Palaeomagnetism and age of the quartz-porphyry intrusions, Isle of Arran

Abstract

Synopsis Most of the quartz-porphyry intrusions of Arran, many of which form the acid member of composite intrusions, have been sampled palaeomagnetically. Though both normal and reversed polarity may be found in a single intrusion it is concluded that the reversed polarities are due to overprinting by later reversely magnetised intrusions and that all the quartz-porphyries were intruded while the magnetic field had normal polarity. The age of the Drumadoon quartz-porphyry dyke and, by inference, that of all the other quartz porphyry intrusions, is 58.5 ± 0.8 (lσ) Ma. It is suggested that normal polarity may provide a stratigraphic ‘horizon’ useful for elucidating the poorly-known succession of Arran Tertiary igenous rocks, most of which are reversely magnetised. This work forms part of a comprehensive study, using palaeomagnetic polarity and radiometric dating, of the timing and duration of activity in the British Tertiary Igneous Province, and the age quoted above lies in the period of peak activity. This is the first time that a correlation of polarity with a distinctive petrographic rock type has been found.