The Shan Tei Tong Rhyodacite (Table 6.4 ) includes all porphyritic rhyodacite and microgranite dykes intruding the Chi Ma Wan and South Lamma plutons. These dykes (Plate 6.18) occur mostly in a zone extending from the Chi Ma Wan Peninsula, across Lamma Island to southern parts of Hong Kong. In thin section, the dykes are difficult to distinguish from those of the Lantau Dyke Swarm. Large feldspar megacrysts are prominent within a fine-grained granular matrix of quartz, feldspar and biotite.

Details

Lamma Island. A prominent dyke crossing southern Lamma is composed of feldsparphyric rhyodacite and is the largest continuous dyke mapped in this district. It averages 100 m in width and in places exceeds 150 m. The dyke is roughly vertical and has been displaced by a major fault at Sham Wan (831800 805200 Kct-1). A similar dyke rock occurs on the eastern side of Sham Wan, where it crosses the Yuen Kok peninsula (832830 805430 Kct-2). Samples collected along this outcrop over a distance of several kilometres show uniformities of texture and appearance. The rock has a fine groundmass with abundant felty biotite patches. In the centre of the dyke (832860 805420 Kct-3) the rhyodacite shows a remarkable similarity to the granodiorite cropping out at Lo So Shing (830500 807440 Kct-4). A similar feldsparphyric rhyodacite dyke occurs 200 m south of the summit of Mount Stenhouse (831320 805660 Kct-5), but there the dyke is only 50 m wide and megacrysts are less prominent (Plates 6.A20 & 6.A21). On the Ngau Tai peninsula, east of Mo Tat Wan (834140 807660 Kct-6), a 75 to 100 m wide dyke of feldsparphyric rhyodacite trends east-west, cutting coarse grained granite. A parallel rhyodacite dyke crops out on the coast 600 m east of Mo Tat Wan (833600 807470 Kct-7). There, the dyke is at least 75 m wide and cuts the coarse grained granite. It is in turn cut by a NW-SE trending quartzphyric rhyolite dyke. The rhyodacite dykes in the Ngau Tai area have a groundmass similar to that of the dykes on southern Lamma Island, but alkali feldspar megacrysts appear to be more common at Ngau Tai.

Numerous feldsparphyric rhyodacite dykes striking roughly east-west cut the granites of central and southern Lamma Island. Most average 20 to 30 m wide and extend over distances varying from 100 m to 2 km. The largest dyke attains a width of 100 m west of Shek Pai Wan (832000 806350 Kct-8), and further west this dyke divides into two branches (831690 806320 Kct-9). A similar branching of a feldsparphyric dyke occurs northwest of Mount Stenhouse (830700 806180 Kct-10). The dyke rock is characterised by a dark grey to greenish grey, very fine grained groundmass, with prominent abundant light grey and light pink alkali feldspar megacrysts averaging 10 mm across. Quartz and plagioclase are found as smaller, less common megacrysts reaching 6 mm across. Biotite is present, usually as individual flakes up to 5 mm in length. On the weathered surfaces, the alkali feldspar megacrysts stand proud of the surface.

Cheung Chau. On the west side (821350 807120 Kct-11) of Morning Beach (Nam Tam Wan), there is a 30 m-wide feldsparphyric rhyodacite dyke trending 080^o within medium-grained granite. The dyke has an irregular chilled margin, contains prominent biotite in the groundmass, and includes many melanocratic xenoliths, up to 0.5 m across. A similar (or possibly the same) dyke, exposed on the coast (822250 807470 Kct-12) further east, incorporates numerous, rounded, feldsparphyric xenoliths ranging from 0.02 to 0.3 m.

Chi Ma Wan. The eastern part of the Chi Ma Wan peninsula is dominated by feldsparphyric rhyodacite dykes, whereas to the west there are only a few, narrow, eastnortheast-trending dykes. On the coast (819210 810470 Kct-13) east of Cheung Sha Wan, a feldsparphyric rhyodacite dyke, which is coarsely porphyritic, varies gradationally from north to south towards its centre, into a porphyritic microgranite with abundant small phenocrysts, up to 10 mm across. The dyke intrudes medium-grained granite. Similar gradations also occur along the coast north of Ha So Pai.