Major shear zones and autochthonous Dalradian in the north-east Scottish Caledonides
The Buchan area within the Grampian Caledonides of north-east Scotland is a classic locality for the study of low-pressure regional metamorphism. Using detailed geophysical surveys, pits and boreholes to augment sparse outcrop data, we have revealed a system of steep shear belts of Grampian age. When combined with a reassessment of the stratigraphy, the new information leads us to the view that the Banff nappe does not exist. We also argue that the Tay nappe is of limited extent in Buchan, and that the tract of uninverted Southern Highland Group (upper Dalradian) is autochthonous.
Precambrian gneisses in the Dalradian sequence of northeast Scotland
In 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 gabbroic complex of Belhelvie in Aberdeenshire
Summary The Belhelvie igneous complex is a mass of basic and ultrabasic rocks, exposed over an area of about seven square miles, which intrudes and metamorphoses sillimanite-cordierite-gneisses and quartzose gneisses of the Dalradian Series. It is younger than the regional metamorphism of the gneisses and older than a patch of Old Red Sandstone conglomerate exposed near the intrusion. The general succession of rock types from west to east is : dunite-serpentine and peridotite, troctolite, hypersthene-gabbro, olivine-gabbro, and end-stage derivatives of olivine-gabbro and hypersthene-gabbro. The main conclusions reached in this paper have been derived from a study of this succession and of small-scale rhythmic banding and allied structures shown by many of these rocks. The evidence indicates that the various rock types have been formed largely by differentiation in place by crystal settling under the influence of gravity, and that at Belhelvie a section is exposed through a layered concordant mass similar in many respects to the large layered intrusions of Bushveld type. Study of the primary banding has led to the conclusion that the mass may have been tilted considerably since its consolidation, so that the bands have a nearly vertical dip. An alternative suggestion—gravity differentiation in another chamber and intrusion in a mushy state—is not supported by the petrological evidence, but the question must remain open until more knowledge is available of the structures of the gneisses of this part of Scotland. At the eastern side of the complex is a large area of uralitized derivatives of olivine-gabbro and hypersthene-gabbro, and the more normal rocks to the west have been uralitized in many places. This uralitization is considered to have been due to the action of end-stage liquids or vapours (introducing water and silica) from the basic magma. Serpentinization of olivines is widespread, and microscopic evidence indicates that the main serpentinization took place before the main uralitization, and that both were preceded by the development of hornblende-spinel reaction rims round the olivines.