Dating of Neogene igneous rocks in the Halmahera region: arc initiation and development SIMON BAKER & JEFFREY MALAIHOLLO SE Asia Research Group, Department of Geological Sciences, University College, London WC1E 6BT, UK Abstract: Potassium-argon ages of Neogene to Recent igneous rocks from the Halmahera region record a history of intra-oceanic arc development since the late Middle Miocene following an earlier phase of collisional plutonism. Arc formation from the Middle Miocene onwards was due to the east-directed subduction of the Molucca Sea plate beneath the Philippine Sea plate as it arrived at the Eurasian margin. The distribution of ages within the Neogene arc indicates a northward migration of volcanic activity during the Late Miocene to Pliocene. Results of the dating work show that after collision with the Australian margin at c. 22 Ma there was a period of volcanic quiescence and limestone deposition before a new arc formed. This arc began erupting at around 11 Ma on Obi as a result of subduction of the Molucca Sea plate. Initiation of subduction is thought to have occurred around 15-17 Ma and may have been responsible for disturbing potassium-argon ages of pre-Neogene rocks. Dates from fresh rocks show that the volcanic front migrated northwards through Bacan and Halmahera throughout the Late Miocene to Early Pliocene. Limestone deposition was curtailed as arc activity migrated north while volcanism died out from the south. No Neogene volcanism younger than 8 Ma is observed in the Obi area while on Bacan subduction-related volcanism ceased at c. 2 Ma. Late Pliocene crustal deformation caused a 30-40 km westward shift of the volcanic front. Quaternary volcanic rocks exposed in Bacan and the extreme south of Halmahera are not direct products of subduction but, rather, display geochemical characteristics of both subduction and fault-related magmatism. These volcanic rocks are distributed along splays of the Sorong fault system. The formation and propagation of the Halmahera arc is a consequence of the clockwise rotation of the Philippine Sea plate as the southern edge moved across the northern Australian margin and impinged on the east Eurasian margin. The ages of initiation of volcanism and subduction track the developing plate boundary as subduction propagated northwards. The principal islands of the Halmahera group The currently active Halmahera arc, at the (Halmahera, Bacan and Obi) lie in northeastern eastern edge of the Molucca Sea (Hamilton 1979; Indonesia in the province of Maluku, straddling the Moore & Silver 1983), formed as a consequence equator between 127°E and 129°E. These islands of eastward subduction of the Molucca Sea plate lie at the junction of three major plates (the beneath the Philippine Sea plate as the latter Philippine Sea plate, the Australian plate and the rotated clockwise. Geophysical evidence from Eurasian plate) where the Alpine-Himalayan and earthquake data indicates a seismic zone dipping the Circum-Pacific orogenic belts meet. The at c. 45 ° to the east to depths of about 200 km present-day tectonics are therefore complex (Fig. (Cardwell et al. 1980). To the south the subduction 1). These belts meet in the region of the Sorong zone appears to be terminated by a strand of the fault system which is responsible for transferring Sorong fault system just north of the eastern tip of crustal fragments of Philippine Sea and Australian Mangole (Sula Platform). Opposing the Halmahera origin into the complex of island arcs and small arc is the Sangihe-north Sulawesi arc approxi- ocean basins that forms the Eurasian margin. mately 250 km to the west which is the product Present-day tectonics are the result of the north- of westward subduction of the Molucca Sea plate; ward movement of continental Australia (Australia beneath the Sangihe arc the slab dips at 55-65 ° and and New Guinea) into the Pacific region throughout reaches a depth of 600 km (Cardwell et al. 1980). the Tertiary. Clockwise rotation of the Philippine The oldest rocks known from the Sangihe arc are Sea plate since c. 25 Ma led to the development of early Middle Miocene age and particularly of the Sorong Fault Zone as it collided with the voluminous arc activity occurred between 5 and northern Australian margin (Hall et al. 1995a, b). 14 Ma (Hamilton 1979). Current motion between these two plates is taken The present Halmahera arc lies north of the up by sinistral movement on the Sorong fault equator and is built upon a Neogene arc that system in northern New Guinea. extended from Obi northwards through Bacan to From Hall, R. & Blundell, D. (eds), 1996, Tectonic Evolution of Southeast Asia, 499 Geological Society Special Publication No. 106, pp. 499-509. 500 S. BAKER & J. MALAIHOLLO I[ " .+.~. " ~+.! 7Z'l/'TZ',/\ ,... /; Euraeian -,- MOROTAI Margnn A N + . + t ~_. • .+ ~_." • + •t ~ • O 0 N.E .~ I A ¥. # ;,- # ......... O ,'-',',-',', Phnlnnnnne -',',- HALMAHEF, A.-I -' ;'." r r .: .'-- +/:# ~ ~= % / < Sea Rate-; :'\-; ~ ~#" Molucca \-~ "-; -~-~-; :-i :w~,J~o.o°:_ - Sea Collision Complex -, "- z- -. "- " ~- \ \ ~.÷~'~"~9~~b~C~UINP_A ~ustra -1,50 km Fig. 1. Location map showing the principal tectonic elements and islands of the study area. north Halmahera (Hall et al. 1988b; Hall & Nichols the region during a period of stability before the 1990). This paper describes the Neogene to development of the Neogene arc and associated Pliocene development of this arc based on sedimentary rocks. Deformation in the Pliocene potassium-argon and petrographic studies of the is marked by another unconformity. Continental volcanic rocks. Other Neogene rocks dated by the metamorphic rocks representing Australian crust potassium-argon method are described and placed and possibly of Palaeozoic or younger age in a plate tectonic context. (Hamilton 1979; Malaihollo 1993) are juxtaposed against ophiolitic rocks in the southern part of the Stratigraphy region. Detailed stratigraphic descriptions of pre- Neogene and Neogene rocks are given in Hall The Tertiary stratigraphy of the region is summa- et al. (1988a, b, 1991), Hakim & Hall (1991) and rized in Fig. 2. The pre-Neogene geology of the Roberts (1993). region consists of a basement of ophiolitic and arc- related rocks (Hall et al. 1988a; 1991; Ballantyne Analytical methods and data presentation 1990) overlain by and imbricated with, arc volcanic, plutonic and sedimentary rocks of late Samples were selected for potassium-argon dating Mesozoic to Eocene age (Hall et al. 1988b; Hakim primarily on the basis of their freshness and the 1989; Ballantyne 1990; Hakim & Hall 1991). presence of suitable potassium-bearing phases. Following an Eocene unconformity (possibly Mineral separates were used; amphiboles and micas linked with the Pacific plate reorganization at this were the preferred minerals. Whole rock analyses time) a Late Eocene arc developed with associated on fresh volcanic rocks provided good quality basaltic pillow lavas and volcaniclastic turbidites results (confirmed by biostratigraphic dating in (Hall et al. 1991; Malaihollo 1993); this arc ceased many areas) with small errors. Biostratigraphic activity in the earliest Miocene and its termination dating gives confidence that young, fresh rocks is marked by a regional unconformity. Middle with a potassium rich, glassy residuum provide Miocene limestones were deposited throughout reliable isotopic ages. Sample locations are given ARC DEVELOPMENT IN THE HALMAHERA REGION 501 HALMAHERA (& KAYOA) BACAN OBI Ma 0 vvvvvvvvvvvvvvvv c;. ~.~. ~.,c:~.. i¢.aya (~a Quaternary Formal;ion Form,l;,o° ~~d 2 Wcd~ Group (rn=intyW~e~) KaFul;u~a~ Format..ion q .L Vo[can'mla~t.ic~' z- "1 Format.ion ~ ~ Pliocene ,4 Ar~ V.olc,,~nic~ ,4 ./" I For'm=l:ion (E=n~) Woi 6, Gu~t.l ]< , volc~nlcla~r.lc~ m I 5 Forrnat.ione ~ubaim Lim¢~t,onc Miocene Formation (Eal~t.) Rut,a Fluk ~ LIm¢~'f.on~e LIm~B1;one Formal;ion Fo~,l;~o~ ~ 23 Tawali Oligocene Tawali Forma'cion Forma¢ion F....... ~ 35 Oha Volcanic. Bacan Format.ion Air I~,,a~i !lIHI)'//.l'it~-~l~ !...... ,!lh Formal;Ion (lh'¢~l;) • ~ri~ ~J I J Wayamli E~SM8 Formal;Ion ? ? ? (Ea~.) 56 I~leocene ~5 Fig. 2. A simplified Tertiary stratigraphy for the three principal islands of the region studied. in Fig. 3 whilst analytical results are presented in Error calculation and data reduction Table 1. methods Samples for potassium-argon analysis were crushed and sieved to yield a fraction between All isotopic ages are quoted with errors at the 95% 300 l.tm or 250 l.tm and 125 ~m (if the sample was confidence level (2o). The major factor affecting small the sieve fraction 500-125 pm was used); the error is connected with the correction for atmos- this was then washed in deionized water. The pheric argon. If the atmospheric component washed fraction was either used directly for (40Aratm) is small, uncertainties in its measurement potassium-argon analysis or further processed to will be minor compared to the total 4°At. However, produce mineral separates by a combination of if 4°Aratm is the major component then the same heavy liquid and magnetic separation techniques. degree of uncertainty will cause large errors in The samples were then split for separate potassium the smaller radiogenic component (40Arrad). For and argon analyses. All potassium-argon analyses samples with low atmospheric contamination a were carried out at the NERC Isotope Geoscience minimum error of 1% is assumed based on the Laboratory (NIGL) at the BGS site in Keyworth, uncertainty in the calculation of the spike volume. Nottinghamshire. Potassium analyses were con- Potassium errors were calculated from the average ducted in duplicate using flame photometry against of at least two analyses and 10 error calculated standard potassium solutions and a pure water from the deviation of the average from the actual blank after sample digestion in excess hydrofluoric values obtained, lo error in % was used for age and perchloric acids.