ATTEMPTED BREAK-UP OF RODINIA AT 850 MA: GEOCHRONOLOGICAL EVIDENCE FROM THE SEVE-KALAK SUPERTERRANE, SCANDINAVIAN CALEDONIDES

Abstract

Lower thrust sheets of the Scandinavian Caledonides derive from the margin of Baltica, which was imbricated during Early Palaeozoic closure of oceans formed during separation of the Baltica. Laurentia and Siberia cratons. At Vistas (Kebnekaise Mts), the Seve Nappe Complex preserves rare lenses of a doleriteintruded granite formed by anatexis during emplacement of gabbro into metasedimentary rocks. Ion microprobe U-Pb dating of prismatic zircons from the granite yielded an age of 845 ± 14 Ma (mean square weighed deviation (MSWD) 1.15), interpreted to date magmatic crystallization. Cores of complex crystals indicate protolith ages of 1778 ± 11 Ma (MSWD 0.86); one core yielded an Archaean age. From narrow rims, a 605 ± 42 Ma metamorphic age is obtained, and is interpreted to reflect the emplacement of the extensive Vendian rift magmatic dolerite dyke swarm. The age pattern allows correlation with a previously dated magmatic complex within the equivalent Kalak Nappe Complex 300 km to the north. Bimodal magmatic complexes with this age pattern do not occur within the basement of the Caledonide foreland, nor elsewhere within the Baltic Shield. However, recent reinterpretations of the Knoydartian event in Scotland as rift related invites correlation with the West Highland Granite Gneiss, which intruded Moine metasedimentary rocks at c. 870 Ma. In Central Taimyr, 850-900 Ma granites of continental crustal derivation and with 1800-1900 Ma Sm-Nd model age occur associated with c. 740 Ma plagiogranites. Using recent palaeogeographical reconstructions, we interpret the evidence of 850-900 Ma magmatism in Scandinavia, Scotland and Taimyr as bimodal rift magmatism in connected arms during an attempted break-up of Rodinia.