The magmatic evolution of young island arc crust observed in gabbroic to tonalitic xenoliths from Raoul Island, Kermadec Island Arc

Lithos 210–211 (2014) 199–208

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The magmatic evolution of young island arc crust observed in gabbroic to tonalitic xenoliths from Raoul Island, Kermadec Island Arc Karsten M. Haase a,⁎, Selma Lima a, Stefan Krumm a, Dieter Garbe-Schönberg b a b

GeoZentrum Nordbayern, Universität Erlangen-Nürnberg, Schlossgarten 5, 91054 Erlangen, Germany Institute of Geosciences, Universität Kiel, Ludewig-Meyn-Str. 10, 24118 Kiel, Germany

a r t i c l e

i n f o

Article history: Received 11 April 2014 Accepted 13 October 2014 Available online 22 October 2014 Keywords: Crust formation Fractional crystallization Magma differentiation O isotopes

a b s t r a c t We provide new geochemical and O isotope data for minerals and whole rocks of a suite of gabbroic to tonalitic xenoliths from Raoul Island in the Kermadec island arc. The plagioclase, olivine and clinopyroxene compositions are similar to those observed in the Raoul Island lavas supporting a close relationship of the plutonic and volcanic rocks by crystal fractionation. Plagioclase in gabbros is significantly more An-rich than in similar rocks from oceanic spreading axes reflecting higher water contents in the island arc magmas. Incompatible element and O isotope data suggest that the gabbroic rocks formed from accumulation of minerals of the ascending magmas whereas the tonalites represent highly evolved magmas after extreme fractional crystallization. Temperatures of the magmas calculated from O isotope equilibria and pyroxene thermometers range from about 1200 °C in the mafic to 800 °C in felsic rocks. Barometry of the rocks suggests that gabbros formed between 12 and 18 km depth and tonalites shallower which is in agreement with seismic models of island arc crustal layering. The xenolith data from Raoul Island support seismic studies indicating that some portions of the TongaKermadec island arc show similar layering of felsic and mafic rocks to the Izu-Bonin and the fossil Talkeetna island arcs. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The crustal structure of oceanic island arcs is highly variable and ranges from relatively thin crust of 15 km thickness, resembling oceanic crust, to thick crust of 30 km thickness, similar to continental crust (e.g. DeBari and Greene, 2011; Holbrook et al., 1999; Suyehiro et al., 1996). The incompatible element composition of the average continental crust implies that it formed in the subduction environment but many young island arcs have a too thin crust and too incompatible elementdepleted rocks to represent average continental crust (Davidson and Arculus, 2005; Jagoutz and Schmidt, 2012). Whereas the average continental crust is believed to be andesitic (Hawkesworth and Kemp, 2006; Rudnick and Gao, 2003), the primary island arc magmas are basaltic in composition and thus far too MgO-rich and SiO2-poor compared to continental crust. Consequently, significant differentiation of basaltic melts from the mantle must take place in order to generate rocks with felsic composition in the upper and middle part of the island arc crust. Such felsic portions have been observed seismically in different island arcs (Holbrook et al., 1999; Kodaira et al., 2007; Kopp et al., 2011; Suyehiro et al., 1996). Fractional crystallization and partial melting of crustal rocks are the accepted mechanisms for the formation of the felsic

⁎ Corresponding author. Tel.: +49 9131 8522616. E-mail address: [email protected] (K.M. Haase).

http://dx.doi.org/10.1016/j.lithos.2014.10.005 0024-4937/© 2014 Elsevier B.V. All rights reserved.

portion of the island arc crust, but the relative importance of each one of these processes is debated (e.g. Haase et al., 2011; Nakajima and Arima, 1998; Smith et al., 2003; Tamura et al., 2009). In any case, an ultramafic lower part of the crust must be formed and transferred back to the mantle in order to add an andesitic average crust to the continents. Processes like delamination, chemical alteration and remelting during collision have been suggested to explain the transition from basaltic island arc to andesitic continental crust (e.g. Arndt and Goldstein, 1989; Draut et al., 2002; Kay and Kay, 1991). Island arcs form building blocks of continents and are accreted to the continents, like e.g. the island arc terranes Talkeetna, Alaska, and Kohistan, Himalaya (Jagoutz and Schmidt, 2012; Pearcy et al., 1990) thus representing large regions of young continental crust. Exposed island arc crustal profiles have been studied in detail (Talkeetna, Bonanza, Kohistan, and South Coast Plutonic Complex; e.g. DeBari and Greene, 2011) but much less is known about the deeper crust in active oceanic island arcs. Crystallization and fractionation of amphibole is believed to play an important role in the evolution of island arc magmas which has significant implications for the incompatible element composition (Davidson et al., 2007; Jagoutz, 2010). Xenoliths represent the only means to get insights into the lower portions of the island arc crust but such inclusions are relatively rare in island arc lavas. Here we describe the composition of gabbroic and tonalitic xenoliths sampled on Raoul Island in the Kermadec Island arc, and their relevance for the formation of young island arc crust. We show that the xenoliths

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are closely related to the fractional crystallization processes yielding the volcanic rocks and that they resemble rock series observed in other island arcs like the Izu-Bonin arc and the fossil Talkeetna arc. The mafic rocks represent cumulates containing mainly olivine, clinopyroxene, and plagioclase but we do not find evidence for significant fractionation of amphibole. Thermobarometry of gabbros and tonalites indicates layering of the Kermadec island arc crust that was observed seismically further north in the Tonga arc.

2. Geological setting The Tonga-Kermadec subduction system produced one of the largest young island arcs on Earth with numerous active volcanoes. Seismic data show that the crust of this arc resembles that of the Izu-Bonin arc, with a total thickness of 20 to 25 km, but varies in its deeper structure. Whereas Crawford et al. (2003) found a 7 km-thick layer with seismic velocities around 6 km/s at about 18.5°S, another study performed further south at 24°S did not reveal the existence of such a layer (Contreras-Reyes et al., 2011). In the Izu-Bonin arc it is believed that this layer consists of tonalites, i.e. relatively evolved plutonic rocks above the mafic to ultramafic cumulates of the lower island arc crust (Kodaira et al., 2007; Suyehiro et al., 1996). However, so far little is known about the presence of tonalitic rocks in the Tonga-Kermadec island arc. Raoul Island is one of the few subaerial volcanic systems in the Kermadec Arc front, rising from the Kermadec Ridge at about 29.3°S (Fig. 1). The island has an area of about 29 km2 and is formed by several volcanic structures, including two large calderas that reveal ages from 1.4 Ma to recent (Lloyd and Nathan, 1981; Smith et al., 2006, 2010; Worthington et al., 1999). Although the island consists of several volcanoes all lavas are very homogeneous in terms of incompatible element and radiogenic isotope ratios (Smith et al., 2010). Previous studies found that the lavas on the island range from basaltic to rhyolitic and are either related to fractional crystallization processes (Barker et al., 2013; Haase et al., 2011) or the silicic magmas resulted from

melting of amphibolite-facies crustal rocks (Smith et al., 2006, 2010). Some of the lava formations contain crustal xenoliths of gabbroic to tonalitic composition as well as cumulate blocks containing abundant glass (Brothers and Searle, 1970). Mineralogical and geochemical compositions indicate that the plutonic xenoliths are parts of the magmatic system of the young Raoul Island volcanoes rather than representing older island crust (Brothers and Searle, 1970; Turner et al., 1997). This is supported by the U–Pb age of 1.25 Ma obtained for zircon in one tonalitic xenolith (Mortimer et al., 2010). Xenoliths are abundant in lavas from Raoul Island (Brothers and Searle, 1970) and the specimen in this study were collected from the 1.4 to 0.6 Ma-old submarine Boat Cove Formation (Smith et al., 2010) in the SE part of the island (Fig. 1). 3. Sampling and methods Nine plutonic xenoliths were collected during a brief visit on Raoul Island in 1998 and most were sampled as loose round blocks on the beach of Boat Cove. The xenoliths probably occur within the submarine Boat Cove Formation consisting of andesitic pillow lavas (Fig. 1). Sample 9802 was recovered near the road from Boat Cove to the hostel (km 9) and 9810 is probably from the Hutchison Formation above the submarine lavas. Thin sections of the samples were studied under the microscope and brief petrographic descriptions are presented in Table 1. The major element concentrations of minerals (Table 2) in polished thin sections were measured with a JEOL JXA 8200 Superprobe electron microprobe at the GeoZentrum Nordbayern (Erlangen). The electron microprobe was operated with an accelerating voltage of 15 kV, a beam current of 15 nA and a focused beam with the exception of plagioclase that was measured with a defocused beam of 3 μm in order to minimize Na loss. Counting times were set to 20 s and 10 s for peaks and backgrounds for most elements, and 40 and 20 s for Cl, respectively (Table 2). Fresh cores from samples were cut with a rock saw, washed in deionized H2O, crushed and pulverized in an agate mill. The powders were analyzed for major and trace element compositions using XRF and ICP-MS methods and results are given in Table 3. The loss on ignition (LOI) was determined by weighing the rock powder after drying first for 12 h at 105 °C in a cabinet dryer and then for 12 h at Table 1 Petrographic description of representative plutonic xenoliths form Raoul Island. Sample Description 9802

9810

9813

9814

9815

9816 9817

9818

Fig. 1. Map of Raoul Island redrawn after Brothers and Searle (1970) and its situation in the Kermadec island arc showing the location of the Boat Cove Formation where most of the xenoliths were sampled. The Kermadec arc map was prepared using GeoMapApp.

9825

Gabbro, medium-grained equigranular adcumulate with 50% plagioclase (An95) up to 2 mm, 42% augite (Mg# 74–81) up to 1 mm, 3% Ol (Fo76), 5% tachylitic matrix Gabbro, medium-grained equigranular adcumulate with 70% plagioclase (An75-88) up to 3 mm, 10% augite (Mg# 71–78) up to 1 mm with enstatite (Mg#65) exsolution, 10% amphibole, 5% oxides and 5% chlorite Tonalite, medium-grained, equigranular, with plagioclase to 2 mm, 50% plagioclase, 42% quartz, 6% amphibole, 2% myrmekite and accessory titanite Tonalite, medium-grained, equigranular, 55% plagioclase (An15–37 but dominantly An80–95) up to 2 mm, 32% quartz, 2% magnetite/ilmenite, 1% titanite, 10% secondary epidote and chlorite Gabbro, medium-grained adcumulate with 40% plagioclase (An93–95) up to 2 mm, 35% augite (Mg# 77–79) to 2 mm, and 25% olivine (Fo71–72) up to 3 mm Gabbro, medium-grained, equigranular adcumulate with 65% plagioclase to 2 mm, 5% augite to 1 mm, 10% oxides, 10% amphibole and 10% chlorite Gabbro, medium-grained, equigranular adcumulate with 70% plagioclase (An93–95), 25% augite (Mg# 77–79) both up to 2 mm, 5% Ol (Fo73) and accessory magnetite Gabbro, medium-grained, equigranular adcumulate with 55% plagioclase (An90-97) up to 5 mm, 30% augite (Mg# 73–84) + enstatite (Mg# 72) up to 3 mm, 10% olivine (Fo68) and 5% magnetite Gabbro, rel. coarse-grained adcumulate with 70% plagioclase (An56–69 but dominantly An79–84) up to 10 mm, 20% augite up to 5 mm, 5% chlorite and 5% amphibole

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Table 3 Element concentrations in the Raoul plutonic rocks. Sample

9802

9810

9815

9816

9817

9818

9825

9813

9814

Rock type

Gabbro

Gabbro

Gabbro

Gabbro

Gabbro

Gabbro

Gabbro

Tonalite

Tonalite

49.34 0.32 17.52 6.86 0.13 7.88 15.77 0.97 0.10 0.04

46.90 0.64 20.77 10.12 0.16 5.49 13.33 1.46 0.07 0.02 0.35 99.3 1.08 41.2 441 37.0 29.6 20.1 26.7 42.4 18.0 0.54 194 9.21 6.81 0.13 0.26 0.36 0.03 0.03 29.6 0.78 2.24 0.39 2.20 0.85 0.46 1.16 0.23 1.55 0.35 1.01 0.16 1.05 0.16 0.31 0.086 0.027 0.48 0.08 0.030

44.11 0.12 14.23 10.77 0.17 14.40 14.93 0.28 0.01 0.01

47.68 0.51 20.07 10.23 0.17 5.78 13.50 1.45 0.09

45.12 0.07 24.07 5.91 0.09 8.01 17.04 0.43 0.01 0.01

46.31 0.14 20.34 6.65 0.14 8.59 16.92 0.35 0.01 0.02

78.92 0.28 12.46 0.57 0.01 0.51 3.70 4.20 0.08 0.05

74.61 0.38 12.20 2.56 0.03 0.68 4.18 3.64 0.15 0.05

47.37 0.97 15.49 11.39 0.17 9.58 13.34 1.81 0.03 0.03

100.76 0.61 30.2 75.4 31.5 43.4 41.2 10.6 24.0 12.5 0.14 131 1.57 1.23 0.17 0.03 0.25 0.01 0.00 4.57 0.16 0.37 0.06 0.31 0.13 0.10 0.20 0.04 0.28 0.06 0.17 0.02 0.16 0.02 0.04 0.014 0.021 0.11 0.02 0.013

99.47 0.80 55.0 141 81.2 36.3 29.0 9.7 35.9 11.5 0.06 114 3.37 1.10 0.03 0.04 0.47 0.01 0.00 3.65 0.10 0.28 0.06 0.44 0.24 0.15 0.39 0.08 0.59 0.13 0.36 0.05 0.36 0.05 0.06 0.006 0.019 0.09 0.01 0.006

43.8 0.11 16.36 10.15 0.18 14.05 15.05 0.30 0.01 0.004 0.51 100.52

100.78 0.66 14.1 15.0 2.55 1.50 0.83 2.12 7.19 17.87 0.27 131 39.9 166 0.76 0.08 0.71 0.03 0.02 104 2.09 7.08 1.34 8.02 3.28 0.88 4.51 0.88 6.18 1.37 4.05 0.63 4.26 0.63 0.71 0.067 0.027 0.40 0.72 0.065

98.48 1.25 12.8 34.0 2.87 3.68 1.25 2.78 10.22 15.42 1.59 155 34.6 81 0.63 0.61 0.90 0.08 0.04 81.9 2.59 8.63 1.61 9.29 3.52 0.92 4.59 0.87 5.92 1.28 3.69 0.55 3.69 0.53 0.39 0.053 0.148 0.80 0.37 0.123

100.18 2.48 41.4 331 376 52.8 168 119 72.6 16.0 0.36 109 14.5 14.5 0.66 0.07 0.72 0.49 0.01 6.39 0.64 1.82 0.37 2.25 1.07 0.50 1.72 0.35 2.47 0.55 1.60 0.25 1.58 0.24 0.59 0.053 0.020 3.05 0.03 0.010

SiO2 (wt.%) TiO2 (wt.%) Al2O3 (wt.%) Fe2O3 (wt.%) MnO (wt.%) MgO (wt.%) CaO (wt.%) Na2O (wt.%) K2O (wt.%) P2O5 (wt.%) LOI (wt.%) Total (wt.%) Li (ppm) Sc (ppm) V (ppm) Cr (ppm) Co (ppm) Ni (ppm) Cu (ppm) Zn (ppm) Ga (ppm) Rb (ppm) Sr (ppm) Y (ppm) Zr (ppm) Nb (ppm) Mo (ppm) Sn (ppm) Sb (ppm) Cs (ppm) Ba (ppm) La (ppm) Ce (ppm) Pr (ppm) Nd (ppm) Sm (ppm) Eu (ppm) Gd (ppm) Tb (ppm) Dy (ppm) Ho (ppm) Er (ppm) Tm (ppm) Yb (ppm) Lu (ppm) Hf (ppm) Ta (ppm) W (ppm) Pb (ppm) Th (ppm) U (ppm)

98.93 2.44 59.5 189 74.3 32.8 29.9 25.1 47.0 12.5 1.59 123 9.57 12.5 0.23 0.42 0.41 0.03 0.13 36.5 0.71 2.01 0.35 2.02 0.80 0.33 1.15 0.23 1.58 0.35 0.98 0.15 1.01 0.15 0.44 0.021 0.090 0.68 0.08 0.040

99.03 0.91 52.9 127 77.4 77.9 87.3 10.8 52.2 9.3 0.14 107 2.97 1.20 0.02 0.05 0.29 0.01 0.01 3.73 0.08 0.27 0.06 0.42 0.23 0.13 0.36 0.07 0.53 0.11 0.31 0.04 0.29 0.04 0.06 0.006 0.019 0.13 0.01 0.005

0.38 99.86 2.02 44.0 388 34.8 33.2 23.3 31.8 48.9 17.6 0.66 192 9.18 5.34 0.09 0.20 0.37 0.02 0.04 31.1 0.69 2.03 0.36 2.09 0.84 0.44 1.15 0.23 1.57 0.35 1.01 0.16 1.07 0.16 0.24 0.011 0.023 0.48 0.06 0.027

BIR-1

Concentrations determined by XRF (bold) and ICP-MS.

1030 °C in a muffle furnace. Whole rock powders were mixed with lithium tetraborate and ammonium nitrate, fused to a homogeneous glass bead, and analyzed using a Philips 1400 XRF spectrometer calibrated against international rock standards. Trace elements of the whole rock powders were analyzed using an Agilent 7500c/s Quadrupole Inductively Coupled Plasma Mass Spectrometer (ICP-MS) at the Institute of Geosciences, Universität Kiel, following procedures described previously (Garbe-Schönberg, 1993). Analytical precision was monitored by the repeated analysis of one sample yielding b3% RSD for most elements except Zr, Th, U (b 7%), and Nb, Ta, (b27%). The average of the analytical results for international rock standard BIR-1 is compiled in Table 3. The reproducibility of the digestion procedure as monitored by duplicate sample digests is better than 3% for all elements. Mineral grains were handpicked from crushed sample material and single grains were analyzed in coarse samples whereas several grains were used for the finer grained samples. The δ18O isotope ratios of olivine, plagioclase, and quartz were measured by laser fluorination

Table 4 Oxygen isotope compositions of mineral separates of the Raoul plutonic rocks. Sample

Rock type

Mineral

δ18O (‰SMOW)

9802

Gabbro

9813

Tonalite

Olivine Plagioclase Clinopyroxene Quartz

9814

Tonalite

Quartz

9815

Gabbro

Olivine

9817

Gabbro

9818

Gabbro

5.23 6.12 5.43 6.73 6.91 7.16 7.23 4.95 4.82 5.05 5.88 4.99 5.93 5.00 5.77

Plagioclase Olivine Plagioclase Olivine Plagioclase

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using a 25 W-Synrad CO2-laser and F2 as reagent, at the GeoZentrum Nordbayern, following the method described in Freund et al. (2013). The results are shown in Table 4. During the period of an entire day, four standard samples (UWG-2 (Valley et al., 1995); NBS-30) were processed and measured together with the samples for accuracy. The long-term reproducibility of the UWG-2 garnet standard is 5.85 ± 0.15‰ (1σ, n = 67), which is similar to the value of 5.74 ± 0.15‰ (1σ, n = 1000) given by Valley et al. (1995). The δ18O raw values of a run were adjusted by the mean difference of the reference values of the standards (δ18O 5.8‰ and δ18O 5.1‰, respectively). Reproducibility of replicate samples, which may reflect internal analytical error but also sample heterogeneity and impurity, varies between 0.05‰ 1σ and 0.15‰ 1σ (mean 0.08‰ 1σ), respectively. 4. Results 4.1. Petrography and mineral compositions of the xenoliths The gabbroic xenoliths are generally medium-grained (crystal diameters 1–5 mm) and contain plagioclase, olivine, clinopyroxene, orthopyroxene, and oxides (Table 1). A few of the mafic xenoliths show glass selvedges around the minerals and are less dense than the bulk of the gabbroic samples. Amphibole is rare in the mafic plutonic rocks and was found only in sample 9825. Olivine crystals range from Fo68 to Fo76 (Table 2). Orthopyroxene with Mg#72 occurs as rims surrounding olivine crystals with Fo68 in sample 9818. Glass of andesitic

composition with SiO2 contents of 55.5 to 58.3 wt.% forms patches and selvedges around minerals in xenolith 9818 (Table 2). Interestingly, some of these glass inclusions have very high S contents up to 800 ppm. Clinopyroxene is mainly unzoned but, in sample 9818, both normally (Mg#81–84 in the core towards Mg#78–76 in the rim) and reversely (Mg#68 in the core towards Mg#71–72 in the rim) zoned grains occur. Plagioclase crystals are large, normally zoned and have compositions of An90–97. In terms of the compositions of olivine, plagioclase and clinopyroxene the Raoul Island gabbros resemble the lavas from the island, the gabbroic rocks from St. Vincent in the Antilles, and the lower crustal gabbronorites from the Talkeetna arc (Fig. 2). However, the An contents in plagioclase are significantly higher than for MORB at given Mg# of olivine and clinopyroxene (Fig. 2). The tonalites are also medium-grained (crystal size 1–5 mm) and consist of plagioclase, quartz, hornblende, epidote, chlorite, titanite, Ti-magnetite, and ilmenite. Tonalite 9814 contains two types of plagioclase (Table 2); crystals with compositions of bytownite–anorthite (An80–93) and others with compositions of oligoclase–andesine (An15–49). Oscillatory zoning is observed in some grains. Titanite and ilmenite occur in close association in a texture similar to that described by Harlov et al. (2006) who suggested that titanite was formed by fluidmediated, subsolidus reaction with magmatic ilmenite. Alteration is significantly more important in tonalites than in gabbros, and considerable amounts of secondary epidote and chlorite formed by alteration of plagioclase and hornblende. 4.2. Geochemical composition of the xenoliths

Fig. 2. Average compositions of a) plagioclase and b) clinopyroxene versus those of olivine in the Raoul Island gabbros compared to those of Raoul Island lavas (Barker et al., 2013), gabbros from the East Pacific Rise (Constantin et al., 1996), the fossil Talkeetna arc (Greene et al., 2006), and St. Vincent in the Antilles (Tollan et al., 2012). The higher An contents in plagioclase at a given Mg# in olivine and clinopyroxene probably reflects higher water contents in the magmas.

One interesting observation is that the two types of xenoliths (gabbroic rocks and tonalites) are, respectively, the least and the most differentiated rocks in the area (Fig. 3). SiO2 contents range between 43.8 and 49.3 wt.% in gabbros, 70.4–78.9 wt.% in tonalites, and 48.8–70.0 wt.% in volcanic rocks from Raoul Island. Although relatively rare, more evolved lavas with SiO2 up to 74.8 wt.% are also found in Raoul Island (Barker et al., 2013; Ewart et al., 1998; Smith et al., 2010). Overall, there is a chemical overlap between plutonic xenoliths and lavas but the plutonic rocks lie at the extreme ends of the lava trends (Fig. 3). Compared to lavas, many plutonic xenoliths are poorer in TiO2 and K2O and a continuous increase in these oxides with increasing SiO2 is observed from gabbros to lavas. Gabbros show large variations in Al2O3 (14.2–30.3 wt.%), MgO (3.2–14.4 wt.%), and FeOT (3.1–9.7 wt.%) at relatively constant SiO2 contents whereas in lavas and tonalites both oxides decrease with increasing SiO2. Although representing both the SiO2-poorest and -richest rocks, xenoliths are depleted or present similar contents in most incompatible elements compared to the Raoul Island lavas (Fig. 4). The fossil Talkeetna Arc section in Alaska represents the wellstudied accreted remnants of an about 35 km thick fossil island arc (Clift et al., 2005; DeBari and Sleep, 1991; Greene et al., 2006). The Raoul Island plutonic rocks chemically closely resemble the plutonic rocks from the fossil Talkeetna island arc (Greene et al., 2006) and the Komahashi-Daini Seamount rocks from the Izu-Bonin island arc (Haraguchi et al., 2003), but the Talkeetna rocks are slightly more enriched in TiO2, K2O, and Ba for similar SiO2 contents (Figs. 3, 4). For example, the maximum content of K2O observed in Raoul Island tonalites is 0.42 wt.%, but values up to 1.65 wt.% and 1.57 wt.% are observed in Izu-Bonin and Talkeetna felsic plutonics, respectively. The Raoul Island gabbros have relatively high contents of Ni but all plutonic rocks from Raoul Island are low in Cu (Fig. 4) compared to the lavas whereas some Talkeetna gabbros display very high Ni and Cu concentrations. In terms of incompatible elements the Raoul Island gabbros lie at the end with low contents but overlap with the increasing trend of the lavas (Fig. 4c to f). The Raoul Island tonalites represent the endmember with the highest Yb and Zr contents of the lava trend but they show lower Ce and Ba contents than highly evolved volcanic rocks. The tonalites from Raoul Island are also depleted in some

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Fig. 3. Diagrams of major element compositions versus SiO2 contents for the Raoul Island plutonic rocks compared to Raoul Island lavas (Barker et al., 2013; Haase et al., 2002; Smith et al., 2010; Turner et al., 1997) and rocks from the accreted Talkeetna island arc (Greene et al., 2006) and to tonalites from the Komahashi-Daini Seamount in the northern Kyushu Palau Ridge, i.e. the rifted part of the Izu-Bonin island arc (Haraguchi et al., 2003).

incompatible elements like Ce, Zr, and Ba relative to the arc tonalites from Talkeetna and Izu-Bonin (Fig. 4). Chondrite-normalized REE patterns are relatively flat (Fig. 5) with slightly negative anomalies of Eu in the tonalites and positive Eu anomalies in gabbros. They are depleted in LREE with (Ce/Yb)N ranging between 0.21 and 0.66, resembling other Kermadec island arc rocks and, specifically, the Raoul Island lavas (Fig. 6a). All plutonic as well as the volcanic rocks have (Dy/Yb)N of about 1 and no change with SiO2 is observed (Fig. 6b). Most mafic plutonic xenoliths from Raoul Island have lower Zr/Sm than lavas whereas the tonalites lie at the high end of the Zr/Sm and there appears to be a coarse systematic increase in Zr/Sm with increasing SiO 2 similar to the Talkeetna rocks (Fig. 6c). Only some of the mafic plutonic rocks

from Talkeetna and one Raoul Island gabbro show Zr/Sm higher than 20 at low SiO2 of about 45 wt.%. Oxygen isotope compositions of olivine and plagioclase in the mafic xenoliths are relatively homogeneous, ranging between 4.95 and 5.23‰ in olivine and between 5.77 to 6.12‰ in plagioclase, respectively (Table 4). These compositions are similar to those found in cumulate xenoliths from St Vincent in the Antilles island arc (Fig. 7). Quartz grains from the two tonalites have δ18O of 6.8 and 7.2‰ which is within the range of quartz from plagiogranites in the Oman ophiolite (Grimes et al., 2013) and also in the range of quartz in equilibrium with zircons from the Oman ophiolite plutonic rocks (Fig. 8). Plagioclase from a tonalite xenolith from Raoul Island yielded δ18O of 5.5‰ (Barker et al., 2013) which is roughly similar to the gabbro plagioclase.

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Fig. 4. Variation of trace element compositions of the Raoul Island plutonic and volcanic rocks and those of the Talkeetna island arc and Komahashi-Daini Seamount versus SiO2. Data sources as in Fig. 3.

4.3. Barometry and thermometry on the xenoliths Two-pyroxene barometry (Putirka et al., 2003) suggests crystallization temperatures of 975 to 980 °C and pressures of 5.7 to 6.0 kbar in sample 9818, and 959 °C and 3.9 kbar in sample 9802. High pressures of origin of cumulate sample 9818 are supported by the high S contents of more than 700 ppm in the glass inclusions in the xenolith (Table 2). The δ18O isotope data of plagioclase and olivine in gabbro samples indicate equilibrium temperatures of 1100 to 1200 °C (Fig. 7), which are more likely magmatic temperatures for these rocks. Mortimer et al. (2010) calculated the whole-rock Zr-saturation temperature and the Ti-in-zircon temperature for the 1.25 Ma tonalite found in Matatirohia

Tephra and obtained temperatures of 748 °C and 715 °C ± 30 °C, respectively. These temperatures are consistent with the temperature of unaltered amphibole cores from another tonalite xenolith from the same area which yielded temperatures of 760–795 °C and pressures of 0.65–0.90 kbar (Barker et al., 2013). Amphibole was not analyzed in our tonalite samples but whole-rock Zr-saturation temperatures (Watson and Harrison, 1983) for these range between 827 and 900 °C which is consistent with the δ18O isotope data of quartz in the two studied Raoul Island tonalites if these are in equilibrium with mantle-like melts or zircons (Fig. 8). The low pressures are supported by the significant hydrothermal alteration of these rocks that suggest a relatively shallow emplacement depth.

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Fig. 5. Chondrite-normalized rare earth element patterns for the Raoul Island plutonic rocks using the chondrite values of Anders and Grevesse (1989). Note the positive Eu anomalies in most of the gabbros and the negative Eu anomalies in the tonalite samples.

5. Discussion 5.1. Differentiation processes in the Raoul Island magmatic system The most primitive lava in the Raoul Island magmatic system has MgO equal to 12 wt.% and Mg# of 67.6, whereas the most Mg-rich olivine has Fo78 (Smith et al., 2010). Thus, so far, no primary magma from Raoul Island has been sampled and all samples indicate significant magma evolution. The lavas seem to be related to the same parental magma, lying along well-defined trends reflecting fractional crystallization of olivine, clinopyroxene, plagioclase, and Fe–Ti oxides (Fig. 3). The high An content of most plagioclase in the Raoul Island xenoliths is characteristic of crystallization from hydrous arc magmas (Beard, 1986; Sisson and Grove, 1993). Fractional crystallization is also supported by the increasing concentrations of incompatible elements (Fig. 4) and by the constant (Ce/Yb)N and (Dy/Yb)N (Fig. 6). The constant and low δ18O isotope data in glasses and plagioclase from Raoul Island (Barker et al., 2013; Haase et al., 2011) as well as the low δ18O isotope data in quartz in the tonalites (Fig. 8) also suggest a magma evolution dominated by fractional crystallization rather than by partial melting of crustal rocks yielding silicic melts. The plutonic xenoliths described in this paper probably also represent products of the crystallization processes and our gabbros are cumulates of olivine, plagioclase, clinopyroxene, and orthopyroxene with higher CaO, Al2O3 and MgO but lower SiO2, K2O and TiO2 concentrations than the associated lavas. Crystallization and fractionation of plagioclase and clinopyroxene occurs at MgO contents less than 8 wt.% in the magmas of Raoul Island and leads to decreasing CaO and Al2O3 contents in the residual liquids (Fig. 3). We suggest that the mafic xenoliths described here represent cumulates that formed during stagnation of magma in deeper parts of the magma system and that were accidentally picked up by later magmas. The abrupt changes in An contents suggest significant modification of the magma composition during crystallization possibly due to mixing of different magmas. The accumulation of plagioclase under reducing conditions results in high Al2O3 and the formation of positive Eu anomalies in the gabbroic rocks whereas the negative Eu anomalies in the tonalites probably result from excessive plagioclase fractionation leading to a slight depletion in Eu relative to the other REE. Given the chemical trends in Figs. 3 and 4, crustal assimilation apparently has a limited effect on the chemical evolution of Raoul Island lavas. An assimilation-fractional crystallization (AFC) mechanism for the Kermadec island arc volcanoes has already been proposed by Barker et al. (2013) and is supported, for example, by the occurrence of An-rich plagioclase in tonalite 9814. Some glasses like the selvedges in sample 9818 have relatively high Cl contents as a possible sign of interaction of the magma with hydrothermally altered country rocks but the low δ18O in the rocks suggests little interaction (Haase et al.,

Fig. 6. Incompatible element ratios of a) (Ce/Yb)N, b) (Dy/Yb)N, and c) Zr/Sm versus the SiO2 contents of the Raoul Island plutonic rocks compared to the Raoul Island lavas, the fossil Talkeetna arc rocks and the Komahashi-Daini Seamount rocks. Note that the Raoul Island plutonic rocks closely resemble the lavas in terms of (Ce/Yb)N, (Dy/Yb)N, and Zr/Sm. Data sources as in Fig. 3 and chondrite from Anders and Grevesse (1989).

2011). The stagnation and fractionation of magma occurred in the deep island arc crust at 4 to 6 kbar as indicated by mineral barometry and high S contents in enclosed glasses. The presence of andesitic glass with MgO of 1.2 to 3.8 wt.% suggests that relatively felsic liquids form in these deep layers of the crust and then ascend to form the upper crust. 5.2. The formation of tonalites in the Raoul Island magmatic system The presence of tonalites at Raoul Island and their geochemical resemblance to the lavas in terms of major and REE compositions indicate that these are related to recent magmatism rather than

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Fig. 7. Oxygen isotope compositions of olivine and plagioclase crystals from four gabbroic samples from Raoul Island compared to mineral compositions of plutonic rocks from St. Vincent in the Antilles (Tollan et al., 2012). The fractionation lines for different temperatures of plagioclase and forsterite are according to Chiba et al. (1989). The plagioclase fractionation factor was assumed to be increased by 0.1 (to 1.78 rather than 1.68) to reflect compositions of An90.

representing a much older magmatic phase related to the formation of the Kermadec ridge. This is also supported by the similar Nd and Pb isotope ratios of about 0.51304 and 18.69 in tonalites compared to 0.51303–0.51307 and 18.61–18.71 in the lavas (Barker et al., 2013; Turner et al., 1997). Zircons from one Raoul Island tonalite sample were dated and yield an age of 1.25 Ma (Mortimer et al., 2010) which is similar to the age of the submarine Boat Cove Formation (1.4 to 0.6 Ma) indicating that at least some tonalites are coeval to the earliest stages of volcanic activity in the area. Previous work on felsic rocks from Raoul Island concluded that the SiO2-rich melts were formed either by fractional crystallization (Barker et al., 2013; Haase et al., 2011; Smith et al., 2010) or by partial melting of older island arc crust (Smith et al., 2006). As discussed above, most of the chemical and isotopic trends (Figs. 3 and 4) support extreme fractional crystallization of a basaltic magma in the absence of significant amounts of amphibole similar to

the processes suggested by Brophy (2009) for plagiogranites in the oceanic crust. The formation of the tonalites by partial melting of mafic rocks that were hydrothermally metamorphosed to amphibolite appears unlikely given the low δ18O of 7.1‰ for quartz from tonalite samples (Fig. 8) that suggest a δ18O of about 6.6 to 6.7‰ for the felsic melts assuming a Δ(Quartz-Melt) of 0.4 to 0.5‰ (Bindeman and Valley, 2002). Because crystal fractionation increases the δ18O of magmas a felsic melt formed from mantle-derived basaltic magmas is expected to result in a δ18O of approximately 6.6‰ at 70 wt.% SiO2 (e.g. Bindeman et al., 2004). Consequently, O isotopes in quartz from Raoul Island tonalite reflect fractional crystallization rather than melting of hydrothermally altered crust. However, the tonalites contain very Ca-rich plagioclase crystals that possibly indicate mixing with less evolved magmas and thus some interaction with mafic melts or rocks probably occurred at depth. Incompatible elements like Ce or Zr vary by a factor of 5 to 8 (Fig. 4) suggesting that 80 to 90% of the initial basaltic magma must have crystallized to form the felsic melts. However, the tonalites are depleted in some highly incompatible elements (like K, Ba, and Ce) compared to the lavas implying the fractionation of phases like relatively K-rich feldspar, zircon, and apatite. Indeed we find plagioclase with up to 0.6 wt.% K2O in tonalite 9814 and K2O contents of 0.68 wt.% were found in feldspars in tonalite by Barker et al. (2013). Because the An-poor plagioclase crystals of Raoul Island lavas also contain relatively high contents of Ba and Rb (Barker et al., 2013) fractionation of such a feldspar can explain the decrease of K2O, Rb, and Ba relative to other incompatible elements in the most evolved tonalitic rocks (Figs. 3 and 4). The variation of Zr concentrations and Zr/Sm in both gabbros and tonalites (Figs. 4e and 6c) probably indicates that zircon crystallized and accumulated in some magmas although the mineral was not observed in thin section. The fact that the tonalites have P2O5 contents of about 0.05 wt.% compared to 0.15 wt.% in the most evolved lavas implies that apatite fractionated from the magmas with more than 70 wt.% SiO2. Because apatite prefers the light and middle rare earth elements to the heavy rare earth elements (Prowatke and Klemme, 2006) the fractionation of apatite may have led to lower contents of Ce in the tonalites (Fig. 4c). We propose that extreme fractional crystallization including rare mineral phases like zircon and apatite resulted in the formation of tonalitic magmas with more than 70 wt.% SiO2. 5.3. Fractionation and accumulation of amphibole and sulfides in the lower arc crust?

Fig. 8. Oxygen isotope composition of quartz in tonalites from Raoul Island compared to quartz from plutonic rocks of the Oman ophiolite and to zircons from different ophiolites (Grimes et al., 2013). The dashed lines indicate the composition of quartz crystals in equilibrium with a zircon from mantle-derived magma with δ18O of 5.0‰ at different temperatures of 700 °C, 800 °C, and 1000 °C using the fractionation factor between quartz and zircon of 2.33 from Trail et al. (2009).

Based on geochemical data Davidson et al. (2007) suggested that amphibole may play an important role during magma fractionation at subduction zones, even though this mineral may be absent in the lavas. Silicic magmas generated by dehydration melting of lower to mid-crust amphibolite leads to negative SiO2–REE correlations, with depletion of HREE abundances with increasing SiO2 (Brophy, 2009), which is the opposite to the observed in the Raoul Island lavas (Fig. 4). Moreover, we find no evidence for primary amphibole as a fractionating phase in the cumulates and we observe constant (Ce/Yb)N and (Dy/Yb)N for the whole range of SiO2 concentrations in the magmas (Fig. 6). Additionally, the slightly lower Zr/Sm ratios in gabbros relative to the lavas can be explained by fractionation of augite and magnetite (Thirlwall et al., 1994) and do not require amphibole fractionation. Thus, crystallization and fractionation of amphibole apparently does not play an important role in the Raoul Island magmatic system. Evolved lavas in island arcs are relatively depleted in chalcophile elements like Ni and Cu which is also observed in the Raoul Island magma series (Fig. 4a and b). It was suggested that these elements are removed from magmas by sulfide fractionation from the magmas (Lee et al., 2012). This would lead to a Ni and Cu enrichment in the lower parts of the crust and it would be expected that gabbroic cumulate rocks show higher contents of these elements compared to lavas. Although higher than the lavas in Ni contents the Raoul Island gabbros

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do not indicate any enrichment in Cu, Co and Zn, implying that sulfides are not typically accumulated with olivine, augite and plagioclase. Only magnetite was found in some of the gabbroic rocks. Therefore, our data do not support the idea of a general accumulation of Cu-rich sulfides in the deeper crust at island arcs although some gabbros from the fossil Talkeetna island arc show Cu enrichment (e.g. Greene et al., 2006). 5.4. Similarity of the Kermadec crust to rocks from Izu-Bonin and the Talkeetna profile Whole-rock and mineral chemistry suggest that the gabbroic rocks from Raoul Island are identical to the lower crustal rocks from Talkeetna island arc (DeBari and Greene, 2011; Greene et al., 2006). The formation of these cumulates in the lower crust allowed the generation of more differentiated compositions of melts by continuous fractionation of olivine, clinopyroxene, plagioclase and Fe–Ti oxides. These relatively evolved melts erupted as lavas whereas the cumulates formed the lower crust as is observed in the seismic studies of Tonga-Kermadec arc crust (Crawford et al., 2003). On the other hand, the Raoul Island tonalites and similar rocks from Talkeetna arc present important differences (Figs. 3 and 4). Although more SiO2-rich, the Raoul Island tonalites are characterized by lower K2O, Ba, Nb, Ce, and Yb contents relative to Talkeetna and also the Izu-Bonin felsic plutonics (Figs. 3 and 4). However, the similarity of mineralogical and major element variations of the Raoul Island plutonic and volcanic rocks and rock series from the Izu-Bonin arc and the fossil Talkeetna island arc (Figs. 2 and 3) indicates that the general processes of melt formation and evolution in these settings are very similar. Thus, the ascending magmas apparently form island arc crust with very similar structure and composition. A seismic line at about 18°S across the Tonga island arc revealed a perhaps 20 km thick crust with a layer of seismic velocities of 6.0–7.0 km/s probably representing felsic rocks between about 5 and 12 km depth (Crawford et al., 2003). At depths greater than about 12 km the seismic velocities increased above 7.0 km/s and were interpreted as more mafic rocks (Crawford et al., 2003). Such a stratification is also known from the Izu-Bonin island arc where the felsic rock layer is believed to be largely composed of diorites and tonalites (Suyehiro et al., 1996; Tatsumi et al., 2008). Our pressure estimates for the gabbroic xenoliths from Raoul Island suggest depths of origin of 3.9 to 6.0 kbar, i.e. 12 to 18 km which agrees with the seismic data at 18°S. Thermobarometry (Barker et al., 2013) and the hydrothermal overprinting of the tonalites formed significantly shallower which is also supported by the seismic data. Thus, the petrological and the seismic data for the TongaKermadec island arc appear to indicate a stratified crust similar to that known from the Izu-Bonin arc and the fossil Talkeetna arc (DeBari and Greene, 2011). However, a seismic study in the Tonga island arc at 24°S did not reveal a layer with velocities between 6.0 and 7.0 km/s and the authors concluded that the crust here consists largely of basalt (Contreras-Reyes et al., 2011). This indicates that the Tonga-Kermadec island arc crust varies in its structure and the thick felsic layer may occur only in some regions with large and long-lived magmatic systems like that of Raoul Island. Geochemical and isotopic data from the Talkeetna arc are consistent with crystallization of the entire arc section along a single liquid line of descent from a homogeneous primary melt, leading to the formation of gabbros and melts represented by lavas and tonalites (Greene et al., 2006). Although chemical and isotopic data for rocks from Raoul Island (Barker et al., 2013; Smith et al., 2010; Turner et al., 1997) suggest a similar mantle-derived primary melt for all the lithologies, a single stage crystal fractionation process cannot explain the higher SiO2 contents but lower concentrations of some incompatible elements in tonalites compared to lavas. More importantly, crystal fractionation processes of one magma batch cannot account for the occurrence of cumulate gabbros and extremely evolved melts represented by tonalites as parts of lower and middle to upper crust, respectively, at the surface. The Boat Cove Formation consists of andesitic lavas with

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ages between 1.4 and 0.6 Ma so at least some of the gabbros and tonalites must represent earlier magmatic phases incorporated and transported to the surface by magmas injected during later phases. The incompatible elements indicate a common source for all the magmatic phases and a cumulate origin for the gabbros closely related to the chemical fractionation of lavas. However, the incorporation of lower crustal gabbros in ascending andesitic magmas implies deep magma stagnation and evolution as well as a continuous or episodic injection of fresh batches of primitive magma. This magma replenishment must occur in all island arcs and may explain the large range on the chemistry of cumulates and plutonic felsic rocks and their overlap with the chemistry of lavas observed in the fossil Talkeetna island arc. Jull and Kelemen (2001) found that due to density contrast and relative viscosity pyroxenites can delaminate almost as quickly as they form so that the injection of primitive batches of magmas may be triggered by delamination of dense ultramafic cumulates at the base of the crust. 6. Conclusions Although plutonic xenoliths are rare in island arc volcanoes they yield important insights into the magma systems and the composition and structure of the crust. The mineral compositions in samples from Raoul Island in the Kermadec island arc presented here show a close petrogenetic relationship of the plutonic and volcanic rocks. Similarly, the geochemical and O isotope data suggest that the gabbroic rocks formed from accumulation of minerals of the melts ascending through the island arc crust whereas the tonalites represent products of extreme fractional crystallization. Barometry indicates that the gabbros formed between 12 and 18 km depth in agreement with seismic profiles whereas the tonalites represent shallower intrusions. The temperatures of the magmas decrease from about 1200 °C in the mafic to 800 °C in the shallow felsic rocks. The absence of amphibole in most mafic cumulates implies that this phase is not important in the crystal fractionation processes beneath Raoul Island. The xenolith data support seismic studies indicating that some portions of the Tonga-Kermadec island arc show similar layering of felsic and mafic rocks as the Izu-Bonin and the fossil Talkeetna island arcs. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.lithos.2014.10.005. Acknowledgments We thank Captain Andresen and his crew of RV Sonne as well as Chief Scientist P. Stoffers for making the visit to Raoul Island possible. We gratefully acknowledge the help of C. Beier and S. Freund with the electron microprobe work. Constructive comments by N. Mortimer and an anonymous reviewer as well as by editor N. Eby helped to improve the quality of this work. GeoMapApp was used for production of Fig. 1 which is gratefully acknowledged. The study was facilitated by the Bundesministerium für Bildung und Forschung research grant 03G0135A to P. Stoffers. References Anders, E., Grevesse, N., 1989. Abundances of the elements: meteorites and solar. Geochimica et Cosmochimica Acta 53, 197–214. Arndt, N.T., Goldstein, S.L., 1989. An open boundary between lower continental crust and mantle: its role in crust formation and crustal recycling. Tectonophysics 161, 201–212. Barker, S.J., Wilson, C.J.N., Baker, J.A., Millet, M.-A., Rotella, M.D., Wright, I.C., Wysoczanski, R.J., 2013. Geochemistry and petrogenesis of silicic magmas in the intra-oceanic Kermadec Arc. Journal of Petrology 54, 351–391. Beard, J.S., 1986. Characteristic mineralogy of arc-related cumulate gabbros: implications for the tectonic setting of gabbroic plutons and for andesite genesis. Geology 14, 848–851. Bindeman, I.N., Valley, J.W., 2002. Oxygen isotope study of the Long Valley magma system, California: isotope thermometry and convection in large silicic magma bodies. Contributions to Mineralogy and Petrology 144, 185–205.

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