The Chek Lap Kok Granite (Table 6.2) forms a subcircular pluton centred on the new airport at Chek Lap Kok (Figure 6.3). Prior to construction of the airport, the granite was exposed on the western half of the former island of Chek Lap Kok and on the former island of Lam Chau. Exposures are now confined to the area between Tin Sum and Hau Hok Wan on the northern coast of Lantau Island. The type locality is designated as Hau Hok Wan.

The Chek Lap Kok Granite was formerly seen on Chek Lap Kok to intrude the porphyritic fine- to medium-grained Lantau Granite. This relationship is still exposed on the northern coast of Lantau Island at Hau Hok Wan, indicating that the Chek Lap Kok Granite post-dates emplacement of the Lantau Granite.

The Chek Lap Kok Granite consists of an equigranular, fine-grained, leucocratic monzogranite (Plate 6.9). Euhedral to anhedral alkali feldspar is composed principally of mesoperthite. Subordinate euhedral to subhedral albite is unzoned and anhedral quartz is weakly strained. Interstitial late stage muscovite is common along with trace amounts of euhedral fluorite. Biotite, zircon, and Fe-oxide may also be present in trace amounts.

Three zircon fractions from the Chek Lap Kok Granite have yielded consistent results that define an age of 160.4 ± 0.3 Ma (Davis et al., 1997)

Details

Chek Lap Kok (prior to development). Inequigranular, non-megacrystic to sparsely megacrystic, fine-grained granite crops out west of a line from Sham Wan to Fu Tei Wan. The granite is light pinkish-grey in coastal exposures, and grey when fresh. Quartz megacrysts, up to 5 mm, are only rarely seen. Single flakes of biotite, mostly from 1 to 3 mm, are a prominent feature of the rock. The grains of the groundmass are between 0.2 and 2 mm, but are mostly around 1 mm.

Sample HK8362 (811010 819760 Jmc-1) from Cheung Sha Lan is typical, displaying single biotite flakes up to 2 mm. The rock exposures at this locality are relatively free of joints and appear to have been extracted for building stone. Sample HK8363 (810690 819050 Jmc-2) from the borrow area for the test embankment is a finer-grained variety of granite (microgranite), and this lithology dominates the borrow. Much of the construction material for the test embankment is fine-grained granite, but blocks of fine- to medium-grained granite, and rarely medium-grained granite, are also present.

Tin Sam. Pinkish-grey, megacrystic fine-grained granite is exposed on the coast west of Hau Hok Wan. Biotite occurs as individual crystals between 1 and 2 mm across, and the rock is cut by thin aplite dykes, numerous thin quartz veins, and occasionally pegmatitic patches.

Southeast of Tin Sam, on the western shores of Tung Chung Wan, fine-grained granite country rock is cut by numerous dykes of feldsparphyric rhyolite, porphyritic microgranite, and basalt. Exposures on the ridge (809930 816270 Jmc-3) to the west comprise slightly weathered, brownish-grey, sparsely megacrystic, fine-grained granite. The inequigranular groundmass varies from less than 1 mm to over 3 mm, and rounded quartz megacrysts up to 10 mm across also occur.

Lam Chau. The small island of Lam Chau (prior to development) is dominantly composed of pinkish-grey, inequigranular, fine-grained granite. A notable feature of exposures on the western tip of the island is the abundance of quartz veins. There are also zones of brecciation, epidotization and silicification in the granite. These zones have a preferred east-west trend and are cut by basic dykes (808580 817810 Jmc-4). The fine-grained granite is remarkably similar to that seen in northwest Chek Lap Kok and along the north Lantau coast, but includes pegmatitic patches and zones of biotite schleiren, as well as irregular aplitic veins.