Reassessment of the origin of the Dun Mountain Ophiolite, New Zealand: Nd-isotopic and geochemical evolution of magma suites

Magmatic suites with contrasting isotopic and geochemical compositions, seq uentially emplaced in different tectonic regimes, comprise the Dun Mountain Ophiolite Belt (DMOB), New Zealand. At D'Urville Island, the northernmost exposure of the DMOB, earliest erupted (stage 1)pillow basalts have epsilon (Nd)(T) = +6.3 to +7.5, and are incompatible element enriched, like basalts from geochemically anomalous ridge segments. Overlying stage 2 basalts (sh eeted flows) show a narrow range of epsilon(Nd)(T) = +8.3 +/- 0.2, with che mical characteristics of depleted backarc basin basalts. These rocks are in truded by mafic to silicic stage 3 magmas, which have high uniform initial Nd-143/ Nd-144 ratios (epsilon(Nd)(T) = +9.3 +/- 0.2) over a wide range of Sm-147/Nd-144 values (yielding a precise Early Pemian Nd-isotope age of 278 +/- 4 Ma (MSWD = 0.48)). Stage 3 magmas show pronounced subduction-related geochemical signatures similar to island are tholeiites (IAT) from immatur e arcs. They are closely analogous to some (boninite)-IAT magmas which char acterise "infant are" eruptive activity in forearc basins of present-day We stern Pacific island are systems. A wide range of stage 3 magma composition s, ranging from near-primary basaltic dikes (Mg# = 74) to extremely fractio nated silicic plagiogranites with uniformly very depleted isotopic ratios, is consistent with slow spreading rates which gave rise to polybaric, close d-system fractionation of magmas and periodic chamber abandonment. Some sta ge 3 rocks with SiO2 levels in the andesite range have low-TiO2 contents an d high Mg#, and may be fractionated equivalents of boninites. High epsilon( Nd)(T) values of stage 3 magmas indicate a lack of subducted sediment with inherited crustal residence signatures, and reflect the extent of supra-sub duction zone (SSZ) mantle wedge depletion. DMOB stage 3 magmas may represen t forearc magmatism that was the precursor to normal subduction-related vol canism established by c. 265 Ma in the Brook Street Are and derived from a SSZ mantle source with identical Nd-isotope characteristics. Less isotopica lly depleted stage 1 and 2 basalts land rocks of the Lee River Group in gen eral) may represent fragments of pre-existing mafic ocean crust sensu stric to, as suggested by significantly older Nd-isotopic ages for the basaltic s eafloor volcanic component of the DMOB. Upon initiation of subduction, pre- existing geochemically anomalous (possibly back-are basin) ocean crust(stag e 1 and 2 magmas) was intruded by the "infant are" (stage 3) magmas, as pro posed for older seafloor remnants in forearc regimes of the Izu-Bonin-Maria na and Tonga are systems.