Okataina Volcanic Centre, Taupo Volcanic Zone, New Zealand: A review of volcanism and synchronous pluton development in an active, dominantly silicic caldera system

Earth-Science Reviews 128 (2014) 1–17

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Okataina Volcanic Centre, Taupo Volcanic Zone, New Zealand: A review of volcanism and synchronous pluton development in an active, dominantly silicic caldera system J.W. Cole a,⁎, C.D. Deering b, R.M. Burt a, S. Sewell c, P.A.R. Shane d, N.E. Matthews e,1 a

Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand Department of Geology, University of Wisconsin–Oshkosh, 800 Algoma Rd, Oshkosh, WI 54901-8649, USA c Mighty River Power, P.O. Box 245, Rotorua 3040, New Zealand d School of Environment, University of Auckland, Private Bag 82019, Auckland, New Zealand e Department of Earth Sciences, University of Oxford, South Parks Road, OX1 3AN, UK b

a r t i c l e

i n f o

Article history: Received 23 July 2013 Accepted 9 October 2013 Available online 17 October 2013 Keywords: Caldera Volcanism Plutonism Structure Petrology Evolution/model

a b s t r a c t The Okataina Volcanic Centre (OVC) is one of eight caldera systems, which form the central part of the Taupo Volcanic Zone, New Zealand. During its ~625 kyr volcanic history, which perhaps equates to ~750 kyr of magmatic history, the OVC has experienced two definite periods of caldera collapse (Matahina, ~322 ka, and Rotoiti, for which dates of 61 and 45 ka have recently been published), one probable collapse (Utu, ~557 ka) and one possible collapse (Kawerau, ~33 ka). Each collapse accompanied voluminous ignimbrite eruptions. Rhyolite dome extrusion and explosive tephra eruptions have occurred throughout the history of OVC. This paper reviews volcanological observations, and geochemical and geophysical data that provides evidence for the nature and evolution of the mid- to upper crustal magma system below OVC. The chemistry of the largely rhyolitic juvenile pyroclastic deposits and lavas (most with 73–78 wt.% SiO2) is reviewed, together with evidence provided by plutonic and mafic lithic blocks found within some pyroclastic deposits to reconstruct reservoir development. Detailed studies of zircon crystals provide age control for the longevity of the supersolidus state of the magmatic system of the OVC, while geophysical measurements, in particular resistivity and magnetotelluric (MT) data, suggest the present day existence of partial melts at depths of between 8 and 15 km. A comparison with older exposed high-level plutonic systems helps explain some of the features found in the erupted plutonic lithic blocks at OVC, and provides an indication of the potential longevity of the system. An integration of these disparate datasets allows a model to be developed in which an extensive, intermediate composition ‘mush’ zone occurs at 8–15 km depth, from which more silicic melt fractions periodically rise to higher level sill or laccolith-like ‘pods’ in the crust. Sometimes one of these pods may erupt to produce lava or pumice of a single composition, while at other times a number of pods are tapped to form large-scale, calderaforming eruptions. Periodically, the magmatic system reaches its solidus or near-solidus, which allows ascending basalt to reach the shallow magmatic system. In the last 50 kyrs, some of these basalts have reached the surface, for example during the 1886 AD fissure eruption from Tarawera volcano. A comparison with other active caldera complex systems in TVZ and overseas suggests that while the general model may apply, there are variations because of different tectonic setting, crustal thickness and age of the system. However, the general model has implications for geothermal reservoir evaluation and studies of epithermal ore deposition. The high crustal level magma system beneath OVC is probably part way through its evolution, so further intrusions and eruptions can be expected in the future, with clear implications for hazard evaluation. © 2013 Elsevier B.V. All rights reserved.

Contents 1. 2. 3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Geologic background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Okataina Volcanic Centre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⁎ Corresponding author. Tel.: +64 3 3642766. E-mail address: [email protected] (N.E. Matthews). 1 Now at: Volcano Science Center, USGS, 345 Middlefield Road, Menlo Park, CA 94025, USA. 0012-8252/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.earscirev.2013.10.008

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3.1. Caldera collapse events . . . . . . . . . . . . . . . . 3.2. Dome complexes and associated tephra fall . . . . . . . 4. Mineralogy and geochemistry . . . . . . . . . . . . . . . . . 4.1. Utu Ignimbrite . . . . . . . . . . . . . . . . . . . . 4.2. Matahina Ignimbrite . . . . . . . . . . . . . . . . . . 4.3. Rotoiti Ignimbrite . . . . . . . . . . . . . . . . . . . 4.4. Earthquake Flat Pyroclastics . . . . . . . . . . . . . . 4.5. Mangaone Pyroclastics (including the Kawerau Ignimbrite) 4.6. Tarawera and Haroharo Dome Complexes . . . . . . . . 5. Plutonic and mafic lithic blocks . . . . . . . . . . . . . . . . 5.1. Rotoiti Ignimbrite . . . . . . . . . . . . . . . . . . . 5.2. Whakatane Tephra . . . . . . . . . . . . . . . . . . 5.3. Kaharoa Tephra . . . . . . . . . . . . . . . . . . . . 6. Depth of crystallisation . . . . . . . . . . . . . . . . . . . . 7. Crystallisation timescales . . . . . . . . . . . . . . . . . . . 8. Comparison with older high-level plutonic systems . . . . . . . 9. Geophysical Data . . . . . . . . . . . . . . . . . . . . . . . 10. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction There have been many studies of caldera structure and evolution (e.g. Smith et al., 1961; Bailey et al., 1976; Walker, 1984; Lipman, 1997, 2000; Cole et al., 2005), and it has been recognised that an understanding of the relationship between caldera volcanism and contemporary plutonism is important (e.g. Hildreth, 1981; Lipman et al., 1997; Metcalf, 2004; Quick et al., 2009; Sewell et al., 2012). However, few single studies can offer comprehensive models of sub-volcanic plumbing systems in the upper crust beneath caldera volcanoes. Magma systems undergo a complex series of processes from their source, commonly in the mantle, to eruption and solidification at the Earth's surface. Our understanding of these processes comes from a study of both volcanic and plutonic rocks. These represent different stages of the process, with plutonic rocks representing either magmas which have completely solidified in the crust or what is left in the crust after a portion of the magma rises to the surface during an eruption (e.g. Bachmann and Bergantz, 2008). The central part of the Taupo Volcanic Zone (TVZ), North Island, New Zealand (Fig. 1), is the most frequently active and productive Quaternary silicic system on Earth (Houghton et al., 1995), characterised by intense, and volumetrically dominant, rhyolitic volcanism associated with large calderas and caldera complexes (Fig. 1). These represent the sites of major ignimbrite eruptions, but rhyolite domes and the products of Plinian explosive eruptions associated with the same magma systems may well extend beyond the boundaries of the calderas. At the northern end of the central TVZ (Fig. 1) is the Okataina Volcanic Centre (OVC), within which is the Okataina Caldera Complex (Cole et al., 2005; Cole and Spinks, 2010). This caldera complex represents several phases of collapse that have created a depression partially filled with intracaldera ignimbrite, younger domes and pyroclastic deposits, and lakes (Fig. 2). OVC includes all vents associated with the Okataina magma system, which extend beyond the structural boundaries of the calderas, and hence the OVC is the appropriate name to use throughout this paper. Despite numerous individual studies of both caldera-related volcanism in the central TVZ (e.g. Brown et al., 1998; Beresford and Cole, 2000; Milner et al., 2002; Wilson et al., 2006; Gravley et al., 2007), and those of the zone as a whole (e.g. Houghton et al., 1995; Bibby et al., 1998; Rowland and Sibson, 2001; Wilson et al., 2009; Cole et al., 2010), only a few studies consider the relationship between TVZ volcanic and plutonic realms (e.g. Brown et al., 1998; Burt et al., 1998; Charlier et al., 2003). A more detailed assessment of this relationship is therefore timely. Volcanic eruptions at the surface provide snapshots in the development of the subsurface magma chamber. Plutons on the other hand,

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3 5 6 6 6 6 6 7 7 9 9 9 9 10 10 10 11 11 14 14 14

represent portions of the magma chamber that were, for a variety of reasons, unable to erupt. Plutons are not exposed at the surface in the TVZ and can only be examined as lithic fragments (e.g. Brown et al., 1998) or in drill core (e.g. Browne et al., 1992). These plutonic rocks are extremely unlikely to represent a simple single-stage cooling event magma chamber; rather they represent the hybridized remnants of repeated magma replenishment, crystallisation, extraction and latestage sub-solidus alteration. This review describes the stratigraphy, geochemistry and geophysics of OVC, evaluates information that can be obtained from lithic fragments brought up from below during explosive eruptions, and briefly compares this modern caldera system with older exposed high-level plutons to produce a model for the shallow, largely silicic, magma system beneath OVC, and compares the system to other active caldera complexes in TVZ and overseas. OVC has been chosen because there has already been extensive study of its volcanic history, shallow crustal structure and petrological and geochemical evolution. It is regarded as active, with the last rhyolite eruption in 1314 AD (Nairn et al., 2001), and basalt eruption in 1886 AD (Sable et al., 2006), and has experienced frequent eruptions over the last ~ 625 kyrs, and hence, a better understanding of its evolution should improve hazard mitigation. The general model for sub-caldera magmatism has implications for geothermal resource evaluation and epithermal ore deposition. 2. Geologic background TVZ is the actively rifting, on-land continuation of the Tonga– Kermadec arc (Fig. 1), formed at the convergent plate boundary between the westward subducted Pacific Plate and the Australian Plate (Cole, 1990; Wilson et al., 1995). Extension rates range from ~15 mm/y at the Bay of Plenty coast to b 5 mm/y at 39°S (south of Lake Taupo) (Wallace et al., 2004). Mesozoic greywacke metasediments outcrop to the east and west of the TVZ, are found in geothermal drill holes on the eastern side of TVZ, and probably underlie the volcanic fill within most of TVZ, either as tectonically stretched greywacke crust, or as rifted greywacke blocks separated by intrusive rocks (Stern et al., 2006). Convective heat output from geothermal systems in the TVZ is exceptionally high at ~4300 MW (Bibby et al., 1995), equivalent to 26 MW/km along strike (Stern et al., 2006) and similar to hot spots such as Iceland and Yellowstone. The high heat flux causes the brittleto-ductile transition to occur at shallow depth (~6 km) within the central TVZ (Bryan et al., 1999; Sherburn et al., 2003). Crystal and melt inclusion data indicate that rising magmas in dike-like structures

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Fig. 1. Regional map showing the location of the Taupo Volcanic Zone (TVZ) and its associated tectonic and volcanic structures (after Deering et al., 2011a). The outline of Old TVZ and Young TVZ, and division into Northern TVZ, Central TVZ and Southern TVZ are from Wilson et al. (1995). Mapped faults in the Taupo Rift are from Gravley et al. (2007); Calderas are as follows: Ma = Mangakino; Ka= Kapenga; Wh = Whakatane; Ro = Rotorua; Rp = Reporoa; Oh = Ohakuri; Tp = Taupo; Ok = Okataina (Okataina Volcanic Centre; with heavier outline). Black triangles show sites of andesite cones. NIFS = North Island Fault System. A-A′ is line of 3-D inversion cross section shown in Fig. 10. Inset shows the location of TVZ and the Coromandel Volcanic Zone (CVZ) within the North Island of New Zealand and their relationship to subduction of the Pacific plate beneath the Australian plate.

stall at the brittle–ductile transition and form sills (Shane et al., 2008a). The depth and nature of the crust–mantle boundary in central TVZ is uncertain. Bannister et al. (2004) has analysed receiver functions in the central TVZ and observed a rapid increase in S-wave velocity at about 25 km depth, which they interpret as the crust–mantle boundary. Harrison and White (2004) examined a seismic refraction profile north of Lake Taupo (~80 km south of the OVC) and concluded that the quartzo-feldspathic upper crust is ~16 km thick and underlain by a heavily intruded or underplated lower crust to a depth of ~30 km. More recently Stern and Benson (2011) have undertaken a wideangle seismic survey across TVZ and have concluded that there is a “rift-pillow” of mafic rocks between 15 and 25 km depth in which Vp values range between 6.8 and 7.1 km/s. They interpret these as partially molten and in various stages of cooling. They also note a reflector at 32 ± 1 km depth, consistent with the top of a body in the upper mantle where partial melts may be as high as 12%. 3. Okataina Volcanic Centre OVC largely comprises silicic volcanism, the younger volcanic history of which is well described by Nairn (2002) and summarized by Cole et al. (2010) (Table 1).

3.1. Caldera collapse events There are two definite, one probable and one possible, periods of collapse within OVC (Cole et al., 2010), to form the Okataina Caldera Complex, each of which has been accompanied by the eruption of a widespread ignimbrite. The probable period of collapse is associated with the ~557 ka Utu Ignimbrite (Deering et al., 2011a; date from Leonard et al., 2010). The eruptive products are not well exposed at the surface, but occur in conglomerates of the Newdick Member at Maketu, on the Bay of Plenty coast (Wehrmann, 2000), in small surface outcrops in the Puhipuhi Basin, and in a drill hole at Rerewhakaaitu (Fig. 3). The unit correlates with the Karaponga Formation in geothermal wells at Kawerau (Milicich et al., 2013). It is described by Nairn (1981; called the Quartz–biotite ignimbrite) as typically non-welded to weakly-welded, with devitrified pumice lapilli, and large crystals of quartz (up to 4 mm) and abundant biotite with a volume of ~90 km3 (Nairn, 2002). Because of subsequent volcanism, there is no direct topographic or subsurface evidence of the Utu caldera in OVC. The two definite periods of collapse are associated with the ~322 ka Matahina Ignimbrite (Deering et al., 2011a; date from Leonard et al, 2010) and the Rotoiti Ignimbrite (Nairn, 2002; Schmitz and Smith, 2004). There has been considerable debate about the age of the Rotoiti

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Fig. 2. Volcanic geology of the Okataina Volcanic Centre. WDC = Wairua Dome Complex. Inset: relationship between the OVC topographic caldera and negative gravity anomaly. Contours in mGals (modified from Cole et al., 2010).

Ignimbrite. Wilson et al (2007) considered it to be ~1 ka, largely on the basis of a 40Ar/39Ar age of obsidian lava bracketing the accompanying Rotoehu Tephra (the fallout correlative of Rotoiti), but Danišík et al. (2012) have recently revised this to ~45 ka on the basis of 238U/230Th disequilibrium and (U/Th)/He dating of zircons from the Rotoiti and Earthquake Flat ignimbrites, plus new high precision carbon dating of wood on either side of the Rotoehu Tephra. If the latter date is correct it raises some questions about the dates on the overlying Mangaone Pyroclastics (45–28 ka), as there is a well-developed paleosol between the two units. It would also mean that the Mangaone magmas would have to develop and erupt very quickly after the larger Rotoiti eruption.

The Matahina Ignimbrite is partially to strongly welded, with characteristic rhyolite and vesicular obsidian lithics. It is exposed mainly to east and north of OVC (Fig. 3), where it ranges in thickness from 5 to 150 m and covers an area of ~2000 km2 (Bailey and Carr, 1994). A basal plinian pumice fall unit overlies a paleosol on Murupara tuffs (Deering et al., 2011a), and is overlain by three ignimbrite sheets, representing a total (intra-caldera and outflow sheet) magmatic volume of at least 160km3 (Deering, 2009). The Rotoiti eruption marks the end of a period of relative quiescence within the OVC that had probably lasted over 150 ka (Nairn, 2002). Rhyolite domes around the southern margin of the OVC, dated at 188 ka and 96 ka (Leonard et al., 2008) are the only

Table 1 Generalised stratigraphy of OVC since ~625 ka (ignimbrites in square brackets found in OVC, but erupted from another caldera/caldera complex). Based on Cole et al. (2010). Name

Mangaone Pyroclastic Subgroup (MaSg) Rotoiti Pyroclastic Subgroup (RoSg) Te Wairoa Pyroclastics

Ignimbrite

Lava Dome (D)/ Dome Complex (DC)

Age (ka)

Volume (magma equivalent)

References

Haroharo & Tarawera Volcanic Complexes

b25 ka

c.85 km3

Nairn (2002)

28–45

N20 km3

Jurado-Chichay and Walker (2000), V.C. Smith et al. (2002), Spinks (2005) Nairn (2002), Charlier et al (2003), Shane et al. (2005a), Wilson et al. (2007), Danišík et al (2012) Bellamy (1991), Nairn (2002)

Kawerau [Earthquake Flat] Rotoiti

c.45/c.61

c.10 km N100 km3 N1 km3

c.240

N145 km3 N100 km3 c.120 km3 c.20 km3

c.322

3

Moerangi DC Tutaeheke DC [Mamaku] [Ohakuri] [Kaingaroa]

Onuku/Pokopoko Pyroclastics Matahina Matawhaura D Murupara Pyroclastic Subgroup (MuSg)

340–322 Wairua DC Maungawhakamana DC Whakapoungaku DC Utu

?

3

c.557

Milner et al (2002), Gravley et al (2007), Beresford and Cole (2000)

?

Nairn (2002), Manning (1996), Bailey and Carr (1994), Nairn (2002), Deering (2009), Deering et al. (2011a) Nairn (2002) Manning (1996), Nairn (2002), Deering (2009), Deering et al. (2011a) Nairn (2002)

c.90 km3

Nairn (2002)

N160 km ? c.30 km3

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Fig. 3. Distribution of ignimbrites erupted from OVC. The Northern Boundary Rd drillhole is the site of the best sequence through the Utu Igninmbrite.

evidence of volcanic activity during this period of quiescence. The Rotoiti Ignimbrite is a compound, non-welded unit, with a combined magma volume of N 100 km3 (Nairn, 2002). The Rotoiti Caldera collapsed during the emplacement of the Rotoiti Ignimbrite (Nairn, 1981, 2002), producing thick lithic lag breccias around lakes Rotoiti, Rotoehu and Rotoma. The Rotoiti Ignimbrite forms a large fan (N850 km2) that extends mainly north of OVC to the Bay of Plenty coastline, with smaller exposures to the east, in the Puhipuhi embayment, and in the axial rift zone to the northeast (Fig. 3). The distribution of the ignimbrite, and the locations of the lithic lag breccias, indicate collapse in the northern part of OVC (Fig. 2). The Rotoiti eruption may have been triggered by regional basalt intrusion (Nairn, 1981; Shane et al., 2005a; Molloy et al., 2008; Smith et al., 2010). Direct evidence for regional basaltic magmatism prior to the Rotoiti eruption is provided by the absence of paleosol development on the basaltic Matahi scoria, which immediately underlies the Rotoiti eruptives (Pullar and Nairn, 1972). Minor collapse may also have occurred with the eruption of the Kawerau Ignimbrite (~33 ka; N20 km3; Spinks, 2005; Unit I of JuradoChichay and Walker, 2000). The distribution, greatest thickness and westward coarsening lithic component at Puhipuhi suggest a source in southern OVC to the west of the embayment (Fig. 2), as suggested by Jurado-Chichay and Walker (2000).

3.2. Dome complexes and associated tephra fall Throughout the history of OVC, rhyolite domes have been extruded, sometimes accompanied by localized block and ash flows, and there have been periodic Plinian eruptions producing widespread tephra deposits. These are fully described in Cole et al. (2010), and here only a summary is given. Manning (1996) noted that the presence of cummingtonite in some distal tephras (dated between 420 and 625 ka; Nairn and Beanland, 1989; Manning, 1995) which is indicative of an OVC source, and suggests rhyolitic volcanism at OVC possibly began as early as ~625 ka. Rhyolite domes are likely to be associated with these tephras, and abundant angular rhyolite lithic blocks (commonly ≥5 cm), which must have been derived from pre-caldera rhyolite extrusions in OVC, are found within the Utu Ignimbrite. Dome extrusion continued after the Utu Ignimbrite eruption, with domes exposed on the NW, NE and SW of OVC (Fig. 2), then at ~340 ka a series of explosive eruptions formed the c.30 km3 Murupara Subgroup (Manning, 1995, 1996; Deering, 2009; Deering et al., 2011a), which are the precursors for the Matahina eruption. Explosive volcanism continued after the Matahina Ignimbrite was erupted, with the deposition of the Onuku Pyroclastics to the south and east of OVC, and the Pokopoko Pyroclastics to the west and north. A number of rhyolite domes were also extruded (Fig. 2). These led up to the Rotoiti

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eruptive sequence from the northern part of OVC, and immediately following, non-welded ignimbrites and widespread tephra fall deposits of the Earthquake Flat Pyroclastics were erupted from a NW-trending series of vents (Fig. 2) located to the southwest of OVC (Nairn, 2002; Molloy et al., 2008). The Mangaone tephra sequence (45–28 ka; N 20 km3; JuradoChichay and Walker, 2000) is bracketed in time by the Oruanui Ignimbrite (25.4 ka; Vandergoes et al., 2013), erupted from the Taupo Volcanic Centre, and the Rotoiti Ignimbrite. Twelve Plinian fall deposits dominate the sequence, with two thin, non-welded, pyroclastic flow deposits associated with specific plinian eruptions (Jurado-Chichay and Walker, 2000). No lavas are recognised from this eruptive phase, but many of the pyroclastic deposits are rich in rhyolite lava lithics, and any lava piles that developed may have been destroyed during larger explosive events. In any case, the young (b25 ka) intra-caldera lavas have buried any evidence for previous dome-building episodes as well as any proximal pyroclastic deposit, making vent location inherently difficult. The style of activity at OVC changed at ~25 ka (perhaps significantly at the same time as the major caldera-forming Oruanui event further south at Taupo; Smith et al., 2005; Wilson et al., 2006) and has been characterised by eruptions from multiple vents along two sub-parallel NE-trending vent zones that transect the caldera complex (Fig. 2). The Haroharo Dome Complex, in the north of OVC (Fig. 2), developed in the last 25 ka during five main eruptive episodes (Nairn, 2002; Smith et al., 2005, 2006). This complex comprises rhyolite lava domes and flows with interbedded pyroclastic flow and fall deposits. Tarawera Dome Complex (Fig. 2) is similar to Haroharo, but occupies the southern part of OVC. It has grown as a result of five eruptive episodes during the last 22 ka, culminating with the 1886 AD basaltic eruption (Nairn, 2002; Sable et al., 2006; Hiess et al., 2007; Shane et al., 2007, 2008b). The OVC occupies a 10 km hard-linked left step in the Taupo Rift, with the northwestern and southeastern boundaries of the caldera complex equally being fault controlled (Seebeck et al., 2010), while the Haroharo and Tarawera Dome Complexes are each controlled by a linear vent zone (Cole and Spinks, 2010; Cole et al., 2010). Eruptive vents within these two zones also occur on the periphery of OVC, associated with embayments of the caldera margin. Vents at Okareka (15.7 ka; Rotorua deposit) and Rotoma lie on the Haroharo linear vent zone, while vents at Puhipuhi and Rotomahana similarly lie on the Tarawera linear vent zone (Fig. 2). 4. Mineralogy and geochemistry Numerous papers have been written on the mineralogy and geochemistry of the silicic rocks associated with OVC (e.g. Burt et al, 1998; I.E.M. Smith et al., 2002; Nairn et al., 2004; Schmitz and Smith, 2004; Shane et al., 2005b, 2008a, 2008b; Deering, 2009; Deering et al., 2010, 2011a; Smith et al., 2010). A summary of key findings is given below. 4.1. Utu Ignimbrite Mineralogically, the ferromagnesian phase assemblage (biotite + hornblende + orthopyroxene) and modal abundance (15–25%) of juvenile pumice clasts from the Utu Ignimbrite are similar to other recently dated volcanics in OVC, including the Whakapoungakau rhyolite dome (531±5ka), exposed along the margin of the OVC and two Plinianstyle fall-out deposits found near the coast north of the OVC (unit 6: 551 ± 4 ka, and unit 8: 537 ± 5 ka; Gravley et al., 2009). 4.2. Matahina Ignimbrite Juvenile clasts from the Matahina Ignimbrite include rhyolite, coarse-grained andesite and fine-grained basalt, which Deering et al.

(2011a) have divided into five groups (1a, 1b, 1c, 2, 3) on bulk pumice major and trace element chemistry (Fig. 4a). The lower and middle member of the ignimbrite is higher-silica rhyolite (Group 1a) and dominantly crystal-poor (b10 vol.%; characterised by orthopyroxene and hornblende), with a lower-silica, slightly more crystal-rich (up to 21 vol.%; with orthopyroxene ± hornblende) upper member (Group 1b). Group 1c is similar to Group 1a, but has higher K2O (Fig. 4a) and is biotite-bearing. Cognate, crystal-rich (up to 50 vol.%) basalt (Group 3) to intermediate andesitic (Group 2) pumice occur on top of lag breccias and within lithic-rich pyroclastic density current deposits along the caldera margin. Collapse of the caldera included all groups within pyroclastic density currents and lag breccias, implying that the foundering crust squeezed out these crystal-rich components. Magma mingling/mixing between the basalt-andesite and the main, slightly compositionally zoned, rhyolitic magma occurred during caldera-collapse, modifying the leastevolved rhyolite at the lower portion of the reservoir and effectively destroying any pre-eruptive gradients. 4.3. Rotoiti Ignimbrite Weak chemical variation in the Rotoiti eruption sequence (Fig. 4b) has been variously attributed to xenocrystic contamination (Davis, 1985), magma mixing (Schmitz, 1995), and a weakly zoned magma chamber (Burt et al., 1998). More recently, systematic analysis of mineral and glass chemistry through the Rotoiti sequence (Schmitz and Smith, 2004; Shane et al., 2005a) has shown that most compositional variation is the result of mingling between a dominant cummingtonite + hornblende + orthopyroxene rhyolitic magma (Assemblage A of Schmitz and Smith, 2004; T1 of Shane et al., 2005a) and a subordinate biotite + hornblende + orthopyroxene rhyolitic magma (Assemblage B of Schmitz and Smith, 2004; T2 of Shane et al., 2005a) which initially appears midway through the eruption sequence and increasing in relative proportion thereafter. While Rayleigh crystal fractionation modeling fits the range in pumice chemistry (Schmitz and Smith, 2004), there is a distinct compositional gap in mineral and glass chemistry separating the two magma types (Shane et al., 2005a). Quartz and plagioclase melt inclusion studies (Smith et al., 2010) provides further evidence for open system behaviour. They suggest that most crystallization occurred in b 2000 years, while mush zones may last much longer (104–105 years). The Matahi Tephra is often correlated with the hypothetical mafic trigger to the Rotoiti eruption and similarly with the Earthquake Flat Pyroclastics (described below in 4.4). Fragments of rhyolite and xenocrysts in the Matahi are from underlying units (Schmitz and Smith, 2004) and, despite no paleosol development between the Matahi and Rotoiti, there is no evidence for mingling between the two magma types (Schmitz and Smith, 2004). Burt et al. (1998), and the larger data set of Matthews (2006), however, showed that rare microdiorite clasts in the Rotoiti lag breccia overlapped with the Matahi in composition and that the microdiorite could represent fragments of the Matahi magma event that were injected laterally, crystallized and then incorporated into the Rotoiti during caldera collapse. These fragments however are almost 100% crystalline (b 10% glass) and the glass remnant contains vesicles suggesting that the plutonic body was disrupted by the caldera collapse and the rapid change in pressure caused the residual glass to vesiculate. 4.4. Earthquake Flat Pyroclastics Eruption of the crystal-rich (~35%) Earthquake Flat (EQF) pyroclastics occurred shortly after the Rotoiti eruption, probably within weeks or months of the latter event (Nairn and Kohn, 1973; Schmitz and Smith, 2004). The eruption took place from six NW–SE trending vents spread over 5 km at the SW margin of OVC (Fig. 2), and produced ~10 km3 of non-welded ignimbrite and phreatomagmatic fall deposits (Rifle Range

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Despite the similarity in age and general chemistry between EQF and Rotoiti Ignimbrite, the two eruptives are thought to come from different high-level magma systems. Charlier et al. (2003) and Charlier and Wilson (2010) show that zircon derived TIMS model ages and Sr isotope ratios preclude a simple relationship. Smith et al. (2010) suggest the EQC magma system may have resided in the upper crust for a longer period than the Rotoiti magma system. 4.5. Mangaone Pyroclastics (including the Kawerau Ignimbrite) Geochemical fingerprinting of the glass and phenocryst phases (V.C. Smith et al., 2002, 2005; Shane et al., 2005b) allows clear subdivision of the Mangaone pyroclastics into two stratigraphic intervals. Units A–G are high-temperature (870–940 °C) rhyodacites and low-SiO2 rhyolites (71–75.5 wt.% SiO2 glass; 68–71 wt.% SiO2 whole rock), while units H–L are lower temperature (755–839 °C), high SiO2 rhyolites (76–78 wt.% SiO2 glass; 71.5–75 wt.% SiO2 whole rock). There are two discrete magmas in the Kawerau Ignimbrite, a dominant rhyolitic magma equivalent to the young Mangaone Subgroup magma of V.C. Smith et al. (2002) and a subordinate dacitic magma (Fig. 4c), which underwent limited mingling during eruption, and provides further evidence for the existence of multiple discrete magmas in the Okataina region. 4.6. Tarawera and Haroharo Dome Complexes

Fig. 4. K2O v SiO2 variation within pumices of a) Matahina Ignimbrite (triangle — type 1a; diamond — type 1b; square — type 1c; cross — type 2 and circle — type 3; date from Deering et al., 2011a); b) Rotoiti Ignimbrite (data from Schmitz and Smith, 2004); c) Kawerau Ignimbrite (data from Spinks, 2005).

Ash; Nairn and Kohn, 1973; Froggatt and Lowe, 1990). The deposits form a low fan around the vents (Nairn, 2002). The eruptives are all high-silica rhyolite dominated by biotite, hornblende and orthopyroxene mafic minerals, but with no cummingtonite. The hornblende crystals are zoned, and Molloy et al. (2008) consider that there was a thermal event immediately prior to eruption that rejuvenated the magma and may have triggered the event.

The eruptive sequence from Tarawera and Haroharo Dome Complexes is given in Nairn (2002) and Cole et al. (2010), with most vents sited along the Tarawera and Haroharo linear vent zones (Fig. 2). There have been 10 eruptive episodes from these complexes in the past 25 ka (Nairn, 2002). Details of the mineralogy and chemistry of all units are given in Smith et al. (2006). Examples of representative eruptions are given below to illustrate the range of petrological and geochemical variation within an episode. Deposits from the Okareka eruption from Tarawera (22.6 ka) came from 3 rhyolite magma batches and a basaltic magma batch (Fig. 5a; Shane et al., 2008b). Volcanism began with a 0.01 km3 basaltic eruption and rapidly changed to a Plinian eruption comprising moderate temperature (750 °C) cummingtonite-bearing rhyolite (T1). Hybrid basalt/ rhyolite clasts indicate intrusion of basalt into T1 magma, initiating eruption. Subsequent eruption deposits contain two additional rhyolitic magmas (T2 and T3) with the abundance of T3 increasing and T1 decreasing with time. Volume estimates of each magma type in the initial pyroclastic eruption are small: 0.3 km3, 0.3 km3 and 0.5 km3 respectively with a further 4.0 km3 of T3 forming domes at the end of the episode. Melt-crystal equilibria indicate depths b8 km and the model of Shane et al. (2008b) has each rhyolitic magma type forming a pod of magma in a large crystal mush zone. Each magma type contains quartz with melt inclusions of more evolved composition than host matrix glass. Shane et al. (2008a) suggests that this resulted from re-melting of the underlying crystal mush, presumably by the same basaltic event that initiated the eruption sequence, with the subsequent melt migrating and hybridising with the residual melts in each of the three magma pods. During the Rerewhakaaitu eruption from Tarawera (17.6 ka), 5 km3 DRE of mainly rhyolitic magma was vented to the surface from a single vent location. Three rhyolite magma types were identified (Fig. 5b; Shane et al., 2007). T1 magma is crystal-poor, orthopyroxenehornblende-bearing and highly evolved (~77% SiO2), while T2, erupted simultaneously from the same vent as T1, is a crystal-rich biotitehornblende rhyolite (~75% SiO2). Temperature of crystallization is also different, with T1 being ~760 °C on the basis of Fe–Ti oxides, while T2 is ~700 °C. T3 is found in Western Dome, a contemporaneous lava dome, extruded from a vent 3 km to the south west. Some T1 and T2 clasts contain vesicular brown blebs (b1 mm to ~10 mm) comprising olivine, clinopyroxene and Ca-rich plagioclase, with glass ranging from

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Fig. 5. K2O v SiO2 variation within pumices of a) Okareka Tephra (data from Shane et al., 2008b); b) Rerewhakaaitu (data from Shane et al., 2007); c) Waiohau (data from Speed et al., 2002); d) Kaharoa (open circles — airfall tephra; open squares — dome rhyolite; open diamonds — rhyolite from block and ash flow; open triangle — granodiorite; plus sign — dacite; inverted open triangle — basaltic andesite; closed circle — basalt; cross — gabbro-diorite; closed square — 1886 basalt average; closed triangle — Okareka basalt average; from Leonard et al., 2002).

andesite to rhyolite in composition. The geothermometry (N1000 °C; estimate from Fe–Ti oxides?) and olivine composition are typical of basaltic magmas and the blebs are believed to represent remnants of the mafic eruption-triggering intrusion (Shane et al., 2007). This may have been part of a dyke injection as large hydrothermal eruptions at Kawerau and Waiotapu (Nairn, 2002), along strike of the Tarawera linear vent zone, also occurred at this time. The 15.8 ka Rotorua eruption from Haroharo began with the rapid ascent of gas-rich actively vesiculating biotite-free rhyolite (SiO2 76.5 wt.% in glass, b10% crystals; 835 °C) magma which encountered a pre-existing cooler and slightly more evolved (SiO2 77.4 wt.% in glass, N20% crystals; 750 °C) biotite-bearing rhyolitic magma (Smith et al., 2004; Kilgour and Smith, 2008). In contrast to other young events from OVC, the 13.8 ka Waiohau eruptive episode from Tarawera came from a single homogenous magma batch, characterised by low crystal contents (plagioclase and orthopyroxene) and high temperatures, lacking gradients in composition (Fig. 5c), and displaying minimal compositional zonation in phenocryst phases (Speed et al., 2002). While Speed et al. (2002) describe no mafic material from this eruption, Nairn (2002) records the presence of rare basaltic and andesitic clasts associated with a satellite vent at Rotomahana. Regional extension is associated with this episode in the form of co-eruption displacement on the Paeroa Fault, at least 17 km to southwest of the main 13.8 ka eruption vent (Berryman et al., 2008). Two periods of displacement occurred on strands of the Paeroa Fault, with a cumulate displacement of 2.6 m (Berryman et al., 2008). Given the general absence of significant mafic material it is likely that this co-magmatic regional tectonism initiated eruption.

The Whakatane eruptive episode (5.6 ka) was the most recent from Haroharo, with a volume of 11.3 km3. Extrusion of lava domes and flows dominated the episode (~75%), but there was also a Plinian eruption (Whakatane Tephra) and other smaller explosive events. Lava and pumice are high silica rhyolite (SiO2 N75%), low crystal content (~15%), with plagioclase, quartz, orthopyroxene, cummingtonite, hornblende and Fe–Ti oxides as main phenocryst phases (Kobayashi et al., 2005). Slight differences in chemistry occur between eruptives from 3 separate vents active during the episode (Smith et al., 2006) suggesting each tapped a different high level reservoir during perhaps a 10 year period. There is no evidence of any basaltic precursor. During the 1314 ± 10 AD (Hogg et al., 2003) Kaharoa eruption from Tarawera, 5 km3 of magma was vented to the surface from multiple sources along the 8 km linear vent zone. Initial phreatomagmatic explosions were followed by a major plinian eruption sequence and the eruption finished with the effusive dome building. Nairn et al. (2004) identified two main rhyolite types — T1 present at the start followed by T2 which became more prevalent as the eruption progressed. A third type, T3, mingled with basalt to form rhyodacite (Fig. 5d). Vent location and deposit stratigraphy were used to provide constraints on the shape of the underlying magma chamber. This was thought to be ~8 km long, 1 km wide and 1.4 km thick, but Nairn et al. (2005) documents the coeval occurrence of large hydrothermal eruptions along strike to the SW at Waiotapu and implies that all the 0.7 ka magmatic and hydrothermal eruptions were driven by intrusion of a 21 km long basalt dike. Finally, a significant basalt eruption occurred on 10 June 1886 (Cole, 1970a; Sable et al., 2006) when ~2 km3 of high-alumina basalt was

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erupted from a 17 km fissure across Tarawera. The basalt comprised microphenocrysts of plagioclase (~17%) and olivine (~5%) in a glassy groundmass (Cole, 1970b; Hiess et al., 2007). 5. Plutonic and mafic lithic blocks Most of the ignimbrites and tephras contain rhyolite lithic blocks, mainly derived from earlier eruptions within OVC, but two eruptions, the Rotoiti and Kaharoa have included both silicic and mafic plutonic blocks, which are considered to be derived from solidified or partly solidified high-level magma chambers and mafic volcanic blocks. The most voluminous of these occur within the lithic lag breccias associated with the Rotoiti Ignimbrite (Brown et al., 1998; Burt et al., 1998; Charlier et al., 2003; Matthews, 2006). The Whakatane tephra also contains rare microdiorite blocks (Smith et al., 2006). Similar blocks have been found in lithic lags associated with ignimbrites of other calderas in the TVZ including: the Kaingaroa Ignimbrite (Reporoa caldera; Beresford and Cole, 2000), Ohakuri Ignimbrite (Ohakuri caldera; Ewart and Cole, 1967; called ‘Atiamuri’; Brown et al., 1998), and as random blocks in the Ongatiti Ignimbrite (Mangakino caldera; Brown et al., 1998), Whakamaru Ignimbrite (Whakamaru caldera; Brown et al., 1998), Earthquake Flat Pyroclastics (vents to the SW of OVC; Brown et al., 1998), and in the Huka Falls Formation from drill core at the Tauhara geothermal field (Taupo Volcanic Centre; Hamish Cattell, pers. comm. 2012). 5.1. Rotoiti Ignimbrite Three types of plutonic lithic blocks can be identified in the Rotoiti lithic lag deposits: 1) Hornblende microgranites, which are porphyritic with very coarse quartz and feldspar phenocrysts (max ~8 mm) in a fine- to medium-grained, equigranular groundmass of quartz and feldspar with a partial granophyric texture (Fig. 6a). They contain interstitial anhedral alkali feldspar (~20%), hornblende (5–9%), biotite, ilmenite, epidote alteration and pyroxene pseudomorphs. The granophyric groundmass and presence of drusy cavities suggest late-stage hypersolvus crystallisation and quenching during ascent/ eruption. 2) Coarse-grained biotite granites, which are holocrystalline with hypidiomorphic granular texture consisting of interlocking crystals of biotite (~10%), quartz (~30%), plagioclase, alkali feldspar, high temperature sanidine, magnetite (~2%), trace orthopyroxene, zircons and minor interstitial glass. Granophyric textures are weakly developed in isolated patches consistent with hypersolvus crystallization of sanidine (Fig. 6b). Three phases of granophyric intergrowth are distinguished radiating out from central phenocrysts, indicative of complex growth histories (Burt et al., 1998). A single layered biotite granitoid found (Fig. 6c) is equigranular, hypidiomorphic, fine- to medium-grained, glass-bearing and leucocratic to mesocratic, with textures ranging from granitic to granophyric. Fine-grained interstitial zones and glass along grain boundaries suggests that some fragments were partially crystallised mush or melt prior to incorporation into the Rotoiti eruption. 3) A single microdiorite fragment is fine-grained, equigranular, hypocrystalline with hypidiomorphic or felsitic texture and incipient flow-banding of acicular groundmass laths with abundant plagioclase, hornblende, quartz, orthopyroxene, ilmenite and magnetite, and a granodiorite lithic fragment is mesocratic, containing biotite (35–40%), magnetite (15%), plagioclase and quartz, and texturally holocrystalline, hypidiomorphic and fine- to medium-grained with minor vesicles. Rare mafic volcanic lithics were also found. 5.2. Whakatane Tephra In the Whakatane tephra deposits are a few small (b8 mm to ~5 cm) microdiorite fragments, which have a mineral assemblage of plagioclase (~50%), calcic amphibole (~35%), quartz (~10%), Fe–Ti oxides (~5%), with traces of cummingtonite and biotite (Smith et al., 2006), and

Fig. 6. Rotoiti lithics: a) hornblende microgranite thin section; b) biotite microgranite thin section; c) layered biotite granite block, showing evidence of slumping within the magma chamber. Darker layers are biotite-rich. a) and b) are from Matthews (2006).

contain some high-silica vesicular glass (b10%; ~77.83 wt.% SiO2), which differs chemically from the host Whakatane Tephra (Smith et al., 2006). 5.3. Kaharoa Tephra The other main occurrence of plutonic and mafic lithic blocks in OVC is within the pyroclastic density current deposit associated with the 1314 AD Kaharoa eruption from Tarawera (Hpdc deposit of Nairn et al., 2001; Leonard et al., 2002). These can be divided into 3 types (Leonard et al., 2002) as follows: 1) Rare, medium-grained biotite-granodiorite inclusions, which contain subhedral to euhedral plagioclase, quartz, alkali feldspar, biotite, Fe–Ti oxide ± hornblende (Fig. 7a). Zircon is a minor accessory mineral. 2) Basaltic inclusions, separated into two types by Leonard et al. (2002) based on the presence or absence of hornblende in the groundmass. Both types have crenulate margins and contain phenocrysts of plagioclase (0.5–12 vol.%) and

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augite (0.75–9 vol.%) ± olivine (b 1 vol.%) and minor Fe–Ti oxides. Augite and olivine are subhedral to euhedral and sometimes corroded; olivine, where present, is fractured. Groundmass is made up of plagioclase (20–40 vol.%) and augite (5–20 vol.%), sometimes swallow-tailed indicating a moderate degree of undercooling. When present, hornblende is lath-like (aspect ratios ~1:10) and commonly has c-axes parallel to augite crystals from which it appears to have grown. Interstitial colourless to brown glass is an important constituent of the groundmass (20–40 vol.%), with ‘plumes’ of brown glass which stream off into the surrounding rhyolite from the crenulate contact with hornblendebearing basaltic inclusions (Fig. 7b). 3) Rare gabbro and olivineclinopyroxenite inclusions are texturally complex, medium-grained, plutonic, porphyritic holocrystalline mafic aggregates. Phenocrysts comprise augite (11–81 vol.%), plagioclase (8–40 vol.%), olivine (6–15 vol.%) and Fe–Ti oxides (2–3 vol.%).

6. Depth of crystallisation There are a number of lines of evidence that suggest crystallisation of the rhyolitic magmas of OVC were very shallow. These include: 1. Cummingtonite is a crystal phase found in a number of OVC magma batches. Experimental data and thermodynamic considerations imply pressures of ~200 MPa for cummingtonite crystallisation (Ewart et al., 1975; Nicholls et al., 1992), equivalent to ~6 km. 2. OVC glass compositions plot on, or close to, the 200 MPa cotectic in the Qtz–Ab–Or-water ternary system (Tuttle and Bowen, 1958; Cashman and Blundy, 2000; Shane et al., 2007; Smith et al., 2010; Shane and Smith, 2013), which would be ~6 km depth. 3. H2O/CO2 contents in OVC quartz melt inclusions (Shane et al., 2007; Shane et al., 2008a) indicate pressures of ~100–260MPa (Smith et al., 2010) or between ~3–8 km, with the majority between 100 and 150 MPa, suggesting 4–6 km (Johnson et al., 2011).

7. Crystallisation timescales Timescales of crystallisation of large silicic magma bodies at OVC have recently been investigated using single- and multi-grain zircon 238 U–230Th disequilibrium dating. Studies of Rotoiti pumices (Charlier et al., 2003) showed that crystallization of most zircon in biotitebearing pumices of T2, and by inference the assembly of the magma body, began at least 50 kyr prior to the caldera-forming event. In contrast there is a pronounced peak coincident with the eruption age for zircon model ages derived from biotite-free T1 pumice (Charlier and Wilson, 2010). The zircon population has a mean age of 83 ± 14 ka, but with outliers up to ~110 ka. The latter probably represent ‘antecrysts’ from earlier crystallisation events ‘quarried’ by the Rotoiti magmas. The model zircon age spectrum (Charlier and Wilson, 2010) suggest that there was a rhyolitic magma system in the SE portion of the OVC operating between 45 and 28ka that was separate to that active in the north of the caldera complex where rhyodacite was erupted. Zircons from pumices within the c. 0.7 ka Kaharoa Tephra from Tarawera and the 5.6 ka Whakatane Tephra from Haroharo, analysed by Klemetti et al. (2011), showed most had U–Th ages between ~15–50 ka, with an apparent peak between 25 and 30 ka. Significantly, the distribution of ages between the two deposits, separated by ~15km, was similar. Storm et al. (2011, 2012) showed from comparative age-depth profiling of zircons from the four rhyolite eruptions from Tarawera since 22 ka that there were periods of continuous and punctuated zircon growth perhaps over ~100kyr. They found that crystal face ages were much older than eruption time, and must therefore have been held in sub-solidus storage conditions for a prolonged period. These results indicate that zircon crystals survive earlier eruptive events and become incorporated in later developed magmas, highlighting the complex evolution of magma systems in this environment. In addition, Storm et al. (2011, 2012) found that co-erupted magmas from the same eruptive episode contained different zircon age spectra. This indicates prolonged separate crystallisation histories over short spatial distances. This is supported by Shane et al. (2012) who analysed 79 zircons from 6 granitoid samples from the Kaharoa Tephra and found a weighted model crystallization event of 208 ± 4 ka, with a few outliers dating back to ~750 ka (pre-OVC volcanism). The zircon studies thus show strong evidence for a long-lived crystal mush beneath OVC. 8. Comparison with older high-level plutonic systems

Fig. 7. Kaharoa: a) granodiorite thin section; b) interface between basalt block and rhyolite host (from Leonard et al., 2002).

In the past 20 years there have been many studies of high-level plutonic systems, many of which are likely to represent sub-crustal equivalents of volcanic rocks (e.g. Barnes et al., 1990; Wiebe and Collins, 1998; Hawkins and Wiebe, 2004; Metcalf, 2004; Quick et al., 2009; Sewell et al., 2012). Other studies have shown the complexity of high-level plutonic systems (e.g. Bachl et al., 2001; Wiebe et al., 2002, 2004; Harper et al., 2004; Westerman et al., 2004; Walker et al., 2007; Turnbull et al., 2010) and the importance of mafic inputs into such systems (e.g. Snyder, 2000; Wiebe et al., 2001, 2004). Features exposed in many of these intrusions are comparable to features identified in plutonic blocks found within the Rotoiti Ignimbrite and the Kaharoa Tephra, described earlier. Some granite lithics within the Rotoiti lithic lag are layered and some have structures resembling load casts (e.g. block RT3; Brown et al., 1998; Burt et al., 1998) (see Fig. 6c), similar to those found in the Vinalhaven Granite, coastal Maine (~420 Ma; Hawkins et al., 2002). Many also have granophyric intergrowths consistent with slow cooling, followed by a period of rapid quenching, perhaps indicating that they came from an incompletely solidified reservoir, while others at Vinalhaven form ‘pillows’ (Fig. 8) which are considered to represent flow fronts of basaltic injections that entered and ponded within the silicic magma chambers (Wiebe et al., 2001). While most of the blocks found in the Rotoiti Ignimbrite are granite, there are also some microdiorite blocks and one biotite granodiorite,

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indicating a range of rock types were quarried during the explosive eruption. Miarolitic and drusy cavities containing epidote are common in the granites, indicating circulation of low-temperature hydrothermal fluids or crystallisation under low-pressure hydrous conditions. Granophyric intergrowths and interstitial glass observed in Rotoiti granitoid lithics also provide evidence for incomplete crystallization, followed by rapid quenching, which most likely occurred during incorporation into the Rotoiti eruption.

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At a depth of between ~8 and 15 km a further region of particularly low resistivity (b1 ohm.m) is centred beneath Haroharo and Tarawera (Heise et al., 2010, Fig. 10). This low resistivity feature may correspond to a zone which contains connected melt within the crust. Receiver function analysis at a site close to the caldera shows a rapid drop of S-wave velocity at ~10 km depth which also suggests the existence of a connected melt fraction at depth (Bannister et al., 2004). 10. Discussion

9. Geophysical Data Gravity modeling of the OVC has been undertaken by Rogan (1982), Stagpoole and Bibby (1999), Davy and Bibby (2005) and Seebeck et al. (2010). Seebeck et al. (2010) has defined primary (structural) margins of the OVC as defined by a 9 x 15km elongated negative gravity anomaly (−45 mGal; Figs. 2, 9), elongate in a NNW direction (175° ± 5°) which represent multiple phases of collapse. Maximum subsidence appears to be in the south-central region of the caldera complex, with maximum depth to the caldera floor of 3.0 ± 0.5 km based on a 2D inversion of MT data and forward modeling of gravity data (Seebeck et al., 2010). In the past 5years, magnetotelluric (MT) data has also become available (Heise et al., 2007, 2010; Bibby et al., 2008). More than 70 broadband MT soundings (300 Hz to 1000 s) have been made in and around the caldera, primarily using five Phoenix MTU-2000 systems (Bibby et al., 2008). Phase Tensor analysis suggests that the conductivity structure over the southern part of the caldera (under Tarawera) is 1-D to about 1 s (~2–3 km depth) but more complex structure is apparent to the north (under Haroharo). Fig. 9 combines the phase tensor information with the gravity data of Seebeck et al. (2010). A change in the phase tensor (from high to low phases, between approximately 1 to 0.1 s, approximately 1–4 km depth) coincides with the geologically mapped boundary of the caldera, particularly on the south and east. The observed behaviour of the phase tensors is consistent with vertical offset of the low resistivity (~10–20 Ω.m) volcanics filling the caldera structure, relative to the high resistivity (N100 Ω.m), greywacke basement. To the north where collapse has offset volcaniclastic rocks, the contrast in resistivity is less obvious. The conductive infill material extends to depths of 2–3 km in the central caldera and shows close agreement with density modeling from the gravity data. Beneath the conductive material the resistivity increases rapidly as the basement rocks are encountered. An apparent contrast from lower to higher resistivity of the basement rocks inside and outside the caldera at around 3–5 km depth suggests that beneath the caldera the basement has been highly fractured, consistent with the eruptive history at OVC.

Fig. 8. Photo of ‘pillows’ from the Vinalhaven Granite intrusion, central Maine (photo: J.W.Cole).

The concept that crystal mushes play an important role in magmatism in the upper crust has been suggested for many years (e.g. Hildreth, 1981; Miller and Mittlefehldt, 1984), although the time scales required to build and maintain these mush bodies was poorly understood. More recent detailed observational and experimental studies; however, have shown that the rheology of magma bodies varies according to crystallinity (Bachmann and Bergantz, 2004), and that ascending mafic magmas impact on this process (Bachmann and Bergantz, 2006). There is no doubt that magma systems beneath OVC are complex. There has been magmatism beneath this region for at least 750 kyrs (Shane et al., 2012), and this magmatism has been strongly influenced by the extensional tectonics of TVZ. Magmatism has ranged from mafic to silicic, with a complex interaction between the two types in the upper crust. Evidence from geothermal drillholes elsewhere in the TVZ (e.g. Ngatamariki; O'Brien et al., 2011) suggests silicic volcanism was preceded by dominantly andesitic volcanism (Deering et al., 2011b). There is no known outcrop evidence of early andesite lavas within OVC, but andesitic lava flows occur in the Kawerau geothermal wells immediately to the east (Allis et al., 1993). Deering et al. (2011b) proposed a model whereby variations in the assimilation and fractional crystallization of mantle-derived primitive magma conditions relate to two different liquid lines of descent. They suggested that a change from a warm-dry (pyroxene–plagioclase dominated) liquid line of descent, to one that was colder and wetter (hornblende–Fe/Ti oxide dominated), resulted in the production of dominantly rhyolitic volcanism. This change appears to be concomitant with an increase in the rate of extension in TVZ, causing rifting, and movement of a silicic mush into a high level in the crust. Slab-derived arc fluid components play an important role in magma types in TVZ (Deering et al., 2012) with geochemical evolution throughout TVZ relating to variation in temperature and fluid composition, producing cycles of cold-wet-oxidising magmas and hotdry-reducing magmas. Superimposed on this is variability caused by periodic caldera collapse. It seems likely that frequent intrusion of basaltic magma into the lower crust, probably as a result of TVZ extension, ‘pre-conditioned’ the crust and provided an environment for fractional crystallisation, plus some assimilation. An intermediate cumulate mush zone (Deering et al., 2010), with ~50% crystals occurs at 8–15 km (Fig. 11), and this provided an intermediate source material for upper (brittle) crust (b8 km) magma systems. In their petrogenetic model crystal accumulation (‘intermediate cumulate’ in Fig. 11) and melt extraction are the dominant processes. The extracted melts from this zone have been continually rising to higher levels (3–8 km depth) throughout the ~750 kyrs duration of the OVC magmatic system. Once magma accumulates in this zone, further crystallisation occurs, with periodic melt extraction forming rhyolitic magma erupted at the surface. Depending on temperature, fluid content, ƒO2 and ƒH2O this will produce the range of crystal contents (5–25 modal %) and compositions seen in OVC rhyolitic lavas and pyroclastics. The non-eruptive fraction will remain in situ or cumulate to produce extensive plutonic systems like those now described in Section 8. Blocks of these periodically become incorporated in the erupted magma during explosive eruptions to form the lithic blocks described in Section 5. Occasionally a microdiorite block is incorporated into an ignimbrite (as in the Rotoiti Ignimbrite — see Section 5.1), where the block contains

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Fig. 9. Map showing phase tensor results across OVC, together with gravity measurements (from Seebeck et al., 2010). + marks on cross section show NW and SE boundary of OVC.

residual glass between crystals, along grain boundaries and in voids, but was almost solid when fragmented by entrainment within the Rotoiti eruption. Other blocks (e.g. the granodiorites from Kaharoa) are petrographically and chemically similarly to their host rhyolites while yet others (e.g. some of the blocks in the Rotoiti Ignimbrite) are quite different. Studies of zircons within many of these blocks (Section 7) confirm their complex history. Magma intrusion into the high level magma chambers or ‘pods’, which are considered to be sill or laccolith-like (Fig. 12), must have

Fig. 10. Resistivity cross section from three-dimensional inversion model across OVC from NW–SE, perpendicular to the strike (N135°E) of the TVZ (‘A-A’ on Figs. 1 and 9). Arrows show NW and SE boundaries of OVC on Fig. 1, and + marks on Fig. 9. After Heise et al. (2010).

Fig. 11. Anatomy of the projected mid-crustal silicic magmatic system under OVC. Figures at right are projected depths (in kms).

J.W. Cole et al. / Earth-Science Reviews 128 (2014) 1–17

been relatively frequent, and from evidence obtained from the eroded high-level systems already described, were largely fed by dikes. Sometimes magma from a single pod was erupted (e.g. Waiohau eruption from Tarawera, and for many other rhyolite lava domes), while particularly for explosive eruptions, multiple bodies contributed, each providing the melt for a pumice composition to air-fall tephras or ignimbrites. Trace element modelling has discounted simple fractional crystallisation links between these different pumice types erupted, and mixing between different bodies. Each magma type at OVC exhibits distinct pressure, temperature and oxygen fugacity suggesting that they are derived from a variable, but long-lived, heterogenous crystal mush zone (e.g. Shane et al., 2007, 2008a, 2008b; Deering et al., 2008; Deering et al., 2011a, 2011b). Melts form at depth within the zone, and undergo varying degrees of crystallisation and regeneration by partial melting prior to ponding in a shallow magma system within the extracted melt zone of Fig. 11, before either stagnation or eruption. The long eruption history (~625 kyrs) and substantial volume (N 300 km3) of volcanic deposits at OVC implies incremental

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emplacement of a large dominantly silicic, complex plutonic body, with each eruption episode drawing on different source sill- or laccolith-like areas within that body (Fig. 12). During ignimbrite eruptions it seems likely that different ‘pods’ within the magma reservoir coalesce. Geophysical experiments across TVZ have found no pluton-scale partial melts in the crust anywhere beneath the present day TVZ, but local partial melts may occur (Heise et al., 2010). Much of the high-level magmatic system must, therefore, be largely crystalline. Although it is likely that voluminous silicic magma at high levels in the crust is derived from a mush at 8–12 km depth, this mush is rarely found in surface deposits of TVZ, suggesting that the mush zone is physically separated from the high-level crustal magma pods, possibly by a largely crystallised dominantly felsic cumulate zone. It is probable that the magmatic system envisaged beneath OVC in Fig. 12 has existed for ~750 kyrs, and by comparison with older granite systems, such as the Spirit Mountain batholiths of Nevada (Walker et al., 2007), which took ~2 Myr to emplace and crystallise, is therefore likely to be only part way through its evolution. This has clear implications for hazard evaluation. Basalt has probably been intruded into the middle

Fig. 12. Schematic cross section of likely features in upper crust below OVC. a) Post Matahina eruption ~322 ka); b) Present day. 1 = Rotokawa basalt; 2 = Haroharo Dome Complex; 3 = Tarawera Dome Complex; 4 = 1886 basalt fissure.

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crust at the same rate over the ~750 kyrs history of OVC, but there does seem to be an increase in basalt eruptions in the last 50 kyrs. This suggests regional extension within the Taupo Rift (Rowland and Sibson, 2001) has allowed the intrusion level to rise in the crust beneath the caldera, with repeated magma emplacement making it easier for the magma to be erupted, as in the 1886 AD eruption of Tarawera. Comparison with other large, active silicic volcanic centres, which have experienced multiple caldera collapses suggests there is a wide variation in the geometry of their subsurface magma systems. Factors that will affect the magma systems include: thickness of crust, rate of extension and age of the magmatic system. For example calderas associated with the Taupo Volcanic Centre (Fig. 1) are younger, with the Oruanui Ignimbrite (from the Oruanui caldera; Wilson et al., 2006) dated at ~25.4 ka (Vandergoes et al., 2013), and the Taupo Ignimbrite (Wilson, 1993) dated at 232 AD (Hogg et al., 2011). There is some evidence of an earlier event, perhaps caldera-forming, producing the Rangitira Point Ignimbrite (Cole et al., 1998), and possibly contributing to the thick pyroclastic deposits found in the geothermal systems of Tauhara and Wairakei (e.g. Bignall et al., 2010). Calderas of the Jemez Mountains volcanic field (Self et al., 1986), New Mexico, USA, also have multiple collapse and extensive silicic volcanism, but the most recent, the Valles caldera, differs in being strongly resurgent (e.g. Kennedy et al., 2012). Once again the tectonic setting differs from the TVZ, although the calderas are sited within the Rio Grande Rift. The area is characterised by relatively thin crust (~35 km), but unlike TVZ, has experienced extensive late Cenozoic basaltic volcanism for the past 16.5 Ma (Riecker, 1979). The Valles caldera is sited where the Rio Grande Rift intersects the Jemez lineament, a NE-trending alignment of young volcanic fields that stretches from Arizona to SE Colorado. A multiple caldera system in a similar tectonic setting to OVC occurs at Lake Toba, Sumatra, Indonesia (Chesner and Rose, 1991). This is one of the largest Quaternary caldera systems on Earth. Here four separate eruptions have occurred from within the current caldera complex over the past 1.2 Ma, with major eruptions and collapse occurring at 1.2 Ma, 0.84 Ma, 0.50 Ma and 0.074 Ma. The northernmost caldera apparently developed on a large andesite stratovolcano (rather like Oruanui caldera in the Taupo Volcanic Centre, New Zealand; Cole et al., 1998). Stankiewicz et al. (2010) have undertaken an ambient noise tomography study at Toba, and this has identified a low velocity zone below ~7 km, which they believe delineates a complex pluton of at least 34,000 km3 in volume. This could well be similar to OVC, but the system differs by having had extensive pre-caldera andesite eruptions, and post-caldera resurgence. 11. Conclusions 1. The Okataina Volcanic Centre (OVC) is predominantly silicic and has experienced multiple caldera collapse, each with widespread ignimbrite eruptions, together with pre- and post-collapse rhyolite dome effusion and accompanying explosive tephra eruptions. OVC is ~ 625 ka old, and probably has a magmatic history spanning ~ 750 kys. 2. Geophysical and geochemical evidence is consistent with a multistage origin of the silicic volcanic, which includes an intermediate composition ‘mush’ zone at 8–15 km depth from which silicic magmas rose to form sill or laccolith like ‘pods’ at 3–8 km depth in the crust. 3. Periodically eruptions occurred from these pods, probably assisted by extension within the Taupo Rift, and at OVC perhaps aided by a hard-linked left step in the Rift. In the past 50 kys most eruptions have been from two linear vent zones — Haroharo and Tarawera, and since 25 ka they have been concentrated within the Haroharo and Tarawera Dome Complexes. 3.1 The proposed model is supported by evidence provided from plutonic and mafic lithic blocks found within some pyroclastic

deposits, and by comparison with with features found in older high-level exposed plutons. 3.2 The general model proposed is probably common to many other caldera systems, but the detail will differ because of variation in tectonic setting, crustal thickness and age of the system. The high-level nature of the magma system in the crust has implications for understanding the heat source for geothermal systems, as well as the formation of epithermal ore deposits associated with silicic caldera systems.

Acknowledgements Initial funding for the study was from Marsden contract UOC0508 (JWC; CD). Thanks go to Ian Nairn for frequent discussions on OVC during the study, to Bob Wiebe for showing JWC the Maine granites and to Calvin and Jonathan Miller for similar visits to the Nevada granites, and for many discussions with all 3 on high-level granite systems. Sam Hampton and Grant Wilson assisted with figure drafting, and Wiebke Heise kindly provided the 3D inversion model cross section used in Fig. 10. Colin Wilson and Olivier Bachmann reviewed an earlier version of this manuscript and their comments are much appreciated. Colin's very thorough discussion on the content and ‘eagle-eye’ spotting of typographic errors is particularly appreciated. We also appreciate the comments about this paper from two anonymous reviewers. References Allis, R.G., Christenson, B.W., Nairn, I.A., Risk, G.F., White, S.P., 1993. The natural state of Kawerau geothermal field. Proceedings of the 15th Geothermal Workshop. University of Auckland, New Zealand, pp. 227–233. Bachl, C.A., Miller, C.F., Miller, J.S., Faulds, J.E., 2001. Construction of a pluton: evidence from an exposed cross-section of the Searchlight pluton, Eldorado Mountains, Nevada. Geol. Soc. Am. Bull. 113, 1213–1228. Bachmann, O., Bergantz, G., 2004. On the origin of crystal-poor rhyolites: extracted from batholithic crystal mushes. J. Petrol. 45, 1565–1582. Bachmann, O., Bergantz, G., 2006. Gas percolation in upper-crustal mushes as a mechanism for upward heat advection and rejuvenation of near-solidus magma bodies. J. Volcanol. Geotherm. Res. 149, 85–102. Bachmann, O., Bergantz, G., 2008. Rhyolites and their Source Mushes across Tectonic settings. J. Petrol. 49, 2277–2285. Bailey, R.A., Carr, R.G., 1994. Physical geology and eruptive history of the Matahina Ignimbrite, Taupo Volcanic Zone, North Island, New Zealand. N. Z. J. Geol. Geophys. 37, 319–344. Bailey, R.A., Dalrymple, G.B., Lanphere, M.A., 1976. Volcanism, structure and geochronology of the Long Valley Caldera, Mono County, California. J. Geophys. Res. 81, 725–744. Bannister, S., Bryan, C.J., Bibby, H.M., 2004. Shearwave velocity variation across the Taupo Volcanic Zone, New Zealand, from receiver function inversion. Geophys. J. Int. 159, 291–310. Barnes, C.G., Allen, C.M., Hoover, J.D., Brigham, R.H., 1990. Magmatic components of a tilted plutonic system, Klamath Mountains, California. In: Anderson, J.L. (Ed.), The nature and origin of Cordilleran magmatismMem. Geol. Soc. Am. 174, 331–346. Bellamy, S., 1991. Some studies of the Te Wairoa ignimbrites and the associated volcanic geology of the SW Okataina volcanic centre, Taupo Volcanic Zone. MSc thesis University of Waikato, Hamilton, New Zealand. Beresford, S.W., Cole, J.W., 2000. Kaingaroa Ignimbrite, Taupo Volcanic Zone, New Zealand; evidence for asymmetric subsidence of the Reporoa Caldera. N. Z. J. Geol. Geophys. 43, 471–481. Berryman, K., Villamor, P., Nairn, I., van Dissen, R., Begg, J., 2008. Late Pleistocene surface rupture history of the Paeroa Fault, Taupo Rift, New Zealand. N. Z. J. Geol. Geophys. 51, 135–158. Bibby, H.M., Caldwell, T.G., Davey, F.J., Webb, T.H., 1995. Geophysical evidence on the structure of the Taupo Volcanic Zone and its hydrothermal circulation. J. Volcanol. Geotherm. Res. 68, 29–58. Bibby, H.M., Caldwell, T.G., Risk, G.F., 1998. Electrical resistivity image of the upper crust within the Taupo Volcanic Zone, New Zealand. J. Geophys. Res. 103, 9665–9680. Bibby, H.M., Heise, W., Bennie, S.L., Caldwell, T.G., Seebeck, H., Tietze, K., Junge, A., Cole, J.W., 2008. Investigation of an active caldera: magnetotelluric survey of Okataina Caldera, New Zealand. The 19th International Workshop on Electromagnetic Induction in the Earth, October 23–29, 2008. Institute of Geology, Beijing, China, p. 239 (abstract S1.2_S11). Bignall, G., Milicich, S., Ramirez, L., Rosenberg, M., Kilgour, G., Rae, A., 2010. Geologiy of the Wairakei-Taupo Geothermal Syste, New Zealand. Proceedings of the World Geothermal Conference, pp. 25–30. Brown, S.J.A., Burt, R.M., Cole, J.W., Krippner, S.J.P., Price, R.C., Cartwright, I., 1998. Plutonic lithics in ignimbrites of Taupo Volcanic Zone, New Zealand; sources and conditions of crystallization. Chem. Geol. 148, 21–41.

J.W. Cole et al. / Earth-Science Reviews 128 (2014) 1–17 Browne, P.R.L., Graham, I.J., Parker, R.J., Wood, C.P., 1992. Subsurface andesite lavas and plutonic rocks in the Rotokawa and Ngatamariki geothermal systems, Taupo Volcanic Zone, New Zealand. J. Volcanol. Geotherm. Res. 51, 199–215. Bryan, C.J., Sherburn, S., Bibby, H.M., Bannister, S.C., Hurst, A.W., 1999. Shallow seismicity of the central Taupo Volcanic Zone, New Zealand: its distribution and nature. N. Z. J. Geol. Geophys. 42, 533–542. Burt, R.M., Brown, S.J.A., Cole, J.W., Shelley, D., Waight, T.E., 1998. Glass-bearing plutonic fragments from ignimbrites of the Okataina caldera complex, Taupo Volcanic Zone, New Zealand: remnants of a partially molten intrusion associated with preceding eruptions. J. Volcanol. Geotherm. Res. 84, 209–237. Cashman, K., Blundy, J., 2000. Degassing and crystallization of ascending andesite and dacite. Philos. Trans. R. Soc. 358, 1487–1513. Charlier, B.L.A., Wilson, C.J.N., 2010. Chronology and Evolution of Caldera-forming and Post-caldera Magma Systems at Okataina Volcano, New Zealand from Zircon U–Th Model-age Spectra. J. Petrol. 51, 1121–1141. Charlier, B.L.A., Peate, D.W., Wilson, C.J.N., Lowenstern, J.B., Storey, M., Brown, S.J.A., 2003. Crystallisation ages in coeval silicic magma bodies: U–Th disequilibrium evidence from the Rotoiti and Earthquake Flat eruption deposits, Taupo Volcanic Zone, New Zealand. Earth Planet. Sci. Lett. 206, 441–457. Chesner, C.A., Rose, W.I., 1991. Stratigraphy of the Toba Tuffs and the evolution of the Toba Caldera Complex, Sumatra, Indonesia. Bull. Volcanol. 53, 343–356. Cole, J.W., 1970a. Structure and eruptive history of the Tarawera Volcanic Complex. N. Z. J. Geol. Geophys. 13, 879–902. Cole, J.W., 1970b. Petrology of the basic rocks of the Tarawera Volcanic Complex. N. Z. J. Geol. Geophys. 13, 925–936. Cole, J.W., 1990. Structural control and origin of volcanism in the Taupo volcanic zone, New Zealand. Bull. Volcanol. 52, 445–459. Cole, J.W., Spinks, K.D., 2010. Relationship between caldera volcanism and structure in the Taupo Volcanic Zone, New Zealand. In: Murphy, B. (Ed.), Ancient Orogens and Modern AnaloguesGeol. Soc. Lond. Spec. Publ. 327, 9–29. Cole, J.W., Brown, S.J.A., Burt, R.M., Beresford, S.W., Wilson, C.J.N., 1998. Lithic types in ignimbrites as a guide to the evolution of a caldera complex, Taupo volcanic centre, New Zealand. J. Volcanol. Geotherm. Res. 80, 217–237. Cole, J.W., Milner, D.M., Spinks, K.D., 2005. Caldera and caldera structures: a review. Earth Sci. Rev. 69, 1–26. Cole, J.W., Spinks, K.D., Deering, C.D., Nairn, I.A., Leonard, G.S., 2010. Volcanic and structural evolution of the Okataina Volcanic Centre; dominantly silicic volcanism associated with the Taupo Rift, New Zealand. J. Volcanol. Geotherm. Res. 190, 123–135. Danišík, M., Shane, P., Schmitt, A.K., Hogg, A., Santos, G.M., Storm, S., Evans, N.J., Fifield, L.K., Lindsay, J., 2012. Re-anchoring the late Pleistocene tephrochronology of New Zealand based on concordant radiocarbon ages and combined 238U/230Th disequilibrium and (U–Th)/He zircon ages. Earth Planet. Sci. Lett. 349–350, 240–250. Davis, W.J., 1985. Geochemistry and Petrology of the Rotoiti and Earthquake Flat pyroclastic deposits. Unpublished MSc thesis, University of Auckland, New Zealand. Davy, B., Bibby, H., 2005. Seismic reflection imaging of the Haroharo Caldera boundary beneath Lake Tarawera, Okataina Volcanic Centre, New Zealand. N. Z. J. Geol. Geophys. 48, 153–166. Deering, C.D., 2009. Cannibalization of an amphibole-rich andesitic progenitor induced by caldera collapse during the Matahina eruption: evidence from amphibole compositions. Am. Mineral. 94, 1162–1174. Deering, C.D., Cole, J.W., Vogel, T.A., 2008. A Rhyolite Compositional Continuum Governed by Lower Crustal Source Conditions in the Taupo Volcanic Zone, New Zealand. J. Petrol. 49, 2245–2276. Deering, C.D., Gravely, D.M., Vogel, T.A., Cole, J.W., Leonard, G.S., 2010. Origins of cold wetoxidizing to hot-dry-reducing rhyolite magma cycles and distribution in the Taupo Volcanic Zone, New Zealand. Contrib. Mineral. Petrol. 160, 609–629. Deering, C.D., Cole, J.W., Vogel, T.A., 2011a. Extraction of crystal-poor rhyolite from a hornblende-bearing intermediate mush: a case study of the caldera-forming Matahina eruption, Okataina volcanic complex. Contrib. Mineral. Petrol. 161, 129–151. Deering, C.D., Bachmann, O., Dufek, J., Gravley, D.M., 2011b. Rift-related transition from andesite to rhyolite volcanism in the Taupo Volcanic Zone (New Zealand) controlled by crystal-melt dynamics in mush zones with variable mineral assemblages. J. Petrol. 53, 2243–2263. Deering, C.D., Horton, T.W., Gravley, D.M., Cole, J.W., 2012. Hornblende, cummingtonite, and biotite hydrogen isotopes: direct evidence of slab-derived fluid flux in silicic magmas of the Taupo Volcanic Zone, New Zealand. J. Volcanol. Geotherm. Res. 233–234, 27–36. Ewart, A., Cole, J.W., 1967. Textural and mineralogical significance of the granitic xenoliths from the Central Volcanic Region, North Island, New Zealand. N. Z. J. Geol. Geophys. 10, 31–54. Ewart, A., Hildreth, W., Carmichael, I.S.E., 1975. Quaternary acid magma in New Zealand. Contrib. Mineral. Petrol. 51, 1–27. Froggatt, P.C., Lowe, D.J., 1990. A review of late Quaternary silicic and some other tephra formations from New Zealand: their stratigraphy, nomenclature, distribution, volume and age. N. Z. J. Geol. Geophys. 33, 89–109. Gravley, D.M., Wilson, C.J.N., Leonard, G.S., Cole, J.W., 2007. Double trouble: paired ignimbrite eruptions and collateral subsidence in the Taupo Volcanic Zone, New Zealand. Geol. Soc. Am. Bull. 119, 18–30. Gravley, D.M., Leonard, G.S., Wilson, C.J.N., Rowland, J.V., Hikuroa, D.C.H., 2009. Build-up to an ignimbrite flare-up: geologic evidence for magmatic–tectonic–volcanic interplay in the Central Taupo Volcanic Zone, New Zealand. Geol. Soc. Am. Abstr. Programs 41, 57. Harper, B.E., Miller, C.F., Koteas, G.C., Cates, N.L., Wiebe, R.A., Lassareschi, D.S., Cribb, J.W., 2004. Granites, dynamic magma chamber processes, and pluton construction: Aztec Wash pluton, Eldorado Mountains, Nevada, USA. Trans. R. Soc. Edinb. Earth Sci. 95, 277–295.

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Harrison, A.J., White, R.S., 2004. Crustal stretching of the Taupo Volcanic Zone, New Zealand: stretching and igneous intrusion. Geophys. Res. Lett. 31 (1-4), L13615. http://dx.doi.org/10.1029/2004GL019858. Hawkins, D.P., Wiebe, R.A., 2004. Discrete stoping events in granite plutons: a signature of eruptions from silicic magma chambers? Geology 32, 1021–1025. Hawkins, D.P., Wobus, R.A., Wiebe, R.A., 2002. Silurian U–Pb zircon dates from the Vinalhaven intrusion and associated volcanic rocks, Penobscot Bay, Maine. Geol. Soc. Am. Abstr. Programs 34, 42. Heise, W., Bibby, H.M., Grant, C.T., Bannister, S.C., Ogawa, Y., Takakura, S., Uchida, T., 2007. Melt distribution beneath a young continental rift; the Taupo Volcanic Zone, New Zealand. Geophys. Res. Lett. 34, L14313. http://dx.doi.org/10.1029/2007GL029629. Heise, W., Caldwell, T.G., Bibby, H.M., Bennie, S.L., 2010. Three-dimensional electrical resistivity image of magma beneath an active continental rift, Taupo Volcanic Zone, New Zealand. Geophys. Res. Lett. 37, L10301. http://dx.doi.org/10.1029/ 2010GL043110. Hiess, J., Cole, J.W., Spinks, K.D., 2007. Influence of the Crust and Crustal Structure on the location and composition of high-alumina basalts of the Taupo Volcanic Zone, New Zealand. N. Z. J. Geol. Geophys. 50, 327–342. Hildreth, W., 1981. Gradients in Silicic Magma Chambers: implications for Lithospheric Magmatism. J. Geophys. Res. 86, 10153–10192. Hogg, A.G., Higham, T.F.G., Lowe, D.J., Palmer, J.G., Reimer, P.J., 2003. A wigglematch date for Polynesian settlement of New Zealand. Antiquity 77, 116–125. Hogg, A.G., Lowe, D.J., Palmer, J., Boswijk, G., Ramsey, B., 2011. Revised calendar date for the Taupo eruption derived by 14C wiggle-matching using a New Zealand kauri 14C calibration data set. The Holocene 22, 439–449. Houghton, B.F., Wilson, C.J.N., McWilliams, M.O., Lanphere, M.A., Weaver, S.D., Briggs, R.M., Pringle, M.S., 1995. Chronology and dynamics of a large silicic magma system: central Taupo Volcanic Zone, New Zealand. Geology 23, 13–16. Johnson, E.R., Kamenetsky, V.S., McPhie, J., Wallace, P.J., 2011. Degassing of the H2O-rich rhyolites of the Okataina Volcanic Center, Taupo Volcanic Zone, New Zealand. Geology 39, 311–314. Jurado-Chichay, Z., Walker, G.P.L., 2000. Stratigraphy and dispersal of the Mangaone Subgroup pyroclastic deposits, Okataina Volcanic Centre, New Zealand. J. Volcanol. Geotherm. Res. 104, 319–383. Kennedy, B., Wilcock, J., Stix, J., 2012. Caldera resurgence during magma replenishment and rejuvenation at Valles and Lake City calderas. Bull. Volcanol. 74, 1833–1847. Kilgour, G.N., Smith, R.T., 2008. Stratigraphy, dynamics and eruption impacts of the dual magma Rotorua eruptive episode, Okataina Volcanic Centre, New Zealand. N. Z. J. Geol. Geophys. 51, 367–378. Klemetti, E.K., Deering, C.D., Cooper, K.M., Roeske, S.M., 2011. Magmatic perturbations in the Okataina Volcanic Complex, New Zealand at thousand-year timescales recorded in single zircon crystals. Earth Planet. Sci. Lett. 305, 185–194. Kobayashi, T., Nairn, I., Smith, V., Shane, P., 2005. Proximal stratigraphy and event sequence of the c. 5600 cal yr BP Whakatane rhyolite eruption episode from Haroharo volcano, Okataina Volcanic Centre, New Zealand. N. Z. J. Geol. Geophys. 48, 471–490. Leonard, G.S., Cole, J.W., Nairn, I.A., Self, S., 2002. Basalt triggering of the c.AD 1305 Kaharoa rhyolite eruption, Tarawera Volcanic Complex, New Zealand. J. Volcanol. Geotherm. Res. 115, 461–486. Leonard, G.S., Calvert, A.T., Wilson, C.J.N., Gravley, D.M., Deering, C.D., Hikuroa, D.C.H., 2008. Coastal uplift linked to sea level and a record of early Rotorua–Okataina eruptions: constraints from new Taupo Volcanic Zone 40Ar–39Ar geochronology. Abstracts, IAVCEI General Assembly, Reykjavik, Iceland, 17–22 August 2008. Leonard, G.S., Begg, J.G., Wilson, C.J.N., 2010. Geology of the Rotorua area. Institute of Geological and Nuclear Sciences 1:250,000 geological map 5. 1 sheet +102p. Lower Hutt, New Zealand. Lipman, P.W., 1997. Subsidence of ash-flow calderas; relation to caldera size and magma chamber geometry. Bull. Volcanol. 59, 198–218. Lipman, P.W., 2000. Calderas. In: Sigurdsson, H. (Ed.), Encyclopaedia of Volcanoes. Academic Press, San Diego, pp. 643–662. Lipman, P., Dungan, M., Bachmann, O., 1997. Comagmatic granophyric granite in the Fish Canyon Tuff, Colorado: implications for magma-chamber processes during a large ash-flow eruption. Geology 25, 915–918. Manning, D.A., 1995. Late Pleistocene tephrostratigraphy of the Eastern Bay of Plenty region, New Zealand. (PhD thesis) Victoria University of Wellington, New Zealand. Manning, D.A., 1996. Middle–Late Pleistocene Tephrostratigraphy of the Eastern Bay of Plenty, New Zealand. Quat. Int. 34–36, 3–12. Matthews, N., 2006. Subcrustal magma reservoirs beneath the Okataina Caldera Complex, Taupo Volcanic Zone. (BSc (Hons) thesis) University of Canterbury, Christchurch, New Zealand. Metcalf, R.V., 2004. Volcanic-Plutonic Links, Plutons as Magma Chambers and Crust Mantle Interaction: A Lithospheric Scale View of Magma Systems. Trans. R. Soc. Edinb. Earth Sci. 95, 357–374. Milicich, S.D., Wilson, C.J.N., Bignall, G., Pezaro, B., Charlier, B.L.A., Wooden, J.L., Ireland, T.R., 2013. U–Pb dating of zircons in hydrothermally altered rocks of the Kawerau Geothermal Field, Taupo Volcanic Zone, New Zealand. J. Volcanol. Geotherm. Res. 253, 97–113. Miller, C.F., Mittlefehldt, D.W., 1984. Extreme fractionation in felsic magma chambers: a product of liquid state diffusion or fractional crystallization? Earth Planet. Sci. Lett. 68, 151–158. Milner, D.M., Cole, J.W., Wood, C.P., 2002. Asymmetric, multiple-block collapse at Rotorua Caldera, Taupo Volcanic Zone, New Zealand. Bull. Volcanol. 64, 134–149. Molloy, C., Shane, P., Nairn, I.A., 2008. Pre-eruption thermal rejuvenation and stirring of a partly crystalline rhyolite pluton revealed by the Earthquake Flat Pyroclastic deposits, New Zealand. J. Geol. Soc. Lond. 165, 435–447. Nairn, I.A., 1981. Some studies of the geology, volcanic history and geothermal resources of the Okataina volcanic center, Taupo Volcanic Zone, New Zealand. (PhD Thesis) Victoria University, Wellington, New Zealand.

16

J.W. Cole et al. / Earth-Science Reviews 128 (2014) 1–17

Nairn, I.A., 2002. Geology of the Okataina Volcanic Centre. Institute of Geological and Nuclear Sciences Geological Map 25, 156p + map. Nairn, I.A., Beanland, S., 1989. Geological setting of the 1987 Edgecumbe earthquake, New Zealand. N. Z. J. Geol. Geophys. 32, 1–13. Nairn, I.A., Kohn, B.P., 1973. Relationship of the Earthquake Flat Breccia to the Rotoiti Breccia, central North Island, New Zealand. N. Z. J. Geol. Geophys. 16, 269–280. Nairn, I.A., Self, S., Cole, J.W., Leonard, G.S., Scutter, C., 2001. Distribution, stratigraphy, and history of proximal deposits from the c.AD 1305 Kaharoa eruptive episode at Tarawera Volcano, New Zealand. N. Z. J. Geol. Geophys. 44, 467–484. Nairn, I.A., Shane, P.R., Cole, J.W., Leonard, G.J., Self, S., Pearson, N., 2004. Rhyolite magma processes of the AD1315 Kaharoa eruption episode, Tarawera volcano, New Zealand. J. Volcanol. Geotherm. Res. 131, 265–294. Nairn, I.A., Hedenquist, J.W., Villamor, P., Berryman, K.R., Shane, P.A., 2005. The AD1315 Tarawera and Waiotapu eruptions, New Zealand: contemporaneous rhyolite and hydrothermal eruptions driven by an arrested basalt dike system? Bull. Volcanol. 67, 186–193. Nicholls, I.A., Oba, T., Conrad, W.K., 1992. The nature of primary rhyolitic magmas involved in crustal evolution: evidence from an experimental study of cummingtonite-bearing rhyolites, Taupo Volcanic Zone, New Zealand. Geochim. Cosmochim. Acta 56, 955–962. O'Brien, J., Mroczek, E., Boseley, C., 2011. Chemical Structure of the Ngatamariki Geothermal Field, Taupo Volcanic Zone, New Zealand. Proceedings of the 2011 Geothermal Workshop, 21–23 November 2011, Auckland, New Zealand (6 pp.). Pullar, W.A., Nairn, I.A., 1972. Matahi basaltic tephra member, Rotoiti Breccia Formation. N. Z. J. Geol. Geophys. 15, 446–450. Quick, J.E., Sinigoi, S., Peressini, G., Demarchi, G., Wooden, J.L., Sbisà, A., 2009. Magmatic plumbing of a large Permian caldera exposed to a depth of 25 km. Geology 37, 603–606. Riecker, R.E. (Ed.), 1979. Rio Grande Rift: Tectonics and Magmatism. American Geophysical Union, Washington, DC, USA (438 pp.). Rogan, M., 1982. A geophysical study of the Taupo Volcanic Zone, New Zealand. J. Geophys. Res. 87, 4073–4088. Rowland, J.V., Sibson, R.H., 2001. Extensional fault kinematics within the Taupo Volcanic Zone, New Zealand: soft linked segmentation of a continental rift system. N. Z. J. Geol. Geophys. 44, 271–283. Sable, J.E., Houghton, B.F., Wilson, C.J.N., Carey, R.J., 2006. Complex proximal sedimentation from Plinian plumes: the example of Tarawera 1886. Bull. Volcanol. 69, 89–103. Schmitz, M.D., 1995. Geochemical studies of the Rotoiti Pyroclastics Eruption, Okataina Volcanic Center, Taupo Volcanic Zone. (MSc Thesis) University of Auckland, New Zealand. Schmitz, M.D., Smith, I.E.M., 2004. The Petrology of the Rotoiti Eruption Sequence, Taupo Volcanic Zone: an Example of Fractionation and Mixing in a Rhyolitic System. J. Petrol. 45, 2045–2066. Seebeck, H., Nicol, A., Stern, T.A., Bibby, H.M., Stagpoole, V., 2010. Fault controls on the geometry and location of the Okataina Caldera, Taupo Volcanic Zone, New Zealand. J. Volcanol. Geotherm. Res. 190, 136–151. Self, S., Goff, F., Gardner, J.N., Wright, J., Kite, W., 1986. Explosive Rhyolitic Volcanism in the Jemez Mountains: Vent Locations, Caldera Development and Relation to Regional Structure. J. Geophys. Res. 91 (B2), 1779–1798. Sewell, R.J., Tang, D.L.K., Campbell, S.D.G., 2012. Volcanic–plutonic connections in a tilted nested caldera complex in Hong Kong. Geochem. Geophys. Geosyst. 13, Q01006. http://dx.doi.org/10.1029/2011GC003865. Shane, P., Smith, V.C., 2013. Using amphibole crystals to reconstruct magma storage temperatures and pressures for the post-caldera collapse volcanism at Okataina volcano. Lithos 156–159, 159–170. Shane, P., Nairn, I.A., Smith, V.C., 2005a. Magma mingling in the 50 ka Rotoiti eruption from Okataina Volcanic Centre: implications for geochemical diversity and chronology of large volume rhyolites. J. Volcanol. Geotherm. Res. 139, 295–313. Shane, P., Smith, V.C., Nairn, I., 2005b. High temperature rhyodacites of the 36 ka Hauparu pyroclastic eruption, Okataina Volcanic Centre, New Zealand: change in a silicic magmatic system following caldera collapse. J. Volcanol. Geotherm. Res. 147, 357–376. Shane, P., Martin, S.B., Smith, V.C., Beggs, K.F., Darragh, M.B., Cole, J.W., Nairn, I.A., 2007. Multiple rhyolite magmas and basalt injection in the 17.7 ka Rerewhakaaitu eruption episode from Tarawera volcanic complex, New Zealand. J. Volcanol. Geotherm. Res. 164, 1–26. Shane, P., Smith, V.C., Nairn, I., 2008a. Millennial timescale resolution of rhyolite magma recharge at Tarawera volcano: insights from quartz chemistry and melt inclusions. Contrib. Mineral. Petrol. 156, 397–411. Shane, P., Nairn, I.A., Smith, V.C., Darragh, M.B., Beggs, K.F., Cole, J.W., 2008b. Silicic recharge of multiple rhyolite magmas by basaltic intrusion during the 22.6 ka Okareka Eruption Episode, New Zealand. Lithos 103, 527–549. Shane, P., Storm, S., Schmitt, A.K., Lindsay, J.M., 2012. Timing and conditions of formation of granitoid clasts erupted in recent pyroclastic deposits from Tarawera volcano (New Zealand). Lithos 140–141, 1–10. Sherburn, S., Bannister, S., Bibby, H., 2003. Seismic velocity structure of the central Taupo Volcanic Zone, New Zealand, from local earthquake tomography. J. Volcanol. Geotherm. Res. 122, 69–88. Smith, R.L., Bailey, R.A., Ross, C.S., 1961. Structural evolution of the Valles Caldera, New Mexico, and its bearing on the emplacement of ring dikes. U.S. Geol. Surv. Prof. Pap. 424, 145–149. Smith, I.E.M., Nairn, I.A., Parker, R.J., 2002. Outline of the petrology and chemistry of the Okataina Volcanic Centre, in: Nairn I.A. (Ed.), Geology of the Okataina Volcanic Centre, scale 1:50,000. Institute of Geological and Nuclear Sciences geological map

25: 146-156. Institute of Geological and Nuclear Sciences Ltd, Lower Hutt, New Zealand. Smith, V.C., Shane, P., Smith, I.E.M., 2002b. Tephrostratigraphy and geochemical fingerprinting of the Mangaone Subgroup tephra beds, Okataina Volcanic Centre, New Zealand. N. Z. J. Geol. Geophys. 45, 207–219. Smith, V.C., Shane, P., Nairn, I.A., 2004. Reactivation of a rhyolite magma body by a new rhyolite intrusion before the 15.8 ka Rotorua eruptive episode: implications for magma storage in the Okataina Volcanic Centre, New Zealand. J. Geol. Soc. Lond. 161, 757–772. Smith, V.C., Shane, P., Nairn, I.A., 2005. Trends in rhyolite geochemistry, mineralogy, and magma storage during the last 50 kyr at Okataina and Taupo volcanic centres, Taupo Volcanic Zone, New Zealand. J. Volcanol. Geotherm. Res. 148, 372–406. Smith, V.C., Shane, P., Nairn, I.A., Williams, C.M., 2006. Geochemistry and magmatic properties of eruption episodes from Haroharo linear vent zone, Okataina Volcanic Centre, New Zealand during the last 10 kyr. Bull. Volcanol. 69, 57–88. Smith, V.C., Shane, P., Nairn, I., 2010. Insights into silicic melt generation using plagioclase, quartz and melt inclusions from the caldera-forming Rotoiti eruption, Taupo Volcanic Zone, New Zealand. Contrib. Mineral. Petrol. 160, 951–971. Snyder, D., 2000. Thermal effects of the intrusion of basaltic magma into more silicic magma chambers, and implications for eruption triggering. Earth Planet. Sci. Lett. 175, 257–273. Speed, J., Shane, P., Nairn, I.A., 2002. Volcanic stratigraphy and phase chemistry of the 11,900 yr BP Waiohau eruptive episode, Tarawera Volcanic Complex, New Zealand. N. Z. J. Geol. Geophys. 45, 395–410. Spinks, K.D., 2005. Rift Architecture and Caldera Volcanism in the Taupo Volcanic Zone, New Zealand. (PhD thesis) University of Canterbury, Christchurch, New Zealand. Stagpoole, V.M., Bibby, H.M., 1999. Residual gravity anomaly map of the Taupo Volcanic Zone, New Zealand, 1:250 000, version 1.0. Institute of Geological and Nuclear Sciences Geophysical Map 13, Institute of Geological and Nuclear Sciences, Lower Hutt, Wellington, New Zealand. Stankiewicz, J., Ryberg, T., Haberland, C., Fauzi, Natawidjaja, D., 2010. Lake Toba volcano magma system imaged by ambient seismic noise tomography. Geophys. Res. Lett. 37, L17306. http://dx.doi.org/10.1029/2010GL044211.2010. Stern, T., Benson, A., 2011. Wide-angle seismic imaging beneath an andesitic arc: Central North Island, New Zealand. J. Geophys. Res. 116, B09308. http://dx.doi.org/10.1029/ 2011JB008337,2011. Stern, T.A., Stratford, W.R., Salmon, M.L., 2006. Subduction evolution and mantle dynamics at a continental margin, central North Island, New Zealand. Rev. Geophys. 44, RG4002 (Paper 2005RG00017). 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 Planet. Sci. Lett. 301, 511–520. Storm, S., Shane, P., Schmitt, A.K., Lindsay, J., 2012. Decoupled crystallization and eruption histories of the rhyolite magmatic system at Tarawera volcano revealed by zircon ages and growth rates. Contrib. Mineral. Petrol. 163, 505–519. Turnbull, R., Weaver, S., Tulloch, A., Cole, J., Handler, M., Ireland, T., 2010. Field and geochemical constraints on mafic–felsic interactions, and processes in high-level arc magma chambers: an example from the Halfmoon Pluton, Stewart Island, New Zealand. J. Petrol. 51, 1477–1505. Tuttle, O.F., Bowen, N.L., 1958. Origin of granite in the light of experimental studies in the system NaAlSi3O8–KAlSi3O8–SiO2–H2O. Mem. Geol. Soc. Am. 74. Vandergoes, M.J., Hogg, A.G., Lowe, D.J., Newnham, R.M., Denton, G.H., Southon, J., Barrell, D.J.A., Wilson, C.J.N., McGlone, M.S., Allan, A.S.R., Almond, P.C., Petchey, F., Dabell, K., Dieffenbacher-Krall, A.C., Blaauw, M., 2013. A revised age for the Kawakawa/Oruanui tephra, a key marker for the Last Glacial Maximum in New Zealand. Quat. Sci. Rev. http://dx.doi.org/10.1016/j.quascirev.2012.11.006. Walker, G.P.L., 1984. Downsag calderas, ring faults, caldera sizes, and incremental growth. J. Geophys. Res. 89, 8407–8416. Walker, B.A., Miller, C.F., Claiborne, L.L., Wooden, J.L., Miller, J.S., 2007. Geology and geochronology of the Spirit Mountain Batholith, southern Nevada: implications for timescales and physical processes of batholith construction. J. Volcanol. Geotherm. Res. 167, 239–262. Wallace, L.M., Beavan, J., McCaffrey, R., Darby, D., 2004. Subduction zone coupling and tectonic block rotations in the North Island, New Zealand. J. Geophys. Res. 109, B12496. http://dx.doi.org/10/1029/2004JB003241. Wehrmann, H., 2000. Lahar deposits and tephrostratigraphy, Maketu Peninsula, Bay of Plenty, New Zealand. MSc thesis in Earth Sciences.University of Waikato, Hamilton, New Zealand. Westerman, D.S., Dini, A., Innocenti, F., Rocchi, S., 2004. Rise and fall of a nested Christmas-tree laccolith complex, Elba Island, Italy. In: Breitkreuz, C., Petford, N. (Eds.), Physical geology of high-level magmatic systemsGeol. Soc. Lond. Spec. Publ. 234, 195–213. Wiebe, R.A., Collins, W.J., 1998. Depositional features and stratigraphic sections in granitic plutons: implications for the emplacement and crystallization of granitic magma. J. Struct. Geol. 20, 1273–1289. Wiebe, R.A., Frey, H., Hawkins, D.P., 2001. Basaltic pillows mounds in the Vinalhaven intrusion, Maine. J. Volcanol. Geotherm. Res. 107, 171–184. Wiebe, R.A., Blair, K.D., Hawkins, D.P., Sabine, C.P., 2002. Mafic injections, in situ hybridization and crystal accumulation in the Pyramid Peak Granite, California. Geol. Soc. Am. Bull. 114, 909–920. Wiebe, R.A., Manon, M.R., Hawkins, D.P., McDonough, W.F., 2004. Latestage mafic injection and thermal rejuvenation of the Vinalhaven granite, Coastal Maine. J. Petrol. 45, 2133–2153.

J.W. Cole et al. / Earth-Science Reviews 128 (2014) 1–17 Wilson, C.J.N., 1993. Stratigraphy, chronology, styles and dynamics of the late Quaternary eruptions from Taupo volcano, New Zealand. Phil. Trans. R. Soc. Series A 343, 205–306. Wilson, C.J.N., Houghton, B.F., McWilliams, M.O., Lanphere, M.A., Weaver, S.D., Briggs, R.M., 1995. Volcanic and structural evolution of Taupo Volcanic Zone, New Zealand: a review. J. Volcanol. Geotherm. Res. 68, 1–28. Wilson, C.J.N., Blake, S., Charlier, B.L.A., Sutton, A.N., 2006. The 26.5 ka Oruanui eruption, Taupo volcano, New Zealand: development, characteristics and evacuation of a large rhyolitic magma body. J. Petrol. 47, 35–69.

17

Wilson, C.J.N., Rhoades, D.A., Lanphere, M.A., Calvert, A.T., Houghton, B.F., Weaver, S.D., Cole, J.W., 2007. A multiple-approach radiometric age estimate for the Rotoiti and Earthquake Flat eruptions, New Zealand, with implications for the MIS 4/3 boundary. Quat. Sci. Rev. 26, 1861–1870. Wilson, C.J.N., Gravley, D.M., Leonard, G.S., Rowland, J.V., 2009. Volcanism in the central Taupo Volcanic Zone, New Zealand: tempo, styles and controls. In: Thordarson, T., Self, S., Larsen, G., Rowland, S.K., Hoskuldsson, A. (Eds.), Studies in Volcanology: The Legacy of George Walker. Special Publications of IAVCEI, 2, pp. 225–247.