The oldest dated rocks from the Acasta gneisses of the western Slave Province, Canada present an igneous age of ~4030 Ma. Following this the detrital zircons from the Jack Hills, Narryer Gneiss Terrane, Yilgarn Craton, Western Australia are identified as 4404 ±8 Ma. These discoveries suggest that crustal formation started as early as the Priscian Eon. Hitherto the Earth has gone through a series of interactions involving the atmosphere, hydrosphere, crust, mantle and core. However, only limited remnants of these early processes remain on the accessible crust due to extensive crustal reworking. The Southern Granulite Terrane (SGT) in the southern part of India represents the most extensive exposure of lower crustal granulite terranes in the world. This study mainly focuses on the characteristics of Archean (~2500 Ma) tectonics and nature of subsequent crustal growth, which led to the formation of Archean Nilgiri Block. Detailed fieldwork in this terrane and subsequent petrographic analysis revealed charnockites, hornblende-biotite gneiss, metagabbro/mafic granulite, websterite, amphibolite, Grt-Ky metasediment, metatuff and banded iron formation as the main rock types in this terrane. Field and petrographic results show a regional trend with garnet-orthopyroxene-biotite-quartz-plagioclase-K- feldspar bearing charnockites in the southern part which gets subsequently enriched in clinopyroxene that forms garnet-absent two pyroxene granulites consisting of orthopyroxene-clinopyroxene-quartz-plagioclase-K-feldspar towards the central part. Further north, metagabbro/mafic granulite is enriched in garnet-clinopyroxene-plagioclase assemblage. Websterite, amphibolite, metasediment, metatuff and banded iron formation are stacked and closely associated within this mafic belt in the north. The metagabbro represents peak P-T conditions of ~850°C and ~14kbar compared to the charnockites, which recorded a peak P-T of ~850°C and 9-10kbar. Petrographic results of oxide minerals show that the southern charnockitic part is abundant in rutile-ilmenite association represent reduced conditions compared to the oxidized magnetite-hematite-ilmenite associations in the mafic rocks. This oxidation trend is followed by pyrrhotite-chalcopyrite enriched southern charnockitic region that transforms to pyrite rich northern mafic belt. Ilmenite¬titanite association with no sulphides characterizes the hornblende-biotite gneiss in the entire Nilgiri Block. The geochemical variations of major, trace and rare earth elements show that the granulite-amphibolite grade felsic rocks evolved in an arc magmatic process leaving behind mafic magma, which later intruded into these rocks, in a subduction related arc magmatic process. The U-Pb LA-ICPMS and SHRIMP dating of charnockite, hornblende-biotite gneiss and met gabbros shows ca. 2550 Ma formation age and ca. 2450 Ma metamorphism in this terrane.

O Complexo Máfico-Ultramáfico Mata Grande (CMG), localizado no município de São Sepé, porção NW do Escudo Sul-Rio-Grandense, é uma intrusão máfico-ultramáfica com cerca de 5 km2 que mantém contatos através de falhas normais com gnaisses do Complexo Cambaí ao SW e ao SE, e com as rochas sedimentares da Bacia do Paraná ao N. O contato com os xistos magnesianos e serpentinitos do Complexo Arroio Lajeadinho situados ao leste é intrusivo. Foram descritas três unidades de rochas cumuláticas: Unidade Máfica (UM), Unidade Ultramáfica (UUM) e Unidade Transicional (UT). A principal estrutura primária é um acamamento composicional/textural milimétrico a centimétrico e uma intercalação de camadas das unidades em escalas de afloramento e regional. As rochas da UM cristalizaram a partir da acumulação de cristais de plagioclásio e, em menor proporção, de olivina, além de fases minerais intercúmulus, que representam de 24% a 41% de líquido intersticial aprisionado nesta acumulação. Na UT, a acumulação de plagioclásio e olivina ocorreu em proporções muito próximas, com uma menor proporção do líquido aprisionado (cerca de 15%). As amostras da UUM evidenciam uma acumulação principal de olivina com plagioclásio intercúmulus mais uma proporção do líquido intersticial (20%). Todas as unidades do CMG são afetadas pelo metamorfismo de contato causado pelo Granito São Sepé, sob condições de temperatura equivalentes às das fácies albita-epidoto hornfels e hornblenda hornfels. Os dados de geoquímica em rocha total mostraram anomalias positivas de Ba e Sr e negativas de Nb para todas as amostras, indicando metassomatismo na fonte. O efeito da acumulação não exerce influência no comportamento destes elementos, tendo em vista a ausência de fases minerais com afinidade química para comportá-los. Os padrões de ETR são mais coerentes com trends cumuláticos. No entanto, a anomalia de Eu conspícua que ocorre nos cumulados de plagioclásio e mais acentuada nos cumulados de olivina sugere um enriquecimento prévio de Eu no magma. Os dados de U-Pb em zircões obtidos por LA-ICP-MS forneceram idades de zircões herdados das rochas (metavulcânicas do Complexo Bossoroca, 800-750 Ma; ortognaisses do Complexo Cambaí, 720 Ma; e granitóides da Suíte Lagoa da Meia-Lua, 680 Ma) e uma idade de cristalização magmática para o CMG (667.8 ± 3.3 Ma). Os dados de geoquímica e geocronologia favorecem a hipótese de um ambiente pós-colisional para a cristalização e colocação do Complexo Mata Grande. Processos de delaminação litosférica tais como slabbreakoff são sugeridos como fonte de calor para o magmatismo máfico pós-colisional. A placa oceanic partiu-se após a subducção abaixo do Arco de São Gabriel e a colisão com o Complexo Encantadas (2,2 Ga), um fragmento do Cráton Rio de La Plata, durante um periodo de extenso magmatismo juvenil associado à amalgamação do Supercontinente Godwana Ocidental. / The Mata Grande Mafíc-Ultramafic Complex (MGC), located at São Sepé municipality, NW portion of the Sul-Rio-Grandense Shield, is a 5 km2 mafic-ultramafic intrusion which maintains contacts by normal faults southwest with the gneisses of the Cambaí Complex and in north with the sedimentary rocks of the Paraná Basin. The contact southeast with magnesian schists and serpentinites of the Arroio Lajeadinho Complex is intrusive. Three cumulatic rock unities were described: Mafic Unit (MU), Ultramafic Unit (UMU) and Transicional Unit (TU). Preserved primary structures are composicional/textural millimetric to centimetric layering with no mineral lineation, outcrop scale intercalation and regional intercalation. UM rocks crystallized from accumulation of plagioclase crystals and, in less proportion, olivine crystals, and also from intercumulus phases, representing 24% - 41% of the interstitial trapped liquid in the accumulation. In UT, plagioclase and olivine accumulation occurred in very close proportions, with a minor trapped liquid proportion (~ 15%). UUM samples shows olivine principal accumulation with intercumulus plagioclase plus trapped liquid (20%). All CMG units are affected by contact metamorphism caused by São Sepé Granite, under albite-epidote hornfels and hornblende hornfels temperature conditions. Geochemical data are presented and Ba, Nb and Sr anomalies indicate previous metassomatism in the source. Accumulation effect on those anomalies is absent or has little influence, since there are no mineral phases capable to hold these elements in studied rocks. REE patterns are more consistent with cumulate trends. However, conspicuous Eu positive anomaly in the plagioclase cumulates and more accentuated in olivine accumulates suggests that there was an Eu enrichment in the magma. U-Pb zircon data obtained by in situ LA-ICP-MS yielded ages of inherited zircons from surrounding igneous and metamorphic rocks (Bossoroca Complex metavolcanic, 800- 750 Ma; Cambaí Complex orthogneiss, 720 Ma; and Lagoa da Meia-Lua Suite granitoids, 680 Ma) and a magmatic crystallization age for the MGC (667.8 ± 3.3 Ma). Either geochemical and isotope data allied with field relationships favor the hypothesis of a post-collisional environment for Mata Grande Complex crystallization and emplacement. Lithospheric delamination process such as slab-breakoff is suggested as source of heat for post-collisional mafic magmatism. The oceanic plate has broken down after subduction under São Gabriel Arc and its collision with 2,2 Ga Encantadas Complex, a Rio de La Plata Craton Fragment, in a extensive period of juvenile magmatism associated to Western Godwana Supercontinent amalgamation.

Fundação Carlos Chagas Filho de Amparo a Pesquisa do Estado do Rio de Janeiro / Nesta dissertação foram estudas rochas máficas dos complexos alcalinos de Morro de São João, Rio Bonito, Tanguá, Gericinó-Mendanha, Morro Redondo, Itatiaia e Passa Quatro. Essas rochas ocorrem na forma de diques e/ou sills. As amostras coletadas foram classificadas como lamprófiros, fonolitos, gabros e diabásios alcalinos. A análise geoquímica permitiu identificar um trend fortemente insaturado e um trend moderadamente alcalino para os complexos estudados. O primeiro é caracterizado por foiditos e fonolitos como membros parentais e mais evoluídos, respectivamente, enquanto o segundo tem basaltos alcalinos como membros parentais e traquitos como os mais evoluídos. Todas as amostras plotam no campo da série alcalina, sendo majoritariamente miaskíticas, sódicas ou potássicas. Adicionalmente, o estudo geoquímico indicou que os complexos alcalinos representam câmaras magmáticas distintas, onde diferentes processos evolutivos tiveram lugar. As modelagens apontaram dois processos de diferenciação distintos nos complexos estudados. Os complexos alcalinos de Morro de São João, Morro Redondo, Gericinó-Mendanha e Itatiaia estariam relacionados a processos de cristalização fracionada. Por outro lado, o Complexo Alcalino de Passa Quatro teria sido diferenciado por processos de cristalização fracionada com esvaziamento e posterior reabastecimento da câmara magmática (RTF). De um modo geral, esses modelos indicaram a presença de mais do que uma série magmática nos complexos estudados e a não cogeneticidade entre as séries agpaíticas e miaskíticas. A discriminação de fontes foi feita com base na análise dos elementos terras raras das amostras parentais de cada um dos complexos (gabro em Morro de São João e lamprófiro nos demais). No entanto, este procedimento não foi aplicado para o Complexo Alcalino de Morro Redondo, uma vez que todas as suas amostras apresentaram valores de MgO muito abaixo do típico para líquidos parentais. O líquido parental do Complexo Alcalino do Gericinó-Mendanha apresentou razões de La/Yb e La/Nb, maior e menor que a unidade, respectivamente, típicas de derivação a partir fontes férteis. Os líquidos parentais dos outros complexos alcalinos tiveram suas razões La/Yb e La/Nb maiores que a unidade, típicas de derivação a partir de fontes enriquecidas. Os modelos desenvolvidos revelaram que os líquidos parentais de cada um dos complexos estudados estariam relacionados a fontes lherzolíticas com granada residual. Além disso, a fusão parcial destas fontes teria ocorrido num intervalo de 1 a 7% dentro da zona da granada. Finalmente, as modelagens petrogenéticas elaboradas permitiram a proposição de um cenário geodinâmico, envolvendo a descompressão adiabática do manto litosférico e sublitosférico anomalamente aquecidos. As características geoquímicas dos líquidos parentais parecem ter sido controladas essencialmente pela mistura desses dois tipos de fontes. / Mafic rocks from the Morro de São João, Rio Bonito, Tanguá, Gericinó-Mendanha, Morro Redondo, Itatiaia and Passa Quatro alkaline complexes were studied based on field, petrographic and lithogeochemical data. The mafic rocks are mostly alkaline lamprophyres and alkaline gabbro (in Morro de São João). The complexes show both strongly undersaturated and mildly alkaline evolutionary trends, having foidites and alkaline basalts as parental compositions. Trachytes and phonolites are the most common evolved rocks. Geochemical modelling has show that miaskitic and agpaitic series are unlikely to be related by fractional crystallization. Enriched mantle sources predominate although a fertile mantle source seems to be related with the Gericinó-Mendanha complex. Parental compositions are related to small amounts of partial melting (1-7%) of garnet lherzolite within the garnet stability field in the mantle. Simple geodynamic models indicate that the alkaline complexes are related to the adiabatic decompression of anomalously hot (~1570C) mantle although unrelated to lithospheric translation over a fixed hotspot or mantle plume. Parental compositions are likely to be strongly controled by mixing of distinctive lithospheric mantle sources and a more homogeneous sublithospheric mantle. The lithospheric components seem to be related with the accreted terranes during the Gondwana amalgamation in Early Proterozoic times.

O Complexo Máfico-Ultramáfico Mata Grande (CMG), localizado no município de São Sepé, porção NW do Escudo Sul-Rio-Grandense, é uma intrusão máfico-ultramáfica com cerca de 5 km2 que mantém contatos através de falhas normais com gnaisses do Complexo Cambaí ao SW e ao SE, e com as rochas sedimentares da Bacia do Paraná ao N. O contato com os xistos magnesianos e serpentinitos do Complexo Arroio Lajeadinho situados ao leste é intrusivo. Foram descritas três unidades de rochas cumuláticas: Unidade Máfica (UM), Unidade Ultramáfica (UUM) e Unidade Transicional (UT). A principal estrutura primária é um acamamento composicional/textural milimétrico a centimétrico e uma intercalação de camadas das unidades em escalas de afloramento e regional. As rochas da UM cristalizaram a partir da acumulação de cristais de plagioclásio e, em menor proporção, de olivina, além de fases minerais intercúmulus, que representam de 24% a 41% de líquido intersticial aprisionado nesta acumulação. Na UT, a acumulação de plagioclásio e olivina ocorreu em proporções muito próximas, com uma menor proporção do líquido aprisionado (cerca de 15%). As amostras da UUM evidenciam uma acumulação principal de olivina com plagioclásio intercúmulus mais uma proporção do líquido intersticial (20%). Todas as unidades do CMG são afetadas pelo metamorfismo de contato causado pelo Granito São Sepé, sob condições de temperatura equivalentes às das fácies albita-epidoto hornfels e hornblenda hornfels. Os dados de geoquímica em rocha total mostraram anomalias positivas de Ba e Sr e negativas de Nb para todas as amostras, indicando metassomatismo na fonte. O efeito da acumulação não exerce influência no comportamento destes elementos, tendo em vista a ausência de fases minerais com afinidade química para comportá-los. Os padrões de ETR são mais coerentes com trends cumuláticos. No entanto, a anomalia de Eu conspícua que ocorre nos cumulados de plagioclásio e mais acentuada nos cumulados de olivina sugere um enriquecimento prévio de Eu no magma. Os dados de U-Pb em zircões obtidos por LA-ICP-MS forneceram idades de zircões herdados das rochas (metavulcânicas do Complexo Bossoroca, 800-750 Ma; ortognaisses do Complexo Cambaí, 720 Ma; e granitóides da Suíte Lagoa da Meia-Lua, 680 Ma) e uma idade de cristalização magmática para o CMG (667.8 ± 3.3 Ma). Os dados de geoquímica e geocronologia favorecem a hipótese de um ambiente pós-colisional para a cristalização e colocação do Complexo Mata Grande. Processos de delaminação litosférica tais como slabbreakoff são sugeridos como fonte de calor para o magmatismo máfico pós-colisional. A placa oceanic partiu-se após a subducção abaixo do Arco de São Gabriel e a colisão com o Complexo Encantadas (2,2 Ga), um fragmento do Cráton Rio de La Plata, durante um periodo de extenso magmatismo juvenil associado à amalgamação do Supercontinente Godwana Ocidental. / The Mata Grande Mafíc-Ultramafic Complex (MGC), located at São Sepé municipality, NW portion of the Sul-Rio-Grandense Shield, is a 5 km2 mafic-ultramafic intrusion which maintains contacts by normal faults southwest with the gneisses of the Cambaí Complex and in north with the sedimentary rocks of the Paraná Basin. The contact southeast with magnesian schists and serpentinites of the Arroio Lajeadinho Complex is intrusive. Three cumulatic rock unities were described: Mafic Unit (MU), Ultramafic Unit (UMU) and Transicional Unit (TU). Preserved primary structures are composicional/textural millimetric to centimetric layering with no mineral lineation, outcrop scale intercalation and regional intercalation. UM rocks crystallized from accumulation of plagioclase crystals and, in less proportion, olivine crystals, and also from intercumulus phases, representing 24% - 41% of the interstitial trapped liquid in the accumulation. In UT, plagioclase and olivine accumulation occurred in very close proportions, with a minor trapped liquid proportion (~ 15%). UUM samples shows olivine principal accumulation with intercumulus plagioclase plus trapped liquid (20%). All CMG units are affected by contact metamorphism caused by São Sepé Granite, under albite-epidote hornfels and hornblende hornfels temperature conditions. Geochemical data are presented and Ba, Nb and Sr anomalies indicate previous metassomatism in the source. Accumulation effect on those anomalies is absent or has little influence, since there are no mineral phases capable to hold these elements in studied rocks. REE patterns are more consistent with cumulate trends. However, conspicuous Eu positive anomaly in the plagioclase cumulates and more accentuated in olivine accumulates suggests that there was an Eu enrichment in the magma. U-Pb zircon data obtained by in situ LA-ICP-MS yielded ages of inherited zircons from surrounding igneous and metamorphic rocks (Bossoroca Complex metavolcanic, 800- 750 Ma; Cambaí Complex orthogneiss, 720 Ma; and Lagoa da Meia-Lua Suite granitoids, 680 Ma) and a magmatic crystallization age for the MGC (667.8 ± 3.3 Ma). Either geochemical and isotope data allied with field relationships favor the hypothesis of a post-collisional environment for Mata Grande Complex crystallization and emplacement. Lithospheric delamination process such as slab-breakoff is suggested as source of heat for post-collisional mafic magmatism. The oceanic plate has broken down after subduction under São Gabriel Arc and its collision with 2,2 Ga Encantadas Complex, a Rio de La Plata Craton Fragment, in a extensive period of juvenile magmatism associated to Western Godwana Supercontinent amalgamation.

O Complexo Máfico-Ultramáfico Mata Grande (CMG), localizado no município de São Sepé, porção NW do Escudo Sul-Rio-Grandense, é uma intrusão máfico-ultramáfica com cerca de 5 km2 que mantém contatos através de falhas normais com gnaisses do Complexo Cambaí ao SW e ao SE, e com as rochas sedimentares da Bacia do Paraná ao N. O contato com os xistos magnesianos e serpentinitos do Complexo Arroio Lajeadinho situados ao leste é intrusivo. Foram descritas três unidades de rochas cumuláticas: Unidade Máfica (UM), Unidade Ultramáfica (UUM) e Unidade Transicional (UT). A principal estrutura primária é um acamamento composicional/textural milimétrico a centimétrico e uma intercalação de camadas das unidades em escalas de afloramento e regional. As rochas da UM cristalizaram a partir da acumulação de cristais de plagioclásio e, em menor proporção, de olivina, além de fases minerais intercúmulus, que representam de 24% a 41% de líquido intersticial aprisionado nesta acumulação. Na UT, a acumulação de plagioclásio e olivina ocorreu em proporções muito próximas, com uma menor proporção do líquido aprisionado (cerca de 15%). As amostras da UUM evidenciam uma acumulação principal de olivina com plagioclásio intercúmulus mais uma proporção do líquido intersticial (20%). Todas as unidades do CMG são afetadas pelo metamorfismo de contato causado pelo Granito São Sepé, sob condições de temperatura equivalentes às das fácies albita-epidoto hornfels e hornblenda hornfels. Os dados de geoquímica em rocha total mostraram anomalias positivas de Ba e Sr e negativas de Nb para todas as amostras, indicando metassomatismo na fonte. O efeito da acumulação não exerce influência no comportamento destes elementos, tendo em vista a ausência de fases minerais com afinidade química para comportá-los. Os padrões de ETR são mais coerentes com trends cumuláticos. No entanto, a anomalia de Eu conspícua que ocorre nos cumulados de plagioclásio e mais acentuada nos cumulados de olivina sugere um enriquecimento prévio de Eu no magma. Os dados de U-Pb em zircões obtidos por LA-ICP-MS forneceram idades de zircões herdados das rochas (metavulcânicas do Complexo Bossoroca, 800-750 Ma; ortognaisses do Complexo Cambaí, 720 Ma; e granitóides da Suíte Lagoa da Meia-Lua, 680 Ma) e uma idade de cristalização magmática para o CMG (667.8 ± 3.3 Ma). Os dados de geoquímica e geocronologia favorecem a hipótese de um ambiente pós-colisional para a cristalização e colocação do Complexo Mata Grande. Processos de delaminação litosférica tais como slabbreakoff são sugeridos como fonte de calor para o magmatismo máfico pós-colisional. A placa oceanic partiu-se após a subducção abaixo do Arco de São Gabriel e a colisão com o Complexo Encantadas (2,2 Ga), um fragmento do Cráton Rio de La Plata, durante um periodo de extenso magmatismo juvenil associado à amalgamação do Supercontinente Godwana Ocidental. / The Mata Grande Mafíc-Ultramafic Complex (MGC), located at São Sepé municipality, NW portion of the Sul-Rio-Grandense Shield, is a 5 km2 mafic-ultramafic intrusion which maintains contacts by normal faults southwest with the gneisses of the Cambaí Complex and in north with the sedimentary rocks of the Paraná Basin. The contact southeast with magnesian schists and serpentinites of the Arroio Lajeadinho Complex is intrusive. Three cumulatic rock unities were described: Mafic Unit (MU), Ultramafic Unit (UMU) and Transicional Unit (TU). Preserved primary structures are composicional/textural millimetric to centimetric layering with no mineral lineation, outcrop scale intercalation and regional intercalation. UM rocks crystallized from accumulation of plagioclase crystals and, in less proportion, olivine crystals, and also from intercumulus phases, representing 24% - 41% of the interstitial trapped liquid in the accumulation. In UT, plagioclase and olivine accumulation occurred in very close proportions, with a minor trapped liquid proportion (~ 15%). UUM samples shows olivine principal accumulation with intercumulus plagioclase plus trapped liquid (20%). All CMG units are affected by contact metamorphism caused by São Sepé Granite, under albite-epidote hornfels and hornblende hornfels temperature conditions. Geochemical data are presented and Ba, Nb and Sr anomalies indicate previous metassomatism in the source. Accumulation effect on those anomalies is absent or has little influence, since there are no mineral phases capable to hold these elements in studied rocks. REE patterns are more consistent with cumulate trends. However, conspicuous Eu positive anomaly in the plagioclase cumulates and more accentuated in olivine accumulates suggests that there was an Eu enrichment in the magma. U-Pb zircon data obtained by in situ LA-ICP-MS yielded ages of inherited zircons from surrounding igneous and metamorphic rocks (Bossoroca Complex metavolcanic, 800- 750 Ma; Cambaí Complex orthogneiss, 720 Ma; and Lagoa da Meia-Lua Suite granitoids, 680 Ma) and a magmatic crystallization age for the MGC (667.8 ± 3.3 Ma). Either geochemical and isotope data allied with field relationships favor the hypothesis of a post-collisional environment for Mata Grande Complex crystallization and emplacement. Lithospheric delamination process such as slab-breakoff is suggested as source of heat for post-collisional mafic magmatism. The oceanic plate has broken down after subduction under São Gabriel Arc and its collision with 2,2 Ga Encantadas Complex, a Rio de La Plata Craton Fragment, in a extensive period of juvenile magmatism associated to Western Godwana Supercontinent amalgamation.

A técnica da Distribuição do Tamanho de Cristais - DTC (Crystal Size Distribution - CSD), que relaciona a densidade de cristais com a distribuição do tamanho, foi aplicada à população de plagioclásio de diques máficos do Enxame de Cabo Frio - Búzios (RJ). Os diques possuem larguras variáveis, de alguns centímetros a 20 metros, e orientação em torno de N45E. A textura dos diques é geralmente fina, localmente microporfirítica e intergranular no centro dos diques mais largos. Bordas resfriadas de alguns centímetros de largura são frequentes nos contatos com as rochas metamórficas encaixantes. Foram estudados dois diques na Praia das Conchas com espessura de 0,8m e 8,2m e, um outro na Praia da Lagoinha, com 2m de largura. As amostras foram coletadas junto às margens (~10 cm do contato) e no centro dos diques. O tamanho médio dos cristais de plagioclásio varia de 0,07 a 0,13 mm na borda dos diques mais finos (<= 2 m de largura) e de 0,09 a 0,20 mm na borda do dique mais largo. No centro do dique de Lagoinha e no dique largo de Praia das Conchas o tamanho de plagioclásio é da ordem de 0,19 ± 0,02 mm e 0,60 ± 0,07 mm, respectivamente. As DTCs nas bordas dos diques, independentemente de sua largura, mostraram um padrão tipicamente encurvado, e que tem sido atribuído na literatura como evidência para misturas de magmas com populações de cristais de tamanhos distintos. No entanto, no centro do dique largo (8,2m) de Praia das Conchas, a DTC é log-linear consistente com uma cristalização magmática simples. A química mineral mostrou que os cristais maiores (precoces) de plagioclásio apresentam um teor mais elevado em An (bytownita-labradorita) que os cristais menores (tardios) da matriz (labradorita-andesina). Além disso, a olivina é mais rica em Fo na borda que no centro do dique e, respectivamente, o piroxênio mais enriquecido em Ca. Esses resultados indicam que as margens resfriadas são mais máficas que o centro sugerindo uma evolução química normal com o resfriamento do magma. Portanto, as DTCs encurvadas provavelmente refletem taxas de cristalização heterogêneas possivelmente induzidas pela despressurização durante a ascensão do magma basáltico seguida de rápido resfriamento. O padrão da DTC log-linear no centro do dique de 8,2m de largura é atribuído ao maior tempo de residência do magma que favoreceria os processos de difusão química e re-equilíbrio textural. Os cálculos da taxa de resfriamento utilizando a inclinação da DTC permitiram estimar que o centro do dique largo da Praia das Conchas estaria completamente cristalizado (~ 900 °C) em torno de 73 dias. O estudo da Orientação Preferencial de Forma (OPF) de plagioclásio mostrou que a petrotrama tende a isotrópica nas margens dos diques com largura menor que 2 metros, o que poderia refletir uma rápida cristalização de plagioclásio por despressurização. Quando a trama é localmente definida, como no dique largo da Praia das Conchas, a lineação de plagioclásio é subhorizontal sugerindo que o fluxo magmático moveu-se predominantemente na lateral do dique. / The method of Crystal Size Distribution (CSD), which relates crystal density with size distribution, has been applied on the plagioclase population of the Mafic Dyke Swarm of Cabo Frio-Búzios (RJ). The dykes are NE-trending with widths from a few centimetres to 20m. The texture is generally fine grained and locally microporphyritic and intergranular at the center of the larger dykes. Chilled margins of a few centimetres in width are common at contacts with the metamorphic basement. Two dykes of 0.8m and 8.2m in width of the Conchas Beach and another of 2m in width at the Lagoinha Beach have been studied. Samples were collected at the margins (~10cm from the contact) and at the center of the dykes. The average characteristic size of the plagioclase crystals varies from 0.07 to 0.13mm at the margins of the narrow dykes (<=2m of width) and from 0.09 to 0.20mm at the margins of the large dyke. At the center of the Lagoinha and Conchas dykes the plagioclase size varies from 0.19 ±0.02mm and 0.60±0.07mm respectively. The CSDs at the dyke margins, irrespective of the dyke width, are typically concave-up, and in literature such patterns have been attributed as evidence of magma mixing with distinct crystal populations. However, at the center of the largest dyke (8.2m) of Conchas Beach, the CSD is log-linear, consistent with simple steady-state crystallization pattern. The mineral chemistry shows that the plagioclase phenocrysts have a high An content (bytownite-labradorite) than the groundmass grains (labradorite-andesine). At the margins olivine is richer in Fo than at the center, and respectively, pyroxene is richer in Ca. These results indicate that the chilled margin is more mafic than the center suggesting a normal chemical evolution in a cooling magma. Nevertheless, the concave-up CSDs probably depict heterogeneous crystallization rates possibly induced by depressurization during the ascent of the basaltic magma followed by rapid cooling. The log-linear CSD pattern at the center of the Conchas dyke (8.2m width) is attributed to a higher residence time of the magma which favors the processes of chemical diffusion and textural re-equilibration. The calculated cooling rates using the CSD slope enables us to estimate that the larger dyke of Conchas would be completely crystallized (at ~900oC) in 73 days. The study of the Shape Preferred Orientation (SPO) in plagioclase shows an isotropic petrofabric at the margins of the dykes <=2m, which could reflect a rapid crystallization of plagioclase by depressurization. When the fabric is defined, as in the larger Conchas Beach dyke, the plagioclase lineation is subhorizontal, suggesting that the magma flow was predominantly lateral to the dyke plane.

Les profils de sismique réflexion de Marges Passives Volcaniques imagent une croûte inférieure réflective formée de deux domaines distincts. Ces réflecteurs sont interprétés comme des intrusions mafiques associées à la formation de la marge. Les observations géophysiques ne permettent pas de comprendre les conséquences associées à la mise en place de ces magmas sur la rhéologie et la déformation effective de la croûte inférieure des marges volcaniques. Cette thèse explore deux exemples géologiques de croûte inférieure à l’affleurement qui ont été affectés par d’importants volumes magmatiques pendant leur histoire extensive : la Zone d’Ivrée (Alpes italiennes) et la province de Seiland (Calédonides finmarkiennes). Une étude approfondie de terrain, associée à des analyses géochimiques et isotopiques ainsi qu’à des expériences de déformation en Presse Paterson permet, (1) de définir la structure synthétique finie d’une croûte inférieure extensive affectée par un ou plusieurs épisodes magmatiques intenses et d'identifier dans la Zone d’Ivrée une croûte moyenne ductile, dans laquelle s’enracinent les failles de la croûte supérieure, (2) de montrer l’évolution de la rhéologie et de la déformation ductile de la croûte inférieure relativement à la mise en place des magmas (3) de caractériser à partir de Seiland l’interface croûte manteau dans un contexte d’extension synmagmatisme mafique. L’ensemble des résultats montre la complexité des relations, à la fois structurales, spatiales et temporelles, entre les magmas et roches mafiques et la déformation en extension dans la croûte inférieure. / Seismic reflection profiles of Volcanic Passive Margins show a two-domain reflective lower crust. The particular reflectors are interpreted as mafic intrusions associated to the margin evolution.From those geophysical observations it is not possible to understand the consequences of mafic magma emplacement on the rheology and effective deformation of volcanic margins lower crust. This thesis explores two geologic examples of outcropping lower crusts affected by important volumes of magma during their extensional story: the Ivrea Zone (Italian Alps) and Seiland (Finmarkian Caledonides). An important field study associated to geochemical and isotopic analyses, and to Patersonapparatus deformation experiences led to (1) define a synthetic structure of an extensional lower crust affected by one or more major magmatic events and to indentify a ductile middle crust in the Ivrea Zone where roots the upper crust faults, (2) show the rheologic and deformation evolution of the lower crust in relation to magma emplacement timing, (3) characterize, from Seiland observations, the crustmantle interface in an extensional System. The whole study shows the complexity of the temporal, spatial and structural relationships between mafic rocks (as magma and as crystallised rock) and extensional deformation in the lower crust.

<p> A petrological and geochemical study has been carried out on some Archean mafic and felsic volcans of Dome Township, Central Red Lake area, Ontario. Included are a discussion on petrographic descriptions along with chemical investigations of the rocks and a discussion concerning the general problem of petrogenesis. Analytical methods used are explained, and di scussions of the results are included. </p> / Thesis / Bachelor of Arts (BA)

<p> The "B" horizon at Tilt Cove occurs in subaqueous mafic volcanics near the base of the Silurian Cape St. John Group. It is 3 metres below a well-banded oxide iron formation ("A" horizon). </p> <p> Mineralization in the "B" horizon is analogous to that of the East Mine in that it is volcanogenic and has resulted in extensive chloritization of the footwall rocks, and in the deposition of banded sulphides or the replacement of the existing mafic volcanics by sulphides. There are differences in the geochemistry mineral textures and mineral types. The East Mine host volcanics are alkali depleted basaltic komatiites to magnesium theleiites. The horizon host volcanics are spillitized magnesium tholeiites. Samples of ore from the East Mine show well-developed colloform and framboidal textures. Pyrite, magnetite, hematite and chalcopyrite are the dominant minerals with minor sphalerite and accessory covellite. Samples from the horizon show relict colloform textures and framboids with less internal structure due to overgrowths. Atoll textures indicating extensive replacement are common. Pyrite is the dominant sulphide followed by sphalerite, chalcopyrite, accessory covellite and gold. The chalcopyrite occurs both as replacement of pyrite and exsolution in sphalerite. The most significant difference between samples from the East Mine and "B" horizon is the greater abundance of gold in the "B" horizon and its correlation with sphalerite. </p> / Thesis / Bachelor of Science (BSc)

Mid-Late Triassic-age alkali-basalt dikes were emplaced along the coast of New England between 240-200 Ma. Known as the Coastal New England (CNE) magmatic province, this dike swarm is the immediate magmatic predecessor to the formation of the Central Atlantic Magmatic Province large igneous province at 201 Ma and the breakup of Pangea. The intent of this study is to determine the melt source and mechanisms for melting which produced the Triassic coastal dikes. To achieve this goal, major and trace element compositions were analyzed for 53 CNE dikes from Maine and Nova Scotia. Radiogenic Nd-Sr-Pb-Hf ratios, representing some of the first 176Hf/177Hf data for CNE, are reported for 12 of the dikes. Taken together, the compositional data implicate melting of a deep mantle source that is relatively enriched in incompatible elements, such as a mantle-plume similar to those hypothesized as the source of melting in modern ocean-island basalts (i.e. Hawaii). Dike compositions are inconsistent with melts generated at typical spreading-center ridges (i.e. MORB). Modeling suggests that CNE melts ascended through thick continental crust, consistent with the incipient stages of rifting of Pangea, as evidenced by a heterogeneous mix of melting and crystallization depths, between 0-70km, with no clear geographic pattern. Radiogenic isotope data are relatively consistent and represent a mixture between HIMU, EMI and DMM mantle reservoirs, implying component consisting of relict subducted oceanic crust (or other similarly evolved material). CNE magmatism may have contributed to the breakup of Pangea by destabilizing the lower crust in the limited local area where it erupted, but its true relationship with the breakup of Pangea and later CAMP event requires more study. / Master of Science / Approximately 200-250 million years ago, hundreds of sheets of lava, called dikes, erupted along what is today the coast of New England. As these volcanic dikes rose up from the Earth’s mantle, they traveled along cracks and weak areas of the Earth’s crust. Today, these dikes are found along the New England coast as far south as Rhode Island and as far north as Nova Scotia, Canada. Based on the similarity of their geochemistry and petrology, as well as their geologic age and geography of their eruption, geologists group these dikes and similar volcanics together as a single, related magmatic event. This magmatic event produced the Coastal New England (CNE) magmatic province. 250 million years ago, the coast of New England was actually an interior part of the supercontinent known as Pangea. Around 250 m.yr. ago, Pangea slowly began rifting apart, which is when CNE volcanism began. By 200 m.yr. ago, Pangea had broken up, and CNE volcanism had ended. Further complicating the story, a large-igneous province (LIP) also erupted 200 m.yr. ago. Known as the Central Atlantic Magmatic Province (CAMP), this volcanism consisted of enormous volumes of lava that flooded over the entire east coast of the United States. The intent of this study is to determine what geological conditions led to the CNE volcanism. By learning which part of the Earth melted and why, CNE volcanism’s role in the breakup of Pangea, and the much larger CAMP eruptions that coincided with it, will become clearer. For instance, did the geologic events that resulted in CNE volcanism contribute to the breakup of Pangea, or did the breakup of Pangea cause CNE volcanism followed by CAMP volcanism? To achieve this goal, the geochemical compositions of 53 CNE dikes from Maine and Nova Scotia were analyzed. Radiogenic Nd-Sr-Pb-Hf ratios for a subset of the dikes (12) were also analyzed. This study presents some of the first radiogenic hafnium data for rocks from CNE. The data indicate that the melting which produced the CNE dikes began in the deep mantle, similar to the melting of mantle plumes beneath modern ocean-islands such as Hawaii. In contrast, shallow mantle melting, like the melting at mid-ocean ridges where oceanic crust is produced, is not consistent with the geochemical evidence presented for CNE in this study. Modeling suggests that CNE magmas rose through thick continental crust, which caused them to begin forming crystals at relatively high depths. Radiogenic isotope data suggests that part of the mantle that melted was old, recycled oceanic crust or similar mantle material. CNE magmatism may have contributed to the breakup of Pangea by destabilizing the lower crust in the limited local area where it erupted, but its true relationship with the breakup of Pangea and later CAMP event requires more study.