THE PRECAMBRIAN ROCKS OF SOUTHERN FINLAND … Finland bedrock_ENG_062011_100dpiS.pdf · PRECAMBRIAN...

THE PRECAMBRIAN ROCKS OF SOUTHERN

FINLAND AND ESTONIA

Institute of Geology at Tallinn University of Technology

University of Turku, Department of Geology

THE PRECAMBRIAN ROCKS OF SOUTHERN FINLAND

AND ESTONIA

Tallinn 2007

Compiled: OlavEklund AlvarSoesoo AriLinna

Acknowledgements:AriLinna,AriBrozinski,HeikkiBauert,OlavEklundandArtoPeltola suppliedpicturestothisbooklet.LeenaLaurilahelpedwiththelayout ofthemaps.PaulReuterandMattiWahlbäckareacknowledgedfor logisticsduringfieldtripsinthearchipelagoofSWFinland.Jeremy WoodardeditedtheEnglishversion.

Edited: MTÜGEOGuideBaltoscandia

Layout: AndresAbe

Figures: OlavEklund

Frontpage: Intensivelyfoldedgraniticveinsinametavolcanite,southwestFinland. ThestructurewasnamedptygmaticbytheFinnishgeologistJ.J.Sederholmin1907. PtygmaticisfromtheGreek,meaning"thefolded".PhotoA.Linna.

Backcover: GeologicmapofsouthwestFinland.

©MTÜGEOGuideBaltoscandia,2007

ReleaseofthisbookletinEnglishwasco-financedbyERDFandbyEstonianMinistry of the Interior under theINTERREG IIIA Southern Finland andEstoniaprogramme.

THE PRECAMBRIAN ROCKS OF SOUTHERN FINLAND AND ESTONIA

MTÜ GEOGuide Baltoscandia. Tallinn, 2007.

ISBN978-9985-9867-4-5

PRECAMBRIAN ROCKS OF SOUTHERN FINLAND & ESTONIA 3

The bedrock of southwestern Finland

By lookingatanyrock,youare lookingatapieceofthehistoryoftheEarth. InFinland, ifyoudrawa linebetweenthetownofRaaheandLakeLadoga,thenanyrock(withafewexceptions)southofthislineyou lookat formedbetween1920and1575millionyearsago.TothenorthofthislineareArchaeanrocks,whicharerocksthatformedmorethan2500millionyearsago.Theserocksall formedeitherdeepwithintheEarth’scrustfromcryst-allisingmagmaorattheEarth’ssurface,eitherdirectlyfrommaterialeruptedfromvolcanoesorfromsediments,forinstancesand or clay, eroded, transported andredepositedattheseafloor.Themajor-ityoftherocksinsouthernFinlandhave,after formation,alsobeensubjectedtoconditionsofextremeheatandpressure,whichresulted inmineralogicalandtex-turalchangesintherock.Thisprocessiscalled metamorphism, from the Greekmeta morphosmeaningchangeofform.ThemetamorphisminSouthernFinlandisrelatedtoorogenic(mountainbuilding)processes,whichaffectedalloftherocksintheregion.Onlytherapakivigranites,Jotnian Satakunta sandstone and dia-bases,whichall formedafterthemeta-morphicevent,areunaffected.

By studying the texture, macroscopicandregionalstructures,mineralogyandmineralparageneses,chemicalcomposi-tionand isotope compositionof rocks,geologistshavebeenabletoidentifytheenvironmentwheretherocksformedandwhenithappened.Thisallowsgeologiststo reconstruct thegeologicalhistoryofourpartoftheEarth.

Why is there an archipelago in southwestern Finland?

WhydowehaveanarchipelagoinfrontofsouthwesternFinland?Whyaretherenotarchipelagoseverywherearoundthecoast?Toanswerthis,wefirstconsidertwomajorgeologicalprocessesthathaveshaped(andcontinuetoshape)thesur-faceof theEarth insouthwestFinland:thosethataffectthebedrockandthosethat affect the soil cover. The bedrockformedduringaprolongedcrustformingand mountain building event between1900 and 1765 Ma (geologists reportagesinanno,meaningyearsago;Ma=mega anno=millionyearsago)thatwecall theSvecofennianorogeny.The soilcover isamixtureofglacial sediments,whichweredepositedonthetopofthiserodedmountainchainattheendofthelasticeage,lessthan10000yearsago.Beforethis,theglaciersscrapedawaythePalaeozoic sedimentary cover plus theweatheredsurfacesofthecrystallinebed-rock.RemnantsoftheformerCambrianandOrdoviciansedimentary rockshaveonlyoccasionallybeenpreservedindeepdepressions,forexamplewithinamete-oritecraterordeepfracturesinthebed-rock.

Thegeologicaleventthathadthemostprofoundeffecton themodernEarth’ssurfacewasthelasticeage,whichendedabout10000yearsago.Strikingfeaturesincludecliffsinthearchipelago,roundedandpolishedontheirnorthern(proximal;towardstheadvancingglacier)side,withgroovesand lineationfromtherocks inthebottomoftheiceandfragmentedon

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thesouthern(distal)side.Thisfragmenta-tiononthedistalsidewascausedbythefreezingofwater,derivedfromicemelt-inginthehigh-pressureconditionsontheproximalside.Meltwaterwouldseepintofracturesandthenrefreeze,andtheresult-ingexpansionbroketherock.Ahugevol-umeofsedimentwastransportedbytheglacieranddepositedinavarietyofdif-ferentsedimentaryformationswhentheglaciersstartedtomelt. Morainefields,end formations, including sandurs anddeltas,andeskerswereformed(seethebookletaboutquaternaryformations inFinlandandEstonia).

Wecanagreethatanarchipelago isanarea with a lot of islands with waterbetween. However, when you travelfromTurkutowardstheeast,youseealotofhillswithfieldsbetween.Imagineifyouweretochangethesefieldstowater,you would also have an archipelagothere.Asamatteroffact,thesenow-dryareasinitiallyformedanarchipelago,butasaconsequenceoflandupliftaftertheendofthelasticeage,partsofthearchi-pelago are now situated on dry land.Thehills (and islands)areusuallymadeofgranitoids,whicharehardandresis-tant toerosion.The rocksbeneath thefields(andthesea-sediments)areusuallymadeofa softer rock, schist,which islessresistanttoerosion.Thegranitichillswereformedfromrocksmelting in theroots of the Svecofennian mountains,1880-1820Maago.Verycommonly inthese rocks, you can see fragmentsofbasic rocks, such as amphibolite, thathavenotmelted,eventhoughtheotherrockmaterialwasalmostcompletelyorcompletelymelted.

The formation of different rock types

To reconstruct the geological history,weneedtoknowwhatmechanismsareactiveintheEarthtoday.Themostimpor-tantconceptisPlateTectonics.

Plate Tectonics

MeileThe Earth’s crust is divided intoeight major lithospheric plates, whichdriftaroundtheEarthonapartlymoltenmantle.TheplateFinlandissituatedoniscurrentlymovingtowardsthenortheastatarateof2cm/year(thesamespeedasyourfingernailsgrow).Theforcesdrivingthismotionareconvectioncurrentsinthemantle, inwhichhot lightmaterialrisesfrom deep within the Earth, cools andsinksdownagain,likeboilingwaterinakettle (Figure1).Someoftheplatesareolder,lightercontinentalplatesandsomeof them are younger, heavier oceanicplates.ThecontinentsNorthandSouthAmericaaredriftingapart fromEuropeandAfrica.Astheydriftapart,newoce-anic crust is forming in the middle oftheAtlanticOcean.Thevolcanicactivityin Iceland isalsoaconsequenceofthis.Icelandissituatedrightonthemid-oce-anicridge,wheretwoplatesdriftapart.

Since the Earth is a sphere, on whichcontinentsaredriftingandnewoceaniccrustisformedallthetime,oceaniccrustmustalsobeconsumedsomewhereandcontinentshavetocollideeverynowandthen. These are geological events thathaveoccurredthroughoutthehistoryoftheEarth.TheHimalayasareamodernexampleof a continent-continent colli-


Faultsystems

Diabasedykes(1590-1570Ma)....Figure17

Rapakivigranite(1590-1570Ma)...Figure15ja16

Microclinegranite(1840-1820Ma)...Figure14

Granodiorite,tonaliteandquartzdiorite(1890-1870Ma)...Figure12

Gabbroanddiorite(1890-1870Ma)...Figure10

Micaschistand-gneisses(about1900Ma)...Figures5a&13

Maficmetavolcanicsandmarble(about1900-1885Ma)...Figures3,4,8,11a&11b

Intermediateandfelsicmetavolcanics,metasedimetsandmarble(about1900-1885Ma)...Figure2

Kallioperäkartta © Geologian tutkimuskeskus

10 km0

Attu

11a

14

Turku

15

16

4

11b

8

6

9

10

1712

Hiertovyöhyke

Intermediääristä ja felsistä metavulkaniittia, metasedimenttiäja kalkkikiveä (noin 1900-1885 Ma) ... Kuva 2

Kiilleliusketta ja kiillegneissiä (noin 1900 Ma) ... Kuvat 5a ja 13

Mafista metavulkaniittia ja kalkkikiveä (noin 1900-1885 Ma)Kuvat 3, 4, 8,11a ja 11b

Rapakivigraniittia (1590-1570 Ma) ... Kuvat 15 ja 16

Mikrokliinigraniittia (1840-1820 Ma) ... Kuva 14

Granodioriittia, tonaliittia ja kvartsidioriittia(1890-1870 Ma)Kuva 12

Gabroa ja dioriittia (1890-1870 Ma) ... Kuva 10

Diabaasijuoni (1590-1570 Ma) ... Kuva 17

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sion. The Himalayas formed when theIndiancontinentalplatecollidedwiththeAsiancontinentalplate.Previously,therewasanoceanbetweenAsiaand India,andsediments fromthesea floorwerepushed upward during this collision.That’swhywecanfindmaterialsformedinoceansonthetopofthemountains.Wemaysaythatwheretherearemoun-tainchains,therehasbeenacollision.ThemountainchaininNorwayandSweden,theCaledonides,formedwhentheNorthAmerican plate collided with Europe

about500millionyearsago.Thehigh-estpointinFinland,Halti,locatedinthewesternmostpartoftheFinnish"arm",isapartofthisextensivemountainchain.

When an oceanic plate collides with acontinental plate, the heavier oceanicplate is pressed under the continentalcrust.Thisprocessconsumestheoldoce-aniccrustand iscalledsubduction.TheoceaniccrustsinksdownintotheEarth’smantle, where, when the temperatureriseshighenough (at thedistal endof

Figure 1. Principles of plate tectonics. 1. Due to convection currents in the deep mantle, hot material from the Earth’s interior rises towards the surface and cau-ses melting in the upper mantle. These magmas intrude the crust and crystallise forming new crust, such as in Iceland. 2. Arrows indicating the motion of crustal plates and their subduction into the mantle. 3. An oceanic plate subducting be-neath another plate. 4. At depth, partial melting takes place above the subduc-ting plate, resulting in the formation of a volcanic chain. 5. When two conti-nents collide, the crust crumples to form a mountain chain. You can imagine that the two continents in the figure represent India and Asia and the mountain chain is the Himalayas.

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figure 1. Plate tectonics.aifigure 1. Plate tectonics.ai

continent

4

3

2

1

5

continent

continentcontinent

the subductingplate), it starts tomelt.Theevolvedhotmagmarisesup,caus-ingmeltingalsointhesurroundingrocksof the continental plate. The magmasformedmigrate towards thesurfaceoftheEarth(numbers3and4inFigure1).Sometimes thesemagmaseruptat thesurface, forming a chain of volcanoes;the"volcanicarc" isanalogoustosub-ductionzones. Theearthquakesresult-ingfromthereadjustmentof litosphericplatesisaninseparablepartinthiscom-plexofprocesses.Atthemoment,thereisalotofgeologicalactivityinThailand,IndonesiaandThePhilippines,becauseseveral plates are fighting for space inthatarea.

Ifwegobackintime1900millionyears,thegeologyofsouthernFinlandwassimi-lartowhatweseeinsoutheastAsiatoday.Volcanic chains were moving towardstoday’snorth,producingvolcanicrocks.Today, most of these have been meta-morphosedand recrystallised toa rocktype called amphibolite (Figure 2). Themajorityof these rocksweredeformedand folded during orogenic processes(Figure3).Still,someprimarystructurescanbeseen intheseoldvolcanic rocks(Figure4).

Beforeyourtourintothegeologyofthearchipelago,somegeologicaltermsmustbeexplained.

Figure 2. Old volcanic rocks showing differences in their composition (black and grey areas). A younger, crosscutting granite is seen on the right side of the pictu-re (Vekara island, west of Uusikaupunki). On a geological map, these rock types are marked in green (numbers 1 and 2 in the map legend). Photo A. Linna.


What are rocks made of?

ArockisasolidpieceoftheEarth’scrust.A rockcanbe formedbyasinglegeo-logicalprocess,oritmayhaveundergoneseveralprocesses.Usually,rocksarebuiltupfromsilicateminerals.Thesecanberoughlydivided intotwoclasses,based

ontheamountofiron-magnesiumbear-ing minerals they contain. Felsic rocks,fromFELdsparandSilica (quartz),havea low amount while mafic rocks, fromMAgnesium and Ferrum (iron) have ahighamount.

reactions.Theircompositionandmeta-morphicreactionsarerelatedtothetem-peratureandpressureintherock.

Themostcommonmineralsofmetamor-phicorigininsouthernFinlandarenamedbelow.Theyareformedinmetamorphic

Typical felsic minerals are: (theseareseeninFigure15)

plagioclase feldspar(Na,Ca)(Al,Si)AlSi2O8 –usuallywhiteonweatheredoutcrops

alkali feldspar(K,Na)AlSi3O8 –usuallyredonweatheredoutcrops

quartzSiO2 –usuallytransparentandpalegreyish

The most common dark iron-magnesium bearing minerals in our rocks are:

olivine(Fe,Mg)2SiO4

clinopyroxeneCa(Fe,Mg)Si2O6

amphiboleCa3(Mg,Fe,Al)(Si,Al)8O22(OH)2

biotiteKAl2(Al,Si3O10)(OH)2 –blackflakes

InFigure5a, theblackmineralsarebiotite, inFigure5bamphibole. InFigure5bthereisasmallamountofolivine,whichisgreenwhenunalteredandbrownwhenweathered.

garnet (almandine)(Fe,Mg)3(Al2(SiO4)3 –red(Figures5a,6a,13)

cordierite(Mg,Fe)2Al3(AlSi5O18) –darkblue(Figure13)

sillimaniteAl2SiO5 –whiteandyellow,fibrous

There are also some common non-sili-catemineralsinthebedrockofsouthernFinland,likeoxidesandcarbonates:

calciteCaCO3 –(Figure11)

magnetiteFe3O4–(Figure10)

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Figure 3. Old volcanic rocks that have been folded in a mountain building pro-cess caused by a plate collision. (Vekara island, west of Uusikaupunki). Photo A. Linna.

Figure 4. Pillow lava from Velkua. Pillow lavas are formed when hot basaltic magma erupts underwater. Because of the temperature difference between the magma (1200ºC) and the water, the magma is quenched, resulting in the pil-low-like shape and the glassy texture at the contact. In this picture, you can see several pillows on top of each other. These pillows are between 30 and 50 cm across. Pillow lavas are marked with black semicircles on geological maps. Photo A. Linna.

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MostofthemineralsintheFinnishbed-rock formed either via crystallisationfrom magma or in metamorphic reac-tionsunderavaried rangeofpressure-temperatureconditions.WhentheseareexposedtheEarth’ssurface,theytendtoreact,albeitslowly, toformnewminer-als thataremorestableunderthenewpressure-temperatureconditions.Partofthisalteration(or lackof it) isvisibleontherocksurfaceasweathering.Mineralshavedifferentcapacitiestoresistweath-ering,suchthat,generallyspeaking,min-eralsthatformedathighertemperaturesweathermorequickly.Thehillsinsouth-ernFinlandaremostlyofgraniticcompo-sition,andtheyconsistofmineralsresis-tant to weathering like alkali feldspar,plagioclasefeldsparandquartz.

Rocks

Rocks are divided into three majorgroups:

1. Igneous rocks

Anigneousrockisarockthathascryst-alliseddirectly frommagma.Rocksthatcrystallisedeepwithin thecrust,wherethemineralshavebeenabletogrowtoasizethattheyareeasilydistinguishablewiththenakedeye,arecalledplutonicrocks (Figures 10, 12, 15, 16). If themagmascrystallise close to the surfaceindykes, they formdykerocks (Figures10,17).IfmagmaeruptsontotheEarth’ssurfaceas lava, itcrystallisesasvolcanicrocks(Figures2,3and4).

Magma. Molten rock below the sur-faceof theEarth iscalledmagma,anderuptedmagmaiscalledlava.Dependingonthecomposition,thetemperaturecanvarybetween700ºand1200ºC.Graniticmagma,whichcontainshighamountofsilicaandgases (usuallysteam), isquiteviscous and tends to erupt explosively.Theseeruptionsproduceavarietyofdif-ferentmaterials,suchasrockfragments(resulting intheformationofagglomer-ates),volcanicash(resultingintheaccu-mulationoftuffsandtuffites)andtephra,which can accumulate rather far awayfrom the volcano. Basaltic magma, ontheotherhand,contains lesssilicaandtherefore flows more easily and rarelyeruptsexplosively.

Partial melting.Partialmeltingisapro-cessthattakesplacewhenanyrockstartstomeltas the temperature risesaboveitscrystallisationtemperature(Figure5).Partialmelting is theprocessbywhichmagmasareproduced.IfpartialmeltingtakesplaceintheEarth’smantle,magmasrichinironandmagnesiumandwithlowamountsofsilica,potassiumandsodiumform.Thesemagmasproducedarkrocks(likebasalt)becauseofthehighamountof iron-magnesium bearing minerals(olivine,pyroxene).Partialmeltingofthecrust,however,generatesmagmaswithhigh amounts of silica, potassium andsodium, but low amounts of iron andmagnesium(minerals likealkali feldsparandquartzform).Apartiallymeltedrockcontainingbothmeltedandnotmeltedmaterialiscalledmigmatite(Figures5,6and13.


2. Sedimentary rocks

Rocks on the surface of the Earth arecontinuouslyexposedtothedirectandindirect effects of the Earth’s atmo-sphere and hydrosphere, topographicgradient and vegetation. Weathering(both physical and chemical break-downof rocks) and erosion (transportofweatheredmaterial)of thebedrockare themostobviousconsequencesofthese phenomena. The weathering ofbedrock produces rocks, gravel, sandand clay, which are transported andredepositedassediments.Thesedimentlithologyandtransportdistancedependonseveralprevailingfactors: the lithol-ogyandprovenanceof thehost rocks,mixingornon-mixing,thegradientandthetransportingmedium(i.e.air,water,etc…). Sedimentsaccumulate in sedi-mentarybasins,whichvaryfrommoun-tain gorges to alluvial plains to waterbasins. Sediments can be transportedovergreatdistances; forexample,sedi-mentsderivedfromFinnishbedrockareencounteredas far awayas theNorthSea near Holland. Sedimentary rocksformwhensoftsediment,suchassand(Figure18)orclay,hardens.Thisprocessiscalleddiagenesis,whichhappensasaresultofthepressurefromtheoverlyingsediment load combined with dissolu-tion of minerals and their reprecipita-tionascement.Limestones (Figure11)andevaporitesalsohaveasedimentaryorigin.Forexample, thesaltweuse ineverydaycookingis,geologicallyspeak-ing,achemical sedimentderived fromtheevaporationofseawater.

3. Metamorphic rocks

Metamorphic rocks are formed whenigneousorsedimentaryrockschangeformduetochangesinpressureandtempera-ture.Metamorphicreactionsconvertthemineralogyoftherockinresponsetothechangedconditions.MostofthebedrockinsouthernFinlandiscomposedofmeta-morphosed igneous and sedimentaryrocks.Duringmetamorphism,whenthetemperaturerisesabovearound700ºC,rocksstarttomelt.Figures5and6showexamplesofmigmatitesfromsouthwestFinland,whichformedasaconsequenceofpartialmelting.

Thesethreerocktypesarepartofagigan-ticrockcycle.Temperatureandchangesintemperature,waterandwinderodecon-tinents,andtheerodedsedimentmate-rialistransportedintosedimentarybasins(e.g.thesea),wheretheyarecompactedattheseabottomandformsedimentaryrocks. Sedimentary and igneous rocksare metamorphosed in collision zoneswhereEarth’slitosphericplatescollide.Iftheyaretransported intothemantle insubductionzones, theymeltandagainbecomemagmasthatcrystalliseasigne-ousrocks.

How do we know the age of a rock?

The relative age of two different rock

typesoccurring together canbedeter-

mined by looking at which one cuts

theprimarystructures intheother; the

youngerrocksalwayscutstheolder.The

stratigraphy,ororderofdeposition,givestherelativeage inthecaseofsedimen-taryrocks.Eveninsomemetamorphosedsedimentary rocks, sedimentary struc-tures,andsometimestheorderofdepo-sition,arestillrecognizable.

Togettheabsoluteageoftherock,iso-topes are used. Isotopes are atoms ofthe same element that have differentnumbersofneutronsinthenucleus.Forexample, the simplestatom,hydrogen,always has one proton in its nucleus,butmayhavezero,oneortwoneutrons(themoreneutrons,theheavierthe iso-tope).Itisstillthesameelement,sinceitistheamountofprotons inthenucleusthatdeterminewhat element it is, buttheelementhydrogenhas threediffer-entisotopes.Someisotopesareradioac-tive, meaning that they spontaneously

changeintonewelementsorisotopesataconstantrate.Mostpeoplehaveheardof radiocarbon dating, which is basedonthedecayoftheradioactive isotopeofcarbon.Ingeology,other,longer-livedradioactiveisotopessuchasuraniumareused.Overtime,uraniumisotopesdecayto lead isotopes. The mineral zircon(ZrSiO4),amineral commonly found ingraniticrocks,isoftenusedindetermin-ingtheageofarock.Whenzirconcrys-tallises,traceamountsofuraniumenterthecrystallattice,whileleadisexcluded.Sinceweknowthedecayrate,andwecanassumethatanyleadistheresultoftheradioactivedecay,wecananalysetheamountofuraniumandleadisotopesinthemineral todayandcalculateback intime towhen themineral formed.Therocks in Finlandare amongst thebest-datedrocks in theworld,which iswhy

Figure. 5. An example of the partial melting that took place during the meta-morphism in southern Finland about 1830 million years ago. a) Partial melting of an old sedimentary rock. b) Partial melting of an old volcanic rock. 1 - the old rock, 2 - the melt produced because of the high temperature, 3 - material not melted. Photo A. Linna.

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3

1

2

1

a

weareabletoreconstructtheevolutionofourpartof theworld insuchdetail.TherocksofsouthernFinland,aswellasinEstonia,formedduringtheearlypartof the Proterozoic Eon, between 1900and1575million yearsago, throughaseriesofgeologicalprocesses.

The geological history of south-ern Finland, focusing on the archipelago in SW Finland

SouthernFinlandwasexposedtointensemetamorphismabout1830Ma.Atthistime,therocksweseenowatthesurfaceweresituatedabout18kmdeepwithintheEarth’scrust,wherethetemperaturewasashighas800°C.Atsuchhightem-peratures, rocksstart tomelt.Oldsedi-mentsandvolcanicrocksunderwentpar-tialmeltingandthemeltscrystallisedasgranitesandtonalites(Figure5a,b).Thisfactmakesitdifficulttotracethehistory

olderthan1830millionyears.However,hereandthereyoumayseetracesofonlyslightlydeformed rocks that tellus theolderhistoryoftherocks.

Let’s lookat thegeologicalevolution insouthwestern Finland in chronologicalorder:

1900-1880 Ma

A volcanic chain above a subductionzone,similartomodernJapan,producedvolcanic rocks (Figure1and7a).Whenthe lava from the volcanoes eruptedunderwater,itwasquenchedbythecoldsea water, forming pillow shapes andtubes,throughwhichthe lavawassub-sequently transported. Figure 4 showspillow lavason thenorthwest shoreofIso-HumasluotoinVelkua.Onthesouth-ernsideoftheisland,thereisanexamplewherelavaintrudedwetcarbonatesedi-ments. Here, the limestone (carbonaterock)iserodedandthelavaproducesthereliefoftherock(Figure8).


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2

b

Figure 6. Examples of typical metamorphic rocks in southern Finland. The pic-ture shows a sequence of alternating sedimentary and volcanic rocks. Brunskär, Korpo (Korppoo). Photo A. Linna.

Other signs of this volcanic activityappearing in the Archipelago includetheamphibolitesandsomeof theacidgneisses. Amphibolite is a metamor-phosedvolcanicrockandiseasytorecog-niseona slightlyweathered surface. Itprincipally contains only two minerals,amphiboleandplagioclase.Onaweath-eredsurface,theamphiboleisblackandtheplagioclase iswhite. The texture isreminiscent of a mixture of black andwhitesugars.

Acid gneiss of volcanic origin is fine-grained(grainsize lessthan1mm)andlight in colour. Usually it also containsvarying amounts of black amphiboles.Figure9 isanexampleofthisrocktypefrom Brunskär, Korpo (Korppoo). The

maficrocksofvolcanicoriginaremarkedingreenonthegeologicalmaps(amfibo-lite,tuff,othermaficvolcanics).Thefelsicvolcanicrocksandotherquartz-feldspargneissesaremarkedinyellow,whilethemicaschistsand-gneissesareinblue.

In the magma chamber beneath thevolcanoes, plutonic rocks like gabbros,diorites and tonalities formed. A goodexampleofsuchmagmachamberisthedioritefoundinKorppoo(Korpo)attheGaltbyharbour(Figure10).

At thesametimeasallof thisvolcanicactivity, erosionalsooccurred. The vol-canoclasticmaterialaccumulatedontheseabottom,togetherwithothermaterialcomingfromthecontinent. Inaddition,carbonatesedimentsalsoaccumulated.

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Figure 7. Schematic cross-sections of the tectonic evolution of southern Finland. North is to the left and south to the right. a. The situation 1900 million yea-rs ago. To the left in the figure, there is a microcontinent, coloured pink. This microcontinent is now situated in central Finland. Green represents the volcanic rocks that formed in the volcanic chain above the subduction zone. The red blob represents partial melting in the mantle. The brown colours are representative of magmas in deep magma chambers. Sediments, formed as the volcanic chain and the continent eroded are shown in blue. b. 1870 million years ago. The vol-canic chain and the microcontinent collide. The volcanic rocks, sediments and plutonic rocks are deformed in the collision (the Svecofennian orogeny). c. 1830 million years ago. The collision is intensified (probably due to the collision of another continent). This increases the metamorphic grade in the crust, which starts to melt at deeper levels (marked in red).

continent

continent

continent

a

b

c

1900 Ma

1870 Ma

1830 Mapresent-dayerosional level

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Figure 8. Lava intruded into wet carbonate sediments. On the right side of the picture, the limestone is reddish and the lava dark. In the central part of the picture is a more extensive area of lava, and around it the limestone has been eroded, giving the rock its particular relief, Velkua. Photo A. Linna.

Figure 9. Acid gneiss, Korppoo (Korpo), Brunskär. Photo A. Linna.

amphibolite

acid gneiss

As thevolcanicarcweathered,calciumwas released. Simultaneously, theCO2content of both the atmosphere andthe sea were elevated because of thevolcanic activity. The calcium and CO2reacted, resulting in the accumulationof calciumcarbonate in the sea.Algaemay have played a role in the deposi-tion process, for example at Tervola innorthernFinland,the2000Mayearoldstromatolite structureswere formedbyblue-green algae. The carbonate rocksformedatanactivecontinentalmargin,inrelativelydeepwateratthebasinmar-gin inpelagicenvironment.During the

regionalmetamorphism,thesecarbonaterocksrecrystallisedasmarble.Marble israthercommoninsouthernFinland.ThemarblequarryinParainen(Pargas)isthebestplacetoseethisrocktype(it isthemetamorphic equivalent to limestone,which is abundant in Estonia). Figure11a isapictureofthemarblequarryatParainen(Pargas),where limestonerawmaterialhasbeenproducedsince1898.Figure11bisfromAhvensaari(Åvensor),Korppoo (Korpo),where it is seenhowthesoftmarble iserodedwhilethesur-roundingvolcanicrockformsthereliefoftherock.

Figure 10. A former magma chamber at Galtby, Korppoo (Korpo). The dark rock is diorite, which crystallised deep within the young crust about 1885 million years ago. This magma chamber was probably located beneath a volcano. This rock type is shown in dark brown on the geological map. The red rocks cross-cutting the diorite are coarse-grained granitic dykes called pegmatites. These formed during the post-collisional uplift in the area around 1800 million years ago. These rocks contain large crystals of the magnetic mineral magnetite. Photo H. Bauert.


magnet

pegmatite cross-cutting diorite

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Figure 11. a. Carbonate sediments accumulated in the sea near volcanoes around 1900 million years ago. Later, these were deformed and recrystallised during the orogeny, forming marble (metamorphic limestone). In places such as Parainen (Pargas), pictured here, the marble became concentrated in the crust. Photo A. Brozinski.

b. Relief formed by erosion of the softer marble around harder volcanic rocks. Note the deformation structures in the volcanic rock. Åvensor, Korppoo (Korpo). Photo A. Linna.

a

b

1880-1860 Ma

The volcanic chain moved toward andcollidedwithamicrocontinent, locatedintoday’snorth (Figure7b).Duringthecollision,slicesofthevolcanicchainandsedimentsfromtheseafloorwerefoldedandthrustedontopofeachother,form-ingamountainchain.Therootsof thechainwerepusheddownward into thehotmantle,wheretheymelted,formingtonaliticmagmas.Figure12showslight-colouredtonalitefromanisletinPerkalafjärden,Iniö.

1850-1810 Ma

Thiswas the timeofhigh-grademeta-morphism in southern Finland. At thistime,rocksnowexposedatthesurface

weresituatedatabout18kmdepth intheEarth’scrust,wherethetemperaturewas between 700 and 800ºC. In thisenvironment, rocks changea lot.Clay-rich (pelitic) formersediments recrystal-lise,resulting inargillites,slates,schists,gneissesandfinally intheformationofpeliticmigmatites.Mineralsthataresta-bleat lowerpressuresandtemperaturesreactandnewmineralsform.Redgarnetanddarkbluecordierite,encounteredalloversouthernFinland,formwhenbiotite(ablackmica)reactswithquartz.Figure13showsanexampleofametamorphic,partiallymoltenrockcontainingrecrystal-lisedgarnetsandcordierites.

Partialmeltingofrocksbeginswiththemelting of pelitic material at around700ºC.The firstmagmasproducedaregranitic incomposition.Thesemagmas

Figure 12. A polished slab of tonalite from an islet south of Perkala, Iniö. This rock type is shown in light brown on the geological map. Photo A. Linna.


20

Figure 13. Garnet-cordierite mica gneiss, a typical rock in southern Finland. The red minerals are garnet and the dark blue minerals are cordierite. This rock type is shown in blue on the geological map.

Figure 14. S-type granite, Kaarina. The majority of the hills in southern Finland as well as the islands in the archipelago are made of this type of granite, which formed via the partial melting of old rocks, particularly old sediments, during the metamorphic peak 1830 million years ago. Photo O. Eklund.

garnet

cordierite


1575 Ma

Aheatpulseofunknownorigincausedextensive melting in the mantle. Hotmagma from the mantle rose into thecrust, causing melting over extensiveareas. This is how the famous rapakivigraniteswereformed.Rapakivigranitescover extensive areas in southwesternFinland.ThemainÅland Island,anareasouthofKökar,theFjälskärbay,VehmaaandLaitilaare coveredby rapakivi (seeback cover). Rapakivi granite is a gen-eralname forgranitesofdifferent tex-ture,whichoccurtogether inthesamebatholith(anextensiveareaofmagmaticrocks).ExamplesfromtheVehmaarapa-kivigraniteareawiborgiticvarietyfromthequarry inMarjuksenranta,Taivassalo(Figure15),andaporphyriticvarietyfromthequarryinHelsinginranta(Figure16).

Atthesametimethattherapakivigran-ites formed, magmas from the mantleintrudedthecrustthroughcracks(figure17a),formingblackdykesofanigneousrockcalleddiabase(figure17b,c).Diabaseisideallysuitedforuseassaunarocks,asitcontainsiron-magnesiumbearingmin-eralssuchasolivineandpyroxene,whichhaveahighheatcapacity.Thesemineralsfillthespaceinathreedimensionalnet-workmadeofplagioclasethatmakestherocksresistanttoalteration(figure17a).

Some younger Precambrian formationsarefoundashortdistancetothenorth,in Satakunta. The topography aroundPori isflat,asthebedrockhere ismadeof Jotunian sandstone. This sandstonewasdepositedinashallowseaaftertheemplacement of the rapakivi granites.Thesandstonecanbeseen intheriver-

formveinsinthehostrock,whichresultsintheformationofdifferenttypesofmig-matites,suchasveingneisses(Figure5a,13).Inadditiontothemigmatitesformedfrompartialmelting,insouthernFinlandthere are also granitoids, which resultfrom the complete or near-completemeltingandrecrystallisationofthehostrocks.ThebedrockofmanyislandsinthearchipelagoandmanyofthehillsalongtheTurku-Helsinkihighwayformedinthismanner.Intheflatareabetweenthehills,softerrockshavebeenerodedtodeeperlevel thanthemoreresistantgranitoidsandgraniticmigmatites.Figure14showsan example of the typical red granite,formed during this intensive phase ofmeltingandrecrystallisation1830Ma.

1800-1760 Ma

This was the time of post-collisionaluplift.Atthistime,thecrustroserapidlyfromdepth,cooledandbecamebrittle.When the crust was brittle, magmaswithacomplicatedoriginwereable torise rapidly through cracks in it. Someweregenerated inthecrustandothersdeepwithintheEarth’smantle.Amongthe rocks formed at this time are themagnetite-bearing pegmatites (coarsegrainedgranitedykes)inthearchipelago(Figure10),andcarbonatites,whicharerarerocksmadeofcalcitethatcrystallisedfrom carbonate magma. A carbonatitedyke can be seen on the southeasternside (before the tunnel) of the Ukko-PekkaBridgeinNaantali.TheredcolouroftherocksatKuparivuorihillinNaantaliisaresultoffluidsderivedfromthecar-bonatitemagma.

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Figure 15. Wiborgite rapakivi granite, Marjuksenranta, Vehmaa. On the left of the picture is a fresh surface and on the right is a slightly weathered surface. The different major minerals stand out on the slightly weathered surface. This rock contains the plagioclase-mantled, egg shaped alkali feldspars called ovoids, typical of rapakivi granites. Rapakivi granites are shown in magenta on the geo-logical map. Photo O. Eklund.

quartz

alkali feldspar

plagioclase

biotite


Figure 16. Porphyritic rapakivi granite, Helsinginranta, Vehmaa. A view of a quarry that produces dimension stones found in buildings all over the world (above). Close-ups of porphyritic rocks (below). Photo O. Eklund.

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Figure 17. Diabase dykes from southern Keistiö, Iniö.a. Microscope picture of a diabase. White and grey laths of plagioclase form a three-dimensional network, wherein crystallises the colourful iron-magnesium rich olivine. Photo A. Peltola, b. A thin, 30-cm-wide diabase dyke cutting the surrounding rock. Photo A. Linna, c. A 2.5-m-wide diabase dyke. These rocks are shown on the geological map as heavy black lines. Photo A. Linna.

a b

c

1275 million years ago, the crust wasagainintrudedbymagmafromtheman-tlethatcrystallisedasolivinediabase.Thisdiabasecanbeseen,forexample,attheSiikaranta camping area at Reposaari,nearPori.Inaddition,theislandofSäppi,southwestofPori,iswhollycomposedofcoarse-graineddiabase.Theserocksaresimilar to the1585Madiabase showninfigure17.Onthegeologicalmaponthebackcover,theserocksaremarkedindarkgreenintheSatakuntaarea.

bedofKokemäenjoki at theHarjavaltapower station. Within this sandstone,tracesofancientripplemarkshavebeenpreserved (Figure 18). When you walkinshallowwateronasandybeachonasunnyday,youcanfeeltheripplemarksinthesoftsandbeneathyourfeet.Theripples in the Jotnian sandstone wereformed in the samewayas the ripplesareformedtoday.ThisisagoodwaytounderstandthatthegeologicalprocessesatworktodayhavebeenactiveoverthewholeevolutionoftheEarth.

Figure 18. The feet of a four-year-old child, standing on a 1300-1500 million year old Jotnian sandstone with ripple marks. Jotnian sandstones are marked in blue on the geological map, and are found in the vicinity of Pori. Photo O. Eklund.


pyrite.Minororeminerals includearse-nopyrite,magnetite,marcasite,löllingite,tetrahedrite, boulangerite, tennantite,bournonite,pyrargyrite,kesterite,sinne-rite, ilmenite,mackinawite,hessite,bre-ithauptite,rutile,molybdenite,electrum,silver,goldandbismuth.

The younger evolution of the crust

ThecrystallinebasementofFinlandwasoncecoveredbyyounger,Palaeozoicfor-mations,justasEstoniaistoday.Althoughmostof thesehavebeenerodedaway,small tracesof theseyounger rocksarefound, for example, in the Lumparn

Metallic ore

Afluid,ingeologicalsense,ishotwatercontaining ions like chlorine, fluorineandsulphur.Hotfluids thatcirculate intheEarth’s crustduring the rock form-ingstagehavethecapacitytoleechele-ments,particularlymetals,fromthesur-roundingrock.Theseelementsmaythenbe transported and precipitated else-where,wherethetemperature is lower.Thisishowsomeofthemostimportantoremineralsareformed.Thefluidstendto follow cracks in the crust, which iswhyoremineralizationareoftenthinandelongated.OnAttuisland,12kmsouthofParainen(Pargas),thereisanorebodythathasbeenminedoffandonsincethe17thcentury.Zinc, lead,copperandsil-verhavebeenproducedhere.Themajororeminerals intheAttuorearepyrrho-tite,sphalerite,galena,pyriteandchalco-

Figure 19. Sulphide ore from collec-tions of Museum of Geology at Tartu University. Photo M. Isakar.

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Figure 20. Dykes filled with Cambrian sandstone. On the left is a close-up picture of the same dyke. Photos A. Linna & O. Eklund.

meteorcraterinÅland.Sometimes,theseyoungerrockscanbefoundfillingcracksinthebedrock,likethedykeofCambriansandstonefromKorppoo(Korpo)shownin Figure 20. In addition, erratic boul-dersofOrdovicianmud-and limestone,as well as some Cambrian sandstone,canbefound inthearchipelago.Theseboulders,manyof them fossil-bearing,originatefromsedimentaryrockdepositsbeneaththeBayofBothnia.

The crystalline Precambrian rocks of Estonia

There are no Precambrian rock out-crops in Estonia. The only crystallinerocksavailableonthesurfaceareerraticboulders–themessengersofcrystallinerocksfromFinlandcarriedbycontinen-taliceduringtheiceage.Thecrystalline

rocksoftheEstonianbasementconsistofearlyProterozoicmetamorphicandigne-ousrocksandarecoveredbyadepositof Palaeozoic sedimentary rocks 100-780mthick.Sincetherearenooutcrops,only drill core material (Figure 21) andgeophysical/geochemicalmethodshavebeen used for the geological study oftheserocks.IntheuppermostsectionoftheEstonianPrecambrianrocks,there isan ancient, dominantly kaolinitic (clay)weatheringcrustwithathicknessrang-ingfromafewmetersupto150meters.Due to intense denudation during thelateProterozoic,thesurfaceofthebase-menthasturnedintoapeneplain,whichdips gently southward (2–3.5 m/km).For example, the crystalline basementliesatthedepthof67.5monVaindloo


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Island (Gulf of Finland), 629.0 m inHäädemeeste (southwest Estonia), and784.1monRuhnuIsland(GulfofRiga)(Figure21).

The Estonian Precambrian basement isdivided intotwomajorgeologicalunits:the North Estonian amphibolite faciesandtheSouthEstoniangranulitefaciescomplexes. These are separated fromeachotherbya tectonicboundary, thePaldiski-Pskov shear zone, and includesixsmallerrockzones.Thesezonesare:South-Estonian, West-Estonian, Tapa,Jõhvi,TallinnandAlutagusezones(Figure22). These zones differ by rock type(metasedimentsormetavolcanites/igne-ousintrusions),degreeofmetamorphismandotherchemical-physicalproperties.

TheEstonian-AustraliangeologistArminÖpikwasthefirst topropose, in1942,that the Estonian basement rocks areanextensionof thesameSvecofennianorogenicsystemastherocks inFinland.WeknowtodaythatatleastthenorthernEstonianrocksareverysimilar to thoseweseeinsouthernFinland,andcancon-cludetheregionshaveasharedgeologi-calhistoryduringtheearlyProterozoic,1920-1575Ma.

The rock formations

From the 1960’s to the 1980’s, mostgeologists believed that the EstonianbasementrocksformedbothduringtheearlyProterozoic (1920-1575Ma)andtheArchean(morethan2500Ma).Later,radioactive isotope geochronological

studieshaveshownthatourbasementismostlyearlyProterozoic,andthereisnodirectevidenceforolder,ArcheancrustinEstonia.

TheSouthEstonianZoneisthenorthernpart of the ca.1000-km-long Belarus-Baltic Granulite Belt, containing pre-dominantly metamorphosed igneousrockswithminormetasedimentaryrocks.These rockshaveexperienced relativelyhigh pressure (up to 6-7 kilobars) andtemperature (700-800ºC), correspond-ing toadepthof18-20km.ThemainrocktypesintheWestEstonianZoneareamphibolitesanddifferentgneisses.TheTapaZonecomprisesamphibolegneisses/amphibolitesand intercalatedpyroxenegneissesinadditiontobiotite-plagioclasegneisses and quartz-feldspar gneisses.ThenarrowJõhviZoneconsistsof inter-layered pyroxene gneisses and quartz-feldspathicgneisses, biotite-plagioclasegneisses,amphibolegneisses,garnet-cor-dieritegneisses, andmagnetite-quartz-ites(Jõhvimagneticanomaly).TheTallinnzoneinnorthernEstoniaiscomprisedofmaficamphibolitefaciesmetavolcanitestometasediments.TheAlutagusezone,alsoinnorthernEstonia,liesinbetweentheJõhviandSouthEstonianZonesandis dominatedbymetasedimentary rocktypes.TheTallinnandAlutagusezonesexperiencedlowertemperatureandpres-sureconditionsthantheotherzones.

Although the majority of the EstonianPrecambrian rocks are metamorphic innature,therearealsosomeoccurrencesof younger, igneous rocks. These rocksaremostlygranites,formingsmall intru-sivebodies,whicharetheproductsofthe


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Metals in Estonian basement

During the Soviet times, the Estonianbasementrockswereextensivelydrilledandstudiedinasearchformetalore.Fromthe metal resources point of view, theJõhviarea innortheastEstoniacontainsgoodoccurrencesofironore(inmagne-titequartzites)andsulphides.TheJõhviironorewasdiscoverednearlyacenturyagoandoccursassubverticalbedswithingneisses.The thicknessof the ironbedisabout100m,andthereserveof ironore (Feover25%) isabout355milliontonnes(calculatedto500mdepth)and629milliontonnes(calculatedto700mdepth).Manyotheroccurrencesofmet-als exist within the crystalline rocks ofEstonianbasement,howeverthesehavenoeconomicvalueatthemoment.

partialmeltingofthesurroundingrocksordeeperones.Thelargestgraniticintru-sionsintheEstonianbasementbelongtothe large-scalerapakivimagmaticevent,around1600Ma.ThelargestoftheseisknownastheRigabatholith.Therearealso at least five minor potassic, por-phyritic granite intrusions: Naissaare,Märjamaa,Taebla,Neeme,Eredaaswellas the Abja quartz-diorite intrusion insouthwestEstonia.Afewsmaller intru-sionsareknown,whicharebasictointer-mediate in chemical composition (con-taining less than 60% SiO2). All theseplutons intruded after the main meta-morphic event, so theyhavepreservedtheiroriginalcrystallisationfeaturesandtextures.

Tallinnzone Alutaguse

zone

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Jõhvi zone

western Estoniazone

southernEstonia

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Gulf of Finland

LakePeipsi

Rapakivi and related granites Rocks of retrograde metamorphism

Microcline granites Rocks of amphibolite facies

Postorogenic rocks Faults and share zones

Mainly granulites or high-temperature amphibolite facies rocks

Figure 22. The major rock units of the Estonian basement.

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REFERENCES

Karhunen,R. (2004)ExplanationofmapsofPre-Quarternary rocksof the IniöandTurkumapsheetareas,sheets1041Iniöand1043Turku.GeologicalsurveyofFinland.76pp.

Koppelmaa, H (Editor) (1998) Põhja-Eesti kristalse aluskorra geoloogiline kaart1:200000 /Geologicalmapof thecrystallinebasementofNorthernEstonia. EestiGeoloogiakeskus/GeologicalSurveyofEstonia.

Lehtinen,M.,Nurmi,P.A.,Rämö,O.T.(eds.)1998.Suomenkallioperä–3000vuosimil-joonaa.GeologicalSocietyofFinland,Helsinki.

Lehtinen,M.,Nurmi,P.A.,Rämö,O.T.(eds.)2005.PrecambriangeologyofFinland.KeytotheevolutionoftheFennoscandianshield.DevelopmentsinPrecambrianGeology14.Elsevier.736pp.

Puura,Väino;Hints,Rutt;Huhma,Hannu;Klein,Vello;Konsa,Mare;Kuldkepp,Reedik;Mänttari,Irmeli;Soesoo,Alvar(2004).Svecofennianmetamorphiczonesinthebase-mentofEstonia.ProceedingsoftheEstonianAcademyofSciences.Geology,53(3),190-209.

Raukas,A.,Teedumäe,A. (eds).1997.GeologyandMineralResourcesofEstonia.EstonianAcademyPublishers,Tallinn.436pp.

Soesoo,A.;Puura,V.;Kirs, J.;Petersell,V.;Niin,M.;All,T. (2004).Outlinesof thePrecambrianbasementofEstonia.ProceedingsoftheEstonianAcademyofSciences.Geology,53(3),149-164.

Suominen,V.1991.ThechronostratigraphyofsouthwesternFinlandwithspecialref-erencetoPostjotnianandSubjotniandiabases.GeologicalSurveyofFinland,Bulletin356,100pp.

Väisänen,M.2002.TectonicevolutionofthePaleoproterozoicSvecofennianorogeninSouthwesternFinland.AnnalesUniversitatisTurkuensis,AII154,143pp.

Rock map © Geological Survey of Finland

km20100

Mica schist and -gneiss (1900 Ma)

Granodiorite, tonalite and quartz diorite (1890-1870 Ma)

Gabbro-anortosite (1590-1570 Ma)

Sandstone and conglomerate

Granite (1800-1765 Ma)

Rapakivi granite (1590-1570 Ma)

Olivine diabase (1275 Ma)

Microcline granite (1840-1820 Ma)

Gabbro and diorite (1890-1870 Ma)

Diabase dyke (1590-1570 Ma)

Fault system

Meteorite crater

Intermediate and felsic volcanics, metasediments and marble (1900-1885 Ma)

Mafic metavolcanics and marble (1900-1885 Ma)

ISBN978-9985-9867-4-5

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