Gabbroic rocks in sills commonly display well organized magnetic fabrics that are generally attributed to magma flow. Yet in a number of cases, magnetic fabrics that are considered proxies for flow fabrics, form at a high angle to the intrusive - host rock wall. A symmetrical oblique magnetic fabric observed in opposite margins of a dike has been interpreted as the result of viscous drag against the margin, a model referred to as fabric imbrication. In this study tested if a similar viscous drag model could also apply to sills. The Karoo Large Igneous Province (LIP) of South Africa offers unparalleled opportunities to investigate fabrics, internal zonation and magma flow in mafic sills of thicknesses ranging from 5 to 50 m. Exceptionally; Karoo sills can reach a 1000 m in thickness (e.g., Insizwa). Geochemical studies suggest that the Karoo gabbro sills have a consistent tholeiitic magma composition across the entire Karoo Basin, especially as far as major elements are concerned. It is also practically undeformed and upon initial inspection unweathered, making it a well suited natural laboratory to test fabric models in gabbro sills. Magnetic fabrics are generally considered a good proxy for magmatic fabrics. The anisotropy of magnetic susceptibility has been measured on oriented hand specimens collected across vertical profiles through two representative sills. Scalar parameters such as magnetic susceptibility, degree of anisotropy and shape factor show variations with height in the intrusion. The variations in these parameters can be interpreted in terms of magmatic dynamic and static processes such as, for example, thermal convection, gravitational crystal settling or magmatic mush compaction. Directional parameters provide additional constrains on the dynamic vs. static nature of fabric-forming processes. Petrographic studies indicate that the primary carrier of the magnetic properties of the Karoo gabbros is titanomagnetite. Accessorily, pyrrhotite and hematite are present, but not in sufficient quantities to account for more than 1% of the sampled intrusions magnetic properties. Hydrous phyllosilicates are present in one sample, suggesting some alteration has occurred at the upper contact of the Country Club Road sill. Magnetic fabrics of the studied sills reveal complex susceptibility and remanence fabrics as a function of stratigraphic height. The proposed fabric development model does not sufficiently describe the observations, thus these fabrics were formed by a combination of mechanisms, both early and late in the cooling history of the intrusions. The observed magnetic fabrics display properties consistent with both the fabric imbrication model and the proposed viscous shear model. It is possible that crystal settling, melt segregation and possibly convection processes also were likely integral to the formation of the magnetic fabrics of the studied Karoo sills, though further studies are necessary.

The ~132 Ma Paraná-Etendeka Large Igneous Province consists of an extensive succession of bimodal flood volcanic rocks and intrusions across Brazil, Paraguay, Uruguay, Argentina, Namibia and southern Angola. This magmatism has been attributed to the impact of the Tristan mantle plume and the associated opening of the South Atlantic Ocean, during the Early Cretaceous breakup of West Gondwana. In southern Africa, this is preserved as the Etendeka flood volcanic succession and dyke suites, and the Damaraland Intrusive Suite, a series of subvolcanic intrusions that occur in a north-east-trending band within the Neoproterozoic Damara Orogenic Belt. The Doros Gabbroic Complex is a relatively small (~3.5 km x 7.5 km) layered mafic intrusion that forms part of the Damaraland Suite in north-western Namibia. It is hosted by deformed Damaran metaturbidites, Damaran granitoids and Karoo-age metasedimentary strata in the Southern Kaoko Zone of the Damara Belt. The intrusion is a shallow lopolith, with an estimated thickness of at least 500 m, which consists of a sequence of roughlyconcordant, sill-like gabbro layers, dipping in towards the centre, cross-cut by dolerite and bostonite dykes. It is undeformed and unmetamorphosed. The fundamental mineralogy is essentially the same throughout the main body of the intrusion (plagioclase + calcic clinopyroxene + oxy-exsolved Fe-Ti oxides ± olivine). However, variations in the modal proportions of these minerals, and in the mineral and rock textures and compositions, define a series of layers. The stratigraphy broadly comprises: 1) a fine-grained gabbroic sill with chilled margin, present mostly as subcrop; 2) a sequence of massive, olivine-cumulate melagabbros (the Lower Zone), with a basal chilled margin, that form the outer ring of the complex; 3) a massive, plagioclase-cumulate olivine gabbro (the Intermediate Zone), comprising the inner ring of the complex; and 4) a sequence of variable, strongly foliated, plagioclase-, olivine- or magnetitecumulate gabbros (the Upper Foliated Series), in the core of the complex. A syenitic (bostonite) phase occurs as cross-cutting dykes or enclaves within the gabbros. In this thesis, I present a detailed petrographic study, an extensive set of whole-rock and mineral major and trace element analyses, whole-rock Sr-, Nd- and Pb-isotopic analyses, melt inclusion analyses and aeromagnetic data for the Doros intrusion. Interpretation of these data, combined with major element and trace element modelling, shows that the stratigraphic order of appearance of cumulus minerals and overall trends in rock compositions in the Doros intrusion are consistent with the fractional crystallisation of olivine, clinopyroxene, plagioclase, magnetite, K-feldspar and apatite, accompanied by variable degrees of accumulation, from a basaltic parental magma. From this combined evidence, I have put forward a compelling argument in favour of an origin for the Doros Complex by multiple, closely-spaced influxes of crystal-bearing magmas (magma mushes). This evidence includes intrusive layer relations, textural evidence for primocrysts, disequilibrium features, and stratigraphic reversals in mineral and whole-rock chemistry and magnetic properties. At least seven distinct major injections of magma have been identified in the stratigraphy, as well as several smaller pulses. Based on this interpretation, a comprehensive emplacement history for the Doros intrusion has been compiled. These findings represent a departure from the traditional model for the formation of such intrusions by the postemplacement differentiation of a single batch of crystal-free melt, and have fundamental implications for our understanding of layered mafic intrusions in general, which I elaborate on. The Doros suite shows depleted ɛNd of +3.67 to +6.46, moderate initial 87Sr/86Sr = 0.703970 - 0.709525, 206Pb/204Pb = 18.17 - 18.47, 207Pb/204Pb = 15.55 - 15.59 and 208Pb/204Pb = 37.93 - 38.42, at 132 Ma. The clustering of isotopic data and trends in incompatible trace element ratios indicate that all the magmas in the complex were derived from the same mantle source. I demonstrate, with the assistance of quantitative isotopic modelling, that this source comprised a dominant depleted MORB-like mantle component (60 - 80%), with a significant Tristan plume-derived, asthenospheric component (20 - 40%) and a minor contribution likely from recycled oceanic crust (< 5%). I further demonstrate that chilled margin to the complex is the only rock type that shows significant evidence of crustal contamination, due to local assimilation of the Damaran host rock on emplacement. In addition, this research shows that the Doros suite has strong geochemical affinities with the Tafelkop group “ferropicrite” lavas of the Etendeka Province. Common features include their near-Bulk Silicate Earth initial Sr ratios, low positive εNd, trends in 207Pb-space and relatively low heavy rare earth element concentrations. Furthermore, trace element modelling of the Doros cumulates from their whole-rock chemistry suggests primary liquid compositions remarkably similar to the Tafelkop magma type. This provides crucial evidence in support of Doros as the eruptive site for the Tafelkop lavas, and thereby links the Doros magmatism to the earliest eruptive phase in the Paraná-Etendeka event. The distinctive chemistry of this magma group is attributed to the early, relatively deep, decompression melting of pyroxenitic material in the heterogeneous Tristan plume starting head, corresponding with the initial impact of the plume on the base of the lithosphere.

<p> A gabbroic intrusion within the Goulburn group of sediments near the west margin of Bathurst trench was studied and mapped using a TV-1 scintillometer. Petrographic examination of the sill was carried out and geochemical whole rock and trace element data were obtained using X.R.F. methods. </p> <p> The 1308 Lake sill was injected conformably between the Western River (argillite/greywacke) and Burnside River (pink quartzite) units of the Goulburn group of sediments. The sill subsequently underwent minor fractional crystallization and differentiation. Generally the sill exhibits poor phase layering, but can be divided into six basic units. </p> 1) Upper Chilled Margin Gabbro 2) Leuco-Micro-Syenite Lens 3) Diabasic Gabbro 4) Pyroxene Granophyre 5) Diabasic Gabbro 6) Basal Sheared Chilled Margin Gabbro These units are evident in this section and are distinguishable both modally and texturally. In most cases, variations in the chemistry and norms reflect the units mapped in the field. Comparisons have been drawn and theories incorporated from other gabbroic intrusions to help explain some features of the 1308 Lake sill. </p> / Thesis / Bachelor of Arts (BA)

No description available.

Mid-ocean ridge hydrothermal systems are known to extend to deep levels of the oceanic crust, including the plutonic section, but little is known about the timing and nature of fluid-rock interactions at these levels. To investigate the temporal and spatial characteristics of hydrothermal alteration in the lower crust, this study investigates a suite of hydrothermally altered (<5 to >20% hydrous alteration) gabbroic rocks recovered from the Hess Deep Rift, where 1.2 Ma fast-spreading East Pacific Rise crust is well-exposed. These samples were altered to amphibole-dominated assemblages with chlorite-rich samples occurring in a restricted region of the field area. Hornfels, indicative of reheated, previously altered rocks, are clustered in the central part of the field area. The entire sample suite has elevated 87Sr/86Sr (mean: 0.70257±0.00007 (2σ), n=16) with respect to fresh rock (0.7024). Bulk rock 87Sr/86Sr is strongly correlated with percentage of hydrous alteration and weakly correlated with bulk rock Sr content. The distribution of Sr in igneous and metamorphic minerals suggests that greenshist-facies alteration assemblages (chlorite, actinolitic amphibole, albitic plagioclase) lose Sr to the fluid while amphibolite-facies secondary assemblages (secondary hornblende, anorthitic plagioclase) take up Sr. The temperature-dependent mobilization of Sr in hydrothermal systems has implications for the 87Sr/86Sr and ultimately fluid/rock ratio calculations based on the assessed 87Sr/86Sr systematics. Considering Sr behaviour, minimum fluid/rock ratios of ~1 were calculated for the plutonic section. Due to the large uncertainty regarding fluid Sr composition at depth and the sensitivity of fluid/rock ratio calculations on this parameter, a model combining the sheeted dike complex and the plutonic section to one hydrothermal system is introduced, yielding a fluid/rock ratio of 0.5. This value may be more realistic since the fluid composition entering and exiting the sheeted dike complex is better constrained. The regional distribution of hornfelsed material with elevated 87Sr/86Sr suggests that fluid ingress into the upper plutonics at Hess Deep occurred on-axis in a dynamic interface of a vertically migrating axial magma chamber (AMC) and the base of the hydrothermal system. / Graduate