The South Lamma Granite (Table 6.3) forms a subcircular, equigranular biotite monzogranite pluton centred on the southern part of Lamma Island (Figure 6.4). The best exposures are found at the type locality on the east side of the island north of Shek Pai Wan where the granite is seen to intrude the Lantau Granite. A body of texturally modified, slightly metamorphosed biotite monzogranite outcropping in the vicinity of Kam Lo Hom may be a related outlier.

South Lamma Granite is typically equigranular and medium grained, although fine-grained equivalents are found adjacent to contacts with older units and as late stage aplite dykes. The granular texture is characterised by euhedral to subhedral oligoclase, subhedral to anhedral perthitic microcline and orthoclase, and subhedral to anhedral slightly strained quartz and minor reddish brown biotite. Perthitic microcline is generally more abundant than orthoclase and may partly be derived from subsolidus recrystallization. Plagioclase often displays weakly sercitized Ca-rich cores and is also slightly strained. Trace muscovite occurs either as a late stage magmatic mineral or replacing biotite. Accessory minerals include fluorite, allanite, zircon, titanite and Fe-oxide.

A U–Pb age of 148.1 ± 0.2 Ma has recently been obtained for the South Lamma Granite (GEO, unpublished data).

Details

Southern Lamma Island. Uniform fine- to medium-grained granite crops out over a wide area of southern Lamma Island. There is little lithological variation between the area southwest of Mount Stenhouse (830500 805500 Jkl-1) and that to the east of Sok Kwu Wan (832700 807700 Jkl-2). The rock is roughly equigranular, with a uniform groundmass and average grain size bordering on 2 mm. It is usually light pink and non megacrystic, and is intruded by numerous feldsparphyric and quartzphyric rhyolite dykes as well as basalt dykes. In the coastal section east of Yung Shue Ha (833170 806840 Jkl-3) the fine- to medium-grained granite has a very irregular contact with the older coarse-grained granite (Plate 6.A13). The younger light pink granite appears to have intruded the older dark grey granite along cooling joints, producing an unusual trellis pattern extending over many metres. On the coast south of Mount Stenhouse (830800 804700 Jkl-4), fine-grained granite has intruded the fine- to medium-grained granite without any apparent infiltration of the latter. Instead, the existence of pegmatite concentrations along the contact suggests that the older granite was already cool at the time of the later intrusion.