Several small stocks and dykes of quartz monzonite crop out discontinuously along the margin of the Lantau Caldera and on the nearby islands of Cha Kwo Chau, Hei Ling Chau and Chau Kung To (Figure 6.6). These intrusions are collectively termed the Tong Fuk Quartz Monzonite (Table 6.5). Major outcrops occur at Sha Lo Wan, at Tong Fuk, and at Fan Lau. At Fan Lau, rounded clasts of alkali feldspar megacrystic quartz monzonite (Luk Keng Quartz Monzonite) are enclosed within the Tong Fuk Quartz Monzonite (Plate 6.19). Elsewhere, the monzonite has intruded the Tai Po Granodiorite, Lantau Granite, and Lantau Volcanic Group (undifferentiated).

The Tong Fuk Quartz Monzonite is characterised by coarse alkali feldspar megacrysts (Plate 6.21). Euhedral alkali feldspar megacrysts (up to 15 mm long) comprise between 40 and 50% of the rock with subordinate (10–20%) smaller (1–3 mm) plagioclase phenocrysts. The orthoclase megacrysts are weakly zoned and commonly display simple twinning. Euhedral to subhedral plagioclase phenocrysts are strongly zoned with sericitized andesine cores and fresh albite rims. The granular fine-grained matrix mostly comprises quartz and alkali feldspar with minor plagioclase. Biotite and minor greenish brown amphibole are the chief mafic minerals. Biotite is commonly altered to chlorite. Accessory minerals include zircon, titanite, allanite, apatite, and Fe-oxide.

Quartz monzonite from Tong Fuk has yielded a U–Pb single zircon age of 140.4 ± 0.3 Ma (Davis et al., 1997).

Details

Luk Keng. At its northwestern extremity, the Tong Fuk Monzonite intrudes porphyritic rhyolite lava of the Lantau Volcanic Group (undifferentiated), although the contact cannot be seen. About 700 m westnorthwest of Shui Hau, a roadside cutting (808560 809460 Klf-1) has rhyolite lava at its base and porphyritic, fine-grained quartz monzonite corestones within 20 m. A trachytoid texture occurs in the monzonite about 150 m to the eastsoutheast, with feldspar crystals no larger than 6 mm, set in an aphanitic groundmass. About 300 m west of Shui Hau, the lava appears to have been hornfelsed by the intrusion of monzonite. Rounded, dark xenoliths are a feature of the quartz monzonite, although they are relatively scarce.

Close to the contact (809010 808680 Klf-2) with the volcanics, feldspar megacrysts show distinct flow alignment. This trachytoid texture is also visible in boulders of monzonite at Shek Lam Chau (808240 808390 Klf-3). In the absence of an exposed contact, this texture indicates that the monzonite at Luk Keng intrudes porphyritic rhyolite lavas of the Lantau Formation. Between Shek Lam Chau and Lo Kei Wan there is an outcrop of granite in fault contact (808520 807770 Klf-4) with the quartz monzonite. The contact gives no clear indication of the relative ages of the two intrusive bodies.

Pui O. The quartz monzonite intrusion at Pui O lies on the northeast-trending Pui O-Sha Tau Kok Fault. The quartz monzonite is well exposed in cut slopes along the South Lantau Road, near Mui Wo, and at the nearby Water Treatment Works. Elsewhere, exposures are limited to corestones and boulders, often set in a distinctive, reddish-brown saprolitic soil. These boulders are particularly well displayed at Ham Tin Village, near Pui O. The adjacent hillside is littered with monzonite boulders, but is also covered in thick vegetation. Closer to Mui Wo, the hillside is more accessible, and the corestones are abundant.

Fresh pinkish-grey monzonite is exposed in cut slopes both at the Mui Wo Water Treatment Works and on the adjacent South Lantau Road. The rock can also be seen in its more typical form as corestones between 0.5 and 2 m across, in reddish-brown, saprolitic soil. The contact to the north with the adjacent feldsparphyric rhyolite dykes and medium-grained granite is not exposed, but the outcrop form indicates that it is vertical. To the south of Luk Tei Tong, there are hill top exposures (816900 813400 Klf-5) of laminated tuffaceous siltstone. This outcrop appears to lie on top of the monzonite, rather than as a sliver within the intrusion, possibly indicating that the monzonite is plug-like in form rather than dyke-like.

About 1 km east of Nam Shan, exposures (817300 812800 Klf-6) of feldsparphyric rhyolite are partly hydrothermally altered within, and adjacent to, the monzonite outcrop, indicating that the monzonite is younger than the rhyolite dykes.

Tong Fuk. Quartz monzonite at Tong Fuk crops out as four discrete bodies. The largest, underlying Ma Po Ping Prison, gives rise to the boulder-strewn hillside, and is well exposed in cuttings on the South Lantau Road. An offshoot of this body occurs on the coast to the south. A third body, north of Tong Fuk, is poorly exposed along the catchwater, and a fourth, very small body is well exposed on the coast west of the prison.

The contact between quartz monzonite, with a trachytoid texture, and banded lava cannot be seen on the road to the prison, although both rock types have been observed within a few metres of each other (810680 809870 Klf-7). The monzonite also has a fine-grained groundmass, probably resulting from chilling at the margin.

Exposures (810650 809640 Klf-8) of typical, fresh monzonite occur on the South Lantau Road, south of the prison. The rock is light grey, with feldspar megacrysts up to 7 mm long, set in a fine-grained groundmass. No xenoliths can be seen, and they have not been observed in corestones in the area either. These corestones are typically set in a reddish-brown, clay-rich saprolitic soil.

Coastal exposures west of the prison (810160 809730 Klf-9) are dominantly composed of sparsely porphyritic, quartz monzonite, contrasting with the markedly porphyritic boulders on the shoreline 100 m to the east. The rock is pale grey, with a marked flow-banding in the western part of the exposure. This flow-banded margin, composed of porphyritic trachyte, has sparse quartz phenocrysts, up to 2 mm, and feldspar phenocrysts up to 4 mm long. The contact with the overlying lavas dips 39^o southwest. The monzonite is intruded by two basalt dykes, respectively 0.1 and 0.3 m wide. These dykes do not cross the adjacent lavas. The trachytic contact clear indicates that the syenite formed a dome-like intrusion in the older lavas.

Fan Lau. Quartz monzonite extends from the north shores of Fan Lau Tung Wan and Fan Lau Sai Wan, through Heung Chung Au to Peaked Hill (Kai Yet Kok). A small body also occurs east of Fan Lau Fort. At the northern end of the pier at Fan Lau Sai Wan (802420 807400 Klf-10), the rock is typically pinkish-grey, mottled black, porphyritic, fine-grained quartz monzonite. Feldspar megacrysts are up to 15 mm long, although most are less than 8 mm, and the groundmass has a grain size of less than 0.5 mm. Boulder-strewn hill slopes occur to the north, and the edge of the intrusion is marked by the change from cuboidal corestones to rubbly debris on the lower flanks of the hill.

Contact relationships with the adjacent, fine-grained granite can be seen 230 m west of the Fan Lau Sai Wan pier. The contact is sharp, trends southeast, and is marked by a biotite enrichment in the monzonite. Quartz veins in the granite do not cross into the quartz monzonite, confirming that the monzonite is the younger of the two. 300 m further along the coast, there are exposures of partially mylonitized granite close to monzonite. The quartz monzonite contains large xenoliths, including fine-grained granite, up to 2 m across. About 300 m to the northwest, there is a 0.6 m-wide dyke of quartz monzonite, trending eastnortheast, which intrudes the fine-grained granite country rock.

Porphyritic, fine-grained quartz monzonite is exposed around high water mark on the northeastern tip of Peaked Hill (Kai Yet Kok). Flow-aligned alkali feldspar laths, up to 15 mm long, define a trachytoid texture. The quartz monzonite is in close proximity to boulders of fractured granite and exposures of fine- to medium-grained granite, and appears to have been intruded along a fault zone within the older granite that trends northwest from Fan Lau to Peaked Hill.

To the east of Fan Lau Tsuen, a contact (803210 807170 Klf-11) between quartz monzonite and granite occurs at the northern end of the beach. About 200 m to the east (803440 807250 Klf-12), a trachytoid texture and contact with adjacent metavolcanics has been observed.

No contact relationships were seen in the outcrop near Fan Lau Fort, although at one locality (802860 806580 Klf-13) the monzonite has a marked north-south trending trachytoid texture.

Sha Lo Wan. The only outcrops of quartz monzonite on the north side of Lantau Island are between Tung Chung Wan to the east and Sha Lo Wan in the west. The outcrop consists of two elongate bodies, each over 1 km long but less than 500 m wide, trending westnorthwest along the contact between volcanics to the south, and granite intrusions to the north. In addition, a small monzonite dyke lies close to the contact between granite and Carboniferous sedimentary rocks at San Shek Wan.

The largest outcrop of monzonite is south and east of Sha Lo Wan Village, but exposures are poor on the steep, thickly vegetated slopes. In a stream bed (807870 816070 Klf-14) south of the village, there are exposures of porphyritic, fine-grained quartz monzonite with tabular, euhedral alkali feldspar megacrysts up to 10 mm across. The monzonite is also exposed close to fine- to medium-grained granite in a stream 300 m to the east (806180 816160 Klf-15),and on the hillside to the southeast (806400 815970 Klf-16).

Exposures of the eastern outcrop of quartz monzonite are very poor, and the strongest evidence for outcrop form comes from the occurrence of a topographic low, rich vegetation and reddish-brown, saprolitic soil on the ridge west of Tung Chung Wan (809700 815700 Klf-17). To the east, towards Tung Chung, scattered exposures have been noted, and quartz monzonite has been recorded close to medium-grained granite at the base of the slope (810460 815390 Klf-18).

On the rocky coast north of San Shek Wan (807140 816110 Klf-19), there is a 3 m-wide, porphyritic, quartz monzonite dyke with abundant feldspar megacrysts, up to 6 mm across. The dyke trends eastnortheast, and has a finer-grained, sheared, or trachytic margin against foliated sedimentary rocks to the north.

Hei Ling Chau. Quartz monzonite crops out as dykes and small bodies on southern Hei Ling Chau, along the east coast of Hei Ling Chau, and on both east and west Sunshine Island (Chau Kung To). The largest bodies are on the southern ends of both islands, where they are up to 700 m long, and less than 200 m wide.

On southern Hei Ling Chau, the quartz monzonite trends northeast with an inferred contact against fine-grained granite. The contact is obscured by boulders, and no age relationships can be determined. However, on the southern tip of Hei Ling Chau (822660 811470 Klf-20), presumably close to the southern margin of the dyke-like intrusion, medium-grained granite and aplite are cut by fine-grained quartz monzonite with a trachytoid texture.

On the southwestern tip of Sunshine Island (823060 813250 Klf-21), the quartz monzonite has a chilled margin about 50 mm wide against feldsparphyric rhyolite. There is a rapid decrease in the feldspar phenocryst content closer to the contact, whereas the older rhyolite appears unchanged at the contact.

Small dykes of quartz monzonite are common along the east coast of Hei Ling Chau. Some have slightly chilled margins against the granite country rock. One dyke (822560 812390 Klf-22) is cut by a flow-banded quartzphyric rhyolite dyke, 10 m wide and trending northeast.

Quartz monzonite was only seen exposed on the southwest tip of the Hei Ling Chau, but its presence is inferred from the abundant, dominant corestones which have toppled onto the beach near the northwest tip of the island (823700 813800 Klf-23). There, the feldsparphyric rhyolite adjacent to the monzonite has been strongly deformed. Feldspar megacrysts are streaked and contorted in a zone over 1 m wide. This is believed to be the result of the younger monzonite deforming the hot, plastic dyke rocks.

Abundant boulders of porphyritic quartz monzonite disguise the generally deeply weathered outcrop in the southwest. However, in one tumbled block (823300 813200 Klf-24), the contact between quartz monzonite and feldsparphyric rhyolite can be seen. The rhyolite appears to be partially assimilated, or absorbed, by the monzonite, and hence is the older rock. The northern margin of the quartz monzonite is exposed in a deeply weathered coastal section. It has a chilled margin, about 50 mm wide, in which there is a rapid decrease in the number of feldspar phenocrysts. The older rhyolite has a distinct margin. Boulders of rhyolite on the point to the south (823000 813200 Klf-25) suggest that the quartz monzonite has a complex, dyke-like form, incorporating slivers of older rock.