Page 98 - Sumatra Geology, Resources and Tectonic Evolution

PRE-TERTIARY VOLCANIC ROCKS 85

Fossil evidence indicates that the Bentaro string of island arcs the collision, the arc was detached from its oceanic basement,
began to grow within Meso-Tethys around the Jurassic Oxfordian ramped onto the Sumatra continent margin, and so overlies thin
stage. Their origin is shrouded in uncertainty, but Barber continental lithosphere. This is demonstrated by the continent
(2000) has suggested that they were generated along transform margin-type mineralogy of the Late Cretaceous (97.7 • 0.7 Ma)
faults. Their formation may have resembled the origin of the intrusion of the Younger Complex of the Sikuleh Batholith into
Izu-Bonin-Mariana island arcs in the Eocene (Stern & the Bentaro Arc and the subsequent (Late Tertiary?) molybdenum
Bloomer 1992). In this model, displacement along translbrm mineralisation and drainage tin anomalies (Bennett et al. 1981b).
faults in the Pacific Plate juxtaposed oceanic crust and litho- The debate over the oceanic or continental origins of arcs
spheres of different ages, densities and thicknesses, which led to is complicated by the discovery of a fragment of a continental
instability relieved by subduction within the ocean. Subduction arc within the Woyla Oceanic and Accretion Assemblage. In the
led to volcanism and the growth of volcanoes, forming Batang Natal section, severely deformed Si Gala Gala Schists
an oceanic island arc, which upon emergence above sea-level represent volcanics with a more acidic (continental) source than
became surrounded by fringing reefs. the intermediate composition volcanics and volcanogenic sedi-
The presence of at least one generation of island arcs within mentary units of oceanic origin in the assemblage. The intense
the Woyla Oceanic Volcanic Arc Assemblage has been deduced deformation in the Si Gala Gala Schists, compared to other
in NW Sumatra. Other large contemporaneous Tethyan oceanic units, may have been the result of a collision of a continental
island arcs include the Kohistan Arc of northern Pakistan island arc with the accretionary margin (Wajzer 1986). Alterna-
(Treloar et al. 1996) which grew in the Mid-Cretaceous and the tively, and believed to be more likely, the Si Gala Gala Schists
Spontang Ophiolite of the Ladakh Himalaya (Pedersen et al. represent a relatively autochthonous fault-sliver of a local
2001). The collision of the Bentaro-Saling Arcs and the associ- Sumatran volcanic centre, deformed as a result of fault move-
ated oceanic crust carrying the Oceanic Assemblage of the ments. The intermediate composition Parlampungan Volcanic
Woyla Group with the West Sumatra margin of Sundaland Unit is adjacent, and may be related to the Si Gala Gala Schists,
had tectonic effects which reached into Peninsular Malaysia and but is not deformed. Wajzer (1986) suggested that it was a fault
beyond. However the Bentaro-Saling Arcs of Sumatra are sliver transported from the continent margin Sumatra Arc by
relatively small and have not been up-ended compared to the strike-slip faulting and became incorporated within the accretion-
contemporaneous giant Kohistan Arc of northern Pakistan which ary complex, but alternatively it is a variably deformed local
represents a deformed crustal section perhaps 40km thick volcanic centre with intermediate volcanics differentiated from
(Hamilton 1988). oceanic basalts.
The debate concerning the nature of the basement of the Bentaro In conclusion, the reconnaissance study of the Pre-Tertiary vol-
Island Arc, whether continental (Cameron et al. 1980 and canics of Sumatra has already provided fascinating data assisting
Pulunggono & Cameron 1984) or oceanic (Wajzer et al. 1991; the understanding of the geological evolution of Sumatra.
Barber 2000), has already been alluded to. The Bentaro Arc was Further study of the volcanic rocks of Sumatra will lead to a
deformed and metamorphosed at low temperatures as a result of better understanding of the history of the break-up of Gondwana,
its forceful collision with the Sumatra margin. To date only a and the rearrangement of crustal blocks during collision and
I%w localities of garnet amphibolite are known believed to be accretion processes throughout the Permian and the Mesozoic,
the exhumed products of subduction metamorphism (see Barber with implications far outside Sumatra.
2000 for details). The simplest explanation is that as a result of