JOURNAL OF PETROLOGY
VOLUME 50
NUMBER 5
PAGES 781^811
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
2
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 07029 to 07052 and from
þ58 to þ83, 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 þ105 to þ146. 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
ß The Author 2009. Published by Oxford University Press. All
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JOURNAL OF PETROLOGY
VOLUME 50
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
NUMBER 5
MAY 2009
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
782
SCHUTH et al.
SOLOMON ISLANDS ARC LAVAS
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
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.
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.
783
JOURNAL OF PETROLOGY
VOLUME 50
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,
NUMBER 5
MAY 2009
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
784
SCHUTH et al.
SOLOMON ISLANDS ARC LAVAS
localities and the full dataset are given as an Electronic
Appendix, available for downloading at http://www.petro
logy.oxfordjournals.org/.
Analytical procedures
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 122 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
0282160 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 07325, 86Sr/88Sr ¼ 01194, and 146Nd/144Nd ¼
07219 for normalization. Repeated analyses of the standards NBS 987 and La Jolla gave mean values of 0710260
(n ¼18) and 0511852 (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. 0045% 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.
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
785
JOURNAL OF PETROLOGY
VOLUME 50
NUMBER 5
MAY 2009
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
wt %
506
479
478
526
521
660
546
613
593
SiO2
062
057
069
079
097
024
043
042
059
TiO2
164
100
118
181
148
166
146
167
171
Al2O3
FeO
446
652
583
559
620
170
437
214
274
483
440
349
404
674
092
384
167
315
Fe2O3
MnO
019
024
016
022
029
008
026
010
013
MgO
577
170
155
470
508
129
782
168
282
CaO
102
907
104
958
557
423
604
499
659
217
091
076
096
110
079
012
173
171
K2O
277
170
185
265
280
451
484
430
347
Na2O
036
019
022
016
007
010
003
014
021
P2O5
LOI
145
120
118
049
310
269
175
328
073
Total
998
998
997
999
996
995
993
989
990
ppm
Sc
293
431
414
305
367
273
389
567
160
V
281
318
204
308
360
365
310
102
169
Cr
81
1278
775
14
934
298
361
589
742
210
419
276
708
153
Co
27
109
50
23
Ni
33
894
444
18
197
243
957
473
169
Cu
495
108
157
295
667
356
517
Zn
94
102
70
96
121
435
108
457
718
Ga
18
146
18
21
177
189
141
200
189
Rb
332
171
115
155
177
368
199
769
167
Sr
887
538
454
397
213
695
991
766
517
470
134
864
210
Y
148
195
145
208
196
Zr
411
364
422
478
376
671
808
607
105
Nb
147
128
109
102
0840
412
0283
518
257
Mo
145
0507
0501
142
0224
0559
0104
0161
0767
Sn
0360
0448
0295
0325
0492
0533
Sb
0037
0020
0026
0036
0766
0070
0359
0043
0060
0118
0126
0192
0080
0182
0288
Cs
0091
0109
0053
Ba
259
101
686
143
996
294
768
754
198
La
103
463
434
480
224
498
162
904
815
Ce
203
883
105
105
635
947
418
139
146
Pr
268
134
164
154
106
131
0683
237
254
Nd
117
649
781
746
553
527
355
953
112
218
229
191
104
123
197
266
Sm
287
176
Eu
0931
0597
0751
0797
0689
0260
0483
0379
0889
Gd
287
206
233
282
263
0974
171
184
307
Tb
0452
0330
0373
0501
0480
0139
0314
0259
0486
Dy
280
208
227
341
331
0795
217
149
315
0158
0475
0295
0679
Ho
0577
0441
0453
0746
0702
Er
164
123
124
219
202
0454
140
0848
200
Tm
0241
0180
0180
0325
0303
0072
0214
0128
0309
Yb
161
119
119
220
205
0510
146
0904
216
Lu
0244
0178
0175
0340
0293
0080
0218
0138
0347
Hf
140
0826
104
156
138
194
0462
184
244
Ta
0074
0068
0055
0061
0055
0204
0020
0275
0144
Tl
0020
0048
0032
0270
0078
0010
0126
0051
Pb
457
154
120
125
289
387
0594
433
327
Th
143
0268
0364
0622
0155
0807
0128
128
0931
U
0575
0112
0142
0253
0094
0527
0067
0661
0394
87
Sr/86Sr
0703851 13
0703642 13 0703458 10 0703901 15 0703776 14 0703770 9
0705221 17 0703892 13 0703596 14
143
Nd/144Nd 0512977 12
0512991 9
0513021 9
0513033 6
0513062 15 0512968 14 0513125 14 0513027 13 0512983 13
176
Hf/177Hf 0283171 9
0283161 12 0283135 10 0283150 9
0283137 10 0283069 10 0283157 7
0283086 9
0283151 8
206
Pb/204Pb 1856
1862
1850
1846
1858
1851
1857
1846
207
Pb/204Pb 1555
1553
1552
1552
1554
1548
1552
1553
208
Pb/204Pb 3841
3835
3823
3830
3838
3823
3835
3823
(continued)
786
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
wt %
496
524
555
622
518
693
535
500
642
SiO2
057
068
065
050
059
035
077
064
030
TiO2
151
151
177
183
139
148
179
140
178
Al2O3
FeO
485
601
376
141
597
102
234
388
120
390
243
346
276
334
089
583
512
151
Fe2O3
MnO
015
019
014
013
017
003
015
016
007
MgO
933
888
425
119
821
173
362
780
110
CaO
114
843
794
587
112
362
812
115
299
128
088
106
137
068
243
134
124
221
K2O
200
281
353
431
218
385
368
265
627
Na2O
022
020
024
018
011
010
026
012
013
P2O5
LOI
012
078
019
033
038
063
141
092
061
Total
992
996
989
988
993
990
992
986
988
ppm
Sc
387
268
206
453
491
517
348
414
213
V
290
219
209
803
280
491
277
301
743
Cr
412
319
661
611
323
389
14
327
114
229
687
370
706
24
389
472
Co
384
544
Ni
173
287
388
497
892
229
19
638
664
Cu
135
662
862
260
140
495
974
111
118
Zn
701
784
828
650
719
246
91
768
426
Ga
160
110
209
211
161
215
17
147
232
Rb
138
164
204
685
116
729
212
179
953
Sr
555
618
664
596
424
773
660
837
1145
Y
128
181
179
146
143
421
231
137
571
Zr
370
682
796
150
395
156
680
477
120
Nb
0658
244
236
684
0688
107
150
0862
369
Mo
0618
0713
146
0286
0668
0104
0461
0991
0386
Sn
0435
0545
0709
0648
0436
0657
0496
0426
0037
0029
0256
Sb
0033
0046
0062
0094
0032
0053
Cs
0122
0169
0160
0164
0151
0084
0195
0293
0831
Ba
184
138
159
171
928
107
277
369
833
La
631
789
107
105
408
107
883
969
691
Ce
139
167
222
198
960
193
185
200
122
Pr
201
269
323
309
145
371
293
302
187
726
Nd
921
115
138
126
695
155
133
126
Sm
236
302
317
263
192
293
323
296
133
Eu
0776
104
0981
0833
0663
0894
0856
0997
0118
Gd
248
344
310
257
221
215
337
298
115
Tb
0377
0512
0480
0388
0374
0228
0532
0439
0162
Dy
228
274
281
236
246
0941
326
235
0949
0665
0564
0480
0513
0148
0669
0553
0195
Ho
0456
Er
128
198
161
141
147
0383
189
164
0575
Tm
0189
0280
0237
0223
0222
0048
0275
0230
0090
Yb
129
193
161
159
152
0322
181
159
0661
Lu
0195
0309
0241
0251
0232
0047
0271
0250
0104
Hf
110
186
184
337
125
387
179
145
295
Ta
0035
0158
0115
0373
0039
0066
0065
0044
0250
Tl
0061
0047
0050
0086
0032
0104
0079
0062
0201
Pb
372
225
223
411
208
497
421
517
127
Th
0875
0805
0713
144
0567
164
107
213
200
0591
0794
U
0421
0307
0333
0578
0199
0744
0451
87
Sr/86Sr
0703950 10 0703682 10 0703589 8
0703845 11 0703767 13 0702887 10 0704098 13 0704046 14
0704157 16
143
Nd/144Nd 0513001 12 0513001 8
0513053 12 0512983 13 0512962 16 0513088 15 0513006 6
0512924 16
0512927 17
176
Hf/177Hf 0283144 8
0283109 6
0283116 9
0283171 11 0283156 10 0283139 9
0283144 9
0283127 11
0283115 8
206
Pb/204Pb 1847
1850
1844
1842
1862
1847
1849
1847
207
Pb/204Pb 1553
1549
1550
1551
1549
1554
1553
1553
208
Pb/204Pb 3825
3821
3815
3823
3811
3835
3835
3831
(continued)
787
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
wt %
615
490
517
498
476
492
506
521
518
SiO2
037
072
057
073
067
135
130
091
112
TiO2
178
181
161
198
113
192
164
179
169
Al2O3
FeO
114
423
513
392
631
616
611
473
673
297
467
403
445
403
376
358
391
465
Fe2O3
MnO
007
018
017
016
019
018
017
014
018
MgO
224
443
674
330
134
452
636
443
360
CaO
558
116
107
952
114
107
105
754
851
133
070
063
133
113
050
060
038
114
K2O
507
225
200
325
152
291
279
428
319
Na2O
012
016
010
023
021
019
020
010
018
P2O5
LOI
035
215
019
178
073
041
012
239
040
Total
988
988
987
988
993
991
992
995
993
ppm
Sc
954
297
512
193
506
288
332
307
339
V
130
349
329
307
284
357
301
301
430
Cr
184
757
180
960
782
364
138
271
715
622
289
312
228
282
Co
116
254
293
241
Ni
116
413
545
131
281
226
470
239
179
Cu
509
197
152
219
125
787
106
109
371
Zn
514
813
702
788
698
840
797
740
101
Ga
212
195
166
221
135
219
189
204
214
Rb
102
830
837
214
228
639
817
864
200
Sr
656
477
356
734
493
383
331
394
397
270
201
250
Y
646
175
211
171
128
252
Zr
846
449
317
622
482
781
108
749
881
Nb
176
0792
0689
120
0982
237
316
117
219
Mo
0269
0400
0958
0963
0384
0521
0639
0310
0999
Sn
0405
0442
0360
0479
0486
0724
0829
0560
0731
Sb
0051
0045
0059
0035
0027
0027
0030
0025
0047
0049
0348
0186
0127
0042
0121
0496
0075
Cs
0265
Ba
420
103
116
248
171
816
102
909
193
La
434
440
357
730
633
521
639
483
886
Ce
672
108
742
177
155
139
164
126
217
Pr
137
170
120
262
236
222
250
195
317
Nd
613
827
602
122
110
111
122
940
146
238
181
310
275
327
350
268
380
Sm
139
Eu
0337
0852
0673
101
0883
123
124
0959
120
Gd
132
276
245
321
278
395
420
314
416
Tb
0188
0466
0423
0496
0412
0677
0723
0526
0678
Dy
109
302
288
303
245
444
472
346
437
0483
0928
0985
0722
0910
Ho
0221
0630
0635
0610
Er
0624
181
185
171
133
262
280
210
261
Tm
0094
0272
0276
0253
0192
0392
0418
0317
0396
Yb
0657
184
186
171
129
262
284
217
271
Lu
0102
0276
0291
0257
0192
0391
0423
0329
0411
190
240
Hf
205
133
1034
174
133
201
257
Ta
0168
0045
0039
0063
0054
0129
0177
0062
0119
Tl
0075
0053
0047
0046
0028
0008
0024
0025
0058
Pb
544
403
312
335
282
0968
177
163
351
Th
0566
0424
0375
0833
0711
0488
0587
0549
127
U
0286
0367
0240
0520
0373
0241
0277
0193
0442
87
Sr/86Sr
0704007 13 0703626 14 0703719 14 0703625 12 0703304 11 0702989 13 0703116 13 0704534 14 0703716 12
143
Nd/144Nd 0512963 12 0513039 14 0513047 13 0513050 12 0513037 14 0513049 13 0513058 15 0512992 13 0512966 14
176
Hf/177Hf
0283127 10 0283159 10 0283188 11 0283141 10 0283171 10 0283157 11 0283128 8
0283129 8
0283115 11
206
Pb/204Pb 1842
1866
1862
1873
1867
1861
1861
1852
1858
207
Pb/204Pb 1550
1552
1553
1556
1553
1550
1550
1549
1552
208
Pb/204Pb 3821
3831
3834
3846
3832
3815
3815
3813
3827
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
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
236
n.d.
335
323
271
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.
509
336
500
329
452
338
14
37
40
51
12
409
406
369
46
Zn
n.d.
n.d.
610
608
574
27
Ga
n.d.
n.d.
158
156
139
58
119
107
109
Rb
Sr
Y
Zr
Nb
105
534
894
256
0610
Mo
n.d.
Sn
n.d.
Sb
Cs
Ba
0011
0025
630
La
411
Ce
935
Pr
140
Nd
Sm
115
527
996
282
532
523
109
105
302
296
521
961
276
46
10
68
57
0662
0719
0709
0708
60
0462
0446
0438
0435
24
0328
0330
0335
0014
0014
0016
18
0052
32
n.d.
0019
0062
672
448
0045
655
0031
623
598
08
41
434
416
414
990
949
957
30
151
150
144
144
29
668
724
686
673
682
29
181
193
182
176
180
31
Eu
0653
0697
0649
0637
0677
32
Gd
184
203
187
181
191
41
Tb
0283
0311
0282
0273
0291
45
Dy
173
183
166
161
174
42
Ho
0346
0367
0323
0317
0344
53
Er
0962
101
0893
0844
0949
61
Tm
0137
0145
0126
0122
0138
62
Yb
0915
0949
0859
0821
0926
53
Lu
0133
0144
0125
0120
0139
65
Hf
0838
0894
0750
0750
0761
Ta
0037
0044
0035
0034
0043
Tl
0013
0017
0015
0014
0016
96
Pb
125
135
124
124
128
31
101
33
72
10
Th
0369
0400
0352
0335
0347
62
U
0138
0151
0133
0130
0134
54
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 122 wt % MgO. The complete dataset for this sample has been given by Schuth
et al. (2004). n.d., not determined.
789
JOURNAL OF PETROLOGY
VOLUME 50
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.
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/Al2O34095 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. 12, 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
505 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
NUMBER 5
MAY 2009
(1990) for adakitic rocks, including all samples with
456 wt % SiO2,435 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 112 to 298 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 017 to 12 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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SCHUTH et al.
SOLOMON ISLANDS ARC LAVAS
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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SOLOMON ISLANDS ARC LAVAS
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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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 029^061). 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. 07). 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. 09^19); 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 05 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.
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 07029 to 07052. eNd
values range from þ58 to þ83. 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 (07052 and 07045, respectively),
indicating alteration. eHf values range from þ105 to
þ146, 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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SCHUTH et al.
SOLOMON ISLANDS ARC LAVAS
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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SCHUTH et al.
SOLOMON ISLANDS ARC LAVAS
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.
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 (519) 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. 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.
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. 07027 to 07052 and from þ55 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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SCHUTH et al.
SOLOMON ISLANDS ARC LAVAS
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)
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.
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
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 0662, orthopyroxene 0191, clinopyroxene 0140,
spinel 0007. Modal mineral composition for PUM ^ 10% melt: olivine 0716, orthopyroxene 0179, clinopyroxene 0102, spinel 0003.
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SCHUTH et al.
SOLOMON ISLANDS ARC LAVAS
(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.
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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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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SOLOMON ISLANDS ARC LAVAS
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).
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. 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.
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. 33 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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SCHUTH et al.
SOLOMON ISLANDS ARC LAVAS
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. 07 (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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SOLOMON ISLANDS ARC LAVAS
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
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. 07 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).
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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(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.
NUMBER 5
MAY 2009
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.
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