Caledonian metamorphism of metasediments from Franz Josef Land

The petrography, detrital zircon age distribution and ⁴⁰Ar–³⁹Ar ages of three samples from the Nagurskaya drill core of westernmost Franz Josef Land have been analyzed and are compared to similar rocks from the Lomonosov Ridge. The analyzed rocks, from near the base of the drill hole, consist of fine-grained psammitic to semipelitic schists, metamorphosed under greenschist-facies conditions. They are lithologically very similar to the recently analyzed metasediments from the Lomonosov Ridge. The detrital zircon age spectra of the samples from both the Lomonosov Ridge and beneath Franz Josef Land span the Meso- to Palaeoproterozoic with a main peak around 1.6 Ga, similar to lower Neoproterozoic metasedimentary rocks in East Greenland, on Svalbard and northern Norway, as well as from Cambrian sediments in Estonia and Silurian–Devonian sediments on Novaya Zemlya. Biotite in metasedimentary rocks from the Nagurskaya drill core indicate an Early Devonian ⁴⁰Ar–³⁹Ar metamorphic age of c. 400 Ma, possibly superimposed on an earlier, Ordovician history similar to that of the metasediments from the Lomonosov Ridge at 470 Ma. Previously published ⁴⁰Ar–³⁹Ar analyses of Nagurskaya muscovite yielded c. 600 Ma ages, characteristic of the Timanian Orogen. Together with the new biotite data, these isotope ages favor the interpretation that the Caledonian suture is located between Svalbard and Franz Josef Land, and the Caledonian deformation front between the latter and Novaya Zemlya.