The Isotope and Trace Element Budget of the Cambrian Devil River Arc System, New Zealand: Identification of Four Source Components

The Takaka Terrane in the South Island of New Zealand contains a well-preserved Cambrian arc system (Devil River Volcanics) that displays a complete assemblage of interbedded low- to high-K arc rocks, back-arc rocks and boninites. Most volcanic rocks are mafic. A coherent dataset was obtained including major elements, trace elements and Sr–Nd–Pb isotope compositions from clinopyroxene and amphibole separates. With time, 207Pb/204Pb in the arc rocks become more unradiogenic and 143Nd/144Nd more radiogenic, and Th/Yb and La/Yb increase. La/Yb values range from one in the boninites and back-arc rocks to 30 in the high-K arc rocks. Corresponding εNd values range from −5 to +6. High field strength element systematics rules out an OIB source component in the mantle wedge. The isotope variations in the volcanic rocks indicate addition of subducted sediments derived from Archaean sources to the mantle source of the magmas. Simple two-component mixing between depleted mantle wedge and melt derived from subducted sediment, however, cannot accommodate all observed trace element and isotope features. Pb–Nd isotope and La/Yb mixing models provide evidence for mixing of at least four components in the mantle sources of the Devil River Volcanics. The components include the mantle wedge, melts from subducted sediment, mid-ocean ridge basalt (MORB)-derived slab fluid, and MORB-derived slab melt. The concomitant increase in εNd, Th/Yb and La/Yb in the arc rocks through time reflects an increasing contribution from MORB-derived slab melts. Addition of up to 0·5% melt from subducted sediment, 3% MORB-derived slab fluid and 6% MORB-derived slab melt (all in wt %) to the mantle wedge is required to explain all magmatic compositions. Such mixing proportions are consistent with those obtained for recent arc systems.