| Summary: | We characterise the geochemistry, zircon Lu-Hf composition, age and the structure of the Uunimaki gabbro (UGB) in south-western Finland to improve the understanding of i) the early Svecofennian (1.92-1.89 Ga) crustal evolution of the central Fennoscandian Shield, ii) the potential role of rift-related magmatism for the build-up of the Paleoproterozoic accretionary orogens and iii) evaluate, which geological features provide the primary control over the localization of an orogenic gold mineralisation. The zircon U-Pb geochronology defines an age of 1891 +/- 5 Ma for the UGB, which is slightly older than most mafic intrusions in south-western Finland. The obtained chondritic initial zircon eHf values with E-MORB type geochemical affinity suggest a sub continental lithospheric mantle source for the UGB. The overall geochemistry indicates that the UGB magma as well as other E-MORB type rocks in the Pirkanmaa and Hame belts were formed in a rift-related environment in a fore-arc region at 1.89 Ga, predated by arc-type magmatism at similar to ~1.90 Ga and back-arc magmatism at similar to ~1.92 Ga in the Tampere belt. Slab retreat due to roll-back is suggested to cause the extension and related magmatism in the forearc region. Moreover, the timing and compositional and isotopic changes of early-orogenic magmatism are broadly compatible with intervals of extension and contraction, i.e., a tectonic switching model, and may provide a perspective to rapid build-up of Paleoproterozoic crust. Structural characterisation provides a framework where gold mineralisations are preferentially located within the high-strain north-eastern domain of the UGB, within fracture networks adjoining the high-strain zones. Our results indicate that neither the geochemical composition nor age of the intermediate-mafic intrusive host rocks play a major role in controlling the formation of gold mineralisation. By contrast, the localization of orogenic gold is controlled by localised structures (shear zones, fractures), and the variation in ...
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