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Ophiolite in Southeast Asia

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Ophiolite in Southeast Asia Ophiolite 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 mafic-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 harzburgite through identification (or rejection) of these rock associations as ophiolite is gabbro to spilite, occurs in northeast Borneo 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 lithosphere, with its thin oceanic crust 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 continental crust 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 subduction 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 suture 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 Japan, greatest part of which has been consumed. The various igneous and California, and the Alps (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 radiometric dating 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 Cyprus (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 "obduction" 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. 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 OPHIOLITE IN SOUTHEAST ASIA 799 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 magma 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 greenschist, almandine- island arcs. Mélange and associated ophiolitic shreds of the sub- amphibolite and hornblende-granulite facies, and serpentinite 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- metasomatism 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, basalt, spreading ocean crust may carry with it a continental plate, plate and metabasalt) with spilite and chert (the "Steinmann trinity" of fragment, or island arc 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
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