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The Geology of Hong Kong (Interactive On-line)
Mafic and intermediate dykes - mnd

Mafic and intermediate intrusive rocks are widespread throughout Hong Kong, cutting many plutonic and volcanic lithologies. They generally crop out as narrow (<1 m wide) dykes (Plate 6.27) but may also occur as small stocks and enclaves in granitoid rocks.

The dykes are dominantly sub-alkaline in composition and can be divided into two main groups. The oldest group comprises dominantly high-K calc-alkaline basaltic andesites to dacites which are found closely associated with rhyolitic dykes of the Lantau Dyke Swarm. In a detailed study of a 20 m-wide zoned composite dyke from the Lantau Dyke Swarm, Li et al. 2000 have showed that the margins of this dyke are composed of basaltic andesite (SiO2 = 54.5 wt%) which then pass inward through a series of sharp transitions to feldsparphyric dacite (SiO2 = 69.2 wt%) and high silica rhyolite- porphyritic microgranite (SiO2 = 76.2 wt%) in the core. Each pulse of new magma injected into the dyke is marked by a more silica-rich composition, and is sometimes accompanied by incorporation of rounded, semi-consolidated inclusions derived from an earlier pulse. The basaltic andesite margins of the dyke are volumetrically minor compared with the rest of the dyke, which is dominantly rhyolitic.

Elsewhere within the Lantau Dyke Swarm, similar composite dykes are seen, although in some cases, only thin (1 m-wide), solitary, basaltic andesite dykes subparallel to the main swarm are present. However, geochemical analyses of these solitary mafic dykes indicate that they lie on exactly the same fractionation trend as those dykes described above.

A younger group of dykes are shoshonitic in composition with higher average abundances of K2O, and lower TiO2 contents, than the older mafic to intermediate dykes. Included among this group are a number of calc-alkaline lamprophyres. These dykes commonly crop out over much of eastern and western parts of Hong Kong. They include a prominent dyke intruding columnar-jointed vitric tuff of the High Island Formation at the East Dam of the High Island Reservoir (see Plate 9.13). Ar–Ar age-dating of the younger group of dykes has returned ages ranging from 108.3 ± 1.6 Ma up to 75.1 ± 0.8 Ma (GEO, unpublished data).

The younger mafic dykes are typically very fine-grained and aphyric, with a groundmass composed of augite ± amphibole ± biotite, and subordinate feldspar and Fe-Ti oxide.

Mafic enclaves are present in several plutonic units including the Lantau Granite, Sok Kwu Wan Granite, and D'Aguilar Quartz Monzonite. These medium-grained rocks are commonly depleted in K2O due to alteration and are classified as quartz diorites.