Volcanic evolution of oceanic crust in a Late Ordovician back‐arc basin: The Solund‐Stavfjord Ophiolite Complex, West Norway

Abstract

The stratigraphy of the well‐preserved Solund‐Stavfjord Ophiolite Complex in the West Norwegian Caledonides documents the volcanic evolution of a spreading center in a Late Ordovician back‐arc basin. Basaltic sheet flows, pillow lavas and volcanic breccias are the main components of the ∼470–800 m thick extrusive sequence, and are organized stratigraphically in a cyclic manner. Cyclic units vary in thickness from ∼5 m to 225 m and are typically composed of basal sheet flows or lava flows with large pillows that are succeeded by flows with progressively smaller pillows and volcanic breccias. Thick, independent breccia units also occur in the stratigraphy. In sheet‐flow dominated parts of the sequence the cyclic units are thicker (average 85 m) than in pillow‐dominated parts (average 20 m). Detailed logging of closely spaced profiles (∼1 km or less apart) shows that the proportions of sheet flows, pillow lavas and volcanic breccias varies laterally. Along an axial segment of less than 10 km, the volcanic products change from predominantly sheet flows, reflecting robust volcanic centers, to pillow lavas to volcanic breccias. Sheet‐flow dominated volcanic centers seem to be spaced at intervals of ∼25–30 km, and we tentatively interpret their regularity as an expression of volcanic segmentation at an intermediate‐ to fast spreading center. Observations of modern ocean crust suggest that sheet flows dominate at fast spreading ridges, while pillow lavas dominate at slow‐spreading ridges. Volcanic breccias are apparently rare in both of these environments. These features contrast with the stratigraphy of the Solund‐Stavfjord Ophiolite Complex, where the proportion of different volcanic products varies laterally and volcanic breccias are common. We emphasize the importance of detailed studies of the volcanic stratigraphy of ophiolites, as complements to those of in‐situ oceanic crust, in order to provide a more complete picture of volcanic evolution of oceanic crust in different spreading regimes.