Contrasting mantle sequences exposed in the Lewis Hills massif: Evidence for the early, arc-related history of the Bay of Islands ophiolite
A major fracture zone is preserved within the Lewis Hills massif (Bay of Islands ophiolite). It separates arc-type crust in the west from 17 m.y. younger, backarc-related oceanic crust in the east. We link mantle peridotites occurring in the Springers Hill locality to the arc crust and use their petrologic characteristics to define the early history of the arc. The Springers Hill mantle displays a weak shape fabric, high Cr/(Cr + Al) ratios in spinel (>60), very low, chondrite-normalized, heavy rare earth element abundances in clinopyroxene (HREE N < 1), and scattered calcic amphibole. It contains two important chromitite deposits and numerous pyroxenite dikes, some of which have a boninitic composition; and it is intruded by a late, large ultramafic body. The Springers Hill mantle peridotites are modeled by 26% melting of a primitive mantle fluxed by a slab melt or sediment (open-system melting with <1% total addition). The melting was outlasted by melt and fluid migration that caused middle rare earth element enrichment through chromatographic effects. This late infiltration event was fabric-destroying by causing orthopyroxene breakdown and recrystallization. In the absence of a clearly identifiable old basement for the arc, and given the extreme degree of depletion of the mantle peridotites underlying the arc, we interpret the Springers Hill mantle as related to upwelling and extension in an infant arc setting. Such a primitive arc setting is supported by strong geochemical similarities of the Springers Hill mantle to peridotites exposed in the Mariana and Bonin forearc systems. In a later event, the infant arc was rifted to give way to mantle upwelling, backarc formation, and associated final exhumation of the Springers Hill mantle within an extensional fracture-zone setting. The exhumation was probably associated with renewed melting at depth and melt and fluid migration in the Springers Hill mantle.