The Tsing Shan Granite (Table 6.2) forms an elliptical pluton in the western New Territories with its outcrop extending from Lung Kwu Chau in the south to Tsim Bei Tsui in the north (Figure 6.3). The western boundary of the pluton is defined by the Deep Bay Fault and the eastern boundary by the Tuen Mun Fault. Contacts with adjacent rocks are faulted making determination of intrusive relationships impossible. However, offshore boreholes drilled between Pillar Point and Lung Kwu Chau suggest that the Tsing Shan Granite has intruded the Tai Po Granodiorite.

The Tsing Shan Granite is composed of variably deformed, equigranular to inequigranular two-mica monzogranite (Plate 6.11). It is typically strongly deformed and recrystallized although southernmost exposures on Lung Kwu Chau are only weakly deformed. The degree of deformation generally increases from southwest to northeast. In the porphyritic lithologies, large (7–9 mm) anhedral phenocrysts (megacrysts) of microcline, orthoclase, quartz and plagioclase are set in a fine-grained, partly recrystallized, granular matrix of quartz, alkali feldspar, plagioclase and biotite. Feldspar megacrysts are commonly veined by quartz. Plagioclase is weakly concentrically zoned and typically has cloudy cores. In the equigranular lithologies, alkali feldspar consists of weakly concentrically-zoned and weakly strained mesoperthite and microcline. Greenish brown biotite is the chief mafic mineral. Plagioclase is oligoclase to albite in composition. Muscovite typically occurs as a late stage primary mineral infilling interstices between quartz and feldspar. Accessory minerals include zircon, allanite, and monazite.

A whole-rock Rb–Sr isochron age of 152 ± 6 Ma has been obtained for the Tsing Shan Granite (Darbyshire, 1993). However, zircon U–Pb age-dating of the granite has indicated an age of <159.6 ± 0.5 Ma (Davis et al., 1997) suggesting a similar emplacement age to the Tai Lam Granite.

Details

Tsing Shan. Medium-grained granite forms the western part of the Tsing Shan area. It is commonly megacrystic, with feldspar up to 30 mm. Xenoliths of basic lava, sometimes containing feldspar megacrysts, are present in parts of the outcrop. The contact with the adjacent fine- to medium-grained granite is gradational. The medium-granied granite generally possesses an equigranular texture, whole progressive modification results in the adjacent megacrystic fine-to medium-grained variety. A distinct megacrystic variety of medium-grained granite was proved in boreholes at Castle Peak Power Station (810000 826000 Jms-1). It is characterized by prominent biotite clots up to 8 mm and alkali feldspar phenocrysts up to 35 mm set in a groundmass of 2 to 6 mm. This variety also contains xenoliths of basaltic lava.

The medium-grained granite is almost invariably microcrystic in thin section, and is often foliated. Typical granite from Lung Kwu Tan (809450 828530 Jms-2) contains feldspar up to 15 mm, but overall has an apparently equigranular texture of 5 mm or less. The biotite forms as felted aggeregates up to 7 mm and well developed single crystals up to 3 mm.

The fine- to medium-grained granite of Tsing Shan is typically megacrystic. In the south, near Mong Hau Shek (812100 825600 Jms-3), the granite contains pegmatite nests and associated layers enriched in biotite; the granite here is also foliated. There are quartz and feldspar megacrysts up to 5 mm, but they are indistinct. The groundmass varies from 1 to 3 mm, with a microcrystic infiltration visible only in thin section. Typical fine- to medium-grained granite from south of Lung Kwu Tan (811300 826300 Jms-4) is microcrystic and slightly foliated.

Fine-grained granite forms the main massif of Tsing Shan, being noticeably more resistant to erosion than adjacent coarser-grained varieties. The rock is generally megacrystic, and on the watershed west of Leung Tin Estate shows a progressive westward increase in megacryst content. A xenolith of basic igneous rock up to 150 mm across was seen about 1 km south of Tsing Shan (813200 826700 Jms-5). This rock, possibly lava, is fine grained and greyish green, with small feldspar phenocrysts. The contact between the granite and country rock to the east appears to have a faulted intrusive nature, with a shear zone typically up to 5 m wide. The fine-grained granite is often metamorphosed to granite schist or mylonite, for example northwest of Po Tong Ha (814800 831300 Jms-6). The contact dips 47^o to the west near Tsing Shan (813480 826900 Jms-7), and to the northwest at 62^o near Leung Tin Estate. About 1 km north of Tsing Shan (813410 828530 Jms-8) the contact between fine-grained granite and sandstone is irregular, with leaves of foliated granite within the sandstone; the contact dips west at about 50^o. A small intrusion of fine-grained granite, separate from the main mass, outcrops within the Tsing Shan Formation west of Shan King Estate (815800 828780 Jms-9). The granite is slighty foliated, with irregular dykes of aplite and small quartz veins.

Typical fine grained granite from Tsing Shan (812700 828200 Jms-10) is megacrystic, with quartz up to 8 mm and feldspar up to 12 mm. Megacrysts are almost invariably abundant, dominating the rock texture, while the matrix is almost very fine-grained. In thin section the groundmass is generally less than 0.1 mm, with pronounced granulation of the edges of the megacrysts. Biotite forms small, felted aggregates up to 3 mm, and rarely as single crystals.