Sr, Nd and Pb isotopes from the basalts delineate relatively simple, almost overlapping, arrays between back-arc basin basalts of the Havre Trough - Ngatoro Basin (87Sr/86Sr = 0.70255; eNd =+9.3; 206Pb/204Pb = 18.52; 208Pb/204Pb = 38.18); island arc basalts from the Kermadec Arc together with basalts from Taupo Volcanic Zone (87Sr/86Sr ~ 0.7042; eNd = +5; 206Pb/204Pb = 18.81; 208Pb/204Pb = 38.61), and sediments derived from New Zealand's Mesozoic (Torlesse) basement (87Sr/86Sr ~ 0.715; eNd ~ -4; 206Pb/204Pb ~ 18.86; 208Pb/204Pb ~ 38.8). Basalts comprising the arc front volcanoes have high Cs, Rb, Ba, Th, U, K and generally high, but variable Ba/La, Ba/Nb ratios, characteristic of subduction related magmas, relative to typical oceanic basalts. These signatures are diluted in the back-arc basins, which are more MORB-like.
Strong chemical correlations in plots of SiO2 vs CaO and LOI for the sediments (fine-grained muds) are consistent with mixing between detrital and biogenic (carbonate rich) components. Other data, such as Zr vs CaO are consistent with the detrital component comprising a mix of arc and continent-derived fractions. In chondrite-normalised diagrams, most of the sediments have LREE- enriched patterns, and all have negative Eu-anomalies. The multielement diagrams have negative spikes at Nb, P and Ti and distinctive enrichments in the LIL elements and Pb relative to mantle. Isotopic measurements of Pb, Sr and Nd reveal restricted fields of Pb isotopes but wide variation in Nd and Sr relative to other sediments from the Pacific Basin.
Rare K-rich basalts from Clark Volcano toward the southern end of the oceanic Kermadec Island Arc show unusual and primitive characteristics (~2% K2O at 50% SiO2, Ba ~ 600 ppm; 9 - 10% MgO and Ni > 100 ppm) but have highly radiogenic Sr, Nd and Pb isotopes, similar to those of basalts from the continental Taupo Volcanic Zone. These oceanic island arc basalts cannot have inherited their isotope signatures through crustal contamination or AFC type processes, leading us to conclude that source processes via bulk sediment mixing, fluid and/or melt transfer or some combination of these are responsible.
Although our results show clear chemical gradients from oceanic island arc to continental margin arc settings (Kermadec Arc to Taupo Volcanic Zone), overlap between the data from the oceanic and continental sectors suggests that the lithospheric (crustal contamination) effect may be minimal relative to that of sediment subduction. Indeed, it is possible to account for the chemical changes by a decrease northward in the sediment flux into the zone of magma genesis. This model receives support from recent sediment dispersal studies in the Southern Ocean which indicate that a strong bottom current (Deep Western Boundary Current) flows northward along the eastern continental margin of New Zealand and sweeps continental derived sediment into the sediment- starved oceanic trench system. The trace element and isotopic signatures of the continental derived component of this sediment are readily distinguished, but also diluted in a south to north direction along the plate boundary.
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