The Tai Po Granodiorite (Table 6.2) is exposed in discontinuous outcrops across central and southern parts of Hong Kong with the largest outcrops in the central New Territories and on Tsing Yi (Figure 6.3). The granodiorite is thought to have been emplaced as a large, high-level, intrusive sheet centred on Tai Mo Shan towards the close of Tsuen Wan Volcanic Group activity. However, it has been widely fragmented by subsequent intrusive activity and faulting. Lithologies vary texturally, without any significant change in bulk composition, from normal subhedral-granular granodiorite, through porphyritic fine-grained granodiorite to densely porphyritic dacite. Porphyritic fine-grained granodiorite and dacite predominate in the type area of Tai Po, and at Shek Kong, Tai Mo Shan, Tsuen Wan and Tsing Yi, and in smaller outcrops at Cape D'Aguilar and Kau Pei Chau. Equigranular medium-grained granodiorite crops out on Stanley Peninsula, Lamma Island, and at Cheung Sha on Lantau Island. Equigranular granodiorite, tentatively correlated with Tai Po Granodiorite, has also been encountered in offshore boreholes in Urmston Road. Texturally modified subequigranular granodiorite is found mainly between Tai Lam Reservoir and Route Twisk, in central Tsing Yi, and on Ngau Chau (Plate 6.5).

Thermal metamorphism has substantially modified the texture of granodiorite close to the contacts with later igneous intrusions. Extensive recrystallization of quartz and biotite at the expense of alkali feldspar and amphibole has led to an increase in grain size, producing a variety of subequigranular lithologies. Original phenocryst minerals in the granodiorite can be recognized as being larger than the surrounding grains but with modified crystal boundaries. Quartz is typically anhedral with serrated grain boundaries. Layers of recrystallized biotite (schlieren) are common, sometimes completely pseudomorphing amphibole grains. Chlorite and epidote alteration in these metamorphosed rocks is ubiquitous. Close to major shear zones, the granodiorite is dynamically metamorphosed, resulting in a greatly reduced grain size. Equigranular granodiorite from Stanley Peninsula, Lamma Island, and Lantau Island generally displays well-developed subhedral-granular texture although the Stanley samples have been partially recrystallized. Non-porphyritic granodiorite, found in Urmston Road boreholes, is typically medium grained with subhedral-granular texture.

Fresh samples of granodiorite contain subhedral grains of quartz, alkali feldspar, and zoned plagioclase, with subordinate grains of greenish-brown biotite and dark green amphibole. Accessory minerals include zircon, titanite, apatite, zoned allanite and Fe-oxide.

Multi-grain zircon fractions from the Tai Po Granodiorite have yielded concordant U–Pb ages of 164.6 ± 0.2 Ma (Davis et al., 1997).

Details

Tai Po. The main mass of granodiorite at Tai Po is exposed only as structureless residual soils with corestones. These soils grade downwards into saprolite with relic quartz veins and joints (834360 833960 Jmt-1). In some deeper valleys, and on some of the steeper slopes, relatively fresh rock may occur (834100 834250 Jmt-2). From the corestones and the saprolitic soil the granodiorite can be seen to be fairly uniform in grain size and mineralogy, but is veined and spotted with epidote in most exposures. Contacts are rarely seen in natural exposures but have been recorded in road cuttings in a number of places. They show little chilling of the granodiorite but are sharp, irregular and commonly sheared (834380 833590 Jmt-3). Exposures in the borrow area at Nam Hang (836200 835500 Jmt-4) show complex relations between granodiorite and the tuffs and tuffites of the Shing Mun Formation; a series of major and minor granodiorite bodies are cut by northeast and northwest trending faults. The outcrops around Yim Tin Tsai are generally homogeneous, but occasionally have feldspar phenocrysts as at (839720 835440 Jmt-5). South of Yim Tin Tsai there is a small outcrop of granodiorite on Centre Island (840950 833340 Jmt-6). An exposure of red saprolitic soil on Harbour Island (844050 835420 Jmt-7) is inferred to be deeply weathered granodiorite. Granodiorite is recorded in submarine boreholes drilled for the Tolo Channel aqueduct in the channel between Harbour Island and Tang Chau.

West of Tai Po. Granodiorite is sporadically exposed in the Lam Tsuen Valley, for example as fresh rock in road cuts near Kadoorie Farm (830030 832670 Jmt-8). An exposure in the river bed north of Chung Uk Tsuen indicates the possibility of an extensive crop under the alluvial deposits of the area. Southeast of Kadoorie Farm, on Kwun Yam Shan (830360 831900 Jmt-9) the granodiorite has been deeply and extensively chloritised to form a rock which consists of stringers and coarse blebs of quartz in a chlorite matrix (HK 332, 333). On the western slopes of Kwun Yam Shan rock types transitional between unaltered granodiorite and quartz chlorite rock can be seen (HK 339, 340) .

South of Tai Po. Granodiorite occurs as dykes and sheets. One intrusion can be traced from the ridge above Sheung Wong Yi Au (835800 832980 Jmt-10) to Tai Po Kau (836800 832990 Jmt-11). Around Tsiu Hang (838500 832450 Jmt-12) and in the Tai Po Kau Nature Reserve (836760 831970 Jmt-13) sheets of granodiorite intrude the lower part of the Repulse Bay Volcanic Group. Similar outcrops are better exposed in the Cove Hill borrow area (838700 830340 Jmt-14), where irregular masses of granodiorite with outcrops from a few metres to 180 m wide are complexly faulted, and intruded by quartzphyric rhyolite.

Shing Mun Reservoir and Kwai Chung. The granodiorite of this area is more variable than around Tai Po. In places the matrix is fine grained; elsewhere it is medium grained. The proportions of feldspar and quartz phenocrysts also vary considerably. However, these variations have not proved to be laterally continuous and have not therefore been delineated on the map. There is an extensive but faulted outcrop of granodiorite in the vicinity of the Shing Mun Reservoir. Contacts exposed in Shing Mun River indicate that the granodiorite body dips to the NW at about 15^o (833230 828860 Jmt-15). The thickness of this intrusion is inferred to be over 100 m. A similar sized, lopolith shaped body extends from the southern part of the Shing Mun Reservoir area through Sheung Kwai Chung (832600 826760 Jmt-16), Sam Tsuen (830600 825750 Jmt-17) and Kwai Shing (830720 825040 Jmt-18). This body is also cut by faults, which are well exposed around Sam Tsuen and Kwai Shing, where granodiorite alternates with numerous sheets of fine grained granite, and feldsparphyric and quartzphyric rhyolites. The contact with the granite on the south eastern flank of the granodiorite demonstrates the younger age of the granite, which penetrates the granodiorite as minor dykes.

Round Island, Stanley Peninsula and Cape D'Aguilar. Exposures of fresh rock in these outcrops are seen only along the coastal cliffs. In every case, the granodiorite intrudes coarse ash crystal tuff of the Yim Tin Tsai Formation. The rock is mesocratic and commonly variable in grain size, ranging from fine to coarse-grained. It is also megacrystic. The average grain size of the groundmass is between 1 and 2.5 mm, and microperthite feldspar megacrysts average 10 mm in length while some reach 25 mm. Quartz is fairly abundant as subrounded megacrysts 6 10 mm in diameter. Patches of fine felty biotite, as aggregations of minute laths, are ubiquitous in the groundmass and give the rock its dark colour. In thin section (HK4225, 844920 807750 Jmt-19, Plate 6.A1) the abundant biotite crystals are associated with light green amphibole and chlorite. Subhedral plagioclase includes zoned and altered crystals and forms an interlocking texture. Subhedral quartz grains fringe some of the larger microperthite megacrysts. The granodiorite contains conspicuous xenoliths of a dark, aphanitic rock, and blocks of coarse ash tuff up to 0.4 m across have been noted (837150 808350 Jmt-20). At Wong Ma Kok Point (839950 806300 Jmt-21) the granodiorite has a 0.25 m wide chilled margin against coarse ash tuff. Nearby, the rock is intruded by dykes of quartz syenite, basalt and quartzphyric rhyolite. On the northwestern part of the Stanley Peninsula (839900 807860 Jmt-22), close to the granite pluton, small dykes of fine grained granite intrude the granodiorite. Much of this western shoreline of Stanley Peninsula is covered by large boulders of granodiorite, up to 5 in across, which generally conceal the bedrock. A distinctive feature of the granodiorite in the cliffs at Cape D'Aguilar is the presence of epidote, both as light green patches and as haphazard veins. Here, the granodiorite is cut by a number of quartzphyric rhyolite dykes, and by several smaller basalt dykes.

Lamma Island. Granodiorite boulders are abundant on the shore between Lo So Shing Beach (830600 807400 Jmt-23) and Tit Sha Long Bay (830500 807720 Jmt-24), but there are no exposures of fresh rock. On the nearby slopes vegetation is sparse and the soil is dark reddish brown. In hand specimen the rock is medium grained with grey quartz and feldspar crystals highlighted by abundant interstitial small black biotite crystals. The average grain size of feldspars and quartz is between 2 and 3 mm, and there are scattered alkali feldspar megacrysts to 10 min in length. In thin section (HK3858, 830510 807440 Jmt-25) subhedral hornblende is common, and present alongside fresh biotite. Plagioclase forms at least 80% of the total feldspars, and quartz makes up 25 to 30% of the rock. Inland from Lo So Shing the granodiorite is flanked by megacrystic, medium grained, biotite rich granite, similar in appearance to the granodiorite. It is possible that the two rocks may have been intruded penecontemporaneously, the granodiorite representing the calcium rich fraction of the pluton.

Tsing Yi. Granodiorite outcrops in northeast Tsing Yi, and is probably contiguous beneath offshore superficial deposits with outcrops around Tsuen Wan. The rock is typically dark grey or composed of contrasting black biotite and pale felsic minerals, giving a speckled texture. West of Fung Shue Wo (828100 824100 Jmt-26) it is markedly porphyritic, varying from fine grained to dacitic. Near Wok Tai Wan on northwestern Tsing Yi is an outcrop of porphyritic dacite. The rock is dark greenish grey to grey, with feldspar phenocrysts up to 20 mm across and quartz up to 10 mm. There are rare recrystallized xenoliths up to 100 mm across set in an aphanitic groundmass. On Nga Ying Chau, a small island east of Tsing Yi which is now joined to the main island by reclamation, there are two varieties of granodiorite which differ from the typical variety seen to the west. One is a dark grey, finely porphyritic granodiorite dominated by a fine grained groundmass of about 0.25 mm (829010 824470 Jmt-27), and the other (829120 824580 Jmt-28) is a speckled black and white variety with phenocrysts of feldspar commonly up to 10 mm, and a groundmass of around 1 mm.

South of Cheung Shue Tau (827750 824320 Jmt-29) there is a contact between granophyric fine-grained granite to the north and granodiorite to the south. A basalt dyke lies on the contact, and a small fault cuts these rocks, displacing the contact by about 5 m. Northeast of Fung Shue Wo (828300 824700 Jmt-30) the granodiorite intrudes tuffs of the Yim Tin Tsai Formation. The contact is highly fractured but has an irregular trend which probably indicates an intrusive rather than a faulted nature. At Fung Shue Wo (828470 824310 Jmt-31) the boundary between megacrystic fine-grained granite and granodiorite was exposed during road construction. Excavations at the Oil Depot south of Tsuen Wan (829700 824700 Jmt-32) revealed highly fractured or jointed fine-grained granite next to prophyritic fine-grained granodiorite. Offshore boreholes between Tsing Yi and Yau Kom Tau revealed granophyric fine-grained granite, strongly fractured and cut by basalt dykes.

Granodiorite forms the lower slopes of a small headland at Kam Chuk Kok (827000 823000 Jmt-33). It is interpreted as a dome-like body with a gently sloping or flat upper surface overlain by tuffs. The granodiorite is cut by north-trending quartzphyric rhyolite dykes (827010 823040 Jmt-34) and easterly-dipping sheets of feldsparphyric rhyolite (827100 822960 Jmt-35). North of Kam Chuk Kok (826920 823360 Jmt-36), a screen of granodiorite occurs between dykes of coarsely feldsparphyric rhyolite. The form of the outcrop around Lo Uk is not clear as the contacts between granodiorite and the older surrounding country rocks are not exposed. The contact with the medium-grained granite near Cheung Hong Estate (828020 823470 Jmt-37) appears to be steeply inclined, sinuous, and northeasterly-trending. Other contacts are faulted, as at Liu To (828020 823470 Jmt-37), or cannot be located with precision. Many exposures show that the granodiorite is cut by northeast-trending dykes of fine-grained granite (828720 823420 Jmt-38), feldsparphyric rhyolite (828560 823540 Jmt-39, 828450 823450 Jmt-40) or quartzphyric rhyolite (828620 823480 Jmt-41).

Tsing Tam Reservoir to Shek Kong Village. This outcrop consists of porphyritic fine grained granodiorite, speckled black and white when unaltered, but often sheared and metamorphosed. The unaltered granodiorite was well exposed in the road widening near Kadoorie Farm and in the borrow area between Lam Kam Road and Route Twisk (829500 832400 Jmt-42). It can also be seen along the catchwater west of Shek Kong Village (828200 831800 Jmt-43), but these exposures are generally deeply weathered. In the borrow area the rock is leucocratic (Plate 6.A2), becoming melanocratic with increasing metamorphism (829460 832450 Jmt-44). Euhedral feldspar crystals are up to 15 mm across, quartz up to 10 mm and biotite clots up to 5 mm, set in a fine grained matrix. Epidotization associated with the metamorphism, particularly in the feldspar nhenocrvsts, gives the rock a greenish colour.

Kam Tin to Shek Kong Camp. Coarsely porphyritic dark grey granodiorite has been found in many boreholes around Kam Tin, Shek Kong Camp and eastsoutheast of Shek Kong. Granodiorite therefore probably underlies most of the Kam Tin Shek Kong alluvial plain. In the small village of Yuen Kong, on a low hill rising above the alluvial plain, exposures of saprolite occur east of the school (826040 831790 Jmt-45). Although no recognizable fresh rock has been recorded, the outcrop is believed to be granodiorite. Kong A Leng hill is covered with corestones and debris of weathered porphyritic fine grained granodiorite. The rock displays typical black and white mottling, with feldspar up to 12 mm, quartz up to 10 mm and a fine grained groundmass. North of Pat Heung, on the footslopes of Kai Keung Leng, is an outcrop of metagranodiorite. Exposures of the rock (827200 834500 Jmt-46) are pale grey, with quartz and relict feldspar phenocrysts up to 4 mm. The characteristic texture of a porphyritic fine grained granodiorite is preserved in the relict crystals, with secondary growth of quartz and sericite dominant in the groundmass.

Chuen Lung to Tin Fu Tsai. One of the largest outcrops of granodiorite and dacite in the area stretches from Route Twisk west of Chuen Lung to Tin Fu Tsai, taking in Lin Fa Shan. This outcrop displays the greatest variety of granodiorite textures, ranging from porphyritic dacite through porphyritic fine grained granodiorite to equigranular fine to medium grained granodiorite. The area of dacite close to Route Twisk is porphyritic, with feldspar up to 20 mm across and quartz up to Tin 8 mm. Biotite crystal aggregates are generally small, up to 4 mm. Two areas of non porphyritic fine to medium grained granodiorite were noted at Pak Shek Kiu (828900 827700Jmt-47) and Lin Fa Shan (826800 828000 Jmt-48). The granodiorite is typically speckled black and white, and where porphyritic contains feldspar up to 20 mm across and quartz up to 10 mm.

Tsing Fai Tong to Tai Lam Chung Reservoir. There are four outliers of granodiorite in this area that can be recognized by their reddish brown clay rich soils, contrasting sharply with the pale granite soils. Rare exposures of fine grained granodiorite occur in road cuttings near Tsing Fai Tong (824000 827400 Jmt-49). To the west the composite outlier of granodiorite and tuff can be seen best in a road cutting (823100 826760 Jmt-50) where equigranular fine grained granite, altered tuffs and fine grained granodiorite are exposed. Elsewhere in the outlier, the fresh granodiorite has the characteristic speckled black and white texture and is typically fine to medium-grained, with feldspar up to 4 mm. Around Yuen Tun and Tai Lam Chung Reservoir, the granodiorite outcrop is inferred from the presence of red saprolitic soil, with boulders of porphyritic and non porphyritic fine grained granodiorite. A sheared contact with mylonite and schistose granite separates the granite and granodiorite on the large island (820340 826460 Jmt-51) in Tai Lam Chung Reservoir.

Yuen Long. Granodiorite was found in two places during construction of the water tunnel. North of Lin Fa Shan the exposures in the tunnel are part of the Chuen Lung to Tin Fu Tsai outcrop. About 2 km west of Yuen Kong the tunnel exposures have no direct surface expression, and are believed to be part of the outcrop beneath the Kam Tin Shek Kong alluvial plain. From 2.0 to 2.8 km south of the portal near Tsing Tam were exposures of fine grained granodiorite. The rock is usually non porphyritic with feldspar less than 4 mm, but rarely contains phenocrysts of white feldspar up to 20 mm across. About 1 km along the tunnel from the portal at Ko Po San Tsuen, exposures of porphyritic fine grained granodiorite or dacite are speckled black and white, sometimes greenish, with feldspar phenocrysts up to 20 mm and quartz up to 10 mm. The groundmass in generally very fine grained, in thin section being less than 0.06 mm. Metagranodiorite has been found in many boreholes to the north and east of Yuen Long. Beneath Yuen Long Industrial Estate (820500 835800 Jmt-52) it occurs with fine grained metagranite and a marble bearing sequence adjacent to an eastnortheast trading fault. Near Yuen Long Old Town (821600 834100 Jmt-53) it overlies white marble, with a steep, westerly dipping contact zone. It may also outcrop on the low hills at Shan Pui (821900 834500 Jmt-54), where the rock has been tentatively ascribed to the Tai Mo Shan Formation. The rock in all these localities is usually foliated, often strongly so; the foliation obscures the original rock texture. When fresh it is pale grey with mafic blotches or streaks, and may possess recognizable feldspar relicts.

Shuen Wan. Granodiorite intrudes block-bearing tuff and tuffite of the Shing Mun Formation to the east (836900 836000 Jmt-55) and west (835800 836000 Jmt-56) of Mak Uk. The host rocks are thermally metamorphosed to hornfels within 100 m of the granodiorite contact at both localities. Granodiorite also intrudes fine ash tuff of the Shing Mun Formation north (839100 837200 Jmt-57, 839250 837300 Jmt-58) and west of (838200 836650 Jmt-59) Shuen Wan, and on the coast at Shuen Wan Chim Uk (839600 836100 Jmt-60). Granodiorite is further exposed on the northern tip of Yuen Chau (840950 836050 Jmt-61) in Shuen Wan Hoi (Plover Cove) and has been recorded in offshore boreholes near Wong Chuk Tsuen (e.g. BHTK2/18, 841890 836960 Jmt-62; BHTK2/19, 841950 836900 Jmt-63).

Cheung Sha. Two small outcrops of porphyritic, fine-grained granodiorite occur on the coast south and east of Cheung Sha. The eastern body is probably less than 300 m long and 100 m wide, and trends eastnortheast between dykes of feldsparphyric rhyolite. The rock is typically speckled black and white, with euhedral plagioclase phenocrysts up to 15 mm long and irregular patches of quartz up to 7 mm across, set in an aphanitic groundmass. Xenoliths are relatively common, and at one locality (814290 810810 Jmt-64) a siliceous, aphanitic xenolith up to 250 mm across can be seen. Porphyritic microgranite is exposed southwest of the granodiorite, but its contact with the latter is not. On the beach (813600 810400 Jmt-65) at Cheung Sha, there is a complex exposure of granodiorite, Yim Tin Tsai Formation tuff and younger rhyolite dykes. The granodiorite is poorly exposed, at or below the high-water mark, and contains many small melanocratic xenoliths. The feldsparphyric rhyolite, which grades into microgranite, is presumed to be younger because of the lack of alteration and fracturing relative to the granodiorite.