U-Pb zircon ages and Rb-Sr whole-rock isotope studies of early Proterozoic volcanic and plutonic rocks near Tampere, southern Finland

Precambrian Research, 45 (1989) 27-43

27

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

U-Pb Zircon Ages and Rb-Sr Whole-rock Isotope Studies of Early Proterozoic Volcanic and Plutonic Rocks near Tampere, Southern Finland YRJO KhHKONEN Department of Geology, University of Helsinki, P.O. Box 115, SF-0017I Helsinki (Finland)

H A N N U HUHMA Geological Survey of Finland, Betonimiehenkuja 4, SF-02150 Espoo (Finland)

KRISTIINA ARO Ministry of the Environment of Finland, P.O. Box 399, SF-00121 Helsinki (Finland) (Received February 18, 1988; revision accepted January 4, 1989)

Abstract Kiihktinen, Y., Huhma, H. and Aro, K., 1989. U - P b zircon ages and Rb-Sr whole-rock isotope studies of early Proterozoic volcanic and plutonic rocks near Tampere, southern Finland. Precambrian Res., 45: 27-43. The Tampere Schist Belt, southern Finland, is a well-preserved volcanic-sedimentarybelt in the early Proterozoic Svecofennian terrain. The metasedimentary rocks of the belt are dominated by turbid±tic greywackes, siltstones and mudstones. The metavolcanics range from basalt to rhyolite in composition, intermediate rocks being the most common. Near Tampere, the belt comprises an east-west trending major syncline. The U - P b zircon ages of the ~ 1.9 Ga old metavolcanics cover an age interval of ~ 15 Ma. The oldest age, 1904 ± 4 Ma, is from the "Intermediate Unit" at 0rives±, which is dominated by calc-alkaline, high-K dacites and andesites. This unit is the lowest in the northern limb of the syncline at 0rives±. At Yliij~irvi,a sample from the Lower Volcanic Unit has an age of 1898 ___4 Ma, while a sample from the Upper Volcanic Unit has the youngest crystallisation age of 1889 _+5 Ma. These ages agree with the structure and stratigraphy of the belt. The implications of the 1891 ± 16 and 1889 + 19 Ma ages of two feldspar porphyries remain obscure; here the error limits are wide and it is not known with certainty whether the rocks are dykes, sills or volcanics. An age of 1901 ± 28 Ma was obtained for the V~ra~ilii granitoid which intrudes the schists. Although the error is large, it is possible that some of the zircon has been inherited from older rocks. The Rb-Sr whole-rock ages of the volcanics agree, within error limits, with the U-Pb zircon ages. The Rb-Sr age of the H~imeenkyrti batholith is, however, about 100 Ma lower than the equivalent U-Pb age. The Isr values of the metavolcanics and the 1.882 Ga old Hfimeenkyr5 batholith range between 0.7020 and 0.7033. These low figures suggest that their protoliths separated from upper mantle sources within a short time interval. The Sm-Nd and Lu-Hf isotopic data available in the literature support the conclusion that the period around 1.9 Ga represents an important episode of continental growth, which is particularly evident in the Svecofennian-KetilidianTrans-Hudsonian terrains.

introduction The Svecofennian terrain is part of the Svecokarelian domain of Finland and Sweden (Fig. 0301-9268/89/$03.50

1). It is dominated by I-type granitoids, but there are also supracrustal belts with variable degrees of metamorphism. The Svecofennian granitoids have U-Pb zircon ages of ~ 1.89-1.86

© 1989 Elsevier Science Publishers B.V.

28

Y. KAHKONEN ET AL

p,. o

<.~

o ,- ~-" \ / \-,~

s ~ ,.

~

-i/

/

~_~

"~

>.

~;>

.~<~

i_~

~

®~

"r, ~

®o

~e

o

~.~

i

/ ~

~<

.~

i !!iiiii

:o ~

i::l

l~

~o~

\

~u

3',j~ i ^

i \

,

~ -

x

-,

. r ~.

u

/ -i

v

f'

\.

I

S ~

N

~

,.,el

',~

~

o

~

5 <+-,

I

/~l

/el

i

"@l.l~l@--"

l X l . \ l \

i\

Z\ I~

:

.

~t

"

t "-. ~ \ ' l i ? i \ t

_

I \ / , ~ 1 1 \ % ~ \ \ I ~

~" /

I I -~

-\i

"\--

-

/xl\

-

k

.

29

EARLY PROTEROZOIC VOLCANICAND PLUTONIC ROCKS

Ga (e.g., Wilson et al., 1985; Huhma, 1986). The few U-Pb zircon ages available for the volcanics range from ~ 1.91 Ga to ~ 1.87 Ga (Patchett and Kouvo, 1986; SkiSld, 1987; Welin, 1987). In this paper, we present U - P b zircon ages for the volcanics of the Tampere Schist Belt (Fig. 1 ), which is one of the best-preserved volcanic-sedimentary belts of the Svecofennian terrain. Data on the Viirmiilii granitoid, which is intrusive to the schists, are also provided. Sr isotope data on the volcanics and the H~imeenkyr5 batholith, originally studied by Aro (1983), are given to demonstrate the absence of Archaean crust in the Tampere area 1.9 Ga ago.

Geological setting The Svecofennian terrain was an area of significant crustal growth 1.9-1.8 Ga ago (Patchett et al., 1981; Wilson et al., 1985; Huhma, 1986; Patchett and Kouvo, 1986). Most of its granitoids have eNd(T) values ranging from --1 to + 3. This suggests only minor involvement of Archaean crustal materials in contrast to the situation in the Karelian terrain (Fig. 1), where {~Nd ( 1.8 Ga ) values as low as - 10 have been recorded (Huhma, 1986; Patchett and Kouvo, 1986). The Svecofennian sedimentary rocks are characterized by turbidites. The volcanics, which range from basalt to rhyolite in composition, often show arc affinities, but features typical of extensional environments have also been observed (Latvalahti, 1979; Miikel~i, 1980; Ehlers et al., 1986; Colley and Westra, 1987; KiihkSnen, 1987 ). Starting with Hietanen (1975), several plate tectonic models have been evoked for the Svecokarelian-Svecofennian evolution (e.g., Pharaoh and Pearce, 1984; Park, 1985; Ga~il and Gorbatschev, 1987). Generally speaking, the Svecofennian terrain is regarded as a convergent plate margin. Svecofennian plutonic activity culminated 1.89-1.87 Ga ago (Huhma, 1986; Patchett and Kouvo, 1986). The deposition of platform-type early Proterozoic Jatu-

lian sediments demonstrates stabilization of the Archaean crust before these events, whereas the ~ 2.1 Ga old Karelian tholeiitic volcanics and diabases (Sakko, 1971) indicate rifting of the continent. The 1.97-1.96 Ga old ophiolite-like complexes at Outokumpu (Koistinen, 1981) and Jormua (Kontinen, 1987) mark the subsequent development of oceanic crust. The roughly 1.93 Ga old tonalitic gneisses near the Archaean-Proterozoic boundary in central Finland (Helovuori, 1979; Korsman et al., 1984) also pre-date the main Svecofennian events. Furthermore, some trondhjemites in the Uusikaupunki region, southwestern Finland, are 1.90 Ga or older (Patchett and Kouvo, 1986). In Finland, the main Svecofennian events were post-dated by the roughly 1.83-1.80 Ga old granitoids (Hopgood et al., 1983; Patchett and Kouvo, 1986).

Tampere Schist Belt The east-west trending Tampere Schist Belt, more than 200 km long, is situated near the southern margin of the granitoid complex of central Finland. To the south, the schists pass abruptly into gneisses and migmatites. The sedimentary rocks of the belt are predominantly turbiditic greywackes, siltstones and mudstones ofa forearc-basin type (Ojakangas, 1986). The fluvial-deltaic arkoses at Mauri form a thick unit 20-40 km west of Tampere (Matisto, 1968). Regional metamorphism in the schist belt commonly peaked under low-pressure amphibolite facies P - T conditions (Campbell, 1980; Miikel~i, 1980). Between Orivesi and Y15jiirvi, the belt comprises a major syncline with subvertical axial planes and subhorizontal fold axes (Fig. 1; K~ihkSnen, 1989; Nironen, 1989). The volcanics are mostly of pyroclastic origin, but lava flows and sills also occur. The Haveri Formation at Viljakkala, about 35 km northwest of Tampere, is the only unit with pillow lavas (Miikelii, 1980). The volcanics vary in chemical composition but are mainly me-

30

Y.

KAHKONEN ET A L .

appear necessary (Fig. 2). Firstly, the Haveri Formation seems to underlie the sedimentary rocks immediately south of this formation (MRkel~i, 1980; K~ihkSnen, 1989) and, secondly, there are volcanics among the lowest greywackes near Lake N~isij~irvi. The Tampere Schist Belt is intruded by the H~imeenkyr5 batholith and the V~irm~il~istock (Fig. 1). The H~imeenkyr5 batholith is composed of calc-alkaline rocks fairly high in alkalis, and features a normal zonation of rock types (Gadl et al., 1981; Nurmi et al., 1984). The V~rm~il~i stock is mainly composed of granodiorites and monzogranites (Nurmi et al.,

dium-K or high-K intermediate rocks of calcalkaline affinity (K~ihkSnen, 1987). In general, the volcanics resemble those in mature island arcs or in active continental margins. Certain mafic units high in Ti may indicate extensional environments. In the conventional stratigraphic scheme of the Tampere Schist Belt (Fig. 2; Simonen, 1980), greywackes, siltstones and mudstones comprise the lowermost strata. They are overlain by felsic schists, mafic and intermediate volcanics, conglomerates and related sedimentary rocks and, finally, by mafic volcanics. However, some revisions of this scheme now

STRATIGRAPHIC SECTIONS OF THE TAMPERE SCHIST BELT

Suoden-

Kankaanp&a

niemi

Villakkala

SIMONEN

YI6j,arvi

N-limb

N&sijtirvi E

Orivesi

S-hmb

N-limb

S-limb

/ / / \ s,,,/ TRACH %ill

S H O S H ~ VVV v V

o ~ o

SUBALK~

VV INTERM VV V

~'VV

~

1 km

HAVERI FM

Arkoses

Predommant ly intermediate volcanic rocks

Conglomerates and associated greywackes sdtstones-mudstones

Predominantly mahc volcanic rocks

Greywackes

siltstones

mudstones

Mafic

and intermediate volcanic rocks in general

Felsic volcamc and sedimentary rocks

Fig. 2. Stratigraphic sections of the Tampere Schist Belt according to Mtikel~i (1980), Simonen (1980), K~ihk6nen ( 1989 ) and the references therein. The Kankaanp~i area is 70-80 km west-northwest, and the Suodenniemi area 50 km west of Tampere. INTERM="Intermediate Unit" at Orivesi; S U B A L K = Subalkaline Basaltic to Rhyolitic Unit at Orivesi; S H O S H = Shoshonitic Unit at Orivesi; T R A C H = Trachytic Unit at Orivesi; LOWER V.U. = Lower Volcanic Unit at Y15j~rvi; U P P E R V.U. = Upper Volcanic Unit at Y15j~rvi; HAVERI FM. = Haveri Formation.

31

EARLY PROTEROZOIC VOLCANIC AND PLUTONIC ROCKS

1984; M. Nironen, personal communication, 1987). The H~imeenkyr5 batholith has a U - P b age of 1882 Ma (Patchett and Kouvo, 1986). The granitoid complex of central Finland has yielded U - P b zircon ages between 1889 and 1879 Ma (Huhma, 1986).

Sample and zircon descriptions

Zircon samples Zircon samples A384- and A385-Koskuenj~irvi are from the 1 km thick "Intermediate Unit", which is the lowest unit in the northern limb of the syncline at Orivesi (Fig. 2; K~ihkSnen, 1989). Sample A384 is from a 5 m thick massive stratum of dacitic crystal tuff. The rock contains ~35 vol.% small albite phenocrysts, many of which are broken crystal fragments. Sample A385 is from a 10 m thick pyroclastic rhyolite stratum. The zircons in both samples are euhedral crystals without distinct cores. At Y15j~irvi, the volcanics are divided into the Lower and the Upper Volcanic Units (Fig. 2; K~ihkSnen, 1989). Zircon samples A386-Sammatinjiirvi is from the Lower Volcanic Unit. The stratum of the parent plagioclase-phyric massive pyroclastic dacite is at least 3 m thick. Zircon sample A137-Takamaa represents the Upper Volcanic Unit. It derives from an andesiticdacitic stratum, which is at least 5 m thick, massive, and contains ~ 10 vol.% plagioclase phenocrysts. Being relatively rich in Ti, it resembles the other andesites and dacites of the Upper Volcanic Unit (K~ihkSnen, 1989). The zircons in both samples are euhedral and lack distinct cores. The petrography and two zircon analyses of samples A7-Kalkku and A8-V~i~iriij~irvi (formerly A8-Aitolahti) have been reported by Kouvo and Tilton (1966). The samples are pink feldspar-phyric rocks. It is not known with certainty whether these porphyries are volcanic rocks, sills or dykes (Kouvo and Tilton, 1966; Matisto, 1977). The zircons in samples A-7 and

A-8 are mostly euhedral, strongly zoned and sometimes with small cores (Kouvo and Tilton, 1966). Sample A341-Kovelahti is from Ikaalinen, about 60 km northwest of Tampere. It contains phenocrysts of quartz and feldspar, and resembles the feldspar porphyries considered above. The zircon analysis given in the present paper was made 15 years ago. Zircon sample A927-Lempiiiniemi is a plagioclase porphyry. Its relationship to the volcanics is not known. Zircon sample All-V~m~ilii represents the V~irm~il~i plutonic stock intruding the schists {Fig. 1). The zircons are mostly euhedral and strongly zoned. Some grains have small cores.

Rb-Sr samples Sample numbers, units and rock types are listed elsewhere in the paper (see Table 3 and Fig. 2). The basaltic trachyandesite samples of the Shoshonitic Unit at Orivesi derive from either massive or fragmental strata. They are porphyritic rocks with a primary phenocrystic assemblage consisting of plagioclase and clinopyroxene. Pseudomorphs after hornblende and mica were also observed (K~hkSnen, 1989). Trachyte sample 6867 is from a 5 m thick stratum of lapilli tuff within the Shoshonitic Unit. The two samples from the overlying Trachytic Unit are of pyroclastic origin and comprise 510 vol.% feldspar phenocrysts. The basalts and andesites representing the Upper Volcanic Unit at Y15j~irvi derive from agglomerates, tuff breccias and lapilli tuffs. The analysed dacites and rhyolites from Y15j~irvi represent strata between the Lower Volcanic Unit and the Upper Volcanic Unit. In places, these strata are rich in K and A1 (K/ihkSnen, 1989). Approaching 20% (Aro, 1983 ), the A1203 contents of the two analysed dacites are relatively high. The rocks display signs of pronounced deformation, but hydrothermal alter-

32

ation and weathering may also have been marked. The major components of the H~imeenkyr5 batholith are equigranular or slightly porphyritic, non-foliated, homogeneous plutonic rocks with plagioclase, potassium feldspar, quartz and biotite as the major minerals. The porphyritic contact rocks of the H~imeenkyr5 batholith belong to two types that cut each other, cut the major components of the batholith and cut the wall-rock schists. The two contact rock types are feldspar porphyries mainly composed of albite phenocrysts, quartz and microcline, and plagioclase porphyries consisting mainly of plagioclase, biotite and quartz.

Analytical methods The U-Pb method The U-Pb zircon datings were performed following the analytical methods described by Krogh (1973). Samples consisting of 5-20 mg of hand-picked zircon were washed in hot 7 N HN03 and rinsed several times with H20 to remove any surface contamination. Some fractions were treated with 5% HF for 4 min in an ultrasonic cleaner. This procedure removes altered outer margins and makes the analyses more concordant (Krogh et al., 1982). The samples were spiked with 235Utracer and, after aliquoting, with 2°SPb tracer. Isotopic measurements were made on a 9 in. radius, 60 ° sector Nier-type mass spectrometer as constructed at the DTM Carnegie Institution of Washington. In most cases, the 2°6pb/ 2°4pb ratios were measured from the chart. The total Pb blanks measured ranged from 0.1 to 1.3 ng. The 2-sigma error estimates based on standard and replicate sample analyses are ~ 0.7% for U / P b ratios and 0.2-0.3% for 2°Tpb/2°6pb ratios. The York (1969) method was used to regress the U - P b data, and the age uncertainties of the concordia intersections are given at the 2-sigma level. Individual data points were

¥. K,~HKONEN ET AL.

weighted according to their precision, and a correlation coefficient of 0.92-0.96 between the 2°6Pbff~sU and 2°TPbff3~U errors was used according to the method of Ludwig (1980). The Rb-Sr method The Rb-Sr whole-rock analyses were made on 100 mg powdered samples. The Rb and Sr fractions were separated by conventional ionexchange chromatography at the University of Rennes, France. The concentrations were determined by standard isotope dilution techniques using spikes enriched in STRb and S4Sr. The isotopic measurements were carried out at the University of Rennes using a 30 cm radius, 60 ° sector TSN mass spectrometer with a 10 kV accelerating voltage. The Sr and Rb blanks were ~ 5 ng and ~ 1 ng, respectively. The measured values of the STSr/S~Srratio were corrected for isotopic fractionation to S6Sr/ SSSr=0.1194. Isochrons were fitted using the method of York (1969). The ages were calculated using a decay constant of 1.42 × 10-11 a - 1 for STRb.For MSWD values above 1, the 2-sigma errors of the ages were multiplied by (MSDW) 1/2 assuming STRb/S6Sr errors of _+2%. For details, see Vidal (1980).

Results U-Pb zircon data Analyses of 31 zircon fractions from nine samples are presented in this paper. The U-Pb analytical results are given in Table 1 and plotted on concordia diagrams in Fig. 3. The regressed data are summarized in Table 2. The three zircon fractions from rhyolite A385-Koskuenjiirvi give an age of 1904 + 4 Ma with a good fit to a straight line (Fig. 3a). The three fractions from dacite A384-Koskuenj~irvi define a trend with a negative lower intercept age, which is impossible for a cogenetic population. This is due to a large analytical error in fraction B and anomalous discordance of the HF-washed fraction C. The abundance of 2°Spb

EARLY PROTEROZOICVOLCANICAND PLUTONIC ROCKS

33

TABLE 1 U - P b analytical results Sample d =dens~y 0 =sizeinpm H F = l e a c h e d in H F

Measured c o n t e n t s ( p p m ) a n d ratios:

Atomic abundances ", 2°ePb = 1000

Atomic ratios

238v

2°ePbra d 206pb 204pb

204pb

207pb

208pb

206pb 23sU

207pb 235U

20Vpb 20epb

207pb 20~pb ± 2 a

107

9153

0.1008

116.79

133.36

0.29838

4.7482

0.11542

1886±3

Age ( M a )

A384-Koskuenj~rvi, Orivesi, dacite A4.2 70 B d > 4.6 0 > 70 C d> 4.2/HF

415 304

80.6

4153

0.2311

118.40

590.17

0.30654

4.8719

0.11528

1884±5

363

89.3

29340

0.0254

116.33

117.59

0.28445

4.5487

0.11598

1895±2

2151 1504 2672

0.4582 0.6578 0.3567

121.30 122.16 120.29

195.45 222.13 174.07

0.28170 0.23086 0.29610

4.4706 3.6045 4.7140

0.11510 0.11325 0.11547

1881 ± 3 1852±4 1887±2

A385-Koskuenjarvi, Or±yes±,rhyolite A 4.04.2

784 1225 416

191 245 107

A386-Sammatinjiirvi, YlSjiirvi, dacite A d > 4 . 2 , 0 > 70 B 4.0 70 D d>4.2, 0>70 E d>4.2, 0> 70/HF F d>4.2, 0> 70/HF

402 527 819

115 148 198

2360 1092 618.2

0.4177 0.9104 1.6131

121.08 127.70 134.11

134.77 172.87 225.24

0.33116 0.32489 0.27970

5.2706 5.1692 4.3278

0.11544 0.11540 0.11223

1886±7 1886±2 1836±4

402 383

116 112

1326 2214

0.7467 0.4428

125.81 122.06

148.95 134.45

0.33203 0.33758

5.2979 5.4014

0.11573 0.11605

1891±3 1896±5

398

116

2320

0.4243

121.44

135.22

0.33583

5.3994

0.11568

1890±3

A137- Tahamaa, YlOjiirvi, dacite A4.3 70 B 4.2 70/HF

316

87.2

2868

0.3401

119.48

102.39

0.31942

5.0598

0.11490

1878±2

441 487 287

118 127 83.6

2464 1873 3500

0.3998 0.5277 0.2740

119.19 120.74 119.14

117.89 126.10 97.56

0.30893 0.30224 0.33673

4.8462 4.7335 5.3584

0.11378 0.11359 0.11542

1860±7 1857±4 1886±3

286

84.1

6264

0.1501

117.47

94.12

0.33938

5.4010

0.11543

1886±2

A 7-Kalkku, Tampere, [eldspar porphyry A Total b B 4.3 < d < 4 . 6 C 4.3 < d < 4.6, HF D4.2
682 394 364

176 106 103

1390 3098 5506

0.58 0.3142 0.1715

121.7 119.58 117.54

126.1 118.72 111.41

0.2972 0.31009 0.32724

i.664 4.9310 5.1985

0.1138 0.11534 0.11522

1861 1885±4 1883±3

607 947

148 235

2270 2372

0.4326 0.4175

120.59 119.55

120.33 118.06

0.28276 4.4733 0.28689 4.5052

0.11474 0.11390

1876±2 1862±3

1740 2905 1868 8569

0.45 0.3379 0.5306 0.1119

120.9 119.70 121.43 116.90

124.4 109.18 122.41 107.21

0.2956 0.31792 0.31073 0.32839

4.6777 0.1148 5.0466 0.11513 4.8947 0.11425 5.2242 0.11538

1876 1882±2 1868±4 1886±2

A8- Vii~riijiirvi, Tampere, feldspar porphyry A Total b B 4.3 < d < 4.6 C 4.2
761 483 682 626

195 133 183 178

34

Y. KAHKONEN ET AL.

T A B L E 1 (continued) Sample d =density = size in/Lm

Measured contents ( p p m ) a n d ratios:

Atomic abundances a, 2°~pb= 1000

Atomic ratios

H F =leached in H F

238U

2°6pbrad 2°~pb 204pb

2°apb

206pb

207pb

238U

235U

207pb 2osp b

20Vpb 2o~pb __2a

817

177

3235

0.2983

117.80

93.03

0.25115

3.9392

0.11376

1860_+2

1138 1712 725

183 228 128

864 824 869

1.1509 1.2066 1.1391

126.80 124.70 128.29

126.60 122.33 148.81

0.18545 0.15454 0.20392

2.8425 2.3068 3.1728

0.11117 0.10827 0.11285

1818_+3 1770_+5 1845_+3

5954 3041

0.1548 0.2242

116.88 117.42

83.74 103.09

0.32697 0.30519

5.1741 4.8124

0.11478 0.11437

1876+2 1870_+4

2259

0.172

117.20

117.7

0.3036

4.806

0.1149

1880

2°7pb 2°8pb

Age (Ma)

A11- Vi~rmtili~, Tampere, granodiorite A4.04.0

A927-Lempi(miemi, YlSji~rvi, plagioclase porphyry Ad>4.2 B4.0
266 561

75.3 148

A341-Kovelahti, Ikaalinen, feldspar porphyry Total zircon

406

107

~238U = 0.15513 × 1 0 - 9 Yr, X236U= 0.98485 × 10-9 Yr. aCorrected for blank a n d fractionation, U blank = 0.7 ng, P b blank = 1 ng, P b fractionation corrected against CIT s t a n d a r d about 0.05 % amu. bFrom Kouvo a n d Tilton (1966). C o m m o n lead correction: 2°epb/2°4pb = 15.49, 2°7pb/2°4pb = 15.30, 2°8pb/2°4pb = 35.14.

is also exceptionally high in fraction B. We regard the obtained age as dating the eruption of the "Intermediate Unit" at Orivesi. Dacite A386-Sammatinj~rvi is from the Lower Volcanic Unit at Y15j~irvi. The fractions, including two nearly concordant HF-leached fractions, give an age of 1898+4 Ma with M S W D - 1 (Fig. 3b). Without the most discordant fraction C, the age is 1894 + 5 Ga. Regression of the four fractions not leached with HF gives an age of 1900 _+4 Ma. Dacite A137-Takamaa is from the Upper Volcanic Unit at Y15j~irvi. Five fractions, including two concordant HF-leached fractions, yield an age of 1889_ 5 Ma with MSWD = 1 (Fig. 3b ). Two zircon U-Pb analyses from feldspar porphyries A7-Kalkku and A8-V~i~ij~rvi were published by Kouvo and Tilton (1966). Seven fractions of these rocks were analysed during the present study. An upper intercept age of 1889 + 19 Ma ( M S W D = 9 ) was calculated for

A7 and an age of 1891 + 16 Ma (MSWD = 3 ) for A8. Pooled regression of all nine fractions yields an upper intercept age of 1887_+ 10 Ma with M S W D = 7 (Fig. 3c). The most concordant fractions are again those washed with HF. However, without these fractions, the age remains about the same. The zircon analysis of feldspar porphyry sample A341-Kovelahti plots on the same line. Two zircon fractions from plagioclase porphyry A927-Lempi'dniemi give an age of 1880 + 7 Ma (Fig. 3b), suggesting closer relations with the plutonic than with the volcanic rocks. The four zircon fractions analysed from sample A l l of the V~-~iilii stock are more discordant than the zircons from the volcanics (Fig. 3d). The best-fit line gives an age of 1901 ___28 Ma with MSWD as high as 9. Because most of these zircons are strongly zoned and some have cores (p.5), they may contain a small inherited component.

EARLYPROTEROZOICVOLCANICAND PLUTONICROCKS

a)

0.35

'

i

I

i

i

MOLCANICS

0RIVESI

35

1904--.411 1800 .

/

[1-

-, /y ~ -

B

+ A385- KOSKUENJARVI 0.288

O.15 I

I

I

i

207p b / 235U

C}

.

.

.

.

.

~'~j/

EF

~

EF

4,4

50

.

• :::L"C

2L

A385C LOWER VOLCANICUNIT ZZ~ N i A3E6- 5AMMATLNJARVI 1/ A3~5.KOSKUEN jARVr ~ - ~ // REFERENC E 150CHRON / ~ ~ R /A6E/190&Z&N@ . . 4,. A927-LEMPIANIEMI .

X A38Z-KOSKUENJARVl

I

A

1,oo

O o4

30

DE

Q0 ('3 (M

,.Q 12_ O

1889±5Mo

1800.

• /'~ A 1600/ --x C / I - AC

,~\t/

I

~

/ -

¢M

"YLOJ~,RV 0335

4.8

207Pb/

5.2 235U

5.6

'

d) A11-VARMALA

GRANODIORITE

18OO.

03~.

1 9 0 1 + 2 8 / " "

g ~0

/1700./

x

g_ (N

+ 0.285

o A7-KALKKU ( 1 8 8 9 ± 1 9 M 0 ) Jr- As-VAARAJARVI [1891±16MQ) X A341- KOVELAHTI

E

i 4.4

I 418

'

512

207Pb/ 235U

I

516

.~ '

'

3!o

'

'

'

51o

'

'

2 0 7 P b / 235 U

Fig. 3. U-Pb zircon isochron diagrams. The salient data are listed in Table 1. In each figure, A-F indicate the fractions in Table 1. (a) "Intermediate Unit" at Orivesi near Koskuenjiirvi. (b) Volcanics of the Lower and Upper Volcanic Units at Y15j~irvi, and the plagioclase porphyry of Lempiiiniemi. The isochron of sample A385-Koskuenj~rvi from Orivesi is shown for reference. (c) Feldspar porphyries of Kalkku, V~iriij~irvi and Kovelahti. (d) V~irmRl~istock.

Rb-Sr data The Rb-Sr analytical data on the 26 wholerock samples are listed in Table 3, and conventional R b - S r isochron diagrams are shown in Fig. 4. A line fitted to the data from the Orivesi volcanics gives an age of 1 8 4 7 _ 9 2 Ma ( M S W D -- 2.6) with an initial 87Sr/SESr ratio of 0.7025 + 0.0003 (Fig. 4a). The eight points from the Y15jiirvi volcanics yield an age of 1898 + 50

Ma ( M S D W = I . 5 ) with an initial ratio of 0.7020 + 0.0003 (Fig. 4b). The samples representing the major rock components of the Hiimeenkyr5 batholith define an isochron of 1775 + 52 Ma ( M S W D = 1.0) with an initial 87Sr/SESr ratio of 0.7030 ___0.0007 (Fig. 4c). An isochron based on samples of contact rocks only gives an age of 1740___ 70 Ma (MSWD=I.3) with an initial ratio of 0.7033 + 0.0009 {Fig. 4d). The ages and initial ratios of the major and contact rock types thus

36

Y. KAHKONEN ET AL.

TABLE 2 S u m m a r y of zircon U - P b ages from the Tampere area (this work ) Sample

Upper intercept age (Ma)

Lower intercept age (Ma)

MSWD

N u m b e r of fractions

202 196

0.1 7

3 6

477 300 498

1 0.9 0.8

6 5 without C 4 without E a n d F

1880+ 5

403

1

5

1889 ± 19 1891 ± 16 1887+ 10

175 297 183

9 3 7

Koskuenj~rvi,'~ntermediateUnit"at Orivesi. A385 A384+A385

1904± 4 1905 ± 15

Lower VolcanicUnitatYlSj~rvi, Sammatinj~rvi A386

1898 ± 4 1894± 5 1900± 4

Takamaa Upper Volcanic Unit at YlSfftrvi, A137

Feldspar porphyries A7-Kalkku AB -V~i~ir~ij~irvi A7+A8

Plagioclaseporphyry, Lempi~niemi, YlSjSvvi A927

1880± 7

178

1901±28

184

V~rm~l~granodiorite All

Errors are at 95% confidence level.

coincide within the limits of error. The age difference between them is presumably small despite cross-cutting rock relationships. U-Pb VERSUS Rb-Sr AGES The zircon data from the igneous rock of the Tampere area presumably mostly indicate the ages of primary crystallisation. This is supported by a concordant titanite age of 1879 Ma from the Hiimeenkyr5 batholith (Patchett and Kouvo, 1986) and the morphology of zircon crystals. Within error limits, the Rb-Sr whole-rock ages of the volcanics are consistent with the UPb zircon ages, whereas the Rb-Sr age of the H~imeenkyr5 batholith is ~ 100 Ma lower than its zircon age. Such discrepancies are common all over the world (e.g., Page, 1978; Welin et al.,

1983; Hoffman and Bowring, 1984), and are usually caused by Sr migration after crystallisat±on. Sr migration can be due to the slow cooling of the igneous bodies (Harrison and Clarke, 1979; Welin et al., 1980, 1983) and/or to subsequent metamorphism (Brooks, 1968; Roddick and Compston, 1977; Page, 1978; van Breemen and Dallmeyer, 1984 ). Present and previous Rb-Sr data indicate that the age discrepancy at Tampere can be attributed both to metamorphism and slow cooling. In the Nokia greywackes (Fig. 1 ), poikilitic flakes of muscovite and biotite have Rb-Sr ages of 1804 and 1726 Ma, respectively (Kouvo and Tilton, 1966; recalculated with new decay constants). These features suggest that late metamorphism had been important. At Orivesi, the felsic volcanics contain muscovite poikiloblasts, while the mafic rocks include non-or±-

37

EARLYPROTEROZOICVOLCANICAND PLUTONICROCKS TABLE 3 Rb-Sr analytical data Sample Unit

Rock type

Rb (ppm)

Sr (ppm)

6688 6689 6690 6691 6692 6693 6694 6695

Major phases of the H~imeenkyr5 batholith

Granodiorite Granodiorite Granodiorite Granodiorite Granodiorite Granodiorite Monzodiorite Granite

130 129 131 127 69.7 73.4 109 144

297 173 268 277 379 400 339 186

6867 6868 6869

Shoshonitic Unit at Orivesi

Trachyte Basaltic trachyandesite Basaltic trachyandesite

6870 6871

Trachytic Unit at Orivesi

Trachyte Trachyte

6872 6873 6874

Felsic and intermediate rocks below the Upper Volcanic Unit at Y15j/irvi

Dacite Rhyolite Dacite

6875 6876 6877 6878 6879

Upper Volcanic Unit at Yl~ij/irvi

Andesite Basalt Basalt Basaltic andesite Basaltic andesite

6925 6926 6927 6928 6929

Porphyritic border phases of the H~imeenkyr5 batholith

Feldspar porphyry P|agioclase porphyry Feldspar porphyry Feldspar porphyry Plagioclase porphyry

ented, apparently late-grown hornblende. The Rb-Sr whole-rock data from both the Y15j~irvi and the Orivesi volcanics show that the most radiogenic samples analysed tend to plot below the isochrons. This feature may be attributed to late metamorphism. The difference between the muscovite and biotite ages indicates regional cooling. The blocking temperature for muscovite is ~ 500 + 50 ° C and for biotite ~ 3 0 0 + 5 0 ° C (J~iger, 1979). The rocks of the H~imeenkyr5 batholith lack evidence of metamorphism. Consequently, the roughly 1.78-1.74 Ga Rb-Sr whole-rock ages of the batholith can probably not be explained this

STRb/s6Sr STSr/S6Sr 1.271 2.177 1.418 1.357 0.533 0.532 0.933 2.250

0.73501 -+ 10 0.75722_+ 4 0.73911 ___ 7 0.73862 _+11 0.71646_+ 7 0.71677_+ 8 0.72651_+ 5 0.76139_+ 3

562 1046 860

0.404 0.109 0.233

0.71302 _+13 0.70528-+ 10 0.70892-+ 7

173 127

511 752

0.982 0.489

0.72792_+ 6 0.71558_+ 6

90.9 81.2 100

158 96.7 186

1.673 2.444 1.552

0.74662_+ 7 0.76794_+ 6 0.74387_+ 6

39.7 38.8 28.5 12.6 27.8

172 202 213 254 162

0.669 0.556 0.386 0.143 0.498

0.72012 + 30 0.71742-+ 8 0.71289+ 8 0.70586_+ 9 0.71616_+16

129 92.0 80.8 157 161

214 441 413 168 329

1.756 0.604 0.566 2.724 1.422

0.74854_+ 7 0.71834 + 9 0.71745_+10 0.76976-+ 4 0.73870_+10

78.5 39.3 69.1

way. The age discrepancy is more likely due to slow cooling of the pluton and associated transport of fluids. Since ~ 1.88 Ga ago, the temperature in the batholith did not exceed the U - P b closure temperature of titanite. However, for 100 Ma, it remained sufficiently high to allow the migration of (radiogenic) Sr on a scale of tens of centimetres. Cooling below ~ 300 ° C took place ~ 1.72 Ga ago, as inferred from the Rb-Sr age of biotite from the Nokia granodiorite (Kouvo and Tilton, 1966). However, in some other areas near Tampere, concordant titanite ages of ~ 1.80 Ga have been measured (e.g., A932-Luoma, Tampere, trachyandesite, and

38

073

Y. K~,HKONENET AL.

a)0RIVESI VOLCAN

I

C

A229-Kankaanp~i~i, synorogenic granite; O. Kouvo, personal communication, 1984). This may indicate resetting of the U-Pb system by local hydrothermal activity.

~

If)

y

071

Zircon geochronology and stratigraphy of the Tampere region

6867

686971

87Rb/86Sr

0~5

110

"b)YLOJ~,RVI VOLCANICS 075 872

LD

6875~ 6879~876

m

~

071

877

AGE = 1898 t 50 Ma

'

Isr

" 0.7020*-3

6878

i c)

87Rb/86Sr

H~,MEENKYRO BATHOLITH ~

6689

075 66~r~90 //~'

~

6688

6694

6 ~

AGE = 1775+-52 Ma

6692

l sr = 0.7030+-7

071 i

1

87Rb/86Sr

d)H~,MEENKYRO PORPHYRITIC BORDER PHASES

i

2 ////~8

075

~o 071

6926

//~927

AGE =

~

1740' 70 Ma

[Sr =0.7033*-9

i

87Rb/86Sr

Fig. 4. Rb-Sr isochron diagrams of the whole-rock samples analysed. Data are from Table 3. Errors are given at the 2sigma level. (a) Orivesi volcanics. (b) Y18j~irvivolcanics. (c) Major rock components of the H~imeenkyro batholith. (d) Porphyritic contact rocks of the H~imeenkyrti batholith.

Although most of the zircon ages coincide within the limits of error, the zircon chronology generally agrees with the stratigraphy. The oldest unit dated so far (1904+4 Ma) is the "Intermediate Unit" at Orivesi. It occupies a low stratigraphic position in the northern limb of the syncline at Orivesi. Considering the limits of error, the Upper Volcanic Unit at Y15j~irvi may be 6-24 Ma younger, provided that its zircon age dates the extrusion. The zircons from the Lower Volcanic Unit at Y15j~irvi are older than those from the Upper Volcanic Unit, although the errors overlap slightly (1898 _+4 vs. 1889 ___5 Ma). The implications of the V~i~ir~ij~irvi and Kalkku zircon ages are not known. Here, the errors are large and the relationships of these rocks to the nearby supracrustal sequences are not clear. The calculated age of 1891 Ma for the V~i~ir~ij~irvi sample is not necessarily the exact age of the nearby volcanic strata. Some zircon crystals in the feldspar porphyries and the granodiorites contain minute cores (Kouvo and Tilton, 1966). Thus the relatively high ages of the V~irm~il~iand Nokia granodiorites (Table 4) may be due to minor inherited zircon. The granodiorites, no doubt, intrude the schists. Considering the analytical errors, the age data are not contradictory. We still do not know the age of deposition of the lowest greywackes in the Tampere Belt and the age relationship of these greywackes to the "Intermediate Unit" at Orivesi. Detrital zircons from seven greywackes near Tampere have yielded 2°7pbF°6pb ages close to 2.3 Ga (Wetherill et al., 1962; Kouvo and Tilton, 1966; cf. Table 4 ). The major source of the greywacke can-

39

EARLYPROTEROZOICVOLCANICANDPLUTONICROCKS TABLE 4 Zircon chronology of the Tampere area Age (Ma)

Ref. a

Rock type, sample number

Remarksa

1882 ± 6

1,6

H~imeenkyr5 batholith A4, A248

1903±9 1901 ±28 ~ 1900 1889±5

1,10 3 2 3

1880± 7 1887± 10

3

1885± 7

6

Nokia granodiorite A2 V~irm~il~igranodiorite A 11 Detrital zircon in the arkose of Mauri, A153 Dacite in the Upper Volcanic Unit at Y15jiirvi, A137 Plagioclase porphyry of Lempi~niemi, A927 Feldspar porphyries of Kalkku A7, V~ifir~ij~irvi A8 and Kovelahti A341 Varissaari gabbro, A684

Isr=0.7030-0.7033 (ref. 3) Titanite U-Pb age 1879 (ref. 6) eNd----+0.1 (ref. 6) Biotite Rb-Sr age 1718 (ref. 1 )

1898±4

3

~ 1890 1904±4

1 3

~ 2300

1,5,10

1890

7

1890 1877 ~ 1780

10 10 8

1,3

Dacite in the Lower Volcanic Unit at Yl(ij~irvi, A386 Plutonic pebbles in conglomerates, A90, A144 Dacites and rhyolites of the Intermediate Unit at Orivesi A384, A385 Detrital zircon in the greywackes of the Tampere schist belt: A57-Siivikkala, A1-Nokia, A6-Alisenj~irvi, A9-Kangasala, A128Mouhij~rvi, A131-Sahalahti, A132-Kuhmalahti Mafic pegmatitoid, Stormi, Vammala, A756, A757 Vihtelj~irvi granodiorite, Kankaanp~i~i, A48 Kankaanp~iiiporphyritic granite, A229 Viitaniemi pegmatite A855

Isr in the Upper Volcanic Unit is 0.7020 (ref. 3)

eHf= +3 (ref. 4) ENd= +1.3 (ref. 6) eNd: --0.7 (ref. 9)

Sample A1-Nokia has Rb-Sr biotite age 1726 and muscovite age 1804 (ref. 1) Sample A57 has eNd (1900) = --0.6 (ref. 9)

U-Pb titanite age 1800

aReferences: 1 Kouvo and Tilton (1966); 2 Matisto (1968); 3 this work; 4 Patchett et al. (1981); 5 Wetherill et al. (1962); 6 Patchett and Kouvo (1986); 7 H~ikli et al. (1979); 8 Lahti (1981); 9 Huhma (1987); 10 Kouvo, personal communication (1985).

not be Archaean, since they have eNd(1.9 Ga) ~ - 0 . 6 (Huhma, 1987). The basalt-rich Haveri Formation may represent the depositional basement of the lowest greywackes (Fig. 2 ). This concept is supported by the ~ 1.99 Ga whole-rock Pb-Pb age of the Haveri Formation (Vaasjoki and Huhma, 1987); however, current evidence is not convincing. Discussion

The most striking feature of the Tampere zircon data is the short duration of volcanism, plu-

tonism and erosion (Table 4). The age range 1904-1877 Ma applies to most of the Svecofennian terrain. The U - P b zircon ages available for the volcanics of the Tampere Belt define a time span of 10-20 Ma. When compared with young volcanic arcs (Gill, 1981; Thorpe, 1982 ), this span is not abnormally short because the Tampere Belt is part of a wider arc-like system and the data may represent only an episode in the evolution of the belt. However, the U - P b zircon ages of the Tampere volcanics cover virtually the whole known age range of the Svecofennian volcanics of Finland.

40

Ages close to the 1904 Ma age of the "Intermediate Unit" at Orivesi have been obtained from Sk~ldS, southern Finland (1895-1903 Ma; Hopgood et al., 1983), from Parkano, western Finland (1907 Ma; Lahti et al., in press) and from Joroinen, eastern Finland (1906 Ma; Vaasjoki and Sakko, 1988). On the other hand, the 1887 Ma age of the Pellinki volcanics in southern Finland (Patchett and Kouvo, 1986) is close to the 1889 Ma age of the Upper Volcanic Unit at Y15j~irvi. Ranging between 1892 and 1867 Ma, the U Pb zircon ages of Svecofennian volcanics in Sweden (SkiSld, 1987; Welin, 1987) tend to be slightly lower than those in Finland, but the range is wider if the pooled age of 1900 + 19 Ma from two rhyolites near Falun, south-central Sweden (Aberg et al., 1984) is included. Furthermore, the U-Pb zircon ages of the felsic volcanics of Kiruna, northern Sweden range from 1909 to 1882 Ma (SkiSld and Cliff, 1984). Relatively low Sr initial ratios of 0.70200.7033 in the Orivesi and Y15j~irvi volcanics and the H~imeenkyr5 batholith indicate only slight, if any, contamination by Archaean crustal materials. This agrees with previously published Sm-Nd, Hf-Lu and Rb-Sr isotope studies inferring the absence of Archaean crust in the Svecofennian terrain (Patchett et al., 1981; Wilson et al., 1985; Huhma, 1986; Patchett and Kouvo, 1986; Patchett et al., 1987; Welin, 1987 ). The bulk of the crust exposed in the 800 X 800 km Svecofennian terrain was formed within a time interval of 40 Ma. The time span of the Tampere volcanism approaches that of volcanism throughout the whole Svecofennian terrain. Such features are not seen at most of the young convergent plate margins. Recent volcanic arcs are typically no more than 100-200 km wide, and subduction-related igneous activity tends to last for more than 10 Ma {Gill, 1981; Thorpe, 1982). Some arc complexes are wide, but they include significant proportions of old crust. For instance, the wide and complex Indonesian region contains several microplates with Palaeozoic or Precambrian rocks (Ham-

Y. K A H K O N E N E T A L .

ilton, 1979). The width of the Svecofennian terrain might be explained by continent-continent collision but the absence of Archaean crust and the scarcity of Proterozoic U-Pb zircon ages older than 1.9 Ga do not favour this model. Park (1985) suggested that the Svecofennian terrain was formed as a succession of 2.01.8 Ga old island arcs accreted as exotic terranes to the Archaean craton. According to this model, the arcs in the south are younger than those in the north. However, there is no evidence of age zoning of this kind (see Patchett and Kouvo, 1986; SkiSld, 1987; Welin, 1987; Vaasjoki and Sakko, 1988; this study). Age data on the volcanics are admittedly scarce and we do not deny the possibility that future U-Pb zircon age determinations may reveal wider age ranges and systematic differences in the eruption times of the Svecofennian volcanics. However, the abundant data already available on plutonic rocks indicate that the formation of Svecofennian crust 1.90-1.87 Ga ago occurred simultaneously over a wide area. This process was complex and may have involved several coexisting spreading-subduction systems with different subduction directions and polarities. Although the subsequent accretion and collision of such units is possible, they were probably not mutually exotic, i.e., the different crustal segments were formed close to each other in space as well as in time. The model of Park (1985) is also disputable because there is no evidence of 2.0 Ga old volcanic arcs close to the Archaean craton. For instance, the 1.96 Ga old Jormua ophiolite represents an incipient oceanic basin rather than a back-arc environment (Kontinen, 1987). The arc-like Svecofennian volcanism probably did not commence until 1.96 Ga ago. However, about 1.91 Ga ago the mantle-derived evolution had proceeded to a stage at which the 1 km thick "Intermediate Unit" at Orivesi with its calc-alkaline high-K dacites and andesites could begin to form. If this development was controlled by plate tectonics, considerable amounts of oceanic

41

EARLY PROTEROZOIC VOLCANIC AND PLUTONIC ROCKS

crust and island arcs must have been formed between ~ 1.96 and 1.91 Ga ago. U - P b zircon ages from this time interval are scarce. However, the tonalitic gneisses near the Proterozoic-Archaean boundary in central Finland and possibly some trondhjemites at Uusikaupunki, southwestern Finland (p. 3) may be indicative of such a process. Related mafic and ultramafic rocks are not abundant. Mafic-ultramafic complexes representing 1.96-1.91 Ga old oceanic crust and island arcs have not yet been identified from southern or central Finland. Crust formation in the Svecofennian terrain took place during a short time interval. Considering, in addition, the Ketilidian (Patchett and Bridgwater, 1984), Wopmay (Hoffman and Bowring, 1984) and Trans-Hudsonian (Chauvel et al., 1987; Van Schmus et al., 1987) orogens, it is evident that the 1.9 Ga orogeny was a brief but major crust-forming event.

Acknowledgements

First of all, we thank Olavi Kouvo for his encouragement and interest in this study. He also provided us with the data on sample A341. Matti Vaasjoki put the data on sample A927 at our disposal. The staff of the Isotope Laboratory of the Geological Survey of Finland also deserve special thanks. The Rb-Sr determinations were made at the initiative of Gabor Ga~il. Kristiina Aro's visit to Rennes was made possible by financial support received from the "Porphyry Project" of the Department of Geology, University of Helsinki and the French Government. Nicole Morin and Herv6 Martin guided her in the Rb-Sr determinations. The paper profited considerably from the comments of Bor Ming-Jahn. The English language was revised by Gillian H~kli and Anthony Meadows. This paper is a contribution to IGCP Project 217 "Proterozoic Geochemistry".

Appendix. Sampling sites No.

Location

Map sheet

Grid coordinates

2123 06 2123 12 2124 10 212404 2122 09 2142 04 2142 04 212404 2124 07

6820.35 6829.90 6832.10 6833.94 6858.5 6839.19 6838.84 6836.69 6834.21

447.53 493. 493. 475.29 447. 513.71 513.44 474.54 482.95

2124 01 2124 01 2124 04 212401 2124 04 2124 04 2124 04 2124 04 2142 04 2142 04 2142 04 2142 04 2142 04 2124 04 2124 04 2124 04 2124 04 2124 04 2124 04 2124 04 2124 04 2124 04 2124 04 2124 04 2124 04 2124 04

6837.78 6837.73 6833.68 6837.88 6833.67 6833.66 6834.46 6834.46 6838.40 6838.40 6838.36 6838.27 6838.26 6835.63 6835.65 6835.66 6835.41 6835.40 6835.39 6835.38 6835.37 6834.37 6834.38 6834.39 6834.44 6834.47

468.16 468.11 473.54 468.26 473.53 473.53 472.79 472.46 513.06 513.02 512.98 512.88 512.86 474.92 474.92 474.92 474.90 474.90 474.90 474.90 474.90 472.79 472.79 472.79 472.79 472.79

Zircon samples A7 A8 All A137 A341 A384 A385 A386 A927

Kalkku, Nokia Vii~ij~irvi, Tampere Viirmiil/i, Teisko Takamaa, Y15jiirvi Kovelahti, Ikaalinen KoskuenjKrvi, Orivesi KoskuenjKrvi, Orivesi SammatinjKrvi, Y18j~rvi Lempiiiniemi, YliSjiirvi

Rb-Sr samples 6688 6689 6690 6691 6692 6693 6694 6695 6867 6868 6869 6870 6871 6872 6873 6874 6875 6876 6877 6878 6879 6925 6926 6927 6928 6929

Joenkulma, H/imeenkyri~ Joenkulma, H/imeenkyri~ ParosjKrvi, Yli~jiirvi Joenkulma, H~imeenkyr5 Parosj/irvi, Y15j~irvi Parosj~irvi, YlSj~irvi Iso Kivijiirvi, H/imeenkyr5 Iso Kivijiirvi, H~imeenkyri5 Orivesi Orivesi Orivesi Orivesi Orivesi Lakiala Y16jKrvl Lakiala Y16jiirvl Lakiala Y15j/irvi Lakiala Y1fj/irvi Lakiala Y1fjiirvi Lakiala Y16j~irvi Lakiala Y16j~irvl Lakiala Y15j~irvl Iso Kivi iirvi, H~meenkyr5 Iso Kivi iirvi, H~imeenkyr5 Iso Kivi ~irvi, H~imeenkyr5 Iso Kivi ~irvi, H~imeenkyr8 Iso Kivi: ~irvi, H~imeenkyr5

References Aberg, G., Levi, B. and Fredrikson, G., 1984. Zircon ages of metavolcanic and synorogenic rocks from the SvKrdsji~ and YxsjSberg areas, south central Sweden. Geol. Foeren. Stockholm Foerh., 105: 199-203. Aro, K., 1983. H~imeenkyrSn batoliitin sekii Y15j~irvenja Oriveden metavulkaniittien Rb-Sr-menetelm~n perustuva kokokivi-iKnmii~iritys. M.Sc. Thesis, University of Helsinki, 73 pp. (unpublished) (in Finnish).

42 Brooks, C., 1968. Relationship between feldspar alteration and the precise post-crystallizationmovement of rubidium and strontium isotopes in granite. J. Geophys. Res., 73: 4751-4757. Campbell, D.S., 1980. Structural and metamorphic development of migmatites in the Svecokarelides, near Tampere, Finland. Trans. R. Soc. Edinburgh, Earth Sci., 71: 185-200. Chauvel, C., Arndt, N.T., Kielinzcuk, S. and Thom, A., 1987. Formation of Canadian 1.9 Ga old continental crust. I: Nd isotopic data. Can. J. Earth Sci., 24: 396-406. Colley, H. and Westra, L., 1987. The volcano-tectonic setting and mineralizationof the early Proterozoic KemiSOrij~rvi-Lohja belt, SW Finland. In: T.C. Pharaoh, R.D. Beckinsale and D. Rickard (Editors), Geochemistry and Mineralization of Proterozoic Volcanic Suites. Blackwell, Oxford. Geol. Soc. Spec. Publ., 33: 95-107. Ehlers, C., Lindroos, A. and Jaanus-J~irkkiil~i, M., 1986. Stratigraphy and geochemistry in the Proterozoic mafic volcanic rocks of the Nagu-Korpo area, SW Finland. Precambrian Res., 32: 297-315. Ga~l, G. and Gorbatschev, R., 1987. An outline of the Precambrian evolution of the Baltic Shield. Precambrian Res., 35: 15-52. Ga~il, G., Front, K. and Aro, K., 1981. Geochemical exploration of a Precambrian batholith, source of a Cu-W mineralization of the tourmaline breccia type in Southern Finland. J. Geochem. Explor., 15: 683-698. Gill, J.B., 1981. Orogenic Andesites and Plate Tectonics. Springer-Verlag, Berlin, 390 pp. H~ikli, T.A., Vormisto, K. and H~inninen, E., 1979. Vammala, a nickel deposit in layered ultramafite, Southwest Finland. Econ. Geol., 74: 1166-1182. Hamilton, W., 1979. Tectonics of the Indonesian region. U.S. Geol. Surv. Prof. Pap., 1078:345 pp. Harrison, T.M. and Clarke, G.K., 1979. A model of the thermal effects of igneous intrusion and uplift as applied to Quottoon pluton, British Columbia. Can. J. Earth Sci., 16: 411-420. Helovuori, O., 1979. Geology of the Pyh~isalmi ore deposit. Econ. Geol., 74: 1084-1101. Hietanen, A., 1975. Generation of potassium-poor magmas in the northern Sierra Nevada and the Svecofennian of Finland. J. Res. U.S. Geol. Surv., 3: 631-645. Hoffman, P.F. and Bowring, S., 1984. Short-lived 1.9 Ga continental margin and its destruction, Wopmay orogen, northwest Canada. Geology, 12: 68-72. Hopgood, A., Bowes, D.R. and Kouvo, 0., 1983. U-Pb and Rb-Sr isotopic study of polyphase deformed migmatites in the Svecokarelides, Southern Finland. In: M.P. Atherton and C.D. Gribble (Editors), Migmatites, Melting and Metamorphism. Shiva Publishing, Nantwich, pp. 80-92. Huhma, H., 1986. Sm-Nd, U-Pb and Pb-Pb isotopic evidence for the origin of the early Proterozoic Svecokarelian crust in Finland. Geol. Surv. Finl. Bull., 337:48 pp.

Y. KAHKONENETAL.

Huhma, H., 1987. Provenance of early Proterozoic and Archaean metasediments in Finland: a Sm-Nd isotopic study. Precambrian Res., 35: 127-143. Jiiger, E., 1979. Introduction to Geochronology. In: E. J~ger and J.C. Hunziker (Editors), Lectures in Isotope Geology, Springer-Verlag, Berlin, pp. 1-12. Kiihk5nen, Y., 1987. Geochemistry and tectonomagmatic affinities of the metavolcanic rocks of the early Proterozoic Tampere Schist Belt, southern Finland. Precambrian Res., 35: 295-311. K~hktinen, Y., 1989. Geochemistry and petrology of the metavolcanic rocks of the early Proterozoic Tampere Schist Belt, southern Finland. Geol. Surv. Finl. Bull., 345:104 pp. Koistinen, T.J., 1981. Structural evolution of an early Proterozoic strata-bound Cu-Co-Zn deposit, Outokumpu, Finland. Trans. R. Soc. Edinburgh, Earth Sci., 72:115158. Kontinen, A., 1987. An early Proterozoic ophiolite - - the Jormua mafic-ultramafic complex, northeastern Finland. Precambrian Res., 35: 313-341. Korsman, K., HSltt~, P., Hautala, T. and Wasenius, P., 1984. Metamorphism as an indicator of evolution and structure of the crust in eastern Finland. Geol. Surv. Finl. Bull., 328:40 pp. Kouvo, 0. and Tilton, G.R., 1966. Mineral ages from the Finnish Precambrian. J. Geol., 74: 421-442. Krogh, T.E., 1973. A low-contaminationmethod for hydrothermal decomposition of zircon and extraction of U and Pb for isotopic age determination. Geochim. Cosmochim. Acta, 37: 485-494. Krogh, T.E., McNutt, R.H. and Davis, G.L., 1982. Two high precision U-Pb zircon ages for the Sudbury Nickel Irruptive. Can. J. Earth Sci., 19: 723-728. Lahti, S., 1981. On the granitic pegmatites of the Er~ij~irvi area in Orivesi, southern Finland. Geol. Surv. Finl. Bull., 314:82 pp. Lahti, S., Kiirkkiiinen, N., Vaasjoki, M. and Kousa, J., in press. Geochemical and tectonomagmatic affinities of the early Proterozoic metavolcanic rocks in southern Pohjanmaa, western Finland. Geol. Surv. Finl. Spec. Pap. Latvalahti, U., 1979. Cu-Zn-Pb ores in the Aijala-Orijiirvi area, Southwest Finland. Econ. Geol., 74: 1035-1059. Ludwig, K.R., 1980. Calculation of uncertainties of U-Pb isotope data. Earth Planet. Sci. Lett., 46: 212-220. Miikel~i, K., 1980. Geochemistry and origin of Haveri and Kiipu, Proterozoic strata-bound volcanogenic goldcopper and zinc mineralizations from southwestern Finland. Geol. Surv. Finl. Bull., 310:79 pp. Matisto, A., 1968. Die Meta-Arkose von Mauri bei Tampere. Bull. Comm. Geol. Finl., 235:21 pp. Matisto, A., 1977. Geological map of Finland 1:100 000, sheet 2123 Tampere. Explanation to the map of preQuaternary rocks. Geol. Surv. Finl., 50 pp. (in Finnish, with an English summary). Nironen, M., 1989. Tampere Schist Belt: structural style

EARLYPROTEROZOICVOLCANICANDPLUTONICROCKS within an early Proterozoic volcanic arc system in southern Finland. Precambrian Res., 43: 23-40. Nurmi, P., Front, K., Lampio, E. and Nironen, M., 1984. Svecokarelian porphyry-type molybdenum and copper occurrences in southern Finland: their granitoid host rocks and lithogeochemical exploration. Geol. Surv. Finl. Rep. Invest., 7:88 pp. (in Finnish, with an English summary). Ojakangas, R.W., 1986. An Early Proterozoic metagraywacke-slate turbidite sequence: the Tampere schist belt, southwestern Finland. Bull. Geol. Soc. Finl., 58: 241261. Page, R.W., 1978. Response of U-Pb zircon and Rb-Sr total-rock and mineral systems to low-grade regional metamorphism in Proterozoic igneous rocks, Mount Isa, Australia. J. Geol. Soc. Aust., 25: 141-164. Park, A., 1985. Accretion tectonism in the Proterozoic Svecokarelides of the Baltic Shield. Geology, 13: 725-729. Patchett, P.J. and Bridgwater, D., 1984. Origin of continental crust of 1.9-1.7 Ga age defined by Nd isotopes in the Ketilidian terrain of South Greenland. Contrih. Mineral. Petrol., 87: 311-318. Patchett, J. and Kouvo, O., 1986. Origin of continental crust of 1.9-1.7 Ga age: Nd isotopes and U - P b zircon ages in the Svecokarelian terrain of South Finland. Contrib. Mineral. Petrol., 92: 1-12. Patchett, P.J., Kouvo, 0., Hedge, C.E. and Tatsumoto, M., 1981. Evolution of continental crust and mantle heterogeneity: evidence from Hf isotopes. Contrib. Mineral. Petrol., 78: 279-297. Patchett, P.J., Todt, W. and Gorbatschev, R., 1987. Origin of continental crust of 1.9-1.7 Ga age: Nd isotopes in the Svecofennian orogenic terrains of Sweden. Precambrian Res., 35: 145-160. Pharaoh, T.C. and Pearce, J.A., 1984. Geochemical evidence for the tectonic setting of early Proterozoic metavolcanic sequences in Lapland. Precambrian Res., 25: 283-308. Roddick, J.C. and Compston, W., 1977. Strontium isotopic equilibration: A solution to a paradox. Earth Planet. Sci. Lett., 34: 238-246. Sakko, M., 1971. Varhais-karjalaisten metadiabaasien radiometrisi~i zirkoni-ikiti. Geologi, 23:117-118 (in Finnish ). Simonen, A., 1980. The Precambrian in Finland. Geol. Surv. Finl. Bull., 304:58 pp. SkiSld, T., 1987. Aspects of the Proterozoic geochronology

43 of northern Sweden. Precambrian Res., 35: 161-167. SkiSld, T. and Cliff, R.A., 1984. Sm-Nd and U - P b dating of early Proterozoic mafic-felsic volcanism in northernmost Sweden. Precambrian Res., 26: 1-13. Thorpe, R.S., 1982. Andesites, Orogenic Andesites and Related Rocks. Wiley and Sons, Chichester, 724 pp. Vaasjoki, M. and Huhma, H., 1987. Lead isotopic results from metabasalts of the Haveri formation, southern Finland: an indication of early Proterozoic mantle derivation. Terra Cognita, 7: 159. Vaasjoki, M. and Sakko, M., 1988. The evolution of the Raahe-Laatokka zone in Finland: isotopic constraints. Geol. Surv. Finl. Bull., 343: 7-32. Van Breemen, O. and Dallmeyer, R.D., 1984. The scale of Sr isotopic diffusion during post-metamorphic cooling of gneisses in the Inner Piedmont of Georgia, southern Appalachians. Earth Planet. Sci. Lett., 68: 141-150. Van Schmus, W.R., Bickford, M.E., Lewry, J.F. and Macdonald, R., 1987. U-Pb geochronology in the TransHudson orogen, northern Saskatchewan, Canada. Can. J. Earth Sci., 24: 407-421. Vidal, Ph., 1980. L'~volution polyorogenique du Massif Armoricain: apport de la ggochronologie et de la g6ochimie isotopique du strontium. Socidtd G~ologique et Min~ralogique de Bretagne, M~moire No. 21,162 pp. Welin, E., 1987. The depositional evolution of the Svecofennian supracrustal sequence in Finland and Sweden. Precambrian Res., 35: 95-113. Welin, E., K~ihr, A.-M. and Lundeg~rdh, P.H., 1980. RbSr isotope systematics at amphibolite facies conditions, Uppsala region, eastern Sweden. Precambrian Res., 13: 87-101. Welin, E., Vaasjoki, M. and Suominen, V., 1983. Age differences between Rb-Sr whole rock and U - P b zircon ages of syn- and postorogenic Svecokarelian granitoids in Sottunga, SW Finland. Lithos, 16: 297-305. Wetherill, G.W., Kouvo, 0., Tilton, G.R. and Gast, P.W., 1962. Age measurements on rocks from the Finnish Precambrian. J. Geol., 70: 74-88. Wilson, W.R., Hamilton, P.J., Fallick, A.E., Aftalion, M. and Michard, A., 1985. Granites and early Proterozoic crustal evolution in Sweden: evidence from Sm-Nd, UPb and 0 isotope systematics. Earth Planet. Sci. Lett., 72: 376-388. York, D., 1969. Least squares fitting of a straight line with correlated errors. Earth Planet. Sci. Lett., 5: 320-324.