Palaeoproterozoic crustal building in NE Utö, southern Svecofennides, Sweden

Abstract

Evidence is presented that the greywackes of Utö were recycled by mass flows in an accretionary prism subducting the floor of an ocean open to the east. The prism built a shelf of siliclastic sediments capped by carbonate banks that were episodically smothered by falls of silicic pyroclastic rocks as intrusions of mixed andesitic to rhyolitic melts from the follow-up magmatic arc began to convert the accretionary prism into crystalline Svecofennia at about 1904 Ma. The NE trending Utö Shear Zone (USZ) juxtaposes greywackes tectonised in two major phases at different structural levels. Two generations of porphyroblasts (andalusite ± cordierite ± garnet) indicate that greywackes south-east of the USZ were never sufficiently deep to melt. Three generations of migmatites (Migs1–3 ± fibrolite) indicate that similar greywackes north-west of the USZ were subducted to temperatures that partially melted them. Mig1 leucosomes fed the older sheet granites that were foliated and deformed by the first nappes that verged westward across N–S axes during open-ocean crustal building. Mig2 leucosomes are axial planar to upright NE trending second folds and fed the graphic pegmatites and younger granites as the region thickened during continental convergence. Mig3 neosomes axial planar to the third folds are only local and probably contemporaneous with the late Li-RE pegmatites (1820–1815 Ga). Lateral tectonic escape along the USZ occurred along gneiss and schist zones hundreds of m wide before localising to metrewide mylonite zones and then millimetre-wide seismic pseudotachylites. Kinking of these structures indicate the onset of tectonic constraint as Utö rose through the ductile-brittle transition zone.