Lewisian complex

Rocks of the Lewisian complex were caught up in the Caledonian orogeny, appearing in the hanging walls of many of the thrust faults formed during the late stages of this tectonic event.

Its presence at seabed and beneath Paleozoic and Mesozoic sediments west of Shetland and in the Minches and Sea of the Hebrides has been confirmed from the magnetic field, by shallow boreholes and hydrocarbon exploration wells.

[1] Lewisian-like granitic gneisses of Paleoproterozoic age of the Rhinns complex are exposed on Islay and Colonsay in the southern part of the Inner Hebrides.

[5][6] In 1951 John Sutton and Janet Watson built on this work by interpreting the metamorphic and structural development of the Lewisian as a series of discrete orogenic events that could be discerned in the field.

[1] The protolith for the Scourian gneisses are thought to be granitic, with subsidiary mafic and ultramafic plutonic rocks giving an overall bimodal character.

Some variation in the age of the protoliths from different parts of the complex and their subsequent tectonic and metamorphic history suggest that there are two or possibly three distinct crustal blocks within the mainland outcrop.

This basic dyke swarm cuts the banding of the Scourie complex gneisses and therefore postdates the main igneous, tectonic and metamorphic events that created them.

[1] The South Harris igneous complex consists mainly of anorthosite and metagabbro, with lesser amounts of tonalitic and pyroxene-granulite gneisses.

Radiometric dating suggests that the complex was intruded over a period from about 2.2–1.9 Ga, comparable to the age of the Loch Maree Group.

The Ness Anorthosite, exposed on the northeastern tip of Lewis, is also found associated with metasediments and yields a similar Sm-Nd model age of about 2.2 Ga.

[1] These two belts of metasediments flank the South Harris igneous complex, and form the largest outcrop of such rocks in the Outer Hebrides.