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Synopsis Sixty-five holes drilled between Huntly and Portsoy have shown that undeformed, basic igneous rocks in this region occupy a smaller area than is shown on existing maps and that the single intrusion previously recognised—the ‘Huntly mass’—does not exist. Three smaller bodies of basic rock, which appear to consist largely of cumulates and xenolithic rocks, can be identified. The structures in the cumulates appear to be nearly vertical in most instances, but no clear evidence of folding has been found in these rocks, although certain of the igneous boundaries appear to have been intensely deformed. Xenoliths are highly metamorphosed, but otherwise there is little evidence of thermal metamorphism in the vicinity of the basic rocks, even when the latter consist of cumulates.

SYNOPSISEvidence of shearing and mylonitization has been found to occur widely within and around the body of mafic and ultramafic igneous rocks that forms the Belhelvie intrusion. It is inferred that the original relations of these rocks were greatly disturbed when this deformation occurred and that the igneous mass has been largely detached from the country rocks into which it was emplaced and now has a complex structure. This disruption appears to have mainly occurred within 30 m.y. of the emplacement of the magma, but does not seem to be simply related to the development of the Buchan Anticline, the major, post-magmatic structure that earlier workers envisaged as occurring in this area.

SYNOPSISA ground magnetic survey and a programme of shallow drilling have enabled considerable revisions to be made to the position of the margin of the Insch mass at its eastern end and revealed that the distribution and attitude of the igneous rock units have been strongly influenced by shear zones trending ENE.New petrological information shows that, except in a restricted area, there is a lack of regular relationships between the cumulate igneous rock units and it is established that the major trends and discontinuities in the contoured magnetic map can be correlated with zones of shearing and mylonitization. Magnetic modelling indicates that the shear zones dip steeply (65-80°) to the south and that the mafic units extend no further than about 2 km in depth. Later faults on N or NW trends have also contributed to the disruption of the intrusion.These findings are discussed in relation to regional structure and it is suggested that such shearing is more important than has been recognized hitherto and may constitute a discrete and extensive episode of deformation about 470 m.y. in age.