svecofennian magmatic and metamorphic evolution in
TRANSCRIPT
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
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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
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).
M. Vaisanen et al. / Precambrian Research 116 (2002) 111–127114
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
M. Vaisanen et al. / Precambrian Research 116 (2002) 111–127 115
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.
M. Vaisanen et al. / Precambrian Research 116 (2002) 111–127116
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
M. Vaisanen et al. / Precambrian Research 116 (2002) 111–127 117
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
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M. Vaisanen et al. / Precambrian Research 116 (2002) 111–127118T
able
1(c
onti
nued
)
Der
ived
ages
aA
naly
zed
Cor
rect
edra
tios
aE
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lda
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mpl
e/sp
otN
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b/(P
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(Th)
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b/(U
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Rho
b�
s�
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206 P
b/20
7 Pb/
Th/
U20
6 Pb
ppm
238 U
ppm
235 U
206 P
bm
eas.
238 U
(%)
204 P
b(%
)23
5 U(%
)(%
)pp
mm
eas.
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rror
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eat
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ror
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elat
ion
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rsus
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8 Ura
tios
.c
Deg
ree
ofdi
scor
danc
eis
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ulat
edat
the
clos
est
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limit
.d
Cor
e/ri
m,
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rco
redo
mai
nw
ith
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ryo
unge
rzi
rcon
phas
e.In
ner/
oute
r,m
ost
prob
ably
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ezi
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phas
ew
ith
anal
yses
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nter
orbo
rder
dom
ains
.H
omog
,gu
ite
hom
ogen
eous
inte
rnal
stru
ctur
e.
M. Vaisanen et al. / Precambrian Research 116 (2002) 111–127 119
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.
M. Vaisanen et al. / Precambrian Research 116 (2002) 111–127120
Tab
le2
Sum
mar
yof
prev
ious
conv
enti
onal
and
new
SIM
Sda
ta
Ion
mic
ropr
obe
U-P
bag
es;
this
stud
yP
revi
ous
conv
enti
onal
Roc
kty
peSa
mpl
eID
ID-T
IMS
age
and
Ref
s.
Pro
cess
App
aren
tag
eA
naly
zed
zirc
onn
type
s/do
mai
ns
Igne
ous
age
Upp
erin
terc
ept
age
12/1
3P
rism
atic
/zon
edG
rano
dior
ite
1891
�13
Ma
A93
3O
rija
rvi
1889
�11
Ma
(Huh
ma,
1986
)Ig
neou
sag
eC
onco
rdia
age
Pri
smat
ic/z
oned
7/13
1898
�9
Ma
Ton
alit
eP
rism
atic
/zon
edan
dri
m10
2.1M
AV
944/
9Ig
neou
sag
eU
pper
inte
rcep
tag
e18
69�
5M
a18
66�
21M
aM
asku
(Van
Dui
n,19
92)
dom
ains
Pri
smat
ic/z
oned
and
rim
Wei
ghte
dav
erag
eag
e4/
9Ig
neou
sag
edo
mai
ns18
54�
18M
a5/
9In
heri
ted
mat
eria
lA
rcha
ean
and
olde
rC
ores
Pal
aeop
rote
rozo
ic
Igne
ous
age
Upp
erin
terc
ept
age
Zon
edco
res
418
79–1
842
Ma
End
erbi
teA
498
Kak
sker
ta18
74�
56M
a(S
uom
inen
,19
91)
Rec
ryst
alliz
atio
nU
pper
inte
rcep
tag
e18
21�
39M
a4
Hom
ogen
eous
core
s18
04�
31M
a(V
anD
uin,
1992
)2
New
zirc
ongr
owth
Con
cord
iaag
eZ
oned
rim
s18
19�
7M
aC
onco
rdia
age
Rec
ryst
alliz
atio
n/ne
w3
Hom
ogen
eous
rim
szi
rcon
grow
th18
76�
5M
a2
07P
b/206P
bag
esH
omog
eneo
usco
re+
rim
2�
1.76
Ga
7/15
Igne
ous
age?
Upp
erin
terc
ept
age
Stub
by/h
omog
eneo
usri
ms
1878
�19
Ma
and
zone
dco
res
6/15
Met
amor
phis
m?
Upp
erin
terc
ept
age
Stub
by/h
omog
eneo
usco
res
and
zone
dri
ms
1804
�31
Ma
6/9
Leu
coso
me
Ton
alit
e,le
ucos
ome
Upp
erin
terc
ept
Pri
smat
ic/b
orde
rdo
mai
ns10
2.2M
AV
94ag
e180
4�
14M
aM
asku
(4)+
hom
og.c
ores
(2)
crys
talli
zati
on3/
9O
lder
core
sR
ough
lyeq
uiva
lent
Pri
smat
ic/c
ore
dom
ains
wit
hig
neou
sag
e
16P
SH93
Lem
u8/
7A
gefo
rm
igm
atiz
atio
nE
quid
imen
sion
al,
Wei
ghte
dav
erag
eag
e:G
arne
t–co
rdie
rite
gnei
ss,
leuc
osom
eho
mog
eneo
usan
don
ezo
ned
1826
�5
Ma
elon
gate
d(s
eete
xt)
6/7
Con
cord
iaag
e18
24�
5M
aH
eter
ogen
eous
zirc
onG
arne
t–co
rdie
rite
4/5
Sedi
men
tary
zirc
ons
Arc
haea
nan
dol
der
94M
AV
97L
emu
gnei
ss,
mes
osom
em
ater
ial
Pal
aeop
rote
rozo
ic
M. Vaisanen et al. / Precambrian Research 116 (2002) 111–127 121
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.
M. Vaisanen et al. / Precambrian Research 116 (2002) 111–127122
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.
M. Vaisanen et al. / Precambrian Research 116 (2002) 111–127 123
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
M. Vaisanen et al. / Precambrian Research 116 (2002) 111–127124
(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
M. Vaisanen et al. / Precambrian Research 116 (2002) 111–127 125
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.
M. Vaisanen et al. / Precambrian Research 116 (2002) 111–127126
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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