Working
Report
2007-96
Paleomagnetism of Diabase Dykes,
Pegmatitic Granites and TGG Gneisses
in the Olkiluoto Area
Satu
Mertanen
March 2008
POSIVA
OY
Olkiluoto
FI-27160 EURAJOKI, FINLAND
Tel
+358-2-8372 31
Fax +358-2-8372 3709
Working
Report
2007-96
Paleomagnetism of Diabase Dykes,
Pegmatitic Granites and TGG Gneisses
in the Olkiluoto Area
Satu
Mertanen
Geological Survey of Finland
March
2008
Base maps: ©National Land Survey, permission 41/MYY/08
Working Reports contain information on work in progress
or pending completion.
The conclusions and viewpoints presented in the report
are those of author(s) and do not necessarily
coincide with those of Posiva.
Paleomagnetism of Diabase Dykes, Pegmatitic Granites and TGG
Gneisses in the Olkiluoto Area
ABSTRACT
Paleomagnetic studies in the Olkiluoto area were carried out on two diabase dykes, on
four sites of pegmatitic granites and on three sites of TGG gneisses. Remanent
magnetizations in the diabase dykes are strong, and two remanence components were
isolated. The other component is regarded as primary, formed during cooling of the
dykes. Based on comparison to previously known Fennoscandian paleomagnetic data,
the pole position of the primary component shows that the dykes are ca. 1560 Ma in
age. The pole position of the poorly defined secondary component of the diabase dyke
gives an age of ca. 250 Ma. Pegmatitic granites are weakly magnetized, but one
pegmatitic granite site gave a rather consistent remanence direction which points to a
remagnetization at ca. 1570 Ma, probably due to emplacement of the nearby rapakivi
granites. The TGG gneisses do not carry any stable remanent magnetization.
Keywords: paleomagnetism, remanent magnetization, Olkiluoto, Finland
Olkiluodon alueen diabaasijuonten, pegmatiittisten graniittien ja TGG
gneissien paleomagnetismista
TIIVISTELMÄ
Olkiluodon alueella on tehty paleomagnettisia tutkimuksia kahdesta diabaasijuonesta,
neljältä pegmatiittiselta graniittipaljastumalta ja kahdelta TGG-gneissipaljastumalta.
Diabaasijuonten remanentti magnetoituma on vahvaa, ja näytteistä on erotettu kaksi eri
komponettia. Toisen remanenssikomponentin on tulkittu edustavan primääriä magnetoitumaa, joka on syntynyt diabaasimagman jäähtyessä. Verrattaessa tämän komponentin remanenssisuunnasta laskettua napaa Fennoskandian tunnettuihin paleomagneettisiin napoihin, saadaan magnetoituman ja juonten iäksi n. 1560 Ma. Sekundäärinen
komponentti on erotettu vain muutamista näytteistä, mutta paleonapa antaisi
remanenssin iäksi n. 250 Ma. Pegmatiittiset graniitit ovat heikosti magnetoituneita,
mutta yhdeltä paljastumalta erotettiin eri näytteistä remanenssikomponentti, joka
osoittaa pegmatiittisen graniitin uudelleenmagnetoituneen n. 1570 Ma sitten. Magnetoituman on tulkittu syntyneen alueen lähellä olevien rapakivien intrudoitumisen
seurauksena. TGG gneisseissä ei esiinny stabiilia remanenttia magnetoitumaa lainkaan.
Asiasanat: paleomagnetismi, remanentti magnetoituma, Olkiluoto, Suomi.
1
TABLE OF CONTENTS
ABSTRACT
TIIVISTELMÄ
1
INTRODUCTION ................................................................................................... 2
2
STUDIED ROCKS AND SAMPLING..................................................................... 3
3
METHODS ............................................................................................................ 6
4
RESULTS ......................................................................................... .................... 7
4.1 Diabase dykes.............................................................................................. 9
4.2 Pegmatitic granites .................................................................................... 11
4.3 TGG gneisses .......................................................................... ................. 12
5
DISCUSSION ................................................................................... .................. 14
6
POSITION OF THE FENNOSCANDIAN SHIELD ....................... ....................... 18
7
CONCLUSIONS .............................................................................. ................... 19
REFERENCES ....................................................................................................... ..... 20
ACKNOWLEDGEMENTS ............................................................................................ 23
APPENDICES 1-11 ...................................................................................................... 24
2
1
INTRODUCTION
Paleomagnetic investigations have been carried out on diabase dykes, TGG gneisses
and pegmatitic granites in the Olkiluoto area. The study forms a continuation for a
preliminary investigation (Mertanen, 2007) where the suitability of rocks of Olkiluoto
for paleomagnetic studies was first tested. The preliminary results from unoriented
samples showed that especially the diabase dykes carry stable remanent magnetizations.
Also the pegmatitic granites and some of the TGG gneisses had strong remanent
magnetizations, whereas the studied potassium feldspar porphyries and sulphidized
rocks from shear zones had no signs of stable ancient remanence. In this study the new
paleomagnetic results from oriented samples are shown.
In paleomagnetic studies the main studied property of rocks is the remanent
magnetization that can be composed of different superimposed components formed in
successive geological events. By applying multicomponent analysis, those components
can be separated. The Olkiluoto area is located in an interesting surroundings where
several possible sources can cause magnetic overprinting and formation of remanence
components of different ages. The basement rock is composed of Svecofennian age (ca.
1.9-1.8 Ga) rocks, but the surrounding Subjotnian rapakivi granites (ca. 1.57 Ga),
Jotnian sandstones (ca. 1.4-1.3 Ga) or Postjotnian diabases (ca. 1.27 Ga) could have
affected the area thermally or hydrothermally. The main aim of the study was to find out
if those events have left their fingerprints in remanent magnetizations.
One of the most expected result of the study was to resolve whether the diabase dykes
of the Olkiluoto area are of Svecofennian or Subjotnian age. U-Pb datings of the dykes
have given Svecofennian ages of ca. 1.80 – 1.86 Ga (Mänttäri et. al., 2005, 2006). Since
other petrological and field evidences suggested that the dykes could be of Subjotnian
age, Mänttäri et al. (2005, 2006) proposed that the zircon could have been inherited. The
remanence direction of Subjotnian age rocks differs distinctly from the Svecofennian
rocks (e.g. Mertanen and Pesonen, 2005). Therefore, the paleomagnetic study can give
new knowledge on the age of the diabase dykes in the Olkiluoto area.
3
2
STUDIED ROCKS AND SAMPLING
Oriented samples for paleomagnetic studies were taken from two diabase dykes (DB, 7
samples and DS, 11 samples), from three sites of tonalitic-granodioritic-granitic gneiss,
TGG (sites GA, GB and GC, 6 samples from each site) and from four sites of
pegmatitic granite (sites PE, PG, PM and PA, 6-8 samples from each site) (Figure 1).
All samples were taken with a portable mini drill, the length of the cores being ca. 5-10
cm. Orientation was done with magnetic compass included in the equipment designed in
GTK. At least two, but typically three standard cylindrical specimens (diameter 2.4 cm
and hight 2.1 cm) were prepared from each core. In the following, the sampled rock
types are described briefly. More detailed descriptions of the rock types can be found in
e.g. Mänttäri et al. (2005, 2006) and Paulamäki et al. (2006 and references therein).
Diabase dyke DB is a narrow, ca. 6 cm at the widest and vanishes at its edges. It can be
seen in about two meters length, and in the middle it shows a stepped structure (Fig. 2a).
The strike of the dyke is 66° (dip direction 346°) and probably it is quite vertical,
although the dip could not be observed properly at the outcrop. The dyke is black and
very fine grained. The dyke was mapped for the first time in 2002 and mentioned in
Paulamäki (2007). Seven samples were taken from the dyke, two of which contained
partially the host rock. For a baked contact test, two samples were taken from the host
migmatitic gneiss at a distance of 0.5 cm from the dyke, and two at a distance 50 cm
from the dyke.
Diabase dyke DS (Fig. 2b) is about 1.5 m wide, and has a strike of 60° and dip
direction/dip of ca. 340°/70°. The outcrop was recently opened, and modelled in 3D as
diabase dyke DB7 in the latest geological model of the Olkiluoto site (Mattila et al., in
prep.). Mänttäri et al. (2005, 2006) have studied two diabase dykes in Olkiluoto area,
one from Olkiluoto 3 construction site (Mänttäri et al., 2006) and one from investigation
trench OL-TK3 (Mänttäri et al., 2005), the latter one being close to dyke DS. Those
dykes are not included in this study. Dyke DS cuts the host mica gneiss sharply on its
northern side, but on the southern side the contact is not so clear being mostly covered.
Samples from the diabase were taken mainly from the finer-grained part in the southern
side of the dyke, and partly from the central and northern side of the dyke. Most
samples are fine-grained black rocks without any clear visible structures. Two samples
looked clearly different in that they contained visible small light-coloured amygdales.
The amygdales probably represent the same type as the quartz- and calcite- filled
amygdales described by e.g. Paulamäki et al. (2006) and Mänttäri et al. (2005, 2006) in
other diabase dykes in the Olkiluoto area. Altogether eleven samples were taken from
the diabase dyke DS. For baked contact test, samples were taken from the host mica
gneiss at distances of 0.2 m, 1 m, 5 m and 8 m from the dyke.
Pegmatitic granite dyke PE cuts the TGG gneisses at site GA, and the pegmatitic granite
PG transects the TGG gneisses of site GB. Both sites are located quite close to each
other in the nortwestern part of the Olkiluoto area (Fig. 1).
4
Figure 1. Paleomagnetic sampling sites in Olkiluoto. Lithological map compiled by
Paulamäki et al., 2006.
Pegmatitic granite samples of site PE (Fig. 2C) were taken mainly from a reddish coarse
grained dyke with a width of about 2.5 m. In the preliminary paleomagnetic study
(Mertanen, 2007) two unoriented test samples (samples OL18) were studied from this
site. The dyke is described by Mänttäri (2007) who has also done the isotope age
determination of the dyke. One sample was taken from a white coloured branch of the
dyke. The pegmatitic granite of site PG contains a network of dykes which are partly
reddish or white in colour. Both types were sampled. At site PM (Fig. 2d) the
pegmatitic granite forms also a migmatizing light-coloured network structure that
intersects the mica gneisses. These pegmatitic granites were studied in the preliminary
paleomagnetic study (samples OL83, Mertanen, 2007) and they are described by
Mänttäri (2007) who has done isotopic datings also from this site. The fourth pegmatitic
granite, PA, was sampled in the northern part of Olkiluoto (north of drillhole OL-KR5,
Paulamäki et al., 2006) close to diabase dyke DS. Here the pegmatitic granite is reddish,
very coarse grained, and it occurs together with mica gneiss. Samples were taken in area
of about 7x12 m.
TGG gneisses at sites GA and GB (Fig. 2e) are quite homogeneous and even-grained
greyish rocks. In some places at site GB a foliation with a SE dipping direction of 4550° is clearly visible. At site GC (Fig. 2f) the TGG gneiss contains plenty of garnet in
some places. The gneiss is more foliated and includes patches and veins of granite.
5
a)
b)
c)
e)
d)
f)
Figure 2. a) Diabase dyke DB, b) Diabase dyke DS, c) Pegmatite granite of site
PE,with the orientation equipments d) Pegmatite granite of site PM, diameter of the
drill hole is 2.5 cm. e) TGG of site GB, with drill core GB2, f) TGG of site GC.
6
3
METHODS
Density and magnetic susceptibility were first measured from each specimen before
paleomagnetic measurements (Table 1). Magnetic susceptibility gives the ability of
rock to magnetize and reflects the amount of ferromagnetic minerals in the samples
(measured as volume susceptibility). Natural remanent magnetization (NRM) was
measured for each such specimen that was also demagnetized (see below). A primary
NRM of magmatic rocks is the permanent thermoremanent magnetization (TRM) that is
acquired when the rock cools below the Curie temperature of its remanence carrying
ferromagnetic minerals. A secondary NRM can be formed subsequently in
metamorphism when the rock can be totally remagnetized, or acquire a partial
thermoremanent magnetization (pTRM) when the original remanence may have
preserved. Hydrothermal activity can also produce a new magnetization in the rock. It is
called chemical or thermochemical remanent magnetization (CRM/TCRM), if new
magnetic minerals that carry the remanence, are formed in a hydrothermal process.
Unlike magnetic susceptibility, NRM exists in the absence of external magnetic field.
Koenigsberger ratio (Q-value) defines the ratio between remanent magnetization
(remanence intensity, J, Table 1) and the induced magnetization (magnetic
susceptibility, Table 1). It was determined for each specimen that was also
demagnetized.
For paleomagnetic studies most of the specimens were stepwise demagnetized with
increasing alternating field (AF) in 12-15 steps up to a field of 100 or 160 mT. Some of
the samples were thermally demagnetized by subjecting the samples in increasing
stepwise temperatures with a maximum temperature of 620°C. The remanent
magnetization was measured with cryogenic three-axes Squid (RF)-magnetometer
between the different steps. By stepwise demagnetization it is possible to isolate
different remanence components on the basis of different coercivities of the
ferromagnetic mineral grains or on the basis of different unblocking temperatures of the
grains. In general, the grain size and form of ferromagnetic minerals is important for the
ability of a rock to preserve an ancient remanence. Small grains occur as single domain
or pseudo single domain (SD/PSD) grains which can retain the remanence much better
than the larger multi domain (MD) grains which tend to easily acquire an unwanted
viscous remanence in time. Even if the magnetization of rock, both induced and
remanent, would be very low, the rock can still have a measurable ancient remanent
magnetization, if the remanence resides in SD grains.
Separation of remanent components was done with principal component analysis
(Kirschvink, 1980) by using Tubefind program (Leino, 1991). Fitting of lines
(Zijderveld, 1967), with the minimum of three demagnetization points, was done
automatically with the maximum angular deviation of the line being 6°. In addition, the
specimens were treated manually, when components with angular deviations as high as
10° were also considered. Mean values of remanence components were calculated
according to Fisher (1953). GMAP program (Torsvik and Smethurst, 1999) was used
for presenting the paleomagnetic poles and for calculating the position of the continent.
No thin section studies were carried out for the diabases or other rock types as the
petrography of the similar rock types in Olkiluoto is already reported in Mänttäri et al.
(2005, 2006) and Paulamäki et al. (2006 and references therein).
7
4
RESULTS
Petrophysical properties of the studied samples are shown in Table 1 and Figure 3, and
the paleomagnetic results in Table 2 and Figure 4. Examples of demagnetization
behaviours are shown in Appendices 1-11. In the following, the results from different
rock types are described. For diabases, also the results of baked contact tests are given.
Table 1. Sampling sites and petrophysical properties.
Site
Rock type
Coordinates
N/n
Density
3
x, y
kg/m
J (NRM)
mA/m
Susc.
x 10
Q
-6
DB
Diabase
6790712, 1527488
6/11
2932
502.7
14405
0.9
DS
Diabase
6792902, 1525345
9/24
2964
808.0
15513
1.3
6792902, 1525345
2/5
2942
110.5
2917
0.9
DS7+8
DB, baked
Gneiss
6790712, 1527488
2/6
2719
2.5
254
0.2
DB, unbaked
Gneiss
6790712, 1527488
2/7
2705
5.6
229
0.6
DS, baked
Gneiss
6792902, 1525345
2/6
2740
110.4
329
7.2
DS, unbaked
Gneiss
6792902, 1525345
2/6
2659
0.6
155
0.5
PE
Pegmatite
6792822, 1523252
6/17
2597
1.4
26
3.0
PG
Pegmatite
6792830, 1523084
5/14
2613
0.8
19
2.8
PM
Pegmatite
6792591, 1525874
6/14
2625
1.2
71
0.4
PA
Pegmatite
6792924, 1525263
6/15
2610
0.9
38
0.8
GA
TGG
6792822, 1523252
6/16
2803
12.3
398
0.8
GB
TGG
6792830, 1523084
6/15
2806
2.9
397
0.2
GC
TGG
6793097, 1524618
6/14
2698
0.7
207
0.1
Note: N/n denotes the number of measured samples/ specimens, the mean values
calculated from samples. Susc. is the magnetic susceptibility, Q denotes Koenigsberger
ratio (Q-value). Sample DB10 of dyke DB was excluded from the mean petrophysical
calculations, because it contains about half of the host rock.
8
Susceptibility x10-6 SI
25000
20000
15000
Diabase
10000
DB
5000
DS
0
0
200
400
600
800
1000
1200
1400
NRM (mA/m)
Susceptibility x10-6 SI
120
100
80
Pegmatite
60
PE
PG
40
PM
20
PA
0
0
1
2
3
4
5
NRM (mA/m)
Suskeptibilty x10-6 SI
500
450
400
350
300
250
200
TGG
150
100
GA
50
0
GC
GB
0
10
20
30
NRM (mA/m)
Figure 3. Petrophysical properties of the studied samples.
40
50
9
4.1 Diabase dykes
The magnetic susceptibilities of dykes DB (individual specimens 13 350 – 19 590 x 10-6
SI) and DS (10 619 – 21 866 x 10-6 SI) are comparable, but dyke DB has lower
remanence intensities (425 – 815 mA/m) than the samples from dyke DS (425 – 1 647
mA/m). Also the Q-values are lower in dyke DB (0.7 – 1.0) compared to that of dyke
DS (0.8 – 2.7) (Table 1 and Fig. 3). Due to higher remanence intensities, most
specimens from dyke DS also gave stable paleomagnetic results while in dyke DB
stable results were obtained only from one specimen of each core.
The characteristic remanence direction (ChRM) of the diabases has a very shallow
inclination that points towards north (Table 2, Fig. 4, Appendicies 1-4). The ChRM was
named as DBp/DSp where p refers to a primary remanence, as will be discussed later.
The component was isolated in AF fields of 30-100 mT, most typically in 30-70 mT. In
higher AF fields, above 70-100 mT, the AF demagnetisation gave rise to false
components (see Appendices 1 and 3), which are attributed to Gyroremanent
magnetization (GRM) (Stephenson, 1980) formed in the demagnetization process.
However, also in those samples carrying the GRM, the DBp/DSp component was
obtained in lower fields. In thermally demagnetized samples the component was
isolated in a quite narrow temperature range of 500-570°C which suggests that the
remanence is carried by SD/PSD magnetite.
In three samples of dyke DS and in two samples of dyke DB another component was
isolated in higher AF fields and in lower temperatures than component DBp/DSp. It was
named as component DBs/DSs where s denotes secondary origin for the component
(Table 2, Fig. 4). It occurs only in a few samples, but as will be discussed later, it
represents a remanent magnetization that has been obtained sporadically all over the
Fennoscandian shield. In most specimens, a third component was typically isolated in
the lowest AF fields and unblocking temperatures. This component has a steep NW
pointing direction close to the Present Earth's Field (PEF) direction and it obviously
represents a recent viscous remanence without any geological meaning. Appendix 4
gives an example of a case where all three components, PEF (0-20 mT), DBp/DSp (3070 mT) and DBs/DSs (70-100 mT) occur together.
Samples DS7 and DS8 which contain visible amygdales show significantly lower
magnetizations, both remanence and susceptibility, compared to other diabase samples
(Table 1, Fig. 3). Also the Q-values are lower. Based on thermal demagnetization
results (Appendix 5), their magnetic mineralogy differs from that of dyke DS. Above
500°C the remanence intensity increases and gets unstable. Also the susceptibility starts
to increase hugely, suggesting that the rock contains pyrrhotite that alters to magnetite
during heating. Although the remanence direction is comparable to the DBp/DSp of the
other samples, the results of those two samples were not included in any mean values.
10
Table 2. NRM components of the diabases DB and DS and pegmatite PE.
Site
N/n
AF Thermal
(mT)
(°C)
DBp
DSp
DBs/DSs
PE
7/7 30-100
7/17 20-100
5/7 60-100
5/6
-
D
(°)
I
(°)
400-570 4.6 -0.7
500-580 5.5 -0.1
0-560 42.0 41.5
0-520 27.9 -30.4
α95
(°)
13.9
6.4
12.2
23.9
k
Plat Plong A95
(°)
(°Ν) (°Ε)
19.8 28.4 196.2
90.6 28.6 195.2
40.2 43.3 144.7
11.1 8.8 175.2
K
dp
(°)
10.7 32.9 7.0
5.6 116.1 3.2
13.2 34.4 9.1
22.7 12.3 14.8
dm
(°)
13.9
6.4
14.9
26.6
Note: N/n = number of samples/specimens, D = declination, I = inclination, AF = alternating
field demagnetization, Thermal = thermal demagnetization, α95 = radius of the circle of 95%
confidence, k = the Fisher's (1953) precision parameter, Plat, Plong = paleolatitude and
paleolongitude of the Virtual Geomagnetic Pole (VGP), A95 = the radius of the circle of 95%
confidence of the mean pole, K = the Fisher's (1953) precision parameter of the VGP, dp,dm =
are the semi-axes of the oval of 95% confidence.
Baked contact tests
Baked contact tests are essential in defining the primary nature of the ChRM. In a baked
contact test the remanence directions of the magma intrusion (dyke) and its baked and
unbaked host rocks are compared. The baked zone of a dyke is defined as half of the
width of the dyke. If the dyke and the baked host rock carry a similar remanence
direction, but that is different from the unbaked host rock, it proves that the remanence
of the dyke is primary, formed during the cooling of the rock. If both the diabase dyke
and the baked and unbaked host rock carry a similar remanence direction, then the
remanence of the dyke is secondary, acquired in a later metamorphic or hydrothermal
event.
Unfortunately, baked contact tests were not succesfull for the studied diabase dykes.
Thermal demagnetizations of the baked and unbaked host gneisses showed pyrrhotite as
the main magnetic mineral. However, the remanence directions of the rocks are very
scattered even between the specimens of a single core. Thus, no conclusive results were
obtained.
Petrophysical properties can also give information about the primary/secondary nature
of the remanence of the dykes. One purpose of the measurements is to see the physical
effects of the dyke to the host rock at various distances from the dyke. It is supposed
that if the remanence intensity and susceptibility are clearly increased at the baked
contact zone of the host rock, closest to the dyke, it may give support that the dykes
carry a primary remanent magnetization. If the whole area has been in increased
temperatures, it is possible that the petrophysical values are smoothened and no clear
difference is seen between the baked and unbaked host rock.
11
DSP
PE
PEF
DBS/DSS
Figure 4. Sample mean remanence directions DBP (circles) and DSP (squares) denoting
the possible primary remanence of the diabase dykes, DBS/DSS (hexagons) the
secondary remanence of the diabase dykes and PE (triangels) the remanence
component of pegmatite granite PE. The cones of 95% confidence are shown. PEF
indicates the Present Earth's Field magnetization direction of the sampling area. Closed
symbols denote positive inclination and open symbols negative inclination.
At the baked contact zone of dyke DB there is no significant change in magnetic
properties between the baked and unbaked host rock (Table 1).
At dyke DS the magnetic susceptibility values of the baked and unbaked samples do not
differ significantly, although the values are slightly higher in the baked rocks (Table 1).
The main difference is seen in the remanence intensity values and Q-values which are
clearly higher in the baked rocks than in the unbaked rocks. This gives the signature that
the dykes have baked the host rocks and both the dyke and the baked host rock have
acquired their remanent magnetization simultaneously during cooling of the rocks. The
lower values of the unbaked host rock would indicate that they are unaffected by the
dykes and carry their original magnetizations.
4.2 Pegmatitic granites
Magnetizations of the pegmatitic granites are very weak. The magnetic susceptibilites of
distinct specimens are within range 0-340 x 10-6 SI, remanence intensities 0.1-12.8
mA/m and Q- values between 0.03 and 12.2. Mean values of samples are shown in
Table 1 and Figure 2.
12
Pegmatitic granite from site PE shows stable paleomagnetic results in part of the
samples although the remanence intensities are weak. One remanence component was
isolated. The component has a NE pointing moderate negative inclination (see
Appendix 6). Although the remanence directions between samples are scattered, the
mean direction (Fig. 4) has a clearly higher negative inclination than the diabases. The
component occurs typically only in one thermally demagnetized specimen of each core
as thermal demagnetization proved to be the more effective method than AF
demagnetization in isolating the remanence component. In some specimens AF
demagnetization showed high coercivities which suggests the existence of pyrrhotite. In
three of the samples the NE pointing component was isolated in a temperature range of
about 200-400°C, also suggesting pyrrhotite as the remanence carrier. However, in two
samples the remanence was unblocked below 500-520°C (Appendix 7) implying that
the remanence resides in titanomagnetite. Since the remanence seems to reside in both
pyrrhotite and titanomagnetite, the mechanism of magnetization is not clear. It has
probably chemical or thermochemical secondary origin. The remanence direction is
clearly different from the known direction of Svecofennian age rocks (Pesonen et al.,
2003) which supports its secondary origin. It is implied that the pegmatite is totally
remagnetized, or that at least those samples with stable remanences are remagnetized
although the unstable samples may have been already originally weak and have not
retained any primary remanence at all.
The pegmatitic granites of sites PG, PM and PA are very weakly magnetized. No stable
results were obtained due to scatter of data in successive demagnetization steps.
However, although principal component analyses could not isolate any stable
component, many specimens do show a negative NE pointing remanence direction
corresponding to that of the DBp/DSp component or the characteristic component of the
PE pegmatite (Appendix 8). The remanence unblocks at 300-400°C, again suggesting
pyrrhotite as the main magnetic mineral (Appendix 9).
4.3 TGG-gneisses
Samples from TGG gneisses were taken from three sites (Fig. 1). Preliminary studies of
the TGG gneisses (Mertanen, 2007) already showed that this rock type did not give
stable paleomagnetic results. However, new studies were carried out because the
remanence intensities and Koenigsberger Q values were high, which in general suggest
prevalence of an ancient remanence. With new sampling it wanted to be tested whether
the TGG could still carry stable remanent magnetizations.
New TGG gneiss samples are from different locations than the test samples. Unlike the
test samples, the new samples have low remanence intensities and Q values in most
cases (Table 1, Fig. 2). The Q-values are typically below 0.5 and the remanence
intensities below 10 mA/m, usually around or below 1 mA/m. As expected, they do not
carry stable consistent remanent magnetizations. This is seen as scattered remanence
directions of different samples. Thermal demagnetizations show that most of the studied
rocks loose their magnetization below 400°C which implies that the ferromagnetic
mineral is pyrrhotite. In few samples there seems to be also magnetite (Appendix 10).
In two samples from site GA (samples GA2 and GA6, Table 1, Figure 3) the remanence
intensity values and Q- ratios are higher compared to other samples. In these samples an
13
equivalent remanence component was isolated in low AF fields of 0-30 mT and
temperatures of 0-350°C. The remanence has a very steep negative inclination which is
reversed to the Present Earth's Field magnetization direction. The direction is not known
from previous paleomagnetic studies. The latest reversal of the Earth's magnetic field
took place about 750 000 – 780 000 years ago (Butler, 1980) so that it cannot be
connected to any Holocene time event, like glaciation that might have affected the
remanent magnetization. In this study the origin of the steep negative inclination
component remains unresolved. It must be related to some unknown secondary
mechanism, even to a drilling induced magnetization at worst.
14
5
DISCUSSION
Two remanence components, DBp/DSp and DBs/DSs were obtained from the diabase
dykes DB and DS, and one remanence component from the pegmatitic granite PE. The
other studied formations did not give stable results, except some hints of a NE pointing
remanence direction. In order to define the ages of the magnetizations, virtual
geomagnetic poles (VGP's) were calculated from the remanence directions. The ages
are obtained by comparing the VGP's with known Fennoscandian paleomagnetic 'key'
poles that are statistically well defined and have precisely dated isotopic ages (Buchan
et al., 2000). The VGP's are shown in Table 2 and Figure 5.
Poles DBp and DSp plot close to the 1560 Ma key pole obtained from the Föglö
(Neuvonen and Grundström, 1969) and Föglö-Sottunga diabase dykes (Pesonen and
Neuvonen, 1981, Pesonen et al., 1987) in Åland archipelago. The dykes have U-Pb ages
(zircon and baddeleyite) of 1577±12 Ma and 1540±12 Ma (Suominen, 1991). The DBp
and DSp poles differ clearly from the Svecofennian 1880 Ma and 1840 Ma key poles
and therefore they cannot be of Svecofennian age, unless the dykes were totally
remagnetized at ca. 1560 Ma (see below). Another source for a remagnetization in the
Olkiluoto area could be the emplacement of Postjotnian 1270 Ma (Suominen, 1991)
Satakunta olivine diabases. However, the DBP and DSP poles differ also clearly from the
1270 Ma key pole (Buchan et al., 2000, Pesonen et al., 2003). Therefore, since no
remanence direction corresponding to the remanence of the Postjotnian magmatism was
obtained in the dykes, the dykes have not been either partially or totally remagnetized
due to the Satakunta diabase. Figure 5 shows also the 1630 Ma key pole obtained from
quartz porphyry dykes in southern Finland (Neuvonen, 1963, Mertanen and Pesonen,
1995). The DBP and DSP poles are also different to that pole, so that the dykes cannot be
of the age of 1630 Ma.
One more key paleomagnetic pole with an age of 1570 is also obtained from Åland,
from the Kumlinge-Brändö diabase and quartz porphyry dykes (Pesonen and Neuvonen,
1981) the latter with U-Pb (zircon) ages between 1571±9 Ma and 1576±13 Ma
(Suominen, 1991). Thus, the ages of the Åland diabase and quartz porphyry dykes are
equivalent within dating limits and therefore, the age difference between the 1560 Ma
and 1570 Ma key poles is not strictly defined. However, based on paleomagnetic
studies, the Föglö dykes have normal polarity and the Kumlinge dykes reversed or
mixed polarity, and the cross cutting relationships suggest that the normal polarity
dykes are slightly younger than the reversed polarity dykes (Bylund and Pesonen,
1987). Based on the similarity of poles DBP and DSP with the poles from the FöglöSottunga dykes and with the occurrence of normal polarity, it is suggested that the
dykes of the Olkiluoto area are of the age of ca. 1560 Ma. It is implied that the remanent
magnetization is of primary origin and the dykes represent Subjotnian magmatism.
Baked contact tests could not properly confirm the primary or secondary origin for the
magnetization as the host rocks at the diabase dykes did not give stable remanence
directions. However, as discussed before, the remanence intensities have increased at
the baked contact zone of dyke DS which is so wide that it have had a more thorough
effect on the host rock compared to the very thin dyke DB. Based on this evidence it is
suggested that the remanent magnetization of the diabase dykes is primary.
15
a)
1840
DBS /DSS
1630
1560 DS
P
1880
DBP
PE
30 N
1570
1270
Equator
180
240
b)
580
600
60
560
500
30
700
480
470
Equator
DBS /DSS
210
750
440
380
-30
460
410
425
320
280
770
250
-60
240
270
300
330
0
30
60
90
120
150
Figure 5. a) Precambrian key poles of the Fennoscandian Shield (Buchan et al., 2000)
are shown as squares with dashed A95 confidence cones and ages (Ma). The shaded
A95 confidence cones are shown for the mean VGP poles of components DBP and DSP
(regarded as primary), DBS/DSS (regarded as secondary) of the diabase dykes and for
the component obtained from pegmatite granite PE. b) Paleozoic APW path of the
Fennoscandian shield (Torsvik et al., 1996) where the pole DBS/DSS is reversed.
16
Petrographic and mineralogical evidences (e.g. Mänttäri et al., 2005, Paulamäki et al.,
2006 and references therein) show that the diabase dykes are highly altered, the original
minerals having been replaced by hydrothermal minerals (e.g. epidote, calcite, sericite).
Strong alteration of the dykes could support a secondary origin for the magnetization
which would then have been formed in a later low temperature hyrothermal event, an
event that has affected the whole Olkiluoto area (Paulamäki et al., 2006). The
remanence would thus represent a total remagnetization and could be of thermal,
chemical or thermochemical remanence type. However, the magnetization is carried by
SD/PSD magnetite which is the typical primary Fe-oxide mineral of diabase dykes.
Therefore, although hydrothemally altered, the primary magnetite seems to have
preserved. Hydrothermal alteration of the dykes may have also taken place in the late
stages of crystallization of the dykes, simultaneously with fluid activity from the nearby
rapakivi granites. Field evidences also support the primary Subjotnian origin for the
dykes.
In Olkiluoto area the studied dykes strike ca. N60°E, although more E-W striking also
occur (e.g. Paulamäki et al., 2006 and references therein). In Föglö the strike is more
NE, striking ca. N20-40E° (Suominen, 1987). In the Rauma and Uusikaupunki map
sheet explanations Suominen et al. (1997, 2006, respectively) have described several
mainly NE-SW (Uusikaupunki) or E-W (Rauma) striking thin diabase dykes. The
dykes in Rauma area also contain amygdales of carbonate and quartz, as described also
in the Olkiluoto diabase dykes (Mänttäri et al., 2005, Paulamäki et al., 2006,).
Suominen et al. (2006) correlate the hornblende diabase dykes in Uusikaupunki area
with the Subjotnian diabase dykes in the Åland archipelago. Further north from the
Olkiluoto and Rauma map sheet area, in the Pori region, Pihlaja (1987) has described
diabase dykes with a similar strike of ca. N60°E as in Olkiluoto area. Those dykes have
corresponding narrow width (0.1-3 m) as in Olkiluoto, they are slightly altered and also
contain quartz and carbonate filled amygdales. Ehlers and Ehlers (1977) have described
corresponding amygdales also in the diabases of Åland archipelago.
To sum up, based on geological evidences from the western coast of SW Finland, from
the Åland archipelago through Rauma up to Pori region, there exists several NE/NNESW/SSW trending diabase dyke swarms. Although not isotopically dated, on the basis
of their petrology, crosscutting relationships and mode of occurrence, they all belong to
the Subjotnian magmatism that is related to the rapakivi granites. Paleomagnetic results
alone from the Olkiluoto area cannot define the extent of ca. 1560 Ma Subjotnian dykes
in other areas, but anyway based on field evidences reported before (Pihlaja, 1987,
Suominen et al., 1997, 2006, Mänttäri et al., 2005, 2006, Paulamäki et al., 2006 and
references teherin), it is probable that there exists ca. 1560 Ma old NE/NNE-SW/SSW
rapakivi granite related dykes or dyke swarms along the rapakivi granite hosted
southwestern coast of Finland.
Virtual geomagnetic pole calculated from the remanence direction of pegmatitic granite
PE is close to the pole with an age of 1570 Ma (Pesonen and Neuvonen, 1981),
although the polarity of the pegmatite is normal (Fig. 5). The main differerence in 1560
Ma and 1570 Ma remanence directions was the higher inclination of the 1570 Ma
component. The data of PE are scattered, obtained only in five samples. Furthermore,
the A95 confidence cone of the pole intersects with the 1630 Ma and 1560 Ma
confidence cones. Therefore, any interpretations about the age of pole PE are quite
uncertain. However, it is suggested that the pegmatitic granites may record a reheating
17
of the Laitila rapakivi batholith and the satellite Eurajoki rapakivi granite stock with UPb ages of 1573±5 - 1548±3 Ma (Vaasjoki, 1977, Vaasjoki and Rämö, in Suominen, et
al., 2006). The diabases did not yield this steeper inclination remanence component
which implies that the remanence component in pegmatitic granites is older than the
remanence in the diabases. This also supports the primary origin for the remanence in
the diabases. If the remanence of the diabases were formed in the same hydrothermal/
thermal event as the one affecting the pegmatitic granites, the dykes would have
acquired the same remanence.
The secondary pole DBS/DSS (Fig. 5) of the diabases was obtained only in five samples
and therefore its importance is doubtful. Anyway, it represents a remanent
magnetization that has been sporadically obtained all over in the Fennoscandian shield
(e.g. Mertanen et al., 2004). When plotted among Precambrian key poles, it doesn't
correspond to any known poles (Fig. 5a). However, when the polarity is reversed and
the pole is plotted along the Paleozoic Apparent Polar Wander Path (APWP) of the
Fennoscandian shield (Torsvik et al., 1996), it gives an age of about 250 Ma. The age
corresponds to a Permian time that was geologically manifested by the occurrence of
supercontinent Pangea. It is possible that the magnetization was formed in the regional
events related to the formation of the supercontinent. However, based on these few data,
no further interpretations are attempted.
18
6
POSITION OF THE FENNOSCANDIAN SHIELD
Figure 6 shows the latitudinal position and orientation of the Fennoscandian shield at
1560 Ma, based on the mean pole position (Plat=28.5°N, Plong=195.7°E) obtained from
the diabase dykes DB and DS. The position is in agreement with previous studies
(Mertanen and Pesonen, 2005, Pesonen et al., 2003). For comparison, the figure also
shows the positions of the Fennoscandian shield at 1830 Ma (Neuvonen et al., 1981),
1570 Ma (Pesonen and Neuvonen, 1981) and 1270 Ma (Neuvonen, 1965, 1966,
Neuvonen and Grundström, 1969). During the emplacement of the 1570-1560 Ma
rapakivi granites and associated dykes, there was some latitudinal movement and
rotation of the shield. At 1560 Ma the Fennoscandian shield was located at the equator.
This could be considered in the interpretations of the alteration processes observed in
the Olkiluoto area (e.g. Paulamäki et al., 2006). Likewise, when taking into account the
occurrence of amygdales in the diabase dykes, which may indicate their emplacement at
shallow depths (Suominen et al., 1997) or even onto the surface (Ehlers and Ehlers,
1977), the equatorial position of the shield may be partly responsible for the high
alteration seen in the dykes. Mänttäri et al. (2005) have described occurrence of
hematite in one of the Olkiluoto diabase dyke, which could possibly be related to
weathering at the equator.
30 N
1830
1560
Equator
1270
1570
30 S
This study
Figure 6. Drift history of the Fennoscandian shield at certain times (see Mertanen and
Pesonen, 2005, Pesonen et al., 2003.)
19
7
CONCLUSIONS
The two studied diabase dykes in the Olkiluoto area yield a consistent remanent
magnetization component that is suggested to be primary. Based on similarity of the
pole positions with the known Fnnoscandian paleomagnetic poles, the age of the
diabase dykes is ca. 1560 Ma. In addition to the primary component the diabases carry
another, secondary remanence component that may be related to the formation of
supercontinent Pangea during Permian time at ca. 250 Ma.
Pegmatitic granite at one site shows a remanence direction that based on its pole
position is ca. 1570 Ma, and may have been acquired due to hydrorthermal events
related to the intrusion of the nearby rapakivi granites. The other studied pegmatitic
granites also gave some hints of a corresponding remanence, but they could not be
properly isolated. No remanence component that would point to a reactivation of the
Olkiluoto area due to Postjotnian magmatism at ca. 1270 Ma was observed. The studied
TGG gneisses did not give any stable paleomagnetic results.
Based on thermal demagnetizations the remanence carrying magnetic mineral of the
diabases is magnetite. In the two samples from dyke DS that show high concentrations
of amygdales, the main Fe-oxide mineral is probably pyrrhotite. In pegmatitic granites
both magnetite and pyrrhotite occur. It is suggested that magnetite can be the original
primary mineral in the pegmatitic granites, but the occurrence of pyrrhotite is related to
the hydrothermal alteration when the pegmatitic granites were remagnetized. Also in
TGG gneisses the main magnetic mineral is pyrrhotite.
The amount of samples that gave reliable paleomagnetic results was statististically very
limited. Therefore, in order to confirm the now obtained results and to assess whether
the whole Olkiluoto area was partially remagnetized due to the rapakivi granites, more
data are required.
20
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23
ACKNOWLEDGEMENTS
Ismo Aaltonen and Jussi Mattila pointed the sampling sites, Fredrik Karell drilled the
samples and helped with the orientations in the field. Matti Kauranne made the sample
preparations, Tuula Laine carried out the paleomagnetic measurements and Matti Leino
took care of the equipments and software. Seppo Paulamäki provided the lithological
map of the Olkiluoto area and made improving comments on the manuscript. All these
people are greatly acknowledged.
24
APPENDICES 1-11
a) Stereoplot shows the movement of NRM vectors during progressive demagnetization.
Declination (0-360°) is read from the stereoplot circle and inclination (0-90°) from
the axis of the stereoplot. Closed circle indicates that the inclination is pointing
downwards (positive inclination), open circle that the inclination is pointing upwards
(negative inclination).
b) Intensity decay curve of AF demagnetization (e.g. Appendix 1) shows the decline of
relative remanence intensity when the specimen is subjected to stepwisely increasing
AF fields. The AF fields (H) are in oerstedts (Oe).
Intensity decay curve of thermal demagnetization (e.g. Appendix 2) shows the
decline of remanence intensity when the specimen is gradually heated in higher
temperatures. Temperatures are in °C.
c) Zijderveld plot shows the vector end points after demagnetization steps, projected on
two perpendicular planes. Horizontal projection on the left, and vertical projection on
the right. In AF demagnetized specimens the large numbers refer to AF fields (in
milliteslas, 1 mT = 10 Oe), in thermally demagnetized specimens to temperatures
(°C). The small numbers show the order of used steps.
25
26
27
28
29
30
31
32
33
34
35