Structural observations and U-Pb mineral ages from igneous rocks at the Archaean-Palaeoproterozoic boundary in the Salahmi Schist Belt, central Finland: constraints on tectonic evolution

The study area in Vieremä, central Finland, contains part of Archaean-Palaeoproterozoic boundary. In the east, the area comprises Archaean gneiss and the Salahmi Schist Belt. The rocks of the schist belt are turbiditic metagreywackes, with well-preserved depositional structures, occurring as Proterozoic wedge-shaped blocks, and staurolite schists, the latter representing higher-strained and metamorphosed equivalents of the metagreywackes. In the west of the area there is an Archaean gneiss block, containing strongly elongated structures, and deformed Svecofennian supracrustal rocks, which are cut by deformed granitoids. These are juxtaposed with the schist belt. The boundaries of these tectonometamorphic blocks are narrow, highly strained mylonites and thrust zones. The metamorphic grade of the supracrustal rocks increases from east to west, the increase being stepwise across the mylonitic block boundaries. The rocks are more deformed from east to west with younger structures overprinting. In the staurolite schists of the Salahmi Schist Belt, the most prominent structure is a lineation (L2) that overprints the bedding and axial plane foliation. In Sorronmäki quarry, at the western boundary of the schist belt, this Palaeoproterozoic lineation dominates all the structures in tonalite gneiss, which gives a U-Pb age of 2731±6 Ma. Southeast of the quarry, at the same boundary, the Salahmi schists have been overturned towards the northeast, suggesting that the Archaean gneiss at Sorronmäki has been thrust towards the northeast over these rocks. In the western part of the study area, the Leppikangas granodiorite that intrudes the Svecofennian supracrustal rocks gives a U-Pb age of 1891+6 Ma. In the granodiorite, a strong lineation formed by the intersection of two foliations, which maybe L2 is associated with thrusting towards the northeast. The monazite age of the Archaean Sorronmäki gneiss is 1817+3 Ma, and the titanite age of the Svecofennian Leppikangas granodiorite, 10 km southwest of Sorronmäki, is 1824±5 Ma. Within the limits of experimental error, these ages are identical and maybe associated with a thermal event that accompanied the D3 deformation phase. D3 structures fold D2 structures in a plastic deformation zone situated between the sample locations. Pegmatitic granite dykes intrude this NW-SE trending zone

U-Pb zircon determinations from the Keikyä Breccia and other sites in the Svecofennides: indications of a Svecokarelian protocrust

Heterogeneous zircon populations from four different localities within the Svecofennian area in Finland demonstrate that the ca. 1890 Ma old syntectonic granitoids contain older material, probably belonging to the ca. 1950 Ma Svecofennian protocrust, the influence of which is also evident in metaturbidites deformed by the Svecokarelian orogeny

Lead and neodymium isotopic results from metabasalts of the Haveri Formation, southern Finland: Evidence for Palaeoproterozoic enriched mantle

Tholeiitic metabasalts and coexisting sulphides have been analysed for their Pb and Nd isotopic compositions from the Proterozoic Haveri Formation, which forms the basal unit of the Tampere Schist Belt in southern Finland. Ten whole rock samples analysed for Pb isotopes form a sublinear array which yields rather uncertain age estimates in the 1900-2000 Ma range and lies on the 207Pb/204Pb vs. 206Pb/204Pb diagram well below the average global lead evolution curve. The initial lead isotopic composition inferred from the whole rock data and measured on chalcopyrite is the least radiogenic obtained from the Svecofennian domain, and precludes involvement of old upper crustal material in basalt genesis. This together with the geochemical composition and initial εNd (1900) of +0.5±0.6 suggest that the Haveri mafic metavolcanic rocks were not derived from convective MORB-type mantle. The source was rather a mantle, which had been enriched in LREE for a considerable time period. Some chalcopyrite trace leads plot close to the whole rock array while others lie above it. This is interpreted as indicating two distinct mineralisation processes. The primary and major process involved lead which was cogenetic with the basalts, while the second mineralising fluid introduced radiogenic (high 207Pb) upper crustal lead scavenged from the adjacent sedimentary rocks. The least radiogenic leads at Haveri and in the Outokumpu ophiolite complex some 300 km NE are similar and the two occurrences can be coeval. The preservation of original mantle material at Haveri may be interpreted as suggesting that continental crust had formed in the Tampere area 1900-2000 Ma ago

Radiometric ages and other isotopic data bearing on the evolution of Archaean crust and ores in the Kuhmo-Suomussalmi area, eastern Finland

The Archaean greenstone-gneiss terrain in the Kuhmo-Suomussalmi district in eastern Finland has been isotopically studied in connection with regional bedrock mapping and local mineral exploration projects. The studies have aimed at testing correlations of lithologic units in partly poorly exposed areas, determining times of ore formation and obtaining ore genetic information in order to better understand the general evolution of the Archaean formations within the Fennoscandian Shield. Isotopic results on 63 zircon and titanite fractions from 13 samples, common lead analyses of 14 sulphide separates from two mineral prospects and 33 whole rock Pb-Pb analyses warrant the following conclusions: 1) Although some dates in excess of 3 Ga have been determined from the Finnish Archaean, most of the granite gneiss terrain was formed between 2.85 and 2.65 Ga with a major period of rock formation from about 2.75 to 2.69 Ga. 2) The majority of the metavolcanic rocks within the Kuhmo-Suomussalmi greenstone belt are 2.79 Ga old, but the meta-andesites of the Luoma Group are distinctly older at 2.97 Ga. 3) The Taivaljärvi Ag-Zn-Pb deposit appears to be syngenetic with the local metavolcanic rocks and is thus 2.79 Ga old. Its lead probably represents the initial lead of both mafic and felsic metavolcanic rocks in the area. 4) When compared to the Abitibi region in Canada, the available Pb-Pb data may suggest a heterogeneous Archaean mantle, but as even this study shows, common lead data is inconclusive evidence, as it may be easily influenced by later hydrothermal processes

Timing of Palaeoproterozoic crustal shearing in the central Fennoscandian Shield according to U-Pb data from associated granitoids, Finland

Granitoids in the Finnish part of the Fennoscandian Shield were created by numerous Archaean and Palaeoproterozoic tectonic events. Late phases of Palaeoproterozoic, Svecokarelian deformation created ductile, crustal-scale shear zones, of which the N-S striking Hirvaskoski Shear Zone and the NE-SW striking Oulujärvi Shear Zone were strongly affected by the latest deformation stages, D3 and D4. These shear tectonic events deformed ductilely prekinematic granitoids and caused intrusion of synkinematic igneous rocks, but the D3 and D4 fault rocks are also penetrated by postkinematic intrusives. Archaean granitoids behaved mostly as rigid blocks unaffected by Palaeoproterozoic deformation. However, inherited Archaean zircons occur in some obviously Proterozoic granitoids, and others contain heterogeneous zircon populations. Granitoids lacking structures of the earliest deformations, D1 and D2, contain inherited zircon populations partially reset during the Svecokarelian orogeny and have been sheared ductilely during stages D3 and D4. Granitic rocks yielding zircon ages of 1.82-1.80 Ga show no features associated with these shearing events and penetrate the D4 fault rocks setting a minimum age for these deformation events. This age marks also the end of convergent tectonics in the central Fennoscandian Shield. Monazite data from a variety of rocks demonstrates a regional cooling below the 600°C isotherm at 1.8 Ga

The age of the ferropicritic volcanics and comagmatic Ni-bearing intrusions at Pechenga, Kola Peninsula, U.S.S.R.

Sm-Nd, Pb-Pb and U-Pb isotopic results are presented for samples from the Pilgujärvi Suite of the Pechenga Series. Whole rock samples and clinopyroxenes of ferropicritic volcanics yield a Sm-Nd isochron age of 1990 ± 66 Ma with εNd + 1.6 ± 0.4. This age is also supported by the Pb-Pb data on ferropicrites, although the Pb isotopic system was partly disturbed by secondary processes. One 90% concordant U-Pb zircon analysis from a felsic metasediment indicates a minimum age of 1970 Ma for the deposition. Our isotopic results along with previous geochemical data support the view that the ferropicritic volcanics and the Ni-bearing gabbro- wehrlite intrusions are coeval and have similar source characteristics. The calculated εNd (1990 Ma) value of + 1.6 suggests that the Pechenga ferropicrites previously had had a long-term depleted mantle source that experiended an enrichment event about 200 Ma before the melt extraction occurred. Comparison with the previously presented Sm-Nd data from northern Finland, Sweden and Norway reveals differences in initial Nd isotopic ratios, which suggests a prolonged geochemical heterogeneity in the early Proterozoic subcontinental mantle below the Baltic Shield