abdullah et al (2007)

The evolution of Sumba Island (Indonesia) revisited in the lightof new data on the geochronology and geochemistry of the

magmatic rocks

C.I. Abdullaha, J.-P. Rampnouxb,*, H. Bellonc, R.C. Mauryc, R. Soeria-Atmadjaa

aTeknik Geologi, Institut Teknologi Bandung, Jalan Ganesa 10, Bandung, IndonesiabUniversiteÂ de Savoie, 73376 Le Bourget du Lac Cedex, France

cUMR 6538, IUEM, UniversiteÂ de Bretagne Occidentale, BP 809, 28285 Brest Cedex, France

Received 27 November 1998; accepted 28 October 1999

Abstract

The island of Sumba, presently located in the southern row of islands of the Eastern Nusa Tenggara province of EasternIndonesia, has a unique position, being part of the Sunda-Banda magmatic arc and subduction system. It represents a

continental crustal fragment located at the boundary between the Sunda oceanic subduction system and the Australian arc±continent collision system, separating the Savu Basin from the Lombok Basin. New data on magmatic rocks collected fromSumba are presented in this paper, including bulk rock major and trace element chemistry, petrography and whole rock and

mineral 40K±40Ar ages.Three distinct calc±alkaline magmatic episodes have been recorded during Cretaceous±Paleogene, all of them characterized by

similar rock assemblages (i.e. pyroclastic rocks, basaltic±andesitic lava ¯ows and granodioritic intrusions). They are: (i) the

Santonian±Campanian episode (86±77 Ma) represented by volcanic and plutonic rock exposures in the Masu Complex inEastern Sumba; (ii) the Maastrichtian±Thanetian episode (71±56 Ma) represented by the volcanic and plutonic units ofSendikari Bay, Tengairi Bay and the Tanadaro Complex in Central Sumba; and (iii) the Lutetian±Rupelian episode (42±31 Ma)of which the products are exposed at Lamboya and Jawila in the western part of Sumba. No Neogene magmatic activity has

been recorded. 7 2000 Elsevier Science Ltd. All rights reserved.

1. Introduction

Sumba Island has a unique position with respect tothe Sunda±Banda arc as it represents an isolated sliverof probable continental crust to the south of activevolcanic islands (Sumbawa, Flores) within the forearcbasin (Fig. 1). It is situated to the north of the tran-sition from the Java Trench (subduction front) to theTimor Trough (collision front). It does not show thee�ects of strong compression, in contrast to islands ofthe outer arc system (Savu, Roti, Timor), while mag-

matic units make up a substantial part of the LateCretaceous to Paleogene stratigraphy.

Bathymetrically, Sumba stands out as a ridge thatseparates the Savu forearc basin (>3000 m depth) inthe east and the Lombok forearc basin (>4000 mdepth) in the west. Seismic refraction studies show(Barber et al., 1981) that it is made up of 24 km thickcontinental crust (Chamalaun et al., 1981). Based ontectonic studies, complemented by paleomagnetismand geochemistry, several workers consider Sumba tobe a microcontinent or continental fragment (Hamil-ton, 1979; Chamalaun and Sunata, 1982; Wensink,1994, 1997; Vroon et al., 1996; Soeria-Atmadja et al.,1998).

Three main geodynamic models for Sumba havebeen proposed by Chamalaun et al. (1982) and Wen-

Journal of Asian Earth Sciences 18 (2000) 533±546

1367-9120/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved.

PII: S1367-9120(99 )00082 -6

* Corresponding author.

E-mail address: [email protected] (J.P. Ramp-

noux).

sink (1994) as follows: (i) Sumba was originally a partof the Australian Continent which was detached whenthe Wharton basin was formed, drifted northwardsand was subsequently trapped behind the eastern JavaTrench (Audley-Charles, 1975; Otofuji et al., 1981); (ii)Sumba was once part of Sundaland which driftedsouthwards during the opening of the Flores Basin(Hamilton,1979; Von der Borch et al., 1983; Rangin etal., 1990); and (iii) Sumba was either a microcontinentor part of a larger continent within the Tethys, whichwas later fragmented (Chamalaun and Sunata, 1982).

The present paper represents an attempt to resolvethis problem.

2. Stratigraphy

The stratigraphy of Sumba has been discussed byseveral workers (van Bemmelen, 1949; Laufer andKrae�, 1957; Burollet and SalleÂ , 1982; Chamalaun etal., 1982; Von der Borch et al., 1983; Fortuin et al.,1992; E�endi and Apandi, 1994; Abdullah, 1994; For-tuin et al., 1994, 1997). The island is composed (Figs.2 and 3) of slightly to unmetamorphosed sediments of

Mesozoic age, unconformably overlain by considerablyless deformed Tertiary and Quaternary deposits; thetotal thickness of which is more than 1000 m (vanBemmelen, 1949). The Quaternary coral reef terraces,which cap the seaward edge of the Neogene SumbaFormation, are almost continuously exposed along thewestern, northern and eastern coasts of Sumba(Hamilton, 1979).

2.1. Mesozoic series

Mesozoic rocks are exposed principally along thecoast immediately south of West Sumba (Patiala,Wanokaka and Konda Maloba) and in the southernpart of the Tanadaro Mountains (Nyengu and Labungrivers). The sediments are typically carbonaceous silt-stones with volcanogenic mudstones, sometimes show-ing signs of low-grade metamorphism, interbeddedwith sandstones, conglomerates, limestones and volca-niclastic debris. They are crosscut by Late Cretaceousintrusions which range in composition from microgab-bro to quartz-diorite, and also by granodioritic andcalc±alkaline dykes of Paleogene age. The sedimentsshow large scale slump structures and strong fractur-

Fig. 1. Tectonic features of the Eastern Indonesia island arc (modi®ed after Hamilton, 1979 and Burollet and SalleÂ ,1982).

C.I. Abdullah et al. / Journal of Asian Earth Sciences 18 (2000) 533±546534

ing. These sediments constitute the Lasipu Formation(Prasetyo, 1981). Microfossil assemblages in somesamples indicate Coniacian to Early Campanian ages(Burollet and SalleÂ , 1982); many Inoceramus sp. arepresent. The detrital materials suggest either a conti-nental origin, or an island arc environment ; it wouldappear to be essentially a Mesozoic submarine fanwith shallow-water deposits (Von der Borch et al.,1983) or an open marine bathyal environment (Burol-let and SalleÂ , 1982).

2.2. Paleogene series

During the Paleogene Sumba was a part of mag-matic arc characterized by calc±alkaline volcanic rockseries (Western Sumba) and shallow marine sediments.The corresponding deposits include tu�s, ignimbrites,greywackes, intercalations of foraminiferal limestones,marls, micro-conglomerates and claystones. Theserocks unconformably overlie Mesozoic rocks and arein turn unconformably overlain by the Neogene Series.

2.3. Neogene series

O�shore seismic re¯ections show that Neogenedeep-water sediments make up the early sedimentarysequence of a forearc basin which laps onto a ridge(Fortuin et al., 1992; Van der Wer� et al., 1994a, b;Van der Wer�, 1995; Fortuin et al., 1997). Theiroccurrence re¯ects the stable position of the SumbaRidge within the forearc since the initiation of theSunda arc±trench system during the late Oligoceneand the early Miocene (Silver et al., 1983; Reed, 1985;Barberi et al., 1987). The Neogene sediments onSumba display two di�erent facies: in the western part,they are represented by mostly reef limestones, bioclas-tic limestones, chalky limestones and marls, inter-bedded with tu�aceous marls, whereas the sedimentsfrom the eastern part of Sumba are dominantly volca-nic turbidites with interbedded pelagic chalks andchalky limestones (Fortuin et al., 1994). In the centralpart of Sumba, these sedimentary facies show inter®n-gering relations. These rocks are undisturbed tectoni-cally.

Fig. 2. Geological sketch map of Sumba (for A, B, C boxes, refer to Fig. 4).

C.I. Abdullah et al. / Journal of Asian Earth Sciences 18 (2000) 533±546 535

Fig. 3. Stratigraphic columns of Sumba.

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2.4. Quaternary series

The whole island has been rapidly uplifted to its pre-sent elevation, as is indicated by the Quaternary ter-races, which reached a height of not less than 500 m(Jouannic et al., 1988), at a rate of 0.5 mm/yr, in thenorthern and central part of Sumba (Pirazzoli et al.,1991). These terraces consist of sandstones, conglomer-ates, marls and prominent reef limestones, whichunconformably overlie gently dipping Neogene sedi-ments along the west, north and east coasts. Locally,Quaternary deposits rest unconformably on Mesozoicrocks along the southwestern coast.

3. Calc±alkaline magmatism

A set of 24 magmatic rock samples representinggranitoid intrusions, lava ¯ows and subvolcanic dykesof ma®c to intermediate composition from various out-crops within the investigated area (Fig. 4) were selectedfor 40K±40Ar dating (Table 1). 40K±40Ar dating as wellas chemical analyses (major and trace elements;Table 2) on these rocks were performed at the Univer-siteÂ de Bretagne Occidentale, Brest, France. Numerousother magmatic rock samples were studied petrogra-phically.

3.1. Analytical procedures

40K±40Ar age dating has been carried out on wholerocks for the volcanic rocks and on whole rocks andcarefully separated minerals (fresh biotite to slightlychloritized biotite, and the non-magnetic fraction,including both quartz and feldspar) from plutonic rockoutcrops of Tanadoro massif. Calculated isotopic agesare presented in Table 1, together with the mostcharacteristic parameters of the measurements i.e. the40ArR (radiogenic argon 40), the percentage of 40ArRversus the total amount of 40ArR and atmospheric40Ar. For whole rock analyses, samples were crushedand sieved to 315±160 mm grain size and then cleanedwith distilled water. This fraction was used (1) forargon extraction under high vacuum by HF heatingand (2) for potassium analysis by atomic absorptionspectrometry after reduction to a powder. Age calcu-lations were carried out using the constants rec-ommended by Steiger and Jaeger (1977) with one serror calculated according to Mahood and Drake(1982). Chronostratigraphical signi®cance of the isoto-pic ages is based on the 1994 geological time scale(Odin, 1994). Major and trace element data wereobtained by ICP±AES methods according to the pro-cedures described by Cotten et al. (1995). The corre-sponding analytical precisions are better than 2% formost major elements and 5% for most trace elements.

3.2. Geochemical and geochronological results

Taking account of duplicate analyses for elevensamples as listed in Table 1, mean 40K±40Ar whole-rock ages2the greater error are reported in Fig. 4 andare discussed below.

Three periods of magmatic activity were recognizedby Abdullah (1994) on the basis of most of these data,at ca 86±77 Ma (Santonian±Campanian), 71±56 Ma(Maastrichtian±Thanetian) and 42±31 Ma (Lutetian±Rupelian) respectively.

These data are in agreement with those published byChamalaun and Sunata (1982), Burollet and SalleÂ(1982), Van Halen (1996) and Wensink (1997).

Fig. 4. Location of sampled magmatic rocks for age determinations

and geochemical analyses (for the key, see Fig. 2).


Table 140K±40Ar isotopic ages of magmatic rocks of Sumba (see text for analytical methods)

Sample No. Location Type Age (Ma)2error 40ArR(eÿ7 cm3/g)

40ArR(%)

K2O

(wt %)

LOI

(wt %)

Analysis No.

Santonian±Campanian episode

Mt Masu

CIA-347 WR Cape Malanggu (Pameti Hawu) Granodiorite 85.421.6 101.60 84.9 3.68 0.58 2794

CIA-348 WR Cape Malanggu (Pameti Hawu) Microdiorite 83.721.8 48.59 70.8 1.76 0.73 2813

CIA-351 WR Cape Malanggu Andesite 85.922.0 71.44 72.0 2.52 1.45 4715

CIA-353 WR Cape Malanggu Bas. andesiteb 78.621.7 32.39 84.2 1.25 0.93 2814

CIA-728 WR Tanarara (Km 5) Basalt 85.422.0 21.71 73.8 0.79 1.67 3933

CIA-733 WR Gunung Kapunduk (Nggongi valley) Bas. andesiteb 80.921.9a 49.11 81.1 1.84 1.48 3922

84.421.9 51.24 87.8 3921

CIA-735 WR Road from Tatunggu to Tanarara (Km 2) Bas. andesiteb 76.921.8a 40.25 74.8 1.59 3.15 4362

77.621.8 40.63 72.1 3947

Maastrichtian±Paleocene episode

Wanokaka area

CIA-317 WR Western side of Wanokaka beach Basalt 71.121.3a 20.35 89.2 0.87 3.48 2792

69.521.5 19.87 90.3 2793

Sendikari and Tengairi Gulfs

CIA-339 WR Eastern part of the Sendikari Gulf (Cape Teki) Microgabbro 70.121.4a 49.11 79.9 2.13 2.95 2857

68.721.4 48.11 79.7 2856

CIA-491 WR Eastern part of the Sendikari Gulf (Cape Teki) Bas. andesiteb 65.221.3 46.27 77.8 2.16 2.81 2880

CIA-481 WR Bottom of the Tengairi Gulf Bas. andesiteb 68.021.3 73.94 82.2 3.31 1.16 2879

CIA-487 WR Bottom of the Tengairi Gulf Bas. andesiteb 65.321.8 55.52 75.8 2.59 2.60 2878

Mt Tanadaro

CIA-133 WR Mt Tanadaro (Western part) Granodiorite 64.321.2 55.78 85.7 2.74 0.68 2848

Bio 66.621.3 173.40 79.0 7.93 2830

Fds 63.621.5 44.03 66.9 2.11 2833

CIA-132 WR Mt Tanadaro (Western part) Diorite 64.321.2a 33.74 82.8 1.60 1.40 2790

61.821.2 32.45 82.7 2791

Chl 62.121.2 69.51 79.0 3.41 2851

Fds 62.921.5 51.04 67.3 1.54 2850

CIA-115 WR Pamalar river (South-western edge of Mt Tanadaro) Diorite 61.821.2 35.09 84.9 1.73 1.49 2846

Bio 63.421.2 128.40 88.1 6.17 2826

Fds 57.721.6 17.96 57.7 0.95 2823

CIA-204 WR Pamalar river (South western edge of Mt Tanadaro) Diorite 57.621.3 33.00 69.4 1.75 1.59 2753

Chl 61.021.2 71.97 81.6 3.60 2852

Fds 61.421.4 32.23 67.5 1.60 2853

CIA-202 WR Pamalar river (South western edge of Mt Tanadaro) Diorite 56.621.2 28.01 75.3 1.51 1.59 2754

CIA-71 WR Nyengu river Basalt 66.521.6 15.73 70.0 0.72 2.29 4722

CIA-73 WR Nyengu river Bas. andesiteb 59.221.2a 16.12 78.8 0.83 1.88 2633

59.221.2 16.11 80.1 2632

Lutetian±Rupelian episode

Mt Lamboya

CIA-62 WR Eastern part of Rua beach (Cape Watumete) Basalt 43.523.2a 8.09 26.4 0.57 2.88 2952

41.222.8 7.66 28.1 2923

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Erupted magmas display the characteristics of a pre-dominantly calc±alkaline (CA) and a minor potassiccalc±alkaline (KCA) series (Fig. 5); they are character-ized by variable K2O contents, relatively high Al2O3

and low TiO2 contents, suggesting a typical island arcenvironment. Such a�nity is consistent with theirmoderately to fairly enriched incompatible elementpatterns (Fig. 6) showing negative anomalies in Nb,Zr, and to a lesser extent in Ti, typical of subduction-related magmas.

Products of the ®rst magmatic episode are rep-resented by the granitoid stock-like body of Mt. Masu(southern coast of East Sumba) accompanied withbasaltic andesites and andesites. To the second mag-matic episode belongs the Tanadaro granitoid intru-sion and subvolcanic basaltic and basaltic andesiticintrusions along the gulfs of Tengairi and Sendikari.Products of the latest magmatic episode are exposedsouth and west of Waikabubak and include the grani-toid, basaltic and dacitic rocks of Mt Lamboya andMt Jawila areas in West Sumba. The petrographic andgeochemical characteristics of each magmatic episodeare discussed successively.

3.2.1. Santonian±Campanian episode

The products from this magmatic episode make upthe Masu Formation of East Sumba (E�endi andApandi, 1994), and consist of an assemblage of pyro-clastic breccias, tu�s and lava ¯ows, intruded by gran-odiorite. The granodiorite is a medium-grainedequigranular rock (CIA-347) which is made up ofmicroperthite, zoned plagioclase, hornblende andquartz. Microperthitic grains are often dusted with ser-icite, clay and opaque particles. Clinopyroxene showspartial alteration to pale green hornblende whereasclusters of dark brown biotite between feldspar, quartzand hornblende, are altered to chlorite, titanite andiron-oxides. The andesites (CIA-351; CIA-426) contain20±25 modal % phenocrysts (plagioclase, clinopyrox-ene, green hornblende, magnetite and apatite) whereasbasaltic andesite (CIA-353) contains no more than 10modal % phenocrysts (plagioclase, green hornblendeand magnetite). Plagioclase phenocrysts are generallyzoned and contain minute sericite, clays, iron-oxides,magnetite and titanite grains. The rock groundmass iscomposed of plagioclase laths, chlorite, hornblendeand magnetite.

The whole rock major trace element compositionsare typically calc±alkaline to potassic calc-alkaline(Fig. 5A) and their incompatible element patternsclearly show the negative Nb, Zr and Ti anomaliestypical of arc volcanics (Fig. 6A). Two K±Ar ageswere obtained from the granitoid intrusion, respect-ively at 83.72 1.8 Ma (CIA-348) and 85.42 1.6 Ma(CIA-347), whereas ®ve volcanic rocks (basalt, basal-C

IA-44

WR

Dassangvalley

Granodiorite

36.7

20.7

a19.48

78.5

1.63

1.55

2629

36.4

20.7

19.35

78.9

2628

CIA

-21

WR

Wanokakavalley

(Praim

aragaarea)

Dacite

35.8

20.7

a19.03

82.5

1.63

0.79

2630

35.7

20.7

18.95

83.3

2631

MtJawila

CIA

-717

WR

Dikiravillage(SouthernedgeofMtJawila)

Bas.andesiteb

37.0

20.9

a7.59

60.5

0.63

1.55

3920

36.1

21.0

7.41

44.2

3919

CIA

-493

WR

KarekaHulu

(EasternedgeofMtJawila)

Dacite

31.5

21.1

a20.22

50.0

1.97

1.77

2864

31.3

20.8

20.04

64.4

2865

aDuplicate

analysesofthesamewhole

rock,WR

fractionforsamples:

CIA

-733,CIA

-735,CIA

-317,CIA

-339,CIA

-132,CIA

-73,CIA

-62,CIA

-44,CIA

-21,CIA

-717andCIA

-493.WR:whole

rock

fractionanalysis.Bio:separatedbiotite.Fds:separatedfeldspar2

quartz.

Chl:separatedchloritizedbiotite.

bBas.andesite=

basaltic

andesite.


Table

2

Majorandtrace

elem

entanalysesofSumbamagmaticrocks.IC

P-A

ESdata,J.

Cotten,Brest

(see

textforanalyticalmethods)

Santonian±Campanianmagmatism

Maastrichtian±Paleocenemagmatism

Late

Eocene±EarlyOligocene

(Lutetian±Rupelian)magmatism

Masu

Mt

South

Coast

Tanadaro

Mt

LamboyaandJawilla

Mts

%CIA

-728

CIA

-735

CIA

-353

CIA

-348

CIA

-733

CIA

-351

CIA

-347

CIA

-317

CIA

-339

CIA

-487

CIA

-491

CIA

-481

CIA

-71

CIA

-204

CIA

-73

CIA

-202

CIA

-115

CIA

-132

CIA

-133

CIA

-62

CIA

-717

CIA

-44

CIA

-493

CIA

-21

SiO

248.8

54

55.3

56.4

56.5

59

64.5

52.6

53

53.5

53.8

53.4

49.35

55.4

56.1

57

58.1

59.4

67

51.5

55

60.8

64.5

67.2

TiO

20.98

0.77

0.84

0.85

0.84

0.71

0.47

1.05

0.84

0.94

0.82

0.91

0.74

0.7

0.63

0.75

0.84

0.62

0.56

0.78

0.94

0.76

0.67

0.9

Al 2O

318.25

17.96

19

16.75

17.25

16.75

16.52

16.95

17.1

18.5

17.25

20.25

21.15

17

20.2

17.21

17.46

16.7

14.8

19.15

17.4

15.44

16.1

126

Fe 2O

310.95

7.65

8.4

8.8

8.5

6.9

4.58

8.8

8.65

8.4

8.56

8.4

8.76

8.22

6.55

7.38

6.9

7.03

4.62

8.12

9.45

7.3

4.52

3.57

MnO

0.19

0.16

0.1

0.21

0.17

0.15

0.1

0.15

0.16

0.13

0.16

0.13

0.15

0.16

0.1

0.13

0.13

0.12

0.08

0.16

0.18

0.13

0.09

0.08

MgO

5.7

3.67

2.61

2.69

3.29

2.9

1.75

3.54

4.46

3.33

4.2

3.32

4.55

4.34

2.07

3.65

3.53

3.26

2.13

5.6

3.37

3.09

1.63

0.38

CaO

9.05

6.4

7.63

7.4

7.3

6.05

4.42

9.16

7.3

5.4

6.35

3.3

9.59

7.43

7.95

7.17

7.22

6.36

4.3

8.1

7.7

6.12

3.45

3.29

Na2O

2.91

3.68

3.27

3.4

2.88

3.13

3.6

2.54

3.81

3.69

3.32

3.95

3.03

3.62

3.88

3.46

3.54

3.49

3.48

3.6

3.53

3.4

4.6

5.74

K2O

0.76

1.54

1.15

1.68

1.75

2.52

3.65

0.83

22.55

2.08

3.2

0.72

1.63

0.83

1.2

1.04

1.63

2.33

0.55

0.59

1.64

1.87

1.4

P2O

50.21

0.18

0.26

0.34

0.2

0.23

0.11

0.24

0.18

0.23

0.17

0.23

00.08

0.06

0.07

0.06

0.04

00.2

0.16

0.02

0.18

0.22

LOI

1.67

3.15

0.93

0.73

1.48

1.45

0.58

3.48

2.97

2.6

2.81

1.16

2.29

1.59

1.88

1.59

1.49

1.4

0.68

2.88

1.55

1.55

1.77

0.79

Total

99.47

99.16

99.49

99.25

100.16

99.79

100.28

99.7

100.47

99.27

99.52

98.25

100.33

100.17

100.25

99.61

100.31

100.05

99.98

100.64

99.86

100.25

99.38

99.57

ppm

Cr

27

27

21

15

68

32

70

19

48

18

32

48

10

48

47

39

34

157

11

24

11

12

Nl

31

14

33

96

629

20

512

926

18

925

26

22

18

85

5.5

14

16

Co

35

21

14

18

22

17

11

32

26

22

25

22

31

26

19

24

22

19

16

30

20

21

11

7

Sc

29

21

23

23

21

20.5

12

30

28

23

27

24

29

29

15

23

21

21

14

23

31

23

17

20

V326

227

190

215

226

170

106

310

250

233

240

222

196

157

110

143

131

142

95

174

269

140

55

56

Rb

10

35.5

33

36

26.5

51

102

20

19

41

18

37

32

45

85

68

39

17.7

28

Ba

184

510

365

390

355

510

560

160

292

382

450

890

191

228

131

229

188

300

350

125

81

134

1010

187

Sr

562

570

653

610

485

490

381

340

490

705

560

760

646

430

525

408

409

404

293

410

302

210

231

282

Nb

2.3

2.6

2.9

2.9

3.4

2.3

3.95

1.4

2.3

3.1

2.2

2.9

25

2.5

45

44

1.8

1.8

52.8

9

La

10.5

12.5

14.3

17.9

14.8

15.35

17.9

9.75

10.1

13.1

9.4

12.5

518

7.5

11

11

14

15

10.2

5.8

9.5

18

15

Ce

24.5

26

32

25

31

22

31

15.5

42

Nd

16

15

19

24

18.4

21

20

15

15

19.3

14

19

916

12

14

13

15

15

13.5

10.5

15

28.5

24

Zr

69

88

26

16

106

60

18

100

89

137

59

125

39

14

27

89

12

883

71

26

148

219

Eu

1.22

1.02

1.2

1.3

1.13

1.35

11.1

1.12

1.12

1.05

1.15

0.5

10.9

11

0.9

0.7

1.2

11

1.95

1.8

Y20

16.8

25

29.5

20.5

22

20

27

22

27

22

25

17

30

20

23

22

27

28

21

22.5

33

55

54

Dy

3.4

2.75

44.9

3.55

3.6

3.2

4.4

3.85

4.4

3.6

4.5

2.2

4.08

2.8

32.9

3.4

3.6

3.1

3.55

4.7

8.6

8.1

Er

1.9

1.5

2.3

3.2

2.05

1.9

2.2

2.5

2.2

2.5

22.6

1.7

2.2

1.7

1.6

2.1

2.4

2.3

2.1

2.35

2.6

5.3

2.7

Yb

1.83

1.6

1.95

2.7

1.97

1.85

1.8

2.4

2.07

2.48

1.92

2.40

1.45

2.13

1.52

1.86

1.73

2.25

2.2

1.9

2.29

2.74

54.86


tic±andesite, andesite) gave ages of 77.2 2 1.8 Ma(CIA-735), 78.6 2 1.7 Ma (CIA-353), 82.6 2 1.9 Ma(CIA-733), 85.422 Ma (CIA-728) and 85.922.0 Ma(CIA-351).

3.2.2. Maastrichtian±Thanetian episodeThe corresponding magmatic rocks are exposed in

the Tanadaro area, in East Waikabubak and along thecoast of southeast Waikabubak. These rocks includemostly granodioritic and dioritic intrusions and volca-nic units of chie¯y basaltic composition (basalts andminor basaltic andesites). The stock-like dioritic togranodioritic body of Tanadaro exhibits medium-grained hypidiomorphic-granular textures consisting oftwinned plagioclase, green hornblende, brown biotiteand quartz. K-feldspar occurs only in the granodiorite.Most plagioclase crystals are fresh, while others arealtered to sericite. Partial replacement by zeolites andcarbonate are also common. Green hornblende andbrown biotite are invariably associated with magnetite,the latter sometimes containing zircon inclusions.Some hornblende crystals contain corroded cores

(relicts) of colorless clinopyroxene. Secondary chloriteaggregates often contain ®ne granular epidote, titaniteand Ti-magnetite.

The basalts are generally porphyritic with 20±30modal % phenocrysts (plagioclase+olivine+clinopyr-oxene+hornblende+magnetite). The rock groundmassmay be intergranular or ¯uidal in texture (CIA-071).Plagioclase phenocrysts often contain aggregates ofsericite, clay, chlorite, actinolite, epidote and opaquegrains. Phenocrysts of clinopyroxene show partialalteration to chlorite, actinolite and calcite, whereasolivine is altered to serpentine (CIA-209, CIA-210,CIA-212) or iddingsite (CIA-071). The rock ground-mass is made up of plagioclase laths, intergranularchlorite, actinolite, hornblende and magnetite, some-times together with calcite and stilpnomelane. Veinletsof epidote have been observed in the basaltic andesiteCIA-073. All these rocks plot within the calc-alkaline®eld in the K2O±SiO2 diagram (Fig. 5B), and theirmulti-element patterns exhibit negative Nb and Zranomalies and enrichment in REE (Fig. 6b and b ').The K±Ar whole-rock ages of the granitoid samples

Fig. 5. K2O±SiO2 diagrams (Peccerillo and Taylor, 1976) for magmatic rocks belonging to the Santonian±Campanian episode (86±77 Ma) (A),

Maastrichtian±Thanetian episode (71-56 Ma) (B), Lutetian-Rupelian episode (42-31 Ma) (C), synthesis from late Cretaceous to Paleogene (D).

CA: calc±alkaline ®eld; KCA: high potassium calc±alkaline ®eld; SH: shoshonitic ®eld.


Fig.6.Mantle-norm

alizedincompatible

elem

ents

plots

(SunandMcD

onough,1989)ofSumbamagmaticrocks(A

)Santonian±Campanianmagmatism

,Masu

Mt.(B)Maastrichtian±Thanetian

magmatism

,Southerncoast

ofCentralSumba.(B

')Maastrichtian±Thanetianmagmatism

,Tanadaro

Mt.(C

)Lutetian±Rupelianmagmatism

,WestSumba.


range from 64.321.2 Ma (CIA-133), to 56.621.2 Ma(CIA-202). Separated minerals from the granitoidsamples yield slightly older ages for biotite than thecorresponding whole rock ages, whereas the feldsparsyield slightly younger ages. The observed chronologicalsequence is in agreement with the respective closuretemperatures of the networks for these minerals, andmay thus be interpreted as representative of the succes-sive cooling stages of the magmatic unit. Those of thevolcanic unit are scattered between 70.3 2 1.5 Ma(CIA-317) and 59.221.2 Ma (CIA-73), but may re¯ectdistinct and short eruption events, before and near theend of the emplacement of the Tanadaro Complex.

3.2.3. Lutetian±Rupelian episodeThe products of this magmatic episode make up the

volcanic complexes of Lamboya and Jawila (Wensinkand Van Bergen, 1995) in Western Sumba. Their K±Ar ages range from 42.323.2 Ma (CIA-062) to 31.421.1 Ma (CIA-493). Three distinct magmatic eventsmay be distinguished at ca 42, 37 and 31 Ma respect-ively. The volcanic units include porphyritic basalt(CIA-062), basaltic±andesite (CIA-717) and dacitic(CIA-021, CIA-493) ¯ows and pyroclastic depositswith 20±30 modal % of phenocrysts. These pheno-crysts include plagioclase, clinopyroxene, hornblende(CIA-493), magnetite and sometimes altered olivine(CIA-062). Partial alteration of pyroxene results inaggregates of chlorite and actinolite, whereas plagio-clase phenocrysts are dusted with sericite, clays, chlor-ite, calcite, epidote and opaque particles. Thegroundmass may show either intergranular texturewith plagioclase, clinopyroxene, olivine (CIA-062) oralternatively ¯uidal texture (CIA-021). A sample ofporphyritic granodiorite (CIA-044) is made up of pro-minent plagioclase, green hornblende and quartz,together with actinolite, chlorite and calcite as altera-tion products. The corresponding rock chemistryshows typical low-K to medium-K calc-alkaline fea-tures (Fig. 5C) with moderate enrichment in light rareearth elements (Rb, Ba, and K) and negative Nb, Zr,Ti anomalies (Fig. 6C).

Corresponding incompatible element patternsinclude nearly ¯at to moderately enriched spectra, andthe bulk of the sequence is clearly less enriched in K2Oand other large ion lithophile elements than the LateCretaceous and Late Cretaceous±Paleocene sequences.Similar features have been described from the Tertiarymagmatic evolution of Java (Soeria-Atmadja et al.,1994; Sutanto, 1993). In Java, although incompatibleelement contents of the magmatic rocks tend toincrease roughly with time, as is usual in most islandarcs (Maury et al., 1998), a temporal gap in magmaticactivity was followed by the emplacement of low-Klavas when volcanic activity resumed.

4. Geodynamic implications and conclusions

Abdullah (1994) distinguished four sedimentarycycles in Sumba. The ®rst cycle (Late Cretaceous±Paleocene) is represented by marine turbidites of theLasipu Formation. It was accompanied by two majorcalc±alkaline magmatic episodes, the Santonian±Cam-panian episode (86±77 Ma) and the Maastrichtian±Thanetian one (71±56 Ma) respectively. The secondcycle (Paleogene) was marked by volcaniclastic andneritic sedimentation accompanied by the third mag-matic episode of Lutetian±Rupelian age (42±31 Ma).The following Neogene sedimentary cycle was a periodof widespread transgression, characterized by rapidsedimentation in a deep sea environment (Fortuin etal., 1992, 1994, 1997). This syntectonic turbiditic sedi-mentation which contains reworked volcanic materialshas also been observed in the neighbouring Lombokand Savu basins. The volcanic centers, providing thesource of the volcaniclastic sediments, were probablylocated on Flores (Hendaryono, 1998). Nevertheless, itis not impossible that some of these magmatic pro-ducts were derived, through uplift and erosion, fromolder Sumba volcanic rocks. During all these events,Sumba was then a part, more or less uplifted, of afore-arc basin within the active Sunda subduction sys-tem. The fourth cycle (Quaternary) was marked by theuplift of terraces, beginning 1 Ma ago.

The distribution of the ages for the K±Ar dated vol-canics in Sumba suggest a westward shift of magma-tism with time (Fig. 3; Table 1). Moreover, noevidence of Neogene magmatic activity has beenrecorded anywhere on Sumba.

However, similarities between Sumba and the South-western Sulawesi magmatic belt (van Leeuwen, 1981;Simandjuntak, 1993; Bergman et al., 1996; Wakita etal., 1996), with respect to both the Late Cretaceous±Paleocene magmatism and the stratigraphy, supportthe idea that Sumba was part of an `Andean' mag-matic arc (Fig. 7A) near the Western Sulawesi mag-matic belt (Abdullah, 1994; Abdullah et al., 1996;Soeria-Atmadja et al., 1998) and near the SoutheastKalimantan coast (Meratus Mountains) (Yuwono etal., 1988; Wensink, 1997; Rampnoux et al., 1997) atthe margin of Asiatic Plate. Thus, during the Paleo-gene, the rate of movement of the Indo-AustralianPlate decreased, leading to the generation of a back-arc basin and the formation of a marginal sea (Hamil-ton, 1979). Back-arc spreading resulted in the south-ward migration of Sumba (Fig. 7B) (Rangin et al.,1990; Lee and Lawver, 1995). Southward migration iscon®rmed by new paleomagnetic data (Wensink,1994). From Neogene to Quaternary times Sumbaisland was trapped within the forearc basin in front ofthe Eastern Sunda volcanic arc (Fig. 7C).

Presently, the collision of Australia with the Banda


Fig.7.Cartoonsdepictingthefourmain

stages

oftectonic

evolutionofSumba:(A

)Late

Cretaceous±Paleocene,

(B)Paleogene,

(C)Middle

Miocene±Pliocene,

(D)Quaternary.


Arc is progressing north-westwards (Fig. 7D) causingSumba to be uplifted at a rate of 0.5 mm/year as evi-denced by the reef limestone terraces (Pirazzoli et al.,1990; Abdullah, 1994; Hendaryono, 1998).

The relatively simple tectonics of Sumba suggeststhat the island has never been subjected to intense de-formation. This implies that from Late Cretaceous±Neogene time Sumba has never been involved in thecollision between the Indian±Australian and Asiaticplates, except during a minor compressive episode inthe Paleogene.

The new data presented in this paper con®rms theAsian (Sundaland) origin of Sumba.

Acknowledgements

J. Cotten and J.C. Philippet from UBO and CNRSare kindly thanked for respectively the performed geo-chemical analyses and the numerous K±Ar ages.

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