The Kowloon Granite (Table 6.5) forms a subcircular biotite monzogranite pluton centred on Kowloon and Hong Kong Island (Plate 6.24). The northern boundary of the pluton is along the southern foot of the hills between Kowloon and Sha Tin districts. In the east and south, the pluton boundary is marked by hills comprising the volcanic roof rocks (Figure 6.6) but volcanic rocks are not generally exposed along the western boundary except at Stonecutters Island. The Kowloon Granite is remarkably uniform in texture and composition and is typically an equigranular medium-grained, biotite monzogranite (Plate 6.22). Plagioclase is strongly concentrically zoned and microcline is the dominant alkali feldspar with subordinate microperthite. The dominant mafic mineral is green to brown biotite, occurring as single flakes (1–3 mm). Accessory minerals include allanite, apatite, zircon and Fe-oxide. Porphyritic fine-grained granite in Kings Park in central Kowloon, fine-grained granite along the northern boundary of the pluton and in the vicinity of Ma On Shan, and small outcrops of coarse-grained granite near Kwun Yam Shan are minor textural variants. The granite typically contains quartz and plagioclase megacrysts set in a matrix of quartz, alkali feldspar and plagioclase with abundant biotite. Titanite and allanite crystals are unusually large. The petrographic features of the Kowloon Granite, particularly the strongly zoned plagioclase, are similar to those of the Lantau Granite. However, there is a notable absence in the Kowloon Granite of amphibole and feldspar megacrysts.
A seven point Rb–Sr whole-rock isochron for the Kowloon Granite has returned an age of 138 ± 1 Ma whereas a U–Pb single zircon age for the King's Park textural variant has yielded an age of 140.4 ± 0.2 Ma (Davis et al., 1997).
Details
Northern Kowloon. The medium-grained granite intrudes coarse-grained granite (Sha Tin Granite) close to the northern boundary of the district, with a roughly vertical contact following the southern slopes of Butterfly Hill, Eagle's Nest and Beacon Hill. Eastwards from Beacon Hill this contact is interrupted by a younger intrusion of fine-grained granite in the vicinity of the Lion Rock (837000 823600 Klk-1). At Lai Chi Kok (832840 822290 Klk-2) the contact is sharp and vertical, with a chilled margin of fine-grained granite up to 10 m wide. Exposure is poor in western and northern Kowloon except in roadside and housing estate cuttings. At Pak Tin, Shek Kip Mei (834930 821910 Klk-3) the granite shows its typical uniformity; a non megacrystic rock with an equigranular groundmass comprising subhedral interlocking grains of quartz light pink alkali feldspar and subordinate, slightly smaller, white plagioclase grains. Biotite, which makes up 3 to 5% of the rock, consists of single, fresh, shiny black flakes. The average grain size of the granite is 4 mm. A chemical analysis of a sample (HK 2180) collected from this locality, which can be regarded as the type location for the equigranular medium-grained granite, is given in Table 6.A1. Pegmatite patches, 0.5 to 0.7 m across, were observed in the fresh granite at Nam Shan Estate, Shek Kip Mei (835800 821850 Klk-4). This pegmatite consists entirely of large quartz and pink alkali feldspar crystals.
The average grain size of the medium-grained granite in the Kowloon Tong and Diamond Hill areas is 5 mm, with biotite generally making up only 2 to 3% of the rock. At Diamond Hill Quarry (838860 822770 Klk-5) the granite is light grey and uniform, but is cut by small aplite dykes, and by a 7 m wide mafic dyke (839060 822750 Klk-6). Medium-grained equigranular granite may be traced up the hill slopes north and east of Diamond Hill to its contact with the volcanic country rock, where no appreciable chilled margin was observed. This granite, and the volcanic rocks in the vicinity of Sha Tin Pass and Grasscutter's Pass (839310 823980 Klk-7), are cut by prominent feldsparphyric rhyolite dykes trending approximately northwest. The Lion Rock (837100 823700 Klk-8) is composed of medium-grained granite, with fine-grained granite forming both a thin capping and dykes intruding the prominent overhanging `Lion's Head' (837070 823610 Klk-9). The presence of these finer-grained intrusions may explain the resistance of this feature to erosion.
Near Hung Hom Power Station (837620 819720 Klk-10) layers of pegmatite are abundant within the medium-grained equigranular granite. These are 2 to 4 m wide but only 50 to 100 mm thick, occurring on joint surfaces dipping 25o to the east northeast. Some 800 m to the north of this site a prominent medium-grained granite (Plate 6.A25) formed To Kwa Wan Island (837850 819500 Klk-11), now joined to the Kowloon Peninsula by reclamation. Most of the Tsim Sha Tsui peninsula is underlain by uniform medium-grained granite except for a younger intrusion of fine- and fine- to medium-grained granite around King's Park (836000 818800 Klk-12) and Gun Club Hill (836200 818300 Klk-13).
The only occurrence of medium-grained granite east of Kwun Tong is found between Yau Tong (842800 817500 Klk-14) and Lei Yue Mun Point (843100 816300 Klk-15). Here, the granite has been extensively quarried, and the contact with the volcanic rocks of the Mount Davis Formation is seen in the quarry face as an irregular surface with an average dip of 25o to the east. The chilled margin of the granite is only 0.3 to 1.5 m thick. Pegmatite patches are common, usually as lenses parallel to the contact or at the contact. The granite of this area is uniform, equigranular and has an average grain size just over 2 mm. It appears identical to the medium-grained granite cropping out at Shau Kei Wan (841900 815900 Klk-16) southwest of the Lei Yue Mun channel.
Southern Kowloon and Tsim Sha Tsui. Distinctive, megacrystic fine-grained granite crops out between Observatory Hill (836000 818100 Klk-17), Gun Club Hill (836300 818200 Klk-18) and King's Park (836100 819100 Klk-19), the best exposures being those behind Queen Elizabeth Hospital (835880 818920 Klk-20) and at the Nathan Road Gascoigne Road Junction (835700 818900 Klk-21). The granite is grey and equigranular, with a fine groundmass of less than 1 mm average grain size and distinctive feldspar megacrysts averaging 8 to 10 mm in length. Biotite occurs as single flakes and in places as clusters of single crystals. This rock grades into a fine- to medium-grained granite with similar characters, but with a groundmass averaging 1.5 to 2 mm. The boundary between these megacrystic rocks and the equigranular medium-grained granite was not seen, but it is probable that the fine-grained megacrystic granite represents a later intrusion. Small bodies of fine-grained granite are intruded into medium-grained granite in Hung Hom (837100 819030 Klk-22 & 837600 818730 Klk-23). They are light pink and sparsely megacrystic, dissimilar to the fine-grained granite of King's Park described above, and their contacts are sharp and steeply dipping.
Tate's Cairn. Numerous bodies of coarse-grained granite, fine-grained granite and quartz monzonite crop out on Man Tau Tun (839300 824610 Klk-24), at Kong Pui Tsuen (839660 824920 Klk-25), Ngau Au Shan (840600 825530 Klk-26) and in Shek Nga Pui (841400 825880 Klk-27). The coarse-grained granite sheets vary from 20 to 180 m wide and are identical in appearance to the altered and discoloured granite bodies, for example, near Amah Rock. The trend of the intrusions is parallel to known northeast-trending faults lying to the northeast, but none of these faults can be traced through these outcrops. In view of this, the intrusive separation of the sheets is thought more likely.
Southeast of the Sha Tin Valley. The main outcrops of fine-grained granite in this area separate medium-grained granite from sedimentary and volcanic rocks. Around Tate's Cairn and Mau Tso Ngam (840660 825000 Klk-28) the outcrops are complex, with numerous sheets of fine-grained granite, screens of tuff and coarse-grained granite, and sheets of quartz monzonite. The fine-grained granites are poorly exposed but appear to be mainly equigranular with scattered megacrysts. The granite which crops out on Tate's Cairn (840500 824290 Klk-29) is very pale grey and strongly recrystallised. The granite here is very close to the roof of the intrusion and it is likely that this recrystallisation is an autometasomatic effect.
Around Ma On Shan and Wu Kai Sha the intrusive relationships are simpler, with fine-grained granite forming two undulating sheets, generally parallel to the contact with the overlying sedimentary rocks. The granite in these outcrops is variably megacrystic.
Stonecutters Island. On Stonecutters Island the rock is uniform, equigranular and medium-grained, with an average grain size of 3 mm. Single, fresh, black biotite crystals are common, and the rock is light grey or light pink. It has been quarried for building stone on the western side of the island (831740 820220 Klk-30). Near the western tip of the island (831590 819980 Klk-31), where volcanic rocks of the Mount Davis Formation have been mapped, the granite becomes finer-grained and probably represents a chilled margin. To the northeast of Stonecutters Island, towards Sham Shui Po, granite formed a chain of small islands that are depicted on the early maps of the Territory. These islands were later removed by blasting to provide a safe passage for shipping. Similar rocky islets were present in Hung Hom Bay prior to its reclamation.
Northern Hong Kong Island. Medium-grained granite crops out along the northern shores of Hong Kong Island between Kennedy Town and Shau Kei Wan. The outcrop is bounded by the overlying volcanic rocks in the western half of the island, and by younger intrusions of fine- and fine- to medium-grained granite (Mount Butler Granite) to the east of Happy Valley. Small areas of medium-grained granite have been mapped in the vicinity of Mount Nicholson and Shouson Hill. In Belcher Street, Kennedy Town (831700 816380 Klk-32), medium-grained granite is found to within a few centimetres of the volcanic country rock contact, with only a very narrow chilled zone. This contact may be traced through the disused, wartime tunnels under Belcher's Gardens (831730 816220 Klk-33), where the contact dips about 25o west and strikes 010o. Thin tongues of fine-grained granite intrude several metres into the contact metamorphosed tuffs and associated sandstones. Between Hong Kong University and Central District the granite is light grey to light pink, uniform, equigranular and medium-grained, with an average grain size of 3 to 4 mm. Tunnelling operations under Statue Square (834560 815830 Klk-34) revealed two chloritic zones within this granite, each about 15 m wide and associated with shear zones, with the biotite completely altered to chlorite. Railway tunnel excavations under Sheung Wan (833830 816440 Klk-35) exposed a northeast striking zone of banded greisen, 7 to 10 m wide, cutting the medium-grained granite, and probably formed by late stage hydrothermal fluids and gases infiltrating a fracture zone (Plate 6.A26).
There are few exposures of fresh medium-grained granite between Central and Causeway Bay except in temporary building site excavations. A 30 m deep section into the hillside at the Ruttonjee Sanitorium (836110 815220 Klk-36) revealed completely weathered granite with fresh equigranular medium-grained granite occurring as isolated corestones at the base. The granite there is pale pink, uniformly equigranular and non megacrystic with an average grain size of 4 mm (Plates 6.A27 & 6.A28). The biotite occurs as shiny, black, single flakes. White plagioclase grains are subordinate to the larger light pink alkali feldspars. A geochemical analysis for a sample (HK 4237) collected from this locality is given in Table 6.A1. Some 200 m to the east, at Morrison Hill (836350 815180 Klk-37), the medium-grained granite is inequigranular in appearance, perhaps a consequence of textural modification by infiltration of fine-grained granite. Similar modification was noted at Kellett Island (836770 816180 Klk-38), where pegmatite lenses are also present. Uniform, equigranular medium-grained granite is exposed on the western side of Leighton Hill (836880 815270 Klk-39), and southeastwards along Happy Valley to Tai Hang Road. 150 m south of the Tai Hang Drive Tai Hang Road junction (837870 814780 Klk-40) molybdenite crystals up to 8 mm long were noted, scattered throughout recently excavated fresh granite. The boundary between the medium- and fine-grained granites is sharp in the Jardine's Lookout area, and was seen as a vertical contact in temporary exposures at the new Mount Butler Government Quarters (837980 814850 Klk-41). However, the contact between the medium- and the fine- to medium-grained granites in the vicinity of Bowen Road and Stubbs Road (836400 814300 Klk-42) is poorly defined and the mapped boundary there is approximate.
From Victoria Park eastwards to Quarry Bay, medium-grained granite occurs close to the original coastline, with later, finer-grained granites occupying the higher ground southwards to Braemar Hill. Along King's Road (837910 816120 Klk-43), fresh, equigranular medium-grained granite is exposed, and the contact with the fine-grained granite is well seen close to the Tsing Fung Street flyover (837900 816260 Klk-44). The best exposures of medium-grained granite are those in the rock cuttings at Healthy Village Estate, North Point (839160 816770 Klk-45), where the rock displays its characteristic uniformity. Major excavations at Kornhill Taikoo Shing (840300 815700 Klk-46) have revealed a sharp vertical contact between the fine- and medium-grained granites. There appears to have been some infiltration of the medium-grained granite by fluids emanating from the intruding fine-grained granite, and within 30 m of the contact (840210 815840 Klk-47) there is some textural modification shown by a marked inequigranularity of the medium-grained rock. Eastwards towards Aldrich Bay the granite grain size diminishes to an average of 2 to 3 mm, as seen in old quarries behind Shau Kei Wan Road (841050 815590 Klk-48). This granite is separated from the fine-grained granite at A Kung Ngam Road (841870 815240 Klk-49) by a 0.2 m wide pegmatite zone. At A Kung Ngam (841880 815930 Klk-50) some diffuse, fine-grained patches were observed in the otherwise uniform, equigranular medium-grained granite.