Petrological Description of the Leucogranite in Örsviken

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).

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 Ga (U. rocks or quartz in oversaturated rocks, mafic to Bergström, personal communication, 13 May felsic plutonic rocks can be classified (Clark, 2013). Some east-west striking dykes of 1999). ultrabasic and intermediate intrusive rocks cut through the area (Geological Survey of Sweden 2.4 Classification of granitoids (Cartographer), 2013). One of the commonly used classifications of granitoids is the S-I-A-M classification (Chappell Geological Survey of Sweden has interpreted & White, 2001). It is mainly a genetic the leucogranite to belong to the Askim granite classification and indicates what kind of source found in the study area (U. Bergström, personal material the granitoid is derived from. There are communication, 13 May 2013). The Askim four (five) classes, one of them is the I-type granite is greyish red to reddish grey and has an granitoids (I in the acronym) (Winter, 2010). The age of 1336±10 Ma. The mafic intrusions chemical composition of I-type granitoids associated with the granite are interpreted to be suggests that they have an igneous source co-magmatic (Hegardt, Cornell, Hellström, & material. The S-type granitoids probably have a Lundqvist, 2007). The Kärra Granite is found melted sedimentary source. M-type granitoids east and north of the Askim granite. The Kärra also have an igneous source but their magma is granite is also known as the RA-granite. derived from partial melting of mantle material. (Lundqvist, Lundqvist, Lindström, Calner, & Anorogenic or A-type granitoids are probably Sivhed, 2011). It is characterised by its high derived from partial melting of granulitic residue uranium and thorium contents. The Kärra granite which is dry and enriched in F and/or Cl. A-type is greyish-red to red due to its high K-feldspar granite is considered to represent the final event content and usually foliated and veined. The of an orogenic belt or rift-related anorogenic Kärra granite has an age of 1311±8 Ma. Both the magmatism. (Whalen, Currie, & Chappell, 1987). Askim and Kärra granites belong to the The fifth type is the H-type (hybrid), which is a Kungsbacka Bimodal Suite (Hegardt et al., combination of two or more granitoid types 2007). mentioned above.

2.3 Classification of volcanic and There are other classifications of granitoids plutonic rocks based on tectonic settings rather than chemical Igneous rocks may have crystallized from compositions. Most of granitoids can be grouped magma but could have been modified by into orogenic, anorogenic and post-orogenic cumulative, deuteric, metasomatic or (transitional) settings (Winter, 2010). metamorphic processes. The primary classification of igneous rocks ought to be based Further classifications are geochemical on their mineral content or mode. Igneous rocks discrimination of tectonic granitoids. De la with an aphanitic texture, i.e. fairly fine-grained Roche, Leterrier, Grandclaude, & Marchal (1980) rocks in which the individual crystals cannot be proposed a classification of plutonic and volcanic

rocks based on major element parameters. oxide wt% (D, Cornell, personal communication, Batchelor and Bowden (1985) used De la 8 April 2013). Roche’s diagram in an attempt to distinguish granitoids from different tectonic settings. Pearce, Harris & Tindle (1984) used trace elements to discriminate between granitoids.

3. Method

3.1 Preparations The ArcGIS 10.1 software was used in advance in order to create a topographic map of the study area. To produce this map data from Sveriges Lantbruksuniversitet were used, analysed and visualized (Sveriges Lantbruksuniversitet, 2013-02-26).

Figure 3. Locations showing where the samples for the 3.2 Field work SEM-analysis and thin sections were collected. The magnetic susceptibility of rocks was In order to do a major element analysis the measured with a hand-held magnetic samples collected were first powdered to smaller susceptibility meter. To get representative than one micron. The rock powder was put in a susceptibility values of the area, 48 readings were molybdenum holder placed between two copper taken across the cape. In the SI system, (Système electrodes, served by a power supply which International) which is used in this report, the delivers about 200 amps at adjustable voltage, magnetic susceptibility is dimensionless. The giving temperatures that are high enough to melt geodetic system used was SWEREF99 TM. quartz at 1700°C. When doing this the chamber Mapping of the bedrock in the study area was is filled with argon gas at about two atmospheres also done. overpressure. Argon is a noble gas and has a low 3.3 Laboratory work chemical reactivity. Before fusion the rock Five samples were collected from the study powder was ignited to oxidize the sample and to area in Örsviken for whole rock and mineral drive off the crystalline water. After the fusion, composition analyses (Figure 3). The major the glass was broken down into smaller pieces element composition was determined on fused from the molybdenum holder and cast into an glasses using an Oxford Instruments energy epoxy disk with all five samples. It was then dispersive spectrometer (EDS) detector diamond-polished and carbon-coated before connected to a Hitatchi S-3400N scanning being analysed in the SEM (D, Cornell, personal electron microscope (SEM), using the Inca communication, 8 April 2013). platform. The instrument setup that was used was Thin sections were made by Mikael Tillberg a 20 keV accelerating voltage, 3.5 nA beam at the Department of Earth Sciences, University current and a working distance of 9.6 mm. Cobolt of Gothenburg in order to perform mineral was measured as a reference standard to correct analyses and determine the mineral composition for system drift. The analyses were performed at in a microscope. Furthermore, two stone slabs the Department of Earth Sciences, University of were polished before the SEM was used to Gothenburg. By using the SEM it can give confirm the mineral composition seen in the detailed analyses of minerals or homogenous microscope. phases such as glass down to levels around 0.1

4. Results 12 4.1 Bedrock n = 48 Two rock types dominate the cape in 10

Örsviken, one coarser-grained and one finer- 8 grained. The contact between the two divides the 6 cape into two main parts, northern and southern.

Number 4 The northern part was mapped as granodiorite 2 and the southern part leucogranite. There is a sharp visible contact between the two rock types 0 in a west-easterly direction across the cape. 0 500 1000 1500 2000

Magnetic susceptibility (10-5[SI])

4.1.1 The granodiorite Figure 4. Histogram of distribution of the magnetic The granodiorite is reddish-grey to grey and susceptibility of the leucogranite (10-5[SI]). has a porphyric texture with small, < 1 cm K- 4.2 Thin sections feldspar augen. It is a medium to coarse-grained The thin sections (Bi01, Bi07, Bi08 & and granite. The magnetic susceptibility values Bi11) from the leucogranite samples all show measured for the granodiorite range from 0 to simillar proportions of mineral content. They are 600 ×10-5 [SI] (see Appendix). At the contact to very rich in quartz and microcline, have low the leucogranite the granodiorite has a strike and precentage of plagioclase, small amounts of dip of around N260/60 and contains intrusions of opaque minerals, muscovite and biotite. Sample pegmatite’s and veins of quartz. Bi07 and Bi08 have a few percent more biotite 4.1.2 The leucogranite than the other samples. The opaque minerals are The rock is fine to medium-grained and red to magnetite which was seen when analysing the greyish-red in colour. The extent of the opaque stone slabs with reflected light in a leucogranite south and west of the cape was not microscope. Sample Bi11 is especially rich in observable at the surface. The magnetic magnetite relative to the other samples. The susceptibility values measured range from 200 to mineral composition seen in the microscope 2000 ×10-5 [SI] (Figure 4 & Figure 5) (see when analysing the thin sections and stone slabs Appendix). The rock has a density around 2.63 was confirmed in the SEM (Figure 9). Zircons g/cm3 and shows planar foliation with strike and were also identified when analysing the dip values of about N270/60 (Figure 6) (Berg & leucogranite in the SEM. Engelbrektsson, 2013). Pegmatite and quartz In the sample collected from the central part intrusions were seen in the study area, often close of the leucogranite (Bi01) the grain size to the contact with the granodiorite, and a few of distribution is more uneven than in the samples the pegmatite intrusions are folded (Figure 7 & closer to the contact (Bi07, Bi08 & Bi11) which Figure 8). Furthermore, signs that might indicate have a finer and more even distribution. cross-bedding and gneissic banding of the rock There seems to be a parallel orientation of the were observed as well as mafic intrusions. minerals in Bi07 and Bi08, but the other samples Fractures are seen all over the leucogranite display a more random orientation of the outcrop with a similar orientation as the strike minerals. and dip.

Figure 5. Susceptibility values for the leucogranite given in (10-5 [SI]). Map made in ArcGIS.

Figure 6. Strike and dip values of planar foliation the leucogranite. Map made in ArcGIS.

Figure 7. Contact between the leucogranite and the Figure 8. Folded pegmatite intrusion in the leucogranite. granodiorite.

Microcline

Magnetite Quartz

Figure 9. SEM backscattered electron image showing magnetite, microcline and quartz in the leucogranite.

4.3 Geochemical analysis - concentrations in the leucogranite samples vary Whole rock major element analysis between 11.35 wt % and 12.57 wt %. The samples collected close to the contact also have There is minor difference in chemical higher Fe O (T) content than the samples from composition of the major elements within the 2 3 the central part of the cape. The samples with the leucogranite samples (Table 1). In comparison highest Fe O (T) content, Bi07 and Bi11, also between the leucogranite (Bi01, Bi05 Bi07, Bi08 2 3 have the highest susceptibility values. All & Bi11), the Askim (askim 1) and Kärra (ra3) samples have high concentrations of Na and K granite (reference samples), the variation is and low concentrations of Mg, Mn, P O , Cr O , bigger. Samples Bi01 and Bi05 collected in the 2 5 2 3 and SO . For the samples collected from the central part of the leucogranite have slightly 2 leucogranite the CaO concentration is around 0.4- higher SiO2 content than the samples collected 0.5 wt %, but the samples from the Askim and closer to the contact between the granodiorite and Kärra granite have a higher content. The leucogranite. The Kärra Granite has similar SiO2 calculated loss of ignition for the samples from content to the leucogranite, but the Askim granite the leucogranite varies between 0.74% and has a few percent lower SiO2 content. The Al2O3 0.42%.

Table 1. Susceptibility values SI [10-5], normalized whole rock major element analysis (wt%), and loss of ignition (wt%) for the leucogranite and the Askim granite and the Kärra granite. The whole rock analysis data for the Askim and Kärra granite are from Anfinset (1999).

Sample Bi01 Bi05 Bi07 Bi08 Bi11 Askim (askim Kärra (ra 3) 1) Susceptibility, SI [10-5] 500 300 700 500 1500 - - Density, g/cm3 2.61 2.63 2.61 2.63 2.65 - - Major element, wt%

SiO2 78.07 77.97 76.73 75.72 77.38 72.97 76.62

TiO2 0.12 0.08 0.25 0.28 0.13 0.42 0.25

Al2O3 11.49 11.61 11.97 12.57 11.35 12.49 11.71

Fe2O3 (T) 1.36 1.33 1.82 1.61 2.18 3.95 2.01 MnO 0.01 0.03 0.01 0.01 0.03 0.03 0.04 MgO 0.02 0.07 0.17 0.13 0.02 0.37 0.12 CaO 0.41 0.56 0.53 0.52 0.42 1.10 0.78

Na2O 2.66 2.63 2.72 3.19 2.68 2.77 3.04

K2O 5.82 5.59 5.66 5.84 5.74 5.84 5.38

P2O5 0.02 0.11 0.09 0.09 0.02 0.07 0.06

Cr2O3 0.01 0.01 0.03 0.03 0.04 - -

SO2 0.00 0.00 0.00 0.00 0.00 - - TOTAL 100.00 100.00 100.00 100.00 100.00 100.00 100.00 LOI, % 0.74 0.58 0.64 0.56 0.42 0.20 0.50 N2O+K2O, wt% 8.49 8.22 8.38 9.03 8.42 8.60 8.42 K/NK, molar 0.69 0.68 0.68 0.65 0.68 0.68 0.64 A/CNK, molar 1.01 1.02 1.03 1.00 1.00 0.97 0.96 NK/A, molar 0.93 0.89 0.89 0.89 0.92 0.87 0.92

The total amount for Na2O and K2O is around samples plot in just the peraluminous field in the 8 wt % to 9 wt % for all the samples. When the aluminium saturation diagram, but quite close to total amount is plotted against the SiO2 wt % all the metaluminous field (Figure 13). This shows five samples plot in the subalkaline field (Figure that the leucogranite contains more Al2O3 than it

10). In the AFM diagram the five samples plot in contains Na2O, K2O and CaO together. When the calc-alkaline field, close to the A corner, the sum (R1) of 4 Si – 11 (Na+ K) – 2 (Fe Ti) is which indicates that the leucogranite is iron- and plotted against the sum (R2) of 6 Ca + 2 Mg + Al magnesium-poor (Figure 11). In the alteration Bi01, Bi05, Bi07 and Bi11 plot close to the post- diagram, all but one sample from the leucogranite orogenic field (Figure 14). This suggests that the and the Askim granite plot in the K-altered field leucogranite was formed shortly after an orogenic (Figure 12). Bi08 and Kärra granite plot in the event. Sample Bi08 plots on the line between the the igneous spectrum. Samples that plot in the syn-orogenic and the post-orogenic field. In the igneous spectrum have not undergone any multicationic diagram for discrimination of metasomatism, but in the Na- and K-altered plutonic rocks the samples from the leucogranite fields the rock is probably enriched in Na or K plot in the alkali granite field (Figure 15). due to metasomatism (Hallberg, 2001). All five

Figure 10. Diagram displaying total alkalis vs. silica for the five samples from the leucogranite from Örsviken and also from the Askim- and Kärra Granite. The line dividing the alkaline and subalkaline series is taken from MacDonald & Katsura, (1964).

Theoleiitic

Calc-alkaline

* Figure 11. AFM diagram showing the five samples from the leucogranite from Örsviken. A = Na2O+K2O, F = FeO , and M = MgO. The Kärra Granite plots in the same place as the samples collected from the leucogranite. The Askim Granite deviates from the other samples. After Irvine & Baragar (1971).

Figure 12. Alteration diagram displaying the five samples from Örsviken and the two samples from the Askim and Kärra Granite. After Hughes (1973).

Figure 13. Aluminium saturation index for the five samples from Örsviken and the two samples from the Askim and Kärra Granite. After Shand (1927).

Figure 14. Multicationic diagram of tectonic-magmatic discrimination for the five samples from Örsviken and the two samples from the Askim and Kärra Granite, after De La Roche, Leterrier, Grandclaude, & Marchal (1980) with geotectonic implications from Bowden & Batchelor (1985).

Figure 15. Multicationic diagram for discrimination of plutonic rock for the five samples from Örsviken and the two samples from the Askim and Kärra Granite. After De La Roche, Leterrier, Grandclaude, & Marchal (1980).

5. Discussion The aim of this report is to identify the cause the redistribution of ferric iron. It is difficult to of the high susceptibility values, to classify and if interpret if the magnetite in the leucogranite is a possible interpret the origin of the leucogranite in result of one or more of the above processes. Örsviken. There is no discrimination between ferrous Many minerals consist of iron, but only a few and ferric iron in Table 1. When comparing of them have enough iron to cause high sample Bi01 and Bi08 they both have similar susceptibility values. Oxides and sulphides are susceptibility values / magnetite content but Bi08 often strongly magnetic while silicate phases are has higher iron content. This is probably due to weakly magnetic. Redox state, hydrothermal “the extra iron” in Bi08 is incorporated into non- alteration, tectonic setting, metamorphism and magnetic minerals. This correlates with the bulk rock composition control the magnetic relative high biotite content seen in thin sections properties of igneous intrusions (Clark, 1999). in Bi08 compared to Bi01. The most common, and one of the most magnetic All the samples taken from the leucogranite minerals, is magnetite (Fe O ). It is therefore 3 4 plot close to the igneous spectrum in the possible to classify a rock’s magnetic behaviour alteration diagram after Hughes (1973) apart by its overall magnetite content (Keary, Brooks, from Bi08 that lies within the igneous spectrum & Hill, 2002). The high susceptibility values (Figure 12). Igneous rocks that lie outside this obtained across the leucogranite are intepreteted igneous spectrum are metasomatic according to be caused by magnetite. This is also confirmed (Hughes, 1973). The samples from the in the thin sections, stone slabs and SEM as leucogranite lie close to the igneous spectrum almost all the opaque minerals found are which indicates that the leucogranite has not magnetite. No apparent pattern of the magnetite suffered metasomatism of any significance, only distribution has been identified, neither in the slightly K-alteration. field when using the magnetic susceptibility meter nor in thin sections. The magnetite appears Accordning to SGU the granodiorite is older to be scattered randomly across the leucogranite than the leucogranite. Also, the mapped xenolith (Berg & Engelbrektsson, 2013). strengthens the idea that the granodiorite is older. The folded pegmatite intrusion suggests that the The iron proportions in magnetite are one leucogranite is older than the last ferrous (Fe2+) and two ferric (Fe3+). The major (Sveconorwegian) orogeny. In summary, the control if iron is in ferrous or ferric state is the leucogranite is older than the Svegonorwegian degree of oxidation i.e. oxygen fugacity. Iron orogeny but younger than the granodiorite. occurs as native metal, (Feo) when the oxygen fugacity is very low. When the oxygen fugacity The genetic S-I-A-M classification by increases, iron is incorporated into magnetite and Chappell and White (2001) is one of the silicates, and occurs in both the ferrous and ferric commonly used classifications of granitoids. The state. At even higher oxygen fugacity the iron is leucogranite in Örsviken is according to this in the ferric state, and is incorporated into classification an A-type granite based on its high hematite (Fe2O3). A good indicator of SiO2 (>77%), Na2O + K2O, Fe/Mg and low CaO. hydrothermal alteration is the redox state of iron As mentioned before the prefix A stands for in rocks. Hydrothermal alteration can both create anorogenic, i.e. this classification indicates that and destroy magnetic minerals when high the granite was formed during a period of no concentrations of unusual reactants (hydrogen or orogenic activity. The cross-bedding seen in the oxygen) or large volumes shift iron (Clark, study area could suggest that the leucogranite in 1999). Also, the degree of metamorphism affects Örsviken is a H-type (hybrid) granitoid that is a

mixture of both an A-type and a S-type granite. and for upcoming students dating of the A classification of granitoids could also be done leucogranite would be of interest, as well as using based on a combination of their tectonic setting the laser ablation inductively coupled plasma and chemical composition. In this way the mass spectrometer (LA ICP-MS) for trace connection between chemical composition and element analyses, to be able to draw further tectonic setting becomes clearer. The conclusions. Also, a more thorough examination leucogranite plots in both the peraluminous and of the thin sections from the leucogranite and calc-alkaline field and has a high K2O content. comparing them with thin sections from the This gives an indication that the leucogranite has Kärra- and Askim Granite would be a good idea. a post-orogenic petrogenesis, which correlates well with the tectonic magmatic discrimination diagram (Figure 15). The origin of a post- 6. Conclusion orogenic magma is partial from melting of lower The questions to be answered in this thesis crust and mantle and mid-crust contribution. were the cause to the high susceptibility values, Post-orogenic granites are usually generated 10 – classification of the leucogranite and if possible 100 Ma after the compressive deformation has to interpret the origin of the leucogranite in ceased (Winter, 2010). Since the leucogranite Örsviken. Magnetite is the most common formed somewhere between 1.52 – 1 Ga, it is magnetic mineral and is likely the reason for the possible that the Gothian orogeny (1.66 – 1.5 Ga) high susceptibility values across the leucogranite. (Lundqvist, Lundqvist, Lindström, Calner, & It is difficult to interpret if the magnetite is a Sivhed, 2011) is the event that preceded and result of one or more of the previous discussed generated the leucogranite. processes. The leucogranite is interpreted as possible A- type alkali granite with an anorogenic The leucogranite in the study area shows quite or a post-orogenic petrogenesis. The leucogranite similar patterns and chemistry to the Askim is older than the Svegonorwegian orogeny but granite (Anfinset, 1999) and Kärra granite (U. younger than the granodiorite. Bergström, personal communication, 24 May 2013) which might indicate that the leucogranite To be able to define the processes that creates, is associated with them. One possible explanation alters and destroys magnetic minerals in rocks it of the leucogranite’s characteristic chemistry is important to incorporate rock magnetism and could be that it is a mobilization and conventional petrology. recrystallization of the already felsic-rich Askim- and/or Kärra granite. Another explanation of the grain size and chemistry of the leucogranite could 7. Acknowledgements be that it is a product of the contact We would like to thank our supervisors Erik metamorphism between the Askim granite and Sturkell and Johan Hogmalm for help during the the granodiorite in the study area, which suggests project and the Department of Earth Sciences in that it has undergone a rapid cooling. the University of Gothenburg for funding this Furthermore, it cannot be excluded that the project. Thanks to David Cornell for helping us leucogranite could be interpreted as an aplitic with laboratory work, and to SGU for answering intrusion in the Askim granite. our questions and giving us great inputs to improve our report. Also, a great thank you to all There are some uncertainties in this work our classmates, family and friends for all the because of the small number of samples collected support and constructive critique during the and analysed. This could be avoided to some thesis writing. Finally, we would like to give a extent by collecting many samples. special thanks to Andreas Inerfeldt, Mikael Further investigations of the study area might Tillberg and Nina Nordgren, without you we be of interest. Zircons were found using the SEM could not have done this.

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9. Appendix Table 2. Minimum, maximum and mean susceptibility values for the granodiorite.

Measuring N -S E-W Susceptibility Susceptibility Susceptibility point Minimum (SI [10-5]) Maximum (SI [10-5]) Mean (SI [10-5]) 49 6384073 316002 0 0 0 50 6384075 315990 - - 0 51 6384101 315941 0 0 0 52 6384075 315984 0 0 0 53 6384077 315989 - - 0 54 6384076 316022 - - 0 55 6384081 316067 - - 0 56 6384079 316080 200 700 600 57 6384062 316165 - - 0 58 6384058 316191 - - 0 59 6384060 316235 - - 200 60 6384062 316228 - - 200 61 6384062 316333 - - 200

Table 3. Minimum, maximum and mean susceptibility values for the leucogranite.

Measuring N -S E-W Susceptibility Susceptibility Susceptibility point Minimum (SI [10-5]) Maximum (SI [10-5]) Mean (SI [10-5]) 1 6384023 316206 200 2000 500 2 6384016 316191 300 1600 600 3 6384016 316178 500 2000 800 4 6384013 316176 600 2000 1500 5 6384015 316162 400 2000 1500 6 6384017 316157 300 1500 800 7 6384013 316144 900 2500 1500 8 6384022 316129 800 2500 1500 9 6384015 316123 1500 5500 2000 10 6384031 316102 400 1500 500 11 6384023 316091 450 2000 800 12 6384038 316087 800 2000 1000 13 6384046 316089 600 1500 1000 14 6384054 316072 200 800 300 15 6384060 316047 300 1000 300 16 6384050 316030 300 500 500 17 6384061 316029 200 400 400 18 6384065 316006 200 500 400 19 6384073 316002 - 600 200 20 6384075 315984 - 400 200 21 6384077 315989 - - 400 22 6384076 316022 - - 400 23 6384072 316037 900 1500 1000 24 6384078 316040 - - 1500 25 6384082 316053 - - 500 26 6384081 316067 - - 200 27 6384079 316080 - - 0 28 6384075 316092 - - 200

29 6384065 316180 400 1500 500 30 6384060 316124 300 1500 800 31 6384063 316153 1000 2000 1500 32 6384061 316156 1000 1500 1000 33 6384062 316165 400 1000 800 34 6384048 316170 600 1500 800 35 6384058 316191 - - 600 36 6384048 316197 800 2000 1500 37 6384061 316112 - - 600 38 6384044 316214 500 4000 1500 39 6384060 316235 - - 200 40 6384041 316239 1000 2000 1500 41 6384062 316228 - - 500 42 6384051 316276 800 2000 1500 43 6384048 316298 800 2000 1000 44 6384047 316314 1000 2000 1500 45 6384062 316333 - - 500 46 6384045 316321 400 1500 600 47 6384030 316305 500 1500 800 48 6384030 316325 800 1000 800