The gabbroic complex of Belhelvie in Aberdeenshire

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Abstract
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.