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 Häme belt (HB) in the south bound the PB.Basaltic to picritic volcanic rocks in the WF supersuite, with a proposed eruption age at ca. 1.91Ga (1.92–1.90Ga), 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 ≥1.91Ga maximum depositional age in the TB, PB and HB have been compared and typically the age distributions of <2.2Ga zircons in different areas favor comparable source characteristics. The major metamorphic event at 1885–1875Ma in the WF supersuite is well dated. Although an earlier metamorphic stage at 1.92–1.91Ga is locally probable most of the <1.94Ga overgrowths in the WF supersuite are considered pre-depositional and recycled. Two models are presented to explain the hypothesis of similar pre-1.91Ga evolution in the WF supersuite and lowermost parts of the TB and HB, and contemporaneous arc magmatism at 1.90–1.88Ga in the Skellefte district (SD), Central Finland Granitoid Complex (CFGC), TB and HB: a) all the 1.90–1.88Ga volcanic rocks have formed in one linear arc system from the SD to the HB; b) there have been two separate arcs SD-CFGC-TB and HB of similar age with double-plunging subduction zones. In both models, orogen parallel shortening and buckling is geometrically viable mechanisms for the origin of oroclinal bending, but slab rollback models should also be evaluated in further studies.
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