Page 77 - Sumatra Geology, Resources and Tectonic Evolution

64 CHAPTER 6

Table 6.1. Pre-Tertiar3, Volcanic and Volcanic-Plutonic Belts, Arcs and occurrences in Sumatra
Ma Duration Description
120-75 Aptian-Campanian Late Cretaceous Plutonic Arc*
Mid-Cretaceous Collision of Bentaro-Saling Oceanic Arcs with West Sumatra*
Early Cretaceous Intrusions in Bentaro-Saling Oceanic Arc*
Late Jurassic-Early Cretaceous Volcanism in Meso-Tethys forms Bentaro-Saling Oceanic Volcanic Arcs
Triassic onwards Mid-Oceanic volcanism forms plateau in Meso-Tethys many of
which grow limestone caps
169-129 Mid-Jurassic-Early Cretaceous Jurassic-Cretaceous Plutonic Arc
Woyla Accretionary Complex forms behind subduction zone
Late Triassic-Early Jurassic West Sumatra Volcanic-Plutonic Arc
224-180 Late Triassic-Early Jurassic Pahang Volcanic Belt
Triassic Medial Sumatra Tectonic Zone formed incorporating sediments and volcanics
Mid-Permian-Mid-Triassic Situtup Fm volcanics (in Miocene thrust zone) (West Sumatra Block)
270-255 Early-Middle Permian West Sumatra Permian Plutonic-Volcanic Belt (West Sumatra Block)
c. 270 Early Permian East Sumatra Permian Plutonic-Volcanic Belt (Sibumasu)
Carboniferous (Vis6an) Kuantan Formation volcanics (West Sumatra Block)
Devonian-Late Permian Accretionary Complex forms behind subduction zone beneath East Malaya and
Riau-Billiton sections of Indochina Block interface with Palaeo-Tethys
Ocean; accretion of volcanics of oceanic origin
*Associated volcanics not identified.

compilation of analyses reported by Rock et al. (1982), but Range Granite Province with its extensive tin mineralization
the initial results of a programme of detailed mapping studies by extends into Sibumasu, but no volcanics are reported.
the Geological Research and Development Centre, Bandung Between about 224 and 180 Ma (Late Triassic-Early Jurassic)
promises an improved understanding of the geochemistry of both the Meso-Tethys commenced subduction along the margin of
volcanic and plutonic rocks in the island (Suwarna et al. 2000). the combined West Sumatra Block and Sibumasu continent and
According to the tectonic synthesis which has been presented in a continental margin volcano-plutonic arc was formed, a small
this volume (Chapter 14), in the late Palaeozoic (Fig. 6.2a) the amount of these volcanics are preserved. Accretion of oceanic
eastern half of Sumatra formed a segment of the margin of materials may have been associated with the formation of this
the southern Gondwana Supercontinent facing the Palaeo-Tethys arc. Accretion between 169 and 129 Ma (Mid-Jurassic-Early
ocean, off NW Australia, while Australia was undergoing glacia- Cretaceous) is better documented in the Oceanic Assemblage of
tion. On the other hand, the western half of Sumatra lay in tropical the Woyla Group, composed of buoyant oceanic volcanics,
latitudes, beyond the Greater Sula Spur of Eastern Indonesia, at sediments, oceanic crust fragments which accumulated in the
the junction zone between Gondwana and the Indochina Block Woyla accretionary complex. Accretion was associated with the
of the northern Cathaysian continent (Fig. 14. I1). Palaeo-Tethys formation of a Jurassic-Cretaceous continent margin plutonic
was subducted beneath the Indochina Block in the Late Palaeozoic arc with its associated volcanics (Fig. 6.2d). This phase of sub-
and Early Mesozoic, accumulating an accretionary complex from duction/accretion was brought to a close by the arrival at the
buoyant oceanic detritus, including ophiolitic fragments, oceanic subduction zone of a large string of oceanic island arcs which
volcanics and oceanic sediments at the margin of the Indochina had originated within the Meso-Tethys Ocean. The arrival of
Block. The deformed remains of this accretionary complex form Bentaro and Saling Oceanic Island Arcs (Fig. 6.2e) terminated
the Bentong-Raub Suture Zone (Metcalfe 2000) and continue subduction, thrust the Woyla Oceanic Assemblage and Volcanic
into the Tin Islands Archipelago. In the Early Permian Gondwana Arc over the margin of the West Sumatra Block in the Woyla
began to move southwards (Fig. 6.2b), and this movement caused Nappe, and caused deformation which penetrated deep into the
extension along the Gondwana margin with Palaeo-Tethys, Malay Peninsula. Subduction of the Meso-Tethys resumed late
accompanied by volcanism and plutonism within the Sumatra in the Cretaceous on the oceanward side of the Bentaro-Saling
blocks. The East Sumatra Block formed part of Sibumasu, a Volcanic Arcs and a new plutonic arc was formed on the Woyla
continental fragment which detached from Gondwana in the Nappe and the margin of the West Sumatra Block.
Early Permian (Sakmarian) and collided with the Indochina
Block later in the Permian or in the Early Triassic (Metcalfe 2000).
Following the collision of Sibumasu with the Indochina
Block, the West Sumatra Block became detached from the Carboniferous volcanism
Gondwana-Cathaysia interface in the Triassic and was translated
by transcurrent faulting along the Medial Sumatra Tectonic Zone Gafoer & Purbo-Hadiwidjoyo (1986) used the term 'Kuantan
to be accreted along the outer margin of Sibumasu (East Sumatra Volcanism' for metavolcanics (Table 6.2) mapped by Silitonga
Block) (refer to Figs 14.11 - 14.14). & Kastowo (1975, 1995) in the Lower and Phyllite and Slate
During the Triassic, after the collision between the Sibumasu members of the Kuantan Formation in West Sumatra. The older
and Indochina blocks, the orogen collapsed, into a system of episode, within the quartzitic Lower Member, is represented by
horsts and grabens parallel to the orogen axis (Fig. 6.2c) and intercalations of volcanic rock and chloritized tuff, which underlie
granites of the Eastern and Main Range Provinces were intruded the Limestone Member, which has been dated as Early or Mid-
into the collision zone. The Pahang volcanics in east Malaya to Late Visdan (Fontaine & Gafoer 1989). The younger episode
represent the volcanic carapaces to Eastern Province granites, is represented by flows of andesite and basalt among the quartzites
preserved along the faulted margins of the grabens. The Main and quartz sandstones of the Phyllite and Shale Member.