Petrogenesis of Lavas along the Solomon Island Arc

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VOLUME 50

NUMBER 5

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2009

doi:10.1093/petrology/egp019

Petrogenesis of Lavas along the Solomon Island Arc, SW Pacific: Coupling of Compositional Variations and Subduction Zone Geometry STEPHAN SCHUTH1,2*, CARSTEN MU«NKER1,2, STEPHAN KO«NIG1,2, CROMWELL QOPOTO3, STANLEY BASI3, DIETER GARBE-SCHO«NBERG4 AND CHRIS BALLHAUS1 1

STEINMANN-INSTITUT, RHEINISCHE FRIEDRICH-WILHELMS-UNIVERSITA«T BONN, D-53115 BONN, GERMANY INSTITUT FU«R MINERALOGIE, WESTFA«LISCHE WILHELMS-UNIVERSITA«T MU«NSTER, D-48149 MU«NSTER, GERMANY

3

DEPARTMENT OF MINES AND ENERGY, GEOLOGICAL SURVEY DIVISION, HONIARA, SOLOMON ISLANDS

4

INSTITUT FU«R GEOWISSENSCHAFTEN, CHRISTIAN-ALBRECHTS-UNIVERSITA«T ZU KIEL, D-24118 KIEL, GERMANY

RECEIVED JANUARY 4, 2008; ACCEPTED MARCH 23, 2009

The Solomon island arc, SW Pacific, is of particular interest for understanding subduction zone volcanism, as magmatism in the active part of the arc is dominated by mafic melts, thus permitting direct insights into mantle processes. Along the Solomon island arc, the Indian^Australian plate is subducting at present beneath the Pacific plate. However, until at least c. 12 Myr ago, the Pacific plate was subducting beneath the Indian^Australian plate until the Cretaceous Ontong Java Plateau collided with the northern Solomon island arc. To evaluate the effects of the changes in tectonic regime on lava compositions, we present a comprehensive Sr^Nd^ Hf^Pb isotope, major element and trace element dataset, covering lavas erupted along the entire island arc (c. 1000 km). Basalts and andesites represent the most abundant rock types. Picrites and ankaramites occur in the New Georgia Group of the Solomon Islands, where they erupted above the subducting Woodlark spreading center, and also in the Santa Cruz archipelago, north of Vanuatu, where the Rennell Fracture Zone is subducting. Recent work has also identified the presence of adakites (Sr/Y up to c. 200), and high-Mg andesites (MgO45 wt %, Sr/Y c. 11^46). Most of the high-Mg andesites are genetically linked to the adakites, but some of the high-Mg andesites show affinities to boninitic compositions. Large ion lithophile element abundances in most Solomon island arc magmas indicate a strong source overprint by subduction components. 87Sr/86Sr and eNd values along the arc range from 0
7029 to 0
7052 and from þ5
8 to þ8
3, respectively. The Sr^Nd values partially overlap the compositions of oceanic basalts from the Indian^Australian plate.

*Corresponding author. Telephone: þ49 228 73 5180. Fax: þ49 228 73 2763. E-mail: [email protected]

Measured eHf values range from þ10
5 to þ14
6. If corrected for contributions from subduction components, combined eHf^eNd systematics also indicate that most of the studied Solomon arc lavas were generated within the Indian-type mantle domain. However, a few samples display eHf^eNd compositions resembling those of the Pacific-type mantle domain. These samples either originate from older Pacific basement (basalts) or represent melts derived from subducted Pacific crust (adakites). Lead isotope compositions, controlled by subduction components, can be used to identify the presence of two distinct types of subduction components that originate (1) from the Pacific plate including Ontong Java Plateau material (46 Myr old) and (2) from the more recently subducted Indian^ Australian plate. Combined Hf^Nd^Pb isotope data also reveal that lower parts of the Ontong Java Plateau entered the mantle wedge, as previously postulated by geophysical models.

KEY WORDS:

adakite; mantle; slab; Solomon Islands; subduction

I N T RO D U C T I O N It is generally accepted that the sources for present-day arc volcanism are located within the metasomatized mantle wedge above a subducting plate. Of particular interest for the understanding of subduction zone magmatism are the mechanisms of mass transport between the subducting

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Islands, thus adding an important new locality to the classic adakite assemblages found in the Central Aleutians (e.g. Kay, 1978), the southern Andes (Stern & Kilian, 1996), and in Costa Rica (e.g. Abratis & Wo«rner, 2001). Depending on rates of ascent, adakitic magmas may preserve the isotope and trace element characteristics of subducted oceanic crust. Slower rates of ascent and small volumes permit reaction of adakitic magmas with peridotite, frequently resulting in relatively high Mg-numbers and disequilibrium textures (e.g. Kelemen, 1995; Yogodzinski et al., 1995; Ko«nig et al., 2007; Sprung et al., 2007). Likewise, mixing with more mafic magmas may result in the generation of Mg-rich andesites and in dilution of the pristine adakitic signatures (e.g. Kelemen, 1995; Yogodzinski et al., 1995; Ko«nig et al.,2007). In the Solomon Islands, such incompatible trace element enriched arc basalts and high-Mg andesites are abundant (e.g. Dunkley,1986; Johnson et al.,1987; Ko«nig et al., 2007). The high-Mg andesites display primitive Mgnumber and elevated Cr and Ni contents. In many cases, adakitic magmas will react entirely with wall-rock peridotite before reaching the surface, thus refertilizing the mantle wedge (e.g. Yaxley & Green, 1998). However, if the conditions for remelting of such refertilized mantle domains are met (e.g. by decompression or heat supply), the melts generated are basaltic in composition but still inherit their trace element and isotope inventory from the ephemeral adakitic melt. Many basalts of the Solomon Islands display relatively low Zr^Nb ratios for arc magmas, suggesting the presence of slab melts in their mantle sources. In this study, we assess the influence of the different subduction components on the petrogenesis of arc lavas in the Solomon Islands. Components originating from the Pacific plate, Indian^Australian plate and possibly subducted fragments of the Ontong Java Plateau are discriminated using isotope and trace element variations in the arc lavas. This approach follows that of an earlier study by Ko«nig et al. (2007) where enigmatic high-Mg andesites on the island of Simbo (located on the subducting Indian^ Australian plate; Yoneshima et al., 2005) were studied. The presence of a fossil fragment of the old subducted Pacific plate beneath the Solomon arc was postulated by Ko«nig et al. (2007). To put young arc volcanism in the Solomon Islands in a broader geodynamic context, we sampled the c. 1000 km long southern island chain of the Solomon island arc. Earlier studies by Cox & Bell (1972), Ramsay et al. (1984), Shimizu et al. (1992), Schuth et al. (2004), Rohrbach et al. (2005), Kamenetsky et al. (2006), and Parkinson et al. (2007) dealt with geochemical and petrological aspects of basalt and picrite petrogenesis in the New Georgia Group. Here, we present a comprehensive major element, trace element and Sr^Nd^Hf^Pb isotope dataset for the complete length of the Solomon island arc. On the basis of the data, a petrogenetic model is developed and

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slab and the mantle wedge. Fluids released by dehydration of subducted oceanic crust and/or sediments overprint the mantle wedge and trigger partial melting of the peridotite (e.g. Ringwood, 1974; Gill, 1981; Nichols et al., 1994). Melting of oceanic crust is widely assumed to be restricted to young or torn oceanic plates (e.g. Kay, 1978; Defant & Drummond, 1990; Peacock et al., 1994; Stern & Kilian, 1996; Abratis & Wo«rner, 2001; Yogodzinski et al., 2001), although more recent geophysical modelling and geochemical observations suggest alternatives to this view (e.g. Kelemen et al., 2003; Macpherson et al., 2006). Recent experimental work has also shown that there is increasing miscibility between slab fluids and slab melts with depth (e.g. Bureau & Keppler, 1999; Kessel et al., 2005). Adakites are regarded as melts that directly originate from subducted oceanic crust and constitute the melt-like endmembers of known subduction components. The effects of subduction components (slab fluids and slab melts) and a variably depleted and hydrated mantle wedge on magma compositions in island arcs are complex and depend on the tectonic framework along the island arc and the geometry of the subducting plate. These parameters can explain the wide compositional variations of arc-related volcanic rock suites worldwide. The Solomon island arc (and the neighbouring Vanuatu arc) in the SW Pacific is located in a tectonically complex region marked by two major plate tectonic events. (1) A reversal in subduction polarity at least c. 12 Myr ago halted subduction of the Pacific plate and triggered the present subduction of the Indian^Australian plate (e.g. Petterson et al., 1999). The cause of this reversal was the docking of the Ontong Java Plateau with the island arc and possibly subduction of Ontong Java Plateau fragments (e.g. Mann & Taira, 2004). (2) Subduction of very young oceanic crust and a mid-ocean ridge system (Woodlark Ridge) beneath the western and central part of the island arc has taken place since c. 4^5 Myr ago (e.g. Weissel et al., 1982). The subduction of the Woodlark Ridge has caused an elevated thermal gradient in the mantle wedge. By analogy to similar tectonic settings in other parts of the world (e.g. Yogodzinski et al., 1995; Peate et al., 1997; Abratis & Wo«rner, 2001), boninitic and/or adakitic lavas would therefore be expected in the Solomon Islands. Boninites occur in, but are not restricted to, various SW Pacific subduction zones such as the Bonin Islands (Petersen, 1891), Cape Vogel (e.g. Dallwitz et al., 1966), or the New Hebrides (Monzier et al., 1993). They are associated with partial melting of a hydrous, refractory mantle wedge at shallow depths of less than 50 km (e.g. Crawford et al., 1989). The unusual hot and possibly longterm depleted mantle wedge beneath the western part of the Solomon island arc would be a suitable setting to generate boninitic melts. A new occurrence of both boninitic and adakitic lavas is reported here for the Solomon

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the petrogenetic significance of the tectonic configuration beneath the arc is evaluated.

GEOLOGICA L A N D G EO P H YS IC A L F R A M E WO R K General tectonic setting

Seismic and volcanic activity As seismic activity is widespread throughout the Solomon island arc, the coupling of magmatism and active tectonic

Fig. 1. Simplified tectonic map of the SW Pacific area (modified after Coleman & Packham, 1976). The inset shows a simplified overview of the SW Pacific area. Within the Indian^Australian plate, different tectonic elements (Woodlark Basin, Pocklington Trough, Santa Cruz Basin, and Rennell Fracture Zone) of different ages are at present being subducted beneath the Solomon island arc. The dashed line in the inset indicates the inactive Vitiaz Trench system. NBT, New Britain Trench; PR, Pocklington Ridge; RT, Rennell Trough; RFZ, Rennell Fracture Zone.

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The Solomon island arc consists of two parallel NW^SEtrending island chains that mark part of the collision zone between the Indian^Australian and the Pacific plates (Fig. 1; Coleman, 1966). This collision zone has probably been active since Eocene times and is characterized by a reversal of subduction polarity during the Neogene (e.g. Petterson et al., 1999; Hall, 2002; Mann & Taira, 2004; Schellart et al., 2006). Before the reversal, the Pacific plate subducted beneath the Indian^Australian plate at the Vitiaz trench and, as a consequence, the northern, older Solomon island chain was formed. After the collision of the c. 30 km thick Cretaceous Ontong Java Plateau with the northern Solomon island chain (e.g. Hussong et al., 1979; Petterson et al., 1999), further subduction of the Pacific plate came to a halt. Because of continuing convergent plate movements, the Indian^Australian plate started to subduct beneath the Pacific plate c. 6 Myr ago, followed by island arc volcanism and formation of the southern island chain. However, geophysical models (Mann &

Taira, 2004; Miura et al., 2004; Taira et al., 2004) postulate continuing subduction of the lower parts of the Ontong Java Plateau beneath the island arc along a thrust detachment. The tectonic setting of the Solomon island arc and its continuation to the east (Vanuatu island arc) that was also affected by the reversal (Peate et al., 1997) is thus highly suited to elucidate the response of arc volcanism to a reversal of subduction polarity. The study area sampled here covers the Solomon island arc over a length of c. 1000 km and comprises (from west to east) the Shortland Islands (Fauro, Shortland), the New Georgia Islands (Vella Lavella, Ghizo, Kolombangara, Kohinggo, New Georgia, Rendova, Vangunu, Nggatokae), the Russell Islands (Mborokua, Pavuvu, Mbanika), Savo, Guadalcanal (eastern Gallego Volcanic Field only), Makira, and the Santa Cruz archipelago (Tinakula, Santa Cruz, Utupua, Vanikoro). For clarity, the Solomon island arc is subdivided below into three provinces: the ‘Western Province’ comprises all islands of the Shortland and the New Georgia Island groups, the ‘Central Province’ includes the Russell Islands, Savo, Guadalcanal, and Makira, and the ‘Eastern Province’ is represented by the Santa Cruz archipelago.

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1982; Ridgway, 1987; Petterson & Biliki, 1994). Absolute age data, however, are still very scarce.

Tectonic elements of the subducting plate A specific tectonic feature of the Solomon island arc is the along-arc subduction of distinct oceanic basins and troughs (Fig. 1). These include the young Woodlark Basin (subducted beneath the New Georgia Group), the Pocklington Trough (subducted beneath Guadalcanal and Savo), and the Santa Cruz Basin that is subducted beneath the Santa Cruz archipelago (Fig. 1). New oceanic crust has been generated in the Woodlark Basin since c. 4^5 Ma (e.g. Weissel et al., 1982; Taylor et al., 1995). Therefore, young and relatively hot crust is subducted at present beneath the western and central parts of the island arc. A thin sediment layer covers the Woodlark Basin in its central and eastern area (e.g. Weissel et al., 1982). The sediments consist of nanofossils and volcanic detritus derived from the rapidly uplifting New Georgia Group islands (see Colwell & Exon, 1988). The Pocklington Trough at present marks the boundary between the Woodlark Basin and the Louisiade Rise (Coleman & Packham, 1976). Presumably, the trough contains volcanic sediments that are erosion products from a remnant, possibly Paleogene island arc (Karig, 1972; Schellart et al., 2006). The sediments also comprise eroded material originating from the arc segment between New Georgia and western Guadalcanal. In the eastern portion of the island arc, the Rennell Fracture Zone is assumed to have formed as a mid-ocean ridge system in the late Cretaceous (Schellart et al., 2006). It crosses the Santa Cruz Basin from SW to NE and is currently subducted west of Tinakula volcano. The Santa Cruz Basin is covered by several hundred meters of possibly volcanogenic sediments that are most probably derived from the Rennell Island Ridge during the late Paleogene (Coleman & Packham, 1976).

S A M P L E S A N D A N A LY T I C A L M ET HODS Sample suite We collected a set of 256 samples from most islands of the southern island chain during two field campaigns in 2001 and 2004 to acquire a representative sample suite covering most volcanic fields. This suite also includes samples previously described by Schuth et al. (2004), Rohrbach et al. (2005), and Ko«nig et al. (2007); all these samples originate from the New Georgia Group. In most cases, samples were taken from river or beach detritus with known source catchments. In situ outcrops are rare because of intense tropical weathering. Samples from the Shortland Islands, Makira, and Santa Cruz are variably altered, these rocks are most probably as old as Paleocene (e.g. Turner & Ridgway, 1982). Details of the sample

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processes can be examined directly. A detailed compilation of across-arc seismic profiles has been provided by Denham (1969) and Mann et al. (1998). A brief overview is given here. In the westernmost portion of the arc, the subducted part of the Solomon microplate has reached a depth of c. 200 km; the Pacific plate shows little seismic activity. Beneath the New Georgia Group (c. 100 km to the east), the Indian^Australian plate is located at depths of less than 90 km and subducts at a steeper angle. In addition, the topography of the New Georgia Group is marked by uplifted Pleistocene coral reefs. This indicates fast uplift rates during the past 50^100 kyr that are caused by the collision of the Coleman seamount with the fore-arc (Mann et al., 1998; Taylor et al., 2005). To the SE of the New Georgia Group, in the centre of the island arc, the subduction angle flattens, resulting in a maximum depth of the Indian^Australian plate of c. 50 km beneath Makira. Here, the Pacific plate has been subducted to a depth of at least 150 km and shows stronger seismic activity than in the westernmost part of the island arc (Mann et al., 1998). In the easternmost part of the arc (Santa Cruz archipelago), the seismicity is comparable with that of the western part, with deep subduction of the Indian^ Australian plate and little seismic activity within the Pacific plate (Denham, 1969). The continuing subduction of the Indian^Australian plate has triggered active volcanism at various localities along the island arc (e.g. Kavachi, Mt. Cook, Savo, and Tinakula volcanoes). Most parts of the southern island chain are of volcanic origin and formed during the last 6 Myr (see, e.g. Coleman et al., 1969; Thompson et al., 1975, 1976; Hackman et al., 1977; Danitofea et al., 1980; Dunkley, 1986; Abraham et al., 1987). The last subaerial volcanic eruption in the Solomon Islands was witnessed in 2002 by local residents at Tinakula volcano. The submarine volcano Kavachi (New Georgia Group) erupted in March 2004. Savo, an active, possibly hazardous volcano is located only c. 30 km away from the capital Honiara. The latest recorded eruption on Savo took place c. 150 years ago. The presence of hot springs, fumaroles and steam eruptions indicates continuing volcanic activity (Petterson et al., 2003; Smith et al., 2006). Hot springs also occur on the islands of Vella Lavella and Simbo, and solfatares and sulfur deposits are visible on Simbo (Dunkley, 1986; Ko«nig et al., 2007). No active volcanism has been reported so far for the area east of Guadalcanal and west of Tinakula. Volcanic edifices are sometimes well preserved; for example, the presumably extinct Kolombangara volcano and the active Tinakula stratovolcano. In some cases they are deeply eroded, such as Mt. Mase (NW New Georgia), and the islands of Utupua, Vanikoro, and Pavuvu. Older, possibly Paleogene volcanic rocks occur in the Shortland Group (NW Solomons), on Guadalcanal, Makira, and on Santa Cruz (see references above and Turner & Ridgway,

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localities and the full dataset are given as an Electronic Appendix, available for downloading at http://www.petro logy.oxfordjournals.org/.

Analytical procedures

P E T RO G R A P H Y All the studied samples are volcanic in origin, with the exception of three that are shallow intrusive igneous rocks. Picritic and basaltic samples have a porphyric texture with occasionally zoned clinopyroxene and olivine phenocrysts in a microcrystalline, partially glassy matrix. Clinopyroxene-rich rocks (425 vol.%) are classified as ankaramites. Olivine crystals are mostly55 mm in diameter and display iddingsite rims in some cases. Olivines showing kink bands and ‘dusty’ regions are abundant in the New Georgia picrites (Rohrbach et al., 2005; Kamenetsky et al., 2006) and were also found in picrites from Utupua (kink bands in sample S 199 Utu). Rohrbach et al. (2005) interpreted these olivines as mantle xenocrysts. Clinopyroxenes of up to 1cm in diameter sometimes occur in glomeroporphyric clusters. Zoning is common and Ti-augite compositions are sometimes present. Orthopyroxene is generally rare and limited to high-Mg andesites; it is occasionally present as reaction rims around olivine (sample S 143 NG and in some high-Mg andesites from Simbo; see Ko«nig et al., 2007). More differentiated samples often contain green and/or brown hornblende, sometimes of sizes up to c. 1cm. Plagioclase is zoned and mostly restricted to the matrix; crystal sizes over 2 mm are rare and typically occur only in differentiated, andesitic and dacitic samples. Altered samples exhibit devitrification and partial sericitization of plagioclase, and cracks are filled with quartz, carbonate and/or epidote. Accessory phases present in the samples

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Prior to grinding, weathering rims were carefully removed; the samples were then crushed in a steel jawbreaker and ground in an agate mill. A subset of 79 representative samples was prepared for thin-section inspection and X-ray fluorescence (XRF) analyses. Whole-rock major element compositions were determined by XRF using a Philips PW-1480 spectrometer at Universita«t Bonn, Germany. The Fe2þ contents were determined titrimetrically (see Heinrichs & Hermann, 1990); the external reproducibility was better than 5%. A further subset of 48 samples was also analysed for their trace element compositions by inductively coupled plasma^quadrupole mass spectrometry (quadrupole ICP-MS), using an Agilent 7500cs at Universita«t Kiel, Germany. The external reproducibility typically ranges around 5% for most elements of interest. Analytical procedures follow those of GarbeScho«nberg (1993). Representative major and trace element and isotope data are given in Table 1. The complete dataset is provided as an Electronic Appendix. Measured trace element concentrations for the BHVO-1 standard mostly agree to within 10% of the literature data (Govindaraju, 1994). Analyses of the in-house standard S E 3 (a picrite with 12
2 wt % MgO from central New Georgia) repeated over a time span of several years support the quoted external reproducibility (Table 2). Whole-rock Sr^Nd^Hf isotope compositions were determined for 62 representative samples. Analyses of Sr^Nd^Hf isotope compositions were carried out on one split of c. 150 mg rock powder. No age correction was applied because of the young age of the rocks (56 Ma). Hafnium separation followed the procedures described by Mu«nker et al. (2001) and Weyer et al. (2002). Strontium and Nd were separated from the matrix left over from the Hf separation step with conventional cation and HDEHP-based ion exchange procedures (e.g. Richard et al., 1976). Lead isotope compositions were determined on a subset of 40 samples. Hand-picked chips were washed with H2O in an ultrasonic bath, and then leached in warm 3M HCl and 6M HCl for 1h each. We employed a HCl^HBr column chemistry using BioRadÕ AG1-X8 anion resin for Pb separation (see, e.g. Korkisch & Hazan, 1965). The procedure was repeated for every sample to ensure a clean Pb fraction. The Pb yield was always higher than 95%. Hafnium was analyzed by multi-collector ICP-mass spectrometry (MC-ICP-MS) using the Micromass IsoProbe system at Universita«t Mu«nster, Germany. All Hf isotope ratios are given relative to a 176Hf/177Hf value of 0
282160 for the JMC-475 standard at a typical long-term external reproducibility of c. 50 ppm. Strontium and Nd were analyzed by thermal ionization mass spectrometry

(TIMS) with a Thermo-Finnigan Triton MC-TIMS system at Universita«t Mu«nster operated in static mode. The long-term external reproducibility is c. 40 ppm for Sr and 30 ppm for Nd. The isotope ratios were corrected for mass fractionation using the exponential law and 179 Hf/177Hf ¼ 0
7325, 86Sr/88Sr ¼ 0
1194, and 146Nd/144Nd ¼ 0
7219 for normalization. Repeated analyses of the standards NBS 987 and La Jolla gave mean values of 0
710260 (n ¼18) and 0
511852 (n ¼17), respectively. All eNd and eHf values are given relative to CHUR values reported by Wasserburg et al. (1981) and Blichert-Toft & Albare'de (1997), respectively. Lead isotope compositions were determined by MC-TIMS on either a VG Sector 54 or a Thermo-Finnigan Triton system in static mode at Universita«t Mu«nster. An external correction for mass fractionation correction was applied based on repeated analyses of the standard NBS 982 that were normalized to the values given byTodt et al. (1996). The external reproducibility was c. 0
045% per a.m.u. (2). Procedural blanks were 522 pg for Hf (n ¼ 6), 535 pg for Sr (n ¼ 5), 570 pg for Nd (n ¼ 4), and 5370 pg for Pb (n ¼ 5). All blanks were negligible relative to the element concentrations in the sample splits.

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Table 1: Representative major element, trace element and Sr^Nd^Hf^Pb isotope data Sample: S 4 Island: Ghizo Rock type: Basalt Subtype:

S 20 NG, Central Picrite

S 23 NG, Central Picrite

SN 2 Nggatokae Bas. andesite

S 88 A þ Fau Fauro Bas. andesite HMA

S 90 Fau Fauro Dacite Adakite

S 95 Sho Shortland Boninite

S 98 Fau Fauro Andesite Adakite

S 100 VL Vella Lavella Andesite

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wt % 50
6 47
9 47
8 52
6 52
1 66
0 54
6 61
3 59
3 SiO2 0
62 0
57 0
69 0
79 0
97 0
24 0
43 0
42 0
59 TiO2 16
4 10
0 11
8 18
1 14
8 16
6 14
6 16
7 17
1 Al2O3 FeO 4
46 6
52 5
83 5
59 6
20 1
70 4
37 2
14 2
74 4
83 4
40 3
49 4
04 6
74 0
92 3
84 1
67 3
15 Fe2O3 MnO 0
19 0
24 0
16 0
22 0
29 0
08 0
26 0
10 0
13 MgO 5
77 17
0 15
5 4
70 5
08 1
29 7
82 1
68 2
82 CaO 10
2 9
07 10
4 9
58 5
57 4
23 6
04 4
99 6
59 2
17 0
91 0
76 0
96 1
10 0
79 0
12 1
73 1
71 K2O 2
77 1
70 1
85 2
65 2
80 4
51 4
84 4
30 3
47 Na2O 0
36 0
19 0
22 0
16 0
07 0
10 0
03 0
14 0
21 P2O5 LOI 1
45 1
20 1
18 0
49 3
10 2
69 1
75 3
28 0
73 Total 99
8 99
8 99
7 99
9 99
6 99
5 99
3 98
9 99
0 ppm Sc 29
3 43
1 41
4 30
5 36
7 2
73 38
9 5
67 16
0 V 281 318 204 308 360 36
5 310 102 169 Cr 81 1278 775 14 9
34 29
8 361 5
89 74
2 27
6 7
08 15
3 21
0 4
19 Co 27 109 50 23 Ni 33 894 444 18 19
7 24
3 95
7 4
73 16
9 Cu 49
5 108 15
7 2
95 6
67 3
56 51
7 Zn 94 102 70 96 121 43
5 108 45
7 71
8 Ga 18 14
6 18 21 17
7 18
9 14
1 20
0 18
9 Rb 33
2 17
1 11
5 15
5 17
7 3
68 1
99 7
69 16
7 Sr 887 538 454 397 213 695 99
1 766 517 4
70 13
4 8
64 21
0 Y 14
8 19
5 14
5 20
8 19
6 Zr 41
1 36
4 42
2 47
8 37
6 67
1 8
08 60
7 105 Nb 1
47 1
28 1
09 1
02 0
840 4
12 0
283 5
18 2
57 Mo 1
45 0
507 0
501 1
42 0
224 0
559 0
104 0
161 0
767 Sn 0
360 0
448 0
295 0
325 0
492 0
533 Sb 0
037 0
020 0
026 0
036 0
766 0
070 0
359 0
043 0
060 0
118 0
126 0
192 0
080 0
182 0
288 Cs 0
091 0
109 0
053 Ba 259 101 68
6 143 99
6 294 7
68 754 198 La 10
3 4
63 4
34 4
80 2
24 4
98 1
62 9
04 8
15 Ce 20
3 8
83 10
5 10
5 6
35 9
47 4
18 13
9 14
6 Pr 2
68 1
34 1
64 1
54 1
06 1
31 0
683 2
37 2
54 Nd 11
7 6
49 7
81 7
46 5
53 5
27 3
55 9
53 11
2 2
29 1
91 1
04 1
23 1
97 2
66 2
18 Sm 2
87 1
76 Eu 0
931 0
597 0
751 0
797 0
689 0
260 0
483 0
379 0
889 Gd 2
87 2
06 2
33 2
82 2
63 0
974 1
71 1
84 3
07 Tb 0
452 0
330 0
373 0
501 0
480 0
139 0
314 0
259 0
486 Dy 2
80 2
08 2
27 3
41 3
31 0
795 2
17 1
49 3
15 0
475 0
295 0
679 0
158 Ho 0
577 0
441 0
453 0
746 0
702 Er 1
64 1
23 1
24 2
19 2
02 0
454 1
40 0
848 2
00 Tm 0
241 0
180 0
180 0
325 0
303 0
072 0
214 0
128 0
309 Yb 1
61 1
19 1
19 2
20 2
05 0
510 1
46 0
904 2
16 Lu 0
244 0
178 0
175 0
340 0
293 0
080 0
218 0
138 0
347 Hf 1
40 0
826 1
04 1
56 1
38 1
94 0
462 1
84 2
44 0
074 0
068 0
055 0
061 0
055 0
204 0
020 0
275 0
144 Ta Tl 0
020 0
048 0
032 0
270 0
078 0
010 0
126 0
051 Pb 4
57 1
54 1
20 1
25 2
89 3
87 0
594 4
33 3
27 Th 1
43 0
268 0
364 0
622 0
155 0
807 0
128 1
28 0
931 U 0
575 0
112 0
142 0
253 0
094 0
527 0
067 0
661 0
394 87 Sr/86Sr 0
703851  13 0
703642  13 0
703458  10 0
703901  15 0
703776  14 0
703770  9 0
705221  17 0
703892  13 0
703596  14 143 Nd/144Nd 0
512977  12 0
512991  9 0
513021  9 0
513033  6 0
513062  15 0
512968  14 0
513125  14 0
513027  13 0
512983  13 176 Hf/177Hf 0
283171  9 0
283161  12 0
283135  10 0
283150  9 0
283137  10 0
283069  10 0
283157  7 0
283086  9 0
283151  8 206 Pb/204Pb 18
56 18
62 18
50 18
46 18
58 18
51 18
57 18
46 207 Pb/204Pb 15
55 15
53 15
52 15
52 15
54 15
48 15
52 15
53 208 Pb/204Pb 38
41 38
35 38
23 38
30 38
38 38
23 38
35 38
23

SCHUTH et al.

SOLOMON ISLANDS ARC LAVAS

Table 1: Continued Sample: S 137 Ren Island: Rendova Rock type: Basalt Subtype:

S 142 NG NG, NW Bas. andesite HMA

S 143 NG NG, NW Bas. andesite Adakite

S 152 Kol Kolombangara Andesite Adakite

S 160 Mba Mbanika Bas. andesite HMA

S 163 Mak Makira Dacite Adakite

SV 78 Savo Bas. andesite

S 171 Sav Savo Basalt

S 176 Sav Savo Dacite Adakite

(continued) 787

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wt % 49
6 52
4 55
5 62
2 51
8 69
3 53
5 50
0 64
2 SiO2 0
57 0
68 0
65 0
50 0
59 0
35 0
77 0
64 0
30 TiO2 15
1 15
1 17
7 18
3 13
9 14
8 17
9 14
0 17
8 Al2O3 FeO 4
85 6
01 3
76 1
41 5
97 1
02 2
34 3
88 1
20 3
90 2
43 3
46 2
76 3
34 0
89 5
83 5
12 1
51 Fe2O3 MnO 0
15 0
19 0
14 0
13 0
17 0
03 0
15 0
16 0
07 MgO 9
33 8
88 4
25 1
19 8
21 1
73 3
62 7
80 1
10 CaO 11
4 8
43 7
94 5
87 11
2 3
62 8
12 11
5 2
99 1
28 0
88 1
06 1
37 0
68 2
43 1
34 1
24 2
21 K2O 2
00 2
81 3
53 4
31 2
18 3
85 3
68 2
65 6
27 Na2O 0
22 0
20 0
24 0
18 0
11 0
10 0
26 0
12 0
13 P2O5 LOI 0
12 0
78 0
19 0
33 0
38 0
63 1
41 0
92 0
61 Total 99
2 99
6 98
9 98
8 99
3 99
0 99
2 98
6 98
8 ppm Sc 38
7 26
8 20
6 4
53 49
1 5
17 34
8 41
4 2
13 V 290 219 209 80
3 280 49
1 277 301 74
3 Cr 412 319 66
1 6
11 323 38
9 14 327 11
4 37
0 7
06 24 38
9 4
72 22
9 6
87 Co 38
4 54
4 Ni 173 287 38
8 4
97 89
2 22
9 19 63
8 6
64 Cu 135 66
2 86
2 26
0 140 4
95 97
4 111 11
8 Zn 70
1 78
4 82
8 65
0 71
9 24
6 91 76
8 42
6 Ga 16
0 11
0 20
9 21
1 16
1 21
5 17 14
7 23
2 Rb 13
8 16
4 20
4 6
85 11
6 7
29 21
2 17
9 9
53 555 618 664 596 424 773 660 837 1145 Sr Y 12
8 18
1 17
9 14
6 14
3 4
21 23
1 13
7 5
71 Zr 37
0 68
2 79
6 150 39
5 156 68
0 47
7 120 Nb 0
658 2
44 2
36 6
84 0
688 1
07 1
50 0
862 3
69 Mo 0
618 0
713 1
46 0
286 0
668 0
104 0
461 0
991 0
386 Sn 0
435 0
545 0
709 0
648 0
436 0
657 0
496 0
426 0
037 0
029 0
256 Sb 0
033 0
046 0
062 0
094 0
032 0
053 Cs 0
122 0
169 0
160 0
164 0
151 0
084 0
195 0
293 0
831 Ba 184 138 159 171 92
8 107 277 369 833 La 6
31 7
89 10
7 10
5 4
08 10
7 8
83 9
69 6
91 Ce 13
9 16
7 22
2 19
8 9
60 19
3 18
5 20
0 12
2 Pr 2
01 2
69 3
23 3
09 1
45 3
71 2
93 3
02 1
87 7
26 Nd 9
21 11
5 13
8 12
6 6
95 15
5 13
3 12
6 2
36 3
02 3
17 2
63 1
92 2
93 3
23 2
96 1
33 Sm Eu 0
776 1
04 0
981 0
833 0
663 0
894 0
856 0
997 0
118 Gd 2
48 3
44 3
10 2
57 2
21 2
15 3
37 2
98 1
15 Tb 0
377 0
512 0
480 0
388 0
374 0
228 0
532 0
439 0
162 Dy 2
28 2
74 2
81 2
36 2
46 0
941 3
26 2
35 0
949 0
564 0
480 0
513 0
148 0
669 0
553 0
195 0
665 Ho 0
456 Er 1
28 1
98 1
61 1
41 1
47 0
383 1
89 1
64 0
575 Tm 0
189 0
280 0
237 0
223 0
222 0
048 0
275 0
230 0
090 Yb 1
29 1
93 1
61 1
59 1
52 0
322 1
81 1
59 0
661 Lu 0
195 0
309 0
241 0
251 0
232 0
047 0
271 0
250 0
104 Hf 1
10 1
86 1
84 3
37 3
87 1
79 1
45 2
95 1
25 Ta 0
035 0
158 0
115 0
373 0
039 0
066 0
065 0
044 0
250 Tl 0
061 0
047 0
050 0
086 0
032 0
104 0
079 0
062 0
201 Pb 3
72 2
25 2
23 4
11 2
08 4
97 4
21 5
17 12
7 Th 0
875 0
805 0
713 1
44 0
567 1
64 1
07 2
13 2
00 0
794 0
591 U 0
421 0
307 0
333 0
578 0
199 0
744 0
451 87 Sr/86Sr 0
703950  10 0
703682  10 0
703589  8 0
703845  11 0
703767  13 0
702887  10 0
704098  13 0
704046  14 0
704157  16 143 Nd/144Nd 0
513001  12 0
513001  8 0
513053  12 0
512983  13 0
512962  16 0
513088  15 0
513006  6 0
512924  16 0
512927  17 176 Hf/177Hf 0
283144  8 0
283109  6 0
283116  9 0
283171  11 0
283156  10 0
283139  9 0
283144  9 0
283127  11 0
283115  8 206 Pb/204Pb 18
47 18
50 18
44 18
42 18
62 18
47 18
49 18
47 207 Pb/204Pb 15
53 15
49 15
50 15
51 15
49 15
54 15
53 15
53 208 Pb/204Pb 38
25 38
21 38
15 38
23 38
11 38
35 38
35 38
31

JOURNAL OF PETROLOGY

VOLUME 50

NUMBER 5

MAY 2009

Table 1: Continued Sample: Island: Rock type: Subtype:

S 185 Gua Guadalcanal Andesite Adakite

S 187 Van Vanikoro Basalt

S 194 Van Vanikoro Bas. andesite HMA

S 200 Utu Utupua Gabbro

S 204 Utu Utupua Picrite

S 207 Tin Tinakula Basalt

S 215 Tin Tinakula Basalt

S 217 Tin Santa Cruz Bas. andesite

S 220 SC Santa Cruz Bas. andesite

Rock type classification after recalculating volatile-free to 100% total. XRF trace element data are shown in italics. Major element data for samples S 20 and S 23 were reported previously by Schuth et al. (2004). XRF Total assumes all iron as Fe2O3. Variations for Sr–Nd–Hf isotope data apply to the last digit/s (2). LOI, Loss On Ignition. NG, New Georgia. 788

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wt % 61
5 49
0 51
7 49
8 47
6 49
2 50
6 52
1 51
8 SiO2 0
37 0
72 0
57 0
73 0
67 1
35 1
30 0
91 1
12 TiO2 17
8 18
1 16
1 19
8 11
3 19
2 16
4 17
9 16
9 Al2O3 FeO 1
14 4
23 5
13 3
92 6
31 6
16 6
11 4
73 6
73 2
97 4
67 4
03 4
45 4
03 3
76 3
58 3
91 4
65 Fe2O3 MnO 0
07 0
18 0
17 0
16 0
19 0
18 0
17 0
14 0
18 MgO 2
24 4
43 6
74 3
30 13
4 4
52 6
36 4
43 3
60 CaO 5
58 11
6 10
7 9
52 11
4 10
7 10
5 7
54 8
51 1
33 0
70 0
63 1
33 1
13 0
50 0
60 0
38 1
14 K2O 5
07 2
25 2
00 3
25 1
52 2
91 2
79 4
28 3
19 Na2O 0
12 0
16 0
10 0
23 0
21 0
19 0
20 0
10 0
18 P2O5 LOI 0
35 2
15 0
19 1
78 0
73 0
41 0
12 2
39 0
40 Total 98
8 98
8 98
7 98
8 99
3 99
1 99
2 99
5 99
3 ppm Sc 9
54 29
7 51
2 19
3 50
6 28
8 33
2 30
7 33
9 V 130 349 329 307 284 357 301 301 430 Cr 18
4 75
7 180 9
60 782 36
4 138 27
1 7
15 28
9 31
2 22
8 28
2 62
2 Co 11
6 25
4 29
3 24
1 Ni 11
6 41
3 54
5 13
1 281 22
6 47
0 23
9 17
9 Cu 50
9 197 152 219 125 78
7 106 109 371 Zn 51
4 81
3 70
2 78
8 69
8 84
0 79
7 74
0 101 Ga 21
2 19
5 16
6 22
1 13
5 21
9 18
9 20
4 21
4 Rb 10
2 8
30 8
37 21
4 22
8 6
39 8
17 8
64 20
0 Sr 656 477 356 734 493 383 331 394 397 27
0 20
1 25
0 Y 6
46 17
5 21
1 17
1 12
8 25
2 Zr 84
6 44
9 31
7 62
2 48
2 78
1 108 74
9 88
1 Nb 1
76 0
792 0
689 1
20 0
982 2
37 3
16 1
17 2
19 Mo 0
269 0
400 0
958 0
963 0
384 0
521 0
639 0
310 0
999 Sn 0
405 0
442 0
360 0
479 0
486 0
724 0
829 0
560 0
731 Sb 0
051 0
045 0
059 0
035 0
027 0
027 0
030 0
025 0
047 0
049 0
348 0
186 0
127 0
042 0
121 0
496 0
075 Cs 0
265 Ba 420 103 116 248 171 81
6 102 90
9 193 La 4
34 4
40 3
57 7
30 6
33 5
21 6
39 4
83 8
86 Ce 6
72 10
8 7
42 17
7 15
5 13
9 16
4 12
6 21
7 Pr 1
37 1
70 1
20 2
62 2
36 2
22 2
50 1
95 3
17 Nd 6
13 8
27 6
02 12
2 11
0 11
1 12
2 9
40 14
6 1
81 3
10 2
75 3
27 3
50 2
68 3
80 2
38 Sm 1
39 Eu 0
337 0
852 0
673 1
01 0
883 1
23 1
24 0
959 1
20 Gd 1
32 2
76 2
45 3
21 2
78 3
95 4
20 3
14 4
16 Tb 0
188 0
466 0
423 0
496 0
412 0
677 0
723 0
526 0
678 Dy 1
09 3
02 2
88 3
03 2
45 4
44 4
72 3
46 4
37 0
928 0
985 0
722 0
910 0
483 Ho 0
221 0
630 0
635 0
610 Er 0
624 1
81 1
85 1
71 1
33 2
62 2
80 2
10 2
61 Tm 0
094 0
272 0
276 0
253 0
192 0
392 0
418 0
317 0
396 Yb 0
657 1
84 1
86 1
71 1
29 2
62 2
84 2
17 2
71 Lu 0
102 0
276 0
291 0
257 0
192 0
391 0
423 0
329 0
411 2
40 1
90 Hf 2
05 1
33 1
034 1
74 1
33 2
01 2
57 Ta 0
168 0
045 0
039 0
063 0
054 0
129 0
177 0
062 0
119 Tl 0
075 0
053 0
047 0
046 0
028 0
008 0
024 0
025 0
058 Pb 5
44 4
03 3
12 3
35 2
82 0
968 1
77 1
63 3
51 Th 0
566 0
424 0
375 0
833 0
711 0
488 0
587 0
549 1
27 U 0
286 0
367 0
240 0
520 0
373 0
241 0
277 0
193 0
442 87 Sr/86Sr 0
704007  13 0
703626  14 0
703719  14 0
703625  12 0
703304  11 0
702989  13 0
703116  13 0
704534  14 0
703716  12 143 Nd/144Nd 0
512963  12 0
513039  14 0
513047  13 0
513050  12 0
513037  14 0
513049  13 0
513058  15 0
512992  13 0
512966  14 176 Hf/177Hf 0
283127  10 0
283159  10 0
283188  11 0
283141  10 0
283171  10 0
283157  11 0
283128  8 0
283129  8 0
283115  11 206 Pb/204Pb 18
42 18
66 18
62 18
73 18
67 18
61 18
61 18
52 18
58 207 Pb/204Pb 15
50 15
52 15
53 15
56 15
53 15
50 15
50 15
49 15
52 208 Pb/204Pb 38
21 38
31 38
34 38
46 38
32 38
15 38
15 38
13 38
27

SCHUTH et al.

SOLOMON ISLANDS ARC LAVAS

Table 2: Replicate analyses of the in-house standard S E 3 over a time span of several years

Year:

S E 3

S E3

S E 3a

SE 3b

S E 3

%

2001

2002

2003

2003

2005

RSD

ppm Sc

23
6

n.d.

33
5

32
3

27
1

V

n.d.

n.d.

198

193

181

Cr

n.d.

n.d.

530

528

485

Co

n.d.

n.d.

Ni

n.d.

n.d.

Cu

n.d.

n.d.

50
9 336

50
0 329

45
2 338

14 3
7 4
0 5
1 1
2

40
9

40
6

36
9

4
6

Zn

n.d.

n.d.

61
0

60
8

57
4

2
7

Ga

n.d.

n.d.

15
8

15
6

13
9

5
8

11
9

10
7

10
9

Rb

Y Zr Nb

534 8
94 25
6 0
610

Mo

n.d.

Sn

n.d.

Sb Cs Ba

0
011 0
025 63
0

La

4
11

Ce

9
35

Pr

1
40

Nd Sm

11
5 527 9
96 28
2

532

523

10
9

10
5

30
2

29
6

521 9
61 27
6

4
6 1
0 6
8 5
7

0
662

0
719

0
709

0
708

6
0

0
462

0
446

0
438

0
435

2
4

0
328

0
330

0
335

0
014

0
014

0
016

18

0
052

32

n.d. 0
019 0
062 67
2 4
48

0
045 65
5

0
031 62
3

59
8

0
8

4
1

4
34

4
16

4
14

9
90

9
49

9
57

3
0

1
51

1
50

1
44

1
44

2
9

6
68

7
24

6
86

6
73

6
82

2
9

1
81

1
93

1
82

1
76

1
80

3
1

Eu

0
653

0
697

0
649

0
637

0
677

3
2

Gd

1
84

2
03

1
87

1
81

1
91

4
1

Tb

0
283

0
311

0
282

0
273

0
291

4
5

Dy

1
73

1
83

1
66

1
61

1
74

4
2

Ho

0
346

0
367

0
323

0
317

0
344

5
3

Er

0
962

1
01

0
893

0
844

0
949

6
1

Tm

0
137

0
145

0
126

0
122

0
138

6
2

Yb

0
915

0
949

0
859

0
821

0
926

5
3

Lu

0
133

0
144

0
125

0
120

0
139

6
5

Hf

0
838

0
894

0
750

0
750

0
761

Ta

0
037

0
044

0
035

0
034

0
043

Tl

0
013

0
017

0
015

0
014

0
016

9
6

Pb

1
25

1
35

1
24

1
24

1
28

3
1

10
1

3
3

7
2 10

Th

0
369

0
400

0
352

0
335

0
347

6
2

U

0
138

0
151

0
133

0
130

0
134

5
4

Most results agree to within around 5% RSD; only Sc, Sb, Cs, Tl, and Ta show larger deviations. The sample S E 3 is a picrite from central New Georgia with 12
2 wt % MgO. The complete dataset for this sample has been given by Schuth et al. (2004). n.d., not determined.

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Sr

10
5

JOURNAL OF PETROLOGY

VOLUME 50

G E O C H E M I S T RY Classification The studied samples were classified following the IUGS classification scheme for volcanic rocks after Le Bas (2000). Classification on the basis of K2O vs SiO2 compositions into low-, medium- and high-K series and the basalt^andesite^dacite^rhyolite (BADR) classification on the basis of MgO vs SiO2 content are shown in Fig. 2. Several sample groups were further subdivided based on their chemical composition. Overall, the compositions range from picrites and basalts to rhyolites. Basalts and andesites are the dominant rock types. Picrites and ankaramites are confined to the New Georgia Group and additional outcrops on the islands of Utupua (Eastern Province) and Mborokua (Central Province). As there is only an imprecise chemical definition of ankaramite based on CaO^Al2O3 variation, only samples with CaO/Al2O340
95 and with clinopyroxene as the dominant phenocryst phase (e.g. Green et al., 2004) were classified as ankaramites. Although some picritic samples also exhibit high CaO/Al2O3 values of up to c. 1
2, they were not classified as ankaramites because of their high olivine phenocryst abundances. Magnesium-rich andesites, also including strongly altered boninitic samples, were sampled at various locations along the entire island arc. As no IUGS classification is so far available for non-boninitic andesites with elevated MgO contents, all andesitic rocks with 452 wt % SiO2 and 45 wt % MgO (volatile-free) are classified as highMg andesites unless they meet the IUGS criteria for boninites (SiO2452 wt %, MgO48 wt %, and TiO2 50
5 wt %; Le Bas, 2000). ‘Adakitic’ andesites and dacites occur in the Western and the Central Provinces of the arc. We have adopted the classification scheme proposed by Defant & Drummond

MAY 2009

(1990) for adakitic rocks, including all samples with 456 wt % SiO2,43
5 wt % Na2O, 415 wt % Al2O3 and 520 ppm Y. Because of the presence of garnet in the sources of adakitic lavas, high Sr/Yand LaN/YbN are typically observed (Martin, 1986; Defant & Drummond, 1990).

Major element variations All major element concentrations plotted have been recalculated on a volatile-free basis. Most samples have typical medium-K compositions (Fig. 2; Gill, 1981), but some lowand high-K rocks were also sampled. The MgO contents of the samples span a range from 1
12 to 29
8 wt %, with Mg-number ranging from 49 to 91 (see also Ramsay et al., 1984; Schuth et al., 2004; Rohrbach et al., 2005; Ko«nig et al., 2007). The concentrations of Ni (up to c. 1400 ppm) and Cr (up to c. 2300 ppm) in the picritic samples (MgO412 wt %) correlate with MgO, and suggest the presence of excess olivine and chromite (Figs 2 and 3). The ultramafic mixing end-member is inferred to be mantle peridotite (see Schuth et al., 2004; Rohrbach et al., 2005). The MgO content of the primary picritic melt was calculated to be c. 13^14 wt % (Rohrbach et al., 2005). Fractional crystallization controls the MgO^Ni^Cr abundances in lavas with MgO513 wt % (Fig. 3). In Harker variation diagrams (Fig. 3), the samples follow typical calc-alkaline fractionation trends with suppressed plagioclase fractionation. The CaO^Al2O3 ratios cover a range from 0
17 to 1
2 with the ankaramites (two samples) close to a ratio of unity. As explained above, a more detailed subdivision was possible for some andesitic and dacitic samples. Thirteen of these samples were classified as highMg andesites with primitive Mg-number of c. 69^76. Three of these samples (S 95 Sho, S 142 NG, S 160 Mba) exhibit boninitic affinities with MgO contents of about 8 wt % and low TiO2 contents, but only S 95 Sho is a type 2 low-Ca boninite following the Crawford et al. (1989) classification. Moreover, the two other high-Mg andesites with boninitic affinities contain plagioclase phenocrysts, a non-typical feature of boninites (Crawford et al., 1989). Possibly, the MgO content in these two highMg andesites has been elevated by olivine assimilation. Eight samples were classified as adakites following the classification of Defant & Drummond (1990). Notably, most picritic and basaltic samples of the New Georgia Group also show unusually high Sr/Y of 35^55 (Fig. 3), probably calling for adakitic components in their sources.

Trace elements Normalized (to normal mid-ocean ridge basalt; N-MORB) trace element and rare earth element (REE) diagrams (Fig. 4) illustrate the typical enrichment of all the studied lavas in mobile elements [large ion lithophile elements (LILE) and light REE (LREE) with LaN/YbN of up to c. 22] as well as typical depletion of the high field strength elements (HFSE) Nb, Ta, Zr, and Hf. Within each of the

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include magnetite, Cr-spinel, and apatite. Partially resorbed quartz crystals were found in one sample (S 90 Fau). Vesicles are rare and mostly small, and sometimes are filled with zeolites and/or carbonate. Some samples from Rendova, Vella and Savo (samples S 100 VL, S 139 Ren, S 176 Sav) contain xenoliths several centimeters in diameter. These xenoliths typically have sharp rims and mainly comprise brown hornblende, plagioclase and clinopyroxene. Shallow intrusive rocks were sampled on the islands of Fauro (samples S 90 Fau, S 98 Fau) and Utupua (sample S 200 Utu). The Fauro samples are andesitic to dacitic in composition with plagioclase, green hornblende, quartz, and secondary chlorite. They were intruded into a basement of altered basaltic andesite. The coarse-grained intrusive rock from Utupua is a gabbro and has a holocrystalline texture. Plagioclase is most abundant, followed by clinopyroxene.

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Fig. 2. Compositional classification of the sample suite after Gill (1981) on the basis of the K2O and SiO2 content (a), and basalt^andesite^ dacite^rhyolite (BADR) classification via MgO vs SiO2 (b; after Arculus et al., 1992; Le Bas, 2000). The unusually K-rich sample (S E 15) is an absarokite from Rendova in the New Georgia Group, and as it is petrographically fresh, secondary alteration (e.g. by seawater) can be ruled out as a process to increase the K content. A wide range in composition is visible in MgO vs SiO2 space as a result of mixing and fractional crystallization (see text).

three geographical provinces, the multi-element patterns are essentially similar (see Schuth et al., 2004; Ko«nig et al., 2007; Fig. 4), suggesting a cogenetic evolution. Because there is no continental basement in the region, contamination

with continental crust during magma ascent can be excluded. Interaction of the ascending magma with the basaltic basement of the island arc, however, may have occurred as mafic xenoliths are observed in some samples.

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Fig. 3. Harker variation diagrams for various major and trace elements. Olivine and clinopyroxene are the major fractionating phases whereas plagioclase fractionation is suppressed (no coupled decrease of Al2O3 with MgO). Open circles (Western Province) include data for mostly Mg-rich picrites from Schuth et al. (2004). The picritic rocks are largely the result of peridotite admixture to primitive basaltic-picritic melts (for details, see Schuth et al., 2004; Rohrbach et al., 2005). A pronounced depletion of Yand Ti in some low-MgO rocks that deviate from the typical fractionation trend should be noted. This probably reflects the presence of residual garnet and the fractionation of magnetite.

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Fig. 4. Normalized trace element patterns for the three provinces. Virtually all samples exhibit typical relative Nb^Ta depletions and LILE enrichments. Largely parallel trends indicate a co-genetic evolution. For clarity, most Western Province data are shown as a grey field, including the picrite data of Schuth et al. (2004). Patterns marked with filled circles depict adakitic samples. Black lines without symbols indicate highMg andesites. Samples with rare compositions (boninite, back-arc basalt, arc basement, picrite) are marked separately. Other lavas are shown as grey lines. Normalization to N-MORB after Hofmann (1988) and to CI chondrite after Boynton (1984).

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Sr^Nd^Hf^Pb isotope compositions Sixty-two representative samples were analyzed for their Sr^Nd^Hf isotope compositions. Strontium isotope compositions of the samples range from 0
7029 to 0
7052. eNd values range from þ5
8 to þ8
3. As shown in Fig. 5, these values partially overlap data reported for the Ontong Java Plateau (OJP), Indian MORB, and volcaniclastic sediments of the North Loyalty Basin (northern Vanuatu island arc; see Peate et al., 1997) in Sr^Nd isotope space (Hofmann, 1997). No Sr^Nd isotope data are available for the volcanogenic sediments in the Woodlark Basin, Pocklington Trough, and Santa Cruz Basin, so we used the data reported for the North Loyalty Basin by Peate et al. (1997) as a proxy. Notably, most of the Solomon Island data overlap the Sr^Nd isotope compositions of Woodlark Basin lavas (Staudigel et al., 1987). Two strongly altered samples (S 95 Sho, S 217 SC) exhibit relatively more radiogenic 87Sr/86Sr (0
7052 and 0
7045, respectively), indicating alteration. eHf values range from þ10
5 to þ14
6, thus most samples plot within the field of the Indian mantle domain in eHf^eNd space as illustrated in Fig. 5 (see Kempton et al., 2002). Only a small number of samples plot close to the discrimination line or, outside analytical uncertainty, within the field of the Pacific mantle domain. In 208Pb/204Pb^206Pb/204Pb space, the data plot in both the fields for Pacific and Indian MORB (Fig. 6). To a lesser extent, this is also visible in 207Pb/204Pb vs 206Pb/204Pb space. No sample overlaps the Pb isotopic composition of pelagic sediments. However, some samples

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overlap the Pb isotope compositions of Ontong Java Plateau rocks (Tejada et al., 2002, 2004) and local volcanogenic sediments (Peate et al., 1997).

DISCUSSION Origin and role of subduction components beneath the Solomon island arc Because a reversal of subduction polarity occurred in the Solomon island arc c. 12 Myr ago (Petterson et al., 1999), the sub-arc at least mantle has been overprinted by different types of subduction component, possibly originating from both the subducted Pacific and the currently subducting Indian^Australian plate. Other possible components include volcaniclastic or pelagic sediments, or Ontong Java Plateau material as suggested from geophysical observations (e.g. Mann & Taira, 2004; Miura et al., 2004). As substantial amounts of volcanogenic sediments occur in the Woodlark Basin, the Pocklington Trough and the Santa Cruz Basin (Karig, 1972; Coleman & Packham, 1976; Colwell & Exon, 1988), their impact on magma compositions will be evaluated below. Of the isotope systems analysed, Sr and Pb are best suited to discriminate between these components, as both elements are highly fluid-mobile and their budget in arc lavas is virtually entirely controlled by subduction components (e.g. McCulloch & Gamble, 1991; Miller et al., 1994; Chauvel et al., 1995; Kessel et al., 2005). In particular Pb isotope ratios allow to discriminate between subducted Pacific and/or Indian^Australian oceanic crust with relatively unradiogenic Sr and Pb isotope ratios on one side and pelagic sediments with radiogenic 87Sr/86Sr and 207Pb/204Pb on the other (e.g. White & Dupre¤, 1986; Peate et al., 1997). In contrast, Hf and Nd are less mobile, thus permitting reconstruction of mantle wedge compositions once possible contributions by subduction components are evaluated (e.g. Pearce & Peate, 1995; Pearce et al., 1999; Woodhead et al., 2001; Tollstrup & Gill, 2005).

Subducted sediments Strontium and 207Pb/204Pb isotope compositions of most Solomon arc lavas are relatively unradiogenic. The low Sr and Pb isotope ratios clearly indicate a negligible influence of subducted pelagic sediments on the trace element inventory of the magmas (see Figs 5 and 6), also in accord with W, Mo, and Ce/Pb systematics (Ko«nig et al., 2008). Subducted sediments particularly cause an increase of W abundances in arc magmas. So far, this effect has not been observed in samples from the Solomon Islands (Ko«nig et al., 2008). In Sr^Nd isotope space, the samples largely overlap values typical of oceanic basalts from the Indian mantle domain and the Woodlark Basin lavas. Lead isotope compositions overlap those of basalts from both the Indian and Pacific domains. These findings are in good agreement with regional sedimentation patterns,

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Nevertheless, the mantle wedge and subducted material largely control the trace element compositions of the magmas. Some samples deviate from the overall sub-parallel trace element patterns. The adakites are characterized by a relative depletion of the medium REE (MREE) and heavy REE (HREE) and in some cases by distinct negative Eu anomalies (Eu/Eu 0
29^0
61). In the Western Province, both the boninite (S 95 Sho) and a high-Mg andesite (S 88 A þ Fau) from the Shortland Group show a weak depletion of LREE relative to other REE (LaN/YbN c. 0
7). The high-Mg andesite (S 88 A þ Fau) is also depleted in HREE. Likewise, an altered Mg-rich basalt (S 3) from a basal conglomerate on Ghizo only displays a very weak enrichment of LILE compared with the general trend. The conglomerate is possibly an erosion product of the back-arc sequence that makes up the sub-arc basement. In the Central Province, basalts from Makira display MORB-like REE patterns (LaN/YbN c. 0
9^1
9); this finding is in agreement with an earlier assumption of Petterson et al. (1999), who suggested MORB-type basement cropping out at Makira. Some samples display significant negative Ce anomalies of up to 0
5 for Ce/Ce (e.g. S 84 Ghi), but these do not correlate with the degree of alteration and are not confined to a certain type of volcanic rock.

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Fig. 5. Sr^Nd^Hf isotope compositions of Solomon Island arc lavas in comparison with compositions of different groups of oceanic basalts and igneous rocks from the Ontong Java Plateau. (a) Sr^Nd isotope compositions of lavas from the Solomon island arc compared with data for Woodlark Basin basalts (Staudigel et al., 1987), pelagic sediments (Hofmann, 1997), the Ontong Java Plateau (OJP, Tejada et al., 2004), and Pacific- and Indian-type MORB (Hofmann, 1997). The dashed line marks the field for volcaniclastic sediments in the North Loyalty Basin close to the Vanuatu island arc (Peate et al., 1997). (b) eHf^eNd values of the lavas in comparison with Indian MORB, Pacific MORB, and OJP data (Kempton et al., 2002; Tejada et al., 2004); discrimination line after Pearce et al. (1999). (See text for discussion.) Symbols are as in Fig. 3.

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Fig. 6. Comparison of (a) 208Pb/204Pb vs 206Pb/204Pb and (b) 207Pb/204Pb vs 206Pb/204Pb for lavas from the Solomon Islands with rocks from the Ontong Java Plateau (Tejada et al., 2004), pelagic sediments, and Indian- and Pacific-type MORB (Peate et al., 1997; Kempton et al. 2002). The dashed line marks the field for volcaniclastic sediments in the North Loyalty Basin (labeled NLB; see Peate et al., 1997). Discrimination line between Indian and Pacific MORB in (a) after Kempton et al. (2002). Symbols are as in Fig. 3.

as all basins and troughs within the subducting Indian^ Australian plate largely contain volcanogenic detritus that is derived from the active island arc (Karig, 1972; Coleman & Packham, 1976; Colwell & Exon, 1988).

Hence, the Sr^Pb isotope compositions of these subducting sediments should be somewhat similar to their source rocks, thus having little impact on the original Sr and Pb compositions of the subarc mantle. As no isotope data for

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volcanogenic sediments in the Woodlark and Santa Cruz basins are available, data for predominantly volcanogenic sediments of the North Loyalty Basin (located west of the northern Vanuatu arc; see Peate et al., 1997) were used as a close representative (Figs 5 and 6). As illustrated in Figs 5 and 6, the compositions of many samples indeed overlap the field of North Loyalty Basin sediments. Hf^Nd isotope systematics shown in Fig. 5 and in Fig. 7 as
eNdP/I vs
Nd values after Pearce et al. (1999) help to further constrain the influence of both subducted volcanogenic and pelagic sediments, ocean island basalt (OIB) material, and Pacific crust. The
eNdP/I value specifies the offset of a sample from the discrimination line between the Pacific and Indian mantle domain in Hf^Nd isotope space (after Pearce et al., 1999, as shown in Fig. 5b). The offset is caused by subducted material, assuming that Nd is more mobile in a subduction zone setting than Hf. The value of
Nd describes the mobility of Nd relative to Hf in an extended REE pattern (see inset in Fig. 7). The Hf^ Nd relationships in Fig. 7 are best explained by addition of subducted volcanogenic sediment, and to a lesser extent, also Pacific crust, again consistent with the

predominance of volcanogenic detritus in the sedimentary basins of the Indian^Australian plate (e.g. the North Loyalty Basin, Peate et al., 1997). In agreement with Sr^Pb isotope and W data (Ko«nig et al., 2008), Hf^Nd isotope systematics also argue against the influence of subducted pelagic sediments on magma compositions. The addition of subducted pelagic sediments to the mantle wedge would have caused a much larger increase in
eNdP/I with
Nd than observed. This is additionally supported by low Th/ Yb (51
9) and high Sr/Nd (up to c. 100) in the Solomon arc lavas, typical for subduction zone regimes dominated by fluids from subducted oceanic crust [not shown; see Woodhead et al. (1998) and Schuth et al. (2004) for the New Georgia Group].

Components from the Woodlark Basin and the Pacific plate Lead isotope data also permit the discrimination between Indian and Pacific oceanic crust (Fig. 6; see also Peate et al., 1997; Kempton et al., 2002). An Indian-type Pb composition of the currently subducting Indian^Australian plate should be expected from the plate teconic

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Fig. 7.
eNdP/I vs
Nd projection after Pearce et al. (1999, 2007). The
Nd values indicate the mobility of Nd relative to that of Hf in subduction components (see inset). Selective Nd enrichment is expressed by positive values. The
eNdP/I parameter represents the distance of a measured eNd^eHf value from the Pacific^Indian discrimination line in Fig. 5b. Positive values indicate an Indian-type signature and negative values a Pacific-type signature. Vectors indicate compositional trends caused by different types of subduction components. Most samples follow a typical vector for admixture of subducted volcanogenic sediments (see text for discussion). Adakitic samples are not shown because Hf and Nd are both similarly compatible in slab-derived melts. The grey horizontal line represents the discrimination line between the two mantle domains as illustrated in Fig. 5b. Symbols are as in Fig. 3.

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configuration and the composition of neighbouring arcs (e.g. Nebel et al., 2007). As evident from Simbo volcano (located on the edge of the subducting Woodlark Basin), the mantle beneath the Indian^Australian plate may locally be overprinted by components originating from the Pacific plate (Ko«nig et al., 2007). Likewise, Pacific-type Pb isotope compositions are reported for Vanuatu arc lavas SE of the Solomon Islands (Peate et al., 1997). A multiple overprint of the mantle wedge by subducted material is also evident from Fig. 8, where LILE enrichment (K/ La) is shown versus distance from the active South Solomon trench system. In Fig. 8, no systematic change in LILE concentrations with distance from the South Solomon Trench can be observed. If the flux of subduction components was entirely controlled by components from the Indian^Australian plate, a more pronounced trend as described for the Kuriles island arc by Ryan et al. (1995) should be visible. The lack of a pronounced trend indicates that the mantle wedge was already locally modified prior to subduction of the Indian^Australian plate, possibly by older subduction components originating from subducted Pacific crust. Hence, the bulk composition of subduction components originating from the subducting Woodlark Basin remains ambiguous. However, Sr^Nd isotope data for Woodlark Basin rocks were reported by Staudigel et al. (1987), Trull et al. (1990), and Dril et al. (1997), ranging

from c. 0
7027 to 0
7052 and from þ5
5 to þ9 epsilon units, respectively. This compositional range overlaps values reported for the Indian^Australian domain and the Solomon island arc (Fig. 5), supporting the assumption that the crust of the Woodlark Basin derives from the Indian-type mantle domain. For the Santa Cruz Basin, no isotope data are available, but its close proximity to the New Hebrides Basin makes a similar composition likely. As is illustrated in Fig. 5, the Sr^Nd^Hf isotope data for the Solomon arc are rather diverse, possibly reflecting local variations in mantle wedge composition and subduction components. Based on Pb isotope compositions both Pacific-type and Indian-type subduction components are present (Fig. 6). The presence of the Indian component can reflect (1) subducted Woodlark Basin crust or (2) Indian-type mantle that has not been overprinted by Pacific-type subduction fluids so far. The lavas with an Indian-type Pb isotope signature are restricted to an area extending from Ghizo (Western Province) to Savo and the Gallego Volcanic Field on Guadalcanal (all in the western part of the Central Province). Interestingly, this region is partially underlain by the subducted Woodlark Ridge, the center of the young and therefore still relatively warm Woodlark Basin (Fig. 1; Weissel et al., 1982; Mann et al., 1998). Hence, the mass flux from the subducting Indian^Australian plate is probably enhanced above the

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Fig. 8. K/La ratios of the samples vs distance from the active South Solomon Trench (SST). For other LILE a similar scatter is observed (e.g. Cs, Rb, Ba, U, Sb, and Ba/La, Rb/Cs, Sr/Nd, Sb/Ce; not shown), pointing to multiple episodes of source overprint. Symbols are as in Fig. 3.

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subducted Woodlark Ridge (see also Perfit & Langmuir, 1984), possibly also including melting of the slab and/or injection of MORB-type material into the mantle wedge (see below). The anomalous thermal gradient in the Western Province is also reflected by the occurrence of picrites that formed at higher degrees of melting (Rohrbach et al., 2005).

The Ontong Java Plateau (OJP)

The mantle wedgeçdepletion and re-enrichment The mafic magmas in the southern Solomon island arc chain originate from the mantle wedge. As illustrated by Hf^Nd isotope data in Fig. 5b, the sub-arc mantle wedge has an Indian-type isotope signature, despite a reversal of subduction polarity at least 6 Myr ago (see Petterson et al., 1999; Mann & Taira, 2004). A similar configuration with an isolated Indian-type mantle wedge was proposed by Crawford et al. (1995) and Turner et al. (1999) for the neighbouring Vanuatu island arc (see also Peate et al., 1997). The interpretation for the Solomon arc relies on Hf^Nd isotope compositions in samples that show the least overprint by subduction components (i.e. low
Nd). There is also little evidence for Hf mobility, as, in contrast to some other intra-oceanic arcs (Woodhead et al., 2001; Tollstrup & Gill, 2005), there are no systematic variations of Hf isotope compositions along and across the arc. Despite its isolated character, it remains enigmatic whether the mantle wedge beneath the Solomon arc was continuously depleted as a result of continuing melt extraction (see Woodhead et al., 1993; Peate et al., 1997), as no age data for the samples exist to track the degree of mantle depletion back through time. As illustrated in Fig. 9a, the

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Subduction of Ontong Java material beneath the Solomon arc was previously suggested based on geophysical criteria (e.g. Mann & Taira, 2004; Miura et al., 2004). Sr^Nd^Hf^ Pb isotope data for igneous rocks from the Ontong Java Plateau (Tejada et al., 2002, 2004) largely overlap our dataset and the fields for the Indian and Pacific domain, thus suggesting Ontong Java material to be an additional possible source of subduction components (Figs 5 and 6). The possible presence of subducted Ontong Java material will be further elucidated below. In summary, the mantle wedge beneath the Solomon island arc was modified by components originating from subducted volcanogenic sediments and Pacific oceanic crust, and locally also by components originating from the Indian^Australian oceanic crust. The addition of subduction components (indicated in Fig. 7 by high
Nd) causes a slight increase of
eNdP/I. Nevertheless, samples displaying very little overprint by subduction components (i.e. low
Nd) still exhibit Indian-type isotope signatures. Therefore, Hf^Nd isotope relationships (Fig. 5b) clearly indicate the presence of a remnant Indian-type mantle wedge beneath the whole Solomon island arc.

mantle beneath the Solomon Islands is highly depleted in some regions (high Zr/Nb of up to c. 80), but has locally been refertilized by slab components. This depletion trend is marked by a moderate decrease in La/Yb with increasing Zr/Nb, reflecting the different compatibility of these elements during partial melting events (Fig. 9a; e.g. Pearce & Peate, 1995; Mu«nker, 2000). The coupled decrease of Zr/Nb with La/Yb in the enriched samples of the Western Province might suggest refertilization of the mantle wedge by slab melt-like components. Fluid-like components would cause a moderate increase in La/Yb, but only little modification of Zr/Nb. This is because Zr, Nb and Yb show a much lower mobility in slab fluids with respect to La (e.g. Kessel et al., 2005). Compositions of adakites from the Solomon Islands confirm this model, as their Zr/Nb and La/Yb values partially overlap those of the enriched basaltic lavas. To assess the degree of depletion and the influence of subducted material on the mantle wedge composition, the effect of melt extraction and later addition of enriched material (melt-like components) was modelled using Zr and Y. Zirconium is more incompatible than Y during partial melting of peridotite, but it is mobile in slab melts and, to a lesser degree, in slab fluids (Kessel et al., 2005). For modelling depletion and re-enrichment of the mantle, we used the data reported by McDade et al. (2003, and references therein) for calculating a ‘pre-subduction’ hydrous mantle wedge composition, assuming variable degrees of depletion. McDade et al. (2003) estimated typical wedge depletions of c. 4^12% relative to primitive upper mantle using South Sandwich and Lesser Antilles island arc data. To assess the mantle wedge composition beneath the Solomon Islands, we estimate theoretical degrees of mantle depletion using Zr/Y and Y contents. A hydrous primitive upper mantle composition was depleted by 5 and 10%, assuming non-modal accumulated fractional melting. Subsequently, melt compositions were calculated for each of these residues, assuming melting degrees of 5^19% (for hydrous PUM up to 30%) and non-modal accumulated fractional melting. Refertilization by slab-derived melts was modelled by adding 5% of an adakitic melt (composition as S 176 Sav) to a 10% depleted mantle residue. The model assumes that the adakitic melt reacted completely with wallrock peridotite as predicted from experimental studies (e.g. Yaxley & Green, 1998; Rapp et al., 1999). Upon reaction with wallrock peridotite, pyroxenite-rich veins are formed (see, e.g. Yaxley & Green, 1998; Rapp et al., 1999; Weyer et al., 2003; Schuth et al., 2004) and partial melting of such enriched mantle domains will generate basaltic melts that have inherited their trace element signature to a significant degree from the adakitic melt (Yogodzinski et al., 1995). In Fig. 9b, the modelling results are compared with the Zr^Y compositions of relatively primitive samples

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Fig. 9. (a) Depletion and refertilization of the mantle wedge illustrated by Zr/Nb vs La/Yb systematics (after Mu«nker, 2000). The grey diamond represents primitive upper mantle (PUM; McDonough & Sun, 1995), and the black triangle N-MORB (Hofmann, 1988).‘Mafic samples’ comprise basalts and picrites, ‘intermediate samples’ the high-Mg andesites, boninites and basaltic andesites (i.e. with SiO2 between 52 and 57 wt %), and ‘felsic samples’ include the andesites, dacites and rhyolites. (b) Zr10/Y10 vs Y10 in Solomon Islands lavas in comparison with modelled concentrations in melts generated by partial melting of variably depleted hydrous mantle reservoirs [represented by PUM and a mantle reservoir depleted by 5 and 10% melting; see McDade et al. (2003) and references therein for PUM modal composition]. Melting degrees modelled (assuming non-modal accumulated fractional melting) range from 5% to 30%. Only samples with MgO contents between 5 and 14 wt % are shown, as their Zr/Y values do not correlate with Mg-number (not shown). To minimize the effect of potential fractional crystallization further, Zr and Ycontents of the samples shown were recalculated to an MgO content of 10 wt % (Zr10 and Y10). Most sample compositions cannot be modelled by simple partial melting of a depleted source because of their elevated Zr10/Y10. To account for their Zr10/Y10, the depleted mantle needs to be refertilized in Zr and the compositions can be explained by addition of up to 5% of an adakitic melt (partial melting curve for refertilized mantle illustrated by the bold black line, Zr and Yconcentrations taken from the adakite S 176 Sav). Source and melting modes of the mantle and partition coefficients were taken from McDade et al. (2003). Tick marks indicate melting degrees. Primitive mantle Zr and Y concentrations are from McDonough & Sun (1995). Vectors schematically indicate compositional changes during partial melting, source enrichment and depletion. Modal mineral composition for PUM ^ 5% melt: olivine 0
662, orthopyroxene 0
191, clinopyroxene 0
140, spinel 0
007. Modal mineral composition for PUM ^ 10% melt: olivine 0
716, orthopyroxene 0
179, clinopyroxene 0
102, spinel 0
003.

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Adakites and adakitic signatures in mafic lavas Adakites are melts originating from subducted oceanic crust that was transformed into garnet-amphibolite or eclogite (e.g. Kay, 1978; Martin, 1986). To melt oceanic crust, the crust must be young and therefore relatively warm or it must be surrounded by hot mantle material; that is, at slab windows or along slab corners (e.g. Stern & Kilian, 1996; Abratis & Wo«rner, 2001; Yogodzinski et al., 2001). Alternative views to this scenario were illustrated by Macpherson et al. (2006). In a case study on the Philippine island arc those workers suggested that adakitic melts may fractionate from a basaltic precursor magma within the garnet stability field or even originate from remelting of mafic arc basement. Along the Solomon island arc, adakites occur in the Western and Central Province (islands of Fauro, Kolombangara, New Georgia, Savo, Guadalcanal, and Makira). Moreover, many mafic samples from all three provinces display adakite-like Sr^Y systematics (Fig. 10a). Regardless of the fact that these samples are not adakites by definition, their trace element signatures can be explained by the interaction of adakites with mantle wedge peridotite. Remelting of such mantle domains generates basalts with a somewhat ‘diluted’ adakitic trace element signature (e.g. Rapp & Watson, 1995; Yaxley & Green, 1998; Yogodzinski et al., 2001). Most mafic lavas from the New Georgia Group with elevated Sr/Y and low Y concentrations were erupted above the subducted part of the Woodlark Ridge (see Schuth et al., 2004), which provides a sufficiently high thermal gradient. This is also the case for slab melt enrichment in the sources mafic lavas on Utupua (Eastern Province), where the subducted part

of the Rennell Fracture Zone may have provided additional heat. Hence, the occurrence of adakites and mafic rocks with adakitic affinities in the Solomon Islands mirrors the tectono-magmatic patterns beneath the arc. As an active spreading ridge is subducting (Woodlark Ridge), it is also likely that slab windows occur along the subducing Indian^Australian plate. Both the Woodlark Ridge and the Pocklington Trough are rheologically weak zones in the subducting plate and might be torn at depth. As a consequence, convective mantle flow around the slab corners might be facilitated, resulting in a higher thermal gradient along the slab edge. These factors might cause partial melting along the subducted Indian^Australian plate and Solomon micro-plate (see Abratis & Wo«rner, 2001; Yogodzinski et al., 2001). In support of such a slab window model, there is an abrupt change in maximum slab depth along the westernmost part of the Solomon arc: whereas the Miocene Solomon micro-plate has reached a depth of c. 200 km, the immediately adjacent Woodlark Basin shows seismic activity only down to depths of c. 80 km (see Denham, 1969; Joshima & Honza, 1987; Mann et al., 1998). The overall similarity in isotope composition to subducted material (Figs 5 and 6) and their generation above a thermally anomalous mantle domain makes fractional crystallization of an adakitic melt from a basaltic magma an unlikely scenario. In addition, our adakitic samples do not follow the trend described by Macpherson et al. (2006) for adakites from the Philippines (see Fig. 10b). In the case of garnet fractionation, an increase of Dy/Yb with SiO2 as observed for the Philippine adakites would be expected. As is evident in Fig. 10b, the Solomon Islands adakites display a slight decrease of Dy/Yb vs SiO2. A schematic illustration (Fig. 11) summarizes our proposed model for melt generation along the proposed Woodlark Ridge slab window in the Western Province. The occurrences of adakites are aligned along the subducted portion of the Woodlark Ridge and the edges of the subducted Woodlark Basin. In the Central Province, magma ascent beneath the Gallego Volcanic Field (western Guadalcanal) and the adjacent Savo volcano was possibly facilitated by NNE-directed fracture zones in the crust of Guadalcanal and the underlying basement (Petterson & Biliki, 1994). In the Eastern Province, samples with elevated Sr^Y ratios are restricted to Pliocene volcanic rocks from Utupua. When Utupua formed, the Rennell Fracture Zone was probably located further south relative to its recent position in the Santa Cruz Basin in close vicinity to Utupua [see, e.g. Hall (2002) and Schellart et al. (2006) for a reconstruction of the plate tectonic situation]. The subducted Rennell Fracture Zone may also have formed a slab window, thus triggering the generation of adakitic melts. A similar setting has been described for the Tonga arc by Falloon et al. (2008).

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(5^14 wt % MgO and Mg-number 465). To further minimize the effect of fractional crystallization, the Zr^Y concentrations of these samples were also corrected to an MgO content of 10 wt % (denoted as Zr10 and Y10; see also Klein & Langmuir, 1987; Mu«nker, 2000). Samples with an MgO content higher than 14 wt % were excluded because their Zr^Y abundances are probably modified by assimilation of peridotite (Rohrbach et al., 2005). The composition of some primitive samples can be explained by melting of variably depleted sources, showing up to c. 5% depletion. Many samples, however, plot above the melting curve for hydrous primitive upper mantle or depleted residues. For these samples, addition of Zr via slab-derived components is clearly required. Addition of up to c. 5% of an adakitic source component into a strongly depleted mantle wedge (10% depletion) can explain all the lava compositions, also consistent with often elevated to ankaramitic CaO^Al2O3 ratios in these samples (see Green & Wallace, 1988; Green et al., 2004). Taking into account this scenario, no unrealistically high melting degrees are required to explain the observed Zr and Y concentrations.

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Fig. 10. (a) Sr/Y vs Y for the Solomon island arc rocks in comparison with compositions of modern adakites and arc basalts (after Defant & Drummond, 1990). Most Solomon Islands samples, including Mg-rich picrites (Sr/Y c. 30^60), plot in the field of adakites despite their mafic compositions. Most mafic samples exhibit even higher Sr/Y than the differentiated samples, therefore ruling out an increase of Sr/Y by fractional crystallization. Altogether, these patterns indicate the presence of slab melt components in the mantle sources. Calculated trends for (1) fractional crystallization of a high-pressure mineral assemblage from a basaltic melt in the garnet stability field (continuous line) and (2) for partial melting of altered basalt in the eclogite stability field (dashed line) are taken from Macpherson et al. (2006). Tick marks indicate the degrees of fractional crystallization and partial melting, respectively. The five most felsic adakites largely follow the line for a slab melt (see text for discussion). (b) Comparison of the Solomon Islands lavas with adakites from Mindanao, Philippines, in terms of their Dy/Yb vs SiO2 compositions, to assess the effect of possible garnet fractionation (after Macpherson et al., 2006). In contrast to the Philippines samples, the Solomon Islands lavas do not follow a trend that would be expected for garnet fractionation. The outlier is an adakite with a Pacific-type Hf^Nd signature, probably indicating a sufficiently slow melt ascent to fractionate garnet.

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The role of the subducted Pacific plate and Ontong Java Plateau material Hafnium-Nd isotope data can help to verify the proposed model for adakite generation, as they are virtually inherited from the mafic source. For most high-Sr/Y samples (including most adakites), Hf^Nd isotope compositions yield an Indian-type mantle signature (Fig. 12), as would be anticipated for partial melts originating from Indiantype oceanic crust. Shifting the Hf^Nd isotope ratios towards an Indian-type signature via melt^wall-rock interaction is unlikely because this would be reflected in strongly increased MgO, Ni, and Cr contents. In marked contrast to most adakitic samples, three samples (S 98 Fau, S 143 NG, S 163 Mak) exhibit a Pacific Hf^Nd signature. This implies that subducted Pacific crust was also partially present and confirms geophysical data indicating that fossil Pacific crust is still present beneath the arc (e.g. Mann et al., 1998). The subducted Pacific plate is Jurassic in age (e.g. Ishikawa et al., 2004), but it is unknown

whether the plate is fragmented. However, high-Sr/Y lavas erupted on Simbo have Pacific-type Pb isotope ratios, pointing towards Pacific-type subduction components beneath the island (see Ko«nig et al., 2007). Simbo is an exception in the Solomon island arc as it is located south of the active arc in the Woodlark Basin and, in a broader context, on the Indian^Australian plate. Consequently, the only way to explain the elevated Sr/Y and a Pacific Pb signature in Simbo lavas is a Pacific slab that was subducted at the former Vitiaz trench beneath the Indian^Australian plate. As suggested from Hf^Nd^Pb isotope systematics (Figs 5, 6 and 12), subducted Ontong Java material may also constitute a suitable source for adakitic melts. Geophysical models developed by Mann & Taira (2004), Miura et al. (2004), and Taira et al. (2004) indeed propose that the lower parts of the Ontong Java Plateau are subducted beneath the Solomon arc along a thrust detachment. Tejada et al. (2002, 2004) published Hf^Nd^Pb isotope

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Fig. 11. Schematic illustration of the plate tectonic configuration beneath the New Georgia Group, Western Province and our proposed model for adakite generation. Subduction of the hot Woodlark Ridge spreading center results in fragmentation of the Indian^Australian plate along the spreading center. Consequently, an additional heat source is provided and possibly MORB-type magma from the Woodlark spreading center may interact with the subarc mantle wedge. Extensive heating along the edges of the fragmented plate triggers generation of adakitic magmas at relatively shallow levels. The subducted parts of the Pacific plate along with fragments of the OJP are shown for comparison (see Mann et al., 1998).

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data for Ontong Java rocks; a comparison of these data with compositions of adakites from the Solomon Islands now permits verification of this model. In Fig. 12, eHf^ eNd values of all adakites and mafic lavas from the Central Province with elevated Sr/Y (i.e. Sr/Y 440) overlap values of both the Ontong Java Plateau and Indiantype basalts. It is noteworthy that all lavas with elevated Sr/Y from the Central Province display a horizontal trend in eHf^eNd isotope space with large variations in eNd of c. 3
3 epsilon units at nearly constant eHf. As illustrated in Fig. 12, the array spans a range covering the Ontong Java Plateau, Indian-type and Pacific-type mantle fields. Hence, the Hf^Nd isotope characteristics of the Central Province lavas could be explained either by a mixture of distinct Indian- and Pacific-type sources or by a mixture of Ontong Java- and Pacific-type sources. As the low-eNd end-members (two samples from Savo) are characterized by slightly lower eNd values than Indian-type mantle, the second model appears to be more likely. This is also corroborated by the Pb isotope compositions of the Central Province samples. Most adakites (except those from Savo) exhibit a Pacific-type Pb isotope signature. The Savo adakite (S 176 Sav) plots closely to the Ontong Java Plateau field in Fig. 12 and has a Pb isotope signature similar to Ontong Java rocks (Tejada et al., 2002, 2004). Thus, it is

likely that at least the Savo adakite originates from subducted Ontong Java Plateau material. Altogether, melting of fossil Pacific crust and subducted Ontong Java Plateau fragments can account for the Hf^Nd^Pb isotope compositions of most adakites and mafic high-Sr/Y lavas in the Central Province. Despite the Cretaceous age of the Ontong Java Plateau (e.g. Tejada et al., 1996), its subducted portions might still be sufficiently hot to be melted. For the Western Province, two patterns can be observed. Most adakites and mafic high-Sr/Y lavas of the New Georgia Group display Indian-type Hf^Nd isotope compositions as shown in Fig. 12. Their Hf isotope compositions are mostly more radiogenic than those of Ontong Java material. Hence, most Western Province adakites can be explained by melting of the subducted Indian^ Australian plate along the proposed slab window. Notable exceptions are samples from Mt. Mase volcano in the New Georgia Group (S 143 NG) and the adakites from Fauro in the Shortland Group. These samples display Pacific-type and Ontong Java Plateau-like Hf^Nd isotope compositions, respectively, suggesting a similar source to that of the Central Province adakites. This observation is in accord with compositions of high-Sr/Y andesites from Simbo (Ko«nig et al., 2007). These lavas are mixtures of adakites originating from the subducted Pacific plate and

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Fig. 12. Hf^Nd isotope compositions (expressed as eHf, eNd) of adakites and mafic lavas with high Sr/Y (440) from the Solomon Islands. The adakite S 176 Sav from Savo is marked by an arrow. Ontong Java Plateau (OJP) field (dark grey) after Tejada et al. (2002); other fields after Kempton et al. (2002). Discrimination line after Pearce et al. (1999). CP, Central Province; WP, Western Province; EP, Eastern Province.

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Fig. 13. Mixing models illustrating the petrogenesis of the non-boninitic high-Mg andesites from the Solomon Islands. (a, b) Plots of SiO2 and La vs Mg-numbers for all analysed high-Mg andesites. Most samples are characterized by relatively high Mg-numbers of c. 0
7 (i.e. in equilibrium with mantle olivine). The lack of correlation between La and SiO2 with Mg-numbers rules out formation of the Mg-rich andesites by fractionation from a REE-depleted boninitic parental magma. (c, d) Modelling of Ni^Cr variations vs LaN/YbN for a mixture of an adakitic melt (represented by sample S 9, chosen because it represents the least modified adakite erupted above the Woodlark Ridge) with typical depleted mantle peridotite (MP; after Workman & Hart, 2004) and basalt (WRB, Woodlark Ridge basalt; after Perfit et al., 1987), respectively. All highMg andesite compositions follow mixing lines between the adakite and basaltic melts. An adakite (S 143 NG) and a high-Mg andesite from Mt. Mase volcano (S 142 NG) are used for trace element modelling in (e). Symbols as in Fig. 3; tick marks indicate 10% mixing steps. (e) Binary mixing model explaining the REE compositions of cogenetic high-Mg andesitic and adakitic lavas from Mt. Mase volcano, New Georgia. The high-Mg andesite composition (sample S 142 NG; thin line) can be explained by a 7:3 mixture between the adakite and typical Woodlark Ridge back-arc basalt [sample KAK820316-029-015 from Perfit et al. (1987)]. Normalization to CI chondrite after Boynton (1984). (f) Thin-section photograph of the adakite S 143 NG from Mt. Mase volcano. Olivine (highlighted by white ellipse) is surrounded by small ortho- and clinopyroxene crystals, indicating disequilibrium. The amphibole phenocryst (right) exhibits a partially resorbed rim. Light grey phases in the matrix are plagioclase crystals.

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Fig. 14. Along-arc variations in Sr/Y, 206Pb/204Pb,
eNdP/I, 87Sr/86Sr, and Zr10/Y10, shown together with a simplified map as reference. The label ‘H’ marks islands with occurrences of high-Mg andesites. Dark grey fields indicate the presence of volcaniclastic sediment piles on the subducting plate. Their extension is uncertain (marked by a ‘?’). It should be noted that the Rennell Fracture Zone (RFZ) was probably located further SE of its present location (Schellart et al., 2006). The abundance of samples with high Sr/Y is linked to the proposed fragmentation zones in the Indian^Australian plate. Elevated Sr/Y values are coupled with those of
eNdP/I and Zr10/Y10, pointing to an enrichment of the mantle wedge by melts with an Indian-type signature. The highest 87Sr/86Sr values are observed in arc sections with increased sediment subduction. Large variations in 206Pb/204Pb are probably the result of mixing of fluids from different subduction components. Lavas that plot in the Indian-type data fields in both 206Pb/204Pb vs 207Pb/204Pb and 208Pb/204Pb are indicated by a light grey field. WR, Woodlark Ridge; dashed line, subducted parts of the Woodlark Ridge and its transform faults (see Mann et al., 1998).

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basalts from the Indian^Australian domain. Altogether, the compositions of the lavas from Simbo, Mt. Mase and Fauro confirm the presence of a fossil Pacific slab beneath the Western Province that must be located beneath the currently subducting Indian^Australian plate (Fig. 11). Again, the proposed slab window in the subducting Indian^ Australian plate would permit the ascent of adakitic melts originating from the deeper Pacific slab.

High-Mg andesites

CONC LUSIONS The southern Solomon island arc provides a unique opportunity to examine the coupling between arc lava compositions and the geodynamic setting along an intra-oceanic island arc. Major element, trace element, and Sr^Nd^Hf^ Pb isotope data for representative samples, covering an along-arc section of c. 1000 km, provide new insights into processes active beneath the southern Solomon island arc and the mantle dynamics along the Indian^Pacific plate boundary. The coupling between trace element and isotope compositions and tectonic features within the subducting Indian^Australian plate is illustrated in Fig. 14. The following conclusions can be drawn from our data.

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Andesitic rocks with elevated MgO contents of 45 wt % occur throughout the younger part of the Solomon island arc. They are relatively abundant in the New Georgia Group, such as on Simbo and at the submarine volcano Kavachi (Johnson et al., 1987; Ko«nig et al., 2007; this study), but have now also been found in the Shortland Group (see Ridgway, 1987), on the Russell Islands (Central Province), and on Vanikoro in the Eastern Province (Table 1). The high-Mg andesites and the boninite from the Shortland Group are presumably of Paleogene age (Ridgway, 1987); therefore, they are not related to young subduction processes along the Solomon island arc. All other analysed high-Mg andesites are most probably of Pliocene or younger age (Thompson et al., 1975; Danitofea et al., 1980; Abraham et al., 1987). The petrogenesis of high-Mg andesites (HMA) is explained either by melting of refractory mantle at low pressures (in the case of boninites; e.g. Crawford et al., 1989) or by the interaction of felsic melts with mantle peridotite or mafic magmas (e.g. Monzier et al., 1993; Kelemen, 1995). With the exception of the Paleogene highMg andesites from Shortland, the enriched trace element patterns of all other samples (Fig. 4) are in support of a mixing model. Boninites display much more depleted incompatible trace element signatures (e.g. Crawford et al., 1989). Furthermore, it is unlikely that the Solomon Islands high-Mg andesites are differentiation products of a boninitic parental magma because of their large variation in radiogenic isotope signatures. In addition, there is no correlation of Mg-number values with REE and SiO2 concentrations of the high-Mg andesites (Fig. 13a and b). This is also the case for other compatible elements such as Cr (not shown). Hence, the major and trace element compositions of the high-Mg andesites rather point to different source compositions. The generally high Mg-numbers values of c. 0
7 and Ni^Cr concentrations indicate equilibration of the andesites with mantle peridotite or primitive basaltic melts. Mixing between silicic magmas and mafic components was previously inferred to explain occurrences of Mg-rich andesites in island arcs (e.g. Monzier et al., 1993; Kelemen, 1995; Yaxley & Green, 1998; Rapp et al., 1999; Ko«nig et al., 2007). In the case of the high-Mg andesites from Simbo, mafic melts originating from the Woodlark Ridge were proposed as a suitable mafic end-

member (Ko«nig et al., 2007). To verify this model for other adakites of the Solomon Islands, we employed a simple binary mixing model based on trace element concentrations (Fig. 13c and d). Mixing end-members are an adakite from Kolombangara, a basaltic melt, and typical mantle peridotite. Basalt from the Woodlark Ridge (Perfit et al., 1987) was used as a mafic melt end-member because the Woodlark Ridge is subducting beneath the New Georgia Group and partial melts from the subducted spreading center may contribute to the high-Mg andesite compositions. As is evident in Fig. 13c and d, mantle peridotite can be excluded as a suitable mixing end-member because the Ni and Cr concentrations in the samples are too low when compared with Ni and Cr vs La/Yb mixing curves with peridotite. The observed variations, however, can be explained by mixing of basaltic magmas with adakitic melts. Mixing relationships for different HMA are best illustrated for the Mt. Mase volcano in NW New Georgia, where both adakitic lavas (S 143 NG) and Mg-rich andesites (S 142 NG) were erupted in one volcanic complex. To investigate mixing relationships for Mt. Mase lavas in detail, we modelled mixing between the Mt. Mase adakite with Woodlark Ridge (WR) basalt with respect to the REE inventory (Fig. 13e). A mixing ratio of 7:3 (adakite:WR basalt) can generate a REE pattern that is almost identical to that of the high-Mg andesite lava. This is also true for the SiO2 and MgO contents of this lava, which can be modelled accordingly employing the same mixing ratio. It can therefore be argued that the Mt. Mase lavas originate from mixing between adakitic and relatively primitive basaltic magmas. This mixing model is also illustrated by the presence of disequilibrium olivine surrounded by orthopyroxene in the adakite S 143 NG that was used as mixing end-member (Fig. 13f). Similar disequilibrium olivines with orthopyroxene rims were also reported for Mg-rich andesites of Simbo by Ko«nig et al. (2007).

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AC K N O W L E D G E M E N T S This study was supported by the DFG (German Research Foundation), grant Mu-1406/2. Heidi Baier from Universita«t Mu«nster and Ulrike Westernstro«er from Universita«t Kiel are thanked for laboratory support, and Paul Lo«bke (Universita«t Mu«nster) for thin-section preparation. Radegund Hoffbauer, Dorothe¤e Dohle and Beate Trenkle from Universita«t Bonn kindly provided XRF and Fe2þ data. The work also benefited from discussions with Peter Sprung, Jo«rg Elis Hoffmann, and Oliver Nebel. Andrew Mason and Thomas Toba from the Solomon Islands Geological Survey in Honiara provided valuable field support. The Geological Survey also helped with maps and reports, and provided sample SV 78. Detailed reviews, comments and suggestions by Julian Pearce and David Peate greatly helped to improve the manuscript. Gerhard Wo«rner and Marjorie Wilson are thanked for editorial handling. We wish to thank the people of the Solomon Islands for their kind assistance and help during the field campaigns.

S U P P L E M E N TA RY DATA Supplementary data for this paper are available at Journal of Petrology online.

R E F E R E NC E S Abraham, D. A., Baekisapa, M., Booth, D. J., Dunkley, P. N., Hughes, G. W., Langford, R. L., Philip, P. R., Ridgway, J., Smith, A. & Strange, P. J. (1987). New Georgia group geological map sheet, 1:250 000. Honiara: Geological Survey Division, Ministry of Natural Resources. Abratis, M. & Wo«rner, G. (2001). Ridge collision, slab-window formation, and the flux of Pacific asthenosphere into the Caribbean realm. Geology 29, 127^130. Arculus, R. J., Pearce, J. A., Murton, B. J. & van der Laan, S. R. (1992). Igneous stratigraphy and major-element geochemistry of holes 786A and 786B. In: Fryer, B. P., Pearce, J. A. & Stokking, L. B. (eds) Proceedings of the Ocean Drilling Program, Scientific Results, 125. College Station, TX: Ocean Drilling Program, pp. 143^169. Blichert-Toft, J. & Albare'de, F. (1997). The Lu^Hf isotope geochemistry of chondrites and the evolution of the mantle^crust system. Earth and Planetary Science Letters 148, 243^258. Boynton, W. V. (1984). Cosmochemistry of the rare earth elements: Meteorite studies. In: Henderson, P. (ed.) Rare Earth Element Geochemistry. Amsterdam: Elsevier, pp. 89^94. Bureau, H. & Keppler, H. (1999). Complete miscibility between silicate melts and hydrous fluids in the upper mantle: experimental evidence and geochemical implications. Earth and Planetary Science Letters 165, 187^196. Chauvel, C., Goldstein, S. L. & Hofmann, A. W. (1995). Hydration and dehydration of oceanic crust controls Pb evolution in the mantle. Chemical Geology 126, 65^75. Coleman, P. J. (1966). The Solomon Islands as an island arc. Nature 211, 1249^1251. Coleman, P. J. & Packham, G. H. (1976). The Melanesian borderlands and India-Pacific plates’ boundary. Earth-Science Reviews 12, 197^233.

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(1) Lead isotope compositions of lavas from the southern Solomon island chain indicate that the sub-arc mantle is overprinted to variable degrees by components derived from the currently subducting Indian^Australian plate, as well as from the Pacific plate that was subducted beneath the Solomon Islands during Eocene to Pliocene times. A large scatter in 206Pb/204Pb (Fig. 14) reflects this variability. The overprint by components from the Indian^Australian plate is most pronounced for the Western Province, where the active Woodlark spreading center is subducted. (2) Coupled Hf^Nd^Sr^Pb isotope systematics rule out a significant influence of subducted pelagic sediments on the magma compositions, whereas significant volumes of volcanogenic sediments were subducted, depending on the alongarc position.The highest 87Sr/86Sr and
eNdP/I are observed in sections of the island arc where considerable amounts of volcanogenic sediments cover the oceanic crust (Fig.14). (3) Subduction of the Woodlark Ridge and the Rennell Fracture Zone provides additional heat sources, resulting in anomalously high thermal gradients within the mantle wedge. Moreover, these two tectonic elements most probably triggered the formation of slab windows, causing partial melting of the subducting plates and generation of adakitic melts. These melts locally overprint the mantle wedge to variable degrees on a kilometer-wide scale, causing enriched trace element signatures in the mafic arc lavas. The enrichment process is visible in elevated Sr/Y and Zr10/Y10 of those lavas that erupted above fracture zones in the subducting plate (Fig. 14). The anomalous thermal gradient also triggered the eruption of the New Georgia picrites. Furthermore, the ocurrence of high-Mg andesites (except for the old boninitic lavas from the Shortland Group) seems to be linked to the hotter regions of the mantle wedge. (4) Coupled Hf^Nd isotope systematics show that the mantle beneath the Solomon arc and the northern Vanuatu arc is an isolated wedge originating from the Indian mantle domain. Disruption of the mantle wedge occurred during the reversal of subduction polarity c. 6 Myr ago. Replenishment of the isolated mantle wedge with Indian-type material is likely to take place via corner flow along the proposed slab windows. (5) Mixing of adakitic and mafic magmas is a likely scenario explaining the compositions of most high-Mg andesites within the island arc. This is supported by REE compositions and the high Mg-number of these atypical arc lavas. (6) As previously proposed based on geophysical models, there is strong evidence for the presence of fossil fragments from the Pacific plate and the Ontong Java Plateau beneath the Solomon island arc. This is supported by the Hf^Nd^ Pb isotope signatures of some adakites that overlap compositions of Pacific oceanic crust and those of Ontong Java Plateau rocks.

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