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The Geology of Hong Kong (Interactive On-line)
East Lantau Rhyodacite - Jkd

Dykes belonging to the initial pulse of the Lantau Dyke Swarm are concentrated mainly in northern and eastern Lantau Island. They are also found in central Lantau Island and on Ma Wan, the Soko Islands and numerous small islands and reefs to the south. In northern and eastern Lantau Island and on Ma Wan, the dykes intrude the Lantau Granite and Tai Lam Granite, whereas in central Lantau Island and the Soko Islands they intrude the Lantau Granite.

The East Lantau Rhyodacite dykes generally have large (5–20 mm) megacrysts of alkali feldspar, bipyramidal quartz and plagioclase, and crystal aggregates of biotite, set in a very fine-grained groundmass containing zoned allanite, zircon and Fe-oxide (Table 6 .3). Close to the margins of the dykes, the feldspar phenocrysts may show a weak preferred orientation.

The rhyodacite dykes commonly contain large xenolithic blocks of the host Lantau Granite. These distinguish them from younger rhyolite dykes (East Lantau Rhyolite). They may also grade internally from porphyritic rhyodacite at the margins to porphyritic microgranite in the interiors.

The East Lantau Rhyodacite dykes have yielded a U–Pb age of 146.5 ± 0.2 Ma (Davis et al., 1997) indicating that they are largely coeval with the emplacement of the Sha Tin Granite.