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.

Popular Abstract

Norway is a country with very interesting geology and contains various types of rocks that were formed during different orogeneses. The rocks in this study were formed during Caledonian orogeny, at the Silurian period ca. 430 million years ago, and are part of the Western Gneiss Region. The rocks in the Western Gneiss Region have been metamorphosed in high-pressure granulite- to eclogite facies. The initial estimation of the metamorphic conditions yields 870 ± 50°C and 14.5 ± 2 kbar, which means that these rocks were buried approximately 50 km deep in Earth’s crust.
The study area is Allmenningen, an island 9 km west of the Roan peninsula in western Norway. The studied rocks are mostly marbles, calc-silicates and mafic knobs that are intercalated within the calcareous succession. The complete mineral assemblage from the three rock types combined, includes calcite, dolomite, clinopyroxene, garnet, amphibole, scapolite, zoisite, epidote, anorthite, phlogopite (Mg-rich biotite), white mica and quartz. Accessory minerals are titanite, apatite, zircon and metallic minerals.
The mineralogy of one mafic knob in a contact marble-amphibolite is peculiar. Thin garnet rims occur that surround the other minerals. This garnet presents different chemistry depending on its textural position. Garnet in the rims is richer in calcium and aluminium (grossular) compared to the core. Based on chemical analyses of garnet and geothermobarometry equations, an attempt for pressure and temperature estimation during metamorphism was conducted. For that calculation, specific mineral phases have been chosen from microscopy observations.
Does the result coincide with the initial estimation?
The mineral equilibria describe grossular-garnet formation. Grossular formed during heating and devolatilization. The minerals which reacted and released fluids are zoisite and calcite. These minerals were consumed constantly, and the fluid phase was maintained in the closed, deep, metamorphic system. At peak metamorphism, temperature reached approximately 875°C and pressure around 13.8 kbar. The fluid phase composition was 25% H2O and 75% CO2. So, what really happens in this CO2? In fact, it is suggested that the CO2 was emitted during prograde metamorphism, because the rocks cannot hold all this fluid phase and some of it must escape from the system. Sometimes, the CO2 quantities are significant and together with other processes like volcanic eruptions and ice cap melting, it contributes to global warming.

• author: Karastergios, Stylianos
• supervisor: Charlotte Möller
• organization: Department of Geology
• course: GEOR02 20201
• year: 2020
• type: H2 - Master's Degree (Two Years)
• subject: Earth and Environmental Sciences
• keywords: Western Gneiss Region, Vestranden, Roan, Caledonides, marble, calc-silicate, metabasic, petrography, SEM-EDS, mineral chemistry, mineral equilibrium, P–T estimate, devolatilization
• publication/series: Dissertations in Geology at Lund University
• report number: 609
• language: English
• id: 9034979
• date added to LUP: 2021-01-09 08:02:51
• date last changed: 2022-12-27 03:40:52