Geochemistry and Nd‐isotope systematics of chemical and terrigenous sediments from the Dun Mountain Ophiolite, New Zealand

Abstract

Two sedimentary associations closely related to temporally discrete ophiolitic magma suites occur within the Early Permian Dun Mountain Ophiolite Belt (DMOB), New Zealand. These are: (1) a suite of diverse chemical sediments and turbidite argillites (TA) that bear an intimate depositional relationship to early‐formed pillow lavas (back‐arc basin basalts); and (2) a younger, lithic‐dominated, bimodal, coarse sandstone‐ophiolitic rudite assemblage of proximal turbidite/mass‐flow origin, rich in clasts of infant‐arc magmas which comprise the bulk of the ophiolite. There are four groups of DMOB chemical sediments. Red hematitic chert (group 1) fills interstices between basalt pillows, and black nodular Fe‐Mn deposits (group 2) occur along pillow lava/sediment interfaces. These facies are overlain by red mudstones (group 3), and mottled orange‐olive brown mudstones (altered hyaloclastites; group 4). Geochemical features (including REE contents, Fe/Ti and transition metal ratios) indicate that the cherts reflect silica and metalliferous contributions to sea water, promoted by low‐temperature hydrothermal alteration of glassy basalt, while group 4 muds represent residual components in halmyrolytically altered volcanic glass. Transition metal and REE enrichments in group 2 nodules (with high Ce/Ce*, Ni/Fe, and Cu/Fe ratios) reflect hydrogenous chemisorption to a hydothermal component (with high Ba and Sr). The nodules possess ϵ Nd(T) values (c. 0) identical to those calculated for Permian sea water. Group 3 red muds have lower ϵNd(T) = ‐1 to ‐2, and Nd model ages (T NdDM) that indicate contributions from continentally derived fluvial particulate fallout of mean Proterozoic age. For the nodules and red muds, strong negative correlations between Mn/Fe, Nd, Ce*/Ce, and ϵNd(T) are attributed to increasing diagenetic influence in the muds. ϵNd(T) values (c. +2) in group 4 muds are transitional toward higher values in their (hyaloclastite) basalt glass precursors. Metalliferous contributions to red and green TA in the overlying terrigenous sedimentary sequence also link these facies to early DMOB back‐arc eruptives. The red and green TA pass upwards into ungraded Atomodesmid‐bearing grey TA, implying turbidite deposition in a shallowing marine environment. The (mainly andesitic) TA show systematic trends of decreasing Eu/Eu* and ϵNd(T) with increasing SREE, La/Y, Th/Sc, and SiO2. Th abundances and Th/Sc ratios (up to 1.6) in the TA are akin to those of calc‐alkaline magmas in continental arcs, and are significantly greater than island‐arc or ophiolitic volcanics. A narrow range of positive ϵNd(T) values (+0.5 to +2.0) for the TA suggests a young differentiated continental arc source, less dissected than the quartzofeldspathic plutonic provenance for the Torlesse (ϵNd(T) ?2). The exclusively terrigenous, younger DMOB sedimentary suite is compositionally bimodal, with coarse mafic sandstones rich in clinopyroxenes identical to those in DMOB infant‐arc magmas, and felsic, matrix‐supported breccias that contain abundant clasts of infant‐arc rocks, including silicic plagiogranites and rare ultramafics. Low Th/Sc ratios (<0.3), and slight LREE enrichment (CeN/YbN= 2.5–2.8) in the sandstones resemble island‐arc tholeiites (IAT). The breccias have low Eu/Eu* (c. 0.72), CeN/YbN = 1.6, Th/Sc (0.1–0.2), and TbN/YbN c. 1.0, features transitional between the sandstones and DMOB plagiogranites. Importantly, the sandstones and breccias show high positive initial Nd ratios (ϵNd(T) = +5 to +8) akin to values for Brook Street IAT (ϵNd(T) = +9). This implies juvenile (mantle‐derived) source rocks for both the mafic and felsic younger terrigenous sediments, compatible with crustal residence (mean provenance) ages less than for the TA. Little continental input is permitted by Nd‐isotopic data for these rocks. Whereas the early DMOB pillow basalt ‐chemical sediment ‐ TA assemblage was emplaced in a back‐arc setting, with influx of detritus in part from an active continental margin (possibly within the New England Orogen), the younger DMOB terrigenous sediment association shows a close kinship with bimodal DMOB magmatism related to infant arc volcanic centres in an extensional forearc regime. Turbidite sands and mass‐flow deposits were shed into proximal fault‐bound basins isolated from continental clastic sources. The data are compatible with a recently reassessed model for the tectonomagmatic evolution of the DMOB, involving initial back‐arc seafloor generation, and later voluminous extensional magmatism during nascent subduction, probably related to initiation of the Brook Street arc.