Page 67 - Sumatra Geology, Resources and Tectonic Evolution

Chapter 5

Granites

E. J. COBBING

Knowledge of the granites of Sumatra has been gathered mainly as on the reliability of the reported isotopic age. This is the case
the result of systematic mapping programmes conducted with the for the Ombilin Granite (Fig. 5.1), cropping out on the western
aim of identifying mineral resources and providing a geological shore of Lake Singkarak, for which Silitonga & Kastowa (1975)
data base for more detailed studies. Mapping programmes were gave an Rb-Sr age of 256 _+ 6 Ma. This body has volcanic arc-
conducted principally by Dutch and Indonesian geologists prior type geochemistry but is very strongly deformed, and shows
to the second world war, mainly in southern Sumatra and the highly anomalous potassium and rubidium values (McCourt &
Tin Islands. In the 1970s a combined Indonesian Directorate of Cobbing 1993). These factors casts doubt on the reliability of
Mineral Resources (DMR)/British Geological Survey (BGS) the reported age, which is at least 50 Ma older than all other gran-
project was set up to map the geology of Sumatra to the north of ites of that affinity.
the Equator. On completion of this project in the mid-1980s geo- A further example of the difficulties in interpreting the isotopic
logical and geochemical maps for the region were published at the ages of the granites of Sumatra is provided by the Sibolga Batholith
scale of 1:250000, together with descriptive sheet bulletins. in northwest Sumatra. This pluton has yielded a wide range of iso-
Another useful compilation which may be refered to is the topic ages from 75 to 264 Ma. It is a very large body, and may well
1:2.5 million scale geological map for the whole of the Indonesian be composite, comprising several distinct units of different ages. In
Archipelago which includes Sumatra (Clarke 1990). the hinterland of Sibolga the granite consists of biotite-hornblende
Subsequently BGS undertook a similar but smaller project in granite and granodiorite with pink K-feldspar megacrysts, mafic
southern Sumatra in order to upgrade geological mapping and enclaves and mafic dykes. These characteristics are typical of the
mineral exploration programmes which were being conducted Eastern Province Granites of Peninsular Malaysia and the Tin
by the Indonesian Geological Research and Development Centre Islands, and distinguish these rocks from the tin-associated granites
(GRDC) and DMR. As part of this programme a specific effort in the same areas (Cobbing et al. 1986, 1992). The position of the
was made to investigate the granites of this region. A combined Sibolga Granite however, is completely anomalous, as it crops
granite workshop/regional mapping programme resulted in the out on the far west coast of Sumatra, 300 km away from the
identification of many granite units within batholiths such as Eastern Province Granites of Peninsular Malaysia.
Lassi, Bungo and Garba, as well as numerous isolated plutons. The isotopic age of 264 Ma (Aspden et al. 1982b) may represent
Full geochemical and isotopic analyses were provided for these the age of emplacement of the Sibolga Granite itself, but the 13
granites (McCourt & Cobbing 1993; McCourt et al. 1996). other ages recorded from this body, ranging from 75 to 264 Ma,
Gasparon & Varne (1995) have provided further geological and cannot represent an emplacement age for the Sibolga Pluton,
geochemical information from selected granites and volcanics and may have been obtained from satellite plutons in the
over the whole of Sumatra. Cobbing et al. (1986, 1992) had pre- Sibolga region.
viously provided full geochemical and isotopic data for the gran- Unlike the Sibolga Batholith there is no question of uncertain
ites of the Tin Islands as part of a comprehensive study of the provenance for the Lassi Batholith (Fig. 5.1) which has yielded
granites of much of SE Asia. a much quoted Early Cretaceous age of 112 Ma (Katili 1974a).
These combined studies confirmed earlier suggestions that the However, this is incompatible with the K-Ar age of 56.3 Ma
granites of Sumatra could be classified into a group of older, reported by Sato (1991). The five K-Ar ages of 57, 55, 54, 53
widely distributed tin-associated granites, and a group of and 53 Ma from different units of this batholith given in
younger, geographically restricted, volcanic-arc granites with a McCourt et al. (1996) and the 4~ ages of 55 and 56 Ma
wide compositional range. (Imtihanah 2000) confirm its Palaeocene age.
The older tin-associated granites crop out throughout the whole The Lassi examples suggests that many of the isotopic ages
of Sumatra, but are concentrated mainly to the east of the Barisan reported from Sumatra do not reflect the age of emplacement,
Range and also within it, but in some areas granite outcrops extend but it is at present impossible to distinguish these from reliable
as far as the west coast. Granites of the volcanic arc suite are con- ages, unless complementary methods of isotopic dating have
fined to the Barisan Range. been used, a requirement which substantially diminishes the
At the present time it is difficult to provide a unified account for value of the currently available data set. For these reasons some
the granites of Sumatra, because much of the earlier work of the isotopic ages quoted in the following acount may be
addressed different aspects of the geological, geochemical and iso- subject to revision. Most of the granite ages considered in this
topic relationships of the granites. This has resulted in difficulties account are those for which there is supporting isotopic and
in interpreting the earlier studies. Consequently the following syn- geochemical data.
thesis is constrained by the different objectives and conditions Until recently the U-Pb zircon age of 264 Ma obtained by Liew &
under which the earlier regional work was carried out. McCulloch (1985) from the Kuantan Granite of the Eastern Province
of Peninsular Malaysia was the oldest recorded age for granites of
the region. This has now been extended to 275 Ma by Schwartz &
Isotopic ages of Sumatran granites Askury (1990) who obtained K-Ar biotite ages from plutons in
the Kuantan-Dungun region ranging from 220 to 275 Ma. Ages
Many of the published isotopic analyses from Sumatra are from the Main Range Province in Peninsular Malaysia are generally
unsupported by petrographic descriptions or whole-rock chemical younger, from 207 to 230 Ma (Cobbing et al. 1992). The peak of
analyses. Moreover, in some cases isotopic ages determined for magmatism for the Main Range Granites in Peninsular Malaysia
particular plutons cover such a wide range that it is impossible and the Tin Islands is 220 Ma, with granites ranging to older ages,
to establish their exact age of emplacement. In other cases the especially in the Tin Islands: e.g. Belinyu 251 _ 10 and Penangas
available geochemistry is sufficiently anomalous to cast doubt 252 _ 8 (Cobbing et al. 1992) (Fig. 5.2).

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