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.

Details

Lamma Island. The fine- to medium-grained granite of Lamma Island is closely related to the intensely modified medium and coarse-grained granites. All these varieties have close affinities and are intermixed, making their distinction on the map difficult. Fine- to medium-grained areas have been mapped at Sok Kwu Wan Quarry (831700 808600 Kls-1), Luk Chau (George Island) (831800 809600 Kls-2) and north of Hung Shing Ye (830400 809200 Kls-3). There are isolated areas of finer-grained granite within the medium-grained granite in inland parts of central Lamma Island, although these cannot be delineated through lack of exposure. The Sok Kwu Wan Quarry (831750 808560 Kls-4) affords the best fresh rock exposures of this granite. The rock here is variable, but characterized throughout by abundant biotite. The average grain size is about 1.5 mm, but alkali feldspar megacrysts to 10 mm and quartz megacrysts to 5 mm are common. The granite is grey and particularly dark in places due to the biotite concentrations. Patches of fine-grained granite pervade this inequigranular rock and have modified the megacrystic medium-grained granite with which it is closely related. Large, angular basalt xenoliths (831800 808330 Kls-5) are recorded in the quarry face (Plate 6.A35).

At Sok Kwu Wan Quarry, medium-grained granite is surrounded by fine- and fine- to medium-grained granite. The finer varieties have infiltrated the medium-grained rock, in particular close to the contact in a zone 25 to 30 m wide (831850 808420 Kls-6). Biotite is abundant in the medium-grained granite and the rock is megacrystic in part, with prominent euhedral alkali feldspar megacrysts. Sometimes they are roughly aligned and similar in appearance to the tabular feldspars that characterise the quartz monzonite intrusions. Another feature of the granites exposed in this quarry is the presence of numerous basaltic xenoliths 0.6 to 1.0 m across and commonly very angular (831640 808370 Kls-7).

Luk Chau. On Luk Chau (831830 809590 Kls-8) the fine- to medium-grained granite has been infiltrated by monzonite. The inequigranular granite has a quartz poor, microcrystic fabric invading a rock which appears to have already undergone some modification from granitic fluids. The resulting fine- to medium-grained granite is very variable and markedly inequigranular. Patches of monzonitic rock displaying the characteristically aligned euhedral feldspar megacrysts can be seen in places (831820 809390 Kls-9). In the vicinity of Hung Shing Ye (830400 809200 Kls-3) the fine- to medium-grained granite is markedly inequigranular, but has a distinctive biotite rich groundmass, with an average grain size of about 1.5 mm. The rock has abundant megacrysts of quartz up to 6 mm across, as well as sparse feldspar megacrysts.