School of Earth Sciences - Theses
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ItemAustralian lineament tectonics: with an emphasis on northwestern Australia( 1994-08)Australia is transected by a network of systematic continental-scale lineaments that are considered to be zones of concentrated, aligned tectonic activity which have apparent continuity over vast distances. The influence of lineaments on the rock record can be identified in many types of data-sets, and existing data reveals previously undescribed basement influences. Several continental-scale lineaments can be traced offshore with apparent continuity for hundreds to thousands of kilometres, two of which are seen to cross the Tasman Sea in offshore eastern Australia. Geological and chronological evidence demonstrates that many of the lineaments have been zones of reactivation since at least the Early Proterozoic (- 1880 Ma) and that they appear to cross major terrane boundaries. Alternative models for their origin are a) a pre-existing lineament network maintained in an ancient basement underlying the entire continent; b) lateral propagation of crustal-scale structures; c) alignment of genetically unrelated lineaments giving the appearance of continuity. Australian deep-seismic profiles show that continental-scale lineaments are zones of crustal-scale structure which in some cases transect the crust-mantle boundary. Lineaments demonstrate many faulting styles, e.g. listric extensional (G3), planar moderate-angle thrusts (G2 l), and sub-vertical thrusts (G 17). In some cases the structural style varies laterally along the length of the lineament. (For complete abstract open document)
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ItemThe geochronology and thermochronology of the Brockman Creek 01, Melita 01 and Melita 02 kimberlites, Western Australia( 2000-10)Due to the difficulties inherent in dating kimberlites, two independent methods are employed to confirm the unusually old ages of approximately 2 Ga for three occurrences in Western Australia. Phlogopite micas from three kimberlites were dated using 40Ar/39Ar and 87Rb/86Sr dating techniques in order to provide an estimate of the intrusion ages. The high reliability of the age estimates is indicated by the similar results produced by the 87Rb/86Sr method. Argon loss is suspected for phlogopite samples, resulting in the apparently reliable 40Ar/39Ar total fusion ages that do not correspond to previously determined emplacement ages. (For complete abstract open document)
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ItemThe geochemistry and petrology of the Enterprise dolerite, Ora Banda, Western Australia( 1998)The Enterprise Dolerite was emplaced as an intrusive tholeiitic sill within the Ora Banda Sequence at Ora Banda in the Eastern Goldfields Province of the Yilgarn Craton. The Enterprise Dolerite is now a metamorphic body with modifications in both the mineralogy and geochemistry of the rocks. Careful analysis of petrographic features integrated with geochemical trends have made it possible to interpret the original igneous characteristics of the sill. It is proposed here that the order of crystallisation in the Enterprise Dolerite is plagioclaseolivine- clinopyroxene-quartz. Furthermore, plagioclase and olivine accumulated through crystal settling before a switch to in-situ crystallisation in the remainder of the sill. The bulk chemistry of the Enterprise Dolerite is equivalent to that of the Mt Ellis Sill which occurs at the same stratigraphic position, and it is proposed here that they are continuations of the same intrusive body. This intrusive body is related to the other mafic members of the Ora Banda Sequence, with all members forming a differentiation trend and in which the Big Dick Basalt represents a primary mantle magma. The Enterprise Dolerite/Mt Ellis Sill has evolved in composition along the trend from this primary magma. Finally, the addition and removal of phases has produced a chemically evolving system with differentiation progressing to maxima in silica and iron concentrations which provide very good conditions tor gold deposition. This study proposes that both the Enterprise Dolerite and the Mt Ellis Sill be examined for future potential gold mineralisation.
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ItemThe thermotectonic evolution of the northern precambrian shield, Western Australia( 2002)Reconnaissance 40Ar/39Ar dating of K-feldspars and muscovites, and fission track and (U-Th)/He thermochronometry on apatites has been applied to ~1 X 106 km2 of the northern Western Australian Shield. The study area includes the Archaean northern Yilgarn Craton and Pilbara Craton, and the intervening Proterozoic basins. The geomorphology of the study area also reveals a subdued relief, which is characteristic for Australian cratonic landscapes. The fact that the shield consists entirely of Precambrian rocks with low topographic relief has led to the widely held view that the West Australian Shield has been tectonically stable for a long period of time. (From Abstract)
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ItemGarnet-bearing metabasic rocks at Mount Joel: an investigation into distribution, petrology and equilibrium thermodynamic modelling( 1998)Garnet-bearing metabasic rocks at Mount Joel, Yilgarn craton, Western Australia, have been studied to determine their distribution, petrography and mineral equilibria. At depth, the orientation of garnet-bearing rocks is approximately 340°N, dipping 60°-70° to the east and mimics that of chloritoid schist and gold mineralisation. Three mineral assemblages at Mount Joel can contain garnet, including: chloritoid-chlorite-plagioclase-quartz-garnet; chlorite-plagioclase-quartz-garnet; chlorite-hornblende-plagioclase-quartz-garnet. Garnets are manganese-rich, composed of up to 23% spessartine. Bulk rock analysis suggests a correlation between manganese enrichment and the appearance of garnet in mineral assemblages. The chemical relationships are consistent with the garnet-bearing rocks being formed from altered basaltic rocks. Thermodynamic calculations have been undertaken using an internally consistent thermodynamic dataset (Powell and Holland, 1990) and THERMOCALC v2.5. Phase diagrams, including Pressure-Temperature (P-T) Projections, P-T Pseudosections and Temperature-Composition (T-X) Pseudosections, have been used to model the mineral equilibria for FeO-MgO-Al2O3-SiO2-H2O (FMASH), Mn-FeO-MgO-Al2O3-SiO2-H2O (MnFMASH)and CaO-Na2O-MnO-FeO-MgO-Al203-Sí02-H2O (CaNaMnFMASH) systems. Upon addition of manganese to a garnet-free system (FMASH), garnet becomes introduced as a new stable phase. As a result, garnet can be present in low pressure and temperature metabasic rocks, such as those at Mount Joel. The variety of mineral assemblages in garnet-bearing rocks at Mount Joel reflects a range in mineral chemistry of the metabasic rocks, possibly due to a range of alteration processes affecting these rocks. The pressure and temperature conditions of formation of garnet-bearing metabasic rocks at Mount Joel have been constrained to about 510 °C at about 3 kbars.