GEOLOGY OF THE PENGKALAN HULU-BETONG TRANSECT ...

3.3.1 Rimba Telui/Si Nakhon granite (Trgrrt/sn)
Distribution
In Malaysia, the name Rimba Telui granite (Trgrrt) was first introduced by Teoh (1992)
to describe the granite body exposed in the Sungai Tiang area, west of the Transect area.
The name was later adopted by Mat Niza & Abdullah (in manuscript) for the granite that
exposed in the Ulu Muda area. The Rimba Telui/Si Nakhon granite (Trgrrt/sn) occurs as a
major proportion of the granite in the Transect area, covering most of the northern part of
the Transect area (approximately 560 square kilometres). The granite body forms hilly
and forested terrains, occasionally reach heights of over 1,000 metres above mean sea
level. The main peaks are Gunung Bubus, Malaysia or Khao Bubut, Thailand (1,121 m),
Bukit Mudim Besar, Malaysia or Khao Nam Kao, Thailand (1,080 m), and Bukit Gajah
Hutan, Malaysia (1,067 m).
On the Thai side, the Rimba Telui/Si Nakhon granite (Trgrrt/sn) is represented by N-S
trending batholith, which extends westward across the border to the Malaysian side,
covering approximately 25% of the Transect area. The granite outcrops are well exposed
along the road-cuts from Ban Nam Pu Ron to Ban Piyamit 1 to Ban Km 7, along the roadcuts
from Ban Sa Kai to Ban Si Nakhon to Ban Din Samoe at the border, and along the
road-cuts from Ban Khok Chang to Ban Km 27.
Petrography
On the Malaysian side, the Rimba Telui/Si Nakhon granite (Trgrrt/sn) is typically grey,
medium- to coarse-grained, porphyritic, and invariably mottled with black biotite (Figure
39A). The coarse-grained varieties are generally porphyritic containing white subhedral
to euhedral phenocrysts of feldspar, but the finer-grained varieties are generally
equigranular. The feldspar phenocrysts vary up to 5 cm in length, consisting of orthoclase
and perthite. Weakly foliated K-feldspar megacrysts are locally observed. Texturally, this
granite is very similar to the Bintang granite (Trgrbt) and Main Range granite.
Petrographically, the rock consists of predominantly quartz, alkali feldspar, plagioclase
and biotite. Quartz is generally anhedral, usually showing only a slight degree of
undulose extinction. Microcline and orthoclase are the common constituents of the alkali
feldspar. Perthite and microcline-perthite are also present. Microcline is generally
subhedral to anhedral and displays fine, sharp, grid-iron twinning. Alkali feldspar
phenocrysts commonly contain inclusions of quartz and biotite as well as laths of
plagioclase. Plagioclase crystals, albite to oligoclase in composition (An0-An30), are
generally subhedral to anhedral, and display typical albite and combined Carlsbad-albite
twinning. The dominant mica is pleochroic brown biotite, generally ranging from 10% or
less. Muscovite is rare. Apatite, sphene, zircon, allanite, magnetite and tourmaline are
present as accessories.
The Rimba Telui/Si Nakhon granite (Trgrrt/sn) is typically granitic in composition.
However, alkali granite and granodiorite also occur locally. The late-phase minor
intrusives consist of mainly microgranite dykes, aplite dykes, tourmaline vein, and quartz
vein.
Xenoliths of sedimentary rock origin are scattered in the granite and they can be seen
throughout the exposures of Rimba Telui/Si Nakhon granite (Trgrrt/sn). The xenoliths are
most commonly observed close to the granite contacts generally exhibiting various
degrees of metamorphism and granitization. The shape of the xenoliths varies from
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