School of Earth Sciences - Theses
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ItemGeology and tectonothermal history of The Fishery Bay Region, Eyre Peninsula, South Australia( 1998)The Fishery Bay region, southern Eyre Peninsula, South Australia, consists of Archaean charnockitic and paragneissic sequences of the Sleaford Complex intruded by Palaeoproterozoic granitoids and two generations of mafic dykes. These rocks preserve the deformational and metamorphic effects of the Kimban Orogeny and the later Wartakan Event. Within the Fishery Bay area, five separate ductile deformation events (D1-D5) are recognised, the dominant of which (D2-D3) are associated with granulite facies metamorphism. The effects of the D3 event are pervasive throughout the Fishery Bay region, with D1 and D2 preserved only in regions of low-D3 strain. The overprinting nature or D3 is recognised in the reorientation of D2 structures. The dominant response of the area to D3 strain is a series of westerly-dipping dextral oblique reverse shears with west block-up movement. Much of the strain is localised within the paragneisses and along the margins of mafic dykes recognised in the development of a NNE-trending D3 high-strain zone termed the Cape Wiles Shear Zone. D3 observations from the Fishery Bay region correlate well with previous studies conducted on southern Eyre Peninsula which lead to the inference that D3 west block-up exhumation is responsible for the positive pressure gradient that exists from west to east across the Kalinjala Shear Zone. The pressure-temperature conditions preserved in the mineral assemblages of the paragneiss units and mafic dykes record two granulite facies metamorphic events, M2 and M3. Mineral assemblages associated with M2 and M3 are similar and passage from M2 to M3 did not result in reaction textures which indicates the proximity of the thermal conditions of these two metamorphic events. M2 corresponds to the second deformational event (D2) where peak metamorphic conditions reached pressures of 8.6±3.2 kbar at 750-900°C, The second thermobaric event correlates with the third deformation event (D3) and a metamorphic peak of 4.1±1.9 kbar at 750-850°C. The decompression of the Fishery Bay region during D3/M3 is synchronous with crustal thickening of the terrain east of the area.
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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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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.