Discovery of diamond in the Tromsø Nappe, Scandinavian Caledonides (N. Norway)

We report the first finding of diamond in crustal rocks from the Tromsø Nappe of the North Norwegian Caledonides. Diamond occurs in situ as inclusions in garnet from gneiss at Tønsvika near Tromsø. The rock is composed essentially of garnet, biotite, white mica, quartz and plagioclase, minor constituents include kyanite, zoisite, rutile, tourmaline, amphibole, zircon, apatite and carbonates (magnesite, dolomite, calcite). The microdiamond, identified by micro‐Raman spectroscopy, is cuboidal to octahedral in shape and ranges from 5 to 50 μm in diameter. The diamond occurs as single grains and as composite diamond + carbonate inclusions. Diamond vibration bands show a downshift from 1 332 to 1 325 cm−1, the majority of Raman peaks are centred between 1 332 and 1 330 cm−1 and all peaks exhibit a full width at half maximum between 3 and 5 cm−1. Several spectra show Raman bands typical for disordered and ordered graphite (sp2‐bonded carbon) indicating partial transformation of diamond to graphite. The calculated peak P–T conditions for the diamond‐bearing sample are 3.5 ± 0.5 GPa and 770 ± 50 °C. Metamorphic diamond found in situ in crustal rocks of the Tromsø Nappe thus provides unequivocal evidence for ultrahigh pressure metamorphism in this allochthonous unit of the Scandinavian Caledonides. Deep continental subduction, most probably in the Late Ordovician and shortly before or during the initial collision between Baltica and Laurentia, was required to stabilize the diamond at UHP conditions.