The earliest (N-S to NE-SW striking) dykes and pillow lavas of the Lower Ordovician Gulfjellet Ophiolite Complex are dominated by N-MORB to IAT-type affinities; the IAT-type seems to become more abundant with time. This development is thought to reftect spreading-related magmatism in a 'supra-subduction' zone environment. Boninitic magmatism, represented by dominantly NE-SW to E-W trending dykes, post-dates the N-MORB and IAT, and suggests formation in a fore-are setting. The occurrence of a few MORB-type dykes cutting boninite dykes may indicate formation of new oceanie crust, possibly associated with are splitting. The rare occurrence of post-MORB calc-alkaline magmatic activity may indicate a maturing stage of are magmatic activity.

B. Heskestad, Amoco Norway Oil Company, P.O. Box 388, 4001 Stavanger, Norway;
N. H. Hofshagen, Den norske stats oljeselskap, a.s., P.O. Box 300, 4001 Stavanger, Norway;
H. Furnes & R.-B. Pedersen, Geologisk Institutt, Allégt. 41, 5007 Bergen, Norway