Early syenites within the Caledonides, Northern Highlands of Scotland

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Johnstone, G S and Mykura, W. 1989. British regional geology: Northern Highlands of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.

The Glendessarry Intrusion[edit]

Glendessarry Intrusion, its psammite envelope and inclusions. P915479.
The rugged terrain typical of the Glenfinnan Division of the Moine. P916023.

The Glendessarry Intrusion (P915479) comprises a complex of felsic and mafic syenitic rocks which was emplaced into Moine schists after the earliest (D1 deformation and metamorphism, and possibly also after the D2 deformation of the region. It has an elliptical outline, elongated NNE–SSW and measuring 4x2 km. The intrusion was largely recrystallised under amphibolite- facies conditions during the Caledonian Orogeny and now shows a well developed mineral lineation which is a function of D3 strain and metamophism, and which is consistent with that developed in the surrounding schists.

First recognised by V. A. Eyles in 1938, it was described by Harry (1951) as an inlier of altered Lewisian rock. The complex was subsequently considered by the Geological Survey (Summary of Progress for 1961; Lambert and others, 1964) to be an igneous intrusion occupying the core of a steep-sided synform. Lambert and his colleagues obtained isotopic ages from the rocks which, while not precisely dating it, showed that the intrusion was unlikely to be Lewisian in age; and van Breemen and others (1979b) give a U-Pb zircon age of intrusion of c.456 Ma. The petrology of the intrusion has been described in detail by S. W. Richardson (1968). Van Breemen and his colleagues suggested that the intrusion was folded in both D2 and D3, and that the present form is the result of the combination of the two periods of folding. In this there are analogies with Carn Chuinneag, but recent research by Roberts and others (1984) interprets the structure of the Complex as a single conical fold. This fold is considered to be D3 in the local sequence.

The intrusion consists of a mafic syenite and a felsic leucocratic syenite, both comprising alkali feldspar with variable amounts of pyroxene, hornblende and magnetite. The mafic syenite contains 50–70% alkali feldspar, while the leucocratic syenite is more uniform with about 85% feldspar. The mafic rock forms the outer part of the complex, while the leucocratic rock has a main central outcrop and a long, thin outcrop near the western margin of the complex (thought by van Breemen and his colleagues to be a ‘tail’ to the main crop folded round in D2). Both syenites have a strong mineral elongation plunging at 70° to the southwest. The less deformed parts of the leucocratic syenite may contain large (1–5 cm) phenocrysts of perthite, commonly showing oscillatory zoning and Carlsbad twinning; in places these are aggregated to produce a spectacular coarse-grained rock. According to S. W. Richardson (1968) the intrusion of the mafic syenite preceded that of the felsic syenite. The only other component of the complex is granite pegmatite occurring in veins and irregular outcrops, the largest of which measures 0.6 x 0.2 km.

The psammitic rocks which form the immediate envelope comprise coarsegrained siliceous psammites and feldspathic, somewhat gneissic, psammites in which potash feldspar seems to have been generally redistributed. Apart from a strong linear fabric (quartz rodding) the rocks adjacent to the intrusion have a ‘wispy’ biotite foliation, which in places probably represents original crossbedding or tight folds, deformed, and now flattened and streaked out. At the north-eastern end of the complex the fabric of the envelope has been completely disrupted to a pseudoconglomerate in which ‘balls’ or ‘hooks’ of hornblende and calc-silicate rock are set in reconstituted quartzofeldspathic matrix resembling coarse psammite; at the south-eastern end of the complex, related gneissic rock contains larger detached fragments of mafic syenite. This disruption may relate to internal shear planes, axial planar to the D3 synform, which affect the syenite and give the north-east and south-west ends of the complex a stepped outcrop.

Within the complex are xenoliths of rocks not represented in the immediate envelope; these include small and large xenoliths of kyanite- and sillimanitebearing metasediment, and marble. One narrow metasediment xenolith measures 0.6 km in length, while the marble forms several outcrops several tens of metres across. The presence of marble suggested to the earlier workers that they were dealing with a Lewisian inlier.

Although the Glendessarry intrusion and its psammitic envelope lie within the Glenfinnan Division of the Moines (P916023) it is thought that the enclosing psammites may represent an outlier of Loch Eil Psammite, such as is found elsewhere in the Glenfinnan Division (Johnstone and others, 1969; Dalziel, 1966; Roberts and Harris, 1974). The more pelitic xenoliths in the Glendessarry Intrusion could, of course, be derived from mica schists of the Glenfinnan Division, but the marbles are uncharacteristic of the Moine Succession. Taken together, the pelitic inclusion, the marbles of the intrusion, and the feldspathic gneiss and calc-silicate-bearing rocks in the pseudoconglomerate of the envelope suggest that the intrusion has been emplaced as a sheet-like body in an unusual lithological group, possibly overlying the Loch Eil Psammite. This assemblage has strong similarities with that into which the Glen Scaddle Intrusion (see below) has been emplaced.

Selected bibliography[edit]