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Characterisation of the Magnetic Mineralogy, Properties and Magnetic Anomaly Responses of Two Garnetiferous Peridotite Bodies in the WGR, Otrøya

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Characterisation of the Magnetic Mineralogy, Properties and Magnetic Anomaly Responses of Two Garnetiferous Peridotite Bodies in the WGR, Otrøya Characterisation of the Magnetic Mineralogy, Properties and Magnetic Anomaly Responses of two Garnetiferous Peridotite Bodies in the WGR, Otrøya Ground Magnetic Surveying, Sampling, Magnetic Characterisation and 2D Modelling Emilie Kolstø Strømøy Geology Submission date: August 2014 Supervisor: Suzanne McEnroe, IGB Norwegian University of Science and Technology Department of Geology and Mineral Resources Engineering i Abstract Information on magnetisation of earth’s crust comes from the study of magnetic anomalies measured at various elevations using ground, airborne and satellite data, as well as studies of deep-seated rocks exposed at the surface. In the Western Gneiss Region (WGR), western Norway, two well exposed bodies of garnetiferous peridotites lay on the island Otrøya. The WGR is a 25.000 km2 window of Proterozoic Baltica continental crust exposed in the Scandinavian Caledonides, and is one of the largest exposures of deeply subducted rocks on earth. The two bodies exposed on Otrøya contain evidence of the early stability of Archean tectonometamorphic megacrystic mineral assemblages that include high P/T enstatites and majoritic garnet (presently exsolved into Proterozoic and Scandian tectonometamorphic assemblages) (Van Roermund, 2008). The magnetic properties and anomaly responses of these peridotites and their immediate enclosing bedrocks were investigated by sampling orientated blocks, conducting magnetic measurements in field and at lab, as well as conducting a ground magnetic survey. This provides us with information on the magnetic properties of deeply subducted mantle fragments, and adds to the knowledge of what is magnetic in the lower crust and lithospheric mantle. The study of the two peridotite bodies at Otrøya displays several morphologies of magnetite, mostly displaying as discrete forms within seams and cracks, but also as well-defined rims around chromites. The remanent intensity and susceptibility of the rocks are low, however the remanent intensity is somewhat varying. The directions of remanent magnetisation within measured specimens are both negative and positive with positive directions commonly displaying the same direction as the ambient field as well as commonly carrying weaker intensities than the negative. All of the above could be indicative of several stages of magnetite creation, and/or several events of recording of natural remanent magnetisation. The immediate constricting bedrocks displayed substantially higher remanent and induced intensities. Within the measured specimens both intensities were varying with the remanence displaying the largest variance in addition to showing both positive and negative directions. This was reflected in the anomaly map created from the ground magnetic survey due to anomalies, both high and low, displaying within these bedrocks. However, the peridotites correlate with wide anomaly lows. With 2D modelling a magnetic profile over the area was investigated with susceptibility and geometry of the bodies as input parameters. It became evident that the susceptibility contrast of the peridotites and the constricting bedrocks, as well as their geometries, could create anomaly lows as displayed in the anomaly map. However, it could not explain the most negative anomaly responses – an indication on remanence being of importance. ii Sammendrag Studier av magnetiske anomalier målt på bakkenivå, fra flymålinger og fra satellittdata, samt studier av dyptliggende bergarter eksponert på jordoverflaten, gir informasjon om magnetisme i jordskorpen. Den vestre gneissregion (WGR), i Vest-Norge, er et vindu i de skandinaviske Kaledonidene, og eksponerer 25,000 km2 med tidligere dyptliggende berg. Disse består av proterosoisk baltisk kontinentalskorpe, og er blant de største vinduene av dypt subduserte bergarter nå eksponert på jordoverflaten. I den WGR befinner det seg blant annet to eksponerte peridotittkropper på øya Otrøya. Disse peridotittene inneholder bevis for tidlig stabilitet av arkeiske tektometamorfoiske megakrystiske mineralsammensetninger med høy P/T enstatitter og majoritiske granater (i dag eksolvert i proterosoiske og skandiske tektometamorfoiske sammensetninger) (Van Roermund, 2008). Ved grundmagnetisk undersøkelse, magnetisk måling i felt, og magnetiske laboratoriemålinger av orienterte steinprøver av disse peridotittene og dets rundtliggende grunnfjell, ble deres magnetiske egenskaper og anomaliresponser undersøkt. Ved å skaffe kunnskap om de magnetiske egenskapene til disse bergartene vil også de magnetiske egenskapene til dypt subduserte mantelfragmenter bli innhentet, og muligens bidra til kunnskap om hva som er magnetisk i den nedre skorpen og i den litosfæriske mantelen. Studiet av de to peridotittkroppene på Otrøya viser flere morfologier av magnetitt, for det meste av diskrete former i sømmer og sprekker, men også som veldefinerte kanter rundt kromitter. Den remanente intensiteten og susceptibiliteten til peridotittene var lav, men med noe varians innenfor den remanente intensiteten. Blant peridotittsteinprøvene var retningen til den remanente magnetiseringen både negativ og positiv med positiv vanligvis i samme retning som det nåværende magnetiske felt, men ofte med svakere intensitet enn remanens med negativ retning. Alle nevnte observasjoner kan være indikasjoner på flere stadier av magnetittdannelse, og/eller flere NRM opptak. Det rundtliggende grunnfjellet viste vesentlig høyere remanent og indusert intensitet. Blant steinprøver av disse varierte begge intensiteter, men den remanente intensiteten viste størst variasjon i tillegg til og vise både positive og negative retninger. Dette illustreres i et anomalikart laget fra de grundmagnetiske undersøkelsene, hvor både positive og negative anomalier kan bli observert over det rundtliggende grunnfjellet. Peridotittene vises i anomalikartet som brede negative anomalier. Et magnetisk profil over området ble undersøkt ved 2D modellering, hvor susceptibilitet og geometri av peridotittene var inngangsparametere. Det fremkom at susceptibilitetskontrasten på peridotittene og det rundtliggende grunnfjellet, samt deres geometri, kan skape negative anomalier slik som observert på anomalikartet. Imidlertid kunne ikke de mest negative anomaliresponsene til peridotittene forklares med 2D modelleringen – en indikasjon på at remanens kan være en medvirkende faktor. iii Acknowledgements I would like to thank Suzanne McEnroe for taking me on as her master student, finding an interesting project, and for helping me and guiding me through the world of rock magnetism. You have always had my best interest at heart. I thank Christine Fichler for helping me with Oasis montaj and modelling in GEM-SYS, being an additional supervisor on my thesis, and giving me feedback on my work. Field work was supported by NFR project 222666 'Sources and nature of remanent magnetization and susceptibility: Keys to interpretation of magnetic anomalies over continental lithosphere'. There were many people at my faculty that helped me. Gerd Inger Sætrom is kindly thanked for her support and guidance throughout the project. Knut Solem is thanked for helping me with general computer issues throughout 2013 and 2014. I thank Nathan Church for help with density and VSM measurements, as well as with discussions of results. I would like to thank Arild Monsøy and Kjetil Eriksen at “slipelaboratoriet” for helping me at short notice and letting me use their machines. I would also like to thank Gunnar Vistnes for helping and learning me the machines at the rock mechanics laboratory. Additionally, I would like to thank Stephen Lippard and Allan Krill for helping me find a suitable supervisor while I was away on exchange in Australia. At NGU I wold like to thank Morgan Ganerød for help with the spinner magnetometer, and Odleiv Olesen and Jomar Gellein for providing me aeromagntic maps over Otrøya. Additionally, I would like to thank Margrét Traustadóttir which also helped me on VSM measurements, as well as the optical microscopy of my thin sections. I would like to thank Silvana and Leo who drove me to my field sight, and Peter Robinson for bringing me back as well as giving me insight into the geology of Otrøya. I would like to thank all the people at Sandneset camping, especially Karin, Arnfinn, Liba, Max and Rhino who took me in and kept me companied, even though they were over twice my age. A special thanks to Aud and Arne which provided me with lunch and a good talk about everything and nothing after finding me measuring susceptibility of rocks in their garden. And last of all I want to thank all of my friends and family which have listened when I have talked about my project, even though they had no idea what I was talking about. Especially Johanne S. Ness, Anine Andresen and Sofie Sjøli which has joined me the last weeks at the student office to keep me companied, and Anniken Mjøen Solli which had to stand my rambling throughout lunchtime at Café Sito for a whole year. iv v Contents Abstract ....................................................................................................................................... i Sammendrag .............................................................................................................................. ii Acknowledgements ................................................................................................................... iii List of Figures ..............................................................................................................................xi
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