New 40Ar/39Ar ages for selected young (<1 Ma) basalt !ows of theNewer Volcanic Province, southeastern Australia

E. Matchan a,*, D. Phillips a

a School of Earth Sciences, University of Melbourne, Parkville, Victoria 3010, Australia

a r t i c l e i n f o

Article history:Received 3 September 2010Received in revised form17 February 2011Accepted 8 March 2011Available online 16 March 2011

Keywords:Basalt40Ar/39Ar geochronologyEruption frequencyVictoriaAustralia

a b s t r a c t

The Pliocene-Holocene Newer Volcanic Province (NVP) of southeastern Australia is an extensive, rela-tively well-preserved, intra-plate basaltic lava "eld containing more than 400 eruptive centres. Thisstudy reports new, high-precision 40Ar/39Ar ages for six young (300e600 ka) basalt !ows from the NVPand is part of a broader initiative to constrain the extent, duration, episodicity and causation of NVPvolcanism. Six fresh, holocrystalline alkali basalt !ows were selected from the Warrnambool-Port Fairyarea in the Western Plains sub-province for 40Ar/39Ar dating. These !ows were chosen on the basis ofpre-existing K-Ar age constraints, which, although variable, indicated eruption during a period ofapparent relative volcanic quiescence (0.8e0.06 Ma).

40Ar/39Ar ages were measured on multiple aliquots of whole rock basalt samples. Three separate !owsfrom the Mount Rouse volcanic "eld yielded concordant 40Ar/39Ar age results, with a mean eruption ageof 303 ! 13 ka (95% CI). An older weighted mean age of 382 ! 24 ka (2s) was obtained for one samplefrom the central Rouse-Port Fairy Flow, suggesting extraneous argon contamination. Two basalt !owsfrom the Mount Warrnambool volcano also yielded analogous results, with an average 40Ar/39Ar age of542 ! 17 ka (95% CI). The results con"rm volcanic activity during the interval of relative quiescence. Mostprevious K-Ar ages for these !ows are generally older than the weighted mean 40Ar/39Ar ages, suggestingthe 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 theNVP.

! 2011 Elsevier B.V. All rights reserved.

1. Introduction

The Neogene-Quaternary Newer Volcanic Province (NVP) ofcentral and western Victoria, Australia, represents one of the moreextensive, well-preserved and diverse continental, intra-platebasaltic lava "elds. The Province incorporates in excess of 400separate eruptive centres distributed over an area of more than15,000 km2 (e.g. Sutalo and Joyce, 2004) (Fig. 1). The NVP wasproduced by intermittent (dominantly tholeiitic to alkalic), low-volume volcanism that initiated at w4.5 Ma and continued torecent times. It has been estimated that at least a dozen volcanoeserupted over the past 20,000e30,000 yr (Joyce, 2005). The youn-gest of these is arguably Mount Gambier, located in the extremewest of the Province, with a 14C-constrainedminimum eruption age

of 5.5 ! 0.1 cal ka BP (1s) (pers. comm., P. DeDeckker). Given theprotracted and recent history of volcanism, Joyce (2004, 2005,2006) argued that the NVP should be considered an activevolcanic province and advocated the development of volcanichazard assessment plans for the region. One method for assessingthe volcanic hazard potential of the NVP is compilation of a preciseand accurate chronology of volcanism to establish eruption dura-tion, episodicity and frequency.

The NVP has been the subject of numerous K-Ar geochrono-logical studies (McDougall et al., 1966; Aziz-ur-Rahman andMcDougall, 1972; McDougall and Gill, 1975, 1981; Singleton et al.,1976; Ollier, 1985; Wallace and Ollier, 1990; Gray and McDougall,2009), with the timing of more recent (<60 ka) volcanism con-strained 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 interms of analytical precision (>1e2%) and assessment of argonloss or gain. These problems may be particularly acute for youngbasalts, where weathering/alteration can cause argon loss, and the

* Corresponding author. Tel.: "61 (0)3 8344 7672; fax: "61 (0)3 8344 7761.E-mail addresses: e.matchan@pgrad.unimelb.edu.au (E. Matchan), dphillip@

unimelb.edu.au (D. Phillips).

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Quaternary Geochronology

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Quaternary Geochronology 6 (2011) 356e368

presence of xenocrysts or volcanic glass may yield extraneous(excess or inherited) argon. As a consequence, K-Ar ages may beunder- or over-estimated, which complicates efforts to establisha precise chronology of volcanic events. The 40Ar/39Ar step-heatingmethod has the potential to ameliorate many of these problems.However, to date, only one 40Ar/39Ar result has been published fora NVP lava !ow (4.19 ! 0.08 Ma; Hare et al., 2005).

Although far from comprehensive, the available geochronolog-ical data indicate that NVP volcanism initiated at ca. 4.5 Ma in theMelbourne region and Central Uplands sub-province (McDougallet 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 timeinterval between 3.0 Ma and 1.4 Ma (Gray and McDougall, 2009).Subsequent eruption episodes are recorded at ca. 1.0e0.8 Ma (Grayand McDougall, 2009) and ca. 60e5 ka (e.g. Blackburn et al., 1982;Stone et al., 1997; Joyce, 2004). The interval from ca. 0.8e0.06 Maappears to represent a period of relative quiescence, with onlythree eruption events yielding K-Ar ages within this interval.However, this interval apparently includes the extensive lava "eldsassociated with the Mount Rouse volcano as well as lava !ows inthe Mount Warrnambool area (Fig. 2). In both cases, reported K-Arages 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 thesevolcanic successions is important for establishing the episodicity ofvolcanism in the NVP for this period.

In the current study, we present new, high precision 40Ar/39Arstep-heating analyses for selected lava !ows from the Mount RouseandMountWarrnambool areas (Fig. 2). The aims of the study are: i)to test the veracity of existing K-Ar age data and con"rm volcanicactivity during a period of apparent quiescence; ii) to assess thepotential affects of argon loss and/or extraneous argon contami-nation in these lavas; iii) to re"ne sample preparation and 40Ar/39Aranalytical techniques applicable to young (<1 Ma) basalts in orderto improve the precision and accuracy of age estimates; and iv) todetermine the duration of Mount Rouse volcanism and the sourceof the Mount Warrnambool area lava !ows. This study constitutesthe "rst phase of a broader initiative to generate high precision40Ar/39Ar data across the NVP, in order to constrain the extent,duration, episodicity and causation of volcanism, enhance strati-graphic correlations, and improve volcanic hazard assessments inthis populous region of south-east Australia.

2. Geological setting

2.1. Regional geology

The Newer Volcanic Province (NVP) of central and westernVictoria overlies reworked Cainozoic sedimentary rocks andPalaeozoic meta-sedimentary rocks of the Lachlan and DelamarianFold Belts (Fig. 1). The NVP is the youngest manifestation of inter-mittent basaltic volcanism in south-eastern Australia that has beenongoing since ca. 190 Ma. Based on available (mostly K-Ar)geochronology, the volcanism appears to have commenced soonafter the breakup of Gondwana, with three main peaks of activityrecognised: 45e37 Ma, 22 Ma, 7e6 Ma, and 4.6e0 Ma (Wellman,1974; Price et al., 1997). The NVP represents the "nal phase ofactivity and has been divided into three sub-provinces based onage: the Mount Gambier, Western Plains and Central Highlands(also known as the Western Uplands) sub-provinces (Fig. 1) (Joyce,1988).

The earliest NVP volcanic activity occurred in the CentralHighlands sub-province and Melbourne region, between ca. 4.5and 2 Ma (McDougall et al., 1966; Aziz-ur-Rahman and McDougall,1972), where more than 250 scoria and lava-producing volcanoeserupted through previously uplifted and dissected Palaeozoicterrain (Joyce, 2004, 2005). Volcanism continued in the volumet-rically dominant Western Plains sub-province, with ca. 130 erup-tion points recognised. The Western Plains sub-province, whichincludes the prominent Mount Warrnambool and Mount Rousevolcanoes, comprises small monogenetic shield volcanoes, lava!ows, scoria cones and maar deposits (Figs. 1 and 2). Many of theeruptive centres have experienced both scoria- and lava-producingeruption phases.

Several K-Ar radiometric dating studies have been conductedon lavas from the Western Plains sub-province (McDougall et al.,1966; Aziz-ur-Rahman and McDougall, 1972; McDougall and Gill,1975, 1981; Singleton et al., 1976; Ollier, 1985; Wallace andOllier, 1990; Gray and McDougall, 2009). This region also boaststhe only 40Ar/39Ar geochronology study published for the NVP,involving a plagioclase separate (plateau age of 4.19 ! 0.08 Ma, 1s)from a !ow located on the Werribee Plains, in the East of theWestern Plains sub-province (Hare et al., 2005). Volcanism isconsidered to have initiated in the sub-province at ca. 4.5 Ma, witha peak of activity at 3.0e1.4 Ma. The most recent volcanic activity,

Fig. 1. Simpli"ed geology map of Victoria showing the distribution of the Newer Volcanic Province, created using the GEOL250 dataset (1:250,000 geology polygons), Department ofPrimary Industries, Victoria. The extent of the Western Plains sub-province is marked by the dashed line, modi"ed from Hare et al. (2005). Study area is indicated by the rectangle.

E. Matchan, D. Phillips / Quaternary Geochronology 6 (2011) 356e368 357

60e10 ka, produced scoria cones, maars and small shield volca-noes with associated vesicular, alkalic lavas that form theconspicuous ‘stony rises’ of the Western Plains (Skeats and James,1937). Located some 50 km west of the nearest volcanoes of the

Western Plains and containing 15 con!rmed eruption points(dominantly scoria cones), the Mount Gambier sub-province, asmentioned previously, records the youngest activity in the NVP(Mount Gambier, w5.5 ka).

As with many intra-plate volcanic provinces, the origin of theNVP has been the subject of considerable debate. Wellman andMcDougall (1974) originally proposed that the province is theproduct of a hotspot trail. Although single/multiple hotspotmodels can account for basaltic and leucititic volcanism along theeastern coast of Australia during the period 31e9 Ma (Cohenet al., 2007, 2008; Knesel et al., 2008), and possibly the 8e5 MaMacedon-Trentham suite in the Central Uplands sub-province(Wellman and McDougall, 1974; Knutson and Nicholls, 1989), it isdif!cult to reconcile such models with the NVP. The main argu-ments against a hotspot origin include the intermittent nature ofthe NVP volcanism and the lack of any clear age progression (e.g.Lister and Etheridge, 1989; Price et al., 1997; Gray and McDougall,2009).

Lister and Etheridge (1989) invoked thermal instability andcrustal thinning caused by the rifting of Australia from Antarcticaduring breakup of Gondwana as the cause of volcanism. Theseauthors proposed that thinning of the continental lithosphereinduced upwelling of underlying asthenosphere, which in turntriggered sub-continental lithospheric melting, partially expressedas volcanism. Demidjuk et al. (2007) proposed that the volcanism isthe result of secondary convection in the upper mantle caused bythe rapid differential motion of variable thickness lithosphere andunderlying asthenosphere, caused by northward migration of theAustralian plate (w6.5 cm/yr).

Fig. 2. Map of the Port Fairy - Warrnambool region indicating sampling localities chosen for this study. Stony rise basalt "ows are shaded in grey. The map was created using theGEOL250 dataset and the ‘Hamilton and Portland area topography’ dataset from the Department of Primary Industries, Victoria.

Table 1Summary of previous whole rock K-Ar ages for Mount Rouse "ows and basalts in theMount Warrnambool area.

Flow Age (ka) Error(ka, 2s)

Reference

Mount Rouse areaMount Rouse-Port Fairy Flow 320a 5 McDougall and Gill (1975)

309a 8 McDougall and Gill (1975)Tarrone Flow 415a 17 McDougall and Gill (1975)

450a 7 McDougall and Gill (1975)Hawkesdale Flow 350 20 Gray and McDougall (2009)

310 20 Gray and McDougall (2009)330 20 Gray and McDougall (2009)

Basalt bounded by scoriaat Mount Rouse

1820 40 Ollier (1985)

Mount Warrnambool areaAllansford basalt 670 30 Gill (1981)

700 30 Gill (1981)580 20 Gill (1981)

Hopkins River Flow 800 60 Henley and Webb (1990)Panmure Quarry Flow 507a 20 McDougall et al. (1966)

549a 22 McDougall et al. (1966)The Sisters 1110 120 Gray and McDougall (2009)Allansford Lava Ridge 2120 80 Henley and Webb (1990)Garvoc Lava Ridge 2190 100 Henley and Webb (1990)

a Data corrected for change in 40K decay constant (Steiger and Jäger, 1977) and40K% atomic abundance (Garner et al., 1975).

E. Matchan, D. Phillips / Quaternary Geochronology 6 (2011) 356e368358

In order to explain the trace element and isotopic signature ofthe NVP lavas, a number of petrogenetic models have beenproposed that require mixing of an OIB-type component with sub-continental lithospheric mantle, with or without assimilation ofcrustal material (e.g. McDonough et al., 1985; McBride et al., 2001).Price et al. (1997) suggested that observed variations in Sr-isotopiccomposition between the eastern and western sections of the NVPare controlled by an underlying Palaeozoic tectonic boundary.However, more recent Sr, Nd, Hf and Pb data reveal that lavasstraddling this apparent lithospheric discontinuity are indistin-guishable (Paul et al., 2005). Furthermore, Paul et al. (2005) foundno evidence for crustal assimilation and proposed that the NVPlavas are derived from a shallow, heterogeneous, asthenosphericmantle source, consistent with the thermal instability models ofLister and Etheridge (1989) and Demidjuk et al. (2007).

2.2. Local geology

2.2.1. Mount Rouse areaMount Rouse is a composite volcano located approximately

60 km northeast of Port Fairy (Figs. 2 and 3). The cone rises 120 mabove the surrounding topography, reaching an elevation of 367 mabove sea level. The associated lava !eld covers an area >450 km2

(Sutalo and Joyce, 2004). TheMount Rouse "ows are the youngest tobe dated by the K-Ar method in the Newer Volcanic Province. K-Arages obtained for these "ows range from ca. 300 ka (McDougall andGill, 1975; Gray and McDougall, 2009) to 1.8 Ma (Ollier, 1985).Detailed mapping of the Mount Rouse "ows (Sutalo, 1996; Sutaloand Joyce, 2004) identi!ed !ve major valley-!lling "ows, thelongest being the Rouse-Port Fairy Flow (Fig. 3). Airborne radio-metric data con!rms that the Rouse-Port Fairy Flow is continuousalong its 60 km length, having been channelled along palaeo-valleysincised into shallowly sloping terrain (w0.35! per km, Fig. 2), beforereaching the coast at Port Fairy.

Based on regolith mapping of "ow units, Sutalo (1996) andSutalo and Joyce (2004) proposed an eruptive sequence that initi-ated with the extrusion of the Rouse-Port Fairy and Spring "owsalong narrow, pre-existing valleys, and was followed by construc-tion of the main scoria cone (Fig. 3). In this interpretation, theTarrone Flow is considered an outbreak from the Rouse-Port FairyFlow (Sutalo, 1996). These authors suggested that the emplacementof "ows and pyroclastic material occurred over a relatively shorttimescale, on the order of a few weeks to months. However, theexisting K-Ar data appear to con"ict with this time-frame.McDougall and Gill (1975) obtained ages of 415 " 17 ka and450 " 7 ka (2s) (corrected for change in 40K decay constant) fora sample from the Tarrone Flow. However, analyses of basaltsamples from Grif!th’s Island at the end of the Rouse-Port FairyFlow yielded younger K-Ar ages of 320 " 5 ka and 309 " 8 ka (2s)(McDougall and Gill, 1975). In addition, K-Ar analyses of Hawkes-dale Flow samples yielded K-Ar ages of 350 " 20, 310 " 20 and330" 20 ka (2s, Gray andMcDougall, 2009), with aweightedmeanof 350 " 50 ka (95% CI, MSWD # 4, p. # 0.02). The Tarrone ages aredistinct from the Hawkesdale and Grif!th Island ages, at the 2slevel, suggesting that these "ows formed at different times or thatthe K-Ar results were affected by argon loss or gain. Extraneousargon contamination from xenocrysts and/or undegassed glass isa relatively common feature of many basalts; however the aboveauthors report that strict sampling criteria were followed to avoidglassy and altered samples. Extraneous argon contamination mightaccount for themuch older K-Ar age of 1.82" 0.08Ma (2s) reportedby Ollier (1985) for a basalt "ow interleaved with scoria and ashdeposits in the cone of Mount Rouse, as the latter result is clearly atodds with previous K-Ar data and stratigraphic constraints.

2.2.2. Mount Warrnambool areaThe Mount Warrnambool area (Fig. 4) hosts a number of lava

"ows of uncertain origin. The area is underlain by undifferentiatedlava plains, estimated to be 2e4 Myrs old. The lava plains thickentowards the north, with their sources now obscured or deeplyeroded (Tickell et al., 1992). A series of lava ridges in close proximityto Mount Warrnambool follow the dominantly north-southdrainage patterns (Fig. 4). One lava ridge to the north-west ofAllansford has a reported K-Ar age of 2.12 " 0.08 Ma (2s, Henleyand Webb, 1990). The Garvoc Lava Ridge, located east of MountWarrnambool (Fig. 4), was likely channelled down a proto-streamfrom an unknown eruption point in the northeast (Tickell et al.,1992). It has a reported K-Ar age of 2.19 " 0.10 Ma (2s, Henleyand Webb, 1990).

More recent eruption centres in the area include the Lake Wan-goom and Dwarroon maars, the Sisters Volcanic Complex and theMount Warrnambool volcano (Figs. 2 and 4). The Lake WangoomandDwarroonmaars are estimated to be>200 ka (Harle et al.,1999)and appear to be single phreatomagmatic eruption centres with noassociated lava "ows. The Sisters is a composite eruptive centre thathas produced scoria, tuff and laterally extensive, deeply weathered

New FlowLava ShieldHawkesdale FlowSpring FlowRouse-Port Fairy FlowTarrone Flow

elddaleowort FFlow

0 10

km

Flow

Fairy Floww

Port Fairy

Penshurst

Hawkesdale

NVP06

PF10 (Mc Dougall & Gill, 1975)

NVP21

NVP20

NVP19

PF6 (Mc Dougall & Gill, 1975)

N

38.2°

38.4°

38.1°

38.3°

142.4°142.3°142.2°142.1°142.0°

Fig. 3. Distribution of main lava "ows from Mount Rouse, modi!ed from Sutalo (1996)and Sutalo and Joyce (2004). Sampling localities of McDougall and Gill (1975) and thepresent study are indicated by white circles and black triangles respectively.

E. Matchan, D. Phillips / Quaternary Geochronology 6 (2011) 356e368 359

basalt !ows (McMullin, 1993), which have a reported K-Ar age of1.11 ! 0.12 Ma (2s, Gray and McDougall, 2009).

MountWarrnambool, which is located to the south of the SistersVolcanic Complex (Fig. 2,), has experienced a complex eruptionhistory involving phreatomagmatic, strombolian and lava-producing activity (Tickell et al., 1992). Field mapping undertakenby McMullin (1993) and Tickell et al. (1992) suggests that basalticlavas from Mount Warrnambool !owed in two lobes extendingapproximately 4.5 km to the west and southwest of the cone(Fig. 4). However, the basalt !ows are not uniformly exposed and inplaces are covered by alluvium. Consequently, the number andextent of lava !ows from Mount Warrnambool is unclear. A basalt!ow attributed to Mount Warrnambool by McMullin (1993) isexposed in the Framlingham Quarry (also known as the PanmureQuarry). The age of two basalt samples from this quarry yieldedK-Ar ages of 507 ! 20 ka and 549 ! 22 ka (95% CI, McDougall et al.,1966; ages corrected for change in 40K decay constant).

Located approximately 8 km southwest of MountWarrnambool,Hopkins Falls comprises an w11 m thick outcrop of columnarjointed basalt (see location of NVP04, Fig. 4). The provenance of thelava !ow is unknown, but it appears to have travelled southwardsdown the deeply incised Hopkins river valley (Tickell et al., 1992).This basalt has a reported K-Ar age of 800! 60 ka (2s) (Henley andWebb, 1990). The relationship between this !ow and the Allansfordbasalt, which outcrops further south in the Hopkins River (Gill,1981; Tickell et al., 1992), is unclear. K-Ar analyses of Allansfordbasalt samples produced discordant K-Ar ages of 670 ! 30 ka,700 ! 30 ka and 580 ! 20 ka (2s, Gill, 1981). The closest lava-producing volcano north of Hopkins Falls is Mount Warrnambool(Rosengren, 1994). However, the !ow margins of the stony rise

basalt !ows linked to Mount Warrnambool do not extend as far asHopkins Falls (Tickell et al., 1992) (Fig. 3). It is possible that theHopkins Falls basalt is part of an older !ow from Mount Warr-nambool, perhaps having travelled down a proto- Emu Creek andnow covered by the stony rise basalt unit from Mount Warrnam-bool and tuff deposits from Dwarroon Maar (Fig. 4). These possi-bilities are tested in the current study.

3. Sample selection and description

Basalt samples were collected from three distinct !ows associ-ated with Mount Rouse, one !ow from Mount Warrnambool andone !ow of unknown origin outcropping at Hopkins Falls (Table 2).Road cuttings and quarries were targeted as these tend to containthick, fresh pro"les suitable for 40Ar/39Ar studies. Approximatelytwo kilograms of sample was extracted from each site usinga masonry chisel and sledgehammer. In order to minimise possibleextraneous argon contamination problems, fresh, holocrystallinesamples of low vesicular content were chosen as per the rigorousK-Ar sample selection criteria outlined byMcDougall andGill (1975).

3.1. Mount Rouse samples

Sample NVP19 was collected from Grif"ths Island, in the samelocality as sampled by McDougall and Gill (1975). Sample NVP06was extracted from a road-cutting 11 km NW of Port Fairy, withNVP20 collected from a road-cutting 8 km north of Hawkesdale.Sample NVP21 was selected from a lava !ow in the southernmostcrater of the Mount Rouse complex; it is unclear whether this is thesame outcrop sampled by Ollier (1985).

Fig. 4. Geology of the Warrnambool area, modi"ed from the Port Campbell Embayment 1:100,000 geological map (Tickell et al., 1992). Sampling localities of NVP03 and NVP04 areindicated.

E. Matchan, D. Phillips / Quaternary Geochronology 6 (2011) 356e368360

In hand-specimen, all Mount Rouse samples are of a blue-grey,!ne-grained appearance. Small plagioclase laths and olivinephenocrysts are discernible. In thin-section, the samples exhibita holocrystalline texture with grain sizes rarely exceeding 1 mm.Plagioclase comprises approximately 50% of the volume, occurringas laths (up to 2 mm in length, averaging 0.5 mm) and interstitialgrains. Olivine comprises 15% of the rock with subhedral pheno-crysts averaging 0.3e0.5 mm, ranging up to 1.5 mm in size. Idding-site alteration of olivine is apparent in NVP19 and, to a lesser extent,in NVP06 and NVP20. Titanaugite comprises 25% of the rock,occurringmainly in the groundmass, but phenocrysts (up to 1.2mm)are also present in NVP20 and NVP21. The dominant iron-oxide inthese lavas is magnetite (Whitehead, 1991) and comprisesw10% ofthe rock. Fine needles of apatite (w1%) occur in interstitial plagio-clase. Vesicles are present in all samples, with contents of w20% inNVP19 (up to 5 mm) and NVP20 (up to 3 mm), 15% in NVP06 (up to8 mm, average 0.5e1 mm) and 5% in NVP21 (up to 2 mm).

3.2. Mount Warrnambool area samples

Sample NVP03 was collected from the Framlingham quarry,approximately 500 m west of the Mount Warrnambool eruptivecentre (Figs. 2 and 4). In hand-specimen, sample NVP03 is grey andhas an average grainsize of 1 mm, with clearly visible plagioclaselaths, pyroxene grains and slightly altered olivine phenocrysts.Small (<2 mm), angular vesicles comprise w1% of the volume. Inthin-section, the basalt is holocrystalline, comprising interlockingolivine and plagioclase phenocrysts set in a matrix of plagioclase,olivine, clinopyroxene (titanaugite), iron-oxides (mostly magnetite,but possibly some ilmenite), apatite and interstitial pools of !nelycrystalline feldspathic material. Modal abundances are approxi-mately 47% plagioclase, 25% olivine, 20% titanaugite, 8% iron-oxides, 0.5% apatite. Plagioclase laths show minor alterationparallel to twinning and range up to 3.5 mm in length. Olivinepheonocrysts average 0.5 mm, ranging up to 1.5 mm. Olivine rimsare altered to iddingsite and plagioclase also shows minor alter-ation. Twinned crystals of titanaugite are common and euhedralgrains commonly exhibit growth-banding.

Sample NVP04 was collected from a basalt column exposed atHopkins Falls (Figs. 2 and 4). In hand-specimen the basalt is blue-grey with an average grainsize of 1 mm, with conspicuous glassyolivine phenocrysts and large plagioclase laths. Fine (<1 mm), sub-rounded vesicles comprise w2% of the volume. In thin-section,NVP04 appears fresh and coarsely crystalline, dominated byplagioclase laths up to 3 mm long (averaging 1 mm) and subhedralolivine phenocrysts, (up to 4mm, averaging 0.8 mm) set in a matrixof plagioclase, olivine, augite, iron-oxides (averaging 0.5 mm) andapatite. Modal abundances are 46% plagioclase, 26% olivine, 22%augite, 5% opaques and 1% apatite. Interstitial pools of plagioclaseshow undulose extinction and complex twinning. Rare anhedral

augite phenocrysts (up to 1.2 mm) are also present and poorlycrystallised brown glassy material comprises w3% by volume.

4. Analytical procedures

4.1. Sample preparation and irradiation

The glass content and extent of weathering for each sample wasdetermined by thin-section examination. Acceptable sample frag-ments were crushed tow2 cm chips using a jaw crusher. Individualchips were then screened for alteration and large vesicles, withacceptable chips crushed manually using a steel piston crusher.Crushed samples werewashed and sieved to a 0.5e2mm grainsize.To minimise possible argon loss and extraneous argon contribu-tions, whole rock chips were handpicked using a binocular micro-scope, avoiding altered fragments, larger vesicles, phenocrysts andxenocrysts. Approximately 1 g of material was handpicked fromeach sample, and further divided into 5 aliquots following irradi-ation. All samples were leached for 10 min in 4% HCl, followed by10 min in 4% HNO3 to remove carbonate and then thoroughlyrinsed with deionised water in an ultrasonic bath.

Samples were loaded into aluminium foil packets and placed inquartz tubes (UM#23 and UM#25) alongwith the "uxmonitor FishCanyon Tuff sanidine (FCTs; 28.02 ! 0.14 Ma, 1s; (Renne et al.,1998)) and irradiated for 2 h in cadmium-lined cans in position5c of the McMaster University reactor, Hamilton, Canada.

4.2. Gas extraction and analysis

40Ar/39Ar step-heating analyses were conducted at the Univer-sity of Melbourne generally following procedures describedpreviously by Phillips et al. (2007). Irradiated samples were splitinto 5 aliquots of w200 mg each, and loaded into tin-foil packets.These packets were placed into a vacuum sample chamber ofa double-vacuum tantalum-resistance-furnace, linked to a VG3600mass spectrometer equipped with Daly and Faraday detectors.The extraction line and contained samples were baked for 48 hat w200". Once acceptable ultrahigh vacuum (UHV) backgroundlevels were achieved, each aliquot was degassed overnight(10e12 h) at w600 "C to reduce atmospheric argon levels prior toanalysis (e.g. Singer and Pringle, 1996). Aliquots were heatedincrementally from an idle temperature of 300 "C up to a maximumof 1450 "C. Heating to the desired step temperature was achievedover 3 min, with the temperature being sustained at each step for20 min. Gas was admitted to the mass spectrometer followingpuri!cation by two Zr-Al getters. The step-heating procedure wasoptimised for each sample by reviewing the age spectra for indi-vidual aliquots and adjusting the heating schedule accordingly.Extraction line blanks were analysed prior to analysis of eachsample, with further outgassing conducted at 1500 "C. Although

Table 2Sampling localities in the Mount Rouse and Mount Warrnambool areas.

Sample no. Eastinga Northinga Elevation (m) Additional locality notes

NVP03 649161 5759239 103 Outcrop near top of Framlingham QuarryNVP04 641491 5755971 25 Columnar-jointed basalt at base of Hopkins FallsNVP06 600148 5763362 51 Road-cutting 11 km NW of Port Fairy located at the

junction of roads C183 and C184NVP19 608704 5749503 7 Grif!ths Island, on beach near pipelineNVP20 614877 5792509 183 Road-cutting 8 km north of HawkesdaleNVP21 614712 5805514 276 Sampled from the eastern "ank of the southernmost crater

in the Mount Rouse complex. This "ow rests on, and is partiallyburied by, scoria and ash deposits

a UTM/UPS co-ordinates utilise the GDA94 datum. Uncertainties in Northings and Eastings are 5e15 m. Uncertainty in elevation is w10 m.

E. Matchan, D. Phillips / Quaternary Geochronology 6 (2011) 356e368 361

the high-temperature furnace blank was highly variable, in almostall cases, uncorrected data yielded ages that are not signi!cantlydifferent from blank-corrected ages. Mass discrimination wascalculated prior to the !rst analysis by measuring multiple (6) airaliquots from a Doer"inger pipette, yielding aweightedmean valueof 1.0075 ! 0.20% (1s) per atomic mass unit (see SupplementaryDataset Table 3). This correction value was applied to all datausing a linear relationship to interpolate across the variousisotopes, assuming that measurement of 39Ar is accurate.

A total of 53 FCT sanidine "ux monitors were fused using an Nd-YAG laser. Fusion was achieved indirectly by heating adjacent,previously outgassed, zero-age basaltic glass beads. Analyses werecarried out on a VG5400 mass spectrometer equipped with a Dalydetector, following gas puri!cation by three Zr-Al getters. Massdiscrimination for this system was determined prior to the "uxmonitor analyses by measuring air aliquots (5), which yieldedaweightedmean value of 1.0075! 0.19% (1s) per atomic mass unit.

Argon isotopic results are corrected for system blanks, massdiscrimination, radioactive decay, reactor-induced interferencereactions and atmospheric argon contamination. Decay constantsused are those reported by Steiger and Jäger (1977), Correctionfactors (!1s) for interfering isotopes were (36Ar/37Ar)Ca "0.000289 ! 1.7%, (39Ar/37Ar)Ca " 0.000680 ! 2.8%, (40Ar/39Ar)K "0.000400 ! 100% and (38Ar/39Ar)K " 0.0130 ! 38.5%. System blanklevels were monitored between analyses and found to be essen-tially atmospheric (40Ar/36Aratm " 295.5 ! 0.5 (Nier, 1950), seeSupplementary Dataset Table 2 for representative blankcompositions).

40Ar/39Ar ages were calculated relative to an FCT age of28.02 ! 0.14 Ma, 1s (Renne et al., 1998), using the decay constantsof Steiger and Jäger (1977). We acknowledge the revised FCT age28.305 ! 0.036 Ma and new decay constants proposed by Renneet al. (2010). However, employing the latter values affects thecurrent age determinations by amaximum of 1.03%, well within theanalytical uncertainties. We also acknowledge the proposedatmospheric 40Ar/36Ar ratios of 298.56 ! 0.31 (Lee et al., 2006) and298.709 ! 0.096 (Valkiers et al., 2010), but note that using eithervalue in the calculation of both the mass discrimination correctionand the blank correction has a negligible effect on the calculatedages (see Renne et al., 2009). Plateau ages were calculated usingISOPLOT (Ludwig, 2003) and are de!ned as including at least 50% ofthe 39Ar, distributed over a minimum of 3 contiguous steps with40Ar/39Ar ratios within agreement of the mean at the 95% con!-dence level (e.g. Lanphere and Dalrymple, 1978). Inverse isochronplots were also constructedwith ISOPLOT (Ludwig, 2003), using thesame data points included in the calculation of the weighted meanages. The inverse isochron regressions reveal that the trappedargon component (40Ar/36Ari) is of atmospheric compositionwithinthe uncertainties, justifying the interpretation of weighted meanages (which have smaller errors than the isochron ages) as erup-tion/crystallisation ages. Calculated uncertainties associated withmean and plateau ages include uncertainties in the J-values, butexclude errors associated with the age of the "ux monitor and thedecay constant. Errors in the J-values ranged from to 0.32e0.87%(2s, mean 0.52%), which were minor compared to errors in thesample weighted mean 40Ar/39Ar ratios, which averaged 5.1% (2s),ranging up to 7.0% (2s).

5. Results

40Ar/39Ar step-heating analyses, obtained for 6 samples from theMount Rouse and Mount Warrnambool area lavas, are summarisedin Table 3 and displayed in age spectra, and inverse isochrondiagrams (Figs. 5 and 6). Detailed 40Ar/39Ar data from individualstep-heating experiments are provided in Supplementary Table 1.

5.1. Mount Rouse samples

Stepwise heating experiments were conducted on !vealiquots of sample NVP19 (from the southern extent of theRouse-Port Fairy Flow), yielding indistinguishable plateau ages of332 ! 36 ka (1s, V19a). 317 ! 22 ka (1s, V19b); 289 ! 19 ka (1s,V19c), 330 ! 24 ka (1s, V19e) and 302 ! 19 ka (1s, V19d).Inverse isochrons indicate that 40Ar/36Ari is atmospheric at the2s level (Table 3), although MSWD values were below unity,suggesting that experimental errors may have been over-estimated. Given that the aliquots represent splits of the samehomogenised sample, analysed under analogous conditions, theplateau-forming steps from all !ve aliquots were combined inthe calculation of a preferred weighted mean age of 309 ! 20 ka(2s). The associated inverse isochron reveals an atmospheric40Ar/36Ari of 298.5 ! 3.5 (2s), and an isochron age of294 ! 27 ka (2s).

Stepwise heating experiments were carried out on four aliquotsof sample NVP06 (central Rouse-Port Fairy Flow) (Fig. 5c). AliquotsV06b and V06d yielded indistinguishable plateau ages of374 ! 26 ka (2s, V06b) and 414! 56 ka (2s, V06d), with respectiveinverse isochrons indicating atmospheric 40Ar/36Ari intercepts(Table 3). Aliquots V06a and V06c yielded statistically discordantage spectra and were excluded from age calculations (seeSupplementary Data Table 1). A weighted mean age of 382 ! 24 ka(2s) was calculated for V06 by combining the plateau age stepsfrom V06b and V06d. The associated inverse isochron (Fig. 5d)indicates an atmospheric trapped argon component (40Ar/36Ari "297.6 ! 3.4 (2s)).

Stepwise heating experiments were conducted on !ve aliquotsof sample NVP20 (from the Hawkesdale Flow) (Fig. 5e). Individualaliquots all yielded plateau ages, four of which were within error atthe 2s level: 326 ! 22 ka (1s, V20a), 250 ! 15 ka (1s, V20b),303 ! 27 ka (1s, V20c), 344 ! 20 ka (1s, V20d), 356 ! 26 ka (1s,V20e. The highest temperature (1450 #C) step of most age spectrayielded younger age results, possibly due to inadequate correctionfor a non-atmospheric 1450 #C furnace blank. Aweightedmean ageof 301! 27 ka (95% CI) was calculated from the plateau data points.The associated inverse isochron (Fig. 5f) indicates that trappedargon is of atmospheric composition (40Ar/36Ari " 295.9 ! 20.8(95% CI)) and gives an age of 313 ! 81 ka (95% CI).

Stepwise heating experiments were conducted on !ve aliquotsof sample NVP21 (from a late-stage basalt associated with scoriaand ash in the southernmost crater of the Mount Rouse) (Fig. 5g).Four analyses yielded essentially "at age spectra (excluding the!rst and last steps), with plateau ages indistinguishable at the 2slevel: 269 ! 14 ka (1s, V21b), 319 ! 20 ka (1s, V21c), 325 ! 23 ka(1s, V21d) and 238 ! 27 ka (1s, V21e). Combining the steps fromthe four age plateaus gives a weighted mean age of 280 ! 19 ka(2s) for sample NVP21. The associated inverse isochron (Fig. 5h)indicates an atmospheric trapped argon component(40Ar/36Ari " 290.7 ! 9.9 (95% CI)) and yields an isochron age of307 ! 77 ka (95% CI).

5.2. Mount Warrnambool area samples

Stepwise heating experiments were conducted on !vealiquots of sample NVP03 from the Framlingham quarry (Fig. 6a).Four aliquots exhibit plateau ages of 593 ! 31 ka (1s, V03a),561 ! 15 ka (1s, V03b), 588 ! 20 (1s, V03c), and 468 ! 37 (1s,V03c) with associated inverse isochrons yielding atmospheric40Ar/36Ari values, The remaining aliquot (V03e), produceda discordant, broadly saddle-shaped, age spectrum. The latterrelease pro!le could indicate the presence of extraneous 40Ar inV03e, possibly due to incorporation of plagioclase phenocrysts in

E. Matchan, D. Phillips / Quaternary Geochronology 6 (2011) 356e368362

larger chips. Excluding V03e, a weighted mean age of 547 ! 23(2s) is calculated by combining the plateau data points. Inclusionof V03c data gives a weighted mean age of 555 ! 25 (2s). Theassociated inverse isochron for the plateau steps reveals a trap-ped argon component of atmospheric composition (300.3 ! 14.495% CI), and an age of 527 ! 76 ka (95% CI).

Stepwise heating experiments were carried out on threealiquots of sample NVP04 from Hopkins Falls (Fig. 6c). Two of thealiquots yielded dissimilar plateau ages of 512 ! 13 ka (1s, V04 ab)and 568 ! 12 ka (1s, V04ef). Aliquot V04ch yielded a plateau age of485 ! 22 ka (1s), although the spectrum is broadly saddle-shaped.A weighted mean age of 535 ! 27 ka (96% CI) was calculated by

Table 3Summary of results from 40Ar/39Ar furnace incremental-heating experiments.a

Sample no. Plateau age Inverse isochron analysis Total gas age K-Ar ages

Aliquot no. Incrementsused "C

39Ar% Age(ka) ! 2s

MSWD p Steps used 40Ar/36Ari ! 2s

MSWD p Age (ka) ! 2s Age (ka) ! 2s Age (ka) ! 2s

NVP19V19a 750e1200 93.9 332 ! 72 0.56 0.73 2e7 299.4 ! 6.2 0.27 0.90 270 ! 110 499 ! 245 320 ! 5; 309 ! 8V19b 750e1200 92.3 317 ! 44 0.23 0.92 2e6 298.6 ! 8.9 0.19 0.90 304 ! 62 373 ! 128 (McDougall and

Gill, 1975)V19c 750e900 61.1 289 ! 38 0.86 0.46 1e4 347 ! 110 0.71 0.49 154 ! 304 393 ! 112V19e 800e1200 88.3 330 ! 48 0.44 0.72 2e5 299.4 ! 8.7 0.33 0.72 305 ! 222 427 ! 186V19d 750e1200 91.7 302 ! 38 1.50 0.19 1e5 286.5 ! 11.5 1.20 0.30 339 ! 58 337 ! 116

Weighted mean(n # 24 of 32)

309 ! 20(2s)a

0.70 0.85 Compositeisochron

298.5 ! 3.5(2s)

0.62 0.91 294 ! 27(2s)

NVP06V06b 750e1200 84.1 374 ! 26 0.70 0.62 2e7 295.9 ! 8.1 0.88 0.48 376 ! 46 593 ! 91 415 ! 17; 450 ! 7V06ca e e e e e e e e e e 529 ! 70 (McDougall and

Gill, 1975)V06aa e e e e e e e e e e 472 ! 93V06d 800e1200 84.6 416 ! 36 1.30 0.21 1e5 297.6 ! 4.6 1.70 0.17 396 ! 77 592 ! 192

Weighted mean(n # 11 of 25)

382 ! 24(2s)b

1.12 0.34 Compositeisochron

297.6 ! 3.4(2s)

1.04 0.40 368 ! 32(2s)

NVP20V20a 700e1200 82.1 326 ! 44 0.88 0.51 3e7 298.5 ! 16.0 1.03 0.38 307 ! 132 371 ! 135 350 ! 20; 310 ! 20;

330 ! 20V20b 750e1200 81.7 250 ! 30 1.10 0.36 2e7 303.0 ! 19.3 1.20 0.29 227 ! 73 282 ! 109 (Gray and McDougall,

2009)V20ca 850e1200 57.5 303 ! 54 0.70 0.63 2e4 305.8 ! 27.1 0.33 0.57 187 ! 294 350 ! 114V20d 740e1200 80.4 344 ! 40 1.90 0.09 2e6 264.6 ! 119.0

(95% CI)3.10 0.03 508 ! 605

(95% CI)373 ! 116

V20ea 800e1200 72.6 356 ! 52 1.40 0.14 1e4 271.0 ! 330.5(95% CI)

2.50 0.08 495 ! 2198(95% CI)

337 ! 121

Weighted mean(n # 25 of 32)

301 ! 27(95% CI)b

2.00 0.00 Compositeisochron

295.9 ! 20.8(95% CI)

1.70 0.02 313 ! 81 ka(95% CI)

NVP21V21aa e e e e e e e e e e 295 ! 94V21b 850e1200 77.5 269 ! 28 0.94 0.42 2e5 271.0 ! 44.1 0.93 0.39 335 ! 126 311 ! 72V21c 800e1200 92.1 319 ! 40 2.00 0.09 1e5 319.5 ! 22.5 0.92 0.43 188 ! 117 315 ! 96V21d 800e1200 88.8 325 ! 46 1.70 0.15 1e5 295.0 ! 10.4

(95% CI)2.20 0.08 322 ! 304

(95% CI)379 ! 117

V21e 800e1200 90.5 238 ! 54 1.40 0.23 2e5 246.3 ! 218.4(95% CI)

1.90 0.15 496 ! 1101(95% CI)

295 ! 145

Weighted mean(n # 18 of 31)

280 ! 19(2s)b

1.60 0.06 Compositeisochron

290.7 ! 9.9(95% CI)

1.70 0.05 307 ! 77 ka(95% CI)

NVP03V03a 800e1200 72 593 ! 62 1.03 0.39 2e4.6 294.1 ! 19.9 0.52 0.59 616 ! 413 666 ! 148 507 ! 20; 549 ! 22V03b 750e1200 78.7 561 ! 30 0.83 0.53 1e6 299.4 ! 9.9 1.03 0.39 552 ! 70 701 ! 85 (95% CI, McDougall

et al., 1966)V03ea e e e e e e e e e e 597 ! 136V03d 850e1200 54 468 ! 74 0.06 0.98 3e6 305.8 ! 52.4 0.02 0.98 424 ! 226 654 ! 140V03c 850e1000 51.9 588 ! 40 1.70 0.15 1e4 349.7 ! 77 1.30 0.28 475 ! 165 643 ! 101

Weighted mean(n # 18 of 36)

547 ! 23(2s)b

1.30 0.20 Compositeisochron

300.3 ! 14.4(95% CI)

4.90 0.00 527 ! 76 ka(95% CI)

NVP04V04 ab 700e1200 93.3 512 ! 26 1.40 0.21 1e7 289.9 ! 9.2 1.40 0.22 530 ! 32 768 ! 163 800 ! 60V04ch 750e800 56.4 485 ! 44 0.61 0.54 1e3 305.8 ! 52.4 1.20 0.27 323 ! 2505

(3 points)838 ! 130 (Henley and Webb,

1990)V04ef 700e1000 84.9 568 ! 24 1.60 0.17 1e6 317.5 ! 28.2 1.20 0.29 491 ! 96 781 ! 119

Weighted mean(n # 16 of 21)

535 ! 27(95% CI)b

2.20 0.01 Compositeisochron

301.2 ! 20.0(95% CI)

2.40 0.00 504 ! 61 ka(95% CI)

a The full analytical dataset is available as Supplementary Table 1.b Contribution of each plateau step weighted by inverse variance.c Statistically discordant age spectrum at 1s level. Data not included in age calculations.d Data from 1450 "C step forcibly excluded from age plateau due to non-atmospheric high-temperature furnace blank.

E. Matchan, D. Phillips / Quaternary Geochronology 6 (2011) 356e368 363

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Age

(Ma)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Age

(Ma)

NVP19weighted mean = 309 ± 20 ka [6.5%] (2!)

MSWD = 0.704 separate experiments (n=24 of 26)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4box heights are 1!

Age

(Ma)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0 20 40 60 80 100

Age

(Ma)

Cumulative % 39Ar

NVP06weighted mean = 382 ± 24 ka [6.3%] (2!)

MSWD = 1.124 separate experiments (n=11 of 25)

NVP20weighted mean = 301 ± 28 ka [9.0%] (95% CI)

MSWD = 2.15 separate experiments (n=23 of 32)

NVP21weighted mean = 280 ± 19 ka [8.5%] (2!)

MSWD = 1.6 5 separate experiments (n=18 of 31)

NVP19age = 294 ± 27 ka (2!)

40Ar/36Ari = 298.5 ± 3.5 (2!)MSWD = 0.6 n= 24 of 32

0.0000

0.0004

0.0008

0.0012

0.0016

NVP21age = 307 ± 77 ka (95% CI)

40Ar/36Ari = 290.7 ± 9.9 (95% CI)MSWD =1.7; n= 18 of 31

36Ar

/40Ar

39Ar/40Ar

36Ar

/40Ar

data-point error ellipses are 1!

36Ar

/40Ar

0.0000

0.0004

0.0008

0.0012

0.0016

0.0020

0.0024

0.0028

NVP20age = 313 ± 81 ka (95% CI)

40Ar/36Ari = 295.9 ± 20.7 (95% CI)MSWD =1.7; n= 25 of 32

0.0000

0.0004

0.0008

0.0012

0.0016

0.0020

0.0024

0.0028

0.0032

0.0036

36Ar

/40Ar

g

NVP06weighted mean = 382 ± 24 ka [6.3%] (2!)

MSWD = 1.124 separate experiments (n=11 of 25)

NVP20weighted mean = 301 ± 28 ka [9.0%] (95% CI)

MSWD = 2.15 separate experiments (n=23 of 32)

NVP21weighted mean = 280 ± 19 ka [8.5%] (2!)

MSWD = 1.6 5 separate experiments (n=18 of 31)

NVP19age = 294 ± 27 ka (2!)

40Ar/36Ari = 298.5 ± 3.5 (2!)MSWD = 0.6 n= 24 of 32

0.0000

0.0004

0.0008

0.0012

0.0016

NVP21age = 307 ± 77 ka (95% CI)

40Ar/36Ari = 290.7 ± 9.9 (95% CI)MSWD =1.7; n= 18 of 31

36Ar

/40Ar

36

Ar/40

Ar

36Ar

/40Ar

0.0000

0.0004

0.0008

0.0012

0.0016

0.0020

0.0024

0.0028

NVP20age = 313 ± 81 ka (95% CI)

40Ar/36Ari = 295.9 ± 20.7 (95% CI)MSWD =1.7; n= 25 of 32

0.0000

0.0004

0.0008

0.0012

0.0016

0.0020

0.0024

0.0028

0.0032

0.0036

36Ar

/40Ar

0.0000

0.0004

0.0008

0.0012

0.0016

0.0020

0.0024

0.0028

0.0032

0.0036

0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.0 4.4

NVP19weighted mean = 309 ± 20 ka [6.5%] (2!)

MSWD = 0.704 separate experiments (n=24 of 26)

NVP06age = 368 ± 32 ka (2!)

40Ar/36Ari = 297.6 ± 3.4 (2!)MSWD =1.04; n= 11 of 25

a b

hg

fe

dc

Fig. 5. 40Ar/39Ar age spectra and inverse isochron diagrams for Mount Rouse samples (NVP19, NVP06, NVP20 and NVP21). Preferred weighted mean ages are indicated in bold print.

E. Matchan, D. Phillips / Quaternary Geochronology 6 (2011) 356e368364

combining the plateau steps from each aliquot. The associatedinverse isochron (Fig. 5d) for these data points has an atmosphericintercept (40Ar/36Ari ! 301.2 " 20.0 95% CI) and gives an age of517 " 33 ka (2s).

6. Discussion

6.1. Mount Rouse lava !ows

Threeof the four samples fromtheMountRouse lava!owsyieldedconsistent 40Ar/39Ar ages of 309" 20 ka (95%CI), 301"27 ka (95%CI)and 294" 25 ka (95%CI), with amean 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,andgave aweightedmeanage signi"cantlyolder than the other threesamples (382 " 24 ka 95% CI). As noted above, McDougall and Gill(1975) reported older, disparate K-Ar ages of 450 " 17 ka and417"7ka (2s) for theTarroneFlow, suggesting intra-!owvariation inextraneous argoncontent.However, the calculated40Ar/36Ari ratio forNVP06 is atmospheric, which implies that the calculated mean40Ar/39Ar age may represent the crystallisation/eruption age of the!ow. Field relationships suggest that the Tarrone Flow is an offshootfromtheMountRouse-Port Fairy!ow, terminatinguponcontactwiththemain lava channel. Although, it is possible that theTarrone Flow isan older !ow whose southern margin was eroded by the MountRouse-Port Fairy Flow, the anomalously old 40Ar/39Ar age yielded bysample NVP06 suggests extraneous argon contamination in theRouse-Port Fairy Flow, supporting the argument that the TarroneFlow is likely a branch of the same !ow.

With some exceptions, the Mount Rouse 40Ar/39Ar ages are inreasonable agreement with the previous K-Ar results of McDougalland Gill (1975) and Gray and McDougall (2009) (Table 1). However,the current results suggest that the K-Ar ages determined for theTarrone Flow (McDougall and Gill, 1975) and the lava shield basalt(Ollier, 1985) are anomalously old, possibly due to the presence ofextraneous 40Ar.

The similarity in the Mount Rouse 40Ar/39Ar ages supports theinterpretation of Sutalo and Joyce (2004) that the series of mosteruptions took place over a short timescale. The ‘youngest’weightedmean age of 294" 25 ka (95% CI) was obtained for sampleNVP21, collected from a late-stage basalt !ow associated withscoria and ash deposits on the !ank of the southernmost crater ofthe Mount Rouse complex.

6.2. Mount Warrnambool area lava !ows

The new 40Ar/39Ar age measurements indicate that the twoWarrnambool area lava !ows, exposed in Framlingham Quarry andHopkins Falls, erupted synchronously at 547 " 23 ka (2s) and535" 27 ka (95% CI), respectively (Fig. 6). This suggests that the two!ows were derived from the same eruption source or, alternatively,from two volcanoes that were active at the same time. Theseoptions are considered below.

As noted above, the 40Ar/39Ar spectra for the FramlinghamQuarry basalt are broadly saddle-shaped, suggesting that thissample may have contained minor extraneous argon. However, theweighted mean 40Ar/39Ar age determined for this !ow, 547 " 23 ka(2s), is indistinguishable from the K-Ar age of 526 " 37 ka (2s)reported by McDougall et al. (1966).

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 20 40 60 80 100

Cumulative % 39Ar

Age

(Ma)

NVP04weighted mean = 535 ± 27 ka [4.9%] (95% CI)

MSWD =2.23 separate experiments (n=16 of 21)

NVP04weighted mean = 535 ± 27 ka [4.9%] (95% CI)

MSWD =2.23 separate experiments (n=16 of 21)

0.0000

0.0004

0.0008

0.0012

0.0016

0.0020

0.0024

0.0028

0.0032

0.0036

NVP03age = 527 ± 76 ka (2 )

40Ar/36Ari = 300.3 ± 14.4 (2MSWD = 4.9; n= 18 of 36

0.0000

0.0004

0.0008

0.0012

0.0016

0.0020

0.0024

0.0028

0.0032

0.0036

0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8

data-point error ellipses are 1

36Ar

/40Ar

36

Ar/40

Ar

39Ar/40Ar

box heights are 1Ag

e (M

a)

NVP03weighted mean = 547 ± 23 ka [4.1%] (2

MSWD = 1.35 separate experiments (n=18 of 36)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

NVP04age = 504 ± 61 ka (95% CI)

40Ar/36Ari = 301.2 ± 20.0 (95% CI)MSWD = 2.4; n= 16 of 21

a b

dc

Fig. 6. 40Ar/39Ar age spectra and inverse isochron diagrams for Mount Warrnambool area samples NVP03 and NVP04. Preferred weighted mean ages are indicated in bold print.

E. Matchan, D. Phillips / Quaternary Geochronology 6 (2011) 356e368 365

The consistency of 40Ar/39Ar results for the Hopkins Falls basalt(Fig. 6) suggests that the mean age of 535! 27 ka (95% CI, NVP04) isa reliable estimate for the time of its eruption. However, this age isdistinctly younger than the K-Ar ages of 670 ! 30 ka (2s),700! 30 ka (2s) and 580! 20 ka (2s) reported for the downstreamAllansford basalt (Gill, 1981), which is considered to be the samelava !ow. It should be noted that the latter two ages reported by Gill(1981), represent replicates of the same sample; therefore, thedisagreement between these K-Ar results may indicate sampleswith variable amounts of extraneous argon, or unspeci"ed analyt-ical problems. Similarly, it seems likely that the signi"cantly olderK-Ar age of 800 ! 60 ka (2s), calculated by Henley and Webb(1990), also re!ects the presence of extraneous argon.

The current 40Ar/39Ar ages provide new insights into the originof the Framlingham Quarry and Hopkins Falls lava !ows. The ageconcordance of sample NVP04 with NVP03 supports shared prov-enance fromMount Warrnambool. Of the other eruptive centres inthe vicinity, most are simple maars and none are known to haveproduced basalt !ows of any noteworthy volume, aside from theapparently older (1.1 ! 0.12 Ma (2s, Gray and McDougall, 2009))Sisters Volcanic Complex. Most of the eruption points of the NVPhave beenwell documented (e.g. Ollier and Joyce, 1964; Rosengren,1994) and it is unlikely that an unrecognised eruption point of sucha young age (w500 ka) exists. Therefore, although it is not possibleto conclude with certainty whether the Hopkins Falls Flow issourced from Mount Warrnambool due to deep incision of theHopkins River north of the Falls, the agreement in 40Ar/39Ar ages,together with a lack of another suitable eruptive centre in thevicinity suggests that this is the most likely scenario. We thereforereport a weighted mean eruption age for Mount Warrnambool of542 ! 17 ka (95% CI, MSWD " 0.46, p. " 0.50).

6.3. Extraneous argon and 40Ar/39Ar analytical protocols

It is possible that the older ages obtained for some aliquots of theMount Rouse samples and the Framlingham Quarry basalt (NVP03)were in!uenced by extraneous 40Ar trapped in phenocrysts orxenocrysts (such as plagioclase containing mantle-derived argon).Although the age spectra are concordant and !at, if the extraneous40Ar was uniformly distributed, it would be dif"cult to detect (e.g.Morgan et al., 2009; Ozawa et al., 2006). Ozawa et al. (2006)observed that in historically erupted basalts, the argon isotopiccompositions of 250e500 mm whole rock and groundmass sepa-rates deviated from the atmospheric mass fractionation line,whereas,180e250 mm fractions weremore concordant. Ozawa et al.(2006) proposed that crushing liberates extraneous 40Ar from !uidinclusions and vesicles,making the "ner,180e250 mmfractionmoredesirable for 40Ar/39Ar dating. Gray and McDougall (2009) useda smaller grainsize (180e300 mm) than this study and were carefulto remove all olivine phenocrysts from their groundmass separates;yet they obtained ages for the Hawkesdale Flow (weighted meanage of 330 ! 50 ka (95% CI)) indistinguishable from the current40Ar/39Ar age of 301 ! 24 ka (95% CI). This suggests minimal excessargon contamination in samples used in the current study.

The alteration of K-bearing phases may lead to anomalouslyyoung apparent whole rock K-Ar ages. Sample NVP19 containssmall amounts of glass (w5%), which was unavoidable at thissampling location. However, the only alteration observable waspartial weathering of olivine to iddingsite, with matrix glassappearing fresh. Due to the low abundance of glass in the sample,together with the fact that most of the loosely-bound gas containedin the glass/montmorillionite would be released during the currenthigh-temperature bake-out (e.g. Baksi, 1974), the effects of glass onthe calculated age of NVP19 are considered to be of negligiblesigni"cance.

6.4. Frequency of NVP eruptions

When assessing eruption periodicity and volumes in a selectedtime period, it is important to include as many relevant volcaniccentres and lava !ows as possible. This requires a reliance onexisting age data, as well as the recognition of previously undatedvolcanic centres and !ows from the speci"ed interval. The lattermay be identi"able using mapping techniques, geochemicalcomparisons and relative erosional levels, although these methodsare best suited to younger or low-volume volcanic provinces.

In the case of the NVP, the assessment of eruption frequency andvolumes is complicated by the paucity of age data and the obser-vation that some previously reported K-Ar ages appear to beerroneously old (e.g. Ollier, 1985; Henley and Webb, 1990). Inaddition, many NVP lava !ows have subdued topographic expres-sions, limited outcrop and similar geochemical signatures, whichlimits efforts to trace individual !ows. Therefore, robust estimatesof NVP eruption periodicity and volumes requires more detailedmapping and 40Ar/39Ar geochronology.

For the period of relative NVP quiescence (0.80e0.06 Ma), thenew 40Ar/39Ar data con"rm volcanic activity at two eruptivecentres in the Western Plains sub-province during this interval. Inaddition, undated eruption centres from this period may be rec-ognised from geomorphology studies in the region. For example,Joyce (2003) sub-divided the Western Plains sub-province intoseveral regolith-landform units, with the youngest being theEccles (0.0e0.2 Ma) and Rouse (0.2e1.0 Ma) units. Gray andMcDougall (2009) analysed samples from several !ows withinthe Rouse regolith-landform unit, and noted a good correlationbetween their K-Ar ages and the regolith age of this unit(0.2e1 Ma). Undated volcanic centres included in the Rouseregolith-landform unit include the Mount Noorat, Mount Menin-gorot and Mount Gellibrand volcanic centres (20 km NE, 35 km NEand 90 km E of Mount Warrnambool respectively). However, asnoted above, tracing individual lava !ows, particularly thoserelated to older centres, is a non-trivial task. For example, theHopkins Falls basalt !ow is not de"ned by stony rises, and wasmapped previously as part of the undifferentiated lava plains,which are considered to be >2.0 Ma (Tickell et al., 1992). There-fore, it is likely that other young (1e0.2 Ma), valley-"lling !ows,not previously dated due to their subdued pro"le and limitedoutcrop, exist in the Western Plains sub-province. Further40Ar/39Ar dating is again required.

Another important factor in!uencing the assessment of erup-tion episodicity in the NVP is sampling bias. In this regard, the NVPK-Ar age data (e.g. Wellman, 1974; Gray and McDougall, 2009),pertain only to lava !ows,w80% of which are located in the CentralHighlands sub-province, thus ignoring single-eruption scoria conesand phreatomagmatic eruptions. Furthermore, the age distributionof lava !ows in the Central Uplands and northern Western Plainsare not well represented, K-Ar studies in these areas are sparse(McDougall et al., 1966; Aziz-ur-Rahman and McDougall, 1972).Although rare across the NVP as a whole, maars are relativelycommon in the Western Plains sub-province, which contains some32 recognised maars/tuff rings (Joyce, 2005). Their presence is dueto underlying water-rich sedimentary rocks of the Tertiary OtwayBasin (Ollier and Joyce, 1964). In general, the eruption ages of thesemaars are poorly constrained, and, where available, are onlyminimum ages. Although many are well preserved and consideredto be younger than ca.60 ka, studies of some crater lake sedimen-tary facies have revealed depositional histories spanning severalhundred thousand years. Examples of these include LakeWangoom(>200 ka; Harle et al., 1999) and Lake Terang (>500 ka; Kershawet al., 2004), in the Mount Warrnambool area. The incorporationof more precise eruption age constraints for older (>60 ka)maars in

E. Matchan, D. Phillips / Quaternary Geochronology 6 (2011) 356e368366

eruption frequency studies would result in a more realistic repre-sentation of volcanic activity across the entire NVP.

7. Conclusions

The 40Ar/39Ar data reported here for Mount Rouse and MountWarrnambool, are in broad agreement with previous K-Ar studies,but are generallymore precise.Wepropose a revised eruption age of303 ! 13 ka (95% CI) for Mount Rouse, although note the possibilityof an earlier phase of activity that produced the Tarrone Flow. ForMount Warrnambool we propose a revised eruption age of542 ! 17 ka (95% CI). The presence of excess argon was detected insome aliquots. It is therefore recommended that a smaller grainsize(180e250 mm) be processed during preparation of future ground-mass separates (e.g. Ozawa et al., 2006), to mitigate this issue. Wenote that our ages are in some cases signi!cantly younger than thepre-existing K-Ar ages, probably due to the presence of undetectedextraneous argon in some of the K-Ar samples.

Our data con!rm the occurrence of volcanic activity duringa period of apparent relative quiescence (0.8e0.06 Ma) in the NVP.It is likely that additional "ows of this age are present in theWestern Plains sub-province. In order to address this, the work ofGray and McDougall (2009) may be advanced by samplingcurrently undated "ows mapped as part of the Rouse regolith-landform unit (Joyce, 2003) for 40Ar/39Ar step-heating analysis. Inaddition, we note the importance of obtaining better ageconstraints on phreatomagmatic deposits of the Western PlainsSub-Province, as neglecting these in eruption frequency studies willskew results. Such work would yield a better understanding oferuption frequency in the Western Plains since 0.8 Ma.

Acknowledgements

The authors thank Bernie Joyce for sharing his invaluableknowledge of the geomorphology and geology of the WesternDistrict and for his assistance with the !eldwork, aerial photog-raphy interpretation and commentary on the manuscript. We alsothank Janet Hergt for discussions on NVP geology and support with!eldwork. We acknowledge Stan Szczepanski for technical assis-tance with 40Ar/39Ar analyses. This study was supported by anAustralian Research Council Discovery Grant (DP0986235) awardedto DP. Reviews by M. Heizler, an anonymous reviewer and P. Renne(editor) have improved the manuscript.

Appendix. Supplementary data

Supplementary data associated with this article can be found inthe online version, at doi:10.1016/j.ejvs.2011.03.002.

Editorial Handling by: Paul R. Renne

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