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The Geology of Hong Kong (Interactive On-line)
East Lantau Rhyolite - Jko

Dykes emplaced during the second main pulse of the Lantau Dyke Swarm are mostly concentrated in northern Lantau Island, and on Ma Wan and Tsing Yi where they intrude East Lantau Rhyodacite dykes, volcanic rocks of the Yim Tin Tsai Formation, and the Tai Lam and Sha Tin granites. They may also form multiple and composite intrusions associated with high–K calc-alkaline mafic dykes.

The rhyolite dykes are mostly characterised by abundant bipyramidal quartz phenocrysts (Plate 6.15), with subordinate euhedral alkali feldspar megacrysts and plagioclase phenocrysts, set in a very fine-grained groundmass (Table 6.3). Many of the narrower rhyolite dykes (<3 m wide) are dominantly quartzphyric. The larger dykes commonly grade internally from rhyolite on the margins to porphyritic microgranite in the centres. In thin section, biotite is generally sparse and accessory minerals include zircon, allanite, apatite, fluorite and Fe–oxide.

A single zircon U–Pb age of 146.3 ± 0.3 Ma (Davis et al., 1997) has been obtained from a composite quartzphyric rhyolite dyke in north Lantau Island indicating that these dykes were emplaced during the same magmatic event that produced the main Lantau Dyke Swarm.