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
aa
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 mineral40K±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.72000 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 tran-sition from the Java Trench (subduction front) to the Timor Trough (collision front). It does not show the eects of strong compression, in contrast to islands of the outer arc system (Savu, Roti, Timor), while
mag-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 (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 have been proposed by Chamalaun et al. (1982) and
Wen-1367-9120/00/$ - see front matter72000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 7 - 9 1 2 0 ( 9 9 ) 0 0 0 8 2 - 6
* Corresponding author.
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; Eendi 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 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 silt-stones with volcanogenic mudsilt-stones, sometimes show-ing signs of low-grade metamorphism, interbedded with sandstones, conglomerates, limestones and volca-niclastic debris. They are crosscut by Late Cretaceous intrusions which range in composition from microgab-bro 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 conti-nental 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 (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 rock series (Western Sumba) and shallow marine sediments. The corresponding deposits include tus, 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.
2.3. Neogene series
Oshore 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 dierent facies: in the western part, they are represented by mostly reef limestones, bioclas-tic limestones, chalky limestones and marls, inter-bedded with tuaceous marls, whereas the sediments from the eastern part of Sumba are dominantly volca-nic turbidites with interbedded pelagic chalks and chalky limestones (Fortuin et al., 1994). In the central part of Sumba, these sedimentary facies show inter®n-gering relations. These rocks are undisturbed tectoni-cally.
Fig
.
3.
Stratig
rap
hic
co
lumns
of
Sumba.
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 the northern and central part of Sumba (Pirazzoli et al., 1991). These terraces consist of sandstones, conglomer-ates, marls and prominent reef limestones, which unconformably overlie gently dipping Neogene sedi-ments along the west, north and east coasts. Locally, Quaternary deposits rest unconformably on Mesozoic rocks along the southwestern coast.
3. Calc±alkaline magmatism
A set of 24 magmatic rock samples representing granitoid intrusions, lava ¯ows and subvolcanic dykes of ma®c to intermediate composition from various out-crops within the investigated area (Fig. 4) were selected for40K±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 Univer-site de Bretagne Occidentale, Brest, France. Numerous other magmatic rock samples were studied petrogra-phically.
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 calcu-lations were carried out using the constants rec-ommended by Steiger and Jaeger (1977) with one s
error 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 were obtained by ICP±AES methods according to the pro-cedures described by Cotten et al. (1995). The corre-sponding analytical precisions are better than 2% for most major elements and 5% for most trace elements.
3.2. Geochemical and geochronological results
Taking account of duplicate analyses for eleven samples as listed in Table 1, mean 40K±40Ar whole-rock 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 Salle (1982), Van Halen (1996) and Wensink (1997).
Table 1
40K±40Ar isotopic ages of magmatic rocks of Sumba (see text for analytical methods)
Sample No. Location Type Age (Ma)2error 40ArR
(eÿ7cm3/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 potassic calc±alkaline (KCA) series (Fig. 5); they are character-ized by variable K2O contents, relatively high Al2O3
and low TiO2 contents, suggesting a typical island arc
environment. Such anity 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 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 with basaltic andesites and andesites. To the second mag-matic episode belongs the Tanadaro granitoid intru-sion 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 grani-toid, 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 (Eendi and Apandi, 1994), and consist of an assemblage of pyro-clastic breccias, tus and lava ¯ows, intruded by gran-odiorite. 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 ser-icite, 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, clinopyrox-ene, 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, respect-ively at 83.721.8 Ma (CIA-348) and 85.421.6 Ma (CIA-347), whereas ®ve volcanic rocks (basalt,
Table 2
Major and trace element analyses of Sumba magmatic rocks. ICP-AES data, J. Cotten, Brest (see text for analytical methods)
Santonian±Campanian magmatism Maastrichtian±Paleocene magmatism Late Eocene±Early Oligocene (Lutetian±Rupelian) magmatism
Masu Mt South Coast 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
SiO2 48.8 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
TiO2 0.98 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
Al2O3 18.25 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
Fe2O3 10.95 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 2 2.55 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
P2O5 0.21 0.18 0.26 0.34 0.2 0.23 0.11 0.24 0.18 0.23 0.17 0.23 0 0.08 0.06 0.07 0.06 0.04 0 0.2 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 2 1 15 6 8 32 70 19 48 18 32 48 10 48 47 39 34 157 11 24 11 12
Nl 31 14 3 3 9 6 6 29 20 5 12 9 26 18 9 25 26 22 18 85 5.5 14 1 6
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
V 326 227 190 215 226 170 106 310 250 233 240 222 196 157 110 143 131 142 95 174 269 140 55 56
Rb 10 35.5 33 36 26.5 51 102 20 19 41 18 37 32 45 85 6 8 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 2 5 2.5 4 5 4 4 1.8 1.8 5 2.8 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 5 18 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 9 16 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 8 9 12 8 83 71 26 148 219
Eu 1.22 1.02 1.2 1.3 1.13 1.35 1 1.1 1.12 1.12 1.05 1.15 0.5 1 0.9 1 1 0.9 0.7 1.2 1 1 1.95 1.8 Y 20 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 4 4.9 3.55 3.6 3.2 4.4 3.85 4.4 3.6 4.5 2.2 4.08 2.8 3 2.9 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 2 2.6 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 5 4.86
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tic±andesite, andesite) gave ages of 77.2 21.8 Ma (CIA-735), 78.621.7 Ma (CIA-353), 82.621.9 Ma (CIA-733), 85.422 Ma (CIA-728) and 85.922.0 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 volca-nic units of chie¯y basaltic composition (basalts and minor basaltic andesites). The stock-like dioritic to granodioritic body of Tanadaro exhibits medium-grained 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
(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+clinopyr-oxene+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 ground-mass is made up of plagioclase laths, intergranular chlorite, actinolite, hornblende and magnetite, some-times 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±SiO2diagrams (Peccerillo and Taylor, 1976) for magmatic rocks belonging to the Santonian±Campanian episode (86±77 Ma) (A),
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.
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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 succes-sive cooling stages of the magmatic unit. Those of the volcanic unit are scattered between 70.321.5 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 respect-ively. 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 pheno-crysts 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 plagio-clase phenocrysts are dusted with sericite, clays, chlor-ite, 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 pro-minent plagioclase, green hornblende and quartz, together with actinolite, chlorite and calcite as altera-tion products. The corresponding rock chemistry shows typical low-K to medium-K calc-alkaline fea-tures (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.
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±Cam-panian 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 mag-matic 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 sedi-mentation 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 pro-ducts 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 sys-tem. 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 vol-canics in Sumba suggest a westward shift of magma-tism 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 South-western 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' 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 Southeast Kalimantan coast (Meratus Mountains) (Yuwono et al., 1988; Wensink, 1997; Rampnoux et al., 1997) at the margin of Asiatic Plate. Thus, during the Paleo-gene, the rate of movement of the Indo-Australian Plate 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 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).
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.
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of
Asian
Earth
Sciences
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(2000)
533±546
Arc is progressing north-westwards (Fig. 7D) causing Sumba 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 suggests that the island has never been subjected to intense de-formation. 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 geo-chemical analyses and the numerous K±Ar ages.
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