The Lantau Granite (Table 6.2) encompasses all strongly megacrystic, hornblende-biotite monzogranite outcropping in southern and western parts of Hong Kong (Figure 6.3). The type locality is at Sz Pak Wan on Lantau Island where particularly large megacrysts are seen (Plate 6.6).
The Lantau Granite is intruded by all known plutonic lithologies except the Tai Po Granodiorite and the Deep Bay Granite. Despite the fact that the Lantau Granite crops out entirely within the outcrop area of the Tai Po Granodiorite, contact relationships between the two rock types have not been observed. However, large rafts of strongly modified Lantau Granite are sometimes found as xenoliths within younger plutons (Plate 6.7). The Lantau Granite crops out from Lamma Island in the east to Fan Lau on Lantau Island in the west, and from the Soko Islands in the south to Tuen Mun in the north (Figure 6.3). Isolated outcrops are found on many small islands including Peng Chau, Kau Yi Chau, Hei Ling Chau, Chau Kung To, Sha Chau and on the former island of Chek Lap Kok.
The Lantau Granite is dominantly medium grained, although it may vary to coarse- and fine-grained lithologies (Table 6.2). Subhedral orthoclase megacrysts (5–15 mm) are typically poikilitic and have irregular grain boundaries whereas smaller (2–5 mm) oligoclase phenocrysts are sieve-textured and strongly concentrically zoned. Cross-hatched twinning is sometimes observed in the alkali feldspar. The feldspar phenocrysts are set in a granular matrix of subhedral alkali feldspar, albite, and quartz. Brown biotite is the chief mafic mineral. Accessory minerals include zircon, zoned allanite, titanite and Fe-oxide.
Two single zircon fractions from the Lantau Granite have yielded concordant and identical U–Pb ages of 161.5 ± 0.2 Ma (Davis et al., 1997), which is likely to date the emplacement of the granite.
The medium-grained granite occupies much of the central part of Lamma Island between Lo So Shing (831000 807400 Jml-2) and Hung Shing Ye (830500 811000 Jml-3). It also occurs on the northern tip of the island at Pak Kok (830200 811300 Jml-4). The granite is extremely variable in grain size, composition and general character. Grain size varies abruptly from fine to coarse grained, and diffuse bounded patches of both fine- and coarse-grained granite may occur within a dominantly medium-grained groundmass, as seen in coastal exposures near the jetty at Pak Kok (830100 811230 Jml-5). Alkali feldspar megacrysts are common, up to 45 mm long, and stand out clearly, particularly on weathered surfaces. Megacrysts are prominent at Pak Kok (830300 811240 Jml-6, Plate 6.A5). In the same area (830280 811200 Jml-7), a chilled margin only 50 mm thick was seen at the irregular contact with the volcanic country rock. Nearby, fine-grained granite appears to have infiltrated the medium-grained rock, producing a patchwork pattern with biotite prominent in the darker, coarser grained patches. South of Hung Shing Ye beach (830430 808730 Jml-8), biotite rich, megacrystic medium-grained granite is exposed in the cliffs. Here, a feldsparphyric rhyolite dyke is seen cutting the granite, and, 200 m further south, a large body of highly altered tuff 25 m across and surrounded by the medium-grained granite is probably a large xenolith (830470 808550 Jml-9). There are few exposures inland, most of the granite outcrop being deeply weathered.
Although there are patches of coarse-grained granite within the main mass of inequigranular medium-grained granite that crops out over much of central and northern Lamma Island, it has not been feasible to delineate them on the 1:20 000 scale map. The main outcrops of coarse-grained granite are found south and east of Mo Tat Wan, and at Yuen Kok in the southeast. Around Mo Tat Wan the coarse-grained granite has an average groundmass grain size of 6-7 mm, a distinctive grey colour, and is biotite rich. The rock is similar in appearance to the mesocratic granodiorite, but contains much less plagioclase. Abundant euhedral alkali feldspar megacrysts averaging 15 mm in length are present, and these are highlighted by the dark biotite rich groundmass. Biotite is present as minute laths which tend to cluster to form distinctive black blotches up to 10 mm in diameter. Fine-grained basaltic xenoliths up to 1 m across are enclosed by the granite (833140 806940 Jml-10). Inland, it weathers deeply to form a dark reddish brown soil containing large rounded corestones. It has been intruded by fine- to medium-grained granite, the irregular contact being well exposed in the cliffs 500 m east of Yung Shue Ha (833170 806840 Jml-11, Plate 6.A13). Basalt dykes intrude the granite nearby (833150 806930 Jml-12). At Yuen Kok (833180 805300 Jml-13) the coarse-grained granite has an average grain size of between 7 and 10 mm, with scattered prominent euhedral or subhedral alkali feldspar megacrysts. Seen in thin section (HK 5100) the megacrysts are generally microperthitic. The rock is intruded by fine- and fine- to medium-grained granites, and intrusive contacts are sharp.
Tuen Mun. Equigranular coarse-grained granite was seen in exposures and boreholes east of Chi Lok Fa Yuen (816100 827600 Jml-14), in part with a small microcrystic content. The granite has alkali feldspar crystals up to 50 mm, pools of interstitial quartz around 5 mm and irregular clots of black biotite up to 10 mm. Zoned sericitized plagioclase crystals are up to 10 mm across and have a characteristic greenish colour; the alkali feldspar is usually white, but can be pinkish. Thin basalt dykes and quartz veins also occur in this area.
On the pre reclamation coastline at Sam Shing Hui (815880 826980 Jml-15) there are exposures of coarse-grained granite with alkali feldspar up to 10 mm, pools of interstitial quartz up to 8 mm, and sericitized plagioclase up to 6 mm. Although most of the irregular clots of biotite are small, some are up to 6 mm across. Although essentially similar to the equigranular coarse-grained granite further north, there is a significant microcrystic content.
Microcrystic coarse-grained granite with dykes or veins of fine-grained granite was intersected by a number of boreholes north and east of Sam Shing Hui (816000 827200 Jml-16). The granite contains alkali feldspar up to 25 mm quartz up to 7 mm and white plagioclase up to 8 mm; black biotite clots are less than 7 mm across. The groundmass is clearly microcrystic, with a grain size visible in hand specimen much less than 0.5 mm. In exposures on the cutting for the Tuen Mun Road (816020 827110 Jml-17), coarse-grained granite with irregular bands of dark, biotite-rich fine-grained granite is cut by aplite dykes up to 150 mm wide and basalt dykes up to 2.5 m (816340 826800 Jml-18). Along the whole of this cutting the coarse-grained granite is characterized by concentrations of biotite, often in bands about 50 mm thick (816250 826890 Jml-19).
Between Castle peak Beach and Ka Fei Wan (Cafeteria Beach) (816100 826400 Jml-20) the coarse-grained granite is cut by numerous, irregular fine-grained granite dykes. On the southeast end of Castle Peak Beach (816060 826560 Jml-21) these dykes are generally flat lying (Figure 6.A1, Plate 6.A6). Southeast of Kadoorie Beach a quartzphyric rhyolite dyke (816220 826320 Jml-22) and a thin belt of mylonite (816260 826330 Jml-23) cut the coarse-grained granite.
East of Wong Ka Wai (816200 828400 Jml-24) the coarse-grained granite grades northwards into medium-grained granite, and no the catchwater (816270 828190 Jml-25) it is cut by thick fine-grained granite dykes. The rock is microcrystic in parts, and the contacts with the fine-grained dykes are sharp in some exposures and gradational in others.
East of Lung Chue To (Pearl Island). On the peninsula (817000 825100 Jml-26) there is an isolated outcrop of equigranular coarse-grained granite bounded to the east by a texturally similar medium-grained granite. In this rock the alkali feldspar crystals are up to 25 mm, quartz pools up to 10 mm and biotite clots up to 7 mm.
Tuen Mun to Siu Lam San Tsuen. There are four outcrops of medium-grained granite in this area, of which three are intimately related to the coarse-grained granite, for example in the development of secondary textures and in the form of the biotite. The most northerly outcrop is an extension to the coarse-grained granite outcrop east of Wong Ka Wai. The rock contains megacrysts of alkali feldspar up to 20 mm across, and biotite clots up to 10 mm, but it has a dominant grain size of 1 to 6 mm; the biotite also occurs as single crystals up to 2 mm.
The outcrop southeast of Tuen Mun, around Perowne Barracks, is very variable, with microcrystic coarse-and medium-grained granite as well as equigranular medium-grained granite. To the west the microcrystic content may be significant, but the dominant megacrysts become generally coarser and the rock grades into coarse-grained granite. The outcrop contains quartzphyric rhyolite and basalt dykes.
To the east of Lung Chue To (Pearl Island) is a small outcrop of equigranular medium-grained granite bounded to the west by equigranular coarse-grained granite and to the east by fine-to medium-grained granite. On the southern side of the peninsula (817760 824950 Jml-27) the rock is cut by irregular fine-grained granite dykes and pegmatite. The granite contains alkali feldspar less than 10 mm across, plagioclase between 2 and 6 mm, quartz between 2 and 8 mm, and biotite crystals between 2 and 4 mm.
Around the Desalting Plant, near Siu Lam San Tsuen, the granite is mostly medium-grained and equigranular. The granite (818220 825150 Jml-28) has quartz which is slightly microcrystic in parts. The form of the biotite clots is reminiscent of that found in the coarse-grained granite and surrounding medium-grained granite at Tuen Mun. On Tuen Mun Road, adjacent to fine grained granite, the contact can be seen to be gradational (818410 825360 Jml-29), with a microcrystic coarse grained granite against sparsely megacrystic fine grained granite.
Tai Lam Chung to Ting Kau. In this area there are four outcrops of medium-grained granite south of Tai Lam Chung, at Tsing Lung Tau, at Sham Tseng, and between Sham Tseng and Ting Kau. The medium-grained granite of these outcrops is mostly equigranular, although there are rare megacrystic and microcrystic varieties. Typically, the alkali feldspar is up to 15 mm across although it can be up to 25 mm, with sericitized plagioclase in the range 2 to 8 mm. Quartz crystals range from 2 to 6 mm in size, with biotite clots in the same range.
South of Tai Lam Chung the granite has a gradational contact with fine- to medium-grained granite to the north, but has a sharp boundary with fine-grained granite to the east, particularly well seen both on the ridge north of Tuen Mun Road (821200 824200 Jml-30) and on Castle Peak Road (821900 824300 Jml-31). The Tai Lam Chung outcrop contains numerous fine-grained granite dykes with sharp margins. Along Tuen Mun Road south of Tai Lam Chung the granite is microfractured, preferentially in the quartz, and this is parallel to extensive southerly dipping sheeting joints. On Castle Peak Road (821020 824080 Jml-32) the granite is noticeably quartz deficient (c. 20%).
At Tsing Lung Tau the equigranular medium-grained granite grades into an inequigranular, microcrystic variety both within and around the outcrop. There are thin fine-grained granite dykes to be seen in exposures on Castle Peak Road (823060 824690 Jml-33). Offshore to the southeast several boreholes have proved medium-grained granite lying between Yim Tin Tsai Formation tuffs to the south and fine- to medium-grained granite to the north.
At Sham Tseng the rock texture is slightly inequigranular, and there is poorly developed microfracturing in the quartz seen in exposures on Castle Peak Road (824030 825210 Jml-34). This microfracturing is parallel to sheet like joints dipping gently east. At the road junction on Tuen Mun Road (824350 825590 Jml-35) an equigranular medium-grained granite grades into megacrystic fine-grained granite over a few metres. The outcrops west of Sham Tseng grade progressively into the surrounding fine- to medium-grained granite, while to the east the outcrop is sharply divided from the fine-grained granite, for example on a ridge north of Tuen Mun Road (825400 825700 Jml-36).
Silver Mine Bay (Mui Wo). At Tung Wan Tau, north of Silver Mine Bay, there are exposures (818540 815150 Jml-37) in a stream bed of slightly weathered porphyritic medium-grained granite, with large pink alkali feldspar and small individual biotite flakes. The granite is cut by thin aplite dykes, and is texturally similar to the granite at Sam Pak Wan. Exposures on the adjacent hillsides and ridges are limited to highly weathered corestones, although the presence of coarsely granular quartz-rich soil is a good indication of medium-grained granite bedrock. This contrasts with the fine, clay-rich soil that is a characteristic of the weathered rhyolite dykes. Extending for up to 400 m to the northnortheast, there are exposures and debris deposits consisting of altered granite and feldsparphyric rhyolite. These rocks are light green when fresh, and are composed of quartz and sericite. The quartz relicts give an indication of the primary texture of the rock. The extent of alteration cannot be accurately mapped, but it is probably coincident with a northeast-trending fault that forms part of the Pui O-Sha Tau Kok Fault Zone.
From Man Kok to Kau Shat Wan, east of Silver Mine Bay, porphyritic medium-grained granite is characterized by patches of pegmatite. East of Kau Shat Wan (820320 814680 Jml-38), a pegmatite vein typically includes alkali feldspar crystals up to 1 m long by 0.1 m across. These are sometimes separated by long, thin patches of grey crystalline quartz.
Between Silver Mine Bay and Chi Ma Wan, there are easterly-trending bands (up to 200 m wide) of altered granite and rhyolite dykes, separated by zones of relatively fresh rock. The altered granite is commonly megacrystic and fine-grained, and is cut by quartz veins along the main ridge of hills (818380 812920 Jml-39).
South of the South Lantau Road there are exposures of equigranular to sparsely megacrystic, fine-grained granite. The rock is typically slightly greenish-white, and weathers to a pale brown. Quartz megacrysts up to 4 mm are prominent on weathered surfaces, and pink feldspar up to 10 mm are also present.
At the east end of Pui O Wan beach, a valley marks a change from equigranular fine- to medium-grained granite in the south, to fine-grained granite in the north. Contact relationships are not exposed. The fine-grained granite at this locality is megacrystic and is cut by feldsparphyric rhyolite dykes.
On the coast south of Silver Mine Bay, from Shui Teng Wan to Wang Tong, there are xenoliths of medium-grained granite in the feldsparphyric rhyodacite dykes. At one locality (818380 812220 Jml-40), the xenolith is 1.5 m wide and over 8 m long, whereas at another (818740 812400 Jml-41), the xenolith is over 15 m long.
Megacrystic fine- to medium-grained granite, with some aplite dykes, is exposed as slivers and narrow belts between the dykes in the Mui Wo area, near Mui Wo Kau Tsuen and Hung Shui. Pink to white feldspar phenocrysts, which are up to 30 mm, are set in a granular matrix of biotite (1-3 mm), quartz (1-8 mm) and feldspar (1-5 mm). Aplite dykes, cutting the granite vary from 0.2 to 0.5 m wide.
Chi Ma Wan Peninsula. On the north side of Chi Ma Wan, there are exposures of coarsely porphyritic, medium-grained granite with pegmatitic patches. Dark aphanitic xenoliths are present but scarce. The granite is pinkish-grey, with white plagioclase and prominent black biotite. Tabular phenocrysts of grey alkali feldspar are up to 40 mm long. On the beach to the east of Shap Long San Tsuen (817460 811620 Jml-42) there are many boulders of altered, brecciated and veined medium-grained granite, which have probably been derived from a deeply eroded northwest-trending fault that extends along the adjacent valley.
Pinkish grey and white, porphyritic medium-grained granite, with prominent biotite crystals, from 1 to 3 mm across, is exposed on the ridge (816680 811400 Jml-43) north of Shap Long Kau Tsuen. Although most of the granite in this area is weathered to a coarsely granular soil, this contrasts with the clayey soil of the quartz monzonite and feldsparphyric rhyolite dykes. The granite also contains thin aplite dykes and occasional, thin quartz veins. These exposures contrast with the outcrop south of unaltered, pink, equigranular medium-grained granite forming the bedrock of Chi Ma Wan Peninsula.
Sha Chau. The island of Sha Chau and neighbouring Tree Island are dominantly composed of greyish-white, equigranular, medium-grained granite. Part of the outcrop, at the northern end of Sha Chau, includes megacrystic fine-grained granite, but field relations between the varieties cannot be seen. The granite is cut by small aplite and basalt dykes, but the most prominent cross-cutting feature is a zone of mylonitization and quartz veins which trends northeast across the island. A 4- to 5 m-wide zone of mylonitised granite is well exposed on the west coast, and can also be seen on the northeast coast (806800 823090 Jml-44). Strongly megacrystic, fine-grained granite is exposed north and west of the temple (806400 823200 Jml-45) on Sha Chau. The granite has tabular feldspar megacrysts, up to 50 mm long, which are set in a fine- to very fine-grained groundmass. Texturally, this granite is similar to that exposed on the east side of Castle Peak.
Peng Chau. Exposures of medium-grained granite on Peng Chau are generally restricted to slivers between feldsparphyric rhyolite dykes. However, more massive outcrops of granite (822400 816300 Jml-46), displaying distinctive textural variations, are present south of Tung Wan. Boreholes sunk for the indoor recreation centre and library are dominated by pinkish, megacrystic, medium-grained granite. The rock is characterized both by abundant pink megacrysts of feldspar, some up to 30 mm across, and black biotite. Some sections of core (D/004/11189, 821958 816206 Jml-47) also contain fine-grained granite and pegmatite patches, which are thought to be younger intrusions within the relatively uniform, medium-grained granite.
On the north side of the island, there is a progressive transition over 100 m through megacrystic and pegmatite-bearing, fine-grained granite, to fine-grained granite (822430 816350 Jml-48). The fine-grained granite is pinkish grey with no obvious megacrysts and an equigranular groundmass whose grainsize is less than 1 mm. Textural variation also occurs along the coast to the east, as far as the island of Ngan Chan, contrasting with the porphyritic, medium-grained granite with pegmatite and aplite that is typically exposed at Nam Wan in the south.
Hei Ling Chau. Porphyritic medium-grained granite, pegmatite and aplite are well exposed on the western and southern coasts of Hei Ling Chau. On the southwest coast (822160 811750 Jml-49), the granite is typically pink and white, mottled black, with pink alkali feldspar phenocrysts up to 40 mm long. The equigranular groundmass contains pink alkali feldspar up to 8 mm and white plagioclase less than 6 mm. Quartz occurs as interstitial crystal aggregates up to 10 mm across. The rock also contains distinctive pegmatitic patches. At a locality (822410 812500 Jml-50) near the southern tip, there are boulders of granite with pegmatite that display good perthitic exsolution in the alkali feldspar (Plate 6.A7). About 100 m to the westnorthwest there is a 1 m-wide, subhorizontal aplite dyke with a pegmatitic core.
The presence of dark aphanitic xenoliths, up to 0.5 m across, has been noted in medium-grained granite on the west coast (821630 812420 Jml-51). Close to the jetty (821000 813050 Jml-52), there are exposures of strongly discoloured, finely-fractured and jointed, essentially fine-grained granite. This is megacrystic, and looks in part like medium-grained granite. Further south, there is a swarm of feldsparphyric rhyolite dykes, between which there are slivers of medium-grained granite up to 8 m wide.
Kau Yi Chau. Porphyritic medium-grained granite is exposed on the west coast of Kau Yi Chau (Plate 6.A8). The rock contains alkali feldspar megacrysts up to 30 mm long with abundant biotite crystals and crystal aggregates from 1 to 10 mm. At one locality (825670 816120 Jml-53) in the southwest, the granite is intruded by fine veins of biotite-rich granite and sub-horizontal pegmatite veins from 0.1 to 0.2 m wide.
Siu Kau Yi Chau. Exposures of porphyritic medium-grained granite on this island are limited, the dominant rock type being feldsparphyric rhyolite dykes. Little of the granite country rock is exposed, but where seen, on the western coast, it is medium grained and porphyritic.
Soko Islands. Porphyritic medium-grained granite (Plate 6.A9) outcrops on both main islands of the Soko Islands group. It is cut by numerous, eastnortheast-trending, feldsparphyric rhyolite dykes.
San Shek Wan to Sha Lo Wan. Exposures of megacrystic, fine- to medium-grained granite adjacent to the pier (808140 817110 Jml-54) at Sha Lo Wan contain feldspar megacrysts up to 10 mm and prominent biotite crystals up to 3 mm. The rock is grey or rarely pinkish-grey, and has a groundmass with grainsize between 1 and 2 mm. Numerous quartz veins cut the granite in coastal exposures to the east, and in one locality (808410 817090 Jml-55) this is accompanied by silicification and brecciation of the granite. There is also a 0.3 m wide fault breccia and finely fractured granite exposed 25 m to the east.
At the northern end of the beacon at San Shek Wan (807140 816120 Jml-56), approximately 100 m north of the jetty, there is an exposed contact between granite and metasedimentary rocks. The granite is a megacrystic, fine-grained variety, and the contact with Middle Jurassic strata to the south strikes eastnortheast and dips 74o south. The contact is marked by a 5 m-wide zone of skarn containing garnet, vesuvianite, diopside and epidote (Peng, 1978). The contact zone also includes quartz monzonite and lamprophyre (vogesite) dykes.
Penny's Bay (Chok Ko Wan). At Sz Pak Tsui there are well exposed contacts between granite and rhyodacite dykes. The granite is medium grained, and contains aplite dykes and pegmatitic patches. At one locality (Plate 6.A16, 821450 818980 Jml-57), it is coarsely porphyritic, with feldspar megacrysts up to 60 mm across, and a groundmass ranging from medium- to coarse-grained. The granite also contains sub-horizontal, fine-grained granite dykes, and has well-defined intrusive contacts with younger feldsparphyric rhyodacite dykes to the north and south.
An exposure (821780 819380 Jml-58) on the southwest side of the bay comprises fine-grained granite which is cut by thin, irregular basalt dykes. Both the granite and the basalt are cut by a feldsparphyric rhyolite dyke. About 200 m to the south, there is a irregularly-shaped xenolith of porphyritic medium-grained granite, with feldspar crystals up to 50 mm long. The granite is cut by thin basalt dykes and by feldsparphyric rhyolite.
Pa Tau Kwu. Slivers of equigranular medium-grained granite are present between feldsparphyric rhyolite dykes on the coast of Lantau Island northeast of Penny's Bay, between Chok Ho Wan Tsui and Pa Tau Kwu Wan. The granite is typically pinkish grey and hypidiomorphic-granular in texture. Small (< 2 m) pods of medium-grained granite may also occur within the feldsparphyric rhyolite dykes. The margins of the dykes are commonly chilled against the granite country rock. Medium-grained granite, very similar to that exposed at Pa Tau Kwu, is present as a small pod on Tang Lung Chau.
Along the coast between Sam Pak Wan and Sz Pak Wan (820580 819000 Jml-59) there is a small outcrop of granite in contact with younger, easterly-trending rhyolite dykes. The granite is porphyritic and medium grained with pinkish alkali feldspar phenocrysts up to 30 mm long. Aplite dykes which cut this granite also cut the adjacent rhyolite dykes. Less than 100 m to the west, the granite contains dark xenoliths.
Discovery Bay. Weathered granite, forming a fine gravel quartz sand, is occasionally exposed between rhyodacite dykes at the golf course. The granite appears to be medium grained and equigranular, at least in part, with prominent, single biotite crystals. However, less weathered exposures show that the main lithology is a fine-grained, conspicuously megacrystic granite, which may grade into fine- to medium-grained granite. Single biotite flakes characterize the rock, and the groundmass crystals are typically less than 1 mm. Larger feldspar crystals are up to 10 mm across, with quartz up to 6 mm.The granite exposed in cut slopes east of the Discovery Bay ferry pier is porphyritic and medium grained, with euhedral white alkali feldspar phenocrysts up to 40 mm long and individual biotite flakes up to 4 mm across. Pinkish porphyritic medium-grained granite is also exposed at the eastern end of the golf course (Plate 6.A17), although the dominant bedrock type is rhyodacite, occurring as numerous dykes cutting the granite. West of the Trappist Haven Monastery, there are exposures of granite and dykes which have been hydrothermally altered and silicified. At one locality (819400 815890 Jml-60), the altered medium-grained granite comprises quartz relicts from 1 to 8 mm across set in a light brown, slightly weathered, aphanitic matrix. This zone of alteration extends east and west for no more than 500 m, but has no obvious structural control.
Weakly megacrystic, medium-grained granite is the main bedrock lithology around Hai Kam Tsui. Minor zones of sparsely megacrystic fine-grained granite, grading into fine- to medium-grained granite have been reported in boreholes of the Discovery Bay Phase V development.
Megacrystic fine-grained granite is exposed in cut slopes (819300 817400 Jml-61) west of the ferry pier at Discovery Bay. The granite is pinkish-white, with alkali feldspar megacrysts up to 40 mm long, and single biotite crystals 1 to 2 mm long. The fine-grained granite is intruded by feldsparphyric rhyolite dykes.
Sunshine Island (Chau Kung To). Exposures of granite are rare along the coast of Sunshine Island where the dominant rock is feldsparphyric rhyodacite. The granite is porphyritic and medium-grained and mainly occurs as slivers between rhyodacite dykes. Feldspar megacrysts up to 25 mm long, and biotite flakes from 1 to 3 mm across, are locally abundant and patches of pegmatite sometimes occur with aplite dykes. At one locality (823680 813980 Jml-62) on the north coast, the medium-grained granite is cut by, and partially assimilated by younger, fine-grained granite dykes.
Siu Ho Wan. Porphyritic medium-grained granite with characteristics similar to that at Sam Pak Wan is exposed along the coast north of Siu Ho Wan. It occurs as pods between dykes of feldsparphyric rhyolite and is thought to intrude crystal tuff exposed on hills to the east. The contact between the tuff and granite is obscured by the rhyolite dykes.
Chek Lap Kok (prior to development). Inequigranular, fine- to medium-grained granite, intruded by feldsparphyric and quartzphyric rhyolite dykes, forms much of the eastern part of the island. It is typically light grey, but may be pinkish in some coastal exposures. Feldspar megacrysts up to 10 mm long are common, and may be up to 45 mm. Glomerophyric aggregates of quartz (up to 10 mm) and biotite (up to 7 mm) are also present, set in a fine- to medium-grained groundmass.