Geological Mapping of the Region Surrounding the Olkiluoto Site

Home , Rapakivi granite

Working Report 2007-30

Seppo Paulamäki

May 2007

POSIVA OY FI-27160 OLKILUOTO, FINLAND Tel +358-2-8372 31 Fax +358-2-8372 3709 Working Report 2007-30

Geological Mapping of the Region Surrounding the Olkiluoto Site

Seppo Paulamäki

Geological Survey of Finland

May 2007

Base maps: ©National Land Survey, permission 41/MYY/07

Working Reports contain information on work in progress or pending completion.

The conclusions and viewpoints presented in the report are those of author(s) and do not necessarily coincide with those of Posiva. ABSTRACT

In 2002 and 2003, a geological mapping was carried out in the surroundings of Olkiluoto (referred here as the Olkiluoto region). The study was divided into two parts: (1) the structural geological mapping of the Olkiluoto region, and (2) the geology of the Eurajoki rapakivi stock with special emphasis on the brittle deformation of the rapakivi granite.

The aim of the structural geological mapping was to determine the history of the ductile deformation in the Olkiluoto region. The bedrock of the region shows structural patterns indicating a polyphase deformational history. On the basis of overprinting relationships, five successive ductile deformational phases have been defined.

The oldest structural feature that has been observed is a primary lithological layering, (S0) with the oldest deformational and metamorphic structure, the penetrative, slightly segregated foliation S1 (sub)parallel to the S0. No F1 folds have positively detected. In the next deformation phase D2, earlier structures were overprinted by tight to isoclinal, overturned F2A folds, with the penetrative S2A axial planar foliation. During the main stage of the D2 deformation phase, the earlier structures were overprinted by a foliation and/or segregation banding S2B, associated with abundant partial melting of the metasediments and the production of granite neosome veins. The migmatites thus produced were intensely flattened and sheared sub-conformably to the composite S0/S1/S2 foliation. The neosome veins were isoclinally folded at the same time, producing F2C folds.

In deformation phase D3, the migmatites were folded by F3 folding, producing tight folds with ductile shears. The formation of granitic melts continued during D3 deformation and granitic neosome veins and patches formed parallel to the axial planes, in the hinge zones of F3 folds and subparallel to the S2 foliation. The fold axes are usually gently plunging to the E or NE/SW. The axial plane is upright or overturned to the NW. Overturned fold structures suggest thrust-related deformation during D3. An axial plane foliation is only rarely developed. In the areas of intense D3 deformation, S2 foliation has more or less rotated into parallelism with the F3 axial plane (S3) so that the foliation therein can be described as S2/3 composite structure.

The earlier structures were re-folded and reoriented in the following deformation phase D4 with E-W compression, which produced F4 folds plunging ca. N-S and have vertical axial planes with associated parallel shearing. During D4 deformation, the S2/3 composite structures and F3 fold axes were zonally reoriented towards the trend of F4 axial plane (S4). The influence of D4 is strong in the central part of Olkiluoto, moderate in the areas east and southeast of Olkiluoto and weak in the area south of Olkiluoto. The youngest identifiable ductile structures are F5 folds, with the fold axes plunging mainly towards the SE. They are mostly very open outcrop-size folds or just small flexures.

The Tarkki granite is cut by partly topaz-bearing quartz-porphyry dykes, and fine- to medium-grained granite dykes with an approximate N-S strike. In the present mapping campaign, new types of dykes, not previously described within the Eurajoki stock, were found in the southern part of the porphyritic Väkkärä granite. The dykes are composite, built up during of several different phases of dyke formation.

The western contact of the Eurajoki stock (Tarkki granite) with the Palaeoproterozoic (Svecofennian) migmatitic country rock is probably gently dipping, the dip of the contact being about 20° to the west/northwest. The contact is very sharp and intact. A local, not previously described breccia zone was found near the western contact of the Tarkki granite with the migmatites. The breccia is composed of abundant fragments of migmatitic mica gneiss, 1–30 cm in diameter, lying in the rapakivi granite groundmass. The randomly oriented fragments are partly rounded, partly angular, most of the fragments being, however, more or less elongated.

Greisen veins and associated quartz (-beryl) veins occur both in the Tarkki and Väkkärä granites. In the Tarkki granite, the veins are mostly subvertical and strike E-W or ENE- WSW. In the Väkkärä granite the veins are more randomly oriented, and also lensoid, rounded and irregular greisen bodies occur.

A total of 933 fractures have been investigated throughout the area. Fracture frequencies (fractures/m) were measured across each outcrop or observation point in N–S and E–W traverses. The surface fractures form a very distinct system with two main fracture strikes, N-S and ENE-WSW. Both the Tarkki and Väkkärä rapakivi granites are “sparsely fractured”, the average fracture frequency being 0.7 fractures/m. Some outcrops may be “slightly fractured” (1 – 3 fractures/m) but “abundantly fractured” (>3 fractures/m) rock is rare and only occurs as narrow zones. The fracture frequency is not controlled by rock type, and the frequencies are the same in both the rapakivi granite types. The lengths of mapped fractures in the study area range from the lower cut-off of 1 m to 20 m, only one fracture being longer than 20 m. About 30% of the fractures are visible in their full length, their average length being 3.8 m.

Keywords: Structural geological analysis, ductile deformation, evaluatory model, rapakivi granite, lithology, rapakivi contacts, composite dyke, brittle deformation, nuclear waste disposal, Olkiluoto, Eurajoki, SW Finland Olkiluodon tutkimusalueen ympäristön geologinen kartoitus

TIIVISTELMÄ

Kesien 2002 ja 2003 aikana Geologian tutkimuskeskus teki geologisen kartoituksen Olkiluodon ympäristössä. Työ jakaantui kahteen osaan: (1) Olkiluodon ympäristön duktiilin deformation selvitys ja (2) Eurajoen rapakivigraniitin kartoitus, jossa pääpaino oli hauraan deformaation selvittämisessä.

Tutkimusalueen kallioperän on tulkittu muokkautuneen monivaiheisessa duktiilissa deformaatiossa. Deformaatiovaiheet ja niiden synnyttämät rakenteet on asetettu ikäjärjestykseen keskinäisten leikkaus- ja poimutussuhteiden perusteella. Varhaisimmat tunnistetut rakennepiirteet ovat primääri litologinen kerroksellisuus (S0) ja lievästi segregoitunut liuskeisuus S1, joka on yhdensuuntainen kerroksellisuuden kanssa. Vaiheeseen mahdolllisesti liittyviä poimuja ei ole varmuudella tunnistettu.

Seuraavassa deformaatiovaiheessa D2, varhaisemmat rakenteet poimuttuivat F2A- poimutuksessa, johon liittyy läpikotainen akselitasoliuskeisuus S2A. D2:n päävaiheessa muodostui liuskeisuus ja metamorfinen raitaisuus S2B. Metasedimentit sulivat osittain ja runsaasti graniittineosomisuonia muodostui raitaisuuden suunnassa. Näin muodostuneet migmatiitit puristuivat ja hiertyivät voimakkaasti. Samanaikaisesti neosomisuonet poimuttivat isoklinisesti (F2C).

Deformaatiovaiheessa D3 migmatiitit poimuttuivat F3-poimutuksessa, johon liittyy duktiileja hiertovyöhykkeitä. Graniitisten sulien muodostuminen jatkui ja neosomisuonia ja osueita muodostui poimujen akselitason suuntaisesti, poimujen taivekohtiin ja S2-liuskeisuuden suuntaisesti. Poimuakselit kaatuvat yleensä loivasti itään tai koilliseen/lounaaseen. Akselitasot ovat jyrkkiä tai ylikääntyneitä luoteeseen. Ylikääntyneet poimurakenteet viittaavat ylityöntöön D3:n aikana. Poimutukseen liittyy vain harvoin akselitasoliuskeisuutta, mutta voimakkaan D3-deformaation alueilla S2- liuskeisuus on kääntynyt yhdensuuntaiseksi F3 akselitason (S3) kanssa, jolloin liuskeisuutta siellä voidaan kuvata S2/3 –komposiittirakenteeksi.

Itä-länsisuuntainen puristus seuraavassa deformaatiovaiheessa D4 tuotti F4-poimuja, joiden akselit kaatuvat eteläänpäin, ja joilla on pystyt akselitasot. Varhaisemmat rakenteet poimuttuivat ja suuntautuivat uudelleen kohti F4-akselitason suuntaa (S4). D4- deformaatio esiintyy vyöhykkeellisesti ja sen vaikutus on voimakkainta Olkiluodon keskiosassa. Nuorimpia havaittuja duktiileja rakenteita ovat F5-poimut, jotka kaatuvat pääasiassa kaakkoon. Ne ovat useimmiten hyvin avoimia, koko paljastuman kattavia poimuja tai vain pieniä taipumia.

Eurajoen rapakivi on Laitilan rapakivibatoliitin satelliittimassiivi ja koostuu kahdesta rapakivigraniittityypistä, Tarkin graniitista ja Väkkärän graniitista. Väkkärän graniitti jakaantuu neljään tyyppiin, kontaktityyppi, tasarakeinen tyyppi, porfyyrinen tyyppi ja karkearakeinen tyyppi, jotka eroavat toistaan rakenteen ja /tai mineraalikoostumuksen perusteella. Eri tyyppien välillä on havaittu sekä teräviä että vaihettuvia kontakteja. Tarkin graniitissa on leikkaavia kvartsiporfyyrijuonia, joista jotkut ovat topaasia sisältäviä, sekä hieno-keskirakeisia N-S-suuntaisia graniittijuonia. Tämän kartoituksen yhteydessä löydettiin Väkkärän graniitin eteläosasta monivaiheisia juonia, joita ei ennen ole kuvattu Eurajoen rapakivestä.

Eurajoen rapakiven ja paleoproterotsooisen migmatiitin välinen läntinen kontakti vaikuttaa loivalta (kaade n. 20° länteen/luoteeseen). Kontakti on hyvin terävä ja ehjä. Tässä kartoituksessa löydettiin ennestään tuntematon paikallinen breksia Tarkin graniitin ja migmatiitin kontaktin läheltä. Breksia koostuu 1-30 cm läpimitaltaan olevista, pitkänomaisista, pyöreähköistä tai kulmikkaista migmatiittisen kiillegneissin kappaleista, joilla ei ole mitään selvää suuntausta. Kappaleiden välinen perusmassa on Tarkin graniittia.

Greisenjuonia ja niihin liittyviä kvartsi-beryllijuonia esiintyy sekä Tarkin että Väkkärän graniitissa. Tarkin graniitissa ne ovat enimmäkseen lähes pystyjä ja E-W- tai ENE- WSW-suuntaisia. Väkkärän graniitissa juonten suuntaus on epämääräisempi ja myös linssimäisiä tai epämääräisen muotoisia greisen-kappaleita esiintyy.

Rakokartoituksessa tutkittiin kaikkiaan 933 rakoa. Rakotiheydet mitattiin kohtisuorilta N-S- ja E-W-suuntaisilta linjoilta paljastuman yli. Rapakiven rakoilussa erottuu kaksi selvää päärakosuuntaa, N-S ja ENE-WSW. Molemmat rapakivityypit ovat harva- rakoisia, rakoluvun ollessa 0,7 rakoa/m. Jotkut paljastumat ovat vähärakoisia (1-3 rakoa), mutta runsasta rakoilua (>3 rakoa/m) esiintyy vain kapeissa vyöhykkeissä. Rakotiheydet ovat samat molemmissa rapakivityypeissä. Mitattujen rakojen pituudet vaihtelevat välillä 1 - 20 m (alle 1 m:n rakoja ei mitattu). Noin 30% raoista on koko pituudeltaan näkyvissä, keskimääräisen pituuden ollessa 3,8 m.

Avainsanat: rakennegeologinen analyysi, duktiilideformaatio, kehityshistoria, rapakivigraniitti, litologia, rapakivikontaktit, monivaiheinen juoni, hauras deformaatio, ydinjätteiden sijoitus, Olkiluoto, Eurajoki, Lounais-Suomi PREFACE

This study was carried out under contract to Posiva Oy (orders 9603/02/LIW, 9673/03/LIW and 9768/03/LIW). On behalf of Posiva Oy the work has been supervised by Liisa Wikström and Jussi Mattila, whereas Seppo Paulamäki was the contact person at the Geological Survey of Finland. The study was divided into two parts: (1) the structural geological mapping of the Olkiluoto region, and (2) the geology of the Eurajoki rapakivi stock with special emphasis on the brittle deformation of the rapakivi granite.

The aim of the first part of study was to determine the history of the ductile deformation in the Olkiluoto region. The mapping was performed by Seppo Paulamäki and he is also responsible of the conclusions and viewpoints presented in the report. The second part of the report was written by Seppo Paulamäki on the basis of the field observations of Aimo Kuivamäki and Antero Lindberg in summer 2002, together with earlier studies of the Eurajoki rapakivi stock by Ilmari Haapala (Haapala 1977), with the exception of Chapter 2.5, which was written by Markku Paananen.

The manuscript of part 1 profited from the discussions with Ph. Lic. Matti Pajunen from the Geological Survey of Finland. The author wishes to thank Prof. Alan Geoffrey Milnes of GEA Consulting, Switzerland, and Dr. Timo Kilpeläinen and Dr. Olav Eklund of the University of Turku for their valuable comments, suggestions and corrections. Prof. Milnes is also thanked for the correction of the English of the text. 1

TABLE OF CONTENTS

ABSTRACT TIIVISTELMÄ PREFACE

PART 1: STRUCTURAL EVOLUTION OF THE OLKILUOTO REGION ...... 3 1 INTRODUCTION ...... 5 2 GEOLOGICAL SETTING...... 7 3 ROCK TYPES...... 11 3.1 Supracrustal rocks ...... 11 3.2 Igneous rocks...... 14 3.3 Greisen veins ...... 16 4 METAMORPHISM ...... 19 5 STRUCTURES ...... 21 5.1 Ductile deformation ...... 21 5.1.1 Deformation phase D1 ...... 21 5.1.2 Deformation phase D2 ...... 21 5.1.3 Deformation phase D3 ...... 27 5.1.4 Deformation phase D4 ...... 31 5.1.5 Deformation phase D5 ...... 33 5.1.6 Deformation features of uncertain position in the sequence ...... 34 5.2 Brittle deformation...... 36 6 EVALUATORY MODEL AND DISCUSSION ...... 39 REFERENCES ...... 43 APPENDICES...... 47

PART 2: GEOLOGY OF THE EURAJOKI RAPAKIVI STOCK ...... 55 1 INTRODUCTION ...... 57 2 LITHOLOGY OF THE EURAJOKI STOCK...... 59 2.1 The Tarkki granite ...... 59 2.2 The Väkkärä granite...... 60 2.3 Dykes ...... 61 2.4 The greisen veins...... 64 2.5 Petrophysics ...... 66 3 RAPAKIVI CONTACTS...... 67 4 FRACTURE PROPERTIES ...... 71 4.1 Fracture orientations ...... 71 4.2 Fracture frequencies and length of fractures ...... 74 4.3 Indications of brittle fault movements...... 78 5 SUMMARY ...... 83 REFERENCES ...... 85 APPENDICES...... 87 2 3

Part 1: STRUCTURAL EVOLUTION OF THE OLKILUOTO REGION 4 5

1 INTRODUCTION

In Finland, two companies utilise nuclear energy to generate electric power – Teollisuuden Voima Oy (TVO) and Fortum Power and Heat Oy (formerly Imatran Voima Oy). The companies are preparing for the final disposal of the spent nuclear fuel waste deep in the bedrock. In 1996, they established a joint company, Posiva, to run the programme of site investigations and other research and development for spent fuel disposal. Posiva will ultimately construct and operate the future disposal facility. On the basis of site investigations at several study sites since 1987, Posiva submitted an application to the Government in May 1999 for a Decision in Principle to build a final disposal facility for spent fuel in Olkiluoto, Eurajoki. The Government made a positive decision at the end of 2000, and in May 2001 the Finnish Parliament ratified the Decision in Principle. The construction of the facility should start after 2010, and the operation of the final disposal facility will start in 2020.

In spring and early summer 2002, the Geological Survey of Finland, GTK, carried out a structural geological mapping in the surroundings of Olkiluoto (referred here as the Olkiluoto region) under contract to Posiva (GTK offer no. 312021, Posiva Oy order 9603/02/LIW). The mapping was supplemented in summer 2003 (order 9678/03/LIW). During the structural geological mapping, observations were made in 324 observation points within the study area. In each observation point, a detailed structural analysis was made, where different structures of ductile deformation were placed into a relative age order on the basis of overprinting and crosscutting relationships. Each new generation of structures deforms the structures of a former generation. Figure 1-2 shows an example of such a structural (geometrical) analysis made on an outcrop.

The present report is a structural interpretation of the Olkiluoto region made on the basis of structural analysis in outcrops. In addition to above observations, observations made in 1:100 000 Rauma map-sheet area (Suominen et al. 1993) and in various occasions at Olkiluoto (see Paulamäki et al. 2006) have been used. Also the 1:100 000 low-altitude aeromagnetic maps (App. 4 and 5), as well as the ground geophysical maps of Olkiluoto have been used to support the structural interpretation presented in App. 6). The aim of the structural geological mapping was to determine the history of the ductile deformation in the Olkiluoto region (Fig. 1-1).

The observations and measurements of the tectonic features are listed as Excel tables, which are delivered to the database of Posiva. The original hand-written observations have been delivered to Posiva's archive. The location of the places mentioned in the text and in the figure captions are shown in App. 2 and 3. 6

Figure 1-1. The study area referred to here as the Olkiluoto region. White = the Olkiluoto migmatite complex, red = the Eurajoki rapakivi stock, brown = olivine diabase sills.

Figure 1-2. Examples of a structural analysis in an outcrop scale. A) Foliation S2 is interpreted to be folded by three successive folding phases Fn+1 to Fn+3. Kivi-Reksaari, x = 6789442, y = 1523975. B) Foliation S2B is folded by two successive folding phases, F3 and F4. S3 and S4 are the axial plane traces of F3 and F4, respectively. Olkiluoto, x = 6791357, y = 1525586. The length of the scale is 12 cm.

A B 7

2 GEOLOGICAL SETTING

The crystalline bedrock of Finland is a part of the Precambrian Fennoscandian Shield. The oldest part of it is formed by ca. 3300 - 2500 Ma old Archaean complex, discordantly overlain by 2500 - 2000 Ma old metasediments and metavolcanics, which are cut by 1970 - 2200 Ma diabase dykes. The SW part of Finnish bedrock is composed of Palaeoproterozoic metamorphic and igneous rocks of the Svecofennian Domain, the long history of volcanism, sedimentation and igneous activity of which culminated in the Svecofennian orogeny 1900 – 1800 Ma ago. Later anorogenic rapakivi granites, 1650 – 1540 Ma in age, intruded the crust with associated mafic dyke swarms. The youngest basement rocks in southern Finland are the 1400 – 1300 Ma old Jotnian (Middle Riphean) sandstones cut by Postjotnian (1270 – 1250 Ma) olivine diabase dykes and sills.

The oldest rocks in southern Satakunta area (Fig. 2-1) are Palaeoproterozoic supracrustal rocks, which consist mainly of migmatitic mica gneisses, being deformed and metamorphosed during the Svecofennian orogeny 1900 – 1800 Ma ago (Suominen et al. 1997; Veräjämäki 1998). In the northeastern part of the area they belong to the psammitic migmatite belt with mainly tonalitic to granodioritic neosome, whereas the southwestern part of the area belongs to the pelitic migmatite belt with mainly granitic neosome. Amphibolites, uralite porphyrites and hornblende gneisses, which are derived from mafic and intermediate volcanics, occur as rare narrow zones. The mica gneisses are intruded by 1890 - 1880 Ma old tonalites and granodiorites, occurring conformably with the structures of the mica gneisses (Pietikäinen 1994, Suominen et al. 1997, Veräjämäki 1998). Coarse-grained granites and pegmatites occur as large bodies and are present in older rocks, either as part of the migmatites or as cross-cutting veins.

The Svecofennian rocks are intruded by Mesoproterozoic Laitila rapakivi batholith, about 1580 Ma in age (Vorma 1976), and its satellite massif, the somewhat younger Eurajoki rapakivi, which can be divided into hornblende-bearing Tarkki granite and younger, light-coloured Väkkärä granite (Haapala 1977).

The Jotnian Satakunta sandstone north of the Laitila batholith is interpreted to have been deposited in a deltaic environment (Kohonen et al. 1993). It has been preserved in a NW-SE trending graben structure, which is bordered by subvertical faults. The sedimentary material of the sandstone derives from the Svecofennian supracrustal and plutonic rocks. The thickness of the sandstone is at least 600 m, and probably as much as 1800 m thick (Elo 1976). The sandstone is cut by Postjotnian olivine diabase dykes and sills, 1270 – 1250 Ma in age (Suominen 1991). Aeromagnetic map suggests that these, in turn, are cut by north-south trending swarm of younger diabase dykes (Veräjämäki 1998). Lake Sääksjärvi in the northeastern part of the map area is an impact structure of early Cambrian age.

The Svecofennian Domain is characterised by a high temperature/low pressure metamorphism. The temperature of the metamorphism is estimated at 650 - 700°C and the pressure 4 - 5 kb, corresponding the upper amphibolite facies. The highest 8

temperature of 700 - 800°C was attained in the migmatite areas under pressures of 4 - 6 kb.

In the psammitic migmatite belt the peak temperature of 800 - 670°C and the pressure of 5 - 6 kb were attained at about 1885 Ma during a thermal pulse caused by the intrusion of the granitoids dated at 1889 - 1880 Ma, and the metamorphic evolution ceased soon after (Korsman et al. 1999). Also the pelitic migmatite belt was metamorphosed at the same time but there the older metamorphism is overprinted by younger metamorphism, caused by a new, strong thermal pulse. This metamorphic event took place 1860 - 1810 Ma ago at the peak temperatures of 700 - 800°C and the pressures of 4 - 5 kb, producing abundant potassium granite melts (microcline granites dated at 1840 - 1830 Ma). In the psammitic migmatite belt, for example, in the Vammala area, the younger metamorphism is only locally visible (Kilpeläinen et al. 1994).

The difference in leucosome composition depends mainly on the primary compositions of the sedimentary protoliths (Korsman et al. 1999). In the Al-rich metasedimentary rocks of the pelitic migmatite belt, the breakdown reactions of muscovite and biotite led first to formation of garnet-cordierite gneisses and later to formation of potassium-rich melts. In the psammitic migmatite belt, with alkalis and calcium in excess over aluminium, no breakdown of biotite and muscovite occurred. Potassium was retained in the biotite and therefore the melts were less potassic than those in the pelitic migmatite belt.

The Svecofennian high-temperature, low-pressure metamorphism is attributed to thinning of the tectonically thickened crust and subsequent intra- and underplating, which led to a strong increase in temperature (Korja et al. 1993; Lahtinen 1994; Nironen 1997; Korsman et al. 1999). 9

Figure 2-1. Geological map of the southern Satakunta region (Anttila et al. 1999). The study area is marked with a black frame. 10 11

3 ROCK TYPES

3.1 Supracrustal rocks

The main rock type of the study area is migmatite (App. 1), where the older component, or the palaeosome, is mica gneiss, and the younger component, or the neosome, is mainly granitic in composition. The type and the intensity of the migmatisation vary considerably (Fig. 3-1). According to the nomenclature used in the Olkiluoto site (Mattila 2006), the migmatites can be divided into stromatic gneisses, veined gneisses and diatexitic gneisses (Fig. 3-1). The amount of neosome granite veins is lowest in the NE part of the area.

The mica gneiss palaeosome is mainly composed of quartz, plagioclase, biotite and potassium feldspar. Cordierite, garnet and sillimanite occur as porphyroblasts. Quartz- feldspar segregation banding and abundant neosome veins have in most cases overprinted the primary bedding. However, in places the alternating quartz-feldspar-rich and biotite-rich bands may refer to the original bedding, representing psammitic (sandy) and pelitic (argillaceous) layers, respectively (Fig. 3-3A). In the southern part of the study area, tuffaceous interbeds have been observed within the mica gneiss (Fig. 3-3B) Investigations of the core samples at Olkiluoto suggest a turbiditic origin of the metasediments (Kärki & Paulamäki 2006). Within the strongly migmatised mica gneisses there are occasionally fragments of fine-grained, homogeneous quartz- and feldspar-rich gneiss, which are most likely remains of psammitic layers. In places, for example on the islands of Kuusisenmaa and Pask-Aikko, as well as in some areas in the southwestern part of Olkiluoto, the mica gneisses are rather homogeneous, lacking in any bedding structures and are close to psammitic in composition.

The granite neosome occurs as coarse-grained veins parallel to the foliation and banding. The width of the veins mostly varies from a few millimetres to several centimetres. The veins are often stretched and elongated, forming boudins and pinch- and swell structures. The principal minerals of the neosome are quartz, potassium feldspar and plagioclase. It often contains garnet and cordierite porphyroblasts.

In the northeastern part of the study area, at Orjansaari, Kaunissaari and Vähä- Kaunissaari, and at Hummatus, in the southern part, the neosome is in places tonalitic to granodioritic in composition. At Hummatus, the composition of the neosome seems to be dependent on the composition of the palaeosome. Where the palaeosome is psammitic, the neosome is tonalitic, whereas in the pelitic rocks the neosome is granitic.

The migmatitic mica gneisses often contain zoned, lensoid, skarn-like inclusions, which occur parallel to the foliation and are composed of quartz-, anorthite-rich plagioclase, amphibole and/or diopside (Fig. 3-2C). The inclusions have two to four different rims. The outer rim is always composed of fine-grained, grey, quartz-feldspar-rich mica gneiss, whereas the centre is brownish or greenish in colour and sometimes has small cavities due to weathering of a Ca-rich mineral (amphibole or pyroxene). The inclusions have been interpreted to be fragments of original calcium-bearing interbeds. The zonality is the result of metamorphic reactions. Such interbeds have been found in the 12

weakly migmatised mica gneisses in the northeastern part of the study area (Fig. 3-3D). In Vähä-Kaunissaari, some thin (10 cm wide) calcareous beds can be followed for tens of metres.

The migmatitic mica gneisses also contain small amphibolite xenoliths, which are mafic dykes or intercalations in origin (Fig. 3-3E). Discontinuous sulphide-rich, weathered interbeds are met with occasionally (Fig. 3-3F).

A B

C D

Figure 3-2. Migmatitic mica gneisses of the Olkiluoto region. A) Weakly migmatised pelitic mica gneiss. Taipalmaa, x = 6787629, y = 1525990. B) Stromatic migmatite. Puulunkulma, x = 6787835, y = 1527938. D) Veined gneiss. Pukkiluoto, x = 6789700, y = 1526522. D) Strongly migmatised diatexitic gneiss. Olkiluoto, x =6791357, y = 1525568. The length of the scale is 12 cm. Photos by Seppo Paulamäki, GTK. 13

A B

C D

E F

Figure 3-3. A) Mica gneiss showing alternating quartz-feldspar-rich and biotite-rich bands, which may represent the original bedding. Pask-Aikko, x = 6790476, y =1522312. B) Banded, tuffaceous interbed in the mica gneiss. Hummatus, x = 6787725, y = 1526547. C) Zoned, lensoid, skarn-like inclusion in the diatexitic migmatite. Voitka, x = 6789346, y = 1526972. D) Carbonate-bearing interlayers in the pelitic mica gneiss. Orjansaari N, x = 6791462, y = 1528664. E) Folded amphibolite layer in the migmatite. Siiliö, x = 6788545, y = 1528405. F) Sulphide-rich, weathered interbed. Pukkiluoto S, x = 6789459, y = 1526380. The length of the scale is 12 cm. Photos by Seppo Paulamäki, GTK. 14

3.2 Igneous rocks

Most of the plutonic rocks among the migmatitic mica gneisses are granodiorites, tonalites, granites and pegmatites. The largest intrusions of the area are composed of trondhjemites, tonalites and granodiorites, the contacts between the different rock types being gradual (Suominen et al. 1997). Trondhjemites are medium-grained and massive or weakly foliated. They are cut and brecciated by pegmatite and in places by aplite. The tonalites and granodiorites (Fig. 3-4A) are fine- to medium-grained and practically even-grained, although the grain size of potassium feldspar and plagioclase is usually greater than the other minerals. Biotite, and often hornblende, occur as mafic minerals.

This group of plutonic rocks includes also gneissose granodiorites, which are fine- or medium-grained, almost always hornblende-bearing, and include a few centimetres wide light- and dark-coloured streaks (Suominen et al. 1997). They sometimes resemble coarse-grained mica gneisses but are lacking in the porphyroblasts typical of mica gneisses. However, the texture of the rocks, occasional concretions and remains of amphibolites parallel to the foliation suggest that they are recrystallised paragneisses (Suominen et al. 1997). According to the geochemical investigation of the core samples, the igneous-looking tonalitic, granodioritic and granitic gneisses at Olkiluoto most likely originated from partial melting and recrystallisation of the mica gneisses (Gehör et al. 1996; Kärki & Paulamäki 2006).

Massive, coarse-grained, reddish or pale-coloured granites and pegmatitic granites occur as large uniform intrusions, containing numerous inclusions and granitised restites of the mica gneiss, or as migmatisation-related veins parallel to, or cross-cutting the foliation (Fig. 3-4B and C). They contain garnet and sometimes also cordierite from the assimilated mica gneisses. The chemical composition of the granites resembles the so- called S-type granites, which derive from partial melting of the sedimentary rocks (Suominen et al. 1997; Kärki & Paulamäki 2006).

Possibly most of the pegmatites were intruded coeval with the migmatisation process and can be considered as neosome. There are, however, also younger, narrow pegmatite dykes, which clearly cross-cut the pegmatitic neosome (Fig. 3-4D and E). 15

A B

C D

Figure 3-4. A) Homogeneous gneissose tonalite. Ilavainen, x =6790922, y = 1527486. The length of the scale is 12 cm. B) Granite pegmatite with mica gneiss inclusions. Olkiluoto, x = 6791900, y = 1525320. C) Wide granite pegmatite dyke parallel to the foliation of the mica gneiss. Pask-Aikko, x = 6790476, y = 1522312. D) Pegmatite dyke cross-cutting the diatexitic migmatite. Heikkilä, x = 6789260, y =1530102. E) Pegmatite dyke (outlined with a hatched line) cross-cutting pegmatitic granite leucosome of migmatite. Road cut 5 km E of Olkiluoto, x = 6789260, y = 1530074. Photos by Seppo Paulamäki, GTK.

A few decimetres wide, dense, black diabase dykes are known from various places in the 1:100000 Rauma map sheet area (Suominen et al. 1993). Two new dykes were found during the structural geological mapping in summer 2002 on the island of Liaskari (Fig. 3-5A), southeast of Olkiluoto and on the mainland south of Hankkilanperä. At Olkiluoto, several dykes have been mapped on the basis of ground 16

geophysical data, and verified by drillings and trenching (Paaanen & Kurimo 1990; Paulamäki & Koistinen 1991; Vaittinen et al. 2001; Gehör et al. 2001; Lindberg & Paulamäki 2004; Talikka 2005; Engström 2006). The dykes are subvertical and trending NE-SW or E-W. The dykes are very strongly altered. They consist of lamellar plagioclase (grain size 0.05-0.2 mm), which lies in a very fine-grained matrix apparently mostly consisting of mixture of very fine-grained secondary alteration minerals, which are frequently covered by a brownish pigment making the identification of the individual minerals impossible (Fig. 3-5B). Primary mafic minerals, amphiboles or pyroxenes, are totally replaced by secondary ones. Opaque minerals (mainly hematite) occur as small (0.01-0.05 mm), randomly oriented needles or skeletal minerals. The rock also contains quartz- and carbonate-filled amygdales, 0.01-0.2 mm in diameter. The contacts between the dykes and the host rocks are very sharp. The dyke has somewhat brecciated the host rocks, as shown by host rock fragments within the diabase near the contact. The origin of the altered dykes cannot be determined on the basis of the present chemical composition. However, the trace element composition indicates a basaltic origin (Gehör et al. 2001, Kärki & Paulamäki 2006). The current geochemical, petrological and U-Pb age data of the dyke in TK3 and in the construction site of OL3 (Mänttäri et al. 2005, 2006) indicate that the Olkiluoto diabase dykes are probably Subjotnian in age.

A B

Figure 3-5. A) Diabase dyke cutting the diatexitic migmatite on the island of Liaskari, 1.5 km SE of Olkiluoto, x = 6790164, y = 1526530. The length of the compass is 11 cm. B) Microphoto of the dyke, showing lamellar plagioclase grains and quartz-carbonate filled amygdales in the fine-grained matrix. Nicols uncrossed.

3.3 Greisen veins

In Ilavainen, in the southeastern part of the Olkiluoto island, the influence of the Eurajoki rapakivi stock is manifested by ENE-WSW-trending hydrothermal, greisen- type veins cutting the tonalite and migmatite host rocks. The veins are composed of a few millimetres to a couple of centimetres wide central quartz veins or quartz-filled fractures surrounded by up to 50 cm wide hydrothermal alteration zone (Fig. 3-6A and D). The contacts between the greisen veins and the tonalite are sharp. In the vein, the 17

plagioclase of the host rock is completely altered to sericite and biotite almost totally to chlorite. Tiny rutile needles occur inside biotite and chlorite. According to Haapala (1977) greisen veins and associated quartz veins occur both in the Tarkki and Väkkärä granites of the Eurajoki rapakivi stock. The greisen veins were caused by hot hydrous fluids, migrating in interstices and fractures of the rapakivi granites, and along the fractures in the country rock (Haapala 1977). Fluids emanated from the Väkkärä granite formed the greisen veins in both granites.

C D

Figure 3-6. A and B) Greisen-type veins with central quartz veins in the gneissose tonalite. Ilavainen, SE Olkiluoto, x =6790922, y = 1527486. The length of the scale is 12 cm. Photos by Seppo Paulamäki, GTK. 18 19

4 METAMORPHISM

The description of the metamorphic conditions is based only on very limited sample data and does not aim to be exhaustive. For more detailed description of metamorphism in the Olkiluoto site, the reader is referred to Kärki & Paulamäki (2006). The rocks of the study area represent high-grade (upper amphibolite facies) metamorphism. The typical mineral assemblage of the studied mica gneiss palaeosomes within the study area is quartz-plagioclase-potassium feldspar-biotite-cordierite±sillimanite or quartz- plagioclase-potassium feldspar-biotite-cordierite±garnet. In some of the mica gneiss palaeosomes in the Olkiluoto core samples, both cordierite and sillimanite are absent, and garnet is the only porphyroblastic mineral (Lindberg & Paananen 1991). Cordierite occurs as large (1-10 mm) grains and is always rather strongly pinitised (fine-grained mixture of sericite and chlorite). In places cordierite occurs as large poikiloblasts, containing abundant roundish quartz inclusions. Sillimanite occurs as fibrous or prismatic grains together with biotite in the dark layers of the palaeosome or as small prismatic inclusions in cordierite. In general, no sillimanite has been identified in mica gneisses, where garnet occurs together with cordierite. However, small sillimanite inclusions in cordierite have sometimes been observed in the samples of the present study, and in some of the quartzitic gneisses at Olkiluoto, garnet and sillimanite coexist (Gehör et al. 1996). Garnet grains are almost inclusion-free, containing only a few quartz inclusions.

At least two generations of cordierite have been identified. According to sample 104/SSP/02 from the island of Pukkiluoto in the southern part of the study area, cordierite seems to be crystallized in breakdown of assemblage of sillimanite, biotite and quartz (biotite + sillimanite + quartz = cordierite + potassium feldspar + vapour). In samples 125/SSP/02 and 155/SSP/02 in the NE part of the area, garnet grains are surrounded by cordierite (+ biotite) and in the former sample cordierite-biotite-quartz symplectites occur. Consequently, cordierite is here a product of garnet and formed in a retrograde reaction garnet + potassium feldspar + vapour = cordierite + biotite + quartz. Large, randomly oriented muscovite grains, occurring in some of the samples, are most likely also produced by retrograde metamorphism. Another retrograde reactions include chloritisation of biotite, saussuritisation of plagioclase and pinitisation of cordierite.

No metamorphic zones can be inferred on the basis of studied samples. The occurrence or absence of sillimanite and garnet in the cordierite-bearing mica gneisses is rather due to different compositions of the host rock than difference in metamorphic grade. Furthermore, there seems to be no difference in metamorphic grade between the weakly migmatised mica gneisses and areas with intensely migmatised gneisses (diatexitic gneisses), which contain more than 50% of granitic leucosome veins. Figure 4-1A and B show examples of weakly and intensely migmatised mica gneisses, respectively. However, palaeosomes of both of the samples have the same mineral assemblage, quartz-plagioclase-potassium feldspar-biotite-cordierite, although the grain size is greater in the latter.

In the coarse-grained or pegmatitic granite leucosomes garnet is, in places, voluminous and can be up to a few centimetres in diameter. In addition to garnet, leucosomes may 20

contain sillimanite or cordierite. Garnet and cordierite are products of an incongruent melting of quartz, plagioclase, biotite and sillimanite (e.g. Otamendi & Douce 2001; Milord et al. 2001).

A B

Figure 4-1. A) Weakly migmatised veined gneiss. Olkiluoto, x = 6792795 , y = 1524271. B) Intensely migmatised diatexitic gneiss. Olkiluoto, x = 6791480, y = 1526073. Lens cover (5 cm) for scale. Photos by Seppo Paulamäki, GTK. 21

5 STRUCTURES

5.1 Ductile deformation

The structural history of the supracrustal rocks has been established on the basis of refolding and crosscutting relationships (overprinting). Also the style and orientation of the structures are used in identification of features of different deformation phases. The successive deformation phases have been designated D1, D2, D3, etc. The foliations and the axial planes, fold axes and lineations of each deformation phase have been designated S1, S2, S3, etc., F1, F2, F3, etc. and L1, L2, L3, etc., respectively. The symbol S0 represents the primary lithological layering.

5.1.1 Deformation phase D1

A rarely seen lithological layering, defined by alternating quartz-feldspar-rich and biotite-rich bands (see Fig. 5-2), has been interpreted to be the primary bedding (S0). The earliest identified metamorphic structure is the bedding-parallel biotite foliation (S1), found in the hinge zones of later F2 folds. In places, S1 foliation is slightly segregated (Fig. 5-2B). No F1 folds have been undoubtedly recognized. In places, however, there are isoclinal, intrafolial folds with flame-like fold hinges, which differ from the later folds with more rounded fold hinges (see also Paulamäki & Koistinen 1991).

5.1.2 Deformation phase D2

The deformation phase D2 can be considered to be the main deformational phase of the study area (see Paulamäki & Koistinen 1991, Paulamäki et al. 2006). It is a complex deformational event with several internal phases of development, during which the migmatites attained most of their present appearance. However, the sequence of sub- phases, 2A-2D, must be seen as a continuous evolution without interruptions.

In the early phase of the D2 deformation, the lithological layering S0 and S1 foliation, were folded by tight to isoclinal F2A folds. The S2A foliation is (sub)parallel to the lithological layering and S1 foliation. It can be separated from S1 only in the hinge zones of the early F2A folds, where it is axial planar to the folds (Fig. 5-2). In thin sections the sillimanite inclusions within cordierite porphyroblasts show a different orientation than the trace of the foliation (S2), and manifest the earlier S1 foliation. The sillimanite needles are also folded by F2A, suggesting that sillimanite is D1 or very early D2 in age. Some of the sillimanite grains are parallel to the axial plane of these folds and may represent a later sillimanite generation.

In the next sub-phase the earlier structures were overprinted by S2B foliation defined by strong segregation banding and abundant granite neosome veins were formed conformably to the foliation. This intense deformation and migmatisation more or less destroyed the early structures, which can be seen only in the areas of a weak migmatisation, in the fragments of competent layers within the migmatites, and, 22

microscopically, as sillimanite inclusion trails in cordierite porphyroblasts. S2B foliation is parallel to S0, S1 and S2A, and therefore the dominant foliation in the area can actually be described as a composite foliation S0+S1+S2A+S2B.

Regionally, this composite foliation shows a very general E-W strike varying between ENE-WSW and ESE-WNW (as shown by the S2 form lines on the structural map, Appendix 4). The contoured stereogram (Fig. 5-1A) shows, however, dip directions both to the S or SSE and to the N or NE, in contrast to the rather constant dips and dip directions shown by a compilation of all foliation measurements at the Olkiluoto site (Fig. 5-1). These dip directions are especially common in the SE and E parts (Orjansaari) of the study are near the Sorkka diabase sill and the Eurajoki rapakivi stock, respectively, and in the Hummatus-Taipalmaa area in the south. The foliation pattern at Olkiluoto is due to thrust-related tectonics during deformation phase D3 (see Chapter 5.1.3; Fig. 5-8E), whereas elsewhere the foliation pattern reflects the large- scale folding (F3 or F2), which is rather subhorizontal and more in a upright position (see Fig. 5.8E). In the Orjansaari area, the foliation changes from south-dipping to north- dipping and again to south-dipping over a distance of about 800 m, when moving from north to south. One explanation to this variation could be a large-scale subhorizontal (F2?) fold structure (synform-antiform structure).

A) B)

Figure 5-1. Distribution of foliation orientations. A) Foliation measured in outcrops during the present study (N = 347). For comparison, B) foliation measurements from outcrops and trencehes at Olkiluoto (N = 1549). Schmidt’s equal area, lower hemisphere projection. 23

S1 S2A

S1 S2A F2A

S2B A B

F2A S0/S1 S2A S2A

C D

Figure 5-2. A) Psammitic mica gneiss band in the migmatitic mica gneiss, showing S1, which is folded by isoclinal F2A. S2A is axial planar to F2A. Olkiluoto, x = 6792745 , y = 1525567. B) Segregation banding interpreted as S1 folded by isoclinal F2A. Foliation S2A is axial planar to F2A. Orjansaari, x = 6791133, y= 1529019. C) Quartz-feldspar-rich bands, probably indicating primary lithological layering S0, are tightly to isoclinally folded by F2A with axial planar foliation S2A. Hummatus, x = 6787725, y = 1526547. D) Segregated S1 folded by tight F2A folds with axial planar foliation S2A. Hepoluoto, x = 6789400, y = 1528690. The length of the scale is 12 cm. Photos by Seppo Paulamäki, GTK.

During the D2C sub-phase the migmatites were intensely flattened as indicated by the granite neosome veins and more competent layers showing boudinage or pinch-and- swell structures. During the later stages (sub-phase D2D) the migmatites were sheared sub-conformably with the foliation. The neosome veins were isoclinally folded, the F2C folds characteristically having thick hinges and attenuated limbs (Fig. 5-3). The sense- of-shear can be deduced, for instant, from the shape of the porphyroblasts, porphyroclasts, boudins and pinch-and swell structures, which have modified into asymmetric lenses (Fig. 5-4).

In addition to localised syn-D2 shearing, which can be noticed all through the area, there are in places distinct shear zones, but due to scanty observation data, the width and length of the zones is difficult to determine. In the western shore of the island of Kivi- Reksaari, a few metres wide D2 shear zone folded during D3 can be followed along the shoreline for about 250 metres (Fig. 5-5). At Olkiluoto, an exposure of a schollen migmatitic mica gneiss, interpreted as shear zone, occurs near the power plant. The rock 24

is characterized by shearing, strong granitisation and the occurrence of abundant potassium feldspar porphyroclasts/large megacrysts (Fig. 5-5B and C). It also contains fragments of mica gneiss, which show early F2 folds indicating that the shearing occurred in the later stages of D2 deformation (or in D3?) (Fig. 3-5C). The width of this foliation-parallel late D2 shear zone is at least six metres and it has a sharp contact with fresh, rather homogeneous mica gneiss. In SW part of Olkiluoto late D2 shear zone occurs, which is cut by narrow shear zones associated with the successive F3 folding (see below) (Fig. 5-5D).

Investigations at Olkiluoto have suggested that in the latest stages of the D2 deformation, some fragmentation of the migmatitic gneisses occurred and these fragmented blocks were rotated (Paulamäki & Koistinen 1991). No such features were, however, identified during the present study.

A B

C D

Figure 5-3. A) Pinch-and-swell structured syn-D2 granite neosome veins. Voitka, x = 6788808, y = 1526194. B) Boudinaged syn-D2 granite neosome vein. Pask-Aikko, x = 6790476, y =1522312. C) Syn-D2 granite neosome veins folded by isoclinal late F2C fold. Kivi-Reksaari, x = 6790006, y = 1523290. D) Isoclinally folded (F2C) syn-D2 neosome veins. Taipalmaa, x = 6787035, y = 1527095. The length of the scale is 12 cm. Photos by Seppo Paulamäki, GTK. 25

A B

C D

E F

Figure 5-4. A) Granitic vein in the mica gneiss is dextrally sheared to form asymmetric boudins. Kaunissaari, x = 6792379, y = 152898. B) Rotated boudins of granitic veins in the mica gneiss showing a sinistral sense of shear. Vähä-Kaunissaari, x = 6791607 , y = 1529185. C) Quartz-feldspar vein and rotated garnet porphyroblasts within the amphibolite interlayer showing dextral sense of shear. Kanni, x = 6788692, y = 1528898. D) Psammitic fragment in the diatexitic migmatite showing dextral sense of shear. Voitka, x = 6788802 , y = 1526758. E) Sheared diatexitic migmatite with boudined syn-D2 granite neosome showing sinistral sense of shear. Ilavainen, x = 6790166, y = 1526988. F) Sheared veined gneiss with elongated/boudined granite neosome veins folded by isoclinal late F2 folds. Rotated swells (boudins) show dextral sense of shear. Kaunissaari, x = 6792379, y = 1528989. The length of the scale is 12 cm. Photos by Seppo Paulamäki, GTK. 26

A B

F2A

D3 shear zones C D

Figure 5-5. A) D2 shear zone in a diatexitic migmatite folded by F3. Kivi-Reksaari, x = 6790006, y = 1523290. B) Schollen migmatitic gneiss, late D2 shear zone with dextral sense-of-shear. Olkiluoto, x = 6792195, y = 1524276. C) Fragment of mica gneiss within the shear zone showing early F2 folds. Olkiluoto, x = 6792195, y = 1524276. D) Foliation-parallel late D2 shearing cut perpendicularly by narrow D3 shear zones. Olkiluoto, x = 6791480, y = 1526113. The length of the scale is 12 cm in A-C and 21 cm in D. Photos by Seppo Paulamäki, GTK. 27

5.1.3 Deformation phase D3

The next deformation phase, D3, is characterised by F3 folds with generally NE-trending axes (Fig. 3-6, contoured stereogram). The uncontoured pole diagram, however, shows a wide distribution of axes plunging towards the NE (between N and E) and also a small number to the SW. The F3 folds are asymmetrical, open to tight, in places chevron-type folds with generally gentle fold axes (Fig 5-7 and 5-8). The variation in the plunge direction indicates dome-and-basin structures (probably due to the influence of later folds belonging to deformation phase D4), although such structures have only rarely been observed from outcrops. On outcrop surfaces, the axial planar traces of the F3 folds have often an acute angle in respect to the trend of the S2 foliation. However, in the areas of intense D3 deformation, the composite foliation S2 has more or less rotated into parallelism with the F3 axial planes (S3) so that the foliation in these areas or zones can be described as an S2/3 composite structure.

The axial planes of the F3 folds are generally subvertical or steeply dipping to the SE (Fig. 5-8D) but in the eastern part of the study area near the contact between the migmatitic mica gneiss and the Eurajoki rapakivi stock, folds with gently dipping axial planes overturned towards the W occur (Fig. 5-8E). On the island of Pask-Aikko, a recumbent F3 fold has been observed (Fig. 5-8F). The wavelength and the amplitude of the folds in the outcrops vary from a few centimetres to several metres (Fig. 3-7 and 3- 8). Axial plane foliation is rarely developed, but can sometimes be seen as a crenulation cleavage in the cores of F3 folds (Fig. 5-9A).

Figure 5-6. Distribution of F3 fold axis orientations measured in outcrops (N = 164). Schmidt’s equal area, lower hemisphere projection.

A characteristic feature of F3 folding is the occurrence of garnet-bearing granite pegmatite veins parallel to the axial plane or as irregular patches in the hinge areas of the F3 folds indicating continuation of the migmatisation during D3 (Fig. 3-9B). Similar granite pegmatite veins also occur parallel to the S2 foliation planes, indicating that the emplacement of the D3 granite melt was also controlled by pre-existing structures. The folding phase is also characterized by narrow, both dextral and sinistral shear zones along the fold limbs and subparallel to the axial plane (Fig. 3-9C and D). In places the axial planar shearing is so intense that the whole D3 structure seems to be "broken" between the ductile shear zones (late-D3 shearing?) (Fig. 3-9E and F). 28

F3 A B F2A

C D

F2C

E F

Figure 5-7. Outcrop photos of F3 folding. A) Foliation S2B outlined by metamorphic quartz-feldspar segregation banding and conformable granite veins folded by F2 are refolded by tight to open F3 folds. Kivi-Reksaari, x =6789296, y = 1524162. B) Amphibolitic layer or dyke in the migmatite folded by isoclinal F2 and refolded by F3. Ilavainen, x = 6790330, y = 1526897. C) Tight F3 folds in the migmatite. Kivi-Reksaari, x = 6789968, y = 1523199. D) Banded mica gneiss folded by F3. Pask-Aikko, x = 6790432, y = 1522105. E) Amphibolitic layer in the migmatite folded by F3. Narrow granite vein subparallel to the axial plane. Siiliö, x = 6788545, y = 1528405. F) Granite neosome veins folded by F2C are refolded by tight F3. Pegmatitic granite also in the hinge zone of F3 fold on the left. Voitka, x = 6788808, y = 1526194. The length of the scale in photos A-C is 12 cm. Photos by Seppo Paulamäki, GTK. 29

A B

C D

E F

Figure 5-8. A) and B) Larger-scale F3 folds folding the banded mica gneiss palaeosome and the pegmatitic granite neosome. Pask-Aikko, x = 6790432, y = 1522105. The hammer marks the trend of the fold axis in A. B) C) Larger-scale F3 fold folding the psammitic mica gneiss palaeosome and pegmatitic granite neosome. Kuusisenmaa, x = 6791800, y = 1522020. D) Upright F3 folds. Taipalinenmaa, x = 6789492 , y = 1525920. E) Overturned F3 fold. Road-cut 5 km E of Olkiluoto, x = 6789189, y = 1530308. F) Recumbent F3 fold. Pask-Aikko, x = 6790432, y = 1522105. Number scale (12 cm) and hammer (60 cm) for scale. Photos by Seppo Paulamäki, GTK. 30

A B

C D

E F

Figure 5-9. A) F3 fold with S3 crenulation cleavage. Siiliö, x = 6788618, y = 1528347. B) Granite pegmatite vein subparallel to the F3 axial plane. Olkiluoto, x = 6791950, y = 1523365. C) Shearing along the limbs of the F3 folds. Pukkiluoto, x = 6789691, y = 1526142. D) Narrow shear zones parallel to the axial planes of the F3 folds. Taipalmaa, x = 6787070, y = 1527128. E) Migmatitic gneiss "broken" by intense D3 shearing. Pukkiluoto, x = 6789688, y = 1526270. F) Migmatitic gneiss with intense D3 shearing. Voitka, x = 6789137, y = 1526672. The length of the scale is 12 cm in A, C, D, E and F and 21 cm in B. Photos by Seppo Paulamäki, GTK. 31

5.1.4 Deformation phase D4

The folds of deformation phase D4 plunge ca. S (Fig. 5-10) and have subvertical N-S striking axial planes. The folds differ from F3 folds in being more open. An axial planar foliation has not been observed. Shearing parallel to the axial plane has been observed at Olkiluoto (Paulamäki & Koistinen 1991). It is possible that the shear zone described in Paulamäki (2005) is in fact a D4 shear zone, supposing that the folds within the shear zone are F3 instead of F2 as suggested in the report. On the outcrop scale, F4 folds can be seen to influence F3 axial traces, causing them to be strongly curved (Fig. 5-11A, C). At Olkiluoto, the S2/3 foliation has zonally been re-oriented towards the F4 axial plane (S4) (Paulamäki et al. 2006). F4 folds have also recognised within the pegmatitic granite. It is uncertain, whether partial melting and melt intrusion has occurred in association with this deformation phase. However, pale-coloured or reddish coarse- grained granite veins trending N-S or NW-SE do occur, which may be related to F4 folding (Fig. 5-11E). In the northwestern shore of the island of Kivi-Reksaari, F3 folded migmatites are cut by a coarse-grained sillimanite-bearing granite dyke (Fig 5-11F). Sillimanite occurs, together with biotite, as narrow ribbons, which are strongly folded. The dyke is 10-15 cm wide, subvertical and trending N-S but the association with F4 folding is speculative, since no F4 folding has been detected in that particular outcrop. Sillimanite-bearing pegmatite granites have formerly been reported from the drill cores of Olkiluoto (Lindberg & Paananen 1991, Gehör et al. 2000, 2001), but it is difficult to say if they are similar to this dyke.

Figure 5-10. Distribution of F4 fold axes orientations measured in outcrops (N = 38). Schmidt’s equal area, lower hemisphere projection. 32

F4 F4

F4 F3

A A B

C D

E F

Figure 5-11. Outcrop photos of F4 folding. A) S2 foliation folded by tight F3 fold, which is refolded by open F4 fold. Kivi-Reksaari, x = 6789442, y = 1523975. B) S2 foliation folded by tight F3 that is refolded by more open F4. Liaskari, x = 6790214, y = 1526640. C) F3 folded migmatite refolded by open F4. Pask-Aikko, x = 6790432, y = 1522105. D) Very open F4 fold in the slightly migmatised, banded mica gneiss. Orjansaari, x = 6791462, y = 1528664. E) N-S striking granite pegmatite vein in the mica gneiss possibly associated with the F4 folding. Vähä-Kaunisaari, x = 6791607, y = 1529185. F) N-S striking coarse-grained granite cutting the migmatite folded by F3. Kivi-Reksaari, x = 6790052, y = 1523736. The length of the scale is 12 cm. Photos by Seppo Paulamäki, GTK. 33

5.1.5 Deformation phase D5

Particularly at Olkiluoto, the youngest identifiable ductile structures are F5 folds, with the fold axes plunging mainly towards the SE (Fig. 5-12, see also Paulamäki & Koistinen 1991). They are mostly very open outcrop-size folds (Fig 5-13) or just small flexures. The folding is often not visible in outcrops as distinct folds, but can be inferred from the curving of F3 and F4 axial traces and F3 shear zones.

Figure 5-12. Distribution of F5 fold axes orientations measured in outcrops during the present study (N = 17). Schmidt’s equal area, lower hemisphere projection.

Figure 5-13. Open F5 fold covering the whole outcrop. Olkiluoto, x = 6791365, y = 1525690. The length of the scale is 21 cm. Photo by Seppo Paulamäki, GTK. 34

5.1.6 Deformation features of uncertain position in the sequence

Under this heading a few features are described, which cannot be directly tied to any specific deformation phase.

On the island of Reksaari and in the western part of Ilavainen, two 1-3 m wide NE-SW striking granitised zones occur parallel to the F3 axial planes, but clearly post-dating the F3 folding (Fig. 5-14A). They are characterized by strong granitisation (K- metasomatism?) and occurrence of large potassium feldspar phenocrysts. On the basis of just two observations nothing precise can be said of their position in the deformational succession. As the migmatite fragments within the zones show F3 folds (Fig. 3-14A), they are tentatively presented as late-D3 zones.

Similar-looking, potassium feldspar porphyritic zones also occur on the island of Pukkiluoto, where they cut the F3 folding (Fig. 5-14B). At Olkiluoto, 30-50 cm wide zone of potassium feldspar porphyric rock cuts F3 folding and may be associated with D4 (Fig. 5-14C).

In the northern shore of the island of Pukkiluoto, four 1-8 cm wide medium-grained granitic veins occur, which are subvertical and strike NW-SE (Fig 5-14D). The veins contain potassium feldspar phenocrysts and the contacts against the migmatite host rock are indistinctive. The granitic material seems to have originated from the migmatitic mica gneiss host rock and not from the outside source (K-metasomatic?). They clearly post-date the F3 folding (Fig. 5-14D) but nothing precise can be said of their position in the deformational sequence.

In the southern part of the study area, in Taipalmaa, several 0.5-2 cm wide medium- grained granitic veins occur, which are trending NE-SW and have indistinctive contacts against the migmatite host rock (Fig. 5-14E). 35

A B C

D E

Figure 5-14. A) Granite showing potassium feldspar phenocrysts and fragments of F3 folded migmatitic mica gneiss. Ilavainen, x = 6790579, y = 1526765. B) Potassium feldspar porphyric granite cutting the migmatite folded by F3. Pukkiluoto, x = 6789695, y = 1526471. C) Potassium feldspar porphyric granite sharply cutting the migmatite with F3. Olkiluoto, x = 6792560, y = 1525865. D) Granitic vein in the migmatite cutting F3 folding. Pukkiluoto, x = 6789688, y = 1526270. E) Granitic veins cutting the migmatitic mica gneiss. Hummatus, x = 6787725, y = 1526547. The length of the scale is 12 cm in A, B, D and E, and 21 cm in C. Photos by Seppo Paulamäki, GTK. 36

5.2 Brittle deformation

Since the main objective of the structural geological mapping was the ductile deformation, no systematic fracture mapping comparable with that carried out at the Olkiluoto site was carried out during the fieldwork. However, on each outcrop fracturing was observed in relation to the structures of the ductile deformation.

As in Olkiluoto (cf. e.g. Anttila et al. 1999), the usual pattern is that the traces of the fractures in one main fracture set trend E-W, subparallel to the trace of the main foliation S2, and the traces of a second set are perpendicular to the first. The traces of a third fracture set intersect these at an oblique angle (NE-SW) (Fig. 5-15). In many of the outcrops, however, it is this fracture trace trend, which is parallel to the foliation traces, since the F3 folding has rotated the latter from ca. E-W to NE-SW. In some outcrops, the fracture traces of this third fracture set are parallel to the axial plane traces of the F3 folds (see Fig. 5-16A).

A) B)

Figure 5-15. A) Distribution of fracture trace orientations measured on outcrops during the present study. B) Photograph of an outcrop showing the main fracture trace orientations. Kaunissaari, x = 6792376, y = 1528837. The length of the hammer is 60 cm.

It has been noticed in the outcrops that the fractures in the wider pegmatitic granite veins or more competent layers with the migmatites are often restricted to these veins and layers and do not continue to the country rock (Fig. 5-16B).

The nature of the magnetic and morphological lineaments presented in Kuivamäki (2005) and Paulamäki et al. (2002) could not be resolved in the outcrop mapping. A few of the interpreted lineaments could be seen as morphological features in the field but 37

they were covered with a thick overburden. The main lineament trend both in the magnetic and morphological interpretations is NW-SE but that trend is only poorly visible in the fracture data. The same applies to the fracturing data from the Olkiluoto site. It is noteworthy, however, that at Olkiluoto the observed brittle faults are show this direction (cf. Paulamäki & Koistinen 1991). The NW-SE trending lineaments have been interpreted to be associated with the emplacement of the Laitila rapakivi batholith.

The observed diabase dykes at Olkiluoto region strike mainly NE-SW or ENE-SSW and dip to the NW or NNW. A question arises whether these dykes have used the existing fracture network or have they created new fractures during the intrusion. The vast majority of the fractures at Olkiluoto with these strikes dip to the SE or SSE. There are, however, fractures with the same strike but which dip to the opposite direction (see e.g. Lindberg & Paulamäki 2004; Paulamäki 2005; Engström 2006). It is possible that the latter fracturing forms a fracture network separate from the former, and that this fracturing and the intrusion of the diabase dykes are interconnected. It is interesting that the recently discovered composite dykes within the Eurajoki rapakivi stock (see part 2 of this report) have the same orientation than the diabase dykes, and that this orientation is one of the main fracture orientations within the rapakivi granite.

In the west of Hankkila two sets of semi-ductile/semi-brittle dextral shears, trending NW-SE and N-S, have been observed (Fig. 5-16C). The age relation between the shears is rather unclear but it seems that the latter set is cutting the former one.

Brittle faulting associated with the fracturing seems to be rare or any displacement is too small to be identified. During the present mapping, only a few brittle faults have been observed. In the west of Hankkila, NNW-SSE striking quartz filled fracture (vein) shows an apparent dextral strike-slip faulting (Fig. 5-16D). In Hankkilanperä, quartz- feldspar veins in amphibolite are apparently displaced dextrally by NNW-SSE striking feldspar-filled fracture veins (Fig. 5-16E). Brittle faults with the same orientation have formerly been observed at Olkiluoto (Fig. 5-16F) (see Paulamäki & Koistinen 1991). 38

A B

C D

E F

Figure 5-16. A) Fractures parallel to the axial plane of the F3 fold trending NE-SW. Voitka, x = 6788808, y = 1526194. B) Fractures restricted to the granitic veins or more competent psammitic layers within the migmatite. Voitka, x = 6788785, y = 1526205. C) Semi-brittle shears cutting the migmatite. Hankkila W, x = 6788129, y = 1530082. D) Migmatite faulted along a quartz-filled fracture, with an apparent dextral sense of movement. Hankkila W, x = 6788191, y = 1530120. E) Feldspar-filled fracture displacing quartz-feldspar veins, with an apparent dextral sense of movement. Neither type of vein is displaced by a later open fracture. Hankkilanperä, x = 6789392, y = 1528618. F) Brittle fault displacing the mafic dyke within the tonalitic gneiss with an apparent sinistral sense of movement. Olkiluoto, x = 6791835, y = 1526715. The length of the scale is 12 cm in A-E and 21 cm in F. Photos by Seppo Paulamäki, GTK. 39

6 EVALUATORY MODEL AND DISCUSSION

The bedrock of the region surrounding Olkiluoto, with the exception of the diabases, shows structural patterns indicating a polyphase deformational history. On the basis of overprinting relationships, five successive ductile deformational phases have been defined at the Olkiluoto region. The interpretations are summarised in Appendix 4 as a map of structural trends.

The oldest structural feature that has been observed in the area is a primary lithological layering, which is interpreted a bedding (S0) with the oldest deformational and metamorphic structure, the penetrative, slightly segregated foliation S1 (sub)parallel to the S0. No F1 folds have positively detected. The importance of this early deformation phase is difficult to evaluate, because the structures have been detected only sporadically.

At the beginning of the next deformation phase D2, earlier structures were overprinted by tight to isoclinal, overturned F2A folds, to which the penetrative S2A biotite foliation is axial planar. The S2A foliation can be separated from S1 only in fold hinges; elsewhere they are subparallel. During the main stage of the D2 deformation phase, the earlier structures were overprinted by a foliation and/or segregation banding S2B, associated with abundant partial melting and the production of neosome veins. The migmatites thus produced were intensely flattened at a later stage, as indicated by the boudinage or pinch-and-swell structures shown by the granite neosome veins and more competent layers. During the later stages of D2 deformation the migmatites were sheared sub- conformably to the composite S0/S1/S2 foliation. The neosome veins were isoclinally folded at the same time, producing F2C folds with characteristically thick hinges and attenuated limbs. In the waning stage of the D2 deformation, some fragmentation of the migmatitic gneisses and rotation of the fragmented blocks occurred.

Only outcrop-scale F2 folds have been observed. The scanty observations of the F2 folding do not allow much discussion of the nature of the folding in the Olkiluoto region. Recumbent F2 folds have been observed both in the Uusikaupunki and Turku regions south of the Olkiluoto area, suggesting over- or underthrusting towards the northwest during D2 (Selonen & Ehlers 1998, Väisänen & Hölttä 1999). However, no major thrust zones have been identified in either of the regions.

A lower bound to the age of the D2 deformation is given by the age of 1890-1870 Ma obtained from the tonalitic to granodioritic granitoids (Suominen 1991, Vaasjoki 1996), which were deformed during D2 and later during D3 deformation (Selonen & Ehlers 1998, Väisänen & Hölttä 1999). Selonen & Ehlers (1998) interpret the foliation of the Uusikaupunki trondhjemite as S2 and suggest that trondhjemite intruded as gently dipping sheets before or during the D2 deformation. According to Nironen (1999) the tonalites and granodiorites in the Loimaa area, ca. 80 km southeast of Olkiluoto, were intruded before the D2 deformation or more likely during the early stages of the deformation at 1890 - 1880 Ma (Nironen 1999). 40

The rocks were metamorphosed for the first time during D2, but this early metamorphism is overprinted by a younger metamorphism dated at 1860-1810 Ma (Korsman et al. 1999).

At Olkiluoto, the recent datings has given an U-Pb age of 1863±3 Ma for the tonalitic gneiss of Ulkopää, which hosts the repository of low-and intermediate level waste (Mänttäri et al. 2006). A similar age is reported by Suominen et al. (1997) from the Lähteenmäki tonalite ca. 16 km SE of Olkiluoto in the Rauma 1:100 000 map sheet area. The tonalitic gneiss is cut by folded pegmatite granite dykes, which have a maximum age of 1865±8 Ma and a minimum age of 1823±3 Ma (Mänttäri et al. 2006). The tonalitic gneiss exhibits a penetrative foliation, which is axial planar to the folded pegmatite dykes. If the interpretation, that the foliation is S2 and the folding F2, is correct, the age of the D2 deformation at Olkiluoto is younger than in the Loimaa area and close to coeval with the D2 deformation in the Turku area. Moreover, this interpretation ensues that the pegmatitic granite dykes were intruded before or during the early phases of the D2 deformation, and that the maximum age is the age of the rock, whereas the minimum age marks the age of the later metamorphism (in D3?) (see Mänttäri et al. (2006). Also the monazite age 1813±4 Ma of the tonalitic gneiss is a metamorphic age (Mänttäri et al., op. cit).

Väisänen (2002) relates the D1/D2 deformation in SW Finland to a collision between two Svecofennian arc complexes and associated metamorphism, which would have taken place at 1.88-1.86 Ma. The age of the Olkiluoto tonalitic gneiss and the structural interpretation within it fits well to this scenario.

In deformation phase D3, the migmatites were folded by F3 folding, producing tight folds with ductile shears. The formation of granitic melts due to partial melting of the metasediments, which began already during D2, continued during D3 deformation and granitic neosome veins and patches formed parallel to the axial planes, in the hinge zones of F3 folds and subparallel to the S2 foliation. During D3 the earlier horizontal structures were folded into subvertical position by a horizontal folding with vertical axial plane and probably an original east-west plunge. The fold axes are usually gently plunging to the E or NE/SW (Appendix 4). The axial plane is upright or overturned to the NW. Overturned fold structures suggest thrust-related deformation during D3. At Olkiluoto, the occurrence of the diatexitic migmatite unit upon the veined gneisses, has interpreted to be due to an overthrusting to the NW (Paulamäki et al. 2006). An axial plane foliation is only rarely developed. However, in the areas of intense D3 deformation, S2 foliation has more or less rotated into parallelism with the F3 axial plane (S3) so that the foliation therein can be described as S2/3 composite structure.

Assumptions of the timing of D3 deformation are variable but, on the other hand, the D3 structures are not necessarily contemporary everywhere. At Loimaa, for instant, the age of the deformation phase is determined by the syn-D3 Pöytyä granodiorite, which is dated at 1870 Ma (Nironen 1999). In the Turku area, the late orogenic potassium granites dated 1840 - 1830 Ma were emplaced during late D3 (Väisänen & Hölttä 1999). Moreover, Väisänen et al. (2002) report an age of 1824±5 Ma for garnet and cordierite- bearing leucosome intruding along the axial plane of regional F3 folding in SW Finland, constraining the age of the F3 folding and regional anatexis. Väisänen et al. (2000) have 41

obtained an U-Pb age of 1814±3 Ma for garnet bearing S-type granite in Masku in SW Finland. Lindroos et al. (1996) have suggested that the D3 deformation in southern Finland could have lasted until 1805 Ma.

At present, there are no age data for the pegmatitic leucosomes related to D3. However, considering the above data from the Turku area, it seems likely that the young metamorphic ages for the tonalitic gneiss and pegmatitic granite in Ulkopää, 1813 and 1823 Ma, respectively, manifest a thermal pulse during D3. In the Turku area the peak of metamorphism is dated at 1824±5 Ma (Väisänen et al. 2000; Väisänen 2002). Väisänen (2002) relates D3 deformation to the period of crustal shortening of the Svecofennian orogeny.

The earlier structures were re-folded and reoriented in the following deformation phase D4 with E-W compression, which produced F4 folds plunging ca. S and have vertical axial planes with associated parallel shearing. During D4 deformation, the S2/3 composite structures and F3 fold axes were zonally reoriented towards the trend of F4 axial plane (S4). The influence of D4 is strong in the central part of Olkiluoto, moderate in the areas east and southeast of Olkiluoto and weak in the area south of Olkiluoto (Appendix 4).

At Loimaa, the ductile folding comparable to the deformation phase D4 varies from open to tight, the axial plane striking north-south (Nironen 1999). According to Nirone (op. cit.) the deformation probably happened within the period 1850 - 1800 Ma. In the Turku area, D4 structures are a few tens of metres wide local shear zones, with the general trend of N-S to NNE-SSW, and they may not be comparable to the D4 deformation in the Olkiluoto region. According to Väisänen et al. (1994) and Väisänen & Hölttä (1999) they may be associated with the post-orogenic granite-intrusions dated at 1770 - 1815 Ma. 42 43

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APPENDICES

Appendix 1: Lithology of the Olkiluoto region (Suominen et al. 1993)

Appendix 2: Location of the places mentioned in the text and figure captions

Appendix 3: Location of the figures

Appendix 4: Low-altitude aeromagnetic map of the Olkiluoto region; illumination from the NE

Appendix 5: Low-altitude aeromagnetic map (vertical gradient) of the Olkiluoto region

Appendix 6: Preliminary structural interpretation of the Olkiluoto region; general trends of the planar ductile deformation elements. 48 49 50 51 52 53 54 55

Part 2: GEOLOGY OF THE EURAJOKI RAPAKIVI STOCK 56 57

1 INTRODUCTION

The aim of the mapping campaign in the Eurajoki stock carried out in 2002 was to investigate all the outcrops within chosen profiles across the whole stock and its contacts (Fig. 1-1). The profiles were chosen so that they cover most of the outcrops. The profile in the contact between the Eurajoki stock and the Laitila rapakivi batholith could not be investigated, because the existed outcrops were covered during the road construction. In the mapping, special emphasis was put on the fracturing of the different rapakivi granite varieties of the stock, the greisen veins cutting the rapakivi granites and the nature and dip of the contact between the rapakivi granites and the migmatitic mica gneiss country rock. During the mapping of the contact, the possible occurrence of the contact metamorphism was also investigated.

A total of 122 outcrops were investigated during the mapping campaign in 2002. The position of each outcrop was determined with GPS equipment. During the fracture mapping all fractures, equal to, or longer than, one metre were investigated. Dip direction and dip, rock type, length, form (straight or curved), type (tight, open or filled), width and infilling, where present, were recorded for each fracture (a total of 933 fractures). Fracture orientations were analysed using Schmidt equal-area, lower hemisphere stereographic projections and rose diagrams. In the diagrams, the north is the magnetic north (geographic north is +5° from magnetic north). In addition, orientation data are also presented using rose diagrams. Fracture frequencies (fractures/m) were measured across each outcrop or observation point in N–S and E–W traverses.

The measured fractures and other tectonic features are listed as Excel-tables, which are delivered to the database of Posiva. The original hand-written observations have been delivered to Posiva's archive.

Figure 1-1. Mapped profiles across the Eurajoki stock and its contacts. Blue circles = observation points. 58 59

2 LITHOLOGY OF THE EURAJOKI STOCK

The Eurajoki rapakivi stock is a satellite massif to the large Laitila rapakivi batholith and is composed of two main rapakivi granite types: hornblende-biotite granite (the Tarkki granite) and topaz-bearing microcline-albite granite (the Väkkärä granite) (Haapala 1977, Suominen et al. 1997). The age of the Laitila batholith is 1583±3 million years, the Tarkki and Väkkärä granite being somewhat younger, 1571±3 and 1548±3 Ma, respectively (Vaasjoki 1996). The petrographic description of the Eurajoki stock is mostly based on the detailed work of Ilmari Haapala (Haapala 1977), unless otherwise cited. The map presented in Appendix 1 is based on Haapala (1977), Suominen et al. (1993), and the observations of Aimo Kuivamäki and Antero Lindberg in 2002.

2.1 The Tarkki granite

The main minerals of the homogenous, medium- and even-grained Tarkki granite (Appendix 1) are alkali feldspar, quartz, plagioclase, biotite and hornblende (Haapala 1977). In its mineral composition, the Tarkki granite is similar to the main rapakivi type of the Laitila batholith. The Tarkki granite contains alkali feldspar ovoids, 3 - 6 cm in diameter, mantled with plagioclase (Fig. 2-1A). The ovoids are sparsely distributed, the distance between individual ovoids being often several metres. Myrmekite and rims of Na-plagioclase are common at the grain boundaries between plagioclase and alkali feldspar.

A dark variety of the Tarkki granite occurs near (50 m) the rapakivi/migmatite contact, containing abundant mafic minerals and inclusions (Fig. 2-1B).

A B

Figure 2-1. A) Alkali feldspar ovoid with a plagioclase mantle within the medium- and even-grained Tarkki rapakivi granite. Location coordinates: x = 6791419, y = 1531202. B) Mafic inclusions (magmatic enclaves?) and an alkali feldspar megacryst within the dark variety of the Tarkki rapakivi granite. Location coordinates: x = 6789912, y = 1530631.The length of the scale is 21 cm. Photos by Aimo Kuivamäki, GTK. 60

2.2 The Väkkärä granite

The Väkkärä granite (Appendix1) consists of several types, which differ from each other in texture and/or mineral composition. Haapala (1977) has distinguished four rapakivi types or magmatic phases in order of formation: (1) contact type, (2) even- grained type, (3) porphyritic, and (4) coarse-grained type. Both sharp and gradual contacts have been observed between the different types (Suominen et al. 1997).

The porphyritic contact type of the Väkkärä granite against the Tarkki granite is exposed along the NW margin of the granite, and has alkali feldspar, plagioclase and quartz phenocrysts, 1 - 10 mm in diameter, in a fine-grained matrix. Within the contact type there are a few fragments of Tarkki granite, which also sends apophyses into it (Haapala 1977).

The even-grained type of the Väkkärä granite (Fig. 2-2A) is, medium-grained (3 – 6 mm) and is mainly composed of alkali feldspar, quartz, albitic plagioclase and biotite, accessory minerals being zircon, ilmenite, anatase and monazite. Topaz and cassiterite occur rarely as secondary minerals.

The porphyritic Väkkärä granite (Fig. 2-2B) occurs in the SE part of the Eurajoki stock (Fig. 2-1). It becomes more fine-grained towards the contact against the Tarkki granite. The coarse-grained granite type occurs in a small area within the porphyritic granite, and it has a relatively sharp contact against the porphyritic granite, the two types grading into each other over a distance of only a centimetres (Haapala 1977).

Both the porphyritic and the coarse-grained granites are topaz-bearing. In the porphyritic granite alkali feldspar and quartz phenocrysts occur in the fine- to medium- grained groundmass. All granite types usually contain 1 – 3% topaz. In addition, fluorine-rich siderophyllite, monazite, ilmenite, Nb- and Ta-rich cassiterite, columbite, xenotime, thorite and bastnaesite occur as accessory minerals (Haapala 1977). Miarolitic cavities, usually filled with kaolinite, sericite and chlorite, are common in the porphyritic granite, indicating the presence of a separate fluid phase during late stages of crystallisation of topaz-bearing granite, and greisen-type Sn-Be-W-Zn mineralisation is closely associated with it (Haapala 1977, 1997). The topaz-bearing Väkkärä granite resembles typical tin granites in chemical composition. It is enriched in fluorine, lithium, gallium, rubidium, tin and niobium, and depleted in strontium, barium and zirconium. 61

A B

Figure 2-2. A) Even-grained Väkkärä rapakivi granite. Location coordinates: x = 6787424, y = 1534812. The length of the scale is 21 cm. B) Porphyritic Väkkärä rapakivi granite. Location coordinates: x = 6786991, y = 1535461. The length of the scale is 21 cm. Photos by Antero Lindberg (A) and Aimo Kuivamäki (B), GTK.

2.3 Dykes

The Tarkki granite is cut by red-coloured quartz-porphyry dykes, some of which are topaz-bearing. The strike of the five observed dykes varies from N-S to NE-SW or ENE-WSW. Both the mineral and the chemical composition of the dykes resemble those of the Väkkärä granite, indicating a close genetic connection between the two (Haapala 1977). The dykes commonly have a darker chilled margin (average grain size 0.01 mm) against the Tarkki granite. The chilled margin is, however, missing in the dyke shown in Fig. 2-3, which according to Haapala (op. cit.) is due to contemporaneous intrusion of the dyke and the Väkkärä granite, which heated the Tarkki granite and prevented the formation of the chilled margin. Towards the central parts of the dykes, the amount of perthitic microcline, albite and quartz megacrysts increases, while the groundmass, composed of the same minerals (average grain size 0.2 mm), becomes coarser.

Three dark-coloured, dark grey or almost black in a fresh surface, porphyritic dykes occur in the northern part of the Eurajoki stock (Haapala 1977). These dark porphyry dykes are composed of plagioclase (An55-60), quartz, alkali feldspar and ilmenite megacrysts in the aphanitic ground mass consisting of plagioclase, brown and green biotite, bluish-green hornblende, alkali feldspar, quartz, apatite, sericite, epidote, sphene, anatase, Fe-Ti oxides, sulphides, prehnite, carbonate, fluorite and zircon (Haapala 1977). Most of the megacrysts are plagioclase, which is commonly largely replaced by sericite, epidote, prehnite, kaolinite and carbonate. Also, the groundmass is intensely altered into sericite, epidote, chlorite and carbonate. 62

Figure 2-3. Quartz-porphyry dyke cutting the Tarkki granite. Location coordinates: x = 6787205, y = 1532989. The length of the scale is 21 cm. Photo by Aimo Kuivamäki, GTK.

During the present mapping campaign, new types of dykes, not previously described within the Eurajoki stock, were found in the southern part of the porphyritic Väkkärä granite (Fig. 2-4). The dykes show zonal structure, suggesting several different phases of dyke formation. The central zone of these composite dykes consists of a fine-grained dark quartz-biotite-apatite rock, which represents the youngest phase of dyke intrusion. Apatite is exceptionally abundant (19.8%) in this dyke. The contacts of the dark zone are toothed, and quartz and potassium feldspar phenocrysts are concentrated to it (Fig. 2-4C). The dark zone is surrounded by zones of grey and reddish, partly fine-grained, partly medium- to coarse-grained granite to granodiorite, indicating several successive intrusive phases. The width of the dykes varies from a few centimetres up to 50 cm and they have an orientation of 323-343/63-68°. Later reddish pegmatite dykes cut the composite dykes. 63

A C

Figure 2-4. A) Composite dyke cutting the Väkkärä granite. B) and C) Detailed pictures of the dyke shown in A. The length of the scale is 21 cm. Location coordinates: x = 6786991, y = 1535461. Photo by Aimo Kuivamäki, GTK.

The Tarkki granite is cut by a few fine- to medium-grained granite dykes, striking approximately N-S (Fig. 2-5A). The width of the dykes varies from a few centimetres to tens of centimetres, and the widest dykes can be followed for tens of metres. In the Tarkki granite, also coarse-grained to pegmatitic dykes also occur, which, in some cases, can be seen cutting the greisen veins. In the SE part of the Eurajoki stock, the porphyritic Väkkärä granite is cut by a few centimetres wide fine-grained, E-W striking, vertical, dark-coloured mafic (?) dyke. This dyke is in turn cut and, in places, dextrally displaced by subvertical, fine-grained, light grey quartz-feldspar dykes striking NE-SW (Fig. 2-5B). 64

A B

Figure 2-5. A) Fine-grained aplite granite vein cutting the Tarkki granite. Location coordinates: x = 6787389, y = 1535175. B) Fine-grained dark dyke in the Väkkärä granite cut by a fine-grained grey quartz-feldspar dyke. Location coordinates: x = 6787068, y = 1536674. The length of the scale is 21 cm. Photos by Antero Lindberg (A) and Aimo Kuivamäki (B), GTK.’

2.4 The greisen veins

Greisen veins and associated quartz (-beryl) veins occur both in the Tarkki and Väkkärä granites (Appendix 1, Fig. 2-6). The greisen veins were caused by hot, hydrous fluids, migrating in interstices and fractures of the rapakivi granites (Haapala 1977). The greisen veins both in the Tarkki and Väkkärä granite were formed by fluids emanated from the Väkkärä granite (Haapala, op. cit.). The width of the veins varies from a few centimetres up to two metres, and the drillings have suggested that at least some of the veins and vein swarms extend over 100 m in depth and over 300 m in a horizontal direction (Haapala 1977). The main minerals of the greisen veins are quartz, micas and iron-rich chlorite. Often they also contain abundant topaz, fluorine, garnet, beryl, genthelvite and bertnandite (Haapala 1977). Many greisen veins have a central quartz vein, which, additionally, may contain beryl, cassiterite, wolframite, molybdenite, sulphides and mica. The most common ore minerals include sphalerite, cassiterite, chalcopyrite, wolframite, gahnite, molybdenite, rutile, secondary iron oxide, pyrite, pyrrhotite, arsenopyrite and galena. The distribution of tin in the greisen veins is uneven, and can locally be as much as 20 weight-% (Haapala 1989). However, ore mineralisations of economic importance have not been found. According to Haapala (1977), erosion has removed the, possibly more strongly greisenised and mineralised, upper parts of the Väkkärä granite

In the Tarkki granite, the veins are mostly subvertical and strike E-W or ENE-WSW (Fig. 2-7). In the Väkkärä granite the veins are more randomly oriented, and also lensoid, rounded and irregular greisen bodies occur. In the western part of the rapakivi stock, the greisen veins are subparallel to the contact between the Tarkki granite and the migmatitic mica gneiss. 65

Figure 2-6. Greisen veins with central quartz-beryl veins in the Tarkki granite. Location coordinates: x = 6787249, y = 1533012. The length of the scale is 21 cm. Photo by Aimo Kuivamäki, GTK.

Figure 2-7. Distribution of greisen vein orientations measured by Aimo Kuivamäki in the Eurajoki rapakivi stock. Left: stereogram of vein poles (Schmidt's lower hemisphere equal area projection); right: rose diagram of vein strikes (N = 17). 66

2.5 Petrophysics

Petrophysical properties of Eurajoki stock and its surroundings were studied based on 12 bedrock samples (Fig. 2-8). The parameters measured were density, magnetic susceptibility and intensity of remanent magnetization. The results show that the strongest magnetization is related to diabase, which is also the densest rock type in the area. Within the Eurajoki stock, the Tarkki granite is also ferrimagnetic and distinguishable from the paramagnetic Väkkärä granite. The Tarkki samples are also denser than those from Väkkärä, indicating greater abundance of mafic silicate minerals. Calculated Q-values reveal that remanent magnetization in ferrimagnetic samples is the same order of magnitude as the induced magnetization, reflected by the susceptibility values. Other rock types (mica gneiss, granodiorite and quartz porphyry) appear to be paramagnetic. From these, mica gneiss is slightly more strongly magnetized than the others. Previous petrophysical studies in Olkiluoto (Lindberg & Paananen 1991) indicated that mica gneisses are often ferrimagnetic, reflecting occurrence of pyrrhotite.

12 samples 100000 SI)

-6 10000

1000

100 SUSCEPTIBILITY (*10 SUSCEPTIBILITY

10 2400 2600 2800 3000 3200

DENSITY (kg/m3)

VÄKKÄRÄ GRANITE TARKKI GRANITE MICA GNEISS DIABASE QUARTZ PORPHYRY DYKE GRANODIORITE

Figure 2-8. Magnetic susceptibility vs. density, Eurajoki stock and its near surroundings. 67

3 RAPAKIVI CONTACTS

On the basis of outcrop observations, the western contact of the Eurajoki stock (Tarkki granite) with the Palaeoproterozoic (Svecofennian) migmatitic country rock seems to be gently dipping, the dip of the contact being about 20° to the west/northwest. The contact is very sharp and intact (Figs. 3-1 and 3-2). No visible features of contact metamorphism have been observed in the migmatitic mica gneiss.

A B

Figure 3-1. Contact between the migmatite and the Tarkki granite. Photos are taken from the opposite sides of the outcrop. Location coordinates: x = 6789925, y = 1530528. The length of the scale is 21 cm. Photos by Aimo Kuivamäki, GTK. 68

A B

Figure 3-2. A) Gently dipping migmatite near the western contact of the Tarkki granite. Location coordinates: x = 6790084, y = 1530398. B) Contact between the mica gneiss (upper left) and the Tarkki granite, which is pegmatitic in the immediate contact. Location coordinates: x = 6789685, y = 1530549. The length of the scale is 21 cm. Photos by Aimo Kuivamäki (A) and Antero Lindberg (B), GTK.

Based on two- and three-dimensional gravity modelling (Elo 2001, see also Paulamäki et al. 2002), the Väkkärä rapakivi granite extends downwards to a depth exceeding 5 km and dips outwards from the present erosion level to every direction (Fig. 3-3). The western contact is interpreted to be dipping 50 - 58° to the west, eastern contact 40° to the east, northern contact 30° to the north and the southern contact 38 - 40° to the south (Fig. 3-4). The outcrop observations, however, indicate that the western contact can, at least in places, be more gentle (10 – 20°) than interpreted from the gravity measurements. The Bouguer anomaly continuously increases from the Väkkärä granite to the Olkiluoto area, and it is estimated (Paulamäki et al. 2002) that the minimum depth of the rapakivi at Olkiluoto is at least ca. 3 km, supposing that the rapakivi extends linearly so far. However, the observed gentle dip of the western contact between the Tarkki granite and the migmatite country rock suggests that the rapakivi granite could be closer to the surface. The influence of the rapakivi granite at and near Olkiluoto is demonstrated by the greisen veins (see Chapter 2.4), hydrothermal alteration and hydrothermally mineralised fractures in the country rock (cf. Blomqvist et al. 1992, Paulamäki et al. 2006). 69

Figure 3-3. Interpreted three-dimensional structure of the Väkkärä rapakivi granite and cross-cutting olivine diabase sills, view from SSE. The marginal Tarkki granite was not modelled (Paulamäki et al. 2002).

N S

Figure 3-4. Interpretation of the Bouguer anomaly profile across the Eurajoki rapakivi stock (Paulamäki et al. 2002). The two-dimensional model of the Väkkärä rapakivi granite is shown in grey, the corresponding three-dimensional model is outlined in green, and the diabases on the northern side of the rapakivi granite are shown in black. The profile starts at the map coordinates of x=6798670 and y=1535700 (N) and ends at the map co-ordinates of x=6781980 and y=1535810 (S). 70

During the present mapping campaign, a local breccia zone, which has not been previously described, was found near the western contact of the Tarkki granite with the migmatites. The breccia is composed of abundant fragments of migmatitic mica gneiss, 1–30 cm in diameter, lying in the rapakivi granite ground mass (Fig. 3-5). The fragments are partly rounded, partly angular, most of the fragments being, however, more or less elongated. The fragments are randomly orientated. In addition to migmatite fragments, quartz fragments also occur. The breccia zone has rather sharp contact against the Tarkki granite. The width of the zone is at least 15 m but the actual thickness of the zone is quite modest due to gentle nature of the contact (at this locality estimated at only ca. 10° to the W).

In the western part of the Eurajoki stock, a ca. 1 m wide zone of dioritic to quartz dioritic rock with potassium feldspar megacrysts occurs between the Tarkki granite and granitised migmatitic mica gneiss. Between it and the Tarkki granite there is 0.5 – 1 cm wide, reddish, fine-grained to very fine-grained seam.

At the contact between the Tarkki granite and the 1270 - 1250 Ma old (Suominen 1991) olivine diabase dykes and sills, the heat from the diabase has caused partial melting of the granite, resulting in quartz intergrowths with alkali feldspar and plagioclase (Kahma 1951; Haapala 1977). At the contact between the Sorkka diabase and the Tarkki granite in the SW part of the stock, the diabase is almost aphanitic, but becomes medium-grained a few metres away from the contact. Large numbers of apophyses proceed from the diabase dykes into the rapakivi granite.

A B

Figure 3-5. A) and B) Intrusive breccia at the contact between the Tarkki granite and the migmatite. The breccia fragments are composed of migmatitic mica gneiss and quartz. Location coordinates: x = 6789768, y = 1530541. The length of the scale is 21 cm. Photos by Aimo Kuivamäki, GTK. 71

4 FRACTURE PROPERTIES

4.1 Fracture orientations

The distribution of all fracture orientations measured in the Tarkki and Väkkärä granites is shown in Fig. 4-1. There are two main fracture sets in both of the granites, NNW-SSE and NE-SW. Additionally, the Väkkärä granite has a third main fracture set, ca. E-W, which is only poorly represented in the Tarkki granite. In the migmatitic country rock, the fractures are slightly more randomly oriented. The main sets of fractures in the different rock units are presented in Table 4-1. Although most of the measured fractures are steeply dipping, it is most likely that also gently dipping and horizontal fractures are present in significant numbers, but due to the flat subhorizontal nature of the outcrop surfaces they are strongly under-represented in the surface fracture data. This kind of fracture system with two vertical fracture sets and one horizontal is characteristic for the rapakivi granites of the Wiborg rapakivi batholith in SE Finland (cf. Simonen 1987; Kuivamäki et al. 1996). It is noteworthy that although NW-SE is the main lineament trend, both in the morphological and the aeromagnetic (see Section 4.3) data), fractures with a NW-SE strike are very poorly represented in the data from the rapakivi granites whereas it is more pronounced in the migmatitic country rock. 72

Table 4-1. Distributions of fracture orientations measured from the outcrops of Tarkki and Väkkärä granites and the surrounding country rock. For observation points, see Appendix 1.

Rock type Main sets of fractures, in terms of Main sets of fractures, in terms Percentage from strike maxima and strike range of mean pole cluster orientation all measured (from rose diagrams, Fig. 4-1) (dip direction/dip) and fractures within orientation ranges (from the rock type stereograms, Fig. 4-1) Tarkki 352° (330-020°) 077/80° (060-098/70-90°) 23.1% granite 270/82° (245-290/77-90°) 17.5% (N=372) 050° (030-073°) 142/85° (122-163/79-90°) 11.8% 318/83° (300-341/80-90°) 16.9% Väkkärä 063° (050-075°) 154/85° (140-168/80-90°) 14.1% granite 332/84° (320-348/80-90°) 12.0% (N= 291) 355° (347-005°) 086/83° (075-092/75-90°) 7.6% 264/82° (254-278/75-90°) 10.3%

275° (263-288°) 184/84° (179-192/80-90°) 5.5% 005/83° (353-018/78-90°) 10.3% Country 353° (342-009°) 262/78° (252-279/67-90° 15.3% rock - 083/75° (077-093/70-90° 6.1% migmatitic mica gneiss 030° (020-046°) 303/75° (290-316/68-85°) 12.2% (N = 131) 115/74° (110-123/63-90°) 7.6%

304° (286-318°) 034/78° (016-048/75-90°) 10.7% 213/76° (202-221/50-90°) 6.1%

068° (060-072°) 158/76° (150-162/70-90°) 6.9%

Figure 4-1. Distribution of fracture orientations measured in A) the Tarkki rapakivi granite (N = 372), B) the Väkkärä rapakivi granite (N = 291) and C) the migmatitic mica gneiss country rock (N = 131). Left: stereograms of fracture poles (Schmidt’s equal area, lower hemisphere projection); right: rose diagrams of fracture strikes. 74

4.2 Fracture frequencies and length of fractures

The fracture frequencies were measured across each outcrop in N-S and E-W traverses (Fig. 4-2A). All fractures equal or longer than one metre were measured in each traverse. The total length of the traverses, the number of measured fractures and the resulting fracture frequencies are presented in Table 4-2. Based on the Finnish engineering geological classification (Korhonen et al. 1974, Gardemeister et al. 1976), both the Tarkki and Väkkärä rapakivi granites can be described as “sparsely fractured” (Fig. 4-2B), the average fracture frequency being 0.7 fractures/m. Some individual outcrops may be “slightly fractured” (1 – 3 fractures/m) (Fig. 4-2C). However, “abundantly fractured” (>3 fractures/m) rock is rare and only occurs as narrow zones (Fig. 4-2D). The fracture frequency is not controlled by rock type but the frequencies are the same in both the rapakivi granite types (Table 4-2). Also, the fracture frequency at the contact between the Tarkki granite and the migmatitic mica gneiss is not different that within the rest of the rapakivi granite.

Table 4-2. Fracture frequencies (fractures/m) measured within the Tarkki and Väkkärä granites of the Eurajoki rapakivi stock across each outcrop in N-S (0°) and E-W (90°) traverses. 0° + 90° = both traverses added together.

Rock type Direction of Total length of the Number of Fracture frequency traverse traverses (m) fractures (fractures/m) Tarkki granite 0° 545 388 0.71 90° 560 417 0.74 0° + 90° 1105 805 0.73 Väkkärä granite 0° 495 363 0.73 90° 537 349 0.65 0° + 90° 1032 712 0.69 Eurajoki stock 0° + 90° 2137 1517 0.71 75

A B

C D

Figure 4-2. A) N-S and E-W traverses in the outcrop within the Väkkärä granite. B) ”Sparsely fractured” Tarkki granite. Location coordinates: x = 6789909, y = 1530632. C) “Slightly fractured” Tarkki granite. Location coordinates: x = 6791033, y = 1530801. D) “Abundant fracturing” within the Väkkärä granite. Location coordinates: x = 6787082, y = 1534833. The length of the scale is 21 cm. Photos by Aimo Kuivamäki, GTK.

The lengths of mapped fractures in the study area range from 1 m (the lower cut-off length) to 20 m, only one fracture being longer than 20 m (Fig. 4-3). About 30% of the fractures are visible in their full length, their average length being 3.8 m. The length of the fractures is not dependent on the fracture orientation but both long and short fractures have approximately the same orientation (Fig. 4-4). 76

300

250

200

150

100

Number of fractures 50

0 1234567891011121314151617181920 Fracture length (m)

Figure 4-3. Distribution of fracture lengths in the Tarkki and Väkkärä granites. Fracture length: 1 = 1-1.99 m , 2 = 2-2.99 m, 3 = 3-3.99 m etc. 77

Figure 4-4. Orientation of fractures A) <5 m, B) 5-9.5 m and C) >10 m in length in the Tarkki and Väkkärä granites. Left: stereograms of fracture poles; right: rose diagrams of fracture strikes. Scale of ring in the rose diagram is 5%. 78

4.3 Indications of brittle fault movements

Lineaments

According to the interpretation of the 1:100 000 low-altitude aeromagnetic map (Paananen & Kuivamäki 2007), lineaments trending NW-SE (130° – 140°) are dominating within the Eurajoki stock and its near surroundings (Fig. 4-5). Other apparent, although magnetically much more poorly visible, directions are 110° – 120°, 60° – 70°, 40° – 50° and 10° – 20°. These lineaments, interpreted as the surface traces of brittle deformation zones, divide the stock into several minor blocks. Outside the rapakivi stock these lineaments can be seen cutting and apparently displacing ca. E-W trending magnetic anomalies at numerous locations (Fig. 4-6). Northwest of the stock within the migmatite area, at least three possible subvertical fault zones can be detected, two of them with a sinistral and one with a dextral component of strike-slip movement (Paananen & Kuivamäki 2007). The apparent horizontal displacements are ca. 100 – 200 metres. However, the displacement directions or amount of offsets are speculative, because the dips of the faults are uncertain. Also a purely vertical movement can induce an apparent horizontal displacement, if the displaced structures are gently dipping.

Figure 4-5. Distribution of magnetic lineament directions within the Eurajoki stock and its near surroundings (Paananen & Kuivamäki 2007). 79

Figure 4-6. Interpretation of the lineaments to the NW of the Eurajoki rapakivi stock in terms of faulting along NW – SE trending fracture zones (Paananen & Kuivamäki 2007). Arrows: apparent horizontal sense of movement.

Greisen veins

In the greisen veins, which cut the Eurajoki rapakivi granite some minor faults have been found. In the approximately E-W trending vein swarm between Hankkila and Lapijoki, within the Tarkki granite in the southwestern part of the stock, the subhorizontal surfaces show an apparent dextral component of strike-slip (Figs. 4-7 and 4-8), i.e., a component of movement to the east of the northern side of the vein in relation to the southern side (Haapala 1977). In the E-W trending vein swarm to the north of the Eurajoki municipality centre, components of sinistral strike-slip movement are dominant. 80

Figure 4-7. Indications of shear movements in greisen veins. a) displacement of quartz- feldspar vein, b) and c) en echelon structure of the minor veins and joints, and d) deformed structure of the central quartz-beryl vein (orientation of beryl crystals and cross joints) (Haapala 1977).

A B

Figure 4-8. Case (a) in Fig. 4-10, except that the displacement component along the quartz-beryl vein is sinistral in this particular outcrop. Location coordinates: x = 6787253, y = 1532950. B) Case(c) in Fig. 4-10: en echelon structure of minor joints interpreted as indicating a dextral component of displacement. Location coordinates: x = 6787266, y = 1532917. C) Case d in Fig. 4-10: deformed structure of the central quartz-beryl vein (orientation of beryl crystals and cross joints), interpreted as indicating a dextral component of displacement. Location coordinates: x = 6787266, y = 1532917. The length of the scale is 21 cm. Photo by Aimo Kuivamäki, GTK. 81

Fractures

The majority of the measured fractures in the Eurajoki stock show no indications of fault movement, i.e. they are joints, which were formed as dilational fractures in extensional deformation. There are, however, a few examples, which suggest that at least a part of the fractures may have been formed in response to shear deformation, or have been reactivated in shear after formation.

Long major fractures have been observed, which terminate in a fan-shaped array of short "horsetail" fractures (Fig. 4-9A). In the literature, such major fractures have been interpreted faults, and according to Segall & Pollard (1983) and Granier (1985), the fractures of the "horsetail" are formed as dilatational fractures, enabling the sense of shear to be estimated (as indicated on Fig. 4-9A).

Two adjacent parallel major fractures are typically linked via short fractures (splay cracks) and often terminate in splay cracks, striking less than ca. 20-30° from the fault plains (Fig. 4-9B). These kinds of fractures could be interpreted as pair of faults, where the movement is transmitted from one fault to another via the splay cracks. Due to lack of good markers, the amount of displacement (if any) could not be recorded. One must admit, however, that the interpretation of Fig. 4-9B is extremely doubtful. Such “side- stepping” is also a typical feature of dilational joint systems.

Figure 4-10 shows what may be a very small fault with splay cracks at the end. However no detectable separation can be seen in the mafic stripes serving as passive markers. Either the fracture is not a fault or the displacement is only a few millimetres or less.

In places NW-SE trending narrow epidote-filled fractures occur, which show apparent sinistral sense of shear movement along the fractures (Fig. 4-10).

The development of a fault needs a pre-existing discontinuity. Since there are no indications of ductile shear zones, the fractures with fault indication were most likely originally extensional joints, which were reactivated by shear movement (cf., for instance, Segall & Pollard 1983; Martel & Pollard 1989). 82

A B

Figure 4-9. A) Long major fractures (faults) with short secondary fractures forming a horsetail-structure, which indicate that they are faults with a sinistral sense of movement. Location coordinates: x = 6791419, y = 1531202. B) Two major fractures (faults?) connected with splay cracks. The apparent sense of movement is dextral. Location coordinates: x = 6789823, y = 1530666. The length of the scale is 21 cm. Photos by Aimo Kuivamäki, GTK.

A B

Figure 4-10. Termination of a possible right-lateral fault, however, with no displacement along the fault visible to the naked eye. Location coordinates: x = 6789585, y = 1530640. B) Epidote-filled shear fractures showing an apparent sinistral sense of movement. Location coordinates: x = 6789280, y = 1531629. The length of the scale is 21 cm. Photos by Antero Lindberg, GTK. 83

5 SUMMARY

The Eurajoki rapakivi stock is a satellite massif to the large Laitila rapakivi batholith and is composed of two main rapakivi granite types: hornblende-biotite granite (the Tarkki granite) and topaz-bearing microcline-albite granite (the Väkkärä granite). The Väkkärä granite consists of several types, which differ from each other in texture and/or mineral composition: contact type, even-grained type, porphyritic type, and coarse- grained type. Both sharp and gradual contacts have been observed between the different types. The Tarkki granite is cut by red-coloured quartz-porphyry dykes, some of which are topaz-bearing, and fine- to medium-grained granite dykes with approximately N-S strikes.

During the present mapping campaign, new types of dykes, not previously described within the Eurajoki stock, were found in the southern part of the porphyritic Väkkärä granite. The dykes are composite dykes showing several different phases of dyke formation. The central and the youngest zone of these dykes consists of a fine-grained dark quartz-biotite-apatite rock, apatite being exceptionally abundant (19.8%). This is surrounded by zones indicating several successive intrusive phases, consisting of grey and reddish, partly fine-grained, partly medium- to coarse-grained granite to granodiorite. The width of the dykes varies from a few centimetres up to 50 cm and they dip ca. 65° to the NW or NNW. The dykes are cut by later pegmatite dykes.

The western contact of the Eurajoki stock (Tarkki granite) with the Palaeoproterozoic (Svecofennian) migmatitic country rock seems to be gently dipping, the dip of the very sharp and intact contact being about 20° to the west/northwest. No visible features of contact metamorphism have been observed in the migmatitic mica gneiss.

A local, previously unknown breccia zone was found near the western contact of the Tarkki granite with the migmatites. The breccia is composed of abundant fragments of migmatitic mica gneiss, 1–30 cm in diameter, lying in the rapakivi granite ground mass. The randomly oriented fragments are partly rounded, partly angular, most of the fragments being, however, more or less elongated. In addition to migmatite fragments, quartz fragments also occur. The breccia zone has a rather sharp contact against the Tarkki granite. The width of the zone is at least 15 m but the actual thickness of the zone is quite modest due to gentle nature of the contact (ca. 10° to the W).

Greisen veins and associated quartz (-beryl) veins occur both in the Tarkki and Väkkärä granites. The greisen veins in both granites were formed by hot, hydrous fluids emanated from the Väkkärä granite. The width of the veins varies from a few centimetres up to two metres, and at least some of the veins and vein swarms may extend over 100 m in depth and over 300 m in a horizontal direction. The main minerals of the greisen veins are quartz, micas and iron-rich chlorite. Many greisen veins have a central quartz vein, which, additionally, may contain beryl, cassiterite, wolframite, molybdenite, sulphides and mica. In the Tarkki granite, the veins are mostly subvertical and strike E-W or ENE-WSW. In the Väkkärä granite the veins are more randomly oriented, and also lensoid, rounded and irregular greisen bodies occur. 84

A total 933 fractures were investigated during the fracture mapping. Fracture frequencies (fractures/m) were measured across each outcrop or observation point in N– S and E–W traverses. The surface fractures form a system with two main fracture directions, N-S and ENE-WSW. Both the Tarkki and Väkkärä rapakivi granites are “sparsely fractured”, the average fracture frequency being 0.7 fractures/m. Some outcrops are “slightly fractured” (1 – 3 fractures/m) but “abundantly fractured” (>3 fractures/m) rock is rare and only occurs as narrow zones. The fracture frequency is not controlled by rock type but the frequencies are the same in both the rapakivi granite types. The lengths of mapped fractures in the study area range from 1 m (the lower measurement cut-off) to 20 m, only one fracture being longer than 20 m. About 30% of the fractures are visible in their full length, their average length being 3.8 m. The length of the fractures is not dependent on the fracture orientation but both long and short fractures have approximately the same orientation. 85

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APPENDICES

Appendix 1: Lithology of the Eurajoki stock.

Appendix 2: Location of the figures. 88 89

Hankkila Eurajoki

Compiled from: Haapala (1977), Suominen et al. (1993)

Lapijoki