Page 69 - Sumatra Geology, Resources and Tectonic Evolution

56 CHAPTER 5

I I
t 06 ~ t08,~ ,
NGAPORE
:am _La~. oi 226Ma
AR~MUN ~ I~-~'~ ~ll BINTAN
5k,'~ ~ J East Bintan
-% \- ~-j oBatholith Eastern Province
X~ Loban 229Ma (I-Type) Granites
Akat ,~ Laut
"~ ~~ ~ Main Range Province
%~ (S-Type) Granites
_ 0 o
_ st Central
Sungai
lsahan [~,~
_ Paku ~-/SINGKEP
"~--"~ BANGKA
\,~ Belinyu Klabat Batholith
25~anjong Layang
~ P e n a n g a s ~---:'~ ...... )\Tanjong Batu o ~
~
9
9
~
-2os S U M A T R A M~e~nu~n~a..lr-- ,. Tanjong Raya Z'-
200Ma 2- ..---<7"z~ ..... \
f _ j ~ % s Tanjong BILLITON
Pluton ~ ~ n n Man
p''
)
'
'
4
~" 213Ma \ .......... ? f 216Ma(~ /-~"~ u ~ g
g
P ALEMBANG 0 . Bukit L ~ /r., 0 %27nong L egau
Toboali ~,~"-5--- 2
Batu 225Ma ~'-~ Fig. 5.2. Main Range and Eastern
/ Nama Parangb~h gP Kelumpang Province granites in the Indonesian Tin
100 200 300km (/ Islands (after Cobbing et al. 1992).
--~ IIIIIII III \ Karimun is a Tin granite, but it does have
104 '~ 106 ~ 108 ~;` A-type affinities. Segal and Akat are both
I /I I l-types. Karimun has affinities with Dabo.
The granite suites ages of 197 • 2 Ma and 193 4- 2 Ma were obtained for muscovite
in greisens in the Sungei Isahan and an age of 198 4- 2 Ma from
The granites of Sumatra form two distinct groups. An older group biotite in K-feldspar megacrystic granite at Bukit Kayumambang
is widely distributed as isolated plutons and batholiths over the 20 km east of Sungei Isahan.
whole island, but mainly in the area to the east of the Barisan The Sijunjung Batholith, which is located on the eastern flank of
Range. Some of these granites are tin-associated and have a the Barisan Range to the northeast of Padang (Fig. 5.1), is a very
narrow compositional range of SiO2 values, generally above large and inaccessible body, but a large sample was dated and
70%. These older granites are related to the Central (Main chemically analysed by Sato (1991). The K-Ar age is 247 Ma
Range) Province of the Southeast Asian Tin Belt of Peninsular and the geochemistry, with a SiO2 value of 72.71%, is similar
Malaysia and Thailand (Figs 5.1 & 5.3). A younger group of gran- to that for the S-type granites of the Main Range Batholith of
ites form the plutonic component of a volcanic arc suite. They are Peninsular Malaysia and the Tin Islands (Sato 1991).
confined to the Barisan Range, where they form small batholiths The Sungei Isahan and Sijunjung occurrences are at present the
and separated plutons with an extended compositional range only examples of the tin-associated granites of Main Range Type
from gabbro to monzogranite. in mainland Sumatra lbr which there is both geochronology and
geochemical analyses. Provisionally these two occurences may
be regarded as representative of the Tin-Associated Suite as a
whole. Although the database for the widespread Tin-Associated
The Tin-associated suite Granites is small, where the writer has inspected them in the
field they were found to bear a striking resemblance to granites
Tin-associated granites are of S-type affinity and are probably of the Main Range (Central) Province in Peninsular Malaysia
mostly of Triassic age. They are widely distributed in Sumatra and the Tin islands.
but are poorly exposed. They are equivalent to the Main Range The Hatapang Granite, which is located to the south of Lake
granites of Peninsular Malaysia and of the Indonesian Tin Toba (Fig. 5.1) was discovered by the investigation of a tin
Islands. There is however, an almost complete lack of geochem- anomaly revealed by reconnaissance geochemical surveying.
ical and isotopic data for these granites. Schwartz (1987) and The geochemical and isotopic study by Clarke & Beddoe-Ste-
Schwartz & Surjono (1990a) reported five major and trace phens (1987) established an Rb-Sr isochron age of 80 i 1 Ma
element analyses from greisens and K-feldspar megacrystic with an initial ratio of 0.7151, which indicates an S-type affinity.
biotite granites from the Sungei Isahan and adjacent areas in the They suggested on the basis of these results, that the pluton was
Tiga Puluh region of South Sumatra (Fig. 5.1). Three of the ana- not representative of the tin-associated granites of Triassic age,
lyses are of greisens and are anomalous in their composition, but was more likely to be one of the Western Province granites
but two are from normal K-feldspar megacrystic monzogranites of mainly Cretaceous-Tertiary age occurring along the Thailand-
with SiO2 values of 71.7 and 71.47% which correspond closely Burma border and the Shan Scarp region of Burma. Representa-
with the geochemical signatures of granites from the Main tives of this suite are present at Phuket in southern Thailand
Range Province of Peninsular Malaysia and Thailand. K-Ar north of the Ban Kram Fault Zone (Garson et al. 1975), and