Thesis (Ph. D.)--University of Rhode Island, 2004. / Typescript. Includes bibliographical references (leaves 263-280).

Th. 3e cycle--Géochimie--Montpellier, 1985. / Bibliogr. p. 175-177.

Cette recherche exploite une combinaison de techniques de terrain, paléomagnétiques et de magnétisme des roches pour corréler et localiser la source de plusieurs ignimbrites de deux régions de Turquie qui appartiennent à deux grands systèmes acides : la région du Cappadoce (Anatolie centrale), qui a été précédemment étudiée par plusieurs auteurs, et la région de Afyon-Eskişehir (Anatolie occidentale), jusqu'à présent peu étudiée. L’examen de l’ignimbrite de Kızılkaya (région du Cappadoce), d'âge Miocène supérieur, a consisté en une étude stratigraphique et de magnétisme des roches de l'unité ignimbritique, qui est considérée comme une seule unité d’écoulement et de refroidissement. La magnétisation rémanente, la fabrique magnétique (AMS, AIRM et AARM) et la minéralogie magnétique ont été analysées pour 35 sites de 7 localités situées à des distances différentes de la source supposée par Le Pennec et al. (1998) et à différentes hauteurs stratigraphiques, pour un total de 444 spécimens. La signature magnétique rémanente n’est pas verticalement homogène dans les dépôts. Deux cas sont distingués : (1) une TRM stable, dont la direction est conforme aux données de la littérature précédente (Piper et al., 2002) et (2) deux composantes de magnétisation caractérisées par une température de blocage et des spectres de coercivité se recouvrant. Ces situations ont été enregistrées respectivement en présence d'un Ti-magnétite primaire, et en présence simultanée de Ti-magnétite et de phases magnétiques secondaires (Ti-magnétite altérée et / ou hématite). La fabrique AMS varie verticalement dans le dépôt, en fonction de l'angle entre la direction de la linéation magnétique K1 et du pôle de la foliation magnétique K3, trois types de fabriques ont été caractérisées : normale, oblique et transversale. Après avoir démontré l’origine primaire de la fabrique magnétique, l'AMS a été accentuée par élimination des échantillons caractérisés par des valeurs de densité différentes de plus de ±} 1σ de la valeur moyenne du site. Cette opération s'est traduite par l'élimination des fabriques obliques, qui sont interprétées par la présence locale de fragments de ponces et clastes lithiques dans les échantillons. Les données d'AIRM et AARM indiquent que la Ti-magnétite MD est le principal minéral magnétique porteur de la fabrique, ce que suggère que la fabrique magnétique peut être utilisée pour déterminer les directions d'écoulement. L’examen de la succession ignimbritique d’âge Miocène précoce-Miocène supérieur, exposée sur une zone de ≈14 300 km2 dans la région d'Afyon-Eskişehir, a consisté en une étude stratigraphique-sédimentologique et de magnétisme des roches, afin de localiser la source des ignimbrites en combinant les données du terrain avec les directions de fabrique magnétique. La mesure des sections stratigraphiques, qui ont été réalisées pour 76 localités, conduit aux corrélations stratigraphiques et à la production de cartes d'isopaques et d'isoplètes. La succession ignimbritique est constituée d'au moins trois éruptions majeures distinctes qui ont produit les ignimbrites de Incik, Sabuncu et Seydiler, différentes en âge, distribution, structure et caractéristiques texturales des dépôts. L’étude magnétique consistait en analyses de la minéralogie magnétique et de la fabrique magnétique (AMS et AIRM), qui a été déterminée pour 22 localités sur un total de 36 sites à différentes hauteurs stratigraphiques, soit 600 spécimens. Les données de terrain et les résultats de la fabrique magnétique indiquent deux zones sources. La source des ignimbrites de Incik et Sabuncu est identifiée dans la caldeira de Kırka, une vaste zone de ≈ 20 x 20 km de diamètre, où la présence d'un dôme résurgent a été détectée. La source de l'ignimbrite de Seydiler est identifiée à proximité du village de Bayat. (...) / This research exploits a combination of field, paleomagnetic and rock-magnetic techniques to correlate locate the source and investigate several ignimbrites from two selected regions of Turkey, belonging to two large silicic ignimbrite systems : the well-investigated Cappadocia region (central Anatolia) and the poorly studied Afyon-Eskişehir region (western Anatolia). Investigation of the Upper Miocene Kızılkaya ignimbrite (Cappadocia region) consisted in a stratigraphic rock-magnetic study of the ignimbrite unit, which is considered to be a single flow and cooling unit. Remanent magnetization, magnetic fabric (AMS, AIRM and AARM) and magnetic mineralogy investigations were performed at 35 sites in 7 areal distributed localities at different distance from the vent position inferred by Le Pennec et al. (1998) and different stratigraphic heights, for a total of 444 specimens. Magnetic remanences are not vertically homogenous through the deposits. Two cases are distinguished: (1) a stable TRM, whose direction is consistent with previous literature data (Piper et al., 2002) and (2) two magnetization components with overlapping blocking temperature and coercivity spectra. These situations have been referred respectively to the occurrence of only primary Ti-magnetite, and primary Timagnetite plus secondary magnetic phases (alterated Timagnetite and/or hematite). The AMS fabric varies vertically throughout the deposit; based on the angle between the direction of the magnetic lineation K1 and that of the foliation plunge K3, three types of fabric are evidenced: normal, oblique and transverse. After have attested a primary origin of the magnetic fabric, the AMS was firstly enhanced by discarding all specimens whose density values differ more than +/- 1σ from the site mean value. This resulted in the elimination of the oblique fabrics, which are interpreted as an orientation disturbance due to local occurrence of pumices and lithic clasts in the specimens. Measurement of the AIRM and AARM pointed out that MD Timagnetite is the main carrier of the fabric, which suggests that the magnetic fabric is a reliable proxy for flow directions. Investigation of the Early-Upper Miocene ignimbrite succession exposed in the region of Afyon- Eskişehir consisted in a stratigraphic-sedimentologic and rock-magnetic study of the ignimbrite deposits over a ≈14,300 km2 extended area, in order to locate the source by combining field data and magnetic fabric flow directions. Measurement of the stratigraphic sections, performed at 76 distributed localities, led to ignimbrite correlations and production of isopach and isopleth maps. The ignimbrite succession consists of at least three distinct eruptions that originated the Incik, Sabuncu and Seydiler ignimbrites, different for age, areal distribution, structure and textural features of the deposits. Magnetic investigation consisted in magnetic mineralogy and magnetic fabric analyses (AMS and AIRM), was performed at 22 distributed localities for a total of 36 sites at different stratigraphic heights and 600 specimens. Both field data and magnetic fabric results concur for two source areas. The source area of the Incik and Sabuncu ignimbrites is identified with the Kırka caldera, a ≈20 x 20 km extended area where the presence of a resurgent dome has been detected; source area of the Seydiler ignimbrite is identified in the proximity of the village of Bayat. Volume calculations based on field data pointed out a VEI index of 7 for the Incik and Seydiler ignimbrite, 6 for Sabuncu ignimbrite, evidencing Plinian eruptions; the correspondent magnitude M (Pyle, 2000) is greater than 7 for each ignimbrite.

Entre 12 e 4Ma, intensas manifestações vulcânicas explosivas ocorrem associadas a formação de grandes caldeiras no platô dos Andes Centrais, lançando mais de 10.000 km3 de material piroclástico e constituindo o Complexo Vulcânico Altiplano Puna (CVAP). O Cerro Pululus, com aproximadamente 178 km2 e 550 m de altura, é uma colina em forma de escudo constituída por três unidades de fluxo ignimbrítico lançadas por um mesmo centro emissor durante os eventos do CVAP. No cume do cerro aflora um corpo intrusivo sub-vulcânico que elevou e deformou os depósitos piroclásticos na forma de um anticlinal. Pululus está localizado na borda sul da Caldeira Vilama, uma das maiores estruturas de colapso do CVAP, que teria sido responsável pela deposição de mais de 1.200 km3 de material piroclástico. O local de emissão dos ignimbritos de Pululus ainda é desconhecido e sua relação com os depósitos da Caldeira Vilama é considerada incerta. A alta concentração de cristais nos púmices evidencia que os depósitos do Cerro Pululus foram originados por um magma porfirítico, apresentando plagioclásio, biotita, quartzo, ferrossilita, enstatita, augita e hornblenda, como minerais principais. As unidades são de composição dacítica e pertencem à série cálcio-alcalina de alto-K, com características meta a peraluminosas e razões A/CNK variando entre 0,9 e 1,04. As feições texturais dos minerais (e.g. texturas de dissolução no plagioclásio) e os dados químicos obtidos (e.g. presença de dois tipos de púmices) evidenciam recorrentes injeções máficas na câmara magmática. A alta taxa de cristalinidade da fusão (50 a 68%) tornou desfavorável o processo de fracionamento e reduziu a ação de correntes de convecção na câmara, evitando a homogeneização do magma. A presença de bordas de reação nos anfibólios, somada a outras evidências observadas no afloramento, indicam que durante o início do evento eruptivo o magma ascendeu a uma velocidade consideravelmente baixa, permitindo a liberação de uma grande quantidade de voláteis e resultando numa erupção de caráter mais explosivo. Com a abertura do sistema e o abatimento da pressão dentro da câmara, ocorre a aceleração na cristalização do fundido, resultando no progressivo aumento de saturação de água em algumas porções da câmara e a conseqüente estabilização e cristalização de anfibólio antes da última erupção. A comparação entre os ignimbritos da Caldeira Vilama e Cerro Pululus é feita, neste trabalho, utilizando-se características deposicionais, petrográficas e químicas. Ambos são compostos por três unidades de fluxo que apresentam similaridades quanto à cor, grau de soldamento, relações estratigráficas, quantidade e tipos de púmices, quantidade e tipos de fragmentos líticos, assembléia mineralógica e composição química. Todos esses elementos, além da inexistência de qualquer conduto vulcânico no cerro, permitem estimar que os depósitos de Pululus representem uma extensão do Ignimbrito Vilama. / About 12 and 4Ma, ago intense explosive volcanic manifestations occurred associated to the formation of the large calderas in the Andean Central Volcanic Zone plateau, launching more than 10.000 km3 of ignimbrites and constituting the Altiplano-Puna Volcanic Complex (APVC). The Cerro Pululus, of about 178 km2 and 550 m high, is a shield-like hill constituted by three units of ignimbritic flows originated from same emission center during APVC\'s events. A sub-volcanic intrusive body, that elevated and deformed the pyroclastic deposits in the form of an anticline, crops out at the hill top. Pululus is located in the south ring of the Vilama caldera, one of the largest of the APVC\'s collapse structures, which would have been responsible for the deposition of more than 1.200 km3 of pyroclastic material. The emission center of Pululus ignimbrites is still unknown and its relationship with the deposits of the Vilama caldera is considered uncertain. The high crystal concentration in the pumices evidences that the deposits of Cerro Pululus were originated from a porphyritic magma, with plagioclase, biotite, quartz, ferrosilite, enstatite, augite and hornblend as main minerals. The units are of dacitic composition and belong to the high-K calc-alkaline series, with metaperaluminous characteristics and A/CNK reasons between 0.9 and 1.04. The textural features of the minerals (e.g. dissolution textures in plagioclase) and the obtained chemical data (e.g. presence of two types of pumices) evidence recurring mafic injections in the magmatic chamber. The high cristallinity rate by the melt (50 to 68%) turned unfavorable the fractionation process and reduced the convection currents action in the chamber, hindering the homogenization of the magma. The presence of reaction borders in amphibole, besides some evidences observed in the outcrops, indicate that during the beginning of the eruptive event the magma ascended of a considerably low speed, which allowed the liberation of a great quantity of volatiles and resulted in an eruption of more explosive character. With the opening of the system and the pressure surcease inside the chamber, an acceleration in the crystallization of the melt occurred, resulting in progressive water saturation increase in some portions of the chamber and consequent amphibole stabilization and crystallization before the last eruption. A comparison between ignimbrites of the Vilama caldera and Cerro Pululus is made, in this work, using depositional, petrographics and chemical characteristic. They are both composed by three flow units that present similarities regarding color, welding, stratigraphic relationships, quantity and types of pumices, quantity and types of lithic fragments, mineralogical assembly and chemical composition. All of these elements, besides the inexistence of any volcanic conduit on the hill, allow to correlate the Pululus ignimbrites directly with those of Vilama.

A região do Cerro Chato, extremo sul do Rio Grande do Sul, é caracterizada por associações de rochas vulcânicas e subvulcânicas de composição ácida, cujo magmatismo tem sido relacionado às suítes graníticas tardias do Batólito Pelotas no Cinturão Dom Feliciano. Os vulcanitos da região agrupam-se em duas feições geomorfologicamente distintas e afetadas por falhas NW e NE: Cerro Chato e Cerro Partido. O Cerro Chato é caracterizado por depósitos piroclásticos e efusivos. Os primeiros são representados por ignimbritos que ocorrem em duas fácies principais: ignimbritos ricos em líticos e ignimbritos ricos em cristais, mal selecionados e constituídos por piroclastos tamanho lápili e uma matriz tufácea. A fácies rica em litoclastos é caracterizada por apresentar fragmentos conatos de riolitos e ignimbritos e, subordinadamente, acidentais. Fragmentos de cristais de K-feldspato e quartzo são comuns e a estrutura eutaxítica é incipiente. A fácies rica em cristais caracteriza-se pela abundância de cristaloclastos e fenocristais de feldspatos e quartzo. Apresentam estrutura eutaxítica e matriz constituída por vitroclastos tamanho cinza. Derrames riolíticos hemicristalinos representam os eventos efusivos, com textura porfirítica, estruturas de fluxo e esferulitos. O Cerro Partido é caracterizado por um corpo subvulcânico, alongado na direção NE-SW, com 8 km de comprimento por 0,7 km de largura aproximadamente. Constitui rochas com textura porfirítica, com fenocristais de quartzo e feldspatos, imersos em uma matriz equigranular fina. Geoquimicamente, são riolitos do tipo alta-sílica, correlacionáveis à série alcalina, próximo ao limite das séries subalcalinas, com um caráter metaluminoso/peraluminoso e teores elevados de álcalis, FeOt/FeOt+MgO e índice agpaítico. Os riolitos do Cerro Partido foram classificados como alto-Ti com elevados teores de CaO, P2O5, FeOt, MgO e K2O se comparados aos riolitos do Cerro Chato, baixo-Ti. O padrão mostrado pelos ETR é definido pelo leve enriquecimento de ETRL em relação à ETRP e uma forte anomalia negativa de Eu, típica de sistemas alcalinos metaluminosos e altamente diferenciados. As características químicas são semelhantes com as de magmatismo granítico do tipo A, relacionado a ambientes pós-colisionais. Dados litoquímicos indicam uma vinculação genética com rochas graníticas mais diferenciadas da Suíte Dom Feliciano, bem como os riolitos do Cerro Ana Dias, associadas ao magmatismo Neoproterozoico pós-colisional do Escudo Sul-Rio-Grandense. Dados geocronológicos U-Pb obtidos em zircões indicam uma idade de 561 ± 2 Ma para os riolitos do Cerro Partido, sugerindo uma contemporaneidade com os granitoides associados a suíte Dom Feliciano. Já os dados U-PB em zircão dos riolitos do Cerro Chato indicam uma idade de 630.4 ± 2.8 Ma. Estas idades são concordantes com as obtidas em clastos vulcânicos na base da formação Maricá, podendo assim haver uma possível relação entre os vulcanitos do Cerro Chato com o vulcanismo sin-sedimentar inicial da Bacia do Camaquã. / The Cerro Chato region is located in the southern portion of Rio Grande do Sul and is characterized by associations of acid volcanic and subvolcanic rocks, whose magmatism has been related to the later Pelotas Batholith suites from the Dom Feliciano Belt. The vulcanites of the region are affected by NW and NE faults and are grouped into two geomorphologically distinct features: Cerro Chato and Cerro Partido. Cerro Chato is represented by ignimbrites that occur in two main facies: lithic rich ignimbrites and crystal rich ignimbrites. They are poorly selected and consist of lapilli-sized pyroclasts in a tuffaceous matrix. The lithoclasts rich facies is characterized by cognate fragments of rhyolites and ignimbrites and, occasionally, accidental fragments. The crystal-rich ignimbrites are characterized by the abundance of crystal fragments and phenocrysts of K-feldspar and quartz. Both facies present eutaxitic structure and a matrix made up of ash-sized vitroclasts. Hemi-crystalline rhyolitic flows represent effusive events, with porphyritic texture, flow structures and spherulites. Cerro Partido is characterized by a subvolcanic body, elongated in the NE-SW direction, of approximately 8 km long by 0.7 km wide.. It consists of rocks with porphyritic/glomeroporphyritic texture, composed of quartz, K-feldspar and plagioclase phenocrysts, within a fine quartz-feldspathic equigranular matrix. Through geochemical data the rhyolitic volcanites were characterized as high-silica type rhyolites, correlated to the alkaline series, but close to the limit of the sub-alkaline series; they present metaluminous to peraluminous character and high contents of alkalis, FeOt / FeOt + MgO and agpaitic index. The Cerro Partido rhyolites were classified as high-Ti with higher CaO, P2O5, FeOt, MgO and K2O contents than the Cerro Chato low-Ti rhyolites. The rhyolites REE pattern is slightly enriched in LREE in relation to the HREE and has a strong negative Eu anomaly, typical of metaluminous and highly differentiated alkali systems. The chemical characteristics are similar to those of A type granitic magmatism, related to post-collisional environments. U-Pb geochronological dating indicates an age of 561 ± 2 Ma for the rhyolites of Cerro Partido, suggesting contemporaneity with the granitoids associated to the Dom Feliciano suite. The zircon U-Pb isotopes dating of the Cerro Chato rhyolites indicates an age of 630.4 ± 2.8 Ma. These ages are in agreement with those obtained in the volcanic clasts at the base of the Maricá formation, which may indicate a possible relationship between the volcanites of the Cerro Chato with the initial sin-sedimentary volcanism of the Camaquã Basin.

Entre 12 e 4Ma, intensas manifestações vulcânicas explosivas ocorrem associadas a formação de grandes caldeiras no platô dos Andes Centrais, lançando mais de 10.000 km3 de material piroclástico e constituindo o Complexo Vulcânico Altiplano Puna (CVAP). O Cerro Pululus, com aproximadamente 178 km2 e 550 m de altura, é uma colina em forma de escudo constituída por três unidades de fluxo ignimbrítico lançadas por um mesmo centro emissor durante os eventos do CVAP. No cume do cerro aflora um corpo intrusivo sub-vulcânico que elevou e deformou os depósitos piroclásticos na forma de um anticlinal. Pululus está localizado na borda sul da Caldeira Vilama, uma das maiores estruturas de colapso do CVAP, que teria sido responsável pela deposição de mais de 1.200 km3 de material piroclástico. O local de emissão dos ignimbritos de Pululus ainda é desconhecido e sua relação com os depósitos da Caldeira Vilama é considerada incerta. A alta concentração de cristais nos púmices evidencia que os depósitos do Cerro Pululus foram originados por um magma porfirítico, apresentando plagioclásio, biotita, quartzo, ferrossilita, enstatita, augita e hornblenda, como minerais principais. As unidades são de composição dacítica e pertencem à série cálcio-alcalina de alto-K, com características meta a peraluminosas e razões A/CNK variando entre 0,9 e 1,04. As feições texturais dos minerais (e.g. texturas de dissolução no plagioclásio) e os dados químicos obtidos (e.g. presença de dois tipos de púmices) evidenciam recorrentes injeções máficas na câmara magmática. A alta taxa de cristalinidade da fusão (50 a 68%) tornou desfavorável o processo de fracionamento e reduziu a ação de correntes de convecção na câmara, evitando a homogeneização do magma. A presença de bordas de reação nos anfibólios, somada a outras evidências observadas no afloramento, indicam que durante o início do evento eruptivo o magma ascendeu a uma velocidade consideravelmente baixa, permitindo a liberação de uma grande quantidade de voláteis e resultando numa erupção de caráter mais explosivo. Com a abertura do sistema e o abatimento da pressão dentro da câmara, ocorre a aceleração na cristalização do fundido, resultando no progressivo aumento de saturação de água em algumas porções da câmara e a conseqüente estabilização e cristalização de anfibólio antes da última erupção. A comparação entre os ignimbritos da Caldeira Vilama e Cerro Pululus é feita, neste trabalho, utilizando-se características deposicionais, petrográficas e químicas. Ambos são compostos por três unidades de fluxo que apresentam similaridades quanto à cor, grau de soldamento, relações estratigráficas, quantidade e tipos de púmices, quantidade e tipos de fragmentos líticos, assembléia mineralógica e composição química. Todos esses elementos, além da inexistência de qualquer conduto vulcânico no cerro, permitem estimar que os depósitos de Pululus representem uma extensão do Ignimbrito Vilama. / About 12 and 4Ma, ago intense explosive volcanic manifestations occurred associated to the formation of the large calderas in the Andean Central Volcanic Zone plateau, launching more than 10.000 km3 of ignimbrites and constituting the Altiplano-Puna Volcanic Complex (APVC). The Cerro Pululus, of about 178 km2 and 550 m high, is a shield-like hill constituted by three units of ignimbritic flows originated from same emission center during APVC\'s events. A sub-volcanic intrusive body, that elevated and deformed the pyroclastic deposits in the form of an anticline, crops out at the hill top. Pululus is located in the south ring of the Vilama caldera, one of the largest of the APVC\'s collapse structures, which would have been responsible for the deposition of more than 1.200 km3 of pyroclastic material. The emission center of Pululus ignimbrites is still unknown and its relationship with the deposits of the Vilama caldera is considered uncertain. The high crystal concentration in the pumices evidences that the deposits of Cerro Pululus were originated from a porphyritic magma, with plagioclase, biotite, quartz, ferrosilite, enstatite, augite and hornblend as main minerals. The units are of dacitic composition and belong to the high-K calc-alkaline series, with metaperaluminous characteristics and A/CNK reasons between 0.9 and 1.04. The textural features of the minerals (e.g. dissolution textures in plagioclase) and the obtained chemical data (e.g. presence of two types of pumices) evidence recurring mafic injections in the magmatic chamber. The high cristallinity rate by the melt (50 to 68%) turned unfavorable the fractionation process and reduced the convection currents action in the chamber, hindering the homogenization of the magma. The presence of reaction borders in amphibole, besides some evidences observed in the outcrops, indicate that during the beginning of the eruptive event the magma ascended of a considerably low speed, which allowed the liberation of a great quantity of volatiles and resulted in an eruption of more explosive character. With the opening of the system and the pressure surcease inside the chamber, an acceleration in the crystallization of the melt occurred, resulting in progressive water saturation increase in some portions of the chamber and consequent amphibole stabilization and crystallization before the last eruption. A comparison between ignimbrites of the Vilama caldera and Cerro Pululus is made, in this work, using depositional, petrographics and chemical characteristic. They are both composed by three flow units that present similarities regarding color, welding, stratigraphic relationships, quantity and types of pumices, quantity and types of lithic fragments, mineralogical assembly and chemical composition. All of these elements, besides the inexistence of any volcanic conduit on the hill, allow to correlate the Pululus ignimbrites directly with those of Vilama.

A região do Cerro Chato, extremo sul do Rio Grande do Sul, é caracterizada por associações de rochas vulcânicas e subvulcânicas de composição ácida, cujo magmatismo tem sido relacionado às suítes graníticas tardias do Batólito Pelotas no Cinturão Dom Feliciano. Os vulcanitos da região agrupam-se em duas feições geomorfologicamente distintas e afetadas por falhas NW e NE: Cerro Chato e Cerro Partido. O Cerro Chato é caracterizado por depósitos piroclásticos e efusivos. Os primeiros são representados por ignimbritos que ocorrem em duas fácies principais: ignimbritos ricos em líticos e ignimbritos ricos em cristais, mal selecionados e constituídos por piroclastos tamanho lápili e uma matriz tufácea. A fácies rica em litoclastos é caracterizada por apresentar fragmentos conatos de riolitos e ignimbritos e, subordinadamente, acidentais. Fragmentos de cristais de K-feldspato e quartzo são comuns e a estrutura eutaxítica é incipiente. A fácies rica em cristais caracteriza-se pela abundância de cristaloclastos e fenocristais de feldspatos e quartzo. Apresentam estrutura eutaxítica e matriz constituída por vitroclastos tamanho cinza. Derrames riolíticos hemicristalinos representam os eventos efusivos, com textura porfirítica, estruturas de fluxo e esferulitos. O Cerro Partido é caracterizado por um corpo subvulcânico, alongado na direção NE-SW, com 8 km de comprimento por 0,7 km de largura aproximadamente. Constitui rochas com textura porfirítica, com fenocristais de quartzo e feldspatos, imersos em uma matriz equigranular fina. Geoquimicamente, são riolitos do tipo alta-sílica, correlacionáveis à série alcalina, próximo ao limite das séries subalcalinas, com um caráter metaluminoso/peraluminoso e teores elevados de álcalis, FeOt/FeOt+MgO e índice agpaítico. Os riolitos do Cerro Partido foram classificados como alto-Ti com elevados teores de CaO, P2O5, FeOt, MgO e K2O se comparados aos riolitos do Cerro Chato, baixo-Ti. O padrão mostrado pelos ETR é definido pelo leve enriquecimento de ETRL em relação à ETRP e uma forte anomalia negativa de Eu, típica de sistemas alcalinos metaluminosos e altamente diferenciados. As características químicas são semelhantes com as de magmatismo granítico do tipo A, relacionado a ambientes pós-colisionais. Dados litoquímicos indicam uma vinculação genética com rochas graníticas mais diferenciadas da Suíte Dom Feliciano, bem como os riolitos do Cerro Ana Dias, associadas ao magmatismo Neoproterozoico pós-colisional do Escudo Sul-Rio-Grandense. Dados geocronológicos U-Pb obtidos em zircões indicam uma idade de 561 ± 2 Ma para os riolitos do Cerro Partido, sugerindo uma contemporaneidade com os granitoides associados a suíte Dom Feliciano. Já os dados U-PB em zircão dos riolitos do Cerro Chato indicam uma idade de 630.4 ± 2.8 Ma. Estas idades são concordantes com as obtidas em clastos vulcânicos na base da formação Maricá, podendo assim haver uma possível relação entre os vulcanitos do Cerro Chato com o vulcanismo sin-sedimentar inicial da Bacia do Camaquã. / The Cerro Chato region is located in the southern portion of Rio Grande do Sul and is characterized by associations of acid volcanic and subvolcanic rocks, whose magmatism has been related to the later Pelotas Batholith suites from the Dom Feliciano Belt. The vulcanites of the region are affected by NW and NE faults and are grouped into two geomorphologically distinct features: Cerro Chato and Cerro Partido. Cerro Chato is represented by ignimbrites that occur in two main facies: lithic rich ignimbrites and crystal rich ignimbrites. They are poorly selected and consist of lapilli-sized pyroclasts in a tuffaceous matrix. The lithoclasts rich facies is characterized by cognate fragments of rhyolites and ignimbrites and, occasionally, accidental fragments. The crystal-rich ignimbrites are characterized by the abundance of crystal fragments and phenocrysts of K-feldspar and quartz. Both facies present eutaxitic structure and a matrix made up of ash-sized vitroclasts. Hemi-crystalline rhyolitic flows represent effusive events, with porphyritic texture, flow structures and spherulites. Cerro Partido is characterized by a subvolcanic body, elongated in the NE-SW direction, of approximately 8 km long by 0.7 km wide.. It consists of rocks with porphyritic/glomeroporphyritic texture, composed of quartz, K-feldspar and plagioclase phenocrysts, within a fine quartz-feldspathic equigranular matrix. Through geochemical data the rhyolitic volcanites were characterized as high-silica type rhyolites, correlated to the alkaline series, but close to the limit of the sub-alkaline series; they present metaluminous to peraluminous character and high contents of alkalis, FeOt / FeOt + MgO and agpaitic index. The Cerro Partido rhyolites were classified as high-Ti with higher CaO, P2O5, FeOt, MgO and K2O contents than the Cerro Chato low-Ti rhyolites. The rhyolites REE pattern is slightly enriched in LREE in relation to the HREE and has a strong negative Eu anomaly, typical of metaluminous and highly differentiated alkali systems. The chemical characteristics are similar to those of A type granitic magmatism, related to post-collisional environments. U-Pb geochronological dating indicates an age of 561 ± 2 Ma for the rhyolites of Cerro Partido, suggesting contemporaneity with the granitoids associated to the Dom Feliciano suite. The zircon U-Pb isotopes dating of the Cerro Chato rhyolites indicates an age of 630.4 ± 2.8 Ma. These ages are in agreement with those obtained in the volcanic clasts at the base of the Maricá formation, which may indicate a possible relationship between the volcanites of the Cerro Chato with the initial sin-sedimentary volcanism of the Camaquã Basin.

A região do Cerro Chato, extremo sul do Rio Grande do Sul, é caracterizada por associações de rochas vulcânicas e subvulcânicas de composição ácida, cujo magmatismo tem sido relacionado às suítes graníticas tardias do Batólito Pelotas no Cinturão Dom Feliciano. Os vulcanitos da região agrupam-se em duas feições geomorfologicamente distintas e afetadas por falhas NW e NE: Cerro Chato e Cerro Partido. O Cerro Chato é caracterizado por depósitos piroclásticos e efusivos. Os primeiros são representados por ignimbritos que ocorrem em duas fácies principais: ignimbritos ricos em líticos e ignimbritos ricos em cristais, mal selecionados e constituídos por piroclastos tamanho lápili e uma matriz tufácea. A fácies rica em litoclastos é caracterizada por apresentar fragmentos conatos de riolitos e ignimbritos e, subordinadamente, acidentais. Fragmentos de cristais de K-feldspato e quartzo são comuns e a estrutura eutaxítica é incipiente. A fácies rica em cristais caracteriza-se pela abundância de cristaloclastos e fenocristais de feldspatos e quartzo. Apresentam estrutura eutaxítica e matriz constituída por vitroclastos tamanho cinza. Derrames riolíticos hemicristalinos representam os eventos efusivos, com textura porfirítica, estruturas de fluxo e esferulitos. O Cerro Partido é caracterizado por um corpo subvulcânico, alongado na direção NE-SW, com 8 km de comprimento por 0,7 km de largura aproximadamente. Constitui rochas com textura porfirítica, com fenocristais de quartzo e feldspatos, imersos em uma matriz equigranular fina. Geoquimicamente, são riolitos do tipo alta-sílica, correlacionáveis à série alcalina, próximo ao limite das séries subalcalinas, com um caráter metaluminoso/peraluminoso e teores elevados de álcalis, FeOt/FeOt+MgO e índice agpaítico. Os riolitos do Cerro Partido foram classificados como alto-Ti com elevados teores de CaO, P2O5, FeOt, MgO e K2O se comparados aos riolitos do Cerro Chato, baixo-Ti. O padrão mostrado pelos ETR é definido pelo leve enriquecimento de ETRL em relação à ETRP e uma forte anomalia negativa de Eu, típica de sistemas alcalinos metaluminosos e altamente diferenciados. As características químicas são semelhantes com as de magmatismo granítico do tipo A, relacionado a ambientes pós-colisionais. Dados litoquímicos indicam uma vinculação genética com rochas graníticas mais diferenciadas da Suíte Dom Feliciano, bem como os riolitos do Cerro Ana Dias, associadas ao magmatismo Neoproterozoico pós-colisional do Escudo Sul-Rio-Grandense. Dados geocronológicos U-Pb obtidos em zircões indicam uma idade de 561 ± 2 Ma para os riolitos do Cerro Partido, sugerindo uma contemporaneidade com os granitoides associados a suíte Dom Feliciano. Já os dados U-PB em zircão dos riolitos do Cerro Chato indicam uma idade de 630.4 ± 2.8 Ma. Estas idades são concordantes com as obtidas em clastos vulcânicos na base da formação Maricá, podendo assim haver uma possível relação entre os vulcanitos do Cerro Chato com o vulcanismo sin-sedimentar inicial da Bacia do Camaquã. / The Cerro Chato region is located in the southern portion of Rio Grande do Sul and is characterized by associations of acid volcanic and subvolcanic rocks, whose magmatism has been related to the later Pelotas Batholith suites from the Dom Feliciano Belt. The vulcanites of the region are affected by NW and NE faults and are grouped into two geomorphologically distinct features: Cerro Chato and Cerro Partido. Cerro Chato is represented by ignimbrites that occur in two main facies: lithic rich ignimbrites and crystal rich ignimbrites. They are poorly selected and consist of lapilli-sized pyroclasts in a tuffaceous matrix. The lithoclasts rich facies is characterized by cognate fragments of rhyolites and ignimbrites and, occasionally, accidental fragments. The crystal-rich ignimbrites are characterized by the abundance of crystal fragments and phenocrysts of K-feldspar and quartz. Both facies present eutaxitic structure and a matrix made up of ash-sized vitroclasts. Hemi-crystalline rhyolitic flows represent effusive events, with porphyritic texture, flow structures and spherulites. Cerro Partido is characterized by a subvolcanic body, elongated in the NE-SW direction, of approximately 8 km long by 0.7 km wide.. It consists of rocks with porphyritic/glomeroporphyritic texture, composed of quartz, K-feldspar and plagioclase phenocrysts, within a fine quartz-feldspathic equigranular matrix. Through geochemical data the rhyolitic volcanites were characterized as high-silica type rhyolites, correlated to the alkaline series, but close to the limit of the sub-alkaline series; they present metaluminous to peraluminous character and high contents of alkalis, FeOt / FeOt + MgO and agpaitic index. The Cerro Partido rhyolites were classified as high-Ti with higher CaO, P2O5, FeOt, MgO and K2O contents than the Cerro Chato low-Ti rhyolites. The rhyolites REE pattern is slightly enriched in LREE in relation to the HREE and has a strong negative Eu anomaly, typical of metaluminous and highly differentiated alkali systems. The chemical characteristics are similar to those of A type granitic magmatism, related to post-collisional environments. U-Pb geochronological dating indicates an age of 561 ± 2 Ma for the rhyolites of Cerro Partido, suggesting contemporaneity with the granitoids associated to the Dom Feliciano suite. The zircon U-Pb isotopes dating of the Cerro Chato rhyolites indicates an age of 630.4 ± 2.8 Ma. These ages are in agreement with those obtained in the volcanic clasts at the base of the Maricá formation, which may indicate a possible relationship between the volcanites of the Cerro Chato with the initial sin-sedimentary volcanism of the Camaquã Basin.

La Cordillère des Andes, issue de la subduction de la plaque Pacifique sous le continent Sud-Américain, est un système orogénique propice à la formation de grands systèmes volcaniques acides, dans lesquels coexistent des produits d’éruptions volcaniques explosives de grandes magnitudes et des laves et dômes, associés à des calderas mono- ou polygéniques. Ce mémoire de thèse apporte de nouvelles connaissances sur certains systèmes volcaniques acides du Sud du Pérou, dont les produits affleurent dans les canyons d’Ocoña-Cotahuasi-Maran (OCM). Ces canyons, les plus profonds des Andes (3 à 3,5 km), résultent de la combinaison des processus de soulèvement tectonique, d’incision, et d’érosion depuis 15 Ma. Les imposantes séries ignimbritiques exposées dans cette région (env. 10000 km²), témoignent de l’existence d’un volcanisme explosif de grande ampleur, associé à des systèmes acides peu connus d’après les études antérieures. Cette étude vise tout d’abord à obtenir une chrono-stratigraphie améliorée des événements ignimbritiques (groupes, unités) dans la région OCM pour mieux connaître les récurrences des super-éruptions dans cette région au cours des derniers 25 Ma. Ensuite, nous cherchons à localiser les sources des grandes unités et à identifier les éventuelles structures d’effondrement associées (calderas). Pour ce faire, nous avons combiné diverses techniques, associant l’étude de la stratigraphie et de la lithologie des dépôts volcaniques, les datations par la méthode 40Ar-39Ar des principales unités ignimbritiques et coulées de lave, l’imagerie satellitaire (Landsat, SPOT) et la pétrologie (assemblages minéralogiques, étude des textures et des compositions chimiques) et enfin, les mesures de densité et d’anisotropie de susceptibilité magnétique des ignimbrites (ASM) pour analyser les directions d’écoulement et tenter de localiser leurs sources. Ainsi, les résultats nous ont permis d’identifier huit événements ignimbritiques, dont six majeurs, datés entre ~24 Ma et ~2 Ma. Les âges des grandes unités (Nazca, Alpabamba, Huaylillas, Caraveli, Sencca inférieure et supérieure) montrent une récurrence moyenne de l’ordre de 4-5 Ma depuis 25 Ma. Les lithologies sont assez semblables, bien que les degrés de soudure varient beaucoup, allant de produits meubles jusqu’aux faciès eutaxitiques. L’étude pétrologique révèle des assemblages de minéraux assez homogènes, les paragenèses étant dominées par le quartz, le feldspath, la biotite, l’amphibole et des oxydes. Les volumes bruts des unités principales que nous avons déterminés se situent entre ~40 et ~500-800 km3. Cependant, il ne s’agit que de valeurs minimales, puisque nous considérons que les volumes initiaux sont au moins le double ou le triple, probablement dans la gamme ~100 à ~2400 km3. La distribution des unités ignimbritiques d’OCM et les résultats des analyses ASM désignent plusieurs zones sources. L’unité de Caraveli provient de la caldera de Trapiche, qui contient le lac Parinacochas. Cette structure que nous identifions, d'un diamètre de ~22 km, est située dans un bassin tectonique allongé selon la direction N-S. Elle est occupée dans sa partie ouest par un dôme résurgent de 800 m de haut, le Cerro Trompo Orjo. Les unités Huaylillas et Alpabamba, ainsi que les groupes Sencca (inférieure et supérieure) seraient issus d’une source entièrement recouverte par le massif volcanique quaternaire du Nevado Coropuna. Des estimations de volumes éruptifs suggèrent une atténuation significative du volcanisme ignimbritique depuis 9 Ma, peut-être liée à la maturation orogénique de l’Altiplano-Puna. En profondeur, le magmatisme a probablement contribué à l’épaississement crustal par adjonction de grands volumes de magma. (...) / The Andes, resulting from the subduction of Pacific plate under South-America continental plate, is an orogenic system suitable for large acidic volcanic systems formation. These structures display at the same time volcanic products from high magnitude explosive eruptions, lavas and domes, and mono- or polygenetic calderas. This manuscript brings a new expertise on some of the acidic volcanic systems in South Peru, which products outcrop in the Ocoña - Cotahuasi - Maran canyons (OCM). This canyons system, one of the deepest worldwide (up to 3.5 km), was created by combined tectonic uplift, incision and erosion processes since 15 Ma. Voluminous ignimbritic series widespread in this region (around 10000 km2) are evidences for a past high amplitude explosive volcanic activity related to little-known acidic systems. The topic of this study is to obtain an enhanced chrono-stratigraphy of the OCM region ignimbritic succession (groups and units). This would provide a better knowledge on the regional super-eruptions recurrence since the last 25 Ma. Then, we focus on localizing the largest ignimbrites sources and identify the possible related collapse structures (calderas). For that purpose we combine several studies, from stratigraphy, deposits lithology to 40Ar-39Ar dating of the main ignimbritic units and lava flows. We also use satellite imagery (Landsat, SPOT) and petrology (mineralogical assemblage, textures and geochemical composition). Finally, we measure the ignimbrites density and anisotropy of magnetic susceptibility (AMS) to determine their flowing directions and estimate their sources locations. Thus, the results let us identify eight ignimbritic events among which, six happened between ~24 Ma and ~2 Ma. The main units ages (Nazca, Alpabamba, Huaylillas, Caraveli, lower and upper Sencca) show an average recurrence of 4-5 Ma since 25 Ma. Lithologies are similar even if welding degrees ranges are spread from loose deposits to eutaxitic facies. Petrological study show quite homogeneous mineralogical assemblages since quartz, feldspar, biotite, amphibole and oxydes dominate the paragenesis. Our estimations of the main units bulk volumes range from ~ 40 to ~ 500-800 km3. However, these are only minimum values since we consider that initial volumes as twice or three times higher, probably in the range of ~100 to ~2400 km3. The OCM ignimbrite flow units distribution and the AMS study results indicate several regions as sources for these units. Caraveli unit flowed from Trapiche caldera in which the Parinacochas lake rests. This structure is estimated to be ~22 km of diameter and lays in a N-S orientated tectonic basin. Its western part is occupied with a 800 m high resurgent dome, named Cerro Trompo Orjo. Alpabamba and Huaylillas units, as Sencca units (lower and upper) would come from a source presently entirely covered with the quaternary massif of Nevado Coropuna. Estimations of eruptive volumes recall an important decrease of ignimbritic volcanism since 9 Ma, which might be related to simultaneous Altiplano-Puna orogenic growing. It is supported that deep magmatism have probably contributed to crustal thickening thanks to the addition of large volumes of magma. These results also involve that canyons erosion and incision would have taken place at a rate ranging from 150 to 500 m/Ma during the last 13 Ma. Given that the large-volume acidic system studied in this work is considerable, further research are necessary to improve these results.

This research exploits a combination of field, paleomagnetic and rock-magnetic techniques to correlate locate the source and investigate several ignimbrites from two selected regions of Turkey, belonging to two large silicic ignimbrite systems : the well-investigated Cappadocia region (central Anatolia) and the poorly studied Afyon-Eskişehir region (western Anatolia). Investigation of the Upper Miocene Kızılkaya ignimbrite (Cappadocia region) consisted in a stratigraphic rock-magnetic study of the ignimbrite unit, which is considered to be a single flow and cooling unit. Remanent magnetization, magnetic fabric (AMS, AIRM and AARM) and magnetic mineralogy investigations were performed at 35 sites in 7 areal distributed localities at different distance from the vent position inferred by Le Pennec et al. (1998) and different stratigraphic heights, for a total of 444 specimens. Magnetic remanences are not vertically homogenous through the deposits. Two cases are distinguished: (1) a stable TRM, whose direction is consistent with previous literature data (Piper et al., 2002) and (2) two magnetization components with overlapping blocking temperature and coercivity spectra. These situations have been referred respectively to the occurrence of only primary Ti-magnetite, and primary Timagnetite plus secondary magnetic phases (alterated Timagnetite and/or hematite). The AMS fabric varies vertically throughout the deposit; based on the angle between the direction of the magnetic lineation K1 and that of the foliation plunge K3, three types of fabric are evidenced: normal, oblique and transverse. After have attested a primary origin of the magnetic fabric, the AMS was firstly enhanced by discarding all specimens whose density values differ more than +/- 1σ from the site mean value. This resulted in the elimination of the oblique fabrics, which are interpreted as an orientation disturbance due to local occurrence of pumices and lithic clasts in the specimens. Measurement of the AIRM and AARM pointed out that MD Timagnetite is the main carrier of the fabric, which suggests that the magnetic fabric is a reliable proxy for flow directions. Investigation of the Early-Upper Miocene ignimbrite succession exposed in the region of Afyon- Eskişehir consisted in a stratigraphic-sedimentologic and rock-magnetic study of the ignimbrite deposits over a ≈14,300 km2 extended area, in order to locate the source by combining field data and magnetic fabric flow directions. Measurement of the stratigraphic sections, performed at 76 distributed localities, led to ignimbrite correlations and production of isopach and isopleth maps. The ignimbrite succession consists of at least three distinct eruptions that originated the Incik, Sabuncu and Seydiler ignimbrites, different for age, areal distribution, structure and textural features of the deposits. Magnetic investigation consisted in magnetic mineralogy and magnetic fabric analyses (AMS and AIRM), was performed at 22 distributed localities for a total of 36 sites at different stratigraphic heights and 600 specimens. Both field data and magnetic fabric results concur for two source areas. The source area of the Incik and Sabuncu ignimbrites is identified with the Kırka caldera, a ≈20 x 20 km extended area where the presence of a resurgent dome has been detected; source area of the Seydiler ignimbrite is identified in the proximity of the village of Bayat. Volume calculations based on field data pointed out a VEI index of 7 for the Incik and Seydiler ignimbrite, 6 for Sabuncu ignimbrite, evidencing Plinian eruptions; the correspondent magnitude M (Pyle, 2000) is greater than 7 for each ignimbrite.