A região de São Félix do Xingu, localizada no centro-sul do estado do Pará e, geologicamente, no contexto da Província Amazônia Central do Cráton Amazônico. Apresenta em seus arredores um registro extremamente preservado das atividades vulcâno-plutônicas ocorridas durante o final do Paleoproterozóico (1870 - 1880 Ma), agrupadas no Grupo Uatumã, que é dividido na região nas formações Sobreiro e Santa Rosa. Estas rochas foram depositadas sobre o embasamento arqueano, representado pelo Terreno Granito-Greenstone do Sul do Pará e pelo Cinturão de Cisalhamento Itacaiúnas, e unidades paleoproterozóicas, tal como o Granito Parauari. Por fim, estas rochas foram invadidas em ~1860 Ma pelos granitos do tipo A da Suíte Intrusiva Velho Guilherme e recobertas pelas rochas sedimentares da Formação Triunfo. A Formação Sobreiro é a unidade basal. Suas rochas tem filiação cálcio-alcalina e são representadas por vulcânicas e piroclásticas predominantemente intermediárias, com componentes ácidos no topo da sequência. Em estudos de campo são reconhecidas ao menos duas sequências de derrames vulcânicos que variam de andesitos basálticos com fenocristais de augita e magnésio-hastingsita nos derrames basais, para andesito e latito com fenocristais de magnésio-hastingsita e de andesina a labradorita e, por fim, quartzo-latito e riolito com fenocristais de plagioclásio sódico e de feldspato potássio, além de quartzo ocasional. Intercalados nestes derames de lava ocorrem corpos de rochas piroclásticas representadas principalmente por tufos máficos de cristais, lapilli-tufo máfico e tufos máficos laminados de cristais. De modo geral essas, são rochas hipocristalinas, maciças, formadas por cristais e fragmentos de cristais líticos e vítreos. Os tufos de cristais máficos laminados apresentam melhor seleção granulométrica, estrutura laminada e arcabouço constituído porcristais e fragmentos de cristais e líticos. A Formação Santa Rosa, como descrita atualmente na literatura, é a unidade superior e representa um vulcanismo intraplaca do tipo A. É composta por rochas vulcânicas, subvulcânicas e piroclásticas com alto teor de \'SiO IND.2\' (> 70% na maioria das amostras). Ao menos três fácies são reconhecidas: a) pórfiros graníticos e riolitos com megacristais de anfibólio, plagioclásio sódico, feldspato potássico e quartzo; riolitos com fenocristais de feldspato potássico e plagioclásio sódico com eventuais megacristais de quartzo; b) álcaliriolitos e pórfiros álcali-riolíticos com fenocristais de feldspato potássico (ortoclásio) e quartzo; c) tufo félsico de cristais hiprocristalino, tufo félsico de cristais levemente soldado com fiamme, tufo soldado laminado, lapilli-tufo acrescionário e tufo vítreo com glass shards. A assembleia de alteração hidrotermal da Formação Sobreiro é composta por epídoto + clorita + clinozoisíta + pirita + quartzo + carbonato + albita + sericita na alteração propilítica; sericita + clorita + quartzo \'+OU-\' pirita \'+OU-\' fluorita \'+OU-\' barita \'+OU-\' alloclasita \'+OU-\' esfalerita na alteração sericítica; e sericita + hematita + quartzo + argilo-minerais \'+OU-\' galena \'+OU-\' ouro na alteração argílica. Indícios de alunita sugerem que a Formação Sobreiro pode hospedar em suas rochas sistemas epitermais do tipo low-e high-sulfidation. Já na Formação Santa Rosa, a assembleia mineral de alteração hidrotermal é composta por feldspato potássico + biotita + quartzo + sericita na alteração potássica; e sericita + quartzo + pirita + clorita \'+OU-\' fluorita \'+OU-\' carbonato na alteração sericítica. Tais assembleias podem hospedar nas rochas da Formação Santa Rosa mineralizações do tipo Intrusion Related Gold Systems. Estudos litoquímicos revelam a naturezacálcio-alcalina de alto potássio da Formação Sobreiro, com enriquecimento em elementos litófilos como K, Ba, Sr, Rb, e baixa concentração de elementos de alto potencial iônico como Nb e Ta. Esta unidade mostra rochas enriquecidas em elementos terras raras leves em relação a terras raras pesados, indicada pela razão \'(La/Yb) IND.N\' ~ 12, sem anomalias de Eu nas rochas menos evoluídas e com anomalias levemente negativas nas rochas mais evoluídas. Tais características são típicas de andesitos orogênicos e estudos comparativos revelam que as rochas da Formação Sobreiro são bastante semelhantes às de alguns arcos magmáticos mais jovens, como o Arco Eólio na região da Sicília. Isto corrobora a hipótese de que a Formação Sobreiro é relacionada a um vulcanismo associado a um evento de subducção. A Formação Santa Rosa mostra resultados mais heterogêneos. Algumas das amostras analisadas apresentam afinidade cálcio-alcalina metaluminosa, com enriquecimento em Ba, Rb e Sr semelhantes às rochas evoluídas da Formação Sobreiro. Nestas amostras são observadas anomalias negativas de nióbio e tântalo em diagramas normalizados de elementos traços, além de suaves anomalias negativas de Eu em diagramas de elementos terras raras, com enriquecimento em terras raras leves em relação aos pesados similar à Formação Sobreiro. Estas características, junto às razões de Ba/Ta > 450 e Rb/Nb > 7, mostram mais semelhanças com as rochas cálcio-alcalinas da Formação Sobreiro do que com as rochas do tipo A da Formação Santa Rosa. O outro grupo de amostras da Formação Santa Rosa apresenta um comportamento completamente diferente, sendo caracterizado por rochas alcalinas, peraluminosas, com enriquecimento em elementos de alto potencial iônico (principalmente Nb e Ta) e fortes anomalias negativas para elementos litófilos (principalmente Ba e Sr, além de CaO, P e Ti) emdiagramas normalizados de elementos traços. Em relação aos elementos terras raras, este grupo apresenta enriquecimento muito mais discreto em elementos terras raras leves em relação aos pesados, evidenciado pela razão \'(La/Yb) IND.N\' ~ 4, com forte anomalia negativa de Eu. As razões Rb/Nb < 7 indicam que estas amostras atendem a maioria dos critérios classificatórios para rochas subalcalinas do tipo A e, portanto devem ser classificadas como pertencentes à Formação Santa Rosa. Por fim, os resultados sugerem que embora agrupados somente na Formação Santa Rosa nos trabalhos anteriores, ao menos uma parte dos riolitos mostra características que apresentam associação ao vulcanismo cálcio-alcalino da Formação Sobreiro / The São Felix do Xingu region, located in the center-south region of the state of Pará - Brazil, under the Central Amazonian province of theAmazonian Craton context, presents in its surroundings extremely well preserved volcano-plutonic activities occurred during the Paleoproterozoic (1870 - 1880 Ma), where units are grouped into the Uatumã Group which is therefore divided into formations Sobreiro e Santa Rosa. These rocks are intrusive in units of the Archean basement represented by the South Pará Granite-Greenstone Terrain and by the Itacaiúnas Shear-belt; and rocks of Paleoproterozoic such as the Parauari Granite. Thus, these rocks are intruded in ~1860 Ma by A-type granites of the Velho Guilherme Intrusive Suite and covered by sedimentary rocks of the Triunfo Formation. The Sobreiro Formation is the basal unity. It is calc-alkaline, composed of volcanic and pyroclastic rocks of mainly intermediate composition, with acid components on the top of the sequence. In field study, at least two sequences of volcanic flows which vary from andesi-basalts with phenocrysts of augite and magnesium-hastginsite in the basal flows, to andesites and latites with phenocrysts of magnesium-hastginsite and plagioclase (andesine to labradorite) and finally, quartz-latites and rhyolites with phenocrysts of sodic plagioclase and potash feldspar, besides occasional quartz, are recognized. Interspersed with these lava flow rocks, pyroclastic rocks represented by mafic crystal tuffs, mafic crystal lapilli-tuffs and laminated mafic crystal tuffs occur. The Santa Rosa Formation, as described in literature nowadays, is the superior unit and represents an A-type intraplate volcanism. It is composed by volcanic, subvolcanic and pyroclastic rocks with high \'SiO IND.2\' content (>70% in most of the samples). At least three facies are identified, the first consisting of granitic porphyry and rhyolite with megacrysts of amphibole, sodic plagioclase, potash feldspar and quartz; the second of rhyolites with phenocrysts of potash feldspar and sodic plagioclase with occasional quartz; and the last of alkali-rhyolites and alkali-rhyolitic porphyries with phenocrysts of potash feldspar (orthoclase) and quartz. Hipocrystalline felsic crystal tuff, lightly welded crystal tuff with fiamme, laminated welded tuff, accretionary lapilli-tuff and vitreous tuff with glass shards represent the pyroclastic rocks associated with Santa Rosa Formation. The hydrothermal alteration mineral assemblage of the Sobreiro Formation is composed of epidote + chlorite + clinozoisite + pyrite + quartz + carbonate + albite + sericite in the propylitic alteration; sericite + chlorite +quartz + carbonate ± pyrite ± fluorite ± barite ± alloclasite ± sphalerite in the sericitic alteration; and sericite + hematite + quartz + clay minerals ± galena ± gold in the argillic alteration. In addition to evidences of alunite, the vii Sobreiro Formation may host low and high-sulfidation epithermal systems in its rocks. The Santa Rosa Formation, on the other hand, presents ahydrothermal alteration mineral assemblage composed of potash feldspar + biotite + quartz + sericite in the potassic alteration; and sericite + quartz + pyrite + sericite ± fluorite ± carbonate in the sericitic alteration. Such assemblages may host Intrusion Related Gold Systemsmineralization type into the rocks of the Santa Rosa Formation. Lithochemistry studies reveal the high potassium calc-alkaline nature for the Sobreiro Formation, with enrichment in lithophile elements such as K, Ba, Sr, Rb and low concentration of high field strength elements such as Nb and Ta. This unit shows rocks enriched in light rare earth elements in relation to heavy rare earth elements indicated by the \'(La/Yb) IND. N\' ~ 12 ratio, with absence of Eu anomalies in the less evolved and lightly negative anomalies in the most evolved rocks. Such characteristics are typical of orogenic andesites and comparative studies reveal that the Sobreiro Formation chemical characteristics are rather similar to some younger magmatic arcs, like the Aeolian Arc in the Sicily\'s region. This data corroborates with the hypothesis that the Sobreiro Formation is related to a calc-alkaline volcanism related to a subduction regime. The Santa Rosa Formation shows more heterogeneous results. Some of the analyzed samples present calc-alkaline affinity, metaluminous, with enrichment in Ba, Rb and Sr similar to the evolved rocks of the Sobreiro Formation. Such samples also present negative Nb and Ta anomalies in normalized trace elements diagrams, besides presenting light negative anomalies in normalized rare earth elements diagrams with enrichment in light rare earth elements in relation to heavy rare earth elements similar to the Sobreiro Formation. These characteristics, allied to Ba/Ta > 450 and Rb/Nb > 7 ratios, show much more similarities to the calc-alkaline rocks of the Sobreiro Formation than to the A-type rocks of the Santa Rosa Formation. The other group of samples of this unity shows a completely different behavior, being characterized by alkaline rocks, peraluminous, withenrichment in high field strength elements (mainly Nb and Ta) and strong negative anomalies for lithophile elements (mainly Ba and Sr, besides CaO, P and Ti) in normalized trace elements diagrams. In relation to rare earth elements, the Santa Rosa Formation presents much more discrete enrichment in light rare earth elements in relation to heavy rare earth elements, highlighted by the \'(La/Yb) IND.N ~ 4 ratio, with strong negative anomalies of europium. The Rb/Nb < 7 ratio indicate that these samples attend to the most of the classificatory criteria for sub-alkaline type-A rocks and, therefore must be classified as belonging to the Santa Rosa Formation. viii All things considered, the results suggest that besides grouped exclusively in the Santa Rosa Formation in the previous works, at least somepart of the rhyolites shows characteristics which presents association to the calc-alkaline volcanism of the Sobreiro Formation.

Abstract A comparison between the two very important sulfide belts Raahe-Ladoga Ore Zone (RLZ) in Finland and Southern Edge of Altay (SEA) in China, including geological setting, metallogenic characters and geochemical exploration has been made. The two sulfide belts share similarities but differ from each other in the tectonic setting and metallogenic epoch. Polymetallic ores in RLZ and SEA are the products of the submarine volcanism, but mainly Zn-Cu type is present in RLZ and Pb-Zn, Cu-Pb-Zn and Cu-Zn types occur in SEA. A main Ni-Cu ore belt related to the mafic-ultramafic intrusions is also present in the RLZ. RLZ is metamorphosed to a higher grade than SEA. The Viholanniemi Zn-Au deposit is a veinlet-disseminated type, possibly beneath the stratabound sulphide ores, and the Keketale Pb-Zn deposit is a stratabound sulphide ore hosted by sedimentary rocks in the volcano-sedimentary formation. They show many differences. It is suggested that stratabound sulphide ores overlie stratigraphically the Viholanniemi stringer ores and Au-bearing stringers underlie the Keketale stratabound ores. Geochemical explorations of the two deposits exhibit different methods, subjects and procedures. Boulder tracing and till geochemical exploration proved to be very effective in finding the Viholanniemi deposit while stream sediment and soil geochemical surveys were the major and effective tools in finding the Keketale deposit. An extensional environment and the intensity of volcanism are the essential conditions for the formation of polymetallic ores related to the volcanism. It is feasible to classify the ores into the ores hosted by volcanics and sedimentary rocks in a volcano-sedimentary formation. The stratigraphical thickness of volcanic rocks and the amount of agglomerates are the two most crucial factors needed to be considered in prospecting. The chemical variations of the host rocks can indicate the sulphide ores hosted by sedimentary rocks in some circumstances.

Master of Science / Department of Geology / Matthew E. Brueseke / The Sonya Creek volcanic field (SCVF) contains the oldest in situ magmatic products in the ~29 Ma–modern Wrangell arc (WA) in south-central Alaska. The WA is located within a transition zone between Aleutian subduction to the west and dextral strike-slip tectonics along the Queen Charlotte-Fairweather and Denali-Duke River fault systems to the east. WA magmatism is due to the shallow subduction (11–16°) of the Yakutat microplate. New ⁴⁰Ar/³⁹Ar and U-Pb geochronology of bedrock and modern river sediments shows that SCVF magmatism occurred from ~29–18 Ma. Volcanic units are divided based on field mapping, physical characteristics, geochronology, and new major and trace element geochemistry. A dacite dome yields a ~29 Ma ⁴⁰Ar/³⁹Ar age and was followed by eruptions of basaltic-andesite to dacite lavas and domes (~28–23 Ma Rocker Creek lavas and domes) that record hydrous, subduction-related, calc-alkaline magmatism with an apparent adakite-like component. This was followed by a westward shift to continued subduction-related magmatism without the adakite-like component (e.g., mantle wedge melting), represented by ~23–21 Ma basaltic-andesite to dacite domes and associated diorites (“intermediate domes”). These eruptions were followed by a westward shift in volcanism to anhydrous, transitional, basaltic-andesite to rhyolite lavas of the ~23–18 Ma Sonya Creek shield volcano (Cabin Creek lavas), including a rhyolite ignimbrite unit (~19 Ma Flat Top tuff), recording the influence of local intra-arc extension. The end of SCVF activity was marked by a southward shift in volcanism back to hydrous calc-alkaline lavas at ~22–19 Ma (Young Creek rocks and Border Lavas). SCVF geochemical types are very similar to those from the <5 WA, and no alkaline lavas that characterize the ~18–10 Ma Yukon WA are present. Sr-Nd-Pb-Hf radiogenic isotope data suggest the SCVF data were generated by contamination of a depleted mantle wedge by ~0.2–4% subducted terrigenous sediment, agreeing with geologic evidence from many places along the southern Alaskan margin. Our combined dataset reveals geochemical and spatial transitions through the lifetime of the SCVF, which record changing tectonic processes during the early evolution of the WA. The earliest SCVF phases suggest the initiation of Yakutat microplate subduction. Early SCVF igneous rocks are also chemically similar to hypabyssal intrusive rocks of similar ages that crop out to the west; together these ~29–20 Ma rocks imply that WA initiation occurred over a <100 km belt, ~50–60 km inboard from the modern WA and current loci of arc magmatism that extends from Mt. Drum to Mt. Churchill.

The emplacement of continental flood basalt provinces is often ascribed to fissure eruption. However, in many provinces the locations of actual vent complexes remains illusive. In southern Africa, the Karoo continental flood basalt province was erupted during the Jurassic between 183 and 179 Ma. The southernmost outcrop of the Karoo continental flood basalt lavas is the Lesotho remnant; and is comprised of the Drakensberg Group which forms the uppermost portion of the Karoo Supergroup. The geochemical stratigraphy for the Drakensberg Group is well established. At the base, there are a number of small volume compositionally diverse units, which form the Barkly East Formation, overlying which, there are larger volume, compositionally less variable units of the Lesotho Formation, which form the bulk of the volcanic sequence. The Lesotho remnant is associated with an abundance of dykes in the adjacent vicinity. This suggests that the lavas were fed from local rather than distal eruption sites. This study presents whole rock major and trace element data for 94 dykes and three sills from the northern Lesotho - northeastern Free State region and demonstrates that on the basis of geochemistry, all 97 intrusions can be correlated with various units of the northern Barkly East and Lesotho Formations. In addition, the petrographical; characteristics, orientation and distribution of the dykes do not correlate with geochemistry. Geochemical discrimination diagrams have been used to identify five compositionally diverse dykes, which are similar to the northern Barkly East Formation units. Three dykes are characteristic of the Letele unit and two are compositionally similar to the Wonderkop unit. Although the geochemical characteristics of the Lesotho Formation units are rather well constrained, the composition of these units is typified by considerable overlap in composition. Therefore, since unambiguous classification of dykes with geochemical similarities to the various units of the Lesotho Formation is unachievable using an empirical approach, the multivariate forward-stepwise discriminant function analysis (DFA) technique was used to facilitate the classification of the remaining 89 dykes and three sills. Forward-stepwise DFA classified 23 dykes as having compositional similarities to the Mafika Lisiu unit, 29 as having compositions of the Maloti or Senqu types; and 32 as having the composition of the Mothae type. In addition, eight dykes are compositionally similar to the Oxbow dykes, which intrude the Senqu unit in northern Lesotho. These results suggest that that the Lesotho remnant was fed from local eruption sites and that long distance magma transport for the bulk of the Lesotho remnant basalt lavas is unlikely.

Abstract Dissimilatory microbial sulfate reduction (MSR) The specific objectives of the study were to provide the first measurements of sulfur isotope fractionation associated with acidophilic sulfate reducing-microorganisms, and to examine whether pH influences sulfur fractionation during MSR. The fractionation associated with the strains investigated was comparable to that of neutrophilic strains with similar metabolisms (4-12‰), but varied with pH. Two fractionation regimes were identified: one regime is consistent with fractionation during exponential growth, while the other – not identified previously - is not linked to active sulfate reduction and may result from internal sulfate accumulation. This would represent the first measurement of sulfur fractionation during sulfate uptake, the first step of MSR. Geological processes at the Cretaceous-Paleogene (KPg) boundary The KPg boundary is associated with one of the largest biological extinctions in the history of our planet. Two major geologic events - the Chicxulub bolide impact with evaporite terrane and the eruption of the Deccan continental flood basalts - coincide with the KPg boundary and have been identified as possible triggers for the extinctions, but their relative timing remains unresolved. The objectives of this study were to identify the contribution of these processes to the sulfur burden in the sedimentary environment of two freshwater KPg sections, and to determine their relative timing. The results demonstrate that the peak of Deccan volcanism post-dates the Chicxulub impact and the associated abrupt KPg mass extinction, thus precluding a direct volcanic causal mechanism, but shedding light on the underlying causes for the delayed recovery of ecosystems in the early Paleogene.

D.Phil. / The Ongeluk Formation is a laterally extensive sequence of ≈2200 Ma tholeiitic basaltic andesites in the upper Griqualand West Sequence of the northern Cape Province. The stratigraphic thickness is about 500 m and the Ongeluk Formation underlies the ore-bearing strata of the Kalahari Manganese Field. The formation comprises massive lavas, pillow lavas and hyaloclastite beds in close association. These rocks were extruded under water in a marginal basin within the continental setting of the Kaapvaal Craton. The Hekpoort Basalt Formation of the Transvaal is magmatically cogenetic with the Ongeluk, having indistinguishable geochemistry and sharing a stratigraphically related hiatus in Cr values. The best age estimate for the two formations is 2193 ± 71 Ma, from Rb-Sr data of two previous workers for Hekpoort samples. The Ongeluk Formation shows a mild "regional" geochemical alteration and a profound "Kalahari" alteration beneath the Kalahari Manganese Field. Geochemical screening was used to reconstruct the magmatic composition from a selected dataset. Three stages in the development of regional alteration are ascribed to sea water-rock interaction at different temperatures, and have distinct geochemical signatures. The pervasive Kalahari alteration is characterised by a purple colouration and the decoupled alteration of alkali and high field strength elements. It is due to the development of major hydrothermal systems close to a volcanic vent which are analogous to modern mid-ocean ridge systems. A multi-system isotopic study showed that most of the isotope systems were modified by sea-floor alteration. The similarity of the 2237 ± 23 Ma Pb-Pb errorchron age with the Rb-Sr Hekpoort age reflects changes in U-Pb ratios with minor changes in Pb isotope ratio. Evidence was found in the Rb-Sr system for a minor disturbance at ≈ 1100 Ma, also reported by previous workers. This event is related to the Namaqua tectogenesis, while no isotopic evidence was found for the enigmatic ≈ 2200 to 1750 Ma Kheis orogeny, regarded as the cause of thrust faulting in the region. A genetic connection between the Ongeluk lava and the Kalahari Manganese deposits was established. The manganese ores contain evidence for both marine and hydrothermal contributions to chemical sedimentation. Negative Ce anomalies characterise an oxygenated sea in which the interaction between global oceanic and continental influences is seen. Heavy rare earth enrichment reflects volcanic hydrothermal exhalations from the Kalahari Ongeluk system. Mass balance calculations show that the entire 9 billion tons of Kalahari Manganese ore could have been derived from the Ongeluk Formation. A new model describing the origin and evolution of the Kalahari Manganese Field places a strong emphasis on the role of the syngenetic hydrothermal exhalation and upgrading.

The influence of volcanoes on climate is very often debated, not only in the scientific community. However, the question remains, how much may the volcanic activity contribute to the significant ganges, that could cause a response of environment, which lies in the immediate vicinity. These changes could arise as a response on prolonged exposure of the negative influence, which would last much longer than decades, like it is at present. From this reason the situation in the České středohoří Mountains, where the volcanic activity lasted for several million years during the Paleogene period, is studied. This area is extraordinally rich in deposits of so-called volcanic floras, which accompany the neovolcanic activity. The age of the localities are between late Eocene to late Oligocene and it is a long time enough for recognizing some changes in the floristic composition to help elucidate the given problem.

L’arc équatorien est caractérisé par un nombre particulièrement élevé de volcans quaternaires. Nos quatre-vingt-dix nouveaux âges K-Ar, réalisés pour une vingtaine d’édifices, montrent que son développement a commencé vers 1 Ma au nord et a migré vers le sud à partir de 600 ka. Ce déplacement pourrait être induit par un changement récent de la géométrie de la plaque Nazca en profondeur et l'activation des failles crustales. Les taux de production magmatique sont assez homogènes à l’échelle de l’arc, et varient entre 0,1 et 5,6 km³.ka⁻¹. Ces données montrent que les volcans se sont construits lors de phases d’activité brèves, séparées par des périodes de repos. Il n'y a pas de corrélation apparente entre l'âge des volcans et leur taux d'émission, mais la subduction de la ride de Carnegie pourrait favoriser la genèse du magma et expliquer la densité de volcans quaternaires en Équateur. Les taux d'érosion varient entre 0,01 et 0,14 km³.ka⁻¹. Les volcans les plus jeunes semblent être érodés par des processus d'altération chimique, tandis que les volcans plus anciens sont principalement démantelés par des processus physiques tels que l'abrasion glaciaire et l'activité tectonique. Enfin, nous montrons que la méthode de datation K-Ar réalisées sur la mésostase ou les esquilles de verre des ponces est bien adaptée pour dater les produits volcaniques quaternaires de l'arc équatorien. / The Ecuadorian arc presents a large number of Quaternary volcanoes. Our ninety new K-Ar ages, carried out on products from twenty volcanic edifices, show that the development of the Quaternary arc began about 1 Ma in northern Ecuador and migrated southward since 600 ka. This migration could be induced by recent change of the slab geometry at depth and by the activation of crustal faults. The magmatic productivity rates are rather homogeneous at the arc scale, and range between 0.1 and 5.6 km³.ka⁻¹. These data highlights that volcanoes grew during sporadic activity pulses separated by quiescence periods. There is no clear correlation between the age of the volcanoes and their emission rates, but the subduction of the Carnegie ridge could favor the magma genesis and explained the increase of the volcanic activity in northern Ecuador during the Quaternary. Erosion rates range between 0.01 and 0.14 km³.ka⁻¹. Youngest volcanic edifices seem to be eroded by a chemical alteration process, while the dismantling of older volcanoes is mostly influenced by physical processes such as glacial abrasion and tectonic activity. Finally, we show that K-Ar ages performed on groundmass or pumice glass shards can be successfully applied to Quaternary volcanic products from the Ecuadorian arc.

Explosion structures occur in flows of Grande Ronde Basalt in the study area near Troy, Oregon. Data from nineteen stratigraphic sites indicate that the maximum number of flows that contain explosion structures at any one site is six. In the informally named Troy flow, explosion structures are widespread.

Greenstone belts are dominated by volcanic rocks with lithogeochemical characteristics that reflect a range of possible geodynamic settings. Many analogies with modern tectonic settings have been suggested. Increasing exploration and comprehensive sampling of volcanic rocks in modern oceans provides the unique opportunity to characterize different melt sources from intraoceanic settings. This thesis examines geochemical data from more than 2850 submarine mafic and more than 2200 submarine felsic volcanic rocks, representing a wide range of settings. The results show significant geochemical variability spanning the full range of compositions of volcanic rocks found in ancient greenstone belts. This diversity reflects complex rift and spreading regimes, variations in crustal thickness, dry melting versus wet melting, mantle mixing and crustal contamination. Highly variable melting conditions are thought to be related to mantle heterogeneities, complex mantle flow regimes and short-lived tectonic domains, such as those caused by diffuse spreading, multiple overlapping spreading centers and microplate breakouts. Systematic differences in the volcanic rocks are revealed by a combination of principal components analysis and unsupervised hierarchical clustering. Rocks from most arc-backarc systems have strongly depleted mantle signatures and well-known subduction-related chemistry. This contrasts with rocks in Archean greenstone belts, which show no, or at least weaker, subduction-related chemistry and stronger mantle enrichment resulting from a less-depleted mantle, less wet-melting, and variable crustal contamination. The geochemistry of the modern volcanic rocks reflects lower mantle temperatures, thinner crust and subduction-related processes of present-day settings. However, rocks that are geochemically identical to those in Archean greenstone belts occur in many modern back-arc basins, such as the Lau Basin. Crustal growth and area-age relationships in the Lau Basin are similar to observed ages and compositions of volcanic assemblages in greenstone belts, such as the Blake River Group of the Abitibi Greenstone Belt. Such settings are recognized as favorable locations for volcanogenic massive sulfide (VMS) deposits, and therefore the particular geochemical signatures of the volcanic rocks are important for enhanced area selection in base and precious metal exploration.