The Hok Tsui Rhyolite (Table 6.3) is the name given to a swarm of quartzphyric rhyolite dykes that intrudes the Tai Po Granodiorite and the Yim Tin Tsai Formation at Cape D'Aguilar (Hok Tsui) in the extreme southeastern corner of Hong Kong Island. The dykes are up to 10 m wide, strongly flow banded, and strike in a northeasterly direction (Plate 6.12).

The Hok Tsui Rhyolite is the only known representative of a magmatic event that occurred around 152 million years ago (GEO, unpublished data). However, inherited components in four other intrusive units belonging to the Kwai Chung Suite consistently give concordant or near-concordant data with ages ranging from 153.8 ± 0.4 Ma to 149.5 ± 0.4 Ma (Period 2, Davis et al., 1997) suggesting that there was a significant magmatic event in the region at this time.

Details

Cape D'Aguilar. The best exposures of quartzphyric rhyolite in the district are found at Cape D'Aguilar (844900 805600 Jkh-1), where a number of dykes cut both the dark grey coarse ash tuff and the granodiorite, and are in turn cut by basalt dykes. The rhyolite dykes are light pink to white in colour, but darker grey to greenish grey when fresh. Distinctive flow banding parallel to the irregular dyke margins is common. The dykes are approximately vertical, but their attitude varies greatly along the strike from northerly dips of only 35^o to southerly dips of 80^o. The rock has a very fine grained groundmass, with scattered quartz and a few feldspar megacrysts usually less than 2 mm in diameter. Biotite is present as tiny specks throughout the rock. In thin section, the groundmass is a microcrystalline aggregate of quartz and feldspar, with some granophyric texture around feldspar crystals. A 0.2 m wide quartzphyric rhyolite dyke cuts the granodiorite on the western side of the Stanley Peninsula (839850 807730 Jkh-2).