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

The Loch Borralan pluton was emplaced ca. 430.6-425 Ma within the Assynt Region of the Moine Thrust zone during the Scandian event (ca. 435-425 Ma) of the Caledonian Orogeny. The pluton consists of two major magma suites, the early suite (431.1 ± 1.2 Ma) is a pre-tectonic syenite, and the later suite (429.2 ± 0.5 Ma) 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. A series of imbricate thrusts between the Sole and Borralan Thrusts juxtapose repeated Cambrian and Ordovician strata. The Loch Borralan pluton is bounded by the Borralan Thrust underneath and the overlying Ben More Thrust.

S>L tectonic fabrics are found throughout. The early suite is only well 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 strike is subparallel to the thrust faults. The foliation dip in the early suite is subparallel to the regional thrusts and the long axes of internal pseudoleucite aggregates. The foliation dip within the later suite is subparallel to the imbricate thrust faults mapped in adjacent sedimentary units. The variation in foliation dips between the two suites could be the result of the spatial relationships, timing, and mineralogical differences between the two suites. The early suite was present prior to movement along the Borralan Thrust and its foliation measurements are near the thrust base. Alternatively, the early syenite may be more competent than the late quartz syenite causing a variance in the dip of foliation similar to the imbrication seen in neighboring units. The lineations are more scarce, but generally plunge shallowly towards the NE in a direction parallel to the strike of the thrusts.

In future work, we will use Anisotropy of Magnetic Susceptibility and a microtextural study to further constrain the relationship between the pluton fabrics and deformation.