Primitive K-rich magmas from Clark volcano, southern Kermadec Arc: A paradox in the K - Depth relationship

Clark volcano is the last significant volcanic edifice in the oceanic segment of the nearly 3000-km-long Tonga - Kermadec - New Zealand volcanic arc system, which becomes continental in the Taupo Volcanic Zone of New Zealand. Clark volcano is a submarine basaltic andesite - dacite edifice with broadly similar petrochemical affinity to the rest of the Kermadec island arc (i.e., lavas are mostly of basalt - basaltic andesite composition, with ∼0.2% K2O at 50% SiO2, MgO in the range 5 to 7%, (CeTYb)n between 100 ppm) and Cr (>200 ppm), and 5 < (Ce/Yb)n < 7; these basalts are unique within the Kermadec - New Zealand subduction regime. Phenocryst assemblages in the K-rich basalts at Clark are olivine (± chromian spinel) + clinopyroxene, which contrast with the plagioclase + olivine ± clinopyroxene assemblages in "typical" Kermadec Arc basalts. In addition to high levels of K and the light rare-earth elements, the K-rich suite shows high concentrations of Ba (∼600 ppm), Rb (40 - 60 ppm) and Cs (∼1 ppm). Radiogenic isotopes of Sr, Nd and Pb in the K-rich basalts are enriched relative to other magmas of the oceanic Kermadec Arc, including those of the more "typical" Clark basalts, and overlap with those of basalts from the continental Taupo Volcanic Zone of New Zealand. We interpret these primitive K-rich lavas as a rare example of near-slab, small-volume melts of a mantle wedge source enriched by sediment and fluid transfer from the descending slab of lithosphere. The nature of this subducting slab is significant, because it comprises crust of the Hikurangi Plateau, an anomalously thick sequence of Cretaceous basalt basement and sedimentary basin fill, which is subducting beneath the present-day continental Taupo Volcanic Zone and the oceanic southern Kermadec Arc.