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TitlePetrogenesis of boninites from the Betts Cove Ophiolite, Newfoundland, Canada: identification of subducted source components
AuthorBédard, J H
SourceJournal of Petrology vol. 40, no. 12, 1999 p. 1853-1889, Open Access logo Open Access
Alt SeriesGeological Survey of Canada, Contribution Series 1999044
PublisherOxford University Press (OUP)
Mediapaper; on-line; digital
File formatpdf
ProvinceNewfoundland and Labrador
Lat/Long WENS -56.0000 -55.5000 50.0000 49.7500
Subjectsgeochemistry; igneous and metamorphic petrology; mineralogy; sedimentology; tectonics; ophiolites; sea floor spreading; basins; lavas; pyroclastics; tholeiites; mantle; partial melting; source areas; subduction zones; titanium; titanium geochemistry; Ordovician
Illustrationslocation maps; geological sketch maps; schematic diagrams; tables; geochemical plots; formulae; graphs
ProgramIndustrial Partners Program
AbstractThe 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 ~0·6 wt%TiO2) 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. Inter-mediate-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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