Plate 22 - Porphyritic Medium-grained Granite (0860 1170), on the North Side of Chi Ma Wan** , *f25 - Porphyritic Medium-grained Granite (085 050), Siu A Chau , Soko Islands
Pui O Wan. Porphyritic medium-grained granite crops out north of the Chi Ma Wan Road. The granite is cut byfeldsparphyric rhyolite dykes and quartz syenite to the north. In deeply weathered areas, the granite produces a finegravel soil composed of quartz crystals. In fresh exposures, it is pinkish-grey and white, with pinkish feldspar up to20 mm, white plagioclase between 1 and 5 mm, and pools of quartz aggregates less than 10 mm. Biotite, occurringas single flakes, is 1 to 3 mm.Chi Ma Wan Peninsula. On the north side of Chi Ma Wan, there are exposures of coarsely porphyritic,medium-grained granite with pegmatitic patches. Dark aphanitic xenoliths are present but scarce. The granite ispinkish-grey, with white plagioclase and prominent black biotite. Tabular phenocrysts of grey alkali feldspar are upto 40 mm long. On the beach to the east of Shap Long San Tsuen (1746 1162) there are many boulders of altered,brecciated and veined medium-grained granite, which have probably been derived from a deeply erodednorthwest-trending fault that extends along the adjacent valley.Pinkish grey and white, porphyritic medium-grained granite, with prominent biotite crystals, from 1 to 3 mmacross, is exposed on the ridge (1668 1140) north of Shap Long Kau Tsuen. Although most of the granite in thisarea is weathered to a coarsely granular soil, this contrasts with the clayey soil of the quartz syenite andfeldsparphyric rhyolite dykes. The granite also contains thin aplite dykes and occasional, thin quartz veins. Theseexposures contrast with the outcrop south of unaltered, pink, equigranular medium-grained granite forming thebedrock of Chi Ma Wan Peninsula.North of Mong Tung Wan, the granite is pinkish grey, equigranular and medium-grained. Black biotite occurs assingle flakes, or in small aggregates offtakes; the individual crystals ranging up to 4 mm. Most of the crystals are 2to 8 mm, although there are some pink feldspar megacrysts up to 20 mm. Dark xenoliths of quartz microdiorite arecommon in this area. They are typically rounded, with a diameter of 0.1 to 0.2 m, but can be up to 0.3 m. Aplitedykes, usually around 0.2 m wide, are also common.On the coast northeast of Lo Yan Shan a rapid textural change can be seen, with equigranular medium-grainedgranite to the south and a finer variety to the north. The mean crystal size is 2 to 3 mm, with biotite flakes less than2 mm. The rock is mapped as fine-, to medium-grained granite, although locally it may be medium-grained.Tung Chung. Porphyritic medium-grained granite has been recorded in offshore boreholes immediately south ofChek Lap Kok. Exposures of similar granite occur on the coast northwest of Lau Fau Sha. The rock is characterisedby feldspar megacrysts, in places up to 25 mm long. Dykes of rhyolite and basalt cut the granite, which is oftenfractured and veined with quartz. Boreholes immediately offshore from Lau Fau Sha have intersected granite withskara mineralization and fault brecciation, cut by feldsparphyric rhyolite dykes. Although no contact has beenobserved between the marble-bearing basement and the granite, it is presumed to be faulted.Sha Chau. The island of Sha Chau and neighbouring Tree Island are dominantly composed of greyish-white,equigranular, medium-grained granite. Part of the outcrop, at the northern end of Sha Chau, includes megacrysticfine-grained granite, but field relations between the varieties cannot be seen. The granite is cut by small aplite andbasalt dykes, but the most prominent cross-cutting feature is a zone of rnylonitization and quartz veins which trendsnortheast across the island. A 4 to 5 m-wide zone of mylonitised granite is well exposed on the west coast (06622292), and can also be seen on the northeast coast (0680 2309).Peng Chau. Exposures of medium-grained granite on Peng Chau are generally restricted to slivers betweenfeldsparphyric rhyolite dykes. However, more massive outcrops of granite (2240 1630), displaying distinctivetextural variations, are present south of Tung Wan. Boreholes sunk for the indoor recreation centre and library aredominated by pinkish, megacrystic, medium-grained granite. The rock is characterized both by abundant pinkmegacrysts of feldspar, some up to 30 mm across, and black biotite. Some sections of core (D/004/11189, 2195816206) also contain fine-grained granite and pegmatite patches, which are thought to be younger intrusions withinthe relatively uniform, medium-grained granite.On the north side of the island, there is a progressive transition over 100 m through megacrystic andpegmatite-bearing, fme-grained granite, to fine-grained granite (2243 1635), The fine-grained granite is pinkishgrey with no obvious megacrysts and an equigranular groundmass whose grainsize is less than 1 mm. Texturalvariation also occurs along the coast to the east, as far as the island of Ngan Chan, contrasting with the porphyritic,medium-grained granite with pegmatite and aplite that is typically exposed at Nam Wan in the south.Sunshine Island (Chau Kung To). Exposures of granite are rare along the coast of Sunshine Island where thedominant rock is feldspaiphyric rhyolite. The granite is porphyritic and medium-grained and mainly occurs asslivers between rhyolite dykes. Feldspar megacrysts up to 25 mm long, and biotite flakes from 1 to 3 mm across,are locally abundant and patches of pegmatite sometimes occur with aplite dykes. At one locality (2368 1398) onthe north coast, the medium-grained granite is cut by, and partially assimilated by younger, fme-grained granitedykes,67
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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
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Chapter 10 Economic Geology 149Intr
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(Plates)Plate 22 - Porphyritic Medi
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Table 1 — Grain Size Description
- Page 18 and 19: 0 1 2 3 4 5 k mCONTOURS AT 100m INT
- Page 20 and 21: properties of the marine deposits b
- Page 22 and 23: Archival DataAll records from the s
- Page 24 and 25: Chapter 2Outline of GeologyThe soli
- Page 26 and 27: 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
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- Page 73 and 74: The granite on Chek Lap Kok is most
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- Page 81 and 82: Chapter 6Minor IntrusionsIntroducti
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- Page 85 and 86: the Lantau Formation, however, and
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- Page 91 and 92: Silver Mine Bay. On the headland so
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- Page 95: Plate 37 - Thin Section ofLamprophy
- Page 98 and 99: Pre-Yensfaanlan StructureWithin the
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- Page 102 and 103: NeotectonicsFault activity in the r
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- Page 107 and 108: the majority of rocks in the distri
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gazetted for recreational purposes.
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DetailsTai O. Extensive intertidal
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Plate 48 - Sandy Beach at Fan Lau S
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Pre-Chek Lap Kok Formation Deposits
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formation. The sequence is usually
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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