K-Ar AGE DETERMINATION on BIOTITE and MUSCOVITE in the PIHTIPUDAS-IISALMI and JOROINEN AREAS, EASTERN FINLAND

K-Ar AGE DETERMINATION ON BIOTITE AND MUSCOVITE IN THE PIHTIPUDAS-IISALMI AND JOROINEN AREAS, EASTERN FINLAND 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 Rantasalmi, 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, Pielavesi, 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 Kuopio 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 ~..

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