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Magnetic Signature of the Leucogranite in Örsviken

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Magnetic Signature of the Leucogranite in Örsviken UNIVERSITY OF GOTHENBURG Department of Earth Sciences Geovetarcentrum/Earth Science Centre Magnetic signature of the leucogranite in Örsviken Hannah Berg Johanna Engelbrektsson ISSN 1400-3821 B774 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 A proton magnetometer is a useful tool in detecting magnetic anomalies that originate from sources at varying depths within the Earth’s crust. This makes magnetic investigations a good way to gather 3D geological information. A field 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. The investigation was carried out with a proton magnetometer and a hand-held susceptibility meter in order to obtain the magnetic anomalies and susceptibility values. High magnetic anomalies were observed on the southern part of the cape and further south and west below the water surface. The data collected were then processed in Surfer11® and in Encom ModelVision 11.00 in order to make 2D and 3D magnetometric models of the total magnetic field in the study area as well as visualizing the geometry and extent of the rock body of interest. The results from the investigation and modelling indicate that the leucogranite extends south and west of the cape below the water surface. Magnetite is interpreted to be the cause of the high susceptibility values. The leucogranite is a possible A-type alkali granite with an anorogenic or a post-orogenic petrogenesis. The leucogranite shows quite similar chemical patterns to the Askim- and Kärra which may indicate that the leucogranite is associated with them. Further investigations in the study area are needed to better understand the connections between magnetic signatures and geology, which in turn can help in the interpretation process. Keywords: Proton magnetometer, leucogranite, susceptibility, magnetic anomalies, Örsviken, magnetite, Surfer11®, Encom ModelVision 11.00. Sammanfattning En protonmagnetometer är ett användbart verktyg för att upptäcka magnetiska anomalier som härrör från olika djup i jordskorpan. Detta gör att magnetiska undersökningar till ett bra sätt att samla geologisk 3D-information. Det var av intresse att göra en fältundersökning av en del av en udde bestående av leucogranite i Örsviken, 20 kilometer söder om Göteborg, efter att höga susceptibilitetsvärden noterats. De magnetiska anomalierna och susceptibilitetsvärdena undersöktes med hjälp av en protonmagnetometer och susceptibilitetsmätare. Höga magnetiska anomalier observerades över de södra delarna av udden samt fortsatt söder- och västerut i vattnet. Insamlad data bearbetades sedan i Surfer11® och Encom ModelVision 11.00 för att skapa 2D- och 3D- magnetiska modeller av det totala jordmagnetfältet i undersökningsområdet samt att visualisera geometrin och utsträckningen för den intressanta bergarten. Resultaten från undersökningen och modelleringen indikerar att leucogranitens utsträckning fortsätter söder- och västerut i vattnet. Magnetit är troligen orsaken till de höga susceptibilitetsvärdena som uppmätts. 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 associerade med dem. Ytterligare undersökningar i området är av intresse för att få en bättre förståelse mellan de magnetiska signaturerna och geologin, som i sin tur kan underlätta tolkningsprocessen. Nyckelord: Protonmagnetometer, leucogranite, susceptibilitet, magnetiska anomalier, Örsviken, magnetit, Surfer11®, Encom ModelVision 11.00. Contents Abstract .................................................................................................................................................... I Sammanfattning....................................................................................................................................... II 1. Introduction ......................................................................................................................................... 1 2. Background ......................................................................................................................................... 1 2.1 Geology ......................................................................................................................................... 1 2.2 Area description ............................................................................................................................ 2 2.3 Magnetic surveying and rock magnetism ...................................................................................... 3 2.4 Proton magnetometer .................................................................................................................... 3 3. Method ................................................................................................................................................ 4 3.1 Working hypothesis ....................................................................................................................... 4 3.2 Preparations ................................................................................................................................... 4 3.3 Field method .................................................................................................................................. 4 3.4 Modelling ...................................................................................................................................... 4 4. Results ................................................................................................................................................. 5 4.1 Bedrock ......................................................................................................................................... 5 4.1.1 The granodiorite ..................................................................................................................... 5 4.1.2 The leucogranite ..................................................................................................................... 5 4.2 Magnetic profiles ........................................................................................................................... 8 4.2.1 Surfer ...................................................................................................................................... 8 4.2.2 ModelVision ........................................................................................................................... 8 5. Discussion ......................................................................................................................................... 12 6. Conclusion ......................................................................................................................................... 13 7. Acknowledgements ........................................................................................................................... 13 8. References ......................................................................................................................................... 14 9. Appendix ........................................................................................................................................... 15 1. Introduction 2. Background During a minor field trip it was discovered that a part of a cape in Örsviken, 20 kilometres 2.1 Geology south of Gothenburg on the county border The bedrock in the Southwest Scandinavian between Halland and Västra Götaland, had higher Domain (SSD) was formed 1.8 – 0.9 Ga ago. It susceptibility values than its surroundings (Figure consists mainly of gneiss, mostly orthogneiss, 1) (E.Sturkell, personal communication, 21 which is partly veined. Parts of the SSD consist January 2013). The aim of this study is to define of rocks with sedimentary and volcanic origin the extent of that part of the cape with high (Sveriges National Atlas, 1998). susceptibility through modelling its appearance and to discuss what causes its high susceptibility The SSD is limited to its east by the Protogine values. Also, if possible to classify and interpret Zone (Figure 2) (Lundqvist, Lundqvist, the origin of the cape. Lindström, Calner, & Sivhed, 2011). The Protogine zone runs in a north–south direction To obtain the necessary information in the from Värmland to Skåne and is a deformation assigned area, a total of four profiles of and/or weakness zone. The southwest approximately 300 meters were measured using a Scandinavian province is divided in two parts, an proton magnetometer. Other data collected were east and a west segment, by the Mylonite zone. susceptibility field values and mapping of the Both the Western Segment and the Eastern bedrock. A proton magnetometer is a useful tool Segment have been deformed, in different in detecting magnetic anomalies that originates degree, by the Gothian orogeny (1.66 – 1.5 Ga) from sources at varying depths within the Earth’s and the Sveconorwegian orogeny (about 1000 crust. This makes magnetic investigations a good Ma). The Eastern Segment has been more way to gather 3D geological information. deformed and displays migmatised gneisses. West of the Mylonite zone
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