The Luk Keng Quartz Monzonite (Table 6.4 ) crops out in two places on the southeastern margin of the Lantau Caldera. A major outcrop occurs at the type locality of Luk Keng and smaller outcrops are found at Fan Lau and Cha Kwo Chau. The monzonite is typically fine grained and dominantly contains subhedral megacrysts of orthoclase (7–10 mm) with subordinate, concentrically-zoned plagioclase phenocrysts (3–5 mm). The matrix mostly comprises plagioclase, quartz, and biotite with trace amounts of zoned allanite, zircon, titanite, apatite and Fe-oxide.

The contact with older intrusive units is not exposed although at Fan Lau, a double enclave relationship confirms that the monzonite post-dates the Lantau Granite (Plate 6.19).

An absolute age for the Luk Keng Quartz Monzonite has yet to be determined. However, field relationships at Fan Lau indicate that it is older than the Tong Fuk Quartz Monzonite (see below), while petrography and geochemistry suggest that it belongs to the same magmatic phase as the Mount Davis Formation (Chapter 5).

Details

Luk Keng. On the coast (810620 808880 Kcl-1) north of Luk Keng Shan, there is a xenolith of dark grey, finely banded lava about 0.3 m across, and 60 m to the southeast is a light grey felsic xenolith, up to 1 m in diameter.

Exposures of the quartz monzonite can be seen along the road heading south from Shui Hau towards Lo Kei Wan. At the end of the road (809800 808300 Kcl-2), there are exposures of porphyritic, fine-grained quartz monzonite with feldspar megacrysts up to 25 mm long. The rock is pinkish-grey, with white plagioclase feldspar and a black biotite-rich groundmass.

On the south side of Shui Hau Wan, the contact between Yim Tin Tsai Formation tuff and the Luk Keng Monzonite can be mapped accurately on the basis of morphology, but it is not exposed. Abundant, flow-aligned feldspars can be seen in the monzonite (810390 808800 Kcl-3). About 80 m to the southwest, the feldspar megacrysts are mostly 2 to 6 mm long, and are set in an aphanitic, possibly trachytic groundmass.

The quartz monzonite is interpreted as extending offshore beneath superficial deposits, and is exposed again on Cha Kwo Chau, a small island 4 km south of Cheung Sha. Most of the island is typically composed of grey to pinkish-grey, porphyritic, quartz monzonite, containing a few dark xenoliths. On the south side of the island a shear zone has developed along the contact between quartz monzonite and granodiorite.