Isotopic evidence for the Precambrian provenance and Caledonian metamorphism of high grade paragneisses from the Seve Nappes,Scandinavian Caledonides

Ion microprobe U-Th-Pb analyses of residual cores and metamorphic mantles of zircons from three high grade paragneisses from the Seve Nappe Complex, north-western Sweden, show that: 1) The sediments comprising the protolith of the Seve Nappes gneisses over a distance of 250 km were probably derived from similarly-aged source terranes. 2) Those source terranes were dominated by rocks with ages in the range 1400 to 1730 Ma, with minor components at least as young as 1000 Ma. The oldest component identified is 1730±12 Ma old. 3) Those rocks had U and Th contents normal for felsic igneous rocks. 4) The gneiss protolith was metamorphosed to granulite grade during the Caledonian. There is evidence to suggest that the peak of metamorphism may not have been synchronous throughout the Nappes. 5) The metamorphism probably included a reduction of about a factor of ten in the gneisses' Th/U.