Tertiary Volcanic Rocks from the Halmahera Arc, Eastern Indonesia

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 ROBERTHALL 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 Recent studies of the tectonic evolution of this area (Hall 1987, Nichols et al. 1990) show that Halmahera is HALMAHERA is over 300 km from north to south and situated at the southern end of the Philippine Sea Plate 125 km from east to west and has a distinctive K-shape. (Fig. 1). The present arc is built on rocks of a Neogene The island is covered by rain forest and the terrain is volcanic arc, related to development of the present generally steep and rugged. Because of the difficulties of east-dipping Molucca Sea subduction at the edge of the access and the remoteness of the island Halmahera has Philippine Sea Plate (Hall and Nichols 1990). The Neo- received relatively little attention. The first comprehen- gene arc developed upon still older arc rocks and the sive geological maps were produced by the Indonesian history of volcanic activity in eastern Halmahera began Geological Research and Development Centre (GRDC) in at least the Late Cretaceous (Hall et al. 1988a, with accounts of the regional geology (Apandi and 1991). Halmahera is situated in a position linking the Sudana 1980, Supriatna 1980, Yasin 1980, Sukamto et Cretaceous-Tertiary arc terranes of the New Guinea aL 1981). The stratigraphy established by these authors margin and those of the Philippines and the history and has been added to and revised in subsequent papers by character of the volcanic activity in this region hence Hall (1987), Hall et al. (1988a,b, 1991a) and Nichols and provides important information on the early history of Hall (1990). the Philippine Sea region. This paper is based on a study Volcanism in the present-day Halmahera Arc (Fig. l) by Hakim (1989) and gives an account of the charac- is the product of subduction at the Halmahera Trench ter of the pre-Quaternary volcanic rocks of western (Hatherton and Dickinson 1969, Katili 1975) of the Halmahera. 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 Previous studies of Halmahera volcanic rocks the NW arm of the island. To the south, the Sorong Fault Zone is a major sinistral strike-slip system which Studies of the volcanic rocks of Halmahera are limited separates this complex zone of east-west convergence to those of the Quaternary-Recent island arc (Kuenen from the Australian Plate and the Banda Sea area 1935, Morris et al. 1983) and the Ophiolitic Basement (Hamilton 1979). South Halmahera lies adjacent to one Complex of eastern Halmahera (Ballantyne 1990, 1991). of the main strands of the Sorong Fault System and There have been no previous studies of the older volcanic continental crust is present on the island of Bacan (Hall rocks of western Halmahera. et al. 1988a) at the southern end of the Halmahera group Kuenen (1935) described Quaternary porphyritic of islands. basalts and andesites from the islands of Tidore and SEA~S6.~--H 271 272 A. SUFNI HAKIM and R. HALL

I I I lot 0 5?0km /

PLATE " )~,£ Yo9 ~o°

5 ~. 0~) ~'v~~".~RIDGE~v ' %~ SNELLIUS o~~ • CAROLINE

:,..~.... ~MO.OTA)~Rouo. ~ o P L AT E :b., 0 .o- '=

0 ° _

5 ~.

110 ° 125 ° 110 ° 135 ° Fig. I. Tectonic setting of Halmahera. Small solid circles mark recent volcanoes of the Sangihe and Halmahera 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 Halma- hera which have produced porphyritic basalts and ande- The 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 inter- mass 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 Base- within the range of 53~3 wt% SiO2 and have high ment Complex) were deformed in the Late Paleogene AI2 03, 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 Halma- Incompatible element concentrations vary with SiO2 hera is composed of volcanic rocks of the Oha For- content, 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 con- Neogene volcanic and sedimentary rocks rest uncon- trast 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 SiO 2 are slightly steeper Oha Formation. Breccio-conglomerates in this for- Tertiary volcanic rocks from the Halmahera Arc 273

! ! ! 12"~ 128"E 12~E Quaternary Alluvium and Limestone MOI~)T Quatemaly-Renent 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~hic Rocks ¢, TERNATE (~ Buff Bay TIDORE o

Major Thrust I ~ Major Fault ~) • Volcano

Weda o". 8 Bay

I Fig. 2. Simplifiedgeological map of Halmahera. mation contain clasts of pyroxene andesites and horn- unconformably upon folded Weda Group rocks. The blende andesites and volcaniclastic rocks containing rocks are petrographically different from most arenites similar debris. Nichols and Hall (1990) suggest that in the Weda Group, although they resemble some of the the Loku Formation represents slope sedimentation youngest volcaniclastic rocks of the Weda Group of close to the Neogene volcanic arc, probably in the central Halmahera; they contain biotite and quartz and forearc. On the eastern side of the SW arm and in cen- abundant lithic volcanic grains including pumice and tral Halmahera the Weda Group, resting unconformably quartz-biotite dacite clasts derived from a nearby on the Oha Formation, is a shallow marine sequence source. Near the northern coast of central Halmahera of conglomerates and sandstones between 2800 and volcaniclastic rocks and pyroxene andesites of the 3800 m thick. Fresh pyroxene andesite and hornblende Tafongo Formation (Hall et al. 1988b) dip at a low andesite debris dominates the clastic component in angle and locally tuff intercalations contain a Pleistocene rocks of the Weda Group (Nichols et al. 1991) and is fauna (Apandi and Sudana 1980). interpreted to be derived from the active Neogene vol- The Tertiary volcanic rocks were investigated in this canic arc. The lower part of the Weda Group contains study as part of a wider investigation of the Halmahera aphyric basalt clasts probably derived from the Oha Arc aimed at determining its history and character. Formation. Samples of volcanic rocks were collected from outcrops The Weda Group was deformed after the Mid-Late and as float in rivers and along the coasts of central and Pliocene. Volcanic activity may have been renewed at south Halmahera. Neogene volcanic rocks were col- about this time, and/or briefly changed character. At the lected from the Tafongo Formation and from the Weda northern end of the SW arm shallow marine tuffaceous Group. The Oha Formation was sampled in central and siltstones and sandstones of the Kulefu Formation rest south Halmahera. 274 A. SUFNI HAKIMand R. HALL

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 ......

KAHATOLAFORMATION Bas;tdc Rio, Lavas & VoicanidasticTudxiles PANITI FORMATION M/dd/e and Late Eocene >500m Ma~inalto ShallowMarine S~menta~ Ro~

OPHIOLmCBASEMENT COMPLEX ARM SEARM BULl GROUP SAGEA FORMATION MiMe Eo~ne >500m Vok:a~cl~ Rocks ReduCed Umesto~s DODAGABRECCIA FORMATION Camp,/an and Maas~ch~an >lO00m OHA VOLCANICFORMATION Deep M~ine VolcanidasticSedimentary GOWONU FORMATION ~te Cretaceousor Eocene? >1000m Rocks and P~agic Modstones Late Cretaceous >lO00m Caical~ine Voltaic Roc~ Ca~a,~ Voice's, Hyr~ys,~ Volcanidastic Breccias, Conglomerates GAU LIMESTONEFORMATION and Volca)idas~¢Rocks Con/ac/an-Campanm >50m Pelagic Mudstones,Lim~sto~ Rede~ted and PelagicLimestones Vok:anidas~cSedimer~a~y Pocks

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

ANALYTICAL METHODS of Walsh (1980). Rare earth elements (REE) were determined by ICP using the method of Walsh et al. (1981). Mineral analyses were made on polished thin sections Other trace elements and some REE (La, Ce, Nd) using a Cambridge Instruments Microscan V electron were analysed using a Philips PW-1400 XRF at Royal microprobe with a Link System energy dispersive system Holloway and Bedford New College London using at University College London. An accelerating potential methods similar to those of Thirlwall and Burnard of 20 kV and probe current of 1 x 10s A were used with (1990). 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. NEOGENE VOLCANIC ROCKS For rocks SiO2 was determined by the method of Shapiro and Brannock (1962) and conventional gravi- The Neogene volcanic rocks can be divided petro- metric methods were employed for CO2 and H20. Other graphically into two groups: (1) two-pyroxene andesites major elements, Li and Co, were analysed by inductively (TPAN) without hornblende, and (2) rocks contain- coupled plasma emission spectrometry (ICP) by methods ing hornblende phenocrysts, which are hornblende- Tertiary volcanic rocks from the Halmahera Arc 275 pyroxene andesites (HPAN) and hornblende andesites NEOGENE ANDESITES (HBAN). Both groups are typically vesicular, massive to Plagioclase amygdaloidal, porphyritic rocks which contain felsic and An 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, clino- Ab An Or pyroxene, 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, 2, 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 Ab An Or 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 ~~desblenditese these textures and zoning patterns may be found in a Groundmass single rock. The inclusion-free phenocrysts and micro- Ab phenocrysts are optically clear crystals. Grains with Or resorption and growth textures are characterized by the Fig. 3. Plagioclase compositions in the Neogene volcanic rocks. presence of zones which are rich in inclusions of glass, fluid inclusions with vapour bubbles, and tiny crystals of Compositionally, the clinopyroxenes are salites and pyroxene and iron oxides. The zoning patterns and augites (Fig. 4) with low to moderate contents of A1 textures of plagioclase grains indicate a complex history (0.75-5.5 wt% A1203) and low Cr (<0.3 wt% Cr203) of growth and reaction with the melt, reflecting possible contents. A slight iron enrichment trend is present in the changes in pressure, PH~O and magma mixing during TPAN. A plot of AltotaI against Si (Fig. 5) shows that crystallization (Dungan and Rhodes 1978). most HPAN and some TPAN clinopyroxenes fall close There is no significant difference in the range of to a straight line above the line Si + A1--2. However, plagioclase compositions of the three groups of andesites many TPAN and some HPAN clinopyroxenes scatter (Fig. 3). Or contents increase with decreasing An. Or- away from this straight line and these have significant enriched rims occur in some samples (TPAN ~2-5%; Fe 3÷ and Ti. Sodium and A! are not correlated. A plot HPAN and HPAN <2%). K-rich alkali feldspar was of Altota~+ Ti + Fe 3÷ against Si (Fig. 5) results in a identified by microprobe in the groundmass of several straight line for all HPAN and TPAN clinopyroxenes. TPAN and one HPAN. These plots indicate two groups of pyroxenes are pre- Clinopyroxene. Clinopyroxene is found in all the sent: (1) those with low Fe a+ and Ti in which Al is pyroxene andesites (TPAN and HPAN). Clinopyroxene present as a Ca-Tschermak (CaTs; CaAI:SiO6) com- constitutes between 3 and 15% of total phenocrysts and ponent; (2) those with higher Fe 3+ and Ti in which there microphenocrysts, is commonly euhedral and less com- is substitution of Fe 3÷ for Al 3+ in Fe-Ts (CaFe3+A1SiO6) monly embayed, and ranges from 0.1 to 3 mm across. In accompanied by Ti as Ti 4÷ entering the M1 site as nearly all samples there are glomerocrysts which may suggested by Deer et al. (1978). Although these pyrox- include anhedral to subhedral grains of clinopyroxene, enes cannot be optically distinguished, the analytical orthopyroxene, plagioclase and opaques. Continuous work suggests that the pyroxenes rich in Fe 3÷ and Ti, normal and sector zoning are present in a few grains; more common in the TPAN, are xenocrysts or early- neither of these types of zoning occurs in the crystal formed higher pressure phases; textural observations of clots. Augite occurs as a rim to colourless orthopyroxene euhedral pyroxenes coexisting with embayed pyroxenes in a few samples and in one HPAN augite occurs with support this interpretation. orthopyroxene, anhedral plagioclase and opaques Orthopyroxene. Orthopyroxene forms less than 10% around a core of hornblende. of total phenocrysts and also occurs as a groundmass 276 A. SUFNI HAKIM and R. HALL NEOGENE ANDESITES: Pyroxenes Di Hd

°mblende'APY~;~:;~\\

En 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 embayment textures are common in most samples. The a weak pink pleochroism and are hypersthenes (Fig. 4). amphibole phenocrysts commonly contain patches of The HPAN orthopyroxenes extend to more iron-rich inclusions including plagioclase, iron oxides and glass. In compositions than those of the TPAN. Temperatures two HPAN fine grained biotite, pyroxene and feldspar calculated from compositions of coexisting TPAN surround hornblende suggesting a crystal-melt reaction. pyroxenes (Wells 1977) fall in the range 959-1096°C; There is no obvious relation of amphibole colour to within a sample variations of +20°C reflect compo- the two types of hornblende andesite (HPAN and sitional range. These temperatures suggest the important HBAN). The majority of HPAN amphiboles are parga- role of water in lowering liquidus temperatures (Green sitic hornblende (Leake 1978) whereas HBAN amphi- and Ringwood 1968). boles fall into a wider field of composition from Hornblende. Brown and green, is present in the horn- pargasitic hornblende to magnesio-hornblende (Fig. 6). blende andesites as phenocrysts and microphenocrysts Although there is considerable overlap in compositions from 0.15 to 3 mm in length. Brown hornblende is much the HPAN amphiboles tend to have higher Mg/Fe 3+ more common than the green variety. Hornblende phe- ratios than the HBAN amphiboles (Fig. 6). No expla- nocrysts and microphenocrysts constitute less than 20% nation emerges from the analytical work for the colour of total phenocrysts. In some samples the hornblendes of the amphiboles. Gill (1981) suggested that a brown have a dark opaque-rich opacite rim (Gill 1981) and in colour is due to a high Fe3÷/Fe2÷ ratio but no clear-cut others brown hornblende phenocrysts are largely or relationship was observed; most of the green amphiboles completely replaced by iron oxides, most commonly in the HPAN. Hornblendes with little or no opacitic alter- NEOGENE ANDESITES: Hornblendes ation occur in the samples containing few or no pyroxene phenocrysts or glomerocrysts. Resorption and 2.5 Tschermakite Pargasite

...... 0.4 2.0 AI total+Ti+Fe 3÷ NEOGENE ANDESITES ~ Clinopyroxenes 1.5 DDD I 0.31 i 1.0 0.2¸ Trernolite Hornblende Edenite 0.5

Na+K 0.1 0.0 Hornblende- ~~ 0 ' o:a ' 014 016 018 1.0 roxotl~ ndesffes HPAN-Brown ~ HPAN-Green ~ HBAN-Brown x HBAN-Green 0.0 3.4 D Two Pyroxene Si D ID AI total Andesltes qz3~ o 3.2 o oo D D~ 0.3- Mg o ~ '~ ~ x 3.0 Z 0.2- X 2.8 ~a~ ~ z 0.1- 2.6 DD D ,, Fe ~÷ o.o 2.4 1.75 1.8 1.85 1.9 1.95 2.0 0 0.2 0.4 0.6 0.8 1.0 .2 Fig. 5. Variations in chemistry of clinopyroxenes from the Neogene Fig. 6. Amphibole compositions in the Neogene volcanic rocks. Fe 3+ volcanic rocks based on formulae recalculated to 6 oxygens. Fe 3÷ estimated for 13 cations excluding Ca + Na + K if Ca + Na > 1.34 or estimated assuming four cations. 15 cations excluding Na + K if Ca + Na < 1.34 (Leake 1978). Tertiary volcanic rocks from the Halmahera Arc 277 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. have relatively high Fe3/Fe3 + A1 and lower Ti but this to trapping of inclusions in early formed silicate grains is not true for all samples. Zoning is present but is or to the presence of xenocrysts of opaque grains. generally slight; there is an increase of Si and Fe/Mg Textural relationships of opaques are particularly from cores to rims in two HPAN amphiboles whereas a difficult to decipher. slight reverse zoning was detected in amphiboles from Groundmass. Varies from a microcrystalline matrix two HBAN. Reverse zoning may reflect an influx of composed of plagioclase, pyroxene and iron oxides with volatiles (Gill 1981). Most amphiboles have FeO/MgO minor glass. Glass predominates in some samples. The ratios typical of island arc environments and medium- to glass in all of the andesites is, like glasses of the andesites high-K acid andesites (Gill 1981). AP contents between of the present day Halmahera Arc (Kuenen 1935), very 0.2 and 0.5 indicate crustal pressures (Gill 1981). The acid in composition. The least acid glasses resemble overlap in compositional fields of the two andesite types feldspars in composition and deviate only slightly from is probably attributable to the initial crystallization of feldspar stoichiometry. As the glasses become increas- amphibole with pyroxene (HPAN), and as the melt ingly silica-rich they deviate more markedly from became increasingly enriched in Si and Fe 3÷, pyroxene feldspar compositions. The most extreme glasses, con- crystallization ceased and amphibole became the only siderably enriched in K20 (~4-10 wt%), are found in mafic silicate phase crystallizing (HBAN). some of the TPAN (Fig. 8). Plots of major elements Accessory minerals. Apatite is present in the majority versus silica for rock and glass compositions (Fig. 9) of samples. It occurs as small grains rimmed by opaques indicate that the andesites include two groups; a suite and in some samples as inclusions in pyroxene and with K-rich glasses (many TPAN, a few HPAN, no hornblende phenocrysts. Quartz is present in one HPAN HBAN), and a suite with glasses containing lower K20 and one HBAN as embayed grains up to 3 mm across. and higher Na20 and CaO (some TPAN, most HPAN, Biotite is present in one HBAN. The rarity of biotite and all HBAN). quartz and their embayed appearance suggest that they Crystallization sequences. The history of the crystal- are xenocrysts. lization of these rocks is complex, indicated by the Titanomagnetite is the most common accessory phase occurrence of xenoliths and crystal-clots, evidence of in the andesites and occurs as inclusions, micropheno- reaction with melt, and indications of multiple gener- crysts and in the groundmass. Titanomagnetite compo- ations of some minerals, especially plagioclase. Textural sitions scatter between ulv6spinel and almost pure observations suggest that plagioclase is an early-crystal- magnetite. Although there is considerable overlap in compositions (Fig. 7), there is a tendency for the titano- CaO magnetites to be relatively enriched in magnetite and NEOGENE ANDESITES/ \ depleted in ulv6spinel in the HBAN, whereas the TPAN Glass/\ ~ Tw~Py~xeno titanomagnetites show the opposite relationship. Chro- Andesltes mium oxide contents are low (< 0.47 wt%). The chemi- Hornblende. Pyroxsl~ cal variation observed is consistent with an increasing Andesites differentiation from TPAN through HPAN to HBAN. [] Hornblende Andesltes 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. / D * The considerable overlap in compositional fields may c3 r~cn [] reflect the complexity of crystallization and magmatic Na20 K20 processes; some of the wide scatter may be attributable Fig. 8. Glass compositions in the Neogene volcanic rocks. 278 A. SUFNI HAKIM and R. HALL NEOGENE ANDESlTES: Major Elements 10 26 x x m [] 8 FeO Al=Os D 22 6 s\. Q [] ~ o c~ • q~ 18 4 x :t :t mmnam C~ o m g 14 2 []

~ B 0 m N 10 10 SIO= = SlO= .;-., m 10 = m m 8 0 n 8 CaO K20 m o i~= m m =~ 6 • o 6 m Az [] c;=c~ c~ 4 m 4 Q ° °'~ ° ° °%1 2 u i~ cl D ~ 2 ® 0 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. lizing phase and pyroxenes are generally later; orthopy- microprobe analyses of glass are available for four of roxene or clinopyroxene may appear in either order. these and all contain a K-rich glass. Hornblende is last. In terms of major element compositions (Table 2, Fig. 9) the Neogene andesites are typical of orogenic andes- Secondary minerals ires. Aiuminium oxide is typically high (17-20 wt%) and TiO, is low (< 1 wt%). FeO* (total Fe as FeO), MgO, Most of the samples examined show no evidence of CaO and TiO2 correlate negatively with SiO2. Sodium alteration, whereas the rest contain some low tempera- oxide and K20 correlate positively with SiOz up to ture alteration minerals and show some devitrification of ~ 60 wt% SiO2 and then decline with further increase in the glassy matrix. Smectite, vermiculite and chalcedony SiO 2 . There is a similar inflection for CaO (a change are the most common secondary minerals. A pale yel- from negative to positive correlation), MnO and P205 lowish-green to yellowish-brown smectite is present in versus SiO2. The ratio Fe203:FeO increases broadly almost all of the altered samples and occurs predomi- with increasing SiO2. The decline of TiO2 and FeO* with nantly as pseudomorphs after orthopyroxene and pla- increasing SiO 2 is attributed to crystallization of mag- gioclase phenocrysts and locally in the groundmass or in netite throughout the calcalkaline series (Kuno 1968, amygdales. In three samples the zeolite mordenite Gill 1981). Major element contents are similar to other occurs as dirty white fibrous rosettes. Mordenite has island arc calcalkaline volcanic rocks such as those from been reported as one of the earliest formed zeolites in active geothermal areas (Honda and Muffler 1970, Kristmannsdottir and Tomasson 1978) and is also com- WEST HALMAHERA VOLCANIC ROCKS 4 monly found as a low-grade product of Tertiary volcan- Basalt ] AndesiteBasal#c ~ Andesite~ l Dacite ism at the Pacific margins (Boles 1984). It is considered K=O ~~ ~ ~ to indicate relatively high thermal gradients. 3 Wt% Chemistry 2 ~ ~ CalcalkalineSeries 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 petro- Low-K Tholeiite Series graphically TPAN. The more acid samples, both HPAN, 0 , fall into the dacite field because of relatively high 45 50 55 60 65 70 contents of secondary chalcedony but are otherwise OHA FORMATION [] NEOGENE M~dlum.K [] NEOGENE High.K similar to the andesites. Most of the samples fall into the Fig. 10. K20-SiO 2 plot for Neogene and Oha Formation volcanic calcalkaline field. Six samples fall into the high-K field; 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

OHA FORMATION NEOGENE VOLCANICROCKS OHA FORI~TION NEOGENEVOLC~C ROCKS Age: ? Late Cretaceous-Eocene Late Miocene -?Pliocene Chemll~-y(Selected Elements): Rock types: Basaits common Andesites common with Major ElementAverages Wt.%: (range in brackets) Rare Basaltic Andesites subordinate Basaits SiOz 51 (47-54) 56 (47-64) Textures: Commonly phyric with Mostly porphyritic and MgO 5.5 (3.0-8.2) 3.2 (1.0-6.2) minor aphyric rocks phenocryst-rloh TiO2 0.8 (0.5-1.1) 0.65 (0.4-0.95) Irrtersertal/intergranular Pilotaxitic very common FeO' 7.6 (6.3-10.5) 6.6 (3.5-9.5) groundmaes, quenched textures wfth abundant fresh glass AI=O~ 16.8 (13.5-19.9) 18.0 (14.6-20,2) and altered glass and some devitrified glass. Alkalis 5.2 (1.6-11,3) 4.6 (2.4-6.9) Trace ElementAverages ppm: (range in brackets) Prlmery Mineralogy: Rb 23 (5-60) 33 (5-94) Plagloclase Plagicclase Sr 438 (186-1120) 559 (368-840) Clinopyroxene Clinopyroxene Ba 179 (21-596) 274 (38-516) Olivine Orthopyroxene V 325 (201-534) 199 (92-367) +Orthopyroxene Hornblende Cr 81 (2-322) 21 (3-81) +Titanomagnetite Titanomagnetite Co 39 (25-98) 36 (20-72) i-,Quartz ± Biotite Ni 42 (5-145) 14 (6-44) SecondaryMineralogy: So 33 (13-46) 21 (8-35) Mesolite Mordenite Zn 79 (60-119) 59 (45-95) Thomsonite Cu 126 (55-216) 95 (35-193) Stilbite Zr 81 (25-162) 94 (37-182) Heulandite Y 23 (15-32) 20 (13-32) Analoite Nb 2 (1-6) 2 (1-4) Low-Si Chlorite Rare Earth Element Averages ppm: (range in brackets) High-Si Chlorite Hk:Jh-Si Chlorite (mostly) La 10 (3-24) 14 (3-27) Srnectite Group Smectite Group Ce 25 (10-51) 32 (11-62) Pumpellyite Nd 15 (8-27) 18 (9-31) Epidote Ce./Yb. 2.5 (1.0-4.8) 4.0 (1.6-9.2) Prehnite Sphene

the present-day Halmahera oceanic segment (Morris exception of K20. The crystallizing mixture of minerals et al. 1983), the Sangihe, Mariana and Tongan arcs initially including plagioclase, titanomagnetite, orthopy- (Morrice et al. 1983, Meijer and Reagan 1981, Bryan roxene and clinopyroxene with later replacement of et al. 1972), and orogenic calcalkaline rocks from the SW pyroxenes by hornblende. Scatter reflects mineral ac- Pacific (Ewart 1982). Major element trends are not cumulation, principally of plagioclase. significantly affected by the minor late stage alteration. (2) The least squares fit is sensitive to the initial choice Of the major elements, K20 shows the weakest corre- of plagioclase composition. In most cases a median lations and the most scatter in plots versus other composition based on inspection of the number and elements suggesting that it could have been mobile. range of compositions for a particular rock gave a good However, Na20 and K20 show a strong positive corre- initial fit. The bulk rock chemical trends are best mod- lation and there is no clear trend of K20 with indicators elled with a plagioclase compositional trend from highly of alteration such as H20 or CO2. calcic (TPAN) to less calcic (HBAN). This trend is not The relationship between rock and mineral compo- detectable in microprobe data probably because of the sitions has been examined using a least-squares method very large range of compositions in a single rock, based on principles discussed by Bryan et al. (1969) and presence of zoning, and mixing of several generations of Wright and Doherty (1970). A common approach is to feldspars. use two different rocks in a given suite as parent and (3) Titanomagnetite was present as a crystallizing daughter compositions to determine if they can be phase throughout the crystallization of the series, as related to one another by fractional crystallization. An found in many other calcalkaline suites. Crystallization alternative approach was also employed since most of of a spinel with increasing magnetite component can the rocks contain a glass phase which may be regarded account for the steep decrease in TiO2 and FeO* with as the result of fractionation. The glass composition was increasing fractionation, as tentatively suggested on the used as the daughter composition and the bulk rock basis of microprobe data. composition as the parent composition. The rocks se- (4) The changing composition of the mineral extract lected for the modelling procedure were chosen from leads to initial enrichment of the liquid in Na20 and those analysed for major elements and for which there depletion in most other elements. A small inflection in were microprobe analyses of each of the phenocryst the modelled curves at about 58-60 wt% SiO 2 for MgO phases and fresh glass. This reduced the number of and less prominently for CaO marks the appearance of samples to ten. A number of conclusions are indicated hornblende instead of pyroxene. This inflection is ob- by the modelled and observed major element trends. served for some major elements (see above). Hornblende (1) The trend TPAN~HPAN~HBAN can be mod- and clinopyroxene or orthopyroxene are stable together elled as the result of low pressure fractionation of a over a small range of intermediate compositions. basaltic initial liquid similar in composition to the most (5) Most of the residual is due to K20. The observed basic TPAN. The bulk rock chemical trends observed variation of K20 with SiO2 suggests two separate groups can be modelled well for all major elements with the of glasses (Fig. 9). Variation of compositions in the 280 A. SUFNI HAKIM and R. HALL

K-poor grasses shows the expected increase of K 20 with NEOGENE ANDESlTES: Trace Elements increasing SiO2 for the modelled minerals. In contrast, 100: --z~-HA147 I the K-rich glasses show a decline in K20 with increasing -~,2~ L two ~rox~e -2 SiO2. Since there is no difference in the modal mineral- /~ ~ -=-..,oo/ ogy of the andesites this trend is interpreted as the result 3z 8 of much greater K enrichment of the liquid for the tr" K-rich suite. At liquid compositions of ~65 wt% SiO2 1 the decrease of K20 results from increase in the K- feldspar component of the plagioclase or crystallization 100 1 of a separate K-feldspar phase; both interpretations are High-K Suite consistent with microprobe observations. Rare earth and trace elements. All the Neogene vol- lol canic 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 abun- 0.1 dances but with diverging LREE ends to the patterns. --x<--HA134 ~ .Horn.~ .~nde- Pyroxer/o '~. The medium-K TPAN have somewhat flatter patterns -o- H261 HornblendeAndesRe ]~ (Ce•/YbN 1.8-2.3) than most of the high-K suite, but 0.01 ...... closely similar in shape and with REE abundances Sr K RbBa NbCe P Zr Sm Ti Y Yb Sc Cr similar to the least ffactionated high-K rocks. The Fig. 12. MORB-normalized trace element plots for the Neogene remaining medium-K HPAN and HBAN have the volcanic rocks. Normalizing values from Pearce (1982). steepest patterns (CeN/YbN 3.1--4.7) with similar LREE abundances to the most fractionated high-K medium-K rocks have very similar patterns but with rocks. lower LIL contents and lower Ce and P. Plots of trace element contents normalized to N-type Summary. The mineral and rock chemical analyses MORB (Fig. 12) show the principal features of the trace indicate that the Neogene volcanic rocks can be divided element chemistry of the Neogene rocks. All show the into two suites. The high-K calcalkaline suite differs humped and sloping patterns typical of island arc vol- from the medium-K suite by the presence of a K-rich canic rocks with a very prominent negative Nb anomaly. glass, higher LIL element and higher HREE abun- The high K suite has the highest contents of large ion dances. Mineralogically, the two suites are similar lithophile (LIL) elements Sr, K, Rb and Ba, is depleted although no HBAN have been identified in the high-K in the high field strength (HFS) elements Nb and Ti, and suite. The mineral and major element chemistry largely is strongly depleted in Cr. The two samples with most reflect the effects of low pressure fractionation of a basic compositions show slight depletion of Zr. All the basaltic or basaltic andesite magma. Possible early higher pressure crystallization is suggested by some NEOGENE ANDESITES: Rare Earth Elements clinopyroxene compositions. The distribution of the 100- high-K rocks indicates that they were produced at O~ ::,7:'t,.o ,ox_ different volcanic centres in the Neogene arc; most are H259 [ Arldesl~s HR1 O0 / from the Plio-Pleistocene Tafongo Formation. The Neo- gene volcanic rocks have not been deeply buried and the

rr' very low grade alteration is consistent with local geothermal effects. 10 100- High-K Suite OHA FORMATION

The Oha Formation volcanic rocks are divided into two groups on the basis of their petrography: phyric and lO aphyric basalts. The phyric basalts have porphyritic textures, with both mafic and felsic phenocrysts set in a -I~E-- HA145 [ Two Pyl'oxene dark brown to greenish-grey microcrystalline ground- --E~HA146 J Andesltes -)y~--HA134 mass, in which patches of pale to dark green secondary -B-H254 } AndesltesH°mblende'Pyr°xene minerals can often be seen. Veinlets and amygdales are -~- H261 HornblendeAndeslte also common. The aphyric basalts are grey, green and red-grey lavas and are fine grained and equigranular La Ce Nd Sm Eu Gd Oy Er Yb Lu with rare microphenocrysts. Some samples contain pale Fig. 11. Chondrite-normalized REE for the Neogene volcanic rocks. green and fine-grained aggregates of secondary minerals. Normalizing values from Wakita et al. (1971). Amygdales are subrounded or irregular in shape and Tertiary volcanic rocks from the Halmahera Arc 281 occur in almost all samples. They typically contain OHA FORMATION: Pyroxenes zeolites in their cores, surrounded by green sheet silicate DJ O Di Hd minerals.

Mineralogy

The modal proportion of phenocrysts and micro- Bmm~ phenocrysts in the phyric basalts is between 10 and 60%. Phenocrysts include plagioclase feldspar, olivine, pyrox- En Fs ene and Fe-Ti oxides. Orthopyroxene phenocrysts are Fig. 14. Pyroxenecompositions in the Oha Formation basalts. present in two samples. In the aphyric basalts plagioclase, clinopyroxene and olivine are the main minerals. Rowbotham (1986), and from the Hess Rise, a Pacific Feldspar. The modal percentages of plagioclase phe- seamount, by Lee-Wong (1981) where it is the result of nocrysts and microphenocrysts in the phyric basalts replacement of plagioclase by potassium-bearing hydro- range from 10 to 30%. In the aphyric basalts plagio- thermal fluids or sea water. The K-rich feldspar in the clase usually constitutes greater than 70o of the mode. Oha Formation is interpreted to be related to similar Plagioclase crystals have elongate, rectangular, acicular, low-temperature alteration. wedge, serrated, fork-edge and swallow-tail shapes; all Clinopyroxene. Clinopyroxene is common in the are typically free of inclusions and unzoned. Embayed phyric basalts. The phenocrysts form from 2 to 7% of crystals are common in almost all phyric samples. the total rock volume. The crystals are generally equant Phenocrysts typically contain a network of cracks and or rectangular and form anhedral and subhedral grains may be altered; the groundmass feldspars are albitized. from 0.2 to 3 mm across. Phenocrysts are occasionally Microprobe analysis reveals that the Ca-rich pheno- weakly zoned and sector zoning is present in two crysts vary considerably in composition but there is no samples. Crystal clots are common in some samples. clear difference between phyric and aphyric basalts They are composed of equant or subrounded grains of (Fig. 13). The smaller number of analyses from the clinopyroxene and orthopyroxene, with or without aphyric basalts reflects the finer grain size and greater plagioclase. There is little evidence of reaction between degree of alteration. In general, zoning is absent. The the clots and the groundmass. In the aphyric basalts wide range of plagioclase compositions (An, to An94) is clinopyroxene forms 15-20% of the mode. Weak con- considered to be due partly to original variation in tinuous zoned and sector zoned crystals are present in primary feldspar composition and partly due to sec- some samples. ondary alteration. Most feldspars fall in the range from Most of the microprobe analyses of clinopyroxenes An41 to An96 which includes two groups, sometimes are core compositions since rims and groundmass pyrox- within a single rock sample: one group with compo- enes are altered. In terms of quadrilateral components sitions of An6j-An96 (median ~Ans0), and a second they scatter in the augite, salite and diopside fields group with compositions from An4, to An6~ (median (Fig. 14). Clinopyroxenes show an iron enrichment trend An53). Those with lower An contents generally have 1-4 which is less obvious for those from the phyric basalts. molar% Or, whereas those with higher An contents The clinopyroxenes have variable A1 contents which are generally have <1 molar% Or. Albite replacement of weakly correlated with Ca. Augites have low AI contents phenocrysts is often recognizable from a dusty appear- (< 0.1 ions per formula unit) whereas salites generally ance and small dark inclusions. In altered samples have higher A1 values (up to 0.35 ions per formula unit). groundmass plagioclase laths and phenocrysts are Diopsidic pyroxenes have a wide range in A1 content and altered to compositions in the range from An] to Anl7. are similar to salites but contain appreciable Cr20 3 K-feldspar has been identified in the groundmass of (0.12~).97 wt%). Weak zoning is present in some phyric two samples. Orthoclase (Or97_98) in alkalic basalt from clinopyroxenes; an increase in A1 and decrease of Cr and the Mariana Basin has been reported by Floyd 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 OHA FORMATION: Plagioclase increase of Ca and decrease of Si, A1, Ti, Fe and Mg An An 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 Ab Or augite, rarely associated with plagioclase. A reaction Fig. 13. Plagioclase compositions in the Oha Formation basalts. with the melt is indicated by the occurrence of the 282 A. SUFNI HAKIM and R. HALL overgrowth of anhedral augite on the rims in some dales, as infilling minerals and as patchy replacements of grains. This feature is rarely observed in Neogene ande- phenocrysts and the groundmass. In addition, plagio- sites containing two pyroxenes. clase is albitized in the majority of samples and calcite Orthopyroxene compositions fall in the bronz- and quartz are common. ite-hypersthenc range (Fig. 14), reflected by a pink pleo- Sheet silicates. On the basis of SiO 2 and alkali con- chroism. Wo contents are ,-~ 3-4 molar%. AI contents in tents, the sheet silicates may be subdivided into two orthopyroxenes and clinopyroxenes are similar, typically types: chlorite- and smectite-type sheet silicates (Fig. 15). < 0.1 ion per formula unit, consistent with the interpret- In general the chlorite-type sheet silicates have less than ation that they are equilibrium low pressure pairs. 40wt% SiO2 and less than 2wt% K20, whereas the Because of alteration of crystal rims temperatures were smectite-type sheet silicates commonly have higher SiO2 calculated from core compositions of coexisting pyrox- and significant K20. enes (Wells 1977) for two rocks containing fresh ortho- Chlorite-type sheet silicates. Chlorites are of two pyroxene and yielded vaues of 1048_+ 14°C and types: I-chlorite type (<30wt% SiO2) and II-chlorite 1016_+90°C; the large error on the latter reflecting type (30-40 wt% SiO2). The I-chlorite types are mostly compositional variation in the sample, possibly due to pale green and show anomalous interference colours, alteration. whereas the II-chlorite types are brown to golden-brown Olivine. Almost all of the phyric basalts contain or greenish-brown and have a distinctive red--orange olivine phenocrysts and microphenocrysts ranging from interference colour. Most of the I-chlorites are bruns- 0.1 to 3.5mm across forming 5-10% of the mode. vigite to diabantite (Foster 1962) with Fe2+/R 2+ ratios Microcrystalline olivine is probable in a few samples but from 0.29 to 0.34. The II-chlorites fall mostly into the the high degree of alteration precludes definite identifi- diabantite compositional field with a range of FeE+/R 2+ cation. Ubiquitous alteration of olivine phenocrysts from 3.2 to 4.0. The principal chemical features of the prevents determination of their compositions. The most two chlorite types are listed in Table 3. Like Icelandic common inclusions in olivine are subhedral to euhedral chlorites (Viereck et al. 1982) cation totals for chlorites Fe-Ti oxides; in one sample dark brown euhedral crys- from the Oha Formation are low and most of the tals of Cr spinel are present as inclusions in altered chlorites have AI vi greater than A1~v. Several samples olivine phenocrysts. In the aphyric basalts olivine mi- contain small quantities of K, Ca, Na and Mn. The crophenocrysts are found in two samples. Olivine is presence of K, Ca and Na is interpreted by Offler et al. replaced by yellowish-brown chlorite goethite, "idding- (1980) as due to minor quantities of illite and smectites, site" and "bowlingite". which are interlayered with chlorite, typical of chlorite Accessory minerals. The principal accessory mineral in produced during sub-greenschist facies metamorphism. all samples is Fe-Ti oxide, forming equant or irregular Smectite-type sheet silicates. Smectite-type sheet sili- grains up to 1 mm across, as aggregates in the ground- cates are found mainly as replacements of phenocrysts mass or as inclusions in the phenocrysts or micro- and interstitial glass, and in amygdales. The colour phenocrysts. Equant grains occur generally as inclusions broadly reflects the chemical composition. Yellowish- in some of the olivine phenocrysts and as the grains in brown varieties have moderate A1203 (12-15 wt%) and crystal clots. The majority of Oha Formation basalts FeO (15-19 wt%) contents. Yellowish-green, pale green contain titanomagnetites which have little Cr and AI. and bluish-green varieties have the low A1203 contents Two samples contain Cr-rich spinel which have Cr # (6-9 wt%), high K20 contents (5-9 wt%) and most have and Mg# in the range of Halmahera harzburgite high FeO* contents (21-22 wt%). Brown varieties have chrome spinels (Ballantyne 1991). the highest Al203 contents (16-27 wt%) and the lowest Groundmass. The microcrystalline groundmass typi- FeO contents (3-8 wt%) with Na20 contents from 1 to cally has quench textures including intergranular and 3 wt%. The principal chemical features of the three branching clinopyroxene, small and radiate plagioclase smectite types are listed in Table 3. These smectite types laths or forked microlites and minute or acicular Fe-Ti compositionally resemble saponite, celadonite and oxides, often set in altered interstitial glass. Crystallization sequence. On the basis of textural OHA FORMATION: Sheet Silicates observations the crystallization sequence is interpreted SiO2 AI203 to have been initiated by the appearance of olivine x // , ( _+ spinel). After the crystallization of olivine, plagioclase / \ formed and was then followed by clinopyroxene and / ~ x "\ / x \ iron oxide• In samples without olivine crystallization was • / \ initiated by plagioclase, followed by orthopyroxene then by clinopyroxene and Fe-Ti oxides. / J = / ,, ~, / / Secondary minerals / / ~ / x [] Low-Si Chlorites ~ High-Si Chlorites z Smectites There are two main groups of secondary minerals in L the Oha Formation: various sheet silicate minerals and MgO FeO a variety of zeolites. They occur in fractures, in amyg- Fig. [ 5. Sheet silicate compositions in the Oha Formation basalts. Tertiary volcanic rocks from the Halmahera Arc 283

Table 3. Chemistryof sheet silicates in the Oha Formation basalts Pumpellyite. This is probably the least abundant secondary mineral in the Oha Formation and occurs in only CHLORITES one sample. It occurs in albitized plagioclase as strongly pleochroic, acicular and bladed crystals, which form Type I II radiating and random aggregates. In places it forms colour Pale Green Green-Brown rosettes of needle-like crystals. Locally, pumpellyite is associated with light yellowish-brown chlorite and wt.% SiO= ~;30 >30-40 colourless epidote. In most cases the pumpellyite exists AI203 14-18 10-16 with chlorite in the matrix. It is an Fe-rich variety, with MgO 11-17 14-22 FeO* generally higher than 10wt% , similar to those FeO 20-30 13-20 MnO 0.3-0,7 0 associated with zeolite or prehnite-pumpellyite facies K~O <1 0.1-2 metamorphism (Liou 1983).

SMECTITES Chem&try

type I II III Major elements. Water and CO2 contents of all colour Yellow-Brown Green Brown samples are high and range from 2 to 8 wt% reflecting extensive secondary alteration. Silicon dioxide contents wt.% AI203 12-15 6-9 16-27 are low and range from 46 to 52 wt%. Titanium dioxide MgO 5-9 4-5 6-8 contents range from 0.7 to 1 wt%. With one exception, FeO 15-19 21-22 3-8 CaO 0-3 <1 4-7 A1203 contents vary from ~ 15 to 19 wt%. On the K20- Na20 0 0 1-3 SiO2 diagram the Oha Formation samples fall in the K20 0.5-3 5-9 1-2 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 beidellite, respectively. Compositions of these minerals secondary minerals. Contents of other major oxides of in the Oha Formation are comparable with those the Oha Formation basalts (Table 2) are comparable to reported by Andrew (1980), Natland and Mahoney tholeiitic basalt samples from the Tongan island arc (1981) and Alt and Honnorez (1984) in rocks from the (Bryan et al. 1972, Ewart 1976), from the Marianas ocean floor and Marianas forearc and interpreted to be island arc (Dixon and Batiza 1979, Stern 1979), the the result of low temperature interaction between basalt Mariana Trough (Hawkins and Melchior 1985), and the and sea water. Aleutian island arc (Kay et al. 1982). The small range of Zeolites. Zeolites identified include analcite, heulandite, chemical composition in the Oha Formation basalts laumontite, mesolite, stilbite and thomsonite. Analcite is precludes modelling but the variation is consistent with the most common zeolite and is usually colourless, typi- olivine-clinopyroxene and plagioclase fractionation. cally replaces plagioclase phenocrysts, and is commonly Rare earth and trace elements. The majority of the Oha associated with calcite, chlorite and other zeolites. Meso- Formation basalts show LREE enrichment similar to lite occurs usually as white brownish fibres and radial most arc volcanic rocks; a small number do not show aggregates in veinlets and amygdales, commonly rimmed this enrichment. The chondrite-normalized patterns fall by chlorite. In most samples, heulandite occurs in veinlets into two groups (Fig. 16). The aphyric basalts and most with mesolite, analcite and calcite. Laumontite is less of the phyric basalts have sloping (aphyric CeN/YbN common and occurs as acicular, columnar and elongated 2.3-3.8; phyric CeN/YbN 1.7~1.8) REE patterns with aggregates with a radiating habit in amygdales together REE contents increasing in most cases with bulk rock with calcite and pale green chlorite. Thomsonite is present SiO2. Most of these rocks have low Ni and Cr contents in veinlets with analcite and carbonate, with chlorite rim- (< 50 ppm) but among the basalts with sloping patterns med by brown cryptocrystalline material, in amygdales in are several with relatively primitive compositions which which laumontite is absent, and as radiating fibrous crys- have low REE, high Cr (50-320 ppm) and high Ni tals around the rim of amygdales infilled by "foliated" stil- (50-150 ppm) contents. Of the remaining phyric basalts bite. Calcium-rich stilbite occurs in only two samples as (Fig. 16), two samples have much flatter and parallel "foliated" and fractured crystals crossed by thomsonite. patterns (CeN/YbN 1.7-1.8) with a slight LREE de- Sphene. Sphene was found in one sample and is an pletion and one sample has a completely flat chondrite Al-rich variety (~6wt%) with a SiO 2 content normalized pattern (CeN/YbN 1.1). (~32wt%) comparable to those in rocks from the The aphyric and phyric basalts with sloping REE Western Andes (Offler and Aguirre 1984). patterns have the humped and sloping MORB-normal- Epidote. Epidote was found in only two samples. It ized trace element patterns (Fig. 17) typical of island arc occurs as fine grained patches in albitized plagioclase volcanic rocks with a prominent negative Nb anomaly. and with pumpellyite and chlorite in one sample. Although the LIL elements may have been mobile and Prehnite. Prehnite is found only as replacement hence an unreliable guide to origin, the patterns also patches of plagioclase in two rocks as radiating and show depletion of the generally immobile HFS elements sheaf-like aggregates with chlorites and albite. Nb and Ti, and in most cases depletion of Zr. The more 284 A. SUFNI HAKIM and R. HALL

OHA FORMATION: Rare Earth Elements HISTORY OF VOLCANISM IN THE 100- HALMAHERA ARC o~ Aphyric Basalt$ --~ HR171 '= ~ HR191 -x<- HR192 The Halmahera region has a long volcanic history ~--~ o--o ~ ~ -B- HG51 (Table 4). The oldest volcanic rocks of Halmahera are those of the eastern Halmahera Ophiolitic Basement Complex in which Ballantyne (1990, 1991) has identified 10 100 a number of distinct and non-cogenetic suites; one of boninitic affinity, two of island arc and one of oceanic Phyric Basalts ~ H269 "~ - ~ H277 island/seamount origin. The boninitic suite is thought to -x<- HR154 ~L ~ --IB-- HR201 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

t Phyric Basalts -~-~ HG88HR129 forearc is proposed as a modern analogue for the ophiolite terrane of eastern Halmahera (Hall et al. 10 ~~ 1988a, Ballantyne 1990). Halmahera was probably situated at the equatorial western Pacific margin at this time , , , , , , , , , ..... , although its exact location is uncertain. La Ce Nd Srn Eu Gd Dy Er Yb Lu Late Cretaceous island arc volcanism is represented in Fig. 16. Chondrite-normalizedREE for the Oha Formation basalts. Normalizing values from Wakita et al. (1971). 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 primitive compositions show no depletion of Cr. The in many ways. They are plagioclase- and clinopyroxene- three samples with flat to slightly sloping patterns have phyric basaltic andesites, lacking hornblende, of tholei- trace element abundances for most elements which are itic to calcalkaline affinities, with LIL and LREE similar to N-type MORB, apart from Cr which is element enrichment. Petrographically similar rocks, but strongly depleted, and the LIL elements which are of Eocene age, are found in eastern Halmahera in the enriched but may reflect alteration, and Nb. All the Sagea Formation but these rocks have not yet been patterns show a pronounced negative Nb anomaly; since chemically studied. No Paleocene rocks have yet been Nb is generally regarded as immobile this feature indi- found in Halmahera and it is not clear if the Late cates an arc volcanic origin. The REE and trace element Cretaceous and Eocene volcanic and volcaniclastic rocks chemistry thus indicate that the Oha Formation basalts are all of island arc character but include more than one OHA FORMATION: Trace Elements 1O0 q suite. Trace element variations suggest most samples are 1 Aphyric Basalts ~- HR171 related by "tholeiitic" fractionation of the more primi- ~ 43= HR191 /,._~¢, "~ -x~ HR192 tive compositions due to crystallization of olivine and /~ \ ~ HG51 pyroxene without magnetite. Summary. 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 100 volcanic rocks, being predominantly basaltic. Unlike the

Neogene rocks, high potassium contents are the result of --H260 10~ ~ \ ~H277 \ 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- HR129\\ 4~-4E- HG88 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, 0.1 Phyric Basalts I~ orthopyroxene is rare and phenocrysts of hornblende and magnetite are absent. In contrast to the Neogene 0.01 ...... volcanic rocks, the Oha Formation has suffered zeolite Sr K RbBaNbCe P ZrSml] YYbScCr and local sub-greenschist facies alteration reflecting deep Fig. 17. MORB-normalized trace element plots for the Oha Formation burial and/or high heat flows. 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 BACAN WAIGEO 0 0 Quaternary J Quaternary-Recent I Quaternary-Recent 2 Pliocene 2 Tafongo Fm/Kulefu F"m''~" 5 U 51Weda Group/L°kuFrn l Obit -- 10 Miocene M

15 -- 15 L 25 l Fm • 25

Oligocene ~ 32 -- 32 L I a 'o'a TRumai Fm 38 38 U 42 -- 42 Eocene M 50 T SageaFm -- 50 L omasFm 55 -- 55 Paleocene 66 66 Upper I GowonliFm ~¢an Fra Cretaceous Olla 98 98 Lower Cretaceous Ma I Ma

represent a long-lived Late Cretaceous-Eocene arc or Oligocene age on the island of Kasiruta at the northern whether volcanic activity ceased for a period at the end end of the Bacan group. Petrographically, these resemble of the Cretaceous. arc basaltic andesites, but they differ from the Oha The age of the Oha Formation is uncertain. Clasts Formation in many petrographic features and in the taken from breccias and conglomerates at the northern degree of secondary alteration. These rocks are not yet end of the SW arm include Eocene limestones (probably chemically characterized. Almost identical rocks are Lower Eocene). Eocene limestones associated with vol- present in NW Halmahera and are interpreted to be the caniclastic rocks were collected in central Halmahera product of rift-related volcanism of similar age to the (Hall et al. 1988a) and similar limestone clasts were opening of the Central West Philippine Basin (Hall et al. found in breccio-conglomerates as float further south in 1991). the SW arm. Since all the dates are from clasts within the Studies in north, central, southern and eastern formation their significance is uncertain. The clasts could Halmahera (e.g. Hall et al. 1988a,b, 1991, in press, be interpreted as reworked at about the period of Nichols and Hall 1990, our unpublished results) indicate volcanism, as seen in volcaniclastic rocks of the Gowonli that there was no arc volcanic activity during most of the and Sagea Formations, or much later. Dating of over- Miocene and the Neogene volcanic arc was initiated at lying sedimentary rocks shows that the Oha Formation the beginning of the Late Miocene by eastward subduc- is older than Late Miocene; a Late Cretaceous or Eocene tion of the Molucca Sea Plate at the Halmahera Trench. age is suspected since petrographically and chemically The Neogene volcanism was marked by an important the volcanic rocks of the Oha Formation closely re- change with the appearance of hornblende-bearing ande- semble the arc volcanic rocks of the Gowonli and Sagea sites. Basaltic compositions are characteristic of all Formations of east Halmahera. pre-Neogene volcanic rocks so far discovered in the The existence of an Oligo-Miocene volcanic arc in the region and hornblende-bearing andesites are rare. Our Halmahera region is widely quoted (e.g. Hamilton 1979, continuing work in the region indicates that this is a Katili 1975) but the evidence for it is weak. Pillow basalts major regional change in the character of volcanic are reported by Yasin (1980) from Bacan, and appear to activity but its cause is unknown. Basaltic compositions be the same sequence as, or an equivalent to, an are characteristic of all pre-Neogene sequences so far extensive undeformed pillow lava sequence of Early discovered in the region and hornblende-bearing andes- 286 A. SUFNI HAKIM and R. HALL ites are rare; in contrast, Neogene-Recent volcanic rocks Related Rocks (Edited by Thorpe, R. S.). John Wiley and Son, New York. are typically andesites, hornblende is a common con- Floyd, P. A. and Rowbotham, G. 1986. Chemistry of primary and stituent, and basalts are rare. secondary phases in intraplate basalts and volcaniclastic sediments, Molucca Sea subduction has continued until the DSDP Leg 89. Initial Reports of the Deep Sea Drilling Project 89, 459-470. present-day, possibly briefly interrupted in the Late Foster, M. D. 1962. Interpretation of the composition and classifi- Pliocene-Early Pleistocene, and the present Quaternary- cation of the chlorites. U.S. Geological Survey Professional Paper Recent volcanic arc is built unconformably upon Neo- 414A, 33pp. Gill, J. B. 1981. Orogenic Andesites and Plate Tectonics. Springer, gene volcanic and volcaniclastic rocks. Comparison of Berlin. the Neogene rocks to those of the present-day arc Green, T. H. and Ringwood, A. E. 1968. Genesis of the calcalkaline indicates that the volcanic products have not changed, igneous rock suite. Contrib. Mineral. Petrol. 18, 105-162. Hakim, A. S. 1989. Tertiary Volcanic Rocks from the Halmahera Arc, with the possible exception of contamination by conti- Indonesia: Petrology Geochemistry and Low Temperature Alter- nental crust at the southern end of the arc. ation. Unpublished M.Phil. thesis, University of London, 292pp. Hall, R. 1987. Plate boundary evolution in the Halmahera region, Acknowledgements--Research in Halmahera and the surrounding eastern Indonesia. Tectonophysics 144, 337-352. region has been supported by The Royal Society, the University of Hall, R. and Nichols, G. J. 1990. Terrane amalgamation at the London Consortium for Geological Research in Southeast Asia, boundary of the Philippine Sea Plate. Tectonophysics 181, 207-222. NERC grant GR3/7149, Amoco International, British Petroleum, Hall, R., Audley-Charles, M. G., Banner, F. T., Hidayat, S. and Enterprise Oil, Total Indonesie and Union Texas (S.E. Asia). Sufni Tobing, S. L. 1988a. The basement rocks of the Halmahera region, Hakim was supported by an award from the Asian Development Bank. east Indonesia: a Late Cretaceous-Early Tertiary forearc. J. geol. We thank GRDC Bandung for their cooperation, our field colleagues Soc. Lond. 145, 65-84. J. AlL M. G. Audley-Charles, P. D. Ballantyne, T. R. Charlton, L. A. Hall, R., Audley-Charles, M. 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