Abstract In central parts of the Scandinavian Caledonides, detrital zircon signatures provide evidence of the change in character of the Baltoscandian crystalline basement, from the characteristic Late Palaeoproterozoic granites of the Transscandinavian Igneous Belt (TIB, c. 1650–1850 Ma) in the foreland Autochthon to the typical, mainly Mesoproterozoic-age profile ( c. 950–1700 Ma) of the Sveconorwegian Orogen of southwestern Scandinavia in the hinterland. Late Ediacaran to Early Cambrian shallow-marine Vemdal quartzites of the Jämtlandian Nappes (Lower Allochthon) provide strong bimodal signatures with TIB (1700–1800 Ma) and Sveconorwegian, sensu stricto (900–1150 Ma) ages dominant. Mid-Ordovician turbidites (Norråker Formation) of the Lower Allochthon in Sweden, sourced from the west, have unimodal signatures dominated by Sveconorwegian ages with peaks at 1000–1100 Ma, but with subordinate components of older Mesoproterozoic zircons (1200–1650 Ma). Latest Ordovician shallow-marine quartzites also yield bimodal signatures, but are more dispersed than in the Vemdal quartzites. In the greenschist facies lower parts of the Middle Allochthon, the Fuda (Offerdal Nappe) and Särv Nappe signatures are either unimodal or bimodal (950–1100 and/or 1700–1850 Ma), with variable dominance of the younger or older group, and subordinate other Mesoproterozoic components. In the overlying, amphibolite to eclogite facies lower part of the Seve Nappe Complex, where the metasediments are dominated by feldspathic quartzites, calcsilicate-rich psammites and marbles, most units have bimodal signatures similar to the Särv Nappes, but more dispersed; one has a unimodal signature very similar to the Ordovician turbidites of the Jämtlandian Nappes. In the overlying Upper Allochthon, Lower Köli (Baltica-proximal, Virisen Terrane), Late Ordovician quartzites provide unimodal signatures dominated by Sveconorwegian ages ( sensu stricto ). Further north in the Scandes, previously published zircon signatures in quartzites of the Lower Allochthon are similar to the Vemdal quartzites in Jämtland. Data from the Kalak Nappes at 70°N are in no way exotic to the Sveconorwegian Baltoscandian margin. They do show a Timanian influence (ages of c. 560–610 Ma), as would be expected from the palinspastic reconstructions of the nappes. Thus the detrital zircon signatures reported here and published elsewhere provide supporting evidence for a continuation northwards of the Sveconorwegian Orogen in the Neoproterozoic, from type areas in the south, along the Baltoscandian margin of Baltica into the high Arctic. Supplementary material: LA-ICP-MS U–Pb analyses are available at http://www.geolsoc.org.uk/SUP18699 .

Abstract Secondary ionization mass spectrometry (SIMS) U–Pb dating of zircons from the Åreskutan Nappe in the central part of the Seve Nappe Complex of western central Jämtland provides new constraints on the timing of granulite–amphibolite-facies metamorphism and tectonic stacking of the nappe during the Caledonian orogeny. Peak-temperature metamorphism in garnet migmatites is constrained to c. 442±4 Ma, very similar to the ages of leucogranites at 442±3 and 441±4 Ma. Within a migmatitic amphibolite, felsic segregations crystallized at 436±2 Ma. Pegmatites, cross-cutting the dominant Caledonian foliation in the Nappe, yield 428±4 and 430±3 Ma ages. The detrital zircon cores in the migmatites and leucogranites provide evidence of Late Palaeoproterozoic, Mesoproterozoic to Early Neoproterozoic source terranes for the metasedimentary rocks. The formation of the ductile and hot Seve migmatites, with their inverted metamorphism and thinning towards the hinterland, can be explained by an extrusion model in which the allochthon stayed ductile for a period of at least 10 million years during cooling from peak-temperature metamorphism early in the Silurian. In our model, Baltica–Laurentia collision occurred in the Late Ordovician–earliest Silurian, with emplacement of the nappes far on to the Baltoscandian platform during the Silurian and early Devonian, Scandian Orogeny lasting until c. 390 Ma.