GEOLOGY OF THE PENGKALAN HULU-BETONG TRANSECT ...

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rectangular, oval and rounded with the size up to 30 cm in diameter. Most of the xenoliths
are dark-coloured, heavily feldspathized with a high ferromagnesian content. However,
some of quartzose sedimentary material origin has been metamorphosed to lightcoloured,
feldspathized, quartz hornfels. Cognate also present randomly throughout the
granite in the forms of oval, rounded, elongate segregation and clot.
On the Thai side, the Rimba Telui/Si Nakhon granite (Trgrrt/sn) is medium- to coarsegrained,
light grey in colour with good primary granitic texture (Figure 39B). K-feldspar
can be observed as large megacrysts of light grey to chalky white colour with elongate to
tabular, euhedral to subhedral habits. Their population varies from sparsely megacrysts to
locally concentrated megacrysts. Weakly foliated K-feldspar megacrysts are locally
observed. Biotite groundmass is observed as euhedral book crystals and subhedral to
anhedral flaky crystals with single to clusters habits. Plagioclase is present in the
groundmass as chalky white colour, euhedral to subhedral crystals with tabular to equant
habits. K-feldspar groundmass forms subhedral to euhedral crystals with tabular habit.
Quartz can be observed as anhedral to subhedral crystals, single and clusters with
subrounded to subangular habits. Cognate and accidental xenoliths also present in these
rocks. Dykes and veins are locally found along the intrusive contact aureoles.
Small granite bodies, which intruded the Carboniferous and Permian sedimentary
country rocks, are also found at Ban Tham Talu abandoned tin mine. The presence of tin
deposits, which are largely distributed in the Rimba Telui/Si Nakhon granite (Trgrrt/sn)
batholith, suggests that tin mineralization may have somewhat been closely associated
with the late stage intrusion of this granitoid.
A
Figure 39: The Rimba Telui/Si Nakhon granite (Trgrrt/sn)
A- Grey, medium- to coarse-grained, porphyritic and invariably mottled
with black biotite at Sungai Sok, Malaysia.
B- Medium- to coarse-grained, porphyritic, unfoliated to weakly foliated,
biotite granite of Si Nakhon granite, with cognate inclusion (right)
exposed at Ban Piyamit 1, Ai Yoe Weng area, Thailand.
B