On the Time Relations between Regional Metamorphism and Permeation in Delting, Shetland

Abstract

Summary The metamorphic rocks of Delting are a thinly bedded succession of pelites, semipelites, impure quartzites, and calcareous rocks, about 36,000 feet in thickness, with a strike approximately north-north-eastward and bedding mostly vertical. Scattered throughout the succession are metamorphosed basic igneous rocks. The earliest recognizable episode after the deposition of the sedimentary rocks was the regional metamorphism. A number of zones of regional metamorphism, equivalent to the Barrovian zones of the South-West Highlands, have been delimited. They show that the intensity of regional metamorphism increased towards the south-west. The next episode was the establishment of two belts of permeation, within which the rocks generally were intensely recrystallized, but not in the same manner in the two belts. The permeation was accompanied by thermal metamorphism of the rocks in the immediate neighbourhood of the belts. The variation in the intensity of this thermal metamorphism, both within and between the two belts, suggests that the permeation took place after the regional metamorphism. From this, and from the absence of any relation between the distribution of the permeation belts and that of the zones of regional metamorphism, the conclusion is reached that in Delting the regional metamorphism was neither the cause nor the result of the permeation, but that both are the consequence of forces acting on a larger scale and at depth. I. Introduction (a) Locality The area of Delting, which is described in this paper, is a part of the Mainland of Shetland. Four long narrow arms of the sea, Olna Firth