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.
It represents a continental crust fragment located at the boundary
between the Sunda subduction system and the Australian collision
system separating the Savu Basin from the Lombok Basin. New data
on magmatic rocks collected from Sumba are presented in this paper
which include bulk rock major and trace element chemistry, petrography
and whole rock and minerals 40K-40Ar ages.
Three distinct calc-alkaline magmatic episodes have been recorded
during Cretaceous - Paleogene, all of them characterized by nearly
similar rock assemblages (i.e pyroclastic rocks, basaltic - andesitic
lava flows and granodioritic intrusions). They are respectively
(i) the Santonian - Campanian episode (86-77 Ma) represented by
volcanic and plutonic rock exposures in the Masu Complex from
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
finally (iii) the Lutetian - Rupelian episode (42-31 Ma) of which
the products are exposed at Lamboya and Jawila in western part
of Sumba. No evidence of Neogene magmatic activity has been recorded
so far.
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 passage from the Java Trench (subduction front) to the Timor Through (collision front). It does not show still the effects of strong compression in contrast to islands of the outer arc system (Savu, Roti, Timor), while the magmatic 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 the results of tectonic studies helped by paleomagnetism and geochemistry, several workers considered Sumba as a microcontinent or a 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 reviewed by Chamalaun et al. (1982) and Wensink (1994) as follows : (i) Sumba was originally a part of the Australian Continent which was detached afterwards when the Wharton basin was formed, drifted northwards and subsequently trapped behind the eastern Java Trench (Audley-Charles, 1975 ; Otofuji et al., 1981), (ii) Sumba was once part of Sundaland which was 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 later was fragmented (Chamalaun and Sunata, 1982).
The present paper tries to make one's contribution to solve this problem.
