Implications of UPb ages of columbite-tantalites from granitic pegmatites for the Palaeoproterozoic accretion of 1.90–1.85 Ga magmatic arcs to the Baltic Shield

Abstract

The Palaeoproterozoic growth of the Baltic Shield involved the accretion of several ∼1.90–1.85 Ga old magmatic arcs to the southwest of the Archaean craton and the deformation and migmatization of sedimentary basins located between the arcs. During crustal thickening after the collision of the arcs, the sedimentary basin fill became intruded by peraluminous two-mica granites. Locally, columbite-bearing pegmatites are genetically associated with these granites. Columbites from lithium-cesium-tantalum-type (LCT-type) pegmatites from the Stockholm area (Sörmland gneisses) yield UPb ages at 1815–1820 Ma, while niobium-yttrium-fluorine-type (NYF-type) pegmatites from the same area are younger (1795±2 Ma, 2σ). Farther to the north, LCT-type pegmatites from the central Bothnian Basin, that correspond geochemically and mineralogically to those of the Stockholm area, yield UPb columbite ages at 1795–1800 Ma, while LCT-type pegmatites in the sedimentary basin between Skellefteå and Luleå yield less well constrained UPb columbite ages at 1765–1775 Ma. LCT-type pegmatites are mainly associated with crustal melts that form during postcollisional thickening of continental crust. They represent markers for the time when the Palaeoproterozoic Baltic Shield suffered sufficient thickening to yield voluminous anatectic melts. The UPb columbite ages from the LCT-type pegmatites indicate that comparable phases of post-collisional crustal thickening of the Svecofennian area of the Baltic Shield occurred later to the north.