Timing of Svecofennian Crustal Growth and Collisional Tectonics in Åland, SW Finland 65

Bulletin of the Geological Society of Finland,Vol. 76, 2004, pp. 63–91

Tiimingming of Svecofennian crustal growth and collisional tectonics in Åland, SW Finland

Carl Ehlers1), Torbjörn Skiöld2)* and Matti Va asjoki3) 1)Department of Geology and Mineralogy, Åbo Akademi University, FIN-20500 Turku, Finland 2)Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden 3)Geological Survey of Finland, P. O. Box 96, FIN-02151 Espoo, Finland

Abstract In an effort to quantify the time parameter in the tectonomagmatic evolution of what has been called the Southern Svecofennian Arc Complex (SSAC) of SW Finland, advanced radiometric dating techniques have here been applied to rock groups of key importance in that area. In this paper we report the results of 131 high-resolution ion microprobe spot analyses (SIMS) of zircons, and 33 measurements using isotope dilution mass spectrometry (ID-TIMS) on zircon, monazite and titanite,and employing both large-sample multi-grain as well as single-grain techniques. The To rsholma area of the Åland archipelago,situated between southern Finland and central Sweden, is a key structural area significant to resolve the time dimension in Svecofennian tectonics.There a collage of imbricated rock slabs was formed by tectonic shortening representing the culmination of large-scale penetrative Svecofenni- an deformation. Another structurally significant feature investigated is the South Fin- land Shear Zone (SFSZ) that transects the southwest-Finnish archipelago and further east follows the southern coast of Finland.This shear zone forms the southern limit of the c. 1830 Ma Late Svecofennian Granite and Migmatite Zone (LSGM) and also features deformations of a later stage when the considered region of Svecofennian crust was consolidated. The obtained age results and theirtectonic analysis can be summarized as follows.The Enklinge volcanic sequence (1885±6 Ma) is within error limits coeval with the intrusion of abundant early-kinematic gneissose granodiorites whose average age of 1884±5 Ma marks the formation of new crust in this region.Some of these geiss- es contain a significant amount of 2000–2080 Ma zircon. Although many Svecofenni- an granitoids are known to contain heterogeneous zircon populations, mainly formed c. 1890 Ma ago but also containing an inherited component, the Kökar gneiss is, to the best of our knowledge,the first case where inheritance from c. 2030 Ma sources has been unequivocally demonstrated in a syntectonic Svecofennian intrusive rock. At To rsholma,granodiorites (1879±6 Ma) have similarly intruded the supracrustal series, but there they were later metamorphosed into granulite orthogneisses.The mesoscopic recumbent folds and subhorizontalschistosityofthesegneisses were transected by a set of steep amphibolitic dykes indicating an episode of extension. During a preceding stage,these gneisses had been sandwiched like an allochthonous slab unit between the supracrustal rocks. Yo unger sheet-like granodioritic intrusions (1861±19 Ma) with associated dykes (1865±7 Ma) reflect an even later stage of collisional thrusting. Altogether this tectonic evolution lasted for approximately 15 Ma (c. 1875–1860 Ma) and records a period of significant deformations during the Svecofen- nian orogeny. Monazites and zircon rims yield concordant U-Pb ages of c.1830 Ma.This is in agreement with previously obtained ages of late Svecofennian granites and migmatites in the LSGM zone in S Finland. Weaklydeformed pegmatites and even-grained granitedykes were emplaced 1790±6 Ma ago and intersect the other rocks of the area.They characterize the vaning stages of shearing along the northwestern part of the SFSZ, and the establishment of a consolidated crust where deformation was about to cease by this time.

Key words: crust, tectonics, plate collision, granites, granodiorites, gneisses, rhyolite porphyry, metadiabase,absolute age,Paleoproterozoic,Åland Province,Finland 64 Ehlers,C., Skiöld,T.and Vaasjoki,M.

1. Introduction Late-Palaeoproterozoic Svecofennian orogeny in tic Shield involving a number of pre-2.0 Ga mi- the Baltic (Fennoscandian) Shield was part of a cro-continents accreted to the Archaean domain worldwide process (cf. Hoffman, 1989) that of- 1920–1880 Ma ago. ten caused amalgamation of newly formed juve- The archipelago of SW Finland represents a nile crust with ancient proto-cratons, simulta- well exposed segment of the Svecofennian crust neously regenerating their border areas. Judging with a demand for modern age determinations. from geochronological and geochemical data, In the present study we recognize zircons rep- similarly timed episodes occured in several now resenting such pre-2.0 Ga crust as well as mi- separate parts of the Laurentian craton, Green- nor Archaean influence in the Åland islands. land and the Baltic Shield. It has been discussed We comparemilli-gram and micro-gram zir- to what extent these shield areas were part of a con isotope dilution mass spectrometrywith pre-Rodinian mosaic of continental crust (Karl- nano-gram technique on homogeneous zircon ström et al., 2001; Elming & Mattsson, 2001). domains within single grains. Our new isotop- The metavolcanic and metasedimentary ic data aim at timing the Svecofennian, c. 1900 belts in southern Finland havebe en consid- Ma,compressional/transpressional stages. In ered to represent parts of two major arc com- particular, we constrain the time of the alloch- plexes; the Central Svecofennian ArcCom- thonous thrusting at Torsholma, Åland (Fig. 1), plex (CSAC) and the Southern Svecofennian which represents the collisional stage of the Sve- Arc Complex (SSAC), which are separated by cofennian tectonic evolution in that region. The an inferred suture zone (Lahtinen, 1996; Niro- timing of later movements within the regional nen, 1997; Nironen et al., 2000; Väisänen et al., ductile South Finland Shear Zone (SFSZ, Tor- 2002). While the crustal evolution of the Baltic vela et al., 2003) is monitored by recrystallized Shield is fairly well constrained when it comes monazite and titanite, and by the dating of zir- to the general framework of crust formation and cons in coeval granitoid intrusions. radiometric ages, the detailed kinematic characteristics of the different tectonic and metamor- 2. General geology phic phases of evolution are less well known. Re- cent works by e.g. Lahtinen & Huhma (1997), The Svecofennian components of the Baltic Nironen (1997), Elliot et al. (1998), Korsman Shield comprise supracrustal continental rock et al. (1999), Vaasjoki et al. (2001), Nironen et accumulations atop and along the borders of al. (2002) and Väisänen (2002) have considera- the Archaean proto-craton as well as successive- bly improved our understanding of these events. ly accreted island arcs and magmatic additions. Zircon ion microprobe studies by Huhma et al. The diverse formation processes have previously (1991) and Claesson et al. (1993), and later by been discussed within the context of cyclic orog- Lindh et al. (2001), Väisänen et al. (2002), Rut- eny but more recently as repeated plate-tectonic land et al. (2001a, 2001b and 2003), and Wei- interactions of crustal segments, which are con- hed et al. (2002), indicate the existence of a pre- sidered to have taken place during the time in- 1.95 Ga Palaeoproterozoic continental crust terval between 2.0 and 1.75 Ga ago (cf. discus- in different parts of the shield. Nironen, et al. sions in Gaál & Gorbatschev, 1987; Nironen, (2002) and Lahtinen et al. (2004) have present- 1997; Nironen et al., 2002). In its present form, ed ”a new tectonic model” based on integrated the crust constitutes domains with slightly dif- research on the early development of the Bal- ferent geochemical and lithological signatures, suggesting sutures between different sub-ter-

* Corresponding author ranes (Ehlers & Lindroos, 1997; Lahtinen et al., e-mail: [email protected] 2002; and references therein). Timing of Svecofennian crustal growth and collisional tectonics in Åland, SW Finland 65

Fig. 1. The mapsketches foliationsinrocks of the Åland archipelagoofSWFinland.Sample numbers and localities areinserted.The dextral South Finland Shear Zone (SFSZ) transects the area in aNWdirection.North of the shear zone the bedrock consists of structurallyoverturned gentlydipping migmatites and supracrustal rocks forming a“dome-and-basin”structure. South of the shear zone granodioritic gneisses appear with sparse supracrustal inclusions. Earlyphases of post-kinematic roundish granite massifs likeMosshaga aretransected by c. 200 Ma younger Rapakivi granites.

The geological structureofSWFinland is parallel transposed layering and early schistos- dominated by early subaqueous mafic and felsic ities. The mainly supracrustal rock sequenc- volcanic rock sequences which were overturned/ es were subsequently intruded by sheets of ear- inverted and thrusted towards the west and the ly-kinematic granodioritic and tonalitic rocks. northwest (Lindroos et al., 1996). They consti- Later metamorphism and intrusion of young- tutearegion of subhorizontal slabs with sub- er migmatitic granites belonging to the c. 1830 66 Ehlers,C., Skiöld,T.and Vaasjoki,M.

Fig. 2. Aschematic historyofthe rocks and acompilation of dated events in the To rsholma area, SW Finland.The dated samples indicate that most of the structural historywith earlyrecumbent folding, extension of the crust,and subsequent thrusting and sandwiching of the gneiss slab took place over arather shortperiod of time,approxi- mately15Ma.The ages arefromthis study. Timing of Svecofennian crustal growth and collisional tectonics in Åland, SW Finland 67

Ma old zone of Late Svecofennian granites and in the gneissic s lab which subsequently was migmatites (the LSGM zone of Ehlers et al., sandwiched in between the sheets of amphib- 1993) dominate the southern coast of Finland. olitic volcanics (Fig. 2:2). An intrusive sheet of All these structures arecut by the large SFSZ granodiorite, less deformed than the surround- (Ehlers &Lindroos, 1990a), while gneissose ing country-rock gneisses and without any traces granodioritic and granitic rocks reappear fur- of metadiabase dykes (Fig. 2:3), occurs between ther to the south (Suominen, 1991; Fig. 1). the deformed gneiss slab and the amphibolitic volcanic outlier. This sheet records deformation 3. Regional geology of the indicating thrusting towards the northwest. We investigated terrains consider therefore that the sheet-like granodiorite was emplaced during a relatively late stage in Mapping and geological correlations in the area the creation of this tectonic collage. havebeen carried out since the early years of The metadiabase dykes cut the deformed the twentieth century, chiefly to produce re- gneiss slab at ahigh angle (Fig. 3). This may gional geological maps and map descriptions. be taken to indicate aphase of c rustal exten- In the southwestern archipelagoofFinland, sion and intrusion of basaltic dykes which, in J.J.Sederholm’s(1934) classical account of the turn, post-dates most of the early deformation geology of the Åland islands is r ich in excel- but pre-dates the stacking of the slabs. The en- lent observations and geological sketches of out- tire collage of juxtaposed rocks was later migma- crops. Later contributions by Edelman and Jaa- tizedand intruded by late Svecofennian gran- nus-Järkkälä(1983), Edelman (1979), Ehlers ites dated at c. 1830 Ma from several localities and Lindroos (1990a and b) describe various as- in southern Finland (Suominen, 1991). In order pects of the regional geology of the areas consid- to establish the timing of the episode of stack- ered in this paper. ing and sandwiching of the different rock slabs, we have sampled the allochthonous gneiss slab (samples A1334, 98033), the intrusive sheet of 3.1.The Torsholma area granodiorite (A1261, A1187), as well as one of The Torsholma area is characterized by gently the metadiabase dykes (00014). An essential- dipping sheets of volcanic rocks and granitoids, ly undeformed cross-cutting pegmatite (Fig. 2: tectonically stacked atop of each other, and sep- 4) represents a post-stacking stage of intrusion arated by tectonic or intrusive contacts (Fig. 2 ). (sample A1185). A thin slab of strongly deformed granulite-facies ortho-gneiss, that appears to have intruded layers of earlier supracrustals and showing multiple stages of deformation, is sandwiched between asequence of gently dipping and folded mafic volcanics (Ehlers et al., 1993). It is the oldest rock in the area, and at the time of original sampling an age in excess of 1900 Ma was considered possible. The gneisses are cut by a set of steep amphibolitic metadiabase dykes which is later than the sub-horizontal early deformation in the gneiss slab (Fig. 3). The appearances of these rocks are visualized in Fig. 2:1 where fold- Fig. 3. Amphibolitic meta-diabases have intruded previ- ed maficbands are cut by relatively undeformed ouslyoverturned and stronglydeformed gneisses with metadiabase dykes. The dykes are present only subparallel transposed amphiboliticlayers. 68 Ehlers,C., Skiöld,T.and Vaasjoki,M.

3.2.The Enklinge area strongly banded, deformed granodiorite was selected. Another sample was collected from the The Palaeoproterozoic volcanic successions matrix between fragments of boudinaged and in the SW Åland archipelago arewell repre- brecciated maficenclav es further southwards on sented on and around the island of Enklinge. the island of Kökar (Fig. 1). Finally, a deformed They consist of subaqueous sequences of mafic layer of granodiorite with a strong gently dip- andfelsic volcanic rocks with primarystruc- ping linear structure was sampled on the north- turesrepresenting lavapillows and pillowbrec- western shore of Kökar. That granodiorite forms cias (Ehlers, 1976; Ehlers &Lindroos, 1990a). a gently dipping layer, a few metres thick, in a Higher strata comprise thin marble layers fol- westwards thrusted and openly folded stack of lowedbyrhyolitic extrusions. Related in time thin layers of metasedimentary rocks embedded but somewhat later aregranodiorite intrusions in deformed granodiorites. surrounding the volcanic rocks at Enklinge, All these samples were collected with the and thereare also intrusivedacitic quartz-por- double aim of determining the intrusion age of phyritic dykes in the volcanics. Chemically,the the granodiorite as well as the age of the SFSZ dykes areidentical with the surrounding gran- deformation. odiorites but differ quite clearly from the ex- trusiverhyolites (Ehlers &Lindroos, 1990b). 4.Analytical Procedure We havecarried out SIMS analyses of zircons from the rhyolite on Enklinge (sample 00013) Mass spectrometric analyses were carried out and from the granodiorite sampled at the small using two types of techniques: (1) Thermal islet of Gräshäran (sample 95020) north of En- Ionization Mass Spectrometry(TIMS) for pre- klinge. cise measurements on Isotope Dilution (ID) treated samples, and (2) SecondaryIonization Mass Spectrometry(SIMS). The ID-TIMS 3.3.The South Finland Shear Zone measurements were performed under differ- at the Sottunga and Kökar islands entconditions and in different isotope labo- The prominent South Finland Shear Zone ratories.The Finnish Geological Survey deter- (SFSZ; Fig. 1) can be traced a couple of hundred minations (byM.V.) were carried out during kilometres along the southern coast of Finland. the late 1980-ties at an early stage of the in- In the southwestern archipelago, it closely fol- vestigation and employing relatively large sam- lows the southern margin of the c. 1830 Ma old ples of >1000 crystals each. The multi-grain belt of Late Svecofennian Granites and Migm- technique described by Vaasjoki et al. (1991) atites (LSGM, Ehlers et al., 1993), but most of was used. The morerecent analyses from the the dextral shear deformation is found in the Swedish Museum of Natural History(by T.S.) surrounding c. 50 Ma older granodiorites. comprised just one or afew mineral grains ac- Subsequently,the SFSZ was intruded by cording to asingle-grain technique. The SIMS even-grained granite dykes which register analyses were carried out on aCameca 1270 the latest phases of shearing along that zone. ion probe which is aNordic research facili- Onesample of these late granite dykes (sam- ty (NORDSIM) at Stockholm. In regardto ple 00010), and three samples of the deformed the various analytical procedures employedin gneissose granodiorites (samples 99021 and Stockholm, we refer to Rutland et al. (2001b) 99022 from Kökar,and sample 99024 from and references therein. The errors and error Sottunga; Fig. 1) were sampled along the SFSZ. calculations in the Tables and the text aregiven The early phases of shear deformation have at the 95 %confidence level, i.e. using 2-sig- not been dated. From the island of Sottunga, a ma uncertainties. Timing of Svecofennian crustal growth and collisional tectonics in Åland, SW Finland 69

5. Results and interpretations associated with the very late stages of Svecofen- 5.1. Samples and analytical data nian deformation.

Our study involved detailed mapping and revi- Samplep A1185 is from ac.one metrewide sion of some key areas in the Åland archipela- pegmatite dyke that cuts vertically across the go with emphasis on the large-scale structural deformed stack of rocks on the islet of Hum- pattern and its evolution. In isotope geology, we melholm, Torsholma (Fig. 2:4). The pegma- have used the most modern techniques to quan- tite is undeformed and obviously post-dates the tify the time dimension for these processes. By collisional tectonic phase. Concordant mona- employing small-scale SIMS spot analyses from zites yield a U-Pb age of 1795±4 Maa (not shown specific domains inside zircon grains, we en- graphically). hanced the possibility of analysing homogenous parts in otherwise complex crystallizations. That Saamplemple A1334 from N. Gräsören, Torsholma, technique is extremely useful for rocks with the represents the allochthonous granulitic gneiss- potential of assimilating xenocrystic parts, and esthat havebeen thrusted eastwards between provides insight in regard to relic provenances the country-rock gneisses. Their zircons are as well as later metamorphic episodes. Because stubby, short-prismatic, and often light-brown of such complexities, we screened/filtered our and translucent with an internal zoning. Some SIMS data to provide the detailed age informa- grains contain turbid cores. The degrees of dis- tion for the specific stage of zircon crystallisa- cordance between zircon populations increase tion that we aimed for. That procedure necessi- with rising uranium contents and decreases in tated repeated analyses and statistical treatment density. By excluding the most discordant and of the data where the separate age determina- darkbrown zircons (fraction E, T able 2, not tion often carried relatively large uncertainties. shown graphically) alinear trend with an up- In comparison, the ID-TIMS analyses provided per intercept age of 1867±15 Ma and a lower excellent analytical precision, but ran the risk of one at 325 Ma is defined.The high MSWD val- sampling diverse crystallizations. ue of 10 indicates scatter in excess of analytical The analytical data and age calculations are error. Yellow anhedral monazite is almost concord- presented in Table 1 for 131 new SIMS and in ant with a 207Pb/206Pb age of 1829±5 Ma. Table 2 for 33 new ID-TIMS determinations. Histograms of age distributions as well as re- Samplep A1261,Räddarskär, Torsholma rep- gressional and other statistical treatments in- resents ageneration of gneissic granitoids that volving these determinations are illustrated in a overlie discordantly and cut across the granu- number of figures. Cathode luminescence (CL) litic gneisses. This subhorizontal sheet is free and transmitted light (TL) photographs tak- from cross-cutting amphibolite dykes and ob- en prior to analysis helped select characteristic viously represents an episode of intrusion into grains and were used to guide the spot analyses. a previously formed collage of rocks. The most They record different crystal textures and mor- abundant zircons areprismatic, of medium phologies relevant to the interpretation and dis- length and with well-developed regular zoning. cussion of the ages significant for specific epi- Some opaque inclusions havebeen excluded, sodes of crystallization. leavingthe lowdensity and moreturbid pop- The investigated rock samples represent a ulation quite discordant. Aregression of all four cross-section of ages from the repeatedly de- populations results in aMSWD of 7.9 and Con- formed granitoid gneisses of the Enklinge, Tor- cordia intercept ages of 1861±19and 221Ma sholma, and Kökar–Sottunga areas and forward (not shown graphically). in time to the gently deformed granitoid dykes 70 Ehlers,C., Skiöld,T.and Vaasjoki,M.

Saamplemple A 1187 Bockholm, Torsholma is from a ing is still the dominant texture in the central granodioritic dyke crosscutting the A1334 gran- parts of the zircon grains. There, the CL images ulite which, based on fieldobser vations, belongs are visible as irregular growth bands of variable to the same phase of intrusion as the Räddar- greyish intensity.Marginal domains of homo- skärsheet (sample A1261). Under oil immer- geneous CL-dark reflection have been analysed sion, the zircons display zoning and multi- in two instances (grains 02a and 04a) and yield ply faceted crystal terminations. The five pop- ages of 1826 and 1828 Ma (see Fig. 4 and Ta- ulations separate into two groups, and aline- ble 1). CL-greyish and zoned interiors, reflect- ar trend with Concordia intercept ages of 1865±7 ing changes in element concentrations associat- and 338 Ma can be calculated (not shown graph- edwith magmatic crystallization, constitute 8 ically). The MSWD of 1.4 indicates little or no of the totally ten analyses. The weighted aver- scatter in excess of analytical errors. The dyke con- age mean of the 207Pb/206Pb ages for these eight tains rounded monazite grains with a 207Pb/206Pb (n=8/10) analyses (thus excluding analyses 02a age of 1827±4 Ma. and 04a) is 1876±11 Ma. The MSWD value for The zircon fractions from samples A1334, this calculation is 3.8 and the age distribution is A1187 and A1261 were all subjected to mul- shown in the histogram of Fig. 4. ti-grain ID-TIMS analysis on large quantities of The analytical SIMS-results can also be ex- crystals and carried out in the later part of the pressed in the form of aConcordiaplot (not 1980-ties. The relatively large MSWD-values shown). A best fit to a straight line of the eight and ± errors are most likely the results of hetero- older analyses yields intercept ages of 1885±14 geneous zircon crystallizations involving signif- and c. 610 Ma. Since the lower intercept of 610 icantly different growth stages. This is probably Ma is rather high for a lead diffusion curve and also true of the numerous multi-grain analyses less likely to mark recent lead loss or a metamor- previously reported. The uncertainty problem is phic event, a lower intercept anchored at 250 particularly severe in the case of analyses aimed Ma would probably result in a more realistic up- at reflecting early stages of development, which per intercept age of 1878.7±5.8 Ma at the 95 % may have been influenced by late-stage anneal- confidence level and a MSWD of 1.3. Similarly ing, metamorphism and the presence of xenoc- to all the other calculations made for this pub- rystic grains from older continental rocks. lication, these ages have been calculated using a 95% confidence level or correspondingly given Saamplemple 98033 represents a granulitic gneiss at with 2-sigma uncertainties in Table 1. Lilla Hummelholm, Torsholma. It is from the We have also carried out some ID-TIMS sin- same rock type as sample A1334. This granulit- gle-grain analyses on selected short-prismatic gneiss carries abundant clino- and ortho-py- ic anhedral zircons which, prior to isotope di- roxenes which cause the brownish appearance. lution (ID), were abraded (Krogh, 1982) to re- The rock has been deformed at least twice be- move potential metamorphic overgrowths and fore reaching its present state, and both TIMS heterogenities in the marginal, sometimes dis- and SIMS ages areavailable. The zircons are turbed parts of the grains. Both these ID-TIMS- mainly short-prismatic and colourless transpar- analyses (samples 98033 D and E in Table 2) are ent to translucent. Irregular and button-like yel- somewhat discordant, yielding 207Pb/206Pb ages low monazites occur. of c. 1869 Ma (Fig. 4 centre). We have not iden- The cathode-luminescence (CL) and trans- tified any inherited/xenocrystic zircon core do- mitted light (TL) images shown in Fig. 4 illus- mains in this material, but any addition from trate the characteristics found among the zir- metamorphic overgrowths would immediate- cons of this granulite sample (98033). Despite ly render the ID-TIMS age determinations of the metamorphic features, magmatic type zon- 1869 Ma to become a minimum age of prima- Timing of Svecofennian crustal growth and collisional tectonics in Åland, SW Finland 71 .6 .1 .9 om 6.0 0 0.5 3.2 -0.6 e, – 0.9 – 5.5 – 1.8 – 2.8 – 3.7 – 8.2 – 3.4 ** sc % – 13.4 Di a) cluded fr 83 84 7 – 6.0 93 7 – 15.3 9 – 50.4 8 – 0.5 12 11 11 12 13 14 15 30 14 15 11 13 11 , cen=centr b) ±2s Ages (M egular Pb 1894 1878 1859 1856 1850 1845 1837 1832 1774 1888 1885 1883 1878 1867 1865 1857 1855 1828 1826 2013 207/206 ystals; x=analysis ex oned, ir=irr . or c) 0.95 0.98 0.98 0.96 0.98 0.97 0.96 0.98 0.97 0.99 0.99 1.00 0.99 0.99 0.95 0.99 0.99 0.99 0.99 0.96 on=z corr err Rho ence (e.g. 5a or 5b for two analyses on efer ea 2.2 2.1 2.1 2.2 2.1 2.1 2.2 2.1 2.1 5.5 5.5 5.5 5.5 5.6 5.7 5.5 5.5 5.5 5.5 2.4 b) ar ±2s% en, w=whitish, z U Pb/ ystal than the best quality cr 235 5.406 5.551 5.456 5.536 4.853 5.354 4.824 4.297 2.350 5.388 5.402 5.360 5.260 5.137 5.303 5.033 4.788 4.878 5.198 5.418 207 , ev=ev ey . and spot analysis r ed cr el 2.1 2.1 2.1 2.1 2.1 2.1 1.1 2.1 2.1 5.4 5.4 5.5 5.4 5.6 5.4 5.4 5.4 5.4 5.4 2.3 b) k, gr=gr ystal no ±2s% e fractur U Pb/ y grain/cr 238 206 omic ratios 0.3383 0.3504 0.3481 0.3538 0.3112 0.3442 0.3116 0.2783 0.1571 0.3382 0.3397 0.3374 0.3321 0.3263 0.3372 0.3214 0.3062 0.3167 0.3378 0.3171 d b At rsholma-Enklinge-Sottunga-Kökar we eviations: da=dar To 0.7 0.5 0.4 0.6 0.4 0.5 0.6 0.4 0.5 0.7 0.7 0.5 0.8 0.8 1.7 0.8 0.8 0.6 0.7 0.6 bbr b) the ±2s% of Pb ounger phase visible, br= mor samples 0.1159 0.1149 0.1137 0.1135 0.1131 0.1128 0.1123 0.1120 0.1085 0.1155 0.1154 0.1152 0.1149 0.1142 0.1141 0.1136 0.1134 0.1117 0.1116 0.1239 207/206 om dia fr Pb 1640 3682 1642 . 13870 90170 99500 72250 87110 23890 21870 37740 54410 22700 54880 40700 26080 11870 12530 34630 con 206/204 110800 zir egion of the spot analysis. A e is at least one y ample number (e.g. 00014), follo of % 0.13 0.02 0.02 0.03 0.02 0.02 0.08 0.09 1.14 0.05 0.03 0.08 0.03 0.05 0.07 0.16 0.15 0.51 0.05 1.14 206 ed 206Pb *f yses e= ther ble 2 anal Ta 133 417 557 304 394 913 443 404 186 272 557 152 132 146 175 213 302 500 158 1315 centage of the total discor [Pb] spot cent of the ratio, and for ages in absolute numbers at 2 sigma lev (ppm) 1270) ranulitic gneiss, grid 6694164-1502371 0.22 0.88 0.55 0.71 1.23 0.61 0.06 0.06 0.19 0.23 0.29 0.31 0.25 0.24 0.22 0.26 0.29 0.12 0.05 0.31 Th/U (calc.) rsholma, grid in To Concentrations Cameca dia denoted as a per 333 874 656 893 465 668 386 341 367 458 579 832 407 rsholma G efers to the CL character and r en at 2 sigma in per 1263 2832 2690 1409 2109 1372 1314 [U] (ppm) To (SIMS, th on at least one older phase, cor e giv ow ummelholm obe rgr on on e in the on on on ve , ogr? , ogr? opr usiv om the Concor centage of common 206Pb in the total measur , cen , bor , bor micr , cen, z , cen , cen, z , cen, z , semicen , bor , cen , cen, z , cen?, z , cen , gr , gr , gr , ogr=o elation for 207Pb/235U vs. 206Pb/238U ratios. The final comment r , da, cen , gr Ion der . rs in atomic ratios ar r corr e1 y to grain analysis and Cathode luminescence (CL) notation: S ro ro enotes the per bl ta-diabase, intr Ke grain 5). the age calculation Er bor=bor Er the deviation fr ain analysis, CL image anulitic gneiss, Lilla H actions 00014, 08a, gr 00014, 05a, da, cen 00014, 03a, ev-gr 00014, 05b 00014, 01a, ir 00014, 04a, ir 00014, 09a, ir x00014, 10a, ir Me x00014, 07a, metamict 98033, 06a, gr 98033, 08a, gr 98033, 05a, da-gr 98033, 07b 98033, 01a, gr 98033, 03a, gr 98033, 07a, gr 98033, 09a, gr Ta Fr x98033, 04a, da, bor a) Gr a) b) Gr c) *d x98033, 02a, da, bor ** x00014, 02a, gr 72 Ehlers,C., Skiöld,T.and Vaasjoki,M. .1 .2 .1 .1 4.0 4.6 3.3 8.1 0.5 1.3 2.0 2.5 5.0 0.2 6.6 2.5 0.1 3.7 2.3 1.5 1.4 ** sc % -5.3 -2.1 -3.9 -2.3 12.6 23.3 -18.6 Di a) 5- 84 74 3- 8- 5- 6- 8- 82 71 11 16 41 24 18 15 15 11 18 19 11 17 16 24 16 11 12 35 b) ±2s Ages (M Pb 1866 1879 1880 1883 1884 1886 1886 1887 1906 2067 1839 1867 1870 1875 1878 1879 1880 1882 1882 1886 1889 1889 1892 1895 1896 1896 1900 1970 207/206 . or c) 1.00 1.00 1.00 1.00 1.00 1.00 1.00 0.99 1.00 0.94 0.93 0.96 0.97 0.97 0.99 0.98 0.96 0.96 0.98 0.96 0.97 0.99 0.93 0.99 0.97 0.98 0.98 0.87 corr err Rho ea ar 3.1 3.1 3.1 1.8 3.2 3.1 3.1 3.1 3.1 3.3 3.7 3.6 3.5 3.5 3.5 3.5 3.6 3.6 3.5 3.5 3.5 3.5 3.7 3.5 3.5 3.5 3.5 4.0 b) ±2s% U Pb/ 235 207 4.612 5.587 5.595 4.134 5.178 5.156 5.847 5.441 5.356 5.418 5.182 5.419 5.570 5.350 5.253 5.720 5.506 5.386 5.580 5.117 5.312 5.538 5.571 5.519 5.550 5.547 5.276 5.821 3.1 3.1 3.1 1.8 3.2 3.1 3.1 3.1 3.1 3.1 3.4 3.4 3.4 3.4 3.4 3.4 3.5 3.5 3.4 3.4 3.4 3.4 3.4 3.4 3.4 3.4 3.5 3.5 b) ±2s% rsholma-Enklinge-Sottunga-Kökar U Pb/ 238 To 206 omic ratios 0.2932 0.3524 0.3528 0.2602 0.3259 0.3241 0.3675 0.3418 0.3329 0.3076 0.3343 0.3442 0.3533 0.3383 0.3316 0.3608 0.3472 0.3394 0.3515 0.3217 0.3333 0.3475 0.3489 0.3453 0.3470 0.3467 0.3292 0.3491 At the of 0.3 0.5 0.4 0.2 0.4 0.3 0.6 0.9 0.3 2.3 1.3 1.0 0.8 0.8 0.4 0.6 1.0 1.1 0.6 1.0 0.9 0.5 1.4 0.4 0.9 0.6 0.7 2.0 b) ±2s% samples Pb om fr 207/206 0.1141 0.1150 0.1150 0.1152 0.1152 0.1154 0.1154 0.1155 0.1167 0.1277 0.1124 0.1142 0.1143 0.1147 0.1149 0.1150 0.1150 0.1151 0.1152 0.1154 0.1156 0.1156 0.1158 0.1159 0.1160 0.1161 0.1163 0.1209 con zir Pb of 8006 1017 3680 9487 22310 75930 26700 44520 38520 17120 15630 18830 67890 40550 17310 12820 94070 14680 27200 28920 34780 91070 55490 49430 206/204 106500 141700 167300 117000 yses % anal 206 0.08 0.02 0.02 0.07 0.04 0.01 0.05 0.11 0.23 1.84 0.51 0.20 0.12 0.10 0.03 0.05 0.11 0.01 0.15 0.02 0.13 0.02 0.07 0.06 0.05 0.02 0.03 0.04 *f spot 600 602 374 298 364 349 458 176 598 424 118 107 176 154 500 370 156 159 381 106 192 337 112 495 279 185 206 825 270) [Pb] (ppm) a1 nklinge, grid 6700731-1491495 Camec 0.08 0.61 0.42 0.55 0.68 0.60 0.26 0.41 0.47 0.76 0.32 0.32 0.44 0.39 0.50 0.40 0.40 0.39 0.51 0.32 0.49 0.51 0.31 0.56 0.47 0.44 0.50 0.65 Th/U (calc.) Concentrations (SIMS, 857 899 852 837 417 294 259 403 370 837 365 381 861 273 457 771 268 644 431 495 800 [U] obe 1799 1322 1047 1435 1024 1196 1124 (ppm) nklinge, grid 6705850-1315465 opr r ä sh ran, NE of E e1 y at E on on on micr on on on on on Ion on on on on on on on on on on on , cen, z , cen, z , cen, z , z on on on , z , bor , bor , z , cen, z , z , cen, z , z , cen, z , z , cen, z , cen, z , z olite porphyr , gr , gr , gr , gr , gr , same as 05a (cont.) . ointed gneiss at G e1 bl ain analysis, CL image actions 95020, J x95020, 04a, rim, br 95020, 01a, cen, z Ta Fr Gr a) 95020, 08a, cen, z 95020, 10a, cen, z 95020, 05a, cen, z 95020, 03a, cen, z 95020, 07a, bor 95020, 09a, cen, z 95020, 02a, cen, z x95020, 06a, bor 00013, Rhy x00013, 10c, gr 00013, 09b 00013, 13a, gr 00013, 11b 00013, 01a, gr 00013, 01b 00013, 12a, gr 00013, 06b 00013, 14a, dagr 00013, 10b 00013, 05a, gr 00013, 03a, dagr 00013, 09a, gr 00013, 04a, dagr 00013, 15a, gr 00013, 08a, dagr 00013, 05b x00013, 07a, da, cen, cor Timing of Svecofennian crustal growth and collisional tectonics in Åland, SW Finland 73 .6 .7 .0 .6 0.8 0.5 8.1 2.4 3.0 0.5 0.7 5.3 2.2 3.2 6.4 1.8 6.6 3.9 1.9 ** sc % -1.5 -0.3 -8.9 -5.6 -0.7 -1.5 -2.3 -0.6 -6.3 -1.9 10.1 Di a) 9- 7- 94 92 56 8- 93 40 23 74 17 11 12 15 68 15 10 10 14 10 13 23 18 18 24 49 18 17 b) 198 107 ±2s Ages (M Pb 1754 1784 1793 1846 1851 1859 1860 1866 1870 1871 1872 1872 1874 1876 1881 1882 1882 1885 1886 1886 1887 1887 1887 1888 1888 1891 1895 1896 1899 1911 207/206 . or c) 0.87 0.54 0.99 0.97 0.68 0.98 0.99 0.98 0.99 0.73 0.99 0.99 1.00 0.99 0.99 0.99 0.99 0.98 0.97 0.97 0.97 0.97 0.99 0.33 1.00 0.99 0.82 0.97 0.99 0.97 corr err Rho ea ar 4.4 7.1 3.9 5.7 5.7 5.5 3.9 3.9 5.5 5.6 3.9 5.6 5.4 3.9 3.9 5.5 3.9 3.9 5.6 4.0 4.0 5.6 4.0 3.9 3.9 4.8 4.0 3.9 4.0 b) 11.8 ±2s% U 235 Pb/ 4.780 4.416 4.640 4.901 5.663 5.210 5.406 5.468 5.339 5.237 5.277 5.367 5.591 5.659 5.257 5.503 5.371 5.579 5.069 5.761 5.313 5.514 5.570 4.555 5.745 5.411 5.820 5.680 5.684 5.672 207 3.9 3.8 3.9 5.5 3.8 5.4 3.8 3.9 5.4 4.1 3.8 5.5 5.4 3.9 3.8 5.4 3.9 3.9 5.4 3.8 3.9 5.4 3.9 3.9 3.8 3.9 3.9 3.8 3.8 3.9 b) 2s% U± 238 rsholma-Enklinge-Sottunga-Kökar Pb/ 0.3178 0.2922 0.2981 0.3140 0.3613 0.3322 0.3436 0.3467 0.3384 0.3318 0.3342 0.3396 0.3534 0.3567 0.3311 0.3465 0.3377 0.3508 0.3186 0.3619 0.3338 0.3463 0.3496 0.3079 0.3606 0.3391 0.3641 0.3551 0.3547 0.3517 To 206 omic ratios At the of b) 2.2 6.0 0.5 1.3 4.2 1.0 0.6 0.7 0.9 3.8 0.4 0.8 0.5 0.6 0.6 0.8 0.6 0.7 1.3 1.0 1.0 1.3 0.5 0.3 0.5 2.7 1.0 0.5 1.0 ±2s% 11.2 Pb samples om 0.10909 0.10960 0.11287 0.11318 0.11370 0.11372 0.11409 0.11440 0.11444 0.11446 0.11450 0.11461 0.11474 0.11508 0.11514 0.11516 0.11535 0.11537 0.11541 0.11544 0.11546 0.11548 0.11553 0.10728 0.11554 0.11572 0.11594 0.11600 0.11621 0.11697 207/206 fr con Pb zir 174 668 654 676 538 1859 2134 3054 9234 5501 1062 2123 9009 of 11470 11160 27070 17040 25110 20350 50680 37360 76280 45580 206/204 134200 233200 111900 232600 102300 224300 515700 yses % anal 206 1.01 0.88 0.61 2.80 0.16 0.17 0.01 0.20 2.86 0.07 3.48 0.11 0.01 0.02 0.01 0.07 0.34 0.02 1.76 0.09 0.88 2.77 0.04 0.05 0.02 0.01 0.04 0.21 0.00 *f 10.77 ble 2 spot Ta 61 270) 381 422 144 291 117 277 210 322 221 328 286 212 231 161 236 211 283 358 167 405 191 276 182 225 270 518 589 [Pb] (ppm) 1285 1928 a1 Camec ottunga, grid in 0.36 0.37 0.28 0.24 0.54 0.45 0.29 0.61 0.43 0.49 0.33 0.10 0.69 0.62 0.42 0.36 0.44 0.67 0.45 0.56 0.35 0.35 0.32 0.30 0.09 0.55 2.28 0.40 0.62 0.13 Th/U (calc.) Concentrations (SIMS, undet, S [U] 988 171 387 633 284 673 470 768 531 811 818 474 505 365 586 491 641 820 411 921 470 659 433 520 617 obe (ppm) 1187 3123 2990 1129 1451 opr on on micr on on on on on on on on on on on on on? on on on , z , z e? , z , z , z , z , z Ion , ogr , cen, z on , cen, z , cen , bor , ogr , cen (cont.) . anded bi-hbl-gneiss, Bogr e1 bl ain analysis, CL image actions x99024, 02a, gr x99024, 07a, gr x99024, 15a, da, ogr x99024, 28a, dagr x99024, 23a, da, cen x99024, 30a, da, cor Ta Fr x99024, 09a, da, bor 99024, 01a, da, cen, z 99024, 29a, da, z x99024, 11a, dagr 99024, 04a, da, bor x99024, 10a, da, bor 99024, 31a, da, cen, z 99024, B 99024, 25a, da, bor 99024, 14a, da, ir 99024, 06a, da, cen z 99024, 27a, da, cen, z 99024, 08a, da, bor 99024, 16a, da, cen, z 99024, 26a, da, bor 99024, 22a, da, cen, z 99024, 18a, da, cen, z 99024, 24a, da, bor 99024, 05a, gr Gr a) 99024, 12a, da, cen, z 99024, 03a, dagr x99024, 13a, da, bor 99024, 21a, da, cen, z 99024, 19a, dagr x99024, 20a, dagr 74 Ehlers,C., Skiöld,T.and Vaasjoki,M. 0.1 3.5 3.9 3.7 1.8 1.3 3.0 1.8 0.4 2.2 0.3 2.5 2.4 0.2 0.1 1.7 2.4 4.7 -3.6 -0.5 -2.2 -3.1 -4.9 -3.4 -6.8 -0.3 -5.6 ** sc % 29.0 -14.1 Di a) 40 91 55 23 34 20 14 30 23 42 14 13 16 12 14 34 14 16 16 29 23 19 24 17 12 63 33 16 16 b) ±2s Ages (M Pb 3163 2081 2045 2041 2034 2026 2014 1999 1907 1893 1891 1887 1887 1887 1884 1884 1883 1882 1879 1879 1879 1877 1877 1877 1876 1870 1862 1862 1856 207/206 . 0.95 0.99 0.98 0.99 1.00 0.99 0.99 0.99 0.96 1.00 1.00 0.99 1.00 1.00 0.97 1.00 0.99 1.00 0.98 0.99 1.00 0.99 0.99 1.00 0.91 0.99 0.99 0.99 0.98 or c) corr err Rho ea ar 5.7 3.9 5.6 5.4 5.4 5.5 5.4 5.5 4.0 5.4 3.9 3.9 5.4 5.4 4.0 5.5 3.9 5.4 3.9 3.9 5.4 3.9 3.9 5.4 4.2 5.5 3.9 3.9 3.9 b) ±2s% 6.998 8.373 6.713 6.647 6.125 6.380 6.127 5.611 5.614 5.532 5.439 5.533 5.289 5.414 5.527 5.518 5.388 5.367 5.451 5.192 5.090 5.478 5.163 5.595 4.559 4.901 5.240 4.927 21.553 U Pb/ 235 207 5.5 3.8 5.4 5.4 5.4 5.4 5.4 5.4 3.9 5.4 3.9 3.8 5.4 5.4 3.8 5.4 3.8 5.4 3.9 3.9 5.4 3.8 3.8 5.4 3.8 5.4 3.9 3.9 3.8 b) ±2s% 0.6340 0.3942 0.4814 0.3869 0.3846 0.3560 0.3733 0.3615 0.3486 0.3514 0.3468 0.3416 0.3476 0.3323 0.3406 0.3478 0.3474 0.3395 0.3386 0.3440 0.3277 0.3215 0.3461 0.3263 0.3537 0.2890 0.3122 0.3339 0.3149 U Pb/ rsholma-Enklinge-Sottunga-Kökar 238 206 omic ratios To At the 1.7 0.7 1.0 0.6 0.4 0.9 0.6 0.6 1.2 0.4 0.4 0.4 0.3 0.4 1.0 0.4 0.4 0.4 0.8 0.6 0.5 0.7 0.5 0.3 1.8 0.9 0.4 0.4 0.7 of b) ±2s% samples Pb 0.24654 0.12874 0.12614 0.12585 0.12537 0.12479 0.12396 0.12290 0.11673 0.11585 0.11569 0.11547 0.11545 0.11542 0.11529 0.11526 0.11520 0.11510 0.11495 0.11493 0.11492 0.11483 0.11481 0.11478 0.11473 0.11440 0.11387 0.11383 0.11348 om fr 207/206 con 649 >1e6 2549 1315 2115 9506 2247 Pb 70970 86430 99300 42750 20600 21980 75530 10270 68780 19290 23670 73910 10150 12650 zir 216500 307700 106400 279600 302100 156300 180400 111000 of 206/204 yses % anal 206 0.03 0.01 2.88 0.02 0.02 0.04 0.01 0.00 0.73 0.09 0.09 0.02 0.01 0.02 0.18 0.01 0.03 0.10 1.42 0.08 0.03 0.88 0.01 0.20 0.01 0.83 0.18 0.15 0.02 *f ble 2 spot Ta 44 84 133 220 299 381 181 125 364 471 372 480 565 586 265 484 670 376 556 132 399 338 388 746 848 419 724 580 482 1270) [Pb] (ppm) , grid in .96 0.29 0.72 0.40 0.29 0.37 0.26 0.25 0.05 0.05 0.10 0.07 0.11 0.02 0.01 0.03 0.02 0.07 0.08 0.44 0.02 0.09 0.38 0.02 0.03 0.09 0.02 0.03 0.08 Cameca Th/U (calc.) Concentrations 70 ells ö K kar 79 44 23 22 91 05 89 15 (SIMS, 923 936 681 976 309 921 1185 1237 1417 1574 1238 1719 1442 1086 2043 2132 1271 2072 1548 1347 [U] (ppm) obe opr e6 e? e3 e9 micr , br e8 e4 e2 e4 e1 e2 on, ogr on, cen on Ion , ogr on, cor on, cor on, cor on, ogr on, cen on, ogr on?, ogr on, ogr on, ogr , z , z , z , cor , cor , cen, br , cor , z , cor , z , cor , cor , w , w cont.) , da, z cciated, banded gneiss at H .( re e1 bl ain analysis, CL image actions x99021, 28a, w x99021, 07b x99021, 26a, w x99021, 08b x99021, 27a, gr x99021, 19a, w x99021, 22a, gr x99021, 24a, w x99021, 02a, da, ogr-cor 99021, 20a, da, z 99021, 04a, dagr 99021, 08a, da, ogr 99021, 16a, da, z 99021, 18a, da, z 99021, 03a, da, ogr 99021, 23a, da, bor 99021, 11a, da, ogr 99021, 15a, da, ogr 99021, 12a, da, z 99021, 05a, dagr 99021, 17a, da, z 99021, 07a, da, ogr 99021, 13b 99021, 21a, dagr 99021, 14a, da, z x99021, 25a, gr x99021, 13a, dagr x99021, 10a, da, ogr?, br 99021, B x99021, 01a, da, ogr Ta Fr Gr a) Timing of Svecofennian crustal growth and collisional tectonics in Åland, SW Finland 75 0 0 1 2 2 1 3 4 om -8 -3 -2 -1 -1 -2 -1 -0 -0 -9 41 e, ** sc % -12 -42 -39 Di a) cluded fr 92 97 9- 17 14 13 17 11 14 21 14 20 14 20 34 23 19 18 16 16 11 21 24 19 b) ±2s , cen=centr Ages (M egular Pb 2033 1946 1888 1859 1835 1818 1813 1801 1798 1798 1796 1796 1794 1791 1788 1788 1783 1781 1779 1778 1773 1761 1749 1360 207/206 ystals; x=analysis ex . oned, ir=irr or c) 0.91 0.94 0.96 0.98 0.91 0.96 0.94 0.98 0.88 0.94 0.89 0.95 0.90 0.75 0.85 0.90 0.90 0.93 0.92 0.96 0.87 0.87 0.97 0.90 corr err Rho on=z ea ence (e.g. 5a or 5b for two analyses on ar efer 2.3 2.4 2.6 2.3 2.3 2.2 2.2 2.2 2.5 2.3 2.4 2.3 2.6 2.8 2.5 2.4 2.3 2.4 2.3 2.2 2.4 2.6 2.2 2.3 b) ±2s% U en, w=whitish, z Pb/ 5.664 5.319 5.264 5.320 4.991 2.902 4.974 5.235 4.835 4.812 4.879 4.907 4.767 4.946 4.792 4.926 4.767 4.816 4.932 4.964 4.725 4.239 2.738 1.712 235 207 ystal than the best quality cr , ev=ev ey . and spot analysis r ed cr b) 2.1 2.3 2.5 2.2 2.1 2.2 2.1 2.1 2.2 2.2 2.1 2.2 2.3 2.1 2.1 2.2 2.1 2.2 2.1 2.1 2.1 2.3 2.1 2.1 ±2s% el k, gr=gr ystal no e fractur U Pb/ 238 0.3279 0.3234 0.3306 0.3394 0.3226 0.1894 0.3256 0.3449 0.3191 0.3176 0.3223 0.3241 0.3152 0.3276 0.3180 0.3269 0.3172 0.3207 0.3288 0.3312 0.3161 0.2854 0.1856 0.1427 rsholma-Enklinge-Sottunga-Kökar 206 omic ratios To y grain/cr At the d b b) 1.0 0.8 0.7 0.5 1.0 0.6 0.8 0.5 1.2 0.8 1.1 0.7 1.1 1.9 1.3 1.0 1.0 0.9 0.9 0.6 1.2 1.3 0.5 1.0 we of eviations: da=dar ±2s% bbr Pb samples 0.1253 0.1193 0.1155 0.1137 0.1122 0.1111 0.1108 0.1101 0.1099 0.1099 0.1098 0.1098 0.1097 0.1095 0.1093 0.1093 0.1090 0.1089 0.1088 0.1087 0.1084 0.1077 0.1070 0.0870 207/206 om fr ounger phase visible, br= mor con Pb 862 206/ dia 1270 3213 1562 8711 1677 zir 204 48800 22500 68400 62620 17890 19460 13920 25160 72100 14360 29570 20530 42320 32980 64940 20720 12180 103500 of . yses egion of the spot analysis. A e is at least one y ample number (e.g. 00014), follo % anal 206 ed 206Pb 2.17 1.47 0.02 0.04 0.58 1.20 0.08 0.03 0.03 0.10 0.10 0.13 0.07 0.03 0.13 0.06 0.09 0.04 0.06 0.03 0.09 0.21 1.11 0.15 *f e= ther spot 99 95 59 53 85 76 77 85 89 78 270) 138 119 662 259 293 514 140 244 234 171 101 179 560 895 centage of the total discor cent of the ratio, and for ages in absolute numbers at 2 sigma lev [Pb] a1 (ppm) 0.61 0.69 0.13 0.26 0.68 0.18 0.43 0.40 0.63 0.64 0.37 0.44 0.47 0.91 0.44 0.50 0.56 0.33 0.70 0.48 0.79 0.57 0.25 0.23 Camec Th/U (calc.) Concentrations (SIMS, ottunga H ä stn s, grid 666582 - 148668 318 274 642 234 236 340 592 151 584 135 383 215 247 188 199 198 434 211 213 1736 1287 1224 2456 5229 [U] dia denoted as a per efers to the CL character and r (ppm) obe en at 2 sigma in per th on at least one older phase, cor opr e giv ow usions at S micr rgr ve e?, gr e? e? Ion on on e? ,gr on, gr on on, gr on, gr on om the Concor centage of common 206Pb in the total measur , z , z e, ev , ogr=o , cen, x elation for 207Pb/235U vs. 206Pb/238U ratios. The final comment r , bor , cen, gr , cen, z , cen , cen, gr (cont.) der . ranitic dyke intr rs in atomic ratios ar r corr e1 y to grain analysis and Cathode luminescence (CL) notation: S ro ro enotes the per bl the age calculation Er Er the deviation fr bor=bor Ke grain 5). ain analysis, CL image actions b) c) *d ** a) x00010, 15a, cen, z x00010, 06b x00010, 13a, cen, z x00010, 04a, x x00010, 12a, cen, x x00010, 03c, rim, me, da 00010, 09a, cen, ?x 00010, 09b 00010, 08b 00010, 05a, cen, gr 00010, 03a, cen, z 00010, 01a, cen, z 00010, 03b 00010, 07a, cen, gr 00010, 14a, cen 00010, 08a, cen, gr 00010, 14b 00010, 02a, cen, gr 00010, 07b 00010, 10a, cen 00010, 16a, bor x00010, 17a, cen, x x00010, 06a, rim, da x00010, 11a, rim, me 00010, G Gr a) Ta Fr 76 Ehlers,C., Skiöld,T.and Vaasjoki,M.

Fig. 4. Granulitic gneissatTorsholma (sample98033). Upper part :cathode luminescence(CL) and transmitted light (TL) imagesofsix selected grainsare shown with the detailed locations of SIMS spot analyses. Metamor- phic and texturallyhomogeneous overgrowth domains yield ages of about 1830 Ma. Central part :these zircon overgrowth ages comparewell with ID-TIMS metamorphic monazite ages. Also shown arediscordant micro- gram analyses whichmost likelyrepresent aminimum age of original zircon crystallization. Bottom part :histogram of SIMS spot analyses yields amean 207Pb/206Pb age of 1876±11Mafor euhedrally zoned domains.

The two youngest SIMS analyses from the overgrowth domains of zircon grains 02a and 04a (1826 and 1828 Ma, and displayed in the CL and TL images of Fig. 4) are very similar to the concordant ID-TIMS single-grain monazite ages of 1828, 1831 and 1829 Maa (Table 2, sample 98033 A, B, C and Fig. 4 centre). Together these ages indicate significantmedium- to high- grade metamorphic recrystallization in the Tor- sholma area very close to 1830 Ma. Because of different blocking temperatures for zircons and monazites, the similarity in the metamorphic ages obtained also suggests that the cooling/ uplift was a relatively rapid process.

Samplep 99021 is from the island of Hell- sö,Kökar.Itrepresents the quartz- and feldspar-rich neosome associated with abrecciat- ed pod of mafic gneiss. The surrounding rocks arebands of mafic boudins within astrong- ly deformed granodioritic-dioritic gneiss sequence. Zircon is abundant and mostly occurs as brownish short-prismatic grains. Occasional- ly, secondary overgrowths create complex crys- tallizationsinovaltorounded grains (Fig. 5). However, our expectations to findmetamorphic zircon among the quartz-feldspatic recrystallization products proved negative. Three short-prismatic light brown(sample 99021 C) and ten ry crystallization. Therefore we consider the pre- long-prismatic zircon crystals (sample 99021 viously calculated ”anchored SIMS concordia” of D) were investigated using the ID-TIMS tech- 1879±6 Ma (Fig 2:1) to best define the time of nique. They yielded, within the limits of error, a primaryzircon crystallization, which also is the combined age of 1889±3 Ma. This result is in- best approximation for the time of magmatic em- distinguishable from that on the zircons from placement. a nearby granodioritic gneiss sample at Kökar. Timing of Svecofennian crustal growth and collisional tectonics in Åland, SW Finland 77

Fig. 5. Brecciated gneissatKökar (sample99021). Upper part :selected images of CL-light (cores) and CL-darker oscilatoryzoned zircon domains with Pb/Pb SIMS spot ages. Central part :SIMS ages separated into twodiffer- ent age groups. Bottom part :aweighted average of sixteen out of atotal thirty spot analyses indicates magmatic crystallization at 1884±3 Ma, while detrital cores fall into the 2080–2000 Ma age interval.

Seven zircon grains from sample 99022 A (see Table 2fraction A) together markanage of about 1891 Ma. The SIMS investigation of sample 99021 showedsome remarkable results: Thus there are no evidences of metamorphic zircon overgrowths from events younger than 1860 Ma. However, metamorphic titanite with ages of c. 1784 Ma (Table 2) indicates less pronounced recrystallization as a result of shearing of the two Kökar gneisses. The kinds of zircon crystallization that are most evident in this sample, constitute zoned magmatic crystallizations which surround cores that arewell distinguished in CL images (cf. zircon grain 26 and 28 in Fig. 5). Eight such coreportions havebeen analysed. Out of these, as many as seven cores yielded 207Pb/206Pb ages within the time interval 2000– 2080 Ma, (cf. the histogram in Fig. 5), while the oldest core is 3163 Ma old. Obviously, this rock sample indicates the presence, not only of mi- nor Archaean material, but also of significant amounts of continental crust formed just before 2000 Ma. The other SIMS spot analyses were directed to parts of zircon crystals where some kind of zoning reaching the grain edges is present. The four youngest of these ages are considered to be the result of grain and/or instrument dis- turbances, and spot 02a may well cover several heterogeneous domains. The remaining sixteen yond the individual uncertainties encompas- analyses define a discordia which intercepts the ing whole grains. At least fraction 99021 B concordia at 1882.6±3.7 Ma and 358 Ma, with most likely contains core materials not seen un- a MSWD of 0.66 at the 95 % confidence level. der the microscope. Therefore, these TIMS ages The weighted average 207Pb/206Pb age for these are considered blended/mixed ages. This leaves sixteen analyses is 1884±3 Ma (Fig. 5 centre). the SIMS based age of 1884± 3 Ma as the best ap- The ID-TIMS ages (Table 2) are precise but proximation for the time of magmatic rock em- significantly older and, above all, scattered be- placement. 78 Ehlers,C., Skiöld,T.and Vaasjoki,M.

Table2. U-Pb TIMS-ID analytical data on zircon and monazite from the To rsholma -Enklinge and Kökar -Sot- tunga areas Fractions Concentrations Atomic ratios Ages (Ma) 206Pb/ U 208Pb/ 207Pb 206Pb/ 207Pb/ 207Pb/ 206 238 235 206 207 206

. Properties weight (ppm) Pb Pb meas- U U Rho Pb/ Pb/ Pb (a) (mg) (b) rad rad. ured (c) ±2s% (c) ±2s% (d) 238U 235U (c) ±2s No 98033, Granulitic gneiss, Lilla Hummelholm Torsholma, grid 6694164 -1502371 A, mon, ye, butt, 1xx 0.003 734 3940 11.64 3514 0.3257 1.1 5.016 1.1 0.99 1817 1822 1827.4 2.7 B, mon, ye, 1xx 0.003 991 3579 11.64 3341 0.3267 0.6 5.041 0.6 0.92 1822 1826 1830.8 4.4 C, mon, ye, an, 1xx 0.009 921 3760 13.32 19000 0.3262 0.4 5.030 0.4 0.97 1820 1824 1829.3 1.6 D, z, ab, >74, 8xx, sp-an 0.011 753 241 0.05 10030 0.3210 0.4 5.059 0.4 0.95 1795 1829 1869.0 2.6 E, z, ab, <74, 9xx, sp-an 0.009 622 203 0.07 8300 0.3219 0.4 5.067 0.4 0.91 1799 1831 1866.8 3.1 99021, Leucosome in brecciated gneiss, Björnsnäs Hellsö Kökar, grid 6665380 -1316074 B, z, oval, an, 9xx 0.012 733 252 0.13 1873 0.3211 0.2 5.215 0.3 0.97 1795 1855 1922.6 1.1 C, z, ab, >74, sp, lbr, 3xx 0.003 961 332 0.15 4160 0.3183 0.4 5.073 0.4 0.98 1781 1832 1889.2 1.2 D, z, ab, <74, lp, 10xx 0.012 899 307 0.18 7000 0.3055 0.5 4.866 0.5 1.00 1719 1796 1887.6 1.0 99022, Granodioritic gneiss, Hamnviken Hellsö Kökar, grid 6665000 -1316060 A, z, ab, >106, sp, 7xx 0.035 288 95 0.11 9030 0.3125 0.5 4.986 0.5 0.97 1753 1817 1891.1 2.1 B, ti, ab, >106, ca.60xx 0.557 80 26 0.09 909 0.3096 0.2 4.658 0.2 0.94 1739 1760 1784.6 2.6 C, ti, ab, lbr, low magnetic 0.907 85 26 0.09 1030 0.3017 0.4 4.541 0.4 0.99 1700 1739 1786.0 2.6 D, ti, ab, <106 0.546 89 29 0.07 1140 0.3101 0.2 4.657 0.2 0.94 1741 1760 1781.2 2.4 99024, Banded Bi+hbl gneiss, Bogrundet Sottunga, grid 6667389 - 1480949 A, z, ab, 6xx 0.059 486 165 0.11 9260 0.3222 0.8 5.115 0.8 1.00 1801 1839 1881.9 1.4 B, z, sp, br, 4xx, 0.018 1159 378 0.05 11100 0.3238 0.2 5.151 0.2 0.96 1808 1845 1885.8 1.2 C, z, ab, >74, lp, 11xx 0.020 649 200 0.07 2700 0.3013 0.1 4.784 0.2 0.80 1698 1782 1882.2 2.1 D, z, ab, sp, 6xx 0.021 539 178 0.08 4200 0.3208 0.3 5.079 0.3 0.91 1794 1833 1877.4 2.4 A1187, Granodioritic dyke, Bockholm Torsholma, grid 6693654 -1503023 A, z, 4.3-4.5 8.5 805 252 0.07 4814 0.3072 0.7 4.780 0.7 0.97 1726 1781 1846.0 2.0 B, z, 4.2-4.3 7.5 754 242 0.08 4612 0.3098 0.7 4.834 0.7 0.97 1739 1790 1851.0 1.0 C, z, 4.0-4.2, >70 7.0 1129 335 0.09 2151 0.2836 0.7 4.369 0.7 0.97 1609 1706 1828.0 2.0 D, mon 2.5 2201 6053 8.49 20939 0.3303 0.7 5.086 0.7 0.95 1839 1833 1827.0 4.0 E, z, 4.0-4.2, <70 7.3 1060 315 0.08 1769 0.2864 0.7 4.433 0.7 0.97 1623 1718 1836.0 2.0 F, z, 4.3-4.5, ab 5.6 802 256 0.07 2954 0.3109 0.7 4.848 0.7 0.97 1745 1793 1849.0 1.0 A1261, Gneissose granite, Räddarskär Torsholma, grid 6692775 -1502759 A, z, +4.5 7.0 630 193 0.07 6578 0.3005 0.7 4.691 0.7 0.97 1693 1765 1851.0 2.0 B, z, +4.5, ab, lbr 5.3 576 201 0.09 1226 0.3284 0.7 5.134 0.7 0.96 1830 1841 1855.0 4.0 C, z, 4.3-4.5 7.0 589 188 0.07 7532 0.3129 0.7 4.888 0.7 0.97 1754 1800 1853.0 2.0 D, z, 4.2-4.3, >200 7.2 888 246 0.07 4583 0.2715 0.7 4.189 0.7 0.96 1548 1671 1831.0 4.0 A1334, Granulitic gneiss, N. Gräsören Torsholma, grid 6694174 -1502649 A, z, 4.0-4.2, ab 9.6 1130 355 0.07 5562 0.3081 0.7 4.811 0.7 0.99 1731 1786 1852.0 1.0 B, z, 4.0-4.2 7.7 1125 354 0.07 5151 0.3064 0.6 4.781 0.6 0.99 1722 1781 1851.0 2.0 C, z, 3.8-4.0, >70 7.0 2037 600 0.08 2165 0.2821 0.6 4.344 0.7 0.97 1601 1701 1827.0 2.0 D, z, 3.6-3.8, lbr 7.1 2415 615 0.01 990 0.2378 0.7 3.598 0.7 0.98 1375 1549 1795.0 2.0 E, z, 3.6-3.8, >70, br 4.6 3473 783 0.13 773 0.1887 0.6 2.824 0.6 0.93 1114 1361 1774.0 4.0 M, mon 1.7 3787 5559 4.01 19262 0.3253 0.7 5.015 0.7 0.97 1815 1821 1829.0 5.0 A 1185, Pegmatite, Hummelholm, Torsholma, grid 6693117 - 1500820 A, mon, ye, butt, 1xx 3853 7356 5.72 26400 0.3226 0.7 4.881 0.7 0.95 1802 1798 1795.0 5.3 (a) all the 98033 and 99XXX sample zircon crystals are transparent, uncoloured and non-magnetic (zero slope, 3 degr of tilt and 1.7 A of Frantz isodynamic separator) unless otherwise indicated; z=zircon; mon=monazite; ti=titanite; >74 and <74=crystal sizes in mi- crons; +4.5, 4.3-4.5 etc refers to the speciphic gravity of the analysed zircons; 4xx=number of crystals analysed; lbr=light brown; br=brown; ye=yellow; sp=short-prismatic (l/w=1-3); lp=long-prismatic(l/w>4); butt=button-like; eu=euhedral; an=anhedral; subround- ed; ab=abraded (Krogh, 1982) (b)Concentrations for the 98033 and 99XXX samples are known to about 15% for sample weights of about 0.01 mg (c) Corrected for lead blank of 5-15 pg in the 98033 and 99XXX samples, fractionation, spike and initial common lead (Stacey & Kram- ers, 1975); errors are at the 2 sigma level and are expressed in per cent of the atomic ratios and in absolute numbers for the ages (d)Error correlation for 207Pb/235U vs. 206Pb/238U ratios.

Samplep 99022 is agranodioritic gneiss with one ID-TIMS zircon analysis was carried out on blastomegacrystic feldspar that was sampled abraded grains of this rock sample. The mean on Hellsö, Kökar. The zircons are often colour- 207Pb/206Pb age of a small number of crystals is less and free from cracks, and display arange 1891±2 Ma (Table 2, not shown graphically). of long-prismatic to round morphologies. Only Since there is a clear possibility that this analysis Timing of Svecofennian crustal growth and collisional tectonics in Åland, SW Finland 79

Fig. 6. Banded biotite-hornblende-gneissatSottunga (sample 99024). Upper part :CLand TL images from five representatives of zircon grains with their locations and corresponding SIMS Pb/Pb age figures. Bottom part :plotting of SIMS spotanalyses in aConcordia diagram where nineteen spots areconsidered to approximate the crystallization age of the rock.Taking the SIMS weighted average and the ID-TIMS data into consideration, an age of 1884±5 Ma is suggested. include at least some inherited radiogenic lead, the obtained age must be considered amaxi- mum age of rock crystallization. Three populations of titanite crystals were also investigated radiometrically from this rock. They yield somewhat discordant results with 207Pb/ 2066 Pb ages of 1786, 1785 and 1781 Ma (not shown graphically).

Sampplle 99024 is from abanded biotite-hornblende orthogneiss on the island of Sottun- ga. The rock has been strongly deformed by ductile shearing, occasionally showing strong sheath folding. There is a range of zircon morphologies that may comprise both xenocrystic and metamorphic parts. The rock was sampled close to a younger granitic dyke intrusion (comparable to the rock type sampled as 00010 described below) which registers significantly later deformations within the shear zone. The thirty SIMS spot analyses performed were mainly di- The ID-TIMS analyses of four zircon popu- rected at domains of zoning and where CL-im- lations with varying morphologies and colour- ages are homogeneously dark or show an inter- ing yield 207 Pb/206 Pb ages between 1886 and play of darker and lighter lamellae. By excluding 1877 Ma.Although the grains were abraded analyses with large instrumental uncertainties, before chemical dissolution, there is some sus- discordancies and/or clearly separate ages (the picion of metamorphic additions in the young ones excluded from the regression are indicated age. Xenocrystic cores with CL-white imag- by an x before their numbers in Table 1 and by es occur in some of the mounted grains (com- their spot numbers in Fig. 6), we end up with a pare CL image of spot 04 in Fig. 6), but none group of nineteen that yield concordia-discordia of these havebeen analysed from this sample intercept ages at 1882.3±4.7 Ma and 158 Ma. and their ages are therefore unknown. The ID- The corresponding weighted average 207Pb/206Pb TIMS ages of the three older populations vary mean age is 1884.1±3.8 Ma with a MSWD of from 1882 to 1886 Ma and are within the un- 2.4. The very young ages, like those of spots 02a certainties of the mean SIMS 207 Pb/206 Pb age. (Fig 6), 15a and 28a which show influences of From a consideration of the various age data, we younger events, were excluded and may have suggest an age of 1884±5 Ma as the best approxi- been related to the emplacement of the adjacent mation for the crystallization of the magmatic zir- intrusive granites. cons and the emplacement of this rock. 80 Ehlers,C., Skiöld,T.and Vaasjoki,M.

Fig. 7. Jointed gneissatGräshäran (sample95020). Upper part :transmitted light (TL) images (texturallyless distinct CL images arenot shown) of zircon grains with spot locations and corresponding Pb/Pb age figures. Central part : plots of spot analyses resulting in aConcordia interinter- cept age based on all but tworejected rim and discordant analyses. Bottom part :histogram of SIMS spot analyses indicating amean Pb/Pb age of 1884±3 Ma.

point of ages and textures, eight out of the ten spot analyses (n = 8/10) make up a homogeneous group. Most of these are shown in transmitted-light (TL) photographs in Fig. 7. They yield a mean 207Pb/206Pb age of 1886.0±6.8 Ma with a MSWD of 7.6. The rather high MSWD value is largely due to the presence of one (spot 02) disparate age, older than the majority of ages obtained (Fig. 7). By using Tukey’sbiweight mean calculation (Ludwig, 1991) instead of the weighted average mean, the distorted distribution is accounted for. The result is a lower, and probably more reliable mean age of 1883.6±2.8 Ma at the 95 % confidence level, approximated to 1884±3 Ma. The contacts between the central domains and the metamorphic overgrowths a re always sharp and follow closely the magmatic zonation of the zircons. Thus there are no signs of crystal absorption of the central parts that would result in indented boundaries crossing over the zonation and formed during the metamorphic overgrowth. Similarly, there are no traces of any erosional processes that could haveworked/ Samplep 95020 represents agrey, strongly de- abraded the zircon grains. Genetically, these fea- formed tonalitic-granodioritic gneiss with an tures confirm a magmatic origin for the rock. axial- plane schistosity trending N40 E. The The homogeneity of the zircon ages can some- sample was collected at the islet of Gräshäran a times be taken to indicate a single source. How- few kilometres north of the island of Enklinge ever,thereisone rather old age of 2061 Ma, (Fig. 1). The zircon grains are generally short- which has been excluded from the calculation. prismatic with curved morphologies (high order This age may well be a reflectionof the presence hkl-indices) in the grain terminations. When of newly formed continental crust at about that immersed in alcohol or mounted in plastic and time. Zircon ages of about 2000 Ma and more polished to expose internal textures, the zircons arefrequent in the Svecofennian sedimentary display marginal rims. Generally, these are rich- rocks distant from the Archaean craton (Claes- er in uranium and more turbid probably due to son et al., 1993; Lahtinen et al., 2002). Further- the development of micro-fractures. From the more, they make up a significantpar t of the zir- Timing of Svecofennian crustal growth and collisional tectonics in Åland, SW Finland 81

Fig. 8. Rhyolite at Enklinge(sample 00013). Upper part : CL and TL images with the location of some zircon spot analyses and corresponding Pb/Pb age figures. Cen- tral part :Concordia diagram forming acluster of SIMS spot determinations. Bottom part :histogram of SIMS spot analyses indicating an average Pb/Pb age of 1887±5 Ma. Considering all the different modes of calculation, the zircon crystalization indicates rock-forming processes at 1885±6 Ma. con population in the Kökar sample (cores in sample 99021; Table 1).

Saamplemple 00013 is from the island of Enklinge. It represents arhyolitic porphyrywith varying concentrations of fragments. According to the stratigraphic section suggested for the En- klinge area by Ehlers and Lindroos (1990b) that rock was deposited atop of the supracrustal pile of mainly mafic volcanic rocks. The homogeneously deformed rocks show a steep linear structure (rodding) with an almost circular XY section of the strain ellipsoid. There are out- crops with well preserved and only weakly distorted primary volcanic structures. The zircon crystals of this porphyry sometimes have radi- al and interlamellar cracks transecting simple prisms which areoften terminated by distorted pyramids (Fig. 8). The CL-images show grey to darker grey domains that follow the internal lamellar textures. In some cases, thin metamorphic rims can be traced, but this is too minor a phenomenon to be analysed. The SIMS spot analyses (i.e. crystals 01, 03 and 05 in Fig. 8) have been directed to what were considered to be homogeneous CL-domains which seemingly match undisturbed domains of lamellar growth zoning seen in the comparable transmitted light el and MSWD of 2.3). The weighted average (TL) images. of the 207Pb/206Pb ages is 1886.5±4.8 Ma, which With the exception of two analyses, one from should be compared to the age of Tu key’sbi- a probable xenocrystic core (07a, Table 1) and weigthed mean of 1884.8±5.4 Ma. The situa- the other from the marginal part of a grain (10c, tion where most analyses are concordant to al- from which we have two additional analyses of most concordant with no significant lead loss the central domains 10a and 10b), the remain- supports a calculation based on the 207Pb/206Pb ing sixteen out of the eighteen (n = 16/18) anal- ages. Thus, an age of 1885±6 Ma is considered the yses yield adiscordia that intersects the con- best approximation for the formation of the En- cordia at 1888±7.5 Ma (95 % confidence lev- klinge felsic porphyry. 82 Ehlers,C., Skiöld,T.and Vaasjoki,M.

Fig. 9. Metadiabase at To rsholma (sample 00014). Upper part :CLimages from representatives of the fewzircon grains availablewith their spot locations and corresponding Pb/Pb age figures. Bottom part :histogram of the limited number of age determinations resulting in aless precise and rather hypothetical age of 1859±19 Ma.

Saamplemple no 00014 is from a meta-diabase inter- secting the granulite gneiss at Torsholma (Fig. 3). The zircons in this rock are few and only ten SIMS analyses were performed. Unfortunately these resulted in a span of crystallization ages indicating the probable presence of heterogeneous grains. Obvious outliers from what might be a main population arethe very discordant and metamict spot nominated 07a (see Table 1) and analysis 02a from the central domain of a CL- heterogeneous grain (Fig. 9). Analyses 05a and 05b seemingly sample similar portions of grain number 05 (Fig. 9), and the diverging age numbers (1856 and 1878 Ma) examplify the limited precision of single SIMS ages as compared with the ages of ID-TIMS. Therefore, weighted mean of SIMS ages or SIMS discordias should be based on a number of spot analyses sufficient to allow statistical treatment. If also analysis 10a isrejected (because of the known effect of an 1830 Ma event in the country gneisses, but also for the relatively high negative discordancy), we may calculate a mean 206Pb/207Pb age for the remaining seven analyses of 1859±19Ma. These seven analyses span an age interval of 1837 to 1894 Ma.Atthe moment thereare no reliable criteria to exclude some of the younger ages apartfromthe noted influence of metamorphism at about 1830 Ma. Having this in mind, however, an age in the older part of the 1859±19 Ma intervalseems to be the morelikely one to match the time of magmatic zircon crystallization and dyke intrusion.

Fig. 10. Stronglydeformed dioritic/granodioritic gneisses (SIMS U-Pb zircon age 1884± 5Ma) with amphib- Saamplemple no 00010 is from a granite dyke at the olitic bands arediscordantly transected by amedium Hästnäs locality on Sottunga island, situated in to fine-grained dykeofmicrocline granite with aSIMS the prominent ductile SFSZ shear zone. The U-Pb zircon age of 1790± 6Ma). The locality is in the middle of the SFSZ on the western shoreofthe island dyke is 5–10 metres wide and cuts the folded of Sottunga. and banded gneisses (Fig. 10). Despite the fresh Timing of Svecofennian crustal growth and collisional tectonics in Åland, SW Finland 83

Fig. 11. Intrusivegranite at Sottunga (sample 00010). Up- per part :CL-imagesofselected zircon grains showing the Pb/Pb age figures forthe spot determinations indicated. Central part :Concordia plotting of all but the most discordant analyses. Bottom part :histogram indicating a mean Pb/Pb age of the morecoherent determinations leadingof1790±6 Ma.

(e.g., grain 03 in Fig. 11) and sometimes irregular central domains. The metamict characters of the three uranium-rich rims that were analysed prohibit age calculation for these late additions. On the other hand, the freshness of the sampled dykes as well as signs of absorption to similarly aged central domains of the zircon grains (cf. grains 03 and 07 in Fig. 11) argue against a metamorphic event significantly separate from the time of intrusion. We prefer a scenario involving the crystallization of rims to create the euhedral outer zircon morphologies (cf. the different phases of grain 03 shown in Fig. 11) during a stage linked to the original rock crystallization. In accordance with an evolving magma chamber, the chemistry of the crystallizing agents had developed into moreuranium-rich concentrations. Twentyfour SIMS spot analyses on various zircon domains from sample 00010 have been performed. Out of these analyses, the spots of relatively young 207Pb/206Pb ages (11a, 06a and 17a in Table 1) havebeen excluded due to a combination of being discordant uranium-rich rims, having poor accuracies, and relatively low 206 Pb/204Pb ratios from the influences of com- monlead. Sixanalyses in the older range of 207Pb/206Pb ages have likewise been put aside because of (1) poor precision and being relatively older than their interior parts (cf. spot analysis 03c in relation to 03a and b also visualized nature of the rock, its zircon grains show com- in Fig. 11); (2) occupying central grain domains plicated crystallization patterns. In some in- of somewhat roundish morphology with poten- stances, possibly xenocrystic cores make up the tial xenocrystic origin (cf. grains 04 and 12a in central parts of the grains (spot 04 in Fig.11), Fig. 11); and (3) exhibiting too old and dispa- while metamict and uranium-richzircon con- rate age figures in order to have a cause of crys- stitutes the outer rims (e.g., spot 07). These late tallization common to the majority of spot anal- crystallizations are readily seen as CL-dark im- yses. Among these, we again note the presence ages against the often zoned CL-greyish-white of plus-2000 Ma,most probably continental 84 Ehlers,C., Skiöld,T.and Vaasjoki,M. zircons in the area (e.g., 00010 and 15a in Ta- gional D2 deformation elsewere in SW Finland ble 1). The remaining fifteenspot analyses make (Ehlers et al., 1993; Väisänen, 2002). That fold- up a seemingly homogeneous group. A best fit- ing occurred well before the regional D3 phase, ted Discordia line yields a Concordia intercept age whichisroughly synchronous with the intru- of 1789±8 Ma while the 207Pb/206Pb ages form a sion of the migmatizing microcline granites similar robust mean at 1790±6 Ma (n=15, 95% within the LSGM zone. The deformed orthog- conf., MSWD=0.91). Either of these age calcula- neiss sequence was extended and intruded by tions are valid. roughly E-W directed maficmetadiabasic dykes (Fig. 2:1 and Fig. 3). The geological structure 6. Discussion and conclusions of the Torsholma area indicates a phase of extension of the crust, and intrusion of steep ma- Zircon is known to surviveseveremetamor- fic dykes which took place after the strongly re- phism, but can readily form secondary coating cumbent fold phases (D2), but before the sub- atop primary grains. Moreover, zircon seeds are sequent stacking and imbrication of the gneiss assimilated during magma ascent, and relict zir- slab between sub-horizontal sheets of mafic ol-v con can survive from a continental provenance. canics (Fig 2:3). Our disparate and rather lim- Together these features result in a complex pat- ited SIMS zircon analyses from the amphibolite tern of crystallizations within a single grain. Ge- dykes (sample 00014) rendered an imprecise re- ologically significant ages are supposed to mon- sult of 1859±19 Ma for the extensional phase itor an isotopically homogeneous part of a grain following upon the subhorizontal D2-folding of or set of grains. It is evident that these con- the gneisses. ditions arenot always met with a nd this fact In the concept of a Svecofennian arc accre- should be considered when comparing age fig- tion, recently reviewed by Väisänen (2002), the ures obtained with different techniques and to formation of this rock collage would represent a different standards. significantphase of collision between the South- ern (SSAC) and Central (CSAC) Svecofennian Arc Complexes. A sheet of granodiorite (sample 6.1.Timing of the imbrication and A1261, 1861±19 Ma) intruded the stack of pre- overthrust in Torsholma viously imbricated rock slabs, between the de- The TIMS and SIMS datings of the rocks in the formed orthogneisses and the overlying outlier Torsholma area give us time constraints as fol- of amphibolite (Fig. 2:3). The granodiorite is lows. The emplacement age of the deformed or- moderately sheared (top towards NW, Fig. 13). thogneiss, c. 1879±6 Ma, is comparable to the Nearby, only slightly deformed granite dykes of ages of the early gneissose granodioritic intru- similar age and composition (sample A1187 sions reported from southern Finland in this with an age of 1865±13 Ma) also transect the study (c. 1885 Ma,and summarised in Table deformed orthogneisses. They seemingly be- 3) and in neighbouring regions of the Tampere long to the same intrusion which apparently SchistBelt (c. 1900–1880 Ma)and elsewhere took place during the vaning stages of the colli- (Kähkönen, 1999; Nironen et al., 2002). The sional episode. studied area forms a sub-horizontal sequence of The emplacement of the orthogneisses at multiply folded supracrustal rocks and gneiss- 1879±6 Ma and the intrusion of the granodi- es with structures (refolded folds in Fig. 12) in- orite dykes at 1865±13 Ma brackets the time of dicating an early overturning and sub-horizon- intrusion of the amphibolite dykes (heredat- tal thrusting of the attenuated and banded se- ed poorly at 1859±19 Ma) into a rather narrow quence towards the northwest. This phase of timeframe (see Fig. 3). Thesedykesintruded deformation conceivably correlates with the re- during what is believed to be a late stage of the Timing of Svecofennian crustal growth and collisional tectonics in Åland, SW Finland 85

thrusting. The extension of the crust and the intrusion of the meta-diabases must have occurred between those two tectonic episodes. This leaves us with an approximate time intervalofc. 1875–1860 Ma for the intrusion of the dykes and the subsequent phase of stacking and imbrication. This is in accordance with the estimation by Väisänen et al. (2002) for the accretion and collission between the SSAC and CSAC. Both in the deformed orthogneisses (sample 98033) and in the cross cutting dykes of a late- kinematic type granodiorite (sample A1187) thereare monazites giving concordant ages of about 1830 Ma. The same age is observed from single grain SIMS spot analyses of zircon overgrowths in the same sample (Fig. 4). It is also the dominant age of the microcline granites belonging to the LSGM zone of southern Finland (Suominen, 1991; Kurhila et al., 2004), and conceivably this signifies an episode of elevat- ed temperatures accompanying metamorphism and intrusion of these granites. The whole collage of rock slabs was later cut by a set of un- Fig. 12. Stronglydeformed granodioriticgneisswith am- deformed pegmatites indicating post-collision- phibolitic bands.The fold interference pattern indicates al collapse (Lindroos et al., 1996; Fig. 2:4) and refolded stronglyrecumbent mesoscopic folds.The fold axes dip gentlytowards the observer(towards W). Lo- yielding a concordant monazite age of 1795±4 cality at To rsholma. Ma (sample A1185).

6.2.The age of the volcanics and the granodiorite of the Enklinge area The Enklinge area features well preserved subaqueous volcanic sequences earlier described by Sederholm (1934), Ehlers (1976), and Ehlers & Lindroos (1990b). Sample 00013 is a rhyolite porphyry from the upper part of the stratigraphic secuence. It overlies a sequence of mafic and intermediate lavas and fragment-rich rocks. The Fig. 13.Vertical outcrop of granodioritic gneissintruding precise SIMS U-Pb zircon age of 1885±5 Ma astack of imbricated rock slabs. Shear lenses indicate reported hereis in agreement with some of the displacement of the top towards the northwest. Local- ity at Räddarskär,Torsholma. published ages on volcanic rocks from the Sve- cofennian domain in Sweden and Finland (e.g., stacking and imbrication processes. On the oth- Kähkönen, 1999). er hand, the original emplacement of the old- The surrounding granodioritic gneiss has a er orthogneiss was followed by early deforma- previously reported multi-grain ID-TIMS zir- tion involving subhorizontal (D2) nappes and con age of 1882±15 Ma (Suominen, 1991). 86 Ehlers,C., Skiöld,T.and Vaasjoki,M.

6.3.The South Finland Shear Zone (SFSZ),

The SFSZ represents a major zone of deformation that can be followed along the south coast of Finland for a couple of hundred kilometres. In its SW part this zone closely follows the con- tact of the LSGM zone (Fig. 1), but most of the deformation is in the older granodiorite-tonal- ite rocks representing early Svecofennian intrusions. Previous ID-TIMS age determinations from these rocks around Sottunga and Kökar cover an approximate age interval of c. 1870– 1895 Ma (Suominen, 1991). Ournew SIMS samples from the strongly banded gneisses on Sottunga and from the deformed tonalites-granodiorites on Kökar both indicate an intrusion age of 1884±5 Ma. The presence of older Palae- oproterozoic and Archaean zircons may explain why some published ages probably represent mixed populations. The gneisses in the area along the SFSZ are intruded by swarms of amphibolitic metadiabase dykes (Fig. 14) reminiscent of those which Fig 14.Amphiboliticmeta-diabases areintrusiveinto gneissose granodiorite.The maficdykes aredeflected appear on Torsholma. This is probably a region- intothe SFSZ shearzone and stronglydeformed and al feature which in most cases is obliterated by transposed parallel to the direction of shearing. Locali- the later phases of metamorphism and granite ty at Rödgrund, Sottunga. intrusions within the LSGM zone. Since the dykes are deflected, they intruded the gneisses These rocks areyounger and structurally dis- previous to the shearing along the SFSZ. In all cordantto the volcanics. However, because of likelihood, the intrusion of these mafic dykes is theirsimilar chemistry, and since several fine- comparable to those on Torsholma and supplies grained quartz porphyrydykes aresubparal- a maximum age of the shearing that post-dates lel with the rhyolitic volcanics, Ehlers and Lin- c. 1865 Ma. droos (1990a) considered the dykes to be hypa- The NW end of the SFSZ around the is- byssal equivalents of the same magma. Our new land of Sottunga is characterized by numerous sample 95020 represents a deformed part of the granitic dykes forming a network discordantly granodioritic gneisses and yields a SIMS zircon crosscutting the shear zone. The granite dykes, age of 1884±3 Ma. This corraborates previous however, haveaschistosity (Edelman, 1979) deductions on the close relationship in time and and showminor deformation and deflection space between the extrusion of the Enklinge suggesting intrusion during the vaning stages of volcanics and the intrusion of the surrounding the shearing. Our SIMS sample (00010) from granodiorite.Moreimportantly,there is asu- one of the late granite dykes in the NW part perb age agreement between these volcanics and ofthe shear zone gives an age of 1790±6 Ma our present SIMS results from the granodiorit- and indicates the time of the last major activi- ic gneisses of the Sottunga, Kökar and Torshol- ties along the zone in this area. On the northern ma areas (Table 3). shore of the island of Sottunga, the SFSZ is cut Timing of Svecofennian crustal growth and collisional tectonics in Åland, SW Finland 87 by the essentially undeformed Mosshaga granite district with continuations to Finland. A time with a U-Pb TIMS zircon age of 1788±11 Ma concept involving c. 2 Ga old micro-continents (Welin et al., 1983). Thus our SIMS dated gran- originally distant to the Archaean regions in the ite dykes in the NW end of the SFSZ are of the NE has recently been proposed by Nironen et same age and extend from the Mosshaga granite al. (2002) and Lahtinen et al. (2002). Although massif some 10 kilometres along the shear zone. many Svecofennian granitoids areknown to The coherence is also noted from petrographi- contain heterogeneous zircon populations (eg. cal and chemical analyses of both dykes and the Vaasjoki et al., 1996), the Kökar gneiss is seem- Mosshaga pluton (e.g., Branigan, 1987). ingly the firstcase where inheritance from c. 2.0 Ga sources has been unequivocally demonstrated in a syntectonic Svecofennian intrusive rock. 6.4. Regional considerations within Thus, increasing evidences for the exitence of a the Svecofennian domain Palaeoproterozoic continental crust before 1.93 Prominent volcanism and igneous activity con- Ga are accumulating. stitute timely separated episodes in the build-up The oldest well-constrained age of ear- of the Svecofennian domain. The granodiorit- ly magmatism from the Tampereregion is c. ic gneisses at Kökar with a zircon SIMS intru- 1904±4 Ma (Kähkönen, 1999). The rock in- sion age of 1884±3 Ma apparently contain sig- terfingers surrounding supracrustals which nificant proportions of continental crust that sometimes havebeen paralleled with the Ha- was formed just before 2000 Ma ago (Fig. 5). veri, and Vammala belts (Kähkönen &Niro- Similarly,thereare anumber of 2.1–1.95 Ga nen, 1994). Magmatism and deposition alter- old zircons in Svecofennian metasedimenta- nate to about 1880 Ma when subsequent met- ry rocks from central Finland (Huhma et al., amorphism and folding took place in central 1991; Claesson et al., 1993; Lahtinen et al., Finland. Towards west in the Bergslagen re- 2002). However, Vaasjoki et al. (2003) have gion of Sweden, magmatism and alternating demonstrated that at least some of the c. 1920 sedimentation areobservedinthe approximate Ma granitoids from central Finland do not con- time interval1904–1890 Ma (Allen et al., tain inherited zircons. Scarce occurrences of 1996; Lundstömetal., 1998; Persson &Pers- rocks close to this age or somewhat older have son, 1999). In comparison, the oldest magma- been identified from central Sweden (c. 1.95 tism in the Åland region is somewhat younger Ga granitoids, Wasström, 1996) while similar than that of both the Tampereand the Bergsla- ages by Welin et al. (1993) and Lundqvist et al. gen region. Thereare so far no modern age de- (1998) run the possibility of including inherit- terminations from Åland indicating granitoid ed grains. About 1.98–2.0 Ga old detrital zircon emplacement ages in excess of 1890 Ma. grains from south-central Finland showboth The earliest metamorphism in the Tampere pre-and post-sedimentarymetamorphism old- schist belt commenced at about 1880 Ma, while er than previously recorded in the Svecofennian the metamorphism of the allochthonous gneiss domain (Rutland et al., 2004). Moreover, Rut- slab of Torsholma is seemingly later. Metamor- land et al. (2001a and b) demonstrate the ex- phic ages of about 1860 Ma have been docu- istence of deformation structures in some Pal- mented from zircon overgrowths and monazites aeoproterozoic supracrustal rocks of the Both- within the Bothnian Basin near the Skellefte nian Basin from central Sweden that arenot district (Rutland et al., 2001b). The comparable found elsewhere. They further postulate (op. metamorphic episode in the Bergslagen region cit.) the existence of a rifting continent making is so far limited to be younger than the c.1856 up the basement to the supracrustal rocks in re- Ma old deformed pegmatites at Stora Vika (We- gions like Bergslagen and south of the Skellefte lin & Stålhös, 1986). 88 Ehlers,C., Skiöld,T.and Vaasjoki,M.

Table3. Summaryofcalculated ages, wheresome indicate rock formation/emplacement, others assimilated crust or metamorphic events.

Sample Rock type and location/ mineral Te chnique No. of Age ± 2s (Ma) No analyses A1185 Torsholma, Hummelholm pegmatite/monazite TIMS 11795±4 LIG00014 Toorsholmarsholma metadiabase/zircon SIMS 71859±19 A1187 Torsholma, Bockholm, granodiorite dyke/zircon/ TIMS Multi 1865±7 monazite TIMS 1 1827±4 A1261 Torsholma, Räddarskär, granodiorite/zircon TIMS Multi 1861±19 A1334 Torsholma, Gräsören, granulite/zirc TIMS Multi 1867±15 /monazite TIMS 1 1829±5 LIG98033 Torsholma, granulite, Lilla Hummelholm /zircon SIMS 8 1879±6 /zircon SIMS 2 c. 1828 /monazite TIMS 3 c. 1830 LIG95020 Enklinge, Gräshäran, granodiorite/zircon SIMS 8 1884±3 /xenocrystic zircon SIMS 1 c. 2061 LIG00013 Enklinge, Rhyolite/zircon SIMS 16 1885±6 LIG99021 Kökar, breccia in banded gneiss Hellsö /zircon SIMS 16 1884±3 /xenocrystic zircon SIMS 7 c. 2040 /oldest xenocrystic zircon SIMS 1 c. 3163 /zircon (a few grains) TIMS 3 1889±3 LIG99022 Kökar, granodioritic gneiss, Hellsö/zircon TIMS 1 1891±2 /titanite TIMS 3 c. 1785 LIG99024 Sottunga, banded gneiss/zircon SIMS 19 1884±5 LIG00010 Soottungattunga granite dyke/zircon SIMS 15 1789±8 Hästnäs dyke/xenocryst. zircon SIMS 1 c. 2033

Metamorphic imprint at about 1830 Ma in ders the Svecofennian domain to the south and the LSGM zone (Ehlers et al., 1993) is also re- west. corded by concordant monazite ages and overgrowths on zircons in several samples from Tor- 7. Concluding remarks sholma. This episode is confined to this zone and is not observed elsewhere in the Svecofenni- The volcanic extrusions at Enklinge (1885±6 an domain. It represents episodes of metamor- Ma) are coeval with the intrusion of the gneis- phism, remelting and formation of migmatit- sose granodiorites in the surrounding areas of ic granites. However, granite intrusions of ap- Enklinge, Sottunga and Kökar.These rocks proximately the same age occur as an older part yield remarkably coherent SIMS ages with an in the Tr ansscandinavian Igneous Belt that bor- average of about 1885±5 Ma (Table 3). They in- Timing of Svecofennian crustal growth and collisional tectonics in Åland, SW Finland 89 dicate a major episode of Svecofennian volcan- place some 1860–1875 Ma ago. Thereafter, ba- ism and crustal formation over large areas. An saltic dyke intrusions mark an episode of crus- older population of zircon grains in the granodi- tal extension. Väisänen (2002) suggests an age orites cluster at around 2040 Ma, in agreement of c. 1870 Ma for this collisional stage based on with similar indications from mainly metasedi- the assumption that this is the youngest true re- mentary rocks in other areas of S. Finland and corded age of the early Svecofennian granodior- Sweden. itic intrusions. The medium- to fine-grained granite dykes In several samples of the Torsholma granodi- along the NW end of the South Finland Shear orites, monazite crystallizations and zircon over- Zone (SFSZ) register the last displacements growths yield concordant U-Pb ages of approxi- along the zone. The dykes are discordant in rela- mately 1830 Ma. This age is in a general accord- tion to the strongly deformed granodiorites but ance with the ages of granites and migmatites in are themselves schistose and gently folded. Their the LSGM zone (Ehlers et al., 1993). Potassium age of c. 1790 Ma dates the later stages of shear- granites belonging to this group occur all over ing along the SFSZ. Petrologically and chemi- the research area in Torsholma, and the mon- cally they are similar to the Mosshaga intrusion azites most probably recordthe overprinting which transects the SFSZ close to the Åland Ra- temperature anomaly of this phase in the struc- pakivi (Fig. 1). The dykes stretch about 10 km tural evolution of southern Finland. southwards along the shear zone, thus substan- A steep undeformed pegmatite dyke with a tiating their close relationship in time to the de- monazite age of 1795±4 Ma transects the whole velopments within the SFSZ. set of imbricated rock slabs in Torsholma (Fig. The collage of thin imbricated rock slabs in 2). The overlapping U-Pb ages of this and oth- the Torsholma area separates two stages of in- er pegmatites (Lindroos et al., 1996) of the post- trusions, at c. 1880 and c. 1865 Ma, in the su- kinematic type intrusions, as well as the timing pracrustal rocks. Gneisses of the older group are of the large-scale shear zones, indicate consoli- sheared and folded together with the supracrus- dation of the Svecofennian crust. This also sug- tals into subhorizontal recumbent folds which gests that deformation was partitioned along are subsequently refolded (Fig. 2 ). The episodes steep discrete crustal shears in contrast to the of folding are followed by an extensional stage earlier subhorizontal crustal shortening and producing a set of steeply dipping metadiabase thickening seen in the Torsholma area. The dykes. The whole collage is subsequently over- transition between the earlier tectonic history of thrusted and sandwiched between pillowed ma- ductile subhorizontal collisional crustal defor- fic lava layers. This episode can be interpreted mation, and the later phases of brittle strain par- as a result of a collisional episode (accretion and titioning into steeply dipping shears must have collision between SSAC and CSAC , Väisänen, taken place after the intrusion of the subhori- 2002 and references therein). zontal granite sheets (c. 1830 Ma) belonging to The c. 1865 Ma granodiorites intrude the the LSGM zone of southern Finland. sandwiched layers as gently dipping sheets and steep dykes in a late stage of this collisional de- Acknowledgements formation, and they still show traces of ”top to- We are grateful to Roland Gorbatschev (Lund), Yrjö wardsW”movements (Fig. 13). These events Kähkönen (Helsinki) and Aulis Kärki (Oulu) for their supply arather limited time intervalfor the thorough and helpful reviews. We thank the staff in the most intensive part of the Svecofennian defor- Isotope Geology units of the Finnish Geological Sur- mational history. They suggest that the main vey (GTK), the Swedish Museum of Natural History (NRM), and in particular Martin Whitehouse and Lev collisional episode, as shown by overthrusting, Ilyinsky of the NORDSIM laboratory, for assistance imbrication and metamorphism, probably took during different stages of sample preparation, analysis 90 Ehlers,C., Skiöld,T.and Vaasjoki,M.

and data reduction. This is contribution #107 of the Elming, S-A. &Mattsson, H., 2001. Post Jotnian basic NORDSIM laboratory which is a joint Nordic facility intrusions in the Fennoscandian Shield and the break in Stockholm sponsored by the research councils, the up of Baltica from Laurentia: apaleomagnetic and GTK and the NRM. We also thank Pernilla Lindroos, AMS study.Precambrian Research 108, 215–236. Gaál, G. &Gorbatschev,R., 1987. An outline of the Pre- Anna Rikberg and Taija Torvela for mapping fieldre la- cambrian evolution of the Baltic Shield. Precambrian tions in the Åland archipelago during their time at Åbo Research 35, 15–52. Akademi University (ÅA). Part of this study was car- Hoffman, P. E., 1989. Precambrian geology and tectonic ried out during a professorship to T. Skiöld at ÅA from historyofNorth America. In:Bally,A.W.and Palmer, the Nordic Academy for Advanced Study, Oslo. A.R. (eds.) The Geology of NorthAmerica–An Over- view. 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