The Tai Lam Granite (Table 6.2) forms a large subcircular pluton in the northwestern New Territories, extending as far north as Yuen Long, east to Tsuen Wan, south to Lantau Island, and west to Castle Peak (Figure 6.3). It intrudes the Lantau Granite in the south, Palaeozoic sedimentary rocks in the north and west, and Jurassic volcanic rocks in the east. An inlier of Tai Po Granodiorite is present within the Tai Lam pluton in the northeast.
The Tai Lam Granite consists of a porphyritic medium-grained to equigranular fine-grained monzogranite (Plate 6.10). Equigranular lithologies are characterised by euhedral to subhedral moderately strained plagioclase (oligoclase to albite), commonly with slightly deformed twin lamellae, anhedral slightly strained quartz, perthitic orthoclase and subordinate microcline in a euhedral granular matrix. Greenish brown biotite is present in aggregates, and there is evidence for subsolidus growth of alkali feldspar and quartz to produce poikilitic textures. Accessory minerals include allanite and zircon, with epidote and chlorite present as alteration products. The inequigranular lithologies show evidence of deformation and partial recrystallization in the form of strained quartz and deformed feldspar lamellae. Muscovite and fluorite are present as late stage crystallization products, and yellowish brown biotite occurs as crystal aggregates.
A textural variant of the Tai Lam Granite (the Ma Wan Granite of Sewell & James (1995)) is present in northeastern Lantau Island and on Ma Wan. Here, the granite is weakly deformed, equigranular, medium- and fine-grained, with local zones of hydrothermal alteration. In thin section, the granite is typically rich in microcline which distinguishes it from typical Tai Lam Granite.
A Rb–Sr whole-rock isochron age of 155 ± 6 Ma has been obtained for fine- to medium-grained lithologies of the Tai Lam Granite. However, single and multigrain zircon fractions have yielded concordant results at 159.3 ± 0.3 Ma (Davis et al., 1997) which is likely to date the emplacement of the granite more precisely.
Details
Tai Lam Chung to Ting Kau. Between Sham Tseng and Ting Kau an outcrop of medium-grained granite is bounded to the north by fine-grained granite. On a ridge above Tuen Mun Road (825400 825750 Jma-1) the contact between the granites can be inferred from the weathered profile with coarse sand to the southwest and fine sand to the northeast. This contact can also be seen on Tuen Mun Road (825450 825440 Jma-2), where the rock grades from megacrystic fine-grained granite to microcrystic medium-grained granite over a distance of 50 m. Megacrysts, relicts of the medium-grained texture, are abundant, with alkali feldspar up to 12 mm, quartz up to 10 mm, plagioclase up to 4 mm and biotite clots up to 3 mm; the average megacryst size is less than 6 mm. Within the outcrop are thin dykes of fine-grained granite and aplite. Boreholes offshore northern Ma Wan indicate that the Tsing Lung Tau and Ting Kau outcrops are contiguous, and only obscured by thin superficial deposits.
Tai Tong to Ma On Kong. In this area, north of Tai Lam Chung Reservoir, a large, irregular outcrop of medium-grained granite is surrounded by, and includes, outcrops of fine-grained granite. The contact with the enclosed outcrop of fine-grained granite is well defined, and where exposed (821650 828930 Jma-3) is gradational, with microcrystic and megacrystic varieties present. In a borrow area 1 km southeast of Tai Tong (821600 829900 Jma-4) the granite is white and equigranular, with alkali feldspar up to 10 mm, greenish plagioclase between 2 and 5 mm, pools of quartz between 2 and 8 mm, and biotite clots from 2 to 5 mm across (Plate 6.A12). Very few fine-grained granite dykes occur in the medium-grained granite outcrop
North of the reservoir the granite is not uniformly equigranular, but contains megacrystic fine-grained and microcrystic coarse-grained varieties. A contact between sparsely megacrystic fine-grained granite below and medium-grains above dips gently northwest to the west of the Country Park Management Centre (822700 828580 Jma-5). The contact is gradational, and there are also patches of medium-grained granite within the fine. North of here, along a footpath, the infiltration of finer material into the medium-grained granite can be seen over a 30 m wide section (822670 828910 Jma-6).
Along the catchwater west of Ma On Kong the medium-grained granite is equigranular. The rock is white, with alkali feldspar generally less than 6 mm, although sometimes up to 12 mm; greenish plagioclase varies from 2 to 4 mm, with quartz pools less than 10 mm and biotite clots less than 7 mm. There are also single crystals of biotite between 1 and 4 mm. The contact with the megacrystic fine-grained granite to the north is well defined, and on the ridge (823410 830680 Jma-7) a microcrystic medium-grained granite is exposed at the contact.
To the southeast of Tai Tong are several isolated outcrops of fine-grained granite within the outcrop of medium-grained granite. A contact between the megacrystic fine-grained granite and overlying thermally metamorphosed tuffs and sedimentary rocks can be defined on a small knoll in the northeast of the outcrop (823800 830900 Jma-8). About 250 m to the south of this intrusive contact an outlier of tuff straddles the fine-grained and medium-grained granite boundary.
At the largest outcrop of megacrystic fine-grained granite in the area the granite is greisenized at its contact with the overlying tuffs (822930 829730 Jma-9). The outcrop is surrounded by medium-grained granite and the contact is well seen on a track to the south (822650 828900 Jma-10). The presence of a microcrystic groundmass to the medium-grained granite becomes progressively more noticeable over a distance of 30 m, until fine-grained granite with many megacrysts is dominant. Along a northnorthwest trending valley (822300 829600 Jma-11) the fine-grained granite contains fault breccia, greisen veins and pegmatite. A schistosity is also developed along this trend in belts up to 200 mm wide. To the north (821100 830400 Jma-12) the contact with Early to Middle Jurassic sedimentary rocks is believed to be intrusive, dipping gently northwards.
Northwest Tai Lam Country Park. A sizeable outcrop of medium-grained granite occurs around the northern end of the catchwater 1 km west of Yeung Ka Tsuen. Small, isolated bodies outcrop about 1.5 km south of Yeung Ka Tsuen, near a small reservoir (819800 828800 Jma-13), and just north of the Country Park. The rocks are white, with alkali feldspar rarely up to 25 mm, greenish plagioclase up to 4 mm and biotite clots between 1 and 4 mm; the groundmass is microcrystic.
Microcrystic medium-grained granite texturally close to the megacrystic fine- and fine- to medium-grained, occurs in isolated outcrops in the northwest of Tai Lam Country Park. Exposures at a tunnel entrance south of a small reservoir (819940 828730 Jma-14) contains alkali and plagioclase feldspar crystals mostly between 2 and 6 mm, but the quartz is microcrystalline and interstitial. Elsewhere the microcrystic texture becomes more noticeable as the rock grades into the surrounding megacrystic fine- to medium-grained granite.
Western Aqueduct. Medium-grained granite is exposed along two long sections of the water tunnel between the portal at Ko Po Shan Tsuen and the shaft at the northeastern end of the reservoir. These exposures correlate with the part of the Tai Tong Ma On Kong outcrop. The granite is white, and contains pink pegmatite nests, for example 2377 m (823410 830790 Jma-15) and 2888 m (823220 830310 Jma-16) from the portal at Ko Po Shan Tsuen. The granite is either equigranular or shows signs of microcrystic infiltration. The equigranular medium-grained granite contains alkali feldspar up to 15 mm across, greenish plagioclase mostly 2 to 4 mm, quartz pools less than 10 mm and biotite clots less than 4 mm.
Megacrystic fine-grained granite was intersected by the tunnel in exposures which can be directly related to outcrop. West of Ma On Kong the granite was found both adjacent to the tuffs of the Shing Mun Formation and within the medium-grained granite. The contact with the tuff (823530 831100 Jma-17) is sharp and intrusive, and dips 18o to the north with a small normal fault displacing it about 0.5 m. Below Lin Fa Shan, megacrystic fine-grained granite is intruded between granodiorite to the north and Shing Mun Formation tuffs to the south.
West Tai Lam Country Park. The fine- to medium-grained granite of this area forms a single, complex outcrop which is aligned with the surrounding fine-grained granite on a northeast trend. The outcrop is divided by sheets of fine-grained granite mostly between 100 and 500 m wide. Most of the outcrop is deeply weathered, but fresh rock has been exposed along catchwaters, in excavations and in boreholes. The contact between the rock types is usually obscure, but in a number of places can be accurately located. For example, east of Chi Lok Fa Yuen (816500 827800 Jma-18), above the catchwater, an abrupt change can be observed between megacrystic fine-grained granite to the north and megacrystic fine- to medium-grained granite to the south; this contact zone also includes pegmatite veins and nests. Similarly, northwest of So Kwun Wat (817890 827350 Jma-19) the contact zone with adjacent fine-grained granite includes fine-grained granite dykes and pegmatite, and forms a dyke-like body 50 to 150 m wide.
Near Wong Ka Wai, eastern Tuen Mun an isolated outcrop is cut both by basalt dykes and a thick quartz vein trending parallel to the contact with the volcanics to the west. Exposures in the stream at Wong Ka Wai (815800 828430 Jma-20) show greenish white granite with megacrysts of feldspar up to 25 mm and quartz up to 10 mm set in an inequigranular groundmass ranging from 1 to 5 mm.
To the west of Lam Tei Reservoir (818600 820200 Jma-21) the fine- to medium-grained granite lies between an area of equigranular medium-grained granite and the surrounding megacrystic fine-grained granite. This outcrop is traversed by quartzphyric rhyolite and basalt dykes, and very thin schist bands. The contact between the fine- to medium-grained and fine-grained granite is not seen, but is probably gradational, for example in the excavations southeast of Tan Kwai Tsuen (818500 831700 Jma-22).
On the catchwater north of a small reservoir (819700 829120 Jma-23) there is a sharp contact between fine-grained granite to the south and inequigranular fine- to medium-grained granite to the north. Fine-grained granite dykes are not common, but a small one, about 50 mm wide, can be seen in the centre of the outcrop (818480 828030 Jma-24), and another can be seen on the catchwater above So Kwun Wat (818150 826670 Jma-25).
Along the catchwater north of So Kwun Wat (818100 826600 Jma-26) the granite is white to grey in colour, with feldspar megacrysts up to 25 mm long, biotite clots up to 7 mm, and quartz megacrysts up to 7 mm. The groundmass mostly falls in the range 1 to 4 mm. In boreholes south of So Kwun Wat the granite varies from pink to white in colour. Alkali feldspar is up to 15 mm, while greenish plagioclase is between 2 and 7 mm. Quartz megacrysts are often clearly defined, and are up to 10 mm across. Biotite clots are generally less than 7 mm, and the groundmass varies from 2 to 4 mm in some parts, to less than 0.5 to 1 mm in others.
Tai Lam Chung to Sham Tseng. There are two outcrops of fine- to medium-granite in this area, both closely associated with the equipgranular medium-granied granite to the south. Around to the south. Around Tai Lam Chung no exposed contacts were noted between the megacrystic fine- to medium-grained granite and the surrounding megacrystic fine-grained granite. However, east of the prison a progressive change can be seen, with the groundmass increasing in proportion and becoming both finer and more equigranular. Near Tai Lam Chung Tsuen (820350 825410 Jma-27), fine- to medium-grained granite is very variable in texture and is believed to lie at the margin of the intrusion. Further north, within the outcrop, (820400 825600 Jma-28) the coarsely megacrystic fine- to medium-grained granite has a relatively uniform texture. The feldspar megacrysts are up to 30 mm, and the groundmass varies from less than 1 to more than 3 mm. There are also thin pegmatite and biotite rich veins.
To the east a continuous outcrop can be seen between Tsing Lung Tau and Sham Tseng. This has also been traced offshore Sham Tseng using borehole information. The granite is pale pink to white in colour and contains megacrysts of alkali feldspar mostly less than 10 mm, with plagioclase less than 6 mm and aggregates of small biotite crystals less than 7 mm, as well as single biotite crystals up to 4 mm. There is a markedly inequigranular groundmass varying from 0.5 to 3 mm. The rock contrasts with the medium-grained granite to the south and with the fine-grained granite to the north, but no sharp contacts have been observed.
At Sham Tseng the outcrop of fine- to medium-grained granite is cut by dykes of basalt and fine-grained granite (823730 825010 Jma-29). To the west of Tsing Lung Tau (822000 824400 Jma-30) a group of basalt dykes cut the contact between the fine-grained and fine- to medium-grained granite.
Tuen Mun to Tan Kwai Tsuen. This outcrop is bounded to the west by a fault separating it from Carboniferous sedimentary rock, and to the east by fine- to medium-grained granite. Well defined contacts with other granite varieties are found in a few places, but sharp contacts have been observed. About 1.5 km east of Wong Ka Wai (817000 828200 Jma-31) the fine-grained granite outcrop extends in a dyke like form, trending northeast and gradually reducing to less than 100 m in width. The rock is megacrystic fine-grained granite, and there is a gradual increase in both megacryst content and groundmass grain size towards adjacent fine- to medium-grained granite outcrops. This same contact with the fine- to medium-grained granite can be seen in a stream to the northeast (817150 828800 Jma-32). About 500 m to the north (816930 828630 Jma-33) the change from megaaystic fine-grained granite is marked by an increase in the grain size of the soil.
In the few places where the fine-grained granite is in direct contact with medium- or coarse-grained granite the boundaries are usually sharp and distinct. About 1 km east of Tan Kwai Tsuen (818690 831910 Jma-34) the contact with a small outcrop of equigranular medium-grained granite is well defined, striking roughly southeast with a vertical dip. The adjacent fine-grained granite is typically uniform and sparsely megacrystic. About 0.5 km east of Wong Ka Wai (816100 828300 Jma-35) there is a marked transition from the megacrystic fine-grained granite to the west into a complex of coarser granite types cut by fine-grained granite dykes.
The intrusive contact of the fine-grained granite with Carboniferous sediments to the west is also faulted. About 500 m northeast of Tuen Mun San Hui, two boreholes intersect highly fractured granite beneath superficial deposits (900D/03410, 815784 829078 Jma-36; 1384D/03546, 815740 829075 Jma-37), and 30 m to the west a borehole intersected phyllitic siltstone. At Wong Fung Lek (816350 829660 Jma-38) a contact between deeply weathered granite and foliated quartzite trends northeast.
A dyke of fine-grained granite trending northeast occurs to the east of Sam Shing Hui (816300 826800 Jma-39). The rock grades into the adjacent coarse-grained granite.
The fine-grained granite is cut by quartzphyric rhyolite and basalt dykes, particularly in a broad belt to the east of Lam Tei (818400 831000 Jma-40). Fault breccia or zones of strong shattering are found in most north trending valleys, and thin bands of mylonite can also be found within the outcrop. Fine-grained granite dykes are very rare, but can be seen in the megacrystic host rock.
Central and Eastern Tai Lam Country Park. One of the largest continuous outcrops of fine-grained granite occupies the centre of Tai Lam Country Park, from So Kwun Wat in the southwest to Ting Kau in the southeast, and extending to the northeast and northwest of Tai Lam Chung Reservoir.
The fine-grained granite is best exposed along the road running parallel to the southeast side of Tai Lam Chung Reservoir (821200 826100 Jma-41). The rock is pink, weathering brown, and is dominated by a fine-grained groundmass of around 0.5 mm. Megacrysts are common, ranging up to 25 mm for the feldspar and 8 mm for the quartz. Biotite occurs as clusters of small crystals up to 3 mm across. The rock is cut by quartz veins, basalt dykes and rhyolite dykes. At one locality (821850 827400 Jma-42) the granite is hydrothermally altered to a dyke-like body about 4 m wide trending southeast.
To the northeast and southeast of the Tai Lam Chung outcrop the granite is in contact with volcanic rocks. North of Ho Pui Reservoir (825550 830110 Jma-43), in the river valley, fine-grained granite lies below thermally metamorphosed tuffs of the Shing Mun Formation. There is an isolated knoll of equigranular fine-grained granite about 20 m across within the tuff, probably indicating the shallow, irregular nature of the contact. This same intrusive contact is also clearly defined on a track west of the reservoir (825330 829370 Jma-44), where fine-grained granite lies below crystal tuffs of the Ngau Liu Member. This contact can be seen again to the west, about 0.7 km north of Tin Fu Tsai (824700 828900 Jma-45), where it is sub-horizontal or dipping gently east.
At Ting Kau, in the southeast of the outcrop, the contact between megacrystic fine-grained granite and tuffs of the Yim Tin Tsai Formation is faulted. Altered and silicified granite can be seen in a valley, with a fault zone dipping steeply east (826250 825910 Jma-46). On the catchwater the granite is strongly jointed (826240 825980 Jma-47), and the, adjacent valley marks erosion along the weaker rocks in the fault.
A sheared contact of megacrystic fine-grained granite and granodiorite can be seen on an island in Tai Lam Chung Reservoir (820900 826500 Jma-48). The shearing is associated with quartz veins and greisen, and follows the dominant northeast structural trend of the faults in the area. Similar shears seen near the main dam (820750 826070 Jma-49) contain basalt dykes and thin mylonite bands.
To the east of the main outcrop are three small inliers of equigranular or megacrystic fine-grained granite. About 1 km east of Tin Fu Tsai (825500 828100 Jma-50) are a few exposures of equigranular or sparsely megacrystic fine-grained granite, largely within granodiorite. Further east (827580 828260 Jma-51), between outcrops of granodiorite and tuffs, is a larger outcrop of megacrystic fine-grained granite. This outcrop is contiguous with exposures in the Western Aqueduct tunnel some 400 m below, and is the expression of a small, steep sided intrusion. About 1 km south of Kap Lung is an outcrop of partially metamorphosed, equigranular fine-grained granite.
Ma Wan. Fine- to medium-grained granite is best exposed on the southeast coast of Ma Wan, around Nam Wan (824700 822880 Jma-52), where it is chilled against an inlier of tuffs, with intrusive contacts dipping gently to the northwest. At the northern edge of the inlier (823660 823920 Jma-53) the contact has a complex structure, suggesting foundering of a polygonally-jointed raft of tuff into the roof of a pluton. Further south, bands of fine-grained granite, within the fine- to medium-grained granite, indicate a flat-lying orientation to the complex (824700 823810 Jma-54, 824500 823720 Jma-55). Fine-grained granite is exposed along the shore between Tai Pai Tsui (824150 822640 Jma-56) and Nam Wan (824460 822730 Jma-57), and on the shore east of Pak Nai Shan (824700 822890 Jma-58). The contact between the fine-grained granite and the country rock to the west of Tai Pai Tsui is faulted, whereas to the east it is intruded by a basalt dyke. This margin may also be faulted. A gently dipping intrusive contact is exposed east of Pak Nai Shan but a little further north, fine- to medium-grained granite is in direct contact with the tuff; the contact is sharp and geometrical as if a large polygonal block had foundered in the granite magma. Fifty metres north of this contact, a fine-grained granite dyke intrudes the fine- to medium-grained granite and can be traced offshore onto rocky islets, where it separates the fine- to medium-grained granite from the tuff. In the north of Ma Wan near Cheung Tsui (824530 823660 Jma-59), Tam Shui Wan (823950 823570 Jma-60) and Shek Tsai Wan (823670 823360 Jma-61) fine-grained granite forms a core facies to sparsely feldsparphyric rhyolite dykes.
Tang Lung Chau. On Tang Lung Chau, the fine- to medium-grained granite forms small outcrops in the west and north of the island. Contacts with fine-grained granite are obscured by feldsparphyric rhyolite dykes.