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Journal of Petrology, Volume 40, Issue 12: December 1999.
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 % TiOCommented out Element2Commented out Element) and Intermediate-Ti (0·3 to ~0·6 wt % TiOCommented out Element2Commented out Element) boninites. The boninites are overlain by arc tholeiites, and then by sequences 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 (~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 of 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 of subducted sediments. The gradation from extremely incompatible-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.
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Pages 1853-1889
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