Basalt and Sediment Geochemistry and Magma Petrogenesis in a Transect from Oceanic Island Arc to Rifted Continental Margin Arc: the Kermadec--Hikurangi Margin, SW Pacific

Sediment mixing and recycling through a subduction zone canbe detected in lead isotopes and trace elements from basaltsand sediments from the Kermadec-Hikurangi Margin volcanic arcsystem and their coupled back-arc basins. Sr, Nd and Pb isotopesfrom the basalts delineate relatively simple, almost overlapping,arrays between back-arc basin basalts of the Havre Trough-NgatoroBasin (⁸⁷Sr/⁸⁶Sr = 0.70255; _Nd=+9.3; ²⁰⁶Pb/²⁰⁴Pb = 18.52; ²⁰⁸Pb/²⁰⁴Pb= 38.18), island arc basalts from the Kermadec Arc togetherwith basalts from Taupo Volcanic Zone (⁸⁷Sr/⁸⁶Sr 0.7042; _Nd= +5; ²⁰⁶Pb/²⁰⁴Pb= 18.81; ²⁰⁸Pb/²⁰⁴Pb = 38.61), and sedimentsderived from New Zealand's Mesozoic (Torlesse) basement (⁸⁷Sr/⁸⁶Sr 0.715; _Nd —4; ²⁰⁶Pb/²⁰⁴Pb 18.86; ²⁰⁸Pb/²⁰⁴Pb 38.8).Basalts from the arc front volcanoes have high Cs, Rb, Ba, Th,U and K, and generally high but variable Ba/La, Ba/Nb ratios,characteristic of subduction-related magmas, relative to typicaloceanic basalts. These signatures are diluted in the back-arcbasins, which are more like mid-ocean ridge basalts. Strongchemical correlations in plots of SiO₂ vs CaO and loss on ignitionfor the sediments (finegrained muds) are consistent with mixingbetween detrital and biogenic (carbonate-rich) components. Otherdata, such as Zr vs CaO, are consistent with the detrital componentcomprising a mixture of arc- and continent-derived fractions.In chondrite-normalized diagrams, most of the sediments havelight rare earth element enriched patterns, and all have negativeEu anomalies. The multielement diagrams have negative spikesat Nb, P and Ti and distinctive enrichments in the large ionlithophile elements and Pb relative to mantle. Isotopic measurementsof Pb, Sr and Nd reveal restricted fields of Pb isotopes butwide variation in Nd and Sr relative to other sediments fromthe Pacific Basin. Rare K-rich basalts from Clark Volcano towardthe southern end of the oceanic Kermadec Island Arc show unusualand primitive characteristics ( 2% K₂O at 50% SiO₂, Ba 600p.p.m., 9–10% MgO and Ni > 100 p.p.m.) but have highlyradiogenic Sr, Nd and Pb isotopes, similar to those of basaltsfrom the continental Taupo Volcanic Zone. These oceanic islandarc basalts cannot have inherited their isotope signatures throughcrustal contamination or assimilation—fractional crystallizationtype processes, and this leads us to conclude that source processesvia bulk sediment mixing, fluid and/or melt transfer or somecombination of these are responsible. Although our results showclear chemical gradients from oceanic island arc to continentalmargin arc settings (Kermadec Arc to Taupo Volcanic Zone), overlapbetween the data from the oceanic and continental sectors suggeststhat the lithospheric (crustal contamination) effect may beminimal relative to that of sediment subduction. Indeed, itis possible to account for the chemical changes by a decreasenorthward in the sediment flux into the zone of magma genesis.This model receives support from recent sediment dispersal studiesin the Southern Ocean which indicate that a strong bottom current(Deep Western Boundary Current) flows northward along the easterncontinental margin of New Zealand and sweeps continental derivedsediment into the sediment-starved oceanic trench system. Thetrace element and isotopic signatures of the continental derivedcomponent of this sediment are readily distinguished, but alsodiluted in a south to north direction along the plate boundary. KEY WORDS: subduction zone basalts; sediments; Sr-, Nd-, Pb-isotopes; trace elements ^*Present address: School of Earth Sciences, University of Melbourne, Parkville, Vic. 3052, Australia.