Report No. : GEO Report No. 206

Report Title : ⁴⁰Ar-³⁹Ar Laser Microprobe Dating of Mafic Dykes and Fault Rocks in Hong Kong (2007), 78 p.

Author : S.D.G. Campbell & R.J. Sewell

Abstract

Following a recommendation in GEO Report No. 118, pilot studies were carried out to investigate the application in Hong Kong of the Argon-Argon (Ar-Ar) method to dating both fault material, and mafic and intermediate dykes. The ⁴⁰Ar-³⁹Ar dating method is based on the same radioisotope decay system as that used in the K-Ar method of dating. The ⁴⁰Ar-³⁹Ar method, however, allows all information needed to calculate a sample's age to be determined from the Ar isotopic composition of irradiated samples. The ⁴⁰Ar-³⁹Ar laser step-heating method was therefore used in this study to date: age(s) of major phases of activity of selected faults; and ages of formation of selected mafic and intermediate dykes.

The study has demonstrated that dating of fault movements is feasible. The main phase(s) of fault activity identified during the study occurred about 70-90 million years ago (Upper Cretaceous). However, the technique appears capable of identifying fault histories in individual samples and younger fault events are also suggested by the data. These include events at 34, 10 and between 3 and 4 million years ago. The technique was not able to identify events more recent than about 3 million years ago. The technique was also successfully applied to the dating of the mafic dykes, the majority of which appear to have been emplaced between 87 and 100 million years ago.

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