THE PETROGENESIS OF HIGH-CALCIUM BONINITE LAVAS DREDGED FROM THE NORTHERN TONGA RIDGE

In 1984 the research vessel 'Natsushima' dredged a fresh suite of MgO- and SiO2-rich lavas from the northern termination of the northern Tonga ridge. These lavas are high-Ca boninites and are characterized by the presence of magnesian olivine (up to Fo94), orthopyroxene, clinopyroxene, Cr-rich spinel and calcic plagioclase (> An90) phenocrysts. Boninite lavas from one dredge site, station 21, range in MgO contents from 3-15 wt% and their major element chemistry appears to be consistent with production of this suite via crystal fractionation. However, large variations in incompatible trace element ratios plus petrographic and mineral chemical evidence demonstrate that magma mixing has been an important process. The isotopic (Sr, Nd) composition of the Tongan high-Ca boninites suggest that their mantle sources are part of a regional OIB mantle domain upwelling beneath the Fiji-Lau-Tonga subduction zone system. The OIB mantle source to the Tongan boninites was of refractory lherzolite composition, depleted in 'basaltic' components by prior generation of Lau Basin back arc crust. The mantle sources of the Tongan high-Ca boninites have been enriched in incompatible elements by one or more metasomatic phases, suggested to be a hydrous fluid from the subducting lithospheric slab, a carbonatite melt and a small-degree silicate partial melt both derived from OIB mantle sources. There is no evidence for the involvement of sediment in the source of the Tonga boninites. The presence of the high-Ca boninite lavas at the northern end of the Tonga ridge can be explained by the presence of a northeast-directed spreading ridge in the northern part of the Lau Basin which is propagating into the north Tonga arc. Upwelling asthenospheric mantle beneath the spreading ridge may cause partial melting of refractory peridotite located in the mantle wedge above the subducting Pacific plate at shallow depths (< 25 km).