An Outline of the Lewisian Complex of the Outer Hebrides in Relation to That of the Scottish Mainland

Abstract

The main structural and metamorphic features of the Lewisian rocks of the Outer Hebrides are outlined and compared with those of the Lewisian on the Scottish mainland. In each area three structural zones are recognized, characterized by folding of different styles and periods. The three zones of the Outer Hebrides are closely analogous to the Scourian and Laxfordian zones of the mainland. Evidence of three periods of folding and metamorphism in varying degrees of intensity may be seen in the Outer Hebrides. The first and second periods were separated in time by the intrusion of a non-orogenic suite of basic sills and dykes of tholeiitic affinities, possibly associated with the plutonic centre in South Harris. This basic suite is regarded as equivalent to the Scourie dykes of the mainland. The first period of folding and (granulite-facies) metamorphism is recognized in the gneisses above the Outer Isles thrust-plane and in a narrow belt along the Sound of Harris, and is correlated with the Scourian of the mainland. After the intrusion of the basic suite, the rocks were affected by a second period of folding and granulite-facies metamorphism which converted the basic intrusions to garnet-pyroxene-amphibolites. The Scourie dykes of the mainland retain evidence of this metamorphism in the form of corona garnets, garnetiferous border zones, and clouded feldspars. The second folding and metamorphism is regarded as Early Laxfordian because the folding has the same trend as that accompanying the third and last period of metamorphism, which is a retrograde type associated with migmatization and intrusion of granites and pegmatites and is known from absolute age-determinations to be equivalent to the Lax-fordian of the mainland. The positions of the Lewisian metamorphic and structural zones on the opposite shores of the Minch do not coincide, and the offset can be adequately explained only by later wrench-faulting. Evidence is presented to support the existence of the Minch fault, a sinistral wrench-fault with an approximate displacement of 77 miles, comparable in direction and magnitude with the Great Glen fault.

Zoning in garnets from the mainland Lewisian

“…However, if one accepts a single dyke suite and since garnets in the latter are affected, the retrogressive event must be Laxfordian in age. Also, garnets from dyke margins are usually destroyed in the narrow Laxfordian shear belts (O'Hara, 1961;Park, 1970;Beach, 1973); and, since the zoning pattern is best developed in the garnet from the 'steep belt', where these narrow shear belts coalesce, and show greatest intensity, it appears that the retrogression leading to zoning, in the garnets, is due to this event (Dearnley, 1962(Dearnley, , 1963Park, op. cit.).…”

A geological interpretation of the aeromagnetic maps of the continental shelf around Orkney and Shetland

“…Their line for the fault lies to the east of Shetland and is unsupported by any other evidence. There is no evidence on either the IGS or the USNOO maps for the existence of the Minch fault (Dearnley 1962), and if this fault exists it has caused no displacement of the Tertiary dykes that cross it.…”

Isotopic age data from Scourian intrusive rocks on the Isle of Barra, Outer Hebrides, north-west Scotland

“…The southern islands of the Outer Hebrides are divided by the Outer Hebrides Thrust into two broad natural units, the Western and Eastern Gneisses (Jehu & Craig, 1923, 1925Dearnley, 1962). These units have more than geographical significance, however, because on Barra rocks which are lithologically of Eastern Gneiss type also occur west of the Outer Hebrides Thrust in the Oitir Mhor zone (Fig.…”

“…The youngest of these sets is a widespread suite of basic dykes which consistently show discordant relationships with the gneisses. It is correlated with the Scourie dyke suite of the mainland (Dearnley, 1962), which is considered to be approximately 2200-1900 m.y. old (Evans & Tarney, 1964;Evans, 1965).…”