Geochemical evolution and tectonic significance of boninites and tholeiites from the Koh ophiolite, New Caledonia

The Central Chain ophiolites in New Caledonia are fragments of a supra-subduction zone (SSZ) ophiolite, now preserved from the upper layered gabbros through to volcanics and overlain by pelagic cherts and a thick Middle Triassic to Upper Jurassic volcaniclastic sequence. Most of the fragments were formed by a single tholeiitic magmatic episode, but one of these, the Koh ophiolite, was formed by two tholeiitic magmatic episodes separated by boninites. The first event in the Koh ophiolite formed cumulate gabbros, dolerites, plagiogranites, and the first pillow lava sequence from a tholeiitic magma with strong depletion in the light rare earth elements (LREE) and abnormally low TiO2 (0.5% at Mg#=60). Shortly after their eruption, these tholeiitic lavas were overlain by a high-Ca boninitic unit with a basal section of boninite pillows, flows, and breccias and an upper section of boninitic dacites and tuffs. The last magmatic phase involved eruption of evolved tholeiitic basalts, as pillows above the boninites and as dykes and sills intruding the older plutonic and volcanic sections of the ophiolite. This second phase of tholeiitic magmatism is compositionally distinct from the first and is closest to back arc basin basalts (BABB) erupted during the early rifting history of modern back arc basins. The boninitic volcanics belong to a high-Ca series with slightly lower SiO2, Al2O3, and TiO2 compared to those from modern island arc systems, and they lack the positive Zr spike relative to adjacent rare earth elements (REE) in normalised element variation patterns. These boninites were formed shortly after the production of back arc basin crust represented by the depleted tholeiites and shortly before a second spreading event which caused 40-60% extension of the initial basin crust and eruption of the upper tholeiites. The dominance of BABB-like tholeiites throughout the Central Chain ophiolites in New Caledonia, the restricted occurrence of boninites, and the stratigraphy and chemistry of the Koh ophiolite suggest that the boninites were erupted in response to an exceptional tectonic situation. We suggest that this boninite generation event was driven by adiabatic decompression of hot depleted mantle residual from the production of the lower tholeiites, during initiation of rifting of young oceanic crust intimately associated with propagation of a back arc basin spreading centre. The occurrence of a thick blanket of calc-alkaline volcaniclastic sediments above the ophiolite indicates proximity to a mature arc and suggests that the Koh boninites were not associated with the initiation of subduction. A close modern analogy for the Koh ophiolite exists on the Hunter Ridge protoisland arc between southernmost Vanuatu (New Hebrides island arc) and the Fijian islands; there, high-Ca boninites lacking positive Zr spikes occur together with low-Ti tholeiites and more typical BABB tholeiites where the southern spreading centre of the North Fiji Basin is propagating into the protoarc crust of the Hunter Ridge.