Academic literature on the topic 'Gawler Range Volcanics'

Volcanic layers within sedimentary basins cause significant problems for petroleum exploration because the attenuation of the seismic signal masks the underlying geology. A test study was conducted for the South Australia Government to map the thickness of volcanics and sub-volcanic geology over a large area in the Gawler Range Volcanics province. The area is covered by good quality magnetic data. The thickness of volcanics and basement configuration was unknown as there has only been a limited amount of drilling. The Automatic Curve Matching (ACM) method was applied to located magnetic data and detected magnetic sources within different rock units, providing their depth, location, geometry and magnetic susceptibility. The magnetic susceptibilities detected by ACM allowed the differentiation of the volcanics and the underlying basement. The base of volcanics and the depth to the top of basement was mapped along 75 km NS profiles, that were spaced 1 km apart over a distance of 220 km. The volcanic and basement magnetic susceptibilities and the magnetic source distribution pattern were used as key determinants to interpret the depth to the two interfaces. The results for each interface were gridded, and images of the base of volcanics and depth to basement were generated. The mapped volcanics thickness was validated by comparison with the results from drilling, with the volcanics thickness matching very well. After project completion, a passive seismic survey was conducted in part of the test area, indicating a base of volcanics of ~4 km, which further confirmed the results.

Abstract. Rhyolite and dacite lavas of the Mesoproterozoic upper Gawler Range Volcanics (GRV) (>30 000 km3 preserved), South Australia, represent the remnants of one of the most voluminous felsic magmatic events preserved on Earth. Geophysical interpretation suggests eruption from a central cluster of feeder vents which supplied large-scale lobate flows >100 km in length. Pigeonite inversion thermometers indicate eruption temperatures of 950–1100 °C. The lavas are A-type in composition (e.g. high Ga/Al ratios) and characterised by elevated primary halogen concentrations (~1600 ppm Fluorine, ~400 ppm Chlorine). These depolymerised the magma such that temperature-composition-volatile non-Arrhenian melt viscosity modelling suggests they had viscosities of <3.5 log η (Pa s). These physicochemical properties have led to the emplacement of a Large Rhyolite Province, which has affinities in emplacement style to Large Basaltic Provinces. The low viscosity of these felsic magmas has produced a unique igneous system on a scale which is either not present or poorly preserved elsewhere on the planet. The Gawler Range Volcanic Province represents the erupted portion of the felsic end member of the family of voluminous, rapidly emplaced terrestrial magmatic provinces.

Abstract. Rhyolite and dacite lavas of the Mesoproterozoic upper Gawler Range Volcanics (GRV) (>30 000 km3 preserved), South Australia, represent the remnants of one of the most voluminous felsic magmatic events preserved on Earth. Geophysical interpretation suggests eruption from a central cluster of feeder vents which supplied large-scale lobate flows >100 km in length. Pigeonite inversion thermometers indicate eruption temperatures of 950–1100 °C. The lavas are A-type in composition (e.g. high Ga/Al ratios) and characterised by elevated primary halogen concentrations (~1600 ppm fluorine, ~400 ppm chlorine). These depolymerised the magma such that temperature-composition-volatile non-Arrhenian melt viscosity modelling suggests they had viscosities of <3.5 log η (Pa s). These physicochemical properties have led to the emplacement of a Large Rhyolite Province, which has affinities in emplacement style to Large Basaltic Provinces. The low viscosity of these felsic magmas has produced a unique igneous system on a scale which is either not present or poorly preserved elsewhere on the planet. The Gawler Range Volcanic Province represents the erupted portion of the felsic end member of the family of voluminous, rapidly emplaced terrestrial magmatic provinces.

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The Gawler Range Volcanics are a Silicic Large Igneous Province that has been extensively studied due to the atypical nature of its widespread felsic lava flows. These low viscosity lavas form the upper sequence of the GRV, termed the Upper Gawler Range Volcanics (UGRV). The older sequence or Lower Gawler Range Volcanics (LGRV) are readily distinguished from the UGRV as they appear as numerous discrete volcanic centres, the best exposed of which are at Kokatha and Lake Everard. A much less discussed volcanic area of the LGRV are the Southern Gawler Ranges Area Volcanics (SGRAV), which form a curvilinear belt along the southern margin of the GRV. The SGRAV are dominantly represented by two volcanic units, the Bittali Rhyolite (BR) and Waganny Dacite (WD) which are exposed discontinuously for ~200km E-W. The SGRAV may be divided into a western section of dominantly effusive volcanism, with elevated temperatures and halogen contents comparable to that of the UGRV, and a central-eastern section where explosive volcanism predominates. Petrogenetic modelling suggests that assimilation fractional crystallization (AFC) processes which played a role in the development of the LGRV, were active in the formation of the SGRAV. However, using AFC modelling, the SGRAV can be reconstructed through a dominant fractional crystallization process with late stage crustal assimilation, as opposed to continual crustal assimilation in the other LGRV magma systems.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Physical Sciences, 2015

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The Gawler Range Volcanics (GRV) have been extensively studied previously, but a source and emplacement mechanism has yet to be agreed upon. This study aims to constrain the source region of the GRV and to make deductions about how the GRV evolved. This has been done through a number of modelling techniques, including AFC modelling and use of the Rhyolite-MELTs program. The εNd values vary widely across the GRV, and these have been used in conjunction with trace element geochemistry to constrain the source region. It is deduced that the most primitive GRV basalts were the result of limited fractionation of a re-enriched refractory harzburgite source in the sub-continental lithospheric mantle. It is then shown that the entire GRV suite can be derived from one fractionation trend, however some assimilation is required.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Physical Sciences, 2014

The Gawler Range Volcanics (GRV), located in the central region of South Australia, represents the eruption of enormous volumes of felsic lava (25,000km3 ) at the beginning of the Mesoproterozoic (~1.59Ga) and is closely associated with the emplacement of the widespread Hiltaba Suite (HS) granites both spatially and temporally. Since the discovery of the Giant Olympic Dam Cu-U-Au-Ag deposit in 1975 (which is intimately related to the emplacement of the HS), both the HS and GRV have attracted vast amounts of attention from research, government and exploration groups. Efforts to understand the HS has been largely hampered by the thick regolith cover which blankets much of the central region of South Australia. The GRV in contrast, is well-exposed and detailed volcanology (Morrow, 1998; Gamer and McPhie, 1999) and geochemical studies (Stewart, 1994) suggest that it represents lava flows, which were produced from the fractionation of a mafic source. One problem associated with the GRV is the wide extent of its homogenous rhyolitic and dacitic lava flows. Creaser and White (1991) reported that the upper GRV, which represents a single emplaced unit >8000 km2 , is characterized by dry (<1 wt% H20), high temperature magma (900-1010°C), which was derived from a single homogeneous source. Gamer and McPhie (1999) advocated that the GRV are lava flows as opposed to ignimbrites (e.g. Stewart, 1994). The notion that these widespread homogenous volcanics are lava flows is at odds with the high viscosity associated with rhyolitic and dacitic lavas. Geochemical studies undertaken by myself however show that rhyolites from the GRV contain 1400-1800ppm ofF and glass inclusions also from the GRV (which are thought to represent melt trapped at depth) contains up to 12,500ppm ofF, which is remarkably high for degassed lava and trapped melt respectively. I propose that F, known as an effective depolymeriser, may have significantly reduced the viscosity of the lava and thus allowed it to flow over an extensive area. Melt inclusions which are hosted within quartz phenocrysts from the upper units of the Eucarro Rhyolite (GRV) were collected from seven samples belonging to three different rock types, which included; 1) plagioclase rhyolite, 2) vesicular rhyolite, 3) quartz rhyolite (also see Morrow et a!., 2000). Each rock type contains excellent examples of large glass inclusions along with magmatic fluid inclusions which were found within the same trapping planes. Samples from the plagioclase rhyolite and the vesicular rhyolite also contain quartz with high-density C02 inclusions, glass inclusions and non-silicate melt inclusions. From the three rock types over fifty homogenized glass inclusions were analyzed for major elements (SX-100 Cameca microprobe) and over eighty for trace elements (laser ablation/IC-PMS). Comparisons were than made between the groundmass and melt inclusions. Each sample showed remarkable similarities in REE (except Eu, which is enriched within the groundmass ). In contrast though, elements such as F, Mo (x1-6), W (xl0-20), U (x1-2.5) and Pb (x3-9) were significantly enriched within melt inclusions. A similar REE concentration, between the groundmass and melt inclusions, suggests that both were derived from the same source, whereas enrichments ofF (x6-7), Mo (xl-6), W (x1 0-20), U (x1-2.5) and Pb (x3-9) within melt inclusions may reflect the concentration of these elements within the melt at depth and therefore concentrations prior to eruption and subsequent degassing. The presence of high-density C02 inclusions, magmatic fluid inclusions and nonsilicate melt inclusions suggests that a significant amount of degassing of the magma may have occurred at depth. This suggests that a significant amount ofF, Mo, W, U and Pb may have been lost as volatiles from the melt and therefore have likely either ended up as mineral deposits within the country rock, or were released into the atmosphere during eruption. If the former was significant, than the location the volcanic vent(s) which link the GRV and HS may be of enormous economic value.

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The Gawler Range Volcanics (GRV) form part of a Mesoproterozoic Silicic Large Igneous Province (Gawler SLIP) within South Australia. The SLIP includes intrusive and extrusive rocks within the Gawler Craton and Curnamona Province that are dominantly felsic. Recent high precision dating of several GRV units has shown that they erupted between 1587 Ma and 1595 Ma allowing for geochemical comparisons with respect to a precise timeline. Trace element geochemistry has shown anomalies to be consistent through the lower and upper GRV demonstrating the main source in the GRV likely did not change. A mafic component is shown to have contributed to both the lower and upper GRV system. Eu anomalies and trace element geochemistry shows that there was a large change in magmatic evolution between the upper and lower GRV within a short time (<1 m.yr). This change is hypothesised to have occurred due to the tectonic regime during the SLIP emplacement. Hydrothermal alteration associated with the emplacement of the Gawler SLIP is known to have contributed to the formation of Iron-Oxide Copper-Gold (IOCG) and shear-hosted deposits in South Australia. More recent discoveries within the Southern Gawler Ranges display epithermal-porphyry characteristics associated with alteration in the lower GRV. Alteration within the GRV is hereby characterised in order to identify alteration associated with mineralisation. Alteration is shown to encompass a sericite – hematite dominated assemblage which has affected most of the GRV. Several other anomalous alteration assemblages exist in localised areas. Using direct evidence, it is suggested that epithermal-porphyry systems may be preserved within the upper GRV, which encompasses a larger outcrop area than the lower GRV which is underexplored.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Physical Sciences, 2019

The Gawler Range Volcanics (GRV) and the co-magmatic Hiltaba Suite (HS) granite form a Mesoproterozoic Silicic-dominated Large Igneous Province (SLIP) cropping out over a vast area in the central Gawler craton, South Australia. Only a few SLIP have been recognised in the world; they occur throughout geological history and in both intraplate and plate margin settings. The igneous province in question, or Gawler SLIP, was emplaced in an intracontinental setting during assembly of the supercontinent Laurentia. Since emplacement, the Gawler craton remained an area of positive relief, allowing a good preservation of the Gawler SLIP. Emplacement of the Gawler SLIP is associated with a major mineralising event in the Gawler craton which includes the super-giant Cu-Au-U Olympic Dam deposit. This thesis focuses on the lower part of the Gawler Range Volcanics. By defi nition, SLIP are formed by large volumes of magma (≥100000 km3) emplaced over a short time, in the order of the millions of years. The emplacement mechanism of large volumes of felsic magma in a short time span is one of the open questions in the study of these igneous manifestations. In this thesis, the volcanic facies in the lower part of the Gawler Range Volcanics have been described in detail, in order to assess the emplacement mechanism of the rocks. The rock units include moderately extensive (up to tens of km in diameter) felsic lavas, associated with pyroclastic deposits (ignimbrite) of comparable extent. Lavas are characterised by evenly porphyritic texture, with medium grained phenocrysts of feldspar ±pyroxene ±quartz in a fi ne microcrystalline quartzofeldspathic groundmass. Flow bands and flow folds, autobreccia domains, elongate vesicles and lithophysae are also present. Some of these large units may be composite lavas, separated by thin breccia layers of possible pyroclastic origin. Ignimbrites are compositionally and texturally homogeneous, and contain mm-scale crystals of feldspar and quartz in fine grained, eutaxitic and vitriclastic matrix. Some of the lavas have previously been interpreted as pyroclastic fl ow deposits in which all evidence of clastic origin have been concealed by welding. This interpretation was mainly based on the extent of the units and inferred high viscosity of felsic lavas. Volumetrically minor mafi c and intermediate lavas are also present and locally form thick piles. The chemical composition of the lower Gawler Range Volcanics has been determined, including major and trace elements. Whole-rock analyses are complemented by melt inclusion analyses, which allowed measuring volatile components (halogens in particular). The rocks are characterised by increasing trends of K2O, REE, Y, Zr, Th and Nb with increasing SiO2, and decreasing trends of CaO, FeO, MgO and TiO2. Fluorine is present in high concentrations in melt inclusions (F ≤1.3 wt.%, more than 20 times the average upper continental crust), whereas Cl has moderate concentrations. High microprobe totals of melt inclusions are compatible with low water contents, in agreement with the anhydrous parageneses and previous estimates based on petrological considerations (method of Nekvasil, 1988). Plots of Zr and the Zr/Hf ratio versus indicators of magma fractionation (e.g. SiO2 and incompatible elements) indicate that source magmas of the most felsic rocks (SiO2 >68 wt.%) were zircon-saturated. Application of the zircon saturation model (Watson and Harrison, 1983) on these samples yields temperatures up to 950-990°C for the lower GRV. The combination of high magmatic temperatures, high F and low water in the magma creates conditions of low viscosity and low explosivity. These conditions are favourable for the effusive emplacement of the units, and help explaining how extensive units were emplaced as lavas. Comparison of whole-rock and melt inclusion analyses allowed assessing the degree of alteration of the rock units. Major element compositions are similar in wholerock and melt inclusion analyses, with the local exception of Na, which showed low concentration in a few whole-rock samples. This indicates that alteration did not affect substantially concentrations of the most “mobile”, water-soluble major elements. This is also true for most trace elements, and whole-rock analyses can be considered as indicative of the magma composition. Notable exceptions are Pb, U and Sn, which were selectively mobilised and variously depleted. Alteration of Pb, U and Sn is evident from scattered whole-rock compositions and lack of correlation between these elements and other elements. In contrast, melt inclusion compositions show good correlations and indicate incompatible behaviour. Quartz textures and trace element content in the lower Gawler Range Volcanics and Hiltaba Suite were studied by scanning electron microscopy cathodoluminescence (CL) and electron microprobe. Mineral zones retain a record of crystallisation conditions through time (“crystal stratigraphy”). Comparisons were made between volcanic units, shallow and deeper intrusions (dykes and granite). Quartz in volcanic units and dykes has sharp-edged CL zones in which CL brightness correlates with Ti content, whereas the granite quartz has “smudged” zones with gradational contacts. This difference is interpreted as the result of poor preservation of Ti zones in the granite, for which slow cooling allowed solid-state diffusion of Ti in the quartz lattice. Intra-granular textures in volcanic units and dykes also include truncation of growth textures and reverse zoning (rimwards increase of Ti content). Intra-granular textures indicate a complex history of crystallisation and resorption, and trace elements suggest varying temperature. These results point to pulsating magmatic conditions, compatible with a non-linear evolution of the lower Gawler Range Volcanics magma chamber(s). The volcanic units have contrasting (non-correlatable) zoning patterns among quartz crystals, each pattern indicating different crystallisation conditions. The juxtaposition of quartz crystals with contrasting zoning patterns are consistent with a dynamic regime (convection, stirring, overturning) of the GRV magma chamber. These results point to pulsating magmatic conditions, compatible with a non-linear evolution of the GRV magma chamber. In contrast, quartz crystals in the dykes have similar zoning patterns, suggesting that all the crystals in each dyke experienced a similar crystallisation history. In some rhyolite samples, aggregates of minerals (including fluorite, epidote, REE-F-carbonate, titanite, anatase, and zircon) have crystallised in “pockets” such as vesicles, micro-miarolitic cavities and lithophysal vugs. These aggregates of minerals contain significant amounts of rare earth (RE), high-fi eld strength (HFS) elements, and base metals (Cu and Mo). In average, concentration of these elements is higher in these aggregates than in the surrounding host rock. These aggregates are interpreted to have crystallised from a late-stage magmatic volatile (F, CO2, H2O, ±S, ±P)-bearing fluid that exsolved and infilled pockets during the final stages of emplacement and crystallisation. The presence of complexing agents such as F and CO2 can explain how low-solubility, “immobile” trace elements were transported in solution. A magmatic, primary origin for this fl uid appears likely, given the F-, REE-, and HFSE-rich composition of the melt shown by melt inclusions. Conversely, the hypothesis of a post-magmatic, secondary origin is considered less likely because of the absence of alteration, mineralisation and veins in these rocks. Magmatic accessory minerals, the alteration of which would have been necessary for secondary remobilisation of RE and HFS elements, appear fresh and unaltered. These data testify the mobility of RE and HFS elements in the lower Gawler Range Volcanics and can have implications in the formation of associated mineral deposits in the Gawler craton, including the Olympic Dam deposit. This deposit is also characterised by high concentrations of F, RE and HFS elements, and a similar F-rich fl uid as the one hypothesised here might have been active in the mineralisation process.

The Tarcoola Goldfield, central South Australia, is one of a number within the Central Gawler Gold Province (CGGP) spatially related to Hiltaba Suite granites. This study investigates the origin of mineralisation at Tarcoola, and the petrogenesis of granites at and around Tarcoola. ‘Hiltaba Suite’ granites in the Tarcoola region are assigned to two supersuites, which is expanded to four once granites from the rest of the Gawler Craton are considered. The term Hiltaba Association Granites (HAG) is introduced as the parental unit of the Jenners, Malbooma, Venus and Roxby Supersuites. These criteria are applied to the felsic parts of the comagmatic Gawler Range Volcanics (GRV). The HAG and GRV are grouped as the bimodal Gawler Ranges–Hiltaba Volcano–Plutonic (GRHVP) Association. The felsic components generally have high K, HFSE, LIL, are fractionated and evolved, have moderate to high Fe/Mg, are slightly alkaline, metaluminous to slightly peraluminous, slightly oxidised and high-temperature. The supersuites of the Tarcoola region are the Malbooma Supersuite, which is more strongly evolved and fractionated than the Jenners Supersuite. Both Supersuites are I-type and evolved from a granodiorite composition by fractional crystallisation. The Pegler and Ambrosia Granites (Jenners Supersuite), and are dated at 1591.7 ± 5.8 and 1575.4 ± 7.8 Ma. The Big Tank, Kychering and Partridge Granites (Malbooma Supersuite), and are dated at 1589.9 ± 7.4, 1574.7 ± 4.3 and 1577 ± 8.5 Ma. The Roxby and Venus Supersuites are A-type granites and volcanics, with higher HFSE, F, and zircon saturation temperatures than the I-types. Nd-isotope data indicate that the felsic GRHVP formed by mixing between evolved mantle and crust. Narrow dykes of the high-K Lady Jane Diorite intrude the Tarcoola Goldfield. This unit, and other basalts of the GRHVP, are interpreted to represent mixing between evolved lithospheric and primitive asthenospheric mantle melts. The Paxton Granite at the Tarcoola Goldfield was dated as older than the HAG at ~1720 Ma. The Tarcoola Formation was deposited in an ensialic basin directly onto the Paxton, with the basal Peela Conglomerate Member contains zircons of 1732.8 ± 5.1 Ma and 1714.6 ± 7.9 Ma, and the middle parts of the Tarcoola Formation being deposited at 1656 ± 7 Ma. Mineralisation at the Tarcoola Goldfield is quartz-vein hosted within the Tarcoola Formation, and comprises Au±Pb-Zn. The veins are structurally-controlled. 40Ar/39Ar geochronology and field relationships show that brittle veining, mineralisation, alteration and intrusion of the Lady Jane Diorite, occurred synchronously at ~1580 Ma. A Pb-Pb isotope study at the Tarcoola Goldfield is consistent with sourcing of Pb from the Paxton Granite, but does not exclude a mixed source. A shift in Nd during alteration may show an input from the relatively primitive Lady Jane Diorite. An atlas shows correlations between the four supersuites and the two defined mineral provinces of the Gawler Craton. Notably the Roxby Supersuite is associated nearly exclusive with iron-oxide copper-gold mineralisation in the Olympic Cu-Au Province in the eastern part of the Craton. The I-type Jenners and Malbooma Supersuites are mostly restricted to the CGGP. A position inboard of a subduction zone (hot continental back-arc) rather than anorogenic setting is proposed.

The Mesoproterozoic Gawler Range Volcanics (GRV) of South Australia include several voluminous felsic lavas that are important extrusive analogues to the coeval Hiltaba Suite (HS) intrusive rocks. These rocks are notable for their voluminous and high temperature nature, the large area over which they occur (850 by 500 km), and because they both host and are implicated in the formation of iron oxide-copper-gold deposits in the Olympic Province. A range of conflicting petrogenetic scenarios have been proposed for the GRV and related HS rocks. These models variously propose fractionation from mantle-derived melts or crustal fusion driven by mantle-derived melts to produce the magmas which formed these rocks; however, the development of a consistent model has been hindered by the large size of the volcanic province, the scarcity of unaltered rock, and the relatively small datasets of these studies. These challenges have resulted in a paucity of research examining the geochemistry and mineralogy of the GRV units in detail, particularly within the recently unified stratigraphic framework and the emergence of a consensus that several GRV units are effusive lavas rather than large, welded ‘ash-flow’ sheets of pyroclastic origin. This thesis addressed this shortfall through evaluation of the textures and compositions of crystals, crystal clusters and clustered distribution of minerals in newly collected, unaltered samples of the Upper GRV lavas and the Roxby Downs Granite (RDG), with the aim of understanding the origins of the crystals and clustered distributions of crystals and appraising their implications for the evolution of their host rocks. The RDG and Upper GRV lavas both contain clusters of touching silicate and oxide minerals. Crystal clusters in the RDG comprise magnetite + apatite ± titanite ± biotite ± zircon that are typically enveloped in a plagioclase crystal-matrix. Consistent Fe-Ti oxide + apatite + zircon + plagioclase assemblages and textures in both the Upper GRV and RDG suggest that for their hot and dry magmas parental magmas, cooling cause pyroxene + Fe-Ti oxide + plagioclase in GRV-type crystal clusters to react with both each other and the melt, forming HS-type magnetite + titanite + biotite + feldspar crystal clusters. Pyroxene and titanomagnetite compositions suggest that the Upper GRV lavas are related through differentiation in the upper crust, and imply that all analysed crystals were formed within, or were at least equilibrated to, the host lavas’ precursors. Different intergrowth textures between crystal clusters in the GRV lavas, and the widespread presence of magmatic enclaves, suggest that the majority of GRV crystal clusters were liberated from closely packed crystal masses through magma rejuvenation and associated crystal resorption. Within- and between-crystal pyroxene compositions suggests some phenocrysts did not grow in situ, adjacent to the other crystals found in individual clusters in the solidified rock, but rather aggregated or settled with other crystals following a stage of growth as independent, melt-bound phenocrysts. The last-erupted lava contains the highest proportion of clusters comprising complex and anhedral intergrowths. This is consistent with a normally zoned reservoir where the crystal:melt ratio and crystal concentration are greatest at depth. Loosely packed frameworks and free crystals are interpreted to occur in the middle and upper sections of the reservoir, respectively, represented by the first erupted Upper GRV lava and quartz-phyric portions of the lavas. Plagioclase is the most abundant phenocryst throughout the lavas but is present in only a subset of crystal clusters. The low abundance and absence of feldspar and quartz in clusters is likely due to mineral resorption and subsequent separation and loss of these components from other cluster components prior to and during the eruption the studied lavas. Once solid-solid crystal contacts were dissolved, the separation of the cluster components was driven by the strain placed on them by melt flow. Zircon is principally associated with Fe-Ti oxides and clusters of touching crystals in these rocks. There are few published reports of concentrations of zircon with Fe-Ti oxides, nor explicit evaluation and explanation of the potential origins of zircon-rich crystal clusters found in the ~1.59 Ga Gawler Craton rocks and similar rocks of western North America and elsewhere. The lack of pre-magmatic zircon, consistent intra-grain and inter-grain zircon compositional trends, the predominance of oscillatory zoned zircon with morphologies indicating growth from hot, evolved silicate melts, and the lack of evidence for zircon recrystallisation, indicates that zircon crystallised in the host GRV and RDG magmas. Variable zircon compositions within individual clusters does not support epitaxial nucleation of zircon on Fe-Ti oxides, however it is likely that some zircon grew from seed crystals formed by exsolution of Zr from Fe-Ti oxides. Aggregation of isolated, liquid-bound crystals is energetically favourable, and the grain size discrepancy between larger crystals (Fe-Ti oxides, pyroxenes) and smaller accessory minerals (zircon, apatite) maximises the disparity in particle velocities and hence enhances the opportunities for collisions and adhesion between these crystals. We propose that zircon adheres to Fe-Ti oxides with greater ease and/or with greater bond strengths, than to other phases present in the parental magmas. It is possible that this association is related to interactions between zircon and Fe-Ti oxide surface sites with opposing charges, presuming the distance between phase surfaces is sufficiently small. This thesis establishes a more precise model for the formation of the Upper GRV lavas and related Hiltaba Suite intrusive rocks, and provides insights into the origins of crystals found within these rocks, crystal cluster formation mechanisms and settings, establishment of magma chamber zonation, quartz-feldspar recycling and mafic magmatism in the genesis of the Upper GRV lavas, and the formation of HS-type clusters from GRV-type clusters. These findings contribute to the theory that voluminous and hot magmatic systems that express a dominant evolved component, such as the GRV and HS, likely depend on a stable connection to actively melting asthenospheric mantle. Furthermore, it is likely that a complete spectrum of mafic to felsic magmas being generated during the Hiltaba event (i.e. any compositional gaps are related to sampling bias and igneous developmental paths that are less favourable for the preservation of intermediate magma compositions), and that the lavas do not accurately reflect the long-term state of the magma reservoir.

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At Toondulya Bluff a sequence of 'older' Gawler Range Volcanics dip in an easterly direction beneath the overlying Yardea Dacite, and are intruded by the comagmatic Hiltaba Granite. The volcanics occur as a series of tuffs and lava flows. Geochemical evidence suggests these volcanics are related to each other by fractional crystallisation, with plagioclase, clinopyroxene, K-feldspar and titan-magnetite, and accessory zircon and apatite controlling differentiation trends. The Si-rich Hiltaba Granite and Yardea Dacite formed from the final, highly fractionated melts. Geothermometry suggests the volcanic and granite crystallised at temperatures within the range 680deg-850degC. The initial magma from which the lithologies were derived, was formed by partial melting of a lower crustal source probably of granulitic composition. Lake Acraman is believed to have been a site of meteoritic impact in the late Proterozoic (~600 Ma ago). Fragments of dacitic ejecta have been identified within the Bunyeroo Formation, Flinders Ranges and dating of these fragments gives an age of c.1575 Ma using single zircon ion probe dating techniques (Gostin et al in prep.). U/Pb dating of the Yardea Dacite at Lake Acraman reveals it to be of comparable age to these fragments (1603-1631 Ma). The lower intercept of the discordia line reveals there has been no resetting of the U/Pb system in response to the postulated meteoritic impact.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Earth and Environmental Sciences, 1985

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Volcanics in the Kokatha region present a wider range of rock types than in other areas of the Gawler Ranges. High temperature Mg rich basalt flows through to rhyolite ignimbrites and air fall tuffs outcrop. Two magmatic cycles are observed with a cycle consisting of initial basalts, followed by voluminous dacites and rhyodacites. The final phase of the cycle following the rhydacites represents a period of more explosive activity resulting in the deposition of rhyolitic ignimbrites, air fall tuffs rhyolitic flows and pyroclastics. Geochemical data indicate both fractionation and mixing of fractionated components were active igneous processes resulting in the formation of layered magma chambers. The layering of the magma chambers being well illustrated in the stratigraphy of the volcanic pile. Further evidence for cyclic fractionation trends exists, with a relative depletion of incompatible elements in the second cycle when compared to the first cycle. Discrimination diagrams plot the rocks from Kokatha in the calc-alkaline field. Calc-alkaline series usually indicate subduction processes however volcanism at Kokatha is intracratonic. Rb-Sr data give an isochron age of 1588.4 ± 14 Ma suggesting the rocks from Kokatha are a part of the lower sequence of the Gawler Range Volcanics. Samples from both cycles produce the isochron indicating a melt from a homogeneous source. Neodymium data suggest a basaltic input from the mantle assimilating with lower crust is a likely source. A possible tectonic model for volcanism is presented. Initially a flux of mantle-derived basalt enters the lower crust. This provides heat for large scale melting. Assimilation of lower crustal melts and mantle-derived basalts may or may not occur however a homogeneous source is formed. Diapirism resulting in upper crustal magma chambers allows the formation of a layered magma chamber. Eruption of the magma results in the stratigraphic sequence of volcanic rock units.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Physical Sciences, 1989