Gravity evidence for diapiric ascent of the Northern Arran Granite

The Northern Arran Granite has a sub-circular form in outcrop with a diamet er of about 11 km. We have acquired six gravity profiles across the margin of the granite with an average station spacing of less than 250 m, two of w hich were extended to give an approximately diametric profile across the wh ole body. Simple modelling indicates that the granite ranges in thickness b etween 0.3 km and 1.2 km close to the margin, and that the base of the gran ite is approximately horizontal. Furthermore. the diametric profile shows n o short-wavelength negative anomaly feature that could be associated with a n abrupt increase in thickness of the granite, We concluded, therefore, tha t the granite probably has the form of a laccolith. More importantly, the s hallow depth inferred for the base of the granite has a significant implica tion concerning its ascent mechanism. Intrusion of the granite led to the d evelopment of the Catacol synform in the Dalradian metasediments around its northwestern margin. and in 1992, England used structural analysis to demo nstrate that the Dalradian rocks presently exposed at the margin were uplif ted by 2-3 km during granite emplacement. Because gravity modelling shows t hat the granite is less than I km thick around its northwestern margin, the granite Could not have uplifted beds from 2-3 km depth if it had been empl aced laterally following dyke ascent. Diapirism therefore appears to be the only ascent mechanism for the granite that is consistent both with its geo metry and with the structures around its margin.