A relict blueschist in meta-ophiolite from the central Norwegian Caledonides—discovery and consequences

Blueschists in ophiolite sequences are meaningful indicators of the kinematics and thermal evolution of ancient subduction zones. Throughout the Caledonian–Appalachian orogen, blueschist occurrences in ophiolites of Early–Middle Ordovician age have been used to decipher the development and destruction of arc-subduction settings around the active Paleozoic Iapetus Ocean margin. We describe herein the first occurrence of glaucophane relics within an Early Ordovician meta-ophiolite sequence from the mainland Scandinavian Caledonides within the Trondheim Nappe Complex. The rock contains a relict blueschist assemblage of glaucophane, epidote, and rutile with pervasive retrograde overprint, first under amphibolite-, and subsequently, greenschist-facies conditions. The existence of blueschist in meta-ophiolite from mainland Scandinavia is consistent with the Ordovician intra-oceanic subduction and arc-continent interactions proposed for the Iapetus oceanic realm, generally, and outboard Baltica, in particular.