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TECTONIC CONTROLS OF GEOCHEMICAL EVOLUTION IN ARC MAGMATISM OF SE ASIA Tectonic controls of Geochemical Evolution in Arc Magmatism of SE Asia C G Macpherson1 and R Hall1 ABSTRACT Subduction zone magmatism is a direct response to tectonic and chemical processes operating at convergent margins. Changes in the tectonic configuration of plate boundaries in SE Asia during the Cenozoic had consequences for the nature and volume of magma produced. Retreat or advance of the trench hinge has probably been an important process in controlling the presence or absence of magmatism in SE Asian subduction zones. This process also provides a means of replenishing the mantle wedge and counteracting the effects of melt extraction in depleting the wedge. Changes in the nature of recycled material and lithologies of the over-riding plate also have the potential to influence the lavas that are erupted in arcs. Several locations in SE Asia have experienced magmatism bearing the geochemical signature of subduction but are remote from coeval subduction zones. Such magmatism requires an earlier period of mantle enrichment by subduction but may result from localised extension. The distribution of such magmatism may provide information of tectonic value. Some adakitic magmatism occurred in tectonic settings where there is no evidence for subduction of young oceanic crust at the time of magma genesis. Slab melting in such settings is not possible and alternative mechanisms are required to explain the occurrence of, at least some, adakitic lavas. INTRODUCTION The recognition that the linear chains of volcanoes in island arcs were related to subduction was very quickly incorporated into tectonic models once the plate tectonic theory of the Earth was accepted at the end of the 1960s. Subsequent research has led to revision of many ideas relating to the causes of melting, and the relative importance of tectonic processes. Early models (eg Oxburgh and Turcotte, 1970) emphasised magmatism due to frictional heating on the Benioff zone, melting of the subducting slab and compressional shortening of the over-riding plate in the arc-trench gap. In contrast, melts are now seen as originating primarily in the mantle wedge above the subduction zone, they are thought to result from the input of volatiles that lower the mantle solidus, and extension of the over-riding plate is now recognised as a common feature in arc settings. Tectonics may influence magmatism at a regional and local scale, for example, by inducing melting during extension, by providing pathways for magmas, or by varying the rate of supply of volatile components. The character of magmatism is of particular interest in tectonic interpretation if, for example, the geochemistry of igneous rocks can be used reliably to infer the tectonic setting (eg Pearce and Peate, 1995). Subduction and volcanic arcs in Southeast Asia Volcanic arcs occur in SE Asia at the boundaries between the Eurasian, the Indian - Australian and the Pacific plates (Figure 1). In addition, the Philippine Sea plate lying between the Pacific and Eurasian plates plays a crucial role in tectonic structures. There are three major zones. The first is between the Eurasian and Indian - Australian plates and extends from Myanmar in the west to the Banda Islands in the east. Within this zone the Sunda arc comprises the islands of the Andamans, Sumatra, Java and Nusa Tenggara and results from subduction of Indian Ocean lithosphere beneath Sundaland. Mesozoic magmatism is known 1. SE Asia Research Group, Department of Geology, Royal Holloway University of London, Egham, Surrey, TW20 0EX, United Kingdom PACRIM 99 in Sumatra and Java (Katili, 1975; Rock et al, 1982) while the oldest magmatic rocks in Nusa Tenggara are of Miocene age (Abbott and Chamalaun, 1978). The eastern part of this convergent zone is the Banda arc where the Australian continental margin is being subducted towards the north (Vroon et al, 1993). The second zone of convergence occurs where lithosphere of the Pacific Ocean is subducted towards the west. A subduction zone extends from Japan, through the Izu Bonin and Mariana trenches to the Ayu Trough. Magmatism has occurred in this arc since the Eocene (Cosca et al, 1998) and involved the opening of several marginal basins within the Philipine Sea plate: the Parece Vela Basin, the Shikoku Basin and the Mariana Trough. The third zone of convergence runs from southern Japan, past the Ryukyu Islands, through Taiwan and the Philippine Islands to the Molucca Sea region in the south. Excepting the Nankai and Ryukyu trenches in the north where the Philippine Sea plate is subducted beneath east Asia, this is a complex region with subduction zones dipping both east and west beneath the Philippine Islands and linked by strike-slip faults. The Philippine Sea plate is subducted towards the west at the Philippine trench. Eastward subduction at the Manila, Negros and Cotobato trenches consumes oceanic lithosphere of the South China, the Sulu and Celebes Seas, respectively. This oceanic crust formed in Cenozoic extensional and backarc basins at the eastern margin of Sundaland. The Celebes Sea is also being subducted towards the south beneath the northern arm of Sulawesi. All of these subduction systems are very young but at the southern end of the Philippines the Molucca Sea plate is being subducted to the east and west beneath the Halmahera and Sangihe arcs, respectively. The greatest ages yet obtained for volcanism associated with subduction of the Molucca Sea are Middle Miocene (Baker and Malaihollo, 1996; Elburg and Foden, 1998), although a Late Cretaceous Eocene phase of subduction magmatism is recorded in the Halmahera basement (Hakim and Hall, 1991). Further to the east young subduction systems and oceanic basins result from complex convergence and strike slip faulting linking the New Guinea margin to the Melanesian arcs at the boundary of the Australian and Pacific plates. In SE Asia most new Cenozoic subduction systems appear to have been initiated close to the boundary between thick and thin crust after collision events or plate reorganisations. From the viewpoint of the present-day region we can distinguish three broad categories of tectonic settings linked to arc magmatism: 1. normal tectonic settings in which there is magmatism associated with a subducting slab; 2. tectonic settings where there is subduction but no magmatism; 3. tectonic settings where there is magmatism of arc character but no subduction. In all cases it is also necessary to consider carefully the history of subduction. Hamilton (1988, 1995) emphasised the necessity of viewing subduction in a dynamic way, as opposed to static concepts which represent only a single instant and usually portray the subducting slab rolling over a stationary hinge and sliding down a slot fixed in the mantle. Viewed in a dynamic way Hamilton (1995) argued that most subduction zones are characterised by retreat of the hinge with time, or rollback, and that the mantle wedge above the subduction zone is constantly replenished by an inflow of hot, undepleted mantle. In this way Bali, Indonesia, 10 - 13 October 1999 Proceedings 4 PACRIM Congress, 1999, Australian Institute of Mining and Metallurgy, 359368. th 359 ug h C G MACPHERSON and R HALL Tr o a w ki na O u Ry Myanmar ky u e Tr nc h PHILIPPINE SEA Mariana Trough Mariana Trench lipp South China Sea 20oN Parece Vela Basin Phi 1 EURASIA Izu-Bonin Trench Shikoku Basin Andamans ine 10oN nc Tre Sulu 2 Sea PACIFIC h s 3 cca S CAROLINE 5 Ayu Trough Su Molu 4 ea s Celebes Sea m at ra s s rc B 90oE 100oE on Tr e nc s aA m h 9 Nusa Tenggara nd INDIA lo Bismarck Sea s Banda Sea Su 0o So s 6 Java N. 8 7 and aA rc 10oS Woodlark Basin INDIA-AUSTRALIA 110oE 120oE 130oE 140oE 150oE 20oS FIG 1 - Map of SE Asia highlighting major tectonic plates and plate boundaries. Subduction zones are indicated by lines with triangles on the over-riding plate. Key to subduction zones not noted in Fig: 1. Manila Trench; 2. Negros Trench; 3. Cotobato Trench; 4. North Sulawesi Trench; 5. Halmahera Trough (also frontal thrust of Sangihe arc); 6. Seram Trough; 7. New Guinea Trench; 8. Manus Trench; 9. New Britain Trench. steady-state arc magmatism can be maintained. In principle there is no reason why there should not be subduction zones characterised by hinge retreat, hinge advance and fixed hinges. However, Figure 2 shows that as far as SE Asia and the SW Pacific are concerned most subduction zones have indeed been retreating as Hamilton suggested. Future improvements in tectonic models may make it possible to examine the tectonic history of different regions in greater detail but from current models there appear to be some general relationships between hinge movement and volcanic activity. For the period since 25 Ma slab rollback was accompanied by significant arc volcanism and in most cases by marginal basin formation. In contrast, periods of hinge advance seem to be marked by reduction or cessation of volcanic activity. Geochemical Characteristics of Arc Magmatism Lavas erupted in subduction zones frequently display geochemical characteristics that can be used to distinguish them from melts generated in other tectonic environments, particularly mid-ocean ridge basalts (MORB). Lavas from many island arcs are often chemically depleted relative to MORB. This is observed as depletions in the relative and absolute concentration of non-mobile trace elements such as the high field strength elements (HFSEs) (Ewart and Hawkesworth, 1987; McCulloch and Gamble, 1991). Such depletion suggests that the mantle wedge has been stripped of a basaltic component to varying degrees prior to the present phase of magmatism (Woodhead et 360 al, 1993) and demonstrates that the wedge exerts important controls on general geochemical characteristics of arc lavas (eg Woodhead, 1989). As a consequence of the cooling effect of the slab and the refractory nature of peridotite in some mantle wedges, volatilerich slab-derived fluid or melt is probably necessary to induce melting beneath the arc front (Morris et al, 1990; Ryan and Langmuir, 1993). Any portion of the sedimentary cover that is subducted with the lithosphere may also be prone to melting at, or close to, the depths at which the mantle wedge melts. Therefore, the recycled component can display dual provenance within individual arcs (Elliot et al, 1997; Turner et al, 1996). The presence of crustal material within the source region of arc magmas provides a reservoir not present during MORB genesis. The nature of this material and the mechanisms of mass transport between slab and wedge can be resolved by examining enrichment of trace elements that are mobile, relative to HFSEs, and characteristic departures of isotope ratios from upper mantle values (Tatsumi and Eggins, 1995; Turner et al, 1996; Hawkesworth et al, 1997). For example, fluid transport from slab to wedge can induce chemical fractionation between elements such as La and Nb that display similar behaviour during magmatic processes (Tatsumi et al, 1986; Thirlwall et al, 1994; Brennan et al, 1995; Keppler 1996; You et al, 1996). Taken together, these observations have caused considerable rethinking of early ideas on the mechanisms and nature of sources operating in subduction zones (eg Woodhead, 1989; Hawkesworth et al, 1991; Elliott et al, 1997). Bali, Indonesia, 10 - 13 October 1999 PACRIM 99 TECTONIC CONTROLS OF GEOCHEMICAL EVOLUTION IN ARC MAGMATISM OF SE ASIA A 25-5 Ma Japan 25-5 Ma Ry uk yu Izu-Bonin 25-5 Ma nes ippi Phil Mariana 25-15 Ma Mariana 15-5 Ma Sangihe Sunda 25-5 Ma Halmahera 15-5 Ma Solomons Sunda-Banda 20-10 Ma New Hebrides 10-5 Ma Tonga 15-10 Ma Hinge Advance Hinge Retreat Kermadec 25-5 Ma B A: Andaman Sea BS: Banda Sea Bi: Bismarck Sea L: Lau Basin M: Mariana Trough NF: North Fiji Basin Wd: Woodlark Basin 5-0 Ma Japan Ry yu Izu Bonin Phil Sa renc ne T ippi Manila Trench Mariana ihe Halmahera North Sulawesi Sunda M h ng A uk New Britain Solomons Bi BS Ban da Wd NF New Hebrides Hinge Advance L Tonga Hinge Retreat Kermadec FIG 2 - A: Regions and intervals of significant movement of subduction hinges in the Neogene based on the reconstructions of Hall (1998). The map represents the interval between the regional plate reorganisations of 25 Ma and 5 Ma. Major continental outlines are shown at 25 Ma for reference. For subduction zones shown without shading there was no significant movement of hinge. B: Regions of significant movement of subduction hinges since 5 Ma based on the reconstructions of Hall (1998). Bold letters indicate areas of young marginal basin formation. Major continental outlines are shown at 5 Ma for reference. PACRIM 99 Bali, Indonesia, 10 - 13 October 1999 361 C G MACPHERSON and R HALL Variation in steady-state settings There are many possible tectonic causes for variation in the character of magmatism in a steady-state setting. Changes in the rate of subduction could vary both the rate at which the mantle wedge is replenished and the fluxes of material (fluids and melts) derived from the down-going slab. The nature of subducted material may influence magmatism and may, in some circumstances, provide a chemical tracer of tectonic history. If magmas interact with the crust, the character of crust in the over-riding plate will be important. For example, in east Indonesia it is possible to identify changes in character of crust from the geochemistry of the Neogene volcanic rocks (see below). In the case of oblique subduction, there may be changes in the character of crust where there are trench-parallel strike-slip faults as in Sumatra and the Philippines. Evolution of Recycled Components Turner and Hawkesworth (1997) have shown that the nature of recycled crust can influence the chemistry of melts erupted through an arc. The nature of recycled materials, and recycling processes, may also change. For example, Elburg and Foden (1998) have suggested that the recycled component in southern Sangihe arc lavas was derived mainly from altered oceanic crust during the Middle Miocene but is presently dominated by a sediment-derived melt. This evolution can be compared with data for Neogene (Forde, 1997) and Quaternary (Morris et al, 1983) magmatism in the Halmahera arc. An orthogonal collision between the Sangihe and Halmahera arcs is consuming the Molucca Sea plate. Therefore, the crust and sediments being subducted beneath both arcs may be similar. Figure 3 shows the changes in Zr/Nb in the Halmahera lavas during differentiation of suites of lavas from Halmahera. The Zr/Nb ratio is considered relatively sensitive to additions of sediment-derived melt to the mantle wedge that produces low values in the erupted lavas (Elliott et al, 1997). There is considerable variation in Zr/Nb between the different Neogene centres investigated by Forde (1997), but most appear to have evolved from (high MgO) parental magmas with Zr/Nb ratios that were similar to, or higher than, N-MORB (Figure 3). None of the phenocryst phases present in Halmahera lavas are able to produce significant changes in the Zr/Nb ratio during fractional crystallisation. Lavas from Obi have relatively constant Zr/Nb close to the MORB range while ratios increase through Central 362 100 C. Halmahera Bacan Obi Quaternary 80 Zr/Nb Following generation, melts must traverse the plate that overrides the subduction zone prior to eruption. This transport provides an opportunity for chemical differentiation, either through melt-solid-vapour partitioning, or through open system processes, such as magma-mixing, crustal contamination or assimilation with fractional crystallisation (AFC). The contaminants involved in these processes can resemble the materials being subducted and care must be taken to fully account for the influence of the overriding plate if recycled fluxes are to be accurately constrained (Davidson and Harmon, 1989; Macpherson et al, 1998). The studies conducted to formulate new theories on melt generation in subduction zones have largely focussed on active and recently active arcs. Much progress has been made by investigating spatial variations within individual arcs that allow evaluation of changes in the balance of different processes. Each of the major processes influencing petrogenesis also has the potential to vary in time (eg Turner and Hawkesworth, 1997; Elburg and Foden, 1998). Therefore, geochemical studies have potential to yield information on the evolution of tectonic events at convergent margins. This is particularly relevant to SE Asia where relatively rapid changes in plate configuration and the rates of tectonic processes have occurred throughout the Cenozoic. 60 40 N-MORB 20 0 0 2 4 6 8 MgO (wt.%) FIG 3 - Plot of MgO (wt. %) content versus Zr/Nb for Neogene and Quaternary to Recent volcanics from the Halmahera Arc. Zr/Nb ratios higher than the value for MORB infer residual mantle wedge while low values can reflect incorporation of subducted sediment. Halmahera data from Forde (1997) and Morris et al (1983) and N-MORB from Sun and McDonough (1989). Halmahera and Bacan. High values for Zr/Nb ratios occur in melts derived from mantle that has experienced previous melt extraction (Woodhead et al, 1993) and probably require a relatively high fluid flux to induce melting. Thus, the Neogene data for Halmahera are consistent with a mantle wedge that was variably depleted, with respect to the MORB source, being fluxed by a slab-derived fluid. The majority of Quaternary lavas from Halmahera also have Zr/Nb ratios that are similar to N-MORB (Figure 3). A few Quaternary lavas also have Zr/Nb in excess of N-MORB. This suggests that, like Neogene magmas, the majority of these lavas were derived from mantle that was similar to or more depleted than the MORB source. However, a number of Quaternary lavas display Zr/Nb lower than any observed from the Neogene arc. Normal mantle processes, such as previous melt extraction or fractional melting, are unable to produce melts with low Zr/Nb ratios suggesting a significant input from a reservoir characterised by low Zr/Nb. Sediments provide such a reservoir (Elliott et al, 1997) and partial melting of sediment, as opposed to bulk additions to the mantle, may further lower the ratio. Therefore, Zr/Nb ratios can be used to infer that Quaternary arc lavas may contain a more significant contribution from subducted sediment than lavas erupted during the Neogene. In this respect, the Neogene to present day evolution of the Halmahera and Sangihe arcs has been similar. However, unlike the Sangihe arc mass transport of recycled components by both fluid and melt appears to have been important in different parts of the Halmahera arc from the Neogene until the present. Figure 4 compares Pb and Nd isotope data for the arcs colliding in the Molucca Sea region. This figure also illustrates the less extreme time dependence of mass transfer processes of the Halmahera arc compared to Sangihe. The younger (Pliocene to Quaternary) Sangihe volcanics are successively displaced closer to the field of sediments than their Neogene counterparts (Elburg and Foden, 1998). In the Halmahera arc there is considerable overlap between lavas of different ages, although the Quaternary rocks are again more concentrated towards the field of sediments. The most conspicuous feature of this plot is the distinct offset between the fields for the two arcs; the Halmahera lavas possess consistently higher 206Pb/204Pb. This may be interpreted in several ways. It may be incorrect to assume that similar sediment is subducted under each arc and that sediment has higher 206Pb/204Pb in the east. Alternatively, the Bali, Indonesia, 10 - 13 October 1999 PACRIM 99 TECTONIC CONTROLS OF GEOCHEMICAL EVOLUTION IN ARC MAGMATISM OF SE ASIA same sediment may enter both subduction zones but mixes with isotopically distinct mantle wedges. The isotopic characteristics of both arcs, throughout their Neogene to Recent history, favour derivation from mantle similar to the source of Indian Ocean (I-) MORB. I-MORB show a sufficiently large isotopic range to accommodate the range of sources required to account for the differences between Halmahera and Sangihe lavas (Figure 4). A final possibility is that the same sediment is being subducted beneath both arcs where it is contaminating mantle wedges that have similar isotopic characteristics but different Pb/Nb ratios, resulting in greater sensitivity to Pb contamination in the low Pb/Nd wedge (Halmahera). 0.5131 Indian Ocean MORB 143 Nd/144Nd 0.5130 0.5129 0.5128 0.5127 0.5126 17.8 Halmahera Neogene Halmahera Quaternary Sangihe Neogene Sangihe Pliocene Sangihe Quaternary 18.0 Sediments 18.2 18.4 206 18.6 18.8 204 Pb/ Pb FIG 4 - 206Pb/204Pb versus 143Nd/144Nd in lavas from the Molucca Sea collision zone. Lavas from the Sangihe arc show a clear temporal progression towards lower 143Nd/144Nd. While there is more overlap between Neogene and Quaternary arc lavas, in the Halmahera data a similar overall progression exists. Halmahera data from Forde (1997), Sangihe data from Elburg and Foden (1998) and Indian Ocean MORB from Michard et al (1986), Ito et al (1987) and references therein. Evolution of Mantle Wedge Progressive melting of the mantle wedge might be expected to result in increasingly depleted lavas being erupted in the later stages of an arcs activity. However, lavas from the Quaternary Halmahera arc show no significant evidence for greater depletion of the mantle wedge when compared to the Neogene arc (Figure 3). This suggests that fresh material has been available to maintain the fertility of the zone of melting beneath the Halmahera arc since the Neogene. Upwelling of asthenosphere from the backarc region is a significant control on the depletion of the mantle wedge. Mantle can be processed through the backarc region before being advected into the mantle wedge (McCulloch and Gamble, 1991). Where melting is minimal, or absent, in the backarc region this will allow the mantle wedge to be replenished with fertile peridotite. Therefore, arcs that are tectonically susceptible to asthenospheric upwelling may be less likely to show a progressive depletion in wedge fertility. Upwelling will be an inevitable tectonic response in subduction zones when hinge retreat occurs (Hamilton, 1995). Figure 2A shows that the boundary between the Molucca Sea plate and the Philippine Sea plate, to the west of Halmahera, migrated relatively rapidly westwards through the Neogene resulting in substantial hinge retreat west of the Halmahera arc. Peridotite from beneath the Philippine Sea plate or deeper in the asthenosphere must have been drawn into the mantle wedge during this process. Recharging of the mantle wedge by asthenospheric peridotite could reduce, or even suppress, the overall wedge depletion throughout the lifetime of an arc. PACRIM 99 This process may also play a factor in the relative volumes of arcs. Volumetrically, the Halmahera arc is considerably larger than the Sangihe arc on the opposing side of the Molucca Sea collision zone. During the Neogene there was negligible movement of the subduction hinge at the Sangihe arc, separating the Molucca Sea plate and the Eurasian plate (Figure 2A). This situation would not favour upwelling of fresh fertile peridotite beneath the Sangihe arc, thus limiting the volume of magmatism. Replenishment of the Halmahera mantle wedge would provide a greater reservoir of fertile mantle from which to construct a larger arc. Evolution of Crustal Interaction The basement upon which island arcs are constructed are relatively poorly known. To a large extent, this is due to the poor exposure of basement rocks that have often been blanketed by the products of the arc. Geophysical techniques can be used to indirectly examine the nature of arc basements, but, interactions between the crust and passing melts can produce distinctive geochemical signatures in the melts. Where local basement rocks have been geochemically characterised such signatures can be exploited to understand the degree and processes of interaction (eg Macpherson et al, 1998). Conversely, where the basement is not exposed, suites of lavas displaying geochemical trends that can be attributed to crustal interactions may reveal something of the nature of the arc basement. Where the age of magmatism is known this may also provide tectonic constraints on the development of crustal domains (Vroon et al, 1996). The island of Bacan, in the Halmahera arc, provides an example. Morris et al (1983) noted that dacitic lavas in southern Bacan, erupted near areas of metamorphic basement, possess exceptionally radiogenic strontium and lead isotopic ratios (87Sr/86Sr: 0.7198-0.7240, 206Pb/204Pb: circa 40.2). In conjunction with elevated alkali contents in the lavas, Morris et al (1983) used the isotopic ratios to suggest that the dacites were contaminated by crust of continental origin. Neogene dacites displaying elevated isotopic ratios indicate that this continental material was present in the arc crust prior to the Late Miocene (Vroon et al, 1996; Forde, 1997). However, the locally exposed basement (Sibela Metamorphic Complex) has lead and strontium isotopic ratios that are too low to produce the values found in the dacites requiring older basement material to be available in the crust (Forde, 1997). In the western part of Bacan isotopic ratios of Late Miocene basaltic andesites show 87 Sr/ 86 Sr and 143 Nd/144Nd ratios typical of oceanic arcs, whereas Pliocene lavas in the same part of the island have higher 87Sr/86Sr and lower 143 Nd/144Nd that are consistent with contamination by the rocks of the Sibela Complex. This implies either: a. a change in magma dynamics leading to interaction between melts and parts of the crust that were previously unaffected by magmatism; or b. tectonic movement - possibly associated with the Sorong Fault Zone (Hall et al, 1991) introducing previously unavailable material. The latter scenario suggests that strike slip faulting in the region would have been active in the late Neogene as implied by palaeomagnatic studies (Ali and Hall, 1995). Subduction without magmatism In some areas there is subduction but no arc magmatism. Changes in rates of subduction are a possible explanation but where subduction has been effectively continuous other explanations are required for temporal changes in arc volcanic activity. It has been suggested that backarc basin formation leads to a cessation of arc volcanism. From early studies of the Philippine Sea plate Karig (1983) suggested arc volcanism Bali, Indonesia, 10 - 13 October 1999 363 C G MACPHERSON and R HALL continued during backarc basin opening whereas ODP drilling in the Mariana arc led other workers (Scott et al, 1980; Scott and Kroenke, 1980; Hussong and Uyeda, 1981) to advocate an alternation of arc activity with backarc basin opening. It now seems that arc volcanism generally does continue during backarc basin opening (Taylor and Natland, 1995). In search of other causes Cambray et al (1995) used ODP and DSDP cores to infer periodicity in volcanic activity based on the distribution of ash deposits in the western Pacific. They interpreted the temporal variations in volcanic activity to reflect variations in shallow stress fields in some arcs (Japan and Bonin), but where this explanation did not apply they appealed to variations in the amount of subducted hydrated sediment in other Philippine Sea arcs. McCourt et al (1996) suggested periodicity in magmatism in Sumatra may mark variations in plate motions and changes in obliquity of subduction but it is not yet entirely clear if the variations they infer, based largely on K-Ar dating, are real or an artefact of the dating method and database. East of Java volcanic activity in the Sunda arc declines significantly in the period between about 20 and 10 Ma although there is no evidence for changes in direction or rates of motion. This may reflect the movement of the subduction hinge and the reconstructions of Hall (1998) suggest that this region experienced hinge advance during this interval (Figure 2A). Carlson and Mortera-Guitierrez (1990) have also related variations in rates of hinge advance to backarc basin formation in the Philippine Sea plate. Elsewhere, hypotheses of slab breakoff (eg Richardson, 1993) or formation of gaps in the subducting slab (Thorkelsen, 1996) are plausible. Until now these types of explanation have been difficult to test because of the complex configurations of slabs under many parts of SE Asia, as is well illustrated by the Molucca Sea region (McCaffrey, 1982; Hall et al, 1995; Lallemand et al, 1998). Continued improvements in the seismic database and detailed tomographic studies may offer the means to test these types of model. Magmatism without subduction It is not unusual to find arc-like magmatism in regions where subduction is not active. Several magmatic provinces in SE Asia, including parts of the Philippines, south Sulawesi and east Indonesia, can be described in these terms. Like similar magmatism in other parts of the world such as the Aeolian Islands, Italy (Ellam et al, 1989) and the Basin and Range province of SW USA (Fitton et al, 1991) - lavas from these settings display distinctive geochemical signatures indicating their origins are linked to subduction processes. The presence of potassic volcanoes at convergent margins and in post-collisional settings argues for some link between subduction and the origin of this type of magmatism. Various models have been proposed including melting of subduction modified mantle (Johnson et al, 1978), melting of subducted sediment (Rogers et al, 1987) and melting of crustal material (Bergman et al, 1996). In several regions, such as western and southern Sulawesi, magmatism clearly post-dates active subduction suggesting that an origin through mantle enrichment or crustal melting is likely. The rapid reorganisation of plate boundaries that are known to have affected SE Asia during the Cenezoic may have caused dispersion of mantle domains that developed distinctive characteristics. Mantle Enrichment Events Bergman et al (1996) used isotopic data to infer that potassic magmatism in the Kalosi region of western Sulawesi is the result of crustal melting during the Middle Miocene to Pliocene. They suggested that the Makassar Strait is a foreland basin between two converging thrust systems. The potassic magmatic province of Sulawesi extends beyond Kalosi to the southwestern tip of the 364 island (Polvé et al, 1997) and plate tectonic reconstructions can be used to show that this portion of Sulawesi was adjacent to Java during from the Paleogene until late Oligocene (Figure 5). At that time, the regions of SW Sulawesi that subsequently experienced potassic magmatism were located along strike from the locations of Plio-Pleistocene ultra-potassic volcanism in northern Java (Figure 5B). A link between potassic magmatism in southern Sulawesi and Java has previously been suggested by Leterrier et al (1990). The plate reconstruction of Hall (1996) provides a tectonic framework for this link showing that the mantle sources of these regions could have been established simultaneously above a single subduction zone and subsequently separated by tectonic processes (Figure 5). A link between potassic magmatism in Java and Southern Sulawesi is further suggested by similarities in trace element and isotopic compositions. Trace element characteristics of lavas in both regions (eg enrichment of large ion lithophile elements relative to high field strength elements) provide clear evidence that the sources are enriched in many elements, with respect to MORB, and that the enrichment event was subduction-related (Edwards et al, 1994; Macpherson, 1994; Elburg and Foden, 1997; Polvé et al, 1997). Strontium and neodymium isotopic ratios are also very similar in lavas from the two areas (Edwards et al, 1994; Macpherson, 1994; Elburg and Foden, 1997). Isotopic ratios of strontium, neodymium and lead in the Javan lavas strongly point towards melting of enriched mantle rather than crustal melting (Figure 6). Melting was probably induced by later tectonic events since the location of potassic magmatism in northern Java coincides with major structural lineaments in the basement (Soeria-Atmadja et al, 1988). This is a feature that the Javan volcanoes share with other post-subduction potassic volcanics, such as the Basin and Range province of the United States (Fitton et al, 1991). These findings imply that a crustal source may not be necessary to explain potassic magmatism throughout western and southern Sulawesi, weakening arguments for a foreland basin origin for the Makassar Strait. Processes such as loading in a collision zone (Bergman et al, 1996) or underthrusting (Priadi et al, 1994), which are necessary to achieve melting of continental crust in western Sulawesi, need not be required in the southwestern arm of the island if the potassic magmatism has a mantle source. In this case, a major discontinuity in magmatic (and tectonic?) processes must be inferred to exist to the south of the Kalosi region (Elburg and Foden, 1997). Polvé et al (1997) suggested that within plate extension might provide a mechanism to induce melting of enriched mantle beneath SW Sulawesi. This may be associated with extension in the Gulf of Bone to the east of the SW Sulawesi peninsula. The axis of inferred extension lies sub-parallel to the alignment of the southern Sulawesi potassic volcanoes and extension is thought to have commenced in the Middle Miocene (Guntoro, 1995) at the same time that magmatism was initiated (Polvé et al, 1997). Adakitic Melts In parts of the Philippines adakitic rocks have been inferred to result from subduction of young, hot crust with resultant slab melting (Defant and Drummon, 1990). However, adakitic lavas in Mindanao interpreted to be related to subduction at the Cotobato trench (Sajona et al, 1994 and 1997) seem unlikely to have been generated in this way. These adakites have ages greater than 1 Ma, the age of the subducted crust is Eocene and the subducted Celebes Sea slab is still currently only at depths of about 80 km. At the time of magmatism the slab would not have penetrated even to this depth. Thus, the availability of a suitable adakite reservoir from the subducted Celebes Sea lithosphere is questionable. In Mindanao and in the Bacan region of east Indonesia (Forde, 1997) there is an obvious relationship of these rocks to strike-slip faults. This suggests that rather than melting Bali, Indonesia, 10 - 13 October 1999 PACRIM 99 TECTONIC CONTROLS OF GEOCHEMICAL EVOLUTION IN ARC MAGMATISM OF SE ASIA s Palawan A: Present Mindanao s r enc ine T s Sulu Sea SUNDA SHELF h s Malaya pp Phili 10oN s Celebes Sea s s Molucca Sea Halmahera s Borneo s Sula Platform assa r Str ait 0o Su nd a Sulawesi Mak Sumatra ng Soro t u Fa l Seram Java Sea Tre n Banda Sea ch da Arc Inner Ban Java Java Tren 10oS Timor ch Sumba h roug or T Tim INDIAN PLATE 100 E 110oE o 120oE 130oE 10oN Proto South China Sea SUNDA SHELF B: 45 Ma Sumatra 0o Borneo a nd Su Celebes Sea Java Sea Tre n Java Jav a Tr 10oS 100oE it tra ch s as k Ma S ar Philippine Sea West Sulawesi enc h AUSTRALIAN PLATE INDIAN PLATE 110oE 120oE 130oE FIG 5 - A: Present day plate configuration of the southern Sundaland margin. Locations of post-Middle Miocene ultra-potassic volcanoes in Java and southern Sulawesi (Letterier et al, 1990; Polvé et al, 1997) are shown are black triangles. B: Plate tectonic reconstruction of the same region during the Eocene (Hall, 1998). The locations of post-Middle Miocene potassic magmatism in Java and southern Sulawesi form a zone parallel to the Java Trench at that time. PACRIM 99 Bali, Indonesia, 10 - 13 October 1999 365 C G MACPHERSON and R HALL REFERENCES 0.5133 Indian MORB 0.5132 0.5131 143Nd/144 Nd 0.5130 0.5129 0.5128 Guntur Cereme 0.5127 0.5126 Muriah Banda Arc 0.5125 0.5124 Sediment 0.5123 0.702 0.704 0.706 0.708 0.710 0.712 87Sr/86Sr Fig 6. 87Sr/86Sr versus 143Nd/144Nd for volcanic rocks from the Sunda Arc. Symbols represent different lava groups from the potassic Ringgit Formation in eastern Java (Edwards et al, 1994; Macpherson, 1994). Although displaced from probable local mantle compositions (Indian Ocean MORB) the Ringgit lavas, and other potassic lavas from the Java (eg Muriah) have isotopic ratios that are not consistent with the involvement of old continental crust. This contrasts with the Banda arc (Vroon et al, 1993). Other data from Michard et al (1986), Ito et al (1987), Ben Othman et al (1989), Stolz et al (1990), Edwards et al (1991), Van Bergen et al (1992), Vroon (1992), Edwards et al (1993). the subducted slab the rocks are produced by melting of the mantle wedge and/or lower crust. Recent slab rollback on the west side of the Philippines (Figure 2B) would have caused an inflow of hot mantle with additional heat provided by ductile shearing in the lower crust or upper mantle as suggested for other major strike-slip zones (Thatcher and England, 1998). A similar explanation would apply in the Halmahera arc where there has been massive rollback of the Molucca Sea slab in the last 10-15 Ma. SUMMARY The timing and location of magmatism in SE Asia can be related to recent models for the evolution of plate boundaries. Hinge retreat appears to be important in controlling the volume of melt produced at an active margin. Furthermore, the dynamic nature of subduction zones suggests that this process may also encourage mantle circulation through the mantle wedge with implications for the chemistry of arc magmas. Changes in the balance between recycled fluxes of slab-derived fluid and sediment melts also appear to be possible and the presence of faults and thrusts in the overriding plate may lead to changes in the crust available to interact with melts during transport to the site of eruption. Finally, mantle that has been enriched by dehydration and/or melting of subducted material may be sampled by later magmatic events. Plate reconstruction models can be used to trace the provenance of such mantle and identify regions that may formerly have comprised part of the same supra-subduction zone complex. ACKNOWLEDGEMENTS Funding from the Southeast Asia Research Group supported by Arco, Canadian Petroleum, Exxon, LASMO, Minorco, Mobil, Union Texas and Unocal is gratefully acknowledged. 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