Geochemical evidence for formation of the Bay of Islands ophiolite above a subduction zone

Abstract

THE Bay of Islands (BOI) complex in western Newfoundland is one of the world's best-known ophiolites, and has generally been interpreted as a fragment of ocean floor emplaced on the eastern margin of North America1,2. The similarities of the BOI to litho-sphere formed at mid-ocean ridges has led many investigators to use the BOI as an onland laboratory in which to study the geochemical, tectonic and geophysical properties of the oceanic lithosphere1–9. There have been, nonetheless, a few suggestions that the BOI ophiolite formed above a subduction zone10–12, as have most other ophiolites13–15. Here I report the existence of a substantial tantalum depletion in BOI magmas; Jenner et al.16 demonstrate that BOI magmas have a niobium depletion as well. These depletions are characteristic of magmas erupted above subduction zones but not at mid-ocean ridges, indicating that the BOI is not, after all, a good analogue for lithosphere formed at mid-ocean ridges.