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the Lantau Formation, however, and since no dykes are known to cross-cut the fault that formsthe eastern side of the Lantau Caldera, it is likely that most predate the main phase of calderacollapse.The presence of xenoliths of basalt within the rhyolites strongly indicates that the feldsparphyricrhyolites post-date some basaltic dykes. Alkali feldspar xenocrysts within some of these xenolithsmay have been derived from a contemporaneous feldsparphyric rhyolite magma, but syenites inthe district (and even quartzphyric rhyolites) also contain similar feldspar megacrysts and couldbe the parental source.Cross-cutting relationships between individual feldsparphyric dykes indicate that they wereintruded over a long period of time. Their gradational relationships with porphyritic micrograniteimply broad contemporaneity of the two magmas, which are essentially end-members of atextural, rather than a compositional spectrum.Some quartzphyric rhyolite dykes, which are described below, cut feldsparphyric rhyolite dykesand are therefore younger. Also, unlike the quartzphyric rhyolites, the feldsparphyric rhyolites donot cut the fine-grained granite on Chek Lap Kok . Some basaltic and lamprophyric dykes clearlycross-cut the feldsparphyric rhyolites and porphyritic microgranites, and these are presumed tobe associated with the phase of basaltic intrusion dated, using K-Ar isotopes, as Tertiary byAllen and Stephens (1971). Some quartz veins are also younger.Quartzphyric RhyoliteDistribution andLithologyQuartzphyric rhyolite dykes are relatively rare in the district. They are usually no more than 5 mwide, are pale grey when fresh and are commonly extensively fractured. Typically, the lithologycomprises small quartz megacrysts and feldspar phenocrysts, both up to 3 mm, and dark specksof chlorite in an aphanitic groundmass. The margins of these narrow dykes are often flow banded(Plate 29), and may be darker and finer grained than the main body of the dyke.Quartzphyric rhyolite crops out mainly as northeast- and eastnortheast-trending dykes, which cutthe granite country rock. Easterly- and northwesterly-trending dykes also occur locally. Theyform part of the dyke swarm that dominates north Lantau Island, and their emplacement wasprobably controlled by major, eastnortheast-trending extensional structures, including the NorthLantau Fault.DetailsCheung Chan. There are a number of thin, flow-banded quartzphyric rhyolite dykes on Cheung Chau, most ofwhich intrude medium-grained granite. At one locality (2170 0897) on Pak Kok Tsui, a strongly flow-banded,quartzphyric rhyolite dyke about 0.5 m wide, trends 030°. It cuts a porphyritic microgranite dyke which has aslightly banded margin (Plate 30). Both dykes intrude medium-grained granite.Chi Ma Wan, On the east side of the peninsula (2008 1015), a grey, fractured, quartzphyric rhyolite dyke, withquartz crystals up to 2 mm across, is exposed on the coast. It includes a large xenolith of medium-grained granite.Sunshine Island (Chan Kwng To), There are several quartzphyric rhyolite dykes on the south coast of the island.One, which is about 12 m wide, trends 040-050° and can be traced for nearly 200 m along the coast (238 137). Itcross-cuts a feldsparphyric rhyolite dyke as well as medium-grained granite,Kau Yi Chau, On the northeast tip (2618 1644), quartzphyric rhyolite dykes vary gradationally into porphyriticmicrogranite, and feldspar megacrystic rhyolite. Flow-banded quartzphyric rhyolite, with abundant small quartzphenocrysts* also grades into porphyritic microgranite about 300 m to the west.83