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UNIVERSITY OF GOTHENBURG Department of Earth Sciences Geovetarcentrum/ Earth Science Centre ISSN 1400- 3821 B606

Bachelor of Science thesis Göteborg 2010

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

Intrusion geometry of Högsbo generation pegmatites:

Gothenburg region, western Sweden

Stina Hallinder

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INTRUSION GEOMETRY OF HÖGSBO GENERATION PEGMATITES: GOTHENBURG REGION, WESTERN SWEDEN

Stina Hallinder, University of Gothenburg, Department of Earth Sciences; Geology, Box 460, SE-405 30 Göteborg

Abstract

The Högsbo generation pegmatites are thought to be late- or post-tectonic and yield a minimum age of the Sveconorwegian metamorphism. The Högsbo pegmatite gives a columbite age of 1030Ma. Gothenburg lies in the Western segment in the Sveconorwegian Province. The intrusion geometry of pegmatites was investigated in the Delsjö area, Kallebäck and Keillers Park, were the direction of the dilation vector, the preferred mineral orientation and other structures were measured. Two distinct directions of the dilation vector, one in the northwest direction and one in the southwest direction, were found in the study area. These are further supported by the preferred mineral orientations, interpreted as mineral growth direction. The dilation vector and the preferred orientation indicate that the pegmatite intrusions were subjected to hybrid shear due to the elevated magmatic or fluid pressure during crystallization. The pegmatites in the Delsjö area and Keillers Park are interpreted as subvertical dikes, emplaced at the critical depth in the brittle domain while the pegmatites in Kallebäck were intruded as subhorizontal sills and could be emplaced at the critical depth or over the critical depth in the brittle domain. The two different pegmatite orientations could be related to different depths in the brittle domain or switching of σ1 and σ3 orientation. The emplacement conditions in the Gothenburg region were interpreted as syn-tectonic and the undeformed pegmatites were influenced by directed tectonic stress from the Sveconorwegian orogeny. Keywords: Pegmatite, dilation vector, preferred mineral orientation, Sveconorwegian orogeny, brittle domain ISSN 1400-3821 B606 2010

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INTRUSION GEOMETRY OF HÖGSBO GENERATION PEGMATITES: GOTHENBURG REGION, WESTERN SWEDEN

Stina Hallinder, Göteborgs Universitet, Geovetarcentrum; Geologi, Box 460, SE-405 30 Göteborg Sammanfattning Pegmatiter från Högsbo generationen tros vara sen- eller posttektoniska och ge en minimiålder för den Sveconorvegiska metamorfosen. Högsbo pegmatiten har en columbit ålder på 1030Ma. Göteborg ligger i västra segmentet i den Sveconorvegiska provinsen. Pegmatiternas intrusions geometri undersöktes i Delsjön, Kallebäck och Keillers park. Där mättes utvidgningsvektorns riktning, föredragen mineralorientering samt andra strukturer. Två olika utvidgningsriktningar påträffades i studieområdet, en i nordvästlig- och en i sydvästligriktning. Dessa styrks ytterligare av de föredragna mineralorienteringarna, vilket tolkas som riktning av mineraltillväxt. Utvidgningsvektorerna och den föredragna mineralorienteringen visar att pegmatiterna utsattes för hybridbrott på grund av förhöjt magma- och vätsketryck under kristalliseringen. Pegmatiterna i Delsjön och Keillers park tolkades som subvertikala dikes, placerade vid det kritiska djupet i den spröda domänen. Pegmatiterna i Kallebäck tolkades som subhorisontella sills, placerade vid eller under det kritiska djupet i den spröda domänen. De två olika pegmatitorienteringarna kan vara relaterat till olika djup i den spröda domänen eller omkoppling av σ1 och σ3 riktningarna. Placeringsförhållandena i Göteborgsområdet har tolkats som syntektoniska och de odeformerade pegmatiterna påverkades av riktad tektonisk stress från Sveconorvegiska orogenesen. Nyckelord: Pegmatit, utvidgningsvektor, föredragen mineralorientering, Sveconorvegiska orogenesen, spröd domän ISSN 1400-3821 B606 2010

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Table of contents 1. Introduction .......................................................................................................................... 6

1.1 Objectives of the project ................................................................................................... 6 1.2 Pegmatites ........................................................................................................................ 6 1.3 Stress and strain ............................................................................................................... 7 1.4 Structures ....................................................................................................................... 10 1.5 Geological setting .......................................................................................................... 11 1.6 Study area ....................................................................................................................... 11

2. Method ................................................................................................................................. 13 2.1 Sampling ......................................................................................................................... 13 2.2 Measuring and plot techniques ...................................................................................... 14

3. Results ................................................................................................................................. 16 3.1 Direction of dilation vector ............................................................................................ 16 3.2 Preferred mineral orientation ........................................................................................ 18 3.3 Main orientation ............................................................................................................. 21 3.4 Other structures .............................................................................................................. 23

4. Discussion ............................................................................................................................ 24 5. Conclusion ........................................................................................................................... 27 6. Acknowledgment ................................................................................................................ 27 7. References ........................................................................................................................... 27

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1. Introduction

1.1 Objectives of the project

The objective of my project is to investigate the intrusion geometry of some pegmatites in the Gothenburg region, western Sweden. My aim is to measure the geometry and orientation of pegmatite structures and to interpret the internal paleo-stress and -strain relationship. My objective is to correlate these and hopefully find a relationship between the pegmatites in the Gothenburg region. A further aim is interpretation of the depth at which the pegmatites was emplaced, if they behaved as horizontal sills or vertical dikes and if they were influenced by the Sveconorwegian orogeny.

1.2 Pegmatites

A pegmatite is a very coarse rock; it’s named based on textural criteria, not necessarily on felsic mineral content. However it is typically of granitic composition. The mineral content is largely alkali feldspars (albite or Na-plagioclase and microcline) and quartz crystals (Blatt & Tracy 1996). Pegmatites are often related to intrusive, late tectonic granite plutons. They are frequently marked by minerals with high volatile components (OH, F, P and B) and accessory minerals. Pegmatite bodies vary greatly in size and shape, from fracture fillings to bigger complex bodies and to swarms forming pegmatite fields (Evans 2005). Pegmatites are classified as simple or complex bodies, the simple ones lacking or with little developed internal zoning. Complex bodies on the other hand have an arrangement of minerals in a zonal sequence (Evans 2005). Mineral zoning means variations in mineral assemblage, grain size and crystal orientation. Pegmatites with internal zoning often have a thin border zone with fine grained plagioclase and quartz followed by an intermediate zone with coarser perthitic microcline, plagioclase and quartz. The intermediate zone develops in the thickest parts of the pegmatite body and the mineral distribution can be either symmetrically or asymmetrically. The innermost unit of the pegmatite body, the core, is often composed of mono-mineralized quartz but cores of quartz, perthitic microcline, albite and lithium have been found (London 2008).

Pegmatites are regarded as rocks that may have been crystallized in the presence of a magmatic aqueous fluid. Many different theories about the origin and the distinct appearance of pegmatites have been presented. Jahns and Burnham (1969) proposed a model based on the influence of an exsolved H2O fluid. The pegmatite crystallization would occur in the presence of an exsolved H2O fluid and the transition from granite to pegmatite would mark the point at which the melt became H2O saturated. The pegmatites would form as two immiscible liquids, a silicate rich melt and a vapour phase. The influence of the vapour phase would depress the granite solidus and permit larger crystals to grow (Robb 2005 and references therein). More recent work by London (1990, 1992 and 1996) implies that pegmatites forms by a H2O undersaturated magma that gets undercooled below its normal liquid temperature (Robb 2005 and references therein). London (2005) further implies that the magma cooled rather quickly, more quickly than the crystallization could keep pace with. The more undercooled the melt got before the crystallization commenced, the more pegmatite like fabric and internal zonation (London 2005). More recent work has though supported Jahns theory about a fluid inclusion and immiscibility between two coexisting phases (Robb 2005 and references therein). But in contrast to a simple H2O fluid, it is more complex with fluxing elements (OH, B, P, and F) that lower the viscosity of the melt and crystallization temperature (London 2005). Therefore,

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a magmatic aqueous phase has to be represented by a H2O fluid that is more complex than a simply exsolved H2O fluid, which means containing fluxing elements (Robb 2005).

1.3 Stress and strain

Stress (σ) means force per unit area acting on a surface and it can deform a body if the strength of the body is exceeded. It is often called traction and is subdivided into components, normal stress and shear stress, the former perpendicular to the plane and the latter parallel to the plane (figure 1).

If the normal stress is positive it is compressive and if it is negative, tensile. Shear stresses are positive when they right handed and negative when they left handed shear stresses. The stress ellipse shows the principal stress directions, σ1 > σ2 > σ3, where σ1 is the greatest principal stress and σ3 is the least principal stress (figure 2).

Stress (σ)

Normal Stress (σ N)

Shear Stress (σ S)

Plane

σ1

σ3

Figure 2: The stress ellipse portrays the stress tensor at a single point in a two-dimensional body with the principal stress axes. Here only the greatest and least principal stresses are showed due to the two-dimensional view (Davis & Reynolds 1996).

Figure 1: A stress (traction) can be determined into normal stress (perpendicular to plane) and shear stress (parallel to plane) components (Davis & Reynolds 1996).

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The Mohr stress diagram picture stress variations in a body and identifies a plane or orientation comparative to σ1 and from the diagram the equivalent normal stress and shear stress. A circle is drawn between normal stress values, σ1 and σ3, and the diameter of the circle is called differential stress (σ1 - σ3). The greater the differential stress, the bigger possibility for fracture (Davis & Reynolds 1996). Figure 3 shows a Mohr stress diagram with three different possibilities, tension fractures, shear fractures and hybrid shear. Both tension fractures and hybrid shear touches the failure envelope in the negative tensile section (Price & Cosgrove 1990).

Figure 3: Mohr stress diagram with different stress conditions. A) Tensile fracture B) shear fracture C) hybrid shear. In hybrid shear both normal stress (tensile) and shear stress will act on the fracture plane. The acute angel (2θ) in hybrid shear will be less than 45º and in the case of shear fracture it will be greater than 45º (Price & Cosgrove 1990). Taken from Price & Cosgrove (1990).

Elevated hydrostatic fluid (Davis & Reynolds 1996) or magmatic pressures (Lennart Björklund, personal communication) will decrease the confining pressure in a rock and it will lead more easily to fracture. The whole stress circle would move to the left and closer to the fracture envelope. This can be the case in the hybrid shear and tensile fractures scenario (Davis & Reynolds 1996). Pegmatites with a magmatic aqueous phase (Robb 2005) will lead to intrusion when the pegmatite’s fluid pressure is sufficiently high to overcome the normal stress conditions. In the crust were the main stresses are compressive, the pegmatite must

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intrude as the product of the fluid (Brisbin 1986) or magmatic pressure (Lennart Björklund, personal communication) overcoming the compressive strengths (Brisbin 1986). Stress produces strain and it is a change in shape or size. Strain is divided into pure shear (coaxial strain) and simple shear (noncoaxial strain). This is best seen in the strain ellipse were pure shear is represented as shortening in one direction and extension in the perpendicular direction and no rotation of the finite stretching axes occur (figure 4). Simple shear is represented as a sheared ellipse with rotated finite stretching axes (figure 5) (Davis & Reynolds 1996).

S3

S1

S

S

E

E

S3

S1

Figure 4: The strain ellipse for pure shear with maximum finite stretch (S1) and minimum finite stretch (S3). The grey area (S) stands for finite shortening field and white area (E) finite extension field (Davis & Reynolds 1996).

Figure 5: The strain ellipse for simple shear with maximum finite stretch (S1) and minimum finite stretch (S3). The grey area (S) stands for finite shortening field and white area (E) finite extension field. In contrast to pure shear, simple shear have rotating finite axes (Davis & Reynolds 1996).

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Strain affect structures differently dependent on if the structures lie in the finite shortening field or finite extension field of the strain ellipse. For example, folds would lie in the shortening field while boudinage would lie in the extension field. The sense of shear also depend on if it is pure shear or simple shear. Simple shear have a clear asymmetry while pure shear would have a distinct symmetry (Davis & Reynolds 1996).

1.4 Structures

Pegmatites form dikes, sills or pods in both igneous and metamorphic rocks (London 2008). Pods occur as bulbous shaped intrusions and are created in the ductile zone in the crust (Brisbin 1986). Dikes are nearly vertical, tabular intrusions (Marshak 2005) that occur below and at the critical depth in the brittle and at the transition zone in the crust during syn-tectonic conditions with directed tectonic stress (Brisbin 1986), lithostatic stress (equal pressure from all directions) (Davis & Reynolds 1996) and fractures with random orientation (figure 6) (Brisbin 1986). Sills are nearly horizontal tabular intrusions (Marshak 2005) and occur during syn-tectonic settings with directed tectonic stress, lithostatic stress and fractures with random orientation in the upper brittle zone (over the critical depth) and at the critical depth (figure 6) At the critical depth the minimum normal stress can be either vertical or horizontal and both dikes and sills have the same minimum normal stress acting across them (figure 6) (Brisbin 1986).

Jogs are structures were the fractures meets at a sharp angle. Horn is a wedge formed termination of a fracture segment at an offset (Hoek 1991). Dilation is strain and means changes in volume and it can either be negative (decreasing) or positive (increasing) (Davis & Reynolds 1996). The dilation direction is the direction of opening in a fracture system and the dilation vector connects the corresponding points on the fracture walls (cp. figure 12) after

Pegmatite orientation

Depth

Normal stress increase Rheologic conditions Min σN

Figure 6: Syn-tectonic conditions with lithostatic stress, directed tectonic stress and fractures with random orientations. σV= vertical normal stress, σH= horizontal normal stress, σH (NS)= σH is arbitrarily N-S with σH (EW) perpendicular. At the critical depth conditions are σV = σH (EW). The minimum normal stress must be overcome by fluid or magmatic pressure at intrusive sites (Brisbin 1986). Taken from Brisbin (1986).

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they have been separated (Hoek 1991). This extensional direction is generally parallel to stress direction σ3 in a mechanically homogenous rock (Davis & Reynolds 1996).

1.5 Geological setting

The Southwestscandinavian Province ranges to the south Norway in the north and to the southern Sweden in the south. The Province has been influenced by two bigger orogenies (Lindström et al. 2000), the Gothian orogeny (1.6- 1.5Ga) (Henderson & Ihlen 2004 and references therein) and the Sveconorwegian orogeny (1.15- 0.90Ga). The Sveconorwegian orogeny influenced the Province by extensive deformation and metamorphism. It also created shear zones that split the bedrock into the Western and Eastern segments and triggered intrusions of the younger granites, e.g. Bohus granite and Blomskogs granite (920Ma). The Eastern and Western gneiss segments represent an increment of the Baltic shield to the west. Gothenburg lies in the western segment and mainly in the Åmål- Horred belt, which is the eastern part of the Western segment. Gothenburg is traversed by a shear zone, the Götaälv zone, with a NNO strike and a W dip. The movement in the shear zone happened at repeated occasions. The southern part of the Götaälv zone has had extensional movements, so that the west (upper) block slided to the west. This happened towards the end of the Sveconorwegian orogeny (Lindström et al. 2000).

1.6 Study area

The study area lies within the Gothenburg region in western Sweden. The pegmatite localities that have been investigated are the Delsjö area, Kallebäck and Keillers Park on Ramberget (figure 7).

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Figure 7: A map over Gothenburg with the sampling areas (red rectangle). A) Keillers Park and Ramberget, B) the Delsjö area and C) Kallebäck. Made in ArcGIS version 9.3.1 with mapdata from Lantmäteriet.

Different generations of pegmatites occur in the Western segment. The older pegmatites are deformed and folded and some have been subjected to boudinage (Lundqvist 1999). The pegmatites studied forms five centimetres to 25 metres wide dikes and sills. They form systems of sub-parallel dikes and sills and they are not deformed by later orogenesis. The Högsbo pegmatite is a large irregular intrusion and was U-Pb dated with columbite to 1030Ma (Romer & Smeds 1996). Romer & Smeds also U-Pb dated two pegmatite localities on the island of Orust and they yielded similar ages and equivalent rare-mineral assemblages. These are not deformed by later orogenesis and are thought to yield a minimum age of the Sveconorwegian metamorphism (Romer & Smeds 1996). In southern Norway, within the Sveconorwegian province, the pegmatites where thought to have been post-tectonic but more recent studies suggest that they took place syn-tectonic with the Sveconorwegian orogeny (Henderson & Ihlen 2004 and references therein). This study concerns the last generations of undeformed pegmatites.

A

B C

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2. Method

2.1 Sampling

The study areas in the Gothenburg region I investigated were the Delsjö area, Kallebäck and Keillers Park. The pegmatite localities were marked on location by a GPS and the location points were thereafter put into a GIS program, ArcGIS 9.3.1, to show all the pegmatite localities (figure 8 and 9).

Figure 8: Pegmatite locales in the Delsjö area, Getryggen in the north and the Peninsula in the south. The single red point in the southwest is Kallebäck. Made in ArcGIS version 9.3.1 with mapdata from Lantmäteriet.

Pegmatite localities in the Delsjö area and Kallebäck

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Figure 9: Pegmatite locales in Keillers Park and Ramberget on Hisingen in Gothenburg. The red points are GPS locations. Made in ArcGIS version 9.3.1 with mapdata from Lantmäteriet.

2.2 Measuring and plot techniques

All the measurements of the pegmatites strike and dip were collected by me and occasionally with the help of Lennart Björklund. This was made with a compass out in the field. The orientation data is noted by the right hand rule together with the extension direction of the dike. The direction of the dilation vector was measured as the true net dilation. This was determined by two formerly juxtaposed points on the opposite walls of the pegmatite (Brisbin 1986). These points were selected where the pegmatites were changed from a main direction to link directions and jogs to find the dilation vector (figure 10).

Pegmatite localities in Keillers Park and Ramberget

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Figure 10: A pegmatite section on Getryggen (Delsjö area) with a distinct link direction, main direction and jog. Photo: Stina Hallinder 2010.

The main and link directions were measured for strike and dip and the corresponding jog-points were only measured for strike in dike-systems or plunge in sill-systems. Thereafter a manual stereographic projection was used to find the true direction of the dilation vector. The manual stereoplot was performed by taking the strike and dips for the main- and link directions and plot them together with the strike or plunge of the jog. The jog direction was moved on the stereo net to fit the great circle with the intercross of the main- and link direction. On this new great circle, with the purpose of a help line, 90º (perpendicular from the intercross of the main- and link direction) was measured from the intercross to find the orientation of the orientation of the dilation vector (figure 11).

Jog

Link

Main

Main

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Figure 11: Stereoplot with the direction of the dilation vector (diamond), help line and main- and link directions. The dilation vector lies on the help line 90º from the intercross (point). See text for more explanation. Made in StereoWin 1.2.

The preferred orientation of the minerals in the inner zones, quartz and alkali feldspar, was also measured. They were measured as strikes and plotted into a stereoplot as rose diagrams. Some quartz veins in Keillers Park, in addition to the pegmatites, were measured for strike and dip. Furthermore offshoots, horns and faults with striae in Kallebäck were measured. The results were all plotted later as equal area stereographic projections in the data program StereoWin 1.2. Also study of previous work concerning the study area and pegmatite intrusion mechanisms has been carried out.

3. Results

3.1 Direction of dilation vector

The measurements of the direction vector were taken in the Delsjö area and Kallebäck. A typical section of the main- and link direction were found in Kallebäck (figure 12).

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Figure 12: A pegmatite step section from Kallebäck with a distinct dilation vector. Photo: Lennart Björklund 2010.

The directions of the dilation vector in the Delsjö and Kallebäck areas are represented in two stereoplots (figure 13A-B). The mean orientation of the dilation vector on Getryggen in the

Direction of dilation vector

Figure 13A-B: The direction of the dilation vectors are represented as lines (trend and plunge). The black diamond represents the mean vector at each cluster and was done by Fisher vector distribution. Made in StereoWin 1.2 A) Dilation vectors from Getryggen in the Delsjö area (green) with mean vector 320º/ 36º. B) Dilation vectors from Getryggen (green), the Peninsula (blue) and Kallebäck (red) with mean vector 225º/ 33º.

13A) 13B)

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Delsjö area is in the northwest and southwest directions (figure 13A-B) respectively with a few points in the opposite direction (northeast and southeast). In Kallebäck the mean direction of the dilation vector is southwest (figure 13B). On the peninsula in Delsjö area only one measurement was made and that also lie in the southwest direction (figure 13B). The two clusters have different mean vectors of 320º/ 36º (figure 13A) in the northwest direction and 225º/ 33º (figure 13B) in the southwest direction.

3.2 Preferred mineral orientation

The pegmatites in the Delsjö area are fine to coarse grained and some are coarse throughout the whole pegmatite and some have a fine grained border zone with a coarser core zone. The pegmatites vary in width, from a few centimetres to tenfold of metres wide. Generally the pegmatites in the Delsjö area contain plagioclase and quartz in the border zone and microcline, quartz and to some degree plagioclase in the core zone. In Kallebäck just southeast of Gothenburg, a 30 meter long pegmatite sill-system is exposed along a vertical road-cut. It has a clear internal zoning with plagioclase and quartz that forms a sharp border zone, 2-4 centimetres thick. This is also apparent in the horns in Kallebäck where a sharp white tip ends the horn. This is further discussed below. The pegmatite in Kallebäck has a very coarse core zone of quartz, plagioclase and microcline, while the border zone of quartz and plagioclase is less coarse. Some of the pegmatites have aligned grains with the gneiss in the border zone but most of the pegmatites in the study area contain border zone grains at angles to the intrusive contact (figure 14). The core zones in both Kallebäck and Delsjö area often had a preferred mineral orientation (figure 14 and 15) of quartz and alkali feldspars skewed in the direction of dilation. The preferred orientation of quartz and alkali feldspar in the pegmatite’s core zones was measured in the Delsjö area (figure 16) and Kallebäck (figure 17). These are generally oriented parallel to the direction of the dilation.

Figure 14: Pegmatite in Kallebäck with border zone grains at angles to the intrusive contact and skewed core zone in the direction of dilation. The lines mark their orientations. Photo: Lennart Björklund 2010.

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Figure 15: Preferred orientation of quartz and alkali feldspars in Kallebäck. The lines mark their orientations. Photo: Lennart Björklund 2010

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Figure 16: A stereoplot over the preferred mineral orientation in the Delsjö area (Getryggen and the Peninsula). This was made as a rose diagram in StereoWin 1.2.

Figure 17: A stereoplot over the preferred mineral orientation in Kallebäck. This was made as a rose diagram in StereoWin 1.2.

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In figure 16 the preferred orientations are scattered with two main directions in the east west and north south but the most lies in the in the northwest southeast direction. In Kallebäck (figure 17) the preferred mineral orientation lies in the southwest northeast direction. Both figures are shown as rose diagrams because only the direction of the preferred orientation was measured, not the dip.

3.3 Main orientation

The pegmatite’s main directions were measured in the Delsjö area (Getryggen and the Peninsula), Keillers Park and Kallebäck. The gneiss orientation in the Delsjö area and some quartz veins in Keillers Park were also measured for comparison. A stereoplot showed as poles from planes illustrate the main directions of the pegmatites, quartz and gneiss in the study area (figure 18A-F).

18A)

Figure 18A-E: Stereoplot of orientations of pegmatites, quartz veins and gneissosity of country rock in the study area. All the plots are plotted from planes to poles (besides from figure 18B) and are made in StereoWin 1.2. Figure 18A: Pegmatites on Getryggen in the Delsjö area. Figure 18B: Pegmatites on Getryggen plotted as Kamb contour (lines) with a mean vector (triangle) of 260º/ 42º.

18B)

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The main orientations of the pegmatites in The Delsjö area (Getryggen and the Peninsula) (figure 18A and C) are quite equal and they strike in the N-S direction with a steep to gentle dip to the E. In figure 18B the pegmatites main orientations on Getryggen were also plotted as Kamb contour with a mean vector of 260º/ 42º. This was only done on figure 18A due to the more amounts of measurements. The gneiss in the Delsjö area also has an N-S strike but with a fairly gentle dip to the E (figure 18F). In Keillers Park (figure 18D) the pegmatites and the

18C) 18D)

18E) 18F)

Figure 18E: Pegmatites in Kallebäck.

Figure 18D: Pegmatites and quartz veins in Keillers Park.

Figure 18C: Pegmatites on the Peninsula in the Delsjö area.

Figure 18F: Gneiss orientation in the Delsjö area.

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quartz veins have equal orientations and they strike in the S-N direction with a steep to gentle dip to the W. The pegmatites in Kallebäck strikes in the northwest direction with a rather gentle dip to the NE (figure 18E).

3.4 Other structures

In the Delsjö area and Kallebäck few offshoots were found in the pegmatites. In Kallebäck a distinct horn was measured to investigate any correlation with the direction of the dilation vector and the preferred mineral orientation. Two faults with striations were also measured to compare any correlations with the dilation vector in Kallebäck (figure 20A-B). Figure 19 shows the horn, two oblique normal faults with striae in Kallebäck.

Figure 19: The pegmatite in Kallebäck has been subjected to faulting. A distinct horn is showed with a sharp white tip of plagioclase and quartz. Photo: Lennart Björklund 2010. Figure 20A shows the situation in Kallebäck with the preferred mineral orientation and dilation vector versus horn. The horn was measured on both the upper and lower line for strike and dip. The preferred orientation and the dilation vector have the same direction, southwest, and they lie perpendicular to the horn. The oblique normal faults strike in the southwest direction with a steep dip to NW. The striations have a northwest direction with a NW plunge (figure 20B).

Horn

Fault and striations

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Figure 20A-B: Stereoplot over Kallebäck with different structures. Made in StereoWin 1.2. Figure 20A: Stereoplot with dilation vectors (red), preferred mineral orientation (black) and horn (blue). The preferred mineral orientation is plotted as lines with trend and plunge. The plunge for the preferred orientation is estimated to zero degrees. The horn is plotted as a great circle. Figure 20B: Stereoplot with oblique normal faults (green) and striations (black). The normal faults are plotted as great circles and the striations are plotted as lines.

4. Discussion The dilation vector measurements (figure 13A-B) in both the Delsjö area and Kallebäck display two distinct directions, one in the northwest direction and one in the southwest direction. The Delsjö area shows both these directions and Kallebäck show southwest direction. The three green points with a southeast and northeast direction maybe be part of the same main cluster of lines but are plotted 90º from them. The Peninsula in the Delsjö area has the same dilation vector as Kallebäck, southwest (figure 13B). It was hard to find distinct jogs in the Delsjö area and maybe some of the jogs in the Delsjö area were really folds instead of jogs. In Kallebäck we found really distinct jogs and these dilation vectors are more reliable than those measured in the Delsjö area. One other reason why there is two distinct dilation directions can be that the stress field has undergone rotation and σ3 and σ1 has changed places (Hoek 1991) in the Delsjö area or Kallebäck. The two mean vectors of direction of dilation are almost at 90º from each other (figure 13A-B).

The preferred mineral orientation in the Delsjö area is scattered over a wide span of directions with the major orientation in the northwest southeast direction (figure 16). This supports the direction of dilation vector in the Delsjö area in the southeast direction. In Kallebäck the preferred mineral orientation is in the northeast southwest direction (figure 17) and this also support the dilation vector in Kallebäck. The general parallelism of preferred mineral

20B) 20A)

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orientation and dilation direction is interpreted as evidence for this also being the direction of mineral growth. The main orientations of the pegmatites (figure 18A-F) in the Delsjö area, both on Getryggen and on the Peninsula, have the same strike, N-S direction. The dip on the other hand varies from steep to fairly gentle, but the most common dip is steep to the E, due to the mean value of 260º/ 42º on Getryggen. The greater amount of the pegmatites in the Delsjö area could be interpreted as dikes (figure 18B-C). The gneiss in the Delsjö area lies in the same strike direction as the pegmatites but the gneiss have a more gentle dip, 10-30º, to the E. This can denote that the pegmatites in the Delsjö area did not follow the gneisses foliation but intruded at an originally steeper dip and the gneiss foliation did not control the orientation of the pegmatites in the Delsjö area. In Keillers Park and on Ramberget the pegmatites and quartz veins main orientations are in the S-N direction with a dip of 20-70º to the W and the pegmatites could occur as both sills and dikes. The difference in strike direction in Keillers Park, from the rest of the study area may be due to the Götaälv shear zone, which dips in the W direction. This faulting may possible have reoriented the pegmatites. In Kallebäck the main orientations of the pegmatites have a strike in the northwest direction with a fairly gentle dip to the NE. The pegmatites in the Kallebäck are interpreted as sills. However, their present orientation may have been influenced by the normal faults (figure 19), if they are listric at depth. In figure 20A the horn, dilation vectors and preferred mineral orientation were plotted. This figure shows how the horn lies perpendicular to the dilation vector and preferred mineral orientation. This indicates that the direction of σ3 and S1 is parallel to the dilation vector plus preferred orientation and perpendicular to the horn (figure 21). The geometry of the horn and its mineral orientation supports the direction of the dilation vector in Kallebäck. The normal faults with striae in Kallebäck (figure 19 and 20B) are younger and cut the pegmatite.

Figure 21: Horn in Kallebäck with the σ3 and S1 direction and dilation vector written out. Photo: Stina Hallinder 2010.

σ3 +S1 + Dilation vector + Preferred mineral orientation

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According to Romer and Smeds (1996) the Högsbo generation pegmatites were not deformed by later orogenesis and thought to yield a minimum age of the Sveconorwegian metamorphism (i.e. syn-orogenic). The pegmatites in the study area are not deformed and they did not follow the rock foliation in the Delsjö area and are thought to be post-peak with the Sveconorwegian orogeny. The pegmatites in the study area may represent emplacement at different depths in the brittle domain or switching of σ1 and σ3 directions. No pods in the study area have been recorded so the ductile domain can be ruled out. The pegmatites were subjected to lithostatic stress and upper crustal and directed tectonic stress (Brisbin 1986). In the Delsjö area the pegmatites did not follow the gneisses rock foliation and this indicates that the emplacement of the pegmatites was not influenced by the host rocks strength anisotropy. The pegmatite sills in Kallebäck could have been emplaced in the brittle domain over the critical depth or at the critical depth (figure 22) and dilated vertically (Brisbin 1986). In the Delsjö area and Keillers Park the pegmatite dikes would have been emplaced in the brittle zone at the critical depth (figure 22), because at this level both vertical, dipping and horizontal fractures have the same amount of minimum normal stress working on them. The changing of the pegmatite orientation in the study area can be related to different depths in the brittle domain where both lithostatic stress and directed tectonic stress control the intrusion (Brisbin 1986). Switching of σ1 and σ3 (directed stress) would explain the occurrence of both dike- and sill-systems in the same rock volume.

Figure 22: Syn-tectonic conditions in the crust. Pegmatites in the study area could have been emplaced in the brittle domain, over and at the critical depth. σV= vertical normal stress, σH= horizontal normal stress, σH (NS)= σH is arbitrarily N-S with σH (EW) perpendicular. At the critical depth conditions are σV = σH (EW). Taken from Brisbin (1986). The results of the dilation vectors and preferred mineral orientations suggest that the pegmatites in the study area probably were subjected to hybrid shear. They were subjected to both extension and simple shear (figure 3). The pegmatites intruded with a small amount of

Kallebäck

Delsjö area Keillers Park Kallebäck

Depth

Normal stress increase Rheologic conditions Min σN Pegmatite orientation

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differential stress aided by elevated magmatic (Lennart Björklund, personal communication) or fluid pressure which decreased the confining pressure (Davis & Reynolds 1996) and moved the stress circle to the left until it touched the fracture envelope on the negative side.

5. Conclusion The pegmatites in the study area have two distinct directions of the dilation vector, one in the northwest direction and one in the southwest direction. These are further supported by the preferred mineral orientations, interpreted as mineral growth directions, in the study area and the horn measurement in Kallebäck. Due to the two dilation vectors in the study area, the stress field may have been rotating and σ3 and σ1 have changed places. Both the dilation vectors and the preferred mineral orientations are parallel to σ3. The different measured structures indicate that the pegmatites were intruded under hybrid shear conditions due to the elevated magmatic or fluid pressure. The pegmatites in the Delsjö area and Keillers Park were interpreted as subvertical dike-systems, emplaced at the critical depth in the brittle domain while the pegmatites in Kallebäck intruded as subhorizontal sills-systems, emplaced at the critical depth or over the critical depth in the brittle domain. The two different pegmatite orientations could be related to different depths in the brittle domain or switching of σ1 and σ3 orientations. The emplacement conditions in the Gothenburg region were interpreted as syn-tectonic and the undeformed pegmatites were influenced by directed tectonic stress and occurred at post-peak with the Sveconorwegian orogeny.

6. Acknowledgment I like to thank Lennart Björklund for all his help, both in the field and as a supervisor. Without his help, this project would not have been carried out. I also want to thank my opponent, Linda Lundgren, for reading and commenting my work and Oskar Bremer for all his help with GIS and other data related problems.

7. References Blatt, H. & Tracy R. J., 1996: Petrology- Igneous, sedimentary and metamorphic. Second edition. W. H. Freeman and Company. 529 pp. Brisbin, W. C., 1986: Mechanics of pegmatite intrusion. American Mineralogist 71, 644-651. Davis, G. H. & Reynolds, S. J., 1996: Structural geology of rocks and regions. Second edition. John Wiley & Sons. 776 pp. Evans, M. A., 2005: Ore geology and industrial minerals – An introduction. Third edition. Blackwell Publishing. 389 pp.

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Henderson, I. H. C. & Ihlen, P. M., 2004: Emplacement of polygeneration pegmatites in relation to Sveco-Norwegian contractional tectonics: examples from southern Norway. Precambrian Research 133, 207-222. Hoek, J. D., 1991: A classification of dyke-fracture geometry with examples from Precambrian dyke swarms in the Vestfold Hills, Antarctica. Geologische Rundschau 80/2, 233-248. Lindström, M., Lundqvist, J. & Lundqvist, T., 2000: Sveriges geologi från urtid till nutid. Andra upplagan. Studentlitteratur. 491 pp. London, D., 2005: Granitic pegmatites: an assessment of current concepts and directions for the future. In: Haapala, I. & Rämö, O. T. (eds.): Granitic systems, 281-303. Elsevier B. V. London, D., 2008: Pegmatites. The Canadian Mineralogist. Special Publication 10. 347 pp. Lundqvist, I., 1999: Kartblad 7A Marstrand SO/7B Göteborg SV och Göteborgs kommun. http://www.sgu.se/dokument/service_sgu_publ/RM_102-marstrand.pdf. Collected 2010-05-25. Marshak, S., 2005: Earth;Portrait of a planet. Second edition. W. W. Norton & Company. 748 pp. Price, N. J. & Cosgrove, J. W., 1990: Analysis of geological structures. Cambridge University Press. 502 pp. Robb, L., 2005: Introduction to ore-forming processes. Blackwell Publishing. 373 pp. Romer, R. L. & Smeds, S.A., 1996: U-Pb columbite ages of pegmatites from Sveconorwegian terranes in southwestern Sweden. Precambrian Research 76, 15-30.