Exposed on Vancouver Island, British Columbia, the Jurassic Bonanza arc is believed to represent the southerly continuation of the Talkeetna arc. Small bodies of mafic and ultramafic cumulates within deeper plutonic levels of the arc constrain the fractionation pathways leading from high-MgO basalt to andesite-dacite compositions. The removal of amphibole from the most primitive non-cumulate compositions controls the compositions of mafic plutons and volcanics until the onset of plagioclase crystallization. This removal is accomplished by the intercumulus crystallization of large amphibole oikocrysts in primitive olivine hornblendite cumulates. Experimental hornblende compositions that crystallize from high-MgO basalts similar to primitive basalts from the Bonanza arc show a good correlation between octahedral Al in hornblende and pressure, and provide a means of estimating crystallization pressures during differentiation of primitive arc basalt. Application of an empirical barometer derived from experimental amphibole data (P = Al(6)/0.056 – 0.143; r2 = 0.923) to natural hornblendes from this study suggests that crystallization of primitive basalts took place at 470-880 MPa. Two-pyroxene thermometry gives a result of 1058 +/- 91 ºC for the only olivine hornblendite sample with both pyroxenes. Lever rule calculations require the removal of 30-45 % hornblende from the most primitive basalt compositions to generate basaltic andesite, and a further 48% crystallization of hornblende gabbro to generate dacitic compositions. Hornblende removal is more efficient at generating intermediate compositions than anhydrous gabbroic fractionating assemblages, which require up to 70% crystallization to reach basaltic andesite from similar starting compositions. There are no magmatic analogues to bulk continental crust in the Bonanza arc; no amount of delamination of ultramafic cumulates will push the bulk arc composition to high-Mg# andesite. Garnet removal appears to be a key factor in producing bulk continental crust.

Exposed on Vancouver Island, British Columbia, the Jurassic Bonanza arc is believed to represent the southerly continuation of the Talkeetna arc. Small bodies of mafic and ultramafic cumulates within deeper plutonic levels of the arc constrain the fractionation pathways leading from high-MgO basalt to andesite-dacite compositions. The removal of amphibole from the most primitive non-cumulate compositions controls the compositions of mafic plutons and volcanics until the onset of plagioclase crystallization. This removal is accomplished by the intercumulus crystallization of large amphibole oikocrysts in primitive olivine hornblendite cumulates. Experimental hornblende compositions that crystallize from high-MgO basalts similar to primitive basalts from the Bonanza arc show a good correlation between octahedral Al in hornblende and pressure, and provide a means of estimating crystallization pressures during differentiation of primitive arc basalt. Application of an empirical barometer derived from experimental amphibole data (P = Al(6)/0.056 – 0.143; r2 = 0.923) to natural hornblendes from this study suggests that crystallization of primitive basalts took place at 470-880 MPa. Two-pyroxene thermometry gives a result of 1058 +/- 91 ºC for the only olivine hornblendite sample with both pyroxenes. Lever rule calculations require the removal of 30-45 % hornblende from the most primitive basalt compositions to generate basaltic andesite, and a further 48% crystallization of hornblende gabbro to generate dacitic compositions. Hornblende removal is more efficient at generating intermediate compositions than anhydrous gabbroic fractionating assemblages, which require up to 70% crystallization to reach basaltic andesite from similar starting compositions. There are no magmatic analogues to bulk continental crust in the Bonanza arc; no amount of delamination of ultramafic cumulates will push the bulk arc composition to high-Mg# andesite. Garnet removal appears to be a key factor in producing bulk continental crust.

No description available.

Les laves d'arc ont une géochimie complexe du fait de l'hétérogénéité des magmas primitifs et de leur transformation dans la croûte. L'identification des magmas primitifs dans les arcs continentaux est difficile du fait de l'épaisseur de la croûte continentale, qui constitue un filtre mécanique et chimique à l'ascension des magmas. En Équateur, cette problématique est particulièrement critique du fait de la grande épaisseur de la croûte (≈ 50-60 km) et de la rareté des magmas primitifs arrivant en surface. Cette thèse a pour but de déterminer la composition des liquides primitifs dans l'arc équatorien, à l'échelle de l'arc entier, et à celle de deux édifices volcaniques : le Pichincha et le Tungurahua. Elle vise également à mieux comprendre comment ces liquides primitifs évoluent à travers la croûte continentale. En Équateur, le pendage, le relief et l'âge de la plaque plongeante varient du nord au sud de l'arc. Ainsi, la première partie de la thèse aborde la question de l'influence de ces paramètres sur la géochimie des magmas, via une étude sur roches totales couvrant la totalité de l'arc. Elle confirme les variations géochimiques décrites à travers l'arc par les études précédentes : augmentation de la teneur en éléments incompatibles et diminution de l'enrichissement en éléments "mobiles" d'est en ouest. L'étude identifie des variations géochimiques le long du front volcanique (e.g., rapport Ba/Th), liées au changement de nature des fluides métasomatiques, qui sont aqueux au centre de l'arc (environ 0,5°S) et silicatés au Nord et possiblement au Sud. Ce changement est attribué à la jeunesse du plancher océanique dans le nord de l'arc, qui pourrait promouvoir la fusion de la plaque plongeante. Enfin, il semble que la contamination par la croûte inférieure augmente vers le sud du front volcanique. Dans un deuxième temps, les produits émis par le Tungurahua lors de ses derniers 3000 ans d'activité sont étudiés. À cette échelle de temps, les paramètres tectoniques de la première étude sont constants. Ce travail détaille le rôle de la croûte dans la production des magmas différenciés, qui sont systématiquement associés à des éruptions plus explosives. Elle conclut que les andésites ont des compositions isotopiques hétérogènes (206Pb/204Pb = 18,834 - 19,038), acquises en profondeur (manteau ou croûte inférieure), qui se restreignent lors de la différenciation des andésites en dacites (206Pb/204Pb = 18,965 - 19,030), par cristallisation fractionnée et assimilation de la croûte supérieure locale (7-9 %). Enfin, la troisième partie de la thèse se focalise sur l'hétérogénéité des magmas primitifs. Des études sur minéraux individuels ont été effectuées au Pichincha et au Tungurahua, et montrent que la majorité des minéraux sont en déséquilibre avec la roche hôte (jusqu'à 8600 ppm en 206Pb/204Pb). Au Pichincha, la diversité des minéraux échantillonnés permet d'identifier la diversité des liquides mantelliques (206Pb/204Pb = 18,816 - 19,007), qui s'alignent dans les espaces Pb-Pb. Comme dans le cas des roches totales du Tungurahua, l'assimilation crustale écrase cette diversité isotopique lors de la différenciation des liquides primitifs, dont la signature n'est pas préservée dans les roches. Au Tungurahua, les minéraux individuels montrent que l'hétérogénéité des signatures est acquise en profondeur. L'analyse de deux lots d'olivines met en évidence une signature radiogénique dans les liquides primitifs du Tungurahua, interprétée comme la présence de croûte délaminée dans la source mantellique du Tungurahua. Enfin, l'étude de lots d'olivines provenant de sept volcans équatoriens montre qu'il n'existe pas de signature primitive unique dans l'arc. La totalité de l'hétérogénéité isotopique des magmas est héritée du manteau (206Pb/204Pb = 18,583 - 19,000). Les compositions des liquides primitifs sont ensuite déviées par la contamination crustale, dans la majorité des cas, vers des signatures plus radiogéniques. / Arc lavas display a complex geochemistry resulting from the heterogeneity of primitive magmas and their transformation within the crust. Identifying primitive magma compositions in continental arcs is challenging because continental crust is thick and acts as a mechanical and chemical filter for ascending magmas. This issue is particularly criticial in Ecuador owing to the great thickness of the continental crust (≈ 50-60 km) and the scarcity of erupted primitive magmas. This thesis aims to determine the composition of primitive silicate melts in the Ecuadorian arc, on the scale of the whole arc, as well as on the scale of two volcanic edifices: the Pichincha and the Tungurahua. This study also intends to better understand how those primitive melts evolve during their journey through the continental crust. In Ecuador, slab dip, relief and age change from north to south. Hence, the first part of the PhD focuses on the influence of those parameters on magma geochemistry, through a whole rock study covering the entire arc. It confirms the across-arc geochemical variations described by previous studies: an increase of incompatible element contents and a decrease of fluid-mobile over fluid-immobile element ratios from west to east. We identify along-arc geochemical variations in the volcanic front (e.g. Ba/Th), related to the changing nature of metasomatic fluids, which are aqueous fluids at the centre of the arc (around 0.5°S) and silicate melts to the north and probably to the south. This change may be due to the subduction of a younger and warmer oceanic crust to the north, which might promote slab melting. Lastly, it seems that deep crustal contamination increases towards the south of the volcanic front. Secondly, volcanic products emitted for the last 3,000 years at Tungurahua are studied. On this timescale, the tectonic parameters of the first study are constant. This work details the role of continental crust in the production of differentiated magmas, which are systematically associated with more explosive eruptions. We conclude that andesites have heterogeneous isotopic compositions (206Pb/204Pb = 18.834 - 19.038), acquired at depth (mantle or deep crust), that homogeneize through andesite differentiation to dacite (206Pb/204Pb = 18.965 - 19.030) by fractional crystallization and assimilation of the local upper crust (7-9 %). Lastly, the third part of the PhD focuses on the heterogeneity of primitive magmas. We study individual minerals from Pichincha and Tungurahua volcanoes and show that most minerals are in disequilibrium with their host rock (up to 8,600 ppm for 206Pb/204Pb). The diversity of Pichincha minerals allows the identification of mantle melt diversity (206Pb/204Pb = 18.816 - 19.007), with compositions forming a tight trend in Pb-Pb isotope spaces. As for Tungurahua whole rocks, crustal assimilation erases the diversity of primitive melt isotope signatures through differentiation, so that primitive melt signatures are not preserved in whole rock samples. At Tungurahua, individual minerals show that the heterogeneity of isotope compositions is acquired at depth. The analysis of two olivine fractions reveals the existence of a radiogenic signature in the mantle source of Tungurahua volcano, interpreted as the presence of delaminated crust within the mantle beneath its edifice. Finally, olivine fractions from seven Ecuadorian volcanoes highlight the fact that no unique primitive signature exists in the arc. Isotopic heterogeneity is entirely inherited from the mantle (206Pb/204Pb = 18.583 - 19.000). Primitive melt compositions are then shifted by continental crust contamination which, in most cases, results in more radiogenic signatures.

Please read the abstract in the section 00front of this document / Thesis (DSc)--University of Pretoria, 1966. / Geology / unrestricted

No description available.

The vast distribution and long duration of the Late Mesozoic magmatism in the eastern part of South China presents a unique case in the world. This offers a natural laboratory to study the process of magma genesis, the magma emplacement mode, the relationship between magmatism and tectonics, the geodynamic role on the magma emplacement and lithospheric evolution. Since 50's, particularly 90's of the last century, geoscientists have made important efforts in geological cartography and carried out numerous studies with remarkable scientific achievements, building a solid background to understand the tectonic evolution of the South China Block (SCB). However, certain fundamental questions mentioned above remain unsolved and/or are in hot debate. In order to make progress in these scientific issues, we have carried out in a multi-disciplinary study in the Late Mesozoic Qingyang-Jiuhua massif, Hengshan massif and Fujian coastal zone according to their distance with respect to the paleo subduction zone of the Paleo-Pacific plate, the ages of granitic massifs and related tectonics, including field observation on the structure geology, micro-observation on thin section, U-Pb dating on monazite, AMS, paleomagnetism, gravity modeling and P condition concern the granite emplacement. In the view of deformation in these granitic massifs and their country rocks, mode and influence of regional tectonics on the emplacement, though each studied zone reveals its distinguished characteristics, they show some intrinsic and common relationships between them. With our new results and integrating previous data, in this thesis, we discuss the tectonic context of emplacement of these Late Mesozoic magmatic massifs and the geodynamic evolution of the SCB., We propose a 3-step geodynamic model: (1) during 145-130 Ma period, the Paleo-Pacific plate subducted northwestwardly, the West Philippines micro-continent, approaching to SCB, important subduction-related arc volcanism was produced in the coastal areas of Southeast China coast (Zhejiang-Fujian-Guangdong), forming a back-arc extension tectonic system in SCB; (2) during 130-110 Ma period, due to the collision between the West Philippines microcontinent and SCB, the compressional tectonic structures were developed in the Changle-Na'ao coastal zone, producing ductile deformation zones. However, the inland of the eastern part of SCB was under a NW-SE extensional tectonic regime; (3) during 105-90 Ma period, a new subduction zone was developed in the SE flank of the West Philippines micro-continent, the subducting slab reached the Changle-Nan'ao tectonic belt, with the possible break-off of slab, the asthenospheric ascent was responsible for the important emplacement of plutonic massifs and dykes. The tectonics of the eastern part of SCB was characterized by a general extensional system in this period. This tectonic pattern has been significantly disturbed by the Oligocene-Eocene opening of the South China sea,and the Miocene shortening of the SCB margin in Taiwan. Of course, this model should be improved by more geological, geophysical and geochemical investigations.

No description available.

Despite increasing concern about the presence of heavy metals, pesticides and other toxins in indoor environments, very little is known about the physical and chemical composition of ordinary household dust. This study represents the first systematic investigation of the mineralogical composition of indoor dust in residential housing in Canada. Specimens of dust were obtained from homes in six geographically separate cities in the Province of Ontario: two located on the metamorphic and igneous rocks of the Precambrian Canadian Shield (Thunder Bay and Sudbury), the other four located on Palaeozoic limestone and shale dominated bedrock (Barrie, Burlington, Cambridge, and Hamilton). Forty samples of household vacuum dust were obtained. The coarse fraction (80 – 300 µm) of this dust was subjected to flotation (using water) to separate the organic components (e.g. insect fragments, dander), natural and synthetic materials (e.g. fibres, plastics) from the mineral residue. The mineral fraction was then analyzed using quantitative point counting, polarizing light microscopy, powder X-ray diffraction and scanning electron microscopy methods. Despite the great distances between the sampling localities and the distinct differences in bedrock geology, the mineral fraction of dust from all six cities is remarkably similar and dominated by quartz and feldspar, followed by lithic fragments, calcite, and amphibole. Some evidence of the influence of local geology can nevertheless be found. For example, a relatively higher proportion of sulphide minerals is observed in the two cities on the Canadian Shield where these minerals are clearly more abundant in the bedrock. Specimens from Sudbury, Canada’s largest mining centre located atop a nickel-sulphide mineral deposit, showed the highest sulphide contents. Quartz is the dominant mineral in all cities. All quartz grains have internal strain features and fluid inclusions that are indicative of a metamorphic-igneous provenance. In all cities, sand is used on the streets as an abrasive for traction during the icy winter season. This sand is obtained in all cases from local glaciofluvial deposits that were ultimately derived principally from the rocks of the Canadian Shield in the last Pleistocene glaciations that affected all of Ontario. Thus, tracking in sand is the most plausible mechanism by which quartz was introduced into these homes since sampling was done, in all cases, in the winter season. The results indicate that glacial deposits dominate the mineral composition of indoor dust in Ontario cities and that nature of the bedrock immediately underlying the sampling sites is relatively of minor importance.

Despite increasing concern about the presence of heavy metals, pesticides and other toxins in indoor environments, very little is known about the physical and chemical composition of ordinary household dust. This study represents the first systematic investigation of the mineralogical composition of indoor dust in residential housing in Canada. Specimens of dust were obtained from homes in six geographically separate cities in the Province of Ontario: two located on the metamorphic and igneous rocks of the Precambrian Canadian Shield (Thunder Bay and Sudbury), the other four located on Palaeozoic limestone and shale dominated bedrock (Barrie, Burlington, Cambridge, and Hamilton). Forty samples of household vacuum dust were obtained. The coarse fraction (80 – 300 µm) of this dust was subjected to flotation (using water) to separate the organic components (e.g. insect fragments, dander), natural and synthetic materials (e.g. fibres, plastics) from the mineral residue. The mineral fraction was then analyzed using quantitative point counting, polarizing light microscopy, powder X-ray diffraction and scanning electron microscopy methods. Despite the great distances between the sampling localities and the distinct differences in bedrock geology, the mineral fraction of dust from all six cities is remarkably similar and dominated by quartz and feldspar, followed by lithic fragments, calcite, and amphibole. Some evidence of the influence of local geology can nevertheless be found. For example, a relatively higher proportion of sulphide minerals is observed in the two cities on the Canadian Shield where these minerals are clearly more abundant in the bedrock. Specimens from Sudbury, Canada’s largest mining centre located atop a nickel-sulphide mineral deposit, showed the highest sulphide contents. Quartz is the dominant mineral in all cities. All quartz grains have internal strain features and fluid inclusions that are indicative of a metamorphic-igneous provenance. In all cities, sand is used on the streets as an abrasive for traction during the icy winter season. This sand is obtained in all cases from local glaciofluvial deposits that were ultimately derived principally from the rocks of the Canadian Shield in the last Pleistocene glaciations that affected all of Ontario. Thus, tracking in sand is the most plausible mechanism by which quartz was introduced into these homes since sampling was done, in all cases, in the winter season. The results indicate that glacial deposits dominate the mineral composition of indoor dust in Ontario cities and that nature of the bedrock immediately underlying the sampling sites is relatively of minor importance.