UNIVERSITY OF GOTHENBURG Department of Earth Sciences Geovetarcentrum/Earth Science Centre Petrological description of the leucogranite in Örsviken Johanna Engelbrektsson Hannah Berg ISSN 1400-3821 B775 Bachelor of Science thesis Göteborg 2014 Mailing address Address Telephone Telefax Geovetarcentrum Geovetarcentrum Geovetarcentrum 031-786 19 56 031-786 19 86 Göteborg University S 405 30 Göteborg Guldhedsgatan 5A S-405 30 Göteborg SWEDEN Abstract For a deeper understanding of the different processes that can create, alter and destroy magnetic minerals in rocks it is important to integrate rock magnetism with conventional petrology. Bulk composition, hydrothermal alteration, redox state, metamorphism and tectonic setting influence the magnetic properties of igneous rocks. An investigation of a part of a cape that consists of a leucogranite in Örsviken, 20 kilometres south of Gothenburg, was of interest after high susceptibility values had been discovered in the area. Five samples from the leucogranite were collected in order to do a whole rock major element analysis, find the cause of the high susceptibility, classify and interpret the origin of the leucogranite. Magnetite is the cause of the high susceptibility values across the leucogranite. The leucogranite is considered to be an A-type alkali granite with an anorogenic or a post-orogenic petrogenesis. The leucogranite shows quite similar patterns and chemistry to the Askim- and Kärra Granites, which may indicate that the leucogranite is associated with them. Keywords: Magnetic minerals, leucogranite, Örsviken, susceptibility, whole rock major element analysis, magnetite, A-type granite. Sammanfattning För att få en djupare förståelse för de olika processer som kan skapa, förändra och förstöra magnetiska mineral i berg är det viktigt att integrera bergarters magnetism och konventionell petrologi. Bulksammansättning, hydrotermal omvandling, redox tillstånd, metamorfos samt tektonisk miljö påverkar magnetiska egenskaper hos magmatiska bergarter. Det var av intresse att göra en undersökning av en del av en udde bestående av leucogranit i Örsviken, 20 kilometer söder om Göteborg, efter att höga susceptibilitetsvärden noterats. Fem bergartsprover från leucograniten samlades in för att göra en ”whole rock major element”-analys, finna orsaken till de höga susceptibilitetsvärdena, klassificera bergarten samt tolka dess ursprung. Magnetit är orsaken till de höga susceptibilitetsvärdena som uppmätts över leucograniten. Leucograniten är tolkad som en A-typ alkali granit med ett anorogent eller ett post-orogent ursprung. Leucograniten har en likartad kemi som Askims – och Kärra graniten, vilket kan innebära att leucograniten är associerad med dem. Nyckelord: Magnetiska mineral, leucogranit, Örsviken, susceptibilitet, “whole rock major element”- analys, magnetit, A-typ granit. Contents Abstract .................................................................................................................................................... I Sammanfattning....................................................................................................................................... II 1. Introduction ......................................................................................................................................... 1 2. Background ......................................................................................................................................... 1 2.1 Geology ......................................................................................................................................... 1 2.2 Area description ............................................................................................................................ 2 2.3 Classification of volcanic and plutonic rocks ................................................................................ 2 2.4 Classification of granitoids ............................................................................................................ 2 3. Method ................................................................................................................................................ 3 3.1 Preparations ................................................................................................................................... 3 3.2 Field work...................................................................................................................................... 3 3.3 Laboratory work ............................................................................................................................ 3 4. Results ................................................................................................................................................. 4 4.1 Bedrock ......................................................................................................................................... 4 4.1.1 The granodiorite ..................................................................................................................... 4 4.1.2 The leucogranite ..................................................................................................................... 4 4.2 Thin sections.................................................................................................................................. 4 4.3 Geochemical analysis - Whole rock major element analysis ....................................................... 7 5. Discussion ......................................................................................................................................... 11 6. Conclusion ......................................................................................................................................... 12 7. Acknowledgement ............................................................................................................................. 12 8. References ......................................................................................................................................... 13 9. Appendix ........................................................................................................................................... 15 1. Introduction 2. Background The aim of this study is to investigate the geochemical composition of a part of a cape that 2.1 Geology has high magnetic susceptibility values, identify The bedrock in the Southwest Scandinavian the cause for the high values, classify and Domain (SSD) was formed 1.8 – 0.9 Ga ago. It interpret a possible origin of the rock body. The consists mainly of gneiss, mostly orthogneiss, cape is located on the county border between which is partly veined. Parts of the SSD consist Halland and Västra Götaland, close to Örsviken, of rocks with sedimentary and volcanic origin 20 kilometres south of Gothenburg (Figure 1). (Sveriges National Atlas, 1998). During a minor field trip it was discovered that a part of the cape in Örsviken had higher The SSD is limited to the east by the susceptibility values than the surrounding Protogine Zone (Figure 2) (Lundqvist, Lundqvist, bedrock in the area (E. Sturkell, personal Lindström, Calner, & Sivhed, 2011). The communication, 21 January 2013). Protogine zone runs in a north–south direction from Värmland to Skåne and is a deformation The focus in this thesis will therefore be on zone. The southwest Scandinavian province is finding the cause of the high susceptibility divided in two parts, an east and a west segment, values. In order to answer the question a whole by the Mylonite zone. Both the Western Segment rock major element analysis, mineral analysis, and the Eastern Segment have been deformed, to susceptibility field values and complementary different degrees, by the Gothian orogeny (1.66 - mapping of the bedrock are needed. 1.5 Ga) and the Sveconorwegian orogeny (about 1000 Ma). The Eastern Segment has been more Results from “Magnetic signature of the deformed and displays migmatised gneisses. leucogranite in Örsviken” (Berg & West of the Mylonite zone the bedrock is less Engelbrektsson, 2013) will be taken in to deformed and veined, and consists mainly of consideration. gneisses and supracrustal rocks. A further shear zone, the Göta Älv zone, cuts the Western Segment in a north-south direction from south of Gothenburg to lake Värnen (Hegardt, 2010). Figure 1. Overview of Örsviken and the study area. Figure 2. Major geological features of the SSD. Map modified from Lundqvist, Lundqvist, Lindström, Calner, & Sivhed (2011). 1 2.2 Area description seen with the naked eye, are termed volcanic The study area and its surrounding are located rocks. Igneous rocks with a phaneritic texture, i.e. in the Western Segment, north and west of the fairly coarse-grained rocks in which the Mylonite zone. The Geological Survey of individual crystals can be seen with the naked Sweden (SGU) has interpreted one part of the eye, are termed plutonic rocks. They are assumed cape in Örsviken as a metamorphic intrusive rock to have formed by slow cooling (Le Maitre, (leucogranite) and the other part as metamorphic 2002). Based upon the relative proportions of extrusive and intrusive rocks (granodiorite). In three major rock forming minerals, plagioclase, the leucogranite a xenolith is found which is alkali feldspar (K-feldspar and albite) and either a interpreted to be a part of the granodiorite. The feltspathoid (usually nepheline) in undersaturated granodiorite has an age of 1.60 – 1.52