Controls on emplacement of the Caledonian Ross of Mull Granite, NW Scotland: Anisotropy of magnetic susceptibility and magmatic and regional structures
ABSTRACT Anisotropy of magnetic susceptibility (AMS) was measured from 139 sites across the exposed portion of the Caledonian Ross of Mull Granite, Argyllshire, NW Scotland, to investigate the internal architecture of the pluton. Field and petrographic observations support the results of the AMS study, and a complementary suite of rock magnetic experiments was used to quantify the magnetic mineralogy of the granite. AMS data reveal a fabric that records a partial tectonic overprint of an emplacement-related magma inflow fabric. The partially preserved inflow pattern indicates a south to north emplacement of subhorizontal sheets that coalesced to form a tabular pluton. The AMS data are remarkably consistent across the intrusion, with clear north-south strikes and trends dominating the magnetic foliation and lineation patterns, respectively. These AMS fabrics are discordant to internal zonation in the granite. In the southernmost part of the granite, AMS data are parallel to bedding-cleavage orientations preserved in many large (>100 m) Proterozoic Moine blocks. The scarcity of Moine blocks in the northern part of the intrusion and the prevalence of randomly oriented stoped blocks suggest that this part of the intrusion is nearer to the ceiling of the pluton. The partial tectonic overprint, consistent with east-west compression, occurred during the latter stages of emplacement and mainly affected the younger granite facies (RM2), while the oldest granite facies (RM1) and certain magmatic structures (diorite enclaves) preserve the original emplacement fabric. An alternative explanation interprets the magnetic lineations as an intersection fabric between a shallowly dipping emplacement fabric and a steep tectonic fabric. We argue that emplacement was not associated with eastward orogenic collapse of the Scandian Moine nappes, as proposed by previous workers, but occurred either before orogenic collapse or during a period of compressional reactivation.