Precise ages and isotopic constraints for the Lewis Hills (Bay of Islands Ophiolite): Preservation of an arc-spreading ridge intersection

The Lewis Hills, the southernmost massif of the Bay of Islands Ophiolite, e xposes a prominent shear zone. The origin of this shear zone is pivotal wit h respect to the interpretation of the tectonic setting of the Lewis Hills and the genesis of the Bay of Islands Ophiolite. Different models suggest t hat the shear zone represents a fracture zone that separates either oceanic lithospheres of similar origin or the lithospheres of an island are and a marginal basin, A contrasting model disputes the fracture zone setting. New U-Pb zircon ages of the Lewis Hills combined with Sm-Nd systematics are us ed to resolve this controversy, In the western part of the Lewis Hills (Wes tern Lewis Hills), magmatic zircons from two trondhjemite bodies yield U-Pb ages of 500.6 +/- 2.0 Ma and 503.7 +/- 3.2 Ma. Initial epsilon(Nd(503 Ma)) values for trondhjemites and gabbros range from -1.5 to +2.0. Both the min eral ages and Nd signature are similar to those of the island-are-related L ittle port Complex, which is located to the north of the Western Lewis Hill s. In contrast, apatite from a gabbro of the eastern part of the Lewis Hill s (Eastern Lewis Hills) yields a concordant U-Pb age of 485.0 +/- 1.0 Ma. T his gabbro has an epsilon(Nd(485 Ma)) of +7.4. The age and isotope signatur e link the Eastern Lewis Hills with the northern marginal-basin-related mas sifs of the Bay of Islands Ophiolite. The significant differences of ages a nd isotope characteristics of the Western and Eastern Lewis Hills support t he interpretation of the shear zone as the major tectonic boundary between an island are and a marginal basin. This complex tectonic setting may have resulted in a wide spectrum of magma compositions that involved both island are and marginal basin sources.