Chapter 5 Laxfordian Intrusive Rocks

Abstract

Igneous intrusions of Palaeoproterozoic age occur within the outcrop of the Loch Maree Group and also within the Archaean gneiss complex, cutting the 'Scourie' dykes. Since they post-date either the dykes or the c. 2.0 Ga Loch Maree Group (LMG), they are considered to be Laxfordian in age. They are broadly granitoid in composition, and fall into three categories: (a) a suite of gneissose granodiorites and tonalites, including the Ard gneiss and Mill na Claise gneiss, which intrude the LMG; (b) thin granite sheets cutting both Archaean basement and 'Scourie' dykes; and (c) granitoid pegmatites. The Ard gneiss sensu stricto (Park 1964) is a gneissose granodioritic to tonalitic body (Fig. 5.1 A) whose outcrop extends from the type locality on the An Ard peninsula [802 751], where it is 600 m wide, southeastwards to Dubh Loch where it narrows to about 150 m. Similar gneisses, which are grouped with the Ard gneiss for convenience, occur in the core of the Mill na Claise fold (the Mill na Claise gneiss), and form a narrow belt along the SW side of the Mill na Claise amphibolite from Shieldaig Lodge to the SW end of Dubh Loch (the Cloiche gneiss) (see map). All these gneisses enclose numerous sheet-like bodies of amphibolite similar to, though coarser than, the amphibolites associated with the LMG metasediments. In the NW, on An Ard peninsula and NE of Lochan Dubh nan Cailleach, these amphibolites form distinct sheets up to 150 m wide. Further SE, at Druim Ruadh [822