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The Geology of Hong Kong (Interactive On-line)
Po Toi Granite - Klp

The Po Toi Granite (Table 6.5) forms a subcircular pluton centred on the southeastern tip of Hong Kong Island Figure 6.6). The pluton encompasses large outcrops of medium-grained granite exposed at Stanley Peninsula and D'Aguilar Peninsula, and granites exposed to the southeast on Sung Kong Island, Beaufort Island and Po Toi. The granite is also exposed on a reef known as Castle Rock between Stanley and the Po Toi Islands. The Po Toi Granite is texturally zoned with coarse-grained megacrystic lithologies predominating in the south and east, and equigranular medium- and fine-grained lithologies in the north and west (Plate 6.23). Pegmatitic veins are common on the Po Toi Islands. The Po Toi Granite intrudes the Tai Po Granodiorite and D'Aguilar Quartz Monzonite on the Stanley and D'Aguilar peninsulas.

Porphyritic lithologies are typically characterised by large (4–10 mm) euhedral to subhedral alkali feldspar megacrysts composed of mesoperthite. Subordinate subhedral to anhedral quartz megacrysts, sometimes bipyramidal, are set in a medium- to fine-grained subhedral-granular matrix of quartz, orthoclase, plagioclase and sparse microcline. Orthoclase megacrysts are commonly sericitized. Subhedral zoned plagioclase (oligoclase to albite) also has sericitized cores and quartz is generally strained. Reddish-brown biotite is commonly altered to chlorite and may occur as crystal aggregates. Accessory minerals include zircon, titanite, allanite and Fe-oxide. Fluorite is sometimes present as a secondary mineral along with muscovite.

Equigranular granites have similar mineral compositions to the porphyritic lithologies except for a generally higher proportion of microcline. Titanite is absent in the fine-grained lithologies and biotite only occurs as sparse reddish-brown flakes. Fluorite and muscovite are commonly present as secondary minerals, and muscovite may also occur.

The Po Toi Granite has not yet been isotopically dated but field relationships indicate that it was emplaced after the D'Aguilar Quartz Monzonite