The Po Toi Granite (Table 6.5) forms a subcircular pluton centred on the southeastern tip of Hong Kong Island Figure 6.6). The pluton encompasses large outcrops of medium-grained granite exposed at Stanley Peninsula and D'Aguilar Peninsula, and granites exposed to the southeast on Sung Kong Island, Beaufort Island and Po Toi. The granite is also exposed on a reef known as Castle Rock between Stanley and the Po Toi Islands. The Po Toi Granite is texturally zoned with coarse-grained megacrystic lithologies predominating in the south and east, and equigranular medium- and fine-grained lithologies in the north and west (Plate 6.23). Pegmatitic veins are common on the Po Toi Islands. The Po Toi Granite intrudes the Tai Po Granodiorite and D'Aguilar Quartz Monzonite on the Stanley and D'Aguilar peninsulas.

Porphyritic lithologies are typically characterised by large (4–10 mm) euhedral to subhedral alkali feldspar megacrysts composed of mesoperthite. Subordinate subhedral to anhedral quartz megacrysts, sometimes bipyramidal, are set in a medium- to fine-grained subhedral-granular matrix of quartz, orthoclase, plagioclase and sparse microcline. Orthoclase megacrysts are commonly sericitized. Subhedral zoned plagioclase (oligoclase to albite) also has sericitized cores and quartz is generally strained. Reddish-brown biotite is commonly altered to chlorite and may occur as crystal aggregates. Accessory minerals include zircon, titanite, allanite and Fe-oxide. Fluorite is sometimes present as a secondary mineral along with muscovite.

Equigranular granites have similar mineral compositions to the porphyritic lithologies except for a generally higher proportion of microcline. Titanite is absent in the fine-grained lithologies and biotite only occurs as sparse reddish-brown flakes. Fluorite and muscovite are commonly present as secondary minerals, and muscovite may also occur.

The Po Toi Granite has not yet been isotopically dated but field relationships indicate that it was emplaced after the D'Aguilar Quartz Monzonite

Details

Stanley, Shek O and Po Toi Island Group. On the Chung Hom Kok and Stanley peninsulas the granite is uniformly equigranular, non megacrystic medium grained, with an average grain size of 3 mm. The rock is usually deeply weathered and exposures of fresh rock are rare except along the coast and in roadside cuttings. On the coast the granite forms cliffs of pink to light brown, slightly to moderately weathered rock. Inland, tors form hilltop features at Chung Horn Kok (838700 808400 Klp-1), and on the Stanley Peninsula (840300 807600 Klp-2).

Aplite dykes, averaging 150 mm in width, are common along the cliffs east of Stanley Prison (840760 808060 Klp-3 & 840620 807890 Klp-4). Wider dykes of aplite were noted on the western side of the Stanley Peninsula (840000 808050 Klp-5) where a 0.8 m wide vertical dyke strikes north south. On the southern side of Stanley Bay the granite contains scattered granodiorite xenoliths (839920 807900 Klp-6); thin stringers of granite intrude the granodiorite, but the contact between the two rock types was not exposed. At Stanley Fort an area of fine- to medium-grained granite cuts the granodiorite and monzonite, and this probably represents a chilled margin of the medium-grained granite pluton. On the D'Aguilar Peninsula, medium-grained granite crops out from the west coast at Shek O Quarry northeastwards to Shek O Village (843700 810200 Klp-7). On both coasts it is bounded gradationally by fine- to medium-grained granite. It is probable the fine- to medium- and the medium-grained granites were derived from the same magma; except for grain size there is no difference in composition or appearance.

Medium-grained granite is found in several places on Po Toi Island, where it intrudes coarse-grained granite, and is in turn intruded by fine-grained granite (844620 802440 Klp-8). Dykes of quartzphyric rhyolite cut the granite near Tai Wan Village (844200 803000 Klp-9). The medium-grained granite is essentially equigranular, non megacrystic and similar in appearance to the granite cropping out at Stanley and Shek O. The medium-grained granite forming the western side of Beaufort Island (Lo Chau) is roughly equigranular, with an average grain size of 2 to 3 mm. Biotite, as shiny black flakes, makes up about 1 % of the rock. Quartz crystals form aggregates up to 8 mm across giving the impression of a coarse-grained rock. There are scattered euhedral alkali feldspar megacrysts up to 7 mm in length. A mineral vein 0.5 m wide and rich in haematite is present in this granite (843310 805010 Klp-10). The contact with the fine-grained granite is sharp (843270 804900 Klp-11).

In the vicinity of Shek O, fine- to medium-grained granite is well exposed in coastal sections between the golf club (844100 810900 Klp-12) and the village. The granite is fairly uniform and equigranular, with an average grain size in the order of 1.5 to 2 mm. In places the rock becomes slightly coarser, verging on medium grained. Westwards from Shek O Village (843800 810130 Klp-13) to the Shek O Quarry (842700 809300 Klp-14) the granite is coarser and has been delineated as medium grained; it is, however, regarded as belonging to the same intrusive phase. Thin aplite dykes are common in the fine- to medium-grained granite, as are quartz veins. Greisenisation associated with several parallel quartz veins is visible in the cliff section 200 m south of the golf club (844100 810540 Klp-15), where greisen bands are present for 0.1 to 0.15 m each side of the thin quartz veins. On Tai Tau Chau (844800 810200 Klp-16), fine-grained granite has intruded fine- to medium-grained granite. Nearby, on Shek O Headland (844430 809960 Klp-17), similar infiltration of the fine- to medium-grained granite has taken place.

There are extensive outcrops of fine- to medium-grained granite on Beaufort Island (Lo Chau), where the rock shows similarities with the granites of Shek O and Stanley. The rock is fairly uniform and roughly equigranular, with an average grain size between 1.5 and 2 mm. The groundmass is light pink to white, with scattered single black biotite flakes throughout. In places, however, the biotite flakes form clusters averaging 4 to 5 mm across. Euhedral alkali feldspar megacrysts reaching 5 mm are randomly distributed. Similar granite occurs at Castle Rock (Lo Chau Pak Pai, 842500 804800 Klp-18) where quartzphyric rhyolite and aplite dykes intrude the rock. Thin quartz veins with associated 0.8 m wide zones of greisenisation affect the granite on the southeastern coast of Beaufort Island (844110 804640 Klp-19), similar to the greisen and quartz veins noted at Shek O. Fine-grained granite intrudes the fine- to medium-grained granite on the southern coast of Beaufort Island (843900 804580 Klp-20), with pegmatite concentrations as a series of parallel subhorizontal layers near the contact (Plate 6.A29). In northwestern Po Toi Island (Figure 6.A3), fine- to medium-grained granite is well exposed, with extensive unvegetated rock surfaces. The granite is light pink and equigranular, with an average grain size of just under 2 mm. There are scattered tabular alkali feldspar megacrysts 5 mm by 3 mm, and pools of quartz crystals to 7 mm across.

Fine-grained granite marginal to the medium-grained granitic intrusions at Stanley and Shek 0 has been delineated on the map. On D'Aguilar Peninsula, fine-grained granite bordering an older quartz monzonite intrusion consists of a fine equigranular groundmass with an average grain size of the about 0.5 mm. The rock is pink, and no biotite is visible. Rounded quartz megacrysts up to 4 mm across are scattered throughout. There are also sparse alkali feldspar megacrysts up to 3 mm. Quartz veining is common (843550 809170 Klp-21) and close to the contact with the monzonite a wide quartzphyric rhyolite band parallels the contact (843730 809000 Klp-22). This is presumed to be a dyke, but it could possibly represent an extremely fine-grained chilled margin to the granite