GEOCHEMICAL EVIDENCE FOR FORMATION OF THE BAY-OF-ISLANDS OPHIOLITE ABOVE A SUBDUCTION ZONE

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 America 1,2 . The similarities of the BOI to lithosphere 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 lithosphere 1-9. There have been, nonetheless, a few suggestions that the BOI ophiolite formed above a subduction zone 10-12, as have most other ophiolites 13-15. 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.