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Journal of Petrology | Volume 40 | Number 12 | Pages 1853-1889 | 1999
© Oxford University Press 1999

Petrogenesis of Boninites from the Betts Cove Ophiolite, Newfoundland, Canada: Identification of Subducted Source Components

J. H. Bédard*

Geological Survey of Canada, Centre GéOscientifique De QuéBec Cp 7500, Ste-Foy, Que.,Canada, G1V 4C7

Received March 9, 1998; Revised typescript accepted May 27, 1999


  Abstract

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.

KEY WORDS: boninite; mantle; seafloor spreading; sediment; subduction

* e-mail: jbedard{at}nrcan.gc.ca


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