Journal of the Geological Society, London, Vol. 156, 1999, pp. 163–175. Printed in Great Britain. The influence of country rock structural architecture during pluton emplacement: the Loch Loyal syenites, Scotland R. E. HOLDSWORTH1, M. A. McERLEAN1 & R. A. STRACHAN2 1Department of Geological Sciences, University of Durham, Durham DH1 3LE, UK (e-mail:
[email protected]) 2Department of Geology, Oxford Brookes University, Headington, Oxford OX3 0BP, UK Abstract: The late Caledonian Loch Loyal syenites comprise three related, but geographically separate, composite intrusions: the Ben Loyal, Ben Stumanadh and Cnoc nan Cuilean bodies. They were emplaced (c. 426 Ma) into Moine and Lewisian rocks of the Moine and Naver thrust sheets previously deformed by Caledonian NW-directed folding and thrusting at amphibolite facies metamorphic conditions. The intrusions lie mainly within a zone of large-scale cross-folds in which the regional foliation trends NW–SE at high angles to regional orogenic strike (NNE–SSW). The cross folds are underlain by a broad, ESE-dipping zone of high strain known as the Ben Blandy shear zone. The NW basal contact of the Ben Loyal body dips generally SE and strikes sub-parallel to the foliation in the underlying Ben Blandy shear zone. The higher parts of this syenite, together with those of the Ben Stumanadh and Cnoc nan Cuilean intrusions, form a series of variably coalesced dykes trending NW–SE sub-parallel to the foliation in the pre-existing zone of cross-folds. There is little evidence for significant country-rock deformation during emplacement of the syenites. Minor components of top-to-the-SE extension occurred along the NW boundary of the Ben Loyal body, whilst minor dextral shearing is associated with the emplacement of the NW–SE, vertical sheet-like units that are thought to form the SE part of this intrusion and the other Loch Loyal syenites.