MULTISTAGE EVOLUTION OF THE PROTEROZOIC CRUST IN THE SAVO SCHIST BELT, EASTERN FINLAND
by KALEVI KORSMAN, REIJO NIEMELÄ and PEKKA WASENIUS
Korsman, K., Niemelä, R. & Wasenius, P., 1988. Multistage evolution of the Proterozoic crust in the Savo schist belt, eastern Finland. Geological Survey 01 Fin land, Bulletin 343, 89-96. 5 figures and I table. The relationship of metamorphic evolution to magmatism and deformation shows that there is a tectono-metamorphic discordance between the northern and sout hern parts of the Savo schist bell. The progressive stage of metamorphism in the northern part was associated with the 0 ,-0, deformations, which star ted to evolve earlier than 1903 Ma ago but came to an end before the emplacement of the 1880 Ma hypersthene granitoids. In the south, the progressive stage of metamorphism was mainly connected with the 0 , deformation, which started after the emplace ment of the 1880 Ma old tonalites. While the northern part was cooling sedimenta tion was still going on in the south, and when the northern part underwent intense erosion the southern part was submitted to strong progressive metamorphism. On the present erosion surface the areas join each other along displacements caused
by the 0 3 deformation, which was active 1830-1810 Ma ago.
Key words: metamorphism, evolution, Proterozoic, Kiuruvesi, Rantasalmi , Finland
K. Korsman, R . Nielllelä and P. Wasenius, Geological Survey 01 Finland, 02150 Espoo, Finland
INTRODUCTION
Many studies (e.g. Neuvonen et al. 1981 & formations have not been encountered west of Koistinen 1981) have shown that the boundary the zone, where, according to Sm-Nd results of the Proterozoic and Archaean formations in (Huhma 1986), the Proterozoic crust is composed eastern Finland is a suture zone along which a predominantly of new mantle material, the Ar Proterozoic island arc system joined the Ar chaean component being in the minority_ chaean continent (Fig. 1). East of the suture zone It has been demonstrated that the tectono the Archaean crust occurs as extensive, con metamorphic evolution of the Proterozoic crust tinuous areas, although in places the Archaean in the southern part of the Savo schist belt is overlain by Proterozoic formations. Archaean (Kiuruvesi-Sulkava, Fig. 1) differed from that 90 Geological Survey of Finland, Bulletin 343 sociated with well-developed zonality, whereas in 26° the north, i.e. in the suture zone proper, the KIURUVESI characteristic feature is metamorphic block structure. In the suture zone, a strong meta morphic pulse accompanied magmatic activity 1880 Ma ago, but the pulse was preceded by in ARCHEAN tense metamorphic evolution. In the southern part of the Savo schist belt metamorphic evolu tion did not start until the tonalites of the 1880 Ma age group had been emplaced. The present study seeks to elucidate the rela tionship of tectono-metamorphic evolution in the northern part of the Savo schist belt to that in the south. The study was conducted largely in the southern part of the belt, in places where the areas are in contact with each other (Figs. 1 and 2). The mapping of the metamorphic zones was done by Reijo Niemelä and Pekka Wasenius, and the study was planned by Kalevi Korsman. The Fig. I. Suture zone between the Archaean and Proterozoie work is based on the following lithological maps: formations in eastern Finland. The Kiuruvesi- Haukivesi complex is marked with oblique ruling. The Rantasalmi- Sulkava (Lehijärvi 1966), Rantasalmi (Korsman Sulkava area is outlined with a dashed line. 1973), Varkaus (Korsman and Pääjärvi 1980), Pieksämäki (Vorma 1971), Haukivuori (Pekka in the north (Korsman et al. 1984). In the south rinen and Hyvärinen 1984) and Mikkeli (Simo the progressive stage of metamorphism was as- nen and Niemelä 1980).
MAIN FEATURES OF THE TECTONO-METAMORPHIC EVOLUTION
The northern part of the belt is characterized Rantasalmi-Sulkava area the D1 deformation by intensely metamorphosed and migmatized was associated with only very weak meta areas often separated from the environment by morphism, which took place below the equilib faults. The northern part is called the rium field of andalusite. The evolution of zonal Kiuruvesi-Haukivesi complex, and the blocks metamorphism visible on the present-day erosion are marked with I in Fig. 2. The zonally meta surface did not start until during the D2 de morphosed Rantasalmi-Sulkava area is denoted formation. Andalusite was crystallized, and with II. muscovite and biotite were decomposed during
The progressive stage of metamorphism in the the D2 deformation. However, the decomposi Kiuruvesi-Haukivesi complex was associated tion reaction of biotite II and the associated equi with the D 1- D2 deformations (Pajunen 1986, libration of garnet and cordierite took place
1988 & Tumanto 1987), which were accompanied mainly after the D2 deformation (Korsman and by granulite facies metamorphism. In the Kilpeläinen 1986 & Kilpeläinen 1988). Geological Survey of Finland, Bulletin 343 91
• VARKAUS le
lb
11 Gar+Cord+Sill+Bio
", --"'''' ..... /""'" -,~:,,// Cord+Gar+Sili -~~",~~,~~,~~ ;,-,- / ' 11 .~ , // : c:, ,/ *-"'~ ,/ : SULKAVA THERMAL DOME // b- / ~~ : " 0' ,/ MIKKELI I ofFig the. 2. Savo Tectono-metamorphic sc hist bell. The metamor- map , ,,.,,"":/CI I":,,, phic blocks in the Kiuruvesi-Haukivesi 20 km complex: la Kyyvesi, Ic Pieksämäki, Id Haukivesi. The Rantasalmi- Sul- 271-- -, -'------'------2 -l030· 00 0 S kava area is marked with II and the Haukivuori area with Ha . The serrated I b 11 line refers to the lineament zones A B caused by the DJ deformation.
Consequently, the evolution of metamorphism titic tonalites of the D2 stage, However, paleo in relation to deformation in the Rantasalmi somes underwent intense metamorphism even be Sulkava area differs c1early from that in the fore migmatization. The U-Pb age on zircon Kiuruvesi-Haukivesi area, although a time lag from the migmatizing tonalite in the Pieksämä• cannot be established in the tectono-metamorphic ki block is 1903 ± 10 Ma (Huhma 1986), in evolution between the areas merely on the basis dicating that the Pieksämäki block was strongly of the relationship of deformation and meta metamorphosed before that date. In the Kiuru morphism, In the Kiuruvesi-Haukivesi complex vesi-Haukivesi complex the 1880 Ma old intru the 1880 Ma old hypersthene granitoids crosscut sions are mainly postmetamorphic in character the structures of metamorphism associated with (Korsman et al. 1984 & Korsman & Pääjärvi the D 1-D2 deformation (Korsman et aL 1984 & 1988). The influence of the D2 deformation, Hölttä 1988), The Pieksämäki block (Fig, 2) is too, ceased before the emplacement of the hyper devoid of hypersthene granitoids but has migma- sthene granites. Observations on crystallization 92 Geological Survey of Finland, Bulletin 343 conditions based on the contact metamorphism The D 3 deformation may have needed a long of the hypersthene granites show that the present time to develop. It reached the active stage day erosion surface in the Kiuruvesi-Haukivesi relatively late, as the deformation also had an ef complex was at a depth of about 15 km. In the fect on the gamet-cordierite isograde. In the Rantasalmi-Sulkava area, the tonalites of the Rantasalmi-Sulkava area, equilibration of 1880 Ma age group underwent progressive me gamet and cordierite marks the culmination of tamorphism and deformation during D2, but the metamorphism, which was reached shortly before relationship of the D 1 deformation and tonalites the emplacement of the granitoids of the 1800 Ma is unknown. age group (Korsman et al. 1984 & Vaasjoki and
The relationship of the D2 deformation to Sakko 1988). The D3 deformation is cut by the magmatism and deformation shows c1early that 1812 Ma old granitoids (Kilpeläinen 1988). deformation both started and terminated earlier Several studies (e.g. Pajunen 1986) have shown in the Kiuruvesi-Haukivesi complex than in the that the D3 deformation was subhorizontal.
Rantasalmi-Sulkava area. The D3 deformation was active 1830-1810 Since there is a time lag in the progressive Ma ago. It is possible that the Kiuruvesi-Hau evolution of metamorphism and deformation kivesi complex was uplifted before that, a con between the Rantasalmi-Sulkava and Kiuruve cept that is corroborated by observations on si-Haukivesi areas, the present-day position of lamprophyre dykes in the Haukivesi block (Neu the areas in relation to each other developed only vonen et al. 1981). The dykes, which are post after the Kiuruvesi-Haukivesi complex had been metamorphic and were emplaced fairly c10se to submitted to progressive metamorphism and the the erosion niveau, are 1836±20 Ma old. When
D2 deformation, i.e. later than 1880 Ma ago. As the lamprophyre dykes intruded, the cooling and early as 1971, Talvitie demonstrated that the Sa erosion were al ready weil under way in the Hau vo schist belt is a major fault zone. Pajunen kivesi block, while in the southem part of the (1986) and Kilpeläinen (1988) attributed the frag Rantasalmi-Sulkava area progressive meta mentation of the crust mainly to the D3 defor morphism was still at the intensive stage. Even mation. This deformation is seen on maps as so, the northem part of the RantasaImi-Sulkava lineament zones along which the Rantasalmi area reached its present vertical position in rela Sulkava and Kiuruvesi-Haukivesi areas join tion to the blocks of Kyyvesi, Pieksämäki and each other (Fig. 2). Haukivesi during the D3 deformation. From the Figure 2 shows that the geometry of the meta above it follows that cooling and erosion were morphic zones in the Rantasalmi-Sulkava area fairly rapid in this part of the Rantasalmi-Sul was affected by the D3 deformation, although kava area. Reactions associated with the cooling as a whole metamorphic zonality is regular as the stage are characteristic of the Kiuruvesi-Hau metamorphic grade increases towards the Sulka kivesi complex (Korsman et al. 1984, Korsman va thermal dome. The most striking metamor and Pääjärvi 1988 & Hölttä 1988). Gamet is often phic discordances are encountered between the enveloped by a cordierite rim, and cordierite has blocks of the Kiuruvesi-Haukivesi complex and further altered into biotite and aluminium sili the metamorphic zones of the Rantasalmi-Sul cate (Fig. 3). In contrast, prograde features pre kava area. The D3 deformation affected the dominate in the Rantasalmi-Sulkava area. metamorphic blocks of the Kiuruvesi-Haukivesi Relics of potassium feldspar generated during the complex so strongly that, on the present-day ero decomposition reaction of muscovite, and relics sion surface, they cut the relatively young meta of cordierite formed due to the first decomposi morphic zonality in the Rantasalmi-Sulkava tion reaction of biotite can still be seen in the gar area. net-cordierite-biotite gneisses (Fig. 4). As a re- Geological Survey of Finland , Bulletin 343 93
Fig. 3. Cordierite altered into biotite and sillimanite. Stipples = corierite, dense ruling = biotite, sparse ruling = sillimanite, colourless = quartz. The Pieksämäki block, x = 6897 .35, y = 531.28.
1mm
Fi g. 4. A sillimanite po rphyroblast formed during th e decomposition reac ti o n of muscovile overgrown by cor dierite. Minerals marked as in Fi g. 3. The garnet-co rdierite-bi otite zone of Rantasalmi- Sulkava area, x = 6876.42, y = 533 .44.
1mm
sult of the D) deformation the cordierite gneis 2), marked IIa on the map, is in a key position ses, which are shown in Figs. 3 and 4 and are in attempts to clarify the tectono-metamorphic entirely different in terms of stages of evolution, relations of the Kiuruvesi-Haukivesi and Ran are located side-by-side on the present-day ero tasalmi-Sulkava areas. Mthough the Haukivuo sion surface. ri area is in isolation in relation to the rest of the The progressively metamorphosed Haukivuori zonally metamorphosed Rantasalmi-Sulkava area in the western part of the study area (Fig. area it clearly belongs to the Rantasalmi-Sul- 94 Geologieal Survey of Finland, Bulletin 343
I I
0.35 -
-
+ ZIRCON 0.15 - o TITANITE -
3.0 5.0 207Pb/ 235U Fig. 5. Coneordia diagram of zireons from granitoid eobbles in the Haukivuori eonglomerate.
Table I. U-Pb analytieal data zireons from granitoid eobbles in the Haukivuori eonglomerate.
Sam pie Fraetion Coneentration 206 Pb !sotopie eomposition of Atom ration and no. (g / m' ) Ilg/g 204 Pb lead 206 Pb = 100 radiometrie ages, Me measured 2J8U 206 Pb 204 207 208 206Pb 20J Pb 207 Pb m u m U 206P b
A464 - Haukivuoren kgl. A464A 4.2 ).mu = 1.55125 x 1O-lo/a ).mu = 9.8485 x 10-lo/ a atomisuhde - atomie ratio m U/2JlU = 137.88 Geological Survey of Finland, Bulletin 343 95 kava system in lithology and metamorphic evo during D 1-D2 stage and became isolated from lution. The D 1 stage was accompanied by very the rest of Rantasalmi-Sulkava areas during the weak metamorphism, and muscovite was decom D3 deformation. posed during the D2 deformation. In the central The relationship of metamorphism to magma parts of the Haukivuori area, subhorizontal tism, to the D 1 deformation and, particularly, to layering and S2 schistosity underwent weak the D2 deformation shows that there is a mar folding during the D 3 deformation (Fig. 2), but ked tectono-metamorphic discordance between at the margins, where the Haukivuori area is the areas of Kiuruvesi-Haukivesi and Rantasal bordered by the Pieksämäki and Kyyvesi blocks, mi-Sulkava. A conglomerate has been encoun the D 3 deformation had a more pronounced im tered in the Haukivuori area whose plutonic pact and it turned the S2 schistosity to a sub elasts have a zircon age of 1885 ± 6 Ma (Fig. 5 vertical position. The D 3 deformation also and Table 1). The conglomerate is deformed and folded the isograde surfaces, as the muscovite metamorphosed as the surrounding schists are. bearing metaturbidites in the central parts of the Like ali the other observations, the conglomerate area have altered into sillimanite-potassium indicates that sedimentation might still have been feldspar gneisses towards the margins. Con going on in the Rantasalmi-Sulkava area when sequently, the Haukivuori area was thrust over erosion was already affecting the Kiuruvesi the blocks of the Kiuruvesi-Haukivesi complex Haukivesi complex. DISCUSSION A elear tectono-metamorphic discordance has going on in the Rantasalmi-Sulkava area, as de been established between the northern and monstrated by the 1885 Ma old plutonic elasts southern parts of the Savo schist belt. In the in the Haukivuori area. During the D 1-D2 de north, in the Kiuruvesi-Haukivesi complex, the formation the Rantasalmi-Sulkava area was D 1-D2 deformation and associated metamor partly thrust over the Kiuruvesi-Haukivesi phism started to evolve before the emplacement complex. Then, 1840 Ma ago, when the cooling of the granitoids of the 1880 Ma age group. of the Kiuruvesi-Haukivesi complex was far According to one age determination, intense advanced and the blocks had been uplifted, in progressive evolution of metamorphism related tense metamorphic recrystallization was still to the D 1-D2 deformation stage took place be under way in the Rantasalmi-Sulkava area. fore 1903 Ma. At the emplacement the hyper Fragmentation of the Savo schist belt mainly sthene granitoids the complex was already occurred 1830-1810 Ma ago, during the D3 de cooling. formation. As a result, the metamorphic blocks The age of the D 1 deformation has not been of the Kiuruvesi-Haukivesi complex crosscut established for the Rantasalmi-Sulkava area in the metamorphic zones of the Rantasalmi-Sul the southern part of the Savo schist belt, but the kava area on the present erosion surface. evolution of neither the D2 deformation nor the As stated in the introduction, the Savo schist associated progressive metamorphism com belt belongs to the suture zone along which the menced until the 1880 Ma old tonalites had been Proterozoic island arc system is bound to the con emplaced. While the Kiuruvesi-Haukivesi tinent. The Kiuruvesi-Haukivesi complex is part complex was cooling, sedimentation was still of this zone. The findings of the present study 96 Geological Survey of Finland, Bulletin 343 suggest that there is a geosuture between the servations. The areas of Rantasalmi-Sulkava northern and southern parts of the Savo sehist and Haukivuori are eharaeterized by MORB-type belt, i.e. between the Kiuruvesi-Haukivesi volcanism with ultramafie lavas (Kousa 1985), eomplex and the Rantasalmi-Sulkava area. This but the Kiuruvesi-Haukivesi eomplex shows eoneept is eorroborated by other geologieal ob- features typical of island are volcanism. ACKNOWLEDGEMENTS The U-Pb analyses on zireons of the Hauki lian Häkli. vuori eonglomerate were done by Dr. Olavi Kou The authors are grateful to the above persons vo. Mr. Timo Kilpeläinen helped us in struetural for the help given them during the study and to geology during the field work. Professor Atso all those not mentioned by name who gave their Vorma read the manuseript eritieally. 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