NORTH TONGAN HIGH-CA BONINITE PETROGENESIS - THE ROLE OF SAMOAN PLUMEAND SUBDUCTION ZONE-TRANSFORM FAULT TRANSITION

High-Ca boninites are characterized by high CaO/Al2O3 values (> 0.75). In the North Tonga forearc, they occur close to the northern terminat ion of the trench, where it swings west into a major transform fault. Contemporaneous OIB-like lavas have been found close to the boninites on the forearc slope into the trench. The Tongan boninites are charact erized by: olivine up to Fo(94); a range of (La/Yb)(N) 0.4-13 in rocks with very similar major element compositions; isotopic composition si milar to Samoan OIB regardless of La/Yb values; and a correlation of H 2O content of primary melts with the degree of enrichment, the most de pleted varieties being most H2O enriched with H2O/K2O > 30. The most e nriched varieties have olivine up to Fo(93) and are identical to adjac ent OIB-like lavas in terms of incompatible element and isotopic ratio s. The primary Tongan high-Ca boninite melts had MgO contents similar to 24 wt% and very high temperatures (similar to 1470 degrees C at the depth of magma generation). We argue that the open edge of the mantle wedge beneath the transform fault has allowed the Samoan plume to int rude above the subducted slab. The intruding plume has been depleted b y extraction of OIB magmas, leaving a hot, dry, refractory peridotite residue. Slab-derived fluids react with the plume residue, producing b oninite primary magmas at 15-17 kbar. During ascent to the surface, so me of these magmas mix with earlier formed OIB-like melts, producing a range of variably-enriched boninite melts. As a result, more enriched boninites have lower H2O contents and Mg# values, but the same isotop ic composition as the more depleted boninites in this magma series.