Page 74 - Sumatra Geology, Resources and Tectonic Evolution

GRANITES 61

I-type and stanniferrous S-type granites termed the Bebulu Suite Province, with the granites of Bangka and Billiton being shown
(Pitfield 1987; Cobbing et al. 1992). The only logical explanation as of mixed affinity. Most of these correlations have been followed
for the mixed granite population of these islands, especially of here, but there are some amendments, and some alternatives have
Bangka and Billiton, is that the contrasted granitic suites have been suggested. Some of the boundaries are of tectonic origin
different source regions. It may be that in the arcuate region to and are well defined, or at least give that impression, others are
the east of Sumatra the suture was imbricated into a m61ange of not, or appear to be 'porous' in that granites of contrasting type
deep crustal wedges derived from adjacent Gondwanan and Cath- or age appear to be mingled together or are 'out of place'.
aysian blocks, providing a complex of compositionally contrasted The only known representative of the Jurassic-Tertiary
source regions for both S and I-type granites. These compositional Western Province on Sumatra is the Hatapang Granite (Clarke
differences are reflected in the geochemical and isotopic character- & Beddoe-Stephens 1987). While more may yet be found, all
istics of the granites derived from them (Cobbing et al. 1992). the other tin-associated granites for which there is data, are of
Pulunggono & Cameron (1984) proposed a similar interpret- Triassic-Jurassic age and suggest that the Main Range (Central)
ation with the Bentong-Raub Line running through Singkep and Province occupies virtually all of Sumatra to the east of the
Bangka, following the southern margin of the Klabat Batholith Barisan Range. Granites of this affinity also occur as tectonic
(Fig. 5.2). They also commented that the suture zone is 'more slices within the range itself, and in the region of Sibolga,
complex than shown and is occupied by lensoid fragments of biotite granites and sedimentary rocks of the the Kluet-Kuantan
both microplates'. Similarly Gasparon & Varne (1995) considered Formations of Upper Palaeozoic age extend as far as the west
that 'the boundary between the Central and the Eastern Granite coast of Sumatra (Clarke 1990), which suggests that the volcanic
Provinces may run through the Tin Islands'. arc was built, at least in part, upon older continental crust. On the
Within the stanniferous granites of the Tin Islands, the Tanjong basis of the occurrence of the Hatapang granite, McCourt et al.
Pandang Pluton on the island of Billiton, is the only body in which (1996) extended the Western Belt through the whole of Sumatra
the tin has behaved as a decoupled element, in that the tin content as a narrow strip east of the Barisan Range. However, in the
does not increase with magmatic differentiation (Lehman & light of the available evidence this may not be the case, perhaps
Harmanto 1990). In this respect it corresponds to granites belong- the Hatapang Granite is the sole representative of that belt
ing to the Kuantan-Dungun stanniferous granites of the Eastern within Sumatra.
Province of Peninsular Malaysia, where tin contents are low and The status of the A-type Bukit Batu granitoids remains enig-
are similarly unrelated to differentiation, but increased during matic. A-type granites have also been identified in the Tin
the hydrothermal stage (Schwartz & Askury 1990). Islands and the islands of Singkep and Karimun (Cobbing et al.
The distribution of stanniferrous and non-stanniferous granites 1986, 1992). The Bukit Batu granitoids are associated in the
on these islands suggests that the Bentong-Raub Line, or perhaps field with stream sediments containing quartz and cassiterite, but
a strand of that structure, runs through or close to central Bangka in view of their unusual composition it is highly unlikely that
and northern Billiton. Moreover, the location of the Main Range they are stanniferous. The sediments may be of alluvial origin,
type S-type granites in the northern half of Bangka and the I- derived from the Tin Islands a short distance to the east (Katili
types of the Bebulu Suite in the southern half (Cobbing et al. 1974a; Pulunggono & Cameron 1984). The geochemical affinity
1992) have a distribution which is the reverse of that in Peninsular and high estimated 86Sr/87Sr ratios of the Bukit Batu granitoids
Malaysia and Thailand. This reversal of the normal pattern pro- suggest correlation with the Tin Islands Suite. However, the esti-
vides additional reason to support the concept of the nearby mated age of 163 + 50 Ma is more compatible with the Volcanic
location of a structurally complex Bentong-Raub Line or Zone. Arc Suite. If the Tin Islands affinity of these granitoids were to be
Host rocks for granites on the islands of Bangka and Billiton confirmed this would have implications for the position of the
include limited outcrops of pebbly mudstone facies and larger Bentong- Raub Line.
occurrences of mainly terrigenous sedimentary rocks of Carbon-
iferous-Permian age, overlain by Triassic sandstones (Ko
1986). According to Priem et al. (1975) country rocks on both Conclusions
these islands are low-grade meta-sedimentary rocks of Stephanian
to Norian age. These sequences are similar to those present in the The granites of Sumatra have developed through two contrasting
Eastern province of Peninsular Malaysia. The host rocks to the tin geological cycles, a Carboniferous-Permian cycle of conver-
granites of the Main Range Province in Peninsular Malaysia gence and collision followed by a younger Triassic-early Jurassic
consist mainly of Lower Palaeozoic formations of Ordovician to cycle in which a new subduction zone was formed along the
Devonian age and consist mainly of pelitic rocks of low to moder- southwestern margin of the new continent (Hutchison 1994;
ate metamorphic grade with subordinate limestones. The observed McCourt et al. 1996). During the first, collisional cycle, the
sequences are essentially the cover to middle and lower crustal different accreted terrains, distinguished by their stratigraphic
material present at depth. and faunal assemblages, were host rocks to granites which,
As noted above the composition of granites within the region is because of their contrasting geochemical and isotopic characters,
not confined to S- and I-types but A-types are also sporadically seemed to mirror the lower crustal regions from which they were
developed. These however, except in the Tin Islands, are not derived. These terrains are distinguished most clearly in
common in Sumatra (Cobbing et al. 1992). Only the Hatapang Peninsular Malaysia and Thailand as contrasting belts which are
and Bukit Batu plutons can be viewed as approaching an A-type additionally characterised by stanniferous S-type and generally
composition and these may be very highly evolved examples of non-stanniferous I-type granites (Beckinsale 1979). The second
S and I-type lineages, respectively. However, the isolated location cycle generated granites having a wide compositional range
of the Bukit Batu Pluton in relation to the main outcrop of the Vol- from diorite to monzogranite, associated with the development
canic Arc Suite at the western margin of the island does not of a late Triassic-early Jurassic volcanic arc along the southern
support such an interpretation for that body. margin of Sundaland. McCourt et al. (1996) suggested that the
Most of the granitic rocks of Sumatra can be accommodated two cycles overlap in Sumatra.
within the framework of granitic belts established in earlier The association of the Main Range Province granites with sedi-
studies, e.g. Mitchell (1977), Hutchison & Taylor (1978), mentary rocks of Gondwana affinity and the Eastern Province
Beckinsale (1979). McCourt et al. (1996) correlated the Volcanic granites with those containing Cathaysian floras provided a
Arc Suite with the Central Valley Province of Burma, the further strand of evidence for the disparate geological histories
Tin-Associated Suite with the Main Range Province of Peninsular of those crustal segments which eventually formed the southern
Malaysia and Thailand, and the Tin islands with the Eastern borderlands of Eurasia during the Permo-Triassic (Hutchison