U-Pb zircon ages and Rb-Sr whole-rock isotope studies of early Proterozoic volcanic and plutonic rocks near Tampere, southern Finland

Abstract

Abstract The Tampere Schist Belt, southern Finland, is a well-preserved volcanic-sedimentary belt in the early Proterozoic Svecofennian terrain. The metasedimentary rocks of the belt are dominated by turbiditic greywackes, siltstones and mudstones. The metavolcanics range from basalt to rhyolite in composition, intermediate rocks being the most common. Near Tampere, the belt comprises an east-west trending major syncline. The U-Pb zircon ages of the ∼1.9 Ga old metavolcanics cover an age interval of ∼15 Ma . The oldest age, 1904 ± 4 Ma , is from the “Intermediate Unit” at Orivesi, which is dominated by calc-alkaline. high-K dacites and andesites. This unit is the lowest in the northern limb of the syncline at Orivesi. At Ylojarvi, a sample from the Lower Volcanic Unit has an age of 1898 ± 4 Ma , while a sample from the Upper Volcanic Unit has the youngest crystallisation age of 1889 ± 5 Ma . These ages agree with the structure and stratigraphy of the belt. The implications of the 1891 ± 16 and 1889 ± 19 Ma ages of two feldspar porphyries remain obscure; here the error limits are wide and it is not known with certainty whether the rocks are dykes, sills or volcanics. An age of 1901 ± 28 Ma was obtained for the Varmala granitoid which intrudes the schists. Although the error is large, it is possible that some of the zircon has been inherited from older rocks. The Rb-Sr whole-rock ages of the volcanics agree, within error limits, with the U-Pb zircon ages. The Rb-Sr age of the Hameenkyro batholith is, however, about 100 Ma lower than the equivalent U-Pb age. The I Sr values of the metavolcanics and the 1.882 Ga old Hameenkyro batholith range between 0.7020 and 0.7033. These low figures suggest that their protoliths separated from upper mantle sources within a short time interval. The Sm-Nd and Lu-Hf isotopic data available in the literature support the conclusion that the period around 1.9 Ga represents an important episode of continental growth, which is particularly evident in the Svecofennian-Ketilidian-Trans-Hudsonian terrains.