Gravity evidence for diapiric ascent of the Northern Arran Granite

Abstract

The Northern Arran Granite has a sub-circular form in outcrop with a diameter 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 which were extended to give an approximately diametric profile across the whole body. Simple modelling indicates that the granite ranges in thickness between 0.3 km and 1.2 km close to the margin, and that the base of the granite is approximately horizontal. Furthermore, the diametric profile shows no short-wavelength negative anomaly feature that could be associated with any abrupt increase in thickness of the granite. We concluded, therefore, that the granite probably has the form of a laccolith. More importantly, the shallow depth inferred for the base of the granite has a significant implication concerning its ascent mechanism. Intrusion of the granite led to the development of the Catacol synform in the Dalradian metasediments around its north-western margin, and England (1992) used structural analysis to demonstrate that the Dalradian rocks presently exposed at the margin were uplifted by 2-3 km during granite emplacement. Because gravity modelling shows that the granite is less than 1 km thick around its north-western margin, the granite could not have uplifted beds from 2-3 km depth if it had been emplaced laterally following dyke ascent. Diapirism therefore appears to be the only ascent mechanism for the granite that is consistent both with its geometry and with the structures around its margin.