ABSTRACT Detrital zircon provenance studies of Precambrian metasedimentary rocks in Wedel Jarlsberg Land and Sørkapp Land, Svalbard’s Southwestern Caledonian Basement Province, were conducted to evaluate local stratigraphic correlations and the role of long-distance strike-slip displacements in assembling the basement of the Arctic Caledonides. The detrital zircon U-Pb age spectra of the late Mesoproterozoic to Neoproterozoic metasediments revealed mainly Mesoproterozoic to Paleoproterozoic age signatures characteristic for a Grenville–Sveconorwegian orogen provenance. These results confirmed a stratigraphic correlation between basement units of southern Sørkapp Land and the Isbjørnhamna Group of Wedel Jarlsberg Land and suggest relocation of the tectonic boundary between the Eimfjellet Complex and the Isbjørnhamna Group above the Eimfjellbreane Formation. Moreover, the results support the Vimsodden Kosibapasset Shear Zone (VKZ) as a major tectonic boundary and highlight the inhomogeneity in the Southwestern Caledonian Basement Province. The detrital zircon age signatures south of the VKZ bear similarities with coeval metasediments of the Northwestern Caledonian Basement Province of Svalbard and other localities in the Greenland and Scandinavian Caledonides. In contrast, the detrital zircon age spectra north of the VKZ are comparable with the high Arctic Neoproterozoic sediments of Baltican affinity. In conjunction with previous studies, the results suggest that the basement units may continue across the traditional boundaries of the Svalbard’s Caledonian basement provinces.

ABSTRACT Eocene Eurekan deformation has proven to be an enigmatic sequence of tectonic episodes dominated by tectonic plate compression and translation in the circum-Arctic region. Prins Karls Forland on western Spitsbergen is composed of Neoproterozoic siliciclastic metasediments of Laurentian affinity regionally metamorphosed to greenschist facies conditions. A crustal-scale ductile to brittle deformation zone, here named the Bouréefjellet fault zone, contains the amphibolite facies Pinkiefjellet Unit exposed between the lower metamorphic grade, upper structural unit of the Grampianfjella Group and the Scotiafjellet Group in the footwall. A preliminary age for the amphibolite facies metamorphism (ca. 360–355 Ma) indicates Ellesmerian tectonism, unlike other higher-grade basement rocks on Svalbard. Ten metasedimentary rocks from within the fault zone were collected for multiple single-grain fusion 40 Ar/ 39 Ar geochronology, with up to ten muscovite crystals dated per sample. High strain in the rocks is evinced by mylonitic structure, mica fish, and C’ shear zones, and dynamically recrystallized quartz with significant grain bulging and subgrain rotation, indicative of >350 °C temperatures. There is notable dispersion in the 40 Ar/ 39 Ar ages between samples, with single muscovite dates ranging from ca. 300 Ma to as young as 42 Ma, reflecting recrystallization and resetting of the muscovite. Younger, reproducible ages were obtained from samples that possess chemically homogeneous muscovite, yielding dates of 55–44 Ma for the Eurekan deformation on Prins Karls Forland. We suggest that Ellesmerian structures on Prins Karls Forland were reactivated during the Eocene (commencing as early as 55 Ma) progressing under warm, yet brittle, conditions that continued to 44 Ma. These 40 Ar/ 39 Ar muscovite dates are the first documented Eurekan deformation ages from Svalbard and enable a better understanding of the stages of Eurekan deformation in the Eocene to improve correlations across the circum-Arctic region.

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 New evidence is presented for ultra-high-pressure metamorphism of kyanite–garnet pelitic gneiss in the Åreskutan Nappe of the Seve Nappe Complex, in the central part of the Scandinavian Caledonides. Modelled phase equilibria for a peak pressure assemblage garnet+phengite+kyanite+quartz (coesite) in the NCKFMMnASH system record pressure and temperature conditions of c. 26–32 kbar at 700–720 °C, possibly up to ultra-high-pressure conditions. Subsequent decompression, simultaneous with an increase of temperature to c. 800–820 °C, led to partial melting largely owing to the dehydration and breakdown of phengite. Based on existing isotope age data, we conclude that the Middle Seve Nappe in central Jämtland experienced deep subduction in the late(st) Ordovician, prior to decompression and partial melting of the pelitic protoliths during Early Silurian extrusion, giving way in the Mid to Late Silurian to thrusting on to the Baltoscandian platform. Nappe emplacement probably continued into and through the Early Devonian.

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.