Dykes emplaced during the second main pulse of the Lantau Dyke Swarm are mostly concentrated in northern Lantau Island, and on Ma Wan and Tsing Yi where they intrude East Lantau Rhyodacite dykes, volcanic rocks of the Yim Tin Tsai Formation, and the Tai Lam and Sha Tin granites. They may also form multiple and composite intrusions associated with high–K calc-alkaline mafic dykes.

The rhyolite dykes are mostly characterised by abundant bipyramidal quartz phenocrysts (Plate 6.15), with subordinate euhedral alkali feldspar megacrysts and plagioclase phenocrysts, set in a very fine-grained groundmass (Table 6.3). Many of the narrower rhyolite dykes (<3 m wide) are dominantly quartzphyric. The larger dykes commonly grade internally from rhyolite on the margins to porphyritic microgranite in the centres. In thin section, biotite is generally sparse and accessory minerals include zircon, allanite, apatite, fluorite and Fe–oxide.

A single zircon U–Pb age of 146.3 ± 0.3 Ma (<a "defaultCursor" href="javascript:;" title="Davis, D.W., Sewell, R.J. & Campbell, S.D.G. 1997. U - Pb dating of Mesozoic igneous rocks from Hong Kong. Journal of the Geological Society, Vol. 154, pp 1067 - 1076.">Davis et al., 1997) has been obtained from a composite quartzphyric rhyolite dyke in north Lantau Island indicating that these dykes were emplaced during the same magmatic event that produced the main Lantau Dyke Swarm.

Details

Tsing Yi. Many of the quartzphyric rhyolite dykes that trend east west across northern Tsing Yi were proven in ground investigation boreholes. The best exposures are along the western coast, for example south of Wok Tai Wan (826460 824060 Jko-1), where an 18 m wide quartzphyric rhyolite dyke with flow banding on the margins cuts a large feldsparphyric rhyolite body. Both rhyolites have subsequently been cut by thin basalt dykes. Quartzphyric rhyolite dykes form part of a complex intrusive suite to the north of Fung Shue Wo (828320 824770 Jko-2). The dykes cut Yim Tin Tsai Formation tuffs and are closely associated with fine-grained granite. Quartzphyric rhyolite forms a dyke up to 2 m wide at Fung Shue Wo (828100 824100 Jko-3) and has characteristic flow banding parallel to the margin.

Quartzphyric rhyolite dykes have been recognised only in north and central Tsing Yi. Isolated dykes intrude granodiorite around Cheung Hong Estate (828640 823460 Jko-4 and 828850 823500 Jko-5), while several others, trending northeasterly within tuff, occur in the cut-slopes east of Sai Tso Wan. The main group of quartzphyric rhyolite dykes is exposed on the headland (826320 823870 Jko-6) north of Kam Chuk Kok, and in the roadway (826520 823900 Jko-7) that crosses the ridge. The dykes strike roughly eastwards. A larger dyke is exposed on the crest of the ridge further east, where it has been intensely kaolinized and was formerly worked for china clay.

Cheung Chau. There are a number of thin, flow-banded quartzphyric rhyolite dykes on Cheung Chau, most of which intrude medium-grained granite. At one locality (821700 808970 Jko-8) on Pak Kok Tsui, a strongly flow-banded, quartzphyric rhyolite dyke about 0.5 m wide, trends 030^o. It cuts a porphyritic microgranite dyke which has a slightly banded margin (Plate 6.A37). Both dykes intrude medium-grained granite.

Chi Ma Wan. On the east side of the peninsula (820080 810150 Jko-9), a grey, fractured, quartzphyric rhyolite dyke, with quartz crystals up to 2 mm across, is exposed on the coast. It includes a large xenolith of medium-grained granite.

Sunshine Island (Chau Kung To). There are several quartzphyric rhyolite dykes on the south coast of the island. One, which is about 12 m wide, trends 040-050^o and can be traced for nearly 200 m along the coast (823800 813700 Jko-10). It cross-cuts a feldsparphyric rhyolite dyke as well as medium-grained granite.

Kau Yi Chau. On the northeast tip (826180 816440 Jko-11), quartzphyric rhyolite dykes vary gradationally into porphyritic microgranite, and feldspar megacrystic rhyolite. Flow-banded quartzphyric rhyolite, with abundant small quartz phenocrysts, also grades into porphyritic microgranite about 300 m to the west.

Peng Chau. On the northern coast of Peng Chau, and on Tai Lei, there are numerous flow-banded quartzphyric rhyolite dykes. They are typically irregular in form, vary from 0-3 m wide along strike, and trend approximately eastwards. The quartzphyric rhyolite dykes cut both feldsparphyric rhyolite and medium-grained granite.

Chek Lap Kok (prior to development). On the coast south of Ha Law Wan, a grey, quartzphyric rhyolite dyke trends northeastwards. It is flow banded near its irregular contact with the granite country rock. The centre of the dyke, which is around 10 m wide, is slightly coarser. It resembles microgranite and also contains abundant small megacrysts.

At Pak Sha Tsui, there are extensive exposures (811000 816900 Jko-12) of quartzphyric rhyolite which include several slivers of granite country rock adjacent to the flow-banded margins. These exposures probably form part of a multiple dyke swarm, up to 150 m wide. Most contacts with the granite are very irregular. South of Kwo Lo Wan, the same quartzphyric rhyolite dyke is exposed on the coast. It is cut by lamprophyre (811560 818270 Jko-13), has flow-banded margins, and intrudes granite.

East of Fu Tau Shan, quartzphyric rhyolite dykes occur in swarms up to 100 metres wide. They include prominent quartz crystals, up to 3 mm. Contacts with megacrystic, fine-grained granite are irregular. Individual dykes, trending eastnortheast, vary from 3-30 m wide. Typically (e.g. HK9040, 811800 818970 Jko-14), they are grey and porcellanous, with pinkish feldspar megacrysts, up to 3 mm across, and quartz, dominantly 0.5 to 2 mm.

Exposures of quartzphyric rhyolite west of the Sham Wan-Fu Tei Wan Fault are rare. One occurrence (810690 818400 Jko-15), about 500 metres southeast of the test embankment, is light greenish-grey, and contains megacrysts of quartz and subordinate feldspars, up to 3 mm, in a very fine-grained groundmass. Intrusive contacts with granite are sharp, and abundant quartz veins occur parallel to the margins. A similar dyke, exposed 200 m to the south (810770 818180 Jko-16), is cut by quartz veins and has a lamprophyre dyke and silicified granite along one margin. This same dyke can be traced to the eastnortheast, and was probably intersected in borehole L27/3427A (811069 818226 Jko-17) near the fault.

Kowloon. A vertical quartzphyric rhyolite dyke, 10 m wide, cuts the coarse-grained granite at Ha Kwai Chung (831180 823360 Jko-18) and parallels nearby feldsparphyric rhyolite and mafic dykes.