Classification and Distribution The intrusive igneousrocks of the district comprisemajor intrusionsof granodioriteand granite,and mi- nor intrusions of porphyritic microgranite,feldsparphyric rhyolite, quartzphyricrhyolite and mafic dykes. Compositionalclassification is basedon Le Maitre (1989) (Figure 8). On the basis of the nomenclature adoptedfor the 1:20 000 mappingprogramme (Strange, 1985), the major intrusivesare divided according to textural characteristicsinto medium- and fine-grainedlithologies. Recentdetailed geochemicaland mineralogicalstudies of the graniteson Tsing Yi haverevealed that the medium-grainedgranites are dis- tinct from the Sha Tin and Needle Hill granitesand probably representa separatepluton. Considerable textural variation is shownby the minor intrusive rocks,particularly the feldsparphyricrhyolites. In some of the largerdykes, there is completegradation from stronglyfeldsparphric textures at the marginsto por-phyritic microgranitetextures in the cores.Quartzphyric rhyolite dykes may also show textural variation from quartzphyricmargins to feldsparphyriccores.

Major Intrusions Granodiorite Porphyritic medium- to fine-grainedgranodiorite is exposedin the northeasternand westernparts of the district (Plate 6). The granodioriteis the oldestintrusive unit, althoughno absoluteages have beendeter-mined. The granodiorite is part of a large, sheet-likebatholithic body known as Tai Po Granodiorite

Q Q = quartz A = alkali feldspar, including albite (AnO-An5) P = plagioclase feldspar, (An5-An100) M < 90% (mafic minerals)

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ALKALI FELDSPAR GRANO-\ GRANITE GRANITE DIORITE alkali feldspar I rhyolite dacite rhyolite J rhyod,acite (SYENO- (MONZO- GRANITE) GRANITE) 20, 20 QUARTZ QUARTZ \ SYENITE MONZONITE DIORITE, GABBRO quartz trachyte quartz latite andesite, basalt 5, \ ~ 5 latite 10 35 65 90 Figure 8 -Generalized Classificationand Nomenclatureof SelectedMajor and Minor Intrusive Rocks (after Le Maitre, 1989)

25 Plate 6 - Porphyritic Fine-grained Grano- diorite Exposedat Tsing Yan (27802430)

which underlies much of central and southern Hong Kong. The granodiorite is typically porphyritic with large euhedral phenocrysts of alkali feldspar set in a medium- to fine-grained matrix of quartz, alkali feld- spar, plagioclase, biotite, and minor amphibole (Plate 7). Accessory minerals include zircon, titanite and apatite. Compositionally, the granodiorite is strongly calcalkaline and varies from 63 to 68 wt% SiOz.

Medium-grained Granite The medium-grainedgranite of Tsing Yi are compositionallydistinct from the Needle Hill and Sha Tin granitesto the north and east.The Tsing Yi graniteforms a subcircularpluton centredon Tsing Yi and is inferred to intrude the Sha Tin granite immediatelyto the east.In centralTsing Vi, the pluton intrudesthe Tai Po granodioriteand rocks of the TsuenWan Volcanic Group. The Tsing Yi granite is intruded by the Needle Hill granite in the vicinity of TsuenWan to the northeast.Non-porphyritic medium-grained gran- ite is the principal lithology in centraland southernparts of the district (Plate 8), although porphyritic texturesare well-developedclose to the contactwith tuff and in the southwest.The granite is character- ised by equalproportions of quartz,alkali feldsparand plagioclase,with biotite asthe chief mafic mineral. The medium-grainedgranite is deeplyweathered in areaswhere the protective cover of tuff has beenre- moved by erosion. Quartz- and feldspar-richresidual soils are the typical weatheringproducts on the lower ridge crestsof centraland southernTsing Vi.

Intrusions IVThe minor intrusions of the district comprise feldsparphyric rhyolite, porphyritic microgranite, quartzphy-ric rhyolite, fine-grained granite and basalt. Most of the felsic dykes are oriented in a WSW-ENE direc- tion, whereas the mafic dykes tend to be variably oriented.

Minor26

Plate 9 -Feldsparphyric RhyoliteExposed at Sai Tso Wan(27562279), Hammer Head 2 cm.

eldsparphyricRhyolite By far the commonestdyke rock encounteredis feldsparphyricrhyolite (Plate 9). The dykes may be up to 5 m thick and may grade laterally into porphyritic microgranite. The margins of the dykes tend to be strongly porphyritic, whereasthe more slowly-cooledinterior is lessporphyritic.

Porphyritic Microgranite Porphyritic micrograniteis a textural variant of feldsparphyricrhyolite but the matrix grainsizeis coarser. In general, porphyritic microgranite dykes are wider than feldsparphyricrhyolite dykes and frequently exhibit prominentbiotite phenocrystsin additionto feldsparand quartz.

Quartzphyric Rhyolite Quartzphyricrhyolite dykesare mostlyfound in the centraland northernparts of the district and post-date emplacementof the feldsparphyricand porphyritic microgranitedykes. These dykes commonly have flow-bandedmargins and may grade laterally into fine-grainedgranite. Quartzphenocrysts are typically bipyramidal. Fine-grained Granite Fine-grained granite occurs chiefly in dykes and sills intruding the older major intrusive bodies (Plate 10). In central Tsing Vi, a 20 m-thick fine-grained granite dyke fed a large sill along the interface between medium-grained granite and tuff of the Vim Tin Tsai Formation. The fine-grained granite is chemically similar to the Needle Hill granite and in thin section is characterised by granophyric textures. Accessory minerals include zircon, monazite, fluorite, and apatite.

Mafic Dykes Mafic dykes are mostly of basaltic andesitecomposition and may be up to 7 m thick. A particularly prominentmafic dyke is presentat the Tsing Ma Bridge anchoragesite (Plate 11). It comprisesa 7 m- thick, E- W trending basaltic andesitedyke with evidenceof minor faulting along the margins.The mafic dykes form the youngestintrusions of the district, althoughthey maybe partly coeval with the quartzphy- ric rhyolite dykes. Absolute agesfor the mafic dykeshave still to be determined,although elsewhere in the Territory Chandy& Snelling (1971)reported K-Ar agesof around65 Ma.

F28 Plate J0 -Fine-grained Granite Sill Exposedabove Tsing Yi Road West(282J 2258)

Plate 11 -Basaltic Dyke Exposedat the Site of the TsingMa Bridge Anchorage,Shek Wan (27002381)

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