Elsevier

Quaternary Geochronology

Volume 6, Issues 3–4, June–August 2011, Pages 356-368
Quaternary Geochronology

New 40Ar/39Ar ages for selected young (<1 Ma) basalt flows of the Newer Volcanic Province, southeastern Australia

https://doi.org/10.1016/j.quageo.2011.03.002Get rights and content

Abstract

The Pliocene-Holocene Newer Volcanic Province (NVP) of southeastern Australia is an extensive, relatively well-preserved, intra-plate basaltic lava field containing more than 400 eruptive centres. This study reports new, high-precision 40Ar/39Ar ages for six young (300–600 ka) basalt flows from the NVP and is part of a broader initiative to constrain the extent, duration, episodicity and causation of NVP volcanism. Six fresh, holocrystalline alkali basalt flows were selected from the Warrnambool-Port Fairy area in the Western Plains sub-province for 40Ar/39Ar dating. These flows were chosen on the basis of pre-existing K-Ar age constraints, which, although variable, indicated eruption during a period of apparent relative volcanic quiescence (0.8–0.06 Ma).

40Ar/39Ar ages were measured on multiple aliquots of whole rock basalt samples. Three separate flows from the Mount Rouse volcanic field yielded concordant 40Ar/39Ar age results, with a mean eruption age of 303 ± 13 ka (95% CI). An older weighted mean age of 382 ± 24 ka (2σ) was obtained for one sample from the central Rouse-Port Fairy Flow, suggesting extraneous argon contamination. Two basalt flows from the Mount Warrnambool volcano also yielded analogous results, with an average 40Ar/39Ar age of 542 ± 17 ka (95% CI). The results confirm volcanic activity during the interval of relative quiescence. Most previous K-Ar ages for these flows are generally older than the weighted mean 40Ar/39Ar ages, suggesting the presence of extraneous 40Ar. This study demonstrates the suitability of the 40Ar/39Ar incremental-heating method to obtain precise eruption ages for young, holocrystalline alkali basalt samples in the NVP.

Highlights

► First 40Ar/39Ar study of <1 Ma basalts from the Newer Volcanic Province, SE Australia. ► A revised eruption age for the Mt Rouse volcanic centre is 303 ± 13 ka (95% CI). ► A revised eruption age for the Mt Warrnambool volcanic centre is 542 ± 17 ka (95% CI). ► Confirms extensive volcanism in the NVP during period of apparent relative quiescence.

Introduction

The Neogene-Quaternary Newer Volcanic Province (NVP) of central and western Victoria, Australia, represents one of the more extensive, well-preserved and diverse continental, intra-plate basaltic lava fields. The Province incorporates in excess of 400 separate eruptive centres distributed over an area of more than 15,000 km2 (e.g. Sutalo and Joyce, 2004) (Fig. 1). The NVP was produced by intermittent (dominantly tholeiitic to alkalic), low-volume volcanism that initiated at ∼4.5 Ma and continued to recent times. It has been estimated that at least a dozen volcanoes erupted over the past 20,000–30,000 yr (Joyce, 2005). The youngest of these is arguably Mount Gambier, located in the extreme west of the Province, with a 14C-constrained minimum eruption age of 5.5 ± 0.1 cal ka BP (1σ) (pers. comm., P. DeDeckker). Given the protracted and recent history of volcanism, Joyce, 2004, Joyce, 2005, Joyce, 2006 argued that the NVP should be considered an active volcanic province and advocated the development of volcanic hazard assessment plans for the region. One method for assessing the volcanic hazard potential of the NVP is compilation of a precise and accurate chronology of volcanism to establish eruption duration, episodicity and frequency.

The NVP has been the subject of numerous K-Ar geochronological studies (McDougall et al., 1966, Aziz-ur-Rahman and McDougall, 1972, McDougall and Gill, 1975, Gill, 1981, Singleton et al., 1976, Ollier, 1985, Wallace and Ollier, 1990, Gray and McDougall, 2009), with the timing of more recent (<60 ka) volcanism constrained from 14C (e.g. Blackburn et al., 1982) and cosmogenic 36Cl (Stone et al., 1997) and 21Ne analyses (Gillen et al., 2010). However, it is well known that the K-Ar dating method has limitations in terms of analytical precision (>1–2%) and assessment of argon loss or gain. These problems may be particularly acute for young basalts, where weathering/alteration can cause argon loss, and the presence of xenocrysts or volcanic glass may yield extraneous (excess or inherited) argon. As a consequence, K-Ar ages may be under- or over-estimated, which complicates efforts to establish a precise chronology of volcanic events. The 40Ar/39Ar step-heating method has the potential to ameliorate many of these problems. However, to date, only one 40Ar/39Ar result has been published for a NVP lava flow (4.19 ± 0.08 Ma; Hare et al., 2005).

Although far from comprehensive, the available geochronological data indicate that NVP volcanism initiated at ca. 4.5 Ma in the Melbourne region and Central Uplands sub-province (McDougall et al., 1966, Aziz-ur-Rahman and McDougall, 1972). Thereafter, volcanism spread throughout southern and western Victoria (Western Plains sub-province), with peak activity in the time interval between 3.0 Ma and 1.4 Ma (Gray and McDougall, 2009). Subsequent eruption episodes are recorded at ca. 1.0–0.8 Ma (Gray and McDougall, 2009) and ca. 60–5 ka (e.g. Blackburn et al., 1982, Stone et al., 1997, Joyce, 2004). The interval from ca. 0.8–0.06 Ma appears to represent a period of relative quiescence, with only three eruption events yielding K-Ar ages within this interval. However, this interval apparently includes the extensive lava fields associated with the Mount Rouse volcano as well as lava flows in the Mount Warrnambool area (Fig. 2). In both cases, reported K-Ar ages vary widely, ranging from 0.3 to 1.8 Ma and from 0.5 to 2.1 Ma, respectively (Table 1). Consequently, accurate age data for these volcanic successions is important for establishing the episodicity of volcanism in the NVP for this period.

In the current study, we present new, high precision 40Ar/39Ar step-heating analyses for selected lava flows from the Mount Rouse and Mount Warrnambool areas (Fig. 2). The aims of the study are: i) to test the veracity of existing K-Ar age data and confirm volcanic activity during a period of apparent quiescence; ii) to assess the potential affects of argon loss and/or extraneous argon contamination in these lavas; iii) to refine sample preparation and 40Ar/39Ar analytical techniques applicable to young (<1 Ma) basalts in order to improve the precision and accuracy of age estimates; and iv) to determine the duration of Mount Rouse volcanism and the source of the Mount Warrnambool area lava flows. This study constitutes the first phase of a broader initiative to generate high precision 40Ar/39Ar data across the NVP, in order to constrain the extent, duration, episodicity and causation of volcanism, enhance stratigraphic correlations, and improve volcanic hazard assessments in this populous region of south-east Australia.

Section snippets

Regional geology

The Newer Volcanic Province (NVP) of central and western Victoria overlies reworked Cainozoic sedimentary rocks and Palaeozoic meta-sedimentary rocks of the Lachlan and Delamarian Fold Belts (Fig. 1). The NVP is the youngest manifestation of intermittent basaltic volcanism in south-eastern Australia that has been ongoing since ca. 190 Ma. Based on available (mostly K-Ar) geochronology, the volcanism appears to have commenced soon after the breakup of Gondwana, with three main peaks of activity

Sample selection and description

Basalt samples were collected from three distinct flows associated with Mount Rouse, one flow from Mount Warrnambool and one flow of unknown origin outcropping at Hopkins Falls (Table 2). Road cuttings and quarries were targeted as these tend to contain thick, fresh profiles suitable for 40Ar/39Ar studies. Approximately two kilograms of sample was extracted from each site using a masonry chisel and sledgehammer. In order to minimise possible extraneous argon contamination problems, fresh,

Sample preparation and irradiation

The glass content and extent of weathering for each sample was determined by thin-section examination. Acceptable sample fragments were crushed to ∼2 cm chips using a jaw crusher. Individual chips were then screened for alteration and large vesicles, with acceptable chips crushed manually using a steel piston crusher. Crushed samples were washed and sieved to a 0.5–2 mm grainsize. To minimise possible argon loss and extraneous argon contributions, whole rock chips were handpicked using a

Results

40Ar/39Ar step-heating analyses, obtained for 6 samples from the Mount Rouse and Mount Warrnambool area lavas, are summarised in Table 3 and displayed in age spectra, and inverse isochron diagrams (Fig. 5, Fig. 6). Detailed 40Ar/39Ar data from individual step-heating experiments are provided in Supplementary Table 1.

Mount Rouse lava flows

Three of the four samples from the Mount Rouse lava flows yielded consistent 40Ar/39Ar ages of 309 ± 20 ka (95% CI), 301 ± 27 ka (95% CI) and 294 ± 25 ka (95% CI), with a mean overall age of 303 ± 13 ka (95% CI; MSWD = 0.45, p = 0.64) The more problematic sample, NVP06 (central Rouse-Port Fairy Flow), yielded two discordant age spectra, and gave a weighted mean age significantly older than the other three samples (382 ± 24 ka 95% CI). As noted above, McDougall and Gill (1975) reported older,

Conclusions

The 40Ar/39Ar data reported here for Mount Rouse and Mount Warrnambool, are in broad agreement with previous K-Ar studies, but are generally more precise. We propose a revised eruption age of 303 ± 13 ka (95% CI) for Mount Rouse, although note the possibility of an earlier phase of activity that produced the Tarrone Flow. For Mount Warrnambool we propose a revised eruption age of 542 ± 17 ka (95% CI). The presence of excess argon was detected in some aliquots. It is therefore recommended that a

Acknowledgements

The authors thank Bernie Joyce for sharing his invaluable knowledge of the geomorphology and geology of the Western District and for his assistance with the fieldwork, aerial photography interpretation and commentary on the manuscript. We also thank Janet Hergt for discussions on NVP geology and support with fieldwork. We acknowledge Stan Szczepanski for technical assistance with 40Ar/39Ar analyses. This study was supported by an Australian Research Council Discovery Grant (DP0986235) awarded

References (55)

  • P.R. Renne et al.

    The isotopic composition of atmospheric argon and 40Ar/39Ar geochronology: time for a change?

    Quaternary Geochronology

    (2009)
  • P.R. Renne et al.

    Joint determination of 40K decay constants and 40Ar∗/40K for the Fish Canyon sanidine standard, and improved accuracy for 40Ar/39Ar geochronology

    Geochimica et Cosmochimica Acta

    (2010)
  • B.S. Singer et al.

    Age and duration of the Matuyama-Brunhes geomagnetic polarity reversal from 40Ar/39Ar incremental heating analyses of lavas

    Earth and Planetary Science Letters

    (1996)
  • R.H. Steiger et al.

    Subcomission on geochronology: Convention on the use of decay constants in geo- and cosmochronology

    Earth and Planetary Science Letters

    (1977)
  • S. Valkiers et al.

    Preparation of argon Primary Measurement Standards for the calibration of ion current ratios measured in argon

    International Journal of Mass Spectrometry

    (2010)
  • P. Wellman et al.

    Cainozoic igneous activity in eastern Australia

    Tectonophysics

    (1974)
  • Aziz-ur-Rahman et al.

    Potassium-argon ages on the Newer Volcanics of Victoria

    Proceedings of the Royal Society of Victoria

    (1972)
  • G. Blackburn et al.

    Further evidence on the age of the tuff at Mt Gambier, south Australia

    Transactions of the Royal Society of South Australia

    (1982)
  • B.E. Cohen et al.

    40Ar/39Ar constraints on the timing and origin of Miocene leucitite volcanism in southeastern Australia

    Australian Journal of Earth Sciences

    (2008)
  • B.E. Cohen et al.

    40Ar/39Ar constraints on the timing of Oligocene intraplate volcanism in southeast Queensland

    Australian Journal of Earth Sciences

    (2007)
  • E.L. Garner et al.

    Absolute isotopic abundance ratios and the atomic weight of a reference sample of potassium

    Journal of Research of the National Bureau of Standards (U.S.)

    (1975)
  • E.D. Gill

    Potassium/argon age of basalt in floor of Hopkins River, Allansford, S.W. Victoria, Australia

    Victorian Naturalist

    (1981)
  • C.M. Gray et al.

    K-Ar geochronology of basalt petrogenesis, Newer Volcanic Province, Victoria

    Australian Journal of Earth Sciences

    (2009)
  • A.G. Hare et al.

    Magnetic and chemical stratigraphy for the Werribee Plains basaltic lava flow-field, Newer Volcanics Province, southeast Australia: implications for eruption frequency

    Australian Journal of Earth Sciences

    (2005)
  • Henley, K.J., Webb, A., 1990. Radiometric dating on various granites and Newer Volcanics basalts. Geological Survey of...
  • B. Joyce

    Western volcanic plains, Victoria

  • B. Joyce

    The young volcanic regions of southeastern Australia; early studies, physical volcanology and eruption risk

    Proceedings of the Royal Society of Victoria

    (2004)
  • Cited by (39)

    • <sup>40</sup>Ar/<sup>39</sup>Ar geochronology of the Pongkor low sulfidation epithermal gold mineralisation, West Java, Indonesia

      2020, Ore Geology Reviews
      Citation Excerpt :

      Five irradiated samples (CGT3.5-159.90, CGT3.5-344.10, CGT5A-107.75, PNG4c and PNGKC6a) which had been grouped into eight aliquots (Po1a, Po1b, Po2_1, Po2_2, Po3, Po5_1, Po5_2 and Po7) were analysed using a new generation ARGUSVI mass spectrometer in the Noble Gas Laboratory of the School of Earth Sciences at the University of Melbourne. The detail analytical techniques and procedures were similar to those reported in previous publications (Phillips et al., 2007; Matchan and Phillips, 2011; Phillips and Matchan, 2013). The aliquots were analysed with a schedule of 3–10 temperature steps from 0.23 to 1.55 W.

    View all citing articles on Scopus
    View full text