Characterization of mineral parageneses and metamorphic textures in eclogite- to high-pressure granulite-facies marble at Allmenningen, Roan, western Norway

Abstract: The Roan peninsula in western Norway, Vestranden, is known for exposing one of the deepest parts of the Scandinavian Caledonian orogen, similar to the Western Gneiss Region. Throughout the Roan area, eclogites and high-pressure granulite- and amphibolite-facies gneisses occur, associated with supracrustal rocks including marbles, calc-silicate rocks and amphibolites. Although extensive research has been conducted regarding Western Gneiss Region and Vestranden, the mineralogical and petrographical data from marbles and calc-silicate rocks are limited. The study materials are from Allmenningen, an island located 9 km west of Roan. The studied rocks belong to the Einarsdalen Supracrustal Unit, which is composed of paragneisses, marbles, calc-silicate rocks with mafic lenses and amphibolites. The aim of this study is to determine the petrography, textural characteristics, and mineral chemistry of the marbles, calc-silicate and associated mafic rocks, and how they are connected to the metamorphic conditions in Roan. Furthermore, calcareous rocks are of environmental importance because they emit significant amounts of CO2 into the atmosphere during metamorphism. Twelve samples collected from Allmenningen have been analysed under petrographic microscope and five further studied in SEM-EDS. Based on the EDS analyses, one sample was used for P–T mineral equilibria calculation using TWQ geothermobarometry. The petrographic data show that the mineral assemblage in the marbles is calcite + dolomite + clinopyroxene + scapolite ± epidote + phlogopite + amphibole + quartz. Calc-silicate rocks are composed of calcite + dolomite + clinopyroxene ± amphibole + scapolite + phlogopite + garnet + quartz. Mafic rocks contain garnet + clinopyroxene + zoisite + plagioclase ± amphibole + calcite. Accessory minerals include titanite, apatite, zircon, and opaques. All three rock types have mineral assemblages that indicate high-pressure granulite-facies metamorphism. P–T estimates using TWQ suggest temperatures ~875°C and pressures ~14 kbar for the formation of garnet rims in a garnet-rich mafic calc-silicate rock in the presence of a fluid phase composed of 75% CO2 and 25% H2O.Diopside and grossular-rich garnet formed by heating and devolatilization in both carbonate and mafic rocks. The dominant minerals that were consumed are zoisite, calcite, plagioclase and quartz. Fluorine is present mainly in apatite, and in minor amounts in phlogopite and amphibole, suggesting that the CO2-rich fluid phase contained minor amounts of dissolved halogens during metamorphism.