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Chapter 5Intrusive Igneous RocksClassification and DistributionThe intrusive igneous rocks of the district comprise major intrusions of granite and quartz monzonite, andminor intrusions of porphyritic microgranite, feldsparphyric rhyolite, quartzphyric rhyolite, aplite, quartzveins and mafic dykes. Compositional classification is based on Le Maitre (1989) (Figure 7). The majorintrusions are divided into medium-grained, fine- to medium-grained, and fine-grained lithologies. On thebasis of geochemical and mineralogical studies, the granites may be further divided into separate plutons orintrusive units. These plutons form the host rock to a swarm of rhyolite dykes (Lantau Dyke Swarm) thatdominates the geology in northeastern Lantau Island. Abundant textural variation is present within bothmajor and minor intrusions.Major IntrusionsGraniteTwo main varieties of granite are present in the district; megacrystic fine- to medium-grained and mediumgrainedgranite belonging to the Lantau Granite, and fine-grained granite belonging to the Chek Lap KokGranite (Sewell et al., 2000). The Lantau Granite crops out intermittently across the northern part of thedistrict between Pak Mong and Sha Lo Wan, whereas the Chek Lap Kok Granite crops out principally alongthe coast west of Tin Sam. U-Pb dating of zircon crystals has returned ages of 161.5 ± 0.2 Ma for the LantauGranite, and 160.4 ± 0.3 Ma for the Chek Lap Kok Granite (Davis et al, 1997). Both granites predateemplacement of the Lantau Dyke Swarm.Q = quartzA = alkali feldspar,including albite(AnO - An5)P = plagioclase feldspar,(An5-An100)QALKALI FELDSPARGRANITEalkali feldsparrhyolite(SYENO-GRANITE)QUARTZSYENITEquartz trachyte(MONZO-GRANITE)QUARTZMONZONITEquartz iatite2010/SYENITE trachyte jMQNZONITE Iatite35 65 90Figure 7 ~ Generalized Classification and Nomenclature of Selected Major and Minor Intrusive Rocks (afte{ JLe Maitre, 1989)28
Lantau GraniteThe Lantau Granite is typically megacrystic medium-grained with large megacrysts of pink alkali feldsparset in a granular matrix of quartz, plagioclase, alkali feldspar, amphibole and aggregates of biotite. Nonporphyriticfine- to medium-grained lithologies are also present (Plates 3 and 4). Accessory minerals includeconcentrically zoned allanite, apatite, titanite, zircon, rare fluorite, and Fe-Ti oxide.Euhedral to subhedral K-feldspar megacrysts (5-15 mm) are dominantly composed of perthitic orthoclasewith subordinate microcline. Two generations of plagioclase crystals (25 - 30%) are commonly present:strongly concentrically zoned species and relatively unzoned species. The strongly zoned plagioclase grainsare euhedral (2-5 mm) to subhedral with andesine-rich cores and oligoclase- to albite-rich rims. The coresof these crystals are commonly altered to sericite and sharp boundaries often exist between successivegrowth phases. In some rocks, residual cores of extensively altered anhedral alkali feldspar are mantled byconcentrically-zoned plagioclase. Unzoned to weakly zoned plagioclase is generally euhedral and composedmostly of oligoclase and albite. The cores are often weakly sericitized. Anhedral quartz (30 - 35%) showsundulose extinction showing that it is typically strained. Yellowish brown to green biotite usually comprisesbetween 2 - 10% of the mode and is sometimes accompanied by subordinate greenish brown amphibole(3-5%).Plate 3 - Fine- to Medium-grained Lantau Granite from a Borehole South of Ha Law Wan (1131 1716)V7Chek Lap Kok GraniteThe Chek Lap Kok Granite (Sewell et al, 2000) forms a subcircular pluton centred on the airport at ChekLap Kok. Prior to construction of the airport, the unit was exposed on the western half of the island Chek LapKok and on Lam Chau (Langford et al, 1995). Exposures are now confined to the area between Tin Sam andHau Hok Wan on the northern coast of Lantau Island. The type locality is designated as Hau Hok Wan.On the former island of Chek Lap Kok, the Chek Lap Kok Granite was seen (Langford et al, 1995) tointrude porphyritic fine- to medium-grained Lantau Granite. This relationship is still exposed on the northerncoast of Lantau Island at Hau Hok Wan indicating that the Chek Lap Kok Granite post-dates emplacementof the Lantau Granite. The U-Pb age-dating results are consistent with the observed field relationships.The Chek Lap Kok Granite consists of an equigranular, fine-grained, leucocratic monzogranite (Plate 5).Euhedral to anhedral alkali feldspar is composed principally of mesoperthite. Subordinate euhedral to subhedralalbite is unzoned and anhedral quartz is weakly strained. Interstitial late stage muscovite is commonly presentalong with trace amounts of euhedral fluorite. Biotite, zircon, and Fe-Ti oxide may be present in trace amounts.29
Page 2 and 3: THE UNIVERSITY OF HONG KONGLIBRARIE
Page 4 and 5: ,11 DEC'£• Government of the Hon
Page 7: CONTENTSPageTitle Page 1Foreword 3C
Page 10: 11 Location Map Showing the Distrib
Page 18 and 19: Seismic reflection line8Onshore are
Page 20 and 21: Table 2 - Summary of the Onshore an
Page 23: These faults offset the caldera bou
Page 27 and 28: Chapter 4Mesozoic Sedimentary and V
Page 29: Tuffite. A prominent outcrop of ban
Page 33 and 34: " .-.": •' - • • *"» JI i I
Page 35 and 36: Quartz VeinsQuartz veins are ubquit
Page 37 and 38: LEGENDOoI2PhotolineamentFaultConcea
Page 39 and 40: No. of Joints measured=82No. of Joi
Page 41 and 42: Chapter 7Metamorphism and Alteratio
Page 43 and 44: Offshore superficial deposits withi
Page 45 and 46: Plate 8 - Tung Chung Wan from the N
Page 47 and 48: Sham Wat FormationThe formation, kn
Page 49 and 50: to infill depressions and channels
Page 52: ased on recommendations contained i
Page 57 and 58: over a wider area. The survey resul
Page 59 and 60: Chapter 10Economic GeologyEconomic
Page 61 and 62: ReferencesAllen, P.M. & Stephens, E
Page 63 and 64: Li, X.C., Sewell, RJ. & Fletcher, C
Page 65 and 66: microgranite, 17, 28, 31, 32, 48 Tu
Page 67 and 68: Marine sedimentDescriptionUnconsoli
Page 73: Plate Ala- Marble. The generally da
Page 76 and 77: Plate A9 - Skarn from Site 3 showin
Page 78 and 79: 1 8Plate Alia - Cavity-fill deposit
Page 80 and 81:
Plate A12b - Pervasively weathered
Page 82 and 83:
RhyoliteDescriptionAcid igneous roc
Page 84 and 85:
Plate A 16a - Pervasively weathered
Page 86 and 87:
1 f < 1 9 2 OPlate A 18b - A Itered
Page 88 and 89:
2 1 2 2Plate A21 - Lamprophyric roc
Page 90 and 91:
Residual soil derived from metasedi
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