NEW AGES CONSTRAIN TIMING OF CALEDONIAN METAMORPHISM ACROSS VÄSTERBOTTEN, SEVE NAPPE COMPLEX, SWEDEN

Multiple ultrahigh-pressure (UHP) metamorphic events occurred during the prolonged Paleozoic closure of the Iapetus ocean and subsequent collision of Baltica and Laurentia. Two of these UHP terranes, southern Norrbotten in northern Sweden and northern Jämtland in central Sweden, are located within the same tectonostratigraphic unit, the Seve Nappe Complex (SNC). The SNC represents the westernmost, extended margin of Baltica and transition to the Iapetus ocean, and is extensively exposed along the strike of the Scandinavian Caledonides. These two UHP terranes are separated by ~250 km (Västerbotten) and differ in age of UHP metamorphism (Norrbotten: ~490–480 Ma; Jämtland: ~460 Ma).

Laser ablation–split-stream analysis (LASS) was used to date zircon, monazite, and titanite from samples across the intervening area of Västerbotten. Only the southernmost samples show evidence for Caledonian metamorphism in zircon, with one meta-igneous sample in southernmost Västerbotten yielding a zircon age of 468.87 ± 0.56 Ma (n=123; MSWD 2.1). Th/U ratios and REE patterns suggest this age is associated with igneous growth of zircon, possibly due to partial melting during a post-(U)HP granulite-facies overprint, as seen further south in Jämtland. However, this age is significantly older, corroborating recently published ages of zircon and monazite from southern to south-central Västerbotten. A new monazite age from a garnet-kyanite schist in central Västerbotten of 468.52 ± 0.73 Ma (n=135; MSWD 2.5) extends this same age range northward. The monazite are not zoned and are found mainly throughout the rock matrix, with a few included in garnet rims. Thermobarometry of the sample indicates granulite-facies rim conditions, leading us to interpret this age also as the timing of granulite-facies metamorphism. Titanite from a sample in south-central Västerbotten yield older ages (~460 to 450 Ma) than titanite from a sample in southernmost Västerbotten, which yields an age of ~432 Ma, similar to exhumation ages from northern Jämtland. Taken together, these new dates indicate a distinct timing for events in Västerbotten, and may suggest a generally decreasing age of (U)HP subduction and exhumation from Norrbotten southward through northern Jämtland.