Arc and back-arc geochemistry in the southern Kermadec arc-Ngatoro Basin and offshore Taupo Volcanic Zone, SW Pacific

Back-arc basin basalts from the Ngatoro Basin (the southern end of the Havre Trough) are similar geochemically to, yet subtly distinct from, basalts of the Havre Trough to the north. Whole rock and glass chemistry are consistent with derivation from a fertile mantle source with subsequent evolution by fractionation of olivine (+ Cr-spinel) + plagioclase, and then clinopyroxene. Basalts from the vicinity of Rumble IV seamount at the southern end of the Kermadec island arc, and the eastern Ngatoro rift escarpment, are strongly porphyritic relative to the back-arc basin basalts and show trace element (high LIL abundances and highly depleted HFS abundances) and isotopic signatures of subduction zone basalts. At its southern end, the Ngatoro Basin penetrates the continental crust of New Zealand creating a major, 3000 m deep bathymetric re-entrant in the slope-break; the slope-break marks the transition from oceanic to continental crust. Basalts from the floor of the Ngatoro Basin re-entrant are isotopically distinct from the basalts of the oceanic sector in that they have higher Sr and correspondingly lower Nd isotope ratios and are comparable to basalts of the Taupo Volcanic Zone (TVZ) to the south. In contrast to the basalt-dominated oceanic sector, basalts from the offshore TVZ, a 100 km long area extending roughly NNE from White Island to the submarine Whakatane arc volcano at the edge of the continental slope-break, occur in association with andesites, dacites and rhyolites. These basalts are generally strongly porphyritic (olivine + plagioclase + clinopyroxene) and show trace element abundances typical of suprasubduction zone rocks. However, offshore TVZ basalts show subtle distinctions from onshore TVZ basalts to the south; the former have more radiogenic Sr isotopes. Furthermore, their high field strength element and transition element systematics appear to overlap with those of basalts from the Kermadec arc to the north. The authors attribute these lateral (along arc) and transverse (across arc) variations to source heterogeneity and variable fertility in the sources of the arc and back-arc basin magmas. Sources of the arc-front magmas are more refractory and also more susceptible to contamination by slab-derived fluids than sources for back-arc basin magmatism, reflecting the dynamic nature of flow from the back-arc into the mantle wedge beneath the volcanic front.