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The Geology of Hong Kong (Interactive On-line)
Tai Lam Granite - Jma

The Tai Lam Granite (Table 6.2) forms a large subcircular pluton in the northwestern New Territories, extending as far north as Yuen Long, east to Tsuen Wan, south to Lantau Island, and west to Castle Peak (Figure 6.3). It intrudes the Lantau Granite in the south, Palaeozoic sedimentary rocks in the north and west, and Jurassic volcanic rocks in the east. An inlier of Tai Po Granodiorite is present within the Tai Lam pluton in the northeast.

The Tai Lam Granite consists of a porphyritic medium-grained to equigranular fine-grained monzogranite (Plate 6.10). Equigranular lithologies are characterised by euhedral to subhedral moderately strained plagioclase (oligoclase to albite), commonly with slightly deformed twin lamellae, anhedral slightly strained quartz, perthitic orthoclase and subordinate microcline in a euhedral granular matrix. Greenish brown biotite is present in aggregates, and there is evidence for subsolidus growth of alkali feldspar and quartz to produce poikilitic textures. Accessory minerals include allanite and zircon, with epidote and chlorite present as alteration products. The inequigranular lithologies show evidence of deformation and partial recrystallization in the form of strained quartz and deformed feldspar lamellae. Muscovite and fluorite are present as late stage crystallization products, and yellowish brown biotite occurs as crystal aggregates.

A textural variant of the Tai Lam Granite (the Ma Wan Granite of Sewell & James (1995)) is present in northeastern Lantau Island and on Ma Wan. Here, the granite is weakly deformed, equigranular, medium- and fine-grained, with local zones of hydrothermal alteration. In thin section, the granite is typically rich in microcline which distinguishes it from typical Tai Lam Granite.

A Rb–Sr whole-rock isochron age of 155 ± 6 Ma has been obtained for fine- to medium-grained lithologies of the Tai Lam Granite. However, single and multigrain zircon fractions have yielded concordant results at 159.3 ± 0.3 Ma (Davis et al., 1997) which is likely to date the emplacement of the granite more precisely.