A re-assessment of the ‘younger’ basic igneous rocks between Huntly and Portsoy based on new borehole evidence

Abstract

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.

Deformation of the Belhelvie Mass, Aberdeenshire

“…There is also evidence from other 'Younger Basic' intrusions in Aberdeenshire and Banffshire that have been affected by zones of shearing and mylonitization in a comparable manner to the Belhelvie mass (e.g. Huntly, Munro 1970 and in preparation; Insch, Read 1951Read ,1956Ashcroft and Munro, in press: Morven-Cabrach, Allan 1970), that the deformation is not confined to the margins of these intrusions and it is difficult to visualise how the development of these zones could have occurred in association with the emplacement of magma. Indeed it is possible that the presence of irregular and impersistent cumulate units and of primary structures displaying local fluctuations in trend in the northern part of the Belhelvie mass (Fig.…”

NW-SE trending monoclines in the Dalradian of Banffshire, Scotland

“…14-31). These structures are found near Keith ( Read, 1923;Munro, 1970;and Ashworth pers. comm., 1971).…”

“…The Durn Hill Quartzite crops out in the western limb of the Boyndie Syncline about 1-3 km W of the 'Newer Basic' intrusions (Munro, 1970). Studies of this formation give some insight into the geology of the western limb of the syncline and its interference with previously undescribed NW-SE trending monoclines.…”

The Belhelvie mafic igneous intrusion, Aberdeenshire—a re-investigation.

“…The procedure and equipment employed was that described by Munro (1970) with the modification that the Holman Universal drilling rig was converted in order that it might be used for diamond drilling. At selected points, on the contacts of the mass, holes were drilled at an inclination of 45 0 to the vertical in order to obtain continuous core across the contact.…”