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