GeochemislryGeochemistry of Proterozoic supracrustalsupracrustal rocksrocks inin Finland Edited by Mikko Nironen andand Yrjö Kähkönen GeologicalGeological Survey ofofFinland, Finland, SpecialSpecial Paper 1919,,85- 85-100,100, 1994.1994 GEOLOGYGEOLOGYAND AND GEOCHEMISTRY OF THE HÄMEENLINNA­HAMEENLINNA- SOMERO VOLCANIC BELTBEI-IL, SOUfHWESTERNSOTJ'THWESTERN FINLAND:FINLAND : A PALEOPROTEROZOIC ISLISLANDAND AARCRC by Gerhard Hakkarainen HakkaraineHakkarainen,n, Gerhard 1994. Geology and geochemistry of the Hämeenlinna•Hämeenlinna- Somero VolcanicYolcanic Belt, southwesternsouthwestem Finland: a PaJeoproterozoicPaleoproterozoic island arc. Geolog­Geolog- ical SurveySun,ey of Finland, Special Paper 1919,,8 85-100,75-100,7 figw-esfigures and 3 tables. The PaJeoproterozoicPaleoproterozoic HämeenlinnaHämeenlinna-Somero-Somero VolcanicYolcanic Belt is situated south oftheof the Bothnian basin in southern Finland, within the South Svecofennian Subprovince. Chemically and lithologically the beltresembles modern island arc environments. phase Three deformational events were recognized in the area. The dominant DD,1 phase folded the supracrustal rocks isoclinally with E-W trending axial planes. The NE trending D and SSE trending D phasesphases vary locally in intensity. Two larlargege D,2 D.3 steeply dipping shear complexes of differentdifl'erent ages are met within the area. The older, W-SW trending Hämeenlinna shear zone predates the intrusion of latelate-­ kinematic potassium granites but postdates the synbnematicsynkinematic granodiorite em­em- placement. The younger, NW trending Painio-Hirsjärvi fault zone shows a sinistral sense of shear and postdates the emplacement of potassium granitesgranites.. The belt is subdivided into two groups. (I)( I ) The (Iower)(lower) Forssa group comprises calc-alkaline metavolcanics and metasediments. The metavolcanic rocks range from basaltic to rhyolitic in composition, with abundant andesites. The area was originallyOIiginally a complex of stratovolcanoes with high erosion ratesrates,, producing large amounts of reworked volcanic matelial.material. (2) The (upper) Häme group is characterized by an association of basalticbasaJtic to andesitic lava flows with interlayered pyroclastics,pyroclastics, erupted in an E-E-WW trending fissure system. The volcanics are layered upon pelitic sediments and graywackes. The earliest exüusivesextrusives in the Forssa group are rhyolitesrhyolites to dacites followed by andesites and basalts. VolcanismYolcanism in thethe Häme group began with basalt eruptions and contemporaneous precipitation of Fe-sulfrdeFe-sulfide formations at the base ofofthe the pile, and volcanismvolcanism subsequently changed intointo andesitic. Major and ffacetrace element geochemistry of thethe volcanic rocks resembles thatthat inin modern mature arcs. In general thethe volcanicsvolcanics are of medium-K type,type, with relatively highabundancesofLILE.high abundances ofLILE. TheForssagroupvolcanicsshowacalc-alkalinefractionationThe Forssagroup volcanics show a caJc-alkaline fractionation trend. The Häme group rocks have more tholeiitic affinities,affmities, with Fe enrichment relativerelative totoMg. Mg. ThePrO.Thepp5 contentcontentis is slightly higherinhigher in thebasaltsthe basalts ofofthe the Häme group thanthan inthoseoftheForssagroup.TheMgnumbersin those ofthe Forssagroup. TheMgnumbers inthein the basalts are moderate (40-52)(40-52) andand thethe Ni content low (20-50 ppm).ppm). Key words (GeoRef Thesaurus,Thesaurus, AGI): volcanic belts,belts, metavolcanic rocks,rocks, basalts,basalts, andesites,andesites, deformation,deformation, geochemistry, stratigraphy, island arcs,arcs, Proterozoic, Paleoproterozoic,Paleoproterozoic , Hämeenlinna,Hämeenlinna , Forssa, Somero, Finland GerhartlGerhard Hakkarainen, DepartmentDepartmen t ofofGeology Geology and MineraktBy.Mineralogy, ÄboAbo Akademi University,University, DomkyrkotorgetDomkyrkotorget I,1, FIN-20500FlN-20500 Turku,Turku, FinlanclFinland PresentPresen( address: Boliden MineralMineral AB , S-77698 Garpenberg, Sweden 85 GeoGeologioallogical SurSurveyvey orof FinlFinland,and , SpSpecialecial PPaperaper 1I 99 GeGerhortlrhard HHukkurairtenakkaraill ell INTRODUCTION ThThee FennoscandianFennoscandian ShieldShield iiss divided intointo threethree and plutonism was aalmostlmost cocontemporaneousntemporaneous with partparts,s, the oldest being the Archean craton (3(3.1-2.'7.1-2.7 Ga)Ga) volcanism (1.89-(1.89-1.831.83 Ga; PPatchettatchett & Kouvo 19861986).). inin the eaeastst (Fig(Fig.. Il).). ItIt iiss bordered to thethe west by thethe TheThe estimated 30-40 Ma duration ofvolcanof volcanicic activity inin Paleoproterozoic Karelian ancland SvecofennianSvecof'ennian terrainsterrains the SvecofenSvecofenniannian resulted inin volcanic bbeltselts of diffdiffer-er­ ((2.5-l2.5-1.7 .7 GaGa).). The yyoungestoungest reregiongion ofProterozoicof Proterozoic age ent chemchemicalical and lithologicallithological characteristics. They iiss the Southwestern Gneiss Complex or the SoutSouth-h­ vary fromfrom bimodal suites in southern Finland (E(Ehlershlers wewestst Scandinavian Domain ((0.9-1.750.9-1 .75 GaGa)) (Gaa!(Gaäl & & Lindroos 1990)1990) to calc-alkaline and alkaalkalineline ssuitesuites GorbatGorbatschev,schev, 19871987).). No ArchArcheanean componentcomponentss have to the north (Kähkönen(Kähkönen 11989),989), whicwhichh indicates a been found inin the central part of the SvecofennideSvecofennides.s. complex development. The Svecofennian ssupracrustalupracrustal rocks were syntecto­syntecto- TheHämeeThe Hämeenlinna-Someronlinna-Somero Volcanic Belt(HSB)Belt (HSB) isis nically intrudedintruded by granodioritegranodiorites,s, tonalitetonalitess and quartz partofthepart ofthe South Svecofennian SubprovinceinSubprovince in sosouth-uth­ diorites, and minor ultramafic and gabbroic bodiesbodies.. western FinFinlandland and forms a large arc-larc-likeike structure The cescessationsation of the SvecofSvecofennianennian orogeny isis postu­postu- striking E-W. It is one of the latlargest"gest exposeexposedd cocoher-her­ latedlated from the largelarge scale anatanatexisexis ofthe supracrustals ent volcanic belts in the Finnish part of the and the emplacement of the latlate-kinematice-kinematic K-rich SSvecofennides. vecofennides. The laterallateral exteextentnt of the belt isis 100I 00 km ggranites.ranites. in the E-E-WW direction and 60 km in the N-S direction. ThThee Svecofennian volcanic rockrockss in Finland yield This study focuses on the central parts ofofthe the bell.belt. ageagess of 1.91l.9l to 11.89.89 Ga (p(Patchettatchett & Kouvo 1986, Simonen (1953(1953,, 19801980)) divided the Svecofennian GaalGaäl & GorbatGorbatschevschev 19871987,, Kähkönen et. al. 1989) supracrustal rocks into three major categories. The SCANDINASCANDINAUAVIA N ,+ Si ,!, I äa6f(=ü a,eI tt ,o Oldonide / 6l!914'- 1- T f+ T + -i- r r ++++++@+++ +}2tJ4r,&=-_-3-1 r * f +++++ llfuns6nlhtra-SomeroHämeenlinna-Somero VolGanlcVolcanic Belt 10to-r- km , , BIffi, 1 I" • EeITJ 9 lllllllllllTlä ,• F=l0 u• F;-]~ ,o10 IM • -1--..---1 7m\7 ~::1 11 • "=1 [-lD .4 F;l@3 s• Fig. I . Fi g. l . CeologicalGeological map ofofthe the ccntralcentral partspan s tll'thcorthe HämeHämee cnlinna-someronlinna-Somero VolcanicVoicanic Belt.Belt. ThcThe thickthic k lineslines inin rhethe upperupper partpan representrepresent thethe Hämeenlinna Shear Shear ZoneZone andand thoscth ose inin thethe southernsOlllhe rn pat'tpan thethe PainioPaini o ShearShear Zone.Zone. 1I = HämeHä me groupgroup basaltsbasalts andand anclesites,andesites, 2 = basiltsbasalts andand andesitesandesites (relationshin(relationship to thc Forssa to the Forssa aandnd HämeHame groupsgroups unknown),un known), 3 = ForssaForssa groupgroup antlesitesandesites andand basalts,4basalts, 4 = dacitesdacites andand rhyolites,rh yolites, J5 = gabbrosgabbros and ultramaficultramafic rocks,rocks, 6 = intercalationsInle rcalatl ons ofquartz-lcldsparof qu art z-reld spa r gneissesg neisses andand micamica gneisscsgneisses oror schists,schi sts, 7 = micaIlli ca schistsschi sts anda nd gneisscsgneisses (originaily(ori g in all y graywackesgraywac kes and pelites),pelites), g8 = volcanic:"o lcanlc conglomerates,9conglomerates, 9 = late-kinematicl a t e~ kln e lll a lJ c K-granites,K-granlles, partlypanly migmatitic,Illiglll ati tic, 10= synkine lllatic granodiorites, tonalites and quartz diorites, I I = = l0 = synkinematic granodiorites, tonalites and quartz dioiites, I I = granodiorites migmatiticIllt glll alilic g ranodlOfIles andancilOll tonalites.aliles. TheThe insetin sel showsshows thethe majormajor provincesprovillces (domains)(domaill s) of thethe FennoscandianFe nnoscalldian Shield.Shield . TheThe dashed lineline renresentsrepresenls thel-adoga-BothnianBavzone.Thestudyareaisshownbythesquare.a=Tampere,b=Enklinge,c=Orijärvi-Lohja.the Ladoga - Bothnl an Bay zone. The stu dy area is shown by the square. a = Ta lll pere, b = Enklinge, c = Orijärvi-Lohja. 86 GeologicalGeological SurSurveyvey ooff FFinland,inland . SpecialSpecial Pappaperer 19I 9 GGeologyeology aandnd geocgeochemistryhemi stry ooff ththee HHämeenlinna-Someroämeenlinna-Somero VolcVolcanicanic BBelt...elt... LowerLower Svecofennian subgroupsubgroup consists of immatureimmature rockrocks,s, while thethe Upper Svecofennian subgroupsubgroup com­com- graywacke-slatesgraywacke- slates and quartz-feldsparquartz-feldspar rocks.rocks. TheThe Mid­Mid- prises argillaceous sediments. Roughly, thethe HSB dIedle Svecofennian subgroup consists of basic and corresponds to the Middle Svecofennian