origin and tectonic significance of the metamorphic rocks ...searg.rhul.ac.uk/pubs/omang_barber_1996...

Origin and tectonic significance of the metamorphic rocks associated with the Darvel Bay Ophiolite, Sabah, Malaysia

SHARIFF A. K. O M A N G 1 & A. J. B A R B E R 2

1 Department of Earth Sciences, Faculty of Science and Natural Resources,

Universiti Kebangsaan Malaysia, Sabah Campus, Locked Bag No 62,

88996 Kota Kinabalu, Sabah, Malaysia

2 SE Asia Research Group, Department of Geology, Royal Holloway,

University of London, Egham TW20 OEX, UK

Abstract: Banded hornblende gneiss, foliated amphibolite, hornblende, chlorite and siliceous schist form lenses in an 8 km wide belt within the Darvel Bay Ophiolite Complex. Foliation in the belt is generally steep to vertical, striking parallel to the trend of the belt and lineations are sub-horizontal. Mineral and geochemical studies show that the metamorphic rocks represent banded and isotropic gabbros, plagiogranites, doleritic and basaltic dykes, basaltic volcanics and cherts formed at a spreading ridge in a supra-subduction zone environment, which were deformed at high temperatures but low pressures along a transform fault. Incorporation of supracrustal cherts indicates that the transform extended for hundreds of kilometres between spreading centres. Garnet pyroxenites and amphibolites found as clasts in Miocene volcanic agglomerates formed at high pressures, and temperatures are interpreted as derived from a metamorphic sole underlying the complex, formed during subduction of ocean crust and the emplacement of the ophiolite complex on Sabah.

Ophiolite outcrops are distributed throughout the eastern part of Sabah, East Malaysia, from Banggi Island in the north to Ranau and Telupid and the Lahad Datu area in the south (Fig.l, inset). The Darvel Bay Ophiolite Complex in the south is the most extensive outcrop, extending 100 km westwards from Darvel Bay. The greater part of the complex consists of peridotite, largely serpen- tinized, but it also includes cumulate pyroxenites, layered and massive gabbros and diorites, metamorphosed to varying degrees. The complex is also cut by dolerite dykes, although these are never so abundant to be termed a sheeted dyke complex. These rock types are closely associated with outcrops of the Chert-Spilite Formation, composed of pillow basalt, banded ribbon chert, turbiditic sandstones, mainly volcaniclastic but with some rare quartz sandstones, and a few occurrences of massive limestone.

The ophiolite complex has been well described in the publications of the Geological Survey of Malaysia (Reinhard & Wenk 1955; Fitch 1955; Dhonau & Hutchison 1966; Koopmans 1967). The complex has been interpreted as a segment of ocean floor, either of a Proto-South China Sea (Holloway 1981; Rangin et al. 1990) or of the Celebes Sea (Hutchison 1988). A wide range of K-Ar age dates has been obtained from the rocks of the ophiolite complex from 210Ma (Leong 1971) to 137 Ma

(Rangin et al. 1990). Cherts from the Chert-Spilite Formation have yielded radiolaria of Lower Cretaceous age (Leong 1977; Rangin et al. 1990; Aitchison 1994). Massive limestones associated with the Chert-Spilite Formation contain Cretaceous foraminifera (Leong 1974). The Chert- Spilite Formation is interpreted as representing ocean floor sediments which were deposited on top of the ophiolite (Hutchison 1975), and carbonate cappings to seamounts. Since the underlying oceanic crust is unlikely to be much older than the oldest overlying sediments, the Early Jurassic K-Ar ages are regarded as spurious (Hutchison 1988).

Fragments of ophiolitic rocks are found as clasts in Eocene sediments (Newton-Smith 1967; Rangin et al. 1990), suggesting that the ophiolite complex had been obducted onto Sabah either in the latest Cretaceous or earliest Palaeogene. In the early Miocene the ophiolite complex formed the basement to a volcanic arc, possibly related to continued subduction of the Proto-South China Sea. Mitchell et al. (1986) and Rangin (1989) have suggested that, as a result of continued compression, the complex was backthrust over the Celebes Sea floor to the south.

The ophiolite complex is surrounded by chaotic melange deposits (Fig. 1) which contain fragments of all the rock units represented in the complex and the Chert-Spilite Formation, as well as

From Hall, R. & Blundell, D. (eds), 1996, Tectonic Evolution of Southeast Asia, Geological Society Special Publication No. 106, pp. 263-279.

263

2014 at Royal Holloway, University of London on March 25,http://sp.lyellcollection.org/Downloaded from

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264 s .A.K. OMANG (~ A. J. BARBER

0 5 10 r' Km Ophio l i te C o m p l e x

E ~ Felsite ~ RecentAIluvium

~ ~'~] Metamorphic rocks E ~ Chert-Spilite • " '* '° "~ "* "* "* "* "° "° "° ". "° " . . . - , ~ '. ". "° '. ". ". "° ". Formation ~ ' . ' ~ * * , ' i**: (foliated metabasites )

~ ~ : : * : i * " []'EI Sheared Serpentinite D Neogene , ,, ,, , , , ,, ,, -° "4 "° . "° "° .o .° .° . . . . . . sediments , ' , ' , ' , ', ', ', "°,:*. :*.:*.:°.:*,:*,:°,:*,:*, ~ Gabbros & Cumulates [ ~ M6langes

"°"°' ' °"°' l Peridotite ~ Thrust fault (mainly serpentinised) - - " Fault

Gabbro

:!:!:!:!:i: l : 8" 20' E. ::::::::::::::::::::::::::::::::::::::::::::::::::::: ~!~:i:ii!i:i:i:!...i.i~.:i:i:i!!iiii!i!iiiiiiiiiiiiiiii!i!iiiii!i!ii!iii!::ii!i!~ii ~~:i.i:i:i:i!ii!i!i!iii!iiiiii!iiiiiiii!!!iii!!!iliiiiiiiiiii =============================================================== *Dolerite dyke PSI2 D~:: PS6

! I ! r l ~ l ! ! ! ! |

~1 i i : • ~. l . l l . l ~ . l ~ . l l . 1.1~.13.11.1~.1~ I ~ . ~ ~

i i i i i i i i - - . ~ . , 1 - . - ~ p , , .z .~ p

, ~ ~ P T 3 Pulau "~;K4

~ , ' %, 9 Katun ruiau • ~l g- / Saddle/ % Kalungan -.L

-.L Cumulate D C rocks

Darvel

SARAWAK 1 " ~

_ / ' f / ~ ,

~':*'i*' 118" 10' E.

I r Pulau Sakar tl

Bay E t

7 4* 58'N Pulau

~ , Bohayan Pulau Tabauwan

P ~ f

V~Silumpat /

E --/- 118" 20' E Dolerite dyke I

Fig. 1. Geological map of the Darvel Bay Ophiolite Complex near Lahad Datu showing location of samples used in this study. Inset map shows distribution of ophiolite complexes in Sabah. B, Banggi Island; R, Ranau; T, Telupid; DB, Darvel Bay.

younger sediments and volcanic rocks. Clennell (1991) considered that the melanges were formed by processes o f s ed imen ta ry s lumping and diapirism, coincident with the collision of micro- continental blocks with the north Sabah margin and the extension of the arc basement related to the development of the Sulu Sea in late early and early mid Miocene times. The ophiolite complex and the melanges are overlain unconformably by Late Miocene to Pliocene sediments of the 'circular basins' and in the Quaternary again fo rmed the basement to a volcanic arc which extends f rom Mindanao in the Philippines, through the Sulu archipelago to the Dent and Semporna peninsulas of Sabah.

M e t a m o r p h i c r o c k s

A l though the rocks fo rming the Darvel Bay Ophioli te Complex are variably metamorphosed throughout , dynamica l ly me tamorphosed rocks are concentrated in an E - W belt c. 8 km wide extending westwards f rom Lahad Datu on the north side of Darvel Bay (Fig. l ) which can be traced for c. 40 km to the west. Metamorphic rocks are well exposed in coastal sections around the shores of Darvel Bay and part icularly around the small islands within the bay, where they have been descr ibed previously by Dhonau & Hutchison (1966) and Hutchison & Dhonau (1969, 1971) f rom the north side of the bay, and by Koopmans (1967)




DARVEL BAY OPHIOLITE, SABAH 265

from the south. Inland exposures are poor, apart from road-cuts, quarries and scarce river sections.

The metamorphic rocks have been described as metagabbros, amphibolites, hornblende gneisses and schists. Reinhard & Wenk (1955) described the dynamically metamorphosed rocks as 'Crystalline Schists' and Koopmans (1967) identified them as the 'Crystalline Basement Complex', with the implication that they were the oldest rock unit in Sabah. However, K-Ar dating reported by Dhonau & Hutchison (1966) showed that these metamorphic rocks are of Cretaceous age, the same age as the Darvel Bay Ophiolite Complex, and form an intrinsic part of the complex

The structure of the dynamically metamorphosed belt has been fully described by Dhonau & Hutchison (1966). The metamorphic rocks are typically banded, schistose and lineated. Through most of the belt the banding and schistosity are vertical or steeply dipping and strike E-W, parallel to the general trend of the belt, swinging round to a more NW-SE trend in the islands of Darvel Bay. The lineation is sub-horizontal, lying in the plane of the schistosity and plunges at low angles either to E or W. From their mapping of the structure in the metamorphic rocks, Dhonau & Hutchison (1966) identified two large-scale open post- metamorphic monoclinal flexures on E-W axes affecting the foliation in the islands of Pulau Saddle, Pulau Bohayan and Pulau Silumpat.

This paper gives an account of the field relations, petrography, mineral chemistry, geochemistry and K-At isotopic dating of the metamorphic rocks associated with the Darvel Bay Ophiolite Complex and discusses their origins and significance in the tectonic evolution of Sabah. A separate occurrence of metamorphic rock is found among Miocene volcanics at Tungku, 50 km to the east of Lahad Datu (Reinhard & Wenk 1955). Garnet amphibolite from this locality was also included in the present study.

Banded hornblende gneiss

The dominant metamorphic rock type in the metamorphic belt within the Darvel Bay Ophiolite Complex is a banded gneiss, the 'Silumpat Gneiss' of Dhonau & Hutchison (1966) and Hutchison & Dhonau (1971), with alternating bands of white feldspathic and black or dark greenish ferromagnesian bands, well exposed on the north shore of Darvel Bay, west of Lahad Datu and in road cuts along the Silam Road. The bands range in thick- ness from tens of centimetres to millimetres. The banding is paralleled by a schistosity, with the alignment of feldspathic and ferromagnesian aggregates which may also be elongated to form a rodding type of lineation. The intensity of the

schistosity increases in localized shear zones. The banding is commonly folded into tight intrafolial isoclinal folds which may show the closed eyed outcrop pattern characteristic of sheath folds. Where folds are present the schistosity is parallel to the axial planes of the folds. In the more intensely deformed shear zones aligned hornblende and feldspar crystals define a mineral lineation, lying in the plane of the schistosity.

Amphibolite

Amphibolites are commonly found associated with the banded gneisses in the field. These may form extensive outcrops, as on the northern side of Sakar Island or as smaller bodies which may show cross- cutting, intrusive relationships to the hornblende gneisses. The grain size of these amphibolitic rocks is variable. Coarse grained amphibolites show the characteristic distribution of white feldspathic and black or green ferromagnesian minerals which indicate that they have been derived from gabbros (metagabbro). Medium and finer grained amphibolites may show relict ophitic textures in thin section and large euhedral plagioclase pheno- crysts or polycrystalline aggregates pseudo- morphing plagioclase which indicate an origin as hypabyssal intrusive rocks (metadolerite). Fine grained amphibolites without these features may represent basaltic volcanic rocks (metabasalts).

Like the banded gneisses the amphibolites show varying degrees of deformation. Where they cross- cut the banding in the gneisses, the amphibolites generally show a less intensely developed schistosity than the associated gneisses. In coarse grained amphibolites the gabbroic texture may be flattened in the foliation, in finer grained rocks minerals may be recrystallized in the schistosity to form hornblende schists. Where the coarser amphibolites cross-cut the banding of the gneisses, no clear chilled margins are evident and the schistosity crosses the contact between the gneisses into the amphibolites, although it is generally not as intensively developed in the latter. Finer grained amphibolites often have a dyke-like form, ranging in width from a few metres to tens of centimetres and show varying angular relationships with the banding of the gneiss. Sometimes they are concordant, and then generally have a well developed schistosity with a similar intensity to that in the adjacent gneisses. Where amphibolite dykes cut the banding at a high angle, the schistosity is generally less well developed than in the surrounding gneiss. Evidently some of the dykes were intruded into already deformed gabbros, but deformation continued after dyke emplacement, indicating that the dykes were intruded into a zone of active deformation.




266 s . A . K . OMANG t~ A. J. BARBER

Mineralogy In the less highly deformed and altered hornblende gneisses and metagabbros the ferromagnesian component includes relict crystals of pale green or colourless pyroxene. These may form the cores to brownish-green amphibole crystals and show alteration along their margins and along internal cracks to aggregates of small blue-green amphibole crystals. Microprobe analysis shows that they are clinopyroxenes of salite to augite composition (En40.5 Fst3 Wo47- En45.5 Fs]2.7 Wo28), with Xrvlg values of 90-97 and an Na20-content of 0.5 wt%.

Plagioclase crystals have a grain size up to 4 ram, and form tabular crystals with albite

twinning. Microprobe analysis shows that where plagioclase is associated with pyroxene it has a composition of An68-An74 (labradorite to bytownite) representing an igneous relic, but more commonly is An 6 -An32 (oligoclase to andesine) of metamorphic origin (Hutchison 1978). In the amphibolites, and especially in highly deformed rocks, the plagioclase is An 3 - A n 5 (albite). In many samples of hornblende gneiss and amphibolite the plagioclase has been completely pseudo- morphed by low grade alteration products (saussurite).

Epidote is a common constituent of the gneisses and amphibolites as an alteration product of plagioclase feldspars, but in occurrences at the western

II

0 (a)

Glaucophane

2.0

1.8

1.6

Na[M4] 1.4

1.2

Winchite a.0

0.8

Tschermakite Pargasite

Tremotite

Edenite Hornblende

• Metagabbro d- Metadolerite

I 0.5 1.0

1

]4)Si(IV) = Ca(M4)Ai(IV) t Sodic-am ~hibole

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I I

Barroisite aramite Sodic-Calcic amphibole . . . .

Calcic-amphibole 0.2 ~ + Metadolerite I

0.0 0 2

Actinolite Hornblende Tschermakite AI [ IV]

(b)

Fig. 2. (a) Composition of amphiboles in metagabbro and metadolerite from Darvel Bay on diagram of Deer et al. (1966). (b) AI[IV] versus Na[M4] plot of amphibole compositions in metagabbro and metadolerite from Darvel Bay, isobars after Brown (1977).





end of Pulau Sakar and in Pulau Katung Kalungan there are amphibolites in which epidote is a major component of the rock. These rocks are distin- guished in the field by their yellowish colour where they occur as bands among more normal amphibolites. In thin section they are foliated, with large crystals of epidote, colourless or pale pink in a matrix of blue-green amphibole and plagioclase. The plagioclase is albite or is represented by an aggregate of sericite and calcite. Microprobe analysis of the epidote crystals show that it is an iron-rich variety (Xps27_33). The Fe 3+ content is very high (1 .5- 2.0 atoms per unit cell) and the XFe 3+ (1.0) gives an estimated temperature of formation of c. 400°C (Nakajima et al. 1977).

Amphiboles in the amphibolites occur either as large brownish-green or as small blue-green crystals which may be randomly oriented or aligned to form foliar and linear structures. The larger amphibole crystals are 2-4 mm in size and are greenish-brown in colour. Some of the brown hornblende has been interpreted by Hutchison (1978) on textural and mineral chemical grounds as of relict igneous origin. However, where they enclose pyroxene relics, or where they are aligned in the foliation and lineation, hornblende crystals are clearly of metamorphic origin. Electron microprobe analyses plotted on the (Na+ K)/A1TM diagram of Deer et al. (1966) show that the amphibole composition lies between the hornblende and pargasitic hornblende fields (Fig. 2a) (cf. Hutchison 1978, fig. 6). On the AlIV/Na (M4) plot of Brown (1977) compositions extend from the calcic amphibole towards the sodic-calcic amphibole field, corresponding to pressures of about 5 kbar (Fig.2b). On the AllY/A1 vI plot of Fleet & Barnett (1978) the amphiboles lie in the low pressure field (Fig. 3a) and on the Na + K/A1TM plot of Jamieson (1981) follow the high temperature/low pressure trend (Fig. 3b). The Ti content of the amphiboles (0.04-0.3 atoms per formula unit) indicates that crystallization of the amphiboles took place in the temperature range 550-650°C under low pressure conditions in the lower amphibolite facies (Spear 1981). Using the semi-empirical geothermometer proposed by Plyusnina (1982) for plagioclase/ hornblende pairs, core compositions of crystals from the amphibolites give temperatures of 580-600°C and pressures of 3-4 kbars for R T conditions at the time of crystallization.

Small blue-green amphibole crystals commonly occur in the banded gneisses and amphibolites marginal to pyroxene or brown hornblende crystals. In the study by Hutchison (1978), microprobe analyses of blue-green amphiboles from the Darvel Bay Ophiolite Complex showed that their composition lay mainly in the actinolitic hornblende field with some actinolite. In highly deformed

hornblende schists these actinolitic hornblendes form a major component of the rock and are aligned to form the schistosity and lineation.

Metamorphic histo:y

As far as may be determined the primary minera- logical composition of the banded and isotropic gabbros, now found as banded hornblende gneiss and amphibolite, was clinopyroxene, hornblende (cf. Hutchison 1978), labradorite/bytownite and ilmenite. Olivine and orthopyroxene may have been present in some rocks, as olivine and noritic gabbros occur elsewhere in the Darvel Bay Ophiolite Complex (Omang 1993), but all evidence of their previous existence has been destroyed. The gabbros were recrystallized in a high temperature, low pressure metamorphic environment where pyroxene was replaced by brownish-green hornblende and plagioclase by oligoclase/andesine. Alignment of brown hornblende and plagioclase crystals with foliar and linear structures indicate that recrystallization took place in a high temperature dynamic environment; continued dynamic metamorphism under lower temperature amphibolite facies conditions are indicated where green amphiboles are aligned with the schistosity and lineation. Later recrystallisation under hydrothermal conditions at lower temperatures resulted in the local replacement of plagioclase by saussurite and the alteration of pyroxene and hornblende to chlorite.

Geochemistry

Whole rock major element and trace element analyses of hornblende gneisses and amphibolites are shown in Table 1. On an AFM plot (Fig. 4) these metabasites lie in the tholeiite and oceanic gabbro (MORB) field of Kirst (1976). AlzO3]TiO2 ratios also fall in the MORB field (Fig. 5) of Sun & Nesbitt (1978). Trace element analyses show that high field strength (HFSE) elements (e.g. Zr, Y) have a high concentration relative to large ion lithophile (LILE) elements. The Cr-Y plot (Pearce et al. 1984b) and Ti-Zr-Y diagram (Pearce & Cann 1973) also suggest a MORB-like character.

On the basis of spider diagrams the samples can be divided into three groups. The metadolerite dykes (Fig. 7a), are a suite of basaltic rocks with moderate LILE enrichment but are with a clear negative Nb anomaly relative to the light rare earth elements (LREE). One group of metagabbros (Fig. 7b) is very similar to the metadolerites, but with more primitive compositions, indicated by high Cr and Ni, and with much lower contents of




268 s.A.K. OMANG (~ A. J. BARBER

2

AI[IV] 1

Pargasite Tschermakite

~-'l'/ High-pressure ~i~' l~ / CT"amphib°lez°ne

E d ~ . ............. Homilende ........................................

Low-pressure

Actinolite ole zone 0 Glaucophane

0 2

(a) AI[Vl] Glaucophane

2.01.5 1 High-pressuretrend

Na+K 1 / Eckermakite 1 Ede ite Pargasite

i + . = [ High-temperature d

0.0 Tremolite/A~tinolit? ~ , Tschermakite

0.0 0.5 1.0 1.5 2.0

(b) Ai[IV] Fig. 3. (a) AI[VI] v. AI[IV], and (b) AI[IV] v. (Na + K) of amphibole compositions in metagabbro and metadolerite from Darvel Bay; boundary between low and high pressure calcic amphiboles in (a) is from Fleet & Barnett (1978); high and low pressure trends in (b) from Jamieson (1981).

K, Rb and Ba. The other group of metagabbros (Fig. 7c) includes samples which have very spiky profiles. They are extremely depleted in most incompatible elements, particularly Nb, LREE, P and Zr; K and Rb are conspicuously low, and only Ba and Sr are at MORB levels in some samples.

In view of their textures and the field relationships, the metabasic rocks are interpreted as part of a single ophiolite suite, with geochemical data indicating MORB-like compositions, but Nb deficient and show a mild LILE enrichment. The metadolerites represent an original liquid, with a chemistry suggesting a supra-subduction zone setting for their environment of formation. The group of metagabbros which resemble the metadolerites on spider diagrams are suggested to be former isotropic gabbros, while the other group of metagabbros probably represent cumulate rocks which have lost an evolved inter-cumulus liquid fraction enriched in incompatible elements.

Felsic rocks

The hornblende gneisses and amphibolites are frequently cut by felsic veins on the outcrop scale. These veins may be concordant, parallel to the foliation and schistosity and are then boudinaged, or are cross-cutting. Cross-cutting veins may be straight or folded in a ptygmatic style, and undeformed veins may cut earlier folded veins. Where the veins are folded, the schistosity in the surrounding rocks has an axial plane relationship to the folds. These relationships provide clear evidence of multiple intrusion of felsic rocks into a zone of active deformation.

At several localities within the metamorphic belt larger bodies of felsic rock are found. On the north shore of Pulau Sakar a felsic body, identified as trondhjemite (Specimen PS12c) has been intruded, with an irregular contact and without a chilled margin, into metadolerites. A weak schistosity is




Table 1. Whole rock geochemical analyses for metagabbros and metadolerites of the Darvel Bay Metamorphic Complex

Metagabbro Metadolerite

Sample Element JS4a JS L JSp KS2 PS2 PS 3a PS6 PS 12d PS 11 PS 11 b PS 12a PS 12b PS 13d PT3a

SiO 2 44.22 48.07 47.55 49.73 46.42 48.82 50.11 48.52 51.42 51.24 49.28 48.82 51.43 51.42 TiO 2 1.70 1.63 1.55 0.62 0.97 0.23 0.57 1.28 1.61 1.64 1.34 1.63 1.61 1.55 A1203 15.86 15.25 15.62 14.17 17.87 19.99 15.49 15.05 14.87 14.30 15.64 15.40 14.67 15.32 Fe203* 12.38 11.70 11.55 8.58 9.83 5.58 6.57 10.60 12.10 12.07 10.96 11.37 11.27 13.94 MgO 9.13 8.60 8.59 10.53 8.83 8.50 9.96 6.52 6.44 6.86 8.45 9.29 7.22 4.64 MnO 0.22 0.18 0.19 0.15 0.18 0.14 0.13 0.18 0.20 0.19 0.17 0.19 0.20 0.23 CaO 15.01 11.40 11.95 13.41 13.52 14.28 14.85 14.74 8.45 9.00 10.22 9.52 8.18 8.18 Na20 0.99 2.94 2.75 2.38 2.01 1.96 2.07 2.26 4.46 3.84 3.58 3.12 4.62 4.10 K20 0.03 0.06 0.04 0.25 0.12 0.12 0.03 0.06 0.28 0.26 0.18 0.15 0.19 0.20 P205 0.13 0.15 0.13 0.02 0.02 0.01 0.02 0.12 0.16 0.15 0.17 0.14 0.15 0.21 Total 99.67 99.98 99.92 99.84 99.78 99.63 99.79 99.33 99.98 99.54 100.0 99.62 99.53 99.79 LOI 3.44 1.83 1.80 1.39 2.56 2.73 1.59 1.31 1.47 1.87 1.51 2.02 1.23 2.60

A1203/TiO2 9.33 9.36 10.08 22.86 18.42 86.91 27.18 11.76 9.24 8.72 11.67 9.45 9.11 9.88 CaO/TiO 2 8.83 6.99 7.71 21.63 13.94 62.09 26.05 11.52 5.25 5.49 7.63 5.84 5.08 5.28 mg c. 59 c. 59 c. 59 c. 71 c. 64 c. 75 c. 75 c. 55 c. 52 c. 53 c. 61 c. 62 c. 56 c. 40

Ni 115 71 105 127 206 159 128 103 52 45 113 136 92 11 Cr 338 364 368 487 413 243 886 269 80 74 282 296 273 5 V 355 336 331 228 392 120 214 303 354 345 308 314 295 386 Sc 56 57 53 60 69 41.6 63 41 49 49 43 43 46 35 Pb 0.7 n.d. n.d. n.d. 0.6 n.d. 0.3 0.6 1.1 0.4 1.3 n.d. 0.3 1.2 Sr 155.6 160.9 168.6 130.0 333.1 493.2 103.1 283.6 105.5 170.6 196.8 126.4 109.0 158.6 Rb 0.6 0.4 0.3 4.9 0.6 0.9 0.2 0.4 2.9 4.1 1.8 1.5 2.0 1.9 Ba 10 1 1 8 36 45 3 8 14 32 22 24 26 20 Th n.d. n.d. n.d. n.d. 0.2 n.d. n.d. 0.4 0.6 0.7 1.7 0.3 0.3 0.8 Zr 77.9 84.5 82.7 25.0 29.1 6.7 21.9 78.8 104.0 105.3 95.0 110.4 107.7 67.6 Nb 1.4 1.2 1.9 0.4 0.8 0.1 0.4 1.0 2.0 1.8 1.7 1.7 1.7 0.9 Y 38.8 34.6 34.7 15.6 19.3 8.8 16.7 27.4 35.9 36.0 29.6 32 35.1 35.1 La 2 2 2 0.3 1 1 1 2 3 2 7 3 2 1 Ce 10 9 11 0 4 1 2 9 12 6 20 10 11 5 Nd 12 10 12 2 6 4 3 7 10 8 13 10 11 8 Cu 60 100 64 5 8 32 60 17 35 64 46 18 83 43 Zn 142 97 158 53 90 42 48 69 102 93 99 98 99 121 C1 n.d. 219 161 359 33 n.d. 614 104 n.d. 44 121 n.d. 33 88 Ga 17 17 17 12 14 14 13 18 16 15 17 16 13 19

Oxides as wt%, trace elements in ppm. Data presented on a volatile-free basis; mg =Mg[Mg +Fe2+], Mg =MgO/40; Fe203 ×0.9/72; total iron as Fe203 t,~ (Fe203* = Fe203 + FeOxI.III); n.d., below detection limit; LOI, loss on ignition at 1100°C.




270 s.A.K. OMANG • A. J. BARBER

F

/ L Me gabr°s 1 I+ M°ta"°l°rT

A M

I ~ Oceanic gabbro (Kirst 1976)~ Amphibolites from Vema | Fracture Zame ]

A M (blonnorez et at. 1984) J

Fig. 4. Metamorphic rocks from Darvel Bay plotted on an AFM diagram compared with gabbros and amphibolites from the Vema Fracture Zone, equatorial Mid-Atlantic; the tholeiite calc-alkaline boundary is from Irvine & Baragar (1971).

developed in the felsic body, parallel to the schistosity in the surrounding metabasites. Thin section study shows that the rock is fine grained with elongated crystals of quartz and feldspar defining the schistosity. Minor constituents are acicular amphibole, epidote, apatite, chlorite, sphene, zircon and Fe-Ti oxides.

A block (1 m × 1.2 m) of felsic rock, identified as tonalite (Specimen PK4a), occurs enclosed in amphibolite on the foreshore on the south coast of Pulau Sakar. The block probably represents a xenolith incorporated in a basic intrusion, but now forms a boudin with a schistosity parallel to that in the surrounding metabasite. Thin section study shows that this rock is composed predominantly of quartz and plagioclase feldspar with subsidiary actinolitic hornblende. The rock has recrystallized, with the growth of porphyroblastic feldspar and hornblende crystals; granophyric texture in the quartz-feldspar matrix may represent an original igneous texture or be due to partial melting of the tonalite. The hornblende porphyroblasts contain trails of small inclusions of quartz, feldspar and Ti-oxides defining an internal schistosity. The porphyroclasts are enclosed in augen structures and the external schistosity in the matrix cuts across the

internal schistosity of the crystal at a high angle, indicating that the rocks have been subjected to multiple deformation (Omang 1993, pl. 5.7D).

Whole rock major element and trace element analyses of felsic rocks are listed in Table 2, and are compared to ocean ridge granites (ORG) on the spider diagram of Fig. 7d. These analytical data show that although the felsic rocks are not true granites they are comparable to ORG in their incompatible element concentrations and resemble acid rocks fore other ophiolites, interpreted as co-genetic with basic rocks (Pearce et al. 1984a).

Metatu f f

Chloritic schists are exposed at Km 134 north of Kampong Silam on Jalan Silam, where they are in contact with sheared serpentinite, and near Kampong Lok Bikin on the coast of the mainland opposite Pulau Sakar where they are interbedded with metacherts. At the locality on Jalan Silam the schists are folded by small-scale open folds on ENE (060°-070 °) axes with a low angle of plunge (10-12 °) and steep axial planes indicating a southerly vergence.




D A R V E L BAY OPHIOLITE, S A B A H 271

100

80

.C..•60

40 .<

2O

(a)

ophiolite

• Arc

!

1

MORB

", !

2 TiO2 (wt .%)

1

3 4

80

70

60

'° ~ 3a

20 ~ ~ Im MORB

A • 4 - _ _ 10 A r c ¥ ~ I I

o I

o ~ 2 (b) TiO2 (wt.%)

ophiolite

!

3 4

Fig. 5. (a).TiO 2 v. A1203frio2; (b) TiO 2 v. CaO/TiO 2 plots of Darvel Bay metamorphic rocks; fields from Sun & Nesbitt (1978).

In thin section the schistosity is defined by fine grained chlorite flakes and small feldspar laths diverging around larger euhedral to subhedral plagioclase crystals showing albite and Carlsbad twinning (Fig. 8a).

These schists have the composition of tholeiitic basalts and are therefore interpreted as tuffaceous rocks belonging to the ophiolite sequence, as hyaloclastic tufts forming part of the ocean floor assemblage, which has been intensely deformed and metamorphosed under greenschist facies conditions.

M e t a c h e r t

Thin bedded red-brown ribbon cherts occur exten- sively in the northern and southern areas of the Darvel Bay Ophiolite Complex (Fig. 1). In the northern part of the area cherts occur as blocks in the Kuamut Melange (Clennell 1991; Aitchison 1994). In the southern area bedded cherts folded and imbricated with basaltic pillow lavas, forming an accretionary complex, crop out along Jalan Silam.

Near Kampong Diam, south of Lahad Datu, dis- continuous exposures of greenish-white chert with a bedded appearance outcrop along the shore. In thin section these cherts are composed almost entirely of microcrystalline quartz. The quartz crystals are elongated and have a strong preferred orientation with a mylonitic texture (Fig. 8b). The apparent bedded appearance is a schistosity pro- duced by deformation. Concordant quartz and epidotic veins extend along the schistosity which is cut by calcite veins.

1000

C r (ppm)

100

.J. JDI A

PK3a f

i/ b

I \~ i I I I | } I : JD6

I h ~ IAT

. . . . . ! . i . ' t , , ,

10 Y(ppm) 100

TfflO0

MORB: Mid-ocean ridge basalts / ~ I AATB ~ sla~d_alI ~ alt~°~e~iatesSalt s

/ / \ I + Metad°'eri e

Zr t / (~ ~o'nAT ~A Amphib°lites,~,3

(a) (b)

Fig. 6. Tectonic discriminant diagrams. (a) Y/Cr after Pearce et al. (1984b); (b) Zr-Ti/100-Y after Pearce & Cann (1973).




lO.00

1.0(3

o.1(

0.0 S r

(a)

i 0 i ! i i i i ! 0 i

K Rb B a Nb La Ce Nd P Zr Ti Y

10.00

1.0£

0

272 s . A . K . O M A N G • A . J . B A R B E R

0,10

0.01 o D i o o o I o a I u Sr K Rb Ba Nb La Ce Nd P Zr Ti Y Sc Cr

(b)

V • ~_~ I • X JSp I

&, PS12d I

1°°/ cR kssllsll 0.00 PS12a

1.

a 0.10 Sc Cr K Rb Ba Th Nb Ce Zr Y

(a)

10.OO Metagabbros (Cumulates)

OlO] ~ T •

0 0 1 1 q i I I I I I I I I I I Sr K Rb Ba Nb La Ce Nd P Zr Ti Y Sc Cr

(e)

Fig. 7. (a)-(c) MORB-normalized chemical plots for metadolerites and metagabbros from the Darvel Bay Ophiolite listed in Table I; normalized values from Pearce et al. (1984b). (d) ORG-normalized ocean-ridge granite chemical plot for felsic rocks listed in Table 2; normalized values from Pearce et al. (1984a).

Epidote crystals from the veins analysed by microprobe are of XpSl8_25. XFe 3+ (0.9) is rela- tively high, corresponding to temperatures of formation of 450-500°C (Nakajima et al. 1977), i.e. in the greenschist facies.

These mylonitic quartz schists have been formed by the deformation of bedded cherts in an active shear zone and affected by calcium metasomatism in a hydrothermal system, in which the fluids were enriched in calcium carbonate, under greenschist facies metamorphic conditions.

G a r n e t a m p h i b o l i t e

Fragments of high grade metamorphic rocks including garnet pyroxenites and garnet amphibolites occur in volcanic conglomerates in the Tungku and Pungulupi Rivers 50 km east of Lahad

Datu (Reinhard & Wenk 1955). Garnet amphibolites from the Tungku River were analysed during the present study. In thin section the amphibolites commonly show mylonitic textures with crystals of garnet, pyroxene and hornblende enclosed in augen structures, enclosed in a fine grained matrix of hornblende and plagioclase (Fig. 8c).

P, T estimates from amphiboles and from garnet-pyroxene pairs in the present study (Omang 1993), and an earlier study of garnet pyroxenite from the same locality by Morgan (1974), give the conditions of formation of these rocks as T > 850°C at P > 5 kbar.

Bulk rock geochemistry, rare earth and trace element geochemistry of the garnet amphibolites show that they are MORB tholeiites and represent oceanic crustal materials. These rocks were metamorphosed as pyroxene granulites and garnet





Table 2. Whole rock geochemical analyses for felsic rocks from the Darvel Bay Metamorphic Complex

Sample No. LD5 PK4a PS 12c

S i t 2 60.14 61.46 67.27 Ti t 2 0.438 1.461 0.370 AI203 17.90 15.38 16.60 Fe203* 5.16 9.43 2.89 MgO 2.01 2.22 1.06 MnO 0.120 0.120 0.044 Cat 8.89 4.77 3.52 Na20 4.73 4.76 7.68 K20 0.333 0.091 0.044 P205 0.250 0.308 0.149 Total 99.98 100.00 99.62 LOI 4.73 1.30 0.73 mg c. 44 c. 32 c. 42

Ni 19 5 5 Cr 13 2 2 V 75 94 26 Sc 10 24 4 Pb 3.4 3.4 0.3 Sr 537 169 125 Rb 7.9 1.2 0.5 Ba 206 23 188 Th 2.9 0.3 2.9 Zr 273.3 169.6 188.0 Nb 3.2 3.9 5.5 Y 22.9 53.9 25.1 La 19 6 15 Ce 40 20 37 Nd 21 17 21 Cu 84 4 38 Zn 56 15 19 C1 8 35 179 Ga 22 19 13

Oxides in wt%, trace elements in ppm. Total iron as Fe203(FeeO 3 + FeOxI.III); mg = [100Mg/(Mg + Fe)], where Mg = MgO/40 and Fe = F203 x 0.9/72. Data presented on a volatile-free basis; LOI, loss on ignition at 1100°C.

amphibolites at temperatures and pressures characteristic of the upper mantle and were deformed and recrystallized with mylonitic textures in the amphibolite facies. These features are consistent with deformation and recrystallization of oceanic crustal rocks carried down in a subduction zone.

Origin of metamorphic rocks in the Darvel Bay Ophiolite Complex

Protoliths of rocks which form the belt of metamorphic rocks in the Darvel Bay Ophiolite Complex extending westwards from Lahad Datu can be identified as mantle peridotites, cumulate pyroxenites and gabbros, isotropic gabbros, plagiogranites, doleritic and basaltic dykes, volcanics

and sediments. This association of rock types is characteristic of the ocean floor and its underlying mantle. The basic rocks have a tholeiitic composition and fall predominantly in the MORB field of discriminant diagrams, showing that they originated by partial melting of mantle peridotite in a mid-ocean ridge environment. Chemical analyses show depletion in HFS elements, enrichment in LIL and LREE, positive Sr and negative Nb and Ce anomalies, characteristic of island arc tholeiites, indicating that the mid-ocean ridge was generated above an active subduction zone.

In map view (Fig. 1), in cross-section (Fig. 9) and also on an outcrop scale the rocks are distributed in lenticular slivers elongated parallel to the E - W trend of the metamorphic belt. The slab of mantle peridotite forming Silam Hill is juxtaposed along the northern shore of Darvel Bay against the 'Silumpat Gneiss', representing deformed cumulate gabbro, and with amphibolite and hornblende schist, representing isotropic gabbro or doleritic and basaltic dykes, from higher levels of the ocean crust. On the northern side of Silam Hill mantle peridotite is juxtaposed with felsic rocks, representing K-deficient silicic differentiates from tholeiitic magma which were intruded into oceanic crust at a high structural level. Chloritic schists at Jalan Silam and Lok Bikin, identified as deformed and recrystallized volcaniclastic rocks, represent tufts or hyaloclastics, and mylonitic quartz-schists, identified as banded cherts, represent the ocean floor and its sedimentary cover. All these rocks are now at the same structural level as the mantle peridotite.

Most of the metamorphic rocks in the belt are foliated, sometimes intensely, to form schists, and frequently show mineral lineations. The foliation or schistosity is generally steeply inclined or vertical and the lineation is subhorizontal. Both foliation and lineation are orientated parallel to the E - W trend of the belt. Preferred orientation of minerals in the gneisses and schists defining foliation, schistosity and mineral lineation, and rotated inclusion trails in hornblende porphyroblasts in the tonalite from Pulau Sakar, indicate that the metamorphic rocks recrystallized syntectonically. Compositions of amphiboles and amphibole/plagioclase pairs indicate that this recrystallization occurred under high temperature but low pressure conditions in the amphibolite and greenschist metamorphic facies. No clear metamorphic gradation across the belt was recognized in our study, different facies being randomly juxtaposed (cf. Hutchison 1975).

Basaltic dykes and felsic veins cutting the metamorphic rocks show varying relationships to the foliation and schistosity. Dykes and veins may be concordant to the foliation or schistosity in the country rocks, or may cut across these structures




t~

( a ) ~)

1 mm 4

(c)

Fig.8. (a) Photomicrograph of chlorite schist, deformed crystal tuff, Jalan Silam Km 134, Darvel Bay, Sabah. (b) Photomicrograph of siliceous schist (metachert) with epidote bands and cross-cutting quartz veins, Kampong Diam, Lahad Datu, Sabah. (e) Photomicrograph of garnet amphibolite (Specimen EKc) showing a fractured garnet crystal with alteration to hornblende enclosed in a fine grained mylonitic hornblende-plagioclase matrix to form an augen structure. Pebble from Tungku River, Dent Peninsula, Sabah. Scale bar 1 mm.





South 3000

2000

1000

0

I S E C T I O N A - A ' ]

Bukit Silam

3000

2000

1000

0

Feet

Distance A-A' ~ 35 Km [

] S E C T I O N B - B' I South Bukit Silam North

3000 " ~ ~ ~ _ SilamDam 2000- Silam Road ~ ~ Sg. Telewas ~ e . . . . . 1000" ~

I SECTION C-C' I South Kampong North South

1,,, ~ P. Gifford Silam Kg. Sepagaya tmo I000

0 0 Feet 0

[ Distance C-C': - 17 Km] I Distance D-I3

South [ SECTION E" E ' i North

10IN) 1000 Feet 0 0 Sea-level

[ Distance E-E' : ~ 11 Km I

3000 Feet

2000

1000

0

Distance B-B' - 29 Km

S E C T I O N D - D ' [

dolerite dyke North _ Pulau Sakar k , v~ Veer

L~. ~" 1000 i o D-D': ~ 17 Kml

Fig. 9. Schematic cross-sections across the metamorphic belt in the Darvel Bay Ophiolite Complex Sabah. Lines of section and key to ornaments are shown on Fig. 1.

with varying degrees of obliquity. Hutch ison & Dhonau (1971) reported a xenol i th o f 'S i lumpa t Gneiss ' (i.e. fol iated banded gabbro) enc losed in a basaltic 'sill ' on Pulau Si lumpat . Within the dykes, schistosity may be deve loped with varying degrees of intensity; in general the more concordant the dyke the more intense the schistosity wi thin it.

These features indicate that some of the dykes were in t ruded into rocks which had already been de formed , but that de fo rmat ion con t inued after dyke emplacement . Deformat ion of the felsic veins is indicated by boud inage where they are concordant, and folding where they are cross-cutt ing. Boud inaged and fo lded veins are f requent ly cut




276 s.A.K. OMANG • A. J. BARBER

by veins which are completely undeformed. The relationships of basaltic dykes and felsic veins to the foliation and schistosity demonstrate that deformation and intrusion of igneous rocks were going on continuously during the development of the metamorphic belt.

Metamorphic rocks associated with ophiolite complexes from many parts of the world have been attributed either to processes of deformation and metasomatism which affect ocean floor materials shortly after their formation in the mid-ocean ridge environment, or to the processes of subduction which lead to the emplacement of the ophiolite on a continental margin. It has been suggested that foliated rocks may be formed at the crust-mantle boundary in the region of a mid-ocean ridge by gravitational spreading of the ridge (Smewing et al. 1984; Gibbons & Thompson 1991). Foliated metamorphic rocks within ophiolites have also been interpreted as having originated along transform fault zones (Karson & Dewey 1978; Saleeby 1978; Simonian & Gass 1978; Prinzhofer & Nicholas 1980; Karson 1984). Foliated metamorphic rocks which occur along the basal thrust of an ophiolite complex have been interpreted as a 'metamorphic sole' formed during subduction and emplacement of the ophiolite on a continent (Davies 1971; Williams & Smyth 1973; Jamieson 1980; Searle & Malpas 1980; Spray & Williams 1980; Ghent & Stout 1981; Moores 1982).

On the basis of the characteristics detailed above, the rocks of the metamorphic belt within the Darvel Bay Ophiolite are interpreted as having formed along a transform fault zone. Throughout the oceans, mid-ocean ridge systems are offset along transform faults at intervals of a few hundred kilometres. In these zones segments of ocean crust are moving past each other between active ridge segments. As the two crustal slices move past each other, oceanic materials along the fault are deformed, producing foliated peridotites, banded gneisses from cumulate gabbros and foliated and schistose amphibolites from gabbros and sheeted dykes. In transform fault zones at the time of their formation the foliation surfaces will be set vertically, parallel to the trend of the fault zone and lineation will be sub-horizontal representing the direction of movement along the fault. In an active fault zone crustal slices on either side of the fault will be of different ages. On one side of the fault the crust will have been formed recently at the ridge axis and will be at a high temperature; on the other the crust will be older and will have cooled and subsided as it moved away from the ridge axis. Deformation and shearing of the oceanic crustal rocks along the fault, with the ingress of water, will induce recrystallization of hydrous phases which are aligned to form schistose and linear structures

parallel to the sense of movement within the fault zone.

At the time of formation the grade of metamorphism will decrease across the fault zone, from pyroxene and hornblende granulite against the hotter younger crustal segment, declining to greenschists and unmetamorphosed rocks towards the older colder segment and vertically towards the ocean floor. As has already been stated, no simple gradation of metamorphic facies has been recognized in the metamorphic belt at Darvel Bay. However, in an active fault zone any such simple arrangement is likely to be disrupted by continual movement along the fault, juxtaposing unrelated fault slices of different metamorphic grades and from different crustal levels, as has been described from Darvel Bay. No sedimentary infills, composed of clastic debris flows formed by submarine erosion, as reported from the Arakapas Transform in Cyprus (Simonian & Gass 1978; McLeod & Murton 1993) have been recognised as associated with the Darvel Bay Transform, suggesting that several kilometres have been removed by erosion since the emplacement of the ophiolite on Sabah.

The mylonitic quartz-schists described from Kampong Diam, and identified as deformed banded cherts, are an unexpected component of an assemblage of rocks incorporated in a fossil transform fault zone. Cherts are deposited on the ocean floor only after it has subsided beneath carbonate compensation depth, some millions of years after the ocean crust formed at the mid-ocean ridge. The presence of chert among the deformed rocks indicates that a much older segment of ocean crust was juxtaposed against an active spreading ridge, implying that the transform fault extended over several hundreds of kilometres. Karson & Dewey (1978) came to a similar conclusion regarding the Coastal Complex transform associated with the Bay of Islands Ophiolite in Newfoundland, and pointed to present day examples where very long transform faults separate short ridge segments, in the Gulf of California and the Andaman Sea. Continued movements over a long distance through transtensional and trans- pressional fault segments increases the likelihood of the juxtaposition of uplifted mantle peridotite, with downfaulted crustal materials including supracrustal volcanics and metasediments, as seen in Silam Hill and along Jalan Silam.

The garnet pyroxenites and garnet amphibolites from the Tungku and Pungulupi rivers described earlier in this account are not compatible with a transform fault origin. As previously concluded the high pressures and temperatures of formation of these rocks, combined with their mylonitic textures are consistent with their formation in a subduction zone. These rocks are therefore considered to





represent fragments of a metamorphic sole which must be present at depth below the Darvel Bay Ophiolite of the Dent Peninsula. A K-Ar age of 7 6 + 21 Ma obtained from garnet amphibolite during the present study coincides with the Late Cretaceous-Palaeogene age of subduction beneath the Darvel Bay Ophiolite inferred from the stratigraphic evidence.

These metamorphic sole rocks, which must underlie the whole of the ophiolite complex, were intersected by Miocene volcanics and carried to the surface, to be incorporated with andesitic fragments, ophiolitic rocks and sandstones in volcanic breccias and conglomerates during the formation of the Sulu Volcanic Arc.

Tectonic setting and emplacement of the Darvel Bay Ophiolite

Evidence given in this account confirms the interpretation of the Darvel Bay Ophiolite Complex as a segment of oceanic crust and upper mantle originating at a mid-ocean spreading ridge. Evidence has also been given that the spreading ridge was developed above a subduction zone in a back-arc basin. The belt of metamorphic rocks, which occurs within the complex and extends westwards from Lahad Datu, is interpreted as a fossil transform fault.

Radiometric ages and biostratigraphic ages from radiolarian cherts and foraminiferal pelagic limestones in the Chert-Spilite Formation indicate a Lower Cretaceous age for the origin of the ophiolite. Shallow-water limestones with Upper Cretaceous fossils, interpreted as the carbonate cappings to seamounts, indicate that the ophiolite still formed part of the ocean floor at this time.

Hamilton (1979), Holloway (1981) and Rangin et al. (1990) have suggested that the Darvel Bay Ophiolite, together with other ophiolite fragments in eastern Sabah, originally formed part of a Proto- South China Sea crust. In this scenario, southward

subduction of Proto-South China Sea crust led to the obduction and uplift of the ophiolite by under- thrusting of Upper Cretaceous and Lower Palaeogene turbidites of the Crocker Formation. Hutchison (1988), from the occurrence of Oligocene granitoid intrusions in the Long Laai area SW of Darvel Bay, suggests that the subduction of continental crust may also have been involved. Fragments of ophiolitic rocks in Eocene conglomerates show that the ophiolite had been obducted, uplifted and was subject to erosion by Eocene times (Newton-Smith 1967).

It has been demonstrated in the foregoing account that the major occurrence of metamorphic rocks associated with the Darvel Bay Ophiolite represents a fossil transform fault, while fragments of high pressure metamorphic rocks in overlying volcanics indicate that the ophiolite is underlain by a metamorphic sole related to subduction and the obduction of the ophiolite in Sabah.

The work described in this paper was presented as a thesis by SAKO for the award of the PhD degree of the University of London, sponsored by the Universiti Kebangsaan Malaysia (UKM) and the Government of Malaysia. Bulk rock geochemistry was determined in the geochemical laboratories at Royal Holloway, University of London using a Philips PW1480 XRF Spectrometer under the supervision of Drs M F Thirlwall and G F Marriner and Mr Ceil Jenkins. Mineral analyses were carried out on a JEOL Superprobe 733 electron microprobe at Birkbeck College under the supervision of Prof Robert Hall, University College and mineral compositions were recalculated using a suite of programs developed by Prof Hall who also gave general advice on the interpretation of the chemical data. K-Ar isotopic dating was carried out at the NERC Isotope Geosciences Laboratory at Keyworth, Nottingham under the supervision of Dr C. C. Rundle. Mr David Lee, Director of the Geological Survey of Malaysia, Kota Kinabalu, provided the samples of garnet amphibolite from the Tungku River used in this study. Attendance at the Conference on the 'Tectonic Evolution of Southeast Asia' held at the Geological Society, London was sponsored by UKM. Presentation has been greatly improved by careful and constructive reviews by Prof. C. S. Hutchison and Dr J. E. Dixon.

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origin and tectonic significance of the metamorphic rocks ...searg.rhul.ac.uk/pubs/omang_barber_1996...

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