U-Pb dating of prograde and retrograde titanite growth during the Scandian orogeny

Abstract

U/Pb ages of metamorphic titanite from high-grade gneisses of the Nasafjallet window in the north-central Scandinavian Caledonides record growth over an ∼20 m.y. interval during the Late Silurian to Early Devonian. Biotite-amphibolite gneisses located near the basement-cover boundary contain metamorphic titanite in two microtextural settings, one as lenticular grains distributed throughout a recrystallized matrix of metamorphic phases, and one as plate-like grains enclosed within biotite grains. These distinct morphological populations exhibit core-overgrowth relationships with compositionally similar cores and overgrowths, but the plate-like titanite grains have a much higher proportion of overgrowth and give slightly younger ages. Cores and overgrowths in both types are inferred to have formed along the prograde (ca. 413 Ma) and retrograde (ca. 395 Ma) paths, respectively, of a single metamorphic cycle associated with the burial and exhumation of a slice of Baltic basement during the Caledonide orogeny (Baltica-Laurentia collision). The existence of polygenic U/Pb systematics in metamorphic titanite demonstrates the potential usefulness of titanite to reveal multiple metamorphic and deformational episodes, and to constrain the duration of metamorphic events.