Note:

München, Univ., Diss., 2001. / Computerdatei im Fernzugriff.

München, Univ., Diss., 2001. / Computerdatei im Fernzugriff.

Ocean Islands Basalts provide important windows into the compositional variations of the Earth's mantle, which in tum constrains models for mantle convection and evolution. The Canary Islands show contrasting styles of eruption and evolution of magmas in an ocean island setting. U-Th-Ra disequilibrium have been used to constrain rates and timescales of melt generation and differentiation beneath ocean islands, and to estimate the buoyancy flux, potential mantle temperature and the depth and degree of melting. The Canary Islands provide a rare opportunity to observe U-Th-Ra disequilibrium, because they are underlain by a region of low buoyancy flux, and were expected to show significant disequilibrium. Tenerife is underlain by numerous magma chambers, in which magmas have time to differentiate from basanites to phonolites, erupting to form large strato-volcano complexes. The fissure and small vent eruptions of unusually primitive basanites and alkali basalts from Lanzarote show little evidence of magma chambers, unless of substantial size and longevity at depth. The U-Th results indicate that lavas underwent rapid transport from the melt region. The historic and recent pre-historic eruptions (1824, 1730-36, Corona) from Lanzarote have some of the most primitive compositions found on oceanic islands with low SiO² contents (< 51 %), Mg numbers of 67-74 and high Cr and Ni contents. The rocks are restricted in Sr, Nd and Pb isotopes, being displaced from MORB towards the HIMU om field. The major and trace elements have been modelled by mixing a deep smaller degree (1 %) melt and a shallower larger degree (4%) melt. Negative K anomalies were observed in the small degree melts indicating that melt generation may have continued at a shallow level, perhaps to within the lithospheric mantle with melting in the presence of residual phlogopite. The Lanzarote source was modelled as a mixture of HIMU and EMIl asthenospheric mantle, with a small contribution from a shallow, lithospheric source. Thermal erosion of the lithospheric mantle is required for melting at depths (58 and 73 km) modelled from the major and trace elements. The Lanzarote lavas exhibit significant (²³⁰Th/²³⁸U) disequilibrium with ²³⁰Th excesses of 6 - 81 %. This was modelled by dynamic melting giving a calculated melt rate of 0.125 x 10^-3 kg.m^-1.yr^-1, a timescale of melt generation (matrix transfer time) of 270 ka for the 1 % melt and 1,100 ka for the 4 % melt. A consistent upwelling rate of I cm.yr^-1 and an assumption that the melting process has remained consistent over tens of km at depth. The Teide-Pico Viejo complex lavas have undergone fractionation and mixing to form compositions from basanite to phonolite. Crystallising phases differ in the Pico Viejo series, where amphibole is dominant in the more evolved lavas, and Pico Teide series, where olivine in the major control. The more evolved lavas require assimilation and fractional crystallisation to explain the range in ⁸⁷Sr/⁸⁶Sr. (²³⁰Th/²³⁸U) ranges from 1.004-1.39 and gives information regarding the timescales of differentiation within the magma chambers, not least because the youngest mafic rocks have the highest (²³⁰Th/²³⁸U) and the most evolved phonolites have the lowest. The timescale of differentiation from basanite to phonolite is of the order of 150,000 years, which links to the periodicity of the eruption cycles on the island. A Ra-Th 'pseudo' whole rock isochron gave an age of fractionation for the Montafia Blanca eruption of 2.3 ka ± 80, which is a maximum of 300 years prior to eruption, indicating that fractionation of plagioclase as a possible trigger of an eruption.

L'objectif de cette thèse est l'étude de l'association magnétite-skarn-magmatisme. Dans ce but, deux gisements de magnétite (Yamansu et Cihai) ont été sélectionnés dans l'Est du Tianshan (NW Chine). Dans les deux gisements, l'assemblage grenat-pyroxène-magnétite a été reconnu, mais les mécanismes de formation de la minéralisation y sont différents. Cet assemblage a ensuite été alteré par un stade hydrothermal. Les études minéralogiques et géochimiques ainsi que les zonations observées suggèrent, que dans le gisement de Yamansu (YMD), le skarn et la minéralisation sont liés à la circulation d'un fluide riche en fer; alors que pour le gisement de Cihai (CMD), la mise en place du minerai est probablement associée à un magmatisme mafique/ultramafique. A YMD, selon l'anisotropie de la susceptibilité magnétique (AMS) et une étude paléomagnétique, le basalte et le skarn massif à grenat, tout deux encaissant les corps à magnétite massive, étaient en position horizontale lors de leur formation. L'attitude actuelle de ce gisement serait donc due à la phase tectonique majeure. La magnétite dans CMD présente les caractéristiques d'une magnetite magmatique, plutôt qu'hydrothermale. Dans ce gisement, la contamination du magma par des formations sédimentaires a pu contribuer à séparer efficacement un liquide riche en oxyde de fer d'un bain silicaté. / The aim of this thesis was to investigate the magnetite-skarn-magmatism association, of which two study objects (Yamansu and Cihai magnetite deposits) were selected from eastern Tianshan, China. Both of the Yamansu and Cihai magnetite deposit illustrate garnet-pyroxene-magnetite assemblage in prograde stage and hydrothermal assemblage in retrograde stage which essentially defined as skarn-related deposit, but with different forming mechanisms. Yamansu magnetite deposit (YMD) is a typical metasomatic skarn deposit confirmed by the mineral assemblage and chemical composition, zonation etc., whereas to Cihai magnetite deposit (CMD) probably associated to sedimentary assimilation into mafic/ultramafic magma and fractionation. According to anisotropy of magnetic susceptibility (AMS) and paleomagnetic study, the basalt and the massive garnet skarn which clamp massive magnetite ore bodies were coincidently horizontal in YMD. In addition, it was the Fe-rich fluid caused extensive skarnization in YMD. The magnetite in CMD is magmatic origin, rather than hydrothermal according to mineralogy and geochemical study. External material contamination of the mafic/ultramafic magma probably efficiently separated the magnetite and silicate at the peripheral part of the large mafic/ultramafic pluton in high temperature.

No description available.

Elizabeth Castle forms part of the South-East Granite Complex of Jersey, Channel Islands and is one of several multi-magma complexes in the region. The rocks have calc-alkaline signatures indicative of a subduction zone setting. In the western half of the Elizabeth Castle complex, the outcrops are wholly granophyre, while to the east, granophyre and minor monzogranite are intimately associated with diorite. The dioritic rocks form part of a layered series which is preserved at several localities. The layered diorites were initially intruded by multiple sub-horizontal granitic sheets. All contacts between the diorite and the granitic sheets are crenulate, indicating that the two were present as coexisting magmas. Fine-grained, dark margins in the diorites contain quench textures such as spherulitic plagioclase and acicular apatite, and are interpreted as chilled margins. At many contacts a narrow tonalitic marginal zone, with acicular amphiboles, is present. Field relationships suggest that this is a hybrid produced by interaction between coexisting dioritic and granitic magmas and this is confirmed by modelling based on geochemical data. It is proposed that within the marginal zones the presence of volatile-rich fluids, increased temperatures and a decrease in viscosity promoted chemical diffusion across the dioritegranite interface. The transfer of elements, together with the presence of volatiles, promoted the growth of hydrous mafic phases and suppressed crystallization of alkali feldspar. At the same time, fluid infiltration modified the composition of the dioritic magma. Field evidence indicates that these processes took place in a narrow time frame prior to further granitic intrusion. Parts of the sheeted complex were extensively disrupted by the later granitic intrusions, producing large areas rich in dioritic enclaves. Within these disrupted areas a grey inhomogeneous rock is encountered. Field and petrographic evidence suggest that this is a hybrid rock produced by the physical mixing of dioritic and granitic magmas. Linear chemical trends confirm this interpretation. Minor intrusions comprising red granite dykes, basic dykes, composite dykes and aplite sheets cut the complex.

No description available.

No description available.

No description available.