Tertiary volcanic rocks from the Halmahera Arc, Eastern ...searg.rhul.ac.uk/pubs/hakim_hall_1991_halmahera.pdfIn terms of major element compositions (Table 2, Fig. 9) the Neogene andesites

Journal of Southeast Asian Earth Sciences, Vol. 6, No. 3/4, pp. 271-287, 1991 0743-9547/91 $3.00 + 0.00 Printed in Great Britain Pergamon Press Ltd

Tertiary volcanic rocks from the Halmahera Arc, Eastern Indonesia

A. SUFNI HAKIM* and ROBERT HALL t

*Geological Research and Development Centre, Bandung, Indonesia and tDepartment of Geological Sciences, University College London, Gower Street, London WCIE 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 comprehen- sive 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.

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 Neo- gene 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

SEA~S 6.~--H 271

272 A. SUFNI HAKIM and R. HALL

lot

5 ~.

0 ° _

5 ~.

I I I

0 5?0 km /

P L A T E " )~,£ Yo9 ~o°

~" o ~ ~ C A R O L I N E 0~) .~ v SNELLIUS ~'v~ %~ RIDGE~ ' • .... ~MO.OTA) P L AT E :,..~ ~Rouo. ~ o

:b., 0 .o- '=

110 ° 125 ° 110 ° 135 °

Fig. I. Tectonic set t ing o f Ha lmahera . Small solid circles ma rk 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 were petrographically similar, the andesitic compositions were due to the presence of an acid glass. Morris et al.

(1983) divided the Quaternary-Recent volcanic arc of western Halmahera into a normal oceanic segment and a continental segment. The normal oceanic segment includes the volcanoes of northern and central Halma- hera which have produced porphyritic basalts and andesites. The rocks are calcalkaline arc lavas with plagioclase, olivine, clinopyroxene, hornblende and magnetite phenocrysts in a glassy to granular groundmass which is variably devitrified. Their chemistry is typical of oceanic island arcs. Most of the rocks fall within the range of 53~3 wt% SiO2 and have high AI2 03, low TiO2 and K20 contents typical of medium-K suites (Gill 1981) with little to moderate iron enrichment. Incompatible element concentrations vary with SiO2 content, and alkali elements, Ba and Sr have typical island arc values. High field strength and compatible elements are characteristically depleted. 87Sr/a6Sr ratios are similar to those of many island arcs but lead isotope ratios suggest minor sediment contamination. In contrast volcanic rocks from south Bacan, at the southern end of the Halmahera Arc, are mostly dacites with minor andesites and are considered to be contaminated by continental crust. Alkali el~ment contents are high and trends of alkali elements versus SiO 2 are slightly steeper

than those in the normal oceanic segment. Ratios of S7Sr/S6Sr and 2°7pb/2°4pb are higher than those of the normal oceanic segment.

STRATIGRAPHY

The oldest rocks of eastern Halmahera (Table 1, Fig. 2) are ophiolitic rocks (Hall et al. 1988a, Ballantyne 1990) overlain unconformably by Upper Cretaceous to Lower Tertiary volcanic and volcaniclastic rocks interpreted as part of a forearc sequence (Hall et al. in press). These ophiolitic and forearc rocks (the Ophiolitic Base- ment Complex) were deformed in the Late Paleogene and form the basement to the Neogene succession in eastern Halmahera. The basement of western Halma- hera is composed of volcanic rocks of the Oha For- mation which forms mountains along the western side of the SW arm extending north into central Halmahera. Similar rocks are reported from Morotai (Supriatna 1980).

Neogene volcanic and sedimentary rocks rest unconformably on the Oha Formation in the SW arm and in a folded belt of central Halmahera. In the western part of the SW arm turbidites and debris flows of the Upper Miocene Loku Formation rest unconformably upon the Oha Formation. Breccio-conglomerates in this for-

Tertiary volcanic rocks from the Halmahera Arc 273

!

12"~ Quaternary Alluvium and Limestone Quatemaly-Renent Volcanic Rocks Rio-Pleistocene Volcanic Rocks

Weda Group

Loku Formation

!

128"E

MOI~)T

Miocene Umestones

Eady-Mid Miocene Sed=mentary Rocks Oligocene Basalts & Sandstones Oha Form~on Volcanic Rocks Ophiolitic Basement Complex Cor~nental Metamoq~hic Rocks

¢,

TERNATE ( ~

TIDORE o

Major Thrust

~ Major Fault ~ )

• Volcano t .

' H A L M A H E R A

Buff Bay

!

12~E

2~N .

I

Weda o".

8 Bay

,D

I

Fig. 2. Simplified geological map 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.


Table 1. Summary of the stratigraphy of Halmahera

CENTRAL AND SW ARM NE AND SE ARMS

Quaternary Vd~ic and Volcaniclas~ P~cks Quatema7 Red Umestones

TAFONGO AND KULEFU FORMATIONS Plio.Pl~stocene 0.1500m Volcanic and Vdcanidastic Ro~s

With Vdcan'¢ Clasts ~ lom~es, Sandsl~, I~dsto~s . . . . . . . . . .

KAHATOLA FORMATION

Bas;tdc Rio, Lavas & Voicanidastic Tudxiles PANITI FORMATION M/dd/e and Late Eocene >500m Ma~inal to Shallow Marine S~menta~ Ro~

OHA VOLCANIC FORMATION ~te Cretaceous or Eocene? >1000m Caical~ine Voltaic Roc~ Volcanidastic Breccias, Conglomerates

OPHIOLmC BASEMENT COMPLEX A R M SEARM

BULl GROUP SAGEA FORMATION MiMe Eo~ne >500m Vok:a~cl~ Rocks ReduCed Umesto~s DODAGA BRECCIA FORMATION Camp,/an and Maas~ch~an >lO00m Deep M~ine Volcanidastic Sedimentary GOWONU FORMATION Rocks and P~agic Modstones Late Cretaceous >lO00m

Ca~a,~ Voice's, Hyr~ys,~ GAU LIMESTONE FORMATION and Volca)idas~¢ Rocks Con/ac/an-Campanm >50m Pelagic Mudstones, Lim~sto~ Rede~ted and Pelagic Limestones Vok:anidas~c Sedimer~a~y Pocks

OPHIOUTIC ROCKS PerkJo~tes, U I ~ ~ Ba,¢ Cumula~, Mic~jaltom Oicri~ Trondh~iles ~kaine Voicadc Rocks, Arc Tholei~, 8onini~c Vol~ic Rocks, Melamorphic Rods

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 10 s 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 gravi- metric 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 volcanic rocks

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 per- centage of phenocrysts or microphenocrysts varies from 35 to 45%.

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. Or- enriched 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.

from the Halmahera Arc

NEOGENE ANDESITES Plagioclase

An

Ab

Ab

Ab

An Or

2, An Or

~~desblend e ites

Groundmass

Or

Fig. 3. Plagioclase compositions in the Neogene volcanic rocks.

275

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 Altota I 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 Al 3+ 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 early- formed 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


En

NEOGENE ANDESITES: Pyroxenes Di Hd

°mblende'APY~;~:;~ \\

Fs Fe3+Ti-poor Cpx o Fe3÷Ti-rich Cpx 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

0.4

0.31 i

0.2 ̧

0.1

0.0

0.3-

0.2-

0.1-

o.o

AI total+Ti+Fe 3÷ NEOGENE ANDESITES ~ Clinopyroxenes

AI total

Hornblende- ~ ~ r o x o t l ~

ndesffes

Two Pyroxene Si Andesltes

Z

DD D

1.75 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.

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 expla- nation emerges from the analytical work for the colour of the amphiboles. Gill (1981) suggested that a brown colour is due to a high Fe3÷/Fe 2÷ ratio but no clear-cut relationship was observed; most of the green amphiboles

NEOGENE ANDESITES: Hornblendes 2.5

2.0

1.5

1.0

0.5

0.0

Tschermakite Pargasite . . . . . . . .

D D D I

Trernolite Hornblende Edenite

N a + K

018 1.0 0 ' o:a ' 014 016 HPAN-Brown ~ HPAN-Green ~ HBAN-Brown x HBAN-Green

3.4 D

D ID

qz3~ o 3.2 o oo D D~

M g o ~ '~ ~ x 3.0

X

2.8 ~a~ ~

z 2.6

, , 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).

Tertiary volcanic rocks from the Halmahera Arc

NEOGENE ANDESITES: Oxides Ti Ti Ti

Two Pyroxene Hornblende-Pyroxene Hornblende Andesites Andesites Andesites

Phenocrysts : o

\ / \ / \ Groundmass ° ° = °

Fe 2+ Fe 3÷

Fig. 7. Oxide compositions in the Neogene volcanic rocks. Fe 3+ estimated assuming 16 cations for 12 oxygens.

277

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. Chro- mium 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-

CaO

NEOGENE ANDESITES / \ G l a s s / \ ~ Tw~Py~xeno

Andesltes Hornblende. Pyroxsl~ Andesites

[ ] Hornblende Andesltes

/ * D c3 r ~ c n []

Na20 K20 Fig. 8. Glass compositions in the Neogene volcanic rocks.


10 NEOGENE ANDESlTES: Major Elements

4

2

0

10

8

6

4

2

0

8 FeO 6

CaO

x x m

s \ .

. ;-. , 0

g

m

SIO=

o i~= • o

Az [] c;= c~ c~

Q

u i ~ cl D ~ ®

Al=Os

x : t : t

[ ]

D

Q

[] ~ o c~ • q~

mmnam C ~ o m

[]

~ B N

K20

= SlO= m

= m

m n m

m m =~

m

m

° ° ' ~ ° ° ° % 1

45 50 55 60 65 70 75 45 50 55 60 65 70 75 80

Calcalkaline Suite [] Glass x TPAN • HPAN • HBAN High-K Suite B Glass ,. TPAN o HPAN

Fig. 9. Major element variations for glasses and rocks from the Neogene volcanic rocks.

26

2 2

18

14

10

10 8

6

4 2

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.

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;

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 andes- ires. 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 SiO 2 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

W E S T H A L M A H E R A V O L C A N I C R O C K S 4

] Basal#c Andesite ~ Dacite Basalt Andesite ~ l K=O ~ ~ ~ ~

3 Wt%

2 ~ ~ Calcalkaline Series

Low-K Tho le i i t e S e r i e s

0 , 45 50 55 60 65 70

OHA FORMATION [] NEOGENE M~dlum.K [] NEOGENE High.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

Table 2. Summary of the mineralogy and geochemistry of the Neogene and Oha Formation volcanic rocks

279

OHA FORMATION NEOGENE VOLCANIC ROCKS

Age: ? Late Cretaceous-Eocene Late Miocene -?Pliocene Rock types: Basaits common Andesites common with

Rare Basaltic Andesites subordinate Basaits Textures: Commonly phyric with Mostly porphyritic and

minor aphyric rocks phenocryst-rloh Irrtersertal/intergranular Pilotaxitic very common groundmaes, quenched textures wfth abundant fresh glass and altered glass and some devitrified glass.

Prlmery Mineralogy: Plagloclase Plagicclase Clinopyroxene Clinopyroxene Olivine Orthopyroxene +Orthopyroxene Hornblende +Titanomagnetite Titanomagnetite

i-,Quartz ± Biotite Secondary Mineralogy:

Mesolite Mordenite Thomsonite Stilbite Heulandite Analoite Low-Si Chlorite High-Si Chlorite Hk:Jh-Si Chlorite (mostly) Srnectite Group Smectite Group Pumpellyite Epidote Prehnite Sphene

OHA FORI~TION NEOGENE VOLC~C ROCKS

Chemll~-y (Selected Elements): Major Element Averages Wt.%: (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 Element Averages ppm: (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)

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 corre- lations 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 TPAN~HPAN~HBAN 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

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 ac- cumulation, 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% SiO 2 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


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 K- feldspar 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 chondrite- normalized 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

100-

O~

rr '

10

100-

lO

NEOGENE ANDESITES: Rare Earth Elements

:: ,7: ' t , .o ,ox_ H259 [ Arldesl~s

/

HR1 O0 /

High-K Suite

-I~E-- HA145 [ Two Pyl'oxene --E~HA146 J Andesltes -)y~-- HA134 -B-H254 } AndesltesH°mblende'Pyr°xene - ~ - H261 Hornblende Andeslte

La Ce Nd Sm Eu Gd Oy Er Yb Lu

Fig. 11. Chondrite-normalized REE for the Neogene volcanic rocks. Normalizing values from Wakita e t al. (1971).

100:

-2

3z 8 tr"

1

100 1

lol

0.1

NEOGENE ANDESlTES: Trace Elements

--z~- HA147 I -~ ,2~ L two ~rox~e

/ ~ ~ -=-..,oo/

High-K Suite

--x<-- HA134 ~ .Horn. ~ .~nde- Pyroxer/o ' ~ .

- o - H261 Hornblende AndesRe ]~

0.01 . . . . . . . . . . . . . . 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 Neo- gene volcanic rocks have not been deeply buried and the very low grade alteration is consistent with local geothermal effects.

OHA FORMATION

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


occur in almost all samples. They typically contain zeolites in their cores, surrounded by green sheet silicate minerals.

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 An

Ab Or

Fig. 13. Plagioclase compositions in the Oha Formation basalts.

OHA FORMATION: Pyroxenes DJ O D i

Bmm~

En

Hd

Fs Fig. 14. Pyroxene compositions in the Oha Formation basalts.

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 Ti 4÷ 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


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 bronz- ite-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 identifi- cation. 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, "idding- site" 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.

Secondary minerals

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-

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 (<30wt% 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 bruns- vigite 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. Yellowish- brown 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 AI203

x / / , / \

/ ~ x "\ / x \

• / \

/ J = / , , ~,

/ /

/ / ~ / x

[] Low-Si Chlor i tes ~ High-Si Chlor i tes z Smect i tes

L

MgO FeO

Fig. [ 5. Sheet silicate compositions in the Oha Formation basalts.


Table 3. Chemistry of sheet silicates in the Oha Formation basalts

CHLORITES

Type I II

colour Pale Green Green-Brown

wt.% SiO= ~;30 >30-40 AI203 14-18 10-16 MgO 11-17 14-22 FeO 20-30 13-20 MnO 0.3-0,7 0 K~O <1 0.1-2

SMECTITES

type I II III

colour Yellow-Brown Green Brown

wt.% AI203 12-15 6-9 16-27 MgO 5-9 4-5 6-8 FeO 15-19 21-22 3-8 CaO 0-3 <1 4-7 Na20 0 0 1-3 K20 0.5-3 5-9 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. Meso- lite 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 (~6wt%) with a SiO 2 content (~32wt%) 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.

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

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 K20- SiO2 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


OHA FORMATION: Rare Earth Elements 100-

o~ Aphyric Basalt$ --~ HR171 '= ~ HR191

-x<- HR192 ~--~ o--o ~ ~ -B- HG51

10 100

Phyric Basalts ~ H269 " ~ - ~ H277

-x<- HR154 ~L ~ --IB-- HR201

t Phyric Basalts ~ HR129 -~- HG88

10 ~ ~

, , , , , , , , , . . . . . ,

La Ce Nd Srn Eu Gd Dy Er Yb Lu

Fig. 16. Chondrite-normalized REE for the Oha Formation basalts. 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 For- mation 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. Miner- alogically, 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 Creta- ceous 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 clinopyroxene- phyric 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

OHA FORMATION: Trace Elements 1 O0 q

1 ~ - HR171 Aphyric Basalts ~ 43= HR191 /,._~¢, "~ -x~ HR192

/ ~ \ ~ HG51

100

- - H 2 6 0 10~ ~ \ ~ H 2 7 7 \

4E- HR129\\ 4~- HG88

0.1 Phyric Basalts I~

0.01 . . . . . . . . . . . . . . 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).


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

Quaternary

Pliocene

U

Miocene M

L

Oligocene ~ L

U

Eocene M

L

Paleocene

Upper Cretaceous

Lower Cretaceous

HALMAHERA BACAN 0 J Quaternary-Recent I Quaternary-Recent 2 Tafongo Fm/Kulefu F"m ' '~"

5 1 W e d a Group/L°ku Frn l Obit

WAIGEO

15

25

32

38 I a 'o'a

l Fm

TRumai Fm

42

50

55

T Sagea Fm omas Fm

66

98

Ma

I Gowonli Fm Olla

I ~ ¢ a n Fra

0

2

5

- - 10

- - 1 5

• 2 5

- - 3 2

3 8

- - 4 2

- - 5 0

- - 55

66

98

Ma

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

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-


ites are rare; in contrast, Neogene-Recent volcanic rocks are typically andesites, hornblende is a common con- stituent, 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 Quaternary- Recent volcanic arc is built unconformably upon Neo- gene 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.

REFERENCES

Alt, J. C. and Honnorez, J. 1984. Alteration of the upper oceanic crust, DSDP site 417: mineralogy and chemistry. Contrib. Mineral. Petrol. 87, 149-169.

Andrew, A. J, 1980. Saponite and celadonite in Layer 2 Basalts, DSDP Leg 37. Contrib. Mineral. Petrol. 73, 323-340.

Apandi, T. and Sudana, D. 1980. Geologic map of the Ternate quadrangle, North Maluku. Geological Research and Development Centre, Bandung, Indonesia.

Ballantyne, P. D. 1990. The Petrology of the Ophiolitic Rocks of Eastern Halmahera, Indonesia. Unpublished Ph.D. thesis, Univer- sity of London. 263pp.

Ballantyne, P. D. 1991. Petrological constraints upon the provenance and genesis of the East Halmahera Ophiolite. J. SE Asian Earth Sci. 6, 259-269.

Boles, J. R. 1984. Zeolites in low-grade metamorphic rocks. Mineral- ogy and Geology of Natural Zeolites. Reviews in Mineralogy: Mineralogical Society o f America 4.

Bryan, W. B., Finger, L. W. and Chayes, F. 1969. Estimating proportion in petrographic mixing equations by least squares approximation. Science 163, 926-927.

Bryan, W. B., Stice, G. D. and Ewart, A. 1972. Geology, petrography and geochemistry of the volcanic islands of Tonga. J. geophys. Res. 77, 1566-1885.

Cardwell, R. K., Isacks, B. L. and Karig, D. E. 1980. The spatial distribution of earthquakes, focal mechanism solutions, and subduction lithosphere in the Philippine and northeastern Indonesian islands. In: The Tectonic and Geological Evolution of South-East Asian Seas and Islands (Edited by Hayes, D. E.). American Geophys- ical Union Monograph 23, 1-35.

Deer, W. A., Howie, R. A. and Zussman, J. 1978. Single-chain Silicates v.2A. Rock-Forming Minerals. Second Edition. Longman.

Dixon, T. H. and Batiza, R. 1979. Petrology and chemistry of Recent lavas in the northern Marianas: implications for the origin of island arc basalts. Contrib. Mineral. Petrol. 70, 167-181.

Dungan, M. A. and Rhodes, J. M. 1978. Residual glasses and melt inclusions from basalts from DSDP Legs 45 and 46: evidence for magma mixing. Contrib. Mineral. Petrol. 67, 417-431.

Ewart, A. 1976. Mineralogy and chemistry of modern orogenic lavas--some statistics and implications. Earth Planet. Sci. Lett. 31, 417-432.

Ewart, A. 1982. The mineralogy and petrology of Tertiary-Recent orogenic volcanic rocks with special reference to the andesitic- basaltic compositional range. In: Andesites: Orogenic Andesites and

Related Rocks (Edited by Thorpe, R. S.). John Wiley and Son, New York.

Floyd, P. A. and Rowbotham, G. 1986. Chemistry of primary and secondary phases in intraplate basalts and volcaniclastic sediments, DSDP Leg 89. Initial Reports of the Deep Sea Drilling Project 89, 459-470.

Foster, M. D. 1962. Interpretation of the composition and classifi- cation of the chlorites. U.S. Geological Survey Professional Paper 414A, 33pp.

Gill, J. B. 1981. Orogenic Andesites and Plate Tectonics. Springer, Berlin.

Green, T. H. and Ringwood, A. E. 1968. Genesis of the calcalkaline igneous rock suite. Contrib. Mineral. Petrol. 18, 105-162.

Hakim, A. S. 1989. Tertiary Volcanic Rocks from the Halmahera Arc, Indonesia: Petrology Geochemistry and Low Temperature Alter- ation. Unpublished M.Phil. thesis, University of London, 292pp.

Hall, R. 1987. Plate boundary evolution in the Halmahera region, eastern Indonesia. Tectonophysics 144, 337-352.

Hall, R. and Nichols, G. J. 1990. Terrane amalgamation at the boundary of the Philippine Sea Plate. Tectonophysics 181, 207-222.

Hall, R., Audley-Charles, M. G., Banner, F. T., Hidayat, S. and Tobing, S. L. 1988a. The basement rocks of the Halmahera region, east Indonesia: a Late Cretaceous-Early Tertiary forearc. J. geol. Soc. Lond. 145, 65-84.

Hall, R., Audley-Charles, M. G., Banner, F. T., Hidayat, S. and Tobing, S. L. 1988b. Late Paleogene-Quaternary geology of Halma- hera, eastern Indonesia: initiation of a volcanic island arc. J. geol. Soc. Lond. 145, 577-590.

Hall, R., Ballantyne, P. D., Hakim, A. S. and Nichols, G. J. (in press). Basement rocks of Halmahera, eastern Indonesia: implications for the early history of the Philippine Sea. Proceedings o f the 5th Circum-Pacific Conference, Hawaii, 1990.

Hall, R., Nichols, G., Ballantyne, P., Charlton, T. and Ali, J. (1991). The character and significance of basement rocks of the southern Molucca Sea region. J. SE Asian Earth Sci. 6, 249-258.

Hamilton, W. 1979. Tectonics of the Indonesian region. U.S. Geological Survey professional Paper 1078, 345pp.

Hatherton, T. and Dickinson, W. 1969. The relationship between andesitic volcanism and seismicity in Indonesia, the Lesser Antilles, and other island arcs. J. geophys. Res. 74, 5301-5310.

Hawkins, J. W. and Melchior, J. T. 1985. Petrology of Mariana Trough and Lau Basin Basalts. J. geophys. Res. 90, 11431-11468.

Honda, S. and Muffler, L. J. P. 1970. Hydrothermal alteration in core from research drill hole Y-l, upper Geyser Basin, Yellowstone National Park, Wyoming. Am. Mineral. 55, 1714-1737.

Katili, J. A. 1975. Volcanism and plate tectonics in the Indonesian island arcs. Tectonophysics 46, 165 188.

Kay, S. M., Kay, R. W. and Citroen, G. P. 1982. Tectonic controls on tholeiitic and calcalkaline magmatism in the Aleutian arc. J. geophys. Res. 87, 4051-4072.

Kristmannsdottir, H. and Tomasson, J. 1978. Zeolite zones in geothermal areas in Iceland. In: Natural Zeolites: Occurrences, Properties and Use (Edited by Sand, L. B. and Mumpton, F. A.), pp. 277-284. Pergamon Press, New York.

Kuenen, P. H. 1935. Contributions to the geology of the East Indies from the Snellius expedition. Part I. Volcanoes. Leids. Geol. Med. 7, 273 331.

Kuno, H. 1968. Origin of andesite and its bearing on the island structure. Bull. Volcan. 32, 141 176.

Leake, B. E. 1978. Nomenclature of amphiboles. Min. Mag. 42, 533-563.

Lee-Wong, F. 1981. Feldspar composition of volcanic flow rocks from Hess Rise, DSDP Leg 62. In: Initial Reports of the Deep Sea Drilling Project (Edited by Thiede, J. et aL) 62, 967-970.

Liou, J. G. 1983. Occurrence, composition and stabilities of some Ca AI hydrous silicates in low grade metamorphic rocks. Mere. Geol. Soc. China 5, 47~56.

McCaffrey, R. 1982. Lithospheric deformation within the Molucca Sea arc-arc collision: evidence from shallow and intermediate earth- quake activity. J. geophys. Res. 87, 3663-3678.

Meijer, A. and Reagan, M. 1981. Petrology and geochemistry of the island of Sarigan in the Manana Arc; calc-alkaline volcanism in an oceanic setting. Contrib. Mineral. Petrol. 77, 337-354.

Morrice, M. G. and Gill, J. B. 1986. Spatial patterns in the mineralogy of island arc magma series: Sangihe Arc, Indonesia. J. geophys. Res. 29, 311-353.

Morrice, M. G., Jezek, P. A., Gill, J. B., Whitford, D. J. and Monoarfa, M. 1983. An introduction to the Sangihe Arc: volcanism accompanying arc-arc collision in the Molucca Sea, Indonesia. J. Volc. Geotherm. Res. 19, 135-165.


Morris, J. D., Jezek, P. A., Hart, S. R. and Gill, J. B. 1983. The Halmahera island arc, Molucca Sea collision zone, Indonesia: a geochemical survey. In: The Tectonic and Geologic Evolution of South-East Asian Seas andlslands. Part 2 (Edited by Hayes, D. E.). American Geophysical Union Monograph 23, 373-387.

Natland, J. H. and Mahoney, J. J. 1981. Alteration in igneous rocks at Deep Sea Drilling Project Sites 458 and 459, Mariana fore-arc region: relationship to basement structure. Initial Reports of the Deep Sea Drilling Project 60, 769-788.

Nichols, G. J. and Hall, R. 1990. Basin formation and Neogene sedimentation in a backarc setting, Halmahera, eastern Indonesia. Mar. Petrol. Geol. 8, 50-61.

Nichols, G. J., Hall, R., Milsom, J. S., Masson, D., Parson, L., Sikumbang, N., Dwiyanto, B. and Kallagher, H. 1990. The southern termination of the Philippine Trench. Tectonophysics 183, 289-303.

Nichols, G. J., Kusnama and Hall, R. 1991. Sandstones of arc and ophiolite provenance in a backarc basin, Halmahera, eastern Indonesia. In: Developments in Sedimentary Provenance Studies (Edited by Morton, A. C., Todd, S. P. and Haughton, P. D. W.). Spec. Publ. geol. Soc. Lond. 57, 291-303.

Offler, R. and Aguirre, L. 1984. Mineral chemistry of low grade metamorphic rocks, Western Andes, Peru. Neues Jahrbuch Mineral. Abhandlangen 150, 229-246.

Oilier, R., Aguirre, L., Levi, B. and Child, S. 1980. Burial metamorphism in rocks of the western Andes of Peru. Lithos 13, 31-42.

Pearce, J. A. 1982. Trace element characteristics oflavas from destruc- tive plate boundaries. In: Andesites (Edited by Thorpe, R. S.), pp. 526-548. J. Wiley and Sons, London.

Peccerillo, A. and Taylor, S. R. 1976. Geochemistry of Eocene calc-alkaline volcanic rocks from the Kastamonu area, northern Turkey. Contrib. Mineral. Petrol. 58, 63-81.

Shapiro, L. and Brannock, W. W. 1962. Rapid analysis of silicate, carbonate and phosphate. U.S. Geol. Sure. Bull. 1144-A.

Stern, R. J. 1979. On the origin of andesite in the northern Mariana island arc: implications from Agrigan. Contrib. Mineral. Petrol. 68, 207-219.

Sukamto, R., Apandi, T., Supriatna, S. and Yasin, A. 1981. The geology and tectonics of Halmahera Island and surrounding areas. In: The Geology and Tectonics of Eastern Indonesia (Edited by Barber, A. J. and Wiryosujono, S.). Geological Research and Development Centre, Bandung, Indonesia, Special Publication 2, 349-362.

Supriatna, S. 1980. Geologic map of the Morotai quadrangle, North Maluku. Geological Research and Development Centre, Bandung, Indonesia.

Thirlwall, M. F. and Burnard, P. 1990. Pb-Sr-Nd isotope and chemical study of the origin of undersaturated and oversaturated shoshonitic magmas from the Borralan pluton, Assynt, NW Scotland. J. geol. Soc. Lond. 147, 259-269.

Viereck, L. G., Griffin, B. J., Schminck, H. and Pritchard, R. G. 1982. Volcaniclastic rocks of the Reydarfjordur Drill Hole, Eastern Ice- land, 2 Alteration. J. geophys. Res. 87, 6459-6476.

Wakita, H., Rey, P. and Schmitt, R. A. 1971. Abundances of the 14 rare earth elements and 12 other trace elements in Apollo 12 samples: five igneous and one breccia rocks and one soil. Proceedings of the 2nd Lunar Science Conference, 1319-1329.

Walsh, J. N. 1980. The simultaneous determination of the major, minor and trace constituents of silicate rocks using inductively coupled plasma spectrometry. Spectrochimica Acta 35B, 107-I 1 I.

Walsh, J. N., Buckley, F. and Barker, J. 1981. The simultaneous determination of the rare earth elements in rocks using inductively coupled plasma source spectrometry. Chem. Geol. 33, 141-153.

Wells, P. R. A. 1977. Pyroxene geothermometry in simple and complex systems. Contrib. Mineral. Petrol 62, 129-139.

Wright, T. L. and Doherty, P. C. 1970. A linear programming and least squares computer method for solving petrologic mixing problems. Geol. Soc. Am. Bull. 81, 1995-2008.

Yasin, A. 1980. Geological map of the Bacan quadrangle, North Maluku. Geological Research and Development Centre, Bandung, Indonesia.

SEAES 6/~-4~-I

Documents

Tertiary volcanic rocks from the Halmahera Arc, Eastern ...searg.rhul.ac.uk/pubs/hakim_hall_1991_halmahera.pdfIn terms of major element compositions (Table 2, Fig. 9) the Neogene andesites