Mineralogy of granites in the Discovery Bay area, east Lantau Island

Abstract

Based on the geological information of Hong Kong from the Civil Engineering Department (CEDD), HKSAR, and studies of the magmatic evolution of rocks in South Eastern China, Mesozoic volcanic and plutonic rocks are the dominant rock types in Hong Kong. They were mainly of Late Jurassic to Early Cretaceous age and emplaced consequently in four magmatic episodes. The eastern trending Lantau Dyke Swarm is one of the special geological settings in Hong Kong. In Discovery Bay area in North East Lantau Island, alternative layers of eastern trending granite and porphyry are observed. The major rock studied is Lantau granite. Information about the mineral compositions and microtextures were obtained by microscopy and Electroprobe microanalyser (EPMA). The granite is found to have high percentage of SiO2 (average about 65%), peraluminous (high Aluminum content) and sodium-rich plagioclase (Albite (Ab), Oligoclase (Ab70-90) and Andesine(Ab50-70)). It is suggested that the major source for Lantau granite came from partial melting of the continental crust, while mixing of mantle magma might have occurred, producing the peraluminous I-type granitoid. The medium grained granite and some perthitic features in feldspar suggested that it was allowed to cooled very slowly over a long period of time. Remelting of the crustal source could be occurred thereafter in the Late Jurassic to Early Crustaceous. The porphyry, which are younger and intruded to the Lantau granite, are found to be more felsic and alkali by fractional crystallization of the crustal source.