JOURNAL OF PETROLOGY
VOLUME 37
NUMBER 2
IftGES 361-394
1996
M. L. G. TEJADA1*, J. J. MAHONEY1, R. A. DUNCAN2 AND M. P. HAWKINS'
'SCHOOL OF OCEAN AND EARTH SCIENCE AND TECHNOLOGY, UNIVERSITY OF HAWAII, HONOLULU, HI 96822, USA
"COLLEGE OF OCEANIC AND ATMOSPHERIC SCIENCES, OREGON STATE UNIVERSITY, CORVALLIS, OR 97JJ1, USA
'BRITISH GEOLOGICAL SURVEY, KEYWORTH, NOTTINGHAM NG12 5GG, UK
Age and Geochemistry of Basement and
Alkalic Rocks of Malaita and Santa
Isabel, Solomon Islands, Southern Margin
of Ontong Java Plateau
Geochemical and ^Ar—^Ar studies of the Malaita Older Series
and Sigana Basalts, which form the basement of Malaita and
the northern portion of Santa Isabel, confirm the existence of
Ontong Java Plateau (OJP) crust on these islands. Sr, Nd, and
Pb isotopic ratios of Malaita Older Series and Sigana lavas
fall within limited ranges [("Sr/^Sr)
T= 0-70369-0-70423,
ENd(T)= + 3-7 to +6-0, and 206Pb/204Pb = 18-25-18-64]
virtually indistinguishable from those found in the three OJP
basement drill sites as far as 1600 km away, indicating a uniform hotspot4ike mantle source with a slight 'Dupal' signature
for the worlds largest oceanic plateau. Three chemical types of
basalts are recognized, two of which are equivalent to two of the
three types drilled on the plateau, and one with no counterpart,
as yet, on the plateau; the chemical data indicate slightly different, but all high, degrees of melting and slight variation in
source composition. All but one of the *°Ar- Ar plateau ages
determined for Malaita Older Series and Sigana Basalt lavas
are identical to those found at the distant drill sites: 121 -3 ±09
Ma and 92-0 +1-6 Ma, suggesting that two short-lived, volumetrically important plateau-building episodes took place ~30
my. apart. Aside from OJP lavas, three isotopically distinct
suites of alkalic rocks are present. The Sigana Alkalic Suite in
Santa Isabel has an ^Ar-^Ar
age of 91-7±0-4 Ma, the
of the 'PHEM' end-member postulated for Samoa, and those of
present-day Rarotonga lavas; one or both of these hotspots may
have caused alkalic volcanism on the plateau when it passed
over them at ~44 Ma. The North Malaita Alkalic Suite in
northernmost Malaita is probably of similar age, but has
isotopic ratios [(87Sr/86Sr)T
K0-7037, £Nd(T)
x + 4-5,
206
Pb/2MPb xl8-8) resembling those of some OJP basement
lavas; it may result from a small amount of melting of aged
plateau lithosphere during the OJPs passage over these hotspots.
Juxtaposed against OJP crust in Santa Isabel is an ~ 62—46Ma ophiolitic (sensu latoj assemblage. Isotopic and chemical
data reveal Pacific-MORB-like, backarc-basin-like, and arclike signatures for these rocks, and suggest that most formed in
an arc—backarc setting before the Late Tertiary collision of the
OJP against the old North Solomon Trench. The situation in
Santa Isabel appears to provide a modern-day analog for some
Precambrian greenstone belts.
KEY WORD& oceanic plateaux; Onlong Java Plateau; Solomon
Islands; Sr-Nd—Pb isotopes; age and petrogenesis
INTRODUCTION
same as that of the younger OJP tholeiites, yet it displays a Oceanic plateaux are the probable oceanic counterdistinct 'HIMU'-type
isotopic signature
[206Pb/™Pb parts of continental and continental-margin flood
x 20 -20, (^Sr/^Sr) T x 0 • 7032, zm( T) x +4 -4], possibly basalt provinces. Plateaux emplaced in an openrepresenting small-degree melts of a minor, less refractory com- ocean environment, such as the Cretaceous plateaux
ponent in the OJP mantle source region. The Younger Series in in the Pacific, provide a means of studying the
southern Malaita has an Ar- Ar age of 44 Ma and isotopic mantle sources and petrogenesis of large-volume
ratios [em(T)=-0-5
to +10, (i7Sr/S6Sr)T=
0-70404- basaltic provinces while avoiding complications
0-70433, ^Pb/^Pb
= 18-57-18-92] partly overlapping those arising from the continental lithospheric influences
•Corresponding author.
© Oxford Univerjity Preu 1996
JOURNAL OF PETROLOGY
VOLUME 37
that plague many studies of continental basalts.
Unfortunately, sampling of oceanic plateaux is, as
yet, very sparse. The world's largest oceanic plateau,
the Ontong Java Plateau (OJP) (Fig. la), is the best
sampled of those in the Pacific, although the volcanic
foundation of the plateau has been drilled at only
three sites: DSDP (Deep Sea Drilling Project) Site
h
D
160'
180" E
158°E
APRIL 1996
289 and ODP (Ocean Drilling Program) Sites 803
and 807, where basement penetration was only 9149 m [Mahoney et al. (1993) and references
therein]. However, unlike most oceanic plateaux,
which tend to be deeply submerged, thick sections of
probable OJP basement are exposed above sea-level
along the southern margin of the plateau, on the
155*E
«*
NUMBER 2
165"
PACIFic
p
«OW/V C e
Mkm
SANXl ISABEL
CHOISBJL
PC^T—^"
RAMOS IS.S.
*•
IQflSLA
EWGEORl
SHORTHAND
O ISLANDS
-Ryssa
/
ISLAND
U.CANAL
SANCWSTDflAL
Fig. 1. (a) Map of the main part of the Ontong Java Plateau and the Solomon Islands chain at its southern margin [boxed area is shown
and magnified in (b)]. Malaita and Santa Isabel are labeled and drill sites 807, 803, 289, and 288 (which did not reach basement) are
indicated with doti. Contour interval ii 500 m. (b) Map of the Solomon Islands [modified from Coulion ft Vedder (1986)] showing
province boundaries [after Coleman (1970)].
362
TEJADA et al. I ONTONG JAVA PLATEAU BASALTS, SOLOMON ISLANDS
islands of Malaita, Ramos, and Santa Isabel, in the
eastern Solomon chain (Fig. 1). These islands, some
1150-1600 km from the ODP drill holes, are
believed to have formed as a consequence of Late
Tertiary obduction and overthrusting of the leading
edge of the plateau against the old North Solomon
Trench (e.g. Kroenke, 1972; Hughes & Turner,
1976, 1977; Coleman & Kroenke, 1981; Kroenke et
al., 1986). In addition to probable plateau basement,
blocks of very different provenance appear to be
juxtaposed in Santa Isabel (e.g. Coleman &
Packham, 1976; Hawkins & Barron, 1991), and this
region provides an excellent example of the effects of
the collision of a large plateau with a subduction
zone, which could be an important early stage in the
formation of (eventual) continental crust (e.g.
Kroenke, 1974; Storey et al., 1991; Cloos, 1993).
We studied the chemical, isotopic and w Ar- 3 9 Ar
characteristics of igneous rocks occurring over a
swath of ~350-km length from southernmost
Malaita (actually a separate island from Malaita,
called Small Malaita) to north-central Santa Isabel,
and including the small island of Ramos, which lies
in the channel between the two much larger islands
(Fig. lb). This is the first such study for these islands;
our results provide new information on the nature,
age and origin of the igneous rocks of the islands, as
well as on the composition and source(s) of the OJP
and on its subsequent history.
GEOLOGIC BACKGROUND,
SOLOMON ISLANDS
Coleman (1965) subdivided the Solomon Islands
into three geologic provinces (Fig. lb): (1) the
Pacific Province, which includes Malaita, Small
Malaita, Ulawa, the northeastern part of Santa
Isabel, and Ramos; (2) the Central Province,
including the southwestern part of Santa Isabel, San
Cristobal, Guadalcanal, the Florida Islands, and
Choiseul; (3) the Volcanic Province, consisting of
westernmost Guadalcanal, the Russell Islands, the
New Georgia Group, and the Shortland Islands.
Islands belonging to the Pacific Province are characterized by an essentially unmetamorphosed
basement of oceanic basalts of probable Late
Mesozoic age, as indicated by the paleontological
age of overlying pelagic sediments (Hughes &
Turner, 1976); islands of the Central Province differ
from those of the Pacific Province in having a partly
metamorphosed and extensively faulted basement of
oceanic basalts of probable Late Mesozoic age
associated with gabbros and alpine-type ultramafic
rocks, and overlain by more recent volcanic and
sedimentary rocks; and islands in the Volcanic Pro-
vince are made up principally of Pliocene to
Holocene volcanoes.
A simplified geologic map of Malaita is presented
in Fig. 2a. In southern and central Malaita, the
basement rocks are tholeiitic basalts (Rickwood,
1957; Hughes & Turner, 1976, 1977), here termed
the Malaita Older Series. Younger alkalic basalts,
referred to here as the Younger Series, occur within
the loosely dated Late Cretaceous to Late Eocene
pelagic sedimentary sequence overlying the Malaita
Older Series in southern and Small Malaita (Hughes
& Turner, 1976). Barron (1993) and McGrail &
Petterson (1995) reported outcrops of basalts that
are mineralogically and texturally similar to the
Younger Series in northernmost Malaita, here
termed the North Malaita Alkalic Suite; stratigraphic relationships and paleontological evidence
suggest it is closely similar in age to the Younger
Series. Small intrusions of 34-Ma alnoite, a rare
ultramafic igneous rock of deep origin with affinities
to kimberlite, crop out in north—central Malaita;
unlike the other rock types, the alnoites have been
studied extensively (e.g. Nixon & Coleman, 1978;
Nixon et al., 1980; Neal & Davidson, 1989) and are
not a focus of this paper.
The general geology of Santa Isabel (Fig. 2b) has
been described by Hawkins & Barron (1991). A
fundamental problem in the study of the basement is
the distinction and recognition of different petrogenetic suites among the basalts present; the problem
arises because of the probable juxtaposition of
igneous terranes from different tectonic environments. The Kaipito-Korighole Fault system
(KKFS) marks the boundary between the Pacific
Province and the Central Province; north of the
KKFS the basement lavas are known as the Sigana
Basalts and are overlain by pelagic limestones and
volcaniclastic sediments. Rare alkalic dikes are also
found intruding the Sigana Basalts; we term them
the Sigana Alkalic Suite. South of the KKFS, the
basement consists of a possible ophiolitic assemblage
referred to here as the Jajao Igneous Suite, which
includes basalts (San Jorge Volcanics), gabbros
(Kolose'eru Gabbros), minor leucocratic rocks, and
ultramafites (San Jorge Ultramafites), overlain by
clastic sediments.
In this paper, based on combined geochemical,
geochronological and field data, we have classified
the rocks broadly into three petrogenetically unrelated suites: (1) Malaita Older Series and Sigana
Basalts; (2) Malaita and Santa Isabel Alkalic Suites;
(3) Jajao Igneous Suite. Significant isotopic,
chemical, and/or age differences exist within the
groups themselves, as we will show.
Both the Sigana Basalts and Malaita Older Series
363
JOURNAL OF PETROLOGY
VOLUME 37
NUMBER 2
APRIL 1996
NORTH MALAITA
a
N
T
SOUTH CENTRAL
MALAITA
SEDIMENTARY
ROCK UNITS
8374(120.8 + 2.4)
P43(121.8 + 2.9)
10444 (44.2 + 0.2)
30 km
SANTA ISABEL: PACIFIC PROVINCE
SEDIMENTARY
ROCK
UNITS
SIGANA
BASALTS
AND
SIGANA
ALKALIC
SUITE
RAMOS
ISLAND
12290(62.5+1.5)
12216(92.5 + 0.4)
12199(90.8 + 0.5)
I2204 (92.0 + 1.6)
12169 (122.9 ±1.5)
12141 (60.9 ± 1.6)
SI74 = 94.5+ 3.1
SI88 = 89 8
SANTA ISABEL: CENTRAL PROVINCE
SN JORGE
ULTRAMAFITES
SI373 = 60.9
SI353 = 61.4
SI346 = 64.0
SI382 = 46.6
SI393 = 46 4
LEUCO
CRATIC
ROCKS
SAN JORGE IS
364
TEJADA el ai.
ONTONG JAVA PLATEAU BASALTS, SOLOMON ISLANDS
crop out as massive and pillowed flows and are
commonly non-vesicular and aphyric, although
minor coarser-grained varieties are present. In both
groups of lavas, clinopyroxene (augite) and plagioclase (An64_76) are the dominant groundmass and
phenocryst phases. However, unlike the Malaita
Older Series, which underlies Early Cretaceous
pelagic sediments, Sigana Basalt outcrops in Santa
Isabel lie below beds of Late Cretaceous to Late
Paleocene limestones. Significantly, no conformable
contacts between the Sigana Basalts and the limestones have been found, but the difference in
sediment age has previously made correlation of the
two suites of basement lavas problematic.
The alkalic suites of Malaita, the Younger Series
in the south and the North Malaita Alkalic Suite in
the north, crop out as pillowed basalts, brecciated
flows, and minor sills and intrusions. Their vesicularity distinguishes them from the Malaita Older
Series in the field but they also differ in having
olivine, titanaugite (vs low-Ti augite in the Older
Series), and more sodic plagioclase (Anss-es) as
phenocrysts (Hughes & Turner, 1976, 1977; Barron,
1993). In Santa Isabel, the intrusive nature of the
Sigana Alkalic Suite, as well as the presence of
basaltic hornblende, titanaugite, and olivine, sets it
apart from the Sigana Basalts (Hawkins & Barron,
1991). Hawkins & Ban-on (1991), Barron (1993) and
McGrail & Petterson (1995) considered it possible
that the Sigana Alkalic Suite was related to the
Malaitan alkalic rocks, based on limited petrological
data.
The Jajao Igneous Suite of Santa Isabel has no
correlatable units in Malaita (Hawkins & Barron,
1991). The San Jorge Volcanics are extrusive rocks
of basaltic to basaltic andesite composition which are
generally distinguished from the Sigana Basalts in
the field by higher vesicle and amygdule content and
a greater proportion of plagioclase-phyric lavas.
They occur as pillowed and massive flows, sometimes
cut by dikes, and grade in texture from aphyric to
porphyritic, with plagioclase as the dominant
phenocryst and with rare to minor clinopyroxene.
The Kolose'eru Gabbros are found adjacent to the
San Jorge Volcanics close to the KKFS and on the
southern coast of San Jorge Island (Fig. 2b). Their
contact with the San Jorge Volcanics is commonly
faulted, but intrusive to gradational contacts were
also noted by Hawkins & Barron (1991). The rocks
range from microgabbros to medium-grained
gabbros, consisting mainly of augite and plagioclase,
with or without olivine and orthopyroxene, and with
apatite, late magmatic hornblende, plus or minus
ilmenite as accessory phases. Finally, the leucocratic
rocks are both extrusive and intrusive, of andesitic to
dacitic compositions, found largely within the area of
the Kolose'eru Gabbros and to a limited extent
within the San Jorge Volcanics outcrops. They are
either aphyric, sometimes with flow texture, or
medium-grained, with plagioclase as the major
mineral phase, accompanied by amphibole (± clinopyroxene), quartz and magnetite.
SAMPLES AND METHODS
Most of the samples for this study were collected by
the Solomon Islands Geological Survey on several
mapping expeditions between 1960 and 1992; Fig. 2
shows sample locations (details can be obtained from
the authors on request). The collection covers most
of the central part of Santa Isabel, where basement
exposures are estimated to be > 1200 m thick
(Hawkins & Barron, 1991), and includes a few
samples from the southern tip of the island. Most of
the Malaitan samples are from southern Malaita,
where a basement section of estimated thickness
>2000 m is exposed (Hughes & Turner, 1976); six
are from the northern tip of the island (Barron,
1993). In addition, two samples are from Ramos
Island. Eight Santa Isabel samples, analyzed here for
40
Ar— Ar only, are from the collection of Ramsay
(1981). It should be noted that, for the most part,
only very crude stratigraphic control exists between
Fig. 2. (a) Simplified geologic map of Malaita [after Rickwood (1957)], showing locations of samples in this study, which are represented by filled squares (Malaita Older Series), filled diamonds (Younger Series), and open diamonds (North Malaita Alkalic Suite).
Samples with w Ar- 3 9 Ar plateau ages (in parentheses) are labeled. The geologic columns (not to scale) for north, south—central, and
Small Malaita illustrate the general stratigraphic subdivision of rock groups into igneous and sedimentary units (e.g. Rickwood, 1957).
Abbreviated labels are for North Malaita Alkalic Suite (NMAS) and Younger Series (YS). (b) Simplified geologic map of Santa Isabel
[after Hawkins & Barron (1991)], including Ramos (inset) and San Jorge islands, showing locations of samples in this study, which are
indicated by open squares (Sigana Basalts and Ramos Island lavas), filled triangles (Sigana Alkalic Suite), open triangles (San Jorge
Volcanics and San Jorge-like lavas), filled circles (leucocratic igneous rocks), and open circles (Kolose'eru Gabbros). Samples with
*°Ar—39Ar plateau ages are labeled. The Si-prefix samples are from the areas enclosed by thin dashed lines (exact sample locations
uncertain) (Ramsay, 1981). The heavy continuous line represents the Kaipito—Korighole Fault system (KKFS), which marks the
boundary between the Pacific and Central provinces in Santa Isabel. The geologic columns (not to scale) for areas north and south
of the KKFS illustrate the general stratigraphic subdivision of rock groups into igneous and sedimentary units (e.g. Hawkins &
Barron, 1991).
365
JOURNAL OF PETROLOGY
VOLUME 37
samples from different locations (Hawkins & Barron,
1991; Barron, 1993).
The Malaita Older Series samples have generally
suffered moderate degrees of alteration, which
includes oxidation and clay mineral formation in
originally glassy matrix and, in some samples, incipient replacement of clinopyroxenes by mixtures of
clays, calcite, and iron hydroxides (chlorophaeite or
bowlingite) and of plagioclases by calcite or zeolite.
Some of the Younger Series and North Malaita
Alkalic basalts are vesicular and some are amygdaloidal, with zeolites and/or calcite and secondary
phosphate minerals filling the vesicles. On the other
hand, zeolite (sometimes with calcite) veins arc
present in some of the samples of Santa Isabel in
addition to minor iron hydroxide staining, and
several samples also have vugs filled with clays,
calcite, phosphate, and/or zeolites. Hematite veining
was observed in one of the samples of leucocratic
rocks, whereas some of the Kolose'eru Gabbro
samples have pyroxenes partially replaced by hornblende. The geochemical characteristics described in
the following sections are based mainly on
alteration-resistant elements in the visibly fresher
samples.
From visual and microscopic examination, a
subset of 22 samples was selected for +0Ar-39Ar
dating (Table 1). Whole-rock pieces were slabbed
and trimmed to obtain the freshest and most holocrystalline material. Relatively altered samples were
crushed to ~0-5-mm size, hand-picked to remove
vesicle and vein alteration, cleaned in an ultrasonic
bath with deionized water, and dried. Mini-cores (3mm diameter, 6-mm length) were drilled from the
freshest, aphyric rocks, then ultrasonically washed
and dried. Approximately 0-7 g of each sample was
sealed in an evacuated fused-silica vial and irradiated for 6 h in the core of the Oregon State
University TRIGA reactor. The efficiency of the
conversion of K to Ar was monitored with
27-7-Ma FCT-3 biotite. Corrections for interfering
K- and Ca-derived Ar isotopes are those from
Dalrymple et al. (1981). Argon extractions were
performed in a conventional high-vacuum glass line
using radio-frequency (RF) induction heating. The
cleanup system consisted of two Ti-furnaces held at
800°C to getter the reactive gases, then cooled to
getter hydrogen. Heating steps, each of 20-min
duration, were set from RF power levels determined
from previous experience to divide the Ar released
into four to seven roughly equal proportions. After
cleanup, the Ar composition of each gas increment
was measured with an AEI MS-10S mass spectrometer using computer-controlled peak switching and
data acquisition. The total system blank was better
366
NUMBER 2
APRIL 1996
than 2x 10 1+ moles of *°Ar during the course of
these experiments.
Major elements and a suite of trace elements
(Table 2) were analyzed on alumina- and tungstencarbide-ground powdered samples by X-ray fluorescence (XRF) spectrometry at the University of
Hawaii, using an automated Siemens 303AS spectrometer with Au and Rh tubes, and following the
methods of Norrish & Chappell (1977). Rare earth
elements (REE) and several other trace elements
(Table 3) were determined on 0-2-g splits of powders
by inductively coupled plasma-mass-spectrometry
(ICP-MS) at Washington State University and, for
two samples, the University of Notre Dame [see
Knaack et al. (1991) for methods]. Agreement
between values obtained by both XRF and ICP-MS
is very good (almost identical for Rb, within errors
for Nb, and within 10% for Y, except for sample
11480).
Samples for Nd, Pb, and Sr isotopic analysis
(Table 4) were chosen to represent the range of
observed major and trace element compositions and
for geographical coverage. Isotopic and isotopedilution measurements were made on a VG Sector
multicollector mass spectrometer at the University of
Hawaii on chips picked from the least altered
interior portions of the samples (e.g. see Mahoney &
Spencer, 1991). Chips selected were briefly cleaned
ultrasonically in an ultra-pure HF-HNO3 mixture,
HC1, and water (in sequence) before being broken
into smaller (~2-mm) pieces, after which the
cleaning procedure was repeated and the pieces were
hand-picked again to avoid any macroscopic
alteration products (calcite, clay, phosphate, and
zeolite veins or vesicle fillings); the pieces selected
were then ground into powder. Sr isotope and, for
some samples, Nd and Pb isotope ratios were
measured on splits of powder subjected to a multistep acid-leaching procedure effective at removing
low-temperature alteration phases such as those
described above (Mahoney, 1987; Mahoney &
Spencer, 1991). For other samples, Pb and Nd isotopes were determined on unleached splits; these two
elements have very low concentrations in seawater
and their isotopic ratios are not affected significantly
by moderate degrees of seawater alteration, such as
those experienced by most of the samples chosen
(e.g. Mahoney, 1987; Mahoney et al., 1993). It
should be noted that because the sample picking and
acid-cleaning procedure, as well as acid-leaching,
modifies a sample's composition, the isotope-dilution
data cannot strictly be compared with whole-rock
XRF or ICP-MS abundances and are used here for
isotopic age corrections only. Age corrections are
based on the ^Ar-^Ar dates in Table 1.
TEJADA et at.
ONTONG JAVA PLATEAU BASALTS, SOLOMON ISLANDS
Table 1: Ar— Ar age determinationsfor Malaita and Santa Isabel basement and alkalic rocks
Sample no.
Age spectrum (plateau)
and location
or group
Integrated
Isochron analysis
age (Ma)
"Ar
Age±1 SD
Age±1 SD(Ma)
Intercept ± 1 SD
SUMS
N
(Ma)
(/V-2)
Malalta Older Series
P43*
79
121 8 ±2-9
127-0
117-412-4
297-5 + 3-6
0-15
8374*
67
120-8 ±2-4
125-4
113314-9
300-6 ±21 0
0-50
100
119-611-6
119-8
120-915 3
284-5116-4
1-50
12169
100
122-911-5
122-0
125-411-2
289-112-1
0-02
Ramos Island
1221
Sigana Basalts
I2204
67
92-011-6
86-7
94-712-5
288-1+6-4
0-69
SI74
68
94-513-1
75-8
86-6117 0
304 9121-8
0-11
SI88
68
89 8 1 1 - 8
81-3
86-717-0
299 619-9
0-39
12141
91
60-911-6
59-2
64-512-6
288-916-2
052
10-4
I262
none developed
107 3
I256
none developed
98-9
11053
none developed
64-9
SI101
none developed
118-8
none developed
111-2
High- Ti Sigana Basalts
11445
Sigana Alkalic Suite
12216
96
92-510-4
92-2
93-311-1
280-3129-4
12199
65
90-810-5
870
89-911-4
399 9194 5
1-44
95
44-210-2
45-1
44-110 6
298-516-5
1-62
5
Younger Series
10444
Sen Jorge Vo/canks and San Jorge-like Lavas
11717
72
61-914-1
625
60-613-8
293-5 + 2-2
0-20
5
I2290
88
62-511-5
56-8
61-6113-3
295-2112-9
0-38
4
SI353
93
61-412-3
67-7
62-517-4
294-816-3
7-20
4
SI346
94
64-011-1
65-2
62 3 1 1 - 8
296-811-6
0-58
4
SI373
100
60-911-2
275-3117-3
3-32
4
SI382
100
SI393
100
11451
67-3
67-015-9
48-611 -9
460
41-813-9
299-914-7
1-32
4
46-411-1
46-8
46-413-1
294-015-5
2-14
4
none developed
34-9
All ages are reported relative to standard monitor FCT-3 biotite (27-7 ±0-2 Ma). Data for samples marked with an asterisk were reported
by Mahoney etal. (1993), and are recalculated here to an Mmhb-1 monitor age of 513 9 Ma. /Vis the number of steps forming the plateau.
367
O
>
r1
O
*d
Pi
O
f
O
o
Table 2: X-rayfluorescencemajor and trace element datafor Malaita and Santa Isabel basement rocks and alhalic basalts
Sample no.
SIO 2
TiO 2 AI 2 O 3
Fe 2 O 3 " MnO MgO
CaO
Na2O K2O P2OS
SUM
LOI
Rb
Nb
Sr
Sc
Zn
Cu
Co
Mn
Ni
182
O
2
to
Malaita Older Series
ML68
48-18
1-10 13-80 14-23 0-21
11-10 1044 1-27
007 0 08
100-46
2-53
3-2
230
8393
50-79
1-34 14-52 12-56 0-20
7-34 11 26 1-77
0-77 0-13
100-68
1-14
4-1
133
8034
5002
1-47 14-18 13-37
825 11-40 1-92
0-10 0-10
100-95
107
4-3
P384
5008
1-51
701
11-49 202
004 0 12
100-45
0-72
P49
51-58
1-55 13-40 15-25 0-16
6-39 10-35 209
003 0-13
100-93
0-71
8027
4908
1-55 13-73 14-50 0-20
720 11-67 1 86
0-17 0-12
10008
1-19
P439
5008
1-57 14-32 1301
6 98 11-44 2-25
0-14 0 16
100-11
P43
49-62
1-58 14-94 13-73 0-18
701
11-68 1-79
0-11 0-13
8374
49-53
7-25 11-79 2-22
8378
50-44
1-59 14-66 13-14 0-23
1-62 14-86 12-80 0-22
8030
50-75
8029
50-88
8028
50-31
7983
50-10
253
18
374
23
145
49
62
72
398
23
51
209
55
75
50
93
88
454
35
46
63
82
0-61
5-8
149
92
357
30
36
87
126
100-77
106
5-7
131
88
375
30
108
113
53
1420
81
164
007 0-12
100-60
1-38
5-5
137
88
405
28
107
117
54
2220
81
186
7-12 11-14 2-23
0-11 0-13
100-67
2-29
1-64 14-90 12-60 0-14
1-66 14-84 12-39 0-14
7-31 11-00 2-20
009 0-13
100-76
1-08
56
145
87
418
28
53
83
165
6-88 11-36 2-29
0-15 0 13
100-72
1-58
5-6
145
87
28
28
87
12
6
1 73 15-32 12-72 0-14
1-90 13-13 1509 0-17
6-58 10-11 2-43
0-18 0-14
99-66
207
605
0-19 0-16
99-87
0-70
6-2
142
96
372
30
45
59
82
48
82
179
85
170
0-14
13-79 14-18 0-21
0-16
10-82 2-26
85
89
81
1540
89
134
48
1600
39
159
S
Ramos Island
1221
48-92
1-50 14-16 1 3 0 2 0-16
7-59 11-61 2-23
0 0 6 0-12
99-37
0-46
<1-5
5-3
136
83
381
26
I223
49-62
1-54 14-44 13-38 0-14
7-25 11-38 2-28
0 16 0-12
100-31
1 65
2
5-7
138
88
383
28
105
125
51 1320
3
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Table 2: continued
Sample no. SIO2
TIO2
f
" MnO MgO
AI2O3
CaO
Na2O
K2O
P2OB
SUM
LOI
Rb
Nb
Sr
Sc
Zn
Cu
Co
Mn
85
113
62
1210
Ni
Cr
High- Ti Sigant Bualti
o
o
<
o
11658
4907
2-22
13-80 16-28
0-16
7-17
9-88
2-14
0 0 8 0-19
100-99
1•32 <1 •5 6-7
128
124
422
38
79
97
11445
49-83
2-27
13-32 16-38
0-18
5-78 10-31
2-17
0-23 0-21
100-68
1•17
4
8-4
140
146
396
43
34
61
80
11123
4802
2-31
13-19 16-89
0-23
6-87
9-61
2-82
0-23 0-22
100-39
1•77
8
8-7
191
135
490
41
46
65
103
s
Sigana Alktlic Suite
12236
47-21
2-80
16-82 11-11
0-15
4-40
805
4-77
3-42 1-24
99-97
3-84
41 120
707
409
151
36
17
65
62
12216
42-71
3-59
13-67 15-42
0-21
10-90
8-82
200
1-95 0-85
100-12
3 56
38 100
980
317
224
30
11
207
232
11853
45-42
3-67
16-64 1205
0-13
5-42 1004
3-18
2-42 0-85
99-82
2-99
34
760
333
247
36
70
52
39
1270
38
13
1-77 0-89
100 34
499
357
237
34
93
34
53
1850
98
90
12199
41-75
3-92
1503 14-50
0-20
6 51 12-73
304
6-96
84
33 107
S
"0
Younger Series
9530
52-80
2-19
13-64 10-31
0-11
7-56
906
2-76
1-31 0-54
100-28
1-39
23
25
604
201
148
25
107
36
43
850 181 324
10016
53-16
2-64
13-96
9-64
0-07
6-64
9-54
2-62
1 36 0-52
100-15
1-11
23
23
620
219
172
27
9538
51-88
2-74
12-80 11-18
0-10
8-38
8-25
2-36
1-55 0-63
99-86
1-79
24
25
643
253
153
24
21
115
43
38
740 144 255
237 358
9959
53-80
2-75
13-60
9-91
0-07
7-60
8-49
2-94
1-50 0-63
101-29
1-10
22
22
670
244
144
27
17
366 333
9591
49-90
2-92
11-54 1185
0-10
10-30
8-28
2-12
2-15 0-83
99-99
223
29
34
810
329
163
23
20
333 370
9562
52-47
2-94
13-42 10-99
009
6-99
8-72
2-27
1-56 0-63
10008
2-15
17
26
620
242
166
27
18
179 240
10444
49-67
2-96
11-76 11-47
0-13
8-89
8-30
207
1-98 0-79
98-02
1-24
P406
50-31
3-62
12-11 11-37
0-14
8-11
9-43
1-89
1-63 0-75
99-36
2-22
17
34
863
342
204
28
8503
4800
3-96
11-17 11-82
0-12
10-05 1008
1-76
2-43 0-91
100-30
2-58
44
52
733
347
203
28
122
14
47
55
1080 247 407
284 325
r
8
North Malaita Alkalic Suite
ML17B
46-96
2-48
11-60 13-73
0-16
6-82
14-36
2-16
1-18 0-62
10004
3 •73
28
44
636
181
247
26
113
75
55
1310
208
332
ML43
45-53
2-54
11-90 11-70
0-13
6-73
17-43
2-48
0-98 1-26
100-68
5 •33
16
47
569
182
274
26
124
82
60
1040
128
242
ML83
46-54
2-61
11-98 12-25
0-20
7-68 14-98
2-11
0-83 0-78
99-94
3•19
18
59
706
209
247
29
121
120
78
1680
207
311
ML19
47-21
2-66
13-08 1301
0-15
0-93 1-54
100-01
2 •16
13
45
574
194
277
29
132
112
51
1180
159
289
ML65
50-79
3-42
12-28 11-45
0-13
1-65 0-77
100-21
6 •15
31
97
1023
294
238
29
109
79
74
1300
435
586
<1 •5 2 0
San Jorge Voicania and San Jorge-like lavas
11898
49-97
1-39
14-73 1003
0-13
9-56 12-61
2-14
0 0 5 0-11
100-72
1-95
123
76
273
30
41
133
685
11717
49-52
1-84
15-96 11-97
0-16
6-40 11-52
2 51
0-19 021
100 28
1 28
7
28
128
153
314
47
40
92
219
11740
53-14
207
15-63 12-46
0-14
3-76
814
3-87
0-58 0-21
10000
0-91
16
2-1
149
117
408
44
37
18
9
12290
49-18
207
14-42 12-56
0-19
738
11-43
2-66
0 0 7 0-18
100-14
2-40 <1 •5
4-1
203
136
399
45
78
284
11451
50-25
2-19
16-11 14-92
0-19
522
8-59
2-37
0-57 0-19
100-60
2-52
7
1-8
126
98
485
41
11720
49-48
2-30 15-42 13-22
0-22
5-14 11-10
309
0-30 0-24
100-51
1 48
8
3-3
152
198
352
64
11582
49-29
2-31
15-38 12-26
0-23
5-45 11-37
2-71
0-34 0-29
99-63
1-16
5
3-5
164
207
380
63
11558
49-80
2-69 14-64 13-85
0-20
4-55 10-77
2-86
0-45 0-31
100-12
1-54
10
40
169
240
417
12217
49-59
2-97
12-76 16-22
0-21
5-93
9-62
2-57
0 0 6 0-28
100-21
0-97
<1 •5
5-1
107
198
11464
50-72
301
14-04 17-72
0-10
4-43
608
3-68
0-22 0-25
100-25
103
I456
48-53
3-39
12-82 17-67
0-22
523
9-53
3-26
0 0 8 0-31
101-04
1-44
<!
50
109
254
•5
95
63
50
1560
32
32
116
30
55
1790
65
135
69
113
71
133
63
61
1540
68
143
546
72
107
56
51
1800
33
26
501
73
39
19
42
38
35
(continued on next page)
o
1
5
O
O
1
Table 2: continued
Sample no.
SI0 2
TiO2 AI 2 O, FejO,' MnO MgO
CaO
Na 2 0 K2O
P 2 C,
SUM
LOI
Rb
<1-5
Nb
Sr
Zr
V
Y
1030
18
Sc
Zn
Cu
Co
Mn
Nl
Cr
162
82
1460
103
Kolota'eni Gabbros
4303
3-45 12-37 18-79 0 2 0
7-68 1304 1-60
006 001
100-23
1-60
46-47
2-47 1609 14-45 0-14
6-20 12-47 2-47
001
005
100-82
0-74
11710
46-73
308
13-85 17-70 0-22
5-48 10-29 3-22
001
0-31
100-89
0-46
1419
47-44
2-85 15-95 15-78 0-15
8-72 3-29
0-17 0-86
100-21
106
11516
48-77
0-23 17-75
6-86
006
10-73 15-35 0-84
000 000
100-59
103
10-71 14-36 1-82
0 07 0-10
100-35
0-10 0-25
100-27
004
100-83
0-92
1-76 0 0 2
10004
on
100-26
5-00
1391
49-14
0-37 16-46
7-22
0-10
11501
49-57
2-75 13-06 1605
0-20
5-47
9-47 3-35
11466
50-88
1-28 14-91 11-31 0-16
897
11-41 1-83
I457
51-21
0-50 15-72
0-13
11-02
7-31 2-80
11599
51-63
2-15 14-82 12-18 0-14
599
10-18 2-92
9-58
004
0-14
10
200
17
78
127
0
<1-5
20
156
54
493
52
72
68
40
1350
28
15
2-66
<1-5 <0-4
102
5
171
0-63
<1-5
2-6
128
191
475
8
48
62
46
1120
122
558
68
103
61
64
1780
42
54
3-93
19
06
203
17
269
11
0-83
<15
1-4
128
110
369
190
521
47
87
63
89
1370
44
101
<1-5
23
162
381
59
61
64
4
71
1100
7
<4
12
5
36
>1
r
O
3
•LOGY
11679
11707
o
Leucocratk Igneous Rocks
I1469B
58-81
103
0-15
1-94
5-87 4-69
0-14 0-41
99 59
087
11470
59-33
1-80 14-17 11-39 0 0 8
3-93
3-13 4-84
004
0-33
9904
3-26
11478
55-56
1-84 14-37 13-22 0-18
3-94
6-73 3-52
0-19 0-16
99-71
1-38
11480
56-95
205
14-27 13-96
0-11
404
3-66 4-67
0-80 0-28
100-79
1 86
9
2-9
104
198
134
67
11489
59-16
0-92 14-29 12-35
0-14
3-40
6 61 2 9 9
0-23 007
100-16
1 32
<1-5
10
109
53
377
26
82
128
51
1380
13
6
11605
59-32
1-80 14-83 10-71
008
3-28
4-36 5-41
0 0 5 0-23
I460
65-68
0-45 15 48
694
004
2-49
301
on
<1-5
10
81
56
148
18
29
520
20
590
16
50
11931
67-95
0-85 1358
8-09
on
0-66
3-26 4 91
17-54
901
5 61
10007
242
005
99-86
2-84
0-56 0-27
100-24
1 28
32
Measured
uo
s
03
M
Standard BCR-1
SD
sm
54-62
0-18
Recommended 5406
2-24 13-64 13-64
0-18
3-55
7 0 4 3-43
1 72 0-38
480
12-2
329
190
407
38
30
126
19
36
1320
10
15
0-22
004
000
007
0 0 6 0-32
001
005
05
1-2
2
5
4
0
1
1
1
2
8
2
1
2-24 13-64 13-41
0-18
3 48
6-95 3-27
1-69 0-36
47 2
330
190
407
38
33
130
19
37
1390
13
16
002
100-44
14
Major element oxides are in weight percent; trace elements are in p.p.m. LOI, weight loss on ignition to 9O0°C for at least 10 h. Fe203*, all Fe as Fe^Oa. Estimated relative errors on major
and minor elements are 1%. For trace elements, estimated average overall errors based on measurements of standards AGV-1, BCR-1, BHVO-1,and W-1 are (in p.p.m.): 0-3 for Rb, 0-8
for Nb, 0-8 for Sr, 4-8 for Zr, 4-8 for V, 0-3 for Y, 1 -3 for Sc, 4-9 for Cr, 1 -3 for Zn, 1 -9 for Ni, 1 -8 for Cu, 0-5 for Co, and 39 for Mn. An indication of accuracy is given by measured values of
standard BCR-1 [average of 101 major and 5 trace element measurements (T. Hulsebosch, unpublished data, 1994)] and recommended working values (Govindaraju, 1989).
*°
"0
Table 3: Inductively coupled plasma mass-spectrometric datafor Malaita and Santa Isabel basement rocks and alkalic lavas
Sample no.
Rb
Ba
Til
U
Nb
La
Ca
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Y
Ho
Er
Tm
Yb
Lu
Malaita Older Senas
ML68
2
n.a.
0-28
007
2-9
32
8-9
1-37
6-63
2-04
084
2-8
0-50
3-4
18
069
1-99
0-30
1-81
0-27
8393
6
33
028
006
3-4
4-3
11 -2
1-62
800
2-66
103
3-3
0-62
4-1
22
0-86
2-43
0-35
2-13
0-33
8034
2
19
0-37
008
4-9
4-4
11-6
1-76
8-85
2-91
1-11
3-6
0-65
4-3
25
0-92
2-68
0-37
2-38
0-37
<1
18
0-40
0-10
5-1
4-6
12-7
201
10-3
3-73
1 -31
4-8
0-91
6-2
40
1-40
412
0-60
3-78
0-61
P439
3
31
0-51
0-11
6-1
6-1
15-9
2-37
11-8
3-88
1-38
4-7
0-84
5-5
32
1-21
3-37
0-48
300
0-47
P43
1
22
0-45
0-10
4-6
51
13-7
207
10-1
3-45
1-29
4-3
0-79
5-2
29
1-12
3-16
0-45
2-83
0-45
8374
1
24
0-44
0-12
4-8
5-1
13 7
2-10
10-3
3-50
1-29
4-4
0-80
6-1
28
108
3-09
044
2-68
0-41
8030
1
27
0-48
0 11
58
4-8
13 5
209
10-5
3-40
1-27
4-2
0-76
50
29
107
3-05
0-43
2-75
0-43
JADA
8029
2
31
0-46
009
5-7
5-4
14-3
2-17
110
3-57
1-33
4-3
0-80
5-2
32
1-15
3-26
0-44
2-91
0-45
««
7983
2
43
0-50
on
6-0
5-7
15-4
2-34
11-6
3-74
1-38
4-5
0-85
5-5
31
1-21
3-43
0-48
302
0-48
P49
O
Ramos It/and
1221
1
17
0-42
011
4-5
4-5
12-7
1-93
9-74
3 31
1-21
4-1
0-76
50
28
105
3-02
0-43
2-68
0-41
Z
O
Sigana Basalts
37
0-27
0-12
3-5
3-4
99
1-56
7-84
2-77
102
3-5
0-63
4-2
24
0-91
2-64
0-37
2-30
0-35
1
21
0-31
008
4-2
4-3
11-9
1-86
9-35
3-24
1-19
4-1
0-76
50
28
108
305
0-43
2-68
0-42
11918
1
38
0-33
008
4-4
4-5
12-5
1-94
9-80
3-42
1-22
4-2
0-80
5-2
29
1-11
3-18
0-46
2-85
0-44
I608
2
45
0-41
0-35
4-6
4-7
13-4
204
10-3
3-50
1-27
4-2
0-80
5-3
29
108
3-13
044
2-77
0-42
I2204
12
37
0-37
0-22
4-9
4-8
13-2
204
10-2
3-46
1-29
4-4
0-81
5-3
31
1-13
3-22
0-44
2-70
0-41
I262
2
27
0-42
009
4-7
4-8
13-1
1-94
9-70
3-19
1-23
40
0-74
50
28
105
303
0-44
2-71
0-43
12118
16
104
0-35
on
4-5
4-4
12-4
2-96
9-92
3-40
1 26
4-2
0-78
5-1
28
1-11
3-14
0-45
2-78
0-43
12141
1
34
028
007
3-4
3-4
100
1-54
7-70
2-73
102
3-5
0-63
4-3
23
0 89
2-60
0-37
2-31
0-36
36
710
7-51
1-83
100
680
9-74
305
8-2
1-16
6-4
31
1-23
3-14
0-42
2-54
0-38
9638
23
347
223
0-60
25
27-6
58-2
7-58
34-4
8-40
2-78
7-3
105
6-7
27
100
2-38
0-29
1-70
0-24
r
9969
22
358
2-11
0-57
22
25-9
54-6
7-24
33-2
8-38
2-89
76
109
5-9
29
105
2-56
0-31
1-78
0-24'
8503
44
561
4-67
1-16
55
52-5
99-3
3-72
9-6
1-27
6-8
31
114
2-65
0-33
1-78
0-24
Sigana Alkalic Su/ta
12216
120
13-2
51 0
Younger Serbs
12-5
54-8
12-0
(continued on next page)
L PLATEAU
2
12169
SOLOMON K
1
I96
Table 3: continued
Sample no.
Rb
BB
Th
U
Nb
La
Ce
Pr
Nd
7-79
33-3
Sm
Eu
Gd
Tb
Dy
Y
Ho
Er
Tm
Yb
Lu
2-60
7-5
107
5-7
25
104
2-47
0 30
1-73
0-24
3-66
9-8
134
6-9
31
M5
2-60
0-30
1-68
0-23
North Malaita Alkalk Suite
ML43
16
395
4-23
0-92
42
38-7
ML65
32
819
7-45
1-34
104
66-6
68-3
121
14-1
57-9
7-78
119
t—
0
c
San Jorge Volcanlcs and San Jorge-like lavas
11717
6
4
0-14
005
20
4-3
15 5
2-72
14-9
5 32
1-82
6-9
1-28
8-6
49
1-86
5-46
0-76
4 72
0-75
I2290
1
21
0-21
008
3-7
4-6
14-6
2-49
13-5
4-76
1-65
6-2
1 -13
7-5
45
1-66
4-77
0-67
4-12
0-65
<1
26
0-26
0-12
4-5
8-7
27-8
4-65
24-7
845
2-80
10-9
1-94
13-2
75
2-88
832
1-16
7-27
1 -13
n.a.
b.d.
b.d.
0-3
0-5
20
0-37
M9
0-59
20
0-37
2-3
16
0-50
1-40
0-19
1-19
0-18
6-69
2-00
8-5
1-40
8-9
49
1-87
5-04
0-66
3-83
0-57
1-85
0-49
1-7
0-32
20
14
0-46
1-33
0-19
1-34
0-22
I456
0
Kolosa'eru Gebbros
11679
n.a.
1419
1
16
0-12
004
20
6-2
20-7
3-59
1457
nj.
n.a.
0-12
006
0-2
1-3
3-4
0-49
2-51
20-1
2-88
9
f1
0
o
o
Leucocratlc Igneous Rocks
11480
0
Tl
-d
pi
78
0-13
005
2-5
7-7
23-4
3-75
20-5
7-23
2-41
90
1-59
10 3
58
2-10
5-80
0-81
5-12
0 76
c
m
S3
Standard BHVO-W
Measured
9
130
M5
0-35
188
±
1
1
002
001
03
10
131
M0
0-30
17-3
Published
16-7
004
16-1
37-4
515
23-9
6-44
2-16
6-2
0-98
5-7
28
107
2-70
0-34
1-99
0-29
0-3
002
0-4
002
001
0-2
001
0-1
1
003
0-09
000
000
001
36 0
4-90
232
6-53
2-26
6-3
104
60
27-9
M0
2-81
0-35
200
0-30
z
c
1
ro
APRIL 199
All abundances are in p.p.m. Our t w o measurements of in-house standard BHVO-W are compared w i t h published values (Garcia et a/., 1993; listed as BHVO-1 by those workers).
Measured La values have been reduced by 0-7 p.p.m. based on an apparent bias. Analytical precision is < 2 % for most elements, except for Rb, T h . T m , Yb, Lu ( < 3 % ) , a n d Nb (-4%)
(Knaack era/., 1991 A. J . Pietruzska, unpublished results, 1994). n.a., not analyzed; b.d., below detection limit.
TEJADA et al.
ONTONG JAVA PLATEAU BASALTS, SOLOMON ISLANDS
Table 4: Sr, Nd, and Pb isotopic compositions and isotope-dilution abundances of the basement rocks
and alkalic lavas ofMalaita and Santa Isabel
Sample no.
,
fSr/"Sr).rWT)
" P W - Pb
-w.pt)
Pb
Rb
0-280
0138
Sr
Sm
Nd
Malaita Older Series (T = 120 Ma)
ML68
LLU
0-70416
43
18-295
15-509
38-263
8393*
LLU
0-70423
40
18-377
15514
38-486
3-20
133 0
1-284
15-502
38-436
0-324
1229
3255
15-498
38-334
P384"
UUU
0-70420
4-8
18-521
P49
LUU
0-70406
60
18-375
P439
LU
0-70414
4-1
P431
LUU
0-70405
4-1
18-245
15-525
38-289
8030
LUU
0-704O8
4-4
18-443
15-495
7983
LLU
0-70405
3-7
18-332
15-513
0-70403
48
18-260
15-507
0-279
0076
41-52
5720
1-599
1 649
3-546
4-571
10 25
4302
7 082
0912
152-4
2322
0-790
0-140
112-2
3-291
38-466
0-241
0 201
160 2
1-772
5062
38-371
0-383
0-028
1 253
2-533
38-363
0-236
0-131
124-7
1-718
4-588
43-90
10-63
Ramos Island (7=120 Ma)
1221
LUU
S/gana Basalts (T= 90 Ma for 196.11918.12141;!= 120 Ma for 156,11445)
196
LUU
0-70369
5-4
18-644
15-543
38-602
0-232
0 453
101-7
2 414
7-143
11918
LLU
0-70374
5-8
18-625
16-540
38-456
0-316
0-408
103-9
1-455
3017
12141
LLU
0-70374
60
18-635
15-570
38-526
0-346
0-274
115-1
1-305
2-952
11445
LLU
0-70394
49
18-488
15-518
38-396
0-798
0-108
108-0
1798
4-244
156
LLU
0-70388
4-5
18-359
15-515
38-309
0 235
102
135-1
1-197
2-676
Sigana Alkalic Suite (1 = 90 Ma)
12216
12199
UUU
0-70336
4-7
20-338
15-634
40-382
3-22
LLL
0-70310
4-5
20-201
15-635
40-249
1 56
(19-98)
(15 63)
(39-95)
LLL
0-70330
37-6
802
4-2
20-202
1 5-632
40-301
1-54
13-3
1038
8-398 44 91
821 2
9-458
42-60
886-1
7-664
35-41
9254
38-90
Younger Series (7 = 44 Ma)
9538
LLL
0-70411
0-6
18-728
15-635
38376
2 50
23-5
9591
LLU
0-70426
-0-4
18-571
15-616
38334
399
360
9562
LLU
0-70412
05
18-791
1 5-636
38-416
1-77
10444
LUU
0-70433
-0-5
18-606
15-616
38-424
351
13-2
808
722-1
1009
10-03
646-9
2-910
952-3
7-142
4906
8-436
33-96
8503
UUU
0-70404
0-9
18-919
15-623
38-606
3-62
47-6
7935
9-305 44-16
8503
LLL
0-70405
10
18-901
15 629
38-610
1-48
21-9
671 5
5001
18-71
North Malaita Alkalic Suite (1 = 44 Ma)
ML43
LUU
0-70367
4-4
18 826
15-561
38-868
2-18
140
614-3
5-360
19-84
ML65
LLU
0-70370
4-7
18-764
16-552
38-817
3-64
13-8
652-0
9031
37-40
San Jorge Volcanic* and San Jorge-tike lavas (7=60 Ma)
11898
LUU
0-70251
9-1
18-567
15-507
37-999
0-330
0-125
77-90
1-792
4000
11717
LLU
0-70248
7-6
18-867
15-536
38-313
0-240
0012
99-90
2-599
5-442
12290
LUU
070289
9-1
18-814
16-529
38-133
0-185
0-488
180-1
2-995
7-710
1456
LLU
0-70260
7-4
18-845
16-537
38-366
0-873
0-105
107-4
4018
8-479
(continued on next page)
375
JOURNAL OF PETROLOGY
VOLUME 37
NUMBER 2
APRIL 1996
Table 4: continued
Sample no.
WO
"Pb/^Pb
Pb/^Pb
""Pb/^Pb
Pb
Rb
Sr
Sm
Nd
Kolose'eru Gabbnu (J=60 Ma)
1457
LLU
0-70400
78
18-799
16-563
38 360
0-514
0-70346
8-2
18-763
16-545
38-318
0-427
17-7
80-20
0-722
1-434
Leucocnticrocks(J =
11480
LUU
6-78
1370
5-349 16-51
'Samples analyzed by Mahoney (1987) and Mahoney & Spencer (1991). L, strongly acid-leached powder; U, unleached but chips
handpicked and acid-deaned; LLU, Sr and Nd isotopes determined on leached split, Pb isotopes determined on unleached split; LUU, Nd
and Pb determined from unleached split, Sr determined on leached split; LLU Sr, Nd, and Pb isotopes determined on leached split; UUU,
Sr, Nd, and Pb isotopes determined on unleached split Pb isotopic values for 12216 LLL enclosed in parentheses are age-adjusted
(Th = 1-53 p.p.m., U = 0-374 p.p.m.; analyzed by isotope dilution). Isotopic fractionation corrections are 1 4 8 Nd0/ 1 4 4 Nd0 = 0-242436
( 148 Nd/ 144 Nd = 0-241572), 8b Sr/ M Sr = 0-1194. Data are reported relative to University of Hawaii standard values: for La Jolla Nd,
T43 N d / i44 N d _ Q 5 1 1 B 5 0 . f ( j r N B S g 8 7 S r 87 Sr/ 86 Sr _ 0 . 7 1 0 2 4 -rj,,, t o t a | r a n g e m e a s u r e c | f or La Jolla Nd is ± 0000012 (0-2 e units); for
N BS 987 it is ± 0-000024 over the last 4 years. Pb isotopic ratios are present-day values, corrected for fractionation using the N BS 981
standard values of Todt et at. (1984); the total ranges measured for NBS 981 are ± 0012 for 208 Pb/ 204 Pb, ± 0012 for 2or Pb/ 2O4 Pb, and
± 0-038 for 208 Pb/ 20 *Pb. Within-run errors on the isotopic data above are less than or equal to the external uncertainties on these standards. Estimated uncertainties on abundances are <0-2% on Smand Nd, <0-5% for Sr, ~1%for Rb, and <1%for Pb. Total blanks are
negligible: < 2 0 pg for Nd; <120 pg for Sn and 5-30 pg for Pb. All abundances are in p.p.m. £Nd = 0 today corresponds to
14i
Nd/ 1 4 4 Nd = 0-51264; f o r ' ^ S m / ^ N d = 0-1967, eNd(T) = 0 corresponds to l 4 3 Nd/ 1 4 4 Nd = 0-512486 at 120 Ma, 0-512524 at 90 Ma,
0-512563 at 60 Ma, and 0-512583 at 44 Ma.
RESULTS
40
Ai>-39Ax age determinations
The incremental heating experiments produced
variably disturbed age spectra. Many developed
nearly ideal plateaux comprising a large majority of
the total Ar released from the samples. Spurious step
ages generally resulted from small amounts of Ar
released at low temperatures, which we attribute to
nonconcordant loss of radiogenic '"'Ar (during
alteration) and 39 Ar (during irradiation) from
alteration minerals.
Table 1 presents both age spectra (plateaux) and
isotopic ratio correlations (isochrons) from the
incremental heating experiments. Errors are
reported as the standard deviation of analytical precision. Plateau ages were calculated as weighted
means of three or more concordant step ages comprising at least 50% of the total 39Ar, where each
step was weighted by the inverse of its variance
(Lanphere & Dalrymple, 1978). Isochron ages were
calculated for 36 Ar/ + °Ar vs s6 Ar/ 4O Ar correlations
(York, 1969); SUMS/N-2 is the F-ratio statistic for
this regression. For all samples, 40 Ar/ 36 Ar intercepts
were near the atmospheric value (2955); however,
two intercept values (for samples 12169 and 12204)
were lower than atmospheric and produced isochron
ages somewhat higher than the plateau ages. There
is no evidence for significant excess w A r in any of the
samples, despite the fact that the lava flows were
erupted in a submarine environment. Integrated
ages were calculated by summing the gas composi-
tions of all steps, as if samples had been heated in a
single step, and provide rough (usually minimum
because of w Ar loss) age estimates in the absence of
plateau and isochron ages.
Because of the often exceedingly low concentrations of K in these basalts, we sometimes decreased
the number of heating steps so as to preserve precision in 39Ar measurements. This led to rather few
steps defining plateau and isochron ages, although
these are concordant in all but one case (sample
12169). Plateau ages almost invariably have smaller
standard errors because the weighting procedure
emphasizes the most precisely determined step ages,
whereas uncertainties in the isochron ages reflect the
analytical errors of the more poorly determined step
ages and the particular distribution of Ar isotopic
compositions in Ar/^Ar vs Ar/ Ar plots. We will
restrict our discussion to the plateau ages, but all
conclusions apply equally well to the isochron ages.
Four whole-rock basalt samples from the Malaita
Older Series, Sigana Basalts of Santa Isabel, and
Ramos yielded indistinguishable crystallization ages
that produced a weighted mean age of 121'3+ 0 9
Ma (la) for the oldest volcanic activity in the Pacific
Province of the Solomon Islands. Data for samples
8374 and P43 were originally reported by Mahoney
et al. (1993); their ages are recalculated here to
reconcile the different monitor standards used.
Figures 3a and 3b present age spectrum and isotope
correlation plots for Sigana basalt 12169; Ramos
sample 1221 had a similar release pattern. Significantly, the mean age for these basalts is identical
376
TEJADA et al.
ONTONG JAVA PLATEAU BASALTS, SOLOMON ISLANDS
to that for basalts drilled on the OJP at ODP Site
807 and DSDP Site 289 [ 1 2 0 6 ± 0 9 Ma (n=8),
recalculated to Mmhb-1 monitor age of 5139 Ma].
On the OJP, in addition to the 121-Ma group of
lavas, basalts from Site 803 gave 40 Ar- 39 Ar ages of
88-93 Ma (Mahoney et al., 1993). Three Sigana
Basalt samples yielded plateau ages in the same
range. Sample 12204 (Fig. 3c and d) produced a
four-step (mid- to high-temperature) plateau age of
9 2 0 ± l - 6 Ma; SI74 and SI88 gave plateau ages of
94-5 ± 3 1 and 89-8 ± 1-8 Ma, respectively. Thus, the
same two age groups found on the northern OJP are
also present along its southern margin in the Pacific
Province of the Solomon Islands. At present, it is
unclear how the ~90-Ma and ~ 121-Ma groups are
related structurally in Santa Isabel (or on the OJP
proper) because detailed structural and stratigraphic
control is lacking.
Discordant age spectra were produced from
experiments on samples 1262, 1256, 11445, and
SI 101, probably reflecting radiogenic *°Ar loss and
Ar recoil. In these cases, crystallization ages are
probably older than the integrated ages (99—119
Ma) because of the petrographic evidence for *°Ar
200
0.004
•125 4 ± 1.2 Ma
0.002
0.02
0.004
,94.7 ±2.5 Ma
0.002
0.02
12216
120
yo
60
f
e
•
0.004
•
,^\
r
0 002
•
30
^-93.3 ± 1.0Ma
-
n
n
0.03
10444
0.004 •
44.2 ± 0.2 Ma
0002
20
40
60
80
100
0.06
released
Fig. 3. Panels (a), (c), (e) and (g) are apparent age-release diagrams for "Ar- m Ar incremental heating experiments on basalt samples.
Horizontal bars indicate estimated analytical error (± la) about each step age. A plateau age (indicated) hat been determined from the
weighted mean of contiguous, concordant step ages. Panels (b), (d), (f) and (h) are the corresponding Ar/i^r vs 9Ar/*°Ar isotope
correlation diagrams for the step Ar compositions measured. The isochron age (indicated) ij calculated from the best-fitting line through
collinear step compositions, after York (1969); analytical uncertainties are shown.
377
JOURNAL OF PETROLOGY
VOLUME 37
loss during matrix alteration to clays. The likelihood
of integrated ages being minima in these cases, combined with compositional data presented below [and
unpublished data of I. Parkinson and R. Arculus for
SI 101 and the other SI Sigana Basalt samples
(1994)], leads us to believe that these samples correlate with the 121-Ma lavas. Sample 11053 yielded
a discordant spectrum with an integrated age of 65
Ma; isotopic and ICP-MS data are lacking but its
major and X R F trace element composition is similar
to that of the ~90-Ma lavas. Sample 12141 produced an apparently good plateau age of 60-9 ± 1 -6
Ma. This age is problematic because 12141 was
taken from a location very close to that of 12169
(122-9 Ma), has an incompatible element signature
closely resembling that of the ODP Site 803 lavas,
and has isotopic ratios that lie within the small field
denned by the Site 803 and Site 807 Units C-G
basalts (see below). Here, in the absence of any other
geochemically OJP-like lavas with post-90-Ma
plateau ages, we discuss this sample with the 90-Ma
Sigana Basalts.
NUMBER 2
APRIL 1996
may be related, and we henceforth refer to them as
'San Jorge-like'.
Malaita Older Series and Sigana Basalts
Nd,Pb and Sr isotopes
The isotopic compositions of the Malaita Older
Series, Ramos and Sigana Basalts are very similar to
those of the OJP basement lavas of ODP Sites 807
and 803 and DSDP Site 289 (Figs 4 and 5). Total
variation in initial £Nd(T) is small, +3-7 to +6-0, and
initial ( 87 Sr/ 86 Sr) T ranges from 070369 to 0-70423;
present-day Pb isotope ratios range from 18-25 to
18-64 for ^ " P b / ^ P b , from 1550 to 1557 for
2072O4
208204
These values largely overlap the ranges observed for
the OJP drillhole lavas. The Site 807 lavas were
subdivided into five units or packets of flows (from
top to bottom): Units A, C, E, F and G; isotopically
and chemically, these units form two slightly different groups, with Unit A composing one and Units
C-G the other (Mahoney et al., 1993). The one flow
recovered
from Site 289 is equivalent to the Units C Surprisingly, the two Sigana Alkalic Suite
G
type,
whereas
the Site 803 lavas have intermediate
samples, 12216 and 12199, also gave results of
91 -7 ±0-4 Ma; their plateau ages are well established incompatible element signatures but are isotopically
(e.g. Fig. 3e and f), and we conclude that these similar to Units C-G. Remarkably, despite the
alkalic intrusions were emplaced concurrently with ~1150-1600-km distances of the ODP drill sites
the ~ 90-Ma group of Sigana Basalts and the Site from the outcrops in the islands, the Pacific Province
803 lavas. In contrast, an alkalic lava from the isotopic results cluster distinctly into Unit A-type
Younger Series in Malaita produced an excellent and Units C-G/803-type groups: data for the
plateau (Fig. 3g and h) with a weighted mean age of Malaita Older Series lavas, Sigana Basalts 156 and
44-2 ±0-2 Ma (Middle Eocene). Thus, the alkalic 11445, and Ramos Island overlap or lie close to those
suites of Malaita and Santa Isabel are of different of Unit A, whereas isotopic ratios of the other three
ages and have different origins. It should be noted Sigana Basalt samples overlap or lie near those of
also that the Younger Series lava is 10 m.y. older Units C-G and Site 803 (Fig. 4). The Malaitan
than the 34-Ma alnoites of north-central Malaita Older Series data extend to slightly lower £ Nd (T)
(Davis, 1978), suggesting the alkalic rocks and than the Unit A lavas and display a somewhat wider
range of Pb isotope values, but the Pb isotopic ratios
alnoites are also unrelated.
are not age-corrected and Unit A at Site 807 consists
Five basalts from the San Jorge Volcanics proof only 46 m of flows, which probably do not
duced Early Tertiary plateau ages of ~ 6 2 and ~ 4 6
represent the full isotopic range of this class of lavas.
Ma. One sample (11451) showed a discordant
spectrum, with an integrated age of 35 Ma. From
compositional data, discussed below, we believe Trace and major elements
these rocks were part of an island arc—backarc basin Alteration-resistant minor and trace element abundcomplex that was tectonically merged with the ances, interelement ratios, and primitive mantleoceanic plateau basalts. Finally, three samples normalized element patterns of the Malaita Older
(12290, 11898, and 12217) from north of the KKFS Series, Ramos lavas, and most Sigana Basalts are
are petrographically, chemically, and isotopically nearly identical to those of the OJP lavas (Fig. 6a).
unlike the Sigana Basalts but similar to the San Incompatible element ratios display only small variJorge Volcanics (see below); we dated one of these, ations (e.g. Fig. 7d), and as with the drillhole lavas
12290, obtaining a plateau age of 6 2 5 ± 1 5 Ma, (Mahoney et al., 1993) ratios such as Zr/Nb (15-18),
equivalent to those of several San Jorge Volcanics Y/Nb (5-7), Ce/Yb (4-3-5-3), and La/Sm (1-5-1-9)
samples. The structural—stratigraphic relationship of are intermediate between values of normal midthese lavas with the San Jorge Volcanics is unclear ocean ridge basalt [MORB; 30-50,15-25, 1-4-3-6,
but their age and geochemical affinities suggest they and 0-6-1-2, respectively; e.g. Sun et al. (1979)] and
378
TEJADA it al.
ONTONG JAVA PLATEAU BASALTS, SOLOMON ISLANDS
+12
+8
Units C-G / Sit» 803
Unit A ^ Samoan Shields
z
CO
0.702
•
Malatta Older Series
D Sigana Basalts
•
Younger Series
o N Malaita AkalK Suite
•
Sigana Altaic Suite
A
San Jorge Volcanlcs and
San Jorge-(Eke Lavas
•
Leucocrattc Rocks
O
Kolose'eru Gabbroa
CO
CD
CO
fe
0.704
Unit A
Units C-G/Stte 803
Padlic
MORB'
0.702
17
18
19
206pb/204pb
20
Fig. 4. Initial eNd(T) vs initial
(a), £ Nd (T) vi 206 Pb/ 204 Pb (b), and ("'Sr/^Sr)-,- vi 206 Pb/ 204 Pb (c), for Malaita, Santa Isabel
and Ramos basement and alkalic rocks. Fields for ODP Site 807 Unit A and for Site 807 Units C-G and Sue 803 on the OJP (Mahoney
et al., 1993) are indicated with heavy continuous outlines. Also shown are fields for Pacific MORB (White et al., 1987; Mahoney el al.,
1994, and references therein), Kerguelen Plateau (H. Davies et al., 1989; Wei« et al., 1989; Salters et al., 1992; Mahoney et al., 1995),
Manihiki Plateau (Mahoney & Spencer, 1991), and selected Pacific OIB fields: Rarotonga (Palacz & Saundcrs, 1986; Nakamura &
Tatsumoto, 1988), Samoan shield and post-erosional (Wright & White, 1986); Pitcairn Seamounts (Woodhead & Devey, 1993); Louisville Ridge (Cheng et al., 1987), and Mangaia Group (VidaJ el al., 1984; Palacz & Saunders, 1986; Nakamura & Tatsumoto, 1988;
Hauri & Hart, 1993). The field for Malaitan alnSitej in panel (a) is from Neal & Davidson (1989). Errors for data of this study are
similar to or smaller than the size of the symbols.
average ocean-island basalts [OIB; 5-8, 0-04, 37, and
3-7, respectively; e.g. Sun & McDonough (1989)].
Three Sigana Basalt samples (11445, 11123, and
11658) have higher alteration-resistant incompatible
element contents (except for Sr) than the drillhole
lavas, the Malaita Older Series and other Sigana
Basalts (Figs 6b and 7), but similar Zr/Nb, Y/Nb,
Zr/Y, and other interelement ratios; here we refer to
them as the 'high-Ti Sigana Basalts'.
The Sigana Basalts, Malaita Older Series, and
Ramos island samples are low-K tholeiites ( 0 0 3 0-19 wt%, with one exception) with major element
compositions closely akin to those of the OJP
drillhole lavas. Ranges of major element composi-
379
tions are relatively small (Table 2 and Fig. 8); e.g.
TiO 2 11-1-9 wt%; A12O3 12-7-15-8 wt%; total Fe
as Fe 2 O 3 11-3-15-5 wt%; CaO 9-5-12-5 wt%; P 2 O 5
008-0-17 wt%; and MgO 5-5-8-4 wt%, except for
one sample with 11-1 wt % found as a clast in a
conglomerate and which has suffered both submarine alteration and significant subaerial weathering. Notably, the major element data cluster into
three groups, two of which correspond approximately to those defined by the Site 807 Unit A and
Site 803 lavas, although the island basalts have
somewhat higher MgO and total Fe, on average.
This grouping is evident in the Al 2 O3/TiO 2 vs
FejO3* and TiO 2 vs MgO diagrams of Fig. 8, in
JOURNAL OF PETROLOGY
I VOLUME 37
40
APRIL 1996
NUMBER 2
c 10
CO
Samoan
Post-arosional
Samoan
CD
•5
E
a.
J2
Q.
§39
o
o
o
Q.
CD
O
CM
_
1
2
I
I
I
I
I
I
I
I
I
I
I
I
I
76
to
re
38
m
to
c
re
g>
15.7
-Q
Q.
o
•
>
o
c
o
O
Mangala
Group
Fig. 6. (a) Average incompatible element patterns of Malaita
Older Series, Sigana Basalts, Site 807 Unit A and Units C-G,
Site 803, and normal (N-) MORB. Site 807 and 803 data are
from Mahoney et al. (1993); N-MORB average and primitivemantle normalizing values arc from Sun & McDonough (1989).
(b) High-Ti Sigana Basalt data and average Malaita Older
Series data normalized to average (non-high-Ti) Sigana Basalt.
(Note that thej^axis scale is linear.)
*
ManWdPI.
CM
JQ
Q_
o
CM
Rarotonga
15.5
F Unto C-G/Slta 803
ManWHPI.
-I—I
h H—f—f-I—I—I I I I I I
1 1 1
TtiNbLaCePrSrNd PZrSmEuTlGdTbDyY HoErTmYbLu
1
represent a distinct chemical group not sampled by
drilling on the plateau; in general, these samples
seem to have undergone greater degrees of differentiation than the other lavas [for example, they lie
18
19
20
near the more fractionated ends of model lowpressure fractionation paths (arrows) in Fig. 8].
206pb/204pb
Overall, the Malaita Older Series and Sigana
Basalts
have similar mg-numbers (47-59) to the
206
204
Fig. 5. Present-day Pb/^^b vs Pb/ Pb (a) and " ' P b / ^ P b
(b) for Malaita and Santa Isabel basement and drillhole lavas (43-64) and, like them, do not form
alkalic rocks. Error bars are for data reported in thij paper. Refer- well-defined trends in most major element plots such
ence* for the fields shown are as in Fig. 4. The dashed line for as those in Fig. 8. The combined data for the
Sigana AlWalic sample 12216 connects present-day and agedrillhole and island lavas cannot be explained solely
corrected values.
by different amounts of low-pressure gabbro fractionation (i.e. olivine + clinopyroxene + plagioclase
which most of the island data resemble those of Site removal) or high-pressure fractionation (i.e. clino807 Unit A, whereas data for several samples fall in pyroxene + orthopyroxene removal; e.g. Bender et al.,
or close to the Site 803 field. Given the large 1978). Some of the major element variation
distances between the drill sites and the islands, and undoubtedly results from variable alteration (see
the shallow basement penetration by drilling, the Mahoney el al., 1993), particularly for CaO, MgO,
similarities suggest that these chemical groups are Na2O, and KjO. Alteration effects are minimized in
widespread. However, lavas with major element diagrams using AI2O3, TiO 2 , and Fe2O3 (e.g.
characteristics corresponding to the Units C-G type Bienvenu et al., 1990) (Fig. 8), where the distinct
of Site 807 are not present in our data set. Con- groupings of data are likely to reflect variations in
versely, the high-Ti Sigana Basalts appear to primary magma composition arising from slight
Pacific MORB
380
TEJADA el at.
ONTONG JAVA PLATEAU BASALTS, SOLOMON ISLANDS
25
120 '
,
-
'
•
•
100
'
80
fit I
/ /\
/
••*•)
;' i
o
60
••'• /
20 -
»
o
15
•'
*
•A"
/
.-•'A .-''
_ . . . . - - < > • • • ,
\
\
\
\
i
'A!
0
10 40
20
5-
a
o- i
—i
—
200
400
1
if
1
H
600
0
Zr
b
1
r—
50 100
Nb
145
MUQEAHTES
•
HAWAIITES
O
/
• 'A
/
.ff
t'~
/}.
••• • " ' A ' / A
Malaita Older Series
D
•
Sigana Basalts
0
N. Mataita AJkafic Suite
Vbunger Series
A Sigana Alkafic Suite
6
CM
CO
A
Jajao Igneous Suite
•
High Tl Sigana
Basalt.
MORB
0- A
40
50
60
70
Zr/Y
SiO 2
10
15
Fig. 7. Plots of Zr and Ti vs Nb (a) and (b), Na 2 O + K 2 O vi SiO 2 (c), and Zr/Nb vs Zr/Y (d) for the different rock groupi of Malaita
and Santa Isabel. Dashed fields in (c) are from Cox et al. (1979) and the line dividing alkalic and tholeiitic fields (strictly for Hawaiian
lavaj) is from MacDonald & KaUura (1964). (Note that several Sigana Basalts fall above this line aj a reiult of relatively high degrees of
alteration.) Fieldi for EPR MORB [Mahoney et al. (1994) and references therein] and the OJP drillholes (Mahoney et al., 1993) are
shown in (d).
account for the difference between Type 2 and Type
3, in particular (Castillo et al., 1986). In addition to
major element similarities, the Nauru Basin basalts
(Floyd, 1986; Saunders, 1986; Castillo et al., 1986)—
and basin-filling Aptian lavas in the East Mariana
basin to the north (Castillo et al., 1991, 1994)—
generally resemble the OJP lavas in their trace
element signatures and overlap isotopically with
the Units C-G/803-type OJP basalts, suggesting
that they are related to the OJP.
differences in the average degree or conditions of
melting, and/or source composition.
Interestingly, the Aptian basalts filling the Nauru
Basin, adjacent to the OJP on the east, also define
three chemical types (Types 1-3) based on major
element data (Castillo et al., 1986). The Type 1
basalts are more primitive than yet found on the
OJP, but Type 2 is similar to the Units C-G-type
lavas, and Type 3 resembles the Unit A-type lavas.
Variable degrees of partial melting were invoked to
381
JOURNAL OF PETROLOGY
VOLUME 37
NUMBER 2
APRIL 1996
limit of the age of the Earth (e.g. White, 1993), the
Pb isotopic values lie close to it (Fig. 5b). Except for
higher 2D7Pb/ Pb, the isotopic compositions of the
Younger Series lavas overlap with those of the
hypothetical 'PHEM' [undegassed (primitive?)
helium mantle] component postulated by Farley et
al. (1992) to be the high- 3 He/ 4 He mantle endmember in Samoan alkalic basalts, although the
pure end-member signature is not seen in modern
products of the Samoan hotspot. Compared with
present-day South Pacific OIB, the Younger Series
isotopic values are most similar to those for
Rarotonga in Sr-Nd, Nd-Pb, and Sr-Pb isotope
plots, but the Younger Series lavas have lower
T
508204
0.5
6
8
MgO
14
12CM
Q
O
cs
10-I
Site 803
Unit A
207204
In contrast to the Younger Series, the Nd and Sr
isotopic compositions of the two North Malaita
Alkalic Suite samples analyzed lie near the fields of
the OJP, Malaita Older Series and Sigana tholeiites:
£Nd(T)= + 4-4, +4-7 (i.e. equivalent to Unit A-type
lavas); ( 87 Sr/ 86 Sr) T = 070367, 0-70370 (equivalent
to Units C-G/Site 803 lavas). Their Pb isotope
values are also roughly similar to those of the
basement tholeiites, although they have higher
206p b/ 204p b
207
a n d
204
208 p b / 204p b
(206 pb/ 2<H pb
208
=
jg.^
204
18-83;
Pb/ Pb= 15-55, 15-56;
Pb/ Pb =
38-82, 38-86). This combination of isotopic ratios is
not within the range of any modern Pacific OIB,
although, for example, some islands of the Cape
Verde group in the North Atlantic Ocean (G. Davies
et al., 1989) have similar compositions. Both the
15
10
20
Younger Series and the North Malaita Alkalic Suite
are isotopically different from the unusual 34-Ma
Malaitan alnoite intrusions; in particular, the alnoites
Fig. 8. TiO 2 vs MgO (a) and Al 2 O 3 /TiO 2 vs total Fe as Fe 2 O 3 *
are characterized by much higher ( 87 Sr/ 86 Sr) T (e.g.
(b) for MaJaita Older Series, Ramos, and Sigana Basalts along
Neal & Davidson, 1989).
with fields for Site 807 and 803 lavaj (Mahoney it al., 1993). The
high-Ti Sigana Basalts and Site 803-like Sigana Basalts are
The two Sigana Alkalic Suite samples analyzed
enclosed by dashed fields. Continuous lines with arrows represent
isotopically
are distinct from both alkalic suites of
low-pressure liquid lines of descent (for 0-30% crystallization) of
Malaita in having markedly high 206 Pb/ 204 Pb
sample 8034 calculated with the CHAOS program of Nielsen &
Dungan (1983).
(20-20), 207 Pb/ 2O4 Pb (15-63, 15-64), and 208 Pb/ 20+ Pb
(40-25, 40-30), and low ( 87 Sr/ 86 Sr) T (0-70310,
0-70330) at £ Nd (T) of+4-2 and +4-5. These values
overlap or lie close to the fields of present 'HIMU'Malaita and Santa Isabel Alkalic Suites
type oceanic islands in the South Pacific, termed the
Nd, Pb and Sr isotopes
Mangaia Group by Nakamura & Tatsumoto (1988).
Our results reveal that the Younger Series, North However, the Sigana Alkalic Suite data do not
Malaita Alkalic Suite and Sigana Alkalic Suite are exactly match the compositions of any single presentall isotopically distinct from one another (Figs 4 and day island in all three isotope systems. For instance,
5). The Younger Series have near-Bulk-Earth Nd the Sigana Alkalic Suite samples have similar
206
Pb/ 28+ Pb but lower e Nd (T) than Rurutu, and
and Sr isotopic compositions [eNd(T)=-0-5 to +1-0;
87
86
87
86
( Sr/ Sr) T = 0-70404-0-70433], but non-Bulk-Earth similar ( Sr/ Sr) T and e Nd (T) but lower
206
204
206
204
Pb/ Pb than Mangaia or Rimatara. More genPb isotopic
ratios
( Pb/ Pb = 18-57-18-92;
207
Pb/ 20+ Pb = 15-62-15-64; and 208 Pb/ 204 Pb = 38-33- erally, the isotopic compositions of the Sigana
38-61) relative to a 4-55-Ga geochron. However, if a Alkalic Suite samples differ from those of the
208
Pb/ 204 Pb for
4-45-Ga geochron is assumed, based on the lower Mangaia Group in having higher
• Older Series
• Sigana Basalts
• HighTi Sigana
Basalts
382
TEJADA el al.
ONTONGJAVA PLATEAU BASALTS, SOLOMON ISLANDS
their 206 Pb/ 204 Pb values, which are at the low end of
the Mangaia Group range.
resembles that of 12216 (Sigana Alkalic Suite)
between Th and Ho, and by that of ML43 (North
Maiaita Alkalic Suite), the shape of which generally
resembles that of 8503 (Younger Series). Overall, the
North Maiaita Alkalic Suite samples have incompatible element ratios (e.g. Zr/Nb = 3-4, Th/
Nb = 0-08-0-13,
La/Nb = 0-70-0-85,
and
Th/
La = 012) rather similar to those of the Sigana
Alkalic Suite, which is surprising in view of the large
contrast in isotopic ratios. Interelement ratios of the
Sigana Alkalic Suite lavas (e.g. Zr/Nb = 3-4, Th/
Nb = 008, La/Nb = 0-69, and Th/La = 0-ll) are
consistent with their isotopic signature in that they
are within the range of values for the 'HIMU'-type
Cook-Austral islands (Dupuy et al., 1988). Likewise,
the Younger Series La/Nb and Th/Nb ratios overlap
with the values for Rarotonga (Palacz & Saunders,
1986), but Rarotongan Zr/Nb ratios are lower and
Major and trace elements
As a group, the alkalic rocks are distinguished from
the Sigana and Maiaita Older Series tholeiites by
much higher total alkalis, TiO 2 and P2O5, and substantial enrichment in the incompatible trace elements (Fig. 7) comparable with most alkalic OIB.
Major and trace element differences among the
alkalic suites agree broadly with the groupings
established on the basis of isotopic differences. For
example, total alkalis are highest and SiO 2 lowest in
the Sigana Alkalic samples, which have basanitic or
near-basanitic compositions, whereas the Younger
Series have the highest SiO 2 (Fig. 7c), with several
reaching basaltic andesite compositions. Likewise,
Nb content (Fig. 7a and b) increases from Younger
Series (22-52 p.p.m.), to North Maiaita Alkalic
Suite (44-97 p.p.m.), to Sigana Alkalic Suite (84120 p.p.m.); this increase correlates with a change
from positive to increasingly negative Nb—La slope
in primitive-mantle-normalized element diagrams
(Fig. 9). The Younger Series show the most distinctive trace element characteristics among the
three alkalic suites, particularly in having a positive
to flat Nb-La slope (La/Nb= 10-1-2) and generally
lesser relative enrichment in the highly incompatible
elements than the other two alkalic suites (e.g. Zr/
Nb = 6-14 vs 3-4; Fig. 7d). However, unlike their
distinct isotopic compositions, incompatible element
characteristics overlap somewhat among the alkalic
suites, as shown in Fig. 9 by the ML65 (North
Maiaita Alkalic Suite) pattern, which closely
Th/La higher.
Jajao Igneous Suite
SanJorge Volcanics and San Jorge-like lavas
Together with their Early Tertiary ages, the isotopic
compositions of the San Jorge Volcanics and the San
Jorge-like samples distinguish them clearly from the
~90- and ~121-Ma Sigana Basalts and Maiaita
Older Series (Figs 4 and 5), compared with which
they have much lower ( 8 7 Srf 6 Sr) T (0-702480-70289), significantly higher e Nd (T) (+7-4 to +9-1),
higher 206 Pb/ 204 Pb (18-57-18-87), and lower
2o6pb/2O4pb (3 8 .oo-38-36). Indeed, the isotopic
compositions of both the San Jorge Volcanics and
the San Jorge-like samples fall within or very near
the Pacific MORB field in all isotope plots; the two
100--
>
•e:
E
Q.
10--
i5
— • — 8503
— o — ML43
Q.
CO
C/3
— D — 9959
— » — ML65
— • — 9538
— f l — 12216
H—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—h
ThNbLaCePrSrNd P ZrSmEuTi GdTbDy Y HoErTmYbLu
Fig. 9. Incompatible element patterns for samples of the three alkalic suites of Maiaita and Santa Isabel with the field for the Austral
Islands (heavy continuous outline) from Dupuy it al. (1988). The P spike on the ML43 pattern is probably a result of secondary phosphate associated with calcite vesicle filling.
383
JOURNAL OF PETROLOGY
VOLUME 37
San Jorge-like samples analyzed isotopically (12290
and 11898) possess higher ENd(T) (Table 4) than the
two San Jorge Volcanics samples analyzed (1456 and
11717), whose almost identical compositions fall at
the less radiogenic Nd end of the Pacific MORB
field.
As a group, the San Jorge Volcanics and San
Jorge-like lavas are characterized by a fairly wide
range of major element variation (e.g. MgO = 3 8 9-6 w t % , TiO 2 = 1-4-3-4 wt%, and Fe 2 O 3 = 1 0 0 17-7 w t % ) . Although their major element compositions are generally MORB-like, the majority of San
Jorge Volcanics samples have slightly elevated
A12O3, similar to backarc-basin basalts (BABB),
whereas the San Jorge-like samples from north of the
KKFS and one of the San Jorge Volcanics show
values similar to normal MORB, as illustrated in
Fig. 10; also, for example, the San Jorge-like samples
have higher, MORB-like CaO/Al 2 O 3 of 0-75-0-86
(cf. 0-66-0-85 for MORB; Sinton et al., 1991),
whereas most San Jorge Volcanics have CaO/Al 2 O 3
of 0-43-0-75, similar to BABB (0-60-0-75; Hawkins
& Melchior, 1985). The San Jorge Volcanics and
San Jorge-like samples also display large variations
in abundances of compatible and incompatible trace
elements, such as Nb (1-8-5-1 p.p.m.), Zr (76—254
p.p.m.), Y (30-73 p.p.m.), V (273-546 p.p.m.), Ni
(18-133 p.p.m.) and Cr (9-685 p.p.m.). They have
markedly greater Zr/Nb (33-168 vs 15-18; Fig. 7d),
Y/Nb (11-27 vs 5-7), and Ti/Nb (2720-7290 vs
1590-2420) than the Sigana Basalts and Malaita
20
MarianaTrough
18 -
•
V*
16 CO
•
o
CM
\rmt/
•
i
O
sZT/ 1 j
a
14 -
•
LSC
or.
San Jorge-like
D
12 •
10
'
1
1
12
MgO
Fig. 10. A12O3 vi MgO for Jajao Igneous Suite. Filled squares are
San Jorge-like and San Jorge Volcanics, open squares are
Kolose'eru Gabbros, and filled diamonds are leucocratic rocks.
Dashed field encloses data for the three San Jorge-like samples.
Fields for Mariana Trough and Lau Spreading Center (LSC) are
from Hawkins (1995); field for MORB is from Hochstaedter et al.
(1990).
NUMBER 2
APRIL 1996
Older Series tholeiites. The samples with more
MORB-like major element characteristics have
essentially normal-MORB-type trace element signatures (see patterns for 12290 and 11898 in Fig.
lla), whereas the more BABB-like lavas (see 11717
pattern) exhibit moderate Nb depletion relative to
La (La/Nb ~ 2 vs ~ 1 in the MORB-like samples),
variable Ti depletion, low Cr and Ni (unrelated to
Mg/Fe values) in some samples, and usually higher
K and Rb relative to the normal-MORB-like lavas.
These characteristics are often taken as an indication
of a supra-subduction-zone affinity in some ophiolitic
rocks (e.g. Jenner et al., 1991; Yogodzinski et al.,
1993).
Kolose'eru Gabbros
The Kolose'eru Gabbros are mafic to intermediate in
composition and, like the San Jorge Volcanics,
exhibit a rather wide range of major and trace
element abundances (Fig. 10). Chemically, some
samples are similar to the broadly BABB- to MORBlike San Jorge Volcanics, whereas others are unlike
the San Jorge Volcanics in having clear affinities to
island-arc rocks (arc-like, e.g. Fig. 12). The broadly
BABB- to MORB-like gabbros are characterized by
significantly lower MgO (as low as 5-0 wt %) and
markedly higher TiO 2 (up to 3-1 wt%) than the
arc-like ones (with MgO up to 110 wt % and TiO 2
as low as 023 w t % ) , which are compositionally
similar, for instance, to the arc ankaramites of Epi,
Vanuatu (Barsdell & Berry, 1990). The sample with
the highest TiO 2 content, 11679, also has very high
Fe 2 O 3 (18-79 wt%) and V (1030 p.p.m.) as a result
of ilmenite accumulation, but its Ti/V ratio of 20
qualifies it as arc-like (Fig. 12). The only BABB-like
gabbro for which we have both ICP-MS and XRF
data (1419) shows marked Nb and Zr depletion (e.g.
La/Nb = 3-4, Zr/Sm = 81). (It also has a high P
content, which appears to reflect the presence of a
small amount of secondary apatite.) On the other
hand, the arc-like gabbros (1457, 1391, and 11679)
show even lower Nb and Zr abundances (Fig. l i b
and c), similar in magnitude to those of, for example,
the Epi arc ankaramites (Barsdell & Berry, 1990)
and arc-like ophiolitic rocks of the Little Port
Complex in western Newfoundland (Jenner et al.,
1991); sample 1457 also shows marked enrichment in
large-ion-lithophile elements (over and above that
probably caused by alteration effects). Another distinctive characteristic of 1457 and 1391 is the low
concentrations, relative to MORB, of moderately
incompatible elements; these features are also shown
by arc-like rocks in ophiolites elsewhere (e.g. Jenner
et al., 1991) and, together with their Ti/V, Ti/Zr and
384
TEJADA et al.
ONTONG JAVA PLATEAU BASALTS, SOLOMON ISLANDS
10"
10-
m
a:
O
0.1-\ i i \ i i i r
Rb K ThNbLaCePr Sr Nd P ZrSmEuTi GdTbDy Y HoErTmYbLu
CO
01-r
1
Rb
I
K
1
I
Nb
Sr
i
P
i
I
1
Zr
Ti
Y
Fig. 11. N-MORB-normalized incompatible element pattern! for different chemical typei of the Jajao Igneous Suite: (a) San Jorge-like
11898 and 12290, San Jorge Volcanics 11717, plus leucocratic rock 11480; (b) arc-like Kolose'eru Gabbroi 1457 and 11679, compared
with representative arc-like Little Port Complex pattern [MB111; data of Jenner it al. (1991)]; (c) arc-like leucocratic rocki 11489 and
1460 and Kolose'eru Gabbroi 1391, and 1457, with representative pattern for western Epi arc ankaramite [W. Epi-12; Barsdell & Berry
(1990)].
Sc/Y ratios, high Sr/Nd, high Al 2 O 3 /TiO 2 (up to 77
in 11516) and low TiOj, indicate a clear island-arc
affinity for these micrograbbros.
Only one of the Kolose'eru Gabbro samples (the
chemically arc-like 1457) was analyzed isotopically;
it is from a dike located close to one of the San Jorge
Volcanics basalts (1456) and has Nd and Pb isotopic
ratios similar to those for the San Jorge Volcanics
but
much
higher
acid-leached
( Sr/ Sr)x
(0-70400), yet the sample is petrographically
relatively fresh and the high ( 87 Sr/ a6 Sr) T is unlikely
to be a result of seawater alteration. The isotopic
composition of this dike is broadly similar to those
found for western Pacific island-arc rocks, such as in
the Marianas (Woodhead & Fraser, 1985) and
Papua New Guinea (Hegner & Smith, 1992), and
385
suggests a slightly different source from that of the
San Jorge Volcanics.
Leucocratic igneous rocks
Although more evolved (Tables 2 and 3), the leucocratic rocks also can be grouped into broadly BABBlike and arc-like classes defined above, the arc-like
samples being characterized by low TiOj ( < 1
w t % ) , Y and Nb contents. MORB-normalized
element patterns of representative samples (11480
and, 11489, both andesites, and 1460, a dacite) are
shown with those of the San Jorge Volcanics and
arc-like Kolose'eru Gabbros in Fig. l l a and c. The
one leucocratic sample analyzed isotopically, 11480,
JOURNAL OF PETROLOGY
1200 -
San Jorge Volcanics,
San Jorge-like Lavas
D Kolose'eru Gabbros
VOLUME 37
B
) d&
800
/UORB,
ARCS
/
/
-
BABB
•
/
m
r
400
/
0
5
•>
10
1£>
20
2^
Ti/1000
Fig. 12. V vs Ti/1000 for Sanjorge Volcania, San Jorge-like, and
Kolose'eru Gabbro samples. Arc and MORB + BABB boundaries
are from Shervaii (1982) and data of Woodhead et al. (1993).
is similar to the Kolose'eru Gabbro sample in having
higher ( 87 Sr/ 86 Sr) T than the San Jorge Volcanics
lavas (although not as high as the Kolose'eru
Gabbro sample: 0-70346 vs 0-70400); again, this
appears unlikely to be a result of alteration. Thus,
the evolved nature of the leucocratic rocks may
simply be a result of differentiation from the more
mafic Kolose'eru-type magmas, as suggested by
Hawkins & Barron (1991).
For the Jajao Igneous Suite as a whole, the close
association in the field of lavas having MORB- or
BABB-like characteristics with a group of hypabyssal
and intrusive mafic to felsic rocks having both
BABB-like and arc-like affinities at least superficially
parallels the examples described by Jenner et al.
(1991) for the Bay of Islands Ophiolite and Little
Port Complex, and by Yogodzinski et al. (1993) for
the backarc-type crust of the Attu Basement Series of
the Western Aleutians, as well as others. Moreover,
the chemical variations observed among the San
Jorge Volcanics, Kolose'eru Gabbros, and associated
leucocratic rocks broadly mimic those of in situ island
arc and backarc basin pairs from the western Pacific
convergent margins (e.g. Woodhead et al., 1993).
DISCUSSION
Malaita Older Series and Sigana Basalts:
on-land outcrops of OJP crust
One important finding of the present study is the
confirmation of previous suggestions (Hughes &
Turner, 1976; Hawkins & Barron, 1991; Mahoney &
Spencer, 1991; Mahoney et al., 1993; Parkinson etal.,
1993; Saunders et al., 1993) that the Pacific Province
basement lavas are correlative with lavas drilled on
the OJP. The combination of ~121-Ma and ~ 9 0 -
386
NUMBER 2
APRIL 1996
Ma ages, close isotopic similarities, and close elemental (e.g. Figs 6a and 7d) similarities appears to
rule out conclusively any possibility that the Pacific
Province is unrelated to the OJP (Musgrave, 1990).
In view of the wide spatial distribution of the
samples (i.e. ~350 km in Santa Isabel and Malaita
to ~1600 km between southern Malaita and Site
807), the limited range of isotopic and incompatible
element ratios implies a uniform, well-mixed mantle
source for the world's largest oceanic plateau. The
occurrence of essentially the same plateau lava types
with exactly the same (within errors) 121-Ma ages in
widely separated areas (Malaita Older Series^
Ramos Island, some Sigana Basalts, and Site 807
and Site 289 basement) is consistent with a cataclysmic outburst of voluminous eruptions above a
large, surfacing plume head (e.g. Richards et al.,
1989). However, the isotopic ratios and incompatible
element characteristics of the ~90-Ma Sigana
Basalts and Site 803 lavas are very similar to those of
the 121-Ma group (particularly to Site 807 Units C G isotopically) and thus pose a complication.
Although they could indicate that the OJP stayed
above a vigorous mantle plume for ~ 3 0 m.y., like
the much smaller Icelandic Plateau, the lack of any
lavas with plateau ages between ~121 and ~ 9 0
Ma, or of a discernible age progression across the
plateau, does not support this possibility. Evidence
on eruptive ages should probably be considered
somewhat inconclusive because basement sampling is
still limited to only a few locations, represents only
the upper parts (9 m to ~ 2 km) of the OJP crust
[which may have an average total thickness of as
much as ~ 3 6 km; Hussong et al. (1979)], and one
isotopically and chemically OJP-like Sigana Basalt
sample (12141) gave a problematic *°Ar—39Ar age of
only 61 Ma; nevertheless, existing data strongly
suggest two relatively short, major eruptive events at
— 121 Ma and at ~ 9 0 Ma, both producing lavas of
very similar composition.
Can these results be reconciled with a plume-head
model for the formation of the OJP? Of particular
interest in this regard is the shape of the OJP, which
consists of two main parts: a large high plateau (the
northern and western part) and an eastern lobe or
'salient' (Fig. la). Bathymetric and seismic reflection
profiles over the OJP reveal two large domal features, one located on the main plateau ~ 200-300
km southwest of DSDP Site 289 and the other on the
eastern lobe ~ 250-300 km east-northeast of Site 288
(Kroenke, 1972). The relative positions of the two
domal areas are consistent with the ~120-90-Ma
Pacific Plate motion proposed by Kroenke & Sager
(1993) over a fixed hotspot source located approximately at 43±5°S and 160±l°W [relatively near
TEJADA el al.
ONTONG JAVA PLATEAU BASALTS, SOLOMON ISLANDS
the proposed pole of rotation (L. Kroenke, personal
communication, 1994)]; and we suggest that the
domal regions may correspond to two major eruptive
foci, the larger northwestern dome being the —121Ma center and the eastern one the ~ 90-Ma center,
reflecting two major pulses of eruption from the same
mantle plume. If true, the ~90-Ma Sigana Basalts
and Site 803 lavas would be either distal parts of
flows of ~300- to > 103-km length emanating from
the eastern center or, perhaps more likely, flows from
closer, secondary ~90-Ma eruption sites. In fact, the
~ 90-Ma activity on the main, high plateau may
have been partially controlled by preexisting
basement structure, particularly if the ~121-Ma
volcanic pile was emplaced near a spreading center
(e.g. Kroenke, 1972; Hussong et al., 1979; Mahoney,
1987; Mahoney & Spencer, 1991; Winterer &
Nakanishi, 1995). The apparent absence of ~ 90-Ma
lavas on Malaita, for example, despite their presence
nearby to the northwest on Santa Isabel, could be a
consequence of different 90-Ma basement paleodepths across a preexisting fault or fracture zone
between the two future islands [e.g. see Hussong et
al. (1979, Fig. 5) for one hypothetical arrangement].
The ~ 90-Ma eruptive pulse itself could partly have
been a response to a change in the stress field of the
plateau caused by the change in Pacific Plate motion
around this time, which was in turn a consequence of
a world-wide plate reorganization (e.g. Rea &
Vallier, 1987). On the other hand, two major pulses
of eruption (with relatively little activity in the
intervening period) could result from large pulsations in the flux of a mantle plume (Bercovici &
Mahoney, 1994).
In either case, the small range of isotopic and
incompatible element ratios observed for the OJP
drillhole and Pacific Province basement lavas suggests that entrainment of non-plume-source mantle
was either very limited or, perhaps more likely, that
entrainment and mixing operated to homogenize the
plume head(s) rather effectively relative to the scale
of melting. Interestingly, although the ~121-Ma
eruptions, in particular, could have occurred in a
near-ridge setting, no evidence of an obviously
normal-MORB-type mantle end-member is yet
present. Nevertheless, the presence of two slightly
different isotopic groups, a broadly Unit A-type and
a Units C—G/803-type, appears consistent with the
prediction by Griffiths & Campbell (1990) of zoning
in plume heads as a result of entrainment of ambient
mantle. Unlike the Unit A-type isotopic signature,
which so far is seen only in ~121-Ma lavas, the
Units C-G/803-type isotopic signature shows up
both at 121 Ma (at Site 289 and Site 807, below
Unit A) and at ~90 Ma (at Site 803 and in Santa
Isabel). To the north and east of the OJP, the isotopic values of the ~ 115-111-Ma East Mariana
Basin and Nauru Basin tholeiites also overlap with
the Units C-G/803 field (Mahoney, 1987; Castillo et
al., 1991, 1994). This signature is closer to the range
of values proposed for the hypothetical 'prevalent
mantle' signature argued by, for example, Stein &
Hoffman (1994) to represent average lower mantle.
On the other hand, isotopic compositions resembling
those of the Unit A type are found in ~ 120-Ma (R.
A. Duncan, unpublished data, 1994) lavas on the
Manihiki Plateau (Figs 4 and 5) well to the east of
the OJP. These considerations suggest that both the
Unit A and Units C-G/803 isotopic groups may
reflect sizable reservoirs in the mantle. However, it is
unclear which type might best reflect plume-source
isotopic composition and which entrained ambient
mantle; indeed, the plume source itself may have
contained both compositions.
The results of REE modeling using the fractional
melting inversion procedure of McKenzie & O'Nions
(1991), with modifications by White et al. (1992),
suggest that the Malaita Older Series, Ramos, and
Sigana tholeiites were produced by high degrees of
partial melting (as high as ~ 2 6 % depending on the
model source assumed), closely comparable with
results obtained for the drillhole lavas (Mahoney et
al., 1993). The procedure requires the assumption of
the REE.characteristics of the mantle source(s), but
acceptable fits in the calculated rare earth patterns
were produced by model sources ranging from
MORB-type to primitive-mantle-type [even though
our ENd(T) data suggest sources with intermediate
REE characteristics, at least on a time-integrated
basis; see Brodie et al. (1994) for a recent discussion
of the use of this procedure]. The high maximum
extents of partial melting inferred from the modeling
[and from major elements; see Mahoney et al.
(1993)] imply that the small range of observed
variation in isotopic and incompatible element compositions in the Pacific Province and drillhole
tholeiites partly reflects the averaging effect of high
degrees of partial melting in the source (see also the
next section). However, lavas of the much smaller
and much more poorly sampled Manihiki Plateau
also appear to represent high-degree melts, yet show
a much wider spread of isotopic values (e.g. 6 ENJ
units; Mahoney & Spencer, 1991), and on the large
Kerguelen Plateau, extreme isotopic variations exist
(e.g. ~13 £Nd units; H. Davies et al., 1989; Weis et
al., 1989; Salters et al., 1992; Storey et al., 1992;
Mahoney et al., 1995). Thus, it is difficult to avoid
the conclusion that the OJP source was intrinsically
rather homogeneous.
The greatest equivalent-melt thicknesses obtained
387
JOURNAL OF PETROLOGY
VOLUME 37
from the modeling ( ~ 19—22 km for a model source
with a primitive-mantle REE pattern) agree with
the estimates of White et al. (1992) for melt generated within plumes in spreading environments.
However, these thicknesses are much less than the
30—40-km OJP crustal thicknesses actually measured
by seismic refraction for the main, high plateau
(Furumoto et al., 1976). The accuracy of the
refraction measurements is open to some question, as
they were based on second arrivals; assuming that
they are accurate and that the modeling results have
at least general validity, the discrepancy could
reflect the likelihood that the lavas sampled so far
represent only the final or near-final episodes of OJP
volcanism (i.e. the top ~ 2 km or less of the thick
crustal section) and, thus, perhaps somewhat smaller
degrees of melting than the bulk of the plateau (see
Mahoney et al., 1993). Furthermore, depending on
the magnitude of ~90-Ma magmatism on the main
part of the plateau, crust formed at ~ 121-Ma could
have been thickened considerably by ~90-Ma
intrusion, extrusion, and underplating.
To each alkalic suite its own mantle source
The isotopic compositions of each alkalic suite are
distinct and indicate different mantle sources. The
Sigana Alkalic Suite reflects a source with a
'HIMU'-type isotopic signature markedly unlike
that of the OJP tholeiites, but the 40 Ar- 39 Ar ages of
the Sigana Alkalic rocks clearly place them within
the ~90-Ma eruptive episode on the plateau. Thus,
the mantle source of the Sigana Alkalic Suite was
probably in, or adjacent to, the OJP plume head
itself. Considering the small volumes (rare dikes) and
alkalic, basanitic to near-basanitic nature of these
rocks, they probably reflect low degrees of partial
melting. Low degrees of melting could have resulted
in the preferential expression of comparatively
fusible, but volumetrically minor, 'HIMU'-type
material present in the OJP mantle source region, if
the predominant lower- Pb/ 204 Pb material was
more refractory (see Mahoney et al., 1993). Whereas
the voluminous tholeiitic eruptions would have
occurred above regions of the plume head where a
high potential temperature and/or thinner lithosphere permitted large degrees of melting and a
swamping or averaging of small source heterogeneities, low-degree melts may have formed in
cooler regions far from the main loci of eruption
(argued above to be the dome of the plateau's
eastern lobe at 90 Ma).
For the Younger Series of Malaita, the 44-Ma age
obtained by Ar— Ar strongly suggests a connection to the Samoan hotspot because plate-tec-
388
NUMBER 2
APRIL 1996
tonic reconstructions show that the OJP passed
directly over this hotspot between ~60 and ~ 3 0 Ma
(e.g. Natland, 1985; Yan & Kroenke, 1993); at 44
Ma, the hotspot is located only a few hundred kilometers north of (the future) Malaita in Yan &
Kroenke's (1993) model. Interestingly, 44 Ma is
very close to the age of a major change in Pacific
Plate motion ( ~ 4 3 Ma at the Hawaiian-Emperor
bend; Epp, 1978; Duncan, 1981); thus, a change in
the stress field of OJP lithosphere may have permitted egress of alkalic melts from the Samoan
plume at this time. Also, although the 'PHEM' endmember for Samoa is speculative, the occurrence of
lavas on the Samoan hotspot track that approximate
the predicted PHEM isotopic composition seems
unlikely to be coincidental. Assuming the Samoan
hotspot existed at 44 Ma, then its earlier signature
may well have been dominated by the PHEM component. In this regard, it would be interesting to
know if the Younger Series lavas have high He/ He.
On the other hand, the Rarotongan hotspot lay
under the eastern limb of the plateau between ~ 5 6
and ~ 4 5 Ma (Yan & Kroenke, 1993); the partial
overlap of the Younger Series isotopic data with the
Rarotonga field in Fig. 4a—c could suggest a role for
the Rarotongan hotspot, if it was active during this
period. Given that the Samoan hotspot appears to
have been much closer to the western plateau, we
favor, at present, a Samoan connection for the
Younger Series.
Because stratigraphic evidence indicates that the
North Malaita Alkalic basalts are closely similar in
age to the Younger Series (McGrail & Petterson,
1995), it is possible that the early Samoan hotspot
was isotopically heterogeneous and produced both
groups of lavas. However, no present-day Pacific
hotspot has isotopic compositions much like those of
the North Malaita Alkalic Suite, and the general
isotopic resemblance of this suite to the OJP lavas
suggests the possibility of a role for OJP lithosphere
in its origin. For example, the North Malaita Alkalic
lavas might represent small-degree melts of the
lithosphere, perhaps of 'fossil' (e.g. Stein & Hoffman,
1992) OJP plume mantle incorporated into the base
of the lithosphere during the waning stages of
plateau construction and later heated during the
plateau's passage over the Samoan hotspot. Indeed,
the North Malaita Alkalic samples have Sr isotopic
ratios essentially identical to those of Units C—Gtype OJP lavas. However, although the Pb isotope
values of the North Malaita Alkalic samples can be
derived by radiogenic ingrowth from ~ 121-Ma
Units C-G-type lithospheric mantle with 238 U/ 2O4 Pb
and 232 Th/ 204 Pb values of 19-26 and 85-115,
respectively, these values are somewhat greater than
TFJADA el al.
ONTONG JAVA PLATEAU BASALTS, SOLOMON ISLANDS
those observed for the ODP drillhole lavas, Malaita
Older Series, and Sigana Basalts ( 238 U/ 204 Pb= 14-18
for fresher samples and 232 Th/ 2O+ Pb = 56-75;
Mahoney et al., 1993; M. Tejada, unpublished
results, 1994), and values in the unmelted source
would be still lower. Most importantly, the eNd(T)
values of the North Malaita Alkalic samples are
~ 1 -5 units lower than Units C—G-type values; for
£
Nd(T) to change this much between 121 and 44 Ma
by radiogenic ingrowth requires an unrealistically
low 147 Srn/ 144 Nd value in the source (i.e. ~ 0-050,
compared with measured values in the tholeiites of
~0-22 and in the alkalic lavas of ~0-14). Unit Atype mantle is likewise unsuitable as a source,
because although the Unit A-type OJP lavas have
e
Nd(T) identical to the North Malaita Alkalic
samples, they also have significantly higher
( 87 Sr/ 86 Sr) T . Thus, if OJP lithosphere was the source
of North Malaita Alkalic Suite magmas it must have
been isotopically slightly different from either of the
types represented by present sampling of OJP
basement tholeiites. It may not be coincidental that
the North Malaita Alkalic Suite data lie approximately between data for the Unit A-type lavas and
the Sigana Alkalic Suite in all panels of Figs 4 and 5
because, as discussed above, the Sigana Alkalic Suite
data imply that the OJP mantle source region may
have contained minor amounts of relatively fusible
'HIMU'-type material; if so, small degrees of
melting of such 'fossil' plume mantle at ~ 4 4 Ma
could have produced magmas with the observed
signature.
Jajao Igneous Suite when the rocks were formed.
The Jajao Igneous Suite rocks may have formed in a
transient backarc basin similar to that associated
with the 55-40-Ma, southwest-facing Papua-New
Caledonia-Rennel Trench. Although the commencement of subduction at the Papua-New Caledonia-Rennel Trench is placed at ~ 5 5 Ma in the
model of Yan & Kroenke (1993), it may have taken
place as early as 65 Ma (L. Kroenke, personal communication, 1994); if so, the ages (~62 and 46 Ma)
determined for Jajao Igneous Suite rocks indicate
that they may be associated with this same subduction system. Subsequent plate reorganization and
collision events would have brought these rocks into
juxtaposition with the OJP, possibly as recently as
~10 Ma (Coleman & Kroenke, 1981).
The San-Jorge-like lavas north of the KKFS have
trace element compositions that are consistently
normal-MORB-like and isotopic characteristics
closely similar to those of modern East Pacific Rise
MORB, suggesting they could be small fragments of
abyssal Pacific Plate seafloor caught up by the
leading edge of the OJP during its emplacement
against the now inactive North Solomons Trench.
Equally possible, given the 62'5-Ma age of 12290, is
that they are related to the San Jorge Volcanics
10 f
A San Jorge Volcanics, San Jorge-like Lavas
QKoloso'eru Gabbros
9 Leucocratlc Rocks
Fiji Rift
Backarc-basin origin for the
Jajao Igneous Suite
There is no conclusive field evidence that the Jajao
Igneous Suite represents a true ophiolite assemblage.
Although Hawkins & Barron (1991) pointed out
that the close physical association of the constituent
rock types could mean that they are petrogenetically
related, the contacts are generally either faultbounded or not exposed. Based on the spectrum of
MORB-like to BABB-like to arc-like geochemical
characteristics, an arc—backarc-basin origin for this
suite of rocks is favored strongly. If true, the Jajao
Igneous Suite provides an important exception to the
observation of Indian Ocean-type isotopic signatures
in the western Pacific backarc basins (e.g. Crawford
et al., 1995; Hickey-Vargas et al., 1995) because the
Jajao Igneous Suite samples lack Indian Ocean-type
isotopic characteristics (Fig. 13). Thus, the Jajao
Igneous Suite data imply that the boundary between
the Pacific- and Indian-MORB-type mantle megadomains must have lain west of the location of the
389
Philippine
Sea
Mariana Trough
10
30
70
A 8/4
Fig. 13. Plot of A7/4 vi A8/4 for Jajao Igneous Suite, Pacific
MORB, and Pacific backarc and marginal basins. A7/4 and A8/4
are measures of vertical deviations from a best-fit line through Pb
isotopic data for Pacific and North Atlantic MORB (Hart, 1984).
Large positive values (larger than the values indicated by the
dashed lines) are typical of Indian MORB. (Note that, in contrast
to modern Pacific marginal basin lavas, the Jajao Igneous Suite
samples do not have an Indian MORB-type signature.) Data
sources: Mariana Trough (Volpe it al., 1990); Philippine Sea
Basin (Hickey-Vargas, 1991); Fiji Rift (Gill, 1984); Papua New
Guinea (PNG) Rift (Hegner &. Smith, 1992); Lau Basin (Jenner
et al., 1987); and Pacific MORB (White it al., 1987; Mahoney it
al., 1994, and references therein).
JOURNAL OF PETROLOGY
VOLUME 37
proper, their patchy occurrence among the Sigana
Basalts a few kilometers north of the K.KFS being a
result of overthrusting of a sliver of OJP crust onto
San Jorge Volcanics basement during the collision
event, followed by erosion and/or faulting leading to
their exposure.
Relevance to continental growth and
greenstone belts
Modern analogs for ancient greenstone belts are significant because greenstone belts hold important
clues to understanding the growth of early continental crust. Normalized incompatible element
patterns of tholeiitic basalts in many greenstone belts
are more or less flat, strongly reminiscent of the OJP
lavas and other oceanic plateau basalts, and it has
been argued that some greenstone belt sequences
contain remnants of Precambrian oceanic plateaux
(e.g. Storey et al., 1991; Kusky & Kidd, 1992; Desrochers et al., 1993). Because of their thickness and
buoyancy, large oceanic plateaux are likely to resist
complete subduction, leading to accretion of plateau
crust along convergent margins (e.g. Kroenke, 1974;
Cloos, 1993); thus, the interaction of oceanic
plateaux with subduction zones could be an
important initial stage in the formation of protocontinental crust (Kroenke, 1974). Recent seismological, stratigraphic and structural studies indicate
an allochthonous origin for many Archean greenstone belts and document the presence of large faults
that mark major stratigraphic boundaries (e.g.
Green et al., 1990; Kusky & Kidd, 1992; Desrochers
et al., 1993). We suggest that Santa Isabel, formed as
a consequence of interaction of a plateau with a
subduction zone, may provide a modern-day
example of the tectonostratigraphic associations
observed in some greenstone belts, such as Belingwe
(Kusky & Kidd, 1992) and Abitibi (Desrochers etal.,
1993). In particular, the KKFS marks a major
structural-stratigraphic boundary, and the subdivision of the principal igneous rocks on Santa
Isabel into mafic plateau lavas and a mafic to felsic
suite with geochemical characteristics ranging from
MORB-like to transitional to arc-like at least superficially parallels the twofold subdivision of some
greenstone-belt volcanic groups into mafic-ultramafic and mafic to felsic calc-alkaline suites (e.g.
Windley, 1979).
This analogy is imperfect in several respects. No
komatiite3 have yet been found in OJP basement,
for instance, although Storey et al. (1991) suggested
they may exist at deeper crustal levels; alternatively, their absence reflects lower temperatures
NUMBER 2
APRIL 1996
in the OJP plume head than in comparable
Archean and Early Proterozoic mantle. Also,
greenstone belts are not obviously connected on
one side to large (originally) oceanic plateaux.
However, the Sigana Basalts and Malaita Older
Series probably represent relatively thin flakes of
OJP basement broken off and overthrust onto
forearc and backarc crust (Coleman & Kroenke,
1981; Kroenke et al., 1986); thus they may be separated rather easily from the body of the plateau in
future plate collision and reorganization events.
Moreover, in some cases, greenstone belts may mark
the edges of larger, originally oceanic-plateau terranes buried by more recent rocks and metamorphosed to form lower crust; the Birrimian crust of
western Africa (Abouchami et al., 1990) is a possible
example of such terranes.
SUMMARY AND
CONCLUSIONS
The igneous rocks of Malaita and Santa Isabel
present a complex juxtaposition of three principal
groups (excluding the late-stage alnoite intrusions on
Malaita): (1) OJP basement (Malaita Older Series,
Ramos Island basalts and Sigana Basalts of Santa
Isabel); (2) a possible ophiolite assemblage (Jajao
Igneous Suite, Santa Isabel); (3) at least three types
of alkalic rocks (Younger Series, North Malaita
Alkalic Suite and Sigana Alkalic Suite).
The Malaita Older Series, Ramos Island and
Sigana tholeiites are remarkably similar in age and
composition to the flows sampled by very limited
basement drilling on the northern and central OJP
as much as 1600 km away. Such lavas may well
make up the bulk of the extrusive products of the
OJP, or at least the upper few kilometers of its crust.
The combined isotopic, major element, and trace
element data reveal a relatively homogeneous
mantle source with a broadly OIB-like, slightly
'DupaP-type isotopic signature, which underwent
high degrees of partial melting. All but one of the
Ar- 3 Ar plateau ages obtained for these lavas and
the OJP drillhole basalts fall into two distinct
periods around 121 Ma and 90 Ma, apparently
indicating two major, short-lived plateau-building
episodes. Despite the ~30-m.y. age difference, the
close isotopic and chemical similarities between the
— 121- and ~90-Ma lavas suggest two major, shortlived pulses of eruption from the same hotspot
source. This possibility appears consistent with the
recent plate reconstructions of Kroenke & Sager
(1993) and the gross morphology of the plateau. In
particular, the earlier pulse of eruption may have
390
TEJADA et al.
ONTONG JAVA PLATEAU BASALTS, SOLOMON ISLANDS
been centered on the large domal area of the main,
high plateau, whereas the focus of the ~90-Ma pulse
may have been on the domal region of the eastern
lobe or salient.
No simple mixing or evolutionary model can
relate all of the alkalic rocks to the OJP. Only the
Sigana Alkalic Suite can be linked directly to the
~90-Ma phase of eruption on the plateau, and may
represent small-degree melts dominated by volumetrically minor but relatively fusible (compared
with the dominant lower I ' 06 Pb/ 204 Pb mantle)
'HIMU'-type material present in the mantle source
region of the OJP. The 44-Ma Younger Series
alkalic lavas of Malaita may record the passage of
the eastern OJP over the Rarotongan hotspot, but
more probably reflect the passage of the main body
of the plateau over the Samoan hotspot, if this
hotspot has been active since the Eocene and if the
'PHEM' component postulated by Farley et al.
(1992) was expressed more fully than in recent
Samoan volcanic products. The North Malaita
alkalic basalts may be small-degree melts of aged
(~ 77-46 m.y. old at the time) OJP lithosphere
melted by the Samoan hotspot around the same
time.
The ~62-46-Ma San Jorge Volcanics of the
Jajao Igneous Suite are not related to the OJP.
Together with the Kolose'eru Gabbros and leucocratic rocks, they probably are fragments of uplifted
ocean floor formed in an arc—backarc system. In
general, they resemble present-day backarc-basin
and arc lavas, except that, unlike the modern Pacific
marginal basins, they lack Indian-MORB-type isotopic characteristics; the absence of such characteristics would appear to mean that the boundary
between the Pacific-type and Indian-type MORB
mantle mega-domains lay (at least locally) to the
west at the time they were erupted.
Three basalts and microgabbros sampled north of
the KKFS in Santa Isabel are not Sigana Basalts,
but isotopically and chemically resemble normal
MORB; they may represent fragments of normal
~62-Ma abyssal Pacific Plate seafloor obducted
onto the leading edge of the OJP during its emplacement against the North Solomon Trench or,
possibly, San Jorge Volcanics overthrust by a sliver
of OJP crust and subsequently exposed by erosion
and/or faulting.
The juxtaposition of OJP crust with a probable
backarc-arc terrane in Santa Isabel may be viewed
as a modern-day example of crustal accretion
resulting from the interaction of an oceanic plateau
and a subduction zone; at least in its general aspect,
this situation may provide an insight into the origin
of some Archean greenstone belts.
391
ACKNOWLEDGEMENTS
We are grateful to the Solomon Islands Geological
Survey for providing most of the samples used in this
study, the University of New England sample
archive for the SI samples, the Program on
Resources: Energy and Minerals of the East-West
Center (through A. Clark) for allowing M.L.G.T use
of facilities, K. Spencer, Z. Peng, N. Hulbirt, B.
Nolte, and P. Wessel for their help, D. McKenzie for
his modeling program, L. Kroenke and M. Petterson
for helpful discussions, and P. Castillo and F. Frey
for their thoughtful reviews. This research was supported by NSF Grant EAR-9219664 and an EastWest Center Scholarship. This paper is SOEST
Contribution 4038.
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