Field relations in the north-eastern part of the Insch mafic igneous mass, Aberdeenshire

Abstract

Synopsis A total field ground magnetic survey, supplemented by a programme of bedrock exposure in shallow pits, has enabled considerable revision to be made to the position of the north-eastern margin of the Insch mass. In addition the distribution and attitude of the basic igneous rocks and their associated hornfelses and the nature of the external contact is shown to be strongly influenced by zones of shearing deformation trending between E–W and NE–SW. Magnetic modelling has shown that the external contact of the mass, and the igneous rock units within the mass dip steeply (60°–70°) southwards, and that the igneous rocks are limited to a depth of about 5 km. New petrological information indicates that narrow but discrete zones of hornfelsed derivatives of the schistose and gneissose country rocks exist, and that major trends and discontinuities observed on the magnetic map correlate with zones of shearing and mylonitization which generate the complex distribution of the basic mass and its country rocks. The external contact is rarely undisturbed. Field relations in NE Scotland indicate a regionally important episode of shearing deformation about 470 Ma in age. Later faulting on NNW-SSE or NW-SE trends has further contributed to the disruption of the basic mass and its environs.