The Geology of Hong Kong (Interactive On-line)

Kowloon Granite - Klk

The Kowloon Granite (Table 6.5) forms a subcircular biotite monzogranite pluton centred on Kowloon and Hong Kong Island (Plate 6.24). The northern boundary of the pluton is along the southern foot of the hills between Kowloon and Sha Tin districts. In the east and south, the pluton boundary is marked by hills comprising the volcanic roof rocks (Figure 6.6) but volcanic rocks are not generally exposed along the western boundary except at Stonecutters Island. The Kowloon Granite is remarkably uniform in texture and composition and is typically an equigranular medium-grained, biotite monzogranite (Plate 6.22). Plagioclase is strongly concentrically zoned and microcline is the dominant alkali feldspar with subordinate microperthite. The dominant mafic mineral is green to brown biotite, occurring as single flakes (1–3 mm). Accessory minerals include allanite, apatite, zircon and Fe-oxide. Porphyritic fine-grained granite in Kings Park in central Kowloon, fine-grained granite along the northern boundary of the pluton and in the vicinity of Ma On Shan, and small outcrops of coarse-grained granite near Kwun Yam Shan are minor textural variants. The granite typically contains quartz and plagioclase megacrysts set in a matrix of quartz, alkali feldspar and plagioclase with abundant biotite. Titanite and allanite crystals are unusually large. The petrographic features of the Kowloon Granite, particularly the strongly zoned plagioclase, are similar to those of the Lantau Granite. However, there is a notable absence in the Kowloon Granite of amphibole and feldspar megacrysts.

A seven point Rb–Sr whole-rock isochron for the Kowloon Granite has returned an age of 138 ± 1 Ma whereas a U–Pb single zircon age for the King's Park textural variant has yielded an age of 140.4 ± 0.2 Ma (Davis et al., 1997).