1 - HKU Libraries

Peng Chau. On the northern coast of Peng Chau, and on Tai Lei, there are numerous flow-banded quartzphyricrhyolite dykes. They are typically irregular in form, vary from 0-3 m wide along strike, and trend approximatelyeastwards. The quartzphyric rhyolite dykes cut both feldsparphyric rhyolite and medium-grained granite.Tsing Yi. Quartzphyric rhyolite dykes have been recognised only in north and central Tsing Yi. Isolated dykesintrude granodiorite around Cheung Hong Estate (2864 2346 and 2885 2350), while several others, trendingnortheasterly within tuff, occur in the cut-slopes east of Sai Tso Wan. The main group of quartzphyric rhyolitedykes is exposed on the headland (2632 2387) north of Kam Chuk Kok, and in the roadway (2652 2390) thatcrosses the ridge. The dykes strike roughly eastwards. A larger dyke is exposed on the crest of the ridge further east(2700 2385), where it has been intensely kaolinized and was formerly worked for china clay.Chek Lap Kok (prior to development). On the coast south of Ha Law Wan, a grey, quartzphyric rhyolitedyke trends northeastwards. It is flow banded near its irregular contact with the granite country rock. The centre ofthe dyke, which is around 10 m wide, is slightly coarser. It resembles microgranite and also contains abundantsmall megacrysts.At Pak Sha Tsui, there are extensive exposures (110 169) of quartzphyric rhyolite which include several slivers ofgranite country rock adjacent to the flow-banded margins. These exposures probably form part of a multiple dykeswarm, up to 150 m wide. Most contacts with the granite are very irregular. South of Kwo Lo Wan, the samequartzphyric rhyolite dyke is exposed on the coast. It is cut by lamprophyre (1156 1827), 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 prominentquartz 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, 1180 1897), they are grey andporcellanous, 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 (1069 1840),about 500 metres southeast of the test embankment, is light greenish-grey, and contains megacrysts of quartz andsubordinate feldspars, up to 3 mm, in a very fine-grained groundmass. Intrusive contacts with granite are sharp, andabundant quartz veins occur parallel to the margins. A similar dyke, exposed 200 m to the south (1077 1818), iscut by quartz veins and has a lamprophyre dyke and silicified granite along one margin. This same dyke can betraced to the eastnortheast, and was probably intersected in borehole L27/3427A (11069 18226) near the fault.PetrographyMany small megacrysts, set in a very fine-grained groundmass, arc typically scon in thin sectionsof the quartzphyric rhyolite (Plate 31, HK904Q, 1180 1897). The quartz megacrysts are euhedralto subhedral, slightly embayed, and are 0.5 to 1 mm across. Sample HK9233 (1069 1840) has asimilar texture and contains microperthitic alkali feldspar, with Carlsbad twins up to 1.5 mmwide. Plagioclase feldspars, of oligoclase composition, can be up to 2 mm across. There arescarce crystals of muscovite, biotite and an opaque mineral,, around 0,2 to 0,5 mm. Thegroundmass is devitrified glass with secondary sericite, 0,02 to 0,05 mm across. Other samplesof quartzphyric rhyolite are also devitrified, with radiating quartz needles and granophyrictexture occurring as reaction haloes around the megacrysts.Age RelationsThe quartzphyric dykes intrude granite, feldsparphyric rhyolite dykes, porpyhyritic micrograniteand the fine-grained granite on Chek Lap Kok. They are in turn intruded by much youngerlamprophyre dykes, and are cut by quartz veins. Elsewhere in the Territory, they cut all otherMesozoic intrusive and extrusive rocks.Basalt, Andesite and LamprophyreDistributionBasic dykes are widespread within the various granites and rhyolitic dykes of the district, They84