We report new geochemical data for boninites and backarc basin‐type basalts recovered from the northern termination of the Tonga trench and Lau Basin. Boninitic pillow lavas, ranging from high‐Mg compositions to andesites and dacites, have been erupted within large submarine volcanic edifices (calderas and volcanoes) associated with active rifting of both the northern end of the Tofua volcanic arc and in a backarc position relative to the arc volcanoes on the northern Tonga Ridge. The mantle sources in the area are a complex mixture of (1) the “normal” Tongan mantle wedge source that has “Pacific”‐type isotopic signature with (2) the plume‐related components (EMI, EMII, and HIMU) and (3) an “Indian”‐type source upwelling beneath the backarc spreading. Some of these sources, such as the “normal” mantle wedge and variably depleted residual plume mantle, are fluxed by subduction components from the slab, which produces boninites, tholeiites, and mixtures thereof. Other mantle sources, such as “Indian”‐type backarc mantle and also some of the plume mantle, produce melts due to adiabatic decompression. These melts are variably mixed with each other and with the slab‐fluid fluxed subduction‐related melts to form the observed spectrum of magma compositions.

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