Chapter 7 Scottish mineral Geological Conservation Review sites – Metamorphic minerals

Abstract

Much of Scotland north of the Highland Boundary Fault has been affected by regional and, to a lesser extent, thermal metamorphism and minor metasomatism which, through variable temperatures and pressures, has produced a range of metamorphic minerals. This paper, the seventh of eight on Scottish Mineral Geological Conservation Review Sites, describes ten sites of the highest conservation value within the Lewisian, Moine and Dalradian terranes of the Scottish Highlands, together with thermally altered inclusions in Paleogene volcanic rocks. The minerals featured include the constituents of pelitic and semipelitic schist, such as kyanite and staurolite, unusually large garnet with orthopyroxene and clinopyroxene in ultramafic rocks, and calc silicate minerals, such as tremolite, actinolite, diopside and epidote, together with phlogopitic mica. One site is noted as having stratabound pyrite, chalcopyrite and sphalerite along with large concentrations of chromium mica. The presence of kyanite and tourmaline on the Ross of Mull is one of the few significant mineral sites within rocks of the Neoproterozoic-age Moine Supergroup.Six of the sites encompass extremely rare minerals, generated by the metamorphism of precursors with unusual rock compositions. Foremost amongst these are the barium minerals celsian, cymrite and hyalophane, and the first recorded occurrence in Great Britain of armenite. Although a range of amphibole species occur in both meta-igneous and metasedimentary rocks of the Scottish Highlands, the iron-magnesium clinoamphiboles of the grunerite-cummingtonite series are extremely rare. Indeed the site described in this paper is one of only two occurrences of cummingtonite within the Precambrian-age Dalradian Supergroup. In some cases, the regional mineral assemblages, including the cummingtonite-bearing rocks, have been overprinted by subsequent high temperature/low pressure thermal events associated with the emplacement of igneous intrusions. Pre-eminent amongst the thermal sites must be the occurrence of mullite and sapphire in aluminous shale xenoliths entrained within Paleogene-age basaltic lava flows on the Isle of Mull.