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Home , Harzburgite, Ophiolite, Spilite

in Southeast Asia

CHARLES S. HUTCHISON Department of Geology, University of Malaya, Kuala Lumpur, Malaysia

ABSTRACT semblages are classified into definite ophiolite, tentatively identified ophiolite, and associations that have previously been named No fewer than 20 belts of -ultramafic assemblages have ophiolite but that are not. Each of the ophiolite or other been named "ophiolite" in the complex Southeast Asia region of mafic-ultramafic associations is listed in Table 1, with brief reasons Sundaland. Fewer than half of these can be confidently classified as for its classification, particularly in regard to the petrography of the ophiolite. The only well-documented complete ophiolite, with con- rock suite and the nature of its sedimentary envelope. The basis for tinuous conformable sections from mantle through identification (or rejection) of these rock associations as ophiolite is to , occurs in northeast and the neighboring discussed in the following section. Philippine Islands. It contains a record of oceanic lithospheric his- tory from Jurassic to Tertiary and has a Miocene emplacement age. OPHIOLITE FORMATION All other ophiolite belts of the region are either incomplete or dis- membered. The Sundaland region probably has examples of several The oceanic , with its thin formed along types of emplacement mechanism and emplacement ages ranging the spreading axes of divergent plate junctures, is thought to be from early Paleozoic to Cenozoic. Key words: Sundaland, plate consumed at arc-trench systems of convergent plate junctures (Fig. tectonics. 2). Minor subtractions of crustal and mantle material from de- scending slabs of oceanic lithosphere are thought to be added to INTRODUCTION belts of mélange, and imbricate slices are caught in crustal subduc- tion zones at the trenches (Dickinson, 1972). These are the ophio- Southeast Asia has been analyzed as an assemblage of several an- lite assemblages. Unfortunately, the preservation of old oceanic cient continental plates separated by géosynclinal fold belts of lithosphere as ophiolite belts is fragmentary at best. Caledonian, Variscan, and Cenozoic orogenesis (Fig. 1). Oceanic- The linear nature of the outcrops of rocks of the ophiolite suite is continental margins that are now active clearly delineate island-arc taken to indicate that ophiolite emplacement into features. is a product of interplate movement. The remnants of once- Many geologists believe that ophiolite, strictly defined by Bailey extensive ophiolitic suites are exposed by isostatic uplift after one and Blake (1973) and by the report on the Penrose Field Confer- or more of the following general modes of structural emplacement ence (1972) as an orderly succession of specific ultramafic and within orogenic belts: mafic igneous rocks with a characteristic pelagic sedimentary en- 1. Incorporation of a few dislocated scraps of ophiolite into the velope, crops out on the land as a linear belt, indicating a former deformed terrane of a zone where the oceanic plate is consuming plate juncture. being consumed (Fig. 2A). In this type of plate contact, there is little In Southeast Asia, the term "ophiolite" has been used by some chance for large ophiolite plates to be uplifted into the sedimenta- geologists to include a variety of rock assemblages, not all of which tion basin. The ophiolite belt is unlikely to qualify as a fully de- would qualify for the currently accepted definition. This work is a veloped ophiolite in the sense of Bailey and Blake (1973) and the review of the ophiolite belts from Burma and North Vietnam to Penrose Field Conference (1972). Disrupted ophiolite masses may Java and Borneo, and it is confined to a discussion of their mean- be tectonically emplaced along the zone and within the as- ing, age of formation, and mode and age of emplacement. sociated schist terrane. The hypothesis that the underlying high- Ophiolite masses exposed on the continents are our only accessi- pressure metamorphic terrane was dragged beneath an overriding ble guide to the history of the ancient oceanic lithosphere, the coherent lithospheric slab is supported by examples from , greatest part of which has been consumed. The various igneous and , and the (Ernst, 1971). Low geothermal gradient metamorphic rocks of the ophiolite suite give us fragmentary rec- metamorphism took place in the downgoing plate simultaneously ords of this history. It is therefore important to combine a pétro- with deformation. The blueschist type of terrane must have re- graphie and structural study of these rocks with bounded isostatically to much shallower depths following cessation if we are to reconstruct the evolution of former oceanic crust. As- of active subduction. It is not certain how these rocks rose chiefly sociated oceanic and pelagic sediments will indicate something of along the former subduction zones. Possibly the blueschist com- the sedimentary history of the oceanic and marginal sea basins. The plexes generally returned toward the surface only where a pro- linear extension of the ophiolite belts indicates the former plate found change in spreading dynamics occurred (Ernst, 1971). contacts, and if we can precisely date the ophiolite belts, then a re- 2. Subduction of light crustal material beneath an intact construction of former plate movements may be possible. ophiolitic slab where the ophiolitic sequence is part of the consum- It is fundamentally important to distinguish truly ophiolitic belts, ing plate margin (Fig. 2B). This mechanism can emplace large, in- which characterize plate junctures, from other mafic-ultramafic tact, complete ophiolite masses in continental crust. Well-known complexes, which could have formed well within a continental examples are the Papuan ultramafic belt of New Guinea (Davies, plate. 1968; Davies and Smith, 1971) and the Troodos massif of (Gass, 1969). The ophiolite belt of North Borneo (Figs. 1, 3) may GEOGRAPHICAL DISTRIBUTION belong to this category. This type of emplacement has been called "" by Coleman (1971). The occurrences of ultramafic-mafic rock assemblages in the In Papua and New Caledonia, high-pressure progressive Sundaland region of Southeast Asia are shown in Figure 1. The as- metamorphic assemblages have been developed in the under-

Geological Society of America Bulletin, v. 86, p. 797-806, 3 figs., June 1975, Doc. no. 50610.

797

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/6/797/3429237/i0016-7606-86-6-797.pdf by guest on 25 September 2021 Figure 1. Outline map of Southeast Asia showing known ophiolite and other mafic-ultramaiic belts. Numbers as in Table 1 and text.

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thrusted subducted continental lithospheric plate (Lillie* 1970). the ophiolite sequence while it is in the ocean or marginal plate Ernst (1971) has concluded that there may be no significant differ- interior. ence between subduction and obduction with regard to the genera- The region around the rise crest continues to be one of a high tion of high-pressure metamorphic belts. geothermal gradient as basaltic continues to evolve and to Uplift of the ophiolite into the zone of erosion might be expected rise through the crust. As it moves away from the spreading axis, to result in ophiolite-rich clast material in associated mélange and previously formed ophiolite may be subjected to high geothermal olistostrome. gradient metamorphism during the formation of new ophiolite in- 3. Collision of sialic crustal blocks of continental fragments or trusions. Metabasalt and metagabbro of , almandine- island arcs. Mélange and associated ophiolitic shreds of the sub- amphibolite and hornblende-granulite facies, and and duction zone mark the suture belt along which the blocks of sialic spilite are therefore to be expected from metamorphism and crust became welded together (Fig. 2C). After a long period of con- in the rise crest area (Dewey and Bird, 1971). tinuing subduction according to the scheme of Figure 2A, the The ophiolitic association of serpentinite (and gabbro, , spreading ocean crust may carry with it a continental plate, plate and metabasalt) with spilite and chert (the "Steinmann trinity" of fragment, or of another arc-trench system. Collision of E. B. Bailey [Bailey and McCallien, I960]) tends to suggest that spi- this plate or arc with the stationary plate will halt subduction and lite represents a rather special rock in this assemblage. Recent work in the process will squeeze up the accumulated sediments of the favors the hypothesis that spilite is low-grade metamorphosed trench along with partly subducted material and ophiolite. This basalt, and Vallance (1965) has shown that the supposed Na-rich orogenic squeezing will give rise to a mountain belt in the plate su- nature of many spilitic pillow lavas is largely illusionary. The sup- ture which is expected to contain abundant ophiolitic fragments position is based on analyses of the Na-rich centers of the pillows; and large complete ophiolite cores to major folds. This process has but the chlorite-rich margins have been enriched in Mg, with a yet to be fully substantiated by examples from the geologic record. complementary depletion in Na. This compositional differentiation The nature and completeness of an ophiolite belt is a result first is not an original magmatic feature but a metamorphic- of its mode of emplacement and second of postemplacement tec- metasomatic feature of the glassy pillow margins caused by the ac- tonics such as faulting. tion of permeating water and low-grade metamorphism (Leake, 1972). Spilite, accordingly, is normal oceanic basalt that has un- OPHIOLITE SUITE dergone metamorphism and metasomatism in the rise crest area, and it should be considered a normal part of the ophiolite suite. The ophiolite suite is considered to consist of fragments of the crust and from oceanic or marginal basin lithosphere. AGES OF OPHIOLITE COMPONENTS When ophiolite occurs as structurally intact slabs, it displays a pseudostratigraphy that resembles the layering known from the The age of an ophiolite belt is defined as the time of its emplace- present-day oceanic lithosphere, as shown by Aumento (1972, ment in the zone of continental and oceanic plate contact, or the 1973) and Dickinson (1972). The fully developed normal ophiolite time of plate collision (emplacement age). Its age Can be deduced succession consists of a basal zone of (typically only by the of its clastic sedimentary envelope. harzburgite with a fabric), a gabbro zone (locally cumu- Radiometric dates on igneous and metamorphic members of the late with monomineralic and bimineralic layers), and a basaltic ophiolite suite will usually indicate their pre-emplacement age (ac- zone (typically of and , and locally with a lower cumulation age), related to events when they were an integral part or "sheeted intrusive" zone). Salic but potash-poor differen- of the oceanic lithosphere, even as far back as to their original tiates such as extrusive keratophyre occur commonly in the upper igneous crystallization or metamorphic recrystallization at an zone, and intrusive trondhjemite may be present in the gabbro zone ocean or marginal basin spreading axis (Fig. 2). (Bailey and Blake, 1973). Oceanic sediments formed a sedimentary cover overlying the Basal , commonly harzburgite, is usually considered to ophiolite when it was in the ocean or marginal basin. These sedi- be residual mantle material from which the oceanic basaltic magma ments, therefore, are also older than the ophiolite belt (emplace- was bled (Dickinson, 1972). Its tectonite fabric is thought to have ment age) in which they are finally incorporated. been imposed during emplacement in the upper lithosphere by The clastic sediments, which are derived from the nearby conti- rheid flowage in the hot regions beneath rise crests or other axes of nental plate, offer the only real guide to the age of an ophiolite belt. crustal spreading. The genetic relationship between mantle tecton- Accordingly, because of the mixture of rocks associated with ite and overlying basalt is now questioned in the light of a growing ophiolite, it is difficult to be precise about the age. A minimum age number of geological and geochemical studies, in particular of the for an ophiolite belt will be indicated by a sedimentary sequence strontium isotope ratios. Sr87/Sr86 disequilibrium between unconformably overlying the mélange that contains the ophiolite ultramafic and the associated basalt minerals is frequently or the associated olistostrome. demonstrated, and it probably indicates a lack of genetic relation- ship between the basaltic and ultramafic parts of many ophiolite SUNDALAND OPHIOLITE BELTS suites and oceanic lithosphere (Stueber and Ikramuddin, 1974; Al- legre, 1973). Mantle ultramafic tectonite may frequently be consid- The ophiolite and other mafic-ultramafic associations of the re- erably older than the overlying basalt. Overlying gabbro and some gion are shown in Figure 1, and the most important features of cumulus peridotite with layered probably result from dif- each are listed in Table 1. ferentiation in basaltic magma chambers near the crust-mantle in- terface beneath a spreading axis (Fig. 2). dike swarms and Definite Ophiolite sills connect with these magma chambers. The pillow basalt and as- sociated lava were erupted at the top of the magmatic crust. Paleozoic Sedimentary rocks associated with ophiolite nearly always in- clude argillite and pelagic rocks such as chert or fine-grained lime- Bl. Bentong-Raub Line. This belt trends north along the eastern stone, and they commonly include turbidite layers and tuff. They foothills of the granitic Main Range. The main ophiolite occur- are added on top of the ophiolite when the mafic igneous crust is rence, as several elongate bodies ranging in length from less than 1 rafted away from the divergent plate juncture into the ocean or km to more than 15 km in steeply dipping lower Paleozoic schist, is marginal basin. Large basaltic piles can also be added to the top of in the Raub district of west Pahang, west and northwest of Cheroh

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village (Fig. 1). An early Paleozoic age is suggested by the presence The Bentong-Raub line can be traced farther north and south of Early Devonian graptolites in overlying south of Karak through additional discontinuous serpentinite bodies (Gobbett, (Jones, 1973). No unaltered peridotite has been reported. Acces- 1972), and the line may continue southward with the Segamat- sory minerals in the serpentinite are magnetite, ilmenite, chromite, Johore-Singapore-Billiton line (B2 in Fig. 1 and Table 1). picotite, pyrite, pyrrhotite, tremolite, chlorite, and The early Paleozoic geography of the Malay Peninsula, complete (Hutchison, 1973a). The associated metabasite is predominantly with a volcanic arc along the western margin of the Main Range actinolite schist, commonly containing , but locally there and a stable early Paleozoic miogeocline to the west of the Malay are occurrences of - and diopside-rich schist and hornfels Peninsula, suggests that the Bentong-Raub line marks the former (Hutchison, 1973b). trench position with westward subduction of an eastern oceanic

TABLE 1. MAFIC AND ULTRAMAFIC ROCK ASSEMBLAGES OF THE SUNDALAND REGION OF SOUTHEAST ASIA

No. Name of belt Locality Ophiolite or Sedimentary Emplacement References Comments and similar rocks envelope rocks time conclusions

A. Confidently Classified as Ophiolite B1 Bentong-Raub Central peninsular Foliated serpentinite, Schist Series: carbona- Ordovician; Hutchison (1973a, Metamorphosed and Malaysia; possible actinolite-epidote schist, ceous schist and phyl- possibly as 1973b, 1973c) dismembered ophiolite extension with B2 diopside schist, gabbro lite, chert, conglom- late as early erate Carboniferous B4 Song Ma North Vietnam, Laos Serpentinized , Lower Paleozoic chert- Permian to Fromaget (1941), Fromaget Dismembered, partly peridotite, ; lower argillite-limestone Early Triassic and others (1971), Gatinsky metamorphosed ophiolite Paleozoic basalt, amphib- sequence; often schistose and others (1972, 1973) in complex fold belt olite C2 Black River North Vietnam Spilite, diabase, gabbro, Triassic argillite-chert Latest Fromaget (1941), Fromaget Dismembered ophiolite (Tonkin) serpentinite sequence with some lime- Triassic and others (1971), Gatinsky in North Vietnam stone; often schistose and others (1972, 1973) suture zone D2 Lupar Sarawak and north- basalt, albitized Rajang Group: phyllite, Late Haile (1957, 1970, 1973) Incomplete ophiolite; west Kalimantan; basalt, gabbro, dolerite, slate, shale, graywacke, Cretaceous parallel igneous arc may extend to tuff, agglomerate; serpen- chert; strongly associated to southwest Natuna tinite, diabase, norite with chert in Natuna 03 Meratus- Southeast Peridotite, pyroxenite, Alino Formation: chert, Jurassic to Bemmelen (1949), Katili Complete ophiolite, Bobaris Kalimantan gabbro, diorite, amphib- siliceous shale, shale, Cretaceous (1971, 1973) partly metamorphlc; olite, hornblende schist, conglomerate with clasts unusual intrusive granite of glaucophane schist breccia pipes El Mandalay Serpentinite, peridotite, Chlorite schist, kyanite Late Chhibber (1934), Incomplete ophiolite gabbro, actinolite schist schist, glaucophane- Cretaceous Soe-Win (1968) in a suture zone epidote schist; uncon- formably overlain by Tertiary rocks E2 Naga Hills Serpentinized Iherzollte, Negrais Series: black Late Cretaceous Chhibber (1934) Incomplete hornblende gabbro, shale, chert, chlorite to early olivine gabbro and schist Tertiary dismembered ophiolite E3 Andaman- Andaman, Nicobar Serpentinized peridotite, Chert, jasper, calcar- Late Cretaceous Chhibber (1934) Nicobar Islands gabbro, diorite, dolerite eous "gneiss," porce- to early Eocene Incomplete dikes laneous limestone dismembered ophiolite Chert-Spilite Formation: E6 Darvel Bay- Sabah, Philip- Banded metagabbro, meta- Hutchison (1968, 1972), Complete ophiolite spilite, chert, argillite, Labuk- pine Islands basalt, spilite, harz- Hutchison and Dhonau (1969, with sequence from Paìawan around Sulu Sea burgite, pyroxenite, some micrite; Tertiary 1971), Kirk (1968) harzburgite to serpentinite, amphibolite mélange with oph1ol1te spilite; high geo- dikes olistoliths thermal gradient metamorphism

B. Tentatively Classified as Ophiolite (More data required for confidence) B2 Segamat- Peninsular Malay- Serpentinite in Uncertain: usually Pre-Triassic Hutchison (1973a, 1973c), Considered as ophiolite Johore, Singa- sia, Singapore, Segamat Basalt; gabbro in associated with granite (pregranite) Aleva (1960) only because of align- pore-Billiton Indonesia Johore, Singapore, Billiton ment with B1 B3 Lampang-Houei- Gabbro, pyroxenite, diabase, Marine chert, sandstone, Late Baum and others (1970), Very likely to be andesite, tuff; metabasite conglomerate, shale Carboniferous Fromaget and others (1971) ophiolite; ultramafic Sai Thailand, Laos (amphibolite) is common to Permian rocks rather lacking CI Uttaradlt- Serpentinized pyroxenite, Sandstone-conglomerate- Carboniferous- Baum and others (1970), Very likely to be gabbro, diorite, dolerite Fromaget (1941) ophiolite; ultramafic Luang Prabang Thailand, Laos shale sequence with Permian to some chert Early Triassic rocks rather lacking C3 Siem Reap- Gabbro, peridotite, dolerite; Sandstone, shale, Carboniferous- Fromaqet (1941), Fromaget Not well defined as belt hornblende is common, perido- 1imestone Permian to and others (1971) Stung Treng North Cambodia tite at Pursat, along with Early Triassic dunite Dl Serabang West Sarawak Albitized basalt, basalt, Serabang Formation: Early Wolfenden and Haile (1963), Not well defined as tuff, agglomerate, meta- argillite, graywacke, Cretaceous Kirk (1968), Haile (1973) belt; suite is ophioli- gabbro, amphibolite, ser- chert, conglomerate tic, but linear exten- pentinite, peridotite, sion is minimal pyroxenite D4 Gumai, Garba South and north Basaltic rocks Lingseng Beds: siliceous Early Bemmelen (1949) Descriptions are Mountains, Sumatra, Indo- marly, clayey shale and Cretaceous lacking; only basalt Atjeh nesia radiolarian chert has been described E5 Tjiletuh Bay- Java, Indonesia Serpentinite, peridotite, Pre-Tertiary: chlorite Bemmelen (1949) Only isolated small Loh Ulo gabbro, diabase, metabasite schist, phyllite pre-Tertiary masses E7 Bukit Merslng Central Sarawak Pillow basalt, spilite, tuff Belaga Formation: shale, Kirk (1968) Incomplete ophiolite graywacke, calcareous red mudstone

C. Previously Called Ophiolite but Now Confidently Discredited Al Kontum South Vietnam Uralitized orthopyroxenite Mica schist with garnet, Late Fromaget (1941), Saurin Small masses inter- massif (amphibolitic gabbro) slllimanite, graphite Precambrian (1956) foliated in schist B5 Cao Beng North Vietnam, Orthoamphibolite (meta- Shale, sandy marl, tuff, Silurian to Gatinsky and others (1973) Associated sedimentary China basite) rich in and sandstone, coal Early Triassic rocks are not pelagic amphibole, pyroxenite N1as, Sipura, Dikes of amphibolite and Tertiary conglomerate, Paleocene or Bemmelen (1949) Minor basic and serpen- Bangkaru serpentinized basic rocks sandstone early Tertiary tinized dikes in (Indonesia) and basalt areni te

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/6/797/3429237/i0016-7606-86-6-797.pdf by guest on 25 September 2021 Figure 2. Diagrammatic sketch ORIGIN- Metomorphism- OPHIOLITE TIME- - DEPOSITION showing generalized layers of ophio- (Igneous age) (if any) RELATIONS IN OPHIOLITE (Metamorphic age ) BELT lite sequence to illustrate formation (Deposition age) of oceanic lithosphere along divergent • plate juncture, and possible mecha- STAGE I STAGE H TIME STAGE HI nisms for ophiolite belt formation at convergent plate junctures.

HIGH LOW HIGH GEOTHERMAL -THERMAL REGIMES GEOTHERMAL GEOTHERMAL GRADIENT GRADIENT GRADIENT

RISE VOLCANIC CREST PLUTONIC ARC

SPREADING

Microaobbro gabbro cumulus T AL Írock, MANTLE PART

OF LITHOSPHERE

LOW AVERAGE GEOTHERMAL GEOTHERMAL GRADIENT GRADIENT

OROGENESIS

BASALT + SPILLI

T D0LERITE + "E. - CONTINENTAL CUMULUS ROC PLATE A N

MANTLE

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plate under a continental plate (Hutchison, 1973c). The sporadic D3. Meratus-Bobaris Line(s). These two parallel belts of occurrence of dismembered, incomplete ophiolite bodies in the ophiolite trend northeast to north, parallel to the coast. The main schist and the absence of glaucophane metamorphism result from one is in the Meratus Mountains; the other is to the north, in the the strongly isoclinal postemplacement folding and metamorphism Bobaris Range (Bemmelen, 1949). that have affected the lower Paleozoic rocks. Closely associated with the ultramafic rocks are the "crystalline B4. Song Ma Line. This northwest-trending occurrence is the schists," which are quartzite and hornblende schist. The latter most prominent line on the map of Fromaget and others (1971), comprise -bearing hornblende schist, hornblende-epidote and it is described as "basic rocks of Song Ma with , schist, albite amphibolite, epidote-albite amphibolite, actinolite bronzites and ." The major occurrence is in the Thahn schist, smaragdite schist, and chlorite-epidote-albite schist. Locally, Hoa Range, where the ophiolite is serpentinized pyroxene and biotite-chlorite and hornblende-epidote rocks occur. Clasts of olivine-rich gabbro and peridotite. Orthopyroxene and gneiss and glaucophane schist occur in conglomerate beds. The clinopyroxene are commonly intergrown, and olivine is usually crystalline schists should therefore be considered, in part at least, to surrounded by orthopyroxene. Chromite occurs in some rocks be metamorphosed oceanic crust and therefore part of the ophiolite (Fromaget, 1941). Serpentinized and peridotite occur in sequence. several places in the Song Ma valley. Gatinsky and others (1972, The peridotite forms two mountain ranges, the Meratus Moun- 1973) showed that the ophiolite crops out among intensely folded tains and the Bobaris Range, in which occur. The perido- marine geosynclinal deposits of early Paleozoic to Permian age that tite, which is mostly serpentinized and sheared, also occurs as dikes contain patches of siliceous shale and basaltic lava of Late Permian 10 to 15 m wide in the crystalline schists and in the overlying strata to Early Triassic age. Unconformably overlying the geosynclinal of the Alino Formation. Pyroxenite occurs in the peridotite. Gab- rocks are less deformed Middle to Upper Triassic coarse red clastic bro and diorite are intimately related and show transitions to rocks associated with acid lava and tuff. Dolerite dikes are abun- quartz gabbro and quartz diorite, with some transition to dant (Saurin, 1956). granodiorite. Locally, gabbro is recrystallized to amphibolite. Dynamothermal and cataclastic metamorphism is common. Late Paleozoic to Early The Meratus-Bobaris ophiolite belt has the typical features of a complete ophiolite as defined by Bailey and Blake (1973). The un- C2. Black River Line. This occurrence trends northwest in the usual feature that casts doubt on this classification, however, is the suture zone of North Vietnam. There is a complete section of Trias- presence of intrusive breccia, which is presumed to be the source of sic geosynclinal strata in the synclinorium of the Black River, ex- diamonds, although most of the breccia pipes and dikes are barren ceeding 8 km, of predominantly black argillaceous schist, chert, of diamonds. Bemmelen (1949) noted that some of the breccia and polymictic, often tuffaceous, sandstone. Limestone occurs in masses contain diamonds. Rotman and others (1973) described the middle part of the section, and the lower part is characterized similar ophiolite masses containing pipes from Kam- by spilitic lava, ophitic diabase, and gabbro (Fromaget, 1941). chatka and the Solomon Islands in the western Pacific. Small Triassic ultramafic bodies are characteristically associated Katili (1971, 1973) has shown that this ophiolite belt continues with the basic volcanic rocks (Gatinsky and others, 1972, 1973). southwestward through central Java and correlates with that of Unconformably overlying the geosynclinal rocks is a sequence of west Sumatra. Hutchison (1973c) has extrapolated the Meratus- molasse of latest Triassic, Jurassic, and Cretaceous age. Bobaris line to continue with the Lupar line. On Pulau Laut, The geosuture of North Vietnam, in which the ophiolite is peridotite with subordinate gabbro is unconformably overlain by localized, narrows along the strike and passes into a major fault in conglomerate. Gabbro and diorite intrude the peridotite. the Chinese region of Yuan-Tsian, where numerous masses of ul- trabasic, granitoid, and alkaline igneous rocks characterize the Tertiary fault line (Gatinsky and others, 1972). El. Mandalay Line. This line trends north, bifurcating to the Mesozoic north. It was named the Igneous Line of the Central Belt by Chhib- ber (1934), and it forms the main ophiolite belt of the region, in- D2. Lupar Line. This line trends northwest in Sarawak and cluding the famous Jade Mines region. The ultramafic rocks occur west in Kalimantan. The possibility of extension seaward through within a metamorphic assemblage which in the Hpakan-Tawmaw Natuna is based on the presence of serpentinite, diabase, tuff, gab- jade tract and elsewhere includes chlorite schist, actinolite schist, bro, diorite, and norite in the Bunguran beds and of shale, sand- kyanite schist, graphite schist, and glaucophane-garnet-epidote stone, and radiolarian chert. The line is presumed to be Jurassic to schist (Soe-Win, 1968). The jadeite occurs as albite-jadeite dikes in- Cretaceous in age (Haile, 1970). truded into the serpentinite along fissures. The peridotite is partly The eastward continuous extension of the Lupar line into or wholly altered to serpentinite. Magnetite and chromite occur in Kalimantan is presumed because of significant outcrops of the concentrations or in ill-defined veinlets. The ophiolite suite and as- Danau Formation as a narrow belt at least 250 km long, extending sociated metamorphic rocks are unconformably overlain by Ter- from Sarawak through the Indonesian region of Upper Kapuas tiary sedimentary rocks (Chhibber, 1934). (Brondijk, 1964). The characteristic assemblage of the Danau For- The presence of high-pressure metamorphic rocks in abundance mation, like the Sarawak occurrence, is diabase-tuff, diabase, and the imbrication of the ophiolite suite suggest a plate contact of radiolarian chert, jasper, and clay-slate. Most fossils have been the type shown in Figure 2C in the geosuture between the Indian dated as Late Cretaceous in age. In both the Lupar and the and Chinese massifs. Kalimantan occurrences, the characteristic association of marine E2. Naga Hills Line. This north-trending line has been referred radiolarian chert with spilitic lava has led to use of the term to as the Western Igneous Line of Burma by Chhibber (1934). In "Chert-Spilite" association for the assemblage. Ultramafic rocks the Naga Hills the serpentinites have a northeast trend, coinciding are generally absent from this association in Borneo, and it appears with the general strike of the country rocks. The line continues that only the upper parts (basaltic layer and pelagic sedimentary southward toward the Andaman and Nicobar Islands. The ophio- layer) of an ophiolite are exposed. lite masses occur in the complex fractured suture line between the The Lupar line represents southwest subduction of oceanic ma- Indian continent to the west and the Chinese massif to the east and terial beneath the Kalimantan continental plate (Haile, 1973), as should be closely correlated with the Mandalay line. indicated by the parallel arc of acidic and intermediate stocks and E3. Andaman-Nicobar Line. This line trends south, swinging laccoliths on the southwest side of the ophiolite belt. The plate con- southeast. The ophiolite occurs mostly on the eastern side of the tact could have been of the type illustrated in Figure 2A. Andaman and Nicobar Islands (Chhibber, 1934). The chief rock is

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NORTH SOUTH SAKAR ISLAND

SE/3 LEVEL SEA LEVEL

Cretaceous —Eocene ? Chert —spilite Formation

Epidote Almandine Hornblende Greenschist Tectonite fabric] Metamorphic Grade Amphibolite Amphibolite Granulite

SOUTH NORTH SILUMPAT TABAWAN ISLAND ISLAND

SEA LEVEL SEA LEVEL

Cretaceous-Eocene ? Chert-spilite Formation

© 10 Figure 3. Cross sections of complete ophiolite sequences in the Darvel amphibolitized dolente sills; 4 = almandine amphibolite-facies banded Bay-Labuk-Palawan ophiolite belt of North Borneo and Philippines. Sec- hornblende-plagioclase gneiss (metagabbro); 5 = hornblende granulite- tions are based on studies of outcrops on Darvel Bay islands of Tabawan, facies pyroxene-hornblende-plagioclase gneiss (metagabbro); 6 = harz- Silumpat, and Sakar. Upper layer of greenschist-facies amphibolite is over- burgite, peridotite, and dunite with tectonite fabric; 7 = serpentinized lain by spilitic pillow lava. Patterns: 1 = greenschist-facies fine-grained am- ultramafite; 8 = sharp, conformable contact; 9 = gradational metamorphic phibolite (metabasalt); 2 = epidote amphibolite-banded metagabbro; 3 = boundary; 10 = Mohorovicic discontinuity; S.Z. = shear zone.

dark-green serpentinite. Peridotite contains bronzite, olivine, and important part of the Crystalline Basement rocks and has been picotite, and an altered diorite mass contains hornblende, chlorite, given the name "silumpat gneiss," a banded hornblende- sericite, and calcite. The main occurrence of the ophiolite is in plagioclase gneiss of metamorphic grade varying from greenschist North Andaman Island. Middle Andaman Island contains ophio- to hornblende-granulite facies (Hutchison and Dhonau, 1971). The lite ranging from - to - to bronzite-peridotite, com- banded gneiss forms the gabbroic part of the ophiolite, and its posed almost entirely of pyroxene and olivine. There are also some thickness ranges from 1 to 2 km. At its lower contact with the feldspathic types, which range from peridotite to gabbro. Many of mantle harzburgite it consists of hornblende granulite, and at its the ophiolite masses are serpentinized. upper contact with the spilitic metabasalt it is of the greenschist E6. Darvel Bay—Labuk—Palawan Line. This line trends north- facies (Fig. 3). The silumpat gneiss is folded with the overlying west, is strongly arcuate, and apparently continues northeastward metabasalt and spilite and the underlying conformable harzburgite along the Sulu Archipelago and along Palawan (Philippines). The into broad open folds with near-horizontal axes. Two major peri- ophiolite suite contains an extensive record of former oceanic crust ods of deformation have been described (Hutchison and Dhonau, and mantle and has been studied in more detail than any others of 1969), the first associated with amphibolite- to granulite-facies the Southeast Asia region. The islands of Darvel Bay provide ex- metamorphism and the second with greenschist facies, with an in- posures of complete ophiolite sequences (Fig. 3), which rival the tervening period of basaltic injection into the upper parts of the best sequences in the world. gabbro layer. Similar formations occur on the Philippine island of The Mesozoic "Crystalline Basement" rocks of Sabah (North Palawan and in the Sulu Archipelago (Hutchison, 1968). A com- Borneo) consist mainly of basic to intermediate igneous and meta- plete ophiolite sequence from underlying serpentinite through igneous rocks, with rare occurrences of metasedimentary rocks and metabasite to overlying spilite of subhorizontal attitude forms the a few occurrences of granitic rocks. Banded metabasite forms an Sulu Archipelago island of Tawitawi, 80 km east of North Borneo

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(E. F. Taylor, in Corby and others, 1951, p. 311-312). The Uncertain Ophiolite silumpat gneiss is considered to be metamorphosed gabbroic oceanic crust similar to that described from the present oceans Paleozoic (Dewey and Bird, 1971), and radiometric determinations on amphiboles from this formation have given K-Ar dates of 101 ± 15 B2. Segamat-Johore-Singapore-Billiton Line. This line trends and 140 ± 20 m.y. B.P. (Hutchison and Dhonau, 1969). northwest through several isolated mafic and ultramafic occur- The Crystalline Basement rocks are overlain by the extensive rences. Normally it would not be considered an ophiolite or even a Late Cretaceous to Eocene Chert-Spilite Formation, characterized single line, because the occurrences are widely separated. But its by an ophiolite suite of pillow spilite, which in many places is actu- alignment with the Bentong-Raub line, offset by the Kuala Lumpur ally a greenschist-facies metabasalt, associated with chert and argil- fault (Fig. 1) suggests that it could be a southern extension of the lite and thin, fine-grained limestone beds. This formation is consid- Bentong-Raub line only sporadically exposed through the overly- ered to form the basaltic layer of the former oceanic crust together ing Triassic sedimentary cover. The serpentinite xenoliths in the with associated oceanic sedimentary deposits. A similar chert and Cenozoic Segamat basalt were recorded by Grubb (1965). I have spilite association characterizes the Philippine area around the Sulu described the Linden Hill and Senai Estate gabbro and eucrite oc- Sea (Hutchison, 1968). The Crystalline Basement rocks, together currences of south Johore and Pulau Ubin (Hutchison, 1973a), as with the Chert-Spilite Formation, represent oceanic crust present well as the Singapore gabbro and norite (Hutchison, 1964) and its from at least Jurassic to early Tertiary time. detailed relation to the Triassic granite. The Billiton gabbro has The Chert-Spilite Formation is overlain by extensive Tertiary been described by Aleva (1960). The country rocks in which these rocks, which have all the characteristics of tectonic mélange and gabbroic rocks were emplaced are unrecognized, because the only olistostrome deposits. contact exposures are with the younger granite. The gabbro of The arcuate outcrop of these rocks is characterized by large Singapore is a composite body, grading from gabbro to norite, in- bodies of serpentinite, harzburgite, pyroxenite, gabbro, and dunite; cluding numerous gabbro sills with chilled margins. The Triassic the latter is commonly chromite bearing (Hutchison, 1972). Many granite has caused a significant replacement of the pyroxene by ultramafic masses are fault bounded, but some are conformable hornblende and biotite. with metabasite in the Crystalline Basement, in which case relict B3. Lampang—Houei-Sai Line. This line trends north, swinging banding and chromite layering in the ultramafic rocks are usually east-northeast northward. Over large areas of a north-trending belt parallel to the folded banding of the metabasite. Some of the harz- lying west of Chiang Rai and east of Chiang Mai, generally due burgite masses are conformable with and in mineralogie equilib- north of Lampang, the lower part of the late Carboniferous succes- rium with the amphibolite- and granulite-facies lower horizons of sion is represented by rocks that Baum and others (1970) classified the silumpat gneiss banded metabasite. On Tabawan and Sakar Is- as as a field term only. These rocks are commonly as- lands, the outcropping conformable transition contact with the sociated with chert. In the gorge of the Mae Kok River, northeast banded silumpat gneiss represents the Mohorovicic discontinuity of Chiang Rai, the rocks are dynamothermally converted to am- between oceanic mantle and overlying gabbroic crust (Fig. 3). phibolite. This is a volcanic and subvolcanic sequence of predom- It is difficult to date the actual ophiolite belt because of the ex- inantly intermediate to basic rocks, and there is no proof that the tensive history of ophiolitic events in this region. The ophiolitic term ophiolite, in present-day usage, is warranted. Fromaget and suite includes many rock members and deformations that date back others (1971) postulated an extension of these rocks into western into Mesozoic time, at least as early as Jurassic time. The overlying Laos, where they are shown associated with Permian to Triassic mélange and olistostrome rocks, which have been mapped as the sedimentary rocks. Ayer Formation and the Kuamut Formation, are generally of Miocene age. The actual emplacement age of the ophiolite belt may Late Paleozoic to Early Mesozoic therefore be taken as Miocene, with possible beginnings in Late Cretaceous or Eocene time. CI. Uttaradit-Luang Prabang-Pak Lay Line. This is a multiple, The extensive outcrops of ophiolite, its common association with northeast-trending line (Fig. 1). In Laos these uncertain ophiolites chert, and its wide age range suggest that this is more complex than have been shown associated with Permian to Triassic sedimentary an ophiolite belt that would have characterized a former trench. In rocks by Fromaget and others (1971), whereas Baum and others my opinion, the ophiolite mass of Sabah and the region of the Sulu (1970) indicated that in northern Thailand they occur in associa- Sea is more like a large flake, as defined by Oxburgh (1972), of tion with Carboniferous rocks but are generally surrounded and oceanic lithosphere that has been thrust over older continental overlain by Late Triassic to Cretaceous continental deposits in the rocks of Sabah. The old continental basement is not exposed or has region east of Nan. remained unrecognized, although the Crystalline Basement rocks Gabbro and dolerite are abundant in western Laos. Around Pak may include oceanic lithosphere and some older crust. A parallel Lay they are essentially composed of labradorite and augite, but andesitic volcanic arc to the south of Darvel Bay on Semporna some are rich in hornblende (Fromaget, 1941). Northeast of Ut- Peninsula (Kirk, 1968) of Pliocene age and the submarine structure taradit, diorite, gabbro, and partly serpentinized pyroxenite form of the Sulu Sea (Murauchi and others, 1973) indicate that the Sulu dikes, small plugs, and stocks in limestone and sandstone in the Archipelago and Darvel Bay part of the Sabah ophiolite resulted neighborhood of the Nan River. In the valley of the Mae Khong from southward subduction of the Sulu Sea oceanic or marginal River in the Loei area, gabbro is associated with diorite and gran- basin crust. The plate contact for this region is considered to be of ite. They were formerly thought to be Triassic in age, but Klompe the type portrayed in Figure 2A. Elsewhere, however, the pattern (1962) considered them to be older, and a Carboniferous age is must change. The westward convexity of the ophiolite outcrop now thought to be more likely (Baum and others, 1970). from Darvel Bay to Labuk Valley would support the hypothesis of C3. Siem Reap—Stung Treng Line. Gabbro, peridotite, and dol- an integral flake dipping eastward toward the oceanic Sulu Sea, erite trend roughly east and occur in a large elongate mass in Per- with a contact more akin to that shown in Figure 2B. The ophiolite mian to Triassic sedimentary rocks. There are also many scattered sheet cannot be of simple geometry, for individual outcrops indi- small masses. The gabbro may be rich in or devoid of olivine. Both cate broad open folds, and the dips may be to the north or to the orthopyroxene and clinopyroxene occur. Complete replacement of south (Fig. 3). the pyroxene by amphibole is common. Pure diallage rock occurs

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south of Treng. Near Pursat and Treng, dunite containing some rich in calcite, chlorite, serpentine, albite, opal, and quartz (Saurin, diopside occurs (Fromaget, 1941; Saurin, 1956). The dolerite often 1956). contains interstitial quartz and sodic amphibole. Amphibolitized labradorite andesite occurs in the Dalat Series. Tertiary

Mesozoic E4. Nias. The line of islands lying west of Sumatra has long been considered an ophiolite belt (Bemmelen, 1949). However, Dl. Serabang Line. This occurrence shows little internal extent; these rocks are in fact minor dikes of amphibolite, serpentinized therefore, any trend is obscure. The line was first named by Haile basic rocks, diabase, basalt, and andesite (Bemmelen, 1949). They (1973). It may extend seaward through Natuna (see D2 below) or are intrusive, with chilled margins, into pre-Miocene sandstone and through the Anambas Islands, where gabbro, andesite, gabbro- conglomerate (Bemmelen, 1949). The dikes occur on Sipurna, porphyry, and diabase occur. The basic igneous rocks are consid- Nias, and Bangkaru Islands. It is important to establish that these ered to predate the granite, which is the main rock type of the is- islands do not contain ophiolite; in the model they land. The age of the basic rocks on Anambas Island is, however, are classified as an accretionary prism of sediments forming the unknown (Haile, 1970; Bemmelen, 1949). The Serabang line has trench-slope break of the outer nonvolcanic arc. These islands offer no eastward extension into Borneo and occurs only within the accessible outcrops of the outer arc of a currently active oceanic- Serabang Formation near the coast. continental consuming margin. The available literature (Bemmelen, D4. Gumai, Garba Mountains, and Atjeh Line. The trend is as- 1949) indicates that the islands are composed of coarse-grained sumed to be northwest parallel to Sumatra. Only for the Gumai sandstone and conglomerate with no ophiolite masses, but that and Garba Mountains does Bemmelen's (1949) description suggest numerous basic to intermediate dikes cut the sedimentary deposits. that this may be an ophiolite. The rocks in question are referred to There was, therefore, some igneous activity. as "basaltic rocks" occurring within bathyal-facies sedimentary rocks called the Lingsing Beds in the Gumai Mountains and the CONCLUSIONS Upper Barba Beds in the Garba Mountains and at Ratai Bay. De- scriptions of the "laccolithic intrusions of basaltic rocks" were not Of the 20 belts of the Sundaland region which have previously given. Bemmelen also said, without giving any details, that similar been referred to as ophiolite, fewer than half appear to be ophiolite rocks occur at Atjeh. by the strict definition. They indicate former plate contacts. The only complete ophiolite, which fills the requirements of the Tertiary present-day definition, is that of North Borneo and the neighboring Philippine archipelagoes. All others are dismembered or incom- E5. Tjiletuh Bay-Loh Ulo Line. Of the isolated occurrences, the plete. The literature indicates that several mafic-ultramafic assem- main one is at Tjiletuh Bay, where metabasic and ultrabasic rocks blages could be established as ophiolite, but more field study is re- occur within chlorite schist and phyllite, unconformably overlain quired. by Eocene arenite (Bemmelen, 1949). Most of the ophiolite belts of the region probably indicate E7. Bukit Mersing Line. This line trends east, through small, former simple subducting plate junctures of the type shown in Fig- isolated occurrences. The possible ophiolite masses are steeply dip- ure 2A. The Darvel Bay—Labuk—Palawan line (E6) may have re- ping massive Paleocene to Eocene spilite and basaltic rocks, basal- sulted from a reversal of subduction direction following collision tic tuff, epidote basalt tuff, and pillow basalt within the Belaga with an island arc, so that a complete and integral ophiolite sheet Formation (Kirk, 1968). has been thrust upward, as shown in Figure 2B. The lines in Tonkin (B4, C2) and in Burma (El, E2) are thought to be of the suture Nonophiolite type, as portrayed in Figure 2C. Correlation of ophiolite belts becomes increasingly difficult the The following rock suites have in the past been classified as older they are. Figure 1 may serve to place some constraint on re- ophiolite, but their petrology and tectonic settings seem to disquali- constructions of the region. For example, Ben Avraham and Uyeda fy them from the currently accepted usage of the term. (1973) have proposed that Borneo formerly occupied a position adjacent to the Kontum Massif and Hainan, requiring that the Precambrian Song Ma and Black River lines (unknown to them) continue with the Serabang and Lupar lines, respectively. This is clearly impossi- Al. Kontum Massif. Uralitized orthopyroxenite occurs sporad- ble because of the different nature and ages of these ophiolite lines. ically. It has been described as quartzitic amphibolitic gabbro with large hornblende crystals (Fromaget, 1941), and as orthopyroxe- ACKNOWLEDGMENTS nite, parapyroxenite, and para-amphibolite (Saurin, 1956). De- scriptions of this region are insufficient for drawing definite geolog- I thank N. S. Haile and P. H. Stauffer for useful discussion, H. ical conclusions, but the general picture obtained from Saurin Craig for suggesting publication of this paper, and M. C. Blake, Jr., (1956) is that what has been called ophiolite in the past are lenses for helpful criticism. and intercalated intrusions of pyroxenite and amphibolite inter- foliated with sillimanite, garnet, and graphite schists, marble, and REFERENCES CITED gneissic granite. Aleva, G.J.J., 1960, The plutonic igneous rocks from Billiton, Indonesia: Paleozoic Geologie en Mijnbouw, v. 39, p. 427-436. Allegre, C. J., 1973, Géochimie des ophiolithes et des roches océanique, re- lations et mode de formation: Réunion Ann. Sci. 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