Lithospheric influences on magma compositions of late Mesozoic and Cenozoic intraplate basalts (the Older Volcanics) of Victoria, south-eastern Australia

Lithos 206–207 (2014) 179–200

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Lithospheric influences on magma compositions of late Mesozoic and Cenozoic intraplate basalts (the Older Volcanics) of Victoria, south-eastern Australia Richard C. Price a,⁎, Ian A. Nicholls b, Arthur Day c a b c

Science and Engineering, University of Waikato, Hamilton, New Zealand School of Geosciences, Monash University, Clayton, Victoria, Australia PO Box 533, Corrimal, NSW, Australia

a r t i c l e

i n f o

Article history: Received 1 May 2014 Accepted 27 July 2014 Available online 7 August 2014 Keywords: Basalt Intraplate volcanism Mantle metasomatism Lithospheric mantle

a b s t r a c t Basaltic volcanism, ranging in age from Late Cretaceous to Holocene and extending across the southern part of the state of Victoria in south-eastern Australia was initiated during the earliest stages of rifting associated with opening of the Tasman Sea and Southern Ocean. Volcanism has continued sporadically since that time with major breaks in activity occurring between 77 and 62 Ma and 18 and 7 Ma. Basaltic rocks with ages in the range 95 to 18 Ma occur in small lava fields scattered across eastern and south-eastern Victoria and they have also been recovered from bore holes in the west of the state. They have been referred to as the “Older Volcanics” to differentiate them from more volumetrically extensive and younger (mainly b 4.6 Ma) lava fields comprising the “Newer Volcanics” of the Western District Province to the west. Older Volcanics vary in composition from SiO2-undersaturated nephelinites, basanites, basalts and hawaiites through transitional basalts to hypersthene and quartz normative tholeiites. Strontium, Nd and Pb isotopic compositions lie between depleted (DM) and enriched (EM1 and EM2) end member mantle components in Sr–Nd– Pb isotopic space. Trace element compositions are generally characterised by enrichment of Cs, Ba, Rb, Th, U, Nb, K and light REE over heavy REE, Ti, Zr and Y and the overall patterns of major and trace element behaviour can be explained in general terms by petrogenetic models involving partial melting of a complex spectrum of mantle compositions with subsequent but limited additional modification by fractional crystallisation with or without assimilation of crust. Among basalts with relatively high Mg# [100 ∗ Mol. MgO/(MgO + FeO) N 65], two distinct end member compositions can be differentiated using primitive mantle normalised extended element patterns. Group 1 basalts have convex upward patterns with enrichment of light over heavy REE and depletion of Rb, Ba, Th and U relative to Nb. Group 2 basalts also have distinctive convex upwards patterns but are characterised by strong depletions of K, Rb and Ba relative to Nb. In both groups there is additional subtle variation with some samples having patterns with relative enrichments in Nb, Sr and Eu and/or depletions in Pb. Group 1 basalt compositions can be approximated by quantitative models involving 2 to 10% partial melting of an originally depleted mantle composition that has been metasomatised by the addition of 2 to 3% of an enriched component with a composition similar to EM1 intraplate basalt. The trace element patterns of Group 2 basalts can be modelled by 2 to 10% partial melting of an originally depleted mantle metasomatised by the addition of 1% of a calci-carbonatite composition. When Sr isotope data for Older Volcanics are projected onto an east–west profile across the state of Victoria, they outline distinctive discontinuities in isotopic composition that appear to be related to surface and subsurface structural features within the basement. One such discontinuity has previously been identified using data for the Newer Volcanics of the Western District Province of Victoria. Lithospheric blocks present beneath southern Victoria range in age from NeoProterozoic or Cambrian to Palaeozoic and some of the lowest 87Sr/86Sr ratios are observed in basalts erupted above an older basement unit (the Selwyn Block). The inference is that there is some form of lithospheric control on basaltic magma chemistry and since a substantial proportion of Older Volcanics have the geochemical characteristics of primary magmas (high Mg# and moderate to high abundances of Ni and Cr), this could indicate that magmas have been sourced from regionally heterogeneous, variably metasomatised, sub-continental lithospheric mantle. Neither the temporal and spatial relationships of the magmatic activity that followed continental breakup nor the uplift history of the south-eastern Australian passive margin are readily explained in terms of deep mantle plume

⁎ Corresponding author at: 13 Parkwood Street, Alfredton, Victoria, 3350, Australia. E-mail address: [email protected] (R.C. Price).

http://dx.doi.org/10.1016/j.lithos.2014.07.027 0024-4937/© 2014 Elsevier B.V. All rights reserved.

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tectonic models. Edge-driven convection across the irregular base of the southern Australian lithosphere, within the asthenosphere offers an elegant explanation for the longevity of the magmatic activity, its distribution, the small magma volumes involved and the uplift history as well as the geochemical variation observed in the eruptives. © 2014 Elsevier B.V. All rights reserved.

1. Introduction From late Jurassic time the eastern and south-eastern margins of Australia were affected by rifting and then slow spreading that heralded the breakup of eastern Gondwana and preceded rapid opening of the Tasman Sea and Southern Ocean (e.g. Gaina et al., 2003; Mutter et al., 1985; Veevers, 1986; Weissel and Hayes, 1977). Magmatism is commonly associated with lithospheric extension at evolving passive margins (Planke et al., 2000) and this was the case during the fragmentation of eastern Gondwana. From the very earliest stages of initial rifting, intraplate basaltic rocks were emplaced adjacent to the margin of the Australian continent (Holford et al., 2012) and this activity has continued to virtually the present day, Magmatic CO2 in mineral springs of central Victoria (Cartwright et al., 2002) and low seismic velocities beneath western Victoria (Graeber et al., 2002; Rawlinson and Fishwick, 2012) could be indications that region still has the potential to be magmatically active. The products of this prolonged magmatic activity are lava fields, shallow intrusions and larger volcanoes and volcanic complexes ranging in age from Mesozoic to Cenozoic and these are exposed in a belt that extends for around 4400 km from far north Queensland to Tasmania (Johnson, 1989; Vasconcelos et al., 2008). Active spreading that led to Gondwanan breakup was preceded by a period of faulting and rifting associated with lithospheric extension and when Australia began to separate from Zealandia and Antarctica, the rate of separation was initially relatively slow (~ 6 mm/year; Weissel and Hayes, 1977). Opening of the Tasman Sea was largely completed between 80 and 50 Ma (Weissel and Hayes, 1977). Along the Southern Ocean segment of the rifted Australian margin, the transition from rifting to spreading is likely to have taken place around 50 Ma (Mutter et al., 1985). Rapid spreading (average around ~5 to 6 cm/year) commenced in the Southern Ocean around 40 Ma ago and continues today. In Victoria and into south-eastern South Australia, one of the consequences of this prolonged tectonic reorganisation was relatively low volume and intermittent magmatic activity over a wide area. Basaltic rocks, ranging in age from Late Jurassic to Holocene are distributed across the state of Victoria (Fig. 1) over an area covering more than 700 km from east to west and around 300 km north to south. Jurassic aged igneous rocks have been identified as surface outcrops in far western Victoria (Day, 1983; McDougall and Wellman, 1976) and in drill holes and seismic profiles from onshore and offshore sedimentary basins (Holford et al., 2012). A breccia pipe with kimberlitic affinities at Meredith in central Victoria also has a Jurassic age (Day, 1983; Ferguson, 1980), as do alkalic dikes that have been found in outcrop and in the subsurface across central and eastern Victoria (e.g. McDougall and Wellman, 1976; Soesoo et al., 1999). This magmatic activity was related to the earliest, rifting stages of Gondwanan breakup. Widespread volcanic activity commenced ~95 Ma ago and continued intermittently through the Cenozoic with the youngest rocks being emplaced b 10 ka ago. A major hiatus occurred between 77 and 62 Ma and another in the time interval 18 to 7 Ma with major peaks in activity at 45 to 37 Ma, 22 Ma and 3 to 2 Ma (Figs. 1 and 2) (e.g. Gray and McDougall, 2009; McDougall et al., 1966; Price et al., 2003b; Wellman, 1974; Wellman and McDougall, 1974). These volcanic rocks have traditionally been subdivided into chronological and spatially different groups (e.g. Edwards, 1938; Hills, 1938; Singleton and Joyce, 1969). Basaltic rocks with ages in the range 95 to 18 Ma have been referred to as the “Older Volcanics” to differentiate them from more volumetrically extensive and younger (b 4.6 Ma) basaltic lava fields and cones of the “Newer

Volcanics” or Western District Province (Fig. 1), 7 to 5 Ma felsic volcanoes and associated mafic lavas of the Macedon-Trentham Province (Fig. 1) (Price et al., 2003b) and leucitites and basalts of the Cosgrove–Euroa area (Fig. 1) in central Victoria, which are estimated to have been emplaced 10 to 5 Ma ago (Paul et al., 2005). Outside the Western Districts Province, Newer Volcanics have been identified at only one locality in eastern Victoria; isolated outcrops of basalt with an age estimated at 4 to 2 Ma have been assigned by Sutherland et al. (2003) to a separate Uplands Province (Fig. 1). The Western District Province, which covers over 15,000 km2 of Victoria, comprises an extensive plain of thin (b 50 m) basaltic lava flows on which basaltic cinder cones, lava shields and maar volcanoes have been emplaced (Boyce, 2013; Hills, 1938; Price et al., 1997; Singleton and Joyce, 1969; Sutherland et al., 2014). The “plains” basalts range in age from 4.6 Ma to b 1 Ma with a volumetric peak in eruptive activity occurring between 3 and 1.8 Ma (Gray and McDougall, 2009). “Cones” basalts are generally less than a few hundred thousand years old and some were erupted b 5 to 10 ka ago. Although this paper is concerned primarily with the older, 95 to 18 Ma eruptives (Older Volcanics), where appropriate and relevant, data for these have been integrated with those available for the plains and cones of the Western District Province (Newer Volcanics), the basalts of the Cosgrove–Euroa area and the Uplands Province of eastern Victoria and this information has been used to evaluate petrogenetic models for the complete suite of Victorian late Mesozoic and Cenozoic basaltic eruptives. Using geochemical data for the Western District Province, Price et al. (1997) suggested that magma chemistry has been influenced by lithospheric structure (see also Price et al., 2003b) and new analyses for the Older Volcanics presented in this paper provide the opportunity to extend both the spatial and temporal scope for assessing the relationship between magma geochemistry and lithospheric influence in the petrogenesis of Victorian intraplate basalts. 2. The Older Volcanics: intraplate basaltic volcanism in Victoria between 95 and 18 Ma On the basis of geographic distribution, geochronology and petrology the Older Volcanics of Victoria have been grouped into a number of provinces and sub-provinces (Day, 1983, 1989; Price et al., 2003b; Wellman, 1974; see Fig. 1). Each of the sub-provinces is a geographic entity comprising the remnants of lava fields, monogenetic cones and/ or shallow intrusions. Age ranges within individual sub-provinces vary from 1 to 19 Ma (Fig. 2). The physical volcanology and petrography of basaltic rocks of the Older Volcanics have been described by Day (1983, 1989) and a summary is provided in Table 1 (see also Price et al., 2003b). The rock classification scheme used here is that of Johnson and Duggan (1989). Older Volcanics vary in composition from nephelinites, SiO2-undersaturated basanites, basalts and hawaiites through transitional basalts to hypersthene and quartz normative tholeiites. In contrast, Newer Volcanics are dominated volumetrically by tholeiitic and transitional types but the youngest eruptives tend to be SiO2-undersaturated (Frey et al., 1978; Irving, 1971; Irving and Green, 1976; Price et al., 1997, 2003b). 3. Geological and tectonic context of Mesozoic and Cenozoic intraplate basaltic volcanism in Victoria The intraplate basalts of Victoria were emplaced on Palaeozoic basement rocks that are part of the Tasman Orogenic system (Glen, 1992; Gray et al., 2003), which is in turn made up of three accreted belts or

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Fig. 1. Distribution of Mesozoic and Cenozoic intraplate volcanic rocks in Victoria, south-eastern Australia. a: Shows distribution of Older (95 to 18 Ma) and Newer (b4.6 Ma) Volcanics and boundaries of provinces defined by Price et al. (2003b). M-T is 7 to 5 Ma Macedon-Trentham Province (Price et al., 2003b and references therein), C–E includes 10 to 5 Ma (?) basalts of the Cosgrove–Euroa area (Paul et al., 2005) and U is the 4 to 2 Ma Uplands Province of eastern Victoria (Sutherland et al., 2003). M is city of Melbourne and Me location of Meredith. b: Shows sub-provinces among Older Volcanics (Day, 1983; Price et al., 2003b). Ge: Gelantipy; Bo: Bogong; To: Toombullup; Ho: Howitt; Ab: Aberfeldy; Ne: Neerim; La: Latrobe; Th: Thorpdale; Po: Poowong; Fl: Flinders; Me: Melbourne; Ba: Ballan Graben; Gel: Gellibrand.

orogens that are progressively younger from west to east. These are: the early Palaeozoic Delamerian, the early to mid-Palaeozoic Lachlan/ Thompson and the early to late Palaeozoic New England Orogens (Glen, 1992; Gray and Foster, 1998; Gray et al., 2003). The exposed basement rocks are largely turbidite fold/thrust sequences and greenstone belts that are metamorphosed to varying degrees and have been extensively intruded by granite (Gray et al., 2003; White and Chappell, 1988). Nine structural zones have been

identified (Fig. 3). Each of these is bounded by major faults or fault zones and each has a particular deformational and metamorphic history (Gray and Foster, 1998; Gray et al., 2003). One of the most significant boundaries, between the Delamerian and Lachlan Orogens (the Glenelg and Stawell structural zones) is defined by the Moyston Fault (Fig. 3), which Price et al. (1997, 2003b) suggested coincides with a geochemical transition (termed the “Mortlake discontinuity”) in the Western District Province.

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at depth of Proterozoic or early Palaeozoic, continent-like lithosphere (Cayley et al., 2002, 2011; Chappell et al., 1988; Glen and VandenBerg, 1987; Gray, 1990; Rossiter and Gray, 2008; Willman et al., 2010). This has been termed the Selwyn Block (VandenBerg et al., 2000) and it has been argued that it formed a rigid massif over which the various Palaeozoic structural zones were thrust and deformed (Cayley et al., 2002, 2011). The proposed location of the Selwyn Block in relation to the distribution of Mesozoic and Cenozoic intraplate basalts and structural elements in Victoria is shown in Fig. 3.

4. Methods

Fig. 2. Age distribution for late Mesozoic and Cenozoic igneous rocks of Victoria. a: Is a histogram showing age distribution of igneous rocks with ages b110 Ma. b: Shows details of ages for individual provinces and sub-provinces (Fig. 1). Geochronological data are from: McDougall et al. (1966); Aziz-ur-Rahman and McDougall (1972); Wellman (1974); Abele and Page (1974); Bowen (1975); McDougall and Gill (1975); McDougall and Wellman (1976); Singleton et al. (1976); Day (1983); Ollier (1985); Ewart et al. (1985); Wallace (1990); Cohen et al. (2008); Gray and McDougall (2009).

Across most of the state, the Palaeozoic basement sequences are postulated to lie on oceanic crust (e.g. Gray and Foster, 1998; O'Halloran and Rey, 1999) but in central Victoria there is evidence for the existence

Major and minor elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P, and S) were determined as oxide components by X-ray fluorescence (XRF) using lithium borate glass discs and methods similar to those described by Norrish and Hutton (1969). For these elements precision is generally better than ±1% (1σ). FeO abundances were measured by direct titration using a standardised CeSO4 solution and H2O and CO2 by a gravimetric method. Selected trace elements were determined by XRF on pressed powder pellets (Norrish and Chappell, 1977) and for these elements, theoretical detection limits are of the order of 1 to 2 ppm and reproducibility is better than ±5% (1σ). Rare earth elements (REE) and Cs, Pb, Th, U, Nb, Hf, Y and Sc were analysed by inductively coupled plasma mass spectrometry (ICP-MS) at the VIEPS Trace Element Laboratory at Monash University. All sample preparation was carried out in class 350 clean air cabinets. Sample powders were dissolved in HF and HNO3 and then refluxed in HNO3 to remove fluorides. The residue was then taken up in 50 ml of 2% HNO3. Aliquots of these solutions were diluted (dilution factor of ~ 2000) and an internal standard added (100 ppb iridium). USGS standards BHVO-1 and AGV-1 were used as calibration standards and additional analyses of BHVO-1 were carried out to assess accuracy during the analysis procedure. Analyses were made using a VG PlasmaQuad PQ2+ in peak-jumping mode. The total analytical blank (chemistry and mass spectrometry) is b10 ppb for all elements. Precision is typically better than 5%, with accuracy, based on analysis of repeat analyses of BHVO1 being, for most elements, better than 5% at the 95% confidence level.

Table 1 Older Volcanics of Victoria, Australia. Provinces and sub-provinces: age, field occurrence, rock types and petrography. Sub-province/province

Age (Ma)

Field occurrence

Rock types

Thorpdale Melbourne Neerim Gellibrand Aberfeldy Gelantipy Bogong Howitt Toombullup Flinders Otway Basin Latrobe Ballan Graben Poowong

22 22–18 26–22 28–27 28–26 42–38 36–30 35–32 43–36 49–39 58, 31 57–55 79, 63–53 95–85

Lava field and valley filling flow remnants Valley filling flow remnants Lava fields and valley filling flow remnants Shallow intrusions (sills and plugs) Valley filling flow remnants Valley filling flow remnants Valley filling flow remnants, dykes, plugs Valley filling flow remnants Valley filling flow remnants 600 m thick lava flow sequences, extensive (1000 km2) lava field, dykes Sub-surface lava flows and intrusions Surface and (mainly) sub-surface lava flows and tuffs 275 m thick lava flow sequence, dykes Remnant lava flows and shallow intrusions

TBa, Ba, Ol-Th, minor Ha and Bas TBa, Ba TBa, Ha, Ol-Th, Bas, minor Neph TBa and Ol-Th Tba, Ba, Ha Q-Th, Ol-Th Bas, TBa, Ba, Ha Ha, Ba, TBa also Neph Ba, Bas, Ha, TBa and Ol-Th Ba and Ha TBa and Ba also Ol-Th and Bas Bas, Ol-Th Ol-Th and TBa then Bas, Neph Ol-Th to Bas

Based on compilations of Day (1983, 1989); Price et al. (2003b). Classification scheme from Johnson and Duggan (1989): Abbreviations: Qz-Th = quartz tholeiite, Ol-Th = olivine tholeiite, TBa = transitional basalt, Ba = alkali basalt, Bas = basanites, Neph = nephelinite, Ha = hawaiite. Olivine and quartz tholeiites: Typical lavas consist of phenocrysts of Ol and Pl and, less commonly Cpx, in a fine grained groundmass of Cpx, Pl, FeOx (Mt ± Ilm) and brown glass. Intrusive rocks are coarser grained (dolerite) and have ophitic textures. Transitional basalt and alkali basalt: In a typical lava flow, Cpx and Ol phenocrysts are contained in a fine groundmass of Ol, Cpx, Pl, Mt and glass. Some examples contain larger crystals or xenocrysts (from ultramafic xenoliths) of Ol and/or Cpx. Basanite: Basanite lavas commonly consist of Ol and (less commonly) Cpx phenocrysts in a groundmass of Ol, Cpx, Pl and Mt. Glass in a minor phase in some examples and Neph is present in others. Xenocrysts and/or megacrysts of Ol and Cpx (also Ilm) occur in some basanites and lherzolite xenoliths occur in others. Nephelinite: A typical nephelinite consists of phenocrysts of Ol and Cpx (Titan-augite) in a groundmass of Ol, Cpx, Mt, Neph and/or analcime and Ap. Some nephelinites contain small amounts of interstitial amphibole. Xenocryst of Ol and Cpx are common and some nephelinites contain small lherzolite xenoliths. Hawaiite/Nepheline hawaiite: Similar to alkali basalt but usually contains less calcic plagioclase. Abbreviations: Ol = olivine, Cpx = clinopyroxene, Pl = plagioclase, FeOx = iron oxide (Mt — magnetite, Ilm — ilmenite), Neph = nepheline, Ap = apatite.

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Fig. 3. Map and cross section showing distribution of late Mesozoic and Cainozoic intraplate volcanics (Older and Newer Volcanics) in Victoria in relation to major faults and Palaeozoic structural zones (Gray et al., 2003). SB indicates the inferred margins of the Selwyn Block (Cayley et al., 2002, 2011). Major faults are: M — Moyston; Av — Avoca; MW — Mt William; G — Governor; KK — Kancoona /Kiewa; T — Tallangatta Creek; GI — Gilmore /Indi; LM — Lucas Point /McLauchlan; CP — Combienbar /Pheasant Creek. Structural zones of western and central Victoria are, from west to east: Glenelg, Stawell (Stw.), Bendigo, Melbourne and Tabberabbera (Tabb.). The cross section is along the line A-A′. Isotopic and other data for the volcanic rocks are projected onto this line in Fig. 9. M is city of Melbourne.

Samples for Sr, Nd and Pb isotopic analyses (approx. 100 mg whole rock powder) were leached with hot hydrochloric acid (6 M HCl, 100 °C, 30 min), followed by dissolution of the rinsed residues with HF, HNO3 and HCl on a hotplate. Pb was extracted first, using a double pass over small anion columns (0.1 ml resin, AG1-X8, 200–400, HBr-HCl chemistry). Pb blanks from this procedure are in the range 50–100 pg. The HBr eluate was collected, fumed with HNO3 and taken up in 1 M HCl for extraction of Sr and light rare earth elements on 4 ml cation exchange columns (AG50-X8, 200–400). The Sr fraction from this column was pure enough for mass spectrometry; Nd was purified using reverse phase ion exchange chromatography on HDEHPcoated Kel-F columns. Blanks of Sr (200 pg) and Nd (100 pg) were negligible. All isotopic data were obtained by thermal ionisation mass spectrometry (TIMS) using a 7-collector Finnigan-MAT 262 spectrometer. Strontium fractions were taken up in dilute phosphoric acid, loaded onto single Ta filaments and briefly glowed to drive off H3PO3. Data were collected in static mode with signals of 2–4 10−11A 88Sr and normalised to 86Sr/88Sr = 0.1194. Typically, 5 to 7 blocks of 10 × 8 s integrations produced in-run precisions of ± 0.003% (2se). Data are reported relative to SRM987 = 0.710230. Neodymium fractions were picked up in 1 M HNO 3 doped with H3PO 3 and loaded onto the Ta side of a Ta–Re double filament assembly. Data were collected in static mode with signals near 2 10− 11 A 144 Nd and normalised to 146 Nd/144Nd = 0.7219. Typically, five to seven blocks of 10 × 8 s integrations produced in-run precisions of ± 0.0025% (2se). Data are reported relative to La Jolla Nd = 0.511860. USGS basalt BCR-1 averaged 0.70500 ± 4 (2σ; n = 6) and 0.512634 ± 18 (2σ, n = 7), while BHVO-1 averaged 0.70348 ± 6 (2σ, n = 19) and 0.512989 ± 13 (2σ, n = 5). This suggests external precisions (reproducibility, 2σ) of ± 0.000040–0.000060 (Sr) and ±0.000020 (Nd). Modern CHUR has the composition 147Sm/144Nd = 0.1967, 143Nd/144Nd = 0.512638. Lead

fractions were loaded onto single Re filaments using silica gel and H3PO4. Data were collected at filament temperatures of 1250 to 1350 °C which produced 208Pb ion currents of 1 to 4 × 10−11A. Typically, three blocks of 10 × 8 s integrations produced in-run precisions of 0.05% (2se). Instrumental mass bias was corrected using a global mass bias factor of 0.109%/amu derived from numerous analyses of SRM981 under similar conditions. External precision (2σ, n = 78) for SRM981 was ± 0.097% (206Pb/204Pb), ± 0.130% (207Pb/204Pb) and ± 0.175% (208Pb/204Pb). The externally-derived mass bias factor given above was confirmed (within uncertainties) for Pb extracted from basalts using the double spike method of Woodhead et al. (1995). Given the 95 to 18 Ma eruption ages of the Older Volcanics, age corrections were applied using parent/daughter ratios calculated from trace element data. The decay constants used were 87Rb 1.395 10−11/yr, 147 Sm 6.54 10−12/yr, 238U 0.155125 10−9/yr; 235U 0.98485 10−9/yr; 232 Th 0.049485 10−9/yr. In general, age corrections result in relatively limited changes to isotopic ratios; the differences between measured and initial (age corrected) 87Sr/86Sr and 143Nd/144Nd isotopic ratios are up to 0.0001 and 0.00008 respectively. 5. Whole rock geochemistry 5.1. Major and trace element compositions The Older Volcanics of Victoria tend to be undersaturated with respect to SiO2 (Table 2 and Fig. 4a) but compositions vary widely from nepheline normative olivine nephelinites and basanites through transitional basalts and hawaiites to hypersthene and quartz-normative tholeiites (classification of Johnson and Duggan, 1989; see Day, 1989; Price et al., 2003b). Around two thirds of analysed Older Volcanics samples are nepheline normative. In contrast, ~30% of analysed samples from the Western District Province (Newer Volcanics) are nepheline

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Table 2 Whole rock analyses of Older Volcanics of Victoria, Australia. 2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

G14

G21

70/1328

B10

B26

H18

H28

H38

T3

T7

T10

69/1468

70/140

TH29

TH39

TH42

70/ 143

70/ 145

N22

TH4

TH18

TH20

TH21

Subprovince

G

G

B

B

B

H

H

H

T

T

T

L

L

L

L

L

N

N

N

TH

TH

TH

TH

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 Mg# (Q + Hy)-Ne Cs Ba Rb Sr Pb Th U Zr Nb Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Sc V Cr Ni Cu Zn Ga

47.50 1.98 13.97 11.87 0.18 10.33 9.09 3.35 1.16 0.55 64.67 −3.0 0.42 223 21 631 2.6 2.3 0.8 170 36.5 20.7 23.4 47.7 6.1 25.4 5.92 1.98 5.75 0.81 4.14 0.72 1.68 0.22 1.36 0.19 17.9 165

51.27 1.81 15.07 10.60 0.17 7.98 8.60 3.54 0.69 0.26 61.29 13.8 0.53 119 15 397 2.6 1.4 0.5 108 13.2 21.9 10.5 22.9 3.2 14.4 4.06 1.44 4.40 0.70 3.92 0.75 1.90 0.27 1.70 0.23 21.3 180

46.65 1.68 14.87 10.43 0.18 10.42 10.66 3.08 1.20 0.82 67.75 −4.7 0.53 504 19 907 7.6 7.5 2.0 205 57.4 28.5 59.3 99.2 10.8 37.5 6.73 2.13 6.38 0.91 4.98 0.96 2.49 0.36 2.32 0.35 24.7 177

46.33 1.76 14.47 10.49 0.18 11.20 10.38 3.26 1.05 0.87 69.17 −5.5 0.65 560 19 902 7.1 7.7 1.9 204 74.2 27.6 61.0 105.6 11.6 40.2 7.16 2.21 6.58 0.91 4.89 0.94 2.40 0.34 2.15 0.32 24.0 172

46.37 1.97 15.14 11.76 0.19 9.75 10.30 2.85 0.94 0.71 63.55 −2.0 0.25 417 13 853 3.0 3.0 0.9 170 50.0 23.2 35.5 69.6 8.3 31.2 6.10 1.99 5.66 0.81 4.32 0.81 2.02 0.28 1.76 0.25 19.9 195

45.11 2.25 14.77 11.71 0.19 9.78 10.61 3.02 1.46 1.09 63.73 −6.1 0.41 628 28 1147 4.8 5.5 1.5 275 81.9 27.8 59.4 112.5 13.0 47.0 8.29 2.64 7.34 0.99 5.19 0.97 2.34 0.33 2.05 0.30 22.2 197

43.89 2.73 14.38 11.06 0.20 9.03 10.00 4.34 2.61 1.75 63.19 −16.6 0.90 1145 45 1677 9.0 11.3 3.0 517 208.1 34.6 108.1 196.0 22.6 78.0 12.89 3.84 10.52 1.38 6.85 1.19 2.82 0.37 2.29 0.33 17.0 168

238 66 117 19

173 74 115 19

44.98 2.34 15.20 10.89 0.21 9.39 10.49 3.47 1.43 1.58 64.45 −7.1 0.18 703 21 1389 5.7 6.9 1.9 266 87.8 28.1 76.7 143.3 16.2 55.9 9.0 2.79 7.80 1.04 5.20 0.97 2.37 0.33 2.04 0.31 21 145 253 109 48 84 19

222 61 89 17

243 69 86 17

207 85 100 18

49.54 1.58 15.50 11.15 0.17 9.00 8.54 3.62 0.62 0.29 62.92 3.5 0.08 236 10 405 2.3 2.0 0.4 138 30.1 20 17.6 35.9 4.46 17.53 3.96 1.42 4.14 0.63 3.51 0.69 1.75 0.25 1.61 0.23 21 146 293 203 101 114 20

45.96 2.24 14.28 11.34 0.18 10.58 10.51 2.74 1.13 1.04 66.25 −2.3 0.40 713 16 1038 5.3 7.2 1.8 230 87.7 26 67.5 120.1 13.74 49.67 8.59 2.78 7.56 1.00 4.98 0.92 2.17 0.29 1.78 0.26 24 178 292 193 73 111 19

47.38 2.15 14.69 10.77 0.18 8.45 9.44 4.24 1.50 1.21 62.27 −7.3 0.96 570 23 1109 4.8 5.7 1.7 290 92.2 28 64.2 122.4 14.40 52.42 9.40 2.97 8.25 1.07 5.42 0.98 2.30 0.31 1.89 0.28 19 123 218 153 78 115 21

48.65 2.00 14.96 10.69 0.18 9.55 9.12 2.73 1.58 0.55 65.26 3.9 0.98 310 36 1118 4.2 3.9 1.7 179 49.7 23.8 28.1 53.7 6.2 25.3 5.75 1.57 5.49 0.85 4.33 8.39 2.02 0.29 1.77 0.29 24.5 194 306 216 67 92 20

46.55 2.25 14.89 11.00 0.19 9.94 9.82 2.96 1.67 0.73 65.52 −4.2 0.68 339 34 1006 3.6 4.4 2.3 249 62.9 24.6 35.8 68.3 7.6 31.0 6.69 1.84 6.41 0.90 4.69 0.87 2.12 0.30 1.90 0.33 22.7 176 265 227 69 87 21

49.11 1.90 14.66 10.44 0.33 10.35 8.85 2.66 1.28 0.44 67.57 9.5 0.62 266 16 520 4.2 3.4 1.1 190 34.8 26.4 25.2 47.9 5.8 24.6 5.30 1.64 5.51 0.83 4.66 0.95 2.47 0.37 2.46 0.40 24.1 186 313 245 64 98 20

48.11 1.90 14.10 11.61 0.18 10.99 8.66 2.76 1.21 0.48 66.56 4.0 6.86 275 22 552 3.0 2.1 0.6 137 28.3 17.4 22.0 42.7 5.2 21.6 4.34 1.39 4.33 0.63 3.23 0.63 1.48 0.23 1.37 0.21 17.8 146 241 215 51 86 19

48.20 1.45 10.98 11.08 0.20 14.32 10.89 1.90 0.65 0.34 73.10 8.5 0.42 377 29 1436 2.7 2.5 0.6 105 25.9 18.2 20.2 38.4 4.6 19.9 4.39 1.43 4.26 0.63 3.60 0.68 1.62 0.25 1.47 0.23 33.3 216 915 255 70 100 21

48.49 2.11 15.96 11.40 0.19 7.52 8.54 3.40 1.55 0.85 58.10 0.1 0.93 388 31 856 4.4 5.1 2.2 275 61.5 25.2 43.8 82.1 8.8 34.7 7.10 1.99 7.15 1.00 5.07 0.96 2.41 0.32 2.06 0.33 20.4 149 149 114 67 109 22

48.58 1.78 13.96 10.76 0.16 9.20 11.21 3.10 0.84 0.40 64.26 −1.9 0.57 305 16 529 2.4 2.6 1.6 152 31.8 23.2 22.7 46.6 5.8 24.5 6.17 1.77 6.01 0.94 4.79 0.91 2.12 0.29 1.76 0.28 22.2 158 315 207 52 108 23

48.32 2.25 15.15 11.20 0.19 8.15 8.54 3.50 1.64 1.07 60.46 −0.8 0.91 534 29 710 4.5 5.4 1.5 289 73.9 31.5 51.5 97.3 11.1 41.9 8.03 2.33 8.67 1.05 5.51 1.01 2.60 0.59 2.29 0.35 23.3 178 230 77 53 83 23

46.60 2.34 14.40 12.73 0.20 8.64 8.88 3.73 1.76 0.72 58.78 −6.9 0.55 300 19 645 3.8 4.0 1.3 224 57.4 23.3 37.4 74.7 8.7 35.1 6.85 2.16 7.02 0.94 5.03 0.88 2.07 0.28 1.68 0.24 17.6 177 172 332 80 75 18

47.43 2.11 13.75 12.45 0.22 10.61 9.46 2.44 1.04 0.49 64.19 6.2 0.18 215 13 558 2.8 1.9 0.6 149 30.8 18.8 21.4 42.6 5.2 22.1 4.82 1.49 4.82 0.71 3.65 0.66 1.67 0.22 1.40 0.20 21.2 194 346 261 70 102 22

48.43 1.73 14.43 11.19 0.18 10.46 9.74 2.39 1.11 0.34 66.27 7.8 0.33 172 13 639 4.3 2.4 0.7 137 25.3 22.0 18.0 34.2 4.2 18.2 4.17 1.33 4.44 0.68 3.94 0.81 1.94 0.30 1.91 0.28 24.3 186 349 316 46 87 15

48.77 1.73 14.29 11.08 0.18 11.08 8.91 2.52 1.03 0.40 67.78 11.0 0.43 241 22 479 3.4 3.0 0.8 128 33.5 22.3 21.9 42.0 5.0 21.1 4.68 1.49 4.72 0.72 4.10 0.84 2.09 0.31 1.96 0.29 24.4 190 329 241 66 90 17

198 73 100 18

177 47 112 22

Italics indicate analyses by inductively coupled plasma source mass spectrometry. Other elements by x-ray fluorescence spectroscopy. Major element analyses normalised to 100%. FeO* = total iron as FeO. [(Q + Hy)-Ne] = normative quartz + hypersthene-nepheline. Mg# = 100 ∗ Mol. MgO / (MgO + FeO) assuming Fe3+/Fe2+ = 0.2. Sub-provinces: G = Gelantipy; B = Bogong; H = Hotham; T = Toombullup; L = Latrobe; N = Neerim; Th = Thorpdale; A = Aberfeldy; P = Poowong; F = Flinders; M = Melbourne; BG = Ballan Graben; O = Otway.

R.C. Price et al. / Lithos 206–207 (2014) 179–200

1 Sample#

Table 2 Whole rock analyses of Older Volcanics of Victoria, Australia. 25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

TH22

A7

A10

F10

69/ 1465

69/ 1466

F2(A)

F3(C)

F5(A)

69/ 1457

M3

BP1

69/ 1454

BG2

BG4

BG10

BG11

BG17

BG35

BG37

16

19

41

54

92

TH

A

A

P

F

F

F

F

F

M

M

M

BG

BG

BG

BG

BG

BG

BG

BG

O

O

O

O

O

47.83 1.69 14.18 11.33 0.18 10.86 9.54 2.94 1.04 0.40 66.85 −1.2 0.33 182 17 489 2.7 2.4 0.8 128 31.3 22.1 21.8 39.1 4.8 20.4 4.54 1.48 4.73 0.71 4.08 0.82 1.99 0.30 1.91 0.29 22.7 174 312 250 59 92 19

48.48 46.17 50.23 48.67 1.89 2.27 2.00 1.98 16.22 14.96 16.31 15.47 10.71 12.41 9.64 10.93 0.18 0.20 0.16 0.17 6.88 9.14 6.47 8.40 10.49 8.97 9.00 9.46 2.97 3.38 4.32 3.30 1.46 1.47 1.45 1.27 0.72 1.03 0.42 0.33 57.47 60.75 58.52 61.78 −0.3 −3.9 −3.8 −1.5 0.50 0.39 0.74 0.58 515 517 249 232 23 17 31 35 978 1017 1019 563 6.9 6.2 2.4 2.4 6.0 6.1 2.4 1.9 2.4 2.4 0.8 0.7 211 206 172 140 59.5 69.6 38.0 32.0 23.9 21.7 24.8 24.9 42.8 54.4 21.8 17.2 76.6 97.3 43.9 35.6 8.1 10.0 5.3 4.4 31.4 39.0 22.1 19.0 6.42 7.69 4.82 4.65 1.87 2.16 1.69 1.61 6.58 7.63 5.00 5.24 0.95 1.04 0.76 0.76 5.02 5.17 4.12 4.42 0.97 0.93 0.82 0.86 2.52 2.35 2.05 2.12 0.35 0.32 0.30 0.30 2.31 1.98 1.85 1.79 0.39 0.32 0.28 0.25 22.5 18.7 23.0 29.4 168 156 223 240 159 190 198 331 57 177 122 136 35 58 53 63 85 105 108 89 21 22 18 22

45.96 2.57 14.30 11.35 0.19 11.36 8.48 3.94 1.33 0.53 67.79 −8.7 0.48 367 31 861 2.8 3.7 2.4 288 56.3 28.6 28.9 57.6 6.8 28.7 7.06 1.77 6.57 0.98 5.05 0.95 2.32 0.32 2.05 0.32 25.1 183 499 389 72 79 20

46.35 2.31 15.48 10.56 0.18 10.58 9.46 3.42 1.09 0.58 67.80 −5.2 0.34 286 21 768 2.6 3.0 1.0 217 48.5 29.3 27.9 56.5 6.8 27.6 5.55 1.87 5.65 0.84 4.58 0.90 2.35 0.34 2.22 0.33 25.7 218 272 234 63 76 19

46.90 47.14 47.43 2.20 2.04 2.08 15.51 14.59 14.76 9.94 10.40 11.86 0.18 0.18 0.19 10.45 12.28 9.42 9.19 9.13 9.17 3.64 2.51 3.01 1.34 1.27 1.54 0.64 0.46 0.54 68.85 71.29 62.55 −6.1 −0.2 −2.1 0.34 0.34 0.56 293 305 188 21 27 23 782 990 664 3.0 2.8 2.9 3.9 3.0 1.6 1.7 1.5 0.3 304 331 165 49.5 32.0 30.8 31.3 31.6 26.5 31.1 22.4 23.8 61.4 47.5 48.3 7.3 3.0 6.1 29.1 26.2 26.4 6.06 6.58 4.99 1.93 1.84 1.76 6.08 6.74 5.73 0.90 0.98 0.76 4.99 5.16 4.66 0.97 0.99 0.90 2.57 2.50 2.22 0.39 0.36 0.32 2.46 2.32 2.09 0.39 0.36 0.29 24.3 22.9 19.9 201 197 202 350 454 301 222 157 204 69 75 80 78 117 111 19 25 22

48.70 1.68 13.83 12.06 0.17 10.80 8.79 2.83 0.81 0.33 65.32 8.3 0.90 197 27 346 3.7 3.5 0.7 121 26.7 25.4 19.2 36.6 4.6 18.9 4.26 1.38 4.81 0.78 4.42 0.92 2.36 0.34 2.18 0.33 27.7 184 327 217 49 85 17

47.32 48.76 48.07 1.92 1.99 1.76 13.69 14.07 15.71 12.00 12.13 10.32 0.19 0.20 0.17 11.63 9.93 9.32 9.73 8.52 9.03 2.25 2.80 3.60 0.86 1.17 1.61 0.41 0.44 0.43 67.08 63.26 65.51 7.1 9.4 −5.1 1.57 0.28 0.38 280 266 341 20 22 23 454 419 625 3.1 3.4 2.4 3.2 2.6 2.1 0.9 0.4 0.7 128 168 140 34.9 30.5 33.4 23.3 23.9 18.2 24.5 24.2 16.9 48.1 45.4 33.9 5.5 6.1 4.3 22.9 23.9 17.9 4.87 5.73 4.17 1.61 1.74 1.39 5.38 5.52 4.05 0.78 0.88 0.61 4.33 4.69 3.41 0.84 0.91 0.68 2.05 2.09 1.62 0.29 0.29 0.23 1.74 1.84 1.54 0.25 0.26 0.23 24.6 13.6 20.3 191 137 179 337 302 298 273 210 204 77 42 71 99 111 86 20 23 21

49.31 47.75 46.61 2.11 2.41 2.11 15.18 14.49 14.52 11.21 11.14 10.56 0.17 0.17 0.19 8.98 9.94 10.37 8.50 8.89 10.73 3.05 3.29 3.17 1.13 1.48 1.11 0.37 0.43 0.63 62.74 65.23 67.38 8.9 −2.9 −5.3 0.22 1.50 0.33 161 226 447 8 15 27 478 572 813 2.8 3.0 4.1 1.6 2.3 4.5 0.6 0.9 1.0 166 204 274 18.0 31.0 91.8 21.8 21.2 26.3 15.8 21.4 45.6 32.9 45.2 82.6 4.3 5.5 9.1 18.2 23.4 36.5 4.73 5.21 6.16 1.43 1.70 2.10 4.71 5.42 6.56 0.70 0.80 0.83 3.78 4.16 4.64 0.73 0.78 0.84 1.79 1.84 2.08 0.25 0.25 0.29 1.59 1.48 1.90 0.25 0.21 0.28 21.6 23.0 28.5 193 204 253 282 309 540 181 242 270 61 67 85 105 97 92 24 23 21

42.51 43.38 42.25 45.41 41.98 2.62 2.77 2.90 2.54 2.83 12.34 13.20 12.77 15.26 13.24 12.09 11.96 12.48 10.51 11.72 0.22 0.21 0.22 0.20 0.20 13.18 12.43 12.53 8.67 12.15 11.30 11.63 11.78 10.49 11.71 3.56 2.77 3.20 4.48 4.10 0.71 0.46 0.69 1.25 0.85 1.47 1.17 1.20 1.19 1.22 69.63 68.60 67.86 63.44 68.57 −11.9 −5.9 −11.0 −12.2 −17.1 1.37 1.60 2.73 0.82 0.58 727 672 794 602 538 10 24 14 10 14 1540 1369 1558 1214 1380 9.0 6.4 6.9 6.5 6.2 16.4 10.9 9.7 8.6 10.9 5.3 4.2 2.8 2.9 2.7 342 258 333 523 300 135.8 111.1 150.7 132.6 122.0 33.8 29.5 35.1 37.9 34.4 98.1 81.3 86.2 72.7 74.6 172.7 142.9 162.9 134.7 127.3 17.1 14.3 17.2 14.3 13.9 65.4 54.8 68.2 56.9 55.5 11.59 16.18 11.46 9.97 9.11 2.99 2.68 3.37 2.87 2.84 11.54 9.77 11.62 9.78 9.49 1.45 1.24 1.44 1.27 1.26 6.69 5.84 6.73 6.40 6.43 1.16 1.00 1.13 1.16 1.17 2.93 2.51 2.82 2.92 2.91 0.38 0.32 0.35 0.40 0.39 2.33 1.96 2.15 2.67 2.42 0.39 0.32 0.32 0.40 0.37 24.9 26.6 24.8 26.7 29.9 179 209 218 179 229 405 337 299 237 301 336 329 274 155 263 50 59 54 57 60 91 89 87 88 82 19 18 19 20 20

47.70 52.70 48.40 2.05 1.72 2.28 15.14 15.84 14.44 10.39 9.11 11.20 0.18 0.14 0.16 10.79 5.92 8.74 8.97 9.84 9.42 2.70 3.71 3.21 1.54 0.75 1.58 0.55 0.27 0.59 68.58 57.73 62.13 −0.2 14.3 −1.6 0.31 0.22 1.99 475 238 387 28 18 36 781 409 760 5.0 2.0 3.0 5.7 4.1 3.1 1.6 0.7 0.8 213 113 184 62.3 18.6 42.6 26.0 19.3 21.8 35.5 18.1 27.6 61.8 36.1 53.0 6.8 5.0 6.5 26.7 23.0 28.1 5.47 7.12 5.89 1.72 2.52 1.94 5.67 8.31 5.93 0.84 1.27 0.85 4.63 7.12 4.29 0.93 1.36 0.76 2.38 3.34 1.82 0.35 0.45 0.25 2.35 2.59 1.50 0.36 0.38 0.23 24.4 23.3 19n8 193 156 172 388 368 250 214 64 199 45 35 48 84 95 119 19 24 23

R.C. Price et al. / Lithos 206–207 (2014) 179–200

24

185

186

R.C. Price et al. / Lithos 206–207 (2014) 179–200

Fig. 4. (a) Normative [Quartz (Q) + Hypersthene (Hy) − Nepheline (Ne)] versus wt% SiO2 for Victorian basalts. Normative compositions have been calculated on an anhydrous basis and assuming an Fe3+/Fe2+ ratio of 0.2. Older Volcanics (95–18 Ma) are compared with plains and cones samples from the Newer Volcanics (b4.6 Ma). (b) Victorian basalt data projected onto the plagioclase saturated Diopside-Olivine-Nepheline pseudoternary phase diagram of Sack et al. (1987). The olivine-calcic pyroxene liquidus phase boundary is shown for pressures of 100 kPa and 0.83 GPa and the nepheline-olivine-calcic-pyroxene cotectic is for 100 kPa. Data for plains and cones basalts are from Frey et al. (1978), McDonough et al. (1985); Ewart et al. (1988); Wallace (1990); Price et al. (1997) and Paul et al. (2005).

normative, with ~ 80% of the volumetrically dominant plains basalts being SiO2 oversaturated. Approximately 60% of the samples from young cones of the Newer Volcanics are nepheline normative. These contrasts and similarities are highlighted when compositions are plotted into the plagioclase saturated diopside-olivine-nepheline pseudoternary projection (Fig. 4b). Collectively the Older Volcanics and the plains basalts define a broad array extending horizontally between the high and low pressure calcic-pyroxene-olivine liquidus phase boundaries across the diopside-olivine join towards the nepheline-calcic-pyroxene-olivine cotectic. Because they are relatively oversaturated with respect to SiO2, the plains basalts define a field overlapping with that defined by the Older Volcanics but extending out of the diagram away from the nepheline apex. Cones basalt compositions overlap the broad field defined by the other two groups but there is a scatter of data points towards the low pressure olivine-calcic-pyroxene phase boundary. Aspects of the major element variation within the Older Volcanics suite are illustrated in selected SiO2 variation diagrams shown in Fig. 5 and comparisons made with representative analyses from the Newer Volcanics. In the analysed Older Volcanics basalts, SiO2 content varies from 39.5 to 51.5 wt.%; a range that extends to lower SiO2 than is observed in either of the plains and cones suites of the Newer Volcanics. The Newer Volcanics data set does however include samples with higher SiO2 abundances (up to 54 wt.%). TiO2 contents in the Older Volcanics suite range from 1.4 to 2.7 wt.% and are negatively correlated with SiO2 content (Fig. 5a). On the TiO2

versus SiO2 variation diagram the Older Volcanics array overlaps with the trend defined by the plains data but it diverges from the field of the cones basalts; the latter have higher TiO2 contents at similar SiO2 contents. A similar divergence is observed on the Al2O3 versus SiO2 variation diagram (Fig. 5b). Al2O3 and SiO2 contents show an overall positive correlation, with Al2O3 contents in Older Volcanics ranging from 10.5 to 15.9 wt.%. The cones basalts define a trend from Al2O3 contents that are relatively low at low SiO2 content to values that are similar to those observed in the Older Volcanics suite at the highest SiO2 values. The plains basalts samples have Al2O3 contents that are generally lower than those observed in the Older Volcanics suite and they scatter along a trend with a slope that differs from those defined by the latter and by the cones suite. Both Mg# [100 ∗ mol.MgO/(FeO + MgO)] and CaO abundance show scattered negative correlations with SiO2 content (Fig. 5 c and d) and in each case the three suites (Older Volcanics, plains and cones) define overlapping trends with different, divergent slopes. Fig. 5e is a total alkalis (Na2O + K2O) versus SiO2 variation diagram in which the three suites define overlapping fields. The Older Volcanics and plains basalts have similar broad ranges of Na2O + K2O (2.4 to 6.7 wt.% for the Older Volcanics and 3.5 to 6.6 wt.% for the plain basalts) but the cones basalts suite includes samples with higher values (over 8 wt.%). The diagram also confirms that the majority of the Older Volcanics are alkalic rather than tholeiitic in composition. The variation of CaO/Al2O3 ratio with SiO2 content is similar for the three suites (Fig. 5d). The Older Volcanics and plains basalts define an overlapping trend of decreasing CaO/Al2O3 ratio with increasing SiO2 abundance but the cones samples lie on a steeper trend that extends to higher CaO/Al2O3 ratios at lower SiO2 and low ratios at higher SiO2 contents. Two Older Volcanics samples plot within the cones array. Four Older Volcanics samples, from the Ballan Graben sub-province, form a distinctive group with low SiO2 contents and, compared with other Older Volcanics samples, relatively high CaO/Al2O3 ratios. The trace element compositions of basaltic rocks of the Older Volcanics are generally characterised by enrichment of Cs, Ba, Rb, Th, U, Nb, K and light rare earth elements (REE) over heavy REE, Ti, Zr and Y but there is subtle diversity within and between particular sub-provinces. Primitive mantle-normalised extended element plots for samples with Mg# greater than 65 are shown in Fig. 6, where comparison is also made with equivalent patterns for Newer Volcanics with similar elevated Mg#. Basalts from the Thorpdale sub-province represent a common type of Older Volcanics trace element pattern (Fig. 6c). In these basalts, Y and Yb abundances are 2 to 3 times and La and Ce contents are 40 to 50 times those of primitive mantle and the normalised extended element pattern is slightly convex upward, with normalised abundances peaking at Nb and progressively falling among the more incompatible elements from Nb through U and Th to Rb and Ba. Basalts from the Gelantipy, Latrobe, Flinders, Melbourne and Otway sub-provinces have normalised extended element plots that are broadly similar to those of the Thorpdale basalts (compare sample G14 in Fig. 6a and b, d and f with Fig. 6c) but there is significant variation on this theme, with some samples having patterns characterised by spikes of relatively higher Zr, Sr, Nb and/or U abundances. In contrast to the trace element behaviour of this dominant group, a different type of pattern is observed in high Mg# basalts from the Bogong and Toombullup sub-provinces (Fig. 6a). These have heavy REE, Ti and Zr abundances that are similar to those observed in other high Mg# Older Volcanics but they are characterised by stronger relative enrichments in the more incompatible trace and minor elements including the middle and light REE, P, Sr, Pb, Nb, U, Th and Ba. Particularly striking features of the normalised extended element patterns for these basalts are strong relative depletions in K and Rb. The strength of the development of negative K anomalies in the normalised extended element plots can be expressed in terms of a parameter K/K* = [(K abundance normalised to primitive mantle) / ((Nb abundance normalised to primitive mantle + La abundance normalised to primitive

R.C. Price et al. / Lithos 206–207 (2014) 179–200

187

Fig. 5. SiO2 variation diagrams for TiO2, Al2O3, Mg#, CaO, Na2O + K2O and CaO/Al2O3 ratio (a to f respectively) for Older (95 to 18 Ma) and Newer (b4.6 Ma) Volcanics of Victoria. All whole rock data have been normalised on an anhydrous basis to 100% and assuming an Fe3+/Fe2+ ratio of 0.2. Mg# or magnesian number is [100*molecular MgO/(FeO + MgO)]. In (e), the line M-K, which is from Macdonald and Katsura (1964), separates alkalic (above the line) from tholeiitic or sub-alkalic compositions (below the line). Data sources are those listed in caption to Fig. 4.

mantle) / 2)]. The Thorpdale/Gelantipy (referred to henceforth as Group 1) basalts have K/K* N 0.7 and the Bogong/Toombullup (Group 2) basalts have K/K* b 0.7. Group 1 and 2 trace element patterns can be identified and clearly distinguished among the Ballan Graben high Mg# basalts (Fig. 6e). The dominant sample population (samples 69/1454, BG2, BG4, BG10 and BG17) has K/K* indicating affinity with Group 2 basalts and is also characterised by higher abundances of the more incompatible trace elements. Two samples (BG35 and BG37) have trace element patterns very similar to Group 1 and, by inference, most other Older Volcanics. When trace element patterns for the high Mg# basalts of the Newer Volcanics are compared with their Older Volcanics counterparts (Fig. 6), the same two end member groupings can be identified although most analysed samples belong to Group 1. The plains basalts are exclusively Group 1. Cones basalts are predominantly Group 1 but a few samples (4 out of 18 higher Mg# basaltic rocks) have the trace and minor element characteristics of Group 2. 5.2. Strontium, Nd and Pb isotopic compositions Basalt samples from the Older Volcanics of Victoria show considerable variation in Sr and Nd isotopic composition with initial (age corrected) 87 Sr/86 Sr ratios varying from 0.70293 to 0.70503 and 143 Nd/ 144 Nd from 0.51262 to 0.51298 or εNd = 0.6 to 7.5 (Table 3

and Fig. 7a and b). The range in Sr and Nd isotopic compositions for basalts with Mg# N 65 is almost the same as for the suite as a whole and is similar to that observed in cones basalts from the Newer Volcanics. The Sr and Nd isotopic compositions of plains basalts overlap with the field of the Older Volcanics but extend to higher 87 Sr/86 Sr and lower 143 Nd/ 144 Nd values (0.705730 and 0.512566 or ε Nd = − 1.4 respectively; Price et al., 1997). (See Table 3.) On a Sr versus Nd isotope diagram (Fig. 7b), the Older Volcanics data are dispersed between two trends that, with increasing 87Sr/86Sr diverge from a depleted, low 87Sr/86Sr (0.70293) and high 143Nd/144Nd (0.512976, εNd = 7.5) component (G14 from the Gelantipy subprovince) towards a spectrum of more enriched isotopic compositions. The array of isotopic data is distributed on the diagram between a relatively steep trend between G14 and composition T7 (Toombullup subprovince; initial 87Sr/86Sr = 0.70426 and initial 143Nd/144Nd = 0.51262 or εNd = 0.6) and a shallower trend from G14 towards composition BG35 (Ballan sub-province; initial 87Sr/86Sr = 0.70492 and initial 143 Nd/144Nd = 0.51268, εNd = 2.5). The isotopic variation shown by the whole suite of Older Volcanics samples is replicated by data for high Mg# samples. Group 1 high Mg# basalts (see above) have Sr and Nd isotopic compositions that lie along the shallower trend (G14 to BG35) whereas the steeper trend (G14 to T7) includes most of the Group 2 high Mg# basalts (Fig. 7b). Newer Volcanics samples (plains and cones)

188

R.C. Price et al. / Lithos 206–207 (2014) 179–200

Fig. 6. Primitive mantle (PM) normalised extended element plots for Older Volcanics (95 to 18 Ma) of Victoria with Mg# [100 ∗ molecular MgO / (FeO + MgO)] greater than 65, grouped according to sub-province (see Fig. 1). Fields defined for cones and plains basalts of Newer Volcanics (b4.6 Ma) include data from Frey et al. (1978), McDonough et al. (1985); Ewart et al. (1988); Price et al. (1997) and Paul et al. (2005). Normalising abundances for primitive mantle and order of element compatibility (most incompatible on the left and less incompatible elements on the right) are from McDonough and Sun (1995).

have Sr and Nd isotopic compositions that overlap with those of the Older Volcanics but they are concentrated along the trend with the shallower slope with only six samples (3 from cones and 3 from the plains) plotting within the steeper trend. Lherzolite and pyroxenite xenoliths from Western District Province eruptives have a much wider range in Sr and Nd isotopic compositions than is observed in either the host basaltic rocks (Newer Volcanics) or the Older Volcanics (Fig. 7a). Lherzolites have 87Sr/86Sr in the range 0.70335 to 0.71053 and 143Nd/144Nd varies from 0.51222 to 0.51312 (Griffin et al., 1988; McDonough and McCulloch, 1987; Stolz and Davies, 1988; Yaxley et al., 1991). In pyroxenite xenoliths 87Sr/86Sr and 143Nd/144Nd vary from 0.70366 to 0.71576 and 0.51214 to 0.51313, respectively (Griffin et al., 1988). The isotopic composition of Pb in the Older Volcanics covers a relatively narrow range that overlaps with but is slightly wider that the compositional range of basaltic rocks of the Newer Volcanics (Fig. 7c

and d). 206Pb/204Pb varies from 18.07 to 18.976, 207Pb/204Pb from 15.506 to 15.620 and 208Pb/204Pb from 38.116 to 39.410 (Table 4). In comparison, data available for Newer Volcanics basalts have values of 18.348–18.712, 15.56–15.60, and 38.470–38.808 for 206Pb/204Pb, 207 Pb/204Pb and 208Pb/204Pb, respectively (Ewart et al., 1988; McDonough et al., 1985; Paul et al., 2005). Lherzolite xenoliths from the Western District Province show a wider range in Pb isotopic composition with 206Pb/204Pb in the range 17.975 to18.911, 207Pb/204Pb varying from 15.507 to 15.675 and 208Pb/204Pb ranging from 38.170 to 39.530 (Stolz and Davies, 1988). Zhang et al. (1999) noted that younger and older Cenozoic intraplate basalts from eastern Australia have subtly contrasting isotopic compositions. Although, Sr, Nd and Pb isotopic ratios show significant overlap, basalts from Queensland and New South Wales with ages between 55 and 14 Ma tend to have less radiogenic Sr and Pb isotopic compositions (e.g. Fig. 7e) than those erupted after 6 Ma. The Older and Newer

R.C. Price et al. / Lithos 206–207 (2014) 179–200

Volcanics of Victoria show the same patterns with the Newer Volcanics having isotopic similarities with younger Queensland and New South Wales eruptives and the Older Volcanics isotopic compositions that resemble those of their northern counterparts of similar age (Fig. 7e). The Sr, Nd and Pb isotope compositions of Older Volcanics samples are compared with global and western Pacific intraplate basalt compositions in Fig. 7a, c, d and e. The Sr and Nd isotopic compositions lie between those of depleted mantle and a range of enriched mantle compositions (DM to EM1 and EM2 of Zindler and Hart, 1986 and Hart et al., 1992) but the Pb isotopic compositions could be indicating limited involvement of a high μ (high U/Pb) component. The Pb isotope compositions are similar to those of Samoan intraplate eruptives but the Sr isotopic compositions are less radiogenic than those observed in Samoan basaltic rocks (Fig. 7). Across the southwest Pacific, intraplate volcanism has been linked into a large magmatic province, which Finn et al. (2005) referred to as a diffuse alkaline magmatic province or DAMP. The southwest Pacific DAMP incorporates Mesozoic and Cenozoic intraplate volcanism in Tasmania, the south Tasman Sea, southern New Zealand, the subAntarctic islands of the southwest Pacific and west Antarctica (Marie Byrd Land and Victoria Land). Intraplate basaltic rocks of the DAMP are characterised by a distinctive isotopic composition that has generally been interpreted to reflect involvement of both depleted and high μ components in the mantle source (e.g. Hoernle et al., 2006; Panter et al., 2006; Price et al., 2003a; Sims et al., 2008). Basalts of both the Older and Newer Volcanics have isotopic compositions that are entirely within the southwest Pacific DAMP field on the Sr–Nd isotope diagram (Fig. 7a) but there is only partial overlap in Pb–Pb isotope space (Fig. 7c and d); the DAMP field extends towards the high μ field and Older and Newer Volcanic rocks tend to have relatively depleted (less radiogenic) Pb isotopic compositions.

6. Discussion Geochemical variation observed in basaltic rocks of the Western District Province, and particularly the cones eruptives, has in the past been explained in terms of models involving the genesis of a range of primary basalts by varying degrees of mantle melting with subsequent modification of these parental magmas by fractional crystallisation (e.g. Irving and Green, 1976; Sun et al., 1989). Major and trace element data and experimental constraints have been used to develop sophisticated, quantitative partial melting models. For example, Frey et al. (1978) argued that partial melting of a pyrolite-like mantle, enriched in strongly incompatible trace elements (Ba, Sr, light REE, U and Th) could produce the complete spectrum of primary basaltic compositions from nephelinite and basanite (5–7% melting) to olivine tholeiite (20–25% melting). As comprehensive Sr, Nd and Pb isotopic data have become available the petrogenetic models have been modified to incorporate multiple, isotopically variable, mantle sources (e.g. McDonough et al., 1985) but variations in the degree of partial melting of the mantle peridotite sources have continued to provide a framework for explaining the broad range in composition, from strongly alkalic to strongly tholeiitic, observed in high Mg# basalts from the Western District Province. Assimilation of continental crust has commonly been invoked to explain more radiogenic isotopic compositions observed in some Cenozoic intraplate basalts from eastern Australia (e.g. Ewart and Menzies, 1989; Ewart et al., 1988; see also McBride et al., 2001) but an alternative proposal is that the range in isotopic compositions observed in the eruptives is largely a reflection of mantle source heterogeneity (McDonough et al., 1985; Paul et al., 2005; Price et al., 1997). Interpretations vary with regard to the spatial distribution of this heterogeneity with alternative models being based on compositionally diverse lithospheric mantle (Ewart et al., 1988), interaction of asthenospheric and lithospheric mantle sources (e.g. McDonough et al., 1985) or chemically variable asthenospheric mantle (e.g. Demidjuk et al., 2007; Paul et al., 2005).

189

6.1. Petrogenesis of Victorian intraplate basalts: source to surface As noted earlier, Older Volcanics with high Mg#s have trace element compositions that are generally characterised by enrichment of Cs, Ba, Rb, Th, U, Nb, K and light REE over heavy REE, Ti, Zr and Y but there is considerable diversity within and between particular lava fields. A feature of the normalised extended element plots for relatively primitive lavas is a group that shows striking relative depletions in K and Rb. The basalts showing these characteristics tend to be relatively undersaturated with respect to SiO2 and they also manifest relatively strong enrichments of light over heavy REE. One possibility is that these eruptives represent magmas generated by relatively low degrees of partial melting with phlogopite being a residual phase (Demidjuk et al., 2007). Under such conditions magma compositions could be expected to be relatively SiO2 undersaturated and enriched in LREE relative to HREE and the K and Rb depletion is also readily explained. There is however an additional level of complexity that can be demonstrated by the variation in Sr and Nd isotopic compositions. On a Sr–Nd isotope diagram (Fig. 7) the primary basalts that do not show relative depletions in K and Rb (Group 1) define a trend between depleted and enriched mantle. In contrast, the K and Rb depleted basalts (Group 2) form an array that diverges towards a different enriched mantle end member. The inference that can be drawn from the trace element and isotopic data is that at least three different mantle source components have been involved in the petrogenesis of Victorian intraplate basalts (c.f. Sun et al., 1989), a conclusion consistent with previously published interpretations of data for lherzolite and pyroxenite inclusions from the Western District Province (e.g. Griffin et al., 1988; O'Reilly et al., 1988; Powell et al., 2004; Stolz and Davies, 1988). Various authors (e.g. Ewart and Menzies, 1989; Irving and Green, 1976; Sun et al., 1989) have suggested that the more evolved Newer Volcanics basaltic compositions have been derived by fractional crystallisation or assimilation fractional crystallisation from more primitive precursors and the possibility that these processes may also have affected Older Volcanics magmas as they moved from source to surface must be considered. Compositions with relatively lower Mg# (down to 57) and Ni, Sc and Cr contents (e.g. 57, 14 and 150 ppm respectively) could represent magmas modified by fractional crystallisation involving olivine, clinopyroxene and spinel. Fractional crystallisation (closed system) and assimilation fractional crystallisation (open system) models for representative Older basalts are illustrated in Fig. 8 where they are also compared with overall variation within both the Older and Newer Volcanics suites. Open and closed system fractional crystallisation can explain some of the scatter on trace element and 87Sr/86Sr diagrams but the dominant trends and patterns are more likely to be related to the mantle source characteristics and partial melting processes controlling the compositions of primary magmas. 6.2. Regional variation in Sr isotope composition The regional spatial variation in Sr isotopic composition of late Mesozoic and Cenozoic basalts across Victoria has been examined by projecting analyses for individual samples onto an east–west profile (Fig. 9a). This approach has previously been applied to the plains basalts of the Western District Province by Price et al. (1997, 2003b), who postulated that a discontinuity (the Mortlake discontinuity) occurs in the isotopic profile adjacent to the southern extension of the Moyston Fault, a major basement structure marking the boundary between the Lachlan and Delamerian orogens (Fig. 3). New data for Older Volcanics basalts have been incorporated into the projection shown in Fig. 9a to extend the Sr isotope profile eastwards across Victoria. Data for basalts from the Cosgrove-Euroa area and Uplands Province of eastern Victoria are also included on the projection. In western Victoria, the available Older Volcanics Sr isotopic data are limited but values overlap with those of the Western District Province. The lowest 87Sr/86Sr ratios observed in Victorian intraplate basalts of

190

Table 3 Strontium and neodymium isotope data for Older Volcanics. 2

3

4

5

6

7

8

9

10

11

12

13

14

G14

G21

70/1328

B10

B26

H18

H28

H38

T3

T7

T10

70-140

69-1468

TH29

TH39

Sub-province

G

G

B

B

B

H

H

H

T

T

T

L

L

L

L

87

0.702980

0.703650

0.703965

0.704100

0.704125

0.703795 0.703824

0.703962

0.704179

0.704154

0.704286

0.703900

0.703629

0.704604

0.704045

0.704137

0.513010

0.512903

0.512755

0.512721

0.512722

0.512705

0.512710

0.512693

0.512647

0.512700

0.703946 0.512735 2.7

0.704073 0.512699 2.0

0.704098 0.512700 2.0

0.703930 0.512682 1.7

0.704144 0.512688 1.8

0.704115 0.512658 1.4

0.704262 0.512620 0.6

0.703867 0.512672 1.6

0.512813 0.512797 0.704529 0.512753 3.8

0.512789

0.703593 0.512862 5.3

0.512871 0.512868 0.703554 0.512824 5.0

0.512806

0.702931 0.512976 7.5

0.512766 0.512764 0.703790 0.512739 2.8

0.703975 0.512758 3.8

0.704047 0.512744 3.5

Sr/86Sr

143

87

Nd/144Nd

Sr/86Srinitial Nd/144Ndinitial

143

εNd

15

Typical in-run precision for Sr and Nd is ±0.0025% (2se). Nd data are reported relative to La Jolla Nd = 0.511860. USGS basalt BCR-1 averaged 0.70500 ± 4 (2σ; n = 6) and 0.512634 ± 18 (2σ, n = 7), while BHVO-1 averaged 0.70348 ± 6 (2σ, n = 19) and 0.512989 ± 13 (2σ, n = 5). These data indicate external precision (reproducibility, 2σ) of ±0.000040–0.000060 (Sr) and ±0.000020 (Nd). Sub-provinces: G = Gelantipy; B = Bogong; H = Hotham; T = Toombullup; L = Latrobe; N = Neerim; Th = Thorpdale, A = Aberfeldy; P = Poowong; F = Flinders; M = Melbourne; GE = Gellibrand; BG = Ballan Graben; O = Otway.

R.C. Price et al. / Lithos 206–207 (2014) 179–200

1 Sample #

Table 3 Strontium and neodymium isotope data for Older Volcanics. 17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

TH42

70-143

70-145

N22

TH4

TH18

TH20

TH21

TH22

A7

A10

69-1458

69-1465

69-1466

F2(A)

F3(C)

F5(A)

L

N

N

N

TH

TH

TH

TH

TH

A

A

F

F

F

F

F

F

0.704766 0.704772

0.703685 0.703694

0.704606

0.704154

0.703471 0.703458

0.703409

0.704179

0.703986 0.703949

0.705068 0.705048

0.703985 0.703979

0.703282

0.703967

0.703242

0.703250

0.703473

0.512755

0.512804 0.512805 0.703662 0.512789 3.3

0.512739 0.512731 0.704576 0.512711 2.1

0.512730

0.512933

0.512851

0.512851

0.512733

0.512748

0.512950

0.512829

0.512959

0.512940

0.512890

0.703444 0.703419 0.703424 0.512928

0.704118 0.512713 2.0

0.703439 0.512916 5.9

0.512898 0.512892 0.703388 0.512876 5.3

0.703728 0.703722 0.703724 0.512872

0.704161 0.512795 3.5

0.703927 0.512832 4.3

0.703694 0.512853 4.7

0.705032 0.512711 2.0

0.703964 0.512727 1.9

0.703245 0.512913 6.4

0.703843 0.512781 4.1

0.703185 0.512921 6.5

0.703203 0.512906 6.3

0.703426 0.512855 5.3

0.703382 0.512885 5.9

0.704723 0.512706 2.6

R.C. Price et al. / Lithos 206–207 (2014) 179–200

16

191

192

Table 3 Table 3 (continued) Strontium and neodymium isotope data for Older Volcanics. 34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

F10

F11

69-1457

BP1

M3

GE5

69-1454

BG2

BG4

BG10

BG11

BG17

BG35

19

54

92

Sub-province

P

P

M

M

M

Ge

BG

BG

BG

BG

BG

BG

BG

O

O

O

87

0.703658

0.703532 0.703567

0.703346 0.703258

0.704234

0.704624

0.704786

0.704579 0.704573

0.704060 0.704066

0.704225 0.704232

0.703945

0.704043 0.704039

0.704052 0.704058

0.705084 0.705167

0.703988

0.704672 0.704682

0.704879

0.512872

0.512901

0.512842

0.512828

0.512723

0.512686

0.512759

0.512737

0.512794

0.512742

0.512742

0.512740

0.512864

0.512721

0.512759

0.703547 0.512794 5.3

0.703430 0.512817 5.8

0.512937 0.512908 0.703278 0.512895 6.1

0.704211 0.512823 4.1

0.704606 0.512806 3.9

0.704684 0.512698 1.8

0.704492 0.512644 1.8

0.704046 0.512714 3.1

0.704184 0.512662 2.1

0.703922 0.512751 3.7

0.704020 0.512698 2.7

0.704029 0.512700 2.8

0.704922 0.512683 2.5

0.703933 0.512830 4.6

0.704602 0.512670 1.7

0.704799 0.512724 2.8

86

Sr/ Sr

143

87

Nd/144Nd

Sr/86Srinitial Nd/144Ndinitial

143

εNd

R.C. Price et al. / Lithos 206–207 (2014) 179–200

33 Sample #

R.C. Price et al. / Lithos 206–207 (2014) 179–200

193

Fig. 7. Strontium, Nd and Pb isotopic compositions of Older and Newer Volcanics of Victoria and comparisons with regional and global data. (a) and (b) show 143Nd/144Nd and 87Sr/86Sr compositions and (c) and (d) variation in Pb isotopic composition. Older Volcanics (95 to 18 Ma) compositions are new data. (e) is after Zhang et al. (1999) and shows a comparison between 87Sr86Sr and 206Pb/204Pb isotope compositions for Victorian Older and Newer Volcanics and Cenozoic intraplate basalts from elsewhere in eastern Australia. N is field for younger (b6 Ma) basalts from north Queensland and S is field for older (55–14 Ma) basalts from Queensland and New South Wales. T is the field for Tasmanian intraplate basalts (46–25 Ma). I and P are fields for Indian and Pacific type mid-ocean ridge basalt (MORB) asthenospheric mantle. Newer Volcanics (b4.6 Ma) data are from McDonough et al. (1985), Ewart et al. (1988), Price et al. (1997) and Paul et al. (2005). Also shown are fields for lherzolite (Griffin et al., 1988; McDonough and McCulloch, 1987; Stolz and Davies, 1988; Yaxley et al., 1991) and pyroxenite (Griffin et al., 1988) inclusions found in basalts of the Western District Province of Victoria. Basalt data in (b) have been differentiated on the basis of Mg# [100 ∗ molecular MgO / (FeO + MgO)]. DM, EM1, EM 2 and Hiμ are depleted, enriched and high μ end member mantle components of Zindler and Hart (1986) and Hart et al. (1992). NHRL is northern hemisphere reference line (Hart, 1984). The field for Samoa is from Wright and White (1987) and Workman et al. (2004). DAMP (Finn et al., 2005) is the field of the Cenozoic diffuse alkaline magmatic province of the south west Pacific (data from Baker et al., 1994; Barriero and Cooper, 1987; Hoernle et al., 2006; Lanyon et al., 1993; Panter et al., 2006; Price et al., 2003a; Sims et al., 2008; Timm et al., 2010).

all ages (Older and Newer Volcanics) occur along the portion of the profile from immediately west to ~ 150 km east of Melbourne. Comparison of Fig. 9a with the structural cross section along the section A-A′ shown in Figs. 3 and 9c indicates that these lower 87Sr/86Sr ratios coincide with the area postulated to be underlain by the Selwyn Block, which is considered to comprise Proterozoic or Cambrian lithosphere (see above). 6.3. Lithospheric influences on basalt chemistry The suggestion that the lithospheric mantle or lithospheric structures have been a significant factor influencing the petrogenesis of

Victorian basalts (Ewart et al., 1988; Price et al., 1997, 2003b) remains contentious. Zhang et al. (1999) explained temporal isotopic variations between intraplate basalts of eastern Australia in terms of changing asthenospheric influences, arguing that opening of the Tasman Sea caused westward migration of Pacific mid-ocean ridge (MORB) asthenosphere, which replaced Indian MORB type asthenospheric mantle (see Fig. 7e). Paul et al. (2005) concluded that the isotopic characteristics of basaltic rocks from central Victoria could be explained in terms of a largely asthenospheric source without the need to invoke a contribution from the lithospheric mantle and Sutherland et al. (2014) suggested that temporal patterns of felsic magmatic activity across Victoria could be

194

R.C. Price et al. / Lithos 206–207 (2014) 179–200

Table 4 Lead isotope data for Older Volcanics of Victoria, Australia. Sample#

Sub-province

206

G14 G21 70/1328 B10 B26 H18b H28 H38 T3 T7 T10

Gelantipy

18.976 18.824 18.847 18.540 18.871 18.076 18.796 18.632 18.823 18.921 18.906

Bogong

Hotham

Toombullup

Pb/204Pb

207

Pb/204Pb

15.565 15.620 15.602 15.582 15.603 15.574 15.596 15.506 15.617 15.618 15.603

208

Pb/204Pb

38.667 38.865 39.161 38.735 39.232 38.116 39.038 38.784 39.421 39.410 39.192

206

Pb/204Pb initial

18.855 18.760 18.744 18.461 18.786 18.042 18.692 18.524 18.748 18.786 18.768

207

Pb/204Pb initial

15.559 15.617 15.597 15.578 15.599 15.581 15.591 15.501 15.613 15.612 15.597

208

Pb/204Pb initial

38.558 38.799 39.037 38.635 39.121 38.106 38.913 38.649 39.305 39.235 39.037

Typical in-run precision is 0 · 05% (2se). Instrumental mass bias was corrected using a global mass bias factor of 0.109%/amu, derived from numerous analyses of SRM981 under similar conditions. External precision (2σ, n = 78) for SRM981 is ±0 · 097% (206Pb/204Pb), ±0 · 130% (207Pb/204Pb) and ±0 · 175% (208Pb/204Pb).

related to long-term asthenospheric upwelling under migrating lithosphere. In contrast to these exclusively asthenosphere sourced models, Gray and McDougall (2009) restated the case for the existence of the Mortlake Discontinuity and argued that variations in Sr isotopic composition across the Western District Province are more likely to have derived from heterogeneity in the lithospheric rather than the asthenospheric mantle. When all of the Sr isotopic data for Victorian intraplate basalts (Newer and Older Volcanics) are considered in a spatial context (Fig. 9) isotopic variation does appear to be related in some way to regional structure. In western Victoria, although data for the Older Volcanics are limited they are consistent with variations previously reported for the plains basalts of the Western District Province (Price et al., 1997). The southward extrapolation of the Moyston Fault, marking the boundary between the Glenelg and Stawell structural zones, coincides with an

abrupt change in the variation of Sr isotopic composition. To the east the 87 Sr/86Sr ratios are more variable and extend to higher values than is the case to the west. The base level 87Sr/86Sr ratio is lower to the west than it is to the east. Further east, in central Victoria the Mount William Fault, which is the boundary between the Bendigo and Melbourne structural zones, marks a second discontinuity in the Sr isotopic data. The lowest 87 Sr/86Sr ratios along the west to east section shown in Fig. 9 occur adjacent to and to the east of the Mount William Fault and they coincide approximately with the area of central Victoria postulated to be underlain by the Selwyn Block. Thus the relationship between regional structure and discontinuities in the regional pattern of basaltic isotopic composition provides a basis on which to argue that there is lithospheric control on basaltic magma chemistry. If there is indeed regional control on the isotopic composition of Victorian intraplate basalts then this could simply reflect contamination

Fig. 8. Comparison of geochemical variation in selected trace/minor element and 87Sr/86Sr ratios for Older (95 to 18 Ma) and Newer (b4.6 Ma) Volcanics of Victoria with fractional crystallisation (FC) and assimilation fractional crystallisation (AFC) models. FC and AFC models are shown for two different primary basalts (compositions G14 and T7). Fraction of melt remaining is marked in 10% intervals on each model trajectory. For models using G14 as the parental magma composition, the fractionating solid comprises 50% olivine (Ol), 29% clinopyroxene (Cpx), 17% magnetite (Mt) and 3% apatite (Ap). For the models involving T7 as the parental magma the proportions of phases in the fractionating solid are: Ol = 52%, Cpx = 42% and Mt = 7%. FC models are based on Rayleigh fractionation and AFC trajectories are calculated using the formulation of DePaolo (1981) and assuming a crustal assimilant with the composition of average eastern Australian Palaeozoic sediment (Ewart and Menzies, 1989). The ratio of assimilated to fractionated solid (r) used in the calculations is 0.3. For both AFC and FC models partition coefficients for the fractionating phases are from Halliday et al. (1995), Ewart and Hawkesworth (1987) and Blundy and Wood (2003).

R.C. Price et al. / Lithos 206–207 (2014) 179–200

195

characteristics of primary magmas that have not been significantly modified by fractional crystallisation; over 50% of analysed samples have Mg#s over 65 and Cr contents N 300 ppm and 40% have Ni abundances N250 ppm. The regional patterns of Sr isotopic composition remain when only less evolved basalts are considered (Fig. 9) and the overall trends and patterns in trace element and isotopic behaviour are not effectively modelled by assimilation fractional crystallisation (see above and Fig. 8). An alternative explanation for the regional variations in Sr isotopic composition is that magmas have been at least in part sourced from regionally heterogeneous sub-continental lithospheric mantle with the link between regional structure and isotopic composition of basaltic magma reflecting variations in lithospheric mantle composition across major lithospheric boundaries. The greatest contrast in lithospheric mantle should be associated with the more significant lateral changes in basement composition. These are: (a) the boundary between the Delamarian and Lachlan Orogens; and (b) the transition to areas underlain by the Selwyn Block. When the K and Rb depleted (Group 2) primary basalts are differentiated on the 87Sr/86Sr regional plot it is clear that although they occur to the east and west along the isotopic traverse they are almost absent in the section of the traverse that lies above the Selwyn Block, reinforcing the suggestion that basalt magma chemistry has been influenced by lithospheric structure. 6.4. Mantle metasomatism

Fig. 9. Regional variation in (a) age corrected 87Sr/86Sr and (b) primitive mantle normalised K/Nb ratio [(K/Nb)n] for Older and Newer Volcanics of Victoria and (c) comparison with structural zones from Fig. 3. C–E is the field for 10 to 5 Ma (?) basalts from the Cosgrove–Euroa area (Paul et al., 2005) and U is a basalt from the 4 to 2 Ma Uplands Province of eastern Victoria (Sutherland et al., 2003). Normalising values used in (b) are from McDonough and Sun (1995). Compositional data are projected onto an east–west profile (A-A′). The line of the profile and spatial distribution of structural zones are shown in Fig. 3. Data are plotted in terms of distance in kilometres east and west of the city of Melbourne, the location of which is marked by M. MD is the Mortlake discontinuity defined by Price et al. (1997) using 87Sr/86Sr data for Western District Province plains basalts. Structural information shown in (c) is from Gray et al. (2003). Major faults are: M — Moyston; Av — Avoca; MW — Mt William; G — Governor; LM — Lucas Point / McLauchlan. Structural zones of western and central Victoria are, from west to east: Glenelg, Stawell (Stw.), Bendigo, Melbourne and Tabberabbera (Tabb.). The postulated location at depth of the Selwyn Block is from Cayley et al. (2002, 2011).

of primary basalts by different crustal compositions within specific lithospheric tectonic zones. The Selwyn Block represents a Cambrian or Proterozoic continental fragment, possibly an oceanic plateau, in which mafic volcanic rocks are a major component (Cayley et al., 2002, 2011; Willman et al., 2010). The component that the Selwyn Block might contribute to crustal contamination would therefore be expected to be isotopically depleted relative to typical Palaeozoic meta-sediments and consequently the impact on the isotopic composition of basaltic magma into which it might be incorporated would be relatively limited. Lower 87Sr/86Sr ratios above the Selwyn Block might therefore reflect a less obvious impact of crustal contamination relative to adjacent areas where the crustal contaminant might be dominantly Palaeozoic sediment. Assimilation should however be accompanied by fractional crystallisation; the thermal energy needed to drive assimilation is generated by crystallisation (Spera and Bohrson, 2001; Taylor, 1980). Basalts that have assimilated substantial quantities of crustal material should be expected to be relatively fractionated. A substantial proportion of Older Volcanics basaltic samples have the geochemical

Lherzolite and pyroxenite xenoliths found in basalts of the Western District Province are samples from the sub-continental lithospheric mantle (e.g. Griffin et al., 1988; Powell et al., 2004; Stolz and Davies, 1988) and it has been demonstrated that their compositions reflect a complex history of fluid and melt metasomatism imposed upon an originally depleted mantle (e.g. Stolz and Davies, 1988). Powell et al. (2004) identified three different end member types of lherzolite each having a different metasomatic history. They argued that “group A” lherzolites have been affected by silicate melts, “group B” by carbonatite metasomatism and “group C” lherzolites have been metasomatised by a fractionated, hydrous silicate melt. Carbonatitic metasomatism has for some time been recognised as a factor influencing the geochemistry and mineralogy of Western District Province lherzolites (Yaxley and Kamanetsky, 1999; Yaxley et al., 1991). Adam and Green (2011) also proposed that the development of metasomatised lithospheric mantle beneath eastern Australia preconditioned the lithospheric mantle so that subsequent, relatively small scale, thermal disturbances could lead to shallow melting and intraplate magmatism. They argued that Cenozoic (28–24 Ma) intraplate basalts from northern Tasmania were generated by partial melting of lithospheric mantle that had been metasomatised by migration of near solidus melts derived from the underlying asthenosphere. The precise timing of these metasomatic events is not readily determined but sequential silicate and carbonatite fluid/melt metasomatism of south-eastern Australian lithospheric mantle has been linked to major tectonic events. Foden et al. (2002) suggested that the subcontinental lithospheric mantle was modified by magmatism associated with NeoProterozoic to Cambrian extension and rifting of the Australian–East Antarctic craton and again during subsequent convergence associated with the Ross-Delamerian Orogeny, which commenced around 514 Ma B.P. These authors also argued that Jurassic aged kimberlites found in South Australia indicate the presence within the sub-continental lithospheric mantle of an ancient alkaline component. There is thus a consensus that the sub-continental lithospheric mantle of south-eastern Australia is dominated by originally depleted peridotitic material that has been complexly modified by metasomatism involving a range of fluid or melt compositions. Metasomatic modification of the lithosphere has been an episodic process dating back to at least the NeoProterozoic, with magmatism associated

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depletion in K and Rb observed in primitive mantle normalised extended element plots (Figs. 6 and 10). The trace element pattern of the model carbonatite metasomatised depleted mantle composition also shows a strong similarity to the patterns observed in apatiteamphibole lherzolites (Griffin et al., 1988) from the Western District Province (Fig. 10). Since the lherzolites are considered to have been derived from the lithosphere, the similarity they have to the source of at least some Older Volcanics basaltic magmas is consistent with the suggestion that the processes by which the latter are generated took place (at least in part) within the lithospheric mantle. The compositions of the carbonatitic and basaltic metasomatising agents used in the models are unlikely to exactly match those that might actually have been involved in the refertilisation of the mantle from which the Older basalts have derived. Nor can it be assumed that the model depleted mantle composition is precisely that of the depleted component involved in the melting process. The role of phlogopite is also debatable. Refertilisation by carbonatitic metasomatism could result in the formation of phlogopite in the mantle source and if this phase was retained in restite during low degree partial melting, the effect would be to exaggerate K and Rb depletion in the derivative melts. Despite these caveats, the models do support the more general argument that the mantle source for the magmas represented by the Older Volcanics is primarily a depleted mantle that has been variously modified by a range of metasomatic processes. Fig. 10. Primitive mantle normalised extended element diagrams illustrating partial melting models for theoretical, metasomatised mantle and comparisons with Older Volcanics high Mg# basalts. DM is a depleted mantle composition from Salters and Stracke (2004). EM1 is an enriched intraplate basalt composition obtained by averaging a compilation of data from Woodhead and Devey (1993), Weis et al. (1993), Barling et al. (1994) and Hermond et al. (1994). Cc is an average calci-carbonatite composition from Woolley and Kempe (1989). In the first model (a), the patterns for 2 to 10% batch melts (grey shaded area) from a composition consisting of DM + 2% EM1 are compared with representative Group 1 basalts and in the second (b) patterns for 2 to 10% batch melts (grey shading) of a composition consisting of DM + 2% Cc are compared with representative Group 2 basalts. In all melting models the residual mineralogy is assumed to be olivine + clinopyroxene + orthopyroxene + garnet in the proportions 0.6:0.2:0.15:0.05 respectively. Partition coefficients used in the models are from Halliday et al. (1995). Also shown for comparison in (b) is the primitive mantle normalised extended element pattern for average Western District Province apatite + amphibole-bearing (AAp) lherzolite (Griffin et al., 1988). Normalising values are from McDonough and Sun (1995).

with each major tectonic episode leaving a particular signature on the south-eastern Australian lithospheric mantle. The most recent of these major events is likely to have been associated with Gondwanan breakup but ongoing magmatic activity since that time is also likely to have further modified the lithospheric mantle, although these more recent metasomatic episodes might be expected to have occurred on a much smaller scale. Partial melting of two possible varieties of metasomatised mantle composition has been modelled and the model melt compositions compared with the two main types of Older Volcanics high Mg# basalt (Fig. 10). In the first model, the mantle source composition is assumed to be a depleted mantle (composition from Salters and Stracke, 2004) that has been refertilised by magma with the composition of intraplate basalt. The basalt composition used in the model is an average EM1 composition (data from Barling et al., 1994; Hermond et al., 1994; Weis et al., 1993; Woodhead and Devey, 1993). The results of the model are shown in Fig. 10a and they indicate that 2 to 10% batch melting of depleted mantle modified by the addition of 2 to 3% EM1 can replicate reasonably well the trace element compositions of Group 1, high Mg# Older Volcanics basalts. The second partial melting model involves a source consisting of depleted mantle to which has been added 1% of a calci-carbonatite composition (average from Woolley and Kempe, 1989). Two to 10% batch melting of this composition produces trace element compositions that have many of the characteristics of Group 2, high Mg# Older Volcanics basalts (Fig. 10). In particular, the model replicates the strong relative

6.5. The relationship between tectonics and Victorian intraplate magmatism Large igneous provinces in intraplate settings have been argued to be the thermal and magmatic manifestations of large plumes, with an ultimate origin in the deep asthenospheric mantle (e.g. Campbell and Griffiths, 1990; Richards et al., 1989). In these models, flood basalt sequences, such as the Deccan Traps or the Ontong Java plateau, represent plume heads and linear, age progressive volcanic chains (e.g. the Hawaiian chain) the trace, on the over-riding plate, of the plume tail (e.g. Richards et al., 1989). This type of “hotspot” or plume model has been proposed to explain some aspects of the temporal and spatial distribution of Cenozoic intraplate volcanism in eastern Australia (Sutherland, 1983; Wellman, 1983; Wellman and McDougall, 1974) but the long time scale over which volcanic activity has taken place and the intricate spatial and temporal relationships of magmatism are such that plume-linked models are perforce extremely complex (Sutherland, 2003; Sutherland et al., 2012). In addition, such models are difficult to reconcile with interpretations of the regional tectonic and uplift history. An alternative to the hotspot models was developed by Lister and Etheridge (1989). They proposed that continental extension associated with the initial stages of the seafloor spreading that ultimately formed the Southern Ocean, was asymmetric and induced uplift of the asthenosphere/lithosphere boundary along the southern Australian continental margin. This caused upward flow of the asthenosphere with the progressive development on various scales of chains and clusters of slow rising mantle plumes or diapirs. Magmatism was argued to be associated with decompression melting in these diapirs as well as thermally induced melting within the lithospheric mantle as the rising plumes impinged on the lithosphere. The model also sought to explain uplift across eastern Australia during the Cenozoic; it was postulated that uplift was associated with rising geotherms caused by the emplacement of relatively hot asthenospheric mantle and magmatic underplating of the lithosphere. Demidjuk et al. (2007) used U–Th disequilibrium data for Newer Volcanics from South Australia to conclude that although mantle upwelling has been involved in the generation of basaltic magmas it has, relative to the conditions postulated to be associated with deep mantle plumes, been limited in scale. This conclusion was based on the following observations: (a) in the Pacific, plume-related intraplate magmatism is

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Fig. 11. (a) Diagrammatic representation of edge-driven convection model (King and Anderson, 1998) of Demidjuk et al. (2007) for south-eastern Australia. Movement of the Australian plate northward at ~ 6 cm/year causes edge-driven convection at a step in the lithosphere/asthenosphere boundary. Mantle upwelling within the convective cell causes topographic uplift at the surface and localised melting in the mantle. Entrainment of lithospheric mantle into the convecting asthenosphere creates a heterogeneous mixed mantle source from which basaltic magmas derive. (b) Pressure (depth) versus temperature profile for Victoria showing pressures and temperatures of basalt melt generation using the geothermobarometer of Lee et al. (2009). The effect of H2O is illustrated by duplication of pressure and temperature estimates for anhydrous melting and for melting with 2% H2O in the melt. The spinel and garnet in phase boundaries and the lherzolite solidus (melt in) are from Gudfinnsson and Presnall (1996). The xenolith geothermal gradient is the geothermal gradient for south-eastern Australian lithosphere estimated by O'Reilly and Griffin (1985) and O'Reilly et al. (1988). The lithosphere–asthenosphere boundary is from Ford et al. (2010).

associated with lithospheric extension whereas south-eastern Australia has been in compression for at least the past 5 Ma (Sandiford, 2003); (b) in plume-related volcanic chains such as Hawaii there are systematic temporal, spatial and geochemical changes in eruptive activity that are not observed in south-eastern Australia; (c) volcanism across southeastern Australia has been associated with only moderate, localised and small scale uplift. Deep mantle plumes do not therefore appear to offer a feasible explanation linking south-eastern Australian Mesozoic and Cenozoic magmatic activity to tectonics and Demidjuk et al. (2007) therefore suggested that south-eastern Australian basaltic magmatism is associated with asthenospheric upwelling driven by differential motion of mantle below the irregular base of the lithosphere (Fig. 11). Tomographic studies and seismic wave imaging (Fishwick et al., 2008; Ford et al., 2010; Kennett et al., 2004) indicate the presence of changes or steps in lithospheric thickness. The most significant of these, an abrupt change in thickness of ~100 km, occurs along an approximately east–west line that has been postulated to correspond with a north to south transition from Proterozoic to Phanerozoic lithosphere (Kennett et al., 2004). Using constraints derived in part from edge-driven convection modelling carried out by King and Anderson (1998), Demidjuk et al. (2007) proposed that as the Australian Plate moves northward, differential flow across the edge of the lithospheric step causes convection within the shallow asthenosphere. Lithospheric mantle is entrained in the edge-driven convection cells and upwelling within the convecting mantle results in uplift and partial melting (Fig. 11). The loci of uplift and magmatism migrate as the plate and the lithospheric irregularity or step move northwards. The Demidjuk et al. (2007) model focused on one major step in the lithosphere/asthenosphere boundary but presumably other smaller irregularities could also have produced similar, albeit less pronounced, edge-driven convection cells elsewhere across south-eastern Australia throughout the Mesozoic and Cenozoic, following continental breakup. The compositions of primary basaltic magmas are in part determined by the physical conditions prevailing in the mantle at the site of partial melting and experimental data for lherzolite melting have been used to calibrate empirical geothermobarometers based on basalt composition (e.g. Lee et al., 2009; Wood, 2004). This approach has been applied to Older and Newer basalts and the pressure and temperature

estimates obtained for the spectrum of basalt compositions can compared with tectonic models. The results are shown in Fig. 11. Only basalts with Mg# N 66 have been used in the calculations and the peridotite source is assumed to be lherzolite. Pressure and temperature estimates obtained using the Lee et al. (2009) geothermobarometer are shown in Fig. 11 along with the lherzolite solidus and garnet and spinel lherzolite stability fields (Gudfinnsson and Presnall, 1996). Calculations have been carried out for anhydrous melting and for 2% H2O in the derivative melts. The Wood (2004) geothermobarometer gives results that are similar but shifted to lower pressure and higher temperature. The data provide an indication that the primary basaltic magmas represented by Older and Newer Volcanics were generated over a wide range in depth, extending well into the garnet lherzolite stability field (Fig. 11). This is consistent with trace element data; REE patterns in many samples are, for example, characterised by high, chondritenormalised Gd/Yb ratios that can be explained by the presence of restitic garnet during partial melting of a lherzolite source. Ford et al. (2010) postulated that the lithosphere–asthenosphere boundary beneath south-eastern Australia is at a depth of 61 ± 11 km and the temperature-depth estimates obtained from the basalts extend across this boundary into the lithosphere (Fig. 11). The wide range in depth and temperature estimated for the partial melting that generated Victorian basalts, from lithosphere to asthenosphere is consistent with the edge-driven tectono-magmatic models shown diagrammatically in Fig. 11 but melting within the lithosphere, as proposed by Lister and Etheridge (1989) is not precluded. The edge-driven convection model has a number of advantages over others. It provides a linkage between current tectonic interpretations, spatial and temporal distribution of magmatic activity and uplift history. The scale of mantle upwelling implied in the model is consistent with uplift rates and melting scales implied from the volumes of erupted material and with the indications from basalt chemistry that magmas were generated across a wide depth range. An innovative aspect is that the model gives a novel explanation for the origin of mantle source heterogeneity implied from studies of basaltic rocks and for the involvement of lithospheric mantle; lithospheric and asthenospheric mantle are mixed at relatively shallow depths within convecting asthenosphere. Lithospheric mantle is not necessarily being melted in situ. If it

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is incorporated into the shallow asthenosphere by edge-driven convection (Fig. 11) then it may also be melted at asthenospheric depths. Wherever the lithospheric mantle is being melted, the chemical heterogeneity of this material is likely to be reflected in subtle variations in the geochemistry of partial melts represented by basalts erupted at the surface. The influence of lithospheric structure on the geochemistry of Mesozoic and Cenozoic volcanics of Victoria could therefore be reflecting both heterogeneity in the sub-continental lithospheric mantle and the differential incorporation of this material into the asthenosphere during dynamic, edge-driven convection. 7. Conclusions Intraplate basaltic volcanism was initiated in eastern Australia in the late Jurassic, during rifting that preceded the breakup of eastern Gondwana. In the state of Victoria, along the south-eastern continental margin, this magmatic activity has continued intermittently since that time to virtually the present day, with breaks in activity occurring between 77 and 62 and 18 and 7 Ma. Basaltic rocks with ages of 95–18 Ma (Older Volcanics) are distributed across eastern and central Victoria as eroded remnants of mostly monogenetic lava fields and cones and they also occur to the west as shallow intrusions. Compositions vary from olivine nephelinite and basanite through transitional basalt to hawaiite and quartz normative tholeiite with 60 to 70% of analysed samples being nepheline normative. Major and trace element and Sr, Nd and Pb isotopic compositional variation observed within the Older Volcanics sample suite is broadly consistent with models whereby primitive magmas are derived by partial melting from a compositionally diverse range of mantle sources with subsequent modification by fractional crystallisation with or without limited assimilation of crust. On the basis of trace element and isotopic compositional variation, two end member groups of relatively primitive (high Mg#) basalts have been identified. Group 1 basalts have: (a) moderate relative enrichment of light over heavy REE; and (b) primitive mantle normalised extended element patterns that are slightly convex upwards with normalised abundances peaking at Nb and progressively falling among the more incompatible elements from Nb through U and Th to Rb and Ba. Among this group there is subtle variation in trace element behaviour with some samples having moderate relative enrichments in Sr, Nb and Eu and depletion of Pb relative to Ce. Group 2 high Mg# basalts are characterised by: (a) heavy REE, Ti and Zr abundances that are similar to those observed in Group 1; (b) stronger relative enrichments in middle and light REE, P, Sr, Pb, Nb, U, Th and Ba; and (c) Strong relative depletions in K and Rb. The isotopic and trace element variations observed among high Mg# basalts of the Older Volcanics suite are consistent with the involvement of at least three different mantle source components: (a) depleted mantle (DM); (b) an enriched (EM) component (or components) similar to EM1 type intraplate basalt; and (c) a calci-carbonatite component. For Group 1 basalts the dominant components are DM and EM whereas the geochemistry of Group 2 basaltic magmas reflects involvement of a carbonatitic component. The primitive mantle normalised extended element pattern for a Group 2 hypothetical mantle source (DM with a small addition of calci-carbonatite) is strikingly similar to the patterns observed in apatite-amphibole bearing lherzolite inclusions from Western District Province eruptives. The latter have been argued by others (e.g. Griffin et al., 1988; Powell et al., 2004; Yaxley et al., 1991) to derive from metasomatised lithospheric mantle. The spatial pattern of 87Sr/86Sr isotopic variation for all Victorian Mesozoic and Cenozoic basalts (Older and Newer) contains discontinuities that appear to be related to surface and subsurface structural features within the basement geology. Although there are only a limited number of Older Volcanic samples from western Victoria, the 87Sr/86Sr compositions of these are consistent with those previously reported for the Western District Province (Price et al., 1997, 2003b) and thus with the definition of the Mortlake discontinuity; a compositionally

defined change in basalt isotopic composition that coincides with a major structural boundary within the basement. The lowest 87Sr/86Sr ratios reported for Victorian Mesozoic and Cenozoic basalts are those from Older Volcanics basalts erupted above the Selwyn Block in central Victoria (Fig. 5). The linkage between the isotopic composition of the basalts and regional structure could reflect contamination of primary basalts by different crustal compositions in each tectonic or structural zone but the patterns persist even when only relatively primitive basalts are considered and this implies that they originate in the subcontinental lithospheric mantle. The longevity, spatial distribution and small magma volumes that characterise intraplate basaltic magmatism in south-eastern Australia cannot be readily explained by deep mantle plume models. Neither the uplift history nor the evidence for a long history of metasomatism affecting the lithospheric mantle is consistent with this type of model. One possibility (Demidjuk et al., 2007) is that magmatism is associated with edge-driven convection caused by differential motion of mantle across the irregular base of the lithosphere as the Australian Plate has been driven northwards. Edge-driven convection within the asthenospheric mantle caused uplift and partial melting and lithospheric and asthenospheric mantle were mixed as the sub-continental lithosphere was incorporated into the convecting asthenosphere. The mantle heterogeneity implied from the variations in basalt chemistry arises from the interaction of compositionally diverse, metasomatised lithospheric mantle and asthenosphere. The model also provides an explanation for the apparent influence of lithospheric structures on basalt compositions. Acknowledgements The technical support of Roland Maas, Jorg Metz and Ian McCabe is gratefully acknowledged. The evolution of the concepts and ideas developed during the research described in this paper benefited considerably from discussion with Roland Maas, Mike Sandiford and Janet Hergt. J.A. Gamble and F.L. Sutherland provided constructive reviews and N. Eby is thanked for editorial handling. The support of La Trobe and Monash Universities and the University of Waikato is gratefully acknowledged. This research was partly funded by an Australian Research Council research grant administered by La Trobe University. References Abele, C.,Page, R.W., 1974. Stratigraphic and isotopic ages of Tertiary basalts of Maude and Aireys Inlet, Victoria, Australia. Proceedings of the Royal Society of Victoria 86, 143–150. Adam, J., Green, T., 2011. Trace element partitioning between mica- and amphibolebearing garnet lherzolite and hydrous basanitic melt: 2. Tasmanian Cainozoic basalts and the origins of intraplate basaltic magmas. Contributions to Mineralogy and Petrology 161, 883–899. Aziz-ur-Rahman, McDougall, I., 1972. Potassium-argon ages of the Newer volcanics of Victoria. Proceedings of the Royal Society of Victoria 85, 61–70. Baker, J.A., Gamble, J.A., Graham, I.J., 1994. The age, geology, and geochemistry of the Tapuaenuku Igneous Complex, Marlborough, New Zealand. New Zealand Journal of Geology and Geophysics 37, 249–268. Barling, J., Goldstein, S.L., Nicholls, I.A., 1994. Heard Island (southern Indian Ocean): characterization of an enriched mantle component and implications for enrichment of the sub-Indian Ocean mantle. Journal of Petrology 35, 1017–1053. Barriero, M.E.,Cooper, A.F., 1987. A Sr, Nd, and Pb isotopic study of alkaline lamprophyres and related rocks from Westland and Otago, South Island, New Zealand. Geological Society of America, Special Paper 215, 115–125. Blundy, J., Wood, B., 2003. Mineral–melt partitioning of uranium, thorium and their daughters. In: Bourdon, B., Henderson, G.M., Lundstrom, C.C., Turner, S.P. (Eds.), Uranium-Series Geochemistry. Reviews in Mineralogy and Geochemistry. 52, pp. 59–123. Bowen, K.G., 1975. Potassium-argon dates — determinations carried out for the Geological Survey of Victoria. Unpublished Report Geological Survey Victoria 1975/3. Boyce, J., 2013. The Newer Volcanics Province of southeastern Australia: a new classification scheme and distribution map of eruption centres. Australian Journal of Earth Sciences 60, 449–462. Campbell, I.H.,Griffiths, R.W., 1990. Implications of mantle plume structure for the evolution of flood basalts. Earth and Planetary Science Letters 99, 79–93. Cartwright, I., Weaver, T., Tweed, S., Ahearne, D., Cooper, M., Czapnik, K., Tranter, J., 2002. Stable isotope geochemistry of cold CO2-bearing mineral springwaters, Daylesford,

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