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, sequentially 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 ε(Nd)(T) = +6.3 to +7.5, and are incompatible element enriched, like basalts from geochemically anomalous ridge segments. Overlying stage 2 basalts (sheeted flows) show a narrow range of ε(Nd)(T) = +8.3 ± 0.2, with chemical characteristics of depleted back-arc basin basalts. These rocks are intruded by mafic to silicic stage 3 magmas, which have high uniform initial ¹⁴³Nd/¹⁴⁴Nd ratios (ε(Nd)(T) = +9.3 ± 0.2) over a wide range of ¹⁴⁷Sm/¹⁴⁴Nd 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 arc tholeiites (IAT) from immature arcs. They are closely analogous to some (boninite)-IAT magmas which characterise 'infant arc' eruptive activity in forearc basins of present-day Western Pacific island arc systems. A wide range of stage 3 magma compositions, ranging from near-primary basaltic dikes (Mg = 74) to extremely fractionated silicic plagiogranites with uniformly very depleted isotopic ratios, is consistent with slow spreading rates which gave rise to polybaric, closed-system fractionation of magmas and periodic chamber abandonment. Some stage 3 rocks with SiO₂ levels in the andesite range have low-TiO₂ contents and high Mg, and may be fractionated equivalents of boninites. High ε(Nd)(T) values of stage 3 magmas indicate a lack of subducted sediment with inherited crustal residence signatures, and reflect the extent of supra-subduction zone (SSZ) mantle wedge depletion. DMOB stage 3 magmas may represent forearc magmatism that was the precursor to normal subduction-related volcanism established by c. 265 Ma in the Brook Street Arc and derived from a SSZ mantle source with identical Nd-isotope characteristics. Less isotopically depleted stage 1 and 2 basalts (and rocks of the Lee River Group in general) may represent fragments of pre-existing mafic ocean crust sensu stricto, as suggested by significantly older Nd-isotopic ages for the basaltic seafloor volcanic component of the DMOB. Upon initiation of subduction, pre-existing geochemically anomalous (possibly back-arc basin) ocean crust (stage 1 and 2 magmas) was intruded by the 'infant arc' (stage 3) magmas, as proposed for older seafloor remnants in forearc regimes of the Izu-Bonin-Mariana and Tonga arc systems.