Mineralogy of porphyritic intrusions in the Discovery Bay area, east Lantau Island

Abstract

A study of the mineralogy of the dyke intrusions in the Discovery Bay area has been carried out because the intrusion contains porphyritic structure and the intrusion formed as successive layers into a large-scale dyke swarm which is special in Hong Kong. The objective of the study was to investigate the petrography and mineralogy of the porphyritic intrusion in the area, to obtain the mineral compositions and microtextures, and eventually to obtain the new data for interpretation of the origin of the granite and porphyry. Data has been obtained from site inspection along the Discovery Bay Valley Road with my Professor Zhou Mei Fu. Rock samples were collected and used to produced thin sections. In total 26 thin sections had been investigated under the optical microscope, with 13 of them were provided by Prof. Zhou and 13 of them were made by myself. 4 thin sections of Lantau Granite and 4 thin sections of feldsparphyric rhyolite had been selected to carry out electron probe micro-analysis. The data is presented in the thin section inspection part with descriptions and photo records. The EPMA data has been included in the two tables attached in the appendix The data has been interpreted in relation to the elemental distribution and the patterns across the mineral grains It is concluded that the plagioclase in the porphyritic dyke intrusions were all albite while in Lantau granite it can exist as albite, oligoclase and andesine. The dyke intrusion was influenced by partial melting and magma mixing during the intrusion period.