K-Ar AGE DETERMINATION ON BIOTITE AND MUSCOVITE IN THE PIHTIPUDAS- AND JOROINEN AREAS, EASTERN

by UELI HAUDENSCHILD

Haudenschild, U., 1988. K-Ar age determination on biotite and muscovite in the Pihtipudas-Iisalmi and Joroinen-Sulkava areas, Eastern Finland. Ge%gica/ Survey of Fin/and, Bulletin 343, 33-50. 6 figures and two appendices. K-Ar ages were determined on biotite and white mica in the two areas of Pihtipudas-lisalmi and Joroinen-Sulkava, along profiles of a length of 70 km each. A gradual decrease in the ages was found in the area of Pihtipudas-Iisalmi. This trend of decreasing ages is twice interrupted by a jump in the age, corresponding to a jump in the metamorphic facies, e.g. a fault system between tectonically moved blocks. Two other jumps in the metamorphic facies cannot be seen in the age pattern. The blocks must have moved at temperatures above the biotite blocking tempera­ ture. The gradual decrease in the measured ages is consistent with the idea of a surface tilted towards the west, where two blocks have been tilted together. In the area of Joroinen-Sulkava the age pattern (1750-1600 Ma) decreases from north to south. The decrease in the measured ages corresponds to tilting of surface, which is also reflected in the progressive increase in the grade of metamor­ phism. The tilting occurred at temperatures below the 300°C blocking temperature of the biotite. A jump in the age pattern can be seen in the north of the area, stopping the gradual trend of the age values. The northern part of the area must have been uplifted alm ost horizontally as an independent block.

Keywords: absolute age, K-Ar, biotite, muscovite, uplifts, block structures, Prote­ rozoic, Finland.

U. Haudenschild, Abteilung für lsotopenge%gie der Universität Bem Er/achstr. 9a, eH-300 Berne 12, Switzer/and.

INTRODUCTION

With the cooperation of the Geological Survey morphic uplift history of the areas using biotite of Finland, sampies for age determination were and white mica K-Ar dating. taken from two profiles in the Pihtipudas-lisal­ Intensive investigative work has been carried mi and Joroinen-Sulkava areas (Fig. 1). The in­ out in the two areas for many years. Both areas tention was to date and compare the post meta- are the same in lithology, consisting mainly of

3 34 Geological Survey of Finland. Bulletin 343

Presvecokarelidic: a Granulite g Schist and Paragneiss _ Orthogneiss Svecokarelidic: ~ Karelidie schist belt Im Svecofennidic schist belt [2J Granite D Tonalit e and Granodiorite Postsvecokarelidian: ~ Rapakivi granites m Jotnian sediments ~ Caledonidic schists

LADOGA

o 150km

Fig. I: Main structural units of Finland (after A. Simonen). The location of the studied areas are marked by black frames. Geological Survey of Finland, Bulletin 343 35 metapeiitic-metavolcanic interlayering sequences bearing monzonitic to granitic intrusion in the (Korsman et 01. 1984), but differ in tectonic middle of the block overprints the metapelitic history. gneisses under granulite facies conditions. The The Pihtipudas-lisalmi area is situated in the Lampaanjärvi block metamorphosed under Raahe- Ladoga (Marttila 1977) or Savo- Lado­ amphibolite facies conditions in the stability field ga (Salop 1977) schist belt, which crosses Finland of muscovite. from northwest to southeast, marking the suture The contacts between the blocks rarely crop zone between the Svecokarelidic metasediments out. Mylonites have been found between the Vaa­ and the underlayering basement gneisses. In the raslahti and the Lampaanjärvi blocks. The con­ west, the area is in contact with the Intrusive tact between the Lampaanjärvi block and the Complex of Central Finland, wh ich is mainly basement is marked by the schist zone of Viere­ built up of granodioritic,to tonalitic, synoroge­ mä, which consists mainly of garnet-mica schists nic rocks (Aho 1979). The easternmost part of and conglomerates (Marttila 1977). the area is in contact with the Archaean basement The area of Joroinen-Sulkava is crossed from gneisses. northwest to southeast by the Kolkonjärvi Between Pihtipudas and Iisalmi five meta­ lineament (Fig. 6). Between , south of morphic blocks can be distinguished that differ Joroinen, and Sulkava the grade of metamor­ in metamorphic overprint (Korsman et 01. 1984). phism increases from north to south from the From west to east the blocks are: Pihtipudas, conditions of amphibolite to granulite facies. In­ Korppinen, , Vaaraslahti al1d Lampaan­ tense migmatite formation is typical, especially järvi (Fig. 2). The small schist zone of Vieremä in the zone of Sulkava, being attributed to the is situated between the Lampaanjärvi block and existence of a thermal dome southeast of Sulka­ the basement. va (Korsman 1977). The southern andalusite­ Metamorphic conditions are lowest in the Pih­ sillimanite iso grade between the zones of Ranta­ tipudas block, where metapelitic rocks are meta­ salmi and Tuusmäki crosses the Kolkonjärvi morphically transformed to andalusite-mica lineament (Korsman et 01. 1984), whilst the schists. The blocks of Pihtipudas and Korppinen northern one turns northwards in the western are separated by the Kinturi fault, a lineament part of the zone to run parallel to it. Amphibolite that crosses the area from northwest to south­ facies occurs once more north of the andalusite­ east. At the contact of the Pihtipudas and Korp­ mica schist zone, in the zone of Joroinen. The pinen blocks, the metamorphic conditions change metamorphie conditions are the same throughout from the stability field of andalusite to that of zone, and therefore block movement is only sillimanite. In the block of Korppinen, musco­ possible in the north, between Rantasalmi and vite was probably not stable at the peak of meta­ Joroinen. morphism but formed during diaphthoresis. The The area between Rantasalmi and Sulkava is block of Pielavesi metamorphosed under granu­ tilted to the north by 1-30 (Korsman 1977) and lite facies conditions (high grade metamorphism is responsible for progressive metamorphism. under dry conditions). Lithologically the block K-Ar age determinations (Korsman et 01. 1984) can be distinguished from its surrounding blocks on biotites show a gradual decrease in age from by the absence of metapelitic rocks and the abun­ north to south, once again possibly because of dance of gabbroic intrusions. A hypersthene- tilting of the surface. 36 Geological Survey of Finland, Bulletin 343

METHODS

For the age determinations, fresh sam pies were Potassium was measured by flame photo­ taken from road cuts or, with aid of a core driller, metry. The standard error corresponds ± 1070. from icepolished surfaces. Sampies taken by Argon was measured by isotope dilution (Kirs­ hand weighted between 10 and 30 kg, drill co res ten 1966) on a VG MM1200 mass spectrometer 0.5 to 2 kg. All sampies were crushed and milled using a 99.9997070 pure 38Ar spike (Schumacher at the department of the Geological Survey of 1975). For the outgassing, the sampies were Finland in Rovanniemi with preliminary sieving packed in aluminium foil and melted using a at the department in into grain sizes of high-frequency generator in a glass high-vacuum < 30,30-120 and > 120 mesh. Further splitting extraction line (Flisch 1986). The extracted gas into 30-60,60-80 and 80-l20 mesh was car­ was purified on Ti and Zr/ AI getters and in­ ried out in our laboratory in Berne. troduced to the mass spectrometer by a direct Biotite and muscovite were concentrated using connection between the line and the mass spectro­ a magnetic separator, then milled for five minutes meter. Biotite B-4B and LP-6 (Flisch 1982) were in pure ethanol in an agate mortar and sieved; used as standards. The measured difference from if necessary the biotites were separated magneti­ the standard value used in spike calibration was cally once more. All sampies were dissolved in < 1070. a mixture of perchloric and hydrofluoric acids, The sam pie ages were calculated using the evaporated and redissolved in hydrochloric acid. constants of Steiger and Jäger (1977).

PIHTIPUDAS-IISALMI

K-Ar ages on biotite

Nineteen sampies for biotite and muscovite age The two grain sizes from the sam pie at Pihtipu­ determinations were collected between the village das (KA W 2560) gave ages of 1759 Ma and 1750 of Pihtipudas and the lake Lampaanjärvi on a Ma. The easternmost sam pie of the block of profile of 70 km (for locality and sam pie descrip­ Korppinen (KA W 2568) gave ages of 1642 Ma tion, see appendices 1 and 2). The profile cor­ and 1646 Ma. A jump in the age of about 130- res ponds approximately to abscissa 7030 of the 150 Ma corresponds to the abrupt change in the Finnish coordinate net and crosses the five tec­ metamorphic facies between the blocks of Korp­ tonic blocks of Pihtipudas, Korppinen, Pielave­ pinen and Pielavesi. The ages of the western part si, Vaaraslahti and Lampaanjärvi (Fig. 2). Six of the Pielavesi block are between 1770 Ma and other sam pies were collected north of the profile 1790 Ma. A decrease in the age from 1790 Ma line. A further six sampies from the Vaaraslahti to 1625 Ma can be observed again between the had been analysed previously (Haudenschild western part of the Pielavesi block and the lake 1985). Lampaanjärvi (Fig. 2; abscissa 7031.50). The Along the first section of the profile, from west locations of the sam pies north and south of the to east, e. g. from Pihtipudas to the contact profile allow the observation of decreasing ages between Korppinen and Pielavesi blocks (Fig. 2; from north to south. The highest ages were abscissa 7030) the biotite ages decrease gradually. measured in the middle of the Pielavesi block, Geological Survey of Finland, Bulletin 343 37

0 Basement []J Muscov it e 1+++ I Gr anitoid [TI Andalusit e 111 Gabbro B Isograd El--- Gneissie tonalit e ~... Hypersthene- granite ~ Epidote ~ Cordierite '.' 8 Sillimanite ',..' I ~ I Hypersthene 0 10 km I@I Garnet

Fig. 2: Metamorphic map of the Pihtipuds-lisalmi area showing location and age of sam pies (after K. Korsman). Ages in million years. Black numbers: biotite age; white numbers: white mica age; *: sampie not used for interpretation. Sampie numbers: a = KAW 2560, b = KA W 2562, c = KA W 2563, d = KA W 2564* , f = KAW 2566, g = KAW 2568 , h = KAW 2571, i = KAW 2572, j = KAW 2576, k = KAW 2574, 1 = KAW 2607 , m = KAW 2587, n = KAW 2588 , p = Haudenschild 1985, q = KA W 2592* , r = KA W 2579, 5 = KAW 2586, t = KA W 2591, u = KAW 2578, v = KAW 2584, w = KAW 2585, x = KA W 2589, Y = KA W 2590, Z = KA W 2860. A-B: profile line of Fig. 3 and 4; C-D: profile line of Fig. 5.

about 5 km west of the Vaaraslahti intrusive. ages between 1696 Ma and 1663 Ma, the northern The sam pies north of the profile were collected sampie being older than the southern. The bio­ from the area of Vieremä and from the Archaean tites of the basement gave ages of 1719 Ma and basement of Iisalmi . The schists of Vieremä give 1740 Ma.

K-Ar ages on white mica

White mica was separated in addition to biotite KA W 2566), one from each block. The third from four sampies. Two sampies were from the sampie was taken near the lake Lampaanjärvi, profile Pihtipudas-Korppinen (KA W 2559, and the fourth from the schist zone of Vieremä 38 Geological Survey of Finland, Bulletin 343

(between the Lampaanjärvi block and the base­ nen is at least twice as high as that in Pihtipu­ ment). das. Areduction of the thermogradient with time Two grain size fractions were analysed on the towards 25 °C/ km would result in an increase of sampies from Pihtipudas and Korppinen. The the uplift rate towards 0.10 mm/ a. results of both fractions of each sam pie are The white mi ca of sam pie KA W 2578 from the identical within the lo-error range. The Pihtipu­ lake Lampaanjärvi (grain size 60-100 mesh) das sampie (KA W 2559) gives ages of 1744 Ma gave an age of 1650 Ma, whilst a second frac­ and 1757 Ma and, that of Korppinen (KA W tion (grain size 130- 200 mesh) gave 1611 Ma; 5266) 1690 Ma and 1695 Ma. the biotite ages lie between these values. Two ge­ The difference between the biotite and white nerations of white mica can be readily identified mi ca ages in Pihtipudas is 54 Ma (difference by microscopic examination of thin sections between the mean values of both fractions of (Haudenschild, 1985). The first generation is in­ biotite and white mica), and in Korppinen from tergrown with biotite, oriented in the same direc­ 9 to 19 Ma. tion, and is folded, ductile or even broken. The The two biotite ages of the Korppinen sampIe size of the flaky grains corresponds to that of (KA W 2566) differ by 20 Ma. The high er age biotite. The second generation is much sm aller (1694 Ma) is nearly identical to the age of white in grain size, the form is that of a needle. The mica. In addition it lies above the line of uplift grains do not show any deformation. The higher (Fig. 3), whilst the lower age lies on the line. Ac­ age corresponds to the cooling to 350°C; the cordingly the age difference of 9 Ma has to be lower age must be influenced by the smaller grain considered a minimum. size fraction. Two explanations are possible for According to the theory of Jäger (1979) used the age of 1611 Ma: for the interpretation of data in the present - Small white mica crystals formed below paper, the blocking temperatures for retention of 350°C. The age would therefore correspond to radiogenic argon in biotite and muscovite are a mixture of the larger and the small white micas. 300°C and 350°C, respectively. By dividing the - The sm aller fraction lost Ar by a late mo­ 50°C temperature difference by the age differ­ vement. ence between the two minerals, a cooling rate in A definitive answer should be possible by Ma can be calculated. lf we suppose, that the analysing Rb-Sr ages on both fractions. thermogradient was about 50°C/ km (Korsman The white mica of Vieremä gives an age of et al. 1984) an uplift rate can be calculated for 1747 Ma corresponding to the cooling to 350°C both sampies: a value of 0.05 mm/ a in Korppi- at the boundary of the basement.

Discussion

Uplift rates were calculated by treating the KA W 2592 have not been used for the interpreta­ results as absolute numbers. Whenever two grain tion. Sampie KA W 2564 was slightly weathered. size fractions of the same sampie were measured, Chloritization seen in the microscope might be the calculation was based on the mean of the two the reason for too young ages. Two low ages of results, provided their difference was not greater sampIe KA W 2592 are not yet understood. The than lo-error of the measurement. In those cases sampIe is taken from a fresh granitic gneiss, and where the difference was larger, the results are Rb-Sr analyses hopefully will help interpretation discussed in detail. The 10 error of all analyses of the results. is about 12-15 Ma. Sampies KA W 2564 and The calculations were made, allowing a numer- Geological Survey of Finland, Bulletin 343 39

3430 3440 Kinturi Fault 3460 3470 1751Ma

Age in Ma 1735

1715 KAW 2563

1695 KAW 2559

1675

T K-Ar ages and me an value 1655 o Biotite age KAW 2568 1635 • Muscovite age

Fig. 3: Age versus location diagram for the profile between Pihtipudas and the contact of Korppinen and Pielavesi block. The abscissa corresponds to the 7030 abscissa of the national Finnish coordinate net (see fig. 2). The circles give the mean value of the ages measured on two different grain size fractions of each sampie. The straight line marks the spread between the measured values.

N (') Ol CD Q) CD CD '" CD CD '"N '"N '"N '"N '"N Temperature ~ ~ ~ oe < < < ~ ~ ~ 300-t~~~~~ __+h~7~1~2~M~a __~r- __ ~ __ -Y ______L-~16~7~4~~_'a~ ____L- ____~1~6~4~4~M~a l~l Abscissa. 7030

350

Fig. 4: Temperature versus location diagram for the profile between Pihtipudas and the contact of Korppinen and Pielavesi block. A-B: profile line see fig. 2.

ical comparison of the uplift histories of the speed of the different components will remain the different blocks. The numerical values calculated same. The same is, of course also valid for the below depend on the accuracy of the age deter­ uplift values calculated for Lampaanjärvi and the mination and the thermogradient used. Con­ area of Joroinen-Sulkava. Therefore the results sidering the I (J analytical error, the uplift rate of mentioned in the discussion are intended to show the Pihtipudas sampie may vary between the two possible trends and are- not rigid numericaI extremes of 0.01 mm/ a and 0.04 mm/ a, the one values. of Korppinen between 0.02 mm/ a and >0.05 A thermogradient of 50°C/km was used for mm/ a, though the relation between the uplift the calculations, allowing an easy comparison of 40 Geological Survey of Finland, Bulletin 343

LO co Ol

"0 Ol J::: Age in Ma t- u co LO cn 0 (\J c LO co (J) t- Ol LO «) Ol "0 co LO (\J co LO co Ol co (\J 3: :::J LO LO (\J t- « co 00 (\J 3: (\J LO ~ I ,,10 "0 « (\J ~ J::: r u I cn I C 00 (J) ,,'\ "0 / ~ I

Ordinate 480

Fig . 5: Biotite age versus location diagram for the profile between the contact Korppinen-Pielavesi block and the lake Lam­ paanjärvi. The black line marks the age distribution of a west-east profile, showing the jump in the biotite age between Vaaraslahti and Lampaanjärvi block. C-D: profile line see fig. 2.

the calculated uplift rates, realising, that the The mean values of the muscovite ages are effective thermogradient probably has been 1751 Ma(KAW2559)and 1693 Ma(KAW2566). somewhat lower. Therefore the term "minimum The locations of the 1751 Ma and 1693 Ma bio­ uplift rate" is used. tite ages can be found using the curve discussed Figure 3 shows the biotite ages of profile above (Fig. 3). Each lies near real measured between Pihtipudas and the border between the values (KA W 2560 and KA W 2559). The connec­ Korppinen and Pielavesi blocks. As mentioned tion between the locations of the biotite and mus­ above, the ages decrease from west to east. If the covite sampies of the same age gives a 1i ne mark­ rate of decrease were gradual (corresponding to ing the temperatures at 1751 Ma and 1693 Ma. gradual uplift), the points of the distance/ age The uplift between 1751 Ma and the biotite age diagram (Fig. 3) would lie in a straight line. This of each sam pie can be calculated with the aid of is the case only east of the third sam pie (KA W the estimated paleothermogradient. Subtraction 2563). West of the sampie the line gradually be­ of the uplift of the oldest sampie (uplift between comes steeper, and the age difference with 1751 Ma and 1736 Ma) from the next youngest distance smaller. The simplest model to explain one (1712 Ma), permits the total upli ft between the resulting curve is that of accelerated uplift 1751 Ma and 1697 Ma to be divided into seg­ movement between 1750 Ma and 1700 Ma, even­ ments. (A graphical picture of the calculation is ing out at about 1700 Ma. given in Fig. 4). With the maximum thermo- Geological Survey of Finland, Bulletin 343 41 gradient of 50°C/ km (see above) the uplift rates at about 1660-1680 Ma, with consequent fault­ of the segments are: ing at the margins of the older blocks and the 1751-1736 Ma: 9.33 m/ Ma corresponding to resulting younger ages, as seen in the age pattern about 0.010 mm/ a (Fig. 2). 1736-1712 Ma: 16.6 m/ Ma corresponding to A jump in the age of at least 100 Ma can be about 0.015 mm/ a seen between Vaaraslahti and Lampaanjärvi, 1712-1697 Ma: 30.0 m/ Ma corresponding to where the blocks of Pielavesi and Vaaraslahti about 0.030 mm/ a have passed the 300°C isotherm together. The jump in the age does not necessarily have to coin­ The uplift rate between 1693 Ma and 1674 Ma eide with the isograd, which parts the two blocks. can be caIculated using the biotite and musco­ Tilting has eventually caused astronger uplift to vite ages of sam pie KA W 2566. The result is the southeast relative to the northwest. about 0.05 mm/ a. After 1700 Ma the uplift rate An uplift rate for the temperature interval should not change any more, and so it has to be 350°-300°C can only be caIculated for the about 0.05 mm/ a. sampie KA W 2578 from Lampaanjärvi. The dif­ The caIculation above cannot be used for the ference between biotite and muscovite ages is 31 profile between Pielavesi and Lampaanjärvi. Ma. The minimum uplift rate (thermogradient Around the Vaaraslahti intrusive the age values 50°C/ km) would be 0.03 mm/ a, corresponding differ between 1750 Ma and 1790 Ma, the highest in order with the values caIculated for the blocks values Iying about 5 km west of the Vaaraslahti of Pihtipudas and Korppinen. intrusive. From Vaaraslahti eastwards, the ag es The Vieremä zone corresponds to the transi­ decrease. The ages also decrease southwards, tion between the Karelides and the Archaean though younger ages can be observed north of basement, the zone between the orogeny and the the profile line. Figure 5 shows the ages in con­ stable craton. The biotite age of Vieremä nection with their localities. The age pattern for (1660-1690 Ma) corresponds to that of Lam­ the blocks of Pielavesi and Vaaraslahti may be paanjärvi. A direct comparison, however, is not explained by the following mechanism: possible for there is a north-south distance of - Uplift of the Pielavesi and Vaaraslahti about 28 km between the two localities. The dif­ blocks followed by joint dipping northwestwards ference between the biotite and muscovite ages explains the gradual decrease in age eastwards allows the caIculation of an uplift rate. With a and southwards. A possible explanation of the thermogradient of 50°C/ km, this would be about low ages north of the profile line is the existence 0.02 mm/ a. The effective thermogradient may of a fault or faultsystem north of the Vaaraslah­ have been lower if the temperature had been buf­ ti intrusive. The blocks moved upwards through fered by the stable craton. The biotite ages of the the 300°C isograd at about 1770-1790 Ma and basement (1720-1740 Ma) show that the border thereafter the middle of the blocks remained of the Archaean craton has also experienced tem­ stable. The adjacent blocks moved upwards later peratures above 300°C.

Summary

The blocks of Pihtipudas and Korppinen about 1640 Ma the whole area was tilted towards passed the 300°C isotherms together. Between the west, the tilting cannot be dated. (North­ 1750 Ma and 1700 Ma the uplift rate in the area south tilting cannot be seen with existing data). increased, after 1700 Ma it was steady. After Between the Korppinen and Pielavesi blocks 42 Geological Survey of Finland, Bulletin 343 there is a jump in the biotite age of about 150 Between the Vaaraslahti and the Lampaanjärvi Ma. blocks there is a jump in the biotite age of at least The Pielavesi and Vaaraslahti blocks were 100 Ma. The Lampaanjärvi block also shows a raised through the 300°C isotherm together. The dip towards northwest. present data can be interpreted by a joint dipping The uplift rate on the borders of the lake Lam­ of the blocks towards the northwest, explaining paanjärvi is of the same order as in the blocks the da ta south of the profile line. A fault north of Pihtipudas and Korppinen; in the zone of Vie­ of the Vaaraslahti intrusive could be the reason remä, the uplift rate probably decreases. of the younger ag es measured north of the in­ The basement of Iisalmi reached temperatures trusive. of above 300°C during the Karelian orogeny.

JOROINEN-SULKA VA

K-Ar ages on biotite and muscovite

Biotite ages have been measured on seventeen Ma) in the area of the thermal dome of Sulkava sampies in the area of Joroinen-Rantasalmi­ (Korsman, 1977). Sulkava (Fig. 1). Muscovite is stable only in the North of the Kolkonjärvi lineament, in the zone of Rantasalmi (Fig. 6). To the north and zone of Joroinen-Tiemassaari, the age values south the degree of metamorphism exceeds the drop to 1705-1717 Ma. An increase can be seen stability field of muscovite, wh ich reacts to K­ towards the northeast. Age differences of 20-30 feldspar and sillimanite. K-Ar were measured on Ma can be seen between sampies taken from as biotite as weil as on muscovite on one sam pie little as 1000 m apart. They represent a realistic from the Rantasalmi zone. assessment of error, 30 Ma corresponding to the Korsman et al. (1984) have published eight 20 analytical uncertainty of the measurements. biotite ages from the Rantasalmi-Sulkava area. Thus there is a gradual progression in ages Two of these sam pies were sieved to different without a break from north to south. A jump in grain size fractions and, for purposes of com­ the age could be possible between Joroinen and parison, were also analysed in Berne. Rantasalmi, but it cannot be established on the The seventeen measured sampies lie on a north­ basis of the present data. south profile 60 km long and 30 km wide which Biotite and muscovite were analysed from the passes through the zones of Joroinen-Tiemas­ sam pie (KA W 2826) in the Rantasalmi zone. The saari, Rantasalmi, Tuusmäki, and Sulka­ biotite age is 1723 Ma and the muscovite age 1772 va, crossing the Kolkonjärvi lineament (Fig. 6). Ma. The thermogradient has been evaluated by Over the zones of Rantasalmi, Tuusmäki, Juva Korsman et al. (1984) for Tuusmäki at 55 °C/ km and Sulkava, e. g. from the andalusite-sillimanite and for Rantasalmi at 45 °C/ km. With a mean isograd in the north to the zone of migmatism thermogradient of 50°C/ km the age difference in the south biotite ages decrease grad ually. The between biotite and muscovite corresponds to an gradual decrease can also be observed from west uplift rate of 0.02 mm/ a. This uplift rate can ob­ to east. The highest age (KA W 2601: 1743 Ma) viously be compared with the rates calculated for lies directly on the border of the andalusite­ the area of Pihtipudas-lisalmi. sillimanite isograd; the lowest (KA W 2825: 1604 In the zones of Tuusmäki, Juva and Sulkava Geologieal Survey of Finland, Bulletin 343 43

the highest uplifL Dipping of the area caused in­ clination of the surface of I-3° (Korsman et al. 1984) towards north-northeast. The maximum temperature of metamorphism was 645 °C in the zone of Tuusmäki and 750°C in Sulkava (Kors­ man et al. op. eiL). On the basis of the thermo­ gradient of 45-55°C/ km the differenee in the degree of metamorphism eorresponds to a dif­ ferenee in depth of 2000 m. With these dates, an uplift rate of 0.004 mm/ a ean be ealeulated for the zone of Sulkava relative to that of Tuusmä• ki . The true uplift will be influeneed by two other parameters: the inclination of the area by temperatures higher than 300°C and the uplift of Tuusmäki zone during dipping. The dip in the area before the passage through the 300°C isotherm reduees the rate of uplift for the ealculated time interval, and thus the eal­ eulated uplift rate has to be eonsidered a maxi­ o mum. Even uplift during dipping increases the uplift rate of the area, the ealculated value is a minimum. The eontinuity of the prograde metamorphism on the one hand, and the weak inelination and regular distribution of the age values on the other are indications of dipping at low temperatures. D Diorit e and Gabbro Eibl Andalusi t e/Mu scovile Possibly dipping occurred much later than the W Tonalite EBl Sillimanitel K-Feldspar uplift through the 300°C isograd. With the D Granil e [Tl Bi ot it e/Cordieril e method used here, it is not possible to evaluate B Isograd ~ Garnet/ Hypersthene the time of dipping. Fig. 6: Metamorphie map of the loroinen-Sulkava area North of the zone of Rantasalmi the trend in showing age and loeation of the sampies (after K. Kors­ biotite ages ehanges and the ages increase towards man). Ages in million years. Blaek numbers: biotite age; northeast. The zone of Joroinen-Tiemassaari white numbers: white miea age; *: age not used for inter­ pretation. Sampie numbers: a = KA W 2596, b = KA W must therefore have had aseparate uplift histo­ 2599, e = KAW 2595, d = KA W 2594, e = KAW 2598, ry. The lowest ages (KAW 2598: 1705-1717 Ma) f = KAW 2600* , g = KAW 2826, h = KAW 2601, i = are directly on the contaet with the Rantasalmi KAW 2822, j = KAW 2823 , k = KAW 2606, 1 = KAW 2605 , m = KAW2824*,n = KAW2825, o = KAW2604, zone. Uplift in the Joroinen zone was therefore p = KAW 2602, q = KAW 2603 . greatest on this contaeL The distribution in age values is puzzling. Over distances of only 2000 m the ages show differenees of 30 Ma. the biotite ages deerease from north to south and The age value varies by order of 20-30 Ma from west to east. The lowest values, about 1600 between the sam pie KA W 2594 and KA W 2595. Ma, ean be found around the thermal dome of Considering the 2o-error and the lack of a clear Sulkava (Fig. 6). Analogous to the highest degree tendency in the age distribution, one ean but of metamorphism, the lowest ages eorrespond to conclude that only minimal dipping is possible 44 Geological Survey of Finland, Bulletin 343 for the Joroinen area, and that uplift was alm ost might have lost argon as weil as potassium dur­ horizontal in the area. ing weathering. The age of sampie KA W 2824 Sampies KA W 2600 and KA W 2824 have not is too high. The sampie is not completely free of been used for the interpretation. Sam pie KA W gamet wh ich may have brought so me excess 2600 shows a very low potassium content, it argon (gamet analysis at the end of appendix 2).

Summary

Decreasing age values from north to south and relative to that of Rantasalmi corresponds to an from west to east correspond to a dip towards uplift rate of 0.04 mm/ a. north to northwest at temperatures below 300°C. The Joroinen zone has aseparate uplift histo­ Uplift was strongest in the area of thermal ry. Uplift there seems to have been stronger in dome of Sulkava. the southwest than in the northeast, possibly Minimum uplift, in the Rantasalmi zone dur­ resulting in a slight dip. The dip could be seen ing the time interval 1770-1720 Ma can be calcu­ only in the two extremes of the five measured age lated to 0.02 mm/ a. Dipping in the Sulkava zone values. If it exist, it is very weak .

Comparison with existing ages

Eight biotite and two hornblende ages have Both biotite concentrates were milled in been published by Korsman et al. (1984) from the ethanol in an agate mill for 15 minutes. The area of Rantasalmi. The trend in biotite ages in the biotite was then further purified by removing any Rantasalmi, Tuusmäki, Sulkava and Juva zones inclusions. Hence the potassium contents of the corresponds to those reported above, that is, the milled fractions are higher. The effect on age is ages decrease from north to south. The absolute small as long as the inclusion do not contain ages of each zone, as far as comparable, are higher argon (overpressure). The ages of both fractions than those presented here. For this reason, two of sampie KA W 2819 are identical and, those of of the published age sampies (l8KAK73/ KA W sam pie KA W 2821 differ by about 1070. The age 2819 and 128KAK73/ KAW2821) were sieved, differences between the sieved and unsieved milled and reanalysed: one sampie in two frac­ sam pies are smaller than the lo-error. tions, and one sam pie in four fractions. The The systematic error between the published measured values of the untreated biotite con­ and the newly calculated ages is therefore made centrations are, as expected, younger than the up of two components: firstly the potassium de­ published ones. Because of a difference in the terminations of the two laboratories differ by potassium value determined (7.86070 and 7.54070 about 1070, secondly the published 40Ar volumes published, 7.94070 and 7.64070 measured) the are about 2070 higher than the measured ones. argon analyses were calculated with the published This is attributed to the error in the spike volume potassium values. In this way, ages of 1700 Ma or, more likely, to the differences between the and 1706 Ma could be calculated, corresponding argon/ air ratio measured and that used for the to 1721 Maand 1716Mapublished. Theagedif­ calculation. The ages were calculated with an ference is smaller than the 2o-error of the argon isotopic ratio for air of 303.3. The ratios measurement: an error of 50 Ma is given for the measured during analysis were between 301.3 and published ages. 304.9. Geological Survey of Finland, Bulletin 343 45

Two other biotite ages have also been reported hornblende, absolute values of uplift must be for the Vaaraslahti intrusive (Wetherill et al. treated with caution. 1962). The age values, 1760 Ma for Rb-Sr and In the Pihtipudas-Iisalmi area V-Pb ages 1730 Ma for K-Ar had been caIculated with old have been determined on the intrusives of Pihti­ constants. RecaIculated to constants of Steiger pudas (Aho 1979) Kotaj ärvi, V aaraslah ti (Salli and Jäger (1977) they give ages of 1723 Ma for 1983) and Tuli-Toiviainen (Marttila 1981). Three Rb-Sr and 1711 Ma for K-Ar. The published er­ other analyses were made in the Vaaraslahti­ ror is 70 Ma. For this reason great care has to Lampaanjärvi area by O. Kouvo (Haudenschild be taken when comparing the age values with the 1985). values measured on the Vaaraslahti block. The zircons of different intrusives give ages Besides the mica ages, V-Pb ages are available about 1880 Ma, the monazite of Pihtipudas (Aho for both areas, and two hornblende ages have op. cit.) lies concordantly at 1800 Ma. The age been published for the Rantasalmi-Sulkava area distribution in the area of Rantasalmi-Sulkava (Korsman op. cit.). is about the same. The granodiorite of Tuusmä• The hornblende analyses, both from the Tuus­ ki has a zircon age of 1888 Ma, whereas paleo­ mäki zone, give ag es of 1770 Ma and 1796 Ma. some and neo so me of migmatites of Säviönsaa• They are slightly older than the ages of the biotite ri (Sulkava) have zircon ages of 1810 Ma and of the same zone (1750 Ma). A blocking tempera­ 1833 Ma. ture for hornblende of 450-500°C is given by While the zircon ages of the intrusives are in­ Jäger and Hunziker (1979) and Mattinson (1982). terpreted as ages of synkinematic intrusion, An uplift rate of 0.1-0.2 mm/ a can be estimated zircon and monazite from the migmatite like the for a time intervai 1790-1750 Ma using a thermo­ monazite of the Pihtipudas intrusives, give the gradient as for the above caIculations. The result age of metamorphism. For the zone of Sulkava allows the concJusion to be drawn that uplift was it must have been high-grade metamorphism that more rapid during the period 1790-1750 Ma formed migmatites. The block of Pihtipudas has than during 1770-1720 Ma. However, because been overprinted by far weaker temperature con­ of the error in the hornblende ages (50 Ma) and ditions. The monazite see m to have been reset at the uncertainty in the blocking conditions of temperatures of around 500°C.

CONCLUSION

In the area of Pihtipudas-Iisalmi, blocks of There is a jump in the biotite age of about 150 different metamorphie facies developed along the Ma between the double-blocks. Another jump in same lines in the final stage of orogenie move­ the biotite ages may exist at the contact of the ments. This applies to the blocks of Pihtipudas Vaaraslahti and the Lampaanjärvi blocks. How­ and Korppinen and to those of Pielavesi and Vaa­ ever, owing to the location of the sampIes the raslahti. The blocks of Pielavesi and Vaaraslah­ numerical value of the jump, cannot be assessed. ti , at least, passed the 300°C isotherm and those The uplift rate of both double-blocks was high er of Pihtipudas and Korppinen also the 350°C iso­ in the east than in the west; uplift rates, calculated therm. Both double-blocks dip towards the west! from the biotite and muscovite ages of the same northwest. A temporal relation between uplift sampIes, are between 0.03 and 0.05 mm/ a . The and dipping cannot be established. time of the retrograde overprint (diaphthoresis) 46 Geological Survey of Finland, Bulletin 343 cannot be solved simply from the K-Ar ages cooling (500°C-350°C). presented in this study. The zone of Joroinen was not tilted with the The biotites of the Archaean base me nt of Iisal­ southern part of the area. Uplift of the northern­ mi have Svecokarelidic ages, but thirty kilometres most zone was nearly horizontal. to the southeast, Paavola (1986) measured Ar­ The following can be concluded about the chaean biotite ages. Accordingly, the basement cooling history: at the contact with the Svecokarelidic sediments - Around 1800 Ma both areas were thermally must have reached temperatures in excess of overprinted. The maximum temperatures in Sul­ 300°C. kava must have been at about 700°C, and in Pih­ In the area of Joroinen-Sulkava, the condi­ tipudas at about 500°C. The following uplift tions of the progressive trend in metamorphism raised different parts at different rates, causing from north to south are also seen in biotite age the juxtaposition of blocks of different meta­ distribution. The zones between Rantasalmi and morphic facies at the same level. The Pihtipu­ Sulkava were tilted towards the northnortheast das-Korppinen block exhibits accelerated uplift by temperatures below 300°C. An uplift rate of between 1750 Ma and 1700 Ma, becoming steady 0.02 mm/a can be calculated for the north with at about 0.05 mm/ a at 1700 Ma. the aid of biotite and muscovite ages from the The present dates it is not possible to establish same sampies. In the zone of Sulkava, however, whether the blocks of Pihtipudas-Korppinen uplift which causes the dipping, must have been and Pielavesi-Vaaraslahti were together during higher. the 1800 Ma overprint or whether they have Hornblende ages make it possible to recognize undergone only the last stage of their cooling a still more rapid uplift at the earlier stage of history together.

ACKNOWLEDGEMENTS

I owe special thanks to Prof. Dr. E. Jäger for for the chemical treatment of sampies, to Mr. U. supervising this work and for helping with the Schaltegger and Mr. M. Soom for the potassium interpretation of the data. I would like to thank determinations, and Mr. R. Krähenbühl and Mr. Dr. K. Korsman for organizing my work in Fin­ R. Bürgi for the encouraging discussions. I am land and for valuable discussions, and Mr. J. Es­ also grateful to Dr. A. J. Hurford, Ms. C. Lewis kelinen and Mr. P. Hölttä for helping in the field. and Mrs. Gillian Häkli for correcting the English Further I owe my thanks to Dr. M. Flisch and manuscript. Mr. R. Brunner for introducing me to mass Finally I thank everyone who made coopera­ spectrometry, to Mrs. 1. Hebeisen and M. Giger tion between Helsinki and Berne possible.

REFERENCES

Aho, L., 1979. Petrogenetic and geochronological studies of F1isch, M., 1982. Potassium-argon analysis. In Numerical metavolcanic rocks and associated granitoids in the Pih­ Stratigraphyed. by G. S. Odin, lohn Wiley, Chichester, ti pu das area, Central Finland. Geol. Surv. Finland, Bull. 151 - 158 . 300. 22 p. Geological Survey of Finland, Bulletin 343 47

Flisch, M., 1986. K-Ar dating of Quaternary sampies. In Surv. Finland, Kallioperäkarttojen selitykset, lehti 3323. Dating young sediments ed. by A. Hurford, E. Jäger & 48 p. J. A. M. Ten Cate, CCOP technical secretariat, Bang­ Mattinson, J. M., 1982. U-Pb blocking temperatures and Pd kok, Thailand, 199-324. 1055 characteristics in young zircon, sphene, and apatite. Haudenschild, U., 1985. Geologie und Petrographie der Sa­ The geological society of America, 95th annual meeting. vo Schieferzone in den Blöcken von Lampaanjärvi• Abstracts with programs, vol. 14. Vaaraslahti- Pielavesi, Svecokareliden, Ost Finnland. Paavola, J., 1986. A communication on the U-Pb and the Lizentiatarbeit, Philosophisch-naturwissenschaftliche Fa­ K-Ar age relations of the Archaean basement in the kultät der Universität Bern. 141 p. -Varpaisjärvi area, Central Finland. Geol. Jäger, E., 1979. Introduction to Geochronology. In Lectures Surv. Finland, Bull. 339, 1-15. in isotope geology ed. by E. Jäger & J. C. Hunzi ker, Salli, 1., 1983. Pielaveden kartta-alueen kallioperä. Geol. Springer Verlag, Berlin, 1-12. Surv. Finland, Kallioperäkarttojen selitykset, Lehti 3314. Kirsten, T., 1966. Determination of radiogenie argon. In 28 p. Potassium argon dating ed. by O. A. Schaeffer & J. Zäh• Salop, L. J., 1977. Precambrian of the nonhern hemisphere. ringer, Springer Verlag, Heidelberg, 7-39. Developments in paleontology and stratigraphy 3. Else­ Korsman, K., 1977. Progressive meta morph ism of the vier Scientific Publishing Company, Amsterdam. 378 p. metapelites in the Rantasalmi-Sulkava area, southeastern Schumacher, E., 1975. Herstellung von >99.9997 per cent Finland. Geol. Surv. Finland, Bull. 290. 82 p. 38Ar für die 4oK /4

Appendix 1. Sites and rock types of mica sam pies.

Sampie Rock type Locality Coordinates Weight No.

2559 Andalusite-micaschist Kivilouhos 7028.65/ 443.40 20 kg Virkamäki 2560 Micaschist Ritovuori 7030.00/428.55 Drillcore 2562 Granodiorite Tahkomäki 7032.35/ 431.10 20 kg 2563 Granite-gneiss Palovuori 7030.85/436.75 10 kg 2564 Granite-gneiss Suoniemi 7029.35/ 451.75 10 kg 2566 Micagneiss Karkeisvuori 7033.25/ 456.65 15 kg 2568 Micagneiss Valkamalahti 7029.50/ 473.95 15 kg 2571 Cordierite-garnet-gneiss Iso Kapustak. 7069.85/469.65 Drillcore 2572 Micagneiss Kiviniemi 7072.75/ 494.10 10 kg 2574 Basement-Orthogneiss Mustapuro 7056.20/ 506.10 20 kg 2576 Micagneiss Petäjäselkä 7066.40/497.50 5 kg 2578 Augengneiss Likosuo 7024.75/ 492.90 20 kg 2579 Granodiorite Ahola 7027.25/ 493.15 20 kg 2584 Hypersthene-Granite Kukkomäki 7028.15/483.45 20 kg 2585 Cordierite-garnet-gneiss Suviniitty 7027.40/ 485.40 20 kg 2586 Cordieri te-garnet -gneiss Hauta-aho 7029.40/ 485.55 20 kg 2587 Micagneiss Itälahti 7029.75/483.90 20 kg 2588 Micagneiss Vörstinaho 7030.10/ 484.75 15 kg 2589 Granodiorite Lehtoniemi 7026.50/ 486.05 15 kg 2590 Granodiorite Tammakaarre 7026.05/ 486 .70 20 kg 2591 Micagneiss Pikkusaari 7027.50/488.65 15 kg 2592 Granite-gneiss Kyntöläinen 7027.45/ 490.35 20 kg 2594 Garnet-mica-gneiss Torasalo 6891.45/ 563.65 Drillcore 2595 Granodiorite Vaajala 6891.10/564.60 5 kg 2596 Garnet-mica-gneiss Teemasaari 6898.40/ 557.50 5 kg 2598 Micagneiss Jaakola 6891.45/ 553.55 Drillcore 2599 Granodiorite Peltola 6893.00/553.50 10 kg 2600 Micagneiss Rettumäki 6892.70/ 544.20 10 kg 2601 Cordierite-garnet-gneiss Sydänmaa 6884.60/ 547.65 2 kg 2602 Cordierite-garnet-gneiss Kietävälä 6837.00/ 572.40 5 kg 2603 Cordierite-garnet-gneiss Kietävälä 6836.10/572.00 15 kg 2604 Cordierite-garnet-gneiss Vahvonen 6841.20/ 565.00 10 kg 2605 Granodiorite Hieta-aho 6864.95 / 542.00 5 kg 2606 Cordierite-garnet-gneiss Rautalehto 6866.70/542.55 5 kg 2607 Granulite Kiuruvesi 7043.05/ 482.20 15 kg 2819 Juva-Zone Biotite-Concentrate; 2821 Juva-Zone Biotite-Concentrate; 2822 Cordieri te-garnet -gneiss Hiidenlampi 6879.65/ 553.95 Drillcore 2823 Cordierite-garnet-gneiss Ronkala 6874.60/ 561.80 Drillcore 2824 Cordierite-garnet-gneiss Sulkava 6854.05 / 565.75 10 kg 2825 Cordierite-garnet-gneiss Säviönsaari 6849.05/ 578.95 10 kg 2826 Cordierite-garnet-gneiss Rantasalmi 6885.35/ 564.65 10 kg 2860 Granodiorite Lötkänmäki 7024.35/ 486.65 10 kg Geological Survey of Finland, Bulletin 343 49 Appendix 2. K-Ar isotopic data.

Sampie Mineral Grain Size D70K Vol40Ar"d. %40Ar Age(Ma) KAW No. (mesh) ccmSTP I gx I O-S

2559 Biotite 80-100 7.40 81.1609 99.82 1699 Biotite 100-120 7.50 81.9440 99.69 1695 White mica 80-100 8.05 91.9956 99.71 1744 White mica 100-120 8.05 93.0677 99.74 1757 2560 Biotite 80-100 7.38 84.8040 99.86 1750 Biotite 100-120 7.30 84.4911 99.81 1759 2562 Biotite 60-80 7.63 86.0300 99.77 1729 Biotite 80-100 7.69 87.7105 99 .88 1742 2563 Biotite 80-100 7.80 86 .3532 99.84 1709 Biotite 100-120 7.74 86.0419 99.74 1714 2564" Biotite 60-80 8.01 78 .2423 99.56 1574 2566 Biotite 80-100 7.52 82.0179 99.31 1693 Biotite 100-120 7.46 79.9789 99.83 1674 White mica 80-100 8.26 90.2996 99.81 1695 White mica 100-120 8.25 89.7582 99.87 1690 2568 Biotite 80-100 7.65 79.6080 99.85 1642 Biotite 100-120 7.53 78.7135 99.83 1646 2571 Biotite 80-100 7.55 81.3509 99.88 1679 Biotite 100-120 7.46 80.0031 99.84 1674 2572 Biotite 80-100 7.27 79.5547 99.86 1696 Biotite 100-120 7.28 79.0137 99.76 1687 White mica 100-120 7.91 90.6305 99.94 1747 2574 Biotite 60-80 7.63 85.3047 99.84 1719 Biotite 80-100 7.81 88 .9277 99.85 1740 2576 Biotite 80-100 7.28 78.3844 99.87 1678 Biotite 100-120 7.25 76.9910 99.91 1663 2578 Biotite 80-100 7.77 79.2158 99.88 1619 Biotite 100-120 7.67 78.4919 99 .80 1624 White mica 60-100 9.02 94.5755 99.75 1650 White mica 130-200 8.42 85.1273 99.76 1611 2579 Biotite 80-100 7.89 80.8905 99.82 1625 2584 Biotite 80-120 7.75 90.2404 99.89 1765 Biotite 60-80 7.82 91.8023 99.92 1775 2585 Biotite 60-80 7.71 90.6947 99.82 1777 2586 Biotite 80-100 7. 78 93 .1040 99 .79 1796 Biotite 100-120 7.80 93 .0740 99.85 1793 2587 Biotite 60-80 7.63 87 .9943 99.86 1755 Biotite 80-100 7.60 87 .6711 99.88 1755 2588 Biotite 60-80 7.80 90.5372 99.85 1762 Biotite 80- 100 7.85 89.3952 99.88 1740 2589 Biotite 80-100 7.29 82.9201 98 .89 1739 2590 Biotite 80-100 7.50 84.1405 99.24 1724 Biotite 60-80 7.22 81.7862 98 .88 1735 2591 Biotite 100-120 7.73 82.3179 99.88 1666 Biotite 80-100 7.66 81.7743 99.75 1669 2592' Biotite 100-120 7.65 77 .1991 99.76 1609 Biotite 80-100 7.74 77.9712 99 .73 1607 2594 Biotite 100- 120 7.64 89.8690 99 .92 1777

4 50 Geological Survey of Finland, Bulletin 343

Appendix 2. (cont.)

Sampie Mineral Grain Size (tloK Vol40Ar,.d . 0J0 4O Ar Age(Ma) KAW No. (mesh) ccmSTP / gx I 0.5

2595 Biotite 60-80 7.57 87 .5618 99.58 1758 Biotite 80-100 7.19 82.3898 99.67 1747 2596 Biotite 60-80 7.75 88.9787 99.72 1750 Biotite 100-120 8.03 90.3880 99.56 1727 2598 Biotite 80-100 7.66 84.4684 99.88 1705 Biotite 100-120 8.02 89.4458 99.75 1717 2599 Biotite 100-120 6.35 72.89 10 99.77 1749 Biotite 80-100 6.95 80.6756 99.77 1762 2600* Biotite 60-1 15 4.42 48.5072 99.03 1699 2601 Biotite 100-120 7.51 85.7472 99.83 1743 2602 Biotite 100-120 7.38 79.2369 99.97 1675 Biotite 80-100 7.56 80.3908 99.38 1665 2603 Biotite 100-120 7. 17 73.9187 99.65 1631 2604 Biotite 100-120 7.75 81.0821 99.67 1647 2605 Biotite 80-100 7.22 77.9421 99.93 1681 2606 Biotite 80-100 7.69 80.9049 99.84 1653 2607 Biotite 100-115 6.83 75.9511 98.86 1714 2822 Biotite 80-100 8.01 89.5653 99.98 1720 2823 Biotite 60-100 7.56 81.0137 99.84 1673 2824* Biotite 60-80 5.58 63.8453 99.97 1746 2825 Biotite 80-100 6.99 70.2653 99.98 1604 2826 Biotite 80-100 7.44 83.4463 99.99 1723 White mica 80-100 8.18 95.7950 99.99 1772 2860 Biotite 60-80 7.68 84.8917 99.09 1705 2861 Gamet (Metapelite Kiuruvesi) 0.1549

Reanalysis 01 !he Juva da!a 2818 Biotite total 7.64 83.4078 99.46 1694 total K-publ. 7.54 » » 1706 Biotite total milled 7.82 85.2917 99.71 1693 Biotite 100-130 7.80 83.9529 99.63 1678 Biotite 80-100 7.86 85.3389 99.66 1688 2821 Biotite total 7.94 86.4750 99.51 1691 total K-publ. 7.86 » » 1700 Biotite total milled 7.98 88.7524 99 .58 1714

Published da!a (Korsman 1984) 2819 Biotite total 7.54 84.166 99.4 1716 83.909 99.2 2821 Biotite total 7.86 88.123 99.0 1721 87.823 99.3