UPb columbite ages of pegmatites from Sveconorwegian terranes in southwestern Sweden

Abstract

Post-kinematic and post-metamorphic rare-mineral pegmatites in the Sveconorwegian Province of southwestern Sweden yield UPb columbite ages at 1041.4 ± 1.6 Ma (2σ), 1038.7 ± 3.4 Ma, 1029.7 ± 1.4 Ma, and 984.3 ± 6.4 Ma. The contrasting age of spatially close pegmatites with equivalent rare-mineral assemblages demonstrates that spatial and mineralogical coincidence does not imply genetic kinship, but rather reflects the repeated prevalence of comparable pressure and temperature conditions favourable for the formation of rare-mineral pegmatites. Furthermore, our data demonstrate that the pegmatites are more than 50 Ma older than the nearby Bohus granite that earlier had been considered to be genetically related to the pegmatites. These new UPb columbite age data together with earlier UPb data from southern Norway and Sweden define a zonal pattern with increasingly younger ages to the east. These pegmatites are interpreted to have formed in relation to the thermal peak in the host or underlying terranes as they were stacked on each other in a piggy-back style during eastward thrusting onto the Baltic Shield. The youngest of these pegmatites (980 Ma) gives a minimum age for the cessation of thrusting. We interpret the 941.6 ± 1.4 Ma old Riddaho pegmatite of the Eastern Gneiss Region to be related to the post-collisional extension of the Sveconorwegian Province. This event exposed high-grade gneisses in the Eastern Gneiss Region and eventually resulted in the emplacement of voluminous melts in the internal allochthonous belt of the Sveconorwegian orogen.