The D'Aguilar Quartz Monzonite (Table 6.4 ) is the collective term given for stocks of quartz monzonite outcropping in the southern part of Hong Kong on D'Aguilar Peninsula, Stanley Peninsula, and Lamma Island (Figure 6.6). The monzonite has previously been referred to as the D'Aguilar Quartz Syenite (Strange, 1990; Sewell et al., 1992). However, based on normative mineralogy, the rock is more correctly classified as a quartz monzonite. The D'Aguilar Quartz Monzonite intrudes the Yim Tin Tsai Formation and Tai Po Granodiorite in the southern part of the D'Aguilar Peninsula and on Stanley Peninsula, and intrudes the Yim Tin Tsai Formation in the northern part of Lamma Island. A broad zone of fine-grained porphyritic quartz monzonite is present at the contact with the Yim Tin Tsai Formation in the southern part of the D'Aguilar Peninsula.

The D'Aguilar Quartz Monzonite is typically porphyritic and fine to medium grained (Plate 6.20). Large (5–10 mm) euhedral to subhedral phenocrysts of oriented mesoperthite are set in a granular matrix comprising euhedral to subhedral (2–3 mm) strongly concentrically-zoned andesine to oligoclase, subordinate anhedral quartz and minor euhedral green amphibole and brown biotite. The matrix comprises quartz, alkali feldspar, and plagioclase with accessory minerals including zircon, allanite, apatite, and Fe-oxide.

Sewell et al. (1992) obtained a Rb–Sr whole-rock isochron age of 147 ± 8 Ma from eight samples of D'Aguilar Quartz Monzonite. However, recent U–Pb zircon age-dating of two samples from this intrusion have returned identical ages of 140.6 ± 0.3 Ma (GEO, unpublished data).

Details

D'Aguilar Peninsula and Stanley. Quartz monzonite crops out over approximately 1.5 sq km, centred on D'Aguilar Peak (843700 808700 Kld-1), and apart from a wide fine-grained zone along its southern margin it is remarkably uniform in texture and appearance. It is a leucocratic medium-grained rock with abundant, prominent euhedral alkali feldspar megacrysts (Plates 6.A22 & 6.A23). These average 7 mm in length and are white or light pink in colour. In places these megacrysts are aligned. Quartz makes up less than 15% of the rock and does not occur as megacrysts. In thin section the groundmass is roughly equigranular, with interlocking subhedral alkali feldspar dominant. Biotite makes up 5 to 6% of the rock, and accessory minerals include minor amphibole, orthite and apatite. The very fine-grained chilled margin of the pluton around Hok Tsui Wan (843700 807850 Kld-2) has alkali feldspar megacrysts set in an indeterminate aphanitic groundmass.

Quartz monzonite is found on the Stanley Peninsula, where it intrudes granodiorite and tuffs of the Yim Tin Tsai Formation, and it is in turn intruded by fine to medium- and medium-grained granites. Northwest of Stanley Village an almost circular outcrop of quartz monzonite (839450 809200 Kld-3) is surrounded by younger granite. This monzonite closely resembles that of the D'Aguilar Peninsula pluton. A similar but smaller outcrop of quartz monzonite is found (838740 809150 Kld-4) in the northeastern corner of Chung Hom Wan Bay, where it intrudes Ap Lei Chau Formation tuffs. It is possible that, in view of the medium-grained nature of this rock, the outcrop represents a much larger pluton at depth.

Lamma Island. Quartz monzonite is found as a number of small intrusive bodies in northern and eastern Lamma Island. To the north and east of Yung Shue Wan, quartz monzonite intrudes both the volcanics and granites. Generally, the groundmass is fine-grained, but abundant alkali feldspar megacrysts are present, and these are commonly aligned. Excavations near Tai Peng (830140 809860 Kld-5) have revealed quartz monzonite at the granite-volcanic contact, with some infiltration of monzonite into the coarse ash tuffs. On Luk Chau (George Island), quartz monzonite intrudes the fine- to medium-grained granite, and infiltration and modification of this granite by monzonitic fluids is common. Monzonitic patches several metres across are present in the granite near the summit at the centre of the island (831940 809560 Kld-6). The weathered quartz monzonite produces rounded corestones that are common on the sparsely vegetated hill slopes east of Hung Shing Ye (830840 808740 Kld-7).

Megacrystic medium-grained quartz monzonite crops out immediately southeast of Mo Tat Wan (833160 807560 Kld-8) in eastern Lamma, where it intrudes coarse-grained granite as a northwesterly striking dyke like body some 250 to 300 m in width.