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Chapter 6Minor IntrusionsIntroductionThe minor intrusions described in this chapter include dykes varying in width from a fewcentimetres to 200 metres. They are best exposed in coastal sections, and most can only be tracedinland for a few tens of metres. However, some have been mapped for more than one kilometre.They are classified using the non-genetic scheme of Streckeisen (1974) and Le Maitre (1989), sothat their identification depends on the mineral content of the rocks (Figure 8). Representativewhole-rock geochemical analyses of the principal minor intrusions are presented in Appendix 2,although chemical analyses are rarely necessary to confimi the composition of the minorintrusions.There are eight main types of dyke: feldsparphyric rhyolite, which is the dominant type;quartzphyric rhyolite; quartz syenite; aplite or fine-grained granite; pegmatite; basalt and gabbro;andesite and dacite; and lamprophyre. Quartz syenites and various granitoids which locally formdykes also form larger plutonic bodies and are therefore described in Chapter 5. Dyke swarmsmapped by Allen & Stephens (1971) as feldspar and quartz porphyry in north Lantau Island(Lamma porphyry and D'Aguilar porphyry) are reclassified here as feldsparphyric andquartzphyric rhyolite. Allen & Stephens (1971) also identified basic dykes (although none wereshown on their map of Lantau Island) and these are renamed as lamprophyre or basalt. Theirdolerites have been remapped as basalt or gabbro, while granite and granodiorite porphyry dykesare renamed as rhyolite and rhyodacite respectively. Some multiple quartz veins are large enoughto be regarded as dykes, and are shown on the published maps, but most quartz veins aresmall-scale.All but the basalt, gabbro and lamprophyre dykes were emplaced during phases of Mesozoicgranite plutonism. Allen & Stephens (1971) interpreted the basic intrusions as being significantlyyounger than the other intrusions and constrained both Upper Cretaceous (syn-plutonic) andTertiary phases of their emplacement. On the 1:20,000 scale maps, however, basic intrusionshave been generalized as Tertiary in age.Feldsparphyric Rhyolite and MicrograniteDistribution andLithologyThe feldsparphyric rhyolites weather to a grey or brown, and are coarsely porphyritic with anaphanitic matrix, They contain abundant alkali feldspar megacrysts up to 20 mm long (Plate 27),as well as crystals of biotite and quartz up to 2 mm across. In common with the quartzphyricrhyolites, they contain central zones of fine-grained, megacrystic granite, Porphyriticmicrogranites, by comparison, are less porphyritic and include smaller feldspar megacrysts(