Dykes belonging to the initial pulse of the Lantau Dyke Swarm are concentrated mainly in northern and eastern Lantau Island. They are also found in central Lantau Island and on Ma Wan, the Soko Islands and numerous small islands and reefs to the south. In northern and eastern Lantau Island and on Ma Wan, the dykes intrude the Lantau Granite and Tai Lam Granite, whereas in central Lantau Island and the Soko Islands they intrude the Lantau Granite.

The East Lantau Rhyodacite dykes generally have large (5–20 mm) megacrysts of alkali feldspar, bipyramidal quartz and plagioclase, and crystal aggregates of biotite, set in a very fine-grained groundmass containing zoned allanite, zircon and Fe-oxide (Table 6 .3). Close to the margins of the dykes, the feldspar phenocrysts may show a weak preferred orientation.

The rhyodacite dykes commonly contain large xenolithic blocks of the host Lantau Granite. These distinguish them from younger rhyolite dykes (East Lantau Rhyolite). They may also grade internally from porphyritic rhyodacite at the margins to porphyritic microgranite in the interiors.

The East Lantau Rhyodacite dykes have yielded a U–Pb age of 146.5 ± 0.2 Ma (Davis et al., 1997) indicating that they are largely coeval with the emplacement of the Sha Tin Granite.

Details

Kwai Chung - Yau Oi Tsuen. The feldsparphyric dykes of Kwai Shing (830800 825000 Jkd-1), around Kam Shan (833700 825000 Jkd-2), Lower Shing Mun Reservoir (835400 826900 Jkd-3) and Yau Oi Tsuen (836100 827750 Jkd-4) are homogeneous in type with a generally uniform northeasterly trend. They intrude all of the plutonic rock types of the area and cross cutting relationships are recorded, although not exposed, as individual dykes clearly intersect the granite boundaries.

Kowloon. Feldsparphyric rhyolite occurs as dykes 2 to 10 m wide in the Lai King area of northwestern Kowloon, where they intrude both the fine-grained and coarse-grained granites. They form part of a wide dyke swarm extending east northeast from Lantau Island through Tsing Yi into the Sha Tin district. The rhyolite is invariably composed of a greyish green, very fine groundmass containing minute black specks of biotite, megacrysts of alkali feldspar averaging 12 mm across, and smaller quartz and plagioclase megacrysts of 3 to 4 mm. Biotite crystals tend to aggregate into felty blotches in a similar manner to those in the coarse-grained granite of this area. At Wah Yiu Road, Lai King (831440 823670 Jkd-5), a 3.2 m wide dyke shows evidence of multiple phases of intrusion (Figure 6.A6). The feldspar megacrysts are commonly rounded and broken, especially where most abundant in the central part of the dyke. A chemical analysis of this dyke rock (HK 2182) shows that the composition is granitic (Table 6.A3) and very similar to the coarse-grained granite host.

Soko Islands. Tai A Chau is dominantly composed of feldsparphyric rhyolite and porphyritic microgranite dykes, which intrude the Lantau Granite. At one locality (808460 802820 Jkd-6), west of Pak Tso Wan, feldsparphyric rhyolite grades northwards into porphyritic microgranite. About 50 m to the south, a feldsparphyric rhyolite dyke, trending 020^o , cuts an older feldsparphyric rhyolite with a similar texture.

There are very few feldsparphyric rhyolite dykes on the other islands of the group. However, one, which is 2 m wide is exposed on the foreshore (809450 804640 Jkd-7) at Sum Wan on Siu A Chau. It trends 020^o, and intersects an earlier feldsparphyric rhyolite dyke which probably trends northeastwards. The younger dyke has an aphyric margin, and resembles aplite.

Tong Fuk. The few feldsparphyric rhyolite and porphyritic microgranite dykes exposed on the north and south sides of Shui Hau Wan and Tong Fuk Miu Wan, all intrude tuffs of the Yim Tin Tsai Formation. Exposures are always poor and field relationships obscure.

On the catchwater (809360 809780 Jkd-8) north of Shui Hau, light grey feldsparphyric rhyolite intrudes lava of the Lantau Volcanic Group (undifferentiated). It contains feldspar and quartz phenocrysts, respectively up to 12 mm and 5 mm across, set in an aphanitic groundmass. The dyke, which is probably up to 70 m wide and trends northeastwards, weathers to form corestones in hillside exposures to the southwest. It is one of very few feldsparphyric rhyolite dykes known to intrude the Lantau Volcanic Group (undifferentiated).

Cheung Sha. Feldsparphyric rhyolite dykes, striking eastnortheast, are exposed between Tong Fuk, Cheung Sha and Pui O. A small group of boulders, exposed 400 m offshore, is an extension of one of these dykes. The country rock is composed of tuff of the Yim Tin Tsai Formation. An eastnortheasterly-trending contact (813870 810490 Jkd-9) is exposed near Cheung Sha Village.

Sunshine Island (Chau Kung To). This island is dominantly composed of feldsparphyric rhyolite, which is intruded by younger quartz syenite. At one locality (823340 813230 Jkd-10) on the south coast, a feldsparphyric rhyolite dyke contains an aphanitic xenolith with sharp edges and few megacrysts.

Hei Ling Chau. The northern part of the island is dominated by feldsparphyric rhyolite and porphyritic microgranite dykes, mostly trending east or eastnortheast. On the northeast coast (822470 813240 Jkd-11), the contact between feldsparphyric rhyolite to the north, and younger porphyritic microgranite to the south, trends 080^o. The microgranite grades southwards into feldsparphyric rhyolite with feldspar phenocrysts up to 40 mm long. A similar relationship also occurs in a dyke trending 085^o, which is exposed (822130 811900 Jkd-12) on the southwest coast. Numerous, narrow, feldsparphyric rhyolite dykes, separated by screens of medium-grained granite as narrow as 2 m, are well exposed on the coastline (820900 812900 Jkd-13) south of the jetty. The dykes mostly trend eastnortheast, and dominate the outcrop.

Barren Hills. In the Barren Hills, south of Silver Mine Bay, feldsparphyric rhyolite dykes (Plate 6.A40) are dominant and intrude the medium-grained Lantau Granite. They trend eastnortheast. On the coast (818380 812220 Jkd-14) between Ngau Kwu Wan and Shui Tseng Wan, the dykes contain large granite xenoliths, up to 1.5 by 8 m across .

Siu Kau Yi Chau. Almost the entire island is composed of feldsparphyric rhyolite and porphyritic microgranite dykes, which locally grade from one to the other. On the south coast (824050 816540 Jkd-15), a flow-banded feldsparphyric rhyolite dyke trends 065^o, and cuts another feldsparphyric rhyolite dyke. To the east, there are rapid textural changes to microgranite, with abundant small phenocrysts, rarely up to 20 mm long, and then back to feldsparphyric rhyolite, whose texture is dominated by large feldspar phenocrysts set in an aphanitic groundmass.

Peng Chau. On Tai Lei, a small island northwest of Peng Chau, good exposures (821550 816800 Jkd-16) of feldsparphyric rhyolite grade, over a width of 2 m, into porphyritic microgranite, but within 5 m the rock texture reverts to feldsparphyric rhyolite. A narrow chilled margin, trending 080^o, is developed along the contact with an adjacent dyke.

A feldsparphyric rhyolite dyke, trending approximately eastwards, cuts fine-grained granite east of Tung Wan. The dyke, which is up to 20 m wide, can be traced for 300 m. On the east coast (822780 816420 Jkd-17), it has aphyric, flow-banded margins to both north and south, whereas its central zone varies from feldsparphyric rhyolite to porphyritic microgranite. Similar relationships occur elsewhere along the east coast of Peng Chau.

Discovery Bay. Feldsparphyric rhyolites dykes, which intrude medium-grained granite, and porphyritic microgranite dykes, are exposed at numerous localities along the coast around Discovery Bay. Complex, composite intrusive relationships occur between the feldsparphyric rhyolites, with or without small feldspar phenocrysts, and porphyritic microgranites in cut slopes (819480 816460 Jkd-18) east of the golf course. The groundmass in the feldsparphyric rhyolites varies locally from aphanitic to microcrystalline.

Between Sam Pak Wan and Sz Pak Wan, north of Discovery Bay, there are good examples of dykes which grade from feldsparphyric rhyolite to porphyritic microgranite. Their margins tend to be coarsely porphyritic, whereas the centres have fewer, smaller phenocrysts.

Siu Ho Wan. Along the coast west of Siu Ho Wan, there are several good exposures of younger feldsparphyric rhyolite dykes chilled against older dykes. In addition, textural gradations occur between light grey feldsparphyric rhyolite, with phenocrysts of feldspar up to 20 mm long, and porphyritic microgranite, with a fine-grained groundmass and phenocrysts of less than 10 mm.

Yam O Wan. A feldsparphyric rhyolite dyke (820360 821810 Jkd-19) on the island of Cheung Sok, north of Yam O Wan, contains phenocrysts which are progressively smaller towards the south. It becomes a porphyritic microgranite with abundant quartz and feldspar phenocrysts up to 2 mm across. The northern margin of the dyke, which intrudes tuff of the Shek Lung Kung Member, is flow banded, and contains feldspar crystals up to 20 mm long.

Complex relationships between feldsparphyric rhyolite and porphyritic microgranite occur on the eastern shores of Yam O Wan (820900 821300 Jkd-20). Gradational contacts between the two occur across both wide (several metres) and narrow zones. In addition, multiple dyke contacts are well exposed.

Tung Chung Wan. On the west side of the bay, a feldsparphyric rhyolite dyke, with aphanitic margins and a porphyritic microgranite centre, trends 055^o, and intrudes fine-grained granite to the north (810380 816250 Jkd-21).

Tsing Yi. Rhyolite dykes, crowded with feldspar megacrysts, are very common on Tsing Yi. They range from a few metres, up to 200 m wide, and extend for up to 1 km. In the north, around Chung Mei (828300 823500 Jkd-22), a number of dykes, up to 40 m wide and trending generally eastnortheastwards, intrude tuff, granodiorite and granites. Rhyolite dykes clearly intrude megacrystic, fine-grained granite southwest of Chung Mei (828270 823270 Jkd-23), and also tuff to its south (828420 823360 Jkd-24), and granodiorite to its east (828440 823470 Jkd-24). Further east, near Ko Tai (829050 823450 Jkd-25), and southeast at Cheung Ching (829210 823000 Jkd-27), feldsparphyric rhyolite intrudes medium-grained granite.

In the southern part of Tsing Yi the rhyolite dykes are generally rich in feldspar megacrysts. They are wider and more extensive than to the north, and trend easterly. A major group of dykes, the largest over 200 m wide, intrudes tuffs and is exposed in road cuts (827850 822160 Jkd-28 and 827590 821910 Jkd-29) south of Nam Wan. In one outcrop (827770 822050 Jkd-30), the rhyolite is intruded by thin dykes of fine-grained granite. This group of dykes can be traced eastwards to the main ridge of Tsing Yi. The dykes, exposed in cut-slopes and quarries in the southwest of Tsing Yi, are dominantly, richly megacrystic; they are narrower than the dykes south of Nam Wan and more irregular in form. Excellent contact relations are exposed in a quarry (827570 821120 Jkd-31), where the rhyolite dyke has a core of sparsely megacrystic, fine-grained granite, and north of the Tsing Yi Road (827850 821330 Jkd-32), where the dyke has a core of richly megacrystic fine-grained granite. Rhyolite dykes with abundant feldspar megacrysts occur on the cut-slope (828400 821530 Jkd-33) north of the Power Station, and at Nam Wan Kok (828820 821220 Jkd-34 and 828830 821670 Jkd-35).

On the slopes (827700 822600 Jkd-36) east of Sai Tso Wan, richly megacrystic dykes are rather narrow, trend in a northeasterly direction, and are usually cut by northerly-trending faults, although at one locality (827920 822420 Jkd-37), a rhyolite dyke deflects along a fault of this trend for a short distance. The dykes on the northern cut-slopes of Sai Tso Wan are wider (up to 60 m) and trend, unusually on Tsing Yi, in a southeasterly direction. Contacts are well exposed at the west end of the cut-slopes (827610 822780 Jkd-38), where a richly feldsparphyric rhyolite intrudes tuff and fine-grained granite.

North of Sai Tso Wan, around Liu To, Kam Chuk Kok and Shek Wan, the feldsparphyric rhyolite dykes trend northeasterly and can be very wide. Although most inland exposures are extremely weathered, the distinctive matrix and megacrystic texture of the rhyolites makes their identification relatively easy on the high ground of northwestern Tsing Yi. However, the dykes are best seen in roadside exposures between Sai Tso Wan and on the northwest headland of Tsing Yi. Around Kam Chuk Kok (827000 823000 Jkd-39), the rhyolites intrude granodiorite, while further north they intrude tuffs.

In some areas, such as south of Cheung Hong Estate (828600 823130 Jkd-40), richly megacrystic, fine-grained granite occurs as textural variants within dykes of feldsparphyric rhyolite which are themselves either richly or sparsely feldsparphyric. Such occurrences were also noted on Ma Wan, for example south of Tung Wan (824610 823270 Jkd-41, 824530 823680 Jkd-42).

Ma Wan. On Ma Wan, richly and sparsely feldspar megacrystic rhyolite dykes occur together throughout the northern and central parts of the island. On the south shore of Tung Wan, there are several rhyolite dykes with feldspar megacrysts up to 15 mm long. One dyke, 8 m wide, has basalt dykes, 1 m wide, along both of its margins (824500 821290 Jkd-43), while a thicker dyke to the east (824610 821720 Jkd-44) grades into fine-grained granite in its core. Similar gradations between richly feldsparphyric rhyolite and fine-grained granite occur around Cheung Tsui (824690 823680 Jkd-45) on the north side of Tung Wan.

On the west coast of Ma Wan, south of Ma Kok Tsui, there are numerous easterly-trending, feldsparphyric rhyolite dykes, but they are narrower than in the outcrops to the north. Several have mafic dykes along their margins (823880 823150 Jkd-46, 823900 822930 Jkd-46). The presence of mafic xenoliths within the rhyolite suggests that the mafic are older.

A dyke complex, exposed on the headland of Ma Kok Tsui (823630 823300 Jkd-48), includes richly and sparsely megacrystic rhyolites. The latter often form cores to the former, but richly megacrystic rhyolite also intrudes a large dyke of sparsely megacrystic rhyolite. On the northern shore (823750 823440 Jkd-49) of the headland, abundantly feldsparphyric rhyolite, with an accompanying mafic dyke, forms the margin of a major, sparsely feldsparphyric rhyolite dyke, which varies to a fine-grained granite.

North of Shek Tsai Wan, the rhyolite dykes are mostly richly megacrystic, and trend northeastwards across Ma Wan. They have been traced inland through isolated exposures and corestones and in boreholes drilled for the Lantau Fixed Crossing Project.

Chek Lap Kok (prior to development). Feldsparphyric rhyolite dykes are scarce, and are restricted to the south of the island, where most occur in the southern peninsula. Texturally they resemble quartzphyric rhyolites more closely than comparable intrusions in north Lantau Island.

Feldsparphyric rhyolite is exposed in coastal sections south of Kwo Lo Wan, where two small dykes, 0.5 and 3 m wide, contain feldspars only up to 3 mm long. Similar dykes, closer to a quartzphyric rhyolite dyke swarm, have small but prominent feldspars. Abundant boulders of feldsparphyric rhyolite with larger megacrysts (around 20 mm), occur on the beach south of Ha Law Wan, and similar corestones are evident on the adjacent hillside.