How old are the rocks of the Lewisian Complex in NW Scotland, and how do we know - Essay Example

This was followed by the new rocks folding and heating, and the intrusion of further magmas during the metamorphism, resulting in almost all the rocks being gneisses (Waters, 2003). The Lewisian Gneiss are a major division of Precambrian rocks (4.6 billion to 542 million years ago) in northwestern Scotland. In the region where the Lewisian rocks are found, they also form the lowermost rocks, also the Outer Hebrides, form the islands of Coll and Tiree, and are exposed along the northwestern coast of Scotland.

“The oldest rocks of the Lewisian have been dated by radiometric technique between 2.4 billion and 2.6 billion years old, whereas the youngest Lewisian rocks have been dated at 1.6 billion years (Encyclopedia Britannica, 2010). Further, the Lewisian Complex form the oldest rocks of the British Isles, and are Mid- to Late- Archaen in age (Jones15). Most of these rocks are deformed and metamorphosed igneous rocks or orthogneisses. The types of rocks found in the Lewisian Comples are broadly subdivided into felsic gneisses which have a high content of light coloured minerals, mafic gneisses with a large proportion of dark mafic minerals, and ultramafic gneisses containing purely mafic minerals.

Usually, the most highly abundant felsic gneisses develop from acidic igneous rocks, and mafic gneisses from basic igneous rocks. “Mafic and ultramafic gneisses are generally found together in this part of the Lewisian Complex, and are considered to be metamorphosed varieties of gabbro and peridotite” (Waters, 2003). The Lewisian Complex has been subjected to successive developments in techniques of age determination. There has been progressive tightening of constraints from Rb-Sr (rubidium/ strontium), K-Ar mineral (potassium/ argon) mineral dating, Rb-Sr and Pb-Pb (lead) whole rock studies through U-Pb dating of bulk zircon fractions, Sm-Nd rock and mineral investigation, to the most