Download Petrology of Olkiluoto

POSIVA 2006-02
Petrology of Olkiluoto
Aulis Kärki
Seppo Paulamäki
November 2006
POSIVA OY
FIN-27160 OLKILUOTO, FINLAND
Phone (02) 8372 31 (nat.), (+358-2-) 8372 31 (int.)
Fax (02) 8372 3709 (nat.), (+358-2-) 8372 3709 (int.)
POSIVA 2006-02
Petrology of Olkiluoto
Aulis Kärki
Kivitieto Oy
Seppo Paulamäki
Geological Survey of Finland
November 2006
POSIVA
FI-27160
OY
OLKILUOTO,
FINLAND
Phone (02) 8372 31 (nat.), (+358-2-) 8372 31 (int.)
Fax (02) 8372 3709 (nat.), (+358-2-) 8372 3709 (int.)
ISBN 951-652-143-6
ISSN 1239-3096
The conclusions and viewpoints presented in the report are
those of author(s) and do not necessarily coincide
with those of Posiva.
Posiva-raportti – Posiva Report
Raportin tunnus – Report code
POSIVA 2006-02
Posiva Oy
FI-27160 OLKILUOTO, FINLAND
Puh. 02-8372 (31) – Int. Tel. +358 2 8372 (31)
Julkaisuaika – Date
November 2006
Tekijä(t) – Author(s)
Toimeksiantaja(t) – Commissioned by
Aulis Kärki, Kivitieto Oy
Seppo Paulamäki, Geological Survey of Finland
Posiva Oy
Nimeke – Title
PETROLOGY OF OLKILUOTO
Tiivistelmä – Abstract
The rocks of Olkiluoto fall into four main groups: 1) gneisses, 2) migmatitic gneisses, 3) TGG-gneisses (TGG =
tonalite-granodiorite-granite) and 4) pegmatitic granites. In addition, narrow diabase dykes occur sporadically. The gneisses
include homogeneous mica-bearing quartz gneisses, banded mica gneisses and hornblende or pyroxene-bearing mafic
gneisses. The migmatitic gneisses, which typically comprise 20 – 40% leucosome, can be divided into three subgroups in
terms of their migmatite structures: veined gneisses, stromatic gneisses and diatexitic gneisses. The leucosomes of the
veined gneisses show vein-like, more or less elongated traces with some features similar to augen structures. Planar
leucosome layers characterize the stromatic gneisses, while the migmatite structure of the diatexitic gneisses is asymmetric
and irregular. The TGG gneisses are medium-grained, relatively homogeneous rocks that can show a blastomylonitic
foliation, but they can also resemble plutonic, unfoliated rocks. The pegmatitic granites are leucocratic, very coarse-grained
rocks, which may contain large garnet, tourmaline and cordierite phenocrysts. Mica gneiss inclusions are typical of the
larger pegmatitic bodies. Gneisses, which are weakly or not at all migmatitic, make ca. 9% of the bedrock. Migmatitic
gneisses make up over 64% of the volume of the Olkiluoto bedrock, with the veined gneisses accounting for 43%, the
stromatic gneisses for 0.4% and the diatexitic gneisses for 21%, based on drill core logging. Of the remaining lithologies,
TGG gneisses constitute 8% and pegmatitic granites almost 20% by volume.
The supracrustal rocks of Olkiluoto can be divided into four series by reference to whole rock chemical composition: a T
series, S series, P series and basic, volcanogenic gneisses. Rocks of the T, S and P series seem to make up 42%, 12% and
26%, respectively, of the volume of central part of the island of Olkiluoto, in addition to which, pegmatitic granites and
diabases form groups of their own that can be identified both macroscopically and chemically. The rocks of the T series are
various veined gneisses and diatexitic gneisses, together with various mica gneisses and quartz gneisses. One typical feature
of this series is the occurrence of strongly pinitized cordierite and sometimes also a small proportion of sillimanite. The T
series is an transition series, the end members of which are relatively dark and often cordierite-bearing mica gneisses and
migmatites with less than 60% SiO2 and quartz gneisses with more than 75% SiO2, representing clay mineral-rich pelitic
materials and greywacke-type impure sandstones, respectively. Certain TGG gneisses that are typically granitic in their
modal mineral composition show a chemical similarity to the members of the T series.
The members of the S series may be identified from their textures and mineral compositions as quartz gneisses, mica
gneisses, migmatites and mafic gneisses. The most essential difference between these and the members of the other series is
their high calcium concentration, the figure typically exceeding 2%, with maximum concentrations over 13%, while those in
the T series are below 2%. A relatively low alkali content and high manganese content are also typical of this series, the
members of which are assumed to have originated from calcareous sedimentary materials.
The members of the P series are TGG gneisses, veined gneisses, diatexitic gneisses, mafic gneisses and mica gneisses
typically with a small proportion of leucosome. These stand out from the other series by virtue of their high phosphorus
content. P2O5 concentrations exceeding 0.3% are characteristic of the members of the P series, whereas the other common
supracrustal rock types at Olkiluoto contain less than 0.2% P2O5.
Mafic gneisses and metadiabases not included in the above-mentioned three series are represented only by a couple of
samples, the characteristic chemical variables of which are high MgO, alkalis, TiO2 and P2O5. The chemical compositions of
these rocks resemble those of picrites or picritic basalts.
Avainsanat - Keywords
Lithology, petrography, whole rock chemistry, nuclear waste disposal, Olkiluoto, Eurajoki, Svecofennian Domain,
SW Finland.
ISBN
ISSN
ISBN 951-652-143-6
Sivumäärä – Number of pages
77
ISSN 1239-3096
Kieli – Language
English
Posiva-raportti – Posiva Report
Raportin tunnus – Report code
POSIVA 2006-02
Posiva Oy
FI-27160 OLKILUOTO, FINLAND
Puh. 02-8372 (31) – Int. Tel. +358 2 8372 (31)
Julkaisuaika – Date
Marraskuu 2006
Tekijä(t) – Author(s)
Toimeksiantaja(t) – Commissioned by
Aulis Kärki, Kivitieto Oy
Seppo Paulamäki, Geological Survey of Finland
Posiva Oy
Nimeke – Title
OLKILUODON PETROLOGIA
Tiivistelmä – Abstract
Olkiluodon alueen kivilajit voidaan jakaa neljään pääryhmään: 1) gneissit, 2) migmatiittiset gneissit, 3) TGG-gneissit
(TGG = tonaliitti–granodioriitti–graniitti) ja 4) graniittipegmatiitit. Näiden lisäksi kapeita diabaasijuonia tavataan
satunnaisesti. Gneissit ovat biotiittia sisältäviä kvartsiittisia gneissejä, kiillegneissejä ja sarvivälke- tai pyrokseenipitoisia
mafisia gneissejä. Migmatiittiset gneissit, joihin sisältyy tyypillisesti 20 – 40% leukosomia, voidaan jakaa
migmatiittirakenteen perusteella kolmeen alaryhmään: suonigneisseihin, stromaattisiin gneisseihin ja diateksiittisiin
gneisseihin. Tyypillisissä suonigneisseissä leukosomi esiintyy pitkänomaisina suonina tai kooltaan vaihtelevina
silmäkkeinä. Stromaattisille gneisseille luonteenomaisia piirteitä on laattamaiset, suoraviivaiset leukosomijuonet, kun taas
diatexiittiset gneissit ovat migmatiittirakenteensa osalta vaihtelevia ja kokonaisuutena epäsäännöllisiä. Tonaliittisetgranodioriittiset-graniittiset gneissit, TGG-gneissit ovat keskirakeisia, verrattain homogeenisia kiviä, joiden rakenteet
vaihtelevat raitaisista tai blastomyloniittisista tyypestä syväkivimäisiin, suuntautumattomiin muunnoksiin. Pegmatiittiset
graniitit ovat leukokraattisia ja hyvin karkerakeisia kivilajeja, joihin voi sisältyä granaatti-, turmaliini- ja kordieriittirakeita.
Etenkin laajimpiin massiiveihin voi sisältyä kooltaan ja muodoltaan vaihtelevia gneissisulkeumia. Tähän asti tutkituista
näytteistä suonigneissit muodostavat 43 %, stromaattiset gneissit 0,4 % ja diatexiittiitiset gneissit 21 %. Pegmatiittisten
graniittien tilavuusosuus on 20 %, TGG-gneissien 8 %, kiillegneissien 7 % ja mafisten gneissien 1 %.
Suprakrustiset kivilajit voidaan jakaa kemiallisen koostumuksensa perusteella neljään sarjaan tai joukkoon, jotka on
nimetty T-sarjaksi, S-sarjaksi, P-sarjaksi sekä mafisiksi gneisseiksi. Suprakrustisista kivilajeista 42 % kuuluu T-sarjaan, 12
% S-sarjaan ja 26% P-sarjaan. Näiden lisäksi graniittipegmatiit ja diabaasit muodostavat makroskooppisten piirteiden ja
kemiallisen koostumuksensa perusteella tunnistettavat ryhmänsä. T-sarjan kivilajit ovat pääosin erilaisia suonigneissejä ja
diatexiittisia gneissejä tai kvartsigneissejä, kiillegneissejä ja TGG-gneissejä. Tälle sarjalle tyypillinen piirre on
voimakkaasti piniittiytyneen kordieriitin ja pienen sillimaniittimäärän esiintyminen siihen kuuluvissa kivilajeissa. T-sarjan
kivilajit muodostavat vaihettumissarjan, jonka päätejäseniä ovat tumat, alle 60 % piidioksidia sisältät kordiittipitoiset
kiillegneissit ja yli 75 % piidioksidia sisältävät kvartsigneissit. Näiden on tulkittu olevan savimineraaleja runsaasti
sisältävien metapeliittien ja epäpuhtaiden, gravakkatyyppisten hiekkakivien metamorfisia vastineita. Tietyt TGG gneissit,
jotka ovat tyypillisesti graniittisia modaalisen mineraalikoostumuksensa perusteella, ovat kemiallisesti T-sarjan kivilajien
kaltaisia.
S-sarjan kivilajit ovat tekstuuriltaan kvartsigneissejä, kiillegneissejä, migmaattisia gneissejä ja mafisia gneissejä.
Kemiallisesti sarjan kivilajit poikkeavat merkittävimmin muista alueen kivilajeista korkean, yli 2 %:n, joissain tapauksissa
jopa yli 10 % CaO-pitoisuutensa ansiosta. Pienet alkalimetallipitoisuudet ja korkeahkot mangaanipitoisuudet ovat samoin
tyypillisiä tämän sarjan kivilajeille, joiden on tulkittu syntyneen kalkkipitoista materiaalia sisältävistä sedimentteistä.
P-sarjan kivilajit ovat TGG-gneissejä, suonigneissejä, erilaisia diatexiitteja, joskus vähän leukosomia sisältäviä raitaisia
kiillegneissejä ja erilaisia mafisia gneissejä. Kemiallisesti ryhmän kivilajit ovat erotettavissa muiden ryhmien
metasedimenttisistä kivilajeista korkean, yli 0,3%:n P2O5-pitoisuutensa ansiosta, sillä muihin Olkiluodon suprakrustisiin
kivilajeihin sitä sisältyy alle 0,2 %.
Kolmeen edellä mainittuun luokkaan kuulumattomia mafisia gneissityyppejä edustaa vain neljä näytettä, joille
luonteenomaisia piirteitä ovat korkeat MgO-, alkali, TiO2- ja P2O5-pitoisuudet. Kemiallisesti nämä kivilajit muistuttavat
pikriittejä tai pikriittisiä basaltteja.
Avainsanat - Keywords
Litologia, petrografia, geokemia, ydinjätteiden loppusijoitus, Olkiluoto, Eurajoki, Svekofenninen pääalue, LounaisSuomi.
ISBN
ISSN
ISBN 951-652-143-6
Sivumäärä – Number of pages
77
ISSN 1239-3096
Kieli – Language
Englanti
1
TABLE OF CONTENTS
ABSTRACT
TIIVISTELMÄ
1
INTRODUCTION .................................................................................................... 3
2
THE GEOLOGICAL SETTING OF OLKILUOTO .................................................... 5
3
MAIN LITHOLOGICAL SUBDIVISIONS ................................................................. 7
3.1
Investigations ...................................................................................................... 7
3.2
Lithological subdivisions ..................................................................................... 8
3.2.1
Main group of gneisses ............................................................................... 9
3.2.2
Main group of migmatitic gneisses............................................................ 11
3.2.3
Main group of TGG gneiss ........................................................................ 16
3.2.4
Pegmatitic granites.................................................................................... 18
3.2.5
Diabases ................................................................................................... 19
3.3
Ductile deformation and deformation textures .................................................. 19
3.4
The regional geology and correlation between the drill core logging and surface
mapping results ............................................................................................................ 21
4
WHOLE ROCK CHEMISTRY AND PETROGRAPHY .......................................... 27
4.1
The T series ...................................................................................................... 27
4.1.1
Chemical character ................................................................................... 30
4.1.2
Petrography and mineral paragenesis ...................................................... 35
4.2 The S series.......................................................................................................... 42
4.2.1
Chemical character ................................................................................... 42
4.2.2
Petrography and mineral paragenesis ...................................................... 43
4.3
The P series...................................................................................................... 47
4.3.1
Chemical character ................................................................................... 47
4.3.2
Petrography and mineral paragenesis ...................................................... 48
4.4
Basic metavolcanics and diabases ................................................................... 51
4.5
Pegmatitic granites ........................................................................................... 55
4.5.1
Whole rock chemistry and petrography..................................................... 55
4.6
Spatial distribution of the members, and volumes of the various rock series ......
.......................................................................................................................... 57
5
METAMORPHIC MINERAL ASSEMBLAGES AND SECONDARY ALTERATION
PRODUCTS.................................................................................................................. 59
2
6
CONCLUSIONS AND DISCUSSION .................................................................... 63
REFERENCES ............................................................................................................. 67
APPENDIX 1................................................................................................................. 75
3
1
INTRODUCTION
According to the Nuclear Energy Act, all nuclear waste generated in Finland must be
handled, stored and permanently disposed of within the country itself. The two nuclear
power companies, Teollisuuden Voima Oy and Fortum Power and Heat Oy, which are
responsible for the safe management of this waste, have established a joint company,
Posiva Oy, to implement the spent fuel disposal programme, whilst other nuclear wastes
are to be handled and disposed of by the power companies themselves.
The plans for the disposal of spent fuel are based on the KBS-3 concept, which was
originally developed by the Swedish SKB. The spent fuel elements will be encapsulated
in metal canisters and emplaced at a depth of several hundreds of metres.
The current stage of development of the Finnish nuclear waste management programme
can be summarised as follows:
x Repositories for the disposal of low and intermediate-level waste are already in
operation at both Olkiluoto and Loviisa.
x A decision in principle regarding the disposal of spent fuel was approved by
parliament in 2002. This allows for the development of a spent fuel repository on
the lines of the KBS-3 concept to be sited at Olkiluoto in the municipality of
Eurajoki.
At present Posiva Oy is preparing to commence construction of an underground rock
characterisation facility at Olkiluoto. The plan is that these underground investigations
and other work should form a basis on which Posiva can submit an application for a
licence to build the disposal facility in the early 2010s, with the aim of starting disposal
operations in 2020.
As a part of these investigations, Posiva Oy is continuing detailed bedrock studies to
form a more precise picture of the lithology and bedrock structure of the site. As a part
of this work, the present report summarises the results of petrological studies carried out
since 1988.
4
5
2
THE GEOLOGICAL SETTING OF OLKILUOTO
The Olkiluoto site is located in southern Satakunta, SW Finland (Fig. 2-1), where the
oldest part of the bedrock is composed of supracrustal, metasedimentary and
metavolcanic rocks deformed and metamorphosed during the Palaeoproterozoic
Svecofennian orogeny ca. 1910 - 1800 million years ago. For the most part, they are
migmatites and high-grade mica gneisses, which may contain cordierite, sillimanite or
garnet porphyroblasts (Suominen et al. 1997, Veräjämäki 1998). In the western part of
the area the leucosome veins in the migmatites are mostly granitic in composition
whereas in the northeast the migmatites may contain trondhjemitic to granodioritic
leucosomes (Pietikäinen 1994, Pajunen et al. 2002). Amphibolites, uralite porphyrites
and hornblende gneisses, which were originally mafic and intermediate volcanics,
occasionally occur as narrow interlayers in supracrustal sequences. The migmatites are
intruded by plutonic rocks consisting of trondhjemites, tonalites and granodiorites,
which occur conformably with the structures of the mica gneisses (Pietikäinen 1994,
Suominen et al. 1997, Veräjämäki 1998). Coarse-grained granites and pegmatites occur
in migmatizing and cross-cutting veins. Except for a few small bodies, more mafic
intrusive rocks, gabbros and diorites, are encountered only as small xenoliths.
Figure 2-1. Geological map of southern Satakunta.
6
Large parts of southern Satakunta area are composed of the Mesoproterozoic,
anorogenic Laitila rapakivi batholith, 1583 million years in age (Vorma 1976, Vaasjoki
1996). The Eurajoki rapakivi stock, about 5 km east of Olkiluoto, is a satellite massif to
this batholith, and can be divided into the marginal hornblende-bearing Tarkki granite
and the younger, central Väkkärä granite (Haapala 1977), both of which are somewhat
younger than the Laitila batholith.
The Satakunta Sandstone, at least 1400 - 1300 million years in age, was deposited
fluvially in a deltaic environment and has been preserved in a graben structure
(Kohonen et al. 1993). It is cut by Postjotnian olivine diabase dykes 1270 - 1250 million
years in age (Suominen 1991). Lake Sääksjärvi, ca. 50 km ENE of Olkiluoto, is situated
in an impact crater of early Cambrian age.
The Palaeoproterozoic rocks were deformed and metamorphosed in two main phases
during the collisional and post-collisional stages of the Svecofennian orogeny. The
earliest observed structural element of the area is the biotite foliation S1 of deformation
phase D1. The dominant foliation is usually penetrative S2 foliation of deformation
phase D2 with metamorphic segregation. The F2 folding is recumbent and isoclinal to
tight. The recumbent attitude of the folds suggests a layer-parallel shearing and over- or
underthrusting during D2. The tonalite-granodiorite intrusions were emplaced before or
during the deformation phase D2 and were deformed during D2, the age of the D2
deformation being, thus, close to the age of these granitoids, 1890 - 1860 Ma.
Both D1 and D2 structures are deformed by the regional F3 folding of the deformation
phase D3. The fold axes are generally horizontal or moderately plunging. Axial planes
of folds are usually vertical but locally also overturned and recumbent folds exist. Fold
limbs are often strongly sheared. Late-orogenic potassium granites were emplaced
during D3 and the age of the deformation phase is close to the age of these granites (ca.
1840 - 1830 Ma).
The Mesoproterozoic rapakivi granites and the related mafic rocks, the Satakunta
sandstone formation and the Postjotnian diabase dykes and sills represent the
cratonisation stages of the Svecofennides.
For a more comprehensive description of the geology and geological development of
the Satakunta area, the reader is referred to Paulamäki et al. 2002 and Väisänen 2002.
7
3
MAIN LITHOLOGICAL SUBDIVISIONS
The bedrock of the Olkiluoto site is composed for the most part of various high-grade
metamorphic and migmatitic rocks of supracrustal origin, derived from various
epiclastic and pyroclastic materials. The most typical lithologies are heterogeneous
migmatites of metapelitic origin. In addition, leucocratic pegmatitic granites are
frequently encountered as veins, vein networks and irregular masses, and the bedrock is
also intruded by a few narrow mafic dykes. The present chapter outlines the way in
which these heterogeneous and gradational crystalline rocks have been subdivided into
a small number of lithological groups, based on macroscopic observations. In the first
section (Section 3.1), an outline of previous investigations in the Olkiluoto area is given,
in order to show the depth of research and experience on which the present subdivisions
are based. The lithological subdivisions at present in use are briefly described in Section
3.2, with reference to the original report on the system of nomenclature for rocks at
Olkiluoto (Mattila 2006). Following sections deal with the ductile deformation and
deformation textures observed at Olkiluoto (Section 3.3) and correlation between drill
core logging and surface mapping results (Section 3.4). The aim of this chapter is to
outline the practical basis for rock classification and nomenclature, not forgetting the
needs to rock engineering and long-term stability. As such, some of the methods and
terminology normal used in scientific work have been modified. More scientific aspects
of the petrology, based on the results of accurate whole rock chemical analysis and
microscopic study, are discussed in Chapters 4 and 5.
3.1
Investigations
During the preliminary stage of investigations, geological mapping was carried out at
Olkiluoto in 1988 and 1991. In the first general geological mapping (Paulamäki 1989)
the outcrops were evaluated with respect to rock types and the main structural features,
with special emphasis to fracturing. The mapping included determination of the rock
types and their description on the basis of visual and microscopic investigations. As
mapping and the detailed observation of structural elements is impossible from a freshly
exposed, wet and dirty outcrop surface, the general geological mapping was followed a
few years later by a detailed structural geological mapping (Paulamäki & Koistinen
1991) The aim of the latter mapping was to describe the deformation history and the
structures of the various phases of ductile deformation and to establish their effect on
subsequent brittle deformation and fracturing. Additional structural observations were
made at Olkiluoto during investigations in 2002 and 2003 (Paulamäki et al. in prep.). In
2004, outcrop mappings were carried out in the central and eastern parts of the Olkiluoto Island by I. Aaltonen and J. Mattila of Posiva Oy.
Since most of the bedrock at Olkiluoto is unexposed, outcrop mapping was
supplemented by the excavation and mapping of the trenches in the central part of the
site (Paulamäki 1995, 1996; Lindberg & Paulamäki 2004; Paulamäki & Aaltonen 2005;
Paulamäki 2005a; Paulamäki 2005b, Engström 2006), with the aim of obtaining more
bedrock and fracture data for areas with few or no outcrops. This enabled lithological
contacts and fracture zones to be located more precisely than on the basis of outcrop
8
observations alone. The trench mappings included the macroscopic determination of
rock types from the bedrock surface, after removal of the remaining loose soil with
compressed air and cleaning with a high-pressure washer. The trenches amounted to a
total length of ca. 3300 m and ranged in width from 0.5 m to 5 m.
Core drillings have been carried out in the area since 1988, and the total number of deep
drill holes exceeds now 40. The results obtained from the earliest 23 drillings were
made use of in this work. The drill cores are mostly 500 – 1000 m in length, and their
combined length is 12,492 m. The locations of the starting points of the drill holes are
indicated in Appendix 1. Technical information on these drillings has been presented by
Jokinen (1994); Niinimäki (2000, 2002a, 2002b, 2002c, 2002d); Rautio (1995, 1996a,
1996b, 1999, 2000a, 2000b, 2002) and Suomen Malmi Oy (1989a, 1989b, 1989c,
1990a, 1990b) and the results of primary petrological studies dealing with material from
the above-mentioned cores have been presented by Gehör et al. (1996, 1997, 2000,
2006) and Lindberg & Paananen (1991). These reports include the results of visual drill
core loggings, polarization microscope examinations and whole rock chemical analyses
of ca. 160 samples which have been carried out in the SGS Minerals Services
laboratory, Canada by X-ray fluorescence analyser, neutron activation analyser,
inductively coupled plasma atomic emission analyser, inductively coupled plasma mass
spectrometer, sulphur and carbon analyser and by using ion specific electrodes. The
results of the whole rock chemical analyses are represented in addition to the original
reports in a CD ROM appendix (Appendix 1) of this paper. The original electron probe
microanalysis results for the most common mineral species are also reported in the
papers mentioned above.
3.2
Lithological subdivisions
The high-grade metamorphic and migmatitic rocks at Olkiluoto were formed under high
amphibolite facies conditions, with or without the production of granitic melts by partial
melting of the epiclastic and pyroclastic protolith. During various kinematic events of
the Svecofennian orogeny, the granitic melts partly remained in place (as multiple
phases of leucosome or neosome veins in the widespread, polyphase migmatites) and
partly migrated into the surroundings (as vein networks and irregular masses of
pegmatitic granite). The degree of migmatization and the types of migmatite structures
vary widely within the study site. However, the concepts of migmatite classification and
the rules proposed, for instance, by the IUGS Subcommission on the Systematics of
Metamorphic Rocks (Schmid et al. 2004) have turned out to be impractical as such in
the case of Olkiluoto. The principal aim of rock classification is to divide the rocks
roughly into groups of similar properties in the sense of rock mechanics, specially
weighting the factors significant for the construction of underground facilities. The
main goal has been to develop a simple, practical classification scheme which can be
performed visually on cores, outcrops and tunnel walls, based on directly observable
structures and textures, and estimates of major mineral composition, without any
reference to the results of instrumental analyses. Nevertheless, although the system is
specially tailored to Olkiluoto, the classification scheme follows the common rules and
methods of migmatite classification to the appropriate extent.
9
The rock nomenclature adopted by Posiva Oy (Mattila 2006) is derived from the
method of British Geological Survey, especially as outlined for igneous rocks by
Gillespie & Styles (1999) and for metamorphic rocks by Robertson (1999). The
migmatitic gneisses are migmatites which contain more than 10% neosome, while the
rocks in the gneiss group may be totally non-migmatitic but they can also contain up to
10% neosome (i.e. the term gneiss is extended to also cover weakly migmatitized rocks,
for reasons of brevity). Each of the main groups is subdivided into a small number of
easily recognized groups, as explained in following Subsections 3.2.1, 3.2.2 and 3.2.3.
With regard to the igneous rocks at Olkiluoto, two main types are recognized,
pegmatitic granite and diabase, as described in Subsections 3.2.4 and 3.2.5.
3.2.1
Main group of gneisses
Certain supracrustal materials at Olkiluoto, e.g. those with an excess of mafic
components and those of quartzitic composition, remained stable without any sign of
partial melting or melted only for a minor part at the peak conditions of metamorphism.
These rocks appear often as different types of gneisses, intruded only by a few, irregular
pegmatite veins or leucosome streaks. All the rocks, in which the proportion of
migmatizing material is less than 10%, have been excluded from the main group of
migmatitic gneisses and are placed into this group. As noted above, however, the gneiss
term is a misnomer, since some members of the group contain small amounts of
neosome, resulting from partial melting, while some are totally non-migmatitic. Strictly,
those weakly migmatized rocks and pure gneisses should be handled separately, but the
mechanical properties for all those rocks have been evaluated so similar that any
separation has considered needless.
The rocks in this group, which vary in structure from homogeneous to banded, include
mica-bearing quartz gneisses, mica gneisses and hornblende or pyroxene-bearing mafic
gneisses. Any subdivision of these rocks has to be based on mineral composition, and
more detailed classification is not always possible without recourse to polarization
microscopy or some other method of instrumental analysis. In the field, however, the
following types can usually be recognized.
Quartz gneisses
The quartz gneisses are fine-grained, homogeneous and typically poorly foliated. They
contain more than 60% quartz and feldspars, and less than, often much less than 20%
micas. Certain variants may contain some amphibole and certain modifications have
some pyroxene in addition to amphibole. Garnet is also a typical constituent of one
subgroup of the quartz gneisses. All these gneiss variants are similar in terms of the
appearance of their fresh, broken faces. They are dark grey, due to the biotite content,
and the large proportion of quartz and feldspars makes their broken faces glossy and
somewhat vitreous in appearance. They make up less than 1% of the bedrock at
Olkiluoto.
10
Mica gneisses
The more mica-rich gneisses are in most cases intensively migmatitized, but fine and
medium-grained mica gneisses with less than 10% leucosome material also occur. The
proportion of micas or their retrograde derivatives exceeds 20% in these rocks.
Cordierite or its retrograde derivative, pinite, is a typical constituent, and large,
roundish, very dark porphyroblasts ca. 5 – 10 mm in diameter can often be seen. The
fine-grained mica gneisses are typically schistose, but the medium-grained variants
show a distinct metamorphic banding. Mica gneisses that are weakly or not at all
migmatitized make up ca. 7% of the total length of the drill cores logged up to now.
Banded and foliated, weakly migmatitized or homogeneous, almost unfoliated micabearing gneisses are randomly encountered also in the outcrops and investigation
trenches (Fig. 3-1).
A
B
Figure 3-1. A. Olivine-phlogopite gneiss (metapicrite); mafic gneiss OL-KR17. B.
Banded, weakly migmatized mica gneiss, Selkänummenharju.
11
Mafic gneisses
Mafic gneisses and schists have been encountered in different variants that have been
referred to as amphibolites, hornblende gneisses and chlorite schists, in which
hornblende or chlorite comprises the dominant mafic mineral phase. Certain exceptional
gneiss variants may contain some pyroxene or olivine in addition to mica and
hornblende (Fig. 3-1). Variable mafic or sometimes even ultramafic gneisses make up
less than 1% of the bedrock of Olkiluoto. These rocks belong to units, which achieve
maximum thicknesses of some tens of metres.
3.2.2
Main group of migmatitic gneisses
The migmatitic gneisses of Olkiluoto are diverse, showing different grades of
migmatization and a very wide range of migmatite structures. The rock classification
could have been based on an evaluation of the degree of migmatization (metatexis,
diatexis, anatexis, see Dietrich & Mehnert 1961, Wimmenauer & Bryhni 2002) or on
the basis of the detailed geometrical properties of the migmatite structures (e.g. phlebitic
structure, nebulitic structure, schollen structure, etc., see Mehnert 1968). Although the
mapping was carried out with these possibilities in mind, the actual subdivision and
classification of the migmatites at Olkiluoto is based on more general characteristics, as
explained above.
The main group of migmatitic gneisses includes migmatites in which the proportion of
neosome exceeds 10 %. In the case of Olkiluoto, these rock mixtures are composed
mostly of a mica-rich older component, palaeosome, and a younger component derived
from the melt, the neosome, the proportion and style of occurrence of which can vary
widely. The neosome is composed of coarse-grained granitic material that can also be
referred to as a leucosome due to its light colour and lack of mafic minerals. The
palaeosome is sometimes referred to as melanosome, since it often appears as dark,
narrow stripes (schlieren) and bands rich in mafic minerals. The typical migmatites of
Olkiluoto contain 20 – 40% leucosome on average, but the proportion can be less than
20% or in excess of 80% in individual samples. In the main group of migmatitic
gneisses the groups of veined gneisses, stromatic gneisses and diatexitic gneisses
(described below) represent distinct end members in a gradational system of gneisses
and migmatites (Fig. 3-2). Veined gneisses and stromatic gneisses belong to the class of
metatexitic migmatites (low grade migmatites) while diatexitic gneisses are, for the
most part, various diatexites (medium grade migmatites). The changes in textures and
migmatite structures typical of a particular migmatite group to those characteristic for
another one take place gradually, so that it is not possible to detect any natural borders
between these end members.
12
Figure 3-2. Textural and structural end members in the migmatite-gneiss system of
Olkiluoto.
Veined gneisses
Veined gneisses are migmatites which contain diverse elongated, folded or stretched
leucosomes of diameters varying from several millimetres up to ten centimetres. Due to
the high grade of deformation, the leucosomes show a distinct lineation and often axial
symmetry and appear as swellings in the veins or roundish quartz-feldspar aggregates
that may be composed of augen-like structures with diameters varying between 1 and 5
cm (Fig. 3-3). The palaeosome is often banded and can show the products of powerful
shear deformation, e.g. asymmetric blastomylonitic foliation. Due to the geometrical
properties of the leucosome and the texture of the palaeosome, the general symmetry of
the veined gneisses is axial, a property that has a significant impact on all the attributes
of these rocks, and not least on the anisotropy of their mechanical properties. Actually,
the migmatite structures of the rocks in this group are variable in reality and show
various modifications of phelibitic structure, folded structure, ophthalmitic structure and
schlieren structure, as described in the scientific literature (e.g. Mehnert 1968). In
addition, features of other structural types, such as diktyonic structure, surreitic structure
13
and ptygmatic structure, have occasionally been observed. Migmatitic rocks that belong
to this group make up 43% of the drill core samples studied so far, and it has been
shown that veined gneisses are the dominating migmatitic gneiss type in the central part
of the Olkiluoto site.
A
B
C
Figure 3-3. Veined gneisses. A. Drill core OL-KR18, B. Drill core OL-KR1, C.
Investigation trench OL-TK4, section P51.
14
A
B
Figure 3-4. Stromatic gneisses. A. Drill core OL-KR11, B. Island of Kuusisenmaa, W of
Olkiluoto.
15
Stromatic gneisses
Stromatic gneisses are characterized by the presence of plate-like, linear leucosome
dykes or layers (Fig. 3-4) varying in width from several millimetres to 10 – 20 cm. This
type of migmatitic gneiss has been referred as “stromatite” in the seminal book on
migmatites by Mehnert (1968). The field investigations at Olkiluoto suggest that the
leucosome dykes in the stromatic gneisses may continue uninterrupted for several
metres, and the widest ones for several tens of metres. The palaeosome in these
migmatites is often well foliated and shows a distinct gneissic banding or schistosity.
The structural character of these rocks means that they show a planar anisotropy of
mechanical and thermal properties even in the context of wider units, and planar
symmetry is typical of all the physical parameters of bedrock units dominated by these
rocks. For instance, breakage along the plane of foliation and leucosome layering during
underground construction can be expected to take place more easily in stromatic
gneisses than in veined gneisses. It should be noted, however, that, because of the
transitional nature of all types of migmatites at Olkiluoto, in addition to a proper
stromatic structure also other migmatite structures may be found in this assemblage.
Stromatic gneisses are relatively rare at Olkiluoto, representing only 0.4% of the length
of the drill cores studied so far.
Diatexitic gneisses
The term diatexitic gneiss is used for more strongly migmatized rocks that show a wide
spectrum of generally asymmetrical and disorganized migmatite structures. Most of
them would fall in the category of “diatexites” (cf. Wimmenauer & Bryhni 2002), and
may be characterized by nebulitic, stictolithic and schollen structures of Mehnert
(1968). However, detailed examination of the diatexitic gneisses at Olkiluoto has shown
that also features of other migmatite structure types can be found. This group includes
variants that may contain more than 70% leucosome, and in which the palaeosome
occurs as fragments of irregular shape and variable size (Fig. 3-5). The margins of the
palaeosome fragments are often gradational, and the fragments may be almost
indistinguishable. Palaeosome fragments can be totally assimilated into cross-cutting
vein materials, or else the border zones of these particles may be gradual, progressive
areas of transformation resembling schollen migmatites. Diatexitic migmatites differ
from stromatic and veined gneisses due to coarser grained, gneissic paleosome and
generally stronger migmatization. All the migmatite variants in which the shapes of the
palaeosome and leucosome are random and which are structurally asymmetric in their
entirety have been classified into this group. Wider units dominated by diatexitic
gneisses may as a whole be relatively homogeneous. A pervasive foliation in these
rocks is not very well developed and physically they can be assumed to be practically
isotropic. Hence, the diatexitic gneiss units show minimal variation in their physical
properties if the dimensions of the units considered are large enough. Diatexitic gneisses
make up 21% of the total length of the drill cores studied so far, and it can be assumed
that the same percentage applies to the bedrock of the central part of Olkiluoto.
16
A
B
Figure 3-5. Diatexitic gneisses. A. Drill core OL-KR1. B. Island of Pukkiluoto, south of
Olkiluoto.
3.2.3
Main group of TGG gneiss
The TGG gneisses, where TGG is an abbreviation for “tonalite-granodiorite-granite”,
are medium-grained, relatively homogeneous rocks that can show a weak metamorphic
banding but can also resemble plutonic, non-foliated rocks (Fig. 3-6), one type
resembling the moderately foliated red granites, and another the weakly foliated grey
tonalites. In places, the TGG gneisses are well-foliated, banded gneisses with features
typical of blastomylonitic rocks in high-grade ductile shear zones (cf. Sibson 1977),
.
17
A.
B.
C.
Figure 3-6. TGG gneisses. A. Drill core OL-KR13. B. Drill core OL-KR5. C.
Promontory of Ulkopää.
18
such as kinematic indicators or other distinctive structures. The TGG gneisses form
homogeneous and typically weakly fractured units that do not always stand out
distinctly from surrounding lithological units, such as medium-grained mica gneisses.
The contacts can be gradual transition zones that may vary in width from several tens of
centimetres to several metres, thus making evaluation of the exact locations of borders
of these units problematic. By contrast, one specific contact type for the TGG gneiss
units resembles the sharp intrusive contacts typical of real igneous rock bodies. Crosscutting pegmatitic granites and leucosome-like veins may comprise up to 20% of the
volume of these gneissic rock units in some places, but totally homogeneous variants
without any leucosome are also typical. The proportion of TGG gneisses in the drill
cores studied so far is 8%.
Some TGG gneisses may resemble plutonic igneous rocks, whereas others seem to be
very high grade migmatitic rocks, anatexites. Typically, the gneisses of granodioritic
and tonalitic composition in this assemblage are more homogeneous and “igneouslooking”, while granitic gneisses may feature structures indicative of extreme highgrade migmatization, with a nebulitic structure and biotite-bearing schlieren. In
addition, porphyritic granite veins and irregular masses known as K-feldspar porphyries
have been found close to certain TGG gneiss units and ductile shear zones. Regardless
of their precise mode of formation, however, the TGG gneisses represent parts of the
bedrock, which have gone through the same deformational and metamorphic history as
the migmatites and gneisses described above.
3.2.4
Pegmatitic granites
Abundant coarse-grained, felsic rocks of granitic composition, in the form of veins, vein
networks and irregular masses, are a typical feature of the Olkiluoto bedrock. These
rocks are referred to as pegmatitic granites and are mainly genetically associated with
the migmatization process. Sometimes it is very difficult to make a distinction between
leucosome and pegmatitic veining, although also post-migmatitic, crosscutting, real
pegmatite dykes from external sources have been observed.
The pegmatitic granites are typically leucocratic, allotriomorphic-granular and very
coarse-grained, granitic rocks. Large garnet phenocrysts, or tourmaline and cordierite
grains of variable size, sometimes occur in these pegmatites, and mica gneiss inclusions
of highly variable sizes, shapes and proportions are also typical constituents of the
larger masses. The widths of these intrusive bodies vary greatly, the narrowest veins
being less than 10 cm in width and the widest bodies several tens or hundreds of metres
in diameters. Pegmatitic granites constitute a fairly large proportion of the bedrock of
Olkiluoto, the pegmatitic granite sections representing 20% of the total length of the
drill cores studied so far. A majority of the pegmatitic granites at Olkiluoto are coeval
with the ductile deformation and migmatization processes and genetically they are
strictly related.
19
3.2.5
Diabases
Dyke rocks classified as diabases have been identified sporadically in investigation
trenches, the shallow and deep boreholes and construction site of OL3 power plant. The
dykes concerned are typically very narrow, of widths varying from 5 to 50 cm, and the
rocks are typically dark or black, dense and fine-grained. The contacts of these dykes
are very sharp, but no evidence of chilled margins has been detected. In places the
dykes contain quartz- and carbonate-filled amygdales, from 0.1- 0.3 mm to ca. 2 mm in
diameter. Microscopy has shown that the diabases are thoroughly altered and the
original mineral phases have been totally replaced by secondary ones, so that large
proportions of the rock are saussurite. Albitic plagioclase is visible in 1 – 2 mm long
plate-like crystals, making the texture similar to the original ophitic texture of diabases.
3.3
Ductile deformation and deformation textures
All the lithological units of Olkiluoto excluding the diabase dykes and youngest
pegmatites have been subject to ductile deformation, including several successive
deformational phases, as determined on the basis of refolding and cross-cutting
relationships (Paulamäki & Koistinen 1991, Paulamäki et al. in prep.). The lithological
layering (S0) with the oldest deformational and metamorphic structure and the
penetrative, slightly segregated foliation S1 of deformation phase D1 mostly
(sub)parallel to S0 are the oldest observed structural features at the site. The subsequent
deformation phase D2 is a complex chain of events characterized by intense deformation
and leucosome production. At the beginning of D2, earlier structures were folded by
stage F2 producing subhorizontal, tight or isoclinal F2A folds, to which the penetrative S2
biotite foliation shows an axial planar relationship. S2 can be separated from S1 only at
the fold hinges, as elsewhere they are subparallel. These early structures have more or
less been overprinted by the later deformation events and only occur in certain more
competent layers within the migmatite. During the main stage of the deformation event
D2, the earlier structures were overprinted by the penetrative foliation and metamorphic
banding S2, associated with abundant production of leucosome veins parallel to this
foliation/banding (Fig. 3-7). The S2 foliation is also often parallel to the lithological
layering and earlier foliations, and can, in fact, be expressed as a composite foliation
S0/1/2. On the course of progressive D2 deformation the production of leucosome veins
continued and the veins formed earlier were folded isoclinally accompanied by
semiconcordant shearing. Some fragmentation of the migmatites and rotation of the
fragmented blocks occurred in the waning stage of the D2 deformation.
In deformation phase D3 the deformed migmatites were refolded or rotated. Zones
dominated by ductile D3 shears and folds were formed, and the S2 foliation was
reoriented parallel to the F3 axial plane (S3). Typically no new foliation was created
during this stage but the foliation can be described as a S2/3 composite structure and a
new granitic leucosome intruded parallel to the F3 axial planes (Fig. 3-7). The fold axes
of the F3 folds usually plunge gently to the NE or SW. The blastomylonitic foliation
sometimes observed in the TGG gneisses is probably a D2 transposition structure, which
20
totally overprints the earlier structures but similar structure have been created by stage
D3, too.
Subsequently the D3 elements were redeformed in the deformation phase, D4, which
produced more open F4 fold axes trending ca. N-S and axial planes dipping to the east.
These structures have been detected normally only in certain outcrops but, according to
latest mappings and interpretations, the eastern and south-eastern parts of the Olkiluoto
area seem to be more strongly affected by it. Due to D4 deformation, the S2/3 composite
structures are locally reoriented towards the F4 axial surface (S4). The latest ductile
structures to be identified are the very open F5 folds, plunging gently to the ESE.
Figure 3-7. Veined gneiss with S2 foliation and parallel granitic leucosome veins
affected by F3 folding. New granite leucosome veins have intruded parallel to the axial
plane of the F3 folds, trench OL-TK1, section P7.
21
3.4
The regional geology and correlation between the drill core logging
and surface mapping results
The practice adopted for naming rock types during the outcrop mapping in 1988 and
1991 and the mapping of the investigation trenches TK1-TK3 was different from that
presented in Section 3.2. In order to co-ordinate the nomenclature, all the old surface
observations were re-assessed and renamed on the basis of the amount of granite
leucosome and its mode of occurrence, to correspond to the nomenclature used for the
rock types in the drill cores. The observations with regard to the outcrops (Paulamäki
1989, Paulamäki & Koistinen 1991) and the investigation trenches (Paulamäki 1995,
1996, 2005a, 2005b, Lindberg & Paulamäki 2004, Paulamäki & Aaltonen 2005) suggest
that migmatite characteristics of the veined gneiss-type predominate in the central parts
of the Olkiluoto site. Investigation trenches OL-TK2, OL-TK3 and OL-TK4, for
example, which together form an almost continuous section more than 1 km long
through the central investigation area, contain migmatite that typically includes 10-30%
granitic leucosome veins. The mica-rich mesosome is medium-grained and in places
clearly banded.
In the southern part of the site, around borehole OL-KR4 and to the south and southeast
of it, strongly migmatized diatexitic gneisses are dominant (Fig. 3-8). The proportion of
leucosome veins often exceeds 0%, and come close to 90% in places, and the
palaeosome usually occurs as narrow biotite-rich schlieren between or within the granite
leucosome veins. Weakly banded, discontinuous mica gneissic mesosome bands occur
in places, however. Diatexitic gneisses are the dominant type down to the level –400 m
in drill hole OL-KR4, but veined gneisses occur below that. Viewed on the map, the
diatexitic gneisses form a rather narrow NE-SW trending strip between the bays of
Liiklanperä and Santalahti, since the veined gneisses are again dominant further to the
southeast (Fig. 3-8).
Rocks classified as the most typical stromatic gneisses have been observed in outcrops
only in the southern part of the island of Kuusisenmaa, west of Olkiluoto, and stromatic
gneisses are similarly very rare in the drill cores. The longest continuous section
mapped as stromatic gneiss, ca. 20 m in drilling length, has been found in core OLKR11, also outside from the central part of the study site.
The surface observations indicate that fairly homogeneous mica gneisses with less than
10% granitic leucosome veins seem to occur mainly in the western, northwestern and
central parts of the Olkiluoto site, so that large parts of trench TK3 and drill core OLKR3, for instance, are composed of mica gneiss, which is only weakly migmatized or
even rather homogeneous. In the central part of the site, mica gneisses occur randomly
in outcrops, drill cores and trenches. Very few mica gneisses have been observed in the
well-exposed southern and southeastern parts of the site, which are dominated by
diatexitic gneisses (Fig. 3-8). The banding in the mica gneisses is metamorphic in
origin, but occasionally the alternation in mineral composition (quartz-feldspar-rich and
biotite-rich bands) and grain size may relate to the primary lithological layering (Fig. 3-1).
In addition to mica gneisses, homogeneous or slightly banded quartz gneisses have occa-
22
Figure 3-8. Lithological map of Olkiluoto.
23
sionally been encountered also in outcrops. These occur either as narrow interlayers or
as small inclusions within the migmatite units.
On an outcrop scale, hornblende and pyroxene-bearing gneisses have been encountered
in markedly extended or lensoid skarn-like inclusions, which mostly consist of several
rims. The outer rim is always composed of homogeneous grey, fine-grained plagioclase
and quartz-rich gneiss, while the nucleus of the inclusion is greenish or brownish
probably depending on the main mafic mineral, hornblende or pyroxene. Between these
there are occasionally narrow rims consisting of quartz or biotite. In addition, lensoid
inclusions occur in which the centre is mostly composed of plagioclase and quartz,
hornblende being just a minor component, thus resembling the quartz gneisses described
in the drill cores. The skarn-like inclusions are most likely fragments or boudins of Carich interlayers, which occur in the northeastern part of the island of Olkiluoto, for
instance (Paulamäki et al. in prep.). Narrow Ca-rich interlayers have also been found in
the central part of the study site (Paulamäki 1989).
The TGG gneisses in the outcrops include a wide variety of grey and reddish, fairly
homogeneous gneisses. The rock on the promontory of Ulkopää in the western corner of
the site is light grey, medium-grained, homogeneous, weakly foliated and somewhat
igneous-looking in appearance. The repository for low and intermediate-level waste is
located within this TGG gneiss unit. On the Selkänummenharju in the middle part of the
site, the TGG gneiss is tonalitic or granodioritic in composition and grey or reddish grey
in colour, but the texture is more clearly gneissose and the foliation is more marked than
at Ulkopää. In both of these areas, especially in the western unit, the TGG gneisses are
cut by pegmatitic granite veins. Both the TGG gneiss and the granite veins cutting it
occasionally include garnet porphyroblasts. The TGG gneisses of Ulkopää and
Selkänummenharju have been connected with the TGG gneiss sections in boreholes
OL-KR5 and OL-KR20, while the TGG gneisses of the outcrops located north of
borehole OL-KR2 in the central area resemble those of the Selkänummenharju area, and
the same unit has most probably been detected in drill cores OL-KR2, OL-KR5, OLKR13, OL-KR14 and OL-KR15, as a stratigraphic unit of thickness 90 – 100 m.
The TGG gneiss unit west of borehole OL-KR8 consists of two kinds of gneisses. The
western part of the unit, intersected by trenches TK1 and TK5, is composed of grey,
medium and even-grained, weakly oriented, homogeneous and tonalitic gneiss of an
igneous appearance, which is cut, or in places brecciated, by pegmatitic granite. The
eastern part of the unit, intersected by trenches TK6 and TK7, consists of a coarsegrained granitic gneiss with large potassium feldspar phenocrysts. This cuts across the
diatexitic gneiss and also includes large and small migmatite inclusions with diffuse
contacts (Paulamäki 2005b). The whole TGG gneiss unit has been connected with the
TGG gneiss sections in boreholes OL-KR8, OL-KR26, OL-KR28, ONK-PH1, as well
as in the ONKALO access tunnel.
The rocks designated as TGG gneisses in the southern part of the site, within the
diatexitic gneiss area, are not intersected by any borehole. These are brownish or dark
grey in colour and well foliated, sometimes resembling coarse-grained mica gneisses,
but are lacking in the porphyroblasts common in the latter. A few coarse-grained granite
veins occur parallel to the foliation, but these gneisses are more commonly brecciated
24
by the pegmatitic granite. In places the gneisses include small, dark patches or streaks,
in which the plagioclase is richer in anorthite than in these gneisses in general (An52 and
An35, respectively), and which have an abundance of opaques and apatite typical for P
type TGG gneisses (see Chapter 4).
Amphibolites and other mafic gneisses have only occasionally been encountered on the
surface, as small inclusions or narrow, discontinuous dykes. Mafic or even ultramafic
metavolcanics have been detected in a comparatively wide unit in drill core OL-KR17,
where the stratigraphic thickness of the mafic gneiss unit seems to exceed 10 m.
Elsewhere the sections composed of mafic gneisses are mostly less than 1 m in drilling
length, and only a couple of intersections varying between 5 and 10 m in length have
been detected.
The pegmatitic granites in the outcrops and trenches occur as veins ranging in width
from a few tens of centimetres to ca. 20 metres, or as large, uniform intrusions, which
include mica gneiss inclusions or restites. The pegmatitic granite veins mostly occur
(semi-)concordantly with respect to the foliation, but cross-cutting veins also occur,
often following the axial plane trend of the F3 folds, striking NE-SW.
The pegmatitic granite veins that are abundant in the TGG gneisses of the Ulkopää area
are a few centimetres to more than one metre in width and range in length from a few
metres to some tens of metres. The veins strike ca. E-W, dip gently to the S or SSE and
are often folded in an E-W direction. Both white and reddish veins occur, separate, in
the Ulkopää area, although their mineral composition is similar in spite of the colour
difference.
Large, homogeneous pegmatitic granite intrusions containing only a few mica gneiss
restites occur west of the power plants, between Ulkopää and Selkänummenharju and
around borehole OL-KR5. The latter pegmatitic granite is connected with the pegmatitic
granite intersections in boreholes OL-KR13, OL-KR2, OL-KR12, OL-KR14, OLKR10, OL-KR4, OL-KR21, OL-KR20 and OL-KR1.The pegmatitic granites in the
middle of the site, west of the Korvensuo reservoir, have been drawn on the basis of
outcrops and the investigation trenches and connected with the pegmatitic granite
intersections in boreholes OL-KR10, OL-KR12, OL-KR14, OL-KR7 and OL-KR16.
The pegmatitic granite unit modelled here is highly heterogeneous and contains
considerable amounts of migmatite material.
The diabase dykes have been observed on the surface in trenches TK3 and TK8 and in
the construction site of OL3 (Lindberg & Paulamäki 2004, Engström 2006, Talikka
2005). The dyke in TK3 is ca. 60 cm wide, has sharp contacts with the country rock and
dips 55° to the NW. A similar diabase dyke observed in the borehole OL-23 is
interpreted on the basis of ground geophysical data as dipping 65 - 75° to the NW or
NNW (Paananen & Kurimo 1990, Vaittinen et al. 2001). Magnetic anomalies
interpreted as diabase dykes also occur north and northeast of boreholes OL-KR5 and
OL-KR6 (Vaittinen et al. 2001). Two of these dykes, dipping steeply to the NNW, have
been observed in TK8 (Engström 2006) and one of them connected to the diabase
sections at 393.7 - 395.8 m and 398.6 - 399.5 m in borehole OL-KR6 (Gehör et al.
2001). Diabase dykes of this kind also occur in the SE part of the island of Olkiluoto
25
and in the surrounding area (Suominen et al. 1997, Paulamäki et al. in prep.), the
average width of these dykes being 20 cm. The current geochemical, petrological and
U-Pb age data of the dyke in TK3 (Mänttäri et al. 2005) indicate that the Olkiluoto
diabase dykes are probably Subjotnian in age.
26
27
4
WHOLE ROCK CHEMISTRY AND PETROGRAPHY
A rock classification based on texture, migmatite structure and mineral composition is
sufficient to describe the basic attributes of the gneisses and migmatites, but it does not
reveal the possible variation in composition and origin of the protolith materials of those
rocks. For that purpose it is necessary to use information yielded by whole rock
chemical analyses, which enables the supracrustal rocks of Olkiluoto to be divided into
four distinct series or groups: the T series, S series, P series and basic, volcanogenic
gneisses which are syngenetic rock classes of Olkiluoto and belong together on the basis
of their chemical character. In addition to these, pegmatitic granites and diabases form
groups of their own that can be identified both macroscopically and chemically.
The phosphorus and calcium concentrations, their mutual ratios and their ratios to other
elements are the most important variables used to identify the members of the different
groups or series. Ternary plots of calcium, phosphorus and aluminium or titanium
provide one basis for this, as the members of the P series are enriched in phosphorus
and the members of the S series in calcium (Fig 4-1). Differences are also visible in
normal Harker diagrams or variation diagrams that show the element oxide
concentrations versus that of SiO2 (Fig. 4-2). The method based on the evaluation of the
phosphorus and calcium concentrations and their ratios to other major element
concentrations constitutes the most reliable and simplest means of discrimination
allowing most of the rock variants to be distinguished. Intermediate types or samples
from different series, i.e. samples in which the SiO2 content is 65 - 70%, may be
chemically very similar, and exact identification of their host groups or source material
type needs information obtainable only from trace element analysis.
4.1
The T series
The rocks in this group are various veined gneisses and diatexitic gneisses, although
various less migmatitized mica gneisses and quartz gneisses also belong to the series.
Certain TGG gneisses that are typically granitic in modal mineral composition also
show a chemical similarity to the members of the T series, and included in it here
despite the fact that they often resemble granitic, slightly foliated rocks. One typical
feature of the migmatites and mica rich gneisses of this group is the occurrence of
variably pinititized cordierite, and sometimes also a small proportion of sillimanite. The
cordierite porphyroblasts are mostly detectable by the naked eye, since they build up
large crystals typically 5 – 10 mm in diameter. Cordierite is totally absent from the
quartz gneisses of this series and rare in the TGG gneisses.
28
Al2O3/5
4
P2O5
1
0.1
40
CaO
50
P2O5*10
60
70
80
SiO2
A.
B.
4
20
High-Ca
P2O5
CaO
1
10
0.1
Mafic
Low-Ca
0
0.1
1
10
40
40
MgO+Fe2O3
C.
50
60
70
80
SiO2
D.
Explanation for the colours: blue = T-series, orange = S-series, violet = P-series, red =
granite, green = mafic metavolcanic rock and black = metadiabase.
= mafic gneiss (S- or P-series),
= veined gneiss,
= diatexitic gneiss,
Symbols:
= mica gneiss,
= quartz gneiss,
= TGG gneiss,
diabase,
= mafic,
volcanogenic gneiss,
= leucocratic pegmatic granite,
= cordierite bearing
= garnet bearing pegmatitic granite and
= pervasively altered
pegmatitic granite,
gneiss or migmatite. Low-Ca = low calcium subgroup of the S-series, high-Ca = high
calcium subgroup of the S-series and mafic = mafic S type gneiss.
Figure. 4-1. Ternary and binary plots used for chemical classification.
29
20
5
TIO2
AL2O3
4
10
3
2
1
0
40
50
60
70
0
40
80
50
SIO2
60
70
80
70
80
70
80
SIO2
30
20
MGO
FE2O3
20
10
10
0
40
50
60
70
0
40
80
50
SIO2
20
NA2O+K2O
CAO
20
10
0
40
60
SIO2
50
60
SIO2
70
80
10
0
40
50
60
SIO2
Symbols:
= mafic gneiss (S- or P-series),
= veined gneiss,
= diatexitic gneiss,
= mica gneiss,
= quartz gneiss,
= TGG gneiss,
diabase,
= mafic
= leucocratic pegmatitic granite,
= cordierite bearing
metavolcanic rock,
pegmatitic granite, = garnet bearing pegmatitic granite and = penetratively altered
gneiss or migmatite.
Explanation for the colours: blue = T-series, orange = S-series, violet = P-series, red =
granite, green = mafic metavolcanic rock and black = metadiabase.
Figure 4-2. Chemical variation diagrams (Harker diagrams, weight percentage values)
for the rocks of Olkiluoto.
30
4.1.1
Chemical character
The members of the T series constitute a transition series, the end members of which are
relatively dark and often cordierite-bearing mica gneisses and migmatites that may have
less than 60% SiO2. The other end of the scale is represented by quartz gneisses in
which the SiO2 content exceeds 75%. These high-grade metamorphic rocks are assumed
to originate from turbidite-type sedimentary materials, and the end members of that
series have been assumed to be developed from greywacke-type impure sandstones at
the one extreme and from clay mineral-rich pelitic materials at the other. The series
consists of material mixtures in which the proportions of these components may vary
without limitations. The SiO2 content of the most typical members of the T series varies
between 58 and 77%, while the concentrations of the other major elements maintain
practically linear control over that variation. TiO2 decreases from 0.8 to 0.4% following
an increase in SiO2 content, Al2O3 from 19 to 11%, total iron (in the form of Fe2O3)
from 9 to 3%, MgO from 3.5 to 1.5% and K2O from ca. 4 to 2% (Fig. 4-2). The Na2O
content increases from 2 to 3% and CaO from 0.5 to 1.5% with increased silicity. These
compositions are quite typical of corresponding recent (Pettijohn 1975, Ishihama &
Kiminami 2003) and ancient metasedimentary rocks (Kähkönen & Leveinen. 1984,
Bhat et al. 2001) of pelitic and greywacke origin as analysed in a large number of
formations all over the world.
The TGG gneisses comprise a distinguishable subgroup in the T series. Several
similarities in chemical composition can be easily seen between the TGG gneisses and
typical migmatitic members of the T series, but also some slight differences (Fig. 4-2).
The TGG gneisses are often richer in aluminium and alkalis, and their titanium, iron and
magnesium concentrations are lower than in typical T type migmatites and gneisses
with similar SiO2 contents. The differences in chemical composition are most probably
caused by metasomatic alteration, which may have affected the content of every major
element. Thus it is not possible to evaluate the enrichment or depletion factors directly.
The Mg, Fe and Ti concentrations seem to have been reduced in this process, however,
and sodium and probably also silicon to have increased.
Trace element concentrations are very close to the average composition of the upper
crust in every member of the T series, and the variations in composition between the
samples in the series are insignificant. Light REE elements are identical in the
migmatites and TGG gneisses of the series, but the TGG gneisses and certain silicic
migmatites are slightly depleted in the heavier elements from Tb to Lu (Fig 4-3).
Other trace element concentrations may be identical in all the members of the series, but
some show increasing or decreasing trends when moving from the darkest variants to
the lighter ones (Fig 4-3). The concentrations of U Ba, Ce, Tb, Th, Y, Zr, Ba and Hf do
not show any systematic change, but Cs, Rb, Cu, Cr, Ni, Zn and Co decrease with
increasing SiO2 content in the migmatites and fine-grained gneisses. Sr is the only
element that increases in concentration with increasing of silicon dioxide. The most
anomalous concentrations of certain elements have been analysed in the TGG gneisses.
U and Th concentrations in the TGG gneisses are without exception higher than in the
31
corresponding migmatites, while the chalcophile elements, Ni, Zn and Co as well as Cr
are correspondingly depleted.
Fluorine is the only anionic element analysed here that shows a systematic variation
with changes in composition, with concentrations around 1500 ppm in the less silicic
migmatites and gneisses, decreasing to 500 ppm as SiO2 increases to close to 80%. The
concentration of Cl in every sample in the group is roughly 100 ppm, and Br varies
unsystematically between 0 and 3 ppm.
La
La
Ce
Ce
Pr
Pr
Nd
Nd
Sm
Sm
Figure 4-3 (1/3)
B.
1
10
100
600
A.
1
10
100
Eu
Eu
Gd
Gd
Tb
Tb
Dy
Dy
Ho
Ho
Tm Lu
Er Yb
Tm Lu
Er Yb
Sample/N-Type MORB
Sample/N-Type MORB
Sample/C1 Chondrite
Sample/C1 Chondrite
0.1
1
10
100
1000
5000
0.1
1
10
100
1000
5000
Sr
Sr
U
U
K
Cs Th P Nb Zr Ti Yb
Rb Ba Ce Ta Sm Hf Y
Cs Th P Nb Zr Ti Yb
Rb Ba Ce Ta Sm Hf Y
Sample/Upper Crust
Sample/Upper Crust
600
0.01
0.1
1
10
100
0.01
0.1
1
10
100
F Br Cl C
F Br Cl C
S Zn Cu Co Ni Cr Sn
S Zn Cu Co Ni Cr Sn
32
Sample/C1 Chondrite
D.
1
10
100
600
C.
1
10
100
La
La
Ce
Ce
Pr
Pr
Figure 4-3 (2/3)
Sample/C1 Chondrite
Nd
Nd
Sm
Sm
Eu
Eu
Gd
Gd
Tb
Tb
Dy
Dy
Ho
Ho
Tm Lu
Er Yb
Tm Lu
Er Yb
Sample/N-Type MORB
Sample/N-Type MORB
0.1
1
10
100
1000
5000
0.1
1
10
100
1000
5000
U
U
Sr
Sr
Cs Th P Nb Zr Ti Yb
Rb Ba Ce Ta Sm Hf Y
Cs Th P Nb Zr Ti Yb
Rb Ba Ce Ta Sm Hf Y
Sample/Upper Crust
Sample/Upper Crust
600
0.01
0.1
1
10
100
0.01
0.1
1
10
100
F Br Cl C
F Br Cl C
S Zn Cu Co Ni Cr Sn
S Zn Cu Co Ni Cr Sn
33
La
La
Ce
Ce
Pr
Pr
Nd
Nd
Sm
Sm
Eu
Eu
Gd
Gd
Tb
Tb
Dy
Dy
Ho
Ho
Tm Lu
Er Yb
Tm Lu
Er Yb
0.01
0.1
1
10
100
1000
5000
0.01
0.1
1
10
100
1000
5000
Sr
Sr
U
U
K
K
Cs Th P Nb Zr Ti Yb
Rb Ba Ce Ta Sm Hf Y
Cs Th P Nb Zr Ti Yb
Rb Ba Ce Ta Sm Hf Y
0.01
0.1
1
10
100
0.01
0.1
1
10
100
F Br Cl C
F Br Cl C
S Zn Cu Co Ni Cr Sn
S Zn Cu Co Ni Cr Sn
Figure 4-3 (3/3). REE-diagrams and multielement diagrams showing the enricment factors of: A. Gneisses and migmatitic gneisses of the T-series, B.
gneisses of the S-series, C. Gneisses and migmatitic gneisses of the P-series, D. Diabases and mafic metavolcanic rocks, E. Leucocratic pegmatitic
granites and F. Cordierite or garnet bearing pegmatitic granites. Symbols as in the Fig. 4-1.
F.
1
10
100
600
E.
1
10
100
Sample/N-Type MORB
Sample/N-Type MORB
Sample/C1 Chondrite
Sample/C1 Chondrite
Sample/Upper Crust
Sample/Upper Crust
600
34
35
4.1.2
Petrography and mineral paragenesis
The mineral assemblages that are most typical of the migmatites and mica gneisses of
the T series include quartz, plagioclase, K-feldspar, biotite, cordierite and sillimanite.
White mica is often present, but mostly to a minor extent. Quartzitic variants contain
more quartz and feldspars and less biotite. Cordierite and sillimanite are not included in
the quartzitic members of the T series but they are common constituents of the other
members. The TGG gneisses are typically richer in K-feldspar, and cordierite is not
typical in them, sillimanite is always absent from the assemblage, and garnet
porphyroblasts have been encountered sporadically even in the T-type TGG gneisses.
The average modal mineral compositions of the samples analysed so far and their
standard deviations are presented in Table 4-1.
The variation in mineral composition is to a certain extent systematic, being controlled
by the chemical composition and silicity of the samples. Quartz concentrations increase
from ca. 20% in the darkest mica gneisses to almost 50% in the quartzitic variants of the
T series, and plagioclase concentrations similarly increase from 10 to 20%, whereas
those of biotite decrease from ca. 40% in the less silicic mica gneisses to below 20% in
the quartz gneiss types. All the gneiss variants may contain up to 10% K-feldspar, but
the proportion may exceed 30% in the migmatite members. Sillimanite has been
detected in the samples containing less than 70% SiO2 and more than 14% Al2O3, and
the highest concentrations have been measured in the less silicic gneiss and migmatite
variants of the series. Cordierite has also been encountered in the samples that contain
less than 70% SiO2, and consequently more than 1.5 MgO. Cordierite is most abundant,
sometimes exceeding 20%, in the less silicic samples, which contain ca. 3.5% MgO.
Typical diatexitic gneisses are medium-grained and their palaeosomes often show
features of a weak metamorphic banding. Allotriomorphic quartz and plagioclase grains
in the leucocratic bands of the palaeosome mostly vary between 0.5 – 1 mm in diameter,
and the texture of the palaeosome as a whole is granoblastic. Biotite is concentrated, at
least in part, in the 0.5 - 2 mm wide melanocratic bands of the palaeosome, and the
biotite scales do not exceed 1 mm in length. The sillimanite crystals are fibrous, so that
they could be referred to as fibrolite, while the cordierite crystals are larger than those of
the other minerals contained in the diatexitic gneisses, the diameters of which vary
between 2 – 5 mm. The large poikilitic crystals have a lot of small felsic mineral
inclusions. The leucocratic neosome contains only a small amount of mafic minerals
and is more coarse-grained than the other parts of the rock, with an allotriomorphicgranular texture similar to that of pegmatitic granites.
The texture of the leucosome in the veined gneisses is allotriomorphic-granular, and the
banding or penetrative foliation of the palaeosome is typically more prominent than in
the diatexitic gneisses, so that it is often possible to outline features typical of
asymmetric, mylonitic foliations. The average grain size varies between samples from
below 0.5 mm to over 1 mm, and the average grain sizes of the quartz and feldspars
vary more widely, being below 0.5 mm in the finest-grained variants but exceeding 2
mm in the coarsest ones. The cordierite crystals are fairly wide, often exceeding 5 mm
in diameter. The large cordierite crystals (Fig. 4-4) are roundish and contain numerous
small inclusions, whereas sillimanite exists in the form of fibrolite.
36
1
4
3
1
2
2
B.
A.
2
4
1
2
1
5
C.
D.
Figure 4-4. Polarization microscope figures: A. Garnet bearing T type TGG gneiss, B.
Cordierite bearing, banded mica gneiss, C. Cordierite and sillimanite bearing paleosome
of diatexitic gneiss, D. Medium grained and banded paleosome of a veined gneiss.
1 = biotite, 2 = plagioclase, 3 = garnet, 4 = cordierite and 5 = sillimanite, scale bar 1
mm. Figures A and B with one polarizer and C and D with crossed polarizer.
37
The mica gneisses are fairly homogeneous, at least when considering the more
extensive units, but on a microscopic scale they show a distinct metamorphic banding.
The dark, biotite-rich bands are typically 1 mm wide and alternate with wider quartzfeldspar-rich bands. The foliations in the dark bands are not rectilinear, but instead the
biotite-rich seams or bands are somehow wavy, and foliation fish-like structures have
often been found in them. The mica gneisses are medium-grained and the average grain
sizes of individual samples vary between 0.5 and 1 mm. The cordierite porphyroblasts
are fairly large and the sillimanite is fibrolitic, as in the migmatites. The quartz gneisses
do not differ dramatically from the mica gneisses, the only difference having been
detected in the mica concentrations, which are lower in the quartz gneisses, making
them typically even and fine-grained, and not strongly foliated.
The TGG gneisses of the T series make up a coherent group, the members of which are
typically granitic or granodioritic in terms of their QAP ratios (Fig. 4-5). They do not
vary greatly in chemical composition, and the variations in modal mineral composition
are similarly fairly strictly limited (Table 4-1). The TGG gneisses typically contain 510% biotite, and the low biotite content is the only notable mineralogical difference
relative to the other silica-rich members of the T series. A few TGG gneiss variants
contain some garnet, but this cannot be considered a characteristic feature of the
subgroup.
The TGG gneisses of the T series are without exception fairly coarse-grained. The
average grain sizes of the quartz and plagioclase in the TGG gneisses are 1 – 2 mm, and
the biotite scales are also at least 1 mm in length, being larger than in the typical mica
gneisses. The TGG gneisses are always not strongly oriented, but they may be
granoblastic or weakly banded in texture. Some TGG gneisses contain garnet
porphyroblasts exceeding 5 mm in diameter at times.
The chemical compositions of the major minerals in the all rock types or textural
variants of the T series are virtually identical. The plagioclases are oligoclases with an
anorthite content between 15 and 28% (Fig. 4-6), and the biotites are similarly almost
identical, with Al IV numbers, i.e. Al numbers at the tetrahedral site, between 2.24 and
2.47, and Fe/(Fe+Mg) ratios, or Fe numbers, between 0.53 and 0.64, which are typical
of the intermediate micas of biotite group. The only difference can be seen in the
compositions of the micas in the TGG gneisses, which have slightly higher Fe numbers
than the other members of the T series (Fig. 4-6).
38
Q
1.
4.
A
2.
3.
5.
P
Symbols:
= TGG gneiss, blue = T-series, violet = P-series;
= leucocratic
pegmatitic granite;
= cordierite bearing pegmatitic granite and
= garnet bearing
pegmatitic granite.
Figure 4-5. QAP-ratios of pegmatitic granites and TGG gneisses of the T- and P-series.
1 = granite, 2 = granodiorite, 3 = tonalite, 4 = quartz syenite and 5 = quartz
monzonite.
39
1
1
Tremolite Tr Hb
Par
Pargasite
Silicic
Ed
Hbl
Tsch
Edenite
Act
Hbl
Fea
Ferroan
Par
Pargasite
Hbl
Fe
Silicic
Ferro-
Ferro-Edenite
Edenite
Ed
Magnesio-Hbl
Tschermakite
Actinolite
Mg/(Mg+Fe2)
Mg/(Mg+Fe2)
Edenite
Hbl
Hbl
Fe-
Fe-
Ferro-
Ferro-
Fe
Act
Ferro-Hbl
Tsch
Actinolite
FerroHbl Par
Tschermakite
Hbl
Hbl
Pargasite
Hbl
0
8.0
7.5
7.0
6.5
6.0
0
8.0
5.5
7.5
7.0
TSi
6.5
6.0
5.5
TSi
A.
B.
Or
Eastonite
Siderophyllite
3
AlIV
Sanidine
Anorthoclase
AlbiteOligoclaseAndesineLabradorite
BytowniteAnorthite
2
0
1
Phlogopite
Fe/(Fe+Mg)
Ab
Annite
An
C.
D.
Wo
Forsterite
1.0
Fayalite
ChrysoliteHyalosideriteHortonolite
0.9
Ferrohortonolite
Diopside
(Mg/(Fe2+Mg))
0.8
Hedenbergite
Augite
0.7
0.6
0.5
0.4
0.3
0.2
Pigeonite
Clinoenstatite
En
E.
0.1
Clinoferrosillite
0.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Fs
(Fe2/(Fe2+Mg))
F.
Figure 4-6. Mineral classification diagrams for amphiboles (A and B), biotites (C),
plagioclases (D), pyroxenes (E) and olivines (F). Rock type symbols as in the Figure 4-1.
Quartz
Plagioclase
K-feldsp.
Biotite
Muscovite
Hornblende
Pyroxene
Chlorite
Cordierite
Pinite
Garnet
Sillimanite
Epidote
Sphene
Apatite
Saussurite
Sericite
Opaques
Mineral
Migmatitic
gneisses
AVG
STD
30.3
8.1
17.0
8.3
8.6
7.4
22.7
9.9
0.9
2.0
0.1
0.5
0.0
0.1
2.6
5.3
4.0
4.8
5.9
6.1
0.0
0.0
1.8
2.6
0.0
0.1
0.1
0.2
0.1
0.1
3.7
4.9
0.8
1.8
1.0
1.4
T series
Mica
Quartz
gneiss
gneiss
AVG
STD 1-samp
31.1
9.5
44.8
17.3
7.1
21.4
6.4
3.7
16.4
21.2
8.4
14.0
0.7
0.7
0.0
0.3
0.4
0.0
0.1
0.2
0.0
3.8
5.2
0.0
0.9
1.6
0.0
9.2
7.6
0.0
0.1
0.2
0.0
1.3
2.9
0.0
0.2
0.2
0.0
0.2
0.2
0.0
0.2
0.2
0.0
5.8
6.1
3.2
0.3
0.4
0.0
1.6
2.4
0.2
TGG
gneiss
AVG
STD
32.9
3.0
23.2
5.9
20.0
9.0
8.2
7.5
1.0
1.2
0.0
0.1
0.0
0.1
1.7
1.9
0.0
0.1
1.3
2.6
1.2
3.6
0.0
0.1
0.1
0.2
0.1
0.1
0.0
0.1
8.9
5.6
0.6
1.8
0.5
0.6
Table 4-1. Average mineral compositions and standard deviations.
Mafic
gneiss
AVG
STD
4.0
3.2
12.4
8.5
0.2
0.2
1.4
2.0
0.2
0.2
71.1
12.3
0.2
0.2
0.5
0.2
0.2
0.2
0.0
0.0
0.2
0.2
0.0
0.0
0.2
0.2
0.3
0.2
0.2
0.2
7.4
11.5
0.3
0.4
1.7
0.8
S series
Low Ca
gneiss
AVG
STD
46.1
11.1
31.6
10.4
0.3
0.2
15.7
6.5
0.1
0.1
1.0
2.2
0.0
0.1
0.5
0.7
0.4
1.1
0.0
0.0
0.4
0.6
0.0
0.0
0.1
0.2
0.0
0.1
0.2
0.3
1.8
1.7
0.1
0.3
1.2
1.2
High Ca
gneiss
AVG
STD
36.0
10.6
25.7
13.5
0.1
0.2
0.6
0.9
2.1
4.3
7.6
6.3
1.6
3.7
0.3
0.3
0.1
0.1
0.0
0.0
1.4
1.3
0.0
0.0
5.6
4.3
0.9
0.6
0.2
0.3
13.4
19.5
3.1
4.2
1.0
0.9
40
16.5
33.9
0.3
17.1
0.0
26.4
0.0
0.0
0.0
0.0
0.0
0.0
0.2
2.9
3.4
2.8
0.2
1.3
Mineral
Quartz
Plagioclase
K-feldsp.
Biotite
Muscovite
Hornblende
Pyroxene
Chlorite
Cordierite
Pinite
Garnet
Sillimanite
Epidote
Sphene
Apatite
Saussurite
Sericite
Opaques
11.6
8.5
0.3
9.3
0.0
13.6
0.0
0.1
0.0
0.0
0.0
0.0
0.5
3.1
1.2
1.4
0.5
1.5
Mafic
gneiss
AVG
STD
20.8
22.6
6.9
20.2
3.9
4.1
0.1
3.7
0.2
5.5
0.1
0.1
0.1
0.2
2.1
2.3
2.0
0.9
12.7
16.6
8.4
14.3
5.4
9.9
0.1
8.3
0.3
9.9
0.1
0.1
0.1
0.4
3.2
2.8
4.7
1.3
Migmatitic
gneiss
AVG
STD
29.0
35.1
0.2
30.6
0.1
0.1
0.1
0.6
0.1
0.0
0.1
0.4
0.1
0.2
1.6
1.6
0.1
0.9
5.7
6.3
0.2
6.8
0.2
0.2
0.2
0.4
0.2
0.0
0.2
0.9
0.2
0.2
0.7
1.2
0.2
1.0
P-series
Micagneiss
AVG
STD
35.2
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
1.6
0.8
62.2
0.0
0.2
23.7
34.4
11.5
22.5
0.3
0.6
0.1
0.3
0.1
0.1
0.4
0.1
0.1
0.1
1.1
4.5
0.3
0.5
5.7
8.3
10.9
7.1
0.5
2.2
0.2
0.5
0.2
0.2
0.8
0.2
0.2
0.2
1.0
3.7
0.6
0.6
Quartz
TGG
gneiss
gneiss
1 sample AVG
STD
Table 4-1, continued. Average mineral compositions and standard deviations.
35.2
16.7
32.8
0.9
2.5
0.0
0.0
0.7
0.1
0.4
0.4
0.4
0.0
0.0
0.0
7.5
1.2
0.4
AVG
14.3
8.5
17.6
1.7
2.8
0.0
0.0
0.9
0.4
1.7
1.0
1.3
0.2
0.0
0.1
5.9
2.9
0.5
STD
Pegmatitic
granite
9.9
2.3
0.0
23.5
0.0
23.3
3.4
20.6
0.0
0.0
0.0
0.0
0.0
0.0
2.0
0.3
0.0
2.1
14.0
1.6
0.0
16.6
0.0
19.8
3.5
29.1
0.0
0.0
0.0
0.0
0.0
0.0
1.3
0.4
0.0
2.5
Metavolcanics
Mafic
gneiss
AVG
STD
41
42
4.2
The S series
In terms of their textures and mineral compositions, the members of the S series are
quartz gneisses, mica gneisses, various migmatites and mafic gneisses. Homogeneous,
often fine-grained gneiss layers, the thicknesses of which vary from tens of centimetres
to several metres, and concretions or roundish boudins differing in composition from
the host rock represent the dominant types for this series, whereas migmatitic rocks are
relatively rare. The members of this group are assumed to have originated from
calcareous sedimentary materials or to have been affected by other processes that
produced the final skarn-type formations.
4.2.1
Chemical character
The most essential difference between the members of the S series and the other groups
lies in the high calcium concentration of the S-type gneisses (Figs. 4-1 and 4-2),
although relatively low alkali concentrations and high manganese are also typical of this
series. The members of the T series are always poorer in CaO, but this difference is not
necessarily distinct with respect to the P series, so that discrimination has to be based on
other variables, too.
Trace element analyses do not indicate any significant differences between the members
of the S and T series. The REE diagrams are almost identical, and most of the other
trace element concentrations are also similar (Fig. 4-3). Concentrations of Rb, Ba and
Nb in the S-type gneisses are as low as the lowest values found in the T series, or
sometimes even lower, while the Y and Sr concentrations are as high as in the T series,
or sporadically a little higher, but the other concentrations fall exactly in the same range,
and both REE and other trace element concentrations are in general very similar to the
averages for the upper crust.
The members of the S series are thought to be mixtures of materials of which one
resembles the rocks of the T series and the other is richer in calcium. The proportions of
the components can vary without limitations and composition of the former can vary
within limits typical of the T-type source. The residual from the carbonate material in
the final metamorphic product may be pure calcium, and thus the S-type gneisses may
be all calcium-enriched derivatives of gneisses of the T series. The similarity in trace
element concentrations between the members of the T and S series supports this
interpretation.
Calcium concentrations typical of the S series exceed 2%, and maximum concentrations
are more than 13%, while those in the T series are less than 2%. The gneisses and
migmatites of the S and T series that contain between 65% and 78% SiO2 but less than
5% CaO are chemically very similar. In addition to the high calcium concentrations, the
only differences are a slightly higher MnO contents and lower alkali content in the S
type gneisses. These gneisses, which are slightly enriched in calcium, constitute the first
subgroup in the S series, which can be designated as the low-Ca subgroup.
The compositions of the other, non-mafic members of the S series deviate more from
those of the ordinary members of the T series. These gneisses, which make up the
43
second subgroup in the S series, have SiO2 concentrations that exceed 60% and high
calcium concentrations, varying between 6 and 13%, in the form of CaO (Fig. 4-1.D).
This can be called the high-Ca subgroup of the S series, in which a high manganese
concentration and low concentrations of titanium, potassium and sodium are also typical
(Fig. 4-2). One outstanding chemical feature concerns iron and magnesium, in that the
concentration of Fe2O3 is permanently 4 – 5% and that of MgO ca. 0.5%, without any
change brought about by fluctuations in SiO2. Heavy REEs are on average slightly more
enriched in the members of the high-calcium subgroup than in the members of the lowcalcium subgroup (Fig. 4-3), whereas the concentrations of Rb, Zr, Ba and Nb are
typically lower.
The third subgroup of the S series consists of the mafic gneisses. These are also basic, in
view of their low SiO2 content, below 52% (Figs. 4-1 and 4-2). By comparison with the
other members of the S series, the mafic gneisses are richer in titanium, iron and
magnesium, but the differences in other major element concentrations are not
conspicuous. The high calcium and manganese concentrations and comparatively low
titanium and alkali concentrations are similar to those in other members of the S series,
and may thus be said to be distinctive properties of the whole series. The REE diagrams
point to low concentrations of light rare earth elements and the absence of any Eu
anomaly (Fig. 4-3). Other trace element concentrations in the more felsic members of
the S series fall very close to the averages for the upper crust, but the mafic members
are depleted in U, Ce, Th and Hf and enriched in Cu, Cr, Ni and Co. Classifications
intended for the discrimination of igneous rock types are not valid for these rocks, as a
chemical classification based on the concentrations of alkalis versus SiO2 (Fig. 4-8)
shows that the compositions of the mafic S-type gneisses are not comparable with any
actual igneous rock class. The trace element ratios resemble those of certain mafic
rocks, however. The Ti/Mn/P ratios are similar to those of island arc tholeites and the
Zr/Ti versus Nb/Y ratios are similar to those of subalkaline basalts (Fig. 4-8). This may
be an indication of a volcanism-related process that has added magnesium and iron to
the material mixture, which metamorphosed to the S-type mafic gneisses now existing
in the bedrock of Olkiluoto.
4.2.2
Petrography and mineral paragenesis
The members of the S series are in general relatively rich in quartz, and even the most
mafic members contain some quartz. The average quartz content is 46% in the quartz
gneisses and mica gneisses of the low-calcium subgroup, 36% in the gneisses of the
high-calcium subgroup and 4% in the mafic gneisses. The average mineral compositions
of these three major subgroups are presented in Table 4-1.
The characteristic mineral assemblage in the members of the low-calcium subgroup is
quartz, plagioclase and biotite, with or without hornblende and garnet. The members of
the high-calcium subgroup are mostly quartzitic, and their typical mineral sequence
includes quartz, plagioclase, hornblende and garnet. Pyroxene may also be present, but
in a lesser amount. The typical paragenesis in the mafic S-type gneisses is hornblende,
plagioclase, quartz, biotite and sometimes pyroxene, although the latter has been
encountered only sporadically.
44
2
2
1
3
3
4
A.
B.
2
1
4
4
3
C.
D.
Figure 4-7. Polarization microscope figures of S type gneisses. A. Garnet bearing
quartz gness, B. and C. Garnet and hornblende bearing gneisses and D. Mafic biotite
bearing gneiss. 1 = biotite, 2 = plagioclase, 3 = garnet, 4 = hornblende. One
polarizator, scale bar 1 mm in figures A, C and D and 0.5 mm in figure B.
45
15
R
K2O+Na2O
T
10
TA
BTA
TB
5
P
0
BA
B
40
50
D
A
60
70
80
SiO2
A.
TiO2
5
Com/Pant
Phonolite
Zr/TiO2*0.0001
1
OIT
Rhyolite
Trachyte
0.1
Rhyodacite/Dacite
MORB
TrachyAnd
IAT
Andesite
0.01
Bsn/Nph
Andesite/Basalt
OIA
CAB
Alk-Bas
SubAlkaline Basalt
0.001
0.01
0.1
1
Nb/Y
B.
10
MnO*10
P2O5*10
C.
Symbols:
= mafic gneiss (S- or P-series),
= TGG gneiss,
metadiabase,
=
mafic metavolcanic rock,
= leucocratic pegmatitic granite,
= cordierite bearing
pegmatitic granite , = garnet bearing pegmatitic granite and = penetratively altered
gneiss or migmatite.
Explanation for the colours: blue = T-series, orange = S-series, violet = P-series, red =
granite, green = mafic metavolcanic rock and black = metadiabase.
Figure 4-8. Petrogenetic classification diagrams for rocks of igneous and volcanogenic
origin. Explanation for the symbols as in the figure 4-1. A = andesine, B = basalt, BTA
= basatic trachyandesine, BA = basaltic andesine, CAB = calcalkaline basalt, D =
dacite, IAT = island arc tholeite, MORB = mid ocean ridge basalt, OIA = ocean island
andesine, OIT = ocean island tholeite, P = picrite, R = rhyolite, T = trachyte, TA =
trachyandesine and TB = trachybasalt.
46
The mineral compositions of the three subgroups in the S series represent a system
similar to that in the T series, although the individual mineral species are different. The
members of the low-calcium subgroup resemble the most silicic variants of the T series,
and quartz concentrations are 30 – 40% in the gneisses that contain ca. 65% SiO2 and
exceed 60% in the most silicic quartzitic variants. Biotite concentrations are close to
30% in the darkest variants but decrease to below 10% in the purest quartz gneisses of
the subgroup. Plagioclase concentrations vary between 20 and 50% and are in general
higher than those in the corresponding gneiss variants of the T series. K-feldspar is
totally missing, and both hornblende and garnet reach a significant but randomly
variable proportion in the quartzitic samples of this subgroup. The members of the lowcalcium subgroup are typically fine-grained, granoblastic rocks that have no orientation
or a poorly developed penetrative foliation. The quartz grains are roundish, of diameters
typically below 0.5 mm, and the biotite scales are also rather small, typically less than
0.5 mm in length. The garnet crystals are larger, poikiloblastic porphyroblasts, with
typical diameters that do not exceed 1 - 2 mm. The name skarn quartzite is commonly
used for garnet or hornblende-bearing quartz-rich gneiss variants of this kind.
The members of the high-calcium subgroup show an excellent system with respect to
their quartz content, which averages ca. 30% in the gneisses that contain 60% SiO2 but
increases to close to 50% in those that contain 70% SiO2. The proportion of hornblende
is 10 – 20% in the less silicic gneisses, but decreases below 10% in the most silicic
ones. The typical plagioclase content varies randomly between 20 and 50% and small
proportions of biotite and garnet have also been encountered (Table 4-1). These
gneisses are even and fine-grained and typically not markedly oriented (Fig. 4-7). The
hornblende, plagioclase and quartz grains are less than 0.5 mm in diameter and
incidental in shape, typically roundish. Garnet is a typical constituent in these gneisses
and sometimes it exists as poikiloblastic porphyroblasts. The diameters of those vary
mostly between 1 - 2 mm and quartz and plagioclase are the most typical inclusions in
garnet.
The mafic S-type gneisses show moderate variations in mineral composition.
Concentrations of hornblende are between 95 and 55%, plagioclase between 0 and 20%
and quartz below 10%. The trends in these concentrations are evidently controlled by
the SiO2 content. The plagioclase content increases and hornblende decreases
systematically as the silica content increases. Pyroxene has been found in one sample,
and biotite seems to belong to the most silicic mafic gneisses (Fig. 4-7), those which
contain over 50% SiO2.
The most basic gneisses of the S series are pure hornblende gneisses or hornblenditic
rocks that contain only a small amount of quartz and plagioclase in addition to
amphibole. The hornblende grains are non-oriented, relatively large crystals with
average diameters from 2 to 5 mm. These gneisses are granoblastic and medium-grained
rocks, but represent a relatively coarse-grained variant in the sequence of supracrustal
rocks at Olkiluoto. Hornblende exists in the form of allotriomorphic crystals, the
average diameters of which vary between 0.5 and 1 mm, while the quartz and
plagioclase grains are often roundish and rather small, with diameters less than 0.5 mm.
In general, most of the S-type mafic gneisses are medium or fine-grained granoblastic
and moderately oriented.
47
The plagioclases of the mica gneisses in the low-Ca-subgroup of the S series are
andesines with anorthite concentrations slightly above 30% (Fig. 4-6), a little higher
than in the gneisses of the T series. The plagioclases of the mafic gneisses and quartz
gneisses of the high-Ca subgroup are bytownites or pure anorthites, with anorthite
percentages above 85%. The micas analysed so far in the members of the S series are
intermediate biotites, like those in the T series, while the amphiboles found in the
quartzitic and mafic members of the series are magnesiohornblendes that are relatively
poor in alkalis and titanium (Fig. 4-6). The pyroxenes in the S-type mafic gneisses and
quartz gneisses are hedenbergites. The proportion of magnesium in these at site M1
varies between 32 and 39%, and is not markedly controlled by the colour index of the
host rock.
4.3
The P series
The members of this series represent a group, which displays variable textures and
migmatite structures. They are encountered quite often in the bedrock of Olkiluoto, and
include veined gneisses, diatexitic gneisses, TGG gneisses and mica gneisses in which
the proportion of the leucosome is small. The TGG gneisses are numerically the largest
subgroup in the series, whereas less than 15% of the samples studied so far represent
mafic gneisses.
4.3.1
Chemical character
The members of this series stand out from the other series by virtue of their high
phosphorus content (Fig. 4-1). Values exceeding 0.3% are characteristic of the P series,
whereas the other common supracrustal rock types at Olkiluoto contain mostly less than
0.2% P2O5. Certain mafic gneisses and diabases show similar features in their chemical
composition, but their phosphorus concentrations are also lower (Fig. 4-2). Another
feature characteristic of the members of the P series is their comparatively high calcium
concentration, which falls between those of the T and S series.
Like the mafic gneisses of the S series, the mafic P-type gneisses are basic. One
systematic difference between the two groups can be seen by the naked eye, however,
since the mafic gneisses of the P series are typically biotite-bearing, whereas those of
the S series do not contain any notable amounts of micas. The SiO2 content of the mafic
P-type gneisses varies between 42 and 52%, so that they constitute a transition series
between the most basic and most silicic end members. The TiO2 content decreases from
4.5 to 1.5%, Fe2O3 from 13 to ca. 10%, CaO from 15 to 5% and P2O5 from 3.5 to 1% as
the concentration of SiO2 increases from 42 to 52% (Fig. 4-2), while Al2O3 increases
from 10 to 16% and K2O quite linearly from 1 to 3%. Magnesium concentrations do not
change in with the SiO2 content, and the alterations in manganese and iron are also
insignificant.
REE concentrations in these mafic gneisses are systematically higher than in the mafic
S-type gneisses, and the REE diagrams also demonstrate differences in element ratios
between the mafic gneisses of the P and S series. In particular, the diagrams for the P-
48
type gneisses dip more steeply than those for the S-type gneisses (Fig. 4-3). Other trace
element concentrations also deviate between the two series. The Ti, Tb, Ce, Y, Zr, Ba,
Hf, Nb, and Sr concentrations in the members of the P series exceed those in the
members of the S series, but of the situation regarding the Cu, Cr and Ni concentrations
is the reverse (Fig. 4-3). Common chemical classification methods based on major
element concentrations are most probably not applicable to these rocks, although the
alkali versus SiO2 concentration ratios of some of these gneisses are similar to those of
basalts enriched in alkalis (Fig 4-8). It is also the case, however, that the Zr/Ti versus
Nb/Y diagrams demonstrate a similarity between the P-type mafic gneisses and basaltic
rocks (Fig. 4-8), while the Ti/Mn/P ratio resembles that of the oceanic island andesites
(Fig 4-8).
The mica gneisses and migmatites constitute the second subgroup of the P series. These
do not vary widely in chemical composition, and their SiO2 content is limited to the
range 55 - 65%. The most reliable identification marks for this subgroup are again the
concentrations of phosphorus and calcium. The P2O5 concentration is 1.2% in the most
basic members of the subgroup and decreases linearly to below 0.4% in the most silicic
variants. Similarly the CaO concentration decreases from 6 to 2%. Other major element
concentrations, except for aluminium, show similar increasing or decreasing trends (Fig.
4-2), the Al2O3 concentrations remaining permanently between 16 and 17% regardless
of changes in SiO2. REE concentrations remain exactly the same in all the typical
members of this subgroup, excluding the samples with the highest degree of
migmatization and the highest grade of secondary alteration (Fig. 4-3), and the same
similarity is visible in the other trace element concentrations (Fig. 4-3). Concentrations
of the light REEs are enriched in these gneisses and migmatites, as is clearly
demonstrated by comparisons with members of the other series. More steeply
descending REE diagrams and minor Eu anomalies are typical of the P series, and also
provide clear evidence of a different origin from the other series.
The third subgroup in the P series includes the P-type TGG gneisses. The variation in
chemical composition is wider for these than among the mica gneisses and migmatites.
The SiO2 concentration varies between 52 and 71% (Fig. 4-2), and the P2O5
concentrations show a clear trend for a decrease from 1.2% in the most basic members
of the subgroup to below 0.3% in the most silicic members (Fig. 4-2). A similar
decrease is visible in the CaO concentration, from 6% to below 2% as the silicity
increases. Exactly the same trends are visible in the other major element concentrations
in these TGG gneisses and other mica gneisses and migmatites of the P series (Fig. 4-2),
so that it is actually not possible to separate the TGG gneisses from the texturally
different but chemically corresponding members of the P series on the basis of their
chemical composition.
4.3.2
Petrography and mineral paragenesis
As in the case of chemical composition, the gneisses and migmatites of the P series can
be divided into three subgroups on the basis of their mineral composition. The typical
mineral sequence for the P-type mafic gneisses is plagioclase, hornblende, biotite,
quartz with some apatite and sphene (Table 4-1). These gneisses form a subgroup, in
49
which SiO2 content varies between 42 and 52%. The average concentration of quartz
increases from ca. 0% to more than 15% with increasing SiO2 content, plagioclase from
20 to nearly 50% and biotite from a couple of percent to ca. 30%. By contrast, the
average concentration of hornblende decreases from 50% to 10%. Apatite is most
abundant in the most basic mafic gneiss variants, which may contain close to 6%
apatite, whereas its concentrations in the more silicic variants decrease to 2 – 3%.
Sphene concentrations show a similar trend, the members at the basic end of the mafic
gneiss subgroup typically containing ca. 5% sphene, whereas those at the other end of
the sequence contain 1% at most.
The mafic P-type gneisses are granoblastic and not markedly oriented (Fig. 4-9), but
they can show a weak metamorphic banding. It may be said on the basis of the samples
investigated so far that the grain size is controlled by the bulk chemical composition, the
most basic gneisses being fine-grained rocks in which the average hornblende and
plagioclase grain diameter is below 0.3 mm, while the grain size in the more acid,
biotite-bearing gneisses is around 1 mm. Due to their weak orientation, the mafic P-type
gneisses are mostly isotropic in their physical properties
The P-type mica gneisses and migmatites form an intermediate subgroup in the P series
both in chemical and mineralogical terms. The SiO2 content varies within rather tight
limits, being typically between 55 and 62%, while the characteristic mineral paragenesis
is plagioclase, quartz, biotite and apatite (Table 4-1). Hornblende is found only in one
migmatite sample. This hornblende-bearing migmatite represents a transitional type
between the mafic gneisses and typical migmatitic mica gneisses, and thus it could also
be associated with the subgroup of mafic gneisses. K-feldspar belongs to the
leucosomes of the migmatites and most probably represents the outcome of a
migmatization process. The average white mica content is 4%, but in reality this mineral
has been detected in the products of strong secondary alteration or in leucosome
material and similarly does not belong to the typical, primary mineral assemblage of the
mica gneisses of the P series. The compositional variation within the members of this
subgroup is not very wide, but some kind of system can be outlined. The average
concentrations of felsic minerals, quartz and plagioclase increase from ca. 50% in the
less silicic members to 70 – 80% in the most silicic ones, and the average biotite
concentration likewise decreases from ca. 45% in the most mafic members to ca. 25% in
the most silicic or felsic ones. Sphene is enriched in the dark members and high
concentrations of apatite, varying between 1.6 and 2.1%, are typical of the all migmatite
and gneiss members of this subgroup.
The leucosomes in all the samples studied are coarse-grained, the average grain sizes
varying between 3 and 5 mm and the texture of the material being allotriomorphicgranular. The palaeosomes are also relatively coarse-grained in the migmatitic members
of the P series, and biotite scales ca. 2 mm in length are typical. The less migmatitized
mica gneisses in the series are fine-grained and typically not markedly oriented, but they
may show a weak metamorphic banding. The average lengths of the biotite scales are
ca. 0.5 mm, coinciding with the diameters of the roundish quartz and plagioclase grains.
Apatite is a common inclusion in the biotite scales, but typical diameters of individual
apatite crystals are below 0.1 mm. On the whole, the mica gneisses are fine-grained and
moderately oriented granoblastic gneisses (Fig. 4-9).
50
1
3
1
2
2
3
A.
B.
5
1
2
1
2
3
4
C.
D.
Figure 4-9. Polarization microscope figures of P type gneisses. A and B. mica gneisses
C. TGG gneiss and D. Mafic, biotite and titanite bearing gneiss. 1 = biotite/phlogopite,
2 = plagioclase, 3 = apatite, 4 = hornblende and 5 = sphene. One polarizator, scale
bar 0.5 mm.
51
The P-type TGG gneisses represent a fairly wide subgroup in which the SiO2 content
varies between 55 and 70%. The characteristic mineral sequence of these gneisses is
plagioclase, quartz, biotite, K-feldspar and apatite (Table 4-1). Hornblende may be
present in the most basic P-type TGG gneisses, but it does not belong to the
characteristic mineral sequence of this subgroup. Garnet has been detected in several
SiO2-rich TGG gneisses. The average quartz content of the most basic TGG gneisses is
15 – 25%, increasing to 25 – 35% in the most acidic ones. The proportion of K-feldspar
is minimal in the most basic types, but varies between 5 and 25% in the acidic variants.
The biotite content is 30 – 40% in the most mafic or basic variants, but decreases to
below 20% in the most felsic ones. No systematic variation in plagioclase content can
be detected, but instead concentrations can vary between 25 and 55%, without being
regulated by silicity. In terms of QAP ratios, the P-type TGG gneisses fall into the
tonalite, granodiorite or granite fields (Fig. 4-5).
Typical dark P-type TGG gneisses are represented by 27 samples from the study site,
the textures of which are sometimes very similar to those of the medium-grained
palaeosomes of the P-type migmatites (Fig. 4-9). The TGG gneisses are in general
granoblastic, but the intensity of the preferred orientation of the micas is variable. Some
of the TGG gneisses show a clear metamorphic banding, and in these the mafic minerals
are totally segregated into 1 - 2 mm wide melanocratic bands (M bands). The biotite
scales are well oriented in the plane of these bands and typically vary in length between
1 and 2 mm. The mass between the dark seams is composed of felsic minerals, and the
textures of these L-bands are granoblastic. The quartz and feldspar grains within them
are roundish and between 1 and 2 mm in diameter. The other textural group of the TGG
gneisses includes the isotropic, non-orientated, granoblastic gneisses, which are fine or
medium-grained, with an average crystal size of ca. 1 mm.
The anorthite content of the plagioclases in the P-type mafic gneisses varies between 28
and 39%, that of the migmatites between 38 and 49% and that of the TGG gneisses
between 19 and 52%. Anorthiticity is controlled by the Ca concentration in the host
rock. The micas in the mafic gneisses of the P series are more phlogopitic than those in
the T and S series, their average Fe number is 0.4 and their AL IV number is 2.1 – 2.2.
The micas in the migmatites and TGG gneisses are more annitic, with average Fe
numbers of 0.57 and 0.65, respectively (Fig. 4-6). The AL IV numbers and Fe numbers
of the micas are controlled by the composition of the TGG gneisses, but variation in the
whole rock composition of migmatites and mica gneisses is insignificant, as is that in
the composition of the biotites. The amphiboles of the mafic P-type gneisses are similar
to those in the S series, and can be classified as magnesiohornblendes.
4.4
Basic metavolcanics and diabases
Basic metavolcanics and diabases are represented by only four samples that are not
included in the above-mentioned three series. One sample of basic igneous rock is
composed of thoroughly altered diabase and another is also intensively altered and
consists only of secondary, retrograde mineral phases. The mafic gneiss samples
probably come from the same layer or layers that had been penetrated by the drill holes
OL-KR13 and OL-KR17. These basic, probably metavolcanic rocks do resemble the
52
mafic gneisses of the P series in many respects, but they are not identical to them. The
alkali versus SiO2 ratios of these gneisses (Fig. 4-8) show a clear similarity to those of
basalts, and in some cases picrites or picritic basalts (Le Bas 2000). The same tendency
is visible in the trace element concentrations, on which the commonly used petrological
classification methods are based. The ratio of Zr/Ti to SiO2 content and ratio of Nb/Y to
Zr/TiO2 (Fig. 4-8) indicate a similarity between the basalts of alkaline affinity and the
mafic P-type gneisses at Olkiluoto. The high concentrations of MgO, TiO2 and P2O5 are
characteristic chemical features of these basic gneisses or mafic metavolcanics (Fig. 42). In all the samples from Olkiluoto analysed so far the alkali-silica ratios and the
magnesium-alkali ratios are similar to those found in high-magnesium basalts,
picrobasalts and picrites (Le Bas 2000).
REE concentrations are systematically lower in these basic gneisses than in the mafic
gneisses of the P series, but the element ratios and the shapes of the patterns are exactly
the same (Fig. 4-3). The absence of any negative Eu anomaly is typical of these rocks.
The N-MORB normalized multielement diagrams demonstrate an enrichment of LILE
(large ionic lithophile elements) but HFSE concentrations are not markedly enriched
(Fig. 4-3). The chalcophile elements Ni, Co and Cr are enriched in some of the basic
gneisses, as also in the mafic gneisses of the S series. Concentrations of the anionic
elements Cl and Br are not controlled by the lithology, but those of S and F are
systematically as high in these rocks as in the P-type mafic gneisses.
Various petrogenetic classification diagrams presenting trace element ratios for the
mafic volcanogenic gneisses of Olkiluoto (e.g. Fig. 4-8) indicate a subduction-related
setting and volcanic arc affinity. The mobility of the elements, especially of K, Rb and
Cs, during secondary alteration (e.g. Ludden et al. 1982) has to be kept in mind when
evaluating the usefulness of LILE concentrations and ratios as indicators of the origins
of these formations. Indications have been reported in similar picritic and phosphorusrich formations, e.g. the Kisko formation, Orijärvi (Väisänen & Mänttäri 2002), which
is a part of the Svecofennian arc complex in southern Finland, but formations of this
type are not common within supracrustal sequences of Palaeoproterozoic, ca. 1.9 Ga old
domain, as is shown by the dominance of tholeiitic and calc-alkaline formations (see
Perdahl & Frietsch 1993, Kousa et al. 1994, Vaarma & Kähkönen 1994, Hakkarainen
1994, Kähkönen, 1994, Stern et al. 1995, Väisänen & Mänttäri 2002).
The textures of the basic gneisses are medium-grained, granoblastic and not markedly
oriented. The olivine-bearing gneiss variant includes roundish olivine crystals of
diameter around 1 mm (Fig. 4-10). The amphibole crystals are about the same size, and
they are also almost isometric. The proportion of apatite is fairly high, and the apatite
crystals are longish, with diameters of up to 0.2 mm. The phlogopitic mica scales are
wider, however, often close to 5 mm in length, and inclusions of quartz, apatite and
other mafic minerals are common in them. Another mafic gneiss variant is composed
for the most part of amphibole and biotite (Fig. 4-10). It is not powerfully oriented but
is fine-grained, the average grain size being ca. 0.5 mm. The rest of the mafic gneisses
are composed entirely of products of secondary alteration, and in their present form they
are either fine-grained chlorite schists or albite-saussurite rocks.
53
The micas detected in these basic gneisses are anomalous, with a composition close to
that of stoichiometric phlogopites (Fig. 4-6). The Fe number of 0.2 is evidently lower
than in the biotites of other groups, but the AL IV number is similar to that found in the
biotites of the mafic P-type gneisses. The amphibole is richer in alkalis and aluminium
than the amphiboles of the other groups, and can be classified as edenitic hornblende
(Fig. 4-6), while the olivine is fairly rich in magnesium, with a magnesium number of
0.67, and can thus be classified as hyalosiderite.
54
2
4
A.
4
5
1
3
B.
Figure 4-10. Polarization microscope figures of metavolcanic rocks. A. Ultramafic
biotite hornblende gneiss. B. Olivine, hornblende and phlogopite bearing gneiss. 1 =
phlogopite, 2 biotite, 3 = apatite, 4 = hornblende and 5 = olivine. One polarizer, scale
bar 0.5 mm.
55
4.5
Pegmatitic granites
Pegmatitic granites are typically very coarse-grained leucocratic rocks. In addition to
texture, their colour variation and the proportion of garnet and sometimes also cordierite
phenocrysts are the only variables that can be evaluated by the naked eye. Pegmatitic
dykes may contain various amounts of inclusions, the sizes of which can vary from cmscale to wide blocks with diameters of more than ten of metres and composed of all the
kinds of gneisses and migmatites to be found in Olkiluoto. The assimilation of such
material into pegmatitic granites is indicated by various restite particles, mostly biotiterich schlieren. The proportion of assimilated materials naturally has a great impact on
the total composition of the pegmatitic granites.
4.5.1
Whole rock chemistry and petrography
Chemically, all the pegmatitic granites are highly acidic granitoids with a SiO2 content
between 70 and 80% and a total alkali content between 4 and 12%. The total alkali silica ratios of the pegmatitic granites are similar to those of rhyolitic extrusive rocks
(Fig. 4-8). Concentrations of aluminium are relatively high and calcium low,
characteristic figures being between 13 and 16% for Al2O3 and below 1% for CaO. On
this basis, the only consistent chemical properties are a high silica content and a
pronounced peraluminous character (Fig 4-11).
Any more detailed geochemical classification of granitoid rocks is often a problematic
process, and the difficulties are increased by the large grain size. Frost et al. (2001)
proposed a geochemical classification for granitic rocks, employing as the parameters
the Fe number, modified alkali-lime index and aluminium saturation index. The iron
number seems to be useful for classifying the Olkiluoto pegmatitic granites, since a
value of over 0.8 is typical of the garnet-bearing pegmatitic granites whereas that for
other variants is smaller.
REE concentrations are systematically lower in the pegmatitic granites than in the
gneisses and migmatites, while the other trace element concentrations or LILEs are
around the same levels, while no marked differences appear to exist in the
concentrations of chalcophile elements or anionic components. One typical feature for
the REE pattern of every pegmatitic granite type is descending trend for light REEs and
a positive Eu anomaly (Fig. 4-3). The patterns show flat or slightly increasing trends for
heavy REEs, except for the garnet-bearing pegmatitic granites, which are characterized
by steeply increasing trends for those. LIL elements that are enriched relative to HFS
elements and extraordinarily high ratios for the enrichment of Cs and U. As
demonstrated by Frost et al. (2001), the use of trace element concentrations for
classifying coarse-grained granitoid rocks is not always very fruitful, and the same can
be seen in the examination of the Olkiluoto pegmatitic granites. The trace element ratios
for the pegmatitic granites nevertheless indicate generation in a volcanic arc
environment (Fig. 4-11). Leucogranites of this kind have mostly been thought to be
generated by partial melting of metasedimentary rocks as a result of isothermal
decompression in the late stage of orogeny (e.g. England et al. 1984), this interpretation
is most probably also acceptable in the case of the Olkiluoto pegmatitic granites.
56
QAP ratios show the pegmatitic granites to be typically granitic (Fig. 4-5), but it is not
possible to detect any systematic variation in mineral composition that is controlled by
silicity, for example. The average quartz content can vary between 10 and 40%,
plagioclase between 15 and 35% and K-feldspar between 25 and 50%. Muscovite does
not exceed 10%, and a small amount of biotite is present in most pegmatitic granites.
The composition of the biotite detected in a pegmatitic granite corresponds to that of the
intermediate biotites typical of the T-type gneisses, while the anorthite content of the
plagioclases is either around 15% or close to 0% (Fig. 4-6). The latter represents a
thoroughly altered type, and the albitic composition is a product of retrograde
metamorphism.
700
Rhyolite Rhyolite or Dacite
600
Dacite
Dacite or Andesite
400
ANK
Zr
500
Andesite
300
Andesite or KB + D
200
100
Komatiite, Basalt + Dolerite Sills
0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
3.0
2.8 Metaluminous
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0 Peralkaline
0.8
0.6
0.4
0.5
1.0
TiO2
Peraluminous
1.5
2.0
ACNK
2000
1000
1000
Syn-COLG
WPG
WPG
100
Nb
Rb
100
VAG+
Syn-COLG
10
10
VAG
ORG
ORG
1
1
1
10
100
Y+Nb
10002000
1
10
100
10002000
Y
Symbols:
= TGG gneiss, blue = T-series, violet = P-series;
= leucocratic
pegmatitic granite;
= cordierite bearing pegmatitic granite and
= garnet bearing
pegmatitic granite.
Figure 4-11. Petrogenetic classifigation diagrams for the granitoid rocks at Olkiluoto.
57
4.6
Spatial distribution of the members, and volumes of the various
rock series
The pegmatitic granites and diabases are the only lithological units that can be identified
without using any results of instrumental analysis, as identification of the other rock
units requires use of the results of whole rock chemical analyses, for instance. Thus it
has not been possible so far to evaluate the spatial distribution or extent of individual
rock units representing the different series, although it has been possible to make some
approximations of the volumes of the units.
The pegmatitic granites represent 20% of the total length of the drill holes examined so
far at the Olkiluoto site, and it is highly probable that the same percentage of the
Olkiluoto bedrock is composed of pegmatitic granites, since the areal distribution of
pegmatitic granites seems to be random, as are the directions and locations of the drill
holes. Thus the lengths determined on the basis of contact intersections should yield
suitable information for evaluating the proportions by volume of the various rock units
in the bedrock of the site.
The rest of the volume of the bedrock is taken up by migmatites and gneisses. The
above arguments suggest that the proportion of veined gneisses must be 43% of the
volume of the domain, that of stromatic gneisses 0.4% and that of diatexitic gneisses
21%. Mica gneisses make up 7% of the bedrock, mafic gneisses 1% and TGG gneisses
the remaining 8%. These proportions were used when sketching the bedrock map of the
Olkiluoto site.
It is not possible to evaluate exactly the volume proportions of the rock series or the
volumes of formations of the T, S and P series, but some calculations can be made on
the basis of the numbers of samples analysed so far and results of the analyses. On the
assumption that the sample material was selected totally at random, it is possible to
estimate that the final ratio of samples analysed must approach the volume proportions
of individual rock series as the number of samples increases. The total number of
migmatite and gneiss samples analysed so far is 155, and members of the T series
account for 53% of these, the S series for 15% and the P series for 32%. If these figures
are assumed to represent the real proportions of the various rock units, formations
belonging to the T series should comprise 42% of the volume of the central part of the
island of Olkiluoto, the S series 12%, the P series 26% and the various pegmatitic
granites 20%. It is possible, however, that the number of S-type samples may be greater
than the original proportion of S-type formations, as the fact that the members of the S
series obviously differ from those of the other series may have caused the selection of
some extra samples from that group. Selective sampling of the other series seems not to
be likely since they are not very easy to discriminate visually. If the surplus number of S
type samples is taken to be 10, the estimate for the proportion of the T series by volume
can be increased to 46% and that of the P series to 28%, whereas that for the S series
decreases to 7%. These figures provide one estimate of the proportions of the total
bedrock volume accounted for by members of the different lithological series.
58
Drill core OL-KR22 was selected as the target for an experiment aimed at testing the
exact proportions of the formations by volume. The core was sampled at intervals of 5
metres and XRF analysis was used to determine the concentrations of the major
elements and selected trace elements. The result differed markedly from the above
calculations, in that pegmatitic granites represented 8% of the bedrock unit intersected
by the drill hole, S-type gneisses 3%, P-type migmatites 7% and T-type migmatites
82%. The domain intersected by drill hole OL-KR22 is composed for the most part of
highly migmatitized diatexitic gneisses, which can be assumed to belong to the T series.
The experiment nevertheless showed that exact evaluation of the source material types
is possible if such information is required.
59
5
METAMORPHIC MINERAL ASSEMBLAGES AND SECONDARY
ALTERATION PRODUCTS
The gneisses and migmatites of Olkiluoto represent relatively high-grade metamorphic
derivatives of different, mostly supracrustal materials. The members of the T series have
been assumed to be composed of turbidite-like pelitic and arenitic materials, as the most
typical mineral assemblages for the migmatites and gneisses of this series include
quartz, plagioclase, biotite, K-feldspar, cordierite and sillimanite (Fig. 4-4). White mica
may also be present but mostly to a minor extent. The TGG gneisses are typically richer
in K-feldspar, while sillimanite is consistently absent, cordierite is not typical and garnet
has been detected only sporadically.
The Ca-rich members of the S series have been thought to have originated from a
carbonate-bearing source material that developed by metamorphism into the present
skarn-like deposits. A typical mineral paragenesis for the mafic S-type gneisses is
hornblende, plagioclase, quartz and biotite. The gneisses of the low-calcium subgroup
of this series contain quartz, plagioclase and biotite, with or without hornblende and
garnet (Fig. 4-7). The mostly quartzitic members of the high-calcium subgroup include
quartz, plagioclase, hornblende, garnet and sometimes also hedenbergitic pyroxene.
The source material for the metasediments of the P series is bimodal. One component
appears to be some kind of turbidite material similar to the source for T-type
metasediments, while the other most probably comes from picritic-type volcanic
deposits that have been affected by the various sedimentary and chemical enrichment
processes that produced the final sediment material rich in phosphorus. A typical
mineral sequence for the most mafic gneisses of the P series consists of plagioclase,
hornblende, biotite and quartz, with a small amount of apatite and sphene (Fig. 4-9). A
typical paragenesis for the P-type mica gneisses is plagioclase, biotite, quartz and
apatite, and for the TGG gneisses plagioclase, quartz, biotite, K-feldspar and apatite.
The mineral assemblage detected for the T-type gneisses is typical of metapelites
crystallized under conditions that produce the cordierite-biotite-sillimanite-K-feldspar
zone of prograde metamorphism. The lower equilibration temperature for a mineral
paragenesis of that kind in a KFMASH system has been calculated to be 620 - 700oC,
thus exceeding the temperature for the muscovite breakdown reaction, which is thought
to take place at a temperature of ca. 650oC (Cheney & Guidotti 1979, Greenfield et al.
1998, Holland & Powel 1990, Mäkitie 1999, Scrimgeour et al. 2001, Pattison et al.
2002). The presence of sillimanite in the metapelites and of sphene in the mafic gneisses
(Frost et al. 2000) limits the possible pressure range between ca. 3.5 and 5.5 kbar. The
garnet-cordierite-K-feldspar assemblage typical of metapelites at the lowest granulite
facies (Waters 1988) has not been detected in the migmatites of Olkiluoto, although
products of a granulite facies metamorphism have been described in surrounding areas,
e.g. in the Turku migmatite belt (Väisänen & Hölttä 1999). Garnet-bearing assemblages
without cordierite have been detected in mica gneisses, but they have been found in the
Ca-rich members of the P and S series, in which lower Fe-numbers and a garnetsillimanite-biotite assemblage are typical. Muscovite belongs to many migmatite
variants and is associated most closely with their leucosomes. It can be classified as a
60
´late´ muscovite, the crystallization of which took place at an early stage of cooling,
thus representing one of the earliest processes in retrograde evolution (Brown 2002).
Formation of the leucosome and of the wider pegmatitic granite dykes may have been
caused by various processes discussed since the beginning of the last century. The
processes by which migmatite-structured rocks may have been created are: injection of
foreign, usually granitic magmas (Sederholm 1907, 1934), metamorphic differentiation
at subsolidus temperatures (Robin 1979), metasomatism, especially the introduction of
potassium, under subsolidus or hypersolidus conditions (Mish 1968) and partial melting
or anatexis with or without segregation of the initial melt (Holmquist 1921, Winkler
1961). The process can be isochemical on a regional scale, as discussed by Milord et al.
(2001). In the case of Olkiluoto, an isochemical character for the migmatization process
is supported by the similarity in chemical composition between homogeneous gneiss
relicts and the corresponding migmatites, which can contain up to 40 – 50% leucosome.
This means that the loss of anatectic melt from the migmatite system is minimal.
Subsequent injection of granitic magmas from an external source is possible, however,
and most probably at least some of the largest pegmatitic granites of Olkiluoto have
originated in that way.
The leucosomes and pegmatitic granites of the Olkiluoto migmatites fall into two
groups. One ideally includes the leucocratic pegmatitic granites, which may contain a
few mica scales as their only mafic species, and the other includes cordierite and/or
garnet-bearing variants. The origin of such cordierite or garnet-bearing materials has
been discussed by various authors during the last ten years or so, and they have most
commonly been interpreted as products of an incongruent melting reaction in which the
melting of quartz, plagioclase, biotite and sillimanite produced a leucogranite melt with
garnet, ilmenite and cordierite crystals (e.g. Otamendi et al. 2001, Jones & Escher 2002,
Milord et al. 2001).
H2O-fluxed melting, near-isothermal decompression and dehydration melting are the
processes that may have caused the melting of pelitic and greywacke-type materials
under the conditions indicated by the metamorphic mineral assemblages typical of
Olkiluoto. The temperature for dehydration melting of a fluidless system is close to
700oC at a pressure of 3 - 4 kbar, but this temperature will be lowered by an increase in
fluid fugacity (Johnson et al. 2001, Scrimgeour et al. 2001, Thomson 2001). Thus the
temperature capable of producing the migmatite structures and metamorphic mineral
assemblages found at Olkiluoto can be estimated to be ca. 650 – 700 oC, representing
the conditions of the highest amphibolite facies at pressures of ca. 3 – 4 kbars.
The first retrograde mineral phases may be the results of crystallization of the residual
melt, which will exsolve H2O, leading after the peak in metamorphism to a reaction
between K-feldspar, sillimanite and water, producing muscovite and quartz (Brown
2002). Analogous reaction model is also possible for the chloritization of biotite. The
pinitization of cordierite and saussuritization of plagioclase and amphiboles are also
common processes in the retrograde evolution of migmatites of the kind found at
Olkiluoto, representing products equilibrated under conditions of lower degrees of
metamorphism. Subsequent evolution at fairly low temperatures has drastically affected
the mineral composition of the gneisses in certain subdomains.
61
One consequence of this alteration is visible in the mineral composition of the diabases
at this site. The original mafic minerals have been penetratively replaced by
microcrystalline saussurite (epidote, calcite and sericite), and the plagioclase has
recrystallized into pure albite. The degree of sericitization and saussuritization of
feldspars, chloritization of mafic minerals and other types of alteration varies widely.
The hornblende is often quite fresh, but has been totally replaced by secondary mineral
species in a mafic gneiss sample from the site (Fig. 5-1A), while biotite shows mostly a
low or moderate degree of alteration in every rock type at Olkiluoto. The feldspars are
often moderately altered, but quite fresh or totally altered modifications are rare in the
samples studied so far. Cordierite is at least moderately pinititized, but totally altered
variants are numerically most common in the rock types of the T series (Fig 5-1B).
In addition to the above-mentioned rather low temperature phases, certain domains in
which alteration products such as illite and kaolinite account for considerable
proportions of the rock volume occur in the bedrock of Olkiluoto. One example of
alteration of that kind has been detected in a diatexitic gneiss unit of the T series.
Microcrystalline saussurite and fine-grained illite make up the majority of the original
cordierite-bearing mica feldspar gneiss. Kaolinite has been found to replace the ordinary
rock in spots or spheres of a diameter which varies from a couple of mm to more than
10 mm. Penetrative alteration of that kind has a drastic impact on all the physical
properties of the rock material. A study of the products of secondary alteration of this
kind has started recently and the first results will be reported in the near future (Front
2006). Low temperature rock alteration and crystallization of low temperature fracture
infills (Gehör et al. 2002, Gehör et al. 2004) have been found to be caused by the same
geological events, and it should be possible to integrate the results of these studies in
order to obtain a more comprehensive understanding of the latest stages of geological
evolution.
62
2
1
A.
2
3
1
B.
Figure 5-1. Polarization microscope figures: A. T-type mica gneiss in which biotite is
totally chloritized and plagioclase saussuritized, B. T-type gneiss in which cordierite
is altered to pinite, biotite to chlorite and plagioclase to saussussirite. 1 = saussurite,
2 = chlorite, 3 = pinite.
63
6
CONCLUSIONS AND DISCUSSION
The bedrock of Olkiluoto is composed for the most part of various high-grade
metamorphic rocks, the source materials of which are epiclastic and pyroclastic
sedimentary deposits. In addition, leucocratic pegmatitic granites have frequently been
encountered, and also some narrow mafic dykes cut across the bedrock of Olkiluoto.
The rocks of Olkiluoto can be divided in terms of their mineral composition, texture and
migmatite structure into four major classes: 1) gneisses, 2) migmatitic gneisses, 3) TGG
gneisses and 4) pegmatitic granites.
The banded or sometimes homogeneous gneisses include mica-bearing quartz gneisses,
mica gneisses and hornblende or pyroxene-bearing mafic gneisses. The quartz gneisses
are fine-grained, often homogeneous and poorly foliated rocks, which are rich in quartz
and feldspars. Some of these contain amphibole and some also pyroxene and garnet.
The more mica-rich metapelites are mostly intensively migmatitized, but mediumgrained mica gneisses with a considerable proportion of cordierite also occur. Banded or
schistose and only weakly migmatitized mica gneisses make up ca. 6% of the bedrock
and quartz gneisses roughly 1%. Mafic gneisses and schists, called amphibolites,
hornblende gneisses and chlorite schists, with certain exceptional gneiss variants that
contain some pyroxene or olivine in addition to dark mica and hornblende, account for
close to 1% of the bedrock.
The migmatitic gneisses can be subdivided on the basis of their migmatite structures.
Ideal veined gneisses, which make up ca. 43% of the bedrock of Olkiluoto, contain
elongated leucosome veins that show a distinct lineation or axial symmetry and have
roundish quartz-feldspar swellings or augen-like structures. The palaeosome is often
banded and may accommodate products of pronounced shear deformation, e.g.
asymmetric blastomylonitic foliation. Stromatic gneisses, which account for 0.4% of the
bedrock of the site, represent one type of stromatic migmatite, for which the most
characteristic feature is the existence of plane-like, linear leucosome dykes or layers.
The palaeosome is often well foliated and shows a linear metamorphic banding or
schistosity. The name diatexitic gneiss is used for migmatites that show a wider
variation in the properties of migmatite structures that are generally asymmetric and
disorganized. The proportion of the leucosome may exceed 70%, surrounding
palaeosome particles of coincidental shape and variable size. The borders of the
palaeosome fragments are often ambiguous, and the fragments may be almost
indistinguishable in these rocks, which comprise ca. 21% of the bedrock of the central
part of Olkiluoto.
The TGG gneisses are medium-grained, relatively homogeneous rocks that can show a
weak metamorphic banding or blastomylonitic foliation or resemble plutonic, nonfoliated rocks. One type resembles moderately foliated, red granites and another grey,
weakly foliated tonalites. In places they resemble well foliated, banded gneisses that
show features typical of high-grade fault rocks. Up to 20% of the volume of a TGG
gneiss unit may consist of leucosome-like veins, but totally homogeneous variants
without any leucosome or distinguishable paleosome schlieren also occur. The
proportion of TGG gneisses by volume in the central part of Olkiluoto is estimated to be
ca. 8%.
64
The pegmatitic granites are leucocratic, very coarse-grained rocks that sometimes have
large garnet crystals and also tourmaline and cordierite phenocrysts. Mica gneiss
inclusions and xenoliths of variable sizes and amounts are also typical constituents of
the wider pegmatitic granite bodies. Pegmatitic granites comprise ca. 20% of the
bedrock of the central part of the Olkiluoto site.
The supracrustal rocks of Olkiluoto can be divided into four distinct series or groups on
the basis of the whole rock chemical compositions: a T series, S series, P series and
basic, volcanogenic gneisses. The pegmatitic granites and diabases can be identified
both macroscopically and chemically.
Certain quartz gneisses, mica gneisses and various migmatites constitute the T series,
one end of which is represented by relatively dark and often cordierite-bearing gneisses
that contain less than 60% SiO2 and the other by quartz gneisses in which the SiO2
content exceeds 75%. The T-type TGG gneisses belonging to the middle part of the
series and having granitic composition are often more rich in aluminium and alkalis, and
their titanium, iron and magnesium contents are lower than those of other T-type
migmatites. These high-grade metamorphic rocks are assumed to have originated from
turbidite-type sedimentary materials, while the end members of that series are assumed
to have developed from greywacke-type, impure sandstones at one end of the series and
from clay mineral-rich pelitic materials at the other. The most typical mineral
assemblages for the members of the series include quartz, plagioclase, K-feldspar,
cordierite and sillimanite. A lower biotite content is a typical feature of the T-type TGG
gneisses, but otherwise they are similar to the other migmatites and the gneisses of the
series.
The members of the S series are quartz gneisses, mica gneisses, mafic gneisses and
migmatites, the most essential difference between these and the members of the other
groups being their high calcium concentration. Calcium concentrations typical of the S
series exceed 2%, and maximum concentrations are over 13%. A relatively low alkali
contents and high manganese content are also typical, but in other respects the S-type
gneisses are very similar to the T-type ones, containing between 65% and 78% SiO2. In
terms of silicity and calcium content, the S-type gneisses have been classified into lowCa, high-Ca and mafic gneiss subgroups. The characteristic mineral assemblage for the
low-Ca subgroup is quartz, plagioclase and biotite, with or without hornblende and
garnet. The members of the high-Ca subgroup are mostly quartzitic and composed of
quartz, plagioclase, hornblende, garnet and sporadically pyroxene. A typical paragenesis
for the mafic S-type gneisses is hornblende, plagioclase, quartz, biotite and sometimes
pyroxene. The members of this group are assumed to have originated from calcareous
sedimentary materials or have been affected by other processes that produced these
skarn-type formations.
The members of the P series are TGG gneisses, diatexitic gneisses, veined gneisses,
mica gneisses and mafic gneisses in which the proportion of the leucosome is typically
small. The TGG gneisses make up the largest subgroup, while the subgroup of mafic
gneisses is the smallest. The members of this series deviate from the others by virtue of
their high phosphorus content, being characterized by P2O5 concentrations that exceed
65
0.3%, whereas the members of other groups contain less than 0.2% P2O5. Certain mafic
gneisses and diabases may show similar chemical features, but their phosphorus
concentrations are lower. Another characteristic feature of the P series is the
comparatively high calcium concentration, mostly falling between the levels for the T
and S series.
A typical mineral sequence for the P-type mafic gneisses is plagioclase, hornblende,
biotite and quartz, with apatite and sphene. Mica gneisses and migmatitic gneisses make
up an intermediate subgroup, and their characteristic mineral paragenesis is plagioclase,
quartz, biotite and apatite. K-feldspar has been found only in the leucosomes. The Ptype TGG gneisses constitute a fairly large subgroup in which the SiO2 content varies
between 55 and 70%. These have a characteristic mineral sequence comprising
plagioclase, quartz, biotite, K-feldspar and apatite. The differences in composition
between the TGG gneisses and other migmatites and gneisses are most probably caused
by metasomatic alteration.
The basic, probably volcanogenic rocks of Olkiluoto resemble the mafic gneisses of the
P series. High magnesium, alkali, titanium and phosphorus concentrations are
characteristic chemical features of these and are similar to the levels found in highmagnesium basalts, picrobasalts and picrites. The Olivine-bearing basic gneisses
include, in addition to dark mica, roundish olivine crystals of diameters ca. 1 mm and
amphibole crystals of about the same size. Another type of mafic gneiss is composed for
the most part of amphibole and biotite.
The chemical similarity between these basic, picrite-type metavolcanic rocks and the
mafic gneisses of the P series is one indication of some kind of genetic link, in that this
volcanogenic material most probably yielded one component for the source material for
the P-type protolith. The other component seems to have been similar to the turbidites
or the protolith for the intermediate T-type gneisses. Mixing of these materials would
not have been sufficient to produce the final P-type compositions, however, as various
sedimentary or chemical enrichment and depletion processes would have been
necessary premises for their achievement. Thus the most plausible assessment of the
origin of the protolith for the P-type gneisses would include mixing of the abovementioned supracrustal components and subsequent physical and chemical enrichment
processes. The final product was affected by a rather high degree of metamorphism and
some kind of metasomatism, especially in the case of the P-type TGG gneisses. One
appraisal of the abundances of the various gneiss and migmatitic gneiss types produced
from the various protoliths would be that presented in Table 6-1.
66
Table 6-1. An appraisal of the abundances of texture/migmatite structure types
produced from the various protoliths and source materials.
Texture / migmatite
structure type
Mafic gneiss/schist/
diabase
Mica gneiss
Quartz gneiss
Stromatic gneiss
Veined gneisses
Diatexitic gneisses
TGG gneiss
Pegmatitic granites
T series
C
R
R
C
C
C
S series
P series
M
M
M
M
R
R
R
M
R
R
M
R
C
Granite
Mafic dyke
R
C
R = rare, M = moderate, C = common.
The mineral assemblage detected in the T-type gneisses is typical of metapelites of the
cordierite-biotite-sillimanite-K-feldspar zone of prograde metamorphism. The
temperature in such an environment would have exceeded 620 - 700oC. The presence of
sillimanite in the metapelites and sphene in the mafic gneisses limits the possible
pressure range to between ca. 3.5 and 5.5 kbar, and thus, the peak metamorphic
conditions for the Olkiluoto gneisses represent the uppermost amphibolite facies. The
age of the peak metamorphism in adjacent areas is ca. 1824 Ma. Subsequent retrograde
metamorphism and alteration under low temperature conditions took place
simultaneously with the following stages of semi-brittle and brittle deformation. These
processes produced the semi-brittle and brittle fault structures and other fractures, but
also again affected the mineral composition of the bedrock. The products of these events
are detectable nowadays as zones or volumes of abundant fracturing and low
temperature mineral assemblages in which the high-grade mineral phases have been
replaced by illite or kaolinite, for instance. The intrusion of rapakivi-type granites ca.
1583 Ma ago and olivine diabase dykes ca. 1270 – 1250 Ma ago markedly increased the
hydrothermal activity and intensity of low temperature alteration. The period from the
intrusion of the olivine diabases to the present nevertheless represents the longest
individual stage in the evolution of the Olkiluoto bedrock and will certainly have had a
distinguishable impact on its properties.
67
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75
APPENDIX 1
Appendix 1 (File: OL_chem.dbf)
Results of whole rock chemical analyses of ca. 160 samples which have been carried
out in the SGS Minerals Services laboratory, Canada by X-ray fluorescence (XRF)
analyser, neutron activation analyser (NAA), inductively coupled plasma atomic
emission analyser (ICP), inductively coupled plasma mass spectrometer (ICPMS),
sulphur and carbon analyser (LECO)and by using ion specific electrodes (ISE).
Descriptive data columns:
NBR = Number of analysis
HOLE_ID = Drill hole number (OL-KR X)
Length = Drilling length of the sample
Lithology:
DB = Diabase
DGN = Diatexitic gneiss
MFG = Mafic gneiss
MGN = Mica gneiss
PGR = Pegmatitic granite
QGN = Quartz gneiss
SGN = Stromatic gneiss
TGG = Tonalite granodiorite granite gneiss
VGN = Veined gneiss
Origin = Original report
1 = Gehör, S., Kärki, A., Määttä, T., Suoperä, S. & Taikina-aho, O. 1996. Eurajoki,
Olkiluoto: Petrology and low temperature fracture minerals in drill core samples (in
Finnish with an English abstract). Work Report PATU-96-42. Posiva Oy, Helsinki. 300
p.
2 = Gehör, S., Kärki, A., Suoperä, S. & Taikina-aho, O. 1997. Eurajoki, Olkiluoto:
Petrology and low temperature fracture minerals in the OL-KR9 drill core sample (in
Finnish with an English abstract). Work Report 97-09. Posiva Oy, Helsinki. 56 p.
3 = Gehör, S., Kärki, A., Paakkola, J. & Taikina-aho, O. 2000. Eurajoki, Olkiluoto:
Petrology and low temperature fracture minerals in the OL-KR11 drill core sample (in
Finnish with an English abstract). Working Report 2000-27. Posiva Oy, Helsinki,. 87 p.
76
4 = Gehör, S., Kärki., A., Määttä, T. & Taikina-aho, O. 2001.Eurajoen Olkiluodon
kairausnäytteiden OL-KR6, OL-KR7 ja OL-KR12 petrologia ja matalan lämpötilan
rakomineraalit. Work report 2001-38, Posiva Oy, Helsinki.
5= Gehör, S., Kärki, A.& Taikina-aho, O.,2005. Eurajoki, Olkiluoto: Petrology and Low
Temperature Fracture Minerals in Drill Cores OL-KR13, OL-KR14, OL-KR15, OLKR16, OL-KR17 and OL-KR18 Work report (in press). Posiva Oy, Eurajoki.
Elements analysed, methods of analysis, mass units and detection limits:
Element
Method.
SiO2
Al2O3
CaO
MgO
Na2O
K2O
Fe2O3
MnO
TiO2
P2O5
Cr2O3
LOI
Rb
Sr
Y
Zr
Nb
Ba
Co
Cr
Cu
S
Cl
C
F
Ag
Ba
Ce
Co
XRF
XRF
XRF
XRF
XRF
XRF
XRF
XRF
XRF
XRF
XRF
XRF
XRF
XRF
XRF
XRF
XRF
XRF
ICP
ICP
ICP
LECO
ISE
LECO
ISE
ICPMS
ICPMS
ICPMS
ICPMS
Mass unit
(Weight proportion)
%
%
%
%
%
%
%
%
%
%
%
%
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
%
ppm
%
ppm
ppm
ppm
ppm
ppm
Detection limit
of analysis
0.01 %
0.01 %
0.01 %
0.01 %
0.01 %
0.01 %
0.01 %
0.01 %
0.01 %
0.01 %
0.01 %
0.01 %
2 ppm
2 ppm
2 ppm
2 ppm
2 ppm
20 ppm
10 ppm
10 ppm
10 ppm
0.01 %
50 ppm
0.01 %
20 ppm
1 ppm
0.5 ppm
0.1 ppm
0.5 ppm
77
Cs
Cu
Dy
Er
Eu
Ga
Gd
Hf
Ho
La
Lu
Mo
Nb
Nd
Ni
Pb
Pr
Rb
Sm
Sn
Sr
Ta
Tb
Th
Tl
Tm
U
V
W
Y
Yb
Zn
Zr
Br
Cs
Th
U
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
ICPMS
NAA
NAA
NAA
NAA
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
0.1 ppm
5 ppm
0.05 ppm
0.05 ppm
0.05 ppm
1 ppm
0.05 ppm
1 ppm
0.05 ppm
0.1 ppm
0.05 ppm
2 ppm
1 ppm
0.1 ppm
5 ppm
5 ppm
0.05 ppm
0.2 ppm
0.1 ppm
1 ppm
0.1 ppm
0.5 ppm
0.05 ppm
0.1 ppm
0.5 ppm
0.05 ppm
0.05 ppm
5 ppm
1 ppm
0.5 ppm
0.1 ppm
5 ppm
0.5 ppm
0.5 ppm
1 ppm
0.5 ppm
0.5 ppm
1 (1)
LIST OF REPORTS
POSIVA-REPORTS 2006
POSIVA 2006-01
Effects of Salinity and High pH on Crushed Rock and Bentonite
-experimental Work and Modelling
Ulla Vuorinen, Jarmo Lehikoinen, VTT Processes
Ari Luukkonen, VTT Building and Transport
Heini Ervanne, University of Helsinki, Department of Chemistry
May 2006
ISBN 951-652-142-8
116 p.
POSIVA 2006-02
Petrology of Olkiluoto
Aulis Kärki, Kivitieto Oy
Seppo Paulamäki, Geological Survey of Finland
November 2006
ISBN 951-652-143-6
77 p.