Page 73 - Sumatra Geology, Resources and Tectonic Evolution

60 CHAPTER 5

distinct plutonic episodes; (3) westward younging of the Miocene Granitoids with volcanic arc characteristics have been
and Pliocene plutons. recovered from oil exploration drilling programmes in NW Java
(1) Persistence of granitic source regions is indicated by the (Patmosukismo & Yayha 1974). These authors report the presence
Sulit Air Suite which consists of three small dioritic plutons of of granitic rocks, described as quartz microdiorite, with a K20
similar lithology, located to the northeast of the Lassi Batholith. content ranging from 1.29 to 4.04% and K-At ages ranging from
Two of these, the Guguchina and Belimbing plutons are close in 94 to 56 Ma, in three exploration wells. These granitoids can be
age at 138 Ma and 141 Ma, but the Sulit Air Pluton gave K-Ar correlated provisonally with the Volcanic Arc Suite of Sumatra.
ages of 203 ___ 6 and 183 + 13 Ma (Table 5.1a) and 192-193
Ma (4~ method, Imtihanah 2000). The suite was evidently
emplaced over a period of 55 million years. An even more
remarkable example is the Rantaupandang Unit of the Bungo The relationship of Sumatran granites to adjacent
Batholith which shows identical lithological and petrographic areas of Sundaland
features in samples from two widely separated localities,
subsequently confirmed by identical major and trace element Sumatra, including the Tin Islands, the southwestern part of
analyses from the two samples. Biotite and hornblende K-Ar geo- Kalimantan, the Malay Peninsula, Thailand and Burma constitute
chronology provided ages of 148 ___ 4 Ma and 137 + 7 Ma for part of Sundaland. The tin-associated granites of Sumatra and the
SSG47 and 54 ___ 2Ma for SSG44a (Table 5.1d). Duplicate stanniferous and non-stanniferrous granites of the Tin Islands can
analyses confirmed these results, which can only mean that the be correlated with the Main Range and Eastern Granite Provinces
source region remained unchanged for nearly 100 million years. distinguished in those areas (Hutchison 1989, 1994) (Fig. 5.3).
(2) The existence of distinct plutonic episodes is suggested by Although there is a paucity of geochemical and isotopic data for
breaks in the sequence of intrusion, with durations of between the tin-associated granites in Sumatra, that which is available,
20 and 34 Ma in the ages of plutons emplaced within the same plu- together with their distinctive field characteristics, leaves little
tonic lineament. Four episodes were recognized 203-130 Ma, doubt that these granites are an expression of the same phase of
117-82 Ma, 60-53 Ma and 20-11 Ma (McCourt et al. 1996). plutonism as that developed in the Main Range (Central Belt) in
Future work may modify these results, but with the present data mainland SE Asia (Mitchell 1977; Beckinsale 1979; Hutchison
they appear to be real. 1989; Cobbing et al. 1986, 1992).
(3) Westward younging of the plutonic arc is indicated by a dis- Similarly, the volcanic arc plutonism of the Barisan Range finds
tinct line of small plutons of Miocene age, extending from Lake a ready analogue in the Central Valley Province of Burma, where
Ranau to Padang (McCourt & Cobbing 1993). the Wuntho Batholith and the Salingyi Complex show a range of
Most of the plutons sampled are characterized by primary mag- lithologies similar to those which are developed in Sumatra, but
matic textures but some, lbr example Sungei Durian in the Lassi which are restricted to the Cretaceous (Cobbing et al. 1992;
Batholith and the Sulan Tonalite, are strongly foliated. In the McCourt et al. 1996).
case of the Sulan Pluton this is clearly a magmatic foliation, The Hatapang Granite of Cretaceous age is stanniferous, and
characterized by evenly deformed mafic enclaves and the align- Clarke & Beddoe-Stephens (1987) have suggested that it may be
ment of mafic and felsic minerals. The most striking example of an outlying representative of the Western Belt, developed in
deformation is seen in the Aroguru Diorite in South Sumatra to Peninsular Thailand and the Shah Scarp region of Burma (Mitchell
the North of Bandar Lampung, where five phases of progressively 1977; Beckinsale 1979).
weaker deformation were recorded. These phases provide a record Most of the regional relationships of the granites of Sumatra
of movement in the region during the emplacement of the pluton, to the geology developed during the geological evolution of
which has been dated at 89 • 2 Ma (McCourt & Cobbing 1993; Sundaland are straightforward, but some are not. Unfortunately,
McCourt et al. 1996; Barber 2000). the most intractable problems are located in the area between
The Lassi Batholith (Table 5. l b) comprises at least nine units, Peninsular Malaysia, eastern Sumatra and the Tin Islands.
five of which were dated. Most of these units are diorites and These problems centre around the southward extension of the
gabbros of varying lithologies and texture, but a distinctive Bentong-Raub Line (Figs 5.1 & 5.3) which, in Peninsular Malaysia
coarse K-feldspar megacrystic granite is present in at least seven and Thailand, divides stanniferrous S-type granites of the Main
small dyke-like intrusions. The foliated and poorly exposed Range (Central) Belt, from non-stanniferous and stanniferous gran-
Sungai Durian granodiorite with an SiO2 content of 68.7% ites of the Eastern Belt. This line is clearly marked in Peninsular
forms a large outcrop in the southern part of the body. The Malaysia by the sporadic occurrence of ophiolites. It can also be
spread of ages from 203 to 55 Ma for the Sulit Air Suite and the followed northwards, across the Gulf of Thailand, as far as the
Lassi Batholith is noteworthy, since their field, petrographic and border with Laos. It cannot, however, easily be followed southwards.
geochemical characteristics are sufficiently similar for them to Whereas some of the islands of the Indonesian Archipelago host
have been initially considered as a consanguineous group stanniferous S-types, most of the granites are non-stanniferous
(McCourt & Cobbing 1993). I-types. There are also both stanniferous and non-stanniferous
The Lolo Pluton (Table 5.1c) is one of the youngest granites A-type granites (Cobbing & Mallick 1984; Cobbing et al. 1992).
with a full geochemical analysis to have been dated, with an There is an extensive literature on this question which is summarised
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39
intrusion age of 15 Ma ( At/~ Ar method. Imtihanah 2000). It by Hutchison (1994) who concludes that the Raub-Bentong Line
is of tonalitic composition and is a component of the belt of probably follows a course near the east coast of Sumatra and lies
very young plutons close to the southwest limit of the plutonic somewhere in the neighbourhood of Bangka and Billiton.
arc (McCourt & Cobbing 1993; McCourt et al. 1996). Granites in most of the northern islands of the Riau Archipelago
There is little doubt that both the Lassi and the Bungo batholiths are non-stanniferous I-types, but stanniferous S-types with Main
are more complex than at present appears to be the case. Most of Range (Central Belt) characteristics are present on the island of
the other granites sampled are simple plutons, consisting of one Kundur and at the southwest tip of Singkep (Fig. 5.2). The pro-
major rock type, but some plutons are zoned, having a compo- longation of this direction leads directly towards the islands of
sitional variation from diorite or tonalite to granodiorite or Bangka and Billiton, and follows an arcuate form leading eastward
monzogranite. from Sumatra towards Kalimantan. Bangka and Billiton contain a
Table 5.1(c-i) show almost the whole compositional range of mixed population of stanniferous S-type granites and non stanni-
the South Sumatra granites and is sufficient to show their essential ferous I-type granites (Fig. 5.2, in which the S and I type granites
similarity to the data of Gasparon & Varne (1995) and, by analogy, are mingled together and are not separated into distinctive belts).
to the entire volcanic arc suite of Sumatra. There is also a suite of intermediate character containing both