Petrogenesis of boninites from the Betts Cove Ophiolite, Newfoundland, Canada:: Identification of subducted source components

The Betts Cove Ophiolite, Newfoundland, Canada, records the initiation of seafloor spreading in an Ordovician marginal basin. Early lavas and sheeted dykes are composed of Low-Ti (<0.3 wt% TiO2) and Intermediate-Ti (0.3 to similar to 0.6 wt% TiO2) boninites. The boninites are overlain by arc theoliites, and then by sequence of calc-alkaline pyroclastics and tholeiitic lavas. Results of trace element melting models suggest that the Betts Cove Low-Ti boninites were extracted from a mantle source residual after 20-22% melting of fertile mantle, subsequently refertilized with minor amounts (<0.25%) of incompatible-element enriched components. Intermediate-Ti boninites were derived from a less depleted source (similar to 12% previous melting), fluxed by similar fertile components. The composition of the source mantle for different end-member boninite magmas is calculated, allowing the composition if the refertilizing components to be derived. The compositions of the refertilizing components are consistent with a mixture of fluid-mobile elements derived from dehydration of the subducting oceanic crust, by partial melting of that same crust, and by partial melting og subducted elements. The gradation from extremely compatible-element depleted boninites to less depleted boninitic and tholeiitic magmas implies the progressive involvement of less depleted mantle sources. This suggests a vertical compositional zonation of the mantle source, with less depleted mantle domains entering the wedge, perhaps in response to slab rollback and extension of the overriding plate.