The structural setting and tectonic significance of the Glen Dessary Syenite, Inverness-shire

Abstract

The Glen Dessary Syenite crops out in the steep belt of the Northern Highland Moine. It is enclosed within a psammite envelope which was previously assigned to the Glenfinnan Division, but which is here correlated with the psammites of the Loch Eil Division exposed to the E of the Loch Quoich Line. Three episodes of deformation have been identified in the Moine metasediments of the Glen Dessary area, but only the latest (D 3 ) is thought to have deformed the syenite, imposing on it a strong, steeply plunging mineral lineation. The syenite is thought to occupy the core of a large, intensely curvilinear, D 3 synform. The D 3 structures which deform the syenite can be correlated with upright structures, which reworked previously flat-lying, already crystalline rocks, recognized at Loch Quoich. Upright reworking must be younger than the Glen Dessary Syenite, dated by U-Pb methods at 456 ± 5 Ma. Two, hitherto unrecorded, small bodies of granitic gneiss, similar to the c. 1020 Ma Ardgour Gneiss, have been identified in psammites of the Loch Eil Division near the W end of Loch Arkaig. Their presence suggests that the Loch Eil Division has a Grenville history, and is part of the 'Old Moine'.