Whole-rock geochemical reference data for Torlesse and Waipapa terranes, North Island, New Zealand

Abstract

Sixteen metasedimentary rocks from the Waipapa and Torlesse composite terranes of New Zealand’s North Island have been analysed for major and trace elements and Sr, Nd and Pb isotope composition. The new data provide a comprehensive analytical dataset that can be used for quantitative trace element and isotopic modelling of the petrogenesis of Neogene North Island volcanic rocks. The major and trace element and isotope data confirm previously recognised systematic compositional differences between Waipapa and older Torlesse (Kaweka Terrane) metasedimentary rocks. Kaweka Terrane samples tend to have higher SiO₂, Al₂O₃, Cs, Rb, Th, U, Hf and total rare earth elements (REE) abundances and chondrite normalised REE patterns characterised by light/heavy REE enrichment. Fe₂O₃, MgO, Sr and Sc contents are lower in Kaweka samples than those observed in their Waipapa Terrane counterparts. ⁸⁷Sr/⁸⁶Sr ratios are lower and ¹⁴³Nd/¹⁴⁴Nd values are higher in Waipapa compared to Kaweka samples, and Pb is isotopically more radiogenic in the Kaweka samples. Younger Torlesse (Pahau Terrane) samples have major and trace element and isotopic compositions that span the Waipapa and Kaweka suites, but overall they are more similar to the former. Mudstones from all terranes converge in bulk composition and there are systematic differences in isotopic composition between mudstones and sandstones from the Kaweka and particularly the Waipapa terranes. Both lithologies need to be considered when estimating the compositions of metasedimentary terranes. Major and trace element and isotope compositions of the metasediments indicate a complex provenance involving subduction-related volcanic and crustal or continental inputs.

Keywords:

• geochemistry
• greywacke
• isotope chemistry
• North Island
• sedimentary basement
• Torlesse Composite Terrane
• Waipapa Composite Terrane

Introduction

The Taupo Volcanic Zone (TVZ) lies in the central part of the North Island of New Zealand and represents the youngest phase of Neogene magmatism associated with westwards subduction of the Pacific beneath the Australian plate. The TVZ is flanked and underlain by basement rocks of the Torlesse and Waipapa composite terranes. These two terranes are dominated by metasedimentary rocks (greywacke and argillite) that accumulated in an accretionary wedge at the convergent margin of Mesozoic eastern Gondwana (Coombs et al. 1976; MacKinnon 1983; Frost & Coombs 1989). Knowledge of depositional age and provenance of these tectonic units has substantially increased as more sophisticated and detailed petrographic, geochemical, isotopic and geochronological information has become available.

In order to understand the petrogenesis of volcanic rocks that have been erupted over the past 25 million years, it is important to establish the geochemical composition and variability of the metasedimentary rocks of the North Island (Whakaari and Horomaka supersuites of Mortimer et al. 2014). It is commonly argued (e.g. Graham & Hackett 1987; Graham et al. 1995; Price et al. 2005; Price et al. 2012) that the generation and evolution of andesitic rocks erupted from volcanoes in the TVZ involves interaction between mantle-derived magmas and metasedimentary basement rocks. Quantitative modelling of this type of process requires a robust and comprehensive estimate of the geochemistry of basement material as well as the magmas represented by the volcanic rocks.

In this context, the geochemical database for North Island basement rocks is neither comprehensive nor complete. There is an extensive dataset of petrographic and X-ray fluorescence major and trace element information (e.g. Graham 1985; Graham et al. 1992; Mortimer 1994; Roser & Korsch 1999; Leverenz & Ballance 2001) but isotope datasets (e.g. Adams & Maas 2004a,b) do not always include linked Sr and Nd isotope analyses and Pb isotope data are sparse. Isotope data are not readily tied to XRF analyses. No high-quality inductively coupled plasma mass spectrometry (ICP-MS) geochemical analyses are available.

This paper provides detailed major and trace element and Sr, Nd and Pb isotope compositional data for a representative suite of basement metasedimentary rocks from the North Island. We use these new data to calculate end-member crustal components that, in turn, can be used to investigate chemical compositional variation in Neogene volcanic rocks of the central North Island.

North Island basement terranes

The Cambrian–Cretaceous basement geology of New Zealand comprises elongate belts of tectono-stratigraphic terranes, intruded by batholiths and recrystallised by structural-metamorphic overprinting events (Mortimer 2004). Only a limited number of these units form known or inferred basement to Neogene volcanic rocks of the North Island (Fig. 1). Spörli (1978) presented a two-terrane model – Torlesse Terrane and Waipapa Terrane – for central and northern North Island. This subdivision is still in use today as Torlesse Composite Terrane and Waipapa Composite Terrane (Edbrooke 2001; Leonard et al. 2010; Mortimer et al. 2014) and each of these units has been subdivided on a regional scale. Waipapa Composite Terrane is subdivided into Permian–Triassic Hunua and Jurassic Morrinsville facies (equivalent to Waipapa Group and Manaia Hill Group; Edbrooke 2001). Detrital zircon dating of Adams et al. (2009) provides a basis on which the Torlesse Composite Terrane in central North Island can be subdivided into Jurassic Kaweka Terrane and Early Cretaceous Pahau Terrane. A Permian–Triassic Rakaia Terrane is also recognised further south in New Zealand. The western parts of Kaweka Terrane in Fig. 1 are weakly schistose and are part of the Haast Schist. The recent recognition of Kaweka Terrane is important as it supersedes former maps and interpretations that showed the east edge of the TVZ to be flanked by Triassic Rakaia Terrane of the Torlesse Composite Terrane (e.g. Mortimer 1994; Mortimer et al. 1997).

Figure 1 A, Map of New Zealand North and South islands showing distribution of litho-stratigraphic tectonic terranes of the Eastern Province. Data compiled from Coombs et al. (1976), Frost & Coombs (1989), Mortimer et al. (1997), Mortimer et al. (1999), Wandres et al. (2004), Adams et al. (2005), Adams et al. (2009) and Leonard et al. (2010); B, Map of part of North Island showing location of samples described in this paper (see Table 1). TVZ indicates the outline of the Taupo Volcanic Zone (Rowland & Sibson 2001). CVZ is the Coromandel Volcanic Zone (Briggs et al. 2005) and NA the Northland Arc (Hayward et al. 2001); C, Schematic cross-section showing relationship between central North Island composite terranes and the TVZ. The dip inferred for the terrane boundaries in central North Island is based on the occurrence of a metamorphosed Pahau xenolith in a young (28 ka) dacite dome from northeast of Taupo (Charlier et al. 2010), which indicates the presence of Pahau at depth to the west of the eastern surface boundary of the Waipapa Terrane.

The position of the boundary between the Waipapa and Torlesse composite terranes is not well defined as it is covered by Quaternary rocks of the TVZ. The discovery just northeast of Lake Taupo of a Pahau Terrane schist xenolith in a 28 ka rhyolite dome (Charlier et al. 2010) raises questions about the deeper crustal distribution of North Island basement terranes. Although somewhat speculative, one schematic possibility is outlined in Fig. 1C. This is based on the Eastern Province accretionary wedge model of Mortimer et al. (2012) in which Torlesse Composite Terrane is underthrust beneath Caples Terrane (the South Island equivalent of Waipapa Composite Terrane) and in which, overall, the Mesozoic terranes form a west-dipping imbricate accretionary stack.

Samples

The samples that form the basis for this study are from the GNS Science Petrology ‘P’ collection and were selected to provide a suite of lithologies representative of the Waipapa and Torlesse composite terranes of the North Island in the vicinity of Neogene subduction-related volcanoes. Sample locations are indicated on Fig. 1B and locality information, lithology and tectonic affiliations of each sample are summarised in Table 1. The analysed sample suite comprises both sandstones and mudstones. Six samples were analysed from the Kaweka Torlesse Terrane (four sandstones and two mudstones), three from Pahau Torlesse Terrane (all sandstones) and seven from Waipapa Terrane (five sandstones and two mudstones).

Table 1 North Island Torlesse and Waipapa terrane sample locality and stratigraphic and petrographic summary. Grain sizes (to nearest 50 µm) and other petrographic data are from visual examination of unstained thin-sections.

Sample	Terrane	Rock description	Accessory minerals	Q:F:L	Lv/L	Latitude	Longitude	Collectors
P74181	Waipapa Hunua	Medium-grained (400 µm) moderately sorted, subangular Ss	Detrital clinopyroxene, hornblende, biotite.	5:15:80	0.8	−35.2646	174.2942	R. Jongens
P73738	Waipapa Hunua	Fine-grained (150 µm) poorly sorted, subangular Ss	Detrital clinopyroxene, hornblende, biotite. Secondary prehnite.	10:20:70	0.9	−36.9415	175.2035	C.J. Adams, N. Mortimer
P16841	Waipapa Hunua	Mudstone (<63 µm)	–	–	–	−36.4398	174.8325	D. Kear
P43647	Waipapa M’ville	Medium-grained (300 µm), poorly sorted, subangular Ss	Detrital hornblende, biotite. Secondary prehnite.	10:20:70	0.9	−39.0911	175.4952	R.D. Beetham
P60553	Waipapa M’ville	Fine-grained (150 µm) poorly sorted, subrounded Ss	–	–		−38.1992	175.6796	G.W. Grindley
P74272	Waipapa M’ville	Very–fine-grained (100 µm) moderately well sorted, subangular Ss	Detrital biotite, epidote.	20:30:50	0.7	−38.7453	175.5263	A.G. O’Loan
P54716	Waipapa M’ville	Mudstone (<63 µm)	–	–	–	−38.9986	175.3972	N. Mortimer
P75218	Torlesse Kaweka	Medium-grained (400 µm) poorly sorted, subangular Ss	Detrital biotite, muscovite.	30:30:40	0.8	−39.3845	175.8442	N. Mortimer
P54704	Torlesse Kaweka	Fine-grained (200 µm) poorly sorted, subangular schistose Ss	Recrystallised secondary stilpnomelane.	15:35:50	?	−39.1775	175.8099	N. Mortimer
P54720	Torlesse Kaweka	Fine-grained (200 µm) poorly sorted, subangular Ss	Detrital biotite, muscovite	25:35:40	0.9	−39.562	176.0364	N. Mortimer
P69955	Torlesse Kaweka	Fine-grained (200 µm) poorly sorted, subangular schistose Ss	Recrystallised secondary stilpnomelane, sericite	15:30:55	0.8	−39.155	175.8376	N. Mortimer
P54700	Torlesse Kaweka	Mudstone (<63 µm)	–	–	–	−39.2437	175.7929	N. Mortimer
P54721	Torlesse Kaweka	Mudstone (<63 µm)	–	–	–	−39.562	176.0364	N. Mortimer
P73599	Torlesse Pahau	Fine-grained (200 µm) poorly sorted, subangular schistose Ss	Detrital clinopyroxene, biotite. Secondary zeolite.	5:15:80	0.8	−38.3056	177.1309	J. G. Begg, N. Mortimer, D.B. Townsend,
P73609	Torlesse Pahau	Fine-grained (150 µm) poorly sorted, subrounded Ss	Detrital muscovite, biotite.	20:30:50	0.7	−38.0832	177.4753	N. Mortimer
P73612	Torlesse Pahau	Very-fine-grained (100 µm) moderately well sorted, subrounded Ss	Detrital biotite, epidote.	30:40:40	0.5	−38.1056	177.5873	N. Mortimer

Q:F:L, quartz:feldspar:total lithics ratio (matrix free); Lv/L, ratio of volcanic lithics to total lithics; Ss, sandstone.

Analytical methods

Samples were crushed using a TEMA swing mill fitted with a tungsten carbide head. Contamination of trace elements during the crushing process is restricted to W and Co. (Martin et al. 2013). Major and minor elements and selected trace elements were determined as oxide components by X-ray fluorescence (XRF) using lithium borate glass discs and methods similar to those described by Norrish & Hutton (1969). H₂O and loss on ignition (LOI) were measured by heating 4 g of sample for 12 hours at 105 °C and 1000 °C respectively. For each sample, 2 g of dried sample powder were fused with 6 g of Spectrachem® 12–22 flux in Pt crucibles and moulded and cooled into a glass disc. Major element concentrations were measured by XRF using a Siemens SR3000 spectrometer at the University of Auckland. The XRF-calibration is based on 34 international standards. Data were processed using the Bruker SpectraPLUS software (V1.51), which uses variable alphas as a matrix correction method and is calibrated for all elements using three multi-element glass beads and a graphite disc. For major and minor elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K and P) the precision is better than ±1% (1σ) of the reported value. For trace elements (Rb, Ba, Pb, Sr, Zr, V, Cr, Ni, Cu and Zn) the Compton scatter of X-ray tube RhKb1 emission was used to correct for mass attenuation. Theoretical detection limits for these elements are 1–2 ppm and reproducibility is <5% (2σ).

A suite of trace elements (Cs, Th, U, Hf, Ta, Nb, rare earth elements, Sc and Ga) was measured by laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at the Research School of Earth Sciences, Australian National University, using an Excimer LPX120 laser (193 nm) and Agilent 7500 series mass spectrometer and following the method of Eggins (2003). For these analyses the same fused glass discs used for XRF were prepared as multi-sample polished mounts. Samples were run in batches of 15 using NST612 glass as a calibration standard. Repeat analyses of standard rock BCR-2 (Table S1), prepared as a fused glass disc in the same way as the samples, indicate precision of <4% (RSD) and accuracy better than 5% at the 95% confidence level for most elements.

Strontium, Nd and Pb isotopic compositions were acquired at the University of Melbourne following methods similar to those described by Maas et al. (2005). Strontium and Nd isotope ratios were normalised to ⁸⁸Sr/⁸⁶Sr = 8.37512 and ¹⁴⁶Nd/¹⁴⁵Nd = 2.0719425, equivalent to the more familiar ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219 (Vance & Thirlwall 2002) using the exponential law; data are reported relative to SRM987 = 0.710230 and La Jolla Nd = 0.511860. Typical two standard error (2se) in-run precisions are ≤0.000020 for ⁸⁷Sr/⁸⁶Sr and ≤0.000012 for ¹⁴³Nd/¹⁴⁴Nd, while external precisions (2σ) are ≤0.000040 and ≤0.000020, respectively. ε_Nd values referred to in text are for a modern chondritic uniform reservoir, CHUR, with ¹⁴³Nd/¹⁴⁴Nd = 0.512638. Typical in-run precisions (2se) in Pb isotope runs with signals near 10 V total Pb are ±0.002 for ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb, and ±0.006 for ²⁰⁸Pb/²⁰⁴Pb (equivalent to 0.010–0.015%). Instrumental mass bias was corrected using the thallium-doping technique of Woodhead (2002), which is expected to produce external precisions of 0.04–0.09% (2σ). Multi-year averages (±2σ) for USGS basalt standard BCR-2 are 0.704997 ± 53, 0.512642 ± 24, 18.759 ± 9, 15.619 ± 10 and 38.726 ± 35; these values agree with thermal ionisation mass spectrometric (TIMS) and multi-collector (MC) ICP-MS reference values.

Whole-rock geochemistry

Major and trace elements

Major and trace element compositions for analysed Torlesse and Waipapa composite terrane whole-rock samples are presented in Tables 2 (Kaweka and Pahau samples) and 3 (Waipapa samples) and the variation of selected elements in relation to SiO₂ abundance is illustrated in Figs 2–3. As major element and XRF trace element variation of North Island basement terrane whole-rocks has previously been explored by Roser & Korsch (1986, 1999) and Mortimer (1994), our comments and observations tend to focus on the rare earth elements (REE) and other elements that have not previously been analysed to high precision.

Figure 2 Variation in abundance of selected major elements (Al₂O₃, MgO and iron as total Fe₂O₃) and K₂O/Na₂O ratio relative to SiO₂ content. All data are normalised to 100% on a volatile free basis. Ss: sandstone; Ms: mudstone. Fields for Waipapa (W) and Torlesse (T) are from the GNS Science PETLAB database. And: average primitive andesite composition from Keleman et al. (2005). L: average Longwood crust representing Median Batholith intra-oceanic arc crust (Price et al. 2011). GL: average global subducting sediment (GLOSS) of Plank & Langmuir (1998). Gr: average composition of Lachlan Fold Belt granites from Chappell & White (1992). UC: estimated upper crustal composition of Rudnick & Gao (2005). Fields shown in D (broken lines) are from Roser & Korsch (1986). PM: passive margin. ACM: active continental margin; ARC: oceanic island arc margin. Broken lines with arrows show mixing trajectories from average granite towards stoichiometric clay, muscovite (Mu) and chlorite (Chl) compositions.

Figure 3 Variation in abundance of selected trace elements relative to SiO₂ content. Acronyms as for Fig. 2.

Table 2 Major and trace element and isotope analyses for Kaweka and Pahau terrane samples. Samples P54704–P54721 are Kaweka and samples P73609–P73599 Pahau. Data shown in italics were obtained by LA-ICP-MS. All other analyses are XRF data. Precision and accuracy for major and trace elements and isotopes are discussed in the text. Gaps in the trace element data indicate that either the element was not analysed or that concentration is below the detection limit. Lithologies are summarised in Table 1.

Sample	P54704	P54720	P69955	P75218	P54700	P54721	P73609	P73612	P73599
	Ss	Ss	Ss	Ss	Mst	Mst	Ss	Ss	Ss
SiO2	68.62	73.98	69.84	70.74	63.41	64.65	64.47	68.38	61.64
TiO2	0.48	0.35	0.50	0.39	0.64	0.62	0.73	0.57	0.81
Al2O3	14.89	13.15	14.47	14.35	16.48	16.69	16.07	14.63	17.10
Fe2O3total	3.88	2.61	3.29	2.91	5.42	5.02	5.37	3.77	5.58
MnO	0.05	0.04	0.06	0.06	0.08	0.05	0.07	0.05	0.08
MgO	1.41	0.73	1.11	1.15	1.72	1.51	1.79	1.16	1.90
CaO	1.21	1.04	1.47	1.54	2.59	1.30	1.82	1.59	2.30
Na2O	5.04	4.59	3.92	3.85	2.07	2.69	4.53	4.66	5.84
K2O	2.16	1.73	3.23	3.42	3.40	3.33	2.70	2.17	1.95
P2O5	0.12	0.08	0.10	0.09	0.16	0.15	0.17	0.14	0.21
H2O^-	0.37	0.28	0.40	0.14	0.80	0.95	0.41	0.67	0.78
LOI	1.47	1.18	1.54	1.27	3.15	2.98	1.85	1.98	1.76
	99.71	99.76	99.93	99.91	99.92	99.94	99.98	99.77	99.95
Cs	6.912	3.089	6.853	4.098	7.459	10.860	2.925	2.006	1.588
Ba	547	376	729	826	626	429	635	499	600
Rb	87	68	110	120	152	171	80	61	57
Sr	277	382	280	405	203	225	463	400	723
Pb	23	27	23	25	32	32	20	14	16
Th	14.483	12.421	19.415	17.991	17.620	19.392	9.544	10.350	6.639
U	2.981	3.226	3.124	4.611	4.260	3.713	2.300	2.003	1.461
Zr	198	182	260	173	190	191	189	235	173
Hf	4.676	5.384	8.257	6.389	5.844	6.184	4.851	6.900	4.024
Ta	1.269	1.365	1.933	1.947	1.361	1.334	0.558	0.858	0.425
Nb	13.511	13.576	16.423	11.712	14.506	15.644	7.556	8.081	6.259
Y	20	23	27	19	34	32	21	19	19
La	35.070	32.695	59.753	36.393	42.757	38.284	19.920	25.782	20.515
Ce	72.331	66.598	107.987	73.608	66.205	78.876	41.650	47.811	41.695
Pr	8.905	7.077	12.746	7.894	9.988	9.320	4.973	5.788	5.006
Nd	31.194	29.474	46.255	28.930	39.840	34.799	19.629	22.778	19.892
Sm	7.379	5.727	9.989	5.050	7.487	7.505	3.940	4.363	4.032
Eu	1.483	1.055	1.116	1.012	1.513	1.133	1.002	1.061	1.279
Gd	4.929	4.505	3.731	3.650	6.244	6.376	3.547	3.707	3.615
Tb	0.556	0.998	0.763	0.670	0.957	0.921	0.532	0.437	0.499
Dy	6.196	4.688	4.868	3.908	5.970	5.974	3.550	3.688	3.210
Ho	0.984	1.080	1.024	0.754	1.277	1.176	0.729	0.639	0.654
Er	2.540	2.409	3.716	2.736	3.649	3.578	2.000	1.955	1.861
Tm		0.529	0.409		0.578	0.549	0.318	0.345	0.251
Yb	2.555		2.767		3.652	3.444	2.173	2.328	1.991
Lu	0.764		0.679		0.548	0.555	0.336	0.356	0.230
Sc	11.778	6.897	9.806	8.872	14.406	12.488	12.705	8.760	14.617
V	63	38	53	54	91	86	101	75	124
Cr	29	18	26	16	41	38	26	26	20
Ni	11	5	5	4	15	15	15	7	13
Cu					9	4	12		16
Zn	63	57	43	50	80	89	64	50	68
Ga	28.408	19.872	23.930	20.945	23.444	22.360	17.928	18.347	18.687
^87Sr/^86Sr	0.708877	0.708006	0.709036	0.708939	0.711819	0.712450	0.705429	0.706148	0.705308
^143Nd/^144Nd	0.512482	0.512412	0.512482	0.512475	0.512404	0.512412	0.512768	0.512690	0.512739
^206Pb/^204Pb	18.872	18.848	18.955	18.890	18.946	18.882	18.814	18.888	18.769
^207Pb/^204Pb	15.641	15.644	15.648	15.642	15.647	15.646	15.612	15.617	15.610
^208Pb/^204Pb	38.743	38.721	38.839	38.773	38.812	38.790	38.663	38.786	38.604

In major element terms, the principal difference between Waipapa Terrane and Torlesse Composite Terrane samples is in SiO₂ abundance (Fig. 2). Kaweka Terrane sandstones analysed as part of this study have SiO₂ abundances ranging from 68.6 to 74 wt%, whereas Waipapa sandstones have relatively lower SiO₂ contents (59.5–66.1 wt%). Although there is significant overlap in abundance ranges for most major components, among the whole suite of analysed samples Al₂O₃, total Fe₂O₃ and MgO contents tend to be negatively correlated with SiO₂ abundance (Fig. 2). Abundances of these major elements therefore tend to be higher in the Waipapa Terrane samples relative to the Torlesse sample suite.

On a K₂O/Na₂O versus SiO₂ plot (Fig. 2D), Roser & Korsch’s (1986) and Mortimer’s (1994) dataset of >300 Older Torlesse (Rakaia and Kaweka) sedimentary rocks plot in the active continental margin (ACM) field with a sandstone/mudstone cut-off of 68 anhydrous wt% SiO₂. On the same diagram, their dataset of >200 Waipapa Terrane and Torlesse–Pahau sedimentary rocks plot in both the ocean island margin (ARC) and ACM fields. Our analysed North Island Torlesse and Waipapa Terrane samples plot within these previously established compositional ranges and also show what has previously been known: that mudstones from all terranes converge in bulk composition (Roser & Korsch 1986). In terms of major elements, Pahau sandstones span the range between Waipapa and Kaweka Terrane samples but mainly overlap the former (Fig. 2). There is no appreciable difference in compositional range between the Hunua facies and Morrinsville facies parts of the Waipapa Composite Terrane.

Trace element behaviour tends to follow the patterns expected from those observed in the major elements. Scandium, for example, behaves similarly to MgO and total Fe₂O₃. The abundance of this trace element is negatively correlated with SiO₂ content (Fig. 2E), with abundances generally higher in Waipapa than in Torlesse sample suites. Strontium abundance tends to show a crude negative correlation with SiO₂ content (Fig. 3A), whereas Rb abundance in sandstones is variable and does not show any obvious correlation with SiO₂ content (Fig. 3B). Th and Hf abundances increase with increasing SiO₂ content (Fig. 3C–D). Mudstones tend to have higher concentrations of Rb and Th and lower Sr abundances than sandstones (Fig. 3).

The general patterns of major and trace element variation are consistent with mixing between relatively mafic subduction-related arc material such as andesite and an upper crustal or granitic component (Fig. 3). For both mudstones and sandstones from the Kaweka sample suite, the major and trace element behaviour appears to be largely controlled by felsic material (both plutonic and volcanic). Even though the samples cover a wide geographic area they define collectively a reasonably coherent grain-size trend (sandstone to mudstone) that, on the major and trace element variation diagrams (Figs 2–3), intersects the andesite–granite mixing lines near the granite end-member. For the Kaweka sample suite, the input of mafic material was therefore, at the time of deposition, comparatively minor.

Chondrite normalised REE patterns are generally similar for all 16 samples, but there are distinct differences between samples from each terrane (Fig. 4). Overall similarities include enrichment of light over heavy REE and relatively flat heavy REE patterns, with variable depletion in Eu relative to the adjacent REE, Gd and Sm. The samples from the Kaweka Terrane are more enriched in total REE (157–256 ppm) compared to their counterparts from the Pahau (105–121 ppm) and Waipapa (95–151 ppm) terranes and they show stronger enrichment in light over heavy REE, (La/Yb)n in the range 7.6–14.7 compared with 6.2–7.5 for the Pahau and 5.3–9.2 for the Waipapa. The Kaweka samples also show more strongly developed Eu anomalies with Eu/Eu* (chondrite normalised Eu/((Gd+Sm)/2)) varying over the range 0.46–0.71 compared to 0.78–1.00 for the Pahau and 0.79–1.02 for the Waipapa Terrane samples (Fig. 3F).

Figure 4 Chondrite-normalised rare earth element patterns for: A, Kaweka; B, Pahau; and C, Waipapa Terrane sandstones and mudstones. These are in each case compared with GLOSS, average global subducting sediment (Plank & Langmuir 1998). Chondritic values are from McDonough & Sun (1995). Filled symbols are mudstones.

Primitive mantle normalised extended element plots provide a basis for graphical comparisons with crustal material or subduction-related volcanic rocks and sediments (Fig. 5), and the patterns for North Island metasediments are characterised by features commonly seen in subducting sediment, upper crust or subduction-related volcanic rocks. Caesium, Rb, Th and U abundances are all enriched relative to the light and medium REE. Niobium is strongly depleted relative to K and La. Lead and (to a lesser extent) Sr are enriched relative to Ce. The patterns also show distinctive negative Ti anomalies. Overall, the patterns are similar to that of the average global subducted sediment (GLOSS of Plank & Langmuir 1998; see Fig. 5). The Kaweka samples show higher abundances of Cs, Rb, Th, U and REE compared to both Pahau and Waipapa counterparts. The primitive mantle normalised extended element patterns for the Waipapa Terrane samples are generally similar to GLOSS (Fig. 5C), although one sample (P74272) is distinctly different with relatively lower Cs, Rb, Ba and K (Fig. 5C). As with major elements, Pahau trace element patterns are more similar to those of the Waipapa than the Kaweka samples (Fig. 5C). The abundance ranges for Ba are similar in sandstones from all three terranes but the primitive mantle normalised patterns for Kaweka samples are characterised by distinctive negative Ba anomalies; Ba is depleted relative to Rb and Th and U. Similarly, Sr tends to be relatively depleted in Kaweka sandstones compared with counterparts from the Pahau and Waipapa terranes. For example, primitive mantle normalised Ce/Sr ratios are in the range 2.2–4.6 in Kaweka compared with 0.7–1.4 for Pahau and 0.8–3.5 for Waipapa sandstones. Mudstones tend to have relatively higher abundances of Cs, Rb, Ba, Th, U and REE compared with sandstones from the same terrane, but the distinctive subduction or continental crustal features of the primitive mantle normalised patterns are present in both lithological types.

Figure 5 Primitive mantle-normalised extended element patterns for: A, Kaweka; B, Pahau; and C, Waipapa Terrane sandstones and mudstones. These are in each case compared with GLOSS, average global subducting sediment (Plank & Langmuir 1998). Primitive mantle values are from McDonough & Sun (1995). Filled symbols are mudstones.

Radiogenic isotopic compositions

Present-day Sr, Nd and Pb isotope compositions for the analysed Torlesse and Waipapa composite terrane samples are reported in Tables 2–3 and isotopic variation is illustrated in Figs 6–7. Kaweka Terrane samples tend to be more isotopically evolved than either Waipapa or Pahau Terrane samples (Figs 6–7). For the Kaweka Terrane sample suite, present-day ⁸⁷Sr/⁸⁶Sr falls within the range 0.70801–0.71245, compared with 0.70482–0.7105 and 0.70531–0.70615 for the Waipapa and Pahau Terrane samples. ¹⁴³Nd/¹⁴⁴Nd falls within the range 0.51240–0.51248 (ε_Nd –4.4 to –2.9) for the Kaweka Terrane samples, which compares with 0.51258–0.51278 (–1.1 to +2.8) for the Waipapa and 0.51269–0.51277 (+1 to +2.7) for the Pahau sample suites. Lead isotopic compositions follow the patterns observed for Sr and Nd isotopes with Kaweka Terrane samples having higher ratios than the Waipapa and Pahau samples (Fig. 7). Ranges for ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios in the Kaweka samples are 18.848–18.955, 15.641–15.648 and 38.721–38.839 respectively, with the equivalent ranges in the Waipapa Terrane samples being 18.739–18.836, 15.610–15.631 and 38.568–38.748.

Figure 6 A, Variation in present-day Sr and Nd isotope composition for North Island greywacke samples; B, ¹⁴³Nd/¹⁴⁴Nd versus Th/Sc for North Island greywacke samples and comparisons with andesite, global subducting sediment and eastern Australian granite averages. Old, upper continental crust has low ¹⁴³Nd/¹⁴⁴Nd and Th/Sc ratios of approximately 1, whereas modern primitive arcs have higher ¹⁴³Nd/¹⁴⁴Nd and Th/Sc ratios <1 (Taylor & McLennan 1985; McLennan et al. 1993). K: Kermadec arc; TVZ: Taupo Volcanic Zone (data from Ewart & Hawkesworth 1987; Sutton et al. 1995; Gamble et al. 1996; Smith et al. 2010; Price et al. 2012). Fields for Waipapa (W) and Torlesse (T) include data from McCulloch et al. (1994), Wandres et al. (2004) and Adams et al. (2005). MB: field for Median Batholith (data from Mortimer et al. 1997; Muir et al. 1998; Mortimer et al. 1999; Price et al. 2006). And: average primitive andesite composition from Keleman et al. (2005). L: average Longwood Complex from Southland representing Median Batholith intra-oceanic arc crust (Mortimer et al. 1999; Price et al. 2006). GL: average global subducting sediment (GLOSS) of Plank & Langmuir (1998). UC: estimated isotopic composition of upper crust exposed to weathering (Goldstein & Jacobsen 1988). LFB: the average composition of Lachlan Fold Belt granites (data from McCulloch & Chappell 1982; Eberz et al. 1990; McCulloch & Woodhead 1993; Elburg 1996; Maas et al. 1997; Soesoo 2000; Waight et al. 2000, 2001; Collins et al. 2006). NEFB: average of New England Fold Belt granites from Hensel et al. (1985). Where required, published Nd isotopic data were re-normalised to ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219 and adjusted to be consistent with modern CHUR = 0.512638. Curves (solid, dashed) depict schematic mixing trends.

Figure 7 Variation in present-day Pb isotope composition for North Island greywacke samples. Fields for Waipapa (W) and Torlesse (T) include data from Graham et al. (1992) and McCulloch et al. (1994). K: Kermadec arc and TVZ: Taupo Volcanic Zone (data from Ewart & Hawkesworth 1987; Graham et al. 1992; Gamble et al. 1996; Smith et al. 2010; Price et al. 2012). MB: field for Median Batholith (unpublished data of A. Tulloch). And: average primitive andesite composition from Keleman et al. (2005). GL: average global subducting sediment (GLOSS) of Plank & Langmuir (1998). LFB: average composition of Lachlan Fold Belt granites (recalculated from feldspar Pb isotope data of McCulloch & Woodhead 1993). UC: estimate of upper crustal Pb isotope composition from Zartman & Doe (1981). Solid lines between the And, LFB and UC model components depict schematic mixing lines which are straight lines in Pb–Pb isotope space.

Table 3 Major and trace element and isotope compositions for Waipapa terrane samples. Data source and precision as for Table 2.

Sample	P43647	P60553	P74272	P54716	P73738	P74181	P16841
	Ss	Ss	Ss	Ss	Ss	Mst	Mst
SiO2	59.54	62.11	66.08	63.02	61.79	63.13	59.55
TiO2	0.98	0.82	0.72	0.80	0.83	0.84	0.90
Al2O3	16.09	15.64	14.89	17.00	16.54	16.04	18.21
Fe2O3	7.37	6.38	6.45	6.03	6.40	5.03	6.34
MnO	0.11	0.11	0.08	0.06	0.10	0.09	0.27
MgO	2.57	2.61	1.60	1.69	2.44	1.86	2.00
CaO	3.13	3.74	1.54	0.68	3.42	2.30	1.42
Na2O	3.96	3.79	4.90	2.78	3.82	5.08	3.52
K2O	2.23	2.04	0.63	3.05	2.09	2.76	3.85
P2O5	0.22	0.20	0.15	0.16	0.23	0.17	0.25
H2O	0.58	0.32	0.33	0.74	0.45	0.56	0.58
LOI	2.36	2.03	1.70	3.33	1.88	1.26	3.07
	99.15	99.80	99.08	99.34	99.98	99.12	99.96
Cs	1.977	1.934	1.076	6.103	1.945	1.622	6.501
Ba	568	481	150	519	634	693	1305
Rb	68	57	24	120	59	72	122
Sr	619	481	393	159	517	392	195
Pb	19	17	17	15	20	13	21
Th	7.700	5.612	6.400	12.653	6.496	7.785	12.642
U	1.882	1.395	1.566	2.655	1.697	1.853	2.596
Zr	178	137	227	199	155	218	203
Hf	4.075	3.454	4.942	4.631	3.893	5.288	5.284
Ta	0.564	0.457	0.525	0.877	0.491	0.614	0.845
Nb	6.936	4.958	6.084	10.980	6.012	8.404	11.294
Y	23	21	18	20	21	21	31
La	20.157	16.930	18.506	19.512	17.861	24.675	24.995
Ce	43.993	36.902	40.455	46.490	39.067	52.283	57.266
Pr	5.162	4.369	4.445	5.207	4.632	5.800	6.515
Nd	20.871	18.737	17.429	19.759	18.696	23.228	35.509
Sm	4.429	3.998	3.460	3.795	4.106	4.295	5.552
Eu	1.199	1.056	1.040	0.988	1.118	1.372	1.422
Gd	4.098	3.406	2.998	3.187	3.403	3.746	5.339
Tb	0.569	0.588	0.424	0.460	0.568	0.485	0.828
Dy	3.554	3.558	2.811	3.323	3.309	3.400	5.221
Ho	0.745	0.759	0.574	0.683	0.684	0.655	1.045
Er	2.034	2.078	1.637	2.116	1.876	1.861	3.293
Tm	0.302	0.326	0.224	0.372	0.286	0.274	0.495
Yb	2.311	2.163	1.603	2.363	1.954	1.819	3.250
Lu	0.298	0.343	0.260	0.371	0.253	0.274	0.461
Sc	18.435	17.574	10.774	16.305	18.556	11.779	17.342
V	142	120	101	124	142	108	135
Cr	25	18	26	40	23	22	34
Ni	14	7	10	12	13	7	19
Cu	23	26	7	10	23	3	35
Zn	88	70	77	74	78	79	100
Ga	17.583	16.952	14.348	21.547	17.453	17.768	20.858
^87Sr/^86Sr	0.705215	0.704818	0.705420	0.710503	0.704993	0.705643	0.708913
^143Nd/^144Nd	0.512778	0.512761	0.512682	0.512577	0.512739	0.512710	0.512608
^206Pb/^204Pb	18.740	18.739	18.784	18.794	18.775	18.836	18.796
^207Pb/^204Pb	15.611	15.612	15.620	15.631	15.621	15.610	15.627
^208Pb/^204Pb	38.591	38.568	38.636	38.722	38.618	38.748	38.705

The Kaweka Terrane samples analysed as part of this study have Sr, Nd and Pb isotopic compositions that partly overlap with the compositional ranges defined by published data for the Torlesse Composite Terrane (Figs 6–7). It is important to realise that existing Torlesse Sr and Nd reference data are dominated by analyses from the Triassic Rakaia Terrane; Kaweka and Pahau terrane are under-represented. Our new data for these terranes, in particular those for Pahau, considerably expands the known isotopic range of the Torlesse Composite Terrane towards juvenile compositions (Fig. 6a). The new Pb isotope data for Waipapa sandstones and mudstones generally overlap published data for the Waipapa Terrane and the new Kaweka data plot close to or within the Torlesse field (Fig. 7). Like their Sr and Nd isotope compositions, the new Pb isotope data for Pahau Terrane samples plot close to the Waipapa data (both new and published, Fig. 7), once again expanding the isotopic range of the Torlesse Terrane in North Island. Overall, on isotope diagrams the Torlesse and Waipapa composite terrane data lie in fields between primitive continental andesite or average Longwood Complex at one extreme, and evolved compositions such as those represented by Palaeozoic granite compositions from SE Australia (Lachlan Fold Belt) at the other (Figs 6–7). The new Sr, Nd and Pb isotope data confirm previously published observations (e.g. Adams & Maas 2004a,b; Adams et al. 2005) that Waipapa and Pahau terrane metasedimentary rocks have similar present-day Sr and Nd isotopic compositions, different from Kaweka Terrane.

Discussion

Compositional ranges

The new major element, trace element and tracer isotope data for Torlesse and Waipapa composite terrane metasedimentary rocks generally lie within fields and along trends established in previously published and cited studies. The Waipapa and Pahau terranes are compositionally less evolved than the Kaweka Terrane (Figs 2–5). Kaweka sandstones and mudstones show moderately pronounced grain-size–composition trends but these trends are much less clear for sandstones and mudstones from the Waipapa and Pahau terranes.

From a purely geochemical point of view, one might postulate compositional control of the entire dataset by mixing between an arc andesite composition and a quartzofeldspathic component similar in composition to either older granitic rocks such as those of eastern Australia or continental crust (Figs 2–5). However, this is probably overly simplistic as petrographic observations (Mackinnon 1983; Mortimer 1994) indicate the presence of grains and clasts of different volcanic rock types (rhyolite-dacite in Kaweka and andesite in Pahau and Waipapa), rather than simple dilution of andesite detritus by addition of plutonic quartz and feldspar in Kaweka sandstones.

Chondrite normalised REE and mantle normalised extended element patterns reinforce the interpretation that both Torlesse and Waipapa sediment packages contain a significant arc andesite and/or continental crustal component. The normalised multi-element patterns of all analysed metasediments (Fig. 5) feature characteristics that are generally interpreted to have a subduction-related igneous origin (e.g. Keleman et al. 2005) or to reflect involvement of continental crust (Rudnick & Gao 2005) or granite (e.g. Kemp & Hawkesworth 2005). These features include: enrichment of Ba, Rb, Th, U, K and light over heavy REE; depletion of Nb relative to K; and enrichment of Pb and Sr relative to La and Ce. McLennan et al. (1993) used Th/Sc and Nd isotope ratios to differentiate various provenance or terrane components for clastic sediments (see also Taylor & McLennan 1985). Old, upper continental crust has low ¹⁴³Nd/¹⁴⁴Nd and Th/Sc ratios of around 1; for average upper continental crust the ratio is 0.75 (Rudnick & Gao 2005). Recycled sedimentary rocks have Th/Sc ratios of 1 or higher and young primitive arc provenances have relatively high ¹⁴³Nd/¹⁴⁴Nd and Th/Sc ratios less than 1 (e.g. the primitive arc andesite average of Keleman et al. 2005 has a Th/Sc ratio of 0.215). Figure 6B shows the variation of present-day ¹⁴³Nd/¹⁴⁴Nd and Th/Sc ratio for Torlesse Composite and Waipapa Terrane metasediments analysed as part of this study. The distribution highlights the compositional differences between greywackes from the different terranes but, more particularly, it emphasises that the provenance of Waipapa Terrane metasediments is strongly influenced by a primitive arc contribution whereas those from the Torlesse Composite Terrane reflect stronger involvement of an old, continental crustal component. Within any one terrane, but particularly the Waipapa terrane, mudstones tend to have more radiogenic isotopic compositions than sandstones. This point was made obliquely by Frost & Coombs (1989), although they did not explicitly name rock types or give grain sizes. It is however worth restating. In Figs 6–7, the Kaweka and Waipapa mudstone analyses are generally offset to higher ⁸⁷Sr/⁸⁶Sr, ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb and lower ¹⁴³Nd/¹⁴⁴Nd ratios.

This paper is focused on providing a dataset for North Island greywackes that can be used in petrogenetic modelling of Neogene magmatic processes; explaining differences in the isotopic compositions of sandstones and mudstones is to some extent outside the scope of the paper. However, it is worth noting that present-day Sr–Nd–Pb isotopic contrasts between sandstones and mudstones may in part be attributed to ageing since sediment deposition. By using broad age constraints based on known stratigraphic ranges for each terrane, such effects can be quantified. Age effects are negligible for ¹⁴³Nd/¹⁴⁴Nd where present-day and depositional isotopic mudstone–sandstone patterns are similar, that is, mudstones preserve the lowest ¹⁴³Nd/¹⁴⁴Nd in both the Kaweka and Waipapa datasets.

Kaweka and Waipapa measured ⁸⁷Sr/⁸⁶Sr ratios overlap because the two Waipapa mudstones have high Rb/Sr and present-day ⁸⁷Sr/⁸⁶Sr. After age correction, the ranges for each suite are smaller and ⁸⁷Sr/⁸⁶Sr ratios no longer overlap. U/Pb and Th/Pb ratios of mudstones and sandstones in the analysed sample sets overlap, as do their present-day and age-corrected Pb isotope ratios. Metamorphic effects on the Rb–Sr and U–Th–Pb isotope systems in the metasediments (e.g. Adams & Maas 2004a,b) are difficult to quantify, but would add complexity to age-corrected Sr–Pb isotope data. It is therefore unclear if Kaweka and Waipapa mudstones and sandstones were deposited with systematically different Sr–Pb isotope ratios. However, the observed grain-size-related Nd isotope contrasts are robust and imply slightly variable provenance signals in mud- and sandstones within the same depocentre.

In a study of modern turbidites, McLennan et al. (1989) concluded that different ε_Nd values in some sandstone–mudstone pairs were probably related to sorting of distinct provenance components during transport and deposition. Mudstones are likely to under-sample coarse-grained contemporary volcanic detritus and can be transported over long distances and across sedimentary basins. They could therefore record provenance signals that differ from those in associated sandstone. This is supported by the results reported here. Relative to the Sr–Nd isotope reference field for the Torlesse Composite Terrane shown in Fig. 6A, the Kaweka and Pahau sandstones measured here show higher ¹⁴³Nd/¹⁴⁴Nd, greatly expanding the known range for the Torlesse Terrane. The reference field shown in Fig. 6 is based on the results of Adams et al. (2005) who used only mudstones, implying that Torlesse mudstones under-sample contemporary high ¹⁴³Nd/¹⁴⁴Nd volcaniclastic input to the depocentre. This has obvious implications for provenance studies and emphasises the importance of collecting data for both sandstones and mudstones.

Implications for TVZ petrogenesis

Igneous geochemists explore the petrogenesis of lavas using diagrams in which various end-member components are plotted, and develop quantitative models that utilise specific end-member compositions for parental magmas and potential contaminants. One of the results of this paper is the geochemical characterisation of three local TVZ crustal end-members that can be used for these purposes (Table 4). Following MacKinnon (1983) we have used a sandstone:mudstone ratio of 2:1 in order to weight the means and calculate Kaweka and Waipapa end-member compositions (Table 4). The geological justification for this is that the Torlesse and Waipapa composite terrane sandstones and mudstones are typically interbedded on a 0.1–10 m scale (MacKinnon 1983) and crustal melting or assimilation processes are likely to operate across longer baselines than this. The Pahau end-member shown in Table 4 is a simple average of the three sandstone analyses from that terrane. We do not feel able to consolidate our data to produce a single crustal end-member for North Island Eastern Province basement, as many weighted average values for specific elements listed in Tables 2–4 vary by close to a factor of two or more (e.g. Ti, La, Lu, Nb, Ta, Th and Cs). Because the deep terrane structure of the TVZ remains speculative (Fig. 1), for some volcanoes it may be preferable to use an average Kaweka composition; for others an average Waipapa or Pahau composition might be more appropriate. In petrogenetic models, the choice of a crustal component will depend on the arc position (e.g. TVZ, Coromandel or Northland arc) as well as the depth at which interaction between evolving mantle-derived magmas and crust is inferred to have taken place. On the basis of the postulated cross-section shown in Fig. 1C, we suggest that a Pahau component might be more suitable for Neogene arcs where crust–mantle interaction takes place at depth. For magmatic systems in which there are grounds for assuming shallower crust–mantle interaction, Waipapa or Kaweka compositions might be more appropriate, depending on the geographic location.

Table 4 Estimated compositions for Kaweka, Pahau and Waipapa crustal (in bold) components and comparison with averages (in bold and italics) for global subducting sediment (GLOSS) and upper crust (UC). Major elements normalised to 100%. Sandstone (Ss) and mudstone (Mst) averages are from Tables 2 and 3. n = number of samples included in each estimate. Sandstone and mudstone are assumed to be in the ratio 2:1 (see text).

	Kaweka			Pahau	Waipapa
	Ss	Mst		Ss	Ss	Mst
	(n=4)	(n=2)	Ss:Mst 2:1	(n=3)	(n=5)	(n=2)	Ss:Mst 2:1	GLOSS^1	UC^2^–4
SiO2	72.12	66.70	70.31	64.83	62.53	63.98	63.01	64.86	66.60
TiO2	0.44	0.65	0.51	0.70	0.84	0.89	0.86	0.69	0.64
Al2O3	14.48	17.28	15.41	15.94	15.84	18.38	16.69	13.19	15.40
Fe2O3total	3.23	5.43	3.97	4.91	6.33	6.46	6.37	6.41	5.04
MnO	0.05	0.07	0.06	0.07	0.10	0.17	0.12	0.35	0.10
MgO	1.12	1.68	1.31	1.62	2.22	1.93	2.12	2.75	2.48
CaO	1.34	2.03	1.57	1.90	2.82	1.10	2.25	6.59	3.59
Na2O	4.43	2.48	3.78	5.01	4.31	3.29	3.97	2.69	3.27
K2O	2.68	3.51	2.96	2.27	1.95	3.60	2.50	2.26	2.80
P2O5	0.10	0.16	0.12	0.17	0.19	0.22	0.20	0.21	0.15
Cs	5.2	9.2	6.5	2.2	1.7	6.3	3.2	3.5	4.9
Ba	619	528	589	578	505	912	641	776	628
Rb	96	161	118	66	56	121	78	57.2	82
Sr	336	214	295	529	480	177	379	327	320
Pb	24	32	27	16	17	18	17	19.9	17
Th	16.08	18.51	16.89	8.84	6.80	12.65	8.75	6.91	10.5
U	3.49	3.99	3.65	1.92	1.68	2.63	1.99	1.68	2.7
Zr	203	191	199	199	183	201	189	130	193
Hf	6.2	6.0	6.1	5.3	4.3	5.0	4.5	4.1	5.3
Ta	1.6	1.3	1.5	0.6	0.5	0.9	0.6	0.63	0.9
Nb	13.8	15.1	14.2	7.3	6.5	11.1	8.0	8.9	12
Y	22	33	26	20	21	25	22	30	21
La	41.0	40.5	40.8	22.1	19.6	22.3	20.5	28.8	31
Ce	80.1	72.5	77.6	43.7	42.5	51.9	45.7	57.3	63
Pr	9.2	9.7	9.3	5.3	4.9	5.9	5.2		7.1
Nd	34.0	37.3	35.1	20.8	19.8	27.6	22.4	27	27
Sm	7.0	7.5	7.2	4.1	4.1	4.7	4.3	5.78	4.7
Eu	1.17	1.32	1.22	1.11	1.16	1.21	1.17	1.31	1.0
Gd	4.20	6.31	4.91	3.62	3.53	4.26	3.77	5.26	4.0
Tb	0.75	0.94	0.81	0.49	0.53	0.64	0.57		0.7
Dy	4.91	5.97	5.27	3.48	3.33	4.27	3.64	4.99	3.9
Ho	0.96	1.23	1.05	0.67	0.68	0.86	0.74		0.8
Er	2.85	3.61	3.10	1.94	1.90	2.70	2.17		2.3
Tm	0.47	0.56	0.50	0.30	0.28	0.43	0.33		0.3
Yb	2.66	3.55	2.96	2.16	1.97	2.81	2.25	2.76	2.0
Lu	0.72	0.55	0.66	0.31	0.29	0.42	0.33	0.413	0.31
Sc	9	13	11	12	15	17	16	13.1	14
V	52	89	64	100	122	129	125	110	97
Cr	22	40	28	24	23	37	28	78.9	92
Ni	6	15	9	11	10	16	12	70.5	47
Cu		6	2	14	17	22	18	75	28
Zn	53	85	64	60	79	87	81	86.4	67
Ga	23.3	22.9	23.2	18.3	16.8	21.2	18.3		17.5
^87Sr/^86Sr	0.708715	0.712135	0.709539	0.705628	0.705218	0.709708	0.705916	0.717300	0.71600
^143Nd/^144Nd	0.512463	0.512408	0.512443	0.512732	0.512734	0.512593	0.512676	0.512180	0.51177
^206Pb/^204Pb	18.891	18.914	18.900	18.824	18.775	18.795	18.782	18.913	19.44
^207Pb/^204Pb	15.644	15.647	15.645	15.613	15.615	15.629	15.620	15.673	15.70
^208Pb/^204Pb	38.769	38.801	38.782	38.684	38.632	38.714	38.661	38.899	39.19

Sources for the average global subducting sediment (GLOSS) and upper crust (UC) data are: ¹Plank & Langmuir (1998); ²Rudnick & Gao (2005); ³Goldstein & Jacobsen (1988); ⁴Zartman & Doe (1981).

There is a general consensus that most primary subduction-related magmas originate where migration of fluids and/or melts from a subducting slab causes melting in the overlying mantle wedge (e.g. Arculus & Powell 1986; McCulloch & Gamble 1991; Hawkesworth et al. 1993). There is however disagreement about the extent to which these mantle-derived magmas are modified by interaction with the mantle and crust during their passage from source to surface. Assimilation or melting of crust is a feature of many petrogenetic models for andesites and rhyolites, particularly for those emplaced in continental settings (e.g. Grove & Kinzler 1986; Price et al. 2005; Annen et al. 2006; Reubi & Blundy 2009). The trace element patterns postulated for slab fluids or melts are so similar to those estimated for the crust that it is not always feasible to differentiate the relative influence of each of these components on the geochemistry of subduction-related eruptives using trace element information alone. Petrologists commonly attempt to resolve the issue by applying a range of isotopic data including, for example, Sr, Nd and Pb isotopes, U-series isotopic disequilibria information and Be isotopes. In the case of North Island andesites and rhyolites the problem for petrogenetic modellers is particularly challenging because, as this study has emphasised, the crustal component represented by the basement greywackes contains a significant, subduction-related terrigenous input. The ‘arc’ trace element signature that is characteristic of andesites and rhyolites, including those of the North Island, is likely to have an ultimate origin from a subducting slab; it is however also probable that it has been enhanced by the involvement of recycled plutonic and volcanic sedimentary material in crustal level processes.

Although extensive discussion is beyond the scope of this paper, it is worth noting the geochemical similarities of our Kaweka, Pahau and Waipapa averaged estimates to other global geochemical averages (Table 4), notably the average global subducting sediment (GLOSS) of Plank & Langmuir (1998) and upper continental crust (UC) of Rudnick & Gao (2005). In major element terms, the analysed Waipapa and Pahau terrane samples are similar to the global subducting sediment average. This is surprising given the sand-dominated nature of the New Zealand terranes compared with the calcium carbonate-, mud- and hydrothermal metal-influenced GLOSS dataset. However, the overall similarity to GLOSS and more particularly to UC is not unexpected. The Torlesse and Waipapa terranes constitute part of the present-day continental crust of Zealandia, and their petrography and geochemistry indicates subduction-related volcaniclastic and plutoniclastic provenance. Their Nd crustal residence ages and detrital zircon ages indicate sediment recycling from the ancient Gondwana supercontinent.

Comparison of present-day Sr, Nd and Pb isotope compositions of North Island basement terranes with modern subduction-related volcanic rocks reveals some striking similarities. This is particularly true for the Waipapa and Pahau terrane samples which, in Sr–Nd isotope space, have present-day compositions that overlap with the field defined by data for TVZ volcanic rocks (Fig. 6A) and, in Pb–Pb isotope space, with both Kermadec arc and TVZ volcanic rocks (Fig. 7). Although we do not attempt any petrogenetic modelling in this paper, the general patterns of isotopic behaviour (Figs 6–7) are consistent with petrogenetic processes involving interaction between crustal material with the isotopic characteristics of Kaweka or Waipapa/Pahau metasediments and relatively primitive Kermadec or TVZ basaltic magmas. In specific cases, the presence or absence of Mesozoic zircons (McCormack et al. 2009; Charlier et al. 2010) in eruptives could provide unambiguous supporting evidence for the involvement of a particular basement terrane in the petrogenetic process.

Conclusions

Systematic major and trace element and Sr, Nd and Pb isotope analyses of a suite of metasedimentary basement terrane samples provide a comprehensive, high-quality estimate of the (present-day) composition of the Waipapa, Kaweka and Pahau terranes from the Taupo Volcanic Zone and environs. These data can be used in quantitative modelling to test hypotheses for processes by which magmas represented by North Island eruptives of the Whakaari and Horomaka supersuites (Mortimer et al. 2014) might have been generated and/or evolved.

Kaweka Terrane samples have higher Si, Al, Cs, Rb, Th, U, Hf and total REE contents, more radiogenic Sr and Pb isotope ratios, stronger negative Eu anomalies, lower Fe, Mg, Sr, Sc and V abundances and lower ¹⁴³Nd/¹⁴⁴Nd ratios than those from the Waipapa and Pahau terranes, at least in the central North Island. Primitive mantle-normalised extended element plots for North Island metasedimentary rocks reveal a similarity to subduction-related suites, and also to average upper continental crust and (in some respects) to average global subducting sediment.

Associate Editor: Dr Richard Wysoczanski.

Supplementary data

Supplementary file: Table S1. Laser ablation inductively coupled plasma source mass spectrometric (LA-ICPM) analyses of standard rock BCR-2 carried out at same time as analyses of North Island greywacke samples.

Acknowledgements

The technical support of John Wilmshurst is gratefully acknowledged. This research was supported by GNS Science, the University of Waikato, the University of Auckland and the University of Melbourne. Barry Roser and an anonymous reviewer provided constructive reviews and these, along with suggestions and comments by associate editor Richard Wysoczanski, significantly enhanced the final paper.