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3j A 4: 5 6' 7 8 9 l o--*'»•"»' t T'i t t «-^'.-T l »-*"*-r"»-~-~-*»--r «''••» I,,-,, .... . -.-„_.,,,,1 .,,., *\ I IV27 - Feldsparphyric Rhyolite(2163 2194), Tai YamPlate 28 - Feldsparphyric Rhyolite (1851 1339), Barren Hills
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(
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UNIVERSITY OF HONG KONGLIBRARIES
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© Government of Hong KongPublished
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CONTENTSTitle pagePage1ForewordCont
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Distribution and Lithology 73Detail
Page 11 and 12:
Chapter 10 Economic Geology 149Intr
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(Plates)Plate 22 - Porphyritic Medi
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Table 1 — Grain Size Description
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0 1 2 3 4 5 k mCONTOURS AT 100m INT
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properties of the marine deposits b
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Archival DataAll records from the s
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Chapter 2Outline of GeologyThe soli
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The Mesozoic volcanic rocks are div
Page 29 and 30: Chapter 3Palaeozoic Sedimentary Roc
Page 31 and 32: Carboniferous metasedimentaiy rocks
Page 33 and 34: Plate 1 - Fining-Upward Sequence of
Page 35 and 36: Plate 3 - Well-bedded Sandstones an
Page 37 and 38: Reef Island. The northern end of th
Page 39 and 40: Table 3. Evolution of Nomenclature
Page 41 and 42: are crudely stratified and welded i
Page 43 and 44: Shing Mun FormationThe type localit
Page 45 and 46: Tai Che Tung. Crystal tuff, interpr
Page 47 and 48: Plate 7 • Lapilli-Ash Crystal Tuf
Page 49 and 50: Plate 11 - Pyroclastic Breccia (086
Page 51 and 52: appears fragmental, and contains mu
Page 53 and 54: exposures in the stream bed consist
Page 55 and 56: Lin Fa Shan. The steep eastern flan
Page 57 and 58: dipping northwest at 46°, is prese
Page 59 and 60: Sunset Peak MemberThe Sunset Peak M
Page 61 and 62: Plate 16- Block and Lapilli-bearing
Page 63 and 64: Chapter 5Major IntrusionsClassifica
Page 65 and 66: in grain size from 0.4 to 1.5 mm, a
Page 67 and 68: 20 - Megacrystic Medium-grained Gra
Page 69 and 70: Pui O Wan. Porphyritic medium-grain
Page 71 and 72: minimum age of c. 145 Ma (see earli
Page 73 and 74: The granite on Chek Lap Kok is most
Page 75 and 76: Southeast of Tin Sam, on the wester
Page 77 and 78: About 1 km east of Nam Shan, exposu
Page 79: Plate 25 - Thin Section of Porphyri
Page 83 and 84: The microgranite grades southwards
Page 85 and 86: the Lantau Formation, however, and
Page 87 and 88: Plate 29 - Flow-banded Quartzphyric
Page 89 and 90: also cut volcanic rocks of the Tsue
Page 91 and 92: Silver Mine Bay. On the headland so
Page 93 and 94: Plate 33 - Core stone Development i
Page 95: Plate 37 - Thin Section ofLamprophy
Page 98 and 99: Pre-Yensfaanlan StructureWithin the
Page 100 and 101: separates areas of different lithol
Page 102 and 103: NeotectonicsFault activity in the r
Page 105 and 106: Chapter 8Metamorphism and Alteratio
Page 107 and 108: the majority of rocks in the distri
Page 109: Chapter 9Superficial GeologyIntrodu
Page 112 and 113: Detailstal O. Alluvial deposits for
Page 114 and 115: trains. The regolith on most steep
Page 116 and 117: Plate 44 - Boulder Debris on the So
Page 118 and 119: gazetted for recreational purposes.
Page 120 and 121: DetailsTai O. Extensive intertidal
Page 122 and 123: Plate 48 - Sandy Beach at Fan Lau S
Page 124 and 125: Pre-Chek Lap Kok Formation Deposits
Page 126 and 127: formation. The sequence is usually
Page 128 and 129: BOREHOLE NO: ESC 17GRID REFERENCE:
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BOREHOLE NO: A5/1GRID REFERENCE: 80
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formation, thinly laminated clays o
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Quinqueloculina seminulum, which is
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Channel and Transgressive Deposits.
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Figure 20 - Areas of Acoustic Turbi
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West Lamma Channel These may be att
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Figure 23 - Depth-Averaged, Maximum
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The formation is Holocene in age. T
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Boreholes (A5/1, A5/2, B2/1, ESC17
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Plate 51 - Deep Weathering Profile
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Chapter 10Economic GeologyIntroduct
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Non-metalliferous MineralsFissure v
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REFERENCESAddison, R. (1986). Geolo
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James, J.W.C. (1993). The offshore
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Sewell, R.J., Strange, PJ., Langfor
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Appendix 1Microfossils identified f
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Appendix 2 (continued)Sample HK9913
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Appendix 2 (continued)Sample HK1054
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Appendix 4aSelective normalized pal
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,Appendix 5 -Radiocarbon and Other
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construction materials 151contact m
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molybdenite 149Mong Tung Hang 151Mo
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talus 107,115Tarn Tsui Wan 71,87Tan
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LB 555.125 G34 LGeology of Lantaii
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