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The Geology of Hong Kong (Interactive On-line)
Needle Hill Granite - Jkn

The Needle Hill Granite (Table 6.3) forms an elliptical biotite monzogranite pluton on the northwestern side of the Shing Mun Valley with the long axis oriented in a northeast direction (Figure 6.4). The granite is largely as described and named by Allen and Stephens (1971), although its distribution is largely restricted to the Shing Mun Valley. The Needle Hill Granite is composed of porphyritic fine-grained monzogranite and equigranular medium-grained monzogranite (Plate 6.16) and includes numerous fine-grained granite dykes which intrude the Sha Tin Granite in the south and west. The granite intrudes the Tai Po Granodiorite in the north.

The fine-grained granites contain abundant subhedral mesoperthite, subhedral to euhedral plagioclase and anhedral quartz in roughly equal proportions. Reddish brown biotite and muscovite are commonly interstitial. Plagioclase is typically unzoned and zircon and fluorite are the main accessory minerals. The fresh appearance of the plagioclase is a distinctive feature of the granite.

In thin section, porphyritic lithologies contain phenocrysts of euhedral to subhedral microperthite, subhedral to euhedral plagioclase, and bipyramidal quartz, set in a granular groundmass of plagioclase, quartz and mesoperthite. Biotite is the dominant mafic mineral and accessory minerals include zircon and fluorite. Equigranular lithologies are characterised by abundant mesoperthite, plagioclase, and quartz, along with minor biotite and accessory minerals. The Needle Hill Granite may be confused with the Tai Lam Granite but the latter is frequently non-porphyritic and chemically different (see above).

A U–Pb single zircon age of 146.4 ± 0.2 Ma (Davis et al., 1997) has been obtained from the Needle Hill Granite. This indicates that it is approximately coeval with the Sha Tin Granite although field relationships suggest that the Needle Hill Granite is younger than the Sha Tin Granite.