Palaeoproterozoic Metasedimentary Rocks in the Tampere-Hämeenlinna Area, Southern Finland: Chemostratigraphical Division into
Arc- and Basement-related Groups

Raimo Lahtinen Geological Survey of Finland, FIN-02150 Espoo, Finland


The study area lies entirely within the Palaeoproterozoic Svecofennian domain and is characterized by two well-preserved volcano-sedimentary belts (TSB, HSB) separated by a sediment-dominated collisional belt (MB) containing mica gneisses and tonalitic LPHT migmatites. Migmatization is seen in increase of Cl, depletion of B and redistribution of sulphide-related elements, but otherwise migmatites appear to have a composition close to the original (Lahtinen, 1996). The lack of clear marker horizons, the complex structural history and the incomplete preservation of sedimentary sequences hinder lithostratigraphical correlations in most of the study area. One exception is the central TSB (Kähkönen et al., 1994) where the detrital zircon data (Huhma et al., 1991; Claesson et al., 1993) from two lower metagreywackes show the occurrence of both Palaeoproterozoic 1.91-2.04 Ga (2/3) and Archaean (1/3) source components. The chemical composition of metasedimentary rocks of the study area was used in making a chemostratigraphical classification to arc- and basement-related rocks. The latter were further divided into groups SG3-SG7, where the SG3-SG5 rocks are correlated with the lower greywackes in the TSB, which are considered to have been deposited before the TSB island arc volcanism (Lahtinen, 1996).

Results and discussion

The characteristic feature of the arc-related metasedimentary rocks is enrichment of Sr, Ba and P, and similar levels of Cr and Ni relative to average Palaeoproterozoic upper crust (PUC (Condie, 1993)), together pointing to a local TSB island arc source (1.9 Ga) enriched in high-K
calc-alkaline and shoshonite volcanics. The low degree of chemical weathering and wide silica variation in the arc-related group indicate rapid erosion of arc material and subsequent deposition in isolated basins in the absence of strong mixing in large river-delta systems. The basement-related group shows smaller variation in chemical composition indicating a more homogenized source. Groups SG3 and SG4, which are characterized by greywackes, show slightly different source characteristiscs, as seen in the Th/Sc ratios (0.8-1.5 and 0.5-0.8, respectively). SG5 rocks have been assumed to be of more distal origin than SG4 rocks, and the minor differences seen in lower Th/Sc ratio (0.5-0.6) are interpreted to be due both to sorting and to the more mafic source. Groups SG6-SG7 have a more pelitic origin than SG3-SG5 rocks, as seen in the higher abundances of elements (K,Rb,Ti,Sc,Cr) enriched in clay minerals. The similar variation in the Th/Sc ratios of the SG4 and SG6 groups (0.5-0.8) and the approximately similar variation in the SG3 and SG7 groups (0.8-1.5 and 0.5-1.1, respectively) could indicate a similar source for each pair and mean that the observed differences are solely due to sorting. Although this may be true for some samples, the difference in the areal distribution pattern of the groups suggests nevertheless a different origin. Basement-related metasediments in general have a more weathered source than arc-related metasediments (CIA; 56-69 and 51-59, respectively). The other major differences relative to the arc-related group, and also to PUC are the higher Rb, Th, Th/U, Sc, Cr and Ni and lower Sr and Ba in the basement-related group. Ba-depletion relative to K and Rb is more pronounced in the more pelitic groups indicating the loss of Ba during weathering (or diagenesis) when K and Rb have been more quantitatively fixed in weathering products. High Th/U ratios are interpreted to result from oxidation and removal of U during weathering and recycling and, thus, the Ba-depleted component could be derived from old recycled component. The high Cr and Ni are assumed to be from the Archaean component (komatiite). High Th and Sc indicate the enrichment of both felsic and mafic components in basement-related rocks, and both quartz depletion in the source and bimodal source are proposed. At least the Archaean component could be in part, derived from the early Palaeoproterozoic cratonic stage exemplified by strong weathering and abundant cratonic quartzites. The mixing with more juvenile source material (1.91-2.0 Ga) has occurred either before (accretionary wedge) and/or during the collision in NE before deposition in hinterland basin (TSB and northern part of MB) (Lahtinen, 1994).


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