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The Geology of Hong Kong (Interactive On-line)
Tai Po Granodiorite - Jmt

The Tai Po Granodiorite (Table 6.2) is exposed in discontinuous outcrops across central and southern parts of Hong Kong with the largest outcrops in the central New Territories and on Tsing Yi (Figure 6.3). The granodiorite is thought to have been emplaced as a large, high-level, intrusive sheet centred on Tai Mo Shan towards the close of Tsuen Wan Volcanic Group activity. However, it has been widely fragmented by subsequent intrusive activity and faulting. Lithologies vary texturally, without any significant change in bulk composition, from normal subhedral-granular granodiorite, through porphyritic fine-grained granodiorite to densely porphyritic dacite. Porphyritic fine-grained granodiorite and dacite predominate in the type area of Tai Po, and at Shek Kong, Tai Mo Shan, Tsuen Wan and Tsing Yi, and in smaller outcrops at Cape D'Aguilar and Kau Pei Chau. Equigranular medium-grained granodiorite crops out on Stanley Peninsula, Lamma Island, and at Cheung Sha on Lantau Island. Equigranular granodiorite, tentatively correlated with Tai Po Granodiorite, has also been encountered in offshore boreholes in Urmston Road. Texturally modified subequigranular granodiorite is found mainly between Tai Lam Reservoir and Route Twisk, in central Tsing Yi, and on Ngau Chau (Plate 6.5).

Thermal metamorphism has substantially modified the texture of granodiorite close to the contacts with later igneous intrusions. Extensive recrystallization of quartz and biotite at the expense of alkali feldspar and amphibole has led to an increase in grain size, producing a variety of subequigranular lithologies. Original phenocryst minerals in the granodiorite can be recognized as being larger than the surrounding grains but with modified crystal boundaries. Quartz is typically anhedral with serrated grain boundaries. Layers of recrystallized biotite (schlieren) are common, sometimes completely pseudomorphing amphibole grains. Chlorite and epidote alteration in these metamorphosed rocks is ubiquitous. Close to major shear zones, the granodiorite is dynamically metamorphosed, resulting in a greatly reduced grain size. Equigranular granodiorite from Stanley Peninsula, Lamma Island, and Lantau Island generally displays well-developed subhedral-granular texture although the Stanley samples have been partially recrystallized. Non-porphyritic granodiorite, found in Urmston Road boreholes, is typically medium grained with subhedral-granular texture.

Fresh samples of granodiorite contain subhedral grains of quartz, alkali feldspar, and zoned plagioclase, with subordinate grains of greenish-brown biotite and dark green amphibole. Accessory minerals include zircon, titanite, apatite, zoned allanite and Fe-oxide.

Multi-grain zircon fractions from the Tai Po Granodiorite have yielded concordant U–Pb ages of 164.6 ± 0.2 Ma (Davis et al., 1997).