Mineralisation around Mount Adrah, New South Wales: new observations by the Geological Survey of NSW

Wang, Yamei; Forster, David; Cronin, Dan; Montgomery, Karen; Blevin, Phil

1Geological Survey of New South Wales, Department of Regional NSW, Maitland, Australia

This study summarises previous exploration in the Mount Adrah area and presents the results of new petrographic, sulfur isotopic and HyLogger hyperspectral analyses of intrusion-related gold mineralisation centred on the Hobbs Pipe intrusion at Mount Adrah, New South Wales. The work was conducted as part of the five-year East Riverina Mapping Project, which improved geological understanding to aid mineral exploration and enable informed land use decisions in the region.

The Hobbs Pipe deposit contains about 770 000 oz of gold and is interpreted to be an intrusion-related gold deposit. Two main styles of mineralisation are present: disseminated pyrite–gold, developed within a cylindrical, zoned intrusion (Hobbs Pipe); and narrow, high-grade gold reefs located nearby. Drillhole GHD009 (total depth 1,312 m) provides a representative section through the Hobbs Pipe deposit and associated rocks.

Reconnaissance petrology was undertaken on twenty thin sections collected from drillhole GHD009. Three main phases have been identified: a felsic monzogranite core, an intermediate quartz monzodiorite to diorite outer core, and an intermediate–mafic diorite to gabbro rim. Disseminated pyrite-gold mineralisation in the Hobbs Pipe deposit is mainly hosted in the felsic monzogranite core.

Twenty-eight sulfur isotope results were obtained; twenty-seven from drillhole GHD009 and one from White Deer Reef, intersected in drillhole GHD011. δ34S results mainly fall within a relatively narrow range and suggest a single, magmatic-dominated source of sulfur.

HyLogger hyperspectral analysis of drillhole GHD009 was conducted at the WB Clarke Geoscience Centre, Londonderry, New South Wales. Chlorite, white mica and dark mica type all show mineralogical changes near lithological contacts and/or alteration which relates to gold mineralisation.

The geology and alteration assemblages in conjunction with new petrology, hyperspectral data and sulfur isotopes suggest that both the disseminated mineralisation within the core of the intrusive stock and the distal high-grade gold-bearing reefs are related to the same mineralising event, and probably the same fluid source.


Biography

Dr Yamei Wang has broad experience in minerals exploration. She also has a strong academic background, with a PhD in structural geology. She joined Geological Survey of NSW in February 2019 and her current role is Senior Geoscientist in Mining & Exploration Assessment team in the Survey.

QEMSCAN and PGE geochemistry to track sulfide saturation, magmatic evolution and fertility of porphyry suites (on the example of Mount Hagen, Papua-New Guinea)

Misztela, Monika1, Campbell, Prof. Ian1

1Research School of Earth Sciences, Australian National University, Canberra, Australia

Sulfide saturation is believed to play an important role in porphyry systems fertility. It can also determine the type of ore in an economic deposit. During the early stages of magma evolution Cu, Au and Pd behave incompatibly, and they are concentrated in the melt by fractional crystallisation. If sulfide saturation occurs early, the chalcophile elements are trapped in sulfide phases and locked in an underlying magma chamber, where they are able to enter the fluid phase, which results in a barren system. However, if sulfide saturation occurs later, after or shortly before volatile saturation, the metals are able to enter the fluid phase and form an economic deposit.

Platinum Group Elements are sensitive indicators of sulfide saturation due to their high partitioning into immiscible sulfide melts. Cu and Au also partition into sulfide melts but with lower partition coefficient, so that PGE are more sensitive to any changes in the system. Furthermore, their solubilities in hydrothermal fluids are lower; they are less mobile than Cu or Au, so their concentration in rocks reflect magmatic rather than hydrothermal processes.

QEMSCAN (Quantitative Evaluation of Materials by Scanning Electron Microscopy) can provide valuable information when studying magmatic suites. It provides high-resolution maps and images of mineral and elemental distributions, porosity structure maps, the density of samples and most importantly, quantitative mineral and elemental analyses.

Mount Hagen is an arc system that could potentially be related to a porphyry deposit. Its favourable location and proximity to other deposits was the initial motivation for undertaking this project. Eighteen samples, covering a compositional range from 2 to 11.5 wt.% MgO, were analysed for the PGE. Thirteen rocks were selected for detailed petrological description and mineral quantification by QEMSCAN. Quantitative reports were used to plot the cumulative abundance of major mineral phases as a function of whole-rock MgO. The results show that significant changes occur at ca. 6 wt.% MgO. Hornblende appears as a primary phase, which is attributed to major input of wet magma into the system at that time. Analysis of olivine cores revealed a reversal in their compositional trend at 6 wt.% MgO, which confirmed this hypothesis. Pd, Pt and Au analysis showed that concentrations of these elements initially increase, up to about 8.5 wt.% MgO, followed by a decrease. The change at 8.5 wt.% Mg is interpreted to mark the start of sulfide saturation, and it correlates well with the similar change in the Cu trend. At about 6 wt.% MgO there is a reversal of the trend, followed by a second decline, which can also be explained by a new magma influx to the system.

Mount Hagen was an open system, with at least one major magma recharge into the chamber. The magma experienced early sulfide saturation, which makes it unlikely that this system produced economic mineralisation.


Biography

Monika Misztela is a PhD student at the Research School of Earth Sciences, Australian National University interested in petrological and geochemical studies. Monika comes from Poland, where she did her Bachelor`s and Master`s degree in Economic Geology at the AGH University of Science and Technology in Krakow.

Corundum Conundrum

Raffan,Nick1

1Unsw, Kensington, Australia

Volcaniclastics and basalts are widely spread across the Anakie Gemfields in Central Queensland, Australia. A principal elevator of sapphires to the surface is widely believed to have been eruptions from Hoy Province volcanoes, followed by disintegration and release of sapphire from lava flows. However, it is noteworthy that volcaniclastic rocks are usually associated with areas where intensive sapphire mining has taken place. Although corundum has been reported from some Hoy plugs, it is extremely rare but is more commonly found by miners in volcaniclastics (called clinker by local miners). Crucially, although numerous basalt clasts of various textural types occur within the mined gravels, despite a hundred years of mining miners have not found any corundum within these clasts. The volcaniclastics from this field vary widely in mineralogy, texture, and chemistry. A surprising result is that some samples comprise over 50% dolomite, with some containing much higher amounts of around 70%. The volcanic origin of these is supported by anomalously high levels of zirconium and titanium. In addition, these alkaline rocks sometimes contain abundant small equant-shaped flakes of biotite, now altered to clays. This raises an important question: were the dolomite-rich volcaniclastics erupted from one or more carbonatite-type volcanos? And were these also the source for the corundums? Also, what were the sources for the diverse suites of zircons and spinels found in the gem gravels? The timing of events is complicated by the range in ages of Hoy plugs from 67 Ma to 14 Ma. The most likely scenario is that lavas flows initially protected earlier volcaniclastics from erosion. After removal of the flows, the volcaniclastics were free to release corundum to the waterways. Trace element geochemistry for seven plugs and four volcaniclastic samples suggests that they are genetically related. Matters are complicated by an undocumented basalt field high in the Drummond Range, west of the sapphire fields where clinker was also observed, along with possible volcaniclastics immediately below the basalt flows.


Biography

Forty six years experience within the resources sector including working as a  petrologist, exploration geologist, equity analyst and portfolio manager, commodity analyst, gold fund manager and minerals economist.

U-Pb ages and aluminium concentrations of colourful sapphire-related zircon megacrysts from Far North Queensland:  Merlot, Rosé and Champagne

Charlotte M Allen1, Erica J Porter1 and Mitchell DeBruyn2

1Central Analytical Research Facility, Institute for Future Environments, Queensland University of Technology, Brisbane, 2School of Mechanical, Medical and Process Engineering, Faculty of Science, Queensland University of Technology, Brisbane

Detrital sapphires are commonly found with megacrystic zircon but it is only co-location that links these free grains.  If related, zircon can constrain the circumstances by which nature creates corundum. Trail and co-workers suggested that zircons from high-Al-activity magmas (peraluminous ones) have Al contents > 4ppm, a step-jump up from Al contents from zircons from metaluminous magmas.  Aluminium is generally not considered to crystallize in zircon because of radius and charge but was included in the 24 isotopes analytical list. Twenty-seven 25 micron laser ablation spots were targeted on 3 colour groups from detrital zircon megacrysts associated with sapphires of Scot’s Camp near Undara National Park, a Lava Plains locality.  Three spots on each of three grains in each colour group were analysed by Agilent 7900 ICPMS.  Zircons were cathodoluminescence and backscatter imaged. Reported dates are concordant 206Pb/238U ages using Temora as calibrant with no Th-disequilibrium correction. Trace element concentrations (TE) are based on NIST 610 with Si as internal standard and assumed 15.22 wt% concentration.

Champagne grain #1, wholly, proved to be older than the rest at 2716 Ma (1stdev, n=3) but its cathodoluminescence, morphology and TE are not distinctive. Two other Champagne spots gave 5.5 and 4.1 Ma, ages older than Merlot grains which are consistent at 2.60.2 Ma (n=7). Rosé grains give the same age (2.90.5 Ma, n=5).

TE among the 9 grains have somewhat grain-specific compositions which is true of aluminium.  Merlot, Rosé and Champagne have average Al contents of 98, 2619 and 3323 ppm (1stdev), respectively.  The 271 Ma grain (Champagne#1) contains variable Al (4 to 78 ppm) whereas #2 is consistent at 812 ppm (n=3).  Compare these averages to our reference zircons and sources: Temora, (quartz-monzodiorite), Plešovice, (alkalic granulite), and 91500, a detrital megacryst provisionally associated with syenite (?). Respectively, they give 1.81.0, 8935 and 7.72.4 ppm Al.  The Lava Plains megacrysts, indeed, were generated from Al-rich sources.

Sapphire-associated zircons tend to be TE-poor as in a 13.6 Ma example from Mt Weldborough, NE Tasmania. Uranium is <100 ppm in Champagne and Rosé but Merlot has U from 200 to 1250 ppm save one and thus gives the more precise age. Except for Al and P, Merlot has greater TE contents.  Zircon classification based on Yb, U, and Hf indicate these are continental and/or kimberlitic zircon types. Features of note are small to zero Eu anomalies and positive Ce anomalies of 100-300 for Merlot and Rosé but 10s for Champagne (method after Blundy and Wood). These Lava Plains zircons are decidedly not-mantle-like, particularly in their Zr/Hf (>>37, the chondritic value).

The dominant age of 2.6 Ma is in the range of the oldest Lava Plains eruption ages, however, to find a morphologically non-distinctive Permian megacryst among them means that nature has repeated herself in providing the odd conditions to generate both sapphire and zircon megacrysts, crystals that have been brought to the surface by alkalic basalts of similar or much younger age. Permian is a common age among Queensland sapphire-related zircons.


Biography

Charlotte is the Director of the Central Analytical Research Facility, IFE.  She is interested in zircons, their ages and trace elements.  As collateral material from a process engineering problem, Mitch identified the zircons as interesting, and Erica named, imaged and analyzed them. Potential reference materials?

Trace element sector zoning in clinopyroxene as a function of undercooling: An experimental evaluation in trachybasaltic magmas

MacDonald, Alice1, Ubide, Teresa1, Masotta, Matteo2, Mollo, Silvio3, Pontesilli, Alessio4

1University Of Queensland, Brisbane, Australia, 2University of Pisa, Pisa, Italy, 3Sapienza University of Rome, Rome, Italy, 4University of Otago, Dunedin, New Zealand

Clinopyroxene chemistry is increasingly being utilised to investigate magmatic processes, due to its ability to record an extensive history of physiochemical changes in the host magma. However, clinopyroxene chemistry is not only influenced by pressure, temperature, and magma composition, but also by kinetic effects that may generate compositional zoning, such as sector zoning. Previous experimental work has highlighted the role of undercooling (ΔT = T­liquidus – Tsystem) on crystal morphology and major element chemistry of sector-zoned clinopyroxene. In this regard, the spatial distribution of trace elements in clinopyroxene remains relatively underexplored.
Here we present trace element data collected by laser ablation ICP-MS mapping of experimental sector-zoned clinopyroxenes crystallised from a trachybasalt melt representative of one of the most primitive magmas ever erupted at Mt Etna volcano (Italy). Experiments were conducted an isobaric pressure of 400 MPa, and a range of temperatures (1050 – 1200°C) and H2O contents (0-4 wt.%). Undercooling was attained by cooling the experiments from a starting temperature (1300°C) above the clinopyroxene liquidus at a rate of 80°C/min to a resting temperature (Tsystem)  resulting in a range of ΔT (23-173°C).
Our results indicate that clinopyroxene crystals show different styles of zoning across the entire range of ΔT, where zones enriched in Al are also enriched in HFSEs and REEs. At ΔT < 40 °C, clinopyroxene is sector-zoned with distinct Al-poor hourglass and Al-rich prism sectors. At ΔT = 75 – 123°C, skeletal morphologies dominate as crystal growth transitions from interface controlled to diffusion limited. These crystals are comprised of Al-rich skeletons and Al-poor overgrowths. At ΔT = 123-173°C, clinopyroxene is primarily dendritic, with subtle Al zoning.
The overall correlation between Al and trace element composition is attributed to charge-balancing mechanisms. Highly charged cations are favourably incorporated into the M1 and M2 sites with increasing Al, to compensate for the substitution of Si for Al in the tetrahedral site. 
Thermodynamic modelling of lattice strain parameters for 3+ cations in the M2 site (REEs + Y) illustrates that the strain-free partition coefficient (D0) is strongly correlated with ΔT. Conversely, the optimum radius (r0) and Young’s modulus (E), remain constant across our dataset.
The application of trace element calibrations to natural samples from Mt Etna supports the growing conception that sector zoning in clinopyroxene is related to low degrees of ΔT, whereas microlites crystallised at moderate degrees of ΔT. Our new experimental data could bring crucial new insights into magmatic processes which occur under polythermal and polybaric crystallisation regimes, in the lead up to volcanic eruptions. 


Biography

I am a current PhD candidate at the University of Queensland, supervised by Dr. Teresa Ubide. My research is focused on investigating sector-zoned clinopyroxenes from volcanic settings, through the analysis of natural and experimental samples using high-resolution techniques.

Zircon trace element geochemistry as an indicator of magma fertility in iron oxide-copper-gold provinces

Wade, Claire1, 2, Payne, Justin3, Barovich, Karin2, Gilbert, Sarah4, Wade, Benjamin4 , Crowley, James5,  Reid, Anthony1, 2 and Jagodzinski, Elizabeth1

1Geological Survey Of South Australia, , Australia, 2Department of Earth Sciences, University of Adelaide, Adelaide, Australia, 3UNISA STEM, University of South Australia, Adelaide, Australia, 4Adelaide Microscopy, University of Adelaide, Adelaide, Australia, 5Department of Geosciences, Boise State University, Boise, United States of America

The trace element signatures of zircon from Phanerozoic porphyry-related magmatic rocks and their mineralising systems have recently been applied as a means to assess potential magmatic suite fertility. The iron oxide-copper-gold (IOCG) and iron oxide-apatite (IOA) deposit family share some genetic attributes with porphyry Cu deposits, including subduction-modified magmatic sources, association with calc-alkaline to mildly alkaline magmas, and highly oxidised magmas. The observed relationship between magma fertility and zircon chemistry in porphyry Cu deposits raises the possibility that the trace element signature of zircon could also be used to assess the fertility of magmatic systems associated with IOCG and IOA systems.

Significant IOCG deposits in southern Australia (Gawler Craton) and IOA deposits in the south-central USA are associated with extrusive and intrusive felsic rocks formed as part of silicic large igneous province magmatism. Zircons from early rhyolitic and granitoid rocks coeval with IOCG mineralisation in the Gawler Craton are distinguished from younger rhyolite and granitoid zircons by their higher Eu/Eu*, Ce/Ce* and Ti values and separate magma evolution paths with respect to Hf. Higher zircon Ce/Ce* and Eu/Eu* correspond to more oxidising magmatic conditions and lower degrees of fractionation and/or crustal assimilation, respectively. Higher zircon Ti contents correspond to higher magmatic temperatures in the magmas coeval with mineralisation. In this respect, we consider higher oxidation state, lower degrees of fractionation and higher magmatic temperatures to be features of fertile magmas in southern Australian IOCG terrains.

Similar zircon REE characteristics are shared between Australian IOCG magmatic rocks and IOA rhyolites from the St Francois Mountains, Missouri. IOCG and IOA magmatic rocks have high Ce/Ce* and high Dy/Yb ratios in zircon, which are indicative of oxidised and dry magmas, respectively. Syn-mineralisation IOCG and IOA magmatic rocks are distinguished from unmineralised ones by their higher Eu/Eu* zircon signature, and higher magmatic temperatures. Zircon Dy/Yb are generally higher and Eu/Eu* are generally lower in IOCG and IOA magmatic rocks when compared with fertile porphyry Cu deposit magmatic rocks. The dry and more fractionated nature of the IOCG and IOA associated magmas contrast with the hydrous and unfractionated nature of fertile porphyry Cu deposit magmas, highlighting differences in setting and magma formation of porphyry Cu deposits and the IOCG-IOA deposit family. As indicated by high zircon Ce/Ce* ratios, the oxidised nature of mineralised IOA magmatic rocks coupled with lower degrees of fractionation and higher magmatic temperatures, are akin to fertile IOCG magmatic rocks and considered to be key elements in magma fertility in IOCG-IOA terrains.


Biography

Claire is an employee of the Geological Survey of South Australia undertaking a PhD at the University of Adelaide. Her background is in igneous geochemistry and isotope geochemistry. Her PhD is investigating the link between magmatism and mineralisation in Mesoproterozoic mineral systems in South Australia.

Transitions in Eruptive Style During the 2012 Deep Submarine Silicic Eruption of Havre Volcano, Kermadec Arc, New Zealand

Clark, Acacia1; Carey, Rebecca1; Jutzeler, Martin1; Mitchell, Samuel2

1University of Tasmania, Hobart, Australia, 2University of Bristol, Bristol, United Kingdom

Submarine eruptions are poorly understood compared to their subaerial counterparts due to challenges accessing and observing them. The 2012 silicic submarine eruption of Havre Volcano in the Kermadec Arc was the largest deep ocean eruption (~900 – 1220 meters below sea level) ever recorded.

The main vent transitioned in eruption style during the event. The current eruption framework describes the onset of magma disruption on the seafloor at high (107 kgs-1) eruption rates, which produced a large pumice raft (~1 km3) accompanied with a giant pumice seafloor deposit. This phase transitioned to an intermediate phase of unknown intensity that produced an ash-lapilli-block (ALB) deposit proximal to the vent. The final eruptive phase was low intensity (104 kgs-1) effusive magma emplacement that produced a 250 m-high dome complex (Dome OP) over the vent. Previous studies have focused on microtextures of these main phases to understand shallow conduit processes.

We have identified lobe deposits around Dome OP which stratigraphically sit above the ALB deposit but were emplaced prior to the end of the effusive phase. These deposits represent a transitional phase between high to low eruption rates. Detailed microtextural studies were conducted on four representative clasts from in-situ Dome OP together with three clasts from surrounding lobe deposits, and two dense end-member ALB clasts not previously studied.

Microlites of the same crystal types and habits are present in lobe deposits and in-situ Dome OP clasts, where they are most abundant. ALB clasts are microlite free with almost spherical vesicles. Clasts from lobe deposits have elongated vesicles with round edges and in-situ Dome clasts have elongated and flattened vesicles. ALB clasts have the highest vesicle number density, followed by lobe deposits and then in-situ Dome clasts. Rounded vapor-phase cristobalite is present in lobe deposits and in-situ Dome O clasts, whereas in-situ Dome P clasts contain an abundance of oblong cristobalite crystals that exist entirely within the groundmass. No discernible correlation could be made between vesicle size and cristobalite crystal size. Silicic submarine domes are morphologically and texturally similar to subaerial domes, indicating hydrostatic pressure has a minor role in outgassing and emplacement processes of lava domes.


Biography

Acacia graduated with a Bachelor of Science from the University of Sydney in 2015. After taking some time to explore Canada she returned to undertake her Honours at the University of Tasmania (UTAS) and graduated with First Class. She intends to begin her PhD at UTAS in late 2020.

Volcanic stratigraphy and eruption mechanisms from the last remaining outcrops at Wiri Mountain, Auckland Volcanic Field, New Zealand

Foote, April1, Németh, Károly2, Handley, Heather1

1Department of Earth and Planetary Sciences, Macquarie University, Sydney, Australia, 2Institute of Agriculture and Environment, Massey University, Palmerston North, New Zealand

Dispersed volcanic fields (commonly labelled as monogenetic volcanic fields) are of great interest in volcanology as they provide a relatively simple volcanic architecture to study regarding magma source to surface processes, edifice growth and subsequent destructions. The Auckland Volcanic Field (AVF) in New Zealand is among the few hundred documented dispersed volcanic fields worldwide that were active through the Holocene and can broadly be defined as a mafic intraplate monogenetic volcanic field. Recent research has highlighted that the transition between monogenetic and polygenetic volcanism is far more continuous than was originally thought, where volcanoes traditionally viewed as monogenetic are commonly found to have had multiple eruptions and complex magmatic plumbing systems.

This study focuses on the last remaining outcrops of Matukutūruru, or Wiri Mountain, one of the southernmost volcanic centres in the AVF. Wiri Mountain presents a unique situation where despite the large extent to which deposits have been removed, the remaining spectacular outcrops allow a clear picture to be formed, including analysis of stratigraphy and facies, vent location and extent of deposits, fragmentation depth, eruptive styles and their transitions and the eruption history of the volcanic centre.

Wiri Mountain has had a complex eruption history, beginning with a pre-existing tuff ring/maar landscape. An initial basal tuff ring was deposited by predominantly pyroclastic density currents with ballistic curtain deposits and some pyroclastic fall, through a debris filled vent that widened mostly at depth through the course of the eruptions. At least two smaller, satellite tuff rings were then deposited on the outer flanks of the first by a combination of pyroclastic density currents and pyroclastic fall, with a transition from phreatomagmatic to Strombolian eruptive style. A central scoria cone was then deposited within the initial tuff ring, followed by lava spatter and lava flows that covered the tuff rings, the scoria cone, and the surrounding area. This complex eruption history highlights the range and transition of eruptive styles leading to the production of multiple types of eruptive products and deposits that can be typical for the AVF.

The small magma volumes typical of monogenetic volcanism allow for significant influence of fragmentation and eruptive products by external water, resulting in a wide variety of volcanic landforms. However, based on the results of this study, volcanic activity at Wiri Mountain and the surrounding area of the southern end of the AVF was potentially more complex than would typically be expected from the textbook definition of monogenetic activity, with a large enough eruptive volume to allow a complex eruptive evolution over time, multiple satellite cones, and potential connections to nearby centres; highlighting the grey area on the concept boundary of monogenetic volcanism.


Biography

April Foote is a PhD candidate at Macquarie University. Her studies are based on the volcanology of the southern end of the Auckland Volcanic Field.

Geophysical and geochemical constraints on the formation of Holocene intraplate volcanism in East Asia

Ward, Jack F.1; Rosenbaum, Gideon1; Ubide, Teresa1; Wu, Jonny2; Caulfield, John T. 1,3; Sandiford, Mike4; Gürer, Derya1

1School of Earth and Environmental Sciences, The University of Queensland, Brisbane, Australia, 2Department of Earth and Atmospheric Sciences, University of Houston, Houston, USA, 3Central Analytical Research Facility, Institute for Future Environments, Queensland University of Technology, Brisbane, Australia, 4School of Earth Sciences, University of Melbourne, Melbourne, Australia

East Asia contains many Holocene volcanic centres, several of which are located far (between 600 and 1500 km) from the Pacific and Philippine Sea plate subduction zones. The origin of these intraplate volcanoes, which include Jeju, Ulleungdo, Tianchi, Jingbohu, Erkeshan and Wudalianchi, remains enigmatic. Geodynamic processes proposed to explain the occurrence of the East Asian Holocene intraplate volcanoes include mantle plume activity, subduction processes with slab fluid involvement, and subduction processes without slab fluid involvement. Here, we evaluate a variety of geophysical datasets and a compilation of geochemical data to assess the feasibility of these mechanisms. High-resolution tomography data provide no evidence for the rise of deep-seated mantle plumes. Instead, the tomographic and seismic data highlight the stagnation of the Pacific slab at the 660 km discontinuity below Tianchi, Longgang, Jingbohu, Erkeshan and Wudalianchi. The geophysical data also provide evidence for the stagnation of the Philippine Sea slab at the 410 km discontinuity below Jeju and Ulleungdo. Although the intraplate volcanoes appear to be located above subducted slabs, the geochemical data do not provide evidence for melt generation due to slab metasomatism. Instead, the intraplate volcanoes are alkaline in composition and display primitive mantle normalised trace element characteristics comparable to those shown by ocean island basalts. In light of the absence of evidence for plume activity or slab metasomatism, we suggest that convective upwellings occurring at the edges of the Pacific and Philippine Sea slabs may be responsible for Holocene intraplate volcanism in East Asia. Because it is likely that the Pacific and Philippine Sea slabs have been stagnant in the mantle transition zone for millions of years, we speculate that slab-edge convection and volcanism may be driven by regional-scale tectonic events. We conclude by discussing possible Neogene–Quaternary tectonic events that may have contributed to the occurrence of East Asian Holocene intraplate volcanism. 


Biography

Jack Ward is a PhD student at the School of Earth and Environmental Sciences, The University of Queensland. He uses geophysical and geochemical data to better understand the processes that cause anomalous subduction-related magmatism.

Timescales of magma ascent recorded by olivine zoning patterns from Mount Leura and Mount Noorat, Newer Volcanics Province, Australia

Didonna, Rosa1, Handley, Heather1, Cas, Ray2, Fidel, Costa3, Murphy, Timothy1

1Department of Earth and Environmental Sciences, Macquarie University, Sydney, NSW, Australia 2School of Earth, Atmosphere and Environment, Monash University, Clayton, Victoria, Australia 3Earth Observatory of Singapore, Nanyang Technological University, Singapore

Intraplate continental basaltic volcanic provinces (ICBVPs) occur on all continents, but the timescales of magmatic processes that lead to eruption in such settings are poorly understood due to the temporal infrequency and lack of spatial pattern in eruptions. Therefore, unravelling the timescale of magma ascent is a critical aspect to advance our understanding of volcanic hazard and risk. Here we focus on the Newer Volcanics Province (NVP) of SE Australia is an active intraplate basaltic province that contains over 400 volcanic centres. Volcanic landforms include maars, tuff rings, scoria cones, lava fields along with more complex eruption centres and the rate of activity has varied in space and time. Despite a large number of studies on the bulk-rock geochemistry and physical volcanology of the deposits (e.g. stratigraphy, eruption styles), few constraints are available on the timescales of magma ascent in the province.

We have investigated the olivine crystals within entrained mantle xenoliths and as individual crystals within the groundmass of basaltic volcanic rocks using compositional X-ray maps, backscattered electron (BSE) images and electron microprobe analyses (EMPA). We focus on NVP samples from Mount Leura (Lehurra kang) and Mount Noorat (Knorart) to shed a light on the dynamic processes that lead to the eruption and the relative timescales of magma ascent. Olivine crystals in mantle xenoliths are mainly unzoned with Mg#90 (Mg# = 100×Mg/[Mg+Fe]) with few crystals normally zoned (<Mg#75 rim). Olivine grains in the groundmass are commonly up to 1 mm in size and are mainly skeletal, with significant variation in Mg#, CaO, MnO and NiO content from core to rim. Olivine grains in the Mount Noorat samples are largely normally zoned with crystal interiors characterised by >Mg#90 and rims by <Mg#75. Olivine crystals from Mount Leura contain complexly zoned olivines suggesting a more complex crystallisation and transport history (Mg#77-79 cores and rims up to Mg#85). We model the chemical zonation patterns in olivine crystals that reveal a short time of magmatic processes before the eruption. The insight of magma storage, ascent and the pathway to the surface at NVP is a crucial information in understanding the volcanic hazard and mitigation risk in the region for which too little consideration is still given at the present.


Biography

Research Fellow in the Dept of Earth and Environmental Sciences at Macquarie University. PhD from Macquarie University in April 2020.

Research interests in igneous petrology and volcanology. Focus on crystal records of magmatic processes and timescales in the lead-up to volcanic eruptions combining field observations and detailed microscopy analytical techniques.

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