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The Geology of Hong Kong (Interactive On-line)
Sha Tin Granite - Jkt

The Sha Tin Granite (Table 6.3) forms an irregular elliptical-shaped biotite monzogranite pluton centred on the Sha Tin district with the long axis oriented to the northeast (Figure 6.4). The outcrop extends from Tsing Yi in the southwest to Wu Kai Sha in the northeast, and from Lion Rock in the southeast to Pak Tin in the northwest. The Sha Tin Granite is intruded in the southeast by the Kowloon Granite and to the northwest by the Needle Hill Granite.

The Sha Tin Granite comprises a coarse-grained central core surrounded by medium- to fine-grained lithologies. The coarse-grained granite is texturally uniform comprising large crystals of quartz, alkali feldspar and plagioclase forming a subhedral granular texture. Interstitial aggregates of brown biotite are a distinctive feature of the granite and are especially conspicuous in the coarse-grained rocks (Plate 6.13). Porphyritic fine-grained lithologies are commonly observed close to the boundaries of the pluton. In thin section, the coarse-grained granite typically contains strongly concentrically-zoned plagioclase, microcline, orthoclase and aggregates of brown biotite. Allanite and zircon are the dominant accessory minerals with subordinate Fe-oxide. Textures are often modified due to later intrusive and hydrothermal activity. Porphyritic fine-grained lithologies are the dominant textural variants along the southwestern edge of the intrusion. Fine- and medium-grained lithologies are commonly mylonitized, or strongly texturally disturbed indicating post-emplacement recrystallization. Biotite is the chief mafic constituent, along with fluorite and secondary muscovite.

A textural variant of the Sha Tin Granite is present on Tsing Yi (the Tsing Yi Granite of Sewell & Fyfe (1995)). The granite is typically equigranular and medium grained but may vary to porphyritic fine-grained lithologies close to the contact with overlying volcanic rocks. Subhedral to anhedral quartz, alkali feldspar, and plagioclase are present in roughly equal proportions, forming a granular texture. Mesoperthite (2–4 mm) typically displays well-developed Carlsbad twins and may be poikilitic. Plagioclase (1–3 mm) is albite in composition, unzoned, and may have sericitized cores. Greenish to brown biotite occurs as single flakes and is commonly altered to chlorite. Accessory minerals include zircon, apatite, titanite, allanite and traces of fluorite and Fe-oxide.

Sewell et al. (1992) obtained a Rb–Sr whole-rock age for the coarse-grained lithologies of 148 ± 9 Ma. More recently, Davis et al. (1997) used three zircon fractions to yield a more precise, and probably more accurate, U–Pb age of 146.2 ± 0.2 Ma for the emplacement age of the granite.