Isotopic investigations of metasedimentary and igneous rocks in the Palaeoproterozoic Bothnian Basin, central Sweden

Abstract

A Palaeoproterozoic Svecofennian sedimentary basin (the Bothnian Basin) extended from central Sweden to eastern Finland. A U-Pb, Rb–Sr, Sm–Nd and Pb isotopic study has been made of plutonic and metasedimentary rocks from the basin. The metasedimentary rocks consist of detritus from Archaean and Palaeoproterozoic rocks. The provenance of Archaean detritus was either the craton to the northeast of central Sweden, or a terrain to the west of the Fennoscandian Shield. The main source area for Palaeoproterozoic detritus appears to have been the volcanic arc systems to the north and to the south of the Bothnian Basin, although early orogenic Svecofennian intrusive rocks and basal sediments in the basin have supplied some detrital material. The sedimentation began at the earliest 2000 Ma ago and continued at least until 1867 Ma.

Syn- and late- to postorogenic magmas were derived from the upper mantle and lower crust and intruded 1890 to 1790 Ma ago. During their ascent through the crust they assimilated crustal material. In particular, the late-orogenic granites exhibit an Archaean component, which presumably originated from Bothnian Basin metasediments.

It is suggested that the basement of the sedimentary basin consisted of continental crust, comprising Palaeoproterozoic acidic and intermediate rocks. Synorogenic granitoids in the adjacent volcanic are terrains have almost no old crustal component, their ε_Nd values are markedly positive.

Key Words:

• Rb–Sr
• U–Pb radiometric ages
• Pb and Sm–Nd isotopic data
• central Sweden