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Quartz syenite is poorly exposed in inland areas but the landform, abundance of corestones, andsoil colour often indicate its presence. Good field relationships are only seen in coastal exposures.The intrusions are generally steep-sided, and some are dyke-like. The quartz syenite islithologically uniform, porphyritic and has a fine-grained groundmass (Plate 26). Alkali feldsparmegacrysts are very prominent and can be up to 40 mm long, but most are less than 15 mm.Plagioclase feldspars are smaller, and white or greenish-white in most exposures, contrastingwith the distinctively pinkish alkali feldspar. The proportion of alkali feldspar is much greaterthan the plagioclase, Grey quartz crystals are infrequent. Visually, the groundmass seemsdominated by mafic minerals, but they only constitute a small proportion of the rock.DetailsLuk Keng. The largest quartz syenite body in the district is exposed between Shek Mun Shan and Luk Keng Shan,south of long Fuk. The margins of the intrusion are well defined in this area, and the quartz syenite probablyextends offshore beneath superficial deposits as far as Cha Kwo Chau, a small island 2 km southeast of Tong Fuk.On the hillside between Shek Mun Shan and Luk Keng Shan, the syenite forms large corestones up to severalmetres across. On footpaths and tracks the typically reddish-brown saprolitic soil of weathered quartz syenitesurrounds these corestones.At its northwestern extremity, the Luk Keng Syenite intrudes porphyritic rhyolite lava of the Lantau Formation,although the contact cannot be seen. About 700 m westnorthwest of Shui Hau, a roadside cutting (0856 0946) hasrhyolite lava at its base and porphyritic, fine-grained quartz syenite corestones within 20 m. A trachytoid textureoccurs in the syenite about 150 m to the eastsoutheast, with feldspar crystals no larger than 6 mm, set in anaphanitic groundmass. About 300 m west of Shui Hau, the lava appears to have been hornfelsed by the intrusion ofsyenite. Rounded, dark xenoliths are a feature of the quartz syenite, although they are relatively scarce. On thecoast (1062 0888) north of Luk Keng Shan, there is a xenolith of dark grey, finely banded lava about 0.3 m across,and 60 m to the southeast is a light grey felsic xenolith, up to 1 m in diameter .Exposures of the quartz syenite canbe seen along the road heading south from Shui Hau towards Lo Kei Wan. At the end of the road (098 083), thereare exposures of porphyritic, fine-grained quartz syenite with feldspar megacrysts up to 25 mm long. The rock ispinkish-grey, with white plagioclase feldspar and a black biotite-rich groundmass.Close to the contact (0901 0868) with the volcanics, feldspar megacrysts show distinct flow alignment. Thistrachytoid texture is also visible in boulders of syenite at Shek Lam Chau (0824 0839). In the absence of anexposed contact, this texture indicates that the syenite at Luk Keng intrudes porphyritic rhyolite lavas of the LantauFormation. Between Shek Lam Chau and Lo Kei Wan there is an outcrop of granite in fault contact (0852 0777)with the quartz syenite. The contact gives no clear indication of the relative ages of the two intrusive bodies.On the south side of Shui Hau Wan, the contact between Yim Tin Tsai Formation tuff and the Luk Keng Syenitecan be mapped accurately on the basis of morphology, but it is not exposed. Abundant, flow-aligned feldspars canbe seen in the syenite (1039 0880). About 80 m to the southwest, the feldspar megacrysts are mostly 2 to 6 mmlong, and are set in an aphanitic, possibly trachytic groundmass.The quartz syenite is interpreted as extending offshore beneath superficial deposits, and is exposed again on ChaKwo Chau, a small island 4 km south of Cheung Sha. Most of the island is typically composed of grey topinkish-grey, porphyritic, quartz syenite, containing a few dark xenoliths. On the south side of the island a shearzone has developed along the contact between quartz syenite and granodiorite.Pui O. The quartz syenite intrusion at Pui 0 lies on the northeast-trending Pui O-Sha Tau Kok Fault. The quartzsyenite is well exposed in cut slopes along the South Lantau Road, near Mui Wo, and at the nearby WaterTreatment Works. Elsewhere, exposures are limited to corestones and boulders, often set in a distinctive,reddish-brown saprolitic soil These boulders are particularly well displayed at Ham Tin Village, near Pui O, Theadjacent hillside is littered with syenite boulders, but is also covered in thick vegetation. Closer to Mui Wo, thehillside is more accessible, and the corestones are abundant.Fresh pinkish-grey syenite is exposed in cut slopes both at the Mui Wo Water Treatment Works and on theadjacent South Lantau Road. The rock can also be seen in its more typical form as corestones between 0,5 and 2 macross, in reddish-brown, saprolitic soil The contact to the north with the adjacent feldsparphyric rhyolite dykesand medium-grained granite is not exposed, but the outcrop form indicates that it is vertical. To the south of LukTei Tong, there are hill top exposures (1690 1340) of laminated tuffaceous siltstone. This outcrop appears to lie ontop of the syenite, rather than as a sliver within the intrusion, possibly indicating that the syenite is plug-like inform rather than dyke-like.74