Extensive Sm-Nd isotope work has been carried out in northern Sweden with the intention of studying crustal reactivation and assimilation processes relative to juvenile arc accretion at an Archaean cratonic margin. Previous regional work has shown a gradual change from negative εNd(t) values in northernmost Sweden to positive values towards the Skellefte sulphide ore district farther south. Isotopic variation and geophysical data show a break in the character of the crust along a WNW-trending zone. It is suggested that this zone, which passes close to the towns of Luleå and Jokkmokk, represents the transition from the Archaean craton in the northeast to Proterozoic juvenile crust in the southwest. A profile across the eastern part of this transition zone has been studied in detail in the Luleå area, where Archaean rocks recently have been discovered. The distribution and character of the Archaean rocks have been studied. These occur as fragments, ranging in size from several km2 to dm in scale, and consist of ca 2.7 Ga orthogneisses of granodioritic to dioritic composition and porphyritic granitoids of quartz monzodioritic to granodioritic composition. Plutonic rocks of ca 1.9 Ga age and Archaean rocks were analysed for major and trace elements, and for Sm-Nd isotopes. The calc-alkaline ca 1.9 Ga granitoids are dominated by granodiorites, tonalites, quartz diorites and quartz monzodiorites, and there is a distinct boundary rather than a transition zone between intrusions with postive εNd(t) values in the southwest and intrusions with negative εNd(t) values in the northeast. The locations of the outcropping Archaean fragments coincides with this distinct boundary, which is interpreted as a terrane boundary separating a ca 1.9 Ga juvenile arc terrane in the southwest from the Archaean continent in the northeast. A possible explanation for this is that the Proterozoic juvenile terrane was obducted onto the Archaean craton during the final stages of the Svecofennian orogeny.