Download Tertiary volcanic rocks from the Halmahera Arc, Eastern Indonesia

Journal of Southeast Asian Earth Sciences, Vol. 6, No. 3/4, pp. 271-287, 1991
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Tertiary volcanic rocks from the Halmahera Arc, Eastern Indonesia
A. SUFNIHAKIM* and ROBERTHALL t
*Geological Research and DevelopmentCentre, Bandung,Indonesiaand tDepartment of GeologicalSciences,
University CollegeLondon,Gower Street, LondonWCIE 6BT, U.K.
(Received 22 August 1990; accepted for publication 5 May 1991)
Abstract--Halmahera is a K-shaped volcanic island arc situated near the junction of the Australian, Eurasian
and Philippine Sea Plates. Recent work on Halmahera has identified two important pre-Quaternary intervals of
volcanism in western Halmahera. Neogene andesites were produced in the Halmahera Arc during subduction of
the Molucca Sea Plate at the western boundary of the Philippine Sea Plate. Pre-Neogene basalts of the Oha
Formation are probably the equivalent of volcanic basement rocks found elsewhere in the Philippine Sea region
and are interpreted to represent the products of Late Mesozoic or Early Tertiary subduction within the Pacific.
Neogene andesites and subordinate basalts contain abundant phenocrysts; plagioclase feldspars, orthopyroxene, clinopyroxene, hornblende and titanomagnetite are common. Like the products of Quaternary volcanism
andesitic bulk rock compositions reflect high proportions of acid glass. The Neogene volcanic rocks have evolved
by plagioclase, pyroxene, hornblende and magnetite fractionation. They are medium-K to high-K rocks of the
calcalkaline series, REE patterns are sloping, typical of arc volcanic rocks, and MORB-normalized element plots
show strong depletion of Nb, similar to other West Pacific arc volcanic rocks. Most samples are very fresh. A
single zeolite (mordenite) is rarely present and chlorites, smectites and chalcedony occur in a few samples. The
local, very low-grade, alteration is typical of geothermal environments.
Volcanic rocks of the Oha Formation, which forms the basement of the western arms, are aphyric and phyric
basalts, typically with textures which reflect rapid cooling. Plagioclase feldspar, olivine and clinopyroxene
phenocrysts are common, orthopyroxene is rare and phenocrysts of hornblende and magnetite are absent. The
Oha Formation basalts evolved by olivine, plagioclase and clinopyroxene fractionation. They are depleted in HFS
elements, and enrichment in LIL elements is partly due to extensive sub-greenschist facies alteration reflecting
deep burial and/or high heat flows. This alteration produced zeolites, chlorites, smectites, and locally, pumpellyite
and sphene. Dating of overlying sedimentary rocks shows that the Oha Formation volcanic rocks are older than
Late Miocene; a Late Cretaceous-Eocene age is suspected since petrographically and chemically similar arc
volcanic rocks of this age are present in the NE and SE arms.
INTRODUCTION
HALMAHERA is over 300 km from north to south and
125 km from east to west and has a distinctive K-shape.
The island is covered by rain forest and the terrain is
generally steep and rugged. Because of the difficulties of
access and the remoteness of the island Halmahera has
received relatively little attention. The first comprehensive geological maps were produced by the Indonesian
Geological Research and Development Centre (GRDC)
with accounts of the regional geology (Apandi and
Sudana 1980, Supriatna 1980, Yasin 1980, Sukamto et
aL 1981). The stratigraphy established by these authors
has been added to and revised in subsequent papers by
Hall (1987), Hall et al. (1988a,b, 1991a) and Nichols and
Hall (1990).
Volcanism in the present-day Halmahera Arc (Fig. l)
is the product of subduction at the Halmahera Trench
(Hatherton and Dickinson 1969, Katili 1975) of the
Molucca Sea Plate (Cardwell et al. 1980, McCaffrey
1982). The Halmahera Arc includes a chain of volcanic
islands offshore of western Halmahera and volcanoes in
the NW arm of the island. To the south, the Sorong
Fault Zone is a major sinistral strike-slip system which
separates this complex zone of east-west convergence
from the Australian Plate and the Banda Sea area
(Hamilton 1979). South Halmahera lies adjacent to one
of the main strands of the Sorong Fault System and
continental crust is present on the island of Bacan (Hall
et al. 1988a) at the southern end of the Halmahera group
of islands.
SEA~S6.~--H
Recent studies of the tectonic evolution of this area
(Hall 1987, Nichols et al. 1990) show that Halmahera is
situated at the southern end of the Philippine Sea Plate
(Fig. 1). The present arc is built on rocks of a Neogene
volcanic arc, related to development of the present
east-dipping Molucca Sea subduction at the edge of the
Philippine Sea Plate (Hall and Nichols 1990). The Neogene arc developed upon still older arc rocks and the
history of volcanic activity in eastern Halmahera began
in at least the Late Cretaceous (Hall et al. 1988a,
1991). Halmahera is situated in a position linking the
Cretaceous-Tertiary arc terranes of the New Guinea
margin and those of the Philippines and the history and
character of the volcanic activity in this region hence
provides important information on the early history of
the Philippine Sea region. This paper is based on a study
by Hakim (1989) and gives an account of the character of the pre-Quaternary volcanic rocks of western
Halmahera.
Previous studies o f Halmahera volcanic rocks
Studies of the volcanic rocks of Halmahera are limited
to those of the Quaternary-Recent island arc (Kuenen
1935, Morris et al. 1983) and the Ophiolitic Basement
Complex of eastern Halmahera (Ballantyne 1990, 1991).
There have been no previous studies of the older volcanic
rocks of western Halmahera.
Kuenen (1935) described Quaternary porphyritic
basalts and andesites from the islands of Tidore and
271
272
A. SUFNI HAKIM and R. HALL
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125 °
110 °
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Fig. I. Tectonic setting of Halmahera. Small solid circles m a r k recent volcanoes of the Sangihe and H a l m a h e r a Arcs.
Ternate (Fig. 2) and observed that although all the rocks than those in the normal oceanic segment. Ratios of
were petrographically similar, the andesitic compositions S7Sr/S6Sr and 2°7pb/2°4pb are higher than those of the
were due to the presence of an acid glass. Morris et al. normal oceanic segment.
(1983) divided the Quaternary-Recent volcanic arc of
western Halmahera into a normal oceanic segment and
a continental segment. The normal oceanic segment
STRATIGRAPHY
includes the volcanoes of northern and central Halmahera which have produced porphyritic basalts and andeThe oldest rocks of eastern Halmahera (Table 1,
sites. The rocks are calcalkaline arc lavas with Fig. 2) are ophiolitic rocks (Hall et al. 1988a, Ballantyne
plagioclase, olivine, clinopyroxene, hornblende and 1990) overlain unconformably by Upper Cretaceous to
magnetite phenocrysts in a glassy to granular ground- Lower Tertiary volcanic and volcaniclastic rocks intermass which is variably devitrified. Their chemistry is preted as part of a forearc sequence (Hall et al. in press).
typical of oceanic island arcs. Most of the rocks fall These ophiolitic and forearc rocks (the Ophiolitic Basewithin the range of 53~3 wt% SiO2 and have high ment Complex) were deformed in the Late Paleogene
AI203, low TiO2 and K20 contents typical of medium-K and form the basement to the Neogene succession in
suites (Gill 1981) with little to moderate iron enrichment. eastern Halmahera. The basement of western HalmaIncompatible element concentrations vary with SiO2 hera is composed of volcanic rocks of the Oha Forcontent, and alkali elements, Ba and Sr have typical mation which forms mountains along the western side of
island arc values. High field strength and compatible the SW arm extending north into central Halmahera.
elements are characteristically depleted. 87Sr/a6Sr ratios Similar rocks are reported from Morotai (Supriatna
are similar to those of many island arcs but lead isotope 1980).
ratios suggest minor sediment contamination. In conNeogene volcanic and sedimentary rocks rest uncontrast volcanic rocks from south Bacan, at the southern formably on the Oha Formation in the SW arm and in
end of the Halmahera Arc, are mostly dacites with minor a folded belt of central Halmahera. In the western part
andesites and are considered to be contaminated by of the SW arm turbidites and debris flows of the Upper
continental crust. Alkali el~ment contents are high and Miocene Loku Formation rest unconformably upon the
trends of alkali elements versus SiO2 are slightly steeper Oha Formation. Breccio-conglomerates in this for-
Tertiary volcanic rocks from the Halmahera Arc
273
!
!
12"~
!
12~E
128"E
Quaternary
Alluvium and Limestone
Quatemaly-Renent
MOI~)T
Volcanic Rocks
Rio-Pleistocene
Volcanic Rocks
Weda Group
2~N.
Loku Formation
Miocene Umestones
Eady-Mid Miocene
' HALMAHERA
Sed=mentaryRocks
OligoceneBasalts
& Sandstones
Oha Form~on
Volcanic Rocks
Ophiolitic
BasementComplex
Cor~nental
Metamoq~hicRocks
¢,
Buff
Bay
TERNATE ( ~
TIDORE
o
~
•
Major Thrust
Major Fault
Volcano
I
~)
t.
Weda
8
o".
Bay
,D
I
Fig. 2. Simplifiedgeologicalmap of Halmahera.
mation contain clasts of pyroxene andesites and hornblende andesites and volcaniclastic rocks containing
similar debris. Nichols and Hall (1990) suggest that
the Loku Formation represents slope sedimentation
close to the Neogene volcanic arc, probably in the
forearc. On the eastern side of the SW arm and in central Halmahera the Weda Group, resting unconformably
on the Oha Formation, is a shallow marine sequence
of conglomerates and sandstones between 2800 and
3800 m thick. Fresh pyroxene andesite and hornblende
andesite debris dominates the clastic component in
rocks of the Weda Group (Nichols et al. 1991) and is
interpreted to be derived from the active Neogene volcanic arc. The lower part of the Weda Group contains
aphyric basalt clasts probably derived from the Oha
Formation.
The Weda Group was deformed after the Mid-Late
Pliocene. Volcanic activity may have been renewed at
about this time, and/or briefly changed character. At the
northern end of the SW arm shallow marine tuffaceous
siltstones and sandstones of the Kulefu Formation rest
unconformably upon folded Weda Group rocks. The
rocks are petrographically different from most arenites
in the Weda Group, although they resemble some of the
youngest volcaniclastic rocks of the Weda Group of
central Halmahera; they contain biotite and quartz and
abundant lithic volcanic grains including pumice and
quartz-biotite dacite clasts derived from a nearby
source. Near the northern coast of central Halmahera
volcaniclastic rocks and pyroxene andesites of the
Tafongo Formation (Hall et al. 1988b) dip at a low
angle and locally tuff intercalations contain a Pleistocene
fauna (Apandi and Sudana 1980).
The Tertiary volcanic rocks were investigated in this
study as part of a wider investigation of the Halmahera
Arc aimed at determining its history and character.
Samples of volcanic rocks were collected from outcrops
and as float in rivers and along the coasts of central and
south Halmahera. Neogene volcanic rocks were collected from the Tafongo Formation and from the Weda
Group. The Oha Formation was sampled in central and
south Halmahera.
A. SUFNI HAKIMand R. HALL
274
Table 1. Summaryof the stratigraphyof Halmahera
NE AND SE ARMS
CENTRAL AND SW ARM
Quaternary Vd~ic and Volcaniclas~P~cks
Quatema7 Red Umestones
TAFONGOAND KULEFUFORMATIONS
Plio.Pl~stocene 0.1500m
Volcanicand VdcanidasticRo~s
With Vdcan'¢ Clasts
~lom~es, Sandsl~, I~dsto~s
..........
KAHATOLAFORMATION
Bas;tdc Rio, Lavas& VoicanidasticTudxiles
PANITIFORMATION
M/dd/eand Late Eocene >500m
Ma~inalto ShallowMarineS~menta~ Ro~
OPHIOLmCBASEMENTCOMPLEX
ARM
SEARM
BULlGROUP
OHA VOLCANICFORMATION
~te Cretaceousor Eocene? >1000m
Caical~ine Voltaic Roc~
VolcanidasticBreccias,Conglomerates
SAGEAFORMATION
MiMeEo~ne >500m
Vok:a~cl~ Rocks
ReduCed Umesto~s
DODAGABRECCIAFORMATION
Camp,/an and Maas~ch~an >lO00m
Deep M~ine VolcanidasticSedimentary GOWONU FORMATION
Rocksand P~agicModstones
Late Cretaceous >lO00m
Ca~a,~ Voice's, Hyr~ys,~
GAU LIMESTONEFORMATION and Volca)idas~¢Rocks
Con/ac/an-Campanm>50m
Pelagic Mudstones,Lim~sto~
Rede~ted and PelagicLimestones
Vok:anidas~cSedimer~a~yPocks
OPHIOUTIC ROCKS
PerkJo~tes,UI~ ~ Ba,¢ Cumula~,Mic~jaltom Oicri~ Trondh~iles
~kaine VoicadcRocks,Arc Tholei~, 8onini~cVol~ic Rocks,MelamorphicRods
ANALYTICAL METHODS
Mineral analyses were made on polished thin sections
using a Cambridge Instruments Microscan V electron
microprobe with a Link System energy dispersive system
at University College London. An accelerating potential
of 20 kV and probe current of 1 x 10s A were used with
a beam spot size of 1/~m. Elements routinely analysed
were Si, Ti, A1, Cr, Fe, Mn, Mg, Ca, Na and K; natural
silicates and pure metals were used as standards.
For rocks SiO2 was determined by the method of
Shapiro and Brannock (1962) and conventional gravimetric methods were employed for CO2 and H20. Other
major elements, Li and Co, were analysed by inductively
coupled plasma emission spectrometry (ICP) by methods
of Walsh (1980). Rare earth elements (REE) were determined by ICP using the method of Walsh et al. (1981).
Other trace elements and some REE (La, Ce, Nd)
were analysed using a Philips PW-1400 XRF at Royal
Holloway and Bedford New College London using
methods similar to those of Thirlwall and Burnard
(1990).
NEOGENE VOLCANIC ROCKS
The Neogene volcanic rocks can be divided petrographically into two groups: (1) two-pyroxene andesites
(TPAN) without hornblende, and (2) rocks containing hornblende phenocrysts, which are hornblende-
Tertiary volcanicrocks from the Halmahera Arc
275
NEOGENE ANDESITES
Plagioclase
pyroxene andesites (HPAN) and hornblende andesites
(HBAN). Both groups are typically vesicular, massive to
amygdaloidal, porphyritic rocks which contain felsic and
mafic phenocrysts and microphenocrysts set in a grey to
brown glassy or microcrystalline groundmass. The percentage of phenocrysts or microphenocrysts varies from
35 to 45%.
An
Mineralogy
The principal igneous minerals are plagioclase, clinopyroxene, orthopyroxene and hornblende which occur
as phenocrysts and microphenocrysts. Plagioclase and
pyroxenes also occur as groundmass phases in most
samples. The main accessory minerals are Fe-Ti oxides,
which occur as microphenocrysts and in the groundmass, and apatite.
Feldspar. Plagioclase constitutes from 35 to 55% of
the total of phenocrysts and microphenocrysts and
forms the largest mineral grains. The size of phenocrysts
ranges from 0.3 to 4.5 mm. Piagioclase phenocrysts and
microphenocrysts can be subdivided on the basis of their
textures, inclusions and zoning patterns. They include
grains which are inclusion-free, and others which have
resorption and growth textures (Morrice and Gill 1986).
Zoned grains include those with normal zoning (most
common), reverse, oscillatory and patchy zoning. All of
these textures and zoning patterns may be found in a
single rock. The inclusion-free phenocrysts and microphenocrysts are optically clear crystals. Grains with
resorption and growth textures are characterized by the
presence of zones which are rich in inclusions of glass,
fluid inclusions with vapour bubbles, and tiny crystals of
pyroxene and iron oxides. The zoning patterns and
textures of plagioclase grains indicate a complex history
of growth and reaction with the melt, reflecting possible
changes in pressure, PH~O and magma mixing during
crystallization (Dungan and Rhodes 1978).
There is no significant difference in the range of
plagioclase compositions of the three groups of andesites
(Fig. 3). Or contents increase with decreasing An. Orenriched rims occur in some samples (TPAN ~2-5%;
HPAN and HPAN <2%). K-rich alkali feldspar was
identified by microprobe in the groundmass of several
TPAN and one HPAN.
Clinopyroxene. Clinopyroxene is found in all the
pyroxene andesites (TPAN and HPAN). Clinopyroxene
constitutes between 3 and 15% of total phenocrysts and
microphenocrysts, is commonly euhedral and less commonly embayed, and ranges from 0.1 to 3 mm across. In
nearly all samples there are glomerocrysts which may
include anhedral to subhedral grains of clinopyroxene,
orthopyroxene, plagioclase and opaques. Continuous
normal and sector zoning are present in a few grains;
neither of these types of zoning occurs in the crystal
clots. Augite occurs as a rim to colourless orthopyroxene
in a few samples and in one HPAN augite occurs with
orthopyroxene, anhedral plagioclase and opaques
around a core of hornblende.
Ab
An
Or
2,
Ab
An
Or
~~desblend
e
ites
Groundmass
Ab
Or
Fig. 3. Plagioclasecompositionsin the Neogenevolcanic rocks.
Compositionally, the clinopyroxenes are salites and
augites (Fig. 4) with low to moderate contents of A1
(0.75-5.5 wt% A1203) and low Cr (<0.3 wt% Cr203)
contents. A slight iron enrichment trend is present in the
TPAN. A plot of AltotaI against Si (Fig. 5) shows that
most HPAN and some TPAN clinopyroxenes fall close
to a straight line above the line Si + A1--2. However,
many TPAN and some HPAN clinopyroxenes scatter
away from this straight line and these have significant
Fe 3÷ and Ti. Sodium and A! are not correlated. A plot
of Altota~+ Ti + Fe 3÷ against Si (Fig. 5) results in a
straight line for all HPAN and TPAN clinopyroxenes.
These plots indicate two groups of pyroxenes are present: (1) those with low Fe a+ and Ti in which Al is
present as a Ca-Tschermak (CaTs; CaAI:SiO6) component; (2) those with higher Fe 3+ and Ti in which there
is substitution of Fe 3÷ for Al3+ in Fe-Ts (CaFe3+A1SiO6)
accompanied by Ti as Ti 4÷ entering the M1 site as
suggested by Deer et al. (1978). Although these pyroxenes cannot be optically distinguished, the analytical
work suggests that the pyroxenes rich in Fe 3÷ and Ti,
more common in the TPAN, are xenocrysts or earlyformed higher pressure phases; textural observations of
euhedral pyroxenes coexisting with embayed pyroxenes
support this interpretation.
Orthopyroxene. Orthopyroxene forms less than 10%
of total phenocrysts and also occurs as a groundmass
276
A. SUFNI HAKIM and R. HALL
NEOGENE ANDESITES: Pyroxenes
Di
Hd
°mblende'APY~;~:;~
\\
En
Fe3+Ti-poor Cpx
o Fe3÷Ti-rich Cpx
Fs
x Opx
Fig. 4. Pyroxene compositions in the Neogene volcanic rocks.
phase in most of the pyroxene andesites; crystals exhibit
a weak pink pleochroism and are hypersthenes (Fig. 4).
The HPAN orthopyroxenes extend to more iron-rich
compositions than those of the TPAN. Temperatures
calculated from compositions of coexisting TPAN
pyroxenes (Wells 1977) fall in the range 959-1096°C;
within a sample variations of +20°C reflect compositional range. These temperatures suggest the important
role of water in lowering liquidus temperatures (Green
and Ringwood 1968).
Hornblende. Brown and green, is present in the hornblende andesites as phenocrysts and microphenocrysts
from 0.15 to 3 mm in length. Brown hornblende is much
more common than the green variety. Hornblende phenocrysts and microphenocrysts constitute less than 20%
of total phenocrysts. In some samples the hornblendes
have a dark opaque-rich opacite rim (Gill 1981) and in
others brown hornblende phenocrysts are largely or
completely replaced by iron oxides, most commonly in
the HPAN. Hornblendes with little or no opacitic alteration occur in the samples containing few or no pyroxene phenocrysts or glomerocrysts. Resorption and
embayment textures are common in most samples. The
amphibole phenocrysts commonly contain patches of
inclusions including plagioclase, iron oxides and glass. In
two HPAN fine grained biotite, pyroxene and feldspar
surround hornblende suggesting a crystal-melt reaction.
There is no obvious relation of amphibole colour to
the two types of hornblende andesite (HPAN and
HBAN). The majority of HPAN amphiboles are pargasitic hornblende (Leake 1978) whereas HBAN amphiboles fall into a wider field of composition from
pargasitic hornblende to magnesio-hornblende (Fig. 6).
Although there is considerable overlap in compositions
the HPAN amphiboles tend to have higher Mg/Fe 3+
ratios than the HBAN amphiboles (Fig. 6). No explanation emerges from the analytical work for the colour
of the amphiboles. Gill (1981) suggested that a brown
colour is due to a high Fe3÷/Fe2÷ ratio but no clear-cut
relationship was observed; most of the green amphiboles
NEOGENE ANDESITES: Hornblendes
2.5
Tschermakite
Pargasite
2.0
0.4
AI total+Ti+Fe 3÷
~
NEOGENE ANDESITES
Clinopyroxenes
. . . . . . . .
1.5
DDD
I
0.31i
1.0
Trernolite
0.2¸
Hornblende
Edenite
0.5
0.1
Hornblenderoxotl~
~~
Na+K
0.0
ndesffes
' o:a
' 014
016
018
1.0
HPAN-Brown ~ HPAN-Green ~ HBAN-Brown x HBAN-Green
3.4
0.0
Two Pyroxene
Andesltes
AI total
0
D
D
Si
3.2
0.3-
o
Mg
o
oo
D
~
o
D~
'~
~ x
3.0
Z
0.2-
ID
qz3~
X
2.8
~a~ ~
z
0.1-
2.6
DD
o.o
1.75
D
,,
1.8
1.85
1.9
1.95
2.0
Fig. 5. Variations in chemistry of clinopyroxenes from the Neogene
volcanic rocks based on formulae recalculated to 6 oxygens. Fe 3÷
estimated assuming four cations.
F e ~÷
2.4
0
0.2
0.4
0.6
0.8
1.0
.2
Fig. 6. Amphibole compositions in the Neogene volcanic rocks. Fe 3+
estimated for 13 cations excluding Ca + Na + K if Ca + Na > 1.34 o r
15 cations excluding Na + K if Ca + Na < 1.34 (Leake 1978).
277
Tertiary volcanic rocks from the Halmahera Arc
NEOGENE ANDESITES: Oxides
Ti
Ti
Two Pyroxene
Andesites
Hornblende-Pyroxene
Andesites
Phenocrysts
Groundmass
Ti
Hornblende
Andesites
:
\/
°
°
o
\/
=
\
°
Fe 2+
Fe 3÷
Fig. 7. Oxide compositions in the Neogene volcanic rocks. Fe 3+ estimated assuming 16 cations for 12 oxygens.
have relatively high Fe3/Fe3 + A1 and lower Ti but this
is not true for all samples. Zoning is present but is
generally slight; there is an increase of Si and Fe/Mg
from cores to rims in two HPAN amphiboles whereas a
slight reverse zoning was detected in amphiboles from
two HBAN. Reverse zoning may reflect an influx of
volatiles (Gill 1981). Most amphiboles have FeO/MgO
ratios typical of island arc environments and medium- to
high-K acid andesites (Gill 1981). AP contents between
0.2 and 0.5 indicate crustal pressures (Gill 1981). The
overlap in compositional fields of the two andesite types
is probably attributable to the initial crystallization of
amphibole with pyroxene (HPAN), and as the melt
became increasingly enriched in Si and Fe 3÷, pyroxene
crystallization ceased and amphibole became the only
mafic silicate phase crystallizing (HBAN).
Accessory minerals. Apatite is present in the majority
of samples. It occurs as small grains rimmed by opaques
and in some samples as inclusions in pyroxene and
hornblende phenocrysts. Quartz is present in one HPAN
and one HBAN as embayed grains up to 3 mm across.
Biotite is present in one HBAN. The rarity of biotite and
quartz and their embayed appearance suggest that they
are xenocrysts.
Titanomagnetite is the most common accessory phase
in the andesites and occurs as inclusions, microphenocrysts and in the groundmass. Titanomagnetite compositions scatter between ulv6spinel and almost pure
magnetite. Although there is considerable overlap in
compositions (Fig. 7), there is a tendency for the titanomagnetites to be relatively enriched in magnetite and
depleted in ulv6spinel in the HBAN, whereas the TPAN
titanomagnetites show the opposite relationship. Chromium oxide contents are low (< 0.47 wt%). The chemical variation observed is consistent with an increasing
differentiation from TPAN through HPAN to HBAN.
Early-formed oxides (TPAN) are relatively lower in
Fe 3+, but with increasing differentiation the Ti content
decreased (HPAN) and as the melt became enriched in
Si and Fe 3+, compositions shift towards pure magnetite.
The considerable overlap in compositional fields may
reflect the complexity of crystallization and magmatic
processes; some of the wide scatter may be attributable
to trapping of inclusions in early formed silicate grains
or to the presence of xenocrysts of opaque grains.
Textural relationships of opaques are particularly
difficult to decipher.
Groundmass. Varies from a microcrystalline matrix
composed of plagioclase, pyroxene and iron oxides with
minor glass. Glass predominates in some samples. The
glass in all of the andesites is, like glasses of the andesites
of the present day Halmahera Arc (Kuenen 1935), very
acid in composition. The least acid glasses resemble
feldspars in composition and deviate only slightly from
feldspar stoichiometry. As the glasses become increasingly silica-rich they deviate more markedly from
feldspar compositions. The most extreme glasses, considerably enriched in K20 (~4-10 wt%), are found in
some of the TPAN (Fig. 8). Plots of major elements
versus silica for rock and glass compositions (Fig. 9)
indicate that the andesites include two groups; a suite
with K-rich glasses (many TPAN, a few HPAN, no
HBAN), and a suite with glasses containing lower K20
and higher Na20 and CaO (some TPAN, most HPAN,
all HBAN).
Crystallization sequences. The history of the crystallization of these rocks is complex, indicated by the
occurrence of xenoliths and crystal-clots, evidence of
reaction with melt, and indications of multiple generations of some minerals, especially plagioclase. Textural
observations suggest that plagioclase is an early-crystal-
NEOGENE
ANDESITES/
Glass/\
CaO
\
~ Andesltes
Tw~Py~xeno
Hornblende.
Pyroxsl~
[]
/
Na20
c3
D
*
r~cn
Andesites
Hornblende
Andesltes
[]
K20
Fig. 8. Glass compositions in the Neogene volcanic rocks.
278
A. SUFNI HAKIM and R. HALL
NEOGENE ANDESlTES: Major Elements
10
26
x
8
x
[]
m
FeO
6
Al=Os
s\.
22
D
Q
[]
•
x :t
4
q~
m
14
[]
~
B
N
m
SIO=
.;-.,
10
CaO
6
=
K20
0
o
i~=
m
n
m
[]
m
m
=~
c~
u
60
®
i~
65
Calcalkaline Suite
High-K Suite
cl
70
10
8
6
m
°
Q
2
10
m
c;=
c~
4
55
m
o
Az
50
SlO=
m
=
•
45
C~ o
g
0
0
18
o c~
mmnam
2
8
~
:t
°'~
° °
°%1
D ~
75
45
[] Glass
B Glass
50
55
x TPAN
,. TPAN
60
65
• HPAN
o HPAN
70
75
4
2
80
• HBAN
Fig. 9. Major element variations for glasses and rocks from the Neogene volcanic rocks.
lizing phase and pyroxenes are generally later; orthopyroxene or clinopyroxene may appear in either order.
Hornblende is last.
Secondary minerals
Most of the samples examined show no evidence of
alteration, whereas the rest contain some low temperature alteration minerals and show some devitrification of
the glassy matrix. Smectite, vermiculite and chalcedony
are the most common secondary minerals. A pale yellowish-green to yellowish-brown smectite is present in
almost all of the altered samples and occurs predominantly as pseudomorphs after orthopyroxene and plagioclase phenocrysts and locally in the groundmass or in
amygdales. In three samples the zeolite mordenite
occurs as dirty white fibrous rosettes. Mordenite has
been reported as one of the earliest formed zeolites
in active geothermal areas (Honda and Muffler 1970,
Kristmannsdottir and Tomasson 1978) and is also commonly found as a low-grade product of Tertiary volcanism at the Pacific margins (Boles 1984). It is considered
to indicate relatively high thermal gradients.
microprobe analyses of glass are available for four of
these and all contain a K-rich glass.
In terms of major element compositions (Table 2, Fig.
9) the Neogene andesites are typical of orogenic andesires. Aiuminium oxide is typically high (17-20 wt%) and
TiO, is low ( < 1 wt%). FeO* (total Fe as FeO), MgO,
CaO and TiO2 correlate negatively with SiO2. Sodium
oxide and K20 correlate positively with SiOz up to
~ 60 wt% SiO2 and then decline with further increase in
S i O 2 . There is a similar inflection for CaO (a change
from negative to positive correlation), MnO and P205
versus SiO2. The ratio Fe203:FeO increases broadly
with increasing SiO2. The decline of TiO2 and FeO* with
increasing SiO2 is attributed to crystallization of magnetite throughout the calcalkaline series (Kuno 1968,
Gill 1981). Major element contents are similar to other
island arc calcalkaline volcanic rocks such as those from
WEST
HALMAHERA
4
Basalt
K=O
3 Wt%
VOLCANIC
] Andesite
Basal#c ~ Andesite
~
~ ~
~
l
ROCKS
Dacite
~
Chemistry
Major elements. On the SiO2-K20 diagram (Fig. 10)
of Peccerillo and Taylor (1976) the majority of samples
plot in the andesite field, two samples are basalts and
two plot as dacites. Mineralogically, there is little difference between these samples. The two basalts are petrographically TPAN. The more acid samples, both HPAN,
fall into the dacite field because of relatively high
contents of secondary chalcedony but are otherwise
similar to the andesites. Most of the samples fall into the
calcalkaline field. Six samples fall into the high-K field;
2
~
~
CalcalkalineSeries
Low-K
0
45
50
55
60
Tholeiite
,
65
Series
70
OHA FORMATION [] NEOGENEM~dlum.K [] NEOGENEHigh.K
Fig. 10. K20-SiO 2 plot for Neogene and Oha Formation volcanic
rocks. Boundaries from Peccerillo and Taylor (1976).
Tertiary volcanic rocks from the Halmahera Arc
279
Table 2. Summary of the mineralogy and geochemistry of the Neogene and Oha Formation volcanic rocks
Age:
Rock types:
Textures:
OHA FORMATION
NEOGENEVOLCANICROCKS
? Late Cretaceous-Eocene
Basaits common
Rare Basaltic Andesites
Commonly phyric with
minor aphyric rocks
Irrtersertal/intergranular
groundmaes, quenched textures
and altered glass
Late Miocene -?Pliocene
Andesites common with
subordinate Basaits
Mostly porphyritic and
phenocryst-rloh
Pilotaxitic very common
wfth abundant fresh glass
and some devitrified glass.
PrlmeryMineralogy:
Plagloclase
Clinopyroxene
Olivine
+Orthopyroxene
+Titanomagnetite
SecondaryMineralogy:
Mesolite
Thomsonite
Stilbite
Heulandite
Analoite
Low-Si Chlorite
High-Si Chlorite
Srnectite Group
Pumpellyite
Epidote
Prehnite
Sphene
Plagicclase
Clinopyroxene
Orthopyroxene
Hornblende
Titanomagnetite
i-,Quartz ± Biotite
Mordenite
Hk:Jh-Si Chlorite (mostly)
Smectite Group
the present-day Halmahera oceanic segment (Morris
et al. 1983), the Sangihe, Mariana and Tongan arcs
(Morrice et al. 1983, Meijer and Reagan 1981, Bryan
et al. 1972), and orogenic calcalkaline rocks from the SW
Pacific (Ewart 1982). Major element trends are not
significantly affected by the minor late stage alteration.
Of the major elements, K20 shows the weakest correlations and the most scatter in plots versus other
elements suggesting that it could have been mobile.
However, Na20 and K20 show a strong positive correlation and there is no clear trend of K20 with indicators
of alteration such as H20 or CO2.
The relationship between rock and mineral compositions has been examined using a least-squares method
based on principles discussed by Bryan et al. (1969) and
Wright and Doherty (1970). A common approach is to
use two different rocks in a given suite as parent and
daughter compositions to determine if they can be
related to one another by fractional crystallization. An
alternative approach was also employed since most of
the rocks contain a glass phase which may be regarded
as the result of fractionation. The glass composition was
used as the daughter composition and the bulk rock
composition as the parent composition. The rocks selected for the modelling procedure were chosen from
those analysed for major elements and for which there
were microprobe analyses of each of the phenocryst
phases and fresh glass. This reduced the number of
samples to ten. A number of conclusions are indicated
by the modelled and observed major element trends.
(1) The trend T P A N ~ H P A N ~ H B A N can be modelled as the result of low pressure fractionation of a
basaltic initial liquid similar in composition to the most
basic TPAN. The bulk rock chemical trends observed
can be modelled well for all major elements with the
OHA FORI~TION
NEOGENEVOLC~C ROCKS
Chemll~-y(SelectedElements):
Major ElementAveragesWt.%: (range in brackets)
SiOz
51 (47-54)
56 (47-64)
MgO
5.5 (3.0-8.2)
3.2 (1.0-6.2)
TiO2
0.8 (0.5-1.1)
0.65 (0.4-0.95)
FeO'
7.6 (6.3-10.5)
6.6 (3.5-9.5)
AI=O~
16.8 (13.5-19.9)
18.0 (14.6-20,2)
Alkalis
5.2 (1.6-11,3)
4.6 (2.4-6.9)
Trace ElementAveragesppm: (range in brackets)
Rb
23 (5-60)
33 (5-94)
Sr
438 (186-1120)
559 (368-840)
Ba
179 (21-596)
274 (38-516)
V
325 (201-534)
199 (92-367)
Cr
81 (2-322)
21 (3-81)
Co
39 (25-98)
36 (20-72)
Ni
42 (5-145)
14 (6-44)
So
33 (13-46)
21 (8-35)
Zn
79 (60-119)
59 (45-95)
Cu
126 (55-216)
95 (35-193)
Zr
81 (25-162)
94 (37-182)
Y
23 (15-32)
20 (13-32)
Nb
2 (1-6)
2 (1-4)
Rare Earth Element Averages ppm: (range in brackets)
La
10 (3-24)
14 (3-27)
Ce
25 (10-51)
32 (11-62)
Nd
15 (8-27)
18 (9-31)
Ce./Yb.
2.5 (1.0-4.8)
4.0 (1.6-9.2)
exception of K20. The crystallizing mixture of minerals
initially including plagioclase, titanomagnetite, orthopyroxene and clinopyroxene with later replacement of
pyroxenes by hornblende. Scatter reflects mineral accumulation, principally of plagioclase.
(2) The least squares fit is sensitive to the initial choice
of plagioclase composition. In most cases a median
composition based on inspection of the number and
range of compositions for a particular rock gave a good
initial fit. The bulk rock chemical trends are best modelled with a plagioclase compositional trend from highly
calcic (TPAN) to less calcic (HBAN). This trend is not
detectable in microprobe data probably because of the
very large range of compositions in a single rock,
presence of zoning, and mixing of several generations of
feldspars.
(3) Titanomagnetite was present as a crystallizing
phase throughout the crystallization of the series, as
found in many other calcalkaline suites. Crystallization
of a spinel with increasing magnetite component can
account for the steep decrease in TiO2 and FeO* with
increasing fractionation, as tentatively suggested on the
basis of microprobe data.
(4) The changing composition of the mineral extract
leads to initial enrichment of the liquid in Na20 and
depletion in most other elements. A small inflection in
the modelled curves at about 58-60 wt% SiO2 for MgO
and less prominently for CaO marks the appearance of
hornblende instead of pyroxene. This inflection is observed for some major elements (see above). Hornblende
and clinopyroxene or orthopyroxene are stable together
over a small range of intermediate compositions.
(5) Most of the residual is due to K20. The observed
variation of K20 with SiO2 suggests two separate groups
of glasses (Fig. 9). Variation of compositions in the
280
A. SUFNIHAKIMand R. HALL
K-poor grasses shows the expected increase of K 20 with
increasing SiO2 for the modelled minerals. In contrast,
the K-rich glasses show a decline in K20 with increasing
SiO2. Since there is no difference in the modal mineralogy of the andesites this trend is interpreted as the result
of much greater K enrichment of the liquid for the
K-rich suite. At liquid compositions of ~65 wt% SiO2
the decrease of K20 results from increase in the Kfeldspar component of the plagioclase or crystallization
of a separate K-feldspar phase; both interpretations are
consistent with microprobe observations.
Rare earth and trace elements. All the Neogene volcanic rocks show light rare earth element (LREE) enrichment typical of calcalkaline suites. The chondritenormalized patterns are shown in Fig. 11. The high-K
suite of rocks have closely grouped sloping (CeN/Ybr~
2.3-3.9) REE patterns with very similar HREE abundances but with diverging LREE ends to the patterns.
The medium-K TPAN have somewhat flatter patterns
(Ce•/YbN 1.8-2.3) than most of the high-K suite, but
closely similar in shape and with REE abundances
similar to the least ffactionated high-K rocks. The
remaining medium-K HPAN and HBAN have the
steepest patterns (CeN/YbN 3.1--4.7) with similar
LREE abundances to the most fractionated high-K
rocks.
Plots of trace element contents normalized to N-type
MORB (Fig. 12) show the principal features of the trace
element chemistry of the Neogene rocks. All show the
humped and sloping patterns typical of island arc volcanic rocks with a very prominent negative Nb anomaly.
The high K suite has the highest contents of large ion
lithophile (LIL) elements Sr, K, Rb and Ba, is depleted
in the high field strength (HFS) elements Nb and Ti, and
is strongly depleted in Cr. The two samples with most
basic compositions show slight depletion of Zr. All the
NEOGENE ANDESITES: Rare Earth Elements
100-
::,7:'t,.o
,ox_
H259 [ Arldesl~s
O~
/
HR1O0/
rr'
10
100-
NEOGENE ANDESlTES: Trace Elements
100:
--z~-HA147I
-~,2~ L two ~rox~e
-2
/~
3z
~
-=-..,oo/
8
tr"
1
100 1
High-K Suite
lol
0.1
--x<--HA134 ~ .Horn.~ .~nde- Pyroxer/o
- o - H261
0.01
HornblendeAndesRe
'~.
]~
. . . . . . . . . . . . . .
Sr K RbBa NbCe P Zr Sm Ti Y Yb Sc Cr
Fig. 12. MORB-normalized trace element plots for the Neogene
volcanic rocks. Normalizing values from Pearce (1982).
medium-K rocks have very similar patterns but with
lower LIL contents and lower Ce and P.
Summary. The mineral and rock chemical analyses
indicate that the Neogene volcanic rocks can be divided
into two suites. The high-K calcalkaline suite differs
from the medium-K suite by the presence of a K-rich
glass, higher LIL element and higher HREE abundances. Mineralogically, the two suites are similar
although no HBAN have been identified in the high-K
suite. The mineral and major element chemistry largely
reflect the effects of low pressure fractionation of a
basaltic or basaltic andesite magma. Possible early
higher pressure crystallization is suggested by some
clinopyroxene compositions. The distribution of the
high-K rocks indicates that they were produced at
different volcanic centres in the Neogene arc; most are
from the Plio-Pleistocene Tafongo Formation. The Neogene volcanic rocks have not been deeply buried and the
very low grade alteration is consistent with local geothermal effects.
High-K Suite
OHA FORMATION
lO
-I~E--HA145
[ Two Pyl'oxene
--E~HA146 J Andesltes
-)y~--HA134
-B-H254 } AndesltesH°mblende'Pyr°xene
- ~ - H261
La Ce
HornblendeAndeslte
Nd
Sm Eu Gd
Oy
Er
Yb Lu
Fig. 11. Chondrite-normalized REE for the Neogene volcanic rocks.
Normalizing values from Wakita et al. (1971).
The Oha Formation volcanic rocks are divided into
two groups on the basis of their petrography: phyric and
aphyric basalts. The phyric basalts have porphyritic
textures, with both mafic and felsic phenocrysts set in a
dark brown to greenish-grey microcrystalline groundmass, in which patches of pale to dark green secondary
minerals can often be seen. Veinlets and amygdales are
also common. The aphyric basalts are grey, green and
red-grey lavas and are fine grained and equigranular
with rare microphenocrysts. Some samples contain pale
green and fine-grained aggregates of secondary minerals.
Amygdales are subrounded or irregular in shape and
Tertiary volcanic rocks from the Halmahera Arc
281
OHA FORMATION: Pyroxenes
occur in almost all samples. They typically contain
zeolites in their cores, surrounded by green sheet silicate
minerals.
DJ
O Di
Hd
Mineralogy
The modal proportion of phenocrysts and microphenocrysts in the phyric basalts is between 10 and 60%.
Phenocrysts include plagioclase feldspar, olivine, pyroxene and Fe-Ti oxides. Orthopyroxene phenocrysts are
present in two samples. In the aphyric basalts plagioclase, clinopyroxene and olivine are the main minerals.
Feldspar. The modal percentages of plagioclase phenocrysts and microphenocrysts in the phyric basalts
range from 10 to 30%. In the aphyric basalts plagioclase usually constitutes greater than 70o of the mode.
Plagioclase crystals have elongate, rectangular, acicular,
wedge, serrated, fork-edge and swallow-tail shapes; all
are typically free of inclusions and unzoned. Embayed
crystals are common in almost all phyric samples.
Phenocrysts typically contain a network of cracks and
may be altered; the groundmass feldspars are albitized.
Microprobe analysis reveals that the Ca-rich phenocrysts vary considerably in composition but there is no
clear difference between phyric and aphyric basalts
(Fig. 13). The smaller number of analyses from the
aphyric basalts reflects the finer grain size and greater
degree of alteration. In general, zoning is absent. The
wide range of plagioclase compositions (An, to An94) is
considered to be due partly to original variation in
primary feldspar composition and partly due to secondary alteration. Most feldspars fall in the range from
An41 to An96 which includes two groups, sometimes
within a single rock sample: one group with compositions of An6j-An96 (median ~Ans0), and a second
group with compositions from An4, to An6~ (median
An53). Those with lower An contents generally have 1-4
molar% Or, whereas those with higher An contents
generally have <1 molar% Or. Albite replacement of
phenocrysts is often recognizable from a dusty appearance and small dark inclusions. In altered samples
groundmass plagioclase laths and phenocrysts are
altered to compositions in the range from An] to Anl7.
K-feldspar has been identified in the groundmass of
two samples. Orthoclase (Or97_98) in alkalic basalt from
the Mariana Basin has been reported by Floyd and
OHA FORMATION: Plagioclase
An
Ab
An
Or
Fig. 13. Plagioclasecompositionsin the Oha Formationbasalts.
Bmm~
En
Fs
Fig. 14. Pyroxenecompositionsin the Oha Formationbasalts.
Rowbotham (1986), and from the Hess Rise, a Pacific
seamount, by Lee-Wong (1981) where it is the result of
replacement of plagioclase by potassium-bearing hydrothermal fluids or sea water. The K-rich feldspar in the
Oha Formation is interpreted to be related to similar
low-temperature alteration.
Clinopyroxene. Clinopyroxene is common in the
phyric basalts. The phenocrysts form from 2 to 7% of
the total rock volume. The crystals are generally equant
or rectangular and form anhedral and subhedral grains
from 0.2 to 3 mm across. Phenocrysts are occasionally
weakly zoned and sector zoning is present in two
samples. Crystal clots are common in some samples.
They are composed of equant or subrounded grains of
clinopyroxene and orthopyroxene, with or without
plagioclase. There is little evidence of reaction between
the clots and the groundmass. In the aphyric basalts
clinopyroxene forms 15-20% of the mode. Weak continuous zoned and sector zoned crystals are present in
some samples.
Most of the microprobe analyses of clinopyroxenes
are core compositions since rims and groundmass pyroxenes are altered. In terms of quadrilateral components
they scatter in the augite, salite and diopside fields
(Fig. 14). Clinopyroxenes show an iron enrichment trend
which is less obvious for those from the phyric basalts.
The clinopyroxenes have variable A1 contents which are
weakly correlated with Ca. Augites have low AI contents
(< 0.1 ions per formula unit) whereas salites generally
have higher A1 values (up to 0.35 ions per formula unit).
Diopsidic pyroxenes have a wide range in A1 content and
are similar to salites but contain appreciable Cr20 3
(0.12~).97 wt%). Weak zoning is present in some phyric
clinopyroxenes; an increase in A1 and decrease of Cr and
Si from core to rim was detected in one grain and in
another sample with low A1 and no Cr the zoning is an
increase of Ca and decrease of Si, A1, Ti, Fe and Mg
from core to rim. Clinopyroxene Si content shows a
negative correlation with Altota I and Ti and positive
correlation with Mg content. Ca-Ts is the principal
non-quadrilateral component with minor substitution of
Fe 3÷ for A13÷ in Fe-Ts and Ti4÷ for Si as discussed
above for the Neogene volcanic rocks.
Orthopyroxene. Orthopyroxene is present in two
phyric basalts where it forms around 10-15% of the
mode. The orthopyroxenes are phenocrysts and microphenocrysts and also form crystal clots with or without
augite, rarely associated with plagioclase. A reaction
with the melt is indicated by the occurrence of the
282
A. SUFNIHAKIMand R. HALL
overgrowth of anhedral augite on the rims in some
grains. This feature is rarely observed in Neogene andesites containing two pyroxenes.
Orthopyroxene compositions fall in the bronzite-hypersthenc range (Fig. 14), reflected by a pink pleochroism. Wo contents are ,-~3-4 molar%. AI contents in
orthopyroxenes and clinopyroxenes are similar, typically
< 0.1 ion per formula unit, consistent with the interpretation that they are equilibrium low pressure pairs.
Because of alteration of crystal rims temperatures were
calculated from core compositions of coexisting pyroxenes (Wells 1977) for two rocks containing fresh orthopyroxene and yielded vaues of 1048_+ 14°C and
1016_+90°C; the large error on the latter reflecting
compositional variation in the sample, possibly due to
alteration.
Olivine. Almost all of the phyric basalts contain
olivine phenocrysts and microphenocrysts ranging from
0.1 to 3.5mm across forming 5-10% of the mode.
Microcrystalline olivine is probable in a few samples but
the high degree of alteration precludes definite identification. Ubiquitous alteration of olivine phenocrysts
prevents determination of their compositions. The most
common inclusions in olivine are subhedral to euhedral
Fe-Ti oxides; in one sample dark brown euhedral crystals of Cr spinel are present as inclusions in altered
olivine phenocrysts. In the aphyric basalts olivine microphenocrysts are found in two samples. Olivine is
replaced by yellowish-brown chlorite goethite, "iddingsite" and "bowlingite".
Accessory minerals. The principal accessory mineral in
all samples is Fe-Ti oxide, forming equant or irregular
grains up to 1 mm across, as aggregates in the groundmass or as inclusions in the phenocrysts or microphenocrysts. Equant grains occur generally as inclusions
in some of the olivine phenocrysts and as the grains in
crystal clots. The majority of Oha Formation basalts
contain titanomagnetites which have little Cr and AI.
Two samples contain Cr-rich spinel which have Cr #
and M g # in the range of Halmahera harzburgite
chrome spinels (Ballantyne 1991).
Groundmass. The microcrystalline groundmass typically has quench textures including intergranular and
branching clinopyroxene, small and radiate plagioclase
laths or forked microlites and minute or acicular Fe-Ti
oxides, often set in altered interstitial glass.
Crystallization sequence. On the basis of textural
observations the crystallization sequence is interpreted
to have been initiated by the appearance of olivine
( _+spinel). After the crystallization of olivine, plagioclase
formed and was then followed by clinopyroxene and
iron oxide• In samples without olivine crystallization was
initiated by plagioclase, followed by orthopyroxene then
by clinopyroxene and Fe-Ti oxides.
dales, as infilling minerals and as patchy replacements of
phenocrysts and the groundmass. In addition, plagioclase is albitized in the majority of samples and calcite
and quartz are common.
Sheet silicates. On the basis of SiO 2 and alkali contents, the sheet silicates may be subdivided into two
types: chlorite- and smectite-type sheet silicates (Fig. 15).
In general the chlorite-type sheet silicates have less than
40wt% SiO2 and less than 2wt% K20, whereas the
smectite-type sheet silicates commonly have higher SiO2
and significant K20.
Chlorite-type sheet silicates. Chlorites are of two
types: I-chlorite type ( < 3 0 w t % SiO2) and II-chlorite
type (30-40 wt% SiO2). The I-chlorite types are mostly
pale green and show anomalous interference colours,
whereas the II-chlorite types are brown to golden-brown
or greenish-brown and have a distinctive red--orange
interference colour. Most of the I-chlorites are brunsvigite to diabantite (Foster 1962) with Fe2+/R 2+ ratios
from 0.29 to 0.34. The II-chlorites fall mostly into the
diabantite compositional field with a range of FeE+/R 2+
from 3.2 to 4.0. The principal chemical features of the
two chlorite types are listed in Table 3. Like Icelandic
chlorites (Viereck et al. 1982) cation totals for chlorites
from the Oha Formation are low and most of the
chlorites have AI vi greater than A1~v. Several samples
contain small quantities of K, Ca, Na and Mn. The
presence of K, Ca and Na is interpreted by Offler et al.
(1980) as due to minor quantities of illite and smectites,
which are interlayered with chlorite, typical of chlorite
produced during sub-greenschist facies metamorphism.
Smectite-type sheet silicates. Smectite-type sheet silicates are found mainly as replacements of phenocrysts
and interstitial glass, and in amygdales. The colour
broadly reflects the chemical composition. Yellowishbrown varieties have moderate A1203 (12-15 wt%) and
FeO (15-19 wt%) contents. Yellowish-green, pale green
and bluish-green varieties have the low A1203 contents
(6-9 wt%), high K20 contents (5-9 wt%) and most have
high FeO* contents (21-22 wt%). Brown varieties have
the highest Al203 contents (16-27 wt%) and the lowest
FeO contents (3-8 wt%) with Na20 contents from 1 to
3 wt%. The principal chemical features of the three
smectite types are listed in Table 3. These smectite types
compositionally resemble saponite, celadonite and
OHA FORMATION: Sheet Silicates
SiO2
x
//
,
\
/
/
~
•
/
Secondary minerals
/
=
/
/
J
/
x
x
"\
\
,,
\
~,
/
/
/
There are two main groups of secondary minerals in
the Oha Formation: various sheet silicate minerals and
a variety of zeolites. They occur in fractures, in amyg-
AI203
/
[] Low-Si Chlorites ~ High-Si Chlorites
~
z Smectites
x
L
MgO
FeO
Fig. [ 5. Sheet silicate compositions in the Oha Formation basalts.
Tertiary volcanic rocks from the Halmahera Arc
Table 3. Chemistryof sheet silicates in the Oha Formationbasalts
CHLORITES
Type
I
colour
Pale Green
wt.%
SiO=
AI203
MgO
FeO
MnO
K~O
~;30
14-18
11-17
20-30
0.3-0,7
<1
II
Green-Brown
>30-40
10-16
14-22
13-20
0
0.1-2
colour
I
II
Yellow-Brown Green
III
Brown
wt.%
AI203
MgO
FeO
CaO
Na20
K20
Pumpellyite. This is probably the least abundant secondary mineral in the Oha Formation and occurs in only
one sample. It occurs in albitized plagioclase as strongly
pleochroic, acicular and bladed crystals, which form
radiating and random aggregates. In places it forms
rosettes of needle-like crystals. Locally, pumpellyite is
associated with light yellowish-brown chlorite and
colourless epidote. In most cases the pumpellyite exists
with chlorite in the matrix. It is an Fe-rich variety, with
FeO* generally higher than 10wt% , similar to those
associated with zeolite or prehnite-pumpellyite facies
metamorphism (Liou 1983).
Chem&try
SMECTITES
type
283
12-15
5-9
15-19
0-3
0
0.5-3
6-9
4-5
21-22
<1
0
5-9
16-27
6-8
3-8
4-7
1-3
1-2
beidellite, respectively. Compositions of these minerals
in the Oha Formation are comparable with those
reported by Andrew (1980), Natland and Mahoney
(1981) and Alt and Honnorez (1984) in rocks from the
ocean floor and Marianas forearc and interpreted to be
the result of low temperature interaction between basalt
and sea water.
Zeolites. Zeolites identified include analcite, heulandite,
laumontite, mesolite, stilbite and thomsonite. Analcite is
the most common zeolite and is usually colourless, typically replaces plagioclase phenocrysts, and is commonly
associated with calcite, chlorite and other zeolites. Mesolite occurs usually as white brownish fibres and radial
aggregates in veinlets and amygdales, commonly rimmed
by chlorite. In most samples, heulandite occurs in veinlets
with mesolite, analcite and calcite. Laumontite is less
common and occurs as acicular, columnar and elongated
aggregates with a radiating habit in amygdales together
with calcite and pale green chlorite. Thomsonite is present
in veinlets with analcite and carbonate, with chlorite rimmed by brown cryptocrystalline material, in amygdales in
which laumontite is absent, and as radiating fibrous crystals around the rim of amygdales infilled by "foliated" stilbite. Calcium-rich stilbite occurs in only two samples as
"foliated" and fractured crystals crossed by thomsonite.
Sphene. Sphene was found in one sample and is an
Al-rich variety ( ~ 6 w t % ) with a SiO 2 content
( ~ 3 2 w t % ) comparable to those in rocks from the
Western Andes (Offler and Aguirre 1984).
Epidote. Epidote was found in only two samples. It
occurs as fine grained patches in albitized plagioclase
and with pumpellyite and chlorite in one sample.
Prehnite. Prehnite is found only as replacement
patches of plagioclase in two rocks as radiating and
sheaf-like aggregates with chlorites and albite.
Major elements. Water and CO2 contents of all
samples are high and range from 2 to 8 wt% reflecting
extensive secondary alteration. Silicon dioxide contents
are low and range from 46 to 52 wt%. Titanium dioxide
contents range from 0.7 to 1 wt%. With one exception,
A1203 contents vary from ~ 15 to 19 wt%. On the K20SiO2 diagram the Oha Formation samples fall in the
medium-K to high-K fields. In contrast to the Neogene
volcanic rocks the K20 content of the samples plotting
in the high-K field is due to the abundance of K-enriched
secondary minerals. Contents of other major oxides of
the Oha Formation basalts (Table 2) are comparable to
tholeiitic basalt samples from the Tongan island arc
(Bryan et al. 1972, Ewart 1976), from the Marianas
island arc (Dixon and Batiza 1979, Stern 1979), the
Mariana Trough (Hawkins and Melchior 1985), and the
Aleutian island arc (Kay et al. 1982). The small range of
chemical composition in the Oha Formation basalts
precludes modelling but the variation is consistent with
olivine-clinopyroxene and plagioclase fractionation.
Rare earth and trace elements. The majority of the Oha
Formation basalts show LREE enrichment similar to
most arc volcanic rocks; a small number do not show
this enrichment. The chondrite-normalized patterns fall
into two groups (Fig. 16). The aphyric basalts and most
of the phyric basalts have sloping (aphyric CeN/YbN
2.3-3.8; phyric CeN/YbN 1.7~1.8) REE patterns with
REE contents increasing in most cases with bulk rock
SiO2. Most of these rocks have low Ni and Cr contents
(< 50 ppm) but among the basalts with sloping patterns
are several with relatively primitive compositions which
have low REE, high Cr (50-320 ppm) and high Ni
(50-150 ppm) contents. Of the remaining phyric basalts
(Fig. 16), two samples have much flatter and parallel
patterns (CeN/YbN 1.7-1.8) with a slight LREE depletion and one sample has a completely flat chondrite
normalized pattern (CeN/YbN 1.1).
The aphyric and phyric basalts with sloping REE
patterns have the humped and sloping MORB-normalized trace element patterns (Fig. 17) typical of island arc
volcanic rocks with a prominent negative Nb anomaly.
Although the LIL elements may have been mobile and
hence an unreliable guide to origin, the patterns also
show depletion of the generally immobile HFS elements
Nb and Ti, and in most cases depletion of Zr. The more
284
A. SUFNI HAKIM and R. HALL
OHA FORMATION:Rare Earth Elements
100-
o~
AphyricBasalt$
'=
~--~
o--o
~
--~ HR171
~ HR191
-x<- HR192
-B- HG51
~
10
100
Phyric Basalts
" ~
~ H269
~ H277
-x<- HR154
--IB--HR201
-
~L
~
t
Phyric Basalts
-~- HR129
HG88
~
10
~~
,
,
La Ce
,
,
Nd
,
,
,
,
Srn Eu Gd
,
.
Dy
.
.
.
.
Er
,
Yb Lu
Fig. 16. Chondrite-normalizedREE for the Oha Formationbasalts.
Normalizing values from Wakita et al. (1971).
primitive compositions show no depletion of Cr. The
three samples with flat to slightly sloping patterns have
trace element abundances for most elements which are
similar to N-type MORB, apart from Cr which is
strongly depleted, and the LIL elements which are
enriched but may reflect alteration, and Nb. All the
patterns show a pronounced negative Nb anomaly; since
Nb is generally regarded as immobile this feature indicates an arc volcanic origin. The REE and trace element
chemistry thus indicate that the Oha Formation basalts
are all of island arc character but include more than one
suite. Trace element variations suggest most samples are
related by "tholeiitic" fractionation of the more primitive compositions due to crystallization of olivine and
pyroxene without magnetite.
S u m m a r y . The volcanic rocks of the Oha Formation
are typical of an intra-oceanic island arc. They have a
more resticted compositional range than the Neogene
volcanic rocks, being predominantly basaltic. Unlike the
Neogene rocks, high potassium contents are the result of
secondary alteration and the majority of the Oha Formation basalts belong to the medium-K calcalkaline
series. The samples with lowest REE element abundances and flatter chondrite-normalized patterns resemble arc tholeiites and are similar to relatively
primitive lavas in the arc basalt series reported from
areas such as the Marianas and New Hebrides. Mineralogically, the Oha Formation volcanic rocks differ
considerably from the Neogene rocks. Plagioclase
feldspar phenocrysts are commonly unzoned and rarely
contain glass or mineral inclusions; olivine is common,
orthopyroxene is rare and phenocrysts of hornblende
and magnetite are absent. In contrast to the Neogene
volcanic rocks, the Oha Formation has suffered zeolite
and local sub-greenschist facies alteration reflecting deep
burial and/or high heat flows.
HISTORY OF VOLCANISM IN THE
HALMAHERA ARC
The Halmahera region has a long volcanic history
(Table 4). The oldest volcanic rocks of Halmahera are
those of the eastern Halmahera Ophiolitic Basement
Complex in which Ballantyne (1990, 1991) has identified
a number of distinct and non-cogenetic suites; one of
boninitic affinity, two of island arc and one of oceanic
island/seamount origin. The boninitic suite is thought to
be the oldest and related to Mesozoic initiation of
subduction. The ocean island/seamount rocks are interpreted as oceanic rocks accreted into the Late Cretaceous forearc and the island arc volcanic rocks as the
product of this Late Cretaceous arc. The Mariana
forearc is proposed as a modern analogue for the
ophiolite terrane of eastern Halmahera (Hall et al.
1988a, Ballantyne 1990). Halmahera was probably situated at the equatorial western Pacific margin at this time
although its exact location is uncertain.
Late Cretaceous island arc volcanism is represented in
SE Halmahera by the Gowonli Formation (Hall et al. in
press). The volcanic rocks of the Gowonli Formation
(Ballantyne 1990) resemble those of the Oha Formation
in many ways. They are plagioclase- and clinopyroxenephyric basaltic andesites, lacking hornblende, of tholeiitic to calcalkaline affinities, with LIL and LREE
element enrichment. Petrographically similar rocks, but
of Eocene age, are found in eastern Halmahera in the
Sagea Formation but these rocks have not yet been
chemically studied. No Paleocene rocks have yet been
found in Halmahera and it is not clear if the Late
Cretaceous and Eocene volcanic and volcaniclastic rocks
1O0
OHA FORMATION: Trace Elements
q
1
Aphyric Basalts
~
/,._~¢,
/ ~
"~
\
~ - HR171
43= HR191
-x~ HR192
~ HG51
100
10~
~
\
H
\R\921
0.1
0.01
Phyric Basalts
--H260
~H277
\
4E- HG88
4~-
I~
. . . . . . . . . . . . . .
Sr K RbBaNbCe P Z r S m l ] Y Y b S c C r
Fig. 17. MORB-normalized trace element plots for the Oha Formation
basalts. Normalizing values from Pearce (1982).
Tertiary volcanic rocks from the Halmahera Arc
285
Table 4. Timing of volcanic episodes in the Halmahera region. Formations sampled in this study are shown in boldface. Shaded units
are of uncertain age
HALMAHERA
Quaternary
Pliocene
U
0
J Quaternary-Recent
2
Tafongo Fm/Kulefu
51Weda
BACAN
WAIGEO
0
I Quaternary-Recent
2
F"m''~"
5
Group/L°kuFrn l Obit
--
10
--
15
Miocene M
L
15
25
Oligocene ~
l
32
I a 'o'a
L
38
U
Eocene
M
TRumai Fm
42
50
T
Upper
Cretaceous
SageaFm
omasFm
55
66
I GowonliFm
•
--
25
32
38
L
Paleocene
Fm
--
42
--
50
- -
55
66
~ ¢ a n Fra
Olla
98
Lower
Cretaceous
Ma
98
I
represent a long-lived Late Cretaceous-Eocene arc or
whether volcanic activity ceased for a period at the end
of the Cretaceous.
The age of the Oha Formation is uncertain. Clasts
taken from breccias and conglomerates at the northern
end of the SW arm include Eocene limestones (probably
Lower Eocene). Eocene limestones associated with volcaniclastic rocks were collected in central Halmahera
(Hall et al. 1988a) and similar limestone clasts were
found in breccio-conglomerates as float further south in
the SW arm. Since all the dates are from clasts within the
formation their significance is uncertain. The clasts could
be interpreted as reworked at about the period of
volcanism, as seen in volcaniclastic rocks of the Gowonli
and Sagea Formations, or much later. Dating of overlying sedimentary rocks shows that the Oha Formation
is older than Late Miocene; a Late Cretaceous or Eocene
age is suspected since petrographically and chemically
the volcanic rocks of the Oha Formation closely resemble the arc volcanic rocks of the Gowonli and Sagea
Formations of east Halmahera.
The existence of an Oligo-Miocene volcanic arc in the
Halmahera region is widely quoted (e.g. Hamilton 1979,
Katili 1975) but the evidence for it is weak. Pillow basalts
are reported by Yasin (1980) from Bacan, and appear to
be the same sequence as, or an equivalent to, an
extensive undeformed pillow lava sequence of Early
Ma
Oligocene age on the island of Kasiruta at the northern
end of the Bacan group. Petrographically, these resemble
arc basaltic andesites, but they differ from the Oha
Formation in many petrographic features and in the
degree of secondary alteration. These rocks are not yet
chemically characterized. Almost identical rocks are
present in NW Halmahera and are interpreted to be the
product of rift-related volcanism of similar age to the
opening of the Central West Philippine Basin (Hall et al.
1991).
Studies in north, central, southern and eastern
Halmahera (e.g. Hall et al. 1988a,b, 1991, in press,
Nichols and Hall 1990, our unpublished results) indicate
that there was no arc volcanic activity during most of the
Miocene and the Neogene volcanic arc was initiated at
the beginning of the Late Miocene by eastward subduction of the Molucca Sea Plate at the Halmahera Trench.
The Neogene volcanism was marked by an important
change with the appearance of hornblende-bearing andesites. Basaltic compositions are characteristic of all
pre-Neogene volcanic rocks so far discovered in the
region and hornblende-bearing andesites are rare. Our
continuing work in the region indicates that this is a
major regional change in the character of volcanic
activity but its cause is unknown. Basaltic compositions
are characteristic of all pre-Neogene sequences so far
discovered in the region and hornblende-bearing andes-
286
A. SUFNI HAKIM and R. HALL
ites are rare; in contrast, Neogene-Recent volcanic rocks
are typically andesites, hornblende is a common constituent, and basalts are rare.
Molucca Sea subduction has continued until the
present-day, possibly briefly interrupted in the Late
Pliocene-Early Pleistocene, and the present QuaternaryRecent volcanic arc is built unconformably upon Neogene volcanic and volcaniclastic rocks. Comparison of
the Neogene rocks to those of the present-day arc
indicates that the volcanic products have not changed,
with the possible exception of contamination by continental crust at the southern end of the arc.
Acknowledgements--Research in Halmahera and the surrounding
region has been supported by The Royal Society, the University of
London Consortium for Geological Research in Southeast Asia,
NERC grant GR3/7149, Amoco International, British Petroleum,
Enterprise Oil, Total Indonesie and Union Texas (S.E. Asia). Sufni
Hakim was supported by an award from the Asian Development Bank.
We thank GRDC Bandung for their cooperation, our field colleagues
J. AlL M. G. Audley-Charles, P. D. Ballantyne, T. R. Charlton, L. A.
Garvie, S. Hidayat, Kusnama, J. Malaihollo, G. J. Nichols, S. L.
Tobing; and F. T. Banner, D. J. Carter, L. Gallagher and A. R. Lord
for their contributions. We thank A. Osborn and M. F. Thirlwall for
help and advice with the geochemistry and I. C. Young for assistance
with microprobe work.
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