The Sha Tin Granite (Table 6.3) forms an irregular elliptical-shaped biotite monzogranite pluton centred on the Sha Tin district with the long axis oriented to the northeast (Figure 6.4). The outcrop extends from Tsing Yi in the southwest to Wu Kai Sha in the northeast, and from Lion Rock in the southeast to Pak Tin in the northwest. The Sha Tin Granite is intruded in the southeast by the Kowloon Granite and to the northwest by the Needle Hill Granite.
The Sha Tin Granite comprises a coarse-grained central core surrounded by medium- to fine-grained lithologies. The coarse-grained granite is texturally uniform comprising large crystals of quartz, alkali feldspar and plagioclase forming a subhedral granular texture. Interstitial aggregates of brown biotite are a distinctive feature of the granite and are especially conspicuous in the coarse-grained rocks (Plate 6.13). Porphyritic fine-grained lithologies are commonly observed close to the boundaries of the pluton. In thin section, the coarse-grained granite typically contains strongly concentrically-zoned plagioclase, microcline, orthoclase and aggregates of brown biotite. Allanite and zircon are the dominant accessory minerals with subordinate Fe-oxide. Textures are often modified due to later intrusive and hydrothermal activity. Porphyritic fine-grained lithologies are the dominant textural variants along the southwestern edge of the intrusion. Fine- and medium-grained lithologies are commonly mylonitized, or strongly texturally disturbed indicating post-emplacement recrystallization. Biotite is the chief mafic constituent, along with fluorite and secondary muscovite.
A textural variant of the Sha Tin Granite is present on Tsing Yi (the Tsing Yi Granite of Sewell & Fyfe (1995)). The granite is typically equigranular and medium grained but may vary to porphyritic fine-grained lithologies close to the contact with overlying volcanic rocks. Subhedral to anhedral quartz, alkali feldspar, and plagioclase are present in roughly equal proportions, forming a granular texture. Mesoperthite (2–4 mm) typically displays well-developed Carlsbad twins and may be poikilitic. Plagioclase (1–3 mm) is albite in composition, unzoned, and may have sericitized cores. Greenish to brown biotite occurs as single flakes and is commonly altered to chlorite. Accessory minerals include zircon, apatite, titanite, allanite and traces of fluorite and Fe-oxide.
Sewell et al. (1992) obtained a Rb–Sr whole-rock age for the coarse-grained lithologies of 148 ± 9 Ma. More recently, Davis et al. (1997) used three zircon fractions to yield a more precise, and probably more accurate, U–Pb age of 146.2 ± 0.2 Ma for the emplacement age of the granite.
Details
Northwest of the Sha Tin Valley. North of Kowloon Reservoir the coarse-grained granite is homogeneous and equigranular, but deeply weathered and poorly exposed. The southern contact with fine-grained granite strikes E-W just north of the reservoir and is complex, with sheets of fine-grained granite intruding the main mass of coarse-grained granite. The fine-grained granite contacts are seldom exposed, but a contact dipping at 70o roughly to the N is recorded northeast of the reservoir (834540 824520 Jkt-1). Immediately to the east of this locality the contact is displaced to the southwest on the Lai Chi Kok - Tolo Channel Fault.
The northwestern contact trends northeastwards through the Lower Shing Mun Reservoir. From there it can be traced in road sections to Fuk Lok Tsuen (835600 827380 Jkt-2). The contact is complex, consisting of somewhat discontinuous interleaves of fine- and coarse-grained granites. In some of these outcrops the megacrystic fine-grained granite may be modified from coarse-grained granite (834950 826760 Jkt-3). Thin sheets of fine-grained granite dip SW at 25o - 30o (834960 826620 Jkt-4) but these clearly diverge from the main granite contact. Further northeast, at Yau Oi Tsuen (836250 827790 Jkt-5), lenses of coarse-grained granite up to 2 m thick, surrounded by fine-grained granite, are probably xenoliths adjacent to the margin. West of Fo Tan (837200 828150 Jkt-6) there are similar complex outcrops of fine-grained granite with common megacrysts of feldspar and quartz, adjacent to coarse-grained granite and containing interstitial fine-grained quartz and biotite. Similarly at Tai Wai (835900 826290 Jkt-7) the coarse-grained granite shows signs of recrystallisation of quartz and biotite (HK 525); in places the rock grades to a megacrystic fine-grained granite.
The coarse-grained granite is occasionally quartz poor, pink and syenitic in appearance, but only minor occurrences of this lithology are recorded, at Ha Wo Che above Fo Tan, (837520 828140 Jkt-8), on the shores of Lower Shing Mun Reservoir (834270 825880 Jkt-9), and near San Tin Wai (837410 825600 Jkt-10).
Southeast of the Sha Tin Valley. The coarse-grained granite here is generally equigranular, occasionally bearing phenocrysts of pinkish grey feldspar, and characterised by large felted clots of biotite, precisely as seen northwest of the Sha Tin valley. Occasional quartz poor monzonitic zones are seen (HK 806). The southern contact can be traced from above the Lion Rock Tunnel entrance (835940 824120 Jkt-11) to Amah Rock (836560 824500 Jkt-12). Northeastwards from Kak Tin the contact extends south of Tsang Tai Uk (837940 825960 Jkt-13) to Sha Tin Wai (838560 826240 Jkt-14), where it passes beneath the alluvium. Numerous intrusions of fine-grained granite and quartz monzonite form outcrops parallel to the contact, separating the coarse-grained granite on the northwest from medium-grained granite on the southeast. These are well exposed in outcrops at the entrance to Lion Rock Tunnel (835820 824180 Jkt-15) and in the catchwater which contours the hills south of the Lion Rock Tunnel Road. Much of the coarse-grained granite here is iron stained and altered, and the quartz : feldspar ratio may be lower than normal; the feldspars are commonly discoloured brown (HK 612). These features are particularly marked where screens of coarse-grained granite are enclosed by quartz monzonite and fine-grained granite as at 834710 824650 Jkt-16, 836300 824070 Jkt-17 and 836620 824050 Jkt-18 near Amah Rock.
The contact zone is well exposed in road cuts at Sha Tin Wai (838500 826400 Jkt-19). Here vertical sheets of quartz monzonite up to 100 m wide intrude the coarse-grained granite and separate it from the medium-grained granite to the south. The monzonite includes xenoliths of, and is chilled against, coarse-grained granite. The coarse-grained granites in these outcrops show signs of recrystallisation, with interstitial fine-grained quartz and biotite. These textures are better developed further west, around San Tin Wai (HK 621, 837070 825620 Jkt-20). An isolated body of coarse-grained granite crops out on Nui Po Shan (Turret Hill), at 841150 828000 Jkt-21, northeast of the main outcrop.
Northeast of the Sha Tin Valley. Around Wu Kai Sha (842500 831500 Jkt-22) and Wu Kwai Sha Tsui (843000 833000 Jkt-23) the medium-grained granite is homogeneous and equigranular.
Northwestern Kowloon. Coarse-grained granite crops out in the northwest of the district, between Lai King, Lai Chi Kok, the Kowloon Reservoirs and Beacon Hill. Exposures of fresh rock are few but can be seen in recent excavations for housing estates and roads around Lai King (831300 823200 Jkt-24) and Lai Yiu (831500 823600 Jkt-25). There the granite is inequigranular, with average grain sizes of between 6 mm and 8 mm (Plate 6.A14). Biotite, as chloritised blotches, makes up less than 3% of the rock. Around Ha Kwai Chung (831200 823500 Jkt-26) east northeast trending dykes of rhyolite are common, and younger basalt dykes are also present. In places the rock is porphyritic, with randomly scattered euhedral alkali feldspars up to 20 mm. The coarse-grained granite is intruded by fine-grained granite in many places, with a very irregular contact well exposed in road cuttings at Lai Yiu Road (831610 823580 Jkt-27) and Cho Yiu Chuen (831340 823270 Jkt-28). In both cases the contacts are sharp, with small tongues of fine-grained granite intruding several metres into the coarse-grained rock. Around Byewash Reservoir (833370 823270 Jkt-29) the coarse-grained granite is faulted against the fine-grained granite by the Lai Chi Kok Tolo Channel Fault (Figure 6.A2). Here, the coarse-grained granite is characterised by large quartz pools, pink alkali feldspars and very little visible biotite. Nearby at Piper's Hill (833470 822800 Jkt-30), the coarse-grained granite is markedly inequigranular and displays distinctive interstitial fine-grained patches (Plate 6.A15).
Between Piper's Hill (833800 823000 Jkt-31) and Beacon Hill (835600 823400 Jkt-32), fine-grained granite intrusions are common, and these form the topographic high points, for example, the summit of Beacon Hill (835580 823430 Jkt-33). On the southern slopes of Beacon Hill (835340 822670 Jkt-34), just north of Beacon Heights, the contact between the coarse-grained and the younger medium-grained granite is seen. Here, for several metres from the contact, the coarse-grained granite takes on a dark bluish grey appearance, with chlorite abundant. Smilar alteration is seen in the road cuttings at the Butterfly Valley Castle Peak Road interchange (832580 822410 Jkt-35), but there the rock has a distinctive pinkish brown colouration, similar to the coarse-grained granite around Amah Rock in the Sha Tin District which Addison (1986) regarded as a quartz poor monzonitic variety. It is probable that the widespread alteration of these rocks results from hydrothermal action in this heavily faulted area.
The medium-grained granite intrudes the coarse-grained granite close to the northern boundary of the district, with a roughly vertical contact following the southern slopes of Butterfly Hill (833200 822400 Jkt-36), Eagle's Nest (834400 822900 Jkt-37) and Beacon Hill (835200 823000 Jkt-38).
Northwestern and Northern Kowloon. The granite is younger than the other granites of the area, and is seen intruding coarse-grained granite north of Lai Chi Kok. Fine-grained granite forms a vertical dyke like body in the Lai King and Cho Yiu estates (831260 823290 Jkt-39 & 831380 823310 Jkt-40), with sharp margins against the coarse-grained granite. Average grain size is less than 1 mm, although there are scattered megacrysts of quartz. Similar rock forms a roughly horizontal contact with the coarse granite, well exposed in fresh rock cuttings in Wah Yiu Road (831600 823580 Jkt-41). Here, several very fine-grained aplitic dykes cut both the coarse- and the fine-grained granites. The contact between the fine- and coarse-grained varieties is sharp everywhere, with no sign of any significant infiltration or modification of the intruded rock.
Fine-grained granite is widespread in Kam Shan Country Park, and is seen faulted against the coarse-grained granite in the stream bed immediately below the Byewash Reservoir dam (833370 823260 Jkt-42). Fine-grained granite associated with the chilled margin of the medium-grained granite intrusion crops out in Butterfly Valley Road (832830 822280 Jkt-43) where it forms a 10 m wide zone against the sharp, vertical contact. A similar fine-grained margin to the medium-grained intrusion is seen near the Carlton Hotel in Tai Po Road (833860 822750 Jkt-44).
It is probable that the fine-grained granite cropping out between Beacon Hill (836000 823480 Jkt-45) and the Lion Rock (837040 823720 Jkt-46) occurs as a sheet like intrusion, dipping gently towards the north.
Tsing Yi. Medium-grained granite on Tsing Yi is typically pale pink and equigranular with euhedral to subhedral flakes of biotite set in a mosaic of subhedral quartz, plagioclase and alkali feldspar. The best exposures are in cut slopes around Leung Ching Estate in North Tsing Yi, and on the southeast and southwest ridges of the southern peak of Tsing Yi. The medium-grained granite intrudes tuff of the Yim Tin Tsai Formation, and is commonly chilled to a fine-grain size within the contact zone. On the main ridge (828480 822740 Jkt-47) of Tsing Yi, the width of the chilled zone is quite narrow, but it is much broader in the area south of Chung Mei owing to the subhorizontal attitude of the contact.
Medium-grained granite is well exposed in the cut slopes along the main roads to the east and west of the Cheung Ching Estate (829200 823030 Jkt-48) and is intruded by northeast-trending dykes of fine-grained granite (829150 822950 Jkt-49) and coarsely feldsparphyric rhyolite (829150 822950 Jkt-49, 829050 823450 Jkt-50). West of Tai Shan Ha (828520 822430 Jkt-51), medium-grained granite is separated from the tuffs which comprise the country rock by a relatively thin (20-25 m) zone of chilled fine-grained granite. The sub-horizontal attitude of the contact is apparent from the manner in which it may be traced along the ridge and around the spurs. The medium-grained granite is equigranular, except in zones close to the contact and in exposures in the Oil Depot (829240 822430 Jkt-52). Exposures of medium-grained granite occur to the west of Nam Wan, beside the Tsing Yi Road (827540 821740 Jkt-53 to 827880 821230 Jkt-54) and in the quarries (827580 821130 Jkt-55) southwest of Nam Wan. The dominant lithology is equigranular medium-grained granite but sheets of megacrystic fine-grained granite have gradational contacts with the medium-grained granite (827550 821720 Jkt-56), or are chilled against it (827530 821590 Jkt-57 and 827490 821190 Jkt-58). In the quarry (827580 821110 Jkt-59) a screen of medium-grained granite is present between dykes of fine-grained granite that have rhyolitic margins.On the peak (828630 821900 Jkt-60) west of Chun Fa Lok, an outcrop of medium-grained granite includes a number of subhorizontal sheets of fine-grained granite. The sheets are chilled against the overlying medium-grained granite but grade down, through richly megacrystic fine-grained granite, into typical equigranular medium-grained granite (828710 821770 Jkt-61). A number of feldsparphyric dykes intrude the granite northeast of Chun Fa Lok (828530 821950 Jkt-62) and around Nam Wan (828460 821570 Jkt-63) and Nam Wan Kok (828860 821230 Jkt-64). The medium-grained granite on Tsing Yi has the overall form of a dome. Contacts in the north and southeast appear to dip steeply (828860 823320 Jkt-65, 827750 821750 Jkt-66), but on the main ridge contacts are horizontal or gently dipping (828510 822410 Jkt-67, 828640 821860 Jkt-68, 828330 821810 Jkt-69). Feldsparphyric rhyolite dykes are common in the southwest of the island.
Fine-grained granite forms a relatively thin (c. 25 m) roof contact zone to the medium-grained granite on the main ridge of Tsing Yi, from northwest of Tai Shan Ha (828270 823000 Jkt-70) to above Chun Fa Lok (828540 822070 Jkt-71) and westwards from there. The contacts are well exposed on the peak (828470 822730 Jkt-72), and to the south (828500 822430 Jkt-73) where gradations from equigranular medium-grained granite to megacrystic fine-grained can be seen.
Southwest of Nam Wan (827490 821190 Jkt-58, 827560 821120 Jkt-74, and 827840 821340 Jkt-75), megacrystic fine-grained granite forms gently dipping sheets, or dykes, which appear to coalesce when traced eastwards.
In northwestern Tsing Yi, sparsely megacrystic fine-grained granite has been noted in numerous localities. Most outcrops appear to be dykes intruding feldsparphyric rhyolite dykes. In cut-slopes (828010 822720 Jkt-76) east of Sai Tso Wan, fine-grained granite dykes are intruded into crystal tuff.
In the southwest of the island, west of the Power Station (827600 821300 Jkt-77), and in the north around Cheung Kong Estate, the medium-grained granite has been modified texturally to form richly megacrystic, fine-grained granite along the contact with sparsely megacrystic, fine-grained granite. Modified textures were also noted in the Oil Depot (829240 822440 Jkt-78) at Tai Shan Ha.