Crustal and mantle influences and U–Th–Ra disequilibrium in andesitic lavas of Ngauruhoe volcano, New Zealand

Chemical Geology 277 (2010) 355–373

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Research papers

Crustal and mantle influences and U–Th–Ra disequilibrium in andesitic lavas of Ngauruhoe volcano, New Zealand Richard C. Price a,⁎, Simon Turner b, Craig Cook a, Barbara Hobden a, Ian E.M. Smith c, John A. Gamble d, Heather Handley b, Roland Maas e, Anja Möbis f a

Science and Engineering, University of Waikato, Hamilton, PB3105, Hamilton, New Zealand GEMOC Key Centre, Macquarie University, North Ryde, Sydney, Australia Geology Programme, School of Environment, University of Auckland, PB2019 Auckland, New Zealand d Department of Geology, National University Ireland, University College Cork, Ireland e School of Earth Sciences, University of Melbourne, Victoria 3010, Australia f Volcanic risk solutions, Massey University, PB11222, Palmerston North, New Zealand b c

a r t i c l e

i n f o

Article history: Received 5 August 2010 Received in revised form 24 August 2010 Accepted 24 August 2010 Editor: R.L. Rudnick Keywords: Andesite U-series isotopes Subduction-related magmatism Ngauruhoe New Zealand

a b s t r a c t The andesitic volcano Ngauruhoe, which is located within the Tongariro Volcanic Complex at the southern end of the Taupo Volcanic Zone in North Island, New Zealand, has been constructed over the past 5 ka and last erupted in 1975. Nearby Ruapehu volcano has a much longer eruptive history extending back beyond 230 ka B.P. The magmas erupted at both volcanoes have been predominantly medium-K basaltic andesites and andesites, which evolved through polybaric crystal fractionation and assimilation processes that took place within complex, dispersed magmatic storage systems. Despite their close spatial proximity, the two volcanoes show geochemical contrasts suggesting that in each case both the mantle-derived parental magmas and the crustal assimilants were different. Variations in major and trace element data for Ngauruhoe lavas indicate control by crystal fractionation and assimilation (AFC) but the data are difficult to reconcile with derivation from a single batch of compositionally unique, mantle-derived parental basalt. Geochemical variation can be approximated by generalised AFC models using average basement or crustal xenolith compositions but precise mathematical modelling is limited because of the sensitivity of models to the selection of a particular parental composition and the difficulty of determining the exact nature of the crustal assimilant compositions that might have been involved in each specific case. U–Th isotopic data for Ngauruhoe volcanic rocks show disequilibrium that defines a positively inclined array lying to the right of the equiline on a 230Th/232Th versus 238U/232Th diagram. U-series data for Ngauruhoe and post 1945 AD Ruapehu eruptives show similar patterns of disequilibrium but the Ngauruhoe data define an array with a different slope and a different intercept on the equiline. 230Th/238U ratios in Ngauruhoe lavas range from close to secular equilibrium (0.979) to 0.864 with the youngest (post 1870 AD) eruptives showing the least disequilibrium. 230Th/238U and 87Sr/86Sr ratios are correlated; the samples with the highest 87Sr/86Sr values show the least disequilibrium. There is also a correlation between eruption age and Nd and Pb isotopic composition; the oldest samples tend to show the least radiogenic isotopic ratios. These data, in combination with major and trace element abundance data indicate that Ngauruhoe U–Th disequilibrium is determined by variations in parental magmatic compositions and AFC. Unlike the mature, long-lived Ruapehu system, Ngauruhoe's eruptive cycles can be directly connected to periodic magma recharge from the lower crust and mantle. For Ngauruhoe eruptives, 226Ra/230Th varies from 1.218 to 1.492. This compares with a range of 0.972 to 1.186 for Ruapehu lavas erupted between 1945 and 1996. Reconciling the 226Ra/230Th data with decay during fractional crystallisation requires relatively low rates of fractionation (1–5 × 10− 5/year) and short (1000–2000 years) fractionation times. © 2010 Elsevier B.V. All rights reserved.

1. Introduction

⁎ Corresponding author. Tel.: +61 3 5334 3811. E-mail address: [email protected] (R.C. Price). 0009-2541/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2010.08.021

There is general agreement that magmas associated with subduction (arc magmas) have a primary source in the mantle wedge above the subducting slab (e.g. Hawkesworth et al., 1979, 1993; Grove and Kinzler, 1986; Brenan et al., 1995; Elliott et al., 1997; Ayers, 1998;

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R.C. Price et al. / Chemical Geology 277 (2010) 355–373

Kessel et al., 2005; Grove et al., 2006) with the geochemical characteristics of primary arc magmas being determined through the interaction of slab components, derived from subducted oceanic lithosphere, with depleted peridotite in the overlying mantle wedge (e.g. Arculus and Powell, 1986; Tatsumi et al., 1986; McCulloch and Gamble, 1991; Hawkesworth et al., 1993; Kessel et al., 2005; Spandler et al., 2007; Straub et al., 2008). The geochemistry of rocks representing primitive magmas derived directly from the interaction of slab derived components and mantle wedge can in theory be used to draw inferences about slab fluxes, lithospheric recycling and crustal evolution (e.g. Tera et al., 1986; Kay and Mahlburg Kay, 1988; Hawkesworth et al., 1991; Plank and Langmuir, 1993, 1998; Regelous et al., 1997) and uranium series (U-series) disequilibrium studies can be applied to constrain the rates at which the processes generating the magmas have taken place (e.g. Condomines and Sigmarsson, 1993; Gill et al., 1993; Hoogewerff et al., 1997; Turner et al., 1997; Clark et al., 1998; Hawkesworth et al., 2000, 2004; Turner et al., 2000a; Zellmer et al., 2000; Sigmarsson et al., 2002). Unequivocal cases of rocks representing primitive magmas derived directly from mantle sources are however rare (Straub et al., 2008). Most subductionrelated volcanic rocks have geochemical and/or petrographic features that suggest modification between source and surface by processes such as crystal fractionation, magma mixing, and crustal assimilation (e.g. Grove et al., 1982; Grove and Kinzler, 1986; Graham and Hackett, 1987; Hobden et al., 1999; Nakagawa et al., 1999; Price et al., 1999; Dungan et al., 2001; Davidson et al., 2005, 2007). This is particularly obvious where magmas have been modified through an interaction with the continental crust (e.g. Dungan et al., 2001; Price et al., 2005; Feeley et al., 2008) but there is also evidence that crustal processes are a significant influence on the petrology of volcanic rocks emplaced in oceanic subduction systems (Smith et al., 2006, 2010; Brophy, 2008; Handley et al., 2008; Tamura et al., 2009). Detailed studies of subduction-related continental volcanic rocks indicate that the interaction of mantle-derived magmas with crust and processes taking place in the crust can have an over-riding influence on geochemistry making it difficult to draw meaningful conclusions about the precise characteristics of mantle source regions. U-series studies of volcanic systems have been used to constrain time scales for the transport of magma from source to surface and the rates at which magmas evolve in crustal and sub-volcanic magma systems (e.g. Volpe and Hammond, 1991; Heath et al., 1998; Vigier et al., 1999; Turner et al., 2000b; Zellmer et al., 2000; Hawkesworth et al., 2004; Blake and Rogers, 2005; Reagan et al., 2005; Yokoyama et al., 2006) but for continental andesitic and dacitic magmas, processes taking place in the crust can have such a major impact on U-series disequilibrium that it becomes difficult to draw unequivocal conclusions about the time scales over which those processes have taken place (Price et al., 2007). Ngauruhoe and Ruapehu are andesitic volcanoes in the central North Island of New Zealand. The two volcanoes are spatially adjacent and both have been frequently active over the past few thousand years (Cole et al., 1986; Donoghue et al., 1995; Gamble et al., 1999; Hobden et al., 2002; Nakagawa et al., 2002) but they have very different geological histories; Ngauruhoe has been constructed over a period of five thousand years (5 ka) whereas Ruapehu has been active for at least 230 ka. There is now a strong body of evidence to suggest that North Island andesites have evolved through assimilation and fractional crystallisation (AFC) processes with the crustal assimilant having a major influence on isotopic and trace element compositions (e.g. Graham and Hackett, 1987; Graham et al., 1995; Gamble et al., 1999; Hobden et al., 1999; Price et al., 2005). The crust or basement immediately beneath the Tongariro Volcanic Centre is composed of Mesozoic metagreywacke of the Torlesse and Waipapa terranes with a thin (b100 m) veneer of tertiary sediments (Gregg, 1961; Roser and Korsch, 1999; Cassidy et al., 2009).

A U-series study of the volcanic rocks erupted from Ruapehu over the past 10 ka (Price et al., 2007) has shown isotopic disequlibrium that is only partly time dependent. For these eruptives, variations in 87 Sr/86Sr, 230Th/238U and 226Ra/230Th are largely controlled by AFC that has taken place in a plexus of small scale crustal magma storages over a time interval of no more than a few thousand years. In this paper we report new Sr, Nd and Pb isotope and whole rock major and trace element analyses for Ngauruhoe lavas and crustal xenoliths and new U-series data for Ngauruhoe eruptives. We apply and test petrogenetic models and compare and contrast the magmatic processes affecting geochemical variation at Ngauruhoe with those involved at Ruapehu. Our data indicate that although the processes by which andesitic magmas evolve are generally similar in these two adjacent andesitic volcanoes, there are geochemical contrasts that reflect significant differences in the magmatic systems. 2. Geological setting and eruptive history of Ngauruhoe 2.1. Ngauruhoe and Ruapehu volcanoes and the Tongariro Volcanic Centre Ngauruhoe and Ruapehu are the most recently active volcanoes of the Tongariro Volcanic Centre (Cole et al., 1986), which lies at the southern end of the Taupo Volcanic Zone (TVZ) in the central North Island of New Zealand (Fig. 1A). The TVZ is the magmatic manifestation of subduction at the south-western margin of the Pacific Plate and is a rifted region of thin crust, and high heat flow (Cole, 1979; Stern, 1987; Hochstein et al., 1993; Bibby et al., 1995; Rowland and Sibson, 2001; Villamor and Berryman, 2001; Sherburn et al., 2003; Stern et al., 2006). The volcanoes of the Tongariro Volcanic Centre are located approximately 80 km above a Wadati–Benioff zone that represents the upper surface of the subducting Pacific plate beneath North Island New Zealand (Adams and Ware, 1977; Stern et al., 2006). Ruapehu has an eruptive history extending over at least 230 ka with the most recent significant eruptions occurring in 1996 (Hackett, 1985; Gamble et al., 1999, 2003). For the past 2 ka, eruptions have been relatively small in volume (b0.1 km3) with events significant enough to cause tephra deposition across the central North Island occurring on average every 50–100 years (Donoghue et al., 1995). Ruapehu eruptives range in composition from basalt to dacite but the vast majority are strongly porphyritic (20–60% phenocrysts) plagioclase and two pyroxene andesites (Graham and Hackett, 1987). Ngauruhoe, which lies 16 km to the north of Ruapehu (Fig. 1B), is the youngest of at least nine cones making up the Tongariro Volcano (Gregg, 1961; Cole et al., 1986; Hobden et al., 1996). Unlike Ruapehu, which is a long-lived, multiple vent complex, Ngauruhoe's classically simple, single vent cone has been built to its present height (2287 m above sea level and ~ 900 m from base to summit) over a period of 5 ka (Möbis et al., 2008; Möbis, 2010). The preserved eruptive volume in Ngauruhoe is ~ 2.2 km3 (Hobden et al., 2002), compared with an estimated volume for Ruapehu of ~ 110 km3 (Hackett, 1985). Most of the lava flows exposed on Ngauruhoe's cone post-date the 1.85 ka Taupo pumice (Hobden et al., 2002). Historical eruptions (post 1870 AD) have been frequent and explosive with an eruption occurring every 2–3 years (Gregg, 1960; Hobden et al., 2002). The last eruptions occurred in 1974 and 1975 (Nairn et al., 1976) and the current quiescence marks the longest period without eruptive activity in the past 180 years (Hobden et al., 2002). Ngauruhoe lavas are strongly porphyritic basaltic andesites and andesites (Hobden, 1997). Plagioclase is the dominant phenocryst (15–30% of the rock) with the modal abundance of clino- and orthopyroxene phenocrysts ranging between 2–9% and 1–10% respectively. Olivine is present in basaltic andesites and is generally b1% of the mode. Magnetite occurs in most samples but commonly makes up less than 1% of the rock.

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Fig. 1. (A) Map showing the location of the Tongariro Volcanic Centre and Ngauruhoe and Ruapehu volcanoes at the southern end of the Taupo Volcanic Zone in New Zealand's North Island. The inset shows the tectonic setting with the Pacific Plate being subducted westward under the Australian plate at the Kermadec Trench. The digital terrane map of the North Island is from Wright et al. (2003). (B) is a view from the west, looking east, towards the Tongariro Volcanic Centre, which is made up of Ruapehu, at right and to the south, Ngauruhoe in the centre and Tongariro, to the left or north of Ngauruhoe.

2.2. Flow and tephra stratigraphy of Ngauruhoe The eruptive history of Ngauruhoe has been established through detailed studies of tephra sequences preserved on the ring plain surrounding the Tongariro Volcanic Centre (e.g. Donoghue et al., 1995; Cronin et al., 1997) and the geochemical evolution of the volcano has been described by Hobden (1997) and summarised in Hobden et al. (2002). On the basis of detailed stratigraphic sampling, petrography, radiocarbon dating and glass chemistry, Möbis (2010) has modified earlier stratigraphic interpretations (e.g. Donoghue et al., 1995) and subdivided the tephra stratigraphy of Ngauruhoe into four chronological stages. Stages 1 to 3 predate the Taupo Pumice, which was emplaced 1.85 ka B.P. (Wilson, 1993). Stage 1 is placed within the Papakai Formation and erupted between 5 and 3.5 ka B.P. Stage 2, which is contained within the Mangatawai Formation, includes tephras that were erupted during the main cone building phase, 3.5 to 2.7 ka B.P. Stage 3, commencing at ~2.7 ka B.P. represents a period of less frequent and less substantial explosive activity preceding the emplacement of the Taupo Pumice and Stage 4 includes all eruptive activity younger than the Taupo Pumice. Mapping and detailed geochemical and petrographic investigations of Ngauruhoe lava flows provided Hobden (1997) with the basis for constructing a flow stratigraphy for the present day cone. The flow sequence was divided into five stratigraphic groups (Hobden et al., 2002) and we will use these subdivisions as a temporal framework in which to describe and discuss the data presented in this paper. For a number of reasons correlation of tephra and lava flow stratigraphy is challenging. Flows tend to be restricted to the upper slopes of the volcano and older flows are commonly concealed under younger lavas. Most of the older flows have been eroded on the unstable and steep upper slopes of the active volcano. Consequently, older flows

that would be correlative with tephras of Möbis's Stages 1 and 2 are not well represented in the flow stratigraphy. In addition, the geochemistry of tephras and flows cannot be directly compared; tephras are generally glassy fragments of relatively evolved melt or magma and their compositions are not directly comparable with less evolved, crystal-rich andesitic magmas represented by lava flows. 3. Analytical methods Major and some trace element data for Ngauruhoe lavas are largely from Hobden (1997) but new major and some new trace element data for Ngauruhoe xenoliths were determined by X-ray fluorescence spectrometry (XRF) on fused glass discs at the University of Auckland. Hobden's (1997) XRF analyses were carried out at the University of Canterbury using either fused glass discs (major components) or pressed powder pellets (trace elements) and the methods of Norrish and Hutton (1969) with modifications by Harvey et al. (1973). XRF major and trace element analyses at the University of Auckland were carried out on fused glass discs using the basic method of Norrish and Hutton (1969) with modifications described in Smith et al. (2006). For XRF major oxides precision is generally better than 1% (at 2sd) and for trace elements it is 1% for Sr and Zr, 1–3% for V, Cr, Zn and Y, 3–5% for Ba, 5–10% for Rb and Nb, detection limits are b2 ppm for Rb, Sr, Y, Zr and Nb, 2–5 ppm for V, Cr, and Zn, and 5–10 ppm for Ba. Accuracy was monitored by repeat analyses of well documented standard rocks (e.g. AGV-1 — Table 1). Additional trace element data were obtained for selected lavas and xenoliths by inductively coupled plasma source mass spectrometry (ICP-MS) at the University of Melbourne and by laser ablation (LA) ICP-MS at the Australian National University. ICP-MS analysis at the University of Melbourne followed the methods of Eggins et al. (1997) and Kamber et al. (2005). 100 mg aliquots of sample powder were

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Table 1 Major and trace element data for representative Ngauruhoe samples. Group

SiO2 TiO2 A12O3 Fe2O3* MnO MgO CaO Na2O K2O P2O5 LO1 Total Cs Ba Rb Sr Pb Th U Zr Hf Nb Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Sc V Cr Ni Zn Ga

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

TG010

P39851

TG042

TG045

TG019

TG001

TG004

TG288

TG289

TG205

TG206

TG014

TG020

TG156

TG163

AGV1

BCR2

BCR2

5

5

5

5

5

5

5

4

4

3

3

2

2

2

1

C

M

ANU

56.93 0.72 17.09 8.49 0.15 4.11 7.27 3.03 1.33 0.15 −0.41 98.86 2.0 263 45 238 8.3 5.5 1.1 132 3.3 5.5 24 14.24 31.35

55.81 0.72 17.28 9.04 0.17 4.93 8.29 2.8 1.04 0.14 −0.43 99.79 1.8 219 32 232 7.1 3.5 0.8 104 2.5 4.1 20 10.69 24.23 3.09 12.89 3.15 0.91 3.38 0.56 3.54 0.77 2.21 0.34 2.25 0.34 30 227 73 24 101 22

55.67 0.73 16.79 8.73 0.15 5.19 8.2 2.88 1.12 0.14 −0.28 99.32 1.9 220 37 238 6.3 3.7 0.9 112 2.6 4.3 19 11.44 25.70 3.30 13.55 3.27 0.92 3.41 0.56 3.51 0.76 2.17 0.33 2.19 0.33 30 219 75 24 81 20

55.77 0.70 16.99 8.92 0.15 5.34 8.39 2.81 1.10 0.14 −0.54 99.77 1.8 217 35 241 7.0 3.6 0.9 109 0.3 4.2 22 11.13 25.25 3.22 13.25 3.22 0.91 3.37 0.55 3.46 0.75 2.13 0.33 2.16 0.33 30 218 81 26 87 21

56.51 0.77 16.97 8.42 0.14 4.6 7.72 2.88 1.25 0.15 −0.2 99.21 2.2 231 40 233 8.1 4.2 1.0 117 2.8 4.7 20 12.33 27.83 3.51 14.32 3.42 0.94 3.58 0.59 3.65 0.80 2.28 0.35 2.29 0.35 28 203 51 21 85 21

55.44 0.77 16.48 9.12 0.16 5.52 8.6 2.83 1.12 0.16 − 0.11 100.09 1.8 232 34 278 6.6 3.7 0.9 110 2.6 4.1 19 12.20 27.61 3.54 14.71 3.46 0.98 3.58 0.58 3.55 0.76 2.17 0.33 2.16 0.33 33 222 89 29 86 22

55.06 0.78 16.44 9.17 0.15 5.57 8.62 2.75 1.10 0.16 − 0.59 99.21 1.8 230 36 282 6.5 3.6 0.9 109 2.5 4.0 20 12.04 27.10 3.49 14.47 3.43 0.98 3.55 0.57 3.52 0.76 2.15 0.32 2.13 0.32 32 234 89 27 82 19

57.3 0.7 16.18 8.08 0.14 5.41 7.71 3.17 1.27 0.14 − 0.1 100 2.2 226 45 225 5.3 3.6 1.3 103 2.7 4.3 21 10.09 20.15

58.62 0.71 16.58 7.78 0.14 4.43 7.37 3.3 1.37 0.15 −0.29 100.16 2.6 243 48 222 6.9 3.9 1.1 111 2.6 4.1 19 10.56 23.68 3.04 12.53 3.07 0.83 3.29 0.54 3.40 0.74 2.11 0.33 2.16 0.33 23 180 108 34 74 23

58.03 0.85 18.99 6.94 0.11 2.17 7.4 3.59 1.44 0.17 − 0.42 99.27 2.6 291 47 280 9.4 4.9 1.2 119 3.4 5.7 22 14.22 31.80 4.04 16.36 3.85 1.06 3.97 0.65 4.03 0.87 2.48 0.38 2.49 0.37 20 176 17 7 80 21

58.3 0.78 17.89 7.7 0.13 3.24 7.44 3.38 1.34 0.16 − 0.33 100.03 2.4 257 45 266 8.8 4.6 1.1 133 3.2 5.3 21 13.47 29.98 3.79 15.47 3.64 1.02 3.78 0.62 3.83 0.83 2.35 0.36 2.36 0.36 23 181 31 13 83 20

55.75 0.94 18.02 9.5 0.15 3.69 8.31 3.18 0.95 0.15 − 0.44 100.2 1.8 184 30 225 5.3 2.9 0.8 105 2.5 3.7 23 9.18 21.28 2.85 12.26 3.22 0.97 3.71 0.63 4.00 0.89 2.54 0.39 2.56 0.39 29 239 17 9 87 20

55.55 0.84 17.98 9.48 0.16 3.77 8.29 3.25 0.98 0.14 − 0.42 100.02 1.7 177 31 220 5.2 2.9 0.8 107 2.5 3.7 22 9.04 20.97 2.81 12.13 3.22 0.97 3.66 0.62 3.98 0.88 2.53 0.39 2.53 0.38 30 239 20 9 85 21

56.92 0.85 17.24 8.37 0.15 4.79 8.16 2.96 1.13 0.15 − 0.23 100.49 2.0 236 36 263 6.5 3.6 0.9 116 2.7 4.2 23 11.06 24.98 3.23 13.38 3.25 0.94 3.47 0.57 3.57 0.77 2.21 0.34 2.21 0.33 28 205 58 19 74 20

55.51 0.99 17.62 9.17 0.15 4.70 8.07 3.10 1.09 0.18 − 0.23 100.35 1.1 237 31 356 5.6 3.4 1.0 130 3.0 5.0 24 11.71 26.78

17.92 4.02 1.02 3.81 0.66

0.38 2.61 0.39 29 185 46 17 82 19

12.34 3.22 0.86 3.30 0.48

0.29 2.07 0.31 26 191 190 57 79 20

15.08 3.55 1.07 4.30 0.67

0.36 2.64 0.39 26 218 31 14 76 19

59.95 1.10 17.34 6.83 0.10 1.51 4.99 4.35 2.96 0.51

1207 66 665 42.0 8.0

1.1

1.0

10.0 5.7 1.7

9.9 5.8 1.7

4.8 11.91 33 24.43 51.97 6.79 27.98 6.43 1.90 6.60 1.04 6.23 1.30 3.57 0.52 3.33 0.49 33

4.4 12.27 32 25.07 53.80 6.84 29.68 6.28 1.81 6.68 1.00 6.05 1.22 3.43 0.48 3.29 0.49 34

221 13 20 43 72 30

107 11 18 82 20

Data in italics are new inductively coupled plasma source mass spectrometry data and instrumental neutron activation data from Hobden (1997). All other data are XRF and Columns 12–14 show data for standard rocks (C = XRF at University of Canterbury, M = ICP-MS at University of Melbourne; ANU = LA-ICP-MS at the Australian National University; see “Analytical methods”).

digested in HF–HNO3 in Teflon pressure vessels over several days. The solute was then dried and dissolved in HNO3. Samples were then spiked with an enriched isotope internal standard. ICP-MS analysis was carried out on a Varian quadrupole spectrometer using W-2 as a calibration standard. Repeat analyses of standard rocks (BCR-2 and BHVO-2; Table 1) indicate precision of 0.3–0.7% (rsd) for the rare earth elements, 0.1% for Sc, 0.6% for Nb, 0.8% for Hf, b0.5% for U and Th, and 0.8% for total Pb. LA-ICP-MS xenolith trace element data were obtained at the Australian National University using fused glass discs and an EXCIMER laser system, operating in the ultra-violet spectrum at a wavelength of 193 nm (Eggins et al., 1998). A standard of NIST 612 was run after every 15 analyses. Si concentrations obtained by XRF were used as the internal standard to account for any variation in ablation yield between samples and calibration standards and the ‘matrix effect’ of variations between counts per second and ppm of different elements. During the course of the analyses period, BCR-2 glass standard was also analysed (every 30 analyses) to provide an independent assessment of accuracy and precision (Table 1).

Sr, Nd and Pb isotope data were obtained at the University of Melbourne, using procedures similar to those described in Maas et al. (2005). After acid leaching (6 M HCl, 100 °C, 60 min), rock powder (60–100 mg) for Sr and Nd analysis and rock chips (60–100 mg) for Pb analysis were dissolved in closed Savillex beakers on a hotplate (2 ml of 3:1 HF–HNO3 for 2 days at 110 °C and 6 M HCl for 1 day). Pb, Nd and Sr were extracted using a combination of anion exchange (AG1-X8, 100–200 mesh, HBr–HCl) and EICHROM RE-, LN- and SRresin chromatography (Manhes et al., 1978; Pin et al., 1994; Pin and Santos-Zalduegui, 1997). Total analytical blanks were all well below 100 pg and therefore negligible. Isotopic analyses were carried out on a Nu Instruments® multicollector (MC) ICP-MS coupled to a CETAC Aridus desolvating nebulizer (Woodhead, 2002; Maas et al., 2005), with a typical sensitivity of 100–150 V/ppm Sr, Nd or Pb. Data were acquired in static multi-collection mode on 60–100 ppb analyte solutions. Instrumental mass bias was corrected by internal normalization to 86 Sr/88Sr = 0.1194 and 146Nd/145Nd = 2.0719425 (equivalent to 146 Nd/144 Nd = 0.7219; Vance and Thirlwall, 2002), using the

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exponential law. Typical internal precisions (2sd) were ±0.000008 (Nd) and ≤±0.000020 (Sr). Data are reported relative to SRM987 = 0.710230 and La Jolla Nd = 0.511860. The Japanese Geol Survey Nd isotope standard J-Nd-1, measured twice with the present sample set and adjusted for lab bias as described above, averaged 0.512109 ± 16, within error of the recommended ratio (0.512117; Tanaka et al., 2000). Other international isotopic standards routinely measured also yield results consistent with reference values, e.g. BCR1 (0.705016 ± 46, 0.512641 ± 18) BHVO-1 (0.703478 ± 36, 0.512998 ± 18) and E&A Sr carbonate (0.708005 ± 47, all quoted errors are external precisions, 2sd). Based on these secondary standard results, we estimate external precisions (±2sd) of ±0.000040 (Sr) and ±0.000020 (Nd). In run precision for all 87Sr/86Sr isotopic measurements is between 0.000014 and 0.000020 and for all 143Nd/144Nd measurements it is between 0.000008 and 0.000010. Mass bias during MC–ICP-MS analysis of Pb isotope ratios was corrected using the thallium-doping technique, which produces data accurate to ±0.03% (2sd), relative to the SRM981 composition reported in Woodhead (2002). In run precision on 206Pb/ 204 Pb and 207Pb/204Pb measurements is b0.001 and for 208Pb/204Pb it is b0.003. U, Th and Ra concentrations and isotope ratios were determined on samples that were spiked with 226U–229Th and 228Ra tracers and dissolved using an HF–HNO3–HCl mix in heated Teflon pressure bombs. The product was converted to chloride using 6 N HCl and then 6 N HCl saturated with H3BO3 to remove residual fluorides. The final product was converted to nitrate using 14 N HNO3 and finally taken up in 7 N HNO3. U and Th purification was achieved via a single pass through a 4 ml anionic resin column using 7 N HNO3, 6 N HCl and 0.2 N HNO3 as elutants. We purposefully avoided the use of ElChrom® resins for the U–Th chemistry as these can bleed organics that lead to interferences during analysis. Concentrations and isotope ratios were measured in dynamic mode on a Nu Instruments® MC–ICP-MS at Macquarie University. 238U and 235U were analysed on Faraday cups, using the 238U/235U ratio to determine the U mass bias, assuming 238U/ 235 U = 137.88, whilst 236U and 234U were alternately collected in the IC0 ion counter that is equipped with a deceleration lens. The IC0 gain was determined during interspersed dynamic analyses of CRM145 assuming a 234U/238U ratio of 5.286 × 10− 5 (Cheng et al., 2000). Methods for Th isotope measurements employed a dynamic routine with 232Th in Faraday cups and 230Th and 229Th alternating on IC0 and using bracketing measurements of the Th″U″ standard to obtain the Th mass bias. Measurements at masses 230.5 and 229.5 were used to derive a linear correction for the 232Th tail as described in detail in Appendix A of Sims et al. (2008). The Ra analysis procedure follows that used by Turner et al. (2000b). Ra was taken from the first elution from the anionic column and converted to chloride using 6 N HCl. This was then loaded in 3 N HCl onto an 8 ml cationic column and eluted using 3.75 M HNO3 and the process repeated on a scaled-down 0.6 ml column. Ra and the REE were then separated using an ElChrom® Ln-spec resin™ using 0.1 N HNO3. Ra and Ba were finally chromatographically separated using ElChrom® Sr-spec resin™ and 3 N HNO3 as elutant in a 150 μl procedure. Samples were loaded onto degassed Re filaments using a Ta–HF–H3PO4 activator solution (Birck, 1986) and 228Ra/226Ra ratios were measured to a precision typically ~0.5% in dynamic ion counting mode on a ThermoFinnigan Triton® TIMS at Macquarie University. Accuracy was assessed via replicate analyses of TML-3 which yielded 226 Ra = 3534 ± 76 fg/g and (226Ra/230Th) = 1.002 ± 0.008 (n = 5). Organic interferences are often noted at low temperatures during TIMS analysis for Ra but were eliminated here by using the instrument fitted with a dry scroll pump instead of the standard rotary pump. This prevents leakage of organic molecules into the source during venting. Ruapehu data used for comparison are from Gamble et al. (1999), Price et al. (2005, 2007) and analytical methods and precision and accuracy applicable to these data are described in those publications.

359

4. Major and trace element variations at Ngauruhoe volcano Ngauruhoe lava flows are medium-K basaltic andesites and andesites (Table 1 and Fig. 2A) with SiO2 contents ranging from 54.2 to 58.6 wt.%, MgO abundances in the range of 2.2–5.6 wt.% and K2O contents between 0.9 and 1.4 wt.%. The oldest of the five flow groupings identified by Hobden et al. (2002) was emplaced before the Taupo pumice (1.85 ka). These Group 1 lava flows, which are exposed on the eastern and north western flanks of the volcano, are likely to be time equivalents of tephra Stage 2. They are olivine-bearing basaltic andesites (Fig. 2) with SiO2 abundances between 54.2 and 55.5 wt.%, Mg-numbers [100 ⁎ Mol. MgO / (MgO + FeOtotal)] ranging from 49.3 to 51.1 and K2O abundances between 0.98 and 1.1 wt.% (Fig. 3). Group 2 lavas were erupted down the north western slopes. Some are older and some are younger than the Taupo Pumice and they could therefore be time equivalents of tephra Stage 3 or Stage 4. Like Group 1 eruptives they are olivinebearing basaltic andesites and andesites and although they have similar or higher SiO2 abundances (55.2–57.6 wt.% versus 54.2– 55.5 wt.% for Group 1), most have lower Mg-numbers (43.4–53.1) and relatively lower K2O abundances (0.92–1.25 wt.%) than are observed in eruptives of Group 1 (Figs. 2 and 3). Groups 3, 4 and 5 were emplaced during tephra Stage 4 (post Taupo Pumice to the present day). Group 3 includes three long (3– 4 km) lava flows on the southern slopes of the volcano. Samples from this group have the highest SiO2 abundances of any Ngauruhoe lavas (58.0–58.3 wt.%) with Mg-numbers between 43.7 and 53.1 and K2O contents ranging from 1.34 to 1.44 wt.%. Group 4 lavas were erupted down the southern slopes but at least one flow descended the north western flank. Samples from these flows have some of the highest SiO2 and K2O abundances (54.6–58.6 and 1.07–1.37 wt.% respectively) observed in Ngauruhoe flows but they also include samples with the highest Mg-numbers (50.9–57.2). Hobden et al. (2002) noted that these flows contain olivine more forsteritic than the composition expected given the Mg-numbers of the host lavas. Group 5 includes all lava and block and ash flows erupted between 1870 and 1975 AD and consequently it is the group that has been most intensively sampled; these younger flows have covered and concealed many of the older eruptives on the upper slopes of the volcano. Group 5 flows are olivine-bearing basaltic andesites and andesites showing considerable compositional variability; SiO2 content is between 54.8 and 58.2 wt.%, K2O abundance in the range of 1.03–1.43 wt.% and Mg-numbers range from 39.4 to 55.2 wt.%. Most of these flows were emplaced on the northern flanks or higher eastern slopes of the volcano. On normalised extended element plots (Fig. 4A and B) all Ngauruhoe eruptives show “arc”-type patterns characterised by enrichment in Cs, Rb, Ba, Th, U, K, and light rare earth elements (REE) relative to Zr, Hf, Ti, Y and the heavy REE all of which have depleted N-MORB type abundances. Lead is enriched relative to Ce, and Nb depleted relative to K. Chondrite-normalised rare earth element patterns (Fig. 4C and D) all show enrichment of La relative to Yb [(La/Yb)n = 2.56–4.10] and pronounced negative Eu anomalies (Eu/Eu* = 0.785–0.860). Light over heavy REE enrichment is not however, systematically related to other conventional indices of fractionation; for example, Group 2 lavas with low Mg-numbers also have relatively low (La/Yb)n and Group 5 eruptives analysed for REE have variable (La/Yb)n (3.07–4.05) even though Mg-numbers for these samples show relatively limited variation (Fig. 3F). In most Ngauruhoe lavas Ni and Cr abundances are at low levels (13–57 ppm Ni and 5–193 ppm Cr). Scandium abundances range between 20 and 33 ppm and V contents vary from 174–234 ppm. In each group of lavas Ni abundance is generally inversely correlated with SiO2 content but different groups define different arrays on the Ni versus SiO2 variation diagram (Fig. 2F). Group 4 includes lavas with the highest Ni and Cr abundances (up to 57 ppm Ni and 193 ppm Cr) whereas Groups 1 and 2 generally show relatively low abundances of

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Fig. 2. Selected silica variation diagrams for samples of Ngauruhoe volcano and comparisons with the Whakapapa formation (b 15 ka and including 1945–1996 eruptives) of Ruapehu volcano (Gamble et al., 1999; Price et al., 2005). Also shown are fields for Taupo Volcanic Zone (TVZ) basalts (Gamble et al., 1993) and high-Mg andesites from White Island in the offshore TVZ (Heyworth et al., 2007). Panels at the right show details for Ngauruhoe data with samples differentiated according to flow stratigraphy (Groups 1–5). (A–B) K2O content versus SiO2 abundance; (C–D) Mg-number [100 ⁎ mol MgO / (MgO + FeOtotal)] versus SiO2 abundance; (E–F) Ni content versus SiO2 abundance; (G–H) 87Sr/86Sr isotopic ratio versus SiO2 abundance. The classification scheme shown in (A) is from Gill (1981). The field labelled “BA” in (A) is that of basaltic andesite.

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361

Fig. 3. Selected Mg-number [100 ⁎ mol MgO / (MgO + FeOtotal)] variation diagrams for samples of Ngauruhoe volcano and comparisons with the Whakapapa formation (b 15 ka and including 1945–1996 eruptives) of Ruapehu volcano (Gamble et al., 1999; Price et al., 2005) Also shown are fields for Taupo Volcanic Zone (TVZ) basalts (Gamble et al., 1993) and high-Mg andesites from White Island in the offshore TVZ (Heyworth et al., 2007). Panels at the right show details for Ngauruhoe data with samples differentiated according to flow stratigraphy (Groups 1–5). (A–B) K2O content versus Mg-number; (C–D) 87Sr/86Sr isotopic ratio versus Mg-number; (E–F) (La/Yb)n versus Mg-number. (La/Yb)n is the ratio of La and Yb abundances normalised to the chondritic abundances of Sun and McDonough (1989).

these elements (13–17 ppm Ni and 28–36 ppm Cr in Group 1 and 9– 19 ppm Ni and 5–58 ppm Cr in Group 2). 5. Sr, Nd and Pb isotopic variations in Ngauruhoe eruptives Representative Sr, Nd, and Pb isotopic analyses are presented in Table 2 and variation for these isotopic ratios is illustrated in Fig. 5. 87 Sr/86Sr isotopic ratios vary from 0.70419 to 0.70613 and 143Nd/ 144 Nd ranges from 0.51264 to 0.51282. Sr and Nd isotopic ratios are negatively correlated (Fig. 5A). 206Pb/204Pb, which varies from 18.236 to 18.883, is crudely correlated with 87Sr/86Sr ratio (Fig. 5B). Group 1 and Group 2 samples have the least and Group 5 the most radiogenic Sr, Nd and Pb isotopic compositions. Group 5 lavas show a wide range in isotopic composition and on the Pb versus Sr isotopic plot samples from this flow group are distributed in two arrays that converge at high 87Sr/86Sr and high 206Pb/204Pb.

Fig. 6A shows 87Sr/86Sr variation as a function of eruption age and/ or stratigraphic position for Ngauruhoe samples. 87Sr/86Sr isotopic ratios increase progressively with time; from a maximum of 0.70419 in Group 1 to 0.70613 in Group 5. The temporal variation of Sr isotopic compositions for tephras, which is more precisely constrained by radiocarbon dating, is compared with time-related variation of 87 Sr/86Sr in lava flows in Fig. 7. 143 Nd/144Nd isotopic ratio decreases systematically from Group 1 through Groups 2 and 3 to lowest values in Group 5 flows (Fig. 6B). Group 4 lavas have 143Nd/144Nd ratios that lie above the overall trend defined by data for the other four groups. Lead isotopic compositions show more complex variability than either Sr or Nd isotopes (Fig. 6C). For example, flow groups 1 to 3 define a trend of increasing 206Pb/204Pb (18.821–18.836 in Group 1 and 18.842–18.846 in Group 3) but Groups 4 and 5 show a wide range of values (18.81–18.85 in Group 4 and 18.78–18.88 in Group 5).

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Fig. 4. Normalised extended element (A–B) and chondrite-normalised rare earth element (C–D) plots for Ngauruhoe lavas (Groups 1–5) and comparisons with Ruapehu lavas of the (b 15 ka) Whakapapa formation of Ruapehu (shaded field). Normalising values are from Sun and McDonough (1989).

Table 2 Sr, Nd and Pb isotopic data for Ngauruhoe samples. Sample

Eruption date

Strat. group

87

Sr/86Sr

TG 010 P39581 TG 042 TG 045 TG 019 TG 001 TG 004 TG 288 TG 289 TG 205 TG 206 TG 014 TG 020 TG 156 TG 163

1975 1974 1954 1954 1949 1870 1870

5 5 5 5 5 5 5 4 4 3 3 2 2 2 1

0.706133 0.705488 0.705540 0.705454 0.705686 0.705305 0.705310 0.704684 0.704887 0.705268 0.705335 0.704785 0.704790 0.705008 0.704529

143

Nd/144Nd

206

0.512643 0.512698 0.512693 0.512704 0.512672 0.512682 0.512720 0.512768 0.512713

Pb/204Pb

207

Pb/204Pb

208

Pb/204Pb

18.883

15.683

38.888

18.855

15.664

38.847

18.882 18.872 18.872 18.823 18.810 18.842 18.846

15.685 15.671 15.670 15.632 15.606 15.627 15.617

38.899 38.850 38.849 38.734 38.653 38.744 38.694

18.836 18.861 18.828

15.639 15.661 15.629

38.767 38.843 38.741

0.512814 0.512719 0.512770

U-series data for Ngauruhoe samples Sample

Eruption date

Strat. Group

U ppm

Th ppm

226

TG 010 P39581 TG 042 TG045 TG 019 TG 001 TG004 TG288 TG 289 TG 205 TG 206 TG 014 TG 020 TG156 BCR-1

1975 1974 1954 1954 1949 1870 1870

5 5 5 5 5 5 5 4 4 3 3 2 2 2

1.095 0.807 0.883 0.849 0.966 0.867 0.829 0.929 1.075 1.156 1.11 0.755 0.736 0.874 1.644

4.615 3.339 3.637 3.508 3.931 3.557 3.428 3.303 3.838 4.663 4.386 2.839 2.769 3.45 5.552

474.10 324.46 354.63 340.52 382.31 382.81 425.43 332.70

Ra fg/g

234

U/238U

1.000 0.999 0.997 0.994 1.000 1.000 0.992 1.001 1.002 1.000 1.000 1.000 0.999 0.988 1.000

238

U/232Th

0.720 0.733 0.737 0.734 0.746 0.740 0.734 0.853 0.850 0.752 0.768 0.806 0.807 0.769 0.898

230

Th/232Th

0.684 0.706 0.709 0.748 0.707 0.718 0.726 0.731 0.728 0.710 0.713 0.719 0.721 0.732 0.866

226

Ra/230Th

1.330 1.219 1.218 1.203 1.219 1.326 1.492 1.228

Sr isotope for TG 010 and TG 288 from Hobden et al. (1999); Nd and Pb isotopes for TG 010 and TG 288 and Sr, Nd, and Pb isotopes for TG 163 are from Hobden (1997). Stratigraphic groups are those of Hobden et al. (2002).

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363

Fig. 5. Variation of 143Nd/144Nd (A), 206Pb/204Pb (C) and 230Th/238U activity ratio (C) versus 87Sr/86Sr ratio for Ngauruhoe lavas (Groups 1–5) and comparisons with the Whakapapa formation (b 15 ka and including 1945–1996 eruptives) of Ruapehu volcano (Gamble et al., 1999, 2003; Price et al., 2005; Price et al., 2007) Also shown are fields for Taupo Volcanic Zone (TVZ) basalts (Gamble et al., 1993) and high-Mg andesites from White Island in the offshore TVZ (Heyworth et al., 2007). Bulk Earth value in (A) is from Faure (1986).

6. U-series results for Ngauruhoe U–Th–Ra isotopic data for Ngauruhoe eruptives are shown in Table 2. 234U/238U, which ranges from 0.997 ± 0.001 to 1.002 ± 0.001 is within analytical error of secular equilibrium for all analyses, indicating that none of the samples has been affected by subsolidus alteration that might modify U-series isotopic behaviour. Fig. 8 is an equiline plot on which the Ngauruhoe data can be compared with data for Ruapehu and TVZ rhyolites (Charlier et al., 2005; Price et al., 2007). Also shown are calculated activity ratios for average basement greywacke compositions. With one exception (sample TG045 which shows slight Th excess) the Ngauruhoe samples show some degree of disequilibrium and collectively they define a sloping array lying to the right of the equiline with 238 U/232Th ranging from 0.720 to 0.853. The samples showing the greatest disequilibrium are Group 2 and 4 lava flows and the youngest eruptives (Group 5) are those closest to secular equilibrium (Figs. 5C and 8); 230Th/238U ratio increases progressively with time (Fig. 6D). There is a positive correlation between 230Th/238U and 87Sr/86Sr (Fig. 5C). Data for Ruapehu lavas erupted over the past several thousand years (b15 ka Whakapapa Formation of Hackett, 1985), including those erupted between 1945 and 1996, define a similar but more scattered array lying to the right of the equiline but displaced to higher 230Th/232Th values (Fig. 8). Eight Ngauruhoe samples have been analysed for Ra isotopic composition and all show significant Ra excesses with 226Ra/230Th ranging from 1.203 to 1.492 (Table 2). Five of these samples are from the 1949, 1954, and 1974 eruptions, two are from the 1870 eruption and one sample (TG288) is Group 4 lava. The age of TG288 is not

precisely known but comparisons with the tephra data and stratigraphic relationships suggest an age between 250 and 1000 years. No age corrections have been made to any of the Ra data. The sample showing the highest 226Ra/230Th activity ratio (1.492) is TG004, a basaltic andesite from an 1870 lava flow. Ngauruhoe samples show significantly more pronounced Ra excesses than is the case for Ruapehu; 20 samples from the 1945–1996 Ruapehu eruptions have 226 Ra/230Th ratios between 0.972 and 1.186 (Price et al., 2007). 7. The crustal sample: xenoliths from Ngauruhoe and Ruapehu Xenoliths and microxenoliths of crustal material are common in both Ngauruhoe and Ruapehu eruptives and their petrography and geochemistry have been documented by Graham (1987) and Graham et al. (1990). They range in size from rare large (up to 30 cm) boulders down to microxenoliths b1 mm across that can be found in almost every thin section. Xenolithic material makes up 2–6% of the mode of Ngauruhoe rocks and is generally less than 2% of the mode in Ruapehu eruptives (Hackett, 1985; Hobden, 1997). In Ruapehu lavas, the xenolith population is dominated by pyroxene granulites representing restitic or refractory meta-igneous and meta-sedimentary crustal lithologies (Graham, 1987; Graham et al., 1990). The meta-igneous types have been interpreted to have derived from an oceanic, lower crustal basement and the metasedimentary xenoliths from the immediate meta-greywacke basement of the Waipapa or Torlesse terranes (Graham, 1987; Graham et al., 1990; Price et al., 2005). In Ngauruhoe lavas, quartzose, feldspathic and calc-silicate metasedimentary xenoliths are dominant (Hobden, 1997); particularly in the youngest flows. They are commonly fine grained (b0.2 mm),

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Fig. 6. Variation of 86Sr/86Sr, 143Nd/144Nd, and 206Pb/204Pb isotopic ratios (A, B, and C respectively) and 230Th/238U activity ratio (D) with emplacement age for Ngauruhoe eruptives. Right hand panels show data for samples arranged in order of stratigraphic flow group (1–5 where 1 is the oldest flow group). The left hand panels show details of samples for which more precise age information is available.

range in size from b1 mm up to 10 cm, are generally sub-angular or rounded and have sugary mosaic textures with triple junction grain boundaries. Quartzose varieties are N90% quartz with plagioclase, orthopyroxene, Fe–Ti oxides, and in some cases minor glass and sanidine. Feldspathic xenoliths are similar to those commonly observed in Ruapehu lavas and are typically N90% plagioclase with orthopyroxene, Fe–Ti oxides, and minor glass. Calc-silicate varieties are dominantly quartz rich (90–60%) but in some cases they also contain calcic plagioclase, wollastonite and minor diopsidic pyroxene (Hobden, 1997).

Table 3 shows representative analyses of xenoliths from Ngauruhoe and Ruapehu and compares these with averages for Waipapa and Torlesse terrane meta-sediments. Fourteen Ngauruhoe xenolith samples were analysed. Thirteen of these are from younger (Group 5) lava flows and are quartzose and calc-silicate types. All but three of the analysed Ngauruhoe xenoliths have low K2O, Rb and Zr abundances (b0.05 wt.%, b8 ppm and b10 ppm respectively) and low total REE contents (b0.2 times the total REE content of N-MORB; data from Sun and McDonough, 1989). There is very limited enrichment of light over heavy REE; 10 of the 14 analysed samples

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365

8. Discussion 8.1. Assimilation and fractional crystallisation processes in Ngauruhoe lavas

Fig. 7. Strontium isotope variation for Ngauruhoe tephra samples (A) and flow groups (B) as a function of time. Tephra stages are from Möbis (2010) and stratigraphic flow groupings are explained in the text. In (B) the shaded boxes give the range in 87Sr/86Sr and an indication of the uncertainty in age for flow groups 1–4.

have (La/Yb)n b 6 and in 8 of these it is b2. Half of the analysed xenoliths show weak to strong positive Eu anamolies with Eu/Eu* between 1.10 and 3.40. Fig. 9 shows plots of wt.% MgO (A) and CaO (B), ppm Sr (C), and logCaO/Al2O3 ratio (D) versus SiO2 abundance on which the compositions of Ngauruhoe xenoliths are compared with those of Ruapehu xenoliths and basement compositions. The xenolith suite from Group 5 Ngauruhoe flows generally has higher SiO2 abundance than their Ruapehu counterparts and collectively they define an array that is characterised by higher relative CaO abundances and significantly higher CaO/Al2O3 ratios.

Fig. 8. 230Th/232Th versus 238U/232Th activity ratio plot for Ngauruhoe eruptives and comparisons with Ruapehu (Price et al., 2007), Taupo Volcanic Zone rhyolites (Charlier et al., 2005) and average basement meta-greywacke terrane compositions; “T” is average Torlesse basement and “W” average Waipapa basement.

For Ruapehu it is now well established that geochemical variation among and between lavas is controlled by AFC processes that involve significant crustal input (Graham and Hackett, 1987; Graham et al., 1995; Gamble et al., 1999, 2003; Nakagawa et al., 1999, 2002; Price et al., 2005). The crystal and lithic cargo carried in Ruapehu andesites derives from multiple mantle and deep and shallow crustal reservoirs and magmas represented by the erupted rocks have evolved on different time scales from magma batches of variable size dispersed throughout the crust (Price et al., 2005). Xenolithic material is ubiquitous in Ruapehu lavas and xenoliths provide a window into the deeper crust and evidence for a significant crustal contribution to the phenocryst and glomerocryst assemblages of the host lavas. In analysed Ngauruhoe samples the positive correlation between 87 Sr/86Sr and SiO2 (Fig. 2) suggests that, as is the case for Ruapehu, AFC processes may have been significant in the evolution of Ngauruhoe magmas. Consequently, Hobden et al. (1999) argued that the complex geochemical variations observed in Ngauruhoe lavas reflect magmatic evolution in a complex feeder system made up of small, discrete magma batches affected by variable amounts of AFC. This explains why Nd and Sr are more radiogenic in younger lavas and why 143Nd/ 144 Nd and 87Sr/86Sr isotopic ratios are inversely correlated (Fig. 5); later lavas would have been more strongly affected by AFC over a longer timeframe. There is, however, evidence to support a more complex scenario, in which the composition of parental magmas changed as AFC proceeded. Firstly, major and trace element data are difficult to reconcile with variable AFC involving a single parental magma. K2O, Mg-number and Ni versus silica variation diagrams (Fig. 2) and the variation of (La/Yb)n with changing Mg-number (Fig. 3) are better explained in terms of different sub-parallel AFC arrays involving variable parental compositions. For example, on the Mg-number and Ni versus silica variation diagrams (Fig. 2D and F) and on the K2O versus Mg-number diagram (Fig. 3B), Group 4 and Group 2 lavas define separate arrays with Group 4 having higher Mg-number and Ni abundance as well as higher SiO2 and K2O contents than Group 2. Secondly, although Sr and Nd isotopic compositions become more radiogenic with time, the change is not continuous. Group 4 lavas have higher 143Nd/144Nd and lower 87Sr/86Sr ratios than Group 3 or most Group 2 flows and although Group 5 includes the most radiogenic Sr and Nd compositions analysed it also incorporates flows with isotopic compositions spanning into the range shown by Groups 2 and 3 (Figs. 5 and 6). Pb tends to become more radiogenic with time but the youngest Group 5 lavas include the full range of Pb isotopic variation from least to most radiogenic (Fig. 5). The isotopic data are consistent with cycles of AFC each initiated with an influx of isotopically different parental magma. This is manifested in the temporally controlled 87Sr/86Sr data for both lava flows and tephras (Fig. 7). The tephra data (Fig. 7A) can be used to infer at least three cycles: Stage 1 with relatively high 87Sr/86Sr ratios (0.70541– 0.70553), Stage 2 to Stage 3 with 87Sr/86Sr rising from 0.70478 to 0.70523 and Stage 4 in which 87Sr/86Sr varied with time from 0.70473 to 0.70543. Within these constraints we have modelled AFC processes for Ngauruhoe lavas (Fig. 10) using De Paolo's (1981) equations. We have assumed that the samples with the higher Mg-numbers in Groups 2, 4 and 5 could be representative of parental magmas within the Ngauruhoe suite and that the modal mineralogy for each of these is in each case representative of the fractionating mineral assemblage (Table 4). The assimilant compositions used are average Torlesse basement and a fertile (higher K2O, Rb, Zr and REE abundances and elevated La/Yb ratio) calc-silicate xenolith composition and in all

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Table 3 Analyses of Ngauruhoe xenoliths and average compositions of Ruapehu xenoliths and basement.

Sample SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI

1

2

Ng TG530

Ng R09/X2 Ng R09/X4 Ng R09/X5 Ng R09/X1 Ng TG-267

75.01 0.22 16.64 2.28 0.08 0.70 2.33 1.57 0.73 0.11 0.08 99.75

ppm Cs 6.54 Ba 266 Rb 34 Sr 429 Pb 10.71 Th 5.95 U 1.74 Zr 106 Nb 5.31 Hf 2.63 Ta 0.45 Y 11.03 Sc 3.60 V 20 Cr 2 Ni 1 Zn 40 La 14.86 Ce 29.53 Pr 3.34 Nd 12.39 Sm 2.24 Eu 0.628 Gd 1.869 Tb 0.304 Dy 1.680 Ho 0.395 Er 1.221 Tm 0.196 Yb 1.504 Lu 0.238 87 86 Sr/ Sr 0.707601 143 0.512772 Nd/ 144 Nd 206 Pb/204Pb 18.818 207 Pb/204Pb 15.630 208 Pb/204Pb 38.743

3

4

79.76 0.02 1.29 0.64 0.11 0.13 17.46 0.08 0.05 0.01 0.03 99.58

83.86 0.02 2.21 0.49 0.14 0.10 12.72 0.18 0.06 0.02 0.05 99.84

81.83 0.02 4.38 0.64 0.09 0.13 12.14 0.16 0.01 0.01 0.04 99.45

0.95 14 9 249 39.64 0.22 0.09 3 0.24 0.13 0.10 1.74 1.53 2 4 bdl 58 1.19 2.64 0.30 1.12 0.23 0.085 0.240 0.054 0.203 0.064 0.198 0.045 0.169 0.027 0.708459 0.512393

0.27 22 5 451 13.15 0.35 0.14 9 0.30 0.14 0.16 2.25 2.15 4 6 bdl 6 0.63 1.60 0.20 0.68 0.15 0.073 0.204 0.045 0.217 0.089 0.336 0.090 0.530 0.103 0.708215 0.512448

0.47 21 5 383 12.02 0.40 0.17 10 0.35 0.18 0.07 1.63 1.66 3 3 bdl 9 0.82 1.74 0.20 0.66 0.08 0.032 0.153 0.028 0.311 0.064 0.254 0.067 0.469 0.082 0.709122 0.512357

18.799 15.641 38.752

18.764 15.652 38.746

18.797 15.657 38.765

5

75.27 0.22 16.58 2.14 0.06 0.70 2.19 1.25 0.77 0.12 0.14 99.44

6.55 407 37 643 36.34 5.80 1.23 103 5.15 2.89 0.52 10.59 3.51 21 3 1 34 14.35 28.95 3.23 12.33 2.04 0.515 1.684 0.288 1.640 0.392 1.216 0.182 1.425 0.226

6

63.29 0.38 8.95 1.75 0.12 0.55 21.96 1.29 0.40 0.12 0.13 98.94

7

8

9

10

Ng TG-269

Ru MIX

Ru Msed

Ru R97/104x Torlesse

52.33 1.03 16.92 13.45 0.17 7.95 8.85 2.03 0.35 0.13

54.06 1.23 20.05 8.26 0.14 3.31 6.39 3.09 1.45 0.45

71.98 0.01 2.02 0.91 0.12 0.17 24.41 0.09 0.04 0.01 0.06 99.82

0.58 0.34 96 8 18 7 707 503 3.71 2.47 4.81 0.34 1.51 0.11 232 9 4.30 0.19 5.28 0.09 0.33 0.03 14.91 2.68 8.04 1.16 37 7 34 9 9 12 38 12 30.16 1.02 53.45 2.05 5.88 0.24 23.21 1.07 3.85 0.30 0.737 0.048 3.256 0.233 0.434 0.032 2.670 0.307 0.534 0.083 1.568 0.189 0.218 0.051 1.613 0.305 0.246 0.041 0.708169 0.708008 0.512431 0.512400 18.853 15.659 38.765

18.756 15.649 38.714

2.74 173 13 378 7.17 1.14 0.45 84 3.34 9.62 1.44 18.53 33.17 248 362 125 115 7.98 18.50 2.69 8.30 2.25 1.215 3.987 0.672 4.173 0.907 2.472 0.379 2.623 0.409 0.706547 0.512851 18.810 15.641 38.725

612 55 455 16 8 3 227

188 194 51 108

0.707818

47.84 1.19 25.82 8.26 0.08 3.27 8.21 3.66 0.41 0.22 1.12 100.08

1.80 394 11 517 6.37 8.48 2.55 266 17.50 7.74 2.19 19 19 162 78 21 143 41.50 75.68 9.18 32.66 5.62 1.679 4.729 0.623 3.305 0.644 1.740 0.248 1.900 0.306 0.707230 0.512420 19.100 15.840 39.340

11

71.84 0.51 14.91 3.57 0.15 1.00 1.42 4.06 2.44 0.11

567 93 305 31 15.0 3.5 197 8

20 8 67 30 12 71 24 51

0.701120 0.512421 18.858 15.645 38.781

12 Waipapa 63.99 0.86 16.90 6.71 0.13 2.46 2.48 3.90 2.32 0.23

538 68 482 19 8.5 1.9 179 6

24 16 141 37 14 84 20 44

0.706460 0.512677 18.784 15.619 38.645

Columns 1–8: new analyses of xenoliths from Ngauruhoe (Ng); majors and some traces (italics) by XRF at University of Auckland, other traces elements by LA-ICP-MS at the Australian National University (see Analytical methods). Columns 8 and 9 are average compositions for meta-igneous (MIX) and meta-sedimentary (Msed) xenolith compositions from Ruapehu (Ru). Data are from Hackett (1985), Graham (1987), Graham et al. (1990) and Price et al. (2005). Column 10 is the composition of a refractory meta-sedimentary xenolith from Ruapehu. Columns 11 and 12 are average compositions of Torlesse and Waipapa basement; data are from an unpublished compilation by N. Mortimer and from Graham et al. (1992), Roser and Korsch (1999), and McCulloch et al. (1994).

models illustrated we have used an r value (ratio of assimilant added to magma fractionated) of 0.25 Partition coefficients are from Halliday et al. (1995), Ewart and Hawkesworth (1987), Blundy and Wood (2003) and Dunn and Sen (1994). Models are presented graphically in Fig. 10A–D, which are variation diagrams showing K2O and Th abundance, 87Sr/86 Sr ratio and 230Th/238U activity ratio as a function of Zr abundance; Zr has been used as fractionation index. Models using average Torlesse meta-greywacke as the assimilant show only minor differences from those in which calc-silicate xenolith is the crustal contaminant; on the 87Sr/86Sr versus Zr diagram the AFC trajectories are virtually the same and on the Th and K2O versus Zr diagrams the models using calc-silicate give paths that are similar but have slightly flatter slopes. For all the models shown in Fig. 10 higher

rates of assimilation (e.g. r values of 0.5) accelerate the rate of change but not the slope of the AFC trajectories. No single model AFC trajectory can explain the variations observed in all Ngauruhoe lavas but collectively they provide a framework that does so. The isotopic and geochemical data are consistent with an overall AFC control but the absence of a consistent coherence between major and trace element abundance patterns and variations in isotopic composition precludes a common, compositionally invariant parental magma. We therefore suggest that over its 5 ka history, the Ngauruhoe magmatic system has been periodically recharged with more primitive magmas that have changed in composition over time. The cyclicity observed in the tephra and flow data is very likely to be a reflection of these recharge events. In this regard the current 35 year

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367

Fig. 9. Silica variation diagrams comparing Ruapehu and Ngauruhoe crustal xenolith populations, showing variation of MgO and CaO abundances (A and B respectively), Sr contents (C), and CaO/Al2O3 ratio (D) with SiO2 content. Also shown are fields (shaded) and average compositions for Torlesse (T) and Waipapa (W) basement (data sources as in Table 3).

hiatus between eruptions may represent a break between recharge events and by inference magmatic cycles. Fig. 10E–F shows AFC models for three TVZ basaltic compositions that are considered to represent primary, mantle-derived magmas (Gamble et al., 1993) and compares these with the variation observed in Ngauruhoe lavas. None of these compositions appears to be an exact match for the primitive magmas feeding Ngauruhoe but the TVZ basalt models do provide a demonstration of how the composition of mantle-derived magma feeding into the Ngauruhoe system could have changed with time. The variability in mantle composition being implied is within the range that can be inferred from primitive basalts within the TVZ. We conclude that all Ngauruhoe eruptives appear to have been affected by crustal level AFC with periodic shifts as the magmatic system has been recharged from the mantle. 8.2. U–Th disequilibrium and AFC models For Ruapehu magmas, U-series disequilibrium is strongly influenced by the AFC process rather than reflecting simple temporal control (Price et al., 2007). The time-dependent variation in 87Sr/86Sr ratios observed in Ngauruhoe lavas and the relationship between 87Sr/ 86 Sr and U-series disequilibrium indicate that similar processes may have operated here. In Ngauruhoe lavas 230Th/238U activity ratio changes with time; U– Th disequilibrium is most strongly manifested in the oldest samples (Fig. 6D) and 230Th/238U activity ratio and 87Sr/86Sr isotopic ratio are positively correlated (Fig. 5). This suggests that, as with Ruapehu, disequilibrium arising from processes taking place at source through the interaction of slab fluid with mantle wedge and from mantle melting processes, is being offset by assimilation of crustal material that is at secular equilibrium so that the slope of the data array on the equiline diagram (Fig. 8) does not have time significance. AFC models are not however entirely consistent with the U-series disequilibrium behaviour observed in Ngauruhoe lavas. Fig. 10D shows variation of 230Th/238U as a function of Zr abundance and

compares this with the quantitative AFC models. Models using a Group 5 basaltic andesite (P39581) as parent and either average Torlesse meta-greywacke or calc-silicate xenolith as assimilant broadly replicate variation for Groups 3 and 5 data; the models involve modest levels of fractionation (15%) and assimilation (r = 0.25). However, samples from Groups 2 and 4 with relatively low Zr abundance have significantly lower 230Th/238U activity ratios (Fig. 10D). Group 2 basaltic andesite TG014 has a Zr content of 105 ppm and a 230Th/238U ratio of 0.892 and for TG288, from Group 4, Zr content is 103 ppm and 230Th/238U activity ratio is 0.857. PG39581 (Group 5) has virtually the same Zr abundance as these samples (104 ppm) but the 230Th/238U is 0.963 and two other Group 5 samples have even higher activity ratios (up to 1.02). These data are consistent with separate AFC trends, which on Fig. 10D produce horizontal scatter, each originating from parental compositions showing different degrees of U–Th disequilibrium. In all likelihood none of the suggested Ngauruhoe parental compositions represents an unmodified, primary, mantle-derived magma; all have relatively low Mgnumbers and Ni contents. Each is in itself the outcome of AFC affecting a subtly different mantle-derived magma. The patterns displayed on the equiline diagram (Fig. 8) are consistent with both changing parental magmas and varying crustal contamination. The overall array reflects primary disequilibrium arising from melting in the mantle that generated primitive parental magmas and modification of these by AFC processes involving crustal materials that were in secular equilibrium. In a relatively short time, assimilation of crust has pulled magmatic compositions with an initial isotopic disequilibrium back towards the equiline. Changes in U-series disequilibrium of Ngauruhoe parental compositions have not been systematic over time. 8.3. Ra disequilibrium and fractionation rates A number of studies have attempted to use 226Ra/230Th to constrain rates of crystal fractionation for a variety of magmatic

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Fig. 10. (A–B) Variation of K2O and Th abundance as a function of Zr content for Ngauruhoe eruptives (stratigraphic Groups 1–5) with assimilation fractional crystallisation (AFC) models. (C–D) Variation of 87Sr/86Sr and 230Th/238U as a function of Zr content for Ngauruhoe eruptives (stratigraphic Groups 1–5) with assimilation fractional crystallisation (AFC) models (see Table 4). Six AFC models are shown; two models for three different starting materials (P39581 from Group5; TG288 from Group 4; TG014 from Group 2). For each starting material AFC is modelled using De Paolo's (1981) equations and average Torlesse and calc-silicate TG267 (Table 3) as assimilants. In all cases r, the ratio of material assimilated to magma crystallised, is assumed to be 0.25. Each interval on the fractionation path represents 5% fractional crystallisation. (E–F) are plots of K2O and Th abundance and variation of 87Sr/86 as a function of Zr on which variation among Ngauruhoe eruptives is compared with AFC models for three Taupo Volcanic Zone (TVZ) basalts; RB is a basalt from Ruapehu, KB is the Kakuki Basalt from the central TVZ (Gamble et al., 1993) and RCB is a basalt from Red Crater in the Tongariro Volcanic Centre (Hobden, 1997; see also Gamble et al., 1993). Each of the basalt AFC models uses De Paolo's (1981) equations and average Torlesse as the assimilant. In all cases r is assumed to be 0.25. Each interval on the fractionation path represents 10% fractional crystallisation.

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369

Table 4 Assimilation fractional crystallisation (AFC) models. A. SiO2, K2O, Mg-number, trace element and isotopic data for TVZ basalts used in AFC models.

SiO2 K2O Mg-number Th U Zr La Yb 87/86

Kakuki Basalt

Red Crater Basalt

Ruapehu Basalt

48.52 0.27 67 0.5 0.1 79 5.73 1.60 0.70388

53.38 0.63 61 2.1 0.5 79 7.63 2.2 0.70456

52.79 0.57 65 1.2 0.33 57 4.97 1.55 0.70475

B. Modal proportions in crystal extract for Ngauruhoe and TVZ basalt AFC models

Olivine Clinopyroxene Orthopyroxene Plagioclase Magnetite

TG014

TG288

P39581

0.22 0.14 0.62 0.02

0.09 0.29 0.11 0.48 0.03

0.01 0.15 0.28 0.56

Kakuki Basalt

0.27 0.02 0.48 0.05

Red Crater Basalt

Ruapehu Basalt

0.15 0.34 0.04 0.47

0.14 0.17 0.04 0.65

All models use De Paolo's (1981) AFC equations with “r” (proportion of material assimilated to magma crystallised) assumed to be 0.25.Partition coefficients are from Halliday et al. (1995), Ewart and Hawkesworth (1987), Blundy and Wood (2003) and Dunn and Sen (1994). Parental materials used for Ngauruhoe AFC models are TG014 (Group 2), TG288 (Group 4) and PG39581 (Group 5). Trace element and isotopic data for these samples are given in Tables 1 and 2. SiO2, MgO, K2O, Mg-number [mol MgO / (MgO + FeOtotal)], trace element and isotopic data for parental compositions used in Taupo volcanic zone (TVZ) basalt AFC models are given in Table 4A. These data are from Gamble et al. (1993), Hobden (1997) and R.C. Price (unpublished data). The modal proportions in crystal extracts for the AFC models are given in Table 4B.

systems (e.g. Vigier et al., 1999; Zellmer et al., 2000; Cooper and Reid, 2003; Turner et al., 2003; Blake and Rogers, 2005; Price et al., 2007). Blake and Rogers (2005) used such data to suggest fractionation rates of 2–6 × 10− 4/year for magmatic systems in tectonic settings that included mid-ocean ridge, oceanic and continental intra-plate, and continental subduction. They concluded that the time scales required for the evolution of intermediate or felsic magmas through closed system crystal fractionation is of the order of 500 to 1500 years. Zellmer et al. (2000) concluded that dacitic eruptives on Santorini evolved through crystal fractionation over a period of approximately 1000 years. For 1945–1996 Ruapehu eruptives, Price et al. (2007) obtained significantly slower (1–5 × 10− 5/year) fractionation rates and longer fractionation times (500–5000 years). The slower fractionation rates observed at Ruapehu may be a consequence of the unique tectonic and thermal setting of the volcano. The TVZ is a region of exceptionally high heat flow (~700 mW/m2 — Hochstein et al., 1993; Bibby et al., 1995) and very high magma production rates (Houghton et al., 1995; Wilson et al., 1995). Slower fractionation rates may reflect slower cooling and crystallisation as a consequence of the crust being thermally preconditioned and hotter. Fig. 11 shows Group 5 Ngauruhoe data plotted on a 226Ra/230Th versus Th plot along with crystal fractionation models at varying rates of fractionation and a field for Ruapehu data. Group 5 lavas represent a coherent magmatic suite derived from a common parental magma (see Figs. 2, 3 and 10). Three sets of crystal fractionation models, representing fractionation rates varying from 1 × 10− 5 to 4 × 10− 4/ year are shown in Fig. 11. The fastest fractionation rate (4 × 10− 4/ year) produces fractionation curves with the shallowest slopes; the models for 5 × 10− 5 and 1 × 10− 5/year produce near vertical curves. An uncertainty in modelling of Ra behaviour is the calculation of Ra partition coefficients. For each of the fractionation rates modelled in Fig. 11, we have compared a fractionation model in which Ra partition

Fig. 11. Crystal fractionation models for 226Ra/230Th activity and Th (abundance) for Ngauruhoe lavas from flow stratigraphic Group 5 and comparisons with 1945–1996 Ruapehu eruptives (Price et al., 2007). The three sets of curves show time integrated fractionation paths for fractionation rates varying from 4 × 10− 4/year to 1 × 10− 5/year. In all three cases only closed system fractional crystallisation has been modelled. Assimilation of crust in secular equilibrium would cause a more rapid return of the evolving magma to secular equilibrium. The starting composition was assumed to have Th = 3.43 ppm, 226Ra = 425.43 fg/g, and 226Ra/230Th = 1.492. For each of the three different fractionation rates two models have been calculated. Those labelled “A” use partition coefficients estimated using the approach of Blundy and Wood (2003) and for those labelled “B” DRa is assumed to equal DBa.

coefficients are calculated using the method of Blundy and Wood (2003) with one for which DRa is assumed to equal DBa (see discussion in Cooper, 2009 and Rubin and Zellmer, 2009). The differences between models using these two different approaches to estimation of Ra distribution coefficients are negligible (Fig. 11). The Ngauruhoe samples show higher degrees of Ra disequilibrium than is the case for Ruapehu lavas, although several Ngauruhoe samples plot close to an extrapolation of the Ruapehu array. The models shown in Fig. 11 can be used to infer that for most Group 5 magmas the fractionation rates have been similar to those involved with Ruapehu eruptives (1–5 × 10− 5/year) but since the Ngauruhoe lavas have major element compositions that are less evolved than many of the 1945–1996 Ruapehu samples, fractionation times must have been lower (1000–2000 years). One of the Ngauruhoe samples (TG010) lies to the right of the main array and this may indicate a faster fractionation rate for this sample. The models shown in Fig. 11 are all for simple closed system fractionation, which is clearly not the case for Ngauruhoe lavas. However, open system crystal fractionation has the effect of accelerating a return to secular equilibrium (e.g. Blake and Rogers, 2005; Price et al., 2007), so the rates obtained from the models shown in the diagram must be regarded as minimal and the actual time scales for magma evolution must be shorter than inferred from these closed system models (Dosseto et al., 2008). 8.4. Geochemical comparisons and contrasts — Ngauruhoe versus Ruapehu The most obvious differences between Ngauruhoe and Ruapehu are their contrasting morphologies and their different histories. Ngauruhoe is a simple and classic cone that has been constructed in less than 5 ka years whereas Ruapehu is a much larger and more complex structure with a history extending back at least 230 ka. It should however be emphasised that during the past several thousand years the two volcanoes have both been active and each has erupted a similar volume of material. Ngauruhoe's cone has a volume of

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~ 2.2 km3 and the post 15 ka lava flows of Ruapehu (Whakapapa Formation of Hackett, 1985) has a volume estimated at 2.6 km3. Medium-K basaltic andesites and andesites dominate both volcanoes (Table 2 and Fig. 2A) but although the ranges overlap, Ngauruhoe eruptives show a narrower spread in SiO2 abundance than is observed at Ruapehu; SiO2 contents in Ngauruhoe lavas range from 54.2–58.6 wt.% whereas all Ruapehu lavas have SiO2 abundances between 52.2 and 64.8 wt.% (Graham and Hackett, 1987; Price et al., 2005). These differences are also observed when the compositions of Ngauruhoe eruptives are compared with those of lavas making up the youngest Ruapehu flow formation. Whakapapa lavas, including those erupted over the past 65 years have SiO2 contents ranging from 55.1 to 64.8 wt.% (Graham and Hackett, 1987; Gamble et al., 1999). Ngauruhoe lavas tend to have lower K2O abundances (K2O = 2.6–4.9 for Ngauruhoe and 3.7–6.2 for Whakapapa flows; see Fig. 2A) and lower Mg-numbers (42.5–60.3 for Whakapapa lavas and 38.2 and 57.2 for Nganuruhoe; see Fig. 2C). More particularly Ngauruhoe lavas show significantly wider variation in 87Sr/86Sr isotopic composition than their Ruapehu counterparts. 87Sr/86Sr ratios for Ngauruhoe vary between 0.70419 and 0.70613 whereas the range observed in Whakapapa eruptives is 0.70510–0.70584 (Figs. 2G and 5). On a 206 Pb/204Pb versus 87Sr/86Sr diagram, Ngauruhoe data overlap with those for Whakapapa but scatter to higher 206Pb/204Pb values (Fig. 5). On normalised extended element plots (Fig. 4) Whakapapa lavas from Ruapehu tend to show stronger enrichment in large ion lithophile elements and greater enrichment in light over heavy REE (Figs. 3 and 4) than their Ngauruhoe counterparts. The distribution of Ngauruhoe and Ruapehu data plotted on K2O (wt.%) versus Mg-number, 87Sr/86Sr ratio versus Mg-number, chondrite-normalised (La/Yb) ratio versus Mg-number diagram, and Mgnumber versus SiO2 abundance diagrams (Figs. 2 and 3) suggests that different basaltic, mantle-derived parental magmas have been involved in each case. On each of these diagrams the data arrays for each volcano define crudely parallel rather than converging trends. The contrasts are consistent with the variability observed in analysed TVZ basalts (Figs. 2 and 3) and with the variability postulated for Ngauruhoe parental magmas. In terms of both major (Fig. 9) and trace elements (Table 3) there are very few Ngauruhoe xenoliths that have compositions approaching Torlesse or Waipapa basement compositions. This is likely to arise from two factors. Firstly, as is the case at Ruapehu (Graham, 1987; Graham et al., 1990; Price et al., 2005), most Ngauruhoe xenoliths are likely to represent refractory lithologies from the original protoliths or restitic compositions from which melt has been extracted. Evidence for this is that most analysed Ngauruhoe xenolith compositions are relatively depleted in K2O, Rb, Zr, and REE with limited enrichment of light relative to heavy REE. In addition, half of the analysed xenolith samples manifest positive Eu anomalies, which may indicate the presence of restitic plagioclase. Secondly, the Ngauruhoe xenoliths sample suite is dominated by quartzose and calc-silicate varieties from Group 5 lavas. The differences between the xenolith populations at Ruapehu and Ngauruhoe appear to indicate that Ngauruhoe magmas have interacted with a different range of crustal compositions. The Ngauruhoe xenolith assemblage indicates that the host magmas, particularly those represented by the Group 5 lavas, have been stored in and have interacted with crust in which calc-silicate and quartz sandstone is locally abundant. One possibility is that this material derives from the Tertiary sediments that overlie the Mesozoic greywacke basement rocks in the central North Island but, if this is the case it implies very shallow (b1 km below the surface on which the volcanic cone has been constructed), pre-eruption storage for the vast majority of xenoliths in Group 5 lavas. Younger (Whakapapa) Ruapehu and Ngauruhoe andesites have evolved through broadly similar magmatic processes but in each case the mantle and crustal inputs have been compositionally different.

The geochemical contrasts reflect differences in the maturity of the respective magmatic systems. The Ruapehu system has evolved to its present state over a period of at least 230 ka. During that time, the magmatic plumbing system beneath the volcano has developed into a complex and vertically dispersed system of dykes, sills and small magma storages in which, over the life span of the volcano, a plethora of magma batches has been evolving and interacting with crust on highly variable time scales (Gamble et al., 1999, 2003; Price et al., 2005). The system has been periodically recharged with magma from the mantle but the last event would appear to have occurred 10– 15 ka B.P. immediately preceding the eruption of the Whakapapa Formation and since that time, the volcano appears to have reached something of a steady state condition with regular (50–100 year) small eruptions apparently related to tapping of magma from relatively shallow crustal level storages. Ngauruhoe has a much shorter history and the magmatic system has not developed to the level of maturity that characterises Ruapehu. The system has been relatively open with periodical replenishment involving small batches of magma from the deeper crust and ultimately the mantle; unlike post 15 ka Ruapehu, these deep crustal or mantle recharge events can be recognised in the geochemical data. On relatively short time scales, small batches of magma have moved through the Ngauruhoe system towards the surface, fractionating and interacting with crust along the way. Ascending magmas have also intersected earlier magma batches that have stalled and been stored within the plumbing system (Hobden et al., 1999). Overall the system shows temporal trends of increasing fractionation, increasing geochemical variability and increasing involvement of crust but, in contrast to younger eruptives of Ruapehu, there is clear evidence in Ngauruhoe eruptives for periodic recharge from the deep crust or mantle. The fact that, on the equiline diagram, the data arrays defined for each volcano are parallel and do not converge either away or towards the equiline is consistent with the involvement of different mantle-derived parental magmas and different crustal components in the evolution of the magmatic suites at each volcano. For both volcanoes the dominant crustal component involved in the AFC process would appear to have approximated the Torlesse meta-greywacke basement but the fact that each array on the equiline diagram (Fig. 8) projects back to a different point on the equiline suggests that there were differences between the bulk compositions of the crustal material involved in each case. A significant uncertainty in AFC models is the precise composition of the crustal assimilant. In our models we have used an average Torlesse basement metagreywacke and a “fertile” or non-restite calc-silicate composition but it is important to stress that these specific compositions are representative of a spectrum of geochemical variability. The average Torlesse meta-greywacke, for example, is calculated using compositions that range from mudstone to sandstone. For each AFC path for each specific magma batch represented by a particular lava flow there will be specific assimilants that will only be approximated by average basement or representative xenolith compositions.

9. Conclusions The processes controlling magmatic compositions at Ngauruhoe and Ruapehu volcanoes are broadly similar, yet subtly distinct. In both cases there is evidence to support the conclusion that andesitic magmas have evolved in a complex system of variable volume reservoirs dispersed throughout the crust with fractional crystallisation, crustal assimilation and magma mixing and mingling being the dominant processes determining whole rock and mineral chemistry and rock textures. The complex crystal and lithic cargo that characterises the texture of each andesite lava is the result of an involved and unique interplay of these processes and an outcome of variable degrees of polybaric magmatic evolution, multiple mantle

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and crustal sources, variable magma batch volumes, and different time scales of evolution. The pattern of U-series isotopic disequilibrium in Ngauruhoe lavas is strongly influenced by fractional crystallisation and crustal assimilation but it also indicates periodic replenishment of the magmatic system with mantle-derived magmas manifesting different degrees of isotopic disequilibrium. This has its ultimate origin in the mantle sources where the primary magmas of the subduction system are generated but andesitic magmas evolve through the interaction of primary magmas with crust and their compositions are largely determined by these processes. In continental subduction systems, the effect of mantle–crust interaction is the efficient and effective restoration of secular equilibrium over relatively short time scales; a few thousand rather than hundreds of thousands of years. Thus, for Ngauruhoe, the slope of the data array on an equiline plot [230Th/232Th versus 232Th/238U] is determined by the U/Th ratios of the crustal components and the disequilibrium isotopic composition of the original primary, mantle-derived magmas feeding the sub-volcanic magmatic system. U-series data can be used in combination with Sr, Nd and Pb isotopic data to characterise the progressive evolution of the Ngauruhoe magmatic system with time and to determine the time scales over which the magmatic processes have occurred. The youngest lavas include those showing the most obvious geochemical impact of crustal assimilation and these are also the compositions showing the lowest levels of U-series isotopic disequilibrium. 226Ra/ 230 Th data for Group 5 lavas, which are considered to have derived from a common parental magma are consistent with relatively short time scales for fractional crystallisation; the range of magmatic compositions at Ngauruhoe has evolved over a period of 1000– 2000 years. The geochemical similarities between younger (Whakapapa Formation) Ruapehu and Ngauruhoe lavas arise because the evolutionary processes (AFC) and the end member components (crustal and mantle) were broadly similar. There are however geochemical and petrological differences, particularly between the younger Ngauruhoe lavas and their post-1945 Ruapehu counterparts. Contrasts in Sr and Pb isotopic compositions and the crustal sample obtained from xenoliths in lava flows are in each case compositionally and petrographically distinct. On the equiline diagram the Ngauruhoe array projects back to a crustal composition with a different U/Th ratio. Unlike the system feeding younger (Whakapapa) Ruapehu eruptions, there is evidence that the Ngauruhoe magmatic system has been recharged several times by new magma from the lower crust or mantle. Major and trace element chemistry and U-series data suggest that different parental magmas were involved at each volcano. Young Ruapehu eruptives (1945–1996) are the end product of a long-lived magmatic system that has reached a steady and mature state. In contrast the short-lived Ngauruhoe system has remained relatively open with cycles of crustal processing being initiated by periodic influxes of new magma from the deep crust or mantle. Over the past few thousand years, these two different magmatic systems have been active simultaneously less than 20 km apart. Acknowledgements This research builds on earlier U-series work on Ruapehu volcano involving Rhiannon George, Chris Hawkesworth, and Rob Hughes and on Barbara Hobden's PhD work, which was carried out at the University of Canterbury under the supervision of Steve Weaver, Bruce Houghton and David Shelley and with the support of Ian Nairn. It has been funded by the Marsden Fund (contract number UOW106), which is managed by the Royal Society of New Zealand and the Foundation for Research, Science and Technology of New Zealand (contract MAUX0401). Simon Turner was supported by an Australian Research Council Professorial Fellowship DP0988658.The instrumen-

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tation used is funded by ARC LIEF and DEST Systemic Infrastructure Grants, Macquarie University and Industry. The manuscript benefited from a thorough and helpful review by Georg Zellmer. We thank Christoph Beier for assisting with the U–Th analyses and Shane, Isabella and Marcel Cronin for their assistance in the field. References Adams, R.D., Ware, D.E., 1977. Structural earthquakes beneath New Zealand; locations determined with a laterally inhomogeneous velocity model. New Zealand Journal of Geology and Geophysics 20, 59–83. Arculus, R.J., Powell, R., 1986. Source component mixing in regions of arc magma generation. Journal of Geophysical Research 91, 5913–5926. Ayers, J., 1998. Trace element modelling of aqueous fluid — peridotite interaction in the mantle wedge and subduction zones. Contributions to Mineralogy and Petrology 132, 390–404. Bibby, H.M., Caldwell, T.G., Davey, F.J., Webb, T.H., 1995. 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