Directory UMM :Data Elmu:jurnal:J-a:Journal of Asian Earth Science:Vol18.Issue5.2000:

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

The evolution of Sumba Island (Indonesia) revisited in the light
of new data on the geochronology and geochemistry of the
magmatic rocks
C.I. Abdullah a, J.-P. Rampnoux b,*, H. Bellon c, R.C. Maury c, R. Soeria-Atmadja a
a

Teknik Geologi, Institut Teknologi Bandung, Jalan Ganesa 10, Bandung, Indonesia
b
Universite de Savoie, 73376 Le Bourget du Lac Cedex, France
c
UMR 6538, IUEM, Universite 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 Eastern
Indonesia, 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 from
Sumba 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 in
Eastern Sumba; (ii) the Maastrichtian±Thanetian episode (71±56 Ma) represented by the volcanic and plutonic units of
Sendikari 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 to
the Sunda±Banda arc as it represents an isolated sliver
of probable continental crust to the south of active
volcanic islands (Sumbawa, Flores) within the forearc
basin (Fig. 1). It is situated to the north of the transition from the Java Trench (subduction front) to the
Timor Trough (collision front). It does not show the
e€ects of strong compression, in contrast to islands of
the outer arc system (Savu, Roti, Timor), while mag-

* Corresponding author.

E-mail address: jean-paul.rampnoux@univ-savoie.fr (J.P. Rampnoux).

matic units make up a substantial part of the Late
Cretaceous to Paleogene stratigraphy.
Bathymetrically, Sumba stands out as a ridge that
separates the Savu forearc basin (>3000 m depth) in
the east and the Lombok forearc basin (>4000 m
depth) in the west. Seismic refraction studies show
(Barber et al., 1981) that it is made up of 24 km thick
continental crust (Chamalaun et al., 1981). Based on
tectonic studies, complemented by paleomagnetism
and geochemistry, several workers consider Sumba to
be a microcontinent or continental fragment (Hamilton, 1979; Chamalaun and Sunata, 1982; Wensink,
1994, 1997; Vroon et al., 1996; Soeria-Atmadja et al.,
1998).
Three main geodynamic models for Sumba have
been proposed by Chamalaun et al. (1982) and Wen-

1367-9120/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved.
PII: S 1 3 6 7 - 9 1 2 0 ( 9 9 ) 0 0 0 8 2 - 6

534

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

sink (1994) as follows: (i) Sumba was originally a part
of the Australian Continent which was detached when
the Wharton basin was formed, drifted northwards
and was subsequently trapped behind the eastern Java
Trench (Audley-Charles, 1975; Otofuji et al., 1981); (ii)
Sumba was once part of Sundaland which drifted
southwards during the opening of the Flores Basin
(Hamilton,1979; Von der Borch et al., 1983; Rangin et
al., 1990); and (iii) Sumba was either a microcontinent
or part of a larger continent within the Tethys, which
was later fragmented (Chamalaun and Sunata, 1982).
The present paper represents an attempt to resolve
this problem.

2. Stratigraphy

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

Mesozoic age, unconformably overlain by considerably
less deformed Tertiary and Quaternary deposits; the
total thickness of which is more than 1000 m (van
Bemmelen, 1949). The Quaternary coral reef terraces,
which cap the seaward edge of the Neogene Sumba
Formation, are almost continuously exposed along the
western, northern and eastern coasts of Sumba
(Hamilton, 1979).
2.1. Mesozoic series
Mesozoic rocks are exposed principally along the
coast immediately south of West Sumba (Patiala,
Wanokaka and Konda Maloba) and in the southern
part of the Tanadaro Mountains (Nyengu and Labung

rivers). The sediments are typically carbonaceous siltstones with volcanogenic mudstones, sometimes showing signs of low-grade metamorphism, interbedded
with sandstones, conglomerates, limestones and volcaniclastic debris. They are crosscut by Late Cretaceous
intrusions which range in composition from microgabbro to quartz-diorite, and also by granodioritic and
calc±alkaline dykes of Paleogene age. The sediments
show 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±546

ing. These sediments constitute the Lasipu Formation
(Prasetyo, 1981). Microfossil assemblages in some
samples indicate Coniacian to Early Campanian ages
(Burollet and SalleÂ, 1982); many Inoceramus sp. are
present. The detrital materials suggest either a continental origin, or an island arc environment ; it would
appear to be essentially a Mesozoic submarine fan
with shallow-water deposits (Von der Borch et al.,
1983) or an open marine bathyal environment (Burollet and SalleÂ, 1982).

2.2. Paleogene series

During the Paleogene Sumba was a part of magmatic arc characterized by calc±alkaline volcanic rock
series (Western Sumba) and shallow marine sediments.
The corresponding deposits include tu€s, ignimbrites,
greywackes, intercalations of foraminiferal limestones,
marls, micro-conglomerates and claystones. These
rocks unconformably overlie Mesozoic rocks and are
in turn unconformably overlain by the Neogene Series.

535

2.3. Neogene series
O€shore seismic re¯ections show that Neogene
deep-water sediments make up the early sedimentary
sequence 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). Their
occurrence re¯ects the stable position of the Sumba
Ridge within the forearc since the initiation of the
Sunda arc±trench system during the late Oligocene
and the early Miocene (Silver et al., 1983; Reed, 1985;

Barberi et al., 1987). The Neogene sediments on
Sumba display two di€erent facies: in the western part,
they are represented by mostly reef limestones, bioclastic limestones, chalky limestones and marls, interbedded with tu€aceous marls, whereas the sediments
from the eastern part of Sumba are dominantly volcanic turbidites with interbedded pelagic chalks and
chalky limestones (Fortuin et al., 1994). In the central
part of Sumba, these sedimentary facies show inter®ngering relations. These rocks are undisturbed tectonically.

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

Fig. 3. Stratigraphic columns of Sumba.

536

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

537

2.4. Quaternary series

3. Calc±alkaline magmatism

The whole island has been rapidly uplifted to its present elevation, as is indicated by the Quaternary terraces, which reached a height of not less than 500 m
(Jouannic et al., 1988), at a rate of 0.5 mm/yr, in the
northern and central part of Sumba (Pirazzoli et al.,
1991). These terraces consist of sandstones, conglomerates, marls and prominent reef limestones, which
unconformably overlie gently dipping Neogene sediments along the west, north and east coasts. Locally,
Quaternary deposits rest unconformably on Mesozoic
rocks along the southwestern coast.

A set of 24 magmatic rock samples representing
granitoid intrusions, lava ¯ows and subvolcanic dykes
of ma®c to intermediate composition from various outcrops within the investigated area (Fig. 4) were selected
for 40K±40Ar dating (Table 1). 40K±40Ar dating as well
as chemical analyses (major and trace elements;
Table 2) on these rocks were performed at the Universite de Bretagne Occidentale, Brest, France. Numerous
other magmatic rock samples were studied petrographically.
3.1. Analytical procedures
40

K±40Ar age dating has been carried out on whole
rocks for the volcanic rocks and on whole rocks and
carefully separated minerals (fresh biotite to slightly
chloritized biotite, and the non-magnetic fraction,
including both quartz and feldspar) from plutonic rock
outcrops of Tanadoro massif. Calculated isotopic ages
are presented in Table 1, together with the most
characteristic parameters of the measurements i.e. the
40
ArR (radiogenic argon 40), the percentage of 40ArR
versus the total amount of 40ArR and atmospheric
40
Ar. For whole rock analyses, samples were crushed
and sieved to 315±160 mm grain size and then cleaned
with distilled water. This fraction was used (1) for
argon extraction under high vacuum by HF heating
and (2) for potassium analysis by atomic absorption
spectrometry after reduction to a powder. Age calculations were carried out using the constants recommended by Steiger and Jaeger (1977) with one s
error calculated according to Mahood and Drake

(1982). Chronostratigraphical signi®cance of the isotopic ages is based on the 1994 geological time scale
(Odin, 1994). Major and trace element data were
obtained by ICP±AES methods according to the procedures described by Cotten et al. (1995). The corresponding analytical precisions are better than 2% for
most major elements and 5% for most trace elements.
3.2. Geochemical and geochronological results

Fig. 4. Location of sampled magmatic rocks for age determinations
and geochemical analyses (for the key, see Fig. 2).

Taking account of duplicate analyses for eleven
samples as listed in Table 1, mean 40K±40Ar wholerock ages2the greater error are reported in Fig. 4 and
are discussed below.
Three periods of magmatic activity were recognized
by 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 by
Chamalaun and Sunata (1982), Burollet and SalleÂ
(1982), Van Halen (1996) and Wensink (1997).

538

Table 1
40
K±40Ar isotopic ages of magmatic rocks of Sumba (see text for analytical methods)
Sample No.

Location

40

ArR
(eÿ7 cm3/g)

40

ArR
(%)

K2O
(wt %)

LOI
(wt %)

Analysis No.

85.421.6
83.721.8
85.922.0
78.621.7
85.422.0
80.921.9a
84.421.9
Bas. andesiteb
76.921.8a
77.621.8
Maastrichtian±Paleocene episode

101.60
48.59
71.44
32.39
21.71
49.11
51.24
40.25
40.63

84.9
70.8
72.0
84.2
73.8
81.1
87.8
74.8
72.1

3.68
1.76
2.52
1.25
0.79
1.84

0.58
0.73
1.45
0.93
1.67
1.48

1.59

3.15

2794
2813
4715
2814
3933
3922
3921
4362
3947

Basalt

71.121.3a
69.521.5

20.35
19.87

89.2
90.3

0.87

3.48

2792
2793

Microgabbro

70.121.4a
68.721.4
65.221.3
68.021.3
65.321.8

49.11
48.11
46.27
73.94
55.52

79.9
79.7
77.8
82.2
75.8

2.13

2.95

2.16
3.31
2.59

2.81
1.16
2.60

2857
2856
2880
2879
2878

64.321.2
66.621.3
63.621.5
Diorite
64.321.2a
61.821.2
62.121.2
62.921.5
Diorite
61.821.2
63.421.2
57.721.6
Diorite
57.621.3
61.021.2
61.421.4
Diorite
56.621.2
Basalt
66.521.6
Bas. andesiteb
59.221.2a
59.221.2
Lutetian±Rupelian episode

55.78
173.40
44.03
33.74
32.45
69.51
51.04
35.09
128.40
17.96
33.00
71.97
32.23
28.01
15.73
16.12
16.11

85.7
79.0
66.9
82.8
82.7
79.0
67.3
84.9
88.1
57.7
69.4
81.6
67.5
75.3
70.0
78.8
80.1

2.74
7.93
2.11
1.60

0.68

43.523.2a
41.222.8

8.09
7.66

26.4
28.1

0.57

Type

Age (Ma) 2error

Santonian±Campanian episode
WR
WR
WR
WR
WR
WR

Cape Malanggu (Pameti Hawu)
Cape Malanggu (Pameti Hawu)
Cape Malanggu
Cape Malanggu
Tanarara (Km 5)
Gunung Kapunduk (Nggongi valley)

CIA-735

WR

Road from Tatunggu to Tanarara (Km 2)

Wanokaka area
CIA-317
WR

Western side of Wanokaka beach

Sendikari and Tengairi Gulfs
CIA-339
WR
Eastern part of the Sendikari Gulf (Cape Teki)
CIA-491
CIA-481
CIA-487
Mt Tanadaro
CIA-133

CIA-132

CIA-115

CIA-204

CIA-202
CIA-71
CIA-73

Granodiorite
Microdiorite
Andesite
Bas. andesiteb
Basalt
Bas. andesiteb

WR
WR
WR

Eastern part of the Sendikari Gulf (Cape Teki)
Bottom of the Tengairi Gulf
Bottom of the Tengairi Gulf

Bas. andesiteb
Bas. andesiteb
Bas. andesiteb

WR
Bio
Fds
WR

Mt Tanadaro (Western part)

Granodiorite

Chl
Fds
WR
Bio
Fds
WR
Chl
Fds
WR
WR
WR

Mt Lamboya
CIA-62
WR

Mt Tanadaro (Western part)

Pamalar river (South-western edge of Mt Tanadaro)

Pamalar river (South western edge of Mt Tanadaro)

Pamalar river (South western edge of Mt Tanadaro)
Nyengu river
Nyengu river

Eastern part of Rua beach (Cape Watumete)

Basalt

3.41
1.54
1.73
6.17
0.95
1.75
3.60
1.60
1.51
0.72
0.83

1.40

1.49

1.59

1.59
2.29
1.88

2.88

2848
2830
2833
2790
2791
2851
2850
2846
2826
2823
2753
2852
2853
2754
4722
2633
2632

2952
2923

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

Mt Masu
CIA-347
CIA-348
CIA-351
CIA-353
CIA-728
CIA-733

Duplicate analyses of the same whole rock, WR fraction for samples: CIA-733, CIA-735, CIA-317, CIA-339, CIA-132, CIA-73, CIA-62, CIA-44, CIA-21, CIA-717 and CIA-493. WR: whole
rock fraction analysis. Bio: separated biotite. Fds: separated feldspar2quartz. Chl: separated chloritized biotite.
b
Bas. andesite=basaltic andesite.

WR
CIA-493

Kareka Hulu (Eastern edge of Mt Jawila)

Dacite

7.59
7.41
20.22
20.04
Bas. andesiteb
Dikira village (Southern edge of Mt Jawila)
WR
Mt Jawila
CIA-717

a

1.77
1.97

3920
3919
2864
2865
1.55
0.63

0.79
1.63
WR
CIA-21

Wanokaka valley (Praimaraga area)

Dacite

37.020.9a
36.121.0
31.521.1a
31.320.8

60.5
44.2
50.0
64.4

1.55

19.48
19.35
19.03
18.95
WR
CIA-44

Dassang valley

Granodiorite

36.720.7a
36.420.7
35.820.7a
35.720.7

78.5
78.9
82.5
83.3

1.63

2629
2628
2630
2631

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

539

Erupted magmas display the characteristics of a predominantly calc±alkaline (CA) and a minor potassic
calc±alkaline (KCA) series (Fig. 5); they are characterized by variable K2O contents, relatively high Al2O3
and low TiO2 contents, suggesting a typical island arc
environment. Such anity is consistent with their
moderately to fairly enriched incompatible element
patterns (Fig. 6) showing negative anomalies in Nb,
Zr, and to a lesser extent in Ti, typical of subductionrelated magmas.
Products of the ®rst magmatic episode are represented by the granitoid stock-like body of Mt. Masu
(southern coast of East Sumba) accompanied with
basaltic andesites and andesites. To the second magmatic episode belongs the Tanadaro granitoid intrusion and subvolcanic basaltic and basaltic andesitic
intrusions along the gulfs of Tengairi and Sendikari.
Products of the latest magmatic episode are exposed
south and west of Waikabubak and include the granitoid, basaltic and dacitic rocks of Mt Lamboya and
Mt Jawila areas in West Sumba. The petrographic and
geochemical characteristics of each magmatic episode
are discussed successively.
3.2.1. Santonian±Campanian episode
The products from this magmatic episode make up
the Masu Formation of East Sumba (E€endi and
Apandi, 1994), and consist of an assemblage of pyroclastic breccias, tu€s and lava ¯ows, intruded by granodiorite. The granodiorite is a medium-grained
equigranular rock (CIA-347) which is made up of
microperthite, zoned plagioclase, hornblende and
quartz. Microperthitic grains are often dusted with sericite, clay and opaque particles. Clinopyroxene shows
partial alteration to pale green hornblende whereas
clusters of dark brown biotite between feldspar, quartz
and hornblende, are altered to chlorite, titanite and
iron-oxides. The andesites (CIA-351; CIA-426) contain
20±25 modal % phenocrysts (plagioclase, clinopyroxene, green hornblende, magnetite and apatite) whereas
basaltic andesite (CIA-353) contains no more than 10
modal % phenocrysts (plagioclase, green hornblende
and magnetite). Plagioclase phenocrysts are generally
zoned and contain minute sericite, clays, iron-oxides,
magnetite and titanite grains. The rock groundmass is
composed of plagioclase laths, chlorite, hornblende
and magnetite.
The whole rock major trace element compositions
are typically calc±alkaline to potassic calc-alkaline
(Fig. 5A) and their incompatible element patterns
clearly show the negative Nb, Zr and Ti anomalies
typical of arc volcanics (Fig. 6A). Two K±Ar ages
were obtained from the granitoid intrusion, respectively at 83.7 2 1.8 Ma (CIA-348) and 85.4 2 1.6 Ma
(CIA-347), whereas ®ve volcanic rocks (basalt, basal-

540

Table 2
Major and trace element analyses of Sumba magmatic rocks. ICP-AES data, J. Cotten, Brest (see text for analytical methods)
Maastrichtian±Paleocene magmatism

Masu Mt

South Coast

Late Eocene±Early Oligocene
(Lutetian±Rupelian) magmatism
Tanadaro Mt

Lamboya and Jawilla 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

48.8
SiO2
0.98
TiO2
Al2O3
18.25
Fe2O3 10.95
MnO
0.19
MgO
5.7
CaO
9.05
2.91
Na2O
K2O
0.76
0.21
P2O5
LOI
1.67
Total
99.47
ppm
Cr
27
Nl
31
Co
35
Sc
29
V
326
Rb
10
Ba
184
Sr
562
Nb
2.3
La
10.5
Ce
24.5
Nd
16
Zr
69
Eu
1.22
Y
20
Dy
3.4
Er
1.9
Yb
1.83

54
0.77
17.96
7.65
0.16
3.67
6.4
3.68
1.54
0.18
3.15
99.16
27
14
21
21
227
35.5
510
570
2.6
12.5
26
15
88
1.02
16.8
2.75
1.5
1.6

55.3
0.84
19
8.4
0.1
2.61
7.63
3.27
1.15
0.26
0.93
99.49
2
3
14
23
190
33
365
653
2.9
14.3
19
26
1.2
25
4
2.3
1.95

56.4
0.85
16.75
8.8
0.21
2.69
7.4
3.4
1.68
0.34
0.73
99.25
1
3
18
23
215
36
390
610
2.9
17.9
24
16
1.3
29.5
4.9
3.2
2.7

56.5
0.84
17.25
8.5
0.17
3.29
7.3
2.88
1.75
0.2
1.48
100.16

59
0.71
16.75
6.9
0.15
2.9
6.05
3.13
2.52
0.23
1.45
99.79

64.5
0.47
16.52
4.58
0.1
1.75
4.42
3.6
3.65
0.11
0.58
100.28

52.6
1.05
16.95
8.8
0.15
3.54
9.16
2.54
0.83
0.24
3.48
99.7

15
9
22
21
226
26.5
355
485
3.4
14.8
32
18.4
106
1.13
20.5
3.55
2.05
1.97

6
6
17
20.5
170
51
510
490
2.3
15.35

8
6
11
12
106
102
560
381
3.95
17.9

32
29
32
30
310
20
160
340
1.4
9.75

21
60
1.35
22
3.6
1.9
1.85

20
18
1
20
3.2
2.2
1.8

15
100
1.1
27
4.4
2.5
2.4

53
0.84
17.1
8.65
0.16
4.46
7.3
3.81
2
0.18
2.97
100.47

53.5
0.94
18.5
8.4
0.13
3.33
5.4
3.69
2.55
0.23
2.6
99.27

53.8
0.82
17.25
8.56
0.16
4.2
6.35
3.32
2.08
0.17
2.81
99.52

53.4
0.91
20.25
8.4
0.13
3.32
3.3
3.95
3.2
0.23
1.16
98.25

70
20
26
28
250

19
5
22
23
233

48
12
25
27
240

18
9
22
24
222

292
490
2.3
10.1
25
15
89
1.12
22
3.85
2.2
2.07

382
705
3.1
13.1
31
19.3
137
1.12
27
4.4
2.5
2.48

450
560
2.2
9.4
22
14
59
1.05
22
3.6
2
1.92

890
760
2.9
12.5
31
19
125
1.15
25
4.5
2.6
2.40

49.35
0.74
21.15
8.76
0.15
4.55
9.59
3.03
0.72
0
2.29
100.33

55.4
0.7
17
8.22
0.16
4.34
7.43
3.62
1.63
0.08
1.59
100.17

56.1
0.63
20.2
6.55
0.1
2.07
7.95
3.88
0.83
0.06
1.88
100.25

32
26
31
29
196
19
191
646
2
5

48
18
26
29
157
41
228
430
5
18

10
9
19
15
110
18
131
525
2.5
7.5

9
39
0.5
17
2.2
1.7
1.45

16
14
1
30
4.08
2.2
2.13

12
27
0.9
20
2.8
1.7
1.52

57
0.75
17.21
7.38
0.13
3.65
7.17
3.46
1.2
0.07
1.59
99.61
48
25
24
23
143
37
229
408
4
11
14
8
1
23
3
1.6
1.86

58.1
0.84
17.46
6.9
0.13
3.53
7.22
3.54
1.04
0.06
1.49
100.31

59.4
0.62
16.7
7.03
0.12
3.26
6.36
3.49
1.63
0.04
1.4
100.05

47
26
22
21
131
32
188
409
5
11

39
22
19
21
142
45
300
404
4
14

13
9
1
22
2.9
2.1
1.73

15
12
0.9
27
3.4
2.4
2.25

67
0.56
14.8
4.62
0.08
2.13
4.3
3.48
2.33
0
0.68
99.98
34
18
16
14
95
85
350
293
4
15
15
8
0.7
28
3.6
2.3
2.2

51.5
0.78
19.15
8.12
0.16
5.6
8.1
3.6
0.55
0.2
2.88
100.64

55
0.94
17.4
9.45
0.18
3.37
7.7
3.53
0.59
0.16
1.55
99.86

60.8
0.76
15.44
7.3
0.13
3.09
6.12
3.4
1.64
0.02
1.55
100.25

157
85
30
23
174
6
125
410
1.8
10.2

11
5.5
20
31
269
8
81
302
1.8
5.8
15.5
10.5
71
1
22.5
3.55
2.35
2.29

24
14
21
23
140
39
134
210
5
9.5

13.5
83
1.2
21
3.1
2.1
1.9

15
26
1
33
4.7
2.6
2.74

64.5
0.67
16.1
4.52
0.09
1.63
3.45
4.6
1.87
0.18
1.77
99.38
11
1
11
17
55
17.7
1010
231
2.8
18
42
28.5
148
1.95
55
8.6
5.3
5

67.2
0.9
126
3.57
0.08
0.38
3.29
5.74
1.4
0.22
0.79
99.57
12
6
7
20
56
28
187
282
9
15
24
219
1.8
54
8.1
2.7
4.86

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

%

Santonian±Campanian magmatism

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

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), 85.4 2 2 Ma (CIA-728) and 85.9 2 2.0 Ma
(CIA-351).
3.2.2. Maastrichtian±Thanetian episode
The corresponding magmatic rocks are exposed in
the Tanadaro area, in East Waikabubak and along the
coast of southeast Waikabubak. These rocks include
mostly granodioritic and dioritic intrusions and volcanic units of chie¯y basaltic composition (basalts and
minor basaltic andesites). The stock-like dioritic to
granodioritic body of Tanadaro exhibits mediumgrained hypidiomorphic-granular textures consisting of
twinned plagioclase, green hornblende, brown biotite
and quartz. K-feldspar occurs only in the granodiorite.
Most plagioclase crystals are fresh, while others are
altered to sericite. Partial replacement by zeolites and
carbonate are also common. Green hornblende and
brown biotite are invariably associated with magnetite,
the latter sometimes containing zircon inclusions.
Some hornblende crystals contain corroded cores

541

(relicts) of colorless clinopyroxene. Secondary chlorite
aggregates often contain ®ne granular epidote, titanite
and Ti-magnetite.
The basalts are generally porphyritic with 20±30
modal % phenocrysts (plagioclase+olivine+clinopyroxene+hornblende+magnetite). The rock groundmass
may be intergranular or ¯uidal in texture (CIA-071).
Plagioclase phenocrysts often contain aggregates of
sericite, clay, chlorite, actinolite, epidote and opaque
grains. Phenocrysts of clinopyroxene show partial
alteration to chlorite, actinolite and calcite, whereas
olivine is altered to serpentine (CIA-209, CIA-210,
CIA-212) or iddingsite (CIA-071). The rock groundmass is made up of plagioclase laths, intergranular
chlorite, actinolite, hornblende and magnetite, sometimes together with calcite and stilpnomelane. Veinlets
of epidote have been observed in the basaltic andesite
CIA-073. All these rocks plot within the calc-alkaline
®eld in the K2O±SiO2 diagram (Fig. 5B), and their
multi-element patterns exhibit negative Nb and Zr
anomalies 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.

542
C.I. Abdullah et al. / Journal of Asian Earth Sciences 18 (2000) 533±546
Fig. 6. Mantle-normalized incompatible elements plots (Sun and McDonough, 1989) of Sumba magmatic rocks (A) Santonian±Campanian magmatism, Masu Mt. (B) Maastrichtian±Thanetian
magmatism, Southern coast of Central Sumba. (B ') Maastrichtian±Thanetian magmatism, Tanadaro Mt. (C) Lutetian±Rupelian magmatism, West Sumba.

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

range from 64.321.2 Ma (CIA-133), to 56.621.2 Ma
(CIA-202). Separated minerals from the granitoid
samples yield slightly older ages for biotite than the
corresponding whole rock ages, whereas the feldspars
yield slightly younger ages. The observed chronological
sequence is in agreement with the respective closure
temperatures of the networks for these minerals, and
may thus be interpreted as representative of the successive cooling stages of the magmatic unit. Those of the
volcanic unit are scattered between 70.3 2 1.5 Ma
(CIA-317) and 59.221.2 Ma (CIA-73), but may re¯ect
distinct and short eruption events, before and near the
end of the emplacement of the Tanadaro Complex.

3.2.3. Lutetian±Rupelian episode
The products of this magmatic episode make up the
volcanic complexes of Lamboya and Jawila (Wensink
and Van Bergen, 1995) in Western Sumba. Their K±
Ar ages range from 42.323.2 Ma (CIA-062) to 31.42
1.1 Ma (CIA-493). Three distinct magmatic events
may be distinguished at ca 42, 37 and 31 Ma respectively. The volcanic units include porphyritic basalt
(CIA-062), basaltic±andesite (CIA-717) and dacitic
(CIA-021, CIA-493) ¯ows and pyroclastic deposits
with 20±30 modal % of phenocrysts. These phenocrysts include plagioclase, clinopyroxene, hornblende
(CIA-493), magnetite and sometimes altered olivine
(CIA-062). Partial alteration of pyroxene results in
aggregates of chlorite and actinolite, whereas plagioclase phenocrysts are dusted with sericite, clays, chlorite, calcite, epidote and opaque particles. The
groundmass may show either intergranular texture
with plagioclase, clinopyroxene, olivine (CIA-062) or
alternatively ¯uidal texture (CIA-021). A sample of
porphyritic granodiorite (CIA-044) is made up of prominent plagioclase, green hornblende and quartz,
together with actinolite, chlorite and calcite as alteration products. The corresponding rock chemistry
shows typical low-K to medium-K calc-alkaline features (Fig. 5C) with moderate enrichment in light rare
earth elements (Rb, Ba, and K) and negative Nb, Zr,
Ti anomalies (Fig. 6C).
Corresponding incompatible element patterns
include nearly ¯at to moderately enriched spectra, and
the bulk of the sequence is clearly less enriched in K2O
and other large ion lithophile elements than the Late
Cretaceous and Late Cretaceous±Paleocene sequences.
Similar features have been described from the Tertiary
magmatic evolution of Java (Soeria-Atmadja et al.,
1994; Sutanto, 1993). In Java, although incompatible
element contents of the magmatic rocks tend to
increase roughly with time, as is usual in most island
arcs (Maury et al., 1998), a temporal gap in magmatic
activity was followed by the emplacement of low-K
lavas when volcanic activity resumed.

543

4. Geodynamic implications and conclusions
Abdullah (1994) distinguished four sedimentary
cycles in Sumba. The ®rst cycle (Late Cretaceous±
Paleocene) is represented by marine turbidites of the
Lasipu Formation. It was accompanied by two major
calc±alkaline magmatic episodes, the Santonian±Campanian episode (86±77 Ma) and the Maastrichtian±
Thanetian one (71±56 Ma) respectively. The second
cycle (Paleogene) was marked by volcaniclastic and
neritic sedimentation accompanied by the third magmatic episode of Lutetian±Rupelian age (42±31 Ma).
The following Neogene sedimentary cycle was a period
of widespread transgression, characterized by rapid
sedimentation in a deep sea environment (Fortuin et
al., 1992, 1994, 1997). This syntectonic turbiditic sedimentation which contains reworked volcanic materials
has also been observed in the neighbouring Lombok
and Savu basins. The volcanic centers, providing the
source of the volcaniclastic sediments, were probably
located on Flores (Hendaryono, 1998). Nevertheless, it
is not impossible that some of these magmatic products were derived, through uplift and erosion, from
older Sumba volcanic rocks. During all these events,
Sumba was then a part, more or less uplifted, of a
fore-arc basin within the active Sunda subduction system. The fourth cycle (Quaternary) was marked by the
uplift of terraces, beginning 1 Ma ago.
The distribution of the ages for the K±Ar dated volcanics in Sumba suggest a westward shift of magmatism with time (Fig. 3; Table 1). Moreover, no
evidence of Neogene magmatic activity has been
recorded anywhere on Sumba.
However, similarities between Sumba and the Southwestern Sulawesi magmatic belt (van Leeuwen, 1981;
Simandjuntak, 1993; Bergman et al., 1996; Wakita et
al., 1996), with respect to both the Late Cretaceous±
Paleocene magmatism and the stratigraphy, support
the idea that Sumba was part of an `Andean' magmatic arc (Fig. 7A) near the Western Sulawesi magmatic belt (Abdullah, 1994; Abdullah et al., 1996;
Soeria-Atmadja et al., 1998) and near the Southeast
Kalimantan coast (Meratus Mountains) (Yuwono et
al., 1988; Wensink, 1997; Rampnoux et al., 1997) at
the margin of Asiatic Plate. Thus, during the Paleogene, the rate of movement of the Indo-Australian
Plate decreased, leading to the generation of a backarc basin and the formation of a marginal sea (Hamilton, 1979). Back-arc spreading resulted in the southward migration of Sumba (Fig. 7B) (Rangin et al.,
1990; Lee and Lawver, 1995). Southward migration is
con®rmed by new paleomagnetic data (Wensink,
1994). From Neogene to Quaternary times Sumba
island was trapped within the forearc basin in front of
the Eastern Sunda volcanic arc (Fig. 7C).
Presently, the collision of Australia with the Banda

544
C.I. Abdullah et al. / Journal of Asian Earth Sciences 18 (2000) 533±546

Fig. 7. Cartoons depicting the four main stages of tectonic evolution of Sumba: (A) Late Cretaceous±Paleocene, (B) Paleogene, (C) Middle Miocene±Pliocene, (D) Quaternary.

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

Arc is progressing north-westwards (Fig. 7D) causing
Sumba to be uplifted at a rate of 0.5 mm/year as evidenced by the reef limestone terraces (Pirazzoli et al.,
1990; Abdullah, 1994; Hendaryono, 1998).
The relatively simple tectonics of Sumba suggests
that the island has never been subjected to intense deformation. This implies that from Late Cretaceous±
Neogene time Sumba has never been involved in the
collision between the Indian±Australian and Asiatic
plates, except during a minor compressive episode in
the Paleogene.
The new data presented in this paper con®rms the
Asian (Sundaland) origin of Sumba.

Acknowledgements
J. Cotten and J.C. Philippet from UBO and CNRS
are kindly thanked for respectively the performed geochemical analyses and the numerous K±Ar ages.

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