JOURNAL OF PETROLOGY

Journal of Petrology, 2015, Vol. 56, No. 4, 735–763 doi: 10.1093/petrology/egv015 Advance Access Publication Date: 6 May 2015 Original Article

Piggy-back Supervolcanoes—Long-Lived, Voluminous, Juvenile Rhyolite Volcanism in Mesoproterozoic Central Australia R. H. Smithies1*, H. M. Howard1, C. L. Kirkland1,2, F. J. Korhonen1, C. C. Medlin3, W. D. Maier4, R. Quentin de Gromard1 and M. T. D. Wingate1 1

Geological Survey of Western Australia, 100 Plain Street, East Perth, WA 6004, Australia, 2Centre for Exploration Targeting - Curtin Department of Applied Geology, Curtin University, Perth, WA 6845, Australia, 3School of Geosciences, Monash University, Wellington Road, Clayton, VIC 3800, Australia and 4School of Earth and Ocean Sciences, Cardiff University, Cardiff CF23 5LG, UK *Corresponding author. Telephone: þ61 8 92223611. E-mail: [email protected] Received September 25, 2014; Accepted March 19, 2015

ABSTRACT The Talbot Sub-basin is one of several bimodal volcanic depositional centres of the Mesoproterozoic Bentley Basin in central Australia. It is dominated by rocks of rhyolitic composition and includes ignimbrites, some forming large to super-eruption size deposits. Ferroan, incompatible trace element enriched, A-type compositions, anhydrous mineralogy and clear evidence for local rheomorphism indicate high eruption temperatures, with apparent zircon-saturation temperatures suggesting crystallization at >900 C. Comagmatic basalt is of mantle origin with minor Proterozoic basement contamination. The rhyolites cover the same range of Nd isotope compositions (eNd(1070) þ124 to –096) and La/Nb ratios (12–21) as the basalts (eNd(1070) þ21 to –11: La/Nb 12–23) and are compositionally far removed from all older basement and country-rock components (average eNd(1070) ¼ –4, La/Nb ¼ 10). The rhyolites and basalts are cogenetic through a process probably involving both fractional crystallization of mafic magmas and partial melting of recently crystallized mafic rock in a lower crustal intraplate, extraction of dacitic magmas to a voluminous upper crustal chamber system, and separation of rhyolite by processes involving rejuvenation and cannibalization of earlier chamber material. More than 230 000 km3 of parental basalt is required to form the >22 000 km3 of preserved juvenile rhyolite in the Talbot Sub-basin alone, which represents one of the most voluminous known felsic juvenile additions to intracontinental crust. Zircon U–Pb age components are complex and distinct from those of basement and country rock and contain antecrystic components reflecting dissolution–regrowth processes during periodic rejuvenation of earlier-emplaced chamber material without any significant interaction with country rock. The overall duration of magmatism was >30 Myr but can be divided into between two and four separate intervals, each probably of a few hundred thousand years’ duration and each probably reflecting one of the distinct lithostratigraphic groups defined in the sub-basin. Neither the composition nor style of felsic and mafic volcanism changes in any significant way from one volcanic event to the next and the range of zircon U–Pb ages indicates that each period utilized and cannibalized the same magma chamber. This volcanism forms a component of the 1090–1040 Ma Giles Event in central Australia, associated with magmadominated extension at the nexus of the cratonic elements of Proterozoic Australia. This event cannot be reasonably reconciled with any putative plume activity but rather reflects the >200 Myr

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legacy of enhanced crustal geotherms that followed the final cratonic amalgamation of central Australia. Key words: Mesoproterozoic supervolcano; central Australia

Musgrave

Province;

bimodal

volcanism;

mantle

derived;

INTRODUCTION

REGIONAL GEOLOGICAL SETTING

The significant hazards to human life presented by large-volume and ‘supervolcano-sized’ eruptions (i.e. 450 km3 of erupted material; Sparks et al., 2005) of rhyolitic magma have prompted numerous studies into recent volcanic systems, particularly aimed at understanding the timescales of magma generation, migration and storage, and storage–eruption lag times (e.g. Reid et al., 1997; Miller & Wooden, 2004; Simon et al., 2008; Schmitt et al., 2010; Storm et al., 2011, 2012; Eppich et al., 2012; Ruprecht & Cooper, 2012; Stelten & Cooper, 2012). These studies have led to a wider understanding of the petrogenetic processes and dynamic sub-volcanic magma chamber processes that combine to produce near-apocalyptic phenomena that are as impressive in the geological record as they are in their recent geomorphological legacy. The circumstances permitting the production, localization and maintenance of the magma- and heat-flux required to create, maintain and ultimately catastrophically destabilize highlevel magma chambers capable of producing large or super-eruptions are rare and extreme. Petrogenetic constraints established from studies of recent systems can be used to better understand the geological and tectonic evolution of ancient terrains containing the remnants of similar systems. Conversely, the deep time perspective offered by ancient systems might help to further understand modern volcanic systems, their durations and possible impacts on the global environment. We present new geological, geochemical, isotopic and geochronological data from the bimodal volcanic successions of the Bentley Supergroup in the Mesoproterozoic Musgrave Province of central Australia (Fig. 1), which include several large to supervolcano-sized rhyolitic deposits. The total duration of this high-volume silicic volcanism was >30 Myr and is one to two orders of magnitude greater than that of recent high-volume silicic volcanic systems. However, it seems that this duration can more probably be separated into two to four distinct, but superimposed, magmatic intervals. Over the eruptive timeframe there was no significant change in magma compositions or in the dominantly mantle source of the rhyolites and comagmatic basalts. The tectonic setting in which these superimposed volcanic systems evolved formed part of a >200 Myr history of high-temperature events that followed the final cratonic amalgamation of central Australia (Smithies et al., 2014; Howard et al., 2015). We investigate the petrogenesis of these rocks, the roles of crustal versus mantle processes and the unusual tectonic regime in which this magmatism occurred.

Extensive bimodal volcanism occurred in the west Musgrave region of central Australia during the 1090–1040 Ma Giles Event (e.g. Edgoose et al., 2004; Smithies et al., 2014; Howard et al., 2015). The basement rocks belong to the Musgrave Province, a Mesoproterozoic belt locked at the junction of the three main Precambrian cratonic elements of Australia. Outcrop in the Musgrave Province is dominated by granites formed during several earlier Mesoproterozoic events, in particular the 1345–1293 Ma Mount West Orogeny and the 1220–1150 Ma Musgrave Orogeny (Fig. 1; e.g. Edgoose et al., 2004; Wade et al., 2008; Kirkland et al., 2013a; Howard et al., 2015). The Mount West Orogeny produced metaluminous, calc-alkaline granites of the Wankanki Supersuite (Howard et al., 2015, and references therein) and is thought to mark final subduction and accretion during the amalgamation of the North, West and South Australian Cratons (Giles et al., 2004; Betts & Giles, 2006; Smithies et al., 2011; Kirkland et al., 2013a). The Musgrave Orogeny has been interpreted to have occurred in an intraplate setting (e.g. Wade et al., 2008; Smithies et al., 2011) and involved widespread emplacement of ferroan, alkali-calcic granites of the Pitjantjatjara Supersuite, which intruded the currently exposed mid-crust for the entire 1220–1150 Ma period. These magmas were emplaced at temperatures up to 1000 C and their intrusion coincided with a 100 Myr period (c. 1220–1120 Ma) of province-scale ultrahightemperature (UHT) metamorphism (e.g. King, 2008; Kelsey et al., 2009, 2010; Smithies et al., 2011; Walsh et al., 2014), forming perhaps the world’s largest and most prolonged known UHT metamorphic belt. Age constraints on the interval separating the Musgrave Orogeny and the Giles Event (c. 65–35 Myr) are minima and near-continuous zircon-forming events reflect widespread migmatization of crustal rocks during this period (Smithies et al., 2014).

The Giles Event Extraordinarily large volumes of mafic and felsic magma were emplaced into and erupted onto the crust of the Musgrave Province during the Giles Event [for a review see Howard et al. (2015)]. These magmatic rocks form the Warakurna Supersuite and include the bimodal volcanic rocks of the Bentley Supergroup, which define the preserved extent of the Bentley Basin (Fig. 2). Much of the Warakurna Supersuite is confined to the Musgrave Province, but it also includes the 1078–1073 Ma Warakurna large igneous province (LIP)

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737

WESTERN AUSTRALIA

130°

CANNING BASIN

ROF F

E

134°

Alice Springs 24°

AMADEUS BASIN

25°

MU

WOO D

132°

Arunta Orogen

NORTHERN TERRITORY

128°

LG A

W OO DR OF

UST THR

FREGON DOMAIN

Yulara

FE

PAR K

?

DOMAIN

NT

TH

MANN

FAULT

SA

RU ST

HINCKLEY FAULT

Inset Fig. 2

Musgrave

27°

SOUTH AUSTRALIA

.

EROMANGA BASIN OFFICER BASIN

RHS449e5

23.09.14

Mission Group

Musgrave Orogeny Mount West Orogeny

Tjauwata Group

Cassidy Group Pussy Cat Group Mount Palgrave and Kaarnka Groups Warakurna Supersuite

Bentley Basin

Tollu Group

Blackstone Sub-basin

Granites

NORTH AUSTRALIAN CRATON

Gabbro (G2) Layered mafic–ultramafic Giles intrusions (G1) Bentley Basin

1220–1150 Ma Pitjantjatjara Supersuite granite 1340–1270 Ma Wirku Metamorphics 1345–1293 Ma Wankanki Supersuite granite

WEST AUSTRALIAN CRATON Rudall Province Hamersley

PA TE ORRSO OG N EN

Arunta

Musgrave Province

Officer

Yilgarn

MSZ

Talbot Sub-basin

Bentley Supergroup

1090–1040 Ma Giles Event

100 km

Kunmarnara Group Musgrave Province

MUSGRAVE REGION

Province

Ngalia Amadeus

Gawler

PINJARRA OROGEN ALBANY–FRASER OROGEN

SOUTH AUSTRALIAN CRATON

Proterozoic basin Proterozoic orogen Archean craton Proterozoic craton margin MSZ Mundrabilla Shear Zone

N

1000 km

Fig. 1. Generalized regional geological map of the Musgrave Province [tectonic map modified from Myers et al. (1996)].

(Wingate et al., 2004), represented by dykes and sills of the Alcurra Dolerite, which were emplaced over c. 15  106 km2 in central and western Australia. The earliest stratigraphic unit of the Bentley Basin comprises medium- to coarse-grained siliciclastic rocks and basalt flows of the Kunmarnara Group. Remnants

of this group are preserved over an area that suggests a once continuous depositional basin of >50 000 km2. Layered mafic intrusions (Giles intrusions) form a series of enormous intrusive bodies dominated by gabbronorite, but ranging from dunitic cumulates to anorthosite. In the west Musgrave Province, Giles intrusions were

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127°00'

127°15'

127°30'

25°45'

50 km Talbot Sub-basin

Finlayson Sub-basin

Blackstone Sub-basin

126°45'

Mount Elsie

26°00'

Mount Waugh Mount Florrie Mount Shaw

Mount Hilda

OW RR BA

Thomas Hill

GE

N RA

Wa rb 4 k urton m

Mount Eliza

26°15'

Frank Scott Hill

Barrow Hill

Ranford Hill

RHS525d

23.09.14

20 km

Permian Buldya Group (Officer Basin) Mission Group

Bentley Supergroup

Warakurna Supersuite

Cassidy Group

Pussy Cat Group

Approximate geophysical outline of possible caldera structures Fault Anticline

Miller Basalt Hilda Rhyolite Warubuyu Basalt Thomas Rhyolite Gurgadi Basalt Gombugurra Rhyolite Wururu Rhyolite Subvolcanic porphyritic rhyolite intrusions Kathleen lgnimbrite Basalt dominated sequence Glyde Fm Volcaniclastic dominated sequence

Dip of bedding

Kaarnka Group

Volcaniclastic sediments Mount Palgrave Group

Subvolcanic rhyolite/dacite Rhyolite Basalt

Mount Waugh rhyolite Scamp Fm Eliza Fm

Winburn granite Layered mafic Giles intrusion Kunmarnara Group

Granofels and metasandstone (Bentley Supergroup)

Fig. 2. Detailed interpreted bedrock geology of the Talbot Sub-basin. Inset shows the preserved extent of the various sub-basins of the Bentley Basin.

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Officer Basin Mission Gp.

Stratigraphic thickness (km)

10

Cassidy

Gp.

Upper Kaarnka Gp.

Ca s s i d y G p. Pussy Cat Gp. 5

Kaarnka Gp.

Mount Palgrave Gp. Mount Palgrave Gp.

0

Winburn Granite

Musgrave Basement RHS981

Kaarnka caldron cluster Miller Basalt Hilda Rhyolite Warubuyu Basalt Thomas Rhyolite Gurgadi Basalt Gombugurra Rhyolite Wururu Rhyolite

Alcurra Dolerite

10.02.15

Unconformity/disconformity Present level of exposure

Fig. 3. Schematic summary of the stratigraphy of the Talbot Sub-basin.

emplaced between c. 1078 and 1076 Ma (Sun et al., 1996; Howard et al., 2011a), locally into the Kunmarnara Group. Bimodal volcanism forms the main component of the Bentley Supergroup and is preserved in several sub-basins. The best preserved sequence is within the Talbot Sub-basin (Figs 2 and 3) in the SW of the province, forming a series of thick, regionally continuous layers of rhyolitic ignimbrite, rheoignimbrite and lava flows (e.g. Smithies et al., 2013; Medlin, 2014). The base of this volcanic pile predates the 1074 6 8 Ma (Kirkland et al., 2010) intrusion of the syn-volcanic Winburn granite. Around 60 km to the east, the Blackstone Sub-basin (Fig. 2) includes rhyolites of the Tollu Group, erupted from c. 1073 to 1067 Ma (Coleman, 2009; Howard et al., 2009). The Alcurra Dolerite has been directly attributed to the Warakurna LIP and its emplacement age is generally regarded to be between 1078 and 1073 Ma (Wingate et al., 2004). However, compositionally equivalent intrusions and volcanic rocks in the Blackstone Sub-basin are as young as 1067 6 8 Ma

(Howard et al., 2009) and compositionally similar basalt occurs in the Talbot Sub-basin (Howard et al., 2011b) overlying rhyolite dated at 1052 6 5 Ma (Kirkland et al., 2013b).

GEOLOGY OF THE TALBOT SUB-BASIN The stratigraphy of the Talbot Sub-basin is summarized in Figs 2 and 3. The thickest preserved continuous stratigraphic section within the basin is c. 12 km. Rhyolite and basalt form c. 48% and 20%, respectively, of the stratigraphy. The rhyolites are typically vitric or cryptocrystalline with well-preserved primary volcanic textures. Feldspar phenocrysts show weak to moderate alteration to sericite and epidote. Magnetite is dusted throughout the otherwise cryptocrystalline groundmass. Magnetite elsewhere occurs as globular phenocrysts rimmed by titanite, and occurs with brown biotite and blue–green amphibole as small anhedral clots. In rare cases, primary clinopyroxene is observed but is largely altered to blue–green amphibole and biotite. Common accessory phases include titanite, apatite and

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Journal of Petrology, 2015, Vol. 56, No. 4

zircon and a rare dark brown, metamict phase which is likely to be chevkinite. The rhyolites typically preserve evidence of a greenschist-facies metamorphic overprint, although a fine- to medium-grained granofelsic texture is developed in the stratigraphically lowest units. Late- to post-magmatic alteration and autometasomatism has also variably resulted in the crystallization of epidote, chlorite, green biotite and pyrite. The detailed stratigraphy and petrography of the volcanic rocks has been described by Daniels (1974), Smithies et al. (2013) and Medlin (2014). The Mount Palgrave Group is the stratigraphically lowest preserved unit and has a strike length of >200 km and maximum thickness of 5 km (Table 1). Depositional layering generally dips shallowly (30 ) to the south and west. The basal formations comprise aphyric to sparsely feldspar-phyric rhyolites, including locally voluminous and laterally continuous massive, perlitic and spherulitic units, with well-developed flowbanding. Basalt and siliciclastic rocks rich in volcanogenic detritus form minor intercalations and clast-rich, non-welded ignimbrite, diamictite, and autoclastic breccia occur in the upper parts. The uppermost formation is composed of flow-banded, sparsely quartz- and feldspar-phyric rhyolite, locally with well-developed flow-banding, fiamme and eutaxitic flow structures. The overlying Kaarnka and Pussy Cat Groups are at least partially time-equivalent (Fig. 3). The Kaarnka Group has a combined stratigraphic thickness of about 3 km and is mainly restricted to a NW-trending 27 km  46 km basin (Fig. 2; Table 1). This forms the extent of the Kaarnka caldera structure (e.g. Daniels, 1974), the result of at least three independent caldera structures (the Kaarnka caldera cluster), and is locally marked by ring-faults filled by sub-volcanic porphyritic rhyolite. A chaotic stratigraphic architecture reflects block-rotation related to caldera formation. Late intrusions into this stratigraphy might be related to minor volcaniclastic deposits (upper Kaarnka Group) that overlie the main lithological units of the Kaarnka Group, reflecting a late stage of caldera activity.

The Kaarnka Group is characterized by massive to flow-banded, feldspar(–quartz)- phyric rhyolite with intercalations of crystal-rich to crystal-poor ignimbrite and lesser siliciclastic rocks and basalt flows. Several distinct rock types are recognized (e.g. rhyolite, quartzrhyolite, crystal-rich ignimbrite), and although all at least locally include what appear to be coherent volcanic units (lavas, sills or lava-like rheomorphic ignimbrites), most also show transitional contacts with bedded depositional units or crude to well-developed grain-size sorting that suggest a pyroclastic origin. The Pussy Cat Group is dominated by the Kathleen Ignimbrite and overlying amygdaloidal basaltic lavas and interlayered volcaniclastic sedimentary rocks. Both the Pussy Cat and Kaarnka Groups have very similar volcaniclastic basal units and a correlation of these units is also supported by geochemical similarities between specific rhyolitic units (i.e. Kathleen Ignimbrite and quartz-porphyritic rhyolite) (Smithies et al., 2013). The Kathleen Ignimbrite is locally over 500 m thick and is mainly composed of a flow-banded and lava-like rheomorphic ignimbrite (Daniels, 1974; Medlin, 2014). The Pussy Cat Group is overlain by the Cassidy Group. Both groups dip shallowly to the south and SW and form continuous packages that can be traced for over 100 km (Figs 1 and 2). The Cassidy Group, in particular, comprises rhyolitic lavas or lava-like rheomorphic ignimbrites and basalt flows and has a maximum preserved thickness of >3 km. Rhyolites of the Cassidy Group are typically dark grey and feldspar-phyric with a very fine-grained groundmass and can be massive, flow banded, or spherulitic. Basal autobreccias are rare and the basal portions of many units show discontinuous flow-banding. At least one rhyolite unit, the Thomas Rhyolite, locally contains abundant flattened pumice showing flow-foliation that wraps around crystals; this unit is the most laterally continuous rhyolite unit with an outcrop strike length of >90 km. The Mission Group conformably overlies the Cassidy Group, and represents the youngest preserved stratigraphic interval of the Talbot Sub-basin. It comprises a

Table 1: Stratigraphy, thickness, extent and volume of units from the Talbot Sub-basin Group

Formation

Mount Palgrave Mt Waugh Pussy Cat Kathleen Ign. Kaarnka Cassidy

Wururu Gombugurra Gurgadi Thomas Warubuyu Hilda Miller

Approx. max. thickness (km)

Strike

Single eruptive

Rhyolite

Basalt

length (km)

volume* (km3)

35 15 09 05 28 081 011

01

>200 >75 >200 35 >74 >30

multiple 3375 multiple 525 multiple 1800† 99

>90

970

>65

1050‡

21 02 05

036 02 054 05

*Assumes an original dip extent of at least 30 km—supported by recent seismic reflection data (e.g. Neumann, 2013). Might reflect up to three separate volumes. ‡ Might reflect two separate volumes (see text). †

Journal of Petrology, 2015, Vol. 56, No. 4

sedimentary lower part and an basalt-dominated upper part and is unconformably overlain by the Neoproterozoic unit of the Officer Basin. Although both the Mount Palgrave and Kaarnka Groups represent several separate eruptive cycles, some units have volumes that reflect ‘supervolcano’ class eruptions (450 km3; e.g. Sparks et al., 2005; Table 1). Some rhyolite formations of the Cassidy Group also show no internal depositional or flow boundaries and almost certainly reflect single eruption volumes. In addition, whereas the lowest rhyolite formation (Wururu Rhyolite) of the Cassidy Group forms three distinct benches, each possibly reflecting a separate depositional package, and exhibits variations in the concentration of Zr (see below) within the upper unit (Hilda Rhyolite) that possibly indicate two separate packages, it remains plausible that each of these five packages is also of ‘supervolcano’ volume (Table 1). High eruption temperatures are indicated by welldeveloped flow-banding and local rheomorphism; zircon-saturation thermometry indicates a wide range of apparent temperatures from 826 to 976 C. Many units are clearly pyroclastic but the origin of others is less clear. This is also the case for compositionally similar high-temperature Miocene rhyolites in the central Snake River Plain area of northwestern America [i.e. Snake River type volcanism of Branney et al. (2008)]. There, ignimbrites with lava-like features, including the absence of vitroclastic or eutaxitic textures, are distinguishable from true lavas only where critical field relationships are well exposed (Branney et al., 2008). One of the most diagnostic features is the lack of widespread basal autobreccias (Branney et al., 2008), a feature shared with rhyolite units of the Talbot Sub-basin. Along the eastern and northern margins of the Talbot Sub-basin, various phases of the Winburn Granite either intrude or are overlain by the Mount Palgrave Group. This intrusion comprises fine- to medium-grained, equigranular to porphyritic (and rapakivitextured), syenogranite and alkali-feldspar granite. Many samples comprise 70 wt % SiO2, and because the assumed parental Alcurra magma had an Mg# 47 indicating it was already considerably evolved. If this reasoning is correct, the initial cumulative volume of parental magma required to produce the total preserved felsic and mafic volcanic pile was >230 000 km3. Such enormous volumes imply petrogenetic processes operating over an area far greater than the region beneath (lower

752

Journal of Petrology, 2015, Vol. 56, No. 4 Table 3: Continued Group:

Alcurra Dolerite

Regional leucogranites

Lithology: Easting:* Northing: Sample:

Dolerite 344628 7120053 189593

Dolerite 354869 7163642 194354

Granite 473626 7116039 174761

Granite 435796 7100732 185583

SiO2 TiO2 Al2O3 Fe2O3T MnO MgO CaO Na2O K2O P2O5 LOI Total F Cr Ni V Sc Cu Zn Ga Cs Ba Rb Sr Pb Th U Nb Ta Zr Hf Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu 143 Nd/144Nd† (2r  10–6) 147 Sm/144Nd‡ eNd (1060 Ma)

4456 319 1406 1821 023 656 836 252 073 049 234 10124 884 139 139 423 27 88 173 252 044 312 23 216 10 30 05 164 10 229 60 506 232 523 69 327 82 260 905 140 845 172 480 430 067 0512367 5 0150824 099

4912 148 2051 930 011 239 1054 322 042 012 268 9988 237 9 46 249 17 100 74 287 017 222 10 355 4 19 03 64 04 98 27 153 119 257 32 1491 356 148 338 056 328 064 178 146 022 0512255 10 0144355 –03

7086 052 1305 384 007 042 155 264 597 011 094 9995 721 7 3 13 6 20 71 210 027 992 236 79 50 469 16 229 b.d. 457 166 805 883 2370 211 8479 1641 219 1406 243 1384 284 812 680 106 0511913 5 0117011 –323

7334 025 1232 222 004 017 107 284 579 004 178 9985 5671 17 7 4 4 2 41 196 244 312 401 49 59 1100 111 195 17 276 86 825 1409 2494 244 7413 1234 096 1058 175 1083 244 709 771 120 0511847 5 0100638 –223

*Alllocations GDA94 Zone 50. † Normalized to nominal La Jolla value of 0511850. ‡ 2r error of 01% on 147Sm/144Nd. n.d., not determined; b.d., below detection.

crust–mantle boundary?) the Talbot and Blackstone Sub-basins alone, as well as processes able to channel evolved magmas into discrete eruption centres.

THERMAL STATE AND HISTORY Large-volume silicic magmas are believed to erupt from melt-rich lenses that segregate within the upper 10 km (e.g. Gualda et al., 2012; Allan et al., 2013) of batholith-scale crystal-rich mush chambers of dacitic bulk composition (e.g. Hildreth, 2004; Bachmann & Bergantz, 2006, 2008; Dufek & Bachmann, 2010). These chambers may approach full crystallization, but

processes such as magmatic underplating, recharge or volatile addition may cause episodes of partial to wholesale remobilization or rejuvenation, and physical separation (e.g. through crystal settling or compaction) may cause melt migration and ponding. For recent felsic volcanic systems, it has been shown that freeze and thaw cycles generally operate on timescales of 104–105 years and that minerals may record several thaw cycles before eventual eruption (Reid et al., 1997; Miller & Wooden, 2004; Simon et al., 2008; Schmitt et al., 2010; Storm et al., 2011, 2012; Eppich et al., 2012; Ruprecht & Cooper, 2012; Stelten & Cooper, 2012).

Journal of Petrology, 2015, Vol. 56, No. 4

753

(a)

Nb ppm

120

80

40

(b) 100

Th ppm

80 60 40 20 0 0

2

RHS540b_BW

TALBOT SUB-BASIN Mount Palgrave Group Pussy Cat Group Cassidy Group Kaarnka Group Winburn granite

4

6

K2O wt%

8

10 04.02.15

BLACKSTONE SUB-BASIN Hogarth Formation Smoke Hill Volcanics

Fig. 6. Variation diagram showing K2O wt % vs Nb and Th (ppm) for magmatic rocks of the Talbot and Blackstone Subbasins.

Batholiths typically accumulate incrementally over periods of several million to a few tens of million years (e.g. de Saint Blanquat et al., 2011; Davis et al., 2012). Because single magma pulses are likely to be separated by sufficient time to cool to near-solidus conditions, only in unusual circumstances could large melt-rich magma chambers be produced or sustained through several freeze and thaw cycles (e.g. Glazner et al., 2004; Leeman et al., 2008; Annen, 2009). Annen (2009) suggested that to sustain the required upper-crustal temperatures long enough for large, or super-eruptive volumes to accumulate, an incoming magma flux of >10–2 to 10–1 km3 a–1 is needed. Similarly, Leeman et al. (2008) showed that raising large volumes of mid- to upper crust to the 900 C appropriate to form rhyolites of the Snake River Plain in North America required unreasonably voluminous intrusion of mafic magma, and that sub-solidus temperatures were re-established in 30 Myr. This system preserves strong

mineralogical (e.g. strongly embayed, disaggregated and multiply resorbed and regrown crystals; Fig. 5), and geochronological evidence that dormant or extinct parts of the same chamber system were repetitively recycled or cannibalized throughout that entire volcanic period. Although intervening non-eruptive periods are too short to be detected within the analytical uncertainty of ion microprobe U–Pb zircon dating [typically 6 5 Ma (1r) at c. 1050 Ma], many of these freeze periods must have been at least an order of magnitude longer than those recorded in modern volcanic systems (Reid et al., 1997; Miller & Wooden, 2004; Simon et al., 2008; Schmitt et al., 2010; Storm et al., 2011, 2012; Eppich et al., 2012; Ruprecht & Cooper, 2012; Stelten & Cooper, 2012), and clearly long enough for the system to cool well below solidus conditions under normal thermal conditions in the upper crust. Similarly, if we apply the incoming magma flux (>10–2 to 10–1 km3 a–1; Annen, 2009) required for super-eruptions to a chamber with a cross-sectional area of 5000 km2 (e.g. enough to enclose the entire eastern half of the Talbot Sub-basin including the Kaarnka caldera cluster), this chamber would fill to a depth of 15 km within only 4–9 Myr. Although this assumes only a single chamber system, it seems inevitable that over much of the >30 Myr volcanic history of the Talbot Sub-basin the magma chamber system was inactive. The models of both Leeman et al. (2008) and Annen (2009) assume a normal geothermal gradient prior to intrusion of heat-providing sills. In the case of the Talbot Sub-basin, however, an enhanced geothermal gradient persisted throughout the Giles Event as a result of the thermal evolution of the earlier (1220–1150 Ma) Musgrave Orogeny (Smithies et al., 2014). This intracratonic orogeny was an extraordinary event characterized by effectively complete removal of the mantle lithosphere, virtually continuous UHT metamorphism for as long as 100 Myr, a geothermal gradient up to 40 C km–1, and a crustal thickness of 30 Myr duration of volcanism within the Talbot Sub-basin, by which stage the chamber system would have expanded to fill virtually the entire upper crustal volume. Column on the right shows the results of thermal modelling. For increments of 100 kyr, rejuvenation of sub-solidus chamber material occurs after three chamber-forming increments and up to 30% of the modelled chamber volume is within the melting zone (i.e. at >830 C) after five increments. It should be noted that this amount would decrease as the chamber system expands and the ratio of sub-solidus chamber material to new magma input increases (i.e. as new inputs become increasingly wider spaced). The grey field denotes thermal models that assumed ‘average’ crustal compositions, a 150 km deep lithosphere–asthenosphere boundary and inferred no residual thermal effects of an earlier UHT metamorphic event.

Table 4: Rock parameters used in thermal modelling Unit Upper crust* Middle crust† Pitjantjatjara Supersuite Lower crust Mantle Dacite magma Dacite (rock) Dolerite magma Dolerite (rock) Upper/middle crust¶

Th (ppm)

U (ppm)

K2O (wt %)

242 42

21 4

43 5

25

33

37

11

04

069

105

27

28

Q (mW m–3)

k (W m–1 K–1)

C (MJ m–3 K–1)

Depth interval (km)

13 33‡ 5‡ 04 002 36‡ 36 034 02 2

2 3 3 277 3 24 3 19 3 3

206 21 21 3 4 206 21 206 4 21

0–10 10–18 18–20 20–46 46–200§

Q, heat production; k, thermal conductivity; C, thermal capacity. *Assumes a conservative, relatively non-radiogenic upper crust. †Composition approximated by 70% average Wankanki Supersuite granite and 30% average Pitjantjatjara Supersuite granite— average compositions from Smithies et al. (2013). ‡Age corrected to c. 1080 Ma. §Lower model boundary fixed at 100 km depth to simulate linear adiabat to 200 km depth. ¶Based on Rudnick & Gao (2003).

or less. The results also confirm that eruptions from the Talbot magma-chamber system cannot have occurred at regular intervals over the >30 Myr duration of volcanism because intervening amagmatic periods of >1 Myr allow sub-solidus portions of the chamber to cool too much. For chamber construction increments of 100 kyr, the residual effects of previous increments

provide a significant advantage that leads to rapidly increasing temperatures that, at a depth of 10 km, exceed 830 C after three increments (Fig. 13). After five increments, temperatures of up to 900 C (and increasing) are locally achieved and the chamber cross-sectional area residing at temperatures >830 C is c. 30% of the total modelled chamber area, of which c. 30%

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represents older (previously sub-solidus) chamber material—as required to form the antecryst-rich Talbot magmas. This translates to a volume of potentially rejuvenated chamber material of 2100 km3, without taking into account any potential mixing between rejuvenated and new magma inputs. To investigate whether the thermal pre-history of the region was a factor in producing and sustaining this volcanic system, we also modelled the effect of the same chamber construction history on the thermal structure of a crustal profile with‘average’ compositions for the upper, middle and lower crust (Table 4), and a lithosphere–asthenosphere transition at a depth of 150 km, and we assumed no residual thermal effects of an earlier UHT metamorphic event. Even assuming only short amagmatic periods, chamber temperatures never exceeded 830 C (grey field; Fig. 13). These are simple models but the primary conclusions are robust. They demonstrate that supereruptions required extreme thermal conditions. In the case of the Bentley Basin, crustal architecture and thermal properties inherited from the Musgrave Orogeny probably had a significant influence on the style and longevity of magmatism. Of particular importance is the likelihood of long-term thin crust and a shallow asthenosphere transition. Within this thin crustal regime, the actual depth of the Alcurra intraplate (hot zone) actually has only a minimal influence on our calculated thermal structure within the Talbot chamber zone, which is more influenced by the regional crustal geothermal gradient. It is perhaps particularly relevant here that pre-Giles evolution left this thin crustal regime enriched in highly radiogenic Pitjantjatjara granite. Even under this elevated regional geothermal gradient, the production of large volumes of eruptible felsic material through rejuvenation of older sub-solidus chamber material at crustal depths of 10–15 km requires a sequence of chamber construction events to occur within a period of only a few hundred thousand years. Consideration of the magma input volumes of each modelled chamber construction event, the number of events needed to elevate large volumes of previously sub-solidus chamber material to >830 C, and the rate at which the system cools between construction events confirms earlier conclusions that voluminous volcanism can have occurred over only a small proportion of the >30 Myr volcanic history of the Talbot Sub-basin. An unavoidable conclusion is that the >11 recorded eruptive events relate to more than one distinct period of volcanism and we can speculate that the four distinct stratigraphic groups within the sub-basin might each relate to one such period. Each distinct period of volcanism would require at least 5–6 chamber construction increments (with at least two increments before any actual eruption) over 500–600 kyr, consistent with estimates from recent systems. Thus, the Talbot Sub-basin actually records at least two, and more probably up to four, distinct volcanic periods of closely spaced and often super-sized eruptive events, spatially

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superimposed, or ‘piggy-backed’ such that they reused the same subvolcanic chamber system, but were separated by time gaps that might normally suggest that they were totally unrelated volcanic systems.

DISCUSSION The Giles Event and the tectonic control on the Talbot volcanic system Although initial studies related the Giles Event to the effects of a deep-mantle plume (Zhao & McCulloch, 1993; Wingate et al., 2004; Godel et al., 2011), other than the large magma volume associated with this event, there is no clear evidence favouring a mantle plume. Evins et al. (2010) and Smithies et al. (2014) suggested that the reasons for the Giles Event are entirely tectonic. Bimodal volcanism within the Talbot Sub-basin is only one component of the overall magmatic inventory of the >1090 Ma to 50 Myr with no time-progressive geographical trend that might reflect the trace of a mantle plume (e.g. Evins et al., 2010; Smithies et al., 2014). In the case of the Talbot Sub-basin, the ‘piggy-back’ sequence of eruptive events is somewhat analogous to compressing or superimposing the entire regional (>600 km) felsic magma track of the Miocene Snake River Plain–Yellowstone Plateau of North America, developed over c. 16 Myr (e.g. Leeman et al., 2008), into a single volcanic centre. The extraordinary conditions required to produce this magmatism, and the reasons why these conditions either persisted or recurred in the same area throughout the formation of the Talbot Subbasin, or indeed, the duration of the Giles Event itself, are intrinsic to the crustal architecture, rather than related to a mantle plume (e.g. Smithies et al., 2014). Such crustal-scale processes must have had a major bearing on the tectonic evolution of the region. The Giles Event can be viewed as a temporal continuation of a much longer history of greatly enhanced thermal gradients extending back at least to the c. 1220 Ma beginning of the Musgrave Orogeny. The Musgrave Province lies at the nexus of the three main cratonic components of Proterozoic Australia (Fig. 1). Final accretion of these cratonic elements probably occurred after the evolution of the Mount West continental arc (i.e. the c. 1293 Ma termination of magmatism related to the Mount West Orogeny) (Giles et al., 2004; Betts & Giles, 2006; Smithies et al., 2011; Kirkland et al., 2013a) but before the UHT Musgrave Orogeny. The Musgrave Orogeny has been interpreted to have evolved in an intraplate setting (e.g. Wade et al., 2008; Smithies et al., 2011) possibly as an ultra-hot orogeny

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(e.g. Chardon et al., 2009) born out of a hot back-arc related to the Mount West Orogeny (Smithies et al., 2014). A critical legacy of this UHT orogeny was that the ambient regional geothermal gradient throughout the Giles Event (and younger) was intrinsically high as a combined result of residual heat, significantly thinned crust and a high mid-crustal accumulation of radiogenic heat producing granites (e.g. Smithies et al., 2011, 2014). Indeed, our thermal models show that Talbot volcanism, and probably the Giles Event itself, would almost certainly not have occurred without the thermal legacy of the preceding Musgrave Orogeny. Mantle-derived melts were a significant contributor to magmatism throughout the Musgrave Orogeny (Smithies et al., 2011; Kirkland et al., 2013a) and continued as the overwhelmingly dominant component of magmas formed during the Giles Event. In this regard, it is possible that both the thermal state and the regional lithospheric architecture—thin crust locked between three cratonic masses—allowed for a longsustained mantle-melt enriched zone beneath the region. Silver et al. (2006) suggested that such melt reservoirs can exist and persist beneath areas with elevated geotherms or favourable lithospheric architecture and that many cratonic flood basalt provinces do not represent mantle melting events but represent events that drain these reservoirs as a result of abrupt changes in stress fields. The Giles Event itself involved the formation of the west-trending Ngaanyatjarra Rift (Evins et al., 2010), which Aitken et al. (2012) viewed as a spectacular example of a ‘magmatism-dominated’ rift system, in which actual extension was relatively limited and controlled more by intrinsic processes related to redistribution of magma and heat rather than extrinsic factors such as regional stress. The western end of the Ngaanyatjarra Rift is truncated by the Mundrabilla Shear Zone (Fig. 1), a north-trending, translithospheric structure (Aitken et al., 2012), and the point of truncation coincides with the Talbot Sub-basin. The age of major sinistral strike-slip displacement along this structure is constrained between the c. 1140 Ma age (Kirkland et al., 2011) of a granite to the south of the Musgrave region, truncated by sinistral movement along the structure, and the age of the Bentley Supergroup, the stratigraphy of which at least partially drapes over the structure. Movement along the translithospheric Mundrabilla Shear Zone potentially had significant implications for the Mesoproterozoic evolution of central Australia (e.g. Aitken et al., 2012). Such structures can juxtapose lithosphere of contrasting thermal and physical properties. Gorczyk et al. (2012), Gessner et al. (2013) and Stern et al. (2013) have highlighted the role that lithospheric heterogeneities play in localizing lithospheric gravitational instabilities that may lead to major asthenospheric upwellings. In the case of the Musgrave Province, the juxtaposition of the Musgrave thermal anomaly against colder cratonic lithosphere might not only trigger complete destabilization of any remaining

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or regrown lithospheric mantle on the Musgrave Province side of the suture, but also lead to gravitational instabilities in, and delamination of, colder cratonic lithosphere on the western side of the shear zone (e.g. Stern et al., 2013). Smithies et al. (2014) suggested that movement along the Mundrabilla Shear Zone initiated the Giles Event by disrupting the post-Musgrave Orogeny juvenile lithosphere and tapping a vast subMusgrave melt reservoir, similar to the process envisaged by Silver et al. (2006). Formation of the Warakurna LIP and emplacement of the giant, layered, Giles intrusions between 1078 and 1073 Ma perhaps represent not the first, but the main phase of movement along the shear zone, catastrophically destabilizing the sub-crustal lithosphere of the Musgrave Province. Subsequent movements could explain the episodic development of superimposed supervolcanic systems within the >30 Myr magmatic history of the Talbot Sub-basin. Evidence for subsequent smaller movements during the Giles Event itself is seen in geophysical images that show the continuous stratigraphy of the Cassidy Group draped over the faulted margins of the Mount Palgrave Group.

CONCLUSIONS The bimodal volcanic stratigraphy of the Mesoproterozoic Talbot Sub-basin represents a truly extraordinary sequence of high-temperature and higheruptive-volume volcanic rocks, perhaps without parallel in the known geological record. The evolution of this sequence has implications in terms of the petrogenesis of high-volume felsic intraplate magmatism and the Mesoproterozoic geological and tectonic evolution of central Australia. The major conclusions of this study are as follows. 1. The intracratonic Talbot Sub-basin bimodal magmatism evolved over an extraordinarily long (>30 Myr) period. This duration is at least one order of magnitude longer than that of recent large, high-volume, rhyolitic systems. However, volcanism was clearly not continuous, but occurred in several (2–4) volcanic periods, each probably 500 kyr in duration, more consistent with that of recent, large, volcanic systems. 2. The intervals separating each of the volcanic periods must have been sufficiently long that each might be considered an independent tectonomagmatic event. However, neither the composition nor style of felsic and mafic volcanism changed in any significant way from one period, or event, to the next, and the locus of bimodal magmatism remained effectively stationary. Zircon U–Pb age ranges are distinct from those of basement and country rocks and apparent mean ages for the rhyolites contradict stratigraphic relationships. However, zircon crystal textures reveal embayed cores, euhedral overgrowths and multiply truncated growth surfaces. Hence, both the zircon

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ages and internal textures indicate an antecrystic cargo within the rhyolites consistent with each volcanic period reworking and cannibalizing the same chamber system over the duration of magmatic activity. Hence, what distinguishes the Talbot Subbasin volcanic system is that it evolved over several separate tectonomagmatic events in exactly the same place over an interval of >30 Myr. 3. Despite the voluminous felsic products, the magmatic system is fundamentally of mantle origin. The rhyolites of the Talbot Sub-basin represent one of the most voluminous known felsic juvenile additions to intracontinental crust. 4. Basement and country-rock Nd isotope compositions and zircon age profiles are distinct from those of the Talbot volcanic rocks and the lack of evidence for any significant contamination indicates an extremely large magma chamber system that effectively shielded any new magma inputs from communication with pre-Giles country rocks. 5. Evolution of the Talbot system reflects processes that occurred at all levels within a crustal profile thinned during the earlier UHT Musgrave Orogeny. These processes would almost certainly not have occurred without the thermal legacy of the preceding Musgrave Orogeny, and the fortuitous and repetitive truncation by the translithospheric Mundrabilla Shear Zone.

ACKNOWLEDGEMENTS We are indebted to the Ngaanyatjarra Council and the Traditional Owners of the Warburton–Wingellina region, for access to their land and for their help and hospitality. Journal reviews from Carol Frost, David Champion and an anonymous reviewer significantly helped improve the paper. Michael Prause and Joyce Peng are thanked for drafting figures. This work is published with the permission of the Executive Director, Geological Survey of Western Australia.

SUPPLEMENTARY DATA Supplementary data for this paper are available at Journal of Petrology online.

REFERENCES Aitken, A. R. A., Smithies, R. H., Dentith, M. C., Joly, A., Evans, S. & Howard, H. M. (2012). Magmatism-dominated intracontinental rifting in the Mesoproterozoic: The Ngaanyatjarra Rift, central Australia. Gondwana Research doi:10.1016/ j.gr.2012.10.003. Allan, A. S. R., Morgan, D. J., Wilson, C. J. N. & Millet, M.-A. (2013). From mush to eruption in centuries: assembly of the super-sized Oruanui magma body. Contributions to Mineralogy and Petrology 166, 143–164. Annen, C. (2009). From plutons to magma chambers: Thermal constraints on the accumulation of eruptible silicic magma in the upper crust. Earth and Planetary Science Letters 284, 409–416.

761 Annen, C., Blundy, J. D. & Sparks, R. S. J. (2006). The genesis of intermediate and silicic magmas in deep crustal hot zones. Journal of Petrology 47, 505–539. Bachmann, O. & Bergantz, G. W. (2006). Gas percolation in upper-crustal silicic crystal mushes as a mechanism for upward heat advection and rejuvenation of near-solidus magma bodies. Journal of Volcanology and Geothermal Research 149, 85–102. Bachmann, O. & Bergantz, G. W. (2008). Rhyolites and their source mushes across tectonic settings. Journal of Petrology 49, 2277–2285. Betts, P. G. & Giles, D. (2006). The 1800–1100 Ma tectonic evolution of Australia. Precambrian Research 144, 92–125. Branney, M. J., Bonnichsen, B., Andrews, G. D. M., Ellis, B., Barry, T. L. & McCurry, M. (2008). ‘Snake River (SR)-type’ volcanism at the Yellowstone hotspot track: distinctive products from unusual, high-temperature silicic super-eruptions. Bulletin of Volcanology 70, 293–314. Bryan, S. E., Ferrari, L., Reiners, P. W., Allen, C. M., Petrone, C. M., Ramos-Rosique, A. & Campbell, I. H. (2008). New insights into crustal contributions to large-volume rhyolite generation in the Mid-Tertiary Sierra Madre Occidental Province, Mexico, revealed by U–Pb geochronology. Journal of Petrology 49, 47–77. Cathey, H. E. & Nash, B. P. (2004). The Cougar Point Tuff: implications for thermochemical zonation and longevity of hightemperature, large-volume silicic magmas of the Miocene Yellowstone Hotspot. Journal of Petrology 45, 27–58. Chardon, D., Gapais, D. & Cagnard, F. (2009). Flow of ultra-hot orogens: A view from the Precambrian, clues for the Phanerozoic. Tectonophysics 477, 105–118. Clauser, C. (2003). Numerical Simulation of Reactive Flow in Hot Aquifers using SHEMAT/Processing Shemat. Berlin: Springer. Coleman, P. (2009). Intracontinental orogenesis in the heart of Australia: structure, provenance and tectonic significance of the Bentley Supergroup, western Musgrave Block, Western Australia. Geological Survey of Western Australia, Record 2009/23, 50 pp. Currie, C. A. & Hyndman, R. D. (2006). The thermal structure of subduction zone back arcs. Journal of Geophysical Research 111, B08404, doi:10.1029/2005JB004024. Daniels, J. L. (1974). The geology of the Blackstone region, Western Australia. Geological Survey of Western Australia, Bulletin 123, 257 pp. Davis, J. W., Coleman, D. S., Gracely, J. T., Gaschnig, R. & Stearns, M. (2012). Magma accumulation rates and thermal histories of plutons of the Sierra Nevada batholith, CA. Contributions to Mineralogy and Petrology 163, 449–465. de Saint Blanquat, M., Horsman, E., Habert, G., Morgan, S., Vanderhaeghe, O., Law, R. & Tikoff, B. (2011). Multiscale magmatic cyclicity, duration of pluton construction, and the paradoxical relationship between tectonism and plutonism in continental arcs. Teconophysics 500, 20–33. Dufek, J. & Bachmann, O. (2010). Quantum magmatism: Magmatic compositional gaps generated by melt–crystal dynamics. Geology 38, 687–690. Edgoose, C. J., Scrimgeour, I. R. & Close, D. F. (2004). Geology of the Musgrave Block, Northern Territory. Northern Territory Geological Survey, Report 15, 48 pp. Eppich, G. R., Cooper, K. M., Kent, A. J. R. & Koleszar, A. (2012). Constraints on crystal storage timescales in mixed magmas: Uranium-series disequilibria in plagioclase from Holocene magmas at Mount Hood, Oregon. Earth and Planetary Science Letters 317–318, 319–330. Evins, P. M., Smithies, R. H., Howard, H. M., Kirkland, C. L., Wingate, M. T. D. & Bodorkos, S. (2010). Devil in the detail; The 1150–1000 Ma magmatic and structural evolution of the

762 Ngaanyatjarra Rift, west Musgrave Province, Central Australia. Precambrian Research 183, 572–588. Frost, B. & Frost, C. (1997). Reduced rapakivi-type granites: the tholeiite connection. Geology 25, 647–650. Frost, B. & Frost, C. (2011). On ferroan (A-type) granitoids: their compositional variability and modes of origin. Journal of Petrology 52, 39–53. Frost, B. R., Barnes, C. G., Collins, W. J., Arculus, R. J., Ellis, D. J. & Frost, C. D. (2001). A geochemical classification for granite rocks. Journal of Petrology 42, 2033–2048. Gessner, K., Gallardo, L. A., Markwitz, V., Ring, U. & Thomson, S. N. (2013). What caused the denudation of the Menderes Massif: Review of crustal evolution, lithosphere structure, and dynamic topography in southwest Turkey. Gondwana Research 24, 243–274. Giles, D., Betts, P. G. & Lister, G. S. (2004). 18–15-Ga links between the North and South Australian Cratons and the Early–Middle Proterozoic configuration of Australia. Tectonophysics 380, 27–41. Glazner, A. F., Bartley, J. M., Coleman, D. S., Gray, W. & Taylor, R. Z. (2004). Are plutons assembled over millions of years by amalgamation from small magma chambers? Geological Society of America, Today 14, 4–11. Godel, B., Seat, Z., Maier, W. D. & Barnes, S.-J. (2011). The Nebo–Babel Ni–Cu–PGE sulfide deposit (West Musgrave Block, Australia): Part 2—constraints on parental magma and processes, with implications for mineral exploration. Economic Geology 106, 557–584. Gorczky, W., Hobbs, B. & Gerya, T. (2012). Initiation of Rayleigh–Taylor instabilities in intra-cratonic settings. Tectonophysics 514–517, 146–155. Gualda, G. A. R., Ghiorso, M. S., Lemons, R. V. & Carley, T. L. (2012). Rhyolite-MELTS: a modified calibration of MELTS optimized for silica-rich, fluid-bearing magmatic systems. Journal of Petrology 53, 875–890. Hildreth, W. (2004). Volcanological perspectives on Long Valley, Mammoth Mountains, and Mono Craters: several contiguous but discrete systems. Journal of Volcanology and Geothermal Research 136, 169–198. Howard, H. M., Smithies, R. H., Kirkland, C. L., Evins, P. M. & Wingate, M. T. D. (2009). Age and geochemistry of the Alcurra Suite in the west Musgrave Province and implications for orthomagmatic Ni–Cu–PGE mineralization during the Giles Event. Geological Survey of Western Australia, Record 2009/16, 16 pp. Howard, H. M., Werner, M., Smithies, R. H., Kirkland, C. L., Kelsey, D. L., Hand, M., Collins, A., Pirajno, F., Wingate, M. T. D., Maier, W. D. & Raimondo, T. (2011a). The geology of the west Musgrave Province and the Bentley Supergroup—a field guide. Geological Survey of Western Australia Record 2011/4, 119 pp. Howard, H. M., Smithies, R. H., Werner, M., Kirkland, C. L. & Wingate, M. T. D. (2011b). Geochemical characteristics of the Alcurra Dolerite (Giles Event) and its extrusive equivalents in the Bentley Supergroup. In: GSWA 2011 Extended Abstracts: Promoting the Prospectivity of Western Australia. Geological Survey of Western Australia, Record 2011/2, 27–30. Howard, H. M., Smithies, R. H., Kirkland, C. L., Kelsey, D. E., Aitken, A., Wingate, M. T. D., Quentin de Gromard, R., Spaggiari, C. V. & Maier, W. D. (2015). The burning heart— the Proterozoic geology and geological evolution of the west Musgrave Region, central Australia. Gondwana Research 27, 64–94. Kelsey, D. E., Hand, M., Evins, P., Clark, C. & Smithies, H. (2009). High temperature, high geothermal gradient metamorphism in the Musgrave Province, central Australia; potential constraints on tectonic setting. In: Abstracts: Specialist

Journal of Petrology, 2015, Vol. 56, No. 4 Group in Geochemistry, Mineralogy and Petrology, Geological Society of Australia; Kangaroo Island 2009 Conference, Kangaroo Island, South Australia, 8–13 November, 2009, p.28. Kelsey, D. E., Hand, M., Smithies, H., Evins, P., Clark, C. & Kirkland, C. L. (2010). What is the tectonic setting of longlived Grenvillian-aged ultrahigh temperature, high geothermal gradient metamorphism in the Musgrave Province, central Australia? Geological Society of America, Abstracts with Programs 42, 516. King, R. J. (2008). Using calculated pseudosections in the system NCKFMASHTO and SHRIMP II U–Pb zircon dating to constrain the metamorphic evolution of paragneisses in the Latitude Hills West Musgrave Province, Western Australia. Geological Survey of Western Australia, Record 2009/15, 67 pp. Kirkland, C. L., Wingate, M. T. D. & Howard, H. M. (2010). 174662: granophyre, Eliza Rocks. Geological Survey of Western Australia, Geochronology Record 909, 4 pp. Kirkland, C. L., Wingate, M. T. D., Spaggiari, C. V. & Tyler, I. M. (2011). 194773: granitic rock, Eucla No. 1 drillhole. Geological Survey of Western Australia, Geochronology Record 1001, 4 pp. Kirkland, C. L., Smithies, R. H., Woodhouse, A. J., Howard, H. M., Wingate, M. T. D., Belousova, E. A., Cliff, J. B., Murphy, R. C. & Spaggiari, C. V. (2013a). Constraints and deception in the isotopic record; the crustal evolution of the west Musgrave Province, central Australia. Gondwana Research 23, 759–781. Kirkland, C. L., Wingate, M. T. D. & Howard, H. M. (2013b). 185415: dacite, Mount Palgrave. Geological Survey of Western Australia, Geochronology Record 1127, 5 pp. Leeman, W. P., Annen, C. & Dufek, J. (2008). Snake River Plain–Yellowstone silicic volcanism: implications for magma genesis and magma fluxes. In: Annen, C. & Zellmer, G. F. (eds) Dynamics of Crustal Magma Transfer, Storage and Differentiation. Geological Society, London, Special Publications 304, 235–259. Ludwig, K. R. (2003). Isoplot 3.00; a geochronological toolkit for Microsoft Excel. Berkeley Geochronology Centre, Special Publication 4, 70 pp. Martin, E. & Sigmarsson, O. (2010). Thirteen million years of silicic magma production in Iceland; Links between petrogenesis and tectonic setting. Lithos 116, 129–144. Medlin, C. C. (2015). The volcanology, geochemistry, and evolution of a shallow marine intra-caldera, rheomorphic ignimbrite succession: The Mesoproterozoic Kathleen Formation, west Musgrave Province, central Australia. PhD thesis, Monash University, Clayton, VIC (unpublished). Miller, J. S. & Wooden, J. L. (2004). Residence, resorption and recycling of zircons in the Devils Kitchen rhyolite, Coso Volcanic field, California. Journal of Petrology 45, 2155–2170. Morris, P. A. (2007). Composition of the Bunbury Basalt (BB1) and Kerba Monzogranite (KG1) geochemical reference materials, and assessing the contamination effects of mill heads. Geological Survey of Western Australia Record 2007/14. Myers, J. S., Shaw, R. D. & Tyler, I. M. (1996). Tectonic evolution of Proterozoic Australia. Tectonics 16, 1431–1446. Neumann, N. L. (ed.) (2013). Yilgarn Craton–Officer Basin–Musgrave Province Seismic and MT Workshop. Geoscience Australia Record 2013/28, 210 pp. O’Nions, R. K., Evensen, N. M. & Hamilton, P. J. (1979). Geochemical modelling of mantle differentiation and crustal growth. Journal of Geophysical Research 84, 6091–6101.

Journal of Petrology, 2015, Vol. 56, No. 4 Pearce, J. A., Harris, N. B. W. & Tindle, A. G. (1984). Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. Journal of Petrology 25, 956–983. Reid, M. R., Coath, C. D., Harrison, T. M. & McKeegan, K. D. (1997). Prolonged residence times for the youngest rhyolites associated with Long Valley Caldera: 230Th–238U ion microprobe dating of young zircons. Earth and Planetary Science Letters 150, 27–39. Rudnick, R. L. & Gao, S. (2003). Composition of the continental crust. In: Rudnick, R. L. (ed.) Treatise on Geochemistry, Volume 3. Elsevier, pp. 1–64. Ruprecht, P. & Cooper, K. M. (2012). Integrating the uraniumseries and elemental diffusion geochronometers in mixed magmas from Volca´n Quizapu, central Chile. Journal of Petrology 53, 841–871. Schmitt, A. K., Stockli, D. F., Lindsay, J. M., Robertson, R., Lovera, O. M. & Kislitsyn, R. (2010). Episodic growth and homogenization of plutonic roots in arc volcanoes from combined U–Th and (U–Th)/He zircon dating. Earth and Planetary Science Letters 295, 91–103. Silver, P. G., Behn, M. D., Kelley, K., Schmitz, M. & Savage, B. (2006). Understanding cratonic flood basalts. Earth and Planetary Science Letters 245, 190–201. Simon, J. I., Renne, P. R. & Mundil, R. (2008). Implications of pre-eruptive magmatic histories of zircons for U–Pb geochronology of silicic extrusions. Earth and Planetary Science Letters 266, 182–194. Smithies, R. H., Howard, H. M., Evins, P. M., Kirkland, C. L., Kelsey, D. E., Hand, M., Wingate, M. T. D., Collins, A. S. & Belousova, E. (2011). High-temperature granite magmatism, crust–mantle interaction and the Mesoproterozoic intracontinental evolution of the Musgrave Province, Central Australia. Journal of Petrology 52, 931–958. Smithies, R. H., Howard, H. M., Kirkland, C. L., Werner, M., Medlin, C. C., Wingate, M. T. D. & Cliff, J. B. (2013). Geochemical evolution of rhyolites of the Talbot Sub-basin and associated felsic units of the Warakurna Supersuite. Geological Survey of Western Australia Report 118, 74 pp. Smithies, R. H., Kirkland, C. L., Korhonen, F. J., Aitken, A. R. A., Howard, H. M., Maier, W. D., Wingate, M. T. D., Quentin de Gromard, R. & Gessner, K. (2014). The Mesoproterozoic thermal evolution of the Musgrave Province in central Australia—plume vs the geological record. Gondwana Research dx.doi.org/10.1016/j.gr.2013.12.014. Solano, J. M. S., Jackson, M. D., Sparks, R. S. J., Blundy, J. D. & Annen, C. (2012). Melt segregation in deep crustal hot zones: a mechanism for chemical differentiation, crustal assimilation and the formation of evolved magmas. Journal of Petrology 53, 1999–2026. Sparks, R. S. F., Self, S., Pyle, D., Oppenheimer, C., Rymer, H. & Grattan, J. (2005). Super-eruptions: Global Effects and Future Threats. Report of a Geological Society of London Working Group, 24 pp. Stacey, J. S. & Kramers, J. D. (1975). Approximation of terrestrial lead isotope evolution by a two-stage model. Earth and Planetary Science Letters 26, 207–221. Stelten, M. E. & Cooper, K. M. (2012). Constraints on the nature of the subvolcanic reservoir at South Sister volcano, Oregon from U-series dating combined with sub-crystal traceelement analysis of plagioclase and zircon. Earth and Planetary Science Letters 313–314, 1–11. Stern, R. A. (2001). A new isotopic and trace-element standard for the ion microprobe: preliminary thermal ionization mass spectrometry (TIMS) U–Pb and electron-microprobe data.

763 Geological Survey of Canada, Radiogenic Age and Isotopic Studies, Report 14, Current Research 2001-F1, 11 pp. Stern, R. A., Bodorkos, S., Kamo, S. L., Hickman, A. H. & Corfu, F. (2009). Measurement of SIMS instrumental mass fractionation of Pb isotopes during zircon dating. Geostandards and Geoanalytical Research 33, 145–168. Stern, T., Houseman, G., Salmon, M. & Evans, L. (2013). Instability of a lithospheric step beneath western North Island, New Zealand. Geology 41, 423–426. Storm, S., Shane, P., Schmitt, A. K. & Lindsay, J. M. (2011). Contrasting punctuated zircon growth in two syn-erupted rhyolite magmas from Tarawera volcano: Insights to crystal diversity in magmatic systems. Earth and Planetary Science Letters 301, 511–520. Storm, S., Shane, P., Schmitt, A. K. & Lindsay, J. M. (2012). Decoupled crystallization and eruption histories of the rhyolite magmatic system at Tarawera volcano revealed by zircon ages and growth rates. Contributions to Mineralogy and Petrology 163, 505–519. Streck, M. J. & Grunder, A. L. (2008). Phenocryst-poor rhyolites of bimodal, tholeiitic provinces: the Rattlesnake Tuff and implications for mush extraction models. Bulletin of Volcanology 70, 385–401. Sun, S.-S., Sheraton, J. W., Glikson, A. Y. & Stewart, A. J. (1996). A major magmatic event during 1050–1080 Ma in central Australia, and an emplacement age for the Giles Complex. AGSO Journal of Australian Geology and Geophysics 24, 13–15. Wade, B. P., Kelsey, D. E., Hand, M. & Barovich, K. M. (2008). The Musgrave Province: stitching north, west and south Australia. Precambrian Research 166, 370–386. Watson, E. B. & Harrison, M. T. (1983). Zircon saturation revisited: temperature and composition effects in a variety of crustal magma types. Earth and Planetary Science Letters 64, 295–304. Walsh, A. K., Kelsey, D. E., Kirkland, C. L., Hand, M., Smithies, R. H., Clark, C. & Howard, H. M. (2014). P–T–t evolution of a large, long-lived, ultrahigh-temperature Grenvillian belt in central Australia. Gondwana Research doi:10.1016/ j.gr.2014.05.012. Whalen, J. B., Currie, K. L. & Chappell, B. W. (1987). A-type granites; geochemical characteristics, discrimination and petrogenesis. Contributions to Mineralogy and Petrology 95, 407–418. Whitaker, M. L., Nekvasil, H., Lindsley, D. H. & Difrancesco, N. J. (2007). The role of pressure in producing compositional diversity in intraplate basaltic magmas. Journal of Petrology 48, 365–393. Whitaker, M. L., Nekvasil, H., Lindsley, D. H. & McCurry, M. (2008). Can crystallization of olivine tholeiite give rise to potassic rhyolites?—an experimental investigation. Bulletin of Volcanology 70, 417–434. Wingate, M. T. D. & Kirkland, C. L. (2014). Introduction to geochronology information released in 2014. East Perth: Geological Survey of Western Australia, 5 pp., http://www. dmp.wa.gov.au/geochron. Wingate, M. T. D., Pirajno, F. & Morris, P. A. (2004). Warakurna large igneous province: a new Mesoproterozoic large igneous province in west–central Australia. Geology 32, 105–108. Zhao, J.-X. & McCulloch, M. T. (1993). Melting of a subductionmodified continental lithospheric mantle: evidence from Late Proterozoic mafic dyke swarms in central Australia. Geology 21, 463–466.