Precambrian gneisses in the Dalradian sequence of northeast Scotland

Abstract

I n this note we draw attention to data which may require major revision of Dalradian stratigraphy and structure in northeast Scotland. The Central Highland sequence of the Dalradian and the Keith Division ( Read 1955 ) of Banffshire are separated from the metasediments of the Banff Division by gneisses. These are referred to as the Cowhythe Gneiss ( Read 1923 ), Queens Hill Gneiss (Harris & Pitcher 1975), Donside and Ellon Gneiss ( Read 1955 ) and Inzie Head Gneiss ( Read & Farquhar 1956 ) after the main areas of outcrop. The reader is referred to Read (1955 , pl. 1) where the general distribution of the gneisses is shown. Their relationships to the Central Highland succession are often complicated by the large granitic intrusions of the post-tectonic Younger Granite suite. The generally accepted model for the region interprets the gneisses as migmatites developed from the Ben Lui and Ben Lawers schists (e.g. Read 1955 , Harris & Pitcher 1975). Further they are considered to represent the migmatitic core of the Tay Nappe (e.g. Ashworth 1975 ). Read (1955) originally considered that the rocks of his Banff Division lay on a lag-surface (the Boyne Line) above the gneisses. The great recumbent fold of which the Banff Division formed the upper limb was called the Banff Nappe, and was considered to be a continuation of the Tay Nappe ( Read 1955 ). Recent investigations by the authors reveal a sharp tectonic break between the Dalradian rocks of the Central Highlands/Keith Division and the gneisses. This break is marked by discordance and a major

The Dalradian rocks of the north-east Grampian Highlands of Scotland

“…Although some authors have also suggested that parts of the area are allochthonous (Sturt et al, 1977;Ramsay and Sturt, 1979), most current interpretations have attempted to correlate the stratigraphical succession in broad terms (i.e. at subgroup level) with Argyll and Southern Highland group successions farther to the south-west (Harris and Pitcher, 1975;Ashworth, 1975;Harte, 1979;Treagus and Roberts, 1981;Ashcroft et al, 1984;Fettes et al, 1991;Harris et al, 1994;Stephenson and Gould, 1995).…”

“…Some structural and Rb-Sr geochronological evidence has been interpreted to infer that the Cowhythe Psammite Formation, along with all the other gneissose units of the North-east Grampian Highlands, represents a pre-Caledonian Neoproterozoic basement gneiss complex (Sturt et al, 1977). Ramsay and Sturt (1979) suggested that all the rocks above the Portsoy Thrust constitute an allochthonous block, and that this consists of a gneissose basement separated from a Dalradian metasedimentary cover by a décollement along Read's Boyne Line.…”

“…Between the putative Boyne Line and the Portsoy Shear-zone, Sturt et al (1977) and Ramsay and Sturt (1979) recognized a complex structural history in the Cowhythe Psammite Formation and allocated the migmatitic gneissose rocks to a pre-Dalradian basement unit formed in the Neoproterozoic. They maintained that the contact of the migmatitic semipelite at the base of the Cowhythe Psammite is visibly discordant with the structurally underlying calcareous psammitic and semipelitic succession to the west along the Portsoy Thrust, thus resurrecting Read's concept of an allochthonous succession in Buchan.…”

“…The sedimentation was studied by Sutton and Watson (1955) and Loudon (1963), but most of the subsequent investigations concentrated upon the structure and metamorphism (Sutton and Watson, 1956;Johnson and Stewart, 1960;Johnson, 1962;Loudon, 1963;Fettes, 1970Fettes, , 1971Treagus and Roberts, 1981;Moig, 1986). Several papers have concentrated upon the position and nature of the western margin of the Buchan Block and its relationship to the ‗main' part of the Grampian Terrane (Elles, 1931;Sturt et al, 1977;Ramsay and Sturt, 1979) leading to speculation on the nature and significance of the Portsoy-Duchray Hill Lineament (Ashcroft et al, 1984;Fettes et al, 1986Fettes et al, , 1991.…”

“…Farquhar visited the coastal sections, as the Inzie Head gneisses were integral to their model of the Banff Nappe (see Read, 1955;Read and Farquhar, 1956). Sturt et al (1977) obtained a Rb-Sr isochron from gneisses at Cairnbulg Point that gave an age of c. 691 Ma, which was accepted by Ramsay and Sturt (1979) as dating the migmatization. They modified Read's hypothesis of the Banff Nappe and described a complex and lengthy history for the gneisses, which they considered to be allochthonous basement.…”