Boninite at a continental margin

Abstract

A lava interbedded with Cretaceous sediments deposited on the continental margin of New Zealand has phenocrystic chromite, olivine, low-Ca and high-Ca pyroxenes with quench morphology, normative quartz, and relatively high MgO, Cr and Ni, but very low Ti and Zr; it is shown here to conform to the term bononite as defined by Cameron et al.¹. Such rocks are usually found in geologically oceanic situations, as in the Bonin, and Mariana arc–trench systems of the Western Pacific, or in ophiolites that represent uplifted, oceanic crust formed close to oceanic plate margins, particularly spreading centres, though not necessarily at mid-ocean ridges^2–6. The New Zealand boninite closely resembles certain ophiolitic lavas, yet its association is not ophiolitic, and this part of New Zealand never was, nor ever became, an oceanic spreading centre⁷. Coeval basalts are alkalic, not tholeiitic as is usual and the occurrence warns against the use of basalt petrochemistry as an indicator of tectonic environment in the absence of supporting geological evidence.