The Needle Hill Granite (Table 6.3) forms an elliptical biotite monzogranite pluton on the northwestern side of the Shing Mun Valley with the long axis oriented in a northeast direction (Figure 6.4). The granite is largely as described and named by Allen and Stephens (1971), although its distribution is largely restricted to the Shing Mun Valley. The Needle Hill Granite is composed of porphyritic fine-grained monzogranite and equigranular medium-grained monzogranite (Plate 6.16) and includes numerous fine-grained granite dykes which intrude the Sha Tin Granite in the south and west. The granite intrudes the Tai Po Granodiorite in the north.

The fine-grained granites contain abundant subhedral mesoperthite, subhedral to euhedral plagioclase and anhedral quartz in roughly equal proportions. Reddish brown biotite and muscovite are commonly interstitial. Plagioclase is typically unzoned and zircon and fluorite are the main accessory minerals. The fresh appearance of the plagioclase is a distinctive feature of the granite.

In thin section, porphyritic lithologies contain phenocrysts of euhedral to subhedral microperthite, subhedral to euhedral plagioclase, and bipyramidal quartz, set in a granular groundmass of plagioclase, quartz and mesoperthite. Biotite is the dominant mafic mineral and accessory minerals include zircon and fluorite. Equigranular lithologies are characterised by abundant mesoperthite, plagioclase, and quartz, along with minor biotite and accessory minerals. The Needle Hill Granite may be confused with the Tai Lam Granite but the latter is frequently non-porphyritic and chemically different (see above).

A U–Pb single zircon age of 146.4 ± 0.2 Ma (Davis et al., 1997) has been obtained from the Needle Hill Granite. This indicates that it is approximately coeval with the Sha Tin Granite although field relationships suggest that the Needle Hill Granite is younger than the Sha Tin Granite.

Details

Northwest of the Sha Tin Valley. The medium-grained granite here forms one northwesterly dipping sheet which attains its maximum thickness of about 300 m south of Needle Hill (834800 827400 Jkn-1). Northeast of Needle Hill the sheet splits into two or three leaves, intercalated with fine-grained granite. Contacts with the fine-grained granite to the north are sharp. They show no evidence of chilling of either facies, and dip NW at angles of around 25^o (834650 827400 Jkn-2, 835900 828430 Jkn-3). At the latter locality coarse- and fine-grained granites are interbanded along the contact. Further northeast, around Ho Tung Lau (838800 829400 Jkn-4) and the Chinese University (839600 831000 Jkn-5), the medium-grained granite is cut by a number of minor sheets of fine-grained granite. Sections recorded in water tunnels northwest of Ho Tung Lau indicate that the contact of this body dips gently to the northwest. Around the Chinese University contacts are steep.

Southwest of Needle Hill, around the Lower Shing Mun Reservoir, contacts between the medium-grained granite and the enclosing fine-grained granite are steep, while on Kam Shan (833300 825100 Jkn-6) and on Ma Tsz Keng (833200 826000 Jkn-7) medium-grained granite forms a pair of lenticular bodies which are sub horizontal or dip gently southeastwards. These changes in attitude along strike are interpreted as original intrusive, as opposed to tectonic, features.

Needle Hill. The main outcrop of fine-grained granite lies between Kwai Chung, Needle Hill and Ho Tung Lau. Around Needle Hill and further north the granite forms northwesterly dipping sheets at the margins of, and within, the medium-grained granite. Dips of the main contacts with the Repulse Bay Volcanic Group are gentle, as can be judged from the effect of topography on the outcrop pattern around Fo Tan, and as is confirmed by the position of the contacts in water supply tunnels beneath Fo Tan. Contacts of fine- and medium-grained varieties also dip gently to the northwest, as seen at 834690 827400 Jkn-8 on Needle Hill. The granite in these outcrops is almost exclusively megacrystic. The Needle Hill outcrops are continuous with those at Kwai Chung but the lithology further south is more variable with both equigranular and megacrystic varieties occurring.

Around Sam Tsuen (831000 825900 Jkn-9) and Kwai Shing (830900 824900 Jkn-10) the fine-grained granite intrudes granodiorite and, in the contact zone, forms vertical dykes up to 20 m wide parallel to the main contact. At Sheung Kwai Chung (832600 826700 Jkn-11) the contact is simple. The contacts with the medium-grained granite east of Kwai Chung on Kam Shan (833200 825100 Jkn-12) and Ma Tsz Keng (833200 825900 Jkn-13) are irregular but generally sub horizontal.

Northwestern and Northern Kowloon. At the main entrance to Princess Margaret Hospital (831580 822720 Jkn-14), parallel zones of biotite concentration, striking 060^o, are prominent in the fine-grained granite. Associated pegmatite patches 0.3 to 0.5 m across are also present, and 200 m to the south (831610 822510 Jkn-15) a 50 m wide N-S striking zone of pegmatite concentration has been mapped. The surrounding fine-grained granite contains 5 mm quartz megacrysts.