Summary In the Caledonides of central Norway, volcano-sedimentary successions, of the Lower and Upper Hovin and Horg Groups (Ordovician to? Lower Silurian), unconformably overlie a variable substrate which includes fragmented oceanic crust and ensimatic immature arc assemblages initially deformed and metamorphosed in earliest Ordovician times. In western areas, remnants of a mid-Ordovician, evolved island-arc complex comprise a volcanic-plutonic suite of clear calc-alkaline affinity. To the E, in the Trondheim district, ...