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and Planetary Science Letters 258 (2007) 269–282 www.elsevier.com/locate/epsl

The deep crust beneath island arcs: Inherited zircons reveal a continental fragment beneath East , ⁎ H.R. Smyth a, , P.J. Hamilton b, R. Hall a, P.D. Kinny b

a SE Research Group, Royal Holloway University of , Egham, Surrey TW200EX, UK b Department of Applied Geology, Curtin University of Technology, Perth 6845, Received 21 November 2006; received in revised form 21 March 2007; accepted 21 March 2007 Available online 1 April 2007 Editor: R.W. Carlson

Abstract

Inherited zircons in Cenozoic sedimentary and igneous rocks of East Java range in age from to Cenozoic. The distribution of zircons reveals two different basement types at depth. The igneous rocks of the Early Cenozoic arc, found along the southeast coast, contain only Archean to Cambrian zircons. In contrast, clastic rocks of north and west of East Java contain zircons, which are not found in the arc rocks to the south. The presence of Cretaceous zircons supports previous interpretations that much of East Java is underlain by arc and ophiolitic rocks, accreted to the Southeast Asian margin during Cretaceous . However, such accreted material cannot account for the older zircons. The age populations of Archean to Cambrian zircons in the arc rocks are similar to Gondwana crust. We interpret the East Java Early Cenozoic arc to be underlain by a continental fragment of Gondwana origin and not Cretaceous material as previously suggested. Melts rising through the crust, feeding the Early Cenozoic arc, picked up the ancient zircons through assimilation or partial melting. We suggest a Western Australian origin for the fragment, which rifted from Australia during the and collided with Southeast Asia, resulting in the termination of Cretaceous subduction. was therefore present at depth beneath the arc in south Java when Cenozoic subduction began in the Eocene. © 2007 Elsevier B.V. All rights reserved.

Keywords: East Java, Indonesia; Gondwana basement; zircon U–Pb SHRIMP

1. Introduction volcanic and sedimentary rocks. Xenoliths studies have proven to be extremely useful in other volcanic arcs [1] The character of the deep crust beneath many vol- where they have shown crustal character and – canic arcs in Southeast Asia is not known. There have crustal interaction but there have been none in Java. In been few seismic refraction studies, and the deep crustal East Java, Indonesia, the basement has long been con- rocks are often covered by thick sequences of young sidered Cretaceous in age and arc and ophiolitic in character. No continental rocks are exposed or reported on East Java. The assumed character of the crust is based ⁎ Corresponding author. Presently of CASP, Department of Earth on small and isolated exposures of basement rocks and a Sciences, University of , West Building, 181a Huntingdon – Road, Cambridge, CB3 0DH, UK. Tel.: +44 1223 277586; fax: +44 handful of basement-penetrating exploration wells. U Pb 1223 276604. dating of zircons in this study provides compelling ev- E-mail address: [email protected] (H.R. Smyth). idence for the presence of a continental fragment of

0012-821X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2007.03.044 270 H.R. Smyth et al. / Earth and Planetary Science Letters 258 (2007) 269–282

Gondwana affinity within the deep crust of East Java. limited exposure. The orientation was determined by This fragment was sampled by melts feeding Early Ce- linking the exposures of subduction complexes in nozoic arc volcanism. to those in the Meratus Mountains in SE (Fig. 2) [2,5]. There is no evidence of a relic 2. Geological background subduction complex cross-cutting the , but this may be due to thick Cenozoic sedimentary cover and 2.1. Regional setting lack of publicly available data from deep penetrating seismic surveys. The island of Java is located within the Indonesian The character of basement beneath most of East Java is between the landmasses of and unknown and surmised from spatially limited data. Australia, on the southeast margin of the . Exposures of basement rocks in Java are rare, and are The southeastern part of the Eurasian Plate is known as found only at Ciletuh in , Karangsambung and (Fig. 1), which is the Mesozoic continental the Jiwo Hills in the western part of East Java (Fig. 2). core of Southeast Asia [2]. Sundaland is not a single These exposures account for less than 1% of the land area. homogenous block of continental crust or a stable con- Lithologies exposed are Cretaceous in age and typical of tinental block; it was formed by the Early Mesozoic arc and ophiolitic , including basaltic pillow lavas, through the amalgamation of continental blocks [3,4]. cherts, limestone, schists, and metasedimentary rocks During the late Cretaceous terranes of arc and ophiolitic [5–8]. In addition to those at Karangsambung there are material of Cretaceous age were accreted to the southern occurrences of high pressure metamorphic rocks such as margin of Sundaland along a northeast–southwest jadeite–quartz–glaucophane bearing rocks and eclogite trending subduction zone [2]. Subduction moved to its interpreted to be diagnostic of subduction zone metamor- present-day position and east–west orientation, south of phism [9]. Cretaceous ages have been determined for the Java, along the Java in the Early Paleogene [4]. basement rocks at Karangsambung using radiolaria within Cenozoic sequences exposed today on East Java rest the cherts [7] and K–Ar dates on muscovite from quartz– unconformably above terranes accreted in the Cretaceous. mica schist ranging from 110 to 124 Ma [5,9].

2.2. Character of the basement 2.3. Volcanic activity

The interpreted NE–SW orientation of the Creta- Java is essentially a volcanic island and two Cenozoic ceous subduction zone is somewhat speculative, due to volcanic arcs are preserved on East Java: the modern arc

Fig. 1. Current plate tectonic setting showing the extent of Sundaland [2] and the location of the study area of East Java. The modern volcanoes of the are shown in black triangles. H.R. Smyth et al. / Earth and Planetary Science Letters 258 (2007) 269–282 271

Fig. 2. Reported basement configuration of the southern margin of Sundaland adapted from Hamilton [2], Parkinson et al. [5] and Miyazaki et al. [9]. The ages quoted are K–Ar dates for metamorphic material from offshore bore holes [2] and outcrop [5,9]. which has been active from the Late Miocene and an arc active from the Middle Eocene until the Early Miocene. of Early Cenozoic volcanoes known as the Southern The erupted products range from to rhyolite and Mountains Arc. The two arcs are separated by a distance form lava flows and domes, volcanic breccias, and of 50 km, the modern arc occupying the central spine of extensive pyroclastic deposits including flow, fall, and the island and the eroded remnants of the Southern surge deposits. The arc rocks are intruded by numerous Mountains Arc exposed along the southern coast (Fig. 3). high-level igneous bodies that range from Eocene to The older arc had been thought to be of Oligo-Miocene Pliocene in age and basalt to granodiorite in composition. age [10], but recent studies [6] indicate that the arc was Some of these intrusions are contemporaneous with arc

Fig. 3. Locations of samples containing populations of ancient zircons. Early Cenozoic volcanic centres are marked as white circles. 272 H.R. Smyth et al. / Earth and Planetary Science Letters 258 (2007) 269–282 activity and others post-date it and are associated with the Supplementary material. Dates greater than 1250 Ma are volcanism in the modern arc. calculated from 207Pb/206Pb ratios, and from this group In addition to the volcanic and volcaniclastic rocks analyses with greater than 15% discordance were ex- exposed in the southern part of East Java, there are thick cluded from any age plots. Dates less than 1250 Ma are Cenozoic sedimentary sequences to the north of the arc. calculated from 206Pb/238U ratios as these yield more The source of these clastic sediments has long been reliable results because of uncertainties in common Pb assumed to be Sundaland to the north and west corrections. All grains older than 100 Ma have been [2,11,12]. corrected for common Pb using 204Pb, and those younger As the erupted products of the Southern Moun- than 100 Ma have been corrected using 208Pb. All errors tains Arc are poorly dated a study was designed to quoted in the text are 2σm. utilise SIMS U–Pb dating of zircons to provide age constraints on the development of the arc. In addition it 3.3. Probability of missing a population of ages was hoped that the zircons would provide provenance information on the sedimentary rocks exposed within As this was the first study of its type in East Java, and to the north of the arc, which are commonly assumed there was some uncertainty about the expected range of to be derived from the continental rocks of Sundaland dates. The strategy adopted was to analyse a relatively [2,11,12]. This paper reports the unexpected results of small number of grains (∼15–50) from a large number this study. of samples, rather than the more conventional approach of analysis of a large number of grains from a small 2.4. Predicted age range number of samples. As a large range of zircon ages was obtained, consideration must be given as to whether the Assuming that the basement of East Java is, as Hamilton population of zircons has been fully sampled. This [2] suggested, Cretaceous arc and ophiolitic fragments, problem is discussed in detail in Dodson et al. [13] and and knowing the history of Cenozoic arc volcanism on Cawood et al. [14] who show that for detrital zircons 60 the island, the predicted zircon age range would be analyses are necessary to sample the entire population Cretaceous to present-day. It was a surprise to discover with greater than 95% confidence. For any population of that in addition to this predicted range there are significant dates that is significant, say greater than 20% of the total numbers of Cambrian and older zircon grains in many of population, there is less than a 3.5% chance that it will the samples analysed. Below we document the range and be missed in a sample set comprising only the smallest distribution of the old zircon grains identified in the number of the grains analysed in this study (n=15 igneous and clastic rocks from East Java, and discuss their grains). We are thus confident that with 15–35 grains potential origin. analysed the probability of sampling any population comprising more than 20% is high. 3. Methods 4. Results 3.1. Samples analysed 4.1. Age range observed We report the results of zircon U–Pb SIMS dating of 17 Cenozoic samples of igneous (6 intrusive bodies and When examining the entire dataset several distinct 4 extrusive rocks), volcaniclastic (n=3) and sedimen- peaks (Fig. 4A) can be identified in the entire popu- tary (n=4) rocks. The samples are listed in Table 1 and lation: a Cenozoic peak (n=55), a Cretaceous peak their locations shown in Fig. 3. In the samples reported (n=77), three Cambrian to Proterozoic peaks (n=166) here more than 350 spot dates were measured and of of 500–750 Ma, 900–1250 Ma, 1600–1850 Ma, and an these 290 were Cretaceous and older. Archean peak of 2500–2800 Ma (n=38). In addition there are 9 zircon ages ranging from to 3.2. Analytical method but these do not form a significant peak. This paper details the Cretaceous and older zircon results. Zircon grains were analysed for Pb isotope compo- sition and U, Th and Pb concentrations using the 4.2. Lithology and zircon ages SHRIMP facility at Curtin University of Technology. The results are summarised in Table 1 and further details The distribution of zircons of different ages is shown of the samples and analytical methods are given in the on Fig. 4 and discussed in detail below. H.R. Smyth et al. / Earth and Planetary Science Letters 258 (2007) 269–282 273

Table 1 Samples and ages of inherited zircons in East Java Position Sample Lithology Cretaceous Ordovician– Proterozoic– Archean Jurassic Cambrian Igneous Central Java Jhs2KaliS3 Nanggulan, West Basaltic sill 0 1 12 8 Progo Mountains Central Java Jhs2AND1 Nanggulan, West Andesite 1 0 12 3 Progo Mountains intrusion Southern Jhs2AN7 Pendul Hill, Bayat, Diorite 0 0 11 2 Mountains Arc Klaten Southern Jhs2PON4 Ponorogo Diorite 0 0 9 5 Mountains Arc Southern Jhs2Treng6 Trenggelek Diorite 0 0 23 1 Mountains Arc Southern Jhs2Jember10 Sukamede, Jember Granodiorite 0 0 14 2 Mountains Arc Southern Jhs3Prambanan , Dacitic ash 0 0 9 1 Mountains Arc Southern Jhs2PAC7 Pacitan Dacitic ash 0 0 15 6 Mountains Arc Southern Jhs2TUN4 Tulunagung Dacitic pumice 0 0 19 8 Mountains Arc breccia Southern Jhs2Turen14 Turen Andesitic breccia 0 1 11 0 Mountains Arc Volcaniclastic Central Java Jhs2KK13 Karangsambung, Reworked andesitic 27 0 1 0 Central Java breccia Central Java Jhs2Langse1 Karangsambung, Volcaniclastic 13 0 0 0 Central Java Core sandstone sample provided by Coparex Indonesia Southern Jhs2Santren1 Batu Agung, Crystal-rich 30 0 0 Mountains Arc Yogyakarta tuffaceous sandstone Sedimentary Shelf Jhs1-012 Lodan Quarries, Quartz-rich sandstone 15 1 0 0 Rembang Basin Jhs2Lutut6 Kali Lutut, Quartz-rich sandstone 6 2 10 0 Southern Jhs2SERMO6 Sermo Quartz-rich sandstone 4 1 13 1 Mountains Reservoir, West Arc Progo Mountains Southern Jhs2NW12 Nanggulan, West 83 7 1 Mountains Arc Progo Mountains Quartz-rich sandstone Total zircons (291) 77 9 166 38 No. of samples (17) 8 6 14 11

4.2.1. Igneous rocks westernmost sample (Jhs2KK13) yielded older zircons, The 6 intrusive and 4 extrusive igneous rocks contain a single grain of Mesoproterozoic age (1079±36 Ma). a range of Cambrian and older zircons (n=155). The All 4 of the sedimentary rocks analysed were quartz-rich samples lack Cretaceous zircons (Fig. 4), with the ex- sandstones. These sandstones yielded varied zircon ception of a sample from Central Java (Jhs2AND1) ages. All of the sandstones contain Cretaceous grains which yielded a single Cretaceous zircon. One sample (n=33) (Fig. 4B). The northernmost sandstone sample from a basaltic sill in Central Java (Jhs2KaliS3) yielded from the edge of the (Jhs1-012) contains a a single Carboniferous age. Of the igneous samples, single Jurassic–Triassic grain, but no older grains. The only one extrusive rock from East Java (Jhs2Turen14) other sandstones from the south contain a range of failed to yield Archean grains. Cambrian and older grains (n=22).

4.2.2. Clastic rocks 4.2.3. Spatial distribution of zircon dates The 3 volcaniclastic rocks yielded a significant The samples containing different zircon dates do number of Cretaceous zircons (n=42) but only the have a clear geographical pattern. The Archean grains 274 H.R. Smyth et al. / Earth and Planetary Science Letters 258 (2007) 269–282

Fig. 4. A. Age range of zircons (bins on the histogram represent 25 Ma and the values are plotted with 2σ errors) B. Samples with Cretaceous zircons. C. Samples with Ordovician to Jurassic zircons. D. Samples with Proterozoic to Cambrian zircons. E. Samples with Archean zircons.

are found only in rocks of the Southern Mountains Arc 5. Discussion (Fig. 4E) and the igneous rocks of this area contain no Cretaceous zircons. Cretaceous zircons are present 5.1. Origin and incorporation of the zircons into the only in rocks located in the west and north (Fig. 4B). rocks of East Java Cretaceous zircons have been identified only in sedi- mentary and volcaniclastic rocks and one The age range identified in the samples from East from the western part of the study area. This distribution Java is much greater than would be predicted whether is interpreted to reveal the character of the crust beneath derived from a Sundaland source or local basement East Java. source of Cretaceous arc and ophiolitic rocks. H.R. Smyth et al. / Earth and Planetary Science Letters 258 (2007) 269–282 275

5.1.1. Possible Sundaland Sources Northern [19]. If these source also Sundaland, the continental core of Southeast Asia, is provided material to East Java, the zircon populations the closest and most obvious source of very old zircons would be similar. The zircon populations of these (Fig. 5). Rocks which could have provided abundant regions are dominated by Cretaceous zircons with age zircons are granites of southwest Borneo [2], the Malay peaks different from those in East Java, there are abun- Peninsula [15], the Thai–Malay Tin Belt [16] and dant –Triassic zircons (Fig. 6C), which are rare [17,18]. These regions could have been a in East Java, and Precambrian zircons are mainly source of Mesozoic and Paleozoic material, but no Paleoproterozoic with very rare Archean zircons [19]. Archean rocks are known, nor is there any indication In addition to the differences in zircon populations, that they are underlain by Archean crust. there are paleogeographic factors to be considered. The There are abundant granites of Cretaceous age island of Java is separated from Borneo by the shallow located in the Schwaner Mountains and the Thai– waters of the Java Sea. Beneath the flat-lying siliciclas- Malay Tin Belt which are known to have contributed tic and carbonate Pleistocene deposits of the Sunda material to the Paleogene sedimentary sequences of Shelf there are numerous broad NE–SW trending ridges

Fig. 5. Potential source areas for Cambrian and older zircons in East Java. 276 H.R. Smyth et al. / Earth and Planetary Science Letters 258 (2007) 269–282

Fig. 6. Comparison of the Cambrian–Archean data set from East Java with sediments of the Perth Basin Australia and Northern Borneo. A. East Java data set. B. Young placer deposits of the Perth Basin, known to be the product of erosion of the Proterozoic and Archean rocks of [34]. C. Range of detrital zircon ages for Paleogene sandstones from Northern Borneo [19]. Bins on the histogram represent 25 Ma and the values are plotted with 2σ errors. H.R. Smyth et al. / Earth and Planetary Science Letters 258 (2007) 269–282 277 flanked by narrow deep Cenozoic basins. During the eroded products of Precambrian basement rocks which earlier part of the Cenozoic the ridges and basins would are currently exposed in Western Australia. The detrital have acted as barriers or traps preventing sediment zircons dated from these placer deposits have three passing to the south and east from Sundaland into East major age peaks and these can be correlated with known Java. Indeed the ridges themselves may have acted as source areas [28,29,34]. The Cambrian to Neoproter- sediment source areas. One such ridge on the edge of ozoic zircons (500–800 Ma) were produced by the Sundaland, the Arch (Fig. 5), is reported erosion of the Pan African Pinjarra , the to have been elevated throughout much of the Late Mesoproterozoic zircons (1000–1300 Ma) were derived Eocene to Late Miocene [20,21]. The elevation of the from the Grenvillian Albany Fraser Orogenic Belt, and arch has been interpreted to be responsible for the the Archean zircons (2500–3200 Ma) were eroded from different sedimentary environments recorded in the East the Yilgarn and Pilbara . There are also a limited and West Java Seas. To the east of the arch, the Lower number of dates which correspond to the formation of Miocene and Oligocene sediments are marine, but to the the Capricorn Orogenic Belt (1600–2000 Ma). west most of the Lower Miocene and all of the A range of Archean and Proterozoic peaks has also Oligocene sediments are non-marine [20,21].The been reported from the Higher Himalayan Gneisses in Karimunjawa Arch is reported to have been a source [35]. Yoshida and Upreti [35] suggest that these of sediment to both the East and West Java Sea Basins peaks represent terranes derived from a Circum-East and potentially provided sediment to the area of present- Antarctic origin, which could have included part of day Java between the Late Eocene and Late Miocene. Greater India. However, the peaks identified do not There are exposures of metasedimentary rocks on small match those reported in this study. islands on the arch but none are dated as they are The Western Australia peaks are remarkably similar terrestrial. They are interpreted to be pre-Cenozoic in to those identified in the Southern Mountains Arc of age and part of the continental basement of Sundaland. East Java (Fig. 6). Given this correlation it is plausible This basement could be the source of the Cretaceous that Western Australia or a region with a similar geo- zircon grains in East Java's clastic sequences. logical history, such as other parts of the Gondwana margin, was the source of material. 5.1.2. Possible Gondwana Sources The Archean–Cambrian peaks identified coincide 5.2. Incorporation of the zircons with periods of major crustal growth represented by the Pan African and Grenville age orogens and the Archean Zircon is a highly refractory mineral. Zircon is known cratons [22,23]. These peaks are characteristic of much to survive numerous sedimentary cycles, metamorphism of the Gondwana margin as recorded in the rocks of and diagenesis [36]. The zircon grains identified in the southern and east India [24], eastern and northern sedimentary rocks of East Java could have been [23,25,26],easternAfrica[22,23] and incorporated through erosion and transport of sediment western Australia [14,27–29]. The rocks of Sundaland from exposed source areas, such as the local basement, are not known to share this history. Plate tectonic the Karimunjawa Arch or from volcanic sources by air- reconstructions of Gondwanaland prior to and following fall and subsequent reworking. The incorporation of break-up indicate that it is unlikely that either Antarctic zircons into the igneous rocks requires more explanation. or African material could have contributed to Sundaland As well as being resistant to weathering, zircon is [30–32]. The largest and closest area of Archean conti- known to survive episodes of melting [37]. Material of nental crust is Australia, which formed part of Gond- Gondwana origin must have been introduced to the melts wana until the Cretaceous. feeding the Southern Mountains Arc. There are several Basement rocks in Western Australia are of Protero- mechanisms whereby this introduction could occur. zoic and Archean age (Fig. 6B). As these ancient rocks have been the focus of detailed zircon dating studies, 5.2.1. Subducted sediment many of the basement blocks have age fingerprints Sediment eroded from western Australia deposited [28,29,33,34]. The rocks at the surface in Western on the Indo- could have been carried Australia are representative of much of the Gondwana north to the Java Trench as Australia moved north. margin. A large published dataset [34] from Western However, even today a fan similar in size to the modern Australia was chosen to compare with that obtained Bengal Fan would be required to bring material from from East Java. The dataset [34] focuses on the young Australia to the site of subduction at the Java Trench placer sediments of the Perth Basin, which are the (Fig. 5). During the Early Cenozoic Australia lay very 278 H.R. Smyth et al. / Earth and Planetary Science Letters 258 (2007) 269–282

Fig. 7. Plate tectonic reconstruction of SE Asia and Australia at 45 Ma [4]. The light grey shaded area around Australia shows the approximate extent of continental crust based on present-day bathymetry and mapping of the continental–ocean boundary. The ornamented area west of western Australia indicates the surmised area of continental crust that rifted from Australia in the Jurassic and early Cretaceous. We suggest that the East Java fragment probably originated in the southern part of this area. far south of its current position (Fig. 7), and at 45 Ma the source [39]. If sediment was produced by the erosion of distance between Australia and East Java would have the Himalaya it would require transport via the Bengal exceeded 1900 km [4]. If the fan model were correct, the Fan to the Java Trench and total transport distances sedimentary sequences on the Indian–Australian plate would exceed 4000 km. In addition to this significant currently entering the Java Trench would correspond to distance, there were topographic barriers at the Inves- a relatively proximal position on the Cenozoic fan. tigator Fracture and the Ninety East Ridge which would However, today there is little sedimentary cover on the have hampered the flow of sediment. The lack of Bengal Indian–Australian plate south of Java [38]. In addi- Fan material today in the Java Trench offshore East Java tion the modern Bengal Fan is fed by erosion of the [38] adds to the argument that this is not a plausible Himalayas, and there is no evidence of a similar moun- mechanism. It is equally unlikely that sediment from tain belt in NW Australia during the Cenozoic. the Himalayan region could travel a distance of over Sediment of Gondwana origin could have been 3500 km from Indochina across Sundaland, by-passing introduced into the Java Trench by axial transport from its deep basins, structural highs and ridges [40]. the Bird's Head microcontinent, New Guinea or from the Himalayas via the Bengal Fan, although both are 5.2.2. Subducted continental fragment unlikely. The basement rocks of the Bird's Head micro- The subduction of a fragment of continental crust were not available for erosion as the Bird's also seems unlikely. Buoyancy forces make subduction Head was the site of active carbonate sedimentation of continental crust difficult and even if these were during most of the Cenozoic and so can be excluded as a overcome, the fragment is required to have supplied H.R. Smyth et al. / Earth and Planetary Science Letters 258 (2007) 269–282 279 melts from the Eocene to the Miocene, a period of doubts about the continental source as many of the around 20 Ma. The fragment either remained stationary sandstones contain considerable acidic volcanic mate- beneath the Southern Mountains or was 1500 km in rial, probably derived from the Southern Mountains Arc length based on the present subduction rate of 75 mm/yr [6]. Thus, although we agree with the interpretation of [41] and Cenozoic plate tectonic reconstructions [4]. Sribudiyani et al. [45] of a continental fragment beneath Neither of these scenarios is likely. East Java, we dispute the evidence on which their interpretation is based, and consequently differ on the 5.2.3. Collision/ of a continental fragment timing of arrival of the continental fragment. An alternative, simpler and more plausible explana- tion is that there is a fragment of continental crust of 6. Character and origin of the crust beneath East Gondwana character beneath the Southern Mountains Java Arc. This fragment must have been in place before subduction began in its current position along the Java 6.1. Character of the crust Trench. We suggest that the fragment collided with the margin of Sundaland terminating the Late Cretaceous The distribution of pre-Cenozoic zircons in samples phase of subduction. The origin of this fragment is from East Java provides new information about the discussed in detail below. rising through, or character of the deep crust about which little was pre- pooling in, the continental crust would lead to partial viously known. We suggest that the most likely expla- melting or assimilation of material. nation for the zircon age range sampled in East Java is that The presence of continental crust is supported by the there is a fragment of continental crust of Archean age, evolved character of the products of the Southern Gondwana affinity and potentially of Western Australian Mountains Arc and the rocks which intrude them. The origin at depth beneath the Southern Mountains. volcanic rocks range from andesite to rhyolite (60–77 wt.% The size of the East Java fragment can be estimated SiO2) with an estimated average SiO2 content of 67 wt.% by utilising the spatial distribution of the different zircon [6], and a significant proportion of the high-level intrusive age populations and other supporting evidence. Cur- bodies are granodiorites. These rocks are considerably rently none of the data suggests that the continental more evolved than the basic-intermediate eruptive products fragment extends north beneath the modern Sunda Arc. of the present-day volcanoes. The chemistry of the modern Therefore, the northern boundary must be located north arc volcanic rocks in East Java [42–44] indicates they are of the Southern Mountains but south of the modern the products of relatively primitive subduction melts Sunda Arc (Fig. 8). The character of the crust beneath having no significant interaction with underlying crust. the modern arc is still unclear, but there is no indication The difference in chemistry of the products of the Southern of a continental contribution to the magmas feeding the Mountains Arc and the Sunda Arc could be explained in a arc [e.g. [42]]. The deep crust is likely to be similar in number of ways: fractional crystallization of basic magma, composition to the Cretaceous arc and ophiolitic frag- or contamination by melting of felsic continental basement ments known from exposures in Central Java. crust (C. Macpherson, 2006, pers. comm.). The evidence The lack of Cretaceous material within the igneous from zircon dating suggests that partial melting of con- rocks of the Southern Mountains Arc to the west of the tinental basement is the most likely explanation. Yogyakarta area suggests that a boundary exists in The distribution of pre-Cenozoic zircons (Fig. 4) and this region between Cretaceous crust and the East Java the character of the arc rocks indicate that the conti- continental fragment. This boundary is interpreted to co- nental fragment is present only beneath the Southern incide with a major northeast–southwest trending struc- Mountains whereas north of the Southern Mountains ture identified at approximately 110.5°E [6] (Fig. 8B). there is no continental basement. The deep crust to the This lineament links several major features onshore and north of the arc is probably Cretaceous arc or ophiolitic offshore, a structural high in the Java Forearc, and the terranes, as suggested by Hamilton [2], representing aligned centres of three Oligo-Miocene volcanoes [10] in material accreted during Cretaceous subduction or the West Progo Mountains, Mount Merapi and the un- underplated during collision. usual K-rich back-arc of Mount Muria. In the Sribudiyani et al. [45] also inferred the presence of a region close to the boundary between the Cretaceous crust continental fragment beneath East Java. Their interpre- and the older Gondwana fragment mixing of zircons of tation is based on the distribution of quartz-rich sand- different ages could easily occur through assimilation or stones assumed to have a continental origin. A detailed partial melting of the crust during magma ascent, or uplift study of the provenance of these sandstones raised and erosion of the basement rocks. 280 H.R. Smyth et al. / Earth and Planetary Science Letters 258 (2007) 269–282

Terrane boundaries are rarely straight or uniform features, but are embayed and irregular, and their shape is dictated by original rift morphology and collision tectonics. Therefore the margins of the East Java frag- ment could be structurally complex.

6.2. Origin and extent of the fragment

Several fragments rifted off the Australian margin during the Mesozoic in a phase of break-up preceding the separation of India from Gondwana [3,46,47]. We sug- gest that one such fragment or fragments collided with the margin of Sundaland during the Late Cretaceous forming the basement to the Southern Mountains Arc. Prior to the collision, arc and ophiolitic terranes were accreted and formed a belt on to the margin of Sundaland (Fig. 8). The ophiolitic blocks may be remnants of crust which preceded the continental fragment on the northward-moving plate. The timing of collision of the fragment pre-dates the Middle Eocene as continental crust was already present beneath the arc when subduction began at that time. We suggest that the collision of this fragment resulted in the termination of Cretaceous subduction. Following the renewal of subduction during the Middle Eocene, the Cenozoic melts feeding the Southern Mountains Arc were contaminated by interaction with the continental fragment. Manur and Barraclough [48] indicate the presence of two fragments (Fig. 8B), in the areas of the Arch and Paternoster Platform, of Pre-Cenozoic conti- nental crust (the shape, orientation and age of which are very poorly constrained) in the Java Sea. These occur between the belt of ophiolitic and arc blocks and the East Java fragment. It is possible that these fragments are a continuation of the East Java fragment, but as there is no evidence of continental crust to the north of the Southern Mountains, beneath the Sunda Arc, it is dif- ficult to envisage how these fragments could be linked. The eastern extent of the East Java fragment is not known. However, in continental rocks of similar age to those identified in the East Java fragment have been reported [49]. Geochemical and zircon dating evidence from the Malino Metamorphic complex of northwest Sulawesi reveals the presence of Archean material with ages up to 3500 Ma [49,50]. We suggest that the East Java fragment extends to the northwest, beneath western Sulawesi, forming a large arcuate block on the southeast Fig. 8. Character of the crust on the southeastern margin of Sundaland. margin of Sundaland (Fig. 8B). Geochemical analysis of A. Extent of the continental fragment onshore East Java. B. Map the modern volcanic products on Java [42–44] suggests showing the proposed extent of the East Java fragment beneath that the magmas are not sampling continental crust. Sulawesi. C. Sketch cross-section. However, the rocks analysed have been basic, and we H.R. Smyth et al. / Earth and Planetary Science Letters 258 (2007) 269–282 281 suggest that a study of the more evolved volcanic rocks References would be profitable. 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