the M2 metamorphism with its thermal peak predating the deformation suggested a counterclockwise P-T-t path, corresponding to the metamorphic evolution of Phanerozoic marginal basins. Some chemical analysis of the metavolcanic rocks permitted the definition of magmatic series and a discussion of petrogenetical modeling. It was possible to recognize three geochemical series, that is, from the older to the younger, komatiitic (UM), tholeiitic (BAS and GB) and calc-alkaline (DAC). The first one corresponds to peridotitic komatiites with MgO>18 weight % (volatile-free basis), with an enrichment trend in Al, such as in Geluk and Munro, and less calcic than the Barberton one. The light rare earth element patterns are irregular with (La/Sm)N ratios between 0.42 and 4.2 and negative Eu anomalies. The heavy rare earth elements seem less affected by post-eruptive processes, being plate or slightly fractionated (1.0<(Gd/Yb)N<2.3). The quantitative models were of hard execution due to the remobilization of several elements. It was possible estimate cumulates rich in olivine and orthopyroxene. With regarding to tholeiites, the BAS and GB showed very similar geochemical signatures, both being low potassium tholeiites comparable to depleted Archean tholeiites. The rare earth elements are almost plate, with values 10X the chondrite, and slight or no Eu anomaly. Preliminary modeling suggested similar cumulates for BAS and GB, composed essentially by clinopyroxene and plagioclase. The magma sources that originated the komatiites and tholeiites would be a garnet lherzolite. The DAC presented geochemical characteristics of modern and Archean metavolcanics and metaplutonics of trondhjemitic nature. The magmatic differentiation would be achieved by fractionation of plagioclase>quartz>hornblende>K-feldspar, with subordinated amount of biotite, magnetite, sphene, allanite and zircon. The source of the dacitic magma would be a tholeiite metamorphosed to the garnet amphibolite facies and somewhat enriched in light rare earth elements. The geodynamical model proposed admit the existence of a gneissic basement prior to 2.96 Ga. Between 2.96 and 2.90 Ga, the interplay of high geothermal gradients and lithospheric extension was responsible for extensive rifting, forming marginal basin systems, where extruded the komatiitic and tholeiitic rocks. At 2.94(?)-2.90 Ga, the DAC were generated from partia' melting of oceanic crust in subduction zone settings, and evolved by low pressure fractional crystallization. The same mechanisms that generated the DAC are extended also to the calc-alkaline plutonism, this one being responsible for the structural inversion of the greenstone, crustal thickening and final shape of the granite - greenstone terrain (dextral transpression ca. 2.88-2.86 Ga). The region still suffered a late episode (end of Eoarchean, 2.69-2.50 Ga) of (re)heating, registered only in sorne mineral, without any evidente of deformation and/or metamorphism. Finally, it occurred the intrusion of felsic (1.60 Ga, Rb/r) and basic dykes at the end of the Paleoproterozoic. The correlation with the actual understanding of the Província Mineral de Carajás permitted envisage that the Rio Maria granite - greenstone terrain was then configured at the moment of implantation of the Itacaiúnas Supergroup (ca. 2.76 Ga) and alkaline granitic plutonism at the Serra dos Carajás. So the sinistrai transpression that inverted that supergroup would correspond to a newer event, very distinct as regards as the dextral transpression of the Rio Maria region.

Thesis (MSc)--Stellenbosch University, 2013.
ENGLISH ABSTRACT: The source of the K2O in the K2O-rich ~3.45 Ga felsic intrusive rocks of the H6 unit in the Hooggenoeg Formation of the Onverwacht Group in the Barberton Granite Greenstone Terrain (BGGT) is examined in this study. This is of particular research interest because the Paleoarchaean rock record is considered to lack K2O-rich magmatic rocks. Previous studies on the felsic igneous rocks of the H6 unit have proposed that these rhyolites are K-metasomatised eruptive equivalents of the sodium-rich ~3.45 Ga TTGs of the BGGT and that the K-feldspar crystals in the rocks formed as a consequence of subsolidus replacement of plagioclase by K-feldspar. Furthermore, the timing of K-metasomatism has previously been related to the formation of the Buck Ridge Chert (BRC), which overlies the H6 unit. However, it has recently been demonstrated from granitic clasts in the conglomerate layer at the base of the Moodies sucession that K2O-rich magmatic rocks formed concurrently with TTG magmas during each of three episodes of TTG magmatism observed in the BGGT. Consequently, the hypothesis of a metasomatic origin for the K2O-rich character of the felsic rocks of the H6 unit requires further examination. Previous studies of the chemistsry of felsic volcanic rocks within the H6 unit were based on relatively low numbers of samples. This study has examined a substantial set of the freshest material available. Two varieties of felsic volcanic rocks were identified; K2O-rich, CaO-poor, Na2O-poor rhyolites and Na2O-rich, CaO-poor, K2O-poor Na-rhyolites. The K2O- rich rhyolite variety is dominant. Consequently, it is possible that the K2O-rich character of these rocks represents a primary magmatic signature. However, this judgment is complicated by the presence of a greenschist-facies metamorphic overprint at 3.2 Ga, which has resulted in complete replacement of micrystalline groundmass and partial replacement of the phenocryst assemblages by greenschist- and sub-greenschist-facies mineral assemblages, which undoubtedly allowed possible shifts in chemical compositions In this thesis, I test the source of K2O in these rocks by using the porphyritic textures of the rocks as an indication of the primary composition of the magmas they were formed from. These textures are typically defined by K-feldspar or albite and quartz phenocrysts within a microcrystalline groundmass. The rocks containing albite are Na-rich (Na-rhyolites) whereas the rocks defined by K-feldspar phenocrysts are rhyolites. XRD study of the structural state of the K-feldspar phenocrysts in the rhyolites indicates that these crystals are orthoclase and intermediate microcline, i.e. medium temperature K-feldspar polymorphs. The modal proportions of K-feldspar, quartz and microcrystalline groundmass in the rhyolites were calculated by using image analysis software. The compositions of the feldspar minerals were determined by electron beam analysis. Minimum bulk rock K2O content of the rhyolites were calculated from the proportions of K-feldspar crystals and their compositions. Even where the proportion of K-feldspar phenocrysts is relatively low (~ 30%), the calculated minimum bulk-rock K2O content is still above 5 wt%. The HREE slope (GdN/LuN) of the felsic porphyritic rocks of the H6 rhyolites is similar to that of ~3.45 Ga TTG plutons and steeper than that of granitic clasts of identical age contained in the basal conglomerate of the Moodies Group. Hence this study has illustrated that the rhyolites of the H6 unit were primary K-feldspar-rich, K2O-rich magmas that formed contemporarily with the ~3.45 Ga TTGs. This implicitly means that rhyolitic volcanism was more wide spread than previously thought in the Paleoarchaean and that it occurred together with the intrusion of the ~3.45 Ga TTGs in the BGGT.
AFRIKAANSE OPSOMMING: Die bron van die K2O in die K2O-ryk ~ 3,45 Ga felsiese vulkaniese rotse van die H6-eenheid in die Hooggenoeg formasie van die Onverwacht Groep in die Barberton Graniet Groensteen Terrein (BGGT) is in hierdie studie ondersoek. Dit is van besondere navorsingsbelang omdat die Paleoargeïse gesteenterekord beskou word as vry van magmatiese K2O ryke gesteentes. Vorige studies oor die felsiese vulkaniese rotse van die H6 eenheid het voorgestel dat hierdie rioliete K-gemetasomatiese eruptiewe ekwivalente van die natrium-ryke ~ 3,45 Ga TTGs van die BGGT is en dat die K-veldspaat kristalle in die gesteentes gevorm is as gevolg van subsolidus vervanging van plagioklaas deur K-veldspaat. Verder is die tydsberekening van K-metasomatisme voorheen gekoppel aan die vorming van die Buck Ridge Chert (BRC) wat die felsiese H6 eenheid bedek. Dit is egter onlangs aangetoon dat K2O-ryke magmatiese rotse gelyktydig met TTG magmas gevorm is tydens elk van drie episodes van TTG magmatisme waargeneem in die BGGT. Gevolglik vereis die hipotese van 'n metasomatiese oorsprong vir die K2O-ryke karakter van die felsiese gesteentes van die H6 eenheid verdere ondersoek. Vorige studies van die felsiese vulkaniese gesteentechemie in die H6 eenheid is gebaseer op 'n relatief klein getal monsters. Hierdie studie het 'n aansienlike stel van die varsste materiaal beskikbaar vir analise ondersoek. Twee variëteite van peralumineuse felsiese vulkaniese gesteentes naamlik 'n K2O-ryk, CaO-arm, Na2O-arm rioliet en Na2O-ryk, CaO-arm, K2O-arm Na-rioliet. Die K2O-ryke rioliet variëteit is meer oorheersend as die Na-rioliete. Dit is dus moontlik dat die K2O-ryk karakter van hierdie rotse 'n primêre magmatiese kenmerke verteenwoordig. Hierdie uitspraak is egter bemoeilik deur die teenwoordigheid van 'n groenskisfasies metamorfe oorprint op 3,2 Ga, wat gelei het tot die volledige vervanging van mikrokrisstalyne grondmassa en gedeeltelike vervanging van fenokrist samestellings deur groenskis en sub-groenskisfasies minerale samestellings en wat ongetwyfeld toegelaat het vir 'n moontlike verskuiwing in chemiese samestelling. In hierdie tesis toets ek die bron van K2O in hierdie gesteentes deur gebruik te maak van die vulkaniese teksture van die gesteentes as 'n aanduiding van die primêre samestelling van die magmas waaruit hulle gevorm het. Hierdie teksture word gewoonlik gedefinieer deur K-veldspaat of albiet en kwarts fenokriste binne 'n grondmassa van wat vroeërglasoorblyfsels was. Die rotse wat albiet bevat is Na-ryk (Na-rioliete) terwyl die rotse gedefinieer deur K-veldspaat fenokriste rioliete is. XRD studie van die strukturele toestand van die K-veldspaat fenokriste in die rioliete dui aan dat hierdie kristalle ortoklaas en intermediêre mikroklien is, dit wil sê die hoër temperatuur K-veldspaat polimorfe. Die modale proporsies van K-veldspaat, kwarts en glasoorblyfsels in die rioliete is akkuraat bereken deur gebruik te maak van beeld analise sagteware. Verder is die samestellings van die veldspaat minerale bepaal deur die elektronstraal analise. Minimum grootmaat rots K2O inhoud van die rioliet is berekén vanaf die fase verhouding van K-veldspaat en hul komposisies. Resultate dui daarop dat selfs waar die verhouding van K-veldspaat phenocrysts is relatief laag (~ 30%), die berekende minimum K2O grootmaat rots samestelling is nog steeds bo 5 wt%. Die REE-helling (GDN / Lun) van felsiese porphyritic rotse van die H6 is soortgelyke relatief tot die REE helling van ~ 3,45 Ga TTGs en steiler REE helling relatief tot granitiese klaste vervat in die basale konglomeraat van die Moodies-groep. Dus het hierdie studie getoon dat die rioliete van die H6-eenheid primêre K-veldspaat-ryke, K2O-ryke en peralumineuse magmas was wat gevorm is terselfdertyd met die ~3,45 Ga TTGs. Dit beteken implisiet dat riolitiese vulkanisme meer wyd verspreid was as wat voorheen gedink is in die Paleoargeïkum en dat dit tesame met die indringing van die ~ 3,45 Ga TTGs in die BGGT plaasgevind het.

Thesis (M.S.) -- University of Maryland, College Park, 2004.
Thesis research directed by: Dept. of Geology. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.