A re-evaluation of the Colonsay gneiss, Inner Hebrides, Scotland, and its bearing on the causes of the late Laxfordian Orogeny

This study is aimed at reconciling certain aspects of the tectonic model[1] for the late Laxfordian orogeny through a re-evaluation of the Colonsay gneiss, Scotland. This model envisages that accretion of an arc terrane, represented by the c. 1780 Ma syenitic Rhinns Complex of Islay and Inishtrahull, deformed and metamorphosed the Archaean gneisses of the Lewisian Complex of Scotland. However, doubt is cast on this model because the timing of the late Laxfordian orogeny and the formation of the Rhinns Complex overlap. Moreover it is difficult to generate large volumes of syenite during subduction. Previous authors considered that the Colonsay gneiss was part of the Rhinns Complex[2], but new field, geochemical and geochronological data shows that it has a separate origin:

• U-Pb zircon geochronology shows that the Colonsay gneiss is 1880 ± 10 Ma, significantly older than the c. 1780 Ma Rhinns Complex.
• Colonsay gneiss has a calc-alkaline chemistry and strongly depleted Hf isotopes consistent with a juvenile arc environment rather than the alkalic and less depleted Hf isotopic character of the Rhinns Complex more commonly associated with anorogenic magmatism.
• Granitoid pegmatites dated at 1800-1794 Ma cut F2 folds of the D1 gneissic fabric, and show that formation of the gneissic fabric on Colonsay occurred prior to c. 1800 Ma, whereas the gneissic foliation affecting the Rhinns Complex formed after c. 1780 Ma.
• Later deformation may be associated with Pb loss of zircon and apatite. In zircon this occurred at c. 735 Ma, and is interpreted to be related to D3 deformation affecting pegmatites at a time similar to that of the Knoydardian Orogeny[3] northwest of the Great Glen Fault.
• Formation of a chlorite foliation is consistent with juxtaposition of the Colonsay gneiss with the Colonsay Group during movement along a splay of the Great Glen Fault.

The lines of evidence listed above permit the Colonsay gneiss to represent the arc that collided with the Lewisian causing the late Laxfordian orogeny, and to act as the host into which the Rhinns Complex intruded. However, the record of Knoydartian events and its tectonic contact with the Colonsay Group allow the Colonsay gneiss to be entirely unrelated to processes outwith the Lewisian Complex, but instead be a displaced part of it.

[1]Park 2022, Scot J Geol 58; [2]Muir et al. 1994, Tr Roy Soc Ed 85; [3]Tanner & Evans 2003, JGSL 160.