GEOLOGY OF THE PENGKALAN HULU-BETONG TRANSECT ...

3.3.2 Bintang granite (Trgrbt)
Distribution
The Bintang granite (Trgrbt) is only exposed on the Malaysian side in the southwestern
part of the Transect area. This pluton continues southward into the Kuala Kangsar and
Taiping areas. The Bintang pluton is in contact with the Kroh formation (SDkr), Kubang
Pasu Formation (Ckp) and Gerik formation (Pgk).
Petrography
The Bintang granite (Trgrbt) is essentially biotite granite which shows surficial variation
from potash alkali granite to granodiorite. However, there is no detailed study carried out
on this pluton to delineate the boundaries of these different varieties. The Bintang granite
(Trgrbt) is typically grey, medium- to coarse-grained, porphyritic with biotite as its
essential mafic constituent (Figure 40). It contains a variety of accessory minerals such as
apatite, sphene, zircon, allanite, magnetite and tourmaline.
The phenocrysts composed of feldspar, which are euhedral, commonly twinned,
measuring in length from 2 to 7 cm, with average length of about 4 cm; set in
xenomorphic to hypidiomorphic granular groundmass of quartz, feldspar and mica. The
average density of the phenocryst is calculated as 15 – 20% by volume. Some parts of the
granite exhibit “flow banding” in which densely porphyritic bands are interlaminated with
bands and tongues of less porphyritic or non-porphyritic material. The feldspar
phenocryst can frequently be seen in varying stages of alteration to kaolin and sericite.
Anhedral and rarely subhedral grains of potassic feldspar and oligoclase are closely
intergrown with quartz to form the groundmass of the granite. Potassic feldspar
(including both microcline and orthoclase) normally predominates over the plagioclase.
In some places the feldspar is pinkish in colour, possibly due to the presence of iron.
Quartz is clear, anhedral grains closely intergrown with the feldspar of the
groundmass. Occasionally, quartz occurs in graphic intergrowth with potassic feldspar
and also in minor myrmekitic developments.
The biotite content of the granite over the area generally varies from 5 – 30%, but
10 – 15% is quite characteristic for the uncontaminated granite. The biotite flakes
measuring about 1.5 mm in width are evenly distributed in the granite as distinct flakes
and loose clusters scattered within quartz-feldspar matrix.
Numerous xenoliths are found scattered over the granite outcrops, occurring as dark
grey to black spots or patches. The xenoliths consist of sedimentary and pyroclastic rocks
that exhibit varying degree of metamorphism and granitization (Jones, 1970). Their
shapes range from circular spots to patches, having elliptical, oval, elongated or
sometimes irregular forms with sizes varies from 2 cm to several metres across. The
elongated forms are usually parallel to the alignment of feldspar phenocrysts. The average
grain-size of these xenoliths is distinctly finer than that of the surrounding granite. The
mineral composition of these xenoliths consists predominantly of biotite and microcline
with subordinate quantities of plagioclase and quartz. Sphene, apatite and opaque iron
oxide are the common accessory minerals. Hornblende is prominent in some of the
xenoliths; it may be as abundant as biotite but it may occur only as accessory minerals or
absent elsewhere. The distribution of these xenoliths is rather haphazard. In some areas
they are very abundant, others are hardly noticeable. Most of these xenoliths are probably
granitized xenoliths that have crystallized together with the surrounding granite whereas
some of them could have originated as segregated bodies from the magma in the course
55