Archaean Proterozoic transition: geochemistry, provenance and tectonic setting of metasedimentary rocks in central Fennoscandian Shield, Finland

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1 Precambrian Research 104 (2000) Archaean Proterozoic transition: geochemistry, provenance and tectonic setting of metasedimentary rocks in central Fennoscandian Shield, Finland Raimo Lahtinen * Geological Sur ey of Finland, P.O. Box 96, FIN Espoo, Finland Received 8 July 1999; accepted 5 May 2000 Abstract The central part of the Fennoscandian Shield in Finland is composed of the Palaeoproterozoic Svecofennian domain and the Archaean Karelian craton with a Palaeoproterozoic allochthonous and autochthonous cover. A cryptic suture separating these areas and another tentative suture dividing the Svecofennian into central and southern parts have been proposed. The chemical composition of sedimentary rocks (N=300) within the study area, including the effects of palaeoweathering, hydraulic sorting, depositional environment and post-depositional processes, have been studied in order to delineate sediment source components. The main proposed source components for the Archaean sedimentary rocks are weathered Ga greenstone+granite TTG and local 2.7 Ga sources. Autochthonous Ga cover rocks were mainly derived from a mixture of chemically weathered palaeosol ( Ga), sedimentary rocks derived from the palaeosol, and mafic dykes and plateau volcanics (mainly Ga) although in places locally derived non-weathered Archaean sources dominated. Archaean crust and low-k bimodal rocks from a primitive island arc are the proposed source for the allochthonous Western Kaleva cover rocks. These formed in a subsiding foredeep during initial collision from orogenic detritus in the same oblique collision zone. The central Svecofennian sedimentary rocks can be divided into local arc-derived rocks ( 1.89 Ga) and older ( 1.91 Ga) rocks from a mixture of Western Kaleva sources and a Ga mature island arc/active continental margin source. Rifting followed by increased subsidence during initial collision in the NE and subsequent arc reversal caused rapid erosion from the mountain belt, exposing diverse source compositions as seen in the large variation of Th/Sc (2 0.5), and deposition into an oblique hinterland basin further developing into a subduction related foredeep. Mature greywackes from the southern Svecofennian in the study area resemble passive margin sediments with a source dominated by inferred alkaline-affinity complexes and Archaean rocks. Less mature rocks also occur and had sources dominated either by island arc/active continental margin rocks or local picritic rocks. In the sedimentary record the Archaean Proterozoic transition up to 2.1 Ga was dominated by input of mainly mafic plateau-type volcanic contribution to the Archaean detritus. Palaeoproterozoic sediments having a crustal component ( 2.1 Ga) show higher Th/Sc, Th/Cr, and lower Sm/Nd and Eu/Eu* relative to the Archaean rocks but locally low Th/Cr ratios complicate the situation. Ba depletion relative to K, Rb and Th is a characteristic feature of the * Fax: address: raimo.lahtinen@gsf.fi (R. Lahtinen) /00/$ - see front matter 2174 Elsevier Science B.V. All rights reserved. PII: S (00)

2 148 R. Lahtinen / Precambrian Research 104 (2000) sedimentary rocks of the central Fennoscandian Shield indicating high amounts of Ba lost from the clastic record during Ga and further recycled back to the mantle forming a subduction component and an enriched mantle component. Ba depletion seems to have been especially characteristic of chemical weathering during Ga under CO 2 -rich and low-o 2 atmosphere. Whether this strong Ba depletion is characteristic of the Archaean Proterozoic transition and quiet supercontinent stages in general remains to be determined Elsevier Science B.V. All rights reserved. Keywords: Archaean; Palaeoproterozoic; Sedimentary rocks; Geochemistry; Provenance; Finland 1. Introduction The geochemistry of clastic sedimentary rocks can be used as an indicator of crustal evolution (e.g. Taylor and McLennan, 1985) or to identify ancient tectonic settings in metamorphic terranes. Sedimentary rocks can be divided into those showing local sources and those having experienced effective mixing in large river marine systems before deposition. The latter types sample large areas providing data for crustal-scale processes. The possibility of different crust-forming mechanisms during Archaean and Proterozoic times emphasizes the importance of the Archaean Proterozoic boundary where there might be a corresponding compositional change in the sedimentary record (e.g. Taylor and McLennan, 1985; McLennan and Taylor, 1991). Selective preservation of sedimentary rocks in the ancient record can on the other hand hamper their use in crustal evolution studies. Along with this limitation, other factors discussed below, should also be taken into account when using ancient sediments to give information on the general provenance of the studied sedimentary unit. The lithology of the provenance area essentially controls the chemical composition of the clastic sediments but other factors such as degree of palaeoweathering, hydraulic sorting (grain-size effects), organic and sulphide input, diagenesis and metamorphism (especially migmatization) may greatly modify or ultimately erase provenance memory. Sediment recycling is a common feature (e.g. Veizer and Jansen, 1985) and produces a buffering effect where a small amount of new input can go unnoticed. Nevertheless, even though the interpretation of their compositions is more controversial than with igneous rocks, the long memory of sedimentary rocks can be quite powerful when modelling the tectonic settings and evolutionary histories of metamorphic terranes. The central part of the Fennoscandian Shield in Finland is composed of the Palaeoproterozoic Svecofennian domain and the Archaean Karelian craton with a Palaeoproterozoic allochthonous and autochthonous cover (Fig. 1). The occurrence of a cryptic suture (Fig. 1; Koistinen, 1981; Huhma, 1986) between the Karelian and Svecofennian domains is favoured by the observation that no Archaean component is found in the Ga gneissic tonalites and related felsic volcanics adjacent to the Archaean craton (Lahtinen and Huhma, 1997). Lahtinen (1994) proposed also the occurrence of a tentative suture (Fig. 1) separating the central part of the Svecofennian domain from the southern Svecofennian. Studies on the geochemistry of sedimentary rocks in the study area are few and include a geochemical and isotopic study from the Archaean Hattu schist belt (O Brien et al., 1993), a major element study from the northern part of the Höytiäinen area (Kohonen, 1995), a regional correlation diagram study from the Savo province (Kontinen and Sorjonen-Ward, 1991) and a research concentrating on black schists (Loukola-Ruskeeniemi and Heino, 1996 and references therein). The study area has been sampled in the course of a regional bedrock geochemical survey undertaken by the Geological Survey of Finland including the 300 metasedimentary samples discussed here. The samples range from Archaean to Palaeoproterozoic, were formed in a variety of tectonic settings, and are thus suitable for studying the Archaean Proterozoic transition and the

3 R. Lahtinen / Precambrian Research 104 (2000) evolution of Fennoscandian Shield. The main source components and implications for the tectonic evolution of the central Fennoscandian shield are given with emphasis on proposed sutures. Notes on the crustal evolution and Archaean Proterozoic transition in general, and on Ba depletion are also given. As all the studied sedimentary rocks are metamorphosed, the prefix meta has been dropped. The data set is available on request from the author. 2. Sampling and analytical methods Sampling was done with a mini-drill with diamond bit. Each sample comprised four to six subsamples (altogether kg) from the same lithological unit, if detection of unit boundaries was possible (sometimes this was impossible, e.g. in some migmatites). In the case of turbidites, the whole Bouma A, AB or ABC was sampled in most cases. A composite sample was taken from Fig. 1. Simplified geological map of Finland and surrounding areas modified from Sorjonen-Ward (1993), Korsman et al. (1997). The study area is outlined (see Fig. 2).

4 150 R. Lahtinen / Precambrian Research 104 (2000) Fig. 2. Simplified geological map of the study area (Fig. 1) modified from Korsman et al. (1997). Sample locations are also indicated. veined migmatites and pelitic rocks where layers were 5 cm thick and a more homogeneous unit was not available. The analytical work was done in the laboratories of the Geological Survey of Finland. Samples were jaw crushed and splits were pulverized in a tungsten carbide bowl for X-ray fluorescence (XRF) analysis, and in a carbon steel bowl for inductively coupled plasma mass spectrometry (ICP-MS). Major elements and Cl, V, Cr, Ni, Zn, Rb, Sr, Y, Zr, Nb and Ba were determined by XRF, C Graf. by Leco CR-12 carbon analyzer, F by ion selective electrode, aqua regia leachable S and Cu by ICP-AES, and aqua regia leachable Au, Pd, Te, As, Ag, Bi, Sb and Se by GAAS (Sandström, 1996). REE, Co, Nb, Hf, Rb, Sc, Ta, Th and U were determined by ICP-MS after dissolution of the sample (0.2 g) with hydrofluoric acidperchloricacid treatment completed by a lithium metaborate/sodium perborate fusion (Rautiainen et al., 1996). The estimated uncertainty is 1 5% for major elements and 3 10% for trace elements. 3. General geology The cratonic part of the study area (Fig. 1) includes rocks from Archaean (mainly Ga; Vaasjoki et al., 1993) and Palaeoproterozoic cratonic stage ( Ga) with coeval and subsequent multiple rifting (e.g. Vuollo, 1994; Kohonen, 1995) in which the latest phase led to formation of ophiolitic sequences (1.95 Ga; Peltonen et al., 1996). The cratonic cover in the Höytiäinen and Suvasvesi areas (Fig. 2) are dominated by autochthonous and allochthonous rocks, respectively. The Höytiäinen area or rift basin (Ward, 1987) includes the Tohmajärvi volcanic complex ( Ma; Huhma, 1986) and associated coarse clastic deposits but is dominated by mica schists representing metamorphosed thinly laminated pelites to massive turbidites (Ward, 1987; Kohonen, 1995). The formal lithostratigraphic procedure has been applied only to the autochthonous Sariola, Jatuli and Ludian groups in the eastern margin of the Höytiäinen area

5 R. Lahtinen / Precambrian Research 104 (2000) (Pekkarinen, 1979; Pekkarinen and Lukkarinen, 1991; Kohonen and Marmo, 1992; Karhu, 1993). Otherwise lithostratigraphy and chronostratigraphy of the Höytiäinen area are not resolved (Kohonen, 1995) but depositional ages from 2.1 to about 1.9 Ga are inferred. The Suvasvesi area is characterized by the Upper Kaleva (Kontinen and Sorjonen-Ward, 1991) or Western Kaleva (Kohonen, 1995 a term adopted in this study) greywackes that occur as allochthonous units in thrust complexes characterized by associated ophiolites and related rocks (Koistinen, 1981 and references therein) though evidence for local deposition upon Archaean basement has also been noted (Ward, 1987). The increase in metamorphic grade from east to west (Fig. 2) is seen as an increase in quartz veins and the onset of segregational banding (quartz+ feldspar) leading finally to migmatites. The boundary zone (BZ) includes migmatitic sedimentary rocks (Korsman et al., 1984) and a Ga volcano-plutonic formation (Lahtinen, 1994 and references therein). The Svecofennian is divided into the central Svecofennian including the Central Finland Granitoid Complex (CFGC) and Bothnian Belt (BB), and the southern Svecofennian including the Rantasalmi Haukivuori area (RH). The tentative sedimentation ages for the central Svecofennian, based on data available from the Tampere Schist Belt (Lahtinen, 1996 and references therein), are 1.91 and Ga for rocks correlated to basement- and arc-related groups in the Tampere Schist Belt, respectively. The southern Svecofennian, including the Rantasalmi Haukivuori area, is characterized by granite migmatites, which is a clear difference to the central Svecofennian, boundary zone and Suvasvesi area, which are characterized by tonalite migmatites (Korsman et al., 1999 and references therein). 4. Results Because lithostratigraphic division of sedimentary rocks is rarely available, division of sedimentary rocks into different groups within domains is based mainly on lithotype and geochemical characteristics. All elements analyzed have been used but the main weight has been put on the REE, Th, Sc, Cr and major elements where the REE, Th and Sc are considered as most reliable elements in monitoring the average source composition (Taylor and McLennan, 1985; McLennan et al., 1990). The arc-related (upper) central Svecofennian rocks of this study (Fig. 2), not discussed in detail, show CaO, MnO, P 2 O 5, Sr, Ba and Sb enrichment, which is characteristic of sedimentary rocks derived from high-k calc-alkaline to shoshonitic volcanics (Lahtinen, 1996). Strongly altered or mineralized samples are excluded from discussion as are minor groups of sedimentary rocks either having undefined origins or a large non-clastic component (e.g. iron formations and carbonate rocks). The group characteristics were also studied by using normalized diagrams (Fig. 3). Archaean sedimentary groups are normalized to Archaean crust (AC1), autochthonous and allochthonous groups to average Karelian craton (KC1) and boundary zone and Svecofennian groups to Western Kaleva WK1 (Table 1). The AC1 is a first approximation of the average composition of Archaean crust in Finland at its present erosion level based solely on the data from the study area. The granitoid-dominated nature of the exposed Archaean part of the study area is seen in higher LILE and LREE and lower MgO, Cr and Ni compared to the Late Archaean ( Ga) restoration model for average juvenile upper continental crust (Table 4 in Condie, 1993). The Karelian craton includes a large contribution from Palaeoproterozoic mafic dykes and volcanics ( Ga; Vuollo, 1994) relative to the Archaean crust average (Fig. 3) Archaean sedimentary rocks The Archaean metagreywackes and mica schists/gneisses have been divided into two main groups (Ar1 Ar2). The Ar1 rocks have a homogeneous composition indicating a thorough mixing of source components. The elevated CIA (Chemical Index of Alteration; Nesbitt and Young, 1982) shows the effects of weathering in the source area and the REE, major and trace

6 152 R. Lahtinen / Precambrian Research 104 (2000) Table 1 Average chemical composition of estimated Archaean crust (AC1) and Karelian craton (KC1), and selected sedimentary rock groups (non-migmatized, except groups BZ1 BZ2) a AC1 KC1 Ar1 H1 H2 H3 WK1 WK1frag WK2 BZ1 BZ2 (N=129) (N=156) (N=4) (N=11) (N=5) (N=9) (N=47) (N=17) (N=6) (N=8) (N=5) SiO 2 (%) TiO 2 (%) Al 2 O 3 (%) FeO (%) MnO (%) MgO (%) CaO (%) Na 2 O (%) K 2 O (%) P 2 O 5 (%) C graf. (%) (0.05) (0.05) (0.10) (0.22) (0.29) (0.07) (0.05) (0.05) S (%) F (%) CIA La (ppm) Ce (ppm) Pr (ppm) Nd (ppm) Sm (ppm) Eu (ppm) Gd (ppm) Tb (ppm) Dy (ppm) Ho (ppm) Er (ppm) Tm (ppm) Yb (ppm) Lu (ppm) Ba (ppm) Cl (ppm) Co (ppm) Cr (ppm) Hf (ppm) Nb (ppm) b Ni (ppm) Rb (ppm) Sc (ppm)

7 R. Lahtinen / Precambrian Research 104 (2000) Table 1 (Continued) AC1 KC1 Ar1 H1 H2 H3 WK1 WK1frag WK2 BZ1 BZ2 (N=129) (N=156) (N=4) (N=11) (N=5) (N=9) (N=47) (N=17) (N=6) (N=8) (N=5) Sr (ppm) Ta (ppm) b Th (ppm) U (ppm) V (ppm) Y (ppm) Zn (ppm) b Zr (ppm) Ag (ppm) b b As (ppm) b Au ppb b Bi (ppm) Cu (ppm) b Pd ppb (0.2) (0.2) (0.79) (0.26) (0.31) (0.39) (0.27) 1.0 Sb (ppm) Se (ppm) Te ppb a WK1frag is the average of mica gneiss fragments in migmatites. Values in parentheses include many determinations below the detection limit (Cgraf 0.05% and Pd 0.2 ppm) and show either the detection limit value or averages excluding values below detection limits. b One to two anomalous analyses have been excluded from some group averages.

8 154 R. Lahtinen / Precambrian Research 104 (2000) elements indicate a more mafic source compared to local Archaean bedrock at the present erosion level (Figs. 4 and 5, and Table 1). The Ar2 samples show variable REE and have higher CaO, Na 2 O and lower K 2 O, Cr and Rb compared to Ar1 (see Fig. 4 for K 2 O and Cr). The lower CIA indicates less weathering relative to Ar1 and low Th/Sc ( ) favours a dominant mafic source Cratonic co er The Jatuli-type quartzites of this study show a strong increase in K 2 O with decreasing SiO 2 (Fig. 4), which is mainly due to variations in sericite/ muscovite content. One subarkose contains fresh K-feldspar also seen in a lower CIA value but otherwise high CIA is a characteristic feature. The sedimentary rocks in the Höytiäinen basin are classified into high- and low-cr groups H1 and H3, respectively (Fig. 4, Table 1). A distinct lithological unit (Huhma, 1975) of high-cr rocks is classified as group H2 and a suspect group of low-cr rocks, possibly related to the Western Kaleva (Kohonen, 1995), is classified as group H4. Samples outside the Höytiäinen area (Fig. 2), but that occur in autochthonous position to Archaean dome rocks or are geochemically similar, are included in these groups. The H1 H3 samples include quartz-rich greywackes and more typically pelites showing thin layering from 1 3 mm to 1 2 cm with thin psammitic interlayers occurring locally. The variation in element abundances inside the H1 group is mainly explained by quartz dilution (Fig. 4). There is evidence of weathering in at least one component (CIA 54 70) and a Fig. 3. Major- and trace-element distributions in Karelian craton 1, Western Kaleva psammites (WK1), Jatuli-type mafics and Kutsu-type granites normalized to Archaean Crust (AC1 in Table 1). The Karelian craton (KC1) and WK1 averages are from Table 1 and the averages for Jatuli-type mafics (N=21) and Kutsu granites (N=8) are from Lahtinen (unpublished data).

9 R. Lahtinen / Precambrian Research 104 (2000) Table 2 Average chemical composition of selected sedimentary groups (non-migmatized, except CF3 average including also mica gneiss fragments in migmatites) a RH1 RH2 RH2mig RH3/lCr RH4/hCr CF1 CF2 CF3 CF3mig (N=4) (N=4) (N=7) (N=6) (N=5) (N=6) (N=14) (N=12) (N=14) SiO 2 (%) TiO 2 (%) Al 2 O 3 (%) FeO (%) MnO (%) MgO (%) CaO (%) Na 2 O (%) K 2 O (%) P 2 O 5 (%) C graf. (%) (0.05) (0.25) (0.09) (0.05) (0.05) (0.05) (0.05) (0.08) 0.15 S (%) F (%) CIA La (ppm) Ce (ppm) Pr (ppm) Nd (ppm) Sm (ppm) Eu (ppm) Gd (ppm) Tb (ppm) Dy (ppm) Ho (ppm) Er (ppm) Tm (ppm) Yb (ppm) Lu (ppm) Ba (ppm) Cl (ppm) Co (ppm) Cr (ppm) Hf (ppm) Nb (ppm) Ni (ppm) Rb (ppm) Sc (ppm) Sr (ppm)

10 156 R. Lahtinen / Precambrian Research 104 (2000) Table 2 (Continued) RH1 RH2 RH2mig RH3/lCr RH4/hCr CF1 CF2 CF3 CF3mig (N=4) (N=4) (N=7) (N=6) (N=5) (N=6) (N=14) (N=12) (N=14) Ta (ppm) Th (ppm) U (ppm) V (ppm) Y (ppm) Zn (ppm) Zr (ppm) Ag (ppm) As (ppm) b Au ppb b Bi (ppm) Cu (ppm) Pd ppb b (0.25) (0.82) 1.02 (0.28) (0.25) (0.2) (0.29) Sb (ppm) Se (ppm) Te ppb b a The RH2mig and BB4mig are the averages of migmatites, respectively. Group RH3 have been divided into low-cr (RH3/lCr) and high-cr (RH/hCr) populations. Values in parentheses include many determinations below the detection limit (C graf 0.05% and Pd 0.2 ppm) and show either the detection limit value or averages calculated excluding values below detection limits. b One to two anomalous analyses have been excluded from some group averages.

11 R. Lahtinen / Precambrian Research 104 (2000) Fig. 4. Harker-type Cr, K 2 O, MgO and CIA (Nesbitt and Young, 1982) variation diagrams for Archaean, autochthonous and allochthonous sedimentary rocks in the study area. Ar1 and Ar2-Archaean, Jqzt Jatuli-type quartzites, H1 H2-autochthonous high-cr, H3-autochthonous low-cr, H4- a low-cr suspect group of Höytiäinen area. WK1 WK2 main field-allochthonous Western Kaleva. AC1 is the average of Archaean crust (Table 1). large mafic component indicated by high contents of HREE, MgO and Pd. The H2 group has many compositional similarities with H1 but the H2 average shows higher levels of most elements (e.g. MgO) and lower SiO 2 (Fig. 4 and Table 1). Some H3 pelites show enrichment of felsic source components manifested as low MgO contents (Fig. 4). The K 2 O, Rb and Bi enrichment (not shown) favour a source dominated by a late-archaean granite (Kutsu; see Fig. 3). The H4 is a heterogeneous group that deviates to some extent from the WK1 main group in having higher K 2 O and lower Cr (Fig. 4). The allochthonous Western Kaleva (WK) sedimentary rocks have been divided into WK psammites and SiO 2 -poor pelitic rocks (WK2). The WK1 psammites (Table 1) form a geochemically homogeneous group (Fig. 4) and most of the variation can be explained by grain size variation. The more pelitic nature of WK2 is seen in enrichment of elements (e.g. Al 2 O 3, MgO, FeO, K 2 O) that characterize clay minerals (Table 1) but the WK2 also seems to be enriched in a mafic source as seen in higher Sc and Cr relative to Th. The WK1 migmatites are mainly psammitic fragments floating in tonalitic (often trondjhemitic) veined

12 158 R. Lahtinen / Precambrian Research 104 (2000) gneisses (WK2 migmatites). Both groups of migmatites only show the systematic depletion of Bi compared to non-migmatitic samples (Table 1) Boundary zone and S ecofennian sedimentary rocks The sedimentary rocks in the boundary zone (BZ; Fig. 2) have been divided into psammitic (BZ1) and pelitic (BZ2) groups. The BZ1 rocks are heterogeneous in chemical composition showing high variation, e.g. in HREE, CaO, K 2 O, Th and Nb and the average (Table 1) should be only considered as an areal average. The southern Svecofennian sedimentary rocks in the Rantasalmi Haukivuori area have been classified into three groups (RH1 RH3). The non-migmatitic RH1 rocks are quartz-rich greywackes and the well-preserved RH2 rocks are more pelitic in character. Both RH1 and RH2 show rather similar patterns in Fig. 6 where the strong effect of weathering is seen in negative peaks of Ba, Sr, CaO, MnO and P 2 O 5, and high CIA values (Table 2). The depletion of HREE, Sc, V, TiO 2 and enrichment of K 2 O, Rb, Th and especially U is the main difference when compared to the Western Kaleva source. A relative Fig. 5. Plots of La vs. Yb and Eu/Eu* vs. Gd N /Yb N for selected sedimentary rocks in this study. Gd N and Yb N are chondrite-normalized values and Eu/Eu* has been calculated using Eu*=(Sm N +Gd N )/2. The Archaean average has been calculated from the average in the Table 1 and Jatuli-type mafics from the average (N=21) in Lahtinen (unpublished data). Ar Ar2-Archaean groups, Jqtz Jatuli-type quartzites, H1 H2-autochthonous high-cr, H3-autochthonous low-cr, RH1 RH2- southern Svecofennian, RH3-southern Svecofennian. CF1 CF3-central Svecofennian.

13 R. Lahtinen / Precambrian Research 104 (2000) Fig. 6. Major- and trace-element distributions in averages of southern Svecofennian sedimentary rock groups RH1 RH3 (Table 2) from the Rantasalmi Haukivuori area normalized to the average of Western Kaleva psammites (WK1 in Table 1). RH3/lCr and RH3/hCr are averages of low- and high-cr populations of RH3. enrichment of Zn to Ni and Co is also a characteristic feature. The RH2 group shows the relative enrichment of CaO, Ba, Nb, V and Sc and low Cr/Sc ratio favouring a new additional mafic component in the RH2. The lower CIA values (Table 2), which are normally higher in more pelitic rocks, indicate that this additional component was less weathered. Compared to the RH1 and RH2 rocks the RH3 samples show lower CIA and higher CaO and Na 2 O with strong variation in the amount of mafic component (Fig. 6 and Table 2). The RH1 RH2 migmatites vary from gneisses with quartz veins and small melt patches cut by pegmatites to veined gneisses with abundant granite leucosome. The main differences (Table 2) can be interpreted to show a more pelitic precursor for migmatites but the slightly lower REE and especially deep negative Eu anomaly in some samples ask for a loss of felsic component. The slight depletion in Ba, K 2 O and K/Rb can be related to a loss of a K-feldspar component and the enrichment of ferromagnesian components to the increased amount of restite. So it seems that these migmatites are mainly in situ migmatites that show a complex mixture of restite and a melt fraction in variable proportion in outcrop scale. The sedimentary rocks in the central Svecofennian have been divided to three groups (CF1 CF3) where the CF1 includes high-sio 2 and high Th/Sc ( 1) psammites, CF2 lower Th/Sc ( 1) psammites and CF3 silt-pelite rocks. The nonmigmatitic CF1 samples show LREE enrichment compared to the Western Kaleva psammites (Fig.

14 160 R. Lahtinen / Precambrian Research 104 (2000) ). The depletion of elements characteristic of mafic components and the relative enrichment of LREE, Sr, Th, U and Zr point to a larger felsic component relative to the WK psammites. The chemical composition of the CF2 group shows an enrichment of mafic components relative to CF1. CF3 is a heterogeneous group characterized by migmatites and thus the average (Table 2) includes also mica gneiss fragments in migmatites. Mineralogically the CF3 rocks differ from the CF1 CF2 in the ubiquitous occurrence of garnet. The more clay-rich nature of CF3 is seen in lower SiO 2 and higher MgO and K 2 O (Table 2). The CF3 migmatites form an inhomogeneous group ranging from samples with HREE enrichment to samples with HREE depletion and Eu enrichment at low total REE abundances compared with less migmatitic CF3 samples. This is interpreted as different amounts of restite and leucosome in sampled outcrops. 5. Discussion 5.1. Palaeoweathering Palaeoweathering in the source area is one of the most important processes affecting the composition of sedimentary rocks. Sedimentary rocks sensu stricto are composed merely of weathering products and reflect the composition of weathering profiles, rather than bedrock (e.g. Nesbitt et al., 1996). Based on CIA values (Nesbitt and Young, 1982) the source rocks affected the most by weathering are those of Archaean group Ar1 (60 65), Jatulian quartzites (58 73), autochthonous groups H1 H3 (54 70) and southern Svecofennian groups RH1 RH2 (57 68) whereas the allochthonous WK1 WK2 mostly show CIA values lower than 55 (Fig. 4). Most of the central Svecofennian psammitic rocks also have low CIA values ( 55) with an increase up to (60 67) in CF3 pelitic rocks. This general increase in CIA with silica-poorer and more pelitic nature is a common feature and readily explained by the higher proportion of clays (weathering products) in pelites. The CIA value is also affected by other processes than the clastic composition of the rock in question. Overestimation of Ca in carbonates can lead to too high CIA values if Mg-bearing carbonates are present. Fortunately only a few samples have over 0.5% CO 2 and thus this is only problematic in limited cases but is especially crucial for quartz-rich samples. The other problem is related to the loss of CO 2 and incorporation of liberated Ca in recrystallizing minerals (e.g. epidote and plagioclase) during metamorphism (cf. Lahtinen, 1996) a situation proposed for some samples in the Höytiäinen area (Fig. 7). The prevailing climatic conditions of the source areas during sediment formation are difficult to estimate especially if we consider the recycled nature of many sediments, possibly having older weathered components. The situation can be thus complex including mixing of a strongly weathered component (older sediments or deeply weathered palaeosol) with immature crust components before deposition, forming a sedimentary rock showing moderate CIA values. Also the degree of weathering is related to the rate of erosion, which is high in tectonically active areas and thus inhibiting extensive weathering even in high rainfall tropical conditions. The extent of weathering is determined primarily by the amount of rainfall (acids) on the weathering profile (Singer, 1980) where as the climatic effect on weathering trends is probably insignificant (Nesbitt and Young, 1989). The REE, Th and HFSE (especially Sc) are considered least susceptible to fractionation by exogene processes including weathering (Taylor and McLennan, 1985; McLennan et al., 1990). REE mobility during weathering has been nevertheless observed (Nesbitt, 1979; Duddy, 1980; Condie et al., 1995) although Nesbitt (1979), Duddy (1980) found no net losses or gains when whole weathering profiles were considered. Depletion of Sc has been postulated during weathering under low-o 2 atmosphere (Maynard et al., 1995). The Palaeoproterozoic autochthonous units above Archaean basement in the study area (Kohonen and Marmo, 1992 and references therein) start with the Ilvesvaara Formation overlain by the glaciogenic Urkkavaara Formation followed

15 R. Lahtinen / Precambrian Research 104 (2000) by Hokkalampi Palaeosol. Sturt et al. (1994) concluded that widespread 2.35 Ga regolith (including the Ilvesvaara Formation) occurred on the Fennoscandian shield and was related to an arid or semi-arid palaeoenvironment. Although this might be the case for the Ilvesvaara Formation, the occurrence of the up to 80 m deep Hokkalampi Palaeosol (not mentioned by Sturt et al., 1994) with a minimum age of 2.2 Ga records intense chemical weathering under a tropical warm and humid climate (Marmo, 1992). The drift of Fennoscandian from 30 S at 2435 Ma to about 30 N at 2100 Ma (Pesonen et al., 2000) shows that Fennoscandian crossed the equator during this time favouring the interpretation of Marmo (1992). It has been suggested that the Hokkalampi Palaeosol and derived formations covered large areas of the stable Karelian craton (Kohonen and Marmo, 1992; Marmo, 1992) where they formed the bulk of detritus for the Palaeoproterozoic rift basins. The chemical and mineralogical data of the Hokkalampi Palaeosol indicate a typical weathering sequence (cf. Nesbitt and Young, 1989; Condie et al., 1995) with an initial decrease in the amount of plagioclase followed by loss of K-feldspar and biotite seen as an increase in CIA values from about (lowermost) to the highest values of in the upper zone (Marmo, 1992). Potassium metasomatism of kaolinite to illite in palaeosol results in lowering of CIA values (Fedo et al., 1995). This possibility has been studied using an A CN K compositional space (Fig. 7) for the data of the Hokkalampi Palaeosol formed upon K-feldspar rich granitoid and sandstone. There is a slight amount of added potassium in lower palaeosol zones probably due to percolation of solutions from the leached uppermost potassium-depleted zone during weathering (Marmo, 1992). However, if the whole mass balance of the weathering profile is considered, no input of external potassium is needed. Fig. 7. A CN K and (A K) C K triangles (see Fedo et al., 1995, 1997) depicting trends in the Hokkalampi palaeosol and autochthonous groups of this study. (A) Data for Hokkalampi palaeosol formed upon a K-feldspar-rich granitoid (granitoid zones 2 3) and sandstone (sandstone zones 1 3), and an average of Archaean crust and Archaean sedimentary rocks (Ar1 Ar2). Trajectories a and b represent weathering trends for sandstone and Archaean average crust predicted from kinetic leach rates (Nesbitt and Young, 1984). (B) Data for Jatuli-type quartzites and autochthonous groups H1 H3. Trajectories a and b same as in Fig. 7A. Dashed line encloses possible source end members for autochthonous sedimentary rocks. (C) Data for Jatuli-type quartzites and autochthonous groups H1 H3. Note the shift of some samples towards the sodium-rich (A K) N-line indicating that albitization has possibly affected these samples. Horizontal arrows for some samples indicate the amount of Ca input due to the inferred occurrence of carbonates followed by CO 2 loss. Averages of palaeosol zones 1 3 in Fig. 7B and C are calculated using mixtures of sandstone zones (50%) and granite zones (50%). J and K are calculated averages of Jatuli-type mafics and Kutsu-type granites, respectively.

16 162 R. Lahtinen / Precambrian Research 104 (2000) The autochthonous Höytiäinen H1 H3 groups show characteristic depletion of CaO, Na 2 O, MnO, P 2 O 5, Sr and Ba, and low K/Rb, which are tentatively proposed to have an ultimate source in the chemically weathered palaeosol. The southern Svecofennian RH1 RH2 groups also show depletion of elements normally lost during weathering (Fig. 6) but the CIA values of other groups are moderately low ( 60) and no clear weathering trends are observable Hydraulic sorting Clay minerals, enriched in most trace elements, and preferentially concentrated in the finer fractions during hydraulic sorting (grain size sorting) produce higher abundances of many elements in pelites relative to associated sands (e.g. Korsch et al., 1993). The situation of pure quartz dilution is the ultimate case and most easily interpreted as a decrease in all other elements and an increase in SiO 2. The situation is more complex when accessory minerals (zircon, monazite, apatite, sphene and allanite), ferromagnesian minerals, feldspars and lithic fragments are also sorted. The Th/Sc ratio remains nearly constant in some cases but often muds can have significantly lower Th/Sc ratios indicating a preferential incorporation of mafic volcanic material in the finer fractions (e.g. McLennan et al., 1990). Considering a simple two-component mixture of mature weathered material (quartz+clays) and immature rock debris (separate minerals+lithic fragments) the result is psammites enriched in immature rocks debris showing complex sorting patterns and pelites enriched in mature weathered material. This preferential sorting can lead to REE fractionation making interpretation of Sm Nd isotope systematics difficult (Zhao et al., 1992) but this is mainly effective when considering sedimentary material from unweathered coarse-grained granitoids with, e.g., allanite hosting LREE and Th. The wide range of SiO 2 (Fig. 4) the Höytiäinen H1 H3 groups exhibit is clearly an effect of sorting (cf. Kohonen, 1995) dominated by quartz dilution seen as abundant quartz clasts. Sorting enhanced enrichment of mafic component was noticed, e.g. in Western Kaleva and southern Svecofennian pelites over psammites. The variation of Zr (normally ppm) found in Western Kaleva psammites indicate zircon sorting but there is no correlation between Zr and HREE or U showing that the zircon control on these elements is minor. The effect of hydraulic sorting is readily observed in the studied samples but in many cases it also sorts different source components into different grain size classes. This is a disadvantage when using only shales (on average more mafic) or psammites (on average more felsic) in crustal evolution studies but is an advantage in characterizing source end members Effects of depositional en ironment Different methods have been applied to the interpretation of the depositional environment of ancient sediments using black shales/schists. These include pyrite formation, S/C ratios, degree of pyritization (Berner, 1984; Berner and Raiswell, 1984; Raiswell and Berner, 1986) and enrichment of U and V (e.g. Jones and Manning, 1994; Breit and Wanty, 1991). The average present S/C ratio of normal marine sediments is 0.36 ( ) but age dependent variation occurs and, for example, early Palaeozoic marine sediments show significantly higher S/C ratio of about 2 (Berner and Raiswell, 1984; Raiswell and Berner, 1986). In fresh or low-salinity brackish water low sulfate level is the limiting factor for pyrite formation and sediments show low S/C ratios without any inter-element correlation (Berner and Raiswell, 1984). According to Thompson and Naldrett (1984) mantle-derived magmatic S/Se ratios are generally lower ( ) than in sedimentary sulphides ( ), which can be used to discriminate hydrothermal influxes of sulphur. Only autochthonous (H1 H3) and allochthonous (WK1 WK2) groups have sufficient samples with carbonaceous matter (graphite) to be plotted in the S vs. C diagram (Fig. 8). The Höytiäinen pelitic samples, especially H2 samples, show good correlation between S and C (S/C about 3.5). There is also a slight increase in U seen in the decrease of Th/U ratios from about 4 to about 2.5 in the H2 samples. These features

17 R. Lahtinen / Precambrian Research 104 (2000) Fig. 8. Plot of C graf. vs S for autochthonous (H1 H3) and allochthonous (WK1 WK2) sedimentary rocks in this study divided into low S/Se ( ) and high S/Se ( ) populations. The S/C ratio 0.36 is for normal marine sediments after Berner and Raiswell (1984). indicate anoxic conditions during deposition and if the S/C ratio of 3.5 is higher than found in the Palaeoproterozoic marine sediments during deposition, it could point out to euxinic environment. The Western Kaleva samples differ from the Höytiäinen basin examples in that they do not show any clear correlation between S and C. The graphite-enriched ( 0.5% C) psammites have low S/C ratios ( 0.15) and S/Se ratios mostly Apart the graphite variation (0 1.6% C) there is no enrichment of studied elements. The occurrence of graphite-bearing thick psammites does not favour a direct hemipelagic origin and indicates mixing of carbonaceous matter into mass flows before deposition. The low S/C ratios could point to fresh water or brackish water environments, or to short intervals between deposition of mass flows preventing significant bacterial sulfate reduction. The lack of U and V enrichment indicates an oxygenated environment while a low S/C excludes an euxinic environment Diagenesis and metamorphism Monitoring the effects of diagenesis in metamorphic rocks is a difficult task due recrystallization requiring that any evaluation of the diagenetic history be based on geochemistry. Ma- jor factors related to the degree of diagenesis are thermal history and time, where rapid burial compacts sediments quickly (dewatering) and blankets any thermal changes (Lee and Klein, 1986). Thus long-lived basins, like the Höytiäinen basin (Kohonen, 1995), should show more pronounced effects of diagenesis compared to allochthonous Western Kaleva-type rocks that were deposited as massive units in an active tectonic setting. The very limited element variation in the WK rocks favours this and although small-scale diagenetic changes within WK samples are possible, a largescale redistribution of elements is not evident. Similar arguments hold for most of the central Svecofennian rocks but, for example, the depositional environment and the elapsed time before dewatering and metamorphism of the Archaean and southern Svecofennian mature rocks are unknown. Diagenetic reactions may include Na-, K-, Mg- and Fe-metasomatism (e.g. Nesbitt and Young, 1989) while REE redistribution and fractionation have also been proposed (Awwiller and Mack, 1991; Milodowski and Zalasiewicz, 1991; Ohr et al., 1991). There is not however consensus about how common the redistribution of REE during diagenesis is (cf. Hemming et al., 1995) and one critical question is that are the proposed diagenetic reactions open or closed systems at sample scale. Redistribution of alkalies during diagenesis has been proposed for the Höytiäinen area rocks (Kohonen, 1994) and to evaluate this possibility, the data are plotted in the A CN K and (A K) C N compositional spaces (Fig. 7; see also Fig. 4 for K 2 O). The data show scatter and there are several factors that may have been responsible for the observed trends: (1) Sedimentary rocks have different source components with different K 2 O/ Na 2 O ratios (see differences in MgO contents and Th/Sc and Th/Cr ratios; Figs. 4 and 9). The problem lies also in the thinly layered nature of pelites where chlorite-rich and biotite-rich layers were noticed, possibly indicating that different layers were derived from different sources in some cases. (2) During grain-size sorting K-rich phases (illite and biotite-vermiculite) are enriched in pelites (K-feldspar is rare in these rocks) and plagioclase in sands forming a trend similar to

18 164 R. Lahtinen / Precambrian Research 104 (2000) that observed in the A CN K compositional space. (3) Albitization of K-feldspar in the sandsize fraction with immediate uptake of liberated K by kaolinite, chlorite, montmorillonite and/or smectite in the clay-rich fraction as proposed by Kohonen (1994). Based on Fig. 7C albite metasomatism has occurred to some degree in some samples favouring Kohonen s (Kohonen, 1994) interpretation. (4) Regional-scale potassic and sodic metasomatism affecting shales and silt-sandsize particles, respectively, has been proposed for the Palaeoproterozoic Serpent Formation (Fedo et al., 1997). The Serpent shales show ultimate potassium variation from 3.3 to 11.2% whereas the H1 H3 pelites show only variation from 3 to 5% (Fig. 4) where the variation is mainly due to the factors 1 3, as discussed above. Thus, the problem in depicting the amount of diagenetic redistribution in the Höytiäinen area rocks is that they show complicated mixing of source components associated with sorting and thus distinguishing purely diagenetic effects is difficult. Although not conclusive it seems that small-scale redistribution of elements has occurred during diagenesis in the Höytiäinen area but no externally derived regional-scale metasomatism, at least for potassium, is observed. Prograde metamorphic effects on REEs, except in areas of partial melting, are minor (Taylor et al., 1986) but the depletion of LILE elements (K, Rb, Ba) has been proposed for granulite terrains (e.g. Weaver and Tarney, 1983; Sheraton, 1984). Fig. 9. Plots of Sm/Nd vs. Th/Sc and Th/Cr for selected sedimentary rocks in this study. The Archaean average has been calculated from the average AC1 in Table 1 and Jatuli-type mafics from the average (N=21) in Lahtinen (unpublished data). See Fig. 5.

19 R. Lahtinen / Precambrian Research 104 (2000) The tonalite migmatites (veined gneisses, schollen migmatites and diatexites) in the study area show variable compositions due to differences in the relative amounts of restite and leucosome in sampled outcrops and those that represent totally melted in situ variants. A depletion of Bi is the main common feature and although migmatites with high proportions of restite component occur there is no area showing large-scale depletion of elements. In many cases the veined gneisses have mostly retained their original composition (cf. Lahtinen, 1996). The southern part of the Rantasalmi Haukivuori area (southern Svecofennian) is characterized by in situ migmatites (RH1 RH2) with variable amounts of restite and granite leucosome components. This difference in the leucosome composition (tonalite granite) has been attributed to the aluminium excess in the source rocks of migmatites having granite leucosomes (Korsman et al., 1999). This interpretation is favoured by the typical CIA values of in the RH1 RH2 rocks compared to the typical CIA values below 60 in the source rocks of tonalite migmatites. On the other hand water-rich conditions during tonalite migmatization favour the formation of plagioclase-enriched melts and water-rich conditions has been considered as the main cause for the formation of tonalite migmatites (Lahtinen, 1996) Main source components The proposed main source components of sedimentary rocks of the Archaean craton and its cover, and Svecofennian domain are mainly based on the geochemical differences but Sm Nd results by Huhma (1986, 1987), O Brien et al. (1993) are also adopted. There are only a few detrital zircon age determinations from the Fennoscandian Shield (Huhma et al., 1991; Claesson et al., 1993) and thus the conclusions presented below are to some extent tentative but serve as a working model for future work. Boundary zone sedimentary rocks (BZ1 BZ2) are probably related to the 1.92 Ga primitive island arc but the occurrence of numerous fault zones, extensive migmatization and complicated shearing precludes further source component interpretation Archaean sedimentary rocks The Archaean sedimentary rocks show very low Th/Cr ratios, which discriminate them from other rocks in this study (Fig. 9). O Brien et al. (1993) concluded that greywackes in the eastern part of the study area (Ar2-type) with T DM ages from 2.83 to 2.99 Ga normally show a local source. The Ar1 samples show a more homogenized source and higher degree of weathering of the source area with higher MgO, Cr, K 2 O and SiO 2. One Archaean sediment has a T DM of 3.24 Ga (Huhma, 1987) favouring also the existence of an older component (cf. Sorjonen-Ward, 1993). Two main ages of source components with variable amounts of intermixing are proposed for the Archaean sediments in the study area: 1. Older main component with Ga average source age. At least three different source rock types are indicated: komatiites (high MgO, Cr, Ni, Cr/Sc), tholeiite (high TiO 2 and Nb/Th) and felsic component (SiO 2,K 2 O and Rb). Intermediate to strong weathering in the source area and thorough mixing has occurred before deposition. Possible sources are greenstone+granite TTG. 2. Local source derived from the Ga (Vaasjoki et al., 1993) magmatic event (cf. O Brien et al., 1993) Cratonic co er Local Archaean craton sources with contributions from Jatuli-type mafic volcanics and dykes has been a common source model for the Höytiäinen basin sedimentary rocks (Huhma, 1987; Ward, 1987; Kohonen, 1995). The results of this study favour this general statement but the composition of the H1 H2 groups is not explained by simple mixing of the presently exposed erosion level of the Archaean crust and Jatuli-type mafics (Figs. 4, 5 and 9) because an additional Cr-rich source is needed. The simplest explanation is higher amounts of Archaean sedimentary rocks (Cr-rich) in the average source area for the H1 H2 group samples. Some sedimentary rocks have high proportions of local Archaean cratonic