Page 96 - Sumatra Geology, Resources and Tectonic Evolution

PRE-TERTIARY VOLCANIC ROCKS 83

West Sumatra Permian Plutonic-Volcanic Belt dolerites and amphibolites from the Dili area of Timor (Berry &
Jenner 1982).
It has been established that the Permian volcanics in the The timing and chemistry of the West Sumatra Permian
Mengkarang Formation in the West Sumatra Block, the Volcanic Plutonic-Volcanic Belt suggest that it was linked both with sub-
Member of the Silungkang Formation, and the Palepat Formation duction and continent margin faulting/seafloor spreading, but
were erupted between the Asselian and the Artinskian and that the chemical data do not discriminate which process was dominant
basaltic volcanics in the Calcareous Member of the Silungkang at any particular time. This might be explained by the palaeogeo-
Formation are probably Roadian. Radiometric dating suggests graphic setting of the West Sumatra Block between Cathaysia and
that some of the volcanics (the andesite-rhyolite sequence in Gondwana, where the Cathaysian margin subduction regime
the Volcanic Member of the Silungkang Formation and at appears to have been affected by the break-up faulting of the
Sibolga) are the extrusive equivalents of plutonic intrusions. Gondwana margin. This palaeogeographic setting ended when
The Ombilin granite is a foliated muscovite (?)S-type granite Sibumasu collided with the Indochina Block of Cathaysia in the
(McCourt et al. 1996) with a K-Ar age of 287 • 3 Ma, corre- Changsingian and Scythian (Metcalfe 2000).
sponding to the Asselian Stage, and a younger Rb-Sr age of
256 _+ 6 Ma. The oldest intrusive phase in the Sibolga Granite
Complex has a Rb-Sr isochron age of 264 • 6 Ma (Aspden Bentong-Billiton Accretionary Complex
et al. 1982b) and may be associated with the volcanics in the
Kluet Formation. The basic and ultrabasic meta-igneous and volcanic lithologies in
Three geological settings for the West Sumatra Permian the Riau-Billiton Permian Volcanic belt in north P. Billiton
Plutonic-Volcanic Belt have been proposed; an island arc, sub- and P. Bangka, and those in west P. Batam and on P. Sugi are
duction-related continental margin arc, or continental breakup. on the strike continuation of the Bentong-Raub collision zone.
The West Sumatra Permian Plutonic-Volcanic Belt is referred These rocks are components of an Accretionary Complex on the
to as the 'Palepat Terrane' by McCourt et al. (1996) who Palaeo-Tethys margin of the Indochina Block, derived from
discuss the suggestion by Wajzer et al. (1991) that the 'Palepat detached slices of the Palaeo-Tethys ocean floor, volcanic rocks,
Terrane' represents an allochthonous oceanic arc which collided intrusions and sediments, all of which were deformed during the
with Sumatra in the Late Permian or Early Triassic. This interpret- collision with Sibumasu. Volcanics in the Tin Islands appear to
ation was adopted by Metcalfe (2000). The Palepat Terrane/ be Permian in age, but the complex as a whole contains sediments
allochthonous oceanic island arc hypothesis is rejected by ranging in age from Late Devonian to Late Permian (Metcalfe
Barber (2000) on the grounds that oceanic volcanics and ophiolites 2000).
have not been identified, nor is the 'Palepat Terrane' bounded
along its eastern boundary by thrusts (Katili 1970), as had been
supposed previously (Tobler 1922; Zwierzijcki 1930a). The Gondwana excursions and the Gondwana Margin
Cretaceous ophiolite outcrops shown within Early Permian
sediments in the Solok Quadrangle by Gafoer et al. (1992a) are, break-up volcanicity
according to Silitonga & Kastowo (1975), basaltic lavas inter-
bedded within phyllites and quartzites of the Phyllite and Shale Charlton (2001) has a novel explanation of the Gondwana margin
sequence of extension, uplift, associated magmatism, fragmenta-
Member of the Kuantan Formation. These basalt outcrops are
now considered to be an outlier of the Calcareous Member of tion and dispersal during the Permian, based on the study of the
palaeomagnetism of Australia and its vicinity by Klootwijk
the Silungkang Formation and are not associated with ultrabasic (1996) (see Fig. 6.2b), Palaeomagnetic data indicate that eastern
rocks, so their ophiolitic association is not established. Gondwana made a northward excursion commencing in the
Katili (1969, 1972, 1981) interpreted the Volcanic Member Early Carboniferous, and reached low to moderate latitudes in
of the Silungkang Formation, the Palepat Formation and the the mid-Carboniferous, before moving southwards again in the
associated granite suite, as relics of a continent margin magmatic later Carboniferous and Early Permian. The return phase of this
arc of subduction origin. This interpretation is supported by the
tholeiitic and calc-alkaline trends in these volcanics (Fig. 6.6) excursion coincides with the rift-faulting, crustal extension,
associated magmatism and fragmentation of Sibumasu from the
(Suwarna et al. 2000). The location of this magmatic arc in the
palaeogeogeographic reconstruction (Fig. 14.11) would have Gondwana (Fig. 6.2a,b & 6.15). In this scenario Sibumasu did
not drift away from Gondwana, as envisaged for example in the
been on the southern margin of the Cathaysian supercontinent
(Fig. 6.2a), where it might have been related to a contemporary reconstructions of Metcalfe (1996), but was abandoned during
the phase of crustal extension which accompanied the southward
Permian magmatic arc in the Indochina Block of East Peninsular return of the Gondwana Supercontinent. The detachment of the
Malaysia described by Cobbing et al. (1992). West Sumatra Block from the area of contact between Cathaysia
A third alternative proposed by Suparaka & Sukendar (1981), is and Gondwana occurred later in the Triassic. By this time
that the volcanics represent igneous activity associated with a Sibumasu had collided with the East Malay Block resulting in
passive continent margin. Charlton (2001), on palaeogeographic the deformation of the Riau-Billiton Accretionary Complex.
reasoning, has suggested that the West Sumatra Permian volcanics
were related to the break-up at the Gondwana-Cathaysia inter- This event was accompanied by a second northward excursion
of Gondwana in the Triassic, during which the West Sumatra
face. In this hypothesis the volcanism was associated with the
thermal uplift of the Gondwana margin (Veevers & Tewari Block was translated along the Medial Sumatra Tectonic Zone
to arrive in its present position alongside the Sibumasu Block.
1995) which coincided in the Asselian with the conclusion of
the Gondwana glaciation and the start of sea-floor spreading in
Meso-Tethys (Fig. 6.15). At this time the West Sumatra Block
lay well to the north of the glaciated area (Fig. 14.11), so that Triassic Plutonic-Volcanic belts in post-collision Sumatra
the thermal uplift resulted in shallow-water deposition under
tropical marine conditions. Extensive igneous activity took place during the Triassic in
The geochemistry of the Silungkang and Palepat Formations Sumatra and Peninsular Malaysia in both of which axial uplifts,
as shown in the rock/chondrite normalized REE plots and the resulting from successive collisions, were followed by extensional
spidergrams of these volcanics (Suwarna et al. 2000) resembles collapse (cf. Dewey 1988). This collapse led to sedimentation in
similar plots for the Gondwana break-up volcanics identified in faulted basins and grabens, beneath and between which, extensive
the Himalayas (Garzanti et al. 1989), and the REE pattern of the granitic plutonism of the Main Range and Eastern Provinces