Parallel Plumbing Systems Feeding a Pair of Coeval Volcanoes in Eastern Australia

Parallel Plumbing Systems Feeding a Pair of Coeval Volcanoes in Eastern Australia Abstract Eastern Australia hosts a long track of Cenozoic age-progressive volcanoes, mostly alkaline in composition. Of these, Warrumbungle and Comboyne are coeval and occur at the same latitude (31°S), but they are ∼300 km apart, on either side of the Great Dividing Range. The lavas from both volcanoes often contain complex crystal assemblages, including plagioclase, olivine and clinopyroxene, which permit a comparative study of pre-eruptive magma histories in a large, complex, continental setting. Here we combine mineral and whole-rock geochemistry with 40Ar/39Ar geochronology to temporally constrain the processes operating in the magma plumbing systems. 40Ar/39Ar geochronology indicates that volcanic activity took place for ∼3 Myr, in two separate stages. The first stage (18–17·5 Ma) is evident only at the larger Warrumbungle volcano. In Stage 2 (∼17–15·5 Ma) the two volcanoes were active contemporaneously. The dominantly porphyritic and relatively evolved (MgO from 7·25 to 0·39 wt %) nature of the lavas suggests that the magmas stalled and differentiated in the crust prior to eruption. At the Warrumbungle volcano, Stage 1 magmas fractionated olivine and minor clinopyroxene and subsequently differentiated during ascent. The crystal cargo in Stage 2 magmas at the Warrumbungle volcano became increasingly more complex with time and the samples have been divided into two subgroups, according to age and petrological variation. Stage 2.1 magmas sampled olivine, clinopyroxene and plagioclase mushes at Moho depths of ∼41 km. Disequilibrium textures in plagioclase and clinopyroxene macrocrysts indicate differences in composition between the mush and the ascending magmas. Stage 2.2 magmas, by contrast, carried a combination of antecrysts and phenocrysts. Clinopyroxene antecrysts show strong disequilibrium textures and are reversely zoned. In plagioclase, anorthite contents increase close to the rim of the crystals, to levels (An60–55) similar to those found at the core of primitive, normally zoned, euhedral antecrysts (An53–50). At the Comboyne volcano mineral phases have a similar complexity to those of Stage 2.2 at the Warrumbungle volcano, with disequilibrium textures and reversely zoned antecrysts providing evidence of magma mixing, only lacking the primitive, normally zoned, euhedral plagioclase crystals. The complex crystal assemblage evident in Stage 2.2 lavas at the Warrumbungle volcano and throughout Stage 2 at the Comboyne volcano indicates a coeval rejuvenation of evolving crystal–melt mushes with the intrusion of more primitive, hotter, and crystal rich or -poor magmas shortly before eruption. Forward modelling using Rhyolite-MELTS replicates the composition of melts and fractionated minerals along a polybaric fractional crystallization path at depths from 24 to 7 km at the Warrumbungle volcano and from 15 to 7 km at Comboyne, supported by barometry estimates on clinopyroxene crystals. This study has identified that the two temporally associated, but spatially discrete, continental alkaline volcanoes were fed by parallel plumbing systems, which become more complex throughout the life of the volcanoes. Multiple mush zones, in which magmas stagnated and fractionated, were periodically replenished with more primitive magmas, triggering eruptions intermittently over a protracted period of ∼3 Myr. INTRODUCTION The understanding of magmatic plumbing systems has drastically improved in recent decades. The traditional view of a magma chamber that consists of a large pool of melt located below the surface (Daly, 1911) has evolved to one that envisages a series of interconnected dyke and sill networks transecting the crust (e.g. Marsh, 1996; Hildreth & Wilson, 2007; Bachmann & Bergantz, 2008; Cashman et al., 2017; Cooper, 2017; Ubide & Kamber, 2018). Magmatic plumbing systems are now understood as open systems that reflect interplay between successive magma recharge, magma mixing, assimilation and crystallization processes (e.g. DePaolo, 1981; Kent et al., 2010; O’Neill & Jenner, 2012; Bohrson et al., 2014; Bragagni et al., 2014; Coogan & O’Hara, 2015; Marzoli et al., 2015; Ubide et al., 2015). The resulting volcanic products may carry a complex assortment of phenocrysts, recycled antecrysts and country-rock or mantle xenoliths that record a plethora of deep processes preceding eruption (e.g. Dobosi & Fodor, 1992; Bachmann & Bergantz, 2004; Hildreth, 2004; Davidson et al., 2007; Ubide et al., 2014a). The identification of magma recharge in the chemical stratigraphy of crystals has attracted major attention, as mafic replenishment of sub-volcanic mushes and subsequent mixing is considered an efficient trigger of volcanic eruptions (e.g. Murphy et al., 1998; Leonard et al., 2002; Kent et al., 2010; Kent, 2013; Ubide & Kamber, 2018). The study of mineral compositional records in volcanic systems has primarily focused on arc settings (e.g. Bachmann et al., 2002; Salisbury et al., 2008; Bragagni et al., 2014) and ocean island systems such as Gran Canaria in the Canary Islands (e.g. Troll & Schmincke, 2002). In continental volcanoes, mineral records have been investigated in certain settings such as the Eifel (Germany) and Mt. Bambouto (Cameroon Line) (Duda & Schmincke, 1985; Dobosi, 1989; Marzoli et al., 2015). In Australia, the eruptive products of Cenozoic volcanoes, concentrated along the eastern margin of the continent, offer a unique opportunity to study the nature of magma plumbing systems feeding continental volcanism. Remnant shield volcanoes have a southward age progression from ∼34 to 6 Ma (Wellman & McDougall, 1974a) and are dominantly confined to the east of the Great Dividing Range (Fig. 1). However, two sets of coeval volcanoes occur on either side of the Great Dividing Range in New South Wales: Nandewar and Ebor, followed in time by Warrumbungle and Comboyne. Here we focus on the Warrumbungle–Comboyne coeval volcano pair (Fig. 1), using an integrated approach that combines 40Ar/39Ar geochronology with mineral and whole-rock geochemistry to investigate the architecture of the magmatic plumbing systems feeding these two temporally associated, but spatially separated, continental intraplate volcanoes through time. Fig. 1 View largeDownload slide (a) Digital elevation model (DEM) of eastern Australia (Geoscience Australia, 2015) that highlights the location of the central volcanoes in blue (after Johnson, 1989). Black squares correspond to the location of the Warrumbungle and Comboyne central volcanoes; grey squares mark the location of other central volcanoes mentioned in the text. Fig. 1 View largeDownload slide (a) Digital elevation model (DEM) of eastern Australia (Geoscience Australia, 2015) that highlights the location of the central volcanoes in blue (after Johnson, 1989). Black squares correspond to the location of the Warrumbungle and Comboyne central volcanoes; grey squares mark the location of other central volcanoes mentioned in the text. GEOLOGICAL BACKGROUND Cenozoic intraplate volcanism in Australia is almost entirely concentrated along the eastern margin of the continent. In the classic work by Wellman & McDougall (1974a), three types of volcanic provinces were established: central volcanoes, lava fields and leucitites. Central volcanoes were defined as containing both mafic and felsic lava flows and intrusive rocks and having a southward age progression from 34 to 6 Ma (Wellman & McDougall, 1974a). Lava fields were considered to encompass mafic flows only, and have no such age progression (Wellman & McDougall, 1974a). The leucitites are leucite-bearing basalts that, like the central volcanoes, show a southward age progression (Cohen et al., 2008). The origin of the age-progressive central volcanoes and leucitites has been linked to a single plume or several mantle plumes (e.g. Wellman & McDougall, 1974a; Sutherland, 2003; Vasconcelos et al., 2008; Davies et al., 2015). By contrast, the generation of lava fields has been related to a number of mechanisms, which include the opening of the Tasman Sea, edge-driven convection, heat flow from warm Pacific mantle, and asthenospheric shearing (e.g. Johnson, 1989; Finn et al., 2005; Demidjuk et al., 2007; Conrad et al., 2011). However, more recent work has shown that some lava fields include basalts with ‘plume-like’ isotopic signatures and ages that correlate with the age-progressive central volcanoes (Sutherland et al., 2012; Jones et al., 2016). The occurrence of central volcanoes in eastern Australia is confined to a strip of thin lithosphere. Surface-wave tomography (Fishwick et al., 2008) shows that the lithosphere thickens from the eastern continental margin westward, in a series of abrupt steps. More recently, Davies et al. (2015) showed that within the strip of thin lithosphere, there are local variations in thickness that could control rates of magma generation in the mantle. The distribution of eastern Australian volcanism follows the Great Dividing Range (Johnson, 1989), the formation of which is still under debate (e.g. Vasconcelos et al., 2008). In its origin, the Great Dividing Range included significant uplift prior to Cenozoic volcanism (Vasconcelos et al., 2008). However, a more recent study of the underlying mantle–crust interactions since 100 Ma suggests a two-stage uplift (Müller et al., 2016). The second stage of uplift occurred later (∼50 Ma) around the southern Highlands as the continental plate passed over the Pacific thermal superswell (Müller et al., 2016). The Warrumbungle and Comboyne central volcanoes are located in New South Wales (NSW), towards the end of the age-progressive track (Fig. 1). The Warrumbungle volcano is located west of the Great Dividing Range and erupted through thinner lithosphere (∼110 km) compared with the craton to the west, in central NSW (Davies et al., 2015). The volcano is ∼50 km in diameter and has an eruptive volume of about 500 km3 (Duggan, 1989). Early K/Ar dating indicated that the lifespan of the Warrumbungle volcano was between 17 and 13·5 Ma (Wellman & McDougall, 1974b, and references therein). More recently, 40Ar/39Ar geochronology showed that volcanic activity ranged from 18·1 ± 0·2 to 15·9 ± 0·1 Ma (40Ar/39Ar ages have been recalculated based on the age of the Fish Canyon sanidine standard adopted in this study: 28·201 ± 0·046 Ma, Kuiper et al., 2008) and that the samples that had yielded younger K/Ar ages, 13·9 ± 0·2 Ma, were actually much older (16·4 ± 0·3 Ma; Cohen, 2007). The products of the volcano define three petrological series (Hockley, 1972): (1) sodic undersaturated alkali basalts, hawaiites, mugearites and trachytes; (2) mildly potassic hypersthene normative trachybasalts, trachyandesites, and trachytes; (3) mildly potassic undersaturated trachyandesites, Fe-rich trachyandesites and nepheline-bearing trachytes, and phonolitic trachytes. Based on geochemistry, the mafic rocks have been subdivided into incompatible-rich and incompatible-poor (Ghorbani & Middlemost, 2000). The Comboyne volcano is located to the east of the Great Dividing Range, closer to the coast. In this area, the lithosphere is slightly thicker (∼130 km; Davies et al., 2015) and Comboyne is significantly smaller (∼20 km diameter, 50 km3 total volume; Knutson, 1989) than Warrumbungle. 40Ar/39Ar geochronology yields ages ranging from 17·2 ± 0·3 to 16·6 ± 0·2 Ma for the Comboyne volcano (Knesel et al., 2008; Sutherland et al., 2012). Sutherland et al. (2012) showed that basalts occurring at lower elevation sites surrounding the Comboyne Plateau are younger (K–Ar ages from 15·7 ± 0·3 to 13·3 ± 0·3 Ma) than the plateau basalts. However, Sutherland et al. (2012) noted that these K–Ar ages need to be confirmed by 40Ar/39Ar geochronology. Volcanic products from Comboyne range from mildly alkaline to sub-alkaline hawaiites, olivine and quartz tholeiites, mugearites, icelandites, benmoreites, dacites, trachytes and peralkaline rhyolites (Knutson, 1989). Intermediate to silicic rocks are volumetrically dominant (Knutson, 1975). SAMPLING AND METHODS Sampling Eighteen mafic to intermediate samples from the Warrumbungle and Comboyne central volcanoes were investigated in this study using petrological, geochemical and geochronological methods. Four of the samples from the Warrumbungle volcano had been previously collected and dated, via 40Ar/39Ar geochronology, by Cohen (2007): one sample from the northern area, one from the southern area, and two from the central area of the volcano. Five additional samples were collected from Warrumbungle volcano: three samples were collected at various elevations along the Observatory Road and two samples were collected from Mt Exmouth. At Comboyne volcano, nine samples were collected from various sites on the Comboyne Plateau. 40Ar/39Ar geochronology Five samples from the Warrumbungle volcano and nine samples from the Comboyne volcano were prepared for 40Ar/39Ar geochronology. Whole-rock samples were crushed to 1–2 mm fragments and treated with 3·5N HCl and 1N HNO3 and subsequently washed repeatedly using distilled water, acetone and ethanol in an ultrasonic bath. Fresh whole-rock grains, neutron fluence monitor Fish Canyon sanidine (28·201 ± 0·046 Ma, Kuiper et al., 2008) and an independent standard (GA1550; 98·5 ± 0·8 Ma, Spell & McDougall, 2003) were loaded into a 21-pit aluminum disk in the pattern described by Vasconcelos et al. (2002). The disk was irradiated for 14 h in a TRIGA-type reactor in the Cd-lined CLCIT facility at Oregon State University. The samples were then dated at the Argon Geochronology Laboratory of the University of Queensland (UQ-AGES) by 40Ar/39Ar incremental heating, following the procedure set out by Vasconcelos et al. (2002). The whole-rock grains were heated incrementally using a 2 mm wide continuous-wave Ar-ion laser beam. The gas was purified in a cryocooled trap (–130°C) and three SAES-50 getter pumps before isotopic analysis using a MAP215-50 mass spectrometer. Air pipettes and full system blanks were analyzed before and after each sample. MassSpec version 8.132 was used to correct the data for mass discrimination, nucleogenic interferences, and atmospheric contamination. Mass spectrometer discrimination was calculated using a 40Ar/39Ar value for atmospheric argon of 298·56 ± 0·31 (Lee et al., 2006). The J factor was determined by analyzing 15 Fish Canyon sanidine crystals (see Supplementary Data Electronic Appendix 1; supplementary data are available for downloading at http://www.petrology.oxfordjournals.org). Results are provided in Table 1 and Supplementary Data Electronic Appendix 1. Table 1 40Ar/39Ar geochronology results of the Warrumbungle and Comboyne volcanoes Sample no. UTM Northing Easting Lab no. Material No. of steps Plateau age ±2σ (Ma)* Steps (% in plateau) Integrated age ±2σ (Ma) Probability density plot age ±2σ (Ma)† Isochron age ±2 σ(Ma)‡ 40Ar/39Ar intercept Warrumbungle W2 55J 6538550 697457 9089-01 WR 12 16·1 ± 0·1 C–J (84·7 %) 16·1 ± 0·1 16·0 ± 0·1 16·0 ± 0·1a 313 ± 16 (MSWD = 0·70) (n = 17) (n = 21) 9089-02 WR 12 15·9 ± 0·1 C–L (92·6 %) 15·9 ± 0·2 (MSWD = 1·14) (MSWD = 1·5) (MSWD = 0·73) W3 55J 6538517 697995 9244-01 WR 12 15·8 ± 0·1 C–H (69·2 %) 15·7 ± 0·1 15·8 ± 0·1 15·8 ± 0·1 305 ± 10 (MSWD = 0·70) (n = 12) (n = 14) 9244-02 WR 12 15·7 ± 0·1 (MSWD = 1·58) D–H (68·0 %) 15·6 ± 0·1 (MSWD = 1·91) (MSWD = 1·6) W7 55J 6537141 698863 9090-01 WR 12 16·9 ± 0·1 C–H (74·6 %) 17·0 ± 0·1 16·9 ± 0·1 16·8 ± 0·1 325 ± 9 (MSWD = 1·14) (n = 15) (n = 16) 9090-02 WR 12 16·8 ± 0·1 C–J (74·3 %) 16·7 ± 0·1 (MSWD = 1·30) (MSWD = 1·6) (MSWD = 1·90) W10 55J 6534326 685343 9091-01 WR 12 15·8 ± 0·2 A–F (71·7 %) 15·4 ± 0·2 15·7 ± 0·1 15·8 ± 0·2 300 ± 13 (MSWD = 1·93) (n = 11) (n = 15) 9091-02 WR 12 n.a. n.a. 15·8 ± 0·2 (MSWD = 1·81) (MSWD = 1·7) W11 55J 6534155 685517 9124-01 WR 12 15·9 ± 0·1 C–L (96·2 %) 15·9 ± 0·2 15·9 ± 0·1 15·9 ± 0·1 296 ± 5 (MSWD = 0·87) (n = 15) (n = 20) 9124-02 WR 12 15·8 ± 0·1 C–I (89·6 %) 15·9 ± 0·2 (MSWD = 0·72) (MSWD = 1·5) (MSWD = 1·99) Comboyne CNB3 56J 6500799 0452447 9129-01 WR 12 16·7 ± 0·1 F–L (74·3 %) 16·7 ± 0·1 16·7 ± 0·1 16·7 ± 0·1 266 ± 8 (MSWD = 0·80) (n = 14) (n = 19) 9129-02 WR 12 16·7 ± 0·1 D–K (67·4 %) 16·6 ± 0·1 (MSWD = 1·17) (MSWD = 0·88) (MSWD = 1·12) CBN4 56J 6500837 0452425 9128-01 WR 12 16·6 ± 0·1 D–H (80·5 %) 16·6 ± 0·1 16·6 ± 0·1 16·6 ± 0·1 400 ± 300 (MSWD = 1·53) (n = 9) (n = 9) 9128-02 WR 12 16·6 ± 0·1 E–H (72·8 %) 16·6 ± 0·1 (MSWD = 1·56) (MSWD = 1·6) (MSWD = 1·89) CBN10 56J 6502929 0445840 9127-01 WR 12 16·5 ± 0·1 F–I (58·5 %) 16·5 ± 0·1 16·5 ± 0·1 16·5 ± 0·1 310 ± 11 (MSWD = 1·03) (n = 14) (n = 21) 9127-02 WR 12 16·5 ± 0·1 C–L (95·7 %) 16·6 ± 0·1 (MSWD = 1·37) (MSWD = 1·4) (MSWD = 1·47) CBN11 56J 6505256 0443116 9081-01 WR 11 16·5 ± 0·1 E–I (53·2 %) 16·7 ± 0·1 16·3 ± 0·1 16·3 ± 0·1 300 ± 40 (MSWD = 0·91) (n = 14) (n = 17) 9081-02 WR 12 16·2 ± 0·1 E–L (58·3 %) 16·4 ± 0·1 (MSWD = 1·32) (MSWD = 1·7) (MSWD = 1·15) CBN12 56J 6505153 0443328 9082-01 WR 12 16·7 ± 0·1 C–I (80·4 %) 16·8 ± 0·1 16·7 ± 0·1 16·7 ± 0·1 340 ± 40 (MSWD = 1·89) (n = 10) (n = 17) 9082-02 WR 12 16·7 ± 0·1§ D–G (53·2 %) 16·7 ± 0·1 (MSWD = 1·95) (MSWD = 1·6) (MSWD = 2·77) CBN13 56J 6505122 0443372 9084-01 WR 12 16·0 ± 0·7 D–G (96·8 %) 16·0 ± 2·0 16·0 ± 0·3 16·0 ± 0·5 298 ± 2 (MSWD = 1·49) (n = 15) (n = 22) 9084-02 WR 12 16·1 ± 0·4 B–I (86·6 %) 15·2 ± 1·4 (MSWD = 0·96) (MSWD = 1·5) (MSWD = 1·60) CBN14 56J 6505069 0443439 9085-01 WR 12 16·7 ± 0·3 D–H (65·5%) 16·4 ± 0·8 16·2 ± 0·2 16·2 ± 0·2 297 ± 5 (MSWD = 2·18) (n = 7) (n = 8) 9085-02 WR 12 16·1 ± 0·2 C–E (54·5%) 15·5 ± 0·2 (MSWD = 1·72) (MSWD = 1·9) (MSWD = 1·53) CBN16 56J 6506672 0440696 9087-01 WR 11 16·7 ± 0·1 C–K (73·2 %) 16·7 ± 0·1 16·7 ± 0·1 16·7 ± 0·1 300 ± 40 (MSWD = 0·98) (n = 16) (n = 20) 9087-02 WR 12 16·7 ± 0·1 A–H (96·6 %) 16·6 ± 0·1 (MSWD = 1·80) (MSWD = 1·5) (MSWD = 1·39) CBN20 56J 6500886 0449412 9088-01 WR 12 16·8 ± 0·1 C–F (53·0 %) 16·8 ± 0·1 16·8 ± 0·1 n.a. n.a. (MSWD = 1·06) (n = 13) 9088-02 WR 12 16·9 ± 0·1 C–L (79·2 %) 16·9 ± 0·1 (MSWD = 1·27) (MSWD = 1·10) Sample no. UTM Northing Easting Lab no. Material No. of steps Plateau age ±2σ (Ma)* Steps (% in plateau) Integrated age ±2σ (Ma) Probability density plot age ±2σ (Ma)† Isochron age ±2 σ(Ma)‡ 40Ar/39Ar intercept Warrumbungle W2 55J 6538550 697457 9089-01 WR 12 16·1 ± 0·1 C–J (84·7 %) 16·1 ± 0·1 16·0 ± 0·1 16·0 ± 0·1a 313 ± 16 (MSWD = 0·70) (n = 17) (n = 21) 9089-02 WR 12 15·9 ± 0·1 C–L (92·6 %) 15·9 ± 0·2 (MSWD = 1·14) (MSWD = 1·5) (MSWD = 0·73) W3 55J 6538517 697995 9244-01 WR 12 15·8 ± 0·1 C–H (69·2 %) 15·7 ± 0·1 15·8 ± 0·1 15·8 ± 0·1 305 ± 10 (MSWD = 0·70) (n = 12) (n = 14) 9244-02 WR 12 15·7 ± 0·1 (MSWD = 1·58) D–H (68·0 %) 15·6 ± 0·1 (MSWD = 1·91) (MSWD = 1·6) W7 55J 6537141 698863 9090-01 WR 12 16·9 ± 0·1 C–H (74·6 %) 17·0 ± 0·1 16·9 ± 0·1 16·8 ± 0·1 325 ± 9 (MSWD = 1·14) (n = 15) (n = 16) 9090-02 WR 12 16·8 ± 0·1 C–J (74·3 %) 16·7 ± 0·1 (MSWD = 1·30) (MSWD = 1·6) (MSWD = 1·90) W10 55J 6534326 685343 9091-01 WR 12 15·8 ± 0·2 A–F (71·7 %) 15·4 ± 0·2 15·7 ± 0·1 15·8 ± 0·2 300 ± 13 (MSWD = 1·93) (n = 11) (n = 15) 9091-02 WR 12 n.a. n.a. 15·8 ± 0·2 (MSWD = 1·81) (MSWD = 1·7) W11 55J 6534155 685517 9124-01 WR 12 15·9 ± 0·1 C–L (96·2 %) 15·9 ± 0·2 15·9 ± 0·1 15·9 ± 0·1 296 ± 5 (MSWD = 0·87) (n = 15) (n = 20) 9124-02 WR 12 15·8 ± 0·1 C–I (89·6 %) 15·9 ± 0·2 (MSWD = 0·72) (MSWD = 1·5) (MSWD = 1·99) Comboyne CNB3 56J 6500799 0452447 9129-01 WR 12 16·7 ± 0·1 F–L (74·3 %) 16·7 ± 0·1 16·7 ± 0·1 16·7 ± 0·1 266 ± 8 (MSWD = 0·80) (n = 14) (n = 19) 9129-02 WR 12 16·7 ± 0·1 D–K (67·4 %) 16·6 ± 0·1 (MSWD = 1·17) (MSWD = 0·88) (MSWD = 1·12) CBN4 56J 6500837 0452425 9128-01 WR 12 16·6 ± 0·1 D–H (80·5 %) 16·6 ± 0·1 16·6 ± 0·1 16·6 ± 0·1 400 ± 300 (MSWD = 1·53) (n = 9) (n = 9) 9128-02 WR 12 16·6 ± 0·1 E–H (72·8 %) 16·6 ± 0·1 (MSWD = 1·56) (MSWD = 1·6) (MSWD = 1·89) CBN10 56J 6502929 0445840 9127-01 WR 12 16·5 ± 0·1 F–I (58·5 %) 16·5 ± 0·1 16·5 ± 0·1 16·5 ± 0·1 310 ± 11 (MSWD = 1·03) (n = 14) (n = 21) 9127-02 WR 12 16·5 ± 0·1 C–L (95·7 %) 16·6 ± 0·1 (MSWD = 1·37) (MSWD = 1·4) (MSWD = 1·47) CBN11 56J 6505256 0443116 9081-01 WR 11 16·5 ± 0·1 E–I (53·2 %) 16·7 ± 0·1 16·3 ± 0·1 16·3 ± 0·1 300 ± 40 (MSWD = 0·91) (n = 14) (n = 17) 9081-02 WR 12 16·2 ± 0·1 E–L (58·3 %) 16·4 ± 0·1 (MSWD = 1·32) (MSWD = 1·7) (MSWD = 1·15) CBN12 56J 6505153 0443328 9082-01 WR 12 16·7 ± 0·1 C–I (80·4 %) 16·8 ± 0·1 16·7 ± 0·1 16·7 ± 0·1 340 ± 40 (MSWD = 1·89) (n = 10) (n = 17) 9082-02 WR 12 16·7 ± 0·1§ D–G (53·2 %) 16·7 ± 0·1 (MSWD = 1·95) (MSWD = 1·6) (MSWD = 2·77) CBN13 56J 6505122 0443372 9084-01 WR 12 16·0 ± 0·7 D–G (96·8 %) 16·0 ± 2·0 16·0 ± 0·3 16·0 ± 0·5 298 ± 2 (MSWD = 1·49) (n = 15) (n = 22) 9084-02 WR 12 16·1 ± 0·4 B–I (86·6 %) 15·2 ± 1·4 (MSWD = 0·96) (MSWD = 1·5) (MSWD = 1·60) CBN14 56J 6505069 0443439 9085-01 WR 12 16·7 ± 0·3 D–H (65·5%) 16·4 ± 0·8 16·2 ± 0·2 16·2 ± 0·2 297 ± 5 (MSWD = 2·18) (n = 7) (n = 8) 9085-02 WR 12 16·1 ± 0·2 C–E (54·5%) 15·5 ± 0·2 (MSWD = 1·72) (MSWD = 1·9) (MSWD = 1·53) CBN16 56J 6506672 0440696 9087-01 WR 11 16·7 ± 0·1 C–K (73·2 %) 16·7 ± 0·1 16·7 ± 0·1 16·7 ± 0·1 300 ± 40 (MSWD = 0·98) (n = 16) (n = 20) 9087-02 WR 12 16·7 ± 0·1 A–H (96·6 %) 16·6 ± 0·1 (MSWD = 1·80) (MSWD = 1·5) (MSWD = 1·39) CBN20 56J 6500886 0449412 9088-01 WR 12 16·8 ± 0·1 C–F (53·0 %) 16·8 ± 0·1 16·8 ± 0·1 n.a. n.a. (MSWD = 1·06) (n = 13) 9088-02 WR 12 16·9 ± 0·1 C–L (79·2 %) 16·9 ± 0·1 (MSWD = 1·27) (MSWD = 1·10) n.a., Failed to produce a valid results (see Results in text). aResults in bold represent the preferred age of the sample. * A plateau age is defined as three or more consecutive steps that consist of at least 50% of the total 39Ar released and the age values overlap within a 95% confidence interval (Fleck et al., 1977). Errors, including errors in irradiation correction factors and errors in J, are reported at the 95% confidence level and are calculated based on the mean weight by inverse variance. All plateau definitions are defined using error-overlap with a 2σ error. † A probability density plot is constructed based on the assumption that a Gaussian distribution occurs for the errors in an age. When each age is plotted the total for every Gaussian curve is taken (Deino & Potts, 1990). ‡ Isochron age errors include the errors in J and irradiation correction factors but not the uncertainty in the potassium decay constant. Isochron ages are measured to the 95% confidence level (2σ). § Plateau ages where the MSWD value is in italics have MSWD >2. Table 1 40Ar/39Ar geochronology results of the Warrumbungle and Comboyne volcanoes Sample no. UTM Northing Easting Lab no. Material No. of steps Plateau age ±2σ (Ma)* Steps (% in plateau) Integrated age ±2σ (Ma) Probability density plot age ±2σ (Ma)† Isochron age ±2 σ(Ma)‡ 40Ar/39Ar intercept Warrumbungle W2 55J 6538550 697457 9089-01 WR 12 16·1 ± 0·1 C–J (84·7 %) 16·1 ± 0·1 16·0 ± 0·1 16·0 ± 0·1a 313 ± 16 (MSWD = 0·70) (n = 17) (n = 21) 9089-02 WR 12 15·9 ± 0·1 C–L (92·6 %) 15·9 ± 0·2 (MSWD = 1·14) (MSWD = 1·5) (MSWD = 0·73) W3 55J 6538517 697995 9244-01 WR 12 15·8 ± 0·1 C–H (69·2 %) 15·7 ± 0·1 15·8 ± 0·1 15·8 ± 0·1 305 ± 10 (MSWD = 0·70) (n = 12) (n = 14) 9244-02 WR 12 15·7 ± 0·1 (MSWD = 1·58) D–H (68·0 %) 15·6 ± 0·1 (MSWD = 1·91) (MSWD = 1·6) W7 55J 6537141 698863 9090-01 WR 12 16·9 ± 0·1 C–H (74·6 %) 17·0 ± 0·1 16·9 ± 0·1 16·8 ± 0·1 325 ± 9 (MSWD = 1·14) (n = 15) (n = 16) 9090-02 WR 12 16·8 ± 0·1 C–J (74·3 %) 16·7 ± 0·1 (MSWD = 1·30) (MSWD = 1·6) (MSWD = 1·90) W10 55J 6534326 685343 9091-01 WR 12 15·8 ± 0·2 A–F (71·7 %) 15·4 ± 0·2 15·7 ± 0·1 15·8 ± 0·2 300 ± 13 (MSWD = 1·93) (n = 11) (n = 15) 9091-02 WR 12 n.a. n.a. 15·8 ± 0·2 (MSWD = 1·81) (MSWD = 1·7) W11 55J 6534155 685517 9124-01 WR 12 15·9 ± 0·1 C–L (96·2 %) 15·9 ± 0·2 15·9 ± 0·1 15·9 ± 0·1 296 ± 5 (MSWD = 0·87) (n = 15) (n = 20) 9124-02 WR 12 15·8 ± 0·1 C–I (89·6 %) 15·9 ± 0·2 (MSWD = 0·72) (MSWD = 1·5) (MSWD = 1·99) Comboyne CNB3 56J 6500799 0452447 9129-01 WR 12 16·7 ± 0·1 F–L (74·3 %) 16·7 ± 0·1 16·7 ± 0·1 16·7 ± 0·1 266 ± 8 (MSWD = 0·80) (n = 14) (n = 19) 9129-02 WR 12 16·7 ± 0·1 D–K (67·4 %) 16·6 ± 0·1 (MSWD = 1·17) (MSWD = 0·88) (MSWD = 1·12) CBN4 56J 6500837 0452425 9128-01 WR 12 16·6 ± 0·1 D–H (80·5 %) 16·6 ± 0·1 16·6 ± 0·1 16·6 ± 0·1 400 ± 300 (MSWD = 1·53) (n = 9) (n = 9) 9128-02 WR 12 16·6 ± 0·1 E–H (72·8 %) 16·6 ± 0·1 (MSWD = 1·56) (MSWD = 1·6) (MSWD = 1·89) CBN10 56J 6502929 0445840 9127-01 WR 12 16·5 ± 0·1 F–I (58·5 %) 16·5 ± 0·1 16·5 ± 0·1 16·5 ± 0·1 310 ± 11 (MSWD = 1·03) (n = 14) (n = 21) 9127-02 WR 12 16·5 ± 0·1 C–L (95·7 %) 16·6 ± 0·1 (MSWD = 1·37) (MSWD = 1·4) (MSWD = 1·47) CBN11 56J 6505256 0443116 9081-01 WR 11 16·5 ± 0·1 E–I (53·2 %) 16·7 ± 0·1 16·3 ± 0·1 16·3 ± 0·1 300 ± 40 (MSWD = 0·91) (n = 14) (n = 17) 9081-02 WR 12 16·2 ± 0·1 E–L (58·3 %) 16·4 ± 0·1 (MSWD = 1·32) (MSWD = 1·7) (MSWD = 1·15) CBN12 56J 6505153 0443328 9082-01 WR 12 16·7 ± 0·1 C–I (80·4 %) 16·8 ± 0·1 16·7 ± 0·1 16·7 ± 0·1 340 ± 40 (MSWD = 1·89) (n = 10) (n = 17) 9082-02 WR 12 16·7 ± 0·1§ D–G (53·2 %) 16·7 ± 0·1 (MSWD = 1·95) (MSWD = 1·6) (MSWD = 2·77) CBN13 56J 6505122 0443372 9084-01 WR 12 16·0 ± 0·7 D–G (96·8 %) 16·0 ± 2·0 16·0 ± 0·3 16·0 ± 0·5 298 ± 2 (MSWD = 1·49) (n = 15) (n = 22) 9084-02 WR 12 16·1 ± 0·4 B–I (86·6 %) 15·2 ± 1·4 (MSWD = 0·96) (MSWD = 1·5) (MSWD = 1·60) CBN14 56J 6505069 0443439 9085-01 WR 12 16·7 ± 0·3 D–H (65·5%) 16·4 ± 0·8 16·2 ± 0·2 16·2 ± 0·2 297 ± 5 (MSWD = 2·18) (n = 7) (n = 8) 9085-02 WR 12 16·1 ± 0·2 C–E (54·5%) 15·5 ± 0·2 (MSWD = 1·72) (MSWD = 1·9) (MSWD = 1·53) CBN16 56J 6506672 0440696 9087-01 WR 11 16·7 ± 0·1 C–K (73·2 %) 16·7 ± 0·1 16·7 ± 0·1 16·7 ± 0·1 300 ± 40 (MSWD = 0·98) (n = 16) (n = 20) 9087-02 WR 12 16·7 ± 0·1 A–H (96·6 %) 16·6 ± 0·1 (MSWD = 1·80) (MSWD = 1·5) (MSWD = 1·39) CBN20 56J 6500886 0449412 9088-01 WR 12 16·8 ± 0·1 C–F (53·0 %) 16·8 ± 0·1 16·8 ± 0·1 n.a. n.a. (MSWD = 1·06) (n = 13) 9088-02 WR 12 16·9 ± 0·1 C–L (79·2 %) 16·9 ± 0·1 (MSWD = 1·27) (MSWD = 1·10) Sample no. UTM Northing Easting Lab no. Material No. of steps Plateau age ±2σ (Ma)* Steps (% in plateau) Integrated age ±2σ (Ma) Probability density plot age ±2σ (Ma)† Isochron age ±2 σ(Ma)‡ 40Ar/39Ar intercept Warrumbungle W2 55J 6538550 697457 9089-01 WR 12 16·1 ± 0·1 C–J (84·7 %) 16·1 ± 0·1 16·0 ± 0·1 16·0 ± 0·1a 313 ± 16 (MSWD = 0·70) (n = 17) (n = 21) 9089-02 WR 12 15·9 ± 0·1 C–L (92·6 %) 15·9 ± 0·2 (MSWD = 1·14) (MSWD = 1·5) (MSWD = 0·73) W3 55J 6538517 697995 9244-01 WR 12 15·8 ± 0·1 C–H (69·2 %) 15·7 ± 0·1 15·8 ± 0·1 15·8 ± 0·1 305 ± 10 (MSWD = 0·70) (n = 12) (n = 14) 9244-02 WR 12 15·7 ± 0·1 (MSWD = 1·58) D–H (68·0 %) 15·6 ± 0·1 (MSWD = 1·91) (MSWD = 1·6) W7 55J 6537141 698863 9090-01 WR 12 16·9 ± 0·1 C–H (74·6 %) 17·0 ± 0·1 16·9 ± 0·1 16·8 ± 0·1 325 ± 9 (MSWD = 1·14) (n = 15) (n = 16) 9090-02 WR 12 16·8 ± 0·1 C–J (74·3 %) 16·7 ± 0·1 (MSWD = 1·30) (MSWD = 1·6) (MSWD = 1·90) W10 55J 6534326 685343 9091-01 WR 12 15·8 ± 0·2 A–F (71·7 %) 15·4 ± 0·2 15·7 ± 0·1 15·8 ± 0·2 300 ± 13 (MSWD = 1·93) (n = 11) (n = 15) 9091-02 WR 12 n.a. n.a. 15·8 ± 0·2 (MSWD = 1·81) (MSWD = 1·7) W11 55J 6534155 685517 9124-01 WR 12 15·9 ± 0·1 C–L (96·2 %) 15·9 ± 0·2 15·9 ± 0·1 15·9 ± 0·1 296 ± 5 (MSWD = 0·87) (n = 15) (n = 20) 9124-02 WR 12 15·8 ± 0·1 C–I (89·6 %) 15·9 ± 0·2 (MSWD = 0·72) (MSWD = 1·5) (MSWD = 1·99) Comboyne CNB3 56J 6500799 0452447 9129-01 WR 12 16·7 ± 0·1 F–L (74·3 %) 16·7 ± 0·1 16·7 ± 0·1 16·7 ± 0·1 266 ± 8 (MSWD = 0·80) (n = 14) (n = 19) 9129-02 WR 12 16·7 ± 0·1 D–K (67·4 %) 16·6 ± 0·1 (MSWD = 1·17) (MSWD = 0·88) (MSWD = 1·12) CBN4 56J 6500837 0452425 9128-01 WR 12 16·6 ± 0·1 D–H (80·5 %) 16·6 ± 0·1 16·6 ± 0·1 16·6 ± 0·1 400 ± 300 (MSWD = 1·53) (n = 9) (n = 9) 9128-02 WR 12 16·6 ± 0·1 E–H (72·8 %) 16·6 ± 0·1 (MSWD = 1·56) (MSWD = 1·6) (MSWD = 1·89) CBN10 56J 6502929 0445840 9127-01 WR 12 16·5 ± 0·1 F–I (58·5 %) 16·5 ± 0·1 16·5 ± 0·1 16·5 ± 0·1 310 ± 11 (MSWD = 1·03) (n = 14) (n = 21) 9127-02 WR 12 16·5 ± 0·1 C–L (95·7 %) 16·6 ± 0·1 (MSWD = 1·37) (MSWD = 1·4) (MSWD = 1·47) CBN11 56J 6505256 0443116 9081-01 WR 11 16·5 ± 0·1 E–I (53·2 %) 16·7 ± 0·1 16·3 ± 0·1 16·3 ± 0·1 300 ± 40 (MSWD = 0·91) (n = 14) (n = 17) 9081-02 WR 12 16·2 ± 0·1 E–L (58·3 %) 16·4 ± 0·1 (MSWD = 1·32) (MSWD = 1·7) (MSWD = 1·15) CBN12 56J 6505153 0443328 9082-01 WR 12 16·7 ± 0·1 C–I (80·4 %) 16·8 ± 0·1 16·7 ± 0·1 16·7 ± 0·1 340 ± 40 (MSWD = 1·89) (n = 10) (n = 17) 9082-02 WR 12 16·7 ± 0·1§ D–G (53·2 %) 16·7 ± 0·1 (MSWD = 1·95) (MSWD = 1·6) (MSWD = 2·77) CBN13 56J 6505122 0443372 9084-01 WR 12 16·0 ± 0·7 D–G (96·8 %) 16·0 ± 2·0 16·0 ± 0·3 16·0 ± 0·5 298 ± 2 (MSWD = 1·49) (n = 15) (n = 22) 9084-02 WR 12 16·1 ± 0·4 B–I (86·6 %) 15·2 ± 1·4 (MSWD = 0·96) (MSWD = 1·5) (MSWD = 1·60) CBN14 56J 6505069 0443439 9085-01 WR 12 16·7 ± 0·3 D–H (65·5%) 16·4 ± 0·8 16·2 ± 0·2 16·2 ± 0·2 297 ± 5 (MSWD = 2·18) (n = 7) (n = 8) 9085-02 WR 12 16·1 ± 0·2 C–E (54·5%) 15·5 ± 0·2 (MSWD = 1·72) (MSWD = 1·9) (MSWD = 1·53) CBN16 56J 6506672 0440696 9087-01 WR 11 16·7 ± 0·1 C–K (73·2 %) 16·7 ± 0·1 16·7 ± 0·1 16·7 ± 0·1 300 ± 40 (MSWD = 0·98) (n = 16) (n = 20) 9087-02 WR 12 16·7 ± 0·1 A–H (96·6 %) 16·6 ± 0·1 (MSWD = 1·80) (MSWD = 1·5) (MSWD = 1·39) CBN20 56J 6500886 0449412 9088-01 WR 12 16·8 ± 0·1 C–F (53·0 %) 16·8 ± 0·1 16·8 ± 0·1 n.a. n.a. (MSWD = 1·06) (n = 13) 9088-02 WR 12 16·9 ± 0·1 C–L (79·2 %) 16·9 ± 0·1 (MSWD = 1·27) (MSWD = 1·10) n.a., Failed to produce a valid results (see Results in text). aResults in bold represent the preferred age of the sample. * A plateau age is defined as three or more consecutive steps that consist of at least 50% of the total 39Ar released and the age values overlap within a 95% confidence interval (Fleck et al., 1977). Errors, including errors in irradiation correction factors and errors in J, are reported at the 95% confidence level and are calculated based on the mean weight by inverse variance. All plateau definitions are defined using error-overlap with a 2σ error. † A probability density plot is constructed based on the assumption that a Gaussian distribution occurs for the errors in an age. When each age is plotted the total for every Gaussian curve is taken (Deino & Potts, 1990). ‡ Isochron age errors include the errors in J and irradiation correction factors but not the uncertainty in the potassium decay constant. Isochron ages are measured to the 95% confidence level (2σ). § Plateau ages where the MSWD value is in italics have MSWD >2. Mineral chemistry Major element compositions of mineral phases from 10 samples (six from Warrumbungle and four from Comboyne) were analyzed by electron microprobe microanalyser (EMPA). Results are provided in Supplementary Data Electronic Appendix 2. Samples BC-151, W10 and CBN12 were measured using a Cameca SX100 electron microprobe with four wavelength-dispersive spectrometers at the Research School of Earth Sciences, Australian National University. The operating parameters were an accelerating voltage of 15 kV, a beam current of 20 nA and a beam diameter of 1 μm. Elemental counting times were 20 s on the peak for Na, Mg, Al, Si and P, 40 s on the peak for K, Ca and Ti, and 30 s on the peak for Ni, Fe, Mn and Cr; we measured for 5 s on each of two background positions for all elements. Ni, Fe, Mn and Cr were measured on two spectrometers. A ZAF procedure was used for matrix correction. Calibration of the major and minor elements was on sanidine (K and Na), MgO (Mg), corundum (Al), SiO2 (Si), andradite (Ca and Fe), TiO2 (Ti), rhodonite (Mn), chromite (Cr) and apatite (P), for all minerals except plagioclase, where Si and Al were calibrated on albite. San Carlos olivine, clinopyroxene and hornblende were used as quality monitors. The remaining samples (as well as additional data on CBN12 and BC-151) were analyzed on a JEOL JXA-8200 EMPA equipped with five wavelength-dispersive spectrometers at the Centre for Microscopy and Microanalysis, The University of Queensland. Analyses were performed using an accelerating voltage of 15 kV, a beam current of 15 nA and a beam diameter of 1 μm. Elemental counting times for all elements were 30 s on the peak and 5 s on each of two background positions. ZAF was used for matrix correction. Calibration of the major and minor elements utilized orthoclase (K), albite (Na), wollastonite (Si and Ca), kyanite (Al), hematite (Fe), chromite (Cr), spessartine (Mn), F-apatite (P), rutile (Ti), P-140 Olivine (Mg) and Ni-olivine (Ni). Springwater Olivine, Kakanui Augite and Lake Co Feldspar were used as quality monitors. In addition, to check the variability between the two EMPA systems used, we compared the analyses on the cores and rims of olivine phenocrysts and found that reproducibility was better than 2% (see Supplementary Data Electronic Appendix 2). Major and trace element geochemistry Eighteen whole-rock samples were powdered in an agate ring and puck mill. Major elements, trace elements and loss on ignition (LOI) were measured in the Geochemistry Laboratory and the Radiogenic Isotope Facility (RIF) at the School of Earth Sciences, the University of Queensland. Results are provided in Table 2. Table 2 Major and trace element results for the Warrumbungle and Comboyne volcanoes Sample: BC-158 BC-167 BC-157 BC-151 W10 W11 W2 W7 W3 Volcano: Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Easting: 697947 717188 697947 682251 685343 685517 697457 698863 697995 Northing: 6538365 6489760 6538365 6558519 6534326 6534155 6538550 6537141 6538517 Elevation: 850 398 1055 372 1159 1096 1979 792 1058 SiO2 49·68 47·72 51·82 48·47 48·84 46·32 50·89 51·74 50·71 TiO2 2·45 2·85 2·35 2·65 2·66 3·06 2·49 1·79 2·48 Al2O3 15·75 14·64 14·7 14·55 14·66 16·61 15·61 17·07 15·43 Fe2O3 10·79 12·74 12·13 11·00 13·27 13·5 12·32 10·16 12·39 MnO 0·12 0·17 0·19 0·14 0·17 0·18 0·19 0·13 0·23 MgO 3·89 5·32 3·06 7·25 4·81 4·05 3·45 2·96 3·03 CaO 6·66 7·09 5·78 7·16 7·20 8·39 6·34 4·84 7·02 Na2O 3·89 4·35 4·53 3·88 4·18 3·95 3·91 4·23 3·9 K2O 2·73 1·87 2·56 2·38 1·82 1·52 2·28 3·70 2·36 P2O5 1·02 0·91 1·25 1·05 0·8 0·82 0·97 0·79 0·91 H2O 1·47 0·59 1·05 0·57 0·69 0·67 0·32 1·50 0·90 LOI 1·38 1·82 0·68 1·185 1·16 0·99 1·12 1·28 1·03 Total 99·83 100·07 100·10 100·28 100·26 100·06 99·89 100·19 100·39 Mg# 44·93 47·90 35·70 59·22 44·39 39·77 38·13 39·10 35·00 Cs 0·37 1·27 0·61 0·59 0·42 0·45 0·57 0·45 0·56 Rb 41·18 51·82 64·29 33·09 40·37 25·23 42·45 62·93 53·62 Ba 776 732 1307 659 651 533 1017 1154 1071 Sr 701 760 554 876 576 666 561 619 571 Pb 3·92 4·13 5·43 4·12 3·21 2·94 4·56 4·5 4·82 Th 6·19 6·79 8·35 5·28 6·74 5·04 7·18 5·56 6·87 U 1·25 1·17 1·77 1·23 1·2 0·92 1·59 1·35 1·62 Zr 351 299 310 358 307 245 301 320 296 Hf 9·32 7·3 7·59 8·39 7·35 5·9 7·08 7·34 7·02 Ta 3·88 3·65 3·75 3·96 3·62 3·16 3·43 3·56 3·41 Y 33·79 37·97 41·91 27·96 43·13 31·45 36·27 27·4 37·36 Nb 54·75 52·62 54·7 54·73 52·2 45·79 51·39 51·57 50·05 Sc 23·40 25·92 27·42 19·99 28·97 27·35 27·29 20·55 28·12 Cr 67·30 96·93 <DL 183 39·31 19·19 19·71 22·73 22·59 Ni 55 67 8 109 33 23 22 32 22 Co 32·21 39·87 21·04 41·61 32·66 35·44 27·6 20·69 26·31 V 111 146 93·6 159 139 194 129 62·68 132 Ga 27·01 25·53 28·16 26 27·35 25·27 27·41 26·67 27 Zn 134 142·6 165 125 151 125 151 151 149 Cu 27·1 38·7 21·0 33·4 30·5 29·7 23·8 23·5 24·6 La 46·89 44·72 51·21 43·86 46·56 36·24 44·97 38·87 43·63 Ce 100 91·24 106 91·17 96·48 74·49 92·83 87·69 91·36 Pr 12·69 11·68 13·26 11·58 12·28 9·51 11·62 10·68 11·36 Nd 52·02 48·16 54·96 46·42 50·76 39·54 47·68 44·29 46·9 Sm 11·18 10·56 12·08 9·92 11·68 9·14 10·7 9·82 10·49 Eu 3·52 3·31 4·58 3·1 3·77 3·15 3·93 3·78 3·91 Gd 9·83 9·81 11·43 8·63 11·35 8·82 10·13 8·67 9·88 Tb 1·39 1·37 1·58 1·22 1·61 1·26 1·42 1·17 1·39 Dy 6·78 7·14 8·11 6·03 8·15 6·28 7·16 5·76 7·18 Ho 1·15 1·23 1·41 1·01 1·39 1·1 1·22 0·95 1·23 Er 3·25 3·57 4·08 2·83 4·00 3·05 3·49 2·64 3·57 Tm 0·45 0·5 0·56 0·39 0·55 0·42 0·49 0·37 0·5 Yb 2·34 2·76 3·05 2·09 2·88 2·23 2·56 1·99 2·7 Lu 0·35 0·43 0·46 0·30 0·43 0·32 0·39 0·30 0·40 Sample: BC-158 BC-167 BC-157 BC-151 W10 W11 W2 W7 W3 Volcano: Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Easting: 697947 717188 697947 682251 685343 685517 697457 698863 697995 Northing: 6538365 6489760 6538365 6558519 6534326 6534155 6538550 6537141 6538517 Elevation: 850 398 1055 372 1159 1096 1979 792 1058 SiO2 49·68 47·72 51·82 48·47 48·84 46·32 50·89 51·74 50·71 TiO2 2·45 2·85 2·35 2·65 2·66 3·06 2·49 1·79 2·48 Al2O3 15·75 14·64 14·7 14·55 14·66 16·61 15·61 17·07 15·43 Fe2O3 10·79 12·74 12·13 11·00 13·27 13·5 12·32 10·16 12·39 MnO 0·12 0·17 0·19 0·14 0·17 0·18 0·19 0·13 0·23 MgO 3·89 5·32 3·06 7·25 4·81 4·05 3·45 2·96 3·03 CaO 6·66 7·09 5·78 7·16 7·20 8·39 6·34 4·84 7·02 Na2O 3·89 4·35 4·53 3·88 4·18 3·95 3·91 4·23 3·9 K2O 2·73 1·87 2·56 2·38 1·82 1·52 2·28 3·70 2·36 P2O5 1·02 0·91 1·25 1·05 0·8 0·82 0·97 0·79 0·91 H2O 1·47 0·59 1·05 0·57 0·69 0·67 0·32 1·50 0·90 LOI 1·38 1·82 0·68 1·185 1·16 0·99 1·12 1·28 1·03 Total 99·83 100·07 100·10 100·28 100·26 100·06 99·89 100·19 100·39 Mg# 44·93 47·90 35·70 59·22 44·39 39·77 38·13 39·10 35·00 Cs 0·37 1·27 0·61 0·59 0·42 0·45 0·57 0·45 0·56 Rb 41·18 51·82 64·29 33·09 40·37 25·23 42·45 62·93 53·62 Ba 776 732 1307 659 651 533 1017 1154 1071 Sr 701 760 554 876 576 666 561 619 571 Pb 3·92 4·13 5·43 4·12 3·21 2·94 4·56 4·5 4·82 Th 6·19 6·79 8·35 5·28 6·74 5·04 7·18 5·56 6·87 U 1·25 1·17 1·77 1·23 1·2 0·92 1·59 1·35 1·62 Zr 351 299 310 358 307 245 301 320 296 Hf 9·32 7·3 7·59 8·39 7·35 5·9 7·08 7·34 7·02 Ta 3·88 3·65 3·75 3·96 3·62 3·16 3·43 3·56 3·41 Y 33·79 37·97 41·91 27·96 43·13 31·45 36·27 27·4 37·36 Nb 54·75 52·62 54·7 54·73 52·2 45·79 51·39 51·57 50·05 Sc 23·40 25·92 27·42 19·99 28·97 27·35 27·29 20·55 28·12 Cr 67·30 96·93 <DL 183 39·31 19·19 19·71 22·73 22·59 Ni 55 67 8 109 33 23 22 32 22 Co 32·21 39·87 21·04 41·61 32·66 35·44 27·6 20·69 26·31 V 111 146 93·6 159 139 194 129 62·68 132 Ga 27·01 25·53 28·16 26 27·35 25·27 27·41 26·67 27 Zn 134 142·6 165 125 151 125 151 151 149 Cu 27·1 38·7 21·0 33·4 30·5 29·7 23·8 23·5 24·6 La 46·89 44·72 51·21 43·86 46·56 36·24 44·97 38·87 43·63 Ce 100 91·24 106 91·17 96·48 74·49 92·83 87·69 91·36 Pr 12·69 11·68 13·26 11·58 12·28 9·51 11·62 10·68 11·36 Nd 52·02 48·16 54·96 46·42 50·76 39·54 47·68 44·29 46·9 Sm 11·18 10·56 12·08 9·92 11·68 9·14 10·7 9·82 10·49 Eu 3·52 3·31 4·58 3·1 3·77 3·15 3·93 3·78 3·91 Gd 9·83 9·81 11·43 8·63 11·35 8·82 10·13 8·67 9·88 Tb 1·39 1·37 1·58 1·22 1·61 1·26 1·42 1·17 1·39 Dy 6·78 7·14 8·11 6·03 8·15 6·28 7·16 5·76 7·18 Ho 1·15 1·23 1·41 1·01 1·39 1·1 1·22 0·95 1·23 Er 3·25 3·57 4·08 2·83 4·00 3·05 3·49 2·64 3·57 Tm 0·45 0·5 0·56 0·39 0·55 0·42 0·49 0·37 0·5 Yb 2·34 2·76 3·05 2·09 2·88 2·23 2·56 1·99 2·7 Lu 0·35 0·43 0·46 0·30 0·43 0·32 0·39 0·30 0·40 Sample: CBN3 CBN4 CBN10 CBN11 CBN12 CBN13 CBN14 CBN16 CBN20 Volcano: Comboyne Comboyne Comboyne Comboyne Comboyne Comboyne Comboyne Comboyne Comboyne Easting: 452447 452425 452425 443116 443328 443372 443439 440696 449412 Northing: 6500799 6500837 6500837 6505256 6505153 6505122 6505069 6506672 6500886 Elevation: 530 527 527 620 583 562 537 605 681 SiO2 59·22 59·48 53·73 54·00 55·03 45·25 46·26 54·49 58·94 TiO2 0·65 0·67 1·36 1·67 1·66 3·30 3·37 1·70 0·96 Al2O3 16·38 16·57 14·63 16·5 15·17 15·54 16·25 16·28 15·49 Fe2O3 8·62 8·51 12·27 10·37 10·72 14·57 13·49 10·79 8·91 MnO 0·18 0·13 0·16 0·16 0·15 0·17 0·16 0·17 0·14 MgO 0·47 0·39 1·38 2·34 2·35 3·9 4·05 2·06 0·88 CaO 2·15 2·33 3·2 4·21 4·62 7·4 6·94 4·47 3·16 Na2O 5·45 4·82 4·08 4·56 5·09 3·63 3·48 4·17 5·15 K2O 5·52 5·24 4·28 3·34 3·02 1·73 1·70 3·33 3·70 P2O5 0·22 0·22 0·85 0·75 0·56 1·26 0·88 0·74 0·41 H2O 0·26 0·44 1·40 0·39 0·24 2·54 2·20 0·42 0·57 LOI 0·71 0·92 2·24 1·62 1·24 0·68 1·10 1·31 1·65 Sum 99·83 99·72 99·58 99·91 99·85 99·97 99·88 99·93 99·96 Mg# 10·79 9·09 19·83 33·23 32·60 37·06 39·77 29·58 17·94 Cs 2·43 2·54 0·51 0·43 0·38 0·32 0·24 0·49 0·873 Rb 187 204 79·52 52·34 50·91 19·63 19·68 60·50 86·17 Ba 507 542 1372 689 677 680 555 678 1301 Sr 38·71 40·61 191 301 332 681 659 300 267 Pb 11·12 10·42 5·91 4·42 4·43 2·49 2·59 8·71 16·26 Th 16·18 15·8 7·57 7·45 7·8 4·25 4·66 6 6·62 U 3·59 3·74 0·74 1·78 1·77 0·87 0·91 1·80 2·29 Zr 832 843 559 363 355 234 260 362 477 Hf 21·51 22·38 14·03 10·08 9·85 5·91 6·51 10·07 12·77 Ta 6·97 6·98 3·81 3·82 3·77 2·82 2·92 3·87 4·33 Y 57·59 57·81 53·90 35·36 33·28 35·84 34·62 24·99 43·89 Nb 103 102 55·92 55·16 54·33 37·18 40·5 55·43 61·22 Sc 25·9 23·3 30·4 19·13 19 22·8 22·3 18·6 27·30 Cr <D.L <D.L <D.L 13·81 11·77 39·05 14·29 13·22 <D.L Ni 6 6. 7 34 18 57 50 19 8 Co 1·97 2·24 10·22 32·65 18·14 43·27 42 18·51 7·123 V 0·41 0·38 6·74 53·38 53·19 152 155 52·06 5·40 Ga 40·6 40·98 34·33 33·26 32·67 25·98 26·23 32·18 32·49 Zn 191 185 201 162 150 141 132 157 194 Cu 8·4 10·9 7·3 15·7 18·3 37·4 31·9 18·6 11·1 La 64·08 71·55 55·78 48·98 51·71 37·21 37·68 43·86 27·97 Ce 116 163 119 108 108 81·57 82·44 96·63 66·01 Pr 17·3 18·72 16·51 12·97 13·37 11·2 11·08 11·79 9·16 Nd 67·59 71·9 68·72 52·18 52·79 49·14 47·52 46·17 39·39 Sm 15·15 15·65 15·53 11·37 11·20 11·65 11·10 9·92 10·03 Eu 2·91 3·00 5·73 3·16 3·17 4·19 3·79 2·69 3·74 Gd 13·48 14·05 14·45 10·2 10·12 10·96 10·35 8·65 9·78 Tb 2·11 2·14 2·03 1·46 1·44 1·51 1·43 1·25 1·52 Dy 11·22 11·69 10·35 7·50 7·34 7·4 7·22 6·43 8·41 Ho 2·02 2·11 1·77 1·3 1·28 1·24 1·2 1·12 1·53 Er 6·14 6·4 5·13 3·68 3·63 3·35 3·33 3·29 4·63 Tm 0·92 0·98 0·71 0·55 0·53 0·46 0·45 0·49 0·70 Yb 5·29 5·49 3·90 3·05 2·89 2·3 2·3 2·63 3·97 Lu 0·82 0·91 0·60 0·49 0·47 0·37 0·35 0·43 0·654 Sample: CBN3 CBN4 CBN10 CBN11 CBN12 CBN13 CBN14 CBN16 CBN20 Volcano: Comboyne Comboyne Comboyne Comboyne Comboyne Comboyne Comboyne Comboyne Comboyne Easting: 452447 452425 452425 443116 443328 443372 443439 440696 449412 Northing: 6500799 6500837 6500837 6505256 6505153 6505122 6505069 6506672 6500886 Elevation: 530 527 527 620 583 562 537 605 681 SiO2 59·22 59·48 53·73 54·00 55·03 45·25 46·26 54·49 58·94 TiO2 0·65 0·67 1·36 1·67 1·66 3·30 3·37 1·70 0·96 Al2O3 16·38 16·57 14·63 16·5 15·17 15·54 16·25 16·28 15·49 Fe2O3 8·62 8·51 12·27 10·37 10·72 14·57 13·49 10·79 8·91 MnO 0·18 0·13 0·16 0·16 0·15 0·17 0·16 0·17 0·14 MgO 0·47 0·39 1·38 2·34 2·35 3·9 4·05 2·06 0·88 CaO 2·15 2·33 3·2 4·21 4·62 7·4 6·94 4·47 3·16 Na2O 5·45 4·82 4·08 4·56 5·09 3·63 3·48 4·17 5·15 K2O 5·52 5·24 4·28 3·34 3·02 1·73 1·70 3·33 3·70 P2O5 0·22 0·22 0·85 0·75 0·56 1·26 0·88 0·74 0·41 H2O 0·26 0·44 1·40 0·39 0·24 2·54 2·20 0·42 0·57 LOI 0·71 0·92 2·24 1·62 1·24 0·68 1·10 1·31 1·65 Sum 99·83 99·72 99·58 99·91 99·85 99·97 99·88 99·93 99·96 Mg# 10·79 9·09 19·83 33·23 32·60 37·06 39·77 29·58 17·94 Cs 2·43 2·54 0·51 0·43 0·38 0·32 0·24 0·49 0·873 Rb 187 204 79·52 52·34 50·91 19·63 19·68 60·50 86·17 Ba 507 542 1372 689 677 680 555 678 1301 Sr 38·71 40·61 191 301 332 681 659 300 267 Pb 11·12 10·42 5·91 4·42 4·43 2·49 2·59 8·71 16·26 Th 16·18 15·8 7·57 7·45 7·8 4·25 4·66 6 6·62 U 3·59 3·74 0·74 1·78 1·77 0·87 0·91 1·80 2·29 Zr 832 843 559 363 355 234 260 362 477 Hf 21·51 22·38 14·03 10·08 9·85 5·91 6·51 10·07 12·77 Ta 6·97 6·98 3·81 3·82 3·77 2·82 2·92 3·87 4·33 Y 57·59 57·81 53·90 35·36 33·28 35·84 34·62 24·99 43·89 Nb 103 102 55·92 55·16 54·33 37·18 40·5 55·43 61·22 Sc 25·9 23·3 30·4 19·13 19 22·8 22·3 18·6 27·30 Cr <D.L <D.L <D.L 13·81 11·77 39·05 14·29 13·22 <D.L Ni 6 6. 7 34 18 57 50 19 8 Co 1·97 2·24 10·22 32·65 18·14 43·27 42 18·51 7·123 V 0·41 0·38 6·74 53·38 53·19 152 155 52·06 5·40 Ga 40·6 40·98 34·33 33·26 32·67 25·98 26·23 32·18 32·49 Zn 191 185 201 162 150 141 132 157 194 Cu 8·4 10·9 7·3 15·7 18·3 37·4 31·9 18·6 11·1 La 64·08 71·55 55·78 48·98 51·71 37·21 37·68 43·86 27·97 Ce 116 163 119 108 108 81·57 82·44 96·63 66·01 Pr 17·3 18·72 16·51 12·97 13·37 11·2 11·08 11·79 9·16 Nd 67·59 71·9 68·72 52·18 52·79 49·14 47·52 46·17 39·39 Sm 15·15 15·65 15·53 11·37 11·20 11·65 11·10 9·92 10·03 Eu 2·91 3·00 5·73 3·16 3·17 4·19 3·79 2·69 3·74 Gd 13·48 14·05 14·45 10·2 10·12 10·96 10·35 8·65 9·78 Tb 2·11 2·14 2·03 1·46 1·44 1·51 1·43 1·25 1·52 Dy 11·22 11·69 10·35 7·50 7·34 7·4 7·22 6·43 8·41 Ho 2·02 2·11 1·77 1·3 1·28 1·24 1·2 1·12 1·53 Er 6·14 6·4 5·13 3·68 3·63 3·35 3·33 3·29 4·63 Tm 0·92 0·98 0·71 0·55 0·53 0·46 0·45 0·49 0·70 Yb 5·29 5·49 3·90 3·05 2·89 2·3 2·3 2·63 3·97 Lu 0·82 0·91 0·60 0·49 0·47 0·37 0·35 0·43 0·654 Table 2 Major and trace element results for the Warrumbungle and Comboyne volcanoes Sample: BC-158 BC-167 BC-157 BC-151 W10 W11 W2 W7 W3 Volcano: Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Easting: 697947 717188 697947 682251 685343 685517 697457 698863 697995 Northing: 6538365 6489760 6538365 6558519 6534326 6534155 6538550 6537141 6538517 Elevation: 850 398 1055 372 1159 1096 1979 792 1058 SiO2 49·68 47·72 51·82 48·47 48·84 46·32 50·89 51·74 50·71 TiO2 2·45 2·85 2·35 2·65 2·66 3·06 2·49 1·79 2·48 Al2O3 15·75 14·64 14·7 14·55 14·66 16·61 15·61 17·07 15·43 Fe2O3 10·79 12·74 12·13 11·00 13·27 13·5 12·32 10·16 12·39 MnO 0·12 0·17 0·19 0·14 0·17 0·18 0·19 0·13 0·23 MgO 3·89 5·32 3·06 7·25 4·81 4·05 3·45 2·96 3·03 CaO 6·66 7·09 5·78 7·16 7·20 8·39 6·34 4·84 7·02 Na2O 3·89 4·35 4·53 3·88 4·18 3·95 3·91 4·23 3·9 K2O 2·73 1·87 2·56 2·38 1·82 1·52 2·28 3·70 2·36 P2O5 1·02 0·91 1·25 1·05 0·8 0·82 0·97 0·79 0·91 H2O 1·47 0·59 1·05 0·57 0·69 0·67 0·32 1·50 0·90 LOI 1·38 1·82 0·68 1·185 1·16 0·99 1·12 1·28 1·03 Total 99·83 100·07 100·10 100·28 100·26 100·06 99·89 100·19 100·39 Mg# 44·93 47·90 35·70 59·22 44·39 39·77 38·13 39·10 35·00 Cs 0·37 1·27 0·61 0·59 0·42 0·45 0·57 0·45 0·56 Rb 41·18 51·82 64·29 33·09 40·37 25·23 42·45 62·93 53·62 Ba 776 732 1307 659 651 533 1017 1154 1071 Sr 701 760 554 876 576 666 561 619 571 Pb 3·92 4·13 5·43 4·12 3·21 2·94 4·56 4·5 4·82 Th 6·19 6·79 8·35 5·28 6·74 5·04 7·18 5·56 6·87 U 1·25 1·17 1·77 1·23 1·2 0·92 1·59 1·35 1·62 Zr 351 299 310 358 307 245 301 320 296 Hf 9·32 7·3 7·59 8·39 7·35 5·9 7·08 7·34 7·02 Ta 3·88 3·65 3·75 3·96 3·62 3·16 3·43 3·56 3·41 Y 33·79 37·97 41·91 27·96 43·13 31·45 36·27 27·4 37·36 Nb 54·75 52·62 54·7 54·73 52·2 45·79 51·39 51·57 50·05 Sc 23·40 25·92 27·42 19·99 28·97 27·35 27·29 20·55 28·12 Cr 67·30 96·93 <DL 183 39·31 19·19 19·71 22·73 22·59 Ni 55 67 8 109 33 23 22 32 22 Co 32·21 39·87 21·04 41·61 32·66 35·44 27·6 20·69 26·31 V 111 146 93·6 159 139 194 129 62·68 132 Ga 27·01 25·53 28·16 26 27·35 25·27 27·41 26·67 27 Zn 134 142·6 165 125 151 125 151 151 149 Cu 27·1 38·7 21·0 33·4 30·5 29·7 23·8 23·5 24·6 La 46·89 44·72 51·21 43·86 46·56 36·24 44·97 38·87 43·63 Ce 100 91·24 106 91·17 96·48 74·49 92·83 87·69 91·36 Pr 12·69 11·68 13·26 11·58 12·28 9·51 11·62 10·68 11·36 Nd 52·02 48·16 54·96 46·42 50·76 39·54 47·68 44·29 46·9 Sm 11·18 10·56 12·08 9·92 11·68 9·14 10·7 9·82 10·49 Eu 3·52 3·31 4·58 3·1 3·77 3·15 3·93 3·78 3·91 Gd 9·83 9·81 11·43 8·63 11·35 8·82 10·13 8·67 9·88 Tb 1·39 1·37 1·58 1·22 1·61 1·26 1·42 1·17 1·39 Dy 6·78 7·14 8·11 6·03 8·15 6·28 7·16 5·76 7·18 Ho 1·15 1·23 1·41 1·01 1·39 1·1 1·22 0·95 1·23 Er 3·25 3·57 4·08 2·83 4·00 3·05 3·49 2·64 3·57 Tm 0·45 0·5 0·56 0·39 0·55 0·42 0·49 0·37 0·5 Yb 2·34 2·76 3·05 2·09 2·88 2·23 2·56 1·99 2·7 Lu 0·35 0·43 0·46 0·30 0·43 0·32 0·39 0·30 0·40 Sample: BC-158 BC-167 BC-157 BC-151 W10 W11 W2 W7 W3 Volcano: Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Easting: 697947 717188 697947 682251 685343 685517 697457 698863 697995 Northing: 6538365 6489760 6538365 6558519 6534326 6534155 6538550 6537141 6538517 Elevation: 850 398 1055 372 1159 1096 1979 792 1058 SiO2 49·68 47·72 51·82 48·47 48·84 46·32 50·89 51·74 50·71 TiO2 2·45 2·85 2·35 2·65 2·66 3·06 2·49 1·79 2·48 Al2O3 15·75 14·64 14·7 14·55 14·66 16·61 15·61 17·07 15·43 Fe2O3 10·79 12·74 12·13 11·00 13·27 13·5 12·32 10·16 12·39 MnO 0·12 0·17 0·19 0·14 0·17 0·18 0·19 0·13 0·23 MgO 3·89 5·32 3·06 7·25 4·81 4·05 3·45 2·96 3·03 CaO 6·66 7·09 5·78 7·16 7·20 8·39 6·34 4·84 7·02 Na2O 3·89 4·35 4·53 3·88 4·18 3·95 3·91 4·23 3·9 K2O 2·73 1·87 2·56 2·38 1·82 1·52 2·28 3·70 2·36 P2O5 1·02 0·91 1·25 1·05 0·8 0·82 0·97 0·79 0·91 H2O 1·47 0·59 1·05 0·57 0·69 0·67 0·32 1·50 0·90 LOI 1·38 1·82 0·68 1·185 1·16 0·99 1·12 1·28 1·03 Total 99·83 100·07 100·10 100·28 100·26 100·06 99·89 100·19 100·39 Mg# 44·93 47·90 35·70 59·22 44·39 39·77 38·13 39·10 35·00 Cs 0·37 1·27 0·61 0·59 0·42 0·45 0·57 0·45 0·56 Rb 41·18 51·82 64·29 33·09 40·37 25·23 42·45 62·93 53·62 Ba 776 732 1307 659 651 533 1017 1154 1071 Sr 701 760 554 876 576 666 561 619 571 Pb 3·92 4·13 5·43 4·12 3·21 2·94 4·56 4·5 4·82 Th 6·19 6·79 8·35 5·28 6·74 5·04 7·18 5·56 6·87 U 1·25 1·17 1·77 1·23 1·2 0·92 1·59 1·35 1·62 Zr 351 299 310 358 307 245 301 320 296 Hf 9·32 7·3 7·59 8·39 7·35 5·9 7·08 7·34 7·02 Ta 3·88 3·65 3·75 3·96 3·62 3·16 3·43 3·56 3·41 Y 33·79 37·97 41·91 27·96 43·13 31·45 36·27 27·4 37·36 Nb 54·75 52·62 54·7 54·73 52·2 45·79 51·39 51·57 50·05 Sc 23·40 25·92 27·42 19·99 28·97 27·35 27·29 20·55 28·12 Cr 67·30 96·93 <DL 183 39·31 19·19 19·71 22·73 22·59 Ni 55 67 8 109 33 23 22 32 22 Co 32·21 39·87 21·04 41·61 32·66 35·44 27·6 20·69 26·31 V 111 146 93·6 159 139 194 129 62·68 132 Ga 27·01 25·53 28·16 26 27·35 25·27 27·41 26·67 27 Zn 134 142·6 165 125 151 125 151 151 149 Cu 27·1 38·7 21·0 33·4 30·5 29·7 23·8 23·5 24·6 La 46·89 44·72 51·21 43·86 46·56 36·24 44·97 38·87 43·63 Ce 100 91·24 106 91·17 96·48 74·49 92·83 87·69 91·36 Pr 12·69 11·68 13·26 11·58 12·28 9·51 11·62 10·68 11·36 Nd 52·02 48·16 54·96 46·42 50·76 39·54 47·68 44·29 46·9 Sm 11·18 10·56 12·08 9·92 11·68 9·14 10·7 9·82 10·49 Eu 3·52 3·31 4·58 3·1 3·77 3·15 3·93 3·78 3·91 Gd 9·83 9·81 11·43 8·63 11·35 8·82 10·13 8·67 9·88 Tb 1·39 1·37 1·58 1·22 1·61 1·26 1·42 1·17 1·39 Dy 6·78 7·14 8·11 6·03 8·15 6·28 7·16 5·76 7·18 Ho 1·15 1·23 1·41 1·01 1·39 1·1 1·22 0·95 1·23 Er 3·25 3·57 4·08 2·83 4·00 3·05 3·49 2·64 3·57 Tm 0·45 0·5 0·56 0·39 0·55 0·42 0·49 0·37 0·5 Yb 2·34 2·76 3·05 2·09 2·88 2·23 2·56 1·99 2·7 Lu 0·35 0·43 0·46 0·30 0·43 0·32 0·39 0·30 0·40 Sample: CBN3 CBN4 CBN10 CBN11 CBN12 CBN13 CBN14 CBN16 CBN20 Volcano: Comboyne Comboyne Comboyne Comboyne Comboyne Comboyne Comboyne Comboyne Comboyne Easting: 452447 452425 452425 443116 443328 443372 443439 440696 449412 Northing: 6500799 6500837 6500837 6505256 6505153 6505122 6505069 6506672 6500886 Elevation: 530 527 527 620 583 562 537 605 681 SiO2 59·22 59·48 53·73 54·00 55·03 45·25 46·26 54·49 58·94 TiO2 0·65 0·67 1·36 1·67 1·66 3·30 3·37 1·70 0·96 Al2O3 16·38 16·57 14·63 16·5 15·17 15·54 16·25 16·28 15·49 Fe2O3 8·62 8·51 12·27 10·37 10·72 14·57 13·49 10·79 8·91 MnO 0·18 0·13 0·16 0·16 0·15 0·17 0·16 0·17 0·14 MgO 0·47 0·39 1·38 2·34 2·35 3·9 4·05 2·06 0·88 CaO 2·15 2·33 3·2 4·21 4·62 7·4 6·94 4·47 3·16 Na2O 5·45 4·82 4·08 4·56 5·09 3·63 3·48 4·17 5·15 K2O 5·52 5·24 4·28 3·34 3·02 1·73 1·70 3·33 3·70 P2O5 0·22 0·22 0·85 0·75 0·56 1·26 0·88 0·74 0·41 H2O 0·26 0·44 1·40 0·39 0·24 2·54 2·20 0·42 0·57 LOI 0·71 0·92 2·24 1·62 1·24 0·68 1·10 1·31 1·65 Sum 99·83 99·72 99·58 99·91 99·85 99·97 99·88 99·93 99·96 Mg# 10·79 9·09 19·83 33·23 32·60 37·06 39·77 29·58 17·94 Cs 2·43 2·54 0·51 0·43 0·38 0·32 0·24 0·49 0·873 Rb 187 204 79·52 52·34 50·91 19·63 19·68 60·50 86·17 Ba 507 542 1372 689 677 680 555 678 1301 Sr 38·71 40·61 191 301 332 681 659 300 267 Pb 11·12 10·42 5·91 4·42 4·43 2·49 2·59 8·71 16·26 Th 16·18 15·8 7·57 7·45 7·8 4·25 4·66 6 6·62 U 3·59 3·74 0·74 1·78 1·77 0·87 0·91 1·80 2·29 Zr 832 843 559 363 355 234 260 362 477 Hf 21·51 22·38 14·03 10·08 9·85 5·91 6·51 10·07 12·77 Ta 6·97 6·98 3·81 3·82 3·77 2·82 2·92 3·87 4·33 Y 57·59 57·81 53·90 35·36 33·28 35·84 34·62 24·99 43·89 Nb 103 102 55·92 55·16 54·33 37·18 40·5 55·43 61·22 Sc 25·9 23·3 30·4 19·13 19 22·8 22·3 18·6 27·30 Cr <D.L <D.L <D.L 13·81 11·77 39·05 14·29 13·22 <D.L Ni 6 6. 7 34 18 57 50 19 8 Co 1·97 2·24 10·22 32·65 18·14 43·27 42 18·51 7·123 V 0·41 0·38 6·74 53·38 53·19 152 155 52·06 5·40 Ga 40·6 40·98 34·33 33·26 32·67 25·98 26·23 32·18 32·49 Zn 191 185 201 162 150 141 132 157 194 Cu 8·4 10·9 7·3 15·7 18·3 37·4 31·9 18·6 11·1 La 64·08 71·55 55·78 48·98 51·71 37·21 37·68 43·86 27·97 Ce 116 163 119 108 108 81·57 82·44 96·63 66·01 Pr 17·3 18·72 16·51 12·97 13·37 11·2 11·08 11·79 9·16 Nd 67·59 71·9 68·72 52·18 52·79 49·14 47·52 46·17 39·39 Sm 15·15 15·65 15·53 11·37 11·20 11·65 11·10 9·92 10·03 Eu 2·91 3·00 5·73 3·16 3·17 4·19 3·79 2·69 3·74 Gd 13·48 14·05 14·45 10·2 10·12 10·96 10·35 8·65 9·78 Tb 2·11 2·14 2·03 1·46 1·44 1·51 1·43 1·25 1·52 Dy 11·22 11·69 10·35 7·50 7·34 7·4 7·22 6·43 8·41 Ho 2·02 2·11 1·77 1·3 1·28 1·24 1·2 1·12 1·53 Er 6·14 6·4 5·13 3·68 3·63 3·35 3·33 3·29 4·63 Tm 0·92 0·98 0·71 0·55 0·53 0·46 0·45 0·49 0·70 Yb 5·29 5·49 3·90 3·05 2·89 2·3 2·3 2·63 3·97 Lu 0·82 0·91 0·60 0·49 0·47 0·37 0·35 0·43 0·654 Sample: CBN3 CBN4 CBN10 CBN11 CBN12 CBN13 CBN14 CBN16 CBN20 Volcano: Comboyne Comboyne Comboyne Comboyne Comboyne Comboyne Comboyne Comboyne Comboyne Easting: 452447 452425 452425 443116 443328 443372 443439 440696 449412 Northing: 6500799 6500837 6500837 6505256 6505153 6505122 6505069 6506672 6500886 Elevation: 530 527 527 620 583 562 537 605 681 SiO2 59·22 59·48 53·73 54·00 55·03 45·25 46·26 54·49 58·94 TiO2 0·65 0·67 1·36 1·67 1·66 3·30 3·37 1·70 0·96 Al2O3 16·38 16·57 14·63 16·5 15·17 15·54 16·25 16·28 15·49 Fe2O3 8·62 8·51 12·27 10·37 10·72 14·57 13·49 10·79 8·91 MnO 0·18 0·13 0·16 0·16 0·15 0·17 0·16 0·17 0·14 MgO 0·47 0·39 1·38 2·34 2·35 3·9 4·05 2·06 0·88 CaO 2·15 2·33 3·2 4·21 4·62 7·4 6·94 4·47 3·16 Na2O 5·45 4·82 4·08 4·56 5·09 3·63 3·48 4·17 5·15 K2O 5·52 5·24 4·28 3·34 3·02 1·73 1·70 3·33 3·70 P2O5 0·22 0·22 0·85 0·75 0·56 1·26 0·88 0·74 0·41 H2O 0·26 0·44 1·40 0·39 0·24 2·54 2·20 0·42 0·57 LOI 0·71 0·92 2·24 1·62 1·24 0·68 1·10 1·31 1·65 Sum 99·83 99·72 99·58 99·91 99·85 99·97 99·88 99·93 99·96 Mg# 10·79 9·09 19·83 33·23 32·60 37·06 39·77 29·58 17·94 Cs 2·43 2·54 0·51 0·43 0·38 0·32 0·24 0·49 0·873 Rb 187 204 79·52 52·34 50·91 19·63 19·68 60·50 86·17 Ba 507 542 1372 689 677 680 555 678 1301 Sr 38·71 40·61 191 301 332 681 659 300 267 Pb 11·12 10·42 5·91 4·42 4·43 2·49 2·59 8·71 16·26 Th 16·18 15·8 7·57 7·45 7·8 4·25 4·66 6 6·62 U 3·59 3·74 0·74 1·78 1·77 0·87 0·91 1·80 2·29 Zr 832 843 559 363 355 234 260 362 477 Hf 21·51 22·38 14·03 10·08 9·85 5·91 6·51 10·07 12·77 Ta 6·97 6·98 3·81 3·82 3·77 2·82 2·92 3·87 4·33 Y 57·59 57·81 53·90 35·36 33·28 35·84 34·62 24·99 43·89 Nb 103 102 55·92 55·16 54·33 37·18 40·5 55·43 61·22 Sc 25·9 23·3 30·4 19·13 19 22·8 22·3 18·6 27·30 Cr <D.L <D.L <D.L 13·81 11·77 39·05 14·29 13·22 <D.L Ni 6 6. 7 34 18 57 50 19 8 Co 1·97 2·24 10·22 32·65 18·14 43·27 42 18·51 7·123 V 0·41 0·38 6·74 53·38 53·19 152 155 52·06 5·40 Ga 40·6 40·98 34·33 33·26 32·67 25·98 26·23 32·18 32·49 Zn 191 185 201 162 150 141 132 157 194 Cu 8·4 10·9 7·3 15·7 18·3 37·4 31·9 18·6 11·1 La 64·08 71·55 55·78 48·98 51·71 37·21 37·68 43·86 27·97 Ce 116 163 119 108 108 81·57 82·44 96·63 66·01 Pr 17·3 18·72 16·51 12·97 13·37 11·2 11·08 11·79 9·16 Nd 67·59 71·9 68·72 52·18 52·79 49·14 47·52 46·17 39·39 Sm 15·15 15·65 15·53 11·37 11·20 11·65 11·10 9·92 10·03 Eu 2·91 3·00 5·73 3·16 3·17 4·19 3·79 2·69 3·74 Gd 13·48 14·05 14·45 10·2 10·12 10·96 10·35 8·65 9·78 Tb 2·11 2·14 2·03 1·46 1·44 1·51 1·43 1·25 1·52 Dy 11·22 11·69 10·35 7·50 7·34 7·4 7·22 6·43 8·41 Ho 2·02 2·11 1·77 1·3 1·28 1·24 1·2 1·12 1·53 Er 6·14 6·4 5·13 3·68 3·63 3·35 3·33 3·29 4·63 Tm 0·92 0·98 0·71 0·55 0·53 0·46 0·45 0·49 0·70 Yb 5·29 5·49 3·90 3·05 2·89 2·3 2·3 2·63 3·97 Lu 0·82 0·91 0·60 0·49 0·47 0·37 0·35 0·43 0·654 Loss on ignition was determined by placing 2 g of sample into a crucible heated at 105°C for 1 h. Samples were removed, cooled and reweighed for loss of water (H2O) before being heated in a furnace at 1000°C for 1 h. Samples were then cooled and reweighed for LOI values. The whole-rock samples were prepared for major element analysis by combining 0·1 g of sample with 0·4 g of lithium metaborate flux in a platinum crucible. The mixture was then placed in a Katanax Automatic Fluxer and fused at ∼1000°C into a glass bead, before being dissolved in 100 ml of 5% nitric acid. Samples, standards, repeats, duplicates, monitors and blanks were analyzed by inductively coupled optical emission spectrometry (ICP-OES) on a Perkin Elmer Optima 8300DV system. Lutetium, Sc, and Y were used as internal standards and synthetic and natural reference materials (HISS-1, JGb-1, BHVO-2, JP-1a, JA-2, AGV2, BCR-2 and GSP-2) were used for calibration. Multiple analyses of BHVO-2 and JA2 standard aliquots were used to determine accuracy and precision. Accuracy was typically better than 2% and always better than 8% (see Supplementary Data Electronic Appendix 3). Precision was typically better than 5%, except for MnO and Al2O3, MgO, SiO2 and K2O (better than 8%; see Supplementary Data Electronic Appendix 3). Samples were prepared for trace element analysis by multi-acid open beaker hotplate digestion. For each sample, a 100 mg aliquot was placed in a Teflon beaker, along with nitric acid, sealed and placed on a hotplate overnight at 140°C. After drying the sample, the sequence was repeated using hydrofluoric acid, followed by hydrochloric acid. After the final drying down, nitric acid was added to the beaker again and heated for 2 h. The solution was then transferred to an auto-sampler tube and made up to 10 ml using milli-Q water. Samples, duplicates, repeats and standards (JA-2, JB-3 and W2a) were diluted 4000 times and 34 elements were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) on an Agilent 7900 system. Internal standards of 6Li, 61Ni, 103Rh, 115In, 185Re, and 235U were used to correct for drift during the analysis, and synthetic and natural reference materials (BIR-1, JA-2, JA-3, W2a) were used for calibration. The accuracy and precision of the trace element analyses was monitored using additional standard aliquots of JA-2, JB-3 and W2a. Accuracy was typically better than 3%, except for Cr, Ni, Pr, Yb and Ta (3–5%) and Rb, Y, Ce and Th (5–10%). Sc and Zn have low accuracy (28–17%; see Supplementary Data Electronic Appendix 3). Precision was generally better than 10%, except for Ta, Hf, Tm, Er and Lu (10–30%; see Electronic Appendix 3). RESULTS 40Ar/39Ar geochronology A summary of 40Ar/39Ar results for 14 whole-rock samples from Warrumbungle and Comboyne central volcanoes is provided in Table 1 and in Figs 2 and 3. The complete results and all step-heating spectra, isochron and age-probability diagrams are given in Supplementary Data Electronic Appendices 1 and 4. Fig. 2 View largeDownload slide 40Ar/39Ar step-heating spectra and combined isochrons for representative samples from the studied volcanoes (sample W11 from Warrumbungle and sample CBN11 from Comboyne). The step-heating spectrum of each sample shows duplicate whole-rock grains: black represents Grain 1; red represents the duplicate grain. Fig. 2 View largeDownload slide 40Ar/39Ar step-heating spectra and combined isochrons for representative samples from the studied volcanoes (sample W11 from Warrumbungle and sample CBN11 from Comboyne). The step-heating spectrum of each sample shows duplicate whole-rock grains: black represents Grain 1; red represents the duplicate grain. Fig. 3 View largeDownload slide 40Ar/39Ar geochronology from the Warrumbungle and Comboyne volcanoes vs whole-rock MgO (wt %) values. Four samples from the Warrumbungle volcano were previously dated by Cohen (2007) and all other ages were determined in this study. Triangles represent samples that were analyzed for mineral chemistry. Fig. 3 View largeDownload slide 40Ar/39Ar geochronology from the Warrumbungle and Comboyne volcanoes vs whole-rock MgO (wt %) values. Four samples from the Warrumbungle volcano were previously dated by Cohen (2007) and all other ages were determined in this study. Triangles represent samples that were analyzed for mineral chemistry. Of the 14 samples analyzed in duplicate, all produced plateau ages. All plateau ages are within error of the ages produced by the combined isochron and age-probability diagrams. The step-heating spectra typically show either Ar recoil or Ar loss in the early steps. Only sample CBN13 yielded large errors in each step as a result of low radiogenic per cent 40Ar. Inverse isochrons combine data for all individual steps analyzed for a given sample into a single age and allow identification and mitigation of excess argon. Therefore, we used inverse isochrons for interpreting geochronological results (Table 1), except for two samples from Comboyne, where the probability-density plot age was preferred (Table 1): sample CBN20 failed to produce an isochron because the gas fractions extracted in each step were nearly 100% radiogenic 40Ar*, and they clustered together and failed to produce the necessary spread in 39Ar/40Ar vs 36Ar/40Ar space; in the isochron for sample CBN3 the 40Ar/39Ar intercept was too low (40Ar/39Ar = 266 ± 8). The age results from the Warrumbungle volcano range from 15·8 ± 0·1 to 16·8 ± 0·1 Ma. These results are similar to those obtained by Cohen (2007) on a smaller sample set: 15·9 ± 0·1 to 18·1 ± 0·2 Ma. The newly dated lavas combined with previous results suggest that the Warrumbungle central volcano formed over at least 3 Myr. This is consistent with the average life span of individual volcanoes in eastern Australia (3–5 Ma: Cohen, 2007). Based on the discontinuous spread in ages, two stages of volcanism were established (Fig. 3): the first stage, from ∼18 to ∼17·5 Ma, includes two samples; the second stage, between ∼17 and ∼15·5 Ma, includes seven samples. Stage 2 samples have ages that generally overlap within error; however, two age subgroups can be identified (2.1 and 2.2) and will be discussed in terms of petrological and geochemical variations through time. Age results for the Comboyne volcano range from 16·0 ± 0·5 to 16·7 ± 0·1 Ma and are compatible with previous geochronological results for the Comboyne Plateau (16·6 ± 0·2 to 17·2 ± 0·3 Ma; Cohen, 2007). These data suggest that the Comboyne volcano has a more limited eruptive period than the Warrumbungle volcano (Fig. 3). However, additional 40Ar/39Ar dating of the flows surrounding the plateau and, in particular, the flows east of the plateau that have been previously dated by K/Ar (Sutherland et al., 2012), would be needed to confirm the more limited age range determined in this study. The current dataset suggests that the larger Warrumbungle volcano erupted in two stages, whereas the smaller Comboyne volcano erupted only during Stage 2 (Fig. 3). Petrography The lavas from the Warrumbungle and Comboyne volcanoes are petrographically diverse, but typically consist of millimeter-sized plagioclase, olivine, clinopyroxene and magnetite macrocrysts set in a fine-grained mafic groundmass. We use the non-genetic term macrocryst rather than the conventional term phenocryst in this section, following the scheme used by Ubide et al. (2014a). Subsequently, we discuss the occurrence of phenocryst and antecryst material in the macrocryst cargo. The Warrumbungle lavas range from aphanitic to glomeroporphyritic in texture (Fig. 4). Porphyritic samples contain macrocrysts of plagioclase and, to a minor extent, olivine and/or clinopyroxene and magnetite. The groundmass consists of plagioclase, clinopyroxene (occasionally ophitic in texture), magnetite ± olivine ± anorthoclase or sanidine. Apatite is also present as an accessory phase. Fig. 4 View largeDownload slide Petrographic thin section scans in plane-polarized light (PPL) and cross-polarized light (XPL): (a) an aphanitic sample from Stage 2.1 of the Warrumbungle volcano; (b) a porphyritic sample from Stage 2.2 of the Warrumbungle volcano; (c) and (d) an aphanitic and a porphyritic sample from Stage 2.2 of the Comboyne volcano, respectively. The lavas were analyzed for mineral chemistry and the crystals highlighted in squares represent the crystals in Figs 5 and 6. Fig. 4 View largeDownload slide Petrographic thin section scans in plane-polarized light (PPL) and cross-polarized light (XPL): (a) an aphanitic sample from Stage 2.1 of the Warrumbungle volcano; (b) a porphyritic sample from Stage 2.2 of the Warrumbungle volcano; (c) and (d) an aphanitic and a porphyritic sample from Stage 2.2 of the Comboyne volcano, respectively. The lavas were analyzed for mineral chemistry and the crystals highlighted in squares represent the crystals in Figs 5 and 6. Plagioclase macrocrysts are common to all lavas, except for one olivine-rich sample in Stage 1 and one aphanitic sample in Stage 2. Plagioclase macrocrysts are up to 5 mm in size and typically have modal abundances of ∼5 to 25 vol. %. There are three distinct types of plagioclase macrocrysts in the Warrumbungle lavas. The first type consists of subhedral to anhedral crystals, up to 5 mm in size, with sieve-textured cores, occasionally embayed, and no distinct rim. This type is most obvious in early Stage 2 lavas. The second are subhedral to anhedral crystals, up to 3 mm in size that show sieve-textured cores and a thin non sieve-textured rim (e.g. Figs 4b, 5a and 6a). This type is most obvious late in Stage 2 lavas, termed Stage 2.2. The third, only observed in Stage 2.2 lavas, are euhedral to subhedral, non sieve-textured crystals (up to 5 mm), which often are oscillatory zoned and sometimes contain melt inclusions (Fig. 4b). Fig. 5 View largeDownload slide Photomicrographs in cross-polarized light of selected macrocrysts found in the lavas from Stage 2.2 of the Warrumbungle (left) and Comboyne (right) volcanoes. Backscattered electron images of these crystals are shown in Fig. 6. Fig. 5 View largeDownload slide Photomicrographs in cross-polarized light of selected macrocrysts found in the lavas from Stage 2.2 of the Warrumbungle (left) and Comboyne (right) volcanoes. Backscattered electron images of these crystals are shown in Fig. 6. Fig. 6 View largeDownload slide Backscattered electron images of zoned plagioclase, olivine and clinopyroxene crystals from Stage 2.2 samples of the Warrumbungle (left) and Comboyne (right) volcanoes. The insets represent core-to-rim compositional transects, marked on the image using the same symbols (see legend in Fig. 7). Fractionation indices used are anorthite (An) for plagioclase, forsterite (Fo) for olivine and Mg# [Mg# = 100 Mg/(Mg + Fe2+) where concentrations are expressed on a molar basis, and Fe2+ is 90% of total Fe] for clinopyroxene. The petrographic context of the crystals is presented in Figs 4 and 5. Fig. 6 View largeDownload slide Backscattered electron images of zoned plagioclase, olivine and clinopyroxene crystals from Stage 2.2 samples of the Warrumbungle (left) and Comboyne (right) volcanoes. The insets represent core-to-rim compositional transects, marked on the image using the same symbols (see legend in Fig. 7). Fractionation indices used are anorthite (An) for plagioclase, forsterite (Fo) for olivine and Mg# [Mg# = 100 Mg/(Mg + Fe2+) where concentrations are expressed on a molar basis, and Fe2+ is 90% of total Fe] for clinopyroxene. The petrographic context of the crystals is presented in Figs 4 and 5. Olivine macrocrysts and microcrysts (0·1–2 mm) are relatively rare, except in one sample from Stage 1 lavas, which has up to ∼9 vol. % olivine macrocrysts. Stage 1 crystals range from subhedral to skeletal and are generally fresh with some minor alteration to iddingsite at the rims and in fractures. Olivine macrocrysts from Stage 2 (up to 5 vol. %) are subhedral to anhedral and range from fresh with alteration (iddingsite and minor serpentine) around the edges and along fractures, to completely altered (iddingsite). There are two types of clinopyroxene macrocrysts and microcrysts. In Stage 1 and early Stage 2 (termed Stage 2.1), clinopyroxene macrocrysts and microcrysts are reddish-pink in colour under plane-polarized light. They become more abundant and larger with time, from Stage 1 (0–1 vol. %; <2 mm) to Stage 2.1 (up to 5 vol. %; up to 1 mm), where they typically exhibit spongy textures. In Stage 2.2 lavas, clinopyroxene macrocrysts are rare (0–1 vol. %) and include reddish-pink as well as greenish crystals. The greenish macrocrysts are relatively more common, slightly larger (but still < 1 mm) and have spongy textures. Magnetite is a minor macrocryst phase in all stages (1–3 vol. %). The crystals are euhedral to anhedral, up to 0·4 mm in size. Magnetite typically occurs in association with macrocrysts of olivine, clinopyroxene and plagioclase. Samples from the Comboyne volcano range from porphyritic to aphanitic (youngest lavas). Macrocryst phases include plagioclase and minor olivine, clinopyroxene and magnetite. Plagioclase macrocrysts are generally large, up to 5 mm in size, and range from euhedral to anhedral and from 2 to 9 vol. %. The types of plagioclase macrocrysts observed in the Warrumbungle samples are also observed in the Comboyne samples, except for the ‘third’ euhedral type that do not have a sieve texture. The rarer olivine macrocrysts (up to 2 vol. %) are subhedral, fresh to completely altered and are up to 1 mm in size (Figs 4d, 5d and 6d). Similar to Warrumbungle samples, certain Comboyne lavas also contain rare (∼1 vol. %), subhedral, greenish clinopyroxene crystals, up to ∼1 mm in size. Two of the clinopyroxene crystals have reddish-pink cores. The pyroxene crystals are often oscillatory zoned and show spongy textures. Magnetite macrocrysts (1–3 vol. %) are euhedral to anhedral and range up to 1 mm in size. Devitrified glass and zeolites are accessory in the groundmass of certain samples, and zeolites make up 3 vol. % of samples CBN13 and CBN14. Mineral chemistry Plagioclase macrocrysts and groundmass microcrysts define a single variation trend in variation diagrams, in which anorthite content is positively correlated with Al2O3 wt % (Fig. 7a and b). In the Warrumbungle volcano, Stage 1 lavas lack plagioclase macrocrysts and the groundmass plagioclase is labradorite (An52–48). In Stage 2.1, plagioclase macrocryst cores have andesine (An47–37) compositions and a spongy texture, whereas rims have very low An contents. Groundmass crystals reach higher calcium concentrations (An54–40) than macrocrysts (Fig. 7a). Stage 2.2 macrocrysts range from labradorite to andesine (An60–35). Euhedral crystal cores have higher An contents (An60–54) than sieve-textured macrocryst cores (An37–35); the euhedral crystals are denoted as ‘primitive’ (P in Fig. 7a). The rims overgrowing sieve-textured cores are richer in calcium (An51–43), similar to the rims of the primitive crystals (An52–42; e.g. Figs 6a and 7a). We denote these rims as ‘enriched’ (E in Fig. 7). Stage 2.2 groundmass plagioclase microcrysts show a large range in An contents (An45–16) and are more evolved than Stage 2.1. Fig. 7 View largeDownload slide Mineral chemistry variation within individual volcanic stages in Warrumbungle (left) and Comboyne (right) volcanoes. (a, b) plagioclase; (c, d) olivine; (e, f) clinopyroxene. In the legend: P, primitive macrocrysts; IC, inner core of macrocrysts; C, cores of macrocrysts; E, enrichment zones close to the rim of macrocrysts; R, rims of macrocrysts; GM, groundmass crystals; OPX, orthopyroxene core. Fig. 7 View largeDownload slide Mineral chemistry variation within individual volcanic stages in Warrumbungle (left) and Comboyne (right) volcanoes. (a, b) plagioclase; (c, d) olivine; (e, f) clinopyroxene. In the legend: P, primitive macrocrysts; IC, inner core of macrocrysts; C, cores of macrocrysts; E, enrichment zones close to the rim of macrocrysts; R, rims of macrocrysts; GM, groundmass crystals; OPX, orthopyroxene core. At the Comboyne volcano, plagioclase macrocrysts have andesine (An47–35) spongy cores overgrown by labradorite to andesine (An53–39) enrichment zones, and andesine to anorthoclase (An43–5) rims (Figs 5b, 6b and 7b). Groundmass crystals have low calcium concentrations (An34–6) and include minor anorthoclase and sanidine. These macrocrysts occur throughout Stage 2 at the Comboyne volcano. The more primitive groundmass compositions (An53–47) were obtained from the aphanitic lava, the youngest of this suite (Fig. 7b). Olivine crystals have magmatic compositions (Fo < 90; CaO > 0·15 wt %; e.g. Sakyi et al., 2012, and references therein) and decreasing forsterite contents correlate with increasing calcium concentrations (Fig. 7c and d). In the Warrumbungle volcano, macrocrysts of Stage 1 lavas have cores ranging from Fo80 to Fo67 and rims ranging from Fo72 to Fo56. Olivine macrocryst from Stage 2.1 also show normal zonation from macrocryst cores (Fo63–57) to macrocryst rims (Fo56–53). Macrocryst cores of Stage 2.2 lavas range from Fo81–62 and the rims are also more evolved (Fo51–43). The cores of Stage 2.2 are similar in Fo composition to Stage 1 cores. Only one crystal from Stage 2.2 shows reverse zoning from Fo76 to Fo79 (Figs 5c and 6c). The outermost rim, however, drops in forsterite content to Fo77 (Fig. 6c). Groundmass olivine ranges from Fo65–53 in Stage 2.1 to Fo49 and the highest CaO concentrations in Stage 2.2 (Fig. 7c). At Comboyne, olivine macrocrysts are consistently normally zoned from cores (Fo73–42) to rims (Fo53–37; Fig. 7d) throughout Stage 2 (Figs 5d and 6d). Groundmass olivine crystals show a wide compositional range (Fo60–28; Fig. 7d) and partially overlap with the olivine macrocrysts. Clinopyroxene crystals, being comparatively rare, show the most significant compositional variations. Reddish-pink microcrysts from Stage 1 at the Warrumbungle volcano range from diopside to augite (TiO2 3·1–1·4 wt % and Al2O3 7·5–3·1 wt %) and are slightly zoned from cores [Mg# 81–74; Mg# = 100 Mg/(Mg + Fe2+), where concentrations are expressed on a molar basis and Fe2+ is 90% total Fe] to rims (Mg# 80–75; Fig. 7e). Reddish-pink clinopyroxene macrocrysts in Stage 2.1 are classified as augite and are unzoned (Mg# 76–70; TiO2 1·6–1·0 wt %; Al2O3 7·0–2·8 wt %; Fig. 7e). Groundmass crystals from Stage 2.2 lavas range from diopside to augite, also ranging in Mg# from 70 to 69 (Fig. 7e). Al2O3 concentrations are lower in the groundmass microcrysts than in the macrocrysts (Fig. 7e). Reddish-pink and greenish augites occur as macrocrysts in Stage 2.2 lavas. Both types show reverse zoning with enriched rims (e.g. Figs 5e and 6e). Greenish cores are corroded and have lower Mg# (64–58) and TiO2 (0·85–0·66) than enriched rims (Mg# 75–71 and TiO2 1·58–0·94 wt %) (e.g. Fig. 7e) and reddish-pink cores (Mg# 70–66 and TiO2 2·20–1·46 wt %) (Fig. 7e). Groundmass augite crystals from Stage 2.2 range from Mg# 73 to 70 and have similar Al2O3 concentrations to enriched rims and also to Stage 2.1 groundmass crystals (Fig. 7e) Similarly, clinopyroxene at the Comboyne volcano is mainly augite with reddish-pink and greenish cores. Greenish cores range in Mg# from 71 to 63, TiO2 from 1·64 to 0·7 wt % and Al2O3 from 3·88 to 1·51 wt %, and are mantled by enrichment zones (Mg# 71–67, TiO2 1·20–0·76 wt %, Al2O3 3·08–1·60 wt %; Fig. 7f). Later in the eruptive sequence the pyroxene macrocrysts develop rims (Mg# 67–63 and TiO2 1·25–0·95 wt %; Fig. 7f). We found that two of the analysed clinopyroxene macrocrysts (one in each of Stage 2.1 and Stage 2.2) have a more primitive resorbed core that we describe as ‘inner core’ (IC in Fig. 7) (Stage 2.1: Mg# 74, TiO2 1·20–1·16 wt %, and Al2O3 3·92–3·47 wt %; Stage 2.2: Mg# 74, TiO2 1·65–1·52 wt %, and Al2O3 6·08–5·75 wt %; e.g. Figs 5f, 6f and 7f). One resorbed core of orthopyroxene was found in Stage 2 lavas (Mg# 79; TiO2 0·36–0·33 wt %; CaO 1·88–1·83 wt %; Fig. 7f). This orthopyroxene is rimmed by augite (Mg# 55–53; TiO2 1·23–1·21 wt %; Al2O3 3·88–3·69 wt %), which is similar to the majority of enrichment zones in this stage (Fig. 7f). Groundmass augite is more evolved than the macrocrysts and also than groundmass crystals in the Warrumbungle lavas (Mg# 70–52 and TiO2 3·32–0·97 wt %: Fig. 7f). Titanium-rich magnetite–ülvospinel is present as macrocrysts and in the groundmass of both volcanoes. At the Warrumbungle volcano, TiO2 concentrations vary weakly across all three stages (19·5–28 wt %). Some microphenocrysts and groundmass crystals are rimmed by ilmenite (TiO2 = 51·5–52·2 wt %). At Comboyne, compositions are similar in TiO2 (23·2–27·1 wt %) throughout the sequence. Ilmenite is found rimming Ti-rich magnetite–ülvospinel microphenocrysts or as a groundmass phase (TiO2 = 49·0–51·5 wt %). Whole-rock geochemistry Major and trace element compositions of 18 samples from the Warrumbungle and Comboyne central volcanoes are shown in Table 2. The lavas are generally fresh, with LOI values between 0·99 and 1·82 wt % for Warrumbungle and between 0·71 and 2·25 wt % for Comboyne. The total alkali vs silica (TAS) diagram shows that the lavas are alkaline and define a common SiO2-saturated trend (Fig. 8). Warrumbungle lavas are trachy-basalts and trachy-andesites (Fig. 8) and have 7·25–3·03 wt % MgO (Fig. 9). Comboyne lavas have a larger compositional range, from trachy-basalt and trachy-basaltic andesite to trachy-andesite and trachyte (Fig. 8). Despite the mafic appearance of all the samples investigated, Comboyne samples reach more evolved compositions (4·05–0·39 wt % MgO; Fig. 9). Our new data fall within the compositional ranges obtained in previous studies (Knutson, 1975; Ghorbani & Middlemost, 2000), and show that the Comboyne volcano reaches SiO2-oversaturated compositions. Fig. 8 View largeDownload slide Major element chemical distribution of Warrumbungle and Comboyne lavas on the TAS diagram (anhydrous basis). The classification fields of the TAS diagram are after Le Maitre et al. (1989) and the alkaline–sub-alkaline line (grey) is after Irvine & Baragar (1971). The green and purple fields represent previous geochemical data from Warrumbungle (Ghorbani & Middlemost, 2000) and Comboyne (Knutson, 1975) volcanoes. The grey outlines represent the compositions from the Buckland volcano (Skae, 1998; Crossingham et al., 2018). The letters represent: B - Basalt, BA - Basaltic andesite, A - Andesite, D - Dacite, R - Rhyolite, Bas - Basanite, TB - trachy-basalt, BTA - Basaltic trachy andesite, TA - Trachy andesite, T - Trachyte. Fig. 8 View largeDownload slide Major element chemical distribution of Warrumbungle and Comboyne lavas on the TAS diagram (anhydrous basis). The classification fields of the TAS diagram are after Le Maitre et al. (1989) and the alkaline–sub-alkaline line (grey) is after Irvine & Baragar (1971). The green and purple fields represent previous geochemical data from Warrumbungle (Ghorbani & Middlemost, 2000) and Comboyne (Knutson, 1975) volcanoes. The grey outlines represent the compositions from the Buckland volcano (Skae, 1998; Crossingham et al., 2018). The letters represent: B - Basalt, BA - Basaltic andesite, A - Andesite, D - Dacite, R - Rhyolite, Bas - Basanite, TB - trachy-basalt, BTA - Basaltic trachy andesite, TA - Trachy andesite, T - Trachyte. Fig. 9 View largeDownload slide View largeDownload slide Major element variation diagrams of bulk-rock compositions from the Warrumbungle (left) and Comboyne (right) volcanoes. Black-filled symbols represent the sample with olivine accumulation; white-filled symbols represent samples with plagioclase accumulation. Rhyolite-MELTS factional crystallization (continuous lines) and equilibrium crystallization (dashed lines) models are plotted for comparison. The models were performed using samples BC167 (Warrumbungle) and CBN14 (Comboyne) as starting compositions. Fig. 9 View largeDownload slide View largeDownload slide Major element variation diagrams of bulk-rock compositions from the Warrumbungle (left) and Comboyne (right) volcanoes. Black-filled symbols represent the sample with olivine accumulation; white-filled symbols represent samples with plagioclase accumulation. Rhyolite-MELTS factional crystallization (continuous lines) and equilibrium crystallization (dashed lines) models are plotted for comparison. The models were performed using samples BC167 (Warrumbungle) and CBN14 (Comboyne) as starting compositions. In major element Harker diagrams (Fig. 9), the samples show relatively linear variation trends in which SiO2, Al2O3, Na2O and K2O increase and TiO2, Fe2O3T (total iron as Fe2O3), CaO and P2O5 decrease with decreasing MgO concentrations. In terms of trace element variations (Fig. 10), there are overall inverse correlations between compatible (e.g. Cr, Ni) and incompatible (e.g. Ba, Sr) elements. The major and trace element compositions of the samples correlate closely with petrographic observations. The sample with highest MgO (wt %), Cr and Ni (ppm) concentrations is a porphyritic lava from the Warrumbungle volcano with abundant olivine macrocrysts (9 vol. %; see Supplementary Data Electronic Appendix 5). Plagioclase macrocrysts are relatively common in both volcanoes, and samples with the highest plagioclase volume fractions (10–20 vol. %; see Supplementary Data Electronic Appendix 5) have low MgO (wt %), Cr and Ni (ppm) concentrations, high Ba and Sr concentrations and slight positive Eu anomalies in chondrite-normalized rare earth element (REE) patterns (Figs 9, 10 and 11a and c). Fig. 10 View largeDownload slide Trace element variation diagrams for bulk-rock compositions from the Warrumbungle (left) and Comboyne (right) volcanoes. Black-filled symbols represent the sample with olivine accumulation; white-filled symbols represent samples with plagioclase accumulation. Diagrams include 20% fractionation vectors of olivine (green), clinopyroxene (orange), plagioclase (blue) and/or magnetite (grey), from the most primitive aphanitic samples in each volcano. The fractionation vectors were calculted using the Rayleigh equation and mineral/melt partition coeffients for basaltic melts from Bougault & Hekinian (1974), Paster et al. (1974), Villemant et al. (1981), Lemarchand et al. (1987), Kloeck & Palme (1988), McKenzie & O’Nions (1991), Nielsen (1992), Beattie (1994), Skulski et al. (1994), Bindeman et al. (1998) and Zack & Brumm (1998). Fig. 10 View largeDownload slide Trace element variation diagrams for bulk-rock compositions from the Warrumbungle (left) and Comboyne (right) volcanoes. Black-filled symbols represent the sample with olivine accumulation; white-filled symbols represent samples with plagioclase accumulation. Diagrams include 20% fractionation vectors of olivine (green), clinopyroxene (orange), plagioclase (blue) and/or magnetite (grey), from the most primitive aphanitic samples in each volcano. The fractionation vectors were calculted using the Rayleigh equation and mineral/melt partition coeffients for basaltic melts from Bougault & Hekinian (1974), Paster et al. (1974), Villemant et al. (1981), Lemarchand et al. (1987), Kloeck & Palme (1988), McKenzie & O’Nions (1991), Nielsen (1992), Beattie (1994), Skulski et al. (1994), Bindeman et al. (1998) and Zack & Brumm (1998). Fig. 11 View largeDownload slide Normalized rare earth element (REE) and multi-element patterns of bulk-rock samples from the Warrumbungle and Comboyne volcanoes. The grey field represents the mafic lavas (4·35–8·81 wt % MgO) from the Buckland central volcano (after Crossingham et al., 2018). Chondrite and primitive mantle compositions are from Sun & McDonough (1989). EMI values are the average for Tristan and Inaccessible OIB from GEOROC (http://georoc.mpch-mainz.gwdg.de/georoc/). HIMU values are from Hoffmann (1997) and references therein. Average upper and lower crustal values are from Rudnick & Fountain (1995). Fig. 11 View largeDownload slide Normalized rare earth element (REE) and multi-element patterns of bulk-rock samples from the Warrumbungle and Comboyne volcanoes. The grey field represents the mafic lavas (4·35–8·81 wt % MgO) from the Buckland central volcano (after Crossingham et al., 2018). Chondrite and primitive mantle compositions are from Sun & McDonough (1989). EMI values are the average for Tristan and Inaccessible OIB from GEOROC (http://georoc.mpch-mainz.gwdg.de/georoc/). HIMU values are from Hoffmann (1997) and references therein. Average upper and lower crustal values are from Rudnick & Fountain (1995). Rare earth element concentrations are enriched up to >100 times over chondritic values (Fig. 11). Rare earth element patterns are similarly fractionated in both volcanoes, with relatively high (La/Yb)N ratios of 8·7–12·8. In primitive mantle normalized multi-element patterns, all samples have negative Sr anomalies, except for the least evolved, aphanitic, youngest lavas of Comboyne. The majority of samples, excluding the most evolved samples of Comboyne, also have positive Ba anomalies (Fig. 11). Negative Th, U, Sr, Ti or K anomalies and small positive Pb, Eu, Zr and K anomalies are present in certain samples. The multi-element patterns of the least evolved samples resemble enriched mantle I (EMI; Zindler & Hart, 1986) signatures. However, the positive peaks in Ba resemble lower crustal signatures. The small positive K and Pb anomalies, evident in the more evolved samples, are characteristic of the crust (Fig. 11). DISCUSSION Mineral zoning and mineral–melt equilibria Porphyritic lavas can carry multiple crystal populations, including phenocrysts, antecrysts and xenocrysts (e.g. Berlo et al., 2007; Cashman & Blundy, 2013; Ubide et al., 2014a, 2014b). Phenocrysts are grown in the host magma, antecrysts represent recycled crystals from an earlier generation of magma and xenocrysts are foreign to the system; only phenocrysts are in chemical equilibrium with their host-rock (e.g. Charlier et al., 2005; Davidson et al., 2007; Jerram & Martin, 2008). At Warrumbungle and Comboyne volcanoes, the lavas are strongly porphyritic, with complexly zoned macrocrysts that often cluster in glomerocrystalline associations (Figs 4 and 5). The chemical stratigraphy of zoned crystals can provide a unique record of pre-eruptive processes, such as magma recharge, magma mixing, fractional crystallization and contamination (e.g. Dobosi & Fodor, 1992; Seaman, 2000; Berlo et al., 2007; Cashman & Blundy, 2013; Ubide et al., 2014a, 2014b). Here we use mineral zoning to investigate magmatic histories in the plumbing system feeding the coeval pair of volcanoes. Plagioclase Plagioclase is a good recorder of pressure, temperature and compositional changes in the magmatic environment (e.g. Nelson & Montana, 1992; Troll & Schmincke, 2002), and preservation of zoning patterns is favoured by slow NaSi–CaAl diffusion rates (Grove et al., 1984). In Stage 2 lavas from the Warrumbungle and Comboyne volcanoes, plagioclase macrocrysts display a variety of textures and zoning patterns (Figs 4–6). These include euhedral An-rich crystals and anhedral, sieve-textured, An-poor crystals. The latter are rimmed by An-enriched zones, except in Stage 2.1 at Warrumbungle. Sieve textures may result from magma mixing or magmatic decompression (Nelson & Montana, 1992). In the cases studied here, the occurrence of sieve textures in anhedral (resorbed) cores, typically rimmed by An-enriched zones, suggests mixing of An-poor cores with a hotter, more primitive recharge magma. Euhedral An-rich crystals occur in Warrumbungle Stage 2.2 lavas and show weak normal zoning, where the rims have similar compositions to the An-enriched zones surrounding sieve-textured cores (Figs 6a-b and 7a-b). It follows that the An-rich cores could have crystallized from the recharge magma before reaching the An-poor mush zone. After recharge, rims crystallizing from the hybrid magma overgrew (variably) the pre-existing An-rich and An-poor (now sieved-textured) cores. We therefore infer the following: (1) a hot, primitive magma, which carried euhedral An-rich cores at Warrumbungle volcano, was injected into a reservoir containing more evolved plagioclase cores; (2) An-rich and An-poor cores represent antecrysts in the studied rocks; the hot magma caused resorption and sieving of the more evolved cores, and both core-types were overgrown by rims crystallizing from hybrid magma; (3) given the occurrence of hybrid zones at the rims of the crystals, mafic recharge is likely to have triggered eruption of Stage 2 lavas; the more evolved nature of the groundmass crystals (Fig. 7) is probably related to fractional crystallization during magma ascent and eruption. Olivine Olivine crystals in the studied samples show simpler zoning patterns than plagioclase. The compositional range in the Warrumbungle volcano is wider than in the Comboyne volcano, and individual crystals are typically unzoned or normally zoned (Fig. 7). To check mineral–melt equilibrium between olivine and host-rock compositions, we followed the Mg–Fe mineral–melt equilibrium diagram of Rhodes et al. (1979) (Fig. 12a and b). Some of the olivine cores are in equilibrium with the whole-rock compositions and therefore can be considered true phenocrysts. However, Stage 2 olivine cores often plot above equilibrium (primitive antecrysts) or below equilibrium (evolved antecrysts). We note that Stage 1 olivine cores from the Warrumbungle volcano seem to be evolved antecrysts, Stage 1 lavas are strongly olivine-rich (up to 9 vol. % olivine macrocrysts) and whole-rock compositions (with high concentrations of MgO, Ni and Cr) are probably affected by accumulation and do not represent true magmatic compositions (e.g. Larrea et al., 2013; Ubide et al., 2014a, 2014b). Indeed, if a primitive aphanitic (macrocryst-free) lava is considered as proxy for the melt in this sample (see vertical line in Fig. 12a), some of the Stage 1 olivine core compositions would plot above equilibrium and therefore should be considered primitive antecrysts. Considering the presence of normal and reverse zoning in the crystals (Fig. 6c and d), we tentatively suggest that the Fo-rich and Fo-poor antecryst populations could have occurred in a similar way to the An-rich and An-poor plagioclase cores; that is, from the recharging magma and the resident mush, respectively. Olivine rims and groundmass microcrysts have fractionated compositions that consistently plot below equilibrium (Fig. 12a and b) and could be related to magma decompression, ascent and eruption. Fig. 12 View largeDownload slide Mineral–melt equilibrium diagrams models for (a, b) olivine and (c, d) clinopyroxene from the Warrumbungle and Comboyne volcanoes. The curved lines, after Rhodes et al. (1979), display the range of equilibrium compositions between mineral and melt using an iron–magnesium distribution coefficient of 0·03 ± 0·03 for olivine (Roeder & Emslie, 1970) and 0·26 ± 0·05 for clinopyroxene (Akinin et al., 2005). In the legend: C, macrocryst cores; IC, primitive inner cores; E, enrichment zones; R, rims of macrocrysts; GM, groundmass. Fig. 12 View largeDownload slide Mineral–melt equilibrium diagrams models for (a, b) olivine and (c, d) clinopyroxene from the Warrumbungle and Comboyne volcanoes. The curved lines, after Rhodes et al. (1979), display the range of equilibrium compositions between mineral and melt using an iron–magnesium distribution coefficient of 0·03 ± 0·03 for olivine (Roeder & Emslie, 1970) and 0·26 ± 0·05 for clinopyroxene (Akinin et al., 2005). In the legend: C, macrocryst cores; IC, primitive inner cores; E, enrichment zones; R, rims of macrocrysts; GM, groundmass. Clinopyroxene Reddish-pink and green-cored clinopyroxene are both common in alkali basalts around the world (e.g. Duda & Schmincke, 1985; Dobosi, 1989; Ubide et al., 2014a, 2014b; Marzoli et al., 2015; Jankovics et al., 2016). The origin of green-cored pyroxenes has been linked to mixing between an evolved magma and a hotter, more primitive magma, or interpreted as xenocryst material from igneous wall-rock or the metasomatized upper mantle (e.g. Barton & van Bergen, 1981; Dobosi & Fodor, 1992; Szabó & Bodnar, 1998; Pilet et al., 2002). Mineral–melt equilibria diagrams were also used to test equilibrium between clinopyroxene and host-rock compositions (Fig. 12c and d). Here we also note that Stage 1 clinopyroxene cores are hosted by an olivine-accumulative whole-rock, and therefore should be considered at the vertical line representing an aphanitic primitive lava. Following this approach, Ti-augites to diopsides in Stage 1 and Stage 2.1 of the Warrumbungle volcano are normally zoned and in equilibrium. By contrast, Ti-augites and greenish augites in Stage 2.2 at the Warrumbungle volcano and throughout Stage 2 at the Comboyne volcano are reversely zoned (Fig. 7). In both locations, the majority of evolved, Mg#-poor cores are in equilibrium and the Mg#-rich ‘enriched’ rims are above equilibrium (Fig. 12c and d). In particular, the green cores are very poor in Mg# and plot below equilibrium, so we propose that they crystallized in an intensely fractionated mush zone. Similar interpretations on green clinopyroxene cores have been reported in other alkaline continental systems (e.g. green-cored clinopyroxene at Mt Bambouto in the Cameroon Line and ‘fassaitic’ green-core clinopyroxene from West Eifel; Duda & Schmincke, 1985; Marzoli et al., 2015). The occurrence of the reversely zoned clinopyroxene with antecryst cores and phenocryst rims supports the interpretation of magma mixing between a hotter, more primitive magma and evolved mush zones. The Mg#-rich inner cores recognized at Comboyne plot above equilibrium and therefore represent early primitive antecrysts in the system. Fractional crystallization Fractional crystallization has been shown to play a major role in magma evolution since the early studies of the Warrumbungle and Comboyne volcanoes (Knutson, 1975; Duggan & Knutson, 1986). The majority of our lavas have MgO contents of less than 8 wt %, Cr values < 200 ppm and Ni values < 100 ppm and, therefore, do not represent primary melts. The complexities in mineral zoning suggest stagnation of magmas in mush zones undergoing fractional crystallization and replenishment events (Figs 6 and 7). In this section, we discuss whole-rock compositional variations in terms of fractional crystallization processes to constrain the spatio-temporal evolution of magmas in the plumbing system. We note that some of our samples show evidence of olivine or plagioclase accumulation (enrichments in MgO–Ni–Cr and Ba–Sr–Eu, respectively; Figs 8–11) and we focus on non-accumulative compositions as representatives of melts. We also restrict our modelling to simpler fractionation and equilibrium crystallization processes. However, we acknowledge that many of the lavas may have also experienced mild crustal assimilation during fractional crystallization, especially given the small positive Pb and K anomalies evident in some lavas (Fig. 11). Strong decreases in Ni with increasing La (Fig. 10), following Rayleigh fractionation vectors, suggest olivine fractionation. The Comboyne lavas show a strong decrease in CaO/Al2O3 with MgO wt % (not shown) and a strong decrease in Cr with increasing La, suggesting additional fractionation of clinopyroxene that is not observed at Warrumbungle (Fig. 10). The evolution of Sr and Ba suggests the presence of plagioclase in the fractionating assemblage, particularly at Comboyne (Fig. 10). Primitive mantle-normalized multi-element patterns for the Warrumbungle samples have minor negative Sr anomalies, supporting minor plagioclase fractionation (Fig. 11b). The majority of the Comboyne samples exhibit progressively larger Sr and Ti anomalies with differentiation (Fig. 11d) and the trachyte samples show a negative Eu anomaly (Fig. 11c), suggesting greater plagioclase and magnetite fractionation than at Warrumbungle. Also at Comboyne, decreases in P2O5 with decreasing MgO suggest fractionation of apatite (Fig. 9). Therefore, the fractionating mineral assemblage appears to include clinopyroxene, olivine, plagioclase, magnetite and apatite, in agreement with observed modal assemblage in the samples. Thermodynamic conditions To constrain the depths of magma stagnation and fractionation, we applied barometric calculations (see summary by Putirka, 2008) to clinopyroxene macrocryst populations. As the majority of the clinopyroxene compositions are not in equilibrium with the host-rocks, our barometric estimates cannot be based on clinopyroxene–melt equilibrium pairs (e.g. Putirka et al., 2003) and instead we used the clinopyroxene-only calibration 32a of Putirka (2008). Temperatures of clinopyroxene crystallization were estimated with the pressure-independent thermometer of Putirka et al. (1996). Results suggest trans-crustal polybaric crystallization at both volcanoes (Fig. 13). Clinopyroxene cores from Stage 1 lavas began to crystallize at 8 kbar (800 MPa). Assuming a crustal density of 2·8 g cm–3, this pressure corresponds to a depth of ∼29 km (Fig. 13a and b). Clinopyroxene cores from Stage 2.1 Warrumbungle lavas began to crystallize at a pressure of ∼12 kbar (∼44 km: Fig. 13a and b), which coincides with the estimated depth of the Moho in this area (Collins et al., 2003). The green, Mg#-poor clinopyroxene cores from Stage 2.2 lavas appear to have crystallized at 6–2 kbar (∼22–7 km), suggesting magma stagnation and fractionation in a shallow mush zone. The enrichment zones began to crystallize at ∼3·5 kbar (13 km) and continued up to the surface (Fig. 13a and b), lending further support to the hypothesis that mafic recharge and mixing triggered the eruption of late Stage 2 lavas. Fig. 13 View largeDownload slide Thermobarometry estimates for clinopyroxene macrocrysts from the Warrumbungle and Comboyne volcanoes. Pressures were obtained with the melt-independent barometer 32a of Putirka (2008) and temperatures were calculated with the pressure-independent thermometer of Putirka et al. (1996). Depths were calculated using a crustal density of 2·8 g cm–3. The depth of the Moho is derived from Collins et al. (2003). In the legend: C, clinopyroxene macrocryst cores; E, enrichment zones; IC, primitive inner cores; R, rims of macrocrysts. Fig. 13 View largeDownload slide Thermobarometry estimates for clinopyroxene macrocrysts from the Warrumbungle and Comboyne volcanoes. Pressures were obtained with the melt-independent barometer 32a of Putirka (2008) and temperatures were calculated with the pressure-independent thermometer of Putirka et al. (1996). Depths were calculated using a crustal density of 2·8 g cm–3. The depth of the Moho is derived from Collins et al. (2003). In the legend: C, clinopyroxene macrocryst cores; E, enrichment zones; IC, primitive inner cores; R, rims of macrocrysts. Clinopyroxene crystallization depths at Comboyne are generally consistent throughout the life of the volcano. The inner cores found in Comboyne lavas (Figs. 6f and 7f) crystallized at the highest pressures and temperatures (Fig. 13d), corresponding to a depth of ∼36 km, the approximate depth of the Moho in this area (Collins et al., 2003). Clinopyroxene cores and enrichment zones began to crystallize at ∼8 kbar (29 km; Fig. 13a), with the bulk of crystallization taking place from 4 kbar (15 km) to the surface. The conditions of magma fractionation were further tested using forward thermodynamic modelling with rhyolite-MELTS (Gualda et al., 2012; Ghiorso & Gualda, 2015; Fig. 9). Both polybaric fractional crystallization and equilibrium crystallization models were attempted. The most primitive aphanitic compositions from the Warrumbungle and Comboyne volcanoes were used as starting compositions. Magma fractionation pressures ranged from 8 to 3 kbar (800–300 MPa) with a dP/dT = 10, as determined from clinopyroxene thermobarometry. Additional parameters included cooling steps of 10°C and fixed oxygen fugacity at the quartz–fayalite–magnetite (QFM) buffer. The lack of hydrous phases (e.g. biotite, amphibole) in the products of both volcanoes suggests low water contents in the magmas. We tested a range of water contents from anhydrous up to the LOI values (2·2 wt %), and found that purely anhydrous conditions produced fractionation trends that did not fit the data. Hence, we present and discuss models with small water contents of 0·2–0·4 wt % (Fig. 9). In addition to comparing modelled liquids with whole-rock compositions, a secondary check on the validity of the models was the comparison of olivine and clinopyroxene compositions fractionated by the models and mineral chemistry results obtained from our samples, following the approach of Kahl et al. (2015, 2017). Equilibrium crystallization models at pressures between 5 and 8 kbar and water contents of 0·4 wt % or higher produce either hydrous mineral phases or mineral phases such as orthopyroxene, which are not observed in the samples. At a pressure of 3 kbar with 0·4 wt % water (Fig. 9), the models for the Warrumbungle volcano produce all the mineral phases observed in the samples, including olivine, clinopyroxene, feldspar, magnetite, and apatite. The compositions of olivine and pyroxene produced in this model overlap with the most primitive compositions observed in the Warrumbungle lavas, but they do not extend across the full range of observed mineral compositions (Fig. 14a and c). The same model parameters, using the starting composition from Comboyne, produced minerals inconsistent with petrographic observations. Models beginning at 4 kbar and 0·2 wt % H2O (Fig. 9) produced all the mineral phases observed at Comboyne. These models, however, failed to reproduce the Fo and Mg# compositions of olivine and clinopyroxene observed in our samples (Fig. 14b and d). Fig. 14 View largeDownload slide (a, b) Olivine and (c, d) clinopyroxene compositions calculated using the Rhyolite-MELTS models (coloured lines) compared with the natural minerals in the Warrumbungle and Comboyne lavas (boxes). FC, fractional crystallization; EQ, equilibrium crystallization. Fig. 14 View largeDownload slide (a, b) Olivine and (c, d) clinopyroxene compositions calculated using the Rhyolite-MELTS models (coloured lines) compared with the natural minerals in the Warrumbungle and Comboyne lavas (boxes). FC, fractional crystallization; EQ, equilibrium crystallization. Fractional crystallization models at high pressures (8 kbar at Warrumbungle and 5–8 kbar at Comboyne) produce extra mineral phases not observed petrographically; they also produce clinopyroxene compositions that are inconsistent with those in the macrocryst assemblage (e.g. Fig. 14c). At lower pressures (e.g. 3 kbar), the clinopyroxene compositions do not extend towards the more evolved compositions observed in Stage 2 at the Warrumbungle volcano (Fig. 14c) and the model fails to produce olivine using the starting composition at Comboyne. The most representative models produced for each volcano differ in pressure, suggesting that the fractionating mush zones were located at different depths. At Warrumbungle, models starting at pressures between 5 and 7 kbar, with water contents between 0·2 and 0·4 wt %, reproduce the major element compositional variations observed in bulk-rock samples (Fig. 9) and the majority of the Fo and Mg# values found in the olivine and clinopyroxene crystals are in equilibrium (Fig. 14a and b). Crystallization initiated at pressures between 7 and 5 kbar is also generally consistent with the thermobarometry results obtained from clinopyroxene with greenish cores. At Comboyne, the fractional crystallization models that produce the necessary mineral phases require pressures ranging from 3·5 to 4·5 kbar (Fig. 9). This is consistent with pressure estimates derived from clinopyroxene thermobarometry and suggests shallower conditions of magma crystallization than those estimated for the Warrumbungle volcano. The modelled Mg# value for clinopyroxene overlaps with the clinopyroxene compositions measured for the Comboyne samples (Fig. 14d). The forsterite content of modelled olivine, however, is generally more evolved than the olivine crystals from Comboyne. The fact that olivine compositions are successfully modelled by fractional crystallization and that primitive olivine compositions are successfully modelled by equilibrium crystallization suggests that the Comboyne lavas represent a combination of equilibrium and fractional crystallization processes. Source constraints Whole-rock major and trace element variations in both volcanoes define similar trends (Figs 8–10) and incompatible trace element patterns (Fig. 11) suggesting a common, or similar, EMI-like enriched source. Detailed discussion of mantle source components is beyond the scope of this study, but we note that previous studies suggested an EMI signature in the lava-fields of New South Wales (Zhang et al., 2001). Zhang et al. (2001) interpreted the enriched signature to be derived from the sub-continental lithospheric mantle. Most recently, a deep EMI-like source has been proposed for the mafic magmas at Buckland central volcano, located further to the north, in Queensland (Crossingham et al., 2018; Fig. 1). Integrating petrology, geochemistry and geochronology: multi-stage evolution of a coeval pair of volcanoes Fractional crystallization and variations in the degree of source melting have been proposed to account for much of the diversity in the Cenozoic volcanic products of eastern Australia (Frey et al., 1978; Ewart et al., 1988). Here, we show that magma rejuvenation events and subsequent magma mixing, together with recycling of crystal assemblages, also play an important role in generating the volcanic products of Warrumbungle and Comboyne central volcanoes. These coeval volcanoes erupted dominantly porphyritic lavas, containing varied antecryst and phenocryst populations. This indicates that the magmas stalled and crystallized in mush-like reservoirs making up complex plumbing systems, reflected in the increasing complexity of antecryst zoning patterns with time. The similarity of crystal cargoes in these two volcanoes is striking considering the large (c. 300 km) distance between them, which would suggest that they were fed by independent crustal plumbing systems. The volcanoes are separated by the Great Dividing Range (Fig. 1), which may have had an effect on magma migration below the surface. On the western side, the Warrumbungle volcano is larger and was active for ∼3 Myr. On the eastern side, the Comboyne volcano is significantly smaller and the main phase of volcanism was restricted to a 1 Myr period. Additional dating of the flows surrounding the Comboyne Plateau would be needed to ensure that volcanism at Comboyne is limited to the 1 Myr period. At the Warrumbungle volcano, Stage 1 (∼18–17·5 Ma) magmas fractionated during ascent and recycled primitive olivine antecrysts at depths of ∼29 km (Fig. 15). During Stage 2.1, ascending magmas sampled olivine, augite and plagioclase mushes at the crust–mantle boundary (∼41 km; Fig. 15). The disequilibrium textures observed in the augite and plagioclase macrocrysts suggest differences in composition between the ascending magmas and the mush. This was followed by the eruption of aphanitic lavas, which suggests that either the mush had been evacuated at that stage or, alternatively, that the magma became reduced in its capacity to carry crystals to the surface (Fig. 15). In Stage 2.2, a shallower mush reservoir (22–7 km), containing fractionated plagioclase, clinopyroxene and olivine crystals, was recharged by a hotter and more primitive magma, which also carried large euhedral, An-rich, plagioclase macrocrysts (Fig. 15). This resulted in the disequilibrium textures of the plagioclase from the mush zone and the growth of enriched rims, and generated reversely zoned olivine and augite crystals (Figs 4–6). Fig. 15 View largeDownload slide Schematic representation of magma dynamics in the plumbing system feeding the Warrumbungle and Comboyne volcanoes through time (not to scale). Fig. 15 View largeDownload slide Schematic representation of magma dynamics in the plumbing system feeding the Warrumbungle and Comboyne volcanoes through time (not to scale). At Comboyne, we have recognized a main mush reservoir, located at 15–7 km depth, throughout the lifespan of the volcano. Within the mush reservoir, we observe that primitive ‘inner core’ clinopyroxene antecrysts formed at the deepest levels, around the crust–mantle boundary, and were transported to shallower depths, where they crystallized further, together with olivine and plagioclase (Fig. 15). A hotter, more primitive magma was subsequently injected into this mush level, forming the disequilibrium textures and enrichment zones in plagioclase and clinopyroxene. We also note that the enriched clinopyroxene zones overgrow an intensely resorbed orthopyroxene core, which may be interpreted as a xenocryst in the system. The continuation of crystal recycling initially observed throughout the majority of Stage 2 suggests that, unlike the Warrumbungle volcano, the mush reservoir was not fully evacuated. The final lavas erupted at the Comboyne volcano, however, have an aphanitic texture and are the least evolved, suggesting that the crystals housed in the mush zone were fully expelled prior to the replenishment of the reservoir by new mafic crystal-poor magma (Fig. 15). Previous geochemical studies of lavas from the Warrumbungle and Comboyne volcanoes proposed either a single magmatic lineage (Knutson, 1975; Duggan & Knutson, 1986) or distinct magma batches not related by a single fractionation event (Ghorbani & Middlemost, 2000). Our new data show that multiple magma differentiation processes occurred and that the magma plumbing system became increasingly complex with time. The mush-style plumbing system at both Warrumbungle and Comboyne can account for the intermittent eruption of diverse crystal-rich and crystal-poor lavas over millions of years. Similar processes occur in each volcano almost concurrently. The main mush reservoir feeding Comboyne Stage 2 magmas from depths of 15 to 7 km was mirrored slightly later in Stage 2 at Warrumbungle (Stage 2.2: 22–7 km). The parallel plumbing systems in Warrumbungle and Comboyne both also show a crystallization episode occurring at the crust–mantle boundary, which could be slightly shallower at Comboyne (30–35 km) than at Warrumbungle (40–45 km: Collins et al., 2003). This crystallization episode occurred in Stage 2.1 at Warrumbungle and slightly later, in Stage 2.2, at Comboyne. In addition, our data record successive events of mush rejuvenation with hotter, more primitive magmas. Mafic recharge is recorded in the rims of the crystals, and by relatively primitive microcryst compositions, indicating that such a process immediately preceded the eruption. We propose therefore that mush rejuvenation was an efficient eruption-triggering mechanism, as observed in volcanic systems worldwide (e.g. Murphy et al., 1998; Leonard et al., 2002; Ubide & Kamber, 2018). Mush rejuvenation is most evident in Stage 2 at Warrumbungle and throughout the life of Comboyne, suggesting that as magma stagnated and fractionated in mush zones at various depths, recharge was required to mobilize the system to erupt. Implications for age-progressive Cenozoic volcanism in eastern Australia Early studies on the mineral assemblages of the volcanic products have dominantly focused on the conditions of crystallization (e.g. Ewart et al., 1980,, 1985; Stolz, 1985; Ewart, 1989) and crystal stratigraphies in the age-progressive volcanoes in eastern Australia have remained relatively underexplored. A few studies on the macrocryst assemblages from various volcanic provinces in eastern Australia, such as the central volcanoes of southeastern Queensland, alluded to the presence of complex plumbing systems, involving magma mixing and magma rejuvenation events (Binns et al., 1970; Ewart et al., 1980; Irving & Frey, 1984). Our study shows that at Warrumbungle and Comboyne, porphyritic lavas contain recycled plagioclase, olivine and pyroxene populations, supporting the idea of complex plumbing systems. In contrast, the feeder systems at Buckland and Springsure, two central volcanoes that formed early in the age-progressive track (Fig. 1), have been considered relatively simple (Ewart et al., 1980; Crossingham et al., 2018). For example, a 278 m near vertical section through the Buckland mafic lavas at Carnarvon Gorge showed that the magmas were derived from a relatively simple plumbing system and did not experience prolonged storage in crustal reservoirs, and that magma mixing and rejuvenation did not appear to play a role in initiating the eruptions (Crossingham et al., 2018). Ewart et al. (1980) reached a similar conclusion for the lavas at Springsure, as no macrocrysts were observed in their samples. However, megacrysts of plagioclase and pyroxene were identified at both Buckland and Springsure by Skae (1998). At Buckland, the macrocrysts were interpreted to have crystallized in the lower crust and to have been randomly incorporated into the erupting lavas (Skae, 1998). At Springsure, the macrocrysts have been interpreted to record high-pressure fractionation (Skae, 1998). Further mineral chemistry would be needed for a more solid understanding of these systems, in particular, and the pathways of magma transport, storage and reactivation in eastern Australian continental age-progressive central volcanoes in general. If complex plumbing systems are a feature of central volcanoes, does the complexity vary with lithospheric thickness, age, and composition? The Buckland and Springsure volcanoes, potentially fed by relatively simple plumbing systems (Ewart et al., 1980; Crossingham et al., 2018), are located on relatively thicker lithosphere (∼130 km: Davies et al., 2015). Comboyne also sits on thicker lithosphere, whereas Warrumbungle sits on thinner lithosphere (∼110 km: Davies et al., 2015); however, both volcanoes were fed by parallel complex plumbing systems. In terms of compositional variations, volcanoes with complex plumbing systems appear logically to have more evolved lava compositions. At Warrumbungle, lavas become more evolved as the plumbing system becomes more complex (Figs 3 and 15). More broadly, the lavas from the Warrumbungle and Comboyne central volcanoes are generally more evolved than the lavas from Buckland (Figs 8 and 11) and Springsure volcanoes, which formed previously along the age-progressive track (Ewart et al., 1980; Skae, 1998; Crossingham et al., 2018). Further investigation into causal links between age and nature of magma transport needs to be carried out. Potential targets for further studies could be the central volcanoes in southeastern Queensland, where more evolved lava compositions have been linked to a period of slower plate motions generating larger and more complex volcanic edifices (Ewart et al., 1980,, 1988; Knesel et al., 2008). CONCLUSIONS By combining 40Ar/39Ar geochronology with mineral and whole-rock geochemistry we have been able to reconstruct the spatial and temporal evolution of the magma plumbing systems that fed two coeval, but spatially distant, continental central volcanoes in eastern Australia: Warrumbungle and Comboyne. 40Ar/39Ar geochronology of porphyritic and aphanitic lavas reveals two stages of volcanism at the Warrumbungle volcano (∼18–17·5 and ∼17–15·5 Ma) and one volcanic episode at Comboyne (∼17–15·5 Ma). Results from the dominantly porphyritic and relatively evolved lavas show that the parallel plumbing systems beneath the two coeval Warrumbungle and Comboyne volcanoes become more complex with time. Fractional crystallization and crystal recycling dominantly occurred early in the life of the Warrumbungle volcano. In later stages at the Warrumbungle volcano and throughout the entire life of the Comboyne volcano, magmas stagnated and fractionated in mush zones at various depths and were subsequently replenished with more primitive crystal-rich and crystal-poor magmas. These rejuvenation events mobilized the magmas, resulting in the intermittent eruptions over 3 Myr at Warrumbungle and over 1 Myr at Comboyne. ACKNOWLEDGEMENTS We thank Benjamin Cohen for providing some of the samples used in this study, and Lucas de Bem for assistance with sample preparation. We thank the NSW National Parks and Wildlife Service (licence number SL101471) for allowing us to sample at the Warrumbungle National Park, and the staff at the National Parks and Wildlife Service at the Warrumbungle volcano for their assistance. We would also like to thank Isabelle Jones, Llyam White and Oliver Turner for their assistance in sample collection; David Thiede for his assistance with the 40Ar/39Ar analysis; and Marietjie Mostert for her assistance with the whole-rock geochemical analysis. We would like to acknowledge the facilities and staff of the Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy and Microanalysis, The University of Queensland. Editorial handling by John Gamble and constructive reviews by Andrea Marzoli, Frederick Sutherland and an anonymous referee helped to improve the original version of the paper. FUNDING This study was funded by the University of Queensland Argon Laboratory (UQ-AGES, to P.V.), The University of Queensland Early Career Researcher Grants Scheme (grant UQECR1717581 to T.U.), the Australian Research Council (grant DE160100169 to G.M), and the International Association of Geochemistry (IAGC PhD Student Research Grant to T.J.C.). 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Parallel Plumbing Systems Feeding a Pair of Coeval Volcanoes in Eastern Australia

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Abstract

Abstract Eastern Australia hosts a long track of Cenozoic age-progressive volcanoes, mostly alkaline in composition. Of these, Warrumbungle and Comboyne are coeval and occur at the same latitude (31°S), but they are ∼300 km apart, on either side of the Great Dividing Range. The lavas from both volcanoes often contain complex crystal assemblages, including plagioclase, olivine and clinopyroxene, which permit a comparative study of pre-eruptive magma histories in a large, complex, continental setting. Here we combine mineral and whole-rock geochemistry with 40Ar/39Ar geochronology to temporally constrain the processes operating in the magma plumbing systems. 40Ar/39Ar geochronology indicates that volcanic activity took place for ∼3 Myr, in two separate stages. The first stage (18–17·5 Ma) is evident only at the larger Warrumbungle volcano. In Stage 2 (∼17–15·5 Ma) the two volcanoes were active contemporaneously. The dominantly porphyritic and relatively evolved (MgO from 7·25 to 0·39 wt %) nature of the lavas suggests that the magmas stalled and differentiated in the crust prior to eruption. At the Warrumbungle volcano, Stage 1 magmas fractionated olivine and minor clinopyroxene and subsequently differentiated during ascent. The crystal cargo in Stage 2 magmas at the Warrumbungle volcano became increasingly more complex with time and the samples have been divided into two subgroups, according to age and petrological variation. Stage 2.1 magmas sampled olivine, clinopyroxene and plagioclase mushes at Moho depths of ∼41 km. Disequilibrium textures in plagioclase and clinopyroxene macrocrysts indicate differences in composition between the mush and the ascending magmas. Stage 2.2 magmas, by contrast, carried a combination of antecrysts and phenocrysts. Clinopyroxene antecrysts show strong disequilibrium textures and are reversely zoned. In plagioclase, anorthite contents increase close to the rim of the crystals, to levels (An60–55) similar to those found at the core of primitive, normally zoned, euhedral antecrysts (An53–50). At the Comboyne volcano mineral phases have a similar complexity to those of Stage 2.2 at the Warrumbungle volcano, with disequilibrium textures and reversely zoned antecrysts providing evidence of magma mixing, only lacking the primitive, normally zoned, euhedral plagioclase crystals. The complex crystal assemblage evident in Stage 2.2 lavas at the Warrumbungle volcano and throughout Stage 2 at the Comboyne volcano indicates a coeval rejuvenation of evolving crystal–melt mushes with the intrusion of more primitive, hotter, and crystal rich or -poor magmas shortly before eruption. Forward modelling using Rhyolite-MELTS replicates the composition of melts and fractionated minerals along a polybaric fractional crystallization path at depths from 24 to 7 km at the Warrumbungle volcano and from 15 to 7 km at Comboyne, supported by barometry estimates on clinopyroxene crystals. This study has identified that the two temporally associated, but spatially discrete, continental alkaline volcanoes were fed by parallel plumbing systems, which become more complex throughout the life of the volcanoes. Multiple mush zones, in which magmas stagnated and fractionated, were periodically replenished with more primitive magmas, triggering eruptions intermittently over a protracted period of ∼3 Myr. INTRODUCTION The understanding of magmatic plumbing systems has drastically improved in recent decades. The traditional view of a magma chamber that consists of a large pool of melt located below the surface (Daly, 1911) has evolved to one that envisages a series of interconnected dyke and sill networks transecting the crust (e.g. Marsh, 1996; Hildreth & Wilson, 2007; Bachmann & Bergantz, 2008; Cashman et al., 2017; Cooper, 2017; Ubide & Kamber, 2018). Magmatic plumbing systems are now understood as open systems that reflect interplay between successive magma recharge, magma mixing, assimilation and crystallization processes (e.g. DePaolo, 1981; Kent et al., 2010; O’Neill & Jenner, 2012; Bohrson et al., 2014; Bragagni et al., 2014; Coogan & O’Hara, 2015; Marzoli et al., 2015; Ubide et al., 2015). The resulting volcanic products may carry a complex assortment of phenocrysts, recycled antecrysts and country-rock or mantle xenoliths that record a plethora of deep processes preceding eruption (e.g. Dobosi & Fodor, 1992; Bachmann & Bergantz, 2004; Hildreth, 2004; Davidson et al., 2007; Ubide et al., 2014a). The identification of magma recharge in the chemical stratigraphy of crystals has attracted major attention, as mafic replenishment of sub-volcanic mushes and subsequent mixing is considered an efficient trigger of volcanic eruptions (e.g. Murphy et al., 1998; Leonard et al., 2002; Kent et al., 2010; Kent, 2013; Ubide & Kamber, 2018). The study of mineral compositional records in volcanic systems has primarily focused on arc settings (e.g. Bachmann et al., 2002; Salisbury et al., 2008; Bragagni et al., 2014) and ocean island systems such as Gran Canaria in the Canary Islands (e.g. Troll & Schmincke, 2002). In continental volcanoes, mineral records have been investigated in certain settings such as the Eifel (Germany) and Mt. Bambouto (Cameroon Line) (Duda & Schmincke, 1985; Dobosi, 1989; Marzoli et al., 2015). In Australia, the eruptive products of Cenozoic volcanoes, concentrated along the eastern margin of the continent, offer a unique opportunity to study the nature of magma plumbing systems feeding continental volcanism. Remnant shield volcanoes have a southward age progression from ∼34 to 6 Ma (Wellman & McDougall, 1974a) and are dominantly confined to the east of the Great Dividing Range (Fig. 1). However, two sets of coeval volcanoes occur on either side of the Great Dividing Range in New South Wales: Nandewar and Ebor, followed in time by Warrumbungle and Comboyne. Here we focus on the Warrumbungle–Comboyne coeval volcano pair (Fig. 1), using an integrated approach that combines 40Ar/39Ar geochronology with mineral and whole-rock geochemistry to investigate the architecture of the magmatic plumbing systems feeding these two temporally associated, but spatially separated, continental intraplate volcanoes through time. Fig. 1 View largeDownload slide (a) Digital elevation model (DEM) of eastern Australia (Geoscience Australia, 2015) that highlights the location of the central volcanoes in blue (after Johnson, 1989). Black squares correspond to the location of the Warrumbungle and Comboyne central volcanoes; grey squares mark the location of other central volcanoes mentioned in the text. Fig. 1 View largeDownload slide (a) Digital elevation model (DEM) of eastern Australia (Geoscience Australia, 2015) that highlights the location of the central volcanoes in blue (after Johnson, 1989). Black squares correspond to the location of the Warrumbungle and Comboyne central volcanoes; grey squares mark the location of other central volcanoes mentioned in the text. GEOLOGICAL BACKGROUND Cenozoic intraplate volcanism in Australia is almost entirely concentrated along the eastern margin of the continent. In the classic work by Wellman & McDougall (1974a), three types of volcanic provinces were established: central volcanoes, lava fields and leucitites. Central volcanoes were defined as containing both mafic and felsic lava flows and intrusive rocks and having a southward age progression from 34 to 6 Ma (Wellman & McDougall, 1974a). Lava fields were considered to encompass mafic flows only, and have no such age progression (Wellman & McDougall, 1974a). The leucitites are leucite-bearing basalts that, like the central volcanoes, show a southward age progression (Cohen et al., 2008). The origin of the age-progressive central volcanoes and leucitites has been linked to a single plume or several mantle plumes (e.g. Wellman & McDougall, 1974a; Sutherland, 2003; Vasconcelos et al., 2008; Davies et al., 2015). By contrast, the generation of lava fields has been related to a number of mechanisms, which include the opening of the Tasman Sea, edge-driven convection, heat flow from warm Pacific mantle, and asthenospheric shearing (e.g. Johnson, 1989; Finn et al., 2005; Demidjuk et al., 2007; Conrad et al., 2011). However, more recent work has shown that some lava fields include basalts with ‘plume-like’ isotopic signatures and ages that correlate with the age-progressive central volcanoes (Sutherland et al., 2012; Jones et al., 2016). The occurrence of central volcanoes in eastern Australia is confined to a strip of thin lithosphere. Surface-wave tomography (Fishwick et al., 2008) shows that the lithosphere thickens from the eastern continental margin westward, in a series of abrupt steps. More recently, Davies et al. (2015) showed that within the strip of thin lithosphere, there are local variations in thickness that could control rates of magma generation in the mantle. The distribution of eastern Australian volcanism follows the Great Dividing Range (Johnson, 1989), the formation of which is still under debate (e.g. Vasconcelos et al., 2008). In its origin, the Great Dividing Range included significant uplift prior to Cenozoic volcanism (Vasconcelos et al., 2008). However, a more recent study of the underlying mantle–crust interactions since 100 Ma suggests a two-stage uplift (Müller et al., 2016). The second stage of uplift occurred later (∼50 Ma) around the southern Highlands as the continental plate passed over the Pacific thermal superswell (Müller et al., 2016). The Warrumbungle and Comboyne central volcanoes are located in New South Wales (NSW), towards the end of the age-progressive track (Fig. 1). The Warrumbungle volcano is located west of the Great Dividing Range and erupted through thinner lithosphere (∼110 km) compared with the craton to the west, in central NSW (Davies et al., 2015). The volcano is ∼50 km in diameter and has an eruptive volume of about 500 km3 (Duggan, 1989). Early K/Ar dating indicated that the lifespan of the Warrumbungle volcano was between 17 and 13·5 Ma (Wellman & McDougall, 1974b, and references therein). More recently, 40Ar/39Ar geochronology showed that volcanic activity ranged from 18·1 ± 0·2 to 15·9 ± 0·1 Ma (40Ar/39Ar ages have been recalculated based on the age of the Fish Canyon sanidine standard adopted in this study: 28·201 ± 0·046 Ma, Kuiper et al., 2008) and that the samples that had yielded younger K/Ar ages, 13·9 ± 0·2 Ma, were actually much older (16·4 ± 0·3 Ma; Cohen, 2007). The products of the volcano define three petrological series (Hockley, 1972): (1) sodic undersaturated alkali basalts, hawaiites, mugearites and trachytes; (2) mildly potassic hypersthene normative trachybasalts, trachyandesites, and trachytes; (3) mildly potassic undersaturated trachyandesites, Fe-rich trachyandesites and nepheline-bearing trachytes, and phonolitic trachytes. Based on geochemistry, the mafic rocks have been subdivided into incompatible-rich and incompatible-poor (Ghorbani & Middlemost, 2000). The Comboyne volcano is located to the east of the Great Dividing Range, closer to the coast. In this area, the lithosphere is slightly thicker (∼130 km; Davies et al., 2015) and Comboyne is significantly smaller (∼20 km diameter, 50 km3 total volume; Knutson, 1989) than Warrumbungle. 40Ar/39Ar geochronology yields ages ranging from 17·2 ± 0·3 to 16·6 ± 0·2 Ma for the Comboyne volcano (Knesel et al., 2008; Sutherland et al., 2012). Sutherland et al. (2012) showed that basalts occurring at lower elevation sites surrounding the Comboyne Plateau are younger (K–Ar ages from 15·7 ± 0·3 to 13·3 ± 0·3 Ma) than the plateau basalts. However, Sutherland et al. (2012) noted that these K–Ar ages need to be confirmed by 40Ar/39Ar geochronology. Volcanic products from Comboyne range from mildly alkaline to sub-alkaline hawaiites, olivine and quartz tholeiites, mugearites, icelandites, benmoreites, dacites, trachytes and peralkaline rhyolites (Knutson, 1989). Intermediate to silicic rocks are volumetrically dominant (Knutson, 1975). SAMPLING AND METHODS Sampling Eighteen mafic to intermediate samples from the Warrumbungle and Comboyne central volcanoes were investigated in this study using petrological, geochemical and geochronological methods. Four of the samples from the Warrumbungle volcano had been previously collected and dated, via 40Ar/39Ar geochronology, by Cohen (2007): one sample from the northern area, one from the southern area, and two from the central area of the volcano. Five additional samples were collected from Warrumbungle volcano: three samples were collected at various elevations along the Observatory Road and two samples were collected from Mt Exmouth. At Comboyne volcano, nine samples were collected from various sites on the Comboyne Plateau. 40Ar/39Ar geochronology Five samples from the Warrumbungle volcano and nine samples from the Comboyne volcano were prepared for 40Ar/39Ar geochronology. Whole-rock samples were crushed to 1–2 mm fragments and treated with 3·5N HCl and 1N HNO3 and subsequently washed repeatedly using distilled water, acetone and ethanol in an ultrasonic bath. Fresh whole-rock grains, neutron fluence monitor Fish Canyon sanidine (28·201 ± 0·046 Ma, Kuiper et al., 2008) and an independent standard (GA1550; 98·5 ± 0·8 Ma, Spell & McDougall, 2003) were loaded into a 21-pit aluminum disk in the pattern described by Vasconcelos et al. (2002). The disk was irradiated for 14 h in a TRIGA-type reactor in the Cd-lined CLCIT facility at Oregon State University. The samples were then dated at the Argon Geochronology Laboratory of the University of Queensland (UQ-AGES) by 40Ar/39Ar incremental heating, following the procedure set out by Vasconcelos et al. (2002). The whole-rock grains were heated incrementally using a 2 mm wide continuous-wave Ar-ion laser beam. The gas was purified in a cryocooled trap (–130°C) and three SAES-50 getter pumps before isotopic analysis using a MAP215-50 mass spectrometer. Air pipettes and full system blanks were analyzed before and after each sample. MassSpec version 8.132 was used to correct the data for mass discrimination, nucleogenic interferences, and atmospheric contamination. Mass spectrometer discrimination was calculated using a 40Ar/39Ar value for atmospheric argon of 298·56 ± 0·31 (Lee et al., 2006). The J factor was determined by analyzing 15 Fish Canyon sanidine crystals (see Supplementary Data Electronic Appendix 1; supplementary data are available for downloading at http://www.petrology.oxfordjournals.org). Results are provided in Table 1 and Supplementary Data Electronic Appendix 1. Table 1 40Ar/39Ar geochronology results of the Warrumbungle and Comboyne volcanoes Sample no. UTM Northing Easting Lab no. Material No. of steps Plateau age ±2σ (Ma)* Steps (% in plateau) Integrated age ±2σ (Ma) Probability density plot age ±2σ (Ma)† Isochron age ±2 σ(Ma)‡ 40Ar/39Ar intercept Warrumbungle W2 55J 6538550 697457 9089-01 WR 12 16·1 ± 0·1 C–J (84·7 %) 16·1 ± 0·1 16·0 ± 0·1 16·0 ± 0·1a 313 ± 16 (MSWD = 0·70) (n = 17) (n = 21) 9089-02 WR 12 15·9 ± 0·1 C–L (92·6 %) 15·9 ± 0·2 (MSWD = 1·14) (MSWD = 1·5) (MSWD = 0·73) W3 55J 6538517 697995 9244-01 WR 12 15·8 ± 0·1 C–H (69·2 %) 15·7 ± 0·1 15·8 ± 0·1 15·8 ± 0·1 305 ± 10 (MSWD = 0·70) (n = 12) (n = 14) 9244-02 WR 12 15·7 ± 0·1 (MSWD = 1·58) D–H (68·0 %) 15·6 ± 0·1 (MSWD = 1·91) (MSWD = 1·6) W7 55J 6537141 698863 9090-01 WR 12 16·9 ± 0·1 C–H (74·6 %) 17·0 ± 0·1 16·9 ± 0·1 16·8 ± 0·1 325 ± 9 (MSWD = 1·14) (n = 15) (n = 16) 9090-02 WR 12 16·8 ± 0·1 C–J (74·3 %) 16·7 ± 0·1 (MSWD = 1·30) (MSWD = 1·6) (MSWD = 1·90) W10 55J 6534326 685343 9091-01 WR 12 15·8 ± 0·2 A–F (71·7 %) 15·4 ± 0·2 15·7 ± 0·1 15·8 ± 0·2 300 ± 13 (MSWD = 1·93) (n = 11) (n = 15) 9091-02 WR 12 n.a. n.a. 15·8 ± 0·2 (MSWD = 1·81) (MSWD = 1·7) W11 55J 6534155 685517 9124-01 WR 12 15·9 ± 0·1 C–L (96·2 %) 15·9 ± 0·2 15·9 ± 0·1 15·9 ± 0·1 296 ± 5 (MSWD = 0·87) (n = 15) (n = 20) 9124-02 WR 12 15·8 ± 0·1 C–I (89·6 %) 15·9 ± 0·2 (MSWD = 0·72) (MSWD = 1·5) (MSWD = 1·99) Comboyne CNB3 56J 6500799 0452447 9129-01 WR 12 16·7 ± 0·1 F–L (74·3 %) 16·7 ± 0·1 16·7 ± 0·1 16·7 ± 0·1 266 ± 8 (MSWD = 0·80) (n = 14) (n = 19) 9129-02 WR 12 16·7 ± 0·1 D–K (67·4 %) 16·6 ± 0·1 (MSWD = 1·17) (MSWD = 0·88) (MSWD = 1·12) CBN4 56J 6500837 0452425 9128-01 WR 12 16·6 ± 0·1 D–H (80·5 %) 16·6 ± 0·1 16·6 ± 0·1 16·6 ± 0·1 400 ± 300 (MSWD = 1·53) (n = 9) (n = 9) 9128-02 WR 12 16·6 ± 0·1 E–H (72·8 %) 16·6 ± 0·1 (MSWD = 1·56) (MSWD = 1·6) (MSWD = 1·89) CBN10 56J 6502929 0445840 9127-01 WR 12 16·5 ± 0·1 F–I (58·5 %) 16·5 ± 0·1 16·5 ± 0·1 16·5 ± 0·1 310 ± 11 (MSWD = 1·03) (n = 14) (n = 21) 9127-02 WR 12 16·5 ± 0·1 C–L (95·7 %) 16·6 ± 0·1 (MSWD = 1·37) (MSWD = 1·4) (MSWD = 1·47) CBN11 56J 6505256 0443116 9081-01 WR 11 16·5 ± 0·1 E–I (53·2 %) 16·7 ± 0·1 16·3 ± 0·1 16·3 ± 0·1 300 ± 40 (MSWD = 0·91) (n = 14) (n = 17) 9081-02 WR 12 16·2 ± 0·1 E–L (58·3 %) 16·4 ± 0·1 (MSWD = 1·32) (MSWD = 1·7) (MSWD = 1·15) CBN12 56J 6505153 0443328 9082-01 WR 12 16·7 ± 0·1 C–I (80·4 %) 16·8 ± 0·1 16·7 ± 0·1 16·7 ± 0·1 340 ± 40 (MSWD = 1·89) (n = 10) (n = 17) 9082-02 WR 12 16·7 ± 0·1§ D–G (53·2 %) 16·7 ± 0·1 (MSWD = 1·95) (MSWD = 1·6) (MSWD = 2·77) CBN13 56J 6505122 0443372 9084-01 WR 12 16·0 ± 0·7 D–G (96·8 %) 16·0 ± 2·0 16·0 ± 0·3 16·0 ± 0·5 298 ± 2 (MSWD = 1·49) (n = 15) (n = 22) 9084-02 WR 12 16·1 ± 0·4 B–I (86·6 %) 15·2 ± 1·4 (MSWD = 0·96) (MSWD = 1·5) (MSWD = 1·60) CBN14 56J 6505069 0443439 9085-01 WR 12 16·7 ± 0·3 D–H (65·5%) 16·4 ± 0·8 16·2 ± 0·2 16·2 ± 0·2 297 ± 5 (MSWD = 2·18) (n = 7) (n = 8) 9085-02 WR 12 16·1 ± 0·2 C–E (54·5%) 15·5 ± 0·2 (MSWD = 1·72) (MSWD = 1·9) (MSWD = 1·53) CBN16 56J 6506672 0440696 9087-01 WR 11 16·7 ± 0·1 C–K (73·2 %) 16·7 ± 0·1 16·7 ± 0·1 16·7 ± 0·1 300 ± 40 (MSWD = 0·98) (n = 16) (n = 20) 9087-02 WR 12 16·7 ± 0·1 A–H (96·6 %) 16·6 ± 0·1 (MSWD = 1·80) (MSWD = 1·5) (MSWD = 1·39) CBN20 56J 6500886 0449412 9088-01 WR 12 16·8 ± 0·1 C–F (53·0 %) 16·8 ± 0·1 16·8 ± 0·1 n.a. n.a. (MSWD = 1·06) (n = 13) 9088-02 WR 12 16·9 ± 0·1 C–L (79·2 %) 16·9 ± 0·1 (MSWD = 1·27) (MSWD = 1·10) Sample no. UTM Northing Easting Lab no. Material No. of steps Plateau age ±2σ (Ma)* Steps (% in plateau) Integrated age ±2σ (Ma) Probability density plot age ±2σ (Ma)† Isochron age ±2 σ(Ma)‡ 40Ar/39Ar intercept Warrumbungle W2 55J 6538550 697457 9089-01 WR 12 16·1 ± 0·1 C–J (84·7 %) 16·1 ± 0·1 16·0 ± 0·1 16·0 ± 0·1a 313 ± 16 (MSWD = 0·70) (n = 17) (n = 21) 9089-02 WR 12 15·9 ± 0·1 C–L (92·6 %) 15·9 ± 0·2 (MSWD = 1·14) (MSWD = 1·5) (MSWD = 0·73) W3 55J 6538517 697995 9244-01 WR 12 15·8 ± 0·1 C–H (69·2 %) 15·7 ± 0·1 15·8 ± 0·1 15·8 ± 0·1 305 ± 10 (MSWD = 0·70) (n = 12) (n = 14) 9244-02 WR 12 15·7 ± 0·1 (MSWD = 1·58) D–H (68·0 %) 15·6 ± 0·1 (MSWD = 1·91) (MSWD = 1·6) W7 55J 6537141 698863 9090-01 WR 12 16·9 ± 0·1 C–H (74·6 %) 17·0 ± 0·1 16·9 ± 0·1 16·8 ± 0·1 325 ± 9 (MSWD = 1·14) (n = 15) (n = 16) 9090-02 WR 12 16·8 ± 0·1 C–J (74·3 %) 16·7 ± 0·1 (MSWD = 1·30) (MSWD = 1·6) (MSWD = 1·90) W10 55J 6534326 685343 9091-01 WR 12 15·8 ± 0·2 A–F (71·7 %) 15·4 ± 0·2 15·7 ± 0·1 15·8 ± 0·2 300 ± 13 (MSWD = 1·93) (n = 11) (n = 15) 9091-02 WR 12 n.a. n.a. 15·8 ± 0·2 (MSWD = 1·81) (MSWD = 1·7) W11 55J 6534155 685517 9124-01 WR 12 15·9 ± 0·1 C–L (96·2 %) 15·9 ± 0·2 15·9 ± 0·1 15·9 ± 0·1 296 ± 5 (MSWD = 0·87) (n = 15) (n = 20) 9124-02 WR 12 15·8 ± 0·1 C–I (89·6 %) 15·9 ± 0·2 (MSWD = 0·72) (MSWD = 1·5) (MSWD = 1·99) Comboyne CNB3 56J 6500799 0452447 9129-01 WR 12 16·7 ± 0·1 F–L (74·3 %) 16·7 ± 0·1 16·7 ± 0·1 16·7 ± 0·1 266 ± 8 (MSWD = 0·80) (n = 14) (n = 19) 9129-02 WR 12 16·7 ± 0·1 D–K (67·4 %) 16·6 ± 0·1 (MSWD = 1·17) (MSWD = 0·88) (MSWD = 1·12) CBN4 56J 6500837 0452425 9128-01 WR 12 16·6 ± 0·1 D–H (80·5 %) 16·6 ± 0·1 16·6 ± 0·1 16·6 ± 0·1 400 ± 300 (MSWD = 1·53) (n = 9) (n = 9) 9128-02 WR 12 16·6 ± 0·1 E–H (72·8 %) 16·6 ± 0·1 (MSWD = 1·56) (MSWD = 1·6) (MSWD = 1·89) CBN10 56J 6502929 0445840 9127-01 WR 12 16·5 ± 0·1 F–I (58·5 %) 16·5 ± 0·1 16·5 ± 0·1 16·5 ± 0·1 310 ± 11 (MSWD = 1·03) (n = 14) (n = 21) 9127-02 WR 12 16·5 ± 0·1 C–L (95·7 %) 16·6 ± 0·1 (MSWD = 1·37) (MSWD = 1·4) (MSWD = 1·47) CBN11 56J 6505256 0443116 9081-01 WR 11 16·5 ± 0·1 E–I (53·2 %) 16·7 ± 0·1 16·3 ± 0·1 16·3 ± 0·1 300 ± 40 (MSWD = 0·91) (n = 14) (n = 17) 9081-02 WR 12 16·2 ± 0·1 E–L (58·3 %) 16·4 ± 0·1 (MSWD = 1·32) (MSWD = 1·7) (MSWD = 1·15) CBN12 56J 6505153 0443328 9082-01 WR 12 16·7 ± 0·1 C–I (80·4 %) 16·8 ± 0·1 16·7 ± 0·1 16·7 ± 0·1 340 ± 40 (MSWD = 1·89) (n = 10) (n = 17) 9082-02 WR 12 16·7 ± 0·1§ D–G (53·2 %) 16·7 ± 0·1 (MSWD = 1·95) (MSWD = 1·6) (MSWD = 2·77) CBN13 56J 6505122 0443372 9084-01 WR 12 16·0 ± 0·7 D–G (96·8 %) 16·0 ± 2·0 16·0 ± 0·3 16·0 ± 0·5 298 ± 2 (MSWD = 1·49) (n = 15) (n = 22) 9084-02 WR 12 16·1 ± 0·4 B–I (86·6 %) 15·2 ± 1·4 (MSWD = 0·96) (MSWD = 1·5) (MSWD = 1·60) CBN14 56J 6505069 0443439 9085-01 WR 12 16·7 ± 0·3 D–H (65·5%) 16·4 ± 0·8 16·2 ± 0·2 16·2 ± 0·2 297 ± 5 (MSWD = 2·18) (n = 7) (n = 8) 9085-02 WR 12 16·1 ± 0·2 C–E (54·5%) 15·5 ± 0·2 (MSWD = 1·72) (MSWD = 1·9) (MSWD = 1·53) CBN16 56J 6506672 0440696 9087-01 WR 11 16·7 ± 0·1 C–K (73·2 %) 16·7 ± 0·1 16·7 ± 0·1 16·7 ± 0·1 300 ± 40 (MSWD = 0·98) (n = 16) (n = 20) 9087-02 WR 12 16·7 ± 0·1 A–H (96·6 %) 16·6 ± 0·1 (MSWD = 1·80) (MSWD = 1·5) (MSWD = 1·39) CBN20 56J 6500886 0449412 9088-01 WR 12 16·8 ± 0·1 C–F (53·0 %) 16·8 ± 0·1 16·8 ± 0·1 n.a. n.a. (MSWD = 1·06) (n = 13) 9088-02 WR 12 16·9 ± 0·1 C–L (79·2 %) 16·9 ± 0·1 (MSWD = 1·27) (MSWD = 1·10) n.a., Failed to produce a valid results (see Results in text). aResults in bold represent the preferred age of the sample. * A plateau age is defined as three or more consecutive steps that consist of at least 50% of the total 39Ar released and the age values overlap within a 95% confidence interval (Fleck et al., 1977). Errors, including errors in irradiation correction factors and errors in J, are reported at the 95% confidence level and are calculated based on the mean weight by inverse variance. All plateau definitions are defined using error-overlap with a 2σ error. † A probability density plot is constructed based on the assumption that a Gaussian distribution occurs for the errors in an age. When each age is plotted the total for every Gaussian curve is taken (Deino & Potts, 1990). ‡ Isochron age errors include the errors in J and irradiation correction factors but not the uncertainty in the potassium decay constant. Isochron ages are measured to the 95% confidence level (2σ). § Plateau ages where the MSWD value is in italics have MSWD >2. Table 1 40Ar/39Ar geochronology results of the Warrumbungle and Comboyne volcanoes Sample no. UTM Northing Easting Lab no. Material No. of steps Plateau age ±2σ (Ma)* Steps (% in plateau) Integrated age ±2σ (Ma) Probability density plot age ±2σ (Ma)† Isochron age ±2 σ(Ma)‡ 40Ar/39Ar intercept Warrumbungle W2 55J 6538550 697457 9089-01 WR 12 16·1 ± 0·1 C–J (84·7 %) 16·1 ± 0·1 16·0 ± 0·1 16·0 ± 0·1a 313 ± 16 (MSWD = 0·70) (n = 17) (n = 21) 9089-02 WR 12 15·9 ± 0·1 C–L (92·6 %) 15·9 ± 0·2 (MSWD = 1·14) (MSWD = 1·5) (MSWD = 0·73) W3 55J 6538517 697995 9244-01 WR 12 15·8 ± 0·1 C–H (69·2 %) 15·7 ± 0·1 15·8 ± 0·1 15·8 ± 0·1 305 ± 10 (MSWD = 0·70) (n = 12) (n = 14) 9244-02 WR 12 15·7 ± 0·1 (MSWD = 1·58) D–H (68·0 %) 15·6 ± 0·1 (MSWD = 1·91) (MSWD = 1·6) W7 55J 6537141 698863 9090-01 WR 12 16·9 ± 0·1 C–H (74·6 %) 17·0 ± 0·1 16·9 ± 0·1 16·8 ± 0·1 325 ± 9 (MSWD = 1·14) (n = 15) (n = 16) 9090-02 WR 12 16·8 ± 0·1 C–J (74·3 %) 16·7 ± 0·1 (MSWD = 1·30) (MSWD = 1·6) (MSWD = 1·90) W10 55J 6534326 685343 9091-01 WR 12 15·8 ± 0·2 A–F (71·7 %) 15·4 ± 0·2 15·7 ± 0·1 15·8 ± 0·2 300 ± 13 (MSWD = 1·93) (n = 11) (n = 15) 9091-02 WR 12 n.a. n.a. 15·8 ± 0·2 (MSWD = 1·81) (MSWD = 1·7) W11 55J 6534155 685517 9124-01 WR 12 15·9 ± 0·1 C–L (96·2 %) 15·9 ± 0·2 15·9 ± 0·1 15·9 ± 0·1 296 ± 5 (MSWD = 0·87) (n = 15) (n = 20) 9124-02 WR 12 15·8 ± 0·1 C–I (89·6 %) 15·9 ± 0·2 (MSWD = 0·72) (MSWD = 1·5) (MSWD = 1·99) Comboyne CNB3 56J 6500799 0452447 9129-01 WR 12 16·7 ± 0·1 F–L (74·3 %) 16·7 ± 0·1 16·7 ± 0·1 16·7 ± 0·1 266 ± 8 (MSWD = 0·80) (n = 14) (n = 19) 9129-02 WR 12 16·7 ± 0·1 D–K (67·4 %) 16·6 ± 0·1 (MSWD = 1·17) (MSWD = 0·88) (MSWD = 1·12) CBN4 56J 6500837 0452425 9128-01 WR 12 16·6 ± 0·1 D–H (80·5 %) 16·6 ± 0·1 16·6 ± 0·1 16·6 ± 0·1 400 ± 300 (MSWD = 1·53) (n = 9) (n = 9) 9128-02 WR 12 16·6 ± 0·1 E–H (72·8 %) 16·6 ± 0·1 (MSWD = 1·56) (MSWD = 1·6) (MSWD = 1·89) CBN10 56J 6502929 0445840 9127-01 WR 12 16·5 ± 0·1 F–I (58·5 %) 16·5 ± 0·1 16·5 ± 0·1 16·5 ± 0·1 310 ± 11 (MSWD = 1·03) (n = 14) (n = 21) 9127-02 WR 12 16·5 ± 0·1 C–L (95·7 %) 16·6 ± 0·1 (MSWD = 1·37) (MSWD = 1·4) (MSWD = 1·47) CBN11 56J 6505256 0443116 9081-01 WR 11 16·5 ± 0·1 E–I (53·2 %) 16·7 ± 0·1 16·3 ± 0·1 16·3 ± 0·1 300 ± 40 (MSWD = 0·91) (n = 14) (n = 17) 9081-02 WR 12 16·2 ± 0·1 E–L (58·3 %) 16·4 ± 0·1 (MSWD = 1·32) (MSWD = 1·7) (MSWD = 1·15) CBN12 56J 6505153 0443328 9082-01 WR 12 16·7 ± 0·1 C–I (80·4 %) 16·8 ± 0·1 16·7 ± 0·1 16·7 ± 0·1 340 ± 40 (MSWD = 1·89) (n = 10) (n = 17) 9082-02 WR 12 16·7 ± 0·1§ D–G (53·2 %) 16·7 ± 0·1 (MSWD = 1·95) (MSWD = 1·6) (MSWD = 2·77) CBN13 56J 6505122 0443372 9084-01 WR 12 16·0 ± 0·7 D–G (96·8 %) 16·0 ± 2·0 16·0 ± 0·3 16·0 ± 0·5 298 ± 2 (MSWD = 1·49) (n = 15) (n = 22) 9084-02 WR 12 16·1 ± 0·4 B–I (86·6 %) 15·2 ± 1·4 (MSWD = 0·96) (MSWD = 1·5) (MSWD = 1·60) CBN14 56J 6505069 0443439 9085-01 WR 12 16·7 ± 0·3 D–H (65·5%) 16·4 ± 0·8 16·2 ± 0·2 16·2 ± 0·2 297 ± 5 (MSWD = 2·18) (n = 7) (n = 8) 9085-02 WR 12 16·1 ± 0·2 C–E (54·5%) 15·5 ± 0·2 (MSWD = 1·72) (MSWD = 1·9) (MSWD = 1·53) CBN16 56J 6506672 0440696 9087-01 WR 11 16·7 ± 0·1 C–K (73·2 %) 16·7 ± 0·1 16·7 ± 0·1 16·7 ± 0·1 300 ± 40 (MSWD = 0·98) (n = 16) (n = 20) 9087-02 WR 12 16·7 ± 0·1 A–H (96·6 %) 16·6 ± 0·1 (MSWD = 1·80) (MSWD = 1·5) (MSWD = 1·39) CBN20 56J 6500886 0449412 9088-01 WR 12 16·8 ± 0·1 C–F (53·0 %) 16·8 ± 0·1 16·8 ± 0·1 n.a. n.a. (MSWD = 1·06) (n = 13) 9088-02 WR 12 16·9 ± 0·1 C–L (79·2 %) 16·9 ± 0·1 (MSWD = 1·27) (MSWD = 1·10) Sample no. UTM Northing Easting Lab no. Material No. of steps Plateau age ±2σ (Ma)* Steps (% in plateau) Integrated age ±2σ (Ma) Probability density plot age ±2σ (Ma)† Isochron age ±2 σ(Ma)‡ 40Ar/39Ar intercept Warrumbungle W2 55J 6538550 697457 9089-01 WR 12 16·1 ± 0·1 C–J (84·7 %) 16·1 ± 0·1 16·0 ± 0·1 16·0 ± 0·1a 313 ± 16 (MSWD = 0·70) (n = 17) (n = 21) 9089-02 WR 12 15·9 ± 0·1 C–L (92·6 %) 15·9 ± 0·2 (MSWD = 1·14) (MSWD = 1·5) (MSWD = 0·73) W3 55J 6538517 697995 9244-01 WR 12 15·8 ± 0·1 C–H (69·2 %) 15·7 ± 0·1 15·8 ± 0·1 15·8 ± 0·1 305 ± 10 (MSWD = 0·70) (n = 12) (n = 14) 9244-02 WR 12 15·7 ± 0·1 (MSWD = 1·58) D–H (68·0 %) 15·6 ± 0·1 (MSWD = 1·91) (MSWD = 1·6) W7 55J 6537141 698863 9090-01 WR 12 16·9 ± 0·1 C–H (74·6 %) 17·0 ± 0·1 16·9 ± 0·1 16·8 ± 0·1 325 ± 9 (MSWD = 1·14) (n = 15) (n = 16) 9090-02 WR 12 16·8 ± 0·1 C–J (74·3 %) 16·7 ± 0·1 (MSWD = 1·30) (MSWD = 1·6) (MSWD = 1·90) W10 55J 6534326 685343 9091-01 WR 12 15·8 ± 0·2 A–F (71·7 %) 15·4 ± 0·2 15·7 ± 0·1 15·8 ± 0·2 300 ± 13 (MSWD = 1·93) (n = 11) (n = 15) 9091-02 WR 12 n.a. n.a. 15·8 ± 0·2 (MSWD = 1·81) (MSWD = 1·7) W11 55J 6534155 685517 9124-01 WR 12 15·9 ± 0·1 C–L (96·2 %) 15·9 ± 0·2 15·9 ± 0·1 15·9 ± 0·1 296 ± 5 (MSWD = 0·87) (n = 15) (n = 20) 9124-02 WR 12 15·8 ± 0·1 C–I (89·6 %) 15·9 ± 0·2 (MSWD = 0·72) (MSWD = 1·5) (MSWD = 1·99) Comboyne CNB3 56J 6500799 0452447 9129-01 WR 12 16·7 ± 0·1 F–L (74·3 %) 16·7 ± 0·1 16·7 ± 0·1 16·7 ± 0·1 266 ± 8 (MSWD = 0·80) (n = 14) (n = 19) 9129-02 WR 12 16·7 ± 0·1 D–K (67·4 %) 16·6 ± 0·1 (MSWD = 1·17) (MSWD = 0·88) (MSWD = 1·12) CBN4 56J 6500837 0452425 9128-01 WR 12 16·6 ± 0·1 D–H (80·5 %) 16·6 ± 0·1 16·6 ± 0·1 16·6 ± 0·1 400 ± 300 (MSWD = 1·53) (n = 9) (n = 9) 9128-02 WR 12 16·6 ± 0·1 E–H (72·8 %) 16·6 ± 0·1 (MSWD = 1·56) (MSWD = 1·6) (MSWD = 1·89) CBN10 56J 6502929 0445840 9127-01 WR 12 16·5 ± 0·1 F–I (58·5 %) 16·5 ± 0·1 16·5 ± 0·1 16·5 ± 0·1 310 ± 11 (MSWD = 1·03) (n = 14) (n = 21) 9127-02 WR 12 16·5 ± 0·1 C–L (95·7 %) 16·6 ± 0·1 (MSWD = 1·37) (MSWD = 1·4) (MSWD = 1·47) CBN11 56J 6505256 0443116 9081-01 WR 11 16·5 ± 0·1 E–I (53·2 %) 16·7 ± 0·1 16·3 ± 0·1 16·3 ± 0·1 300 ± 40 (MSWD = 0·91) (n = 14) (n = 17) 9081-02 WR 12 16·2 ± 0·1 E–L (58·3 %) 16·4 ± 0·1 (MSWD = 1·32) (MSWD = 1·7) (MSWD = 1·15) CBN12 56J 6505153 0443328 9082-01 WR 12 16·7 ± 0·1 C–I (80·4 %) 16·8 ± 0·1 16·7 ± 0·1 16·7 ± 0·1 340 ± 40 (MSWD = 1·89) (n = 10) (n = 17) 9082-02 WR 12 16·7 ± 0·1§ D–G (53·2 %) 16·7 ± 0·1 (MSWD = 1·95) (MSWD = 1·6) (MSWD = 2·77) CBN13 56J 6505122 0443372 9084-01 WR 12 16·0 ± 0·7 D–G (96·8 %) 16·0 ± 2·0 16·0 ± 0·3 16·0 ± 0·5 298 ± 2 (MSWD = 1·49) (n = 15) (n = 22) 9084-02 WR 12 16·1 ± 0·4 B–I (86·6 %) 15·2 ± 1·4 (MSWD = 0·96) (MSWD = 1·5) (MSWD = 1·60) CBN14 56J 6505069 0443439 9085-01 WR 12 16·7 ± 0·3 D–H (65·5%) 16·4 ± 0·8 16·2 ± 0·2 16·2 ± 0·2 297 ± 5 (MSWD = 2·18) (n = 7) (n = 8) 9085-02 WR 12 16·1 ± 0·2 C–E (54·5%) 15·5 ± 0·2 (MSWD = 1·72) (MSWD = 1·9) (MSWD = 1·53) CBN16 56J 6506672 0440696 9087-01 WR 11 16·7 ± 0·1 C–K (73·2 %) 16·7 ± 0·1 16·7 ± 0·1 16·7 ± 0·1 300 ± 40 (MSWD = 0·98) (n = 16) (n = 20) 9087-02 WR 12 16·7 ± 0·1 A–H (96·6 %) 16·6 ± 0·1 (MSWD = 1·80) (MSWD = 1·5) (MSWD = 1·39) CBN20 56J 6500886 0449412 9088-01 WR 12 16·8 ± 0·1 C–F (53·0 %) 16·8 ± 0·1 16·8 ± 0·1 n.a. n.a. (MSWD = 1·06) (n = 13) 9088-02 WR 12 16·9 ± 0·1 C–L (79·2 %) 16·9 ± 0·1 (MSWD = 1·27) (MSWD = 1·10) n.a., Failed to produce a valid results (see Results in text). aResults in bold represent the preferred age of the sample. * A plateau age is defined as three or more consecutive steps that consist of at least 50% of the total 39Ar released and the age values overlap within a 95% confidence interval (Fleck et al., 1977). Errors, including errors in irradiation correction factors and errors in J, are reported at the 95% confidence level and are calculated based on the mean weight by inverse variance. All plateau definitions are defined using error-overlap with a 2σ error. † A probability density plot is constructed based on the assumption that a Gaussian distribution occurs for the errors in an age. When each age is plotted the total for every Gaussian curve is taken (Deino & Potts, 1990). ‡ Isochron age errors include the errors in J and irradiation correction factors but not the uncertainty in the potassium decay constant. Isochron ages are measured to the 95% confidence level (2σ). § Plateau ages where the MSWD value is in italics have MSWD >2. Mineral chemistry Major element compositions of mineral phases from 10 samples (six from Warrumbungle and four from Comboyne) were analyzed by electron microprobe microanalyser (EMPA). Results are provided in Supplementary Data Electronic Appendix 2. Samples BC-151, W10 and CBN12 were measured using a Cameca SX100 electron microprobe with four wavelength-dispersive spectrometers at the Research School of Earth Sciences, Australian National University. The operating parameters were an accelerating voltage of 15 kV, a beam current of 20 nA and a beam diameter of 1 μm. Elemental counting times were 20 s on the peak for Na, Mg, Al, Si and P, 40 s on the peak for K, Ca and Ti, and 30 s on the peak for Ni, Fe, Mn and Cr; we measured for 5 s on each of two background positions for all elements. Ni, Fe, Mn and Cr were measured on two spectrometers. A ZAF procedure was used for matrix correction. Calibration of the major and minor elements was on sanidine (K and Na), MgO (Mg), corundum (Al), SiO2 (Si), andradite (Ca and Fe), TiO2 (Ti), rhodonite (Mn), chromite (Cr) and apatite (P), for all minerals except plagioclase, where Si and Al were calibrated on albite. San Carlos olivine, clinopyroxene and hornblende were used as quality monitors. The remaining samples (as well as additional data on CBN12 and BC-151) were analyzed on a JEOL JXA-8200 EMPA equipped with five wavelength-dispersive spectrometers at the Centre for Microscopy and Microanalysis, The University of Queensland. Analyses were performed using an accelerating voltage of 15 kV, a beam current of 15 nA and a beam diameter of 1 μm. Elemental counting times for all elements were 30 s on the peak and 5 s on each of two background positions. ZAF was used for matrix correction. Calibration of the major and minor elements utilized orthoclase (K), albite (Na), wollastonite (Si and Ca), kyanite (Al), hematite (Fe), chromite (Cr), spessartine (Mn), F-apatite (P), rutile (Ti), P-140 Olivine (Mg) and Ni-olivine (Ni). Springwater Olivine, Kakanui Augite and Lake Co Feldspar were used as quality monitors. In addition, to check the variability between the two EMPA systems used, we compared the analyses on the cores and rims of olivine phenocrysts and found that reproducibility was better than 2% (see Supplementary Data Electronic Appendix 2). Major and trace element geochemistry Eighteen whole-rock samples were powdered in an agate ring and puck mill. Major elements, trace elements and loss on ignition (LOI) were measured in the Geochemistry Laboratory and the Radiogenic Isotope Facility (RIF) at the School of Earth Sciences, the University of Queensland. Results are provided in Table 2. Table 2 Major and trace element results for the Warrumbungle and Comboyne volcanoes Sample: BC-158 BC-167 BC-157 BC-151 W10 W11 W2 W7 W3 Volcano: Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Easting: 697947 717188 697947 682251 685343 685517 697457 698863 697995 Northing: 6538365 6489760 6538365 6558519 6534326 6534155 6538550 6537141 6538517 Elevation: 850 398 1055 372 1159 1096 1979 792 1058 SiO2 49·68 47·72 51·82 48·47 48·84 46·32 50·89 51·74 50·71 TiO2 2·45 2·85 2·35 2·65 2·66 3·06 2·49 1·79 2·48 Al2O3 15·75 14·64 14·7 14·55 14·66 16·61 15·61 17·07 15·43 Fe2O3 10·79 12·74 12·13 11·00 13·27 13·5 12·32 10·16 12·39 MnO 0·12 0·17 0·19 0·14 0·17 0·18 0·19 0·13 0·23 MgO 3·89 5·32 3·06 7·25 4·81 4·05 3·45 2·96 3·03 CaO 6·66 7·09 5·78 7·16 7·20 8·39 6·34 4·84 7·02 Na2O 3·89 4·35 4·53 3·88 4·18 3·95 3·91 4·23 3·9 K2O 2·73 1·87 2·56 2·38 1·82 1·52 2·28 3·70 2·36 P2O5 1·02 0·91 1·25 1·05 0·8 0·82 0·97 0·79 0·91 H2O 1·47 0·59 1·05 0·57 0·69 0·67 0·32 1·50 0·90 LOI 1·38 1·82 0·68 1·185 1·16 0·99 1·12 1·28 1·03 Total 99·83 100·07 100·10 100·28 100·26 100·06 99·89 100·19 100·39 Mg# 44·93 47·90 35·70 59·22 44·39 39·77 38·13 39·10 35·00 Cs 0·37 1·27 0·61 0·59 0·42 0·45 0·57 0·45 0·56 Rb 41·18 51·82 64·29 33·09 40·37 25·23 42·45 62·93 53·62 Ba 776 732 1307 659 651 533 1017 1154 1071 Sr 701 760 554 876 576 666 561 619 571 Pb 3·92 4·13 5·43 4·12 3·21 2·94 4·56 4·5 4·82 Th 6·19 6·79 8·35 5·28 6·74 5·04 7·18 5·56 6·87 U 1·25 1·17 1·77 1·23 1·2 0·92 1·59 1·35 1·62 Zr 351 299 310 358 307 245 301 320 296 Hf 9·32 7·3 7·59 8·39 7·35 5·9 7·08 7·34 7·02 Ta 3·88 3·65 3·75 3·96 3·62 3·16 3·43 3·56 3·41 Y 33·79 37·97 41·91 27·96 43·13 31·45 36·27 27·4 37·36 Nb 54·75 52·62 54·7 54·73 52·2 45·79 51·39 51·57 50·05 Sc 23·40 25·92 27·42 19·99 28·97 27·35 27·29 20·55 28·12 Cr 67·30 96·93 <DL 183 39·31 19·19 19·71 22·73 22·59 Ni 55 67 8 109 33 23 22 32 22 Co 32·21 39·87 21·04 41·61 32·66 35·44 27·6 20·69 26·31 V 111 146 93·6 159 139 194 129 62·68 132 Ga 27·01 25·53 28·16 26 27·35 25·27 27·41 26·67 27 Zn 134 142·6 165 125 151 125 151 151 149 Cu 27·1 38·7 21·0 33·4 30·5 29·7 23·8 23·5 24·6 La 46·89 44·72 51·21 43·86 46·56 36·24 44·97 38·87 43·63 Ce 100 91·24 106 91·17 96·48 74·49 92·83 87·69 91·36 Pr 12·69 11·68 13·26 11·58 12·28 9·51 11·62 10·68 11·36 Nd 52·02 48·16 54·96 46·42 50·76 39·54 47·68 44·29 46·9 Sm 11·18 10·56 12·08 9·92 11·68 9·14 10·7 9·82 10·49 Eu 3·52 3·31 4·58 3·1 3·77 3·15 3·93 3·78 3·91 Gd 9·83 9·81 11·43 8·63 11·35 8·82 10·13 8·67 9·88 Tb 1·39 1·37 1·58 1·22 1·61 1·26 1·42 1·17 1·39 Dy 6·78 7·14 8·11 6·03 8·15 6·28 7·16 5·76 7·18 Ho 1·15 1·23 1·41 1·01 1·39 1·1 1·22 0·95 1·23 Er 3·25 3·57 4·08 2·83 4·00 3·05 3·49 2·64 3·57 Tm 0·45 0·5 0·56 0·39 0·55 0·42 0·49 0·37 0·5 Yb 2·34 2·76 3·05 2·09 2·88 2·23 2·56 1·99 2·7 Lu 0·35 0·43 0·46 0·30 0·43 0·32 0·39 0·30 0·40 Sample: BC-158 BC-167 BC-157 BC-151 W10 W11 W2 W7 W3 Volcano: Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Warrumbungle Easting: 697947 717188 697947 682251 685343 685517 697457 698863 697995 Northing: 6538365 6489760 6538365 6558519 6534326 6534155 6538550 6537141 6538517 Elevation: 850 398 1055 372 1159 1096 1979 792 1058 SiO2 49·68 47·72 51·82 48·47 48·84 46·32 50·89 51·74 50·71 TiO2 2·45 2·85 2·35 2·65 2·66 3·06 2·49 1·79 2·48 Al2O3 15·75 14·64 14·7 14·55 14·66 16·61 15·61 17·07 15·43 Fe2O3 10·79 12·74 12·13 11·00 13·27 13·5 12·32 10&midd