Page 97 - Sumatra Geology, Resources and Tectonic Evolution

84 CHAPTER 6

took place in Malaya and in the Tin Islands off Sumatra. The only volcanic rocks from the Saling Formation of the Gumai Mountains
volcanic units related to this phase of plutonism which have sur- include examples from the oceanic crust (high-Ti contents) while
vived, form the Pahang Volcanic Belt associated with the low-Ti samples represent volcanics of subduction origin, some
Eastern Granite Province of Peninsular Malaya. high in Si and another high in Mg. Analyses of the Nabana
At the same time Meso-Tethys commenced subduction beneath Volcanics in the Batang Natal and from the Tapaktuan Formation
Western Sumatra creating the continental margin West Sumatra are high in Ti, confirming the field identification of ocean-floor
Triassic Plutonic-Volcanic Arc. Some of these Triassic arc volcanics within these units (Fig. 6.14).
plutons were intruded into the (formerly) extensive limestone Other volcanic units in the Woyla Accretionary Complex are
platform which formed at the Meso-Tethys ocean margin suggested to be the remnants of volcanic arcs (Tambak Baru,
but few associated volcanics have been recognized (Cubadak Parlumpangan) but the absence of collision deformation suggests
Formation). an alternative origin as volcanic centres intruded into the complex
which were subsequently broken up by faulting. A reconstruction
of the different depositional and volcanic environments within
the oceanic assemblage of the Woyla Accretionary Complex is
Jurassic-Cretaceous plutonism and volcanism attempted in Fig. 6.16. The environments of the sedimentary
units (Table 6.9) were appraised by Wajzer et al. (1991). Sub-
Towards the end of the Jurassic, before the accretionary margin of sequent oblique subduction beneath the Woyla Accretionary
western Sumatra was firmly established, the Mid-Jurassic-Early Complex caused transcurrent faulting, which broke up and
Cretaceous was a time of extensive plutonism associated with vol- dispersed the component sediment and volcanic units as described
canism of the continental margin Jurassic-Cretaceous Plutonic by Wajzer et al. (1991).
Arc. This magmatic pulse in Sumatra coincides with the rapid The large serpentinite bodies are fragments of the basal harzbur-
formation of the Pacific Plate (c, 175-170Ma, Bartolini & gite layer of the ocean crust which have become detached from
Larson 2001), which led to a world-wide flare-up of subduction their volcanic and dyke carapaces as a result of their emplacement
magmatism. The rapid growth of the Pacific Plate (15 cm a -I) across the subduction complex and subsequent strike-slip faulting.
continued until the Oxfordian, when it reduced to 10 cm a -l. In The majority of serpentinite bodies in the Aceh area are of this
Sumatra the Mid-Jurassic-Early Cretaceous Plutonic Arc dates type, but others, like the Pasaman Complex (Rock et al. 1983),
from 169-129 Ma (McCourt et al. 1996) in the Meso-Tethyan and the various serpentinites in the NW corner of the Takengon
Ocean and the Woyla Accretionary Complex incorporated Quadrangle (Cameron et al. 1983), are associated with large
oceanic seamounts dating from the Triassic and volcanic units limestone outcrops. Such serpentinites may be the remnants of
derived from oceanic and continental sources (Figs. 6.16 & 14.16). the foundations of uplifted oceanic plateaus with limestone caps
Limited chemical data (Table 6.11) hints that on the basis (Wajzer et al. 1991) which collided with the subduction zone
of separation into high (> 1%) and low (< 1%) TiO2 contents, and were fragmented.

OCEANIC ACCRETIONARY COMPLEX To the margin of >
ISLAND ARC (oceanic assemblage) SUNDALAND - subducted
(arc assemblage) beneath the Woyla Nappe
Andesitic volcanics UPPER TRENCH in the mid-Cretaceous
and volcaniclastic SLOPE BASIN
sediments greywackes LOWER TRENCH COLLAPSING
SLOPE BASIN
SEAMOUNT
(Tambak Baru and (Muarasoma (Belok Gadang with olistostrome
Turbidite
Parlumpangan Formation) Siltstone (Panglong
Volcanic Units) Formation) Melange
TRENCH Formation)
(Simarobu I
Turbidite ]
Forr~ation) $

r ' . . . , . . . . 9
" ' ' " .
, ' v v v
/ (Triassic- mid-Cretaceous)
FOREARC BASIN
volcaniclastic sediments oceanic lithosphere, ocean floor
and reefs and pelagic sediments
(Nabana Volcanic Unit
(Rantobi Sandstone and Pasaman Ultramafic Complex)
Jambu Baru Formations)

Fig. 6.16, Cartoon reconstruction of environments of sediment and volcanic units within the Woyla Accretionary Complex of the Natal area. Sediment environments are
as interpreted by Wajzer et al. (1991) and in Table 6.9, but do not represent a specific time frame.