The Chi Ma Wan Granite (Table 6.4) forms a subcircular, biotite monzogranite pluton centred on the Chi Ma Wan Peninsula on the eastern side of Lantau Island. The granite extends south and east to the islands of Shek Kwu Chau and Cheung Chau respectively (Figure 6.5). The Chi Ma Wan Granite intrudes the Lantau Granite and East Lantau Rhyodacite dykes at Chi Ma Wan. However, on Cheung Chau, the granitoid is cut by rhyolite dykes suggesting a coeval phase of dyke emplacement.

The Chi Ma Wan Granite is dominantly equigranular and medium grained (Plate 6.17). In thin section, mesoperthite is the dominant alkali feldspar although sparse microcline is also present. Subhedral plagioclase is strongly concentrically zoned. Brown biotite is the dominant mafic mineral. Accessory minerals include zircon, titanite, allanite, apatite and Fe-oxide.

U–Pb single zircon geochronology has established an age of 143.7 ± 0.3 Ma which is considered to be an upper limit on the age of crystallization of the rock (Davis et al., 1997).

Details

Chi Ma Wan Peninsula. North of Mong Tung Wan, the granite is pinkish grey, equigranular and medium-grained. Black biotite occurs as single flakes, or in small aggregates of flakes; the individual crystals ranging up to 4 mm. Most of the crystals are 2 to 8 mm, although there are some pink feldspar megacrysts up to 20 mm. Dark xenoliths of quartz microdiorite are common in this area. They are typically rounded, with a diameter of 0.1 to 0.2 m, but can be up to 0.3 m. Aplite dykes, usually around 0.2 m wide, are also common.

On the coast northeast of Lo Yan Shan a rapid textural change can be seen, with equigranular medium-grained granite to the south and a finer variety to the north. The mean crystal size is 2 to 3 mm, with biotite flakes less than 2 mm. The rock is mapped as fine-, to medium-grained granite, although locally it may be medium-grained.

Cheung Chau. Equigranular medium-grained granite is well exposed on the coast of Cheung Shan, often displaying well-developed honeycomb structure on weathering (Plate 6.A18). The granite is light pink, weathers to a yellowish brown, and is very uniform in texture and appearance over the whole island. Numerous east-, to eastnortheast-trending feldsparphyric rhyolite dykes cut the granite. They often have well-developed chilled margins, for example, on the coast at Italian Beach (Pak Tso Wan) in southwest Cheung Chau (819990 806540 Kcc-1), and on the coast south of Fa Peng in southeast Cheung Chau (822080 807200 Kcc-2). In addition, the granite is cut by many small aplite (Plate 6.A19) and basalt dykes, and by thin quartz veins. South of the cemetery at Kau Kung Tong (820430 806640 Kcc-3), in southwestern Cheung Chau, there is an eastnortheast-trending zone of quartz veins alternating with aplite, up to 20 m wide. Localized kaolinization, probably the result of weathering, can be seen in an isolated exposure northeast of Cheung Kwai Estate (820820 808460 Kcc-4).

Shek Kwu Chau. Equigranular medium-grained granite with a distinctive pinkish colour in fresh exposures can be seen over most of the island. Only rarely does the rock vary texturally. Mafic xenoliths, up to 0.15m across, occur on the northwest coast (816340 806660 Kcc-5), along with biotite concentrations and small pegmatite patches (816460 806800Kcc-6).

Cheung Chau (North). On the northwest coast (820720 809080 Kcc-7), there is a small patch of fine-grained granite and pegmatite less than 1m by 3m. The fine-grained granite grades into the adjacent medium-grained granite. Pegmatite veins in the medium-grained granite are exposed out about 120 m to the south, as are thin veins of finer granite and pegmatite in a cut slope (820720 808590 Kcc-8) south of Tai Kwai Wan.

Shek Kwu Chau (Southeast). Exposures of megacrystic fine-grained granite are present in the southeast of the island, adjacent to outcrops of medium-grained granite, although the contact between the two is not exposed. Feldspar megacrysts in the fine-grained granite are up to 25 mm long, whereas quartz megacrysts are up to 6 mm across. The megacrysts are set in an equigranular groundmass. Aplite and pegmatite veins cut the granite, and there are localized biotite concentrations. Exposures on the coast, about 120 m east of the jetty (817430 806270 Kcc-9), comprise pinkish-grey, megacrystic, fine-grained granite which are cut by aplite dykes up to 0.3 m across. Both the aplite and the granite are cut by a 1 m-wide feldsparphyric rhyolite dyke.