As rochas vulcânicas ácidas da região de Piraju - Ourinhos fazem parte da grande manifestação vulcânica, de natureza predominantemente básica, ocorrida na Bacia do Paraná no Cretáceo, em decorrência da quebra do continente de Gondwana, dando origem à Província Magmática do Paraná. Estas rochas estão agrupadas no Membro Chapecó que, junto com o Membro Palmas, constituem os litotipos ácidos da Formação Serra Geral, perfazendo 3% do volume total do material vulcânico da Província. As rochas vulcânicas ácidas de Piraju - Ourinhos afloram seguindo a direção do Rio Paranapanema, numa área de 65 por 20 km, totalizando 1300 km2 de superfície, e assentam-se sobre os arenitos da Formação Botucatu, sendo recobertas pelos basaltos da Formação Serra Geral. Há controvérsias na literatura sobre os modos de erupção e colocação de certas unidades vulcânicas ácidas extensas e de grande volume, relacionadas a grandes províncias basálticas, se lavas extensas ou ignimbritos reomórficos de alto grau. O objetivo deste trabalho foi caracterizar as rochas vulcânicas ácidas de Piraju - Ourinhos, dando enfoque aos aspectos vulcanológicos, especialmente físicos, através de levantamento de perfis de detalhe, descrições de estruturas observadas e micropetrográficas, além de estimativas de viscosidades, de forma a fornecer subsídios para um melhor entendimento da origem e evolução do vulcanismo ácido da Formação Serra Geral na região em questão. Quimicamente estas rochas foram classificadas como traquidacitos, sendo divididos, segundo características texturais, em cinco tipos: chocolate, cinza vítreo, bandado/laminado, sal e pimenta e granular. Os traquidacitos são porfiríticos com fenocristais, principalmente de plagioclásio e subordinadamente de clinopiroxênios (augita e pigeonita), minerais opacos (titanomagnetita e magnetita) e apatita. A matriz é vítrea a holocristalina conforme a localização no perfil do corpo vulcânico e a sua espessura, e exibe devitrificação acentuada e de alta temperatura verificada pela presença de esferulitos com fibras longas e textura micropoiquilítica, além de feições de resfriamento rápido (quenching) como cristais de plagioclásio ocos ou com terminações em cauda de andorinha. Foram observadas estruturas como juntas de baixo ângulo cerradas paralelas à laminação ou bandamento no traquidacito, juntas do tipo lápis, brechas de interação de lava com sedimentos e vitrófiro de topo de derrame, isto aliado à ausência de fenocristais quebrados, shards, púmices, fragmentos líticos, fiammés, zonas soldadas e à ausência de vestígios de caldeira na região. Estas feições sugerem que os traquidacitos de Piraju - Ourinhos foram colocados na superfície através de fissuras, como fluxos de lava de baixa viscosidade, altas temperaturas e altas taxas de efusão, o que permitiu fluírem para longe do conduto. Na porção inferior do pacote vulcânico, correspondente aos primeiros pulsos, com o predomínio de traquidacito chocolate vesiculado a escoriáceo alternado com o traquidacito cinza vítreo, a correlação entre os derrames individuais é difiícil devido à influência do paleorelevo irregular. Em direção ao topo do pacote os corpos vulcânicos estão estruturados na forma de derrames extensos e tabulares, apresentando zonas basais, centrais e superiores bem definidas. / The acid volcanic rocks of the Piraju - Ourinhos region are part of the predominantly basic volcanic manifestation that occurred in the Paraná Basin in the Cretaceous, due the breakup of the Gondwana continent, giving rise to the Paraná Magmatic Province. These rocks are grouped in the Chapecó Member which, together with the Palmas Member, constitute the acid lithotypes of the Serra Geral Formation, accounting for 3% of the total volume of the Provinces volcanic material. The Piraju - Ourinhos acid volcanic rocks outcrop following the valley of the Paranapanema River, occupying an area of 65 by 20 km with an 1300 km2 surface, and overlie the Botucatu Formation sandstones, being capped by the Serra Geral Formation basalts. There is a controversy in the literature about the eruption styles and emplacement of certain extensive acid volcanic units, related to large basaltic provinces, whether as extensive lavas or high temperature rheomorphic ignimbrites. The aim of this work was to characterize the Piraju Ourinhos acid volcanic rocks, focusing on volcanological aspects, specially the physical ones, through detailed profiles survey, structural and micropetrographic descriptions, as well as viscosity estimates, to provide basis for a better understanding of the origin and evolution of the Serra Geral Formation acid volcanism in the region. These rocks were classified chemically as trachydacites, being divided, according to textural characteristics, in five types: chocolate, gray glassy, banded/laminated, salt and pepper and granular. The trachydacites are porphyritic with mainly plagioclase fenocrystals and subordinately clinopyroxenes (augita and pigeonite), opaque minerals (titanomagnetite and magnetite) and apatite. The groundmass is glassy to holocrystalline depending on the position in the profile and thickness of the volcanic body and display high temperature devitrification features such as spherulites with long fibers and micropoikilitic texture, as well as quench textures such as hollow or swallow tail plagioclase crystals. Structures observed include sheeting joints parallel to flow lamination or banding, pencil joints, lava-sediment interaction breccias and lava flow top vitrophyre. Broken phenocrysts, shards, pumices, lithic fragments, fiammés, welded layers and caldera structures were not observed in the area. These features suggest that the emplacement of the Piraju - Ourinhos trachydacites occurred as low viscosity, high temperature and high effusion rate fissural lava flows, allowing the lava to flow large distances from the vent. In the lower part of the volcanic pile, corresponding to the first pulses, vesicular to scoriaceous chocolate trachydacite alternated with gray glassy trachydacite predominate and the correlation between single lava flows is difficult due to the irregular paleorelief. Towards the top of the pile volcanic bodies are structured as extensive and tabular lava flows, exhibiting well defined basal, central and superior zones.

Próximo à cidade de São Félix do Xingu, centrosul do Estado do Pará, no contexto da Província Mineral de Carajás, ocorre um amplo vulcanoplutonismo Paleoproterozóico (1,88 1,87 Ga) excepcionalmente preservado e agrupado nas formações Sobreiro e Santa Rosa. Estas unidades são genericamente correlacionadas ao vulcanoplutonismo do Supergrupo Uatumã, um magmatismo com abrangência estimada de 1.500.000 km2 e registrado em praticamente todo o Cráton Amazônico. Essas rochas vulcânicas encontram-se sobrepostas ao Granito Parauari, do Paleoproterozóico, e às unidades do embasamento arqueano, dos domínios do Cinturão de Cisalhamento Itacaiúnas e do Terreno GranitoGreenstone do Sul do Pará. Maciços Granitóides mineralizados a estanho da Suíte Intrusiva Velho Guilherme de 1,86 Ga invadem as unidades supracitadas. Apesar da evolução do conhecimento nos últimos anos nesta região, há uma grande carência de dados geológicos, geoquímicos e isotópicos mais detalhados e precisos acerca dessa associação vulcânica no Cráton Amazônico como um todo. Essa escassez de informações é decorrente, em parte, das dificuldades de acesso às áreas e da densa cobertura vegetal e de solo, características peculiares da região Amazônica; descontinuidade lateral e vertical das unidades; e principalmente pelos poucos grupos de pesquisa especializados nesta temática. O mapeamento geológico intensivo realizado neste trabalho revelou que a unidade basal Formação Sobreiro é composta por fácies coerente de fluxo de lavas predominantemente andesítica, com subordinados dacito e riodacito; bem como por fácies vulcanoclástica caracterizada por tufo, lapilli-tufo e brechas polimítica maciça. Estas rochas exibem fenocristais de augita, magnesiohastingsita e plagioclásio de variável composição em uma matriz microlítica ou traquítica. Magnetita e apatita figuram como os principais acessórios primários. A variação sistemática da mineralogia de andesito basáltico para riodacito e dacito, bem como as características petrográficas destes litotipos, sugerem que as rochas dessa unidade diferenciaram-se por cristalização fracionada com provável assimilação crustal. Análises litoquímicas mostram que possui assinatura geoquímica de granitóides de arco vulcânico, enquadra-se na série magmática cálcio-alcalina de alto potássio e tem composição metaluminosa. A associação superior, Formação Santa Rosa, é formada por fácies coerente maciça de riolitos e subordinadamente riodacitos com variáveis conteúdos modais de feldspato potássico, plagioclásio e megacristais de quartzo envoltos por matriz constituída de quartzo e feldspato potássico intercrescidos, comumente esferulítica. Localmente ocorrem esferulitos de até 10 cm de diâmetro. Biotita é uma fase varietal, embora de abundância reduzida, apontando para uma unidade extremamente evoluída. Zircão, apatita e, subordinadamente óxidos de Fe e Ti, são acessórios primários. Fácies vulcanoclásticas de ignimbritos, lapilli-tufos, tufos de cristais félsicos e brechas polimíticas maciças representam um ciclo de vulcanismo explosivo nesta unidade. Essa associação vulcanoclástica possui mineralogia e características geoquímicas muito similares à fácies coerente. Diques métricos e stocks de pórfiros graníticos e granitóides equigranulares completam esta suíte. A deposição desta foi controlada por grandes fissuras crustais de até 30 km de comprimento de direção NESW, e subordinadamente NW-SE, materializado por fluxo magmático predominantemente vertical. Estas rochas exibem afinidade geoquímica intraplaca, composição peraluminosa e características transicionais entre subalcalina e alcalina. Zonas hidrotermalmente alteradas foram identificadas nestas suítes, sugerindo um potencial metalogenético para as mesmas. A grande quantidade de blocos isolados em uma topografia plana lembra um sistema eruptivo monogenético na Formação Sobreiro. Os altos topográficos podem ter sido originados pela acumulação de lava do tipo scutulum. Os depósitos vulcanoclásticos que ocorrem na porção superior dos fluxos de lavas estão associados à fragmentação autoclástica, embora possam estar associados também ao regime de fluxo de piroclástico originado nas elevações. O modelo de erupção da unidade superior é muito semelhante ao da seqüência ignimbrítica Sierra Madre Ocidental, localizada na América do Norte. A presença deste vulcanismo fissural na região de São Félix do Xingu poderia estar relacionada a um batólito ou um conjunto de batólitos formados em um regime distensivo. A integração de dados de isótopos de Nd com o possível zonamento metalogenético que ocorre na porção sul do Cráton Amazônico, entre as regiões do Gráben da Serra do Cachimbo e São Félix do Xingu, sugere que evolução desta porção está vinculada ao desenvolvimento de uma orogênese oceanocontinente orientada aproximadamente lesteoeste, e materializada pela geração de arcos magmáticos gradativamente mais jovens entre 2,1 1,88 Ga. A geração do vulcanismo cálcio-alcalino de 1,88 Ga na região de São Félix do Xingu estaria relacionada à suavização do ângulo de subducção e posterior migração do arco magmático, a exemplo do Cinturão Andino e Montanhas Rochosas. Neste cenário, o vulcanismo exclusivamente crustal de 1,87 Ga representado pela Formação Santa Rosa estaria vinculado a um evento distensivo identificado em várias regiões do Cráton Amazônico e que extendeu-se até o Mesoproterozóico. / Near the São Félix do Xingu city, centersouth portion of the Pará state, in the context of the Carajás Mineral Province, occur extensive Paleoproterozoic volcanoplutonism (1.88 1.87 Ga) exceptionally well-preserved and grouped in the Sobreiro and Santa Rosa formations. These suites are generically correlated to Uatumã Supergroup volcanoplutonism, a magmatic event that covers approximately 1,500,000 km2 and is registered in several areas of the Amazonian craton. These volcanic rocks overlap the paleoproterozoic Parauari Granite, and units of the archean basement included in the Itacaiúnas Shear Belt and South Pará GraniteGreenstone terrain. Later tin-bearing 1.86 Ga A-type granitoid massifs of the Velho Guilherme Intrusive Suite intrude above units. Although knowledge improvement in the last years in this region, there is necessity of more detailed and precise geologic, geochemical, and isotopic data for this volcanic association in the Amazonian craton. This gap of information is partially explained by access difficulties to the studied areas and dense forest cover; lateral and vertical discontinuities of the units; and especially by few researchers groups focalized in this thematic. The intensive geologic mapping developed for this thesis showed that the basal Sobreiro Formation has coherent lava flow facies mainly andesitic, with subordinate dacite and rhyodacite; as well volcaniclastic facies with tuff, lapilli-tuff, and massive polymictic breccia. These rocks show augite, magnesiohastingsite, and plagioclase of variable compositions phenocrysts set in microlithic or traquitic groundmass. Magnetite and apatite are the primary accessories. The systematic mineralogic variation from basaltic-andesite to rhyodacite and dacite, and the petrographical characteristics of these rocks, suggest that the fractional crystallization was the prevailing differentiation process with probable crustal contamination. Their geochemical signature is similar to those of volcanic-arc related granitoids, with high-K calc-alkaline affinity, and metaluminous composition. The upper Santa Rosa Formation is formed by massive coherent facies of rhyolite and subordinate rhyodacite with variable modal contents of K-feldspar, plagioclase and quartz megacrysts in groundmass formed by intergrowth of quartz and potassic feldspar, commonly spherulitic. Spherical spherulites until 10 cm diameter occur locally. Biotite is a varietal phase modally reduced. Zircon, apatite, and Fe-Ti oxides are primary accessories, suggesting an extremely evolved unit. Volcaniclastic facies of ignimbrites, lapilli-tuffs, felsic crystal tuffs, and massive polymictic breccias represent an explosive volcanism cycle in the Santa Rosa Formation. This volcaniclastic association has mineralogic and geochemical characters very similar to the coherent facies. Metric dikes and stocks of granitic porphyries and equigranular granitoids complete this formation. The deposition was driven by large ~ 30 km length NESW crustal fissures, and subordinate NWSE, where magmatic flow are predominantly vertical. These upper volcanic rocks and associated porphyries and granites exhibit intraplate geochemical affinity, peraluminous composition, and transitional subalkaline to alkaline characteristics. Hydrothermally altered zones were identified in these suites, pointing to a metallogenetic potential for them. The presence of several isolated blocks in a flat topography resembles monogenetic eruptive system in the Sobreiro Formation. The topographic highs could have been originated scutulum-type lava accumulation. The volcaniclastic deposits that occur in the top of the lava flow are related to autoclastic fragmentation processes, although can be related to pyroclastic flow regime originated in the hills. The eruption model for the upper unit is very similar to Sierra Madre Occidental ignimbritic sequence, located in North America. The fissure-controlled volcanism in the São Félix do Xingu region could have been related to a batholith or series of batholiths generated in extensional tectonic regime occurred in the Amazonian craton. The integration of Nd isotope data with the possible metallogenetic zoning that occurs in the south portion of the Amazonian craton, between Serra do Cachimbo Graben and São Félix do Xingu region, suggests that the evolution of this portion is related to an approximately eastwest oceancontinent orogenesis, materialized by progressively younger 2.1 1.88 Ga magmatic arcs. The generation of 1.88 Ga calc-alkaline volcanism in the São Félix do Xingu region can be explained by flattening in the subduction angle and following arc migration, as occurs in the Andes Belt and Rocky Mountains. In this scenario, the exclusively crustal 1.87 Ga volcanism of the Santa Rosa Formation could have been linked to extensional tectonic identified in several regions of the Amazonian craton that extended until Mesoproterozoic.

O mapeamento detalhado de uma área de ocorrência de rochas vulcânicas na borda sul da Provincia Magmática Paraná Etendeka (PMPE), entre as cidades de Gramado Xavier e Barros Cassal, RS, permitiu estabelecer a relação estratigráfica de três sequências vulcânicas ácidas geradas por eventos eruptivos associados a magmas-tipo quimicamente distintos. A sequência Caxias do Sul corresponde à primeira manifestação de vulcanismo ácido e é formada por diversos fluxos de lava e lava-domos, emitidos de forma continua, sem intervalos significativos entre as erupções, o que resultou em um espesso pacote de até 140 m de espessura. O final do magmatismo se deu de forma intermitente, com a deposição de arenito entre os últimos derrames. Estas rochas têm composição dacítica (68-70% SiO2) e textura inequigranular hipohialina afanítica a fanerítica fina, sendo compostas por microfenocristais (<2,3 mm) e micrólitos de plagioclásio (\'An IND.55-67\"), piroxênios (hiperstênio, pigeonita e augita) e Ti-magnetita imersos em matriz vítrea ou desvitrificada. Modelos de fracionamento sugerem que seu magma parental pode ter evoluído a partir de um líquido fracionado de basaltos tipo Gramado. As assinaturas geoquímicas e isotópicas (\'ANTPOT.87 Sr\'/\'ANTPOT.86 \'Sr\'IND.(i)\' 0,7192-0,7202) indicam que a evolução pode ter ocorrido em um sistema fechado, com participação, ao menos localmente, de um contaminante crustal mais oxidado. Estima-se que, previamente à erupção, apresentavam temperaturas próximas ao liquidus, de 980-1000ºC, 2% de H2O, fO2 \'10 POT.10,4\' bar, e devem ter residido em reservatórios localizados na crosta superior, a P~3 kbar. Um evento de recarga na câmara pode ter disparado o início da ascensão, que ocorreu com um gradiente dP/dT de 100bar/ºC e velocidades de 0,2 a 0,5 cm \'s POT.-1\' , propiciando a nucleação e crescimento de feno e microfenocristais. O magma teria alcançado a superfície a temperaturas de ~970ºC e viscosidades de \'10 POT.4\' a \'10 POT .5\' Pa.s. A segunda sequência vulcânica, aqui denominada Barros Cassal, é composta por diversos fluxos de lavas andesito basálticas, andesíticas e dacíticas (54-56; 57-58 e 64-66% SiO2, respectivamente), com frequentes intercalações de arenito, que atestam o comportamento intermitente deste evento. Estas rochas apresentam uma textura hipohialina a hipocristalina afanítica a fanerítica fina, cor preta a cinza escura e proporções variadas de vesículas e amígdalas. Todas são compostas por microfenocristais (<0,75 mm) de plagioclásio, augita e Ti-magnetita subédricos, anédricos ou esqueléticos, imersos em matriz vítrea ou desvitrificada. As assinaturas isotópicas das rochas que compõem esta sequência (e.g., \'ANTPOT.87 Sr\'/\'ANTPOT.86 \'Sr\' IND.(i)\' = 0,7125-0,7132) encontram-se dentro do campo dos basaltos toleíticos tipo Gramado, que pode ter sido o magma parental a partir do qual derivaram por cristalização fracionada. Estimativas baseadas nas condições de equilíbrio cristal-líquido indicam que os magmas mais evoluídos da sequência Barros Cassal, de composição dacítica, apresentavam temperaturas de 990 a 1010 ºC, 1,4 a 1,8% de H2O e viscosidades de \'10 POT.4\' Pa.s. As pequenas dimensões dos cristais e cálculos barométricos indicam que a cristalização se deu durante a ascensão, entre 2 e 3 km de profundidade (0,5 a 0,7 kbar de pressão), enquanto o magma ascendia a uma velocidade de 0,12 cm \'s POT.-1\' . Com o fim deste evento vulcânico, desenvolveu-se regionalmente uma expressiva sedimentação imatura (espessura >10 m) de arenitos arcosianos e conglomerados. O último evento vulcânico corresponde à sequência Santa Maria, composta por fluxos de lava e formação de lava domos de composição riolítica (70-73% SiO2), que atingiram espessuras totais de 150 a 400 m. Na base ocorrem feições de interação lava-sedimento (peperitos) e autobrechas (formadas na base e carapaça dos derrames, que constituem lobos nas porções mais distais). Obsidianas bandadas e outras feições indicativas de fluxo coerente são características da unidade. No centro da pilha, a sequência de riolitos constitui uma camada mais monótona de rochas dominantemente cristalinas com marcante disjunção vertical que correspondem à parte central de corpos de lava-domos, no topo predominam as disjunções horizontais. Estas rochas contém < 6% de fenocristais e microfenocristais (<1,2 mm) de plagioclásio (An40-60), Ti- magnetita e pigeonita imersos em matriz vítrea ou cristalina (maciça ou bandada) com até 20% de micrólitos. Modelos de fracionamento são consistentes com modelos em que o magma parental do riolito Santa Maria teria composição similar ao dacito Barros Cassal. As variações nas razões \'ANTPOT.87 Sr\'/\'ANTPOT.86 \'Sr\'ind.(i)\' (0,7230-0,7255) sugerem evolução em sistema aberto, envolvendo contaminação crustal. O magma teria evoluído em câmaras magmáticas localizadas a <12 km de profundidade (<3 kbar), a temperaturas entre 970 e 1000ºC, com fO 2 de ~\'10 POT.10-11\' bar e até 1% de H2O. A cristalização, que se iniciou dentro do reservatório, teria prosseguido durante a ascensão, que ocorreu em gradientes dP/dT de 100 bar/ºC e velocidades médias de 0,2 cm \'s POT.-1\' . O processo de nucleação de micrólitos ocorreu quando o magma ultrapassou o limite de solubilidade a 200 bar de pressão, apresentando temperaturas de 940-950ºC e viscosidades de \'10 POT.5\' a \'10 POT.7\' Pa.s. A alimentação por condutos fissurais, associada a altas taxas de extrusão, teriam elevado a tensão cisalhante próximo às paredes do conduto, gerando bandamentos com distintas concentrações de água. As bandas hidratadas funcionaram como superfícies de escorregamento, diminuindo a viscosidade efetiva, favorecendo a desgaseificação e aumentando a eficiência do transporte do magma desidratado até a superfície. A identificação de estruturas associadas à efusão de lavas, como dobras de fluxo, fluxos lobados, auto-brechas, além da identificação de estruturas de lava domos, contraria interpretações que propõem origem dominantemente piroclástica para o vulcanismo ácido na região, a partir de centros efusivos localizados em Etendeka, na África. / The detailed mapping of an area in the southern edge of the Paraná Etendeka Magmatic Province (PEMP), between the cities of Gramado Xavier and Barros Cassal, Rio Grande do Sul, Brazil, revealed three stratigraphic sequences generated by silicic volcanic eruptions associated to chemically distinct magma-types. The Caxias do Sul sequence corresponds to the first volcanic manifestation of silicic magmatism in the PMPE. It consists of several lava flows and lava domes which erupted continuously, without significant gaps between the events, and resulted in a thick deposit of up to 140 m. The deposition of layers of sandstone between the last lava flows show the intermittent ending of this volcanic event.. These rocks present dacitic composition (~68 wt% SiO2) and hipohyaline to phaneritic texture with microphenocrysts (<2.3 mm) and microlites of plagioclase (\'An IND.55-67\'), pyroxene (hypersthene, pigeonite and augite) and Ti-magnetite surrounded by vitreous or devitrified matrix. The fractionation models suggest that their parental magma may have evolved from a liquid which fractionated from Gramado-type basalts. Geochemical and isotopic signatures ( \'ANTPOT.87 Sr\'/\'ANTPOT.86 \'Sr\' IND.(i)\' 0.7192 to 0.7202) indicate that evolution may have occurred in a closed system, with the participation, at least locally, of a more oxidized crustal contaminant. It is estimated that prior to the eruption the magma might have reached a near-liquidus temperature (980-1000°C), with 2%H2O, fO2 \'10 POT.10.4\' bar, in the reservoirs located in the upper crust, at P~3 kbar. A recharge event in the camera may have triggered the ascension, which occurred with a dP/dT gradient of 100bar/°C and speeds from 0.2 to 0.5 cm.\'s POT.-1\' , leading to nucleation and growth of pheno and microphenocrysts. The magma may have reached the surface at a temperature of ~970 °C and viscosity of \'10 POT.4\' -\'10 POT.5\' Pa.s. The second volcanic sequence, Barros Cassal, is composed of several andesite basaltic, andesitic and dacitic lava flows (54-56, 57-58 and 64-66% SiO2, respectively), with frequent intercalations of sandstone, proving the intermittent behavior of this event. These rocks present aphanitic hipohyaline to hipocrystaline phaneritic texture, black to dark gray color and varied proportions of vesicles. They are all composed of microphenocrysts (<0.75 mm) of plagioclase, augite and subhedral, anhedral or skeletal Ti-magnetite, immersed in glassy or devitrified matrix. The isotopic signatures of the rocks that make up this sequence (eg. \'ANTPOT.87 Sr\'/\'86 ANTPOT. \'Sr IND.(i)\' = 0.7125 to 0.7132) are within the field of tholeiitic Gramado- type basalts, which may have been the parental magma from which they derived by fractional crystallization. Estimates based on the conditions of crystal-liquid equilibrium indicate that the most evolved magmas of the Barros Cassal Sequence, of dacitic composition, reached a temperature of 990-1010°C, 1.4 to 1.8% H2O, and viscosity of \'10 POT.4\' Pa.s. The small size of the crystals and the barometric models indicate that crystallization occurred during the rise, between 2 and 3 km depth (0.5 to 0.7 kbar pressure), while the magma ascended at a speed of 0.12 cm \'s POT.-1\' . With the end of this volcanic event, a significant immature sedimentation (thickness> 10 m) of feldspathic sandstone and conglomerates developed regionally. The last sequence corresponds to Santa Maria, composed of lava flows and lava domes of rhyolitic composition (70-73% SiO2). These deposits can be 150-400 m thick. Features as lava-sediment interaction (peperites) and autobreccias (formed at the base of the flows, which are lobated in the more distal portions) are common in the base of the volcanic pile. banded obsidian and other distinctive features of effusive flows are common in this unit. In the center of the stack, a more monotonous body flow predominates, with hipocrystalline textures and vertical disjunction (corresponding to the central portion of the lava dome). on the top, horizontal disjunctions predominate. These rocks contain <6 % of microphenocrysts and phenocrysts (<1.2 mm) of plagioclase (\'An IND.40-60\'), Ti-magnetite and up to 20% of pigeonite microlites. all these mineral phases occur immersed in glassy or crystalline (massive or banded) matrix. The fractionation models are consistent with models in which the parental magma of the Santa Maria rhyolite and the dacites of Barros Cassal Sequence have similar composition. Variations in \'ANTPOT.87 Sr\'/\'ANTPOT.86\'Sr IND.(i)\' (0.7230 to 0.7255) suggest open-system evolution, involving crustal contamination. The magma might have evolved into dacitic composition in magma chambers located at a depth of <12 km (< 3 kbar), at temperatures between 970 and 1000°C, fO2 of ~\'10 POT.10\'-\'10 POT.11\' bar and 1% of H2O. The crystallization began in the reservoir and might have continued during the ascent, which occurred in dP/dT gradients of 100 bar/°C, with average speeds of 0.2 cm s -1 . The microlites nucleation process occurred when the magma exceeded the solubility limit at 200 bar and displayed a temperature of 940-950°C and viscosity of 10 5 -10 7 Pa.s. The feeding through fissure conduits, associated to high- rate extrusion, might have increased the shear stress near the conduit walls, generating banding with different concentrations of water. Hydrated bands acted as slip surfaces, decreasing the effective viscosity, favoring degassing and increasing the efficiency of transport of dry magma to the surface. The identification of structures associated with lava effusion - like folds of flow, lobed flows, autobreccias, as well as lava dome structures - contradicts the current interpretation, which proposes one single pyroclastic origin, eruptive centers located in Etendeka, Africa, for all deposits of silicic composition in the PEMP.

Cyprus comprises three tectono-stratigraphic terranes: first, the Troodos Massif made up of Late Cretaceous oceanic lithosphere and its sedimentary cover in the centre of the island; secondly, the Mamonia Complex (and Moni Melange) a passive margin lithological assemblage in the west (and south) and thirdly, the Kyrenia Range, an active margin lithological assemblage in the north. This study focuses on the sedimentary cover of the Troodos Ophiolite in W Cyprus, the Triassic-Cretaceous sedimentary rocks of the Mamonia Complex and Late Cretaceous-Miocene igneous and sedimentary rocks in the Kyrenia Range, mainly based on combined sedimentology, geochemistry and geochronological dating. The Late Triassic-Early Cretaceous Mamonia Complex, SW Cyprus (and the Moni Melange, S Cyprus) represent parts of the emplaced passive continental margin of the S Neotethys. Late Triassic sandstones are characterised by a predominantly felsic source, with a subordinate mafic contribution. Jurassic-Early Cretaceous sandstones have a polycyclic felsic origin. Geochemical analyses are suggestive of progressive weathering and sediment recycling/sorting. The dominance of Ediacaran-Cryogenian and Tonian-Stenian-aged detrital zircon populations is suggestive of an ultimate north Gondwana source, probably recycled from Palaeozoic siliciclastic sedimentary rocks within Anatolia to the north. Similar detrital zircon populations characterise Early Cretaceous deltaic sandstone of the Moni Melange, S Cyprus. Sporadic Late Cretaceous subduction-related magmatism, represented by a Campanian volcaniclastic sequence (80.44±1.0 Ma) inWCyprus and a Late Campanian felsic volcanogenic sequence (72.9±1.0 Ma) in N Cyprus, represents early and more advanced stages of northward subduction during closure of the S Neotethys. Specifically, the Kannaviou Formation in W Cyprus (up to 750 m thick) is made up of deep-marine volcaniclastic sandstones that were mostly deposited by gravity flows and as air-fall tuff, interbedded with clay and radiolarian mudstones. Petrographic and geochemical analyses are indicative of a volcanic arc source, with deposition in a fore-arc basin. Petrographic evidence of terrigenous input (e.g. muscovite, muscovite schist, polycrystalline quartz) points to a subordinate continental source. Mineral chemistry is consistent with a volcanic arc origin. Elevated trace-element ratios in undevitrified volcanic glass (e.g. Th/Nb, Th/La) are indicative of involvement of continental crust or subducted terrigenous sediments in source-arc melting. Felsic volcanogenic rocks (Fourkovouno (Selvilitepe) Formation) in the Kyrenia Range, N Cyprus, occur as an up to 400 m-thick sequence of felsic tuffs, felsic debris-flowdeposits and rhyolitic lava flows. Geochemical analyses are indicative of evolved high-K and shoshonitic compositions, similar to those of the Andean active continental margin. Subduction continued to affect the northern continental margin of the S Neotethys in the Kyrenia Range during the Maastrichtian. This lead to the accumulation of Late Cretaceous sandstone turbidites and related basaltic volcanics, possibly in a back-arc setting. The volcanism took place in two phases (Late Cretaceous and Palaeogene-Early Eocene) during pelagic carbonate accumulation. These lavas have within-plate affinities, but with a variable subduction influence in some areas (e.g. western Kyrenia Range), which may be contemporaneous or inherited from previous subduction. The sedimentary sequences in the Kyrenia Range, N Cyprus, document diachronous closure of the S Neotethys. Late Cretaceous and Eocene sandstone turbidites, and the lower part of the overlying Oligocene-Miocene succession exhibit enrichment in ultramafic components that was probably sourced from ophiolite-related rocks in the Taurides to the north. In contrast, Miocene sandstone turbidites higher in the sequence show an increasing input of continent-derived siliciclastic material (and sorting effects). The terrigenous-influenced sediments are likely to represent erosion of thrust sheets that were emplaced from the S Neotethys onto the Arabian foreland in SE Turkey related to continental collision. Ediacaran-Cryogenian and Tonian-Stenian-aged zircons dominate the Late Cretaceous and Eocene sandstone turbidites, consistent with derivation from the Tauride micro-continent to the north and/or NE. Overlying Miocene sandstones include minor populations of Neoproterozoic-aged zircons, suggestive of reworking from source rocks of ultimately Gondwanan origin (e.g. NE Africa/Arabian-Nubian Shield). In summary, the thesis results exemplify the interaction of tectonic processes associated with the evolution of the S Neotethys Ocean. This began in the area studied with passive margin development (Triassic-Cretaceous), and was followed by multi-stage subduction-related volcanism and sedimentation (Late Cretaceous-Miocene). Final closure of the S Neotethys in this area took place during the Late Miocene-Recent.

Andesitic volcanoes are notorious for their rapid and unpredictable changes in eruptive style between and during volcanic events, a feature normally attributed to shallow crustal and intra-edifice magmatic processes. Using the example of eruptions during the last 1000 yrs at Mt. Taranaki (the Maero Eruptive Period), deposit sequences were studied to (1) understand lava dome formation and destruction, (2) interpret the causes of rapid shifts from extrusive to explosive eruption styles, and (3) to build a model of crustal magmatic processes that impact on eruption style. A new detailed reconstruction of this period identifies at least 10 eruptive episodes characterised by extrusive, lava dome- and lava flow-producing events and one sub- Plinian eruption. To achieve this, a new evaluation procedure was developed to purge glass datasets of contaminated mineral-glass analyses by using compositional diagrams of mineral incompatible-compatible elements. Along with careful examination of particle textures, this procedure can be broadly applied to build a higher degree of resolution in any tephrostratigraphic record. Geochemical contrasts show that the products of the latest Mt. Taranaki eruption, the remnant summit dome (Pyramid Dome) was not formed during the Tahurangi eruptive episode but extruded post-AD1755. Its inferred original maximum volume of 4.9×106 m3 (DRE) was formed by simultaneous endogenous and exogenous dome growth within days. Magma ascent and extrusion rates are estimated at =0.012 ms-1 and =6 m3s-1, respectively, based on hornblende textures. Some of the Maero-Period dome effusions were preceded by a vent-clearing phase producing layers of scattered lithic lapilli around the edifice [Newall Ash (a), Mangahume Lapilli, Pyramid Lapilli]. The type of dome failure controlled successive eruptive phases in most instances. The destruction of a pressurised dome either caused instantaneous but short-lived magmatic fragmentation (Newall and Puniho episodes), or triggered a directed blast-explosion (Newall episode), or initiated sustained magmatic fragmentation (Burrell Episode). The transition from dome effusion to a sustained, sub- Plinian eruption during the Burrell Lapilli (AD1655) episode was caused by unroofing a conduit of stalled magma, vertically segregated into three layers with different degrees of vesiculation and crystallisation. The resultant ejecta range from brown, grey and black coloured vesicular clasts to dense grey lithics. Bulk compositional variation of erupted clasts can be modelled by fractionation of hornblende, plagioclase, clinopyroxene, and Fe-Ti oxides. Pre-eruption magma ascent for the Maero Period events is assumed to begin at depths of c.9.5 km.

Mt. Morning is a 2,732 m high, Cenozoic, alkaline eruptive centre situated in the south-west corner of McMurdo Sound in the Ross Sea, Antarctica. Mt. Morning is approximately 100 km south-west of Mt. Erebus, the world's southernmost active volcano. Several Cenozoic, alkali eruptive centres in this region make up the Erebus Volcanic Province. The region is currently undergoing continental extension. Regional-scale, north-striking faulting on the northern flank of Mt. Morning has offset vertical dykes, as young as 3.9 Ma, by up to 6 m dextrally. This is consistent with the trans-extensional regime in the region. The faults also have a dip-slip component, downthrown to the east. These faults define part of the western boundary of the West Antarctic Rift System. Mt. Morning straddles the boundary between the continental rift shoulder of the Transantarctic Mountains in Southern Victoria Land, and the perceived oceanic crust of the Ross Sea. Age determination of the youngest offset dyke constrains movement in the last 3.88 � 0.05 m.y., to an average rate of 0.0015 mm per year. Volcanism on Mt. Morning is divided into two phases. Phase I was erupted between 18.7 � 0.3 and 114 � 0.2 Ma and Phase II between 6.13 � 0.20 and 0.15 � 0.01 Ma. The two phases are separated by a 5.3 m.y. period of quiescence. The geochemistry of Phase I is mildly alkaline; it is composed of volcaniclastic deposits, dykes, sills, and volcanic plugs of nepheline-basanite, nepheline-trachyte, quartz-mugearite, quartz-trachyte, and rhyolite. Phase I rocks evolved along at least two trends: a quartz normative trend, and a nepheline normative trend. Chemical variation in Phase I can be explained in part by crystal fractionation, which has been modelled using major element multiple linear regression. Phase I quartz-mugearite can fractionate to quartz-trachyte after 44% crystallisation. Quartz-trachyte can fractionate to rhyolite after a further 6% erystallisation. The models indicate that clinopyroxene + plagioclase + opaque oxides � alkali feldspar � apatite are the dominant fractionated phases. Many of the Phase I quartz normative volcanic rocks have relatively high ⁸⁷Sr/⁸⁶Sr ratios (0.70501), suggesting that assimilation, most likely of crustal material, has modified them. Phase I nepheline-basanite can fractionate to nepheline-trachyte after 68% crystallisation. Modelling indicates clinopyroxene + nepheline + olivine + opaque oxides are the dominant fractionated phases. Phase II volcanic rocks are strongly alkaline and are mapped as flows, volcaniclastic deposits, dykes, and sills. They have been erupted mainly from parasitic scoria vents and rarely from fissure vents. Rock types include picrobasalt, basalt, basanite, tephrite, hawaiite, mugearite, phonotephrite, tephriphonolite, benmoreite, and phonolite. Chemical variations in the Phase II volcanic rocks can be explained by simple fractionation. Phase II picrobasalt can fractionate to phonotephrite after 78% crystallisation. Phonotephrite can fractionate to phonolite after at least 35% crystallisation, depending on which of several multiple linear regression models are selected. Fractionation is dominated by the removal of clinopyroxene + plagioclase + nepheline + olivine + opaque oxides � apatite � kaersutite. Volcanic rocks in the Erebus Volcanic Province are strongly alkaline on a silica versus total alkalis plot, similar to the Phase II volcanic rocks from Mt. Morning. Mildly alkaline rocks of Phase I are, to date, unique within the Erebus Volcanic Province. Bulk rock isotope ratios of ⁸⁶Sr/⁸⁷Sr (0.70307 - 0.70371 and 0.70498 - 0.70501), �⁴�Nd/�⁴⁴Nd (0.512650 - 0.512902), and �⁰⁶Pb/�⁰⁴Pb (18.593 -20.039) show that the majority of Mt. Morning volcanic rocks lie on a mixing line between HIMU (high-[mu]; enriched in �⁰⁶Pb and �⁰⁸Pb and relatively depleted in ⁸⁶Sr/⁸⁷Sr values) and DM (depleted mantle; high �⁴�Nd/�⁴⁴Nd, low ⁸⁶Sr/⁸⁷Sr, and low �⁰⁶Pb/�⁰⁴Pb). This is similar to the majority of volcanic rocks from the SW Pacific, including Antarctica and New Zealand. Mt. Morning volcanic rocks have tapped this broadly common mantle reservoir. There are variations in radiogenic isotope ratios between Mt. Morning and Mt. Erebus. There are also differences between the incompatible element ratios in volcanic rocks from Mt. Morning, Mt. Erebus, and White Island (a third eruptive centre in the Erebus Volcanic Province), suggesting heterogeneity in the mantle beneath the Erebus Volcanic Province. Significant chemical differences are also noted between ultramafic xenoliths collected from Mt. Morning and from Foster Crater only 15 km away. This suggests a deca-kilometre, possibly even kilometre-scale, heterogeneity in the mantle. Such small-scale chemical differences appear difficult to reconcile with large-scale plume hypotheses for the initiation of volcanism in the Erebus Volcanic Province. Instead, volcanism is much more likely to be related to numerous small plumes, or the preferred hypothesis, metasomatism and amagmatic rifting, followed by decompression melting of upwelling mantle and volcanism during transtensional lithospheric rifting. This latter model is supported by a lack of regional updoming expected with a plume(s), and fits models of localised extension proposed in this thesis. Calc-alkaline and alkaline igneous xenoliths, of felsic to mafic crustal material, have been collected from Mt. Morning. U-Pb geochronology (545.4 � 3.7 Ma and 518.6 � 4.4 Ma) on crustal xenoliths from Mt. Morning illustrate that the basement is Cambrian. Bulk rock chemistry of crustal xenoliths has similarities to compositions reported for Ross Orogen rocks, suggesting the Mt. Morning volcanic edifice is built on a basement that is composed of Cambrian Ross Orogen rock types. Quartz-bearing felsic granulite xenoliths with greater than 70 weight percent silica, collected from Mt. Morning, suggest that part of the basement is felsic. This is the only occurrence of felsic xenoliths reported to date east of the present day coastline of Victoria Land. Mt. Morning crops out less than 25 km from the known northern end of the Koettlitz Glacier Alkaline Province in the Transantarctic Mountains. The partially alkaline basement beneath Mt. Morning suggests the province may continue beneath part of Mt. Morning. The mantle beneath Mt. Morning can be characterised as anhydrous and otherwise largely unmetasomatised, which is atypical of xenoliths collected from the western Ross Sea. Only a handful of Mt. Morning xenoliths show petrographic evidence of metasomatism, these include modal phlogopite, apatite, Fe-Ni sulphide, and plagioclase (in pyroxenite xenoliths), suggesting metasomatising fluids occur discretely in this region. Where present, the metasomatic fluid(s) beneath Mt. Morning are enriched in Ba, LREEs, Th, U, P, Fe, Ni, S, and K, and depleted in Ti relative to the metasomatic fluid composition described at nearby Foster Crater. Oxygen fugacity (fO₂) of the Antarctic shallow mantle has been measured from xenoliths collected from Mt. Morning, where fO₂ was demonstrated to be strongly dependant upon spinel Fe�⁺ content that was measured using Mössbauer spectroscopy, and calculated from the olivine-orthopyroxene-spinel oxybarometer. fO₂ in the rifted Antarctic mantle varies between 0.1 and -1 log units relative to the fayalite-magnetite-quartz buffer and is coupled to melt depletion, with increasing degrees of melt extraction resulting in a more oxidised mantle. This range of upper mantle fO₂ is commonly observed in continental rift settings worldwide. The mantle beneath Mt. Morning is composed of, in increasing degree of fertility, dunite, harzburgite, and lherzolite. Xenoliths representing discrete samples of this mantle have mostly crystallised in the spinel stability field of the mantle at pressures of approximately 15 kb and temperatures between 950 - 970 �C. Symplectites of spinel and pyroxene have been interpreted as petrographic evidence that some of the spinel peridotite originated in the garnet stability field of the mantle. Rare plagioclase-bearing spinel lherzolite (plagioclase lherzolite) is also present in the mantle beneath Mt. Morning, which crystallised at temperatures of between 885 and 935 �C at 5 kb. The Mt. Morning peridotite xenoliths plot along the pre-defined geotherm for the Erebus Volcanic Province, strongly supporting it as the appropriate choice of geothermal gradient for south-west McMurdo Sound. Mineral and bulk rock compositions are nearly identical between the plagioclase lherzolite xenoliths and spinel lherzolite xenoliths. Mineral and bulk rock chemistry suggest it is unlikely that the plagioclase is due to metasomatism. Petrographic evidence and mass balance calculations suggest that the plagioclase lherzolite has crystallised via a sub-solidus (metamorphic) transition from spinel lherzolite upon decompression and upwelling of the mantle. The occurrence of plagioclase lherzolite beneath Mt. Morning could be explained by lithospheric scale uplift along faults that define the western margin of the West Antarctic Rift System. Plagioclase lherzolite has also been collected and described from White Island. White Island is also interpreted to straddle lithospheric scale faults. Rifting and buoyant uplift is sufficient to explain the presence of plagioclase lherzolite in the Erebus Volcanic Province. Plagioclase lherzolite has also been described from Mt Melbourne, an eruptive centre in Northern Victoria Land. Known occurrences of plagioclase lherzolite from the western shoulder of the Ross Sea now cover an area 430 km long and 80 km wide. This is one of the largest provinces of plagioclase peridotite worldwide so far reported.

The Altiplano-Puna Volcanic Complex in the Central Andes is one of the youngest large silicic volcanic fields (LSVFs) in the world, erupting over 13,000 km³ of material during multiple supereruptions from 11 to 1 Ma. Understanding the timescales over which magma is stored in the crust prior to eruption is crucial to understanding the development of LSVFs such as the APVC. The residence time of a magma is defined as the time between magma formation and its eruption. While the eruption age of a volcanic system is generally well constrained through ⁴⁰Ar/³⁹Ar dating of sanidine and biotite crystals, determining the time of magma formation offers a bigger challenge. U-Pb dating of zircon—an early crystallizing, ubiquitous phase in silicic systems—is a commonly used method for determining the timing of magma formation. U-Pb zircon ages were collected for 16 ignimbrites representing the temporal and spatial distribution of the APVC. Zircon crystallization histories show significant overlap between eruptive centers of similar age separated by as much as 200 km. Ignimbrites erupted from the same multicyclic caldera show little relationship. This suggests that ignimbrites may share a deeper, regional source. Timescales of zircon crystallization for individual ignimbrites range from ~400 ka to more than 1 Ma, with little correlation with age or erupted volume. Ignimbrites with longer crystallization timescales frequently exhibit a stepped age distribution and highly variable U contents, suggesting that these ignimbrites likely formed in a very crystalline, low melt fraction environment while ignimbrites with short crystallization times and constrained U concentrations crystallized in high melt fraction systems. Zircon crystallization histories record periods of continuous zircon crystallization in the APVC that extend over 1.5-2 Ma pulses and correlate well with eruptive pulses recognized by previous studies. Overall, zircon crystallization histories of the magmas feeding ignimbrite eruptions in the APVC record long timescales of magmatic activity from a shared regional source, likely the Altiplano-Puna Magma Body currently detectable underlying the APVC. / Graduation date: 2012

Im Rahmen der Dissertation wurden an Wässern und freien Gasen aus Thermalquellen sowie an weniger als 5 Millionen Jahre alten basischen Vulkaniten des zentralandinen Puna-Hochplateaus (NE-Argentinien) umfangreiche element- und isotopengeochemische Untersuchungen durchgeführt und die Edelgasgehalte und -isotopensignaturen in diesen Medien bestimmt. Damit soll ein Beitrag zum besseren Verständnis der jüngeren Subduktionsgeschichte im Bereich der südlichen Zentralanden geleistet, die Wechselwirkungen zwischen ozeanischer Unter- und kontinentaler Oberplatte sichtbar gemacht und die Edelgassystematik verbessert werden. Wie die Ergebnisse der Untersuchungen an Gasen aus den Thermalquellen der Puna-Region zeigen, ist der Anteil an Mantel-Helium in den Thermalquellen dieser Region mit bis zu 67 % wesentlich höher als in der westlich gelegenen vulkanisch aktiven Westkordillere und den anderen angrenzenden Gebieten. In einigen Quellen konnten sogar Anteile an Mantel-Neon nachgewiesen werden, was aufgrund von Überlagerungen mit Neon atmosphärischen und krustalen Ursprungs weltweit bisher nur vereinzelt gelungen ist. Für kontinentale Bereiche mit großer Krustendicke ist ein solch starker Mantelgasfluss äußerst ungewöhnlich und bedeutet, dass Mantelschmelzen bis in die Kruste aufgedrungen sind und tief reichende Wegsamkeiten existieren, so dass die Mantelgase aufsteigen können, ohne stark krustal beeinflusst zu werden. Dass im Bereich der Puna rezent Mantelmaterial in die Kruste aufsteigt, zu diesem Ergebnis kommen auch aktuelle seismologische Untersuchungen. Zudem wurden junge, vorwiegend monogenetische Basalte bis basaltische Andesite geochemisch auf ihre Haupt-, Neben- und Spurenbestandteile sowie ihre Gehalte an Seltenenerdenelementen hin untersucht. Auch wurden die Isotopenverhältnisse von Sr, Nd und Pb in den Gesteinen bestimmt und petrographisch-mineralogische Analysen der darin enthaltenen Olivine und Pyroxene durchgeführt. Wie die Resultate belegen, haben die Magmen bei ihrem Aufstieg durch die Erdkruste insbesondere Material aus der Oberkruste assimiliert und sind zudem durch Fluide aus der abtauchenden Platte beeinflusst worden. Damit konnte gezeigt werden, dass einfache geochemische Methoden allein nicht ausreichen, um die Mantelquelle der Magmen ermitteln oder Aussagen über die Asthenosphärendynamik in der Region machen zu können. Im Gegensatz dazu zeigen die Messungen der Edelgasisotopenverhältnisse in den Fluideinschlüssen der Olivine und Pyroxene, dass deren Edelgaszusammensetzung nicht durch Krustenkontamination beeinflusst wurde, weil die Magmen erst nach der Olivin- bzw. Pyroxen-Kristallisation Schmelzen aus der Oberkruste assimiliert haben. Darüber hinaus konnten durch die Edelgasisotopenmessungen die bisher höchsten magmatischen He- und Ne-Isotopenverhältnisse von ganz Südamerika nachgewiesen werden. Aus der unterschiedlichen Höhe der Messwerte ist zu schließen, dass die im Osten der Puna vorkommenden älteren Laven aus einem nichtkonvektiven (lithosphärischen) Mantel stammen, während die am vulkanischen Bogen und Westrand der Puna gelegenen jüngeren Laven, ihren Ursprung in einer konvektiven (asthenosphärischen) Mantelquelle haben. Zudem konnte gezeigt werden, dass der Mantelgasfluss in der Region in den letzten 5 Millionen Jahren stark zunahm und sich die Eruption von mantelstämmigen basischen Laven in dieser Zeit kontinuierlich in westliche Richtung zum aktiven Vulkanbogen hin verlagerte. Im daraus abgeleiteten Modell beruht dieser Prozess (1) auf einer an die Kontinentalverschiebung gekoppelten W-Drift des Kontinents und (2) auf einem mit der Versteilung der Unterplatte verbundenen Vordringen des subkontinentalen asthenosphärischen Mantels nach W, nach dem Ende der Subduktion des unterseeischen aseismischen Juan Fernández-Rückens in der Region. Zudem gibt es starke Argumente dafür, dass die asthenosphärischen Magmen aus einer fluidreichen Zone in 500 – 600 km Tiefe parallel zur subduzierten Platte aufsteigen und nicht, wie bisher angenommen, durch Schmelzbildung in Bereichen unter 200 km Tiefe, allein durch Entwässerung der abtauchenden Platte erzeugt werden. Zu diesem Resultat führt vor allem die Kombination der He-Isotopenverhältnisse mit Ergebnissen seismologischer Untersuchungen. / This study has determined the concentrations and isotopic composition of noble gases in water and gas samples from thermal springs and in samples of post Miocene basic volcanic rocks from the central Andean Puna Plateau (NW Argentina). The aim of this study is to shed light on questions related to the Neogene subduction history, the geochemical relationship between the oceanic and continental plate and on the distribution of noble gases in mantle-derived rocks of the Central Andes. The results of the geothermal water study show that the Puna plateau has higher values of mantle-derived He between 22° and 26° S compared to the neighbouring Western and Eastern Cordilleras. The highest 3He/4He ratio (5,4 Ra) was obtained close to the Tuzgle volcano, and this is rather high for back-arc gases considering they have ascended through a relatively thick 65 km crust, enriched in crustal 4He. In some cases it was also possible to detect the presence of mantle-derived Ne, which has so far only been demonstrated in a few locations around the world, because of the ubiquitous contamination by atmospheric- and crustal-Ne. Hence, this study clearly demonstrates a higher flux of noble gases from the mantle in the Puna Plateau region than in nearby regions of the Altiplano, the Salta-Rift and the Eastern and Western Cordilleras. In addition to the water study, a series of samples from post Miocene basic volcanic rocks in the Puna back-arc region were analyzed for major, minor and trace element composition as well as Sr, Nd and Pb isotopic ratios. Mineralogical analysis of olivine and pyroxenes from the lavas show that the rocks compositions have signatures that vary depending on the distance from the volcanic arc. Accordingly, magma compositions reflect processes that took place in the subduction-modified mantle wedge and the overlying continental plate. During their ascent, most of the magmas were contaminated with acid crustal melts that mask the geochemical signature of their mantle sources. This makes it difficult to accurately reconstruct the specific geotectonic setting for the magmas and their related mantle sources from the whole rock compositions. However, it is possible to put limits on the origin and amount of contamination from the Pb, Nd and Sr isotopic data. The results show that contaminants are mainly from the upper mantle. Mixing models suggest degrees of crustal assimilation on the order of 10 % and less. Given the problems of interpreting conventional geochemical studies on the origin of the back-arc magmas as just described, the combination of whole rock geochemical results with the corresponding noble gas data is potentially very important. For this purpose, olivine and pyroxene mineral separates mechanically and thermally degassed in order to measure their noble gases content and isotopic ratios. As the results show, the He isotopic signatures in the rocks are quite variable (4,5 - 8,1 Ra), depending on the age and distance to the volcanic arc. Whereas the samples nearest to the arc have MORB-type 3He/4He > 7 Ra, those farthest from the arc have 3He/4He < 7 Ra similar to the typical signature of a subcontinental lithospheric mantle (SCLM). The youngest Puna lavas have the highest primordial 3He/4He ratios and therefore the highest values for mantle-derived He yet found in the whole Andes chain. This implies that the mantle gas flux in the Puna region has increased since the Pliocene while during the same time interval, the focus of back-arc volcanic activity migrated progressively west towards the position of the present volcanic arc. This migration can be correlated with steepening of the subducted slab and advance of the asthenospheric mantle wedge to the west as a consequence of the drift of the Juan Fernández ridge towards the south. A consequence of the westward drift of the subduction zone at ca. 26 km/Ma is the ascent of asthenosphere into the mantle wedge. Geophysical studies suggest that this material ascends parallel to the slab (return flow). The He isotopic signatures from this study show that the asthenospheric ascent was not spacious but took place along certain channels or branches that may relate to deep-reaching weak lithospheric zones but not to a widespread delamination in the SCLM. The isotopic He, Ne and Xe relations of the lava samples collected in the back-arc region far from the volcanic arc indicate the presence of SCLM during back-arc volcanism in this region, which is a grave argument against a crustal delamination. Crustal contamination could not have been responsible for these values as the assimilation of crustal melts essentially took place after the olivine crystallization, as indicated by the variations in Sr- and He-isotope data and the coexistence of quartz and olivine in the samples.

In the Central Anatolian Crystalline Complex (CACC) the Late Cretaceous post-collisional granitic magmatism is followed by Eocene extension, resulting in formation of roughly E-W trending transtensional basins. Formation of these basins was accompanied by calc- alkaline- mildly alkaline volcanism. The volcanic rocks, mainly subaques lava flows and subareal domes are concentrated along these basins and associated with Middle Eocene (Bartonian) Mucur Formation. They are basic to intermediate and are classified as basalt, basaltic andesite and rarely alkali basalt and trachy-andesite. All studied samples are strongly and variably LREE enriched relative to chondrite with the (La/Sm)N ratio of 2.26- to 6.17. They have negative Nb-Ta and Ti anomalies in the primitive mantle normalized diagram, and are characterized by low Nb/La (0.21 to 0.62), Ce/Pb (3.70-34.90) and Nb/U ratios (1.11-30), which may indicate an interaction with the Late Cretaceous granitic host rocks in the course of their ascent. The volcanic rocks display similar but variable ranges of Sr, Nd and Pb isotope values. Relatively high values of &epsilon / Nd (0.53 to 4.33) indicate an isotopically depleted mantle source. Combined trace element and isotope compositions of the Eocene samples suggest that they were derived from a heterogeneous lithospheric mantle source that had been metasomatized by subduction related agents such as fluids and/or melts during a previous geodynamic event. Geochemistry and geotectonic setting point out that lithospheric delamination was the most likely mechanism to generate these calc-alkaline to mildly alkaline volcanic rocks in the CACC.

Mafic magma is the most common magma erupted at the surface of the earth. It is generated from partial melting of the mantle, which has been subdivided into end-members based on unique geochemical signatures. One reason these end members, or heterogeneities, exist is subduction of lithospheric plates back into the mantle. The amount of elements, such as Cl and K, removed during subduction and recycled into the deep mantle, is poorly constrained. Additionally, the amount of volatiles, such as Cl, that are recycled into the deep mantle will strongly affect the behavior of the system. I have looked at Cl and K in HIMU source melts to see how it varies. Cl/Nb and K/Nb suggest that elevated Cl/K ratios are the result of depletion of K rather than increased Cl recycled into the deep mantle. After the mantle has partially melted and mafic melt has migrated to the surface, it usually erupts effusively or with low explosivity because of its low viscosity, but it is possible for larger eruptions to occur. These larger, Plinian eruptions, are not well understood in mafic systems. It is generally thought that basalt has a viscosity that is too low to allow for such an eruption to occur. Plinian eruptions require fragmentation to occur, which means the melt must undergo brittle failure. This may occur if the melt ascends rapidly enough to allow pressure to build in bubbles without the bubbles expanding. To test this, I have done decompression experiments to try to bracket the ascent rate for two Plinian eruptions. One eruption has a fast ascent, faster than those seen in more silicic melts, whereas the other eruption is unable to be reproduced in the lab, however it began with a increased viscosity in the partly crystallized magma. After fragmentation and eruption, it is generally thought that tephra do not continue to crystallize. We have found that crystallinity increases from rim to core in two basaltic pumice. Textural data along with a cooling model has allowed us to estimate growth rates in a natural system, which are similar to experimental data. / text