Multiple felsic events within post-10 Ma volcanism, Southeast Australia: inputs in appraising proposed magmatic models

Felsic episodes in young SE Australian volcanism were studied using new combined zircon U–Pb, feldspar 40Ar–39Ar and fission-track dating. Trachytes, xenocrysts in basalts and derived detrital crystals yielded an 8 Ma range for felsic sequences in the Macedon–Trentham (ca 8–5 Ma) and Western District (< 5–0.0 Ma) provinces of Victoria. At Newham, zircon and feldspar ages of 6.3–6.1 ± 0.1 Ma agree with the local basalt stratigraphy, while near Trentham zircon dating suggests felsic activity at ca 8.3 Ma and 6–5 Ma. Zircons crystallised in high-temperature crustal trachytes that evolved from alkali basalts, following amphibole crystallisation in the mantle (6.3 Ma Brimbank complex). The 8–5 Ma felsic episodes are attributed to lithospheric passage over an asthenospheric plume-like upwelling, now centred under Bass Strait. The Western District Province includes quartz-normative trachyte near Creswick (40Ar–39Ar age ca 2.4 ± 0.4 Ma), zircon xenocrysts in basalt near Daylesford (U–Pb age 1.8 ± 0.3 Ma) and zircon megacrysts in tuff at Bullenmerri maar (U–Pb zircon age 0.28 ± 0.04 Ma). The Creswick and Daylesford felsic phases may represent fractionation of basaltic icelandites during peak Western District volcanic activity. Bentonitic beds of trachyandesite affinities in NW Victoria–SE New South Wales lie in strata dated at ca 2 Ma and may mark a separate distal phase of peak Western District felsic volcanism. The E–W trend of post-5 Ma Western District basaltic activity has been attributed to lithospheric edge-driven or Tasman Fracture Zone fault-driven magmatic up-wells. However, new tomographic modelling of sublithospheric upper mantle suggests that Bassian asthenospheric inputs may explain young felsic components in adjacent basalts. Multiple felsic inputs allow greater appraisal of the young volcanic genesis and eruptive risks for the area.


INTRODUCTION
The late Cenozoic (<10 Ma) intraplate volcanic fields in Victoria and SE South Australia (Figure 1 The genesis of Australian-Tasman Sea volcanism has been widely debated. Some researchers advocate a role for asthenospheric mantle plume-like inputs related to the age-progressive volcano migrations, especially in continental basaltic shields with evolved felsic cores (central volcanoes) and offshore basaltic seamount chains Sutherland et al. 2012). Plume-like inputs, however, become less obvious where felsic activity is limited and basaltic lavas are widespread (Price et al. 2003). Some studies may support plume-like asthenospheric inputs, such as the Os-isotope and trace-element data on alkaline basalts that are consistent with either with an oceanic island basalt (OIB)-type mantle plume source or a veined lithospheric mantle source (McBride et al. 2001;Sutherland 2003). Other workers, however, downplay plume involvement and instead propose repeated long-term magmatic events triggered by regional tectonic causes with shallower asthenospheric /lithospheric sources (Price et al. 2003;Demidjuk et al. 2007;Lesti et al. 2008;Farrington et al. 2010). However, some Victorian volcanism such as the felsic rocks in the Macedon-Trentham area (K-Ar ca 6 Ma) and the leucite-bearing basalt at Cosgrove ( 40 Ar-39 Ar ca 9 Ma), lie close to the latitudinal expression of the East Australian Plume (EAP) trace through SE Australia Vasconcelos et al. 2008; Figure 2). Further evidence for a potential recent plume component in this area was advanced by Matsumoto et al. (1997), who identified a primitive neon component in apatite from a metasomatised mantle xenolith in young Victorian basalt. This interpretation was disputed by Gautheron & Moreira (2003), based on a different modelling of Ne isotope fractionation. The initial finding of a plume-like Ne-isotopic composition, however, remains as a robust conclusion after the isotope fractionation model was re-examined by Matsumoto et al. (2004).
The region where suggested plume-like magmatic interactions exist below Victoria has present surface expression through mantle signatures for He, Ne and CO 2 detected in spring waters around Daylesford (Cartwright et al. 2002). The underlying mantle exhibits seismic velocities that reach their greatest reductions (slowest values) below the young basalt fields to depths of at least 200 km, suggesting anomalously high heat flows (Graeber et al. 2002). The depths and positions of asthenospheric thermal anomalies relative to the basalt fields may provide insights into the underlying magmatic processes. High surface heat flows in western Victoria may be linked to mantle and tectonic features (Purss & Cull 2001;O'Neill et al. 2003; Geothermal Heat Flow Atlas of Victoria 2010, www.dpi.vic.gov.au/minpet/ geovic), while the young volcanic fields have been linked to high advective mantle-crust geotherms, the SE Australia (SEA) geotherm, based on xenolithderived pressure-temperature data from the Bullenmerri and Gnotuk maars Sutherland et al. 2005a). In a wider context, high heat flow regions along the southern Australian margin were strongly correlated with present seismic and neotectonic zones, suggesting that thermal weakening has localised intraplate deformations (Holford et al. 2011a). The neotectonic deformations, including those in western Victoria, have been linked to enhanced stress fields related to Indo-Australian and Pacific Plate couplings (Sandiford et al. 2004;Robson & Webb 2011;Holford et al. 2011b).  (Table 1). Note Locality 5 includes two related sites tabled as 5a and 5b. The box outlines the trachytic sites detailed in Figure 3.
Several models have been developed to explain the younger (post-10 Ma) volcanism in SE Australia. Models include a system of transpressive-faults and mantle melting (TFMM; Lesti et al. 2008); a lithospheric-step, mantle convection (LSMC) system (Demidjuk et al. 2007) and an irregular EAP interactive system ). The TFMM model involves stress effects along deep crustal faults and the onshore termination of the oceanic Australia-Antarctica Tasman Fracture Zone (TFZ), as influences on Victorian basalt distribution. The LSMC model invokes a known lithospheric-step north of the basalt fields (Fishwick et al. 2008), being a cause of trailing-edge mantle convection along the wake of Australian northerly plate motion. The convective cell enables uptake of asthenospheric components. The EAP model involves a deep asthenospheric 'thermal' anomaly detected under Bass Strait by seismic studies (Montelli et al. 2006;Ford et al. 2010;Kennett & Abdullah 2011), as a dormant but previously active site of a plume system. The plume site lies east of the TFZ and southeast of the young Victorian basalts and was not part of the older Tasmanian basalt province farther south. The plume inputs may include mantle plume upwelling deflected westward by asthenospheric flow . Such westerly mantle flow appears at the Australia-Antarctica depth anomaly, where Pacific-signature oceanic basalts have invaded Indian Ocean-signature basalts for at least 28 Ma (Whittaker et al. 2010).
Within the younger SE Australian basalt fields, further dating now suggests a wider distribution and age range of felsic episodes than previously considered and allows a fresh approach into assessing the different petrogenetic models for this regional volcanism. Younger and more westerly trachyte than the dated Macedon-Trentham trachytes (8-5 Ma) was reported near Creswick with a K-Ar age of ca 2.4 Ma (Gibson 2007). Zircon megacrysts shed from central-western Victorian volcanic fields (Graham et al. 2003;Birch & Henry 2013) within the last 10 Ma (based on U-Pb formation and reset fission track ages) suggest widespread generation of felsic melt events under the volcanic fields.
This paper reports more precise ages on the wider range of felsic components within the late Cenozoic SE Australian basaltic volcanism. An updated, integrated inventory of felsic episodes has been assembled as a guide for assessing the prevailing models of magma generation below the volcanic fields. New results are based mostly on zircon U-Pb and alkali feldspar 40 Ar-39 Ar dating techniques for felsic mineral formation ages, supplemented by zircon fission track (FT) and feldspar K-Ar dating, which although subject to thermal resetting and Ar loss/excess effects, provide back up support. The U-Pb method now produces reliable results for quite young zircons (Cocherie et al. 2009), so can be extended to the youngest zircon sites. Minor discrepancies between zircon U-Pb and alkali feldspar 40 Ar-39 Ar ages from the same host rock owing to using different techniques and mineral standards can be reconciled with appropriate corrections  and are discussed further within this study.
Another aim is to provide representative major and trace-element analyses of the dated rocks, to facilitate discussion on potential processes of basaltic fractionation that evolved felsic magmas within these basalt fields. A final overall aim is to give a detailed picture of late Cenozoic SE Australian volcanism than was outlined in the broad study of Cenozoic eastern Australian volcanism.

GEOLOGICAL SETTING
The volcanic rocks studied here lie within the Macedon-Trentham and Western District Provinces of the post-10 Ma Western Victoria-SE South Australian volcanic fields (Price et al. 2003;Boyce 2013). The bulk of younger basaltic activity forms the Western Districts Province spread across the Central Highlands, western Plains and Mount Gambier subprovinces. At least 704 eruptive points from 416 centres and possibly 785 eruptive points from 491 centres are mapped, with $88% of centres being simple rather than complex volcanoes.
The felsic rocks along with some associated basalts lie in the Macedon-Trentham Province and represent at least 32 eruption points related to 24 centres (Boyce 2013). Two separate exposures ( Figure 3) include an eastern group in the Romsey-Woodend area (6.0 AE 0.5) and a western group near Trentham (5.9 AE 0.1 Ma) and both lie within error in K-Ar age (Gibson 2007;Vasconcelos et al. 2008 et al. 2003). This eastern isotopic signature, however, is not a consistent feature, as more northeastern basalts exhibit yet another set of isotopic signatures (Paul et al. 2005).
Victorian basaltic magmas were generated from partial melting of various asthenospheric and lithospheric mantle sources, with some modifications through crystal fractionation and crustal assimilation processes (Price et al. 2003). The felsic rocks include both Na-rich and K-rich types, with the more K-rich types developing higher 87 Sr/ 86 Sr ratios probably as a result of greater crustal contamination. Some evolved basalts were fractionated at mantle depths and experimental runs on such Victorian hydrous basanitic compositions suggest such magmas evolved into at least nepheline-mugearite compositions (Irving & Green 2008). Such fractionation proceeded by crystallisation of Mg-rich olivine, Ca-rich clinopyroxene, kaersutitic to pargasitic amphibole, Mgrich, K-rich mica and ilmenite between 0.8 and 1.5 GPa. This may have implications for zircon formation from evolved magmas at mantle depths.
The volcanic and placer sample sites in this study mostly lie within the Bendigo and Stawell structural zones of the Paleozoic western Lachlan Orogen, bounded by the Mt William Fault on the east and Moyston Fault on the west and partly underlain by an emplaced extension of the Tasmanian (Gardam et al. 2008;Rawlinson et al. 2008;Whitehouse 2009). The bentonites mark felsic ash fallouts (ca 3 Ma) into near-shore waters and at the Arumpo Mine, NSW, have compositions suggesting an intermediate trachyandesitic origin. The bentonites were first related to clusters of pipe-like bodies imaged along a 270 km EWE-WSW swathe in the underlying basement (Gardam et al. 2008), but later geophysical modelling suggested the pipes were probably late Paleozoic emplacements (Carlton 2009(Carlton , 2010. The close correspondence between bentonite age and peak volcanism in western Victoria (Gray & McDougall 2009) will form a connective point in this paper.

Sample sites
Zircon megacrysts were sampled for fission track (ZFT), U-Pb dating and geochemical analysis from trachytic pyroclastic deposits and lavas, sub-basaltic placers and basaltic plugs and maar deposits across the Macedon-Trentham and Western District provinces (Sample sites 1-9; Figure 1; Table 1). A composite mineral inclusion ($0.25 mm across) in a Bullenmerri zircon megacryst was studied by back-scattered imaging (BSE) and electron microprobe analysis, using a 3 spectrometer WDS JEOL JXA 8600 Superprobe in the School of Science, University of Western Sydney. Apatite megacrysts were sampled for FT dating from basaltic breccia at Brimbank Hill (Graham et al. 2003), 5 km NW of Blampied (Site 7; Table 1). Whole-rock samples were collected for 40 Ar/ 39 Ar dating and major/trace-element analysis of trachytes from Deep Creek, Newham (Locality 2) and Niggl Road, Creswick (Locality 8) and for analysing evolved alkali basalt from Brimbank Hill (Site 7).
The Deep Creek trachyte (Site 2) is poorly exposed in Sheltons Creek, within the Smokers Creek Volcanic Subgroup sequence. The flow that descends north from 660 to 500 m asl, overlies a porphyritic benmoreite flow at least 60 m thick and also trachytic tuffs of the Yangabulla Formation (Site 1). The Niggl Road trachyte (Site 8) was sampled from blocks of porphyritic trachyte up to several metres across in the weathered soil profile on a low hill. Some blocks exhibit patches of brecciated trachyte made up of angular fragments up to 2 Â 1.5 m across, Ordovician slate and vein quartz. The exposure represents a trachyte body, partly brecciated by a later disruptive event, which has shed zircon into the weathered soil profile. Figure 3 Macedon-Trentham and Western District Provinces showing felsic volcanic locations (stippled areas) and eruptive centres (filled circles) in relation to significant faults (saw tooth lines, with teeth on down thrown side) and local granite bodies (hachured areas). The numbers refer to sample sites ( Table 1). The dashed lines indicate the likely boundaries to the Macedon and Trentham felsic eruptive centres.

Mineralogy and petrography
The sampled zircons include a range of crystallographic types described from Victorian volcanic fields (Hollis & Sutherland 1985;Graham et al. 2003). The crystals range up to 25 mm in size and exhibit near-equal developments of prismatic {100} and pyramidal ({101} and {211}) forms, in some cases with other prismatic {110} and pyramidal {331} forms. These combinations typify crystals from higher temperature alkaline melts, based on a proposed simple temperature-magmatic alkalinity index scale (Pupin 1980). Variants were termed by Hollis & Sutherland (1985) as Newberry Group (mostly red, short prismatic {100} and pyramidal {211} types, often high in U þ Th, ranging up to 10 600 ppm), Lyonville Group (euhedral {110, 100, 211} forms, with abundant rutile inclusions) and Daylesford Group (elongate prisms with length/width ratios >4 and abundant tubular fluid inclusions). Dated zircons are cognate euhedra with abundant inclusions related to the host mineralogy. Zircons from a palagonitic ashy clay below ca 6 Ma two basalt flows at Trentham Falls were partly resorbed, subhedral to rounded red-orange to pale pink crystals. Similar zircons came from basaltic hosts in the South Bullarto (Site 3) and Daylesford (Site 5b) areas. Zircons from a tuff within the young Bullenmerri maar (Site 9) show a different habit to the other crystals and range up to 17 mm across, with many being fractured or crazed. Most exhibit {101} or {101} þ {301} pyramidal forms, sometimes combined with a {110} prism. Such crystal combinations mostly characterise lower temperature alkaline melts (Pupin 1980). Six Bullenmerri crystals were selected for LA-ICP-MS U-Pb dating and selected trace-element analysis. Another was chosen for a more refined LA-ICP-MS multi-element analysis of its zoned structure across a detailed traverse.
The apatite crystals from Brimbank Hill vent (Site 7) are partly resorbed, prismatic grey xenocrysts several millimetres long. They are associated with resorbed kaersutitic amphibole and dark mica megacrysts up to 3 cm long and intergrowths (up to 10%) with pyropealmandine in small garnet-rich xenoliths.
Petrographic descriptions of age-dated felsic rocks ( Figure 4) in this study (Appendix 1) include those for a ca 6 Ma trachyte (Deep Creek) and a ca 2.4 Ma trachyte (Niggl Road).

ZIRCON DATING
The laser ablation-inductively coupled plasma-mass spectroscopy (LA-ICP-MS) method is now widely used for measuring U, Th and Pb isotopic data (e.g. Kosler & Sylvester 2003;Black et al. 2004;Jackson et al. 2004;Harley & Kelly 2007;Cocherie et al. 2009;Abduriyim et al. 2012). In this study, zircon crystals were placed on double-sided sticky tape, and then epoxy glue was poured in a 2.5 cm diameter mould over them. The mount was dried for 12 h and polished, finally, using a clean polishing lap and washed in distilled water in an ultrasonic bath. The age analyses (Tables 2, 6; Supplementary Table 1) were performed using an Agilent 7500 csa quadrupole ICP-MS, modified at the University of Tasmania to date such young zircons (e.g. Meffre et al. 2007Meffre et al. , 2008, and a 193 nm solid-state New Wave Laser with a custom lowvolume ablation cell. A detailed run on a selected Bullenmerri zircon employed a Coherent excimer laser and Resonetics M50 ablation cell. The down-hole fractionation, instrument drift and mass bias correction factors for Pb/U ratios on zircons were calculated using two analyses on the primary standard and two analyses on the secondary standard zircons (Mud Tank and Temora;Black & Gulson 1978;Black et al. 2003) between samples. The 91500 standard of Wiedenbeck et al. (1995) was used as the primary standard analysed at the same spot size, fluence (3 J cm À2 ) and repetition rate (5 Hz) as those used to analyse the zircons on the samples. A 32 mm spot size was used on all of the analyses except those on the Bullenmerri zircons where a 100 mm spot size was used. Uncertainty calculations were undertaken using techniques similar to those by used by Paton et al. (2010) where the variances in the ages of standards are taken into account to calculate the uncertainty. However, owing to the young age of the zircons, we used the 'ratio of the means' method for age calculations rather than the 'mean of the ratios' method (see Fisher et al. 2010), as it provides much more precise ages on Cenozoic zircons.
FT dating of zircons (Table 3) was performed at Geotrack International, Brunswick, Melbourne, with sample preparation, etching, track counting and statistical procedures following the methods outlined in Sutherland & Fanning (2001).

FELDSPAR DATING
The K-Ar isotopic age determinations were carried out at CSIRO ESRE and JdL, Curtin University, Western Australia (Table 4). Sample preparation, potassium concentration measurement (Heinrichs & Hermann 1990), argon extraction, isotopic Ar determinations using spiked Ar calibrated against biotite standard GA1550 and monitoring measurements (blanks and mass Creek trachyte containing composite anorthoclase crystals, with reaction rim, in fine-grained matrix (crossed nicols); (b) Niggl Road trachyte containing alkali feldspar overgrowth on fine-grained trachyte core (crossed nicols); (c) Niggl Road trachyte, showing fluidal matrix containing abundant alkali feldspar laths, sparse oxidised Fe-rich olivine grains and scattered small pyroxene grains (plane polarised light); (d) Ridge Road basalt containing a mantled feldspar xenocryst containing a sieved alkali feldspar core (part view) and glomero-microphenocrysts of clinopyroxene in a fluidal basaltic matrix. discrimination factor determined by airshots) followed procedures given in Sutherland et al. (2012). The error for the argon analysis is below 1%. The K-Ar calculation used constants after Steiger & J€ ager (1977). The age uncertainty includes the errors during sample weighing, 38 Ar/ 36 Ar and 40 Ar/ 38 Ar measurements and K analysis.
The 40 Ar-39 Ar analyses were performed at the Western Australian Argon Isotope Facility at Curtin University (Table 5; Supplementary Table 4). From the Deep Creek trachyte sample (Site 2), hundreds of micrometresized sanidine crystals that were unaltered and transparent were handpicked under a binocular microscope after Pb corrected ages, with 2s errors. Analyst S. Meffre.

ZIRCON MULTI-ELEMENT ANALYSIS
Selected elements were analysed by LA-ICP-MS during the isotope dating runs for comparisons of zircon compositions (  Figure 6). Each analysis on the zircon began with a pre-ablation (5 laser pulses) followed by a 30 s blank gas measurement followed by a further 30 s of analysis time when the laser switched on. Zircons were sampled on 32 micron spots using the laser at 5 Hz and a density of $2 J cm À2 providing a drill rate of 0.5 mm s À1 . A flow of He carrier gas at a rate of 0.5 L/min carried particles ablated by the laser out of the chamber to be mixed with Ar gas and carried to the plasma torch. Isotopes measured include 49 Ti, 96 Zr, 178 Hf, 202 Hg, 204 Pb, 207 Pb, 208 Pb, 232 Th and 238 U with each element being measured sequentially every 0.14 s.
A more extended elemental array, including rare earth elements (REE) was analysed by LA-ICP-MS on a zoned Bullenmerri zircon megacryst, analysing the crystal at 8 spots on different zones along a traverse from its rim into its inner core (Table 7; Figure 7). The analysis

WHOLE ROCK GEOCHEMISTRY
Major, minor and selected trace elements of the dated evolved rocks were analysed at the XRF and XRD Facility, University of Pretoria, Pretoria, South Africa (Maggi Loubser), using routine procedures. More extended trace and REE analyses on these rocks were made using inductively coupled plasma mass spectrometry (ICP-MS) at the Geology Department, University of Cape Town, Rondebosch, South Africa (Andreas Spath) along procedures given in Appendix 3. Additional traceelement analyses utilised ICP-MS facilities at AMDEL laboratories, Adelaide and the University of Melbourne for further comparisons of associated rocks. Representative analyses for the evolved rocks are listed in Table 8.

U-PB AGES
The zircon U-Pb ages (6.3-0.     The youngest zircon U-Pb age has a mean age of 0.187 AE 0.22 Ma (MSWD ¼ 1.08, P ¼ 0.37) for five megacrysts (core and rim analyses) from Lake Bullenmerri maar (Supplementary Table 1). This result is within error of a U-Pb age estimate of 0.24 AE 0.04 on zircon from this locality obtained by isotope dilution mass spectrometry (Hiess et al. 2012). To refine the Bullenmerri zircon age on such young U-Pb ages, Th-U disequilibrium corrections are required (Cocherie et al. 2009). The results (Table 6) involve assumptions on the Th/U ratio in the original melt composition (f) that crystallised this zircon (Scharer 1984). Using an average younger western Victorian basalt composition (f ¼ $0.26) gave a corrected age of 0.27 AE 0.06 Ma (MSWD ¼ 0.64, P ¼ 0.76). Crystallisation from a felsic magma, however, is more likely (f ¼ $0.17; Blundy & Wood 2003) and gave a corrected age of 0.28 AE 0.06 Ma (MSWD ¼ 0.59, P ¼ 0.80). Thus, melt activity that formed zircons below Bullenmerri took place at <0.3 Ma, with ejection sometime afterwards.
Older zircon is a rare component among the Bullenmerri megacrysts and is reported here. It has a mean age of 93.8 AE 2.5 Ma and distinctly lower U, Th and higher Ti content than in the young zircons (Table 2; Supplementary Tables 1, 2). Rare zircon groups with FT ages in the 106-70 Ma range appear in alluvial sequences in western Victoria, e.g. Stony Creek Basin, Daylesford (Willman et al. 2002) and gravels at Carrapooee (Birch et al. 2007), which with the older Bullenmerri zircon suggest minor felsic magmatic/volcanic events across western Victoria during that period.

ZIRCON/APATITE FT AGES
The oldest pooled FT ages came from red-orange (8.3 AE 0.5 Ma, n ¼ 3) and pale pink (8.1 AE 0.5 Ma, n ¼ 9) sub-basaltic zircon groups (Site 4) at Trentham Falls (Table 3). These ages resemble the 8.4 AE 0.5 Ma (n ¼ 10) FT age found for zircon within the sedimentary infill in Stony Creek Basin to the west (Willman et al. 2002) and suggests an earlier zircon crystallisation event than that is defined by the 5-7 Ma U-Pb zircon ages in the area. A younger Trentham Falls red-orange zircon FT group at 5.7 AE 0.4 Ma (n ¼ 4), however, falls within the later interval, within error of the K-Ar dating on the overlying Trentham Falls basalt sequence (Graham et al. 2003). The FT age of 6.0 AE 0.3 Ma (n ¼ 9) for zircon in Allens Creek breccia pipe (Site 6) is within error of and probably genetically related to the nearby Trentham felsic intrusions (K-Ar ages, 5.8-6.0 AE 0.1 Ma; Graham et al. 2003). Apatite FT ages on crystals from breccia in the Brimbank vent (Site 7) with a mean age at 6.3 AE 1.1 Ma suggest that this is a relatively old centre related to the Macedon-Trentham Province (Table 3).
Significantly younger zircon FT ages characterise zircons recovered from vents in the Daylesford-Creswick region (

AR-39 AR AGES
The dating of alkali feldspars from two representative trachytes gave significantly different ages (Table 5, Supplementary Table 4; Figure 5). The Deep Creek trachyte (Site 2) has a weighted plateau age of 6.14 AE 0.08 Ma (MSWD ¼ 0.96, P ¼ 0.49) within error of a zircon U-Pb age from this rock (Tables 2, 5; Figure 5a). The Niggl Road trachyte (Site 8) age lies within error of a feldspar K-Ar age and FT age of the oldest zircon group from this rock (Tables 3-5; Figure 5b). This result represents a perturbed age spectrum with increasing ages converging toward an apparent age of ca 2.4 Ma. This kind of profile is typical of 40 Ar Ã loss by thermally active diffusion and suggests a minimum age of >2.4 Ma for crystallisation of the Niggl Road trachyte. The results suggest that felsic eruptive events not only characterise the older Macedon-Trentham Province but also form younger smallvolume events within peak (ca 2.5 Ma) Western District Province basaltic eruptions.
The Hf values are highest in the trachytic Sheltons Road zircons (8443) and lowest in the young Bullenmerri megacrysts (5737). Average values (ppm) for Th (4305) (Figure 6). Within the basaltderived zircon fields, the young Bullenmerri field with much higher Th/U is well separated from the older Bullenmerri field.

DETAILED TRACE-ELEMENT TRAVERSE RIM TO CORE, BULLENMERRI ZIRCON
A Bullenmerri zircon, with well-developed part-oscillatory zoning around a subrectangular core zone was analysed at different spots across from its outer zones into the core area (Supplementary Table 3; Figure 7). The REE profiles are all similar with well-developed positive Ce anomalies and negligible negative Eu anomalies (Figure 7). The core is largely depleted in REE relative to the outer zones, while the outer zones suggest oscillatory enrichment/depletion, with greater enrichments in darker zones than in paler zones. The average Hf ($0.6 wt%) and Y ($870 ppm) contents for the Bullenmerri zircon (Table 7), in relation to a zircon geochemical affinity diagram (Belousova et al. 2002), suggest a parental intermediate felsic magma was involved in its crystallisation.
The more precise LA-ICP-MS results for Ti determined in the traverse from rim to core (2.7-1.5 ppm) is considered precise enough for Ti-in-zircon thermometry (Ferry & Watson 2007;Ferriss et al. 2008) and suggests T increased overall during megacryst growth. However, absolute T values have uncertainties related to corrections for Ti and Si activities and pressure (P) in the crystallising process (Stepanov et al. 2011;Tailby et al. 2011), values not easily constrained for isolated megacrysts. At 750 C, changing TiO 2 activity from 1.0 to 0.5 would generally increase T by 60-70 and changing SiO 2 activity likewise would decrease T by a similar amount, while the P correction is normally $100 C/GPa (>1 GPa) and $50 C/GPa (<1 GPa). As TiO 2 activities in silicate melts are rarely <0.5, the activity corrections should be relatively small for calculated TiO 2 :SiO 2 at 1:1. Such thermometry gives a T estimate of 600-640 AE 50-100 C for the megacryst growth, which with uncertainty errors suggests thresholds of 540-700 C. This overlaps the range based on the Pupin zircon crystallographic temperatures for some 85% of the Bullenmerri megacrysts studied by Hollis & Sutherland (1985), providing some correlative support for the projected T ranges from the two methods.

SILICATE INCLUSION IN BULLENMERRI ZIRCON
A composite, mineral inclusion in a Bullenmerri megacryst (Figure 8) was composed of $30 vol% euhedral diopside ($wo 46 fs 54 ), an altered Al, Fe-rich silicate matrix ($60 vol%) and scattered clusters of elongate baddeleyite crystals ($10 vol%) along its reaction margin. The inclusion suggests incorporation of a globule of early-crystallising Zr-bearing silicate melt within the growing zircon megacryst. Small minerals in the host zircon included apatite, ilmenite and P-bearing hedenbergite (wo 46 fs 54 ; P 2 O 5 3.3 wt%), suggesting Fe and P enrichment in the zircon-crystallising silicate melt.

WHOLE-ROCK CHEMISTRY
Multi-element (including REE) results for the 6 Ma and 2.3 Ma trachyte samples are compared in Table 8. In major element analyses, the trachytes have similar SiO 2 ($59 wt%), alkali (10-11 wt%), and total FeO contents (5.7-6.1 wt%) on a 100 wt% comparison. The Niggl Road trachyte, however, has 2 wt% higher Al 2 O 3, 1.5 wt% lower CaO and is significantly low in Mg # (6) in comparison with the Ol ($3%)-normative Deep Creek trachyte (Mg # 26). At Niggl Road, the trachyte represents a Q ($5%), Hy ($8%) and C ($4%)-normative rock. The Brimbank evolved mugearite is similar in age to the Deep Creek trachyte although significantly lower in Si, Al, alkalis and higher in Ti, Fe, Mg, Ca and Mg#. It is similar in composition to the mantle-derived ne-mugearite from Victoria used for experimental studies on high pressure amphibole crystallisation from such magmas (Irving & Green 2008), although more depleted in Mg and slightly enriched in Ti and Fe relative to the experimental rock. An enriched P content produces 4.5% Ap in the norm and is presumably related to crystallisation of the large apatite megacrysts used in dating the host eruption. The Ol-Di-Ne-normative nature of the rock make it potentially a parent mantle magma for evolution into Ol-normative trachytic magmas of Deep Creek composition, after further crystal fractionation of suitable mineral phases at higher crustal levels (Irving & Green 2008).
In primitive mantle-normalised analyses (Figure 9), the trachytes are enriched in large ion lithophile (LIL) and high field strength (HFS) elements such as K, Rb, Nb, Ta, U, Pb, Zr and Hf, but are depleted in Sr and P, relative to moderate to strongly evolved western Victorian basalts (Brimbank mugearite, Newlyn hawaiite). The Brimbank mugearite is highly enriched in Ba, Th, Sr and middle REE (Ho to Lu), whereas the Newlyn hawaiite (Sutherland et al. 2004) is moderately evolved basalt of similar provenance to the Niggl Road trachyte. In chondrite-normalised REE plots (Figure 10), the trachyte arrays are higher in Pr and heavy REE and exhibit negative Eu anomalies compared with the basalt arrays.
Such Eu depletions are typical of considerable plagioclase/alkali feldspar fractionation during basaltic magma evolution, as found in the late Cenozoic bi-modal basalt/felsic sequences in central Patagonia (Espinoza et al. 2008). Such fractionation is only specific to the mantle, if supported by further data such as the presence of mantle xenoliths as found in the Brimbank mugearite.

DISCUSSION
The zircons and rocks studied here from this SE Australian volcanic region suggest a wider felsic distribution, particularly from underlying sources intersected by later basalts. This needs discussion within the overall context of the prevailing basaltic magmatism and its tectonic context. Previous modelling will be supplemented by further modelling involving the deeper mantle below the volcanic fields.

Zircon-felsic magmatic relationships
An older zircon U-Pb age group (6.5-5 Ma) represents high-level crystal types allied to Macedon-Trentham Province trachytic eruptions and magma chambers. Zircon chemistry suggests probable evolution within fractionating magma chambers (Clairborne et al. 2006), giving increasing Th and U, Zr/Hf and Th/U trends within the Shelton Road-Deep Creek sequence and among Rat Hole Track xenocrysts ( Figure 6). An intermediate U-Pb age group (2.5-1.5 Ma), with typical elongated prism forms, characterises the Niggl Road trachyte, and xenocrysts in the Ridge Road basalt and Leonards Hill volcano. The Th and U contents and Th/U ratios are lower than in zircons from the Macedon Trentham trachytes and represent felsic fractionation within peak basaltic activity in the Western District Province. A young U-Pb age group (<1 Ma) is represented by Bullenmerri zircon from a minor felsic fractionation event where the zircons (av. Zr/Hf 55-85) do not attain the low Zr/Hf (<40) found in highly fractionated rhyolites in eastern Australian central volcanoes . The young Bullenmerri zircons formed from silicate magma (Figures 8, 11-12) and provide an important key to understanding any present felsic magma generation under the western Victorian volcanic field.

BULLENMERRI ZIRCON RELATIONSHIPS
The detailed multi-element traverse within a Bullenmerri zircon revealed oscillatory zoning around a core zone, more depleted in most REE elements (Table 7; Supplementary Table 3; Figure 7). In its REE profile, the depleted core mimics the enriched outer profiles, with positive Ce anomaly and negligible Eu depletion, although showing a distinct gap in absolute values. This suggests two-stage growth with the outer zones growing from more fractionated parental melt. The Ce anomalies are larger in the outer zones, with the La-Ce lengths twice those in the core, although the Ce-Pr lengths are similar. Recent experimental investigation of Ce and Eu anomalies in zircon at 1 GPa and 800-1300 C (Trail et al. 2012) suggests that Ce/Ce Ã increases not only with rising oxygen fugacity but also with decreasing crystallisation temperatures. Although these factors interplay, a crystallising core of the zircon normally would grow at a higher initial temperature than the later growth zones within cooling melt. If the later growth forms from fractionating magma under higher oxygen fugacity, then both controlling factors would reinforce and increase Ce 4þ entry in the crystallising zircon. Whether this applies to the Bullenmerri zircon is uncertain, as exact conditions of growth are difficult to establish for isolated megacrysts. The Bullenmerri zircon exhibits negligible Eu depletion core to rim and would discount high oxygen fugacities, inherited Eu depletions in the parent melt and any Eudepleting processes such as plagioclase crystallisation, based on the studies of Trail et al. (2012).
The depth of origin of Bullenmerri megacrysts needs consideration. From the 238 U/ 235 U systematics, Hiess et al. (2012) assigned the high Bullenmerri ratio (137.86 AE 0.02) to a probable mantle origin, while Ce/Ce Ã -Eu/Eu Ã plots for Bullenmerri zircons also match a mantle zircon array for Australian and other mantle-derived zircon ( Figure 13). Nevertheless, the crystallisation T range based on the Pupin index and Ti-in-zircon thermometry (540-700 C) seems relatively low for a parental mantle melt evolved below Bullenmerri maar. In regard to a late Quaternary xenolith-derived geotherm here Sutherland et al. 2005a), the ambient geotherm T in a Moho transitional zone (25-35 km;Fichtner et al. 2010; would range between $920 and 970 AE 100 C. This apparent T discrepancy for a mantle origin may involve uncertainties in initial Bullenmerri zircon growth. The Pupin T index results only reflect the final external crystal growth T, while the core Ti-in-zircon T estimates may need a P correction of 100 C upwards if it crystallised at mantle depth ($1 GPa). Thus, initial age, growth from mantle melt then ascent to a crustal-level melt that had fractionated with trace-element enrichment, may explain the zircon characteristics. The parental melt probably had a significant hydrous content, as mantle xenoliths at Bullenmerri maar carry hydrous metasomatic phases (Powell et al. Figure 11 Hf vs Y plots for Bullenmerri zircon (filled square) in relation to zircon plots and fields from East Australia (filled circles, light grey area), New Zealand (filled triangles, dark grey areas) and Italian mantle basalts (filled triangles, medium grey area). The comparative data comes from Sutherland & Meffre (2009).   2004) that would generate cooler melts than for melts derived from anhydrous mantle.
The average REE profile for Bullenmerri zircon is compared with those for other zircon megacryst suites from eastern Australia in Figure 12. Bullenmerri occupies a mid-range position, more enriched than low-U mantle zircon suites (Barrington, NSW; Mount McLean, Queensland) and similar in profile to the older Weldborough, Tasmania and Yarrowitch, NSW megacryst suites. It is distinct from the high-U and REE-rich suites that exhibit Eu depletions (Barrington, NSW; younger Weldborough, Tasmania). The REE profile and abundances for the Bullenmerri depleted core (Figure 7), however, are similar to these on low-U Mount McLean suite (Figure 12), which typifies mantle zircon.

Relationships of felsic components
The felsic events mostly stemmed from basaltic fractionation and assimilation processes (Price et al. 2003). The Brimbank ne-mugearite dated at ca 6.3 Ma suggests high-pressure mantle evolution based on its mantle xenoliths and xenocrysts that foreshadowed the nearby 6 Ma Macedon-Trentham felsic eruptions. Such evolution from hydrous basanitic magma probably took place by crystal fractionation between 1.43 and 1.35 GPa, based on experimental runs on similar Victorian basaltic compositions (Irving & Green 2008). A hydrous amphibolemetasomatised mantle was probably involved, as related xenoliths are common in Victorian basalts. Two types of amphibole in Victorian mantle samples carry trace-element signatures that suggest silicate melt and hydrous silicate fluid activity, from both earlier subduction and later intraplate magmatic events (Powell et al. 2004). While undersaturated mantle magmas potentially could evolve to ne-normative trachytes, fractionation beyond mugearite probably proceeded at crustal levels (<0.8 GPa) and involved feldspar fractionation (Irving & Green 2008). A crustal stage for Deep Creek trachyte generation is likely, as it overlies porphyritic benmoreite with abundant large phenocrysts of alkali feldspar ( Van-denBerg 2005). The Deep Creek trachyte is marginally sodic in composition, suggesting that crustal contamination effects were limited, based on Macedon area Na/K trachyte isotope studies (Price et al. 2003).
The Niggl Road trachyte with an age related to peak Western District volcanism (ca 2.4 Ma) and a K-rich, quartz-normative composition clearly differs from the Macedon-Trentham felsic eruptives. The magma probably fractionated from tholeiitic basalt progenitors, as evolved basaltic icelandite and icelandite appear in nearby basaltic sequences of similar age (Price et al. 2003). The icelanditic sequences exhibit isotopic characteristics that suggest their magmas were derived from complex mantle subcontinental lithospheric and asthenospheric sources. In the Niggl Road region, these tholeiitic and transitional basalts carry higher 87 Sr/ 86 Sr isotopic ratios than basalt sequences derived from mantle sources west of the Mortlake Discontinuity (Price et al. 2003).
Felsic outcrop in the Western District Province at present is limited to the Niggl Road trachyte plug, within the Central Highlands subprovince, although other felsic bodies may be obscured by erosion and/or burial under later soils, sediments and basalts. Zircon megacrysts with reset FT ages matching peak Western District volcanism are common in alluvial deposits in basaltic areas east of the Mortdale Discontinuity (Graham et al. 2003;present authors' unpublished data). Some may represent further felsic eruptions. The Spring Hill centre is of particular interest here with flows extending towards Coliban Dam and includes crustal meta-xenoliths (Allchurch et al. 2008). A minimum K-Ar age for the Spring Hill centre is 3.3 Ma (Graham et al. 2003) and suggests its fractionated lavas mark a more extended felsic eruptive span within Western District activity of at least a million years duration. Other potential felsic activity includes transitional to tholeiitic parental magma evolution towards felsic compositions within the Gisborne volcanic complex (Heyworth et al. 2005) where Fe-rich enstatite and quartz-bearing trachyandesite lavas require precise age-dating to confirm their Western District or Macedon-Trentham Province age-status (Price et al. 2003). Nevertheless, they add to the overall western Victorian felsic spectrum.
Alkali feldspar megacrysts (K-rich albite-sanidine series) within Western District basaltic centres also need evaluating in terms of underlying felsic sources. These xenocrysts occur across the Central Highland and Western Plains subprovinces and some sites were recently studied using 40 Ar-39 Ar geochronology (Ismail et al. 2013). The precise origins of these megacrysts are debated, but they are widespread in eastern Australian and other intraplate alkali basalt fields, where some studies relate them to disaggregated, small-volume syenitic bodies (Zhang et al. 2002;Upton et al. 2009). If so, the dating of Western District eruptive events (<5 Ma) suggests young felsic crystallisation processes under the region are more widespread, than just represented by rare surface trachytes and zircon xenocryst distribution.
Outlying felsic ash beds are represented in the bentonites in the upper Murray Darling sedimentary sequence in NW Victoria-SW New South Wales (Gardam Wenem Mine within the SW-end of the bentonite field suggest a proximal source with present considerations favouring a source near Mildura, Victoria, compatible with ash transport under a likely prevailing south-westerly wind system. The many vents interpreted north of the Mount Gambier-Portland region by Lesti et al. (2008), if confirmed, would be ideally placed as a potential bentonite source region. Whatever the actual source, the bentonites add to the felsic inventory within the young volcanic fields of SE Australia.

Origin of post-10 Ma volcanism, SE Australia
Several models exist for the origin of young SE Australian basaltic volcanism since 10 Ma; they include plume-like asthenospheric upwellings, convective cells produced by lithospheric edge-driven plate motion and stress-field effects along major transform and onshore fracture systems (outlined in Introduction). The new data on ages of felsic components within the basaltic magmatism will assist appraisals of these models. However, the accompanying host petrological data, while locally relevant, is not detailed enough to delineate the complex asthenospheric and lithospheric sources producing the mantle melting. To further consider underlying inputs producing the magmatism, particularly plume-like processes, additional tomographic modelling was enlisted. This modelling uses geophysical parameters within the upper mantle to distinguish areas of seismically slower (hotter) upwelling below the Tasman An age-progressive component extended from 9 Ma leucite-bearing basalt in northern Victoria southwards to the 8 to 6 Ma Macedon-Trentham basalt-trachyte sequences. The timing matches projected trends from migratory felsic centres erupted from 21 to 12 Ma in eastern-central NSW . After underpinning the central Victorian felsic activity, with further northerly plate motion, the projected asthenospheric thermal anomaly (EAP) is presently sited within a 700 km-diameter, 150 km-deep dormant thermal anomaly now under Bass Strait (Figure 14). Initial northern Victorian activity involved low degrees of partial mantle melting within heterogeneous mantle (Paul et al. 2005), while succeeding central Victorian activity marked basaltic fractionation and a high level felsic surge typical of EAP line volcanism. The Macedon-Trentham felsic event compares closely in size and character with the small Belmore central volcano, NSW, where felsic eruptions dominated rare basalts (Sutherland et al. 2005b). A 10 km-diameter 'gabbroic' magma chamber injected into the crust below the Macedon felsic field area (Figure 3), based on thermal modelling would cool exponentially through its 80% solidus in the first million years of fractionation before becoming immobile, but would take 10 million years to cool to the pre-intrusion geotherm (R. B. Kitch, pers. comm., October 2013).
The Australian progressive plume surges are variously attributed to plate motion changes or westerly deflections of an upwelling plume Sutherland et al. 2012). The westerly location of the central Victorian episode within the asthenospheric source region may suggest plume deflection but deflection of a massive mantle mass has to overcome considerable gravitational and thermal inertia, so is unlikely to respond quickly or be driven by thin-skin crustal events and structures. Such mantle deflection, however, may reflect westward flow of Pacific asthenosphere, evident since 28 Ma within the Australian Antarctica Discordance (Whittaker et al. 2010), and possibly be reflected in the 6 Ma mantle upwelling structure modelled below central-western Victoria (Figure 15).
A western Victorian basaltic component may mark outlying activity that accompanied the central felsic episodes, based on a 9-5 Ma K-Ar ages for an emergent submarine volcano at Lady Julia Percy Island and flows near Hamilton and Linton (Edwards et al. 2004;Gibson 2007). Accepting these ages as reliable, this activity predates most Western District activity (<5 Ma;Price et al. 2003;Gray & McDougall 2009) and would represent sufficient degrees of partial mantle melting to produce tholeiitic magmas, although their precise source affinities remain uncertain without detailed geochemistry. This melting event lies outside the present asthenospheric thermal anomaly confines, but will be examined in the mantle upwelling modelling (9-6 Ma slices).
The time-depth slices (Figures 15-17) show an intense upwelling region that underpinned the original 65 Ma Coral Sea triple point rift zone, particularly along the main western rift arm. This upwelling has persisted below the EAP flare-ups, and edged below north-central Victoria in the 9 and 6 Ma time slices at 113 km depth. It persisted downwards through the 284 km and 655 km depth slices indicating a deep upper mantle origin. At 113 km depth, only an outer periphery zone of reduced thermal effect lies below SW Victoria, but beneath this zone upwelling extends westward intensifying below 200 km, merging into the main upwelling at $600 km depth.

LATER PHASES OF SE AUSTRALIAN MAGMATISM
The post-5 Ma components form the most prolific and extended basaltic volcanism in the region. Apart from minor basaltic eruptions at ca 4 and 2 Ma in eastern Victoria (Kershaw 2004), activity was concentrated west of 145 E, across W Victoria, SW New South Wales and SE South Australia (Figure 14). The activity marks a pronounced shift in the axis of volcanism from the earlier N-S plume-like volcanism and largely extends along an ENE-WSW-trending axis. The model of Lesti et al. (2008) may answer some of these questions. This study interpreted volcano distribution from Landsat 7 scenes using protocols for volcano recognition. It expanded volcano density, particularly north of the Burr Range-Mount Gambier and Portland areas, into domains not hitherto included in the younger basalt fields and linked all the post-10 Ma fields to magmatism triggered by onshore reactivations of transtensional structures through the dynamic role of the Tasman Fracture Zone in the Southern Ocean spreading process.
At 3 Ma, just prior to peak Western District and outlying bentonitic eruptive activity, the mantle thermal modelling ( Figure 15) shows that at 113 km depth, the intense centre of the anomaly lies just south of western Victoria, but the northern part remains under central Victoria. The western side of Western District basalt fields overlies a less pronounced thermal zone, which intensifies at depths >250 km and merges towards the main anomaly at lower depths (384-655 km).

MODIFIED SE AUSTRALIAN MODEL
Based on the wider felsic interactions and underlying mantle upwelling story outlined in this paper, a modified genetic model is advanced to explain the causes of the underlying basaltic magma generation. Whether the magmatism involved edge-driven convection cells, tectonic fracture reactivations, or both processes, this modified model introduces greater interaction with a deep asthenospheric upwelling zone, now centred in the Bass Strait thermal anomaly. The previous suggested models, individually, may have limitations in their scope, to cover the full range of lithospheric-asthenospheric magmatic processes operating under this complex region. Many variables can produce shear-driven upwellings from asthenospheric situations, using wider modelling that also included young SE Australian volcanism  ( Bianco et al. 2011;Conrad et al. 2011). After activation of 9 to 6 Ma north to central Victorian magmatism as part of the EAP line, the trailing thermal aureole of this mantle-melting anomaly then passed under the eastern side of developing Western District activity. Extra inputs from this enhanced mantle-melting source would account for the plume-like OIB components, a young metasomatic xenolithic mantle plume signature and the prominent basaltic icelandite-felsic magmatic components, all features largely identified within eastern segment basalts, east of the Mortlake mantle discontinuity.
After peak activity, by 2 Ma northern plate motion would keep moving Victorian lithosphere away from the centre of main asthenospheric upwelling, although a peripheral part still persisted below the SE side of the volcanic fields, where a strong thermal anomaly was identified by seismic tomography (Graeber et al. 2002). A complex mix of magma source models, including input from a decaying EAP-like asthenospheric upwelling anomaly, may reconcile some points of previous debates on the precise nature of the sources contributing to Western District volcanism (McBride et al. 2001;Price et al. 2003).
The present mantle thermal regime modelled here under SE Australia is given in Figures 16 and 17. At 113 km depth, the main thermal anomaly resides within Bass Basin lithospheric mantle, but a segment underlies the young Western Districts basalt fields on their east side. A weak thermal region under the more western basalt fields forms a bridging ridge between the Bassian anomaly and a sharp thermal peak in SE South Australia. Below 200 km depth, the Bassian anomaly broadens to extend under the whole Western District volcanic zone, which continues into deep upper mantle (384-655 km depth).
The post-10 Ma SE Australian volcanic region embroiders the edge of a broad SW Pacific thermal upwelling below the north-migrating Australian plate and resides over a western lobe of the upwelling (Figures 15-17). This asthenospheric lobe not only cradled the initial Coral Sea-Cato Trough triple-point spreading rift, but also the plume-like sources for the subsequent migratory East Australia central volcanoes and Tasmantid seamount chains. Considerable debate now questions whether Lower Mantle plumes fed such 'hot spot' intraplate volcanism and rift zones, or whether other models apply (Anderson 2013; Smith 2013). This paper suggests that young SE Australian volcanism was influenced by Australian passage over 'thermal' asthenospheric upwelling from the base of the transition zone at $650 km in depth. Support for injections of deep Lower Mantle 'plumes' would require more conclusive data than presently available. Nevertheless, the apparent western incursion into an increased upwelling zone after 5 Ma would probably enhance injections of enriched asthenospheric melts into the lithosphere, based on experimental petrological modelling of similar Australian basalts (Adam & Green 2011). This would aid a spread of basaltic volcanism across the Western District Volcanic Province.

EPILOGUE
Although basalts dominate these SE Australian fields, they differ from young Queensland basalt fields in a Knowledge and modelling of the young Australian basalt fields and their recent eruptive characteristics were advanced considerably in the last decade (Blaikie et al. 2012;Boyce 2013;Holt et al. 2013;Jordan et al. 2013), assisting in understanding the volcanic evolution and potential volcanic risks (Joyce 2004(Joyce , 2005(Joyce , 2006. Basaltic eruptions are forefront in this modelling. Improved statistical re-analysis in such young intraplate basalt fields can modify inferred risk, as found for the <1 Ma Auckland basalt field, New Zealand that gave a more random pattern (Beddington & Cronin 2011). Whether such randomness applies to the young Victorian activity remains for further testing, but the field has a greater magmatic input and recent dating suggests clustered activity at ca 100 ka (Ismail et al. 2013). The wider geographic and temporal spread in felsic episodes found in this study raises the stakes for new felsic outbursts, even in a waning magmatic system with dying hydrothermal activity (Cartwright et al. 2002;Gray & McDougall 2009

CONCLUSIONS
Combined zircon and alkali feldspar dating and host geochemical analysis in central-western Victorian volcanic fields suggest felsic magmas were generated over an 8-Ma period. Trachytic eruptions at ca 6 Ma and ca 2.4 Ma include prominent undersaturated (Macedon-Trentham) and saturated (Creswick) basaltic fractionation, with other zircon dating suggesting other felsic magmatism at ca 8, 5, 1.8 and 0.3 Ma. Zircon megacrysts at Bullenmerri maar grew from a core with depleted mantle-like trace elements and then enriched silicate melt. A corrected U-Pb age of <0.3 Ma suggests future felsic eruption in Victoria remains feasible. New modelling of upper mantle regions, based on seismic wave speeds and tectonic plate motion parameters, suggest mantle upwelling contributed to magmatic processes. Upwelling asthenosphere and stress field changes from interactions on the bounding SW Pacific plate margins focused magmatism in SE Australia over the last 10 Ma.
Progressive N-S volcanism over a plume-like upwelling formed a felsic surge in north-central Victoria (9-5 Ma) and then peripheral inputs into younger E-W basaltic activity. Felsic activity remains a potential risk in the region.

ACKNOWLEDGEMENTS
Fons VandenBerg, Geological Survey of Victoria, supplied background data on the Trentham and Lancefield 1: 50 000 and Romsey 1: 100 000 mapping and stratigraphic notes. D. H. Taylor and C. E. Wilman and associates provided data from the 1: 100 000 Creswick and Castlemaine map sheet projects. Bernie Joyce, Geography Department, University of Melbourne provided data on Victorian volcano distribution, regolith and eruptive risk. Astrid Carlton, Geological Survey of NSW, Maitland, assisted with airborne geophysical survey reports and unpublished data from SW New South Wales. Alan Reid and Anthony Mason, Arumpo Bentonite Pty Ltd, Mildura, provided data and discussion on the bentonite deposits. Hugo Corbella, National Natural Sciences Museum, Buenos Aries, Argentina assisted with fieldwork in central Victoria.
The Australian Museum Trust supported fieldwork, collection use and analytical work. Jo-Anne Wartho assisted with 40 Ar/ 39 Ar data for the Niggl Road trachyte sample (413) analysed at Curtin University. The geology departments of the University of Pretoria and the University of Cape Town, South Africa, assisted with analytical results through Maggie Loubser and Andreas Spath. Greg Yaxley, Research School of Earth Sciences, Australian National University, Canberra, provided data on East Australian zircon megacrysts and Ti-in-zircon thermometry. The School of Biological, Earth and Environmental Sciences, University of New South Wales, helped with microscope photography and chemical analyses (Irene Wainwright, UNSW Analytical Centre). School of Science, University of Western Sydney provided EMPA access through Simon Hager. Ben Cohen, Earth Sciences, University of Queensland gave access to his 2007 PhD thesis on Ar-Ar dating of East Australian volcanic rocks.
Sabin Zahirovic, Earth Byte Group, University of Sydney, was instrumental in developing tomographic mantle models.