FABRIC ANALYSIS OF THE LOCH BORRALAN PLUTON, NORTHWEST HIGHLANDS, SCOTLAND

The Loch Borralan pluton was emplaced within the Assynt Region of the Moine Thrust zone during the Scandian event of the Caledonian Orogeny. The pluton consists of two major magma suites, the early suite is a pre-tectonic syenite, and the later suite is a quartz syenite, which is generally thought to be syn- to post-tectonic. The region is characterized by a series of in-sequence thrust faults that strike NE-SW and dip approximately 20° to the SE, including (from lower to upper): the Sole Thrust, the Borralan Thrust, the Ben More Thrust, and the Moine Thrust.

S>L fabrics are found throughout the pluton, and foliations in the field are defined by preferred orientation of alkali feldspars. The early suite is only exposed in the south, and its foliations strike approximately 030 and dip 20° SE. Foliations in the late suite have a similar strike orientation, but dip roughly 50-60° to the SE and NW. The foliation orientations of both the early and late suites are subparallel to the regional imbricate thrust faults in adjacent sedimentary units. The difference in foliation dips between the two suites could be the result of the spatial relationships, timing, and/or mineralogical differences between the two suites. The early suite’s shallow foliation dips may be influenced by the proximity to the Ben More Thrust. Alternatively, the early syenite may be more competent than the late quartz syenite, resulting in a variation in the dip of foliation similar to the imbrication seen in neighboring units. Field lineations are scarce, but generally plunge shallowly towards the NE in a direction parallel to the strike of the thrust faults, suggesting a component of strike parallel elongation.

Anisotropy of Magnetic Susceptibility (AMS) results show principal susceptibilities (k1>k2>k3), with k1 oriented approximately parallel to thrust motion, k2 parallel to the strike of the field foliations, and k3 normal to the foliations. The AMS fabrics (recording fabric of multi-domain magnetite) are rotated 90° clockwise relative to our field measurements (recording the fabric of feldspars). Shape Preferred Orientation (SPO) analyses using thin sections will be conducted to establish the discrepancy in lineation orientation between the field fabrics and the AMS results.