Svecofennian magmatic and metamorphic evolution in

Precambrian Research 116 (2002) 111–127

Svecofennian magmatic and metamorphic evolution insouthwestern Finland as revealed by U-Pb zircon SIMS

geochronology

Markku Vaisanen a,*, Irmeli Manttari b, Pentti Holtta b

a Department of Geology, Uni�ersity of Turku, FIN-20014 Turku, Finlandb Geological Sur�ey of Finland, P.O. Box 96, FIN-02151 Espoo, Finland

Received 1 June 2001; accepted 8 February 2002

Abstract

Zircons from six samples collected from igneous and metamorphic rocks were dated using the NORDSIM ionmicroprobe, in order to investigate the tectonic evolution of the Palaeoproterozoic Svecofennian Orogen insouthwestern Finland. These rocks represent pre-collisional, collisional and post-collisional stages of the orogeny. Theion microprobe results reveal two age groups of granodioritic–tonalitic rocks. The intrusions have different tectonicsettings: the Orijarvi granodiorite represents pre-collisional 1.91–1.88 Ga island-arc-related magmatism and yieldedan age of 1898�9 Ma, whereas the collision-related Masku tonalite was dated at 1854�18 Ma. The latter ageaccords with more accurate previous conventional zircon age data and constrains the emplacement age of collisionalgranitoids to �1.87 Ga. This is interpreted to reflect the collision between the Southern Svecofennian Arc Complexwith the Central Svecofennian Arc complex and the formation of a suture zone between them during D2 deformation.Granulite facies metamorphism in the Turku area was dated at 1824�5 Ma using zircons from leucosome in theLemu metapelite. This age constrains D3 folding related to post-collisional crustal shortening in this area. Crustalmelting continued until �1.81 Ga, as indicated by the youngest leucosome zircons and metamorphic rims ofenderbite zircons. New metamorphic zircon growth took place in older granitoids at granulite facies, but not atamphibolite facies. Detrital zircons with ages between 2.91 and 1.97 Ga were found in the mesosome of the Lemumetapelite and 2.64–1.93 Ga inherited cores were found in the 1.87 Ga Masku tonalite. © 2002 Elsevier Science B.V.All rights reserved.

Keywords: Svecofennian Orogen; Palaeoproterozoic; Ion microprobe; Tectonics; Crustal evolution

www.elsevier.com/locate/precamres

1. Introduction

Since the early pioneering works of Kouvo(1958), Wetherill et al. (1962) and Kouvo andTilton (1966), the number of U-Pb datings onzircons has increased enormously in Finland (seee.g. Huhma, 1986; Patchett and Kouvo, 1986;

* Corresponding author. Fax: +358-2-3336-580.E-mail addresses: [email protected] (M. Vaisanen),

[email protected] (I. Manttari), [email protected] (P.Holtta).

0301-9268/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.

PII: S0 301 -9268 (02 )00019 -0

mailto:markku.vaisanen@utu.�

mailto:irmeli.manttari@gsf.�

mailto:pentti.holtta@gsf.�

M. Vaisanen et al. / Precambrian Research 116 (2002) 111–127112

Vaasjoki, 1996), paving the way for a clearerunderstanding of the evolution of the Precam-brian crust in the Fennoscandian shield. The ionmicroprobe studies have revealed that even a sin-gle zircon may have a multiple growth history,making the interpretation of conventional analy-ses difficult or even misleading in some cases.Although ion microprobe analyses are not quiteas accurate as conventional analyses, the spatialresolution of ion microprobe is the only methodcapable of dating multiply grown domains in asingle zircon (see Williams, 1998 for review). Therecent works of Mezger and Krogstad (1997) andLee et al. (1997) have shown that zircons cansurvive temperatures in excess of 900 °C withoutresetting the U-Pb system. This suggests that in-herited, primary and metamorphic zircons of dif-ferent ages can survive and be distinguished byion microprobe.

In this work, we have attempted to constrainthe temporal evolution of granitoid magmatismand high-grade metamorphism using the sec-ondary ion mass spectrometer (SIMS) for selectedsamples collected in southwestern part of the LateSvecofennian Granite–Migmatite zone (LSGM,Fig. 1), which is a high temperature– low pressureamphibolite to granulite facies migmatite zone(Ehlers et al., 1993).

The aims of the study are:(i) dating of the early (syn-)orogenic granitoids.

The main questions regarding the granitoidsin the LSGM are: (a) Are some of theplutonic rocks related to the island–arc vol-canism in southern Finland? (b) Is the syn-orogenic magmatism younger in the LSGMthan in the Tampere area north of theLSGM? (Fig. 1) (c) Are the previous con-ventional datings of synorogenic granitoidsin the LSGM a mixture of magmatic zirconsand younger metamorphic zircons and howmuch inherited zircons the granitoidscontain?

(ii) dating of the granulite facies meta-morphism.

(iii) dating of any possible metamorphism thatpredated the peak metamorphism.

(iv) dating of the deformation events. Accordingto Vaisanen and Holtta (1999), high grade

metamorphism in the LSGM took placesyntectonically with D3 deformation thatoverprinted the synorogenic granitoids em-placed during D2 deformation. By datinganatectic leucosomes that are syntectonicwith D3, we obtain the age of D3 as well.By dating synorogenic granitoids that aresyntectonic with D2, we also know the ageof this deformation.

For these purposes zircons from three granitoidsamples, two leucosome samples and one micagneiss sample were dated in this study.

2. Regional geology

The Svecofennian Orogen was formed by accre-tion of island arcs and ophiolites against theArchaean craton between 2.0 and 1.75 Ga (Gaaland Gorbatchev, 1987; Kontinen, 1987; Nironen,1997). Collisional tectonics and possible magmaticunderplating thickened the crust to its maximumthickness of 65 km in central Finland and �50km in the LSGM (Korja et al., 1993), where thepeak metamorphic conditions were reached laterthan in central Finland (Korsman et al., 1984). Inthe central Finland Granitoid Complex (CFGC;Fig. 1) regional metamorphism was coeval withthe main phase of synorogenic magmatism at1.89–1.88 Ga (Vaasjoki and Sakko, 1988; Hau-denschild, 1995; Holtta, 1995).

In the Tampere Schist Belt (TSB; Fig. 1) and inthe CFGC the volcanic rock are dated at 1.90–1.89 Ga (Kahkonen et al., 1989), synorogenicgranitoids are dated at 1.89–1.88 Ga (Nironen,1989) while the 1.88–1.87 Ga granitoids are evi-dently post-kinematic (Nironen et al., 2000). Agesof volcanic and synorogenic rocks are, therefore,partly overlapping. The age of metamorphism isaccurately determined in this area by conventionalU-Pb dating of leucosome and mesosome monaz-ites yielding a concordant age of 1878.5�1.5 Ma(Mouri et al., 1999). This is coeval with the syn-orogenic magmatism in that area (Nironen, 1989).

In the LSGM, the previous conventional dat-ings of the synorogenic granitoids span between1.90 and 1.87 Ga (Hopgood et al., 1983; Huhma,1986; Patchett and Kouvo, 1986) but most clusteraround 1.87 Ga (Patchett and Kouvo, 1986; Van

M. Vaisanen et al. / Precambrian Research 116 (2002) 111–127 113

Duin, 1992; Nironen, 1999). The magmatismdated at �1.87 Ga is syntectonic in relation todeformation (Vaisanen and Holtta, 1999). In theeastern part of the LSGM (�400 km east fromthe present study area), zircons from leucosomesand mesosomes yielded ages between 1850 and1800 Ma (Korsman et al., 1984). Considering thepossible inheritance and large uncertainties in pre-vious datings, the age of peak metamorphism hasremained unclear. Anatectic granites in the

LSGM are dated at �1840–1810 Ma (Huhma,1986; Suominen, 1991; Vaisanen et al., 2000).

On the basis of these age determinations, hightemperature metamorphism (�800 °C/4–6kbars) culminated in the LSGM during the lateorogenic or post-collisional stage of the Svecofen-nian Orogen between 1840 and 1810 Ma. It over-printed and, in places, effectively obscured theearlier features of the orogeny (Korsman et al.,1984; Schreurs and Westra, 1986; Vaisanen and

Fig. 1. Main lithological units of SW Finland. Some of the conventional U-Pb zircon ages of granitoids are numbered: (1)=Huhma(1986), (2)=Kilpelainen (1998), (3)=Lindberg and Bergman (1993), (4)=Nironen (1989), (5)=Nironen (1999), (6)=Patchett andKouvo (1986), (7)=Suominen (1991), (8)=Vaasjoki (1977), (9)=Van Duin (1992), (10)=Vaisanen et al. (2000). Sample locationsin this study are shown by black arrows. CFGC, Central Finland Granitoid Complex; CSAC, Central Svecofennian Arc Complex;HSB, Hame Schist Belt; KMB, Kemio–Mantsala Belt; LSGM, Late Svecofennian Granite–Migmatite zone; SSAC, SouthernSvecofennian Arc Complex; TSB, Tampere Schist Belt. Solid line marked by ‘S’ is approximate location of suture zone proposedby Lahtinen (1996). Map compiled after Hietanen (1947), Kilpelainen (1998) and Korsman et al. (1997).


Holtta, 1999; Vaisanen et al., 2000). Only rela-tively small areas, such as that of the Orijarviarea, remained unmagmatized. Because of thehigh heat flow, zircons in pre-granulite faciesrocks might have been affected by this late meta-morphism, as suggested by the very heterogeneouszircon age data on enderbites in the Turku gran-ulite area (Suominen, 1991; Van Duin, 1992; Fig.1).

In the LSGM, high grade metamorphism andlate orogenic S-type granites at 1.84–1.81 Ga areinterpreted to be related to post-collisional maficintraplating (Vaisanen et al., 2000). An exceptionto this is the western part of the volcanic Kemio–Mantsala Belt (west of Orijarvi in Fig. 1), wheremetamorphism did not reach melting conditionsand was probably related to earlier orogenic stage(e.g. Ploegsma and Westra, 1990). Post-collisional�1.80 Ga shoshonitic intrusions (Eklund et al.,1998) and �1.60 Ga rapakivi granites (Vaasjoki,1977) represent the latest magmatic events (Fig.1).

Deformation in the LSGM is attributed to atleast two main stages. The first stage was thecollision of the Southern Svecofennian Arc Com-plex with the Central Svecofennian Arc Complexduring D2 north-vergent thrusting and intrusionof synorogenic granitoids. This produced a suturezone between the two arc complexes. The secondstage involved continued crustal convergence pro-ducing the main regional, upright, E–W trendingD3 folds simultaneously with crustal anatexis andthe formation of migmatites and granites (Lahti-nen, 1994; Korsman et al., 1999; Vaisanen andHoltta, 1999; Fig. 1).

3. Sample descriptions

The six samples chosen for SIMS analyses aredescribed below. The sampling locations are dis-played in Fig. 1.

3.1. A933: Orijar�i granodiorite

The Orijarvi granodiorite is a compositebatholith ranging in composition from gabbro totonalite with minor hornblendites and granodior-

ites (sample A933 in this study, Fig. 1). It waschosen for dating for two reasons. Firstly, previ-ous conventional U-Pb dating of the granodioriteat 1891�13 Ma (Huhma, 1986) did not unam-biguously distinguish between volcanic versusorogenic origin of the batholith because of largeuncertainties. Secondly, regional metamorphismin this area is of andalusite–cordierite grade(Schreurs and Westra, 1986). This is the lowest insouthern Finland. On this basis, the metamorphicimprint on zircons should be absent allowingcomparison for the other samples from the highergrade Turku area. The rock is medium- to coarse-grained, weakly foliated and contains mafic en-claves. This sample is the same used by Huhma(1986). National grid coordinates are 6678800–2474700.

3.2. 102.1MV94: Masku tonalite

The hornblende– tonalite sample from Maskuwas collected from the same locality as the sampledated at 1869�5 Ma by Van Duin (1992). Thereason for choosing this sample was to check ifthere is any metamorphic overprint on the zir-cons. The outcrop is situated on the transitionalgranulite–amphibolite facies boundary. The rockcontains mafic enclaves and shows a weak D2fabric (Vaisanen and Holtta, 1999). National gridcoordinates are 6719150–1567550.

3.3. A498: Kakskerta enderbite

The sample is from medium- to coarse-grainedorthopyroxene bearing enderbite from Kakskerta,Turku. The rock type has been previously de-scribed as charnockite by Hietanen (1947), pyrox-ene granodiorite by Suominen (1991) andleuconorite by Van Duin (1992). Two conven-tional U-Pb zircon datings have been carried outon the same intrusion. Suominen (1991) con-cluded that his sample yielded an age interval of1842–1879 Ma with probably two zircon genera-tions and Van Duin (1992) obtained an age of1821�39 Ma. On the structural basis, however,the rock belongs to the early orogenic groupshowing the D2 fabric (Vaisanen and Holtta,1999) suggesting that substantial metamorphic


zircon growth has occurred. The sample used inthis study is the same used by Suominen (1991).National grid coordinates are 6694370–1565120.

3.4. 102.2MAV94: leucosome in the Maskutonalite

This sample is collected from the same exposureas the sample 102.1MAV94. The sample is a pinkgranitic leucosome in the hornblende– tonalitehost. It occurs as patches and heterogeneous veinscutting the D2 fabric. The reason for samplingwas to date the age of leucosome formation.

3.5. 16PSH93: leucosome of the Lemugarnet–cordierite gneiss

The sample is collected from garnet- andcordierite-bearing granitic leucosome from thelow P-high T garnet-cordierite gneiss of Lemu.The sample is taken from the hinge of a subhori-zontal, E–W trending F3 fold where the leuco-some intrudes its the axial plane. The amount ofleucosome is about 50% at this locality. The rea-son for sampling was to date the age of leucosomeformation. National grid coordinates are6716100–1552700.

3.6. 94MAV97: mesosome of the Lemugarnet-cordierite gneiss

The sample is from mesosome of the low P–high T migmatite, Lemu. The mesosome is bi-otite-rich and contains garnet and cordieriteporphyroblasts. The texture is distinctly foliatedand segregated with millimeter-scale alternatingdark and light bands parallel to foliation. Leuco-some, represented by the light bands could not beavoided. The purpose of this sample was to datedetrital and metamorphic zircons. National gridcoordinates are 6714750–1551850.

4. Analytical methods

The ion microprobe analyses were run at theNORDSIM laboratory using the Cameca IMS1270 secondary ion mass spectrometer (SIMS)

located at the Swedish Museum of Natural His-tory, Stockholm. The spot diameter for the 4nAprimary O2− ion beam was �30 �m and oxygenflooding in the sample chamber was used to in-crease the production of Pb+ ions. Four countingblocks comprising a total of twelve cycles of thePb, Th and U species were measured at each spot.The mass resolution was (M/�M) of 5400 (10%).The raw data were calibrated against a zirconstandard (91,500; Wiedenbeck et al., 1995) andcorrected for background (204.2) and moderncommon lead (T=0; Stacey and Kramers, 1975).For further details of the analytical procedures seeWhitehouse et al. (1997, 1999). The plotting of theU-Pb data, the fitting of the discordia lines andthe calculation of the concordia intercept ageswere carried out using the Isoplot/Ex program(Ludvig, 1998).

5. Results

Ion microprobe isotope analyses for respectivesamples are presented in Table 1 and Fig. 2illustrates the back-scattered electrons (BSE) im-ages of selected zircon crystals. Analytical uncer-tainties in the text and in the concordia diagramsare presented with 2� errors. Summary of theresults is presented in Table 2.

5.1. A933: Orijar�i granodiorite

Zircons from the Orijarvi granodiorite are typi-cal magmatic zircons with euhedral elongatedprismatic shapes (100–250 �m, length/width ra-tios between 5:1 and 2:1) and clear magmaticzoning (Fig. 2a). Generally, the zircons arecolourless and transparent, but a few have turbidcore domains. In BSE images presumably oldercores are visible in a few zircons (Fig. 2b).

A total of 13 spots were analysed using the ionmicroprobe. The U and Pb concentrations aremoderate and vary in a reasonable range, exceptfor very high U, Th and Pb concentrations in theinner domain of zircon 16. Most of the age dataplot on a discordia line with an upper interceptage of 1889�11 Ma (MSWD=2.9; n=12). Out-side this line plots analysis number n207–17b.


Tab

le1

Ion

mic

ropr

obe

U-P

bda

taon

zirc

ons

Der

ived

ages

aC

orre

cted

rati

osa

Ele

men

tal

data

Ana

lyze

dSa

mpl

e/sp

otN

o.zi

rcon

dom

aind

207 P

b/�

s20

6 Pb/

�s

�s

207 P

b/20

7 Pb/

�s

207 P

b/20

6 Pb/

�s

Th/

U20

6 Pb/

(Pb)

�s

(Th)

Rho

bD

isc.

c(U

)(%

)pp

m(%

)pp

m23

8 Um

eas.

ppm

(%)

(%)

204 P

b23

8 U23

5 U23

5 U20

6 Pb

206 P

bm

eas.

A93

3,O

rija

r�i

gran

odio

rite

1897

1818

8733

0.11

680.

45.

475

2.1

0.34

002.

00.

9832

787

132

0.27

2.55

E19

07n6

71-0

1a8

Zon

ed+

0420

4236

0.11

560.

65.

939

2.1

0.37

272.

00.

97−

4.3

1967

263

Zon

ed63

116

0.24

4.07

E18

8919

n671

-07a

10+

04Z

oned

1819

0534

0.11

720.

65.

559

2.1

0.34

392.

00.

9621

483

900.

397.

67E

1915

10n6

71-1

2a19

10+

03n6

71-1

7b18

Hom

og-

1915

340.

1157

0.6

5.51

62.

10.

3458

2.0

0.96

301

7012

40.

234.

06E

1890

1119

03+

04co

re18

1662

300.

1136

0.6

4.60

62.

10.

2941

2.1

0.96

1857

7.4

1117

144

600.

266.

64E

1750

n671

-22a

Zon

ed-

inne

r+

03Z

oned

-19

0718

1935

340.

1148

0.5

5.54

22.

10.

3501

2.0

0.98

246

5610

20.

233.

56E

1877

n671

-22b

8+

04ou

ter

1987

350.

1146

0.5

5.70

52.

10.

3611

2.0

0.97

n671

-25a

−2.

1Z

oned

235

5210

00.

225.

07E

1874

919

3218

+04

1819

5735

0.11

540.

35.

642

2.1

0.35

472.

10.

996

1885

537

204

232

0.38

5.58

EZ

oned

-19

23n6

71-2

6a+

04in

ner

Zon

ed-

1945

1819

8435

0.11

650.

45.

792

2.1

0.36

042.

00.

98−

0.2

237

5410

10.

232.

98E

1904

n671

-26b

8+

04ou

ter

1116

50n2

07-1

6a19

Zon

ed-

0.11

410.

24.

590

1.3

0.29

171.

30.

9811

3284

2742

1295

0.83

7.84

E18

664

1748

inne

r+

0428

1436

290.

1139

2.7

3.91

93.

50.

2496

2.2

0.64

1863

22Z

oned

-34

864

103

0.18

1.50

En2

07-1

6b48

1618

oute

r+

04Z

oned

-19

0714

1907

250.

1167

0.5

5.54

21.

60.

3443

1.5

0.95

430

253

192

0.59

5.03

E19

07n2

07-1

7a9

+04

rim

1414

1321

0.10

441.

43.

528

1.8

0.24

511.

70.

9416

n207

-17b

551

Zon

ed-

112

164

0.20

6.50

E17

0426

1533

+02

rim

102.

1MA

V94

,to

nalit

e22

1882

340.

1179

1.5

5.51

32.

50.

3391

n673

-02a

2.1

Hom

og-

0.81

147

5863

0.39

4.78

E19

2526

1903

+02

core

1118

8918

1903

340.

1146

0.6

5.42

52.

10.

3435

2.0

0.96

278

123

118

0.44

3.96

En6

73-1

7aZ

oned

-18

73+

04in

ner

1920

7438

0.12

070.

36.

317

2.1

0.37

96n6

73-2

0a2.

1Z

oned

-0.

99−

1.8

914

1739

10.

021.

87E

1966

520

21+

04co

re28

1716

510.

1137

0.5

4.78

03.

40.

3049

3.4

0.99

1859

38

312

123

118

0.39

1.12

E17

81n2

01-0

7aZ

oned

-ou

ter

+04

Zon

ed-

1807

617

7710

0.11

260.

34.

927

0.7

0.31

740.

60.

963

1167

178

431

0.15

7.01

E18

41n2

01-0

7b5

+03

inne

r25

8267

0.17

810.

412

.099

3.2

0.49

273.

10.

99n2

01-1

3aC

ore

120

8682

0.72

3.16

E26

357

2612

30+

0416

9529

0.11

370.

24.

714

2.0

0.30

081.

90.

997

1770

820

489

283

0.11

2.52

E18

5916

n201

-13b

Rim

+04

2051

Zon

ed-

5121

3910

50.

1205

0.2

6.53

55.

80.

3934

5.8

1.00

501

401

334

0.80

1.04

E19

634

n201

-20a

core

+05


Tab

le1

(con

tinu

ed)

Der

ived

ages

aC

orre

cted

rati

osa

Ele

men

tal

data

Ana

lyze

dSa

mpl

e/sp

otN

o.zi

rcon

dom

aind

(Th)

�s

206 P

b/�

s�

s20

7 Pb/

(Pb)

�s

207 P

b/20

7 Pb/

Th/

U�

s20

6 Pb/

�s

206 P

b/R

hob

Dis

c.c

(U)

207 P

b/23

8 U20

6 Pb

(%)

235 U

(%)

238 U

(%)

235 U

(%)

ppm

206 P

b20

4 Pb

ppm

mea

s.pp

mm

eas.

595

76

0.10

510.

62.

319

0.8

0.16

000.

60.

8247

1736

665

339

0.38

n201

-20b

2.48

EZ

oned

-17

1610

1218

oute

r+

03

102.

2MA

V94

,le

ucos

ome

inM

asku

tona

lite

1783

210.

1157

0.4

5.08

31.

40.

3186

1.3

n199

-23a

0.96

Cor

e4

324

108

125

0.33

1.58

E18

917

1833

12+

057

1807

140.

1106

0.1

4.93

70.

90.

3236

0.9

0.99

n199

-23b

Zon

ed-

2456

135

895

0.05

2.25

E18

102

1809

+05

rim

917

5316

0.10

960.

24.

721

1.1

0.31

251.

00.

9817

931

410

2634

359

0.03

1.19

E17

71n1

99-2

4aH

omog

-co

re+

05H

omog

-18

4813

1824

240.

1147

0.4

5.17

41.

60.

3270

1.5

0.97

025

710

110

30.

392.

19E

1876

n199

-28b

7+

05co

re17

2820

0.10

960.

24.

645

1.3

0.30

741.

30.

99n1

99-2

8c2

Rim

2475

112

855

0.05

1.17

E17

933

1757

11+

0511

1769

210.

1102

0.2

4.80

01.

30.

3158

1.3

0.99

418

0312

2191

436

0.07

2.56

EH

omog

-17

85n1

99-2

9a+

05co

reH

omog

-17

6010

1730

180.

1097

0.2

4.65

71.

20.

3078

1.2

0.99

221

0410

572

80.

051.

88E

1795

n199

-29b

3+

05ri

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Fig. 2. BSE images of selected zircons. Sites of the spot analyses and corresponding 207Pb/206Pb ages (2�) are marked on the figures.(a, b) A933 Orijarvi granodiorite; (c, d) 102.1MAV94 Masku tonalite; (e, f) A498 Kakskerta enderbite; (g) 102.2MAV94 Maskutonalite leucosome; (h) 16PSH93 Lemu garnet–cordierite gneiss leucosome.


Tab

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Fig. 3. Concordia plot for sample A933 Orijarvi granodiorite.

1865�47 Ma (MSWD=7.3). We also calculatedthe weighted average of the 207Pb/206Pb ages fromthe four most concordant data points. The result-ing age is 1854�18 Ma. (95% C.I., MSWD=4.1). These ages, the same within errors, areconsidered to reflect the time of emplacement ofthe Masku tonalite. The other four analyses fromthe core domains give clearly older Palaeoprotero-zoic and Archaean 207Pb/206Pb ages (Table 1 andFig. 4).

5.3. A498: Kakskerta enderbite

Zircons from the Kakskerta enderbite areslightly elongate to stubby crystals (100–200 �mlong, length/width ratios between 2:1 and 1:1)with partly smooth surfaces. The zircons havebrown inner domains surrounded by colourlesstransparent rims. In BSE images (Fig. 2e,f), theinner domains and often fractured outer rims areseparated by fractures which are filled with anunknown material.

A total of 15 zircon spots were dated using theion microprobe. On the concordia diagram theU-Pb data scatter between 1.9 and 1.8 Ga. Fouranalyses from the magmatically zoned core do-mains give an upper intercept age of 1874�56Ma and four analyses from the homogenous coredomains give an upper intercept age of 1804�31

The fact that this particular analysis is stronglydiscordant and has 206Pb/204Pb ratio clearly lowerthan the second analytical attempt on the samezircon domain indicates that the first, discordantanalyse was probably on a fissure. Because theage data forms a cluster on the concordia curve,the intercept age can also be calculated as aconcordia age of 1898�9 Ma (MSWD=4.5; n=7) defined by seven concordant data points (Fig.3).

5.2. 102.1MAV94: Masku tonalite

Zircons from the Masku tonalite are almostcolorless and transparent. They are euhedral pris-matic crystals with sharp surface angles andbipyramidal edges (200–350 �m long, length/width ratios between 5:1 and 2:1). Their internalstructure seen in BSE-images is zoned and occa-sionally older cores and small nucleus are found(Fig. 2c,d).

Nine zircon spots were dated using the ionmicroprobe. Except for one rather discordantdata point, the others are concordant or showonly slight discordance. The U-Pb age data can bedivided into two groups according to analyzeddomains of the zircons. Ages from zoned zirconsand border domains of zircons with cores plotroughly on a line with an upper intercept age of1866�21 Ma (MSWD=7.4). When excludingthe discordant point the upper intercept age is

Fig. 4. Concordia plot for sample 102.1MAV94 Maskutonalite. Solid lines prismatic zoned zircons and rims, dashedlines inherited cores.


Fig. 5. Concordia plot for sample A498 Kakskerta enderbite.

Nine zircon spots were dated using the ionmicroprobe. The ion microprobe ages from threecore domains indicate roughly similar ages as theage of the Masku tonalite. All the four dated rimgrowths as well as two structurally homogeneouscore domains give meaningfully younger ages, theupper intercept age being 1804�14 Ma(MSWD=0.94; Fig. 6). This is interpreted as theage of the leucosome crystallization. It is notewor-thy that the zircon phase crystallized during theleucosome formation is rich in uranium and theTh/U-ratios are clearly lower compared to theother tonalite zircon domains.

5.5. 16PSH93: leucosome of the Lemugarnet-cordierite gneiss

Zircons from the Lemu leucosome are darkbrown, translucent to transparent, mainly equidi-mensional and multifaceted. A few elongategrains also exist. Zircons are 200–400 �m longwith a length/width ratios between 3.1 and 2:1.Some zircons have a turbid core domain and tinyinclusions are common. In BSE images, core do-mains and fractures with light alteration materialare common, especially in the elongate zircons.The equidimensional zircons typically have a ho-mogeneous internal structure (Fig. 2h).

Seven of the eight dated zircons give similarages and one zircon gives an Archaean age. If the

Ma (MSWD=0.94). The three dated structure-less rim domains give a concordia age of 1876�5Ma and the two zoned rims have an approximateage of 1.82 Ga. Although this age data is difficultto interpret, it is suggested that the ages from thezoned core domains reflect the time of enderbitecrystallization. In addition, the three homoge-neous rim domains either recrystallized or grewduring the enderbite crystallization. Then, the re-sulting age for the Kakskerta enderbite is 1878�19 Ma (MSWD=1.9). The ages from theinternally homogeneous core domains must thenreflect a later recrystallization of some older coresand the ages from zoned rim domains new zircongrowth on some older grains. The age for thesubsequent metamorphism can then be approxi-mated at 1804�31 Ma (Fig. 5).

5.4. 102.2MAV94: leucosome of the Maskutonalite

Zircons from the leucosome of the Maskutonalite are prismatic with sharp surface anglesand bipyramidal edges. They are 100–200 �mlong with length/width ratios between 5:1 and 2:1.Under a stereomicroscope, the colorless inner do-mains are enveloped by brown, translucent zirconmaterial. In BSE images, the zircons are highlyfractured and show structurally more homoge-neous outer rim domains (Fig. 2g).

Fig. 6. Concordia plot for sample 102.2MAV94 Maskutonalite leucosome. Solid lines rims, dashed lines inheritedcores.


Fig. 7. Concordia plot for sample 16PSH94 Lemu garnet–cordierite gneiss leucosome.

207Pb/206Pb age of 2.38 Ga. Zircon number 10gives the same age (�1.98 Ga) from its core andthe zoned major part of the grain. Elongate zirconnumber 06 gives a concordant age of 1.83 Ga thatis identical to ages from the leucosome zircons.Therefore, it is considered that this zircon comesfrom the leucosome material.

6. Discussion

6.1. Age results

The tectonic setting of the Orijarvi granodioritehas been controversial. Latvalahti (1979) arguedthat the intrusive rocks in the middle of thevolcanic belt are synorogenic and rose diapiricallypushing earlier volcanic rocks aside. Colley andWestra (1987), based on field relationships andthe similarity of geochemistry between intrusiveand extrusive rocks, concluded that the intrusiverocks are pre-kinematic and cogenetic with vol-canic rocks, i.e. they were magma chambers feed-ing volcanoes. Previous conventional U-Pb zircondating of the granodiorite yielding the age of1891�13 Ma (Huhma, 1986), failed to distin-guish between volcanic versus orogenic origin forthe batholith. Published ages of both volcanic andorogenic igneous rocks are alike within errors(Patchett and Kouvo, 1986). The preferred intru-sion age of 1898�9 Ma now obtained in our newSIMS data has only marginally smaller errorsthan the previous conventional age. However,most of the zircons clearly show a magmaticgrowth zoning without any evidence of latermetamorphic overgrowths, except for one withirregular core domain with slightly older apparentage (Fig. 2b). This older core might be an inher-ited relict from volcanic rocks below erosion sur-face, or it might represent earlier crystallisedzircon within a long-lived magma chamber.Within errors, however, core and rim ages are thesame. The obtained age still does not reveal theorigin of the batholith because of the small differ-ences between the volcanic and orogenic ages.Therefore, we made two conventional U-Pb zir-con analyses from felsic volcanic rocks, one sam-ple from the lowest stratigraphic level

youngest zircon from the mesosome material isadded to the age calculations (see below), theweighted average of 207Pb/206Pb ages for the eightzircon spots is 1826�5 Ma (MSWD=2.1; n=8). Leaving the mesosome zircon out, no interceptage can be calculated for these data, but theconcordia age for the five concordant and oneslightly discordant data points is 1824�5Ma(MSWD=0.62; n=6; Fig. 7). This age deter-mines well the age for migmatization.

5.6. 94MAV97: mesosome of the Lemugarnet–cordierite gneiss

The number of zircons obtained from the gneissafter separation was very small, only ten zircons.In addition, the zircon population is quite hetero-geneous. The zircons are either equidimensionalor elongate and they have clearly rounded facets.Their diameter range from 70 to 150 �m. Onelong grain with a length to width ratio of �3 alsoexists. In BSE images, almost all of the zirconsshow oscillatory zoning and some show clear coredomains.

Because of the shortage of zircons and theheterogeneity of the zircon material only fivespots were analyzed from this sample. Zirconnumber 01 has a zoned core domain with a 207Pb/206Pb age of 2.91 Ga surrounded by a narrowzoned new zircon phase with a probably mixed


(1895.3�2.4 Ma) and another from the highestone (1878.2�3.4 Ma). The results (Vaisanen andManttari, submitted) show that the sample fromthe Orijarvi batholith is coeval with the volcanicrocks, and the volcanite on the highest strati-graphic level is younger than the batholith, evenwith errors included. Therefore, as suggested byColley and Westra (1987), the Orijarvi batholith iscogenetic with the volcanic rocks which itintruded.

The previous conventional U-Pb age of thesynorogenic Masku hornblende tonalite at1869�5 Ma yields rather small uncertainties(Van Duin, 1992). However, BSE images andSIMS analyses indicate the existence of olderPalaeoproterozoic and Archaean cores but nometamorphic overgrowth on the oscillatorilyzoned zircons. Evidently, metamorphic tempera-tures at the amphibolite–granulite boundary (�750 °C) were not enough to start a newmetamorphic zircon growth. The new age(1854�18 Ma) now obtained by SIMS has quitelarge errors, but combined with the conventionalage of 1869�5 Ma, 1.87 Ga or less can beconsidered as a true intrusion age. In addition,Nironen (1999) dated two synorogenic granitoidsin adjacent area and obtained similar results (seeFig. 1). The inherited older Palaeoproterozoic andArchaean cores clearly indicate incorporation ofolder crustal material in the samples. Patchett andKouvo (1986) also pointed out, that �Nd values introndhjemites in southern Finland suggest a mi-nor older crustal precursor to some of the igneousrocks. However, there is too little data to arguewhether older material was incorporated by as-similation (cf. Nironen, 1997) or sediment-derivedzircons (cf. Claesson et al., 1993).

The Kakskerta enderbite is situated within theTurku low pressure/high temperature granulitesthat reached temperatures of 800 °C or more(Vaisanen and Holtta, 1999). The spatial connec-tion between granulites and enderbites combinedwith conventional U-Pb zircon ages led Van Duin(1992) to propose that the intrusions were respon-sible for the granulite facies metamorphism.Vaisanen and Holtta (1999), however, challengedthis idea and argued, on a structural basis, thatenderbites were already deformed by D2 while the

peak metamorphism took place some 30 Ma laterduring D3. Suominen (1991) analyzed the samesample used in this study and obtained two ageson different types of zircons, 1880 and 1840 Ma.The zircons analyzed in this study showed verycomplex age patterns. The core ages (1874�54Ma) from the zoned zircons, as well as the con-cordant rim ages of 1876�5 Ma together areinferred to represent the intrusion age of 1878�19 Ma, while the �1.82 Ga ages from the zonedrims and recrystallized cores are interpreted asmetamorphic overgrowths. The youngest �1750Ma core age must represent metamict lead loss atthat time. In summary, the field observations(Vaisanen and Holtta, 1999) and SIMS data indi-cate that the Kakskerta enderbite belongs to thesynorogenic intrusive suite that has been meta-morphosed during granulite facies metamorphismresulting in new zircon growth. Previous, hetero-geneous conventional U-Pb zircon datings were aresult of mixture of at least two generations ofzircons.

The patchy leucosome of the Masku tonaliteyielded an age of 1804�14 Ma. One of theanalysis was concordant and yielded an age of1810�4 suggesting that the age of migmatizationis closer to the older than the younger end of theerror limits. Within error limits, these analysesagree with the conventional U-Pb zircon age of1814.3�2.7 Ma obtained from a garnet-bearing,S-type, anatectic granite in the same area(Vaisanen et al., 2000). The other leucosome sam-ple was from a garnet- and cordierite-bearingleucosome intruding along the axial plane of re-gional F3 folding. The age of 1824�5 Ma fromthis sample accurately determines the age of re-gional anatexis. Since the leucosome fills F3 foldhinges and intrudes their axial planes, it alsoconstrains the age of F3 folding (Vaisanen andHoltta, 1999). The garnet–cordierite gneiss meso-some sample contained strongly abraded detritalzircons and four analyses yielded 207Pb/206Pb agesof 2.91, 2.38, 1.98 and 1.98 Ga. These ages are inaccordance with ages obtained by the SHRIMPmethod from detrital zircons from Orijarvi, 100km to the east (Claesson et al., 1993). One euhe-dral elongate zircon yielded an 207Pb/206Pb age of1829�6, i.e. the same age as in the leucosome


sample. It was probably derived from the tinyleucosome stripes present in the sample. The ab-sence of island-arc-related detrital zircons (1.90–1.88 Ga) indicate that the sedimentary materialcame outside the local volcanic arcs, which mayhave been subaqueous and incapable of producingerosional detritus. In summary, the leucosomesamples provide the age of anatexis and the peakmetamorphism. Anatexis began at �1.83 Ga andcontinued at least until �1.81 Ga, possibly to 1.80Ga. These ages are in accordance with the meta-morphic zircon ages from the Kakskerta enderbite.It is noteworthy that no metamorphism predatingthe granulite facies was detected in the samples.Using the timing of metamorphic mineral growthin relation to structures, Vaisanen and Holtta(1999) concluded that crystallization of garnet andcordierite and the onset of melting began beforeD3 deformation. However, this could not be ver-ified in this study. Perhaps pre-existing zirconswere totally dissolved to produce new zirconsdetected in this study, or alternatively, the temper-ature of the previous metamorphism was not highenough to produce zircons. 100 km southeast ofthe present study area, Hopgood et al. (1983)report U-Pb ages of 1.88–1.87 Ga derived frommonazites in garnet-bearing gneiss and apliticveins. They interpret these as metamorphic ages.Together with 1.88–1.86 Ga ages of synorogenicintrusions, this suggests that metamorphism of thatage also took place in southernmost Finland.

6.2. Regional implications

The geological history of the Southern Svecofen-nian Arc Complex (SSAC) differs in many waysfrom the geology of the Central SvecofennianGranitoid Complex (CSAC). Northeast of theCFGC (Fig. 1) peak metamorphism was coevalwith intrusion of 1885 Ma enderbites (Holtta,1995). Peak metamorphism at the southern marginof the CFGC in the Tampere Schist Belt is nowdetermined at 1878.5�1.5 Ma (Mouri et al.,1999), coeval with 1.88 Ga collision-related mag-matism. Soon after the collision, crust was stabi-lized at 1.88–1.87 Ga when post-kinematic,bimodal, magmatism took place (Nironen et al.,2000).

In the SSAC the earliest intrusive magmatism(1.91–1.88 Ga) is coeval with volcanism. Thisbelongs to the original idea of Hietanen (1975) ofan island–arc origin for Svecofennian formations.It is often difficult to know whether intrusions arearc- or orogen-related. Unequivocal crosscuttingfield relationships or accurate age determinationsare needed. Early orogenic magmatism, dated at1.88–1.86 Ga, is �10 Ma younger in the SSACthan in the CSAC and is coeval with post-kine-matic magmatism in that area. Therefore, thesetwo areas do not share a common tectonic evolu-tion, and the idea of a suture zone between theseareas (Lahtinen, 1996) is supported. Collision ofthe SSAC with the CSAC probably took place at1.88–1.86, i.e. �10–20 Ma later than the collisionof the CSAC arc with the Archaean craton.

The high heat flow that produced granulitefacies metamorphism is restricted to the LSGM.Peak metamorphism that triggered crustal anatexisstarted at 1.84–1.83 Ga as indicated by zircongrowth in enderbites and leucosomes within thegranulite area. Melt formation continued 20–30Ma to 1.81 Ga, probably because continued mafic,mantle derived magmatism provided additionalheat during that time period. Vaisanen et al. (2000)suggested that convective removal of the subconti-nental lithospheric mantle during upwelling as-thenosphere caused melting of the enriched partsof the mantle under this part of the FennoscandianShield during post-collisional stage. These wereintruded into the middle crust causing the post-col-lisional high grade metamorphism.

7. Conclusions

From the ion microprobe (SIMS) dating ofzircons, we draw the following conclusions:1. The Orijarvi granodiorite belongs to the 1.91–

1.88 Ga arc magmatism.2. Synorogenic magmatism is dated at 1.88–1.86

Ga, i.e. 10 Ma younger than related magma-tism in the Central Finland Granitoid Com-plex and Tampere Schist Belt.

3. Collision-related D2 deformation is �1.87 Gain age.


4. Peak metamorphism took place at 1824�5Ma in the Turku area. This also dates the ageof the D3 folding. Crustal melting continueduntil at least 1.81 Ga.

5. Zircons in the synorogenic �1.88–1.86 GaKakskerta enderbite obtained a new metamor-phic overgrowth in the Turku granulite area,but the coeval zircons in tonalite in the amphi-bolite–granulite boundary area were notaffected.

6. Some zircons in the synorogenic Masku intru-sion contain older Palaeoproterozoic and Ar-chaean cores.

7. Sedimentary detritus in metapelites came out-side the local volcanic arcs.

Acknowledgements

Many thanks are owed to Hannu Huhma forproviding samples A498 and A933 for our use.The helpful assistance of the staff in the Nordsimlaboratory, Kerstin Linden, Torbjorn Sunde, Jes-sica Vestin and Martin Whitehouse are greatlyacknowledged. Paul Evins and DmitryKonopelko reviewed the early version of themanuscript and both are acknowledged. J.D.Kramers and Torbjorn Skiold reviewed and sug-gested many valuable improvements to themanuscript. This study was funded by the Litho-sphere Graduate School and the Academy ofFinland (project No. 40553). This is NORDSIMcontribution number 60.

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