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 Yolcanic Volcanic 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 granites granites.. 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 Subprovincein Subprovince 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 1987 1987,, 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 represent represent 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 subgroup of intermediateintermediate volcanics and intercalatedintercalated sedimentary Simonen (1980).(1980).

METAMORPHISM

The study area areawas was metamorphosed to amphibolite study area the intrusiverockintrusive rockss comprise granodiorites facies but the original volcanic textures are still and quartz diorites. preserved. ComrnonCommon mineral assemblages in the mafic The higher metamorphic grade in the southernsouthern part rocks are plagioclase and hornblende. Some of the is shown by a higher degree of melting of the hornblende phenocrysts are augite pseudomorphs supracrustal rocks and formation of migmatites. StillStill,, and relict zonation in plagioclase is common. The the volcanic and sedimentary rocks inin the southern sedimentary rocks contain assemblages of biotitebiotite,, partpart,, situated in adepressiona depression adjoined by migmatite garnet and cordierite. rocksrocks,, show lower-grade metamorphic features such The metamorphic grade in the northern part of the as stable muscovite-cordierite assemblages. The study area is slightly lower than in the south. This is boundary between the migmatite zone and the less shownshownby by locally persisting assemblages of cordierite­cordierite- metamorphosed northern zone with granodiorites andalusite-chlorite-white mica in the northern pelitic and quartz diorites runs in a WSW-ENE direction schists. Retrograde reactions are manifested in the near Forssa (Fig. I).l). Further southwards, the alteration of Ca-rich plagioclase to Na-rich plagio-plagio­ amphibolite facies rocks in Orijärvi-Lohja yield tem­tem- claseclase,, zoisite and epidote and in alteration of horn­horn- peratures and pressures of about 650650'° and 3 to 5 kbar blende to biotite. Mäkelä (1980) estimated the prespres-­ (Schreurs & Westra 1986) and the grade of metamor­metamor- sure and temperature to be 3-4 kbar and 600600'C°C at phism was highest in the West-Uusimaa thermal Kiipu in the Forssa areaarea.. In the northern part of the domedome..

SUBDIVISION OF THE STUDY AREA

The volcanic belts in southwestern Finland are Based on lithological differences (Table I,1,Fig.2), Fig. 2), characterizedcharacterized by various lithologies. The unique the belt is divided into two groups: (1) the Forssa characteristic features of different belts are summa­summa- group is dominated by andesites and (2) the Häme rized below. The OrijärviOrijärvi-Lohja-Lohja area (Schreurs & group by basalts.basalts. Furthermore,Furthermore , there are two domains Westra 1986, Colley & Westra 19871987,, Mäkelä 1989, whose relationship to these two groups is uncertain,uncertain, Väisänen 1991) 50 km south of the study area (Fig(Fig.. the Nuutajärvi and Koijärvi domains. 1) consists of bimodal volcanic rocks, and abundant Northwards the Häme group isis in contact with limestones and iron formations.formations. The present study migmatites and gneisses. The basalts rest on pelites,pelites, area is composed of tholeiitic to calc-alkaline rhyolitic and inin addition some of the extrusives in the upper to basaltic volcanic rocks,rocks, containing few Fe-oxide part of the group occur as massive interlayersinterlayers in the formations and limestones. The Tampere volcanics, sedimentogenic migmatites and gneisses to the north. 70 km north of Hämeenlinna, consist mainly of calc-calc­ Within the Forssa group no basement below the alkalinealkali ne high-K dacites and andesites and shoshonitic volcanic rocks has been found (Table I)l).. Since both rocks associated with conglomerates (Kähkönen of the groups are embraced by large granitoid 1989). The Haveri tholeiitic basalt formation also batholiths the only areas where they actually are inin occurs inin this area (Mäkelä 1980, Kähkönen & mutual contact are the Humppila and Nuutajärvi Nironen, this volume).volume). domains (Fig. 1).

87 GeologicalGeological Survey Survey of of Finland, Finland, Special Special Pap Paperer 1919 GerlwrdGerhard HakkarainenHakkarainen

TableTablc I.l. LithologicalLithological differences differences belween between thethe HHämeäme andand ForssaForssa groupsgroups..

ForssaForssa groupgroup HämeHäme groupgroup

NoNo clearlycleally discediscerniblernible sedsedimentaryimentary basement.basement. BBasementasement ofpeliticof pelitic scschistshists aandnd volcanogenicvolcanogenic conglomerates. conglomerates.

TheThe mostmost abuabundantndant rockrock ttypeype i iss andesiteandesite.. TheThe mmostost ababundantundant rockrock ttypeype i iss babasalt.salt.

AcAcidid vovolcaniclcanic rocksrocks aarere abu abundant.ndant. AcidAcid vovolcaniclcani c rocksrocks areare spa sparse.rse.

BothBoth uralite-uralite- aandnd plagioclase-porphyriticplagioclase-porphyritic rocks.rocks. MoMostlystl y ururalite-porphyriticalite-porphyritic rockrocks.s.

IntIntermediateermediate aandnd maficmafic llavasavas withwith pillowpillow sstructurestructures andand TheThe mmainain ppartart consistsconsists ofof mamassivessive lavalava f1owsflows,, thethe offshootsoffshoots aautobrecciation.utobrecciati on. ((Kalvola)Kalvola) areare aautobrecciatedutobrecciated andand pillowedpillowed..

Sedimentar'ySedimentary ((pelitic)pelitic) intercalationintercalationss inin thevolcthevolcanisani s formations.formations. NoNo peliticpelitic intercalationsintercalations inin thethe volcanicvolcanic formations.formations.

FOFORSSARSSA GGROUPROUP

The ForForssassa group includesincludes threethree synformal EE-W-W plagioclase-porphyritic basalts and andesites to var­var- trending sstructures,tructures, which in order fromfrom south to ious plagioclase-porphyritic dacitic and rhyolitic lavas north are the Somero, For Forssassa and Humppila domainsdomains.. and pyroclastic rocks, andesite being the most abun­abun- ThThee volcanic rocks range from uralite- and dant rock type.

HHumppilaumppila ddomainomain

Here the Forssa group consists of basaltic uralite-uralite­ Häme group basalts close to the depositional contact porphyritic agglomerates and lavas, and of thick withtheForssagroup.TheFe-formationsoccuras3-with the Forssa group. The Fe-formations occur as 3- andesitic to dacitic tuffstuffs inin the southernsouthern part. In the 4 m thick disseminated layers between basalt units or northern part therethere is a heterogenous association of as more dispersed inin bodies of mica-rich altered laminated intermediateintermediate and acid tuffs,tuffs, tuffites,tuffites, ag-ag­ basalt. Dacitic toto rhyolitic pyroclastic interlayersinterlayers or glomerates, lavaslavas and minor pelitic interlayers.interlayers. The fragments are also found inin thethe basalts. The relation-relation­ Häme group basaltsbasalts lielie inin a NE trendingtrending syncline, and ship betweenbetween thethe twotwo groups isis based on the fact thatthat thebasaltsareexposedonthepresenterosionsurfacethe basalts are exposed on the present erosion surface thethe Fe-formations elsewhere also occupy a inin thethe eastern part of thethe domain only. At Humppila stratigraphical positionposition inin thethe lowestlowest partpart oiof thethe Fe-sulfideFe-sulfide formationsformations are as intercalationsintercalations inin thethe Häme group.group.

ForssaForssa domaindomain

InIn thethe ForssaForssa domaindomain severalseveral differentdifferent lithologicallithological limestonelimes tone layerslayers (Lindroos(Lindroos 1980,1980, MäkeläMäkelä 1980).1980). entitiesentities cancan bebe distinguished.di stinguished. SuccessionsSuccessions ofof thickthick InIn thethe areaarea southsouth ofof ForssaForssa gradinggrading intointo distaldistal andesiticandesitic lavalava flowsflows andand agglomerätesagglomerates areare thethe mostmost faciesfacies volcanicvolcanic andand volcaniclasticvolcaniclastic rocksrocks indicatesindicates abundantabundant ones.ones. TheThe lavaslavas areare partlypartly pillowedpillowed andand increasingincreasing distancedistance fromfrom thethe volcanicvolcanic centers.centers. TheThe autobrecciated.autobrecciated. SeveralSeveral acidacid eruptiveeruptive centerscenters withwith centralcentral ventsvents werewere identifiedidentified byby successionssuccessions ofof thick thick rhyoliticrhyolitic andand daciticdacitic lava lava domes, domes, enclosedenclosed in in agglom-agglom­ lavas,lavas, coarsecoarse agglomeratesagglomerates andand poorlypoorly sortedsorted tuffs.tuffs. erates eratesand and laminatedlaminated tuffs, tuffs, occuroccur within within thethe andesites. andesites. AtAt the the western western marginmargin of ofthe the Forssa Forssa domain, domain ,lavas lavas are are TheThe KiipuKiipu areaarea inin thethe northernnorthern partpart ofof thethe ForssaForssa rarerare andand laminatedlaminated fine-grainedfine-grained tuffitestuffites areare abun-abun­ domaindomain containscontains aa successionsuccession withwith reworkedreworked acidacid dant.dant. Accordingly,Accordingly, the the volcanicvolcanic centers centers were were situatedsituated volcanicvolcanic rocks,rocks, sulfidesulfide mineralizationsmineralizations andand sparsesparse inin thethe centralcentral partpart ofof the the ForssaForssa domain,domain , wherewhere thethe

8888 Geological Survey ofof Finland, SpeciSpecialal Paperpaper 19l9 GeologyGeology aandnd geochemisgeochemistrytry ofof thethe HHämeenlinna-Someroämeenlinna-Somero Volcanic BBelt...ell... largelargestst sheets of basalt and andesite lavaslavas occur and es relativelyrelatively maficrocksmafic rocks withwithbasaltic basaltic autobrecciated where the rhyoliterhyolite domes are found.found. TheThe narrow neck and partly pillowed lavas,lavas, lapillilapilli tuffstuffs and agglomer­agglomer- between the Somero and the ForForssassa domains compris- ates interlayeredinterlayered with sparse andesitic tuff horizonhorizons.s.

Somero domain

The Somero domain was described by Mäkelä HirsjärviHirsj?irvi faultfaultby byMäkelä Mäkelä (1989). The WNW toESEto ESE (1989) and Stel et. al. (1989). The geology iiss control­control- trending sshearhear complex dividedividess the area into two ledled by a sysynformalnformal depresdepressionalsional structure oriented lithologicallylithologically different parts (Fig. 1).1). South of the subparallel with the Painio sshearhear zone, also called the shearshear complex the rocks consist momostlystly of andesites

HÄMEHAME GROUPGRoUP HÄMEHAME GROUP KALVOLA f::-= CENTRAL PARTS .. FFFFFH t---l C2J 55ffi 9gFA ~== [-lwHÄME GROUP ,,, .. l:-.'-l "*'-:'":: 2,hl:l ~... I'" .... ,I' 66E •~ 10ß%l M kMzi l WESTERNwESTERN PARTS- I I fl3m 3 G1J, /. 7?m ~ 11na ~: r .il f?qrl 44r 8I ~- - 122l%l ~ =z--l t"''',1r::::t:-] ILdI I FORSSAFORSSAGROUP GROUP • E I I lxl I I HUMPALAHIJMPPil-A - - r^-^l -I - I d NUUTAJÄRVlrrevr^udvl -rr=tr------= - - - FORSSA GROUP I -:: FORSSA ffiffiFffiE'$g+: + : SQMEROsoilERo r: FORSSAFoRSsA GROUPGRouP : : ffi ig€i - : ffi l==:l N-FORSSA FORSSA=o=*--- GROUP : -=ffi ü- rJ--:r S-FORSSA -:ftHE*+H* FHlts++f, EEgtrl l-:-_ | -^ Ir l.:--: _l ffiffiffi H_ r""'"1 flI I E=J I l.-'r]-l1 ffiffiiffi I +:l ffiK'l E I t.1'l

W ffi00000 ffil- --' -*HE]- --

The Fig.Fi g. 2. StratigraphicStratigraphie columnscolumns fiomfrom thethe Forssa and the HämeHäme groups. KalvolaKalvola representsrepresents anan oft.shootoffshoot inin thethe gneissesgnei sses and schistsschists toto the north. The western partsparts of thethe HämeHäme group group constituteconstitute thethe Lautaporras area (Fig.(Flg. 1).I). The centralcentral partsp~rts.of of thethe HämeHarne groupgroup constitutecon~tttute thethe areasare~s easteast of Lautaporras.Lautaporras. The intermediateintermediate andand mafic volcanic rocksrocks inin thethe uppermostuppermos t part ofoftheNu the NuutajärviutaJarv l columncolumn belongbelong toto thethe HämeHarne group.group. 1I =- rhyolite'rhyollte, 2=daciticfeldsparporphyry,3=andesiticuraliteiplagioclaseporphyry,4=basalticuralite/plagioclaseporphyry,5=gabbro,6=graywacke2 = dacitic feldspar porphyry, 3 = andesiti c uralite/plagioclase porphyry, 4 = basaltlc uraltte/plaglOc lase porphyry, 5 = gabbro, 6 = graywacke formation / limestone, 124 /I pelite,pelite,1 7 = conglomerate, 8 = tuffitetuffite /I tuff,tuff, 9 = agglomerate,agglomerate, 1010= = pillowpiliow lavalava /I autobrecciatedautobrecciated lava,lava, I11 I -= Fe-sulfideFe-sulfIde formatIOn I ltmestone, 12A = "onglo*".u1", = Mg-metasomaticMg-metasomatic rock,rock, 12D12D = diopside-bearingdiopside-bearing rock.rock.

89 GeGeologicalological Survey Survey ofof FinlandFinland,, SpecialSpecial PaperPaper 1919 GGerharderhard HakkarainHakkarainenen

andand these belongbelong to to thethe ForssaForssa group.group. ThisThis sequencesequence tufftuff formationformation lieslies southsouth ofof thethe SomeroSomero town.town. TheThe containscontains aa Zn-AgZn-Ag sulfidesulfide mineralization describeddescribed successionssuccessions northnorth ofof thethe shearshear complexcomplex showshow aa widerwider byby MäkeläMäkelä (1989)(1989).In. In additionaddition,, aa300 300 mmthickrhyolitic thick rhyolitic rangerange ofof rocksrocks includingincluding basalts,basalts, quartz-feldsparquartz-feldspar toto daciticdacitic plagioelase-quartz-porphyriticplagioclase-quartz-porphyritic stratifiedstratified gneissesgneisses,, andand turbiditicturbiditic pelitic schistsschists andand gneisses.gneisses.

HÄMEGROUPHAME GROUP

TheThe HämeHäme groupgroup consistsconsists oftholeiitic of tholeiitic plagioclase­plagioclase- by plagioelase-porphyriticplagioclase-porphyritic andesites,andesites, which inin turnturn and uralite-porphyritic basalts gradinggrading intointo even­even- areare cutcut andand brecciated by younger uralite-porphyritic grained or plagioclase-porphyritic andesites.andesites. InIn itsits basalt dikes.dikes. present outline itit isis a 5 km wide irregular zone On the southernsouthern margin of thethe groupgroup the basalts striking E-E-WW forfor approximately 100 km. At Lauta­Lauta- restrest upon a 100-200100-200 m thick sequence of volcanic porras the volcanic rocks are foldedfolded intointo a largelarge conglomerates and interlayeredinterlayered tuffs. The conglom­conglom- northeastwardly plunging synfQrm,synform, which is partly erates are poorly sorted and contain subrounded intruded and split by the Forssa gabbro (Figs. 1I and elastsclasts of plagioclase- and uralite-porphyritic and 2). East of Lautaporras, in the central partpart,, the belt even-grained basalts. They have a matrix-supported bends from NE towards EE.. The group isis built up of fabric and resemble debris flow deposits. Carbonate thick lavaflowslava flows and interlayered basaltic pyroelastics.pyroclastics. breccia pipes and a 1-2l-2 m thick banded Fe-oxide­Fe-oxide- The most abundant rock type is a uralite porphyritic chert layer occur in the lowermost part of the con­con- rock wiwith th 2-3 mrnmm hornblende phenocrysts and smsmaller aller glomerates. Also inin the lowermost part of the con­con- plagioelaseplagioclase phenocrysts which occasionally show glomerates at Lautaporras dacitic feldspar-porphyritic oscillatory zoning. However, some of the andesitic fragments are common, and their shapes vary from end members contain only plagioclaseplagioelase phenocrysts. angular to weilwell rounded. Similar rocks are found in Augite is seldom found as phenocrysts due to the the ForsForssa a groupgroup.. The conglomerates are located strong uralitization. The pyroclasticpyroelastic rocks are mostly between two basalt formations but this arrangement poorly sorted, although some reworked tuffs in the is not depositional, rather it is caused by folding. (see upper part of the volcanic pile are graded. page 92.) Deformed basaltic feeder dikes occur throughout Approximately 2-5 m thick laminatedlaminared Fe-sFe-sulfideulfide the Häme group. Most ofthemof them are fine-grained with formations occur in connection with the basal con­con- small- or medium-sized hornblende phenocrysts, glomerates. Their occurrence is not restricted to the but coarse-grained porphyritic hornblenditic dikes depositionalleveldepositional level of the conglomerates, but they are and sills, containing more than 507o50% phenocrysts, partly interlayered withbasaltswith basalts higherinhigher in stratigraphy. also occur in the volcanic pile and in the underlying South of the volcanics and conglomerates there pelitic schists. are pelitic and laminatedlaminated schists containing corroded Several branches of the upper part of the Häme cordierite pseudomorphs and a metamorphicmetamorphie differ-differ­ group extend intointo the gneissose sediments to north.north. ential banding. Top of strata directions were deter-deter­ At Kalvola theythey consist of several generations.ofgenerations· of mined from scour surfaces, graded bedding, slump-slump­ autobrecciated and pillowed uralite-porphyritic ing and load casts in several locations alonsalong the basalts. Stratigraphically thesethese rocksrocks are succeeded contact between thethe pelitespel ites and thethe basalts.

KOIJARVIKOIJÄRVI AND NUUTAJARVINUUT AJÄRVI DOMAINS

The Koijärvi domain divides thethe Häme group intointo by thethe Forssa gabbro inin thethe east. The observations of two parts. two parts. The NW corner of thethe domain consistsconsists of Neuvonen (1954)(1954) suggest thatthat thethe Koijärvi domain a syncline containing graded a syncline containing graded cordierite-mica schists,schists, waswas uplifteduplifted relativerelative toto thethe Häme group. It seems interlayered interlayered with rhyoliticrhyolitic lavalava flowsflows and basaltic appropriate toto inferinfer thatthat thethe KoijärviKoijärvi domaindomain repre-repre­ tuffs. gradation tuffs. The gradation betweenbetween mica schists and sents a blockblock thatthat waswas juxtaposedjuxtaposed verticallyvertically by faultsfaults volcaniclasticsvolcaniclastics suggests a locallocal origin forfor thethe toto thethe same levellevel asas thethe HämeHäme group.group. sediments.sediments. TheThe SE partpart consistsconsists ofof aa thickthick sequence TheThe NuutajärviNuutajärvi areaarea formsforms an ellipsoid-shapedellipsoid-shaped of rhyolitic of rhyolitic lavalava flows,flows, reworkedreworked pyroclasticspyroclastics andand aa anticline.anticline . TheThe internalinternal stratigraphystratigraphy isis deduceddeduced fromfrom bandedbanded chert-Fe-sulfidechert-Fe-sulfide formation.formation . TheThe domaindomain isis structuralstructural criteriacriteria andand toptop ofof stratastrata determinations.determinations. bounded by bounded by thethe KoijärviKoijärvi shearshear zonezone inin thethe westwest andand AccordingAccording toto NeuvonenNeuvonen (1954),(1954), thethe supracrustalsupracrustal

9090 GeologicalGeological SurveySurvey ofof Finland, Finland, SpecialSpecial paperPaper l919 GeologyGeology andand geochemistrygeochemistry ofof the the Hämeenlinna-someroHämeenlinna-Somero Volcanic Volcanic Belt...Belt. .. rocksroeks canean bebe divideddivided intointo threethree majormajor units.units. TheThe graywackesgraywaekes andand pelitespelites withwith minorminor basalticbasaltie toto daciticdaeitie author'sallthor's observationsobservations areare broadlybroadly compatibleeompatible withwith tuffs.tuffs. ItsIts lowermostlowermost part part isis dominateddominated byby pelitespelites withwith thosethose ofof Neuvonen,Neuvonen, withwith slightslight modifications.modifieations. (l)(1) localloeal sulfide-richsulfide-rieh intercalations.interealations. (3)(3) TheThe uppermostuppermost TheThe oldest oldest unit unit consists, eonsists, fromfrom thethe lowermostlowermost member member unitllnit compriseseomprises basalticbasaltie toto intermediateintermediate pyroclasticspyroclasties upwards,upwards, ofof basalticbasaltie pillowpillow lavas,lavas, mudstonesmudstones withwith andand lavas,lavas, whichwhieh belongbelong toto thethe HämeHäme group.group. ItIt startsstarts interlayersinterlayers ofof basalticbasaltie andand rhyoliticrhyolitie toto rhyodaciticrhyodaeitie withwith andesiticandesitie agglomeratesagglomerates andand lavas,lavas, whichwhieh areare tuffs,tuffs, calcareousealcareous precipitates,preeipitates, andand rhyodaciticrhyodaeitie lavaslavas succeededsueeeeded byby basalticbasaltie tuffstuffs andand lavas.lavas. andand reworkedreworked tuffs.tuffs. (2)(2) TheThe middlemiddle unitunit consistseonsists ofof

DEFORMATIONDEFORMATION

TheThe structuralstruetural stylesstyles ofof thethe Orijärvi-LohjaOrijärvi-Lohja areaarea MAp oF HAMEENLTNNA.SoMERo volcANtc BELT andand thethe Tampere SchistSehist BeltBelt havehave beenbeen investigatedinvestigated byby several authors. The HSB isis situated betweenbetween thesethese twotwo areas.areas. NironenNironen (1989)(1989) proposed forfor thethe centraleentral partsparts of thethe Tampere SchistSehist Belt an initialinitial deformation D,D 1 with subverticalsubvertieal axial planes and horizontal fold axes.axes . D,D2 and D.D3 are non-penetrative low-angle shears inin relationrelation toto D,D 1 with dextral and sinistral asymmet-asymmet­ ricrie folding, respectively.respeeti vely. Ploegsma and Westra ((1990) 1990) describeddeseribed in the Orijärvi area a recumbentreeumbent D,D folding 1 Dl-STRUCTURESD1 - STRUCTURES event with subhorizontalsub horizontal E-WE-W trending axial planes N r-_l= D2-srRUcruREsD2 - STRUCTURES thateollid beeorrelated with theD eventnearTampere. A Fr D3 - srRucruREsSTRUCTURES thatcould be correlated with the D,I event nearTampere. il- It F SHEARZoNESSHEARZONES The NE-SWNE-SW oriented DD,2 dominates the tectonicteetonie style. [ä- tg lal ANnroRMsANTIFORMS The locallyloeally oeeuringoccuring D,0 3 eonsistsconsists of open folds with ßn- ^ TI-l SYNFORMS axial planes in NW-SENW-SE to NE-SW directions.direetions. The deformational styles in the northern part of the HSB, whiehwhich eomprisescomprises aB all domains exeeptexceptSomero, Somero, Fig. 3. Structural map oftheof the Hämeenlinna-SomeroHämeenlinna-Somero Volcanic Belt. Only the major tecttectoniconic features are illustrated. Plutonic rocks are shown as are illustrated in Fig. 3 and listed in Table 22.. The DD, 1 are illustrated sshadedhaded areas.

Table 2.2. Record of ththee deformational hihistorystory in ththee HämeenlinnHämeenlinna-Someroa-Somero Volcanic Belt.

pre-D, S. Slumpings,Slumpings, loadload casts,casts, sedimentarysedimentary dykedykes,s, ripple marks

Dr Fr TightTight toto openopen foldingfolding with subhorizontalsubhorizontal foldfold axes.axes. s Dominant penetrativepenetrative axial-plane axial-plane scschistosity,histosity, uusuallysually oriented oriented E-W.E-W. MetamorphieMetamorphic differentiation.differentiation. -l Dominant

dextral sense D2 '2F Small-scaleSmall-scale asymmetrieasymmetric dragdrag foldsfolds toto large-scalelarge-scale openopen foldsfolds withwith dominantly dominantly dextral sense ofof shear.shear. SteeplySteeply plungingplunging foldfold axesaxes (>(> 6060").°). q '2 AxialAxial-planecrenulationcleavagedeformingSnandS,,oriented-plane crenulation cleavage deforming So and SI' oriented 20-6020-60"withsubverticalaxialplanes'° with subvertical axial planes.

plutonic emplacement. D. ^3 LocalLocal large-scalelarge-scale undulatoryundulatory upliftinguplifting associatedassociated withwith plutonic emplacement. OpenOpen foldsfolds withwith steepsteep axialaxial planesplanes strikingstriking 115-I l5-130".I 30°. S. WeaklyWeakly developeddeveloped axialaxial planeplane schistosity.schistosity.

D1 F1 Non-penetrativeNon-penetrative brittlebrittle toto plasticplastic shearshear deformation.deformation. s NW-SENW-SE toto NE-SWNE-SW orientedoriented shearshear zoneszones locallylocally bendingbending thethe earlierearlier S-planes.S-planes.

919l GeologicalGeological Survey Survey ofof Finland Finland,, SpecialSpecial PaperPaper 19l9 GerhardGerhard HakkarainenHakkarainen eventevent waswas anan E- E-WW orientedoriented isoelinalisoclinal toto openopen folding,folding, foundfound inin thethe TampereTampere Schist Schist Belt,Belt, butbut thethe moremore flat­flat- lying S I planes in Somero may reflect the regional withwith initiallyinitially subhorizontalsubhorizontal foldfold axes.axes. TheThe DD,2 eventevent lying S, planes in Somero may reflect the regional waswas regionallyregionally aa non-penetrativenon-penetrative asymmetricalasymmetrical shearshear changechange fromfrom recumbentrecumbent foldingfolding in in OrijärviOrijärvi toto moremore folding,folding, predominantlypredominantly dextraldextral withwith subverticalsubvertical axialaxial uprightupright foldingfolding northwardsnorthwards toto thethe Forssa-Hämeen•Forssa-Hämeen- linna area. planesplanes orientedoriented 4040'to° to 6060'.°. DD22 developeddeveloped locallylocally aa linna area. penetrativepenetrative S2S, eleavagecleavage,, whichwhich overprintsoverprints thethe SoSo SeveralSeveral complexescomplexes ofof megashearsmegashears transecttransect thethe beddingbedding andand thethe SIS, axialaxial planeplane schistosity.schistosity. SS,2 isis SvecofennianSvecofennian bedrockbedrock inin southernsouthern Finland.Finland. Two Two largelarge markedlymarkedly well-developed well-developed inin thethe westernwestern partpart ofof thethe shearshear complexescomplexes areare detecteddetected inin thethe studystudy area,area, thethe HämeHäme groupgroup andand inin thethe northernnorthern partpart oftheof the HumppilaHumppila PainioPainio shearshear zonezone inin SomeroSomero (Mäkelä(Mäkelä 1989,1989, StelStel et.et. domain.domain. AtAt LautaporrasLautaporras thethe basalbasal conglomeratesconglomerates ofof al.al. 1989)1989) andand thethe HämeenlinnaHämeenlinna shearshear zonezone (Fig.(Fig. 3).3). thethe HämeHäme groupgroup areare structurallystructurally betweenbetween thickthick basaltbasalt TheTherocks rocks inin thethe shearshearzones zones varyvary fromfromprotomylonites protomy lonites formations.formations. TheyThey areare exposedexposed inin thethe corecore ofof aa DD,I andand mylonitesmylonites toto pseudotachylytes.pseudotachylytes. UsuallyUsually aa mylonitic foliation developed around the crushed antieline.anticline. DuringDuring DD22 thethe SIS, andand SSoo structuresstructures werewere mylonitic foliation developed around the crushed foldedfolded intointo aa large-scalelarge-scale openopen synformsynform plungingplunging elastsclasts andand changedchanged them them to to resembleresemble protomylonites.protomylonites. towardstowards the the NE.NE. TheThe HämeenlinnaHämeenlinna shearshear zonezone trendstrends E-WE-W andand TheThe DD, eventevent isis observedobserved locallylocally andand itit isis alsoalso formsforms anan incoherentincoherent lineament lineament withwith irregularirregular bound­bound- 3 recorded (1989) Somero. is ariesaries andand consisting of several branches. AtHumppila recorded byby StelStel etet al.al. (1989) inin Somero. DD,3 is consisting of several branches. At Humppila defineddefined byby largelarge subhorizontalsubhorizontal open open foldsfolds andand thethe itit branches outout inin aa duplexduplex whichwhich comprisescomprises thethe associatedassociated schistosityschistosity isis developeddeveloped onlyonly inin narrow Nuutajärvi andand HumppilaHumppila domains.domains. ObservationsObservations onon zones on the fold limbs. prominent feature both dextral and sinistral sense of shear have been zones on the fold limbs. A prominent DD,3 feature both dextral and sinistral sense of shear have been occurs inin thethe migmatized gneisgneisses ses east of Forssa, made. TheThe Koijärvi sedimentary-acidsedimentary-acid volcanogenic where SS,2 and small-scale asymmetric foldsfolds ofDof D,2 are basin was juxtaposed against the Häme group during folded by F..• S, is here expressed as a metamorphic this faulting.faulting. The diversity of movement directions folded by F 3 SI is here expressed as a metamorphic diversity of movement directions segregation in 0.5-1 mm thick mafic and felsic lam­lam- indicates that the shear zone was affected by inaeinae or as a strong planar fabric in the aluminous mica displacements both horizontally and vertically. gneiss layers. The regional consistency of SS,2 implies In the northern part of the Humppila domain the that the D., event was not regionally prominent. extremely well-developed S2 is the dominant that the D 3 event was not regionally prominent. extremely well-developed S, is the dominant Do comprises n.urow N-S oriented brittle shears schistosity. The areas where S2 dominates over SI D4 comprises narrow N-S oriented brittle shears The areas where S, dominates over S, with local folding of the earlier schistosities and the and where the shears of the Hämeenlinna shear zone bedding around the fracture planes. form duplex structures coincide fairly weilwell with each The southern migmatite zone and the northern other. In these areas both structures are oriented NE part, dominated by granodiorites, show slightly dif-dif­ and hence these structures may have a genetic rela-rela­ ferent deformational patterns. The lineations are gen-gen­ tionship. erally steep in the granodiorite zone. In contrast, the The Hämeenlinna shear zone cross-cuts and de-de­ lineations in thethe migmatite zone tendtend to be forms thethe intermediateintermediate synkinematic granitoids. subhorizontal plunging lessless thanthan 45'.45°. This may be However,However, within thisthis zone therethere isis a totally un-un­ explained by subparallelism of S,S land and S'S2' resultingresulting inin deformed latelate kinematic K-granite. In contrast, thethe typetype 0 fold patterns of Ramsay & Huber (1987).(1987). Painio shear zone regularlyregularly deforms thethe latelate kine-kine­ Transposition of S,SI towards S,S2 causes thethe intersec-intersec­ maticmatic pegmatitic and granitic veins. Hence thethe NW-NW­ tiontion lineationslineations toto bebe subhorizontal whenwhen correspond-correspond­ SE trendingtrending lineamentslineaments tendtend toto bebe youngeryounger and thethe E-E­ inging axialaxial planesplanes havehave differentdifferent dips.dips. NoNo observationsobservations WW toto NE-SWNE-SW trendingtrending zones,zones, likelike thethe HämeenlinnaHämeenlinna recumbent ofof recumbent foldingfolding duringduring D,D I werewere mademade inin thethe shearshear zone,zone, areare slightlyslightly older.older. TheThe movementsmovements inin thethe northernnorthern parts,parts, butbut inin SomeroSomero thethe S,S Iaxial axial planesplanes tendtend HämeenlinnaHämeenlinna andand thethe PainioPainio shearshear zoneszones areare ofof dif-dif­ toto bebe moremore horizontal,horizontal, causingcausing thethe differencesdifferences inin ferentferent agesages andand theythey areare closelyelosely relatedrelated toto thethe em-em­ lineationslineations mentionedmentioned above.above. Generally,Generally , thethe placementplacement ofof thethe plutonicplutonic rocksrocks inin respectiverespective areas.areas. deformationaldeformational stylesstyles ofof thethe HSBHSB resembleresemble thosethose

GEOCHEMISTRYGEOCHEMISTRY OFOF THETHE VOLCANICVOLCANIC ROCKSROCKS

InIn general,general, thethe rocksrocks ofof thethe ForssaForssa groupgroup rangerange RepresentativeRepresentative analysesanalyses ofof the the HSBHSB volcanicsvolcanics areare inin fromfrom basaltsbasalts toto rhyolites,rhyolites, butbut intermediateintermediate rocksrocks areare TableTable 3.3. TheThe classification elassification basedbased onon alkaliesalkalies isis ques-ques­ thethe mostmost commoncommon ones.ones. TheThe HämeHäme volcanics,volcanics, inin tionabletionable toto somesome degreedegree sincesince thesethese elementselements areare contrast, are dominated basalts contrast, are dominated byby basalts andand basalticbasaltic sensitivesensitive toto alteration.alteration. However,However, thethe K,O+Na.OK20+Na20 vs.vs. andesites, andesites, whilewhile dacitesdacites andand rhvolitesrhyolites areare scarce.scarce. SiO,Si02 diagram diagram showsshows thatthat thethe rocksrocks ofof the the ForsiaForssa and and

9292 GeologicalGeological SurveySurvey of Finland,Finland, SpecialSpecia l PaperPaper 1919 GeologyGeology and geochemistrygeochemistry of thethe Hämeenlinna-SomeroHämeenlinna-Somero VolcanicVolcanic Belt..Bell..

Table 3. RepresentativeRepresentati ve analyses oftheof the volcanic rocksrocks fromfrom thethe HämeHäme group (H)(H) andand thethe Forssa groupgroup (F).(F).

Numb 1.1. 2.2. 3.3. 4.4. 65. 6. 7.7. 8.8. 9.9. 10.10. 11.11 . Label 459459.2.2 109 215 65 144.2 323 1848 459-1459.1 338 12'l121 37 _G_r_o_upGroup______H______H______H______F ______H ______F______~F F______~H~ H ____~F FF______~F ______F

Si0 sio,2 45.9545.95 48.3048.30 49.9049.90 50.4050.40 53.9053.90 54.9054.90 58.5058.50 59.2059.20 59.2659.26 61.50 70.9070.90 Tio.Ti02 1.611.61 1.121.12 0.830.83 1.11 1.091.09 1.63 'r1.01 -o1 0.860.86 0.840.84 0.550.55 0.300.30 AlP"AI 20 , 17.4517.45 15.3015.30 18.3018.30 15.7015.70 15.7015.70 14.40 15.60 15.1015.10 17.6917.69 17.fi17.50 14.70 FeO'FeO· 11.6011 .60 11.3611 .36 9.819.81 '11.2311.23 10.44 11.2711 .27 8.698.69 6.336.33 8.71 6.146.14 3.25 MnO 0.180.18 0.200.20 o170.17 0.200.20 o.170.17 0.'180.18 o.170.17 0.130.13 0.07 o.120.12 0.060.06 McoMgO 6.676.67 6.746.74 5.625.62 4.674.67 3.973.97 3.183.18 2.962.96 2.872.87 2.172.17 2.672.67 0.640.64 CaO 8.438.43 10.2010.20 8.598.59 8.798.79 7.36 7.377.37 6.306.30 9.489.48 4.394.39 5.665.66 1.771.77

Na20Na 20 2172.17 2.912.91 2.842.84 2.312.31 2.742.74 3.253.25 2.812.81 3.303.30 3.793.79 3.773.77 4.O24.02 KrOK,o 2.832.83 0.730.73 0.75 1.40 1.801.80 1.391.39 1.83 1.24 2.682.68 0.910.91 3.453.45 P9"P20 . o170.17 0.300 .30 0.30 0.240.24 0.370.37 0.270.27 0.250.25 0.200.20 0.190.19 0.18 0.09 Sum 97.0697.06 97.1697.16 97.11 96.0596.05 9797.54.54 97.84 98.1298.12 98.7198.71 99.79 99.0099.00 99.1899.18 CrCr(ppm) (ppm) 42 240 167 50 70 20 <10 Ni 70 90 32 30 30 10 14 20 26 2124 16 Co 23 - 11 V 165 290 224 270 220 320 188 180 316 103 39 Cu 20 60 320 190 60 150 38 20 893138 93 13 Zn 150 'f110 10 117 140 120'120 140 95 90 63 73 59 Ss 90 60 220 170 80 190 79 50 420 1330 90 Rb - <20 7'l71 <20 27 90 Ba 353 192 22s225 242 373sr; 3s;393 234 343 272 257297 729 Sr 270 530 576 316 47047o 2902eo 214 440 163 217 194191 Zr 90 60 91 90 130 150 144 150 149 113 244241 Mg-value (0,(,)(%) 50.6050.60 51.3951 .39 50.5150.51 42.5642 .56 40.39 33.45 3737.77.77 44.6844.68 3030.74.74 43 43.65.65 25.97 Sum of alkalies 5.005.00 3.643.64 3.593.59 3.713.71 44.54.54 4.64 4.644.64 4.544.54 6.47 4.684.68 7.477.47 CIPW Normative composition (wt%),(wt%), Fe calculated as 0.85o,60.85% FeO and 0.15%0.150,6 Fe,o,FerO! oQ 00.00.00 00.oo.00 0 0.00.00 2.81 66.20.20 7.7'l7.71 1313.48.48 12 12.58.58 1010.58.58 16.8916.89 2727.90.90 Or 1616.73.73 4.31 4 4.44.44 8 8.27.27 10 10.64.64 88.22.22 10.84 77.33.33 15.84 55.39.39 2020.43.43 Ab 1414.76.76 24.62 24 24.03.03 19.54 2323.1A.18 2727.5.5 2323.78.78 27 27.92.92 3030.53.53 3131.9.9 3333.84.84 An 26.13 26 26.56.56 35 35.01.01 2828.36.36 25 25.26.26 20.63 2421.56.56 22.76 2020.66.66 2727.07.07 8.49 Ne 1.95 0000.0.0 00 00.0.0 00 00.0.0 00.0 00.0 0000.0.0 00 00.0.0 0000.0.0 0000.0.0 00.0 Di 1212.17.17 18.57 4 4.81.81 111 1 ..5656 77.61.61 12',12.',13.13 4.36 19 19.28.28 o0 .00.OO 00.00.00 00.00.00 Hy 0o.oo.00 5 5.72.72 24 24.01.01 20.35 1919.44.44 1515.45.45 16.18 5.39 1616.12.42 1414.39.39 55.77.77 01ol 1818.29.29 12',t2.24.24 0.31 0.00 00.00.00 00.00.00 00.00.00 00.00.00 00.00.00 00.00.00 00.00.00 tl 22.91.91 22.13.13 1 1.58.58 22.'.11.11 22.O7.07 33.1.1 11.42.42 11.63.63 11.60.60 11.04.04 00.5E.58 Mt 22.80.80 22.75.75 2 2.37.37 2 2.71.71 2 2.52.52 2.72 22.10.10 1 1.s3.53 2.11 11.46.48 0.79 Sum 9696.19.19 9797.51.51 97.44 96 96.35.35 97 ..8787 9898.06.06 98.36 98 98.96.96 9999.99.99 9999.34.34 9999.45.45 11.. Uralite-porphyritic basalt dykedyke,, Häme groupgroup,, Kalvola (x-6775.18,y-2505.08)(x=6775.18,y=2505.08). . 22.. Uralite-porphyriticUrafite-porphyritic lava,lava, Häme groupgroup,, Lautaporras (x=6754(x=6754.37,.37, y=2494y=2494.2O)..20). 33.. Even-grainedEven{rained metametabasalt, basalt, Häme groupgroup,, SärkijärviSärküärvi (x=6764(x=6764.45,.45, y=2478.15).y=2478.15). 44.. MetabasaltMetabasalt,, Forssa groupgroup,, Forssa domain (x= 67356735.80,.80, y=2467.89r.y=2467.89). 5. Uralite-plagioclase-porphyriticUrafite-plagioclase-porphyritic lavalava,, Häme groupOroup,, KotkajärviKotkaiärvi (x=6767(x=6767.53,.53, yy=2499.77r.=2499.77). 66.. Meta-andesMeta-andesite,ite , Forssa groupgroup,Forssa-Somero,Forssa-Somero domain (x= 67276727.93,y=247o.6o)..93, y=2470.60). 77.. Even-grainedEven{rained andesitic metalava metalava,, Forssa groupgroup,, Forssa domain (x=6738(x=6738.20,.20, y=2469.82).y=2469.42r. 88.. Plagioclase-porphyriticPlagiocla8e-porphyritic andesite metalava, Häme group, Kalvola (x=6775(x=6775.19,.19, y=2505y=2595.9tr..08). 99.. Uralite-plagioclase-porphyritic andesiteandesite,, ForssaFo6sa group, Forssa domain, Kiipu (x=6747(x=6747.07,.07, y=2467y=2467.06)..06). 10.'t0. AndesAndesilicitic metalava metalava,, Forssa groupgroup,, ForssaFoßsa domain (x=6737(x=6737.82,y=2465.82t..82, y=2465.82). 11. Rhyolitic metalava metalava,, Forssa group, Humppila domain (x=6755(x=6755.43,y=2469.r0r..43, y=2469.24).

Analytical equipmenl:equipment: Ph Philipsilips PW 1400 XRF-spectrometer (powder(powder briquettes)briquettes).. LaboratorLaboratories:ies: Outokumpu Oy and Rautaruukki Oy

93 GeolGeologicalogical Survey of FinlandFinland,, Special P Paperaper 19l9 Gerhard HHakkarainenakkarail1el1 thethe Häme grogroupsups representrepresent momostlystly subalkaline types. Forssa group constant FeOFeO./MgO*/MgO ratios were main­maln- In vs. SiO2 diagram, they vary from low-K tained. According to MiyaMiyashiroshiro (1974),(1974), Ti0TiO, de­de- In thethe KpKrO vs. Si02 diagram, they vary from low-K to 2 toto high-K rockrockss but are dominated bbyy medium-K ccreasesreases inin calc-alkaline seseriesries aandnd increaseincreasess inin typestypes.. The scatter inin the total alkalis relrelativeative to silica tholeiitic seseriesries during fractionation. The Ti0TiO,2 con­con- is mainly due to variation inin the potassium content tents inin thethe HSBHSB basalts show a decrease with in­in- and lessless dependent on the variation of sosodiumdium (Fig.(Fig. creasing ssilicailica contentcontent,, and in andesites (Si0(SiO,2 = 55- 4b-c). 6060Vo)%) tittitaniumanium decreadecreasesses lelessss rapidly than forfor the The rockrockss ofthe Forssa group sshowhow a general calc­calc- other silica values (F(Fig.ig. 6a). Also ppPrO,s values de­de- crease with increasing silica in both groups. The aalkalinelkaline fractionation trend in the AFM and Si0SiO.2 vs. crease with silica in both The PrO,pps FeOFeO./MgO*/MgO diagrams ((Figs.Figs. 4a and 5b)5b),, butbutthe the basaltbasaltss contents in the babasaltssalts of the Häme group (0(0.27-.27- and a few andeandesitessites sshowhow a diverdiversifiedsified trend with OAI0.417o) %) are sslightlylightly hihighergher than in those of the ForForssassa tholeiitic affinities. With a few exceptions, the rockrockss ggrouproup ((0.13-0.317o;0.13-0.31 %; FigFig.. 6b). of the Häme group have tholeiitic rather than calc­calc- The trace elementelementss NiNi,, Cr, Co,Co, Zr, Sr and Ba alkalialkaline ne affinities. However, the tholeiitic affinities distinguidistinguishsh to a lesser extent the two groups and the are not pronounced in group and the vs. are not pronounced in either group and in the Ti0TiO,2 vs. didistributionstribution fields overlapoverlap.. The Ni content in the FeOFeO./MgO*/MgO diagram (Fig. 5a) the babasaltssalts are trantransi-si­ babasaltssalts and basaltic andesites are 10wlow,, ranging be­be- tional between tholeiitic and calc-alkaline rocksrocks.. The tween 20 andand 60 ppmppm;; neverthelesneverthelesss in the Häme general trend is an enrichment in Fe relative to Mg in group two sasamplesmpies contain 90 ppm Ni (Fig. 6d)6d).. The the early stages of crystacrystallizationlli zation in both the Häme Häme basaltsbasalts show sslightlyli ghtly elevated Sr values com­com- and the Forssa groups. DurinDuringg progreprogressivessive crystalli­crystalli- pared to the ForForssassa babasaltssalts (F(Fig.ig. 6c)6c).. The Häme zationzation of dacitedacites,s, rhyodaciterhyodacitess and rhyolitesrhyolites in the babasaltssalts are mostly olivine-normative in contrast to

a) FeO' b) 12tz r--,--,--;--v-;---;-----r-;---.,..--.,

Irvine & Baragar 1971 ~ o :.:N + o o shlt J' 4 ^umt%-l tu Z

Ql-x" Calc-Alkaline o 0D 40 so 60 70 80 SiO, (wt%)

Na20+Na2O + K2OK20 MgO c) B=basaJ' BA=basaltic andesite A=andesite 0 5 0 =dacite and rh olile

0 high-K 4 0 Forssa group ti n0 Humppila and Forssa domain ! 3 \ "" K ·. 1Sl ON ,f1 Nr:;J High-K andesites, Kiipu :.: '. X Somero domain medium-K * 2 Al:::. HämeHämegroup group *

: ""

0 45 55 65 75 SiO2SiO, (wt%)(wt%)

Fig. 4. Geochemical characteristics of the Hämeenlinna-Somero Fig. 4. Geochemical characleri stics ofthe Hämeenlinna-Somero VolcanicVol canic Belt.Belt. AFM diagramdiagram afterafter IrvineIrvine & BaragarBaragar ( l97l197 1); KrOK 0 vs.vs . SiO,SiO and. total alkalis and total alkali s vs. SiO,SiO, diagramsdiagrams after LeLeMailr Maitree( ( 1989).1989).Th Thee pointpoinlm markedarkedwi withth "K".. K .. isi s aaTiO TiOr--a andndKO K,O-rich- richba b-asaltsa llfromK~lvola.Th from fälvota. fne~ dash-dotteddash-dotted lineline inin Fig.4cFig. 4c delineatesdelineates thelhe high-Khi gh-K andesires.andesites. ' ,

94 GGeologicaleological SurSurveyvey ofof FFinland,inland, SpSpecialecial P Paperaper 1199 GGeologyeology aandnd geocgeochemistryhemistry of ththee HHämeenlinna-Someroämeenlinna-Somero VVolcanicolcanic B Belt...elt... thosethose of thethe Forssa groupgroup,, whiehwhich areare quartz-norma­quartz-norma- patipatible ble elementselements,, whiehwhich are sstabletable duriduring ng metasomatiemetasomatic tive.tive. TheThe normative An eontentcontent rangesranges between 50 alteration. andand 6060Vo% (see(see TableTable 3)3).. Some of the basalts in KalvolaKalvola,, whiehwhich formform an There are some exeeptionsexceptions to thethe summarysummary given offshoot from the Häme group, offshoot from the Häme group, are fairlyfairly riehrich in Ti0TiO,2 above. The andesites at and poor (Table above. The andesites at Kiipu and partly those inin and KpKrO and poorinin Si0SiO,2 and ppPrO.s (Table 3, Number Somero are high-K roeksrocks (traehyandesites).(trachyandesites). ReeentRecent 1)1).. The sampIe sample isis takenfromtaken from a 1010 mmthick thiekdyke dyke whiehwhich orogenieorogenic high-K andesites are relatively riehrich inin P"O.,pps' isis ssituatedituated inin an unaltered andesite. InIn most of the ranging from 0.32-0.540.32-0.547o% (Baker 1982).1982). SineeSince-the the diagrams the sampIesample deviates markedly fromfrom the high-K andesites in Somero and Kiipu have lowlow PrO,pps general fraetionationfractionation trend of the HSB. The high eontentscontents of ea.ca. 0.20.2Vo,%, theirtheir high K eontentscontents are alkali eontentcontent is due to K-enriehment,K-enrichment, while the Na­Na- probably due to alteration. At Kiipu also the FeOFeO./*/ eontentcontent iiss similar to that of the other basalts of the MgO ratios and the V eontentscontents are high (Figs. Sb5b and area. The KalvolaKalvolabasalts basalts eontaincontain normative nepheline 7b). In the V vs. MnO plot (Fig(Fig.. 7b) the Kiipu but the eontentcontent of Ba and Sr is lower than in average traehyandesitestrachyandesites defleetdeflect from the general differentia­differentia- high-K roeksrocks with similar alkali eontentscontents from eon­con- tion trend. Both inin Somero and Kiipu the high-K vergent plate boundaries (see Baker 1982). The high roeksrocks are loeatedlocated close to sulfide mineralizationsmineralizations.. K and low P eontentscontents suggest that the elevated K­K- Thus pre-metamorphiepre-metamorphic redistribution of alkalis and values are eausedcaused by metasomatiemetasomatic proeesses.processes. The partly of ferromagnesian elements by ore-forming Kalvola basalts are interpretedinterpreted,, due to their strati­strati- fluids is suggested to explain the deviations. The Ti­Ti- graphiegraphic position,position, as magmas that erupted during a Zr relations in Fig. 7a show that these roeksrocks do not late stage in the evolution of the Häme groupgroup,, possi-possi­ differ from the main trend on behalf of these ineom-incom- bly from a different magmatiemagmatic souree.source. The low Mg and Ni values indieateindicate that the HSB voleanievolcanicss underwent signifieantsignificant fraetionationfractionation during a) aseentascent to the surfaee.surface. The fast deereasedecrease in the Ni 2.5 .------r---r--....---~---.--__, eontentscontents suggests an early olivine removal (Fig. 6d). The abundaneeabundance of plagioclase phenocrystsphenoerysts indieatesindicates low-pressure fraetionationfractionation eonditionsconditions and the lower S r< Sr eontentcontent in the Forssa group basalts reflectsrefleets a ; greater involvementofinvolvement of plagioclase precipitation.preeipitation . Also, FI the abundaneeabundance of uralite showshowss that clinopyroxene was a major erystallizingcrystallizing phase.phase. .5 The HSB volcanicsvolcanies are eomparedcompared to Japanese volcanoes in Fig. 5. The Amagi volcano belongs to

1.5 2 22.5.5 3.5 the Izu-Bonin arcare and the Asama volcano belongs to FeO*FeO*/ / MgO the Northeast Japan are.arc. The ehemistrychemistry of eaeheach of them represents a single trend eale-alkalinecalc-alkaline b) them a 75 / eompositionalcompositional variation. The single trend variation . -\ calc'Alkatine F6saForssa A-) \' i _ur reflectsrefleets a magmaticmagmatie evolution from a single magmat-magmat­ 70 ,/-x! ry'as Asma icie souree,source, hencehenee these volcanoesvolcanoes should be good for 0 comparisoneomparison purposes.purposes . The ForssaForssa group displays two 65 different trends; one follows the calc-alkalineealc-alkaline Amagi ~ I trend and the otherfollows the more tholeiitictholeiitie Hachijo-Haehijo­ ö 60 jima trend in the Miyashiros (1974) FeO*/MgO vs. ö; trend in the Miyashiros FeO./MgO vs. 'n;ffi TiO,Ti0 diagram.diagram . In the FeO./MgOFeO*/MgO vs. SiOrdiagramSi0 diagram the 2 2 55 Forssa group also displays two different trends. Some of the samplessampIes defining the subhorizontal trend are 50 altered,altered, but this trend also embracesembraees unaltered sam-sam­

Häme {j, ples.pIes. Wilson (1991, p. 181) reportsreports relatively high 45 11.522.533.544.551.5 2.5 3.5 4.5 recent island arcs. High FerO.,/FeO-ratiosFe20/FeO-ratios in reeent island ares. High oxygen FeO* // MBOMgO fugacitiesfugaeities in magmas imply that magnetite would be a near liquidus phase. Varying oxidation conditionseonditions Fig.5.Fi g.S. GeochemicalGeochemi cal variationvari ation diagramsdi agrams forfor thethe rocksrocks of thethe liquidus oxidation Hämeenlinna-SomeroHämeenlinna- Somero VolcanicVolcanic BeltBelt (after(after MiyashiroMiyashiro 1974).1974). a) during low-pressure fractionationfraetionation playplaya a fundamen-fundamen­ TiOrvs.TiO, vs. FeO*/MgO.FeO*/MgO. b) SiOrvs.SiO, vs. FeO+/MgO.FeO*/MgO. SymbolsSymbols as inin Fig.Fig. taltal rolerole in determining whether thethe magmas fractionatefraetionate 4. The shadedshaded fi eld s outline theth e rocksrocks oftheof the HämeHäme group andand thethe 4. The fields outline along the tholeiitie or the calc-alkalineealc-alkaline path. In Fig. dasheddashed lineline outlinesoutlines rocksrocks of thethe ForssaForssa group.group. LightLi ght dotted lineslines tholeiitic areare fractionationfractionation trends.trends. LinesLines inin Fig.Fig . a: T= Hachijo-jimaHachijo-jima tholeiites;tholeiites; 5b,Sb, thethe steep trendtrend of thethe Forssa group isis causedeaused by H= Hachijo-jimaHac hijo-jima lessless typicaltypical tholeiites.th oleiites. magnetite/ilmenite precipitationpreeipitation during high oxygen

95 GeoloGeologicalgical SurveySurvey ofof Finland,Finland, SpecialSpecial PapPaperer 1l99 GerhardGerhard HakkHakkarainena ra il1 el1

c) 1000lom a) 0

1.5 Semero

800t00 Häme 1.2 a- tr:\ D. D. \ -&* e D. ~ a \- 0.ä 600600 D.D. .9 a .e ~ \ \.J ~ D. Forssa Fo6s --0- Q \o ra , '" -z 0 t:i. --..,( E- , , 4004{n "- , .6 , ,i-\ D.AKK , B " \n , ,0 0 "& '~\- 200 0 .3 LQ \ ~ ..------O#J O/^q - , 0'" " ' O ___ SV ~..- / "J

0 45 SO 55 6060 65 10 15 45 SO 55 60 65 10 15 SiO 2 (wt(wt%)%) 2 Si~ (wt%)

dd)) 100 r------r----,-----,---,----r-----, b) ', .4 D. :,^A I!A: t:i Häme 8080 t:i .3 D./ O Forssa ,f D. ,/' 'AK 0' , 't ~ , 60 n'. rtr l\ \ "-. D.. ~ "- o~ .2 "- , Zz o!" , 40 .>!> t:iA A D. K o , 40 oD "- "- "'--a- 0tr !;;] N"..". "- .^--^e'.t:i-U 0 t:i^ ~ ..... "- '. . 1 • ISJEl )I; v.+ 0D .. . .. ---'----.. Ö 20 '... §t:iEa^ 0tr Gä(^1SJ~t:it:i a_ cf " . \ . '. t:i)l;X"3 0u D. o QJ '. 0-- -4 Semero ".X.··.lIfE 0c 0o OoIMJ lotrtr ....'--...... ,.x, * ..~ .... 0 45 SO50 55 60 6S65 10 70 15 45 SO50 55 60 65 107o 15 Si0 (wt%) Si0sio (wt%) sio 22(wt%) 22(wt%)

Fig. 6. HHarker-typearker-type vavariationri ati on diagrams for ththee rockrockss oftof thehe HHämeenlinna-Someroämeenlinna-Somero Volcanic Bell.Belt. SSymbolsymbols as in Fig.Fi g. 44.. The lineliness ddefineefi ne the fields for ththee HHämeäme aandnd Forssa ggroupsroups aandnd the Somero domain.

fugacities. In Fig. 5a most of the intermediate rocks fractionation conditions as in the South Sandwich of the Forssa group follow a trend whichwhieh is interme­interme- are.arc. diate between the tholeiitic and ealc-alkalinecalc-alkaline trendtrend.. In recent orogenic belts,belts, occurence of andesites is This trend may be caused by less magnetite/ilmenitemagnetitelilmenite mainly restricted to subduction settisettings.ngs. In mature precipitation during loweroxygen lower oxygen fugacities. Amagi-Amagi­ arcs, like Japan, the andesite/basalt ratios tetend nd to be type fractionation depletes the liquids from titanium greater thanthan2 2 (Gill(GillI981, 1981, p.95,p. 95, Aramaki & Ui 19821982,, more rapidly. The scatter of the samplessampies and the Hall 19891989,, p.p. 391). The Forssa group has a high deviating trends in the FeOFeO./MgO-SiOr-TiO,*/MgO-Si0 -Ti0 plotsplots andesite/basaltandesitelbasalt ratio (estimated ratio = 2-4), while in 2 2 = 2-4),wh1lein reflect interaction of replenishment, eruption and the Häme group the andesite/basalt ratio is similar to extensive differentiation, that in primitive arcsares (<1).«1). Also, the Ti0 eontent in extensive differentiation, which resulted in varia-varia­ Also, the TiO,2 contenr in tions of the oxygen fugacities in the magma cham-cham­ the HSB is typicaltypieal for recent volcanicvolcanie arcs. The high bers. ZrlTi-ratios,Zr/Ti-ratios, low Ni and high LILE concentrationsconcentrations Variable conditions of low-pressurelow-pressure fractionation classify the HSB as a calc-alkaline island arcare com-eOffi­ may occur in individual arc segments. For example, plex. Due to the monogenetic rock association in the in the South Sandwich arc the volcanic rocksrocks vary vary,, in Häme group the magmatic evolution may there be terms of their potassiumpotassium content, from low-Klow-K to calc-calc­ different from that in the Forssa group.group. SinceSinee basalts alkalinealkali ne types, but in terms of their FeO*/MgO-FeO*/MgO­ dominate in thethe Häme group,group, it could be regarded as ratiosratios theythey can be classified as tholeiitestholeiites (Wilson (Wilson a tholeiitic trench-sidetreneh-side assemblage,assemblage , but the relativelyrelatively 1991,] 991, p.p. 173).173). The HSB volcanics are mostlymostly calc-calc­ high Zr, LILE and P.O.pps values discriminate thethe alkaline, but thethe FeO*-MgOFeO*-MgO characteristics showshow Häme group basalts from primitive arcare tholeiites. thatthat part of the basalts are tholeiitic.tholeiitic. This diversity Some of thethe basalts lacklack normative quartz.quartz. Accord-Accord­ could be explainedexplained by similar variation in the ing to Miyashiro (1914),(1974), thisthis is unusualun usual even for

96 6 CeologicalGeological Survey of Finland,Finland, Special PaperPaper I1 9 Geology andand geochemistrygeochemistry of theth e Hämeenlinna-SomeroHämeenlinn a-Somero Volcanic Belt...Bell.. a) basalts oftheof the innerinner volcanic zones inin islandisland arcs. 12000 DB = OFB D arc volcanics vary AB= LKT The chemical characteristics of arc volcanics vary )I; AC= CAB inin space and time.time. The alkalinity of arcs isis regardedregarded E 9000 ,e,0. as an indicatorindicator of theirtheir maturity. Studies from the i= mature arcs in Japan suggest an increaseincrease in alkalinity 6000 mature arcs Japan c away from the trenchtrench and several investigations showshow

3000 that a significant change inin the geochemistry may 00 0 also occuroccuralongthearcs along the arcs (Gill(Gill1981, 1981, p. 213). Based on 0 the geochemical similarities between the volcanic 50 100 150 200 250 rocksrocks of the HSB and evolved, mature island arc Zr (ppm) rocks,rocks, such as those in the Honshu and Sunda arcs, it b) 400 appears feasible toto suggest that the formation of the . /::, HSB volcanics was preceded by crustal thickening.thickening . 350 ISI x)I; 300 3m o _aA.6.6. ISI _^fE-06. 00 6.. ISI U A,'^ ^E 250 0 0. al!. na.'-06., " ,e, . ' 0 - 200 _4|K.',fl K . · J,,> 6. o .'

150 150 ISI .'0 0 - 100100 Fig.Fig. 7. a) Ti vs. Zr diagram forfar thethe rocks of thethe Hämeenlinna-SomeroHämeenlinna-Somero Volcanic Belt (after(after PearcePearce & Cann 1973). RhyolitesRh yolites and dacites are 50 Gü0 0 .' aalsolso plotted.ploned. CAB = calc-acalc-alkaline lkaline basalts, LKT = low-K low-K tholeiites,th oleiites, OFB 6. ' = ocean floorf100r basalts, D = wiwithinthin plate basalts.basalts. b)b) V vs.vs . MnO diagram. 0 0 .05 .1 ..t515 .2 .25 . 3 Symbols as in Fig. 4.4 . MnO (wl%)(wt 7o)

STRATIGRAPHY AND EVOLUTION

The complexity of the volcanic and sedimentary fissure system. Volcanism started with explosiexplosive ve successions in ththee Forssa domain makes it difficult shallow-water eruptions that formed volcanic breccias to establish a common stratigraphic record. The and local pillow lavas. Contemporaneous erosion complexities may be explained by contemporaneous resulted in the development of debris flows, which eruptions of different types of magmas from differ­differ- formed volcanic conglomerates at the base of the ent volcanic centers or different magma batches Häme group. The lavas in the central parts of the from one center. Häme group do not exhibit pillow structures, and the In the southemsouthern Forssa domaindomain,, andesites brecciated poor stratification of the pyroclastics suggests that rhyolitic tuffstuffs,, and in the Somero domain andesitic the central parts were formed partly in subaerial fragments have been encountered in basalt lalavas.vas. In conditions. The Fe-sulfide formations at the base of the Forssa domain basaltic to andesitic uralite-por-uralite-por­ the volcanic pile suggest that extension of the ccrustru st phyry dikes cross-cut or are emplaced as sills in caused fissures which provided pathways for mag­mag- andesitic and dacitic plagioclase-porphyries. Fur­Fur- mas. During the extensional period small anoxic thermorethermore,, there are two thick coarsely plagioclase­plagioclase- basins were formed which favored the precipitation porphyritic basalt dikes containing fragmentsfragments of of strata rich in Fe-sulfidesFe-sulfides.. intermediate volcanic rocksrocks.. Evidently the volcanic From the above data,data, a complex magmatic evolevolu-u­ activity started with acid eruptionseruptions,, which were tion can be deduced. The lowermost exposed se­se- succeeded by intermediateintermediate and finally, by basaltic quences comprise the Forssa group rhyolitic to basal­basal- eruptions. The Häme group volcanics were deposit­deposit- tic volcanics. They were followedfollowed by the extrusion of ed on sedimentary rockrocks,s, which may partly represent the Häme group basalts. The relatively K- and Ti-rich erosion products from the Forssa group. The wide basalts of Kalvola cross-cut the intermediate end distribution of basaltic feeder dikes suggests that the members oftheof the Häme group and they commenced the Häme group basalts were not erupted through a latestlatest extrusional event inin the areaarea.. central vent; ratherrather the eruptions took place along a

97 Geological Suwey of Einland, Special Paper 19 Gerhard Hakkarainen

DISCUSSION

The lithostratigraphic correlation and the tectonic derivatives. This kind of monogenetic volcanism can setting of the volcanic-sedimentary sequences in be ascribed to tensional stress fields (Nakamura & southern Finland is principally unresolved. Interpre- Uyeda 1980),which are found in the rear of subduction tation of the Svecofennian stratigraphy has been zones. The tholeiitic Hame group could have been done in some key areas; however the relationships produced at a short stage of tension (decoupling) between different belts is obscure and the tecto- during the generally compressional arc volcanism. nostratigraphic relationship between, for example, Cyclic variation between plate decoupling and con- the supracrustals of the Hämeenlinna-Somero (this pling in oceanic island arcs is described by Hawkins study), Tampere (Kähkönen 1987,1989)andorijärvi- et. al. (1984). Kemiö (Colley & Westra 1987, Mäkelä 1989) (Fig. 1) The Japanese and Izu-Mariana arcs have high areas is yet to be solved. Most of the volcanics in the convergence rates of 7-9 cmlyr and together they Svecofennides are calc-alkaline with arc affinities form one of the fastest consuming plate margins (Gaál & Gorbatschev 1987). In general, the volcanic active today. Japan represents an evolved stage of arc belts in southwestern Finland show a northward volcanism. The existence of several volcanic zones transition from bimodal sequences in Enklinge- with geochernical island arc affinities in the Sveco- Orijärvi (Ehlers & Lindroos 1990, Lindroos 1990) fennian Domain and their formation within a few through calc-alkaline associations (Hämeenlinna- tens of millions of years suggests fast movement Somero; Hakkarainen 1989, this study) to more alka- rates in the pre-Svecofennian plate collage. There- line volcanics in Tampere (Kähkönen 1987). The fore, the magma generation in the HSB may show tholeiitic Haveri basalts in the Tampere region are sirnilarities to the magmatic evolution of the Japan- considered to represent an initial-stage island arc Mariana arcs. The volcanic rocks in the HSB show environment (Mäkelä 1980). The concept that the both tholeiitic and calc-alkaline affinities. Andesites, southern Svecofennian volcanic belts were formed dacites and rhyolites are abundant and the basalts in an oceanic environment (Gaál 1990) was mainly have higher potassium contents than the basalts in based on the absence of evidence of an older conti- prirnitive island arcs. These characteristics suggest a nental basement. Huhma et. al. (1991) concluded subduction site like that of Group II in Gil1 (1981, p. that many of the zircons in the graywackes at Tampere 220), with relatively high convergence rates. The are older (2.0-1.9 Ga) than the volcanic rocks (1.904- magmas in the HSB are chemically more diverse 1.89 Ga) from the same area. This implies the exist- compared to the primitive, more depleted and iron- ence of an earlier crust predating the formation of the enriched magmas of Group 1formed in arcs with thin arc volcanics. crust. More than 80% of the erupted magmas in If the abundances of LIL-elements in the HSB Group 1 are tholeiitic, which is not the case in the basalts are primary, an enrichment process in the HSB. mantle would be indicated. This could not likely take More precise geochronological data are needed to place in larger extents in a thin oceanic plate. For establish a detailed and accurate model for the evo- example, the Mariana-Izu arcs, with a 20-30 km thick lution of the Svecofennian crust, and the discussion crust, are primitive and dominated by tholeiitic low- presented below should be seen as tentative. The K basalts. K-rich lavas occur in the Mariana arc (Lin complex tectonostratigraphic relations of the vol- et al. 1989), but volumetrically in smaller amounts, canic belts at Orijärvi, Tampere and Hämeenlinna relative to the bulk volume, than the medium-K lavas could be consistent with a model with several small in the HSB. Furthermore, in the HSB calc-alkaline tectonic entities. They vary in size from small rocks are more abundant than tholeiitic rocks. microcontinents to accretionary prisms, and interact Miyashiro (1974) stressed that the ratio of calc- witheach other through thrusting, subduction, dock- alkalineltholeiitic rocks tends to increase with in- ing etc. The amalgamation of primitive arcs, fore-arc creasing thickness of the crust. Crustal thickening basins, back-arc basins, and rifts upon each other restrains prirnitive magmas from ascending to the provides materia1 for crustal thickening and evolu- surface. They are collected in shallow magma cham- tion to mature calc-alkaline volcanism. The forma- bers where they evolve to calc-alkaline successions. tion of arc volcanics in southern Finland could have Hence, the HSB volcanics were probably formed in been predated by a rift-related bimodal magmatism, a mature arc milieu. such as in Orijärvi (Colley & Westra 1987, Väisänen Although the high andesitehasalt ratio is a general 199 1) and Enklinge (Ehlers & Lindroos 1990, Lind- feature in the HSB, the Häme group volcanic rocks roos 1990). A similar type of bimodal magmatism show a monotonous evolution with mainly basaltic may have contributed to the crustal thickening that GeologicalGeological SurveySurvey ofof Finland,Finland, SpecialSpecial PaperPaper I1 99 GeologyGeology andand geochemistrygeochemistry ofof thethe Hämeenlinna-SomeroHämeenlinna-Somero VolcanicVolcanic Belt...Bell. .. precededpreeeded thethe arcare volcanismvolcanism andand thethe thickeningthiekening result-result­ plate"plate" causedeaused aa consumingeonsuming subductionalsubduetional eventevent onon thethe eded inin aa continentaleontinental geochemicalgeoehemieal signature ofof thethe arcare borderborder toto thethe ArcheanArehean craton.eraton. ContinentalContinental marginsmm'gins magmas.magmas. tendtend toto havehave slowerslower closingelosing ratesrates thanthan intra-oceanicintra-oeeanie Generation of oceanicoeeanie subductionsubduetion zones involvesinvolves marginsmargins (Sugisaki(Sugisaki I976).1976). Therefore, fastfast movementmovement earlierearlier imbricationimbrieation andand fracturingfraeturing of thethe lithosphere.lithosphere. of thethe "proto-Svecofennides""proto-Sveeofennides" northeastwardsnortheastwards couldeould Early fracturesfractures and faultsfaults formform zoneszones of weakness inin havehave exceededexeeeded thethe consumptioneonsumption of thethe oceanicoeeanie plateplate thethe lithospherelithosphere and thesethese zones are apt toto develop at thethe Ladoga-BothnianLadoga-Bothnian Bay zone. The compressioneOlnpression intointo newnew subductionsubduction zones when thethe plate margins causedeaused byby thethe onpushing plate would have beenbeen are subjectedsubjeeted toto compressionaleompressional stress (Hawkins et al. releasedreleased inin migrationmigration ororformation formation ofofnew new subductionsubduetion 1984). The bimodalbimodal magmatism inin southern Finland zones further away from thethe craton.eraton. If thethe motionmotion of couldeould be a resultresult of an early imbricationimbrieation and exten-exten­ thethe SvecofennianSveeofennian collage was inin a NE-ENE-direc-NE-ENE-diree­ sion ofofthe the oceanicoeeanie crust.erust. Some ofofthe the fracturesfraetures couldeould tiontion (Gaäl(Gaal & GorbatschevGorbatsehev 1987),1987), the angle between laterlater have developed into arcsares inin southern Finland,Finland, thethe movement directiondireetion and thethe orientation of thethe when thethe stress field changedehanged toto compressional.eompressional. The Bothnian-Ladoga arcare (NNW-SSE) supports thethe in-in­ assumed subduction zone on the border toto thethe ArcheanArehean terpretationterpretation ofofthe the conversioneonversion ofofthe the Bothnian-Ladoga cratoneraton is earlier or of the same age with thethe earliest arcare intointo a dextral strike slip complex.eomplex. This concepteoneept isis phases of thethe SvecofennidesSveeofennides (Park 1985, GaälGaal & favored by the lockingloeking event ofofthe the Bothnian-Ladoga GorbatschevGorbatsehev 1987). A plausible explanation couldeould arcare describeddeseribed by Park (1985). be that a NE movement of the "proto-Svecofennian"proto-Sveeofennian

ACKNOWLEDGEMENTS

The author is grateful to Dr. Y. Kähkönen, Dr. T. manuscript.manuseript. The English language was revised by BrewerBrewer,, Prof. C. Ehlers and Dr. M. Nironen for Christopher Cunliffe. eritieallycritically reading and thereby greatly improving the

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