Magmatic and volcanic evolution of a silicic udlarge igneous province (SLIP): the Gawler Range udVolcanics and Hiltaba Suite, South Australia ud

摘要

udThe Gawler Range Volcanics (GRV) and the co-magmatic Hiltaba Suite (HS)udgranite form a Mesoproterozoic Silicic-dominated Large Igneous Province (SLIP)udcropping out over a vast area in the central Gawler craton, South Australia. Only a fewudSLIP have been recognised in the world; they occur throughout geological history and inudboth intraplate and plate margin settings. The igneous province in question, or GawlerudSLIP, was emplaced in an intracontinental setting during assembly of the supercontinentudLaurentia. Since emplacement, the Gawler craton remained an area of positive relief,udallowing a good preservation of the Gawler SLIP. Emplacement of the Gawler SLIPudis associated with a major mineralising event in the Gawler craton which includes theudsuper-giant Cu-Au-U Olympic Dam deposit. This thesis focuses on the lower part of theudGawler Range Volcanics.udBy defi nition, SLIP are formed by large volumes of magma (≥100000 km3) emplacedudover a short time, in the order of the millions of years. The emplacement mechanism ofudlarge volumes of felsic magma in a short time span is one of the open questions in theudstudy of these igneous manifestations. In this thesis, the volcanic facies in the lower partudof the Gawler Range Volcanics have been described in detail, in order to assess theudemplacement mechanism of the rocks. The rock units include moderately extensive (upudto tens of km in diameter) felsic lavas, associated with pyroclastic deposits (ignimbrite) ofudcomparable extent. Lavas are characterised by evenly porphyritic texture, with mediumudgrained phenocrysts of feldspar ±pyroxene ±quartz in a fi ne microcrystalline quartzofeldspathicudgroundmass. Flow bands and flow folds, autobreccia domains, elongateudvesicles and lithophysae are also present. Some of these large units may be compositeudlavas, separated by thin breccia layers of possible pyroclastic origin. Ignimbrites areudcompositionally and texturally homogeneous, and contain mm-scale crystals of feldsparudand quartz in fine grained, eutaxitic and vitriclastic matrix. Some of the lavas haveudpreviously been interpreted as pyroclastic fl ow deposits in which all evidence of clasticudorigin have been concealed by welding. This interpretation was mainly based on theudextent of the units and inferred high viscosity of felsic lavas. Volumetrically minor mafi cudand intermediate lavas are also present and locally form thick piles.udThe chemical composition of the lower Gawler Range Volcanics has been determined,udincluding major and trace elements. Whole-rock analyses are complemented by meltudinclusion analyses, which allowed measuring volatile components (halogens in particular).udThe rocks are characterised by increasing trends of K2O, REE, Y, Zr, Th and Nb withudincreasing SiO2, and decreasing trends of CaO, FeO, MgO and TiO2. Fluorine is presentudin high concentrations in melt inclusions (F ≤1.3 wt.%, more than 20 times the averageudupper continental crust), whereas Cl has moderate concentrations. High microprobeudtotals of melt inclusions are compatible with low water contents, in agreement with theudanhydrous parageneses and previous estimates based on petrological considerationsud(method of Nekvasil, 1988). Plots of Zr and the Zr/Hf ratio versus indicators of magmaudfractionation (e.g. SiO2 and incompatible elements) indicate that source magmas ofudthe most felsic rocks (SiO2 >68 wt.%) were zircon-saturated. Application of the zirconudsaturation model (Watson and Harrison, 1983) on these samples yields temperatures upudto 950-990°C for the lower GRV. The combination of high magmatic temperatures, highudF 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 helpudexplaining how extensive units were emplaced as lavas.udComparison of whole-rock and melt inclusion analyses allowed assessing theuddegree of alteration of the rock units. Major element compositions are similar in wholerockudand melt inclusion analyses, with the local exception of Na, which showed lowudconcentration in a few whole-rock samples. This indicates that alteration did not affectudsubstantially concentrations of the most “mobile”, water-soluble major elements. Thisudis also true for most trace elements, and whole-rock analyses can be considered asudindicative of the magma composition. Notable exceptions are Pb, U and Sn, which wereudselectively mobilised and variously depleted. Alteration of Pb, U and Sn is evident fromudscattered whole-rock compositions and lack of correlation between these elements andudother elements. In contrast, melt inclusion compositions show good correlations andudindicate incompatible behaviour.udQuartz textures and trace element content in the lower Gawler Range Volcanicsudand Hiltaba Suite were studied by scanning electron microscopy cathodoluminescenceud(CL) and electron microprobe. Mineral zones retain a record of crystallisation conditionsudthrough time (“crystal stratigraphy”). Comparisons were made between volcanic units,udshallow and deeper intrusions (dykes and granite). Quartz in volcanic units and dykesudhas sharp-edged CL zones in which CL brightness correlates with Ti content, whereasudthe granite quartz has “smudged” zones with gradational contacts. This difference isudinterpreted as the result of poor preservation of Ti zones in the granite, for which slowudcooling allowed solid-state diffusion of Ti in the quartz lattice. Intra-granular textures inudvolcanic units and dykes also include truncation of growth textures and reverse zoningud(rimwards increase of Ti content). Intra-granular textures indicate a complex history ofudcrystallisation and resorption, and trace elements suggest varying temperature. Theseudresults point to pulsating magmatic conditions, compatible with a non-linear evolutionudof the lower Gawler Range Volcanics magma chamber(s). The volcanic units haveudcontrasting (non-correlatable) zoning patterns among quartz crystals, each patternudindicating different crystallisation conditions. The juxtaposition of quartz crystals withudcontrasting zoning patterns are consistent with a dynamic regime (convection, stirring,udoverturning) of the GRV magma chamber. These results point to pulsating magmaticudconditions, compatible with a non-linear evolution of the GRV magma chamber. Inudcontrast, quartz crystals in the dykes have similar zoning patterns, suggesting that all theudcrystals in each dyke experienced a similar crystallisation history.udIn some rhyolite samples, aggregates of minerals (including fluorite, epidote,udREE-F-carbonate, titanite, anatase, and zircon) have crystallised in “pockets” such asudvesicles, micro-miarolitic cavities and lithophysal vugs. These aggregates of mineralsudcontain significant amounts of rare earth (RE), high-fi eld strength (HFS) elements, andudbase metals (Cu and Mo). In average, concentration of these elements is higher in theseudaggregates than in the surrounding host rock. These aggregates are interpreted to haveudcrystallised from a late-stage magmatic volatile (F, CO2, H2O, ±S, ±P)-bearing fluid thatudexsolved and infilled pockets during the final stages of emplacement and crystallisation.udThe presence of complexing agents such as F and CO2 can explain how low-solubility,ud“immobile” trace elements were transported in solution. A magmatic, primary origin forudthis 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, secondaryudorigin is considered less likely because of the absence of alteration, mineralisationudand veins in these rocks. Magmatic accessory minerals, the alteration of which wouldudhave been necessary for secondary remobilisation of RE and HFS elements, appearudfresh and unaltered. These data testify the mobility of RE and HFS elements in theudlower Gawler Range Volcanics and can have implications in the formation of associatedudmineral deposits in the Gawler craton, including the Olympic Dam deposit. This depositudis also characterised by high concentrations of F, RE and HFS elements, and a similarudF-rich fl uid as the one hypothesised here might have been active in the mineralisationudprocess.ududud