PRIMITIVE K-RICH MAGMAS FROM CLARK-VOLCANO, SOUTHERN KERMADEC-ARC - APARADOX 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 volc anic are system, which becomes continental in the Taupo Volcanic Zone of New Zealand. Clark volcano is a submarine basaltic andesite - dacit e edifice with broadly similar petrochemical affinity to the rest of t he Kermadec island are (i.e., lavas are mostly of basalt - basaltic an desite composition, with similar to 0.2% K2O at 50% SiO2, MgO in the r ange 5 to 7%, (Ce/Yb)(n) between <1 and 2), but also present is a suit e of unusual K-rich basalts. The latter show similar to 2% K2O at 50% SiO2, similar to 9% MgO, high levels of Ni (>100 ppm) and Cr (>200 ppm ), and 5 < (Ce/Yb)(n) < 7; these basalts are unique within the Kermade c - New Zealand subduction regime. Phenocryst assemblages in the K-ric h basalts at Clark are olivine (+/- chromian spinel) + clinopyroxene, which contrast with the plagioclase + olivine +/- clinopyroxene assemb lages in ''typical'' Kermadec Are basalts. In addition to high levels of K and the light rare-earth elements, the K-rich suite shows high co ncentrations of Ba (similar to 600 ppm), Rb (40 - 60 ppm) and Cs (simi lar to 1 ppm). Radiogenic isotopes of Sr, Nd and Pb in the K-rich basa lts are enriched relative to other magmas of the oceanic Kermadec Arc, including those of the more ''typical'' Clark basalts, and overlap wi th those of basalts from the continental Taupo Volcanic Zone of New Ze aland. We interpret these primitive K-rich lavas as a rare example of near-slab, small-volume melts of a mantle wedge source enriched by sed iment and fluid transfer from the descending slab of lithosphere. The nature of this subducting slab is significant, because it comprises cr ust of the Hikurangi Plateau, an anomalously thick sequence of Cretace ous basalt basement and sedimentary basin fill, which is subducting be neath the present-day continental Taupo Volcanic Zone and the oceanic southern Kermadec Arc.