Mafic and intermediate intrusive rocks are widespread throughout Hong Kong, cutting many plutonic and volcanic lithologies. They generally crop out as narrow (<1 m wide) dykes (Plate 6.27) but may also occur as small stocks and enclaves in granitoid rocks.
The dykes are dominantly sub-alkaline in composition and can be divided into two main groups. The oldest group comprises dominantly high-K calc-alkaline basaltic andesites to dacites which are found closely associated with rhyolitic dykes of the Lantau Dyke Swarm. In a detailed study of a 20 m-wide zoned composite dyke from the Lantau Dyke Swarm, Li et al . 2000 have showed that the margins of this dyke are composed of basaltic andesite (SiO 2 = 54.5 wt%) which then pass inward through a series of sharp transitions to feldsparphyric dacite (SiO 2 = 69.2 wt%) and high silica rhyolite- porphyritic microgranite (SiO 2 = 76.2 wt%) in the core. Each pulse of new magma injected into the dyke is marked by a more silica-rich composition, and is sometimes accompanied by incorporation of rounded, semi-consolidated inclusions derived from an earlier pulse. The basaltic andesite margins of the dyke are volumetrically minor compared with the rest of the dyke, which is dominantly rhyolitic.
Elsewhere within the Lantau Dyke Swarm, similar composite dykes are seen, although in some cases, only thin (1 m-wide), solitary, basaltic andesite dykes subparallel to the main swarm are present. However, geochemical analyses of these solitary mafic dykes indicate that they lie on exactly the same fractionation trend as those dykes described above.
A younger group of dykes are shoshonitic in composition with higher average abundances of K 2 O, and lower TiO 2 contents, than the older mafic to intermediate dykes. Included among this group are a number of calc-alkaline lamprophyres. These dykes commonly crop out over much of eastern and western parts of Hong Kong. They include a prominent dyke intruding columnar-jointed vitric tuff of the High Island Formation at the East Dam of the High Island Reservoir (see Plate 9.13). Ar–Ar age-dating of the younger group of dykes has returned ages ranging from 108.3 ± 1.6 Ma up to 75.1 ± 0.8 Ma (GEO, unpublished data).
The younger mafic dykes are typically very fine-grained and aphyric, with a groundmass composed of augite ± amphibole ± biotite, and subordinate feldspar and Fe-Ti oxide.
Mafic enclaves are present in several plutonic units including the Lantau Granite, Sok Kwu Wan Granite, and D'Aguilar Quartz Monzonite. These medium-grained rocks are commonly depleted in K 2 O due to alteration and are classified as quartz diorites.
Details
Mafic dykes
Jurassic mafic dykes
Kwai Chung. Around Kwai Chung there are a few mafic dykes up to 2 m wide. North of Cheung Shan Estate (831240 826700 mnd-1 and 831660 826770 mnd-2) they intrude tuffs of the Tsuen Wan Volcanic Group, whereas at Shek Lei they intrude fine grained granite (832200 824740 mnd-3). At Kwai Shing, in road cuts near the Texaco Road roundabout (830120 824780 mnd-4), a mafic dyke carries xenocrysts of quartz and feldspar up to 15 mm long. This dyke is also cut by quartz veins.
Tsing Yi. South of Cheung Shue Tau (827750 824320 mnd-5), in deeply weathered temporary exposure, an irregular mafic dyke up to 2 m wide has been intruded along the contact between fine-grained granite to the north and porphyritic fine grained granodiorite to the south. The dyke is displaced sinistrally about 5 m by two small north trending faults.
Wide mafic dykes occur at only two localities on Tsing Yi. Around Lo Uk (828160 823760 mnd-6), mafic rock was intersected in several site investigation boreholes, and a substantial intrusion is inferred to strike northeasterly. To the south, in cut-slopes (827670 822820 mnd-7) north of Sai Tso Wan, a dyke about 8 m wide, with a northeasterly strike cross-cuts southeasterly- trending dykes of fine-grained granite and feldsparphyric rhyolite. In both cases, the mafic dykes vary texturally to fine-grained gabbro in their cores, and it is possible they are parts of the same extensive intrusion.
Elsewhere on Tsing Yi, mafic dykes form only minor bodies. Thin dykes crop out in shear zones on the cut-slopes (827900 822410 mnd-8) west of Sai Tso Wan, and intersect quartzphyric rhyolite at Kam Chuk Kok (827000 823040 mnd-9). On the headland west of Shek Wan, several thin dykes intrude tuffs and quartzphyric rhyolite, and they are accompanied by a narrow zone of Fe-Cu-Pb mineralization.
Ma Wan. On Ma Wan, mafic dykes are relatively numerous, but they are usually narrow, and intruded parallel to dykes of feldsparphyric rhyolite. Several occur on the west coast (823900 823000 mnd-10) of the island, north of Lung Ha Wan, where mafic dykes, 0.5-1 m wide, form the margins to rhyolite dykes which are up to 10 m wide. The same group of mafic and rhyolite dykes appears to be exposed along strike on the opposite coast of the island, at Tai Lung (824600 823270 mnd-11). It could be inferred that the rhyolite dykes are older than the mafic dykes, but at Lung Ha Wan (823900 822940 mnd-12), they contain mafic xenoliths, although these in turn contain xenocrysts of alkali feldspar. Hence, in some cases, the two magmas could have been generated broadly at the same time.
Other minor mafic dykes on Ma Wan, occur around Nam Wan (824350 822700 mnd-13), where they intrude along the contact zone of the tuffs and the fine-grained granite. Thin dykes also occur in the north of the island, around Tam Shui Wan (823920 823780 mnd-14) and Pak Wan (824500 823950 mnd-15), where they intrude tuffs of the Yim Tim Tsai Formation.
Sha Lo Wan. A mafic dyke, over 10 m wide and trending eastnortheast, is exposed (807350 816970 mnd-16) on the coast west of Sha Lo Wan. It is mafic-rich, has an aphanitic margin, and contains abundant hornblende, up to 4 mm long, set in a felsic groundmass (HK9309). This mafic dyke is cut by a thin aplite dyke. About 90 m to the south, an easterly-trending mafic dyke, about 2.4 m wide, cuts the megacrystic, fine-grained granite. It contains needle-like phenocrysts of amphibole, and quartz amygdales up to 7 mm across, set in a glassy groundmass.
A garnet-bearing, dark green, metabasic dyke crops out parallel to the eastnortheast-trending contact between megacrystic fine-grained granite and Jurassic pre-volcanic sedimentary rocks at San Shek Wan (807120 816130 mnd-17). The dyke, which intrudes granite, is about 1 m wide. It comprises pseudomorphs after feldspar, up to 3 mm long, set in an aphanitic groundmass. The garnet crystals indicate skarn-type mineralization, and are up to 3 mm across.
Tin Sam. A greenish-grey, eastnortheasterly-trending mafic dyke, 1.5 m wide, cuts weathered, fine- to medium-grained granite on the ridge (809930 816280 mnd-18) south of San Tau. The mafic rock is poorly foliated, sub-parallel to the dyke margins, and also contains pyrite.
On the coast (809570 817050 mnd-19) northeast of Tin Sam, there is a northnortheast-trending mafic dyke that cuts fine-grained granite. The dyke is 0.6 to 1 m wide, and contains quartz amygdales and small garnet crystals. A small xenolith of altered garnetiferous granite was also noted.
An eastnortheast-trending mafic dyke cuts fine-grained granite country rock on the east side of Hau Hok Wan (808920 816910 mnd-20). It is 2.4 m wide, and greenish-grey, with black augite phenocrysts, up to 3 mm across, set in a felsic groundmass. 80 m to the southwest, there is a dark green, aphanitic, mafic dyke, 0.4-1 m wide, which bifurcates towards the north. It trends northeastwards and is closely jointed.
Tung Chung . Northwest of Tung Chung (813740 817510 mnd-21), near Kei Tau Kok, a bifurcated, 0.1 to 0.3 m wide mafic dyke cuts feldsparphyric rhyodacite dykes. The dyke is grey and aphanitic, and dominantly consists of plagioclase and epidote, with some interstitial quartz (HK8576). It trends eastnortheast, parallel to the dominant trend of the older East Lantau Rhyodacite.
Pak Mong. Along the coast (814620 817670 mnd-22) west of Pak Mong, there is an exposed mafic dyke, 0.3 m wide, trending eastnortheast, and parallel to the porphyritic microgranite country rock. The mafic rockis dark greenish- to bluish-grey, partially epidotized, and varies from aphanitic margins to a porphyritic centre. Feldspar phenocrysts are up to 10 mm long, and quartz xenocrysts are up to 4 mm. There are also chloritized mafic phenocrysts, set in a groundmass dominated by small feldspar laths, but including chlorite and opaque minerals (HK8573).
On a ridge about 1 km southwest of Pak Mong, there is a bluish-grey mafic dyke, which intrudes feldsparphyric rhyolite. The dyke, whose thickness cannot be determined, probably trends east, varying to southeast, across the ridge. Petrographically, the dyke is not porphyritic, and has a well-developed, sub-ophitic texture of plagioclase and augite. It also contains chlorite and opaque minerals, and small quantities of interstitial quartz. Minor amounts of pyrite can be seen in hand specimens (HK8318).
Yam O Wan. Rarely, mafic dykes intrude the tuffs and rhyolite dykes around Yam O Wan. They do so especially at the contact between these two rock types or, between two rhyolite dykes. For example, a 0.3 m wide mafic dyke (820940 821320 mnd-23) occurs along the contact between two northeast-trending feldsparphyric rhyolite dykes on the east side of Yam O Wan.
In the hills east of Yam O Wan, there is a 300 m long, 20 to 30 m wide basalt, or lamprophyre dyke (821700 821200 mnd-24). The hillside is strewn with debris from the dyke, which cuts both tuffs and a rhyolite dyke. The rock is light grey, with a thin, light brown-weathering rind around exhumed corestones. Petrographically, the rock contains abundant mafic minerals and subordinate plagioclase laths, and includes a small proportion of quartz in the groundmass. The dyke trends roughly east-west, parallel to the dominant dyke trend in the area.
Tai Che Tung. A large, easterly-trending maficdyke, crops out on the summit of Tai Che Tung (818800 819500 mnd-25). The greenish-grey dyke is only seen as hard, well-rounded corestones with exfoliation shells. It contains prominent feldspar laths, up to 1 mm, set in a groundmass with a grain size of around 0.1 mm. The dyke cuts feldsparphyric rhyolite and porphyritic microgranite dykes, which also trend dominantly east to eastnortheast.
Discovery Bay. There are few mafic dykes exposed in the Discovery Bay area. Slope formation in 1989, for the Phase IV development at Hai Tei Wan, exposed a deeply weathered, 2 m-wide mafic dyke, trending eastnortheast (820610 817410 mnd-26). The rock is bluish grey when fresh, brown when weathered, and cuts the East Lantau Rhyodacite.
Hei Ling Chau. Mafic dykes have been recorded at only two localities on Hei Ling Chau. One trends eastnortheast, and is only 0.5 m wide. It is foliated, and contains small granite xenoliths (822420 813280 mnd-27). The other (822630 812640 mnd-28) trends eastsoutheast and is 2 m wide. Both dykes cut the East Lantau Rhyodacite.
Silver Mine Bay. On the headland southeast of Silver Mine Bay, a 15m-wide mafic dyke cuts feldsparphyric rhyolite dykes. The mafic dyke is exposed on the beach and probably lies in the east- to northeast-trending depression behind the beach. The slightly weathered exposures comprise hard, greenish-brown, equigranular basalt, with a grain size of less than 0.5 mm.
At Man Kok Tsui, at the eastern end of Silver Mine Bay, a wide mafic dyke intrudes feldsparphyric rhyolite. The basalt is hard and bluish-grey when fresh, and weathers to form well-developed corestones in a brown, clayey matrix (Plate 6.A45). The dyke, which trends northeast, is up to 10 m wide.
Peaked Hill (Kai Yet Kok). On the northern shores of Peaked Hill - a small island west of Lantau Island, a thin, foliated mafic dyke cuts an irregular, impersistent, quartzphyric rhyolite dyke. Both dykes cut the medium-grained granite. The basalt trends southeast and is about 0.1 m wide.
Soko Islands. West of Siu A Chau Wan (808510 804570 mnd-29), there is a 5 m wide, northerly-trending mafic dyke cutting coarse-grained granite. The dyke is grey when fresh, and has a sharp, non-foliated margin to the west, and a fractured margin to the east. Petrographically, the rock (HK9562) is dominated by laths of sericitized plagioclase, but also contains augite, biotite, chlorite and opaque minerals. The grain size is generally 0.1 to 0.5 mm, although there are some mafic phenocrysts up to 2 mm across and some secondary pyrite crystals. The even-textured groundmass also contains xenocrysts of quartz, up to 6 mm long.
A 5 m-wide mafic dyke, trending approximately northwards, is exposed on the north coast of Ma Chau (807570 803750 mnd-30), a small island southwest of Siu A Chau. The dyke, dipping 80o to the west, comprises fresh, dark grey corestones set in a deeply weathered matrix. The grain size of the mafic is mostly less than 0.2 mm, and it displays a good ophitic texture in thin section (HK10018).
Chi Ma Wan. Mafic dykes have been noted at a few localities on the Chi Ma Wan peninsula. Most are in the east of the area, where the mafic dykes cut both rhyolite dykes and granite. At Ha So Pai (819840 809680 mnd-31), a 0.6 m-wide, eastnortheast-trending mafic dyke intrudes equigranular, medium-grained Chi Ma Wan Granite. The dyke is parallel to a feldsparphyric rhyolite dyke to the north. A thin sliver of medium-grained granite, 0.3 to 1 m wide, occurs between the two dykes.
Near the easternmost tip of the peninsula (819980 810090 mnd-32), an 8 m-wide, dark, greenish-grey mafic dyke cuts a quartzphyric rhyolite dyke and medium-grained granite. The mafic dyke lies both along the contact between these two older rocks, and cuts across them. The grain size of the mafic rock ranges from 0.1 to 1 mm, and feldspar laths and augite crystals form a well-developed sub-ophitic texture (HK8351, Plate 6.A46). The rock also contains quartz amygdales, or xenocrysts.
Cheung Chau. A few easterly-trending mafic dykes, cutting medium-grained granite, crop out on the eastern coast of Cheung Chau, and a bluish-grey, aphanitic to very fine-grained mafic dyke, about 1.7 m wide, is exposed on the south coast near Chi Ma Hang (822110 807580 mnd-33). North of Coral Beach (Tung Wan Tsai), there are two, dark grey mafic dykes. The smaller of the two (821650 809090 mnd-34) is only 1.5 m wide, whereas the larger, exposed 110 m to the north, is about 10 m wide. Both dykes have a grain size of 0.05 to 0.1 mm, with plagioclase feldspar laths up to 0.3 mm long. Although there is some augite, the dominant mafic minerals in these slightly altered basalts are chlorite, epidote and opaque minerals.
Chek Lap Kok (prior to development). There are many small mafic dykes which cut granite, and rhyolite dykes on the northeastern end of the peninsula. A spheroidally weathered mafic dyke, 2.5m wide and trending southsoutheast (811560 818270 mnd-35), clearly cuts both megacrystic, fine-grained granite, and a flow-banded, quartzphyric rhyolite dyke that intrudes the granite. About 100 m to the north, a swarm of mafic dykes also intrudes the granite (811510 817380 mnd-36), as well as an easterly-trending feldsparphyric rhyolite dyke. These mafic rocks are dark greenish-grey, cariously weathered, mafic-rich, contain altered feldspar megacrysts, and are 0.5-1.5 m wide. Similar mafic dykes, about 0.5m wide, crop out on the north coast of Fui Yiu Wan. They trend variably east to eastsoutheast (Plate 6.A47, 812030 819640 mnd-37), following the dominant joint trend in the area. Two thin mafic dykes, about 0.3 m wide, and deeply weathered and foliated, cut fine-grained granite west of Fu Tei Wan. They are themselves cut by a small, sinistral fault (810830 818140 mnd-38), with an offset of about 0.3 m (Plate 6.A48). The dykes are also cut by an aplite dyke in the granite.
Lamma Island. At Sok Kwu Wan Quarry (831800 808320 mnd-39) large, angular and sub angular xenoliths of mafic rock are found in the granite, suggesting that at least some mafic rock predates the Upper Jurassic to Lower Cretaceous granites (Plate 6.A35).
Po Toi Island Group. Mafic dykes extending less than 200 m and less than 2 m wide cut granites on Lo Chau and Po Toi Island; they generally trend between ENE and NE. At Tai Pai (846050 803580 mnd-40) a number of mafic dykes cut the coarse-grained granite and fine-grained granite as well as faults and a quartzphyric rhyolite dyke.
Ma On Shan . Thin dykes have been recorded intruding the medium grained granite southeast of the Sha Tin valley in Tai Shui Hang (841980 828380 mnd-41), and at Wu Kai Sha (842500 831420 mnd-42).
Cretaceous mafic dykes
Tai Po. The mafic dykes of Tai Po are not well exposed in natural sections but can be seen in road excavations. Near Kam Shan (834100 834530 mnd-43) deeply weathered, thin ( c . 1 m) dykes intrude saprolitic granodiorite. At Wai Tau Tsuen (833220 835800 mnd-44) bridge excavations revealed a dyke about 50 m wide which was homogeneously aphanitic (HK 412). Thin dykes intrude siltstones and tuffs of the Tsuen Wan Volcanic Group in borrow areas south of Tai Po (835400 833420 mnd-45) and in roadside exposures at Tai Po Kau (836860 833170 mnd-46).
Kowloon. Mafic dykes have been mapped in several localities, especially in the Lai King (e.g. 831000 823410 mnd-47) and Kwun Tong areas. One large dyke cuts medium grained granite at Diamond Hill, and this can be traced over 1.4 km striking ENE. The roughly vertical dyke is best exposed in Diamond Hill Quarry (839060 822750 mnd-48, Plate 6.A41), where it can be seen as a series of sub parallel dykes which merge and divide, enclosing screens of granite. The total dyke width amounts to about 7 m. The margins are chilled for several centimetres, and the central part of the dyke is coarser grained and is classified as fine grained gabbro. An unusual characteristic of the mafic rock at Diamond Hill is the vesicular nature of part of the dyke. The thin section (HK424, 839060 822750 mnd-48, Plate 6.A42) shows a very fine, equigranular groundmass dominantly of plagioclase crystal laths, with partially altered mafic minerals and much chlorite. The dyke follows a dominant joint trend, as do the mafic dykes mapped in the Junk Bay Road area of Kwun Tong (842000 819080 mnd-49 & 842880 819410 mnd-50). There, a number of mafic dykes less than 1 m wide cut fine- and fine- to medium-grained granite and strike in a similar direction to the Diamond Hill dyke. Other mafic dykes in the Jordan Valley and Yau Tong area have a dominant northwesterly strike. In some places, dykes have been noted cutting volcanic rocks, but these are usually less obvious due to lack of colour contrast with their host rock. On the western shores of Junk Bay, near Lei Yue Mun Point (843280 816590 mnd-51), a mafic dyke about 2.5 m wide follows a N-S striking fault zone. This dyke rock was named a diorite lamprophyre or spessartite by Ruxton (1957). Mafic dykes intrude fine-grained granite at many localities; these were formerly well exposed in Junk Bay Road (842910 819450 mnd-52), where three prominent vertical dykes cut both the fine- and the fine- to medium-grained granite. Similar dykes were noted along the coast from Lei Yue Mun Point, northeastwards to Junk Bay, including a lamprophyre dyke following a fault trace striking 010o (843270 816600 mnd-53).
Hong Kong Island. Mafic dykes usually less than 1.5 m wide have been noted intruding granite near the Hong Kong Stadium (837380 814880 mnd-54), the Kornhill development at Quarry Bay (840230 815680 mnd-55), at Sai Wan Ho (840600 816080 mnd-56) and in Chai Wan (841970 813900 mnd-57). In all cases the dykes are impersistent and seldom extend more than 300 m. Mafic dykes have also been noted intruding the volcanic rocks in the Aberdeen Country Park (835470 813280 mnd-58) and near the Cape Collinson Training Centre (844190 812590 mnd-59). Mafic dykes cut volcanic rocks, granodiorite and quartzphyric rhyolite on the D'Aguilar Peninsula (844100 807700 mnd-60, 844500 808040 mnd-61 & 844700 807680 mnd-62). Allen & Stephens (1971) recognized vogesite lamprophyre at Cape D'Aguilar, but in this survey the separation of lamprophyre and basalt dykes, both very similar in appearance when weathered, has proved impractical. Where these dykes are exposed in the shoreline, deep erosion has taken place along the dyke, producing caves and in one case (844810 807740 mnd-63) a spectacular blowhole feature.
Lamma Island. Mafic dykes have been noted at several localities on Lamma Island, and in many cases the intrusions appear to be related to and follow faults. Southwest of Mount Stenhouse (830720 804760 mnd-64) a 3.5 m wide mafic dyke follows a major N-S striking fault, and 1 km to the east (831620 804960 mnd-65) another mafic dyke, 1.5 m wide, follows an important NNE striking fault.
Tsing Shan (Castle Peak). Mafic dykes are common features of the southern part of the Tsing Shan range. In most exposures they are weathered and their precise rock type is difficult to determine.
A swarm of foliated mafic dykes individually trending east to northeast, and associated with quartz veins, cuts the medium-grained granite and the fine- to medium-grained granite northeast of Tai Lang Shui (811500 829000 mnd-66). Mafic dykes cutting granite were reported during the site investigation and construction of the Castle Peak Power Station at Tap Shek Kok. The dykes were exposed trending mainly east west, and extend to the granite hills to the east. They are mostly less than 3 m wide, although a dyke up to about 15 m wide with an irregular trend was observed (810380 826310 mnd-67). Further east, near Mong Hau Shek (Pillar Point), dykes of basaltic andesite were noted intruding fine- to medium-grained granite. Northerly trends are common in this area, with thicknesses ranging up to 3 m; foliation is not recorded, although the basalt exposed in a stream north of Mong Hau Shek (812390 826660 mnd-68) is brecciated and sheared along a fault zone.
A 1.6 m wide mafic dyke with an easterly trend intersects the contact of the granite pluton of Tsing Shan and foliated metavolcanics of the Tuen Mun Formation at the western end of Wu Tip Wan (Butterfly Beach) (813150 825640 mnd-69). The host rocks are veined with quartz. The dyke carries no quartz veins and displays no sign of displacement across the pluton metavolcanic contact, presumably postdating both events.
A group of foliated lamprophyre dykes associated with vein quartz forms the spine of the headland of Lan Kok Tsui (Black Point), cutting medium-grained granite. Individual dykes range up to about 5 m wide and trend between northnortheast and eastnortheast.
Tuen Mun. A group of northeast trending mafic dykes is exposed east of Sam Shing Hui both on Tuen Mun Road and on the catchwater above. These dykes lie parallel to a dyke like outcrop of fine-grained granite. They vary in width from 0.4 to 3 m and may have foliated margins. The contact with the surrounding coarse-grained granite is irregular. Further north, at Wong Ka Wai (815800 828300 mnd-70), an irregular 0.1 to 1 m wide mafic dyke can be traced for 200 m. The dyke is fresh, black and aphanitic in Borehole 1445D (815870 828220 mnd-71). About 0.8 km south of Tan Kwai Tsuen (817930 831490 mnd-72) is a mafic dyke up to 1 m wide and very irregular, with branches extending into the granite. This dyke has thin quartz veins within it and parallel to the margins.
Siu Lam to Tai Lam. Mafic dykes are well exposed in a road cutting on Tuen Mun Road at Siu Lam. Five dykes were noted, ranging in thickness from 0.2 to 3 m, all trending roughly north south. All the dykes have irregular foliated margins, with small splays penetrating the surrounding granite (Plate 6.A43). To the northeast, along the southern shores of Tai Lam Chung Reservoir, a number of mafic dykes lie parallel to or slightly oblique to northeasterly trending faults. The widest dyke is about 5 m and contains a lens or xenolith of the granite country rock (820220 826190 mnd-73).
Three dykes of fine-grained olivine gabbro outcrop in the rock slope behind the Desalting Plant (818200 825000 mnd-74); they are between 1 and 1.8 m thick, dip steeply east and strike southsoutheast. The rock at the centre of the dykes is olive green, with black phenocrysts up to 3 mm, while in a narrow band at the margins it is darker, finer and slightly foliated.
Tsing Lung Tau to Ting Kau. A northeasterly trending group of deeply weathered mafic dykes was temporarily exposed in site formation work at Tsing Lung Tau (822200 824700 mnd-75). Two of the dykes are 1 to 2 m wide, while the largest is 5 m wide and extends for at least 150 m. The dykes have foliated margins with some quartz veining. A northeasterly trending mafic dyke up to 1.5 m wide is exposed north of Ting Kau in a narrow gorge above the catchwater (825890 826270 mnd-76). This dyke is dark greenish grey, strongly jointed and sinistrally sheared, indicating a fault or shatter belt. A northerly trending mafict dyke can be traced for 200 along a valley at Tiu Yue Wan (Angler's Beach) (823700 825000 mnd-776). The dyke is at least 0.5 m wide inland and divides into dykes 2 m and 1 m wide on the coast (823710 824990 mnd-78).
A northerly trending andesite dyke was temporarily exposed at Tsing Lung Tau (822410 824740 mnd-79). The dyke is 2 to 2.5 m wide and pale greenish grey, with lath like mafics up to 3 mm and feldspar up to 4 mm. It was identified as an andesite on its petrography and geochemistry (HK6047, Table 6.A4).
There were exposures of lamprophyre in the construction site for the Western Aqueduct at Yau Kom Tau (827380 825810 mnd-80). The dyke trends north, and is black with an aphanitic groundmass and abundant maficphenocrysts around 1 to 2 mm; there is no visible feldspar. Further north (826900 826500 mnd-81) there is localized boulder debris of lamprophyre within a debris flow deposit consisting of tuff. The rock is greenish grey with mafic phenocrysts up to 5 mm set in an aphanitic groundmass. The dyke from which the debris is derived probably trends east-west.
Clear Water Bay Peninsula. Mafic dykes are usually less than 2 m in width and are laterally impersistent. They are found cutting most rock types and usually strike roughly north south. At Tai Miu Wan (847630 814680 mnd-82), several basalt dykes, less than 400 mm in width cut Kau Sai Chau Volcanic Group (undifferentiated) tuff. A 4 m wide dyke with a northerly strike outcrops along the shoreline 500 m south of Clear Water Bay Second Beach (847700 815970 mnd-83). At Tai Chik Sha (846610 816770 mnd-84) a mafic dyke has been dislocated by a northeast striking fault.
Sai Kung Country Park. Mafic dykes have been noted cutting most volcanic strata, but are usually only seen in coastal exposures, especially in the High Island area where they are usually less than 1.5 m in width. One well exposed dyke at the High Island east dam (Plate 6.A44) intrudes the columnar jointed tuffs following a well defined kink band. Another dyke was exposed in the foundation excavations for the west dam, where it has been dislocated some 30 m by a westnorthwest trending fault.
Sha Chau. A completely weathered, sheared mafic dyke is exposed in the middle of Sha Chau (806590 823040 mnd-85), trending parallel to an earlier quartz vein and mylonitized zone in the granite.
Lam Chau (prior to development). An altered mafic dyke crops out on the western side of Lam Chau (808470 818010 mnd-86). It intrudes a fine-grained, partly epidotized segment of the Chek Lap Kok Granite. The dyke is greenish-grey, with altered feldspar phenocrysts up to 2 mm long, set in a chloritized, quartz-sericite groundmass HK9211). It is 0.3 m wide and trends northnortheast, cutting a thin quartz vein in the granite. There is no apparent displacement of the vein, indicating that the dyke was not intruded along a fault zone.
Tung Chung. In cut slopes 400 m eastnortheast of the ferry pier, there was a temporary exposure (811720 816560 mnd-87) of a completely weathered mafic dyke. The dyke, approximately 3 m wide, trends northnorthwest, and cuts the East Lantau Rhyodacite. Similar dykes, trending northnorthwest, occur on the eastern side of Chek Lap Kok, near Kwo Lo Wan.
Yam O Wan . At Yam Tsai Wan, there are a number of thin mafic dykes cutting both the Shek Lung Kung Member tuff and feldsparphyric rhyolite dykes. The mafic dykes are typically 0.3 to 0.5 m wide, and trend generally between northwest and northnorthwest. In a 0.5 m wide, easterly-trending dyke, within tuff (819820 821170 mnd-88), there is a small, medium-grained granite xenolith.
Tai Ho Wan. A swarm of narrow, northerly-trending mafic dykes crops out on the east side of Tai Ho Wan (815900 818100 mnd-89). The dykes are 0.2 to 1 m wide, and intrude rhyolite dykes. They are typically olive green and aphanitic, with quartz-filled amygdales 3 to 4 mm across. The swarm is probably intruded along the northerly trending fault zone that marks the eastern margin of the Lantau Caldera and which extends through Tai Ho Wan.