The origin of the rocks of Ach'uaine and Appinite types in the Rogart area, Sutherland (a petrological and geochemical study of granitisation phenomena in the Rogart District, Sutherland)

Abstract

As long ago as 1912 (pp. 128 -9) the late Sir John Flett made reference to the occurrence in the Moine series of the Northern Highlands of Scotland of a peculiar suite of basic rocks in which high proportions of ferromagnesian minerals, including hornblende, biotite, pyroxene and olivine, are associated with alkali feldspars (orthoclase and sodic oligoclase) and quartz. In concluding his description of a particular member of the suite (connected with the scyelite of Carn Cas nan Gabhar), he directed attention to the abnormal character of the rock in the following words: "The rock is far from being a normal diorite; neither is it a syenite. The abundance of ferromagnesian minerals in a mesostasis rich in silica and alkalies is a feature which recalls the lamprophyres ".Read encountered a large number of these curious basic rock masses during his regional geological investigation of the Northern Highlands (1925, pp. 45 51; 1926, pp. 154-166; 1931, pp. 165 -172) and he gave them the name Hybrid rocks of Ach'uaine Type after the district of Ach'uaine, 2.5 miles north-north-east of Bonar Bridge, where they are typically developed. According to Read these rocks constitute a group of minor intrusions of widely varied composition ranging from ultrabasic to acid; he attributed the various members of the series to the mixing of a granitic magma either with an ultrabasic magma or with solid ultrabasic rock". (1925, p. 45; 1926, p. 155). He dated the "hybrid" rocks (i.e. the time of hybridisation of the earlier ultrabasic intrusions by the later granitic intrusions) as later than the regional metamorphism of the Moines, but earlier than the deposition of the local Old Red Sandstone. The granite intrusions of the region are of two main types: (a) Migdale granite; (b) Lairg -Rogart granodiorite (1926, pp. 9 and 148). Later, after studying both the Loch Choire and Strath Halladale Complexes and the rocks of Ach'uaine Type associated with them, Read entertained the view that The acid end of the series is possibly derived from the granitic injections of Loch Choire or Strath Halladale, so that these hybrids may be a special facies of injection-rocks ". (Read, 1931, p. 165).MacGregor and Kennedy (1932, pp. 105-119) drew attention to the close resemblance between the Lairg-Rogart granodiorite and its associated rocks of Ach'uaine Type and the Morvern-Strontian 'Granites' and its associated Appinitic suite. They established the time -sequence as follows (p. 119): (1) Regional injection of acid pegmatitic magma. (2) Intrusion of ultrabasic to acid igneous rocks (Appinite-lamprophyre suite). (3) Intrusion of Morvern-Strontian 'Granite' of Caledonian, probably Lower Old Red Sandstone, age.In his summary of the regional geology of the Northern Highlands, J. Phemister (1936, pp. 60-1), following MacGregor and Kennedy, grouped the rocks of Ach'uaine Type with the rocks of the Appinitic suite and dated them as members of the Newer Igneous Rocks of Caledonian age. Phemister also stressed the important facts that the rocks of Ach'uaine and Appinitic types are definitely associated, in their distribution, with the 'Newer Granite' stocks and that they appear to be absent from central and southern Rossshire where the 'Newer Granites' are not represented.In the course of the present investigation the writer has made a detailed study of the rocks of Ach'uaine and Appinitic types in the Rogart area. In addition to the masses of Ach'uaine type recorded by Read (1925, pp. 45-51) from this area, he has mapped many more newly discovered occurrences of such rocks. Besides mapping the basic bodies on the six-inch scale, he has also made large -scale drawings of typical exposures. He has, moreover, visited the type localities of the so- called 'Hybrids' in the Ach'uaine region itself near Bonar Bridge under the guidance of miss J. Watson, and has examined the rocks of the Bettyhill area, the "Hornblendic Complex" of which has already been described in great detail by Y.C. Cheng (1942, p. 67; 1944, p. 107).In a description of an agmatite formed from a hornblende-biotite-pyroxene-schist in the Rogart area, the writer (Ma, 1948,pp.8-14; and pp.153-9 of this thesis) has already shown that hornblendite developed as a basic front in the schist as a result of introduction and fixation of cafemic constituents displaced from parts of the rock body which were undergoing granitisation. The subsequent granitisation of the initially basified rocks gave rise to the series: hornblendite ---> appinitic rocks ---> dioritic migmatite ---> granodiorite. Some of these rock types resemble the 'Hybrids of Ach'uaine Type' so closely as to provide a clue towards the solution of the problem of the origin of this curious suite of rocks.