Geochronological and Geochemical Constraints on the Magmatic Evolution of the Dun Mountain Ophiolite Belt, New Zealand

Abstract

New whole-rock major and trace element geochemical, zircon U-Pb geochronological, and Hf isotopic data from gabbroic rocks in New Zealand’s mid-Permian Dun Mountain ophiolite belt (DMO) provide insight into the evolution of subduction systems and early stages of intraoceanic arc development. Fe-oxide-bearing gabbros yielded high εHf(t) values (+10.3 to +13) and zircon U-Pb ages of 271.6 ± 0.6 Ma. In contrast, Fe-Ti-oxide-bearing gabbros of 268.1 ± 0.6 Ma show more enriched geochemical characteristics, including a wide range of εHf(t) values (+15.5 to +6.8). New findings strengthen the evolutionary model for the DMO and place constraints on its youngest known magmatic episode. We infer that late magmatism fingerprinted by these gabbros, including consistent negative Nb-Ta anomalies, reflects early stages of arc development and formation of island arc tholeiites on the DMO. Our model is consistent with other existing regional geochronological and geochemical data, implying that the DMO had an early stage of normal-mid-ocean ridge basalt crustal accretion followed by an influx of slab-derived components and maturity of the subducting system between ca. 271.6 and 268 Ma. These results extend our understanding of the evolution of distinct intraoceanic systems.