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The Geology of Hong Kong (Interactive On-line)
Sok Kwu Wan Granite - Kls

The Sok Kwu Wan Granite (Table 6.5) is the name given to a small porphyritic monzogranite pluton outcropping in the vicinity of Sok Kwu Wan in the northern part of Lamma Island (Figure 6.6). The granite is exposed on Luk Chau off the northeastern coast of the main island and extends as far south as Lo So Shing Beach on the west side of the island. At Pak Kok, on the northern tip of the main island, the granite intrudes coarse ash crystal tuff of the Yim Tin Tsai Formation with abundant evidence for assimilation. Dykes of alkali feldsparphyric monzogranite extend upward into the overlying tuff which displays evidence for incipient partial melting. The tuff clasts show segregation of mafic and felsic minerals on the margins grading inward to more normal granular textures. Biotite schlieren, commonly seen in thin section, are almost certainly derived from assimilated tuffaceous material. Also present in the granite at Pak Kok are large blocks of fine-grained granite with chilled margins.

The Sok Kwu Wan Granite is dominantly non-porphyritic fine- and medium-grained monzogranite, but may vary locally to porphyritic medium-grained lithologies with large megacrysts (5–10 mm) of alkali feldspar (Plate 6.26). Euhedral to subhedral perthitic microcline megacrysts and crystal aggregates of quartz are set in a subhedral-granular, fine- to medium-grained matrix of quartz, microcline, albite and greenish brown biotite. Quartz is commonly weakly strained and plagioclase may be zoned with Ca-rich cores. Accessory minerals include zircon, titanite, fluorite and Fe-oxide.

An absolute age for the Sok Kwu Wan Granite has not been determined. However, based on geochemical criteria and the associated mafic enclaves, the granite is interpreted as having been emplaced during the final pulse of Mesozoic magmatic activity.