1 - HKU Libraries

in grain size from 0.4 to 1.5 mm, and includes biotite, most of which has been replaced bychlorite. Accessory minerals include hornblende, zircon, apatite, titanite and allanite.Age RelationsThe granodiorite belongs to a large sheet-like intrusion known as the Tai Po Granodiorite (Allen& Stephens, 1971) which crops out over a broad area in the New Territories. It is thought to bethe oldest intrusive unit in the Territory (Addison, 1986) but has not yet been isotopically dated.Medium-grained GraniteDistribution andLithologyMedium-grained granite forms the principal bedrock on central Lantau Island and Tsing Yi, andin many of the offshore islands including Peng Chau, Sha Chau, Kau Yi Chau, Shek Kwu Chau,Cheung Chau, and Siu A Chau. The granite is pink to pinkish-grey and varies texturally fromequigranular to strongly megacrystic. The medium-grained granite is commonly intruded, excepton Sha Chau, by feldsparphyric rhyolite dykes of the Lantau dyke swarm. Xenoliths have beennoted at several localities.DetailsTsing Yi. Medium-grained granite on Tsing Yi is typically pale pink and equigranular with euhedral to subhedralflakes of biotite set in a mosaic of subhedral quartz, plagioclase and alkali feldspar. The best exposures are in cutslopes around Leung Ching Estate in North Tsing Yi, and on the southeast and southwest ridges of the southernpeak of Tsing Yi. The medium-grained granite intrudes tuff of the Yim Tin Tsai Formation, and is commonlychilled to a fine grain size within the contact zone. On the main ridge (2848 2274) of Tsing Yi, the width of thechilled zone is quite narrow, but it is much broader in the area south of Chung Mei owing to the subhorizontalattitude of the contact.Medium-grained granite is well exposed in the cut slopes along the main roads to the east and west of the CheungChing Estate (2920 2303) and is intruded by northeast-trending dykes of fine-grained granite (2915 2295) andcoarsely feldsparphyric rhyolite (2915 2295,2905 2345).West of Tai Shan Ha (2852 2243), medium-grained granite is separated from the tuffs which comprise the countryrock by a relatively thin (20-25 m) zone of chilled fine-grained granite. The sub-horizontal attitude of the contactis apparent from the manner in which it may be traced along the ridge and around the spurs. The medium-grainedgranite is equigranular, except in zones close to the contact and in exposures in the Oil Depot (2924 2243).Exposures of medium-grained granite occur to the west of Nam Wan, beside the Tsing Yi Road (2754 2174 to2788 2123) and in the quarries (2758 2113) southwest of Nam Wan. The dominant lithology is equigranularmedium-grained granite but sheets of megacrystic fine-grained granite have gradational contacts with themedium-grained granite (2755 2172), or are chilled against it (2753 2159 and 2749 2119). In the quarry (27582111) a screen of medium-grained granite is present between dykes of fine-grained granite that have rhyoliticmargins.On the peak (2863 2190) west of Chun Fa Lok, an outcrop of medium-grained granite includes a number ofsubhorizontal sheets of fine-grained granite. The sheets are chilled against the overlying medium-grained granitebut grade down, through richly megacrystic fine-grained granite, into typical equigranular medium-grained granite(2871 2177). A number of feldsparphyric dykes intrude the granite northeast of Chun Fa Lok (2853 2195) andaround Nam Wan (2846 2157) and Nam Wan Kok (2886 2123).The medium-grained granite on Tsing Yi has the overall form of a dome. Contacts in the north and southeastappear to dip steeply (2886 2332, 2775 2175), but on the main ridge contacts are horizontal or gently dipping(2851 2241,2864 2186,2833 2181). Feldsparphyric rhyolite dykes are common in the southwest of the island.Pa Tan Kwu. Slivers of equigranular medium-grained granite are present between feldsparphyric rhyolite dykeson the coast of Lantau Island northeast of Penny's Bay, between Chok Ho Wan Tsui and Pa Tau Kwu Wan. Thegranite is typically pinkish grey and hypidiomorphic-granular in texture. Small (< 2 m) pods of medium-grainedgranite may also occur within the feldsparphyric rhyolite dykes. The margins of the dykes are commonly chilled63