The Deep Bay Granite subcrops offshore to the west of the Deep Bay Fault in the western New Territories (Figure 6.1). Contacts with older rocks in Hong Kong have not been observed. Fresh rock recovered from fourteen marine boreholes in Deep Bay consists of weakly deformed, fine- to medium-grained, two-mica monzogranite. In thin section, quartz, alkali feldspar, and plagioclase form a subhedral-granular texture with subordinate reddish-brown biotite and muscovite. Fluorite, zircon and monazite are the main accessory minerals.

Whole-rock major and trace element geochemistry (Sewell & Campbell, 1997) and an isotope analysis (Darbyshire & Sewell, 1997) indicate that the Deep Bay Granite has S-type affinities. In this regard, it is compositionally distinct from all of the Jurassic and Cretaceous I- and A-type granites (see below). Two single zircon fractions and a grain of monazite from the Deep Bay Granite (808046 832130 Td-1) have yielded concordant results that indicate a magmatic crystallization age of 236.3 ± 0.8 Ma (Davis et al., 1997), i.e. Triassic.