The metamorphic rocks of the southern part of the Mainland of Shetland

Abstract

AbstractThe paper presents a one mile to the inch geological map of the southern part of the Mainland of Shetland (excluding areas underlain by Old Red rocks) and an account of the metamorphic rocks occurring there.The metasediments mostly belong to the East Mainland Succession which is about 14 km thick. They dip vertically and strike north‐south. The metasediments in the west were originally laminated sandstones devoid of current bedding and grading and composed of poorly weathered material. They include several limestones up to half a kilometer thick. In the east the metasediments were originally gritty quartzites and sandstones together with shales which are usually finer grained and composed of more weathered material than those to the west. On two occasions the deposition of these rocks was interrupted by outpourings of spilites.The rocks show signs of four metamorphisms. The first metamorphism was accompanied by a penetrative deformation causing elongation of the rocks in a north‐south direction, shortening in an east‐west direction and elongation and shortening in different areas in a vertical direction. As a result of this metamorphism the pelitic rocks over the whole area became schists containing such minerals as biotite, chlorite, muscovite, quartz and feldspar, and possibly garnet. In a later metamorphism a belt of these rocks a kilometer or so wide became gneissified, probably due to the passage of high temperature fluids. In various parts of the area such minerals as biotite, chlorite, garnet, staurolite, chloritoid, and kyanite grew as porphyroblasts in a series of episodes referred to collectively as the porphyroblast metamorphism. After these metamorphisms the area was intruded, 400 m.y. ago, by a number of granites. In places these have produced extensive thermal aureoles containing such minerals as staurolite, chloritoid, andalusite, kyanite, sillimanite, garnet, muscovite, biotite and chlorite in pelitic rocks. The emplacement of one of the granites was accompanied by large and small scale folding.Several major faults are described including the Nesting tear fault with a 16 km dextral displacement and a mélange zone beneath a nappe.