Thesis (M.Sc. Earth and Ocean Sciences)--University of Waikato, 2007. / Title from PDF cover (viewed February 25, 2008) Includes fold out pages. Includes bibliographical references (p. 153-160)

Volcanism associated with the onset of Karoo flood basalt eruptions (c. 180 Ma) at Sterkspruit, South Africa, began with emplacement of thin lava flows before abruptly switching to phreatomagmatic and magmatic activity that formed a nest of craters, spatter and tuff rings and cones that collectively comprise a crater complex >40 km� filled by 9-18 km� of volcaniclastic debris. Phreatomagmatic activity driven by interaction of Karoo magma with groundwater hosted in country rock and crater-filling debris quarried broad, mainly shallow craters (hundreds of metres, but not kilometers deep) into wall-rock. Closely spaced individual vents, the consequence of magma emplaced over a broad area through a network of feeder dikes and stocks, were active at the same time or over short periods of time. Highly ephemeral access of external water to vents drove repeated and reversible switches between explosive to effusive magmatic and explosive phreatomagmatic activity, resulting in vents and craters that grew laterally and vertically into adjacent ones through quarrying and vent migration. Deposits within the Sterkspruit crater complex are dominated by 7-15 km� of massive, unsorted polymict lapilli tuff and tuff breccia juxtaposed with localised fountain-fed lava and strombolian spatter deposits. Transport within the complex was dominated by jets and fountains of volcaniclastic debris and by mass movement. Country-rock breccias indicate that craters grew via a combination of mechanical fragmentation, granulation and mass-movement of 7-12 km� of wall-rock, adding mass and previously locked-up pore-water to the volcanic system. Ash and lapilli, the deposits of plumes 5-15 km high, form a 50-110 m-thick ejecta blanket mantling Clarens Formation country rock that thins gradually away from the crater-complex margins. Explosive volcanism was succeeded by brief fluvial and eolian reworking of volcaniclastic debris and formation of a shallow crater lake 12 km� in extent, and then by voluminous effusion of flood basalt that inundated the Sterkspruit crater complex with lava. Flood basalt magmas involved in Sterkspruit eruptions were chemically heterogenous. This study documents the rapid (perhaps simultaneous) eruption of multiple, chemically distinct basaltic magmas, which cannot be simply related to one another, from one vent site, and possibly many others, within the Sterkspruit crater complex. Five distinct magma types were involved in eruptions at Sterkspruit, indicating that in the early stages of flood basalt eruption (i) magma batches may be small and not simply related to one another, (ii) heterogeneities in the magma source region may be close to each other in time and space, and (iii) eruptions of chemically distinct magmas may take place over short intervals of space and time without significant hybridisation. Formation of the Sterkspruit Complex, and many others like it in South Africa, confirms that the opening phases of Karoo flood basalt volcanism were explosive, and that the volume of the products of explosive volcanism may have important implications for climate change and landscape development associated with the emplacement of large igneous provinces.

Volcanism associated with the onset of Karoo flood basalt eruptions (c. 180 Ma) at Sterkspruit, South Africa, began with emplacement of thin lava flows before abruptly switching to phreatomagmatic and magmatic activity that formed a nest of craters, spatter and tuff rings and cones that collectively comprise a crater complex >40 km� filled by 9-18 km� of volcaniclastic debris. Phreatomagmatic activity driven by interaction of Karoo magma with groundwater hosted in country rock and crater-filling debris quarried broad, mainly shallow craters (hundreds of metres, but not kilometers deep) into wall-rock. Closely spaced individual vents, the consequence of magma emplaced over a broad area through a network of feeder dikes and stocks, were active at the same time or over short periods of time. Highly ephemeral access of external water to vents drove repeated and reversible switches between explosive to effusive magmatic and explosive phreatomagmatic activity, resulting in vents and craters that grew laterally and vertically into adjacent ones through quarrying and vent migration. Deposits within the Sterkspruit crater complex are dominated by 7-15 km� of massive, unsorted polymict lapilli tuff and tuff breccia juxtaposed with localised fountain-fed lava and strombolian spatter deposits. Transport within the complex was dominated by jets and fountains of volcaniclastic debris and by mass movement. Country-rock breccias indicate that craters grew via a combination of mechanical fragmentation, granulation and mass-movement of 7-12 km� of wall-rock, adding mass and previously locked-up pore-water to the volcanic system. Ash and lapilli, the deposits of plumes 5-15 km high, form a 50-110 m-thick ejecta blanket mantling Clarens Formation country rock that thins gradually away from the crater-complex margins. Explosive volcanism was succeeded by brief fluvial and eolian reworking of volcaniclastic debris and formation of a shallow crater lake 12 km� in extent, and then by voluminous effusion of flood basalt that inundated the Sterkspruit crater complex with lava. Flood basalt magmas involved in Sterkspruit eruptions were chemically heterogenous. This study documents the rapid (perhaps simultaneous) eruption of multiple, chemically distinct basaltic magmas, which cannot be simply related to one another, from one vent site, and possibly many others, within the Sterkspruit crater complex. Five distinct magma types were involved in eruptions at Sterkspruit, indicating that in the early stages of flood basalt eruption (i) magma batches may be small and not simply related to one another, (ii) heterogeneities in the magma source region may be close to each other in time and space, and (iii) eruptions of chemically distinct magmas may take place over short intervals of space and time without significant hybridisation. Formation of the Sterkspruit Complex, and many others like it in South Africa, confirms that the opening phases of Karoo flood basalt volcanism were explosive, and that the volume of the products of explosive volcanism may have important implications for climate change and landscape development associated with the emplacement of large igneous provinces.

No description available.

Thèse (D.Ress.Min.) -- Université du Québec à Chicoutimi, programme extensionné à l'Université du Québec à Montréal, 2003. / Bibliogr.: f. 267-278. Document électronique également accessible en format PDF. CaQCU

The Strawberry Volcanics of Northeast Oregon are a group of geochemically related lavas with a diverse chemical range (basalt to rhyolite) that erupted between 16.2 and 12.5 Ma and co-erupted with the large, (~200,000 km3) Middle Miocene tholeiitic lavas of the Columbia River Basalt Group (CRBG), which erupted near and geographically surround the Strawberry Volcanics. The rhyolitic lavas of the Strawberry Volcanics produced the oldest 40Ar/39Ar ages measured in this study with ages ranging from 16.2 Ma to 14.6 Ma, and have an estimated total erupted volume of 100 km3. The mafic and intermediate lavas of the Strawberry Volcanics include both tholeiitic and calc-alkaline compositions; calc-alkaline andesite is the dominant type by volume. 40Ar/39Ar ages of the mafic and intermediate lava flows range from 15.6 Ma to 12.5 Ma, and volume estimates of the intermediate lavas are approximately 1,100 km3. The magmas that gave rise to the Strawberry Volcanics traveled to the surface through numerous dikes, some of which have been exposed at the surface and supplied lava to fissure – style eruptions and/or shield volcanoes. Herein, we show that the Strawberry Volcanics are related to the CRBG in both time and space and share a chemical affinity, specifically to the Steens Basalt. Chemical similarities are observed in normalized trace element patterns, selected trace element ratios, and radiogenic isotopes. Comparison of the Strawberry Volcanic rhyolites to the other Middle Miocene rhyolites of eastern Oregon associated with the initiation of the Yellowstone – Snake River mantle plume reveals similar eruption ages, trace element compositions, including the rare earth elements (REEs), and "A-type" rhyolite characteristics. These data suggest that the Strawberry Volcanics are part of the regional volcanism (basalt to rhyolite) of the Columbia River Basalt Province. The petrogenesis of the Strawberry Volcanics can be explained as follows: 1) The tholeiitic, intermediate magmas were produced by fractional crystallization of mafic magmas, which have a commonality with the surrounding Columbia River Basalt Group; 2) The calc-alkaline magmas are a result of mixing between tholeiitic basalt, rhyolite, and crust. The arc-like signature of the calc-alkaline lavas (elevated large ion lithophiles) is a result of both the melting source region and the end-members with which the mafic magmas mixed/contaminated. Other authors have produced similar findings from within the Basin and Range/Oregon-Idaho graben and CRB province. The difference at the Strawberry Volcanics is that there is no need for a primitive calc-alkaline magma or extensive fractional crystallization to generate the calc-alkaline andesites. Alternatively, the calc-alkaline magmas of the Strawberry Volcanics were generated by a more primitive tholeiitic magma than erupted at the surface, which interacted with crustal melts and assimilated crustal lithologies from the complex zone of assimilated terranes that make up the basement of eastern Oregon.

No description available.

Mémoire (M.Sc.T.)-- Université du Québec à Chicoutimi, 1991. / Bibliogr.: f. 83-86. Document électronique également accessible en format PDF. CaQCU

Mémoire (M.Sc.A.Geologie)--Université du Québec à Chicoutimi, 1985. / Une carte pliée, en pochette. "Mémoire présenté en vue de l'obtention de la maîtrise en sciences appliquées en géologie" CaQCU CaQCU Document électronique également accessible en format PDF. CaQCU

Mémoire (M.Sc.T.)--Université du Québec à Chicoutimi, 1991. / Document électronique également accessible en format PDF. CaQCU