Several quartzphyric rhyolite dykes cut the youngest volcanic and plutonic lithologies in the eastern New Territories, Kowloon, Hong Kong Island and the Po Toi Islands. The dykes vary in trend from northnorthwest to eastnortheast. Little is known about their geochemical characteristics and they have not yet been dated radiometrically.

Details

Kowloon. In central, northern and western Kowloon, quartzphyric rhyolite has not been mapped, but in East Kowloon there are isolated occurrences. The most prominent is a N-S vertical dyke 60 m wide, cutting volcanic rocks of the Mount Davis Formation on Fei Ngo Shan (841100 822370 qrd-1). The rhyolite displays distinctive flow banding, well exposed 50 m east of the radio transmitter tower (841130 822400 qrd-2), where the flow fabric is highly contorted. Between Ngau Tau Kok and Shun Lee (840620 821140 qrd-3), two roughly parallel quartzphyric rhyolite dykes intrude the fine- to medium-grained granite. Trial adits have been dug into the larger of the dykes, presumably in an exploration for metalliferous minerals.

In the vicinity of Grasscutter's Pass (839300 824000 qrd-4) and Fei Ngo Shan, prominent NW-SE trending feldsparphyric rhyolite dykes cut both the granite and the volcanic rocks. The largest of these is 120 m wide at Grasscutter's Pass and 90 m wide west of Middle Hill (840570 823180 qrd-5). In the Ho Chung Valley (842580 823340 qrd-6) an isolated outcrop of feldsparphyric rhyolite was noted in the stream bed, but its relationship with the country rock was obscured by superficial deposits.

Hong Kong Island. Quartzphyric rhyolite has been exposed in temporary excavations at Queen Mary Hospital (831510 814680 qrd-7), where it forms a NNW trending dyke several metres wide. Similar trending dykes occur at Shouson Hill (836600 812300 qrd-8, Plate 6.A36) and on the southern flanks of Mount Nicholson. There (836780 813170 qrd-9), one dyke is 40 m wide and cuts both fine-grained granite and Ap Lei Chau Formation volcanic rocks. Prominent vertical flow banding parallel to the dyke trend is seen on weathered surfaces. At Bennet's Hill (835270 813000 qrd-10) a quartzphyric rhyolite dyke up to 60 m wide strikes N-S. Some flow banding was observed, but the rock is deeply weathered. Feldspar megacrysts have been eroded out, leaving euhedral hollows often stained purple, but the quartz megacrysts, up to 3 mm across, are unaffected by the weathering. Near South Bay (838030 809510 qrd-11) a 30 m wide flow banded rhyolite forms a prominent coastal rib. This dyke has sharp contacts with the fine ash tuffs of the Ap Lei Chau Formation, and both rocks, weather to a very similar appearance.

Feldsparphyric rhyolite dykes were mapped in only two locations, at the hill above Belcher's and northwest of Mount Cameron. The rhyolite forms a prominent N-S ridge named the Hill Above Belcher's (831960 815660 qrd-12), where the dyke is approximately 25 m wide and cuts coarse ash tuffs of the Mount Davis Formation. The rock is highly weathered, but distinct alkali feldspar megacrysts can be seen, often more weathered than the groundmass. A crude flow banding was noted in places. A similarly trending N-S dyke, approximately 10 m wide, crops out in the stream bed 200 m northwest of Mount Cameron summit. Here, the groundmass is almost aphanitic, and the megacrysts average 5 mm.

Lamma Island. Light grey, sparsely megacrystic quartzphyric rhyolite crops out as a dyke almost 200 m wide northeast of Mo Tat Wan (833600 808050 qrd-13) on eastern Lamma. This dyke intrudes both the fine- and the coarse-grained granites, and appears to be cut by a thin feldsparphyric rhyolite dyke. Locally (833740 808080 qrd-14), the central part of the dyke consists of a monomictic breccia and has the appearance of a tuff breccia. This zone, some 10 m wide, may be an auto brecciated core to the dyke; brecciation in dyke intrusions has not been observed elsewhere in the district. The dyke can be traced along the coast on the eastern side of the bay as far as Wong Chuk Kok (834490 807860 qrd-15). Along this coast the southern margin of the dyke crops out in the cliffs, with the rhyolite part of the cliffs being particularly resistant to erosion by the sea. 500 m ESE of Mo Tat Wan (833540 807440 qrd-16) a 20 m wide quartzphyric rhyolite dyke cuts obliquely across a feldsparphyric rhyodacite dyke, suggesting that at least some of the quartzphyric rhyolite intrusions post date the feldsparphyric varieties. South of Lo So Shing, several 20 to 30 m wide quartzphyric rhyolite dykes crop out on the coast (830560 806850 qrd-17 & 830490 806030 qrd-18) striking 070 to 090^o, parallel to the main feldsparphyric rhyolite dyke swarm. Inland, one quartzphyric rhyolite dyke (831000 806430 qrd-19) reaches a width of 100 m and consists of light pink very fine-grained rock that forms a clear positive feature on the more readily eroded granite landscape.

Po Toi Island Group. A number of quartzphyric rhyolite dykes are found on Po Toi and Sung Kong islands (Figure 6.A3). One dyke (844420 803440 qrd-20) forms a strong negative photogeological lineament north of Tai Wan Village on Po Toi. Flow banded margins to the dykes are common and particularly well seen on Lo Chau Pak Pai (Castle Rock, 842440 804070 qrd-21).

Southeast Tuen Mun. Quartzphyric rhyolite dykes are exposed along the coast southeast of Tuen Mun, in Perowne Camp and at Siu Sau. The largest of these dykes, at Siu Sau (817700 825510 qrd-22), is about 10 m wide and trends eastnortheast; the centre of the dyke is grey, with quartz megacrysts up to 1 mm and a fine scattering of biotite, while the margins are slightly flow banded and much paler. Close by to the south is a similar dyke 1 m wide.

East of Ka Fei Wan (Cafeteria Beach) a 17 m wide quartzphyric rhyolite dyke has sheared margins and trends roughly northeast. A quartzphyric rhyolite dyke lying on the same trend outcrops to the northeast in Perowne Camp (817320 826350 qrd-23). The centre of the dyke is pale greenish grey, while a narrow zone seen on one margin is a darker bluish grey.

Clear Water Bay Peninsula, Junk Island and Tung Lung Island. A northeast striking quartzphyric rhyolite dyke reaches 90 m in width near Lam Tong (847580 812780 qrd-24). The pale grey to pink rhyolite is homogeneous but contains small scattered quartz and feldspar megacrysts. Distinctive flow banding, striking 65^o, is prominent in the weathered coastal exposures. The dyke forms a well defined positive topographic feature inland. Small quartzphyric rhyolite dykes averaging 2 m in width, are seen in a number of coastal sections cutting Che Kwu Shan Formation tuff on Tung Lung Island, Tit Cham Chau and at Tin Ha Shan.

Flow banded quartzphyric rhyolite forms a wide margin to a small elongate fine-grained granite pluton on Junk Island (prior to reclamation) (845485 815810 qrd-25). This intrusive body can be traced northeast on to the mainland at Shek Miu Wan (846480 816170 qrd-26). On the north side of Junk Island the rhyolite margin reaches 100 m in width, with distinctive flow banding throughout. The megacryst content increases gradually towards the granite, where the boundary is gradational. A raft of tuff breccia, 2 m by 500 mm was noted in the centre of the rhyolite body.

Sai Kung and Kau Sai Chau. A 40 m wide quartzphyric rhyolite dyke with prominent flow banded margins strikes north through Long Harbour Formation tuff on Yeung Chau (846900 826800 qrd-27). One rhyolite dyke was identified at Kwai Tau Tam, northern Kau Sai Chau, striking roughly east west and cutting the High Island Formation tuff.

Long Harbour and Sharp Peak. A number of quartzphyric rhyolite dykes cut coarse ash tuff of the Long Harbour Formation. Many parallel the east west striking Cheung Sheung - Chek Keng Fault, as for example at Chek Keng Hau, but others strike northeast on the Kau Lo Wan peninsula in the same direction as numerous parallel photolineaments. All these dykes are impersistent and, although they may reach 20 m in width, rarely extend laterally for more than 200 m. In all cases the rhyolite is pale grey and flow banded with sparse quartz megacrysts. Immediately west of Sharp Peak (856650 832340 qrd-28) a feature forming rhyolite dyke strikes north south and averages 40 m in width. This dyke bifurcates near the coast east of Nam She Wan (856700 833230 qrd-29) and several branches are seen cutting Long Harbour Formation tuff in the cliffs. The rhyolite is atypical in that it is high in silica and abundant quartz veining is present. It seems probable that there has been later injection of silica rich fluids along the dyke. To the east of Sharp Peak, quartz veins parallel this major dyke (859000 832900 qrd-30). Another dyke also intrudes the Long Harbour Formation at Chuen Lo Kok Tsui (Tide Pole Point) (852100 836900 qrd-31).

Kwai Chung - Yau Oi Tsuen. Around Nim Au (838250 830900 qrd-32) and near Tsiu Hang (837950 832540 qrd-33) rhyolite dykes are sinistrally offset by the northwest-trending faults.

Sha Tin Pass - Shui Ngau Shan. The dykes of Tsz Wan Shan (839100 824100qrd-34 ) and Tiu Tso Ngam (839800 824100 qrd-35) are up to 60 m wide and strike in a northwesterly direction. Although they mostly have the normal aphanitic groundmass with scattered phenocrysts of quartz and feldspar (840050 824050 qrd-36), in some instances they are crowded with feldspar phenocrysts (838800 824130 qrd-37). At these localities the rhyolites intrude fine- and medium-grained granites, and tuffs of the Mount Davis Formation.

A substantial, 15 m wide, northeast trending quartzphyric rhyolite dyke crops out in roadside exposures at Pak Wai (844440 824500 qrd-38) intruding tuffs of the Long Harbour Formation.

Tsing Shan (Castle Peak). A dyke of feldsparphyric rhyolite trends approximately north-south through medium-grained granite at the Castle Peak Power Station site at Tap Shek Kok. This dyke was formerly exposed in rock cuts forming the back wall of the site (810050 826630 qrd-39), where it is recorded as being up to 10 m thick; it was also found in excavations for chimney foundations (810000 826070 qrd-40).