The influence of country rock structural architecture during pluton emplacement; the Loch Loyal Syenites, Scotland

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doi: 10.1144/gsjgs.156.1.0163
Authors:Holdsworth, R. E.; McErlean, M. A.; Strachan, R. A.
Author Affiliations:Primary:
University of Durham, Department of Geological Sciences, Durham, United Kingdom
Volume Title:Journal of the Geological Society of London
Source:Journal of the Geological Society of London, Vol.156(Part 1), p.163-175. Publisher: Geological Society of London, London, United Kingdom. ISSN: 0016-7649
Publication Date:1999
Note:In English. 51 refs.; illus., incl. geol. sketch maps
Summary: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. Generally contact-parallel magmatic-state deformation fabrics are dominant in most of the intrusions and are thought to reflect limited magma pressure-driven, internal deformation of the igneous bodies during and immediately after emplacement. The geometry of these fabrics and the Loch Loyal syenites as a whole has been directly controlled by the pre-existing structural architecture of the surrounding country rocks. This appears to be a previously overlooked feature of many plutons emplaced at mid- to upper crustal depths below the region influenced by the free surface. The siting and emplacement of the Loch Loyal syenites may have been initially facilitated by late Caledonian gravity-driven extension of the Moine Thrust sheets. Following the initial phase of intrusion, it is also possible that the presence of magma actually promoted further nappe collapse in the adjacent country rocks. There is no compelling reason to link the upper crustal extension observed during emplacement to the genesis of the alkaline magmas in the underlying mantle. The siting and ascent of the syenites at depth may be linked to the presence of steeply-dipping structures in the Lewisian autochthon buried beneath the Moine Nappe.
Subsections:Igneous petrology
Subjects:Alkalic composition; Country rocks; Deformation; Emplacement; Fabric; Faults; Folds; Geometry; Igneous rocks; Intrusions; Magmas; Paleozoic; Petrography; Plutonic rocks; Plutons; Shear zones; Silurian; Structural analysis; Syenites; Tectonics; Textures; Thrust sheets; Europe; Great Britain; Highland region Scotland; Moine thrust zone; Scotland; Sutherland Scotland; United Kingdom; Western Europe; Ben Blandy shear zone; Ben Loyal Intrusion; Ben Stumanadh Intrusion; Cnoc nan Cuilean Intrusion; Loch Loyal Syenites; Naver thrust sheet
Coordinates:N575000 N583000 W0033500 W0052500
Abstract Numbers:99M/2139
Record ID:1999019875
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute.
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