LAUSR

Abstract The Western Finland (WF) supersuite, characterized by supracrustal rocks and intruding plutonic rocks, forms the core component of the coupled Bothnian oroclines in the central part of the Svecofennian orogen. We present here geochemical-isotope data, detrital zircon age data on sedimentary rocks and age data on granitoid plutons within the WF supersuite in the Pirkanmaa (PB) and Pohjanmaa (PoB) belt representing the southernmost limb of the southern and the hinge zone of the northern Bothnian orocline, respectively. The Tampere belt (TB) in the north and Hame belt (HB) in the south bound the PB. Basaltic to picritic volcanic rocks in the WF supersuite, with a proposed eruption age at ca. 1.91 Ga (1.92–1.90 Ga), show locally pillow-structures and are typically associated with black shales ± cherts ± carbonate-bearing rocks. Based on geochemistry and Sm-Nd data a variable MORB-WPB-type origin is evident and an evolution scenario from earlier WPB-type (rifting) to more MORB-affinity (ocean/sea opening) is proposed. Detrital zircon age distributions from sedimentary rocks with ≥1.91 Ga maximum depositional age in the TB, PB and HB have been compared and typically the age distributions of