1 - HKU Libraries

idge 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. Excellentcontact relations are exposed in a quarry (2757 2112), where the rhyolite dyke has a core of sparsely megacrystic,fine-grained granite, and north of the Tsing Yi Road (2785 2133), where the dyke has a core of richly megacrysticfine-grained granite. Rhyolite dykes with abundant feldspar megacrysts occur on the cut-slope (2840 2153) north ofthe Power Station, and at Nam Wan Kok (2882 2122 and 2883 2167).On the slopes (2770 2260) east of Sai Tso Wan, richly megacrystic dykes are rather narrow, trend in a northeasterlydirection, and are usually cut by northerly-trending faults, although at one locality (2792 2242), a rhyolite dykedeflects along a fault of this trend for a short distance. The dykes on the northern cut-slopes of Sai Tso Wan arewider (up to 60 m) and trend, unusually on Tsing Yi, in a southeasterly direction. Contacts are well exposed at thewest end of the cut-slopes (2761 2278), where a richly feldsparphyric rhyolite intrudes tuff and fine-grainedgranite.North of Sai Tso Wan, around Liu To, Kam Chuk Kok and Shek Wan, the feldsparphyric rhyolite dykes trendnortheasterly and can be very wide. Although most inland exposures are extremely weathered, the distinctivematrix and megacrystic texture of the rhyolites makes their identification relatively easy on the high ground ofnorthwestern Tsing Yi. However, the dykes are best seen in roadside exposures between Sai Tso Wan and on thenorthwest headland of Tsing Yi. Around Kam Chuk Kok (2700 2300), the rhyolites intrude granodiorite, whilefurther north they intrude tuffs.Ma Wan. On Ma Wan, richly and sparsely feldspar megacrystic rhyolite dykes occur together throughout thenorthern and central parts of the island. On the south shore of Tung Wan, there are several rhyolite dykes withfeldspar megacrysts up to 15 mm long. One dyke, 8 m wide, has basalt dykes, 1 m wide, along both of its margins(2450 2129), while a thicker dyke to the east (2461 2172) grades into fine-grained granite in its core. Similargradations between richly feldsparphyric rhyolite and fme-grained granite occur around Cheung Tsui (2469 2368)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 rhyolitedykes, but they are narrower than in the outcrops to the north. Several have basalt dykes along their margins(2388 2315, 2390 2293). The presence of basalt xenoliths within the rhyolite suggests that the basalts are older.However, xenocrysts of alkali feldspar occur in the basalt xenoliths. These could have been derived from a parentalfeldsparphyric magma, but both syenites and quartzphyric dykes in the area also contain feldspar megacrysts.A dyke complex, exposed on the headland of Ma Kok Tsui (2363 2330), includes richly and sparsely megacrysticrhyolites. The latter often form cores to the former, but richly megacrystic rhyolite also intrudes a large dyke ofsparsely megacrystic rhyolite. On the northern shore (2375 2344) of the headland, abundantly feldsparphyricrhyolite, with an accompanying basalt 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 MaWan. They have been traced inland through isolated exposures and corestones and in boreholes drilled for theLantau Fixed Crossing Project.Chek Lap Kok (prior to development). Feldsparphyric rhyolite dykes are scarce, and are restricted to the south ofthe island, where most occur in the southern peninsula. Texturally they resemble quartzphyric rhyolites moreclosely 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 mwide, contain feldspars only up to 3 mm long. Similar dykes, closer to a quartzphyric rhyolite dyke swarm, havesmall but prominent feldspars. Abundant boulders of feldsparphyric rhyolite with larger megacrysts (around 20mm), occur on the beach south of Ha Law Wan, and similar corestones are evident on the adjacent hillside,PetrographyFeldsparphyric rhyolite (HK9234, 1103 1764), from north of Ha Law Wan, contains subhedral,embayed quartz crystals, up to 5 mm, and smaller feldspars, up to 3 mm, set in a veryfine-grained groundmass.Age RelationsThe feldsparphyric dykes intrude tuffs of both the Yim Tin Tsai and Shing Mun formations, andall granite phases in the district except for the granite on Chek Lap Kok . They are rare within82