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The Geology of Hong Kong (Interactive On-line)
Mount Butler Granite - Klb


The Mount Butler Granite (Table 6.5) forms small, subcircular leucocratic monzogranite intrusions on the southeast and east margins of the Kowloon Granite on Hong Kong Island and in eastern Kowloon (Plate 6.24; Figure 6.6). The granite is dominantly equigranular and fine grained, but varies to fine to medium grained (Plate 6.25). The Mount Butler Granite intrudes the Kowloon Granite and the Ap Lei Chau and Mount Davis formations. In the vicinity of Kwun Tong, the Mount Butler Granite is locally greisenized, producing a saccharoidal-textured equigranular rock which is cut by narrow (0.1–1.5 m) westnorthwest-trending quartz veins. These typically contain minor quantities of fluorite, chalcopyrite, molybdenite, wolframite and beryl. Pegmatite patches and miarolitic cavities are common, particularly close to the granite–volcanic contact.


In thin section, the granular texture consists of subhedral poikilitic alkali feldspar (orthoclase), albite and quartz with sparse interstitial single flakes of greenish brown biotite. Plagioclase is sometimes weakly zoned. Accessory minerals include zircon, apatite and Fe-oxide. Miarolitic cavities and pegmatite are evidence for shallow emplacement and post-magmatic hydrothermal activity.


A Rb–Sr whole-rock isochron age of 136 ± 1 Ma has been obtained for the Mount Butler Granite (Sewell et al., 1992), but the Rb–Sr system may have been affected, as described elsewhere by Evans (1995), by interaction with water during post-emplacement hydrothermal activity.




Details

East Kowloon. Fine-grained granite crops out from Pik Uk (843110 822080 Klb-1), southwards and roughly parallel to the volcanic contact, to Rennie's Mill and Lei Yue Mun. Smaller areas occur at Ping Shek (840070 821370 Klb-2), north of Jordan Valley (840920 821340 Klb-3), and between Shun Lee (841500 821500 Klb-4) and Sau Mau Ping (841900 820600 Klb-5). The granite is light pink, uniform, equigranular and occasionally megacrystic. Average groundmass grain size is 0.5 to 1 mm, with quartz megacrysts up to 5 mm (Plates 6.A31 & 6.A32). Megacrysts of feldspar are not common. Biotite occurs as single flakes, and constitutes less than 3% of the rock. There is a gradational margin with fine to medium-grained granite, the transition occuring over a distance of several metres. This is well seen at Junk Bay Road (842880 819470 Klb-6) and in the Anderson Road Quarries (842220 820820 Klb-7). Geochemical analyses from samples collected each side of the boundary (842880 819490 Klb-8) give almost identical results, and it is probable that the fine- and fine- to medium-grained granites are parts of the same intrusion. Pegmatite patches are common, usually consisting of large crystals of alkali feldspar and quartz; these are well seen at or near the granite volcanic contact, for example, near the Clear Water Bay Road (842750 821600 Klb-9) at Tseng Lan Shue Village. Nearby (843110 822080 Klb-10), a borehole in the fine-grained granite showed considerable amounts of pyrite present as discrete crystals scattered throughout the rock. Thin mineral fissure veins crop out above Anderson Road Quarries (842250 821000 Klb-26), where wolframite was seen in the quartz vein rock. A number of parallel fissure veins containing mainly quartz, but with small amounts of beryl, fluorite and wolframite, cut the fine-grained granite between Rennie's Mill and Lei Yue Mun, and have been extensively prospected in the past. (Details of this mineralisation are given in Chapter 11). The fine-grained granite of this area is extensively hydrothermally altered, and a sheet of greisenised fine-grained granite, probably up to 100 m thick, lies parallel to the gently dipping volcanic contact. The greisenisation is considered to result from infiltration of late stage hydrothermal fluids and gases contained within the granite by the overlying impervious fine ash tuffs. Ruxton (1957) regarded the volcanics as having been greisenised and not the granite. Evidence from this survey suggests, however, that the greisen has developed mainly in the granitic rocks. The greisenised granite has a sacharroidal texture, is equigranular, and has an average grain size of 1 mm (Plate 6.A33). It is composed largely of quartz and light coloured micas; mainly muscovite, but also some light purple lepidolite. At the surface the rock is extensively weathered and takes on a reddish brown or purple coloration, as seen on the top of Chiu Keng Wan Shan (843380 817890 Klb-11). In trial adits nearby (843190 818070 Klb-12), the fresh rock is fairly soft, white to light brown, with abundant quartz and muscovite. The alteration here is patchy, and in places relatively unaltered fine-grained granite occurs. The outcrop of greisenised fine-grained granite is delineated on 1:20 000 map Sheet 11.


Central Hong Kong Island. Medium-grained granite crops out on the slopes between Mount Nicholson Road and Black's Link (836600 813500 Klb-13). Corestones of fresh equigranular medium-grained granite in highly to completely weathered granite are exposed in cuttings at Black's Link (836560 813490 Klb-14). The medium-grained granite grades into fine-grained granite within 2 m of the contact with the volcanic country rock (836600 813470 Klb-15). Small areas of medium-grained granites occur on the southwestern slopes of Mount Nicholson (836510 813100 Klb-16) and along Deep Water Bay Road close to its junction with Nam Fung Road. There (837220 812780 Klb-17), the granite is equigranular and averages 2 to 3 mm in grain size, and contains distinctive pegmatitic patches surrounded by 0. 1 m wide biotite concentration haloes (Plate 6.A30). This granite grades imperceptibly into fine- to medium-grained granite towards Shouson Hill (837070 812610 Klb-18). The valley slopes southeast of Deep Water Bay Road are covered by superficial deposits, but medium-grained granite is again exposed in the stream bed (837610 812750 Klb-19), where quartz monzonite is also seen, probably forming a dyke.


Between Stubbs Road and Wong Nai Chung Gap (837400 813630 Klb-20), fine- to medium-grained granite is present, but its relationship with the medium-grained granite to the north is unclear. At Stubbs Road (836470 814130 Klb-21) the fine- to medium-grained granite has a microcrystic texture; very fine-grained granite infiltrating the fine- to medium-grained groundmass, producing a markedly inequigranular rock. At Deep Water Bay Road (837200 812730 Klb-22) the fine- to medium-grained granite passes gradationally into medium-grained granite, with both rocks similar in character apart from grain size. Fine- to medium-grained granite predominates between Wong Nai Chung Gap (838230 813200 Klb-23) and Tai Tam Reservoir (839680 813130 Klb-24). In places, for example 450 m southeast of the main dam at Tai Tam Reservoir (840100 812890 Klb-25), infiltrations of fine grained granite are common, and diffuse patches of fine-grained granite up to 0.5 m across also occur. At Mount Butler Quarry (839000 814400 Klb-26) the granite is light grey to light pink, slightly inequigranular and appears to be medium-grained. This appearance, however, is due to clotting of quartz crystals, creating the impression of a coarser-grained rock. Closer inspection reveals a dominant groundmass of 1.5 to 2 mm average grain size. Quartz crystals form aggregations up to 5 mm. Biotite as single flakes stands out clearly in this leucocratic granite, but makes up less than 2% of the rock. Outcrops have been distinguished on the map where the average grain size reaches 2 mm, for example, 200 m southwest of the Country Park Management Centre (838830 813300 Klb-27) and on the eastern shore of the Tai Tam Reservoir (839920 813330 Klb-28).


Fine- to medium-grained granite crops out from the flanks of Mount Nicholson and the Tai Tam Country Park, northwards to Braemar Hill. There are small outcrops at Lei Yue Mun Bay (842530 815770 Klb-29) and Chai Wan (841920 813760 Klb-30). The granite has close affinities with the fine-grained granite and, in many places, infiltration by the fine-grained granite has produced a modified texture.


Northern and Eastern Hong Kong Island. Fine-grained granite crops out over wide areas on the hills between Happy Valley and Chai Wan. It is quarried at Mount Butler (839000 814400 Klb-26), where a gradational boundary with the fine- to medium-grained granite runs through the northern part of the workings (839050 814530 Klb-31). Mount Butler can be regarded as the type locality of the fine-grained granite of the district. The rock is leucocratic, light pink to light grey and roughly equigranular, with a groundmass grain size between 0.5 and 1 mm. It is sparsely megacrystic, with quartz grains up to 4 mm and occasional alkali feldspar crystals to 5 mm in length. In places, finer diffuse patches occur up to several metres across, with an average grain size of less than 0.5 mm. Biotite in the form of fresh black shiny crystals, is scattered throughout but constitutes less than 2% of the rock. Pegmatite is common, usually as irregular shaped pods 0.1 to 0.3 m across. In the light grey granite, small specks of pyrite are visible in hand specimen.


At Jardine's Lookout, Mount Butler Drive (837970 814180 Klb-32), the granite contains scattered large feldspar megacrysts and is similar in appearance to the fine-grained granite at King's Park in Kowloon. The section along Mount Butler Drive and Price Road (837820 814020 Klb-33) shows variation from a non megacrystic, markedly fine-grained granite with a sacharroidal texture (837950 814120 Klb-34) to a fine-grained granite containing diffuse patches of medium-grained granite (837820 814020 Klb-33). Pegmatite pods are common in these sections. Similar variation is seen along Tai Hang Road near its junction with Mount Butler Road (837660 814040 Klb-35), southwards to the Tai Tam water tunnel portal (837740 813750 Klb-36). Thin aplite dykes are common, and these stand out as ribs on the highly weathered granite surfaces, for example, along Sir Cecil's Ride, (838520 815310 Klb-37) and at Braemar Hill (838680 816000 Klb-38).


Between Victoria Barracks (834620 815420 Klb-39) and Bowen Road, Wan Chai (835940 814690 Klb-40), fine-grained granite crops out as a margin to the main medium-grained granite pluton. It is usually equigranular, and only sparsely megacrystic. A similar chilled margin is recorded at Middle Gap (836490 813620 Klb-41). Fine-grained granite intrusions into the main medium-grained pluton body occur at Morrison Hill (836420 815440 Klb-42) and at the Hong Kong Stadium (837390 814810 Klb-43), where a weathered basalt dyke (837380 814880 Klb-44) cuts the fine-grained granite. Here, the granite is equigranular with an average grain size of 0.5 to 1 mm. It is sparsely megacrystic, with scattered megacrysts of quartz, alkali feldspar and biotite up to 4 mm. The rock is light grey when fresh, with small, diffuse pink patches, probably resulting from iron staining of the alkali feldspar. Fine-grained granite crops out to the north, east and southeast of Mount Parker. The granite outcrop at the latter location forms a 150 to 200 m wide body with steeply dipping sides. Exposures near the junction of Chai Wan Road and Tai Tam Road (842070 814580 Klb-45) show the typical rock. It has a light pink colouration and is roughly equigranular, with an average grain size of 0.8 to 1 mm. There are quartz megacrysts up to 2 mm, but these are not prominent. Biotite is present as scattered fresh, single, black flakes. In places, however, the flakes have clustered to form clots of crystals 2 to 3 mm across. Pyrite also occurs, as crystals up to 3 mm across, with a 2 to 3 mm surrounding halo of dark green to black mineral, probably chlorite.


The fine-grained granite grades imperceptibly into a fine- to medium-grained granite at Hing Wah Estate, Chai Wan (842000 813860 Klb-46). The only intrusions cutting the fine-grained granite of this area are thin aplite (841970 813770 Klb-47) and mafic (841980 813910 Klb-48) dykes.


The contact between the fine- and medium-grained granites is clearly defined between Tai Hang Road, King's Road, North Point and the Kornhill site at Quarry Bay. In this area, centred on Mount Butler, the fine-grained granite and associated fine- to medium-grained granite have an almost circular outcrop of some 9 sq km. Cross cutting relations, as seen at Kornhill (840310 815650 Klb-49), demonstrate that the fine-grained body forms a younger intrusion into the medium-grained granite pluton of Kowloon and northern Hong Kong Island. Much of the area covered by this younger intrusion is designated as country park (Tai Tam and Quarry Bay country parks), and there are few exposures except in road cuttings around the fringe.


Western Hong Kong Island. Isolated outcrops of fine-grained granite occur in the Pok Fu Lam area and also on Green and Little Green islands (Figure 6.A4). The largest lies between Pokfulam Road and Baguio Villas, where the fine-grained granite grades into fine- to medium-grained granite (831670 813490 Klb-50). It is probable that all the isolated outcrops form parts of a single intrusive body at depth. The contact with the volcanics is very irregular, as seen in the small granite intrusions at the northern end of Sandy Bay (830310 814840 Klb-51, Figure 6.A5). Everywhere the granite is light pink or light brown and characterized by its fine grain size, less than 0.5 mm. It appears equigranular, with a few scattered quartz megacrysts up to 2 mm. In thin section these granites possess a granophyric texture not apparent in hand specimen (Plate 6.A34). No dykes were seen intruding this fine-grained granite, but pegmatite lenses were noted on Little Green Island (830130 816280 Klb-52), and on the western side of Telegraph Bay pegmatite concentrations occur at the granite - volcanic contact (831310 813670 Klb-53). In the grounds of Grantham Hospital, Wong Chuk Hang (835830 812470 Klb-54), fine-grained granite very similar to the variety at Pok Fu Lam occurs. This small body is bounded to the north by quartz monzonite and to the south by volcanic rocks, but its precise relationship with the monzonite is not clear.