Chapter 2 The Early Stages - Scourian and Inverian

Abstract

Archaean gneisses occupy large areas on each side of the central NW-SE-trending belt formed by the outcrop of the Loch Maree Group (Fig. 2.1). They are cut by numerous amphibolite dykes of the 'Scourie dyke' swarm (see Chapter 3). The gneisses are predominantly granodioritic to tonalitic, quartzo-feldspathic biotite gneisses but large areas of more mafic hornblende gneiss occur in the NE, and small bodies of amphibolite are enclosed within the gneisses in all parts of the area. The gneisses have undergone a long and complex history, having experienced Scourian, Inverian, and Laxfordian thermotectonic events. Although Scourian structures have been preserved locally, little of the original Scourian mineral assemblage remains, and the mineral assemblages mainly reflect Inverian and Laxfordian recrystallizations (see Section 2.4). The quartzo-feldspathic biotite gneisses are pale grey to pinkish-weathering, banded or massive, granodioritic (or less commonly tonalitic) gneisses containing biotite as their main mafic mineral. The banded varieties show partial or complete segregation of micas into seperate layers or lenses (Fig. 2.2). Typical examoles contain oligoclase, quartz, microcline and a dark brown biotite, in varying proportions. Muscovite, chlorite or epidote may be present in addition, together with traces of opaque ore and apatite. The gneisses within several kilometres of the outcrop of the Loch Maree Group exhibit evidence of partial recrystallization from a coarsergrained assemblage, the larger feldspar grains being surrounded by granular aggregates of smaller grains feldspar and quartz. Chlorite and epidote are clearly replacive. Retrogressive recrystallization to an epidote-bearing assemblage is particularly marked on the SW