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

Abstract

The Betts Cove Ophiolite, Newfoundland, Canada, records the INTRODUCTION initiation of seafloor spreading in an Ordovician marginal basin. The origin of the distinctive geochemical signature that Early lavas and sheeted dykes are composed of Low-Ti (<0·3 wt characterizes arc magmas is a contentious issue. Geo% TiO2) and Intermediate-Ti (0·3 to ~0·6 wt % TiO2) boninites. chemical and isotopic data imply that the depleted periThe boninites are overlain by arc tholeiites, and then by sequences dotites of the supra-subduction mantle wedge are of calc-alkaline pyroclastics and tholeiitic lavas. Results of trace refertilized by the influx of a slab-derived component element melting models suggest that the Betts Cove Low-Ti boninites (SZ component), which is either an aqueous fluid or a were extracted from a mantle source residual after 20–22% melting hydrous silicate melt enriched in incompatible trace of fertile mantle, subsequently refertilized with minor amounts elements such as Rb, Ba, Cs, Th, U, Pb, Sr and the light (<0·25%) of incompatible-element enriched components. Interrare earth elements (LREE) (Hawkesworth et al., 1993; mediate-Ti boninites were derived from a less depleted source Pearce & Parkinson, 1993; Arculus, 1994; Iwamori, (~12% previous melting), fluxed by similar fertile components. 1998). However, considerable uncertainty remains conThe composition of the source mantle for different end-member cerning the extent of prior wedge depletion, and with boninite magmas is calculated, allowing the composition of the regard to the composition and source of this slab-derived refertilizing components to be derived. The compositions of the SZ component. Boninites are thought to originate as refertilizing components are consistent with a mixture of fluid-mobile low-pressure partial melts of extremely depleted mantle elements derived from dehydration of the subducting oceanic crust, wedges (Cameron et al., 1979; Coish et al., 1982; Hickey by partial melting of that same crust, and by partial melting of & Frey, 1982; Kostopoulos & Murton, 1992; Sobolev & subducted sediments. The gradation from extremely incompatibleDanyushevsky, 1994; Taylor et al., 1994). As such, the element depleted boninites to less depleted boninitic and tholeiitic trace element signatures of the SZ component should magmas implies the progressive involvement of less depleted mantle stand out more clearly in boninites than in normal arc sources. This suggests a vertical compositional zonation of the magmas. This paper presents field, petrographic and mantle source, with less depleted mantle domains entering the wedge, perhaps in response to slab rollback and extension of the overriding whole-rock geochemical data for a suite of boninites from plate. the Ordovician Betts Cove ophiolite of Newfoundland, Canada (Fig. 1). The composition of the mantle sources of these boninites is calculated, allowing the degree of prior wedge depletion and the composition of the added SZ component to be modelled, so addressing some of the controversies related to arc magma genesis.