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Abstract

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.

Item Type:	Thesis - PhD
Authors/Creators:	Agangi, A
Keywords:	volcano, Mesoproterozoic, rhyolite, Australia
Additional Information:	Copyright © the author
Item Statistics:	View statistics for this item

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