Field relationships and emplacement of the Caledonian Ross of Mull Granite, Argyllshire

Abstract

Synopsis The Ross of Mull granite (ROMG) occupies the largest part of the pre-Tertiary inlier of the Ross of Mull peninsula. Previous ideas that the Moine Thrust crops out in the Sound of Iona have now been discounted. Recent models have proposed that: (a) the Sound of Iona Fault and the associated mylonites on Iona are part of a steeply inclined extensional fault throwing down to the ESE, bringing high grade Sgurr Beag or Knoydart Nappe Moine rocks to the same erosion level as the foreland units on Iona; and (b) one of the main Caledonian thrusts must extend to a point no more than 10–15 km from the projected trace of the Great Glen Fault. A hypothetical intersection would occur SW of Mull and it is proposed here that the intersection of these two structures has been the dominant control on the siting of the ROMG. In this model, magmas using this intersection as an ascent site were emplaced eastwards as flattish sheets within an overall east-dipping extensional zone that has high-grade Moine lithologies in its hanging wall. The present erosion level provides a section through a granitoid complex from a roof zone with flattened diorite enclaves and abundant country rock xenoliths to a clearly demonstrable westward-dipping floor; a situation which may be unique in the British Caledonian granitoid complexes.